As sprayed coatings - Heriot

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As sprayed coatings - Heriot
Tribo-Mechanical Evaluations of HIPed
Thermal Spray Cermet Coatings
Rehan Ahmed
Heriot Watt University, UK
V. Stoica
T. Itsukaichi
Heriot-Watt University, UK
Fujimi Inc., Japan
S. Tobe
R. Gadow, M. Escribano
Ashikaga Inst. of Tech., Japan
University of Stuttgart, Germany
INTRODUCTION
•Theaim
The
aimofofthis
thisinvestigation
investigationwas
wastotointegrate
integratethe
thepotential
potentialbenefits
benefits
of two process technologies of thermal spraying and HIPing to
POWDER
improve
coatings tribo-mechanical performance.
HVOF
HIPing POSTMANUFACTURE
Thespecific
specificobjectives
objectiveswere
were
toimprove
improvecoating
coating
strengthand
andwear
SPRAYING
TREATMENT
The
to
strength
(WC-NiCrBSiFeC)
wear
resistance
resistance
by : by :
•Improve intersplat
•Improved
intersplatcohesion.
cohesion by HIPing post-treatment.
•Transformthe
themechanism
mechanismof
ofcoating
coatingadhesion
adhesionfrom
frommechanical
mechanical
•Transform
interlock
tometallurgical
metallurgical
bonding.
•Coating
Microstructure
(SEM, XRD)
interlock
to
bonding.
•Improve
the homogeneity
and crystallinity
of coating
•Mechanical
Strength (Modulus,
Hardness,
Toughness)
•Improve
the homogeneity
and crystallinity
of coating
microstructure.
microstructure.
•Sliding Wear Resistance (Ceramic and Metallic couples)
•Residual Stress
STARTING POWDER
WC-10%NiCrBSiFeC
WC-40%NiCrBSiFeC
Alloy composition: Cr 7.6%, Si 3.6%, Fe 2.4%, B 1.6%, C 0.25%, Ni Bal.
•
Pre-alloying of WC-NiCrBSiFeC powders.
•
Two different compositions: WC-10%NiCrBSiFeC and WC40%NiCrBSiFeC were produced by the agglomeration and sintering.
THERMAL SPRAYING
•
•
Functionally graded coatings were produced by the HVOF (JP5000)
process on 440-C bearing steel substrate to minimise the mismatch
of thermal and elastic properties.
The spraying parameters were as follows:
Oxygen flow – 893 lit/min
Kerosene flow – 0.321 lit/min
spraying distance – 380 mm
Spraying rate – 50 g/min
WC-10%Ni alloy (400mm)
Shot-blasting
WC-40%Ni alloy (100mm)
440C steel substrate (8mm thick)
32 mm diameter
Grinding and
polishing
HIPing POST-TREATMENT
• Two Different HIPing temperatures of 850oC and 1200oC
were adapted at a pressure of 150 MPa.
• Cooling and heating rates were optimised to 4oC/minute.
• Holding time was 60 minutes.
• Uncapsulated HIPing conditions.
COATING MICROSTRUCTURE
10µm
10µm
HIPed at 850oC coatings
As sprayed coatings
WC-10NiCrBSi
WC-10NiCrBSi
WC-40NiCrBSi
Substrate
WC-40NiCrBSi
Substrate
7µm
7µm
10µm
HIPed at 1200oC coatings
Substrate
7µm
XRD EVALUATIONS
10
20
30
40
50
60
70
-WC
-WC
-WC
-WC
-WC
-WC
-WC
-WC
-W 2C
-WC
-WC
-WC
-Ni
-W 2C
-WC
Starting powder
-Ni
-NiB
-Ni
As-sprayed coating
-FeW 3C
-Ni3B
-W C
-W 2
-WC
WC
WC
Powder vs. Sprayed Coating
80
90
As-sprayed coating
10
20
30
40
50
60
70
-WC
-WC
-WC
-WC
-WC
-W2C
-WC
-W2C
-Ni
W
-WC
-WC
-WC
-WC
-WC
-WC
-Ni
-Ni2W 4C
-FeW 3C
-W 2C
HIPed at 850oC
-FeW3C
-Ni3B
-W2C
HIPed at 1200oC
-WC
-WC
-WC
-WC
-Ni3B
-WC
-WC
-Ni5Si2
-Ni2W 4C
-Ni
-Ni2Si
-Ni4B3
-Ni2W 4C
-FeW 3C
-FeW 3C
-Ni4B3
WC
-WC
-WC
XRD EVALUATIONS
Sprayed and HIPed coatings
80
90
MIROHARDNESS EVALUATIONS
WC-40%NiCrBSi
WC-10%NiCrBSi
Substrate
1400
Vickers Hardness
1200
1000
800
600
As-sprayed
400
HIPed at 850
200
HIPed at 1200
0
0
50
150
250
350
Distance from Surface (µm)
