Vacuum thermal treated electroless NiP–TiO2 composite coatings

Composite NiP–TiO2 layers were prepared by simultaneous electroless deposition of Ni–P and TiO2 on steel substrate, from a solution in which TiO2 particles were kept in suspension by stirring. Deposits were characterized for its structure, morphology and hardness. It was found that the chemical composition of Ni–P matrix has been influenced by the incorporation of TiO2 particles. TiO2 particle incorporation increases with increase in their bath concentrations (0.5–2.0 g/l). An improvement (up to 20%) in microhardness was observed in both as plated and vacuum heat-treated composite coatings compared to Ni–P coatings. Electroless deposited composite coatings exhibit an amorphous structure of the nickel matrix in which crystalline titanium oxide is incorporated. Vacuum heat treatment leads to the formation of a crystalline layer in which the Ni and Ni3P crystallites appear apart from those of the TiO2 (anatase). Potentiodynamic polarization measurements made on these deposits in 3.5 wt.% sodium chloride solution showed decrease in the corrosion resistance for the as-plated and heat-treated composite coatings.