Ultrahard C2-BN nanocomposite

Figure (a) Vickers hardness of new C<sub>2</sub>-BN nanocomposite is 85(±5) GPa, placing it in the ultrahard class of materials. (b) synthesis at 7.5 GPa and 2273 K leads to bulk samples up to 10 mm in diameter, which can be made more transparent (c) through annealing at 18 GPa and 2100 K. The new C<sub>2</sub>-BN nanocomposite consists of nanotwinned diamond and cBN domains sutured by 5 nm wide B-C-N solid solution, leading to bulk p-type semiconductivity.
Figure (a) Vickers hardness of new C2-BN nanocomposite is 85(±5) GPa, placing it in the ultrahard class of materials. (b) synthesis at 7.5 GPa and 2273 K leads to bulk samples up to 10 mm in diameter, which can be made more transparent (c) through annealing at 18 GPa and 2100 K. The new C2-BN nanocomposite consists of nanotwinned diamond and cBN domains sutured by 5 nm wide B-C-N solid solution, leading to bulk p-type semiconductivity.

Materials combining the hardness and strength of diamond with the higher thermal stability of cubic boron nitride (cBN) have broad potential value in science and engineering. Reacting nanodiamond with cBN at high pressures and high temperatures provides a pathway to such materials. Postdoctoral associate Dr. Xiaobing Liu, with CDAC partner Steve Jacobsen at Northwestern University, report the fabrication of C2-BN nanocomposite (2:1 ratio of diamond to cBN), measuring up to 10 mm in longest dimension. The nanocomposite consists of randomly-oriented, diamond and cBN domains of 50-250 nm, stitched by sp3 hybridized C-B and C-N bonds. B-C-N solid solution within the sutures leads to bulk p-type semiconductivity with an activation energy of 6.2 meV. The new study, published in the 27 July issue of Scientific Reports, shows that dislocations near the sutures accommodate lattice mismatch between diamond and cBN. Nanotwinning within both diamond and cBN domains further contributes to a bulk hardness ~50% higher than sintered cBN. The nanocomposite of C2-BN is stable in air up to 1183 K and becomes transparent when re-annealed at higher pressures of 18 GPa and 2100 K. High-pressure X-ray diffraction experiments were conducted at HPCAT.

X. Liu et al. (2016) Sci. Rep. 6, 30518.  http://www.nature.com/articles/srep30518