Mechanical and Tribological Properties of Ceramic-Reinforced Copper-Based Composites: Analysis of Single and Multi-Component Synergistic Mechanisms

1.   Introduction

The development history of Cu-based composite materials can be traced back to the early research on metal composite materials. Initial studies primarily focused on simple alloying of Cu with other metals, such as Cu-Ag alloys and Cu-Ni alloys^[1-3]. However, with the advancement of materials science, researchers began to introduce non-metallic components into the Cu matrix to improve its mechanical properties and wear resistance^[4-6]. These non-metallic components, due to their high hardness, high-temperature stability and excellent corrosion resistance, significantly enhance the mechanical and wear properties of Cu-based composite materials^[7-8]. Notably, their machinability and design flexibility not only reduce production costs, but also allow for the realization of multifunctionality by adding different types of reinforcing phases to meet the demands of various application scenarios^[9-14]. These advantages have led to the wider application and development of Cu-based composite materials across various fields. From their early applications in electrical engineering and electronic devices, they have expanded to the automotive, aerospace, high-speed rail, and mechanical manufacturing industries. In the automotive industry, they are used as brake pads, clutch plates, and cylinder liners to enhance the safety and reliability of vehicles^[15-20]. In the aerospace sector, Cu-based composite materials serve as high-temperature structural materials and friction materials to withstand extreme high-temperature and high-pressure environments. In the mechanical manufacturing field, they are utilized in bearings, gears and cutting tools to improve the lifespan and precision of equipment^[21-27].

Despite the many advantages of Cu-based composite materials, their drawbacks cannot be ignored^[28-30]. Firstly, the high density of Cu-based composites limits their application in fields that require lightweight materials. Secondly, the interfacial bonding strength between the ceramic particles and the Cu matrix is relatively low, which may lead to interface debonding and delamination under high-stress conditions^[31-33]. Additionally, the significant difference in thermal expansion coefficients can cause thermal stress concentration and deformation of the material at high temperatures. To address these shortcomings, researchers have conducted extensive studies on the preparation methods and compositions of Cu-based composite materials^[34-38]. Common preparation methods include powder metallurgy^[29], internal oxidation^[28], mechanical alloying^[9] and high-temperature sintering^[7].

Powder metallurgy involves mixing Cu powder and ceramic powder uniformly, followed by pressing and high-temperature sintering to produce composite materials^[39-41]. This method results in good uniformity and density of the composite materials. However, it has the drawback of higher porosity in samples and weaker interface bonding between ceramic particles and the Cu matrix, which may lead to interface cracking and delamination phenomena. Internal oxidation method enhances the strength, hardness and high-temperature stability of composite materials by generating uniformly distributed oxide particles within the Cu matrix^[42-46]. Although this method requires high alloy composition and oxidation processes, its advantages in improving material performance make it an effective method for preparing high-performance Cu-based composite materials. However, it is limited by composition restrictions and complex processes^[46-48]. Mechanical alloying involves thoroughly mixing Cu powder and ceramic powder using a high-speed ball mill, where mechanical bonding occurs between the two during milling, forming the composite material^[49-51]. This method enables the uniform distribution of ceramic particles in the Cu matrix, achieving nanoscale particle size and distribution, thus enhancing the material’s strength and hardness. Its drawback lies in higher energy consumption and the risk of sample contamination by impurities. High-temperature sintering method creates a stable bond between the Cu matrix and ceramic components through diffusion and chemical reactions at high temperatures. It effectively eliminates pores, improving the density and mechanical properties of the composite material, and enhancing the interface bonding strength between ceramic particles and the Cu matrix^[52-54]. However, it has the drawback of complex processes, high costs of high-temperature equipment, and potential issues with thermal stress and cracking during high-temperature sintering. In addition to these traditional preparation methods, other methods for preparing Cu-based composite materials include hot isostatic pressing, electroplating, casting, sol-gel method and in-situ synthesis method^[55-56].

In the research on the composition of Cu-based composite materials, the focus is mainly on improving the low interface bonding strength between ceramic particles and the Cu matrix, which easily leads to interface debonding and delamination under high-stress conditions. Common methods to address this include multi-component ceramic composites^[1], multi-scale particle composites^[4], surface/interface modification techniques^[2] and the formation of functionally graded materials^[20]. Multi-component ceramic composites involve introducing two or more different types of ceramic particles into the Cu matrix to utilize the synergistic effects of these particles to enhance the overall performance of the material. By carefully selecting and combining different ceramic materials, better mechanical properties, wear resistance and high-temperature stability can be achieved in the composite material. Different ceramic particles contribute their respective advantages, forming a synergistic enhancement effect that improves the comprehensive performance of the material^[5]. This approach can also endow the material with various functions, such as simultaneously increasing hardness, wear resistance and oxidation resistance, thus providing it with broader applications^{[27, 57]}. However, it requires precise control over the distribution and proportion of various ceramic particles, making the process complex and challenging. Multi-scale particle composites involve introducing both micron-sized and nano-sized ceramic particles into the Cu matrix to leverage the characteristics of particles at different scales to enhance material performance. Micron-sized particles provide reinforcement, while nano-sized particles fill microstructural voids, increasing the material’s density and strength. The advantage is that nano-particles can improve the interface bonding between micron particles and the matrix, reducing interface defects. The drawback is that achieving uniform dispersion of multi-scale particles is difficult, often leading to agglomeration, which affects the uniformity of the material. Surface/interface modification techniques involve chemical or physical treatment of the surfaces or interfaces of the Cu matrix and ceramic particles to improve their bonding strength and interface characteristics. These treatments can include coatings, surface activation, and the introduction of interface layers, effectively enhancing the interface bonding strength between ceramic particles and the Cu matrix, thus improving the mechanical properties and durability of the material. However, potential issues include the introduction of new interface problems or residual stress, which can affect the overall performance of the material. Functionally graded materials refer to materials with continuously varying composition and structure in space, achieving different properties in different regions^[48]. By controlling the distribution and concentration gradient of ceramic particles, gradient performance changes can be realized within the material. The gradual design of composition and structure can reduce thermal stress and interface stress, enhancing the thermal stability and durability of the material. However, the preparation process is complex and costly^{[6, 11]}.

Therefore, this article takes the synergistic enhancement of Cu-based composites by single/multi-component ceramics as a starting point. It systematically explains the mechanisms of different types of ceramics and their applications in improving the performance of Cu-based composites, and explores the synergistic effects and comprehensive advantages brought by multi-component ceramic composites, providing strong support for the development of Cu-based composites. The development of Cu-based composites with synergistic enhancement by single/multi-component ceramics not only leverages the advantages of single ceramic components in improving material performance, but also overcomes the shortcomings of single ceramic components through the synergistic effects of multiple ceramics, achieving comprehensive performance enhancement. This multi-component composite strategy offers broad prospects and infinite possibilities for the application of Cu-based composites. In the future, with continuous advances in materials science and processing technology, multi-component ceramic composites will demonstrate their unique performance advantages in more fields, driving further development in related industries and technologies.

