In-depth and in-plane thermal diffusivity measurements of thermal barrier coatings by IR camera: evaluation of ageing

Ceramic thermal barrier coatings (TBCs) are widely used for protect- ing hot path components from combustion gases in gas turbines for both aero- and land-based applications. TBCs undergo degradation and eventually detach from the substrate. Forecasting of the detachment of TBCs for timely maintenance is an open problem in gas turbine technology. It is known that sintering happens in the TBCs when exposed to high temperature. Sintering affects the mechanical properties of TBCs and mainly their strain compliance for which degradation causes the detach- ment. As sintering strongly affects the thermal diffusivity of TBCs also, the idea is to measure the latter parameter to account for the former. Pulsed thermography is the technique selected to monitor the thermal diffusivity variation due to TBC ageing. In perspective, it should be applied to monitor the gas turbine during the normal stop for maintenance. This article reports preliminary laboratory tests carried out on a set of metal samples coated with TBCs. The samples were aged during cyclic oxidation tests at various percentages of their estimated life, the end of life being the time of the TBC detachment from the substrate. The identification of the thermal diffusivity in the coating layer is carried out for the general case of anisotropic conductivity.