Mechanical properties and electronic structures of borides in Ni-Co-B alloy system

Ni-Co-B ternary coating was obtained by electrodeposition with an microhardness of 285–360 HV. After heat treatment, the hardness reaches up to 953–1100 HV, and CoB, Co2B, and Ni3B second phases were observed. The mechanical properties and electronic structures of NiCo and borides (NiB, Ni2B, Ni3B, Ni4B3, CoB, Co2B, and Co3B) were then systematically investigated by first-principles calculations. Among them, CoB possesses the highest bulk modulus (294 GPa), shear modulus (143 GPa), Young's modulus (370 GPa), and hardness (12.73 GPa), but the weakest ductility. In addition, the hardness of borides gradually increases with increasing boron content. The electronic structures indicate that B-B and Co/Ni-B covalent bonds play an important role in the mechanical properties of borides. Overall, CoB has the largest elastic modulus and hardness, which is expected to increase the rigidity of Ni-Co-B alloys. © 2021 Elsevier B.V.