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

Abstract

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™) and the three different experimental set-ups in terms of spectral range and frame rate of the IR cameras, heating source and data reduction.

Thermal conductivity of ZrO2-4 mol%Y2O3 thin coatings by pulsed thermal imaging method

“…The graphite coat was observed to be well bonded to the TBC coating and its thickness was estimated to be within 1~2 µm; such coat has been verified to have negligible effect to measurement accuracy [14]. Detailed description of this and two other graphite coats, their application and removal, and their effect to the thermal property measurement for various types of TBCs can be found in [25]. The experimental data provide surface temperature transients as a function of time for all surface positions of a TBC specimen.…”

Microstructural Evolution and Sintering of Suspension Plasma-Sprayed Columnar Thermal Barrier Coatings

“…Specific density and thermal diffusivity measured for Hastelloy X was 0.469 J/g K and 2.889 mm 2 /s, respectively, whereas for the bond coat, the values were 0.453 J/g K and 3.283 mm 2 /s, respectively. The detailed procedure of thermal diffusivity and conductivity measurement can be found in the previous literature (Ref 32). Thermal conductivity was then calculated using following Eq 3:…”

Can TBC porosity be estimated by non-destructive infrared techniques? A theoretical and experimental analysis

“…For experiments lasting within very few seconds or less, convective heat losses do not take place; moreover, for small temperature increases caused by the heating pulse, radiative heating losses resulted sufficiently small to adopt the adiabatic model. A thin graphite layer (b 4 μm) on the TBC surface can be painted to prevent the laser radiation to penetrate within the TBC without altering significantly the final result [31][32][33]. In the specific case when an IR camera is used to detect the surface temperature decay after the pulse, the uncertainty is related to the frame rate and the integration time (the time period where the detectors are active in receiving the signal) of the camera.…”