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 Al2O3–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 Al2O3–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.