Hardness and thermal stability of cubic silicon nitride

The hardness and thermal stability of cubic spinel silicon nitride (c-Si 3 N 4 ), synthesized under high-pressure and high-temperature conditions, have been studied by microindentation measurements, and x-ray powder diffraction and scanning electron microscopy, respectively. The phase at ambient temperature has an average hardness of 35.31 GPa, slightly larger than SiO 2 stishovite, which is often referred to as the third hardest material after diamond and cubic boron nitride. The cubic phase is stable up to 1673 K in air. At 1873 K, α- and β-Si 3 N 4 phases are observed, indicating a phase transformation sequence of c-to-α-to-β-Si 3 N 4 phases.