Friction and Wear Properties of Copper/Carbon/RB Ceramics Composite Materials under Dry Condition

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1 Tribology Online, 3, 4 (28) ISSN DOI /trol riction and Wear Properties of /Carbon/RB Ceramics Composite Materials under Dry Condition Kei Shibata 1)*, Takeshi Yamaguchi 1), Junichiro Mishima 2) and Kazuo Hokkirigawa 1) 1) Graduate School of Engineering, Tohoku University Aramaki Aza-Aoba, Aoba-ku, Sendai, Miyagi , Japan 2) East Japan Railway Company 2-2 Yoyogi 2-chome, Shibuya-ku, Tokyo , Japan *Corresponding author: shibata@gdl.mech.tohoku.ac.jp ( Manuscript received 19 March 28; accepted 19 June 28; published 15 July 28 ) In order to realize reducing wear of both overhead wires and pantograph sliders, the authors have developed new composite materials for pantograph sliders by using hard porous carbon materials RB ceramics. The new composite materials were developed by sintering compounds of copper, carbon and the RB ceramics particles. riction and wear properties of these copper/carbon/rb ceramics composite materials (Cu/C/RBC composites) sliding against a copper alloy pin under dry condition were investigated. The friction coefficient for the Cu/C/RBC composites was lower and more stable than that for the conventional pantograph slider material,. The friction coefficient for the Cu/C/RBC composites was about 1/2 ~ 3/5 of that for the. The specific wear rate of the Cu/C/RBC composites was extremely lower than that of the. The specific wear rate of the Cu/C/RBC composites was decreased with a decrease of the weight fraction and the mean particle size of the RB ceramics particles. The specific wear rate of the Cu/C/RBC composites was about 1/22 ~ 1/7 of that of the conventional. urthermore, the specific wear rate of a copper alloy pin sliding against the Cu/C/RBC composites is about 1/17 ~ 1/15 of that sliding against the conventional. Keywords: pantograph slider, friction, wear, RB ceramics, composite 1. Introduction In recent years, a reduction of maintenance costs of pantograph as well as other railway equipments has been desired. Several characteristics such as low electrical resistivity, high strength, high wear resistance, low aggressivity to overhead wires and high economic efficiency, etc. are required for pantograph sliders for railways in current collector system as shown in ig. 1. A copper/carbon composite material () has mainly been used as the pantograph sliders in conventional lines as shown in Table 1. However, maintenance costs of the sliders are still a majority of those of conventional railway-lines facilities. Based on the above background, development of a new pantograph slider material which has higher wear resistance and lower aggressivity to overhead wires than the conventional material has been required 1). On the other hand, Hokkirigawa, et al. developed hard porous carbon materials RB ceramics made from rice bran 2-5). The RB ceramics particles are prepared by carbonizing the mixture of defatted rice bran and phenol Overhead wire Sliders 5 mm ig. 1 Schematic diagram of pantograph and overhead wire Table 1 Conventional pantograph slider materials Railway lines Conventional railway lines Shinkansen Others Pantograph slider materials /carbon composite, Sintered alloy (copper) Sintered alloy (iron) Sintered alloy (copper), Pure carbon, etc. Copyright 28 Japanese Society of Tribologists Tribology Online, Vol. 3, No 4 (28) / 222

2 riction and Wear Properties of /Carbon/RB Ceramics Composite Materials under Dry Condition Table 2 Composition ratio and mechanical or electrical properties of Cu/C/RBC composites Sample number A B C D E G H (conv.) Mean particle size of RBC d m, μm RBC α Weight fraction, Carbon β wt.