Prospects of "Colorit" Technology of Color Laser Labeling
It is known that when certain metals are heated in air, an oxide film is formed on their surface, due to the interference of light where the color changes. Based on this effect, the technology of color metal laser of  (commercial name "Colorit" ) was developed. The technology is an analog of monochrome laser engraving in color and makes it possible to obtain a color stable image with a high resolution on a metal surface with a high degree of protection against falsification. As a rule, color laser labeling is carried out using a pulsed fiber laser [4–6], which provides the necessary stability of the space-time radiation characteristics. An example of such a system is a compact precision marker "MiniMarker 2" by LLC "Laser Center" . The integral color of the surface of metals after laser exposure depends both on interference effects in the upper oxide layer (e. g., TiO2 in the case of titanium treatment) and on the intrinsic color of the lower oxides (e. g., TiO, Ti2O3) [8–9]. Thus, only metals oxidized in air and alloys, e. g., stainless steels, titanium and copper alloys, refractory metals, can be stained.
IS THE LASER STAINING OF PRECIOUS METALS POSSIBLE?
There is a need to adapt the laser staining technology to non-oxidizing metals, e. g., precious metals. One possible solution to this problem is the staining of precious metals due to surface plasmon resonance (SPR) on the nanoparticles formed by laser irradiation in the air, which was well demonstrated in  by means of ultrashort laser pulses. In this paper, to implement the proposed mechanism, it was decided to use a fiber laser of nanosecond duration. After laser heating of the metal surface above the evaporation temperature, spherical nanoparticles are formed, deposited from the vapor cloud on the surface being treated. Due to the SPR effect in the obtained nanoparticles (10–50 nm), selective absorption of the incident white light occurs, while the reflected wavelengths give the surface the desired color (Fig. 1A). The images from the scanning electron microscope (Fig. 1B) allows us to see that the shape and size of the nanoparticles, as well as the distance between them depend on the color of the silver surface. And with a decrease in the distance between nanoparticles, the minimum of the reflection spectrum shifts to the ultraviolet region (Fig. 1B). Examples of the images obtained on the silver surface are shown in Fig. 1D.
IS IT POSSIBLE, WITH THE HELP OF A LASER, TO CREATE A PROTECTIVE "HOLOGRAPHIC" IMAGE DIRECTLY ON THE METAL SURFACE?
One of the labeling functions is to protect the product from falsification. The technology of direct laser structuring of metals by excitation of a surface electromagnetic wave (SEW) allows creating color images similar to protective holograms.
The recording of the "hologram" can be implemented due to the formation of surface periodic structures (SPS) with a period on the order of the wavelength of the laser radiation directly on the metal surface due to interference of the incident light with the excited SEW . The direction of the structures formed by this method is perpendicular to the direction of polarization of the laser radiation. In this paper, we demonstrate the recording of a SPS using a fiber-optic pulse laser with nanosecond duration. A change in the direction of polarization of the laser radiation directly during processing makes it possible to obtain the effect of the motion of individual elements (this effect is shown in Figure 2 as a displacement of the horizontal line (region Б) with a change in the angle of view). When recording individual elements of a pattern with a different direction of the SPS, the effect of the presence of several images when the angle of view is changed (region A) is achieved. Thus, a laser-induced security identification tag contains both visually observable and hidden features for authenticity verification. The visual signs include: changing the type and color of the image, as well as its displacement when changing the angle of view. The hidden signs of authenticity include the presence of different types of periodic structures (grooves, recessions) when viewed under a microscope with magnification of more than 100Ч and the exact coincidence of colorimetric coordinates with a previously developed template as a result of reading images on an office scanner (for more details see ).
IS IT POSSIBLE TO PROVIDE COLORED LASER LABELING EFFICIENCY COMPARABLE TO THAT OF THE MONOCHROME ENGRAVING?
There is a problem of introducing color laser labeling technology into the industrial production due to its low efficiency. For example, the logo labeling time with an area of 3 cm2 at a pulse repetition rate of 90 kHz is 6 minutes 55 seconds (Figure 3B), while the time for labeling the same logo using already widely used laser engraving is 30 seconds (Figure 3A). In order to bring the color labeling performance closer to monochrome engraving, the authors proposed to switch to quasi-continuous mode of operation and use modes with increased frequencies (up to 1 MHz). Currently, it was possible to reproduce a wide range of colors in the mode of operation with high frequencies, which significantly reduced the processing time of the metal surface. For example, the time for labeling the logo under consideration at a pulse repetition rate of 900 kHz was 1 minute 9 seconds (Figure 3B).
IS IT POSSIBLE TO CORRECT THE COLOR OF THE IMAGE IN THE CASE OF FAULTS DURING LABELING?
During the laser labeling of the product, a fault may occur (the color of the image will have incorrect colorimetric coordinates). Due to the layer-by-layer build-up of the oxide film (repeated passages of the laser), it is possible to correct the colors in the course of the laser exposure (see Figure 4: 1 designated the desired color, 2 designates the color obtained after laser exposure, in the second pass in mode No. 3 it is possible to obtain color No. 4 according to the colorimetric coordinates similar to those of color No. 1). Thus, by the method of layer-by-layer build-up of an oxide film, it is possible to change the color of the image during the laser staining process.
WHAT IS THE CHEMICAL AND TEMPERATURE RESISTANCE OF COATINGS OBTAINED BY "COLORIT" TECHNOLOGY?
Research has been carried out to determine the chemical resistance of color coatings obtained using the "Colorit" technology. As a material for the study, titanium (Grade 2) and steel (AISI 304) plates were chosen, as the most commonly used construction materials, with six primary colors. During the tests, the samples were placed for 48 hours in the following substances: H2SO4 (40%), NaOH (10%), C2H5OH (98%) and surfactant "New ultrasil 69". It was found that the coating on titanium is not resistant to alkali, and the coating on steel is not resistant to acid (Fig. 5). In the same manner, the investigations of the stability of coatings in a thermal chamber were carried out. The coating on titanium is stable at any temperature from –40 °C to + 100 °C and high humidity from 70% to 90%. The coating on steel proved to be unstable at 100 °C and 90% humidity. Thus, the results of this research should be taken into account when implementing this technology in the actual production.
The proposed technology is of great practical importance for applying a color wear-resistant image to the surface of metals (stainless steels, titanium, copper alloys, precious and refractory metals, etc.) with good resolution without using dyes, labels, etc. The applied identification mark has several degrees of protection both visually observable (changing the type and color of the image, its displacement when changing the angle of view) and hidden ones (availability of different types of periodic structures when viewing them under a microscope, the exact coincidence of colorimetric coordinates with a previously developed template) that meet the basic requirements for protective holograms. Thus, the labeling provides multi-level protection from product falsification. It should be noted that the technology of color laser labeling is implemented with the help of fiber-laser complexes widely implemented in industrial production, and its efficiency is comparable to that monochrome laser engraving, which is used by many Russian enterprises.
The technology already finds its application in many areas of industry: staining of components and labeling instrument panels in machine and instrument engineering (Fig. 6A), printing of logos of enterprises on souvenir products (Fig. 6B), creating a protective non-toxic coating on medical products made of titanium (Fig. 6B), as well as the creation of products with a unique design in jewelry (Fig. 1D), modern art (Fig. 6D) and much more.