The Growing Behavior of the Ca2Gd8(SiO4)6O2 Dense Reaction Layer in Molten CMAS at High Temperatures

Calcium–magnesium–alumina–silicate (CMAS), as an environmental deposit, deposits on engine components and causes serious damage to traditional thermal barrier coatings (TBCs) at high temperatures. The rare-earth silicate apatite dense reaction layer is regarded as a promising strategy to prevent TBCs from molten CMAS penetration and corrosion. The interactions between the Gd2O3 ceramic and CMAS are discussed at various temperatures and times in the study. The main reaction products are gadolinium silicate apatite (Ca2Gd8(SiO4)6O2, Gd-apatite) and melilite phases. Within the first 15 min of interaction, a thin, continuous and dense reaction layer (DRL) consisting of Gd-apatite comes to form, and it thickens with increasing exposure temperature and time. The thickness of the DRL is ~0.8 μm after 15 min of the reaction at 1250 °C and it slowly increases to ~9.1 μm after a duration of 24 h at 1400 °C. This is attributed to CMAS infiltration along the grain boundaries of the Gd-apatite phases in the DRL. The growing rates of the Gd-apatite DRL decrease with reaction time and are significantly influenced by the temperature and the ability of the DRL to inhibit CMAS infiltration.