The Researchment Of The Thickness Measurement Techniques For Special Composite’s Coating By Eddy-current Testing

In order to prevent the aviation parts which in the work environment ofultra-high temperature from being ablated and oxidized, we usually coated a layer ofsilicon carbide material (thickness of about50~160μm) on the substrate of thecarbon fiber composite material. The accurate measurement of the coating layer is notonly the basis of safety working of aviation parts but also the critical evaluation ofcoating quality. So far, there are lots of Non-destructive testing methods for coatingthickness measurement such as metallographic microscopy method the ray detector infrared method the ultrasonic test magnetic induction eddy-current testing and soon. Each method has its own advantages and limitations,but the best methods ofmeasurement for such ultra-thin coatings thickness (tens of μm) is the eddy-currenttest.According to the special carbon fiber composite material s properties，this articleestablish a theoretical model of ultra-low conductivity material’s measurement byeddy-current test.On the based of this, to simulate and analysis the influent of thematerial s inhomogeneity conductivity to the measurement of the coating s thickness;Researching the signal characteristics of the eddy-current testing underthe condictionof materials inhomogeneity conductivity. Puting forward the program of theexcitation parameters for the eddy-current system. Research and analysis theeddy-current’s signal characteristics under the effects of the diferent thickness of thecoating and inhomogeneity conductivity of the matrix. On the basis of using existingspecimens and the existing equipment to carry out part of the conductivitymeasurements and experimental study of coating’s thickness measurements, Study andwork out the minimum conductivity and the range of itsinhomogeneity and thetheoretical measurement range and measurement accuracy of the coating. Thenestablish the basic model (the main steps and testing parameters,etc) for specialcarbon fiber composite matrix s coating thickness measurement. The results showthat the probe coil impedance and the coating thickness has a good linear correlationand detection resolution and also can inhibit the material conductivity heterogeneityon the thickness measurement while selecting the appropriate frequency of testing. This method is well reflected the characteristics of the material properties andimproves the accuracy of measuring the carbon fiber composite coating s thicknesssimulation by the eddy-current testing. Thus implementation of the carbon fibercomposite coating thickness measurement is feasible by using the eddy-currenttesting.This article completes the researchment of the feasibility theoretical andexperimental study, determines the theoretical measurement range and resolutionthrough carrying out the technology research of thickness measurement for the surfacecoating (anti-oxidation protective coating of the silicon carbide) on the specialcomposite material matrix(special carbon fiber composite material). The experimentalthickness measurement of the special carbon fiber composite materials coating showsgood agreement with the theoretical simulation, thus it provides a reliable theoreticalbasis and technical support for the precision measurement for the special carbon fibercomposite materials coating’s thickness by eddy-current testing.