2.   The impact of single-phase ceramic components on the performance of Cu-based composites

2.1   Oxides

In Cu-based friction materials, the incorporation of ceramic components significantly enhances the overall performance through various mechanisms, particularly in terms of wear resistance, thermal stability, and oxidation resistance. The wear resistance mechanism of Al₂O₃ in Cu-based friction materials primarily involves its high-hardness particles reinforcing the matrix, forming a hard protective layer that reduces direct wear. However, due to its high brittleness, Al₂O₃ may fracture under high stress conditions^[58-59]. As shown in the Fig. 1(a),the load-bearing friction mechanism of Al₂O₃ in Cu-based materials is illustrated. Li et al.^[60] demonstrated that using an internal oxidation method to prepare Cf-Al₂O₃/Cu composites could further enhance friction stability, reduce arc erosion and optimize the tribological performance of the material. The wear resistance mechanism of silicon oxide in Cu-based friction materials primarily relies on the dispersion of hard particles, enhancing the material’s hardness and wear resistance. An appropriate amount of silicon oxide can form a dense protective layer, significantly reducing the friction coefficient. However, an excessive SiO₂ content may lead to braking noise and severe wear of the counterpart material. Therefore, it is necessary to optimize the SiO₂ content to balance friction performance and material durability^[61-62]. ZrO₂, as a relatively new wear-resistant component, holds significant promise for applications in Cu-based friction materials. Its high hardness and excellent wear resistance enable zirconium oxide particles to effectively strengthen the Cu matrix, forming a robust protective layer that resists friction and wear^[63]. The unique feature of ZrO₂ lies in its higher thermal stability and resistance to thermal shock, which can inhibit the propagation of cracks and maintain the structural integrity of the material^[64-65]. Y₂O₃, as a wear-resistant component used under specific conditions, exhibiting excellent chemical stability and high-temperature stability, enabling it to maintain stability under extreme conditions^[66]. Therefore, under specific operating conditions requiring long-term material stability, Y₂O₃ can demonstrate its unique wear-resistant properties. Y₂O₃ dispersion-strengthened Cu alloys prepared by sol-gel synthesis and spark plasma sintering exhibit high strength and high thermal conductivity, offering potential application prospects in high-heat flux components of fusion reactors^[67]. Its excellent thermal stability is mainly attributed to the pinning effect of nanoscale Y₂O₃ particles in the Cu matrix. Of course, in Cu-based wear-resistant materials, in addition to the above, there are also other oxides such as TiO₂ and ZnO. Among them, TiO₂ is particularly suitable for high-strength engineering applications^[68], while ZnO is suitable for improving oxidation resistance and corrosion resistance materials^[69]. These oxides play important roles under different conditions, providing a variety of optimization choices for the performance of Cu-based wear-resistant.

Figure  1.  Friction mechanism diagram of (a) Al₂O₃^[60], (b) SiC^[70], (c) B₄C^[71]and (d) TiC^[72]

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2.2   Carbide

Carbides also play a crucial role in Cu-based friction materials, among which SiC stands out due to its high hardness and high-temperature resistance. It can form a hardness gradient structure, effectively resisting wear on friction surfaces, reducing the friction coefficient, and is suitable for high-temperature and high-load friction conditions^[73-75]. The wear mechanism diagram shown in Fig. 1(b) is for Cu-Cr-SiC coatings prepared using laser cladding. Jiang et al.^[70] demonstrated that the C element generated by the thermal decomposition of SiC partially combines with Cr, while the generated Si element diffuses into the Cu matrix, forming a unique bilayer-microhardness region. This further reduces wear and enhances electrical conductivity. The application of B₄C in Cu-based friction materials is crucial, because it not only provides wear resistance but also acts as a lubricant, making it particularly suitable for extreme friction conditions^[76-77]. Its wear resistance primarily manifests in high hardness, while its lubricity is evident in its ability to form an effective B₂O₃ lubricating film on the friction surface. This lubricating film can reduce the wear and heat generation of Cu-based materials, thereby enhancing the durability and performance stability of friction materials. As shown in Fig. 1(c), this is an illustrative diagram of the phase separation effect of B₄C in Cu-based composite materials. Song et al.^[72] found that the addition of B₄C could adjust the mixed enthalpy and entropy, refine phase distribution and reduce performance differences caused by macroscopic segregation, thereby further enhancing wear resistance and magnetism. TiC as another wear-resistant component particularly suitable for extreme friction conditions is renowned for its outstanding chemical stability and oxidation resistance. It can maintain stability in high-temperature friction, thereby extending the service life of Cu-based friction materials^[78-79]. Fig. 1(d) illustrates the wear mechanism of TiC in Cu-based composite materials. Research by Yan et al.^[71] on TiC-modified Cu-graphite composites prepared using direct current in situ interface resistance sintering method revealed that the dense TiC layer significantly strengthens the Cu matrix. It optimizes interface bonding and stress transfer, effectively preventing graphite from cracking and peeling off. Additionally, the inclusion of TiC effectively protects the worn surface, reduces damage to the mechanical mixing layer, and lowers wear rates and friction coefficients by simultaneously increasing strength and ductility. The wear resistance mechanism of WC in Cu-based friction materials primarily lies in its high hardness and excellent wear properties^[80-81]. When WC particles are embedded in the Cu matrix, they form a hard reinforcing phase that effectively increases hardness and wear resistance, enabling excellent stability and durability in high-speed, high-temperature environments^[82-83]. Additionally, Cr₇C₃ exhibits excellent heat resistance and corrosion resistance, performing remarkably well in high-temperature environments^[84]. TaC possesses high hardness and heat resistance, making it suitable for use in high-temperature friction materials^[85]. ZrC exhibits excellent wear resistance and corrosion resistance, making it suitable for applications in specific friction environments^[86].

2.3   Nitride

The application of nitrides in Cu-based friction materials is essential, primarily due to their significant ability to enhance material hardness and wear resistance. Among them, boron nitride acts as a high-temperature lubricant by forming a thin layer on the friction contact surface, effectively preventing direct contact and wear between metal surfaces. It is commonly used in high-temperature, high-load and high-speed friction conditions^[87-89]. As shown in Fig. 2(a), this is a schematic diagram illustrating the role of h-BN in Cu-based friction materials. Chen et al.^[31] studied the tribological behavior and mechanism of h-BN modified Cu-based brake pads paired with C/C-SiC. Their researches indicated that an increase in h-BN content leaded to a change in the wear mechanism. Cu-based brake pads containing 2% h-BN exhibited better stability during high-speed braking. Si₃N₄ possesse excellent high-temperature stability and chemical inertness^[90-91]. It maintains good friction performance under high-temperature and high-pressure conditions. Its mechanism of action primarily involves forming a robust Si₃N₄ film that protects the friction surface, reducing friction wear and heat generation from friction pairs. This enhances the wear resistance and thermal performance of the friction material. However, it is relatively brittle at room temperature and prone to cracking or breaking, making it mainly suitable for materials requiring high-temperature wear resistance. TiN exhibits good interfacial bonding with Cu-based materials^[92], which can enhance the material’s adhesion and stability. During friction, it reduces interfacial delamination and damage. However, it has the drawback of an unstable friction coefficient. AlN, with its excellent thermal conductivity, can effectively dissipate and conduct frictional heat, reducing heat accumulation and improving the wear resistance and chemical stability of Cu-based friction materials^[93-95]. Of course, there are also some complex and less common nitride wear-resistant components, such as SiCN^[96]. By using in-situ processes, polymer-derived SiCN nanoparticles can be dispersed into Cu, employing multiple channels to crosslink polymer particles and form bulk particulate metal matrix composites. Although this approach enhances wear resistance and high-temperature stability, extending the lifespan of friction materials, the preparation process is complex and costly.

