DIRECT COLOR MARKING OF METALS WITH FIBER LASERS

Transcription

VTT-R-02403-09
RESEARCH REPORT
DIRECT COLOR MARKING OF
METALS WITH FIBER LASERS
Authors:
Petri Laakso, Saara Ruotsalainen, Heikki Leinonen, Aino Helle, Raimo
Penttilä, Anni Lehmuskero, Jouni Hiltunen
Confidentiality:
Public
RESEARCH REPORT VTT-R-02403-09
1 (66)
Report’s title
Direct color marking of metals with fiber lasers
Customer, contact person, address
Order reference
Project name
Project number/Short name
Direct color marking of metals with fiber lasers
17383 / DIME
Author(s)
Pages
Petri Laakso, Saara Ruotsalainen, Heikki Leinonen, Aino Helle,
Raimo Penttilä, Anni Lehmuskero, Jouni Hiltunen
Keywords
66/
laser, marking, fiber, color
VTT-R-02403-09
Report identification code
Summary
In this project laser color marking was further developed and the marked areas were examined
on wear, corrosion and color wise. During project couple different strategies was examined
how to make the oxide grow. Biggest effort was put to research work around stainless steel
due it was already known that titanium is easier to color mark with laser. From the corrosion
results we can see that the marking speed has a big effect on corrosion resistance even if heat
input remains the same. Strategy used in marking has also big effect due line by line marking
there is base material always present between lines and this make the surface more prone to
corrosion. Wear tests showed clearly that the laser color marking parameters have a significant
influence on the wear behaviour of both the color marked stainless steel surface and of the
counter part surface.
The colors of the surfaces can be explained by thin film interference. The thickness of the film
together with the illumination angle determines the visible color. Colors are best defined if
measured spectrally under some CIE standard geometries. From the spectra it is always
possibly to calculate any desired color coordinate values, Lab or others, under different
illuminations.
As a final conclusion of we can state that project covers wide range of color marking with
fiber lasers and laser parameter effects to the oxide growth and so on to the corrosion and wear
properties. With the project results end user should have comprehensive knowledge how to set
parameters to successful color marking.. Used laser in the project showed that correct
equipment will help in the finding of colors a lot. Of course the more the parameters the more
work has to be done when optimizing the result. Also different optical setups are a necessity in
successful process.
Confidentiality
Public
Lappeenranta 30.3.2009
Written by
Reviewed by
Accepted by
Petri Laakso
Research Scientist
Ilkka Vanttaja
Team Leader
Timo Määttä
Technology Manager
VTT’s contact address
Tuotantokatu 2, 53850 Lappeenranta, Finland
Distribution (customer and VTT)
Iittala Group, Nanofoot Finland, Nokia, Kone, TEKES, Savcor Alfa, SPI Lasers and VTT
The use of the name of the VTT Technical Research Centre of Finland (VTT) in advertising or publication in part of
this report is only permissible with written authorisation from the VTT Technical Research Centre of Finland.
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Preface
Project “Direct color marking of metals with fiber lasers” (DIME) was carried out between
2007 and 2009. Project was lead by VTT Laser processing group in Lappeenranta. Color
measurements and modelling was done in Joensuu by Joensuu University. Corrosion testing
was done in Espoo by VTT Power plant environment. VTT friction and wear solutions in
Espoo made the wear testing. In addition project was participated by five companies. Industry
had the major role in steering the project.
Project was funded by TEKES with participating companies and VTT.
Steering group consisted of following people
Iittala Group
Nokia Oyj
Savcor Alfa
Nanofoot Finland
Kone Oyj
SPI Lasers Ltd
TEKES
VTT
Juha Pimiä
Tiina Moisio
Anssi Jansson
Vamshi Bukimi
Anne Stenroos
Jack Gabzdyl
Kari Kuokkanen
Jari Karjalainen to 2.4.2008 and after that Timo Määttä
Carrying out the project was in charge project manager Henrikki Pantsar until 1.11.2007 and
after that project manager Petri Laakso from VTT. In addition project involved a number of
different researchers from VTT and Joensuu University.
Writers of the this report would like to acknowledge the steering group and all companies in
the project for active steering and good cooperation through out the project.
Lappeenranta 30.3.2009
Authors
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Contents
Preface ........................................................................................................................2
1 Introduction.............................................................................................................5
2 Goal........................................................................................................................5
3 Limitations ..............................................................................................................6
4 Experimental...........................................................................................................6
4.1 Laser equipment .............................................................................................7
4.1.1 SPI G3 20W pulsed fiber laser.............................................................7
4.1.2 IPG YLP 1/100/20 ................................................................................8
4.1.3 Diode pumped Nd:YVO4 Laservall Violino 3 ........................................9
4.1.4 Continuous wave fiber laser SPI SP-100C...........................................9
4.1.5 Rofin RS-100D diode pumped q-switched Nd:YAG ...........................10
4.1.6 Fraunhofer CLT fiber laser .................................................................11
4.2 Color measuring............................................................................................13
4.2.1 Color definitions .................................................................................13
4.2.2 Color measurements for laser marked samples.................................15
4.3 Surface characterization ...............................................................................17
4.3.1 Surface roughness measurements ....................................................17
4.3.2 Oxide thickness measurements .........................................................17
4.3.3 Elementary analysis...........................................................................18
4.4 Marked materials ..........................................................................................18
4.4.1 AISI 304L ...........................................................................................18
4.4.2 Titanium Commercial grade 1............................................................19
4.5 Test methods for corrosion resistance ..........................................................19
5 Results from laser marking...................................................................................20
5.1
5.2
5.3
5.4
5.5
Introduction to color marking.........................................................................20
Principle of laser color marking .....................................................................21
Laser parameters in color marking................................................................24
Different processing strategies in color marking with fiber lasers..................27
Actual laser tests...........................................................................................28
5.5.1 Color marking line by line...................................................................28
5.5.2 Color marking with small spot size when overlapping........................33
5.5.3 Color marking with big spot size when overlapping ...........................33
5.5.4 Color marking of titanium ...................................................................34
5.5.5 Marking in oxygen atmosphere ..........................................................37
5.5.6 Marking with 100W pulsed fiber laser ................................................38
5.5.7 Marking with 100W CW fiber laser.....................................................40
5.5.8 Marking with 20W pulsed Nd:YAG laser ............................................41
5.5.9 Common error causes in laser color marking.....................................41
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6 Results from color measuring ...............................................................................43
6.1
6.2
6.3
6.4
6.5
Spectral color measurements .......................................................................43
Thickness of the oxide layers........................................................................45
Surface roughness........................................................................................46
Elementary analysis......................................................................................47
Modelling of the colors ..................................................................................49
7 Results from corrosion tests .................................................................................52
7.1 Effect of processing parameters on corrosion resistance .............................52
7.2 Effect of interface on corrosion resistance ....................................................56
7.3 Corrosion resistance of the laser colour marked titanium specimens ...........58
8 Results from wear tests ........................................................................................59
9 Combination of different results............................................................................62
10 Conclusion and summary .....................................................................................64
References ................................................................................................................66
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1
Introduction
Laser marking is in most applications fastest and cheapest method. Flexibility of
laser marking is based on writing with laser beam when there is no need for
chemicals or tools. Laser marking has had its limits on producing graphics and
colours. Color marking on stainless steel and titanium has been available for
sometime but still it has not been used widely in consumer products. Some
industrial applications have been seen. New MOPA fiber lasers allow independent
tuning of the pulse width and the marking process can be optimized for producing
colors with better quality and visual appearance. Laser marking of metal surfaces
with correct parameters creates an oxide layer on the surface. The thickness of this
layer defines how white light is reflected from the sample. What in principle is
only a thin oxide layer on the surface can be seen as different colors by the
viewer. In this study, the visual appearance of laser marked surfaces was
optimized by varying the pulse width, laser power, pulse energy and scanning
velocity. The aim was to create uniform oxide layers on the surface, which would
appear as high quality color marking. Surface properties of selected samples were
measured using SEM-EDS and the effect of the viewing angle was examined
using a spectral camera. If this oxide layer is thick and solid it will have good
corrosion and wear properties. Consumer products are constantly under certain
corrosion and wear. In this study, the corrosion and wear properties of laser color
marking on AISI Type 304L (EN1.4307) austenitic stainless steel were evaluated
based on laser process parameters and colors. Also CP1 grade titanium was
briefly tested. The results show that the corrosion properties of marked surface
expressed as critical pitting temperatures (CPT) depend on heat input: coarsely the
lower heat input the higher CPT. Also scanning velocity has a remarkable effect
on CPT. The laser color marking parameters have a significant influence on the
wear behaviour of both the surface itself and that of the counter body surface.
Laser color marking has high potential to become a widely used technique for
product marking and decoration.
2
Goal
Projects aims were four supplementing entities:
1. Developing of direct metal marking using newest fiber lasers
2. Research of properties of optical surfaces and research of colours and also
optimizing of optical properties using calculative methods
3. Corrosive testing of surfaces in conditions that simulate end user conditions
4. Further development of colour marking and optimizing of fabrication
techniques of surfaces using equipment which are still to come to market.
Materials which were to be investigated are limited to AISI 304L stainless steel
1 and 2 mm and commercial purity 1 titanium with 0.5 mm thickness which were
selected in the project steering group. Wear and corrosion properties were tested
with commonly used method to sufficient extent. Surface optimizing with
calculative method is limited to surfaces which are done straight with laser.
Though in some cases using multiple production methods with or without laser
one can bring more value to product. If this need realized then it will be evaluated.
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3
Limitations
If one thinks laser color marking there is a lot of other ways to make the same
marking. For example excimer lasers have been used and there is literature
available but of course excimers are a lot more expensive and the flexibility due
imaging optics is limited. Also some other laser sources might be possible to use
but the project focus was on fiber lasers.
A lot more comprehensive tests with oxygen atmosphere could have been done
but normally marking has to be done in ambient air so too much effort was not
used on that. Materials selection was also quite narrow and it is obvious that
different base material has effect to the outcome. Preheating or cooling of the
samples might give also new process areas but they have not been tested.
Ambient moisture in air might be one issue in laser color marking but it was not
tested.
Long term tests should be also done in different environments. Due color is
formed in thin layer all the other layers on top of oxide does have an effect to the
visible color. For example greasy fingerprints will change the outlook of visible
color but when grease is wiped away the original color will appear.
Corrosion testing could also be more comprehensive but the focus was on the
items used by consumers in normal daily use. Demands for example for some
process industry might be different on corrosion wise. Same thing is for wear
testing that consumer products are not used in too abrasive environment and that
is why only basic testing was done.
The obtained oxide thicknesses in the color modelling are dependent on the
refractive indexes of the chromium oxide. There exist several values for the
optical constants of chromium oxide, both for amorphous and crystalline
chromium oxide that are produced in different techniques. Each of the refractive
indexes result in slightly different oxide thicknesses. Therefore, the oxide
thicknesses presented in this report might not be exact and they could be scaled
down or up to some nanometers.
Pre-polished surfaces could have been examined more thoroughly. The corrosion
and wear properties could have been examined for surfaces that were polished
before the laser marking. The surface polishing could have had effect on the
colors as well, since the oxidation depends on the surface properties such as the
roughness.
4
Experimental
Experimental part consist introduction to used laser marking, corrosion testing,
wear testing, color measurement and SEM equipment. Also used materials are
presented in this chapter.
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4.1
Laser equipment
Early stages of the project the IPG 20W pulsed fiber laser was used to start
making preliminary tests. After SPI joined the project laser was changed to more
tunable SPI pulsed fiber laser. During project a 20W Nd:YVO4 laser was tested to
see the difference between traditional and fiber lasers.
In the end of the project really brief tests with continuous wave fiber and Nd:YAG
laser was done.
4.1.1
SPI G3 20W pulsed fiber laser
The SPI G3.0 pulsed fiber laser module is a pulsed fibre MOPA (master oscillator
power amplifier) laser with maximum average power of 20W. Laser central
wavelength is 1061.44 nm according to SPI test report. Laser can be operated also
in CW mode if needed. Pulsing range is from 0 to 500 kHz. Laser pulse width can
be tuned from 9 to 200ns with 29 different variants. Laser was connected to
detachable isolator to prevent back reflections damaging the laser. Laser was also
equipped with high-capasity heat sink to enable stability if room temperature
would change during marking tests.
The output beam from the laser is 3.2mm and after isolator it is enlarged with
variable expander 1-4x to be able to focus beam as small as possible. After
expander beam is steered and focused with Scanlab HurryScanII with 14mm
aperture. Scanhead can be equipped with f100, f160, f254 f-theta lenses. Picture
of the system is presented on the figure 1. SCAPS SamLite was used to control
the scanhead and in making the marking programs. Behind of the setup is the
Rofin Nd:YAG laser.
