Crystallographic effect of elastic recovery on surface roughness of ultra-precision diamond turning machined polycrystalline ZnS

Abstract

Polycrystalline zinc sulfide (ZnS) is a widely used optical material especially for infrared applications. After ultra-precision diamond turning (DT) machining of the material in ductile regime, surface roughness of a few nanometers is achievable. However, the traces of crystal structure were not only visible on the DT machined surface but it also showed the steep ups and downs on the surface profile, although the same depth-of-cut was applied to the whole surface. In this study, it was hypothesized that the ups and downs on the surface profile might be affected by the crystallographic orientation and elastic recovery of each grain on the surface of ultra-precision DT machined ZnS. The relationship between the crystallographic orientation and elastic recovery of the material was investigated using electron microscopy with electron back-scattered diffraction (EBSD) and nanoscale scratch tester. The results showed certain grain orientations exhibit higher levels of elastic recovery, leading to increased surface roughness. These results highlight the importance of considering crystallographic effects in the machining process of ZnS to achieve desired surface quality.