Strengthening Tungsten Diboride toward a Superhard Material by Ordered Vacancy Pairs

Tungsten diboride (WB_{2+x}) has been predicted to be a superhard material. It, however, has yet to be practically realized, because of its intrinsically low toughness, without involving favorable dislocation slip systems. Here, we report a viable strategy to effectively strengthen both the toughness and hardness of WB_{2+x} by introducing ordered atomic vacancies to increase dislocation mobility along certain directions. By doping with rhenium atoms, the ordered metal-vacancy pairs are revealed to occur extensively in the optimally doped sample with a composition of (W_{0.9}Re_{0.1})_{1-δ}B_{2+x} synthesized under high pressure. Such vacancy pairs are found to mainly reside in the {210} and {102} planes, along which the long-range dislocations are kinetically favored for improving its toughness and plasticity to achieve a load-invariant superhardness of ∼40  GPa. In addition, its thermal stability is drastically promoted and rivals that of cubic boron nitride (cBN). These discoveries not only experimentally identify a superhard material but also provide powerful insights into how the mechanical properties of transition-metal diborides can be improved by tailoring atomic deficiencies.