A Composite Approach to Al2O3-based Plasma-Sprayed Coatings

Thermally-sprayed ceramic coatings such as plasma-sprayed alumina show a composite microstructure actually due to the presence of defects such as pores, inter-lamellar and intra-lamellar cracks. These 2nd phase-typed features influence the mechanical behavior and electrical insulation of the coating dramatically. In this study, a composite approach to the microstructure of plasma-sprayed alumina was developed for the optimising of component properties such as electrical gaps used in the oil industry. This approach consisted of a Finite Element Analysis (FEA) of thermo-mechanical and electrical properties from simulated microstructures. Series of composite microstructures were tested, i.e that of air plasma-sprayed (APS) alumina basically plus those obtained by addition of glass or resin using co-spraying and impregnation respectively. Various degrees of porosity and cracks could be obtained from different spraying conditions and by subsequent laser surface remelting. Every composite microstructure was studied using quantitative image analysis of series of SEM cross-sections. Electrochemical Impedance Spectroscopy (EIS) in NaCl solution was also performed to characterize the level of connected pores and the resulting electrical insulating properties. From experiments, a Finite Element Model (FEM) based on the actual microstructure was developed. The latter was simulated with involving of all significant features, such as phase distribution, porosity and defects. This simulation was developed to optimize the composite microstructure to meet industrial applications