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Review

Research Progress on Current-Carrying Friction with High Stability and Excellent Tribological Behavior

by
Peng Wei
1,
Xueqiang Wang
2,
Guiru Jing
2,
Fei Li
2,
Pengpeng Bai
1 and
Yu Tian
1,*
1
State Key Laboratory of Tribology in Advanced Equipment, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, China
2
Aerospace System Engineering Shanghai, Shanghai 201109, China
*
Author to whom correspondence should be addressed.
Lubricants 2024, 12(10), 349; https://doi.org/10.3390/lubricants12100349
Submission received: 14 September 2024 / Revised: 9 October 2024 / Accepted: 11 October 2024 / Published: 13 October 2024
Browse Figures
Versions Notes

Abstract

:
Current-carrying friction affects electrical contact systems like switches, motors, and slip rings, which determines their performance and lifespan. Researchers have found that current-carrying friction is influenced by various factors, including material type, contact form, and operating environment. This article first reviews commonly used materials, such as graphite, copper, silver, gold, and their composites. Then different contact forms like reciprocating, rotational, sliding, rolling, vibration, and their composite contact form are also summarized. Finally, their environmental conditions are also analyzed, such as air, vacuum, and humidity, on frictional force and contact resistance. Additionally, through experimental testing and theoretical analysis, it is found that factors such as arcing, thermal effects, material properties, contact pressure, and lubrication significantly influence current-carrying friction. The key mechanisms of current-carrying friction are revealed under different current conditions, including no current, low current, and high current, thereby highlighting the roles of frictional force, material migration, and electroerosion. The findings suggest that material selection, surface treatment, and lubrication techniques are effective in enhancing current-carrying friction performance. Future research should focus on developing new materials, intelligent lubrication systems, stronger adaptability in extreme environments, and low friction at the microscale. Moreover, exploring stability and durability in extreme environments and further refining theoretical models are essential to providing a scientific basis for designing efficient and long-lasting current-carrying friction systems.

1. Introduction

Friction accounts for 30% of the world’s primary energy consumption, and 80% of mechanical component failures are attributed to friction [1,2]. Current-carrying friction refers to the frictional behavior and characteristics between contact surfaces when an electrical current passes through a frictional contact interface [3,4]. Unlike friction that occurs without current, current-carrying friction involves not only mechanical contact and relative motion but is also influenced by the electromagnetic and thermal effects caused by the current [4,5]. During this process, the current can alter the temperature, resistance, and friction coefficient at the contact interface, leading to wear, material transfer, and changes in the microstructure of the contact surface [6,7]. The primary objective of studying current-carrying friction is to understand and control the mechanisms that reduce friction and wear while maintaining low contact resistance [8,9,10]. This knowledge is crucial for improving the performance and lifespan of equipment in applications such as electrical contactors [11,12], sliding contacts [13,14,15], and microelectromechanical systems [16,17]. The challenge in this field lies in the need to efficiently transmit current while minimizing interfacial friction, making it a significant concern in tribology.
In terms of materials, conductivity and wear resistance are key factors influencing friction behavior [11,18,19]. Additionally, contact forms [20,21,22], applications [23], and environmental conditions, such as current intensity [24,25], temperature [16,26], and humidity [27,28,29], can significantly impact this behavior. Other influencing factors include contact resistance [30,31], interface oxidation [32,33], and arc discharge [31,34], which are particularly relevant in applications like brush slip rings and switch contactors. The primary mechanism involves the thermal and electric field effects induced by current at the friction interface, which alter the interface properties, leading to changes in the friction coefficient and wear rate [23,35]. Thermal and electric field effects significantly affect the reliability and lifespan of equipment, as illustrated in Figure 1. This review will provide an overview of six aspects: materials, contact types, environment, influencing factors, application scenarios, and mechanisms.
The development of the theory of current-carrying frictional interface electrical transmission is an interdisciplinary field that integrates tribology, electrical transmission, materials science, and quantum mechanics [6,7]. Below is a brief review of the development history of this field, as depicted in Figure 2. The first stage, early research (early to mid-20th century), primarily focused on the macroscopic phenomena of tribology, particularly the relationship between friction and surface roughness [38]. As electrical technology advanced, researchers began to emphasize the significance of electrical transmission at contact interfaces, especially concerning electrical contact resistance. During the 1940s and 1950s, scholars like Holm [38] and Greenwood [39] laid the foundation for electrical contact resistance theory by proposing the contact point theory, which explained the relationship between the true contact area and contact resistance. However, research in this period was mainly centered on static contact interfaces, with limited attention given to the electrical properties of dynamic friction interfaces.
The second stage, the discovery and preliminary theory of frictional electrical phenomena (the 1960s to 1980s), saw the identification of frictional electrical phenomena, such as frictional current and frictional potential, during frictional contact [40]. This finding attracted quite a lot of attention in the scientific world, leading to inquiries into how friction relates to electrical transmission. It was the 1970s when advances in semiconductor and microelectronics technology made it possible for researchers to look into frictional interfaces at the nanoscale with electrical transmission, which is not quite like the classical theory of electrical contact because of quantum effects and tunneling phenomena happening. By the 1980s, early friction current models started being established, and researchers began suggesting quantum mechanical reasons for how friction current is generated.
The third stage, nanotribology along with quantum effects (1990s going to early 21st century), showing nanotechnology rise, brought methods such as AFM and STM, used in nanoscale frictional interface electrical transmission studies [41,42]. At this time, many researchers have found quantum effects in frictional interface electrical transmission. Entering the 2000s, quantum friction theories and frictional potential understanding show how electronic states and coupling at frictional interfaces are affecting electrical transmission details. More advanced theoretical models, like non-equilibrium Green’s function method (NEGF) [43] and density functional theory (DFT) [44], are to be applied to study nanoscale frictional interface electrical transport.
The fourth phase, modern development (21st century till now), came to bring research where multiple physical fields got coupled together [45]. Thermal, electrical, and mechanical behaviors at friction points started being considered by researchers, and this led to multi-scale simulations being developed. At the same time, the study of two-dimensional materials such as graphene in transmitting electricity through frictional points got attention because of their special properties for friction and electricity. These properties give options for both lower friction and lower resistance during contact. In the 2020s, the research has moved more toward dynamic adjustments and adaptive points of contact by using outside fields like electrical and magnetic, and smart materials started to adjust characteristics in real-time. Also, machine learning and artificial intelligence began to show up for predicting and making better optimizations of those characteristics. The development of the theory on current-carrying interfaces for electricity and friction goes on, from the old macroscopic ideas to the newer ways that look at quantum mechanics and connections between different physical fields. As the experiments and theory methods improve, this area keeps on growing and changing.
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Figure 2. Four stages of current-carrying friction development history [45]. Reproduced with permission from Ref. [45]; copyright 2024, Springer Nature).
Figure 2. Four stages of current-carrying friction development history [45]. Reproduced with permission from Ref. [45]; copyright 2024, Springer Nature).
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Summarizing the four stages of the development of current-carrying friction discussed above, the interface electrical transmission theory of current-carrying friction can be categorized into two main types: macroscopic theories based on electrostatic and electromagnetic principles and microscopic theories grounded in quantum mechanics. The classic macroscopic theories of electrical transmission include Ohm’s law and Holm’s constriction resistance formula. In contrast, the microscopic theories include the scattering theory proposed by Landauer et al. [40]. For field emission, the probability D of electron transmission through an interface can be approximated by the Wenzel–Kramers–Brillouin (WKB) method [41,45]. Moreover, this study aimed to explore current-carrying friction in terms of material properties, friction types, applications, environmental conditions, influencing factors, and underlying mechanisms, providing a scientific basis for the design of efficient and long-lasting current-carrying friction systems.

