Brittle-ductile transition during diamond turning of single crystal silicon carbide

Single crystal silicon carbide (SiC) is an ultra hard ceramic material which possesses extremely high hardness following diamond and CBN. In this experimental study, diamond turning of single crystal 6H-SiC was performed at a cutting speed of 1 m/sec on an ultra precision diamond turning machine (Moore Nanotech 350 UPL) to elucidate the microscopic origin of ductile-regime machining. Distilled water (pH value 7) was used as a preferred coolant during the course of machining in order to improve the tribological performance. A high magnification scanning electron microscope (SEM) (FIB- FEI Quanta 3D FEG) was used to examine the cutting tool. An optimum matrix of machining parameters was used to optimize the material removal rate and the machined surface roughness. A surface finish of Ra 9.2 nm, better than any previously reported value on SiC was obtained. Also, tremendously high cutting resistance was offered by SiC resulting in the observation of significant wear marks on the cutting tool just after 1 Km of cutting length. It was found out through a DXR Raman microscope that similar to other classical brittle materials (silicon and germanium etc.) an occurrence of brittle-ductile transition is responsible for the ductile-regime machining of 6H-SiC.