Hard-Chrome Coatings: Advanced Technology for

Advanced Coatings Technology Group
Northwestern University
BIRL
1801 Maple Avenue
Evanston, IL 60201
Defense Advanced Research Projects Agency
October 28,1997
Hard-Chrome Coatings:
Advanced Technology
for Waste Elimination
Final Report
Grant #MDA972-93-1-0006
Team: BIRL, Corpus Christi Army Depot, Hill AFB,
Cummins Piston Ring Division, GE Aircraft Engines, Dover Plating
Principal Investigator and Technical POC:
Keith O. Legg
BIRL, Northwestern University
Phone: (847) 467-1572
Fax:
(847) 467-1022
e-mail: kdlegg@ nwu.edu
Contributors: Jerry Schell (GE Aircraft Engines), Freidoon Rastegar (Cummins),
George Nichols, Robert Altkorn, Peter Chang (BIRL, Northwestern University)
i
ABSTRACT
Electrolytic hard chrome (EHC) is used in DoD both as a protective coating to reduce wear and fretting in original
components and for rebuilding worn components in maintenance depots. In this project we have evaluated the
relative merits of HVOF, PVD, and laser coating compared with EHC. The work done under this program has shown
that DoD could almost completely eliminate the Cr6+ waste produced by EHC operations by replacing EHC at the
OEM level for original components and at the depot level for component rebuilding. At the OEM level thick thermal
spray coatings (primarily high velocity oxy -fuel, HVOF) are excellent replacements for many applications, especially
on large components or components where only a small portion of the item is to be coated. Thin, high-performance
vacuum physical vapor deposited (PVD) coatings are useful for small components, especially where extreme
hardness and wear resistance is important or where only small changes are permissable in tolerance and surface
finish. At the depot maintenance level, where thick coatings are needed for rebuilding, EHC can be replaced with
HVOF and other thermal sprays. HVOF is commercially available from several equipment makers and many job
shops, and is consistent with existing DoD maintenance procedures, which commonly use other thermal sprays such
as flame and plasma spray. Depot electrolytic chrome cannot, however, be replaced by thin coating processes such
as PVD.
HVOF coatings are found to offer superior performance and processing costs from 50% to 150% that of EHC. Diesel
engine tests and cost calculations for HVOF replacements on piston rings, for example, have shown that not only do
these coatings cost about half as much to deposit, but perform better than hard chrome, giving them a far lower life
cycle cost. Furthermore HVOF coatings use only about 20% of the factory floor space. When applied to high
strength steels such as those used in hydraulics and landing gear, HVOF avoids the problems of hydrogen
embrittlement because is not an electrolytic process. Consequently it avoids the need for the post-deposition heat
treatment that is essential after chrome plating. This reduces production and maintenance time and cost.
For piston rings, a combination of plasma nitriding of the underlying steel, with PVD CrN coating offers a costeffective alternative to chrome plating, whose cost is similar and whose performance in general matches that of a
production Japanese product currently on the market.
Laser cladding and laser CVD have been shown to be unsafe for depot use. They work well at the OEM level, but
process control is extremely critical, and the result of surface overheating is loss of critical surface temper and
hardness. Consequently, we conclude that laser coating is not a suitable replacement technology for depot use.
Given DoD’s widespread reliance on EHC technology, it will realistically be many years before EHC can be
completely eliminated. Our evaluation and development of methods for cleaning up the current EHC technology
shows that a combination of recyclable stripping, surfactant fume suppression, bath chemistry monitoring and
control, and new anode materials could reduce the most concentrated EHC waste streams (air emissions and bath
solids) by more than 90%, while reducing the use of acids, bases, and water by 25 - 50%.
ii
Table of Contents
Abstract ..............................................................................................................................................................................................ii
Introduction .......................................................................................................................................................................................1
Section 1 .............................................................................................................................................................................................1
Hard Chrome Replacement...............................................................................................................................................................1
1.
Choice of Chrome Replacement Technologies for DoD Use...........................................................................................1
1.1.
Technical requirements for chrome replacements...................................................................................................1
1.2.
The growing use of HVOF for chrome replacement ...............................................................................................3
2.
Alternative Approaches and their Shortfalls .....................................................................................................................3
3.
Results - hard chrome replacement coatings......................................................................................................................4
3.1.
Summary of results ......................................................................................................................................................5
3.2.
Coating methods..........................................................................................................................................................8
3.2.1.
HVOF coating method ...........................................................................................................................................8
3.2.2.
PVD coating method ..............................................................................................................................................9
3.2.3.
Laser coatings - deposition methods ................................................................................................................10
3.3.
Coating performance .................................................................................................................................................11
3.3.1.
HVOF coating structure and mechanical properties .......................................................................................11
3.3.2.
PVD coating - structure and mechanical properties ........................................................................................12
3.3.3.
Laser coating - structure and mechanical properties ......................................................................................13
3.3.4.
Laser coating - applicability to OEM and maintenance operations..............................................................15
3.3.5.
HVOF coating fatigue [turbines, diesels, general aerospace]........................................................................16
3.3.6.
HVOF coating - compressive creep tests [turbines].......................................................................................16
3.3.7.
HVOF and PVD coating scuffing [diesels, general aerospace].....................................................................17
3.3.8.
PVD coating wear - piston rings [diesels, general aerospace].......................................................................18
3.3.9.
Comparative fretting wear of hard chrome, HVOF and PVD coatings [turbines].......................................19
3.3.10.
4.
3.4.
Waste streams from hard chrome and alternative coatings ................................................................................22
3.5.
Demonstration HVOF and PVD coatings on DoD maintenance components..................................................22
3.6.
Costs of alternative coating methods.....................................................................................................................24
Recommendations ................................................................................................................................................................29
4.1.
5.
HVOF and PVD coatings for piston rings - diesel engine tests [diesels]...............................................20
Hard chrome replacement .........................................................................................................................................29
Bath Chemistry Analysis .....................................................................................................................................................34
5.1.
Introduction................................................................................................................................................................34
5.2.
Background.................................................................................................................................................................35
iii
5.3.
Copper.........................................................................................................................................................................36
5.3.1.
Direct Near-Infrared Determination of Copper.................................................................................................36
5.3.2.
Determination of Copper Using NaBr/HBr Method........................................................................................40
5.4.
Iron...............................................................................................................................................................................43
5.4.1.
Introduction...........................................................................................................................................................43
5.4.2.
Development of Analytical Procedure ..............................................................................................................45
5.4.3.
Automated Iron Analyzer....................................................................................................................................48
6.
Mist Minimization.................................................................................................................................................................51
7.
Stripping.................................................................................................................................................................................53
8.
Alternative electrode materials ...........................................................................................................................................55
9.
References .............................................................................................................................................................................56
Appendix 1. GEAE Report............................................................................................................................................................58
Hard Chrome Coatings: Advanced Technology for Waste Elimination................................................................................58
10.
Materials/Processes For Replacement of Electroplated Hard Chrome (EHC)........................................................59
10.1.
HVOF WC-Co .............................................................................................................................................................59
10.2.
HVOF Triballoy 400 ...................................................................................................................................................59
10.3.
Laser Clad Triballoy 400............................................................................................................................................59
10.4.
Unbalanced Magnetron Sputtered Chrome Nitride (CrN) ...................................................................................60
10.5.
Plasma Nitrided Substrates + Chrome Nitride (PN+CrN) .....................................................................................60
10.6.
Cathodic Arc Titanium Aluminum Nitrides [(Ti,Al)N] .........................................................................................60
10.7.
Electroplated Hard Chrome (EHC) Baseline:..........................................................................................................60
11.
HVOF Coating Process Studies ....................................................................................................................................60
11.1.
General Description ...................................................................................................................................................60
11.2.
Coating Process Response Measurements ...........................................................................................................61
11.2.1.
Coating Deposition Rate................................................................................................................................61
11.2.2.
DPH Microhardness .......................................................................................................................................61
11.2.3.
4.2.3 Rockwell 15N superficial Surface Hardness.......................................................................................61
11.2.4.
Tensile Bond Strength....................................................................................................................................62
11.2.5.
Substrate Temperature ...................................................................................................................................62
11.2.6.
Almen Strip Deflection ...................................................................................................................................62
11.3.
WC-17Co Process Development .............................................................................................................................64
11.4.
Triballoy 400 HVOF Process Development............................................................................................................73
11.5.
DOE Confirmation Runs............................................................................................................................................78
12.
Other Coating Properties Tests and Results...............................................................................................................79
12.1.
CSEM ReveTest Scratch Adhesion ........................................................................................................................79
12.2.
Fretting Wear Tests...................................................................................................................................................85
iv
12.3.
Fatigue Tests ..............................................................................................................................................................90
12.4.
Chrome and HYOF Coatings Compressive Creep Tests......................................................................................93
12.5.
Selected Component Demonstration Trials for HVOF Coatings ........................................................................97
List of Tables
Table 1. Typical requirements for hard chrome replacement for various applications....................................................... 2
Table 2. Technologies and materials evaluated in the DARPA program. ............................................................................ 5
Table 3. Summary of alternative coating evaluations.............................................................................................................. 7
Table 4. Typical PVD deposition conditions.......................................................................................................................... 10
Table 5. HVOF coating properties obtained in confirmation runs....................................................................................... 11
Table 6. Chemical composition (wt %) of T400 coatings, powder, and IN718................................................................... 15
Table 7. Waste streams from coating processes.................................................................................................................... 22
Table 8. Surfactant additions to EHC tanks............................................................................................................................ 52
Table 9. Operating conditions for stripping............................................................................................................................ 54
Table 10. Matrix for Site A HVOF WC-Co DOE...................................................................................................................... 63
Table 11. Site A HVOF WC-Co DOE average responses...................................................................................................... 64
Table 12. Typical statistical analysis table for Site A HVOF WC-Co Almen data. ........................................................... 66
Table 13. Site A WC-Co DOE statistical analysis summary................................................................................................. 67
Table 14. Matrix for Site B HVOF WC-Co DOE. ..................................................................................................................... 69
Table 15. Site B HVOF WC-Co DOE average responses...................................................................................................... 70
Table 16. Site B WC-Co DOE statistical analysis summary.................................................................................................. 71
Table 17. Matrix for HVOF Triballoy 400 DOE. ....................................................................................................................... 74
Table 18. HVOF Triballoy 400 DOE average responses........................................................................................................ 75
Table 19. Triballoy 400 DOE statistical analyses summary................................................................................................... 77
Table 20. confirmation runs data............................................................................................................................................... 78
Table 21. Fretting wear test conditions and results............................................................................................................... 87
Table 22. Fatigue test results..................................................................................................................................................... 91
Table 23. Compressive creep test results for coatings.......................................................................................................... 94
Table 24. Hard chrome replacement demonstration components........................................................................................ 97
v
List of Figures
Figure 1. Schematic of HVOF coating method.......................................................................................................................... 8
Figure 2. HVOF spraying of a landing gear inner cylinder. (Courtesy Southwest Aeroservice)...................................... 8
Figure 3. Schematic of PVD coating system (sputtering)........................................................................................................ 9
Figure 4. The Hauzer ABS production PVD coating system. Chamber is approx. 1m cube.............................................. 9
Figure 5. Laser cladding of a tube............................................................................................................................................. 10
Figure 6. Cross section of HVOF Cr3C2-NiCr coating........................................................................................................... 11
Figure 7. PVD CrN surface at 5000x. ......................................................................................................................................... 12
Figure 8. Hardness and structure of PVD CrN vs N2 partial pressure................................................................................. 12
Figure 9. Hardness vs depth profiles of plasma nitrided metals (PVD chamber nitriding)............................................... 12
Figure 10. Hardness vs. depth profile for duplex plasma nitride/PVD CrN coating on ASL81 stainless steel.............. 13
Figure 11. Cross section of laser clad steel. ............................................................................................................................ 14
Figure 12. Detail of heat-affected zone..................................................................................................................................... 14
Figure 13. Depth profile of hardness and cross section of laser clad T400 on IN718, showing limited mixing............ 14
Figure 14. Hardness and cross-section of IN718 laser clad with T400, showing interface mixing.................................. 14
Figure 15. Fatigue performance of HVOF coated vs. chrome plated and uncoated Inconel. .......................................... 16
Figure 16. Compressive creep of HVOF and EHC coatings - 426C, 50 and 100 ksi load................................................... 16
Figure 17. Piston ring scuffing apparatus................................................................................................................................ 17
Figure 18. Friction and load scuffing test output................................................................................................................... 17
Figure 19. Scuffing of various coated rings against gray iron............................................................................................. 18
Figure 20. Wear coefficients for different PVD coating designs vs EHC. .......................................................................... 18
Figure 21. Fretting wear coefficients for various coatings.................................................................................................... 20
Figure 22. Wear in individual cylinders during 500 hr abuse test........................................................................................ 21
Figure 23. Ring and liner wear in 500 hr abuse test................................................................................................................ 21
Figure 24. Wear of EHC and CrN coated rings - 350 thermal cycling engine test............................................................. 21
Figure 25. UH60 helicopter tail landing gear fork (7175 aluminum) - as sprayed............................................................... 22
Figure 26. T700 helicopter engine power turbine shaft - IN718 (OEM GEAE)................................................................... 23
Figure 27. T700 helicopter engine #4 bearing support, as sprayed - AM355 stainless steel (OEM GEAE)................. 23
Figure 28. CH47D helicopter swivelling hydraulic rod - 4340 steel...................................................................................... 24
Figure 29. Costs of producing a stack of piston rings with different coatings.................................................................. 25
Figure 30. Total costs for coating and finis hing a stack of rings, broken down by cost category................................ 25
Figure 31. Cost breakdown for a stack of piston rings - standard chrome plate. .............................................................. 26
Figure 32. Cost breakdown for a stack of piston rings - HVOF coating............................................................................. 26
Figure 33. Cost breakdown for a stack of piston rings - duplex PVD coating, 3 micron thick sputtered....................... 27
Figure 34. Cost breakdown for a stack of piston rings - PVD coating, 15 micron thick sputtered.................................. 27
vi
Figure 35. Costs of coating 1" diameter hydraulic pistons of two different lengths........................................................ 28
Figure 36. Basic steps needed to bring hard chrome alternative coatings to use at OEMs and maintenance depots.30
Figure 37. Results of bath chemistry evaluations.................................................................................................................. 32
Figure 38. Waste streams from EHC process by applying various waste stream reduction methods........................... 33
Figure 39. New Chrome Plating Solution at Various Levels of Dilution in Water. ............................................................ 35
Figure 40. Spectrum of aqueous copper sulfate solution containing 2500 mg/l copper metal in 1-cm pathlength
cuvette. ................................................................................................................................................................................. 37
Figure 41. Comparison of copper concentrations determined by direct absorbance at 800 nm with those of two
independent laboratories using ICP................................................................................................................................. 38
Figure 42. Schematic of optical system. ................................................................................................................................... 38
Figure 43. Schematic of LED and detector/amplifier circuits................................................................................................ 39
Figure 44. Results of copper measured using LED-based photometer compared with determinations by TEI
Analytical and Northwestern’s Analytical Services Lab.............................................................................................. 40
Figure 45. Experimental Apparatus Used to Determine Copper by Bromide Method...................................................... 41
Figure 46. Spectrum of Plating Solution Containing 1.57 g/l Copper Mixed with NaBr/HBr Solution........................... 42
Figure 47. Comparison of Copper Concentrations Determined by NaBr/HBr method with those of two independent
laboratories using ICP. ....................................................................................................................................................... 43
Figure 48. Spectrum of 0.06 g/l Fe(NO3)3.9H2O (1.48 x 10-4M) in 1-cm cuvette. The molar extinction coefficient is
approximately 2.7 x 103 at 300 nm. ..................................................................................................................................... 44
Figure 49. Iron - thiocyanate complex. The peak absorbance occurs at 473 nm. As discussed in the text, the
extinction coefficient decreases with time....................................................................................................................... 45
Figure 50. Schematic of thiocyanate analytical procedure.................................................................................................... 45
Figure 51. Spectra of iron-thiocyanate complex in chromium plating solution.................................................................. 46
Figure 52. Comparison of Iron Concentrations Determined by BIRL/KSCN method with those of two independent
laboratories using ICP. TEI is TEI Analytical, Niles IL, and ASL is the Analytical Services Laboratory at
Northwestern University.................................................................................................................................................... 48
Figure 53. Schematic of Iron Analyzer. .................................................................................................................................... 49
Figure 54. Spectrum of light emitted by LED/photodiode..................................................................................................... 50
Figure 55. Spectra of iron-free plating solution mixed with KSCN solutions in 2-mm pathlength cell. Trace A is
plating solution diluted with water (1:55). Traces B and C are plating solutions diluted with 5% and 10% KSCN
solutions, respectively. Trace D is plating solution diluted with 10% KSCN solution, pump running at half
speed..................................................................................................................................................................................... 51
Figure 56. SEM photos of scratch test paths. Top: Hard chrome; Bottom: (Ti,Al)N-L................................................. 80
Figure 57 SEM photos of HVOF coating scratch test paths. Top: WC-17Co; Bottom: Triballoy 400. ......................... 81
Figure 58. Scratch test data chart for (Ti,Al)N-H.................................................................................................................... 82
Figure 59. Scratch test data for hard chrome........................................................................................................................... 83
Figure 60. Scratch test data for T400........................................................................................................................................ 84
Figure 61. Scratch test data for WC-17Co............................................................................................................................... 85
Figure 62. Fretting of HVOF and laser clad coatings compared to hard chrome............................................................... 88
Figure 63. Fretting of PVD coatings compared with hard chrome. ...................................................................................... 89
vii
Figure 64. Low cycle fatigue results......................................................................................................................................... 92
Figure 65. High cycle fatigue results........................................................................................................................................ 92
Figure 66. Main power shaft for T700 turbine engine - GEAE. The shaft has four HVOF-sprayed journal areas (light
bands). .................................................................................................................................................................................. 93
Figure 67. Compressive Creep Data at 800 oF and 100 ksi for Hard Chrome and HVOF Coatings................................. 95
Figure 68. Compressive Creep Data at 800 oF and 50 ksi for Hard Chrome and HVOF Coatings................................... 95
Figure 69. Average creep measured by extensometer readings........................................................................................... 96
Figure 70. Average creep measured by direct micrometer readings.................................................................................... 96
Figure 71. T700 engine power turbine shaft............................................................................................................................ 97
Figure 72. Number 4 bearing support - T700 engine turbine shaft...................................................................................... 98
Figure 73. CH47 actuator cylinder piston................................................................................................................................ 98
Figure 74. UH60 landing gear tail fork...................................................................................................................................... 98
viii
INTRODUCTION
Cr6+ is a major source of toxic waste from military hard chrome plating operations, and is produced both at the OEM
level (military contractors) and at the base level (parts rebuilding in maintenance operations). Replacement of hard
chrome is becoming increasingly urgent, not just to reduce toxic waste in the DoD production and maintenance
system, but to keep weapons systems operational. In January 1997 the new EPA Clean Air Act Rules for hexavalent
chrome emissions came into full operation, mandating mesh filters and packed bed scrubbers to reduce stack
emission limits by 95% to 0.015 mg/dscm. This had the effect of increasing capital cost and operating costs for hard
chrome plating, as well as increasing the risks of violations.
Upcoming OSHA regulations, expected to be announced in 1998 and to become final in 2000 or 2001, are likely to
lower the permissible exposure limit (p.e.l.) for workers in chrome plating plants from the current level of 52
micrograms m-3 to somewhere in the range 0.5 - 5 micrograms m-3 - a reduction by a factor of 10 to 100. At this level,
chrome workers are likely to be required to wear respirators, and the costs and risks of chrome plating will become
significantly higher. Cherry Point NAD has said that they do not expect to be able to meet these new limits without
significantly reducing throughput, requiring hard decisions on which weapons systems are to be kept operational.
Even at the lower level there appears still to be a measurable number of excess deaths per thousand, and it is
therefore likely that the levels will become even more stringent in the future.
The DARPA program made a two-pronged attack on eliminating Cr6+ waste currently produced by the electrolytic
hard chrome (EHC) electroplating process used by DoD and many commercial vendors:
1.
Replace hard chrome by modern technologies that totally eliminate hexavalent chrome.
2.
Recognizing that, despite our efforts in 1 above, EHC will be around for some time because it is currently so
widely used in DoD, find ways to minimize hexavalent chrome emission from the existing process.
The approach used in the DARPA program recognizes the differences between the requirements of the OEM and
military base environments and aims at almost completely eliminating hard chrome from both places by the use of
modern, clean, dry alternatives that are ready to use now. At the same time, since the realities of the testing and
approval process will retain EHC for some time to come, we are developing methods to reduce Cr6+ emissions from the
existing process. These two approaches are covered as two sections in this report, followed by several appendices
covering details of specific areas.
1
SECTION 1
HARD CHROME REPLACEMENT
The DARPA chrome replacement program and the HCAT (Hard Chrome Alternatives Team) program that has
evolved from it have become quite well known and well received among various DoD agencies. A great deal of
interest has been generated among DoD agencies and commercial vendors and there is clearly need, interest, and
momentum for putting replacement and clean-up technologies into use in DoD.
1.
Choice of Chrome Replacement Technologies for DoD Use
For a coating to be a viable alternative to hard chrome it is not sufficient that the coating be hard. One must
remember that chrome plating is not just a hard coating but part of a system where it has a variety of uses and
therefore must possess a variety of properties, depending on the application. Hard chrome has been in commercial
production for over 50 years. While it is often far from the best material available, it is well-known, well-defined, and
the systems in which it is used are designed to take account of both its strengths and weaknesses. The properties of
hard chrome are quite remarkable - hardness, wear resistance, adhesion, lubricity, color, ease of deposition and of
stripping. Even some of its problems, such as its microcracks, have not only been worked around, but are sometimes
treated as virtues, for example for retaining oil. No single alternative will be able to match chrome for all applications.
However, by judicious use of several different modern alternative technologies and/or materials we should be able to
exceed the performance of hard chrome for most applications.
1.1.
Technical requirements for chrome replacements
Typical requirements for a hard chrome replacement for a variety of DoD and commercial uses are summarized in
Table 1. Note that, while most of the items in this table are technical properties, in many ways those are not usually
the only (or often even the most critical) factors. For example, in the depot maintenance environment hard chrome is
used primarily to rebuild worn shafts and other components, often for use on military aircraft (a particularly sensitive
and demanding application). Unlike most OEMs, depots handle used components in an enormous variety of shapes,
sizes, materials, and conditions. Depot personnel are experienced process engineers and machinists (quite often
more experienced than those in industrial production facilities), but they are not PhDs and cannot realistically be
expected to deal with highly complex coating systems. Indeed, processes for depot (and especially aerospace) use
need to be as simple as possible and highly robust both mechanically (for reliability) and in their process windows (to
ensure reproducibility). Deposition systems that must be constantly retuned for each component will not produce
sufficiently reliable products for aerospace use.
1
Table 1. Typical requirements for hard chrome replacement for various applications.
Application
Requirements
All
Well defined, commercial process
Generally hard, wear resistant, corrosion resistant
Existing aerospace specifications in use (military or commercial)
Hydraulics
Wear resistance against rubber seals
Low friction against seals
Low wear of seals and guides
Abrasion resistance against trapped sand and other particles from outside the cylinder
Corrosion resistance outside cylinder
Industrial rolls and
process cylinders
Rolling contact fatigue resistance
Wear resistance
Corrosion resistance against water and materials being processed
Correct friction against materials in process
Correct wetting properties for inks and glues
Rebuilding worn
components
Good mechanical properties (including adhesion and cohesion) when up to 0.010” thick
Able to be deposited efficiently and inexpensively on large areas, or small areas of large
objects
Able to be finished to proper surface roughness
Able to be stripped
Aircraft components
No hydrogen embrittlement
Fatigue debit no worse than hard chrome
Must be able to be used on shot-peened surfaces
Good process definition, producibility, and quality control
Acceptable risk for production engineers
Depot maintenance
Good fit with depot maintenance environment:
Able to be used and maintained in depot using typical depot personnel and equipment
Compatible with typical depot maintenance operations and equipment
Applicable to a wide variety of components, materials, and applications
Given the ongoing changes in the chrome industry regulations, the long lead time for validation and acceptance of
alternatives, and the risks (both real and perceived) of making changes in aircraft components, the only processes
that can reasonably be ready in time are those that are already well known, well-defined commercial processes,
preferably already specified for use in aircraft. The alternative must not only work technically, but it must also work
politically - i.e. it must be broadly acceptable to production engineers and PMs, who are understandably very
conservative in permitting changes to flight-critical aircraft parts.
