DFT Study of Selected Double Transition Metal MAX Phases M ₂ M ^′ AC ₂ for Thermal Barrier Coatings

Abstract Double transition metal MAX phases () are emerging as next‐generation thermal barrier coating (TBC) candidates because they uniquely combine ceramic‐like stiffness with metallic‐like ductility. While 24 quaternary configurations are possible form M, (Mo, Ti, Cr, Nb), this study focuses on six representative compounds, , , , , , and to capture key bonding and property trends. Density functional theory (DFT) calculations confirm their thermodynamic stability (formation energies –0.38 to –0.70 eV per atom) and mechanical robustness via Born criteria. Metallic band structures, dominated by transition‐metal d states, ensure high electrical and thermal transport. achieves the highest stiffness (B = 290 GPa, E = 461 GPa) and hardness ( 22.5 GPa), rivaling TiN coatings, whereas Ge‐containing phases (e.g., ) display exceptional ductility (B/G = 2.17) and the lowest thermal conductivity ( = 0.68 W m −1 K −1 ). All compounds exhibit Debye temperatures up to 893 K and melting points 2100 K, indicating strong high‐temperature endurance. Compared to Yttria‐stabilized zirconia, YSZ ( 1.22–1.85 W m −1 K −1 ), these MAX phases combine lower or comparable thermal conductivity with higher stiffness and superior damage tolerance. Thermodynamic and mechanical stability are evaluated through formation energies and Born criteria, while dynamical stability is not addressed. Full phonon analyses are left for future work.