Influence of annealing on the microstructure and mechanical properties of MTCVD TiC0.79N0.21 coating

TiC0.79N0.21 coating was deposited on WC-Co cemented carbide by moderate temperature chemical vapour deposition technique and then annealed in vacuum or air in the temperature range of 700-900 °C for 1 h. The morphology, composition, phase component, residual stress, hardness and plastic deformation resistance of the coating before and after annealing were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe micro-analyzer (EPMA), X-ray diffractometer (XRD) and nano-indentator. The results show that after annealing in low vacuum, the grain boundaries of the coating surface are still clearly visible and small capsule or granular oxide particles are uniformly distributed on the columnar surfaces of the coating. Coarse irregular particles enriched with W element are randomly located at the grain boundaries. After annealing in air, the coating is, however, oxidized to rutile TiO2 and oxidation rate increases with temperature. Dramatic coating oxidation is observed at 900 °C. With the increase of temperature, the residual tensile stress increases and compressive stress decreases after annealing in air and vacuum, respectively. The hardness and plastic deformation resistance on the cross section of coating are improved with temperature after vacuum annealing while opposite tendency is obtained for annealing in air.