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reduced graphene oxide filler
Author’s Accepted Manuscript Fabrication and tribological properties of copper matrix composite with short carbon fiber/reduced graphene oxide filler Xinjiang Zhang, Pengyu Dong, Yong Chen, Wenchao Yang, Yongzhong Zhan, Kaifeng Wu, Yueyu Chao www.elsevier.com/locate/jtri
PII: DOI: Reference:
S0301-679X(16)30245-6 http://dx.doi.org/10.1016/j.triboint.2016.07.027 JTRI4305
To appear in: Tribiology International Received date: 12 April 2016 Revised date: 14 July 2016 Accepted date: 29 July 2016 Cite this article as: Xinjiang Zhang, Pengyu Dong, Yong Chen, Wenchao Yang, Yongzhong Zhan, Kaifeng Wu and Yueyu Chao, Fabrication and tribological properties of copper matrix composite with short carbon fiber/reduced graphene oxide filler, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2016.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and tribological properties of copper matrix composite with short carbon fiber/reduced graphene oxide filler Xinjiang Zhanga, Pengyu Dongb, Yong Chena, Wenchao Yangc, Yongzhong Zhanc, Kaifeng Wua,c, Yueyu Chaoa a School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224002, PR China b Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments, Yancheng Institute of Technology, Yancheng 224002, PR China c College of Materials Science and Engineering, Guangxi University, Nanning, Guangxi 5 30004, PR China Abstract Copper matrix composite with short carbon fiber(CF)/reduced graphene oxide(rGO) hybrid filler have been fabricated by freeze-drying and spark plasma sintering. Microstructure and tribological properties of as-prepared composites were characterized. The microstructural observation shows that the rGO exhibits both agglomerated and dispersed states, and the uniform CF presents the various space orientations in the Cu matrix. The composite with CF/rGO hybrid filler exhibited the lower friction coefficient (0.32), and whose wear rates decreased respectively by 45.3 % and 86.0 % in comparison with pure Cu and the composite with rGO filler. The CF/rGO hybrids act as solid lubricant and play the major role in the improved tribological performance. The formation, friction-reducing and anti-wear mechanisms of the composite were discussed.
Keywords: Metal matrix composites; Friction; Wear; Lubrication
Corresponding author: Tel./fax: +86-515-88298871, E-mail: [email protected] (X. Zhang) 1
1. Introduction Copper matrix composites (CMCs) have attracted much attention during recent years in applications as bearing, electrical sliding contacts, resistance welding electrodes, and so on [1]. As attractive reinforcements, carbon materials like graphite, carbon nanotube and carbon fiber were popularly employed due to high thermal conductivity, low coefficient of thermal expansion and good self-lubricant property [2,3]. And most important, they attain the enhanced mechanical, thermal, electrical and tribological properties of CMCs [3,4]. Still, the performance enhancement of CMCs is increasingly required to explore new reinforcing methods in order to extend their application. Besides properties such as thermal conductivity [5], high modulus (1 TPa) [6] and fracture strength (125 GPa) [6], the unusual friction and wear properties at nano to micro scales were possessed by the graphene [7,8]. Interestingly, the few-layer graphene also exhibited the extraordinary macrotribological properties, which can last for 47000 cycles despite rather high contact pressure of 0.5 GPa [8]. Most previous investigations focused on the improved tribological properties of graphene/metal nanocomposites (< 0.3 wt.% graphene) [9-11]. More graphene addition further reduced the friction coefficient of metal matrix, but resulted with the increased wear rate [10]. Recently, attempts of hybrid CMCs have been made to further enhance the mechanical and/or tribological properties by the incorporation of both type and dimensions of the reinforcing medium [12,13]. Carbon fiber, a conventional microscale fiber, is a excellent potential reinforcement in the composites. Some reports [14,15] shown that the addition of carbon fiber significantly improved the metallic and polymeric properties. Herein, to combine the merits of two-dimensional lamellar graphene and
2
conventional microscale carbon fiber, the objectives of the curren work are to synthesize copper matrix composite with hybrid CF/rGO filler and investigate microstructure and tribological properties of the resultant composites. 2. Experimental procedure For this investigation, GO was firstly prepared from purified natural graphite by a modified Hummer’s method [16]. Expandable graphite powder (8 g) was mixed with a mixture of concentrated 1.5 L H2SO4 and 70 g KMnO4. After reaction at 55 °C for 6 h, the mixture was cooled to room temperature, poured into an ice bath (1.5 L) and further treated with H2O2 (30%, 10 mL). The mixture gradually became the bright-yellow suspension. For purification, the mixture was washed by repeated rinsing and centrifugation with 10% HCl and DI water several times. As-synthesized GO was suspended in water to give a brown dispersion, which was subjected to dialysis to completely remove residual salts and acids. Fig. 1 shows the schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler. Firstly, the short carbon fiber (T700, 1~2 mm) was added into as-prepared GO dispersion, and their mixtures were mechanically stirred for 3 hours. After that, the hydrazine hydrate (80 %) was added drop by drop, and then the electrolytic Cu powder (99.9 % pure) was added and formed a powder slurry under the mechanical stirring. The composite slurry was frozen until totally solid. The frozen composite slurry was maintained under vacuum (1 Pa) and removed the water by sublimation to obtain the freeze-dried composite powder. Finally, the composite as a powder was compacted at a pressure of 450 MPa, and then sintered under vacuum at 750 °C for 8 minutes under a pressure of 45 MPa to synthesize Cu–2.5rGO–1.0CF (wt.%)
