Surface Modification of Aluminum Alloys by Plasma Electrolytic Oxidation
Published on Jan 1, 2014
Plasma Electrolytic Oxidation (PEO) is a surface treatment for the production of ceramic oxide coatings with great properties, such as high wear and corrosion resistance, on metal substrates, particularly aluminum and magnesium alloys. Formation of PEO coatings involves complex processes and mechanisms that are difficult to study. Currently, the PEO process is in a transition phase from research to commercial application, with a primary focus on the corrosion and wear protection of light alloys, and has recently generated interest as a promising surface treatment for biomedical applications. To justify the industrial application of PEO, a more systematic and in-depth study of the influence of various parameters on the process is required. The control of the PEO process to yield the desired morphology and microstructure for specific applications is a key requirement if the process is to be industrially applied. The aim of the research in this thesis is to study the influence of electrical parameters, so they can be optimized to produce coatings with enhanced properties mainly for tribological applications. Alumina coatings were deposited on 6061 aluminum alloy substrates in an alkaline silicate electrolyte using a unipolar pulsed DC current mode. The influence of processing conditions, mainly electrical parameters (applied frequency, duty cycle, and current density), on the formation, growth behaviour and properties of PEO coatings were investigated. Different characterization methods including scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffractometry, microhardness tests, electrochemical impedance spectroscopy, and linear polarization resistance measurements were used to study the microstructure, morphology and properties of coatings. The correlation between the stage of the PEO process and the properties of the coating has been shown. The voltage-time response was found to be important since it provided readily measurable and useful information about these stages. It was found that using different electrical conditions the microstructure, morphology, growth rate, phase distribution and composition of coatings could be changed. To some degree corrosion performance could be tailored by adjusting processing parameters.