Plasma Electrolytic Oxidation (PEO) Process on Commercially Pure Ti Surface: Effects of Electrolyte on the Microstructur
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LOW density and elastic modulus, high specific strength, high corrosion resistance, and good biocompatibility properties are among the important factors that have led to extensive applications of Ti and its alloys in many industries such as aerospace, automotive, chemical, and biomedical.[1–3] In addition, formation of a thin passive layer (1.5 to 10 nm in thickness) on the surface of Ti and its alloys, as a result of exposure to the air or moisture at room temperature, protects these materials against oxidation and corrosion.[4,5] On the
ARASH FATTAH-ALHOSSEINI, MARYAM MOLAEI, and SEYED OMID GASHTI are with the Department of Materials Engineering, Bu-Ali Sina University, 65178-38695, Hamedan, Iran. Contact e-mail: [email protected] MOHSEN K. KESHAVARZ is with the Department of Mining & Materials Engineering, McGill University, Montreal, QC, H3A 0C5, Canada. Manuscript submitted November 20, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
other hand, tribological applications of these materials are limited due to low surface hardness and wear resistance and high friction coefficient.[6] Different methods such as electrodeposition, laser cladding, thermal spray, plasma spray, plasma immersion implantation, physical and chemical vapor deposition (PVD and CVD), and conversion coatings can be used for materials and thus form wear- and corrosion-resistant coatings.[7] However, the high cost and long duration of the process and weak adhesion of coating to the substrate in most such modification methods are undesirable.[8] Therefore, in recent years, plasma electrolytic oxidation (PEO) or microarc oxidation (MAO) technique has attracted great interest among the researchers due to some significant features like making compact, hard, and thick oxide coatings on the surface of light metals (Al, Mg, Zr, and Ti).[8–10] In general, weak alkaline and environmental-friendly electrolytes containing silicate, phosphate, and aluminate anions are used in PEO processes.[11] The PEO process is performed at higher voltages than the dielectric breakdown voltage of the coating. The breakdown of the oxide layer
happens at the same time as that taken for reaching the breakdown voltage. Consequently, plasma discharge channels, with local temperature range from about 2000 to 10,000 K, are formed.[12–14] Then, short-lived microdischarges with incidence of light and gas emission are produced steadily across the coating surface during the coating growth. These events allow the oxide coating to be formed.[12,15] Due to the involvement of the electrolyte ions into the oxide film by diffusion and electrophoresis processes during the oxidation,[16] PEO coatings contain mainly predominant oxide of the substrate metal and more complex compounds of the electrolyte components.[17] Also, due to the high pressure and temperature at the discharge channels, PEO coatings may contain phases which are not formed in the conventional anodizing.[18] In PEO method, coatings with high wear and corrosion resistance, high mechanical strength, strong adhesion to
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