APS Y2O3 Environmental Barrier Coatings with high CMAS-Resistance

Abstract

Al2O3/Al2O3 ceramic matrix composites (CMCs) are candidate materials for combustor walls of next generation gas turbines. The maximum CMC operation temperatures, however, are limited since Al2O3 is prone to thermochemical recession in combustion environments. The application of environmental barrier coatings (EBC) is considered a suitable way to overcome the recession problem. Y2O3 is a potential EBC material for Al2O3/Al2O3 CMCs due to its high thermal and chemical stability and a thermal expansion close to Al2O3. Different types of continuous fiber reinforced Al2O3/Al2O3 CMC substrates were coated with Y2O3 by air plasma spraying (APS). Upon isothermal and thermocyclic heat treatments APS Y2O3 coatings exhibit good adherence to Al2O3/Al2O3 CMCs. This is due to the low thermal mismatch and the formation of an approximately one micron thick reaction zone at the Y2O3-Al2O3 interface, comprising a thermodynamically stable sequence of garnet-type Y3Al5O12, perowskite-type YAlO3 and monoclinic Y4Al2O4. The response of APS Y2O3 to high-temperature attack by molten mineral dusts was studied using synthetic CMAS powders. It turns out that melt CMAS melt infiltration of APS Y2O3 is low, which is explained by the dense microstructure and a rapid formation of a continous Ca,Y-oxyapatite.