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resin composite
Composites Part B 168 (2019) 572–580
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of MWCNT content on conductivity and mechanical and wear properties of copper foam/resin composite
T
Pei Wanga,b, Guanyu Dengb, Hongtao Zhub, Hongbo Zhanga, Jian Yina,c,∗, Xiang Xionga, Xiaoguang Wua a
Science and Technology on High Strength Materials Laboratory, Central South University, Changsha, 410083, PR China Faculty of Engineering and Information Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522, Australia c Swanson School of Engineering, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, 15261, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper foam Carbon fiber Wear Current-carrying friction Carbon nanotube
Multi-walled carbon nanotube (MWCNT) and carbon fiber and flaky graphite reinforced copper foam/furan resin composite for a novel sliding contact material was fabricated using an acid purify and ultrasonic dispersion technology, followed by a mold pressing process. The microstructure, electrical resistivity, impact strength and current-carrying friction performances of the composites were measured to investigate the effect of the different MWCNT contents. The results showed that the MWCNTs had a good dispersion state in the composite and the percolation threshold was approximate 0.074–0.185 vol%. As addition of the MWCNTs, the impact strength and wear resistance of the composites could be raised by 45% and 22%, respectively.
1. Introduction In the field of tribology, polytetrafluoroethylene, polyoxymethylene, polyamide and other polymer materials with good tribological characteristics are widely used as solid lubricating materials [1–3]. As one of them, thermosetting polymer has good mechanical properties, chemical/corrosion resistance, low density, low shrinkage during forming, simple fabrication, low cost and so on, which can meet the requirements of the pantograph strip and electrical brush during the current-carrying friction, so it is often used to fabricate the sliding contact materials in the railway transport systems and electric machinery [4,7,8,11]. However, polymer composite has some shortcomings, such as insufficient toughness and poor thermal and electrical conductivities. All these factors lead to high wear losses and poor friction behaviors when the material is used as a contact sliding material. To improve these weaknesses, researchers have tried several methods, such as the additions of the fiber and copper powder and have gained certain achievements [5,6]. Tu [7] and Yuan [8] investigated the modified resin-matrix pantograph contact strip with copper power and carbon fibers prepared by hot repressing, solidification and dipping treatment processes. Meanwhile, it was reported that a new sliding contact material was able to be prepared using by the carbon fibers and copper fibers reinforced the epoxy resin carbon [9,10]. The results showed that these composites had nice friction and wear performances
∗
as well as mechanical strength. However, the improvement of thermal and electrical conductivities of the copper alloys were limited, due to one-dimensional or two-dimensional structures isolated in the matrix. Open-cell copper foam has a porous structure with the interconnected 3D metallic network and possesses many special properties, such as high conductivity, low density, specific mechanic performance and high specific surface area, which has been used for supports, electrodes and heat radiators [11,12]. In our current study, a novel sliding contact material was fabricated by a mold pressing process, using the copper foam as the composite preform with mixed short carbon fiber, flaky graphite and furan resin. The results show that addition of the carbon fiber and flaky graphite can improve the overall performance of the composite (including conductivity, strength and tribology property). However, they were found not to increase monotonically with the carbon fiber and flaky graphite contents and no more improvement has been found after an optimum addition, which is attributed to the agglomeration problem. Similar phenomenon has also been reported by Qu and co-authors [24]. It has been well known that carbon nanotube (CNT) has the high aspect ratio and high specific surface area, and the percolation threshold of CNTs-reinforced polymer nanocomposites is much lower than that of the composites containing carbon fibers and flaky graphite [13–15]. Moreover, owing to their unique mechanical, thermal and electrical properties, CNTs was usually used to develop advanced
Corresponding author. Science and Technology on High Strength Materials Laboratory, Central South University, Changsha, 410083, PR China. E-mail address: [email protected] (J. Yin).
https://doi.org/10.1016/j.compositesb.2019.03.067 Received 18 April 2018; Received in revised form 18 March 2019; Accepted 30 March 2019 Available online 01 April 2019 1359-8368/ © 2019 Elsevier Ltd. All rights reserved.
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60 mm × 4 mm × 4 mm, and their end-sections were polished to remove the isolate place. As shown in Fig. 2, the sample resistance R at room temperature was measured using a UIN-T UT620D Digital Micro Ohm Meter. The electrical resistivity ρ was calculated as follows
structural materials with high strength and wear resistant [16–18]. To further improve the comprehensive performances of the copper foam/ furan resin composite with some amount of the short carbon fiber and flaky graphite, the multi-walled carbon nanotubes (MWCNTs) with high specific surface area were added in the composite using a dispensations technology of the acid purify and ultrasonic treatment. In this paper, a novel sliding contact strip material of MWCNTsreinforced copper foam/resin composite was fabricated using an acid purify and ultrasonic treatment of MWCNTs dispersed in the furan resin, and the mixing and addition technology of short carbon fiber and flaky graphite, then a conventional molding technique. Namely SEM image of the fracture surface reveals the dispersion state of the MWNCTs in the local region of the composite as an experimental method, and percolation threshold indicates the dispersion state of the MWNCTs in whole region of the composite as a theoretical calculation. The electrical conductivity, impact strength, current-carrying friction and wear behaviors have been investigated to understand the effect of MWCNTs content on the comprehensive performances of the composite.
ρ=
R·S L
(1)
where R is the resistance of the composite and L is the distance of the sample and S is the section area of the sample. Five specimens for each condition were measured to obtain the average electrical resistivity. 2.4. Measurement of impact property
2. Experimental
Impact tests were carried out on a ZY-3003 miniature pendulumtype impact tester at room temperature, with hammer energy of 1 J and the impact velocity up to 2.9 m/s. It was necessary to check the impact machine and make sure the friction and wind age losses are with in allowable tolerances prior to testing. The effective size of the samples was 55 mm × 10 mm × 10 mm, and the span of the test was 40 mm. The results for each composite were determined by averaging from at least five specimens.
2.1. Materials
2.5. Measurement of friction and wear
Furan resin containing furfural-acetone resin, epoxy resin (Modifier) and phosphoric acid (Curing agent) was purchased from Hunan Boyun New Materials Co., Ltd., China. Short carbon fibers with a diameter of 7 μm and a length of 50–100 μm were purchased from Nanjing WeiDa Composite Material Co., Ltd., China. Open-cell copper foam with an aperture of 15 ppi and a porosity of 96% was purchased from Kunshan Jianyisheng Electronics Co., Ltd., China. Multi-walled carbon nanotubes (MWCNTs) with a diameter of 20–30 nm, a length of 5–15 μm, and a purity of > 97% were purchased from Shenzhen Turing Evolutionary technology Co., Ltd., China. Flaky graphite with a diameter of 150 mesh and a purity of > 99% was purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD., China.
The friction and wear tests with electric current were conducted using a HST-100 multifunction friction and wear tester with a pin-ondisc tribometer, as shown in Fig. 3a. The test rig allowed testing at speed up to 100 m/s with DC electrical current of 300 A. The contact strip used under normal circumstances was replaced with a pin. The chemical composition of the disc (wt%) was 99.9% copper and 0.1% microelements as well as impurities. As demonstrated in Fig. 3b, the size of the pin was 14 mm × 9 mm × 20 mm, and a facade of 14 mm × 9 mm was used as the worn surface. The sliding tests were achieved at speeds of 20 m/s, with a constant load of 70 N and an DC electric current of 10 A. Prior to each electrical sliding test, the friction couple (pin and disc) was first polished using #1200 sandpaper, and the pre-worn without electric current for 3 km to ensure a conformal contact, then the current-carrying friction tests were finished. The friction coefficient was defined as friction force divided by normal force, and the measurement data were recorded automatically by the computer control system during testing. Wear rate ω was determined as the mass loss after each test:
2.2. Preparation of MWCNTs-reinforced copper foam/resin composite WMCNTs were oxidized in a mixed acid (HNO3:H2SO4 = 3:1, vol%) at 100 °C for 4 h and the excess of acid and amorphous carbon impurities were removed by centrifugation. The residual solids were cleaned with deionized water several times, and then vacuum dried in an oven at 60 °C. Meanwhile, copper foam was pretreated in acetone and dilute hydrochloric acid solution to remove oil and oxide on the surface, then it was washed with deionized water several times and vacuum dried at 60 °C. To prepare the MWCNTs-reinforced copper foam/resin composites, the treated MWCNTs were firstly dispersed in ethanol by an ultrasonic process for 1 h, then furan resin was poured into the mixture and magnetic stirring 1000 rpm for 1 h, followed by sonicating for 2 h at 60 °C. Secondly, the short carbon fiber and flaky graphite were added respectively to the resultant mixture in the ratio of 10 wt% and 15 wt%, and then stirred 150 rpm for 2.5 h at 80 °C till completing the evaporation of alcoholic. Finally, the treated copper foam was put in a preheated open mold at 80 °C, and the final mixture was poured in and casted. The blends were cured at 135 °C and 36 MPa for 2.5 h, then at 190 °C and 48 MPa for 3 h. After curing, the specimens were cooled naturally to room temperature. With this method (As shown in Fig. 1), the copper foam/resin composites containing the MWCNTs of 0 wt%, 0.1 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt% and 1 wt% were prepared, and the densities of the composites were 1.61–1.66 g/cm3.
