The Evolution of Brake Disc Materials for Trains: A Review

Abstract

As a key component of the train braking system, the comprehensive performance of brake discs plays a vital role in ensuring the operational safety of trains. With the advent of high-speed and heavy-haul trains, the thermal energy generated by braking systems has significantly increased. The resulting rapid temperature rise can easily exceed the material limits of brake discs. Consequently, research focused on enhancing brake disc performance in high-temperature environments, improving thermal fatigue resistance, and optimizing tribological properties has become increasingly critical. Brake disc materials have undergone substantial evolution, transitioning from traditional iron and steel to lightweight aluminum matrix composites and carbon matrix composites. While iron and steel benefit from mature manufacturing processes and proven reliability, their high mass density poses challenges in meeting the demands for lightweight and high-speed development in modern rail transit. Although aluminum matrix composites and carbon matrix composites offer advantages like low density and high heat capacity, they still face several technical challenges in practical applications. This paper outlines the key characteristics of train brake disc materials, emphasizing the application status and research progress of iron and steel, aluminum matrix composites, and carbon matrix composites. Additionally, it briefly introduces surface modification technologies for iron and steel brake discs, with the goal of providing insights and references to guide the innovation and development of train brake disc materials.

1. Introduction

With the continued advancement of the technological revolution and industrial transformation, the rail transportation industry is encountering unprecedented development opportunities. Driven by both technological innovation and theoretical breakthroughs, train technology is rapidly evolving toward cutting-edge areas such as high-speed operation, lightweight design, heavy-haul capacity, and environmental sustainability. For instance, the braking energy of high-speed trains has increased several times over that of traditional trains, leading to a significant rise in energy conversion and consumption, as well as notably extended braking distances and times [1]. While this technological evolution improves operational efficiency, it also introduces greater complexity and operational uncertainties, placing more demanding requirements on the design and optimization of braking systems. Under such circumstances, ensuring the safety and efficiency of braking systems has emerged as a critical technological bottleneck in the high-quality development of modern rail transportation, necessitating urgent collaboration between academia and industry to achieve breakthroughs.

The train braking system is a highly integrated system composed of several key components, including the electrical braking system, mechanical braking system, anti-skid device, and intelligent braking control system. Among these, the mechanical braking system serves as the fundamental braking unit, ensuring safe deceleration or stopping within specified distances during emergency braking conditions, highlighting its critical role. The disc brake, a core of the mechanical braking system, efficiently converts the kinetic energy of trains into thermal energy through friction between the brake disc and the brake pad, achieving smooth braking via rapid heat dissipation [2]. This braking method offers significant advantages, such as high braking torque, excellent thermal stability, and superior reliability, all of which provide strong safeguards for train safety operations. According to the International Railway Union (UIC) Standard UIC CODE 541-3 [3], the disc brake of trains traveling at speeds of 350 km/h must maintain the stability of the friction coefficient (fluctuation range ± 0.05) under thermal shock conditions ranging from 800 to 1200 °C, with thermal conductivity exceeding 40 W/(m·K). In the case of emergency stop-braking of an 8-car group high-speed train with axle loads of 17 tons at 350 km/h, the kinetic energy of 2.571 × 10³ MJ is converted into thermal energy via friction between the brake discs and pads, causing the brake disc surface temperature to exceed 500 °C, with local temperatures at the interface potentially rising up to 900 °C [4]. To ensure the functional integrity of the disc brake under these extreme conditions, train brake disc materials must offer high and stable friction coefficients, excellent wear resistance [5,6,7], low density, high specific heat capacity, superior thermal conductivity [8,9], high hardness, and outstanding fatigue resistance [10,11,12]. Currently, train brake disc materials are primarily categorized into two types—traditional iron and steel materials and emerging composites—as shown in Figure 1 [13]. Each of these materials has distinct characteristics that directly influence braking efficiency and safety, making them crucial factors in determining braking performance.

Most previous research has focused on a single type of brake disc material, often with limitations, and has not conducted comparative studies on the advantages and disadvantages of various disc materials. This paper not only provides an overview of the material characteristics and surface treatments of traditional iron and steel brake discs (Section 2) but also analyzes the preparation processes and material properties of two new types of composites: aluminum-based composites and carbon-based composites (Section 3 and Section 4). Additionally, this article concludes with a discussion of the technical indicators and future development prospects for brake disc materials (Section 5). By comparing and analyzing the characteristics of these materials, it provides forward-looking guidance for the future development of brake disc materials, promoting the rail transportation industry toward greater safety, efficiency, and environmental sustainability, and driving optimization and upgrading within the industry.

To ensure the comprehensiveness and objectivity of the analysis, this paper adopts a systematic literature collection framework. In terms of database selection, priority is given to major databases such as Scopus, Web of Science, Google Scholar, and CNKI. These databases comprehensively cover over 95% of SCI- and EI-indexed journals in the field of engineering materials and include international standard database resources such as UIC and ISO. Keywords were selected and optimized through iterative combinations. For example, terms like “train brake disc material”, “iron brake disc”, “steel brake disc”, “surface modification technologies”, “composite material brake disc”, “anti-thermal fatigue”, “friction coefficient stability”, and “characterization of brake disc material” were employed to strike a balance between the uniqueness of the search and the coverage of relevant vocabulary. The time range for the literature review spanned from 2000 to 2025, considering both foundational research and cutting-edge developments in the field. The inclusion criteria focus on peer-reviewed papers addressing the performance of train brake disc materials (e.g., wear resistance and thermal stability), experimental studies with quantitative performance indicators (e.g., friction coefficient and thermal conductivity), technical standards (e.g., UIC and EN), and industry white papers. Exclusion criteria involve conference abstracts lacking methodological details, non-peer-reviewed reports, and studies not related to railway transportation applications. The retrieved literature was screened and retained based on the relevance of the title or abstract. The full text was then evaluated to select papers that met the inclusion criteria. Finally, additional reference sources were supplemented through reference tracing of the key literature.

