Electroless deposition of nickel is interesting from both the scientific as well as technological viewpoint. Electroless nickel (EN) is not pure but an alloy of nickel and phosphorus when deposited from baths containing sodium hypophosphite as the reducing agent. Depending on the bath chemistry and operating conditions, it is possible to vary 'P' content from 1 wt.% to about 15 wt.%. The phosphorus incorporated in EN plays a key role and drastically affects the structure and properties of EN. In the as deposited condition, the deposits with low 'P' content are crystalline and those containing more than 7 wt.% of 'P' are amorphous or nanocrystalline. These deposits are in a metastable state and on heat treatment, undergo structural changes. Electroless nickel is a promising engineering coating with unique properties. It exhibits better corrosion resistance compared to electroplated nickel and has comparable wear resistance as that of hard chromium, which exhibits excellent uniformity. Though the process was first reported about five decades ago, its penetration into the engineering industry is rather slow. Only recently (within a decade) fascination with EN has turned into an industrial excitement. The unique properties are being tapped for a large number of applications. Aerospace/aviation, automotive, chemical processing, food processing, oil and gas, electronics and a host of other industries have already turned to EN. The development of ternary alloys, EN composites and low phosphorus EN has widened the prospects for EN. It is also possible to co-deposit boron along with nickel, by using amino boranes or borohydride as reducing agent. Ni-B deposits are very hard and highly wear resistant. These coatings exhibit excellent solderability and bondability, and find applications in electronics industries. In this paper, the trends in this most exciting and potential research area are reviewed.
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