Figure  2.  Mechanism diagram of (a) BN^[31], (b) TiB₂^[97], (c) ZrB₂ ^[98] and (d) CaF₂^[99]

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2.4   Borides and others

Borides have recently gained attention as wear-resistant components in Cu-based friction materials, especially with the application of in-situ synthesis technology. Through this technique, boride particles can be directly formed within the Cu matrix, significantly enhancing the material’s hardness and wear resistance. Among them, TiB₂ stands out as a wear-resistant component with multiple advantages. Its extremely high hardness effectively resists external abrasive forces, extending the material’s lifespan^[100-101]. The microstructure of TiB₂ is uniformly dense, with tightly bonded particles that effectively prevent micro-abrasion and spalling on the material surface, thus enhancing wear resistance^[102]. As shown in in Fig. 2(b)^[97], the Electron Back-Scattered Diffraction (EBSD) and particle size distribution of the TiB₂ particle-reinforced Cu surface indicate that TiB₂ particles prepared using in-situ deposition technology can improve the hardness and tensile strength of Cu-based composites through two mechanisms: dislocation multiplication caused by differences in thermal expansion coefficients and Orowan strengthening. This enhancement effectively increases deformation resistance and wear resistance. ZrB₂ also possesses high hardness and excellent wear resistance, along with good thermal conductivity and high-temperature stability, making it suitable for high-temperature friction conditions. However, its significant brittleness can result in insufficient toughness for the material^[103]. In Fig. 2(c), Shaik et al.^[98] found that the addition of ZrB₂ significantly enhanced the hardness and yield strength strength of Cu-based composites. With the addition of ZrB₂, the relative density of Cu-ZrB₂ composites ranged from 96.0% to 99.7%, with the Cu-10ZrB₂ composite exhibiting the highest hardness (1.25 GPa) and yield strength (261 MPa). This enhancement was attributed to the ultrafine ZrB₂ particles, which tended to segregate at Cu-Cu grain boundaries, thereby strengthening resistance to plastic deformation in the matrix. As the ZrB₂ mass fraction reached 3%, plastic deformation of the material decreased, resulting in improved wear resistance. Additionally, AlB₂ also exhibits good wear resistance and high-temperature stability. Compared to other materials, AlB₂ has a lower density, which can effectively reduce the overall weight of the material, thereby improving lightness and energy efficiency in applications^[104].

Certainly, in addition to the four major types of wear-resistant ceramic components mentioned earlier, there are other wear-resistant components used in Cu-based friction materials, such as NbSe₂^[105], CaF₂^{[99, 106]} and sepiolite^[107-108]. NbSe₂ can effectively reduce the plastic deformation of the Cu matrix, while sepiolite provides dual benefits of high-temperature lubrication and wear resistance, significantly improving the friction and wear performance of the composite material. CaF₂ can form a stable lubricating film during friction, reducing the friction coefficient between contact surfaces and decreasing adhesion and wear at the friction interface, thus extending the service life of Cu-based friction materials. It also exhibits good lubricating performance under high-temperature conditions, effectively preventing adhesive wear and reducing adhesion and peeling caused by high temperatures. As shown in Fig. 2(d), the wear mechanism of CaF₂-reinforced Cu-based composites demonstrates that the formation of a friction layer by reciprocating sliding can lead to new oxidation sites. Subsurface cracks tend to propagate parallel to the surface, resulting in delamination during sliding wear. The primary wear mechanisms include adhesive wear, frictional oxidation wear, delamination and minor abrasive wear. Studies have shown that the friction layer cracking induced by reciprocating sliding results in the generation of new oxidation sites, and subsurface cracks tend to propagate parallel to the surface, leading to delamination during sliding wear.

3.   Effects of dual-phase friction components on the properties of Cu-based composites

Cu-based composites face multiple challenges in enhancing overall performance. Firstly, the limitations of single ceramic reinforcement phases constrain their effectiveness in composites. Secondly, the uneven distribution of ceramic reinforcement phases within the Cu matrix may result in unstable mechanical properties. Additionally, the interface bonding strength between the ceramic reinforcement phase and the Cu matrix is often relatively low, which can lead to interface delamination under stress, thereby affecting the material’s durability and reliability. These factors collectively limit the development potential and performance enhancement of Cu-based composites in practical applications. To overcome the drawbacks of single ceramic phase reinforcement in Cu-based composites, a multi-component synergistic reinforcement design approach can be introduced. This design leverages the physical properties and spatial distribution of multiple reinforcement phases to fully exploit their advantages, ultimately achieving Cu-based materials with superior comprehensive performance.

3.1   Synergistic effects of oxides and other components

Oxides, as a type of synergistic reinforcement phase, offering significant advantages in wear-resistant applications, including enhanced hardness, improved wear resistance, chemical stability, temperature stability, and multifunctionality. These characteristics make oxides play a crucial role in the design and application of wear-resistant materials. Zhou et al.^[109] utilized a combined technology of chambered melting and efficient solidification of the melt to prepare TiB₂ and Al₂O₃ dual-phase synergistically reinforced Cu-based composites, significantly improving tensile strength and hardness. Under low deformation states, the fracture mechanism of the Cu matrix is primarily particle-matrix interface fracture, which gradually transitions to particle shear fracture with increased deformation. As shown in Fig. 3(a~p), the typical TEM micrographs of TiB₂ and Al₂O₃ in Cu-based composites reveal that the interface between TiB₂ and the Cu matrix is semi-coherent, while there is a distinct transition between Al₂O₃ and the Cu matrix with tight interface bonding, free of voids and impurities. Both phases exhibit good interfacial bonding and uniform distribution within the Cu matrix, which helps fully utilize their synergistic reinforcement effects, thereby enhancing the comprehensive performance of the composite materials.

Figure  3.  The typical TEM micrographs of TiB₂ phase in as cast samples: (a) bright field-TEM micrograph of TiB₂; (b) high resolution-TEM micrograph of TiB₂; (c) SAED pattern of the yellow area in figure 3(a); (d) high resolution-TEM micrograph of Cu/TiB₂ interface; (e~g) Fast Fourier transformation patterns of areas in figure 3(d) that corresponding (e) white, (f) red and (g) yellow; (h) Inverse fast Fourier transformation micrograph of TiB₂; (i) schematic diagram^[109]. The typical TEM micrographs of Al₂O₃: (j) bright field-TEM micrograph of Al₂O₃ phase; (k) high-resolution TEM micrograph of Al₂O₃; (l) selected area electron diffraction pattern of red area in Fig. 3(k), (m) yellow area and (n) white area; (o) inverse fast Fourier transformation image of Al₂O₃^[109]; (p) TEM micrograph of SiC_w- Al₂O₃-Cu; (q) electron back-scattered diffraction map of SiC_w- Al₂O₃-Cu^[110]

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Lin et al.^[110] prepared a Cu-based composite material synergistically reinforced with nano-Al₂O₃ particles and micron-sized SiC whiskers (SiC_w) using powder metallurgy combined with internal oxidation, further enhancing tensile strength and arc erosion resistance. The Al₂O₃ particles helped regulate the spatial distribution of SiC_w in the Cu matrix, reducing the aggregation of SiC_w. Meanwhile, the addition of SiC_w leaded to a more uniform distribution and better spatial configuration of Al₂O₃. As shown in Fig. 3(q), the HR-TEM and EBSD micragraphs of the SiC_w and Cu matrix interface revealed that Si atoms from SiC diffused into the Cu matrix, resulting in the formation of a distinct amorphous transition layer at the interface. This transition layer reduced the interface bonding strength, facilitating the pull-out of SiC whiskers, thereby improving tensile strength and toughness. Additionally, the nano-Al₂O₃ particles generated Zener resistance by migrating along the Cu matrix grain boundaries, inhibiting grain growth and achieving fine-grain strengthening.

