Micro-thermoreflectometry as a quantitative microscopic and full-field method for tracking local thermo-optical property variations of MCrAlY coatings induced by high temperature oxidation

This study focuses on detecting and characterizing local damage and chemical failure induced by high-temperature oxidation in MCrAlY coatings. Such damage manifests as complex and heterogeneous oxide formation at the sample surface, notably the growth of a bi-layer oxide composed of semi-transparent layers of Al A 2 O A 3 and Cr A 2 O A 3 . An original method, termed micro-thermoreflectometry, is proposed for monitoring post-mortem local oxide formation based on the evolution of the surface’s thermo-optical properties, using an innovative active- and full-field imaging device. The micro-reflectometer is based on high-resolution cameras and illumination sources to capture full-field bidirectional reflectivity with a sub-micron spatial resolution of 0.625 μ m . p x − 1 across the visible and near-infrared spectral ranges. Particular attention was paid to calibrating the micro-reflectometer and validating it with Lambertian diffuse Spectralon®standards, compared to reference measurements acquired on a Fourier-transform infrared spectrometer (FTIR). This experimental setup was successfully validated at room temperature with an off-line test that allowed for the production of a multispectral map of bidirectional reflectivity of the pre-oxidized surface, from which spectral regions were extracted using a segmentation process. The spectral signature for each identified region was defined by analyzing the reflectivity transformation within a multispectral space, which enables the introduction of the concept of spectral strain to evaluate the optical path of each chemical product during the oxidation process. The proposed mathematical framework enables the phenomenological characterization of oxidation processes through surface observation of their thermo-optical properties.