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Thermal protection of the hot-end components of ultra-high-flying vehicles requires the microwave absorption of thermal barrier coating (TBC). In this work, the microwave-absorbing functionalization of LaMgAl11O19 (LMA) TBC was successfully realized by adding FeSiAl (FSA) absorber to the LMA thermal barrier ceramic matrix to adjust electromagnetic parameters. Due to the formation of the layered lamellae structure during atmospheric plasma spraying (APS), LMA–FSA composite TBCs have better electromagnetic wave (EMW) absorbing properties than feed powder. EMW absorption of TBCs is mainly controlled by the magnetic loss, and the natural resonance is the main mechanism of magnetic loss. TBCs exhibit a minimum reflection loss (RL) value of −13.4 dB, and effective absorption bandwidth (EAB) of RL < −10 dB is up to 3.11 GHz at a simulated thickness of 2 mm. Phase and structure stability of the TBCs and microwave absorption property could be relatively well preserved even after heat treatments at 600–1000 °C for 3–50 h. Thermal conductivity of the LMA–FSA composite TBCs with FSA contents of 30–50 wt% are about 2.84–3.05 W·m−1·K−1 at 800 °C. LMA–FSA composite TBCs with heat-resistant, heat-insulation, and EMW absorbing properties might find attractive potential applications in the thermal protection for the light alloy hot-end components in civil and military industry.

Investigations of surface structure, thermo-physical properties and mechanical properties of a thermal barrier coating (TBC) fabricated from NiCrAlY/8YSZ subjected to a laser remelting process have been conducted. The NiCrAlY/8YSZ was deposited onto an Inconel 718 substrate using the atmospheric plasma spay (APS) process. A CO2 laser was then used to remelt the TBC to effect a change in structure. Thereafter, the surface structure, thermal properties, and mechanical properties like hardness and roughness were evaluated. It was found that as-sprayed TBC develops a splat layer structure with few loosely bound particles with the presence of defects. The laser remelting process modifies the TBC surface producing a smooth region with dense hexagonal equiaxed grains. The as-sprayed TBC surface showed defects and loose partially melted particles contributing to a rough area. In contrast, laser remelting improved the TBC surface structure, resulting in a dense grain structure forming a smooth region. Compared to the as-sprayed TBC, the laser remelted TBC surface is reduced by about 10% in thermal conductivity. Laser remelting also improved thermo-physical and mechanical properties of the TBC, providing better thermal insulation to enhance the TBCs lifetime. [ABSTRACT FROM AUTHOR]

