Valence-dependent TiO2 inhibition for enhancing oxidation resistance in Ti2AlC via Zr/Nb solid solution

Ti₂AlC, as a typical MAX phase material, combines the excellent properties of both metals and ceramics for protective coatings used in harsh high-temperature conditions. However, concurrent existence of TiO₂ (rutile) and Al₂O₃ easily causes the rapid growth of oxide scales and poor oxidation durability. In this study, we explored the pivotal concept of vacancy-dependent Zr/Nb solid solution to improve the oxidation resistance of Ti₂AlC coating through the comprehensive atomic-scale calculations and microstructural characterizations. Results showed that the incorporated Zr and Nb concentration was about 5.3 and 5.2 at.% in high-purity Ti₂AlC coating, respectively, presenting the homogeneous substitutions for M-site Ti atoms within the nanolayered structure. Comparing with the pristine Ti₂AlC coating, both Zr and Nb solid solution distinctly inhibited the growth of TiO₂ and simultaneously promoted a continuous Al₂O₃ layer, enhancing oxidation resistance during 650 °C exposure. The density functional theory calculations revealed that either solid solution of Zr or Nb in Ti₂AlC significantly increased Ti vacancy formation energy, suppressing the generation of Ti vacancies that are essential for Ti diffusion. In addition, the higher valence state of Nb⁵⁺ , compared to Zr⁴⁺ constituent was found to be more effectively inhibiting the growth of non-protective TiO₂ film. This study elucidates the vacancy-dependent oxidation resistance of MAX phase coatings with solid solutions used for high-temperature fields.