Abstract
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.
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