Laser Color Marking of Metal Surfaces
Also it should be noted that the protective effect generated by the oxide film on the surfaces of some metals can be increased manyfold with the help of additional laser oxidation . CLM technology makes it possible to obtain both various colors on the metal surfaces and transparent protective coatings (Fig. 2b). Also modification of the optical properties of material surfaces can be implemented using the method of laser micro- and nanostructuring [5-10]. Such structuring can be applied in solar power engineering for the increase of receivers’ absorptance and in other types of detectors. In this paper the technology of change of surface optical properties is considered only at the expense of oxidation (without structuring).
Currently, the laser marking facilities are produced in batches. But the main problem which prevents from the implementation of CLM technology in industry is the absence of clear-cut and scientifically-based physical representations of the CLM process. Thus, in order to implement this method actually it is necessary to reveal the controlling parameters of the process of laser coloring.
In turn, it will make it possible to formulate the technical recommendations, algorithms and SW for the control of formation process and growth of surface structures with the required colorimetric (color) characteristics.
The objective of this paper is to find the controlling parameters of the process of metal surfaces laser coloring (oxidation) providing the one-to-one association with the surface color in given local region.
MATERIALS AND METHODS
In order to carry out experiments on study of the laser radiation action on metal surfaces the facility "Минимаркер–2" based on Yb pulse fiber laser was used (Table 1) . Choice of the experimental facility was determined by the considerable coefficient of radiation absorption by metals on the wavelength of 1.06 µm and its batch production. Irregularity of energy distribution in beam cross-section of the fiber laser is removed at the expense of homogenization of radiation density upon large overlap of laser spots when scanning on x axis (Lx>98%). Polished 10Х18Н10Т stainless steel plates (Table 2) (71% Fe; 0,1% C; 18% Cr; 10% Ni and small amounts of other addition elements) [14,15] with thickness of 0.7 mm and ВТ1 0 commercial titanium plates [16, 17] with thickness of 2 mm were used as the samples. Irradiation of the samples was performed under the regular laboratory conditions in air atmosphere. The properties of obtained samples were studied by the methods of optical microscopy (Zeiss Axio Imager A1M) and spectrophotometry (Ocean Optics CHEM4-VIS-NIR USB4000).
RESULTS OF WORK AND THEIR CONSIDERATION
Parameter of Action Which Determines the Surface Color
Results of the study confirmed the facts known to the experts – the color of processed metal surface, in other words, the thickness and chemical composition of the formed oxide film depends on the temperature to which the surface is heated up and time of action on the sample.
Using the method of sources in order to estimate the temperature of metal surface Т(Nх) upon laser irradiation with the series of Nх pulses the following formula can be attained .
where q0 is the density of radiation power,
τ is the pulse duration,
f is the repetition rate,
Тн is the initial sample temperature,
R is the reflectivity on wavelength of 1.06 µm,
k is the material thermal conduction,
a is the material thermal diffusivity.
When scanning the focused radiation beam with the diameter d the action time tx,y can be calculated on the basis of the following formula:
where Nx, Ny are the amounts of pulses in the focus region taking into account the overlap on х and y axes, Vck is the scanning speed, N is the resolution (lines by 1 mm).
Certain integral heating characteristic which takes into account the sample surface temperature T (Nх) generated by the action of pulses train and total heating time tx,y must be the parameter which characterizes the formation of oxide films with different structures and colors. In general case this characteristic has the form of the function Ц = f (T(Nx),tx,y). According to the results of experiments the specific form of chosen function for laser coloring is well described by the semiempirical temperature-time combination "Ц" :
Research suggested that change of the parameter "Ц" characterizes the order of occurrence of colors upon the laser coloring. Correspondence of color tones with the parameter "Ц" as well as composition of oxides generated on the stainless steel surface (calculated by the method of chemical thermodynamics ) and commercial titanium (according to )) are given in Table 3. Those intervals where the oxide films are formed with irregular coloring along the area are not specified in the table.
Mechanisms of Color Formation on Metal Surfaces under the Action of Laser Radiation
Two mechanisms of color formation on metal surfaces under the action of laser radiation are known: oxidation of surface with the formation of transparent interferential film coatings or new substances – color pigments [11, 22-24] and ablation of the metal causing the formation of micro- and nanostructures [5-10]. In our paper such laser modes were used where the beam power density was lower than the material vaporization threshold. Therefore, the coloring mechanism connected with the formation of regular reliefs or other microstructures can be excluded from the considerations.
