This paper summarizes the basic properties of ceramic materials for thermal barrier coatings. Ceramics, in contrast to metals, are often more resistant to oxidation, corrosion and wear, as well as being better thermal insulators. Except yttria stabilized zirconia, other materials such as lanthanum zirconate and rare earth oxides are also promising materials for thermal barrier coatings.

Ceramic materials for thermal barrier coatings

This paper presents the full generalization of the Cartesian coordinate-based higher-order theory for functionally graded materials developed by the authors during the past several years. This theory circumvents the problematic use of the standard micromechanical approach, based on the concept of a representative volume element, commonly employed in the analysis of functionally graded composites by explicitly coupling the local (microstructural) and global (macrostructural) responses. The theoretical framework is based on volumetric averaging of the various field quantities, together with imposition of boundary and interfacial conditions in an average sense between the subvolumes used to characterize the composite's functionally graded microstructure. The generalization outlined herein involves extension of the theoretical framework to enable the analysis of materials characterized by spatially variable microstructures in three directions. Specialization of the generalized theoretical framework to previously published versions of the higher-order theory for materials functionally graded in one and two directions is demonstrated. In the applications part of the paper we summarize the major findings obtained with the one-directional and two-directional versions of the higher-order theory. The results illustrate both the fundamental issues related to the influence of microstructure on microscopic and macroscopic quantities governing the response of composites and the technologically important applications. A major issue addressed herein is the applicability of the classical homogenization schemes in the analysis of functionally graded materials. The technologically important applications illustrate the utility of functionally graded microstructures in tailoring the response of structural components in a variety of applications involving uniform and gradient thermomechanical loading.

Higher-order theory for functionally graded materials

Recent development in modeling and analysis of functionally graded materials and structures

Reduction of thermal stresses by developing two-dimensional functionally graded materials

Functionally graded ZrO2/NiCoCrAlY coatings were produced by plasma spraying using pre-mixed and spheroidized powders as the feedstock. The microstructure, density, elastic modulus, thermal conductivity/diffusivity, microhardness and coefficient of thermal expansion were found to change gradually through the five-layer functionally graded coatings which was beneficial for the improvement of mechanical and thermal properties of the coatings. The residual stresses of the as-sprayed coatings with different graded layers and different thicknesses, as well as the changes of residual stresses during thermal cycling were simulated by finite element analysis (FEA). Results showed that residual stress was the lowest for the five-layer functionally graded coating compared to that of the duplex coating and three-layer coating with the same thickness, and the residual stresses increased with a decrease in coating thickness. For the coatings with the same thickness, the bond strength and thermal cycling resistance were found to increase with an increase in the number of graded layers which is due to the decrease in the residual thermal stresses. The bond strength of the five-layer functionally graded coating was about twice as high as that of the duplex coating and the number of thermal cycles of functionally graded coating was five times higher than that of the duplex coating. Results also showed that the bond strength decreased with an increase in the coating thickness.

Effects of residual stress on the performance of plasma sprayed functionally graded ZrO2/NiCoCrAlY coatings

The initiation of surface cracks in zirconia based multilayer thermal barrier coatings is related to stress relaxation which occurs at the top surface of the coating at high temperatures. An analytical model is used to show that the compressive stresses decrease over time and become tensile upon cooling of the coating, thus resulting in cracks. Mullite, which exhibits significantly reduced stress relaxation behavior, is shown to remain in compression, thus preventing the initiation of surface cracks. This behavior is compared for the cases of equal heat flux and equal surface temperature conditions.

Surface thermal cracking of thermal barrier coatings owing to stress relaxation: zirconia vs. mullite

The objective of this research was to study the thermal fracture of ceramic thermal barrier coatings under high heat flux laser heating and investigate the effect of time-dependent behavior of the ceramic. Continuously plasma-sprayed zirconia coatings with thicknesses varying from 0.26 to 1.5 mm were heated with a CO 2 laser to maximum surface temperatures varying from 700 to 1700°C. Temperature differences from 600 to 1300°C across the ceramic coating were applied. High heat flux heating was found to cause changes in the material, leading to a denser microstructure and relaxation of compressive stresses. These changes lead to the development of surface and interface cracks during ambient air cooling following laser heating. Increasing the coating thickness was found to decrease the number of surface cracks developed and increase the distance between cracks. Surface cracks were found to extend further into thin coatings than thick coatings. Increasing the maximum surface temperature was found to increase the surface crack length. The thinnest and thickest coatings developed fewer and shorter interface cracks than coatings with intermediate thicknesses.

Thermal fracture of ceramic thermal barrier coatings under high heat flux with time-dependent behavior.: Part 1. Experimental results

Ceramic thermal barrier coatings represent an attractive method of increasing the high temperature limits for systems such as diesel engines, gas turbines and aircraft engines However, the dissimilarities between ceramics and metal, as well as the severe temperature gradients applied in such systems cause thermal stresses which can lead to cracking and ultimately spalling of the coating This paper reviews the research which considers initiation of surface cracks, interfacial edge cracks and the effect of a transient thermal load on interface cracks The results of controlled experiments together with analytical models are presented The implications of these findings to the differences between diesel engines and gas turbines are discussed The importance of such work for determining the proper design criteria for thermal barrier coatings is underlined

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Thermal fracture mechanisms in ceramic thermal barrier coatings

Surface thermal fracture of functionally graded ceramic coatings: Effect of architecture and materials

The objective of this research was to study the response of thick thermal barrier coatings to high heat flux environments. High heat flux environments subject the coating to large transient thermal loads. In this paper, the thermal fracture process of coatings manufactured in different ways under controlled thermal loading conditions is presented. The transient thermal loads were applied, using a 1.5 kW CO 2 laser, to steel beam specimens plasma sprayed with yttria partially stabilized zirconia. Laser heating was found to sinter the heated regions of the coating. A 0.76 mm thick coating resulted in multiple surface cracks, whereas a thicker 1.09 mm coating resulted in surface and interface cracks. The effect of a common procedure of interrupting plasma spraying, reheating, and continuing plasma spraying was investigated. If the stress field due to a thermal heating event was of sufficiently high intensity, and it coincided with a plane of weakness within the coating, it was possible to cause delamination at that plane.

B.D. Choules

Papers

Surface thermal cracking of thermal barrier coatings owing to stress relaxation: zirconia vs. mullite

Thermal fracture of ceramic thermal barrier coatings under high heat flux with time-dependent behavior.: Part 1. Experimental results

Thermal fracture mechanisms in ceramic thermal barrier coatings

Surface thermal fracture of functionally graded ceramic coatings: Effect of architecture and materials

Thermal fracture of thermal barrier coatings in a high heat flux environment