Study on the microstructure and properties of copper/graphite composites with two-step method and copper-modified graphite

In this experiment, copper was coated on the surface of graphite by chemical process. Besides, the microstructure and crystal structure of copper-coated graphite were characterized by SEM and XRD. Copper-plated C/Cu composite was prepared by SPS sintering technique of powder mixture by two-step method. In order to test the dispersion state of graphite in the composites and the phenomenon of interfacial dispersion, the effect of graphite content on the properties of copper-coated C/Cu composites was also an important point. Therefore, the microscopic characterization and performance test were carried out. The results show that magnetic stirring and high-energy ball milling ensure the uniformity of copper-coated graphite in copper matrix, and conducted structural grain refining. The grain size of copper, and the properties of composite materials decreased with the increase of graphite content, such as relative density, conductivity and hardness. However, the further increase of graphite content leads to excessive grain size, the relatively long time of ball milling makes the fine grain become larger after sintering. When the graphite content is 1.0wt.%, the relative density, conductivity and hardness are 99.91%, 92.1%IACS and 67.7 HV, respectively. It is proved that copper-coated graphite and two-step powder mixing play an important role on improving the properties of the composites.


Introduction
Copper rank only second to silver in the electrical conductivity, even have lower cost.The conductivity of pure copper at room temperature is 5.8×10 6 S/m.The copper and its alloys are widely used in power equipment, aerospace, machinery manufacturing and other industrial fields [1] .However, the ever-expanding application scenarios and the complex working environment put forward higher requirements for the safe and efficient operation of the equipment.The mechanical properties of copper alloy components affect the service life of equipment and directly determine whether the mechanical equipment can operate safely.A single electrical performance index is difficult to meet the requirements for safe operation.
Adding graphite with self-lubricating properties to copper and copper alloys can significantly improve the wear resistance of graphite/copper composite materials while maintaining good electrical conductivity of copper [2] .Hou et al [3] .found that as the dihedral angle between the graphite flake and the sliding surface increased, the friction coefficient decreased first and then increased, while the wear rate continued to decrease.Wei et al [4] added the second lubricating component TiSnC to the composite material using the method of ball milling and sintering, and the results showed an improvement in both impact toughness and hardness.The research improved the friction and wear performance of the composite material, but did not address the interface problem between graphite and copper.The low mutual solubility and poor wetting between the two materials, the absence of chemical reaction between C and Cu, the failure to generate carbides under high temperature conditions, while the absence of significant decrease in the contact angle all indicate weak interface bond that will affect the overall performance of the composite material.
By adding other alloying elements to the copper matrix for matrix alloying treatment, the wettability of graphite and matrix can be effectively improved and the interfacial bond between graphite and matrix can be enhanced [5] .Ren et al [6] added powdered Cr to the copper matrix to form the carbide transition layer at the interface between copper and graphite, thus increasing the bending strength to 93 MPa and reducing the coefficient of thermal expansion to one-third of the original.Zuo et al [7] constructed a Cu-Fe alloying system where Fe reacted with graphite to form Fe3C layer and the contact angle was reduced from 124°to 21°.Lin et al [8] investigated the wettability of molten Cu-xSn-yCr ternary alloy with graphite at 1373 K and found that both Sn and Cr could reduce the contact angle.Besides, the addition of Cr can significantly improve the wettability.
Alloying treatment can improve the interfacial bonding strength of graphite and substrate, But there are no guarantees good dispersion of graphite in the substrate, and agglomeration in the substrate can easily occur.Researchers found the surface modification of carbon materials can not only improve the wettability between copper and carbon, but also make the dispersion of graphite more uniform.Zeng et al. [9] used chemical plating method to coat Ni on the surface of natural scaled graphite and prepared composites by vacuum hot pressing method, and found good interfacial bonding and significant improvement in flexural strength.Wang et al. [10] deposited 1.52 um Ag layer on the graphite surface by Tollens reaction, the results showed that the hardness was increased by about 50% after the modification.Wang et al. [11] used immersion reduction method to prepare W-coated graphite flakes and hot pressed them at 900℃, and finally obtained composites with 22.3% improvement in thermal conductivity.Bai et al. [12] metallized the copper-plated EG/Cu (EG@Cu/Cu) composites, the friction coefficient and the wear mass are 22.9% and 7.9% of pure copper, respectively.
The preparation process of C/Cu composites mainly includes powder mixing and powder sintering forming, in which the mixing process play an important role in the powder mixing effect.Generally, the homogenous distribution of graphite in copper matrix can be attributed to the long duration of high-energy ball milling [13] .At the mean time, despite its enhancement of the homogenous distribution of graphite, there is also a negative effect on the properties of composite materials due to the defects of composite powder and the formation of large spherical powders during the milling process [14 15] .On this basis, the appropriate dispersion process was combined with high-energy ball milling are significant to obtain high-homogenous distribution C/Cu composites.
To further improve the wettability and dispersion between copper and graphite, the interfacial bond between them was improved and the performance of C/Cu composites was enhanced.In this study, the graphite surface was modified by chemical process, and copper was plated on the graphite surface to improve the wettability between copper and graphite; and the powder was mixed by two-step mixing method using magnetic stirring combined with high-energy ball milling, and then the copper-plated C/Cu composites with excellent performance were prepared by SPS sintering.SEM and XRD were used to analyze the microscopic morphology and physical phase changes of the copper-plated C/Cu composite powder, and BSE/EDS was applied to analyze the graphite dispersion and interfacial bonding in the copper-plated C/Cu composites, while their properties such as relative density, electrical conductivity and hardness and friction wear were tested to analyze the mechanism of the effect of copper-plating on the graphite surface on the organization and properties of the composites.

