The effect of temperature on the current-carrying tribological behevior of C/Cu contact pairs in high humidity environments

The effect of temperature on the current-carrying tribological behevior of C/Cu contact pairs in high humidity environments | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article The effect of temperature on the current-carrying tribological behevior of C/Cu contact pairs in high humidity environments De-Hui Ji, Li Xiao, Qiang Hu, Siyang Chen, Qiuping Li, Mingxue Shen This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3872711/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 07 May, 2024 Read the published version in Tribology Letters → Version 1 posted 7 You are reading this latest preprint version Abstract The environmental temperature alters the frictional behaviour by changing the state of the current-carrying contact interface, which makes the electrical contact invalid. In this work, the effects of three different temperatures (-20 ℃, 0, 20 ℃) on the current-carrying tribological behaviour of C-Cu tribo-pairs in high humidity environment (85%) were discussed. The evolution laws of friction coefficient, wear volume, contact surface properties, and contact resistance of C-Cu contact pairs under the coupling effect of temperature and current were studied, and the current- carrying wear mechanism of C-Cu at low temperature was analyzed in depth. The friction coefficient at each temperature exhibits a similar changing rule before and after current-carrying, demonstrating that the friction coefficient increases as temperature falls. However, the average friction coefficient at each temperature is lower than that without current. Although it will hasten the material surface's oxidation, a drop in ambient temperature will effectively lessen the transfer behavior of copper to carbon surface and reduce the wear volume of carbon material. The amount of copper transferred increases as current rises. Compared with the current, the change of temperature has a greater impact on the damage of tribo-pairs. At room temperature, the contact resistance under high current is greater than that of low current, the low temperature is just the opposite. In addition, at 0℃, although the contact resistance of low current (5 A) decreases significantly in the early stage of friction, its average resistance and fluctuation amplitude are the largest. As the temperature decreases, the current-carrying wear mechanism of C-Cu contact pairs gradually changes from adhesive wear to fatigue wear. Temperature high humidity Ice Current-carrying C/Cu Wear Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 1. Introduction The current-carrying friction pair not only has mechanical functions such as transmission and bearing, but also has the function of electric energy transmission. Current-carrying friction and wear exist in various fields such as railway, aerospace, electronics, circuits, weapons and equipment, such as carbon strips and contact wires of electrified railways, armature and guide rail materials of electromagnetic railguns, electrical contacts of high-voltage switches, etc. [ 1 , 2 ] . Due to its good self-lubricating and friction reducing properties, and excellent arc resistance, carbon materials with layered structure are commonly used as lubricating components and matrix materials of current-carrying tribo-pairs, and are widely used in pantograph strips of electric locomotives, graphite brushes and other current-carrying composites as lubricating components. In the pantograph-catenary system or brush system, the most prevalent and well-matched materials are the sliding current-carrying tribo-pairs made of copper material and graphite blocks [ 3 , 4 ] . Unlike traditional mechanical wear, current-carrying friction is a unique and complex working state caused by the presence of an electric field. Numerous research have examined how the friction and wear properties of C-Cu contact pairs are affected by various working conditions, including relative speed, contact pressure, polarity, and current [ 5 – 7 ] . However, varied service environments (temperature, humidity, atmosphere, etc.) also play a role in influencing current-carrying friction and wear in addition to operational conditions [ 8 – 10 ] . Taking the pantograph catenary system supplying current to high-speed train through sliding contact as an example, it is an open and complex current-carrying tribological system. The interface friction, current-carrying capacity and damage failure between pantograph and contact wire will be strongly disturbed by the external environment, and the complicated operating environment will have a major impact on the locomotive's ability to operate safely and steadily [ 11 , 12 ] . The replacement amount of carbon strips for Stockholm electric locomotive in winter is much larger than that in summer, which is about 9–10 times; In addition, the lower the temperature, the greater the proportion of electrochemical wear of the carbon strips, while there is almost no electrochemical wear in summer [ 13 ] . The frictional and electrical properties are greatly affected by temperature due to the synchronous occurrence of friction and conduction at the current-carrying friction contact interface [ 14 ] and the strong interference of ambient temperature on the surface/interface characteristics. Ding et al. [ 15 ] evaluated the electric sliding friction and wear behavior of carbon strips at different temperatures (room temperature − 300℃), and obtained that the wear amount of carbon strips increased slowly as the contact temperature increased. They also concluded out that appropriate cooling measures for the friction pair were an effective way to reduce wear. Similarly, J Bu et al. [ 16 ] discovered that, above 200°C, adhesive wear replaces abrasive wear as the primary current-carrying wear mechanism of carbon strips. The friction interface temperature is the key factor affecting the friction products and electrochemical reaction. When the external temperature is applied to different current-carrying contact pairs, the electrical transmission characteristics will be affected due to the different oxidation degree. For example, Liu et al. [ 17 ] obtained by studying the tribological behavior of copper-graphite brush at 30–60℃ that the surface film primarily made up of Cu, O and C elements acts as a barrier and lubrication on the contact surface, while its integrity and stability decrease as the increasing temperature, and the wear rate rises accordingly. Wang et al. [ 18 ] investigated the contact resistance of different metals from a microscopic perspective by changing the temperature (room temperature ~ 300℃). They discovered that high temperatures promoted the production of metal oxide films and the oxidation of wear debris, becoming the insulating layer. Therefore, metals that are easy to oxidize usually have high contact resistance at high temperatures. Unfortunately, the test temperature selected in the above research is far from the actual service temperature, so it is challenging to assess the current-carrying tribological properties of carbon materials below room temperature. Nevertheless, it is confirmed that temperature has an impact on the wear degree and current-carrying properties of tribo-pairs. The tribological characteristics of various current-carrying materials were investigated at -20°C by Liu et al. [ 19 ] , but they did not confirm if the wear state of carbon strips at low temperatures differed noticeably from room temperature or if there was a temperature correlation between the tribological behavior and temperature. Therefore, most of the work ignored the effect of low temperature on the damage and current-carrying tribological properties of carbon strips. On the other hand, the service behavior of the current-carrying system caused by water dew and ice frost under low temperature and high humidity environment is still unclear. Consequently, it is important to confirm the above properties at various low temperatures in order to provide theoretical basis and technical support for the current-carrying tribological behavior in extreme environments in the future. Based on the above discussion, the effects of different temperatures (20, 0, -20℃) on the current-carrying tribological behavior of C/Cu tribo-pairs in high humidity environment (85% RH) were studied. It mainly involves the influence law of friction coefficient (COF) and wear volume, and the damage morphology of the third body medium (water, ice, oxide film, etc.) acting on the carbon material, so as to comprehend and characterize the materials’ current-carrying wear mechanism under the synergistic effect of temperature and current. 2. Test materials and methods The tribological tests were carried out on a reciprocating friction and wear tester (CETR UMT-3, Bruker, USA). The tester is equipped with a humidity regulation system and temperature control elements to form a temperature and humidity controllable current-carrying friction test bench. The schematic diagram of the device is presented in Fig. 1(a). The humidity regulation system is composed of a humidity generator, a humidity sensor, an environmental chamber and a conducting tube. The refrigeration plate for temperature control is composed of two semiconductor refrigeration sheets surrounded by water pipes, and then connected to the temperature control system. Fig. 1(c) displays its schematic diagram. During the experiment, the circulating water enters the water pipe, so that the low temperature generated by the refrigeration plate can be extended. A specimen clamp is placed on the refrigeration plate, and the friction environment in the environmental chamber is maintained at the set temperature through contact and air conduction. The humidity adjustment range is 10-90%, and the temperature adjustment range is -25℃ -+25℃. The fluctuation of temperature and humidity can be controlled at ±2℃ and ±5% RH respectively. The current-carrying circuit mainly includes DC regulated power supply, variable resistance, upper sample, lower sample, current sensor, voltage sensor, data acquisition card and several wires. Fig. 1(b) depicts its circuit diagram. It is worth mentioning that the clamps of the upper and lower samples are made of insulating materials, which can ensure the insulation between the current circuit and the testing machine. The data acquisition system continuously measures the load and shear force in real time, and sends the data directly to the computer. The materials used in the current-carrying friction test are carbon rod samples and copper rod samples. The cylindrical carbon rod ( R a =0.2 μm) with a diameter of 10 mm and