Conductivity Of Thermal Barrier Coatings Research Articles

Estimation of Thermal Conductivity of Thermal Barrier Coatings through Measured Electronic and Structural Characteristics

Estimation of Thermal Conductivity of Thermal Barrier Coatings through Measured Electronic and Structural Characteristics

Effect of Heat Treatment on Microstructure and Thermal Conductivity of Thermal Barrier Coating.

Thermal barrier coatings (TBCs) are essential for increasing the inlet temperature of gas turbines to improve their thermal efficiency. Continuous exposure to flames is known to affect the thermal properties of TBCs, degrading the performance of gas turbines as a consequence. In this study, we quantified the changes in the thermal conductivity of yttria-stabilized zirconia coatings with respect to various heat treatment temperatures and times. The coating exhibited an increase in thermal conductivity after heat treatment, with higher heat treatment temperatures resulting in greater thermal conductivity. The coatings were analyzed by X-ray diffraction and scanning electron microscopy before and after heat treatment. Results showed that there was little change in thermal conductivity due to phase changes and grain size. We conclude that pore structures, i.e., circular and lamellar pores, affected the change in thermal conductivity. Specifically, we confirmed that the change in thermal conductivity depends on the size of the lamellar pores.

Thermal Conductivity of Multi-Sized Porous Thermal Barrier Coatings at Micro and Nano Scales after Long-Term Service at High Temperatures

Thermal barrier coatings with multi-sized porous structure at micro and nano scales were prepared with hollow spherical YSZ powders and polypropylene powders through atmospheric plasma spraying. The thermal conductivities of the multi-sized thermal barrier coatings after a long-term serving at high temperature were tested through laser flash heating method. Meanwhile, the physical models of thermal barrier coatings with multi-sized porous structure at micro and nano scales were constructed through Ansys Workbench. The evolutions of thermal conductivity of thermal barrier coatings with multi-sized pores after long-term service at 1100 °C were investigated through computation. It was found that the thermal conductivity of the coating increased with the extension of the serving time. When the serving time reached 60 days, the thermal conductivity of the coating tended to be stable and close to the compacted bulk. The computational results were consistent with the tested ones, which could be helpful to explain the thermal conducting evolution in thermal barrier coatings with multi-sized porous structure at nano and micro scales after long-term serving at high temperature.

Effects of Splat Interfaces, Monoclinic Phase and Grain Boundaries on the Thermal Conductivity of Plasma Sprayed Yttria-Stabilized Zirconia Coatings

Microstructure has a significant influence on the thermal conductivity of thermal barrier coating (TBC) systems. In this work, the microstructures including splat interface, monoclinic phase and grain boundaries in the YSZ air plasma spraying (APS) TBC systems are investigated. A finite element simulation model based on electron backscatter diffraction (EBSD) images is established. It is found that the simulation results of thermal conductivity are in good agreement with the experimental results. Using this model, the effect coefficient of splat interface, monoclinic phase and grain boundaries on thermal conductivity are calculated. Results show that the splat interface influences the thermal conductivity of the TBCs. Those results provide important guidance for reducing the thermal conductivity of thermal barrier coatings.

Tailoring the heat transfer on the injection moulding cavity by plasma sprayed ceramic coatings

Inhomogeneous material shrinkage in injection moulding can cause warpage in thermoplastic components. To minimise the deformations of the injection moulding parts, the heat transfer during the cooling phase can be adjusted according to the local cooling demand on the surface of the mould cavity by means of plasma sprayed coatings with locally variable thermal resistance over the surface of the mould. Thermal resistance is a function of thermal conductivity and thickness of the coatings, where thermal conductivity of thermal barrier coatings can be adjusted by altering the chemical composition and the microstructure, which is depending on the thickness. This work evaluates the application of plasma sprayed coatings with variable thickness as thermal barrier coatings in the mould cavity. The thermal resistance of the coating and thereby the heat transfer from the melt into the mould will be influenced locally by varying the coating thickness over the cavity area according to the local cooling demand. Using the laser flash method, the thermal conduction of coatings with different thicknesses will be determined. On the basis of the experimentally determined thermal conduction, the effect of the coatings on the temperature field of the mould cavity will be numerically calculated and the required thickness distribution of the coating for an optimal temperature gradient will be determined.

