Preparation and microstructure of superhard Al2O3-cubic boron nitride ceramic composites

Cubic boron nitride (cBN) is valued for industrial applications such as cutting tools and abrasives due to its excellent thermal and mechanical properties. To kinetically suppress the phase transformation from cBN to the stable hexagonal modification (hBN), Spark Plasma Sintering (SPS) was applied to AlO-cBN ceramic composites using high heating rates (50 K/min) and short isothermal holding times (5 min) at temperatures up to 1600 °C. X-ray diffraction (XRD), scanning electron microscopy (SEM), hot gas extraction, and differential scanning calorimetry (DSC) were used to study the microstructure of composites with an AlO matrix and an AlO matrix containing 5 vol.% of a glassy phase. Effective densification is achieved once the glassy phase is fully melted. However, this accelerates the transformation of cBN into hBN at temperatures 1350 °C (1400 °C in the pure AlO matrix). Microcracks, due to residual stress, can be mitigated by reducing the cBN grain size.