2. Iron and Steel Brake Discs

Iron and steel, including cast iron, cast steel, and forged steel, have become widely used in train brake discs due to their high strength, hardness, and excellent heat resistance. These properties enable brake discs to endure heavy loads, high temperatures, and resist impacts, ensuring structural integrity and dimensional stability under extreme conditions while minimizing the risk of deformation and fracture.

2.1. Cast Iron Brake Discs

Cast iron can be primarily categorized into gray cast iron, vermicular graphite cast iron, and nodular cast iron, which holds a significant position in brake disc manufacturing due to its cost-effectiveness and outstanding tribological performance [14]. However, with the continuous increase in train speeds, the limitations of traditional cast iron brake discs in tensile strength and wear resistance have become more pronounced, making it challenging to meet the operational demands of high-speed and heavy-haul trains. During high-energy braking, disc wear becomes particularly severe, significantly reducing the reliability of the braking system and exacerbating environmental pollution from brake dust.

To address these issues, various effective strategies to improve the performance of cast iron brake discs through systematic experimental studies and theoretical analysis have been explored. Introducing nano-scale additives can significantly enhance the wear resistance of nodular cast iron [15]. Surface quenching can effectively reduce the wear rate of gray cast iron and lower particulate emissions [16]. The addition of vanadium (V) can refine carbides, optimizing the wear resistance of cast iron [17]. Turu et al. [18] verified that the appropriate V addition notably improves the wear and corrosion resistance of nodular cast iron, with distinct variations in load and sliding speed affecting material wear rates, as shown in Figure 2a. Furthermore, adding niobium (Nb) can substantially improve the hardness and wear resistance of gray cast iron [19,20]. As illustrated in Figure 2b, increasing the Nb content leads to higher hardness and a significant reduction in the wear rate, clearly reflecting Nb’s beneficial effect on the wear resistance of gray cast iron. Tonolini et al. [21] further confirmed that adding 0.3 wt% Nb effectively enhances the wear resistance of gray cast iron brake discs. Furthermore, the addition of niobium (Nb) to nodular cast iron has been demonstrated to improve tensile strength and wear resistance, attributed to the formation of NbC carbides and refined graphite nodule morphology [22,23]. Notably, heat treatment, another effective method for enhancing the performance of cast iron, demonstrates synergistic effects with alloying approaches. Sckudlarek et al. [24] found that adding Nb to nodular cast iron, combined with isothermal tempering treatment, can further improve yield strength and tensile strength, highlighting the importance of rational alloy selection and optimized heat treatment processes.

In terms of addressing thermal crack issues in cast iron brake discs under thermal and mechanical loads, Ding et al. [25] discovered that the addition of molybdenum (Mo) significantly improves the mechanical properties and thermal conductivity of gray cast iron, offering a promising solution for thermal crack prevention. Goo et al. [26] enhanced thermal crack resistance and fatigue life by optimizing material composition and structural design. Moreover, surface corrosion, a major factor accelerating wear, is closely linked to the oxide layer thickness [27]. Studies indicate that the addition of V and Mo can effectively enhance the corrosion resistance of cast iron brake discs [28]. These research findings provide important theoretical foundations and technical support for enhancing the comprehensive performance of cast iron brake discs.

The selection of alloying elements and the optimization of heat treatment processes are crucial for enhancing the performance of cast iron brake discs. Further exploration of alloying elements and additives could enhance the tensile strength, wear resistance, and corrosion resistance of cast iron. On the other hand, optimizing heat treatment processes will be essential, enabling precise control of the microstructure through accurate regulation of heating and cooling cycles to improve comprehensive properties. Furthermore, as environmental concerns grow, the environmental performance of cast iron brake discs will become a key area of research. For instance, reducing particulate emissions during braking and minimizing noise pollution will drive ongoing innovation in the material design of cast iron brake discs.

2.2. Steel Brake Discs

With the continuous increase in the operational speeds and carrying capacities of trains, the limitations of cast iron brake discs in braking systems have become more apparent. This has led to a gradual transition from cast iron to steel brake discs. The widespread preference for steel brake discs is primarily due to their superior thermal load-bearing capacity, high mechanical strength, and fatigue resistance. Steel brake discs are currently divided into two main categories: forged steel and cast steel. Forged steel brake discs exhibit outstanding braking performance, thanks to their superior microstructural uniformity, plasticity, and toughness. However, their ability to incorporate complex heat dissipation structures is limited by the forging process, which results in poor thermal dissipation performance and an increased susceptibility to thermal cracking. In contrast, cast steel brake discs have become the predominant choice for braking systems due to their excellent thermal fatigue resistance, favorable mechanical properties, and flexibility in heat dissipation structure design [29].

