Electrical Performance of Insulating Ceramic Coatings in Different Environmental Conditions

Thermally sprayed insulating ceramic coatings are typically utilized to protect material against hot corrosion or wear. However, the ceramic coatings are also used as an electrical insulation e.g. in applications where the operating conditions are demanding, e.g. very high temperatures or harsh environments. A coating is formed layer-by-layer within a process where a raw material, e.g. powder, is heated, accelerated and melted by high temperature and high-velocity gas stream towards a substrate. The layered microstructure of the coating differs strongly from the sintered bulk alumina, and thus their dielectric properties differs as well. Since the previous studies in the literature are typically focused on the dielectric properties of sintered alumina, there is a need to study dielectric properties of ceramic coatings comprehensively. This is also required to increase the use of alumina and spinel coatings as electrical insulation systems.

Atmospheric plasma spraying (APS) is a typical method to deposit oxide coatings, e.g. alumina and spinel coatings. However, a more dense and well-adhered coatings with better dielectric properties can be deposited by utilizing high-velocity oxygen fuel (HVOF) spray method, which makes this interesting method compared to traditional plasma spraying. In this thesis, most of the coatings were sprayed by utilizing HVOF method but some of the studied coatings were deposited by atmospheric plasma spraying or by rod flame spraying. Several different alumina and spinel raw materials were studied to form wide perspective of the coating properties.

This thesis focuses on the dielectric properties of the alumina and spinel coatings at different ambient conditions. In addition, suitable measurement methods for the dielectric characterization of ceramic coatings were studied and developed. The dielectric characterizations included following test series: i) short-term breakdown performance, ii) long-term breakdown performance, iii) DC resistivity at low and high electric fields, and v) relative permittivity and dielectric losses at low and high electric fields.

For all the studied coatings, a comprehensive dielectric characterization (short-term DC breakdown performance, DC resistivity, relative permittivity and dielectric loss) were made at controlled conditions (20 ◦C/RH 20%). According to the results, the HVOF sprayed coatings have superior dielectric properties in comparison to the rod flame sprayed coatings. HVOF spinel coatings have in general better performance than HVOF alumina coatings. The properties of the plasma sprayed coatings were at quite similar level with the HVOF coatings but the breakdown performance of the plasma coatings were lower. The differences between the different spray methods can be linked to different particle velocities since the HVOF coatings have the highest particle velocity and thus more dense structure while in rod flame spray process the velocity is the lowest.

Due to the microstructural differences between a bulk and a ceramic coating, the DC resistivities of the studied coatings are similar with that of bulk alumina only at the lowest electric fields (typically below 0.5 V/μm). At higher electric fields, resistivity of the coatings decreases gradually indicating strong non-linear resistivity. According to the high field DC conductivity studies, it is proposed that the coatings follow only partly the space charge limited conduction (SCLC) theory as opposite to bulk alumina which has been reported following the SCLC theory fully. The difference in conduction behavior can be explained by the layered structure of coatings consisting of crystalline and amorphous regions which exhibit different conductivities resulting in uneven DC electric field distribution.

The role of microstructure can also be seen in the relative permittivity and dielectric losses of the coatings. At the highest frequencies, the relative permittivities of coatings are more or less constant being at similar level with bulk alumina. However, at the lowest frequencies the relative permittivity rapidly increases which most probably indicates interfacial polarization occurring at the crystalline-amorphous interfaces in the coating. Also, the role of microstructure can be seen in the dielectric losses since the losses of the coatings were higher than those of bulk ceramic. In addition, low-frequency dispersion/quasi-DC conduction contribution can be seen in the dielectric losses in the low frequency region which also can be linked to the layered microstructure.

The effect of temperature and humidity on the dielectric properties of HVOF, APS and rod flame sprayed alumina and spinel coatings was studied. The breakdown strength of HVOF sprayed alumina coating is at a similar level from 20 ◦C to 300 ◦C. From 300 ◦C to 800 ◦C the breakdown strength decreases gradually reaching a value which is only 14% of the breakdown strength at 20 ◦C. In addition, the results show that increasing humidity does not directly affect the DC breakdown strength but it decreases the DC resistivity several orders of magnitude and increases the relative permittivity and dielectric losses notably. The DC resistivities of alumina coatings increased more significantly with increasing humidity than those of spinel coatings. It can be concluded that both ceramic coating materials exhibit moisture sensitive nature which is in coherence with literature. In addition, this moisture sensitive nature of the coatings can be seen in the breakdown voltage measurements when the oil immersion was utilized since the breakdown strength in oil is in the order of two times higher than the breakdown strength without oil immersion. In order to increase the use of thermally sprayed alumina and spinel coatings, their long-term reliability needs to be known. When the long-term performance is known, a possible service stress levels can be estimated. Long-term breakdown performance of HVOF sprayed spinel and alumina coatings were studied with a long 1000 h test and several shorter 168 h tests. According to the test series, a possible maximum service stress level for the studied coatings could be in the range of 10 V/μm which is approximately 25–30% of the short-term breakdown strength (Weibull α).