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The continuous improvement of aero-engines, driven by industrial and environmental motivations, implies an ever-increasing level of thermal and mechanical stresses within the turbines. structural materials (titanium, nickels-based superalloys, TBCs, etc.) are stressed to their performance limits and altered by environmental degradation mechanisms involving changes in surface and subsurface properties [1]. A better understanding of these damages and their initiation requires the development of characterization methods capable of microscopic, in-situ, and continuous monitoring of thermochemical-mechanical mechanisms, such as oxidation-assisted failure. Non-contact imagery techniques, such as digital image correlation (DIC) and infrared (IR) thermography, offer promising solutions for the in-situ and non-intrusive monitoring of these dynamic scenes. Experimentally, determining thermo-optical properties is complex, particularly for heterogeneous, dynamic surfaces composed of multiple materials—typical of oxidation processes—as it depends on variables factors such as material composition (metal, oxide, intermetallic…), microstructures, surface roughness, temperature, and wavelength.
Although originally intended for mapping temperature, some IR thermography methods are promising candidates for the spatial and temporal determination of the surface’s thermo-optical properties. Particularly, near-infrared (NIR) thermography techniques (0.7-2.5 µm spectral range) can exploit the semi-transparent properties of oxides and are also compatible with temperature determination in most high-temperature processes where oxidation occurs, ranging from 600 °C up to more than 1100 °C. Based on preliminary research [2, 3], thermoreflectometry emerges as a technique capable of in-situ and full-field monitoring of the reflectivity on heterogeneous and dynamic surface. The technique is based on two multi-spectral measures of both bidirectional reflectivity and radiance temperature, allowing the resolution of a radiometric system of equations for the determination of the emissivity and the true temperature. Such analysis unveils the potential to map the optical signature—through emissivity/reflectivity determination—enabling the simultaneous identification and spatial localization of oxidation products. Consequently, offering the perspective of monitoring local kinetics of oxidation reactions, as well as the evaluation of surface temperature.
This work details the thermoreflectometry method and describes the design of the micro-thermoreflectometer capable of performing thermo-optical measurements in the NIR spectral band with sub-micron spatial resolution (0.625 µm.pixel-1) up to 1000 °C. A particular attention given to its calibration on Spectralon® white, diffuse Lambertian standards. Validation of the method is then proposed, first on diffuse gray-level samples and subsequently on a real oxidized surface, with particular emphasis placed on the quantitative optical identification of oxides coupled with spatially resolved detection. The suggested device demonstrates the feasibility of detecting and correlating thermo-optical properties with microstructural and oxidation evolution at the surface and subsurface through a spectral signature. The proposed signature—based on the analysis of the resulting system’s output from an innovative framework—allows to identify and quantify oxidation damages at the surface from their thermo-optical response.
[1] Young, D. J., 2016. High Temperature Oxidation and Corrosion of Metals Ed. 2 978-0-08-100101-1, (2016), https://doi.org/10.1016/C2014-0-00259-6
[2] Javaudin, B., Gilblas, R., Sentenac, T., Le Maoult, Y., Experimental validation of the diffusion function model for accuracy-enhanced thermoreflectometry, Quantitative InfraRed Thermography Journal, vol. 18 (1): 18—33, (2021), https://doi.org/10.1080/17686733.2019.1665297
[3] Lafargue-Tallet, T., Vaucelle, R., Caliot, C. et al. Active thermo-reflectometry for absolute temperature measurement by infrared thermography on specular materials. Sci Rep 12, 7814, (2022), https://doi.org/10.1038/s41598-022-11616-8
