Effects of cubic BN addition and phase transformation on hardness of Al₂O₃-cubic BN composites

The Vickers hardness of dense Al₂O₃-cubic BN (cBN) composites prepared by spark plasma sintering under a moderate pressure of 100 MPa at 1200-1600 °C was investigated at indentation loads of 0.098-19.6 N. The BN grains in the Al₂O₃-BN composite prepared at 1300 °C showed no transformation from the cBN to hBN phase, and the hardness was 59 GPa at 0.098 N. The hardness of the Al₂O₃ matrix in the Al₂O₃-BN composites containing 10-30 vol% cBN prepared at 1300-1400 °C was around 25 GPa at 0.098 N, which was higher than monolithic Al₂O₃ bodies prepared at the same temperatures. The hardness of the Al₂O₃ matrix in the Al₂O ₃-BN composites decreased with increasing sintering temperature. The increase in the hardness of the Al₂O₃ matrix may be due to the decrease in the size of Al₂O₃ grains in the Al ₂O₃-BN composites owing to the addition of cBN particles and the decrease in sintering temperature. The Meyer exponents of the monolithic Al₂O₃ bodies and Al₂O₃-BN composites were 1.90-1.94 independent of cBN content.