During the emergency braking of trains, the intense friction between brake discs and brake pads generates a substantial amount of frictional heat. This leads to rapid temperature increases on the surface of the steel brake disc, resulting in significant temperature gradients and the distribution of thermal stress. This thermal stress can easily induce thermal cracks on the surface of the steel brake disc, posing a serious threat to the operational safety of trains. Additionally, the accumulation of frictional heat triggers a chain reaction, which includes reduced braking performance, accelerated wear, the propagation of thermal cracks, and changes in disc thickness [30]. The formation and propagation of thermal cracks are especially critical as they weaken the effective friction between discs and pads, leading to extended braking distances and times.

To understand the mechanisms by which thermal load affects the performance of steel brake discs, Suo et al. [31] systematically investigated the temperature and stress field distributions in forged steel brake discs. Their research revealed that harsh operating conditions exacerbate thermal stress generation on the surface of the steel disc, which is the primary cause of surface crack initiation and structural deformation. Further investigation into the damage mechanisms of thermal load was conducted by He et al. [32], who performed detailed microstructural observations and analysis of failed forged steel brake discs. They found that the alternating thermal stress causes the cracking and spalling of oxide films, which accelerates material wear and shortens the discs’ service life. Figure 3 illustrates the crack morphology of a forged steel brake disc under thermal load, clearly revealing the microscopic characteristics and evolution of thermal damage.

In tribological systems, the third-body film (TBF) refers to the intermediate layer formed between two contacting surfaces, typically composed of wear particles, oxides, and reaction products. This film not only prevents direct contact between the surfaces—thereby reducing wear during the friction process—but also helps transfer braking pressure through shear plastic deformation and accommodates speed differences. Liu et al. [33] conducted friction tests on the performance of steel brake discs with thermal cracks (TCS) and without thermal cracks (WTCS). They found that thermal cracks compromised the integrity of the TBF, leading to a significant reduction in the friction coefficient and increased fluctuation. As shown in Figure 4a, the surface of the disc without cracks formed a continuous and complete TBF, with only a few adhered pits and micrometer-scale furrows, and the wear form was primarily abrasive wear. Under these conditions, the brake disc exhibited a stable friction coefficient and low wear rate. In contrast, as shown in Figure 4b, the surface with thermal cracks exhibited localized fracture and peeling of the TBF, creating a discontinuous film layer with significant surface damage.

In research aimed at optimizing the performance of cast steel brake discs, Harada et al. [34] found that adding an appropriate amount of V to Ni-Cr-Mo cast steel brake discs significantly enhances thermal shock resistance, toughness, and hardness, effectively extending their service life. Additionally, Wu et al. [35] demonstrated that the synergistic effects of Nb and V greatly improve the thermal fatigue performance of Cr-Mo-V cast steel brake discs. Notably, the addition of chromium (Cr) and V helps form dense protective films on crack surfaces, effectively inhibiting crack propagation and enhancing the overall reliability and safety of brake discs [36].

Steel brake discs experience extreme frictional heat during high-energy braking, which significantly affects both braking performance and safety. To improve the performance of steel brake discs, appropriate elemental additions can enhance their thermal shock resistance and thermal fatigue properties. Future research should focus on deepening the understanding of microstructure–property relationships and achieving precise control over alloy compositions to attain superior thermal shock resistance, toughness, hardness, and thermal fatigue resistance, thereby significantly improving overall performance.

2.3. Surface Modification of Iron and Steel Brake Discs

With the rapid advancements in braking technology, the surface modification of iron and steel brake discs has become a mainstream trend in the industry. This approach is designed to extend service life by enhancing surface performance. During surface strengthening treatment, ensuring the uniformity, density, and interfacial bonding strength of the treated layers is critical to avoid defects such as voids and peeling. Podgornik et al. [37] demonstrated that localized surface strengthening technology can significantly improve wear resistance and optimize the frictional properties of cast iron brake discs through surface hardening effects. Nowadays, advanced surface engineering technologies, including electrochemical processes, physical/chemical vapor deposition, thermal spraying, plasma spraying, and laser cladding, are widely adopted [38]. These technologies enable the preparation of high-performance, wear-resistant coatings by regulating microstructure, interfacial bonding mechanisms, and residual stress distribution, thereby enhancing both the frictional performance and service reliability of brake discs.

Laser cladding, a fast and environmentally friendly coating technology, has been increasingly exploited to improve the surface properties and prolong the lifetime of components [39,40]. A high-precision powder-feeding system delivers specific alloy powders to substrate surfaces in a controlled manner. Under instantaneous high-energy laser irradiation, the powder and substrate undergo rapid melting and cladding, forming coatings with predetermined compositions and internal structures. This process allows for precise control of the coating thickness and ensures a robust metallurgical bond between the coating and the substrate, significantly enhancing both stability and durability. Laser cladding is advancing toward the development of multi-functional protective coatings that combine superior wear resistance with enhanced corrosion and high-temperature resistance, offering comprehensive protection under various operating conditions [41]. As such, laser cladding holds significant potential for improving surface performance and extending the service life of brake discs. However, the performance of materials prepared by laser cladding technology is highly sensitive to process parameters. Some studies [42,43,44] have shown that even small deviations in core parameters such as the laser power, scanning speed, and powder feeding rate can significantly alter the thermodynamic behavior of the molten pool, leading to issues such as cracks, pore retention, and incomplete fusion. Therefore, effective monitoring, control, and optimization of the process parameters are essential to achieving an ideal microstructure and service performance for the cladding layer [45,46,47]. Notably, the laser cladding process involves at least 19 independent process parameters [48,49], including not only core parameters but also factors such as spot diameter, gas flow rate, powder carrier gas ratio, and lap rate. This makes the laser cladding process complex.

In research focused on improving the wear resistance of brake discs, Shi et al. [50] innovatively applied laser cladding technology to fabricate high-performance wear-resistant coatings on cast iron brake discs using a nickel-based alloy powder. As shown in Figure 5a, within a wide temperature range, the volume wear of the nickel-based coating was reduced by more than 30% compared to that of cast iron. Further analysis revealed significant differences in wear mechanisms between the cast iron and the Ni-based coating at room temperature and 300 °C. At room temperature, cast iron displayed typical groove morphology (Figure 5b), while at 300 °C, severe delamination occurred (Figure 5c). In contrast, the Ni-based coating maintained excellent interface bonding performance at both temperatures. At room temperature, the surface exhibited continuous grooves and oxides (Figure 5d), and at high temperatures, a dense oxide layer formed on the surface, effectively preventing direct contact between the two contacting surfaces and providing real-time repair of the wear area, significantly reducing crack density and presenting oxidation wear (Figure 5e). The dense oxide film formed on the surface of the Ni-based coating during friction demonstrated excellent oxidation resistance, providing essential technical support for the reliable operation of brake discs under extreme working conditions. Tonolini et al. [51] found that introducing hard particle powder coatings can significantly enhance the wear resistance of cast iron brake discs. Lin et al. [52] discovered that treating composite coatings with selective laser technology not only effectively improves the microhardness and tensile strength of coatings on cast iron but also significantly inhibits the initiation and propagation of surface cracks.

High-performance coatings on steel brake discs were prepared through laser cladding technology with Stellite 21 cobalt-based alloy powder, as reported by Zhao et al. [53]. The coating meets the operational demands of high-speed trains and contributes to extending the service life of brake discs. Liu et al. [54] employed laser cladding technology to deposit cobalt-based alloy powder onto the surface of steel brake discs. The resulting Co06 coating demonstrated remarkable wear resistance, high-temperature oxidation resistance, and excellent anti-friction properties during braking, further substantiating the effectiveness of laser cladding technology. Xiao et al. [55] successfully fabricated Co06 coating on 30CrMoSiVA steel—a material used for high-speed train brake discs—using laser cladding technology. To thoroughly evaluate the performance of this coating, an extensive analysis was conducted on the tribological properties of copper-based composites against both 30CrMoSiVA steel and the Co06 coating at temperatures ranging from 25 °C to 800 °C. As clearly shown in Figure 6, at 800 °C, the friction curves and wear morphology of the Co06 coating and 30CrMoSiVA steel display significant differences. The study revealed that Co06 coating exhibited exceptional tribological performance at high temperatures, with considerably less fluctuation in the friction curve compared to 30CrMoSiVA steel, as well as superior high-temperature wear resistance.

To develop new materials, Zhang et al. [56] employed laser cladding technology to fabricate an AlCrFeMnNi high-entropy alloy (HEA) coating on a steel substrate. This coating exhibited exceptional performance in high-temperature wear tests thanks to its dense and uniform microstructure, as well as the superior properties imparted by its protective oxide film. This breakthrough not only broadens the material system for laser cladding technology but also opens new research avenues for improving the high-temperature performance of brake discs. Rajaei et al. [57] developed an iron-based coating on gray cast iron brake discs and found that this coating effectively enhanced the wear resistance of brake discs, offering another promising technical solution for the surface modification of brake discs. As research continues to advance, researchers are focused on developing coating materials with specialized functionalities, with the goal of achieving a dual reduction in wear rates and particulate emissions in braking systems [58]. In addition, Shi et al. [59] utilized selective laser melting technology to successfully develop brake discs with excellent overall performance using 24CrNiMo alloy steel powder.

In conclusion, surface modification technology for iron and steel brake discs has emerged as a key research focus aimed at extending their service life. Among these technologies, laser cladding stands out as a highly promising surface engineering method due to its efficient, eco-friendly processing capabilities, precise control over coating morphology, and strong metallurgical bonding at the interface. The future development of this technology is likely to follow several key directions: First, the development of multi-functional composite coatings with properties such as wear resistance, corrosion resistance, and thermal protection will address issues related to coordinated failure under complex operating conditions. Second, optimizing the thermodynamic compatibility between the coating and the substrate will help mitigate residual stress at the interface. Third, integrating machine learning algorithms and digital twin technology will enable multi-objective optimization of process parameters and facilitate real-time quality monitoring. Finally, establishing a clean production system based on life cycle assessment (LCA) will minimize the environmental impact through strategies like energy field distribution optimization and waste recycling. These technological innovations will not only enhance the performance of brake disc materials but also drive the industry toward greater intelligence and sustainability, fostering continuous progress in the manufacturing of brake disc materials.

3. Aluminum Matrix Composite Brake Discs

The lightweight design of trains has become a pivotal trend in modern rail transit. It not only significantly improves the running speed, energy efficiency and noise reduction effect of the train, but also greatly enhances the stability and safety of the train’s operation. Amidst the ongoing wave of technological innovation, a key approach focuses on using novel low-density materials for brake disc manufacturing. Aluminum matrix composites (AMCs) have attracted considerable attention due to their outstanding overall properties [60].

3.1. Manufacturing Process of AMC Brake Discs

The manufacturing of AMC brake discs primarily involves solid-state and liquid-state processing methods. Liquid-state forming technology, particularly casting, offers distinct advantages, including lower production costs and suitability for mass production while maintaining precision in shaping complex components [61,62,63]. Currently, the main manufacturing processes for AMC brake discs include stir casting, pressure casting, pressureless infiltration, and powder metallurgy, each offering diverse technical pathways for production.

Stir casting has emerged as the dominant method for the large-scale production of AMC brake discs due to its simplicity, technical maturity, and cost-effectiveness. This method ensures the uniform dispersion of ceramic particles within a molten aluminum matrix through mechanical stirring [64]. Laden et al. [65] developed a specialized stir casting process for manufacturing SiC-reinforced AMC brake discs, which enhances the material performance and advances technical progress. Figure 7 illustrates the detailed stirring casting process for producing AMC brake discs: The aluminum alloy is melted in resistance furnaces to remove impurities, followed by thorough stirring and mixing with SiC particles in graphite crucibles to ensure uniform distribution. After secondary heating, the homogeneous mixture is poured into a mold for solidification, ensuring uniform cooling and shape stability during the process, resulting in AMC brake discs with excellent mechanical properties and dimensional accuracy [66].

While traditional pressure casting can improve the bonding between particles and the aluminum matrix, its complex equipment requirements and high costs have limited its applications. In response, innovative ultrasonic extrusion-assisted stir casting technology has been developed [67]. This advancement leverages ultrasonic cavitation effects and acoustic streaming to enhance the uniform dispersion of nanoparticles within the aluminum matrix, effectively reducing porosity and cavity formation while significantly improving wear resistance and mechanical properties [68,69,70,71]. This breakthrough provides a new technical pathway for the manufacturing of nano-ceramic-particle-reinforced AMCs, emerging as a key development direction in metal matrix composite brake disc production. Pressureless infiltration utilizes capillary action for molten aluminum to penetrate reinforcement preforms, requiring no external pressure and involving simple equipment. However, it requires precise control over the matrix melting points and preform structures. The powder metallurgy method involves mixing aluminum and reinforcement particle powders, followed by cold pressing and sintering to fabricate AMCs. This method enables the production of AMC brake discs with high density and stable performance, making it particularly suitable for manufacturing components with complex shapes or precise dimensions. Nevertheless, it requires sophisticated powder processing equipment, resulting in higher production costs.

Each manufacturing method presents distinct advantages and limitations, with the choice of process depending on specific production requirements, cost considerations, and desired material characteristics. Practical applications require flexible process selection to optimize the performance of AMC brake discs. Future developments in AMC brake discs will focus on technological innovation and cost optimization. Enhanced stir casting techniques, with ultrasonic assistance, will aim to improve material properties and production efficiency while reducing costs. Concurrently, pressureless infiltration and powder metallurgy methods will explore new process parameters and material combinations to expand their application scope.

3.2. Materials Design for AMC Brake Discs

AMCs have demonstrated considerable potential for lightweight brake discs due to their low density, excellent thermal conductivity, and good corrosion resistance. However, the material’s limitations in hardness and wear resistance hinder its applications under high-energy braking conditions. To address this issue, ceramic particles such as SiC or Al₂O₃ are integrated into the aluminum matrix, resulting in ceramic-particle-reinforced AMCs, which have brought significant performance improvements to train brake discs and have attracted extensive attention in the field of brake discs [72]. Nevertheless, compared with iron and steel brake discs, AMC brake discs suffer from reduced wear resistance at elevated temperatures, and surface cracks are prone to occur under cyclic thermal stress, which can adversely affect their service life [73].

The morphology, size, and content of the reinforcement phase in AMCs directly influence the density of the composites. Specifically, the density decreases as the reinforcement phase content increases, which is of great significance for promoting the lightweight application. However, while the introduction of the reinforcement phase enhances the strength and modulus of AMCs, it often results in a reduction in plastic deformation capacity. The wear resistance of AMCs is closely related to the content of the reinforcement phase, while the hardness may decrease due to the aggregation of the reinforcement phase. This principle provides important guidance for optimizing the wear resistance of AMCs [74,75,76,77]. Mann et al. [78] found that, compared with the aluminum matrix, the addition of the reinforcement phase can significantly enhance the hardness of the material while reducing its wear rate and friction coefficient. Vencel et al. [79] conducted a systematic investigation into the effects of various reinforcement phases, such as Al₂O₃, SiC, and graphite, on the performance of the composite. The results demonstrated that the wear resistance and friction coefficient of particle-reinforced composites are superior to those of the aluminum matrix, with the addition of graphite particles further enhancing the tribological properties, as illustrated in Figure 8.

