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Prediction of superstrong tau-boron carbide phase from quantum mechanics
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Searching for ultrahard materials is of great interest in scientific research and for industrial applications. Boron carbide (B4C ) is one of the hardest known materials, but its Vickers hardness (30 GPa) is much less than diamond (115 GPa) and cubic boron nitride (48 GPa). A new B4C phase with twinlike character, denoted as ? -B4C , is characterized using density functional theory. This ? -B4C is based on the Cmcm orthorhombic space group and is slightly less stable than the known rhombohedral B4C (R -B4C ) by 3.87 meV /B4C . Resulting shear deformation along the least stress slip system shows that ? -B4C has an ideal shear strength of 45.1 GPa, which is 15.6% higher than R -B4C , suggesting that ? -B4C is much stronger than R -B4C . However, under biaxial shear deformation to mimic indentation stress conditions, the critical shear stress for ? -B4C is 28.8 GPa, which is similar to that of R -B4C (28.5 GPa), indicating that the intrinsic hardness is similar for these two phases. The failure mechanism of the ? -B4C is the deconstruction of the icosahedra arising from the B-C bond breaking within the icosahedron. It may be worth exploring how to synthesize ? -B4C experimentally because of its high strength.
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