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A jet flame occurs when the release of flammable gas or liquid ignites, resulting in a long, intense, and highly directional flame. This type of fire is commonly associated with industrial incidents involving pipelines, storage tanks, and other pressurized equipment. Jet fires are a significant concern in the oil and gas industry due to the handling and processing of large volumes of flammable hydrocarbons under pressure. The new computational method presented here includes several aspects of hydrogen jet flame accidents and their mitigation: the CFD simulation of a hydrogen jet flame using the HyRAM code and Fire Dynamics Simulator (FDS) software 5.0 using a large eddy simulation (LES) turbulence model; the calculation of the gaseous mixture's thermo-physical properties using the GASEQ thermochemical code; the calculation of convective and radiative heat fluxes using empirical correlation; and a heat transfer simulation on the pipe thermal barrier coating (TBC) using COMSOL Multiphysics software 4.2a during the heating phase. This method developed for the ceramic blanket was validated successfully against the previous experimental results obtained by Gravit et al. It was shown that a jet fire's maximum temperature obtained using FDS software was similar to that obtained using GASEQ thermochemical software 0.79 and HyRAM software. The TBC's surface temperature reached 1945 °C. The stainless steel's maximal temperature reached 165.5 °C. There was a slight decrease in UTS at this temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] Thermal barrier coatings (TBCs) have garnered significant attention as crucial protective components for turbine blades. However, the current use of TBCs is limited by their singular functionality and the inability to accurately obtain the temperature gradient distribution within the coatings. Addressing the aforementioned issues, this paper proposes an intelligent thermal barrier coating embedded with thin-film thermocouples. This method not only provides effective thermal protection but also facilitates the precise measurement of the internal temperature gradient within the coating. To mitigate the thermal mismatch in TBCs under high-temperature environments, which can compromise their lifespan, this study employs multi-objective optimization of structural parameters to design an optimal coating thickness. This strategy ensures both superior thermal protection and extended service life. The intelligent temperature-sensing TBCs were fabricated using atmospheric plasma spraying and magnetron sputtering, followed by comprehensive characterization. To validate the performance of the intelligent temperature-sensing TBCs, static tests were conducted in a muffle furnace. The results demonstrated that the sensors exhibit excellent repeatability and high-temperature durability. Furthermore, a test platform replicating the thermal shock conditions of an engine environment was developed. This platform confirmed that the intelligent temperature-sensing TBCs are capable of accurately measuring the internal temperature gradient within the coating under engine-like conditions, offering a novel methodology for engine monitoring and diagnostics. [ABSTRACT FROM AUTHOR]

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.

Thermal barrier coating (TBC) is applied to protect the hot-section components of a gas turbine engine. In the present work, a La2(Zr0.2Ti0.2Y0.2YB0.2Nb0.2)2O7 high entropy oxide was designed, fabricated by a solid-phase reaction at 1400 °C, and subjected to an investigation of its properties. The results indicate that the La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7 high entropy oxide had a pyrochlore structure. During synthesis, a small amount of La2TiO5 was produced. The thermal conductivity of the La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7 high entropy oxide was approximately 0.8855 W·m-1·K-1 at room temperature. The coefficient of thermal expansion of the La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7 high entropy oxide was approximately 9.374 × 10−6 K−1 at 1000 °C. To improve the service life of the TBC composed of the La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7, a double-layer structural coating composed of La2(Zr0.2Ti0.2Y0.2YB0.2Nb0.2)2O7 and yttria-stabilized zirconia (YSZ) layers was designed and produced on a NiCoCrAlY alloy by laser cladding. The thermal cycling behavior of the double-layer coating was studied at 1050 °C. The results indicate that a La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7 single-phase defective-fluorite high entropy oxide was produced after preparation and thermal cycling. The spallation of the double-layer coating composed of La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7/YSZ occurred after 70 thermal cycles at 1050 °C. The double-layer coating composed of La2(Zr0.2Ti0.2Yb0.2Y0.2Nb0.2)2O7/YSZ can increase the thermal cycling lifetime of TBCs containing high entropy oxides. [ABSTRACT FROM AUTHOR]

