Micro-thermoreflectometry applied to in situ monitoring of changes in the optical properties of a MCrAlY coating during high-temperature oxidation
Résumé
The paper deals with the characterization of the formation of high-temperature oxides formed on the surface and subsurface in MCrAlY alloys, and more generally nickel-based single-crystal superalloys, using an in-situ method of measuring thermo-optical properties. The suggested method, denoted micro-thermoreflectometry, involves fullfield measurement of the bidirectional reflectivity at the microscale using high-resolution cameras operating in the visible and near-infrared (NIR) spectral range, with a spatial resolution of 0.625 µm. Prior to measurement, a reflectometric calibration of the thermo-reflectometry method was carried out to determine the model between the intensity delivered by each pixel and the reference value of the bidirectional reflectivity of a Lambertian surface given by a Spectralon® standard. This model was validated on Lambertian surfaces with attenuations of 80 and 50 % relative to Spectralon® by comparison with the bidirectional reflectivity measurement performed by Fourier's transform infrared spectrometer (FTIR). Both measurements are coherent and perfectly fitted for the three micro-thermoreflectometry measurement wavelengths. By measuring bidirectional reflectivity fields, the detection and the identification were performed on the various oxidation compounds that are formed either at the surface or beneath the semi-transparent Al 2 O 3 -layer. A multispectral image was provided revealing differences between oxides formed at the surface and subsurface of the MCrAlY alloys.
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