Comparison of the Thermal Cycling Performance of Thermal Barrier Coatings under Isothermal and Heat Flux Conditions

Abstract

Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The integrity and reliability of these coatings is of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. The main cause of failure of coatings is through debonding which develops as a consequence of thermally induced strains between the metallic bondcoat and the alumina TGO layers due to the differences in the thermal expansion coefficients of the individual layers. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach here employs a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Tests have been conducted on a TBC system consisting of an IN738 substrate with a CN334 bondcoat and EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. This paper reports on the performance of the flame rig equipment and the results from the exposures on the TBC system.