This study uses atmospheric plasma spraying (APS) technology to prepare thermal barrier coatings (TBCs) with yttrium-stabilized zirconia (YSZ) and Yb2O3-Y2O3-co-stabilized ZrO2 (YbYSZ) materials at different spraying powers. It analyzes the differences and changes in the microstructure, thermodynamic properties, and mechanical properties of the TBCs. The CaO-MgO-Al2O3-SiO2 (CMAS) resistance of coatings was tested using thermal cycling-CMAS experiments and isothermal corrosion experiments. Compared to YSZ coatings, YbYSZ coatings have lower thermal conductivity, a higher hardness and elastic modulus, a longer lifetime under thermal cycling-CMAS conditions, and lower penetration and degradation depths. Under thermal cycling-CMAS coupling conditions, the optimal power range for the longest thermal cycling lifetime for both coatings is 39–40 kW. Overall, compared to the YSZ material, the YbYSZ material exhibits superior properties. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings to protect and extend the life of hot-end parts of gas turbine engine have come a long way from their introduction almost half a century ago, having been the subject of constant innovations determined to push their capabilities. The need for even more efficient next generation of gas turbines will require higher operating temperatures that results in an increased near-infrared radiative output. Unfortunately, the state-of-the-art yttria-stabilized zirconia is transparent to near-infrared leading to direct radiative heating of the substrate, significantly reducing its life. This opens up the requirement for advanced thermal barrier coatings that can optically reflect the incident radiation reducing overall thermal transport to the underlying super-alloy. The present review gives a thorough overview of the current state of research into the approaches employed to achieve thermal radiative barrier coatings for reflection of near-infrared radiation; either by modification of existing infrastructure or via completely novel approaches. Moreover, to complement the experimental research in this up-and-coming domain, an emphasis is laid on the modeling approaches to expedite further advancements. [ABSTRACT FROM AUTHOR]

The effects of rare earth dopants on the CMAS resistance of zirconia thermal barrier coating materials were systematically investigated using ceramic pellets and CMAS. Yb, Er, Gd and Sm elements were assessed as stabilising agents of zirconia at increasing concentrations for CMAS reactions at 1300 °C across timeframes of 1 to 60-minutes. Two distinct microstructures of the ceramic pellet –CMAS reaction layer were observed then characterised as a dense layer microstructure and a non-dense layer microstructure. The presence of each microstructure was dependent on the RE ionic radius and concentration. The thickness of the reaction layer and overall volume of precipitated reaction products increased with increasing RE ionic radius. Therefore, an optimal RE element would exhibit dense layer forming microstructure with the lowest overall infiltration depth. CMAS loading volume significantly impacted the rate of reaction product precipitation. The volume of apatite precipitate was inversely proportional to the CMAS loading quantity. [Display omitted] [ABSTRACT FROM AUTHOR]

Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high-velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high-velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process. [ABSTRACT FROM AUTHOR]

Thermal barrier coating (TBC) materials can improve energy conversion efficiency and reduce fossil fuel use. Herein, novel rare earth tantalates RETaO4, as promising candidates for TBCs, were reassembled into multi-component solid solutions with a monoclinic structure to further depress thermal conductivity via an entropy strategy. The formation mechanisms of oxygen vacancy defects, dislocations, and ferroelastic domains associated with the thermal conductivity are demonstrated by aberration-corrected scanning transmission electron microscopy. Compared to single-RE RETaO4 and 8YSZ, the intrinsic thermal conductivity of (5RE1/5)TaO4 was decreased by 35%–47% and 57%–69% at 1200 ℃, respectively, which is likely attributed to multi-scale phonon scattering from Umklapp phonon–phonon, point defects, domain structures, and dislocations. r¯RE3+/rTa5+ and low-temperature thermal conductivity are negatively correlated, as are the ratio of elastic modulus to thermal conductivity (E/κ) and high-temperature thermal conductivity. Meanwhile, the high defects’ concentration and lattice distortion in high-entropy ceramics enhance the scattering of transverse-wave phonons and reduce the transverse-wave sound velocity, leading to a decrease in the thermal conductivity and Young’s modulus. In addition, 5HEC-1 has ultra-low thermal conductivity, moderate thermal expansion coefficients, and high hardness among three five-component high-entropy samples. Thus, 5HEC-1 with superior thermal barrier and mechanical properties can be used as promising thermal insulating materials.

Rear-earth doped advanced thermal barrier coatings (TBCs) have been developed by in-situ doping of Gd into YSZ matrix by atmospheric plasma spray coating of YSZ and Gd 2 O 3 (6 to 15 mol %) premixed powders. These modified TBCs are deposited over CoNiCrAlY bond coated Inconel 718 substrate. Diffusion of Gd or Gd3+ into YSZ matrix resulted in further stabilization of its tetragonal phase which is confirmed by detailed X-Ray diffraction analysis. Microstructure and chemical compositions of doped TBCs are examined by scanning electron microscopy and energy dispersive spectroscopy, respectively. Incorporation of Gd 2 O 3 resulted in relatively denser and smoother coating compared to pure YSZ coating for identical process parameters due to finer particle size and complete melting of Gd 2 O 3 powders. However, in-situ doped coatings are sinter resistant at elevated temperature unlike pure YSZ coating which aids in retention of porosities during high temperature exposure and provide better thermal performances. Modified TBCs showed higher hardness and Young's modulus, as revealed by micro and nano hardness measurements. High temperature isothermal oxidation studies carried out between 1173 K and 1273 K allowed evaluation of the efficacy of modified TBCs for oxidation resistance and related kinetics. The results show that modified TBCs provide better oxidation resistance compared to pure YSZ coating. Erosion tests of TBCs both at ambient and elevated temperature (773 K) showed that modified coatings are more resistant to erosion compared to YSZ coating. [ABSTRACT FROM AUTHOR]