Increasing the content of SiC particles improves the wear resistance of AMCs, as reported by Tan et al. [80], and effectively inhibits the formation of cracks in the brake disc. This discovery provides a theoretical basis for the development of new brake disc materials. Ferraris et al. [81] innovatively fabricated AMC brake discs with wear-resistant SiC particles embedded on the surface. These discs exhibited excellent tribological and thermal properties, showing promising application potential. Ahmad et al. [82] conducted research on the wear behavior of alumina particles reinforced Aluminum Matrix Composites (AMCs) and brake disc material, and found the coefficient of friction decreased as the surface of the AMCs become rougher at higher load. This was considered due to removal of alumina particles from the surface of AMCs during wear test and the remaining aluminum alloy without alumina particles made the material softer. Alloying modifications have also proven effective in enhancing the properties of AMCs. Lattanzi et al. [83] discovered that the addition of alloying elements like nickel (Ni), copper (Cu), lanthanum (La), and cerium (Ce) not only improves the mechanical properties of AMCs but also regulates their thermal conductivity and enhances rigidity at high temperatures, paving the way for the multifunctional design of AMCs. Liu et al. [84] further investigated the influence of rare earth elements La and Ce on SiC-particle-reinforced AMCs. The experiments revealed that the addition of these two elements refines the grain structure of the aluminum matrix and increases the density of AMCs, significantly improving both the mechanical properties and thermal stability of AMCs.

A promising and cost-effective approach to developing lightweight brake discs involves preparing wear-resistant coatings on aluminum alloys as replacements for iron and steel. Zhang et al. [85] fabricated TiC-enhanced aluminum-based composite coatings using laser direct deposition technology. Their experiments demonstrated that, at 40% TiC content, the uniformly distributed TiC particles effectively resisted indentation from external hard particles, exhibiting superior wear resistance and shear strength. Shrestha et al. [86] found that ceramic coatings on aluminum brake discs significantly improved their heat dissipation and corrosion resistance, providing a robust solution for the stable operation of brake discs in complex environments. Additionally, the initial braking velocity and the thickness of the wear-resistant layer are crucial factors affecting the friction temperature of brake discs [87]. The high thermal conductivity and excellent cooling performance of aluminum brake discs result in relatively low surface temperatures and more uniform thermal stress distribution. Li et al. [88] prepared SiC/Al composites using the infiltration method and discovered that the continuous distribution of SiC, as an efficient thermal conductive material, significantly enhanced the thermal conductivity of the composite. Moreover, the geometric shape of SiC reinforcement also plays a notable role in improving the thermophysical properties of SiC/Al composites, offering valuable insights for optimizing the thermal performance of these materials.

Aluminum-based composite brake discs, due to their low density, offer significant potential in lightweight applications within train braking systems, effectively reducing energy loss and enhancing braking safety. However, compared to traditional iron and steel brake discs, this material system still faces challenges, including insufficient high-temperature wear resistance and surface crack propagation caused by thermal cycling stress under extreme conditions, which directly affect its service life [73]. To address the high-temperature challenges faced by aluminum-based composite brake discs under railway braking conditions, SiC or Al₂O₃ particles can be added to the aluminum matrix to increase hardness and high-temperature stability [76,77,78,79]. Additionally, ceramic coatings like Al₂O₃-TiO₂ can be sprayed on the aluminum brake disc surface to isolate oxygen, reducing friction interface temperatures and minimizing oxidation and thermal degradation [77]. Finally, optimizing the heat dissipation structure, such as incorporating a ventilation heat dissipation system, can enhance convective heat dissipation, accelerate cooling, and prevent local overheating [89]. Notably, research on thermal property changes during braking and the interaction between reinforcing phases and the aluminum matrix provides a solid theoretical foundation for designing advanced aluminum-based composite brake discs, ultimately achieving a lightweight, highly reliable, and environmentally friendly braking system, meeting modern rail transportation demands for safe operation, energy efficiency, and sustainable development.

4. Carbon Matrix Composite Brake Discs

To address the growing demand for high-performance materials in modern train braking systems, the development of brake disc materials with exceptional overall performance has become a significant focus in materials science. Ideal brake disc materials must not only possess outstanding wear resistance and corrosion resistance but also maintain superior thermal stability under extreme conditions. Among the numerous candidate materials, carbon matrix composites have emerged as a promising next-generation solution for high-performance brake discs. A representative of these composites, carbon/carbon–silicon carbide (C/C-SiC), has been innovatively developed by introducing SiC components into carbon/carbon (C/C) composites. This material outperforms iron and steel brake discs in terms of high-temperature tolerance, showing excellent braking stability at temperatures above 1000 °C [90]. It holds broad application prospects in train brake discs.

4.1. Manufacturing Process of C/C-SiC Brake Discs

The key to the preparation process of C/C-SiC composites lies in minimizing damage to the fibers, optimizing the interface bonding between the fibers and carbon matrix, and controlling production costs. The primary preparation techniques in this field include hot-pressing sintering (HPS), liquid polymer infiltration (LPI), chemical vapor infiltration (CVI), and reactive melt infiltration (RMI) [91]. HPS achieves material densification under high-temperature and high-pressure conditions; LPI allows for precise control of matrix composition and microstructure through liquid polymer precursors; CVI forms a uniform and dense matrix on the fiber surface via gas-phase reactions; and RMI significantly improves the mechanical properties and high-temperature stability by utilizing the chemical reaction between molten silicon and the carbon matrix.

During the preparation process of high-performance C/C-SiC brake discs, the formation of three-dimensional needled carbon-fiber-reinforced C/C-SiC dual-matrix composites is of paramount importance. Li et al. [92,93] successfully prepared three-dimensional needled C/C-SiC brake discs by combining CVI and LSI techniques. Figure 9 clearly illustrates the manufacturing process of C/C-SiC brake discs: first, a three-dimensional needle-punched fabric preform is constructed; then, CVI is used to densify it into a porous C/C composite; this is followed by the formation of the SiC ceramic matrix through molten silicon infiltration; and finally, the brake disc is precisely processed and a surface coating is prepared.

