Influence of the Two-Stage Femtosecond Laser Processing on AISI 321 Surface Roughness and Optical Parameters

The study is devoted to determining the degree of influence of the first and second stages of AISI 321 steel surface treatment with a femtosecond laser, with unchanged laser parameter characteristics, on the blackening parameters and parameters characterizing the distribution of surface heights according to the ISO 25178 standard. Surface blackening is important for production automation for better visibility of markers by optical sensors. The assessment is carried out from the point of view of changing the degree of blackening of the studied surface. During the experiment, it was found that secondary surface treatment without changing the processing mode leads to an insignificant, up to 5%, increase in the degree of surface blackening. Secondary laser processing revealed a diminishing returns effect, where doubling the energy input in perpendicular scanning resulted in only a marginal (~5%) gain in blackening. This phenomenon stems from surface morphological saturation, as the primary roughness parameters Sq and Sdq attain their plateau values, without contributing to the further formation of hierarchical light-trapping structures. It was also found that during the blackening process, such parameters as the maximum peak height Sp, the ten-point surface height S10z, and the asymmetry Ssk increased more than others. After repeated treatment, the values of the parameters maximum valley depth Sv and the root mean square slope Sdq increased the most. At the same time, the nature of the normal Gaussian surface height distribution was preserved. As the Sdq value increases, the number of randomly located reflecting surfaces increases, and this leads to better scattering of the directed beam. On the other hand, the absence of random fragments on the vertices of the periodic surface structure, which corresponds to a high Sku index, allows such a surface to scatter light more effectively.