Breaking the Hardness Limit of WB₄: Transformation From β-B to Harder T-B

Transition metal borides as a kind of novel multifunctional superhard material remain inferior in hardness compared with traditional superhard material. To this end, this study proposes a novel approach to enhance their hardness through a synergistic hard mechanism. The large-scale superhard WB₄ bulk is synthesized by modulating the boron isomers (β-B→T-B phase transition) at mild temperature and press conditions using a spark plasma sintering technique. The results show that WB₄-TB bulk is composed of nanosized WB₄ grains and T-B grains with a high density of stacking faults and grain boundary distortions, which exhibits a Vickers microindentation hardness of 63.1 GPa (0.49 N load) that is a 37% enhancement over the conventional WB₄ (≈46.2 GPa), surpassing all reported transition metal borides and approaching the performance of covalent superhard materials. The synergistic effects of fine grain hardening, generation of superhard T-B phase, and grain boundary strengthening inhibit dislocation motion and crack extension. This work provides a promising paradigm for designing new materials with both superhard and functional properties.