Degradation of Ytterbium Disilicate Environmental Barrier Coatings in High Temperature Steam Atmosphere

The use of Ceramic Matrix Composites (CMCs) in the hottest part of an aero engine promises great improvements in fuel efficiency by decreasing component weight and allowing higher gas inlet temperatures. However, an environmental barrier coating (EBC) is required to protect the CMC from the corrosive water vapour contained in the combustion environment.
Here, CMC specimens were coated with a silicon bond coat and ytterbium disilicate (Yb2Si2O7) layer using air plasma spraying. The specimens were subsequently exposed to a water steam environment at 1350°C for hundreds of hours. Stress evolution and phase stability were measured throughout to observe possible degradation. Cross-sectioning of the samples revealed the occurrence of sintering, the formation of a thermally grown oxide along the silicon/EBC interface, and a reaction between the ytterbium disilicate and silica. However, no coating failure was observed, even after 750 h of isothermal exposure to the hot steam environment.