Designing thermal barrier coatings (TBCs) with varied thicknesses on surface of a turbine vane has practical significance. The present study focused on reducing the residual stress levels through optimally designing the interface morphology in TBC at the junction between zones with different coating thicknesses. The two-dimensional FEM was employed to estimate the interface stress level. The combined influences of the internal interface morphology parameters and the external geometrical parameters of TBCs on the interface stress behaviors were discussed deeply. The TBC with unchanged thickness was chosen as the reference. The comparisons revealed that the maximum increment in harmful tensile stress level due to the variation of thickness can reach 60%. Relative to the other phases, the interface morphology featuring zero phase can acquire a higher harmful stress level and larger high-stress region. The proposed interface with nonuniform morphologies can reduce the harmful stress by about 60%. The criteria in the amplitude variations and the location of each period with the external geometrical parameters were acquired. The thickened thermally grown oxide can reduce properly the benefits of the modifications of interface morphology on the reduction of stress; however, the maximum decrement can be controlled below 15%. [ABSTRACT FROM AUTHOR]

Chemical doping is a normal strategy to tune thermal expansion coefficient (TEC) of ceramics in engineering applications, but the resultant TEC values usually follow Vegard’s law, as doping does not modify the nature of chemical bonding in ceramics and its anharmonicity. In this paper, we report abnormal TEC behavior in (Nd1−xDyx)2Zr2O7 ceramics, where the TEC values remarkably exceed the values predicted by Vegard’s law and even exceed the values obtained for two constituents Nd2Zr2O7 and Dy2Zr2O7. In addition to a reduction in lattice energy with an increasing molar fraction of Dy (x) value, we attribute the additional increase in the TEC to the high concentration of Dy dopants in a pyrochlore (P) region, which can soften low-lying optical phonon modes and induce strongly avoided crossing with acoustic phonon branches and enhanced anharmonicity. We believe that this finding can provide a new route to break through the restriction imposed by the conventional Vegard’s law on the TEC values and bring new opportunities for thermal barrier coatings (TBCs) or ceramic/metal composites towards realizing minimized thermal mismatch and prolonged service life during thermal cycling.

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]

Columnar structured thermal barrier coatings (TBCs) have been intensively investigated due to their potential to enhance the durability and reliability of gas turbine engine components. These coatings consist of vertically aligned columns that provide excellent resistance to thermal cycling. In this study, the lifetime of columnar suspension-plasma-sprayed (SPS) TBCs was evaluated using burner rig tests. The tests were carried out under high-temperature conditions. Significantly, the pre-oxidation of the bondcoat during diffusion bonding treatment was found to have a substantial impact on the performance of the SPS TBCs. The optimized treatment resulted in columnar SPS TBCs demonstrating excellent thermal stability and resistance under the test conditions. The lifetime of the coatings was significantly extended compared to conventional TBCs by pre-oxidation of the CoNiCrAlY bondcoat in argon, which suggests that columnar SPS TBCs have great potential for use in gas turbine engines. [ABSTRACT FROM AUTHOR]

Rare-earth aluminates (REAlO3) are potential thermal barrier coating (TBC) materials, but the relatively high thermal conductivity (k0, ~13.6 W·m−1·K−1) and low fracture toughness (KIC, ~1.9 MPa·m1/2) limit their application. This work proposed a strategy to improve their properties through the synergistic effects of high-entropy engineering and particulate toughening. High-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)AlO3 (HEAO)-based particulate composites with different contents of high-entropy (La0.2Nd0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 (HEZO) were designed and successfully prepared by solid-state sintering. The high-entropy feature of both the matrix and secondary phases causes the strong phonon scattering and the incorporation of the HEZO secondary phase, remarkedly inhibiting the grain growth of the HEAO phase. As a result, HEAO–xHEZO (x = 0, 5%, 10%, 25%, and 50% in volume) ceramic composites show low thermal conductivity and high fracture toughness. Compared to the most commonly applied TBC material—yttria stabilized-zirconia (YSZ), the HEAO–25%HEZO particulate composite has a lower thermal conductivity of 0.96–1.17 W·m−1·K−1 (298–1273 K), enhanced fracture toughness of 3.94±0.35 MPa·m1/2, and comparable linear coefficient of thermal expansion (CTE) of 10.5×10−6 K−1. It is believed that the proposed strategy should be revelatory for the design of new coating materials including TBCs and environmental barrier coatings (EBCs).

