Role of substrate on formation process and properties of plasma electrolytic oxidation coatings
This thesis is aimed at revealing the influence of substrate microstructure/properties on PEO coating formation process, morphology, microstructure, composition and properties, as well as the evolution behavior of the coatings in presence of additional phases and intermetallics.The results show that, the presence of additional phases and intermetallics (caused by alloying elements), can be responsible for fluctuation of the voltage response and modified characteristics of the discharges during the processing. Moreover, the coatings are not uniform especially for short treatment duration due to the local morphology and microstructure of coatings on the intermetallics, such as Mg17Al12, Al-Nd, Al-Mn-Nd in AZ91Nd alloy, and Al2Cu and Al5FeSi in AlSi9Cu3 alloy. This heterogeneity of the coating can be fully eliminated by either increasing the current density or prolonging the processing time. However, it could be detrimental to the quality of the resultant coating. New phases eg. MgAl2O4 in the coating on Mg composite are found due to the incorporation of the phases/alloying elements reacting with other coating forming species. Before the breakdown phenomena of the process, the oxidation of the substrate is controlled by the electrochemical stability of different phases in the substrate, for instance, Mg17Al12/α-Mg and Al2Cu/α-Al. When the discharges are generated, the reaction of the substrate/coating is controlled by the electrochemical property together with the chemical stability and melting point of the additional phases and intermetallics, regarding their reactive and/or inert incorporation into the coatings. In addition, the in-situ PEO study of Ti alloy demonstrated a different oxidation behavior of Ti in comparison to Al and Mg, featured by the lattice expansion due to the oxygen penetration.