Investigation of CMAS Resistance of Sacrificial Suspension Sprayed Alumina Topcoats on EB-PVD 7YSZ Layers

Abstract

Molten calcium-magnesium-aluminum-silicate (CMAS) mineral particles cause significant degradation of thermal barrier coatings (TBCs) in aero-engines. One approach to protect the TBC coating against the CMAS attack is the application of a sacrificial coating on top of the TBC coating. In this work, Al2O3 coatings were deposited on EB-PVD 7YSZ layers using suspension plasma spraying (SPS) and suspension high velocity oxy-fuel spraying (SHVOF), in order to produce sacrificial topcoats with two different microstructures and porosity levels. The coating systems were tested under CMAS attack with one natural volcanic ash and two artificial CMAS powders by conducting infiltration tests at 1250 °C in the time intervals between 5 min and 10 h. It was found that the porosity and morphology of suspension sprayed alumina topcoats, the chemical composition of the deposits and the infiltration conditions strongly influence the CMAS infiltration, reaction kinetics and formation of the reaction products. While the porous SPS coatings offer limited resistance against CMAS infiltration, the dense SHVOF coatings show promising CMAS sealing behavior. Among the formed reaction products, only (Fe, Mg) Al spinel acted as an efficient barrier against CMAS infiltration. However, the formation of uniform spinel layers strongly depends on the pore morphology of the sacrificial coating and the CMAS chemistry.