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

Abstract

The present work investigated the O2-adsorption behaviors on the {110} 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.