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The surface fracture toughness is an important mechanical parameter for studying the failure behavior of air plasma sprayed (APS) thermal barrier coatings (TBCs). As APS TBCs are typical multilayer porous ceramic materials, the direct applications of the traditional single edge notched beam (SENB) method that ignores those typical structural characters may cause errors. To measure the surface fracture toughness more accurately, the effects of multilayer and porous characters on the fracture toughness of APS TBCs should be considered. In this paper, a modified single edge V-notched beam (MSEVNB) method with typical structural characters is developed. According to the finite element analysis (FEA), the geometry factor of the multilayer structure is recalculated. Owing to the narrower V-notches, a more accurate critical fracture stress is obtained. Based on the Griffith energy balance, the reduction of the crack surface caused by micro-defects is corrected. The MSEVNB method can measure the surface fracture toughness more accurately than the SENB method. [ABSTRACT FROM AUTHOR]

Porous ceramic materials are popularly accepted as thermal barrier coatings (TBCs) in insulating gas turbine parts working at high temperatures. In this research, three different types of Scandia stabilized zirconia (ScSZ) systems, a nanometric 10 mol% ScSZ (10-ScSZ), a micrometric 8 mol% ScSZ (8-ScSZ) and a combination of these two powders (10-8-ScSZ) have been developed by Spark Plasma Sintering (SPS) process. Varying SPS parameters, for instance, temperature, pressure and dwell time were applied to develop the different volumes of porosity in the materials. Subsequently, the microstructure of the materials has been studied and mechanical properties have been evaluated. All three materials demonstrate a reduced porosity level at high sintering temperature and pressure. However, the nanometric 10-ScSZ material shows a higher reduction of porosity from 51.8% to 11.1% at 30 MPa pressure and 40.2%–8.5% at 60 MPa pressure within the temperature range of 1000–1200 °C. Besides, the 10-8-ScSZ composite exhibits substantially increased porosity in comparison to its constituent parts. The results also show that the nanometric 10-ScSZ material exhibits a greater mechanical strength including Vickers microhardness of 81 HV, flexural strength of 361 MPa and elastic modulus of 187 GPa at 5% porosity level, as compared to the other two materials. Additionally, it is observed that all the mechanical properties for all three materials consistently decrease with the increase in porosity levels. While compared with the traditional atmospheric plasma spray (APS) processed ceramic coating, the porous ScSZ coating materials exhibit a larger elastic modulus. Therefore, the porous ScSZ developed by the SPS process could be a prospective alternative thermal barrier coating (TBC).

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]

The aerospace components operating in hot sections of aero-engines and combustors experience extreme environments. Typically, the components are subjected to high service temperatures exceeding 1100°C and oxidizing conditions. Protective coatings are essential for preventing oxidation-induced dimensional degradation of the components and enhancing their high temperature capability as well as durability. Defence Metallurgical Research Laboratory (DMRL) has developed a variety of metallic and ceramic Thermal Barrier Coating (TBC) systems for Ni-base superalloys, and refractory Nb-alloys for strategic aerospace applications involving ultra-high temperatures and high flow velocities. These coatings have demonstrated significant effectiveness against thermal degradation at temperatures as high as 2000 °C during oxidation in static air as well as in dynamic conditions involving high flow velocities (Mach > 2). The present article provides an overview of the advanced oxidation resistant and thermal barrier coatings developed in DMRL. The effectiveness of the TBCs in preventing dimensional degradation of the metallic and composite substrate materials has been evaluated at the laboratory scale. The developed TBCs have the potential for use in aero-engines and propulsion systems of hypervelocity vehicles. [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]

Nondestructive testing (NDT) of thermal barrier coatings (TBCs) is a critical and ongoing topic of research and development. In particular, inspection techniques that determine the thickness of ceramic topcoat and thermally grown oxide (TGO) are of interest. This work investigates the utility of open-ended rectangular waveguide probes in the millimeter wave frequency range of 26.5–110 GHz for evaluation of topcoat and TGO thicknesses through a compressive set of electromagnetic (EM) simulations. In addition, these EM simulations are used to illustrate the influence of probe size and TBC substrate curvature on the complex reflection coefficient properties and the subsequent thickness estimation. The impact of volumetric porosity level on the same is also investigated. A standing-wave probe at V-band (50–75 GHz) is constructed and used to measure the topcoat thickness on three button-type TBC samples. This probe eliminates the need for using expensive and bulky vector network analyzers (VNA), which is quite desirous from a practical point-of-view. The experimental results indicate the capability of estimating the topcoat thickness to within ±15 μm (0.6 mils). [ABSTRACT FROM AUTHOR]

