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Thermal Barrier Coatings (thermal + barrier_coating)
Selected AbstractsModeling of Coating Process, Phase Changes, and Damage of Plasma Sprayed Thermal Barrier Coatings on Ni-Base Superalloys,ADVANCED ENGINEERING MATERIALS, Issue 3 2010Tilmann Beck The paper gives an overview on the modeling activities on plasma sprayed thermal barrier coating in the frame of TFB 63. In the first part, through-process modeling of the APS deposition of a ZrO2 based TBC is described. Starting from simulation of the plasma jet, heat transfer into the powder particles, particle melting, particle impact on the substrate surface, and solidification is simulated. A homogenization method is introduced to describe the mechanical properties of the resulting TBC. The second part shows simulation of interdiffusion and phase transformations of MCrAlY and intermetallic oxidation protection coatings on several cast Ni-base alloy substrates. Finally, FEM-based damage simulation of oxidation protection coatings by transversal fatigue cracks during thermomechanical fatigue loading as well as by delamination of the TBC during thermocyclic loading is discussed. [source] Microstructure Tailored Functionally Graded Alumina/Lanthanum Hexaaluminate Ceramics for Application as Thermal Barrier Coatings,ADVANCED ENGINEERING MATERIALS, Issue 12 2009Zahra Negahdari The thermal and mechanical properties of a functionally graded lanthanum hexaaluminate-alumina ceramic are described. The gradation of functionality is based on different volume fraction of lanthanum hexaaluminate, varying from 0 to 80 vol% and corresponding porosity from 2 to 32 vol%. The highest volume fraction of lanthanum hexaaluminate enables a five time reduced thermal diffusivity as compared to alumina. The fracture toughness and elastic modulus is highest for a 20 vol% lanthanum hexaaluminate-alumina composite. [source] Monitoring Delamination Progression in Thermal Barrier Coatings by Mid-Infrared Reflectance ImagingINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2006Jeffrey I. Eldridge Mid-infrared (MIR) reflectance imaging is shown to be a reliable diagnostic tool for monitoring delamination progression in thermal barrier coatings (TBCs). MIR reflectance imaging utilizes the maximum transparency of TBCs in the 3,6 ,m wavelength region to probe below-surface delamination crack propagation that is typically hidden from visible wavelength inspection. The image contrast that identifies delamination progression arises from the increased reflectance produced by a large component of total internal reflection at the TBC/buried-crack interface. Imaging was performed at a wavelength of 4 ,m to take advantage of the relatively high transmittance of plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs along with a desirable relative insensitivity to potentially interfering absorptions by atmospheric constituents at that wavelength. A key advantage of MIR reflectance imaging over competing techniques is that it is sensitive to delamination progression even at very early stages before delamination cracks start linking together; therefore, TBC health assessment can be achieved throughout the life of the TBC well before TBC failure is imminent. Examples are presented to demonstrate monitoring delamination progression by MIR reflectance imaging in 8YSZ TBC-coated specimens subjected to furnace cycling to 1163°C. The experimental results were in good agreement with reflectance values predicted by a four-flux Kulbelka,Munk approximation applied to the extreme cases of a completely adherent and a completely detached TBC. Practical considerations, including potential interfering effects from surface contamination, sintering, and erosion are discussed. [source] Temperature-Gradient Effects in Thermal Barrier Coatings: An Investigation Through Modeling, High Heat Flux Test, and Embedded SensorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Yang Tan The harsh thermal environment in gas turbines, including elevated temperatures and high heat fluxes, induces significant thermal gradients in ceramic thermal barrier coatings (TBCs), which are used to protect metallic components. However, the thermal conductivity of plasma-sprayed TBC increases with exposure at high temperatures mainly due to sintering phenomena and possible phase transformation, resulting in coating performance degradation and potential thermal runaway issues. An analytical thermal model and experimentally obtained coating thermal conductivity data are used to determine the coating through-thickness temperature profile and effective thermal conductivity under gradient conditions at high temperatures. High heat flux tests are then performed on TBCs to evaluate coating thermal behavior under temperature gradients close to service conditions. Coating internal temperature during the tests was also measured by thermally sprayed embedded thermocouples within the top coat. This combined approach provides a sintering map with a new model and allows for the assessment