% γ Density ρ, g/cm 3 Electric resistivity R, μωm Bending strength σ B, MPa Shore hardness HS I particles Mixing Sintering RB ceramics particles 2 mm Cu/C/RBC composites Carbon particles ig. 2 Schematic diagram of manufacturing process of Cu/C/RBC composites resin at 9 C in an atmosphere of nitrogen. The RB ceramics are composed of a soft amorphous carbon corresponding to carbonized defatted rice bran and a hard grassy carbon corresponding to carbonized phenol resin with many pores. The RB ceramics show superior tribological properties such as low friction, high wear resistance and low aggressivity to mating materials under dry condition. In addition, they have positive velocity dependency with a friction coefficient resulting in the reduction of a frictional vibration and a friction noise due to stick-slip motion. By using such superior tribological properties of the RB ceramics, new composite materials by mixing the RB ceramics particles with thermoplastic or thermosetting resins 6,7), rubbers 4,5), steels or ceramics such as silicon carbide 8,9) have been developed. Thus, an improvement of wear resistance of the pantograph sliders and the overhead wires would be expected by developing new pantograph slider materials using the RB ceramics. The purpose of this study is to develop new composite materials composed of copper, carbon and the RB ceramics particles (Cu/C/RBC composites), and to clarify their friction and wear properties under dry condition. 2. Development of Cu/C/RBC composites igure 2 shows a schematic diagram of manufacturing process of the Cu/C/RBC composites. Compounds of copper, RB ceramics and carbon particles were sintered. Table 2 shows composition ratio and mechanical or electrical properties of the Cu/C/RBC composites. Three kinds of the RB ceramics particles with the mean particle size of 4.9, 3.2 and 82.8 μm were used respectively. The weight fraction of copper particles was fixed to 6 wt.%. (a) (conventional material) Pin specimen (φ = 2mm) riction coefficient μ ig. 3 SEM images of disk specimens Stage 1μm Normal loadw = 9.8 N Disk specimen Pin specimen : alloy Disk specimen : Cu/C, Cu/C/RBC composites Normal load : W = 9.8 N Sliding velocity : v = 3. m/s A () D C (b) Cu/C/RBC composite (α = 5 wt.%, d m = 4.9 μm) Sliding velocity v = 3. m/s Torque meter Amplifier Recorder ig. 4 Schematic diagram of pin on disk friction apparatus.2 I E B H Number of repeat passages N, 1 4 cycles ig. 5 Relationship between number of repeat passages and friction coefficient under dry condition G Jig 1μm Japanese Society of Tribologists ( Tribology Online, Vol. 3, No 4 (28) / 223

3 Kei Shibata, Takeshi Yamaguchi, Junichiro Mishima and Kazuo Hokkirigawa riction coefficient μ Weight fractions of RBC particles : α = 5, 1, 2 wt.% Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles α = 5 wt.% α = 1 wt.% α = 2 wt.% riction coefficient μ Mean particle size of RBC particles : d m = 4.9, 3.2, 82.8 μm Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles d m = 4.9 μm d m = 82.8 μm d m = 3.2 μm Mean particle size of RBC particles d m, μm (a) Weight fraction of RB ceramics particles (b) Mean particle size of RB ceramics particles ig. 6 Relationship between weight fraction or mean particle size of RB ceramics particles and friction coefficient Specific wear rate of disk ws_disk, mm 2 /N Mean particle size of RBC particles : d m = 4.9, 3.2, 82.8 μm Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Cu/C composite d m = 82.8 μm d m = 4.9 μm Number of repeat passages : N = cycles d m = 3.2 μm Specific wear rate of pin ws_pin, mm 2 /N d m = 4.9 μm d m = 82.8 μm Mean particle size of RBC particles : d m = 4.9, 3.2, 82.8 μm Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles d 1-1 m = 3.2 μm (a) Disk specimen (b) Pin specimen ig. 7 Relationship between weight fraction of RB ceramics particles and specific wear rate of disk and pin specimens Specific wear rate of disk ws_disk, mm 2 /N Weight fractions of RBC particles : α = 5, 1, 2 wt.% Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles α = 2 wt.% α = 1 wt.% α = 5 wt.% Mean particle size of RBC particles d m, μm (a) Disk specimen Specific wear rate of pin ws_pin, mm 2 /N Weight fractions of RBC particles : α = 5, 1, 2 wt.% Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles α = 5 wt.% α = 1 wt.% α = 2 wt.