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composite, with a heating rate of 50 °C/min, using a spark plasma sintering (SPS-211Lx, Japan). For comparison, pure copper and Cu–2.5rGO (wt.%) specimens were fabricated using the same processing parameters as those for the composite. X-ray diffraction (XRD) was carried out to determine phase constitutions of the specimens via Rigaku D/max-2200 diffractometer. Microstructural observation was studied by transmission electron microscope (TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, FEI Quanta 200FEG) equipped with backscattered electron detector (BSE). The tribology test of this study were carried out using a pin-on-disc tribometer (Fig. 2) operating in dry sliding condition. In the tests, cylindrical pins with dimension of 6 mm in diameter and 10 mm in height were used from the as-prepared samples. As a counterpart, the bearing steel GCr15 disc hardened to 62 HRC was used. Prior to test, all contact surfaces were metallographically polished with 800 grit size SiC paper and cleaned with acetone. The applied load and sliding speed were maintained respectively at 20 N and 1.6 m/s at a constant sliding distance of 2000 m. The coefficient of friction was continually recorded during the tests, and the average value was calculated for each test within the distance of 2000 m. Wear rate was calculated by the formula of w = v/pl, where w is the wear rate, v is the worn volume of the specimen, p is the normal load applied and l is the sliding distance of specimen. Microhardness tests were conducted on a HVS-1000 Vickers sclerometer with the load of 29.4 N and a load-dwell time of 10 s, and eight indentations were taken for each sample to obtain an average value. The worn surfaces at the end of tests were examined and analyzed using SEM with energy dispersive X-ray spectroscopy (EDX).
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3. Results and discussion 3.1 Microstructural characterization For the freeze-dried composite powders and sintered composite, their XRD peaks (Fig. 3a) at 2θ = 43.3°, 50.4°and 74.2°were assigned to the (111), (200) and (220) planes of face-centered cubic (fcc) structured Cu phase. No characteristic peaks of CF and rGO were detected, possibly due to the strong peaks of Cu. The CF/rGO mixture and rGO were separated and collected from freeze-dried composite powder, and investigated by XRD together with GO and starting CF, as shown in Fig. 3b. A strong peak of GO appeared at 2θ = 9.8°corresponding to (001) reflection peak, which is due to the formation of intercalated water moieties and oxygen functionalities groups between the layers of GO [17]. However, the peak at 2θ = 9.8°disappeared completely in rGO or CF/rGO mixture, a broad (002) reflection peak presented at 2θ = 27.1°, which was consistent with starting CF. It indicated that most of intercalated water and oxygen functional groups were removed and the formation of graphite-like structure. Microscopic observation of dried composite powder (Fig. 4) shown that the layered rGO sheets and CF appeared clearly in the composite powders. Furthermore, the massive corrugated rGO sheets coated tightly on the surface of electrolytic Cu particles and CF, and some rGO sheets embed into the hole of Cu particles. The mutual lap joint CF exhibited the different spatial orientation. It demonstrated that the CF and rGO were randomly and completely mixed with the Cu particles in the dried composite powders, using the combination of mechanical stirring and freeze-drying of the composite slurry. SEM-BSE micrographs of the sintered composite are shown in Fig. 4(a,b). From low magnification observation (Fig. 5a), it can be seen that the abundant CF/rGO hybrid filler
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(dark phase) distributes in the whole microstructure. Further observation (Fig. 5b) showed both dispersed (marked by white arrow) and agglomerated (marked by dark arrow) rGO distributions. The agglomerated rGO mainly exhibited the irregular lamellar shape, which might be due to the adsorption interaction of rGO flakes during the mechanical stirring of the composite slurry. Moreover, the expulsion from growing crystals of the water might result into its aggregation during freeze-drying process [4,18]. Especially, the CF shown the various orientations in the three-dimensional space. Further examination (Fig. 5c) revealed that there is no interfacial reaction at the rGO/Cu interfaces. Thus, it can be deduced that their interfaces are stable between rGO flakes and Cu matrix. The distinct CF/rGO distribution (Fig. 5d ) formed in the Cu matrix could be attributed to the mechanical blending of composite slurry, freeze-drying and SPS solid-phase sintering. On the one hand, the mechanical blending was the benefit of CF/rGO/Cu intensive mixing in the slurry. On the other hand, the freeze-drying removed the solidified water by sublimation to retain the space distribution state of CF/rGO. Finally, SPS solid-phase sintering leaded to faster densification of various composite powders during the rapid heating. 