Mechanisms of the formation of low spatial frequency LIPSS on Ni/Ti reactive multilayers

The present paper aims at investigating the mechanisms of imprinting LIPSS (laser-induced periodic surface structures), arrangements of parallel ripples with a periodicity slightly smaller than the radiation wavelength, on metallic surfaces. To this end, Ni/Ti multi-layered samples produced by magnetron sputtering were textured with LIPSS using a 1030 nm, 560 fs pulse duration laser and pulse frequency of 1 kHz, and the resulting surfaces were investigated by scanning and transmission electron microscopies. The results obtained show that the core of the ripples remains in the solid state during the laser treatment, except for a layer of material about 30 nm thick at the valleys and 65-130 nm thick at the top of the crests, which melts and solidifies forming NiTi with an amorphous structure. A layer of ablation debris composed of amorphous NiTi nanoparticles was redeposited on the LIPSS crests. The results achieved indicate that the periodic variation of the absorbed radiation intensity leads to a variation of the predominant ablation mechanisms and, consequently, of the ablation rate, thus explaining the rippled surface topography. The comparison with theoretical predictions suggests that in the intensity maxima (corresponding to the valleys) the material is removed by liquid spallation, while at its minima (the crests) the predominant material removal mechanism is melting and vaporization. These results support Sipe et al LIPSS formation theory and are in contradiction with the theories that explain the formation of LIPSS by convective fluid flow or self-organized mass transport of a laser-induced instability.