Thermal diffusivity of TBC: Results of a small round robin test and considerations about the effect of the surface preparation and the measuring approach

Among the techniques used to measure the thermal diffusivity of TBC, the Laser Flash is a standard. Nonetheless, this technique shows two main limitations related to the size and the well-defined geometry of the specimens. Furthermore the most reliable data can be typically obtained only on freestanding coatings. On the contrary, other photothermal and thermographic techniques in reflection configuration (the same side is heated and temperature detected) can overcome these limitations. One aspect, only partially studied in the literature, is common to most of the photothermal and thermographic techniques. It concerns the effect of the blackening coating used for guaranteeing the absorption of the heating radiation just within a very shallow outer layer and to make opaque the TBC in the sensitivity range of the IR detector/camera.For this purpose, an inter-laboratory round robin has been promoted for comparing the thermal diffusivity in dependency of the blackening layer deposition technique, the TBC microstructure (in particular porous APS, columnar EB-PVD and PS-PVD(TM)) and the three different experimental set-ups in terms of spectral range and frame rate of the IR cameras, heating source and data reduction.