Femtosecond laser induced selectively surface ablation in Ni-Al bilayer film: Experiments and simulations

Metals with high thermal conductivity and deformability often form micro-nano structures during femtosecond laser ablation, making it challenging to achieve high-quality surface processing. This study investigates Ni-Al bilayer films, consisting of an 80 nm Ni surface layer and Al layers of 0, 40, and 80 nm thickness, as Ni and Al have different thermo-physical properties. Using pump-probe imaging and molecular dynamics coupled two-temperature model (MD-TTM) simulation, we analyze the ultrafast thermomechanical behavior and structure formation. Ultrafast reflectivity shows nearly identical melting rates for different Al thicknesses, while the Newton ring cycle decreases with increasing Al thickness, indicating reduced mechanical removal. MD-TTM reveals stress concentration at the Ni-Al interface and destructive stress interference in the 80 nm Ni-40 nm Al, leading to higher Al potential energy. The 80 nm Ni-40 nm Al bilayer shows a smooth Al surface with minimal damage, while the 80 nm Ni-80 nm Al bilayer shows more severe Al damage and microcraters. So that there forms a smoother surface in 80 nm Ni-40 nm Al than that of 80 nm Ni-80 nm Al. This work emphasizes the importance of material design to reduce damage, enhance the quality of multilayer metal processing, and achieve selective ablation, which has significant implications for film design, heat transfer, and precision machining.