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A technology of laser-induced coloration of metals by surface oxidation is demonstrated. Each color of the oxide film corresponds to a technologic chromacity coefficient, which takes into account the temperature of the sample after exposure by sequence of laser pulses with nanosecond duration and effective time of action. The coefficient can be used for the calculation of laser exposure regimes for the development of a specific color on the metal. A correlation between the composition of the films obtained on the surface of stainless steel AISI 304 and commercial titanium Grade 2 and its color and chromacity coordinates is shown.
© 2014 Optical Society of America
1. Introduction
Laser pulse heating of metal surfaces in air gives a unique possibility to control the composition and topology of formed oxides. It allows the modification of varied properties of the surface, such as optical, tribological and others, and can find different applications [1]. Laser structuring (by ultrashort pulses) or oxidation of the surface can induce change of its optical properties in the visible range [2,3]. A technology of laser processing by femtosecond pulses provides an opportunity to create various colors at metals, resulting in the control of optical properties in the range from UV to THz [2]. Formation of polarization dependent structures, which can generate specific color patterns, was presented in [3] and a technology of laser coloration was developed. Enhancing of laser marking technology (high resolution with greater color palette) can be used for identification of metal products and its counterfeit protection [3], information encoding, as well as extra security features of banknotes and bonds papers [4].
Under the action of pulse fiber [5,6] and Nd:YAG [4] lasers a surface color is governed by an oxidation. Such lasers are preferable to use for industrial introduction of the technology of color laser marking (CLM) rather than surface structuring by ultrashort laser pulses. Among the key factors, designating this option, it is possible to mention a lower cost of such devices along with its robustness, compactability and simplicity of operation. Surface oxidation of some metals (for example, titanium) not only leads to its coloration but provides for a good adhesion of resulting oxide films as well [7], which produces a constant corrosion-preventing effect. It should be noted that recently a great attention is paid in scientific periodicals to an effect of surface laser coloration [2–7].
In this paper, we present the change of the metal surface reflectance in the visible range due to oxide films formation when exposed to radiation of fiber laser (CLM technology). Although there is a considerable amount of works on the specified subject, the following major problems remain. For different papers, there are substantive divergences at determining of composition of resulting films while they are produced on the same metal and correspond to the same color. The main reason for such disagreement is a difficulty of an assignment of results obtained by various experimental techniques. In principle, the well-defined process parameter responsible for metal color after laser processing is absent. For this reasons, the goal of the present paper is research and development works on the method of a controllable change of a metal surface color by means of a local laser oxidation.
2. Experimental section
Stainless steel AISI 304 and commercial titanium Grade 2 50 mm x 50 mm plates with thickness 0.7 mm and 1 mm, correspondingly, were used. The laser exposure was made in air with the use of a commercially available machine (Minimarker) on the base of ytterbium pulse fiber laser with λ = 1.06 µm wavelength, which generates pulses of τ = 100 ns duration at repetition rates in the range of f = 20-99 kHz (Fig. 1a). The laser spot with a focal diameter of d0 = 50 µm was moved over the sample surface with a velocity Vsc from 1 to 250 mm/s. The distance to the sample surface is variable due to automatic move of the focusing lens, but such adjustment can be done before the scanning process only. Laser intensity I0 was from 0.85 to 2.91 × 107 W/сm2. The choice of the setup is based on the sufficient absorption of radiation at λ = 1.06 µm by metals and its serial production. Energy nonuniformity in the beam of fiber laser (М2 = 2) was averaged by means of high value of overlapping along the X-axis (Lx>98%). Laser coloring of metal can be done by line-by-line scan of its surface by the sequence of laser pulses at given intensity (I) and overlapping along the X- (Lx) and Y-axis (Ly) (Fig. 1b). It should be noted that the same color could be made with different combinations of above-mentioned characteristics.
Fig. 1 a) Experimental setup on the base of ytterbium pulse fiber laser. b) Line-by-line strategy of the laser beam scanning on the metal surface and layout view of overlapping along the X- (Lx) and Y-axis (Ly). Dashed line – laser generation is absent.
Obtained samples were examined by means of optical microscopy (Zeiss Axio Imager A1M). In order to characterize the color of the samples, reflectance spectra in the range 300–800 nm were measured with a spectrophotometer. From these spectra, the standard CIE 1931 chromaticity coordinates of the different colors were calculated.
