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

Abstract 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–Al 2 O 3 ) 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–Al 2 O 3 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 (Ni 3 P) 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–Al 2 O 3 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.