Yeon-Gil Jung

Yttria-stabilized zirconia (YSZ) is the benchmark thermal barrier topcoat, but its sintering response to water vapor is unclear. YSZ pellets of three initial porosities and free-standing plasma-sprayed coatings were heat-treated at 1300 °C in dry air, steam, or dry nitrogen. Contrary to reported promotion in simple oxides, steam retarded microstructural evolution. After 100 h the most porous pellet showed a 31% smaller grain size in steam, with apparent rate constants lower by factors of 2.8–4.2. A dry-nitrogen control in the same furnace changed the grain size by only 5.5%, isolating water vapor. Coatings likewise retained higher porosity and, at intermediate exposures, lower thermal conductivity. DFT calculations showed preferential OH binding at a grain-boundary Y ³⁺ site (−2.73 eV); a defect-equilibrium sensitivity analysis predicted 8–22% oxygen-vacancy depletion. These results support a mechanism in which OH-mediated grain-boundary stabilization through Y ³⁺ anchoring and vacancy-related defect modification leads to the retardation of sintering.

Traditional rare-earth zirconates are limited by mismatched thermophysical properties for advanced thermal barrier coating (TBC).

Traditional rare-earth zirconates are limited by mismatched thermophysical properties for advanced thermal barrier coating (TBC). In this work, two novel high-entropy rare-earth zirconates ( \mathrm{HE-RE_2Zr_2O_7} >), namely <![CDATA[ ( \mathrm{La_{0.2}}\mathrm{Sm_{0.2}}\mathrm{Nd_{0.2}}\mathrm{Y_{0.2}}\mathrm{Yb_{0.2}})_2 <![CDATA[ \mathrm{Zr_2O_7} > (HE-Yb) and <![CDATA[ ( \mathrm{La_{0.2}}\mathrm{Sm_{0.2}}\mathrm{Nd_{0.2}}\mathrm{Y_{0.2}}\mathrm{Lu_{0.2}})_2 > \mathrm{Zr_2O_7} > (HE-Lu), were designed by tailoring size and mass disorder and prepared via spark plasma sintering, with <![CDATA[ \mathrm{La_2Zr_2O_7} > (LZO) and <![CDATA[ \mathrm{Gd_2Zr_2O_7} >(GZO) prepared as control samples. Microstructural and phase characterizations reveal that HE-Yb and HELu form dual-phase structures with uniform elemental distribution and high relative density (> 98%). In terms of thermophysical properties, HE-Yb and HE-Lu exhibited low thermal conductivity, high thermal expansion coefficients and excellent phase stability. Among them, HE-Lu presented the optimal thermal insulation performance owing to enhanced Umklapp and point defect scattering, with its modified thermal conductivity reaching 1.244W·<![CDATA[ m^{-1} >·<![CDATA[ K^{-1} > at room temperature. Additionally, HE-Lu achieved the highest thermal expansion coefficient (10.96 × <![CDATA[ 10^{-6} >/K at 1200°C) due to the synergistic effects of lattice energy, electronegativity and dual-phase composition. In terms of mechanical properties, HE-Lu showed the lowest Young’s modulus (159.3GPa), highest Vickers hardness (11.37 ± 0.28GPa) and fracture toughness (1.72 ± 0.05MPa·<![CDATA[ m^{{1/2}} >). This work provides a novel design strategy for <![CDATA[ \mathrm{HE-RE_2Zr_2O_7} >, offering a reference for the development of novel TBC materials with improved properties. 2026 175 193 http://creativecommons.org/licenses/by/4.0/ 10.2298/PAC2602175L https://doiserbia.nb.rs/Article.aspx?ID=1820-61312602175L J.H. Liu, Y.Y. Wang, Z. Lu, Y.W. Zhou, Y. Li, J. Zhang, G. Lyu, Y.G. Jung, “A review of the high-entropy ceramic materials for thermal barrier coatings: Design, synthesis, sintering and properties”, Process. Appl. Ceram., 19 [1] (2025) 1-25. B. Gleeson, “Thermal barrier coatings for aeroengine applications”, J. Propul. Power, 22 [2] (2006) 375-383. Z.H. Zhang, G.J. Jiang, Y.Z. Jin, Z.M. Zhao, D.D. Wu, W.D. Ma, “Microstructure, mechanical and thermal properties of (Gd1/4Dy1/4Y1/4Er1/4)TaO4 high-entropy ceramics sintered from nanopowder prepared by reverse chemical co-precipitation and freeze drying methods”, Ceram. Int., 52 [7] (2026) 8582-8593. L.Q. Liu, R.Z. Wang, W.Z. Wang, S.L. 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Zeng, “A novel (Ho0.2Er0.2Tm0.2Yb0.2Lu0.2)2Zr2O7...

