Hardening Effects in Superhard Transition-Metal Borides

Mechanical hardness is a physical property used to gauge the applications of materials in the manufacturing and machining industries. Because of their high hardness and wear resistance, superhard materials (Vickers hardness, H-v >= 40 GPa) are commonly used as cutting tools and abrasives. Although diamond is the hardest known material used for industrial applications, its synthesis requires both high pressure and high temperature. Interest in the field of superhard materials research has led to the search for alternatives with high hardness and thermal stability at low cost. The discovery of novel ultraincompressible, superhard materials has largely developed through trial and error along two paths. In one approach, researchers combine light elements, such as boron, carbon, nitrogen, and oxygen, often at high pressure, to replicate the highly directional, dense, covalent bonds of diamond. In the second approach, these light elements (B, C, N, and O) are combined with highly incompressible, electron-rich transition metals to form dense covalently bonded networks at ambient pressure. In this Account, we highlight our progress in developing superhard transition-metal borides through solid solution effects and grain boundary strengthening. We begin with a review of the factors that contribute to a material's hardness and guide our design parameters of high electron density and high covalent bond density in the search for new materials. In subsequent sections, we examine various metal boride systems with increasing bond covalency and structural complexity, from metal-rich mono- and diborides to boron-rich tetra- and dodecaborides. The metal borides discussed in this Account are formed at ambient pressure using high-temperature solid-state techniques such as arc melting and molten flux synthesis. By characterizing these materials through both Vickers hardness testing and high-pressure experiments, we gain insight into the coupled effects of bonding and grain morphology on mechanical properties. Finally, we provide an outlook into the expedited discovery and accessible compositions for future materials. We hope that the materials and methods discussed in this Account offer new opportunities for the design and synthesis of the next generation of superhard materials for industrial applications.