A Novel Electrical Contact Material with Improved Self-lubrication for Railway Current Collectors

Da Hai He^1*, Rafael Manory² ,

¹ Visiting Researcher, Current Collection Laboratory, Railway Technical Research Institute

2-8-38, Hikari-cho Kokubunji-shi, Tokyo 185-8540 JAPAN

² Joining and Welding Research Institute, Osaka University

11-1 Mihagaoka, Ibaraki, Osaka 567-0047 JAPAN

Abstract

A new series of copper-graphite composite materials (CGCMs), which show improved electrical conductivity and tribological properties without the need for lubrication, were developed. The materials reported herein were prepared by the powder metallurgy (P/M) route and present a higher density (6.3 ~ 7.6 g/cm³) than other P/M prepared contact materials. The new materials differ from other sintered carbon-copper composite material (CCMs) used as contacts, in which an intermediate alloy is used in order to introduce carbon into the copper phase, resulting in high electrical contact resistance and/or high wear of the counterpart due to hardening of the copper matrix.

The CGCMs exhibit a self-lubricating function, which works by formation of a carbonaceous layer (believed to be graphite) onto the counterpart surface, as confirmed by Auger analysis The coefficient of friction was reduced during the wear test under 13.5N constant normal load without lubrication from 0.220 to 0.185. The wear mechanism of the CGCMs vs. Cu was identified as a combination of mechanisms which changes with variations in composition. The wear on contact wire was below a measurable rate, and the wear rate of CGCM samples against pure copper was also very low, in the range of 3.2x10^-6 to 2x10^-8 mm³/N m.

The high electrical conductivity of the new series of materials is attributed to ‘network conduction’ channels which result in electrical conductivity values of 60% IACS (International annealed copper standard) in some samples.

Based on these properties, the new materials show a clear advantage for use in applications such as pantographs, contact brushes and other electrical contact components.

Keywords:
Contact material, Pantographs, Contact wire, Metallic composite, Friction coefficient. Contact Resistance

1. Introduction
Development of contact materials, including contact wires and pantograph strips for railway power collection systems is aimed to reduce the wear of facilities and the cost of maintenance, as well as to induce efficiency improvements in power transmission. A relatively large number of studies were conducted on contact materials, both strips and wires, and on wear phenomena in the field of railways in the past ten years. In order to meet the requirements of high speed trains, the contact strip for pantographs were subject to increased demands to improve their wear properties. The requirement was that the strip materials should possess a combination of hard particles and solid lubricants in the form of sintered alloys [1]. This opinion has been dominant for a long period in the railway field; however, after several tests on the actual contact strip materials, with a combination of wear couples, such as Cu-based and Fe-based sintered alloys against hard drawn copper and copper alloy wires, it was found that the effects observed in wear characteristics are mainly due to arcing current and adhesion [2]. For the real application, reduction of maintenance cost is a strong requirement, whereas low wear rate of contact wires and contact strip materials is mainly desired [3]. It is well known that to reduce the wear of contact wire the use of carbon as contact strips is effective, but the strength of the carbon strips is not suitable for the application. Therefore, metal/carbon composites are considered the ideal materials to be used in high speed railway [4]. The difference between ‘carbon’ and ‘graphite’ lies in the synthesis process. The graphitic form of carbon is obtained after a heating treatment under 2500°C and the two appear to have different performance in wear response and electrical conductivity. Graphite has a lamellar structure and is commonly used as solid lubricant and its electrical resistivity is relatively low, about 0.001% IACS..Most of the carbon block strips, which are widely used in the world, especially in European lines, are using composites in carbon (not graphite) form.

The task of increasing the speed of the trains has been made difficult by problems of wear and electricity collection capability. In various studies, the wear of contact strips increased with increasing power collection, and it was therefore necessary to suppress this wear as much as possible [5]. It was also found that the effective methods to be used to decrease the wear rate of the contact wire involve decreasing the heat generated on the sliding surface and contact pressure in a range which does not cause contact loss with the trolley wire [6].

