Unveiling the Impact of Substitutional Doping on the Oxidation Behavior of MoAlB: A Comprehensive First-Principles Study

This study utilized density functional theory to examine how Si, Zr, Cr, and C doping affects the oxidation behavior of the (010) and (111) surfaces of MoAlB. Si doping at the Al site weakens Al-O bonds, while Zr doping at the Mo site significantly strengthens the Zr-O bonds, enhancing the stability of the doped structure. Zr and C doping cause notable variations in Al, B, and Mo defect formation energies on the surface layer, while Si has a lesser impact. Zr doping reduces Al vacancy diffusion barriers by 0.2 eV, promoting Al migration toward the surface and enhancing the formation of a protective Al2O3 layer on the MoAlB surface. In contrast, Si doping increases the Al vacancy diffusion barrier energy by 0.1 eV, potentially slowing oxidation on the surface. Zr increases oxygen penetration barriers into subsurface layers, strengthening surface oxidation but limiting deeper migration, while Si lowers these barriers, allowing more extensive oxygen diffusion. Our molecular dynamics simulations at 1200 K showed that Zr attracts more oxygen atoms on the surface but prevents deep penetration of O atoms. These findings underscore the distinct effects of different dopants on the oxidation mechanics and stability of MoAlB, with implications for high-temperature applications.