The hardness and mechanical properties of cfc C 8 are close to those of diamond and lonsdaleite, as expected. The corresponding values for the new BN polytypes are significantly lower and are at the level of the mechanical properties of wurtzite boron nitride, while the density, hardness and elastic moduli are practically the same for all three polytypes. As for hexagonal BC 2 N, in terms of mechanical properties, it also occupies an intermediate position between C 8 and BN polytypes, with a hardness 12% lower than that of experimentally synthesized cubic BC 2 N , and 7% lower than calculated H V values of hypothetical orthorhombic and trigonal BC 2 N .