Tribological studies of epoxy-carbon nanofiber composites – Effect of nanofiber alignment using AC electric field

Tribology International 138 (2019) 450–462

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Tribological studies of epoxy-carbon nanofiber composites – Effect of nanofiber alignment using AC electric field

T

Amit Chanda, Sujeet K. Sinha, Naresh V. Datla∗ Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi, 110016, India

ARTICLE INFO

ABSTRACT

Keywords: CNF/epoxy composites Electric field alignment Friction and wear Self-lubrication

In this study, an electric field was applied to align carbon nanofibers (CNF) in an epoxy matrix and the tribological performances were evaluated. The epoxy/aligned CNF composites were fabricated with different weight percentages of non-functionalized CNFs (0.2–2 wt%) with which friction and wear tests were carried out keeping sliding direction normal to fiber orientation. The wear resistance of 1 wt% random and aligned composites improved by 3.5 and 25 times, respectively, compared to pure epoxy (20 N load). Moreover, the friction coefficient reduced from 0.6 for pure epoxy to 0.44 and 0.24 for random and aligned CNF/epoxy composites, respectively. Improved thermal stability, normal load carrying capacity and lubricating effect of CNFs contributed to the superior wear resistance of aligned composites.

1. Introduction Thermosetting epoxy resin is one of the widely used polymeric materials in applications such as coatings, adhesives and composites owing to its superior adhesive properties, mechanical strength, chemical resistance and thermal stability [1,2]. However, epoxy is brittle due to cross-linked structure thus results in poor friction and wear performances that restrict its use in tribological applications [2]. Addition of carbon-based nano-fillers such as carbon nanotubes (CNT) [3,4], graphite [5], graphene [6,7] and fullerene [8] have already been proven as essential ways to improve tribological and mechanical properties of epoxy-based composites. Interestingly, being an important member of carbon nanoparticles family, use of carbon nanofiber (CNF) to enhance the tribological performance of epoxy composites is rare in literature. CNFs are suitable to manufacture advanced polymer-based composites because of their outstanding mechanical strength, electrical and thermal conductivity [9,10]. Extensive research has been carried out on epoxy/CNF composites to find the contribution of CNFs towards improvement in mechanical strength [11,12], electrical [11,13] and thermal [14] properties compared to pure epoxy. However, very few research articles can be found on the tribological properties of CNF reinforced epoxy. Barrena et al. [9] and Zhu et al. [15] explored the wear performance epoxy nanocomposites with surface functionalized CNFs. Combined effects of CNFs and polyimide fiber was found to improve the wear resistance of epoxy by 18 times with 1.5 wt% CNF concentration [10]. However,

∗

improvement in friction coefficient was found to be limited to only 10% in that case. Comparatively, detailed study along with a quantitative evaluation of the influence of CNF on the tribological properties of CNF/epoxy system was done by Khanam et al. [16]. Using 0.5 to 3 wt% CNF concentration they observed significant wear rate reduction in epoxy, though the effect of CNFs on friction coefficient was found to be insignificant. Chen et al. have indicated that the synergistic effect of CNF and Molybdenum disulfide (MoS2) and their interaction with epoxy can reduce the friction coefficient and wear rate of EP/MoS2/ CNF coatings by 5.6 and ∼16 times respectively (for 1.25 wt% loading). However, the wear resistance improvement was found as ∼10 times when only CNF was used with epoxy [17]. Recent studies revealed that along with the dispersion and agglomeration of carbon nanoparticles in polymer matrix, proper orientation of the particles could help to get further improvements in mechanical strength and tribology [11,18]. Different approaches have been reported based on the effect of fiber alignment on wear performance of epoxy, mainly with long continuous carbon fibers [19] and long aligned CNTs [4,18]. Improved tribological performances were reported in case of horizontally aligned long and short carbon fiber composites with different polymer matrices [19,20]. Liu et al. also reported a similar kind of result with horizontally aligned graphene oxide-Fe3O4 polyphosphazene inside bismaleimide resin, indicating the shape anisotropy and orientation as the major factor behind improved wear resistance [21,22]. However, completely different trend was noticed in case of one-dimensional aligned nanoparticles. A significant improvement in wear performance

Corresponding author. E-mail address: [email protected] (N.V. Datla).