2. Progress on Current-Carrying Friction

The challenge of achieving both low friction and low contact resistance at an interface stems from the conflicting strategies required to meet these two objectives. Numerous theoretical and experimental studies have shown that reducing friction necessitates weakening the electronic coupling between interfaces [46]. This is because strong electronic coupling intensifies interfacial interactions, thereby increasing friction. By weakening electronic coupling, the interaction between interfaces is reduced, leading to a lower friction coefficient. Conversely, reducing contact resistance requires enhancing electronic coupling at the interface to lower the electron tunneling barrier. Enhanced electronic coupling facilitates easier electron passage through the interface, thereby reducing contact resistance [47]. The inherent contradiction is that weakening electronic coupling to reduce friction simultaneously increases contact resistance while strengthening electronic coupling to reduce contact resistance results in higher friction. The key to resolving this contradiction lies in identifying a mechanism that can dynamically adjust electronic coupling based on varying conditions or in precisely regulating the microstructure to locally achieve different functionalities.

2.1. Materials of Current-Carrying Friction

Materials play a crucial role in this context, as the selection and engineering of materials at the interface can offer tailored electronic properties that allow for the fine-tuning required to balance friction and contact resistance [12,48]. Advanced materials, with their customizable microstructures and adaptive characteristics, provide the foundation for developing interfaces that can meet these dual demands under varying conditions. Methods to improve current-carrying friction performance primarily focus on material selection and optimization. Different materials exhibit various advantages and disadvantages in current-carrying friction due to their conductivity [49,50], wear resistance [51,52], oxidation resistance [53], and other characteristics. To enhance current-carrying friction performance through material improvements in two main categories: metal or non-metal materials and composite materials.
Firstly, we discuss metal and non-metal materials. Copper is a commonly used conductive material with good conductivity and high mechanical strength, suitable for high current-carrying conditions [53,54]. N. Argibay et al. [11] used two copper brushes (the positive and negative) paired on a copper disk and tested them on the slip-ring. The results showed that when the current density reached 180 A/cm2, the COF of the positive brush was 0.3–0.4, while the value for the negative brush was only 0.18. Debris was observed on the surface of the negative brush, remaining as separate particles. In contrast, on the surface of the positive brush, the debris began to fuse together, as shown in Figure 3a, highlighted with red circles in the SEM images. Although copper exhibits a moderate COF in current-carrying friction, it is prone to surface oxidation, leading to increased contact resistance and intensified friction and wear [55,56]. Another most conductive metal is silver, which offers extremely low contact resistance and excellent oxidation resistance [57]. Pure materials can reveal the fundamental mechanisms of current influence on friction and wear behavior, as well as the surface electromigration effects in current-carrying friction. Their compositional uniformity and high data repeatability make them ideal candidates for theoretical model studies. However, due to their poor wear resistance, future research should prioritize a deeper understanding of current-friction coupling effects and the development of novel composite materials to address the limitations in wear resistance and electrical corrosion resistance.
Material surface treatment technologies, such as electroplating and coating technologies, can significantly improve the wear resistance and oxidation resistance of materials [58]. Techniques like silver plating, gold plating, or applying wear-resistant coatings on metal surfaces can reduce friction coefficients and contact resistance, thereby extending the service life of current-carrying friction components. In current-carrying friction, silver effectively reduces contact resistance and friction coefficient, making it ideal for highly sensitive electrical contact equipment, and it can also be used as a coating to enhance the wear performance as displayed in Figure 3b [57]. However, due to its low hardness and poor wear resistance, which makes it difficult to meet the requirement of high-load conditions, it is therefore used in applications requiring light loading, high conductivity, and low friction [59]. Gold, a precious metal, has excellent conductivity, corrosion resistance, and a low friction coefficient [60,61]. In current-carrying friction, gold provides extremely low contact resistance and excellent COF, maintaining stable electrical contact performance even in harsh environments, as displayed in Figure 3c [62]. However, due to its high cost, gold is typically used only in critical components or electrical equipment with special requirements [58], such as the contact points of high-end electronic devices. Graphite, a non-metallic material, exhibits unique advantages in current-carrying friction. It has a low coefficient of friction and excellent self-lubricating properties, which can effectively reduce friction and wear. Additionally, graphite has good conductivity and oxidation resistance, making it suitable for current-carrying contact environments [13,63]. Hu et al. [64] used two different kinds of graphite materials to test their tribological performance under different current density and sliding velocity. The results showed that the lower hardness graphite exhibited better adaptability while the higher hardness graphite had a lower wear rate and good resistance as the increase in current density and sliding velocity increased, as shown in Figure 3d. However, graphite’s low mechanical strength makes it prone to wear, so it is commonly used in low-load, high-conductivity applications such as electric brushes and slip rings. Surface treatment technology can significantly enhance the wear resistance, electrical corrosion resistance, and interface stability of materials under current-carrying friction conditions. However, its applicability is often constrained by specific processing techniques and environmental limitations. Future research should focus on investigating the effects of various surface treatments on friction-current coupling and developing more efficient, low-cost treatment methods.