Experimental preparation
Preparation of copper-plated graphite: The surface activity of graphite is poor.Reduced graphite powder is obtained after oil removal, coarsening, sensitization, activation, reduction and drying.The prefabricated chemical plating solution is 60℃ and the duration is 60 min.The PH value of the plating solution is adjusted by 40g/L sodium hydroxide solution.The composition of the chemical plating solution is as follows: copper sulfate 10 g/L, formaldehyde 20 mL, sodium potassium tartrate 25 g/L, Edetate disodium 10 g/L, Sodium acetate 0.03 g/L, potassium ferrocyanide 0.02 g/L.
The different weights of copper plated graphite powde was added to the appropriate amount of anhydrous ethanol, ultrasonic dispersion for 1 h, adding an appropriate amount of electrolytic copper powder, magnetic stirring at room temperature for 3 h to obtain composite materials.The composite powder slurry is obtained after 8h high-energy ball milling, and the composite powder is obtained after filtration and air drying,the ball milling speed is 200 r/min, the ball material ratio is 10:1.With the composite powder as raw material, it was poured into the mold, preformed under 35 Mpa pressure and then sintered to prepare copper-coated C/Cu composite material with different graphite content.The sintering parameters are as follows: sintering temperature is 800℃, hot pressing pressure is 50Mpa, heating rate is 100℃/min, holding time is 5 min, vacuum degree is less than 3 Pa.

Microstructure analysis of copper-plated graphite powder
The copper-plated graphite powder prepared by chemical plating was selected for SEM observation.As shown in Figure 1, the continuous and uniform coating layer was attached to the surface of the irregularly scaled graphite particles; and the selected area was analyzed by energy spectroscopy.The results show that the copper content in the selected area is high, which indicates that the copper plating process is able to achieve a good copper coating on the surface of graphite particles.

Figure 1. SEM and EDS of copper-coated graphite powder
The XRD spectra of graphite and copper-plated graphite were tested as shown in Figure 2. As can be seen from the Figure2 graphite powder was detected at diffraction angles of 26.5°, 54.6°and 44.5°f or graphite phase (003), ( 101) and (004) characteristic peaks, while copper-plated graphite was also detected at diffraction angles of 43.3°, 50.4°and 74.1°for copper phase (111), ( 200), (220) characteristic peaks were also detected at 43.3°, 50°and 74.1°, and the intensity of the characteristic peak at graphite phase (003) was significantly weakened.This indicates that the graphite surface is covered by a copper layer, resulting in a significant decrease in the intensity of the diffraction peaks of the graphite phase, and no other diffraction peaks occurred, these indicate that this chemical copper plating process can effectively coat the graphite surface with a copper layer without introducing impurities [16] .

