Studies on Electroless Coatings
Since the introduction of Electroless (EL) coating in 1946 by Brenner and Riddle, the process has been the subject of steady growth. It is one of the most elegant methods available for the production of protective coatings on surface. The technique involves the autocatalytic reduction, at the substrate/solution interface, of cations released from suitable chemical reducing agents with in the EL bath. EL coating technique is simple one, as can be manifested just by controlling pH and temperature of the coating bath. Such coatings are reported to provide excellent physical and mechanical properties. The nickel, silver and copper based alloy and composite EL coatings are being studied at Indian Institute of Technology Roorkee since 1985 and this paper deals with the gist of the same. The structural behavior of Ni-P coatings for different phosphorus contents has been extensively studied. Transmission electron microscopy (TEM) and magnetic movement studies have been used as tools for structural and kinetic studies, respectively. Submicron size coating islands and their transformations have been deduced. The metallography studies using hot stage within TEM to follow the phase transformations occurring at various temperatures have been studied for Ni-B, EL coatings. As a forward step towards composite coatings, Ni-P-C, Ni-P-Al2O3, Ni-P-ZrO2 etc. have been developed by co-deposition techniques. Ag-graphite composite coatings produced by EL technique exhibits ~5 times higher wear resistance and ~ 2 times better corrosion resistance apart from being a good electrical conductor. The tribological behavior of EL Ni-P-X and Ni-P coatings on steel and aluminium substrates in different conditions i.e., as-coated, heat treated at various temperatures at different extents of time with different normal loads, have been studied in terms of dry sliding friction and wear against counter face of case hardened steel. In Ni-P-X nanocoatings (X= ZrO2-Al2O3-Al3Zr), X has been produced in-situ and are composed of nano-sized particles. Such coating could be produced uniformly on carbon fibre of 7µ diameter. Ni-P and Cu are also coated successfully on graphite/ oxide powders of ~ 120µ sizes. Micro-thickness coatings are paving ways to nanocoatings. These are the coatings in which either the thickness of the coating is in nano level or the second phase, that dispersed in the coating matrix is of nano-size. Ni-P-Ferrite nano coatings with thickness less than ~0.1mm thick, is found to exhibit about 20db of absorption of microwave in the rage of 12-18 GHz which can be exploited for radar applications. Here the nano-sized ferrite particles are co-deposited along with the Ni-P EL coatings.
M. Danielewski, R. Filipek, R. Kozubski, W. Kucza, P. Zieba, Z. Zurek
R. C. Agarwala "Studies on Electroless Coatings", Defect and Diffusion Forum, Vols. 237-240, pp. 952-957, 2005