Abstract
Pure alumina coating obtained by thermal spraying can find applications as electrical insulating layer. Thermally-sprayed ceramic coatings exhibit a complex lamellar structure with a network of interconnected pores, inter-lamellar and intra-lamellar cracks. In this work, the influence of the microstructure on electric properties for plasma-sprayed alumina coatings was investigated. Coatings have been sprayed with different pressures and gases using a CAPS (‘Controlled Atmosphere Plasma Spraying’) as well as different alumina feedstock powders. Detailed quantitative image analysis of cross-section views allowed to select six microstructures with different porosity levels and cracks orientation distributions. In order to assess the behaviour of the electrical insulation and the influence of local defects on electric properties, the so-called Scanning Electron Microscope Mirror Effect (SEMME) method has been applied on outer surfaces and on cross-sections of the different selected alumina coatings. This method, originally developed to study the ability of a bulk insulating material in trapping of charges from an electron beam irradiation in a SEM, revealed to be successfully feasible for porous materials such as thermally-sprayed ceramic coatings. It has been shown that cracks orientation modified both propagation and trapping of charges and therefore the electric properties of plasma-sprayed alumina coatings.