3.2 Tribological properties Table 1 summarizes the tribological properties of reinforced and un-reinforced Cu. As expected, the friction coefficient was decreased from 0.84 (pure Cu) to 0.38 (Composite with rGO filler) and 0.32 (Composite with CF/rGO hybrid filler), respectively. That is to say, the rGO or CF/rGO filler obviously decreased the friction coefficient of Cu matrix. According to the previous reports about rGO/Cu [11], rGO/Al nanocomposites [10] and CF/Cu composites [19], the single rGO or CF filler in metal matrix had the
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friction-reducing effect. Moreover, for the composite with CF/rGO hybrid filler, its volume wear rate is as low as 0.64 × 10-4 mm3/N*m, decreased respectively by 45.3 % and 86.0 % in comparison with pure Cu and the composite with rGO filler. The CF/rGO hybrid filler has superior friction-reducing and anti-wear effect to Cu matrix. From Table 1, it can be also seen that the microhardness of the composite enhances significantly with the introduction of CF/rGO hybrid filler. After wear tests, the morphologies of worn surface of the composite with CF/rGO hybrid filler are displayed in Fig. 6. The worn surfaces have parallel grooves with varying shallow depth and narrow width in the direction of sliding, and the scuffing and spalling phenomena was observed in Fig. 6a. Some fibers oriented parallelly and perpendicularly to the relative sliding direction on the surface, as shown in Fig. 6b. The morphologies of scratched surfaces of parallel oriented CF are presented in the inset of Fig. 6b. There is no noticeable track on the CF surface, and no micro-cracks appears at the interface between CF and matrix. The turnup rGO film was further found, and the crescent debris is produced accompanied by some micro-cracks (Fig. 6c). In Fig. 6d, the strong peak of carbon on the worn surface indicates the effective operating of rGO/CF solid lubricant at contact surface. The CF/rGO hybrid filler were ground at applied loads, and the resulting CF together with the crushed, turnup rGO creates the transfer film over the contact area. Moreover, the low intensity oxygen peak was observed. It indicates that some oxide formation occurs on the worn surface during dry sliding of composite with steel counter surface. The cause of observed enhancement could be sought in the spatially geometrical distribution and characteristic of CF and rGO second-phases, and the CF/rGO hybrid
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filler played a vital role in the tribological response of the composites. The CF and rGO consist of the carbon atoms layer, resulting in low coefficient [8,19]. The reports [19,20] pointed out that the lubricant films formed on CMCs with the single short carbon fibers or graphene nanoplatelets. In this work, a lubrication transfer film could be formed on the worn surface of the composites due to the rGO/CF hybrid addition, promoting the lubricating effects to reduce the material removal. The rGO nanosheets, due to their smaller size, could be squeezed out and penetrated into the very narrow grooves and gaps between the asperities of sliding contact easily, improving the lubricating film continuity [3,21]. Moreover, the CF and rGO with high strength and good ductility are all the anisotropic materials, but their hybrid filler presents the various space orientation in the microstructure to improve the microhardness obviously, which can hinder the deformation of the matrix and hinder the propagation of the micro-cracks and wear debris during the sliding process; The good interfacial relationship between CF/rGO and Cu matrix enhances the enduring ability to sustained external pressure during a sliding process. 4. Conclusion (1) Copper matrix composite with CF/rGO hybrid filler were successfully fabricated by the combination of freeze-drying and spark plasma sintering. For such hybrid composite, the agglomerated and dispersed rGO distributed in Cu matrix, and the CF exhibited various space orientations. (2) Compared to as-sintered pure Cu and composite with single rGO filler, the hybrid composite presented the lower friction coefficient and wear rates due to the cooperative effect offered by both the reinforcements. The CF/rGO hybrid filler improve tribological
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behavior of copper matrix composites by hindering deformation of copper matrix and forming a lubrication transfer film on the worn surface.
Acknowledgments This work was supported by National Natural Science Foundation of China (21403184, 51361002, 51161002), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015A025) and Talent Introduction Project of Yancheng Institute of Technology (No. XJ201529).