Yttria-stabilized zirconia (YSZ), a thermal barrier coating material characterized by low thermal conductivity, is typically deposited via electron beam-physical vapor deposition. Notably, oxygen depletion occurs during this process, causing color changes in YSZ. Therefore, YSZ is speculated to undergo phase transformation during this process, which demands careful consideration owing to its effect on the life of thermal coatings. To study this phenomenon, bulk samples were prepared, subjected to vacuum heat treatment to induce oxygen depletion, and followed by oxidative heat treatment. Experimental results showed that the color of the samples changed to black after the vacuum heat treatment and to a lighter color after the oxidative heat treatment. In addition, X-ray diffraction and Raman analyses were performed. The monoclinic phase formation was confirmed during the vacuum heat treatment; however, it disappeared after the oxidation heat treatment. The coating obtained in a high vacuum atmosphere exhibited a black color and cubic phase, which changed to a bright color and tetragonal phase after the oxidation heat treatment. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) are used to shield from heat and wear of components which are used at high temperature situation, for example, the blades of gas turbines. There is a pressing need to evaluate the thermal conductivity of TBCs in the thermal design of advanced gas turbines with high energy efficiency. In the case of gas turbines, these TBCs consist of a ceramic-based top-coat (TC) and a bond-coat (BC) on a superalloy substrate. TBCs are commonly prepared by plasma spraying process. The TC is expected to play the role of a thermal shield. Thus, thermal conductivity of thickness direction of TBC has been investigated. However, plasma-sprayed TCs were known that they have anisotropic microstructure. We have been investigating in-plane and cross-plane thermal diffusivities of plasma-sprayed Yttria-stabilized zirconia (YSZ) TCs at 300 K in detail in a previous study. From the results of thermal diffusivity measurements for four kinds of TCs, it was found that plasma-sprayed TCs had significantly anisotropic thermal diffusivity depending on their microstructures. In this study, we measured in-plane and cross-plane thermal diffusivities of typical two plasma-sprayed TCs from 300 K to 1200 K in vacuum. And we also measured in-plane and cross-plane thermal diffusivities of annealed TC with different aging time. It was found that anisotropy estimated from in-plane and cross-plane thermal diffusivities did not change from 300 K to 1200 K. There was a tendency that thermal diffusivity value and anisotropy slightly increased by thermal aging effects. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) are essential for improving the heat resistance of materials operating in high-temperature environments. This paper proposes a new method for manufacturing double-layered TBC with graded porosity for carbon fiber-reinforced plastic (CFRP) composites. The TBC was created by a flame spraying process, consisting of relatively dense and porous layers: (1) a dense layer was produced by spraying yttria-stabilized zirconia (YSZ) particles directly onto neat carbon fabric substrate and (2) a porous layer was prepared by co-spraying YSZ particles with sacrificial polyetheretherketone (PEEK) particles. The porosity of the porous layer was controlled by varying a PEEK injection distance (D) and a PEEK feed rate (R). The correlation between porosity and thermal conductivity of the TBC layer was investigated to assess its thermal barrier performance. The TBC fabricated with the D = 5 cm and the R = 1.0 g/min offered the optimal porosity and conductivity. The 640μm-thick TBC with 34% porosity and 0.27 W/m·K thermal conductivity protected the CFRP substrate remarkably under the subjected torch at 500°C, as the TBC layer reduced the surface temperature of CFRP to 230°C. Thermomechanical analysis, following thermal shock tests, revealed that the double-layered TBC/CFRP composite retained 87% and 75% of its pristine flexural strength and modulus, respectively, while the neat CFRP composite was completely burnt out. This study explored the application of flame spray technology to develop highly effective double-layered TBCs with tunable porosity to maximize their thermal barrier performances. All results from the current study provide new insights into the design and development of TBC and CFRP composites, which will benefit a wide range of lightweight high-temperature applications. [ABSTRACT FROM AUTHOR]

Firstly, the yttria-stabilized zirconia (YSZ) coating and gadolinium zirconate (GZO) coating with the quasi-columnar structure were manufactured by plasma spray physical vapor deposition. At the same time, a novel three-dimensional geometrical model was established that could satisfactorily reflect such quasi-columnar structural characteristics. Then, based on this model, the three-dimensional spatial distribution of pores and porosity of coatings and the thermal resistance behaviors of the quasi-columnar structured coating were analyzed. Later on, the thermodynamic model was established to estimate the thermal conductivity of the quasi-columnar structured coatings at different temperatures. Finally, a model for predicting the effective thermal conductivity of the GZO/YSZ double-layer coating with quasi-columnar structure was validated to account for the effects of the variable thickness ratios of GZO top layer to YSZ inner layer. [ABSTRACT FROM AUTHOR]

Abstract HfO2 alloying effect has been applied to optimize thermal insulation performance of HoTaO4 ceramics. X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy are employed to decide the crystal structure. Scanning electronic microscopy is utilized to detect the influence of HfO2 alloying effect on microstructure. Current paper indicates that the same numbers of Ta5+ and Ho3+ ions of HoTaO4 are substituted by Hf4+ cations, and it is defined as alloying effect. No crystal structural transition is introduced by HfO2 alloying effect, and circular pores are produced in HoTaO4. HfO2 alloying effect is efficient in decreasing thermal conductivity of HoTaO4 and it is contributed to the differences of ionic radius and atomic weight between Hf4+ ions and host cations (Ta5+ and Ho3+). The least experimental thermal conductivity is 0.8 W·K−1·m−1 at 900 °C, which is detected in 6 and 9 mol%-HfO2 HoTaO4 ceramics. The results imply that HfO2–HoTaO4 ceramics are promising thermal barrier coatings (TBCs) due to their extraordinary thermal insulation performance.

Abstract High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE1/5)2Zr2O7 have been formed after heated at 1000°C. The (5RE1/5)2Zr2O7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE1/5)2Zr2O7 high-entropy ceramics are below 1 W·m–1·K–1 in the temperature range of 300–1200°C. The (5RE1/5)2Zr2O7 can be potential thermal barrier coating materials.