Figure 1. SPI fiber laser optical setup.
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Beam quality of the laser is bit better specified than M2 = 2. Measured spot sizes
(2nd. moment) with different expander settings and optics are presented on the
table 1. 20W average power and waveform 0 (pulse width was 200ns) was used.
More information about the beam measurements can be found from appendix x.
Beam measurements was made with Primes micro spot monitor.
Table 1. Measured spot sizes with different optical setups.
Ex1
Ex2
Ex3
Ex4
4.1.2
f254
177 µm
103 µm
71 µm
61 µm
f160
104 µm
64 µm
44 µm
38 µm
f100
69 µm
41 µm
28 µm
25 µm
IPG YLP 1/100/20
IPG pulsed fiber laser uses a Q-switched MOPA configuration and delivers
maximum average power of 20 W. Pulse repetition rate can be adjusted between
20 and 80 kHz when pulse duration is 112.3 ns according to IPG test results.
Laser has also an isolated output with the output beam of 7.5 mm. Laser is straight
connected to Scanlab HurryScanII with 14 mm aperture.
SCAPS SamLite software was used to control laser and scanhead. Beam quality is
around M2 = 1.2. With IPG the f160 optic was mainly used and the spot size is
39µm. Spot size measurement data is shown in figure 2. Measurement was done
with 4W average power and 80 kHz repetition rate.
Figure 2. IPG laser beam analysis with f160 optics.
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4.1.3
Diode pumped Nd:YVO4 Laservall Violino 3
In September 2008 Nd:YVO4 laser was briefly tested at VTT in Lappeenranta.
Laser was delivered by Savcor Alfa. Laser was a part of complete system which
made the testing easy. Laser unit was Laservall Violino 3 with 20W average
power and scanhead from Laservall with f160 mm f-theta optic. Spot size around
110 µm. In figure 2 is the whole system.
Figure 2. Savcor Alfa CUBE.
4.1.4
Continuous wave fiber laser SPI SP-100C
In December 2008 continuous wave fiber laser was tested to see if CW laser could
be used to make color marking. Laser is 100W fiber laser from SPI lasers with
1090nm wavelength. Laser was connected to ARGES fiber rhino 16 scanhead
with f300mm focusing optics. Optics produced 90µm spot. System is the figure 3
with f825 optic and not the f300. Laser beam analysis is shown in the figure 4.
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Figure 3. SPI 100W CW laser setup.
Figure 4. SPI 100W CW beam analysis.
4.1.5
Rofin RS-100D diode pumped q-switched Nd:YAG
Rofin RS-100D is diode pumped q-switched Nd:YAG laser with capability to use
CW mode. CW mode is produced so that q-switch is opened for maximum of 2.5
seconds at a time. This is fairly enough to make short lines due beam is rapidly
switched on and off to make color marking. Due this behaviour 2.5 s time is never
reached during these color marking tests. Laser can be also used in pulsing mode
up to 65 kHz with maximum of 100W average power.
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Beam intensity profile with f=160 mm f-theta-optics is in Figure 5. Spot size is
163…210 µm depending on the used power.
Figure 5. Beam intensity at 25 kHz repetition rate. Left: P=10 W, Right P=88 W.
4.1.6
Fraunhofer CLT fiber laser
Fraunhofer Yb-doped fiber laser was based on the Master Oscillator Power
Amplifier (MOPA) approach consisting of a seeder and three amplifier stages, as
shown in Figure 6. Single-mode beam of 1064 nm wavelength was emitted by a
diode laser. The first two amplification stages consisted of single-mode fibers.
The third stage used LMA fiber. The output beam of the first two amplification
stages was coupled into the LMA fiber via an adiabatic taper in a monolithic
setup.
Figure 6. Schematic set-up of the fiber laser.
The laser was capable to produce both continuous wave (CW) mode and pulsed
mode from 10 kHz up to 1 MHz. The nominal maximum output power was 11
W but this time the laser was adjusted to hand out 6 W. Table 2. summarizes the
operating data of the laser.
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Table 2. Operating parameters of the fiber laser.
Measurements of an identical polarization maintaining amplifier revealed
circular geometry but only to result in M² = 1,42, shown in Figure 7. Multiple
imperfect surfaces to attenuate the beam and the molded aspherical lens used for
collimation are the main reasons for the imperfect measured beam quality.
Figure 7. The caustic and beam quality of the identical polarization maintaining amplifier.
The output beam was guided to scanner. The scanner was Scanlab SK1020 with
Linos F-Theta lens which has a nominal focal length of 163 mm. The marking
board underneath the scanner was manually adjustable along z-axis. Focal spot
size of the beam was 30 µm. System is presented in figure 8.
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Figure 8. Pictures of the laser (left) and the scanner (right).
4.2
Color measuring
The color of an object is not a physical quantity to measure. Color is a matter of
perception and subjective interpretation. We use the term color to quantify some
object but actually it is only a sensation in our brain. So we can not directly
measure the color but we can measure the signal reaching to the eye. In the eye
there is three different kind of cones sensitive to different regions of visible
spectrum which is only a small part of electro-magnetic spectrum. The coming
signal into an eye is a product of spectral irradiance of the light source and
reflection properties of an object. By knowing behaviour of the human vision
system i.e. properties of the cone sensitivities, we can model color sensation and
that way define colors. There are three basic things that affect to our color
sensation: illumination, reflectance and observer. All these have their spectral
distributions of electro-magnetic radiation. When one of these factors changes,
changes also color sensation.
4.2.1
Color definitions
In 1931 International Color Comission (CIE, Commission Internationale de
l’Eclairage) introduced three color model, called tristimulus system. These
formed the basis of modern colorimetry. In this system the color matching
functions was defined by experimental tests with a small number of people which
correspond to cone sensitivity curves. These functions are then used to define
color coordinate systems. The most accurate way to define the color of an object
is to measure its spectrum. The spectrum is however unconvenient to use in
general discussions that’s why three coordinate systems are developed. The most
well known coordinate systems are CIE xyY and Lab systems. In Lab color space
colors are expressed with three coordinates: L, a and b. L indicates lightness
between 0 to 100, 0 = black and 100 = white. Chromaticity coordinates a and b
indicates chromatic values in red–green and yellow–blue axis, respectively.
Bigger absolute value between a and b defines primary color. If a is positive the
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color is reddish, if a is negative the color is greenish. Same way if b is positive the
color is yellowish, and if b is negative the color is bluish.
4.2.1.1
Measuring geometries
The CIE recommends that reflectance-factor measurements should be made under
few illuminating and viewing conditions, such as 45/0, 0/45, d/0 or 0/d. In these
geometries illuminating angle from the normal to the sample surface is defined
first and then viewing angle. In all these geometries, 45º = 45º ± 2º and 0º = 0º to
10º degree. Notation d means the diffuse illuminating or viewing condition. So
geometry 0/45 means that we are illuminating sample from the normal (or under
10º from the normal) to the sample surface and viewing at the angle of 45º ± 2 º
from the normal. Geometry 0/d means that we are illuminating object by a beam
whose axis at an angle not exceed 10º from the normal of the sample and we are
viewing diffusely reflected rays from the sample surface. Diffuse viewing
conditions are obtained with an integrating sphere. In Figure 9 are shown
geometries 45/0 and 0/45 and in Figure 10 are shown geometries d/0 and 0/d.
Measuring geometries should be always mentioned while introducing color values
based on spectral measurements.
Figure 9. CIE standard geometries 45/0 and 0/45.
Figure 10. CIE standard geometries d/0 and 0/d.
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4.2.1.2
Surface properties
Objects may be seen in different colors even they are composed exactly the same
material. This is due to differences in the gloss of the surfaces. On coarce
divisions surfaces can be divided in to two groups, matt and gloss. The absolute
matt surface will reflect coming rays in the same way in all directions. In a glossy
reflection all rays are reflected with the same angle respect to the surface normal.
This is the reason, why measuring geometries should always be mentioned while
giving some color values based on measurements. Also results between same
measuring geometries should only be compared.
Figure 11. Diffuse and specular reflectances.
In real world there is no perfect gloss surface or matt surface. There will be
always some portion of both specular and diffuse components. The specular
reflectance component is reflected as if reflected by the mirror. The rays that are
not reflected specularly, but scattered in many direction, are called diffuse
reflectance. The sum of the diffuse reflectance plus the specular reflectance is
called the total reflectance. In Figure 11. diffuse and specular reflectances are
shown.
4.2.2
Color measurements for laser marked samples
Three different instruments were used to measure colors of the laser marked
samples. First the PerkinElmer Lambda 18 spectrophotometer was used to
measure total reflectance of the samples. Second the Hamamatsu PMA-11 fiber
spectrometer was used to measure reflectance in multiangle measurements. Third
the CRi Nuance spectral camera was used to measure spectral images through a
microscope.
4.2.2.1
Total reflectance
PE18 is a two monochromator scanning type reference beam spectrophotometer.
It is equipped with a 150 mm integrating sphere with 8/d measuring geometry.
This satisfies CIE 0/d geometry requirements, Figure 12.
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Figure 12. Integrating sphere with a 8/d geometry.
4.2.2.2
Multiangle measurements
Hamamatsu PMA-11 fiber spectrometer was used as a detector in multiangle
measurements. In figure 13 is shown measuring angles in this setup.
Figure 13. Measuring geometries in multiangle measurements.
A 50W halogen lamp with a daylight filter was used as a light source in this setup.
It was powered by stabilized DC power supply and it was attached in 15 degree
angle to the sample normal. Fiber optic detector head was installed in to the
measuring arm with a constant distance to the sample. This detector arm was then
rotated by a computer controlled rotary unit. The reflectances of all samples were
measured with a 1 degree steps between 0 degree to 45 degree compared to the
sample normal. So the measuring geometry was 15/0-45 using the CIE notations.
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4.2.2.3
Spectral camera measurements trough microscope
The visible colour of the samples consists of several different colours that are
indistinguishable with the naked eye. Therefore we wanted to measure also the
microscopic colours of the surface. For this purpose, the Nuance multispectral
imaging system by CRi (Cambridge Research & Instrumentation, Inc.) was
attached with a C-mount ring to an optical microscope Leica LM DM.
Spectral images in Nuance camera are obtained by chancing the transmittance of a
LCTF (Liquid Crystal Tunable Filter) as a function of wavelength. Spectral
bandwidth in this camera was 10 nm and its spectral range goes from 420 nm to
720 nm in wavelength direction. So actually when we are collecting image cube
where in every image pixel is spectral information of 31 channels of different
wavelengths, we are taking spectral images.
The resolution of the camera together with 20 times magnification of the optical
microscope enabled the spectral measurements for 500 x 500 nm² sized areas on
the stainless steel surfaces. The obtained spectra of the small areas are referred as
the microscopic spectra in this report. The microscopic spectra were the most
important measurement results that were used in the modelling of the colours in
section 6.1.5.
4.3
Surface characterization
The surface characterization includes surface roughness and oxide thickness
measurements and elementary analysis. The characterization was made for the
same IPG and SPI laser marked stainless steel samples that were investigated in
the colour measurements.
4.3.1
Surface roughness measurements
The surface roughness of the samples was determined with Tencor AlphaStep 500
surface profiler. The surface roughness was measured because it has an effect on
the way that light reflects from the surface, and thus on the brightness of the color.
The roughness is also believed to be in connection with wear and corrosion
properties of the surface.
Tencor AlphaStep 500 is a metrology tool that generates a two dimensional
profile of the surface of a sample. This is done by a stylus, which touches the
surface of a sample and runs across a prescribed length. Step heights and widths
of structure can be measured in this way.
4.3.2
Oxide thickness measurements
The oxide thicknesses were measured by breaking the samples and taking the
cross-section images with a scanning electron microscope (SEM) Zeiss, LEO
1550 Gemini. The samples were cut with high-speed steel blade and diamond
blade that were attached to sawing machine. We tried also cooling the samples in
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liquid nitrogen before the sawing but it didn’t help in breaking the samples.
Example of a cross-section image is given in Figure 14.
The oxide thickness measurements were only suggestive because of the limited
resolution of the measurement equipment: less than 100 nm thick layers were hard
to distinguish with SEM. Furthermore, the oxide thickness was usually not
constant on the surface which resulted in the fact that the thickness of each sample
could be only roughly estimated.
Figure 14. Cross-section image of a stainless steel sample taken with scanning
electron microscope. Oxide layer is seen between the green lines (486.7nm).
4.3.3
Elementary analysis
Elementary analysis was done to the samples to obtain better understanding on the
laser oxidation process at a chemical level. It also helped the modelling of the
colours by giving knowledge on the composition of the thin films on the surface.
