Development of nanocrystalline Ni-Al coatings and its thermal stability

Abstract

In this paper, Ni-Al compound coatings were electrodeposited from a nickel sulphate electrolyte containing metallic Al particles (1-3um). The deposition conditions adopted were: compound content 50 g/l, pH 4.0, and present density 1.6A/dm'. At electrode position, the particles were held in suspension by magnetic stirring. The deposition was carried out on a stainless steel substratum and the coating was mechanically Segregated out of the substratum as an electroform after completion of the deposition. The thermal sustainability of the Ni-Al coatings was observed by annealing electroforms at temperatures of 200°, 400°, 600°, and 800 °C for 6hrs in an electric furnace under normal conditions. To further enhance the thermal stability of Ni-Al coatings, the cobalt element has been code posited with the addition of cobalt sulphamate in Ni electrolytic bath to obtain Ni-Co alloy matrix containing Al particles. The electrodeposited coatings were characterized for their shape and the phases formed during annealing using the X-ray diffraction technique (XRD). The Al particle distribution was studied using optical microscopy (OM) and field emission scanning electron microscopy (FESEM). The Ni-Aland Ni-Co-Al coatings were also characterized by their cross-sectional micro hardness. As these Ni-Al coatings tend to form Al2O3; upon exposure to high temperatures, a comparative study of the structure, microhardness, thermal stability has also been made between Ni-Al. Ni-Co-Al and Ni-Al2O3; coatings. The Ni-Al2O3; (um) composite coating was obtained under the same conditions as that of the Ni-Al coating. The properties of the three coatings were compared and discussed.