The depot requirement for thick coatings eliminates most vacuum processes, which typically produce only thin
coatings (0.0001’ or 2.5 microns). Thick coatings (0.010”) cannot generally be made by vacuum processes, both
because of time and cost and because of the stresses that tend to build up in such coatings. Heat treatments such as
plasma nitriding, are generally not acceptable because of their deleterious effect on the properties of most steels and
other alloys. While one could choose a specific coating technology and material as the optimum for each application,
this would be disastrous from the practical point of view. The depots cannot realistically become experts in all
technologies, and the validation costs would be exorbitant. All of the requirements of the depots must therefore be
met with the minimum number of technologies and materials. This leaves thermal spray methods as the broadest
category of standard coating processes, in which most applications can be met with a limited set of coating materials.
Most depots already use plasma spray, flame spray, or arc spray, because they are specified for a large number of
2
engine equipment and other repairs. In recent years Detonation gun (D-gun)1 and High Velocity Oxy -Fuel (HVOF)
sprays have become widely used in industry, especially for aircraft components. Several depots have now installed
HVOF equipment to meet the requirements for coating aircraft parts after maintenance or to evaluate for use in
chrome replacement.
Thermal spray coatings can be made to meet all of the requirements in Table 1. HVOF and D-gun coatings have
become widely used in aerospace because the coatings have the highest density, best adhesion, and consequently
the best mechanical properties of all the thermal sprays. They are also easier to machine without chipping and
delamination, so that they can be ground or machined to the proper surface tolerance and finish.
Thermal sprays are best used on large objects and situations where thick coatings are needed. For smaller objects, or
applications where hard, very high quality, thin coatings are needed, vacuum coatings can be a more cost-effective
alternative. In general this would be at the OEM level. Applications for these types of coatings include actuators
used in turbine engines, small hydraulics, some piston rings, and bearings (replacing thin dense chrome). There are
many different coating technologies and materials for thin coatings. Chemical Vapor Deposited (CVD) coatings
require high temperatures, making them unsuitable for most applications. The alternative is Physical Vapor
Deposited (PVD) coatings. There are many PVD coating technologies, but the most widely used commercially are arc
evaporation, sputtering, and electron beam evaporation, all under the influence of a plasma, and the most common
hard coating materials are metal nitrides and carbides.
1.2.
The growing use of HVOF for chrome replacement
Several approaches are have been examined or are currently being evaluated to reduce hexavalent chrome in DoD
operations, some funded by DoD agencies and some being undertaken at the depot level through the initiative of
depot engineers. There are also similar replacement efforts in the commercial sector, mostly carried out by individual
companies.
Apart from the DARPA program, several other military programs at individual vendors or bases have been
developing or evaluating HVOF chrome replacements. For example,
l
Oklahoma City and Ogden ALCs have both installed and tested HVOF. Oklahoma City is working on
obtaining GE approval for coating turbine engine components.
l
Jacksonville Naval Air Station is HVOF-coating components and moving toward approval on some.
l
Sacramento ALC has tested HVOF and other coatings and had intended to purchase an HVOF system, until
prevented from doing so by the base closure.
l
Sikorsky has tested D-gun (similar to HVOF) WC-Co and chrome oxide for helicopter landing gear, and
found both to be good alternatives.
l
Lufthansa is currently flight testing HVOF-coated landing gear main pistons.
l
GE Aircraft Engines now uses HVOF WC-Co and Tribaloy instead of chrome on many of their turbine main
shafts.
l
Praxair and other commercial vendors now coat with HVOF or plasma spray coatings many of the large rolls
that were previously chrome plated, such as Anilox print rolls and mill rolls.
2.
Alternative Approaches and their Shortfalls
Apart from HVOF there are three primary approaches to chrome reduction being undertaken with government
funding at present. One of these (at NIST) involves the development of trivalent chrome baths. This approach
1
Until recently D-gun coatings have been available only from Praxair’s shops and the equipment was not available
commercially. HVOF coatings were developed as a general commercial equivalent that produces the same coatings
with essentially equivalent properties.
3
appears to be showing success in increasing the deposition rate and allowing the creation of thick coatings (the
process has heretofore been self-limiting, making it unsuitable for anything but decorative applications). Other
trivalent processes are also being developed commercially. However, wet chemical methods inevitably produce large
waste volumes (even though they are not Cr6+-contaminated) in the form of rinse water. Furthermore, the old chrome
must still be stripped, producing large volumes of chrome-contaminated waste acid or base solution. An alternative
to this method, which is in increasing commercial use, is electroplated or electroless nickel with an additive such as
SiC. This approach eliminates chrome and retains the same basic technology, but it is really an interim solution since
nickel is also on the EPA list of 17 most toxic common substances. Therefore, it is most probable that nickel will be
next on the list for strict regulation or elimination, just as chrome is now.
There is an Army -funded program being undertaken at NDCEE that concentrates on reducing chrome through the
use of ion beam methods - ion implantation and ion beam assisted deposition (IBAD). This is a long-range approach
that uses very clean technology. It is a good way of reducing the problem at the OEM level. However, at the base
level (where the primary use of chrome is in rebuilding worn components), it relies primarily on rebuilding using nickel
or chrome electroplate, and increasing its life by ion implantation or IBAD methods. This still retains a reliance on
electroplating - either hard chrome or nickel, which is also on the EPA list of 17 toxic substances - although it reduces
their use if the implantation or IBAD overcomes the primary failure mechanisms (wear in use and scoring on
disassembly). Furthermore, the cost of using two surface treatments in place of one is likely to be a significant
problem.
Apart from these approaches, there are many other coating and surface treatment methods that are being put forward
as chrome replacements. Many of these do not make good commercial or technical sense, or are too early in their
development to be good alternatives on a large scale, and most are at best niche products. They include
l
Heat treatments such as plasma nitriding and gas carbonitriding, which can be good for large commercial
hydraulics and are being used for some standard hydraulic rod (carbonitriding) and very large hydraulics,
such as those used for lift bridges (plasma nitriding).
l
Diamond-like carbon and diamond, which usually have good lubricity
l
Jet Process (metal vapor entrained in a gas stream). There is not yet enough data to evaluate the usefulness
of this technology
l
CVD, metalorganic CVD, Combustion CVD. Most CVD techniques require too high a temperature for most
applications, requiring post-deposition heat treatment of the component.
l
Ion implantation and ion assisted coating or IBAD do not permit thick coating build-up and are relatively
expensive and complex for most depot use.
l
A variety of other dry hard coatings.
l
Wet chemical coatings, which are nearly all based on nickel electroplate, usually with a hard material
entrained within it, such as boron, SiC, diamond, etc. These approaches can fit well with existing depot
maintenance operations, but nickel is also on the EPA’s “17 toxic enemies” list, and is likely to be the next
material to be severely restricted. Some work well on a small scale, but are difficult to scale up reliably, as
Boeing has found with the Takada process, which works quite well on hydraulics at the laboratory scale, but
fails on the production scale because of the difficulty of properly mixing the filler materials uniformly into the
Ni coating.
Each of these approaches has something to recommend it for certain applications. Most, however, are limited in
application, and none fit well with depot maintenance operations, or only solve the problem in the short term.
3.
Results - hard chrome replacement coatings
The purpose of this program was to evaluate several technologies for their performance, cost, and fit with the DoD
prime contractor and depot maintenance environments. These technologies were chosen based on an initial
evaluation of their fit with the DoD usage of hard chrome (see above). The HVOF, PVD, and plasma nitriding
(although not the duplex process) are standard industrial technologies that have been in quite widespread
4
production use for more than ten years. Laser coating is used industrially, but on a limited basis for items such as
turbine blades. The duplex process is now beginning to be offered commercially, although usually as two separate
processes - nitriding followed by coating. The coating methods examined are summarized in Table 2.
Table 2. Technologies and materials evaluated in the DARPA program.
Technology
Materials
Usage
HVOF
WC-Co, Tribaloy, Cr3C2-NiCr
Supersonic thermal spray coatings for shafts, landing gear,
rebuilding worn parts.
PVD
TiN, CrN, TiAlN
OEM, use to minimize wear on original components
Duplex PVD
PVD + plasma nitride
OEM use on relatively soft materials (such as 4340 steel) able to
withstand the 500C nitriding temperatures.
Laser coating
WC-Co, Tribaloy
Very highly adherent coatings for localized build-up of new and
worn components, requiring no masking and no overall heating.
All these methods are modern, dry (i.e. non-electrolytic), and Cr6+-free, even where they use Cr as an alloying or
compounding element. As mentioned above, some of these materials contain chrome. However, the chrome is not in
its toxic hexavalent form, and in most cases it can be completely eliminated by the choice of materials.
The different technologies were evaluated for OEM or depot use by considering
l
Deposition methods - whether the method is suitable for depot or for OEM production use
l
Coating properties - whether the basic mechanical properties of the coating are suitable for replacing hard
chrome plate
l
Performance of the coated system - whether the coated component has sufficient wear resistance and other
critical mechanical properties. This is a function of the entire system, not just the coating, and is a question
that is much broader than simply the hardness or wear resistance of the coating itself.
l
Other concerns, such as coating removal, finishing, and other issues associated with the fit with OEMs and
depots.
l
Cost of coating - how the total cost of producing coated parts compares with the cost of producing chrome
plated parts. This includes preparation, finishing, waste disposal, and legal and regulatory costs.
l
Demonstration coating of some helicopter components typical of those that CCAD refurbishes.
3.1.
Summary of results
The coating methods were evaluated for fit with OEMs and maintenance depots, mechanical properties, and
structure. It was concluded that
•
the HVOF coatings fitted well with both OEMs and maintenance depots
•
the PVD coatings fitted well with OEM usage on relatively small components, and particularly on piston
rings, and
•
the laser coatings were problematical due to the lack of control in actual use.
Consequently, while deposition methods and coating mechanical properties and structure were measured for all the
coating methods, only the HVOF and PVD coatings were evaluated for performance and cost. Reflecting this, we
describe the HVOF and PVD coating findings in detail below, and separate out the laser coatings to the end of the
section.
The main results are summarized below and in Table 3. The results with PVD and HVOF coatings have been very
good, and we have developed new technologies to fit them better into the DoD environment.
5
1.
HVOF:
l
l
2.
3.
Cummins has developed a co-spraying method for composite Cr2C3MoNiCr piston ring coatings
â
Engine tests show that the HVOF coating performs better than hard chrome.
â
Calculations at Cummins and BIRL show that HVOF coating of piston rings can be done in 20% of the
factory space and at 50% of the cost of hard chrome.
â
Cummins management decided to develop the technology to pilot production, with success dependent
on detailed materials evaluation and testing. However, in 1997 the Piston Ring Division was sold, and
Cummins is no longer involved with the manufacture of rings.
HVOF Tribaloy 400 and WC-Co perform better than hard chrome for aerospace use
â
Wear tests, GEAE fretting tests, and calculations indicate an expected coating life of 2 - 4 times that of
hard chrome, often eliminated the need for depot rebuilds.
â
Triballoy 400 can be lathe-turned to a good bearing finish rather than requiring grinding - a significant
reduction in cost and complexity.
â
GE operational experience is that the cost of HVOF is about 1 - 1.5 times that of hard chrome. Even at
the higher typical cost and the lower typical life, the life cycle cost of HVOF coating is below that of
hard chrome.
PVD and Duplex plasma nitride/PVD:
l
Very hard and more wear-resistant than hard chrome
l
High quality duplex PVD coatings can be used on typical DoD components made of relatively soft steel
Laser coatings:
l
Excellent hardness and adhesion, and rapid build-up
l
Can easily cause overheating damage to substrate
l
Good for OEM use on well-defined components such as turbine blades, but not recommended for general
use, either at OEMs or in depots.
6
Table 3. Summary of alternative coating evaluations.
Process
Waste
Characteristics
streams
HVOF thick coating
(0.001 - 0.050”)
Overspray,
grindings,
cleaners, grit
blast medium
•
•
•
•
PVD thin coating
(0.0001 - 0.001”)
Laser thick coating
(0.020 - 0.100”)
Coated
shields,
cleaners
Overspray,
grindings,
cleaners, grit
blast medium
•
•
•
•
•
•
•
•
Usage
Components
More wear resistant than EHC
Simple process similar to those
already used by OEMs and depots
Good for large areas and for small
areas on large items
Can be ground to finish (or
sometimes single point turned)
Hardest of the commercial
coatings
Process more complex than HVOF
Good for small/medium parts
No finishing required
OEMs
Hydraulics*
Piston rings
Turbine components*
Depot
rebuilding
Hydraulics
Turbine components
Landing gear
Turbine components*
Hydraulics
Piston rings*
Hard coatings, very strong bond
Good for small area on large part
Can be ground to finish (or
sometimes single point turned)
Difficult to control - can easily
damage substrate
OEMs
No rebuilding - possible
recoating
Turbine components*
Not recommended for
general use
Depots
Not recommended
7
OEMs
Depots
3.2.
Coating methods
3.2.1.
Fuel
HVOF coating method
Powder
Supersonic flame
Axial feed
powder
Shock fronts
Oxygen
Radial feed
powder
Figure 1. Schematic of HVOF coating method.
surface being coated at high speed. The speed is sufficient to flatten the
softened powder particles into “splats” at the surface of the component,
forming a dense, well-adhered coating. The gun is moved back and forth and
the part rotated as necessary to obtain an even coating (just as one would in
spray painting), typically built up to a thickness of 10-15 mils, which is ideal for
rebuilding worn components. Because the flame puts out a great deal of heat,
the substrate is usually cooled with jets of compressed air.
The High Velocity Oxy -Fuel (HVOF)
gun (Figure 1) is essentially a
supersonic oxy -hydrogen flame into
which one injects powder. In most
HVOF guns, combustion begins
inside the gun and the powder is
injected at the back of the device.
(In the Tafa gun the powder is
injected at the front of the gun, as is
commonly done in plasma spray
guns.) The flame heats and softens
the powder while the high velocity
gases accelerate it so that it hits the
The fuel for an HVOF gun is commonly hydrogen, but acetylene or propylene
can be used, or even kerosene, in the case of the Tafa gun (which is used at
Ogden ALC). The powder can be a metal or alloy (such as Tribaloy), a low
melting point ceramic, or a cermet (such as WC-Co). The gun is not capable of
softening or melting oxides, which can only be done with a plasma spray gun
(and in some cases a detonation gun).
The form of the powder is critical to the process. It would usually be a
spheroidized powder, 30-60 microns in diameter obtained from a well-qualified
supplier. Since the powder is critical to the success of the HVOF process, the
process needs to be optimized and done in production with
l
a particular powder chemistry,
l
made with a well-defined production process,
l
with a consistent particle size distribution,
l
and with consistent powder flow characteristics.
Figure 2. HVOF spraying of a
landing gear inner
cylinder. (Courtesy
Southwest Aeroservice)
Because the flame is supersonic, the process must be done in an enclosed, sound-proofed room, usually using an
industrial robot, and the excess powder (or overspray) ducted out and collected. Figure 2 shows a Boeing 737
landing gear inner cylinder being HVOF sprayed. This type of item is typically about 4 feet long and 3” in diameter.
It is rotated at about 60 rpm while the main shaft is sprayed (as shown here). The gun is on the left, mounted on a
robotic arm. On the top of the “Tee” are chromed bearing surfaces, which can be HVOF coated in the same manner,
except that the cylinder must be rotated about the “Tee” (90 degrees to the above position) with a counterweight to
balance the table.
For evaluation in this program, HVOF coatings were developed using statistical design of experiment (DOE) methods,
which not only optimize the deposition parameters, but also give a good measure of the sensitivity of the process to
the various parameters, and hence the width of the process window. The details of the DOEs are given in Appendix
1. The general finding was that the HVOF process is less sensitive in general than plasma spray, which is the other
primary thermal spray process used in aerospace. In particular, HVOF is fairly insensitive to the “stand-off” - the
8
distance between the gun and the component. This makes it a generally more reliable process than plasma spray, and
more suited to depot maintenance operations, where a large variety of components must be processed.
3.2.2.
PVD coating method
magnetrons
vacuum
chamber
substrates
-500V
-125V
Figure 3. Schematic of PVD coating system
(sputtering).
PVD coatings were done in two ways:
1.
Sputter deposition (at BIRL)
2.
Arc deposition (at Praxair, one of GEAE’s aerospacequalified vendors).
The choice of deposition technology depends on the application.
In general the higher deposition rate makes arc coatings cheaper to
deposit, but they tend to contain “macroparticles” of unmelted and
unreacted metal, which creates a rough surface. Sputter coatings
are smooth, but slower and more expensive to deposit.
Figure 4. The Hauzer ABS production PVD
coating system. Chamber is approx.
1m cube.
Sputter coatings were optimized by a computerized design of experiment method. It was found that several chrome
nitride phases could be produced, depending on the partial pressure of the nitrogen during deposition. Coating
adhesion was best when the coating was designed as a layered system, with soft Cr-N adjacent to the substrate, with
hard CrN at the surface for optimum wear resistance.
The thin (typically only 5 microns - 0.0002” - thick) PVD coatings need the support of a hard underlying surface in
order to provide good wear resistance to the component. PVD coatings are generally used on hardened tool steel or
cobalt cemented tungsten carbide, and a substrate hardness below about 55Rc is insufficient. Most DoD
components are made of materials such as high strength steels, stainless steels, or inconels, whose hardness seldom
exceeds about 45Rc .
In order to provide support to thin PVD coatings, a process was therefore developed in which the substrate was
plasma nitrided for one hour at 500C in the deposition chamber prior to PVD coating. The plasma nitriding (or ion
nitriding) process produces a surface containing hard nitrides of the alloying elements (typically Mo, Co, or Cr), and
a surface hardness of about 60-70Rc . The advantage of the plasma nitride is that it can be done at about 500C, which
is below that needed for gas nitriding. Furthermore, it can be done in the same equipment used for the PVD
deposition, provided that equipment is properly configured. This approach is viable for nitridable steels (including
4340 high strength steel and stainless steels), on which it produces a thin case (typically less than 0.001” as against
the standard 0.005-0.010” case produced by standard 8-12 hour cycles).
Because of our prior experience with various PVD coatings, we chose TiAlN and CrN for the PVD coating materials,
because both of these materials are hard, oxidation resistant, and suitable for turbine engine use. CrN was deposited
by sputtering and TiAlN by arc deposition. In both cases, the deposition is carried out in a vacuum chamber at a
9
Table 4. Typical PVD deposition conditions.
Parameter
Value
Target Power
2.5 kW per target
Total Pressure
3 µBar
Partial Pressure, N2
0.75 µ Bar
Substrate Bias
-125 V
Plasma Potential @ Target
-30 V
Sputter-Etch Time
15 min.
Sputter-Etch Voltage
-1200 V
Ion-Current Density @ Target
4-6 mA/cm2
Temperature at Start of Run
125ºC
Temperature at End of Run
220ºC
pressure of a few millitorr. The metal is removed from a
source (or “target”) by a plasma (sputtering) or an electrical
arc. Nitrogen gas fed into the chamber simultaneously
reacts with the atoms as they land on the surface, forming a
nitride. The entire process takes place with the surface
surrounded by a plasma to provide the energy needed to
create a dense, well-adhered compound coating. To ensure
uniformity, the substrate components are rotated within the
chamber. A Response Surface design of experiment (DOE)
was used to optimize the coatings for hardness, adhesion,
and structure in a minimum number of deposition runs.
Plasma nitriding (which is carried out prior to coating)
requires that the surface of the component be bombarded
with nitrogen ions and atoms in a plasma. Since PVD
involves the use of a plasma, the plasma nitriding process
can quite naturally be done in a PVD chamber (albeit under
conditions somewhat different from those typically used in
industrial plasma nitriding.
A typical resulting hardness depth profile is shown in Figure 10. Note that the hardness of the material rises from the
bulk value of the steel to about the hardness of chrome at the steel surface, while the PVD coating (which actually
comprises a soft substoichiometric CrNx layer beneath a harder stoichiometric CrN layer) blends this nitrided layer
into the final hard surface.
3.2.3.
Laser coatings - deposition methods
The primary laser coating method evaluated in this
program was laser cladding. In this process a metal
powder is dropped in a continuous stream onto the
surface while the point of impact is irradiated with
an infrared laser. The powder particles are welded
onto the surface (while some, as we see in Figure 5,
bounce off and are lost). Because of the necessity
for precise coincidence between the beam and the
powder, the component is generally moved beneath
the beam. In Figure 5 one can see that the
component is rotating clockwise from the viewer’s
direction, leaving a trail of material at red heat
around the surface to the right of the coating zone.
The method rapidly builds up a very strongly
adhered coating, and is capable of deposition on
small areas of large items without the need for masking, since the laser beam defines the coated area quite precisely.
As an alternative to powder, the coating material can be applied in the form of a wire, which is somewhat easier and
more reliable for production. In either case, the excess heat is removed by a gas jet.
Figure 5. Laser cladding of a tube.
Attempts were made to evaluate the use of laser chemical vapor deposition for localized coating. Unlike PVD this
vacuum method could build up a high quality coating in a small area of a large part. However, the method was found
to be so critically dependent on laser stability that it was abandoned as not viable using our standard CO2 infrared
laser, although the method may be possible using modern excimer lasers.
10
3.3.
Coating performance
In this section we shall discuss the performance of the coatings in terms of type of test or end use, rather than
coating technology. This is because in a number of cases we have directly compared the PVD, HVOF, and EHC
coatings in the same test, a comparison that allows us to draw conclusions as to the best approach for different
applications.
3.3.1.
HVOF coating structure and mechanical properties
The properties of the HVOF coatings are shown in Table 5. These were properties for the optimized coatings, made
at two different sites. There are differences from one site to the other, but these differences are not large, except in
the area of compressive stress. Since site B had no data (ND) for the substrate temperature, it is possible that the
stress difference is due to a different temperature, which is strongly related to stress. For the WC-Co the bond
strength was in general greater than the strength of the glue used in the test, so that the adhesive failed rather than
the coating. The almen deflection is a measure of stress in the coatings, with negative numbers being compressive.
All the coatings exhibited compressive stress, which is essential to improve fatigue. (EHC, on the other hand, always
has a large tensile stress - the origin of its microcracking - which creates the large fatigue debit that is a particular
problem for aerospace components.)
Table 5. HVOF coating properties obtained in confirmation runs.
Coating
Site Run Dep. Rate DPH
R 15N Tensile Bond Substrate
Almen
Number mils/pass kg/mm2 kg/mm2 Strength, psi Temp oF Deflection, mils
WC-17 Co
A53
0.416
1065
94.4
11,486
158
-27.5
WC-17 Co
A54
0.521
1145
94.3
11,821
130
-39.5
WC-17 Co
B22
ND*
1136
93.0
12,701
ND
-15.4
WC-17 Co
B23
ND
1167
94.2
12,081
ND
-12.1
Triballoy 400
A59
0.416
568
89.4
9,534
162
-5.0
Triballoy 400
A60
0.416
598
89.2
9,127
ND
-5.5
Triballoy 400
B18
ND
602
87.8
8,273
ND
-2.4
Triballoy 400
B19
ND
588
89.2
8,736
ND
-1.6
Note that the hardness of WC-Co is higher than is typical of EHC (800 - 1,000 kg/mm2), while Triballoy is softer. This
would mean that in abrasive wear situations the tungsten carbide would be the better material. However, in other
types of wear, this is not necessarily true at all.