Previous studies indicated that big globular pores in air plasma sprayed (APS) thermal barrier coatings (TBCs) are more likely to remain after calcium-magnesium-alumino-silicates (CMAS) attack than are small ones. In this research, therefore, the CMAS attack behavior of an APS TBC with more big globular pores was investigated. It was demonstrated that the more-big-globular-pores APS TBC exhibited better resistance to exfoliation and intergranular corrosion, and chemical degradation than normal APS TBCs, while its pores can be easily filled by CMAS. Analysis on pore structures and CMAS infiltration characteristics indicated that the morphologies of the transition regions between the globular and the crack network pores may play an import role in determining the CMAS filling behavior for globular pores in APS TBCs. [ABSTRACT FROM AUTHOR]

Residual stresses' magnitude generated by deposition, quenching stress, thermal stress operation temperature, and infiltration in the thermal barrier coating (TBC) of gas turbines was determined. A thermal barrier coating was manufactured by the deposition of two layers, CoNiCrAlY and yttria-stabilized zirconia (YSZ), on an AISI 304 stainless steel substrate. The CoNiCrAlY was deposited by using an HVOF gun and the YSZ by an atmospheric plasma spray (APS). The TBCs were heat-treated at 1250 °C, with a CMAS (CaO, MgO, Al2O3, and SiO2) attack with a concentration of 10 mg/cm2 for 6 h in order to evaluate the evolution of the state of residual stresses in the coating at a high temperature. Residual stresses were determined by employing the modified layer removal method for duplex coatings (MLRMDC), ANSYS Version R19.2, and the equations proposed by Noda et al. In the YSZ, the total maximum residual stresses were 139 MPa in compression, and in the CoNiCrAlY, the maximum residual stress was 214 MPa in compression. The factor that has the largest effect on the magnitude of residual stresses was the infiltration of the CMAS in the YSZ. [ABSTRACT FROM AUTHOR]

Current paper comprises the inter-relationship between second-order regression modeling and genetic algorithm (GA)-based optimization of air plasma deposited with improved thermal barrier coating (TBC) systems structures. For developing the regression model, experiments were performed as per L 2 7 orthogonal array, and models were established by MINITAB software. The regression models have been found satisfactory for predicting the responses at 99% confidence level. GA optimization showed a 14.86% improvement in hardness and a 15.99% reduction in roughness. The optimal level of air plasma spraying (APS) parameters was obtained as 8 number of spraying layers, 70 V of accelerating voltage, 600 A of Arc current, 30 mm/s of travel speed, 100 mm of spray distance, 25 g/min of powder feed rate, 4 J/min of carrier gas flow rate, and 55 L/min of primary gas flow rate for maximum hardness and minimum roughness. [ABSTRACT FROM AUTHOR]

The superposed structure of double ceramic layer (SDCL) could be an effective means to develop long-life thermal barrier coating (TBC) at high temperatures. In this study, YSZ/LaMgAl 11 O 19 TBC system with double-ceramic layer (DCL) and SDCL structures were prepared on nickel-based superalloy substrates by atmospheric plasma spraying. The thermal cycling behavior of the coatings was investigated using a furnace at 1000 °C and burner-rig facility at 1375 ± 25 °C on the coating surface. Results showed that the thermal cycle life of the SDCL structure was increased by 7.2% for the furnace and 13.2% for the burner-rig facility compared with that of the DCL structure. The relatively long thermal cycle life of the SDCL structure was attributed to the blocking of the propagation of cracks in the LMA layers by the YSZ ceramic layer and the release of residual thermal stresses by the formation of cracks in the LMA layers. [ABSTRACT FROM AUTHOR]

Abstract Gadolinium zirconate (GZ) is a promising candidate for next-generation thermal barrier coating (TBC) materials. Its corrosion resistance against calcium-magnesium-alumino-silicate (CMAS) needs to be further increased for enhancing its in-service life. As the Gd element plays an important role in the CMAS resistance, three GZ coatings (GZ-0.75, GZ-1.0, and GZ-1.2) with different Gd/Zr atomic ratios are designed and deposited by laser enhanced chemical vapor deposition (LCVD) in this work. It is found that the generated Gd-apatite in GZ-1.2 would block micro-cracks inside the column structure and the inter-columnar gap more efficiently. Thus, the CMAS penetration rate (5.2 μm/h) of GZ-1.2 decreases over 27% comparing with GZ-1.0 and GZ-0.75, which is even lower than the Gd2Zr2O7 coatings fabricated by electron-beam physical vapor depositions (EB-PVDs). This work provides a feasible way to adjust the coating’s corrosion resistance and may guide the development of future coating for long in-service life.

