Boron Suboxide and Boron Subphosphide Crystals: Hard Ceramics That Shear without Brittle Failure

Boron suboxide (B6O), boron carbide (B4C), and related materials are superhard. However, they exhibit low fracture toughness, which limits their engineering, applications. Here we show the shear deformation mechanism of B6O using density functional theory along the most plausible slip system (0 (1) over bar 11)/<10<(1)over bar>1>. We discovered an unusual phenomenon in which the highly sheared system recovers its original crystal structure, which indicates the possibility of being sheared to a large strain without failure. We also found a similar structural recovery in boron subphosphide (B12P2) for shearing along the same slip system. In contrast, for components of B4C, we found brittle failure. These novel deformation mechanisms under high shear deformation conditions suggest that a key element to designing ductile hard materials is to couple the icosahedra via one- or two-atom chains that allow the system to shear by walking the intericosahedral bonds and chain bonds alternately to accommodate large shear without fracturing the icosahedra.