High-pressure synthesis of Al2O3-cBN composites: Effect of thermodynamic condition and cBN volume fraction on their microstructure and properties

Alumina with different amounts of Cubic boron nitride (cBN) was synthesized by a high-pressure sintering method. By adjusting the sintering temperature to a relatively low level, the undesirable reverse transformation from cBN to hBN was avoided, and almost entirely dense composites were obtained. The sintering behaviors, phase compositions, microstructures, and mechanical properties of Al2O3-cBN composites were investigated. The cBN phase significantly improved the modulus of elasticity and hardness, but the fracture toughness was only slightly increased. Vickers hardness of Al2O3–40 vol% cBN composites sintered at 850 °C showed a maximum value of 30 GPa. The Al2O3–30 vol% cBN composite sintered at 1250 °C had the highest fracture toughness of 5.7 MPa·m1/2.