Microstructure and mechanical properties of TiC-TiB₂ composite cermet tool materials at ambient and elevated temperature

TiC-TiB₂ composite cermet tool materials were fabricated by the hot pressing process in vacuum at 1450 °C for 30 min under 32 MPa. The mechanical properties at ambient temperature, the high temperature flexural strength and the high temperature fracture toughness were tested in air. The content of Ni-Mo had significant influence on the microstructure and mechanical properties of the composites at ambient temperature. TBNM8 (TiC+TiB₂+8 wt%Ni-Mo) had the optimum comprehensive mechanical properties at ambient temperature with flexural strength of 921.2±140 MPa, fracture toughness of 7.18±0.5 MPa·m^1/2 and Vickers hardness of 22.4±1.0 GPa. The great mechanical properties of TBNM8 were ascribed to its fine and homogeneous microstructure as well as the mixed mode of intergranular and transgranular fracture. This paper focused on the investigation of the flexural strength and the fracture toughness increment/degradation mechanisms of the materials at elevated temperatures. The flexural strength of TBNM8 decreased gradually with increasing temperature. The strength degradation was ascribed to the softening of grain boundary, the surface oxidation and the elastic modulus degradation. The improvement of the fracture toughness of TBNM8 at 900 °C was attributed to the grain boundary sliding which resulted in plastic deformation. But the fracture toughness of TBNM8 at 1000 °C decreased due to the serious oxidation of TiC and TiB₂.