The Effects of Heating on the Microstructure and Surface Topography of Gold-Cobalt Electrodeposits

Abstract

Gold-cobalt deposits produced from an acid citrate bath have been examined after annealing in the temperature range 20–600°C. Microhardness measurements have shown that deposits soften with increased temperature, particularly beyond 250°C. Transmission electron microscopy (TEM) observations and X-ray diffraction measurements indicated that grain and pore sizes increased appreciably as softening was proceeding, whilst internal stress virtually receded to zero. After quenching from the annealing temperature to room temperature it has been shown that a small increase in hardness takes place with time. Deposits treated in this manner contained strain contrast effects as observed in the TEM; these entities could have resulted from the formation of fine cobalt precipitates. Scanning electron microscope examination of gold after annealing in air showed that the originally flat featureless surface progressively changes to one incorporating pores, ripples, blisters, craters and flakes of debris as the treatment temperature rises. Such deterioration in the surface could result from gas evolution and the flow of low viscosity solids onto the surface, for example from the partially decomposed polymer phase always found in these deposits.