Superhard nanocomposite of dense polymorphs of boron nitride: Noncarbon material has reached diamond hardness

The authors report a synthesis of unique superhard aggregated boron nitride nanocomposites (ABNNCs) showing the enhancement of hardness up to 100% in comparison with single crystal c-BN. Such a great hardness increase is due to the combination of the Hall-Petch and the quantum confinement effects. The decrease of the grain size down to 14nm and the simultaneous formation of the two dense BN phases with hexagonal and cubic structures within the grains at nano- and subnanolevel result in enormous mechanical property enhancement with maximum hardness of 85(5)GPa. Thus, ABNNC is the first non-carbon-based bulk material with the value of hard-ness approaching that of single crystal and polycrystalline diamond and aggregated diamond nanorods. ABNNC also has an unusually high fracture toughness for superhard materials (K1C=15MPam0.5) and wear resistance (WH=11; compare, for industrial polycrystalline diamond, WH=3–4), in combination with high thermal stability (above 1600K in air), making it an exceptional superabrasive.