Regardless of the model used, all three BP polymorphs are characterized by very close hardness values. As for the absolute H V values, the thermodynamic model seems to be the most reliable, because it shows the best agreement with experimental data for c-BP . It is worth mentioning that all three polymorphs have virtually equal bulk moduli, while their shear and Young's moduli differ noticeably, decreasing in the c-BP -w-BP -rh-BP row (see Table ). Fracture toughness of all three polymorphs is the same and almost twice lower than that of single-crystal cubic BN .