ω=
composites
were
cut
into
the
samples
v⋅t
(2)
where minitial is the sample mass before the current-carrying friction test, mpost is the sample mass after testing and v is the speed and t is the time during each test. A high precision digital scale (SHIMADZU, AUY220) with an accuracy of ± 0.0001 mg was used for the mass measurements. The microstructures of the MWCNTs-reinforced copper foam/resin composites were investigated using an optical microscope (OM, DM4000 M). Scanning electron microscope (SEM) images for fracture morphology and worn surface were obtained using a Nova NanoSEM230 microscope. Transmission electron microscopy (TEM) images were carried out on a Tecnai G2 20 S-Twin instrument in bright field. 3. Results and discussion 3.1. Morphology of MWCNT and MWCNT-reinforced copper foam/resin composite Fig. 4 shows the transmission electron microscopy (TEM) images of the untreated and ultrasonic treated multi-walled carbon nanotubes (MWCNTs). The raw MWCNTs have slender shapes and hollow
2.3. Measurement of electrical resistivity The
minitial − mpost
of 573
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Fig. 1. Schematic diagram of MWCNTs-reinforced copper foam/resin composite.
treatment, most of the agglomerations and entanglements are opened and dispersed evenly (Fig. 4b), and new defects appear in the MWCNTs. As shown in Fig. 4b, the MWCNTs have some new fractures and the length is lower than that of the untreated MWCNTs, which are easy to be dispersed due to the low length and diameter ratio [19,20]. Fig. 4c shows XRD pattern of the ultrasonic treated MWCNT, and the presence of diffraction peak at 2θ = 26° corresponds to (002) graphite. There is a sharp (002) peak at 26°, and the crystal orientation of the MWCNT is relatively ordered along the z axis (Fig. 4a), which confirms the carbon of the MWCNT has a high crystallinity. The high crystallinity is not only good for its conductivity property, it also endows the material high strength and modulus. Fig. 5 shows the morphology images of the copper foam and the MWCNTs-reinforced copper foam/resin composite. As shown in Fig. 5 (a), the copper foam has a porous structure with an interconnected 3D metallic network, which seems to be a hexagon. After the densifying progresses of the impregnation and mold pressing, the optical
Fig. 2. Schematic of polished sample for the electrical resistivity measurement.
channels, with the diameter and wall thickness of approximately 20–30 nm and 5–8 nm, respectively. Before the acid purifying and ultrasonic treatments (Fig. 4a), there are some agglomerations and entanglements existing in the MWCNTs. Through the ultrasonic
Fig. 3. (a) Schematic of the HST-100 friction and wear tester with electric current. 1. pin specimen, 2. disc specimen, 3. insulated material, 4. photosensitive triode, 5. AC driving motor, 6. computer, 7. DC power supply, 8. hydraulic device, 9. load shaft, and 10. collector pin frame; (b) MWCNTs-reinforced copper foam/resin composite for pin sample.
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Fig. 4. TEM images of (a) untreated MWCNTs, (b) ultrasonic treated MWCNTs; (c) XRD image of ultrasonic treated MWCNTs. Typical dimensions of the MWCNTs are about L = 5–15 μm and d = 20–30 nm and L/d ≤ 500.
microscope images (Fig. 3c and d) show that the MWCNTs-reinforced copper foam/resin composite has a dense matrix, and the interfaces between the furan resin and the reinforcing fillers (flaky graphite, carbon fiber and copper foam) have good bonding condition with very few defects being found. Meanwhile, the flaky graphite and carbon fibers are dispersed evenly in the matrix, which is beneficial to achieve their full potential of the reinforcement. However, it is difficult to observe the dispersion state of the MWCNTs, so the percolation threshold and theoretical formula and fracture surface morphology are used to solve this problem in this study.
3.2. Electrical resistivity and percolation threshold A sliding contact material with well comprehensive properties requires sufficient electrical conductivity, mechanical strength, good lubricating and wear resistance properties [21,22]. To improve electrical conductivity of the composite, the copper foam and MWCNTs were added as the reinforcing fillers. As shown in Fig. 6, the electrical resistivities of the composites are plotted as a function of the MWCNTs contents. All the MWCNTs-reinforced copper foam/resin composites have low electrical resistivity (less than 4 μΩ m) due to the improvement of the copper foam with the interconnected 3D metallic network. The electrical resistivity is lower than the other sliding contact materials, including the pure carbon composite (about 25 μΩ m) and copper/ polymer composite with the same copper content (more than 12 μΩ m)
Fig. 6. Electrical resistivities of the composites with different MWCNTs contents: 0 wt% MWCNTs, 0.1 wt% MWCNTs, 0.25 wt% MWCNTs, 0.5 wt% MWCNTs, 0.75 wt% MWCNTs, 1 wt% MWCNTs.
[8,9,23]. Due to the high aspect ratio and high specific surface area of the MWCNTs, the low content filler can form an electrical network in the composite, which transforms easily in the resin matrix from the insulator to conductor [24,25]. Percolation threshold is the value of MWCNTs content when the MWCNTs form the interconnected network in the composite, which is usually used to evaluate the dispersion state
Fig. 5. Morphologies of the composites: (a) SEM images of copper foam; (b) (c) Optical microscope of the MWCNTs-reinforced copper foam/resin composite. 575
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of the MWCNTs [26,27]. As reported in the literatures [28–31], the percolation threshold of typical CNT-epoxy composites varies from 0.002 wt% to 4 wt% depending on the type of CNTs and the processing techniques. For the MWCNTs-reinforced copper foam/resin composite in Fig. 6, as the MWCNTs content increases from 0 wt% to 1 wt%, the electrical resistivity of the composite decreases sharply from 3.582 μΩ m to 2.884 μΩ m (about 19.49% decrement) and then tends to be stable. Meanwhile, the standard deviation of the electrical resistivity also decreases with increasing of the MWCNTs content. According to the variation tendency, the percolation threshold of the MWCNTs is approximate 0.1–0.25 wt% (or 0.074–0.185 vol%) in the composite, which is relatively low compared to the previous reports. On the basis of the aspect ratio and percolation threshold, the descriptive dispersion parameters are calculated using a model to analyze the disperse state of the MWCNTs in the composite. Suppose that the individual CNT is cylindrical with length l and diameter d , ε is the localized volume content of CNTs in an agglomerate, and ξ is the volume fraction of agglomerated CNTs (0«1, 0«1). The percolation threshold, pc , is given by [32].
pc =
ξεπ (1 − ξ )27πd 2 + 6 4l 2
Table 1 Impact strength of the composites with different MWCNTs contents.
(
I
d
Impact strength, kj/m2
MWCNT-0% MWCNT-0.1% MWCNT-0.25% MWCNT-0.5% MWCNT-0.75% MWCNT-1%
0.382 0.464 0.525 0.553 0.549 0.538
± ± ± ± ± ±
0.021 0.015 0.019 0.010 0.018 0.020
+ + + + + +
0% 21.47% 37.43% 44.76% 43.72% 40.84%
content increases, the impact strength of the composite increases firstly and then tends to be stable. The addition of MWCNTs into the composite results in a rapid increase of the impact strength when the content is less than 0.5 wt%. The maximum reinforcement is obtained for the case of 0.5 wt%, and the impact strength is improved by 44.76%. The impact strength (0.553 J/cm2) of the composite with 0.5 wt% MWCNTs is higher than those of the sliding contact material of the copper/resin composites [7,41], and close to the national standard (0.55 J/cm2) of the carbon brush in China. However, as the MWCNTs content is more than 0.5 wt%, the impact strength will be stable or even decrease. These phenomena may be explained in terms of the disperse state of the MWCNTs in resin matrix. Fig. 8 shows SEM images of the fracture surfaces of the composites with different MWCNTs contents, and the MWNCTs characterization can be clearly observed, which reveals the dispersion state of the MWNCTs in the composite as an experimental method. Based on the morphological analysis, fracture mechanisms of the composites are identified. As shown in Fig. 8a, the composite without MWCNTs exhibits a smooth and mirror-like fracture surface representing brittle failure, which explains that this composite has relative low impact strength. For the composite with 0.1 wt% MWCNTs (Fig. 8b), the potholes are formed on the fracture surface as the MWCNTs are pulled out during the fracture process, whose morphology is like the dimple on fracture theory of the metal. With further increasing MWCNTs content (Fig. 8c and d), the fracture surfaces of the composites reveal a systematic increase in the number of the potholes and the corresponding surface roughness, which means that the fracture mechanism changes to a ductile fracture. It appears that the increasing number of the potholes corresponds to the MWCNTs content. It is because the MWCNTs of the composite forces the cracks to propagate by bypassing and taking a long path, which makes the composite break require more energy according to the well-known pinning and crack tip bifurcation mechanisms. Therefore, the impact strength of the composite increases with the MWCNTs content. Meanwhile, a higher MWCNTs (more than 0.5 wt%) content results in an increasing probability of the agglomeration. As showed in Fig. 8e and f, the MWCNTs dispersions are uneven and there are some agglomerations in the composites with 0.75% and 1% MWCNTs. Therefore, when the MWCNTs content is more than 0.5 wt%, the more filler would not result in further increasing of the impact strength.
