As turbine inlet temperatures in advanced aeroengines and heavy-duty gas turbines continue to rise, conventional thermal and environmental barrier coatings (TBCs/EBCs) are increasingly confronted with critical operational bottlenecks. They are increasingly plagued by deleterious high-temperature phase transformations, thermal expansion mismatch with substrates, and catastrophic degradation induced by molten calcium–magnesium–aluminosilicate (CMAS) and water vapor corrosion. To decisively break these inherent limitations, high-entropy rare-earth oxides (HEREOs) have rapidly emerged as a revolutionary materials paradigm. Driven by configurational entropy stabilization, severe lattice distortion, and sluggish diffusion effects, HEREOs uniquely synchronize ultralow thermal conductivity, tailorable thermal expansion coefficients, and exceptional chemical inertness within a single crystal lattice. This comprehensive review systematically navigates the cutting-edge advancements of HEREOs for next-generation hot-section protection. It first demystifies the intricate process–microstructure relationships during coating deposition, highlighting the nonequilibrium phase evolution induced by thermal spraying. Subsequently, it critically dissects high-temperature phase stability, multiscale defect-engineered thermophysical properties, and intricate failure mechanisms under CMAS and water vapor attack. Notably, to counteract the intrinsic brittleness and improve the inferior fracture toughness of HEREOs, advanced structural engineering—incorporating multiphase synergistic toughening and gradient architectures—is highlighted as a crucial strategy for enhancing thermal shock durability. Finally, transitioning from empirical trial-and-error to a predictive framework, this review envisions a machine-learning-empowered inverse design paradigm, offering a data-driven roadmap for multiobjective optimization and lifetime prediction of highly robust HEREO coatings. This contribution also statistically outlines the latest research trends, offering researchers forward-looking guidance and evidence-based references.

Hafnium-containing ultra-high temperature ceramics refer to hafnium carbide, hafnium boride, hafnium nitride, and their multiphase ceramics, which have extremely high melting point, high hardness, high stability, excellent mechanical properties, oxidation resistance, and ablation resistance. They are ideal candidate materials for high-temperature applications such as an aerodynamic surface of high-speed aircraft, propulsion system components, plasma arc electrodes, high-temperature furnace heating elements, and high-temperature shielding. It is difficult to prepare high-purity hafnium-containing ceramic powder. The self-diffusion coefficient of ceramics is low, and conventional powder sintering has some bottlenecks in preparing large-size and complex-shaped ultra-high temperature ceramic materials. Polymer-derived ceramic is an efficient method for preparing high-performance hafnium-containing ultra-high temperature ceramic fibers and composites because of the advantages of design-able composition and structure, diversity of molding shapes, low preparation temperature, and uniform distribution of the prepared ceramics.

According to the bonding method, organic hafnium polymers can be divided into a single source and polymer-modified precursor containing Hf-O, Hf-C, and Hf-N bonds, which can be used to prepare ultra-high temperature ceramic powders, coatings, fibers, and composites. Polymers containing Hf-O bonds can be obtained via de-HCl condensation of hafnium-containing chlorides (such as HfCl4, etc.) and hydroxyl compounds (such as sucrose, etc.) with di(multi)functionality or modification of organosilicon polymers with organic hafnium compounds (such as Hf(OC₄H₉)₄, etc.), which have the advantages of cheap and easy to obtain raw materials, simple synthesis process conditions, easy industrial production, and good stability of the prepared ceramics. However, the low densification ability restricts its applications. Polymers containing Hf-C bonds are mainly obtained via de-chlorination condensation of hafnium-containing chlorides (such as Cp₂HfCl₂, etc.) and organometallic reagents, which have high ceramic yield, good ultra-high temperature performance of the pyrolytic ceramics and high hafnium content that can be introduced by a single-source precursor route. However, cyclopentadiene rigid structures limit their melting and dissolution performance, and there is still a need to improve their formability. There are also some problems, such as the synthesis process is complex. The complex network structure and large molecular weight also limit the precursor’s melting and dissolution properties synthesized by a polymer modified precursor route. The type of precursor and heat treatment process affect the composition, structure, and properties of organic hafnium polymers solidified by thermal cross-linking, gas-phase reaction cross-linking, catalytic cross-linking, and irradiation cross-linking, as well as polymer-derived HfC, HfN or HfB₂-based ceramics. Compared with the conventional inorganic powder sintering method, the hafnium-containing ultra-high temperature ceramics derived from organic hafnium polymer precursors (especially non-oxygen precursors) have the advantages of smaller grain size, more uniform distribution, lower oxygen content, and lower synthesis temperature, as well as more superior ultra-high temperature resistance, oxidation resistance, and ablation resistance.

For the wide application as thermal protection materials, it is very necessary for mullite ceramics to improve fracture toughness. In this paper, the laminated and stitched carbon fiber cloth preform reinforced mullite (C/mullite) composites were prepared through the route of sol impregnation and heat treatment using the Al₂O₃-SiO₂ sol with a high solid content as raw materials. The C/mullite composites showed a flexural strength of 228.9 MPa that was comparable to that of dense monolithic mullite although the total porosity reached 13.4%. Especially, a fracture toughness of 11.2 MPa·m^1/2 that was 4-5 times that of dense monolithic mullite was obtained. Strength deterioration due to the carbothermal reduction between carbon fiber and the residual SiO₂ in matrix was found above 1200 ℃. A pyrolytic C (PyC) coating was deposited on carbon fibers as interfacial coating. The chemical damage to carbon fibers was obviously alleviated by the sacrifice of PyC coating. Accordingly, the C/PyC/mullite composites kept strength unchanged up to 1500 ℃, and showed much higher strength retention ratio than C/mullite composites after annealing at 1600 ℃.