Material removal behavior analysis of ZnSe crystal during side-forward nanoscratching

Zinc selenide (ZnSe) crystal is difficult to cut due to its high brittleness. A better understanding of the material removal mechanism of ZnSe crystal is indispensable to help achieve a high-quality machined surface. In this paper, a theoretical model is first developed for analyzing the material removal behavior of brittle crystals during side-forward nanoscratching using the Berkovich indenter. The model correlates scratching parameters with scratch behavior parameters, morphology parameters, material removal modes, and material mechanical properties. To validate the model, the nanoindentation and side-forward nanoscratch tests of ZnSe crystal are carried out, and the effects of load and strain rate on the mechanical properties and scratch behavior of ZnSe crystal are investigated. The results show that the hardness decreases first and then stabilizes as the indentation load increases; the elastic modulus generally increases with the indentation load while it increases sharply at a very small strain rate. The results also show that with an increasing scratching normal load, the elastic recovery decreases first and then stabilizes while the friction coefficient increases slightly first and then stabilizes; with the increase of strain rate, the elastic recovery, scratching load, and friction coefficient decrease quickly at first and then increase by a similar power function. The model is proven to be applicable to determining the scratching load, friction coefficient, scratching depth, and elastic recovery with pile-up parameters, groove parameters, indenter orientation angle, and mechanical properties of ZnSe crystal, which has an average relative error of less than 14%. Furthermore, the effects of scratching direction on the scratching load and friction coefficient are discussed using the model. The results show that for side-forward scratching, the maximum normal load always occurs which is independent of the indenter orientation angle, while a relatively small friction coefficient exists. This research provides insights into the material removal behavior and mechanism in ultra-precision machining of ZnSe crystal.