Dense nanocrystalline ReB2 bulk with ultrahigh hardness

Microcrystalline ReB 2 with a high hardness of 48 GPa measured under a small load of 0.49 N has been reported as a superhard material. Nanocrystalline ReB 2 is expected to achieve better mechanical properties than its microcrystalline counterparts. Here we report dense nanocrystalline ReB 2 bulks synthesized by modified mechanical 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 ReB 2 powders with an average grain size of 6.6 ± 2.0 nm. The extrinsic high-pressure (6 GPa) and the intrinsic nanocrystalline structure greatly promote the densification process of the ReB 2 bulks during sintering. Our optimized nanocrystalline ReB 2 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 m 1/2 . The high hardness can be ascribed to the synergistic effect of high density and nanocrystalline structure of ReB 2 bulks. This work provides promising strategies to largely enhance the mechanical properties of different ceramics for practical applications.