Brittle failure of orthorhombic borides from first-principles simulations

Abstract

The orthorhombic boride family X M B14 , where X and M are metal atoms, have been of great interest in hard coating applications because of such novel properties as high thermal stability, low density, chemical stability, and a low friction coefficient. However, the brittle failure of orthorhombic borides limits their mechanical stability under working environments and prevents their extended engineering applications. To provide guidelines of improving their stability, we employed density functional theory (DFT) to examine the bonding character and mechanical response of X M B14 under pure shear, biaxial shear, and tensile loading conditions. Two typical X M B14 compounds, AlLiB14 and A l0.75M g0.78B14 , were examined to illustrate the effects of intrinsic metal vacancies. We find that the ideal strength for AlLiB14 is higher than that for A l0.75M g0.75B14 , suggesting that AlLiB14 is intrinsically stronger than A l0.75M g0.75B14 . The failure mechanism of both AlLiB14 and A l0.75M g0.78B14 arises from deconstructing B12 icosahedra under pure shear and biaxial shear conditions, while the structural failure under tensile deformation arises from breaking interlayer bonds between icosahedral layers.