References [1] Chen J, Cheng J, Li F, Zhu SY, Qiao ZH, Yang J. The effect of composition altailoring and sintering temperature on the mechanical and tribological properties of Cu/AlMgB14 composite. Tribo Int 2016; 96: 155–162. [2] Gautam RK, Ray S, Sharma SC, Jain SC, Tyagi R. Dry sliding wear behavior of hot forged and annealed Cu–Cr–graphite in-situ composites.Wear 2011; 271: 658–664. [3] Moghadam AD, Omrani E, Menezes PL, Rohatgi PK. Mechanical and tribological properties of self-lubricating metal matrix nanocomposites reinforced by carbon nanotubes (CNTs) and graphene: A review. Compos Part B 2015; 77: 402–420. [4] Lloyd JC, Neubauer E, Barcena J, Clegg WJ. Effect of titanium on coppe-titanium/carbon nanofibre composite materials. Compos Sci Technol 2010; 70: 2284–2289. [5] Balandin AA, Ghosh S, Bao W, Calizo I, Teweldebrhan D, Miao F, Lau CN. Superior thermal conductivity of single-layer graphene. Nano Lett 2008; 8: 902–907. [6] Lee C, Wei XD, Kysar JK, Hone J. Measurement of the elastic properties and 9
intrinsic strength of monolayer graphene. Science 2008; 321: 385–388. [7] Deng Z, Smolyanitsky A, Li Q, Feng XQ, Cannara R. Adhesion-dependent negative friction coefficient on graphite. Nat Mater 2012; 11: 1032–1037. [8] Berman D, Deshmukh SA, Sankaranarayanan S, Erdemir A, Sumant AV. Extraordinary macroscale wear resistance of one atom thick graphene layer. Adv Funct Mater 2014; 24: 6640–6646. [9] Algul H, Tokur M, Ozcan S, Uysal M, Cetinkaya Y, Akbulut H, Alp A. The effect of graphene content and sliding speed on the wear mechanism of nickel-graphene nanocomposites. Appl Surf Sci 2015; 359: 340–348. [10] Tabandeh-Khorshid M, Omrani E, Menezes PL, Rohatgi PK. Tribological performance of self-lubricating aluminum matrix nanocomposites: Role of graphene nanoplatelets. Eng Sci Tech 2016; 19: 463–469. [11] Jia ZF, Chen TD, Wang J, Ni JJ, Li HY, Shao X. Synthesis, characterization and tribological properties of Cu/reduced graphene oxide composites. Tribo Int 2015; 188: 17–24. [12] Akbarpour MR, Salahi E, AlikhaniHesari F, Simchi A, Kim HS. Fabrication, characterization and mechanical properties of hybrid composites of copper using the nanoparticulates of SiC and carbon nanotubes. Mater Sci Eng A 2013; 572: 83–90. [13] Zhou SF, Wu C, Zhang TY, Zhang ZZ. Carbon nanotube- and Fep-reinforced copper-matrix composites by laser induction hybrid rapid cladding. Scripta Mater 2014; 76: 25–28. [14] Zhang JJ, Liu SC,
Lu YP, Dong Y, Li TJ. Fabrication process and bending
properties of carbon fibers reinforced Al-alloy matrix composites. J Mater Process Tech
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2016; 231: 366–373. [15] Lv M, Zheng F, Wang QH, Wang TM, Liang YM. Friction and wear behaviors of carbon and aramid fibers reinforced polyimide composites in simulated space environment. Tribo Int 2015; 92: 246–254. [16] Marcano DC, Kosynkin DV, Berlin JM, Sinitskii A, Sun Z, Slesarev A, Alemany LB, Lu W, Tour JM. Improved Synthesis of Graphene Oxide. ACS Nano 2010; 4: 4806–4814. [17] Pei SF, Cheng HM. The reduction of graphene oxide. Carbon 2012; 50: 3210–3228. [18] Deville S. Ice-templating, freeze casting: Beyond materials processing. J Mater Res 2013; 28: 2202–2219. [19] Tang YP, Liu HZ, Zhao HJ, Liu L, Wu YT. Friction and wear properties of copper matrix composites reinforced with short carbon fibers. Mater Des 2008; 29: 257–261. [20] Chen FY, Ying JM, Wang YF, Du SY, Liu ZP, Huang Q. Effects of graphene content on the microstructure and properties of copper matrix composites. Carbon 2016; 96: 836–842. [21] Huang H, Tu J, Gan L, Li C. An investigation on tribological properties of graphite nanosheets as oil additive. Wear 2006; 261:140–144.
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Figures and table captions Figures: Fig. 1. Schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler. Fig. 2. Schematic illustration of pin-on-disc tribometer Fig. 3. (a) XRD patterns of the freeze-dried composite powders and sintered composite by SPS; (b) XRD patterns of GO, rGO, CF and isolated CF/rGO mixture from freeze-dried composite powder. Fig. 4. SEM micrographs of the dried CF/rGO/Cu composite powder, where (b) is the higher magnification image from the rectangle area in (a). The black arrow in inset marks the rGO nano-sheet. Fig. 5. (a,b) Low (a) and high (b) magnification SEM-BSE micrographs of sintered CF/rGO/Cu-matrix composite, where the dark and white arrows in (b) mark respectively the agglomerated and dispersed rGO; (c) TEM image of rGO/Cu interface; (d) Schematic illustration of CF/rGO distribution. Fig. 6. (a,b) SEM micrograph of the worn surface with sintered CF/rGO/Cu-matrix composite, where the inset in (b) shows the CF on the worn surface; (c) SEM micrograph of rGO on the worn surface; (d) EDX result from the rectangle area in (b).
