A direct method for the simultaneous characterization of thermal diffusivities of a bi-layer material consisting of a thin coating deposited on a substrate

Abstract This work presents a method dedicated to the simultaneous identification of the thermal diffusivities of coatings or thin film materials. To ensure non-destructive thermal characterization of the coating, the present method also implies the identification of the substrate thermal properties, which may be orthotropic. The estimation of thermal diffusivities is based on the resolution of an inverse problem by minimizing the quadratic difference between the response of a 3D semi-analytical model and the measurements resulting from a single ’3D flash method’ type experiment, using a stochastic based optimization algorithm. A unique non-intrusive test, that consists in recording the temperature change cartography on one face of the sample by means of an infrared camera, is required. The evolution of the temperatures within the material is generated by a non-uniform and almost instantaneous excitation imposed by a CO2 laser on the measured face. The difficulties related to the control of the excitation, in terms of the distribution of the imposed flux or the energy absorbed by the material, are overcome by estimating the parameters associated to the excitation simultaneously with the thermal diffusivities. The developed method is applied to estimate the thermal diffusivity of a coating used in thermographic phosphor thermometry to measure wall surface temperatures and heat fluxes in combustion environments. The method is first evaluated on simulated data as a function of the measuring/excitation face in order to ensure the feasibility, the robustness and accuracy of the current method and to establish the best experimental configuration. Experimental results are then exploited and the estimation results are compared with other methods results.