Oxidation and microstructure evolution of cobalt aluminide coatings on directionally solidified superalloys during long term exposure at 1000°C

Abstract

Using low pressure chemical vapour deposition, cobalt aluminide coatings were deposited onto directionally solidified Ni based superalloys (DZ466 and DS GTD111). Isothermal oxidation testing of the coated specimens was performed at 1000°C under atmospheric pressure for times up to 2500 h. Oxidation and interdiffusion behaviour of the coating superalloy systems was examined. It was observed that the oxide growth mode of both coating systems conformed to a staged parabolic law, with differences in the transient stage related to the growth of θ-Al2O3. Coating degradation and formation of chromium containing precipitates occurred during exposure because of severe interdiffusion between the coating and superalloy. The composition of the superalloy substrate played an important role in the microstructure evolution of the aluminide coating, and the corresponding mechanism is discussed.