The elementary analysis was made with Energy Dispersive X-ray Spectroscope
(EDS) that was attached to scanning electron microscope Hitachi, S4800 + EDS,
Thermo electron. The method relies on the investigation of a sample through
interactions between electromagnetic radiation and matter, analyzing x-rays
emitted by the matter in response to being hit with charged particles. Its
characterization capabilities are due in large part to the fundamental principle that
each element has a unique atomic structure allowing x-rays that are characteristic
of an element's atomic structure to be identified uniquely from each other.
4.4
Marked materials
4.4.1
AISI 304L
Stainless steel materials where ordered from BE Group in Finland and 2 mm
plates had melting number C2700675. Steel is AISI grade 304L even though
plates are marked with EN 1.43001/2B. Correct grade was confirmed by the steel
provider. Surface quality was 2B which means cold rolled surface. The typical
composition of the AISI Type 304L stainless steel grade according to the
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EN1.4307 steel specification is 0.02 wt% carbon (C), 18.1 wt% Cr and 8.1 wt%
(Ni).
4.4.2
Titanium Commercial grade 1
Titanium was decided in the steering group to be the commercial purity 1 grade.
Thickness was also decided to be 0.5 mm. Titanium was ordered from Apollo
Materials which is nowadays known as ThyssenKrupp Aerospace Finland Oy.
Titanium manufacturer is KOBE steel LTD. Actual grade is ASTM B265-99
G1/ASTM F67-00 G1. Inspection certificate is in the appendix x. The typical
composition of the commercial purity 1 (CP1) titanium grade according to the
ASTM Gr1 titanium specification is maximum of 0.08 wt% carbon (C), 0.2 wt%
Fe, 0.03 wt% (N), is 0.015 wt% hydrogen (H) and is 0.18 wt% oxygen (O).
4.5
Test methods for corrosion resistance
Corrosion resistance of the laser colour marked stainless steel flat specimens (50
mm * 60 mm * 2 mm) was tested by defining critical pitting temperature (CPT)
according to ASTM G 150-99a “Standard Test Method for Electrochemical
Critical Pitting Temperature of Stainless Steels”. The test specimens were fully
marked to have the whole plate surface covered with oxide layer. Finally, some
test specimens were covered with a mesh of 2 mm * 2 mm oxide layer.
The solution used in the CPT tests was 0.01 M NaCl ( 0.59 g/l NaCl) prepared
from distilled water and high purity reagents. The specimens were polarised to +
400 mV against saturated calomel electrode (SCE) in order to keep oxidation
similar to the practical climatic use of the coloured surfaces in economic goods.
The CPT tests simulated the effect of regular surface cleaning with tap water. The
starting temperature was + 5ºC in all corrosion tests. During the tests the
temperature of the solution increased stepwise 1ºC/min until the measured
corrosion current density exceeded the limit 100 A/cm2.
Titanium has no local corrosion risk because of its high oxidation capability.
Thus, CPT tests mentioned above are not suitable for titanium but its general
corrosion can be harmful. General corrosion resistance of the laser colour marked
titanium specimens (50 mm * 60 mm * 1 mm) was tested by defining anodic
polarisation behaviour according to ASTM G 61-86 “Standard Test Method for
Conducting Cyclic Potentiodynamic Polarization Measurements for Localised
Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys”. The solution
used in titanium corrosion tests was aggressive 2 M NaCl ( 117 g/l NaCl) at
temperature of + 50ºC. The scanning rate of the potential was 20mV/min starting
from – 550 mV SCE and ending to + 7000 mV SCE.
The corrosion tests of the stainless steel and titanium specimens were performed
in Floating Sample Cell (FSC) developed by VTT. FSC was used instead of
Avesta cell in order to avoid crevice corrosion on the specimen edges. The
electrochemical measuring system CMS100 (Gamry Instruments, Inc.) was used
in all corrosion measurements.
20 (66)
5
Results from laser marking
5.1
Introduction to color marking
Color marking of metal surfaces is conventionally done by printing, anodization
or emulsion coating. However, the wear properties of printing are limited and
anodizing more than a single color precisely is not easy. Emulsion coatings are
more expensive and they require one step more to produce colors. Lasers can be
used to create a permanent color mark on a metal surface as a one step process
with a high throughput. Laser color marking of metals has been used for more
than ten years with a variety of different laser sources /1, 2, 3, 4/.
Despite the possibility to vary processing parameters typically in a wide range,
some lasers are more suitable for color marking than others and the pulse width
among other parameters seems to be an important issue in defining the marking
quality and contrast. Typical laser of choice for marking is a q-switched crystal
laser, which produces pulses in the nanosecond regime. These lasers do not allow
independent adjustment of the processing parameters, but the pulse width is
dependent on the repetition frequency and as the frequency changes, so does the
pulse width. Therefore lasers that allow adjustment of the pulse width regardless
of the frequency might give an advantage in marking.
In most applications laser marking is the fastest and the cheapest method.
Flexibility of laser marking is based on writing with the laser beam, which
interacts with the material surface creating the mark. Unlike most of the other
marking techniques, laser marking does not use any chemicals or tools. Some
metals can be marked in a way that the surface appears colored. This is based on
oxidation and the following thin film effect. In order to create a uniform and high
quality mark, the used laser must have a good enough beam quality and stability.
High quality mark results in a smooth and uniform oxide layer. In consumer
products the color marked surface is usually always subjected to conditions
causing corrosion and wear. Before this kind of marking method can be applied as
a real production line method, it is essential to verify that the marked surfaces will
have sufficient resistance against corrosion and wear under normal usage
conditions to ensure a long life time for the product. Hence, it is important to
investigate the corrosion and wear properties of laser color marked surfaces. The
surface properties including corrosion and wear resistance of the laser color
marked stainless steels depend on the oxide layer produced during laser process.
Oxide formation during laser process involves among others transport of oxygen
from medium to the solid surface, adsorption of molecular oxygen and electric
field enhanced diffusion of species through the oxide layer [5]. The laser-induced
temperature rise enhances the diffusion flux and the reaction rate within the
scanned area. In corrosion the formed oxide layer can weaken or vanish in
reduction or dissolution depending on the environments, where the laser color
marked item is used.
During use the surfaces of consumer products are frequently in sliding contact
with other surfaces or particles, e.g. fingers, fabrics, abrasive particles. This
results in wear of the thin oxide film on the marked surface which appears as
more or less visible worn areas or scratches.
21 (66)
In this study, at first the principles of laser color marking are described including
laser processing strategies. The corrosion and wear properties of laser color
marking on Type AISI 304L (EN1.4307) austenitic stainless steel were evaluated
using standardized electrochemical corrosion tests and reciprocating sliding wear
tests in order to simulate conditions of use.
5.2
Principle of laser color marking
Surface oxidation of metals in an oxidizing agent is a well known phenomenon.
Clean surfaces of many materials spontaneously react in air to form thin native
oxide layers. These native oxide layers are very dense and terminate further
oxidation and their thickness is typically 10-100 Å thick. Light-enhanced and in
particular laser-enhanced material oxidation is based on thermal or non-thermal
molecule surface excitations /5, p. 535/.
Native oxide formation involves a number of consecutive steps:
- transport of oxygen from ambient medium to the solid surface
- adsorption of molecular oxygen
- electron transfer to adsorbed O2
- electric-field-enhanced diffusion of species through the oxide layer.
Adsorption of (strongly electronegative) O2 on a metal surface favours electron
transfer. Dissociative chemiabsorption is enhanced for O2- because it requires only
3.8eV compared to 5.1 eV for O2. Chemiabsorbed oxygen reacts with surface
ions/atoms and forms an ultrathin oxide layer. Further oxide growth can proceed
via electron tunnelling and diffusion of species through the growing layer. This
process is self terminating, because it comes less likely with increasing layer
thickness. Oxide growth beyond a single or few monolayers is controlled by the
transport of electrons, ions, atoms, or molecules through the oxide layer (figure
15).
22 (66)
Figure 15. Model for surface oxidation of metals. With most metals, the metal
ions diffuse to the surface to react with oxygen. Diffusion is enhanced if cracks
within the oxide layer are formed /5, p.536/.
Laser-enhanced pyrolytic (photo thermal oxidation can be understood along
similar lines as ordinary thermal oxidation. the influence of oxide-layer thickness,
h, on the laser-induced temperature rise, T, is related to changes in the thermal
and optical properties of the irrated material. Changes in the absorptivity due to
laser-beam interferences yield oscillations in the laser induced temperature
distribution and thus in the oxidation rate.
The laser-induced temperature rise enhances the diffusion flux and the reaction
rate of species within the irrated area. The enhancement is based on various
different mechanisms: the temperature dependence of ordinary diffusion, the
thermal deneration of defects such as vacancies, etc., and the thermal excitation of
electrons. The latter increases the rate of electron transfer to oxygen and thereby
the electric flied enhances the diffusion flux. Furthermore, strong temperature
gradients induced by the laser light will enhance the transport of species via
thermal diffusion and via formation of stresses, strains, cracks and other defects.
Finally, with the dwell times involved, in particular in pulsed laser oxidation,
thermodynamically unstable phases remain, while other phases cannot nucleate or
form larger crystallites within short times. Thus it is not astonishing that laser
fabricated oxide layers differ significantly from those observed with films
fabricated under equilibrium conditions in an oven.
Photolytic (photochemical) mechanisms have been proved to be important, in
particular in cases where the photon energy matches the energy for selective
excitation of a particular transition.
At high intensities, laser light initiates a breakdown at or near the solid surface.
This is a regime of pulsed-laser plasma chemistry (PLPC or also laser-pulsed
23 (66)
plasma chemistry LPPC). The laser-generated plasma contains excited reactants
which can combine with substrate atoms to form oxide over layers.
Thin oxide films can be formed by heating the surface uniformly using a laser
beam. Oxygen must be present in the ambient atmosphere when marking with a
laser. Air is sufficient for thin film growth, but a higher concentration of oxygen
can be used to enhance film growth. Oxide film growth cannot be started in an
inert atmosphere.
Most important parameters in laser marking are the focal spot diameter, power on
sample, marking speed, line spacing, marking direction, repetition rate and pulse
length. Gaussian beam profile of these fiber lasers might not be the best option for
marking, but it would be modified into a top hat mode beam using a simple beam
homogenizer.
The surface temperature and the thermal load, or the input energy over an area,
can be adjusted by varying the beam power, scanning velocity or the line spacing.
The aim is to maintain a constant surface temperature in order to create a uniform
color surface. The following oxide layer will grow to a certain thickness and
create the thin film effect. The process is self terminating because it comes less
likely with increasing layer thickness /5, p. 536/.
The thickness of this layer defines how white light is reflected from the surface.
Also surface roughness of the oxide and surface roughness under oxide has their
effect to visible color. What in principle is only a thin oxide layer on the surface
can be seen as different colors by the viewer. If this oxide layer is thick and solid
it will have also good corrosion and wear properties, which are essential in
consumer products. Variation in the oxide layer thickness and the surface
roughness will have an effect to the resulting color seen by the viewer. Depending
on the oxide layer quality, the color may change when viewed from different
angles.
Composition of the oxide has also effect to the color seen by the viewer due
different oxides have different index values. Oxide composition can vary with in
one sample if base material has too inhomogeneous composition. Different
material will for sure have different oxide composition. Literature also shows that
Oxide layer may have different composition layers on top of each other and this
also will have effect to color seen by the viewer (figure 16).
24 (66)
Figure 16. Laser induced oxide formation on different material surfaces. The
change in concentration with depth can be described by multiple layers. The
composition and thickness of layers depends on the laser parameters /5, p.539/
5.3
Laser parameters in color marking
Laser color marking is made so that beam is moved along sample surface line by
line with certain marking speed. Beam displacement from line to line is also
important. If separation is too big lines will be apart from each other and then the
oxide layer would not be even. Displacement should be less than 50µm because
human eye can see individual lines if value is larger. Marking direction has also
effect to out come. One can mark the lines to one direction e.g. left to right or to
both directions which means that every other line is marked from left to right and
every other from right to left. Depending on the marking speed and the size of the
marked area the heat cycle can to really different when using these two different
methods. When marking is done to one direction only the end of the lines have
time to cool down before the next line is marked but when the marking is done in
both directions the line ends tend to heat up a little bit more due this small area is
marked twice within a small time frame. Normally marking is done with pulsed
lasers but it can be also done with CW (continuous wave) lasers but then color
formation is not as good. Parameters for color marking are shown in figure 17.
25 (66)
Figure 17. Principle and basic parameters for color marking.
