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 alloy coatings on surface. The technique involves the autocatalytic reduction, at the substrate/solution interface, of cations by EL bath released from suitable chemical reducing agents. 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 electroless coatings are being studied at Indian Institute of Technology Roorkee since 1985. The structural and morphological behavior of Ni-P coatings for different phosphorous contents has been extensively studied. Sub-micron 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. The realization of mechanical bonding along with chemical bonding between EL coating and the substrate has been explained by coated copper on ceramic powder. As a forward step towards composite coatings, Ni-P-C, Ni-P-Al2O3, Ni-P-ZrO2 has been developed by EL co-deposition technique. Ag-graphite coatings produced by EL technique exhibits nearly five times higher wear resistance and nearly two times better corrosion resistance apart from a good electrical conductor. The tribological behavior of electroless 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 nanocoating (X= ZrO2-Al2O3-Al3Zr), X has been produced in-situ and are of nanosize particles. Such coating could be done on carbon fibre of 7µ diameter uniformly. Ni-P-ferrite nanocoatings with thickness less than nearly 1mm thick, is exhibiting good absorption of microwave in the range of 12-18 GHz which can be exploited for radar applications. Micro-thickness coatings are paying ways to nanocoatings. Nanocoatings are the coatings in which either the thickness of the coating is in nanolevel or second phase that dispersed in the coat matrix is of nanosize. To further explore the field of EL nanocomposite coatings, now days, a work on EL Ni-P-ZnO, TiO2, Al2O3, ZrO2 and Ni-B-ZrO2 for its mechanical properties has been carried out.
FUNCTIONAL COATINGS APPLICATION FOR STRENGTHENING AND RESTORATION OF AVIATION PRODUCTS
