Effect of CeO₂ on microstructure and oxidation resistance of silicide coatings prepared on Nb-silicide-based ultrahigh temperature alloy

Si-Ce co-deposition coatings were prepared on an Nb-silicide-based ultrahigh temperature alloy by pack cementation processes at 1 150°C for 8 h. The effects of CeO₂ content in pack mixtures on the microstructure, constituent phases and high temperature oxidation resistance of the coatings were studied. The results show that all coatings prepared with different contents of CeO₂ in the pack mixtures are mainly composed of a (Nb, X)Si₂ (X represents Ti, Hf and Cr) outer layer, a (Ti, Nb)₅Si₄ transitional layer and an Al-rich diffusion zone, which are similar to that of purely siliconized coatings. EDS analysis reveals that the distribution of Ce in the co-deposition coatings is not uniform. The content of Ce in the Hf-rich (Nb, X)Si₂ phase transferred from (Nb, X)₅Si₃ in the base alloy is higher than that in other phases. The addition of CeO₂ in the pack mixtures not only refines the microstructure of the coatings, but also obviously catalyzes the coating growth, especially when 3% CeO₂ (mass fraction) is added in the pack mixtures. Both scales formed on the Si-Ce co-deposition coating and purely siliconized coating after oxidation at 1 250°C for 50 h are mainly composed of TiO₂ and SiO₂. However, much finer TiO₂ rods are observed distributing evenly in the scale of the Si-Ce co-deposition coating after oxidation. Thus, the Si-Ce co-deposition coatings enhance the high temperature oxidation resistance due to the improved compactness of the protective oxide scale.