The mechanical properties of the HVOF and PVD
coatings (hardness and adhesion) were both high. HVOF
WC-Co was typically 1,200 - 1,500 HV, while CrN was
typically 1,600 - 2,000 HV, as against the typical hard
chrome of 800 - 1,000 HV. Both HVOF and PVD coatings
generally exceeded 10,000 psi bond strength in adhesive
pull tests (i.e. greater than the bond strength of the
adhesive).
100 microns
Figure 6. Cross section of HVOF Cr3C2-NiCr coating.
11
3.3.2.
PVD coating - structure and mechanical properties
The PVD coatings were generally fine-grained, smooth structures with a surface finish close to that of the underlying
metal. Figure 7 shows the surface of such a coating, where the grains
are clearly visible.
Cr2 N
1600
Cr-N
CrN
1400
1200
Vicker’s
Hardness
kgf/mm2
Cr 2N
+
CrN
1000
Cr-N
+
Cr2 N
800
600
Target Power = 8 kW
Substrate bias = -125 V
Total pressure = 8 mTorr
Alpha System
400
200
Figure 7. PVD CrN surface at 5000x.
0
The hardness of the different phases of
0
0.5
1
1.5
2
chrome nitride (produced by changing the
Nitrogen Partial Pressure, mTorr
partial pressure of nitrogen in the
deposition chamber) varied from 800 to
Figure 8. Hardness and structure of PVD CrN vs N2 partial pressure.
1600 Vicker’s (see Figure 8). Therefore,
layering the coating, with Cr-N beneath and CrN on top produced a coating system with exellent adhesion because of
the elimination of strong discontinuities in mechanical properties at the interfaces.
The concept of plasma nitriding the underlying metal
to provide sufficient support to the hard coating was
tested with several alloys, including 4340 high
strength steel (used in aircraft hydraulics), ASL81 steel
(a Japanese stainless used for piston rings), and
Inconel 718 (a high temperature alloy used in turbine
engines). In each case the nitriding was done in the
PVD deposition system using the following sequence:
l
HV (kg/mm2)
900
800
4340
IN718
ASL81
700
600
500
400
300
200
Radiant heaters brought the substrate to
400C.
100
0
l
The temperature was increased to 500C by
bombardment with an argon plasma, which
also cleaned the surface. (Plasma-only
heating overetched the surface.)
0
50
100
150
200
Depth (microns)
Figure 9. Hardness vs depth profiles of plasma nitrided
metals (PVD chamber nitriding).
l
The surface was then plasma nitrided at a
pressure of 3-10 mTorr for one hour at 500520C.
l
At the end of the nitriding step, the coating process was begun immediately to prevent denitriding of the
surface and to ensure the cleanest surface for coating adhesion.
Our process differs from conventional plasma nitriding in two primary ways:
l
The nitriding pressure was far lower than the several Torr used by commercial nitriders, but the ion
bombardment current, which defines the input rate for nitrogen, was similar.
l
Nitriding is usually done for 10-24 hrs, but our process limits it to one hour both for cost considerations and
because only a thin case is needed to support the coating.
12
CrN Cr-N
Plasma nitride
case
Underlying steel
HV (kg/mm2 )
1600
1400
1200
1000
800
600
400
200
0
-20
0
20
40
60
80
100
120
Depth (microns)
10X
Figure 10. Hardness vs. depth profile for duplex plasma nitride/PVD CrN coating on ASL81 stainless steel.
The resulting hardness depth profiles (measured by microhardness indentation on a metallurgical cross section) are
shown in Figure 9. The largest effect was found with the ASL81 stainless steel used for piston rings, whose surface
hardness was more than double that of the bulk. The surface hardness of the 4340 steel increased about 50%, but
there was little or no effect on the inconel.
Combining the plasma nitriding with the two-layer Cr-N/CrN PVD coatings produces the composite material shown
schematically in Figure 10. We see that this material grades the hardness from the low bulk steel value to the high
level of the ceramic coating to produce a well-supported, well adhered surface structure. This coating structure was
used in diesel engine tests and fretting wear tests.
3.3.3.
Laser coating - structure and mechanical properties
Control of the laser power delivered to the surface was critical to the coating structure and mechanical properties, as
well as to the structure of the underlying metal. Initial coatings deposited by laser cladding tended to crack if laid
down in more than one pass due to the thermal shock and changes in stress. Early coatings also created a deep heataffected zone at the surface, often with complete alloying of the coating with the surface of the metal.
13
Figure 12. Detail of heat-affected zone.
An example of this is shown in Figure 11 and Figure 12.
In Figure 11 it is clear that the surface of the underlying
steel is affected to a depth of several hundred microns.
In Figure 12, the details of the surface structure show
Figure 11. Cross section of laser clad steel.
that the surface has recrystallized near the coating
boundary. Such extensive crystal structure changes are a serious concern for most comp onents, since they affect
the mechanical properties of the component - especially properties such as hardness and fatigue, and quite possibly
also corrosion.
Figure 14. Hardness and cross-section of IN718 laser
clad with T400, showing interface mixing.
Figure 13. Depth profile of hardness and cross section
of laser clad T400 on IN718, showing limited
mixing.
The other primary effect of the heat-affected zone is the intermixing of coating and substrate. If the laser power
density at the surface is too high the result is as shown in Figure 14. The coating mixes completely with the
14
underlying surface, forming a relatively soft alloy of indeterminate properties and poor wear resistance. With careful
control of the laser power, it is possible to prevent this from happening, creating a surface such as that shown in
Figure 13. Here, as the cross section shows, there is a distinct hard surface layer on top of an intermediate layer
alloyed with the substrate. Beneath this the substrate remains unaffected. In each cross section, the diamonds are
Vickers indentations, whose size reflects the hardness readings.
Chemical analysis of the surface (done by EDAX) shows the same picture (see Table 6). The coating of Figure 14is
clearly a mixture of substrate inconel and the Tribaloy powder. The coating of Figure 13, on the other hand, is very
close in composition to the original powder.
Table 6. Chemical composition (wt %) of T400 coatings, powder, and IN718.
Element
T400 Powder
(specified)
IN 718
(specified)
Coating in
Figure 14
Coating in
Figure 13
Co
56.6
0
27.08
58.75
Mo
Cr
Si
Fe
Ni
28.5
8.5
2.6
1.5
1.5
0
18.3
1.36
19.6
55.4
10.54
13.93
2.53
12.09
33.82
20.17
9.28
4.51
2.18
4.71
3.3.4.
Laser coating - applicability to OEM and maintenance operations
While OEMs tend to produce large volumes of a limited number of items, maintenance depots must coat a wide
variety of components which they receive in a state that is far from pristine. Consequently the coating conditions at
OEMs are better controlled, and for this reason OEMs have been able to use such coatings successfully (such as
GE’s use of it on engine components). However, laser coating can easily lead to substrate overheating, through a
variety of causes:
l
Coating materials typically have higher melting points than steels, making control of the laser power critical
to ensure proper melting of the coating material and surface layer without overheating below this area.
l
Laser spot instability, such as changes in laser mode, can significantly alter the power density and over- or
under-heat the surface.
l
Interruptions in coating powder flow, which are quite common, deposit excess power at the surface.
l
Any misalignment of the laser beam and the powder stream will change the heat load at the surface.
A heat-affected zone will always be present at the surface, making it very difficult to be sure of the heat treated state
of the alloy. This is especially problematic for the heat-sensitive high strength steels and aluminum alloys frequently
used for aircraft components. Given these concerns, we believe that the process cannot be sufficiently well
controlled at this point to ensure the safety of flight-critical parts.
We therefore recommend against the use of laser coating in maintenance depots.
Once this conclusion was reached, further work on laser coatings was halted in order to concentrate on HVOF and
PVD coatings.
15
3.3.5.
HVOF coating fatigue [turbines, diesels, general aerospace]
LCF Data
HCF Data
1000
IN718 Min LCF
IN718 Min HCF
IN718 Avg LCF
IN718 Avg HCF
Hard Chrome LCF
WC-17 Co LCF
Hard Chrome HCF
Alt Sress (ksi)
Alt Stress (ksi)
1,000
Triballoy 400 LCF
100
10
1,000
10,000
100,000
WC-17 Co HCF
Triballoy 400 HCF
100
10
10,000
1,000,000
100,000
1,000,000
10,000,000
Nf, Cycles
Nf, Cycles
Figure 15. Fatigue performance of HVOF coated vs. chrome plated and uncoated Inconel.
In aerospace applications the fatigue properties of coated materials are critical to system performance and safety. It
is known that hard chrome plating causes a fatigue debit. A successful replacement cannot cause a larger debit than
hard chrome, and a lower debit (or no debit at all) would be valuable. A limited amount of fatigue data were obtained
for HVOF-coated inconel (Figure 15).
In this data we can clearly see the fatigue debit created by chrome plating. The WC-Co causes a fatigue debit
somewhat less serious than that of hard chrome, while the Tribaloy causes a small debit, which is within the
engineering range of the uncoated material. This is not unreasonable, given that we would expect the WC-Co to be
somewhat more brittle because of its ceramic phase. Given that these coatings were not optimized for fatigue, it is
quite probable that better fatigue performance can be obtained, perhaps with some loss of hardness, but still with
better tribological performance than chrome.
3.3.6.
HVOF coating - compressive creep tests [turbines]
In turbine engines creep of engine materials and
coatings under high temperature and pressure is
very important. Creep tests were run by depositing
thick (0.025-0.030”) HVOF and EHC coatings on thin
steel, and then removing the steel backing by
mechanical and chemical machining. A stack of
three coating thicknesses was then subjected to
high temperature and pressure, while measuring the
changes in stack height versus time. Tests were run
at 426C and 50-100ksi for times from 300 to 1000 hrs.
Creep was measured both by direct micrometer
(shown in Figure 16) and by extensometer readings.
The WC-Co had the lowest creep rate - in fact it was Figure 16. Compressive creep of HVOF and EHC coatings 426C, 50 and 100 ksi load.
so low that low stress creep measurements were not
done on this material. The T400 creep was significantly lower than the chrome. However, while the chrome creep
doubled with a doubling of stress, the T400 increased fourfold. This may be due to a change in creep mechanism in
the two phase T400 under increasing load.
16
3.3.7.
HVOF and PVD coating scuffing [diesels, general aerospace]
The aim of replacing chrome plating on piston rings is
partly to avoid hexavalent chrome pollution issues,
and partly to obtain better performance. There is a
drive toward the million mile diesel. However, current
rings, with their chrome coatings, cannot reach this
goal. Therefore attempts are under way to find
alternatives that can survive for a million miles. In
diesel engines, the primary failure mechanism of
concern to Cummins is scuffing against the ductile
iron cylinder liner.
upper wear plate
piston ring sections
Piston Ring Segments
In order to assist in coating development, we
therefore designed a lubricated scuffing test in which
coated ring segments were run against an uncoated
ductile iron plate. The test was run on a high speed,
high load Falex machine, capable of measuring friction
and temperature under a defined load and speed. The
Figure 17. Piston ring scuffing apparatus.
force between the rings and the disk was increased
stepwise until the “scuffing load” at which scuffing failure occurred, evidenced by a rapid rise in friction and oil
temperature, and sometimes by smoking of the oil.
1000
0.5
Plasma
Iron Based
Normal Load (N)
800
700
Load
0.4
Chrome
CKS36
600
0.3
Plasma - I
500
Taikuku
400
0.2
PVD CrNO
300
PVD CrN
200
Friction Coeff.
900
0.1
BIRL
100
0
0
0
15
30
45
Test Duration (min)
Figure 18. Friction and load scuffing test output.
The details of this test as contained in a Society of Automotive Engineers paper, included as Appendix 2.
Results of typical runs for different coating materials are shown in Figure 18. The Taikuku material is a chrome oxy nitride developed in Japan and used commercially on piston rings. Clearly, the chrome plate performs the worst,
while the commercial Taikuku product and the two-layer (Cr-N/CrN) material developed by BIRL perform the best.
Performance in scuffing tests appeared to be a reasonable predictor of performance in actual engine tests, giving
confidence in the test as a coating development tool. The test results for a number of coatings are summarized in
Figure 19. As before, the chrome performs the worst, various plasma and HVOF coatings perform better, while the
PVD coatings perform the best. In fact, neither of the PVD coatings evidenced scuffing at all. Note that the Taikuku
material (CrNO) was highly polished, whereas all the other coatings had a 10 microinch finish, more typical of
production coatings.
17
PVD(CrN)
PVD(CrNO)
Plasma -I
Plasma -III
Plasma -II
Plasma Iron
Based
Polished Chrome
HVOF(Cr3C2)
CKS36
Ground Chrome
850
Failure Load (N)
750
650
550
450
350
250
Figure 19. Scuffing of various coated rings against gray iron.
The PVD coatings are clearly the best performers in scuffing. However, wear rate is also critical, since it is this that
will determine the longevity of a coated piston ring system.
3.3.8.
PVD coating wear - piston rings [diesels, general aerospace]
CE/SF 15W40 Oil
Coated steel vs. Grey Iron
Fresh Oil
@ 350C
2.5% Soot
Cr-N
Oil @ 200C
TEST ENVIRONMENT
Data Courtesy of Malcolm Naylor, Cummins Engine
Cr2N/Cr-N
CrN
CrN/Cr-N
EP Cr
Cr-N
1.00E-13
Cr2N/Cr-N
CrN
CrN/Cr-N
EP Cr
1.00E-12
1.00E-11
1.00E-10
1.00E-09
WEAR COEFFICIENT (mm3/mm/N)
Figure 20. Wear coefficients for different PVD coating designs vs EHC.
In developing the PVD CrN coatings difficulties were at first encountered in obtaining adequate adhesion to the
substrate. Several coating structures were examined during the development process, and tested for adhesion,
hardness, and wear. Figure 20 shows the results of tests runs of coated rings in wear tests at two different
temperatures. (These wear tests, while not part of the program, were run on the same coating materials.) The oil at
18
the higher temperature was also contaminated with soot. The wear coefficients of the PVD coatings are in all cases
less than that of the chrome, with the best performance being obtained from a bilayer stoichiometric CrN on top of
substoichiometric Cr-N (which was the structure ultimately adopted). The wear coefficient of this material is one to
two order of magnitude less than that of hard chrome. To meet the requirements for a million mile engine, Cummins
estimates that the PVD CrN coating thickness would need to be about 15 microns. This is thick compared with most
PVD coatings, but thin compared with the 60 micron CrON coating developed for piston rings by Taikuku.
3.3.9.
Comparative fretting wear of hard chrome, HVOF and PVD coatings [turbines]
At GEAE fretting wear is a critical issue at the roots of turbine blades, and for that reason GE uses a fretting test (low
stroke, high frequency oscillating wear). The test consists of a coated “shoe” rubbing against an uncoated block.
Two substrate materials were tested - 4340 steel and Inconel 718. The following coating materials were tested:
l
EHC
l
PVD sputter coated CrN
l
Duplex plasma nitride plus PVD sputter coated CrN
l
PVD arc deposited (Ti0.6Al0.4)N, (Ti0.5Al0.5)N, (Ti0.4Al0.6)N, designated (TiAl)N-L, -M, -H respectively
l
HVOF WC-17Co from two sites
l
HVOF Tribaloy 400 from two sites
l
Laser clad Tribaloy 400
The data are summarized in Figure 21. The exceptionally high data points for (TiAl)N-H occurred because the test
inadvertently ran through the coating and began to wear the substrate, which has a much higher wear coefficient.
Zero wear values occurred in those cases where material transferred from the uncoated 4340 counterface onto the
coating, protecting it from wear. This is the case for the EHC-coated 4340 steel. In general, the wear coefficients for
the various coatings are similar. Of the PVD coatings the duplex material shows the most promise. As one might
expect, the HVOF WC-Co generally shows low wear but causes increased wear on the uncoated counterface. On the
other hand, the HVOF T400 wears faster but causes less counterface wear.
It is important to note that a low wear coefficient does not imply a long wear life, since wear life depends on coating
thickness. The PVD coatings, which are typically 3-5 microns thick, will generally have shorter lives than the HVOF
coatings, which are 2-5 mils thick (25x as thick). Therefore the choice of coating for fretting wear depends upon the
application. For example, one might choose duplex PVD for small precision parts, HVOF WC-Co where the wear of
the coated component is critical, or T400 where the wear of the uncoated component is more important.
19
1.00E-07
Wear Coeff.
1.00E-08
1.00E-09
1.00E-10
1.00E-11
1.00E-12
Block Wear Coeff
Shoe Wear Coeff
Shoe - coated
Block - uncoated
EHC
L
M
TiAlN
H
CrN
CrN HVOF
T400
T400
+ PN WCSite A Laser
17Co HVOF T400
Site A WCSite B
17Co
Site B
Figure 21. Fretting wear coefficients for various coatings.
3.3.10.
HVOF and PVD coatings for piston rings - diesel engine tests [diesels]
PVD and HVOF coatings have been run in full scale diesel engine tests. For these tests differently coated rings were
placed in different cylinders of the same engine to make direct comparisons. There are various types of engine tests,
with conditions varied to assess the performance of the rings and cylinder liners under different conditions.
20
scuffed
scuffed
scuffed
7
6
5
Wear (x0.0001")
Engine tests were run and
the wear of both the ring and
liner were measured. The
results of a 500 hr abuse test
are shown in Figure 22 and
summarized in Figure 23.
Note that the EHC coating
scuffed in each cylinder,
while the HVOF-coated rings
did not. The HVOF
Cr3C2MoNiCr coating not
only wears at a lower rate
than hard chrome, but also
causes less liner wear.
4
Max. Cylinder Wear
Average Cylinder Wear
3
Average Ring Wear
2
1
coatings as thick as the EHC in any costeffective manner. Unfortunately, establishing
the curve requires a long term test, which is
very time consuming and expensive.
Average Wear (x0.0001")
The same is true of CrN
0
coated rings. Figure 24
1 (Cr)
2
3 (Cr)
4
5 (Cr)
shows the average cylinder
(HVOF)
(HVOF)
and liner wear for a 350 hr
Cylinder #
thermal cycling engine test
in which PVD coated rings
were tested with EHC rings. Figure 22. Wear in individual cylinders during 500 hr abuse test.
In engine testing Cummins has established that the wear follows a curve, with the highest wear rate at the beginning
of the test, falling off as the test progresses. A
4.0
factor of two change such as we see here is
3.0
likely to lead to a much larger total reduction in
Cylinder
wear over the life of an engine. This is
2.0
Wear
important since we cannot make the PVD
1.0
0.0
-1.0
1
-2.0
-3.0
Chrome
2
Ring
Wear
HVOF
-4.0
-5.0
-6.0
Figure 24. Wear of EHC and CrN coated
rings - 350 thermal cycling engine
test.
scuffed
Figure 23. Ring and liner wear in 500 hr abuse test.
The conclusion of Cummins’ engine testing is that in general the
HVOF and PVD coatings wear at a lower rate than hard chrome.
This leads to better wear life, both for the coated rings and for the
liners they run against. For the HVOF coatings, whose thickness
is similar to chrome, ring and cylinder wear life will both be
enhanced. For PVD coatings, which are generally a great deal
thinner, it is not possible to predict wear life on the basis of the
existing data.
21
3.4.
Waste streams from hard chrome and alternative coatings
Because both HVOF and PVD are dry coating processes there are no high volumes of plating solutions, air
emissions, or rinse water, as there are for chrome plating. The waste streams are summarized in Table 7.
Table 7. Waste streams from coating processes.
Hard chrome
HVOF
PVD
Preparation
Aqueous cleaners
Aqueous cleaners
Aqueous cleaners
Stripper solution (Cr6+
contaminated)
Stripper solution (e.g. Rochelle
salt, not Cr6+ contaminated)
Blasting grit
Cr6+ air emissions
Coating
Overspray powder
Coated shields within vacuum
chamber
Grindings
None
6+
Cr plating solution
Cr6+ contaminated rinse water
Finishing
Cr6+ contaminated wax, paint,
tape
Chrome grindings
Note that none of the HVOF or PVD processes incur the production of any Cr6+ waste. This is true even when the
coating itself includes Cr (as in T400, CrN, or Cr3C2/NiCr), since the chromium is not in solution as CrO3, the normal
hexavalent chrome form.
3.5.
Demonstration HVOF and PVD coatings on DoD maintenance components
Several aerospace components were HVOF-coated to demonstrate the capability of the HVOF method for depot
maintenance. All thermal sprays were done under the direction of Jerry Schell at GEAE and were finished to final
dimensions and an Ra of 32-16 microinches, as specified in the maintenance orders (DMWRs - Depot Maintenance
Work Requirements).
The UH60 fork (Figure 25) frequently
poses quality control problems in
attempting to chrome plate the two
bearing areas on the shaft. The double
zincate process that that the
specification calls out for proper EHC
adhesion on aluminum frequently fails,
requiring that the part be stripped and
replated. A previous attempt at testing
a plasma spray coating failed because
of difficulties in obtaining a large
enough diamond wheel for finishing,
since the part is about 12” long.
HVOF T400 coating
The fork was thermal sprayed with T400
Figure 25. UH60 helicopter tail landing gear fork (7175 aluminum) - as and ground with a standard grinding
wheel at CCAD. Note that simple
sprayed.
mechanical masking made it possible to
avoid filling in the retaining ring groove at the top of the shaft, thus reducing the extent of post-spray rework.
Furthermore, the HVOF spray easily reaches into the join of the shaft with the fork - the type of area that often
requires extensive overplating to obtain an adequate chrome thickness.
22
approx 40”
T400
WC-17Co
ground
as-sprayed
identifying labels
Figure 26. T700 helicopter engine power turbine shaft - IN718 (OEM GEAE).
The turbine shaft (Figure 26) is an example of an item that requires coating over a small portion of a large object - the
bearing journals along its length. HVOF can do this quite efficiently, while EHC requires that the entire shaft be
masked except the area to be plated. The specified finish and tolerance were readily accomplished with standard
grinding methods, as the finished brightness of the journal areas indicates.
~2.5” ID
HVOF
WC-17Co
HVOF
T400
Figure 27. T700 helicopter engine #4 bearing support, as sprayed AM355 stainless steel (OEM GEAE).
23
The bearing supports of Figure 27 are an
example of the ability of HVOF to coat
thin walled items without the warpage
which can easily result from overheating
on a relatively delicate structure. In this
case the upper ID was coated. IDs can
be coated, but not inside high aspect
ratio holes. Not easily visible in the
pictures are oil weep holes through the
sprayed area that are only about 0.010”
diameter. The spraying did not clog
these holes, showing how finely detail
can be reproduced. Note also the clear
definition of the coated areas around the
mechanical masking.
T400
Turned
WC-17Co
Ground
14.5”
Figure 28. CH47D helicopter swivelling hydraulic rod - 4340 steel.
The hydraulic rod of Figure 28 is typical of most aerospace hydraulics. It is made of 4340 high strength steel and
must be finished to close tolerance and well-defined finish. One half of this rod was sprayed with WC-17Co and then
ground to finish, while the other was sprayed with T400 and then single point turned (which is cheaper than grinding
and can be used on many alloy sprays). With hydraulics the primary concern is leakage due to seal damage, and it is
unclear what surface finish will be needed. EHC usually has a fairly rough finish to hold the hydraulic fluid, but some
thermal spray companies recommend a superfinished surface, which is a finishing technique widely used in Europe,
but not widely used in the US. Application of HVOF to hydraulics may therefore require a change in finishing
procedures and specifications.
3.6.
Costs of alternative coating methods
Cost evaluations have been made for piston rings, based on data from Cummins Piston Ring Divis ion. What is
important is not the cost of coating but the cost of production, which incorporates all the finishing, waste disposal,
legal, and regulatory costs. We have incorporated the capital cost of the facilities into the model for consistency,
and because most chrome platers are finding that meeting increasingly stringent regulations is mandating capital
expenditures. (Corpus Christi Army Depot has spent many millions of dollars on new plating shops, while Tinker
AFB has had to make multi-million dollar improvements to their existing shop.) The model incorporates the following:
l
Capital (15yr straight line depreciation)
l
Labor (various types of workers)
l
Materials
l
Maintenance (including up-time)
l
Waste treatment/removal
l
Utilities (water, electricity, floor space)
l
Legal/regulatory
Some of these are well-defined (such as cost for waste disposal or typical labor costs), while others are difficult to
gauge (such as future legal and regulatory costs). We therefore do not see our costings as accurate absolute
numbers, but as pointers to the important cost factors and the generally expected costs of the different technologies.
