Vapour phase codeposition of Al and Si to form diffusion coatings on γ-TiAl

Abstract

Thermochemical analyses were undertaken for a series of pack powder mixtures for codepositing Al and Si to form diffusion coatings on γ-TiAl by the pack cementation process. Based on the results obtained, experimental studies were carried out to identify suitable pack powder mixtures for the codeposition process. The thermochemical calculation results suggested that codeposition of Al and Si is possible with CrCl 3 · 6H 2O packs using elemental Al and Si as depositing sources. But, experimental results obtained at 1100 °C revealed that CrCl 3 · 6H 2O is not a suitable activator for codepositing Al and Si on γ-TiAl. It caused a significant degree of degradation, as indicated by the weight losses, instead of coating deposition to the substrate. However, it was demonstrated that codeposition can be achieved at 1100 °C using pack powder mixtures activated by AlCl 3. The coatings obtained had a multiple layer structure consisting of an outer silicide layer, an inner aluminide layer of TiAl 3 and a transition zone at the boundary between the deposited coating and the substrate that ensured integrity of the coating during cooling. It was suggested that such a coating forms via a sequential deposition mechanism through inward diffusion of aluminium and silicon. The oxidation resistance of the coating was evaluated in air at 850 °C by intermittently measuring the weight change at room temperature. The coating showed good oxidation resistance and thermal stability. The conditions that need to be effectively controlled to ensure the codeposition of Al and Si from the vapour phase to form a composite silicide and aluminide diffusion coating on γ-TiAl with a coherent structure free from microcracking and spallation by the pack cementation process were discussed.