Microstructural characterization of rotating Cu-Cu electrical contacts in vacuum and wet CO 2 environments

Abstract

The chemical, electrical and wear properties of the rotating interface between OFHC copper slip rings and two high purity copper wire brushes were investigated in situ in ultrahigh vacuum (UHV) and in 1 atm wet CO 2. The chemical composition of the slip ring surface was determined by Auger electron spectroscopy. The contact resistance was measured by a potentiometric four-point probe technique while the wear properties of the interface and the morphology of the debris were studied by frictional force, scanning electron microscopy (SEM), transmission electron diffraction (TED) and X-ray diffraction (XRD) measurements. Rotation in UHV of a conventionally cleaned (CC) slip ring produced a much cleaner surface. The contact resistance of both brush interfaces decreased and the frictional force increased with increasing number of revolutions. After many revolutions the brush and slip ring welded. The decrease in contact resistance with the number of slip ring revolutions more or less paralleled the decrease in total impurities. Rotation in wet CO 2 of a CC slip ring and brushes also produced much cleaner surfaces. In contrast, initially argon ion sputter-cleaned surfaces became slightly contaminated (mainly carbon and sulfur) when rotated. The contact resistance at both interfaces and the coefficient of friction decreased with increasing number of slip ring revolutions, finally reaching steady state values. After each experiment, SEM examination of vacuum rotated surfaces showed deep ridges and broken pieces of material on the slip ring surface and badly deformed brush wire ends. Wet CO 2 rotated surfaces were relatively smooth and shallow ridges were seen. SEM examination of wear particles collected during rotation indicated that they may have come from both the slip ring and brush wire materials and were rolled in the regions between the brush and slip ring. XRD and TED from individual particles showed a randomly oriented polycrystalline microstructure. The particles collected from the wet CO 2 experiments were much smaller in size than those collected in vacuum experiments. In wet CO 2, the contact resistance was interpreted as being predominantly due to an electron tunneling mechanism through the CO 2-H 2O molecular layer at the interface. As expected, the thickness of the layer appeared to vary with the contact pressure. Friction would then arise largely when the molecular layer was occasionally broken, allowing intimate contact and temporary welding of the brush and slip ring surfaces. Subsequent fracture of these welds during continued rotation would initiate the formation of wear particles.