High-Cycle Fatigue Behavior of Microarc Oxidation Coatings Deposited on a 6061-T6 Al Alloy
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ALUMINUM and its alloys have found extensive use in the textile, food processing, aerospace and automobile industries due to their high specific strength and stiffness; however, they exhibit relatively poor corrosion and tribological properties. Therefore, for many applications, coatings with good corrosion/wear resistance are specifically applied on Al-alloy components. Among the coating options, hard-anodized (HA) (porous, amorphous Al2O3) coating is the most popular.[1] The HA coatings, although largely used for cosmetic purposes, impart reasonable corrosion and wear resistance.[2–7] Microarc oxidation (MAO) done is an ecofriendly coating technique used for depositing hard crystalline Al2O3 coatings on Al alloys. These coatings provide outstanding wear and corrosion resistance, especially when compared to HA, and thus can potentially replace HA coatings in applications requiring substantial wear/corrosion resistance.[8–11] In many applications, the bulk Al-alloy components over which HA or MAO coatings are applied have been chosen based on their bulk properties. Under such circumstances, it is important that either the coating process or the coating itself does not result in the degradation of the bulk properties of the underlying Al alloy. One such bulk property, appropriate for many applications using Al alloys, is the high-cycle fatigue NITIN P. WASEKAR, N. RAVI, P. SURESH BABU, and L. RAMA KRISHNA, Scientists, and G. SUNDARARAJAN, Director, are with the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), Hyderabad 500 005, India. Contact e-mail: [email protected] Manuscript submitted April 28, 2009. Article published online October 30, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
(henceforth called fatigue) resistance. In this regard, it has been observed that the fatigue life of the HA Al alloy is considerably lower (even up to 75 pct) than that of an uncoated Al alloy.[12] It was also observed that shot peening of the base Al alloy prior to hard anodizing improves the fatigue life.[13] Limited data are available in the literature on the influence of the MAO coating on the fatigue life of Al alloys. Lonyuk et al.[12] have carried the first study in this direction comparing the influence of 65-lm-thick HA and MAO coatings on the fatigue life of the Al alloy. Their results indicate that the HA reduced the fatigue life by 75 pct while the MAO coating was marginally better and reduced the fatigue life by 58 pct. Asquith et al.[14] observed that shot peening of the Al alloy followed by MAO coating improved the fatigue life compared to the coating without shot peening. The fatigue data, specifically on the hard MAO coatings formed on the softer Al alloys, are quite limited. However, considerably more fatigue data are available with regard to the generic case of a hard coating on a soft substrate. For example, Suresh et al.[15] noted that when the fatigue crack is propagated from the stronger to the weaker steel, crack growth occurs unimpeded through the interface. In co
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