Rhenium diboride is an established superhard compound that can scratch diamond and can be readily synthesized under ambient pressure. Here, we demonstrate two synergistic ways to further enhance the already high yield strength of ReB2. The first approach builds on previous reports where tungsten is doped into ReB2 at concentrations up to 48 at. %, forming a rhenium/tungsten diboride solid solution (Re0.52W0.48B2). In the second approach, the composition of both materials is maintained, but the particle size is reduced to the nanoscale (40–150 nm). Bulk samples were synthesized by arc melting above 2500 °C, and salt flux growth at ∼850 °C was used to create nanoscale materials. In situ radial X-ray diffraction was then performed under high pressures up to ∼60 GPa in a diamond anvil cell to study mechanical properties including bulk modulus, lattice strain, and strength anisotropy. The differential stress for both Re0.52W0.48B2 and nano ReB2 (n-ReB2) was increased compared to bulk ReB2. In addition, the lattice-preferred orientation of n-ReB2 was experimentally measured. Under non-hydrostatic compression, n-ReB2 exhibits texture characterized by a maximum along the [001] direction, confirming that plastic deformation is primarily controlled by the basal slip system. At higher pressures, a range of other slip systems become active. Finally, both size and solid-solution effects were combined in nanoscale Re0.52W0.48B2. This material showed the highest differential stress and bulk modulus, combined with suppression of the new slip planes that opened at high pressure in n-ReB2.

Conspectus 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.

Materials with superior hardness can be categorized as ultrahard (Vickers hardness, H v  ≥ 80 GPa) and superhard ( H v  ≥ 40 GPa). These materials are commonly used as cutting tools and abrasives in the machining and manufacturing industries. With its extreme hardness, diamond is the best known and most used ultrahard material for industrial applications. However, it is ineffective at cutting and drilling ferrous alloys due to diamond's high reactivity with iron and poor thermal stability in air. Additionally, the synthesis of diamond requires both high pressure (HP) and high temperature, making it an expensive process. These limitations have driven the search for alternative superhard materials that are capable of cutting steels and other materials at lower costs. This article reviews the concept of hardness and summarizes advancements in the synthesis and mechanical properties of hard materials. It begins with a review of methods to measure hardness, adding HP diffraction methods to more conventional hardness measurements. It then considers new ultrahard materials that exist within the B–C–N ternary system, with hardness approaching diamond but improved chemical stability. Finally, it surveys superhard nitrides, oxides, and borides as potential alternative materials, focusing on transition metal boride systems where the synthesis can be readily achieved at ambient pressure and scaled for industrial applications. We hope that this article serves as an overview of hard materials and guide for the comparison of data reported in the literature.

Rhenium diboride (ReB2) exhibits high differential strain due to its puckered boron sheets that impede shear deformation. Here, we demonstrate the use of solid solution formation to enhance the Vickers hardness and differential strain of ReB2. ReB2-structured solid solutions (Re0.98Os0.02B2 and Re0.98Ru0.02B2, noted as “ReOsB2” and “ReRuB2”) were synthesized via arc-melting from the pure elements. In-situ high-pressure radial x-ray diffraction was performed in the diamond anvil cell to study the incompressibility and lattice strain of ReOsB2 and ReRuB2 up to ∼56 GPa. Both solid solutions exhibit higher incompressibility and differential strain than pure ReB2. However, while all lattice planes are strengthened by doping osmium (Os) into the ReB2 structure, only the weakest ReB2 lattice plane is enhanced with ruthenium (Ru). These results are in agreement with the Vickers hardness measurements of the two systems, where higher hardness was observed in ReOsB2. The combination of high-pressure studies with experimentally observed hardness data provides lattice specific information about the strengthening mechanisms behind the intrinsic hardness enhancement of the ReB2 system.

Tungsten diboride (WB 2 ) solid solutions with increasing molybdenum (Mo) substitution were synthesized by resistive arc-melting from the pure elements and characterized for their mechanical properties. The WB 2 -type structure is maintained up to 30 atomic percent (at%) Mo substitution. W 0.70 Mo 0.30 B 2 achieved a maximum Vickers hardness of 45.7 ± 2.5 GPa at 0.49 N, resulting in the hardest WB 2 solid solution to date. In agreement with this fact, high-pressure radial diffraction studies indicate that substitution of Mo into WB 2 strengthens metal−boron bonding, as the solid solution supports high differential stress and has a bulk modulus of 355 ± 2 GPa. WB 2 and W 0.70 Mo 0.30 B 2 composites were then synthesized with increasing additive content (0−30 wt%) of B 4 C or SiC to study extrinsic hardening effects through multiphase formation. These composites show extrinsic effects on the Vickers hardness because of secondary-phase precipitation. While WB 2 −30 wt% B 4 C exhibited the highest hardness (53.8 ± 6.0 GPa at 0.49 N), WB 2 −30 wt% SiC demonstrated the slowest oxidation rate. This work offers new insights for tailoring transition-metal boride systems with optimized hardness, grain morphology, and thermal stability.