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Table: Table 1. Friction coefficient, volume wear rate and microhardness of sintered pure Cu, rGO/Cu-matrix composite and CF/rGO/Cu-matrix composite Samples Pure Cu rGO/Cu-matrix composite CF/rGO/Cu-matrix composite
Friction coefficient
Volume wear rate (×10-4 mm3/N*m)
Microhardness (HV)
0.84
1.17
62.2
0.38
4.57
65.3
0.32
0.64
68.7
Figures-revised
Fig. 1. Schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler.
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Fig. 2. Schematic illustration of pin-on-disc tribometer
Fig. 3. (a) XRD patterns of the freeze-dried composite powders and sintered composite by SPS; (b) XRD patterns of GO, rGO, CF and isolated CF/rGO mixture from freeze-dried composite powder.
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Fig. 4. SEM micrographs of the dried CF/rGO/Cu composite powder, where (b) is the higher magnification image from the rectangle area in (a). The black arrow in inset marks the rGO nano-sheet.
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Fig. 5. (a,b) Low (a) and high (b) magnification SEM-BSE micrographs of sintered CF/rGO/Cu-matrix composite, where the dark and white arrows in (b) mark respectively the agglomerated and dispersed rGO; (c) TEM image of rGO/Cu interface; (d) Schematic illustration of CF/rGO distribution.
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Fig. 6. (a,b) SEM micrograph of the worn surface with sintered CF/rGO/Cu-matrix composite, where the inset in (b) shows the CF on the worn surface; (c) SEM micrograph of rGO on the worn surface; (d) EDX result from the rectangle area in (b).
Research highlights ►Copper matrix composites with novel hybrid filler were facilely fabricated. ►The short carbon fiber presents the various space orientations. ►The rGO or rGO/CF fillers effectively reduced the friction coefficient of Cu matrix. ►The best excellent wear resistance of the composite with rGO/CF hybrid filler.
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PII: DOI: Reference:
S0301-679X(16)30245-6 http://dx.doi.org/10.1016/j.triboint.2016.07.027 JTRI4305
To appear in: Tribiology International Received date: 12 April 2016 Revised date: 14 July 2016 Accepted date: 29 July 2016 Cite this article as: Xinjiang Zhang, Pengyu Dong, Yong Chen, Wenchao Yang, Yongzhong Zhan, Kaifeng Wu and Yueyu Chao, Fabrication and tribological properties of copper matrix composite with short carbon fiber/reduced graphene oxide filler, Tribiology International, http://dx.doi.org/10.1016/j.triboint.2016.07.027 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fabrication and tribological properties of copper matrix composite with short carbon fiber/reduced graphene oxide filler Xinjiang Zhanga, Pengyu Dongb, Yong Chena, Wenchao Yangc, Yongzhong Zhanc, Kaifeng Wua,c, Yueyu Chaoa a School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224002, PR China b Jiangsu Collaborative Innovation Center for Ecological Building Materials and Environmental Protection Equipments, Yancheng Institute of Technology, Yancheng 224002, PR China c College of Materials Science and Engineering, Guangxi University, Nanning, Guangxi 5 30004, PR China Abstract Copper matrix composite with short carbon fiber(CF)/reduced graphene oxide(rGO) hybrid filler have been fabricated by freeze-drying and spark plasma sintering. Microstructure and tribological properties of as-prepared composites were characterized. The microstructural observation shows that the rGO exhibits both agglomerated and dispersed states, and the uniform CF presents the various space orientations in the Cu matrix. The composite with CF/rGO hybrid filler exhibited the lower friction coefficient (0.32), and whose wear rates decreased respectively by 45.3 % and 86.0 % in comparison with pure Cu and the composite with rGO filler. The CF/rGO hybrids act as solid lubricant and play the major role in the improved tribological performance. The formation, friction-reducing and anti-wear mechanisms of the composite were discussed.
