Research On Thickness Measurement Of Coatings On RCC Composite Substrates By Eddy-current Method

Reinforced carbon-carbon(RCC) composites as advanced materials have been widely used in the production of key components of aviation and aerospace.This kind of materials often work in the environment of ultra-high temperature. In order to prevent the materials from being oxidized and ablated, the surface of the composite materials is coated with an anti- oxidation layer of silicon carbide, and the thickness of coatings is about 40 ~ 200μm. The thickness of anti- oxidation coatings can affect the stability of the components and is of great significance to ensure the normal operation of aerospace vehicle. Eddy-current testing method is one of the most effective methods to measure such thin anti-oxidation coatings’ thickness.Firstly, based on the theory, this paper states the theoretical basis of eddy-current testing and thickness measurement of coatings. According to the properties of carbon fiber composite substrates and coatings, the influence of lift-off effect of eddy-current and the inhomogeneity conductivity of substrates and coatings is analyzed by finite element method simulation. The results show that the eddy-current method can be used for measuring the thickness of coatings by selecting appropriate parameters. Then, on the basis of theoretical analysis the appropriate eddy-current instrument is selected and the special probe used for measureing thickness is developed. By using the eddy-current instrument and probe, research on thickness measurement of coatings of carbon fiber composite materials is carried out. The optimum parameters of eddy current testing equipment is measured, the detection resolution, sensitivity and measuring range of the instrument are researched. The quantitative relationship between the eddy current testing signals and the thickness of coatings is detected through the polynomial fitting analysis. The equivalent thickness of coatings can be gotten by measureing the composites specimens which are covered with different thickness of coatings. Then according to the data processing and the analysis of measurement results, the coatings distribution of specimens could be evaluated. Finally, the microstructure of specimens section and the thickness of coatings are also analyzed by using the scanning electron microscopy. Experiments about substrate inhomogeneity are carried out to explore the precision of the measurement by eddy-current method.Theoretical analysis and experimental study results show that the thickness of Si C coatings on RCC composite substrates can be measured and evaluated effectively by using eddy-current instrument and probe. It provides a reliable method to measure the coatings’ thickness of such materials.