High pressure sintering of cubic boron nitride compacts with Al and AlN

Cubic boron nitride (cBN) compacts, using 15 wt.% Al and 20 wt.% AlN respectively as additives, were sintered in the temperature range of 1300–1700 °C for 20 min under high pressure of 5.0 GPa. The hardness, microstructure, phase composition and cutting performance of the high pressure sintered samples were investigated. A liquid phase sintering and reaction process was observed in the cBN–Al system, which leads to the formation of AlN and AlB2 as confirmed by X-ray diffraction (XRD) in the sintered compacts. Scanning electron microscopy (SEM) analysis shows that the samples have a homogeneous microstructure. The hardness decreases with increase of sintering temperature and reaches the highest Vickers hardness of 32.1 GPa at 1350 °C. While in the cBN–AlN system, AlN grains agglomerate heavily at temperature below ~ 1500 °C. As the sintering temperature increasing, Al2O3 appeared and the AlN agglomeration disappeared gradually. A highest cBN–AlN composite hardness of 29 GPa was achieved when sintered at 1600 °C. Turning tests showed that cBN compacts with 15 wt.% Al as the additive has a longer tool life as compared to that with 20 wt.% AlN. Our results indicated that cBN–Al system is more favourable to obtain well-sintered cBN compacts comparing with the cBN–AlN system.