a length of 30 mm, which is processed from the carbon strips in service on the high-speed railway, is the upper sample. The lower sample is a heat-treated cylindrical copper bar ( R a =0.05 μm) with a diameter of 15 mm and a length of 30 mm made of T2 red copper ( R a =0.05 μm). Fig. 2 presents the microstructure and morphology of the test material. The operation mode of the two samples is: the lower sample moves back and forth while the top sample is subjected to the load. Before the experiment, the carbon rod and copper rod are assembled in the clamps to achieve contact, and the ambient chamber temperature is adjusted to the set value. The parameter settings of the current-carrying friction experiment are displayed in Table 1. Table 1 Details of test parameters Cycles Frequency Sliding displacement Normal load Tempreature Electric current Humidity 10000 4 Hz ±5 mm 10 N 20℃,0℃,-20℃ 0 A，5 A，10 A 85% The morphology of the worn samples were investigated using a scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and element distribution were dectected using a Flash 6I60 X-ray energy dispersive spectrometer (EDS, Xflash 6160, Bruker, USA). The 3D morphology and wear depth of the samples could be obtained by a 3D surface profilometer (Bruker, Contour GT-K, USA). The wear volume is used to measure the degree of carbon specimens’ wear. Due to the fact that the wear scar of the carbon specimen is a concave ball pit, the wear region is estimated as a ball crown body in order to compare the wear losses. Therefore, the wear volume formula of the carbon rod can be converted using the ball crown volume formula [20]: Where, V represents the wear volume (mm 3 ), h represents the height of the ball crown body, which is the depth of the wear scar (mm), and r represents the radius of the ball crown section, which is half of the width of the wear scar (mm). 3. Results and discussion Fig. 3 (a-c) displays the time-varying curve of COF between contact pairs at different service temperatures (i.e., 20, 0, and -20 ℃) and currents of 0, 5, and 10 A, respectively. Fig. 3 (d) shows the average COF at the stable stage (2000-8000 cycles) under various parameters. By observing Fig. 3 (a-c), it is found that the COF basically shows an upward trend with the decrease of temperature, whether current is applied or not. Under non-current-carrying conditions (Fig. 4 (a)), The COF is higher at -20 °C because of resistance to the ice layer that forms on the surface and to the ice fragments that are left behind when the ice layer breaks. 0℃ is the critical point of water freezing, and the water vapor in the air condenses on the cold contact pair to form a thin ice crystal layer. However, when the contact pair slides relatively, the COF will decrease slightly due to the ice crystal layer moving and melting under the action of friction heat. The minimum COF is achieved at 20°C due to an improvement in the lubricating state and an increase in water film thickness. When current is applied (Fig. 4 (b-c)), the difference of COF between contact pairs at different temperatures increases. The combined action of friction heat and Joule heat leads the ice crystals around the tribo-pairs at 0°C to melt into water very quickly when the current increases to 10 A, and the COF progressively drops. Then the water molecules in the air condense on the cold copper surface, and the wear debris and water molecules form a wetting equilibrium state. Therefore, the COF is less than the COF at 20℃ when the contact surface is relatively dry. The COF decreases at various temperatures after applying current as shown in Fig. 4(d). At 20°C, the oxidation reaction on the surface is accelerated by the combined effect of friction and Joule heat, and the resulting oxide film provides a slight lubricating effect. On the other hand, the softening degree of the copper contact point increases, which reduces its shear effect on the carbon micro-protrusions and decreases the the COF. At -20 ° C, there is a liquid like layer between the substrate and ice, as well as on the free ice surface, composed of an amorphous layer of water molecules [21] . It arises from the pre-melting phenomenon on the material surface. The flow behavior of ice and snow, substances adsorption on the ice surface, and the low friction of materials on the ice surface are all significantly influenced by this liquid-like layer [22] . Similar to the wetting material between the ice and the substrate surface, the liquid-like layer can increase the effective contact area between the two by changing its thickness, thus affecting the adhesion strength of ice, namely the tangential ice strength [23] . As the temperature of the contact surface rises, the thickness of the liquid-like film increases. This decreases the tangent line's strength to create ice, making the ice layer easier to remove. Meanwhile, the ice layer becomes thinner with increasing current and is more easily crushed by external forces, resulting in a decrease in COF. Fig. 4 presents the three-dimensional topography of the carbon samples’ worn surface. It can be seen that under no-current conditions, there are more buildups around the wear scar compared to the worn surface after applying current. SEM observation reveals that the buildups on the edges are formed by debris accumulation, and the debris accumulated before and after applying current exhibit different topographic states. Taking 20°C as an example, the buildups at 0 A are black dense films, which are presumed to be carbon debris accumulated around the wear scar under the push and squeeze of reciprocating motion. After applying current, the buildups are dominated by white dense films. It is important to note that at -20°C, the wear scar boundaries of carbon samples under 0 and 5 A are not very clear. Additionally, material stacking around the scar edge forms a wave-like ring structure, which causes the wear scar diameter under various currents at -20°C to vary significantly. The composition, morphology, and formation process of this ring-shaped deposit will be discussed in Fig. 6. The average wear volume of carbon samples at various temperatures is displayed in Fig. 5. It is evident that non-current-carrying conditions cause minimal damage to the material, while the current accelerates the deterioration. The wear volume reduces with a drop in the service temperature. Consequently, the wear volume is lowest at -20°C under non-current conditions. When current is applied, the contact points between the tribo-pairs not only bear high stress, but also bear high current and high temperature, which leads to the high-current carbon material contact points at 20°C being prone to wear. A paste that sticks to the contact surface and engages in friction is created when the worn debris from previous friction combines with the water film and ice slag at 0°C and -20°C. The paste builds up around the wear scar under reciprocating action, which is why the wear scar profile diameter at the two temperatures is substantially less than at 20°C (see Fig. 4). At the same time, the mixture of wear debris and water participates in secondary wear, avoiding direct damage to the friction pair. On the other hand, the presence of the blend increases the number of contact points, and the friction surface can be cooled in time without the micro-bumps being softened and cut. It is worth noting that the influence of different temperatures under the same current on the wear volume is greater than the influence of changing current under the same temperature, that is, compared to current, temperature changes have a more serious effect on the damage to carbon materials. In order to investigate the ring-shaped structure of the the wear scar’s edge at -20°C, EDS analysis was performed under various current-carrying conditions (as shown in Fig. 6). It was found that the ring-shaped structure at the edge was basically a concentrated distribution area of copper elements (region I in Fig. 6(a-c)), indicating that material transfer occurred during the friction process. Various current-carrying conditions resulted in various copper element distributions. When there is no current, copper elements are mainly distributed in the outer ring-shaped region in the form of cellular particles (Fig. 6(a)), which are composed of countless nanoscale particles, as shown in Fig. 6(d). After applying 5 A current, copper elements were found to be concentrated in both the center and edge rings (Fig. 6(b)), and the ring-shaped structures on both sides perpendicular to the sliding direction were more pronounced and surrounded by multiple rings, as shown in Fig. 6(e). It was found that the ring-shaped structures were composed of small particles and carbon film coverings. The center of the wear scar was dominated by ploughing, and covered with fine debris, as shown in Fig. 6(b) part II. As illustrated in Figs. 6(c) and 6(f), the copper element is concentrated on both sides of the sliding direction in the form of dense films after applying a current of 10 A. From the morphology and element distribution, it was inferred that during the sliding wear process, the copper surface froze at -20°C, and the contact pair broke the ice under the action of shear force and material transfer occurred. A mixture of small copper particles, a small quantity of carbon shavings, and ice slag is heaped up on the worn surface and then scattered under pushing around the wear scar. During the continuous accumulation, ice condenses around the wear scar and mixes with the debris to form a ring-like structure. Under current-carrying conditions, copper transfers to the carbon wear scar and mixes with carbon debris to adhere to the center of the wear scar. When the thickness increases to a certain extent, it is pushed away during reciprocating action. It is also because some copper remains in the center that the proportion of copper elements in region I decrease (see Fig. 6(h)). As the current continues to increase, Joule heating thickens the liquid-like film between the surface and the ice around the contact area, making the ice layer easier to remove. The mixing flow of debris and water in the contact area is enhanced, and it is easily pushed by the friction pair to the two sides along the direction of the ploughing, and is compacted by reciprocating rolling to form dense films. The element composition analysis of Fig. 6(d-f) reveals that the oxygen element progressively increases with applied current, suggesting that an increase in current encourages the production of copper oxide layer. The surface morphology of the carbon samples’ wear scar center at different service temperatures is displayed in Fig.7. Significant ploughing features