Water Vapor Effects on the CMAS Degradation of Thermal Barrier Coatings
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A new damage mechanism of CMAS degradation of TBCs involving delamination through the thermally grown oxide (TGO) has been observed on burner rig tests of EB-PVD thermal barrier coatings. This mechanism operates in the absence of CMAS melting and is mediated by hydroxylation and volatilization of the constituent CMAS hydroxides, followed by vapor transport to the vicinity of the TGO through the TBC porosity. Analyses reveal the formation of spinel (MgAl2O4) and anorthite (CaAl2Si2O8) at the TGO/TBC interface, with concomitant evolution of porosity at the spinel/alumina interface. Tests were conducted in a controlled atmosphere tube furnace to validate the hypothesis and assess the factors that affect the evolution of the deleterious oxides. The microstructure of the reaction products formed in the lab test is compared to that formed in the burner rig.
KeywordsThermal barrier coatings CMAS Thermally grown oxide Spinel
Research is supported by the Honeywell-UCSB Alliance for Thermal Barrier Coatings, monitored by W. Baker and N. Conklin. The authors are grateful to Deryck Stave, Maxwell Fisch, and Duncan Campbell for their assistance in building the controlled atmosphere furnace apparatus. Funding was provided by Honeywell Aerospace (PO4204813245E).
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