Study On The Preparation Of Ag-Graphite Self-Lubricating Electro-Contact Composite Coatings

Silver is widely used in industry as electro-contact materials. However, pure silver shows low hardness, weak wear resistance and weak resistance against welding; on many occasions, especially on the occasion of sliding contact, a new kind of materials which own both great electro-contact properties and great performance friction are needed. Ag-graphite composites not only show great lubricating properties, but also improve the resistance against oxidation and welding of pure silver significantly. The Ag-graphite composites have advantages especially on the occasions of sliding contact. Ag-graphite composite coatings process used in industry from cyanide electrolyte was developed by Siemens in the 1980s. But the composite coatings are very rough and porous, which leads to bad wear resistance and short product life. Moreover, Cyanide is highly toxic, causing huge environmental pollution, will soon face elimination.Non-cyanide silver plating method is found and then improved to be suitable in the process of electroplating composite coatings in this research. Ultrasonic solves the problems of roughness and agglomeration of conductive particles when they are co-deposited with metal. The composite coatings fabricated under the influence of ultrasonic are compact, in which graphite particles disperse uniformly. Non-cyanide electroplating process and Ag-silver composites coatings are tested and studied with modern analysis technology. The microstructure and phase of those materials are observed. And the effect of techniques parameters on the microstructure and the effect of microstructure on the electric and friction properties are also discussed. Those researches results confirm that the composite coatings fabricated under the influence of ultrasonic show both great electro-contact properties and friction performance.The results of methanesulfonic-succinimide non-cyanide silver plating process study show that ultrasonic eliminates the passivation layer on the surface of cathode and increases the limited current density from 2 A/dm2 to 6 A/dm2. The influence of ultrasonic on the deposition of silver has been studied, the result shows ultrasonic could decrease the diffusion layer and increase the rate of mass transfer on the cathode surface significantly.The influences of techniques parameters on the volume percentage of graphite in composite coatings have been studied. The results show: Ultrasonic solves the problems of roughness and agglomeration of conductive particles when they are co-deposited with metal. Ultrasonic can choose the size of the graphite co-deposited into the silver matrix. The size of the particles co-deposited in composite coatings decreased as the ultrasonic intensity increased. The volume percentage of graphite for composite coatings depends on the current density and ultrasonic intensity. It increases with the increase of current density and ultrasonic intensity firstly, and then decreases considerably. An increased concentration of graphite in the bath corresponds to an increased volume percentage of graphite for composite coatings, which declines slightly when the concentration of graphite in the bath is above 30g/L.The relation between the structures and properties of composite coatings has been studied. The results show: The friction coefficient of composite coatings decreases with the increase of graphite volume percentage; however, when the volume percentage of graphite in the composite coating is above 9%, the friction coefficient remains constant of about 0.2. With the increase of graphite volume percentage, the wear of composite coating decreases first, and then increases when the percentage is above 9%. The contact electric resistance of silver-graphite composite coatings remains the same with pure silver coatings.With graphite particles uniformly dispersed, the compact Ag-graphite composite coatings are fabricated under the influence of ultrasonic. Those coatings show great electric and friction performance. The method of composite plating under the influence of ultrasonic offers a new way for co-depositing conductive particles with metal matrix.