Life Prediction and Mechanical Behavior of Thermal Barrier Coatings

University dissertation from Div. Materials Engineering, Lund University, P O Box 118, SE-221 00 Lund, Sweden

Abstract: Delamination and spallation of thermal barrier coating (TBC) systems, used in hot parts of gas turbines, have been studied with respect to thermal fatigue. TBCs with and without pre-existing cracks were investigated. Thermal shock tests were performed until coating spallation. The experimental results revealed that the cracks primarily responsible for spallation grew horizontally, close to the interface between ceramic top coat and bond coat. In order to predict life of the TBC systems, finite element (FE) modeling of the coatings with sinusoidal interface geometry was performed. The results show, as expected, high stress levels close to the asperity peaks. Life prediction models, based on in-plane stress range and on inelastic strain range, were investigated. It was found that the in-plane stress range best correlates to the coating experimental life data. With this choice, the correlation between actual life and calculated life falls within a three times scatter band. FE modeling of TBCs with pre-existing cracks, in the range of 0.01~2.36 mm long, undergoing thermal shock loading, was performed using a fracture mechanics approach. The stress intensity factors of the cracks were calculated during thermal cycling. The influence of thermally grown oxide (TGO) was considered by incorporating an alumina layer between the ceramic/bond coat interface. Micro-cracks, with the length up to 200 ƒÝm, were found to keep safe when the TGO growth is not severe and temperature is not high enough. An edge crack has a higher risk to propagation than a centrally located crack. TGO redistributes the stresses around the interface and makes the stress intensity factors of micro-cracks sensitive to the undulation of the interface. Macro-cracks are more dangerous than the micro-cracks in according to the FM modeling, which was confirmed by the thermal shock tests as well.

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