Root causes of the performance of boron carbide under stress

The absence of a plastic phase in boron carbide and its failure at shock impact velocities just above the Hugoniot elastic limit (H_EL) has been the subject of several experimental investigations. Furthermore, the common presence of contaminants, such as disordered graphitic inclusions, oxygen, etc., needs to be addressed. Further, a theoretical picture accounting all these phenomena is still lacking. In the present work, using self-consistent field density functional simulations we are able to account for many experimental observations by noticing that several boron carbide polytypes [e.g. (B ₁₁C)CBC, (B₁₂)CCC, ...] coexist without significant lattice distortions. Our analysis also indicates that above a threshold pressure all such polytypes are less stable than a phase involving segregated boron (812) and amorphous carbon (a-C) but the energy barrier for the transformation into a segregated phase of boron and carbon, is by far lower for the B ₁₂(CCC) polytype. For such a configuration, segregation of carbon occurs in layers orthogonal to the (113) lattice directions, in excellent agreement with recent transmission electron microscopy (TEM) analysis. We will also, in the actual preparation conditions of the material, show that the Gibbs free energy per site in the B₁₂ + a-C segregate phase, in B ₁₂O₂ and B₄C_1-xSi_x is not significantly lower than in most of the B₄C polytypes. Silicon inclusions, however, should strongly reduce the formation of the (B ₁₂)CCC phase.