Moisture Dependence of Silver-Graphite Brushes in Air, Nitrogen, Helium, and Carbon Dioxide

Wear tests were conducted on silver-graphite brushes sliding against copper slip rings in order to determine moisture levels below which the brushes would dust (wear rapidly). It was found that the brushes would not dust in carbon dioxide at the lowest attainable moisture concentration of 9 to 10 ppm, whereas critical moisture levels were found for the other three gases. The order of increasing moisture dependence for prevention of dusting was as follows: carbon dioxide, air, helium, and nitrogen. Savage´s work on graphite adsorptive properties provides the basis for an explanation of these results.