Spot size on sample together with average power and pulse width have an effect
to energy density on sample. If energy density is too big the marking is not
heating anymore and material starts to melt. Melting is not a problem because
almost always with ns lasers material is melted a little bit to be able to create
oxide on top of material. If energy density is too high the material melts but also
starts to evaporate. Evaporation needs a lot of power and this way the power used
for evaporation is not helping in color marking. Evaporation also makes the
sample surface rougher. Beam spatial profile has also effect how laser pulse
energy behaves. Normally fiber laser have a really good beam quality and that is
why beam has the highest intensity in the middle of the beam. This has the side
effect that in the center of the marked line beam might evaporate material and
further away from the center of marked line the process is just heating. This is
why only one line might have different color on sides and center. Top hat mode
would maybe be beneficial but then also the average power has to be higher. So
intensity can be adjusted by changing the spot size on sample or by average
power. Spot size tuning is made by defocusing or by using different raw beam
diameter (numerical aperture is different). If spot diameter is doubled the area is
quadrupled which means that intensity is only one fourth that it was (figure 18.).
Figure18. Effect of spot size to beam intensity.
26 (66)
If 3D surfaces are marked one has to take care of the focal point position to be
able to produce same quality. If focal point position changes depending on the
optic used the spot size will change accordingly and it will have a big effect if
spot size on sample is changed too much. Spot size position and its efficiency to
intensity on samples are presented in Figure 19. From appendix 1 one can see
the measured data on the used optics.
Size of focal spot
Intensity
Power distribution on sample surface
Figure 19. Effect of spot size to focus position and power distribution on sample
surface.
Repetition rate of the laser has a connection to marking speed and spot size. If
continuous line is wanted then marking speed divided by the spot size has to be
smaller than repetition rate. Normally pulse overlapping has to be quite big to get
really smooth marking quality. Pulse width of the laser has influence to color
marking so that if the pulse is too long (µs or ms region) the process is too slow
for oxidation. If the pulse is in the ps or fs region heat conduction to surface is
minimal and oxidation does not happen.
It is commonly known that incident angle has effect to absorption but using
optimal incident angle would make marking a lot more difficult because marking
would be distorted and when using scanhead the focus plane not would be correct
only on one line on the marking area. This would result changes in the marking
quality.
Depending on the thickness of the marked sample the parameter window is
limited by the heat input due to high energy input will result in distortions if
sample is thin. Also is marked area is large the temperature rise in the whole
marked area will have effect to the outcome.
Atmosphere does have effect to color marking due oxygen is needed in the
process. Is marking is done in an inert atmosphere there is no oxide formation
27 (66)
present. If marking is done in oxygen atmosphere it will have accelerating effect
to the outcome and then needed laser power is lower.
5.4
Different processing strategies in color marking with fiber lasers
Laser color marking can be done using different processing strategies. Figure 19
shows four different surfaces, which have been marked using different techniques.
In Figure 1a each line produces a certain thermal load to the material and the color
is produced line by line. With this approach, the mark could also be single lines
instead of a marked area. The laser power and the scanning velocity are adjusted
in a way to create a proper oxide layer thickness in the illuminated region. In
order to create a surface which appears to have a uniform color, the line width has
to be in the range of 20 to 50 µm. If the line is wider, the direction of the scan
lines becomes visible. A narrow band of the base material can be seen between the
colored lines, but these are not visible for the human eye. Overall, the surface in
the Figure 19a appears blue.
A
B
N
C
D
Figure 19. Micrographs from different laser techniques used to produce oxide
layer on AISI Type 304L.
Figure 19b shows a surface produced by another technique. This second way to
make a color reflecting oxide layer on the surface is to scan the area in a way that
the marking lines overlap. The oxide layer is created by the accumulated energy
of more than one line. The thickness of the oxide layer, i.e. the visible color, can
be altered by varying the power, scanning velocity or the line spacing. The input
energy required to create a certain color is expressed in the units of J/mm2. The
down side to this approach is that the last marked line often appears to be different
in color, due to the different input energy in that area. However, if the line width
is small, it is hard to recognize this without magnification.
In the two previous techniques, the surface has melted during laser processing and
the oxide layer is formed during solidification and cooling. If the scanning
velocity is high enough, it is possible to heat the surface uniformly and for a
sufficiently long time to form the oxide layer without significant melting. In this
28 (66)
approach the line spacing has to be small enough to bring enough energy to a
surface area for oxide formation. As the scanning velocity is very high and the
same area is scanned numerous times due to the small line spacing, the laser beam
does not act as a point source of energy but more as an area source. The resulting
oxide layer is very even and the marking lines cannot be seen even with a
microscope, Figure 19c.
In addition to these three techniques it is possible to create a grating with the
oxide. In this technique each pulse creates a similar melt pool and when
resolidified, the melt pool edges create a diffracting surface. Such a surface is
presented in Figure 19d (note the very fine scale as opposed to a, b and c). The
surface does not have a red color in real life as the figure has, but the color is
caused by the DIC optics used for taking the microscopic image. When viewed
with bare eye the surface reflects all colors depending on viewing angle.
Some lasers, such as the excimer laser, can deliver a beam which has a highly
uniform intensity distribution. Consequently a larger area can be heated evenly
without moving the laser beam. Small color marks can be created with such lasers
using a mask which defines the geometry of the illuminated area on the sample
surface. /7/
All the previous methods can be made faster by doing the marking in an oxygen
atmosphere. As the formed oxide layer is a reaction product of oxygen and a
metallic element, higher oxygen content, and therefore a larger quantity of oxygen
available for the reaction, enhances the film growth rate and makes the marking
process faster. The required laser power for producing an oxide film of certain
thickness can therefore be reduced. It is also possible that the faster oxide growth
rate increases the absorptivity. Oxygen can also create an exothermic reaction
which brings more energy into the process.
Recently, an additional laser technique to make colors to metal surfaces using fslaser has been reported. In this case the color is not a result of an oxide layer, but
nanostructures on surface of metal plate /6/.
5.5
Actual laser tests
After reading available literature color marking seems to be quite straight forward
process if proper parameters are chosen. But if high speed marking is wanted then
the process is not so simple and cannot be predicted 100% sure. For example if
one wants to make 5 by 5 millimeter square and he or she has available data for
color marking on that size and base material and laser setup process can be
replicated really well. If square size is altered the process might end up into
different color. With the same parameters larger box allows the marked line to
cool down a bit longer time before next mark. Also really thin areas might be
quite difficult to replicate with same set of parameters due heated area is really
small compared to large cool area around.
5.5.1
Color marking line by line
Color marking line by line is most reliable way to make colors because each line
will produce the same color and will not affect to the other lines. In this method
the line thickness i.e. spot size has to be thin enough to fool the human eye not to
29 (66)
see the actual lines. This ends up into quite low average powers due heat input
cannot be too high because otherwise lines start to accumulate heat and it will
make larger area hard to stay even in color. Lines are marked side by side so no
overlapping is done. Due low average powers the sample to be marked does not
heat up. If visible lines are accepted marking can be speeded up by increasing spot
size and average power so this way marking is faster due smaller amount of
marked lines.
In these tests the 2 mm x 2 mm boxes in the 10 x 5 matrix was marked. In the one
matrix the scanning velocity was varying and other parameters were kept
constant. The distance of the marked lines were 30 µm and the size of the laser
beam in the work piece was 38 µm (f160, expander setup 4). Matrixes with
different parameter setups were marked. Parameters varied were laser power,
pulse width and repetition rate.
As can be seen in figure 20 when the marking speed is increased the formed color
changes. When other parameters are constant and marking speed is increased the
energy input is decreased. With highest marking speed, lowest energy input no
color or only silver color was achieved. When the marking speed was decreased
and energy input is increased different colors were formed. Finally brown and
black colors were formed and if the energy input is still increased the material
starts to evaporate/burn and no oxide is formed any more. Colors were formed in
certain order when the scanning speed is decreased, first color was silver (glossy
melted metal surface with really thin oxide), then gold, violet, blue, green, yellow,
orange, red, brown and black. In the figure 20 is colors from three different
matrixes, the used power and repetition rate was varied. It can be seen that with
same energy input, the same color was achieved despite the marking speed used.
In this case the repetition rate was different in the different matrixes and there
with higher energy inputs the colors formed with same energy are not exactly
same.
30 (66)
A
6
silver
5
Energy input [J/mm2]
gold
violet
4
blue
green
3
yellow
A2
A3
2
A4
1
0
0
50
100
150
200
250
Marking speed [mm/s]
Figure 20. Pulse width 120 ns, A2) laser power 5 W, repetition rate 75 kHz, A3)
laser power 6 W, repetition rate 100 kHz, A4) laser power 8 W, repetition rate 150
kHz. Marking done with SPI laser.
Repetition rate
In one test setup other parameters were kept constant and repetition rate was
changed. Nine different repetition rates were used from 50 kHz to 500 kHz, laser
power was 6 W and the pulse width was 120 ns. With 50 kHz markings were
burned and no color was formed. It was noticed that with lower repetition rates
colors were more brownish and darker than with higher repetition rates. When the
repetition rate is increased colors were more yellow/orange and lighter. Despite of
the repetition rate used same colors were formed with same energy input even if
the other colors were darker and other lighter (figure 21). With higher energy
input red orange and brown colors were difficult to determine which color is
which and that’s why there is more differences in what energy the certain color is
formed. As we can see from figure 21 the color formed is not depending on the
repetition rate use, only the energy input. But we have to remember that the
brightness of the colors is dependent of the repetition rate used.
31 (66)
6 W, 120 ns
30
1
25
Pulse energy [µJ]
gold
violet
2
20
blue
15
3
green
4
yellow
orange
6
10
red
7
brown
5
0
0
1
2
3
4
5
2
Energy input [J/mm ]
Figure 21. Laser power 6 W, pulse width 120 ns, repetition rates 1) 150 kHz, 2)
200 kHz, 3) 250 kHz, 4) 300 kHz, 5) 350 kHz and 6) 400 kHz
Pulse energy
Two different test setups were tested. In all tests the peak power was same
0,34 kW. In the first tests the pulse width and repetition rate was changed, laser
power was 6 W. In the second tests the laser power and pulse width was changed,
repetition rate was 100 kHz. Pulse energy (µJ) was varied from 31 µJ to 61 µJ. In
both cases the pulse energy has not a significant effect to color formation pulse
energy effected more to the brightness of the color. With low pulse energy no
color or very light colors were formed, and if the pulse energy was increased the
colors turn to darker and more brownish until the material starts to evaporate. In
both cases the best colors was achieved with pulse energies 31 µJ and 41µJ. Also
in this test setup the energy input has more effect to the color formation than the
pulse energy, nearly same colors were achieved with same energy input (figure 22
and figure 23).
32 (66)
B
Pulse energy input [µJ ]
70
60
1
50
2
silver
gold
violet
3
40
blue
4
30
green
yellow
brow n
20
10
0
0
1
2
3
4
5
6
Energy input [J/m m 2]
Figure 22. Laser power 6 W, 1) pulse width 90 ns, repetition rate 133,3 kHz, 2)
pulse width 120 ns, repetition rate 100 kHz 3) pulse width 150 ns, repetition rate
80 kHz 4) pulse width 180 ns, repetition rate 66,66 kHz
D
60
silver
gold
1
50
Pulse energy [µJ ]
violet
40
2
30
3
blue
green
yellow
orange
red
20
brow n
90 ns
10
120 ns
150 ns
0
0
1
2
3
4
5
6
2
Energy input [J/m m ]
Figure 23. Repetition rate 100 kHz, 1) pulse width 90 ns, laser power 5 W, 2)
pulse width 120 ns, laser power 6 W 3) pulse width 150 ns, laser power 7 W
33 (66)
5.5.2
Color marking with small spot size when overlapping
When overlapping is used with small spot size overlapping percentage is not high
but still line to line spacing cannot be too big. Small spot size tends end up into
high intensities if pulse energy used is too high. If high average powers are
wanted to be used i.e. fast marking is preferred then high repetition rate option
should be used. If repetition rate is doubled the pulse energy is only half of what it
was. This way excess evaporation of material can be avoided and unnecessary
power loss due evaporation is also avoided.
For example a set of experiments was carried out using the full scale of repetition
rates ranging from 50 kHz up to 500 kHz (figure 24.). The average power was
kept constant at 4 W and thus the pulse energy varied between 80 and 8 µJ,
respectively. Pulse width was 120 ns. Lowest repetition rates started to ablate
material from the surface and resulted in poor marking quality. Higher repetition
rates resulted in better colors, each of them having different tones. The highest
repetition rate also formed some colors, but the quality of these was limited.
Figure 24. Repetition rates on matrix 1-8 are 1)50 kHz, 2) 150 kHz, 3) 200 kHz,
4)250 kHz, 5) 300 kHz, 6) 350 kHz, 7)400 kHz, 8) 450 kHz, 9) 500 kHz.