Effect of graphite content on the microstructure of copper plated C/Cu composite powder 3.2.1 SEM/EDS analysis of copper-plated C/Cu powder
The graphite content (1.0 wt.%, 1.5 wt.%, 2.0 wt.% and 2.5 wt.%) of the copper-plated C/Cu composite powders prepared by the two-step mixing method were selected for SEM observation, and the morphology is shown in Figure 3.As can be seen from Figure 3, the powder particle size tends to decrease with the gradual increase of graphite content.It could be due to the good self-lubricating property of graphite particles, which can effectively inhibit the cold welding bonding between copper powder particles during the high-energy ball milling process, making the powder particle size gradually decrease [17] .This also indicates that increasing the graphite content can obtain finer particle size and more uniformly dispersed composite powder in a shorter period of time.The selected area in Figure 3(c) was observed by high magnification SEM/EDS, and it can be seen from Figure 4 that the flake graphite surface is still covered with a continuous layer of copper nanoparticles.These indicate that the copper-plated layer on the graphite surface was not peeled apart when the copper-plated C/Cu composite powder was prepared by the two-step mixing method, and illuminates that the copper plating layer prepared by this process is denser and more uniform.Compared with the C/Cu composite powder with the same graphite content, the dispersion of graphite particles in the copper plated C/Cu composite powder has been improved, especially when the graphite content exceeds 2.0 wt.%.Owing to the copper plating on the graphite surface, which improves the density difference between the two phases and facilitates more diffuse and uniform graphite distribution under the same mechanical energy provided by the equipment.

XRD analysis of copper-plated C/Cu powder
Figure 5 shows the XRD patterns of the original copper powder and the copper-plated C/Cu composite powder prepared by the two-step mixing method.The copper-plated C/Cu composite powder not only detects the characteristic peaks of copper phase (111), ( 200) and (220) at 2θ=43.3°, 50.4°and 74.1°, but also detects the diffraction peak of graphite phase (003) at 2θ=26.5°, and no other diffraction peaks are detected.These indicate that the introduction of impurities can be effectively avoided by using this mixing method.In comparison with the diffraction peaks of the original copper powder, the diffraction peaks were significantly broadened and weakened, where the broadening of the diffraction peaks reflected the refinement of the grain size and the increase of the microstrain, the decrease of the intensity of the diffraction peaks reflected the increase of the lattice defects, and the shift of the diffraction peaks reflected the change of the lattice constants, which indicated the lattice distortion of the powder.Due to the copper powder has good ductility, it is constantly subjected to the collision and extrusion of the high-speed turning grinding balls during the high-energy ball milling process, which causes serious plastic deformation of the powder and generates the large number of lattice distortions, forming dislocation cells, and when the sub-grains between the dislocation cells are oriented to a certain degree, the sub-grains are transformed into grains, prompting the refinement of the grains [18] .

Effect of graphite content on the microstructure of copper plated C/Cu composite powder after SPS sintering technique 3.3.1 Surface topography analysis
In order to study the dispersion of graphite after SPS sintering, the surface morphology of copper-plated C/Cu composites with different graphite contents was observed by BSE, as shown in Figure 6. Figure 6(a) to (d) show the low magnification BSE plots of copper-plated C/Cu composites with graphite contents of 1.0 wt.%, 1.5 wt.%, 2.0 wt.% and 2.5 wt.%, respectively.As can be seen from Figure 6, the gray continuous microstructure is copper and the black dispersion is graphite particles, which are mainly diffusely distributed in the copper matrix in a ribbon shape, and no obvious pores and graphite agglomeration are found.This indicates that the copper-plated C/Cu composites prepared by the two-step mixing method combined with SPS sintering are highly dense and have good graphite dispersion [19] .With the increase of graphite content, the difficulty of graphite dispersion and aggregation in copper-plated C/Cu composites gradually increased, while no obvious agglomeration occurred.These are mainly because of the increase of graphite content , the more difficult the dispersibility of graphite particles at the same mixing process conditions, the possibility of forming aggregates increases.

