UV-laser ablation of ductile and brittle metal films

Single-shot laser damage of Ni and Cr films on fused silica substrates has been studied as a function of film thickness, utilizing 248 nm/14 ns pulses and detection by probe beam deflection. Threshold fluences for visible damage and vaporization are compared to predictions of the heat diffusion model. The model fits thresholds for visible damage well and identifies their origin, which is melting for Ni films and brittle-to-ductile phase transition for Cr films. When predicting thresholds for vaporization, the diffusion model is of limited success in case of Ni films but fails completely for Cr films, indicating that transient thermal properties of the material should be taken into account. Microscopic inspection shows that Cr films rupture at low fluences before entering the common sequence of melting and vaporizing with increasing fluence.