Hardness and Mechanical Properties of Wurtzite B–C–N Compounds

Diamond, cubic BN (c-BN), and cubic B–C–N compounds, i.e., c-BCXN, are well-known ultrahard materials. While cubic phases of these materials are relatively stable and have been widely studied, recent experimental and theoretical studies have reported evidence that indentation hardness can be significantly enhanced in wurtzite phases of BN (w-BN) and diamond, i.e., lonsdaleite. However, the mechanical properties and hardness of wurtzite B–C–N compounds have been less explored. In this study, we systemically investigated wurtzite B–C–N compounds (w-BCXN), such as wurtzite BCN (w-BCN), BC2N (w-BC2N), and BC4N (w-BC4N), and compared their properties with those of c- and w-BN, diamond, and lonsdaleite. Although w-BC X N shows relatively high formation energies compared with the others, configurational entropy can assist the compounds to be stabilized at elevated growth temperatures (over 2700 K). In addition, w-BCXN exhibits a relatively superior hardness, which increases with the C ratio. Our results provide a basic understanding of w-BCXN as a possible family of superhard materials.