Mechanisms of Ytterbium Monosilicate/Mullite/Silicon Coating Failure During Thermal Cycling in Water Vapor

An air plasma spray process has been used to apply a model tri-layer Yb2SiO5/Al6Si2O13/Si environmental barrier coating system on SiC test coupons. Significant differences in the thermal expansion of the component layers resulted in periodically spaced mud cracks in the Yb2SiO5 and Al6Si2O13 layers. Upon thermal cycling between 1316°C and 110°C in a 90% H2O/10% O2 environment flowing at 4.4 cm/s, it was found that partial delamination occurred with the fracture plane located within a thermally grown oxide (TGO) at the Al6Si2O13–Si interface. Delamination initiated at test coupon edges where the gaseous environment preferentially oxidized the exposed Si bond coat to form b-cristobalite. Simultaneous ingress of the gaseous environment through mud cracks initiated local formation of b-cristobalite (SiO2), the thickness of which was greatest directly below mud cracks. Upon cooling, cristobalite transformed from the b to a phase with a large, constrained volume contraction that resulted in severe microfracture of the TGO. Continued thermal cycling eventually propagated delamination cracks and caused partial spallation of the coatings. Formation of the cristobalite TGO appears to be the delamination life-determining factor in protective coating systems utilizing a Si bond coat.