The sealing treatment of plasma-sprayed Al2O3 insulating coatings and its effect on the microstructure and dielectric properties were systematically investigated. To compare the penetration depth of sealants, various sealing processes were employed, utilizing two types of sealants and under two different ambient pressures. The microstructure of coatings was analyzed using scanning electron microscopy, energy dispersive spectrometer, and isothermal adsorption/desorption tests. Breakdown strength, resistivity, dielectric constant ( \({\varepsilon }_{r}\) ), loss tangent ( \(\text{tan}\delta\) ), and complex impedance were measured for both unsealed and sealed coatings. The results indicated that the maximum penetration depth percentage (50.8%) was achieved with the diluted sealant under low ambient pressure (20 Pa). A penetration model, that includes the ambient pressures, pore diameters, and physical properties of the sealant, was proposed and validated. The sealing treatment nearly doubled the breakdown strengths of coatings under DC, 50 Hz, and 500 Hz voltages and increased resistivity by nearly 100 times in environments with varying relative humidity (RH) levels. Additionally, the sealing treatment decreased the \({\varepsilon }_{r}\) and \(\text{tan}\delta\) of coatings in a 20% RH environment and increased their impedance in both 20% RH and 80% RH environments. This study provides a theoretical basis for the design and application of sealing treatments.