% Mean particle size of RBC particles d m, μm (b) Pin specimen ig. 8 Relationship between mean particle size of RB ceramics particles and specific wear rate of disk and pin specimens The conventional pantograph slider material, Cu/C composite, was also prepared. igure 3 shows SEM images of the Cu/C and the Cu/C/RBC composites. As shown in ig. 3, it can be observed that each particle was uniformly dispersed. These composites were cut into 5 mm 5 mm 1 mm disk, and then their test surfaces were finished by grinding. The surface roughness of the disk surface R a was 3.8 ~ 6.62 μm. 3. Experimental procedure igure 4 shows the pin on disk friction apparatus used in this study. The and the Cu/C/RBC composites were used as disk specimens, and a copper alloy (Cu:99.7±.5%, Sn:.3±.5%) used for actual overhead wires was used as a pin specimen (φ = 2 mm). Pantograph slider is slid continuously against overhead wires, thus pantograph slider materials should be used for pin specimens. However, it was difficult to cut the actual overhead wire (φ = 12 ~ 16 mm) into disk geometry (5 mm 5 mm 1 mm). Therefore, a copper alloy pin specimen was cut from the actual overhead wire into pin geometry in this study. The surface roughness of the pin surface R a was.37 μm. Each specimen was ultrasonically cleaned for 15 minutes in hexane. The specimens were subsequently deaerated for 5 minutes and then subjected to testing. A normal load W was 9.8 N (an apparent contact pressure P was 3.12 MPa), a sliding Japanese Society of Tribologists ( Tribology Online, Vol. 3, No 4 (28) / 224

4 riction and Wear Properties of /Carbon/RB Ceramics Composite Materials under Dry Condition Mean particle size of RBC particles dm, μm Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles < ws_disk ws_disk < ws_disk Unit : mm 2 /N < ws_disk < ws_disk < ws_disk ws_disk (a) Specific wear rate of disk specimen (b) Specific wear rate of pin specimen ig. 9 Distribution of specific wear rate as a function of weight fraction and mean particle size of RB ceramics particles velocity v was 3. m/s (a rotation speed was 191 rpm, a rotation radius was 1.5 mm). Thus, Pv value was 9.36 MPa m/s which is equivalent to actual use of pantograph slider. riction tests were conducted under dry condition at room temperature (21±3 C) and 65±5% of relative humidity. The number of repeat passages was cycles. 4. Experimental results igure 5 shows the relationship between the number of repeat passages and the friction coefficient for each disk specimen. As shown in ig. 5, the friction coefficient for the (sample A) takes high value over.5 at the initial stage, then increases linearly with respect to the number of repeat passages. At 34 cycles, the value of friction coefficient for the is.62. At this moment, a severe vibration of the apparatus occurred, so the friction test was stopped. On the other hand for the Cu/C/RBC composites, the friction coefficient decreases at the initial stage of friction, then gradually increases with the number of repeat passages. It can be seen in ig. 5 that the friction coefficient for the Cu/C/RBC composites takes lower value than that for the. igure 6 shows the relationship between the weight fraction or the mean particle size of the RB ceramics particles and the friction coefficient obtained just before the friction tests terminated. As shown in ig. 6(a), the friction coefficient for the Cu/C/RBC composites takes lower value than that for the and keeps a constant value irrespective of the weight fraction of the RB ceramics particles. It can be seen in ig. 6(b) that the friction coefficient for the Cu/C/RBC composites slightly increases with an increase of the mean particles size of the RB ceramics particles. igure 7 shows the relationship between the weight fraction of the RB ceramics particles and the specific wear rate of a disk or a pin specimen. It can be seen in ig. 7(a) that the specific wear rate of a disk specimen made of the Cu/C/RBC composites disk takes lower Mean particle size of RBC particles dm, μm ws_disk, mm 2 /N Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles < ws_pin Unit : mm 2 /N < ws_pin < ws_pin < ws_pin 1-9 < ws_pin ws_pin 1-9 (conventional material) < ws_pin 1-9 < ws_pin ws_pin 1-9 Cu/C/RBC composite (α = 5wt.%, d m = 4.9μm) ws_pin, mm 2 /N.8 μ ig. 1 Values of friction coefficient, specific wear rate of disk specimen and that of pin specimen for three composite materials.2 Cu/C/RBC composite.4 (α = 5wt.