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Top1. Introduction
Electroless coating is a type of coating method in which metallic coatings are formed as a result of a chemical reaction between the reducing agent present in the coating solution and the metallic ions. For electroless nickel deposition, nickel ions are added to the solution in the form of nickel salts. One of the main uses of electroless nickel coating has been in the aerospace and automotive industries for its splendid tribological properties. Electroless nickel coatings have been applied to engine-related applications which include coating of compressor blades, piston heads, main shafts, bearing liners, carburetors, fuel pumps, etc. Electroless nickel coating has certain procedural advantages over the conventional electrolytic coatings. These mainly include uniformity of the coating even on intricate geometries and their ability to coat non-conductors. Moreover, the electroless nickel coatings are found to possess improved protection against corrosion and wear. The primarily used variety of electroless nickel is the Ni-P coating, though expanding needs have given rise to coatings viz. Ni-B, Ni-W-P, Ni-P-SiC, Ni-P-TiO2, Ni-P-PTFE and so on (Sahoo and Das 2011; Sahoo 2008a-d; Sahoo and Pal 2008).
One of the unique characteristics of electroless Ni-P deposition is the superior wear resistance of the coatings. Theoretically, there is a correlation between wear resistance and hardness of a surface. However, the wear properties of a surface are affected by numerous other parameters such as the nature of the applied stress and the surface morphology. The wear resistance of electroless Ni-P deposits depends on both phosphorus content and the type of heat treatment applied. It has been observed that the highest wear resistance and hardness were obtained for samples heat treated at 400ºC for 1h (Staia et al. 1996). In general, heat treatment increases hardness and hence the wear resistance of the coating but the grain coarsening at higher heat-treatment temperatures negatively affect the wear resistance of the coating.