Effects of activators on formation of Si-Zr-Y co-deposition coatings on Nb-Ti-Si-Cr base ultrahigh temperature alloy

Si-Zr-Y co-deposition coatings on an Nb-Ti-Si-Cr base ultrahigh temperature alloy were prepared by pack cementation processes at 1250°C for 8 h with different halide activators in the packs. The structure, constituent phases and formation process of the coatings were investigated. The results show that all the coatings are mainly composed of an (Nb, X)Si₂ (X represents Ti, Cr and Hf elements) outer layer, a (Ti, Nb)₅Si₄ transitional layer, and an Al, Cr and Y-rich (Nb, X)₅Si₃ inner layer located between the coating and the substrate. In the five kinds of (NaF, NH₄F, NH₄Cl, NaBr and NaCl) activators investigated, the coatings prepared respectively with NaF and NH₄F activators are thicker and denser, and furthermore, the contents of both Zr and Y elements are larger in the coating prepared with NaF as the activator. However, the coatings prepared respectively with NH₄Cl, NaBr and NaCl as the activators are thiner, and lots of holes and ZrO₂ and HfO₂ particles are observed in the surface layer of the coatings. The higher equilibrium partial pressures of gaseous halide silicides in the packs resulted in thicker and denser co-deposition coatings, while the equilibrium partial pressures of gaseous halide Zr and Y in the packs have less effect on their contents in the coatings.