Thermal barrier coatings (TBCs) are ceramic-based materials, which are used in gas and aircraft turbines in order to protect the metallic components from the operating temperatures of aforementioned turbines. The thermal performance of these coatings is characterized by investigating established thermophysical properties such as emissivity and thermal conductivity. Furthermore, the adhesion properties between TBC and substrate are of great importance. Usually, local delamination occurs first as a precursor to large-scale delamination. This leads in its final stage to a complete coating failure, which can cause considerable damage to the turbine during operation.
Therefore, the development of non-destructive and contactless methods for detecting local delamination in situ is of major interest. Existing investigation methods are either based on destructive testing (e.g. cutting the specimen) or require contact sensors (e.g. ultrasonic testing). Contactless and non-destructive methods are not sufficiently reliable up to now.
Hence, within this work a method for detecting delamination by a non-contact measurement technique has been developed, which uses a heating setup in combination with a thermographic device. For contactless inspection, the surface of the investigated object is thermally excited with a suitable non-contact heating method, e.g. laser heating. The resulting temperature distribution on the surface is recorded using a thermographic camera. Since an existing delamination increases the thermal contact resistance between the coating and the substrate, the heat transfer from the coating to the underlying substrate is locally reduced, which results in a locally higher temperature.
The infrared-optical properties of the ceramic based TBCs – especially spectral emissivity and transmissivity – were derived as a function of temperature by characterising specifically prepared samples. Finally, the developed method for detecting delamination will be presented together with the obtained results.

Acknowledgement: This Project (funding code: 03ET7082A) has been supported by the Federal Ministry for Economic Affairs and Climate Action (BMWK) on the basis of a decision by the German Bundestag.