FRICTION AND WEAR PROPERTIES OF 2 TYPES OF COPPER-GRAPHITE BRUSHES UNDER SEVERE SLIDING CONDITIONS

High speed dry friction experiments using two copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities up to 230 m s-1 and at current densities up to 526 A cm-2. One brush was a commercial powder metallurgy (PM) specimen and the other was a graphite fiber-Cu/Sn matrix composite material. The composite brush was prepared by a proprietary process of liquid-metal infiltration and was run with the graphite fibers perpendicular to the rotor surface. The coefficient of friction was determined as a function of velocity, the wear rates were determined as a function of velocity and the voltage drop was determined as a function of velocity and current. The results show that the coefficient of friction exhibited by the PM brush is lower than that of the composite material at any velocity tested. The wear rates without current are much higher for the PM than for the composite brush, but they are of the same order when a current of 600 A is passed through. The voltage drop at the brush-rotor interface shows a similar variation with velocity for the two brushes, but the variation of the voltage drop as a function of current is different for the two specimens. The voltage drop increases almost linearly with increasing current for the PM brush. For the composite brush it exhibits a sharp increase up to about 50 A and then varies very little up to the maximum current of about 600 A. The damage done to the rotor surface in the case of heavy sparking is more pronounced with the PM brush than with the composite brush. It appears that the difference between the high temperature mechanical properties of the two types of brushes is responsible for their different behavior under severe sliding conditions. © 1978.