Kim et al.^[111] utilized plasma discharge sintering technology to fabricate Cu-based friction materials reinforced with Al₂O₃ and SiO₂. They conducted performance tests and mechanism analyses under various friction conditions. As shown in Fig. 4, the 3D super depth, SEM micragraph, and friction performance images of the Cu-based friction materials under different braking conditions demonstrated the significant enhancement in friction performance due to the dual ceramic reinforcement. Based on the surface morphology shown in Fig. 4(a) and (b), the wear surface retained low wear characteristics even during high-speed braking, which could be attributed to the roles of Al₂O₃ and SiO₂ in the wear mechanism. During friction, wear debris from these ceramic constituents adhered to the Cu matrix, forming a thin lubricating film. In addition to the ceramic components, the SPS sintering temperature and pressure also significantly impacted the friction performance, as shown in Fig. 4(c) and (d). Cu-based composites sintered at higher temperatures and pressures exhibited increased friction coefficients. Notably, in low-pressure friction tests, samples sintered at 800 ℃ and 400 MPa displayed a significantly higher friction coefficient, likely due to increased densification achieved during sintering. Rajkovic et al.^[112] found that the simultaneous presence of nano-Al₂O₃ and micron-sized Al₂O₃ particles in the Cu matrix could impede recrystallization, leading to grain refinement. This significantly enhanced the strength, hardness, and softening temperature of the Cu-based composites, while maintaining sufficient thermal and electrical conductivity at high temperatures.

Figure  4.  (a) 3D micrographs of worn surfaces of 600 ℃~20 MPa, 800 ℃~20 MPa, and 800℃~40℃ samples; (b) BSE micrographs of worn surface after test at speeds of 60, 80, and 100 km/h; (c) Friction coefficient of Cu-based brake pads under different loads of 3, 5, and 10 N; (d) Friction coefficient of 600 ℃~20 MPa, 800 ℃~20 MPa, and 800 ℃~40 MPa samples^[111]

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Tong et al.^[113] utilized spray pyrolysis to obtain composite powders of SiO₂ quantum dots (SiO₂ QDs) and Cu₂O, which were used as reinforcements to fabricate Cu-based composites with excellent tensile strength and electrical conductivity. SiO₂ QDs contributed to the strengthening of the composite through three mechanisms: load transfer, dislocation and grain refinement. As shown Fig. 5, the TEM micrograph of the interface, XRD patterns, and performance comparison chart of the Cu-based composite revealed that the SiO₂ quantum dots embeded within the Cu₂O transition phase. This effective combination enhanced the interface bonding strength and the dispersed interconnection structure remained intact during the sintering process, further preventing continuous strain hardening. Consequently, the mechanical properties of the Cu-based composite were significantly improved.

Figure  5.  (a) Low magnification TEM micrograph and (b, c) HRTEM micrographs; (d) Schematic illustration of the interface between SiO₂, Cu₂O and Cu; (e) XRD patterns; (f) Performance comparison with related literature^[113]

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Research has also found that SiO₂ and ZrO₂ have certain complementary effects on the friction mechanism^[114], with their synergistic reinforcement significantly improving the friction performance of Cu-based composites. SiO₂ ceramic components can increase the friction coefficient, but cause higher wear on the counterpart material, whereas ZrO₂ ceramic components reduce counterpart wear. Particularly, when the mass ratio of SiO₂ to ZrO₂ is maintained at 2:4, the Cu-based composite exhibits not only smooth braking, but also lower counterpart wear. Additionally, there are some less common oxide ceramic powders used in specific scenarios. For instance, CeO₂ and Al₂O₃ synergistically reinforced Cu-based functionally graded composites^[115]can be used as brake pads in wind turbines due to their high interface bonding strength and excellent wear resistance. Moreover, Pr₂O₃ and Y₂O₃ reinforced Cu-based composites with gradient layer structures, known for their high hardness and wear resistance are used in high-performance brake pads^[116].

3.2   Synergistic effects of carbides and other components

Carbides, as a type of synergistic reinforcement phase, offer significant advantages in the wear resistance field, primarily due to their high hardness and high-temperature stability. These properties are often utilized to enhance the high-temperature friction performance of Cu-based composites. Usca et al.^[117] used powder metallurgy to prepare Cu-based composites synergistically reinforced with SiC and WC, with the preparation process shown in the diagram. They found that the reinforcement phases were uniformly distributed within the Cu matrix and had good interfacial bonding, significantly enhancing the hardness and flexural strength while reducing wear. Zhao et al.^[118] successfully prepared Cu-based composites synergistically reinforced with ZrB₂ and SiC using laser cladding combined with in-situ techniques, as shown in Fig. 6(a~e). They discovered that the reinforcement phases existed as nanoscale particles and microscale needle-like structures. The ZrB₂ needle-like phase bonded well with the Cu matrix, with no defects at the interface, while the SiC mainly existed as nanoscale particles. The combined effect of these reinforcement phases endowed the Cu-based composite with high wear resistance and electrical erosion resistance. Moreover, in current-carrying friction tests, the wear decreased with an increase of the content of the reinforcing phases, and the wear mechanism transitioned from adhesive wear to abrasive wear. Şap et al.^[119] utilized powder metallurgy techniques to prepare Cu-based composites reinforced with Ti-B-SiC powder, as shown in Figs. 6(f~g), depicting friction surface temperature and wear surface mechanisms. They found that when the composite powder was added at a mass fraction of 6% and sintered at 1 050 ℃, the synergistically reinforced Cu-based composite exhibited the lowest friction coefficient and wear temperature.

Figure  6.  (a) Preparation process diagram; (b) TEM micrograph; (c) Selected area diffraction patterns of ZrB₂; (d) High-resolution TEM micrograph of the interface; (e) TEM micrographs^[118]; (f) Temperature graphs generated in the wear test of composites; (g) Uniform surface formation after wear process^[119]

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Zheng et al.^[105] studied the friction mechanism of Cu-based composites synergistically reinforced with SiC and NbSe₂. They found that the addition of NbSe₂ could effectively reduce the plastic deformation of the matrix, while SiC could enhance the wear resistance of the matrix. The combined action of both significantly improved the friction performance of the Cu-based composite. Under certain friction pressure and sliding speed conditions, the wear mechanism involved mild adhesive wear and abrasive wear. Wu et al.^[120] utilized powder metallurgy to prepare Cu-based composites co-reinforced with B₄C and SiC. They discovered a strong complementary effect between the two reinforcements in enhancing friction performance. Specifically, SiC significantly improved friction performance at medium to low temperatures, but tended to detach at high temperatures. On the other hand, B₄C at high temperatures generated a B₂O₃ ceramic film, maintaining stable friction performance. The optimal mass ratio of addition was found to be 5% B₄C and 3% SiC. Wu et al.^[121] also enhanced Cu-based composites for improved wear resistance and stability by pre-treating B₄C and SiC ceramic powders through high-energy ball milling to form B₄C-SiC composite powders. As shown in Fig. 7, from TEM micragraphs of the friction film formed on the worn surface and the morphology of detached debris, it was observed that when the matrix surface experienced frictional resistance, the B₄C particles in the composite ceramic powder were oxidized to form a B₂O₃ film. A small amount of SiC adheres to the friction film, enhancing the friction coefficient. Meanwhile, the detached debris accumulated around the friction film, continuously increasing its thickness to ensure friction stability. The study found that Cu-based composites reinforced with 10% B₄C-SiC composite ceramic powders not only exhibited significantly improved mechanical properties, but also showed great improvements in wear resistance and stability.

