| In this dissertation, AT/Al2O3composite coatings with the Ni transition layer weredeposited on the Cu substrate by atmospheric plasma spraying （APS） using commercialNi-based alloy powder, AT （Al2O3-TiO2instead） composite powder and pure Al2O3powder as feedstock materials. Microstructure, phase composition, component distributionof composite coatings were characterized by scanning electron microscopy （SEM）, energydispersive spectrometer （EDS）, X-ray diffraction （XRD）, etc. The coating properties, suchas porosity, microhardness, fracture toughness, bond strength, DC breakdown voltage andresidual stresses, were measured. The microstructure and properties of four different ATcoatings were examined and compared. The stress distribution of Al2O3coating wasdiscussed and AT/Al2O3coating was optimized based on the analysis results of stressdistribution.The results show that AT20（Al2O3-20wt.%TiO2） coating demonstrates well bondingand dense microstructure. It is consisted by less γ-Al2O3phase content, relatively moreα-Al2O3and Rutile-TiO2phases. Tensile strength of AT20coatings is1.6times that ofAl2O3coatings. Its hardness value is83%that of Al2O3coating and the lifetime of itsbeginning crack is two times that of Al2O3coating. AT20coating was selected as themiddle layer to enhance the synthetical performances of AT/Al2O3composite coating.The surface stress of free-standing Al2O3coating is about168.3MPa, and the stress ofAl2O3coatings with Cu substrate is about28.5MPa. After optimizing, the thickness of Nilayer, AT20layer and Al2O3layer are75±5μm,185±20μm and265±15μm respectively.The bonding strength of optimized AT20/Al2O3coating is over15MPa.The dielectric mechanism investigation of Al2O3coating shows that breakdownmorphology of Al2O3coatings is typical corona breakdown morphology, the insulationmechanism can be explained by intrinsic insulation mechanism and the thermalmechanism. Moreover, the energy release rate during breakdown processes is obeyingGriffith rule. The dielectric properties of Al2O3coatings can be improved by both of thecoating thickness and sealing treatment, and the overlap of sealants can strongly reduce sensitivity to relative air humidity.Under the conditions of100Hz and25℃, the resistivity of optimized AT20/Al2O3coating is1.18×108·m, relative dielectric constant is1.25, dissipation factor is0.6andthe breakdown strength is about11.72±0.47KV/mm after the sealing treatment. |