(3)
To simplify the prediction, the dispersion paremeters (ξ , ε ) are assumed to be identical, and the effect of aspect ratio on the percolation thresholds can be showed in Fig. 7. As a result of previous analysis on the electrical current, the percolation threshold of the MWCNTs for the composite is found to be approximate 0.074–0.185 vol%. Excluding the affections of the flaky graphite, carbon fiber and copper foam (Total volume ≈ 23.5 vol%), the actual percolation threshold is about 0.097–0.242 vol%. Meanwhile, for the aspect ratio of the MWCNT with not more than 500 in Fig. 4, the dispersion paremeters (ξ = ε ) of the MWCNTs in the composite can be calculated and are less than 0.07 in accordance with equation (3) and Fig. 7. It means that the volume fraction of the agglomerated MWCNTs in the composite and the localized volume content of the MWCNTs in an agglomerate are all about 7 vol%. This value about the dispersion state of the MWCNTs is relative low. It is similar to that in the literatures [33,34] (ξ = ε ≈ 0.06) for the aspect ratio of 500
MWCNT concentration in composite, wt %
)
= 500 under the ultrasonic dispersion but lower
than that of the literatures [35,36] (ξ = ε ≈ 0.1). 3.3. Impact properties Table 1 shows the impact properties of the composites with different MWCNTs contents measured from the impact tests. As the MWCNTs
3.4. Friction coefficient and wear rate Fig. 9 shows the evolutions of the friction coefficient and the average friction coefficients of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N. After a running-in process (about 400s), the friction coefficients gradually stabilize instead of fluctuating with the time. Meanwhile, the friction coefficients of the composites with 0.5 wt% and 1.0 wt% MWCNTs are more stable than those of the composites with 0 wt% and 0.1 wt%. Contrasting with the average friction coefficients of the composites, it tends to decrease with increasing the MWCNTs content. When the nanofiller consists of only 0.1 wt% MWCNTs, the average friction coefficient of the composite is 0.30, which is not much different from that of the composite without
Fig. 7. Effect of aspect ratio on the percolation threshold of the composites with varying dispersion states. 576
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Fig. 8. Fracture surface morphologies of the composites with different MWCNTs content: (a) 0 wt% MWCNTs; (b) 0.1 wt% MWCNTs; (c) 0.25 wt% MWCNTs; (d) 0.5 wt% MWCNTs; (e) 0.75 wt% MWCNTs; (f) 1 wt% MWCNTs.
the MWCNTs (0.31). However, as the MWCNTs content is more than 0.25 wt%, the average value and amplitude of the friction coefficients have a considerable decrease. With further increasing the MWCNTs content from 0.5 wt% to 1 wt%, the average friction coefficient still tends to decrease. It is because that the MWCNTs with high crystallinity has a good lubricating performance. These results certainly infer that the friction behavior of the nanocomposite could be improved effectively by the MWCNT during the current-carrying friction. Mohammed et al. [37] and Meng et al. [38] have also found similar results. They reported that the friction behaviors the of nanocomposites were improved using the MWCNTs under friction tests without electrical current. Fig. 10 shows wear rates of the composites with different MWCNTs contents. With increasing the MWCNTs content, the wear rate decreases quickly and then tends to be stable. The wear rate of the composite without MWCNTs is 6.25 mg/km. As the incorporation of a small amount of MWCNTs (0.25 wt%) into the resin matrix, the wear rate of the composite decreases by 9%. With increasing the MWCNTs content to 0.75 wt%, the wear rate reaches to the minimum value 5.11 mg/km, which is less than those (0.58 mg/km) of the pure carbon composite under similar test condition [9]. The 0.75 wt% MWCNTs addition causes a 22.31% decrease of the wear rate. However, with further
Fig. 10. Wear rates of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N.
increasing the MWCNTs content to 1 wt%, the wear rate begins to increase. It can be explained by the agglomeration of the MWCNTs in the composite (Fig. 8f). Based on the above analysis, it is obvious that the composites with the MWCNTs have lower friction coefficient and wear rate, and MWCNT is an effective filler in improving tribology
Fig. 9. Typical friction curves and average friction coefficient of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N. 577
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Fig. 11. SEM morphologies of the worn surface of the MWCNTs-reinforced copper foam/resin composites: (a, d, g) 0 wt% MWCNTs; (b, e, h) 0.5 wt% MWCNTs; (c, f, i) 1 wt% MWCNTs.
Fig. 12. SEM morphologies of the wear debris of the MWCNTs-reinforced copper foam/resin composites: (a, c) 0 wt% MWCNTs; (b, d) 1 wt% MWCNTs. 578
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Fig. 13. Physical models of wear mechanism for the composites without MWCNTs and with MWCNTs.
found as a cross among the wear debris in Fig. 12d. Under the reinforcement of the MWCNTs, the crack propagation is arrested, and the formation of the big debris is difficult for the composite with 1 wt% MWCNTs. Therefore, the composite with more MWCNTs has better wear resistance.
performance for the copper foam/resin composite during current-carrying friction.
3.5. Morphology of worn surface and wear debris Fig. 11 shows SEM morphologies of the worn surfaces of the MWCNTs-reinforced copper foam/resin composites with 0 wt%, 0.5 wt % and 1 wt% MWCNTs. As shown in Fig. 11(a, d and g), the composite without the MWCNTs has a coarser worn surface, where the resin matrix has been broken and some furrows, spalling pits and cracks are formed. For the composites with the 0.5 wt% (Fig. 11b and c) and 1 wt % MWCNTs (Fig. 11e and f), the furrows are smaller and there is a comparatively smooth film on the worn surfaces. Meanwhile, the worn surface of the composite with 1 wt% MWCNTs is the most smooth and continuous among the three composites. It can be explained that: (1) Because MWCNT has good thermal and electrical conductivities (Fig. 6), the composites with the MWCNTs have lower resistance heat during the current-carrying friction, and the friction and resistance heats spread more quickly on the worn surfaces, which could prevent overheating and decrease wear loss during the current-carrying friction test. (2) The MWCNT improves the mechanical strength of the composite matrix by blocking the cracks propagation. As shown in Fig. 11(h and i), although the resin matrixes are destroyed, some debris are still connected with the worn surface by the MWCNTs and will not drop out immediately, which benefits that the broken surface is restored to a smooth surface during the current-carrying friction test. (3) After the MWCNTs are broken during the current-carrying friction, the debris has a small size due to the nanometer characterization and the high crystalline degree (Fig. 4c), which is usually used to fill in the spalling spits and restore the worn surface. Therefore, by contrasting with the composite without the MWCNTs, the composites with 0.5 wt% and 1 wt% MWCNTs have a smoother and more continuous worn surface. These results are consistent with the previous conclusions that the composite with more MWCNTs has the lower friction coefficient (Fig. 9) and wear rate (Fig. 10). Fig. 12 shows SEM images of the wear debris of the composites with 0 wt% and 1 wt% MWCNTs collected after the current-carrying friction tests. For the wear debris of the composite without the MWCNTs as shown in Fig. 12 (a, c), the diameter of the particles varies from several nanometers to 50 μm, which includes two main types of the wear debris. One is the wear debris with a large size and flaky shape (Fig. 12c), and most of them fall directly from the worn surface caused by the crack extension. The other has a small size and oval shape, which attributes to the shearing and grinding actions between the friction couple. However, for the wear debris of the composite with 1 wt% MWCNTs (Fig. 12b, d), most of the sizes are below 10 μm and there is a high proportion of small wear debris. Meanwhile, many MWCNTs are
3.6. Wear mechanism In fact, the process of forming friction film on the worn surface is a dynamic balance during the friction tests [39,40]. Under the combined effects of the shearing stress and normal force, the cracks grow and propagate along the defects on the worn surface, and the particles finally fall off from the matrix, leading to the formation of the spalling pits. The particles are grinded into the wear debris between the friction couple. Some of the wear debris will escape from the friction surface driven by acentric force, while the others will be filled in the spalling pits and compacted to restore the friction film. Fig. 13 shows two wear models of the composites with MWCNTs and without MWCNTs, which are used to schematically illustrate the wear mechanism. Because the composite without the MWCNTs has the brittle resin matrix, the cracks grow easily under the combined actions of shearing stress and friction heat during the current-carrying friction, resulting in the large resin particles breaking off from the matrix and the formation of the spalling pits. It is difficult for the big spalling pits to be filled up, so there are some spalling pits existing on the worn surface (Fig. 11d). For the composites with the MWCNTs, the crack propagation requires more energy due to the reinforcement of the MWCNTs in the resin matrix, and the spalling pits are smaller during the current-carrying friction test. Meanwhile, the small wear debris with the MWCNTs is easy to be left and fill up the spalling pits to form a uniform and compacted film on the worn surface (Fig. 11b and c). Therefore, the composites with the MWCNTs exhibit better friction and wear behaviors than the composite with pure resin matrix during the current-carrying friction test. 4. Conclusions In this paper, a novel sliding contact strip material of MWCNTsreinforced copper foam/resin composite was fabricated using an acid purify and ultrasonic treatment of MWCNTs dispersed in the furan resin, and the mixing and addition technology of short carbon fiber and flaky graphite, then a conventional molding technique. Due to the high specific surface area of the MWCNT, the low content of the filler can form an electrical network and improve the electrical conductivity of the composite, and the percolation threshold of the MWCNTs is found to be approximate 0.074–0.185 vol% in the composite. Because of the MWCNTs blocking the cracks propagation in the resin matrix, the 579
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impact strength and wear resistance of the composite could be increased respectively 45% and 22% by the MWCNTs addition. Therefore, the MWCNT is a potentially reinforcing filler for the sliding contact material, which not only increases the electrical conductivity and mechanical performances but also improves the tribology behavior during the current-carrying friction test.
