ANALYSIS OF COATING INTERLAYER BETWEEN SILICON-NITRIDE CUTTING TOOLS AND TITANIUM CARBIDE AND TITANIUM NITRIDE COATINGS

Silicon nitride (Si3N4) cutting tools exhibit excellent thermal stability and wear resistance in the high-speed machining of cast irons, but show poor chemical wear resistance in the machining of steel. Conventional chemical vapour deposition (CVD) coating of Si3N4 tools has not been very successful because of thermal expansion mismatch between coatings and the substrate. This problem was overcome by developing a CVD process to tailor the interface for titanium carbide (TiC) and titanium nitride (TiN) coatings. Computer modelling of the CVD process was done to predict which phases would form at the interface, and the results compared with analyses of the interface. Three Si3N4 compositions were considered, including pure Si3N4, Si3N4 with a glass phase binder, and Si3N4 + TiC composite with a glass phase binder. Results of machining tests on coated tools show that the formation of an interlayer provides superior wear resistance and tool life in the machining of steel as compared to uncoated and conventionally coated Si3N4 tools.