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

Boron suboxide (B_6O), boron carbide (B_4C), and related materials are superhard. However, they exhibit low fracture toughness, which limits their engineering applications. Here we show the shear deformation mechanism of B_6O using density functional theory along the most plausible slip system (0111)/. 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 (B_(12)P_2) for shearing along the same slip system. In contrast, for components of B_4C, 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