Effects of pore microstructure on the effective thermal conductivity of thermal barrier coatings

The pore microstructure of thermal barrier coatings (TBCs) has great effect on its heat transfer performance. However, it is difficult to establish structure–property relationships of TBCs due to inhomogeneous geometry microstructures. In this work, various anisotropic microstructures of TBCs were reconstructed using the developed software based on four parameter stochastic growth method. Heat transfer performance of these coatings was numerically investigated using the developed numerical calculation software based on the thermal resistance network method. The effective thermal conductivity and the temperature field at the steady state were obtained. The results show the effect of size and shape of pores on the effective thermal conductivity of columnar and layered TBCs is different, and these effects increase with the porosity. At the same porosity, the effective thermal conductivity of the layered structure TBCs increases with the decrease of pores size and logarithmically decreases with the elongated pores. In contrast, the effective thermal conductivity of columnar structure TBCs decreases with the decrease of pores size and logarithmically increases with the elongated pores. The effective thermal conductivity of TBCs continuously increases with increasing of the horizontal angle of the pores from 0° to 90°. The direction of pores has greater effect on the effective thermal conductivity of the coating with high porosity of 20%.

A novel low-thermal-conductivity plasma-sprayed thermal barrier coating controlled by large pores

Abstract In order to reduce the thermal conductivity of thermal barrier coatings (TBCs) for land-based gas turbine applications, porous TBC (P-TBC) with a microstructure consisting of a distribution of large open pores was developed. A powder mixture of zirconia and polyester was used to form such open pores within the microstructure. The results showed that the thermal conductivity of the P-TBC monotonically decreased as the porosity increased. In addition, compared with conventional TBC with a thermal conductivity value of 1.2 W/(mK), the thermal conductivity of P-TBC with a polyester content 15 wt.% achieved a value as low as 0.3 W/(mK). This implies that the presence of large open pores leads to a dramatic reduction in the thermal conductivity, and the thermal conductivity can be freely controlled by adjusting the polyester content of the powder. A SEM-image-based finite element model was constructed. The FE analysis revealed that the presence of pores disturbed the heat flow. Thus, compared with the relatively straight lines that occurred for the TBC, which possessed few pores, it was considered that the presence of open pores within the P-TBC increased the length of the heat flow lines. This provides an explanation for the reduction in the apparent thermal conductivity of the P-TBC.

Morphology and Size Evolution of Interlamellar Two-Dimensional Pores in Plasma-Sprayed La2Zr2O7 Coatings During Thermal Exposure at 1300 °C

La2Zr2O7 (LZO) is widely expected to be one of the promising thermal barrier coating materials for application in high-temperature conditions (1200 °C). However, high-temperature exposure causes sintering which heals interlamellar two-dimensional (2D) pores and intrasplat pores. This sintering effect increases the stiffness and thermal conductivity of thermal barrier coatings, consequently reducing their durability. In this study, to reveal the possible critical opening of 2D pores above which they are free from sintering, LZO coating and splat were deposited by atmospheric plasma spraying and were exposed to 1300 °C for different durations. Thereafter, the evolution of the parameters of residual 2D pores in the coating and the surface morphology of LZO splat were characterized. It was found that there is a critical opening width for 2D pores above which grain bridging does not occur across the gaps. Accordingly, pores with an opening larger than this critical width are free from sintering across the 2D pores despite surface roughening of splats, whereas pores with an opening less than the critical width sinter rapidly at the early stage of thermal exposure through the formation of grain bridges.

OS11-1 Thermal Conductivity of Thermal Barrier Coatings with Porous microstructure(Characterization and Inspection of Coatings,OS11 Reliability of heat resisting alloys and coatings,STRENGTH OF MATERIALS)

OS11-1 Thermal Conductivity of Thermal Barrier Coatings with Porous microstructure(Characterization and Inspection of Coatings,OS11 Reliability of heat resisting alloys and coatings,STRENGTH OF MATERIALS)

Detection of Surface Crack on the Substrate under Thermal Barrier Coatings Using Microwave Non-Destructive Evaluation

