On the Thermal Cycling and Evolution of Surface Morphology for Thermally Cycled NiCoCrAlY Bondcoats

Morphological instabilities such as rumpling and ratcheting on bond coat surfaces in thermal barrier coatings (TBCs) have recently received significant attention since these phenomena can be directly linked to system failures. Recent observations from thermo-mechanically cycled TBC-samples reveal that, if the ceramic top coat has spalled, the morphological evolution of a NiCoCrAlY bond coat can be linked to the presence or absence of a thermal gradient over the coating. Morphological instabilities develop during cycling with a thermal gradient while the surface remains smooth for cyclic conditions without a gradient. This paper discusses an initial numerical model to simulate this behavior, lending itself to some insight in the thermo-mechanical response. In particular, it is shown that not only is the thermal gradient important, but the relative rate between the inside and outside heating/cooling sequence plays a significant role in driving the morphological instabilities.