Hardness of cubic silicon nitride

We report that polycrystalline cubic-Si3N4 with a spinel structure and low oxygen concentration (<0.5 wt%) shows Vickers hardness of 43 GPa when measured with the indentation load of 10 mN. The hardness decreases with the increase of the indentation load, which can be ascribed to the presence of weak grain boundaries. The high hardness can be well explained by its large shear modulus as predicted by first-principles calculations.