The microstructure of three-dimensional needled C/C-SiC composites, as shown in Figure 10, mainly consists of carbon fibers, pyrolytic carbon (PyC), Si, and SiC [92]. The needle-shaped fiber design enhances the interlaminar shear strength, optimizes thermal conductivity, and improves friction stability. In addition, the introduction of C/C fiber bundles enhances the toughness of the material, reduces the risk of brittle fracture, and ensures its reliability and durability under extreme conditions. By optimizing the spatial distribution of carbon fibers, the uniformity of each phase in the friction layer is improved, helping to maintain stable braking characteristics. Notably, the pore structure on the friction surface is conducive to capturing wear particles, reducing braking noise, and enhancing the overall performance of the material.

To mitigate the adverse effects of high-temperature sintering on the performance of carbon fibers, Li et al. [94] developed a novel preparation method for C/C-SiC composites. This method employs carbon fibers as the reinforcement phase, combined with a dual-phase matrix of carbon and silicon carbide, to fabricate C/C-SiC composites through a combined warm compaction and in situ reaction (WCISR) process. Figure 11 provides a detailed illustration of the WCISR preparation process. Firstly, the carbon fiber, graphite, Si powder, resin, and agglutinant are uniformly mixed, then pressed into a green body preform under medium-temperature conditions, followed by a carbonization treatment to convert the resin into resin carbon, and finally, a high-performance C/C-SiC composite is developed through a high-temperature in situ reaction. This innovative method not only significantly improves production efficiency but also effectively ensures the long-term stability of the material’s performance.

To further explore the influence of the preparation process on the friction performance of C/C-SiC composites, Mor et al. [96] found that the friction film formed on the surface of ZrB₂-SiC ultra-high-temperature composites reinforced with short-cut carbon fibers can effectively improve the braking efficiency and extend service life. Goo innovatively developed a manufacturing process for full-scale C/C-SiC brake discs [97]. By optimizing the material compositions and structural design, brake discs with excellent mechanical properties and stable friction coefficients were obtained. Wang et al. [98] proposed a novel surface protection technology, preparing an anti-oxidation protective coating on the surface of C/C-SiC composites. After oxidation testing, it was verified that this coating can significantly reduce the weight loss of the material during high-temperature oxidation. This technology is not only applicable to the surface protection of new brake materials but also holds promise for brake disc coating repairs, offering extensive practical value.

The preparation processes for carbon matrix composite brake discs are diverse, with a key focus on minimizing fiber damage and optimizing interface bonding. Consequently, more efficient, environmentally friendly, and cost-effective preparation methods must be explored to meet the increasing demand for high-performance brake disc materials. Furthermore, in-depth research into the performance optimization and surface treatment technology of C/C-SiC composite brake discs will be essential for enhancing their durability, stability, and oxidation resistance.

4.2. Materials Design for C/C-SiC Brake Discs

C/C-SiC composites are primarily composed of carbon fibers, dark PyC surrounding the carbon fiber, gray SiC, and grayish-white Si. Carbon fibers serve as the reinforcement phase, enhancing the mechanical properties and fracture resistance of the material, while SiC contributes to its hardness. Compared with metal brake discs, C/C-SiC composite brake discs offer superior braking performance at high temperatures, making them ideal for applications in rail transit, high-end automobiles, etc. [99]. Additionally, C/C-SiC composite brake discs have emerged as strong candidates for replacing steel brake discs, with their braking performance potentially supporting the development of high-speed and heavy-haul trains. However, the inherent brittleness of the ceramic phase can lead to sudden fractures under external stress, which negatively affects the impact resistance of C/C-SiC composites. Furthermore, porosity defects introduced by powder metallurgy processes may reduce the uniformity of the material [100]. The relatively high wear rate and friction coefficient of C/C-SiC composites may cause unstable braking and abnormal wear of brake discs, potentially compromising the operational safety of trains [101]. Therefore, to expand the application of C/C-SiC composite brake discs, it is essential to conduct in-depth research on their brittleness, uniformity, and wear issues to identify practical solutions.

Improving fracture toughness is crucial for enhancing the performance of C/C-SiC composite brake discs. For instance, introducing ductile phases like FeSi2 can improve the friction coefficient and wear resistance of C/C-SiC composites [102]. Structurally, incorporating SiC ceramic phases into carbon fiber bundles can significantly enhance their wear resistance, thereby reducing the wear rate of the composite [103]. Additionally, modifying the material matrix to optimize its composition and structure can stabilize the friction coefficient, improve oxidation resistance, and enhance mechanical properties. Notably, a combined modification process involving Fe-Si alloy and Cu has shown a significant reduction in the wear rate of C/C-SiC composite brake discs, which holds great importance for the design and development of train brake discs [104]. Optimizing the Si content in C/C-SiC composites can significantly improve their tribological properties, reducing the wear rate and stabilizing the friction coefficient [105]. Further research into the surface structure of C/C-SiC composite brake discs has revealed that SiC plays a crucial role in enhancing the friction coefficient of the material [106,107].

