The Effect of Water Absorption in Ceramic Coatings on High Frequency AC Resistance

Thermally sprayed ceramic coatings can be used for wear protection, thermal and electrical insulation. When exposed to environments with high humidity, the water absorption of the ceramic coating has a tremendous impact on the electrical insulation. In thermally sprayed ceramic coatings, water can easily be absorbed by the porous microstructure of the coating. A general result of the water absorption is the reduction of the DC resistivity. However, in the high frequency regime of AC loads, contrary results were observed for sealed Al₂O₃-coatings on steel substrates. Specimens exposed to high air humidity have shown an increased AC resistance compared to dry specimens, if frequencies above 1 MHz are considered. To analyse this phenomenon, a novel measurement technique was developed to investigate the influence of the water absorption of detached ceramic coatings on the AC resistivity at high frequencies. Moreover, the water absorption of the ceramic is measured gravimetrically. To ensure the results are also applicable to ceramic coatings on substrates, the coatings’ morphology and phase composition was analysed using light microscopy, electron microscopy and XRD-analysis and compared to reference specimen deposited on steel substrates.