Study On Preparation And High Temperature Properties Of Ag/Silver-Graphite Composite Coating For Outdoor High-voltage Isolation Switch

The silver coating and Ag/ silver-graphite composite coating were prepared on copper substrate by electroplating in cyanide system, respectively. In this paper,atmospheric corrosion resistance, machine life, thermal conductivity, high temperature wear resistance, high temperature corrosion resistance and high temperature contact resistance of composite coatings on the substrate of fine copper were investigated by scanning electron microscopy(SEM), field emission scanning electron microscopy(FESEM), energy dispersive spectroscopy(EDS), atmospheric exposure test, wear test in actual working conditions, thermal conductivity measurement, high temperature friction test, Tafel curve, contact resistance test and so on.The results showed that self-made Ag / silver-graphite composite coating were composed of silver coating and composite coating, being similar to the coatings prepared on contact finger by Siemens AG, but being not similar to the coatings prepared on contact finger by Shandong Tai Kai. In atmospheric exposure test, tarnish was observed on the surface of these coatings prepared by Siemens AG, the coaring of Shan Dong Tai Kai company and ourselves. Meanwhile, all of the coating’s weight increased, due to the existence of oxides, sulfides, dust and impurity attaching on the coating surfaces in natural environment. The weight gain of self-made Ag/silver-graphite composite coating was not too much, but the weight gain of self-made fine silver coating was the most and the corrosion of self-made fine silver coating was the most serious. The Ag/silver-graphite composite coatings were prepared on the contact terminal and contact finger. After contact terminasl and contact fingers with the Ag/silver-graphite composite coatings made the off-on operations of 10000times(the clamping force was equal to 500N), the migration amount of coating thickness for contact terminal in the planar low vice was less than 2 μm and the abrasion loss in the linear high vice friction of contact finger was less than 10 μm. The coating achieved the highest level of isolation switch mechanical life, namely M2 level standard.The thermal conductivity measurement of oxygen-free copper, fine Ag on oxygen-free copper substrate and Ag / silver-graphite composite coating on oxygen-free copper substrate showed that the order of for the composite coatings on the oxygen-free copper was fine Ag n oxygen-free copper substrate(402.528 W·m-1·k-1)> Ag /silver-graphite composite coating on oxygen-free copper substrate(372.577 W·m-1·k-1)> oxygen-free copper(340.395 W·m-1·k-1).The wear characteristics of the Ag/silver graphite composite coating in the friction test of high temperature were divided into two stages. In the first stage of room temperature to 120 ℃, the Ag/silver-graphite composite coating was dominated by the mechanism of abrasive wear and slight adhesive achieving the values of the friction coefficient of about 0.1 and the wear rate of about 10×10-14m-3/N·m(under load of 260g), the wear-resistant property of the coating was high. In the second stage of 240 ℃ to480 ℃, with temperature increasing, the desorption of Water vapor and oxygen ingraphite of the coating occurred, and “dust wear” had happened, meanwhile the friction coefficient was 0.44 and the wear rate was 56×10-14 m-3/N·m at 480 ℃.The composite layer was dominated by obvious abrasive wear and adhesive from 240 ℃ to480 ℃.The Tafel curve showed that self-corrosion current density of fine Ag coating and Ag /silver-graphite composite coating decreased, and self-corrosion potential of that increased, When the temperature increased from 120 ℃ to 600 ℃.At room temperature, the contact resistance of 24.9 μΩ(the torque is 5 N·m) for fine Ag coating was slightly larger than that of 19.4 μΩ and 19.9 μΩ for the silver-graphite composite coating( the torque is 5 N·m) and Ag/ silver-graphite composite coating(the torque is 5 N·m). At the range of 120 ℃ to 240 ℃, the contact resistance of three coatings had not big difference. When the heating temperature increased to 300 ℃, the contact resistance of Ag-graphite composite coating enhanced to the value of 40.3 μΩ. However, the contact resistance of the other two coatings had not nearly big change. At the range of 48 ℃ to 600 ℃, there had been the continuous decomposition in the silver-graphite composite coating, and the contact resistance was increased to the value of 123.3 μΩ. The contact resistance of fine Ag coating and Ag/Ag-graghite increased to 54.6 μΩ. and 42.6 μΩ, due to the existence of oxides and sulfides on the coating surface.