Characterization of copper slip ring-wire brush electrical contacts

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The chemical composition and the electrical and mechanical properties of the interface between a Cu slip ring that rotates in contact with two Cu wire brushes were investigated by Auger electron spectroscopy (AES), scanning electron microscopy (SEM), contact resistance, and frictional force measurements. The experiments were carried out in an ultra-high vacuum system and in an environment of 1 × 10 −4 torr of wet C0 2 . The contact resistance at both the positive and negative interfaces decreased with increasing number of slip ring revolutions while the frictional force increased. Under the wet 10 −4 torr C0 2 environment the increase in frictional force was smaller than that in high vacuum, which suggests that wet CO 2 has a lubrication effect even at these relatively low pressures. In situ AES measurements showed that the composition of the slip ring surface, which was initially covered by about 50 atm% of carbon, changed drastically during rotation. After many revolutions it approached that of a clean Cu surface (total impurities < 10 atm%). The decrease in contact resistance with the number of slip ring revolutions more or less paralleled the decrease in total impurities in the high vacuum experiments. This parallelism suggests that the contact resistance is caused predominantly by the surface impurities. No systematic relationship between contact resistance and brush current density was observed. SEM observations showed that the surface material was smeared out in the brush track areas and that the initial surface impurities were buried during rotation.