Variable thermochemical stability of RE 2 Si 2 O 7 (RE = Sc, Nd, Er, Yb, or Lu) in high‐temperature high‐velocity steam

Abstract Five rare‐earth (RE) disilicates (RE 2 Si 2 O 7 , RE = Sc, Nd, Er, Yb, or Lu) were synthesized and exposed to high‐velocity steam (up to 235 m/s) for 125 hours at 1400°C. Water vapor reaction products, mass loss, average reaction depths, and product phase microstructural evolution were analyzed for each material after exposure. Similar to steam testing results in the literature, RE 2 Si 2 O 7 (RE = Er, Yb, Lu) underwent silica depletion producing gaseous silicon hydroxide species, RE 2 SiO 5 , and RE 2 O 3 product phases. Sc 2 Si 2 O 7 reacted with high‐velocity steam to produce only a Sc 2 O 3 product layer with no stable Sc 2 SiO 5 phase detected by X‐ray diffraction or microscopy techniques. Further, Nd 2 Si 2 O 7 rapidly reacted with steam to produce with no Nd 2 SiO 5 or Nd 2 O 3 reaction products. All RE 2 Si 2 O 7 that produced a silicate reaction product (RE = Nd, Er, Yb, Lu) showed densification of the product phase at steam velocities above 150 m/s that resulted in enhanced resistance. The results presented in this work demonstrate that rare‐earth silicates show diverse steam reaction products, reaction product microstructures, and total reaction depths after high‐temperature high‐velocity steam exposure. Of the materials in this study, RE 2 Si 2 O 7 (RE = Yb, Lu) were most stable in high‐temperature high‐velocity steam, making them most desirable as environmental barrier coating candidates.