Superhard CubicBC2NCompared to Diamond

Recent experiments claimed successful synthesis of cubic boron-carbonitride compounds ${\mathrm{B}\mathrm{C}}_2\mathrm{N}$ with an extreme hardness second only to diamond. In the present Letter, we examine the ideal strength of cubic ${\mathrm{B}\mathrm{C}}_2\mathrm{N}$ using first-principles calculations. Our results reveal that, despite the large elastic parameters, compositional anisotropy and strain dependent bonding character impose limitation on their strength. Consequently, the hardness of the optimal ${\mathrm{B}\mathrm{C}}_2\mathrm{N}$ structure is predicted to be lower than that of cubic BN, the second hardest material known. The measured extreme hardness of ${\mathrm{B}\mathrm{C}}_2\mathrm{N}$ nanocomposites is most likely due to the nanocrystalline size effect and the bonding to the surrounding amorphous carbon matrix. This may prove to be a general rule useful in the quest for new superhard covalent materials.