The role of relatively humidity in friction and wear behaviors of carbon sliding against copper with electric current

Abstract

During the train operation, the pantograph/catenary system is subjected to an extremely harsh service environment. Relative humidity has a great influence on the current-carrying tribological behaviors of carbon strips. The identification and understanding of the wear mechanism are extremely important in wet and dry conditions. This study was carried out to investigate the humidity effect on the service properties of carbon sample rubbing against copper (Cu) with and without electric current using a home-made wear tester, and the humidity ranging from 10% RH to 80% RH. The results indicate that the sliding wear behavior of the friction interface is drastically affected by the intervention of water vapor and electric current. The coefficient of friction (COF) without current is obviously lower than current-carrying sliding when the humidity is constant. However, the increased humidity led to a decreasing trend. After the current increases to 10 A, all the COF values are closed to each other ultimately. These phenomena mainly result from the formation and destruction of water lubrication film. Furthermore, the tribo-pairs worn surface appears the most sensitive to the current effect under dry conditions. The reverse transfer of Cu and carbon is greater on account of the current agglomeration effect, and the oxidation degree is more severe. The wear mechanisms of carbon are mainly material transfer accompanying with oxidation erosion. However, the wear degradation is weakened under water lubrication and uniform current distribution which improves the conductive quality. This is the coupling effect of humidity environment, current and heat concentration. This experiment provides technical support for the operation of an electric locomotive under an extremely harsh service environment.

Study on current-carrying tribological characteristics of C-Cu sliding electric contacts under water accumulation from rainfall condition

“…The percentage of Cu 2+ content on the surface of the carbon slider after the test was obtained by quantitative Cu 2+ analysis of figures 8(a)-(c), as shown in figure 8(d), it also proves that the presence of water promotes oxidation at the C-Cu interface under the low rainfall levels, but inhibits the copper oxidation under high rainfall levels. This is consistent with the conclusions drawn from the EDS spectra of points A, B, and C. Obviously, when the rainfall level is low, the high degree of oxidation leads to a larger film resistance, but the contraction resistance is obviously reduced(specific reasons have been proposed in 3.2), and the contact resistance is reduced instead [36]. Under the high rainfall level, on the one hand, the water current has a stronger impact on the interface, and a large number of conductive abrasive debris are washed away; on the other hand, the excessively thick water film seriously isolates the C-Cu contacts and the direct contact with air, both of which lead to a serious obstruction of the current transmission channel, and the increase of the contraction resistance is significant.…”

Study on Current-Carrying Tribological Characteristics of C-Cu Sliding Electric Contacts under Different Water Content

“…The main factors affecting ECR include the number and size of contact spots and the interfacial film layer. In the water film, carbon/copper debris and positive/negative ions form a new current transmission channel under the action of electric field force, and the actual number of conductive spots increases, which is helpful in reducing ECR [29]. However, when the coverage area and thickness of the water film increase further with the increase in water content, a large number of C-Cu spots cannot be directly contacted, and the velamen resistance increases significantly, which makes the ECR rise.…”