of temperature-gradient effects on the thermal performance of plasma-sprayed TBCs. [source] Design, Preparation, and Characterization of Graded YSZ/La2Zr2O7 Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2010Hongfei Chen Large-area spallation and crack formation during service are big problems of plasma-sprayed thermal barrier coatings (TBCs), owing to their weak bond strengths and high residual stresses. Functional gradient TBCs with a gradual compositional variation along the thickness direction are proposed to mitigate these problems. In this paper, a six-layer structured TBC composed of Y2O3 partially stabilized ZrO2 (YSZ) and La2Zr2O7 (LZ), was prepared by plasma spraying with dual powder feeding ports. This coating had a gradient composition and function. Thermal conductivity of the coating was comparable with that of a single LZ coating while the coefficient of thermal expansion was nearly equal to that of YSZ single coating. The experiment was conducted to compare the thermal shock resistance of a graded coating with a conventional YSZ/LZ double-layer system. Changes in weight and morphology of specimens before and after thermal shock tests were analyzed. Results demonstrated that the thermal shock resistance of the graded coating was superior to the double-layer coating. Typically, a barely visible pimple-like spallation was present on the surface of the graded coating after 21 cycles. On the other hand, obvious delamination was observed for a double-layer coating after six to seven cycles. Special focus was also placed on a comparative investigation of stresses that are closely related to spallation via the use of numerical simulation. [source] Thermophysical Properties of Complex Rare-Earth Zirconate Ceramic for Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2008Liu Ling Two complex rare-earth zirconates (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 for thermal barrier coatings (TBCs) were synthesized by the coprecipitation method. Their phase composition, microstructure, and thermophysical properties were investigated. X-ray diffractometry results revealed that single-phase (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 with pyrochlore structure were prepared, and the scanning electron microscopy results showed that the microstructures of the products were dense and no other phases existed among the grains. With the temperature increasing, the thermal expansion coefficient (CTE) of the ceramics increased, while the thermal conductivity decreased. The results indicated that the CTE of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 was slightly higher than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and the thermal conductivity of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 was lower than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4. These results imply that the thermophysical properties of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 are better than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 as the material for the ceramic layer in the TBC system. [source] Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2008Jeffrey I. Eldridge Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directional-hemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 ,m. Spectra were collected over a wide range of coating thickness from 60 to almost 900 ,m. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surface-temperature pyrometer measurements and on (2) applying mid-infrared reflectance to monitor TBC delamination are discussed. [source] Creep Behavior of Plasma-Sprayed Zirconia Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2007Reza Soltani Thermally sprayed ceramic coatings deposited from nanostructured feedstock powder have often demonstrated improved properties relative to coatings produced from conventional powders. This type of coating has been reported to exhibit better wear resistance and higher adhesion strength compared with conventional deposits. Powder consisting of hollow spherical particles has been reported to produce coating with lower unmelted particles and lower thermal conductivity. In this study, the thermo-mechanical properties of plasma-sprayed yttria-stabilized zirconia coatings deposited using each of these types of powder were investigated. Creep strain and creep rate were measured using free-standing thick coatings loaded in a four-point bend configuration at temperatures ranging from 800° to 1200°C in air under a range of loads. The creep exponent and activation energy were determined. [source] Microstructure,Property Correlations in Industrial Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004Anand A. Kulkarni This paper describes the results from multidisciplinary characterization/scattering techniques used for the quantitative characterization of industrial thermal barrier coating (TBC) systems used in advanced gas turbines. While past requirements for TBCs primarily addressed the function of insulation/life extension of the metallic components, new demands necessitate a requirement