Effect of Environmental Humidity on Tribological Behavior of Carbon/Copper Current-Carrying Sliding Contact Pairs

The pantograph-catenary system relies on the sliding electrical contact between the pantograph slide plate and the catenary wire to transport electric energy for the electric train. As an open tribology system, its service behavior is strongly affected by the external environment. In this study, the effects of environmental humidity (10% to 80% RH) on the tribological behavior of carbon/copper sliding contact pairs under non-current and current-carrying condition opper rod as a tribo-pair under sliding electric contact condition by adding humidity control module. The time-varying characteristics of the friction coefficient of the tribo-pair, the cumulative arc energy and the average contact resistance under the current-carrying condition, and the wear morphology, the form of copper particles and the damage mechanism under different conditions were discussed. The results showed that the friction coefficient under the current-carrying condition was higher than that under the current-free condition; Under the current-free condition, the average friction coefficient decreased monotonously with the increase of environmental humidity. However, due to the positive correlation between cumulative arc discharge energy, average contact resistance and relative humidity, the friction coefficient after 35% RH under current-carrying condition was hardly affected by environmental humidity. It was further found that the adhesive wear and oxidation wear on the carbon sample gradually decreased with the increase of relative humidity under the current-free condition. Under the current-carrying condition, there was an optimal humidity value with the lowest adhesive wear, which appeared near 55% RH; There were many forms of copper particles on the friction surface of carbon rods: under the current-free condition; most of them were large particle debris attached to the worn surface; Under current-carrying conditions, it was mainly very small copper particles directly embedded in the worn surface or peeled carbon coated on the worn surface; When the humidity was increased to 35% RH. copper particles underwent electrochemical reaction to form copper oxide film adhered to the worn surface; Under high humidity, the change of molecular structure of carbon sample on the worn surface during carbon/copper current-carrying sliding was accelerated. Friction and wear mechanism of carbon/copper sliding contact current depended on the increasing of environmental humidity. In the low humidity environment (10% RH), there was low amount of water vapor, and a large number of fine copper particles and debris were adhered to the worn surface. The main wear mechanisms were adhesive wear and abrasive wear. In the medium humidity environment (35%~55% RH), the amount of water vapor increased, and an incomplete adsorption water film was gradually formed in the contact area. The water film acted as a lubricating film to reduce the adhesive wear and abrasive wear. At the same time, induced electrochemical oxidation under current, and the wear mechanism was mild adhesive wear, abrasive wear and oxidation wear. In the high humidity environment (80% RH), the continuous increase of water vapor further improved the integrity of the water film, intensified the electrochemical oxidation reaction, and made the copper oxide film more complete. At this time, the main wear mechanism was oxidation wear accompanied by adhesive wear.