Development of internal cavities in platinum-aluminide coatings during cyclic oxidation

The microstructural evolution of platinum-modified nickel-aluminide coatings, which takes place in the course of high temperature cyclic oxidation, results in two major types of coating degradation. One type, extensively studied and reported in the literature, is the development of surface undulations, also known as rumpling. The other type of degradation is the formation of internal cavities in the coating, which occurs concurrently with rumpling. Unlike the pores or voids that are commonly observed at the oxide-metal interface in many alumina-forming alloys, the cavities in NiPtAl coatings nucleate and grow entirely within the coating. Examination of the coating microstructure after various exposures shows that both the size and the number of cavities increase with the oxidation time and eventually lead to complete disintegration of the coating after prolonged cyclic oxidation at 1150 °C. In this work, various aspects of this phenomenon are examined, and the effect of temperature regime on the propensity of this type of coating degradation is discussed.