are seen on the worn surface at 20°C (Fig. 7(a)). When an electric current is applied, large areas of dense white patches appear (Fig. 7(b and c)), which adhere to the substrate surface and integrate with it (as shown in Fig. 7(k)). Around worn scars are the areas where the white spots are predominantly concentrated. Through EDS analysis, it is found that the white areas are mainly composed of copper and oxygen elements (Fig. 8(a)). In addition, the white patches under 10 A are more dense than those under 5 A, and delamination and debris also appear. As a result of continuous friction and current flow across the contact site, friction heat and Joule heat builds up on the worn surface. A significant area of copper transfer film is formed and oxidized as a result of the contact pair's temperature rising [24] . Because of the combined effects of thermal stress, shear strain, and normal force, brittle cracks are generated as the amount of copper transfer rises. The expansion of cracks promotes the delamination of layers and the production of small amounts of wear debris [25] . The good solid lubricating effect of this copper oxide debris is another significant factor contributing to the COF drop when compared to the current-free situation [18, 26] . However, uneven oxidation thickness can cause current concentration at the contact interface, resulting in local current suddenly increasing and fusion occurring, which increases the wear amount of the material [27] . At 0°C, many scattered small debris and shallow ploughing can be observed on the worn surface, but no large oxide film is found, but rather scattered white patches (Fig. 7(d)). The worn surface is covered with a large amount of peeling debris and small particles under 5 A (Fig. 7(e)), and magnified images reveal scattered and embedded white particles in the carbon film, with a size of approximately 1 μm (Fig. 7(m)). EDS testing reveals that the white particles in Fig. 7(m) EDS A contain more copper elements than other regions (Fig. 7(m) EDS B and C). Therefore, in low temperature environments, the friction surface does not form a complete and dense copper oxide film, but exists in the form of dispersed or embedded copper particles, which are evenly dispersed and embedded in the carbonaceous material. After repeated wrapping and extrusion, they integrate with the matrix. However, the worn surface smooths out and the amount of white spots decreases when the current reaches 10 A. There is no visible oxide film and the wear center is scattered with peeling and pitting when the temperature falls below -20°C. There is also a layered structure that is in line with the usual traits of delamination wear (inset in Fig. 7(h)). When hard copper shear carbon surface at low temperature, the material at the interface is prone to block breakage and brittle fracture, accompanied by spreading cracks, and the cracks extend horizontally to the surface and form a layered structure [28,29] . After applying a current of 5 A, the wear scar surface is distributed with cracks and a small number of peeling pits, and the layered structure disappears. This indicates that the copper contact point is sufficiently softened by the Joule heat, and the shearing impact on the carbon micro-protrusions is nearly eliminated. At a current of 10 A, the surface of the wear scar is relatively smooth and has cracks, with local oxide films formed by the stacking of nanoscale particles (see Fig. 7(j, l)). The above phenomena indicate that the material undergoes fatigue wear at -20°C. After applying a current, the degree of fatigue wear decreases. It is important to note that the transfer behavior of copper to the carbon surface is effectively reduced by the interface temperature drop, as Fig. 8(b) illustrates. On the one hand, it reduces the possibility of the contact pair from carbon-copper lubrication friction to copper/copper high adhesion friction. On the other hand, abrasive wear is decreased because fewer particles of copper and copper oxide are involved in the current-carrying friction process. In addition, as the temperature decreases, the Cu-O ratio also gradually decreases, indicating that in a high humidity environment, the ability of the carbon worn surface to adsorb water molecules increases as the temperature decreases, and the presence of ice water layers promotes oxidation reactions on the worn surface, including both carbon oxidation and copper oxidation. In general, the degree of surface oxidation increases with current. The distribution of EDS elements and the worn surface morphology of copper samples at varying service temperatures with currents of 0 A and 10 A are displayed in Fig. 9. At 20°C, the worn surface of the copper without current adheres to a black film, which is identified as element C by EDS surface scanning (Fig. 9(a)).After applying a current of 10 A, the copper surface exhibits severe peeling at the ploughing groove, with debris mainly concentrated in the groove (Fig. 9(b)). The worn copper surface is comparatively smooth at 0°C, with the debris dispersed in granular form (Fig. 9(c)). The peeling of the copper surface is shown to increase at -20°C (Fig. 9(e)), especially when a current of 10 A is applied (Fig. 9(f)). It is noteworthy that at -20°C, splashes of copper particles are observed on the carbon worn surface (Fig. 9(g)), indicating that the presence of ice water results in erosion of the electric arc. Water under the action of ionization in the electric arc will produce H + dissolved in the metal, making the surface brittle and supersaturated [30] , thus exacerbating the flaking of the copper surface. Comparing the roughness of the copper worn surface under different current conditions at the same temperature (Fig. 9(h)), the copper with a current of 10 A has the highest roughness and the most severe surface damage. It implies that the intervention of electrical variables considerably deteriorates the friction and wear properties of the material, which is consistent with the severe peeling occurrence of the copper surface under 10 A. From the above carbon’s worn surface, it is observed that copper undergoes severe material transfer at room temperature, resulting in a surface roughness greater than that at -20°C and 0°C. The contact resistance of the current-carrying tribo-pairs at various service temperatures is displayed in Fig. 10. Generally, the contact resistance consists of the film resistance on the surface of the material and the contraction resistance caused by the current contraction effect [18] . As can be seen that, under a current of 5 A, the contact resistance at -20℃ and 0℃ both have a large initial value at the beginning of the cycle (within 2000 cycles), and then a sharp decrease. This is due to the presence of water film or ice layer on the surface during the initial stage of friction, and undergoes changes from a mixed layer of ice and water to a mixture of ice slag and wear debris. The state of the contact surface becomes extremely complex, and the participation of wear debris greatly improves the conductivity of the water film, resulting in a sharp decrease in contact resistance until the blending reaches equilibrium. Comparing the average contact resistance values in the stable stage (2000-10000 cycles) in Fig. 10(b), it can be found that the contact resistance at room temperature under 10 A is higher than that under 5 A. When at low temperatures, the contact resistance under 10 A current decreases instead. Upon applying 10 A, the C sample's worn surface morphology shows that the contact surface developed an oxide layer and turned into a poor conductor, which causes the film resistance to rise quickly as the film thickness grows [31] . When the temperature drops to 0℃, water molecules in the air encounter the cold copper surface and continuously condense into small water droplets that participate in the friction process, greatly affecting the transmission of electrical signals. The water film's impact is lessened as the current is increased, which lowers the contact resistance. Although the oxide film has not formed at -20°C, the additional film resistance generated by the ice film and liquid-like film is still relatively large. However, the additional film resistance decreases due to the melting of the ice film when the current increases. The wear mechanism of C/Cu tribo-pairs with changes in temperature and current is schematically diagrammed in Fig. 11. Under non-current-carrying conditions, after the C/Cu sample undergoes wear in a room temperature environment, a small amount of debris accumulates on the wear edge and repeatedly plows the contact interface during reciprocating motion. Applying current causes the microconvexity to soften, enhancing copper transfer and progressively generating a dense and complete copper transfer film. Meanwhile, chemical oxidation and electrochemical oxidation occur, and the characteristics of abrasive wear and adhesive wear are significant, leading to increased wear loss of carbon. A decrease in COF and an increase in contact resistance result from the thickening of the copper oxide film, which has a greater adsorption capacity for water molecules than pure copper [32] . The variation of the COF with the current is consistent with the results of literature [33]. As the temperature decreases to -20°C, the worn surface of carbon increases its shear strength to resist the adhesion strength of ice, resulting in an increase in COF and surface peeling. Delamination wear is typically characterized by dislocations and cracks occurring beneath the carbon surface that expand horizontally to the surface. Meanwhile, although the sustained low temperature has a suppressive effect on the accumulation of interfacial Joule heat after applying current [28] , there is still a temperature rise on the surface of the material compared to pure mechanical friction. The liquid-like film between the ice film around the contact pair and the material gradually thickens, reducing the ice formation strength of the tangent line and and acting as a lubricant to reduce COF. As a lubricant and coolant, the water film that forms between the contact pairs lessens the amount of copper that is transferred to carbon and lowers the wear loss of carbon. The amount of water molecules adsorbed on the cold copper increases compared to room temperature. The oxidation reaction of the worn surface is enhanced by the coupling effect of current and water. As the temperature decreases, the wear mechanism shifts from adhesive wear to slight fatigue wear. This finding is consistent with the conclusion of the coupling effect of water and current in literature [31]. The aforementioned findings show that the interplay of heat exchange, water state, and current alters the wear mechanism of the contact pair at low temperatures. 