Accurate Microwave Measurement of Coating Thickness on Carbon Composite Substrates

Abstract

This paper describes the utility of a microwave-based nondestructive evaluation (NDE) method for accurate thickness measurement of thin dielectric coatings (e.g., paint and primer) on a carbon composite (CC) substrate and the results are compared with those using a conducting (aluminum) substrate. The small thickness and the diverse dielectric properties associated with these coatings, along with the anisotropy associated with CC substrates, significantly limit the use of traditional NDE methods. This method not only overcomes these limitations but also provides for simultaneously evaluating the thicknesses of multiple coating layers (and also their dielectric constants). The electromagnetic simulation results for simultaneously estimating the thicknesses of a top and a bottom coating (i.e., primer and paint) as well as experimentally evaluating thicknesses of several coatings on such substrates are presented. In addition, thickness evaluation sensitivity to several critical parameters such as CC fiber orientation and variation in the measured coating dielectric constant used in the thickness recalculation is also investigated and presented.