Effect of synthesis conditions and post-deposition treatments on composition and structural morphology of medium-phosphorus electroless Ni-P films

The optimized plating bath composition (mol%) of 0.47NiCl₂:0.23NaH₂PO₂:0.13C₄H₄Na₂O₄:3.47NaCl:95.70H₂O produced Ni-P films with ~ 10 wt.% P at 85 °C over copper and steel substrates. The presence of chloride ions (Cl⁻) in the plating solution was found to facilitate initial deposition without any need of surface activation. Film growth rates as high as 7-8 μm/h were achieved with minimal or no deposition on surfaces other than the substrates. Scanning electron microscope (SEM) and atomic force microscope (AFM) studies revealed cauliflower-like morphology with submicron size grains. Increasing the amount of reducing agent increased film smoothness and refined grain size with corresponding increase in P content. X-ray diffraction studies of the deposit revealed semi-crystalline nature of the film which underwent transition to fully crystalline form upon heat treatment with an associated increase in average HK value from ~ 390 to 807 HK. High temperature provided driving force for surface and volume diffusion that subsequently led to reduced porosity and more homogeneous composition through rearrangement of atoms to form certain Ni_xP_y phases. Acid etching of the deposits transformed shiny films into matte black surfaces due to preferential removal of nickel leading to formation of porous structure with stalagmite-like morphology.