Current research on C/C-SiC composite brake discs mainly focuses on enhancing friction performance and braking stability to improve braking efficiency and ensure train safety. To this end, full-scale bench tests and finite element simulations have been conducted to systematically evaluate the adaptability of C/C-SiC composites in high-speed and high-energy load braking systems. Qu et al. [108] confirmed that C/C-SiC brake discs maintain stable performance during emergency braking and meet the braking requirements for trains at speeds above 400 km/h. Zhao et al. [109] demonstrated that C/C-SiC brake discs for high-speed trains exhibit excellent friction coefficients and braking stability under various braking conditions. Hui. et al. [110] found that although the surface temperature of C/C-SiC brake discs is relatively high during cyclic braking, their friction performance is superior to that of cast iron brake discs, with the friction film formed at high temperatures further enhancing braking performance. Chen et al. [111] showed that C/C-SiC composites exhibit superior friction coefficients, stability, and wear resistance in comparison to traditional steel under high-energy braking conditions. Xiao et al. [112] conducted a comparative study on the tribological properties of copper-based composites paired with cast steel and C/C-SiC composites. As shown in Figure 12a, the braking curves indicate that the copper-based composite paired with the C/C-SiC composite maintains a more stable friction coefficient during braking, outperforming its pairing with cast steel. Figure 12b further illustrates that the wear rate of the copper-based composite significantly decreases when paired with the C/C-SiC composite compared with cast steel. Notably, the negative wear rate observed in the C/C-SiC composite suggests the formation of a stable friction film on its worn surface during braking. Regarding surface modification of the C/C-SiC brake discs, Fan et al. [113] ingeniously utilized phosphate solution treatment technology to prepare a functional coating on the disc surface, improving the braking stability and significantly reducing the wear rate, thereby extending the service life of the brake disc.

The frictional behavior and wear mechanisms of C/C-SiC composite brake discs under various conditions have been extensively investigated, providing valuable insights into enhancing the performance of brake discs [114,115]. Regarding the oxidation behavior and wear mechanism of C/C-SiC composites, Deng et al. [116] conducted full-scale bench tests to deeply analyze the oxidation behavior and wear mechanism of C/C-SiC brake discs during braking, emphasizing the roles of friction film formation and temperature variations on the oxidation reaction. Fan et al. [117] identified that the wear mechanisms of C/C-SiC brake discs primarily include abrasive wear, oxidative wear, fatigue wear, and adhesive wear. Given their exceptional braking performance at high temperatures, C/C-SiC composites have become the preferred material for train brake discs. However, issues such as brittleness, material uniformity, and wear have limited their widespread applications. Through further research into friction and wear mechanisms, as well as the optimization of material composition, structure, and preparation technologies, the scope of their application can be expanded.

5. Conclusions and Prospects

As demands for high-speed operation, a lightweight design, large load capacity, and environmental sustainability continue to increase in train technology, materials used for train brake discs are shifting from traditional iron and steel to high-performance composites. Under actual operating conditions, train brake discs must meet several strict technical standards: excellent wear resistance, maintaining a stable friction coefficient during millions of braking cycles; high-temperature tolerance, withstanding temperatures exceeding 800 °C during braking; high thermal conductivity, with a coefficient greater than 50 mm²/s for efficient heat dissipation; a tensile strength above 600 MPa to ensure structural stability under complex stresses; an excellent thermal fatigue life, withstanding more than 10⁷ cycles under thermal shock; and a composite material density of less than 3.0 g/cm³, reducing weight by over 60% compared to traditional iron and steel.

Table 1. Advantages and limitations of various brake disc materials.

Material Class	Advantages	Limitations
Iron and steel brake discs [7,14,26,29]	High mechanical strength (>600 MPa); Stable braking performance; Mature manufacturing.	Excessive density (about 7.8 g/cm3); Poor thermal fatigue resistance (<106 cycles); Poor high-temperature stability (degradation > 800 °C).
Aluminum matrix composite brake discs [60,72,73,90]	Lightweight (2.7–3.0 g/cm3); High thermal conductivity (>50 mm2/s); Good wear resistance.	Moderate high-temperature capability (degradation >600 °C); Complex fabrication processes increase cost.
Carbon matrix composite brake discs [90,101,108,109,110]	Lightweight (1.6–2.2 g/cm3); Exceptional thermal stability (>1000 °C); Superior thermal fatigue life (>107 cycles).	High brittleness (fracture risk under impact); High production costs; Moderate tensile strength (400–550 MPa).

summarizes the advantages and limitations of iron and steel brake discs, aluminum-based composite brake discs, and carbon-based composite brake discs, systematically comparing key performance indicators among these three material categories.

Looking ahead, as railway transportation trends toward high-speed, lightweight, and heavy-duty components, train brake discs are undergoing revolutionary advancements in materials and technology. Regarding material innovation, nano-enhanced biomimetic composites enable stable control over the friction coefficient, while gradient functional materials integrated with phase-change energy storage technology can withstand instantaneous thermal loads of up to 1500 °C. Additionally, three-dimensional heterogeneous heat conduction networks enhance heat diffusion efficiency by more than 300%, and carbon-fiber-reinforced metal matrix composites maintain a strength of 1800 MPa while reducing the density to 60% of traditional iron and steel. In terms of technological integration, AI-driven material genomics will accelerate the development of friction materials, while advanced composite manufacturing techniques allow for precise microstructure control. Biomimetic intelligent coatings provide brake discs with self-regulating properties. Digital twin technology enables real-time predictive maintenance. These innovations not only extend the service life of train brake discs but also enhance the safety performance of braking systems. They ensure that train brake discs can meet increasingly stringent technical standards while effectively managing production costs.