Correlation between hardness and bond orientation of vanadium borides

The relationship between hardness and bond characteristic of vanadium borides was investigated by first-principles approach. The calculated lattice parameters of V–B system are in good agreement with previous experimental data. The convex hull indicates that the VB are most stable at ground state. The vanadium borides have higher bulk modulus, shear modulus and Young's modulus, and lower B/G ratio. These vanadium borides are brittle. We predict that the V5B6 and VB2 are potential superhard materials. The nature of hardness is related not only to covalent bonding but also to bond orientation. The B–B and V–B covalent bonds parallel to the load plane are the origin of high levels of hardness.

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