Effects of testing temperature and thermal treatments on some mechanical properties of a Si3N4?TiN composite

Strength and fracture toughness of an electroconductive hot-pressed Si3N4_/35vol.% TiN ceramic composite were evaluated in air as a function of testing temperature up to 1200 8C. The toughness already shows a clear decrease at 800 8C and then remains almost constant, and the flexural strength steadily decreases with increasing testing temperature. At 1200 8C, the strength value is about 40% of that measured at room temperature. After thermal treatments in air (800, 1000 and 1200 8C) and argon (1200 8C) for 100 h, the Young’s modulus, hardness, fracture toughness and flexural strength were measured at room temperature and compared to the baseline material. Young’s modulus and hardness remain unchanged. The fracture toughness does not show any clear trend with the treatment temperature, while the strength, which is unaffected by the thermal treatment in argon, decreases with increasing treatment temperature in air. The long-term oxidation involves microstructural changes at the surface and in the bulk, such as the formation of a surface oxide layer and a porous sub-layer. In the bulk, the main modification is the partial crystallization of the grain boundary phase.