4. Conclusion In this work, the tribological properties of C-Cu under current-carrying conditions were analyzed in detail at ambient temperatures of -20, 0, and 20℃.The evolution laws of COF, wear volume, contact surface characteristics, and contact resistance of the C-Cu tribo-pairs at different ambient temperatures were investigated, and the current-carrying wear mechanism of C-Cu at low temperatures was deeply explored. The main findings of this study can be summarized as follows: (1) The COF at various temperatures exhibits a similar variation pattern before and after applying current. That is, the COF increases with decreasing temperature. However, the average COF after applying current at each temperature was lower than that without current. The fluctuation and average value of contact resistance is large under low temperature conditions, with a more significant change at low currents (5 A). (2) The application of current accelerates the damage to carbon materials. An increase in current at room temperature can facilitate copper transfer and the development of an oxide film, both of which lower the COF of the C/Cu contact pair. However, it can increase the copper loss and the contact resistance, and the wear mechanism is dominated by adhesive wear. (3) The transfer behavior of copper is effectively reduced by the drop in interface temperature, but the water produced at this time can function as a catalyst to promote the occurrence of oxidation reactions and shield the carbon material from wear and damage. The wear and damage of the friction materials are more affected by temperature changes than by applied current. The coupling effect of low temperature and current is beneficial for the wear mechanism of the C/Cu contact pair to shift from adhesive wear to slight fatigue wear. Declarations Acknowledgements This work was supported by National Natural Science Foundation of China (nos. 52365022 and 52375181), Natural Science Foundation of Jiangxi Province (no. 20224ACB204012) and Postgraduate Innovation Special Fund Project in Jiangxi Province (no. YC2022-B177), General Subject of State Key Laboratory of Performance Monitoring and Protecting of Rail Transit Infrastructure (no. HJGZ2023208). Author Contributions All authors contributed to the study conception and design. Material preparation and results analysis were performed by D.H. Ji, M.X. Shen and Q. Hu. The manuscript draft was written by D.H. Ji and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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Shangguan, Tribological properties of pure carbon strip affected by dynamic contact force during current-carrying sliding, Tribol. Int. 123 (2018) 256–265. https://doi.org/10.1016/j.triboint.2017.12.032. J.Q. Wu, Study on the characteristics of electric contact between pantograph and overhead contact line, Sichuan, Southwest Jiaotong Univ. (2010) 1-108. X. long Liu, C. wei Zhou, X. jian Zhou, M. jie Hu, D. yun Wang, Q. Xiao, X. Guan, W. lue Zhang, S. Zhang, Z. biao Xu, Influence of different arc erosion durations on the wear properties of carbon skateboards/contact wires under low temperature, Wear. 516–517 (2023) 204600. https://doi.org/10.1016/j.wear.2022.204600. Y.T. Zhao, S.Q. Wang, Z.R. Yang, M.X. Wei, A new delamination pattern in elevated-temperature oxidative wear, J. Mater. Sci. 45 (2010) 227–232. https://doi.org/10.1007/s10853-009-3923-8. J. Songshan, Current collection tribology of pantograph, Electr. Tract. Rep. (1997)52-59. Y. Sun, C. Song, Z. Liu, J. Li, Y. Sun, B. Shangguan, Y. Zhang, Effect of relative humidity on the tribological/conductive properties of Cu/Cu rolling contact pairs, Wear. 436–437 (2019) 203023. https://doi.org/10.1016/j.wear.2019.203023. Y. Sun, C. Song, Y. Zhang, M. Li, Y. Zhang, Oxidation on the current-carrying rolling surface and its subsequent impact on the damage of Cu contact pairs in O 2 / N 2 mixture, Mater. Lett. 288 (2021) 129349. https://doi.org/10.1016/j.matlet.2021.129349. X. Zhi, N. Zhou, Y. Cheng, X. Wang, H. Wei, G. Chen, W. Zhang, Effect and behaviors of ambient humidity on the wear of metal-impregnated carbon strip in pantograph-catenary system, Tribol. Int. 188 (2023) 108864. https://doi.org/10.1016/j.triboint.2023.108864. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 07 May, 2024 Read the published version in Tribology Letters → Version 1 posted Editorial decision: Revision requested 26 Feb, 2024 Reviews received at journal 20 Feb, 2024 Reviewers agreed at journal 05 Feb, 2024 Reviewers invited by journal 22 Jan, 2024 Editor assigned by journal 19 Jan, 2024 Submission checks completed at journal 19 Jan, 2024 First submitted to journal 17 Jan, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3872711","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":268506369,"identity":"02cf3269-7f32-4a9e-b319-6c632b233619","order_by":0,"name":"De-Hui Ji","email":"","orcid":"","institution":"East China Jiaotong University","correspondingAuthor":false,"prefix":"","firstName":"De-Hui","middleName":"","lastName":"Ji","suffix":""},{"id":268506370,"identity":"81083731-0faf-4ba4-b12f-1becf80dba35","order_by":1,"name":"Li Xiao","email":"","orcid":"","institution":"East China Jiaotong University","correspondingAuthor":false,"prefix":"","firstName":"Li","middleName":"","lastName":"Xiao","suffix":""},{"id":268506371,"identity":"14bc576f-ca65-4590-882b-2038dd3d3744","order_by":2,"name":"Qiang Hu","email":"","orcid":"","institution":"Jiangxi Academy of Sciences","correspondingAuthor":false,"prefix":"","firstName":"Qiang","middleName":"","lastName":"Hu","suffix":""},{"id":268506372,"identity":"caad8294-9901-4bfb-9ef4-cded38ae9231","order_by":3,"name":"Siyang Chen","email":"","orcid":"","institution":"East China Jiaotong University","correspondingAuthor":false,"prefix":"","firstName":"Siyang","middleName":"","lastName":"Chen","suffix":""},{"id":268506373,"identity":"14cf6c8a-e824-4c1a-aae2-03bb8be71b7f","order_by":4,"name":"Qiuping Li","email":"","orcid":"","institution":"East China Jiaotong University","correspondingAuthor":false,"prefix":"","firstName":"Qiuping","middleName":"","lastName":"Li","suffix":""},{"id":268506374,"identity":"d91b0929-208f-42f9-9eda-00d3ba75e8df","order_by":5,"name":"Mingxue Shen","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1UlEQVRIiWNgGAWjYDACCRBhAGYyPkiosCFNC7PBgzNpxGqBADbJh22HCOuQn9187NGNgjt2G24kH6tIYDvAwN/enYBXi8GdY+nGOQbPkmfOSEu7kcBzh0HizNkN+LVI5JhJ5xgcTuYHMm4kSDwDiuTi1yI/I/8bWAubRP63ggSDw4S1MNzIYQNpsQPawsaQkECEFoMbaWCHJUj2PDOWSDiQxkPQL/Izkp9J5/w5bG9wPPnhx5//bOT423sJOAwKEhugDB6ilIOAPdEqR8EoGAWjYOQBAM1JSatZZhLiAAAAAElFTkSuQmCC","orcid":"","institution":"East China Jiaotong University","correspondingAuthor":true,"prefix":"","firstName":"Mingxue","middleName":"","lastName":"Shen","suffix":""}],"badges":[],"createdAt":"2024-01-17 11:14:16","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3872711/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3872711/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11249-024-01856-2","type":"published","date":"2024-05-07T21:17:41+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":50101955,"identity":"6fbb0d17-2195-414d-8caa-f21f76f37773","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":96085,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of current-carrying friction test bench with controllable temperature and humidity\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/3bfb4a37a310f0801ae5bd3a.png"},{"id":50102508,"identity":"e7b359ab-a3eb-455a-82cf-915ad8912edf","added_by":"auto","created_at":"2024-01-24 15:07:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":94657,"visible":true,"origin":"","legend":"\u003cp\u003eMicrostructure and structure of test material (a) microstructure of carbon rod (b) metallographic structure of copper rod after heat treatment\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/0eeab18ab276e2ef96218875.png"},{"id":50101951,"identity":"a42bc306-e6ab-4304-8f5b-64349f9ca7b3","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":87675,"visible":true,"origin":"","legend":"\u003cp\u003eCOF at high humidity (85%RH) at different service temperatures: (a) 0 A; (b)5 A; (c)10 A; (d) Average COF\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/a353dc60ec0ac835d79d3127.png"},{"id":50101949,"identity":"a8a2a962-f434-4efa-8031-c04eb60e05a5","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":260032,"visible":true,"origin":"","legend":"\u003cp\u003eThe 3D morphology of wear region of carbon samples at different temperatures\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/1923a3a91128df4124b757ff.png"},{"id":50101946,"identity":"789b0d7c-6b39-4e99-8112-fcbd0abb62ca","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":26219,"visible":true,"origin":"","legend":"\u003cp\u003eWear volume of C samples at different service temperatures\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/d8303f698cf73ab194eaef8a.png"},{"id":50101948,"identity":"918da576-1786-40a3-98f9-880137bc19ac","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":312010,"visible":true,"origin":"","legend":"\u003cp\u003eThe morphology and element analysis of carbon worn surface at -20℃\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/946cd3196db66fa57bb70b17.png"},{"id":50101956,"identity":"11aa705c-264f-4bb8-9060-969dcbf719d0","added_by":"auto","created_at":"2024-01-24 14:59:04","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":254307,"visible":true,"origin":"","legend":"\u003cp\u003eWorn surface morphology of carbon samples at different service temperatures: (a-c) 20°C;(d-f) 0°C;(h-j) -20°C;(k) 20°C-10A;(l) -20°C-10A;(m) 0°C-5A.