Modulating Hardness in Molybdenum Monoborides by Adjusting an Array of Boron Zigzag Chains

Hardness is an essential but complex property of materials. Although it is generally accepted that a high hardness is related to a high covalent bond density, the determinant of hardness is always unclear. To overcome the restriction of the high density of covalent bonds, the low-boron content transition metal borides (TMBs) are chosen to explore a new way to enhance hardness. We fix the density of covalent bonds and modulate the hardness by designing perpendicular boron zigzag chain (Bzc) skeletons (pe-Bzcs) and parallel Bzc skeletons (pa-Bzcs) in α-MoB (I41/amd) and β-MoB (Cmcm). We utilize pe-Bzcs in α-MoB to enhance the shear modulus via less slippage than in pa-Bzcs. Pe-Bzcs generate a higher grain boundary density in α-MoB than in β-MoB to create a nano-polycrystal morphology under high pressure and high temperature. Hence, the hardness of α-MoB (18.4 GPa) is greater than that of β-MoB (12.2 GPa), which can be attributed to the higher shear modulus and higher density of the grain boundary caused by pe-Bzcs. This work suggests a new idea that modulating boron covalent bond substructures is an effective way to enhance hardness even in low-boron content TMBs. This finding is significant for the design new hard or superhard functional materials.