Oxidation resistance of diffusion coatings formed by pack-codeposition of Al and Si on γ-TiAl

Oxidation resistance of the aluminide and silicide diffusion coatings pack-deposited on γ-TiAl were studied in air over the temperature range of 800 and 850°C for up to 4596 h. The oxidation kinetics of the coatings was monitored by intermittent weight gain measurement at room temperature. The XRD and SEM/EDS techniques were used to identify the oxide scales formed during the oxidation process and to assess the thermal stability of the coatings at the oxidising temperatures. It was revealed that the TiAl3 coating underwent preferential Al oxidation to form the Al2O3 scale in the early oxidation stage, which resulted in Al depletion and formation of TiAl2 in the subsurface of the coating. The Al depletion could not be sufficiently compensated by Al diffusion from the inner layer of the coating and eventually, in the late oxidation stage, led to the Ti oxidation and formation of the TiO2 phase in the scale. The preferential Si oxidation was the main oxidation mechanism for the coatings with an outer silicide layer and an inner TiAl3 layer with the formation of SiO2 as the stable oxide scale. The thermal stability of the coatings over the temperature range up to 850°C was discussed in relation to the high-temperature stability of diffusion couples of different coating layers.