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Top1. Introduction
As a chemical inquisitiveness, in the year 1844 Wurtz revealed that the metallic nickel could be deposited from aqueous solution in the presence of sodium hypophosphite. Eventually, Breteau (in the year 1911) and Roux (in the year 1946) obtained the bright coatings on all surfaces in the reaction vessels, from aqueous solution in the presence of the reducing agent. In the year 1946, Brenner and Riddell rediscovered electroless nickel coating, essential method of coating technique, and did extensive investigations on various aspects of the very interesting and attractive finding to coat the material using the supposed technique (Agarwala, Agarwala, & Sharma, 2006). Electroless nickel plating, also acknowledged as autocatalytic nickel plating, is produced by the catalytic reduction of nickel ions using sodium hypophosphite, which acts as the reducing agent and without the use of electric current (Brenner, & Riddell, 1946; Mallory, & Hajdu, 1990). Electroless coating can be broadly classified into four categories viz, pure nickel and black nickel coating, alloy and poly-alloy coatings, composite coatings and electroless nano coatings (Sudagar, Lian, & Sha, 2013). Electroless processes are grouped according to the use of reducing agent. For Ni-P coatings sodium hypophosphite is used as reducing agent whereas, for Ni-B coatings sodium borhydride and for pure nickel based coatings hydrazine is used as a reducing agent in the electroless bath. Electroless plating have nearly constant coating thickness across all surfaces, including edges and complex interior geometry. These coatings are more popular due to their properties like excellent wear, corrosion and frictional resistance, and hardness (Balaraju, Sankara Narayan, & Seshadri, 2003). An advantage of electroless nickel plating is, ability to coat interior surface of pipes, valves, and other parts of various materials, including metals (ferrous and nonferrous), plastic, glass and ceramics etc. These coatings have found wide applications in various industries such as, aviation, aerospace, electronics, petroleum, chemical, textile, machinery, and automotive (Momenzadeh, & Sanjabi, 2012).
The present review highlights the development of electroless nickel composite coatings and improvement in performance of the coating by incorporating composite (hard/soft) particles in the Ni-P matrix. The present work deals with the bath preparation for EN composite coatings, methods to incorporate second phase particles into electroless bath, investigation of influence of particle incorporation on the coatings and its effect on hardness, wear resistance, corrosion resistance, and friction coefficient. Correspondingly, the present study tries to investigate the effect of composite particles and heat treatment on the microstructure and phase transformation of coatings with reference to the valuable work on it by well-versed researchers to meet the ever-increasing demand in various challenging areas.