[17]
[18]
[19]
Acknowledgment [20]
The authors thank Prof. Yongzhen Zhang from the Department of Material Science and Engineering of the Henan University of Science and Technology for providing the friction tester. The authors are very thankful for 2017 Natural Science Foundation of Hunan Province [No. 2017JJ3514] and National Natural Science Foundation of China [No. 51302322 and No. 51602351]. Additionally, the first author would like to greatly acknowledge financial support from China Scholarship Council [No. 201706370190].
[24]
Appendix A. Supplementary data
[25]
[21]
[22]
[23]
Supplementary data related to this article can be found at https:// doi.org/10.1016/j.compositesb.2019.03.067.
[26]
References
[27]
[1] Ray BC, Rathore D. Durability and integrity studies of environmentally conditioned interfaces in fibrous polymeric composites: critical concepts and comments. Adv Colloid Interface Sci 2014;209:68–83. [2] Mohammed AS, Fareed MI. Improving the friction and wear of poly-etherketone (PEEK) by using thin nano-composite coatings. Wear 2016;364–365:154–62. [3] Hoskins TJ, Dearn KD, Chen YK, Kukureka SN. The wear of PEEK in rolling sliding contact-simulation of polymer gear applications. Wear 2014;309:35–42. [4] Williams J, Graddage N, Rahatekar S. Effects of plasma modified carbon nanotube interlaminar coating on crack propagation in glass epoxy composites. Compos Part A 2013;54:173–81. [5] Fredi G, Dorigato A, Luca Fambri, Pegoretti A. Wax confinement with carbon nanotubes for phase changing epoxy blends. Polymers 2017;405:1–16. [6] Sebastian R, Noll A, Zhang G, Burkhart T, Wetzel B. Evaluation of the role of functionalized CNT in glass fiber/epoxy composite at above- and sub-zero temperatures: emphasizing interfacial microstructures. Compos Part A 2017;101:215–26. [7] Tu CJ, Chen ZH, Xia JT. Thermal wear and electrical sliding wear behaviors of the polyimide modified polymer-matrix pantograph contact strip. Tribol Int 2009;42:995–1003. [8] Yuan H, Wang CG, Zhang S, Lin X. Effect of surface modification on carbon fiber and its reinforced phenolic matrix. Appl Surf Sci 2012;259:288–93. [9] Wang P, Zhang HB, Yin J, Xiong X, Tan C, Deng CY. Wear and fiction behaviors of copper mesh and flaky graphite-modified carbon/carbon composite for sliding contact material under electric current. Wear 2017;380–381:59–65. [10] Deng CY, Yin J, Zhang HB, Xiong X, Wang P, Sun M. The tribological properties of Cf/Cu/C composites under applied electric curren. Tribol Int 2017;116:84–94. [11] Ji KJ, Xu YS, Zhang J, Chen J, Dai ZD. Foamed-metal-reinforced composites: tribological behavior of foamed copper filled with epoxy–matrix polymer. Mater Des 2014;61:109–16. [12] Yan SL, Ren J. Research on the mechanics and friction behaviour of a foam Ni/ polytetrafluoroethylene composite. Mater Des 2012;39:425–31. [13] Gao S, Villacorta BS, Ge L, Rufford TE, Zhu ZH. Effect of sonication and hydrogen peroxide oxidation of carbon nanotube modifiers on the microstructure of pitchderived activated carbon foam discs. Carbon 2017;124:142–51. [14] Farrash SMH, Shariati M, Rezaeepazhand J. The effect of carbon nanotube dispersion on the dynamic characteristics of unidirectional hybrid composites: an experimental approach. Compos Part B 2017;122:1–8. [15] El MA, Tarfaoui M, Lafdi K. Mechanical characterization of carbonnanotubes based polymer composites using indentation tests. Compos Part B 2017;114:1–7. [16] Ürk D, Demir E, Bulut O, Çakıroglu D, Cebeci FC, Öveçoğlua LM, et al. Understanding the polymer type and CNT orientation effect on the dynamic
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38]
[39]
[40]
[41]
580
mechanical properties of high volume fraction CNT polymer nanocomposites. Compos Struct 2016;155:255–62. Bouhamed A, Müller C, Choura S, Kanoun O. Processing and characterization of MWCNTs/epoxy nanocomposites thin films for strain sensing applications. Sens Actuators, A 2017;257:65–72. Ma HL, Jia Z, Lau K, Li X, Hui D, Shi S. Enhancement on mechanical strength of adhesively-bonded composite lap joints at cryogenic environment using coiled carbon nanotubes. Compos Part B 2017;110:396–401. Krause B, Mende M, Potschke P, Petzold G. Dispensability and particle size distribution of CNTs in an aqueous surfactant dispersion as a function of ultrasonic treatment time. Carbon 2010;48:2746–54. Zhong J, Isayev AI. Ultrasonically assisted compounding of CNT with polypropylenes of different molecular weights. Polymers 2016;107:130–46. Xiong XZ, Tu CJ, Chen D, Zhang JQ, Chen JH. Arc erosion wear characteristics and mechanisms of carbon strip against copper under arcing conditions. Tribol Lett 2014;53:293–301. Kubo S, Kato K. Effect of arc discharge on the wear rate and wear mode transition of a copper impregnated metallized carbon contact strip sliding against a copper disc. Tribol Int 1999;32:367–78. Yang L, Ran LP, Yi MZ. Carbon fiber knitted fabric reinforced copper composite for sliding contact material. Mater Des 2011;32:2365–9. Qu MC, Nilsson F, Qin YJ, Yang GD, Pan YM, Liu XH, et al. Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black. Compos Sci Technol 2017;150:24–31. Yang H, Gong J, Wen X, Xue J, Chen Q, Jiang ZW, et al. Effect of carbon black on improving thermal stability, flame retardancy and electrical conductivity of polypropylene/carbon fiber composites. Compos Sci Technol 2015;113:31–7. Gojny FH, Wichmann MHG, Fiedler B, Kinloch IA, Bauhofer W, Windle AH, et al. Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites. Polymer 2006;47:2036–45. Moisala A, Li Q, Kinloch IA, Windle AH. Thermal and electrical conductivity of single- and multi-walled carbon nanotube-epoxy composites. Compos Sci Technol 2006;66:1285–8. Dumas L, Bonnaud L, Olivier M, Poorteman M, Dubois P. Multiscale benzoxazine composites: the role of pristine CNTs as efficient reinforcing agents for high-performance applications. Compos Part B 2017;112:57–65. Bryning MB, Islam MF, Kikkawa JM, Yodh AG. Very low conductivity threshold in bulk isotropic single-walled carbon nanotube-epoxy composites. Adv Mater 2005;17:1186–91. Yasser Z, Rhee KY. A power model to predict the electrical conductivity of CNT reinforced nanocomposites by considering interphase, networks and tunneling condition. Compos Part B 2018;155:11–8. Drakonakis VM, Aureli M, Doumanidis CC, Seferis JC. Modulus–density negative correlation for CNT-reinforced polymer nanocomposites: modeling and experiments. Compos Part B 2015;70:175–83. Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim JK. Correlations between percolation threshold, dispersion state and aspect ratio of carbon nanotubes. Adv Funct Mater 2007;17:3207–15. Bai JB, Allaoui A. Effect of the length and the aggregate size of MWNTs on the improvement efficiency of the mechanical and electrical properties of nanocomposites—experimental investigation. Compos Part A 2003;34:684–94. Hoseini AHA, Arjmand M, Sundararaj U, Trifkovic M. Significance of interfacial interaction and agglomerates on electrical properties of polymer-carbon nanotube nanocomposites. Mater Des 2017;125:126–34. Kim YJ, Shin TS, Choi HD, Kwon JH, Chung YC, Yoon G H. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005;43:23–30. Allaoui A, Bai S, Cheng HM, Bai JB. Mechanical and electrical properties of a MWNT/epoxy composite. Compos Sci Technol 2002;62:1993–8. Mohammed AS, Fareed MI. Improving the friction and wear of poly-ether-etherketone (PEEK) by using thin nano-composite coatings. Wear 2016;364–365:154–62. Meng H, Sui GX, Xie GY, Yang R. Friction and wear behavior of carbon nanotubes reinforced polyamide 6 composites under dry sliding and water lubricated condition. Compos Sci Technol 2009;69:606–11. Han JH, Zhang H, Chu PF, Imani A, Zhang Z. Friction and wear of high electrical conductive carbon nanotube buckypaper/epoxy composites. Compos Sci Technol 2015;114:1–10. Gonzalez JJ, Datye A, Wu KH, Schneider J, Belmonte M. Robust and wear resistant in-situ carbon nanotube/Si3N4 nanocomposites with a high loading of nanotubes. Carbon 2014;72:338–47. Tu CJ, Chen D, Chen ZH, Xia JT. Improving the tribological behavior of graphite/Cu matrix self-lubricating composite contact strip by electroplating Zn on graphite. Tribol Lett 2008;31:91–8.