https://doi.org/10.1016/j.triboint.2019.06.014 Received 28 March 2019; Received in revised form 6 June 2019; Accepted 9 June 2019 Available online 10 June 2019 0301-679X/ © 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. (a) Schematic of preparation for aligned CNF/epoxy nanocomposites; (b) Alignment studies of uncured 0.1 wt% CNF/epoxy in between parallel electrodes with AC electric field, observed under optical microscope.

of epoxy was observed with CNTs aligned vertically to sliding direction [18]. This trend was also supported by numerical analysis of aligned CNT/epoxy composites under different sliding direction by RodríguezTembleque et al. [23]. They showed that the normally aligned CNT yielded stiffer response and the horizontally aligned CNT yielded maximum tangential forces i.e., high friction coefficient in case of horizontal alignment. The same phenomenon was also explained in polyurethane based composites by Randall et al., where they indicated that strongly locked normally aligned particles prevent indentation and yields better wear resistance than horizontally aligned particles [24]. Based on these findings, we decided to focus on the vertical alignment effect of carbon nanofibers in the epoxy matrix. Recently, it has become favored to tailor the properties of epoxy matrix by aligning carbon nanoparticles inside it through external force such as mechanical stretching [25], electric field [11] or magnetic field [21,22]. Liu et al. [22] used a magnetic field to align graphene oxideFe3O4 polyphosphazene particles parallel to surface inside bismaleimide resin and studied tribological properties. However, there is no report comprising electric field induced alignment of discontinuous carbon nanoparticles and their alignment effect on tribological properties of epoxy. These limited findings have motivated us to work on non-functionalized CNF's effect on tribological performances in epoxy resin matrix in the domain of fiber alignment. This study contains a detailed study on the effects of load and speed on the tribological performances of randomly dispersed and aligned CNF/epoxy nanocomposites. AC electric field was used to align CNFs along the thickness direction within the epoxy matrix. The effects of alignment on tribological properties were investigated and compared with randomly dispersed nanocomposites. Both aligned, and randomly dispersed nanocomposites were characterized through morphological study, DC electrical conductivity, thermal conductivity, Vickers microhardness test, and tribological tests. The aligned composite showed significant improvement in terms of electrical, thermal and tribological performances.

2.2. Preparation of random and aligned CNF/epoxy composites

2. Experimental

2.3. Characterization

2.1. Materials

2.3.1. Alignment studies and morphology The parallel electrode arrangement (Fig. 1b) was placed under an optical microscope (MM-400, Nikon India Pvt. Ltd.) to observe the insitu alignment of the CNF particles within the epoxy matrix. This study was conducted with the 0.1 wt% uncured CNF/epoxy solution placed between the two aluminum electrodes separated by a distance of 2.5 mm, across which an AC electric field of 20 kHz and 40 V/mm was applied. The dispersion and alignment of CNF particles within both random and aligned CNF/epoxy composites weres studied by freezefracturing the cured samples in liquid nitrogen to obtain a planar surface. These fractured surfaces were gold coated prior to observation under scanning electron microscopy (SEM, Zeiss EVO 18). Microstructure of CNF was also analyzed by using transmission electron microscopy (TEM, Tecnai G2 20, ThermoFisher Scientific).

To allow proper de-agglomeration, as received CNFs were immersed in absolute ethanol (1:15 mass ratio) followed by drying by first placing in a vacuum desiccator for 40 h and then placing in a vacuum oven for 2 h at 80 °C. After cooled to room temperature, the CNFs were added in acetone and dispersed by ultrasonication for 15 min. 10 CMC (critical micelle concentration) of non-ionic surfactant Triton X-100 was then added in the mixture and then sonicated for another 20 min. Epoxy resin, preheated at 40 °C to reduce the viscosity, was then added to the suspension which was followed by sonication for another 45 min. Sonication was applied at 20 kHz in pulse mode (2 s on, 5 s off) to keep the solution temperature in control. After getting a homogeneous mixture, it was kept on a heating pan overnight at 50 °C to remove acetone and then degassed in a vacuum oven for 2 h at the same temperature. The above method was used to prepare CNF/epoxy composites with CNF weight fractions of 0.2, 0.4, 0.6, 0.8, 1.0 and 2.0 wt%. Pure epoxy and randomly dispersed CNF/epoxy composites were prepared by adding hardener with the pure resin and the sonicated resin solutions (mix ratio of epoxy: hardener is 100:10, parts by weight), respectively, followed by hand mixing with a stirrer for 5 min to ensure proper mixing. The mixtures were degassed in vacuum for 5 min and then cast into a Teflon mold that was followed by a curing schedule of 24 h at room temperature and then 5 h at 100 °C. Aligned CNF/epoxy composite mixtures were also prepared in the same procedure. The CNF/epoxy mixture was placed between parallel aluminum electrodes separated by glass spacers of 2.5 mm and then an electric field was applied for the initial 5 h of curing cycle to ensure proper alignment. CNFs were aligned in the thickness direction inside the epoxy matrix as shown schematically in Fig. 1a. AC signal generator (33220A, Agilent Technologies, USA) and a custom-built power amplifier were used to generate an electric field of 20 kHz and 40 V/mm strength. All the aligned samples followed the same curing cycles as applied in the case of random CNF/epoxy composites.

