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High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior
Authors:Lucas A. Herweyer  Elizabeth J. Opila
Affiliation:Department of Materials Science and Engineering, School of Engineering and Applied Sciences, University of Virginia, Charlottesville, VA, USA
Abstract:The high-temperature interaction between ~2.5 mg/cm2 of Na2SO4 and an atmospheric plasma sprayed (APS) Yb2Si2O7 topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO2–O2 gaseous environment. Yb2Si2O7 reacted with Na2SO4 to form Yb2SiO5 and an intergranular amorphous Na-silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb2SiO5 and Yb2Si2O7 phases. Na2SO4 deposits infiltrated the Yb2Si2O7 topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb2SiO5 area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO2 thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na-silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb2SiO5 from the topcoat at 1316°C for a single applied loading of Na2SO4.
Keywords:corrosion/corrosion  environmental barrier coatings (EBC)  infiltration  oxidation  resistance  silicates
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