High-pressure synthesis of tungsten carbide–cubic boron nitride (WC–cBN) composites: Effect of cBN particle size and volume fraction on their microstructure and properties

WC-cBN composites without any additives were synthesized by High Pressure-High Temperature (HPHT) method. The effect of sintering temperature, cBN content, and particle size on the densification behavior, microstructure, and mechanical properties of WC-cBN composites were investigated. It was found that the low sinterability of cBN particles resulted in poor bonding between WC grains and difficulty in forming grain boundaries, leading to difficulties in synthesizing fully dense WC-cBN composites at a pressure of 5.5 GPa. Under high pressure conditions, the phase transformation from cBN to the low-hardness hexagonal hBN was inhibited. The low sintering temperature and the inhibition effect of WC grain growth by the large cBN particles can deteriorate the hardness and fracture toughness of the composite. However, the hardness and toughness of the WC-cBN composites increased significantly with increasing cBN content. Excellent mechanical properties can be obtained by sintering composites containing cBN with small particle size and large content at high temperature with the hardness of 32.8 GPa and fracture toughness of 9.95 MP·m1/2.