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Thermal barrier coating (TBC) materials employ ceramics with low thermal conductivity and excellent thermal durability. Recently, the gas turbine market has been increasing demand for a thermally insulating material applicable at higher operating temperatures above 1200 °C for greater thermal conversion efficiency. In this study, synthesized high-entropy A 2 Zr 2 O 7 zirconate ceramics incorporated five cations of a similar ionic radius into the A3+ cation site through a conventional solid-state reaction, and their superior thermophysical properties demonstrate. The HEO ceramics exhibited overall low thermal conductivity (from room temperature to 1000 °C) due to inducing lattice distortion in the crystal structure. In addition, divalent cations were added to the A3+ cation site to increase the oxygen vacancy at the structure, thereby suppressing the heat transfer caused by phonon. A 2 Zr 2 O 7 zirconate HEOs show enhanced thermal barrier performance than yttria-stabilized zirconia (YSZ), conventional TBC material, demonstrating a possibility of application as next-generation TBC materials. [ABSTRACT FROM AUTHOR]

The effects of substituting the B cation in A3BO7 ceramics on their thermal physical properties were investigated by applying a large mass difference. Y3(Nb1‐xTax)O7 (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y3(Nb1‐xTax)O7 ceramics showed defective fluorite structures and glass‐like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta5+ enhanced the sintering resistance, leading to superior thermal‐insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y3(Nb1‐xTax)O7 as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials. [ABSTRACT FROM AUTHOR]

Herein, we propose a new multidisciplinary approach for investigating the corrosion behavior of thermal barrier coatings, combining the thermal-gradient mechanical fatigue method with hot corrosion tests. Corrosive salts (Na 2 SO 4 ; V 2 O 5) of varying concentrations (10−20 mg/cm2) were deposited on the surface of yttria-stabilized zirconia (YSZ) coatings to evaluate the microstructural changes occurring during the reaction, via thermal graded mechanical fatigue tests. The tests were conducted at a surface temperature of 1150 °C in 10 min cycles and underwent applied uniaxial tensile loads. The corrosion tests show that the ZrV 2 O 7 plays an important role as an intermediate in the collapse of the lamellar structures during the reaction, which results from the repetitive precipitation of V 2 O 5 and m -ZrO 2 from the ZrV 2 O 7. The microstructure of corroded YSZ coatings exhibited a different degradation mechanism under a thermomechanical environment, compared with the testing under isothermal atmospheric conditions. [ABSTRACT FROM AUTHOR]

In this study, a high-entropy approach was employed to design a new single-phase A 2 B 2 O 7 oxide for thermal insulation applications. Multicomponent high-entropy oxides, containing up to seven different cations, were successfully synthesized in a single defective fluorite structure. We observed that the incorporation of the functional cation, Zn2+, effectively blocked the heat radiation phenomenon by reducing the photon mean free path, and further reduced the high-temperature thermal conductivity owing to the larger free carrier concentration originated from the large number of oxygen vacancies. Although a large concentration of Zn2+ caused a slight reduction in thermal expansion, this study suggests that functional cations can be easily incorporated; thus, expanding the material diversity beyond the typical doping levels to develop new thermal barrier materials. [ABSTRACT FROM AUTHOR]

Calcium magnesium alumina-silicate (CMAS) corrosion on thermal barrier coatings (TBCs) is one of the reasons for the early delamination of TBCs. In this study, the effects of CMAS on the microstructure evolution of TBCs were investigated for yttria-stabilized zirconia (YSZ), lanthanum zirconate (LZ), and composite coating with a 50:50 volume ratio of LZ and YSZ (LZ-YSZ). The LZ showed the mitigated penetration of CMAS by the formation of the reaction products produced by the high reactivity between the LZ and the molten CMAS. The LZ-YSZ showed a denser microstructure on the surface, resulting in retarding the penetration of CMAS. Therefore, the LZ-YSZ composite TBC is expected to prolong the lifetime performance and protect against further corrosive degradation of TBCs in high-temperature environments. The infiltration behavior and reaction mechanism of molten CMAS were discussed based on the microstructures after the corrosion test. [ABSTRACT FROM AUTHOR]

