Characterisation of Ni–P electroless deposits with moderate phosphorus content

Abstract

The influence of the phosphorus content on hardness, structure, and morphology of electro less Ni–P deposits on an AISI 1020 steel substrate was investigated. The deposits were prepared from acidic plating solutions based on nickel chloride, as the source of nickel cations, and sodium hypophosphite, as the reducing agent. Coatings with an average of 4·68 and 6·65 wt-%P were obtained, which were analysed using glow discharge optical emission spectroscopy. Variation of phosphorus content with coating thickness was observed when the pH of the solution was not controlled rigorously. The coated samples were annealed at 170°C for 1 h in order to eliminate hydrogen, then post-treated for 25 h at 260°C or 1h at 400°C. X-ray diffraction studies indicated that the as deposited coatings presented a microcrystalline/amorphous or an amorphous structure depending on the phosphorus content. After both heat treatments, a strong (111) preferred orientation was found and coarsening of the grain sizes occurred with increase in temperature. The maximum hardness achieved was 916 HK(100 g) (0.63 GPa). Scanning electron microscopy studies have revealed a nodular, cauliflower type morphology. Extended cracks through the coating surface were observed when the heat treatment was performed at 400°C. A crack free surface was obtained when the heat treatment was performed at 260°C.