for spallation resistance/strain tolerance, i.e., prime reliance, on the part of the TBC. In an extensive effort to incorporate these TBCs, a design-of-experiment approach was undertaken to develop tailored coating properties by processing under varied conditions. Efforts focusing on achieving durable/high-performance coatings led to dense vertically cracked (DVC) TBCs, exhibiting quasi-columnar microstructures approximating electron-beam physical-vapor-deposited (EB-PVD) coatings. Quantitative representation of the microstructural features in these vastly different coatings is obtained, in terms of porosity, opening dimensions, orientation, morphologies, and pore size distribution, by means of small-angle neutron scattering (SANS) and ultra-small-angle X-ray scattering (USAXS) studies. Such comprehensive characterization, coupled with elastic modulus and thermal conductivity measurements of the coatings, help establish relationships between microstructure and properties in a systematic manner. [source] Strength Degradation and Failure Mechanisms of Electron-Beam Physical-Vapor-Deposited Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2001James A. Ruud Failure mechanisms were determined for electron-beam physical-vapor-deposited thermal barrier coating (TBC) systems from the degradation of mechanical properties and microstructural changes in a furnace cycle test. Bond strength degradation for TBCs resulted from the initiation and growth of interfacial delamination defects between the yttria-stabilized zirconia topcoat and the thermally grown alumina (TGO). It is proposed that defects started from concave depressions in the bondcoat surface created by the grit-blast-cleaning process and that defect growth was driven by the reduction in compressive strain in the TGO as the alumina deformed into and displaced the bondcoat during the cooling cycles. Inclusion of yttrium in the substrate resulted in a doubling of the furnace cycle life of the TBCs because of enhanced fracture toughness of the TGO-bondcoat interface. [source] Piezospectroscopic Analysis of Interface Debonding in Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2000Xiao Peng One of the principal modes by which electron-beam-evaporated thermal barrier coatings fail is via the nucleation of local regions of debonding, which grow and link together until reaching a critically sized flaw for spontaneous buckling and spalling. This progressive-failure mode is used as a basis for analyzing the changes that can occur in photostimulated luminescence spectra that have been recorded from the thermally grown oxide. This process also provides a basis for the quantitative determination of the extent of local damage prior to spalling from an analysis of the shape of the luminescence spectra. [source] Advanced Homogenization Strategies in Material Modeling of Thermally Sprayed TBCs,ADVANCED ENGINEERING MATERIALS, Issue 7 2006K. Bobzin Thermal barrier coatings (TBC), obtained by atmospheric plasma spraying (APS), have a complex microstructure (lamellar, porous, micro-cracked). Process parameters take an influence on this microstructure. Two methods based on the homogenization for periodic structures are presented in this article. The methods are used to calculate the effective material behavior of APS-TBCs made of partially yttria stabilized zirconia (PYSZ) depending on the microstructure. [source] Effect of the Starting Microstructure on the Thermal Properties of As-Sprayed and Thermally Exposed Plasma-Sprayed YSZ CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2009Yang Tan Thermal barrier coatings (TBCs) experience thermal gradients, excessive temperature, and high heat flux from hot gases in turbines during service. These extended thermal effects induce sintering and significant microstructure changes, which alter the resulting thermal conductivity of the TBCs. To study the effects of different starting microstructures on the sintering behavior, plasma-sprayed yttria-stabilized zirconia (YSZ) TBCs produced from different starting powders and process parameters were subjected to thermal aging at several temperatures and time intervals, after which their thermal conductivity was measured at room temperature. The thermal conductivity results were analyzed by introducing the Larson,Miller parameter, that describes the creep-like behavior of thermal conductivity increase with annealing temperature and time. One set of coatings was also annealed under the same conditions and the thermal conductivities were measured at elevated temperatures. The temperature-dependent thermal conductivity data were analyzed and used to predict the long-term thermal property behavior for a general YSZ coating design. [source] Microstructure,Property Correlations in Industrial Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2004Anand A. Kulkarni This paper describes the results from multidisciplinary characterization/scattering techniques used for the quantitative characterization of industrial thermal barrier coating (TBC) systems used in advanced gas