These works have generated a vast body of knowledge regarding the wear and electrical properties of contact strips, and they still serve as guidance for further research work on contact materials. . To meet the requirements of high electrical conductivity, low friction coefficient, and high abrasion resistance, a metal matrix of copper/graphite composite can provide a reasonable good electrical conductivity and self lubrication, but there is a difficulty in mixing these two elements, namely the insolubility of graphite by copper [7].

As a primary result of the present work conducted in Melbourne, Australia, with the clear aim of improving wear and electrical conduction performance, a new series of copper and graphite composite materials (CGCMs) was developed. These new materials also differ from the sintered carbon-copper composite material (CCM) which use iron as an intermediary material in the copper alloy used to introduce carbon into copper by carbide decomposition [7]. The new series of materials exhibit a self-lubricating function, which works by formation of a carbonaceous graphitic layer onto the counterpart’s surface.

Using a simple powder metallurgy (P/M) route under non-oxidizing conditions, a production process was conducted based on the concept of using the combined properties of composite, copper matrix for electric conduction and self-lubricant storage (pores) for wear response. Also, special methods, which will not be detailed here, were used to achieve a high bulk density of the materials..

This paper deals with the microstructure-properties relationship of the new series of materials and presents laboratory test results for friction, wear and electrical conduction. The new series of materials (CGCMs) presented here were developed and optimised under specific parameters of wear resistance, electrical conductivity, thermal conductivity and physical stability for the purpose of use as current collectors in trains and trams (pantographs and pole shoes).

2. Experimental details
2.1 Specimens

2.1.1 Preparation
The samples of CGCMs used in this study were made by powder metallurgy and the process included a special powder treatment in non-oxidizing conditions, compacting and sintering in protective atmosphere. The route of the production is simple and is based in principle on a method to mix graphitic form of carbon, into the copper matrix in non-oxidized condition. The compacts of CGCMs were consolidated in a complex process related with the physical and chemical reactions detailed elsewhere [8] by using a control method of semiliquid phase sintering, and partial gas pressure for copper oxide reduction with hydrogen and carbon monoxide. To encourage the metal (copper) to flow during the sintering stage, a small amount of Sn (or Zn - another low melting temperature metal) was added in the mixing stage before the compacting stage [9]. Given in Table I are three typical chemical compositions of CGCMs and their measured properties.

TABLE I

2.1.2 Materials
The specimens were 16.1mm in diameter and about 30.0mm in length, which was the size after sintering. A flat end surface was kept in clean condition for the wear test. The carbon-copper composite materials (CCMs) used in this study are currently used on trains in Victoria, Australia, and the chemical composition of CCM is shown in Table II. A large CCM block used for pantograph strip was cut to the size of 25.0mm x 15.0mm x10.0mm to be fitted into the sample holder of the wear tester.

TABLE II

The contact wire used in this study was a hard-drawn pure copper trolley wire of 1.61x10-4 m² cross-sectional area in accordance with the product standard BS 23:1970. Also some pure copper blocks (99.99% purity, with 100% IACS) cut in right size were used for the contact counterparts in this study. All the samples were cleaned by using #600 sand paper in dry condition.
The CALTEX TL750 grease used for lubrication in this study is commonly used as a high viscosity mechanical lubricant. A specially designed graphite grease used as a lubricant in this study is made of 1-2mm graphite powder in 85%wt mixed with a commercially available common grease in 15%wt to form a solid block.

2.3 Wear test
The wear tests for this study were performed on a specially designed horizontal sliding wear tester and its schematic layout is shown in Fig. 1a. This wear tester has been described in detail elsewhere [10]. The tester is designed to simulate as closely as possible the relative condition of the actual tribo-system of a contact material sliding on a static wire. For metallic contact counterparts, it is capable of examining wear and electrical properties at the same time.
In this design the contact wires are fixed between the fixtures under a tension of 25% rupture strength of the wire. No support is applied at the back of the contact wires during the wear test. The contact materials are fixed in the specimen holders which are joined with a weight applied by a steel plate driven back and forth by a moving arm.. All the specimens had a flat contact surface sliding on the round contact wire surface. This wear tester was operated in a slow horizontal sliding speed of 0.25m/s. Due to restrictions on power supply in the Laboratory, the wear couples were charged with 15A/6V power. The normal load applied for the wear test was constant at about 13.5N. The friction force was measured by a strain gauge bridge placed on a spring plate placed in between each individual sample holder and the loading plate. The signals from strain gauges of each wear couple were collected by a data acquisition system, and the friction coefficient of each wear couple was calculated. The measuring response rate was about 1/100 s, and the accuracy was about 1/1000.

2.3 Electrical contact resistance measurements
The electrical contact resistance between the interfaces was measured in accordance with ASTM B539-90, the standard test method for measuring contact resistance of electrical connections (static contacts). This method is particularly suitable to deal with the difficulty of measuring small contact resistance. The circuit was calibrated in accordance with a standard series of resistors with an accuracy of 1/1000.
The method is difficult to use in wear test conditions, because of the low response rate of the current directional switch. Therefore, the circuit was modified by using a variable frequency (20 ~ 50Hz) AC power supplier ( Fig. 1b). This was done by using a high speed data logging system to collect bi-directional values of the voltage between the test interfaces, after which the contact resistance calculation data was processed by a computer. The details of this device are described in reference [11].The resistivity of materials in this study was calculated from the voltage and current measurements in accordance with ASTM B193-95.

2.4 Inspection
The microstructures of the CGCMs were observed under a Zeiss optical microscope after metallographic preparation. An SEM (HITACHI S-520) was used for observation of the worn surfaces. These observations were performed without cleaning (in ‘as is‘ condition) in order to observe all the features on the surface including the wear scar and wear debris on the tribo-faces. 3D surface modelling was scanned using Strata 3D computer software. Auger Electron Spectroscopy (AES) was used in this work to analyse the chemical composition of the top surface of worn contact wires. The measurement of wear rate of CGCM blocks was based on the weight loss measured by 1/1000 gr high precision scale during wear test period.

3. Results and discussion

3.1 Microstructure of CGCMs
Fig. 2 shows the microstructure of CGCMs taken after metallographic preparation. As shown, the CGCMs’ microstructure consists of an irregular copper matrix, with graphite or/and MoS2 fine powders stored in the pores (the black spots). During the sintering process, the flowing copper particles were connecting with each other to increase the neck region [9,12] and this effect pushed the graphite powder to be stored into pores, resulting in a microstructure which resembles nodular cast iron, but with smaller pores.
These microstructures show the key elements for improved tribological and electrical performance of these material: The copper matrix remains basically unalloyed in the P/M process, thus maintaining its high conductivity, and the graphite stored in the pores provides lubrication during sliding.

3.2 Friction behaviour of CGCMs
The friction coefficient of CGCMs against copper wire compared with other couples in the same test condition in previous study [13] is shown in Fig. 3.
For the friction test the data was collected over a short sliding distance in static condition, i.e in constant movement in one direction, and the result presented is averaged . As shown in Fig. 3, the friction coefficient of CGCM against copper, which is about 0.185 in the steady-state stage, is lower than the rest of the contact couples at later stage of friction. Although, there was a loss of graphite powder due to the curved contact wire surface, a dark layer on contact wire surface has still formed during the short distance sliding. The friction coefficient of CGCMs against copper was similar to that obtained by using graphite grease as lubricant in a previous study [13]. The similarity of the CGCM result with that obtained under lubrication with graphite powder, combined with Auger analysis results (to be shown later) is a strong indication on the presence of graphite at the contact interface. It is worth noting that the coefficient of friction of CGCM against Cu is lower than in the case of Cu vs Cu lubricated with TL750 grease. The low friction coefficient is attributed to the microstructure which provides a mechanism of self lubrication with graphite during sliding [14].

