Effect of reducing agent and nano Al2O3 particles on the properties of electroless Ni-P coating

This work is an experimental study on the formation, characteristics and properties of electroless nickel phosphorous (Ni-P) coatings and electroless nickel alumina (Ni-P-Al2O3) coating with varying reducing agent concentration. The results obtained indicate that the deposition rate and surface roughness of both Ni-P coating and Ni-P-Al2O3 coatings are highly influenced by reducing agent (sodium hypophosphite). With increase in sodium hypophosphite Ni forms amorphous phase and as a result the micro hardness of the coating gets reduced. Heat treatment was carried out at 400 °C for 1 h after the coating, resulted in the formation of an intermetallic nickel phosphide (Ni3P) phase which improved the hardness of the Ni-P coating from 400 ± 25 to 700 ± 25 HV. A composite coating is formed due to the incorporation of nano alumina in the Ni-P coating leading to an increase in the hardness. The chemical composition of nickel gets reduced from 85.3 to 77.8 wt.% due to the presence of alumina which resists the deposition of Ni. The uniform distribution of alumina particles are observed using a scanning electron micrograph and confirmed by X-ray diffraction techniques. The specific wear rate of Ni-P-Al2O3 coated and post coating heat treated specimens was observed to be less when compared with that obtained in the case of conventional Ni-P coating.