Improved Ductility of Boron Carbide by Microalloying with Boron Suboxide

Boron carbide (B₄C) is the third hardest material in nature, but applications are hindered by its brittle failure under impact. We found that this brittle failure of B₄C arises from amorphous shear band formation due to deconstruction of icosahedral clusters, and on the basis of this model we suggest and validate with quantum mechanics (QM, PBE flavor of density function theory) that a laminated B₄C-B₆O composite structure will eliminate this brittle failure. Using QM to apply shear deformations along various slip systems, we find that the (001)/[100] slip system has the lowest maximum shear strength, indicating it to be the most plausible slip system. We find that this composite structure has a shear strength of 38.33 GPa, essentially the same as that of B₄C (38.97 GPa), indicating the same intrinsic hardness as B₄C. However, the critical failure strain for (001)/[100] slip in the composite is 0.465, which is 41% higher than B₄C, indicating a dramatically improvement on ductility. This arises because incorporation of B₆O prevents the failure mechanism of B₄C in which the carbene formed during shear deformation reacts with the C-B-C chains. This suggests a new strategy for designing ductile superhard ceramics.