However, our numbers compare very well with Cummins’ internal cost estimates for piston ring coatings, which gives
us a reasonable degree of confidence in the model.
24
The results from our model applied to piston ring
production are shown in Figure 29. Note that in
each case the alternative coatings are more
expensive than chrome plate. However, the
savings in finishing, waste disposal, and
regulatory costs are expected to more than make
up for the increased processing cost. In this
case, large savings derive from reduction of
finishing cost. The hard chrome plated rings
must be ground and finished in a large number of
steps, while the HVOF coated rings can be simply
lapped, and the PVD coatings require no further
finishing. For this reason, piston rings are a
special case - most other applications will require
about the same finishing cost for HVOF as for
EHC. However, we see that even if the same
finishing cost is added into the other coating
technologies, the HVOF and thin PVD will still be
cost competitive, while the thick PVD will become
a little more expensive.
Figure 29. Costs of producing a stack of piston rings with
different coatings.
In Figure 30 we break out the costs by general
cost category in order to
determine the total costs for
$350
different coating processes. It is
$300
clear that chrome plating is labor
intensive. Again, this might be
$250
surprising, since in chrome
$200
plating, the items are simply
Other
deposited in a bath and left until
Materials
$150
properly coated, then moved
Labor
$100
(usually automatically) to the
Depreciation
various rinse baths. However,
$50
the plating time is often many
hours, it is preceeded by
$0
EHC
HVOF
3 um
15
15 um
masking (which is often
Sputter
micron
Arc PVD
extensive in DoD depots), and
PVD
Sputter
followed by extensive grinding.
In HVOF the throughput is very
Figure 30. Total costs for coating and finishing a stack of rings, broken down
high (typically 5-15 minutes per
by cost category.
item), and masking is usually
simple (using mechanical masks). In DoD set-up will generally be a larger factor, since maintenance depots must coat
many different items, each of which may require careful setting up with respect to the gun. The capital cost of HVOF
is also quite small, of which the largest cost is typically the physical installation (the sound proof room and its
associated exhaust and dust collection systems. The deposition times for PVD coatings are typically 4 hours floor to
floor, requiring extensive operator supervision.
In order to understand the primary cost factors we have broken out the cost by category for several types of
coatings on a pis ton ring stack. These are shown in Figure 29 to Figure 34. Note that the major cost for the hard
chrome plating in this instance is the grinding. For HVOF the coating materials (primarily powder) dominate. Other
major materials costs for HVOF are the gases (large volumes of hydrogen and oxygen, typically). Surprisingly, given
the high capital cost of PVD coating, the largest factor is the cost of materials. This is true when one uses arc or
sputter deposition, which require a specially manufactured, high purity “target” (plate of deposition material). If one
can use evaporative methods the cost drops dramatically. Unfortunately the evaporative techniques generally
25
require higher deposition temperatures, which can damage the heat treat of sensitive steels and aluminum alloys. As
the coatings become thicker the cost of materials for PVD coating becomes increasingly high.
These calculations are, of course, simply calculations of production cost. Much bigger life-cycle cost savings ensue
from the fact that the HVOF (and in some cases the thicker PVD) coatings are predicted to outlast hard chrome,
typically by a factor of three (according to GEAE’s experience). There are some components whose disassembly for
inspection is the primary source of damage, and for these alternative coatings are unlikely to offer longer service life.
For most other components, however, increased wear life should translate into less frequent recoating and lower lifecycle cost. There have been other calculations of life-cycle cost
Other
35%
$30.00
Deprec.
8%
$25.00
$20.00
Labor
35%
$15.00
Finishing/QA
$10.00
Pre-treating
Materials
23%
Materials
$0.00
Labor
Coating
Depreciation
$5.00
Figure 31. Cost breakdown for a stack of piston rings - standard chrome plate.
$45.00
$40.00
$35.00
$30.00
$25.00
$20.00
$15.00
$10.00
$5.00
$0.00
Other
6%
Labor
31%
Finishing/QA
Coating
Materials
Labor
Depreciation
Pre-treating
Materials
58%
Figure 32. Cost breakdown for a stack of piston rings - HVOF coating.
26
Deprec.
5%
$60.00
Other
9%
$50.00
$40.00
Deprec.
10%
$30.00
Finishing/QA
$20.00
$0.00
Pre-treating
Labor
32%
Materials
49%
Materials
Labor
Coating
Depreciation
$10.00
Figure 33. Cost breakdown for a stack of piston rings - duplex PVD coating, 3 micron thick sputtered.
$300.00
Other
6%
$250.00
Deprec.
5%
$200.00
$150.00
Finishing/QA
$100.00
Coating
Pre-treating
Materials
75%
Materials
Labor
Depreciation
$50.00
$0.00
Figure 34. Cost breakdown for a stack of piston rings - PVD coating, 15 micron thick sputtered.
27
Labor
14%
$25.00
$20.00
$15.00
$10.00
HVOF
$5.00
PVD
$0.00
18"
5"
Figure 35. Costs of coating 1" diameter
hydraulic pistons of two different
lengths.
When should one consider PVD rather than HVOF? The primary
purpose of PVD chrome replacements is for OEMs, and is dictated by
performance and cost. Depots cannot use PVD coatings for
maintenance since they cannot be used to rebuild dimensions. Depots
could, however, use PVD coatings to recoat previously PVD-coated
components that have suffered small wear (less than a few tenths of a
thousandth). In most cases, PVD coated parts can in principle be
ground down and recoated with HVOF. PVD coatings become costeffective for small components. As Figure 35 shows, HVOF coatings
become very inefficient for small components because of the
overspray at the ends of the part (i.e. the torch must be traversed
beyond each end to ensure even coverage). PVD coatings are also
thinner and harder than most HVOF coatings. Therefore some wear
situations may call for PVD rather than HVOF, as has been done in
some turbine engines.
28
4.
4.1.
Recommendations
Hard chrome replacement
This program has demonstrated the gains to be made by replacing hard chrome with modern alternatives. We
recommend the following:
•
HVOF coatings (primarily WC-Co for simplicity) should be demonstrated and validated for replacement
of hard chrome in depots and OEMs. The method is particularly appropriate for large components,
permits reliable build-up of worn areas, and fits well with existing depot maintenance procedures and
capabilities.
•
PVD coatings show a great deal of promise for piston rings and may be viable for small aerospace
components as well – in fact, they are already being used in some engine components and turbine
blades. They should not be considered for use at depots because they cannot be used for rebuilding
worn components. A comparison of duplex coating and thick coating is needed to determine which
offers the more cost-effective solution for relatively soft substrate materials.
•
Laser coating (cladding) should not be considered for depot use, since it is too easy to damage the
underlying component.
In order to bring these alternatives into use by OEMs and depots, it will now be necessary to demonstrate and
validate them to the satisfaction of users - the program managers in charge of the military’s weapons systems. Such
demonstration and validation needs to produce the following data:
•
Desired coating structure and general deposition methods.
•
Reliably attainable coating properties – hardness, adhesion, stress, etc.
•
Coated component performance – wear, corrosion, fatigue
•
System performance – hydraulic fluid leakage, engine testing, etc.
•
Manufacturability issues – stripping, finishing, QC testing, etc.
•
Cost determinations for specific DoD production or maintenance requirements – total production cost,
life-cycle cost, capital cost
The political aspects of a successful demonstration and validation program will be at least as important as the
technical aspects, and should include
•
Forming a comprehensive team of OEMs and depots
•
Incorporating weapons system PMs and engineers
A block diagram of the basic steps that will be required is given in Figure 36.
29
HVOF: Piston Rings
Optimize coating method
Optimize ring/coating system design
Analysis
500 hr engine
test
Rings: Integrate plasma nitride
with heat shape
PVD/Duplex PVD:
Pilot production set-up
Pilot tests
Production
1000 hr engine test
500 hr engine
test
Define final process
specs
Optimize process
DoD components: Transfer to
commercial vendors
HVOF: DoD components - Transfer to
maintenance depots, ALCs
Test and validate on typical OEM components
Modify and transfer composite
process for DoD components
Validate for
depot use
Adequately test critical propertie/performance
Training system development
Similar
programs for
Army, Navy,
Air Force
Install or modify
HVOF systems at
bases as needed
Identify components, define
processes - OEMs and bases
Base personnel training
Test and validate on specific maintenance
components
Test/transfer process
parameters to vendors
Approved
uses
QC testing/evaluation
Figure 36. Basic steps needed to bring hard chrome alternative coatings to use at OEMs and maintenance
depots.
30
Section 2.
Electrolytic Hard Chrome Clean-up Technologies
It is clear that it will take some years to bring about replacement of EHC in DoD, regardless of how good the
alternatives are, since aerospace engineers and program managers are, of necessity, very careful and conservative in
modifying flight critical components. In the meantime, regulations will continue to tighten and military plating
operations will continue to be a source of hazardous wastes. Therefore, we must also find ways of reducing waste
streams from existing plating operations without affecting product quality.
Approach: Various DoD operations have made significant strides in recent years in reducing plating wastes, usually
by a combination of modernizing waste treatment (e.g. OC-OLC) and installation of new plating facilities (e.g. CCAD).
A careful evaluation of how DoD uses EHC showed that the primary unaddressed problems are waste generation by
stripping of old chrome, monitoring and control of the plating process to keep it efficient (minimal waste and
emissions), reduction of lead chromate anode sludge, and reduction of hexavalent air emissions. Our approach has
addressed each of these problems.
Results: The results are summarized in Figure 37:
l
Hexavalent emissions have been reduced by >95% by using a fluorosurfactant bath additive, and it has
been shown that this additive is not deleterious to bath performance or coating quality. By reducing air
emissions it also reduces usage of plating solution and rinse water for scrubbers.
l
We have developed a simple flow injection bath chemistry monitor that is very much more cost-effective for
maintaining plating bath control than the complex chemical methods in current use. This instrument takes a
few minutes to measure concentrations of Cr3+, Cr6+, SO4-, Fe and Cu. Cummins currently has to send out
samples for analysis - a process that takes about a week.
l
A method of chrome stripping has been developed that is very effective for many DoD steels. Since it uses
chromic acid, the stripped chrome can be directly returned to the plating bath, which cannot be done with
HCl or anodic stripping methods. This is very important for DoD since each component to be plated must
first be stripped.
l
New anode materials have been evaluated to replace lead and lead-tin. For some plating operations lead
chromate sludge from anode dissolution is the major bath solid waste, and replacing the anode would
eliminate this waste stream.
Each of these approaches is described below. We estimate that very large reductions are possible in all plating waste
streams by the combined use of these approaches, as summarized in Figure 38.
31
STRIPPING
SOLUTION
Method: Chromic
acid solution
Eliminates Cr-contaminated
waste acids and bases
Primary use in
depot rebuilidng
Waste streams:
Reduces waste by
direct chrome recycling
PLATING AIR
EMISSIONS
FLOW
INJECTION
ANALYSIS
Method: Add
surfactant to
plating bath
Reduces chrome
consumption
Can be used for hard and
decorative chrome
Waste streams:
No adverse effect on
plating quality
Reduces waste by
suppressing mist
Does not affect tank access
Method: Solution
drawn off and
analyzed optically
Waste streams:
Very simple, easy to use requires no special skills
Estimated
reduction in
depot Cr6 +
waste 50%
OEM production
Estimated
reduction in air
emissions -
Depot rebuilding
97%
Meets
proposed
Clean Air rules
OEM production
Estimated
reduction in
Cr 6+ waste -
Reduces analysis time from
hours or days to minutes
Depot rebuilding
25%
Monitors
Cr 3+
, Cr 6+platers’
,Fe, Cu,
Reduces
some
SO
primary
solid waste source
4
OEM production
Estimated
reduction in
solid waste -
Depot rebuilding
90%
Reduces waste by
greater process
efficiency, less replating
NEW
ANODE
MATERIALS
Method: Replace
Pb with Pt-Ti or
conducting ceramic
Waste streams:
Eliminates lead
chromate sludge
Figure 37. Results of bath chemistry evaluations.
32
140.0
120.0
100.0
Hexavalent chromecontaminated waste
(lb/engine)
80.0
60.0
Rinse Water
40.0
Plating Solution (x1000)
Air Emission (x1000)
20.0
Acids. bases (x10)
HVOF or PVD alternatives
All electrochemical
Fluorosurfactant
Bath Solids (x10)
Non-lead anodes
Tank monitor/control
Chromic strip/recycle
Present
0.0
Figure 38. Waste streams from EHC process by applying various waste stream reduction methods.
33
5.
5.1.
Bath Chemistry Analysis
Introduction
A plating bath analysis system can help to reduce waste in hard-chrome plating operations in several ways. The
efficiency of a chrome plating bath is a function of its chemistry. If the ratio of hexavalent chromium to sulfate is
incorrect, or if concentrations of trivalent chromium or certain impurities become too high, the efficiency of the bath
will drop. Decreased efficiency leads to longer plating times and therefore greater amounts of chromium-containing
mist and lead chromate sludge. If the concentrations of impurities becomes great enough to adversely affect the
quality of the deposit, a new source of waste associated with regrinding and recoating improperly plated parts is
created. In extreme cases it can become necessary to dispose of a bath entirely, leading to yet another source of
waste.
Over the course of this program we asked platers at government and large and small commercial plating facilities
about their bath analysis methods. Most platers seemed to fall into one of three categories: those who do little
routine bath analysis, those who regularly send samples out, and those who regularly do analysis in house.
However regardless of the methods they use, all platers seemed to have similar opinions about bath analysis. All
platers agreed that it is important; those who choose not to have their plating solutions regularly analyzed do so for
reasons of cost. Platers who send their solutions out expressed concerns about long turnaround times (generally a
minimum of two or three days) and reliability. Those who send samples to independent laboratories (as opposed to
another division within the organization) were also concerned about high cost. Those who wish to do analysis inhouse must hire a trained chemist and make a significant investment in equipment. All platers we spoke with
indicated that an inexpensive instrument capable of rapidly analyzing plating solutions and requiring relatively little
operator training would be extremely desirable.
In the approximately 70 years since hard chrome plating became an industrial technique, a great many methods have
been developed for bath analysis. These range from simple techniques such as hygrometry and visual assessment of
color to a great many wet-chemical procedures to newer instrumental methods such as atomic absorption
spectroscopy and ion chromatography. The goal of this program was to develop a system that would be less
expensive and easier to use than the more sophisticated techniques, yet more accurate, versatile, and easily
automated than the simple methods. We chose to base this method on continuous flow analysis with colorimetric
detection. There are several reasons for this choice. First, the equipment required is inexpensive and/or robust.
Continuous flow analyzers are based on pumps, tubing, connectors, and valves. Colorimetric detection requires light
bulbs/filters or LED’s, flow cells, and detectors. These components range in price from a few cents (for some LED’s)
to at most $300 (for some cells). Second, this technique lends itself well to computer control, and third, instruments
are relatively easy to operate and (much easier than atomic absorption, ion chromatography, or ICP, for example), and
should require no formal training in analytical chemistry.
On the basis of discussions with platers and initial analyses of numerous plating bath samples, we came to the
conclusion that it is most important to analyze for two bath constituents, hexavalent chromium and sulfate, trivalent
chromium, and two impurity metals, copper, and iron. Hexavalent chromium and sulfate are the only two components
of sulfate-catalyzed (Sargent) baths, which are widely used in both industrial and government facilities. Trivalent
chromium, iron, and copper can all have a deleterious effect on both bath efficiency and deposit quality. Trivalent
chromium is formed at the cathode as metal is deposited. Although it is ideally reoxidized to hexavalent chrome at the
anodes, initial analyses of bath samples indicates that very high levels of trivalent chrome can build up (as high as 27
g/l, or more than one-fifth of the total chromium). Iron is generally introduced into the bath through dissolution of
the plated part (which is generally made of an iron-based alloy), or, in cases where conformal iron anodes are used,
through dissolution of the anodes. Copper is introduced into the bath primarly through dissolution of copper bus
bars, copper brushes, or copper-containing base metals. Although trivalent chromium, copper, and iron can usually
be easily removed using methods such as porous pots, it is in general necessary to analyze the baths to determine
when impurity levels are becoming too high.
34
5.2.
Background
Any system for the colorimetric analysis of chromium plating baths must take into account the extremely strong color
of the plating bath itself. Common uncatalyzed hard chrome plating baths typically contain only two components chromium trioxide (approximately 250 g/l or 2.5M), and sulfuric acid (approximately 2.5 g/l or 0.026M). The main
chromophores in the bath are hexavalent-chromium-containing species, of which four are believed to exist (CieslakGolonka 1991, Hoare 1979, Hoare 1989). These are chromate ion (CrO42-), dichromate ion (Cr2O72-), trichromate ion
(Cr3O102-), and tetrachromate ion (Cr4O132-). Each of the ions has a different spectrum, with absorption bands shifting
to the red as the molecular weight of the ion increases. The relative concentration of the different ions depends on a
number of factors, including total concentration of hexavalent chromium in solution. Figure 39 shows spectra of new
plating solution (250 g/l chromic acid, 2.5 g/l sulfuric acid) taken in a standard 1 cm pathlength cuvette at various
levels of dilution.
4
new plating solution
10:1 dilution
100: 1 dilution
1000:1 dilution
10,000:1 dilution
absorbance
3
2
1
0
200
300
400
500
600
700
800
900
1000
wavelength (nm)
Figure 39. New Chrome Plating Solution at Various Levels of Dilution in Water.
Any photometric method of determining components in hard-chrome plating baths must either contend with (or use)
the the strong visible and UV absorption bands of the chromium-containing species, reduce the chromium to its
trivalent form (in which case one must contend with weaker but still prominent absorption bands in the visible region
of the spectrum), or remove the chromium from solution prior to analysis. Although the latter option might seem the
most desirable, removing chromium from solution generally requires precipitation using solutions containing lead (or
other highly toxic heavy-metal ions) and then filtration. Because of the danger of this approach, as well as the
necessity of creating hazardous wastes as part of the analysis procedure, we chose methods of analysis that do not
require the removal of chromium from solutions prior to analysis. Below we describe the analysis methods used for
copper, iron, sulfate, hexavalent chromium, and trivalent chromium.
35
5.3.
Copper
Analysis of chrome-plating solutions obtained from Cummins Engine, Corpus Christi Army Depot, and several
industrial hard-chrome plating facilities in the Chicago area indicate that copper is one of the most common
transition-metal impurities in hard-chrome plating solutions. It is most likely introduced through the dissolution of
bus bars, brushes, and/or copper-containing base metals. If concentrations of copper and/or other transition metals
(usually iron) become too high (several grams/liter), both current efficiency and deposit quality are adversely
affected. As the current efficiency drops, plating times must be increased. This leads to greater release of hexavalent
chromium through misting and generation of more lead chromate sludge through decomposition of the anodes. It
also raises plating costs by increasing energy consumption and reducing the number of parts that can be plated per
day. In the worst cases, it can sufficiently degrade deposit quality that regrinding and replating of the part become
necessary.
We tested two methods of determining copper. The first was based simply on direct absorption of copper in the near
infrared, and the second on the use of a concentrated NaBr/HBr solution to produce a strongly colored copper bromide complex. The first method has the advantage of simplicity while the second offers greater accuracy.
Chronologically, the method based on direct detection was tested after the method based on NaBr/HBr had been
developed. The two methods are described in detail below.
5.3.1.
Direct Near-Infrared Determination of Copper
It might appear somewhat surprising that direct absorption is a suitable method for determining copper in hardchrome plating solutions. The reason for this is that hard-chrome plating solution is very strongly colored while
copper solutions are only weakly colored even at high metal-ion concentration. Direct absorption had been used
previously to determine copper concentration in copper plating solutions (Freeman et al., 1985), where copper metal
concentrations are extremely high and other colored species exist only as impurities. However we were unable to find
mention of the use of direct absorption to determine copper in any other work on either chromium plating, analysis of
plating solutions, or spectrophotometric/colorimetric determination of the elements (see, for example, Foulke and
Crane, 1963, Lowenheim, 1978, Irvine, 1970, Morriset et al., 1954, Metal Finishing Guidebook 1995, Marczenko et al.,
1976, Onishi et al., 1986). Nevertheless, further consideration suggests that direct absorption in the near-infrared is
indeed a suitable method for determining copper in hared-chrome plating solutions.
As seen in Figure 39, the primary absorption bands of the hexavalent chromium species present in hard-chromium
plating solution appear in the ultraviolet region of the spectrum; the red color of the solution is due to the longwavelength tail of these bands. On the other hand, the primary absorption band of copper, shown in Figure 40, is
located in the near infrared and the blue color of copper is caused by the short-wavelength tail of this band.
36
0.6
0.5
absorbance
0.4
0.3
0.2
0.1
0.0
400
500
600
700
800
900
1000
1100
wavelength (nm)
Figure 40. Spectrum of aqueous copper sulfate solution containing 2500 mg/l copper metal in 1-cm pathlength
cuvette.
Therefore, there is virtually no interference at the main copper absorption peak due to hexavalent chromium. Further,
most of the other primary colored species commonly found in hard-chrome baths, including trivalent chromium and
iron, also have absorption bands widely separated from that of copper. Consequently, it is reasonable to assume that
a simple absorption sensor using a light source having the same wavelength as the copper absorption maximum
should be well suited to measuring copper in chromium plating solution. To evaluate the accuracy of absorptiometric
determination of copper samples of a number of different solutions were analyzed and the results compared to those
of two independent laboratories, TEI Analytical (Niles, IL), and Northwestern University’s Analytical Services
Laboratory (ASL). All absorption measurements were performed on a Varian 2400 spectrophotometer, while both of
the analytical laboratories used ICP to analyze for copper. The results of these determinations are shown in Figure
41.
37
6000
Copper (mg/l) TEI
Copper (mg/l) ASL
Copper (mg/l) direct abs @ 800 nm
Copper Concentration (mg/l)
5000
4000
3000
2000
1000
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sample
Figure 41. Comparison of copper concentrations determined by direct absorbance at 800 nm with those of two
independent laboratories using ICP.
As seen in Figure 41, the concentrations determined by absorption at 800 nm are in reasonably good agreement with
those obtained by the two analytical laboratories. We therefore proceeded to fabricate a stand-alone photometric
module suitable for use in determining copper either alone or as part of an automated flow analyzer. A schematic of
this device is shown in Figure 42.
FromPump
Return
Detector/
Amplifier
LED
FlowCell
Figure 42. Schematic of optical system.
An 808 nm light-emitting diode (LED) (Quantum Devices
QDDH80001) is used as a light source. This LED was chosen
because its emission wavelength corresponds approximately to the
copper absorption maximum (see Figure 40). The detector is an
integrated photodiode/transimpedance amplifier (Burr-Brown
OPT201). A 1-cm pathlength absorption cell (Starna 75-G-2) is
positioned between the LED and the detector amplifier. As shown in
Figure 43, the diode is powered by a 6-volt lantern battery and the
detector/amplifier by two 9-volt batteries. Because of the low
currents drawn by the LED/resistor and detector/amplifier, the
batteries are suitable power sources even for extended use. A
computer is used to record voltage from the detector/amplifier and
calculate copper concentrations. Since no power supplies are used,
the cost of the electronics (other than the computer) is extremely low
and the module is well suited to use as an inexpensive stand-alone
copper detector.
38
Burr-Brown OPT201
detector/amplifier
7620 ohms
Quantum Devices
QDDH80001
LED
2
4
5
+
Vo
8
6-volt
lantern battery
1
+V
3
-V
9-volt batteries
Figure 43. Schematic of LED and detector/amplifier circuits.
