High-Current Brushes Part VI: Evaluation of Slip Ring Surface Films

The thin film that is formed on the surface of an oxygen-free high-conductivity (OFHC) Copper slip ring sliding against monolithic silver-graphite brushes in a humidified carbon dioxide atmosphere has been investigatéd experimentally. The object was to study the nature and method of formation of the film since the films play a major role in the electrical, frictional, and wear performance of sliding electrical contacts. The techniques used included optical and electron microscopy and Auger electron spectroscopy. With the latter, two techniques were employed, both of which involved placing complete (83-mm diameter) slip rings within an Auger high vacuum chamber. In the first case externally operated slip rings were transferred into the Auger chamber for film analysis. In the second case a complete rotatable slip ring assembly, including brushes, was mounted within the Auger chamber. Low-speed rotational operation of this ring then permitted brush/slip ring sliding to take place so that film build-up could be analyzed in situ. The nature of the film formed on a copper slip ring was found to be dependent on the surface preparation and the ambient atmosphere, but it was found that the thin films of adsorbed gases did not form an oxide of appreciable thickness. Water vapor was identified and is known to play a critical role in the successful operation of brushes containing graphite. The adsorbed thin films were also found to contain significant quantities of sulfur and chlorine. In the presence of water vapor the adsorbed film on copper surfaces appears to have a special character. A hypothesis is suggested to explain the nature of this adsorbed film and the way in which it is bonded to the slip ring surface.