Anisotropic plastic deformation and damage evolution of sapphire under nanoindentation

Sapphire has extensive applications in advanced manufacturing fields, including electronics and semiconductors. However, its pronounced anisotropy poses significant challenges for ultra-precision machining and effective damage control. This study systematically investigates the anisotropic mechanisms of plastic deformation and subsurface damage (SSD) evolution across different sapphire crystal planes. This is achieved using molecular dynamics simulations, nanoindentation experiments, and transmission electron microscopy characterization. The results definitively show that distinct slip system behaviors depend on crystal orientation: the basal plane acts as the primary slip plane for the A/M/R-planes, whereas the rhombohedral slip is dominant for the C-plane. Subsurface damage of the A/M-planes is dominated by the formation of dislocation loops, whereas the C/R-planes primarily exhibit cross-slips. It was also identified that crack initiation and propagation mechanisms are closely linked to twinning behaviors. Cracks preferentially nucleate and propagate along the twinning planes, with basal and rhombohedral twinning playing key roles. The formation and evolution of SSD are significantly influenced by the activation and interaction of slip systems, leading to variations in damage depth, with a clear trend of M-plane > A-plane > R-plane. On the C-plane, basal slip initially minimizes damage; however, increased indentation depth activates rhombohedral slip, exacerbating SSD. Furthermore, a theoretical slip system activation model was successfully developed and validated, accurately predicting SSD evolution. These findings provide a robust theoretical basis for optimizing low-damage ultra-precision machining processes for sapphire and other anisotropic crystalline materials.