IntroductionThermal barrier coatings (TBCs) are capable of effectively isolating high-temperature gases, protecting the turbine blade matrix, reducing blade surface temperatures, and consequently enhancing aero engine thrust. TBCs with a thickness ranging from 100 to 500 μm in conjunction with internal turbine blade cooling can decrease the surface temperature of the superalloy by 100-300 ℃. In recent decades, yttrium-stabilized zirconia (YSZ) containing 7%-8% (by mass) is widely utilized as a material for thermal barrier coatings (TBCs) due to its superior properties. With the increasing demands for thrust-to-weight ratio, weight reduction, and stability in aviation turbine engines, the surface temperature of blade is expected to surpass 1300 ℃. When the operating temperature exceeds 1200 ℃, the YSZ coating undergoes the phase transformation and sintering. For cooling, it transitions from the tetragonal phase to the monoclinic phase, leading to coating volume expansion. Moreover, calcium- magnesium-alumino-silicate (CMAS) compounds derived from dust, sand, and volcanic ash can melt on the TBC surface, causing a severe erosion and ultimately leading to coating failure. The existing CMAS erosion becomes a challenge for TBCs. It is thus necessary for the evolving requirements of future turbine engine development to develop novel thermal barrier coating materials. Recent studies focus on new TBCs materials, including rare-earth zirconates, phosphates, and hafniates, which exhibit an enhanced CMAS erosion resistance, compared to YSZ. In this paper, the CMAS erosion resistance of rare-earth zirconates was investigated.MethodsY₂Zr₂O₇ (H1), (Y_0.5Gd_0.5)₂Zr₂O₇ (H2), (Y_0.33Gd_0.33Er_0.33)₂Zr₂O₇ (H3), (Y_0.25Gd_0.25Er_0.25Yb_0.25)₂Zr₂O₇ (H4), (Y_0.2Gd_0.2Er_0.2 Yb_0.2Lu_0.2)₂Zr₂O₇ (H5) were synthesized by a solid-state synthesis method at 1650 ℃ for 10 h. The glass phase was prepared according to the classical composition of 33CaO-9MgO-13Al₂O₃-45SiO₂. In the CMAS erosion resistance test, CMAS powder was uniformly applied to ceramic discs at a density of 20 mg/cm2. The discs were then placed in a crucible and maintained in an air atmosphere at 1300 ℃ for 5 h to conduct the erosion experiment. Subsequently, the samples were sectioned along the centerline, and the cross-sections were ground and polished to achieve a surface roughness of 1 μm, facilitating the observation of the morphology and thickness of the eroded layer. To analyze the reaction products of different zirconate ceramics during CMAS erosion, CMAS powder was mixed with five types of zirconate ceramic powders at a mass ratio of 1:1 and heated in an air atmosphere at 1300 ℃ for 5 h.Results and discussionAll the five rare-earth zirconate ceramics exhibit a single, defective fluorite structure. After five rare-earth zirconate ceramics are eroded at 1300 ℃ for 5 h, the CMAS erosion depths are ranked as H2 > H3 > H1 > H4 > H5. Despite the fact that the CMAS corrosion resistance of rare-earth zirconate ceramics does not improve with increasing entropy, the CMAS corrosion resistance of high-entropy rare-earth zirconate ceramics is superior to that of low- and medium-entropy ceramics. After heating at 1300 ℃ for 5 h, the erosion depth of H5 is only 22 μm. The mechanism of resistance to CMAS erosion in rare-earth ceramics can be elucidated through the "dissolution-reprecipitation" mechanism. Upon interaction between rare-earth zirconate ceramics and CMAS, RE3⁺ and Zr₄⁺ dissolve from the ceramic matrix into the CMAS melt. RE3⁺ ions with larger ionic radii (such as Gd3⁺ and Y3⁺) react with Ca2⁺ and Si₄⁺ in the CMAS to form RE-apatite. Conversely, RE3⁺ ions with smaller ionic radii (such as Er3⁺, Yb3⁺, and Lu3⁺) preferentially incorporate into ZrO₂ lattice, forming a fluorite-structured RE-ZrO. When the precipitation rates of RE-apatite and RE-ZrO₂ exceed the erosion rate of CMAS, a dense protective layer forms, effectively preventing further CMAS erosion. The average rare-earth ion radius and optical basicity both affect the corrosion resistance of five rare-earth zirconate ceramics to CMAS.ConclusionsY₂Zr₂O₇, (Y_0.5Gd_0.5)₂Zr₂O₇, (Y_0.33Gd_0.33Er_0.33)₂Zr₂O₇, (Y_0.25Gd_0.25Er_0.25Yb_0.25)₂Zr₂O₇, and (Y_0.2Gd_0.2Er_0.2Yb_0.2Lu_0.2)₂Zr₂O₇ were synthesized by a high-temperature solid-state reaction method. These five rare-earth zirconate ceramics exhibited a defective fluorite crystal structure. The analysis of the mechanism of resistance to CMAS erosion in these ceramics revealed that the erosion layer consisted of two distinct layers, i.e., an infiltration layer and a reprecipitation layer. The reprecipitated layer primarily comprised near-spherical fluorite-structured RE-ZrO₂ and RE-apatite crystalline pillars, forming a dense barrier that prevented further CMAS penetration into the substrate. The impact of configurational entropy on the CMAS corrosion resistance of these five rare-earth zirconate ceramics was investigated. The CMAS corrosion depths were found to be H2 > H3 > H1 > H4 > H5, and H5 exhibited the optimum CMAS corrosion resistance, showing a corrosion depth of only 22 μm after heating at 1300 ℃ for 5 h. The findings indicated that while the resistance to CMAS erosion in rare-earth zirconate ceramics did not strictly increase with increasing configurational entropy, high-entropy rare-earth zirconate ceramics demonstrated a superior resistance. compared to low- and medium-entropy counterparts. In addition, the CMAS corrosion resistance of rare-earth zirconate ceramics was influenced by both configurational entropy and the radius of rare earth ions. A concept of optical basicity was introduced to evaluate the corrosion resistance of five rare-earth zirconate ceramics against CMAS. Under identical conditions, a larger optical basicity difference (ΔΛ) between the rare earth zirconate ceramics and CMAS resulted in a higher reaction rate and deeper erosion by CMAS. The optical basicity could serve as a valuable reference for selecting new thermal barrier coating materials.