The effect of the α/γ phase on the dielectric properties of plasma sprayed Al2O3 coatings

Alumina coatings with different ratio of alpha- Al2O3 to gamma- Al2O3 were prepared from the crystalline alpha- Al2O3 powder using atmospheric plasma spraying at various arc current. The presence of an increasing ratio of gamma- Al2O3 was observed with increasing the plasma arc current. Dielectric constants of alumina coatings were measured from the capacitors consisting of alumina as dielectric and steel substrate and aluminum as electrodes. Dielectric constant increased as the ratio of gamma- Al2O3 phase increased in the coatings. The capacitance is not only related to the phase of alumina in the coating but also the measured frequency. The phase state presented in the coatings was the primary determinant of the coatings' dielectric constants compared with the porosity of the coating. In addition, heat treatment at 1300 A degrees C for 20 min was conducted to change the phase to complete alpha- Al2O3 and its capacitance was measured.