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The thick film that is formed on the surface of a copper slip ring sliding against monolithic silver-graphite brushes in a humidified carbon dioxide atmosphere has been investigated experimentally. The object was to study the nature and method of formation of the thick film since the films play a major role in the electrical, frictional, and wear performances of sliding electrical contacts. The morphology and composition of the thick films were studied by a combination of metallographic and electron optical techniques. The principal mechanism by which the films are formed was found to be weld formation between the silver in the brush and the copper in the slip ring followed by the rupture of a weaker interface in the brush material. The "film" deposited on the copper slip ring was, therefore, a composite of individual lumps of material, rather than a smooth layer-by-layer atomic deposit. The thickness of the film was highly variable from location to location; values as thin as 0.5 µm and as thick as 150 µm were observed. The collected wear debris were much smaller than the original particles used to form the brush composite, and the graphite content of the wear fragments was higher than that of the brush. It is believed that the thick film initially had the same composition as the brush material, but an enrichment in silver occurred during operation. This is attributed to the preferential removal of exposed graphite from both the thick film itself and from the brush surface during continued wear.