Effect of Laser Fluence and Scanning Strategy on Topography and Morphology of Femtosecond Laser-Structured Stainless-Steel Surfaces

Laser-induced, self-organized surface structures offer substantial functional advantages across a wide range of applications, including tribological, biomimetic, and medical applications. Among the key processing parameters governing the formation of laser-induced self-organized surface structures, laser fluence, pulse overlap (PO), and line overlap (LO) are particularly important. This study systematically investigates their influence on the morphology and topography of femtosecond laser-structured stainless-steel surfaces. For this purpose, the samples were irradiated with a 300 fs laser emitting linearly polarized light (λ = 1030 nm). Scanning electron microscopy revealed four surface morphologies: laser-induced periodic surface structures (LIPSS), micrometric ripples (MR), micro-crater structures, and pillared microstructures. Surface roughness measurements and morphological analyses revealed that an increasing PO promotes the transition from LSFL- and MR-dominated morphologies to crater and pillared microstructures at substantially lower fluences than increasing LO. The strong-ablation regime was therefore reached at lower fluences, resulting in earlier formation of complex hierarchical microstructures and a maximum surface roughness of Sa = 13.46 µm, compared with Sa = 6.90 µm obtained by varying LO. These findings demonstrate that PO has a stronger influence on local heat accumulation than LO and thus plays a more prominent role in governing the evolution of surface morphology and roughness.