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Figure 3. Current−carrying friction with high stability and excellent tribological behavior achieved with metal or non−metal materials: (a) copper [11], (b) silver [57], (c) gold [62], and (d) graphite [64] Reproduced with permission from Refs. [11,57,62,64]; copyright 2010, Elsevier; copyright 2024, Elsevier; copyright 2023, Elsevier; copyright 2023, Elsevier.
Figure 3. Current−carrying friction with high stability and excellent tribological behavior achieved with metal or non−metal materials: (a) copper [11], (b) silver [57], (c) gold [62], and (d) graphite [64] Reproduced with permission from Refs. [11,57,62,64]; copyright 2010, Elsevier; copyright 2024, Elsevier; copyright 2023, Elsevier; copyright 2023, Elsevier.
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Secondly, we discuss composite materials. This composite material can maintain a low friction coefficient and significantly improve wear resistance, making it suitable for high-load and high-current applications [14,65,66,67]. It is commonly used in electric brushes, slip rings, and electrical contact systems that require long lifespans. By combining the high conductivity of copper with the low friction properties of carbon, copper-carbon composites exhibit better interfacial bonding [68] as well as high mechanical performance, and their microscopic images can be found in Figure 4a [69]. Additionally, silver composites combine the excellent conductivity of silver with the low friction properties of WS2 and MoS2, making them suitable for applications requiring high conductivity under light loads [70]. Silver significantly reduces contact resistance, while WS2 and MoS2 reduce friction and wear, extending the equipment’s service life. In order to improve its wear resistance, Xiao Kang et al. [71] used spark plasma sintering (SPS) to fabricate Ag-WS2-MoS2 nanocomposite and Ag-WS2-MoS2 composite. The Ag-WS2-MoS2 nanocomposite has the lowest wear rate and is displayed in Figure 4b.
The application of Au/MoS₂ composites in current-carrying friction primarily lies in their excellent conductivity and lubricating properties, making them a critical material for enhancing the performance of electrical contact interfaces. Pei et al. [72] developed Au/MoS₂ composites that form a stable solid lubricating film during friction, effectively enhancing the lubricating properties of the material while preserving the ductility and conductivity of Au as displayed in Figure 4c. This composite demonstrates exceptional performance in harsh environments, such as vacuums. The combination of the inertness of Au and the layered structure of MoS₂ endows the composite with strong anti-adhesion and anti-arc properties during electrical contact, minimizing the risk of contact surface damage.
The application of graphite composites in current-carrying friction is advantageous due to their unique layered structure and excellent lubrication performance, making them a crucial material for enhancing electrical contact performance while reducing friction and wear [73,74,75]. Graphite’s layered structure facilitates interlayer slip during friction, forming a stable solid lubricating film that significantly lowers the friction coefficient and wear rate, as shown in Figure 4d [76]. Additionally, graphite exhibits good conductivity and chemical inertness, providing stable electrical contact and minimizing fluctuations in contact resistance [77]. Furthermore, graphite’s high-temperature resistance can be significantly enhanced by compounding with metals or other materials, enabling it to maintain excellent lubrication and conductivity in high-temperature current-carrying friction environments. Its conductive properties and lubricating effects help mitigate the arc effect during electrical contact, reducing damage to contact surfaces and material transfer, thereby extending equipment service life. These exceptional properties make graphite composites an effective solution for enhancing equipment performance and reliability. In addition, the layered structure of h-BN, which is similar to that of graphene, allows it to easily slide along the lattice during friction, forming a solid lubricating film that reduces adhesion and lowers the friction coefficient of the contact surface. Moreover, the high hardness and wear resistance of h-BN can significantly enhance the overall wear resistance of the composite material, thereby extending the service life of friction pairs. As a result, incorporating h-BN into the matrix material not only improves the stability of the composite under current-carrying friction conditions but also mitigates performance degradation caused by oxidation or chemical reactions [78,79]. Composite material processing technology can enhance the conductivity, wear resistance, and electrical corrosion resistance of materials in current-carrying friction. However, the preparation process is complex, and the stability of performance is not stable.
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Figure 4. Current-carrying friction with high stability and excellent tribological behavior achieved composite materials: (a) C/C-Cu composite [69], (b) Ag-WS2-MoS2 nanocomposites [71], (c) Au/MoS2 [72] and (d) copper-graphite composites [76]. Reproduced with permission from Refs. [69,71,72,76]; copyright 2015, Elsevier; copyright 2020, Elsevier; copyright 2024, IOP Publishing; copyright 2021, Elsevier.
Figure 4. Current-carrying friction with high stability and excellent tribological behavior achieved composite materials: (a) C/C-Cu composite [69], (b) Ag-WS2-MoS2 nanocomposites [71], (c) Au/MoS2 [72] and (d) copper-graphite composites [76]. Reproduced with permission from Refs. [69,71,72,76]; copyright 2015, Elsevier; copyright 2020, Elsevier; copyright 2024, IOP Publishing; copyright 2021, Elsevier.
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This section provides an overview of the current-carrying friction behavior and performance of both pure and composite materials, highlighting the substantial impact of material composition, internal structure, and surface properties on frictional force and contact resistance. By selecting and optimizing materials in combination with appropriate surface treatment techniques, the performance of current-carrying friction systems can be significantly enhanced, the service life of equipment extended, and the needs of different application scenarios met. Future research should focus on investigating its long-term reliability under varying current densities and environmental conditions while optimizing the preparation process to improve overall performance.

2.2. Contact Types of Current-Carrying Friction

Current-carrying friction exhibits different characteristics under various forms of contact. The following sections provide a detailed introduction to current-carrying friction in several typical forms of contact, including reciprocating friction, rotational friction, and more.
The first is reciprocating friction. This type refers to the periodic back-and-forth sliding of two contact surfaces in a straight-line direction during relative motion [80,81]. It is commonly found in linear actuators and some micromotors. The characteristic of reciprocating friction is that the repetitive friction of the contact surface can lead to increased wear of the surface material as the increase in normal load, which was conducted on the ball-on-flat tribometer as displayed in Figure 5 [82]. When current passes through a frictional interface, periodic fluctuations in contact resistance occur due to frequent changes in the direction of friction. The contact resistance in reciprocating friction is easily influenced by oxide layers, contaminants, or surface roughness, which can cause the contact surface to deteriorate and increase COF over time. Especially under high-frequency reciprocating motion, thermal effects and current concentration may further exacerbate changes in contact resistance.
The second type is rotational friction. This type involves the relative rotational motion of one contact surface around another, commonly seen in electric motors, brushes, bearings, and slip rings. The friction characteristic of rotational friction is that it leads to a relatively uniform frictional force at the contact interface, although centrifugal force during high-speed rotation may alter contact pressure and affect the magnitude of frictional force [83,84]. When current passes through, the rotational motion can cause an increase in the temperature of the friction interface and may even lead to electrical corrosion. Three kinds of graphites, including hard-carbon graphite, electrographite, and polymer-bonded graphite, were fabricated by Mitjan Kalin et al. [63]. The results showed that graphite tribofilm was formed at the interfaces of these graphites, while polymer-bonded graphite has the lowest wear rate and best contact resistance, as displayed in Figure 6. The contact resistance in rotational friction is typically stable but may gradually increase under high-speed or high-current conditions due to factors such as temperature rise and oxide accumulation. High-speed rotational friction can also accelerate the wear of brushes or contact surfaces.
The third type is sliding friction. Sliding friction occurs between two contact surfaces during relative sliding, with applications in many electrical and mechanical systems such as sliding switches and motor slides. The characteristic of sliding friction is that it typically produces a large frictional force, which can lead to thermal effects when current passes through, resulting in increased contact interface temperature, intensified material deformation, and wear [85,86,87]. Under sliding friction, the local temperature at the contact point may cause melting or electro-corrosion of the friction surface. The contact resistance in sliding friction changes with the increase in sliding distance, and surface contaminants and material wear can also affect the resistance value. Zhang et al. used pure carbon trip paired with chromium bronze on the sliding tester, and they found that arcing played an important role in pure carbon and changed mechanical wear into arc erosion as shown in Figure 7 [88]. Prolonged sliding friction may cause degradation of the contact surface, leading to a significant increase in contact resistance. Sliding friction is commonly found in track sliders, sliding switches, and electrical connectors.
The fourth type is rolling friction. Rolling friction occurs during the rolling process of an object along another surface and is typically applied in bearings, motor rotors, and stators. The characteristic of rolling friction is that it generally produces less frictional force compared to sliding friction, but high loads in the contact area may cause local plastic deformation, which in turn affects the frictional force. When current passes through rolling contact, the temperature rise at the friction interface is relatively low, but under high current-carrying conditions, it may still cause local electrical corrosion or arc phenomena. The contact resistance in rolling friction is usually low and stable, but with changes in time and load, the contact surface may experience pitting or wear, leading to a gradual increase in resistance. And it can be found that carbon film was formed at the contact interface, as displayed in Figure 8 [89]. The contact resistance in rolling friction is also affected by the material of the rolling elements and the lubrication state. Rolling friction is suitable for high-precision motion systems such as motor bearings, ball screws, and rotary joints.
The fifth type is vibration friction. Vibration friction occurs on the contact surface during high-frequency vibration [88,90] and is common in ultrasonic motors or certain precision control devices. The characteristic of vibration friction is that it can significantly reduce static friction force, thereby helping to reduce energy loss. When current passes through, the thermal effect of the vibration friction interface is low, but it may cause micro-motion wear or contact fatigue. Meanwhile, even at lower COF, the vibration can still be formed at their interface, as displayed in Figure 9 [91]. The contact resistance in vibration friction is relatively stable, but intermittent electrical contact may occur under high-frequency vibration, leading to resistance fluctuations. The frequency and amplitude of vibration have a significant impact on contact resistance. Vibration friction is applied in ultrasonic motors, vibration sensors, and high-precision electrical drive systems.
The sixth type is composite friction. Composite friction involves a combination of various forms of friction, such as rotation with sliding or rolling with sliding. This complex form of friction is common in intricate mechanical systems and multifunctional electrical devices. The characteristic of composite friction is that the frictional force and contact resistance may vary depending on the combination of friction forms. When current passes through, the friction interface under composite friction may exhibit complex temperature distributions and friction behavior. The contact resistance in composite friction is difficult to predict and requires a detailed analysis based on the specific friction forms and material properties. The design of composite friction systems typically needs to consider the balance between current load, friction form, and material properties. Composite friction is commonly found in multi-axis motion platforms, complex electrical and mechanical systems, and precision equipment requiring high reliability. Through detailed analysis of different contact forms, the behavior of current-carrying friction can be better understood and controlled, thereby optimizing the design and performance of related systems.
This section examines common types of current-carrying friction, including reciprocating, rotational, sliding, rolling, and vibrational friction, as well as their combinations. It elaborates on the unique characteristics and suitable application scenarios of each type under various contact forms and load conditions.