Thermal barrier coatings (TBCs) have been widely used in gas turbine engines in order to protect the substrate metal alloy against high temperature and to enhance turbine efficiency. Thermal effects in the work process will produce a certain stress to induce the metal substrate surface defects such as cracks, which affect thermal conductivity of TBCs. Therefore, it is necessary to detect the cracks in order to ensure the quality of TBCs. In this paper, an approach for microwave non-destructive evaluation of surface crack on the substrate under thermal barrier coatings will be presented. The relationship between reflection coefficient and defects of TBCs were studied, based on the propagation feature of microwave isonified through multi-layer. A network analyzer operated in the frequency range of 26.5 GHz to 40 GHz was used to measure the reflection coefficient. Scanning images were obtained by using phase difference and amplitude at the optimum detection frequency. The results indicate that the amplitude, and the phase difference of the reflection coefficient, can represent the defects under TBCs using microwave non-destruction evaluation when the stand-off distance is suitable. The experimental setup that validates the microwave non-destructive inspection of TBCs is presented, and the feasibility to evaluate the cracks on the substrate surface using microwave non-destructive evaluation is verified in this paper.

Knudsen Effect on the Estimation of the Effective Thermal Conductivity of Thermal Barrier Coatings

A numerical model based on the use of cross-sectional micrographies and a 3D image of thermal barrier coatings for the estimation of the material effective thermal conductivity is presented. The case of a YSZ thermal spray coating consisting of a 2 phase network, namely, the coating material and pores, is considered. The variation of the thermal conductivity of pores caused by their small size was considered by taking the Knudsen effect into account. The quantification of this effect on the effective thermal conductivity of the coating was achieved with the help of image analysis combined with an in-house program coded in C language. Finite-difference (FD) and finite-element (FE) models were applied using both 2D images and a 3D image. Despite the differences in the computed values obtained with these two numerical methods, the decrease of the computed thermal conductivity caused by the Knudsen effect was found to remain quite moderate for both methods (i.e., about 3-5% for the 3D results).

A 3D finite-difference model for the effective thermal conductivity of thermal barrier coatings produced by plasma spraying

Effective properties of thermal barrier coatings produced by plasma spraying can be quantified via different measurement techniques. Numerical modeling applied on 2D cross-sectional images of the coating represents an alternative method often applied in the literature. In the present study, a three-dimensional finite-difference-based model was developed for analyzing the heat transfer mechanisms through a porous structure and also for investigating the differences between 2D and 3D results. An artificial pore network was first specially generated from the microstructural information measured for a real coating cross-sectional image captured by scanning electron microscopy. The computed effective thermal conductivity based on 2D calculations performed on cross-sectional images of the 3D artificial structure was found to be nearly the same compared with the thermal conductivity evaluated from cross-sectional images of the real coating. In a second time, the 3D computed value of the effective thermal conductivity was found to be in better agreement with the measured value, in comparison with that computed on the basis of 2D cross-sectional images. In addition, the effective thermal conductivity was also evaluated with two different packages based on the finite element method (namely OOF2 and ANSYS): the computed thermal conductivity derived with these tools was found to be somewhat larger than the one obtained with the present FD model. Finally, the thermal conductivity computed for different artificial pore networks were compared with those obtained from 2D computations performed on their cross-sections, revealing the differences between 2D and 3D image-based modeling: a correlation was then derived between the results computed with 3D and 2D models.

Effects of defects on the effective thermal conductivity of thermal barrier coatings

It is difficult to establish structure-property relationships in thermal barrier coatings (TBCs) because of their inhomogeneous-geometry microstructures caused by defects. In the current research, the effects of pores and cracks on the effective thermal conductivity of TBCs are studied. Based on the law of the conservation of energy, mathematical formulations are proposed to indicate the relationship between defects and the effective thermal conductivity. In this approach, detailed equations are illustrated to represent the shape and size of defects on the effective thermal conductivity of TBCs. Different from traditional empirical analyses, mixture law or statistical method, for the first time, our results with the aid of finite element method (FEM) and strict analytical calculation show the influence of pore radius and crack length on effective thermal conductivity can be quantified. As an example to a typical microstructure of plasma sprayed TBCs, the effects of defects on the effective thermal conductivity of TBCs are expressed by the influence parameter, which indicating that the longest transverse crack dominates the contribution of the effective thermal conductivity along the spray direction compared with any individual defect.

Thermal conductivity of thermal barrier coatings

The results of experimental studies of the thermal conductivity coefficient of thermal barrier coatings (TBC) are presented. A technique for its determination is proposed and tested. The data of design evaluation of the TBC thickness distribution along the outline of a cooled turbine blade are given.