Microstructure analysis
Figure 7 shows the microstructure of copper plated C/Cu composites with different graphite contents.
As can be seen from the Figure, the gray-white continuous microstructure is the copper matrix, and the black dispersion is graphite, which is mainly distributed at the position of the grain boundary edge, and no obvious pores are observed in the copper matrix, and the pores mainly exist in the area with graphite aggregation.In view of the two-step dispersion of the composite powder, there will still be little graphite aggregation phenomenon.During the SPS sintering process, these aggregated graphite particles can effectively hinder the migration of copper grain boundaries, inhibit the growth of grains, and leave channels at the interface where the graphite aggregates are not completely fused, providing effective conditions for the formation of pores [20] .
The copper grain size of the copper coated C/Cu composites with different contents was counted as shown in Figure8.The copper grain size in the copper coated C/Cu composites showed a trend of decreasing and then increasing with the increase of graphite content.During the SPS sintering process, the graphite particles diffusely distributed in the copper matrix can inhibit the migration of copper grain boundaries, prevent the growth of copper grains, and play a role in refining the grains of the copper matrix.In addition, in the composite powder prepared by the two-step mixing method, the particle size and grain size gradually decrease with the increase of graphite content, which can refine the copper grain size in the final sintered material to a certain extent.However, when the graphite has high content , it is beneficial to obtain the composite powder with finer and more uniform grain size distribution in shorter ball milling time.The same ball milling time for the composite powder means that the exceed longer ball milling time leads to a closer contact of the composite powder and easier heat transfer and atomic diffusion during the sintering process, resulting in a faster grain growth rate and an increase in copper grain size [21] .In order to further investigate the interfacial bonding of copper-plated C/Cu composites, high magnification SEM observation and corresponding surface scanning EDS were performed on copper-plated C/Cu composites with graphite content of 1.0 wt.% and 2.5 wt.%, respectively.In Figure 9(a) and (b), no obvious pores were found at the interfacial bonding of mutual contact, the organization was relatively dense, and the graphite particles were distributed at the copper grain boundaries intersections.The surface scanning energy spectroscopy of the contact area between graphite and copper matrix shows that there are more copper elements in the graphite aggregation area, which indicates that the copper atoms penetrate or diffuse into the graphite area to enhance the interfacial bonding between copper and graphite and improve the comprehensive performance of the composite.

Effect of graphite content on the performance of copper plated C/Cu composite powder after SPS sintering technique 3.4.1 Relative density
Figure 10 shows the trend of relative density of copper plated C/Cu composites with graphite content with 1.0wt.%,1.5wt.%,2.0wt.% and 2.5wt.%graphite content, respectively.In Figure10, the relative density showed gradual decrease with the increase of graphite content.At the graphite content of 1.0wt.%, the relative density reaches maximum of 99.91%; with the gradual increase of graphite content to 2.5wt.%,its relative density decreases to 97.64%, a decrease of 2.3%.It could be the diffusely and uniformly distributed graphite particles hinder the migration of copper grain boundaries during the sintering process, inhibit the growth of copper grains, and easily form pores in the contact area of the two phases, thus reducing the relative density of the composite.However, at a graphite content of 2.5 wt.%, a slight agglomeration of graphite particles occurred in the composites (Figure 7d), and the agglomerated graphite was able to effectively inhibit the growth of copper grains, thus introducing a large number of pores and increasing the decrease in relative density [22] .
The relative densities of copper-plated C/Cu composites were 99.91%, 99.56%, 98.73%, and 97.64% when the graphite content was 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, and 2.5 wt.%.This is an increase of 1.14%, 1.22%, 0.93% and 1.12% compared to the pure C/Cu composites with relative densities of 98.78%, 98.36%, 97.82% and 96.56%, respectively.The copper plating on the graphite surface improves both the wettability between copper and graphite and the degree of dispersion distribution of graphite particles in the composite powder, which promotes the sintering densification process of the powder material, enhances the interfacial bond between the two phases, reduces the porosity, and increases the relative density of the composite [23] .