Epoxy resin (Araldite AY 103) and hardener (Aradur HY 951), procured from Huntsman International (India) Pvt. Ltd., India, were used as the matrix. The low viscosity epoxy resin was based on bisphenol A and the hardener was of aliphatic amine (triethylenetetramine) type. Graphitized (iron free), vapor-grown CNF (PR-25-XTHHT) procured from Sigma-Aldrich, India, with an average diameter of 130 nm and an average length of 20–200 μm was used as reinforcements in the present study. Triton X-100, a non-ionic surfactant, supplied by Bio Instruments and Chemicals, New Delhi, India, was used to avoid CNF agglomeration in epoxy. Acetone and ethanol were procured from SRL chemicals, India. All reagents were used as received without further purification. 451

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Fig. 2. Optical micrographs of in-situ alignment of 0.1 wt% CNF/epoxy (liquid form) under AC electric field of (a) randomly oriented CNF at time t = 0 min, (b) Aligned CNFs at t = 2 min (c) Aligned CNFs at t = 3.5 min. (d) TEM micrograph of Carbon nanofiber. SEM images of fracture surface from (e) 0.4 wt% random CNF/epoxy nanocomposites and (f) 0.4 wt% Aligned CNF/ epoxy nanocomposites. Direction of applied electric field is shown by the arrow.

2.3.2. Electrical conductivity, thermal conductivity and surface energy measurements DC conductivity values of both random and aligned CNF/epoxy composites as well as pure epoxy were measured by a two-probe method using Keithley 4200-SCS parameter analyzer. Typical current vs. voltage (I–V) characteristics curves were recorded between −5 and +5 V and resistances were calculated from the slope of the Ohmic region. Circular specimens of 12 mm diameter and 2.5 mm thickness were cut from all samples. These samples were polished and then coated with conductive silver paint before electrical measurements. Thermal diffusivity measurements were performed using a xenon light flash diffusivity system (LFA 447 Nanoflash, Netzsch India) at 25 °C and the Cape-Lehman + pulse correction model was adopted. Square samples with 3 × 13 × 13 mm size were cut and coated with graphite before testing. Specific heat capacity of samples was measured using hot disk thermal constant analyzer (TPS 500, KAN-THT India Pvt. Ltd.) at 25 °C and the thermal conductivity was calculated by the following equation.

k=

Cp,

2.3.4. Friction and wear behavior Friction and wear tests of pure epoxy as well as random and aligned CNF/epoxy composites were performed on a ball-on-disc tribometer (Invogineering Pvt. Ltd, New Delhi, India) under dry sliding condition at room temperature (25 ± 1 °C). Circular disc-shaped specimens were rotated against a 316 stainless steel ball (with initial surface roughness, Ra of 0.3 μm) of 12 mm diameter with a normal load varying from 10 to 30 N, sliding speed varying from 0.05 m/s to 0.2 m/s and for test duration of 120 min. Before the test, all of the specimens were polished with 1000 grit sandpaper and cleaned properly with acetone to remove surface contaminants. Friction force was measured with a load cell in contact with the ball holder. Weight loss test was carried out using an electronic balance with an accuracy of ± 0.0001 g (HR-250AZ, A&D Instruments India Pvt. Ltd.). The specific wear rate (SWR) was calculated using the following equation:

