Densification, phase transformation and hardness of mullite-cubic BN composites prepared by spark plasma sintering

Mullite-cubic boron nitride (cBN) composites were prepared using mullite and cBN powders by spark plasma sintering at temperatures of 1500-1700°C for a holding time of 600 s under a pressure of 100 MPa. Densification, microstructure and hardness of the mullite-BN composites and the phase transformation of cBN to hexagonal BN (hBN) in the mullite-BN composites were studied. Fully dense mullite-cBN composites originally containing 10-30 vol% cBN with relative density of over 95% were obtained at 1500°C without the phase transformation of cBN. At the cBN content of 10 vol%, the densification of the mullite-BN composite was not inhibited. When more than 20 vol% cBN was added to mullite, the densification of the mullite-BN composites was suppressed. The phase transformation of cBN in the mullite-BN composites started at 1600-1700°C. The hardness of mullite-BN composites originally containing 20 vol% cBN sintered at 1500°C without the phase transformation of cBN reached a maximum value of 16 GPa.