Effect of Current on Friction Performance of Flexible Ring under Atmospheric and Vacuum Environments

The rolling current-carrying ring, as a key component for electrical connection between rotating parts, needs to face various complex working environments. In this article, the influence of current on the tribological behavior of flexible rings under both air and vacuum environments was studied. The friction pair used in the experiment consisted of an outer ring, a flexible ring, and an inner ring. The flexible ring was elastically clamped between the inner and outer rings by its own deformation. During the experiment, the inner cylinder drives at a speed of 600 r/min, and the total number of rotations for each test was 72 000 rotations. The vacuum degree was better than 1 \times 10⁻³ Pa, and the preload force of the flexible ring was 20.4 N. The results showed that as the current increased from 0 A to 20 A, the wear amount and contact area of the wear zone increased under both atmospheric and vacuum environments. The widening of the wear mark was beneficial to reducing the contact resistance, but would cause an increase in friction. EDX testing showed that the atomic ratio of O:Cu on the worn surface decreased from 0.379 7 to 0.144 3 under atmospheric conditions, while it decreased from 0.093 9 to 0.029 9 under vacuum conditions. Due to the oxidation effect, material peeling could be promoted in the wear region, resulting in a larger wear mark width and higher friction coefficient, with more severe subsurface plastic deformation under atmospheric conditions. The peeling of oxide products under atmospheric conditions could reduce the oxidation degree of the wear surface. Under vacuum conditions, oxide wear was suppressed, and the degree of material peeling and plastic deformation were relatively lighter. Microhardness testing was carried out on the depth direction of the worn surface. At the same depth position, the hardness of different samples firstly increased and then decreased with the increase of current. When the current reached 5 A, the hardness of the friction surface was maximum. For the same sample, the position closest to the wear surface had the most significant plastic deformation, and its hardness gradually decreased with depth direction and tended to be similar to that of the matrix. When the current exceeded 5 A, the current density of a single contact point had exceeded 140 A/mm², and high-density current could help reduce internal dislocation slip and annihilation in metal materials, reducing friction-hardening effect. Under vacuum conditions, the wear mark width of the flexible ring was relatively small, and current density was higher, resulting in stronger electrical softening effect. Therefore, surface hardness of wear zone under current-carrying conditions was a competitive result between friction-hardening and electrical-softening. In future work, we would investigate the tribological behavior of gold-plated flexible rings to eliminate oxidation effects. The research results in this article could provide references for the design of high-power rotating conductive joints under different environments.