The electrical insulation behavior and sealing effects of plasma-sprayed alumina–titania coatings

Two commercial sealants based on polymers were studied and the electrical insulation properties of ceramic coatings were tested, before and after the impregnation treatment, using a plasma-sprayed Al2O3–13% TiO2 coating as the reference coating. The second part of this paper reports on the effects of curing temperature and exposure time of the sealants on the penetration depth of the ceramic coating. The preliminary test results with a DC puncture tester imply that the dielectric breakdown voltage mechanism of plasma-sprayed ceramic coatings has been determined to be a corona mechanism. Dielectric breakdown voltage of the as-sprayed and as-ground samples have shown a linear trend with regard to the thickness, showing an average dielectric strength of 20 kV/mm for the thickness scale studied. The penetration depth of sealants to the ceramic coating can be clearly recognized with addition of fluorescent dye (EpoDye). It is also shown that grinding the coating before sealing and adding fluorescent dye (EpoDye) does not affect the penetration depth of sealants. For epoxy-based sealant, the effects of temperature on the penetration depth of the sealant show that an optimum temperature can be determined. The balance between the beneficial and the detrimental effects is obtained at approximately 100°C. Moreover, the effect of curing time on the penetration depth is beneficial. However, for urethane-based sealant, it seems that the impregnation process may be complex because of the high pressures of entrapped gases occurring during the process. Although the penetration of sealant to the ceramic coating has reached over 300 μm, the effects of curing time and temperature have not been clearly established.