Influence of defect orientation on electrical insulating properties of plasma-sprayed alumina coatings

The influence of the microstructure (pores and cracks) on electric properties of plasma-sprayed alumina coatings was investigated using the so-called Scanning Electron Microscope Mirror Effect (SEMME) technique. Coatings were sprayed with different alumina feedstock powders on various atmospheres using a CAPS ('Controlled Atmosphere Plasma Spraying'). Microstructures with various amount of porosity and cracks orientation distributions were analysed. Both outer surfaces and cross-sections of alumina coatings have been analysed by SEMME technique using two complementary modes (measurement of absorbed current and mirror methods). Originally developed to study the behaviour of injected electrons and related phenomena, such as trapping ability, detrapping process and relaxation phenomena in bulk insulating materials, the SEMME technique was successfully applied, in this study, to porous coatings. It is proved that cracks orientation modifies both motion and trapping of charges and therefore the dielectric properties of plasma-sprayed alumina coatings.