Kinetics and Properties of Micro Arc Oxidation Coatings Deposited on Commercial Al Alloys
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THE microarc oxidation (MAO) technique has, in recent years, emerged as a unique technique capable of depositing the dense, ultrahard ceramic composite coatings on Al and its alloys. The process for depositing the oxide coating on aluminum anode under an arc discharge condition was first reported by Markov and co-workers in the 1970s.[1] Subsequently, Snezhko et al.[2,3] in the early 1980s, Kurze et al.[4,5] in the mid 1980s, Gordienko et al.[6,7] in the late 1980s, and Klapkiv et al.[8] in the early 1990s contributed to the development of the MAO process. However, the MAO process gained worldwide recognition as an eco-friendly technology for depositing the tribologically superior ceramic coatings on aluminum and its alloys by the pioneering research contributions made by Yerokhin et al. of Tula State University (Russia) in the 1990s.[9,10,11] The research related to development of the MAO process has intensified in the last 5 years. Voevodin and co-workers in the United States,[12] Matthews and co-workers in the United Kingdom,[13] Wang and co-workers in China,[14] Sundararajan et al. in India,[15,16] and Mecuson et al. in France[17] have contributed to the further development of the MAO technique in conjunction with improving our underL. RAMA KRISHNA, and NITIN P. WASEKAR, Scientists, A. SUDHAPURNIMA, Junior Research Fellow, and G. SUNDARARAJAN, Director, are with International Advanced Research Centre for Powder Metallurgy and New Materials, Hyderabad 500005, India. Contact e-mail: [email protected] Manuscript submitted May 23, 2006. 370—VOLUME 38A, FEBRUARY 2007
standing of the mechanism of coating formation and coating properties. The overall wear resistance offered by the MAO coatings under the abrasion, erosion, and sliding wear modes was found to be comparable to that of the bulk alumina and better than the thermal-sprayed alumina and hard anodized coatings.[18] In this context, the MAO coating technology has been proven to be an affective, eco-friendly substitute to the conventional acid-based anodizing techniques. Several independent investigations have been carried out to investigate the tribological performance of MAO coatings deposited on almost all commercial Al alloys. However, the majority of studies have been concerned with the coating deposition on one or two alloys only. There exists only one study wherein 12 different aluminum alloys have been used for coating deposition through the MAO technique.[19] However, most of these alloys are not of commercial grade and the reported phases contain metastable d-Al2O3, which is not generally observed in the present, state-of-theart MAO coatings.[15] In addition, the microhardnesses of coatings deposited on different alloys were compared with each other even though the final coating thickness was not identical on all alloys. It is well known that the MAO coating properties (e.g., peak hardness) are largely dependent on the final coating thickness, because the proportion of the a- and cAl2O3 phases in the coating is itself dependent on the coating thickn
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