Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel
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JMEPEG DOI: 10.1007/s11665-016-2413-9
Corrosion and Nano-mechanical Behaviors of Magnetron Sputtered Al-Mo Gradient Coated Steel A. Venugopal, J. Srinath, P. Ramesh Narayanan, S.C. Sharma, and P.V. Venkitakrishnan (Submitted April 6, 2016; in revised form July 25, 2016) A gradient three-layer Al-Mo coating was deposited on steel using magnetron sputtering method. The corrosion and nano-mechanical properties of the coating were examined by electrochemical impedance spectroscopy and nano-indentation tests and compared with the conventional electroplated cadmium and IVD aluminum coatings. Electrochemical impedance spectroscopy was performed by immersing the coated specimens in 3.5% NaCl solution, and the impedance behavior was recorded as a function of immersion time. The mechanical properties (hardness and elastic modulus) were obtained from each indentation as a function of the penetration depth across the coating cross section. The adhesion resistance of the coatings was evaluated by scratch tests on the coated surface using nano-indentation method. The results show that the gradient Al-Mo coating exhibits better corrosion resistance than the other coatings in view of the better microstructure. The impedance results were modeled using appropriate electrical equivalent circuits for all the coated systems. The uniform, smooth and dense Al-Mo coating obtained by magnetron sputtering exhibits good adhesion with the steel substrate as per scratch test method. The poor corrosion resistance of the later coatings was shown to be due to the defects/cracks as well as the lesser adhesion of the coatings with steel. The hardness and elastic modulus of the Al-Mo coating are found to be high when compared to the other coatings. Keywords
EIS, low-alloy steel, pitting corrosion, sputtered film
1. Introduction Cadmium coatings are being used in the aerospace industry for the corrosion protection of steel fasteners due to its high electrical conductivity, lubricity and solderability added with sacrificial nature of the coating (Ref 1-4). The toxicity and environmental health hazards related to cadmium handling are well known and hence alternate coatings based on aluminum are considered in place of cadmium (Ref 5-8). Although aluminum coating by ion vapour deposition (IVD) has emerged as a suitable alternate coating, this coating needs a glass bead peening treatment along with a coating of MoS2 for improved corrosion and frictional properties (Ref 9-13). In the absence of this treatment, the as-deposited IVD coating exhibited inferior corrosion resistance due to the porous and columnar nature of the coating. Even with peening treatment, it is difficult to control the thickness and uniformity of the coating and hence the barrier property of the coating also found to vary depending on the surface quality of the coating. Aluminum coatings by other methods such as arc spray/hot dipping process also was found to give better properties only after post-coat peening treatment/additional top coating (Ref 14-16). Recently, coatings based on mag
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