Non-destructive microwave evaluation of plasma sprayed TBCs porosity

Porosity of top coating (TC) in thermal barrier coatings (TBCs) is one of the main factors affecting the thermal conductivity and causing failure of TBCs. To ensure the quality of the TBCs, the evaluation of TC porosity was carried out using a microwave non destructive technique. The reflection coefficient at the interface of TC and waveguide probe was studied with regard to the TC porosity using computer simulation technology-microwave studio (CST-MWS). The results showed that the greater the porosity of the TC the larger the phase difference when the microwave was operated at the sensitive frequency. And the sensitive frequency is different when TC thickness changes. Besides, a network analyzer was used to measure the reflection coefficient phase which can be used in the evaluation of the TC porosity. The results showed that the phase difference increases linearly with increasing porosity of TC when the network analyzer operating over the frequency range of 30 GHz to 40 GHz. Therefore, the porosity of TC can be calculated by measuring the phase of the reflection coefficient. The experimental results are consistent with the theory simulation ones.