The Constitutive Behavior of Thick Thermal Barrier Coatings
Plasma-sprayed thick thermal barrier coatings on the order of a millimeter are currently being developed for use in diesel engines with operating temperatures of about 800°C. In this environment, coatings will be subjected to thermomechanical compressive strain cycles that correspond to the on/off cycle of the engine. The focus of this study was to assess the deformation behavior of a plasma-sprayed ZrO-25wt%CeO2-2.5wt%Y2O3 coating material and to determine the critical features that will affect its durability in service. Using a novel specimen design that allowed testing independent of a substrate, it was found experimentally that the ceramic coating material exhibits considerable irreversible deformation and that this behavior can result in detrimental tensile stresses upon unloading from compression. This irreversible behavior is caused by the combined closing and sliding along the numerous microcrack surfaces that exist in the as-sprayed coating material. A model based on four deformation mechanisms---intrinsic elasticity, microcrack opening and closure, sliding along microcrack surfaces, and tensile microcracking---was proposed. It is shown that by conducting a few simple experiments, the model can be used to quantify the effects of each of these mechanisms and can predict the cyclic deformation behavior for a wide variety of loading conditions. In addition, the model is demonstrated to have promising potential for predicting tensile failures that occur during compressive unloading.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign