Research Status and Progress on the Current-Carrying Friction and Wear Performance of Conductive Slip Rings in Harsh Environments

Abstract

With the rapid development of aerospace, industrial automation, and weapons manufacturing, the performance requirements for conductive slip rings have become more stringent, and their operating environments have become increasingly harsh, making the study of the effects of harsh environmental conditions on slip rings particularly important. This paper systematically reviews the effects of harsh environments on the current-carrying friction and wear behavior of slip rings, with a detailed discussion on the mechanisms by which environmental factors such as high temperature, humidity, corrosive gases, and vacuum influence the tribological properties of slip ring materials. Research has shown that these harsh environments significantly change the friction coefficient, wear rate, and electrical contact performance of slip rings, causing degradation of material properties. By reviewing current experimental studies and numerical simulations, this paper analyzes the performance variations and failure mechanisms of slip rings in various environments, summarizing the key technological progress in enhancing slip ring performance under such conditions, particularly the application of material modification and surface coating technologies. Additionally, concerning the lifetime prediction and monitoring of slip ring systems, this paper explores the potential of multiphysics simulation technology and intelligent monitoring methods. Finally, this paper looks forward to future research directions, including optimization design based on multiphysics simulation, the development of high-temperature coatings, the improvement of lifetime prediction models, and the optimization of thermal management strategies, aiming to provide theoretical support and technical guidance for enhancing the reliability and durability of slip rings in extreme environments.

1. Introduction

As a core device for energy and signal transmission between fixed and rotating components, the electrical slip ring (hereinafter referred to as the “slip ring”) is widely used in high-tech fields such as aerospace [1,2,3], military equipment [4,5,6], wind power generation [7,8,9], and industrial automation [10,11]. It is an essential component of unrestricted continuous rotation systems in modern industry (Figure 1). The slip ring achieves electrical conduction through sliding electrical contact, and its typical structure includes conductive rings, brushes, a bracket, and a bearing system (such as bearing end caps and shields). Based on structural design, slip rings can be classified into coaxial slip rings and planar slip rings, suitable for different application scenarios (Figure 2).

Slip rings usually operate in complex and dynamic environments, where they must endure harsh conditions including high loads, high temperatures, high humidity, corrosive gases, and vacuum settings [12]. In these harsh environments, the friction and wear behavior of slip rings becomes particularly complex. When current passes through the slip ring interface, the phenomenon of current-carrying friction becomes the dominant factor in tribological behavior, with associated thermal and electrochemical effects accelerating the deterioration of the material surface [13]. For instance, high temperatures facilitate material oxidation, where the formed oxide layer may act as a lubricant in certain situations. However, excessive oxidation can fail the oxide film, further accelerating wear [14]. Humidity and corrosive gases exacerbate electrochemical corrosion, significantly increasing the rate of material surface degradation, and thereby affecting the reliability and service life of the slip ring. Moreover, in vacuum conditions, the drastic reduction of gas molecules can render conventional lubrication mechanisms ineffective, exacerbating adhesion and wear issues on material surfaces. These environmental factors interact and couple with each other, making the current-carrying friction behavior of slip rings even more complex.

In recent years, substantial advancements have been achieved in the research on the current-carrying friction behavior of slip rings. On the one hand, in materials science, novel high-performance materials (e.g., nanocoatings, ceramics, and advanced alloys) have exhibited superior wear and corrosion resistance, offering innovative approaches to improving the wear resistance of slip rings [12]. On the other hand, surface modification technologies (such as laser treatment, electroplating, and spraying) have effectively enhanced the stability of slip rings in harsh environments by optimizing interface performance. Furthermore, two-dimensional materials (such as graphene and MoS₂) have become a research focus as novel solid lubricants due to their ultra-low friction properties and excellent durability [13]. The application of these new materials and surface technologies can significantly improve the friction performance of slip rings and extend their service life.

Nevertheless, there are still some limitations in the current studies on slip rings. For example, the majority of research focuses on the influence of individual environmental factors, with insufficient systematic investigation of slip ring friction and wear behavior under coupled conditions like high temperature, high humidity, and corrosive gases. In addition, the applicability of existing slip ring life prediction models is relatively limited, making it difficult to meet the needs of performance evaluation under complex working conditions [14].

This paper is intended to systematically summarize the impact of harsh environments on the current-carrying friction and wear behavior of slip rings, elucidate the mechanisms of various environmental factors (e.g., high temperature, humidity, corrosive gases, and vacuum) on the tribological characteristics of slip ring materials, identify technical challenges in current studies, review advancements in materials and protective technologies, and discuss future research directions. It is hoped to provide theoretical foundations and technical support for researchers and engineers in related fields, promoting the application and development of slip rings under extreme conditions.

2. Effects of Harsh Environments on the Current-Carrying Friction and Wear Behavior of Slip Rings

Harsh environments have a complex and profound impact on the current-carrying friction and wear performance of slip rings. Environmental factors such as high temperature, humidity, corrosive gases, and vacuum significantly alter the tribological behavior and electrical contact performance of slip ring materials [15,16]. Therefore, in-depth research on the effects of these environmental factors on current-carrying friction will provide an important basis for material selection and design optimization of slip rings.

2.1. Effects of High Temperature

High-temperature conditions are one of the most demanding challenges for slip ring operation, as they have a substantial impact on the physical properties, frictional behavior, and wear mechanisms of slip ring materials [17]. Elevated temperatures lead to material softening and oxidation while also modifying the microstructure of the frictional interface, which exacerbates wear and diminishes electrical conductivity.

2.1.1. Effects of High Temperature on the Physical Properties of Materials

High-temperature environments significantly affect the physicochemical properties of slip ring materials. As the temperature rises, the electrical conductivity, hardness, and oxidation resistance of the material change, which directly determine the stability and durability of slip rings under high-temperature conditions [18,19,20].

Copper-based materials are widely used in slip rings due to their excellent electrical conductivity and thermal conductivity, but they exhibit significant performance degradation under high-temperature conditions. Studies have shown that as the temperature increases, the hardness and yield strength of copper decrease significantly, thereby increasing the risk of plastic deformation on the surface [21]. This material softening phenomenon not only alters the mechanical response of the contact interface but also directly affects the stability of current-carrying capacity. Meanwhile, high temperatures promote the rapid formation of Cu₂O and CuO oxide films on the copper surface. The role of these oxide films is quite complex, as they can effectively reduce direct contact wear in the initial stages and provide some protective effect. However, due to the brittle nature of the oxides themselves, these films are prone to breaking under mechanical stress and forming wear debris, which further exacerbates interface wear [22]. This dual effect makes the wear behavior under high temperatures exhibit nonlinear characteristics.

High temperature also adversely affects the electrical contact performance of the slip ring [19]. The increase in contact temperature not only enhances the Joule heating effect but also exacerbates the frequency of arc discharge, both of which contribute to an increase in contact resistance and cause localized thermal damage. More importantly, the continued thickening of the oxide film further increases the resistance at the contact point, reducing the conductivity of the slip ring. Wang et al.’s research [23] revealed an interesting phenomenon, that the effect of temperature on the contact resistance of the C-Cu friction pair is closely related to the current size. When the current is less than 20 A, the average contact resistance increases as the interface temperature rises, and the amplitude of dynamic contact resistance also increases; whereas, when the current exceeds 60 A, the situation is the opposite. This finding suggests that there is a complex coupling effect between high temperature and current, and their mechanism of action cannot be simply understood from the perspective of a single parameter.

In contrast, graphite-based materials, while exhibiting good thermal stability and self-lubricating properties, face unique challenges under high-temperature conditions. Its layered structure may degrade due to oxidation, leading to a significant reduction in lubrication effectiveness, which in turn increases the friction coefficient and wear rate [24,25,26]. Turel et al.’s research [27] quantified this effect, finding that within the temperature range of 90 °C ± 5 °C, the wear rate of copper-graphite brushes increased fivefold, and this dramatic change is believed to be closely related to the alteration of the material’s dynamic properties. Moreover, high temperatures may also trigger dynamic recrystallization of the metal matrix, leading to grain growth and further weakening the mechanical strength of the material [28,29].

Overall, the impact of high temperature on the physical properties of slip ring materials manifests as the combined effect of several interrelated degradation mechanisms. Material softening reduces load-bearing capacity, the formation and rupture of oxide films alter the interface characteristics, and the presence of current amplifies the effect of temperature through Joule heating and electrochemical effects. The complexity of this multi-factor coupling requires the integrated consideration of thermal, mechanical, and electrical performance for synergistic optimization during material selection and system design.

2.1.2. Effects of High Temperature on Tribological Behavior

High temperature has a seemingly contradictory but actually interconnected effect on the tribological behavior of the slip ring friction interface. Wang et al.’s systematic study [30] provided important clues for understanding this complex phenomenon. They found that as the interface temperature of the C-Cu friction pair gradually increased from 25 °C to 300 °C, the friction coefficient exhibited a continuous decreasing trend. This phenomenon is mainly attributed to the synergy of three mechanisms. First, material softening reduces the interface shear strength, making relative sliding more likely; second, the solid lubrication provided by the formed oxide film; third, enhanced plastic deformation increases the real contact area, dispersing the load.

Ding et al.’s research [31] further enriched our understanding of the temperature effect. They observed that the friction coefficient slightly increased with temperature at first, then drastically decreased, reflecting a transition of the dominant mechanisms across different temperature ranges. However, perplexingly, although the friction coefficient decreased, the wear volume significantly increased with temperature, as shown in Figure 3. This “low friction–high wear” paradox indicates that friction and wear are actually controlled by different physical processes. Friction is primarily determined by interface shear strength, while wear is more influenced by the depth of sub-surface material plastic flow.

A deeper analysis of wear behavior reveals that the wear mechanisms induced by high temperature exhibit diverse characteristics. The plastic deformation caused by material softening is no longer limited to the surface layer but extends to deeper layers, resulting in a greater volume of material loss. Although the oxide film reduces friction, its repeated formation and mechanical rupture process itself constitutes a form of wear, and the generated oxide particles can lead to additional abrasive wear. It is also noteworthy that cyclic thermal stresses can cause the initiation and propagation of microcracks within the material, ultimately leading to fatigue spalling. Wang et al. [30] specifically pointed out that although high interface temperatures can suppress arc discharge erosion and delamination wear, they may induce surface cracking, and once these cracks form, they become the starting point for accelerated wear.

These findings are of great significance for understanding the high-temperature service behavior of slip rings. The traditional belief that reducing the friction coefficient can reduce wear does not apply under high-temperature conditions. On the contrary, a balance between friction and wear needs to be sought in material design and operating condition control. Simply pursuing low friction may come at the cost of material durability, while overly emphasizing wear resistance could result in excessive friction resistance, affecting system efficiency.

2.1.3. Oxidative Wear Mechanism Under High Temperature

In high-temperature environments, oxidation becomes one of the primary wear mechanisms of slip ring materials. The effect of oxidative wear is dual in nature, depending on the formation rate, thickness, and mechanical stability of the oxide film. On one hand, high-temperature oxidation leads to the accumulation of spalled materials at the contact interface, destroying surface films, causing lubrication failure [32], and resulting in abrasive wear [30,33], which increases the friction coefficient. On the other hand, moderate temperature increases caused by frictional and Joule heating promote surface oxidation, forming oxide films that effectively prevent direct contact between current-carrying friction pairs, reducing wear and contact resistance, thereby providing a certain antifriction and lubrication effect [34], which decreases the friction coefficient.

Azevedo et al. [35] found that, under high-temperature conditions, oxides formed by the reaction of copper with oxygen (e.g., Cu₂O) may exist as abrasive particles, increasing the wear rate and undermining the stability of electrical contact. Under conditions of high current density and sliding speed, a rise in interfacial temperature further aggravates oxidative wear. The study by Wang et al. [36] on copper-impregnated brushes indicates that elevated interfacial temperature arising from electrical and mechanical wear significantly promotes oxidative wear. Liu et al. [37] showed that, compared with brushes with low Cu content, high-Cu brushes perform better in terms of friction coefficient, wear rate, and contact resistance, yet they respond more strongly to temperature variations and suffer more severe oxidative wear. The SEM and EDS analyses of the worn surfaces clearly reveal the distribution and morphological features of the oxidation products (as shown in Figure 4).

Overall, the influence of oxidative wear depends on the dynamic balance between the formation rate of the oxide film and its mechanical removal rate. When the formation rate dominates, the oxide film can offer a certain degree of protection and lubrication; However, when mechanical removal becomes dominant, continuous rupture of the oxide film generates abundant wear debris, thereby accelerating wear. Understanding this balance mechanism is essential for optimizing the high-temperature operating parameters of slip rings.

2.1.4. Adhesive Wear and Arc Erosion at High Temperatures

High temperatures can also significantly intensify other thermally induced wear mechanisms, particularly arc erosion and adhesive wear [38]. These mechanisms are coupled under conditions of high temperature and high current, posing a serious threat to the service life of slip rings.

Arc erosion is one of the most destructive failure modes under high-temperature conditions. High temperatures facilitate the escape of electrons from the material surface, and this thermionic emission phenomenon lowers the threshold for arc initiation, leading to a significant increase in arc discharge frequency. The local transient high temperatures generated by arc discharges can reach several thousand degrees Celsius, causing melting, evaporation, sputtering, and resolidification of the material [39,40,41]. This process not only causes direct material loss but also produces cracks, pits, and oxidized corrosion products on the surface, severely compromising the material integrity and electrical contact performance, as shown in Figure 4.

Adhesive wear also exhibits a pronounced intensification trend at elevated temperatures. The current-carrying friction experiments conducted by Mei et al. [29] on carbon sliders demonstrated that, under high temperatures, material transfer increased significantly due to interfacial softening and local melting. The softening of the material reduces its shear strength, making adhesion more likely under the same load, while the enhanced plastic deformation capacity leads to more severe tearing after adhesion. This effect not only increases the friction coefficient but, more importantly, causes a sharp rise in wear rate, significantly shortening the service life of the contact pair.

The Joule heating effect plays a key amplifying role in this process. The rise in interfacial temperature leads to an increase in contact resistance, and the elevated resistance, in turn, generates more heat through Joule heating, further increasing the temperature [40]. This positive feedback mechanism may result in the so-called thermal runaway phenomenon, where the local temperature rises sharply within a short time, leading to catastrophic failure. The study by Lyu et al. [29] demonstrated that, under high-temperature conditions, the stability of electrical contact in Cu–graphite friction pairs decreases significantly due to the accumulation of wear debris and the increased frequency of arc discharges. They found that the application of active cooling measures can effectively interrupt this positive feedback loop, reduce the temperature rise, thereby improving the electrical contact performance of the slip ring system and extending its service life [22].

A systematic review of high-temperature effects reveals that the influence of temperature on the current-carrying friction behavior of slip rings is essentially a thermally activated, multi-mechanism synergistic evolution process. Existing studies have clearly identified key degradation pathways such as material softening, oxide film evolution, and arc intensification; however, a unified theoretical framework describing the quantitative coupling among these mechanisms is still lacking. It is noteworthy that the frequently reported “low friction–high wear” paradox in the literature actually reflects the scale-dependent responses of tribological interfaces: The friction coefficient is mainly governed by surface shear strength, whereas wear is controlled by the volume of subsurface plastic deformation; their respective temperature response functions are inherently different. However, most studies remain at the phenomenological level, lacking in-depth investigation into how temperature alters the contact stress field distribution and consequently affects the depth of plastic deformation. Another issue worthy of attention is the material dependence of the temperature threshold effect. Although researchers have identified characteristic temperature points such as 90 °C, 150 °C, and 200 °C, these thresholds are in fact closely related to material composition, microstructure, and loading conditions; simple numerical generalization may lead to misleading conclusions in practical applications. Future research should develop predictive models based on intrinsic material parameters and operating conditions, rather than relying on empirical temperature values. Furthermore, current understanding of the current–temperature coupling effect remains limited. Although Wang et al. observed that current magnitude modulates the temperature–contact resistance relationship, it is still unclear whether this arises from a current-induced change in interfacial energy distribution or the introduction of new electrochemical reaction pathways. In situ characterization and multiphysics simulations are required to clarify this issue.

2.2. Effect of Humidity

The presence of moisture in the environment affects the electrical conductivity and the degree of chemical reactions at the friction interface, making humidity another important environmental factor influencing the current-carrying friction behavior of slip rings [42]. Variations in humidity not only change the lubrication state but also intensify electrochemical reactions at the interface [43,44], exerting a profound impact on the stability and lifespan of the slip ring system. This influence is particularly pronounced under extremely high or low humidity conditions [45,46].

2.2.1. Effects of Humidity on the Formation of the Lubricating Film

Environmental humidity profoundly influences the tribological performance of slip rings by modulating the adsorption and diffusion of water molecules at the tribological interface [43]. This influence is not a linear relationship but exhibits a distinct interval characteristic, with different humidity ranges corresponding to markedly different interfacial states and wear modes.

When the environmental humidity falls below 10%, the interface is nearly in a completely dry state. Under such conditions, the lack of sufficient water molecule adsorption makes it difficult for the metal surface to form a complete protective oxide film [45]. Direct contact between exposed metal surfaces leads to a strong adhesive effect, resulting in wear behavior dominated by adhesive wear and a significant increase in wear rate. This phenomenon is particularly pronounced in low-humidity environments such as aerospace applications, becoming a key factor limiting the reliability of slip rings.

As humidity rises to a moderate level, the interfacial state undergoes a qualitative transformation. Wu et al. [47] found that within a relative humidity range of 30%–60%, a unique composite lubricating film spontaneously forms on the surface of Cu–C friction pairs. This film, composed of Cu, CuO, and carbon elements, exhibits excellent lubricating properties, significantly reducing the friction coefficient and wear depth [44,46]. Its formation mechanism involves a series of interrelated processes. Water molecules first promote mild oxidation of the copper surface, forming copper hydroxide, which subsequently transforms into copper oxide; Meanwhile, carbon materials are more readily transferred to the contact interface under the influence of water molecules; Ultimately, these components mix under frictional action to form a stable composite layer. More importantly, an appropriate amount of water molecules adsorbed on this composite film can provide an additional boundary lubrication effect, further enhancing interfacial performance.

However, the stability of this composite film is highly dependent on specific dynamic equilibrium conditions: (1) Continuous frictional action is required to supply fresh wear debris, with a typical wear rate of about 10⁻⁶–10⁻⁵ mm³/N·m [48,49]; (2) A relatively stable humidity environment is necessary to maintain the adsorption of water molecules [50]; (3) A moderate contact pressure is needed to prevent excessive compaction or rupture of the film, typically within the range of 0.5–7 MPa [34]. When these conditions are met, the composite film can remain stable for several hundred to several thousand hours [51], exhibiting excellent self-healing capability. However, once the environmental parameters deviate from the optimal range—especially when humidity support is lost—the friction coefficient rises immediately, and the film rapidly deteriorates within minutes to hours, causing a sharp degradation in tribological performance.

When the humidity continues to rise above 70%, the situation becomes more complex. On one hand, excessive moisture may compromise the stability of the existing lubricating film, causing it to dissolve or peel off, thereby leading to a renewed increase in wear rate [51]. An excessively thick water layer can also hinder the formation of a solid lubricating film while intensifying electrochemical corrosion. On the other hand, a high-humidity environment also introduces certain positive effects. Studies have shown that increased humidity promotes interlayer sliding of carbon materials; water molecules inserted between graphite layers reduce the interlayer bonding strength, thereby improving the self-lubricating properties of the interface and, to some extent, lowering the friction coefficient [47].

This humidity-dependent nonmonotonic behavior reveals a key fact: there exists an optimal humidity window within which both the tribological and electrical contact performances of the slip ring reach their best state. For most copper-based slip ring systems, this optimal window lies between 40% and 60% relative humidity. In practical applications, maintaining this humidity range through environmental control or selecting appropriate material systems for different humidity conditions are both effective ways to enhance the reliability of slip rings.

2.2.2. Effects of Humidity on Electrochemical Corrosion

Humidity not only affects frictional performance but also significantly influences the electrical conductivity and corrosion behavior of materials through electrochemical reactions. In high-humidity environments, water molecules act as an electrolyte medium, markedly promoting electrochemical reactions at the interface. This electrochemical oxidation is fundamentally different from the thermal oxidation discussed in the previous high-temperature section, mainly in terms of reaction temperature, product morphology, and influencing mechanisms.

Under high-humidity conditions, oxidation of copper surfaces is accelerated via electrochemical pathways. In the anodic region, copper atoms lose electrons and oxidize to copper ions; in the cathodic region, oxygen gains electrons with the participation of water, and the resulting hydroxide ions combine with copper ions, ultimately forming copper oxides. This electrochemical oxidation proceeds efficiently even at room temperature, without requiring high-temperature thermal activation, which is a key distinction from thermal oxidation. Moreover, oxide films produced by electrochemical oxidation typically exhibit higher porosity and lower density, affording inferior protection compared with the slowly grown thermal oxides formed at elevated temperatures.

The study by Pompanon et al. [52] provides important data for understanding how humidity affects electrical contact performance. They found that increasing humidity reduces electrical contact resistance while improving wear life. This seemingly beneficial effect exhibits different characteristics across humidity ranges. Below the 50% relative humidity (RH) threshold, the lifetime is extended but increases only slowly. Above 50% RH, high humidity markedly alters the rheological properties of the debris layer, making it easier to expel from the contact region and thereby reducing the wear rate.

However, beyond the moderate humidity range (e.g., 40%–60%), the negative effects of excessive humidity begin to emerge. Enhanced electrochemical reactions under high humidity accelerate the formation and accumulation of surface oxides. Although these oxides can partially reduce the friction coefficient by providing some lubrication, their semiconducting properties cause a significant increase in contact resistance [51], thereby affecting the stability and conductivity of the electrical contact. More seriously, under current-carrying conditions, the presence of electric current further intensifies the electrochemical corrosion process, significantly accelerating the rate of material degradation [53,54].

The degree of humidity influence varies among different substrate materials. Copper-based materials are the most sensitive to humidity and are prone to electrochemical corrosion; Silver-based materials show slightly better moisture resistance but can still form poorly conductive oxides under high humidity; Gold-based materials, due to their chemical inertness, can maintain stable electrical contact performance under various humidity conditions. This material dependence provides guidance for selecting suitable slip ring materials under different humidity environments.

2.2.3. Effects of Humidity on Wear Modes

Variations in humidity not only affect the lubrication state and electrochemical behavior, but also fundamentally alter the dominant wear mechanisms, driving the system from one failure mode to another. The study by Song et al. [55] vividly illustrated this transition. They found that as the environment gradually shifted from low humidity (10% RH) to extreme conditions with liquid water present, the current-carrying friction coefficient increased by a remarkable factor of seven, while the dominant wear mechanism fundamentally changed—from adhesive and abrasive wear to fatigue wear—and the fatigue morphology evolved from fine, scaly features to large-area spallation.

As shown clearly in the SEM images of Figure 5, the worn surface morphologies vary under different humidity conditions. At low humidity, the surface exhibits typical features of adhesive tearing and scratching; At moderate humidity, the surface appears relatively smooth, with the protective effect of the lubricating film being evident; Under high humidity, especially in the presence of liquid water, the surface shows numerous fatigue spalling pits and crack networks. The evolution of contact resistance also confirms this trend: at moderate humidity, the contact resistance is the lowest and most stable, while under high humidity or liquid-water conditions, it increases markedly and fluctuates intensely.

The physical mechanism behind this transition is closely related to the additional forces induced by electric current. Under low-humidity conditions, the interface is relatively dry, and wear is primarily governed by mechanical factors. The friction coefficient remains at a moderate level (approximately 0.15), and wear is mainly characterized by adhesive tearing and abrasive scratching of the surface layer. As humidity increases to a moderate range, the formed lubricating film effectively reduces the friction coefficient (to about 0.12), and wear reaches its minimum level. However, when humidity increases further, especially approaching or reaching conditions with liquid water, a dramatic change occurs. The friction coefficient rises sharply to about 0.25 or even higher, up to around 1.0, and the wear mode shifts to severe fatigue spalling.

The mechanism of this transition involves the synergistic interaction of multiple factors. Firstly, the influence of the meniscus force: excessive moisture forms liquid bridges at the contact interface, generating significant capillary adhesion forces that increase the effective normal load on the interface. Secondly, the electrochemical effects induced by current within the water film not only accelerate material dissolution and redeposition but may also cause side effects such as hydrogen embrittlement, thereby reducing the material’s fatigue resistance. Thirdly, the lubricating effect of water transforms into a hydrodynamic pressure effect under high-speed sliding conditions; the resulting pressure fluctuations cause cyclic contact stresses that accelerate the initiation and propagation of fatigue cracks.

Studies on the influence of humidity on slip ring performance have revealed the critical regulatory role of interfacial water molecules in tribochemistry; however, the current understanding remains significantly limited. Firstly, the universality of the 40%–60% relative humidity “optimal window” is open to question. This conclusion is primarily derived from experiments on copper–carbon friction pairs under specific load and speed conditions, and its applicability to other material systems and operating combinations has yet to be systematically validated. A deeper issue lies in the fact that this nonmonotonic relationship involves multiple competing mechanisms—such as lubricating film formation, electrochemical reactions, and capillary forces—yet their relative quantitative contributions as a function of humidity remain unclear. Secondly, the humidity-induced transition in wear modes (from adhesive to fatigue wear) is an important finding in this field; however, the mechanism proposed by Song et al. [55]—involving meniscus force, electrochemical effects, and hydrodynamic pressure—is largely qualitative speculation rather than experimentally validated. In particular, under current-carrying conditions, how local Joule heating, electrolysis, and electromigration induced by current through the water film affect interfacial mechanical responses has been scarcely explored in existing studies. Furthermore, the existing literature offers only a superficial discussion of the synergistic effects between humidity and temperature. In real service conditions, temperature rise alters interfacial relative humidity, while humidity variation affects heat dissipation efficiency; however, how this bidirectional coupling influences the system’s stability boundary has not been systematically investigated. From a methodological perspective, most current studies employ constant-humidity experiments; however, in real environments, humidity fluctuates dynamically, and the influence of such transient humidity histories on lubrication film stability may be more critical but has been largely overlooked.

2.3. Effects of Corrosive Gases

Corrosive gases (e.g., SO₂, H₂S, Cl₂, etc.) significantly affect the performance of the slip ring. These gases generate corrosive products through electrochemical reactions, and their synergistic effect with mechanical wear accelerates material degradation, thereby altering the friction behavior and electrical contact performance of the slip ring.

2.3.1. Formation and Characteristics of Corrosion Products

Under the influence of corrosive gases, a series of complex corrosion products usually form on the surface of metal slip rings. For copper- or silver-based materials, these products mainly include compounds such as sulfates, sulfides, or chlorides. The formation of these corrosion products not only alters the chemical composition of the material surface but, more importantly, fundamentally changes the physical and electrical properties of the interface. Studies have shown that corrosion products may form poorly lubricating films, resulting in increased friction coefficients and contact resistance; On the other hand, due to their inherent brittleness, these products are prone to fracture under mechanical loads, thereby further aggravating wear [56].

In sulfur-containing environments, the reactions of silver with H₂S and SO₂ are particularly typical. Silver reacts directly with hydrogen sulfide to form black silver sulfide, whereas in the presence of oxygen, it reacts with sulfur dioxide to produce white silver sulfate. Although these two products have distinct properties, both are detrimental to slip-ring performance. Although silver sulfide possesses some electrical conductivity, it is far lower than that of pure silver, significantly increasing contact resistance; silver sulfate exhibits pronounced brittleness, tending to flake off under friction to form abrasive particles, thereby increasing the wear rate [57]. For copper-based materials, copper chloride compounds can form in chlorine or salt-spray environments. These chlorides not only exhibit poor electrical conductivity but also strong hygroscopicity, which under high humidity further accelerates electrochemical corrosion.

As a typical representative of corrosive gaseous environments, the salt-spray atmosphere contains chloride ions with extremely strong corrosive properties. Chloride ions are small in size and have strong penetrating ability, allowing them to destroy the passive film on the material surface and accelerate localized corrosion. Green corrosive compounds such as copper chloride readily form on the slip ring surface; these not only significantly weaken the material’s durability and electrical conductivity [58], but also continuously spall and regenerate during friction, creating a persistent material degradation cycle. This corrosion–wear synergistic effect severely impacts the current transmission efficiency of slip rings, not only reducing power generation efficiency but also greatly increasing the frequency of equipment failures.

2.3.2. Synergistic Mechanism of Corrosion and Wear

The influence of corrosive gases on slip ring performance extends far beyond simple chemical reactions; more importantly, there exists a strong synergistic interaction between corrosion and mechanical wear [59]. This synergistic effect causes the overall degradation rate to be much higher than the simple sum of pure corrosion and pure wear. In gaseous environments containing SO₂ and H₂S, gas-induced electrochemical reactions significantly enhance adhesive and three-body wear effects. Particularly under high-humidity conditions, water molecules act as electrolytes that accelerate the corrosion process while simultaneously serving as an additional medium for wear, making the chemical–mechanical coupling more pronounced [60].

From a mechanistic perspective, this synergistic effect manifests as several mutually reinforcing processes [61]. The first is the acceleration of wear by corrosion. Corrosion products generally have low hardness and mechanical strength; the presence of the corrosion layer reduces the deformation resistance of the surface material, making it more easily worn away under the same load. More importantly, the bond between corrosion products and the substrate is often weak, making them prone to detachment as a whole during friction. The second is the promotion of corrosion by wear. Mechanical wear continuously removes the corrosion products that have already formed on the surface. Although this may temporarily improve electrical conductivity, it actually keeps exposing fresh metal surfaces, providing new reaction interfaces for corrosion. The repeated formation and removal of the corrosion film itself constitute an efficient material loss mechanism.

The three-body abrasive effect is particularly pronounced in corrosive environments. After detaching from the surface, corrosion products remain at the contact interface as hard particles, acting as abrasives during relative sliding. These abrasive particles not only scratch the material surface, producing grooves and scars, but also damage newly formed lubricating or protective corrosion films, accelerating further material degradation. Studies have shown that the accumulation of corrosive products significantly increases interfacial friction, leading to higher wear rates [57]. The accumulation of corrosion products may also hinder normal lubrication at the interface. When these products occupy the space where the lubricating film should exist, the contact interface loses its effective protective layer, further aggravating electrical contact failure and contact resistance instability.

2.3.3. Corrosion-Resistant Materials and Protection Strategies

In response to the severe challenges posed by corrosive gases to the performance of slip rings, researchers have proposed and validated a series of material optimization and surface protection strategies. The core concept of these strategies is to block or mitigate the corrosion process through material design or surface modification while maintaining favorable tribological and electrical contact properties.

Copper-based composites doped with carbon nanotubes (CNTs) and molybdenum disulfide exhibit remarkable corrosion resistance in salt-spray environments [62]. The superior performance of these composites arises from multiple synergistic mechanisms. The incorporation of carbon nanotubes not only enhances the mechanical strength of the matrix but also establishes a conductive network within the material, ensuring that slight surface corrosion does not significantly affect overall conductivity. As a solid lubricant, molybdenum disulfide—with its layered structure—provides excellent self-lubricating properties, effectively reducing the friction coefficient and wear rate. More importantly, these added phases can hinder the diffusion of corrosive ions into the matrix, serving as an effective barrier to corrosion. Experimental results show that such materials not only effectively reduce wear but also maintain low contact resistance in corrosive environments, making them an effective solution for combating corrosive gas exposure.

Noble metal coating technology provides another effective protective approach. Silver- or nickel-based coatings, owing to their relatively good corrosion resistance, have been widely employed to enhance the reliability of slip rings in harsh environments [63]. The experimental study by Pompanon et al. [56] provided an in-depth understanding of the protective mechanism of silver coatings in corrosive gas environments. In atmospheres containing H₂S and SO₂, silver coatings indeed react with these corrosive gases to form Ag₂S and Ag₂SO₄ films. Surprisingly, these reaction products are not entirely detrimental; under certain conditions, they even exhibit protective effects. Although the resulting Ag₂S and Ag₂SO₄ films increase contact resistance, they partially block further interaction between the gases and the underlying silver layer, thereby slowing the progression of deep corrosion. More importantly, these films possess certain lubricating properties that reduce the friction coefficient and wear rate.

Humidity plays a crucial regulatory role in the interaction process between corrosive gases and materials. The study by Pompanon et al. [56] showed that when humidity is below the 60% threshold, variations in the friction coefficient and contact resistance of silver coatings in corrosive atmospheres differ little from those in clean air, indicating that corrosion effects are somewhat suppressed within this humidity range. However, when humidity exceeds this threshold, the situation changes; unexpectedly, the durability of contact resistance increases significantly, as shown in Figure 6. This anomalous phenomenon may be related to changes in the rheological properties of corrosion products under high humidity, making them easier to expel from the contact area and thereby maintaining relatively stable electrical contact.

Experimental studies by Pompanon et al. [56] further validated the effectiveness of these materials and technologies. In atmospheres containing H₂S and SO₂, silver coatings react with corrosive gases to form Ag₂S and Ag₂SO₄ films. The formation of these films not only reduces the friction coefficient but also decreases the wear rate. When the humidity is below the threshold (60% RH), the changes in friction coefficient and contact resistance are similar to those in uncontaminated air; however, when the humidity exceeds the threshold, the durability of the contact resistance significantly improves (Figure 6). Similarly, in salt mist environments, CNTs-MoS₂/Cu composite materials exhibit excellent tribological performance and electrical conductivity stability, with significantly better friction coefficients and wear rates than conventional copper-based materials [62]. These findings indicate that the proper selection of high-performance materials, combined with surface protection coating technologies, can effectively enhance the lifespan and reliability of slip rings in corrosive gas environments.

Research on the effects of corrosive gases on slip ring performance highlights the complexity of chemo-mechanical synergistic degradation; however, the current understanding remains at an early stage. Although the existence of corrosion–wear synergism has been widely recognized, its quantitative description still relies on empirical models and lacks first-principles explanations based on interfacial physicochemical mechanisms. In particular, the dual role of corrosion products (such as Ag₂S and CuO)—providing partial lubrication while increasing resistance—remains mechanistically unexplained at the microscopic level. A more critical issue is that most existing studies focus on performance changes after steady-state exposure, while the dynamic cycle of corrosion product formation, rupture, and regeneration has not been thoroughly characterized. This dynamic process is key to determining long-term service reliability, but in-depth investigation requires breakthroughs in in situ observation techniques. The finding by Pompanon et al. regarding the modulation of corrosion effects by a humidity threshold (60% RH) is valuable, but the proposed mechanism—change in the rheological properties of corrosion products—remains hypothetical. A deeper question arises: why does contact resistance durability improve under high humidity. This may relate to the dissolution–redeposition process of corrosion products within the water film, but direct evidence is lacking. From a materials strategy perspective, composites such as CNTs–MoS₂/Cu show promising corrosion resistance, but their long-term stability (e.g., interfacial bonding strength of the solid lubricating phase, oxidation behavior in corrosive environments) still requires long-term validation. In addition, although noble metal coatings can offer short-term protection, their failure mechanisms under cyclic loading and electrical current—whether coating fatigue, interfacial delamination, or diffusion-induced failure—have not been systematically investigated, limiting reliability design in engineering applications.

2.4. Effects of Vacuum Environment

The vacuum environment of aerospace slip rings poses unique challenges to their tribological behavior and electrical contact performance. In atmospheric environments, the presence of oxygen and moisture facilitates the formation of an oxide film, which, although increasing contact resistance, protects the material surface to some extent and reduces adhesive wear caused by direct metal-to-metal contact. However, in vacuum environments, the extreme absence of oxygen and moisture makes it difficult for such a protective oxide film to form or persist, resulting in a fundamental change in tribological and wear characteristics.

2.4.1. Adhesive Wear Caused by the Absence of Oxide Film

The most direct impact of the vacuum environment on slip ring performance lies in the absence of the oxide film. Under atmospheric conditions, even a surface oxide film only a few nanometers thick can effectively prevent direct contact between metal atoms and inhibit strong metallic bonding. In vacuum, when bare metal surfaces come into direct contact, interatomic interaction forces increase sharply, leading to intensified adhesive wear and surface plastic deformation [64].

The study by Kang et al. [65] revealed that under vacuum conditions, the friction coefficient between the slip ring and brush materials is generally higher than that in conventional atmospheric environments. The physical essence of this phenomenon lies in the absence of the oxide film, which significantly increases the real contact area between metals and correspondingly raises the interfacial shear strength. In atmospheric environments, even surfaces that appear smooth are actually covered by a layer of oxides and adsorbed water molecules; although this “contamination film” is detrimental to electrical contact, it unexpectedly serves as a friction-reducing layer. In vacuum, the disappearance of this protective layer exposes the inherently high-friction nature of metallic materials.

Adhesive wear in vacuum exhibits characteristics that are distinctly different from those in atmospheric environments [66]. Firstly, direct metallic bonding can form between metal atoms, whose strength is much higher than that of contacts mediated by oxides, resulting in a significant increase in adhesion. Secondly, the depth of plastic deformation increases significantly; the softened metal undergoes severe plastic flow under high contact stress, making material transfer extremely prevalent. Thirdly, the nature of wear debris changes: in vacuum, the debris remains highly active and adhesive due to the absence of oxidation passivation, making it more prone to accumulate on contact surfaces, increasing interfacial roughness and causing frictional instability.

2.4.2. Performance of Solid Lubricants in Vacuum

While vacuum conditions generally deteriorate the tribological behavior of most metals, some specialized solid lubricants demonstrate outstanding, even superior, performance under vacuum relative to ambient environments. This observation offers valuable guidance for selecting appropriate slip-ring materials for vacuum applications.

The outstanding performance of layered-structure solid lubricants in vacuum has attracted extensive attention. In-depth studies by Yang et al. [64] and Pei et al. [67,68] have shown that materials such as NbSe₂, MoS₂–Ti, and Au/MoS₂ films exhibit not only no deterioration but even superior sliding electrical contact lubrication performance under vacuum compared with that in air. The origin of this phenomenon lies in the unique layered crystal structure of these materials. Taking NbSe₂ as an example, its crystal consists of two-dimensional layered units stacked together by weak van der Waals forces; atoms within each layer are bonded by strong covalent bonds, while adjacent layers are held only by weak intermolecular forces. This structure allows easy interlayer sliding under tangential stress, providing an intrinsic self-lubricating effect.

More importantly, the lubricating performance of such layered materials does not depend on the presence of oxygen and moisture in the environment. Conventional metal-based lubrication relies on the formation of an oxide film, whereas the lubrication mechanism of layered solid lubricants is entirely based on their intrinsic crystalline structural characteristics. Therefore, in vacuum environments, when other materials lose lubrication due to the absence of an oxide film, layered materials can instead fully exert their inherent advantages. In fact, the vacuum environment provides a certain protective effect for these materials, preventing oxidative degradation caused by atmospheric oxygen and moisture. In air, oxygen atoms and water molecules may intercalate between the layers of materials such as molybdenum disulfide, disrupting their perfect layered structures; in contrast, under vacuum, these structures remain intact, resulting in more stable and durable lubrication performance.

2.4.3. Arc Discharge Effects in Vacuum

The phenomenon of arc discharge in vacuum environments is another challenge that cannot be ignored, especially for slip ring systems operating under high current density conditions. Compared with atmospheric environments, arc discharges in vacuum exhibit distinctly different characteristics and cause unique types of material damage.

The most prominent feature of arc discharge in vacuum is the reduction of arc ignition voltage. In atmospheric environments, air molecules act as insulators, requiring a relatively high voltage to break down the air gap and initiate an arc. In vacuum, due to the extreme scarcity of gas molecules, electrons can more easily accelerate under the electric field to gain sufficient energy, making arc initiation easier. However, the behavior of the arc after ignition becomes more complex. In the absence of gas molecules as the discharge medium, vacuum arcs primarily sustain their conductive channels through metal vapor. The electrode material evaporates under the high temperature of the arc, and the vapor rapidly ionizes to form plasma. This metal-vapor-based discharge alters both the energy distribution and the duration of the arc [69].

The energy of a vacuum arc tends to be more concentrated, causing a sharp rise in local temperature at the contact point and a significant increase in thermal stress. Such highly localized energy deposition may induce localized melting, evaporation, and even sputtering of the material. The molten metal is ejected from the contact point under the combined effects of electromagnetic force and vapor pressure, and upon cooling and resolidification, forms molten beads, pits, and irregular undulations on the surface. These surface defects not only directly cause material loss but also degrade the morphology of the contact interface, increasing contact resistance instability during subsequent operation.

For copper-based and silver-based materials, arc erosion in vacuum is particularly severe. These two metals possess high thermal conductivity and relatively low melting points, making them prone to melting and ablation under the high temperatures generated by arc discharge. Silver has a relatively high vapor pressure, leading to more pronounced evaporation losses under vacuum arc conditions. Although copper has a slightly lower vapor pressure, repeated arc impacts cause its surface to form a porous and loose oxide layer (even in vacuum, the high temperatures generated by the arc are sufficient to drive localized oxidation of residual oxygen), further compromising material integrity.

However, from the perspective of electrical contact lifespan, the vacuum environment presents an apparently paradoxical advantage. Research indicates that with increasing vacuum degree, the service life of electrical contacts markedly extends [70], as illustrated in Figure 7. The underlying reason for this phenomenon lies in the improved stability of contact resistance in vacuum. In atmospheric environments, oxide films continuously form on contact surfaces, and their thickness and distribution fluctuate randomly, leading to continual variations in contact resistance. In vacuum, although mechanical wear may be more severe, the variation in resistance is more predictable and stable. Moreover, the absence of corrosive gases in vacuum eliminates the impact of chemical corrosion on electrical contact, thereby extending service life from this perspective.

Research on slip ring performance in vacuum environments reveals a physical picture fundamentally different from other environmental conditions; however, the current understanding remains notably incomplete. Although the mechanism by which the absence of oxide films increases the friction coefficient has been clarified, quantitative descriptions of interfacial contact behavior across different vacuum levels remain insufficient. In particular, few studies have addressed how the adsorption–desorption dynamic equilibrium of residual gas molecules affects interfacial shear strength across the transition from low to ultra-high vacuum. More importantly, the paradoxical phenomenon of “reduced resistance but increased wear” in vacuum reflects a fundamental conflict between electrical and mechanical performance; achieving their synergistic optimization through material design remains an unsolved challenge. The superior performance of layered solid lubricants in vacuum indeed offers promise for applications, yet understanding of their lubrication mechanisms remains insufficiently deep. It is overly simplistic to attribute the low friction coefficient of MoS₂ in vacuum (as low as 0.02) compared to its value in air (around 0.15) solely to “oxidation avoidance.” Such an explanation neglects the essential changes in surface energy state, charge transfer, and interfacial bonding occurring in vacuum. Moreover, the service life of such materials is governed by the stability of the transfer film, yet the critical conditions for its formation, its evolution patterns, and its failure mechanisms have not been systematically studied. Vacuum arc discharge is another underestimated issue. Although researchers have recognized the mechanism of metal vapor discharge, little is known about the spatial–temporal energy distribution of the arc, the redeposition behavior of evaporated material, and the cumulative damage effects under repeated arc events. The “reduced contact resistance” observed by Xiao et al. is an important phenomenon; however, whether this improvement can offset the reliability degradation caused by increased wear requires system-level trade-off analysis rather than evaluation by a single indicator. From a materials selection perspective, Au/MoS₂ composite systems have been recommended, yet key engineering issues—such as balancing gold’s softness with MoS₂’s brittleness, maintaining interfacial bonding strength under cyclic loading, and ensuring phase stability during long-term vacuum exposure—remain insufficiently investigated.

2.5. Summary

This chapter systematically reviewed the mechanisms by which harsh environments—such as high temperature, humidity, corrosive gases, and vacuum—affect the current-carrying friction and wear behavior of slip rings, revealing the complex laws by which environmental factors regulate tribological performance and electrical contact reliability through changes in interfacial physicochemical states. Harsh environments exert complex and far-reaching effects on the current-carrying friction and wear performance of slip rings.

Table 1. Effects of different environmental factors on the performance of slip rings.

Environmental Factor	Reference	Material System	Effect on Friction Coefficient	Effect on Wear Rate	Effect on Contact Resistance	Main Wear Mechanism
High temperature (>150 °C)	[71]	Cu/Cu	Moderate (↓)	Strong (↑)	Moderate (↑)	Thermo-oxidative coupled wear (tribo-oxidation)
High humidity (>70% RH)	[72]	C/Cu	Strong (↓)	Moderate (↑)	Strong (unstable)	Electrochemical–mechanical coupling (tribocorrosion)
Corrosive gases (SO2, Cl2)	[56]	Silver coating/copper alloy	Moderate (↓)	Moderate (↑)	Strong (↑)	Chemical film–induced brittle spalling
Vacuum (<10−3 Pa)	[65]	Ag–C	Strong (↑)	Strong (↑)	Low (↓)	Adhesive wear/mechanical wear

compares the impacts of different environmental factors on slip ring performance. Existing studies have provided a relatively comprehensive understanding at the phenomenological level, identifying key physical processes such as material softening, oxide film evolution, lubrication film regulation, corrosion–wear synergy, and adhesion enhancement, which offer valuable guidance for material selection and operating condition optimization. However, in terms of scientific depth and engineering application needs, the current body of knowledge still exhibits significant limitations.

It is worth noting that these factors do not act independently. High temperature accelerates the reaction rates of corrosive gases, humidity affects the formation and stability of oxide films, and vacuum fundamentally alters the formation mechanism of surface films. Moreover, the relative influence of these factors varies: corrosive gases exert the strongest effect on electrical contact performance, whereas vacuum conditions have the most pronounced impact on mechanical wear. Therefore, in practical applications, the synergistic effects of multiple factors must be comprehensively considered.

3. Simulation and Modeling Research

Although the influence of environmental conditions on slip-ring performance and failure mechanisms has been widely recognized, the complexity of electromechanical coupling and the harshness of experimental conditions make it difficult to obtain precise experimental data. These limitations have prompted researchers to increasingly adopt numerical simulation techniques that integrate experimental observations with theoretical models, using commercial software such as COMSOL, ANSYS, and ABAQUS to predict wear patterns, optimize material compositions, and improve design configurations. As the most commonly used simulation technique in studies of current-carrying friction, the finite element method (FEM) demonstrates unique advantages in revealing the mechanisms of multiphysics coupling [73,74,75,76,77,78,79].

3.1. Multiphysics Modeling

Multiphysics modeling, by coupling current-induced thermal effects, mechanical contact forces, and surface evolution processes, provides a theoretical tool for understanding the synergistic mechanisms among different forces in current-carrying friction. The core advantage of this approach lies in its ability to capture coupling effects that cannot be revealed by single-physics analyses.

3.1.1. Temperature Field Modeling

In the field of current-carrying friction, temperature accumulation and electromagnetic effects significantly influence the performance of friction pairs, and temperature field modeling provides an important basis for studying these complex phenomena. Shen et al. [80] established a transient heat transfer model to evaluate the surface temperature rise induced by Joule heating, while Blanchet [81] developed a dimensionless temperature rise model to describe temperature evolution during reciprocating sliding friction. Yu et al. [82] used COMSOL finite element software to build a simulation model of the friction pair and analyzed the surface temperature distribution. Liu et al. [83] explored the influence of the velocity skin effect on the maximum temperature of frictional sliding characteristics using a multiphysics finite element model and optimized relevant parameters through an improved Kriging optimization method.

However, these studies share a common limitation: model validation often relies on a limited number of experimental data points, and full-field verification of the spatiotemporal evolution of the temperature field remains insufficient. Particularly under high-speed and high-current conditions, accurate prediction of transient temperature peaks imposes stringent demands on experimental measurement techniques, and whether existing methods such as infrared thermography possess sufficient spatiotemporal resolution for model validation remains questionable. Although progress has been made in the methodology of temperature field modeling, the spatiotemporal accuracy of model predictions and their extrapolation capability to varying operating conditions still require more rigorous experimental validation.

3.1.2. Electromagnetic–Fluid Coupled Modeling

For the transient mixed behavior of electrical contact and interfaces, Liao et al. [84] coupled magnetic and temperature fields using the finite difference method to establish a transient mixed hydrodynamic model. The study found that the action of the electromagnetic field caused an increase in temperature peaks and led to a nonuniform distribution of the metallic liquid film, thereby affecting friction and wear. Morris et al. [85] developed an electric field intensity model based on electrostatics and elastohydrodynamic theory, further exploring the influence mechanism of the lubricant film under electrical contact conditions. Zhang et al. [86] established a magneto-electro-thermo-mechanical coupling model for the lubricating film in electromagnetic launchers, and the study revealed that viscous heating within the film was more significant than Joule heating.

These models are all built upon a series of simplifying assumptions—such as neglecting interfacial chemical reactions and assuming material parameters are temperature-invariant; when the interfacial temperature approaches the material’s melting point or a phase transition occurs, the validity of these assumptions is called into question. Electromagnetic–fluid coupled modeling exhibits methodological innovation, but the applicability of its simplifying assumptions under extreme conditions still requires thorough evaluation.

3.1.3. Rough Surface Contact Modeling

Guo et al. [87] developed a sliding electrical contact temperature-field simulation model with rough contact surfaces using COMSOL, introducing a rough-surface contact model based on the Weierstrass–Mandelbrot fractal function (Figure 8). The study showed that the contact temperature first decreases and then increases with increasing average surface roughness. Moreover, under high-speed and high-current conditions, the effect of average roughness slope on contact temperature is relatively small, while the contact temperature continuously decreases with increasing fractal dimension (D) or decreasing fractal roughness (G). This is because, with the other parameter held constant, an increase in D or a decrease in G smooths the contact surface, thereby increasing the real contact area in the contact zone and reducing the contact resistance and the associated Joule heating.

The value of this work lies in incorporating the statistical features of surface topography into macroscopic models; however, the inherent limitations of fractal models should not be overlooked. Whether the multiscale features of real rough surfaces can be adequately represented by a single fractal dimension remains uncertain. How the dynamic evolution of surface morphology during friction influences contact behavior remains an open question. These issues have not been effectively addressed in current models. Rough-surface contact modeling represents an important advance, but the description of the dynamic evolution and multiscale characteristics of surface morphology still requires further refinement.

In summary, multiphysics modeling has played a vital role in optimizing slip ring design and has provided theoretical support for understanding the mechanisms of current-carrying friction. However, current simulation and prediction studies mainly focus on temperature field and wear volume prediction, while simulations addressing material friction and wear behavior under multiple interacting factors remain relatively scarce. Existing simulations primarily focus on operating conditions (such as current, load, and speed), while paying insufficient attention to environmental factors (such as atmosphere, vacuum, temperature, humidity, and corrosive gases), resulting in a clear mismatch with practical application requirements.

3.2. Wear Models and Prediction

Wear prediction is one of the core objectives of current-carrying friction modeling; researchers have proposed various models to analyze the wear behavior of brush contacts under current-carrying conditions, yet they commonly face challenges of incomplete physical mechanisms and strong parameter dependence.

3.2.1. Modeling of Brush–Slip Ring Systems

For brush–slip ring systems, researchers have conducted multilevel numerical modeling. Suchan et al. [88] used Simulink and Flux software to simulate brush torque and iron losses in electrically excited synchronous machines. Houenouvo et al. [89] developed a finite element model of the carbon brush–slip ring contact using ANSYS software. Zuo et al. [90] established a friction–wear model with current effects using COMSOL to investigate the temperature characteristics of slip rings. Deeva et al. [91] proposed a model of wear particle flow in the sliding contact layer based on gas dynamics theory. Sincero et al. [92] performed finite element simulations of brush–slip ring systems based on an arc discharge model.

These models provide tools for understanding the physical phenomena of brush–slip ring systems, but each focuses on a relatively single physical process and lacks a systematic multi-mechanism coupling framework. Although modeling studies of brush–slip ring systems cover multiple physical aspects, the nonlinear synergistic effects among arc discharge, oxidative wear, and mechanical wear have not yet been adequately characterized.

3.2.2. Contact Resistance Modeling

Wu et al. [93] and Zhang et al. [94] each developed contact resistance models to analyze the effect of contact conditions on brush performance. The model developed by Qiu et al. [95] revealed the relationship between interfacial contact pressure and contact resistance. Most of these models are grounded in classical contact mechanics and describe the relationship between the real and apparent contact areas via the Hertzian contact theory or the Greenwood–Williamson contact model.

However, when oxide films or corrosion products exist at the contact interface, the contact resistance is no longer governed solely by mechanical contact but instead depends on quantum transport processes such as tunneling or local breakdown. Current models handle these microscopic mechanisms in a rather simplified manner. Contact resistance modeling has revealed the pressure–resistance relationship, but the description of interfacial oxide films and quantum transport effects still needs to be strengthened from a microscopic mechanism perspective.

3.2.3. Development of Wear Prediction Models

In the field of wear prediction, Collina et al. [96] were the first to propose a predictive procedure for the wear process; subsequently, Bucca et al. [97] proposed a model aimed at improving system design. Derosa et al. [77,98], Mei et al. [99], and Zhou et al. [78] successively developed heuristic wear prediction models under various conditions. Existing tribological models, such as the Archard wear model, combine mechanical contact area, sliding distance, and wear rate through simple mathematical relationships. Although practical for preliminary estimation, they fail to capture the complex coupling effects in current-carrying friction.

Zhou et al. [100] proposed a computational approach based on an enhanced Holm–Archard adhesive wear model, which significantly improved the accuracy of wear rate prediction. Balakrishna et al. [101] established a predictive model for temperature rise in the contact region during dry sliding. However, the Archard model is essentially an empirical relation, and it remains uncertain whether this linear relationship still holds when the wear mechanism shifts from adhesion-dominated to oxidation- or arc-erosion-dominated modes. Existing models remain inadequate regarding the criteria for wear-mechanism transitions and their corresponding mathematical descriptions. Wear-prediction models have evolved from empirical formulas to physics-based models, yet quantitatively describing the conditions for multi-mechanism transitions remains a key challenge.

3.2.4. Digital Twin and Life Prediction

Hansen et al. [102] developed a digital twin model of a pin-on-disk wear test rig; Zhao et al. [103] used MATLAB to construct a brush wear prediction model based on an improved Coulomb–viscous friction model to estimate service life. These studies attempt to establish a closed-loop system from real-time monitoring to life prediction, but the fundamental challenges lie in online parameter identification and uncertainty quantification.

Material parameters, environmental conditions, and operational loads are time-varying during actual service, and ensuring the reliability of model predictions over long timescales remains an unresolved challenge. Digital twin technology represents a new direction in modeling research; however, breakthroughs are still needed in the online identification of time-varying parameters and in ensuring long-term predictive reliability.

3.2.5. Molecular Dynamics Simulation

Yin Nian et al. [104] investigated the friction and wear behavior of Au-coated slip rings under ultra-high vacuum (10⁻⁶ Pa), thermal cycling (−70 to 30 °C), and sliding speeds of 10, 50, and 100 m/s using molecular dynamics simulations, revealing that the wear of the coating is primarily governed by an adhesive wear mechanism (Figure 9). The study found that increasing the temperature significantly accelerated the mechanical response rate at the slip ring and ring brush interface, thereby accelerating the wear process. Additionally, lower relative motion speeds exacerbated adhesive wear, suggesting that wear behavior is highly dependent on the interface mechanical characteristics under varying operational conditions.

The strength of the molecular dynamics (MD) approach lies in its ability to directly simulate atomic-scale contact, deformation, and fracture processes, but its application is severely constrained by spatiotemporal scale limitations. Conventional MD simulations are restricted to nanometer length scales and nanosecond timescales, and reliable multiscale strategies for extrapolating these results to macroscopic engineering applications are still lacking. MD offers a unique lens for understanding microscopic wear mechanisms, yet cross-scale linkage methods from the nanoscale to practical macroscopic applications remain to be established.

3.3. Summary

Simulation techniques have made substantial progress in studies of current-carrying tribology: multiphysics modeling has successfully captured electro-thermal-mechanical coupling effects; wear-prediction models have shown practical value for quantitatively assessing material lifetime; and microscopic approaches such as molecular dynamics have opened new avenues for mechanistic exploration. Nevertheless, from the perspectives of scientific rigor and engineering reliability, current simulation studies still exhibit systemic limitations.

The primary issue is the incompleteness of the physical models. Although existing multiphysics models encompass key quantities such as current, temperature, and stress, their treatment of critical processes—chemical reactions, phase transformations, and arc discharges—remains rudimentary. A deeper problem lies in the absence of scale bridging: a multiscale modeling framework connecting atomic bonding to macroscopic wear has yet to be established. Second is the reliability of parameter acquisition; under extreme service conditions, materials’ mechanical, thermal, and electrical parameters are often difficult to measure precisely, and how their uncertainties propagate into predictions is scarcely addressed. Third, experimental validation is insufficient: although most simulation studies include comparisons with experiments, validation is typically confined to a few operating points and a single output variable. Fourth, research priorities are imbalanced: existing simulations focus primarily on operating parameters, while attention to environmental factors is seriously lacking, leading to a clear disconnect from practical needs.

From a methodological standpoint, current simulation studies tend to over-rely on commercial software, and when the physics of the problem exceeds the software’s preset scope, their applicability becomes limited. Looking ahead, simulation studies of current-carrying tribology should focus on: building comprehensive physical models that encompass multiple processes—including electro-thermal-mechanical-chemical interactions and phase transitions; developing multiscale modeling approaches that bridge atomic to macroscopic systems; constructing material-parameter databases for extreme service conditions; advancing synergistic techniques that integrate in situ characterization with model validation; and establishing uncertainty quantification and reliability-based design methodologies for engineering applications. Only through improved physical modeling, enhanced parameter acquisition, and strengthened validation methodologies can simulation evolve from an auxiliary analysis tool into a truly reliable means for predictive design.

4. Research on Improving Current-Carrying Friction and Wear Performance of Slip Rings Under Harsh Environments

In current-carrying friction and wear, surface protection technologies and material optimization are crucial methods for enhancing equipment wear resistance and reducing friction loss. Especially in harsh environments, the proper selection of materials and surface coating technologies is vital for enhancing the service performance of slip rings.

4.1. Material Modification Methods

The selection and improvement of materials are the foundation for enhancing the wear resistance of current-carrying friction systems. Commonly used slip ring materials include copper-based, silver-based, and gold-based materials,

Table 2. Types and properties of friction pair materials [105].

Materials		Reference Number	Advantages	Disadvantages
Copper-based electrical contact materials	Copper-base alloys	[106]	Excellent physical and mechanical properties	Strength and conductivity are difficult to reconcile
	Fiber reinforced (carbon fiber, boron fiber…)	[107]	Self-lubricating, wear-resistant, high strength and temperature-resistance, etc.	Large brittleness, uneven microstructure, anisotropy, higher cost
	Ceramic reinforced (SiC/WC/TiN)	[72]	Good wear-resistant and temperature mechanical properties, low coefficient of thermal expansion, lower cost	Weak dispersion and interfacial bonding
	New types (Cu-WS2, Cu-G-MoS2…)	[108,109]	Excellent self-lubricating, wear- resistant and environmental adaptability	Reduced mechanical strength
Silver-based electrical contact materials	Silver -base alloys (Ag-Cu, Ag-Cu-Ni…)	[106]	high mechanical strength and wear-resistant, stable contact characteristics under low contact pressure	Poor oxidation resistance and corrosion resistance, lower conductivity than silver
	Ag/C series	[65]	High weld-resistant, low contact resistance	Low hardness, poor anti-arc erosion capability
	Ag/WC series	[110]	Anti-melt welding, heat and wear resistance	Producing WO3 resulting in contact resistance
	Ag/Ni series	[111]	Wear-resistant, saving silver	Low weld resistance at high current
	Ag/MeO series (Ag/CdO, Ag/SnO2, Ag/ZnO…)	[112]	Better weld and arc resistance, conductivity at high current	Ag/CdO pollutes the environment
	New types (Ag-MoS2, Ag-MoS2-G-CNTs…)	[113]	Excellent wear-resistant, anti-vulcanization and environmental adaptability	Reduced mechanical strength
Gold-based electrical contact materials	Gold-base alloys	[106]	Excellent conductivity and mechanical properties	Expensive
	Lanthanon reinforced	[114]	High melting point, hardness, anti-arc and chemical stability	Expensive
New electrical contact materials (TiNiC, Graphene…)		[115]	Integrating electrical conductivity and lubrication	-

presents a comprehensive classification of tribo-pair materials used in slip-ring applications. Research has shown that adjusting the microstructure of materials or introducing other elements into the matrix can significantly enhance their wear resistance.

Copper-based materials are widely used in the current-carrying friction field due to their excellent electrical conductivity and corrosion resistance. Due to the poor heat resistance and tendency to soften and deform at high temperatures, copper is often improved by adding other materials. For example, metal alloying can improve the strength and oxidation resistance of materials [116,117,118]; Adding layered lubricants, such as graphite [119,120,121,122], WS₂ [69], and MoS₂ [123,124], can enhance the material’s self-lubricating properties, thereby reducing the friction coefficient and wear rate in current-carrying friction [125]; Adding fiber-reinforced materials (such as carbon fiber [107,126] and boron fiber can improve the material’s impact resistance and durability [126,127]; In addition, adding ceramic particles (such as WC, SiC, and TiN [128]) can enhance the material’s wear resistance and high-temperature mechanical properties [129,130].

Silver-based materials stand out in high-conductivity applications due to their low resistivity and high electrical conductivity. However, their low wear resistance, susceptibility to sulfidation, and tendency to undergo welding and adhesion under high loads limit their use under high-load and high-current conditions. Therefore, they are often improved through alloying or surface coating techniques. Fukuda et al. [131] investigated the contact voltage drop and wear characteristics of silver-graphite brushes with varying silver content in slip ring systems, finding that the best contact voltage drop and wear performance occurred with silver content between 60% and 80%. Additionally, the study indicated that as the carbon content decreased below 40%, the lubricating effect of carbon in the contact area of the brush significantly reduced, leading to a rapid increase in wear. Silver-based alloys such as Ag/Ni and Ag/Pd have lower contact resistance and better stability, allowing for up to a 40% reduction in silver usage. Meanwhile, Ag/MeO (including Ag/CdO, Ag/ZnO, and Ag/SnO₂ [132]) exhibits better electrical conductivity, thermal conductivity, anti-welding properties, and resistance to arc erosion under high-current conditions.

Gold-based materials, due to their excellent oxidation resistance and electrical conductivity, are typically used only in high-end precision systems such as aerospace, medical devices, and high-end electronic components, despite their high cost. To improve their hardness, elasticity, wear resistance, and electrical erosion resistance, these materials are typically alloyed with elements such as silver, copper, palladium, or platinum. Xie et al. [133] studied the current-carrying friction process of AuAgCu brushes and Au coatings under vacuum conditions. In the CuNiAu slip ring they developed, gold serves as the surface coating providing chemical stability and spot-welding performance, copper as the bonding layer offers conductivity and ductility, and nickel as the intermediate layer provides wear resistance and high-temperature stability, resulting in excellent properties of each constituent metal.

In addition to traditional copper-based, silver-based, and gold-based materials, researchers are exploring the potential of novel materials and have developed a series of new composite films with superior overall performance [134], such as TiNiC [135], ZrN [136], Nb [137,138], and graphene [139,140,141]. Shiri et al. [142] prepared a Cu/NbC composite material that demonstrates excellent mechanical and electrical properties. The Ti-Ni-C nanocomposite coating developed by André et al. [135] features low contact resistance and a low friction coefficient with silver, making it ideal for high-performance electrical contacts.

4.2. Surface Coating Technology

Surface coating technology is one of the main methods to improve the current-carrying friction stability of electrical contact materials [143,144,145]. Typically, the materials mentioned above are used to prepare metal, self-lubricating, or composite coatings through cold spraying technology [132], plasma spraying technology [146,147], magnetron sputtering technology [64], laser cladding technology, electroplating technology [148] (

Table 3. Comparison of Common Surface Coating Technologies.

Coating Technology		Reference Number	Combination Method	Material Type	Substrate Heating	Single Layer Thickness/mm	Advantage	Disadvantage
Plating technology	Chemical vapor deposition	[154]	Physical bonding	Metallic, ceramic materials	Large	<0.01	High-quality coating, uniform and controllable thickness, high heat and chemical resistance, high deposition efficiency	High temperature process, toxic reaction gas, slow deposition rate
	Physical vapor deposition	[155]		Metallic, ceramic materials	Lesser	<0.01	Low temperature process, high bonding strength, high hardness and wear resistance, no pollution	Slow deposition rates, limited coating thickness, and limited material selection
	Electrodeposit	[156]		Metallic	Nothing	<0.1	Easy operation, fast deposition speed, no limitation of substrate shape and size, low internal stress and controllable thickness of the plated layer.	Low dimensional accuracy and environmental pollution
Surface spraying technology	Cold spray technology	[157]	Mechanical bonding	Alloys, ceramic powders	Little	0.1–9	Low heat input, high material utilization, high bond strength, coating density, low porosity, and high deposition rate	Difficulty in ensuring coating uniformity, coating thickness limitations, material limitations, need for post-processing
	Thermal spray technology	[158]	Metallurgical bonding	Alloys, ceramic powders	Large	0.1–1	Small heat-affected zone, high deposition efficiency, good coating performance, thickness control	Limited coating bonding, high porosity, high surface roughness, low material utilization
Laser cladding technology		[159]	Metallurgical bonding	Alloy powders, wires, plates	Little	0.02–2	High deposition rate, small heat affected zone, high coating quality	Complex process debugging, limited material selection, high cost
Electron beam additive manufacturing technology		[160]	Metallurgical bonding	Alloy powders	Large	0.001–9	High precision and complexity, high material utilization, superior mechanical properties, high reparability power	Slower production speeds, heat-affected zones, material selection limitations, high vacuum requirements
Arc Additive Manufacturing Technology		[161]	Metallurgical bonding	Alloy powders, wires, plates	Large	2–4	High deposition rate, material adaptability, economy, superior mechanical properties, uniform structure, high production efficiency	Large heat affected zone, poor surface quality, difficult to control, lack of refinement

). Moreover, researchers have explored other methods to fabricate coatings, including rotary spray deposition [149], electrodeposition [150,151,152], and electron beam evaporation [153].

Cold spraying technology accelerates metal or alloy particles to the substrate surface using high-speed airflow, depositing the coating at low temperatures. Since the coating is deposited at a low temperature, it avoids phase transformation or oxidation of the coating material at high temperatures, resulting in a coating layer with good conductivity and wear resistance. Cold spraying is commonly used to prepare copper-based coatings, which maintain high conductivity while improving surface oxidation resistance and wear performance, making it one of the key technologies in current-carrying friction surface engineering [157].

Plasma spraying technology uses high-temperature plasma to melt the coating material and sprays it at high speed onto the substrate surface to form the coating. This technology offers high temperatures, controllable coating thickness, and high deposition efficiency, making it suitable for various materials, including metals [158], ceramics [162], and composites. Coatings produced by this technology exhibit good adhesion and wear resistance, making them suitable for applications under current-carrying friction conditions, especially in scenarios that require high wear resistance and thermal shock resistance [163].

Magnetron sputtering technology uses a magnetic field to constrain plasma, causing charged particles to collide with the target material and deposit atoms onto the substrate surface to form a thin film. Magnetron sputtering allows precise control over film thickness and uniformity, with a dense coating structure, making it suitable for current-carrying friction applications that require high precision and thin-film thickness control, such as the preparation of gold-based or silver-based coatings [117,164]. Research on this technology in the field of slip ring surface treatment is growing, particularly in precision electronic components. Wang et al. [123] used unbalanced magnetron sputtering to prepare Ti/MoS₂, Pb/MoS₂, and MoS₂ coatings, and the experiments showed that the coatings became denser after the current passed through, reducing the friction coefficient of the material.

Laser cladding technology uses high-power lasers to melt the coating material and deposit it onto the substrate surface, forming a dense and strongly bonded coating. This technology has precise and controllable energy input, making it suitable for the surface of parts with complex shapes. The coating materials for laser cladding are diverse, including metals, ceramics, and composite materials, and they provide excellent wear resistance, corrosion resistance, and high-temperature performance. In recent years, Cu-based coatings and composite materials have been widely used in laser cladding, further enhancing their wear resistance under current-carrying friction conditions. Zhang et al. [159] used laser cladding technology to prepare CrB₂/Cu composite coatings, and the material exhibited excellent wear resistance and high-temperature stability, significantly improving its service life.

Electroplating technology is a process that uses electrolysis to deposit metal coatings onto the substrate surface. Electroplated coatings typically have good adhesion and a smooth surface, and are widely used in current-carrying friction applications to enhance conductivity and corrosion resistance. Ren et al. [165] showed in their study that gold-coated copper alloys in current-carrying friction experiments at different temperatures played an important role in softening and lubricating, promoting the expansion of the low-contact resistance region and reducing the failure rate at higher temperatures. In recent years, researchers have actively explored composite electroplating technology by adding nanoparticles or other functional materials to the electroplating solution to further enhance coating performance [152].

Scholars have adopted multi-layer coating designs to alleviate thermal and mechanical stresses in current-carrying friction [166]. Common multi-layer coating structures include hard coatings as the transition layer and functional coatings as the outer layer. Transition layer materials such as TiN and CrN can enhance the hardness and adhesion of the coatings.

Cao et al. [167] used laser surface texturing and magnetron sputtering techniques to prepare Ag coatings with different micro-pit structures on copper substrates, and developed multi-layer graphene lubricant (MGLG) with multi-layer graphene as an additive (Figure 10). The properties of textured Ag coatings and MGLG were characterized, and the tribological and electrical properties of textured Ag coatings under MGLG lubrication were further investigated. The results indicate that, compared with non-textured Ag coatings lubricated by MGLG, textured Ag coatings with appropriate pit diameters (244 and 307 μm) exhibit superior tribological and electrical properties. This indicates that the synergistic effect of surface texturing and multi-layer graphene lubricant can effectively improve the functional performance of Ag coatings.

In conclusion, surface modification and alloy coatings have significant advantages in improving the wear resistance and corrosion resistance of slip rings, and materials science demonstrates broad application prospects in slip ring technology. Through the exploration of different materials (such as copper-based, silver-based, gold-based, and new materials), we recognize the advantages and disadvantages of each material in current-carrying friction. This systematic analysis guides material selection and design and reveals the competitive and collaborative relationships between different materials. At the same time, by combining advanced surface coating technologies, we find that optimizing coatings can effectively improve the wear resistance and conductivity of materials, pointing the direction for future research.

4.3. Summary

Surface modification and coating technologies possess significant advantages in enhancing the wear resistance and corrosion resistance of slip rings, and materials science shows great potential for application in slip ring technology. Through a systematic review of different materials (copper-based, silver-based, gold-based, and novel materials), the performance trade-offs in current-carrying friction for each material are revealed: the contradiction between electrical conductivity and mechanical strength in copper-based materials, the conflict between electrical conductivity and wear resistance in silver-based materials, and the constraint of high cost despite excellent properties in gold-based materials. This systematic analysis provides guidance for material selection but also highlights the fundamental dilemma that a single material system is difficult to simultaneously meet multidimensional performance requirements.

The diversified development of surface coating technologies provides greater degrees of freedom for performance optimization. Cold spray maintains the advantages of low temperature but compromises density; plasma spraying provides high deposition efficiency yet faces challenges in oxidation control; magnetron sputtering achieves precise control but is constrained by low efficiency; laser cladding attains high performance but must contend with residual stress; and electroplating is mature and economical but entails environmental concerns. Each technique has its appropriate application scenarios and inherent limitations; there is no “universal” coating method. Although multilayer coatings and surface texturing show potential for synergistic optimization, interfacial compatibility, process complexity, and cost control remain key barriers to engineering implementation.

Nevertheless, current research still suffers from systemic limitations. First, material modification and coating design are largely guided by empirical trial-and-error, lacking theoretical predictive models that bridge microscopic mechanisms to macroscopic performance. Second, most studies focus on performance optimization under a single operating condition, while assessments of long-term stability in complex environments (temperature fluctuations, humidity variations, corrosive gases) remain insufficient. Third, high-performance materials prepared in the laboratory often face practical challenges during scale-up, including reproducibility, uniformity, and cost control. Fourth, the absence of a systematic comparative evaluation framework among coating technologies leaves technology selection in engineering applications without a solid scientific basis.

Looking ahead, the development of materials and surface protection technologies should focus on: establishing material performance prediction models based on microscopic mechanisms to guide rational design rather than blind trial-and-error; developing accelerated aging evaluation methods under the synergistic effects of multiple environmental factors to ensure long-term service reliability; breaking through the scale-up preparation technologies for high-performance materials to address the bottleneck from laboratory to engineering; and building a full life-cycle evaluation system for coating technologies that comprehensively considers performance, cost, and environmental impact. Only through the deep integration of materials science, surface engineering, and manufacturing technology can the highly reliable application of current-carrying slip rings in extreme environments be truly achieved.

5. Conclusions

This paper systematically reviews the mechanisms by which harsh environments affect the current-carrying friction and wear behavior of slip rings, modeling and simulation methodologies, and material optimization strategies. Through critical analysis, the following principal conclusions are drawn.

• The extent to which environmental factors exert influence varies significantly. High temperature and humidity are the most critical environmental factors. Elevated temperature (>150 °C) can increase the wear rate by 200%–500% through the synergistic action of multiple mechanisms, including material softening, accelerated oxidation, and arc intensification. Humidity exhibits a distinctive non-monotonic effect: 40%–60% relative humidity is the optimal operating window, and deviations from this range lead the wear mechanism to shift from adhesive wear to fatigue spalling. The corrosion–wear synergistic effect of corrosive gases is irreversible and is markedly intensified under high humidity. Vacuum conditions increase the coefficient of friction by a factor of 2–3, yet enhance the stability of contact resistance, necessitating entirely different material solutions.
• The synergistic effects of multiple factors far exceed the simple superposition of single factors, yet quantitative descriptions remain severely inadequate. The coupling of high temperature and humidity accelerates electrochemical corrosion, while the coupling of high temperature and current may induce thermal runaway. However, current research mainly relies on single-factor experiments, lacking systematic studies on multi-factor coupling, making it difficult to accurately predict material behavior under real service conditions.
• The depth of mechanistic understanding is uneven, and cross-scale linkages are lacking. The macroscopic phenomena influenced by high temperature and humidity are relatively well documented, but the elucidation of microscopic mechanisms is weak. Fundamental questions—such as the nature of the “low-friction–high-wear” paradox, the dual role of oxide films, and the dynamic cycling of lubricant films—still lack clear answers. A cross-scale theoretical framework linking atomic bonding to macroscopic wear has yet to be established.
• The practicality of simulation technologies is limited. Multiphysics models have successfully captured electro-thermal-mechanical coupling effects, but their descriptions of key processes such as chemical reactions, phase transitions, and arc discharges are overly simplified. Most wear prediction models are based on empirical correlations, and their applicability during mechanism transitions remains questionable. More critically, parameter acquisition is difficult and validation is insufficient, leaving extrapolation capability and long-term predictive reliability unverified. Simulation studies addressing environmental factors are far fewer than those focusing on operational parameters, creating a gap between research and practical needs.
• Material optimization strategies are diverse but lack systematic design principles. Copper-, silver-, and gold-based materials, as well as novel composite materials, each possess their own advantages and disadvantages. Although numerous surface coating technologies exist, there is still a lack of systematic design criteria and long-term stability validation for the rational selection of materials and coating systems according to specific environments.

To address the systematic gaps in existing research, future work should focus on the following key breakthroughs:

• Cross-scale mechanistic linkage and dynamic process tracking. Establish a multiscale integrated framework spanning from first-principles calculations to macroscopic finite-element analysis; develop in situ multiphysics characterization techniques to track in real time the dynamic processes of lubricant-film formation–rupture, oxide-film growth–spallation, and arc initiation–extinction, thereby elucidating the quantitative links between microscopic mechanisms and macroscopic performance.
• Quantitative theory of multi-factor coupling effects. Systematically conduct studies on multi-factor interactions and establish physical models for quantitatively describing coupling effects. Develop phenomenological models grounded in mechanistic understanding, integrate machine learning to extract patterns from big data, and advance failure criteria and lifetime prediction models under multi-factor conditions.
• Extreme-environment parameter databases and high-fidelity modeling. Systematically measure material parameters under high temperature–high current, extreme humidity, and corrosive atmospheres, and establish an open, shared database. Develop comprehensive physical models encompassing chemical reactions, phase transformations, and arc discharge to improve the accuracy and reliability of simulation predictions. Prioritize breakthroughs in modeling methods for environmental factors.
• Materials genome engineering and intelligent response systems. Integrate computational materials science, high-throughput experiments, and machine learning to establish a rapid iterative process of “computational screening–experimental validation–performance optimization.” Develop performance-driven inverse design methods for materials, and explore intelligent material systems such as temperature-responsive self-lubricating materials and humidity-adaptive coatings, achieving a transition from passive adaptation to active control.
• Digital twins and intelligent health management. Integrate physics-based models with data-driven approaches to establish real-time condition monitoring and remaining useful life (RUL) prediction systems for slip ring systems. Develop adaptive algorithms that dynamically update model parameters based on real-time data, and construct intelligent decision-making systems to optimize operating conditions.

The reliable operation of conductive slip rings under harsh environments is a complex scientific issue involving multiphysics coupling, multiscale mechanistic correlations, and multifactor synergies. Only through the coordinated advancement of cross-scale theoretical breakthroughs, the development of advanced characterization techniques, innovations in high-fidelity modeling, and the construction of intelligent material systems can the leap from fundamental scientific understanding to reliable engineering application be achieved, supporting the major demands of aerospace, new energy, and advanced manufacturing fields.