The apparatus is first calibrated using the following procedure. The cell is filled with new plating solution containing
no copper and a background voltage, Vbackground, is measured. Next the cell is filled with a sample of plating solution
containing a known amount of copper and a second voltage, Vstandard, is measured. (It is important that all solutions
be at the same temperature when measurements are performed.) The output voltages of the detector/amplifier,
Vbackground and Vstandard, are proportional to the intensity of light transmitted through the respective solutions. They can
be related to the concentration of copper through Beer’s law4, which in our case takes the form
 Vbackground 
log 10
 = ε lc.
 Vstandard 
Here l is the pathlength of the cell, c is the concentration of copper and D is the extinction coefficient. Since l and c
are both known, Beer’s law is solved to give D. Once D is known, samples of plating solution can be analyzed using
essentially the same procedure. The cell is first filled with new plating solution to obtain a background voltage, and
next with a bath sample to obtain a sample voltage. Once these voltages have been measured, Beer’s law is solved to
give the copper concentration, c.
The results of the copper determinations using the photometer described above are shown in Figure 44. If each of the
measurements is assumed accurate to within ± 10% (the level of accuracy estimated by one of the analytical labs),
then the results of all measurements are in good agreement. More importantly, since transition-metal concentration
measurements are generally used to form the basis of a “go/no-go” decision on bath usage or cleanup, the
measurement accuracy is more than adequate for hard-chromium plating applications
39
5000
4500
Copper Concentration (mg/l)
4000
Copper (mg/l) Lab A
Copper (mg/l) Lab B
Copper (mg/l) This work
3500
3000
2500
2000
1500
1000
500
0
1
2
3
4
5
6
Sample
Figure 44. Results of copper measured using LED-based photometer compared with determinations by TEI
Analytical and Northwestern’s Analytical Services Lab.
5.3.2.
Determination of Copper Using NaBr/HBr Method
The second method of determining copper tested was based on the formation of a copper-bromide complex that is a
relatively strong absorber. The use of bromide to determine copper has been mentioned previously (Marczenko,
1976). A schematic of the experimental apparatus is shown in Figure 45.
40
Pump
To Waste
NaBr/HBr Solution
M ixing Coil
Cuvette/
Spectrophotometer
Pump
Plating Solution
Figure 45. Experimental Apparatus Used to Determine Copper by Bromide Method.
In this technique, a mixture of 90% saturated NaBr/concentrated HBr is mixed with new plating solution in the ratio of
approximately 5.3 : 1. The solution is then passed through a mixing coil and into a 1-cm pathlength cuvette inside a
Varian 2400 spectrophotometer. A typical spectrum of the copper/bromide complex is shown in Figure 46. The
spectrum is characterized by a very broad peak having a maximum at approximately 900 nm. As seen by comparison
with Figure 40, the maximum extinction coefficient of the copper/bromide complex is not only considerably higher
than that of aqueous copper, but is also shifted further into the near infrared where it is less subject to interference
from hexavalent chromium and other species having absorption in the visible.
41
absorbance
1
0
700
800
900
1000
1100
1200
wavelength (nm)
Figure 46. Spectrum of Plating Solution Containing 1.57 g/l Copper Mixed with NaBr/HBr Solution.
Copper concentrations in various plating solutions were determined based on absorption at 900 nm. These results,
along with those of two independent analytical laboratories, are shown in Figure 47. As seen in the figure, the
agreement between the determinations based on the NaBr/HBr method and those of the two independent laboratories
are excellent. However, because the simpler method based simply on direct absorption at 800 nm proved adequate,
we chose not to develop the NaBr/HBr method for use in the automated analyzer.
42
6000
Copper (mg/l) TEI
Copper (mg/l) ASL
Copper (mg/l) NaBr/HBr
5000
Copper Concentration
4000
3000
2000
1000
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Sample
Figure 47. Comparison of Copper Concentrations Determined by NaBr/HBr method with those of two
independent laboratories using ICP.
5.4.
Iron
5.4.1.
Introduction
Analysis of chrome-plating solutions obtained from Cummins Engine, Corpus Chris ti Army Depot, and several
industrial hard-chrome plating facilities in the Chicago area show iron to be the most common transition-metal
impurities in hard-chrome plating solutions. It is most likely introduced through the dissolution of ferrous base
metals. As is the case with copper, if iron concentrations become too high (several grams/liter), both current
efficiency and deposit quality are adversely affected. As the current efficiency drops, plating times must be
increased. This leads to greater release of hexavalent chromium through misting and generation of more lead
chromate sludge through decomposition of the anodes. It also raises plating costs by increasing energy
consumption and reducing the number of parts that can be plated per day. In the worst cases, it can sufficiently
degrade deposit quality that regrinding and replating of the part become necessary.
Due to the strongly oxidizing environment of chromium plating baths, iron is present as ferric ion. Although ferric ion
is a strong absorber, its absorption bands overlap with those of hexavalent chromium to a sufficiently large degree
that direct absorption cannot be used to determine iron concentration (see Figure 48). An alternative method must
therefore be used.
43
absorbance
2
1
0
200
300
400
500
600
700
800
wavelength (nm)
Figure 48. Spectrum of 0.06 g/l Fe(NO3)3.9H2O (1.48 x 10-4M) in 1-cm cuvette. The molar extinction
coefficient is approximately 2.7 x 103 at 300 nm.
A large number of reagents have been used for the spectrophotometric determination of iron [Onishi 1986,
Marczenko 1976]. The method chosen here is based on the use of thiocyanate ion, which forms strongly colored
complexes with ferric ion (primarily Fe(SCN)2+ - see Figure 10). We selected this method for several reasons. First,
iron thiocyanate is a very strong absorber that has its primary visible absorption band to the red of hexavalent
chromium absorption peaks (a molar extinction coefficient of 8500 at has been reported [Marczenko 1976]). Therefore,
iron can be determined even in the presence of the large concentration of CrO3 present in chromium plating baths.
Second, there seems to be little interference due to common impurity ions such as copper, nickel, calcium, trivalent
chromium, etc., and third, thiocyanate is inexpensive, stable, and safe to handle, and therefore attractive from the
standpoint of practicality. There are some drawbacks to the use of thiocyanate, however. The chemistry of the iron thiocyanate system is somewhat complicated, involving a number of different complexes [Lewin and Wagner 1953,
Marczenko 1976, Onishi 1986, Perrin 1958, Perrin 1964, Sultan and Bishop 1982]. Further, thiocyanate is oxidized by
ferric ion, resulting in a reduction in absorption strength as a function of time. This problem is exacerbated in
chromium plating solution, as CrO3 is not only a stronger oxidizing agent than ferric ion, but also present in much
greater concentration. Nevertheless, as discussed below, thiocyanate proves to be well-suited to the determination
of iron in chrome-plating solutions.
44
0.8
0.7
iron thiocyanate
absorbance
0.6
0.5
0.4
0.3
0.2
0.1
0.0
400
500
600
700
800
wavelength (nm)
Figure 49. Iron - thiocyanate complex. The peak absorbance occurs at 473 nm. As discussed in the text, the
extinction coefficient decreases with time.
5.4.2.
Development of Analytical Procedure
The analytical procedure for iron
was developed using the
apparatus shown in Figure 50.
Plating solution is sampled using
one channel of a two-channel
Pump
To Waste
peristaltic pump (ColeKSCN Solution
Parmer/Ismatec 7334-50). The
Cuvette/
or Water
Mixing Coil
Spectrophotometer
second channel of the pump
delivers either potassium
thiocyanate solution (50 g/l or
.51M) or water (used for
background measurements). The
Pump
ratio of KSCN solution or water to
Plating Solution
plating solution is approximately
55:1. Tygon tubing is used in
both channels of the pump. The
liquids pass through a mixing coil
consisting of a length of Tygon
tubing tied into an extended knot
Figure 50. Schematic of thiocyanate analytical procedure.
(similar devices are sometimes
referred to as knitted coil reactors [Karlberg and Pacey, 1989]) and then into a 0.1 mm-pathlength flow cell located
45
inside a Varian 2400 spectrophotometer. Spectra of the plating solution/KSCN mixtures obtained using this
apparatus are shown in Figure 51 for plating solutions of several different iron concentrations.
new plating solution (no iron)
0.28 grams/liter iron
0.57 grams/liter iron
0.82 grams/liter iron
absorbance
1
0
500
600
700
wavelength (nm)
Figure 51. Spectra of iron-thiocyanate complex in chromium plating solution.
During the development phase of this work, iron determinations were based on measurements of the absorbance of
the plating solution/thiocyanate mixture was measured at 473 nm, which corresponds to the absorption peak of the
iron - thiocyanate complex (Figure 49). The following procedure was used. First, new plating solution containing no
iron was diluted with water and the absorbance at 473 nm, A new,water, was measured. (As seen in Figure 1, plating
solution has significant absorbance at 473 nm.) Next, the same iron-free new plating solution was diluted with KSCN
solution and the absorbance A new,KSCN measured. This step was required because the reduction of plating solution
by KSCN causes a slight shift in the absorbance of the plating solution even in the absence of iron. (The procedure
described later for the apparatus used with the automatic analyzer eliminates the need for this step.) Next the new
plating solution is replaced with a standard plating solution containing a known amount of iron. This solution is first
diluted with water and then with KSCN solution and the absorbances A std,water and A std,KSCN are measured. The change
in absorbance in the standard due to formation of the iron thiocyanate complex alone is then calculated as
¦A std = (A std,KSCN - A std,water) - (A new,KSCN - A new,water).
(The second term in this expression, A new,KSCN - A new,water, is meant to correct for the change in absorbance due to the
reduction of CrO3 by KSCN. This term is strictly correct only if all plating solutions contain the same amount of CrO3.
However this term is small relative to A std,KSCN - A std,water, and corrections due to changes in the concentration of CrO3
appear to be unimportant.) Once ¦A std has been calculated, the procedure is repeated for the sample solutions. The
change in absorbance of the sample due to the formation of iron thiocyanate complex alone is calculated as
46
¦A sample = (A sample,KSCN - A sample,water) - (A new,KSCN - A new,water),
and the iron concentration of the sample is calculated using the formula
 ∆A

[ Fe ]sample = [ Fe]std  sample ∆ A 

std 
where [Fe]sample is the iron concentration in the sample, and [Fe]std is the iron concentration in the standard. Tests
using samples of known iron concentrations showed good agreement between calculated and known iron
concentrations. However actual plating solutions can contain a variety of impurities. Therefore to test the accuracy
of our method under more realistic conditions, we compared iron concentrations determined using our method to
those determined by two independent analytical labs using ICP for a number of samples obtained from plating shops.
A graph comparing the iron concentrations these solutions determined by our method with those determined by the
two independent laboratories is shown in Figure 52. This graph indicates that the accuracy of the KSCN method is
comparable to that of the methods used by the independent labs. More importantly, since iron concentration is
generally used only as the basis of a “go/no-go” decision on whether to use or treat the bath, the iron concentrations
determined by the KSCN method are more than accurate enough for the application.
47
8000
7000
Iron (mg/l) TEI
Iron (mg/l) ASL
Iron (mg/l) KSCN
6000
Iron Concentration
5000
4000
3000
2000
1000
0
1
2
3
4
5
6
7
8
9
10
11
Sample
Figure 52. Comparison of Iron Concentrations Determined by BIRL/KSCN method with those of two
independent laboratories using ICP. TEI is TEI Analytical, Niles IL, and ASL is the Analytical Services
Laboratory at Northwestern University.
5.4.3.
Automated Iron Analyzer
The apparatus and procedure described above was modified somewhat for use in the automated determination of
iron. A schematic of the automated iron analyzer is shown in Figure 53.
48
Computer
Valve
Pump
Water
Mixing
Coil
To waste
Filter
KSCN solution
LED
Detector
Cuvette
Pump
Plating solution
Figure 53. Schematic of Iron Analyzer.
The most significant difference between this apparatus and that used initially to test the KSCN method is that the
spectrophotometer has been replaced by a light emitting diode (LED), interference filter, and detector. The light
source is a Ledtronics T 1-3/4 5mm BP280CWB1K-3.6vf-050T ultra bright blue LED. The light is passed through a 510
nm interference filter (ESCO S915100). It then passes through a 0.1 mm-pathlength cuvette and onto a
photodiode/transimpedance amplifier (Burr-Brown OPT-201 - the same detector/amplifier used previously in the
copper detector). A spectrum of the light produced by the LED/interference filter is shown in Figure 54.
49
relative intensity
1
0
490
500
510
520
530
wavelength (nm)
Figure 54. Spectrum of light emitted by LED/photodiode.
50
4
A
B
C
D
absorbance
3
2
1
0
450
460
470
480
490
500
510
520
530
540
550
wavelength (nm)
Figure 55. Spectra of iron-free plating solution mixed with KSCN solutions in 2-mm pathlength cell. Trace A is
plating solution diluted with water (1:55). Traces B and C are plating solutions diluted with 5% and
10% KSCN solutions, respectively. Trace D is plating solution diluted with 10% KSCN solution, pump
running at half speed.
6.
Mist Minimization
The EPA recognizes airborne chromium as the most significant source of hexavalent chromium emissions in
chromium plating operations in part because of the toxicity of this form of chromium (EPA 1993). They estimate that
approximately 200 tons of chromium are released into the air annually. Along with Dover Industrial Chrome, we
attempted to estimate the magnitude of the reduction in airborne chromium that can be achieved through the use of
fluorinated surfactant additives, as well as the nature and magnitude of problems (if any) associated with the use of
these additives.
51
Table 8. Surfactant additions to EHC tanks.
Tank Number Tank Capacity (gal) Additive Amount (gal)
1
800
1
2
900
1
3
1500
2
6
800
1
7
1200
1.5
8
400
0.5
Beginning in January 1994, Dover began adding a perfluorinated ionic surfactant (sodium perfluorooctylsulfonate or
similar perfluorinated alkyl sulfonate) to each of their plating tanks. The manufacturer claims that this mist
suppressant is effective in reducing more than 99% of the airborne emissions from chromium tanks. The product was
marketed in Europe prior to its introduction in the United States. Surfactants offer significant advantages over other
methods of reducing mist. They avoid the cost and effort associated with elaborate air handling Systems and the
significant inconvenience of polypropylene balls or "UFO's" used to cover tanks.
The additions to Dover tanks are summarized below.
It was qualitatively apparent that the surfactant was extremely effective in suppressing mist. In order to estimate the
magnitude of the effect, measurements of surface tension reduction and mist reduction were performed. Surface tension
was measured using a Cenco tensiometer. Measurements were performed on the #6 bath prior to the addition of
surfactant and at the same time on the #1 bath to which surfactant had already been added. Substantial reductions in
surface tension were confirmed. These are summarized in the table below:
Sample
Surface Tension (dynes/cm)
Water
72
Bath #6 (no surfactant)
53
Bath #1 (0.13% surfactant)
28
To assess the magnitude of mist reduction, we sampled air above the number 6 tank before and after the addition of the
surfactant. Time did not permit the implementation of a standard test such as NIOSH 7600/7604/7024, nor was it clear that
such a test, designed for air sampling in the workplace, would be appropriate for an in-tank test where concentrations of
chromium are much higher. Our test consisted of placing small glass sampling tubes approximately six inches above the
surface of the liquid in the number six tank, and drawing air through the chambers at a flow rate of 340 cc/mm. In all cases,
tests were run under actual plating conditions, while parts were being plated in the tank. In all cases identical parts were
plated at identical current densities. Tests were run prior to the addition of surfactant and begun again 15 minutes after
the addition of surfactant. Air was used to stir the plating solution during the interim period.
52
Water was used to flush the interior of each sampling tube and the amount of chromium oxide in each tube was
determined spectrophotometrically. The measurements performed prior to the addition of surfactant indicated
concentrations of 9 and 26 mg/m of hexavalent chromium. (Although these are significantly above the OSHA ceiling limit
of 0.1 mg/m3, it should be emphasized that they were conducted in the plating tank and bear no correlation to the amount
of chromium in the shop.) Measurements performed subsequent to the addition of surfactant indicated no measurable
chromium. Because limited time was available for sampling, the total amount of chromium oxide collected was small.
Therefore we are only able to verify that the surfactant reduced chromium-containing mist by at least 95%. It is likely that
the actual reduction in emissions was significantly greater.
Periodic additions of surfactant are needed to tanks, with the frequency increasing with the extent of use. We do not
know the breakdown products of the surfactant. There has been no observable effect on the plating characteristics of
any of the baths since the additions.
Some platers have apparently experienced pitting in the hard chrome plate when using surfactants. This has not been
seen at Dover. However, Atochem (one of the main manufacturers of surfactant fume suppressant) reports that the use
of fume suppressant sometimes enhances pits that may already be in the surface.
There have been occasional reports of exp losions related to fume suppressant use, due to the ignition of the hydrogen
foam at the surface. Dover has not experienced any such ignitions in the 3 years that they have been using the
suppressant.
As a result of its generally trouble-free operation combined with its high effectiveness and low cost, surfactant fume
suppression is now standard operating procedure at Dover and many other plating shops.
7.
Stripping
In the chrome plating industry, significant efforts are routinely employed to strip chromium from worn or improperly
plated parts. Currently the bulk of chrome stripping is being done using caustic baths, resulting in an enormous amount
of mainly chromium hydroxide precipitate which is subsequently processed as waste material. This project studied the
feasibility of performing the stripping using anodic dissolution of chromium in the plating solution itself or in a solution
of similar composition. This has the effect of recycling stripped chrome directly into the plating solution, making it into a
resource rather than a toxic waste.
48
The potential problems associated with this approach are:
l
More frequent reconditioning of the plating solution will likely be required because base metals and trivalent
chrome will be introduced into the tank as the stripping process proceeds;
l
Excessive dissolution of the base metal may occur, causing unacceptable damage to the part; and
l
Using the plating bath for stripping may limit the time available for plating.
The second approach - using a dedicated bath of composition similar to the plating bath (ideally with no sulfate or
other catalyst) - will eliminate this last problem. The stripping bath could be reconditioned periodically and chromium
solution returned to the plating baths for reuse. A second advantage of a dedicated hexavalent chrome stripping
bath is that the bath conditions can be optimized for stripping as opposed to plating.
Some platers (Dover, for example) use in-tank stripping on a limited basis and find it to be rather efficient. There are
several concerns associated with expanding the scale of chromic-acid-based stripping operations, including the rate
of stripping, unevenness of stripping, attack of the base metal by the plating solution and adjustment of the plating
solution chemistry for subsequent use. In this effort, we addressed these issues through lab-scale experiments
coupled with collaborations of Dover.
Our approach is based on monitoring two events during the stripping process. In one case, based on the difference
in the electrolytic dissolution characteristics of chromium and the base metal, at constant voltage, a sudden change
in the current passing through the stripping cell is expected at the time when the chrome is stripped out and the base
metal begins to dissolve. A plot of current versus elapsed time can convey the information required to note the
critical event. In collaboration with Dover, we have verified this effect; a decrease in current of 40% was observed
during stripping of chrome from large steel rollers. In another case, it may be possible to identify the dissolving
species of the base metal using fiber-optic based spectrometric techniques.
During anodic stripping, it is still unclear whether Cr dissolves as Cr3+ or Cr6+ into the plating solution. There are
reports published that support both possibilities. It is well known that the trivalent/hexavalent chromium content in
the plating solution critically affects the plating efficiency. Therefore, adjustment of the solution chemistry will be
necessary prior to reusing the plating solution. Porous pots consisting of alumina and lead electrodes have been
used in the past to recondition the plating baths.
Preliminary stripping experiments were carried out using conventional plating solution (chromic/sulfate bath) on Crplated Inconel 718 and 4340-steel coupons (both base metals are commonly used in a variety of military applications).
The operating conditions were set as shown below. Weight losses observed during the experiments were in excess of
what would be expected if the dissolution was solely controlled by the hexavalent species. We suspect the weight
losses to correspond to a mixture of trivalent and hexavalent chromium species. The bath operating conditions are
summarized in Table 9.
Table 9. Operating conditions for stripping
Temperature of bath
44-500C
Current density
0.25 to 0.5 amperes per inch-square
Plating solution
100:1 chromic to sulfate
Cathode
lead
Anode
Cr-plated Inconel
Anode to cathode ratio
1:1
54
In the case of Inconel, it was observed that upon stripping the chromium, the base metal was attacked by the plating
solution quite dramatically, leading to a "black deposit". However, the 4340-steel coupon remained stable in the
presence of the plating solution. It required a higher voltage to register passage of current through the 4340 sample,
which seems encouraging. This correlates well with the observation at Dover where at the end of the stripping
process an approximately 40% drop in the current is observed.
In order to monitor the dissolution of the base metal spectrometrically, preliminary experiments were conducted using
solutions containing Fe and Cr. UV-VJS Spectra were obtained from solutions of FeCl~ and Fe(N03)3 in deionized
water and a mixture of Fe(N03)3 and Cr-plating solution. The characteristic absorbance of Fe around 294 nm was
observed in the spectrum obtained from the pure solution. It was also observed from the solution mixture of Fe3+ and
Cr6+ that the absorbance from Fe3+ can be easily isolated from the peaks corresponding to chromium.
The results obtained showed that anodic stripping of chromium using chromium plating solution is feasible. Under
these stripping conditions, it was observed that Inconel was severely attacked by the chromic acid solution, whereas
4340-steel was stable in its presence. Results obtained from the 4340-steel stripping experiments also show that it
should be possible to identify the time at which most of the Cr is stripped from the part using the current-time plot.
This will be useful to the plater so that the stripping process can be arrested at the appropriate time. The
characteristic absorbance of Fe at a wavelength of about 294 nm can serve to be useful in the identification of the end
of the stripping of Cr so that the stripping process can be stopped.
8.
Alternative electrode materials
Two alternative materials have been evaluated – Ti metal and barium plumbate ceramic. Titianium has been used
commercially on a small scale, while barium plumbate is a material suggested by us. Because of limitations in money
and time the evaluation was somewhat cursory.
The titanium metal does permit EHC deposition without itself dissolving. However, it does not oxidize Cr3+ to Cr6+ in
solution. Since the operation of the tank relies on the proper amount of Cr6+, this means that an additional cell must
be added to effect the oxidation, such as in a porous pot. Titanium is very expensive compared to lead, and very
difficult to shape, which is another reason for its lack of commercial use.
Using his extensive electrochemical knowledge, Sankar Sambusivan at BIRL devised an electrode comprising barium
plumbate plasma sprayed onto a titanium anode. The coated anodes were tested at Dover Industrial Chrome, where it
performed quite well:
l
It permitted the deposition of hard chrome
l
It oxidized oxidize Cr3+ to Cr6+
l
It did not dissolve
l
It did require a small excess voltage, probably because of contact resistance between the coating and the
substrate
We conclude that barium plumbate has potential as an alternative electrode material, but there is insufficient data to
know whether it would perform well in production.
55
9.
References
M. Cieslak-Golonka, “Spectroscopy of Chromium (VI) Species,” Coordination Chemistry Reviews, vol. 109, pp. 223249, 1991.
J.P. Hoare, “On the Mechanism of Chromium Electrodeposition,” J. Electrochem. Soc., vol. 126, pp. 190-199, 1979.
J.P. Hoare, “An Electrochemical Mystery Story: A Scientific Approach to Chromium Plating,” Plating Surface
Finishing, vol. 76, pp. 46-52, 1989.
D.G. Foulke and F.E. Crane, Electroplaters’ Process Control Handbook, Reinhold, NY, 1963
J.E. Freeman, A.G. Childers, A.W. Steele, and G.M. Hieftje, Analytica Chimica Acta, 177, 121 (1985).
