Effect of coating on the deformation behavior of single-crystal sapphire during ultra-precision machining*

Sapphire has found a wide range of applications due to its superior thermal, electrical, and mechanical properties. To fulfill the requirement of shape, size, precision, and surface quality for each application, ultra-precision machining process is commonly used. However, due to its brittleness and hardness, machinability has been a major challenge to fabricate parts from single crystal sapphire. In experiments, coating with materials such as mineral oil, wax, glue, and other adhesives have been commonly applied to assist the machining process of sapphire and have shown significant improvement in machinability by increasing the critical depth of cut (CDC), promoting chip formation and suppressing the propagation of cracks. In this study, we developed a thin film coating model in molecular dynamics (MD) simulations using finitely extensible non-linear elastic (FENE) potential with controllable adhesion energy and adhesion strength and applied it in the ultra-precision cutting simulations of coated sapphire substrate. Cutting simulations were conducted on a variety of crystallographic planes of sapphire. With this model, we investigated the effects of coating materials on the deformation mechanisms of sapphire during machining. The effects of several input parameters of the film on deformation and fracture mechanisms, such as the molecular weight per polymer chain and the adhesion strength between the film and the sapphire substrate were investigated and analyzed at atomistic levels.