3. Results and discussion
3.1. Connection between a surface color and laser processing parameters
Obtained surface color is an “aggregated” color of several separate microscopic regions. The reason of this phenomenon consists in a nonuniform local heating, generated by inhomogeneous distribution of intensity in the beam and temperature change during a scanning time. For all experiments maximum values of overlapping along both the X-axis and Y-axis (from 80.0 to 99.9%) and optimal values of intensity (I ~2.91 × 107 W/сm2 for steel, and I ~1.24 × 107 W/сm2 for titanium) which allow exposure below an evaporation threshold was used.
Metal oxidation in air is a heterogeneous physicochemical process that includes several stages from which counter diffusion of oxygen and metal atoms through initial oxide film is the slowest. It is known, temperature and time govern the diffusion. For this reason it is possible to assume that surface color is defined by a certain physical parameter C, which is a function of the temperature T(Nx) and the effective time of action teff_x,y: C = f (T(Nx),teff_x,y), where Nx is a number of pulses during the scanning along the X-axis (there is no heat accumulation during the scanning along the Y-axis since next line has enough time to cool down before beginning of an exposure).
Maximum surface temperature (along the X-axis) of the sample after an exposure by sequence from N pulses during a scanning was estimated from the solution of the heat conduction equation [8]. The following formula corresponds to the multishot irradiation by rectangular light pulses with a uniformly distributed intensity in the spot and disregards the change of material reflectivity during exposure [Eq. (1)]:
where t(Nx) = Nx/f + τ; R - reflectivity (at λ = 1.06 μm); a - thermal diffusivity; k – thermal conductivity.Effective time of action per unit of a surface coincident with the area of a laser beam in a focus of an optical system can be calculated by formula [Eq. (2)]:
where Ny – number of pulses per spot with diameter d0 during the scanning along the Y-axis.It was found from preliminary tests that a formation of the same surface color is possible due to heating at a lower temperature but with a longer duration or a smaller duration but at a higher temperature. On this basis the technologic chromacity coefficient Сtech, which corresponds to the product of temperature and exposure duration was introduced empirically [Eq. (3)]:
Consequently, it is possible to calculate the treatment regimes (pulse duration, intensity, frequency and scanning velocity) for the formation of a defined color of the metal on the base of the value of the technologic chromacity coefficient Сtech for a given metal. It should be mentioned that the coefficient is applicable for pulse exposure by fiber laser, its upper value is limited by the material evaporation threshold, lower limit is specified by the requirement of material heating up to oxidation point. In Fig. 2, there are sample colors with corresponding ranges of the chromacity coefficient Сtech for stainless steel (a) and commercial titanium (b).
3.2. Spectral and color properties of the films
Material reflectance spectra before and after laser treatment was measured by Ocean Optics CHEM4-VIS-NIR USB4000 spectrophotometer (signal-to-noise ratio - 300:1; integration time - 20 ms; slit - 5 μm x1 μm) and presented in Fig. 3 for color identification. A halogen lamp served as a light source (15 V, 360-2000 nm), and reflection probe (light and probe fiber diameters are 200 μm) was used for illumination and light collection. International standard CIE 1931 chromaticity coordinates [9] of the different colors at the illumination of the A-type light source are calculated and presented in Fig. 3. Also, the criterion of color reproduction accuracy ΔEab is calculated similar to [5] (see Fig. 2).
Fig. 3 Reflectance spectra and samples of AISI 304 steel (St, (a)) and titanium Grade2 (Ti, (b)) before (St1, Ti1) and after laser treatment. For the main colors CIE 1931 chromaticity coordinates and corresponding Сtech values are: St2 - Ctech = 2.82 °C ×ms (X = 100.00, Y = 100.00, Z = 99.99, x = 0.33, y = 0.33), St3 - 6.83 °C × ms (81.62, 81.27,70.46, 0.35, 0.35), St4 - 14.72 °C × ms (29.72, 29.43, 36.08, 0.31, 0.31), St5 - 53.25 °C × ms (22.62, 19.89, 16.01, 0.39, 0.34), St6 - 78.62 °C × ms (7.74, 8.27, 6.53, 0.34, 0.37); Ti2 - 23.89 °C × ms (25.82, 25.32, 20.11, 0.36, 0.36), Ti3 - 43.64 °C × ms (20.65, 18.93, 6.60, 0.45, 0.41), Ti4 - 72.78 °C × ms (8.40, 6.02, 0.60, 0.56, 0.40), Ti5 - 102.32 °C × ms (3.63, 2.68, 4.27, 0.34, 0.25), Ti6 - 198.63 °C × ms (17.00, 17.01, 18.60, 0.32, 0.32).
