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 (01Ì...11)/<101Ì...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.