Graduation Semester and Year

2017

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

First Advisor

Efstathios I. Meletis

Second Advisor

Harry F Tibbals

Third Advisor

Pranesh Aswath

Fourth Advisor

Ali Abolmaali

Fifth Advisor

Choong-Un Kim

Abstract

Plasma electrolytic oxidation (PEO) is an environmentally friendly technology capable of forming coatings with excellent adhesion strength at high deposition rates. The PEO process is particularly attractive for forming desirable oxide-based coatings in transition metals (Al, Ti, Mg) to improve their surface sensitive properties. At present, the PEO mechanism is not fully understood since the process includes complex processes that are difficult to study. However, understanding the PEO mechanism is essential to produce coatings with desirable characteristics for a variety of new applications. One critical PEO process parameter is the applied total current to the workpiece. The total applied current in this process is composed of electronic current caused by sparking and ionic current caused by diffusion of electrolyte ions into the oxide. In the present work, a wide spectrum of current densities was applied on commercially pure titanium in alkaline electrolytes to investigate its effect on the voltage response and produced coating characteristics. The growth mechanism and oxide characteristics were investigated by studying the correlation between the ionic/electronic current contribution rate at different current densities during the PEO stages. The voltage-time response was found to be essential because it enables the quantification of the information about different PEO phases and correlate that information with the growth mechanism. It was found that at low current densities (30 and 40 mA/cm2), the contribution of electronic current is dominating and a large number of discharge channels develop in the oxide. However, in this case there would not be enough ions (low ionic current) to diffuse through the discharge channels for reaction. High density of plasma discharges at this condition, forms large number of discharge channels and increases the porosity and surface roughness of the coating. Also, these discharges provide enough energy to raise the temperature facilitating formation of both stable rutile and metastable anatase phases. By increasing the current density, the incorporation rate of ionic current increases which results in the formation of dense anatase coatings. It was found that in order to achieve high growth rates, an equal or balanced contribution from ionic (diffusion of ions for reaction) and electronic (generation of channels) charges is required. Transmission Electron Microscopy studies showed that all coatings are composed of two layers: an amorphous layer on the top produced by quenching from the electrolyte and a composite layer close to the substrate. The bottom composite layer has a complex microstructure consisting of nanocrystalline phases due to fast nucleation rate, amorphous structure and nano pores. The thickness ratio between the bottom complex layer and the top amorphous layer increases by increasing the applied current density. Thus, the present research revealed clearly that both the electronic and ionic current play a critical role in the PEO process and a balanced contribution is needed to realize the benefits in the oxide growth process. Furthermore, it is shown that electrolyte properties including composition and conductivity have significant effects on breakdown voltage and contribution of the ionic charge and as a result coating characteristics.

Keywords

plasma electrolytic oxidation, titanium, coating growth mechanism, discharge

Disciplines

Engineering | Materials Science and Engineering

Comments

Degree granted by The University of Texas at Arlington

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