Vapor-Phase CMAS-Induced Degradation of Adhesion of Thermal Barrier Coatings
An investigation of the interface degradation and failure of EB-PVD thermal barrier coatings (TBCs) during burner rig cyclic oxidation in the presence of calcium–magnesium–alumino-silicate (CMAS) deposits has been conducted. The formation of two compounds of major CMAS constituents, specifically MgAl2O4 (spinel) and CaAl2Si2O8 (anorthite), was observed at the interface between the alumina scale and the TBC. Spallation occurred as a result of cracking and delamination primarily through the spinel layer. The tests showed more than a tenfold decrease in TBC life relative to the oxidation test without CMAS at the same temperature. It is demonstrated that this particular form of coating degradation occurs below the melting temperature of this particular CMAS composition and hence does not involve TBC infiltration by molten silicates. The likely mechanism of interface degradation, which involves vapor-phase transport of CMAS constituents as hydroxides, is proposed and discussed.
KeywordsThermal barrier coatings (TBCs) Calcium magnesium alumino-silicate (CMAS) Alumina scale Adhesion
This work has been supported by Honeywell Advanced Technology IR&D funds. The assistance of J. Cobb and J. Christopher in burner rig experiments is greatly acknowledged. The author is grateful to Mr. W. Baker (Honeywell) and Prof. C.G. Levi (UCSB) for critical reading of this manuscript.
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