T.H. Irvine, The Chemical Analysis of Electroplating Solutions, Chemical Publishing Co., Inc., N.Y., 1970
F.A. Lowenheim, Electroplating, McGraw-Hill, NY 1978
Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood, Sussex, England, 1976
Metal Finishing 63rd Guidebook and Directory Issue, 93, Elsevier Science, NY, 1995
P. Morisset, J.W. Ostwald, C.R. Draper, and R. Pinner, Chromium Plating, Robert Draper Ltd., Teddington,
Middlesex, England, 1954
H. Onishi, Photometric Determination of Traces of Metals, Pt. IIA, 4th Ed., Wiley, New York, 1986
B. Karlberg and G.E. Pacey, Flow Injection Analysis - A Practical Guide, Elsevier, New York, 372 pp., 1989.
S.Z. Lewin and R.S. Wagner, J. chem. Ed.,vol 30, p. 445, 1953
Z. Marczenko, Spectrophotometric Determination of Elements, Ellis Horwood, Sussex, England, 1976
H. Onishi, Photometric Determination of Traces of Metals, Pt. IIA, 4th Ed., Wiley, New York, 1986
D.D. Perrin, "The Ion Fe(CNS)2+. Its Association Constant and Absorption Spectrum," J. Am. Chem. Soc., vol. 80,
pp. 3852-3856 (1958).
D.D. Perrin, Organic Complexing Reagents, Interscience, NY 1964.
S.M. Sultan and E. Bishop, Analyst vol. 107, p. 1060, 1982.
EPA Reports 453/R-93-030a and 453/R-93-030b, Chromium Emissions from Chromium Electroplating and Chromic Acid
Anodizing Operations (July 1993).
EPA Report 453/R-93-031, Technical Assessment of New Emission Control Technologies Used in the Hard Chrome
Plating Industry (July 1993).
Cardone, MJ, and Compton, J, "Spectrophotometric method for following dichromate oxidations,'1 Anal. Chem., vol.
24, pp. 1903-1908, 1952.
Gao, Ruo-Mei, Zhao, Zhi-Qiang, Zhou, Qing-Ze, and Yuan, Dong-Xia, "Simultaneous determination of hexavalent
and total chromium in water and plating baths by spectrophotometry," Talanta, vol. 40, pp. 637-640, 1993.
Lingane, J.J, and Collat, J.W., "Chromium and manganese in steel and ferroalloys," Anal. Chem., vol. 22, pp. 166-169,
1950.
De Lippa, M.Z., "A new method for the absorptiometric determination of chromium in low alloy steels by oxidation
with potassium bromate," Analyst, vol. 71, pp. 34-37, 1946
Marczenko, Z., Spectrophotometric Determination of the Elements, Wiley, New York, 1976, 643pp.
56
Roehl, R. and Alforque, M.M, "Comparison of the Determination of Hexavalent Chromium by Ion Chromatography
Coupled with ICP-MS or with Colorimetry," Atomic Spectrosc. vol. 11, pp. 210-215, 1990.
Wood, AAR, "The absorptiometric determination of chromium in steels and alloys," Analyst, vol. 78, pp. 54-60, 1953.
Hirsch, S., and Rosenstein, C., "Chemical Analysis of Plating Solutions," in Metal Finishing Guidebook Directory,
56th Ed., Metals and Plastics Publications, Inc., Hackensack NJ. 1988,
57
APPENDIX 1. GEAE REPORT.
HARD CHROME COATINGS: ADVANCED TECHNOLOGY FOR
WASTE ELIMINATION
Subcontractor Final Report to
Northwestern University/BIRL
Contract No. NU 0650525A417GE
November 1, 1993 - September 30, 1997
Jerry D. Schell
GE Aircraft Engine
1 Neumann Way
Cincinnati, Ohio 45215
58
10.
Materials/Processes For Replacement of Electroplated Hard
Chrome (EHC)
Hard chrome plating is used in a wide variety of applications for several different purposes. Amongst these
are tribology (wear and friction), interference fits such as journal surfaces, and dimensional buildup/restoration. Thus, a number of potential replacements have been investigated with the expectation no
one material/process could cover all EHC uses due to this wide variety of uses and types of parts. The
following sections describe the replacements investigated as part of this program. Most of the candidates
were evaluated for adhesion and wear. The HVOF coatings were selected for more extensive evaluations of
their effects on fatigue properties of the substrate materials and resistance to compressive load creep. This
was based on favorable economics for HVOF coatings and the large percentage of applications where HVOF
coatings could replace EHC in DoD maintenance depots.
10.1.
HVOF WC-Co
The HVOF WC-Co coatings investigated were all produced using WC-17Co powders. The powders used
were sintered agglomerates with a nominal size of -270 mesh. The HVOF spray work was carried out on
JetKote II and JetKote IIA equipment at two different spray sites. The WC-17Co coatings at Site A were
applied using oxygen-hydrogen parameters and the WC-17Co coatings at Site B were applied using oxygenpropylene parameters. Argon was used as the powder carrier gas for all spray studies. Surface preparation
prior to coating included solvent cleaning and grit blast to about 100-150 microinches AA with 60 or 80 grit
aluminum oxide. The final WC-17Co coatings selected for the wear, fatigue effects, compressive creep test
evaluations, and component coating demonstrations were those based on the nominal center point DOE
results of the Site A process.
Note: All HVOF studies were conducted using statistically designed experiments methodology to
identify the relative importance of various process factors. These DOEs (design of experiments)
and their findings will be further described in a subsequent section.
10.2.
HVOF Triballoy 400
Triballoy 400 has a nominal composition of Co-28 Mo-8 Cr-3 Si and was selected as a candidate because of its low
chrome content and good tribological properties. The Triballoy 400 powder was gas atomized for Site A and water
atomized for Site B; both powder varieties were nominally -325 mesh. The HVOF Triballoy 400 coatings were applied
at both Sites A and B using oxygen-hydrogen parameters using JetKote II and JetKote IIA equipment. Argon was
used as the powder carrier gas for all spray studies. Surface preparation prior to coating included solvent cleaning
and grit blast to about 100-150 microinches AA with 60 or 80 grit aluminum oxide. The final Triballoy 400 coatings
selected for the wear, fatigue effects, compressive creep test evaluations, and component coating demonstrations
were those based on the best DOE results of the Site A process.
10.3.
Laser Clad Triballoy 400
Triballoy 400 was also applied by a CO2 laser cladding process at Northwestern University BIRL. The size range for
the laser cladding Triballoy 400 powder was 45-150 microns. Two features were noted as important to avoiding
compositional changes due to substrate material solutioning in the cladding and microcracking in the final overlay
deposit; a) powder was injected into the CO2 laser beam above the substrate surface, and b) the substrate was
preheated. The only additional details on processing conditions as provided by BIRL included that the laser was a
gaussian beam profile, 500 watts, and a 1 mm/sec traverse rate was utilized. The Triballoy 400 laser cladding which
was evaluated achieved a microhardness of 650-700 HV. This process was evaluated in wear only.
59
10.4.
Unbalanced Magnetron Sputtered Chrome Nitride (CrN)
UBM sputtered CrN was applied by Northwestern University BIRL to a thickness of 3-4 microns in a Hauzer vacuum
sputter system. This was accomplished by biased DC reactive sputtering using a pure chromium target and nitrogen
reactive gas which was introduced separately from the argon plasma gas. These thin CrN coatings typically had a
microhardness of about 1600 HV. The application parameters included 3 mbar total gas pressure, 1 mbar nitrogen
partial pressure, 2.5 kw target power, 125 volts bias on workpiece, and plasma potential of -30 volts at 4-6 ma cm-2 in
front of the target. The workpieces were sputter etched for 15 minutes at 1200 volts prior to coating deposition. The
substrate temperatures were 125 C at the start of deposition and 220 C upon completion. These conditions resulted
in a 0.5 micron/hr CrN deposition rate. These coatings were evaluated for adhesion and wear.
10.5.
Plasma Nitrided Substrates + Chrome Nitride (PN+CrN)
These coatings were provided by Northwestern University BIRL. The plasma nitride process for this
evaluation was applied by Advanced Heat Treat, Inc. in a separate operation prior to depositing the CrN
coatings as described above. The nitrided layer was several mils deep for the 4340 steel, but less than 0.001
inches for the IN718. No information was available on the plasma nitriding parameters.
10.6.
Cathodic Arc Titanium Aluminum Nitrides [(Ti,Al)N]
Three compositions of (Ti,Al)N coatings were applied by Praxair Surface Technologies to a thickness of 3-4
microns in a standard MultiArc, Inc. 48 inch box vacuum coater. The (Ti,Al)N coatings were applied to
IN718 and 4340 steel substrates by a cathodic arc process which utilized opposing cathode targets of pure
titanium and pure aluminum in a reactive gas (nitrogen) coating process. The specimens being coated were
rotated in the coating chamber with a negative bias voltage such that they were exposed alternately to the
metal ion fluxs of the opposing titanium and aluminum cathodes in the presence of the nitrogen gas at a
partial N2 pressure on the order of several millitorr. The cathode to substrate distance and/or cathode
power were used to control the relative concentrations of arriving metal ion species at the specimen
surfaces, hence the composition of the coatings. The nominal compositions evaluated were (Ti0.6Al0.4)N,
(Ti0.5Al0.5)N, and (Ti0.4Al0.6)N. These nominal compositions are designated with a -L, -M, or -H
respectively for the aluminum content in the coating {e.g., (Ti,Al)N-L} throughout the rest of this report.
Scratch adhesion and wear test evaluations were conducted on these coatings.
10.7.
Electroplated Hard Chrome (EHC) Baseline:
All EHC specimens were coated to a thickness of 0.003-0.005 inches per the requirements of AMS 2406 at an
approved GEAE vendor using a standard chromic acid plating bath.
11.
11.1.
HVOF Coating Process Studies
General Description
All HVOF coating work was carried out on JetKote II and JetKote IIA equipment at two sites. The Triballoy
400 coatings at Sites A and B and the WC-17Co coatings at Site A were applied using oxygen-hydrogen
parameters. The WC-17Co coatings at Site B were applied using oxygen-propylene parameters. Argon was
used as the powder carrier gas for all spray studies. Surface preparation prior to coating included solvent
cleaning and grit blast to about 100-150 microinches AA with 60 or 80 grit aluminum oxide. The coatings
were all deposited to a 0.009 - 0.011 inch thickness for all coupon types.
All HVOF studies were conducted using statistically designed experiments methodology to identify the relative
importance of various process factors. A series of pre-DOE (design of experiments) runs or previously known
60
process experience was always utilized in designing the actual DOE matrices to assure that viable coatings would be
produced for each individual set of process parameters that resulted in the DOE matrices. The author highly
recommends the general practice of conducting sufficient pre-DOE trials for the selected design factor levels to verify
they work in the combinations that result in the DOE as a necessary step for any DOE. Another necessary
consideration that was made in the selection of design factor levels was that there had to be a wide enough range in
the values selected to assure adequate discrimination in the chosen responses. These DOEs and their findings will
be further described in subsequent sections.
The gun operating parameters were electronically monitored by a portable computerized data acquisition package
during all spray work. This unique process monitoring equipment, was connected to the vendor equipment between
the JetKote control console and the spray gun. Use of the monitoring equipment will allow subsequent transfer of
the HVOF coating process(es) to other sites with a high degree of confidence irrespective of any unique facility
features or differences in plumbing, gas supply designs, etc.
11.2.
Coating Process Response Measurements
A number of process responses were tracked for the deposited coatings. These included deposition rates, DPH
microhardness, R15N superficial surface hardness, tensile bond strength by a modified ASTM C633 method, Almen
strip arc height as an indication of coating residual stresses, and substrate temperature during spraying. The
specimens for these measurements were rotated on a turntable while mounted on a cylinder as the gun traversed
parallel to the cylinder axis of rotation in an oscillating stroke.
11.2.1.
Coating Deposition Rate
The deposition rate was determined as the measured coating thickness divided by the recorded number of gun
passes. The final coating thickness was measured by hand held flat anvil micrometers on the coating microstructure
coupons, typically a 0.25 inch cube or one square inch piece of 0.060 inch or thicker sheet metal. The number of gun
passes were recorded as determined by an automatic cycle counter during the spray operation. One cycle consisted
of an initial stroke and its return stroke, thus two gun passes for any given position along the cylinder’s length.
11.2.2.
DPH Microhardness
The microhardness measurements were taken on a polished cross-section of the microstructure coupon. A
standard commercially available diamond pyramid indentor and test machine were used with a 300 gram load.
The specimen was sectioned parallel to the rotation direction and perpendicular to the gun traverse
direction, then mounted and polished by standard procedures. Ten readings were taken in a standard
pattern which diagonally traversed the coating thickness from the substrate to the surface and back to the
substrate again taking care to maintain adequate distances from coating edges and between indentations.
11.2.3.
4.2.3 Rockwell 15N superficial Surface Hardness
This data was taken by making indentations directly on the coated coupon surface after lightly hand polishing the
surface on 400 grit SiC metallographic polishing paper to remove any loosely adherent particles or small asperities.
Testing was don with a standard commercial tester with a dial indicator or automatic printout of the hardness reading
which used a 120 degree diamond cone indentor and a 15 kilogram load. It is possible that at the coating thickness
used, there is some influence of the substrate hardness on the absolute values obtained for the coating hardness, but
since the substrates were all IN718 typically of Rockwell C 40+/-2 and the coatings all .010+/-1 in thickness, it was
assumed that changes in hardness were reflections of process effects. In general, the Rockwell 15N hardness
showed minimal variability in most of the HVOF process studies so statistical analyses were not performed for these
data.
61
11.2.4.
Tensile Bond Strength
Tensile bond strengths were determined by a modified ASTM C633 method. The coatings were deposited on one
inch diameter buttons which were 0.25 inches thick. This allows buttons to be conveniently mounted along with the
other specimens, or in the case of coating actual parts, often lends itself to locating buttons right on or adjacent to
the part. The button s were then epoxy glued between the normal ASTM C633 mandrels and tested per the
specification. The HVOF WC-Co always resulted in epoxy breaks and the Triballoy 400 gave almost all coating
breaks. However, a few epoxy breaks were seen for the Triballoy 400 coatings.
11.2.5.
Substrate Temperature
The temperatures measured for the spray process trials were taken by a hand held touch probe which
utilized a thermocouple (iron-constantan). The probe was touched to the coated coupon surface within one
minute after the gun, turntable, and cooling jets were shut down. Past experience comparing this method to
IR pyrometers used on rotating specimens and embedded thermocouples on stationary specimens which
were XY traversed has shown general agreement within about +/- 50 oF on the absolute values and
agreement in trends between individual process trials.
11.2.6.
Almen Strip Deflection
The Almen strip was a standard type N SAE 1070 steel strip 3”x0.75”x0.030” held by 4 round head screws
(see Mil-S-13165C for more details). The Almen strip was grit blasted on one side only and the arc height
due to the induced bending stresses recorded before and after spraying of the HVOF coating. The
difference in the two measurements was reported as the Almen strip deflection. A negative sign indicated a
change representing a compressive stress in the coating and a positive sign indicated a tensile stress in the
coating. (Note: Shot peening work at GE has demonstrated the validity of taking the difference in curvature
without the need to return the Almen strip to a perfectly flat condition; i.e. the deflection is cumulative in
linear fashion in the elastic region. The single side grit blast procedure typically resulted in a starting arc
height of -0.004 inches; i.e. compressive.) The Almen strip is restrained in the flat position during coating.
No effort was made to calculate actual coating stresses since the exact coating modulus was unknown.
62
Table 10. Matrix for Site A HVOF WC-Co DOE.
Site Run L8 Std.
Random
Stoichiometry Total Flow
Number
Order
Run Order
scfh
CO2
Powder
Feed Rate,
Cooling Jets
gm/min
Surface
Speed,
ipm
A 39
1
1
1.8
1700
2 @ 5"
50
2400
A 41
2
3
1.8
1700
1 @ 7"
50
1200
A 44
3
6
1.8
2200
2 @ 5"
25
2400
A 42
4
4
1.8
2200
1 @ 7"
25
1200
A 43
5
5
2.5
1700
2 @ 5"
25
1200
A 46
6
8
2.5
1700
1 @ 7"
25
2400
A 40
7
2
2.5
2200
2 @ 5"
50
1200
A 45
8
7
2.5
2200
1 @ 7"
50
2400
Stoichiometry
& Total Flow
H2 Flow
O2 Flow
Ar Carrier
Site Run
scfh
scfh
Gas, scfh
Numbers
sd @7"
1.8,1700
1093
607
68
A39, A41
H2ODelta
T=40°F
1.8,2200
1414
786
88
A42, A44
cg@4%
TF
2.5,1700
1214
486
68
A43, A46
6" stroke
2.5,2200
1571
629
88
A40, A45
Surface
Speed, ipm
[email protected]"
Diameter
Traverse
Rate
FIX:
9"x.25"no
zzle
Site Run
Numbers
inch/sec
2400
118
0.19
A39, A44, A45, A46
1200
59
0.09
A40, A41, A42, A43
63
11.3.
WC-17Co Process Development
The WC-17Co DOEs were individual eight run, two level, five factor, precision III experiments for the two
vendor sites A and B. Site A utilized a JetKote IIA control console and hydrogen as the fuel gas. Site B
utilized a JetKote II control console and propylene as the fuel gas. The factors studied included the total
gas flow, the gas stoichiometry (mix ratios) of oxygen and fuel gas (hydrogen or propylene), substrate
cooling, part surface speed as determined by the part rotation and spray gun traverse speeds, and powder
feed rates. Other important parameters, which were held constant to the extent possible, included spray
distance, gun nozzle, and gun cooling water flows and temperatures.
Site A - Hydrogen/Oxygen DOE Matrix Design - The DOE matrix for Site A was as specified in Table 10.
The combustion gases stoichiometry ratios used were 1.8 and 2.5 moles of hydrogen per mole of oxygen so
one ratio was reducing (1.8) and the other oxidizing when full combustion takes place. The total gas flows
(sum of the hydrogen and oxygen flows) were 1700 and 2200 scfh. The resultant DOE matrix combinations
and the individual gas flow parameters which resulted have been summarized in the insert blow the matrix
design. The JetKote IIA console at Site A used gas mass flow controllers so the gas pressures were not
considered although they were recorded. The hydrogen pressures ranged from 79 to 112 psi and the oxygen
pressures ranged from 66 to 107 psi as measured at the control console. The author chose to use gas total
flow and stoichiometric mix ratios because of their intuitive physical significance to the combustion process
instead of individual gas pressures and flows. Stoichiometric ratio signifies available heat content per unit
volume which combined with total flow per unit time determines the heat flux from the gun.
There was not a convenient way to measure the CO2 cooling flows directed at the specimens as they were coated so
a qualitative method was devised which used either 2 cooling jets at 5 inches (“high” cooling) or 1 cooling jet at 7
inches (“low” cooling) for the two levels in the DOE. The powder feed rates were a straightforward selection of rates
higher and lower than known typical production parameters to give a 2X difference.
Surface speed was chosen based on the typical range of speeds used for wide ranging part sizes. It is the vector
resultant of the rotational speed and gun traverse speed. It was important that the gun feed per turntable revolution
be maintained at values which prevented “barber pole striping” effects. This was best accomplished using
combinations which gave multiple revolutions within the time required for the gun to traverse one spot size diameter
of the spray pattern.
Hydrogen/Oxygen DOE Results - The resultant coating process data for this DOE was compiled in Table 11. The
coating deposition rates ranged from 0.00012 to 0.00100 inches/pass, a factor of 8.5X between the minimum and the
maximum. The coating thickness actually obtained for the DOE process trials ranged from about 0.009 to 0.012
inches. The DPH microhardness range was very tight, 1017-1098 kg/mm2, for 7 of the 8 DOE trials. The one exception,
run A39 which gave 849 kg/mm2, turned out to be a parameter set that combined all the design factor levels favoring
Table 11. Site A HVOF WC-Co DOE average responses.
Site Run
Dep. Rate
DPH
Rockwell 15N
2
kg/mm
2
Tensile Bond
Substrate
Almen
Strength, psi
Temp., °F
Deflection, mils
Number
mils/pass
kg/mm
A 39
0.27
849
93.0
10121
86
-8.5
A 41
0.75
1041
93.2
10472
136
-6.3
A 44
0.12
1025
93.7
11469
120
-21.0
A 42
0.42
1096
94.1
10389
167
-21.5
A 43
0.50
1017
93.3
10923
122
-16.5
A 46
0.12
1034
93.4
10936
164
-18.0
A 40
1.00
1098
94.5
11119
160
-29.0
A 45
0.29
1074
94.5
11148
226
-26.5
64
the “coldest” coating deposition conditions. The Rockwell 15 N hardness likewise had a narrow range from 93.0 to
94.5 kg/mm2, or less than 2 % difference for all 8 DOE runs. (No analysis of the Rockwell 15N data was made for this
reason.) The tensile bond data was all over 10,000 psi with epoxy breaks; i.e., no information of process effects on
the actual coating strength can be obtained due to a limitation of the evaluation method. This would lead one to
expect approximately equal contributions for all the design factors in a statistical analysis, which indeed happened,
thus validating the methods used. (See Table 13.) The substrate temperatures ranged from 86-226 oF, or 26%
difference on the Rankine scale between the minimum and maximum, thus, a significant response range. The Almen
deflection ranged from -6.3 to -29.0 mils, a factor of 4.6X over the 8 DOE matrix, hence a significant level of response
variation.
65
Table 12. Typical statistical analysis table for Site A HVOF WC-Co Almen data.
Design Factors:
Std Site Run Almen
Order
Number Deflect
Stoiciometery
a1
a2
Total Flow
b1
b2
1
A 39
-8.5
-8.5
-8.5
2
A 41
-6.25
-6.25
-6.25
3
A 44
-21
-21
-21
4
A 42
-21.5
-21.5
-21.5
5
A 43
-16.5
-16.5
-16.5
6
A 46
-18
-18
-18
7
A 40
-29
-29
-29
8
A 45
-26.5
-26.5
-26.5
Total
No. of Values
Average
-147.25 -57.25
8
4
-18.406 -14.313
Effect
% Effect
Rank
c1
c2
Powder Feed Surface Speed
Rate
d1
-8.5
d2
e1
-8.5
-6.25
-21
-6.25
-16.5
-18
bc1
-8.5
-6.25
-21
-21.5
e2
Interactions
-21
bc2
cd1
8.5
8.5
6.25
6.25
21
21
-21.5
-21.5
21.5
-16.5
-16.5
16.5
-18
-29
-18
-29
-26.5
-29
-26.5
cd2
21.5
16.5
18
18
29
29
-26.5
26.5
26.5
-90
-49.25
-98
-75
-72.25
-77
-70.25
-73.25
-74
74.25
73
77
70.25
4
4
4
4
4
4
4
4
4
4
4
4
4
-22.5
-12.313
-24.5
4
Preferred Level
CO2 Cooling
4
-18.75 -18.063 -19.25 -17.563 -18.313
4
4
-18.5
4
-18.563 -18.25 -19.25 -17.563
4
4
-8.1875
-12.188
0.688
1.688
-0.1875
0.313
1.688
32.8
48.9
2.8
6.8
0.8
1.3
6.8
2
1
4
3
6
5
3
66
Table 13. Site A WC-Co DOE statistical analysis summary.
Dep. Rate
DPH
2
Tensile Bond
Substrate
Almen
Strength, psi
Temp., °F
Deflection, mils
Design Factor
Level
mils/pass
kg/mm
Total DOE
Mean
0.434
1029
10,822
148
-18.4
Stoiciometery
a1
0.390
1003
10,613
127
-14.3
a2
0.477
1056
11,032
168
-22.5
b1
0.410
985
10,613
127
-12.3
b2
0.458
1073
11,031
168
-24.5
c1
0.472
997
10,908
122
-18.8
c2
0.396
1061
10,736
173
-18.1
d1
0.292
1043
10,929
143
-19.3
d2
0.575
1016
10,715
152
-17.6
e1
0.668
1063
10,726
146
-18.3
e2
0.199
996
10,919
149
-18.5
bc1
0.499
1050
10,999
145
-18.6
bc2
0.368
1009
10,645
150
-18.3
cd1
0.453
1019
10,641
144
-19.3
cd2
0.414
1039
11,003
151
-17.6
Total Flow
CO2 Cooling
Powder Feed Rate
Surface Speed
Interaction
Interaction
Main Effects’
1
Percentages
2
3
Surface speed
Total flow
Total flow
CO2 cooling
41.5%
24.4%
19.6%
32.7%
Powder feed rate 25.0%
Surface speed
Stoiciometery
Total flow
18.7%
19.6%
26.3%
bc interaction
CO2 cooling
bc interaction
Stoiciometery
11.5%
17.8%
16.9%
26.0%
Stoiciometery
cd interaction
14.7%
16.6%
75.4%
72.7%
4
Sum
78.0%
67
85.0%
Total Flow 48.9%
Stoiciometery 32.8%
81.7%
Statistical Analyses of Hydrogen/Oxygen DOE Results - Statistical analysis of the data was performed for each
response type and the average response values and percent of data variation determined for each experimental
design factor. A typical response analysis for the Almen strip deflection data from the Site A HVOF WC-Co DOE has
been shown in Table 12 as an example of the statistical analysis methodology. The design factors, designated a
through e were arranged in columns for each level , 1 or 2 in the DOE, of the design factor and summed by column to
determine the average value for each level. The difference between the design factor level averages was a measure of
the effect for that design factor. By summing the absolute values of the effects and taking the ratio of any one
design factor effect, the percentage of variation in the response data for that design factor was determined. Thus,
one has determined the quantitative importance each design factor contributed to the response data and they were
ranked by percent effect.
