High temperature corrosion of multilayer (Gd_0.2Dy_0.2Ho_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ thermal/environmental barrier coatings exposed to CMAS

To enhance the performance of hot-section components in gas turbine engines under extreme environments, a novel multilayer thermal/environmental barrier coating (T/EBC) system, consisting of Si-15 mol% HfO₂/Yb₂SiO₅-50 mol% Yb₂Si₂O₇/(Gd_0.2Dy_0.2Ho_0.2Tm_0.2Yb_0.2)₄Hf₃O₁₂ (15SH/50YbMDS/(5RE)₄Hf₃O₁₂) was fabricated by air plasma spraying (APS). The corrosion behavior and underlying mechanisms of the coating in high-temperature CMAS environment were systematically investigated. At 1350 ºC, minor apatite and fluorite grains initially precipitated on the coating surface after 1 h of corrosion. With prolonged exposure, the number of precipitated grains increased and they interconnected with each other to form a reaction layer whose thickness increases with time following a parabolic trend. Increasing temperature to 1450 °C significantly accelerated fluorite precipitation. Meanwhile, the infiltration of CMAS melt was enhanced. The designed (5RE)₄Hf₃O₁₂ layer, with a gradient distribution of cationic radii (0.938–0.868 Å), created a unique kinetic environment wherein cations of different sizes displayed distinct diffusion behaviors and reaction selectivity within the CMAS melt, leading to enhanced CMAS resistance. These findings are expected to provide valuable insights for the design of high-entropy hafnate-based T/EBCs with enhanced durability under extreme service conditions.