Mechanism of Ultra-precision Machining of Different Crystal Planes of Zinc Selenide: Molecular Dynamics Simulation and Experimental Verification

Zinc selenide (ZnSe) convex aspherical optical components have been widely used because of their excellent optical properties. Single-point diamond turning (SPDT) is the mainstream method for machining of ZnSe aspherical optical surfaces. However, the high brittleness and extremely low fracture toughness of ZnSe make it prone to surface defects such as cracks and pits during the SPDT process, which seriously impair the quality of ultraprecision machined ZnSe surfaces. At present, remarkable achievements have been made in the research on inducing ductile-brittle transition (DBT) to improve the crystal surface quality via the precise control of cutting parameters during the ultraprecision SPDT of ZnSe crystals. Nevertheless, few studies have reported the influence of the ZnSe crystal orientation on the surface quality. The anisotropy of ZnSe crystal grains also exerts a significant impact on the machined surface quality. Therefore, it is crucial to explore the anisotropic cutting mechanism of ultraprecision turning for ZnSe crystals and improve the surface and subsurface quality of ZnSe. In this study, molecular dynamics simulations of different ZnSe crystal planes were combined with gradient ultraprecision turning experiments to systematically investigate the material removal mechanisms of ultraprecision turning for the ZnSe (100), (110), and (111) crystal planes and polycrystalline ZnSe, as well as the effect of cutting depth on the surface quality of different ZnSe crystal planes. The results show that the surface morphologies of different ZnSe crystal planes exhibit distinct differences at the same cutting depth and that the morphologies of different crystal planes also vary at an identical cutting depth. The machined surface quality of ZnSe crystals is significantly affected by the crystal orientation. Among the four crystal planes investigated in the experiments, the machined surface quality is ranked as (110) > (111) > polycrystalline ZnSe > (100).