Figure  7.  (a) SEM micrograph of sampling location; (b) SEM micrograph of the Focused Ion beam (FIB) specimen; (c) an Bright field-TEM micrograph; (d) The typical load-displacement curve of friction surface at 6 000 r/min; Wear debris morphologies of Cu-based powder metallurgy (PM) brake pads at 6 000 r/min: (e) SB1; (f) SB2; (g) SB3; (h) SB4; (i) Typical braking curve at different braking speeds, and the value of the friction stability of Cu-based PM brake pad during 10 repeated braking processes^[121]

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Kannan et al.^[122] successfully prepared a Cu-based functionally graded material reinforced with SiC and Al₂O₃ using powder metallurgy techniques. They found that the friction performance was stable with low wear under both low and high load pressures. However, under an intermediate load of 50 N, severe wear occurred due to the lack of a stable oxide layer. Zhang et al.^[123] used a melting method to fabricate Cu-based composite materials reinforced with TiC and Ti₅Si₃. They observed that the reinforcement phase effectively withstood the load and prevented crack propagation, thereby enhancing tensile strength and ductility. With the combined effects of these phases, the Cu-based composite material could withstand friction experiments under higher pressures, and a more stable wear-resistant layer formed on the worn surface. Hamid et al.^[124] successfully prepared Cu-based nanocomposites reinforced with TiC and Al₂O₃ using the spark plasma sintering (SPS) process. They utilized the diffusion effect at high temperatures and pressures to achieve a tight interface bonding between ceramic particles and the Cu matrix. The study found a significant increase in hardness and ultimate compressive strength of the matrix, while maintaining high levels of ductility and electrical conductivity. Wu et al.^[125] studied the mechanical, thermal and frictional properties of Cu-based composites reinforced with TiC and B₄C. They found that the mass ratio of TiC/B₄C at 3:5 resulted in a more continuous and intact friction film on the surface of the composite, along with higher friction stability coefficients and lower wear rates. On the other hand, a ratio of 5:3 provided excellent mechanical strength and thermal conductivity. Additionally, they observed that the synergistic strengthening effect of both components was generally effective, especially under low-speed and low-temperature conditions, where their wear resistance effects were similar, indicating substitutability. Moreover, Wu et al.^[126] discovered that TiC-B₄C composite ceramic powders formed after high-energy ball milling were beneficial for the formation of friction films during braking processes. As shown in Fig. 8, they prepared Cu-based composites using high-energy ball milling combined with conventional powder metallurgy [(Fig. 8(a)]. The results of friction experiments, as shown in Fig. 8(b) indicated that when the composite ceramic powder mass fraction reached 6%, the Cu-based composites exhibited improved structural compactness and enhanced mechanical strength. However, with a ceramic powder content of 10% or more, the effect of the friction film in enhancing friction stability became significantly apparent. Additionally, the friction film’s coverage on the matrix led to insufficient oxidation of Fe elements presented on the surface. The presence of Fe²⁺ and Fe³⁺ peaks detected via XPS, as shown in Fig. 8(c) further supported this observation.

Figure  8.  (a) Preparation chart of Cu-based powder metallurgy brake pads (PMBP); (b) The value of the friction stability of Cu-based PMBP and the braking curves; (c) Fe core-level XPS of worn surface at 6 000 r/min^[126]

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TiC and WC particle synergistic reinforcement has been found to induce a phenomenon of weak matrix-particle interface bonding in Cu-based composites. The presence of carbide particles inhibit grain growth and generates high-density dislocations, effectively strengthening the mechanical properties of Cu-based composites^[127]. Yusoff et al.^[128] successfully prepared WC and W₂C-reinforced in-situ nanostructured Cu-based composites using mechanical alloying combined with powder metallurgy. Higher energy and longer milling times favored the formation of WC, progressive milling reduced grain size and increased internal strain, and the sintering process successfully relieved local stresses in the composite while maintaining its nanostructure, thereby enhancing mechanical strength. Wu et al.^[129-130] studied the preparation of B₄C-SiC composite powder and sepiolite-reinforced Cu-based composites, exploring the synergistic reinforcement mechanisms of both. Friction performance and mechanical properties, as shown in Fig. 9, it could be seen that the prepared Cu-based composites not only exhibited higher friction coefficients and better friction stability, but also demonstrated excellent mechanical strength and compressive reinforcement, fully show-casing the significant performance enhancement resulted from the synergistic interaction of these two materials.

Figure  9.  (a) Friction performance comparison chart; (b) Brinell hardness-shear strength performance comparison chart; (c) Density-compressive strength performance comparison chart; (d) Schematic diagram of sepiolite particle strengthening; (e) XPS curve fittings of worn surface^[129-130]

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Şahin et al.^[131] used powder metallurgy to prepare Cu-based composites reinforced with B₄C and Al₂O₃ particles, and studied their friction performance and mechanisms in terms of lubrication, load, sliding distance and hardness. They found that the two materials had a synergistic effect, with B₄C particles significantly promoting wear resistance compared to Al₂O₃ particles. Additionally, Li et al.^[132] synthesized TaC and Cr₃C₂ co-reinforced Cu-based composites using in-situ synthesis techniques, significantly enhancing their hardness and wear resistance. TaC particles were evenly distributed in the Cu matrix, while Cr₃C₂ existed in the form of needle-like phases.

3.3   Synergistic effects of nitrides and other components

Nitrides, as a type of synergistic reinforcing phase, have several advantages in the field of wear resistance. Their ultra-high hardness significantly enhances the wear resistance of composite materials, making them suitable for high-pressure, high-speed and harsh environments. They also perform well in high-temperature conditions, making them ideal for applications requiring high-temperature wear resistance, maintaining stable performance and structure. Additionally, nitrides exhibit good lubricity on the surface, which helps reduce frictional losses, thereby improving the material’s lifespan and efficiency. Thankachan et al.^[133] used the stir friction method to prepare Cu-based composite materials synergistically reinforced with AlN and BN particles. They found that due to the effective distribution of ceramic particles and the occurrence of dynamic recrystallization processes, the hardness and ultimate tensile strength of the base material were greatly enhanced. The addition of dual-component ceramic particles reduced wear and improved surface corrosion resistance. This was partly because the presence of nitrides formed a thin oxide layer on the sample surface, which had a wear-resistant and corrosion-inhibiting effect. Furthermore, the different mechanisms of action of AlN and BN contributed to this effect. Specifically, AlN particles reduced electron migration, thereby lowering the corrosion rate, while BN particles, due to their insulating properties and hindered electron transfer, slowing down the corrosion rate. Chen et al.^[134] explored the frictional properties and mechanisms under different pairing conditions by incorporating h-BN and ZrSiO₄ into Cu-based composite materials, as shown in the TEM micrographs of the friction surface and wear area Fig. 10. During friction, ZrSiO₄ became a component of the nanoscale oxide layer in the friction film, effectively preventing further oxygen diffusion and direct contact on the friction surface, thereby improving wear resistance. h-BN did not undergo oxidation, but become part of the friction film, mainly relying on its structural advantages to provide high-temperature lubrication. With the combined action of these two components, compared to the pairing with 30CrMnSi, the prepared Cu-based composite material showed more significant advantages when paired with C/C-SiC, including an increase in stability coefficient, a decrease in wear rate, and a reduction in the tailing phenomenon of the friction curve.

Figure  10.  Bright field TEM micrographs with the corresponding selected area electron diffraction (SAED) and High resolution-TEM patterns of the interface between Cu matrix and solid lubricants: (a~c) Cu-graphite; (d~f) Cu-h-BN; (g) SAED pattern from the region D; (h~i) High resolution-TEM lattice micrograph of region D and the corresponding FFT micrograph of (j) and (k)^[134]

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Luo et al.^[135] studied the frictional properties and mechanisms of h-BN and B₄C reinforced Cu-based composite materials by adjusting the B₄C content. They found that the formation of the B₂O₃ film on the wear surface mainly resulted from the oxidation of B₄C, while h-BN maintained its particle morphology, providing high-temperature lubrication and wear resistance. Increasing the B₄C content enhanced the material’s resistance to thermal degradation and changed the wear mechanism. Nautiyal et al.^[136] investigated the frictional properties and mechanisms of h-BN and Al₂O₃ co-reinforced Cu-based composite materials under different loads. They observed that the friction coefficient and wear rate at low loads increased with load, while at high loads, a friction film formed on the friction surface, reducing wear and strengthening the frictional performance. They also noted a relatively poor synergistic effect between h-BN and Al₂O₃, with h-BN easily detaching during friction, leading to a decrease in friction stability. Chen et al.^[137] used powder metallurgy combined with in-situ synthesis techniques to prepare Cu-based composite materials reinforced with TiN and TiB₂, significantly increasing hardness and wear resistance. The formation of TiN might be attributed to nitrogen atoms entering the interstitial sites of Ti.

