Testing and evaluation of thermal-barrier coatings
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Introduction Testing thermal-barrier coating (TBC) systems and evaluating their performance in service presents major challenges. First and foremost, the conditions under which they operate are often extremely harsh, combining high temperatures, steep temperature gradients, fast temperature transients, high pressures, and additional mechanical loading, as well as oxidative and corrosive environments. These are difficult to reproduce in the laboratory. The coating system also changes with time and temperature as interdiffusion occurs, microstructures evolve, and the properties of the constituent multilayer materials change. For instance, the oxide topcoat sinters,1,12 increasing both its thermal conductivity and elastic modulus, but the rate of sintering depends on its purity. Furthermore, the properties that need to be evaluated are rarely those of the constituent bulk materials themselves. For instance, while the intrinsic fracture toughness of the ceramic topcoat—typically made of 7 wt% yttriastabilized zirconia (7YSZ)—is important, it is the toughness that a delamination crack experiences as it extends in or near the interface with the thermally grown oxide (TGO) that directly influences the lifetime under thermal cycling conditions. As coatings become prime-reliant, meaning that they can be implemented into the design of the engine with reliable performance criteria, it is also essential to develop sensors and nondestructive evaluation methods to monitor TBC temperatures, the extent
of sub-critical delamination in service, as well as identifying manufacturing flaws, while also creating an artificial intelligence supervisory system that can be implemented in the field to provide feedback to the manufacturing and design sectors for product improvement. Several sensor approaches are being explored, including infrared imaging, Raman spectroscopy, thermography, impedance spectroscopy, acoustic emission, and luminescence sensing.2–7
Mechanical properties One of the fundamental problems in discussing and evaluating the mechanical properties of coatings is establishing what the appropriate value of a particular property should be and at what microstructural scale it should be determined. This is especially true of the oxide topcoat since considerable variability and uncertainty arise from the porous nature of the coating as well as its anisotropy and microstructural evolution at elevated temperatures. For simple properties, such as the overall thermal expansion mismatch stresses on thermal cycling and the available elastic strain energy release rate, the macroscopic biaxial Young’s modulus, such as determined by a macroscopic mechanical test, is generally adequate, recognizing that it can be expected to be different under tension than compression. In contrast, the local modulus obtained by nanoindentation, for instance, pertains to the modulus of the intrinsic material
Robert Vaßen, Institute of Energy and Climate Research, Forschungszentrum Jülich GmbH, Germany; [email protected] Yutaka Kagawa, Research Center for A
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