Electronic structure, bonding characteristics, and mechanical behaviors of a new family of Si-containing damage-tolerant MAB phases M₅SiB₂ (M = IVB–VIB transition metals)

MAB phases are layered ternary compounds with alternative stacking of transition metal boride layers and group A element layers. Until now, most of the investigated MAB phases are concentrated on compounds with Al as the A element layers. In this work, the family of M₅SiB₂ (M = IVB–VIB transition metals) compounds with silicon as interlayers were investigated by density functional theory (DFT) methods as potential MAB phases for high-temperature applications. Starting from the known Mo₅SiB₂, the electronic structure, bonding characteristics, and mechanical behaviors were systematically investigated and discussed. Although the composition of M₅SiB₂ does not follow the general formula of experimentally reported (MB)_2zA_x(MB₂)_y (z = 1, 2; x = 1, 2; y = 0, 1, 2), their layered structure and anisotropic bonding characteristics are similar to other known MAB phases, which justifies their classification as new members of this material class. As a result of the higher bulk modulus and lower shear modulus, Mo₅SiB₂ has a Pugh’s ratio of 0.53, which is much lower than the common MAB phases. It was found that the stability and mechanical properties of M₅SiB₂ compounds depend on their valence electron concentrations (VECs), and an optimum VEC exists as the criteria for stability. The hypothesized Zr and Hf containing compounds, i.e., Zr₅SiB₂ and Hf₅SiB_2, which are more interesting in terms of high-temperature oxidation/ablation resistance, were found to be unfortunately unstable. To cope with this problem, a new stable solid solution (Zr_0.6Mo_0.4)₅SiB₂ was designed based on VEC tuning to demonstrate a promising approach for developing new MAB phases with desirable compositions.