Abstract:Thermally sprayed ceramic insulating coatings are widely applied to provide workpiece insulation due to their excellent dielectric and mechanical properties. The dielectric performance is determined by the coating materials and preparation techniques. Firstly, the dielectric and mechanical property characteristics of several ceramic materials, including high-purity Al 2O3 , doped Al 2O3 ( doping MgO, TiO 2 , or ZrO 2 ) , and other materials ( Y2O 3 , MgAl2O4 ) , were summarized. Among these, as the most widely used coating material, high-purity Al2O3 has great dielectric strength and volume resistivity and its purity, phase compositions and defect orientations have a significant impact on the dielectric properties of coatings. However, high-purity Al2O 3 coating has poor impact resistance and high preparation cost, and we can dope the Mg or Ti element in Al 2O3 to reduce the powder melting temperature and improve the coating's deposition efficiency and density. Moreover, mixing partial-stabilized tetragonal ZrO2 in Al2O3 can also improve the impact toughness with the mechanism of phase transformation toughening. Doping and mixing other elements in Al2O3 will damage the dielectric property more and less, but enhance the comprehensive performance. Furthermore, Y2O3 can be used for insulating protection in ion etching environments and MgAl2O4 is appropriate for some harsh service environments due to its excellent dielectric and mechanical performances. Secondly, the effect on coating microstructure of different thermal spraying methods ( including plasma spraying, high-velocity oxygen fuel spraying and detonation gas spraying ) was analyzed and it was found that powder characteristics and spraying parameters ( such as spraying power、particle flight speed and flame atmosphere ) have a significant impact on the porosity, crack density and phase composition of the coating, which further affect the dielectric properties. Pores and cracks would seriously damage the dielectric properties of coatings, but they can be filled with the sealing process in which different sealers, sealing techniques and the physical and chemical characteristics of high-performance sealers were introduced. Additionally, heating treatment above transition temperature allows the metastable phase to transform to the high dielectric stable phase. Thirdly, the dielectric breakdown, resistivity, polarization, and environmental adaptability of insulating coatings were described. The dielectric breakdown strength is related to the electrode structure, voltage rise mode, and environment humidity and the test data can be processed by Weibull Distribution. Constantly reinforced partial discharging caused the dielectric breakdown. The three-electrode method was adapted to avoid the interference of stray currents in the insulation resistance testing. And the coating has an obvious “absorbing electrical charge” phenomenon due to its multi-hierarchies and abundant grain boundaries. At low-frequency voltage, space charge polarization is the main polarization manner of the ceramic insulating coatings, in which conductivity and polarization loss both exist. In a high-humidity environment, adsorbed water on the pore and crack surface will significantly reduce the resistivity and increase the dielectric constant. At last, the future research directions of thermally sprayed ceramic insulating coatings were proposed based on the true service environment.