Study on Subsurface Damage in Sapphire During Nano Indentation Using Molecular Dynamics Simulation

Advanced single crystalline ceramics such as sapphire (α-Al₂O₃), silicon carbide (SiC) and potassium dihydrogen phosphate (KH₂PO₄) are being used widely in a many different industries such as electro-optics, aerospace, and biomedical devices due to superior mechanical, thermal, optical and electronic properties and the ability to perform in harsh environments over prolonged periods of time. Apart from the hard and brittle nature, a major challenge in processing ceramics results from crystal anisotropy which requires careful toolpath selection and several post processing operations such as polishing and annealing to produce a crack free component which significantly adds to the cost. Additionally, processing ceramics causes residual stresses and subsurface damage that can lead to premature failure of the component as the flaws generated during processing can act as crack initiators on application of a load. As these ceramics are adopted into more applications, damage free processing is extremely important to ensure proper functionality of the ceramic components. However, as the mechanics of nano-scale deformation in single crystalline ceramics have not been fully understood, further study is required on these topics. In this study, molecular dynamics is employed to understand the extent of subsurface damage on four crystallographic planes of sapphire (prismatic A-, basal C-, prismatic M- and rhombohedral R-) during nano indentation using LAMMPS software. The influence of crystal anisotropy on subsurface damage is evaluated by examining the types of flaws generated and the depth up to which subsurface damage extends on the different crystallographic planes. It is hypothesized that the amount and types of subsurface damage is anisotropic and dependent on the crystallographic orientation of the sample as different slip and fracture mechanisms are activated in a crystalline material based on the characteristics of the load applied which is thought to influence the deformation in the subsurface regions as well. The results are analyzed and correlated to the activation of specific slip/twinning and fracture systems based on the crystal orientation and direction of application of load by examining the load vs. deflection graphs and visualization of the material deformation. Preliminary results indicate that under identical conditions of indentation, the deformation and subsurface damage in the basal and rhombohedral planes extend  deeper than in the case of the prismatic planes. This understanding will be key to developing efficient strategies to process ceramics such as sapphire with minimal residual stresses and subsurface damage and possibly eliminate the need for any post processing.