The Effects of Carbon Content on the Anisotropic Deformation Mechanism of Boron Carbide.

The effects of carbon content on the mechanical properties and deformation mechanisms of boron carbides were investigated by first-principles calculations, based on the density functional theory. The B⁻CBC (13.33 at % C) and B C 2 P ⁻CC (28.75 at % C) were studied and then compared with the deformation of regular BC⁻CBC (20.0 at % C). The results show the B C 2 P ⁻CC, which has the lowest carbon content, has the highest strength and hardness as well as the lowest toughness. With the increase of carbon content, the rhombohedral symmetry will be broken and the three-atoms chains will be replaced by diatomic carbon chains. These changes may have an influence on their anisotropic deformation mechanisms. For the B⁻CBC, the destruction of icosahedra without bending three-atom chains causes structural failure for compression along the axis; while for compression along the axis, new B⁻B bonds are formed, causing an unrecoverable deformation; then it is gradually destroyed until full destruction. For the B C 2 P ⁻CC, the anisotropic deformation mechanism is not obvious. For both loading directions, the breakage of B⁻C bonds causes the stress to drop, suggesting that the structure is beginning to be destroyed. Finally, the icosahedra are fully destroyed, resulting in structural failure.