Study On The Preparation Process And Properties Of Silver-graphite Composite Coating By Non-cyanide Electroplating For Outdoor High-voltage Isolation Switch

The silver-graphite composite coatings combine excellent characteristics of pure silver(i.e.high conductivity and low contact resistance)and graphite(i.e.self-lubrication,high corrosion resistance and anti-arc erosion),which have been widely applied in the electrical contact materials for outdoor high-voltage isolating switch.In this paper,the dispersion behavior on different additives to flake graphite in aqueous medium as well as the ingredient of dispersant and assistant agent was investigated.The silver-graphite composite coatings on the substrate of pure copper were prepared by a method of electro-deposition in a mixed electrolyte of the non-cyanide system of potassium iodide(KI)with the as-prepared the dispersant and assistant agent.The properties(i.e.wear resistance,corrosion resistance,sulfur resistance,thermal conductivity,conductivity)of composite coatings were investigated by friction and wear experiment,microscopic observation,scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),Tafel curve,X-ray diffraction(XRD),sulfur resistance experiment,thermal conductivity test and resistivity test,respectively.The results showed that the optimal formula of the dispersant(i.e.lauryl sodium sulfate of 7.7 wt%,N-acetyl-L-proline of 2.7 wt% and tributyl phosphate of 1.6 wt%)and the assistant agent(i.e.sodium dodecyl benzene sulfonate of 2.2 wt%,ethyl acetate of 2.0 wt%,ethylene glycol of 0.8 wt% and isooctyl acetate of 0.7 wt%),were obtained through the optimized experiments.The flake graphite that was dispersed by using the as-prepared dispersant and assistant agent,was uniformly distributed in the silver-graphite composite coating,which ran up to the dispersion effect compared with that of German Siemens Corp.With increasing the amount of graphite in the mixed electrolyte for the non-cyanide KI system,the graphite content in the silver-graphite composite coating increased.Meanwhile,wear rate for the composite coating firstly decreased and then increased.As the deposition current density increased,the graphite content in the silver-graphite composite coating increased.At the same time,wear rate for the composite coating firstly decreased and then increased.With increasing the stirring speed,the graphite content in the silver-graphite composite coating firstly increased and then decreased.Meanwhile,wear rate for composite coating increased at first,decreased later and then increased.The optimum preparation process for the preparation of the composite coating was graphite of 60 g/L in the mixed electrolyte,deposition current density of 0.35 A/dm2 and stirring speed of 700 r/min.The optimalsilver-graphite composite could achieve the wear rate of 2.457×10-11 mm3·N-1·m-1 and average friction coefficient of 0.178 which were far less than that of 6.501×10-11mm3·N-1·m-1 and 0.515 of pure silver coating.Moreover,wear resistance and thermal conductivity were better than that of the pure silver coating,and the binding force of the composite coating on the pure copper substrate met the qualification of GB/T5270-2005.