Platinum-modified-βNiAl is a bond coat (BC) material for thermal barrier coatings (TBCs) applications applied on aeroengine hardware to reduce their surface temperatures. However, it is desirable to minimize its production and material costs by the low-cost alternatives of similar performance. It has been acknowledged that the small concentration of the reactive elements (REs), such as Zr, Hf, and Y, could tremendously enhance the oxide adhesion even in some cases better than Pt modified counterparts. The aim of this study was to design and fabricate the Zr-modified-βNiAl bond coat on CMSX-4 superalloy using an aluminizing method. Moreover, the study focused on the development of a systematic understanding of underlying mechanisms behind the beneficial effects of REs. Initially, three sets of BCs were prepared: Zr-free βNiAl (undoped), Al and Zr co-deposited in a single-step process (1SP), and Zr and Al, which were individually deposited in two processing steps (2SP): zirconizing and aluminizing. Such three sets of BCs helped to understand the processing, as well as Zr and Al effects on scale adhesion. In particular, 1SP/2SP BCs showed uniformity of Zr in the form of precipitates and networks that caused hardness enhancement. All BCs were isothermally oxidized at 1150°C for 100 hours wherein 2SP revealed the best spallation resistance, microstructural stability, and its Zr-oxide pegs were extended to substrates. In addition to the Zr effect, BC Al content was found to affect the oxide adhesion equally. Under identical Zr contents (of 1SP and 2SP = 1at %), the higher Al showed the better spallation resistance while lower Al caused the inverse effect of Zr owing to its reactive nature that was termed as over doping. Moreover, it was established that over-doping either local or into entire BC, accelerated the Al depletion that destabilized the βNiAl into γ’-Ni3Al phase. An extensive discussion is presented in the light of the observed results.

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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.

Experimental results based on L 2 7 orthogonal arrays (OAs) of Air Plasma Spraying (APS) parameters for deposition of thermal barrier coatings (TBCs) onto bond coated Inconel 718 (IN718) superalloys were used for mathematical models, using artificial neural network (ANN), for hardness and roughness. Thereafter, it was optimized from ANN-coupling Genetic Algorithm (GA). The developed ANN-based models showed optimal level of responses as 6.61 and 1209.8, for roughness and hardness, respectively. The average percentage prediction error (APPE)/mean error percentage (MEP) and root mean square error (RMSE) for the roughness were found as 0.07834% and 0.00754%, respectively, while these APPE/MEP and RMSE for hardness were found as 0.456% and 0.00765%, respectively. [ABSTRACT FROM AUTHOR]

Under steady-state conditions, the high-temperature oxidation of a thermal barrier coating (TBC) leads to the formation of a thermally grown oxide (TGO). This growth of TGO is a major cause of damage to the TBC, as it results in delamination of the top coating, which decreases the adhesion strength of the bond coating. In this study, an isothermal degradation test was performed to obtain the growth curve of the TGO of a TBC. The growth behavior of the TGO differs with respect to the microstructure of the top coating. Accordingly, if the growth of the TGO can be predicted universally for air plasma-sprayed (APS) TBCs, the time and economic costs can be saved as additional tests would not be required. In this study, a model that can predict capable of predicting the growth of the TGO on an APS TBC with various microstructures. An isothermal degradation test was performed on three specimens featuring substrates and top coatings with different microstructures. The results confirmed that the growth of the TGO was affected by the microstructure of the top coating, regardless of the type of substrate. A structure constant term accounting for the thickness and porosity of the top coating was defined. Furthermore, the reaction rate constant was derived based on the measured TGO thickness. Using these constants, a novel equation for predicting the growth behavior of the TGO with respect to temperature and the microstructure of the top coating was proposed. The validity of this prediction model was verified through comparisons with the previously reported results. [ABSTRACT FROM AUTHOR]

Numerical simulation was performed to investigate adiabatic film cooling effectiveness of cooling holes over thermal barrier coated superalloy substrate. Divergent, convergent-divergent and curved configuration of cooling holes were compared with the inclined cylindrical hole configuration. The coolant flow was strongly attached with the surface for 35º inclination. The film cooling effectiveness at different blowing ratios of 0.4, 0.8 and 1.2 was analysed. The study was carried out using realizable k-ϵ (RKE) model. The curved hole configuration provided enhanced cooling effectiveness even at 0.8 blowing ratio in comparison with other cases. The presence of curvature reduced the coolant velocity thereby preventing the detachment of jet from the surface and forming a very strong longitudinal film. The coolant jet starts detaching from the surface for blowing ratio higher than 1.2; the high momentum of the film cooling jet causes a lift off the surface. At 1.2 blowing ratio, a greater spread of coolant flow along the test surface was observed. Better film cooling effectiveness was evident at 0.8 and 1.2 blowing ratios. At 0.8, the enhancement in the overall film cooling effectiveness of divergent, convergent-divergent and curved hole configuration was about 14 %, 50 % and 59 %, respectively. [ABSTRACT FROM AUTHOR]