SWR =

m mm3/N.m, FL

(2) 3

where m is the mass loss (g), ρ is the density of the samples (g/cm ), F is the normal load (N) and L is the sliding distance (m). Density (ρ) of the composite materials was measured using the rule of mixture. Temperature at the trailing end of the contact zone was continuously monitored using non-contact infrared temperature gun during sliding. The infrared temperature gun was kept fixed at a distance of 10 cm from the contact zone. Room temperature was 25 ± 1 °C for all the tests. Surface roughness of samples was observed under 3D optical profilometer (Zeta 20 optical profiler, USA). Worn surfaces of composite samples and the steel ball counterfaces were studied under Scanning Electron Microscopy and Optical Microscopy to find out the possible wear mechanisms. At least three repetitions from each batch were performed and the mean of results from three different batches were reported.

(1)

where k is thermal conductivity (W/mK), ρ is density (kg/m3), α is the thermal diffusivity (m2/s) and Cp is specific heat capacity (J/kgK). In the case of aligned samples, thermal conductivity was measured in the direction parallel to alignment of CNFs. The surface energy values of pristine CNF and surfactant treated CNF were evaluated to obtain their wettability, dispersibility and interfacial adhesion with epoxy. Besides, surface energies of pure epoxy and different wt% CNF/epoxy composites were also calculated using the Fowkes model. De-ionized water (polar liquid) and n-hexane (nonpolar liquid) were used as the test liquids. Static contact angles using both polar and non-polar liquids were measured within 2 s of liquid deposition (1 μl) in Krṻss drop shape analyzer (Krṻss GmbH, Germany).

3. Results and discussion 2.3.3. Hardness and thermomechanical properties Hardness of the samples was determined using a microhardness tester (Invogineering Pvt. Ltd., India) using square pyramidal diamond indenter at a dead weight of 1 kgf and a loading duration of 15 s. At least ten indentations were conducted on each sample and the average values were reported. The thermomechanical spectra of the pure epoxy and 0.6 wt% CNF/epoxy samples were obtained using a dynamic mechanical analyzer (DMA Q-800, TA instruments, USA). The samples with dimensions of 35 mm × 12.75 mm × 3 mm were tested under single cantilever mode with heating ramp 4 °C/min (Temp range 30–110 °C, frequency 1 Hz).

3.1. In situ alignment studies and morphology Before composite fabrication, the alignment of CNF in epoxy was verified experimentally under optical microscope (as shown in Fig. 1b) and the micrographs are presented in Fig. 2a,b,c. Fig. 2a represents randomly dispersed CNFs in liquid epoxy before application of an electric field. It can be seen that efficient de-agglomeration and homogeneous dispersion occurred during the ultrasonication cycle using the surfactant. On the application of electric field, within 2 min the nanofibers rotated and aligned along the direction of the field (as shown in Fig. 2b), but beyond 2 min the nanofibers formed a chain-like 452

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facilitate the rotation of CNFs in the direction of electric field. Almost similar kind of behavior with CNF/epoxy system was observed in the literature with the same CNF concentration [11]. Fig. 3b compares thermal conductivity of random and aligned CNF/ epoxy composites with pure epoxy. Randomly oriented composites showed a continuously increasing trend whereas in the case of aligned, the trend became almost constant after 1 wt% CNF content. Thermal conductivity became more than double (∼0.48 W/m.K) of pure epoxy at 2 wt% random CNF concentration. Alignment of CNFs got a significant effect on thermal conductivity too. An improvement of 24% compared to random composites and 74% compared to pure epoxy was found in case of 0.6 wt% aligned composites. As the CNF content increased beyond 1 wt%, thermal conductivity difference between aligned and random composites started reducing. That reduction is likely because of lesser alignment of particles at a higher concentration which could be observed in the case of electrical conductivity measurements.