In this work, Gd 2 Hf 2 O 7 ceramics were synthesized and investigated as a potential thermal barrier coating (TBC) material. The phase composition, microstructure and associated thermal properties of Gd 2 Hf 2 O 7 ceramics were characterized systematically. Results show that the thermal conductivity of Gd 2 Hf 2 O 7 ceramics is 1.40 Wm−1K−1 at 1200 °C, ~25% lower than that of 8 wt% yttria partially stabilized zirconia (8YSZ). Gd 2 Hf 2 O 7 ceramics also present large thermal expansion coefficients, which decrease from 12.0 × 10−6 K−1 to 11.3 × 10−6 K−1 (300–1200 °C). Besides, the hot corrosion behaviors of Gd 2 Hf 2 O 7 ceramics exposed to V 2 O 5 and Na 2 SO 4 + V 2 O 5 salts at temperatures of 900–1200 °C were discussed in great detail. We pay much attention on the corrosion process, corrosion mechanism and corrosion damage of Gd 2 Hf 2 O 7 ceramics subjected to molten V 2 O 5 and Na 2 SO 4 + V 2 O 5 salts at different temperatures. [ABSTRACT FROM AUTHOR]

Rare-earth doped zirconates are promising candidate materials for high-performance thermal barrier coatings (TBCs). The phase and microstructure stability is an important issue for the materials that must be clarified, which is related to the long-term stable work of TBCs at high temperatures. In this work, La 2 (Zr 0.75 Ce 0.25) 2 O 7 (LCZ) ceramic coatings prepared by atmospheric plasma spraying present a metastable fluorite phase, which can transform into stable pyrochlore under high-temperature annealing. The detailed structure evolution of the ceramic coatings is characterized systematically by SEM, XRD and Raman. The associated thermal properties of LCZ ceramics were also reported. Results show that LCZ ceramic has an ultralow thermal conductivity (0.65 W/m·K, 1200 °C), which is only 1/3 of that of yttria-stabilized zirconia (YSZ). The thermal expansion coefficients of LCZ ceramic increase from 9.68 × 10-6 K-1 to 10.7 × 10-6 K-1 (300 - 1500 °C), which are relatively larger than those of La 2 Zr 2 O 7. Besides, Long-term sintering demonstrates that LCZ ceramic coating has preferable sintering resistance at 1500 °C, which is desirable for TBC applications. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBC) are multilayered systems comprising a metallic oxidation protection layer or so-called bond coat, a ceramic topcoat, and a thermally grown aluminum oxide developing at the interface. The coating systems fail typically by delamination cracking near this interface, which has a complex three-dimensional morphology influencing the crack path. This study combines laser shock adhesion test to introduce an artificial interfacial crack, known in size and location, and focused ion beam coupled with scanning electron microscopy 3D serial sectioning tomography. Methods for proper segmentation of cracks and adjacent materials and quantitative evaluation of the complex crack system are proposed and applied for analyzing the crack tip region. Finally, derived from the three-dimensional segmentation, a finite element model has been achieved and used for thermal analysis highlighting the crucial role of local damage on thermal conductivity of the TBC. [ABSTRACT FROM AUTHOR]

