In situ graphite lubrication of metallic sliding electrical contacts

Abstract

Decoupled graphite lubrication of monolithic silver brushes on a copper rotor was studied in an ambient air environment under current varying from 0A/cm2 to 200A/cm2 using a custom designed electrical contact tribometer. Bifurcation of the positive and negative brush wear rates was observed at a current density of 200A/cm2. Energy dispersive spectroscopy showed transfer of copper from the rotor to the lower wear negative brush. Scanning electron microscopy of worn brush surface cross-sections created by focused ion beam milling revealed a fine-grained metallic layer below the graphite transfer layer on the negative brush surface; no such layer was found on the positive brush surface. At 40A/cm2, steady-state brush wear rates were very low (<10−11m/m). Friction coefficient at steady state was measured to be 0.15±0.02 and was independent of current direction. Using a scanning white light interferometer, the thickness of the graphite transfer layer on the rotor surface was estimated to be ∼5μm. Ultimately, the goal is to model lubricant buildup and removal as a competitive rates problem.