Brittle-to-Ductile Transition of Single-Crystalline Sapphire During Ultra-Precision Machining

Although sapphire has a wide range of applications due to its superior thermal, electrical, and mechanical properties, the machinability of sapphire is a major challenge due to its inherent brittle nature. Ultra-precision machining has provided a promising solution by enabling ductile-mode cutting, but there remain several problems such as residual stress and surface and subsurface damages after the machining process. In this study, atomic-scale cutting mechanisms of sapphire are investigated using the molecular dynamics (MD) simulation method. The MD simulations are performed on various crystallographic planes and directions of sapphire and the critical depth of cut (CDC) representing the transition from ductile-mode machining to brittle fracture is determined in each case. During the ductile-mode machining, plastic deformation is observed and the corresponding slip systems are identified. When the depth of cut exceeds CDC, fracture is initiated after the small-scale initial slip and twinning occur at the intersection point of different dislocations. The physical origin and mechanisms leading to these different cutting modes are investigated through the analysis of the simulation results.