Abstract Forsterite-type Mg 2 SiO 4 was investigated systematically for thermal barrier coating (TBC) applications. Results showed that Mg 2 SiO 4 synthesized by solid-state reaction possessed the good phase stability up to 1573 K. The thermal conductivity of Mg 2 SiO 4 at 1273 K was lower ˜20% than that of yttria stabilized zirconia (8YSZ). Mg 2 SiO 4 also presented moderate thermal expansion coefficients, which increased from 8.6 × 10−6 K−1 to 11.3 × 10−6 K−1 (473˜1623 K). Mechanical properties including hardness, fracture toughness, and Young's modulus of Mg 2 SiO 4 were comparable to those of 8YSZ. The sintering results indicated a promising low-sintering activity of Mg 2 SiO 4. Mg 2 SiO 4 samples were subjected to water quenching test at 1573 K and showed a superior thermal shock resistance compared to 8YSZ. Mg 2 SiO 4 coating with stoichiometric composition was produced by atmospheric plasma spraying. The thermal cycling test result showed that Mg 2 SiO 4 coating had a lifetime more than 830 cycles at 1273 K, which is desirable for TBC applications. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) are used to shield the blades of gas turbines from heat and wear. There is a pressing need to evaluate the thermal conductivity of TBCs in the thermal design of advanced gas turbines with high energy efficiency. These TBCs consist of a ceramic-based top coat and a bond coat on a superalloy substrate. Usually, the focus is on the thermal conductivity in the thickness direction of the TBC because heat tends to diffuse from the surface of the top coat to the substrate. However, the in-plane thermal conductivity is also important in the thermal design of gas turbines because the temperature distribution within the turbine cannot be ignored. Accordingly, a method is developed in this study for measuring the in-plane thermal diffusivity of the top coat. Yttria-stabilized zirconia top coats are prepared by thermal spraying under different conditions. The in-plane and cross-plane thermal diffusivities of the top coats are measured by the flash method to investigate the anisotropy of thermal conduction in a TBC. It is found that the in-plane thermal diffusivity is higher than the cross-plane one for each top coat and that the top coats have significantly anisotropic thermal diffusivity. The cross-sectional and in-plane microstructures of the top coats are observed, from which their porosities are evaluated. The thermal diffusivity and its anisotropy are discussed in detail in relation to microstructure and porosity. [ABSTRACT FROM AUTHOR]

Low emission in diesel engines increases the importance of low fuel consumption and high efficiency. In this experimental study, the parts forming the combustion chamber were coated with ceramic materials, which had low thermal conductivity and resistance to high operating temperature, in order to reduce the loss energy and harmful emissions in a diesel engine. This study was conducted in 3 stages. In the first stage, cylinder liner and the valves were coated by using boronizing method. In the second stage, piston surface was coated by using plasma spray method. In the third stage, tripropylene glycol + diesel mixtures (added as 2%, 4%, and 6% by volume) were used in coated (COE) and uncoated (STE) engines. The results determined that in coated engine, carbon monoxide (CO), hydrocarbon (HC), smoke emissions and brake specific fuel consumption (BSFC) values of diesel fuel and tripropylene glycol + diesel mixtures decreased; whereas, nitrogen oxide (NO x ), exhaust gas temperature (EGT) and brake thermal efficiency increased. [ABSTRACT FROM AUTHOR]

Axial suspension plasma spraying (ASPS) is a new, innovative plasma spray technique using a feedstock consisting of fine powder particles suspended in a liquid. With ASPS, thermal barrier coating (TBC) topcoats with columnar structures have been produced that are built up by fine powder particles. The microstructure consists of micro-, meso-, and macro-pores. Due to the wide pore size range including nano-porosity it is challenging to measure porosity and pore size distribution in TBC topcoats. However, it is important to characterize the porous structure as it affects the thermal conductivity. Nuclear magnetic resonance (NMR) cryoporometry is a promising method for performing such measurements because of its capability of measuring pores down to nanometer size and providing information about the pore geometry. The aim of this paper is to introduce NMR cryoporometry as a new characterization technique for determining porosity, pore size distribution and pore geometry of TBC topcoats produced by ASPS. The study includes the comparison of two different yttria-stabilized zirconia topcoats and NMR cryoporometry is complemented by microstructural characterization using scanning electron microscopy. [ABSTRACT FROM AUTHOR]

The effective high-temperature operation limit of a ZrO-7-8 wt.%YO (YSZ) thermal barrier coating (TBC) manufactured via air plasma spray (APS) is considered to be ~1300 °C. This is related to the metastable tetragonal t′-phase formed during the rapid quenching of the YSZ particles during spraying. The t′-phase transforms into the equilibrium tetragonal and cubic phases at temperatures ≥~1300 °C, which can lead to the formation of the monoclinic phase of YSZ upon cooling to room temperature. This formation of the monoclinic phase is accompanied by a volume expansion that leads to TBC failure due to extensive micro-cracking. To further investigate this limitation, an APS YSZ TBC was sprayed on a CMSX-4 substrate. By using a thermal (laser) gradient cyclic testing, a temperature gradient was generated across the TBC/substrate system. The YSZ T- front and substrate backside T- back temperature levels were ~1500 and ~1000 °C, respectively. In cycle conditions (5-min or 1-h hot and 2-min cool), no TBC failure has been observed. This behavior was partially attributed to the unexpected absence of the monoclinic phase of the YSZ in the cycled coatings. Although preliminary, these results are promising regarding increasing the effective high-temperature operational limits of APS YSZ TBCs. [ABSTRACT FROM AUTHOR]