3.3 The graphitic layer formation
The topography of a worn contact wire surface after 106 cumulative cycles sliding with the CGCM samples is shown in Fig.4 (a) and a 3D computer model showing the topography of the worn surface in the vertical direction of SEM micrograph is shown in Fig.4 (b).
As shown in Fig.4 (a), the actual wear scar covered a small curve area of the top contact wire, and the width of the wear scar is only about 5.00mm, less than 1/3 of the height of the contact wire. Numerous grooves can be observed on the worn surface that are attributed to ploughing and scratching in abrasive wear. The 3D computer model in Fig. 4 (b) shows the depth of the groove to be less than 40 mm. This 3D model is based on the contrast of the topography and contains a certain unavoidable error , but it gives an image of the worn surface. Although, there were many small scattered particles over the entire sliding surface appearing in bright contrast, it is still hard to see a surface layer. One fact is clearly shown, that the top surface was soft and was easily cut into by copper particles. Fig.5 shows the surface of the worn contact wire in cross sectional view in large magnification taken by SEM, and proves these grooves are just within the soft non-metallic surface layer (shown in bright contrast) which is a carbonaceous layer believed to be graphite.
This layer structure of graphite formed with the pattern similar to the grooves could be observed on the micrograph of the worn wire shown in Fig.4 (a). It should be noted that the contact wire has preserved its original dimension and the sample has a perfectly round and clear boundary line of the contact wire surface. The wear loss of the contact wire was too small to weigh.
Hence, the layer protects the contact wire while in the same time providing full electrical contact through in two ways – i) it can be scratched easily and maintains the metallic contact by copper asperities ii) graphite itself is considered a partially conductive material (has high resistivity, but is better than various greases). A number of studies have indicated that a self lubricant material can be developed in the way that produces a low surface binding energy to adhere to its substrate [15–17]. In this case, the requirement of reduction in surface energy is achieved by the absorption of a carbonaceous, believed to be graphitic (probably also containing some MoS2) layer on the contact wire surface, thus the sliding between layers occurs at low friction and low shear stress. It should be noted that the adhesion versus cohesion balance is often not well defined [17].
Auger electron spectroscopy (AES) analysis was performed over a small worn area of the tested contact wire. Fig 6 shows the Auger analysis result of the sub-surface area of the same specimen of the contact wire (copper) taken after 106 cumulative wear cycles sliding against the CGCM sample. As is common in this technique, a sputter ion gun (using argon as the sputtering gas) was used to sputter away successive layers of the worn surface to obtain the concentration-depth profile [18] shown in Fig.6 .
As shown, the chemical composition in a small sub-surface area of the contact wire contains the following elements: C, Mo (very small amount, from MoS₂), N, O (from air contamination in small amount), and the balance is Cu. In the spectrum presented here carbon clearly covers the surface: it initially constitutes about 90%wt of the top substrate in the early stage of sputter time and eventually stabilizes at a constant concentration below 20%wt. which extends beyond the analysed spectrum. The AES result indicates the presence of a carbon layer on the top surface . Combined with the static friction coefficient results shown in the previous section, there is no doubt that the low friction is due to the thin carbonaceous layer which is believed to be graphitic because of a number of reasons: a) its origin is the graphite content of the pores, and b) because graphite itself has a layered structure with a well documented tendency to smear on the surface and c) the low friction coefficient observed with CGCMs is similar in value to that obtained for copper vs copper with addition of graphite lubricant. The tribological effect of the carbonaceous layer was to reduce both friction and wear. The behavior of these materials in prolonged wear tests is discussed in the following section.

