Effects of Surface Films on the Performance of Silver-Graphite (75 wt% Ag, 25 wt% C) Electric Brushes

The brush resistance of silver graphite (Stackpole SG142, 75 wt% Ag, 25 wt% C) has been investigated over a range of speeds up to 35 m/s and a range of current densities up to 46.5 x 10
⁵
A/m
²
. The data show conclusive evidence for the buildup and breakdown of surface films due to the graphite lubrication and also for a persistent surface film of about 10
^-12
\Omegam
²
film resistivity. Film breakdown appears to be triggered at some critical combination of current density and speed. In the absence of the lubricating film the brushes run at much lower electrical loss and with a reduced coefficient of friction. This result suggests that the optimum performance of the brush is at high speeds and current densities.