2.3. Applications of Current-Carrying Friction

Current-carrying friction plays a crucial role in various application scenarios, particularly in electrical and mechanical systems [92]. Below are detailed introductions to several key application scenarios. First, the brush and commutator system is fundamental in DC motors and some AC motors, where they are used to transmit current to the rotating armature winding as displayed in Figure 10a [93]. In this system, the brush, usually made of graphite or metallic graphite [94], maintains contact with the rotating surface of the commutator under pressure from a spring. As the motor operates, friction occurs between the brushes and the commutator, allowing current to flow into the armature through the contact points [85]. Due to continuous wear of the contact surface during high-speed rotation, friction and contact resistance change over time. Brush wear, thermal effects, arcing, and friction noise are key influencing factors. To reduce friction and wear, lubricants are often employed, or material selection is optimized by using highly conductive composite materials. Additionally, the design can minimize arc generation by controlling the contact pressure and surface roughness of the brush.
Next is the application of sliding contactors (slip rings) as displayed in Figure 10b [32], which are used to transmit electrical signals or power in rotating machinery, such as transferring electrical energy between the rotor and stator of a wind turbine. Slip rings consist of metal rings in contact with brushes. As the brush slides on the surface of the slip ring, it generates friction while maintaining stable current transmission. During current-carrying friction, thermal effects and material wear are key concerns. Over time, the heat generated by friction may cause surface oxidation or material degradation, leading to increased contact resistance. To mitigate these effects, wear-resistant and high-temperature-resistant materials, such as precious metal alloys, are used along with lubrication or cooling systems to reduce thermal effects. Additionally, the design should consider more uniform contact pressure to minimize friction fluctuations.
Electrical switches and relays are widely used in circuit control to achieve current connection or disconnection through physical contact, as displayed in Figure 10c [90]. During the switching process, the contact surface experiences wear due to friction and thermal effects, particularly in high-frequency switching operations. Repeated operation may cause material transfer or localized melting due to arcing. The lifespan and reliability of switches are affected by friction, wear, and arcing. To enhance performance, contact points are typically made from precious metals or alloys and designed with self-cleaning structures to minimize the impact of arcing. Moreover, appropriate lubricants are applied at contact points to reduce friction and heat accumulation.
Finally, current-carrying friction in aerospace equipment involves electrical contacts in extreme environments, as displayed in Figure 10d [95], such as vacuum, high temperature, low temperature, strong radiation, and other harsh conditions. In these environments, friction interfaces are prone to failure due to temperature changes, differing thermal expansion coefficients of materials, or material degradation caused by external radiation. The arc effect is more pronounced in vacuum conditions, where it has a stronger destructive impact on the contact surface. To address these challenges, aerospace equipment typically uses materials with high heat resistance, radiation resistance, and oxidation resistance, designed to maintain stable performance under extreme conditions. Equipment operating in a vacuum may use frictionless contact methods or high-performance lubricants to reduce wear and arcing effects. By analyzing current-carrying friction behavior in different application scenarios, targeted strategies can be developed to ensure the long-term reliability and stability of electrical equipment.
This section introduces electric brushes and commutation systems, conductive slip rings, electrical switches, and relays, as well as current-carrying friction in aerospace equipment that involves electrical contact under extreme environmental conditions. Furthermore, it highlights the key technical challenges these systems face under high temperatures, heavy loads, high speeds, and complex working conditions.

2.4. Environment in Current-Carrying Friction

The characteristics of current-carrying friction are influenced not only by the contact form but also by the usage environment, which can cause significant changes. Below is a detailed discussion of current-carrying friction in different environments, such as atmospheric, vacuum, and humid conditions. In an atmospheric environment, which primarily contains oxygen, nitrogen, and water vapor, oxygen and water vapor tend to react with friction surfaces, generating oxides or adsorbing moisture to form oxide layers as displayed in Figure 11a. However, significant abrasive debris was observed on the surface, and the fractured oxide film on the wear surface generated large fragments as well as numerous small particles of abrasive debris, as shown in Figure 11b [96]. Additionally, the formation of surface oxide films can have an insulating effect, increasing contact resistance. As friction progresses, the oxide layer may be damaged, leading to resistance instability. Moisture in the air can also cause fluctuations in contact resistance, especially under high humidity conditions. Therefore, this environment is generally suitable for most conventional electrical equipment and mechanical systems, such as switches, connectors, and brush systems [53,86,97].
In a vacuum environment, where there is almost no gas present, such as in spacecraft or certain high-tech experimental equipment, the absence of oxygen and water vapor prevents the formation of oxide layers or water films on friction surfaces [36,98]. As a result, friction can be high, and cold welding may occur, where the contact surfaces adhere directly without lubrication. The wear rate of friction materials in a vacuum is usually high, and surface materials may peel off. However, contact resistance tends to be relatively stable, and due to the lack of a surface oxide layer, direct material contact can result in lower resistance, as shown in Figure 12 [14]. In vacuum environments, the current may cause surface arcing, especially under high current conditions, leading to spark discharge or vacuum arcing. This environment is commonly encountered in spacecraft, electric vacuum devices, and precision electrical connections in high vacuum settings.
A humid environment, characterized by high relative humidity and the presence of water vapor, mist, or direct exposure to liquid water, can lead to water molecules easily adsorbing onto friction surfaces, forming a liquid film that provides lubrication and reduces friction, as displayed in Figure 13 [27]. However, the water film can also cause electrochemical corrosion, particularly in current-carrying friction situations. The current passing through the water film may trigger electrolytic reactions, further deteriorating the friction interface. Additionally, contact resistance in humid environments is usually unstable, as the conductivity of the water film can cause current short circuits or uneven distribution. Under high humidity conditions, contact resistance may significantly decrease, but this can also increase corrosion and lead to contact point failure [99,100]. This environment is typically found in marine environments, electrical connectors under humid conditions, outdoor electrical equipment, and similar situations.
This section systematically analyzes the impact of atmospheric, vacuum, and humidity conditions on current-carrying friction performance. It further proposes the underlying mechanisms by which environmental factors affect current distribution and contact stability in friction pairs, thereby providing a theoretical basis for predicting and controlling friction behavior in specific environments.

