Lubrication of high current density metallic sliding electrical contacts

Proper lubrication of sliding electrical contacts is necessary for efficient operation of electric motors and generators. In many cases, the electric brush, which is responsible for transferring current between the stationary and moving parts of the circuit, is composed of a lubricant, such as graphite, to aid in reduction of friction and brush wear. For high current applications, the presence of a lubricating film can be detrimental to the electrical conductivity. Decoupling of graphite lubrication from the electric brush was investigated in ambient air environments. Low brush wear rates (on the order of 10-11 m/m and lower) and low friction coefficients(0.15) were achieved at 40 A/cm2, but brush wear increased at a current densityof 200 A/cm2.;Copper fiber brushes offer improvements in electrical conductivity due to low bulk resistivity and large numbers of independent contact points. However, copper fiber brushes sliding on a copper rotor require humid non-oxidizing operating environments to reduce friction and wear. In the present study, copper fiber brush wear rates in humid carbon dioxide environments were comparable to graphite lubricated monolithic silver brushes, while brush electrical losses per ampere conducted were significantly lower with vapor phase lubricated copper fiber brushes (0.07 W/A) than with graphite lubricated silver brushes (0.7 W/A). From X-ray photoelectron spectroscopy analysis, chemisorbed carbon dioxide on the sliding surfaces reacted to form a carbonate species. Multiple monolayers of water adsorbed on to the sliding surfaces from the environment also aided in lubrication. Proper cooling of the sliding bodies was critical to maintaining the adsorbed water layers on the metal surfaces in humid environments. Brush wear showed a general trend of higher wear rates for positive brushes compared to negative brushes at high sliding velocities (greater than 1 m/s). Worn copper brush fibers examined by electron microscopy showed gross plastic deformation and shearing at fiber surfaces after sliding in humid carbon dioxide. The importance of tribofilm composition in the copper-copper humid carbon dioxide system suggested that other vapor phase additives could potentially be used to form and replenish thin carbon films on the metal sliding surfaces. (Full text of this dissertation may be available via the University of Florida Libraries web site. Please check http://www.uflib.ufl.edu/etd.html)...