Failure of thermal barrier coating systems under cyclic thermomechanical loading

The failure mechanisms of thermal barrier coating (TBC) systems applied on gas turbine blades and vanes are investigated using thermomechanical fatigue (TMF) tests and finite element (FE) modeling. TMF tests were performed at two levels of applied mechanical strain, namely five times and three times the critical in-service mechanical strain of an industrial gas turbine. TMF testing under the higher mechanical strain of air plasma-sprayed (APS) and electron beam–physical vapor deposition (EB-PVD) coated samples showed that both systems failed after a similar number of cycles by cracks that initiated at the bond coat/thermally grown oxide (TGO) interface and propagated through the bond coat to the substrate. When the applied mechanical strain was decreased, cracking of the bond coat in EB-PVD coated systems was suppressed, the life of the coated system increased significantly and delamination of the top-coat was observed. A subsequent FE analysis showed that, by subjecting the system to the higher mechanical strain, significant tensile stresses develop in the TGO and the bond coat that are thought to be responsible for the observed crack initiation and propagation. The FE model also predicts that cracking initiates at specific geometric features of the rough interface of a PS coated system, which was confirmed by metallographic examination of failed samples. The decrease of the applied mechanical strain and hence of the developed stresses led to the suppression of failure by bond coat cracking and activate delamination. These results outline the importance of designing TMF tests and selecting the appropriate mechanical loading in order to accelerate testing and still trigger the same failure mechanisms as observed in-service.