Strong anisotropy in strength and toughness in defective hexagonal boron nitride

Abstract

Using a combination of density functional theory calculations and molecular dynamics simulations we show that the strength and toughness of hexagonal boron nitride (hBN) containing isolated vacancy defects are strongly anisotropic, regardless of the size of the defect core. The degree of anisotropy is preserved for a number of defect structures including monovacancy, tetravacancy, tridecavacancy, triheptacontavacancy, or heptatriacontavacancy defects. The chirality-dependent effects are strongly nonlinear and well characterized by close-form mathematical equations indicating pronounced strength and toughness along the zigzag direction compared to the strength and toughness along the armchair direction. Also, the size dependence of the strength and toughness of the defective lattice shows an inverse relationship with the effective diameter of the defect core. An atomistic analysis of the deformation fields reveals that nonuniformity in bond length, bond strain, and force distribution in the nonlinear regime of mechanical deformation surrounding the defect cores forms the physical basis for the observed anisotropy. The anisotropic character of the lattice is governed primarily by the nearest-neighbor covalent interactions (dominated by the first-nearest neighbors). Consequently, as soon as a set of bonds rupture at the defect core, the entire lattice undergoes catastrophic failure underscoring the brittle nature of the fracture state in hBN. Results also suggest that chirality-dependent elastic effects are dominated by the third-order elastic modulus which stiffens the lattice at higher chiral angles, whereas the second- and fourth-order elastic moduli soften the lattice affecting the strength and toughness of the lattice in an intricate manner.