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/54ba1213e04b6a2c4328e14f.png"},{"id":50102510,"identity":"4cb80262-31f6-4f14-a482-fe26de79ed4f","added_by":"auto","created_at":"2024-01-24 15:07:03","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":71074,"visible":true,"origin":"","legend":"\u003cp\u003eDistribution of elements on the carbon samples’ worn surface: (a) distribution cloud map of elements at 20℃-10A; (b) percentage of elements at different temperatures\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/50158e88188fc1905ad82d96.png"},{"id":50102849,"identity":"2eba1c5b-653d-49c4-93c2-f86ea9c68104","added_by":"auto","created_at":"2024-01-24 15:15:03","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":124323,"visible":true,"origin":"","legend":"\u003cp\u003eSurface morphology and roughness of copper samples: (a) 20℃-0A；(b) 20℃-10A；(c) 0℃-0A；(d) 0℃-10A；(e) -20℃-0A；(f) -20℃-10A；(g) copper sputtering particles; (h) copper surface roughness\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/fa7dfab30864e0fbba8621a0.png"},{"id":50101954,"identity":"3b1a0109-c78f-466b-9fd9-acc0827aaebc","added_by":"auto","created_at":"2024-01-24 14:59:03","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":13849,"visible":true,"origin":"","legend":"\u003cp\u003eContact resistance between contact pairs at different service temperatures\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/28f6dc40d307a7295dee9a92.png"},{"id":50102509,"identity":"cc0acd2f-f555-4e46-acae-ce413498f74a","added_by":"auto","created_at":"2024-01-24 15:07:03","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":53929,"visible":true,"origin":"","legend":"\u003cp\u003eWear mechanism of current-carrying contact pairs at different service temperatures\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/a998e90c354848db8903de9b.png"},{"id":56488074,"identity":"79052419-8335-4096-8b7c-1efd352131ca","added_by":"auto","created_at":"2024-05-14 21:28:38","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1692908,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3872711/v1/0ad279f8-2848-4ea8-a0af-05607e025d8d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"The effect of temperature on the current-carrying tribological behevior of C/Cu contact pairs in high humidity environments","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe current-carrying friction pair not only has mechanical functions such as transmission and bearing, but also has the function of electric energy transmission. Current-carrying friction and wear exist in various fields such as railway, aerospace, electronics, circuits, weapons and equipment, such as carbon strips and contact wires of electrified railways, armature and guide rail materials of electromagnetic railguns, electrical contacts of high-voltage switches, etc. \u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. Due to its good self-lubricating and friction reducing properties, and excellent arc resistance, carbon materials with layered structure are commonly used as lubricating components and matrix materials of current-carrying tribo-pairs, and are widely used in pantograph strips of electric locomotives, graphite brushes and other current-carrying composites as lubricating components. In the pantograph-catenary system or brush system, the most prevalent and well-matched materials are the sliding current-carrying tribo-pairs made of copper material and graphite blocks \u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eUnlike traditional mechanical wear, current-carrying friction is a unique and complex working state caused by the presence of an electric field. Numerous research have examined how the friction and wear properties of C-Cu contact pairs are affected by various working conditions, including relative speed, contact pressure, polarity, and current \u003csup\u003e[\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. However, varied service environments (temperature, humidity, atmosphere, etc.) also play a role in influencing current-carrying friction and wear in addition to operational conditions \u003csup\u003e[\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. Taking the pantograph catenary system supplying current to high-speed train through sliding contact as an example, it is an open and complex current-carrying tribological system. The interface friction, current-carrying capacity and damage failure between pantograph and contact wire will be strongly disturbed by the external environment, and the complicated operating environment will have a major impact on the locomotive's ability to operate safely and steadily \u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. The replacement amount of carbon strips for Stockholm electric locomotive in winter is much larger than that in summer, which is about 9\u0026ndash;10 times; In addition, the lower the temperature, the greater the proportion of electrochemical wear of the carbon strips, while there is almost no electrochemical wear in summer \u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe frictional and electrical properties are greatly affected by temperature due to the synchronous occurrence of friction and conduction at the current-carrying friction contact interface \u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e and the strong interference of ambient temperature on the surface/interface characteristics. Ding et al. \u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e evaluated the electric sliding friction and wear behavior of carbon strips at different temperatures (room temperature \u0026minus;\u0026thinsp;300℃), and obtained that the wear amount of carbon strips increased slowly as the contact temperature increased. They also concluded out that appropriate cooling measures for the friction pair were an effective way to reduce wear. Similarly, J Bu et al. \u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e discovered that, above 200\u0026deg;C, adhesive wear replaces abrasive wear as the primary current-carrying wear mechanism of carbon strips. The friction interface temperature is the key factor affecting the friction products and electrochemical reaction. When the external temperature is applied to different current-carrying contact pairs, the electrical transmission characteristics will be affected due to the different oxidation degree. For example, Liu et al. \u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e obtained by studying the tribological behavior of copper-graphite brush at 30\u0026ndash;60℃ that the surface film primarily made up of Cu, O and C elements acts as a barrier and lubrication on the contact surface, while its integrity and stability decrease as the increasing temperature, and the wear rate rises accordingly. Wang et al. \u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e investigated the contact resistance of different metals from a microscopic perspective by changing the temperature (room temperature\u0026thinsp;~\u0026thinsp;300℃). They discovered that high temperatures promoted the production of metal oxide films and the oxidation of wear debris, becoming the insulating layer. Therefore, metals that are easy to oxidize usually have high contact resistance at high temperatures. Unfortunately, the test temperature selected in the above research is far from the actual service temperature, so it is challenging to assess the current-carrying tribological properties of carbon materials below room temperature. Nevertheless, it is confirmed that temperature has an impact on the wear degree and current-carrying properties of tribo-pairs. The tribological characteristics of various current-carrying materials were investigated at -20\u0026deg;C by Liu et al. \u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e, but they did not confirm if the wear state of carbon strips at low temperatures differed noticeably from room temperature or if there was a temperature correlation between the tribological behavior and temperature. Therefore, most of the work ignored the effect of low temperature on the damage and current-carrying tribological properties of carbon strips. On the other hand, the service behavior of the current-carrying system caused by water dew and ice frost under low temperature and high humidity environment is still unclear. Consequently, it is important to confirm the above properties at various low temperatures in order to provide theoretical basis and technical support for the current-carrying tribological behavior in extreme environments in the future.\u003c/p\u003e \u003cp\u003eBased on the above discussion, the effects of different temperatures (20, 0, -20℃) on the current-carrying tribological behavior of C/Cu tribo-pairs in high humidity environment (85% RH) were studied. It mainly involves the influence law of friction coefficient (COF) and wear volume, and the damage morphology of the third body medium (water, ice, oxide film, etc.) acting on the carbon material, so as to comprehend and characterize the materials\u0026rsquo; current-carrying wear mechanism under the synergistic effect of temperature and current.\u003c/p\u003e"},{"header":"2. Test materials and methods","content":"\u003cp\u003eThe tribological tests were carried out on a reciprocating friction and wear tester (CETR UMT-3, Bruker, USA). The tester is equipped with a humidity regulation system and temperature control elements to form a temperature and humidity controllable current-carrying friction test bench. The schematic diagram of the device is presented in Fig. 1(a). The humidity regulation system is composed of a humidity generator, a humidity sensor, an environmental chamber and a conducting tube. The refrigeration plate for temperature control is composed of two semiconductor refrigeration sheets surrounded by water pipes, and then connected to the temperature control system. Fig. 1(c) displays its schematic diagram. During the experiment, the circulating water enters the water pipe, so that the low temperature generated by the refrigeration plate can be extended. A specimen clamp is placed on the refrigeration plate, and the friction environment in the environmental chamber is maintained at the set temperature through contact and air conduction. The humidity adjustment range is 10-90%, and the temperature adjustment range is -25℃ -+25℃. The fluctuation of temperature and humidity can be controlled at \u0026plusmn;2℃ and \u0026plusmn;5% RH respectively. The current-carrying circuit mainly includes DC regulated power supply, variable resistance, upper sample, lower sample, current sensor, voltage sensor, data acquisition card and several wires. Fig. 1(b) depicts its circuit diagram. It is worth mentioning that the clamps of the upper and lower samples are made of insulating materials, which can ensure the insulation between the current circuit and the testing machine. The data acquisition system continuously measures the load and shear force in real time, and sends the data directly to the computer.\u003c/p\u003e\n\u003cp\u003eThe materials used in the current-carrying friction test are carbon rod samples and copper rod samples. The cylindrical carbon rod\u0026nbsp;(\u003cem\u003eR\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e=0.2 \u0026mu;m)\u0026nbsp;with a diameter of 10 mm and a length of 30 mm, which is processed from the carbon strips in service on the high-speed railway, is the upper sample. The lower sample is a heat-treated cylindrical copper bar (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e=0.05 \u0026mu;m)\u0026nbsp;with a diameter of 15 mm and a length of 30 mm made of T2 red copper (\u003cem\u003eR\u003c/em\u003e\u003csub\u003ea\u003c/sub\u003e=0.05 \u0026mu;m). Fig. 2 presents the microstructure and morphology of the test material. The operation mode of the two samples is: the lower sample moves back and forth while the top sample is subjected to the load. Before the experiment, the carbon rod and copper rod are assembled in the clamps to achieve contact, and the ambient chamber temperature is adjusted to the set value. The parameter settings of the current-carrying friction experiment are displayed in Table 1.