The effects of microstructure design on the lifetime performance of lanthanum zirconate (La 2 Zr 2 O 7 ; LZO)-based thermal barrier coatings (TBCs) were investigated through various thermal exposure tests, such as furnace cyclic thermal fatigue, thermal shock, and jet engine thermal shock. To improve the thermal durability of LZO-based TBCs, composite top coats using two feedstock powders of LZO and 8 wt.% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing in different volume ratios (50:50 and 25:75, respectively). In addition, buffer layers were introduced in layered LZO-based TBCs deposited using an air-plasma spray method. The TBC with the double buffer layer showed the best thermal cycle performance among all samples in all tests. For applications with relatively slow cooling rates, the thermal durability in single-layer TBCs is more effectively enhanced by controlling a composition ratio in the blended powder, better than introducing a single buffer layer. For applications with relatively fast cooling rates, the thermal durability can be effectively improved by introducing a buffer layer than controlling a composition in the top coat, since the buffer layer provides fast localized stress relief due to its high strain compliance. These research findings allow us to control the TBC structure, and the buffer layer is efficient in improving thermal durability in cyclic thermal environments. [ABSTRACT FROM AUTHOR]

In this study, we designed a thermal barrier coating (TBC) system—Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2 (YGYZ) coating—with a Lewis neutral B–SiO 2 layer applied on its surface via a sol–gel-based glassification process and investigated its long-term corrosion resistance under corrosive environments. The corrosion behavior of the B–SiO 2 -covered YGYZ coating was assessed after 20, 40, 80, 160, and 320 h exposure to a hot corrosive environment of Na 2 SO 4 + V 2 O 5 at 1100 °C. Results indicated that the B–SiO 2 layer effectively prevented the penetration of corrosive salts and formation of m -ZrO 2 phase in the coating layer as well as generated vertical cracks that could release the residual stress. Therefore, it is concluded that depositing a Lewis neutral B-SiO 2 layer on YGYZ coatings is highly beneficial for extending the lifetime of the TBCs. • Lewis neutral B–SiO 2 layer was coated on Yb 2 O 3 –Gd 2 O 3 –Y 2 O 3 co-stabilized ZrO 2. • B–SiO 2 -covered YGYZ coatings effectively blocked the penetration of corrosive salts. • With increasing exposure time, B–SiO 2 -covered YGYZ coating generate vertical cracks. • B–SiO 2 -covered YGYZ coatings have better thermal durability than existing systems. [ABSTRACT FROM AUTHOR]

• Thermal-insulating performance of fluorite ceramic as a new TBC material. • Synthesis of YYNT ceramics via a solid-state reaction. • Glass-like low thermal conductivity and excellent thermophysical properties compatible with YSZ. The effects of substituting A and B cations in A 3 BO 7 compounds on their thermophysical properties were studied by imposing a significant mass difference on the substituents. Specifically, (Yb 1−x Y x) 3 (Nb 1−y Ta y)O 7 (YYNT) ceramics were synthesized, and their structural characteristics were investigated. All the YYNT ceramics had defective fluorite structures and exhibited glass-like low thermal conductivity (1.19 W/m K at 25 °C), which was derived from the highly distorted crystal structure and cation substitution. In addition, their excellent thermophysical properties, such as compatible thermal expansion, high elastic modulus, and hardness, make them promising candidates for new thermal barrier materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel high-entropy oxide (YYHEO) was engineered with significant mass disorder. • YYHEO exhibited glass-like low thermal conductivity (1.27 W/m∙K). • YYHEO exceeded typical substitutional solid solubility limit even with a significant atomic size difference (>50%). • YYHEO shows thermal stability even at elevated temperature (1500 ° C) without any phase degradation. • Thermophysical properties were compatible with YSZ (CTE: 10.08 × 10−6 K−1, E : 218.21 ± 3.64 GPa and H : 14.42 ± 0.5 GPa). An entropy-stabilization strategy was applied to engineer a novel thermal insulating material by establishing a single-phase solid solution with the formula of (Y 1/2 Yb 1/2) 2 (Ti 1/3 Zr 1/3 Hf 1/3) 2 O 7 (YYHEO). Structural analysis revealed that this material exceeded the typical substitutional solubility limit, even with a significant atomic size difference, where the conventional determination rule was defied. YYHEO exhibited a lower glass-like thermal conductivity due to its highly disordered crystal structure and the specific design strategy. The coefficient of thermal expansion and mechanical properties (e.g. elastic modulus and hardness) were comparable to conventional materials, suggesting that YYHEO is a promising candidate for application as a new thermal insulating material. [ABSTRACT FROM AUTHOR]

