APS Y2O3 Environmental Barrier Coatings For Oxide Ceramic Matrix Composites

Abstract

Similar thermal expansion, microstructural stability, good adherence, and corrosion resistance are key issues for environmental barrier coatings (EBC) for oxide ceramic matrix composites (CMC) such as Al2O3/Al2O3. Laboratory-scale specimen as well as prototypic engine components were coated with Y2O3 by means of air plasma spraying (APS). Cubic alpha-Y2O3 exhibits favorable properties such as thermal expansion close to that of Al2O3 and phase stability up to melting. The high thermal stability and low diffusivity of Y2O3 provides an exceptional microstructural stability of APS Y2O3 coatings. Although Y2O3 and Al2O3 are not thermodynamically compatible, the interdiffusion and reaction zone consisting of Y-aluminates is growing only slowly under relevant thermal conditions and does not seriously affect coating adherence. Consequently, APS Y2O3-coated Al2O3/Al2O3 CMC exhibited a high durability versus thermal cycling. APS Y2O3 coatings showed a high recession resistance in water vapor-rich, high-velocity combustion atmospheres. Moreover, APS Y2O3-coatings exhibited a high resistance against thermos-chemical attack by inorganic particles commonly referred to as CMAS corrosion. Infiltration of molten CMAS is mitigated by a dense reaction layer as well as by a quasi-impermeable coating microstructure. The outstanding combination of properties was the rationale for selecting APS Y2O3 as EBC material for Al2O3/Al2O3 combustor liners. Two can-type demonstrators with 0.5 and 1 mm thick APS Y2O3 EBC were manufactured and tested under engine conditions. Both EBCs passed high-pressure combustor rig tests without visible damages, emphasizing the fundamental viability of the concept.