\u003c/p\u003e\n\u003cp\u003eTable 1 Details of test parameters\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"584\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd width=\"9.246575342465754%\" valign=\"top\"\u003e\n \u003cp\u003eCycles\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.013698630136986%\" valign=\"top\"\u003e\n \u003cp\u003eFrequency\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.267123287671232%\" valign=\"top\"\u003e\n \u003cp\u003eSliding displacement\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.301369863013699%\" valign=\"top\"\u003e\n \u003cp\u003eNormal load\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.34931506849315%\" valign=\"top\"\u003e\n \u003cp\u003eTempreature\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.80821917808219%\" valign=\"top\"\u003e\n \u003cp\u003eElectric current\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.013698630136986%\" valign=\"top\"\u003e\n \u003cp\u003eHumidity\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd width=\"9.246575342465754%\" valign=\"top\"\u003e\n \u003cp\u003e10000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.013698630136986%\" valign=\"top\"\u003e\n \u003cp\u003e4 Hz\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"16.267123287671232%\" valign=\"top\"\u003e\n \u003cp\u003e\u0026plusmn;5 mm\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"11.301369863013699%\" valign=\"top\"\u003e\n \u003cp\u003e10 N\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"19.34931506849315%\" valign=\"top\"\u003e\n \u003cp\u003e20℃,0℃,-20℃\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"17.80821917808219%\" valign=\"top\"\u003e\n \u003cp\u003e0 A，5 A，10 A\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd width=\"13.013698630136986%\" valign=\"top\"\u003e\n \u003cp\u003e85%\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe morphology of the worn samples were investigated using a scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and element distribution were dectected using a Flash 6I60 X-ray energy dispersive spectrometer (EDS, Xflash 6160, Bruker, USA). The 3D morphology and wear depth of the samples could be obtained by a 3D surface profilometer (Bruker, Contour GT-K, USA). The wear volume is used to measure the degree of carbon specimens\u0026rsquo; wear. Due to the fact that the wear scar of the carbon specimen is a concave ball pit, the wear region is estimated as a ball crown body in order to compare the wear losses. Therefore, the wear volume formula of the carbon rod can be converted using the ball crown volume formula [20]:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eWhere, V represents the wear volume (mm\u003csup\u003e3\u003c/sup\u003e), h represents the height of the ball crown body, which is the depth of the wear scar (mm), and r represents the radius of the ball crown section, which is half of the width of the wear scar (mm).\u0026nbsp;\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eFig. 3 (a-c) displays the time-varying curve of COF between contact pairs at different service temperatures (i.e., 20, 0, and -20 ℃) and currents of 0, 5, and 10 A, respectively. Fig. 3 (d) shows the average COF at the stable stage (2000-8000 cycles) under various parameters. By observing Fig. 3 (a-c), it is found that the COF basically shows an upward trend with the decrease of temperature, whether current is applied or not. Under non-current-carrying conditions (Fig. 4 (a)), The COF is higher at -20 \u0026deg;C because of resistance to the ice layer that forms on the surface and to the ice fragments that are left behind when the ice layer breaks. 0℃ is the critical point of water freezing, and the water vapor in the air condenses on the cold contact pair to form a thin ice crystal layer. However, when the contact pair slides relatively, the COF will decrease slightly due to the ice crystal layer moving and melting under the action of friction heat.\u0026nbsp;The minimum COF is achieved at 20\u0026deg;C due to an improvement in the lubricating state and an increase in water film thickness. When current is applied (Fig. 4 (b-c)), the difference of COF between contact pairs at different temperatures increases. The combined action of friction heat and Joule heat leads the ice crystals around the tribo-pairs at 0\u0026deg;C to melt into water very quickly when the current increases to 10 A, and the COF progressively drops. Then the water molecules in the air condense on the cold copper surface, and the wear debris and water molecules form a wetting equilibrium state. Therefore, the COF is less than the COF at 20℃ when the contact surface is relatively dry.\u003c/p\u003e\n\u003cp\u003eThe COF decreases at various temperatures after applying current as shown in Fig. 4(d). At 20\u0026deg;C, the oxidation reaction on the surface is accelerated by the combined effect of friction and Joule heat, and the resulting oxide film provides a slight lubricating effect. On the other hand, the softening degree of the copper contact point increases, which reduces its shear effect on the carbon micro-protrusions and decreases the the COF. At -20 \u0026deg; C, there is a liquid like layer between the substrate and ice, as well as on the free ice surface, composed of an amorphous layer of water molecules \u003csup\u003e[21]\u003c/sup\u003e. It arises from the pre-melting phenomenon on the material surface. The flow behavior of ice and snow, substances adsorption on the ice surface, and the low friction of materials on the ice surface are all significantly influenced by this liquid-like layer \u003csup\u003e[22]\u003c/sup\u003e. Similar to the wetting material between the ice and the substrate surface, the liquid-like layer can increase the effective contact area between the two by changing its thickness, thus affecting the adhesion strength of ice, namely the tangential ice strength \u003csup\u003e[23]\u003c/sup\u003e. As the temperature of the contact surface rises, the thickness of the liquid-like film increases. This decreases the tangent line\u0026apos;s strength to create ice, making the ice layer easier to remove. Meanwhile, the ice layer becomes thinner with increasing current and is more easily crushed by external forces, resulting in a decrease in COF.\u003c/p\u003e\n\u003cp\u003eFig. 4 presents the three-dimensional topography of the carbon samples\u0026rsquo; worn surface. It can be seen that under no-current conditions, there are more buildups around the wear scar compared to the worn surface after applying current. SEM observation reveals that the buildups on the edges are formed by debris accumulation, and the debris accumulated before and after applying current exhibit different topographic states. Taking 20\u0026deg;C as an example, the buildups at 0 A are black dense films, which are presumed to be carbon debris accumulated around the wear scar under the push and squeeze of reciprocating motion. After applying current, the buildups are dominated by white dense films. It is important to note that at -20\u0026deg;C, the wear scar boundaries of carbon samples under 0 and 5 A are not very clear. Additionally, material stacking around the scar edge forms a wave-like ring structure, which causes the wear scar diameter under various currents at -20\u0026deg;C to vary significantly. The composition, morphology, and formation process of this ring-shaped deposit will be discussed in Fig. 6.\u003c/p\u003e\n\u003cp\u003eThe average wear volume of carbon samples at various temperatures is displayed in Fig. 5. It is evident that non-current-carrying conditions cause minimal damage to the material, while the current accelerates the deterioration. The wear volume reduces with a drop in the service temperature. Consequently, the wear volume is lowest at -20\u0026deg;C under non-current conditions. When current is applied, the contact points between the tribo-pairs not only bear high stress, but also bear high current and high temperature, which leads to the high-current carbon material contact points at 20\u0026deg;C being prone to wear. A paste that sticks to the contact surface and engages in friction is created when the worn debris from previous friction combines with the water film and ice slag at 0\u0026deg;C and -20\u0026deg;C. The paste builds up around the wear scar under reciprocating action, which is why the wear scar profile diameter at the two temperatures is substantially less than at 20\u0026deg;C (see Fig. 4). At the same time, the mixture of wear debris and water participates in secondary wear, avoiding direct damage to the friction pair. On the other hand, the presence of the blend increases the number of contact points, and the friction surface can be cooled in time without the micro-bumps being softened and cut. It is worth noting that the influence of different temperatures under the same current on the wear volume is greater than the influence of changing current under the same temperature, that is, compared to current, temperature changes have a more serious effect on the damage to carbon materials.