Characterization of Ag Wire Brush/Cu Slip Ring Electrical Contacts Rotating in Wet CO<inf>2</inf>

Current-carrying silver wire brushes were run on the same track against a rotating copper slip ring in a conventional stainless steel ultra high vacuum system. The experiments were performed after sputter cleaning of the slip ring in a H20-CO2ambient held at one atmosphere of pressure. The electrical contact resistance and the frictional coefficient were measured in situ. Subsequently, scanning electron microscopy, electron probe microanalysis, and elemental mapping studies were carried out ex situ on the slip ring, brush end, and wear particle surfaces. Both low (50 mA) and high (40 A) currents were used for comparison. Interestingly, it was found that not only was silver transferred to the copper slip ring, but copper was also transferred to the wire brush ends. Moreover, the wear particles were mainly of two types, one being rich in silver and the other rich in copper, the relative concentrations depending on the current flowing through the contacts. These results demonstrate clearly that mechanical mixing of brush and slip ring material and the incorporation of both materials in wear particles play important roles in rotating current-carrying electrical contact phenomena.