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.

Date of Award	1973
Original language	English