The development of thermal/environmental barrier coatings (T/EBCs) is significantly constrained by the stringent material requirements for their topcoats. This study explores the implications of hafnium oxide (HfO₂) doping on the crystal structure and thermophysical properties of high-entropy (Y_0.25Ho_0.25Er_0.25Yb_0.25)₂O₃ oxide, with the goal of designing and selecting novel topcoat materials. (RE_1−xHf_x)₂O_3+xX_1−x (RE = Y_0.25Ho_0.25Er_0.25Yb_0.25; X = oxygen vacancy) coatings with 5, 10, 20, and 50 mol% hafnium oxide were deposited by atmospheric plasma spraying (APS). Systematic investigations were conducted on their hardness, Young’s modulus, phase composition, phase stability, thermal conductivity, and coefficient of thermal expansion. At lower HfO₂ doping contents (5–20 mol%), the (RE_1−xHf_x)₂O_3+xX_1−x coatings retained the bixbyite structure, whereas the higher HfO₂ doping level (50 mol%) induced a phase transition to the fluorite structure. The structural evolution is attributed to the ordered arrangement of oxygen anions and vacancies in the crystal structure, which enhances the phase stability and simultaneously reduces the thermal conductivity but increases the hardness, Young’s modulus and thermal expansion coefficient. The thermophysical properties of the (RE_1−xHf_x)₂O_3+xX_1−x coatings were strongly dependent on the HfO₂ doping content. The establishment of composition–structure–property relationships in the (Y_0.25Ho_0.25Er_0.25Yb_0.25)₂O₃–HfO₂ system provides critical insights for the optimization of T/EBC materials in extreme environments.

Ultrahigh-temperature ceramics (UHTCs) have a unique combination of high melting points, high strengths, and high chemical stabilities, which makes them unique materials for a wide range of ultrahigh-temperature (> 2000 °C) applications. Herein, we first report a novel highly porous dual-phase high-entropy UHTCs material composed of a high-entropy boride (HEB) phase and a high-entropy carbide (HEC) phase, which was fabricated via foam-gelcasting-freeze drying technology and high-temperature sintering with mixed borides and carbides as raw materials. The as-fabricated samples have a uniform pore structure and a firm skeleton that consists of random alternating distributions of HEB and HEC particles. The porous dual-phase high-entropy UHTCs samples have ultrahigh porosities of 96.4%–90.1%, low densities of 0.31–0.87 g/cm³, high strengths of 0.45–4.17 MPa and low thermal conductivities of 0.202–0.281 W/(m·K), as well as better oxidation resistance than single-phase HEC. The present results highlight the potential of as-prepared porous dual-phase high-entropy UHTCs as promising materials for ultrahigh-temperature thermal insulation applications.

The search for new materials with reliable molten calcium–magnesium–alumino–silicate (CMAS) resistance at elevated temperatures is important for the development of advanced aeroengines. In the present study, a novel Y₄Al₂O₉ (YAM)/Y₂O₃ composite was designed and fabricated from dense samples via the hot-pressing method. The interactions and mechanisms between the Y₄Al₂O₉/Y₂O₃ composite and CMAS at 1300 and 1500 °C for durations of 1, 4, 25, and 50 h were thoroughly explored. The results revealed that Y₄Al₂O₉/Y₂O₃ exhibited substantial resistance to CMAS infiltration at both temperatures, without notable grain-boundary penetration by CMAS glass. More importantly, the incorporation of reaction-active components in the composite accelerated the consumption of molten CMAS constituents and reduced their corrosive activity, which is recognized as the crucial principle for the composition design of anti-CMAS materials. This work provides valuable insights that can guide the design of the composition and advancement of superior CMAS-resistant materials.

Rare earth (RE) silicate is one of the most promising environmental barrier coatings for silicon-based ceramics in gas turbine engines. However, calcium–magnesium–alumina–silicate (CMAS) corrosion becomes much more serious and is the critical challenge for RE silicate with the increasing operating temperature. Therefore, it is quite urgent to clarify the mechanism of high-temperature CMAS-induced degradation of RE silicate at relatively high temperatures. Herein, the interaction between RE₂SiO₅ and CMAS up to 1500 ℃ was investigated by a novel high-temperature in-situ observation method. High temperature promotes the growth of the main reaction product (Ca₂RE₈(SiO₄)₆O₂) fast along the [001] direction, and the precipitation of short and horizontally distributed Ca₂RE₈(SiO₄)₆O₂ grains was accelerated during the cooling process. The increased temperature increases the solubility of RE elements, decreases the viscosity of CMAS, and thus elevates the corrosion reaction rate, making RE₂SiO₅ fast interaction with CMAS and less affected by RE element species.

Model composites consisting of SiC fiber and Yb₂SiO₅ were processed by the spark plasma sintering (SPS) method. The mechanical compatibility and chemical stability between Yb₂SiO₅ and SiC fiber were studied to evaluate the potential application of Yb monosilicate as the interphase of silicon carbide fiber reinforced silicon carbide ceramic matrix composite (SiC_f/SiC CMC). Two kinds of interfaces, namely mechanical and chemical bonding interfaces, were achieved by adjusting sintering temperature. SiC_f/Yb₂SiO₅ interfaces prepared at 1450 and 1500 ℃ exhibit high interface strength and debond energy, which do not satisfy the crack deflection criteria based on He–Hutchison diagram. Raman spectrum analyzation indicates that the thermal expansion mismatch between Yb₂SiO₅ and SiC contributes to high compressive thermal stress at interface, and leads to high interfacial parameters. Amorphous layer at interface in model composite sintered at 1550 ℃ is related to the diffusion promoted by high temperature and DC electric filed during SPS. It is inspired that the interfacial parameters could be adjusted by introducing Yb₂Si₂O₇–Yb₂SiO₅ interphase with controlled composition to optimize the mechanical fuse mechanism in SiC_f/SiC CMC.