The preferred level for each design factor was determined, also, based on the criteria “larger is better” for Almen
deflection. The basis of the criteria was that larger meant higher compressive coating residual stresses which would
offset tensile cracking stresses as a part was loaded in tension or bending. As an example of the application of this
knowledge, if one desired to improve the compressive Almen value, the first thing to try would be increasing the total
gas flow and once that was optimized, the next thing for further improvement would be increase the stoichiometry.
The rankings of design factors can be expected to (and did as we shall soon see) vary for different responses so each
response was analyzed separately. The results for all analyzed responses were as shown in Table 13. The deposition
rate was affected most strongly by the surface speed (41.5% of the data variation) and the powder feed rate (25% of
the data variation) as might be expected. The significance of not analyzing the Rockwell 15N data and of analyzing
the tensile bond data even though all data were epoxy breaks have been discussed above. The coating DPH
microhardness responded most to Total Flow (24.4 % of the data variation) with a fiarly equal distribution amongst
the rest of the parameter effects. It could be argued that except for the one DPH data point in the Site A DOE results,
a similar equi-partitioning of main effects as seen for tensile data would be expected due to the narrow range of the
data set. However, the one outlying data point has pushed the strongest response towards the total flow (i.e., total
heat) which made sense in light of the earlier observation about “coldest parameters”.
The substrate temperatures seen during spraying were most strongly affected by the CO2 cooling (32.7% of the data
variation) and the total flow and stoichiometry (26% each; or 52% taken together as heat per the earlier discussion of
the physical significance of these design factors). Thus, the heat available ranked as the main effect and the amount
of cooling ranked as the second strongest effect determined how hot the substrates got as would be expected.
Again the statistical analyses validated the expected results based on a physical model. Finally, the Almen results
were shown to be highly dependent on heat content with the combined 81.7% of data variation due to total flow
(48.9%) and stoichiometry (32.8%).
Site B - Propylene/Oxygen DOE Matrix, Results, and Statistical Analyses - Having carefully completed a
discussion of the design, results, and statistical analyses for the hydrogen/oxygen process in the preceding sections
for the reader, a much briefer discussion will be presented for the Site B WC-17Co propylene/oxygen process. The
reader is referenced to Table 14 to Table 16, which have summarized the matrix design, results and statistical
analyses, respectively, for the Site B WC-17Co DOE. The same matrix design (see Table 14) was used for Site B as for
Site A, but there were three obvious differences; (1) propylene was used as the fuel gas instead of hydrogen, (2) the
total flows were much lower, and (3) air was used for the cooling jets.
The chemical reaction for the burning of propylene is C3H6 + 4.5 O2 à 3 CO2 + 3 H2O so 4.5 is the stoichiometric
combustion ratio. The values selected for the stoichiometric mix ratio in the Site B matrix were both oxidizing, 6.2 and
8.1, which differs from the Site A selections of one reducing and one oxidizing condition. The total gas flows for site
B were nominally half those of Site A, but the gas mass flow through the nozzle for Site B was roughly twice that for
Site A. However, the available heats from these gas flows were quite similar for the two sites with a range of 243,100
to 383,960 BTU/hour for Site B and 292,924 to 378,952 BTU/hour for Site A. Finally, the air cooling used for Site B
was expected to be less efficient cooling than that of the CO2 used for Site A.
68
Table 14. Matrix for Site B HVOF WC-Co DOE.
Site
Run No.
L8 Std. Stoiciometery Total Flow
Order
Cooling Air
scfh
Set-up
Powder Feed
Rate, gm/min
Surface
Speed,
ipm
17
1
8.1
1262
2 gun@50 psi
50
2400
15
2
8.1
1262
2 gun +2 side@90 psi
50
1200
11
3
8.1
1000
2 gun@50 psi
25
2400
12
4
8.1
1000
2 gun +2 side@90 psi
25
1200
10
5
6.2
1262
2 gun@50 psi
25
1200
13
6
6.2
1262
2 gun +2 side@90 psi
25
2400
14
7
6.2
1000
2 gun@50 psi
50
1200
16
8
6.2
1000
2 gun +2 side@90 psi
50
2400
FIX:
Stoiciometery C3H6 Flow,
& Total Flow
scfh
O2 Flow, scfh
Run Nos.
sd @7"
8.1,1262
138.9
1123.7
15,17
Delta
T=20°F
8.1,1000
109.9
890.1
11,12
4.6"
stroke
6.2,1262
175.9
1093.4
10,13
6"x.32"no
zzle
6.2,1000
138.9
861.1
14,16
Surface
Speed, ipm
[email protected]"
Diameter
Traverse Rate
Run Nos.
2400
118
0.19
11,13,16,17
1200
59
0.09
10,12,14,15
inch/sec
69
Table 15. Site B HVOF WC-Co DOE average responses.
Run
Dep. Rate
DPH
Rockwell 15N
2
kg/mm
2
Tensile Bond
Substrate
Almen
Strength, psi
Temp., °F
Deflection, mils
Number
mils/pass
kg/mm
17
0.300
1123
92
12740
332
-14
15
0.600
1094
92
12392
266
-6.75
11
0.450
840
91
11366
270
-4.5
12
0.900
984
90
11908
240
-1.5
10
0.667
1126
93
12555
400
-12.3
13
0.346
1173
93
11670
325
-12
14
0.625
1136
92
12541
357
-3.75
16
0.310
1117
93
12090
315
-3.5
70
Table 16. Site B WC-Co DOE statistical analysis summary.
Dep. Rate
DPH
2
Tensile Bond
Substrate
Almen
Strength, psi
Temp., °F
Deflection, mils
Design Factor
Level
mils/pass
kg/mm
Total DOE
Mean
0.525
1074
12,158
313
-7.3
Stoiciometery
a1
0.563
1010
12,101
277
-6.7
a2
0.487
1138
12,214
349
-7.9
b1
0.478
1129
12,339
331
-11.25
b2
0.571
1019
11,976
296
-3.3
c1
0.511
1056
12,301
340
-8.6
c2
0.539
1092
12,015
287
-5.9
d1
0.591
1031
11,874
309
-7.6
d2
0.459
1118
12,441
318
-7.0
e1
0.698
1085
12,349
316
-6.1
e2
0.352
1063
11,967
311
-8.5
bc1
0.505
1060
11,992
305
-6.75
bc2
0.544
1088
12,323
322
-7.8
cd1
0.543
1104
12,214
314
-7.8
cd2
0.507
1044
12,101
313
-6.75
Total Flow
CO2 Cooling
Powder Feed Rate
Surface Speed
Interaction
Interaction
Main Effects’
1
Percentages
2
3
Surface Speed
Stoichiometry
Powder feed rate
Stoiciometery
Total Flow
46.2%
27.3%
26.3%
37.5%
46.9%
Powder feed rate
Total flow
Surface Speed
Cooling Air
Cooling Air
17.6%
23.4%
17.8%
27.6%
15.9%
Total flow
Powder feed rate
Total flow
12.4%
18.5%
16.8%
4
Total flow Powder feed rate 14.4%
18.3%
bc interaction
15.3%
Sum
76.2%
69.2%
71
76.2%
83.4%
77.2%
72
The Site B results (Table 15) were not much different than those of Site A for most factors. The difference in volume
fractions of retained carbides appeared to be larger than for the Site A DOE. This may represent a shift in phases
present as a very light gray third phase can be detected. It could be attributed to a different powder being used
and/or differences during metallographic preparation which changed the contrast between phases. The substrate
temperatures were higher, in the 240-400 oF range for Site B, as expected due to the less efficient cooling. The range
of deposition rates was slightly tighter, 0.300-0.900 mils/pass and the DPH range wider a little, 840-1173 kg/mm2. The
one significantly different response was Almen deflection which ranged from -1.5 to -14 mils or about half that of the
Site A results. This effect could be related to the less efficient cooling scheme, also, with residual stresses lower due
to slower quenching of the coating material as it was deposited.
The statistical analyses results for the Site B DOE have been summarized in Table 16. The main effects inset at the
bottom of the table indicated similar trends in the main effects to those for the Site A DOE for deposition rate,
substrate temperatures during spray, and the Almen deflection data. The tensile bond analysis again had fairly equal
responses except powder feed was somewhat higher. Remember that there should have been equal distribution of
effects since all data was epoxy breaks. Given the narrow range of tensile data, it appears to be just a fluke that the
powder feed rate effect turned out somewhat higher than the other effects. The DPH main effects turned out different
for Site B than Site A. This was attributed to a more uniformly distributed data set for Site B, hence the resultant
effects were more realistic. The main effects for DPH for the Site B DOE were heat (stoichiometry and total flow) and
the amount of powder being heated, i.e., heat per unit weight of coating, a plausible result from the physical model
viewpoint It could be argued that except for the one DPH data point in the Site A DOE results, a similar equipartitioning of main effects as seen for tensile data would be expected due to the narrow range of the data set.
11.4.
Triballoy 400 HVOF Process Development
DOE Matrix Design - The approach taken to the Triballoy 400 process development was different from that for the
WC-Co in that it was desired to determine more directly if the site had a significant effect on the process response
data. The design used, shown in Table 17 was a single eight run, three factor, two level, precision IV fractional
factorial design with blocking by the spray site. Additional runs for the center point of each site were made as the
first and last run for the site. Inserts in the bottom half of Table 17 show the actual gas flows, turntable rpm, and
gun traverse speeds used at each site. The selected design factors for the DOE included gas mix stoichiometry, total
flow, and powder feed rate. The fixed conditions for each site were as shown at the right side of the DOE matrix.
The design resulted in two significant factors being associated with the site blocking in addition to the actual site.
These were the different cooling schemes and different total flow levels. It was important to recognize this when the
DOE response data was analyzed and was ultimately the driving force behind the DOE matrix used. These site
differences defined the need for a site blocked design and from there the degree of precision required to isolate the
effects of blocking and all of the design factor effects on the response data required limiting to just three design
factors, as given in the preceding paragraph.
DOE Results - The resultant response data obtained for the Triballoy 400 DOE have been summarized in Table 18.
The DPH and Rockwell 15N hardnesses and the tensile bond strength results were very similar for the two sites. The
tensile bond strength results were all failures in the coating or mixed coating/epoxy failures so the statistical analysis
of the tensile results were meaningful for the Triballoy 400 DOE. The remaining responses showed separations by
site. The Almen deflection data ranged from 6.5 mils (i.e., tensile coating residual stresses) to -2 mils (compressive
coating stress) for Site B, but were -9.5 to -15 mils except for one run that gave -0.5 mils for Site A. The substrate
temperatures for Site B ranged from 334 to 499 oF while those for Site A were 166-240 oF, a result that might have been
expected due to the less efficient part cooling scheme for Site B. Deposition rates ranged from 0.386 to 1.643
mils/pass for the Triballoy DOE. The deposition rates for Site B were higher than those for Site A, although the
ranges did overlap in the 0.6-0.9 mils/pass region. These site differences showed up quite clearly in
73
Table 17. Matrix for HVOF Triballoy 400 DOE.
FIXED:
Site
A
A
A
A
A
A
B
B
B
B
B
B
Full
Expt.
1
3
8
4
11
6
7
5
2
10
9
12
L8 Std.
Run
Order
Order
Center Point
1
2
2
5
3
3
4
8
5
Center Point
6
Center Point
7
4
8
1
9
7
10
6
11
Center Point
12
Stoiciometery
2.26
2
2.52
2.52
2
2.26
2.26
2
2.52
2.52
2
2.26
Total Flow
scfh
2000
1800
1800
2150
2150
2000
1550
1700
1400
1700
1400
1550
Powder Feed
Rate, gm/min
45
30
60
30
60
45
32
20
20
44
44
32
Individual Gas Flows
Site
A
A
A
A
A
B
B
B
B
B
Stoic. &
H2
Total Flow
2.26,2000
1387
2.0,1800
1200
2.0,2150
1433
2.52,1800
1289
2.52,2150
1539
2.26,1550
1072
2.00,1400
933
2.00,1700
1133
2.52,1400
1002
2.52,1700
1217
Speed
rpm/dia
inch/min
m.
(inch-1)
2400
109/7.0
2400
118/6.5
O2
Argon
613
600
717
511
611
475
467
567
398
483
Traverse,
in/sec
80
72
86
72
86
62
56
68
56
68
Runs
0.19
(6mm/sec)
0.19
(5mm/sec)
Site B
Site A
74
Run
Order
1,6
2
5
3
4
7,12
11
8
9
10
sd @7"
Delta T=40°F
cg@4% TF
6" stroke
6"x.25"nozzle
ipm=2400
CO2=1@5"
sd @7"
Delta T=40°F
cg@4% TF
4.5" stroke
9"x.25"nozzle
ipm=2400
2 side air at
90psi,7.25"
and 2 gun air at
50 psi,14"
Table 18. HVOF Triballoy 400 DOE average responses.
Site
Run
Dep. Rate
DPH
R15N
2
kg/mm
2
Tensile Bond
Substrate
Almen
Strength, psi
Temp., °F
Deflection,
in.
Number
mils/pass
kg/mm
A
1
0.492
626
89.3
9,385
198
-10.5
A
2
0.386
582
88.6
9,627
166
-12
A
3
0.929
631
88.5
8,071
240
-0.5
A
4
0.391
621
89.4
10,478
243
-15
A
5
0.698
645
89.8
10,391
240
-10
A
6
0.555
668
89.3
10,786
207
-9.5
B
7
1.000
626
89
10,532
437
2
B
8
0.588
657
89
8,470
378
-2
B
9
0.733
569
88
12,480
402
6.5
B
10
1.643
558
90
12,403
499
3.75
B
11
1.050
535
88
8,425
334
2.5
B
12
1.100
584
88
11,462
424
2.5
the line entries of response effects for the design factor, Site Blocking, in Table 10, the summary of statistical
analyses.
The range of coating microstructures obtained for the Triballoy 400 at Sites A and B, respectively, were as shown in
Figures 4 and 5. It was interesting to note the individual coating splats in the microstructures from Site A trials
appeared to be flattened out more than those for Site B, but the coatings looked very similar in other respects such as
the amounts of oxides and porosity present. This feature of the microstructures was consistent with the higher
kinetic energy that would be expected for the higher gas flows at Site A and helped the physical understanding of the
lower deposition rates for the Site A coatings. Although somewhat more powder particles arrived per pass for the
higher powder feed rates used at Site A than Site B, the higher kinetic energy contribution at Site A would have
caused more plastic deformation of the powder particles as they impinged on the substrate; hence the greater
flattening effects seen in the microstructures for Site A which results in the apparent lower deposition rates.
Statistical Analyses of the DOE Results - The statistical analyses results for the various response data were
summarized in Table 19. The deposition rate was seen to depend most strongly on powder feed rate (33.0 % of the
data variation) as might be expected. The Site Blocking dependence (23.9 %) may be partially due to heat effects
related to the less efficient cooling at Site B. The heat /cooling theory as the reason behind Site Blocking as a main
effect made further sense when stoichiometry (14.5 %), i.e., heat per unit volume of combustion gases, turned up as
the third ranked main effect, also. For DPH, the heat factors of the bc interaction of Total Flow and Stoichiometry
(26.5 %), Total Flow (21.1 % and more was better for higher DPH hardness) and Site Blocking (20.6 %; Site A had
higher flows) all turned up again for DPH hardness with high energy (kinetic and/or thermal) favored for higher
hardness.
The main effects for tensile bond strength were the ab interaction (powder feed rate and total flow), the
stoichiometry (available heat per unit volume), and Site Blocking . In the case of tensile bond strength effects for this
DOE, the mix of preferred levels and main effects were sufficiently daunting to discourage the author from trying to
relate them to any physical model implications. The substrate temperature effects were strongly dominated by Site
75
Blocking (47.4 %, interpreted as cooling effect differences between sites based on the outcomes in the WC-Co DOEs
discussed earlier) and direct heat effects as Stoichiometry plus Total Flow made a combined 31.7 %
76
Table 19. Triballoy 400 DOE statistical analyses summary.
Design Factor
Total DOE
Powder Feed
Rate
Total Flow
Stoichiometry
Interaction
Interaction
Interaction
Site
Blocking
Main Effects’
Percentages
Level
Mean
a1
a2
b1
b2
c1
c2
ab1
ab2
ac1
ac2
bc1
bc2
B
A
1
2
3
4
Sum
Dep. Rate
mils/pass
0.80
0.525
1.08
0.775
0.83
0.924
0.681
0.740
0.865
0.887
0.718
0.868
0.737
1.004
0.601
DPH
kg/mm2
600
607
592
579
620
595
605
611
589
607
593
574
626
580
620
Tensile Bond
Strength, psi
10,043
10,264
9,823
9,651
10,436
10,858
9,228
8,861
11,225
9,643
10,444
10,233
9,853
10,445
9,642
Substrate
Temp., °F
313
297
328
286
340
346
280
299
327
321
305
311
315
403
222
Almen
Deflection, mils
-3.3
-5.6
-1.1
-0.9
-5.8
-1.3
-5.4
-3.8
-2.9
-2.7
-4.0
-5.2
-1.5
+2.7
-9.4
Powder feed rate
33.0%
Site
23.9%
Stoichiometry
14.5%
bc Interaction
26.5%
Total flow
21.1%
Site
20.6%
ab Interaction
32.8%
Stoichiometry
22.6%
Site
11.1%
Site
47.4%
Stoichiometry
17.4%
Total flow
14.3%
Site
38.4%
Total Flow
15.7%
Powder feed rate
14.5%
71.4%
68.2%
66.5%
79.1%
68.6%
77
contribution to data variation. Finally, Almen deflection data variation was dominated by Site blocking (38.4 %)
which implied both heating and kinetic effects of total flow and quenching in of compressive coating residual
stresses for the efficient cooling at Site A. Total flow (15.7 %) and powder feed rate (14.5 %) were the next two
strongest factors in controlling Almen deflection results. Most of these results seemed reasonable and were not
terribly different from those of the WC-Co DOEs.
11.5.
DOE Confirmation Runs
Duplicate runs of each coating for each site were made to confirm the predicted HVOF process capabilities.
The parameters were optimized primarily for best (highest compressive residual stresses) Almen deflection
without undue sacrifices in other properties such as deposition rate, DPH microhardness, or tensile bond
strength. Only values of spray parameters utilized in the DOEs were used in the confirmation runs, but in
the preferred combinations to achieve the desired coating properties. The results of the confirmation runs
have been compiled in Table 11. Note that the desired out come for high compressive stress Almen
deflections was obtained in for 3 out of 4 processes. The one exception was for the Site A Triballoy 400
coatings where a mid-range result was obtained. The reason for that was because it was decided to use mid
range parameters on this particular run which were similar to an existing practice for the Triballoy 400, thus
tying in known industrial experience. The results in Table 20 verified that none of the other coating
properties were compromised in the confirmation runs. All results obtained during confirmation trials were
within the statistically predicted ranges of coating properties for all properties.
Originally, it had been planned that all subsequent HVOF characterization for wear, fatigue effects, and compressive
creep be conducted with the optimized parameters of the best processes for one site. However, this plan was
modified to use the DOE mid point processes at Site A because these provided a good match in properties to the
optimized properties at Site B. The justification for this decision was that the mix of coating properties thus obtained
were determined to be more representative of overall industrial experience for what could be expected on a repeatable
basis as opposed to committing to the capabilities of a particular site.
Table 20. confirmation runs data.
Site Run Dep. Rate
Coating
DPH
Number mils/pass kg/mm
R 15N
2
kg/mm
Tensi le Bond Substrate
2
o
Almen
Strength, psi
Temp F
Deflection, mils
WC-17 Co
A53
0.416
1065
94.4
11,486
158
-27.5
WC-17 Co
A54
0.521
1145
94.3
11,821
130
-39.5
WC-17 Co
B22
ND*
1136
93.0
12,701
ND
-15.4
WC-17 Co
B23
ND
1167
94.2
12,081
ND
-12.1
Triballoy 400
A59
0.416
568
89.4
9,534
162
-5.0
Triballoy 400
A60
0.416
598
89.2
9,127
ND
-5.5
Triballoy 400
B18
ND
602
87.8
8,273
ND
-2.4
Triballoy 400
B19
ND
588
89.2
8,736
ND
-1.6
* ND - no data reported
78
12.
12.1.
Other Coating Properties Tests and Results
CSEM ReveTest Scratch Adhesion
The hard chrome, three (Ti,Al)N coatings, and two HVOF coatings were evaluated by the CSEM Revetest
scratch tester. These automated scratch tests were run using a 0.2 mm radius 120° cone diamond indentor.
The load was ramped up from 0 to 200 newtons at a rate of 100 newtons/minute as the specimen was
traversed at a speed of 10 mm/minute. The test output consisted of a chart with an acoustic emission signal
and a friction force plotted against the applied load. The resultant scratches were viewed optically at 200X
and selected representative scratches examined on the SEM in the back-scattered electron (BSE) mode.
Typical scratch test charts and SEM photographs of selected areas of the actual scratches are shown for
hard chrome, the HVOF Triballoy 400 and WC-17Co, and one of the PVD (Ti,Al)N coatings in Figure 56 and
Figure 57.
Scratch test results are typically reported in terms of a critical load, Lc, but care must be taken to understand how the
Lc is defined and what the scratch test conditions were since these affect the values of Lc. The scratch test has been
most widely used on thin (less than 6 microns) PVD hardcoatings where one of the easiest to define Lc criteria exists.
This would be a visual criteria sometimes called the "clear path" criteria wherein the the load becomes sufficiently
large to undercut the coating completely exposing the underlying metal substrate. Applying the coating "clear path"
criteria to the (Ti,Al)N-L scratch tests of the SEM photo (Figure 56, bottom) gives Lc=63-69 N, values comparable to
those obtained for TiN coatings currently used on GE production hardware. Examination of the acoustic emission
and friction force traces for the same (Ti,Al)N-L coating shows sharp increases at the 45-60 N range.
The (Ti,Al)N-M and (Ti,Al)N-H coatings were similar in their scratch test results. They gave scratch clear path
values of Lc=49-60 N. The scratch test charts for the acoustic emission and friction force traces gave Lc=20-34 N or
Lc=60-78 N depending on the criteria applied. A typical scratch test chart for these coatings is seen in the data plot
(Figure 58) for (Ti,Al)N-H.
The thick (75-125 micron) hard chrome plating exhibited transverse cracking in the scratch path (see Figure 56, top)
even at low (<20 N) loads, yet did not show evidence of spalling or side lateral cracks at the full 200 N load. The data
chart for the hard chrome was as seen in Figure 59. The friction force trace contained no abrupt changes incicative of
an Lc. The acoustic emission trace showed a raise at about Lc=30-40 N.
