Influence of unidirectional polishing on the formation of laser-induced periodic surface structures on steel

We have performed a detailed analysis of the dependence of the regularity of laser-induced periodic surface structures (LIPSS) on the surfaces of steel samples that have been ground (polished) to achieve unidirectional surface roughness. Two low spatial-frequency LIPSS data sets were created by scanning the steel surface with femtosecond laser pulses using field polarisation perpendicular and parallel to the direction of the initial linearity of the surface roughness. The images were analysed by SEM, and a database of LIPSS characteristics was created using a digital image processing module. It was found that the regularity of the LIPSS depends on the polarisation direction of the scanning beam with respect to the linearly polished surface. A statistically significant result was evaluated: scanning perpendicular to the direction of the initial roughness leads to more regular low spatial frequency LIPSS. Compatibility with electromagnetic scattering theories is demonstrated.