Comparison in the Oxidation and Corrosion Behavior of Aluminum and Alumina-Reinforced Ni/Ni-Co Alloy Coatings
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TRODUCTION
NICKEL aluminides are excellent coating materials for high-temperature protection due to their ability to form thermodynamically stable Al2O3 scale.[1–4] These aluminides have attracted a lot of interest due to their diverse applications in the aerospace and power industries on account of their properties like low density, high strength, and good oxidation resistance at elevated temperatures. These coatings are deposited by techniques like physical vapor deposition, magnetron sputtering, or by low-pressure plasma spraying.[5] The electrodeposition technique has attracted a lot of interest to develop composite coatings due to the inherent advantages like simple processing, ease of fabrication, cost effectiveness, high productivity, good compositional control, excellent metallurgical bond, extremely low porosity, and no thermal effect on the base material.[6,7] Researchers have reported different aspects of electrodeposited Ni aluminide coatings.[1,4–11] Susan et al.[6] and Bilnik et al.[10,11] obtained a Ni-Al electrodeposited coating with a uniform distribution of Al particles in the Ni matrix with a fine, columnar MEENU SRIVASTAVA, Senior Scientist, J.N. BALARAJU, Principal Scientist, and V.K.WILLIAM GRIPS, Consultant, are with the Council of Scientific and Industrial Research, Surface Engineering Division, National Aerospace Laboratories, Bangalore 560 017, India. B. RAVISANKAR, Associate Professor, is with the Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620015 TN, India. Contact e-mail: [email protected] Manuscript submitted March 6, 2012. Article published online October 10, 2012 696—VOLUME 44A, FEBRUARY 2013
morphology. Defects like cracks or porosity were not present in the coating.[3,11] They observed a maximum microhardness of 400HVN for the Ni-Al coating, and with a further increase in Al particle content, the softness of the particles played a role and resulted in decrease in the values to 300-350HVN.[3,11] Zhou et al.[7] emphasized that the codeposition of small Al particles in Ni matrix results in the formation of fine grain deposits, thereby imparting a better oxidation resistance. Liu and Chen[8] investigated the effect of porosity on the oxidation behavior of annealed [1123 K (850 °C)] electrodeposited Ni-Al coatings. Coatings with the porosity levels less than 7 pct showed the formation and retention of a thin, compact Al2O3 film on the surface. An increase in the porosity levels to 11 pct resulted in cyclic oxidation behavior with an initial low weight gain followed by a quick formation of protective oxide film, and subsequently, pore healing reduces the chances of fracture of the oxide film. Zhou et al.[9] have reported that the oxidation kinetics of Ni-28Al composite is almost as good as that of magnetron-sputtered CoCrAlY coating. Although enormous information is available on the development and behavior of electrodeposited Ni aluminide coatings, scarce literature is available on the Ni-Co aluminide coating. The authors have ob
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