Consequently, with the knowledge of a range of the chromacity coefficient Сtech connected with the selected color for specific metal it is possible to choose laser processing parameters to obtain the color. It also should be mentioned that in the case of stainless steel brown and purple colors are not shown between gold and blue colors. The films of such colors do not form a coating providing a sensation of an integral color. There are no red and green colors on the surface of titanium. It appears that these phenomena can be attributed to the film thickness, which becomes too great for the origin of interference effects inside the film.
3.3. Composition of obtained films and mechanism of the color formation
Previously authors performed a thermodynamic calculation at a temperature Tmax upon which the composition of films obtained on the steel [10] and titanium [11, 12] under laser action with a periodic pulse train of nanosecond durations at sufficiently high overlapping of pulses (80-100%) in the normal atmosphere was determined. Usually such approach is applicable for continuous processes. Certainly, in the case of pulse-frequency processes reaction yield determination accuracy will be constrained by temporal and spatial temperature variations in the reaction space. At the same time it was shown in [13], that in the sense of the resulting oxide thickness a laser pulse heating is equivalent to a heating by continuous radiation at a maximal temperature Tmax on the given time duration, which is maintained through a corresponding equivalent time. Simulated results were in line with the experimental data provided by energy-dispersive x-ray spectroscopy [10] as well as with results published in other papers (please, look at the references 15-18 at [10]).
At the initial stage of an exposure (after several passes of laser radiation), there are no limitations for interaction between air components and metal. In the following layer-by-layer growth of the generated film takes place with an increase of a number of passes. It is obvious that the thicker film, the more limited diffusion of air components to a metal becomes. For this reason, the composition of the top layer differs from underlying layers (multilayer films formation becomes possible). It was shown [10], that at the initial stage of the exposure chrome (III) oxide grew on the surface of AISI 304 steel since chrome had a greater affinity than iron (Table 1).
Table 1. Composition and structure of the films obtained on the surface of steel and titanium after pulsed laser exposure
A yellow color (St3, Fig. 3) is accounted for interference effects at thin layer of chrome oxide. With the increase of a number of passes, there is a tendency for the formation of a double layer oxide structure, which lower layer consists of a spinel oxide FeCr2O4 with Fe2O3 and NiO admixtures, and upper layer is a ferric oxide. It should be noted, that FeCr2O4 oxide at lower layer has much greater attenuation coefficient (lower transmittance and grey color), than the ferric oxide at the upper layer. Therefore, both interference effects at upper oxide layer and the intrinsic color of the lower oxide have an impact on the integral color (St4-6, Fig. 3). In the case of titanium almost the same mechanism takes place (Table 1). It seems that the silver color (St2 and Ti2, Fig. 3) has another way of an origin. It is an intrinsic color of the metal which becomes apparent after the laser cleaning of the metal surface from biological and organic impurities (there is no formation of noticeable amounts of oxides due to low temperature during an exposure for the case under consideration) [14].
4. Color laser marking technology by oxidation
CLM technology based on the oxidation of metal by laser pulses at λ = 1.06 µm wavelength with nanosecond durations and software for Minimarker equipment were developed [15]. The software algorithm is based on the known principle [3]. For the formation of a defined color of a metal surface, it is possible to calculate the treatment regimes on the base of the value of the technologic chromacity coefficient Сtech for the given metal or the software of Minimarker equipment can do it automatically.
CLM technology can be easily adapted for another metals and alloys, where oxide films formation under laser exposure is possible. Examples of images marked on the surface of stainless steel and titanium by means of the CLM technology are presented in Fig. 4.
Fig. 4 Images obtained on stainless steel (a - QR-code generated by Microsoft Tag software, b - surface of penknife blade with the logo where there are microscopic-sized identification mark “ITMO”) and titanium (c) by the CLM technology.
5. Conclusion
Technology of a controllable deposition of a color image on the metal surface by its oxidation is presented. The paper introduces the chromacity coefficient Сtech, which makes it possible to determine parameters of laser processing to get a necessary color on the surface. Surface temperature Т(Nx) and effective time of action teff_ x,y govern it. Reflectance spectra of the main colors that can be generated by the technology are presented. Color properties of the processed surface are measured for the different light sources as well. The CLM technology has a high potential to implement in industrial-scale production.
Acknowledgment
Authors are very grateful to Prof., Dr. Phys.-Math.Sci. Shakhno E.A. and Sinev D.A. for consulting about conducting of temperature calculations. The research is supported by RSF agreement Nº 14-12-00351.
References and link
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