A novel CMAS‐resistant material based on thermodynamic equilibrium design: Apatite‐type Gd <sub>10</sub> (SiO <sub>4</sub> ) <sub>6</sub> O <sub>3</sub>
Di Wu, Han Zhang, Xiao Shan, Fan Yang, Fangwei Guo, Xiaofeng Zhao, Ping Xiao, Shengkai Gong
Abstract
Abstract Calcium‐magnesium‐alumino‐silicates (CMAS) melt attack has been a critical issue for the thermal barrier coatings (TBCs) with ever‐increasing engine operating temperature. In this study, a novel CMAS‐resistant material apatite‐type Gd 10 (SiO 4 ) 6 O 3 is developed for TBCs application based on thermodynamic equilibrium design. The chemical reaction of Gd 10 (SiO 4 ) 6 O 3 bulk and CMAS melt is investigated at 1300°C. The CMAS corrosion resistance of Gd 10 (SiO 4 ) 6 O 3 bulk is evaluated and compared with the well‐studied CMAS‐resistant material Gd 2 Zr 2 O 7 (GZO). It is found that Gd 10 (SiO 4 ) 6 O 3 shows a significantly enhanced CMAS resistance, including lower intrinsic CMAS infiltration rate (~1.09 μm/h 1/2 ) and smaller infiltration upper limit (50‐62 μm) for a 20 mg/cm 2 CMAS deposition. More importantly, for Gd 10 (SiO 4 ) 6 O 3 , the CMAS infiltration only alters the composition but does not change the crystal structure or destroy microstructural integrity. The reaction mechanism is elucidated as following two stages: (a) surface Gd 10 (SiO 4 ) 6 O 3 quickly transforms into Ca 2 Gd 8 (SiO 4 ) 6 O 2 in suit by interdiffusion with CMAS melt and then is thermodynamically stable with CMAS melt, thereby effectively inhibiting the further CMAS infiltration and (b) with the ongoing interdiffusion of Gd/Ca, the CMAS‐infiltrated layer slowly thickens and follows a parabolic law. Meanwhile, the CMAS melt gradually precipitates Ca 2 Gd 8 (SiO 4 ) 6 O 2 and CaAl 2 Si 2 O 8 (anorthite) until the melt is exhausted.