3.4 Wear tests and dynamic friction coefficients
Fig. 7 shows the dynamic friction coefficients of these specimens of CGCMs during 106 cumulative wear cycles.
These results are higher than the static condition (sliding in one direction for a short distance) due to the effect of dynamic variation during data collection in forward and backward sliding period. The data was collected in positive and negative values, but the final data is presented only in the positive direction. Fig.7 shows that the CGCMs have had a lower friction coefficient than CCM in the same test condition. Increasing the graphite content in CGCMs results in a significant reduction of friction due to increase in the self-lubrication function and the dynamic friction coefficient value varies from 0.256 to 0.185. The data fluctuation of CGCMs against copper is smaller than CCM Vs Cu in the same condition.
The wear loss of contact wire was in unmeasurable amount during sliding, therefore it is difficult to present the wear rate of the tested contact wires. Fig. 8 shows the wear rate of CGCMs sliding against contact wires in accordance with the weight loss of the specimen collected from the wear test. The data was calculated by Archard’s equation [19] and the weight loss of CGCMs during wear test.
There is a geometric effect of the round shape of the wire which increases the normal stress at the beginning, and removal of the material is more severe initially. The data shown in Fig.8 also indicates that high graphite content samples may not have a low wear rate due to brittle behaviour. The wear rate of CGCMs was from 3.2 x 10^-6 to 2 x 10^-8 mm³/N m, which is significantly lower than expected, and is similar to a wear results in good oil lubrication observed for hard metals. This superior performance of CGCM is quite surprising and unique and is attributed to a low shear stress between the surface (graphitic) layers on the counterparts and to the strong bonding of the layer with the metallic surface. Analysis of the wear mechanisms in this low wear situation will be presented in the next section.

3.5 Wear mechanisms
Fig. 9 shows SEM micrographs of the worn surface of the contact wire after 106 wear cycles against the sample of 92%Cu; 7%Graphite; 0.5%MoS2; 0.5%Sn and of flake like wear debris collected during the wear test.
Fig.9 (a) shows a large particle which was pulled off from the contact wire surface, and also a number of cracks can be noticed on the worn surface. The worn surface shows grooves containing debris of metallic and graphitic particles spread over the entire interface. A metallic particle lifted off the surface as a result of metallic contact can also be observed. Overall however, the appearance of the worn surface is not like that of a pure metallic contact such as Cu vs Cu in dry condition [13], but is only partly metallic. In Fig.9 (b), flake-like debris can be observed, in which some particles appear to be covered by graphitic powder. This appearance of wear debris, particularly the pulled-out particle, can only be generated by an adhesive wear mechanism. The grooved appearance on the other hand indicates an abrasive behaviour during sliding. Abrasion has also occurred on the soft non-metallic (graphitic) layer by copper asperities and the presence of the layer itself, which was discussed above, evidently shows lubrication between counterparts. Therefore, it can be stated that the appearance of the worn contact wire surface was the result of a combined wear mechanism, consisting of adhesion and abrasion. With varying C content in CGCM the mechanisms remain the same but their relative percentage in the combined mechanism changes. As shown in Fig.7, the dynamic friction coefficient is slightly higher in this high copper content sample than in other samples due to an adhesive wear mechanism domination. The dynamic friction coefficient is still lower than the value for unlubricated metal-to-metal (copper) case, under the same test condition in previous study [10]. This result also indicates that the higher metallic content (copper) in CGCMs will cause higher friction coefficient during sliding.
Fig. 10 shows an SEM micrographs of the worn surface of the contact wire after 106 wear cycles against the sample of 87%Cu; 11%Graphite; 1.5%Zn; 0.5%Sn and agglomerate debris collected during sliding.