turbines. While past requirements for TBCs primarily addressed the function of insulation/life extension of the metallic components, new demands necessitate a requirement for spallation resistance/strain tolerance, i.e., prime reliance, on the part of the TBC. In an extensive effort to incorporate these TBCs, a design-of-experiment approach was undertaken to develop tailored coating properties by processing under varied conditions. Efforts focusing on achieving durable/high-performance coatings led to dense vertically cracked (DVC) TBCs, exhibiting quasi-columnar microstructures approximating electron-beam physical-vapor-deposited (EB-PVD) coatings. Quantitative representation of the microstructural features in these vastly different coatings is obtained, in terms of porosity, opening dimensions, orientation, morphologies, and pore size distribution, by means of small-angle neutron scattering (SANS) and ultra-small-angle X-ray scattering (USAXS) studies. Such comprehensive characterization, coupled with elastic modulus and thermal conductivity measurements of the coatings, help establish relationships between microstructure and properties in a systematic manner. [source] Strength Degradation and Failure Mechanisms of Electron-Beam Physical-Vapor-Deposited Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2001James A. Ruud Failure mechanisms were determined for electron-beam physical-vapor-deposited thermal barrier coating (TBC) systems from the degradation of mechanical properties and microstructural changes in a furnace cycle test. Bond strength degradation for TBCs resulted from the initiation and growth of interfacial delamination defects between the yttria-stabilized zirconia topcoat and the thermally grown alumina (TGO). It is proposed that defects started from concave depressions in the bondcoat surface created by the grit-blast-cleaning process and that defect growth was driven by the reduction in compressive strain in the TGO as the alumina deformed into and displaced the bondcoat during the cooling cycles. Inclusion of yttrium in the substrate resulted in a doubling of the furnace cycle life of the TBCs because of enhanced fracture toughness of the TGO-bondcoat interface. [source] The role that bond coat depletion of aluminum has on the lifetime of APS-TBC under oxidizing conditionsMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 7 2008D. Renusch Abstract Bond coat oxidation as well as bond coat depletion of Al are still believed to be a major degradation mechanism with respect to the lifetime of thermal barrier coating (TBC) systems. In this study the top coat lifetime is described as being limited by both bond coat depletion of Al and mechanical failure of the top coat. The empirical results are introduced by considering three spallation cases, namely, Al depletion failure, thermal fatigue failure, and thermal aging failure. Al depletion failure occurs when the Al content within the bond coat reaches a critical value. In this paper bond coat depletion of Al is modeled by considering the diffusion of Al into both the thermally grown oxide (TGO) and substrate. The diffusion model results are compared to Al concentration profiles measured with an electron beam microprobe. These measured results are from oxidized air plasma sprayed TBC systems (APS-TBC) with vacuum plasma sprayed (VPS) bond coats for exposures up to 5000 h in the temperature range of 950,1100,°C. This paper focuses on the Al depletion failure and how it relates to top coat spallation. [source] An investigation of the effect of thermal cycling on plasma-sprayed zirconia/NiCoCrAlY thermal barrier coatingMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 1 2004A. El-Turki Abstract The microstructural change, crack initiation and spallation of a vacuum plasma sprayed (VPS) thermal barrier coating on an INCONEL-738 superalloy substrate were investigated after successive 300 h thermal cycles at 1050°C. The coating was characterised using Raman spectroscopy, scanning electron microscopy (SEM) energy dispersive X-ray analysis (EDX) and Auger electron spectroscopy (AES). Localised micro-cracks at the yttrium (III) oxide stabilised zirconium (IV) oxide (YSZ) ceramic coating/thermally grown oxide (TGO) interface were observed after 8 cycles. Spallation of the YSZ coating occurred after approximately 21 cycles. Significant amounts of the elements titanium, tantalum and chromium were found within the TGO together with the formation of nickel, cobalt and chromium-rich oxides at this TGO/YSZ interface. [source] Thermal Barrier Coatings Design with Increased Reflectivity and Lower Thermal Conductivity for High-Temperature Turbine ApplicationsINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2006Matthew J. Kelly High reflectance thermal barrier coatings consisting of 7% Yittria-Stabilized Zirconia (7YSZ) and Al2O3 were deposited by co-evaporation using electron beam physical vapor deposition (EB-PVD). Multilayer 7YSZ and Al2O3 coatings with fixed layer spacing showed a 73% infrared reflectance maxima at 1.85 ,m wavelength. The