YbPO₄: A novel environmental barrier coating candidate with superior thermochemical stability

YbPO₄ was synthesized to study thermomechanical and thermochemical stability as a novel EBC material candidate. Thermal expansion coefficients were measured by high temperature XRD and determined to be a = 5.4 ± 0.4 × 10⁻⁶ / °C, c = 7.3 ± 0.1 × 10⁻⁶ / °C for the tetragonal structure with a linear CTE = 6.0 ± 0.3 × 10⁻⁶ / °C for the 100 °C – 1200 °C temperature range. High-velocity water vapor exposure at 1400 °C for 60, 125, and 250 h resulted in the formation of Yb₂O₃ product layer at a rate of 8.7 µm²/h for the 80–200 m/s steam velocity range, which was a slower reaction rate than state of the art EBC Yb₂Si₂O₇ under similar conditions. YbPO₄ – molten CMAS exposures at 1300 °C for 4, 24, and 96 h resulted in a compositionally variant reaction product composed primarily of a continuous Ca₈MgYb(PO₄)₇ layer that inhibited CMAS penetration into the bulk of the samples. YbPO₄ was additionally found to be chemically stable in the presence of SiO₂ at 1400 °C, which is important for compatibility with SiC/Si substrates at elevated temperatures. Initial thermomechanical and thermochemical analysis indicate that YbPO₄ should be considered a viable material candidate for next-generation EBC systems.