Low wear metal sliding electrical contacts

The efficiency and performance of modern brushed DC electric motors, generators, and communication slip-rings are typically limited by the tribological behavior of the sliding contacts designed to transport current between the components in relative motion. It is argued that metal fiber brushes may provide unprecedented performance gains, along with complex design challenges, that are the source of ongoing multi-disciplinary research in the search for novel alternatives in current transfer applications.

This work describes an experimental study of copper as a design material for sliding electrical contacts, in the process providing fundamental insight on the mechanisms of wear, friction, and contact resistance governing metal sliding electrical contacts at high current density. A conceptual model correlating wear, electrochemically enhanced oxidation, and dislocation trapping near a passivated sliding surface is described. Perhaps the most remarkable experimental result presented is the demonstration that a beryllium copper fiber sliding on a rotating copper disk immersed in an oxidation inhibiting liquid medium, a hydrofluoroether (HFE), was able to reliably sustain more than 400 A/cm ² of electrical current transport across a lightly loaded sliding contact while maintaining relatively low wear (K_vol ∼ 2x10^-5 mm³/N-m) and friction (μ ∼ 0.18). The wear rate was insensitive to current density up to a maximum tested 14 value of 440 A/cm², corresponding to 50 mA through a 120 μm diameter fiber cross-section, in contrast with equivalent sliding conditions in an oxidative medium (water) where the system showed current dependent degradation of the tribological and electrical properties. The wear rate of the metal fiber at 400 A/cm² operating in an HFE was equivalent to the wear rate without current flow. These results highlight the importance of electrochemically enhanced oxidation at high current density and identified in numerous publications on the subject as a primary challenge in the design of increasingly efficient and inexpensive electrical machinery.

Sliding experiments were performed on copper substrates using alumina as the countersurface material, where a liquid cell used for contact immersion was engineered into a potential controlled three electrode cell for in situ measurement of friction as a response to a prescribed electric field across the copper disk surface. In this way it was possible to decouple current transport from electrochemically enhanced oxidation and to study tribological behavior as a function of oxide composition and rate of formation. Sliding experiments with alumina on copper were also performed in oxidation enhancing electrolytic solutions without the aid of an externally supplied electric field by immersing the sliding contact in 3wt% hydrogren peroxide (oxidation enhancing) and in 2M acetic acid (oxide reducing/dissolving). Three distinct friction regimes were observed. The results of these experiments with copper reveal a high friction response for alumina on copper when the principal reaction byproducts actively forming on the surface are CuO, Cu(OH)₂, and CuCO ₃ of μ ∼ 0.9. When the primary species was Cu₂O, the native copper oxide, the friction coefficient was μ ∼ 0.4. When sliding in 2M acetic acid, nominally in the absence of a passivating oxide, friction was μ ∼ 0.2.

Subsurface sliding damage on unpassivated copper (μ ∼ 0.2) was characterized at the completion of a sliding experiment for a sapphire ball sliding on a single crystal copper disk 15 immersed in an oxidation inhibiting medium. Focused ion beam (FIB) milling was used to mill cross-sections from inside the wear tracks which were extracted and characterized using scanning and transmission electron microscopy. These experiments focused on establishing the role of grain refinement near the surface of a copper disk due to friction damage.