Electrodeposition of Metastable Au–Ni Alloys

Abstract

The electrodeposition of Au-Ni alloys from near-neutral, sulfite-based electrolytes derived from a commercial bath for soft gold plating is investigated. Alloy compositions ranging from 0 to 90 atom % Ni were obtained by varying the deposition potential, with Ni content increasing with overpotential. Cathodic efficiency was lower than 50% due to concurrent parasitic reactions, including the reduction of products from the decomposition of sulfites and the hydrogen evolution reaction. As-deposited films form a continuous series of metastable solid solutions and exhibit a nanocrystalline morphology, with grain size decreasing with increasing Ni content and a possible Ni enrichment at the grain boundaries. Thermal annealing at 200°C was sufficient to start the relaxation of the metastable solid solution toward the thermodynamically stable biphasic configuration of pure Au and Ni phases; however, 400°C was necessary to complete the phase separation process within ∼1 h. The formation of a metastable structure is interpreted in terms of the limited surface diffusivities of adatoms at the growing interface and atomic volume differences. The excess free energy of the as-deposited alloys with respect to the stable, phase separated configuration is estimated between 6 and 18 kJ/mol, consistent with what can be expected in electrochemical processing. © 2010 The Electrochemical Society.