Co-deposition of aluminide and silicide coatings on γ-TiAl by pack cementation process

Abstract

Thermochemical analyses were carried out for a series of pack powder mixtures for deposition of aluminide and for co-deposition of aluminide and silicide coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were undertaken to identify optimum pack powder mixtures for depositing adherent and coherent aluminide and silicide 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 of TiAl3 with a coherent structure free from microcracking was deposited on the substrate surface via inward diffusion of aluminium. The results of thermochemical calculations indicated that co-deposition of Al and Si is possible with CrCl3 · 6H2O and AlCl3 activated pack powders containing elemental Al and Si as depositing sources. Experimental results obtained at 1100°C revealed that CrCl3 · 6H2O is not suitable for use as an activator for co-depositing aluminide and silicide coatings on γ-TiAl. It caused a significant degree of degradation instead of coating deposition to the substrate. However, adherent coatings with excellent structural integrity consisting of an outer TiSi4 layer and an inner TiAl3 layer were successfully co-deposited at 1100°C and 1000°C using pack powder mixtures activated by AlCl3. IT is suggested that such coatings were formed via a sequential deposition mechanism through inward diffusion of aluminium and silicon. Discussion is presented on the issues that need to be considered to ensure the deposition of aluminide and silicide coatings with coherent structure free from microcracking on γ-TiAl by the pack cementation process.