Influence of Microstructure on CMAS Resistance and Lifetime of EB-PVD Gadolinium Zirconate TBCs

Abstract

Thermal barrier coatings (TBCs) used in high-temperature sections of aero-engines and gas turbines are often subjected to severe degradation by molten calcium-magnesium-alumina-silica (CMAS) minerals mainly found in volcanic ashes (VA), runway dust, desert sands and air pollution. The application of reactive TBCs such as gadolinium zirconate (GZO) on the affected parts is one of the successful approaches to protect the TBCs against CMAS attack. Being exposed to molten CMAS, the coatings form crystalline reaction products that seal pores and gaps and hinder further infiltration of the deposit. However, the microstructure of these coatings plays a significant role on both the infiltration and reaction kinetics of the sacrificial layers with CMAS. GZO layers with different columnar microstructure and porosity were deposited by varying the electron beam physical vapor deposition (EB-PVD) process parameters. Infiltration experiments with CMAS or VA were carried out at 1250°C for different time intervals between 5 min and 50 h. It was found that microstructural features influence greatly the infiltration depth and reaction kinetics. Lifetime of these coatings was tested by doing thermal cyclic testing and a correlation with their microstructure is drawn.