2 Z Hongyu, Laser-Induced Colours on Metal Surfaces

SIMTech Technical Report (PT/01/005/AM)

Laser-induced Colours on Metal Surfaces

Dr Zheng Hongyu

(Advanced Machining Group, Process Technology Division, 2001)

Laser-induced Colours on Metal Surfaces PT/001/005/AM

Keywords: Metal colouration; Laser colour marking; Laser surface decoration 1

1 INTRODUCTION Colour marking on metal surfaces is conventionally achieved by printing and emulsion coating techniques. Scratch and poor wear resistance of such coatings and fading of colours with time, are recognized problems associated with these coatings. Decorative coatings on metal surfaces may also be prepared by electro-chemical treatments in aqueous electrolytes. In these processes, changing the metal ions in the electrolyte produces different colours on the metal surface [1]. The flexibility of obtaining different colours and patterns is limited. Conventional use of lasers to produce identification marks on metals is by engraving. The engraved area is usually rough and dark brown in colour. Such marking appearance does not meet the requirements of product decoration and personalisation. Recently, a patent on laser colour marking has been filed [2], whereby a laser beam is used to induce a wide spectrum of colours on metals such as titanium and stainless steel by simply changing the processing parameters. The process incurs no visible surface material removal and the colours are found to be resistant against normal wear, solar irradiation and solutions. 2 OBJECTIVE The project was to develop a laser-induced colour marking process on metal surfaces and to understand the colour-change mechanism.

3 METHODOLOGY A krypton-fluoride excimer laser was used in this study. The laser operates at 248 nm with pulse duration of 20 ns and is homogenised to have a flat-top beam profile as shown in Fig. 1. The processing environment was controlled with shrouding gases of O2 and N2, which were introduced through nozzles to the laser-irradiated area at the same time as the laser beam was fired. Substrates of titanium (ASTM B265 Grade 1) and stainless steel (SU303) were used.

Fig. 1 Homogenised beam profile of the KrF

Excimer laser. 4 RESULTS Examples of the laser-induced colours on the titanium and stainless steel substrates are shown in Fig. 2 and Fig. 3 respectively. Different colours were produced by varying the processing parameters in a controlled oxygen environment. The colours vary from light yellow to dark blue. When viewed at angles, the colours vary slightly indicating the light interference effect.

Fig. 2 Laser-induced colours on Ti

Fig. 3 Laser-induced colours on stainless steel


2

The colours were found to be durable against ultrasonic rinsing in various solutions including alcohol, acetone, light acids and alkaline. Solar light exposure also did not cause noticeable colour degradation. 4.1 Formation Mechanism of Thin

Film The laser beam acts as a localised heating source and promotes the formation of an oxide layer on the metal surface. The thickness of the oxide layer is determined by the processing parameters, which in turn determines the resultant colours. An illustration of the interference effect in a thin film is shown in Fig. 4. Line ab is represent a light ray incident on the upper surface of a thin film. This incident beam is partially reflected at the first surface, as indicated by ray bc, and the remainder, represented by bd, is transmitted. At the second surface ray bd is partially or wholly reflected and of this, it partially emerges out into the air again as represented by ray ef. Lines at right angles to bc and ef represent wave surfaces. The wave surfaces may overlap to produce interference effects. If λ is the wavelength of the incident light waves in air, the wavelength in the film is λ/n (n is the refractive index of the film), and the number of waves contained in the path length through the film and back, or 2t, is 2t/(λ/n). This assumes that light is

incident at right angles to the film. When a train of waves is reflected at the surface of a medium of higher index, the reflected wave train loses (or gains) half a wavelength [3]. Hence if the path length through the film and back contains some integral number of waves plus half a wave, conditions are right for constructive interference (bright colours). If number of waves is some integral number, then waves reflected from the two surfaces would be in the correct correlation to interfere destructively, that results in dark colours. When the film is illuminated by white light, its colour at any point is formed by the mixture of those wavelengths that are reflected and interfered constructively. The colour for the reflected wavelength that interfered destructively will be absent [3]. The thickness of the oxide layer thus determines the colour spectrum. To determine the layer thickness and its optical constants, the technique of ellipsometry was employed. The measurement results are expressed in terms of Psi (polarisation orientation) and Delta (polarisation phase). The optical constants, which describe how the light propagates through a material, are determined using the WVASE32 software (J.A. Woollam Co. INC.). Constants for the refractive index (n) and the extinction coefficient (k) describing light absorption

