Pack deposition of coherent aluminide coatings on γ-TiAl for enhancing its high temperature oxidation resistance

Thermochemical analyses were undertaken for a series of pack powder mixtures for deposition of aluminide coatings on γ-TiAl by the pack cementation process. Particular attention was focused on the effects of halide activators on the deposition characteristics of the pack powder mixtures. The activators investigated included the halide salts of NH4F, NH4Cl, AlF3 and AlCl3. Based on the results obtained, experimental studies were carried out to identify optimum pack powder mixtures for depositing adherent and coherent aluminide coatings. Pack powder mixtures activated by 2 wt.% AlCl3 was used to aluminise γ-TiAl at 1000 °C. With proper control of pack compositions and coating conditions, an aluminide coating consisting of an outer TiAl3 layer and an inner TiAl2 layer with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of an oxidation study showed that the aluminide coating produced in this study can provide a significant degree of protection against oxidation in air at temperatures up to 850 °C due to its ability to form a continuous Al2O3 scale on the coating surface. A discussion is presented on the parameters that need to be effectively controlled to ensure the deposition of aluminide coatings with a coherent structure free from microcracking on γ-TiAl by the pack cementation process.