Dense nanocrystalline ReB2 bulk with ultrahigh hardness

Microcrystalline ReB2 with a high hardness of 48 GPa measured under a small load of 0.49 N has been reported as a superhard material. Nanocrystalline ReB2 is expected to achieve better mechanical properties than its microcrystalline counterparts. Here we report dense nanocrystalline ReB2 bulks synthesized by modified me-chanical alloying and high-pressure sintering techniques. The modified mechanical alloying technique reduces the content of WC impurity from 7.75 wt% to 1.59 wt% in as-synthesized nanocrystalline ReB2 powders with an average grain size of 6.6 & PLUSMN; 2.0 nm. The extrinsic high-pressure (6 GPa) and the intrinsic nanocrystalline structure greatly promote the densification process of the ReB2 bulks during sintering. Our optimized nanocrystalline ReB2 bulk is-99% dense and with an average crystallite size of 40 nm, resulting in a high hardness of-33 GPa under a load of 9.8 N and indentation fracture toughness of-5 MPa m1/2. The high hardness can be ascribed to the synergistic effect of high density and nanocrystalline structure of ReB2 bulks. This work provides promising strategies to largely enhance the mechanical properties of different ceramics for practical applications.