2.5. Influence Factors of Current-Carrying Friction

Current-carrying friction is a complex process influenced by multiple factors. Below is a detailed analysis of some key factors, including arcs, thermal effects, lubrication, material properties, and contact pressure. An arc is a discharge phenomenon caused by the ionization of air or other media due to high current density and temperature between two electrical contact surfaces. Arcs typically occur when the contact surface is separated or unstable [101]. The generation of an arc can cause serious thermal damage to the contact surface, leading to material melting, vaporization, and redeposition [102]. These phenomena can alter the morphology of the friction surface, causing fluctuations in the friction coefficient, increased friction, and even surface damage, as displayed in Figure 14 [103]. Arcs may also induce surface micro-welding, further increasing friction. Regarding resistance, material damage and increased surface roughness caused by arcs can lead to significant fluctuations and increases in contact resistance [104]. Over time, arcs exacerbate the degradation of the contact points, ultimately leading to contact failure [25,37].
The thermal effect refers to the temperature rise caused by joule heating when current passes through a frictional interface, affecting the material properties and friction behavior of the contact surface. The thermal effect can cause the material on the friction surface to soften or melt, leading to changes in friction, usually manifesting as an increase in the wear rate as displayed in Figure 15 [105]. High temperatures can also cause thermal expansion of materials, altering contact pressure and further affecting friction behavior. Long-term high-temperature friction may lead to oxidation, increased wear, and material transfer. As temperature increases, the conductivity of the material may decrease, resulting in increased contact resistance. Additionally, high-temperature environments may lead to thermal stress concentration at contact points, causing local material fracture or ablation, thereby increasing the instability of contact resistance [86,106].
Lubrication is a critical method for controlling current-carrying friction, as lubricants can reduce friction and wear. Lubricants form a protective film between contact surfaces, reducing friction and preventing direct contact, thus decreasing wear [21,87,98,107]. However, in current-carrying friction, lubricants may undergo electrochemical reactions under the influence of current, altering their physical and chemical properties and may even fail. Some lubricants may produce insulation effects under high current conditions, affecting electrical contact performance. While lubricants can fill micro surface irregularities and reduce contact resistance, their deterioration may also increase insulation at contact points, resulting in unstable resistance, especially under lower current, while it would reach a higher wear rate when the current density reaches a high level. At this time, it would break the lubrication film formed by the lubrication, as shown in Figure 16 [108].
The material properties of the frictional interface, such as hardness, conductivity, and chemical stability, directly impact current-carrying friction behavior. The hardness and toughness of the material determine the wear resistance of the friction surface. Materials with good conductivity produce lower contact resistance in current-carrying friction, but insufficiently hard materials may experience plastic deformation or increased wear due to thermal effects caused by the current. The chemical stability of materials also affects the degree of oxidation on the friction surface, thereby altering the friction coefficient. Materials with good conductivity, such as copper and silver, typically have lower contact resistance but are prone to oxidation at high temperatures, leading to increased resistance. Heat-resistant and oxidation-resistant materials, such as gold or palladium, can maintain stable resistance values under harsh current-carrying friction conditions. Contact pressure refers to the normal force between two contact surfaces, which directly affects the frictional force and contact resistance. High contact pressure usually increases friction as the peak areas of surface roughness are flattened, increasing the actual contact area. However, excessive contact pressure may lead to plastic deformation or fatigue damage of the material, affecting its friction performance. Conversely, low contact pressure may lead to unstable contact and increased friction force fluctuations. Under high contact pressure, the actual contact area increases, and contact resistance usually decreases. However, excessive pressure may cause cold welding of materials or adhesion at contact points, leading to a sharp increase or fluctuation in resistance. Low contact pressure may result in higher resistance due to poor contact.
This section explores the effects of arcs, thermal effects, lubrication, material properties, and contact pressure on current-carrying friction. It suggests that surface treatment technology can enhance the conductivity and wear resistance of materials, optimize the contact pressure and surface lubrication of friction pairs, and thereby effectively reduce both contact resistance and frictional force.

2.6. Mechanism of Current-Carrying Friction

The mechanism of current-carrying friction varies significantly with the magnitude of the current, and it was tested in Figure 17a. The following is a detailed analysis of current-carrying friction mechanisms under three conditions: no current, low current, and high current [35]. Under the condition of no current, frictional behavior is primarily determined by the mechanical interaction of the contact surfaces, as displayed in Figure 17b. Friction arises from the adhesion between contact surfaces, the interlocking of surface asperities, and the influence of any wear debris. The frictional force between contact interfaces is generally related to factors such as the hardness, surface roughness, and contact pressure of the materials involved. When no current is present, the resistance at the contact point is mainly governed by the intrinsic resistivity of the material and the microscopic real contact area. In the absence of current, there are no additional thermal effects or electrochemical reactions. The friction mechanism is thus purely mechanical. The wear rate and friction coefficient of the contact surfaces are relatively stable, and the contact resistance is solely determined by the mechanical contact conditions.
When a small current passes through a frictional contact point, the current generates a small amount of Joule heat at the micro-contact areas, which may not be sufficient to cause significant temperature rises or material phase transitions as displayed in Figure 17c. However, even this small current can cause a slight temperature increase at these micro-contact points, altering the local properties of the material, such as reducing hardness or forming surface oxide layers. At this point, the presence of current may trigger minor electrochemical reactions, especially in oxygen-rich environments, which can change the chemical properties of the contact surface, such as the formation or removal of oxide films, thereby affecting both friction and contact resistance. Typically, under low current conditions, friction and contact resistance experience slight variations due to the influence of current. This often manifests as minor fluctuations in contact resistance and slight increases or decreases in friction, depending on the nature of the electrochemical reactions occurring.
When a high current passes through the frictional contact point, the current density significantly increases, generating substantial Joule heat at the contact point, as displayed in Figure 17d. The resulting high temperature can cause material softening, melting, or even vaporization, leading to severe damage to the contact surfaces. High temperatures can also induce phase transformations in the contact materials, such as hardening or oxidation, which in turn affects frictional behavior. Under high current conditions, arcing becomes a significant concern. Due to unstable contact or local overheating, discharges may occur between the contact surfaces, with the arc further exacerbating wear and material transfer. The presence of an arc causes severe fluctuations in contact resistance and significantly affects frictional force. Electrochemical reactions intensify under high currents, potentially leading to electrochemical corrosion or electromigration effects, accelerating the degradation of contact materials or causing local element migration, which further influences friction behavior. Common consequences include severe wear of the contact surface, marked fluctuations in frictional force, and a substantial increase in contact resistance. Contact points may suffer from melting, erosion, or material transfer due to arcing or overheating, resulting in contact failure or irreversible damage.
In summary, under no current, friction behavior is predominantly controlled by mechanical factors, resulting in relatively stable frictional force and contact resistance. With low currents, slight Joule heating and electrochemical reactions occur, leading to minor changes in friction and contact resistance. Under high currents, significant heat generation and arc effects cause substantial fluctuations in friction and contact resistance, potentially leading to severe damage and failure of the contact surfaces.

