Investigating and understanding the effects of multiple femtosecond laser scans on the surface topography of stainless steel 304 and titanium

Abstract

The majority of studies performed on the formation of surface features by femtosecond laser radiation focuses on single scan procedures, i.e. solely manipulating the laser beam once over the target area to fabricate different surface topographies. In this work, the effect of scanning stainless steel 304 multiple times with femtosecond laser pulses is thoroughly investigated over a wide range of fluences. The resultant laserinduced surface topographies can be categorized into two different regimes. In the low fluence regime (F Σline,max < 130 J/cm 2 ), ellipsoidal cones (randomly distributed surface protrusions covered by several layers of nanoparticles) are formed. Based on chemical, crystallographic, and topographical analyses, we conclude that these ellipsoidal cones are composed of unablated steel whose conical geometry offers a significant degree of fluence reduction (35-52%). Therefore, the rest of the irradiated area is preferentially ablated at a higher rate than the ellipsoidal cones. The second, or high fluence regime (F Σline,max > 130 J/cm 2 ) consists of laser-induced surface patterns such as columnar and chaotic structures. Here, the surface topography showed little to no change even when the target was scanned repeatedly. This is in stark contrast to the ellipsoidal cones in the first regime, which evolve and grow continuously as more laser passes are applied. Keywords

Experimental explanation of the formation mechanism of surface mound-structures by femtosecond laser on polycrystalline Ni60Nb40

“…Furthermore, the lack of significant heating is consistent with BSG-mound formation via the PVA mechanism. 6,34,35,37 PVA occurs due to higher laser fluence at the valleys than the peaks of precursor mounds. This difference in fluence is a combination of different incident area and reflection of incident photons toward the valleys.…”

“…49,50 Besides fluid flow, VLS growth, where laser-processed structures receive deposition of target material from a vapor cloud produced during the ablation process, could have contributed to forming this top amorphous portion. 6,33,34,51 Utilizing FIB milling to observe the microstructure within the mounds, we provided direct evidence of the formation process of BSG-and ASG-mounds created on polycrystalline Ni 60 Nb 40 during FLSP. Cross sectional imaging of ASG-mounds revealed an amorphous portion at the top of the mound and the bulk polycrystalline Ni 60 Nb 40 microstructure at the bottom.…”

“…NC-pyramids form when additional laser pulses on LIPSS create small precursor cones that are believed to originate from localized increases in ablation threshold. 30,33,34 These precursor cones increase steadily in height as the valleys deepen through preferential valley ablation (PVA). During the ablation process, nanoparticles are generated and redeposited on the surface, increasing the height of these cones.…”

“…30,33,34 These NC-pyramids have a height-to-width aspect ratio of about 1:1. They can grow taller than 50 lm and are covered with a layer of nanoparticles typically more than 2 lm thick.…”

“…6,34,35 A commonly studied type of mound can be called below-surface growth (BSG) mounds, which are more tightly packed and have peaks that are beneath the original target surface. A second class of mounds which form at higher laser fluence than BSGmounds can be called above-surface growth (ASG) mounds.…”