Fabrication and Characterisation of Regular and Complex Shapes Alumina from Aluminium Foil by PEO Process

Alumina thick and thin films can be used for a variety of purposes, including the semiconductors, electronics, dielectric, piezoelectric and ferromagnetic devices. The films can be produced by conventional methods such as tape casting and roll compaction from ceramic slurry. However, these methods offer limited part geometries and cause defects in the film structure. Plasma electrolytic oxidation (PEO) is a surface engineering technique that converts the surface of light metals and alloys into oxide ceramics layers. In this study, ceramic alumina films were produced by conversion of different shapes aluminium foil (50 μm) substrate using PEO technique.
The influence of processing conditions, such as (treatment time, electrolyte composition, current density and sample shape), on the formation, growth behaviour and properties of PEO coatings were investigated. Optical emission spectroscopy was used to investigate the composition of plasma discharge and evolution of its main components during the PEO process. The plasma electron temperature was calculated using two independent peaks of aluminium in the near ultraviolet band.
COMSOL Multiphysics software was used to model the distribution of electric field in the electrolyser and investigate a possibility of achieving a uniform coating thickness on complex shape in several electrode configurations. The surface morphology and phase composition of ceramic coatings were analysed using SEM and XRD techniques. The breakdown voltages, thickness of ceramic coatings in different electrolyte were comparatively studied.
Depending on the treatment parameters the metal-to-ceramic conversion ratio varies from 10 to 80 %. However, after a treatment for 12 min, complete conversion of aluminium foil into alumina was successfully achieved. The shape of working electrodes strongly influenced the coating thickness. Gamma alumina was the prominent phase in thin coatings, however formation of alpha-alumina was observed when treatments lasted longer than 6 min. Correlations have been studied between characteristics of plasma discharge, such as plasma electron temperature, and phase transitions in the surface layer to develop in situ diagnostic methods for the PEO processes.
The L and U shaped thin-walled 3D ceramic-metal composite structures were successfully fabricated using PEO treatments in specific electrolyte with uniform PEO coatings without cracks grown on the edges and corners at outer surfaces of L and U shape Al foil samples. However, the coating thickness at the inner surfaces was around 30 % lower.