Homogeneous functional Ni-P/ceramic nanocomposite coatings via stable dispersions in electroless nickel electrolytes

Stable nanoparticle dispersions in concentrated electrolytes are prerequisite for a variety of advanced nanocomposites prepared by deposition techniques. In this work we investigate the synthesis of electroless Ni-P/functional ceramic coatings from concentrated electrolytes containing functional nanoparticles such as TiO2, alpha-Fe2O3, ITO, and CeO2. Stable nanoparticle dispersions in both low and high phosphorus electrolytes are achieved at plating temperatures (80-90 degrees C) by a generalized scheme employing comb-polyelectrolyte and antifreeze additives. Dispersion stability at room temperature is achieved in both low and high phosphorus EN media using anionic comb-polyelectrolyte surfactants with polyether side chain of 1100 g/mol. The optimal surfactant concentration is determined by zeta-potential and thermo-gravimetric analysis. Without additives the dispersions flocculate and sediment between 65 and 80 degrees C. Such phenomenon is believed to be associated with a critical flocculation temperature (CET). The CFT is also weekly dependent on the particle type and the high ionic strength media. Addition of antifreeze additives such as propylene glycol and urea to the dispersions restores stability and increase the CET for all particles. We estimate an average increase of the CFT by 1.5-2 degrees C per 1% additive for all particles and electrolytes. While the particle stabilization scheme is generalized in this work, the composite EN plating proved highly dependent on particle type. Baths containing ITO nanoparticles showed no plating reactions and those containing alpha-Fe2O3 no nanoparticle co-deposition. In contrast, homogeneous Ni-P/TiO2 and Ni-P/CeO2 nanocomposites with up to 22 vol.% nanoparticles are produced. The possible application of the stabilization principles developed here for other functional nanocomposite systems is discussed. (C) 2011 Elsevier Inc. All rights reserved.