Nanocrystalline Cubic Silicon Carbide: A Route to Superhardness

Superhard materials other than diamond and cubic boron nitride have been actively pursued in the past two decades. Cubic silicon carbide, i.e., β-SiC, is a well-known hard material with typical hardness <30 GPa. Although nanostructuring has been proven to be effective in enhancing materials' hardness by virtue of the Hall-Petch effect, it remains a significant challenge to improve hardness of β-SiC beyond the superhard threshold of 40 GPa. Here, the fabrication of nanocrystalline β-SiC bulks is reported by sintering nanoparticles under high pressure and high temperature. These β-SiC bulks are densely sintered with average grain sizes down to 10 nm depending on the sintering conditions, and the Vickers hardness increases with decreasing grain size following the Hall-Petch relation. Particularly, the bulk sintered under 25 GPa and 1400 °C shows an average grain size of 10 nm and an asymptotic Vickers hardness of 41.5 GPa. Boosting the hardness of β-SiC over the superhard threshold signifies an important progress in superhard materials research. A broader family of superhard materials is in sight through successful implementation of nanostructuring in other hard materials such as BP.