Thermal barrier coatings (TBCs) are crucial for protecting aero-engine components against high-temperature and corrosive environments. Conventional yttria-stabilized zirconia (YSZ) coatings undergo phase transformation and degradation at high temperatures of > 1250 ℃, while accelerating their failure process particularly under Ca-Mg-Al-Si (CMAS) attack. The emergence of high-entropy ceramics can give an effective pathway for the development of advanced thermal barrier coating materials. Some studies demonstrate that the incorporation of multiple rare-earth elements endows high-entropy rare-earth zirconates with a lower thermal conductivity, a better phase stability, and a superior corrosion resistance, compared to YSZ. However, the existing research predominantly focuses on rare-earth zirconate materials or process optimization of conventional YSZ coatings, while studies on the atmospheric plasma spraying (APS) preparation process of high-entropy rare-earth zirconate coatings as well as their CMAS erosion behavior and dynamic reaction mechanisms remain insufficient. This study was to thus investigate the CMAS erosion behavior of atmospheric plasma-sprayed high-entropy rare-earth zirconate (Eu_0.2Gd_0.2Er_0.2Tm_0.2Yb_0.2)₂Zr₂O₇ thermal barrier coatings. In addition, the regulatory mechanism of high-entropy design on the CMAS erosion resistance of the coatings was also revealed via systematically analyzing the microstructural evolution, phase transformation, and chemical reaction kinetics under high-temperature CMAS environments.

High-entropy (Eu_0.2Gd_0.2Er_0.2Tm_0.2Yb_0.2)₂Zr₂O₇ (EGETYZ) powder was synthesized via solid-state reaction and subsequent spray granulation to produce spherical particles suitable for APS. Coatings were applied on nickel-based alloy substrates with varying spraying distances (i.e., 80, 100 mm and 120 mm) to optimize the microstructure. The CMAS corrosion tests were conducted at different temperatures (i.e., 1300, 1400 ℃ and 1500 ℃) with a synthetic CMAS composition (33CaO-9MgO-13AlO_1.5-45SiO₂). The phase composition was analyzed by X-ray diffraction (XRD), the surface roughness was determined by laser confocal microscopy (LCM), and the microstructure/elemental distribution was characterized by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS).

The results show that the high-entropy zirconate coating exhibits a defect-fluorite structure with a minimal surface roughness (2.72 μm) and a low porosity (10.10%) at an 80 mm spraying distance. At 1300 ℃, the CMAS corrosion forms a dense reaction layer of apatite and fluorite, effectively limiting a penetration (i.e., 19.7 μm after 5 h). At 1400 ℃, a three-layer reaction structure emerges, with an increased penetration (i.e., 95.2 μm). At 1500 ℃, a penetration deepeneds significantly (i.e., 321.1 μm), with sparse apatite and prevalent gehlenite and transitional phases. Higher temperatures reduce the CMAS viscosity (by up to 77.57%), accelerating infiltration along coating defects. The dense reaction layer at lower temperatures is a key to corrosion resistance, while a rapid CMAS penetration at higher temperatures hinders its formation.

The EGETYZ coating exhibited the superior resistance to the CMAS corrosion. The corrosion temperature directly affected the viscosity change of CMAS, thereby exerting a significant influence on the corrosion behavior and reaction products. At 1300 ℃, the reaction layer was mainly composed of fluorite and apatite, forming a dense structure that effectively inhibited the CMAS infiltration. At 1400 ℃, the EGETYZ at the bottom of the reaction layer was not completely melted. The elements Al/Mg enriched in the gaps, in synergy with elements Ca/Si/RE, form a rare-earth gehlenite solid solution, resulting in a three-layer structure of the reaction layer (i.e., pure fluorite at the top, fluorite and apatite in the middle, and gehlenite-containing phase at the bottom). At 1500 ℃, only a minimal amount of apatite phase appeared, the degree of EGETYZ melting was relatively low, and it was not completely transformed into the fluorite phase, with the gehlenite phase distributing more extensively. To enhance the CMAS corrosion resistance of high-entropy rare-earth zirconate thermal barrier coatings, the key could lie in promoting the reaction with the CMAS to form a continuous and dense reaction layer. All the formed reaction products could effectively consume the CMAS components.