A Molecular Dynamics Study on the Orientation Dependence of Nano-Scale Scratching of Sapphire

Sapphire is a promising material for various optical, electronic, and mechanical applications because of its excellent optical transparency, high thermal and chemical stability, and superior mechanical strength. However, like silicon, sapphire is a brittle material with very high hardness so that it is difficult to apply machining processes to obtain high-quality surfaces with ultra-precision features. In this study, the molecular dynamics (MD) methodology using Vashishta and Lennard-Jones potentials is employed to unveil the nano-scale cutting mechanisms of sapphire. The MD simulations reproduce the nano-scale indentation and scratching tests of sapphire substrates using a spherical diamond tool along various slip directions on several crystallographic planes (c, a, m, r planes). The results show that the hardness and the cutting force exhibit a non-trivial dependence on the crystal orientation. Moreover, a systematic analysis using a novel scheme to measure the atomic-scale plastic strain and the orientation change reveals the localized atomic-scale deformation mechanisms that are responsible for characteristic behaviors of sapphire in the scratching process on various crystallographic orientations.