%, d m = 3.2μm) Sliding velocity : v = 3. m/s Normal load : W = 9.8 N Number of repeat passages : N = cycles value than that of the and increases with an increase of the weight fraction of the RB ceramics particles. The specific wear rate of the takes a high value of around 1 mm 2 /N which is greater than that of an actual pantograph slider made of the ( ~ mm 2 /N). It is considered that an apparent contact pressure in this study (3.12 MPa) higher than in actual use condition (.3 MPa) causes an increase of real contact area, especially an increase of contact area between Cu and Cu resulting in a severe adhesion. Thus, the shows high specific wear rate. However in such a severe condition, the specific wear rate of the Cu/C/RBC composites shows lower value than that of the. As shown in ig. 7(b), the specific wear rate of a pin specimen made of copper alloy sliding against the Cu/C/RBC composites takes lower value than that of the. igure 8 shows the relationship between the mean particle size of the RB ceramics particles and the specific wear rate of a disk or a pin specimen. As shown in ig. 8, the specific wear rate of the Cu/C/RBC Japanese Society of Tribologists ( Tribology Online, Vol. 3, No 4 (28) / 225

5 Kei Shibata, Takeshi Yamaguchi, Junichiro Mishima and Kazuo Hokkirigawa SEM SEM 1 μm 1 μm 1 μm C : Blue Cu : Yellow 1 μm C : Blue Cu : Yellow P : Red (a) (b) Cu/C/RBC composite (α = 5 wt.%, d m = 4.9 μm) ig. 11 SEM images and elemental analysis of worn surface of and Cu/C/RBC composites disks 6 μm 1 μm 6 μm 1 μm (a) Sliding against (b) Sliding against Cu/C/RBC composite (α = 5 wt.%, d m = 4.9 μm) ig. 12 SEM images of worn surface of pin specimen sliding against and Cu/C/RBC composites composites increases with an increase of the mean particle size of the RB ceramics particles and that of a pin specimen sliding against the Cu/C/RBC composites takes the lowest value at the mean particle size of 3.2 μm irrespective of the weight fraction of the RB ceramics particles. igure 9 shows the distribution of the specific wear rate of a disk or a pin specimen as a function of the weight fractions and the mean particle sizes of the RB ceramics particles. As shown in ig. 9(a), the specific wear rate of the Cu/C/RBC composites decreases with a decrease of the weight fraction and the mean particle size of the RB ceramics particles. It can be also clarified that the specific wear rate less than 1 mm 2 /N can be obtained when the weight fraction and the mean particle size of the RB ceramics particles were 5 wt.% and 4.9 μm, respectively. On the other hand for the specific wear rate of a pin specimen (ig. 9(b)), the specific wear rate takes a low value around or less than 1 mm 2 /N when the mean particle size of RB ceramics particles is smaller than 3.2 μm. In addition, it can be seen that extremely low value of the specific wear rate of a pin specimen less than mm 2 /N can be obtained when the mean particle size and the weight fraction of RB ceramics particles are 3.2 μm and 1 wt.%, respectively. igure 1 shows the values of friction coefficient, the specific wear rate of disk specimen and that of a pin specimen for three composite materials. As shown in ig. 1, the friction coefficient for the Cu/C/RBC composite is about 1/2 of that for the, the specific wear rate of the Cu/C/RBC composite disk is about 1/22 of that of the, and the specific wear rate of a pin specimen sliding against the Cu/C/RBC composite is about 1/15 of that of the when the mean particle size and the weight fraction of the RB ceramics particles are 4.9 μm and 5 wt.%. Additionally, the friction coefficient for the Cu/C/RBC composite is about 3/5 of that for the, the specific wear rate of the Cu/C/RBC composite disk is about 1/7 of that of the, and the specific wear rate of a pin specimen sliding against the Cu/C/RBC composite is about 1/1,7 of that of the when the mean particle size and the weight fraction of the RB ceramics particles are 3.2 μm and 5 wt.%. Based on these results, it was clarified that a lower friction, an extremely higher wear resistance and an extremely lower aggressivity to a pin specimen can be achieved for the Cu/C/RBC composites as compared with the conventional. 5. Discussion igure 11 shows SEM images and elemental analysis of the worn surface of the and the Cu/C/RBC composite (α = 5 wt.%, d m = 4.9 μm). It can be observed in ig. 11 that the exposed area of copper for the is larger than that for the Cu/C/RBC Japanese Society of Tribologists ( Tribology Online, Vol. 3, No 4 (28) / 226