variable 7YSZ and Al2O3 multilayer coatings showed an increase in reflection spectrum from 1 to 2.75 ,m. Preliminary results suggest that coating reflectance can be tailored to achieve increased reflectance over a desired wavelength range by controlling the thickness of the individual layers. In addition, microstructural enhancements were also used to produce low thermal conductive and high hemispherical reflective thermal barrier coatings (TBCs) in which the coating flux was periodically interrupted creating modulated strain fields within the TBC. TBC showed no macrostructural differences in the grain size or faceted surface morphology at low magnification as compared with standard TBC. The residual stress state was determined to be compressive in all of the TBC samples, and was found to decrease with increasing number of modulations. The average thermal conductivity was shown to decrease approximately 30% from 1.8 to 1.2 W/m-K for the 20-layer monolithic TBC after 2 h of testing at 1316°C. Monolithic modulated TBC also resulted in a 28% increase in the hemispherical reflectance, and increased with increasing total number of modulations. [source] Monitoring Delamination Progression in Thermal Barrier Coatings by Mid-Infrared Reflectance ImagingINTERNATIONAL JOURNAL OF APPLIED CERAMIC TECHNOLOGY, Issue 2 2006Jeffrey I. Eldridge Mid-infrared (MIR) reflectance imaging is shown to be a reliable diagnostic tool for monitoring delamination progression in thermal barrier coatings (TBCs). MIR reflectance imaging utilizes the maximum transparency of TBCs in the 3,6 ,m wavelength region to probe below-surface delamination crack propagation that is typically hidden from visible wavelength inspection. The image contrast that identifies delamination progression arises from the increased reflectance produced by a large component of total internal reflection at the TBC/buried-crack interface. Imaging was performed at a wavelength of 4 ,m to take advantage of the relatively high transmittance of plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs along with a desirable relative insensitivity to potentially interfering absorptions by atmospheric constituents at that wavelength. A key advantage of MIR reflectance imaging over competing techniques is that it is sensitive to delamination progression even at very early stages before delamination cracks start linking together; therefore, TBC health assessment can be achieved throughout the life of the TBC well before TBC failure is imminent. Examples are presented to demonstrate monitoring delamination progression by MIR reflectance imaging in 8YSZ TBC-coated specimens subjected to furnace cycling to 1163°C. The experimental results were in good agreement with reflectance values predicted by a four-flux Kulbelka,Munk approximation applied to the extreme cases of a completely adherent and a completely detached TBC. Practical considerations, including potential interfering effects from surface contamination, sintering, and erosion are discussed. [source] Temperature-Gradient Effects in Thermal Barrier Coatings: An Investigation Through Modeling, High Heat Flux Test, and Embedded SensorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Yang Tan The harsh thermal environment in gas turbines, including elevated temperatures and high heat fluxes, induces significant thermal gradients in ceramic thermal barrier coatings (TBCs), which are used to protect metallic components. However, the thermal conductivity of plasma-sprayed TBC increases with exposure at high temperatures mainly due to sintering phenomena and possible phase transformation, resulting in coating performance degradation and potential thermal runaway issues. An analytical thermal model and experimentally obtained coating thermal conductivity data are used to determine the coating through-thickness temperature profile and effective thermal conductivity under gradient conditions at high temperatures. High heat flux tests are then performed on TBCs to evaluate coating thermal behavior under temperature gradients close to service conditions. Coating internal temperature during the tests was also measured by thermally sprayed embedded thermocouples within the top coat. This combined approach provides a sintering map with a new model and allows for the assessment of temperature-gradient effects on the thermal performance of plasma-sprayed TBCs. [source] Design, Preparation, and Characterization of Graded YSZ/La2Zr2O7 Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2010Hongfei Chen Large-area spallation and crack formation during service are big problems of plasma-sprayed thermal barrier coatings (TBCs), owing to their weak bond strengths and high residual stresses. Functional gradient TBCs with a gradual compositional variation along the thickness direction are proposed to mitigate these problems. In this paper, a six-layer structured TBC composed of Y2O3 partially stabilized ZrO2 (YSZ) and La2Zr2O7 (LZ), was prepared by plasma spraying with dual powder feeding ports. This coating had a gradient