Effect of microstructure and environmental conditions on the dielectric properties of plasma sprayed Al2O3 insulating coatings

The effect mechanism of microstructure and environmental conditions on the dielectric properties of plasmasprayed Al2O3 insulating coatings was systematically investigated in this study. Two kinds of Al2O3 powders with different particle sizes were used to deposit the coatings under the same spraying parameters. The microstructures of the coatings (including porosity, pore size, crack density, and phase composition) were analyzed using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and X-ray diffraction (XRD). The dielectric properties (including breakdown strength, resistivity, dielectric constant, dielectric loss, and complex impedance) were characterized by a two-electrode measurement cell. The results indicated that the coating deposited with the larger-sized powder (D50=45.8 mu m) exhibited higher porosity, alpha-Al2O3 content, and connected crack density, than the coating with the other kinder of powder (D50=22 mu m). The DC and AC breakdown strengths were found to be related to the porosity of the coatings and the partial discharge in the pores results in the eventual breakdown. The resistivity of the coatings decreased significantly with increasing humidity due to the moisture adsorbed on the pores and cracks. In high-humidity environments, the water molecule enhanced space charge polarization (WESCP), as the main polarization mode, contributed to the dramatic increase of the dielectric constant and dielectric loss. Moreover, the complex impedance and phase angle showed frequency-dependent changes in different environments, suggesting a combined effect of humidity and temperature on the polarization and conductance of coatings. The obtained results of this study could pave the way for the design and application of high-performance plasma-sprayed insulating coatings.