When small spot size is used can small details be done very accurately. Last line
to be marked will always be different in color because next to it is the cool base
material. Again when marked line is thin enough it is hard to notice with bare eye.
Beam profile closer to top hat than Gaussian would be advantageous due small
spot has high intensity already.
5.5.3
Color marking with big spot size when overlapping
With a big spot size like 100µm in diameter marking has to be done with high
overlapping percentage. Large spot means smaller intensity and also small amount
of melted material. Evaporation is not necessary produced at all. Colors made
with this method have more matte outlook than made with small spot. One big
disadvantage is that last line to be marked will be quite easy to be noticed by
human eye. When marking small features 100µm spot size can be a problem and
also the issue with last line maybe a problem also in every corner.
34 (66)
In these tests on samples were the marked with 3 mm x 3 mm areas in the 7 x 7
matrix. In one matrix the scanning velocity was varying and also the distance
between the marked lines was varying from 1 µm to 30 µm, other parameters
were kept constant. The f254 f-theta was used and four different expander setups
were used, so spot sizes on the work piece were Ø60 µm, Ø70 µm, Ø104 µm and
Ø178 µm. Repetition rate in these tests was 200 kHz and the pulse width was 65
ns. Two different laser powers were used, 20 W and 15 W.
With biggest spot size Ø178 µm and biggest overlap more energy was needed to
achieve same colors than with smaller spot size (figure 25). With smaller spot
sizes nearly same colors were achieved with same energy input. When the
brightness of the colors were compared, it was noticed that with biggest spot size
the colors have matte outlook. Best colors were achieved with the beam size of
104 µm, colors were quite bright, slightly matte outlook was achieved with
smallest scanning speeds and overlaps. With the smallest beam sizes Ø70 µm and
Ø60 µm the marked colors become more yellow, red and brown. In these tests the
best colors were achieved with the small line to line distance (5 and 10 µm).
8
Energy input [J/mm 2]
7
gold
violet
blue
green
yellow
orange
red
brown
60 µm
70 µm
104 µm
178 µm
6
5
4
3
2
1
0
0
20
40
60
80
100
120
140
160
180
200
Overlap [µm]
Figure 25. Repetition rate 200 kHz, pulse width 65 ns, line distances from 1 µm
to 30 µm and beam sizes from 60 µm to 178 µm.
5.5.4
Color marking of titanium
Laser marked colors on titanium differs a bit of colors on stainless steel. Blue
colors on titanium are better than on stainless steel and more blue colors are
formed to titanium. Colors on titanium are also more yellow/brown than in
stainless steel. To titanium colors formed in different order than to stainless steel.
On titanium first comes silver and gold color, after that, when the laser energy is
35 (66)
increased the orange/red/brown color is formed. When the energy increases more
the violet and blue color is formed and after that the black color appears.
Titanium was marked with different ways. In the first tests the area was marked
multiple times. In these tests on samples were marked with 3 mm x 3 mm areas in
the 7 x 6 matrix. In one matrix six different laser powers (10 – 20 W) were used
and 7 different marking times (1-7) were used, other parameters were kept
constant. Tests were done with three different scanning speeds (500, 1000 and
2000 mm/s) and four different pulse widths (15, 30, 65 and 200 ns). The f254 ftheta optic was used and four different expander setups (beam sizes were Ø 60
µm, Ø 70µm, Ø 104µm and Ø 178 µm) were used. Repetition rate was 200 kHz
and the distance between marked lines was 20 µm in all tests.
In these tests best colors was achieved with marking speed 500 mm/s. With
highest marking speed 2000 mm/s no good colors were achieved, colors were
light and only some silver and gold colors appeared. With 1000 mm/s some gold,
yellow and orange color was achieved with few parameter combinations. Spot
size has also an effect to color formation. With bigger spot size colors were darker
than with smaller spot size. With smaller spot size more gold and orange colors
were achieved and when the spot size was increased the colors tuned to more
blue. Pulse width has also effect to color formation, but can be said that best
colors were achieved with spots size Ø 70 µm. With smallest spot size Ø 60 µm
marking was vaporised material when short pulse widths were used. With biggest
spot size colors were very dark or black and no good colors were achieved.
Pulse width effects also to color formation. With longest pulse width 200 ns
colors were brighter and blue colors were achieved only with 6 and 7 scanning
times and biggest laser power 20 W used. When the pulse width became shorter
the colors became darker and more blue colors were achieved until the shortest
pulse width was used. With shortest pulse width blue colors disappeared but still
violet and brown colors were achieved.
When the marked area was scanned multiple times the multiple scanning has an
effect to color formation. When number of scanning times increases the energy
input is increased. With used parameters and one scan colors were light and only a
silver or gold colors was achieved. If the number of scanning time was increased
the color started to turn more orange until the color turned to violet and blue.
Multiple scanning affects same way to color formation as increasing the power or
decreasing the marking speed because the heat input increases. As can bee seen in
figure 26. same colors were not formed exactly at same energy. Because material
cools down little bit between the scanned lines seems like more energy is needed
with multiple scanning to achieve same colors.
36 (66)
14
12
Heat input [J/mm 2]
silver
10
gold
orange
8
brown
6
violet
blue
4
black
2
0
0
1
2
3
4
5
6
7
8
Number of scanning times
Figure 26. The effect of number of scans and heat input to color formation.
Repetition rate 200 kHz, pulse width 15 ns.
In the second tests on samples were marked with 2 mm x 2 mm areas in the 10 x
4 matrix. In one matrix scanning speed was varied and other parameters were kept
constant. Scanning speeds varied from 20 mm/s to 1720 mm/s. Tests were done
with four different laser powers (3,5 W – 20 W), six different repetition rates (50500 kHz) and five different pulse widths (9-200 ns). The f160 f-theta optic was
used and four different expander setups (beam sizes were Ø 38 µm, Ø 44µm,
Ø 64 µm and Ø 114 µm) were used. Distance between marked lines was 25 µm in
all tests.
Spot size and pulse width had a same kind of effect to marked colors in these tests
as they had in the multiple marking tests of titanium. With big spot sizes colors
were darker and with smaller spot size more gold and orange (figure 27) colors
was achieved. With long pulse width colors were lighter and became darker when
short pulse widths were used. The effect of pulse width can not be seen so
evidently in these tests as in the multiple marking tests.
As can bee seen from figure 27. nearly the same color can be achieved with same
energy input despite of the beam size and other parameters used. With ~1 to
2 J/mm2 gold/orange color was formed and with ~3 to 4 J/mm2 blue color was
formed.
Heat input [J/mm2]
37 (66)
silver
Marking speed [mm/s]
gold orange violet blue
black
Figure 27. The effect of marking speed and heat input with different beam sizes
(ex1 = Ø 114 µm, ex2 = Ø 64 µm, ex3 = Ø 44µm and ex4 = Ø 38 µm). Laser
power was 3,5 W – 20 W, repetition rates 100-500 kHz and pulse width 9-200 ns.
5.5.5
Marking in oxygen atmosphere
As stated before the oxygen atmosphere helps in the creation of oxide on top of
base material. It also makes the process more efficient and this way marking may
change to evaporation if wrong set of parameters are chosen. In one test it was
tested that can one just by lowering average power obtain the same color as with
marking done in normal air. Outcome was that only average power is not the key
parameter and also other parameters have to be changed. On figure 28 one can see
on the upper row on left marking made in 100% oxygen atmosphere and on the
right in normal air. In air marking produces different colors which are
unfortunately not shown very well on the picture. In oxygen marked sample is
burned with the heaviest set of laser parameters and all the other colors are also
totally different. On the left sample the laser average power is lowered by 10% on
each line number line is 50% average power. The lowest one is made in normal
air with original parameters. On the right sample numbers are marked so that the
top one is made in air and the next 100% with oxygen, 80%, 60%, 50% and 70%.
Parameters for different numbers are shown on table 3.
38 (66)
Figure 28. Marked numbers in oxygen and air atmosphere with different set of
parameters.
Table 3. Parameters for oxygen test.
Number
2
3
4
5
6
7
8
9
Color
Orange
Blue
Silver
Dark Red
Green
Gold
Red
Black
Pulse
width [ns]
30
200
65
30
30
65
65
30
Speed
[mm/s]
1000
400
1600
1000
1000
150
100
50
Power
[W]
15
20
20
15
15
13
20
8
Repetition rate
[kHz]
150
125
125
150
150
125
125
200
Hatch
[µm]
3
30
30
2
5
30
30
30
One issue is that color marking in oxygen is that it may end in different
composition of oxide on top of base material. Then this different composition on
top of base material will have different refractive index which make surface look
different in color even though the oxide thickness could be the same.
5.5.6
Marking with 100W pulsed fiber laser
100W IPG pulsed fiber laser was tested at Savcor Alfa within one day in
December 2008 and the performance of the laser was found out roughly. During
the day it came obvious that using full 100W average power when pulsing the
maximum 100 kHz was a challenge due high intensity laser pulses. With full
power and 100 kHz the pulse energy is 1mJ. If beam was focused to too small
spot it resulted in evaporation of material without any or little oxidation. With
larger spot size it was possible to use higher powers but also then in focus the
beam started to evaporate the base material. Series of tests was made so that
variables were the de focusing value, line to line spacing and laser power. Test
matrix consisted five rows with marking speeds from 200 mm/s to 2120 mm/s.
and with 25µm line to line spacing. First test was done in focus with 10W average
power and 10x lower speeds. Colors produced quite ok (Figure 29).
39 (66)
Figure 29. Example of color marking line by line with low average powers.
Then it was decided that with same setting but speeds from 200 to 2120 mm/s and
50W average power. After couple iterations small defocusing helped colors to
form. When taking more and more defocusing it was possible to use higher
average power and get almost the same colors. Problem in defocusing is the quite
visible last marked line. Also high accuracy marking is difficult with large beam
size.
After making more tests it was possible to make marking faster by optimizing the
spot size and line to line spacing. Fairly good blue was done using expander
setting 1 and using 15 mm de focusing with full 100W average power and
marking speed ~800mm/s with 50µm line to line spacing. This means that 2 by 2
mm area was marked in 0,12 seconds.
Figure 30. Test plate with color marking with full power and with big spot.
Focus measurement from 15mm defocus is shown in figure 31. This reveals that
spot size is around 340µm in diameter which is really large.
40 (66)
Figure 31. Beam analysis from used defocus setting.
5.5.7
Marking with 100W CW fiber laser
100W CW fiber laser was tested at VTT with scanhead equipped with f300 ftheta. Preliminary testing of color marking parameters with CW fiber laser was
evaluated. It seems that color marking could be possible but quite limited manner.
It was noticed that oxide starts to grow in the beginning of marking but it fades
away quite fast and the result is not color marking. Easiest colors to obtain were
silver and gold like with pulsed fiber lasers. Other colors are hard to get. In figure
32 there is two different colors shown which are made with CW laser. Beam
analysis of the laser was shown in section 4.1.4. Spot size was 90µm. Used laser
power 25W. Marking speeds 350 and 250mm/s and line to line separation was
30µm.
Figure 32. On left 350mm/s and or right 250mm/s with 30µm bi directional
hatch. Square size 3 x 3mm.
41 (66)
5.5.8
Marking with 20W pulsed Nd:YAG laser
Pulsed Nd:YAG was tested as reference for fiber lasers due they are commonly
used in the industry. Several different tests were done. Here in figure 33 are four
different test matrixes where marking speed is 300 – 175mm/s on vertical axes.
Repetition rate was from 50 kHz to 120 kHz on horizontal axes. Line to line
distance was 10µm on every marked area. On left the sample was kept in focus
and on right next to it 1mm below focus. Second on the right is 2 mm from focus
and the last on right 7mm below focus. What is quite nice is that with this 163mm
optic even 7mm difference in focal point does not totally ruin the process.
Figure 33. Four different tests matrixes with four different focal setups.
These tests showed that Nd:YAG is also suitable for color marking but due the
laser pulse length changes if repetition rate is changed it might be not se easy to
find parameters for every color. Also normally beam characteristics does change
if power is increased and this also has an effect to output which might be even
advantageous. One thing is also that with Nd:YAG poorer beam quality ends up to
larger spot size which in some cases forces to use shorter focal lengths and then
smaller work areas.
5.5.9
Common error causes in laser color marking
Due laser color marking is based on the thin film effect it is crucial to have all
affecting parameters normalized as well as possible. Common causes for wrong
color are: unclean surface, different material, different surface finish, wrong focus
setting, problems with laser stability and different material thickness.