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of MWCNT content on conductivity and mechanical and wear properties of copper foam/resin composite
T
Pei Wanga,b, Guanyu Dengb, Hongtao Zhub, Hongbo Zhanga, Jian Yina,c,∗, Xiang Xionga, Xiaoguang Wua a
Science and Technology on High Strength Materials Laboratory, Central South University, Changsha, 410083, PR China Faculty of Engineering and Information Sciences, University of Wollongong, Northfields Avenue, Wollongong, NSW, 2522, Australia c Swanson School of Engineering, Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, PA, 15261, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Copper foam Carbon fiber Wear Current-carrying friction Carbon nanotube
Multi-walled carbon nanotube (MWCNT) and carbon fiber and flaky graphite reinforced copper foam/furan resin composite for a novel sliding contact material was fabricated using an acid purify and ultrasonic dispersion technology, followed by a mold pressing process. The microstructure, electrical resistivity, impact strength and current-carrying friction performances of the composites were measured to investigate the effect of the different MWCNT contents. The results showed that the MWCNTs had a good dispersion state in the composite and the percolation threshold was approximate 0.074–0.185 vol%. As addition of the MWCNTs, the impact strength and wear resistance of the composites could be raised by 45% and 22%, respectively.
1. Introduction In the field of tribology, polytetrafluoroethylene, polyoxymethylene, polyamide and other polymer materials with good tribological characteristics are widely used as solid lubricating materials [1–3]. As one of them, thermosetting polymer has good mechanical properties, chemical/corrosion resistance, low density, low shrinkage during forming, simple fabrication, low cost and so on, which can meet the requirements of the pantograph strip and electrical brush during the current-carrying friction, so it is often used to fabricate the sliding contact materials in the railway transport systems and electric machinery [4,7,8,11]. However, polymer composite has some shortcomings, such as insufficient toughness and poor thermal and electrical conductivities. All these factors lead to high wear losses and poor friction behaviors when the material is used as a contact sliding material. To improve these weaknesses, researchers have tried several methods, such as the additions of the fiber and copper powder and have gained certain achievements [5,6]. Tu [7] and Yuan [8] investigated the modified resin-matrix pantograph contact strip with copper power and carbon fibers prepared by hot repressing, solidification and dipping treatment processes. Meanwhile, it was reported that a new sliding contact material was able to be prepared using by the carbon fibers and copper fibers reinforced the epoxy resin carbon [9,10]. The results showed that these composites had nice friction and wear performances
∗
as well as mechanical strength. However, the improvement of thermal and electrical conductivities of the copper alloys were limited, due to one-dimensional or two-dimensional structures isolated in the matrix. Open-cell copper foam has a porous structure with the interconnected 3D metallic network and possesses many special properties, such as high conductivity, low density, specific mechanic performance and high specific surface area, which has been used for supports, electrodes and heat radiators [11,12]. In our current study, a novel sliding contact material was fabricated by a mold pressing process, using the copper foam as the composite preform with mixed short carbon fiber, flaky graphite and furan resin. The results show that addition of the carbon fiber and flaky graphite can improve the overall performance of the composite (including conductivity, strength and tribology property). However, they were found not to increase monotonically with the carbon fiber and flaky graphite contents and no more improvement has been found after an optimum addition, which is attributed to the agglomeration problem. Similar phenomenon has also been reported by Qu and co-authors [24]. It has been well known that carbon nanotube (CNT) has the high aspect ratio and high specific surface area, and the percolation threshold of CNTs-reinforced polymer nanocomposites is much lower than that of the composites containing carbon fibers and flaky graphite [13–15]. Moreover, owing to their unique mechanical, thermal and electrical properties, CNTs was usually used to develop advanced
Corresponding author. Science and Technology on High Strength Materials Laboratory, Central South University, Changsha, 410083, PR China. E-mail address: [email protected] (J. Yin).
https://doi.org/10.1016/j.compositesb.2019.03.067 Received 18 April 2018; Received in revised form 18 March 2019; Accepted 30 March 2019 Available online 01 April 2019 1359-8368/ © 2019 Elsevier Ltd. All rights reserved.
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60 mm × 4 mm × 4 mm, and their end-sections were polished to remove the isolate place. As shown in Fig. 2, the sample resistance R at room temperature was measured using a UIN-T UT620D Digital Micro Ohm Meter. The electrical resistivity ρ was calculated as follows
structural materials with high strength and wear resistant [16–18]. To further improve the comprehensive performances of the copper foam/ furan resin composite with some amount of the short carbon fiber and flaky graphite, the multi-walled carbon nanotubes (MWCNTs) with high specific surface area were added in the composite using a dispensations technology of the acid purify and ultrasonic treatment. In this paper, a novel sliding contact strip material of MWCNTsreinforced copper foam/resin composite was fabricated using an acid purify and ultrasonic treatment of MWCNTs dispersed in the furan resin, and the mixing and addition technology of short carbon fiber and flaky graphite, then a conventional molding technique. Namely SEM image of the fracture surface reveals the dispersion state of the MWNCTs in the local region of the composite as an experimental method, and percolation threshold indicates the dispersion state of the MWNCTs in whole region of the composite as a theoretical calculation. The electrical conductivity, impact strength, current-carrying friction and wear behaviors have been investigated to understand the effect of MWCNTs content on the comprehensive performances of the composite.
ρ=
R·S L
(1)
where R is the resistance of the composite and L is the distance of the sample and S is the section area of the sample. Five specimens for each condition were measured to obtain the average electrical resistivity. 2.4. Measurement of impact property
2. Experimental
Impact tests were carried out on a ZY-3003 miniature pendulumtype impact tester at room temperature, with hammer energy of 1 J and the impact velocity up to 2.9 m/s. It was necessary to check the impact machine and make sure the friction and wind age losses are with in allowable tolerances prior to testing. The effective size of the samples was 55 mm × 10 mm × 10 mm, and the span of the test was 40 mm. The results for each composite were determined by averaging from at least five specimens.
2.1. Materials
2.5. Measurement of friction and wear
Furan resin containing furfural-acetone resin, epoxy resin (Modifier) and phosphoric acid (Curing agent) was purchased from Hunan Boyun New Materials Co., Ltd., China. Short carbon fibers with a diameter of 7 μm and a length of 50–100 μm were purchased from Nanjing WeiDa Composite Material Co., Ltd., China. Open-cell copper foam with an aperture of 15 ppi and a porosity of 96% was purchased from Kunshan Jianyisheng Electronics Co., Ltd., China. Multi-walled carbon nanotubes (MWCNTs) with a diameter of 20–30 nm, a length of 5–15 μm, and a purity of > 97% were purchased from Shenzhen Turing Evolutionary technology Co., Ltd., China. Flaky graphite with a diameter of 150 mesh and a purity of > 99% was purchased from Shanghai Aladdin Bio-Chem Technology Co., LTD., China.
The friction and wear tests with electric current were conducted using a HST-100 multifunction friction and wear tester with a pin-ondisc tribometer, as shown in Fig. 3a. The test rig allowed testing at speed up to 100 m/s with DC electrical current of 300 A. The contact strip used under normal circumstances was replaced with a pin. The chemical composition of the disc (wt%) was 99.9% copper and 0.1% microelements as well as impurities. As demonstrated in Fig. 3b, the size of the pin was 14 mm × 9 mm × 20 mm, and a facade of 14 mm × 9 mm was used as the worn surface. The sliding tests were achieved at speeds of 20 m/s, with a constant load of 70 N and an DC electric current of 10 A. Prior to each electrical sliding test, the friction couple (pin and disc) was first polished using #1200 sandpaper, and the pre-worn without electric current for 3 km to ensure a conformal contact, then the current-carrying friction tests were finished. The friction coefficient was defined as friction force divided by normal force, and the measurement data were recorded automatically by the computer control system during testing. Wear rate ω was determined as the mass loss after each test:
2.2. Preparation of MWCNTs-reinforced copper foam/resin composite WMCNTs were oxidized in a mixed acid (HNO3:H2SO4 = 3:1, vol%) at 100 °C for 4 h and the excess of acid and amorphous carbon impurities were removed by centrifugation. The residual solids were cleaned with deionized water several times, and then vacuum dried in an oven at 60 °C. Meanwhile, copper foam was pretreated in acetone and dilute hydrochloric acid solution to remove oil and oxide on the surface, then it was washed with deionized water several times and vacuum dried at 60 °C. To prepare the MWCNTs-reinforced copper foam/resin composites, the treated MWCNTs were firstly dispersed in ethanol by an ultrasonic process for 1 h, then furan resin was poured into the mixture and magnetic stirring 1000 rpm for 1 h, followed by sonicating for 2 h at 60 °C. Secondly, the short carbon fiber and flaky graphite were added respectively to the resultant mixture in the ratio of 10 wt% and 15 wt%, and then stirred 150 rpm for 2.5 h at 80 °C till completing the evaporation of alcoholic. Finally, the treated copper foam was put in a preheated open mold at 80 °C, and the final mixture was poured in and casted. The blends were cured at 135 °C and 36 MPa for 2.5 h, then at 190 °C and 48 MPa for 3 h. After curing, the specimens were cooled naturally to room temperature. With this method (As shown in Fig. 1), the copper foam/resin composites containing the MWCNTs of 0 wt%, 0.1 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt% and 1 wt% were prepared, and the densities of the composites were 1.61–1.66 g/cm3.
