Rumpling instability in thermal barrier systems under isothermal conditions in vacuum

Bond coat (BC) surface rumpling has been identified as one of the important mechanisms that can lead to failure of thermal barrier coatings. The driving force behind rumpling - whether the stresses in the thermally grown oxide over the BC or the stresses in the BC - remains to be clarified. Also, the mass transport mechanisms in the BC leading to rumpling are not clearly identified. In the present investigation, we subjected two types of BC-superalloy systems, nickel aluminide and platinum aluminide BCs on a Ni-based superalloy, to isothermal exposure at temperatures ranging from 1150°C to 1200°C in vacuum. The results show that the nickel aluminide BC rumples at 1200°C and at 1175°C in the absence of significant oxidation. The wavelength of the rumpled surfaces was 60-100 μm, with an amplitude of 5-8 μm. The rumpling was insensitive to the initial BC surface morphology. At 1150°C, no clear rumpling was observed, but some surface undulations could be seen related to the BC grains. The platinum aluminide BC with an initially polished surface showed the formation of dome-like structures corresponding to the BC grains at 1200°C, indicating a strong influence of BC grain boundary diffusion on the BC rumpling. The above observations indicate that large-scale mass transport manifested in the form of BC rumpling can occur in the absence of a significant oxide layer. The stresses in the BC appear to be sufficient to cause the rumpling behaviour. The current rumpling results are discussed in the context of the possible mechanisms. It is concluded that various diffusive processes (grain boundary, surface and bulk diffusion) in the BC driven by the BC stresses lead to the rumpling behaviour observed in the current study.