To realize direct reduction by hydrogen in iron and steel making industry, hydrogen resistance of refractory bricks should be sufficient. However, the effects of hydrogen on refractory bricks have not been studied intensively. In this study, the physical and chemical changes in mullite‐based refractory bricks were investigated for use in a hydrogen atmosphere. After exposure to hydrogen at 1100°C for 72 h, the mechanical strength was reduced by up to ∼ 15%, with a change in color. Multi‐aspect analysis indicated possible causes: the reduction of SiO ₂ , that is, SiO(g) formation, for the strength degradation, and the reduction of Fe ₂ O ₃ and/or TiO ₂ for the optical changes. Both were identified from defects at the interface of the mullite particles observed using scanning electron microscopy and the lower‐valence states detected via X‐ray photoelectron spectroscopy. In addition, thermodynamic analysis revealed that the dominant reduction mechanism of the oxides was direct reduction due to the hydrogen reaction rather than indirect reduction due to low oxygen partial pressure.

This study investigates the enhancement of thermal durability in multilayer yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBC) with porosity-controlled structures. Conventional single-layer YSZ and multilayer TBCs with dense and porous layers were fabricated by air plasma spraying and the TBC specimens were subjected to furnace cyclic testing. The single-layer TBC suffered from catastrophic delamination under cyclic thermal loading, driven by the mismatch in thermal expansion, while the multilayer TBCs exhibited a significant increase in thermal durability, by up to 50%. The relevant delamination mechanism was suggested with microstructural analysis, showing that the multilayer structure effectively relieved residual stresses by forming horizontal cracks, thereby mitigating crack propagation. This study emphasizes that the multilayer design in TBC with controlled porosity significantly enhances thermal durability, improving the operational lifespan of gas turbine hot components.

The outstanding thermophysical properties and mechanical properties are crucial for the application of RE2Zr2O7 in thermal barrier coatings (TBCs). To simultaneously optimize the thermal conductivity, thermal expansion behaviour and mechanical properties of rare-earth zirconate ceramics, in this work a novel highentropy (Sc0.2La0.2Sm0.2Er0.2Yb0.2)2Zr2O7 (REZO) ceramics was designed with significant mass and size differences based on the thermal properties tailoring theory. Structural analysis revealed that the REZO ceramics prepared by conventional solid-state reaction exhibits a dual-phase structure with coexisting pyrochlore and fluorite phases, and the five rare-earth cations were uniformly distributed throughout REZO without compositional segregation. In terms of thermophysical properties, compared to La2Zr2O7 and Gd2Zr2O7, the REZO exhibits a glass-like thermal conductivity (1.31?W?m?1?K?1, at room temperature) and a high thermal expansion coefficient (11.054 ? 10?6/K, 1200?C). Additionally, the REZO demonstrates excellent high-temperature phase stability from room temperature to 1600?C. In terms of mechanical properties, the REZO exhibits a lower Young?s modulus, higher Vickers hardness and higher fracture toughness compared to La2Zr2O7 and Gd2Zr2O7. In summary, the thermal properties tailoring theory employed in this work provides a novel design approach for developing RE2Zr2O7 ceramics with tunable thermophysical and mechanical properties, enhancing the application prospects of RE2Zr2O7 in advanced TBCs.

Dual-phase high-entropy rare-earth zirconates (RE2Zr2O7) have become an important development direction of new thermal barrier coating materials due to their excellent properties. Currently, dual-phase high-entropy RE2Zr2O7 is mainly prepared by conventional solid-state sintering. In contrast, little research has been reported on the rapid sintering of dual-phase high-entropy RE2Zr2O7 using ultrafast high-temperature sintering (UHS). Therefore, in this work, a novel dual-phase high-entropy (La0.2Sm0.2Gd0.2Y0.2Yb0.2)2Zr2O7 (LSGYY) ceramic was designed and prepared by UHS at 1700?C for 5 min. Structural analysis showed that LSGYY consisted of both pyrochlore and defective fluorite structures, and the rare-earth cations were uniformly distributed without segregation. Compared with Gd2Zr2O7 and La2Zr2O7, LSGYY exhibited excellent mechanical and thermal properties, including higher hardness (11.73 ? 0.58 GPa), higher fracture toughness (1.55 ? 0.26 MPa?m1/2), glass-like thermal conductivity (1.37 - 1.45 W?m-1?K-1), and higher coefficient of thermal expansion (11.67 ? 10-6 K-1), suggesting that LSGYY is a promising candidate for application as a novel thermal barrier coating material.

Thermal barrier coatings are used as a surface technology for hot-end components of turbine engines, and the continuous development of aerospace technology puts higher requirements on ceramic materials for thermal barrier coatings. High-entropy ceramics, as a novel material developed recently, have received wide attention due to their particular structure and high-entropy effect. In recent years, researchers have shown interest in various high-entropy ceramics due to their exceptional thermodynamic properties and outstanding thermal insulation capabilities. These ceramics are being considered for potential application in thermal barrier coatings. This paper reviews the component design theory, synthesis methods, sintering methods and functional performance of high-entropy ceramics for thermal barrier coatings. The future development direction of high-entropy ceramics for thermal barrier coatings is also prospected.