Improved ductility of B12 icosahedra-based superhard materials through icosahedral slip

Abstract

Boron carbide (B_4C) is superhard but suffers from brittle failure because shear stress leads to formation of amorphous shear bands in which the icosahedral clusters fracture, leading to amorphous regions with higher density than the crystal that results in tension induced cavitation and brittle failure. Based on our previous studies of related systems, we speculated that replacing the C-B-C chains with Si_2 and replacing cage C with Si might reduce or eliminate this amorphous shear band formation responsible for brittle failure, In this paper we consider (B_(10)Si_2)Si_2, using density functional theory (DFT) to examine its shear deformation. We find that the stress accumulated as shear increases is released by slip of the planes of icosahedra through breaking and then reforming the Si-Si chain bonds without fracturing (B_(10)Si_2) icosahedra. This is because the (B_(10)Si_2) icosahedra are more stable than the chain under highly stressed conditions. This chain disruption deformation mode prevents amorphous shear band formation, providing a strategy to dramatically improve the ductility of B_4C based materials. Our results suggest that making the icosahedra more stable than the chains structure can lead to slipping of the icosahedral planes while avoiding the icosahedral fracturing that leads to brittle failure.