Nanoscale indentation and scratching tests of single crystal sapphire using molecular dynamics simulation

Sapphire is a crystalline aluminum oxide, which has been used for various applications due to its superior thermal, chemical, mechanical, and optical properties. Even though sapphire is a brittle material, its plastic deformation modes have also been studied for decades, mostly in experiments. In this study, we apply the molecular dynamics simulation method to investigate the atomic-scale deformation mechanisms of single crystal sapphire during the indentation and scratching tests. The indentation simulations are carried out in four crystallographic planes (C, A, M, R) and reveal various slip deformations. In particular, the rhombohedral twin structures are formed in the C-plane indentation, and the R-plane indentation triggers most slip systems including the C-, A-, N-, R-, S-planes. Hardness on the four slip planes is calculated during the indentation test and compared with the experimentally measured values. The scratching tests are performed in six different crystallographic directions at three scratching depths of 10 Å, 20 Å, and 30 Å. The simulation results show the deformation modes similar to those found in the indentation tests. In the scratching tests in the two slip directions of the C-plane the basal slip deformation is observed. The normal and scratching forces that are calculated in the scratching test exhibit the orientation dependence such that the C-plane and A-plane have the largest and smallest forces. The atomic displacements and configurations in the rhombohedral twin and basal slip are discussed.