Variable thermochemical stability of RE2Si2O7 (RE = Sc, Nd, Er, Yb, or Lu) in high-temperature high-velocity steam

Five rare-earth (RE) disilicates (RE2Si2O7, 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 degrees 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, RE2Si2O7 (RE = Er, Yb, Lu) underwent silica depletion producing gaseous silicon hydroxide species, RE2SiO5, and RE2O3 product phases. Sc2Si2O7 reacted with high-velocity steam to produce only a Sc2O3 product layer with no stable Sc2SiO5 phase detected by X-ray diffraction or microscopy techniques. Further, Nd2Si2O7 rapidly reacted with steam to produce Nd9.330.67Si6O26 with no Nd2SiO5 or Nd2O3 reaction products. All RE2Si2O7 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, RE2Si2O7 (RE = Yb, Lu) were most stable in high-temperature high-velocity steam, making them most desirable as environmental barrier coating candidates.