\u003c/p\u003e\n\u003cp\u003eIn order to investigate the ring-shaped structure of the the wear scar\u0026rsquo;s edge at -20\u0026deg;C, EDS analysis was performed under various current-carrying conditions (as shown in Fig. 6). It was found that the ring-shaped structure at the edge was basically a concentrated distribution area of copper elements (region I in Fig. 6(a-c)), indicating that material transfer occurred during the friction process. Various current-carrying conditions resulted in various copper element distributions. When there is no current, copper elements are mainly distributed in the outer ring-shaped region in the form of cellular particles (Fig. 6(a)), which are composed of countless nanoscale particles, as shown in Fig. 6(d). After applying 5 A current, copper elements were found to be concentrated in both the center and edge rings (Fig. 6(b)), and the ring-shaped structures on both sides perpendicular to the sliding direction were more pronounced and surrounded by multiple rings, as shown in Fig. 6(e). It was found that the ring-shaped structures were composed of small particles and carbon film coverings. The center of the wear scar was dominated by ploughing, and covered with fine debris, as shown in Fig. 6(b) part II. As illustrated in Figs. 6(c) and 6(f), the copper element is concentrated on both sides of the sliding direction in the form of dense films after applying a current of 10 A. From the morphology and element distribution, it was inferred that during the sliding wear process, the copper surface froze at -20\u0026deg;C, and the contact pair broke the ice under the action of shear force and material transfer occurred. A mixture of small copper particles, a small quantity of carbon shavings, and ice slag is heaped up on the worn surface and then scattered under pushing around the wear scar. During the continuous accumulation, ice condenses around the wear scar and mixes with the debris to form a ring-like structure. Under current-carrying conditions, copper transfers to the carbon wear scar and mixes with carbon debris to adhere to the center of the wear scar. When the thickness increases to a certain extent, it is pushed away during reciprocating action. It is also because some copper remains in the center that the proportion of copper elements in region I decrease (see Fig. 6(h)). As the current continues to increase, Joule heating thickens the liquid-like film between the surface and the ice around the contact area, making the ice layer easier to remove. The mixing flow of debris and water in the contact area is enhanced, and it is easily pushed by the friction pair to the two sides along the direction of the ploughing, and is compacted by reciprocating rolling to form dense films. The element composition analysis of Fig. 6(d-f) reveals that the oxygen element progressively increases with applied current, suggesting that an increase in current encourages the production of copper oxide layer.\u003c/p\u003e\n\u003cp\u003eThe surface morphology of the carbon samples\u0026rsquo; wear scar center at different service temperatures is displayed in Fig.7. Significant ploughing features are seen on the worn surface at 20\u0026deg;C (Fig. 7(a)). When an electric current is applied, large areas of dense white patches appear (Fig. 7(b and c)), which adhere to the substrate surface and integrate with it (as shown in Fig. 7(k)). Around worn scars are the areas where the white spots are predominantly concentrated. Through EDS analysis, it is found that the white areas are mainly composed of copper and oxygen elements (Fig. 8(a)). In addition, the white patches under 10 A are more dense than those under 5 A, and delamination and debris also appear. As a result of continuous friction and current flow across the contact site, friction heat and Joule heat builds up on the worn surface. \u0026nbsp;A significant area of copper transfer film is formed and oxidized as a result of the contact pair\u0026apos;s temperature rising \u003csup\u003e[24]\u003c/sup\u003e. Because of the combined effects of thermal stress, shear strain, and normal force, brittle cracks are generated as the amount of copper transfer rises. The expansion of cracks promotes the delamination of layers and the production of small amounts of wear debris \u003csup\u003e[25]\u003c/sup\u003e. The good solid lubricating effect of this copper oxide debris is another significant factor contributing to the COF drop when compared to the current-free situation \u003csup\u003e[18, 26]\u003c/sup\u003e. However, uneven oxidation thickness can cause current concentration at the contact interface, resulting in local current suddenly increasing and fusion occurring, which increases the wear amount of the material \u003csup\u003e[27]\u003c/sup\u003e. At 0\u0026deg;C, many scattered small debris and shallow ploughing can be observed on the worn surface, but no large oxide film is found, but rather scattered white patches (Fig. 7(d)). The worn surface is covered with a large amount of peeling debris and small particles under 5 A (Fig. 7(e)), and magnified images reveal scattered and embedded white particles in the carbon film, with a size of approximately 1 \u0026mu;m (Fig. 7(m)). EDS testing reveals that the white particles in Fig. 7(m) EDS A contain more copper elements than other regions (Fig. 7(m) EDS B and C). Therefore, in low temperature environments, the friction surface does not form a complete and dense copper oxide film, but exists in the form of dispersed or embedded copper particles, which are evenly dispersed and embedded in the carbonaceous material. After repeated wrapping and extrusion, they integrate with the matrix. However, the worn surface smooths out and the amount of white spots decreases when the current reaches 10 A. There is no visible oxide film and the wear center is scattered with peeling and pitting when the temperature falls below -20\u0026deg;C. There is also a layered structure that is in line with the usual traits of delamination wear (inset in Fig. 7(h)). When hard copper shear carbon surface at low temperature, the material at the interface is prone to block breakage and brittle fracture, accompanied by spreading cracks, and the cracks extend horizontally to the surface and form a layered structure \u003csup\u003e[28,29]\u003c/sup\u003e. After applying a current of 5 A, the wear scar surface is distributed with cracks and a small number of peeling pits, and the layered structure disappears. This indicates that the copper contact point is sufficiently softened by the Joule heat, and the shearing impact on the carbon micro-protrusions is nearly eliminated. At a current of 10 A, the surface of the wear scar is relatively smooth and has cracks, with local oxide films formed by the stacking of nanoscale particles (see Fig. 7(j, l)). The above phenomena indicate that the material undergoes fatigue wear at -20\u0026deg;C. After applying a current, the degree of fatigue wear decreases. It is important to note that the transfer behavior of copper to the carbon surface is effectively reduced by the interface temperature drop, as Fig. 8(b) illustrates. On the one hand, it reduces the possibility of the contact pair from carbon-copper lubrication friction to copper/copper high adhesion friction. On the other hand, abrasive wear is decreased because fewer particles of copper and copper oxide are involved in the current-carrying friction process. In addition, as the temperature decreases, the Cu-O ratio also gradually decreases, indicating that in a high humidity environment, the ability of the carbon worn surface to adsorb water molecules increases as the temperature decreases, and the presence of ice water layers promotes oxidation reactions on the worn surface, including both carbon oxidation and copper oxidation. In general, the degree of surface oxidation increases with current.\u003c/p\u003e\n\u003cp\u003eThe distribution of EDS elements and the worn surface morphology of copper samples at varying service temperatures with currents of 0 A and 10 A are displayed in Fig. 9. At 20\u0026deg;C, the worn surface of the copper without current adheres to a black film, which is identified as element C by EDS surface scanning (Fig. 9(a)).After applying a current of 10 A, the copper surface exhibits severe peeling at the ploughing groove, with debris mainly concentrated in the groove (Fig. 9(b)). The worn copper surface is comparatively smooth at 0\u0026deg;C, with the debris dispersed in granular form (Fig. 9(c)). The peeling of the copper surface is shown to increase at -20\u0026deg;C (Fig. 9(e)), especially when a current of 10 A is applied (Fig. 9(f)). It is noteworthy that at -20\u0026deg;C, splashes of copper particles are observed on the carbon worn surface (Fig. 9(g)), indicating that the presence of ice water results in erosion of the electric arc. Water under the action of ionization in the electric arc will produce H\u003csup\u003e+\u003c/sup\u003e dissolved in the metal, making the surface brittle and supersaturated \u003csup\u003e[30]\u003c/sup\u003e, thus exacerbating the flaking of the copper surface. Comparing the roughness of the copper worn surface under different current conditions at the same temperature (Fig. 9(h)), the copper with a current of 10 A has the highest roughness and the most severe surface damage. It implies that the intervention of electrical variables considerably deteriorates the friction and wear properties of the material, which is consistent with the severe peeling occurrence of the copper surface under 10 A. From the above carbon\u0026rsquo;s worn surface, it is observed that copper undergoes severe material transfer at room temperature, resulting in a surface roughness greater than that at -20\u0026deg;C and 0\u0026deg;C.\u003c/p\u003e\n\u003cp\u003eThe contact resistance of the current-carrying tribo-pairs at various service temperatures is displayed in Fig. 10. Generally, the contact resistance consists of the film resistance on the surface of the material and the contraction resistance caused by the current contraction effect \u003csup\u003e[18]\u003c/sup\u003e. As can be seen that, under a current of 5 A, the contact resistance at -20℃ and 0℃ both have a large initial value at the beginning of the cycle (within 2000 cycles), and then a sharp decrease. This is due to the presence of water film or ice layer on the surface during the initial stage of friction, and undergoes changes from a mixed layer of ice and water to a mixture of ice slag and wear debris. The state of the contact surface becomes extremely complex, and the participation of wear debris greatly improves the conductivity of the water film, resulting in a sharp decrease in contact resistance until the blending reaches equilibrium. Comparing the average contact resistance values in the stable stage (2000-10000 cycles) in Fig. 10(b), it can be found that the contact resistance at room temperature under 10 A is higher than that under 5 A. When at low temperatures, the contact resistance under 10 A current decreases instead. Upon applying 10 A, the C sample\u0026apos;s worn surface morphology shows that the contact surface developed an oxide layer and turned into a poor conductor, which causes the film resistance to rise quickly as the film thickness grows \u003csup\u003e[31]\u003c/sup\u003e. When the temperature drops to 0℃, water molecules in the air encounter the cold copper surface and continuously condense into small water droplets that participate in the friction process, greatly affecting the transmission of electrical signals. The water film\u0026apos;s impact is lessened as the current is increased, which lowers the contact resistance. Although the oxide film has not formed at -20\u0026deg;C, the additional film resistance generated by the ice film and liquid-like film is still relatively large. However, the additional film resistance decreases due to the melting of the ice film when the current increases.