Keywords: Metal matrix composites; Friction; Wear; Lubrication
Corresponding author: Tel./fax: +86-515-88298871, E-mail: [email protected] (X. Zhang) 1
1. Introduction Copper matrix composites (CMCs) have attracted much attention during recent years in applications as bearing, electrical sliding contacts, resistance welding electrodes, and so on [1]. As attractive reinforcements, carbon materials like graphite, carbon nanotube and carbon fiber were popularly employed due to high thermal conductivity, low coefficient of thermal expansion and good self-lubricant property [2,3]. And most important, they attain the enhanced mechanical, thermal, electrical and tribological properties of CMCs [3,4]. Still, the performance enhancement of CMCs is increasingly required to explore new reinforcing methods in order to extend their application. Besides properties such as thermal conductivity [5], high modulus (1 TPa) [6] and fracture strength (125 GPa) [6], the unusual friction and wear properties at nano to micro scales were possessed by the graphene [7,8]. Interestingly, the few-layer graphene also exhibited the extraordinary macrotribological properties, which can last for 47000 cycles despite rather high contact pressure of 0.5 GPa [8]. Most previous investigations focused on the improved tribological properties of graphene/metal nanocomposites (< 0.3 wt.% graphene) [9-11]. More graphene addition further reduced the friction coefficient of metal matrix, but resulted with the increased wear rate [10]. Recently, attempts of hybrid CMCs have been made to further enhance the mechanical and/or tribological properties by the incorporation of both type and dimensions of the reinforcing medium [12,13]. Carbon fiber, a conventional microscale fiber, is a excellent potential reinforcement in the composites. Some reports [14,15] shown that the addition of carbon fiber significantly improved the metallic and polymeric properties. Herein, to combine the merits of two-dimensional lamellar graphene and
2
conventional microscale carbon fiber, the objectives of the curren work are to synthesize copper matrix composite with hybrid CF/rGO filler and investigate microstructure and tribological properties of the resultant composites. 2. Experimental procedure For this investigation, GO was firstly prepared from purified natural graphite by a modified Hummer’s method [16]. Expandable graphite powder (8 g) was mixed with a mixture of concentrated 1.5 L H2SO4 and 70 g KMnO4. After reaction at 55 °C for 6 h, the mixture was cooled to room temperature, poured into an ice bath (1.5 L) and further treated with H2O2 (30%, 10 mL). The mixture gradually became the bright-yellow suspension. For purification, the mixture was washed by repeated rinsing and centrifugation with 10% HCl and DI water several times. As-synthesized GO was suspended in water to give a brown dispersion, which was subjected to dialysis to completely remove residual salts and acids. Fig. 1 shows the schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler. Firstly, the short carbon fiber (T700, 1~2 mm) was added into as-prepared GO dispersion, and their mixtures were mechanically stirred for 3 hours. After that, the hydrazine hydrate (80 %) was added drop by drop, and then the electrolytic Cu powder (99.9 % pure) was added and formed a powder slurry under the mechanical stirring. The composite slurry was frozen until totally solid. The frozen composite slurry was maintained under vacuum (1 Pa) and removed the water by sublimation to obtain the freeze-dried composite powder. Finally, the composite as a powder was compacted at a pressure of 450 MPa, and then sintered under vacuum at 750 °C for 8 minutes under a pressure of 45 MPa to synthesize Cu–2.5rGO–1.0CF (wt.%)
3
composite, with a heating rate of 50 °C/min, using a spark plasma sintering (SPS-211Lx, Japan). For comparison, pure copper and Cu–2.5rGO (wt.%) specimens were fabricated using the same processing parameters as those for the composite. X-ray diffraction (XRD) was carried out to determine phase constitutions of the specimens via Rigaku D/max-2200 diffractometer. Microstructural observation was studied by transmission electron microscope (TEM, Tecnai G2 F30) and scanning electron microscopy (SEM, FEI Quanta 200FEG) equipped with backscattered electron detector (BSE). The tribology test of this study were carried out using a pin-on-disc tribometer (Fig. 2) operating in dry sliding condition. In the tests, cylindrical pins with dimension of 6 mm in diameter and 10 mm in height were used from the as-prepared samples. As a counterpart, the bearing steel GCr15 disc hardened to 62 HRC was used. Prior to test, all contact surfaces were metallographically polished with 800 grit size SiC paper and cleaned with acetone. The applied load and sliding speed were maintained respectively at 20 N and 1.6 m/s at a constant sliding distance of 2000 m. The coefficient of friction was continually recorded during the tests, and the average value was calculated for each test within the distance of 2000 m. Wear rate was calculated by the formula of w = v/pl, where w is the wear rate, v is the worn volume of the specimen, p is the normal load applied and l is the sliding distance of specimen. Microhardness tests were conducted on a HVS-1000 Vickers sclerometer with the load of 29.4 N and a load-dwell time of 10 s, and eight indentations were taken for each sample to obtain an average value. The worn surfaces at the end of tests were examined and analyzed using SEM with energy dispersive X-ray spectroscopy (EDX).
