A novel test approach for plasma-sprayed coatings tested simultaneously under CMAS and thermal gradient cycling conditions

Plasma-sprayed ceramic coatings, used as thermal barrier or abradable coatings in high-pressure stages of gas turbines, are exposed to high thermo-mechanical loading due to harsh operating conditions. Under certain conditions they also have to withstand attack by calcium-magnesium-alumino-silicate (CMAS) deposits resulting from the ingestion of siliceous minerals with the intake air. Resistance to this kind of attack becomes more important at higher temperatures, when the melting temperature of the CMAS deposits is exceeded and a penetration into the coating microstructural features (cracks and pores) can take place. During cooling the CMAS solidifies and the coating loses its strain tolerance, which can lead to coating failure. Although the basic principles of failure seem to be understood, a detailed analysis of the mechanisms and the possibilities of avoiding delamination are still lacking, especially because there are as yet no adequate test beds. This paper investigates the possibility of testing such coatings in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. The CMAS rig was established because the test conditions simulated here are closer to actual engine conditions, as compared to previous tests with primarily CMAS deposition and subsequent thermal furnace testing. The experimental setup of this new test approach is described and the applicability is confirmed. Furthermore, a first evaluation for plasma-sprayed coatings is presented. A significantly reduced lifetime was found for samples tested with CMAS attack in comparison to tests with water deposition only. The lifetime was also reduced compared to older results without any corrosive media. A decrease of nearly two orders of magnitude was found. A microstructural evaluation of the coatings is presented and the mechanisms and reasons for the very early failure are discussed. (C) 2010 Elsevier B.V. All rights reserved.