Cubic ${\mathrm{BC}}_{\mathrm{2}}\mathrm{N}$ was synthesized from graphite-like ${\mathrm{BC}}_{\mathrm{2}}\mathrm{N}$ at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of $\mathrm{c-}{\mathrm{BC}}_{\mathrm{2}}\mathrm{N}$ at ambient conditions is 3.642(2) Å, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of $\mathrm{c-}{\mathrm{BC}}_{\mathrm{2}}\mathrm{N}$ is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and $\mathrm{c-}\mathrm{BN}.$ The hardness of $\mathrm{c-}{\mathrm{BC}}_{\mathrm{2}}\mathrm{N}$ is higher than that of $\mathrm{c-}\mathrm{BN}$ single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

Here, we report the synthesis of cubic ${\mathrm{BC}}_5$ ($\mathrm{c}\mathrm{\text{−}}{\mathrm{BC}}_5$), the diamondlike B-C phase with the highest boron content ever achieved, at 24 GPa and about 2200 K, using both a laser-heated diamond anvil cell and large-volume multianvil apparatus. The synthesized phase is low compressible (bulk modulus of 335 GPa), conductive, and exhibits extreme Vickers hardness (71 GPa), unusually high for superhard materials fracture toughness ($9.5\mathrm{MPa}{\mathrm{m}}^{0.5}$), and high thermal stability (up to 1900 K); this makes it an exceptional superabrasive and promising material for high-temperature electronics.

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 $14\mathrm{nm}$ 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 $85\left(5\right)\mathrm{GPa}$ . 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 $(K_{1C}=15\mathrm{MPa}{\mathrm{m}}^{0.5})$ and wear resistance ($W_H=11$ ; compare, for industrial polycrystalline diamond, $W_H=3–4$ ), in combination with high thermal stability (above $1600\mathrm{K}$ in air), making it an exceptional superabrasive.