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Experimental and Finite Element Study of an Air Plasma Sprayed Thermal Barrier Coating under Fixed Cycle Duration at Various Temperatures


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Cyclic furnace tests were conducted on Air Plasma Sprayed (APS) Thermal Barrier Coating (TBC) samples which were coated with NiCoCrAlY on a nickel-based superalloy at three temperatures. From these tests the following were determined: oxidation kinetics and activation energy, bond coat rumpling rate, and cyclic failure lives. All failures occurred in the top coat consistent with engine experience. The measured rumpling rate and oxidation kinetics were used as input for a finite element model that utilized a highly realistic top coat constitutive model that included creep and relaxation behavior as well as tension/compression flow stress asymmetry consistent with available experimental data. The modeling results based on these measured inputs show that the primary sources of stress and strain in descending order are rumpling, oxide growth and finally thermal expansion mismatch between the top coat and underlying metal. Three important implications of these results are; predicted stresses are too low to predict failure based on single crack fracture mechanics, cycling increases accumulated inelastic strain in a fixed time interval providing an additional reason why cycling is damaging and finally to get realistic stresses and strains it is necessary to accurately characterize the geometry changes associated with rumpling.

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