Impact on surface topography of interference patterned steel and brass using an ultrashort pulsed laser source

Abstract

According to the Organization for Economic Cooperation and Development in Paris, the overall economic damage caused by counterfeiting worldwide is estimated at around 250 billion euros per year. In addition to the economic damage, counterfeit drugs or safety-relevant components endanger costumers. Individual forgery-proof laser marking of each product in a batch enables a digital product identification for all consumers in the future. Direct laser interference patterning, also known as laser interference structuring, can be used to apply a unique pattern matrix to surfaces, in particular, products. Each element of this matrix with diameters of an ablation spot could be provided with a defined periodic micropattern before shipment, resulting in a pattern-dependent interaction with incident light. The application on different surfaces can be realized in one process step using an automated spatial period variation under ambient conditions. In the case of two-beam interference structuring using two partial beams, line-like structures can be patterned. The spectrum of incident light, the angle of view, and the spatial periodicity in the nano- and micrometer range determine the color perception of the human eye. In this study, an ultrashort pulsed laser radiation of 10 ps and a wavelength of 532 nm are used for ablation of line-like spatial periods of 1.00, 1.25, and 1.50  μm under ambient conditions. The texture quality is analyzed by atomic force microscopy and scanning electron microscopy. Two materials with similar densities but different thermal conductivities and thermal diffusivities, stainless steel and brass, are structured and analyzed. Scanning electron microscopy images show the influence of heat-affected zones on texture. Electron backscatter diffraction analyses characterize the influence on the microstructure in the marginal regions.