A Two-Dimensional Heat Transfer Model for Thermal Barrier Coating Average Thermal Conductivity Computation

ABSTRACT The present article is devoted to quantifying the contribution of pores and cracks in the decrease of the effective thermal conductivity of thermal barrier coatings (TBCs). A finite-difference-based model is used for the computation of heat transfer through a porous structure. Each pixel of a binary picture describing the pore network is interpreted as a cell of integration of the heat conduction equation. A thermal gradient is applied and a large system of linear Equations is subsequently derived. Since it has a strong influence on the required CPU time, particular attention is given to the solving procedure.

Thermal conductivity of zirconia‐based ceramics for thermal barrier coating

AbstractLowering the thermal conductivity of thermal barrier coatings used to protect blade and vane airfoils represents an important challenge for gas turbine designers and manufacturers. Dense zirconia‐based materials have been prepared by solid state reaction methods to determine their thermal properties up to 1000 °C. Partially stabilised zirconias having a thermal conductivity 40 % lower than the thermal conductivity of the most widely used system (ZrO2‐8wt.%Y2O3) have been obtained.

HITS - High Insulation Thermal Barrier Coating systems

The European programme HITS constitutes the first attempt to reduce the thermal conductivity of Thermal Barrier Coatings (TBCs) for industrial applications through engineered coating microstructures and new chemistries. This article provides an overview of the work carried out and presents examples of the results achieved.

Thermal barrier coatings for aircraft turbine airfoils : thermal challenge and materials

In this paper, the importance of the challenge associated with the control of the thermal conductivity of thermal barrier coatings for aircraft turbine hot stages is reviewed. It is firstly illustrated by the description of a practical aeronautic coated and uncoated turbine blade design exercise. The various contributions to TBC thermal conductivity are then exposed. Their low conductivity finds its source not only in the nature of the ceramic layer, but also in the morphology of the insulating layer, closely linked to its fabrication process parameters. The influence of various factors (such as yttria content, temperature, porosity content and distribution, etc.) on the thermal conductivity is examined, largely based on a literature review. In this field of investigation, the modelling tool should allow to predict, to a certain extent, which morphology would lead to the lowest values of thermal conductivity. Eventually, attempts are made to identify research domains where further understanding is needed, and to formulate several suggestions concerning possible ways to lower the thermal conductivity.

Thermal conductivity of thermal barrier coatings

In thermal barrier coatings and other ceramic oxides, heat is conducted by lattice waves, and also by a radiative component which becomes significant at high temperatures. The theory of heat conduction by lattice waves is reviewed in the equipartition limit (above room temperature). The conductivity is composed of contributions from a spectrum of waves, determined by the frequency dependent attenuation length. Interaction between lattice waves (intrinsic processes), scattering by atomic scale point defects and scattering by extended imperfections such as grain boundaries, each limit the attenuation length in different parts of the spectrum. Intrinsic processes yield a spectral conductivity which is independent of frequency. Point defects reduce the contribution of the high frequency spectrum, grain boundaries and other extended defects that of the low frequencies. These reductions are usually independent of each other. Estimates will be given for zirconia containing 7wt% Y 2O 3, and for yttrium aluminum garnet. They will be compared to measurements. The effects of grain size, cracks and porosity will be discussed both for the lattice and the radiative components. While the lattice component of the thermal conductivity is reduced substantially by decreasing the grain size to nanometers, the radiative component requires pores or other inclusions of micrometer scale.

Microstructure and Thermal Conductivity of Thermal Barrier Coatings Processed by Plasma Spray and Physical Vapor Deposition Techniques

AbstractThe temperature dependence of the thermal conductivity of multilayer coatings made by a plasma spray technique as well as some coatings made by physical vapor deposition (PVD) was investigated. The multilayer coatings consisted of a varying number of layers of Al2O3 and ZrO2 stabilized by 8%Y2O3. Plasma sprayed coatings exhibited a large reduction in thermal conductivity at all temperatures when compared to the bulk monolithic materials. This reduction was found to be due to porosity as well as thermal resistance brought about by interfaces in the coatings. A comparable reduction in thermal conductivity was achieved in monolithic ZrO2 as well as in a composite coating deposited by the PVD technique. Microstructural factors that may be responsible for this reduction are discussed.