In this study, we investigate the laser-induced structural coloring of titanium nitride (TiN)-coated stainless steel substrates via direct laser writing. As a noncontact surface modification method, laser processing offers several advantages over conventional coating techniques, such as ease of operation, high processing efficiency, and eco-friendliness. However, research on the laser processing of thin-film-deposited metal surfaces is at its early stage and the color formation speed remains slow. Unlike the conventional coating technologies physically depositing film layer, the magnetron sputtering coating technology can form uniform nanoscale films on stainless steel surfaces. We use a nanosecond fiber laser to prepare coloring patterns directly onto a TiN-coated stainless steel surface, creating a distinctive structural color. Through appropriate experiments, the effects of three factors—laser scanning speed, scanning interval, and output power—on pattern colors are assessed independently. The reflectance spectra and energy spectrum of structural color pattern and temperature analysis provide insights into the interaction between the laser and stainless steel surface. The mechanism of the laser-induced structural colors on the coated stainless steel surface is identified. The structural colors are not angle-dependent.

In this study, we investigated the effects of the scanning speed, scanning interval, and output power on the sample surface while setting the other parameters as constant. We calculated and analyzed the variation trends of the pattern color with respect to these three parameters. First, by varying the scanning speed in the range of 100?300 mm/s, we analyzed its effect on the surface structural color. Second, by setting scanning intervals of 0.02, 0.05, 0.07, and 0.10 mm, we observed the micro-nanostructures of the surface and discussed the effect of scanning intervals on the surface structural color. Next, for a fixed focal length of 156 mm, scanning speed of 100 mm/s, repetition frequency of 20 kHz, scanning interval of 0.05 mm, and an output power range of 46%?66% (in 4% steps), the trend of the structural color changes was investigated and the effects of output power were analyzed. Based on the experimental data, we determined the parameter range of the surface structural colors. By examining the microstructure and energy spectrum of the sample surface, we investigated the mechanism of color formation. Finally, we verified the color stability.

First, we discuss the effect of the laser parameters on the sample surface color. At low scanning speeds, a transition from yellow to violet, then to blue, and finally to silver-gray is observed, and the color transition is relatively fast. With the increase in the scanning speed, the color becomes lighter and the color transition speed decreases owing to a decrease in the total laser output energy (Fig. 2). Second, at a small scanning interval, the iridescence of surface structures at a scanning interval of 0.02 mm is observed under a microscope (Fig. 3). Under the conditions of the other parameters being set as constants while varying the output power, only the color brightness changes with increasing output power (Fig. 4), and yellow, violet, and blue are observed. This is the reason why the nonoverlapping laser beam part reduces during pattern marking. Based on the above experiments, different color-mixing techniques are used to obtain coloring patterns on the sample surface (Figs. 5 and 6). Table 1 summarizes the coloring parameters obtained from the above experiments, and the reflectance spectra of the five colors are measured (Fig. 7). Finally, based on the experiments, the microstructure of the sample surface is observed via scanning electron microscopy (Fig. 8). According to the elemental content analysis (Table 2), the presence of Cr, Mn, Fe, and Ni in the energy spectroscopy test results confirms the laser radiation penetrating the TiN coating and interacting with the stainless steel surface, which results in the melting of the stainless steel surface and its penetration into the TiN coating (Fig. 9). Thus, we conclude that the color formation on a metal surface is primarily dependent on the color of surface oxides or nitrides.

Basic structural colors are obtained via direct writing on the surface of a TiN-coated stainless steel plate using a nanosecond fiber laser. The results show that the metal surface color changes regularly with the scanning speed, scanning interval, and laser output power. The main colors are yellow, pink, blue, purple, and green. Various color-mixing techniques and unique color patterning can be used to produce consistent and clear images. However, the wide range of wavelengths for each color can result in color mixing, which affects the purity and saturation. In the green region, fine patterns obtained by laser scanning are observed, particularly for the laser-induced periodic surface structures (LIPSSs) with a period of approximately 1 μm. Subsequent energy spectrum analysis and temperature field calculations imply the presence of Ti, N, O, Cr, Mn, Fe, and Ni on the metal surfaces, which confirms that the laser radiation interacts with the stainless steel surface through the TiN coating, thereby melting and penetrating it. The color of the TiN coating or stainless steel surface under laser irradiation is primarily dependent on the color of surface oxides or nitrides and the LIPSS. The LIPSS occupies a smaller area than the oxide color area and is not the primary coloring mechanism that is affected by the angle or light source.