composition and function. Thermal conductivity of the coating was comparable with that of a single LZ coating while the coefficient of thermal expansion was nearly equal to that of YSZ single coating. The experiment was conducted to compare the thermal shock resistance of a graded coating with a conventional YSZ/LZ double-layer system. Changes in weight and morphology of specimens before and after thermal shock tests were analyzed. Results demonstrated that the thermal shock resistance of the graded coating was superior to the double-layer coating. Typically, a barely visible pimple-like spallation was present on the surface of the graded coating after 21 cycles. On the other hand, obvious delamination was observed for a double-layer coating after six to seven cycles. Special focus was also placed on a comparative investigation of stresses that are closely related to spallation via the use of numerical simulation. [source] Thermal Conductivity of the Rare-Earth Strontium AluminatesJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2010Chunlei Wan The thermal conductivity of a series of complex aluminates, RE2SrAl2O7, with different rare-earth (RE) ions, has been measured up to 1000°C. There is a strong dependence on the atomic number of the RE ion, ranging from an approximately 1/T dependence for the lanthanum strontium aluminate to an almost temperature-independent behavior of the dysprosium strontium aluminate. The latter conductivity is comparable with that of yttria-stabilized zirconia, the current material of choice for thermal barrier coatings. The temperature dependence of the thermal conductivities of all the aluminates studied can be fit to a standard phonon,phonon scattering model, modified to account for a minimum phonon mean free path, in which the difference in behavior is attributed to increased phonon,phonon scattering with the atomic mass of the RE ion. Although a satisfactory parametric fit is obtained, the model does not take into account either the detailed layer structure of the aluminates, consisting of alternating rock-salt and perovskite layers in a natural superlattice structure, or the site preferences of the RE ion. This suggests that further model development is warranted. [source] Thermophysical Properties of Complex Rare-Earth Zirconate Ceramic for Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 7 2008Liu Ling Two complex rare-earth zirconates (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 for thermal barrier coatings (TBCs) were synthesized by the coprecipitation method. Their phase composition, microstructure, and thermophysical properties were investigated. X-ray diffractometry results revealed that single-phase (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 with pyrochlore structure were prepared, and the scanning electron microscopy results showed that the microstructures of the products were dense and no other phases existed among the grains. With the temperature increasing, the thermal expansion coefficient (CTE) of the ceramics increased, while the thermal conductivity decreased. The results indicated that the CTE of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 was slightly higher than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 and the thermal conductivity of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 was lower than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4. These results imply that the thermophysical properties of (Sr0.1La0.3Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.3 are better than that of (La0.4Sm0.5Yb0.1)2(Zr0.7Ce0.4)2O7.4 as the material for the ceramic layer in the TBC system. [source] Determination of Scattering and Absorption Coefficients for Plasma-Sprayed Yttria-Stabilized Zirconia Thermal Barrier CoatingsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 5 2008Jeffrey I. Eldridge Prediction of radiative transport through translucent thermal barrier coatings (TBCs) can only be performed if the scattering and absorption coefficients and index of refraction of the TBC are known. To date, very limited information on these coefficients, which depend on both the coating composition and the microstructure, has been available for the very commonly utilized plasma-sprayed 8 wt% yttria-stabilized zirconia (8YSZ) TBCs. In this work, the scattering and absorption coefficients of freestanding plasma-sprayed 8YSZ coatings were determined from room-temperature normal-incidence directional-hemispherical reflectance and transmittance spectra over the wavelength range from 0.8 to 7.5 ,m. Spectra were collected over a wide range of coating thickness from 60 to almost 900 ,m. From the reflectance and transmittance spectra, the scattering and absorption coefficients as a function of wavelength were obtained by fitting the reflectance and transmittance values predicted by a four flux model to the experimentally measured values at all measured 8YSZ thicknesses. While the combined effects of absorption and scattering were shown in general to exhibit a nonexponential dependence of transmittance