High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

The high-temperature interaction between similar to 2.5 mg/cm(2) 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 degrees C-1316 degrees C in a 0.1% SO2-O-2 gaseous environment. Yb2Si2O7 reacted with Na2SO4 to form Yb2SiO5 and an intergranular amorphous Na-silicate phase. Below 1200 degrees 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 degrees 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 degrees 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 degrees C for a single applied loading of Na2SO4.