Scratch tests on the two HVOF coatings showed typical stick slip behavior in the friction force trace and similar
appearing acoustic emission traces (see Figure 60 and Figure 61). Small amplitude peaks typically started at loads in
the 15-20 N range and significantly larger peaks started in the 30-40 N range. Thus, one could choose Lc = 15 or 30 N
depending on the criteria. SEM photos (Figure 57) of the HVOF coating scratch paths showed a significant amount
of smeared metal along the entire scratch length, but no transverse cracking like that seen for the hard chrome. It was
not ruled out that such cracks actually were present and associated with the spikes of the data plots, but were
concealed by the smearing.
79
Figure 56. SEM photos of scratch test paths. Top: Hard chrome; Bottom: (Ti,Al)N-L.
80
Figure 57 SEM photos of HVOF coating scratch test paths. Top: WC-17Co;
Bottom: Triballoy 400.
81
Figure 58. Scratch test data chart for (Ti,Al)N-H.
82
Figure 59. Scratch test data for hard chrome.
83
Figure 60. Scratch test data for T400.
84
Figure 61. Scratch test data for WC-17Co.
12.2.
Fretting Wear Tests
85
Fretting wear tests were conducted on all coatings under evaluation and hard chrome as a baseline for
comparison. The fretting wear test stand is located in the GE Aircraft Engine Evendale plant. It utilizes an
MTS Model 810 microprocessor controlled mechanical test stand which controlled the fretting wear motion
in the stroke control mode. The test regimen utilized a 0.040 inch stroke for 786,000 wear cycles with a one
inch wear debris cleanout stroke every 1,000 wear cycles. The test specimens consisted of a flat uncoated
block and a coated shoe with a 0.060 inch wide flat ground on a 0.25 inch radius cylindrical nose which was
oscillated along the block surface under load. The coatings were applied to the fretting wear shoes to an
excess thickness and then ground back to leave a nominal coating thickness of 0.010 inch. The ground
coatings were left with a surface finish of Ra 32 micro-inches.
The wear test matrix and resultant wear data have been summarized in Table 21 and presented in bar graph form in
Figure 62 and Figure 63. The wear data was normalized by the widely used empirical relation, Wv = K P L where Wv
is the wear volume, P the normal load, L the sliding distance, and K the wear coefficient. Figure 17 compared the wear
results obtained for hard chrome coatings to those for the PVD alternative coatings. The (Ti,Al)N-H coating was the
only test that gave significantly different wear from that obtained for hard chrome. However, that test was
inadvertently allowed to continue well past the point at which the coating wore through so the wear coefficient
reflected an average of coating and substrate wear. (Uncoated substrates would have higher wear coefficients more
typical of most metals). The absence of a shoe wear coefficient for some of the tests was due to transferred material
form the block to the shoe for tests against 4340 steel blocks. In general, the PVD coatings gave comparable wear
coefficients to hard chrome, but had shorter wear lives because they were thinner as seen from the number of wear
cycles in Table 21.
The wear coefficients for the HVOF and laser clad coatings have been compared to those of hard chrome in Figure 62.
The WC-Co which wore against IN718 showed comparable wear of the IN718 to that caused by the hard chrome, but
the WC-Co itself wore less than the hard chrome. However, for the case of WC-Co against 4340 steel, the WC-Co
coating did exhibit a small amount of wear unlike the hard chrome which remained protected by the adhesive transfer
layer of 4340 steel. The entire Site A WC-Co DOE series was wear tested against the IN718 substrate, but very little
difference was seen for the differing spray parameters so no statistical analysis was performed.
The Triballoy 400 was tested in two forms, HVOF thermal spray and laser clad, against an opposed IN718 surface. It
showed coating wear comparable to the hard chrome for both forms of Triballoy 400. The HVOF Triballoy 400
resulted in less wear of the opposing surface than was seen for the hard chrome. The laser clad Triballoy 400
resulted in comparable wear to the opposing surface as seen for the hard chrome. In retrospect, it would have been
prudent to test the Triballoy 400 against 4340 steel, also.
It appears on the basis of the limited testing to date that one needs to closely watch the differing material
combinations. One should also keep in mind the coating thickness and wear coefficients of the various coatings. If
one coating has 0.05X the thickness of a second coating, then the first coating needs 20X the wear resistance of the
second coating for an equivalent wear life. The use limits of the various material combinations also needs to be
properly understood as indicated by the lower wear coefficient obtained in the one low load test, 94-17. Thus,
whether or not a given coating material alternative to hard chrome is acceptable may be dependent on the particulars
of the proposed application.
86
Table 21. Fretting wear test conditions and results.
Fretting Block Matl. Shoe Matl.
Wear
Test No.
(Bare)
Shoe Coating
Ambient
Temp.
Contact
Stress
(psi)
Frition Coeff.
Coefficient
Coefficient
94.19
94.20
94.17
94.18
94.21
94.22
95.03
94.23
94.27
94.24
94.29
94.25
94.26
95.01
95.04
94.28
94.64
94.47
94.49
94.51
94.52
94.53
94.54
94.55
94.48
94.50
94.56
94.57
94.60
94.58
94.59
94.61
94.62
94.63
94.65
IN718
IN718
4340 steel
4340 steel
4340 steel
IN718
4340 st eel
IN718
4340 steel
IN718
4340 steel
4340 steel
IN718
4340 steel
IN718
4340 steel
4340 steel
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
4340 steel
4340 steel
IN718
IN718
IN718
IN718
4340 steel
IN718
IN718
IN718
4340 steel
IN718
4340 steel
IN718
4340 steel
IN718
4340 steel
4340 steel
IN718
4340 steel
IN718
4340 steel
4340 steel
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
IN718
4340 steel
IN718
Hard Chrome
Hard Chrome
Hard Chrome
Hard Chrome
Hard Chrome
(Ti,Al)N-L
(Ti,Al)N-L
(Ti,Al)N-M
(Ti,Al)N-M
(Ti,Al)N-H
(Ti,Al)N-H
CrN
CrN
CrN
CrN
PN +CrN
PN +CrN
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF WC-17Co Site B
HVOF WC-17Co Site B
HVOF WC-17Co Site B
HVOF WC-17Co Site A
HVOF WC-17Co Site A
HVOF T400 Site A
HVOF T400 Site A
HVOF T400 Site B
Laser T400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
400
5000
2000
400
1000
2000
2000
2000
2000
2000
2000
400
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
2000
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
200,000
125,000
1,000,020
1,000,020
125,000
34,979
125,040
35,000
35,000
35,000
625,000
125,000
35,001
125,040
125,040
125,000
125,040
125,000
125,000
125,000
125,040
125,040
125,040
125,040
125,000
125,000
125,040
125,040
125,040
125,040
125,040
125,040
125,040
125,040
125,040
0.48-0.77
0.40-0.86
0.19-0.70
0.37-0.84
0.34-0.71
0.44-0.86
0.61-0.87
0.77-0.95
0.55-0.94
0.74-0.98
0.75-1.17
0.54-0.82
0.76-0.90
0.57-0.79
0.79-1.06
0.59-0.90
0.59-0.80
0.74-1.06
0.76-1.02
0.80-0.99
0.77-1.01
0.73-1.00
0.75-0.98
0.80-1.00
0.79-0.99
0.75-1.01
0.81-0.97
0.74-1.01
0.84-1.00
0.60-1.12
0.62-1.08
0.54-0.91
0.48-0.81
0.52-0.85
0.74-1.33
4.88E-11
1.35E-10
9.37E-12
4.37E-11
6.50E-11
2.14E-10
1.00E-10
2.86E-10
8.93E-11
2.14E-10
5.00E-09
2.00E-11
1.96E-10
9.50E-11
1.55E-10
7.00E-11
2.50E-11
1.70E-10
1.65E-10
1.65E-10
1.75E-10
1.80E-10
1.80E-10
1.65E-10
1.80E-10
1.60E-10
1.70E-10
1.65E-10
1.50E-10
1.75E-10
1.65E-10
2.00E-11
3.50E-11
3.00E-11
1.30E-10
1.00E-10
1.35E-10
0.00E+00
0.00E+00
0.00E+00
2.68E-10
2.50E-11
1.96E-10
1.79E-11
2.68E-10
1.50E-08
0.00E+00
2.86E-10
5.00E-11
2.10E-10
0.00E+00
1.00E-11
4.50E-11
4.50E-11
4.00E-11
5.50E-11
5.00E-11
4.50E-11
4.50E-11
5.50E-11
4.00E-11
4.00E-11
3.00E-11
3.00E-11
1.50E-11
2.00E-11
2.65E-10
2.50E-10
2.95E-10
1.95E-10
95.02
IN718
IN718
Laser T400
400
2000
0.04
10
125,040
0.76-1.07
2.00E-10
2.00E-10
87
Wear
Stroke
(mils)
Freq.
Number of
Range of
Block Wear
Shoe Wear
(Hz)
Wear Cycles
Figure 62. Fretting of HVOF and laser clad coatings compared to hard chrome.
88
Figure 63. Fretting of PVD coatings compared with hard chrome.
89
12.3.
Fatigue Tests
The HVOF WC-Co and Triballoy 400 coatings were selected for further evaluation in Fatigue tests to determine their
effect on the substrate material, IN718. hard chrome applied to IN718 served as the baseline of comparison with the
criteria that the fatigue stregth for the HVOF coated specimens had to be equal to or better than that for the hard
chrome coated specimens. The fatigue specimen test goemetry consisted of a rectangular gage section and was a
specimen type frequently used in crack growth tests known as a Kb bar. The fatigue specimen manufacture was
standardized as low stress ground plus electropolished to remove surface residual stresses which might interact with
coating stresses.
The specimens had a one inch long coating patch applied to each of the larger flat surfaces and centered in the gage
section. The coatings were applied to a thickness of 0.003-0.005 inches, typical of many hard chrome applications.
The specimen preparation prior to coating deposition was solvent clean with acetone and grit blast to about Ra 100150 micro-inches using 60 grit alumina.
The fatigue tests were conducted with an A-ratio of +1.0 as strain controlled LCF tests at 0.5 Hz at 750°F or load
controlled HCF tests at 30 Hz at 800°F' (temperatures selected on basis of existing uncoated comparison data).
Twelve specimens of hard chrome were tested, six in LCF and six in HCF, and eight specimens each of the WC-Co
and Triballoy 400 until failure or runout (105 and 107 cycles) at varied loads. The results have been summarized in
Table 22 and plotted in Figure 64 (LCF) and Figure 65 (HCF) as typical S/N curves.
The WC-Co coated IN718 curve was slightly higher (i.e., better) than the liud chrome plated lN718 curve at all except
the highest stresses where the data becomes indistinguishable. The Triballoy 400 coated IN718 curve was
considerably higher than the hard chrome curve at all stresses evaluated. Historic curves of uncoated IN718 showing
typical average and 3σ data were superimposed on the plots and indicated that the 'l'ribilloy 400 data fell within the
expected range for uncoated IN718.
90
Table 22. Fatigue test results.
Bar # Coating
Type test
Result
Nf cycles
13 Hard Chrome
7 Hard Chrome
3 Hard Chrome
11 Hard Chrome
1 Hard Chrome
9 Hard Chrome
2 Hard Chrome
4 Hard Chrome
12 Hard Chrome
10 Hard Chrome
8 Hard Chrome
14 Hard Chrome
LCF
LCF
LCF
LCF
LCF
LCF
HCF
HCF
HCF
HCF
HCF
HCF
R/O
Failed
Failed
Failed
Failed
Failed
R/O
R/O
R/O
Failed
Failed
Failed
473,808
75,367
56,177
19,926
8,467
8,148
10,038,600
14,065,700
10,260,800
117,200
103,200
172,900
Alt stress
(ksi)
35
38
45
60
90
120
28
29.5
30
30.5
31.5
33
20 WC-17Co (LW90B)
17 WC-17Co (LW90B)
15 WC-17Co (LW90B)
19 WC-17Co (LW90B)
18 WC-17Co (LW90B)
21 WC-17Co (LW90B)
22 WC-17Co (LW90B)
16 WC-17Co (LW90B)
LCF
LCF
LCF
LCF
HCF
HCF
HCF
HCF
Failed
Failed
Failed
Failed
R/O
R/O
Failed
Failed
120,126
53,943
24,803
3,043
11,596,000
10,166,700
120,400
88,500
45
52
70
140
28
31.5
35
42
5 Triballoy 400 (LT91)
27 Triballoy 400 (LT91)
23 Triballoy 400 (LT91)
25 Triballoy 400 (LT91)
26 Triballoy 400 (LT91)
24 Triballoy 400 (LT91)
28 Triballoy 400 (LT91)
6 Triballoy 400 (LT91)
LCF
LCF
LCF
LCF
HCF
HCF
HCF
HCF
R/O
Failed
Failed
Failed
R/O
Failed
Failed
Failed
438,177
38,355
23,458
9,386
10,040,300
1,877,900
896,300
153,300
55
85
100
140
40
47
52
60
91
1000
Hard chrome
100
WC-17Co
T400
10
1,000
10,000
100,000
1,000,000
Nf Cycles
Figure 64. Low cycle fatigue results.
100
Hard chrome
WC-17Co
T400
10
10,000
100,000
1,000,000
10,000,000
Nf Cycles
Figure 65. High cycle fatigue results.
92
100,000,000
12.4.
Chrome and HYOF Coatings Compressive Creep Tests
One function of hard chrome has been to ensure proper interference fits between parts, such as a bearing journal surface and
the bearing itself. The shaft in Figure 66 has four such journal areas, shown as light bands. An important aspect of this type
application has been the ability of the hard
chrome to maintain a tight fit without
relaxation of the compressive interference
loads due to creep. If the chrome creeps the
holder will become loose, spin, and gall the
shaft. A compressive creep test was utilized
to test the hard chrome and the HVOF
coatings, WC-17 Co and Triballoy 400.
Figure 66. Main power shaft for T700 turbine engine - GEAE. The shaft has
Tests were conducted at 800 oF under 50 and
four HVOF-sprayed journal areas (light bands).
100 ksi loads for 300 or 1000 hour hold times.
The 800 oF temperature was selected as a representative upper temperature limit where hard chrome might be used and the
high load, high temperature tests represented a worst case set of conditions for compressive creep.
Specimens were prepared by depositing 0.030 inches minimum of the hard chrome and 0.025 inches maximum of the two HVOF
coatings, WC-Co and Triballoy 400, on flat sheets of 0.060 inch SAE 1020 steel. The coated sheets were cut into smaller
pieces and the coated surfaces of the three sheets ground flat with minimal coating removal. The reverse side of the sheets
was then carefully machined away to leave coating only. It was found that a thin layer, less than 0.005 inches, of the steel
sheet had to be left intact for the hard chrome to prevent it from fracturing into tiny unusable pieces during machining. The
final steel removal from the chrome was accomplished by selective acid dissolving of the steel backing. Specimens 0.25 inches
square were then cut for testing. The testing was accomplished using stacks of 3 specimens to allow for greater accuracy in
measuring the height changes due to creep. Alumina platens were utilized for loading the coating specimens to assure the
creep deformation was in the coating specimens and not the platens. Measurements were obtained by two methods; 1) direct
flat anvil micrometers measurements of the individual test specimen stacks before and after testing, and 2) dial gage
extensometer readings during the test.
The test results for both measurement methods have been summarized in Table 23, below. Some differences were seen for the
two measurement methods. The cause of these differences appeared to be due to a “seating in” of the stacked specimens
after the load was applied. For the direct micrometer measurements of the specimen stack heights before and after testing, the
hard chrome gave the largest average compressive creep strain rates at both loads, 0.000278 in/in/hr at 50 ksi and 0.000563
in/in/hr at 100 ksi. Triballoy 400 gave the next largest compressive creep strain rates by the micrometer measurement method,
0.000043 in/in/hr at 50 ksi and 0.000181 in/in/hr at 100 ksi. WC-Co gave the lowest compressive creep strain rate at 100 ksi,
0.000057. The WC-Co showed such low creep after the planned 300 hours at 100 ksi that the time was extended to 1000 hours
and the low load test abandoned.
Plots of the extensometer test data can be seen in Figure 67 for the 100 ksi tests and Figure 68 for the 50 ksi tests. Most tests,
to varying degrees, showed high short time thickness changes compared to the overall change. After correcting for this effect
believed to be due to seating in of the specimens, the strain rates based on the extensometer data were calculated and have
been included in Table 14, also. Overall, the hard chrome compressive creep strain rates were again the largest (0.000126 at 50
ksi and 0.000322 at 100 ksi), Triballoy 400 second (0.000084 at 50 ksi) or about equal (0.000364 at 100 ksi), and WC-Co the
smallest (0.000016 at 100 ksi). The lower range of strain rates for the extensometer data may be indicative of steady state creep
since the exclusion of the early portion of the data would have contained any rapid initial creep as well as the specimen
seating in phenomena.
It was interesting to note that regardless of which set of measurement data was examined, the hard chrome strain rates were
approximately doubled when the load was doubled while the Triballoy 400 strain rates increased by about 4X. The hard
chrome was a homogeneous single phase material and gave linear behavior with load. However, the Triballoy 400 material
contained a dispersed hard phase (Laves phase) in an fcc alloy matrix. The Triballoy 400 behavior probably occurred due to a
creep mechanism change with load for the two phase structure not seen for the single phase structure of hard chrome. In the
final analysis, it was clear the HVOF coatings gave at least equal or better compressive creep performance than that of the
hard chrome.
93
Table 23. Compressive creep test results for coatings.
Summary of Height Change of Three (3) Stacked Specimens
Direct micrometer reading measurements
Amb. Initial
Test
No.
Init. Hgt. of
Final Hgt.
Change in
Specimen Temp. Stress Duration Specimen
Specimen
Height
Number
Test
Extensometer reading measurements
Avg. Strain
Change in
Avg. Strain
Strain
Rate
Height
Strain
Rate
(°F)
(ksi)
(hours)
Stack (in.)
Stack (in.)
(in.)
(in./in.)
(in./in./hr.)
(in.)
(in./in.)
(in./in./hr.)
67947 Chrome #1
800
50
1108.1
0.13655
0.09443
0.04212
0.30846
0.000278
.01920
.14061
0.000126
67946
800
50
1108.4
0.05805
0.05529
0.00276
0.04755
0.000043
.00540
.09302
0.000084
67852 WC-Co #1
800
100
1003.5
0.06891
0.06499
0.00392
0.05689
0.000057
.00110
.01596
0.000016
67853
800
100
334.8
0.05585
0.05246
0.00339
.06070
0.000181
.00680
.12175
0.000364
800
100
308.2
0.08273
0.06837
0.01436
0.17358
0.000563
.00820
.09912
0.000322
T400 #1
T400 #2
67868 Chrome #2
94
Coatings Compressive Creep: 800 F, 100 ksi
0.018
Height Change, inches
0.016
0.014
0.012
0.01
0.008
Chrome #2
0.006
T400 #2
0.004
WC-Co#1
0.002
0
0
100
200
300
400
500
600
700
800
900
1000
Time, hours
Figure 67. Compressive Creep Data at 800 oF and 100 ksi for Hard Chrome and HVOF Coatings.
Coatings Compressive Creep: 800 F, 50 ksi
0.14
Height Change, inches
0.12
0.1
Chrome#1
0.08
T400#1
0.06
0.04
0.02
0
0
100
200
300
400
500
600
700
800
900
Time, hours
Figure 68. Compressive Creep Data at 800 oF and 50 ksi for Hard Chrome and HVOF Coatings.
95
1000
Graphs of the average strain for the two different measurement methods are shown in Figure 70 and Figure 69.
0.0004
Avg. strain rate
(in/in/hr)
Avg. strain rate
(in/in/hr)
0.0006
0.0005
0.0004
0.0003
0.0002
0.0001
0
100ksi
0.0003
0.0002
100ksi
0.0001
0
50ksi
WC17Co
T400
50ksi
WC17Co
T400
Chrome
Chrome
Figure 69. Average creep measured by extensometer
readings.
Figure 70. Average creep measured by direct micrometer
readings.
96
12.5.
Selected Component Demonstration Trials for HVOF Coatings
Four different aircraft components which used hard chrome were selected for demonstrations of HVOF
coatings as replacements of the hard chrome. Repairable parts were supplied by Corpus Christi Army depot
for the demonstrations. Information on these four parts has been summarized in Table 24. The
Table 24. Hard chrome replacement demonstration components.
Part Number
Part Description
Material
6043T34, 6032T67
T700 helicopter engine power turbine shaft
IN718
6035T26P01, P02
T700 helicopter engine No. 4 bearing support
AM355 stainless steel
I45H6700
CH47D helicopter swiveling dual actuating cylinder
4340 high strength steel
70250-I3130-041
UH-60 helicopter tail fork landing gear
7175 aluminum
demonstrations consisted of applying the WC-l7Co and Triballoy 400 HVOF coatings to the required areas
of the parts and grinding the HVOF coatings to the final dimensions and Ra 32 or 16 rnicro-inch (max)
surface finish as would he required for the final finish on the hard chronic being replaced. The HVOF spray
process parameters employed were the results of the earlier confirmation runs for the DOEs and the same as
those used in all of the coating properties evaluations. The fixturing and masking techniques employed for
the HVOF coating applications were makeshift for budgetary reasons, but representative of methods used in
production. The grinding was accomplished with standard machine set-ups, but the grinding wheel type and
the details of grinding parameters were not documented. The grinding demonstrations on some of these
components showed very precice tolerances and good surface finishes could simultaneously be met.
Meeting both requirements with plasma sprayed coatings (as opposed to HVOF) can be difficult and was a
significant factor in selecting HVOF processes for this effort.
Figure 71. T700 engine power turbine shaft.
The first component was a power turbine shaft from a GE T700 military helicopter engine. It had five bearing
journal locations which were HVOF coated. Figure 23 showed the shaft with the as sprayed journals. The
coated journals were the raised rnatt finish appearing surfaces. Starting at the right hand end of the shaft,
the first two journals to the left of the circular flange were WC-l7Co coated and the remaining three as you
progress to the left end of the shaft were Triballoy 400 coated. Figure 71 shows the power turbine shaft after
finish grinding the HVOF coated journals. The four main journals have been identified with tape labels as 14; 1-2 were Triballoy 400 and 3-4 were WC-l7Co. The Triballoy 400 and WC-l7Co both exhibited the ability to
obtain good finishes and precise tolerances.
97
The second demonstration component was the No.4 hearing
support, also from a GE T700 military helicopter engine. Figure
72 shows this item in the as sprayed coating condition. A pair
of these components have been documented – one sprayed
with HVOF WC-l7Co and the other with HVOF Triballoy 400.
The coated areas were the cylindrical bands on the l.D.
surfaces above the ribbed cage structure with the matt
appearing finish. One point worth noting was the small oil
weep holes in the coated band area did not require any special
masking to maintain an open through hole. The surface finish
obtained after grinding was beginning to approach mirror
quality as evidenced by the reflections of the scalloped ring
structure where the support ribs terminated.
Figure 72. Number 4 bearing support - T700
engine turbine shaft.
The third component was a CH47 actuation cylinder piston.
The particular piece supplied by CCAD for the demonstration
had a small bend in it and was not reusable so one half was
Figure 73. CH47 actuator cylinder piston.
coated with WC-l7Co and the other half with Triballoy 400. The darker colored coating near the flange in
Figure 29 was the WC-l 7Co. The ground coatings are seen in Figure 30. The transition area between the
coatings was not masked, was where the slight bend occurred., and thus gave some difficulty in obtaining a
perfect blend due to run out differences. In practice of course, there would be only one or the other coatign
selected for use and no bend to be contended with.
The last component was the UH60 tail fork
landing gear which was coated with HVOF
Triballoy 400. This component afforded an
opportunity to demonstrate detail masking
capability for the HVOF coatings at the
piston ring groove at the top of the cylinder.
Figure 74 shows an overall view of the
landing gear. This part had to be returned to
CCAD for machining of the Triballoy
coatings due to its size and the requirement
for counter balance tooling to obtain true
running of the cylinder surface as it was
machined. Thus, no photos were available
for the report
Figure 74. UH60 landing gear tail fork.
These parts demonstrated the robustness of
the HVOF coating process for chrome replacement on a variety of typical military component materials,
geometries, sizes, for two types of coating materials., and were sprayed at two sites for transferability of the
processes.
98