To enhance efficiency of gas turbines, new thermal barrier coatings (TBCs) must be designed which improve upon the thermal stability limit of 7 wt.% yttria stabilized zirconia (7YSZ), ~ 1200 °C. This tenant has led to the development of new TBC materials and microstructures capable of improved high temperature performance. This study focused on increasing the erosion durability of cubic zirconia based TBCs, traditionally less durable than the metastable t′ zirconia based TBCs. Composite TBC microstructures composed of a low thermal conductivity/high temperature stable cubic Low-k matrix phase and a durable t′ Low-k secondary phase were deposited via APS. Monolithic coatings composed of cubic Low-k and t′ Low-k were also deposited, in addition to a 7YSZ benchmark. The thermal conductivity and erosion durability were then measured and it was found that both of the Low-k materials have significantly reduced thermal conductivities, with monolithic t′ Low-k and cubic Low-k improving upon 7YSZ by ~ 13% and ~ 25%, respectively. The 40 wt.% t′ Low-k composite (40 wt.% t′ Low-k — 60 wt.% cubic Low-k) showed a ~ 22% reduction in thermal conductivity over 7YSZ, indicating even at high levels, the t′ Low-k secondary phase had a minimal impact on thermal conductivity in the composite coating. It was observed that a mere 20 wt.% t′ Low-k phase addition can reduce the erosion of a cubic Low-k matrix phase composite coating by over 37%. Various mixing rules were then investigated to assess this non-linear composite behavior and suggestions were made to further improve erosion durability. [ABSTRACT FROM AUTHOR]

A 2 B 2 O 7 -type rare earth zirconates have been intensively investigated as potential thermal barrier coating (TBC) materials due to their low thermal conductivities and high stability at high temperature. In this work, mixing La 3+ and Yb 3+ with a large difference in atomic radii at A site formed a quasi-eutectoid mixture of La 2 Zr 2 O 7 -rich pyrochlore phase and Yb 2 Zr 2 O 7 -rich fluorite phase. The competition between these two phases in grain growth effectively reduces the grain size down to 0.8–1 μm, giving rise to improved mechanical properties comparing with the single-phase materials, including a higher hardness, a higher fracture toughness and a lower elastic modulus. Furthermore, a significantly low thermal conductivity was found, as the small and heavy Yb 3+ ions that dissolve in the La 2 Zr 2 O 7 -rich pyrochlore could “rattle” in the lattice and strongly scatter the heat-carrying phonons. Considering the comprehensively improved mechanical and thermal properties, the quasi-eutectoid (La 1− x Yb x ) 2 Zr 2 O 7 ceramic can be considered as a potential TBC material. [ABSTRACT FROM AUTHOR]

Abstract: The effects of La2O3 addition on thermal conductivity, phase stability and thermal cycle life of Y2O3 stabilized ZrO2 plasma sprayed coatings were investigated. Although low thermal conductivity as well as high resistance to sintering was achieved by La2O3 addition, it tended to also result in lower phase stability and thermal cycle life of the coatings. Optimization of the composition and structure of the coatings improved these properties, and the optimized coatings showed prolonged thermal cycle life. [Copyright &y& Elsevier]

Thermal conductivity and phase stability of La2O3,Y2O3 stabilized ZrO2 plasma sprayed coatings were investigated. La2O3 was selected as a stabilizer because it had a significant effect on reducing densification of Y2O3 stabilized ZrO2 (YSZ). The developed coating showed low thermal conductivity even after high temperature exposure due to its high resistance to sintering. However, phase stability and thermal cycle life of the coating decreased as the amount of La2O3 was increased. It was concluded that optimum amount of La2O3 addition was about 1 mol% to suppress sintering with little degradation of other properties. [Copyright &y& Elsevier]