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

Abstract

The high-temperature interaction between ~2.5 mg/cm 2 of Na 2 SO 4 and an atmospheric plasma sprayed (APS) Yb 2 Si 2 O 7 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% SO 2 -O 2 gaseous environment. Yb 2 Si 2 O 7 reacted with Na 2 SO 4 to form Yb 2 SiO 5 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 Yb 2 SiO 5 and Yb 2 Si 2 O 7 phases. Na 2 SO 4 deposits infiltrated the Yb 2 Si 2 O 7 topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb 2 SiO 5 area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO 2 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 Yb 2 SiO 5 from the topcoat at 1316°C for a single applied loading of Na 2 SO 4 .

Interactions of Na2SO4 and yttrium‐silicate environmental barrier coatings at 825°C

“…This dry salt loading technique differs from usual procedures that involve salt loading by a liquid vehicle. Typically Na 2 SO 4 is dissolved in water and administered onto a sample surface by pipetting 26 . Liquid loading may cause salt infiltration through the specimen before testing, reducing its measured salt load at the surface and increasing the depth of the reaction.…”

“…Typically Na 2 SO 4 is dissolved in water and administered onto a sample surface by pipetting. 26 Liquid loading may cause salt infiltration through the specimen before testing, reducing its measured salt load at the surface and increasing the depth of the reaction. Samples were placed onto an Al 2 O 3 boat wrapped with a 94% Pt/ 6% Rh wire to suspend the sample and prevent reactivity between the yttrium-silicates and Al 2 O 3 , as shown in Figure 1.…”

“…These materials and temperatures were selected due to their relevance for RE‐EBCs for SiC‐CMC systems. It was shown that SiO 2 in the coating reacted with Na 2 O from decomposed Na 2 SO 4 26 27–29 .…”

“…It was shown that SiO 2 in the coating reacted with Na 2 O from decomposed Na 2 SO 4 . 26 Reports of the NaY 9 Si 6 O 26 reaction product phase show this to occur from interactions between yttrium-silicate and Na-containing compounds such as Na 2 CO 3 and Na 2 SO 4 +V 2 O 5 . [27][28][29] These reactions are reported to occur at temperatures ranging from 1000-1200…”

Holistic comparison of environmental barrier coating material candidates through design of a figure of merit

“…Lower left. Yb 2 Si 2 O 7 reacting to form Yb 2 SiO 5 after reaction with Na 2 SO 4 at 1316°C for 96 h 124 . All exposures were conducted in O 2 + 0.1% SO 2 (g).…”