3. Future Prospects

As a critical issue in electrical equipment and mechanical systems, current-carrying friction is advancing towards more refined and efficient research and application with technological progress. The following outlines the future development directions and application prospects of current-carrying friction.
Firstly, future research will increasingly focus on the development of materials with high conductivity and wear resistance, such as nanocomposites, precious metal alloys, and carbon-based materials (e.g., graphene, carbon nanotubes) [109]. Equipment lifespan increases because they lessen contact resistance and improve wear resistance, and reliability also becomes better. Application of self-repairing materials, which fix the internal damage during friction and help reduce wear overtime, and such materials, especially in extreme places, maintain low contact resistance.
Secondly, advanced lubrication in the future will tend to be more sophisticated and intelligent. Lubricants with the ability to automatically change their viscosity and have better conductivity will be investigated. Lubricants having particles on the nanoscale or ionic liquids help in keeping low friction levels and low contact resistance even under high current densities [110,111]. Nanoscale surface treatments, which will be used to control the morphology and chemical properties of the contact surfaces precisely, should also reduce friction and wear much more effectively. The changes in current generated during the friction process can reflect the real-time state of the contact interface, such as frictional force, temperature, and wear. By processing and responding to these signals, materials can adaptively adjust the contact pressure or self-repair the worn areas. In the future, new intelligent materials with self-lubricating functions, anti-arc erosion properties, and self-healing capabilities can be developed. For example, coatings structured at the nano-level can notably reduce the friction coefficient while making the contact surface more durable.
Moreover, human activities need more extreme environments, like space exploration and developing deep-sea sites [112,113], and the systems that have current-carrying friction are adapting to temperatures that are extreme, vacuum areas, and conditions with radiation. Future research focuses on making materials and technologies for current-carrying friction that are fit for these extreme places, aiming for stable workings under harsh situations. The creation of high-frequency and high-current density use, which rapid changes in power electronics and technologies for communications are causing, needs developments in materials that have low contact resistance and can manage high-frequency currents. Optimizing designs to reduce electromagnetic interference and thermal effects will be another key area.
Lastly, with more electric vehicles and smart grids growing, current-carrying friction technology is crucial in high-current, high-frequency contact systems [114]. Future use centers on maintaining low friction rates, low contact resistance, and high reliability in these cases. In microelectronics fields and nanotechnology, current-carrying friction is very important, especially for microcontactors and in MEMS (Micro Electro Mechanical Systems). Researching in the future aims at making technologies ensure the current transmission is stable and low-friction at the microscale [115,116,117]. Through continuous innovation and optimization, current-carrying friction technology is poised to play a key role in a broader and more profound array of applications in the future.

4. Conclusions

Current-carrying friction has been happening in systems of electrical contact, it affects the performance of types of equipment and its reliability. Multiple factors influence its behavior, like the material involved, the form of contact, or the environment. In these things, the material plays an important role in deciding the performance of current-carrying friction. Graphite, copper, silver, gold, and their composites are used quite often, and they all show various behaviors of friction and erosion when currents change. Copper and silver have good conductivity and friction characteristics, which makes them used in electrical contacts a lot, but the problem with them is electrical corrosion occurs. Plus, silver is expensive relatively. Graphite is considered better because it is cheaper, and its lubrication is superior and stands up to electric corrosion, which better suits high currents and difficult conditions. Pure materials elucidate current-friction mechanisms, but their inadequate wear resistance necessitates exploring novel composites to enhance both wear and electrical corrosion resistance. Composite materials help improve properties at the interface by improving surface hardness and resisting oxidation, which helps cut down friction and loss in electrical contact. However, due to its complex preparation process, further research is needed to address performance stability and ensure long-term reliability under varying conditions.
The contact type also matters a lot in how current-carrying friction behaves. Electrical corrosion and damage to surfaces often result from reciprocating friction, which is rather serious. Mechanical movement, along with electrical currents, affects rotational friction. Friction and erosion are pronounced in sliding, quite noticeably. In rolling friction, due to point contact, there is localized current concentration. Vibration friction increases micromotion wear, which causes contact to be quite unstable. Different operating environments, like atmospheric vacuum and humid, play a vital role in the stability of friction interfaces. When in an atmospheric environment, oxide forms on contact surfaces, making friction behavior unstable and wear increase more. In the vacuum, less oxidation on surfaces means more friction, wear is more severe, and cold welding risk is higher. The humid environment changes contact resistance due to the presence of water vapor, promoting electrical corrosion. Various factors like arc and thermal effects, material properties, contact pressure, and lubrication have significant influence on current-carrying friction behavior, particularly in extreme conditions.
The key mechanisms of current-carrying friction involve material migration, localized heating, and electroerosion. In the absence of current, friction is primarily controlled by mechanical interactions, such as surface adhesion, asperity interlocking, and the properties of the contact materials; the frictional behavior is solely influenced by these mechanical factors. Under low current conditions, the generated Joule heat and minor electrochemical reactions can alter the surface properties of the contact area, such as by changing hardness or forming oxide layers. These alterations can result in slight variations in friction and contact resistance. High electric conditions speed up material migration and get local temperatures higher in places where electricity touches, causing more wear and contact resistance. In real use, current-carrying friction is often in things like motors, switches, and slip rings, where the stability of performance depends on the electrical and mechanical characteristics of the contact interfaces.
A comprehensive review of current-carrying friction, including materials, contact types, applications, environmental conditions, influencing factors, and mechanisms, is crucial for optimizing performance, minimizing wear, and enhancing the reliability of electrical contacts across various industrial and technological applications. Future research should focus on developing new wear-resistant and corrosion-resistant materials, surface modification techniques, and exploring friction behavior under extreme conditions.

