Nano-indentation and nano-scratching of pure nickel and NiTi shape memory alloy thin films: an atomic-scale simulation

Abstract Issues with contact and friction in thin-film devices are inevitable in the field of micro-engineering, especially in micro-electro-mechanical systems (MEMS). In this study, the nano-indentation and nano-scratching behaviours of pure nickel and NiTi shape memory alloy (SMA) nano-films, two common materials used in MEMS, were comparatively investigated using large-scale molecular dynamics simulations. For the nickel nano-film structure, the load–displacement (P–h) curve demonstrated distinct elasto-plastic characteristics, and visible permanent deformation remained after unloading. In contrast, for the NiTi SMA, the P–h curve formed a hysteresis loop, and no evident deformation remained on the surface after unloading. It is noteworthy that during scratching, despite a lower applied normal pressure on the nickel nano-film than on the NiTi SMA, the friction and friction coefficient of the former were much higher. These differences in performance are closely related to the super-elasticity induced by the martensitic transformation within the NiTi SMA.