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3. Results and discussion 3.1 Microstructural characterization For the freeze-dried composite powders and sintered composite, their XRD peaks (Fig. 3a) at 2θ = 43.3°, 50.4°and 74.2°were assigned to the (111), (200) and (220) planes of face-centered cubic (fcc) structured Cu phase. No characteristic peaks of CF and rGO were detected, possibly due to the strong peaks of Cu. The CF/rGO mixture and rGO were separated and collected from freeze-dried composite powder, and investigated by XRD together with GO and starting CF, as shown in Fig. 3b. A strong peak of GO appeared at 2θ = 9.8°corresponding to (001) reflection peak, which is due to the formation of intercalated water moieties and oxygen functionalities groups between the layers of GO [17]. However, the peak at 2θ = 9.8°disappeared completely in rGO or CF/rGO mixture, a broad (002) reflection peak presented at 2θ = 27.1°, which was consistent with starting CF. It indicated that most of intercalated water and oxygen functional groups were removed and the formation of graphite-like structure. Microscopic observation of dried composite powder (Fig. 4) shown that the layered rGO sheets and CF appeared clearly in the composite powders. Furthermore, the massive corrugated rGO sheets coated tightly on the surface of electrolytic Cu particles and CF, and some rGO sheets embed into the hole of Cu particles. The mutual lap joint CF exhibited the different spatial orientation. It demonstrated that the CF and rGO were randomly and completely mixed with the Cu particles in the dried composite powders, using the combination of mechanical stirring and freeze-drying of the composite slurry. SEM-BSE micrographs of the sintered composite are shown in Fig. 4(a,b). From low magnification observation (Fig. 5a), it can be seen that the abundant CF/rGO hybrid filler
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(dark phase) distributes in the whole microstructure. Further observation (Fig. 5b) showed both dispersed (marked by white arrow) and agglomerated (marked by dark arrow) rGO distributions. The agglomerated rGO mainly exhibited the irregular lamellar shape, which might be due to the adsorption interaction of rGO flakes during the mechanical stirring of the composite slurry. Moreover, the expulsion from growing crystals of the water might result into its aggregation during freeze-drying process [4,18]. Especially, the CF shown the various orientations in the three-dimensional space. Further examination (Fig. 5c) revealed that there is no interfacial reaction at the rGO/Cu interfaces. Thus, it can be deduced that their interfaces are stable between rGO flakes and Cu matrix. The distinct CF/rGO distribution (Fig. 5d ) formed in the Cu matrix could be attributed to the mechanical blending of composite slurry, freeze-drying and SPS solid-phase sintering. On the one hand, the mechanical blending was the benefit of CF/rGO/Cu intensive mixing in the slurry. On the other hand, the freeze-drying removed the solidified water by sublimation to retain the space distribution state of CF/rGO. Finally, SPS solid-phase sintering leaded to faster densification of various composite powders during the rapid heating. 3.2 Tribological properties Table 1 summarizes the tribological properties of reinforced and un-reinforced Cu. As expected, the friction coefficient was decreased from 0.84 (pure Cu) to 0.38 (Composite with rGO filler) and 0.32 (Composite with CF/rGO hybrid filler), respectively. That is to say, the rGO or CF/rGO filler obviously decreased the friction coefficient of Cu matrix. According to the previous reports about rGO/Cu [11], rGO/Al nanocomposites [10] and CF/Cu composites [19], the single rGO or CF filler in metal matrix had the
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friction-reducing effect. Moreover, for the composite with CF/rGO hybrid filler, its volume wear rate is as low as 0.64 × 10-4 mm3/N*m, decreased respectively by 45.3 % and 86.0 % in comparison with pure Cu and the composite with rGO filler. The CF/rGO hybrid filler has superior friction-reducing and anti-wear effect to Cu matrix. From Table 1, it can be also seen that the microhardness of the composite enhances significantly with the introduction of CF/rGO hybrid filler. After wear tests, the morphologies of worn surface of the composite with CF/rGO hybrid filler are displayed in Fig. 6. The worn surfaces have parallel grooves with varying shallow depth and narrow width in the direction of sliding, and the scuffing and spalling phenomena was observed in Fig. 6a. Some fibers oriented parallelly and perpendicularly to the relative sliding direction on the surface, as shown in Fig. 6b. The morphologies of scratched surfaces of parallel oriented CF are presented in the inset of Fig. 6b. There is no noticeable track on the CF surface, and no micro-cracks appears at the interface between CF and matrix. The turnup rGO film was further found, and the crescent debris is produced accompanied by some micro-cracks (Fig. 6c). In Fig. 6d, the strong peak of carbon on the worn surface indicates the effective operating of rGO/CF solid lubricant at contact surface. The CF/rGO hybrid filler were ground at applied loads, and the resulting CF together with the crushed, turnup rGO creates the transfer film over the contact area. Moreover, the low intensity oxygen peak was observed. It indicates that some oxide formation occurs on the worn surface during dry sliding of composite with steel counter surface. The cause of observed enhancement could be sought in the spatially geometrical distribution and characteristic of CF and rGO second-phases, and the CF/rGO hybrid
