Improved Ductility of Boron Carbide by Microalloying with Boron Suboxide

What is Boron carbide?

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 the deconstruction of icosahedral clusters, and based on this model, we suggest and validate with quantum mechanics (QM, PBE flavor of density functional 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. 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 dramatic improvement in flexibility. This arises because incorporating 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.

Improved Ductility of Boron Carbide by Microalloying with Boron Suboxide

We investigate a laminated composite structure of B4C–B6O in which layers of B4C alternate with B6O. This is meant to model 5–10 nm particles, which would be essentially flat at our scale. We then sheared the composite structure along various slip systems both parallel to and perpendicular to the ordered planes to determine which configurations require the least shear stress. We find that the shear strength of the most plausible slip system is very close to the ideal shear strength of B4C, indicating an intrinsic hardness similar to that of B4C. More importantly, the critical failure strain for this slip system is 41% larger than that of perfect B4C, indicating a dramatically improved toughness. We describe the deformation mechanisms along various slip systems to provide an atomistic understanding of the deformation mechanisms and their coupling to the mechanical properties of these composite ceramics.

B6O and B4C have the same space group and similar lattice parameters

Among these superhard materials, B6O and B4C have the same space group and similar lattice parameters; therefore, we speculated that a solid–solution interlayer between B6O and B4C crystals could be formed during the sintering reaction that might increase the hardness and toughness properties. For example, the fracture toughness of the nanostructured B6O–B4C improves to 8.72 MPa m1/2 in contrast to 1.3 MPa m1/2 of the single phase B4C. In these systems, the B6O and B4C grains were both well-dispersed concerning each other, suggesting that interlayer bonding is critical to the improved properties. To validate such concepts, we used QM to examine how the atomistic level bonding structure between these two components might affect the mechanical properties.

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