structure. Owing to opposite charges at their ends, a dipole-dipole attraction forms between CNFs that brings them closer and forms end to end connection termed as ‘chaining’. Chaining and network formation can be visible within 3.5 min after applying electric field. Similar behavior has been noticed in prior research works for CNFs and other carbon nanoparticles in epoxy matrix upon electric field application [12,22]. Fig. 2d shows a TEM image of CNFs. The analysis of TEM images by NIH Image-J software revealed that the diameter of the CNF varied from 50 nm to more than 200 nm, with maximum values in the range of 100–150 nm (see supporting information S1). Fig. 2e and f shows SEM images of freeze-fractured surfaces of nanocomposites (0.4 wt% CNF/epoxy) cured both without and with applied electric field, respectively. Since the specimens were frozen before fracture, they show the actual nature of fiber orientation. It is seen that the application of electric field resulted almost all the CNFs to be oriented within ± 30° of the direction of electric field (Fig. 2f, angle orientation comparison between random and aligned composites can be found in supplementary information, S2). However, the ‘chaining’ was not found in cured aligned composites, which is due to the movement restriction of CNF in high viscosity epoxy/hardener system. In both random and aligned composites, the CNFs were homogeneously dispersed and deagglomerated.

3.3. Hardness and thermomechanical properties Hardness of polymer composites is one of the most influencing factors on their tribological properties. Increased hardness prevents plastic deformation, increases the load bearing capacity and possibly hinders stick-slip phenomena [22]. Vickers micro-hardness data (Fig. 4a) of pure epoxy, randomly oriented and aligned CNF/epoxy composites were evaluated. Progressive increase in hardness could be observed with increased CNF content. However, the increment becomes very small beyond 0.6 wt% CNFs and an overall increase of ∼10% was found for 2 wt% CNF/epoxy composite compared to pure epoxy samples. The increment in hardness may be attributed to good CNF and epoxy matrix interaction and the ability to prevent plastic deformation. An improvement in hardness was also observed in the case of aligned composites, though it can be clearly visible only for lower CNF concentration of 0.4 or 0.6 wt%. At higher CNF content, the hardness difference between random and aligned composites is very small thus no conclusion could be drawn from that data. As no literature was available on hardness of electric field aligned CNF/epoxy composites, we tried to compare it with the Rockwell hardness result of aligned CNT/epoxy composites studied by Felisberto et al. [28]. However, they found that the hardness decreased in both random and aligned composites with increasing CNT content

3.2. Electrical and thermal conductivity Fig. 3a shows the DC electrical conductivity measured across the thickness direction for both randomly-dispersed and aligned composites as well as for pure epoxy. It can be seen that all of the nanocomposites show significantly higher conductivity compared to the pure epoxy sample. Moreover, the conductivity of the aligned nanocomposites compared to the randomly-oriented nanocomposites increased at all concentrations of CNFs. The increment is largest for 0.6 wt% and 0.8 wt % CNF where the conductivity increased by approximately 33 and 15 times, respectively. The changes in conductivity became minimal beyond 0.6 wt% CNF concentration in the case of aligned composites. This behavior can be attributed to saturated percolation network formation in aligned composites at a lower CNF concentration (0.4 wt% to 0.6 wt %). Similar kind of behavior for CNFs and graphene nanoplatelets was also reported in the literature [26,27]. Possible reasons for this could be the viscosity increase with CNF addition, chance of agglomeration of CNFs at higher concentration and lack of free space available to

Fig. 3. (a) DC electrical conductivity and (b) Thermal conductivity as a function of CNF wt% for both Random and Aligned CNF/epoxy Composites (measured in the through thickness direction i.e., in the direction of alignment). 453

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Fig. 4. (a) Vickers micro-hardness as a function of CNF wt% for both randomly oriented and aligned CNF/epoxy Composites (In case of aligned composites the hardness was measured in the direction of alignment). (b) Effect of CNF addition on storage modulus and glass transition temperature of epoxy (solid line – storage modulus; dotted line – tan delta).

