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

Highlights: • Femtosecond laser processing of glass-forming Ni₆₀Nb₄₀ produce surface structures. • Cross sectioning, imaging, & TEM sample preparation with dual-beam SEM. • Low laser fluence surface structures' form by ablation. • High laserfluence surface structures form by ablation and fluid flow. - Abstract: Femtosecond laser surface processing is a technology that can be used to functionalize many surfaces, imparting specialized properties such as increased broadband optical absorption or superhydrophilicity/superhydrophobicity. In this study, two unique classes of surface structures, below surface growth (BSG) and above surface growth (ASG) mounds, were formed by femtosecond laser surface processing on amorphous and polycrystalline Ni₆₀Nb₄₀ with two different grain sizes. Cross sectional imaging of these mounds revealed thermal evidence of the unique formation processes for each class of surface structure. BSG mounds formed on all three substrates using the same laser parameters had similar surface morphology. The microstructures in the mounds were unaltered compared with the substrate before laser processing, suggesting their formation was dominated by preferential valley ablation. ASG mounds had similar morphology when formed on the polycrystalline Ni₆₀Nb₄₀ substrates with 100 nm and 2 μm grain size. However, the ASG mounds had significantly wider diameter and higher peak-to-valley heights when the substrate was amorphous Ni₆₀Nb₄₀. Hydrodynamic melting was primarily responsible for ASG mound formation. On amorphous Ni₆₀Nb₄₀ substrates, the ASG mounds are most likely larger due to lower thermal diffusivity. There was clear difference in growth mechanism of femtosecond laser processed BSG and ASG mounds, and grain size does not appear to be a factor.