Evolution of microstructures on stainless steel induced by ultra-short pulsed laser ablation

Ultra-short pulsed laser ablation of stainless steel is accompanied by the evolution of different microstructures. Depending on the fluence, accumulated energy and number of laser passes cones from impurities, laser induced periodic surface structures, cone-like protrusion (CLP), and thermal bumps evolve at the surface. These often unwanted morphologies can be induced or inhibited by carefully choosing the strategy and laser parameters. The investigated range reveals a small processing window for defined 515 nm sub 1 ps ablation leading to low surface roughness using circular polarization. Hitherto, the origin and dependencies of CLP are still not well understood and for the first time a precursor ripple structure reported. These precursor ripples reveal supra-wavelength periodicity with about 2 mu m spacing and evolve earliest after the second layer of ablation. Potentially, low spatial frequency laser-induced periodic surface structure generated with the first laser pass with pulse and hatch overlap are the root cause of CLP evolution. Moreover, the CLP growth is grain orientation and strongly polarization state dependent. Preferentially, CLP start to evolve at the {110} planes of the face-centered cubic crystals of the inspected austenitic stainless steel and linear polarized laser radiation revealing a 1:1 aspect ratio of 10 mu m. A nanoindentation study at the interface near region on cross-sections reveals robust mechanical properties of this CLP structure.