If marked surface is not cleaned before marking the dirt will affect marking. For
example fingerprint on stainless steel surface will make different color on the
areas where finger has touched. Also oil or moisture etc. will probably ruin the
wanted outlook. Because formed oxide consists of different oxides the
composition may change if base material has different composition and this may
lead to different color. This can be also used to produce really high resolution
marking due for example only on places were fingerprint is present the finger
print will be marked blue and everything else non colored. Still good control of
surface additives might be challenging.
Depending on the strategy used in color marking the surface finish will affect the
outcome. If small line to line separation is used the melting of material could be
42 (66)
big enough to smoothen the surface enough. If surface is rough the marking can
also be made to smoothen the surface. Surface is marked once first with such
parameters which will produce silver like outlook and then second time makes the
color. This makes the process a lot slower and heat input will be higher.
Wrong focal setting will change the spot size on sample and this way laser
intensity on sample will be different. Focal setting is crucial with shorter focal
lengths because with longer ones the problem is not that big. When marking logos
and text the stability of the laser plays a big role due thinner areas will have
different heat load if the all the pulses are not equal in pulse energy and this may
lead to different outlook.
Different material thickness affects to the cooling rate due thicker material will
cool the marked area down quickly but if material is thin the cooling is only
coming from the sides and then output will be different.
In figure 34 is couple test samples with different error sources on plate before
marking. Both plates have the normal marking on upper row and then on left the
second one from top was made so that surface was smeared with normal soap
before marking. Next one was smeared with gearbox oil and the last with oily
finger prints. On the right second form top is grinded with 80 grade sandpaper
quite roughly. The next was treated with pit saw file. Second from bottom was
blown with normal glass abrasive blasting. Bottom one was blasted second time
quickly after marking also.
Figure 34. Test samples with different error sources.
From previous picture we can see that uncleaned surface will end up into
unwanted result. Also different surface finish will have dramatic effect on the
result. Good parameters on some material might not work on the other material if
too many factors change.
43 (66)
6
Results from color measuring
6.1
Spectral color measurements
Total reflectance was measured using Perkin Elmer lambda 18 spectrophotometer
with an integrating sphere. In Figure 35 total reflectances are shown for the
measured samples. In Table 4 and Table 5 calculated CIE Lab color coordinates
are show using CIE 1964 observer and D65 illuminant. In Figure 36 calculated
RGB representations from the spectra are shown. The spectra is very smooth and
the level is quite low. This means no saturated colors.
1
0.9
0.8
reflectance
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
400
450
500
550
600
wavelength [nm]
650
700
750
Figure 35. Total reflectance measured with the Perkin Elmer Lambda 18
spectrophotometer.
Table 4. CIE Lab color coordinates for the samples I2, I4, I1, E1, E3, E4 and
H1.
I2
I4
I1
E1
E3
E4
E2
H1
L*
55.6
55.4
54.4
55.6
54.3
51.5
51.6
54.2
a*
10.7
9.8
14.9
2.2
-9.4
-5.4
21.5
13.5
b*
-7.3
-8.6
-7.1
0.2
-0.6
-5.1
-7.3
-7.0
44 (66)
Table 5. CIE Lab color coordinates for the samples F2, F3, A2, G1, H2, H3, H4,
F1 and base material.
F2
F3
A2
G1
H2
H3
H4
F1
steel
L*
54.8
60.4
64.3
46.7
59.5
67.3
71.4
82.2
82.8
a*
25.1
3.9
2.6
-0.2
6.8
3.1
2.0
0.5
0.2
b*
-4.6
8.1
13.4
-9.3
21.6
21.7
17.7
8.3
2.9
Figure 36. Calculated RGB representations of the measured total reflectance
spectra of the samples with a 0/d geometry.
Multiangle measurements were with the Hamamatsu PMA-11 fiber spectrometer
with a rotary unit. Samples were illuminated in a 15 degree angle to the sample
normal. Spectral measurements were made from 0 to 45 degree angles compared
to the sample normal, totally 46 spectra were measured from each samples. In
Figure 37 an example of an angular dependence on a wavelength of 500 nm are
shown as a function of measuring angle. The reflectance of the metal plate is
very high in a specular angle, here 15 degree. Reflectance also changes very
rapidly as a function of viewing angle. In Figure 38. RGB representation is
shown under different measuring angles. One can see very high angular
dependence of the color as a function viewing angles. Colors are highly
saturated with the specular angle, so in right side RGB representations spectra
are divided by factor 5.
45 (66)
Figure 37. Angular dependence of one wavelength as a function of measuring
angle.
Figure 38. Calculated RGB representation with different measuring angles from
0 to 45 degree when illumination angle was solid 15 degree.
Results with the spectral camera trough a microscope are reported in chapter 6.5.
6.2
Thickness of the oxide layers
The thicknesses of the oxide layers on stainless steel samples were in order of few
hundred nanometres. The minimum value was 100 nm and the maximum value
was 565 nm. However, it must be reminded that SEM cannot distinguish film
thicknesses much smaller than 100 nm. Therefore, the results do not exclude the
possibility of thinner layers as well. All together, the measurement results
correlate well with the results given in journal articles /7, p.215/ and also with the
modelling results, given in section 6.1.5.
The tables 6 and 7 for the oxide layer thicknesses are given below. The table 6
represents the stainless steel sample marked with IPG laser. The table 7 represents
samples marked with SPI laser. IPG laser power was 18 W on the sample surface.
The temporal width of the pulse was 112.3 ns, spot size 38.8 m, and pulse
repetition rate 85 kHz.
46 (66)
Table 6. Thicknesses of the oxide layers for IPG marked stainless steel samples.
color
dark
green
dark blue
light blue
silver
green
red
gold
Scanning speed [mm/s]
Line spacing [ m]
2000
Thickness of the oxide layer
1,2 300-470 nm
450
350
130
30 100-200 nm
30 330-565 nm
50 380-500 nm
80
60
50 200-500 nm
50 200-300 nm
Table 7. The thicknesses of the oxide layers for SPI laser marked stainless steel
samples
Sample
A2
E1
E2
E3
E4
F3
G1
H1
H2
H3
H4
I1
I2
I4
6.3
Thickness of oxide [nm]
100-150
260-360
240-320
220-290
240-340
110-170
60-105
120-240
130-250
120-200
140-200
260-325
110-130
200-450
Surface roughness
Three different surface roughnesses were able to be obtained from the surface
profiler measurements. The most common roughness is the arithmetic average of
the absolute vertical deviations of the profile. Another roughness value is
Skewness that is a measure of the asymmetry of the surface profile curve.
Negative skewness indicates porous surfaces with deep valleys on smooth plateau.
Positive skewness thus means high spikes above flatter average. The third
roughness value, Kurtosis, is a measure of the spikiness of the profile.
The arithmetic mean surface roughness values, Ra, for samples marked with IPG
laser and SPI laser are given in tables 8 and 9 below, respectively. Also skewness,
Rsk, and kurtosis, Rku, are given for the SPI marked samples.
47 (66)
Table 8. The surface roughness of IPG marked stainless steel surfaces
Sample
Ra [nm]
dark
340.5
green, B3
dark blue 203.9
light blue 240.6
Silver
558.8
green
Red
227.4
Gold
246.3
Table 9. The surface roughness of SPI marked stainless steel surfaces
A2
E1
E2
E3
E4
F3
G1
H1
H2
H3
H4
I1
I2
I4
Ra [µm]
0.8336
0.2733
0.7346
0.1426
Rsk
0.2414
-0.1932
-0.0798
0.1134
Rku [1/µm2]
2.3214
2.8889
3.2308
2.5516
0.2086
0.2964
0.2183
0.1807
0.1913
0.1753
-0.5650
-0.0344
0.2168
0.1458
-0.2569
0.2138
2.2828
3.1415
3.3599
2.5455
2.9338
2.9385
0.1664
0.2649
0.3354
0.3260
0.1066
0.3284
-0.1018
0.3308
2.8560
2.7915
2.7587
0.3308
As can be seen from the tables above, the average arithmetic roughness was few
hundreds of nanometres. This roughness value is the same order as the oxide layer
thicknesses.
6.4
Elementary analysis
The energy dispersive x-ray spectroscope measures the number of atomic counts
of each element in the examined material. Thus, the number of the counts is
directly proportional to the quantity of the atoms of an element.
The results show that the atomic counts are related to the surface texture. For
example, the elementary analysis made for IPG samples show clear periodicity
which is the same periodicity that on the surface. If we consider the results for the
IPG silver green sample in Figure 39, we see that the counts of chromium and
oxygen follow the same trend: the counts are the lowest at the laser grooves (light
vertical lines) and as counts for chromium increase between the grooves, so do the
counts for oxygen. The counts for iron show an opposite trend.
48 (66)
Figure 39. The atomic counts for IPG silver green sample. The green arrow in
the picture on the left indicates the scanning line of the EDS. The laser beam has
scanned the areas that are seen as the lighter vertical lines.
The SPI laser marked samples that were examined were E1, E3, E4, F1, F2, F3,
and H4. They were chosen according to their corrosion properties. Samples F1
and H4 had the best corrosion properties while samples E4 and F2 had the worst
corrosion properties. The results for SPI samples were not as local as for the IPG
samples. The periodicity of the SPI samples was not strong and therefore the
counts were approximately unchangeable also in the direction parallel to the
laser grooves. F1 and H4 were the best corrosion resistant samples and
according to the elementary analysis they had the lowest oxygen counts, both
absolute values and relative values. An example of the atomic counts is given in
Figure 40. for sample E3.
Figure 40. The atomic counts for SPI marked sample E3.
49 (66)
6.5
Modelling of the colors
From the optical microscope images it was found out that the colours of the
samples that are seen with naked eye are effective. This means that the visible
colour is formed when several differently coloured small areas are so close
together on the surface that cannot be distinguished by the resolution of the
human eye. Example of coloured surface on stainless steel viewed through an
optical microscope is given in Figure 41.
Figure. 41. Laser marked surface on stainless steel viewed through an optical
microscope. Sample looked as silver green when seen with naked eye.
Colours of these small areas were modelled to be resulting from thin film
interference. The composition of the thin films on stainless steel was assumed to
be chromium and iron oxides because of the results from the elementary analysis
and knowledge from similar studies. Different colours were obtained by varying
the oxide thickness. During the modelling it was noticed soon that only
chromium oxide, Cr2O3, produced matching spectra with the measured
microscopic spectra. Therefore, the colours were concluded to be formed by thin
film interference in chromium oxide films.
Cr2O3
Stainless steel
Figure 42. The model for the microscopic colours.
An example of the modelling of the microscopic colours is given in table 10.
The IPG silver green sample, that is illustrated also in figure.41, was divided
into four colours: purple, turquoise, green and yellow. The modelled spectra that
had the best correspondence with the measured microscopic spectra are shown
in the table. The chromium oxide thicknesses that gave the best matching
spectra with the measured ones were 290 nm, 200 nm, 460 nm, and 117 nm
corresponding to purple, turquoise, green and yellow areas on the sample.
50 (66)
Table 10. Measured and modelled microscopic reflection spectra for IPG silver
green sample. The wavelength range on the x-axis is 420-720 nm.
Purple
Turquoise
Green
Yellow
Measured
Modelled
The chromium oxide layer on stainless steel -model was supported by the
elementary analysis: in the laser groove, where the atom counts for chromium
and oxygen were the lowest, the model suggested thin oxide layers, whereas
between the grooves, where the chromium and oxygen counts were high, the
modelling resulted in thicker oxide layers. Also the film thickness measurements
agree well with the modelling results. Both suggest thin film thickness in order
of few hundreds of nanometres.
We also wanted to see, if the modelled microscopic spectra produced the same
visible colour as was measured. For the calculation of the visible (effective)
spectra we took the average of the modelled microscopic spectra weightened by
the surface area. The resulting calculated effective spectrum is compared to the
measured effective spectrum in table 11. The spectra match reasonably well with
each other and the colours that are produced by the spectra are both green.
Table 11. Measured and modelled visible (=effective) spectrum and colour of
the spectrum for IPG silver green sample. The x-axis wavelength range is 420720 nm.
Effective spectrum
Color
Measured
Modelled
The colour that is formed during the laser marking process depends on the laser
parameters, as shown before. More detailed look reveals that the fundamental
origin of the colour is the energy that is absorbed by the surface. Therefore, we
51 (66)
have defined the connection between the energy density and the colour of the
surface. Different colours corresponds certain thickness of the chromium oxide,
therefore also thickness map of the surface is presented.
The measurement of the colour-energy relation was done with SPI laser with
f254 optics, spot size of 108µm, repetition rate 500 kHz, pulse width 200 ns.
