DC conduction and breakdown behavior of thermally sprayed ceramic coatings

In this study, the DC conductivity from low electric fields up to breakdown fields is studied for several different thermally sprayed ceramic coatings. Although the DC conductivity of bulk alumina ceramic has been observed to follow the space charge limited current conduction mechanism, the studied ceramic coatings do not follow or follow only partly this mechanism. Possible reason for this is their different microstructure since bulk alumina exhibits fully crystalline microstructure while the ceramic coating consists of crystalline and amorphous regions as well as voids, defects and numerous interfaces. A possible conduction mechanism of the ceramic coatings based on the different conductivities of the amorphous and crystalline regions of the coatings is proposed. The microstructural features (e.g. volumetric porosity) are found to affect the breakdown strength for some of the studied coatings. The step-test breakdown strengths of the coatings were lower than the ramp-test ones due to the longer stress durations in step tests giving an indication of effects of electrical stress duration and possible short-term degradation of the coatings.