High-temperature corrosion of sintered RE2Si2O7 (RE = Yb and Ho) environmental barrier coating materials by volcanic ash

Rare-earth silicates are promising environmental barrier coatings (EBCs) that can protect SiCf/SiCm substrates in next-generation gas turbine blades. Notably, RE2Si2O7 (RE = Yb and Ho) shows potential as an EBC due to its coefficient of thermal expansion (CTE) compatible with substrates and high resistance to water vapor corrosion. The target operating temperature for next-generation turbine blades is 1400 degrees C. Corrosion is inevitable during adhesion to molten volcanic ash, and thus, understanding the corrosion behavior of the material is crucial to its reliability. This study investigates the high-temperature corrosion behavior of sintered RE2Si2O7 (RE = Yb and Ho). Samples were prepared using a solid-state reaction and hot-press method. They were then exposed to volcanic ash at 1400 degrees C for 2, 24, and 48 h. After 48 h of exposure, volcanic ash did not react with Yb2Si2O7 but penetrated its interior, causing damage. Meanwhile, Ho2Si2O7 was partially dissolved in the molten volcanic ash, forming a reaction zone that prevented volcanic ash melts from penetrating the interior. With increasing heat treatment time, the reaction zone expanded, and the thickness of the acicular apatite grains increased. The Ca:Si ratios in the residual volcanic ash were mostly unchanged for Yb2Si2O7 but decreased considerably over time for Ho2Si2O7. The Ca in volcanic ash was consumed and formed apatite, indicating that RE3+ ions with large ionic radii (Ho > Yb) easily precipitated apatite from the volcanic ash.