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filler played a vital role in the tribological response of the composites. The CF and rGO consist of the carbon atoms layer, resulting in low coefficient [8,19]. The reports [19,20] pointed out that the lubricant films formed on CMCs with the single short carbon fibers or graphene nanoplatelets. In this work, a lubrication transfer film could be formed on the worn surface of the composites due to the rGO/CF hybrid addition, promoting the lubricating effects to reduce the material removal. The rGO nanosheets, due to their smaller size, could be squeezed out and penetrated into the very narrow grooves and gaps between the asperities of sliding contact easily, improving the lubricating film continuity [3,21]. Moreover, the CF and rGO with high strength and good ductility are all the anisotropic materials, but their hybrid filler presents the various space orientation in the microstructure to improve the microhardness obviously, which can hinder the deformation of the matrix and hinder the propagation of the micro-cracks and wear debris during the sliding process; The good interfacial relationship between CF/rGO and Cu matrix enhances the enduring ability to sustained external pressure during a sliding process. 4. Conclusion (1) Copper matrix composite with CF/rGO hybrid filler were successfully fabricated by the combination of freeze-drying and spark plasma sintering. For such hybrid composite, the agglomerated and dispersed rGO distributed in Cu matrix, and the CF exhibited various space orientations. (2) Compared to as-sintered pure Cu and composite with single rGO filler, the hybrid composite presented the lower friction coefficient and wear rates due to the cooperative effect offered by both the reinforcements. The CF/rGO hybrid filler improve tribological
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behavior of copper matrix composites by hindering deformation of copper matrix and forming a lubrication transfer film on the worn surface.
Acknowledgments This work was supported by National Natural Science Foundation of China (21403184, 51361002, 51161002), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015A025) and Talent Introduction Project of Yancheng Institute of Technology (No. XJ201529).
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Figures and table captions Figures: Fig. 1. Schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler. Fig. 2. Schematic illustration of pin-on-disc tribometer Fig. 3. (a) XRD patterns of the freeze-dried composite powders and sintered composite by SPS; (b) XRD patterns of GO, rGO, CF and isolated CF/rGO mixture from freeze-dried composite powder. Fig. 4. SEM micrographs of the dried CF/rGO/Cu composite powder, where (b) is the higher magnification image from the rectangle area in (a). The black arrow in inset marks the rGO nano-sheet. Fig. 5. (a,b) Low (a) and high (b) magnification SEM-BSE micrographs of sintered CF/rGO/Cu-matrix composite, where the dark and white arrows in (b) mark respectively the agglomerated and dispersed rGO; (c) TEM image of rGO/Cu interface; (d) Schematic illustration of CF/rGO distribution. Fig. 6. (a,b) SEM micrograph of the worn surface with sintered CF/rGO/Cu-matrix composite, where the inset in (b) shows the CF on the worn surface; (c) SEM micrograph of rGO on the worn surface; (d) EDX result from the rectangle area in (b).
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Table: Table 1. Friction coefficient, volume wear rate and microhardness of sintered pure Cu, rGO/Cu-matrix composite and CF/rGO/Cu-matrix composite Samples Pure Cu rGO/Cu-matrix composite CF/rGO/Cu-matrix composite
Friction coefficient
Volume wear rate (×10-4 mm3/N*m)
Microhardness (HV)
0.84
1.17
62.2
0.38
4.57
65.3
0.32
0.64
68.7
Figures-revised
Fig. 1. Schematic illustration of fabrication process of copper matrix composites with CF/rGO hybrid filler.
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Fig. 2. Schematic illustration of pin-on-disc tribometer
Fig. 3. (a) XRD patterns of the freeze-dried composite powders and sintered composite by SPS; (b) XRD patterns of GO, rGO, CF and isolated CF/rGO mixture from freeze-dried composite powder.
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Fig. 4. SEM micrographs of the dried CF/rGO/Cu composite powder, where (b) is the higher magnification image from the rectangle area in (a). The black arrow in inset marks the rGO nano-sheet.
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Fig. 5. (a,b) Low (a) and high (b) magnification SEM-BSE micrographs of sintered CF/rGO/Cu-matrix composite, where the dark and white arrows in (b) mark respectively the agglomerated and dispersed rGO; (c) TEM image of rGO/Cu interface; (d) Schematic illustration of CF/rGO distribution.
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Fig. 6. (a,b) SEM micrograph of the worn surface with sintered CF/rGO/Cu-matrix composite, where the inset in (b) shows the CF on the worn surface; (c) SEM micrograph of rGO on the worn surface; (d) EDX result from the rectangle area in (b).
Research highlights ►Copper matrix composites with novel hybrid filler were facilely fabricated. ►The short carbon fiber presents the various space orientations. ►The rGO or rGO/CF fillers effectively reduced the friction coefficient of Cu matrix. ►The best excellent wear resistance of the composite with rGO/CF hybrid filler.
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