Structural deformation, strength, and instability of cubic BN compared to diamond: A first-principles study

Abstract

Cubic boron nitride ($c$-BN) is the second (only to diamond) hardest material with superior thermal stability. Despite its wide range of applications as a superhard material, the structural deformation modes of $c$-BN at the atomistic level are still not well understood. In this paper, we report first-principles calculations on its structural deformation, strength, and lattice instabilities under large tensile and shear strains. Calculations are also performed for diamond to extend previous results for a systematic comparison with $c$-BN. We examine the atomistic bonding structural change and analyze the calculated stress-strain relations for a microscopic understanding of the deformation modes. Both $c$-BN and diamond show essentially isotropic elastic response at small strains under tensile and shear deformation. At larger strains, anisotropies in the stress response develop, yielding significantly different peak stresses along different tensile and shear directions. It results in a strong tendency for tensile fractures in the (111) planes in both materials. The local bonding structural relaxation modes are analyzed to understand the large anisotropies in the tensile peak stresses in different crystallographic directions and to explain the quantitative differences between $c$-BN and diamond in their stress-strain relations. A simple rule is suggested for determining the direction of the weakest tensile strength for similar covalent solids. Under large shear deformation, the bond breaking in $c$-BN leads to a graphitic phase with an orientation different from that in diamond. Its atomistic origin and possible consequences on the mechanical property are discussed.