Effect of Ambient Temperature and Humidity on the Current-carrying Tribological Properties of Carbon Brushes/Collector Rings

ZHAO Xinze; LI Yang; WU Hailin; LI Wanting; LI Chenshi; XU Xiang; ZHAO Meiyun; YANG Wei

doi:10.16490/j.cnki.issn.1001-3660.2026.11.004

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Surface Technology ›› 2026, Vol. 55 ›› Issue (11) : 38-49. DOI: 10.16490/j.cnki.issn.1001-3660.2026.11.004

Effect of Ambient Temperature and Humidity on the Current-carrying Tribological Properties of Carbon Brushes/Collector Rings

ZHAO Xinze^1a,1b, LI Yang^1a, WU Hailin^1a, LI Wanting^1a, LI Chenshi², XU Xiang^1a,1b, ZHAO Meiyun^1a,1b, YANG Wei^1c,*

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Abstract

The work aims to optimize the operating environment of carbon brush/slip-ring assemblies in hydroelectric generator units to improve the operational reliability of the carbon brush/slip-ring current-carrying friction system. Current-carrying friction and wear experiments were conducted with a controlled-atmosphere tribological test apparatus. A friction pair composed of 45 steel (slip ring) and carbon material (brush) was selected to simulate practical operating conditions. The tests were performed under ambient temperatures ranging from 20 ℃ to 40 ℃ and relative humidity levels from 40% RH to 60% RH. During the experiments, key tribological and electrical parameters, including the friction coefficient, wear loss, wear rate, contact resistance, and contact temperature, were continuously measured. In addition, the coefficients of variation of the friction coefficient and contact resistance were calculated to evaluate the stability of the current-carrying friction process. The experimental results were further analyzed in terms of effect significance and surface damage characteristics. Based on the experimental observations, a mechanistic schematic was established to illustrate the coupled effects of environmental temperature and humidity on current-carrying friction and wear, and an equivalent contact resistance model was proposed.
The results demonstrate that ambient temperature and humidity have significant and coupled effects on the current-carrying tribological characteristics of the carbon brush/slip-ring pair. Under a relative humidity of 50% RH and an ambient temperature of 35 ℃, the minimum average friction coefficient of 0.129 7 was obtained, representing a reduction of 46.6% compared with the maximum average friction coefficient of 0.242 7 observed at 25 ℃ and 55% RH. The average contact resistance reaches its minimum value of 1.52 Ω at 25 ℃ and 50% RH as well as at 40 ℃ and 45% RH, which corresponds to a 49.3% reduction compared with the maximum average contact resistance of 2.27 Ω at 25 ℃ and 40% RH. The coefficient of variation of the friction coefficient is lowest 23% at 25 ℃ and 55% RH, while the coefficient of variation of contact resistance reaches its minimum value of 3.7% at 25 ℃ and 60% RH, indicating enhanced operational stability under these conditions. The wear rate exhibits a strong dependence on environmental humidity and temperature. The minimum wear rate of 47.35 mg/(m²·s) occurs at 50% RH and 30 ℃, whereas both the wear rate and wear volume reach their maximum values at 60% RH and 30 ℃, with a wear rate of 378.79 mg/(m²·s) and a wear volume of 3 mg. At constant humidity, the contact temperature of the friction pair shows an "N"-shaped variation with the increasing ambient temperature. The lowest contact surface temperature of 50.1 ℃ is recorded at 20 ℃ and 55% RH, which is 29.3% lower than the maximum value of 70.9 ℃ at 25 ℃ and 55% RH.
From a mechanistic perspective, increasing ambient temperature under constant humidity promotes oxidation reactions at the friction interface. However, excessive temperature reduces water vapor content and induces desorption of water molecules from the friction surfaces, thereby inhibiting water-film formation. At constant temperature, increasing humidity reduces the surface roughness of the carbon brush, leading to a smoother contact interface. Water vapor is identified as a key factor governing abrasive and adhesive wear, and adhesive wear is minimized near an optimal humidity of approximately 50% RH. Overall, excessively high temperature or high humidity conditions are detrimental to the current-carrying tribological performance of carbon brush/slip-ring assemblies.

Key words

current-carrying frictional wear / hydroelectric generator sets / ambient temperature and humidity / carbon brushes/collector rings / damage mechanism.

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ZHAO Xinze, LI Yang, WU Hailin, LI Wanting, LI Chenshi, XU Xiang, ZHAO Meiyun, YANG Wei. Effect of Ambient Temperature and Humidity on the Current-carrying Tribological Properties of Carbon Brushes/Collector Rings[J]. Surface Technology. 2026, 55(11): 38-49

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Funding

National Natural Science Foundation of China (52475202); Open Fund Project of the State-Local Joint Engineering Laboratory for Advanced Bearing Tribology Technology and Applications (202607).