(0–1 wt%) and the reason was attributed to low adherence between matrix and filler. In the present study, hardness increase of random composites could be thought as a result of better surface interaction between CNFs and epoxy (explained in supporting info section S4, Table S1). The minimal improvement in hardness for aligned composites may be attributed to the better load carrying ability of bonded aligned nanofibers when it was loaded in the direction of alignment [24]. The thermomechanical spectra of the pure epoxy and CNF/epoxy composite obtained from the DMA test is shown in Fig. 4b. The storage modulus of 0.6 wt% CNF/epoxy composite was found to be significantly higher than pure epoxy at the glassy region. Moreover, the addition of CNF also increased the glass transition temperature (Tg increases by 7.5 °C at 0.6 wt% CNF content, measured from the tan delta peak) of the composites. The CNFs were dispersed using a non-ionic surfactant Triton X-100, which interacts with CNF through its hydrophobic segment, while its hydrophilic segment interacts with the epoxy through hydrogen bonding. In the glass transition regime, molecular segmental motion of polymer occurs. This kind of molecular-scale movement of polymer chains can be significantly affected by the presence of reinforcements. Being dimensionally similar to the polymer chain segments, carbon nanoparticles can influence the alignment of surrounding polymer chains and thus restrict their movement through interfacial interaction. The immobilization of polymer chains requires more temperature to supply the necessary thermal energy in order to induce the glass transition in the composite [29,30].

was almost 3.5 times reduction. These results of friction coefficient and specific wear rate reduction with increased CNF content were found to be consistent with the previous reports on CNF/epoxy nanocomposites [9,10,16]. Similar trend of decrease in friction coefficient with CNF addition was found under different loads and speeds. Details of the load and speed effect on CNF content will be discussed in sections 3.4.2. and 3.4.3. The reduction in friction coefficient and specific wear rate with increased CNF concentration could be explained from additional characterization results including SEM investigations of the wear track after conducting the tribology test. For pure epoxy, the material removal pattern indicates fatigue induced wear, which can be attributed to the repeated loading during sliding. Along with this, a much rougher surface with plucking marks on the track was observed (Fig. 6a), that is a sign of dominant adhesive wear and ploughing. Pure epoxy is softer compared to nanocomposites due to the absence of reinforcing nanoparticles. Moreover, pure epoxy at the point of contact becomes more compliant due to frictional heat generation and poor heat dissipation. This relatively softer pure epoxy is one of the main reasons for more adhesion during sliding. These mechanisms led to higher friction coefficient and specific wear rate for pure epoxy. Fig. 6a shows that as CNF concentration was increased the wear track width decreased that indicated that with CNF wt% the stiffness increased and thereby the deformation at the contact decreased. This could be attributed to the structural enhancement and improved hardness of nanocomposites through efficient interfacial interaction of CNFs and epoxy. Tribological properties of CNF/epoxy composites could be significantly affected by interface interaction. Surfactant treatment of CNF de-agglomerated the particles properly and it also helped in better wettability in epoxy through hydrogen bonding (Explained in supporting information, S4). Moreover, the surface roughness (Table 1b) and surface energy of cured epoxy and composite samples are related to the reactivity of the surface. With increasing CNF concentration, the surface energy reduced (Fig. 5c) that also suppressed surface reactivity and adhesion during sliding wear. In the case of relatively high CNF concentration epoxy composites, the increased thermal conductivity helped to dissipate the friction generated heat and keep the contact zone temperature significantly lower than that of pure epoxy (Fig. 5d). Less heat generation and increased glass transition temperature of composites offered comparatively larger safe operating temperature zone than pure epoxy. As composites induce increased storage modulus and lower temperature at

3.4. Friction and wear behavior 3.4.1. Effect of CNF content (wt%) Fig. 5a,b shows the friction response of pure epoxy and nanocomposites with different CNF content under 20 N normal load and 0.05 m/s sliding speed. It can be clearly seen that the friction coefficient was highest for pure epoxy and that it decreased as CNF concentration was increased. The decrease in the friction coefficient is highest of about 35% for 2 wt% CNF nano-composite. Specific wear rate too mostly showed a gradual decreasing trend with increased CNF content, except at few conditions where it increased slightly beyond 1 wt% CNF. Least wear rate result could be found in the case of 1 wt% CNF content, where it reduced to 1.12 × 10−4 mm3/Nm (for 1 wt% random CNF/ epoxy composites) from 3.95 × 10−4 mm3/Nm (for pure epoxy); which 454

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Fig. 5. (a) Friction response vs. number of cycles; (b) variation of friction coefficient and specific wear rate with CNF wt%; for CNF/epoxy random nanocomposites under 20 N load and 0.05 m/s sliding speed; (c) Surface energy of pure epoxy, randomly-dispersed and aligned CNF/epoxy nanocomposites with respect to CNF content; (d) Variation in generated temperature at contact vs. CNF concentration.