The integrity and properties of ceramic coatings produced by atmospheric plasma spraying are highly controlled by the splat morphology and splat interconnection. In this study, the influence of selected parameters (spray angle, surface velocity of the spray gun, and substrate temperature) on splat morphology and coating microstructure was investigated. A favorite set of spray gun parameters, of which their effects on splat morphology and coating microstructure have been verified by previous experiments, were used to conduct the experiments for the present work. It was found that depositing fully molten particles on a hot substrate increases the fraction of disk-like splats by about 60% at the expense of the fraction of irregular splats. Preheating the substrate also increases the pore count and level of coating porosity, while it does not influence the density of segmentation cracks. In contrast, the surface velocity of the spray gun does not affect the splat morphology while a slow speed decreases the coating porosity and plays a significant role in generating segmentation cracks. Shifting the spray angle by 15° distorts up to 20% of disk-like splats and slightly decreases the porosity level. However, changing the spray angle does not affect the generation of segmentation cracks. [ABSTRACT FROM AUTHOR]

In this work, we present the effect of the amount of CMAS infiltration into YSZ of Thermal barrier coatings (TBC) on the magnitude of residual stresses. The (TBC) were deposited by thermal spraying of CoNiCrAlY (Bond Coat-BC) and YSZ (Top Coat-TC) powders. The deposition of the BC was through the high velocity oxygen fuel (HVOF) system. The TC was deposited via an atmospheric plasma-spraying gun (APS). The TBCs were heat treated at 1250 °C, with a CMAS attack at a concentration of 10 mg/cm².The attack exposure was for 2 and 4 hours respectively. In this evaluation, the measured parameter was the magnitude of the residual stress state in Yttria Stabilized Zirconia (YSZ). The residual stress profiles were obtained using the Modified Layer Removal Method for Duplex Coatings (MRCMRB) and the Noda equations. An increase of 26.446 MPa was determined for 2 hours of thermal treatment and 30.743 MPa for 4 hours.

Functionally graded materials (FGMs) are engineered materials that are inhomogeneous and can be purposefully processed to obtain discrete or continuously varying compositions/microstructures over a definable geometrical length. FGMs can be used in a number of applications, such as aircrafts, combustion nozzles, gas turbines, energy conversion cells, biomaterials, etc. The objective of this paper is to review the new developments in production processes and their prospects in the creation of next-generation FGMs. Traditionally, four potential methods were used for fabrications of FGMs, i.e. powder metallurgy, chemical vapor deposition, self-propagating high-temperature synthesis, and plasma spraying. Some of the recently developed methods are the cast-decant-cast process, friction stir processing, and laser-engineered net shaping, which are usually cost-effective and used to make a quiet change in properties. An effective production method for conversion of the concept of gradient into practice is still a challenge for the research community. In this paper, research works toward meeting these challenges will be highlighted, and the future scopes of investigation in this area will be explored.

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity. [ABSTRACT FROM AUTHOR]

Thermal barrier coating (TBC) systems reduce the temperature of the metallic substrate of aerospace engines, resulting in improved component durability, better fuel economy and performance. Scientific research and innovation on TBC systems evolves rapidly and diversely over the years. Hence, an understanding of the research and innovation pathways on TBC systems is extremely important for the future development of aerospace engines. This study examines research publications and patents on TBC during 1980–2019. While USA led the TBC publication output from 1980 to 2010, China takes the lead after 2011 and India too is emerging noticeably on TBC research. The citation impact of publications reveal key topic shifts. Distinct peaks identified for (1) materials sciences during 1998–2003; (2) energy during 2001–2007, and (3) chemical engineering during 2009–2018. The visual mappings of author keywords of the publications reveal the progress of TBC technology. Results show shifts in deposition techniques for TBC coating preparation, and thermal management in the engines. The journal papers cited by patents have low correlation to the highly cited papers. This paper introduced an innovation relevance indicator to gauge the relevance of an institution's scientific research to technological innovation. This novel indicator reveals the industry relevant work among the active institutions and companies with patents in TBC technologies. Universities with significant number TBC patents fall into two broad categories, (1) those who have high (> 70%) collaborations on patents filed, and (2) those that filed patents without existing external collaborations. [ABSTRACT FROM AUTHOR]

