Electroless Ni-P-Al 2 O 3 composite coatings have found wide range of applications due to their high hardness and wear resistance. The present study triesto investigate the influence of coating process parameters on the microhardness of electroless Ni-P-Al 2 O 3 composite coating with the help ofTaguchi analysis. Four parameters, namely, concentration of nickel sulphateas a nickel source, concentration of sodium hypophosphite as a reducing agent, concentration of Al 2 O 3 particles as concentration of second phase particles, and annealing temperature, are considered and fitted into anL 27 orthogonal array to find out the optimized condition for improvedhardness of the coating. The optimized condition is found to yield about 20.47% improvement in hardness of the coating comparedto the initial condition. The significance of the process parameters and their interactions on the hardness of electroless Ni-P-Al 2 O 3 composite coating isstudied with the help of analysis of variance, which revealed that annealing temperature and concentration of second phase particles (Al 2 O 3 particles) have significant influence on the hardness characteristics of electroless Ni-P-Al 2 O 3 composite coating. The compositional, microstructural, andphase structure analyses are conducted with the help of energy-dispersive X-ray analysis, scanning electron microscope, and X-raydiffraction analyzer, respectively.

Electroless nickel composite coatings possess excellent mechanical and tribological properties such as, hardness, wear and corrosion resistance. Composite coatings can easily be coated not only on electrically conductive materials but also on non-conductive materials like as fabrics, plastics, rubber, etc. This review emphasizes on the development of electroless nickel composite coatings by incorporating different types of hard/soft particles (micro/nano size) in the electroless Ni-P matrix to improve the mechanical and tribological properties of the coatings. The preparation of electroless bath for nickel-phosphorus composite coating, methods to incorporate hard and/or soft particles in the bath, factors affecting the particle incorporation in the coating and its effect on coating structure, hardness, wear resistance, friction behavior, corrosion resistance, and mechanical properties are discussed thoroughly.

The present study investigates the e®ect of titania particles on the micro-hardness, wear resistance, corrosion resistance and friction of electroless Ni-P-TiO 2 composite coatings deposited on mild steel substrates at di®erent annealing temperatures. The experimental results con¯rmed that the amount of TiO 2 particles incorporated in the coatings increases with increase in the concentration of particles in the electroless bath. In presence of TiO 2 particles, hardness, wear resistance and corrosion resistance of the coating improve signi¯cantly. At higher annealing temperature, wear resistance increases due to formation of hard Ni 3 P phase and incorporation of titania particles in the coated layer. Charge transfer resistance and corrosion current density of the coatings reduce with an increase in TiO 2 particles, whereas corrosion potential increases. Microstructure changes and composition of the composite coating due to heat treatment are studied with the help of scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDXA) and X-ray di®raction (XRD) analysis.

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