The worn surface of the contact wire shown in Fig.10 (a) has little difference from Fig.9(a). A large removed particle and cracks can be seen on the grooved worn surface, but the material removed appears to be the result of adhesion from a soft substrate, and there is no metallic pull-off. In Fig.10 (b). a large agglomerate debris is observed which matches the large removed particle shown in Fig. 10 (a). It shows many small flake like particles that have agglomerated to form a large ‘pile’ of wear debris. The appearance of the worn surface of the contact wire supports the wear mechanism mentioned before, but agglomeration has been involved. The shape of this agglomerate of debris is not like ‘cylindrical with a hemispherical tip’ model suggested by Oktay and Suh [20], , but shows the weak strength of the agglomerate mixed with graphite. This CGCM contains a higher amount of graphite and here it seems that copper asperities abraded the soft non-metallic (graphitic layer) causing the grooved appearance, and in the same time some asperities made contact in adhesion causing metallic particle removal. Metallic and non-metallic particles were mixed together during sliding to form agglomerate and digging a large groove on the interface. Because of the continuos graphite supply, the layer builds again at the interface aand the process is repeated This results in low static and dynamic friction coefficients (a low value 0.190 of dynamic coefficient is shown in Fig. 7) as well as a . Once again, the appearance of the worn surface shows the influence of graphite content on the wear mechanism.

Fig. 11 shows SEM micrographs of the worn surface of the contact wire after 106 wear cycles against the sample of 75%Cu; 15%Graphite; 10%MoS₂ and collected wear debris. Fig. 11 (a) shows the wear behaviour result of increasing the content of graphite and MoS₂. Large particles removed from interface and deep grooves appear on the worn surface. Also, a large amount of small graphite powder and wear particles are scattered all over the entire interface. It should be noted that MoS₂ performs the same function as solid lubricant as graphite. This appearance of the worn surface is similar as the result of the wear mechanism mentioned above, but deep grooves point out that the non-metallic layer (which is now made of graphite + MoS₂ ) is now thicker due to high graphite and MoS₂ content. Therefore, the friction coefficient of this couple could be lower than for other couples because metal-to-metal contact is prevented. Fig. 7 shows a value of 0.185 for the dynamic friction coefficient . In Fig.11 (b), flake like wear debris is observed. This also indicates that the wear mechanism is close to abrasion. This result is very encouraging, as it indicates that the self-induced lubrication reduces the friction and maintains it at a constant level.

3.6 Electrical performance
Fig.12 shows the approximate volume resistivity of CGCMs, calculated by the data from the electric contact resistance collected during wear test. The approximate volume resistivity of CGCMs presented in the chart was averaged over more than twenty measurements performed by varying the applied voltage and current intensities. As shown in Fig.12, the approximate volume resistivity of CGCMs is equivalent to IACS values between 55-80%. The same device was used to measure the resistivity of a pure copper block with a similar size to that of CGCM blocks, and the result was reasonably close to the standard value (100%IACS).
It can therefore be stated that CGCMs have a much lower resistivity than other composites made by metallic impregnation and measured by the same method:the resistivity of CCMs used on pantographs in Victoria, Australia, is about 10.5mWm. The CCM block used on pole shoe collectors is about 32.0mWm. The improved low resistivity is attributed to a unique electrical conductive mechanism termed ‘network conduction’, in which the conduction occursthrough the matrix of pure copper, with very little effect from the second phase of graphite.
Fig. 13 shows the dynamic contact resistance between contact wires and CGCMs, monitored during the wear test. In general, as shown in Fig.13, CGCMs exhibit good electrical conduction and electrical performance. The data has some noise due to vibration during data collection in the sliding test, and might also be the effect of wear debris formation on the interfaces. This dynamic contact resistance reflects the wear condition:, as indicated in Fig.13, the higher copper content CGCM is closer to pure copper performance, but the data fluctuation is much more severe than that for CGCM with higher C content, and this is attributed to an adhesive wear mechanism and to the effect of oxidized wear debris which has a much higher resistivity than copper. A CGCM with high copper-content will have less pores to transfer the graphitic layer onto the interface (copper surface), and the metal-to-metal contact will exhibit an adhesive wear behaviour. Such behaviour was shown to have a major role in pure copper sliding in previous study [10, 11, 13].
The high-graphite-content CGCM presents a number of high peaks due to the contact loss at the metallic surface, and the high contact resistance of the graphitic layer. The metallic contact was interrupted during the wear cycle due to coverage of the metal by the graphitic layer which has a much higher contact resistance. The graphitic layer does affect the contact resistance due to its high resistivity, but overall the low electrical contact resistance and good conductivity of CGCMs are attributed to the fact that the highly conductive copper matrix is continuos and that the bulk copper phase remains practically unalloyed by the graphite addition.