Fig. 4 Interference between light waves reflected from the upper and lower surface of a thin film

t n Film

Substrate

a

b

c f

e

d

Ai


3

are generated. Each sample was measured at three incident angles to maximise the measurement sensitivity over the selected spectral range. Measurement results of a blue spot and a brown spot on the stainless substrate are presented below. Blue spot on stainless steel The Psi and Delta for the blue spot on the stainless steel are shown in Fig. 5, where interference oscillations over the measured spectral region are observed. This indicates the formation of a semi-transparent film, which allows the beam to transmit through the layer and to reflect at the interlayer and thus to interfere with the reflected beam from the top surface. The film thickness and optical constants were determined using a Cauchy dispersion model incorporated with thickness non-uniformity. It is seen from Fig. 6 that the film absorbs well at short wavelengths (higher k values) and transmits partially the visible lights (k is equal to zero for transparent film). The blue film thickness was determined as 73 nm, which is about 1/7 of the wavelength of a blue beam (490nm). Strong depolarisation was observed (Fig. 7), which indicates that the reflected beam is partially polarised due to the rough substrate surface or/and non-uniform film structure.

Generated and Experimental

Wavelength (nm)200 400 600 800 1000 1200

Ψ in

deg

rees

0

20

40

60

80

100

Model Fit Exp E 55°Exp E 65°Exp E 75°

(a)


Wavelength (nm)200 400 600 800 1000 1200

∆ in

deg

rees

-100

0

100

200

300

Model Fit Exp E 55°Exp E 65°Exp E 75°

(b)

Fig. 5 Ellipsometry measurements of blue spot on stainless teel, a) Psi, b) Delta

Fig. 6 Optical constants of blue on stainless steel, n – index,; k - extinction coefficient


Wavelength (nm)200 400 600 800 1000 1200

%D

epol

ariz

atio

n

0

10

20

30

40

50

60

Model Fit Exp dpolE 55°Exp dpolE 65°Exp dpolE 75°

Fig. 7 Depolarisation of blue spot on stainless

steel Brown spot on stainless steel For the brown spot, the depolarisation was found not significant, which indicates that the film is either scattering or absorbing sufficient enough to reduce the depolarisation. The model was a graded layer varying between two different Lorentz oscillators. The film absorbs well


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in short wavelengths between 400 nm and 500 nm as indicated by the high k values in Fig. 8. It is observed that the optical constants increase toward the surface (Fig. 9) indicating a complex film microstructure. The film thickness is determined to be 88 nm, which is about 1/7 of the wavelength of a brown beam (590nm). The visible spectrum is given in Fig. 10 as reference.

Optical Constants

Wavelength (nm)200 400 600 800 1000 1200

Inde

x of

refra

ctio

n 'n

'

2.1

2.4

2.7

3.0

3.3

3.6

3.9

brown_bottombrown_top

(a)

Optical Constants

Wavelength (nm)200 400 600 800 1000 1200

Extin

ctio

n C

oeffi

cien

t 'k '

0.0

0.3

0.6

0.9

1.2

1.5

1.8

brown_bottombrown_top

(b)

Fig. 8 Optical constants of brown spots on

stainless steel , a)n ; b) k

Depth Profile of Optical Constants at 500nm

Distance from Substrate in Å-200 0 200 400 600 800 1000

Inde

x of

refra

ctio

n 'n

'

Extinction Coefficient 'k'

2.1

2.4

2.7

3.0

3.3

3.6

3.9

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

nk

Fig. 9 Depth profile of optical constants of brown spot on stainless steel at 500nm

Fig. 10 The visible spectrum Stronger light interference happens with thicker transparent films (e.g. at ½ or ¼ λ). In the case of the laser-induced colours, the film is semi-transparent and thin (around 1/7 λ), and therefore the colour is observed with only slight interference effect. This is supported by the fact that the colours change only slightly when they are viewed at different angles. The strong depolarisation observed for the blue area and the changing refractive of index and optical extinction coefficients through the brown layer indicate that scattering, non-uniform and complex film microstructures may have been formed. It is natural to examine the microstructures of the coloured areas. 4.2 Film Microstructure 4.2.1 SEM analysis The surface morphologies of the original stainless steel and titanium as well as the coloured areas are given in Fig. 11 and 12 respectively. Laser treated-areas in both titanium and steel indicate that the surface has been molten and re-solidified to form larger clusters of crystals. As the photos give only 2-D views, the height variations in the new clusters cannot be seen.


