Brittle‒ductile transition behavior of 6H-SiC in oblique diamond cutting

Abstract

6H-SiC is a typical hard-brittle material, and the challenge of performing ductile-regime machining in diamond cutting necessitates an in-depth understanding of the brittle‒ductile transition (BDT) mechanism. In this work, a novel machining method named oblique diamond cutting is employed to improve the BDT behavior of 6H-SiC. First, the tool edge ovalization effect is proposed, which provides deeper insight into the change in cutting edge shape evaluated in the cutting plane. An empirical model under the combined influence of the nominal tool rake angle, tool sharpness and tool inclination angle is established to effectively describe the equivalent cutting edge radius of the elliptic-shaped cutting edge in the cutting plane. When the tool inclination angle was increased from 0° to 45°, the equivalent cutting edge radius increases gradually, and the error of the prediction model remained at a low level of less than 7.5%. Second, an oblique plunge cutting experiment and finite element (FE) simulation were carried out under the consideration of tool edge ovalization effect. The simulation results indicate that the BDT depth grew gradually as the tool inclination angle increased, and shows the same changing trend as the experimental results with an average error of 14.9%. When the tool inclination angle was increased to 45°, the BDT depth obtained in experiments doubled (dc = 42.4 nm) compared to the results of traditional orthogonal cutting including from the references and this work. Such a huge improvement in the BDT behavior in cutting is of great significance for the successful execution of ductile-regime machining on a SiC optical component in practice. Finally, the reason for the increase in the BDT depth is revealed to result from changes in the stress distribution in the cutting area due to the increasing equivalent cutting edge radius of the elliptic-shaped cutting edge arc, especially the tensile stress, which is particularly sensitive to brittle materials with insufficient tensile strength. Compared to traditional orthogonal cutting, a high compressive stress zone is formed beneath the tool because more materials undergo the squeezing of the elliptic-shaped cutting edge. Thus, the tensile stress concentrated on the machined surface behind the cutting edge is suppressed, which is beneficial for promoting the plastic deformation of brittle materials with insufficient tensile strength. This is of great significance not only for SiC but also for the processing of other hard-brittle materials.