The dusting wear of carbon brushes is studied in dry nitrogen at atmospheric pressure. The effect of water, oxygen, and carbon dioxide on dusting wear is described, and the results compared with those of investigations carried out under partial vacuum conditions. No effect of current, up to 114 amp per sq in. is found under the conditions studied. It is shown that a highly oriented natural graphite specimen or an oriented graphite brush, operated so that the faces of the cleavage planes rub on a plane rotating copper surface, neither dusts nor produces wear on the copper surface in dry nitrogen. When operated so that the edges of the cleavage planes rub against the plate, rapid dusting wear takes place and the copper surface is deeply scored. The dusting rate of carbon brushes in dry nitrogen against several metals is given. The rate is shown to decrease as the hardness of the metal increases, the behavior of brass being exceptional. It is found to be zero on smooth chromium and rhodium surfaces. On the assumption that an adsorbed lubricant film prevents scoring of the plate by the edges of the graphite crystals, allowing them to be oriented, the results are shown to be consistent with recent theories of solid friction.

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