4. Conclusions
In this paper, a new type of copper-graphite composite materials, CGCMs, was presented. These materials use the concept of a network structure of copper matrix for the electrical conduction channel, and the tribological response, which is the self-lubrication, depends on the graphite and MoS₂ powders stored in the pores.
The experimental results lead to the following conclusions:

• For the first time a process has been designed to successfully form an alloy of copper with graphite.
• The unique microstructure of CGCMs, which is composed of a graphite islands in a copper matrix achieves the solid lubrication function with very little effect on electrical conduction during sliding.
• A carbonaceous layer believed to be graphite is formed on the contact wire surface by sliding, and its thickness is correlated with the graphite content in CGCMs.
• The layer transfer function of CGCM can reduce wear and provide protection to contact wires.
• CGCMs are self lubricating materials which also exhibit a special electrical conduction mechanism.
• A special electrical conductive mechanism – namely network conduction plays a major role in maintaining the low resistivity of CGCMs.
  

Acknowledgements
Some information contained in this paper is proprietary to M&H Materials (Australia). The authors are grateful to Drs. Hiroki Nagasawa, and Shunichi Kubo from RTRI, Japan, for their help in proofreading this manuscript and providing help with the literature survey of the field.

Figures

Fig. 1a. The schematic layout of the wear tester

Fig. 1b.Layout of electrical tests

Fig.2. Micrographs of CGCMs at x50 magnification

Fig.3. Friction coefficients of current collectors against contact wires

Fig.4. Micrographs of worn scar and layer’s 3D computer model after 10⁶ cycles sliding (a)    Worn scar. (b) Worn surface of contact wire. (c) 3D computer model.

Fig.5. SEM micrograph of graphite (and MoS₂) layer structure, showing the surface zone of contactwire at large magnification indicating that the grooves are just within the graphite layer.

Fig.6. Auger analysis result of worn wire surface after 10⁶ cycles sliding with CGCMs a) surface b) depth profile. The high C concentration on the surface is evident.

Fig. 7. The dynamic friction coefficient of CGCMs

Fig. 8. The wear rate of CGCMs sliding against pure copper contact wires

(a)                                                                    (b)

  Fig.9. SEM micrographs of worn surface of the contact wire after 10⁶ cycles against CGCM (92%Cu sample) and collected wear debris

(a)     Worn surface of the contact wire; (b) Wear debris collected form the wear couple (92%Cu CGCM Vs Cu).

(a)                                                                                                                                         (b)

Fig.10. SEM micrographs of worn surface of the contact wire after 10⁶ cycles against CGCM (87%Cu sample) and collected wear debris

(a)     Worn surface of the contact wire; (b) Wear debris collected from the wear couple (87%Cu CGCM Vs Cu).

Fig.11. SEM micrographs of worn surface of the contact wire after 10⁶ cycles against CGCM (75%Cu sample) and collected wear debris

(a) Worn surface of the contact wire; (b) Wear debris collected from the wear couple (75%Cu CGCM Vs Cu).

Fig. 12. The electrical property of CGCMs

Fig. 13. Dynamic contact resistance of CGCMs against contact wires during wear compared with pure copper. Conditions: 13.5N normal load at 0.25m/s dry sliding speed, room temperature (20^°C).

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