First-Principles Investigation of Oxidation Mechanisms in MoAlB

MoAlB's remarkable oxidation resistance, attributed to a protective Al2O3 scale, necessitates computational analysis for deeper insights into its oxidation mechanisms. We examined the initial oxidation of low-energy MoAlB surfaces under varying conditions. The (010) surface, Al-terminated and the most stable, was identified through surface energy calculations. We explored defect formation, noting the sensitivity of Al, Mo, and B vacancies to surface termination and position. Different surfaces displayed distinct defect energetics, influencing oxidation reactions and products. For instance, Al vacancy formation on the (111) surface was less favorable than that in bulk MoAlB but more favorable on the (010) surface. Mo vacancy formation was enhanced near the surface. O2 dissociatively adsorbed on all surfaces, with strong binding energies. We calculated diffusion barriers for Al vacancies and O atoms, revealing the crucial role of Al vacancies in oxidation. Energetics of the oxidation reactions indicated that Al2O3 formation involves sequential reactions. Molecular dynamics simulations at various temperatures highlighted the complex, termination-dependent oxidation of MoAlB surfaces. For instance, the (010) surface may favor MoO2 and MoO3 formation. Our study provides valuable atomic-level insights into MoAlB surface stability, structure, and oxidation behavior.