The hot corrosion behavior of plasma-sprayed Yb 2 O 3 -Gd 2 O 3 -Y 2 O 3 co-doped ZrO 2 (YGYZ) coating against Na 2 SO 4 + V 2 O 5 molten salts was investigated and its detailed hot corrosion mechanism was suggested. The chemical reactivity order between the stabilizers was proposed based on the electronic structure and Lewis acid-base mechanism. After hot corrosion, the YGYZ coating exhibited the maintenance of tetragonality ratio and relatively lower extent of destabilization less than 40% of conventional yttria-stabilized ZrO 2 coating. These great chemical and phase stabilities imply that YGYZ is promising candidate for high-temperature thermal barrier material. • Hot corrosion behavior of Yb 2 O 3 -Gd 2 O 3 -Y 2 O 3 co-doped zirconia (YGYZ) was investigated. • YGYZ exhibit great chemical and phase stability, proved by the higher tetragonality and low extent of degradation. • The chemical reactivity order is proposed with the illustration of electronic structure and Lewis acid-base mechanism. [ABSTRACT FROM AUTHOR]

• B-SiO 2 exhibits unique properties, such as Lewis neutrality, a uniform and dense morphology, and solubility in YSZ. • B-SiO 2 Lewis neutral layer was successfully applied on conventional YSZ by a sol–gel-based glassification process. • B-SiO 2 layer significantly improved the corrosion resistance and chemical stability of conventional YSZ. • A protection mechanism based on the electronic structure and the Lewis acid–base mechanism was suggested. A boron-doped silica (B-SiO 2) layer was coated on the surface of conventional plasma-sprayed yttria-stabilized zirconia (YSZ) coating to improve the hot corrosion resistance and chemical stability against Na 2 SO 4 + V 2 O 5 salts by glassification based on sol-gel process. The YSZ surface was successfully covered by the glassificated B-SiO 2 with Lewis neutrality. Further destabilization of YSZ was prevented by the formation of a solid solution. Consequently, the B-SiO 2 layer improved the hot corrosion resistance and chemical stability of YSZ. [ABSTRACT FROM AUTHOR]

The objective of this work is to suggest the thermal decomposition mechanism at the subsurface of plasma-sprayed La 2 Zr 2 O 7 (LZO) before and after isothermal exposure. The free-standing LZO coatings were prepared and suffered from volatile thermal decomposition on the subsurface during thermal exposure. The La 2 O 3 was evaporated during thermal exposure from the subsurface because of its high vapor pressure, producing the de-mixed layer consisting of La-rich and La-deficient areas in the shape of an isolated island and a lamellar structure. Some of the pre-existed pyrochlore LZO phase was transformed to monoclinic ZrO 2 because of the absence of the La 2 O 3. The thermal decomposition behavior is discussed in detail, and the possible decomposition mechanism is suggested. • The thermal decomposition behavior of plasma-sprayed LZO was investigated. • The volatilization of La 2 O 3 transformed p -LZO to m -ZrO 2 and t -ZrO 2 during isothermal exposure. • The decomposed layer exhibits poor mechanical properties, resulting in the bad resistance from erosion and the impact of foreign objects. • A schematic diagram is suggested, illustrating the detailed decomposition mechanism at the LZO subsurface. [ABSTRACT FROM AUTHOR]

• Hot corrosion behavior of YSZ/LZO blended TBC was investigated. • YSZ/LZO blended layer produced LaVO 4 , m -ZrO 2 , and t -ZrO 2 in a hot corrosive environment. • The blended layer showed a shallow infiltration depth and a severely degraded microstructure. • The corrosion mechanism has been suggested in the blended TBC with a heterogeneous interface. In the present study, layered thermal barrier coatings (TBC) were fabricated with different surface layers with lanthanum zirconate (LZO), yttria-stabilized zirconia (YSZ), and YSZ/LZO blended layer. Their hot corrosion behaviors were investigated in the molten salt of Na 2 SO 4 + V 2 O 5 at 1100 °C. TBC with the YSZ/LZO blended layer showed a different hot corrosion behavior with a relatively shallow depth and densified reaction layer compared with those with the LZO and YSZ layers. The blended layer showed the most severely degraded microstructure above the planar reacted area, which can be more appropriate for a sacrificial layer to protect the substrate. [ABSTRACT FROM AUTHOR]

