SYNTHESIS, POST-DEPOSITION TREATMENT, AND CHARACTERIZATION OF NICKEL-PHOSPHORUS THIN FILMS USING ELECTROLESS DEPOSITION TECHNIQUE

Abstract

The utilization of Electroless Nickel-Phosphorus (EN) films has witnessed a staggering increase over the last two decades. Many outstanding characteristics of EN deposition technique have generated tremendous interest in various industries including oil and gas, electronic, chemical, automotive, aerospace, and mining. Some of the unique characteristics of EN films are superior corrosion and wear resistance especially in hostile environments containing H2S and CO2, superior mechanical properties, uniform film thickness, excellent surface finish, and superb adhesion. The EN deposition process is based on a redox reaction in which a reducing agent is oxidized and cations (Ni2+, P3+, H+ ions) are reduced on the substrate surface. Once a monolayer is produced, the deposited layer acts as a catalyst for subsequent deposition. If the reducing agent used is sodium hypophosphite, the deposit obtained will be a nickel-phosphorus (Ni-P) alloy with upto 15 weight percent phosphorus. During this research, the plating bath composition was formulated and optimized to obtain Ni-P films at reasonable deposition rates on copper, steel, and aluminum substrates. The molar composition of 0.47NiCl2: 0.23NaH2PO2: 0.13C4H4Na2O4: 3.47NaCl: 95.70H2O produced significant amount of film deposits on polished copper substrate at 80–90oC. The presence of chloride ions (Cl-) in the plating solution facilitated deposition without any need of surface activation. The films produced had typical thicknesses of the order of 1–2 μm with corresponding growth rate in the range of ~0.4–0.8 μm/h. Once experimental setup was further improved to ensure better temperature control and bath circulation, film growth rates as high as ~7–8 μm/h were achieved with minimal or no deposition on surfaces other than the substrate. Microstructural examination of the film surfaces using scanning electron microscope (SEM) and atomic force microscope (AFM) revealed granular, also referred to as cauliflower-like, morphology with average grain size of ~800 nm. Increasing the amount of reducing agent was found to increase the film smoothness and refine the grain size. EDS analysis of the film indicated deposit composition (by weight percent) of 91Ni–9.0 P. X-ray diffraction studies of the deposit revealed predominantly amorphous character with some degree of crystallinity. The average values of Knoop microharndess was ~390 KHN, which increased to ~800 KHN upon heat treatment at 400oC for 1 hour. This improvement in hardness can be attributed to deposit crystallization upon heat treatment, as corroborated by XRD spectrum of the heat treated sample.Films with such characteristics hold great potential for use in applications that require better wear and hardness properties. Acid etching of the deposits transformed shiny films into matte black surfaces. This can be attributed to the preferential removal of nickel leading to formation of porous structure with stalagmite-like morphology. EDS spectra of the etched surface showed an increased P content of ~17wt%. Etched electroless nickel is also referred to as nickel black due to its appearance. Such films can be used for application involving spectral or electronic absorption.