ω=
composites
were
cut
into
the
samples
v⋅t
(2)
where minitial is the sample mass before the current-carrying friction test, mpost is the sample mass after testing and v is the speed and t is the time during each test. A high precision digital scale (SHIMADZU, AUY220) with an accuracy of ± 0.0001 mg was used for the mass measurements. The microstructures of the MWCNTs-reinforced copper foam/resin composites were investigated using an optical microscope (OM, DM4000 M). Scanning electron microscope (SEM) images for fracture morphology and worn surface were obtained using a Nova NanoSEM230 microscope. Transmission electron microscopy (TEM) images were carried out on a Tecnai G2 20 S-Twin instrument in bright field. 3. Results and discussion 3.1. Morphology of MWCNT and MWCNT-reinforced copper foam/resin composite Fig. 4 shows the transmission electron microscopy (TEM) images of the untreated and ultrasonic treated multi-walled carbon nanotubes (MWCNTs). The raw MWCNTs have slender shapes and hollow
2.3. Measurement of electrical resistivity The
minitial − mpost
of 573
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Fig. 1. Schematic diagram of MWCNTs-reinforced copper foam/resin composite.
treatment, most of the agglomerations and entanglements are opened and dispersed evenly (Fig. 4b), and new defects appear in the MWCNTs. As shown in Fig. 4b, the MWCNTs have some new fractures and the length is lower than that of the untreated MWCNTs, which are easy to be dispersed due to the low length and diameter ratio [19,20]. Fig. 4c shows XRD pattern of the ultrasonic treated MWCNT, and the presence of diffraction peak at 2θ = 26° corresponds to (002) graphite. There is a sharp (002) peak at 26°, and the crystal orientation of the MWCNT is relatively ordered along the z axis (Fig. 4a), which confirms the carbon of the MWCNT has a high crystallinity. The high crystallinity is not only good for its conductivity property, it also endows the material high strength and modulus. Fig. 5 shows the morphology images of the copper foam and the MWCNTs-reinforced copper foam/resin composite. As shown in Fig. 5 (a), the copper foam has a porous structure with an interconnected 3D metallic network, which seems to be a hexagon. After the densifying progresses of the impregnation and mold pressing, the optical
Fig. 2. Schematic of polished sample for the electrical resistivity measurement.
channels, with the diameter and wall thickness of approximately 20–30 nm and 5–8 nm, respectively. Before the acid purifying and ultrasonic treatments (Fig. 4a), there are some agglomerations and entanglements existing in the MWCNTs. Through the ultrasonic
Fig. 3. (a) Schematic of the HST-100 friction and wear tester with electric current. 1. pin specimen, 2. disc specimen, 3. insulated material, 4. photosensitive triode, 5. AC driving motor, 6. computer, 7. DC power supply, 8. hydraulic device, 9. load shaft, and 10. collector pin frame; (b) MWCNTs-reinforced copper foam/resin composite for pin sample.
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Fig. 4. TEM images of (a) untreated MWCNTs, (b) ultrasonic treated MWCNTs; (c) XRD image of ultrasonic treated MWCNTs. Typical dimensions of the MWCNTs are about L = 5–15 μm and d = 20–30 nm and L/d ≤ 500.
microscope images (Fig. 3c and d) show that the MWCNTs-reinforced copper foam/resin composite has a dense matrix, and the interfaces between the furan resin and the reinforcing fillers (flaky graphite, carbon fiber and copper foam) have good bonding condition with very few defects being found. Meanwhile, the flaky graphite and carbon fibers are dispersed evenly in the matrix, which is beneficial to achieve their full potential of the reinforcement. However, it is difficult to observe the dispersion state of the MWCNTs, so the percolation threshold and theoretical formula and fracture surface morphology are used to solve this problem in this study.
3.2. Electrical resistivity and percolation threshold A sliding contact material with well comprehensive properties requires sufficient electrical conductivity, mechanical strength, good lubricating and wear resistance properties [21,22]. To improve electrical conductivity of the composite, the copper foam and MWCNTs were added as the reinforcing fillers. As shown in Fig. 6, the electrical resistivities of the composites are plotted as a function of the MWCNTs contents. All the MWCNTs-reinforced copper foam/resin composites have low electrical resistivity (less than 4 μΩ m) due to the improvement of the copper foam with the interconnected 3D metallic network. The electrical resistivity is lower than the other sliding contact materials, including the pure carbon composite (about 25 μΩ m) and copper/ polymer composite with the same copper content (more than 12 μΩ m)
Fig. 6. Electrical resistivities of the composites with different MWCNTs contents: 0 wt% MWCNTs, 0.1 wt% MWCNTs, 0.25 wt% MWCNTs, 0.5 wt% MWCNTs, 0.75 wt% MWCNTs, 1 wt% MWCNTs.
[8,9,23]. Due to the high aspect ratio and high specific surface area of the MWCNTs, the low content filler can form an electrical network in the composite, which transforms easily in the resin matrix from the insulator to conductor [24,25]. Percolation threshold is the value of MWCNTs content when the MWCNTs form the interconnected network in the composite, which is usually used to evaluate the dispersion state
Fig. 5. Morphologies of the composites: (a) SEM images of copper foam; (b) (c) Optical microscope of the MWCNTs-reinforced copper foam/resin composite. 575
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of the MWCNTs [26,27]. As reported in the literatures [28–31], the percolation threshold of typical CNT-epoxy composites varies from 0.002 wt% to 4 wt% depending on the type of CNTs and the processing techniques. For the MWCNTs-reinforced copper foam/resin composite in Fig. 6, as the MWCNTs content increases from 0 wt% to 1 wt%, the electrical resistivity of the composite decreases sharply from 3.582 μΩ m to 2.884 μΩ m (about 19.49% decrement) and then tends to be stable. Meanwhile, the standard deviation of the electrical resistivity also decreases with increasing of the MWCNTs content. According to the variation tendency, the percolation threshold of the MWCNTs is approximate 0.1–0.25 wt% (or 0.074–0.185 vol%) in the composite, which is relatively low compared to the previous reports. On the basis of the aspect ratio and percolation threshold, the descriptive dispersion parameters are calculated using a model to analyze the disperse state of the MWCNTs in the composite. Suppose that the individual CNT is cylindrical with length l and diameter d , ε is the localized volume content of CNTs in an agglomerate, and ξ is the volume fraction of agglomerated CNTs (0«1, 0«1). The percolation threshold, pc , is given by [32].
pc =
ξεπ (1 − ξ )27πd 2 + 6 4l 2
Table 1 Impact strength of the composites with different MWCNTs contents.
(
I
d
Impact strength, kj/m2
MWCNT-0% MWCNT-0.1% MWCNT-0.25% MWCNT-0.5% MWCNT-0.75% MWCNT-1%
0.382 0.464 0.525 0.553 0.549 0.538
± ± ± ± ± ±
0.021 0.015 0.019 0.010 0.018 0.020
+ + + + + +
0% 21.47% 37.43% 44.76% 43.72% 40.84%
content increases, the impact strength of the composite increases firstly and then tends to be stable. The addition of MWCNTs into the composite results in a rapid increase of the impact strength when the content is less than 0.5 wt%. The maximum reinforcement is obtained for the case of 0.5 wt%, and the impact strength is improved by 44.76%. The impact strength (0.553 J/cm2) of the composite with 0.5 wt% MWCNTs is higher than those of the sliding contact material of the copper/resin composites [7,41], and close to the national standard (0.55 J/cm2) of the carbon brush in China. However, as the MWCNTs content is more than 0.5 wt%, the impact strength will be stable or even decrease. These phenomena may be explained in terms of the disperse state of the MWCNTs in resin matrix. Fig. 8 shows SEM images of the fracture surfaces of the composites with different MWCNTs contents, and the MWNCTs characterization can be clearly observed, which reveals the dispersion state of the MWNCTs in the composite as an experimental method. Based on the morphological analysis, fracture mechanisms of the composites are identified. As shown in Fig. 8a, the composite without MWCNTs exhibits a smooth and mirror-like fracture surface representing brittle failure, which explains that this composite has relative low impact strength. For the composite with 0.1 wt% MWCNTs (Fig. 8b), the potholes are formed on the fracture surface as the MWCNTs are pulled out during the fracture process, whose morphology is like the dimple on fracture theory of the metal. With further increasing MWCNTs content (Fig. 8c and d), the fracture surfaces of the composites reveal a systematic increase in the number of the potholes and the corresponding surface roughness, which means that the fracture mechanism changes to a ductile fracture. It appears that the increasing number of the potholes corresponds to the MWCNTs content. It is because the MWCNTs of the composite forces the cracks to propagate by bypassing and taking a long path, which makes the composite break require more energy according to the well-known pinning and crack tip bifurcation mechanisms. Therefore, the impact strength of the composite increases with the MWCNTs content. Meanwhile, a higher MWCNTs (more than 0.5 wt%) content results in an increasing probability of the agglomeration. As showed in Fig. 8e and f, the MWCNTs dispersions are uneven and there are some agglomerations in the composites with 0.75% and 1% MWCNTs. Therefore, when the MWCNTs content is more than 0.5 wt%, the more filler would not result in further increasing of the impact strength.
