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 superalloy 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