High-Current Brushes, Part II: Effects of Gases and Hydrocarbon Vapors

The performance of metal-graphite brushes sliding against a copper ring while carrying high current loads (to 80 A/cm²current density) has been evaluated under five nonoxidizing gas atmospheres (CO2, SF6, N2, He, and A) in the presence of constant water partial pressure (600 Pa) and under a dry CO2atmosphere in the presence of n-paraffinic hydrocarbon vapors (n-heptane to n-hexadecane, 0.2-1300 Pa) as vapor additives. Of the nonoxidizing atmospheres, a lowest friction coefficient (0.06) and lowest wear rate (0.7 mm³/Mm) were measured in file wet argon gas. Low wear rate was achieved in dry CO2with each of the hydrocarbon vapors, but the voltage drop, which was nearly constant for all, is higher than that observed in wet CO2. The latter phenomenon was implicated by static electric resistance measurements using graphite rod contacts under vacuum and under low partial pressures of the hydrocarbon vapors. The film resistances derived from the measurements remained near the same magnitude regardless of the carbon chain length of the vapor molecule. A discussion of absorption of these gases and vapors is presented.