Thermomechanical response and strengthening effect of femtosecond laser-irradiated CoCrFeNi high-entropy alloy films

Films can enhance surface corrosion resistance and mechanical properties when applied as a protective layer. However, these films have poor crystallinity and weak bonding and need improvement by developing a treatment strategy. In this study, we used a femtosecond laser to treat a CoCrFeNi film and examined the ultrafast thermomechanical response and its effects on surface hydrophilicity and microhardness. Pump-probe images show that the way the material is removed changes from spallation to phase explosion as the laser fluence increases. This finding is further supported by the molecular dynamics coupled two-temperature model (MD-TTM). The surface heat accumulation creates uneven bubbles and results in the formation of a rough recast layer. As a result, the ablation depth remains relatively constant as the surface roughness increases, while the laser intensity increases from 1.35 to 3.82 J/cm2. Considering the comprehensive thermomechanical effect, we achieved a preferred [111] orientation after recrystallization, and the surface hardness increased by more than 32 %. The contact angle also changed from 96 degrees to 108 degrees at 1.35 J/cm2. In summary, the strengthening effect of femtosecond laser direct writing on CoCrFeNi was verified, and the related mechanism was systematically explained. This approach offers valuable insights for the study and application of other thin-film systems.