The Electrical and Mechanical Properties of Electrographite-Metal Interfaces

Abstract

Experiments are described on the sliding of an electrographitic brush material against a number of counterface materials, using a pin and disk machine. The electrical and mechanical characteristics of the interface; frictional force, wear rate and contact resistance, were measured, and have been explained by physical changes occurring at the interface. Three wear regimes were encountered, mild wear, severe wear and catastrophic wear The wear mechanisms involved in these regimes were attributed to fatigue, adhesion and abrasion; which of these mechanisms predominated was dependent upon brush load and current. The transition from mild wear to severe wear was caused by mechanical disruption of any protective oxide layer, when a critical load was exceeded. A general wear theory is proposed, but this is limited by the necessity of considering each metal individually to determine whether or not oxide breakdown will occur. The use of aluminium as an alternative to copper for sliprings and commutators was investigated. Pure aluminium was found unsuitable because of a low, mild wear-severe wear transition load, due to the onset of plastic deformation. The use of aluminium is also severely limited by catastrophic failure of the brushes when carrying relatively small electric currents, caused by the production of abrasive particles. The contact resistance of the brushes sliding on all the metals investigated was controlled by the formation of oxide films upon the metal surface. The variations in contact resistance with electric current were attributed to electrical breakdown of the oxides. Finally, the frictional force behaviour was investigated and was explained in terms of the formation of lubricating, thin graphite films upon the brush surface.