Crack-growth behavior in yttria-stabilized zirconia-based thermal barrier coatings (TBCs) is investigated through a cyclic thermal fatigue (CTF) test to understand TBCs' failure mechanisms. Initial cracks were introduced on the coatings' top surface and cross section using the micro-indentation technique. The results show that crack length in the surface-cracked TBCs grew parabolically with the number of cycles in the CTF test. Failure in the surface-cracked TBC was dependent on the initial crack length formed with different loading levels, suggesting the existence of a threshold surface crack length. For the cross section, the horizontal crack length increased in a similar manner as observed in the surface. By contrast, in the vertical direction, the crack did not grow very much with CTF testing. An analytical model is proposed to explain the experimentally-observed crack-growth behavior. [ABSTRACT FROM AUTHOR]

In this work, a novel thermal barrier coating (TBC) system is proposed that embeds silicon particles in coating as a crack-healing agent. The healing agent is encapsulated to avoid unintended reactions and premature oxidation. Thermal durability of the developed TBCs is evaluated through cyclic thermal fatigue and jet engine thermal shock tests. Moreover, artificial cracks are introduced into the buffer layer's cross section using a microhardness indentation method. Then, the indented TBC specimens are subject to heat treatment to investigate their crack-resisting behavior in detail. The TBC specimens with the embedded healing agents exhibit a relatively better thermal fatigue resistance than the conventional TBCs. The encapsulated healing agent protects rapid large crack openings under thermal shock conditions. Different crack-resisting behaviors and mechanisms are proposed depending on the embedding healing agents. [ABSTRACT FROM AUTHOR]

Sintering behavior and phase transformations in yttria-stabilized zirconia (YSZ)-based thermal barrier coatings (TBCs) control their applications in gas turbines operated at high working temperatures required for improved fuel efficiency. In this work, to control the sintering behavior and reduce phase transformations in YSZ-based TBCs, lanthanum zirconate (LZ) powder was blended with the YSZ feedstock powder, and YSZ-LZ composite coatings were fabricated using the air plasma spraying method. The influence of mixture weight ratio of YSZ to LZ (75:25, 50:50, and 25:75) on the sintering behavior and phase stability of the composite coatings was investigated through the isothermal exposure test at 1100, 1300, and 1400 °C. The as-coated composites showed the pyrochlore and tetragonal phases, indicating that the phases are LZ and YSZ, respectively. As the exposure temperature was increased, the phase transformation of YSZ from the tetragonal phase to the monoclinic phase was accelerated. The content of monoclinic phase was changed with the increasing LZ content after thermal exposure at 1300 and 1400 °C. In addition, the composites showed different sintering and bridging behaviors at the adjacent splats with the LZ content. The composites prepared with the blended feedstock powders of LZ and YSZ produced an obvious effect on the phase stability and mechanical properties. [ABSTRACT FROM AUTHOR]

The effects of structural design on the thermal durability and fracture behavior of Yb-Gd-Y-based thermal barrier coatings (TBCs) were investigated through thermal cyclic exposure tests, such as furnace cyclic thermal fatigue (FCTF) and jet engine thermal shock (JETS) tests. The effects of composition in the bond coat and feedstock purity for the buffer layer on its lifetime performance were also examined. To overcome the drawbacks of Yb-Gd-Y-based material with inferior thermal durability due to poor mechanical properties and low coefficient of thermal expansion, a buffer layer was introduced in the Yb-Gd-Y-based TBC systems. In FCTF tests, the TBCs with the buffer layer showed a longer lifetime performance than those without the buffer layer, showing the longest thermal durability in the TBC with the Co-Ni-based bond coat and the buffer layer of regular purity. In JETS tests, the TBC with the Ni-based bond coat and the buffer layer of high purity showed a sound condition after 2000 cycles, showing better thermal durability for TBC with the Co-Ni-based bond coat rather than that with the Ni-based bond coat in the single layer coating without the buffer layer. The buffer layer effectively enhanced the thermal durability in slow temperature change (in the FCTF test), while the bond-coat composition and the feedstock purity for the buffer layer were found to be important factor to improve the thermal durability of the TBC in fast temperature change (in the JEET test). Finally, these research findings allow us to control the structure, composition, and feedstock purity in TBC system for improving the thermal durability in cyclic thermal environments. [ABSTRACT FROM AUTHOR]