Upon the laser heating of titanium or steel in air the transparent oxides will be formed on their surface (Fe2O3, Cr2O3, NiO and TiO2 respectively) (Table 3). Conditions of the formation of each oxide and thickness of occurred oxide films correspond to its temperature range. Therefore, the most probable reason for "laser" coloring of metal surfaces is the formation of interference films (commensurable with the wavelength by thickness) as a result of the chemical interaction of metals or alloys components with atmospheric gases upon their heating. It should be noted that upon the heating up to the temperature 1438–1497 °С titanium oxide (II) which has non-transparent golden-yellow color occurs on titanium surface .
Colorimetric Characteristics of Generated Films
In order to standardize the results of research the international system of color quantitative estimation XYZ was used . The following color equation is the foundation of this system: , where S is the color of the subject under study, XYZ are the main colors of the system, x´,y´,z´ are the color coordinates . In order to determine the color it is necessary to calculate the values of coordinates (defines the color brightness), , , (lie in the plane of zero brightness) and relative values of color coordinates and (characterize the radiation chromaticity: wavelength and color purity), is the normalizing factor. For this reason the spectral aperture reflectivity was measured within the visible range for each color. The light sources were selected, their spectral energy distributions S(λ) were found: source of A-type – standard of artificial light (color temperature of radiation of 2850 K, makes it possible to characterize the subjects colors correctly in premises); source of D-type – standard of natural light (color temperature of radiation of 6500 K, is the closest to natural day light). Thus, using the method of weighed coordinates the coordinates of color and chromaticity were determined by the example of stainless steel (Table 4).
Software Used for Implementation of the Color Laser Marking Technology Based on the Technological Facility "Минимаркер–2"
Algorithm and relevant SW which interacts with the software of the laser facility were developed for the marking process automation. There is capability of viewing in colors which were obtained for this material experimentally; also division of the image into black and white layers is provided for each color which can be input in SinMark program (software for Минимаркер–2). It relieves user from the necessity to apply external images-editing programs such as Adobe Photoshop, for instance. Examples of marking of the stainless steel and commercial titanium surface obtained using the suggested technology and software are shown in Fig. 1, 2 and 3.
COMPARISON OF CLM TECHNOLOGY WITH EXISTING COLORING TECHNOLOGIES
When practically applying the graphic information on the surface of metal components theremoprinting [1, 3], powder coating , anodic coloring  and CLM technology are used. Since the marking technologies first of all are designated for the batch production it is reasonable to compare them from the point of view of marketing. For this reason we will select some parameters which should correspond to the acceptable standards. First of all, the parameters which are stringent by construction and technological criteria: minimum size of the obtained element, amount of production stages, chromaticity (amount of potential colors and tones), capability of non-contact processing of material, stability of obtained image. They are characterized by the index of stringent parameters Iж. Secondly, flexible consumer parameters which reflect the psychological, esthetic or economic requirements of costumers (efficiency of the technological process). They are characterized by the index of flexible parameters Iм. Thirdly, it is necessary to take into account the market value of the technological facility, ecological cleanness of the technological process considering the volume and amount of chemical waste in the process of production and harm caused by them with respect to environment. Assuming that flexible and stringent parameters are equally important for the consumer the total index of competitiveness Iкс is calculated as the arithmetic mean value of indices Iж and Iм. Calculation methods are given in the paper  in details. Calculation data is given in Table 5.
Carried out marketing analysis shows that the CLM technology is capable to compete with modern technologies of colored image application on metal surface. It yields only to the thermoprinting technology but if there is necessity to attain the long-life image with high resolution and non-contact action on material and minimum environmental impact the CLM technology is absolute leader.
The technology of color laser marking of metal surface was considered by the example of stainless steel and titanium which makes it possible to change their optical properties within the visible range. This technology can be applied for the coloring of metals when it is necessary to attain the long-life image with high resolution upon non-contact action on material. The following application fields can be marked out among the suggested areas: advertising, jewelry production, engineering industry where it can be used as the mean for protection against corrosion (simultaneous surface color change and formation of oxide protective layer), as well as for protection against production falsification or information encoding etc.
Authors express gratitude to E.A. Shakhno, Professor, Doctor of Physical-Mathematical Sciences, and D.A. Sinev for the valuable consulting when performing the temperature calculations.
The work was performed with the support of Grant of the President of the Russian Federation on leading scientific school НШ–619.2012.2. and grant of the Russian Foundation for Fundamental Research 12–02–00974а.