Defect evolution in thermal barrier coating systems under multi-axial thermomechanical loading

In service, gas turbine components with thermal barrier coatings experience high cyclic mechanical and thermal loading. Important, but not yet considered sufficiently, are the multi-axial stresses arising from thermal gradients.In this study, multi-axial stresses were simulated in laboratory experiments using a specially designed test rig. Cyclic thermomechanical fatigue experiments with radial thermal gradients (TGMF) were performed on tubular specimens consisting of a directionally densified super-alloy substrate, a NiCoCrAlY bond coat, and a ceramic thermal barrier coating (TBC). The test setup enables surface temperatures of 1000 °C, temperature differences over the whole wall thickness of the specimen of about 170 °C, and high heating and cooling rates.The resultant defects have specific features consisting of cracks parallel to the bond coat/TBC interface. They are located within the bond coat close to this interface. Weakening thus this interface, the defects enhance TBC spallation.Finite element analyses, calculating stress distributions during the quasi-stationary condition of the TGMF test cycle at high temperature and the transient stress distributions during cooling, were used to discuss the evolution of these specific defects.