3.4   Synergistic effects of borides and other components

As a synergistic reinforcing phase, one of the advantages of nitrides in the field of wear resistance lies in their high hardness, which enhances the wear resistance of composite materials. Additionally, under specific conditions, they exhibit lower friction coefficients, which are beneficial for reducing frictional wear. Chen et al.^[138] used liquid phase in-situ reaction technology to prepare TiB₂ and Al₂O₃ dual-phase reinforced Cu-based composite materials, as shown in Fig. 11. The Cu matrix contained a large number of dispersed particles, mainly spherical and irregular polygonal particles, with no obvious intermediate transition layer observed near the interface, indicating good bonding between the particles and the matrix. TEM micrographs showed that the spherical particles were Al₂O₃, ranging in size from 50 nm to 500 nm, while the irregular polygonal particles were TiB₂, ranging in size from 50 nm to 1.5 μm. When the Cu-based composite material was subjected to high pressure, it leaded to severe shear strain between the ceramic reinforcement particles and the Cu matrix, causing particle clustering separation, improved uniform distribution, rapid increase in dispersion strengthening and subgrain strengthening, resulting in enhanced strength.

Figure  11.  The microstructure evolution of TiB₂ and Al₂O₃ dual-phase reinforced Cu-based composite materials: (a) TEM micrographs of ingot; (b) selected area diffraction analysis; (c) dislocation entanglement −20%; (d) particles hindered the dislocation movement −20%; (e) dislocation cells −60%; (f) shear band −90%; (g) dislocation cells inside the grains −90%; (h) dislocations and dislocation cells accumulated near grain boundaries −90%; (i) a high magnification micrograph of a particle −90%; SEM fracture micrographs of the composite ingot sample and cold rolled samples with different reductions: (j, k) ingot; (l, m) CR 20%; (n, o) CR 60%^[138]

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Qiu et al.^[139] successfully prepared Cu-based composite materials reinforced with TiC and TiB₂ through combustion synthesis and hot pressing, with Zr serving as a sintering aid and variable. The strain curve and wear trend of the Cu-based composite materials indicated that the addition of Zr might cause significant lattice distortion in the solid solution, promoting the refinement of ceramic particles, thereby enhancing the compressive strength and hardness of the matrix. Additionally, the finer and more uniform distribution of ceramic particles on the worn surface could provide a stronger barrier effect, protecting the Cu matrix from damage and thereby reducing wear. Pugacheva et al.^[140] used the self-propagating high-temperature synthesis method to prepare Cu-based composite materials reinforced with TiB₂ and TiC. They found that in regions where ceramic particle phases coexisted, the microhardness was significantly higher than in single-phase regions and the overall hardness. Additionally, abrasive wear could lead to the cutting of areas where single-phase ceramics existed on the surface layer, forming smooth planes.

Yan et al.^[141] used the reactive hot pressing method to in-situ prepare Cu-based composite materials reinforced with TiB₂ and TiC, and combined it with finite element analysis to explore the influence of dual-phase ceramic particles on the thermal, mechanical and frictional properties of Cu-based composite materials. As shown in Fig. 12, the crystal orientation relationships of low interfacial spacing and atomic spacing mismatch indicated that the crystal mismatched between Cu and TiB₂ was lower than that between Cu and TiC, which was conducive to the formation of a stable Cu-TiB₂ interface. Additionally, finite element analysis showed that spherical TiC particles could optimize the heat transfer path, improve stress distribution and plastic deformation, reduce stress concentration at particle tips, and significantly improved the compressive strength of Cu-based composite materials. Although hexagonal prism-shaped TiB₂ particles were more prone to initiate initial cracks at the tips, they were less likely to detach from the matrix, thereby contributing to overall wear resistance. The complementary advantages of both types of particles could better optimize the heat transfer path and the uniformity of stress distribution, thereby demonstrating excellent comprehensive performance of Cu-based composite materials. Zhang et al.^[142] prepared Cu-based composite materials reinforced with ZrC and ZrB₂ by adding Cu elements to Cu-Zr-B₄C. The solid-solid reaction between Zr and B₄C particles, along with the liquid displacement reaction between Cu-Zr could produce ZrC and ZrB₂. The amount of added Cu could control the transformation process and pathway of ZrC and ZrB₂.

Figure  12.  (a) XRD patterns; (b~f) Atomic configuration on the close-packed/nearly close-packed planes of Cu, TiB₂ and TiC; (g) Orientation relationship of interplanar spacing mismatch (f_d) and interatomic spacing mismatch (f_r) between ceramic and matrix; Finite element SEM micrographs of phases distribution of (h) TiB₂/Cu and (k) (TiC + TiB₂)/Cu composites; Representative volume element finite element of the (i) TiB₂/Cu and (l) (TiC + TiB₂)/Cu configurations; Grey represents Cu matrix (transparent), green TiC, and orange TiB₂; Finite element meshes for (j) TiB₂/Cu and (m) (TiC + TiB₂)/Cu configurations^[141]

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4.   Effects of multi-component friction components on the properties of Cu-based composites

The incorporation of multiple ceramic phases in Cu matrix composites (CMCs) offers synergistic effects that enhance mechanical and tribological performance. These effects arise from interactions among ceramic components and between each component and the Cu matrix, improving properties such as wear resistance, frictional stability and structural integrity under load.

4.1   Synergistic effects of multi-ceramic component

Dinaharan et al.^[143] investigated CMCs reinforced with B₄C fly ash, and W via powder metallurgy (PM). Fly ash comprises ceramic phases like SiO₂, Al₂O₃ and Fe₂O₃, all of which distribute uniformly within the composite, creating a refined Cu grain structure. This fine microstructure, alongside the good interfacial bonding observed in XRD spectra (showing no extraneous compounds or oxides), significantly reduced the wear rate and friction coefficient. Notably, reinforcement particles limited subsurface damage and reduced debris size, suggesting that their presence shifted wear mechanisms away from the severe abrasive wear typical of unreinforced Cu. Calli et al.^[144] also highlighted the influence of reinforcement particles (B₄C, TiB₂ and TiC) on the wear and corrosion resistance of Cu-based coatings fabricated by cold gas dynamic spraying. While these composites showed slightly reduced wear performance compared to pure Cu coatings, the presence of ceramic particles inhibited continuous oxide layer formation during wear, shifting the wear mechanism to third-body wear. This interaction between the reinforcements and the wear surface altered debris formation, demonstrating the importance of specific reinforcement phases in shaping wear behavior even in dense, well-bonded coatings. The results suggested that simply emphasized the type and content of ceramic components, while overlooking the synergistic effects between these components, might not lead to Cu-based composites with optimal friction performance. Vaibhav et al.^[145] explored the enhancement of Cu-based composites by incorporating ceramic components with oxidation lubrication properties alongside frictional ceramic components, including silicon carbide powder, aluminium oxide powder, calcium carbonate powder, graphite and talcum powers. The pin-on-disc test was performed on brake composite material to analyse their tribological properties namely friction and wear. From tribotest, it was observed that all composites give the friction coefficient in the range of 0.33~0.51 and the loss of materials in the range of 79~131 mg. Further, the mechanical, thermal stability and surface characterization were also carried out on brake composites using universal testing machine, vicker’s hardness tester, thermogravimetric analyser and scanning electron microscope respectively.

