Influence of SiO2 nanoparticles on hardness and corrosion resistance of electroless Ni-P coatings

Nickel-phosphorus (Ni-P) coatings with controllable P content and desirable characteristics can be produced by tuning bath chemistry and processing conditions during the electroless process. Although pure medium- and high-P coatings offer a good combination of mechanical and corrosion properties, their performance attributes can be further enhanced through incorporation of silica (SiO2) nanoparticles. Using 15 and 30g/L of sodium hypophosphite as reducing agent in the plating bath, Ni-P coatings with respective P content of 10.8 and 14.3wt.% were produced. While the surface roughness and grain size increased with an increase in the P content, the average hardness value and corrosion resistance of medium-P coatings were superior to their high-P counterparts. The average hardness of the Ni-P- SiO2 coatings based on medium-P Ni-P matrix was determined to be ~11.0GPa. Electrochemical impedance spectroscopy (EIS) studies of these coatings indicated area-impedance values on the order of 108Ω·cm2, whereas the double-layer capacitance per unit area (Cdl) and charge transfer area-resistance (Rct) values, computed from theoretical fit of the EIS data, were in the range of 1.18×10-10F·cm-2 and 3.14×108Ω·cm2, respectively. The findings confirmed that addition of SiO2 nanoparticles modifies deposit morphology through grain refinement, reduction in the surface roughness and minimization of surface porosity in the nanocomposite coatings.