Two Regimes of Current Conduction in Metal-Graphite Electrical Brushes and Resulting Instabilities

Two distinct regimes of brush behavior in regard to brush resistance and coefficient of friction are observed in silvergraphite brushes. These are explained in terms of two different surface films, One a lubricating layer composed of a mixture of fine silver and graphite particles, the other an adsorption layer of only a few atomic diameters thickness, overlaying a sintered silver layer of negligible resistance. The lubricating film is destroyed through loss of graphite Within some critical temperature range above 100°C. It is characterized by relatively high film resistivity (in the Order of 3 x 10^-11 m²) and a coefficient of friction between about 0.15 and 0.2. The conduction through this film is nonohmic. The adsorPtiOn film, with a film resistivity of about 10^-12 m², exhibits ohmic conduction, identifying electron tunneling as the major current transport mechanism. The coefficient of friction associated with.it ranges between about 0.3 and 0.4, which is somewhat characteristic for dry sliding of fcc metals, in the high resistance regime brush noise is high, but it is low in the absence of the lubricating layer. Finally, when the lubrication layer is present, the resistance of the cathodic brush is markedly lower than that of the anodic brush (in the order of 2/3).