Enhancing the Vickers hardness of Yttrium borides through bond optimization

Abstract

The hard nature of transition metal boride reminds a conundrum. Compared to the diboride, we propose that the high hardness of transition metal boride strongly depends on the structural feature and bonding state (bond type, bond number and bond direction) in addition to the network B-B covalent bond. To demonstrate our idea, we symmetrically study the structural configuration, hardness, elastic properties and bonding state of the B-rich Y-B borides by using the first-principles method. The hexagonal YB2, tetragonal YB4, cubic YB6 and cubic YB12 are studied. The convex hull implies that YB4 has better thermodynamic stability in comparison to the YB2, YB6 and YB12. Importantly, it is found that the calculated Vickers hardness of the cubic YB12 is 41.4 GPa, indicating that YB12 is a potential superhard material. However, the Vickers hardness of YB2, YB4 and YB6 is lower than 40 GPa. Naturally, the high harness of YB12 is determined by the network B-B covalent bond. For non-superhard materials (YB2, YB4 and YB6), the hardness is related not only to the 3D network B-B covalent bond but also to the structural feature and bond state, particularly for TM-B bond. Compared to the strong B-B covalent bond, the Y-B bond plays an important role in harness because the Y-B bond is not surrounded by the 3D network B-B covalent bond. Therefore, we believe that the bond type and bond arrangement play an important role in hardness. This work can provide new design rule to adjust the structural feature and bond state to enhance the Vickers hardness of transition metal borides superhard materials.