on specimen thickness, it was shown that for sufficiently high absorption and optical thickness, an exponential dependence becomes a good approximation. In addition, the implications of the wavelength dependence of the plasma-sprayed 8YSZ scattering and absorption coefficients on (1) obtaining accurate surface-temperature pyrometer measurements and on (2) applying mid-infrared reflectance to monitor TBC delamination are discussed. [source] Depth-Resolved Porosity Investigation of EB-PVD Thermal Barrier Coatings Using High-Energy X-raysJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 2 2004Anand A. Kulkarni Demands for designing prime reliant, energy-efficient, and high-performance thermal barrier coatings (TBCs) in gas turbines have led to a growing interest toward comprehensive microstructural characterization. Here we investigate the novel use of high-energy X-rays for small-angle X-ray scattering (SAXS), together with wide-angle scattering and radiography, for the depth-resolved characterization of TBCs grown by electron beam physical vapor deposition (EB-PVD). The coating microstructure is found to consist of columns perpendicular to the substrate, extending through the thickness, with a [001] growth texture and significant intercolumnar porosity. In addition, overshadowing effects during deposition together with gas entrapment give rise to nanoscale intracolumnar porosity consisting of featherlike and globular pores. Radiography showed an increase in the total porosity, from 15% near the substrate to 25% near the coating surface, which is ascribed to an increase in the intercolumnar spacing at the top of the coating. By contrast, the small-angle scattering studies, which are sensitive to fine features, showed the pore internal surface area to be greatest near the substrate. [source] Thermal Stability of Lanthanum Zirconate Plasma-Sprayed CoatingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2001X. Q. Cao Lanthanum zirconate (La2Zr2O7, LZ) is a newly proposed material for thermal barrier coatings (TBCs). The thermal stability of LZ coating was studied in this work by long-term annealing and thermal cycling. After long-term annealing at 1400°C or thermal cycling, both LZ powder and plasma-sprayed coating still kept the pyrochlore structure, and a preferred crystal growth direction in the coating was observed by X-ray diffraction. A considerable amount of La2O3 in the powder was evaporated in the plasma flame, resulting in a nonstoichiometric coating. Additionally, compared with the standard TBC material yttria-stabilized zirconia (YSZ), LZ coating has a lower thermal expansion coefficient, which leads to higher stress levels in a TBC system. [source] Investigation on the oxidation behaviour of gamma titanium aluminides coated with thermal barrier coatingsMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 7 2008R. Braun Abstract In the present study, the applicability of thermal barrier coatings (TBCs) on ,-TiAl alloys was investigated. Two alloys with the chemical compositions of Ti-45Al-8Nb-0.2B-0.15C and Ti-45Al-1Cr-6Nb-0.4W-0.2B-0.5C-0.2Si were used. Before TBC deposition, the specimens were pre-oxidised in laboratory air or low partial pressure oxygen atmosphere. Yttria partially stabilised zirconia top coats were then deposited using electron-beam physical vapour deposition (EB-PVD). The oxidation behaviour of the ,-TiAl specimens with TBC was studied by cyclic oxidation testing in air at 850 and 900,°C. Post-oxidation analysis of the coating systems was performed using scanning electron microscopy with energy-dispersive X-ray spectroscopy (EDS). No spallation of the TBC was observed for pre-oxidised specimens of both alloys when exposed to air at 850,°C for 1100 cycles of 1,h dwell time at high temperature. SEM micrographs of the thermally grown oxide scale revealed outer mixed TiO2/Al2O3 protrusions with a columnar structure. The protrusions contained small particles of zirconia and a low amount of about 0.5 at% zirconium was measured by EDS analysis throughout this outer oxide mixture. The TBCs exhibited excellent adherence on the oxide scale. Intercolumnar gaps and pores in the root area of the TBC were filled with titania and alumina. Below the outer columnar oxide scale, a broad porous zone of predominant titania was observed. The transition region between the oxide scale and substrate consisted of a discontinuous nitride layer intermixed with alumina particles and intermetallic phases rich in niobium formed at the nitride layer/substrate interface. When thermally cycled at 900,°C, the oxide scales on the alloy Ti-45Al-8Nb-0.2B-0.15C pre-oxidised in low partial pressure oxygen spalled off after 540 cycles. For the sample with TBC, spallation was observed after 810 cycles. Failure occurred in the thermally grown oxide near the oxide/nitride layer interface. Microstructural examinations revealed again oxide scales with columnar structure beneath the zirconia top coat and good adherence of the TBC on the thermally grown oxides formed at 900,°C. [source] | ||||||||||