On the two modes of operation of monolithic Ag-C brushes

The transition from low-temperature to high-temperature behavior in silver-graphite brushes may be explained either by changes of constriction resistance due to the temperature dependence of electrical resistivity and hardness at negligible film resistivity and one to three contact spots, or by loss of graphite lubrication within the interfacial film. These two interpretations were tested by controlling the contact spot temperature by (1) heating in an oven, and (2) local heat input through friction. Correlated studies of contact resistance, friction, wear rate, wear chip size, and wear chip microstructure, and calculations of contact spot temperatures, favor the second hypothesis. Micrographic evidence suggests that the loss of lubrication occurs through desorption of water vapor from the graphite. It is found that primary wear particles form because of the wedge mechanism and because of cutting by graphite fragments steeply inclined to the interface. Silver fragments may consolidate into secondary wear particles.<>