6 riction and Wear Properties of /Carbon/RB Ceramics Composite Materials under Dry Condition During sliding W alloy pin During sliding alloy pin W Carbon Severe adhesion (a) Carbon Cu/C/RBC composite RB ceramics Carbon or RB ceramics particles (b) Cu/C/RBC composite ig. 13 Schematic diagram of sliding contact interface between disk and pin specimen composite. This is because the worn surface of the Cu/C/RBC composite is covered by carbon or the RB ceramics particles. igure 12 shows SEM images of the worn surface of the pin specimen sliding against the and the Cu/C/RBC composite (α = 5 wt.%, d m = 4.9 μm). As shown in ig. 12, strong plastic flows of copper can be obtained on the worn surface of the pin specimen sliding against the. On the other hand, plastic flows of copper on the worn surface of the pin specimen sliding against the Cu/C/RBC composite are smaller in amount than that sliding against the. Thus, it can be considered that a severe adhesion of copper/copper might occur in the contact interface between the and the copper alloy pin during sliding. However, a favorable solid lubrication in the sliding contact interface between the Cu/C/RBC composite and the copper alloy pin was realized by the thin film composed of carbon or the RB ceramics as shown in ig. 13. In this study, it can be clarified that the Cu/C/RBC composites have superior friction and wear properties under dry condition. However, the wear of pantograph slider materials caused by arc-discharge is also important, so these friction and wear properties under arc-discharge condition should be investigated in the future. 6. Conclusions (1) The new composite materials Cu/C/RBC composites were developed by sintering compounds of copper, carbon and the RB ceramics. (2) The friction coefficient for the Cu/C/RBC composites was about 1/2 ~ 3/5 of that for the Cu/C composite which is used for the conventional pantograph slider. (3) The specific wear rate of the Cu/C/RBC composites took a low value less than 2 mm 2 /N and was about 1/22 ~ 1/7 of that of the. (4) The specific wear rate of a pin specimen sliding against the Cu/C/RBC composites took a low value less than 2 mm 2 /N and was about 1/1,7 ~ 1/15 of that sliding against the. 7. References [1] Kubo, S., Latest Trend in Material for Pantograph s Contact Strip, Journal of Japanese Society of Tribologists, 5, 3, 25, [2] Hokkirigawa, K., Shikano, S. and Takahashi, T., Development of Hard Porous Materials RB Ceramics Made of Rice Bran, Proc. 3 rd Int. Conf. on Ecomaterials, 1997, [3] Hokkirigawa, K., Development and Application of Rice Bran Ceramics as a New Tribo-Material, Proc. Int. Conf. Nagasaki, 2, [4] Yamaguchi, T. and Hokkirigawa, K., Development of Hard Porous Carbon Materials RB Ceramics and Their Tribological Application, Proc. Third Asia Int. Conf. Tribol., 26, [5] Yamaguchi, T. and Hokkirigawa, K., Tribological Properties and Applications of Hard Porous Carbon Materials RB Ceramics, Journal of Japanese Society of Tribologists, 52, 2, 27, [6] Akiyama, M., Matsumoto, K., Yamaguchi, T. and Hokkirigawa, K., Development of Thermoplastic Resin/RB Ceramics Composites and Their riction and Wear Properties under Dry and Oil Lubricated Conditions, Book of Synopses of Int. Tribol. Conf. Kobe 25, 25, 381. [7] Akiyama, M., Matsumoto, K., Yamaguchi, T. and Hokkirigawa, K., Tribological Properties of PA66/RB Ceramics Composites under Dry and Oil Lubricated Conditions, Proc. 3 rd Asia Int. Conf. Tribol., 26, [8] Yamaguchi, T., Zhou, Y., Hirao, K., Kino, K., Matsuura, H. and Hokkirigawa, K., riction and Wear Properties of SiC/RB Ceramics Composites, Proc. Third Asia Int. Conf. Tribol., 26, [9] Zhou, Y., Hirao, Yamaguchi, T. and Hokkirigawa, K., Preparation and Tribological Properties SiC/RBC Composite Ceramics, Journal of Material Research, 2, 12, 25, Japanese Society of Tribologists ( Tribology Online, Vol. 3, No 4 (28) / 227