(3)
To simplify the prediction, the dispersion paremeters (ξ , ε ) are assumed to be identical, and the effect of aspect ratio on the percolation thresholds can be showed in Fig. 7. As a result of previous analysis on the electrical current, the percolation threshold of the MWCNTs for the composite is found to be approximate 0.074–0.185 vol%. Excluding the affections of the flaky graphite, carbon fiber and copper foam (Total volume ≈ 23.5 vol%), the actual percolation threshold is about 0.097–0.242 vol%. Meanwhile, for the aspect ratio of the MWCNT with not more than 500 in Fig. 4, the dispersion paremeters (ξ = ε ) of the MWCNTs in the composite can be calculated and are less than 0.07 in accordance with equation (3) and Fig. 7. It means that the volume fraction of the agglomerated MWCNTs in the composite and the localized volume content of the MWCNTs in an agglomerate are all about 7 vol%. This value about the dispersion state of the MWCNTs is relative low. It is similar to that in the literatures [33,34] (ξ = ε ≈ 0.06) for the aspect ratio of 500
MWCNT concentration in composite, wt %
)
= 500 under the ultrasonic dispersion but lower
than that of the literatures [35,36] (ξ = ε ≈ 0.1). 3.3. Impact properties Table 1 shows the impact properties of the composites with different MWCNTs contents measured from the impact tests. As the MWCNTs
3.4. Friction coefficient and wear rate Fig. 9 shows the evolutions of the friction coefficient and the average friction coefficients of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N. After a running-in process (about 400s), the friction coefficients gradually stabilize instead of fluctuating with the time. Meanwhile, the friction coefficients of the composites with 0.5 wt% and 1.0 wt% MWCNTs are more stable than those of the composites with 0 wt% and 0.1 wt%. Contrasting with the average friction coefficients of the composites, it tends to decrease with increasing the MWCNTs content. When the nanofiller consists of only 0.1 wt% MWCNTs, the average friction coefficient of the composite is 0.30, which is not much different from that of the composite without
Fig. 7. Effect of aspect ratio on the percolation threshold of the composites with varying dispersion states. 576
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Fig. 8. Fracture surface morphologies of the composites with different MWCNTs content: (a) 0 wt% MWCNTs; (b) 0.1 wt% MWCNTs; (c) 0.25 wt% MWCNTs; (d) 0.5 wt% MWCNTs; (e) 0.75 wt% MWCNTs; (f) 1 wt% MWCNTs.
the MWCNTs (0.31). However, as the MWCNTs content is more than 0.25 wt%, the average value and amplitude of the friction coefficients have a considerable decrease. With further increasing the MWCNTs content from 0.5 wt% to 1 wt%, the average friction coefficient still tends to decrease. It is because that the MWCNTs with high crystallinity has a good lubricating performance. These results certainly infer that the friction behavior of the nanocomposite could be improved effectively by the MWCNT during the current-carrying friction. Mohammed et al. [37] and Meng et al. [38] have also found similar results. They reported that the friction behaviors the of nanocomposites were improved using the MWCNTs under friction tests without electrical current. Fig. 10 shows wear rates of the composites with different MWCNTs contents. With increasing the MWCNTs content, the wear rate decreases quickly and then tends to be stable. The wear rate of the composite without MWCNTs is 6.25 mg/km. As the incorporation of a small amount of MWCNTs (0.25 wt%) into the resin matrix, the wear rate of the composite decreases by 9%. With increasing the MWCNTs content to 0.75 wt%, the wear rate reaches to the minimum value 5.11 mg/km, which is less than those (0.58 mg/km) of the pure carbon composite under similar test condition [9]. The 0.75 wt% MWCNTs addition causes a 22.31% decrease of the wear rate. However, with further
Fig. 10. Wear rates of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N.
increasing the MWCNTs content to 1 wt%, the wear rate begins to increase. It can be explained by the agglomeration of the MWCNTs in the composite (Fig. 8f). Based on the above analysis, it is obvious that the composites with the MWCNTs have lower friction coefficient and wear rate, and MWCNT is an effective filler in improving tribology
Fig. 9. Typical friction curves and average friction coefficient of the composites with different MWCNTs contents under the test conditions of the electric current of 10 A, the sliding speed of 20 m/s, the normal load of 70 N. 577
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Fig. 11. SEM morphologies of the worn surface of the MWCNTs-reinforced copper foam/resin composites: (a, d, g) 0 wt% MWCNTs; (b, e, h) 0.5 wt% MWCNTs; (c, f, i) 1 wt% MWCNTs.
Fig. 12. SEM morphologies of the wear debris of the MWCNTs-reinforced copper foam/resin composites: (a, c) 0 wt% MWCNTs; (b, d) 1 wt% MWCNTs. 578
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Fig. 13. Physical models of wear mechanism for the composites without MWCNTs and with MWCNTs.
found as a cross among the wear debris in Fig. 12d. Under the reinforcement of the MWCNTs, the crack propagation is arrested, and the formation of the big debris is difficult for the composite with 1 wt% MWCNTs. Therefore, the composite with more MWCNTs has better wear resistance.
performance for the copper foam/resin composite during current-carrying friction.
3.5. Morphology of worn surface and wear debris Fig. 11 shows SEM morphologies of the worn surfaces of the MWCNTs-reinforced copper foam/resin composites with 0 wt%, 0.5 wt % and 1 wt% MWCNTs. As shown in Fig. 11(a, d and g), the composite without the MWCNTs has a coarser worn surface, where the resin matrix has been broken and some furrows, spalling pits and cracks are formed. For the composites with the 0.5 wt% (Fig. 11b and c) and 1 wt % MWCNTs (Fig. 11e and f), the furrows are smaller and there is a comparatively smooth film on the worn surfaces. Meanwhile, the worn surface of the composite with 1 wt% MWCNTs is the most smooth and continuous among the three composites. It can be explained that: (1) Because MWCNT has good thermal and electrical conductivities (Fig. 6), the composites with the MWCNTs have lower resistance heat during the current-carrying friction, and the friction and resistance heats spread more quickly on the worn surfaces, which could prevent overheating and decrease wear loss during the current-carrying friction test. (2) The MWCNT improves the mechanical strength of the composite matrix by blocking the cracks propagation. As shown in Fig. 11(h and i), although the resin matrixes are destroyed, some debris are still connected with the worn surface by the MWCNTs and will not drop out immediately, which benefits that the broken surface is restored to a smooth surface during the current-carrying friction test. (3) After the MWCNTs are broken during the current-carrying friction, the debris has a small size due to the nanometer characterization and the high crystalline degree (Fig. 4c), which is usually used to fill in the spalling spits and restore the worn surface. Therefore, by contrasting with the composite without the MWCNTs, the composites with 0.5 wt% and 1 wt% MWCNTs have a smoother and more continuous worn surface. These results are consistent with the previous conclusions that the composite with more MWCNTs has the lower friction coefficient (Fig. 9) and wear rate (Fig. 10). Fig. 12 shows SEM images of the wear debris of the composites with 0 wt% and 1 wt% MWCNTs collected after the current-carrying friction tests. For the wear debris of the composite without the MWCNTs as shown in Fig. 12 (a, c), the diameter of the particles varies from several nanometers to 50 μm, which includes two main types of the wear debris. One is the wear debris with a large size and flaky shape (Fig. 12c), and most of them fall directly from the worn surface caused by the crack extension. The other has a small size and oval shape, which attributes to the shearing and grinding actions between the friction couple. However, for the wear debris of the composite with 1 wt% MWCNTs (Fig. 12b, d), most of the sizes are below 10 μm and there is a high proportion of small wear debris. Meanwhile, many MWCNTs are
3.6. Wear mechanism In fact, the process of forming friction film on the worn surface is a dynamic balance during the friction tests [39,40]. Under the combined effects of the shearing stress and normal force, the cracks grow and propagate along the defects on the worn surface, and the particles finally fall off from the matrix, leading to the formation of the spalling pits. The particles are grinded into the wear debris between the friction couple. Some of the wear debris will escape from the friction surface driven by acentric force, while the others will be filled in the spalling pits and compacted to restore the friction film. Fig. 13 shows two wear models of the composites with MWCNTs and without MWCNTs, which are used to schematically illustrate the wear mechanism. Because the composite without the MWCNTs has the brittle resin matrix, the cracks grow easily under the combined actions of shearing stress and friction heat during the current-carrying friction, resulting in the large resin particles breaking off from the matrix and the formation of the spalling pits. It is difficult for the big spalling pits to be filled up, so there are some spalling pits existing on the worn surface (Fig. 11d). For the composites with the MWCNTs, the crack propagation requires more energy due to the reinforcement of the MWCNTs in the resin matrix, and the spalling pits are smaller during the current-carrying friction test. Meanwhile, the small wear debris with the MWCNTs is easy to be left and fill up the spalling pits to form a uniform and compacted film on the worn surface (Fig. 11b and c). Therefore, the composites with the MWCNTs exhibit better friction and wear behaviors than the composite with pure resin matrix during the current-carrying friction test. 4. Conclusions In this paper, a novel sliding contact strip material of MWCNTsreinforced copper foam/resin composite was fabricated using an acid purify and ultrasonic treatment of MWCNTs dispersed in the furan resin, and the mixing and addition technology of short carbon fiber and flaky graphite, then a conventional molding technique. Due to the high specific surface area of the MWCNT, the low content of the filler can form an electrical network and improve the electrical conductivity of the composite, and the percolation threshold of the MWCNTs is found to be approximate 0.074–0.185 vol% in the composite. Because of the MWCNTs blocking the cracks propagation in the resin matrix, the 579
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impact strength and wear resistance of the composite could be increased respectively 45% and 22% by the MWCNTs addition. Therefore, the MWCNT is a potentially reinforcing filler for the sliding contact material, which not only increases the electrical conductivity and mechanical performances but also improves the tribology behavior during the current-carrying friction test.
