YTTRIA RICH TBCS AS CANDIDATES FOR CMAS RESISTANT TOP COATS

Abstract

State of the art thermal barrier coatings (TBC) commonly made of 7-8 wt. % yttria stabilized zirconia (7YSZ) are used in modern gas turbines to generate a thermal protection to the underlying super alloy components. TBCs allow higher operating temperatures for hot gas path components, thus, generating higher engine efficiency. The infiltration of molten glassy mineral deposits composed of CaO-MgO-Al2O3-SiO2 (CMAS) represents one of the major threats in reducing performance and service life in aero and land based gas turbine engines. The CMAS deposits are ingested into the engine carried commonly in sand, runways debris, fly ash and volcanic ash. The ingested particles melt on the engine’s hot gas path components infiltrating the porous TBC, this infiltration generates detrimental effects on the coating such as microstructural degradation and strain tolerance reduction. The present work studied in detail the CMAS infiltration resistance of EB-PVD –yttria (Y2O3) coatings which can be used as a top coat on standard 7YSZ TBC layer. Studies were made with Y2O3 rich ZrO2 coatings with an yttria content ranging in 65 wt. % (rest is zirconia). CMAS infiltration experiments were performed at 1250 ºC -at time intervals from 1 to 20 hours for long term infiltration and 5 minutes for rapid infiltration using real volcanic ash and a model CMAS source for results comparison. The results show that the studied yttria coatings induced the crystallization of the CMAS melt by forming stable oxyapatite and garnet phases. These layers exhibit a minimal growth making yttria rich coatings promising candidates for CMAS resistant multilayer TBCs. The results also proved a dependency of coating microstructure for infiltration resistance suggesting that the infiltration resistance can be further improved by refining the TBC microstructure by maintaining a densified morphology.