\u003c/p\u003e\n\u003cp\u003eThe wear mechanism of C/Cu tribo-pairs with changes in temperature and current is schematically diagrammed in Fig. 11. Under non-current-carrying conditions, after the C/Cu sample undergoes wear in a room temperature environment, a small amount of debris accumulates on the wear edge and repeatedly plows the contact interface during reciprocating motion. Applying current causes the microconvexity to soften, enhancing copper transfer and progressively generating a dense and complete copper transfer film. Meanwhile, chemical oxidation and electrochemical oxidation occur, and the characteristics of abrasive wear and adhesive wear are significant, leading to increased wear loss of carbon. A decrease in COF and an increase in contact resistance result from the thickening of the copper oxide film, which has a greater adsorption capacity for water molecules than pure copper \u003csup\u003e[32]\u003c/sup\u003e. The variation of the COF with the current is consistent with the results of literature [33]. As the temperature decreases to -20\u0026deg;C, the worn surface of carbon increases its shear strength to resist the adhesion strength of ice, resulting in an increase in COF and surface peeling. Delamination wear is typically characterized by dislocations and cracks occurring beneath the carbon surface that expand horizontally to the surface. Meanwhile, although the sustained low temperature has a suppressive effect on the accumulation of interfacial Joule heat after applying current \u003csup\u003e[28]\u003c/sup\u003e, there is still a temperature rise on the surface of the material compared to pure mechanical friction. The liquid-like film between the ice film around the contact pair and the material gradually thickens, reducing the ice formation strength of the tangent line and and acting as a lubricant to reduce COF. As a lubricant and coolant, the water film that forms between the contact pairs lessens the amount of copper that is transferred to carbon and lowers the wear loss of carbon. The amount of water molecules adsorbed on the cold copper increases compared to room temperature. The oxidation reaction of the worn surface is enhanced by the coupling effect of current and water. As the temperature decreases, the wear mechanism shifts from adhesive wear to slight fatigue wear. This finding is consistent with the conclusion of the coupling effect of water and current in literature [31]. The aforementioned findings show that the interplay of heat exchange, water state, and current alters the wear mechanism of the contact pair at low temperatures.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eIn this work, the tribological properties of C-Cu under current-carrying conditions were analyzed in detail at ambient temperatures of -20, 0, and 20℃.The evolution laws of COF, wear volume, contact surface characteristics, and contact resistance of the C-Cu tribo-pairs at different ambient temperatures were investigated, and the current-carrying wear mechanism of C-Cu at low temperatures was deeply explored. The main findings of this study can be summarized as follows:\u003c/p\u003e\n\u003cp\u003e(1) The COF at various temperatures exhibits a similar variation pattern before and after applying current. That is, the COF increases with decreasing temperature. However, the average COF after applying current at each temperature was lower than that without current. The fluctuation and average value of contact resistance is large under low temperature conditions, with a more significant change at low currents (5 A).\u003c/p\u003e\n\u003cp\u003e(2) The application of current accelerates the damage to carbon materials. An increase in current at room temperature can facilitate copper transfer and the development of an oxide film, both of which lower the COF of the C/Cu contact pair. However, it can increase the copper loss and the contact resistance, and the wear mechanism is dominated by adhesive wear.\u003c/p\u003e\n\u003cp\u003e(3) The transfer behavior of copper is effectively reduced by the drop in interface temperature, but the water produced at this time can function as a catalyst to promote the occurrence of oxidation reactions and shield the carbon material from wear and damage. The wear and damage of the friction materials are more affected by temperature changes than by applied current. The coupling effect of low temperature and current is beneficial for the wear mechanism of the C/Cu contact pair to shift from adhesive wear to slight fatigue wear.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by National Natural Science Foundation of China (nos. 52365022 and 52375181), Natural Science Foundation of Jiangxi Province (no. 20224ACB204012) and Postgraduate Innovation Special Fund Project in Jiangxi Province (no. YC2022-B177), General Subject of State Key Laboratory of Performance Monitoring and Protecting of Rail Transit Infrastructure (no. HJGZ2023208).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. Material preparation and results analysis were performed by D.H. Ji, M.X. Shen and Q. Hu. The manuscript draft was written by D.H. Ji and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of competing interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData supporting the findings of this work are available upon request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eS.B. Li, X.Z. Yang, Y.Q. Kang, Z.Y. Li, H.W. Li, Progress on current-carry friction and wear: An overview from measurements to mechanism, Coatings. 12 (2022) 1345. https://doi.org/10.3390/coatings12091345.\u003c/li\u003e\n \u003cli\u003eW. 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(1997)52-59.\u003c/li\u003e\n \u003cli\u003eY. Sun, C. Song, Z. Liu, J. Li, Y. Sun, B. Shangguan, Y. Zhang, Effect of relative humidity on the tribological/conductive properties of Cu/Cu rolling contact pairs, Wear. 436\u0026ndash;437 (2019) 203023. https://doi.org/10.1016/j.wear.2019.203023.\u003c/li\u003e\n \u003cli\u003eY. Sun, C. Song, Y. Zhang, M. Li, Y. Zhang, Oxidation on the current-carrying rolling surface and its subsequent impact on the damage of Cu contact pairs in O 2 / N 2 mixture, Mater. Lett. 288 (2021) 129349. https://doi.org/10.1016/j.matlet.2021.129349.\u003c/li\u003e\n \u003cli\u003eX. Zhi, N. Zhou, Y. Cheng, X. Wang, H. Wei, G. Chen, W. Zhang, Effect and behaviors of ambient humidity on the wear of metal-impregnated carbon strip in pantograph-catenary system, Tribol. Int. 188 (2023) 108864. https://doi.org/10.1016/j.triboint.2023.108864.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Temperature, high humidity, Ice, Current-carrying, C/Cu, Wear","lastPublishedDoi":"10.21203/rs.3.rs-3872711/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3872711/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe environmental temperature alters the frictional behaviour by changing the state of the current-carrying contact interface, which makes the electrical contact invalid. In this work, the effects of three different temperatures (-20 ℃, 0, 20 ℃) on the current-carrying tribological behaviour of C-Cu tribo-pairs in high humidity environment (85%) were discussed. The evolution laws of friction coefficient, wear volume, contact surface properties, and contact resistance of C-Cu contact pairs under the coupling effect of temperature and current were studied, and the current- carrying wear mechanism of C-Cu at low temperature was analyzed in depth. The friction coefficient at each temperature exhibits a similar changing rule before and after current-carrying, demonstrating that the friction coefficient increases as temperature falls. However, the average friction coefficient at each temperature is lower than that without current. Although it will hasten the material surface's oxidation, a drop in ambient temperature will effectively lessen the transfer behavior of copper to carbon surface and reduce the wear volume of carbon material. The amount of copper transferred increases as current rises. Compared with the current, the change of temperature has a greater impact on the damage of tribo-pairs. At room temperature, the contact resistance under high current is greater than that of low current, the low temperature is just the opposite. In addition, at 0℃, although the contact resistance of low current (5 A) decreases significantly in the early stage of friction, its average resistance and fluctuation amplitude are the largest. As the temperature decreases, the current-carrying wear mechanism of C-Cu contact pairs gradually changes from adhesive wear to fatigue wear.\u003c/p\u003e","manuscriptTitle":"The effect of temperature on the current-carrying tribological behevior of C/Cu contact pairs in high humidity environments","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-24 14:58:58","doi":"10.21203/rs.3.rs-3872711/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-02-26T13:55:17+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-02-20T09:10:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"2ed62b1f-e9ef-4e71-94cd-6534ed54797e","date":"2024-02-05T07:51:15+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-01-22T08:16:12+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-01-20T02:57:42+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-01-20T02:57:41+00:00","index":"","fulltext":""},{"type":"submitted","content":"Tribology Letters","date":"2024-01-17T11:10:52+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"tribology-letters","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"tril","sideBox":"Learn more about [Tribology Letters](https://www.springer.com/journal/11249)","snPcode":"11249","submissionUrl":"https://submission.nature.com/new-submission/11249/3","title":"Tribology Letters","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"bef2a323-96b4-4e24-9768-f1354e1fae46","owner":[],"postedDate":"January 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-05-14T21:21:00+00:00","versionOfRecord":{"articleIdentity":"rs-3872711","link":"https://doi.org/10.1007/s11249-024-01856-2","journal":{"identity":"tribology-letters","isVorOnly":false,"title":"Tribology Letters"},"publishedOn":"2024-05-07 21:17:41","publishedOnDateReadable":"May 7th, 2024"},"versionCreatedAt":"2024-01-24 14:58:58","video":"","vorDoi":"10.1007/s11249-024-01856-2","vorDoiUrl":"https://doi.org/10.1007/s11249-024-01856-2","workflowStages":[]},"version":"v1","identity":"rs-3872711","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3872711","identity":"rs-3872711","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}