Number of the pulses on the surface was 175 and they were all focused at the
same spot on the surface. That is, no scanning of the laser was made. The energy
distribution was calculated from beam intensity measurements made with
Primes micro spot monitor. The thickness of the chromium oxide was calculated
with the same modelling procedure as described above. The obtained energy
distributions, the thickness map of chromium oxide, and the microscopic picture
are given in Fig. 43 (a)-(d). The energy values are for area of ~8 m².
(a)
(b)
(c)
(d)
Figure 43. Three dimensional energy distribution (a), cross-section of the energy
distribution (b), view through optical microscope (c), chromium oxide thickness
map (d).
It can be seen from Figure 43 that the colors are produced with energy density
values between approximately 2 - 12 J / 8 m². The thickness of the chromium
oxide increases along the energy density. However at the places with the highest
energy density, the thickness is very small or zero. This can be explained by the
fact that some of the stainless steel surface is probably evaporated with higher
energies. Therefore, in these high energy regions, the oxide layer has not been
able to be formed or only a very thin layer has been grown. This could explain
why the thickness of the oxide layers seem to be the thinnest also at the laser
52 (66)
grooves of the colored stainless steel samples. The energy has likely been enough
high to evaporate small layers from the surface.
7
Results from corrosion tests
The results of corrosion tests are presented separately for the austenitic stainless
steel and for the titanium. The main interest focused corrosion resistance of the
colour marked stainless steel, which was fully or partially covered by laser.
Effect of processing parameters on corrosion resistance
An example of the corrosion test for defining critical pitting temperature (CPT)
according to ASTM G 150-99a is presented in Figure 44. The CPT for the tested
specimen in the figure is 76ºC.
80
0,9
0,8
70
0,7
TEMPERATURE (ºC)
60
0,6
50
0,5
40
0,4
30
0,3
TEMPERATURE (°C)
CURRENT (mA)
CURRENT (mA)
7.1
20
0,2
10
0,1
0
0
1000
2000
3000
4000
5000
0
6000
TIME (s)
Figure 44. Corrosion current as a function of testing time and temperature in CPT
testing for the laser colour marked stainless steel specimen F1 (20W, 800 mm/s,
200 kHz, 30 m, 0.8 J/mm2). Visible colour is gold.
The CPT for the base stainless steel EN 1.4307 was measured 57ºC. Thus, the
corrosion resistance of the laser colour marked specimen was clearly higher than
in the base steel.
The summary of the CPT measurements as a function of heat input is in Figure
45. The high heat input of the laser colour marked specimens produce low
corrosion resistance as low CPT values. If the heat input is less than 1.4 J/mm2 ,
the CPT value is higher than the CPT of the base stainless steel. The summary of
the CPT measurements as a function of heat input is in Figure 6, where laser
parameters were the following:
- for F1 0.8 J/mm2 800 mm/s, 30µm, 15 ns, 18 W, 200 kHz and 55 µm spot size;
53 (66)
- for A2 1.4 J/mm2 300 mm/s, 30µm , 120 ns, 15 W, 85 kHz and 45 µm spot size;
- for F3 2 J/mm2 300 mm/s, 30µm, 30 ns, 15 W, 200 kHz and 83 µm spot size;
- for G1 3 J/mm2 1200 mm/s, 5 µm, 65 ns, 18 W, 200 kHz and 83 µm spot size;
- and for I4 5 J/mm2 120 mm/s, 30 µm, 65 ns, 18 W, 150 kHz and 165 µm spot
size.
80
70
60
CPT (oC)
50
40
30
20
10
0
0
1
2
3
4
5
6
HEAT INPUT (J/mm2)
Figure 45. CPT of the laser colour marked stainless steel specimens as a
function of heat input.
The colour marked surfaces after the CPT test were destroyed in the different
way depending their laser process parameters and CPT values. Few round pits
were located randomly in the laser colour marked specimens, which had higher
CPT than in the base steel, Figure 46. The CPT for the specimen in figure is 59
ºC and the diameter of the pit is 25 m.
54 (66)
Figure 46. Optical microscopic view from the laser colour marked stainless steel
specimen A2 (15 W, 300 mm/s, 85 kHz, 30 m, 1.4 J/mm2) after the CPT test.
Visible colour is dark blue.
Corrosion in the specimens, where CPT was proven very low, was totally
different, Figure 47. The whole tested surface of the specimen, which has CPT as
low as 6 ºC is covered by a crack net.
Figure 47. Optical microscopic view from the laser colour marked stainless steel
specimen E1 (20 W, 240 mm/s, 85 kHz, 10 m, 7.5 J/mm2) after the CPT test.
Visible colour is light black.
55 (66)
How does affect the other laser process parameters on the CPT values? By fixing
the heat input 5 J/mm2, where CPT was measured 11 ºC (Figure 45), and by
increasing the scanning velocity the corrosion resistance is increasing remarkably,
Figure 48. The limiting factor for the elevated scanning velocities is finally the
laser power capacity. The laser parameters of the specimens of Figure 48 were the
following:
- for H1 240 mm/s, 15 m, 65 ns, 20 W, 150 kHz, and 165 m spot size;
- and for H4 720 mm/s, 5 m, 65 ns, 20 W, 150 kHz, and 165 m spot size.
70
60
CPT (oC)
50
40
30
20
10
0
0
100
200
300
400
500
600
700
800
SCANNING VELOCITY (mm/s)
Figure 48. CPT of the laser colour marked stainless steel specimens ( 5 J/mm2)
as a function of scanning velocity. All samples have almost black outlook.
Different heat inputs are needed to form different colours. The lowest heat input
is needed for silver or polished surface and there is only thin oxide layer present.
When increasing the heat input it gives next the gold colour with less than
1 J/mm2. After gold there is blue colour with bit more than 1 J/mm2. Then
around 3 and 5 J/mm2 there is red available. Between 5 and 7 J/mm2 green is
produced. Black looking oxide layer form around 7.5 J/mm2. These heat input
windows are not exact values due for example pulse width and marking speed
among others has a big effect to the outcome. Depending on the final application
of the laser colour marked product including environmental corrosion factors
each wanted colour can be produced by studying the best laser process
parameter to achieve at least the corrosion resistance of the base stainless steel.
From the results we can see that the marking speed has a big effect on corrosion
resistance even if heat input remains the same. From the Figure 48 we can see
56 (66)
that just by increasing the marking speed the resistance can be improved a lot.
With 5 J/mm2 and 120 mm/s and line spacing of 30 µm CPT is only +6 °C.
When same heat input is made with 720 mm/s and line spacing 5 µm the CPT is
+65° which is more than the CPT of the base steel. Both of these have almost
the same black outlook. Pulse length was 65 ns and the calculated spot size
around 120 µm. If spot size would be changed to 60 µm the outlook of surfaces
would be red or light red. Again playing with spot size and pulse width oxide
colour will result to green with otherwise same parameters.
7.2
Effect of interface on corrosion resistance
Some test specimens were covered with a checkered pattern of 2 mm * 2 mm
oxide layer (Figure 49) in order to test corrosion resistance on the colour-base
steel interface.
Figure 49. Checkered laser colour marked pattern (20 W, 800 mm/s, 200 kHz,
30 m, 0.8 J/mm2) on the stainless steel specimen, same parameters as specimen
F1.
During the CPT test corrosion attacks are located visibly on the pattern (small
light damages on the colour surface) and on the base steel between the patterns
(Figure 50).
57 (66)
Figure 50. Corrosion damages on the checkered laser colour marked pattern (20
W, 800 mm/s, 200 kHz, 30 m, 0.8 J/mm2) of the stainless steel specimen (F1).
The CPT results of the checkered laser colour marked pattern are collected in
Table 12 , where for the comparison purposes the corresponding CPT values of
the fully colour marked specimens and the base stainless steel are presented,
too.
Table 12. Critical pitting temperatures (ºC) of the laser colour marked stainless
steel specimens and the base steel EN1.4307.
laser colour marking
F1 20W 200kHz 800 mm/s 30
m
20W 200kHz 800 mm/s 5 m
H4 20W 150kHz 720 mm/s 5
m
checked pattern
46
fully covered
76
base steel
57
45
47
65
57
57
The CPT values of the tested checkered marked stainless steel specimens
diminished drastically from the CPT values of the corresponding fully covered
marking specimens. Actually, the CPT value of the base stainless steel is higher
than all CPT values of the tested checkered marked specimens. It becomes
obvious, that the heating due to the checkered laser colour marking effects on
the stability of the native oxide layer on the base stainless steel.
58 (66)
Corrosion resistance of the laser colour marked titanium
specimens
General corrosion resistance of the laser colour marked titanium specimens (50
mm * 60 mm * 1 mm) was tested by defining anodic polarisation behaviour of the
specimens. The solution used in titanium corrosion tests was 2 M NaCl ( 117 g/l
NaCl) at temperature of + 50ºC. The scanning rate of the potential was 20mV/min
starting from – 550 mV SCE and ending to + 7000 mV SCE. The results of the
anodic polarisation tests for titanium are shown in Figure 51.
8000
Potential (mV)
7.3
7000
Base titanium
6000
Ti : 15kW 1000mm/s 85kHz 2 m
5000
4000
3000
2000
1000
0
-1000
1.E-08
1.E-06
1.E-04
1.E-02
1.E+00
1.E+02
Current (mA)
Figure 51. Anodic polarisation curves of the laser colour marked titanium
specimen (15kW 1000mm/s 85 kHz 2 m) and the base titanium specimen.
The anodic current of the laser colour marked titanium specimen in Figure 51 is
clearly lower than the current of the base titanium when potentials are on the
common use range 0…+ 2 V. Because of the logarithmic current scale the
difference can be even two orders of magnitude. In the transpassive potential
range, i.e., over + 2 V, the current of the base titanium is about one order of
magnitude less and more stabile than the current of the laser colour marked
titanium specimen. The fluctuation of the transpassive current in the laser colour
marked titanium specimen can be caused by the cyclic damages of the colour
marked surface.
Based on the performed general corrosion tests the laser colour marked titanium
specimen seems to have higher general corrosion resistance than the base titanium
specimen in the practical potential range.
59 (66)
8
Results from wear tests
Reciprocating sliding wear tests were performed on four samples which had one
half of the surface been color marked and the other half was untreated,
representing the original base material. The samples were made using four
different parameter combinations resulting in different color appearances, see
Table 13. The sliding direction in all tests was perpendicular to the laser marking
direction, see Figure 52. The test method and parameters are described in chapter
4.5.
Table 13. Laser marking parameters of the samples tested.
dark green blue
gold (not
uniform)
2
Heat input, J/mm
7.5
1.44
5
Marking speed, mm/s
2000
350
720
Line spacing, µm
1.2
30
5
Pulse width, ns
120
120
65
Average power, W
18
18
18
Repetition rate, kHz
85
85
150
Spot size, µm
45
45
177
Specimen
B3
A2
H4
gold
0,8
800
30
15
18
200
71
F1
Laser marking
direction
Laser marked surface
Original surface
a)
b)
Figure 52. a) schematic presentation of the position and sliding direction of the
wear tests on the laser marked samples. b) photograph of two samples tested, left
appearing gold, right also gold but somewhat darker and not uniform near the
upper edge of the sample.
Since the surface roughness may often be of importance with respect to friction
and wear, the surfaces of the samples were characterised before the wear tests.
Figure 53 shows 3D optical profilometry images across the edge of the color
marked area of the samples so that the topography of both the marked and the
unmarked surface area of the sample can be seen. Table 14 gives certain surface
roughness parameters measured with the 2D stylus profilometer from both the
color marked and the untreated surfaces.
60 (66)
The most commonly used surface roughness parameter is the Ra-value, i.e. the
arithmetic average of the roughness profile. However, it is not sufficient alone to
describe the differences in the surface roughness profile. Other values such as
skewness (Rsk) and kurtosis (Rku) as well as the peak height (Rp) and valley depth
(Rv), can give a clearer indication of the effect of the laser marking on the surface
topography of stainless steel. Rsk is a measure of the asymmetry of the surface
profile curve, and a negative value indicates a surface with good bearing
properties. Rku is a measure of peakedness of the surface profile, and has a value
of 3 for a Gaussian distribution of profile values. For a profile with sharper peaks
the value is greater than 3.
a)
b)
c)
d)
Figure 53. 3D profilometry image of the surface of the samples with color
marked area on the left side and untreated original surface on the right. a) visible
color dark green, b) blue, c) gold (non-uniform) and d) gold. The size of the
imaged area is 1000 x 800 µm.
Table 14. Surface roughness parameters measured from the wear test samples on
the original surface and on the marked surface.