Conductivity analysis
Figure 11 shows the trend of conductivity of copper-plated C/Cu composites with graphite content.
From the Figure, it can be seen that the conductivity of copper-plated C/Cu composites decreases with the increase of graphite content and the decrease increases.When the graphite content was 1.0wt.%,1.5wt.%,2.0wt.% and 2.5wt.%, the conductivity of copper-plated C/Cu composites was 92.1%IACS, 91.2%IACS, 86.9%IACS and 85.0%IACS.The conductivity of the more pure C/Cu composites was 89.7%IACS, 86.3%IACS, 85.1%IACS and 81.3%IACS by 2.7%, 5.7%, 7.2%, 2.1% and 4.6%, respectively.Compared to the pure copper conductivity of 96.4% IACS, the reductions were 4.5%, 5.4%, 9.9% and 11.8%, respectively.With the increase of graphite content, some graphite aggregation or even agglomeration occurs, bringing in a large number of pores, and the pores and agglomerated graphite can seriously weaken the conductivity of the composite [24] , As the increase of graphite content, the particle size of the copper-plated C/Cu composite powder prepared by the two-step mixing method gradually decreases, which increases the powder bonding interface.The increase of interface will increase the degree of electron scattering, which makes the conductivity decrease further [24] .The conductivity of copper-plated graphite is lower than that of pure copper, and the conductivity decreases gradually with the increase of copper-plated graphite content.Compared with the C/Cu composites with the same graphite content, the conductivity of copper-plated C/Cu composites is significantly increased, thus demonstrating the dispersion and relative density of graphite particles in the composites were improved after copper plating on the graphite surface, which contributed to the increase of the conductivity of the composites.

Hardness analysis
Figure 12 shows the trend of hardness of copper-plated C/Cu composites with graphite content with 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, and 2.5 wt.% graphite content, respectively.From the Figure, it can be seen that the hardness of copper-plated C/Cu composites shows a gradual decreasing trend with the graphite content.When the graphite content was 1.0wt.%,1.5wt.%,2.0wt.% and 2.5wt.%,respectively, the hardness of copper-plated C/Cu composites was 67.7 HV, 62.8 HV, 60.1 HV and 58.3 HV.Compared with the hardness of pure C/Cu composites of 63.9 HV, 58.6 HV, 57.0 HV and 54.8 HV, the hardness increased by 5.9%, 7.2%, 5.4% and 6.4%, respectively.The copper plating on the surface of graphite particles reduces the density difference between the two phases and improves the dispersion distribution of graphite particles in the composite powder, which helps to promote the densification process of the composite powder and increase its relative density, thus enhancing the hardness of the composite.Meanwhile, the copper plating of graphite improves the wettability between copper and graphite, which makes the interfacial bond between the two phases stronger, enhances the interfacial bond strength, also improves the hardness of the composite [25] .

Friction and wear analysis
Figure 13 shows the trend of the average friction coefficient of copper-plated C/Cu composites with graphite content.From the Figure 13, it can be seen that the average friction coefficient of copper-plated C/Cu composites decreases gradually with the increase of graphite content and decreases slowly.When the graphite content increased from 1.0wt.% to 2.0wt.%, the average friction coefficient decreased by 40% from 0.114 to 0.069.As the graphite content increased from 2.0wt.% to 2.5wt.%, the average friction coefficient decreased by 13% from 0.069 to 0.06.When the graphite content was 1.0wt.%,1.5wt.%,2.0wt.% and 2.5wt.%,respectively, the average friction coefficients of copper-plated C/Cu composites were 0.114, 0.091, 0.069 and 0.060.Compared with the average friction coefficients of pure C/Cu composites of 0.119, 0.097, 0.077 and 0.062, which were reduced by 4.2%, 6.2%, 10.4% and 3.2%, respectively.The above datas demonstrate the improved dispersion of graphite particles after copper plating on the graphite surface, which is more uniformly distributed on the surface of the copper matrix.And the better the dispersion of graphite in the copper matrix, the lower its friction coefficient.In addition, the copper plating on the graphite surface improves the interfacial wettability and enhances the interfacial bond strength.During the mutual friction between the composite surface and the counter-abrasive parts, the copper-plated layer can play a role of protecting the graphite, so that the graphite on the contact surface is not extruded, but it can form a continuous and constantly replenished lubrication mechanism, which can effectively enhance the friction coefficient of the composite [26] .As indicated in Table 1, average friction coefficient of the copper-coated C/Cu composites prepared in this work and other literature [27][28][29][30][31] .Cf/Cu/C composites 0.26 [27]   Cu-graphite-Cr3C2 Composites(Air) 0.4644 [28]   Ti2SnC-copper/graphite 0.15 [3]   foam copper-copper/graphite(60%graphite) 0.162 [29]   Ag-Cu-Ni-graphite(1wt%graphite) 0.183 [30]   Cu-Ni-graphite(1wt%graphite) The three-dimensional profile analysis of the wear scar morphology of copper-plated C/Cu composites with different graphite contents is shown in Figure 14.From the Figure 14, the width and depth of the abrasion marks of copper-plated C/Cu composites with different graphite contents can be observed, and the data corresponding to the width and depth of the abrasion marks were obtained after Origin fitting as shown in Table 2. From the table, it can be seen that with the increase of graphite content, the width and depth of the abrasion marks show a trend of increasing and then decreasing.At the graphite content of 2.0 wt.%, the width and depth of abrasion marks of the composites reached the lowest level, 54.1 μm and 6.8 μm, respectively, which indicates that the addition of a small amount of graphite particles can both reduce the friction coefficient and improve the wear resistance of the composites; however, the wear resistance of the composites decreased when the graphite content was higher.It could be the fact that the relative density and hardness of the composites are also important in affecting the wear resistance of the composites The relative density and hardness of the composites decreased more when the graphite content exceeded 2.0 wt.%, and the wear resistance of the composites was proportional to the hardness of the composites, the greater the hardness, the smaller the wear and the better the wear resistance [32] .The surface morphology of the copper-plated graphite composite with graphite content of 2.5 wt.% was selected for the observation of the wear scar morphology, and the surface morphology is shown in Figure15.As can be seen from the Figure 15, the flaky abrasive chips are sporadically distributed on the surface of the wear scar, and the surface of the wear scar is smooth and flat, and the wear mechanism is fatigue wear.Point-scan energy spectrum analysis of the abrasive chips and matrix in the abrasive marks, the energy spectrum results show that: the presence of C, O, Cu and other elements in the abrasive chips and matrix, where the content of carbon elements is high, indicating that in the round-trip friction process to form a complete and continuous carbon film on the surface of the abrasive parts and composite materials, it can effectively reduce the direct contact of the friction sub to the abrasive parts and copper matrix, so that the friction coefficient gradually reduced, so as to play a role of composite material lubrication and friction reduction effect [33] .

