Environmental stability of the TBC system of a serviced EB-PVD coated high-pressure turbine blade

Abstract

Degradation of thermal barrier coatings via reactive hot corrosion products imposes a potential threat to the lifetime of engine hardware. This issue is addressed for a serviced EB-PVD coated 1st stage HPT airfoil which has experienced complete CaSO4 infiltration of the TBC layer prior to the deposition of CMAS-type hot corrosion deposits. The blade has been retained from service prior to spallation thus offering a fully intact YSZ TBC layer for this case study. The hot corrosion process has introduced several reactive interfaces with the TBC top coat and the TGO layer which were investigated by scanning electron microscopy (SEM) and focused-ion-beam (FIB) assisted analytical transmission electron microscopy ( TEM). Leaving aside minor elements effects the reactivity of the TBC system is primarily controlled by the high lime-to-silica ratio of the deposits. Upon partial dissolution of the TBC the CMAS/TBC interface typically consists of a CaZrO3 layer followed by fine-grained Ca-FSZ particles which envelop the formerly faceted, now rounded YSZ column tips. This reaction zone can be rationalized by phase compatibilities in the systems CaO-Y2O3-ZrO2 and CaO-ZrO2-SiO2 respectively. Possible terms for the formation of this unique CaSO4/CMAS microstructure are evaluated. Destabilization of the top coat is put into perspective with previous work on both, natural and synthetic CMAS deposits.