450
INDENTATION MODULUS
Surface
=E(1-2)
Indentation Modulus (GPa)
WC-10%NiCrBSi
WC-40%NiCrBSi
450
400
350
As-Sprayed
300
HIPed at 850oC
250
HIPed at 1200oC
200
150
100
50
0
0
50
100
150
200
250
Distance from Surface (µm)
300
350
400
SEM observations: HVOF coatings
Cryogenic fractured coatings
micro-cracks
As sprayed coatings
pores
pores
HIPed at 850C coatings
HIPed at 1200C coatings
INDENTATION TOUGHNESS
Cracks
200 mm
As-sprayed coating
200 mm
200 mm
HIPed at 850oC coating
HIPed at 1200oC coating
SLIDING WEAR TESTS
Test conditions
Normal load
Ball
Coating
Counter Body
(balls)
440C Steel
Si3N4 ceramic
Load
12 and 22 N
Sliding Speed
0.012m/s
Dry/Lubricated
Dry
Sliding direction
Reciprocating ball on plate apparatus
SLIDING WEAR:
COATING VOLUME
LOSS
Coatings Vs steel, 12N load
Volume loss (mm3)
0.014
0.012
Coatings Vs steel, 22N load
0.01
Coatings Vs ceramic, 12N load
Coatings Vs ceramic, 22N load
0.008
0.006
0.004
0.002
0
As-sprayed
HIPed at 850oC
HIPed at 1200oC
SLIDING WEAR: WEAR SCARS
Three dimensional interferometric plots of the coatings tested against
ceramic balls (load – 22N)
As-sprayed coating
HIPed at 850oC coating
HIPed at 1200oC coating
SLIDING WEAR: TOTAL VOLUME LOSS
Total volume loss – Coatings Vs steel balls
1
Total volume loss (mm3)
0.8
Total volume loss, 12N load
0.6
Total volume loss, 22N load
0.4
0.2
0
As-sprayed
HIPed at 850oC
HIPed at 1200oC
SLIDING WEAR: TOTAL VOLUME LOSS
Total volume loss – Coatings Vs ceramic balls
Total volume loss (mm3)
0.1
Ball volume loss, 12N load
Coating volume loss, 12N load
0.08
0.06
Ball volume loss, 22N load
Coating volume loss, 22N load
0.04
0.02
0
As-sprayed
HIPed at 850oC
HIPed at 1200oC
Why improvement in wear resistance?
1.4
1.2
Friction coefficient
FRICTION
1
0.8
0.6
As-sprayed
0.4
HIPed at 850oC
0.2
HIPed at 1200oC
0
100
200
300
Time (mins)
400
600
500
Coatings Vs steel balls (load - 22N)
1.4
1.2
Friction coefficient
1
0.8
0.6
As-sprayed
0.4
HIPed at 850oC
0.2
HIPed at 1200oC
0
100
200
300
Time (mins)
400
500
Coatings Vs ceramic balls (load - 22N)
600
SLIDING WEAR: WEAR MECHANISMS
SEM micrographs within the wear tracks of the coatings tested
against steel (load – 12N)
As-sprayed coatings
HIPed at 850oC coating
HIPed at 1200oC coating
RESIDUAL STRESS MEASUREMENT
Distance from the surface (µm)
0
0
Residual stress (MPa)
-200
-400
20
40
60
80
100
120
140
160
180
200
As-sprayed
HIPed at 850C
HIPed at 1200C
-600
-800
-1000
-1200
-1400
220
CONCLUSIONS
1. Uncapsulated HIPing can be successfully applied to post-treat
thermally sprayed coatings.
2. HIPing post-treatment can improve the sliding wear resistance of
thermal spray cermet coatings.
3. Wear resistance improves with the increase in HIPing temperature.
4. Improvement in sliding wear resistance is thought to originate from
the increase in coating’s hardness, elastic modulus and fracture
toughness.
5. HIPed coatings show WC recovery and formation of complex
carbides.
6. Results indicate higher elastic modulus after HIPing due to higher
bonding between lamellas.
WORK IN PROGRESS
 Influence of HIPing pressure, HIPing vs. Vacuum
Heat Treatment.
 Influence of Coating Materials, especially WC-Co
 Coating Substrate Bonding Mechanism.
 Measurement of Adhesive and Cohesive strength.
 Optimisation of HIPing Parameters
 Influence on Fatigue and Impact performance

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