The research difficulty of multi-component ceramic-reinforced Cu-based materials is substantial due to the extensive interactions among their internal components. However, the self-diffusion effect among ceramic components significantly enhances friction performance. Gao et al.^[146] reported the self-diffusion reaction between TiN and B₄C within the TiN-ZrO₂-B₄C ternary system. Their research results were depicted in Fig. 13. The study revealed that internal diffusion bonding chemical reactions (TiN-B₄C) occurred among the ceramic components, enhancing their retention within the Cu matrix [Fig. 13(c)]. The authors highlighted that the three friction components contributed significantly at different stages of friction [Fig. 13(d)]. ZrO₂ primarily contributed at the low-speed stage and provided effective friction. TiN played a major role at the medium-speed stage, with its strong bonding capability with the Cu matrix resulting in a high friction coefficient, its diffusion bonding with B₄C aided in B₄C retention. B₄C particles mainly contributed at the high-speed stage, where they oxidized under high temperatures to form a B₂O₃ film, offering high-temperature lubrication.

Figure  13.  Performance of TiN-ZrO₂-B₄C ternary ceramic particle-reinforced powder metallurgy Cu-based composites: (a) the friction performance graph of the Cu-based composite; (b) the element distribution map of the worn surface; (c) a schematic of atomic diffusion between B4C and TiN; (d) the wear mechanism diagram^[146]

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4.2   Synergistic effects of MAX phase ceramic component

Up to now, the study of ternary ceramic-reinforced Cu-based composites has been challenging due to their complex composition, resulting in relatively few reported studies. However, recent attention has turned to a new class of layered ternary ceramics known as MAX phases. These phases exhibit a unique combination of metallic and ceramic properties due to their thermodynamically stable nano-lamellar structures. MAX phases are described by the formula M_n+1AX_n, where n can be 1, 2, or 3, M is an early transition metal, A is a group A element, and X is either carbon or nitrogen. They are notable for their high damage tolerance, thermal shock resistance, machinability, and Vickers hardness ranging from 2 to 8 GPa. Some MAX phases also show a ductile-to-brittle transition at temperatures exceeding 1 000 ℃, while maintaining excellent mechanical properties at these high temperatures. The layered nature of these materials also suggests significant potential as solid lubricants.To date, over 50 different MAX phase materials have been discovered and are being applied, particularly as solid lubricants and reinforcing particles.

(1) Ti₃SiC₂

The introduction of Ti₃SiC₂ into Cu matrices has shown promise for enhancing tribological performance over a wide temperature range. Yang et al.^[147] used cold spraying to prepare Cu-Ti₃SiC₂ composites with varied Ti₃SiC₂ content. They found that the composite’s ultimate tensile strength (UTS) significantly increased with Ti₃SiC₂ addition, while conductivity was slightly reduced. Notably, annealing at 800 ℃ enhanced composite performance due to a solid solution of Ti₃(Si₁-δCu)C₂ at the Ti₃SiC₂-Cu interface, verified by TEM micrographs in Fig. 14. This solid solution, together with interface strengthening, contributed to the composite’s high UTS and stable conductivity, showing how Ti₃SiC₂ could effectively reinforce the Cu matrix through both mechanical and electrical pathways.

Figure  14.  TEM bright-field micrographs, diffraction patterns of the red dotted areas, and EDS results of Cu-20Ti₃SiC₂ composite annealed at (a, c) 500 ℃ and (b, d) 800 ℃, respectively^[147]

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Similarly, the chemical reactions between Cu and Ti₃SiC₂ also enhance the tribological performance of Cu-based composites. Zhang et al.^[148] used spark plasma sintering to synthesize a Ti₃SiC₂/Cu composite (TSC-Cu) with enhanced lubrication across a broad temperature spectrum. Their findings showed that Cu addition increased the composite’s hardness and compressive strength, while reducing flexural strength. The TSC-Cu composite, containing evenly dispersed Cu₃Si along the Ti₃SiC₂ grain boundaries demonstrated a temperature-dependent tribological behavior: it had a higher friction coefficient than polycrystalline Ti₃SiC₂ at temperatures up to 400 ℃ but exhibited a lower coefficient at 600 and 800 ℃, likely due to the formation of a lubricating oxide film (TiO₂, SiO₂ and CuO). This shift in friction coefficient, alongside a significantly lower wear rate than pure Ti₃SiC₂, underscored Ti₃SiC₂’s complex role in high-temperature lubrication and surface strengthening. Rreating Ti₃SiC₂/Cu composites with varied Cu content through mechanical alloying and spark plasma sintering. Consistent with Zhang’s findings, Dang et al.^[149] created a composites with lower friction coefficients and wear rates than monolithic Ti₃SiC₂, attributed to hard phases like TiC_x, Ti₅Si₃C_y, and Cu₃Si, formed by Ti₃SiC₂ decomposition. These phases stabilized the wear surface and mitigated abrasive friction. However, at higher temperatures (up to 600 ℃), friction and wear rates increased, likely due to plastic deformation and oxidative wear, illustrating how elevated temperatures challenged Ti₃SiC₂/Cu composite stability. Yang et al.^[150] indicated that the interaction between Cu and hard phases like TiC and SiC increased the composite’s friction coefficient while reducing wear rates, underscoring Ti₃SiC₂’s role in enhancing load-bearing capacity and wear resistance.

Finally, Dang et al.^[151] explored a Ti₃SiC₂/Cu/Al/SiC composite (TCASc) prepared by powder metallurgy and spark plasma sintering, examining its tribological properties from room temperature to 800 ℃. The composite contained AlCu, SiC and Al₂O₃, which stabilized Ti₃SiC₂ grains under cyclic stress. At high temperatures, a balanced frictional oxide layer reduced both friction and wear, as observed in Fig. 15. Although TCASc’s wear rate increased due to adhesive wear, the dynamically stabilized oxide layer at elevated temperatures illustrated Ti₃SiC₂’s complex interaction within Cu-based composites.

Figure  15.  The contact mode of the pin and disk (Ti₃SiC₂-Cu) and the mechanism diagrams of wear process at different temperatures^[148]

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(2) Ti₃AlC₂

Lian et al.^[152] synthesized Cu/graphene oxide (GO)/Cu core-shell structured composite powders and Cu-decorated Ti₃AlC₂ particles using hot-press sintering, resulting in a novel Cu-based composite with a low friction coefficient and high wear resistance. This structure enabled uniform dispersion of GO within the Cu matrix, along with strong interfacial bonding between Cu and both GO and Ti₃AlC₂. Their findings showed that the GO and Ti₃AlC₂ synergistically reinforced the composite, achieving lower friction coefficient and wear rates compared to reinforcement with either component alone. The improvement was attributed to the load-bearing capacity of GO and Ti₃AlC₂, and the formation of a dense, lubricating tribofilm on the worn surfaces. Wei et al.^[153] modified Cu-graphite composites with Ti₃AlC₂ to enhance hardness, lubricity and wear resistance. Notably, Cu-coated Ti₃AlC₂ prevented particle agglomeration, achieving superior density and mechanical properties, while promoting interfacial bonding with the graphite matrix. Su et al.^[154] investigated TiC_x-reinforced Cu/graphite composites with varying Ti₃AlC₂ contents, as shown in Fig. 16. In-situ formation of TiC_x particles improved density, hardness and wear resistance by enhancing deformation resistance and reducing crack propagation, while maintaining a stable friction coefficient around 0.14 across tests. Further, Liu et al.^[155] reported that Ti₃AlC₂ content in Cu matrix composites affected the transition of wear mechanisms, with higher content resulting in enhanced hardness and strength. The in-situ formation of TiC_x at the interface improved interfacial bonding, shifting wear from delamination to a combination of adhesive, abrasive and oxidative wear.

