First-Principles Modeling and Evolutionary-Algorithm Prediction of Superhard B-C and B-N systems*

Superhard materials with a Vickers hardness larger than ~40 GPa have a wide range of industrial applications such as cutting tools and protective coatings. Superhard boron-carbon (B-C) and boron-nitride (B-N) composites are especially important because of their superior high-temperature performance as compared to diamond and their low reactivity with ferrous metals. Here we employ evolutionary algorithm to predict the crystal structures of superhard B-C and B-N composites. We also apply density functional theory to study mechanical properties, electronic structures, phonon and Raman spectra of different B-C and B-N systems, including boron-doped cubic diamond and icosahedral boron subnitrides. Comparison of our calculations and recent experimental data will be discussed as well.