Effect of Temperature and Humidity on Dielectric Properties of Thermally Sprayed Alumina Coatings

Breakdown strength, DC resistivity, permittivity and loss of thermally sprayed alumina coatings were studied at various temperatures and relative humidities. The studied coatings were sprayed by utilizing three different spray techniques: flame, high-velocity oxygen fuel (HVOF) and plasma spraying. Breakdown behavior of HVOF sprayed alumina were studied up to very high temperatures (800 degrees C). At 20-180 degrees C, no significant trend could be seen in the breakdown strength of HVOF and plasma sprayed alumina coatings. The breakdown strength of alumina coatings decreased gradually from 300 to 800 degrees C reaching a value which was only 14% of the breakdown strength measured at 20 degrees C/RH 20%. Increasing humidity (from 20 to 90%) decreased the DC resistivity of the alumina coatings five orders of magnitude. Correspondingly, permittivity and losses increased with the humidity; in most cases with a notable contribution due to DC conduction. The material behavior may be linked to the microstructure of coatings consisting of amorphous and crystalline regions with interfaces in between. Moreover, the alumina coatings exhibited notable amount of highly hygroscopic gamma-phase which also affected the moisture sensitivity of the coatings.