Dark green, B3
Blue, A2
Gold (not uniform),
H4
Gold, F1
parameter
original
marked
original
marked
original
marked
original
marked
Ra (µm)
Rsk
Rku
0.17
-1.48
6.41
0.16
-0.20
3.31
0.15
-1.71
7.24
0.19
-0.02
3.07
0.17
-1.32
6.08
0.19
-0.85
4.96
0.17
-1.75
8.83
0.24
-0.49
4.87
Rp (µm)
Rv (µm)
Rz* (µm)
0.32
1.11
1.43
0.50
0.57
1.07
0.27
1.05
1.32
0.66
0.63
1.29
0.41
1.05
1.46
0.64
1.08
1.72
0.37
1.27
1.64
0.82
1.32
2.14
*Rz=Rp+Rv
61 (66)
As can be seen from the surface roughness parameters in Table 14, the laser
marking process reduced the peakedness and gave a more symmetric surface
roughness profile to the samples when compared to the original untreated stainless
steel surface. The same is indicated by the change in the values of the peak
heights and valley depths: even though the peak height increased in each sample,
the values of the peak heights and peak depths had become closer to each other as
the profile was more symmetric especially in the case of the dark green and blue
color.
Wear tests were carried out with a steel ball sliding against the stainless steel
surface both on the laser color marked surface areas and on the untreated original
surface area, see Figure 8.1 (short arrows). The test was repeated using different
test durations, i.e. 1 unidirectional slide, 5, 10, 20, 30 and 50 reciprocal sliding
cycles with the blue and dark green sample, and 1 unidirectional slide and 10 and
50 resiprocal sliding cycles with the gold colored samples. After the wear tests the
surfaces of the samples were examined visually and with an optical microscope.
Figure 54 shows optical micrographs of the wear scars on both the sample plate
surface and on the counter part steel ball after 50 cycles of reciprocal sliding. The
steel ball made a clearly visible wear mark on all the samples. However, clear
differences could be observed between the wear behaviour of the color marked
surfaces made using different laser processing parameters. The sample with the
visible color blue had the best wear resistance of the laser marked surfaces and it
also caused least wear on the counter part steel ball of all the samples examined.
The blue colored surface was also clearly less damaged than the original untreated
stainless steel surface. The wear resistance of the dark green surface was the
second best but the steel ball sliding against it suffered heavy wear and formation
of wear debris. The samples with gold appearance had the worst wear behaviour
of the laser marked samples tested and their wear behaviour was comparable to
that of the untreated stainless steel. Considering the surface roughness parameters
it seems that the best wear behaviour would be obtained with a surface roughness
profile with good symmetry and low peakedness.
Figure 54. Micrographs of the wear scars of the tested surfaces and the counter
bodies after 50 cycles resiprocal sliding. The lower pictures show the steel ball
and the upper pictures show the sample surface. Samples from left to right: dark
green oxide, blue oxide, gold (non uniform) oxide, gold oxide and the untreated
base material after the test. Scale 200µm on the upper pictures and 100µm on the
lower ones.
Wear tests were also carried out with a fabric covered steel pin (or cylinder)
sliding longitudinally a short distance forth and back against the stainless steel
62 (66)
sample for test durations of 15 and 60 minutes. The pin was 40 mm long and it
was placed on the sample so that it was sliding both on the laser marked and the
untreated surface at the same time (long arrows in Figure 52 a). The sliding
distance to one direction was 5 mm so that only a small portion of the pin was
actually in contact with both surfaces. Only a slight wear mark was visible even
after 60 minutes sliding and it was more easily seen on the untreated original
surface than on the laser color marked surfaces. Wear debris formation increases
the visibility of the wear track. If wear particles or other foreign particles get into
the contact, they can quickly result in the formation of a visible scratches.
The tests showed clearly that the laser color marking parameters have a significant
influence on the wear behaviour of both the color marked stainless steel surface
and of the counter part surface. However, since only a limited number of tests
have been made, it is difficult to determine which are the most significant
parameters affecting the wear resistance and how strong the influence is. In
addition, the correlation between the corrosion resistance and the wear behaviour
of samples made with different laser processing parameters remains still to be
clarified. Corrosion and wear, when acting simultaneously, may also have an
accelerating effect on each other. Hence further testing would still be needed to
increase the knowledge of the influence of the laser processing parameters in
order to be able to produce surfaces with desired colors together with optimised
corrosion and wear properties.
9
Combination of different results
Project had four supplementing entities:
Developing of direct metal marking using newest fiber lasers
Research of properties of optical surfaces and research of colours and also
optimizing of optical properties using calculative methods
Corrosive testing of surfaces in conditions that simulate end user
conditions
Further development of colour marking and optimizing of fabrication
techniques of surfaces using equipment which are still to come to market.
During project a lot of laser marking work was done. To join all different research
work a set of test samples (table 15.) was chosen and the results have been shown
in different sections of this report.
63 (66)
Table 15. Parameters for different specimens
Specimen
Marking
speed mm/s
Average
power W
Repetition
rate kHz
Pulse
width
ns
Spot
size
µm
Hatch
µm
CPT
I2
144
18
150
65
177
25
11
I4
120
18
150
65
177
30
6
I1
180
18
150
65
177
20
19
E1
240
18
85
120
103
10
6
E2
400
18
85
120
103
6
5
E3
600
18
85
120
103
4
5
E4
1200
18
85
120
103
2
5
H1
240
18
150
65
177
15
18
H2
360
18
150
65
177
10
32
H3
514
18
150
65
177
7
37
H4
720
18
150
65
177
5
65
F1
800
18
200
15
71
30
76
F2
120
18
200
65
177
30
12
F3
300
18
200
30
103
30
47
A2
300
13
85
120
45
30
59
G1
1200
18
200
30
71
5
34
From this set of parameters and different colors it is possible to see that it is hard
to make straight forward conclusion that how different parameters affect to
corrosion, wear and color properties due with totally different parameter
combinations lead to same color but totally different behaviour on corrosion or
wear testing.
Looking by the outlook of laser marked sample one cannot say whether it good or
bad on corrosion wise. Smooth oxide surface may look good but on wear test
oxide may be damaged seriously. Roughness measurements does not reveal any
explanation on which surface could be good or bad. So it is not easy to predict
which color or parameters setup would lead to good combination on all properties.
When some color is wanted on surface it might be so that laser parameter has to
be set into some parameter range and there might not be any alternative parameter
for doing that if speed is wanted to be maximized. If one can use a little bit slower
marking speeds it is easier to tune parameters and different marking strategies so
that also wear and corrosion is kept in mind.
If someone would like to buy or sell color marked samples and would need to
specify the wanted color they should use the CIE Lab method which was
introduced on section 6.1. If marked color is wanted to be specified it is possible
to measure the marked areas with equipment which was introduced in the section
4.2.
Corrosion testing of marked samples is time consuming and that is why the
amount of tested samples was kept at adequate level. Still if color marked areas
are wanted to be tested it is possible to do so at VTT or some other facility with
similar equipment introduced in section 4.5.
Wear testing was also done for only for few samples but it showed easily the
differences on wear properties with different laser parameters.
64 (66)
During project a wide variety of different lasers were used. Commercially
available lasers kept getting better and better during project. Now higher average
powers are available than year 2007. Still the Fraunhofer CLT’s fiber laser is
unique with it’s capabilities on pulse width tuning which is evidently a key factor
to successful color marking. SPI fiber laser used in the project is a good tool also
due it has quite wide field on pulse width and repetition rate tuning. High average
powers are already commercially available which could be utilized in color
marking also.
In some cases even a continuous wave laser could be used in color marking but
then the colors are really limited and heat input a lot higher when compared to
pulsed operation.
10
Conclusion and summary
In this project laser color marking was further developed and the marked areas
were examined on wear, corrosion and color wise. Before this project color
marking was available but colors could be produced limitedly with one laser
system and one would maybe need another laser for doing other colors. Also the
fiber lasers entering the market has pushed laser prices down and enabled new
parameter areas for marking. Good beam properties and tunability of the laser
make it possible to do high quality marking with affordable equipment. During
project couple different strategies was examined how to make the oxide grow.
Different strategies end up into different results.
Marking made in the project was fairly fast but it should be faster. Now with 20W
laser the color marking speed was from 0.1 mm2/s to 1 mm2/s and industry would
maybe need something like 10 mm2/s. This speed could maybe attained with
different optical means and with high power lasers. At least higher power lasers
are needed.
Biggest effort was put to research work around stainless steel due it was already
known that titanium is easier to color mark with laser. Still both materials had a
bit different behaviour but the basic mechanism is the same.
During project a wide variety of colors were found out. Almost all colors can be
marked on stainless steel surface but the different shades of one color might be
challenging to do. Really dark red was maybe the most difficult to make due it is
produced with the thickest oxide layers. Depending on the used laser parameters
the viewing angle might be really narrow or even quite large. Surface quality was
seen the key issue to have larger viewing angle. Due the nature of process laser
beam is always in the tens of microns at least and when one is doing marking line
by line the effects are in the micrometer scale. Oxide growth smoothens up the
surface but still the surface roughness can be more than the oxide thickness. This
leads to problem on viewing angle and if the surface could be smoothened
somehow it would most probably give better viewing angle. Another thing is the
surface underneath oxide and its roughness.
65 (66)
From the corrosion results we can see that the marking speed has a big effect on
corrosion resistance even if heat input remains the same. Strategy used in marking
has also big effect due line by line marking there is base material always present
between lines and this make the surface more prone to corrosion. Normally color
marking is marking of logos and texts and then there is a significant amount of
interface between base material and oxide layer. Even though the oxide would be
good without cracks the interface is the most crucial point. This is why scanner
delay and first pulse suppression should be optimized due same color on surface
may be different on corrosion wise at the interface. Tests showed that the interface
is always a little bit worse than the base material. Depending on the application
this is not a problem. Titanium is a lot better on corrosion wise due the base
material has already really good properties against corrosion.
Wear tests showed clearly that the laser color marking parameters have a
significant influence on the wear behaviour of both the color marked stainless
steel surface and of the counter part surface. However, since only a limited
number of tests have been made, it is difficult to determine which are the most
significant parameters affecting the wear resistance and how strong the influence
is. In addition, the correlation between the corrosion resistance and the wear
behaviour of samples made with different laser processing parameters remains
still to be clarified. Corrosion and wear, when acting simultaneously, may also
have an accelerating effect on each other. Hence further testing would still be
needed to increase the knowledge of the influence of the laser processing
parameters in order to be able to produce surfaces with desired colors together
with optimised corrosion and wear properties.
The colors of the surfaces can be explained by thin film interference. The
thickness of the film together with the illumination angle determines the visible
color. According to the results from the modelling, it can be concluded that the
thin film is composed of chromium oxide, Cr2O3.
Colors are best defined if measured spectrally under some CIE standard
geometries. From the spectra it is always possibly to calculate any desired color
coordinate values, Lab or others, under different illuminations. However laser
marked samples give their best colors when viewing under specular angles, and
that geometry is not recommended in CIE measuring geometries. Furthermore
colors change very rapidly when viewing angle is changed. This makes color
definitions of the samples quite demanding since little change in illumination or
viewing angle change the color appearance of the laser marked sample. Of course
it is easy to measure the total reflectance of the sample accurately and repeatably
but this will give little bit dimmer results than seen in specular angle.
As a final conclusion of we can state that project covers wide range of color
marking with fiber lasers and laser parameter effects to the oxide growth and so
on to the corrosion and wear properties. With the project results end user should
have comprehensive knowledge how to set parameters to successful color
marking. When marked colors are wanted to quantify the CIE Lab method will be
the tool for doing it. Used laser in the project showed that correct equipment will
help in the finding of colors a lot. Of course the more the parameters the more
work has to be done when optimizing the result. Also different optical setups are a
necessity in successful process.
66 (66)
References
[1] A.M. Carey et al., Laser Surface Ornamentation, Proceedings of International Congress on
Application of Lasers & Electro-optics, ICALEO 1998, D170-178.
[2] Z Hongyu, Laser-induced colours on metal surfaces. SIMTech Technical Report
PT/01/005/AM 2001.
[3] R. Rusconi, J. Gold, Color marking. Industrial Laser Solutions,p. 16-18, Dec 2005.
[4] L Ming et al., Colour marking of metals with fiber lasers. Proceedings of the 3rd Pacific
International Conference on Application of Lasers and Optics 2008.
[5] D. Bäuerle, Laser Processing and Chemistry 3rd edition, Springer, Berlin-Heidelberg 2000.
[6] J. Sherwood, Researchers Create Gold Aluminum, Black Platinum, Blue Silver, WWWpage: http://www.rochester.edu/news/show.php?id=3106 (cited on 1st July 2008)
[7] H.Y. Zheng, G. C. Lim, X. C. Wang, J. L. Tan, Process study for laserinduced surface coloration, Journal of Laser Applications, vol 14, number 4,
p.215-220.

DIRECT COLOR MARKING OF METALS WITH FIBER LASERS

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