Effects of temperature and humidity on the dielectric properties of thermally sprayed ceramic coatings
Permanent address of the item is
Electrical insulation is indispensable in electrical equipment. Thermally sprayed ceramic coatings are promising materials, offering insulating capabilities in areas where other materials do not fulfil the requirements. For instance their chemical stability, a high wear resistance and an ability to withstand high temperatures are significant advantages. This thesis is a part of the HICC project (Highly Insulating Ceramic Coatings), in which a new class of thermally sprayed ceramic coatings, with significantly improved dielectric properties, will be developed. It contributes to that project in two ways. First by the development of a measurement program to measure the relative permittivity and the loss index in thermally sprayed coatings. Secondly by the analysis of the dielectric properties of three Al2O3 coatings, coated by respectively ethylene high velocity oxygen fuel (HVOF) spraying, atmospheric plasma spraying (APS) and flexicord spraying. In the theoretical part of this thesis, a thorough literature study regarding general insulator behaviour and thermally sprayed ceramic coatings is performed. In the empirical part, the dielectric properties of the coatings were studied using dielectric spectroscopy measurements. It was found that at temperatures below 100°C the permittivity and the losses are strongly dependent on the frequency, the temperature and the humidity. Regarding permittivity and dielectric losses, the flexicord sprayed sample showed the most promising results. On the other hand, the plasma sprayed sample showed the highest values for those quantities. These variations are found to be related to differences in microstructure and phase composition, due to the distinctions between the various spraying techniques. At temperatures above 100°C, an unexpected permanent decrease in relative permittivity and loss index was observed during heat treatments. The outcome of this thesis provides a good base for the continuation of the HICC-project. More recommendations for further research and the advice to use stainless steel substrates for future samples, are provided as well.