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

Abstract

Femtosecond laser surface processing (FLSP) is an emerging technique for creating functionalized surfaces with specialized properties, such as broadband optical absorption or superhydrophobicity/superhydrophilicity. It has been demonstrated in the past that FLSP can be used to form two distinct classes of mound-like, self-organized micro/nanostructures on the surfaces of various metals. Here, the formation mechanisms of below surface growth (BSG) and above surface growth (ASG) mounds on polycrystalline Ni60Nb40 are studied. Cross-sectional imaging of these mounds by focused ion beam milling and subsequent scanning electron microscopy revealed evidence of the unique formation processes for each class of microstructure. BSG-mound formation during FLSP did not alter the microstructure of the base material, indicating preferential valley ablation as the primary formation mechanism. For ASG-mounds, the microstructure at the peaks of the mounds was clearly different from the base material. Transmission electron microscopy revealed that hydrodynamic melting of the surface occurred during FLSP under ASG-mound forming conditions. Thus, there is a clear difference in the formation mechanisms of ASG- and BSG-mounds during FLSP.

Creation of micro/nano surface structures on silver using collinear double femtosecond laser pulses with different pulse separation

“…Due to the minimized heat affected zone associated with femtosecond ablation the subsurface microstructure would remain unaltered during the preferential ablation process (Zuhlke et al 2013). The small grain sizes and large number of voids in the upper portion of the mound structure indicate either fluid flow of the surface melt induced by the laser pulses or redeposition of ablated material on top of the structure resulted in upward growth of the structure similar to what has been reported in previous work on nickel (Zuhlke et al 2013), polycrystalline Ni60Nb40 (Peng et al 2016(Peng et al , 2017b and titanium (Peng et al 2017a). While voids in the subsurface microstructure have been previously observed for single-pulse FLSP of titanium (Peng et al 2017a), the dual-pulse silver mound structure contains a higher density of voids.…”

“…The subsurface microstructure, with a balance between original grains sizes at the base of the structure and smaller grain sizes near the surface is similar to what has been observed through single-pulse FLSP on other materials (Peng et al 2016(Peng et al , 2017a. These similarities indicate similar formation processes for the dual-pulse structures on silver as single pulse structures on other materials.…”

Micro/nanostructures formation by femtosecond laser surface processing on amorphous and polycrystalline Ni60Nb40

“…The alloy utilized in this study was the well-characterized, easy glass-forming Ni 60 Nb 40 [57,58]. Arc-melting and melt spinning formed amorphous Ni 60 Nb 40 ribbons as described in previous published study [46]. Ribbons were sealed in quartz tube under Ar atmosphere after repeated evacuation cycles to prevent oxidation.…”

“…One of the formation processes associated with mounds is preferential valley ablation. This is the process where the valleys, or low areas of the surface, are ablated more than the peak or top of precursor mounds [5,38,46]. As illustrated in Fig.…”

“…This is the process where, during ultrafast laser pulse irradiation, the substrate surface melts and the resulting liquid layer flows away from the valleys up the sides of the structures and onto the peaks [48][49][50]. Resolidification of this layer increases the height of the peaks, and is thought to be a significant part of the ASG mound formation process [5,46]. Fluid flow could be the result of higher laser fluence at valleys versus sides, or could be due to laser-induced shock waves [51,52].…”