Exploring oxidation behavior of Nb₄AC₃ MAX phases (A = Si and Ga): Ab initio calculations

MAX phases have the potential for high-temperature applications due to good oxidation resistance. However, the micro-mechanisms of oxidation behavior of MAX phases on the atomic scale have not been widely studied yet. To explore physical insight into the oxidation process, we investigated the equilibrium structural feature, energy change, electronic structure, and bonding feature of oxidized Nb₄AC₃ MAX phases (A = Si and Ga) using ab initio calculations. Two potential O-doped sites, A and B, were taken into consideration. It was found that the O-doped energy of Nb₄SiC₃ was lower than that of Nb₄GaC₃. According to the electronic structure and Mulliken population analysis, when site B was occupied by the O atom, the good oxidation resistance of Nb₄SiC₃ may be derived from the formation of the Si-O bond without the Nb-O bond. Interestingly, the formed Si-O bond was a mixture of the covalent bond and ionic bond, while the formed Ga-O bond was purely ionic. In summary, our work can provide an interesting perspective to understand the oxidation mechanism of Nb₄AC₃ MAX phases.