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Acknowledgment [20]
The authors thank Prof. Yongzhen Zhang from the Department of Material Science and Engineering of the Henan University of Science and Technology for providing the friction tester. The authors are very thankful for 2017 Natural Science Foundation of Hunan Province [No. 2017JJ3514] and National Natural Science Foundation of China [No. 51302322 and No. 51602351]. Additionally, the first author would like to greatly acknowledge financial support from China Scholarship Council [No. 201706370190].
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Appendix A. Supplementary data
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Supplementary data related to this article can be found at https:// doi.org/10.1016/j.compositesb.2019.03.067.
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References
[27]
[1] Ray BC, Rathore D. Durability and integrity studies of environmentally conditioned interfaces in fibrous polymeric composites: critical concepts and comments. Adv Colloid Interface Sci 2014;209:68–83. [2] Mohammed AS, Fareed MI. Improving the friction and wear of poly-etherketone (PEEK) by using thin nano-composite coatings. Wear 2016;364–365:154–62. [3] Hoskins TJ, Dearn KD, Chen YK, Kukureka SN. The wear of PEEK in rolling sliding contact-simulation of polymer gear applications. Wear 2014;309:35–42. [4] Williams J, Graddage N, Rahatekar S. Effects of plasma modified carbon nanotube interlaminar coating on crack propagation in glass epoxy composites. Compos Part A 2013;54:173–81. [5] Fredi G, Dorigato A, Luca Fambri, Pegoretti A. Wax confinement with carbon nanotubes for phase changing epoxy blends. Polymers 2017;405:1–16. [6] Sebastian R, Noll A, Zhang G, Burkhart T, Wetzel B. Evaluation of the role of functionalized CNT in glass fiber/epoxy composite at above- and sub-zero temperatures: emphasizing interfacial microstructures. Compos Part A 2017;101:215–26. [7] Tu CJ, Chen ZH, Xia JT. Thermal wear and electrical sliding wear behaviors of the polyimide modified polymer-matrix pantograph contact strip. Tribol Int 2009;42:995–1003. [8] Yuan H, Wang CG, Zhang S, Lin X. Effect of surface modification on carbon fiber and its reinforced phenolic matrix. Appl Surf Sci 2012;259:288–93. [9] Wang P, Zhang HB, Yin J, Xiong X, Tan C, Deng CY. Wear and fiction behaviors of copper mesh and flaky graphite-modified carbon/carbon composite for sliding contact material under electric current. Wear 2017;380–381:59–65. [10] Deng CY, Yin J, Zhang HB, Xiong X, Wang P, Sun M. The tribological properties of Cf/Cu/C composites under applied electric curren. Tribol Int 2017;116:84–94. [11] Ji KJ, Xu YS, Zhang J, Chen J, Dai ZD. Foamed-metal-reinforced composites: tribological behavior of foamed copper filled with epoxy–matrix polymer. Mater Des 2014;61:109–16. [12] Yan SL, Ren J. Research on the mechanics and friction behaviour of a foam Ni/ polytetrafluoroethylene composite. Mater Des 2012;39:425–31. [13] Gao S, Villacorta BS, Ge L, Rufford TE, Zhu ZH. Effect of sonication and hydrogen peroxide oxidation of carbon nanotube modifiers on the microstructure of pitchderived activated carbon foam discs. Carbon 2017;124:142–51. [14] Farrash SMH, Shariati M, Rezaeepazhand J. The effect of carbon nanotube dispersion on the dynamic characteristics of unidirectional hybrid composites: an experimental approach. Compos Part B 2017;122:1–8. [15] El MA, Tarfaoui M, Lafdi K. Mechanical characterization of carbonnanotubes based polymer composites using indentation tests. Compos Part B 2017;114:1–7. [16] Ürk D, Demir E, Bulut O, Çakıroglu D, Cebeci FC, Öveçoğlua LM, et al. Understanding the polymer type and CNT orientation effect on the dynamic
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36] [37] [38]
[39]
[40]
[41]
580
mechanical properties of high volume fraction CNT polymer nanocomposites. Compos Struct 2016;155:255–62. Bouhamed A, Müller C, Choura S, Kanoun O. Processing and characterization of MWCNTs/epoxy nanocomposites thin films for strain sensing applications. Sens Actuators, A 2017;257:65–72. Ma HL, Jia Z, Lau K, Li X, Hui D, Shi S. Enhancement on mechanical strength of adhesively-bonded composite lap joints at cryogenic environment using coiled carbon nanotubes. Compos Part B 2017;110:396–401. Krause B, Mende M, Potschke P, Petzold G. Dispensability and particle size distribution of CNTs in an aqueous surfactant dispersion as a function of ultrasonic treatment time. Carbon 2010;48:2746–54. Zhong J, Isayev AI. Ultrasonically assisted compounding of CNT with polypropylenes of different molecular weights. Polymers 2016;107:130–46. Xiong XZ, Tu CJ, Chen D, Zhang JQ, Chen JH. Arc erosion wear characteristics and mechanisms of carbon strip against copper under arcing conditions. Tribol Lett 2014;53:293–301. Kubo S, Kato K. Effect of arc discharge on the wear rate and wear mode transition of a copper impregnated metallized carbon contact strip sliding against a copper disc. Tribol Int 1999;32:367–78. Yang L, Ran LP, Yi MZ. Carbon fiber knitted fabric reinforced copper composite for sliding contact material. Mater Des 2011;32:2365–9. Qu MC, Nilsson F, Qin YJ, Yang GD, Pan YM, Liu XH, et al. Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black. Compos Sci Technol 2017;150:24–31. Yang H, Gong J, Wen X, Xue J, Chen Q, Jiang ZW, et al. Effect of carbon black on improving thermal stability, flame retardancy and electrical conductivity of polypropylene/carbon fiber composites. Compos Sci Technol 2015;113:31–7. Gojny FH, Wichmann MHG, Fiedler B, Kinloch IA, Bauhofer W, Windle AH, et al. Evaluation and identification of electrical and thermal conduction mechanisms in carbon nanotube/epoxy composites. Polymer 2006;47:2036–45. Moisala A, Li Q, Kinloch IA, Windle AH. Thermal and electrical conductivity of single- and multi-walled carbon nanotube-epoxy composites. Compos Sci Technol 2006;66:1285–8. Dumas L, Bonnaud L, Olivier M, Poorteman M, Dubois P. Multiscale benzoxazine composites: the role of pristine CNTs as efficient reinforcing agents for high-performance applications. Compos Part B 2017;112:57–65. Bryning MB, Islam MF, Kikkawa JM, Yodh AG. Very low conductivity threshold in bulk isotropic single-walled carbon nanotube-epoxy composites. Adv Mater 2005;17:1186–91. Yasser Z, Rhee KY. A power model to predict the electrical conductivity of CNT reinforced nanocomposites by considering interphase, networks and tunneling condition. Compos Part B 2018;155:11–8. Drakonakis VM, Aureli M, Doumanidis CC, Seferis JC. Modulus–density negative correlation for CNT-reinforced polymer nanocomposites: modeling and experiments. Compos Part B 2015;70:175–83. Li J, Ma PC, Chow WS, To CK, Tang BZ, Kim JK. Correlations between percolation threshold, dispersion state and aspect ratio of carbon nanotubes. Adv Funct Mater 2007;17:3207–15. Bai JB, Allaoui A. Effect of the length and the aggregate size of MWNTs on the improvement efficiency of the mechanical and electrical properties of nanocomposites—experimental investigation. Compos Part A 2003;34:684–94. Hoseini AHA, Arjmand M, Sundararaj U, Trifkovic M. Significance of interfacial interaction and agglomerates on electrical properties of polymer-carbon nanotube nanocomposites. Mater Des 2017;125:126–34. Kim YJ, Shin TS, Choi HD, Kwon JH, Chung YC, Yoon G H. Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites. Carbon 2005;43:23–30. Allaoui A, Bai S, Cheng HM, Bai JB. Mechanical and electrical properties of a MWNT/epoxy composite. Compos Sci Technol 2002;62:1993–8. Mohammed AS, Fareed MI. Improving the friction and wear of poly-ether-etherketone (PEEK) by using thin nano-composite coatings. Wear 2016;364–365:154–62. Meng H, Sui GX, Xie GY, Yang R. Friction and wear behavior of carbon nanotubes reinforced polyamide 6 composites under dry sliding and water lubricated condition. Compos Sci Technol 2009;69:606–11. Han JH, Zhang H, Chu PF, Imani A, Zhang Z. Friction and wear of high electrical conductive carbon nanotube buckypaper/epoxy composites. Compos Sci Technol 2015;114:1–10. Gonzalez JJ, Datye A, Wu KH, Schneider J, Belmonte M. Robust and wear resistant in-situ carbon nanotube/Si3N4 nanocomposites with a high loading of nanotubes. Carbon 2014;72:338–47. Tu CJ, Chen D, Chen ZH, Xia JT. Improving the tribological behavior of graphite/Cu matrix self-lubricating composite contact strip by electroplating Zn on graphite. Tribol Lett 2008;31:91–8.