Electroless Deposition of Composite Materials: From Fundamental Mechanisms to Applications

Abstract

Electroless nickel phosphorus (Ni−P) plating is an autocatalytic reduction process that has been used for various engineering applications. High phosphorus metallic matrix materials produced by electroless deposition are examined to exploit the good corrosion behavior of such coatings and the high mechanical properties of the composites. In order to obtain the desired properties, it is important to deeply understand the deposition mechanism and its influence on the final characteristics of the layer. The kinetic mechanism of the electroless deposition of NiP matrix from hypophosphite solutions is investigated from first principles with the intent of determining a set of elementary reactions that could effectively identify the most relevant steps of this complex process. The outcome of the computational approach is discussed in terms of experimental results obtained by electrochemical methods and surface analysis techniques [1,2]. The interaction of Ni++ and H2PO2 - with the Ni surface was studied using Ni clusters of different sizes (from 4 to 12 atoms), introducing solvation in water through the implicit polarizable continuum model. Though several theories were proposed to describe this key step in the literature, our search was guided by the original insight published several years ago [3], according to which the reduction of Ni++ requires the formation of an intermediate species containing the NiOH group. Such species would then take active part in the transfer of the OH group to H2PO2 -, thus favoring its oxidation to H2PO3-. In order to understand completely the adsorption phenomena on the electrode surface during the electroless deposition process, in situ-Raman analyses were performed. The effect of each complexing agent dissolved into electroless NiP bath on complexes adsorbed on the electrode surface and the comparison with the DFT computational analysis will be presented, highlighting the effect of additives, e.g. tungsten salts, added to the electrolyte to modify the properties of the matrix. A tool based on computational fluid dynamics CFD simulation for the co-deposition of the dispersed phase is also presented, showing the correlation between particles’ load in the bath and the amount of co-deposited particles as a function of hydrodynamic conditions. The results are discussed in light of the theoretical models proposed to describe the co-deposition processes. Following the theoretical description, composite coatings are produced and characterized in terms of particles content and distribution, morphology and mechanical properties. Erosion resistance is specifically investigated with respect to sand particles fluxes, discussing the optimal combination between the metallic matrix and the dispersed phase. Corrosion behavior is presented both in electrochemical tests and in accelerated tests in salt spray chamber. The effect of the applied thermal treatment to modify the mechanical properties is investigated and discussed as a function of the dispersed phase. [1] P.L. Cavallotti, L. Magagnin, Influence of added elements on ACD electroless NiP, ECS Transactions, 50 (53) 1-8 (2013). [2] P.L. Cavallotti, L. Magagnin, C. Cavallotti, Influence of added elements on autocatalytic chemical deposition electroless NiP, Electrochim. Acta (2013), http://dx.doi.org/10.1016/j.electacta.2013.09.083. [3] P.L. Cavallotti, G. Salvago, Electrochimica Metallorum III, 239 (1968)