Phase stability and mechanical properties of Al2O3-cBN composites prepared via spark plasma sintering

The sintering behaviours, phase compositions, microstructures, and mechanical properties of Al2O3-based matrix composites reinforced with up to 20 vol% of cubic boron nitride (cBN, super-hard phase) were investigated. The materials were prepared via the method of spark plasma sintering (SPS). Additionally, pure cBN powder was treated under identical SPS conditions for the purpose of comparison. The temperatures at which hexagonal boron nitride (hBN, graphite-like phase) was detected in pure cBN and in cBN dispersed in the Al2O3 + 10 TiN matrix were 1600 °C and 1300 °C, respectively. The presence of additives in the Al2O3 matrix had a pronounced influence on the formation of hBN, with TiN > ZrO2 > pure Al2O3. All the composites exhibited quite good improvements in fracture toughness, of about 1.5 MPa·m1/2 on average over that of Al2O3. Crack deflection on cBN grains was identified as the main toughening mechanism of the ceramics investigated.