Electrical Brushes and Sliprings

Transfer of electric signals or power from rotating components is commonly achieved by the use of stationary brushes in sliding contact with rotating rings. 

Traditional materials are carbon brushes against copper commutators or sliprings, but carbon brushes rely on graphite content for lubrication of the sliding contact, and graphite requires the presence of moisture or other vapours, or sometimes more exotic materials, to ensure low friction. They cannot therefore be used in vacuum or 

Early attempts to overcome this problem included the addition of hygroscopic compounds, liquid lubricants, metal halides and solid lubricants, including PTFE and molybdenum disulphide ° to the carbon to provide lubrication. However, one of the basic reasons for using carbon for brushes is its good lubricant properties, so that when these are lost and must be supplemented by the addition of solid lubricants, there is no dominant reason for continuing to use carbon. 

For space use, the voltages and current densities are usually low, and there has been extensive study of systems in which the brush and slipring materials are metallic, and lubrication is provided by molybdenum disulphide or other dichalcogenides. 

Molybdenum disulphide itself is most reliably considered as a semi-conductor (see Chapter 4), and if used as a continuous film over the contact surfaces it forms a high-resistance layer. Attempts to overcome this problem have included coatings containing metal powder and molybdenum disulphide and very thin molybdenum disulphide films formed in by burnishing or by sputtering . The in situ and 

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