Formation of linked nanostructure-textured mound-shaped microstructures on stainless steel surface via femtosecond laser ablation

Linked nanostructure-textured mound-shaped microstructures (termed N-mounds) were induced by femtosecond laser on AISI 443 stainless steel surface in air via multiple raster scans. The formation mechanism of the linked N-mounds is discussed in this paper. With increasing number of raster scans, four stages of the linked N-mounds' formation are identified. First, femtosecond laser-induced periodic surface structures (FLIPSSs) are produced on the stainless steel surface. Then precursor protrusions are generated on the FLIPSSs because of unhomogeneous spatial distribution of laser energy caused by modifications of surface morphology. Further increasing the number of raster scans, precursor protrusions will grow into randomly distributed N-mounds owing to preferential ablation resulted from geometric effect. Finally, linked N-mounds gradually form as N-mounds' growth reduces the width of the valleys between N-mounds. Surface morphology will not change significantly with increasing number of raster scans once the linked N-mounds are formed. Besides, the N-mounds are covered with concentric ripples and irregular nanostructures as a result of redeposition of ablated materials which is also the reason why the linked N-mounds rise above the surface formed in the third stage. (C) 2014 Elsevier B.V. All rights reserved.