Thick ceramic coatings deposited by plasma spraying techniques are widely used as heat, wear and corrosion resistant coatings at high temperature. The characterization of the thermal diffusivity and the thermal conductivity of these materials is especially important for coatings used in aeronautics, automotive and power generation industries. It is therefore necessary to have a reference facility for the measurement of these thermal properties over a wide range of temperature with reliable measurement uncertainties. LNE reference diffusivimeter, which is used for several years for measuring thermal diffusivity of homogeneous materials, is based on the principle of the rear-face flash method. This method is not convenient for the study of coatings or multilayered materials. The existing LNE facility has been thus set-up in order to enable the determination of the thermal diffusivity from the temperature of the front face or the rear face of the tested specimen. It also allows to measure simultaneously the thermal diffusivity and thermal effusivity of these coatings at high temperature by front-face flash method. This method has been first validated from room temperature to 1 400 °C by comparing the obtained results of thermal diffusivity of two wellknown homogeneous materials (Armco iron and Poco graphite) with those determined by using rear-face technique. The measured thermal diffusivity values have relative deviation of less than 3.5% depending on temperature and materials, with an expanded uncertainty less than 5.5%. The front-face laser flash method has then been applied to the determination of thermal conductivity of chromium oxide coating deposited on iron alloy substrate from 23°C to 800°C, by using simultaneous measurements of thermal diffusivity and thermal effusivity. The results were compared to thermal conductivity values determined by indirect method from thermal diffusivity, specific heat and density. The thermal conductivity values measured by using both methods are in good agreement, with a relative deviation of less than 7%, and are within the range of uncertainty of measurement.

Key words

ceramic coating
chromium oxide
high temperature
flash method
thermal diffusivity
thermal conductivity