Promoteur : Prof. Fabienne Delaunois
The electroless nickel deposit is formed by the chemical reduction of nickel, which depends on the reducing agent used. Baths reduced by borohydride compounds provide nickel boron (NiB) alloys. The reaction must be managed to ensure that the reduction only takes place on the substrate and not on the tank and other equipment used for plating or in the solution far from the samples. In order to control this mechanism, additives can be added to the baths. Complexing agents, pH regulators and stabilizers are some of these additives. Stabilizers are the most effective agents in order to operate under adequate and stable conditions over extended periods.
Different bath compositions are nowadays successfully used to prepare NiB coatings for various applications. However, lead has been the industry standard as stabilizer for the full range of electroless nickel processes over the last years. Lead is a high toxic element whose wastes produced during processing and recycling of alloys can contaminate the water, air and soil.
In the present work, NiB deposits reduced by borohydride are produced from a lead-free bath. The replacement of lead as the bath stabilizer is effectuated, the properties of the lead-free baths are characterized, and the stabilization mechanism is studied.
• First, the stability of the lead stabilized bath and the morphological, chemical, mechanical and corrosion properties are characterized. Before replacing the lead salts stabilizer, the influence of the anionic part of heavy metals salt stabilizers (like WO4-2 as NO3-2) on the bath stabilization process is investigated.
• The optimization of an electroless NiB bath exempt of stabilizer is reached by the addition of a specific concentration of complexing agents and pH regulator. This concentration is determined by a design of experiment scheme. • The stabilization properties of a large number of metallic cations are measured and a new theory about the stabilization of the electroless nickel-boron bath is presented. Additionally, electroless NiB coatings were prepared using two new environmentally friendly baths stabilized by bismuth or tin. These systems do not use either lead or thallium and can successfully replace traditional lead-containing chemistry for different applications. These new coatings, when compared with the classical bath (NiB-Pb),
present modified chemical composition, growth mode, morphology and mechanical properties.
• Finally, the properties of all the coatings presented in this work are compared and the influence of the stabilizer on the coatings properties is discussed.
Keywords: Electroless nickel; nickel-boron; stabilizer; bismuth; tin.