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(a) (b)

Fig. 11 SEM photo of (a) original stainless steel, and (b) brown spots

(a) (b)

Fig. 12 SEM photo of (a) original titanium, and (b) brown spots 4.2.2 AFM analysis An Atomic Force Microscope (AFM) was used to map and analyse the surface structures of the laser-treated areas. Brown spot on titanium It is seen from Fig. 13a that the laser-treated area (left) has a granular structure with islands of grains. The islands are inhomogeneous in size with a mean value of 4.0 µm2 and a standard deviation of 4.3 µm2. The average roughness, Ra, of the treated surface is 31.5 nm. The peak to valley distance is around 400 nm (Fig. 13b). An enlarged image of an island is shown in Fig. 14, where the roughness (Ra) is 1.884 nm. These islands may be viewed as micro-mirrors that reflect light beams. The reflected beams are from different surfaces and angles due to the granular structure and surface imperfections and thus interfere to produce colours.

Green spot on titanium Similar to the brown spot, the laser-induced green area (left in Fig. 15a) has also granular structures (islands). The mean grain size is 4.1 µm2 with a standard deviation of 6.4 µm2. The islands have larger size variations than the brown islands as indicated by the larger standard deviation. The average roughness, Ra, of the treated surface is 91.3 nm. The peak to valley distance is around 551 nm (Fig. 14b). The more granular surfaces are the expected results of the increased number of laser pulses and thus the higher laser energy input. Grey-blue spot on titanium The laser-treated blue (left in Fig. 16a) has an average roughness, Ra, of 90.9 nm. The mean grain size is 6.4 µm2 with a standard deviation of 6.9 µm2. The peak to valley distance is around 889 nm (Fig. 16b). The increased number of laser pulses has resulted in the more granular surfaces than the green.


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(a) (b)

Fig. 13 (a) 3D view of laser-induced brown on titanium; (b) cross-sectional view of the brown area

Fig. 14 An enlarged view of an island, Ra =1.884nm

(a) (b)

Fig. 15 a) 3D view of laser-induced green on titanium; b) Cross-sectional view of the green area.

(a) (b)

Fig. 16 a) 3D view of laser-induced grey-blue on titanium; b) Cross-sectional view of the grey-blue area.


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The peak to valley distance was plotted against the colours as shown in Fig. 17. The value increases with darker colours (shorter wavelengths) and with the increasing number of laser pulses. At the given processing and substrate conditions, colours were determined by varying the number of laser pulses. As the surface becomes more granular with the increase in the number of laser pulses, light is more scattered or absorbed causing probable destructive interference or dark colours (e.g. blue). With the reduced number of laser pulses, the laser-treated area is less granular and therefore light is less scattered or more reflected causing probable constructive interference or bright colours (e.g. yellow brown and light green). AFM results for laser-induced colours on stainless steel are similar to those on titanium. The 3D profiles of a blue spot and the untreated surface are shown in Fig. 18. 5 CONCLUSIONS KrF excimer laser has been shown to

induce multiple colours on both titanium and stainless steel substrates.

Laser-treated surface has granular

structures as revealed by AFM. These structures are causes for light scattering and interference.

Semi-transparent films are formed as

revealed by ellipsometry. The thickness of the film affects the resultant colours due to constructive and destructive interference.

6 INDUSTRIAL SIGNIFICANCE The developed technology can be used for product identification and decoration such as marking company logos and creating colour patterns on personal items.

0200400600800

1000

1 2 3

Wavelength (nm)

Peak

to v

alle

y (n

m)

Blue490 nm

Green520

Brown590 nm

Fig. 17 Correlation of colour and degree of surface imperfections

(a)

(b) Fig. 18 a) 3D view of laser-induced blue, and

b) original stainless steel surface. REFERENCES 1 US patent 4,869,789. 2 Singapore Patent Application 200001597-

4. 3 Modern University Physics, James A.

Richards Jr., Francis Weston Sears, M. Russell Wehr, and Mark W. Zemansky, Addison-Wesley Publishing Company, INC., 1966, pp674 – 676.

Documents

2 Z Hongyu, Laser-Induced Colours on Metal Surfaces