Asymmetric wear behavior of self-mated copper fiber brush and slip-ring sliding electrical contacts in a humid carbon dioxide environment

Abstract

This study examines wear phenomenon occurring in high-current density self-mated metal copper sliding contacts in humid carbon dioxide environments. A custom tribometer built to test macroscopic (lightly loaded) brush and slip-ring sliding electrical contacts at relatively high speed and current density is described in the text. Data is presented as further evidence that at some elevated current density (between 120 and 180A/cm2) there is a transition in the wear mechanism as the brush wear rates, friction coefficients, and contact resistances bifurcate. In situ white light interferometry provides new insight on the evolution of slip-ring surfaces as a function of current flow direction through the contact. Ex situ SEM of the brush fiber bundle surfaces shows a strong dissimilitude in the wear of contacting fibers as a result of the current flow direction. Steady-state friction coefficients in the range of 0.20–0.25, linear wear rates of 3–5×10−11m/m, and contact resistances of 0.6mΩ are reported for copper fiber brushes in water saturated carbon dioxide environments at 2.5m/s sliding speed and up to 120A/cm2. A bifurcation occurs at 180A/cm2 where the electron receiving brush (positive bias) exhibits an increase in friction coefficient to a range of 0.3–0.4, and the electron donating brush (negative bias) a drop to 0.18. The positive brush also exhibits an increase in wear rate to 2.3×10−10m/m and a sharp decrease in contact resistance to 0.2mΩ. The negative brush shows an increase in wear rate but only to half the value of the positive brush, and the negative brush contact resistance increased sharply to 0.6–0.8mΩ. An uncertainty analysis of wear rate is presented that demonstrates the need for long (100s of km) sliding distances to achieve a reasonable measurement uncertainty for such low wear contacts.