Nondestructive evaluation of dielectric layers on conductive substrates by microwave surface electromagnetic waves

The thickness and permittivity (or dielectric constant) of dielectric layers on electrically conductive substrates can be determined by suitable measurements using surface electromagnetic waves. The approach used in the paper is to measure the cutoff frequency of the TM1 SEW mode and, in effect, the propagation constant of the TM1 SEW mode as a function of frequency. The theory of the method and some preliminary results obtained using 8-12GHz SEW supported by a layer of polypropylene on an aluminum substrate are presented. By going to higher frequencies the method can be extended to thin protective coatings on metals, e.g., ceramic coatings on jet engine and coal utilization components.