MICROSTRUCTURE, MECHANICAL PROPERTIES AND FRICTION/WEAR BEHAVIOR OF HOT-PRESSED Si₃N₄/ BN CERAMIC COMPOSITES

Nano-scale and micro-scale hexagonal boron nitride (hBN) powders, respectively, were added to silicon nitride (Si₃N₄) matrix powder, and Si₃N₄/BN ceramic composites with different content of solid-lubrication hBN were produced by hot pressing. The combined effect of starting powder and hBN content on the microstructure, mechanical properties and tribological properties of Si₃N₄/BN ceramic composites has been studied. The results showed that the nano-sized starting powder was beneficial to the densification of Si₃N₄/BN ceramic composites, and resulted in a Si₃N₄/BN nano/nano ceramic composite with slightly higher density and improved mechanical properties compared to the corresponding micro/nano ceramic composite. The physical and mechanical properties of Si₃N₄-based ceramic composites were found to be affected by addition of BN. Increasing BN content led to a decrease of bulk density, hardness, bending strength and fracture toughness. On the other hand, the starting powder has no significant influence on the tribological properties of Si₃N₄/BN ceramic composites. The friction properties and wear resistance continuously improved with increasing BN content up to 20 vol.%, due to the formation of tribochemical film composed of H₃BO₃, SiO₂ and metal oxides.