Conclusion
( 1 ) The copper-plated graphite was prepared by chemical plating.Besides, the continuous and uniform distribution of the copper-plated layer was found by SEM inspection, and the presence of the characteristic peaks of copper phases (111), ( 200) and (220) was characterized by XRD diffraction patterns, indicating the successful preparation of copper-plated graphite without the introduction of new impurity phases.
(2 )The two-step mixing method using magnetic stirring combined with high-energy ball milling both refined the powder particle size and was able to promote the uniform dispersion of graphite in the copper powder particles, and the graphite copper plating layer did not peel off, indicating the tight bonding of the plating layer.The graphite dispersion in the copper-plated C/Cu composite is relatively uniform, while the grain size is gradually refined and more uniform with the increase of graphite content.Besides, there is elemental diffusion in the interfacial bonding area, and the interfacial bonding is tight.
(3)The relative density, electrical conductivity and hardness of Cu/Cu-plated composites showed a gradual decrease with the increase of graphite content.At 1.0 wt.% graphite content, the relative density, conductivity and hardness were 99.91%, 92.1%IACS and 67.7 HV, respectively.1.1%, 2.7% and 5.9% higher than those of C/Cu composites with the same graphite content, respectively.The average friction coefficient of copper-plated C/Cu composites showed a gradual decrease with the increase of graphite content.Compared with the unplated Cu/Cu with the same graphite content, the average coefficient of friction decreased.

Figure 2 .
Figure 2. XRD of raw graphite and copper-coated graphite

Figure 4 .
Figure 4. EDS of copper-coated C/Cu composite powder

Figure 5 .
Figure 5. XRD of raw copper powder and copper-coated C/Cu composite powder

Figure 10 .
Figure 10.Relative densities of the C/Cu composites and the copper-coated C/Cu composites with different graphite content

Figure 11 .
Figure 11.Conductivity of the C/Cu composites and the copper-coated C/Cu composites with different graphite content

Figure 12 .
Figure 12.Hardness of the C/Cu composites and the copper-coated C/Cu composites with different graphite content

Figure 13 .
Figure 13.Average friction coefficient of the C/Cu composites and the copper-coated C/Cu composites with different graphite content

Figure 15
Figure 15Energy spectrum analysis of wear marks of copper-coated C/Cu composites with graphite content of 2.5wt.%

Table 1 .
Average friction coefficient of the copper-coated C/Cu composites prepared in this work and other literatures.

Table 2 .
Wear depth and width of copper-plated C/Cu composites with different graphite content