Crystallographic Effects on Microscale Machining of Polycrystalline Brittle Materials

Abstract

This paper studies the effects of crystallography on the microscale machining characteristics of polycrystalline brittle materials on a quantitative basis. It is believed that during micromachining of brittle materials, plastic deformation can occur at the toolworkpiece interface due to the presence of high compressive stresses which leads to chip formation as opposed to crack propagation. The process parameters for such a machining process are comparable to the size of the grains, and hence crystallography assumes importance. The crystallographic effects include grain size, grain boundaries (GB), and crystallographic orientation (CO) for polycrystalline materials. The size of grains (crystals), whose distribution is analyzed as a lognormal curve, has an effect on the yield stress of a material as described by the Hall–Petch equation. The effects of grain boundary and orientation have been considered using the principles of dislocation theory. The microstructural anisotropy in a deformed polycrystalline material is influenced by geometrically necessary boundaries (GNB) and incidental dislocation boundaries (IDB). The dislocation theory takes both types of dislocations into account and relates the material flow stress to the dislocation density. The proposed analysis is compared with previously reported experimental data on polycrystalline germanium (pGe). This paper aims to provide a deeper physical insight into the microstructural aspects of polycrystalline brittle materials during precision microscale machining.