A mechanistic understanding on rumpling of a NiCoCrAlY bond coat for thermal barrier coating applications

We present a series of experimental observations on surface rumpling of an initially flat NiCoCrAlY coating deposited on a Ni-based superalloy during cyclic oxidation at 1150 °C. The extent of rumpling of the coating depends on the thermal history, coating thickness and exposed atmosphere. While the coating surface progressively roughens with cyclic oxidation, the bulk NiCoCrAlY alloys with the same nominal composition are much less susceptible to rumpling under the same oxidation conditions. The coatings, especially the thin ones, experience substantial degradation (e.g. β to γ phase transformation) induced by oxidation and coating/subsatrate interdiffusion. The observations together suggest that rumpling of the NiCoCrAlY coating is driven by a combination of the lateral growth of the thermally grown oxide and coating/substrate thermal mismatch. The results in this work are further discussed and compared with the rumpling behaviour of a β-(Ni,Pt)Al bond coat reported in the literature to illustrate the importance of possible factors in governing the development of rumpling in the NiCoCrAlY coating.