CMAS Infiltration Studies on Thermal Barrier Coatings under Thermal Gradients Using Synchrotron X-Ray Diffraction

Abstract

High temperature ceramic protective coatings, also called thermal barrier coatings (TBCs), are used to protect components, which are internally or back-side cooled. Calcium-magnesiumalumino-silicate (CMAS) particulates, such as sand, are ingested by these aero-engines and travel through the hot gas stream, eventually depositing onto the TBCs. The subsequent degradation is detrimental to the lifetime of the coatings with the risk of engine failure. In this work, synchrotron X-ray diffraction measurements were used to capture effects CMAS has on a 7-8 wt% yttria-stabilized zirconia (YSZ) TBC applied on an internally cooled hollow dogbone sample under thermal gradient conditions. Thermochemical degradation of the YSZ coating was observed, after 1 hour of CMAS-infiltration, through the formation of monoclinic phase. The monoclinic phase concentration was observed to be around 18% at the surface of the YSZ coating and declining towards zero halfway through the coating. This monoclinic concentration is roughly 2 times less than under isothermal conditions at similar annealing temperatures and timescales. These results showcase the successful capture and monitoring of coating degradation throughout a replicated aircraft flight cycle. These results can be further expanded upon with other coating systems, to elucidate the transitory processes of in-situ coating degradation throughout an operational cycle. The results may be used in strategies to mitigate the detrimental effects of CMAS on high temperature ceramics.