Broad band microwave probe for nondestructive test of dielectric coatings

We report on nondestructive characterization of Thermal Barrier Coatings (TBC: YSZ/MCrAlY/Ni-superalloy). To test TBC we have employed the method of electromagnetic defectoscopy. We have modernized this method by applying microwave probe instead of RF coil: an open-ended coaxial probe was used to measure complex permittivity of dielectric material ε = ε\´ + ε". The probe is basically a coaxial transmission line with an open end put into tight mechanical contact with the sample under test. Complex permittivity of the sample material was calculated from the coefficient of microwave reflection from the probe-sample interface. Using the probe the complex permittivity can be measured in a wide frequency range. Such broadband spectroscopy enables detection of foreign phase inclusions, delamination of dielectric layer and cracks on the ceramics/metal substrate interface, etc. Microwave probe enables noninvasive and nondestructive test of both small-area and large-area surfaces. Defects in the ceramic layer under test are revealed as anomalies of the dielectric permittivity on the background of almost constant ε, which is characteristic for the rest of the material.