In this work, we present the effect of the amount of CMAS infiltration into YSZ of Thermal barrier coatings (TBC) on the magnitude of residual stresses. The (TBC) were deposited by thermal spraying of CoNiCrAlY (Bond Coat-BC) and YSZ (Top Coat-TC) powders. The deposition of the BC was through the high velocity ox ygen fuel (HVOF) system. The TC was deposited via an atmospheric plasmaspraying gun (APS). The TBCs were heat treated at 1250 ° C, with a CMAS attack at a concentration of 10 mg/cm?. The attack ex posure was for 2 and 4 hours respectively. In this evaluation, the measured parameter was the magnitude of the residual stress state in Yttria Stabilized Z irconia (YSZ). The residual stress profiles were obtained using the Modified Layer Removal Method for Duplex Coatings (MRCMRB) and the Noda equations. An increase of 26.446 MPa was determined for 2 hours of thermal treatment and 30.743 MPa for 4 hours. [ABSTRACT FROM AUTHOR]

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al 2 O 3 -SiO 2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation. [ABSTRACT FROM AUTHOR]

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]

Ceramics coatings produced by atmospheric plasma spraying (APS) on steel alloys parts involve high residual stress level that can affect the coating properties and integrity. Most of the experimental studies were focused on the stress evaluation into APS-produced coatings onto static targets. However, these studies have not been extended to stresses induced on rotating samples. Indeed, the thermal background and the deposition way onto a static target is different from a rotating one, resulting in indifferent coating residual stresses. The aim of this work is to estimate and understand the real-time stress evolution from the sandblasting process to the coating formation while the substrate is fixed at a rotating support thanks to a new system developed in the laboratory. Three different sources of stresses are analyzed in this paper: stresses induced by sandblasting, substrate heating, and splat quenching followed by the cooling down of the coating/substrate composite. Resulting stresses measurements are compared with after test ones made by incremental hole drilling. [ABSTRACT FROM AUTHOR]

The effects of TaO 2.5 additions to Y 4 Zr 3 O 12 (YZO) on its interactions with calcium-magnesium alumino-silicate (CMAS) melts are investigated. YZO is a candidate material for the protection of novel ZrO 2 -YO 1.5 -TaO 2.5 based thermal barrier coatings (TBCs) against CMAS attack, but thermochemical considerations dictate that YZO contain TaO 2.5 for interfacial compatibility. Oxides based on Y 4 Zr 3-x Ta x O 12+0.5x (x = =0, 0.7, 1.4) were mixed with various ternary and quinary silicate compositions ranging in Ca:Si ratio from 0.35 to 1.13 and equilibrated at 1300 °C. The results show that all combinations form the desired apatite phase except for the mixture of the higher Ta5+ content with the lowest Ca:Si melt. It is found that the addition of Ta5+ increases the capture of Y3+ by the reprecipitated fluorite, decreasing the amount available to form apatite and eventually suppressing it at low Ca:Si. The effect is ascribed to the attractive interactions between Y3+ and Ta5+ in solid solution, similar to that found in tetragonal zirconia, but with concurrent incorporation of anion vacancies associated with excess Y3+ that stabilize the cubic form. As the Ca2+ content of the melt increases, so does the amount of Ca2+ incorporated in fluorite. The latter also interacts attractively with Ta5+ and leads to a reduction of captured Y3+ in fluorite as the Ca content in the reacting deposit increases. This serves to increase the amount of Y3+available in the melt to form apatite in TBC-CMAS interactions. Image, graphical abstract [ABSTRACT FROM AUTHOR]

The integrity of thermal barrier coatings (TBCs) plays an important role in the performance of turbine blades, nevertheless the effect of cooling hole on fracture of TBCs have been less studied. The present work aimed to study the stress evolution and fracture behavior of TBCs with cooling hole under cyclic thermal loadings. Cyclic thermal tests were conducted and the cracking behavior of TBCs around cooling hole was examined using scanning electron microscope (SEM). TBCs exhibited two types of cracks near the hole edge, i.e. surface crack in the top coat and interfacial crack, which could lead to local TBC spallation and shorten the TBC lifetime. To disclose the fracture mechanism, finite element (FE) analysis was also performed. The computed residual stress values were consistent with those measured by Raman spectroscopy tests. FE simulations indicated that the free-edge effect facilitated interfacial peeling and shear stresses near the cooling hole, and hence promoted the initiation of interfacial crack in TBCs. With the increase of thermal loading cycles, the interfacial crack propagated and then coalesced with the surface cracks in the top coat, leading to the final spallation of TBCs at the cooling hole. [ABSTRACT FROM AUTHOR]