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

Rare earth silicates are promising candidates for environmental barrier coatings (EBCs) to protect SiCf/SiCm substrates in next-generation gas turbine blades. Notably, RE₂Si₂O₇ (RE = Yb, Ho) shows potential due to a coefficient of thermal expansion (CTE) that is compatible with the substrates, as well as its excellent water vapor corrosion resistance. The target operating temperature for next-generation turbine blades is set at 1400 °C. Therefore, the corrosion from adhering molten volcanic ash becomes inevitable, making it crucial to understand this corrosion behavior to ensure reliability. This study investigates the high-temperature corrosion behavior of sintered RE₂Si₂O₇ (RE = Yb, Ho). The samples were prepared using a solid-state reaction and hot press method. They were then exposed to volcanic ash at 1400 °C for 2, 24, and 48 h. After 48 h of exposure, volcanic ash did not react with Yb₂Si₂O₇ but penetrated through its interior, causing damage. Meanwhile, Ho₂Si₂O₇ was partially dissolved in the molten volcanic ash, forming a reaction zone that preventing the penetration of volcanic ash melts into the interior. With increasing heat treatment time, the reaction zone expanded, and the apatite acicular-grain became thicker. The Ca:Si ratios present in the residual volcanic ash were mostly unchanged for Yb₂Si₂O₇ but decreased significantly over time for Ho₂Si₂O₇. The Ca in volcanic ash was consumed to form apatite, indicating that RE³⁺ ions with large ionic radii (Ho > Yb) easily precipitated apatite from the volcanic ash.