First-principles study on the surface oxidation behavior of ternary M₆C₆ (M₆ = Zr₅Ti, Zr₅Ta, Hf₅Ti, Hf₅Ta) carbides

The present work investigated the O₂-adsorption behaviors on the {1 1 0} surface of binary MC and ternary M₆C₆ (M₆ = Zr₅Ti, Zr₅Ta, Hf₅Ti, Hf₅Ta) carbides via the first-principles calculations. Several structural units of ternary carbides are first constructed for the calculations, in which the Zr₅TiC₆ structure with the substitutional Ti atoms in an ordered packing is the most stabilized configuration for O₂-adsorption due to the most negative adsorption energy. The additions of Ti and Ta into ZrC and HfC could result in different distributions of electric structures and more negative primary O₂-adsorption energies in ternary carbides, indicating that Ti and Ta have much stronger interactions with O₂ than the base element Zr or Hf. For the secondary O₂-adsorption, the addition of Ti renders the Zr₅TiC₆ and Hf₅TiC₆ carbides with more positive adsorption energies compared with ZrC and HfC, which would prohibit effectively the further oxidation into the matrix, leading to an improvement of oxidation resistance of carbides. However, the total O₂-adsorption energies in Ta-added Zr₅TaC₆ and Hf₅TaC₆ carbides are the most negative, indicating that the addition of Ta is not conducive to anti-oxidation.