Evaluating oxide and boride chemistries for mitigating CMAS corrosion and infiltration

This study investigates the interactions between calcia-magnesia-alumino-silicate (CMAS) glass and seven T/environmental barrier coating (EBC) materials, including five oxides—7YSZ, Gd₂O₃, Yb₂Si₂O₇, (Y_1/2Yb_1/2)₂Si₂O₇, and (Y_1/2Yb_1/2)₂SiO₅—and two borides, ZrB₂ and HfB₂. Ex situ powder X-ray diffraction on oxide–CMAS pellets elucidated the crystalline products formed at 1350°C and revealed apatite and disilicate phases in Gd₂O₃, (Y_1/2Yb_1/2)₂Si₂O₇, and (Y_1/2Yb_1/2)₂SiO₅, but no reaction in 7YSZ or Yb₂Si₂O₇. Complementary diffusion couple experiments at the same temperature evaluated CMAS infiltration kinetics, reaction product formation, and microstructural changes. Results show the critical role of rare earth concentration and cation size along with the importance of Ca/Si ratio in the glass in determining CMAS resistance. After a 36 h anneal at 1350°C, 7YSZ and Yb₂Si₂O₇ showed significant CMAS infiltration (∼1 mm) due to the absence of reactive products, while Gd₂O₃, (Y_1/2Yb_1/2)₂Si₂O₇, and (Y_1/2Yb_1/2)₂SiO₅ demonstrated reduced infiltration (∼50 µm), attributed to the formation of dense apatite and garnet protective reaction layers. Borides exhibited CMAS interaction proportional to their substantial oxidation, which emphasizes the necessity for oxidation resistant coatings to realize their potential. This analysis provides insights into CMAS–ceramic interactions and establishes a framework for designing resilient coatings to enhance the durability of next-generation gas turbine engines.