Author Contributions

P.W.: Writing—original draft preparation, Conceptualization, Formal analysis, Methodology; X.W.: Methodology, Formal analysis, writing—original draft preparation; G.J.: Methodology, Formal analysis; F.L.: Methodology, Formal analysis; P.B.: Conceptualization, Formal analysis, Funding acquisition; Y.T.: Conceptualization, Formal analysis, Funding acquisition, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 92266206, No. 52275198).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Latest research progress on current-carrying friction from the aspects of materials [33], contact types [21], applications [23], environment [36], factors [31], and mechanisms [37]. Reproduced with permission from Refs. [21,23,31,33,36,37]; copyright 2020, Elsevier; copyright 2024, Springer Nature; copyright 2019, Elsevier; copyright 2024, Elsevier; copyright 2016, Elsevier; copyright 2024, Elsevier.
Figure 1. Latest research progress on current-carrying friction from the aspects of materials [33], contact types [21], applications [23], environment [36], factors [31], and mechanisms [37]. Reproduced with permission from Refs. [21,23,31,33,36,37]; copyright 2020, Elsevier; copyright 2024, Springer Nature; copyright 2019, Elsevier; copyright 2024, Elsevier; copyright 2016, Elsevier; copyright 2024, Elsevier.
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Figure 5. Current-carrying friction tested with reciprocating sliding tribometer: (a) actual image, (b) friction curves under different current values, and (c) temperature increased with the increase in current [82]. Reproduced with permission from Ref. [82]; copyright 2023, Elsevier.
Figure 5. Current-carrying friction tested with reciprocating sliding tribometer: (a) actual image, (b) friction curves under different current values, and (c) temperature increased with the increase in current [82]. Reproduced with permission from Ref. [82]; copyright 2023, Elsevier.
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Figure 6. Current-carrying friction tested with rotational friction tribometer: (a) schematic of the rotating tester, (b) COF, contact resistance, contact temperature, and graphite specific wear [63]. Reproduced with permission from Ref. [63]; copyright 2018, Elsevier.
Figure 6. Current-carrying friction tested with rotational friction tribometer: (a) schematic of the rotating tester, (b) COF, contact resistance, contact temperature, and graphite specific wear [63]. Reproduced with permission from Ref. [63]; copyright 2018, Elsevier.
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Figure 7. Current-carrying friction tested with sliding friction tribometer: (a) schematic of sliding wear tester, (b) failure mechanism [88]. Reproduced with permission from Ref. [88]; copyright 2021, Elsevier.
Figure 7. Current-carrying friction tested with sliding friction tribometer: (a) schematic of sliding wear tester, (b) failure mechanism [88]. Reproduced with permission from Ref. [88]; copyright 2021, Elsevier.
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Figure 8. Current-carrying friction tested with rolling friction tribometer: (a) schematic of rolling tester, (b) friction mechanism [89]. Reproduced with permission from Ref. [89]; copyright 2021, Springer Nature.
Figure 8. Current-carrying friction tested with rolling friction tribometer: (a) schematic of rolling tester, (b) friction mechanism [89]. Reproduced with permission from Ref. [89]; copyright 2021, Springer Nature.
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Figure 9. Current-carrying friction tested with vibration friction tribometer: (a) schematic of vibration tester, (b) vibration signals with different currents [91]. Reproduced with permission from Ref. [91]; copyright 2023, Elsevier.
Figure 9. Current-carrying friction tested with vibration friction tribometer: (a) schematic of vibration tester, (b) vibration signals with different currents [91]. Reproduced with permission from Ref. [91]; copyright 2023, Elsevier.
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Figure 10. Current-carrying friction in different applications: (a) brushes [93], (b) slip rings [32], (c) switches [90], and (d) aerospace equipment [95]. Reproduced with permission from Refs. [32,90,93,95]; copyright 2021, Springer Nature; copyright 2023, Elsevier; copyright 2024, Elsevier; copyright 2024, Elsevier.
Figure 10. Current-carrying friction in different applications: (a) brushes [93], (b) slip rings [32], (c) switches [90], and (d) aerospace equipment [95]. Reproduced with permission from Refs. [32,90,93,95]; copyright 2021, Springer Nature; copyright 2023, Elsevier; copyright 2024, Elsevier; copyright 2024, Elsevier.
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Figure 11. Oxides layer formed in the atmospheric environment for the current-carrying friction at the contact surface and generated abrasive debris: (a) oxide layers, (b) abrasive debris. Reproduced with permission from Ref. [96]; copyright 2024, Elsevier.
Figure 11. Oxides layer formed in the atmospheric environment for the current-carrying friction at the contact surface and generated abrasive debris: (a) oxide layers, (b) abrasive debris. Reproduced with permission from Ref. [96]; copyright 2024, Elsevier.
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Figure 12. Current-carrying friction of (a) COF and contact resistance and (b) worn surface of coin-silver and Au in vacuum and air [14]. Reproduced with permission from Ref. [14]; copyright 2016, Elsevier.
Figure 12. Current-carrying friction of (a) COF and contact resistance and (b) worn surface of coin-silver and Au in vacuum and air [14]. Reproduced with permission from Ref. [14]; copyright 2016, Elsevier.
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Figure 13. Current-carrying friction under humid environment and liquid-like layer were formed at the interface [27]. Reproduced with permission from Ref. [27]; copyright 2024, Springer Nature.
Figure 13. Current-carrying friction under humid environment and liquid-like layer were formed at the interface [27]. Reproduced with permission from Ref. [27]; copyright 2024, Springer Nature.
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Figure 14. Arc erosion appeared on the surface of current-carrying friction as the increase in current: (a) schematic illustration and (b) 3D morphologies [103]. Reproduced with permission from Ref. [103]; copyright 2024, Elsevier.
Figure 14. Arc erosion appeared on the surface of current-carrying friction as the increase in current: (a) schematic illustration and (b) 3D morphologies [103]. Reproduced with permission from Ref. [103]; copyright 2024, Elsevier.
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Figure 15. Temperature increased as the current in current-carrying friction: (a) infrared images, (b) temperature, and (c) wear rate [105]. Reproduced with permission from Ref. [105]; copyright 2024, Spinger Nature.
Figure 15. Temperature increased as the current in current-carrying friction: (a) infrared images, (b) temperature, and (c) wear rate [105]. Reproduced with permission from Ref. [105]; copyright 2024, Spinger Nature.
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Figure 16. Dry friction and lubrication in no current-carrying friction compared with current-carrying friction [108]. Reproduced with permission from Ref. [108]; copyright 2022, Elsevier.
Figure 16. Dry friction and lubrication in no current-carrying friction compared with current-carrying friction [108]. Reproduced with permission from Ref. [108]; copyright 2022, Elsevier.
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Figure 17. Mechanism of current-carrying friction: (a) schematic of tribometer with different current, (b) zero current density, (c) lower current density, and (d) high current density [35]. Reproduced with permission from Ref. [35]; copyright 2020, Elsevier.
Figure 17. Mechanism of current-carrying friction: (a) schematic of tribometer with different current, (b) zero current density, (c) lower current density, and (d) high current density [35]. Reproduced with permission from Ref. [35]; copyright 2020, Elsevier.
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Wei, P.; Wang, X.; Jing, G.; Li, F.; Bai, P.; Tian, Y. Research Progress on Current-Carrying Friction with High Stability and Excellent Tribological Behavior. Lubricants 2024, 12, 349. https://doi.org/10.3390/lubricants12100349

AMA Style

Wei P, Wang X, Jing G, Li F, Bai P, Tian Y. Research Progress on Current-Carrying Friction with High Stability and Excellent Tribological Behavior. Lubricants. 2024; 12(10):349. https://doi.org/10.3390/lubricants12100349

Chicago/Turabian Style

Wei, Peng, Xueqiang Wang, Guiru Jing, Fei Li, Pengpeng Bai, and Yu Tian. 2024. "Research Progress on Current-Carrying Friction with High Stability and Excellent Tribological Behavior" Lubricants 12, no. 10: 349. https://doi.org/10.3390/lubricants12100349

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