Figure  16.  Schematic diagram of the effect of Ti₃AlC₂ addition on material wear: (a) sample without Ti₃AlC₂ addition; (b) sample with Ti₃AlC₂ addition^[154]

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Friction processes are often accompanied by high temperatures. Zhou et al.^[156] focused on temperature effects on Cu/Ti₃AlC₂ composites across 25~700 ℃, highlighting a shift in wear behavior, as shown in Fig. 17. At low temperatures, a tribofilm of Ti₃AlC₂ and Cu₂O formed and provided lubrication. However, at 300 ℃, significant material transfer and plastic deformation were observed, resulting in higher wear rates. At elevated temperatures (500~700 ℃), an oxide layer stabilizing the wear rate and reducing material degradation formed. In particular applications for armature materials, conditions involving the simultaneous presence of force, electricity, and friction are commonly encountered. Zhao et al.^[157] explored Cu-Ti₃AlC₂ composites as sliding electrical contacts with a Cu-5% Ag alloy. They found that increased current density led to higher friction coefficients, wear rates, and contact voltage drops. The main wear mechanisms were adhesive and arc erosion wear, with a lubricating film observed under all conditions, which enhanced tribological performance by reducing frictional degradation. In the context of Cu-Ti₃AlC₂ content optimization, Huang et al.^[158] demonstrated that increasing Ti₃AlC₂ content led to an initial increase in hardness and subsequent arc erosion resistance. After arc erosion, oxidation to CuO, TiO₂, and Al₂O₃ was noted, with Ti₃AlC₂ protecting the Cu matrix by localizing arc erosion on its particles.

Figure  17.  The worn surface the Cu–30% Ti₃AlC₂ composites for different sliding velocity at constant apparent contact pressure and current density (2.5 N/cm², 5A/cm²), each experiment condition including three cicrographs: the topography, a 3D simulated diagram and a sectional view: (a) 2.5 m/s; (b) 5 m/s; (c) 10 m/s; (d) 15 m/s^[157]

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Together, these studies illustrated that Ti₃AlC₂, whether through direct reinforcement or in-situ TiC_x formation, significantly enhanced the wear performance, interfacial bonding and overall tribological properties of Cu-based composites, particularly by enabling protective tribofilms or oxide layers across different temperatures and wear conditions. Therefore, Ti₃AlC₂ not only strengthens the friction behavior of Cu-based composites under general conditions, but also demonstrating excellent friction performance in force-electric coupling environments. This greatly expanded the application range of Cu-based composites.

(3) Ti₂SnC/ Ti₂AlN

Ti₂SnC and Ti₂AlN, as 211-type MAX phase ceramics, possess relatively simple crystal structures that contribute to their more stable properties. However, due to their high cost, research on these materials is still relatively limited. Wei et al.^[159] prepared Cu/graphite composites and Cu/graphite/Ti₂SnC composites using ball milling, pressing, and sintering techniques. They investigated the effect of Ti₂SnC as a secondary lubricating component on the mechanical properties, wear resistance and lubricating performance of Cu/graphite composites. The results indicated that the Cu/graphite/Ti₂SnC composites exhibited superior hardness, impact toughness, wear resistance and lubricating performance compared to Cu/graphite composites^[160].

Zhang et al.^[161] systematically studied the effect of Ti₂AlN content on the microstructure, resistivity and mechanical properties of the composites. The tensile strength of the Ti₂AlN/Cu composites was approximately 380 MPa with a reinforcement phase mass fraction of 7%, and the electrical conductivity was maintained at around 61.5% international annealing copper standard (IACS). Additionally, wear tests indicated that Ti₂AlN-reinforced materials effectively bored the load compared to Cu, and the main tribological mechanism shifted from adhesive wear to abrasive wear, significantly enhancing friction and wear resistance. Gao et al.^[162] further analyzed the friction behavior of Cu-based composites reinforced with varying Ti₂AlN contents under emergency braking conditions at different rotational speeds. Combining the composition and morphology analysis of the worn surfaces, they proposed a relatively complete tribological mechanism, as shown in Fig. 18. The Ti element in Ti₂AlN attracted Cu elements from the matrix, forming Cu_xTi_y solid solutions, thereby enhancing the interfacial bonding strength between Ti₂AlN and the matrix. During friction, Ti₂AlN acted as an anchor and trapped more friction debris, and at high temperatures, it promoted the formation of a tribofilm, helping to improve lubrication performance.

Figure  18.  Friction mechanism diagram of Ti₂AlN-Cu composite materials.

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5.   Summary and prospect

Cu-based composites are widely used in applications such as electrical brushes, wind turbines, and high-speed rail braking systems due to their excellent mechanical and tribological properties. As the service environments for Cu-based materials become increasingly demanding, enhancing Cu-based composites with ceramic components has proven to be a highly effective method. Ceramic particle-reinforced Cu-based composites combine the high thermal and electrical conductivity of metal Cu with the high specific strength and high-temperature stability of ceramic phases, offering outstanding overall performance^[163-165]. This paper reviews the mechanical (including compressive strength and shear strength), thermal (thermal diffusivity and thermal conductivity) and electrical properties of Cu-based materials reinforced with single ceramic components, binary or ternary ceramic components, and novel ternary MAX phases. It also focuses on the tribological behavior of Cu-based materials with multiple ceramic components, discussing the effects on friction coefficient, wear rate, and wear mechanisms.

The inclusion of specific amounts of TiC, B₄C, SiO₂, ZrO₂ and other ceramic components in a single or combined form into a Cu matrix can significantly enhance the overall friction coefficient and reduce the wear rate of the material. On the one hand, these hard phases directly contribute to provide friction during the wear process, increasing the friction coefficient of the Cu-based material. Additionally, the introduction of fine ceramic particles into the metal matrix offers strong grain boundary migration resistance, effectively hindering dislocation movement. This results in significant improvements in mechanical properties due to grain refinement, dislocation strengthening, Orowan strengthening and load transfer strengthening. These improved mechanical properties also play a crucial role in the tribological performance of the composite, leading to complex friction mechanisms^[166-168]. Therefore, the wear mechanisms of composites reinforced with different ceramic components need to be discussed individually.

However, studies on Cu-based materials reinforced with three or more ceramic components are still relatively scarce, as the interactions between multiple components increase the complexity of investigations. Nevertheless, synergistic enhancement of Cu-based materials through the reaction combination, size effects, and shape effects of multiple components is still of great interest. The size and morphology of ceramic particles affect stress accumulation, deformation within the matrix and conduction paths, leading to variations in composite performance. Generally, small, uniformly dispersed and near-spherical ceramic particles are more effective in enhancing the composite’s strength and limiting its thermal expansion behavior. Larger particles can reduce the hindrance to electron flow by ceramics, thereby improving the composite’s thermal and electrical conductivity.

Furthermore, diffusion reactions between ceramic components can improve their interfacial bonding with the Cu matrix, thereby enhancing the friction performance of Cu-based materials. Currently, the friction-lubrication effects of ternary ceramic MAX phase compounds that exhibiting characteristics of both ceramics and metals have garnered much attention. Research on reinforcing Cu-based materials with solid lubricant MAX phases improving mechanical and tribological properties has been widely conducted and may be a key focus for future studies.

The current focus of researchers is on the types, morphology and sizes of ceramic components. Future research should also prioritize as 1) achieving uniform dispersion of ceramic particles within the matrix through rational structural design; 2) employing advanced powder metallurgy techniques, such as mechanical alloying, spark plasma sintering and internal oxidation to fabricate advanced Cu-based composites; 3) developing novel ceramic components and utilizing their properties to achieve synergistic enhancement of Cu-based composites; 4) mixing high-performance materials like carbon nanotubes, carbon fibers, and advanced MAX phase ceramics into Cu-based composites to improve tribological properties.

In the foreseeable future, more precise design and control of the microstructure of composites, as well as the morphology and composition of frictional components are anticipated to enhance overall performance and meet the increasing demands in the field of Cu-based composites.