the contact zone, they would have less compliant structure than pure epoxy, which could contribute as a key factor during sliding. Effect of CNF content could be well understood from Fig. 6a. Very rough surface of pure epoxy was replaced by a comparatively smooth surface with less plucking marks, furrows and microcracks perpendicular to sliding direction. Less number of plucking indicates lowering in adhesive wear and microcracks proved existence of fatigue wear. The furrows are usually sign of abrasive wear. In the case of CNF/epoxy composites, CNF disengagement happened, and the matrix was also worn out. These disengaged fibers and worn materials could act as abrasive particles and caused the furrows on wear tracks as well as deep scratches on steel ball counterface. Interestingly, those loose nanofibers and worn materials could fill the abrasive induced scratches on the counterface that minimized the counterface irregularities and formed a thin transfer film on the steel counterface which reduced the direct contact between composite and steel surface. The existence of transferred materials was supported by optical micrographs (Fig. 6b, also see supporting information S5) and EDAX element analysis (Table 1a) of steel ball counterfaces. The increased carbon content at worn counterface than that of pure steel surface indicates materials transfer from wear track. The area covered on the counterface by transfer film became significant as the CNF content increased (Fig. 6b, S4). The transfer film and corresponding self-lubricating effect of CNFs sharply decreased

both of friction coefficient and wear rate. However, addition of CNF beyond a certain limit can partially limit the cross-linked structure of epoxy and thereby decrease the shear strength [31]. This could induce more abrasive particles generation on the track that disturbed the balance between adhesive and abrasive wear and increased the wear rate. However, the wear rate and friction coefficient for CNF/epoxy composites were found to be still lower than those of pure epoxy in almost all cases. 3.4.2. Effects of load Effects of load variation on friction coefficient and wear rate of pure epoxy and different CNF/epoxy nanocomposites at 0.05 m/s sliding speed are presented in Fig. 7. It can be seen that the friction coefficient decreased as load increased for almost all samples. Moreover, this decreasing trend in friction coefficient with normal force was more prevalent at lower CNF concentration than at the higher CNF concentrations. Increasing the load from 10 to 20 N showed a significant decrease in the specific wear rate for all samples. However, increasing the load from 20 to 30 N showed negligible differences in the wear rate at lower concentrations (0–0.8 wt% CNF) and increase in the wear rate at the higher concentrations (1 and 2 wt% CNF). When load increased, the real contact area between the steel ball and composite samples also increased (Fig. 6). Besides this, the 455

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Fig. 6. (a) Effect of CNF wt% and load on friction and wear behavior; SEM images of wear tracks of pure epoxy, 0.6 wt%, 1 wt% and 2 wt% CNF/epoxy random nanocomposites samples at 0.05 m/s constant sliding speed (1st row) under 10 N load (2nd row) under 20 N load (3rd row) under 30 N load. (b)Optical micrographs of wear tracks and counterfaces for pure epoxy, 0.6 wt% and 1 wt% CNF/epoxy under 20 N load, 0.05 m/s sliding speed; (All the photos were taken after test duration of 2 h or ∼ 6000 cycles).

temperature generation (Fig. 5d) was also found to be more at the contact point which can induce increased compliance in samples. Becoming more compliant helps the bond to become more shock, vibration and wear resistant that results in lower friction coefficient and wear rate initially [18]. Though the friction coefficient decreased for all samples, the mechanism is a bit different for pure epoxy and CNF/ epoxy nanocomposites. Temperature-induced softening was attributed as the major reason for better friction behavior at higher load in the case of pure epoxy. However as the CNF content increased, temperature effect was not prevalent for CNF/epoxy composites due to higher thermal conductivity. SEM image (Fig. 6a) showed that, with increased load, the high fatigue wear almost diminished in the case of pure epoxy. However, the plucking effect was still found which proved adhesion as major wear mechanism in pure epoxy. The mechanisms for CNF/epoxy composites can be understood more clearly from Figs. 6a and 8. Plucking effect, adhesion and fatigue induced crack at lower load (10 N) were reduced

as the load increased. Further, a few furrows and microcracks were observed on the wear tracks which were the results of abrasive wear and mild fatigue wear. The abrasive wear could be initiated by disengaged CNFs on wear track and worn materials. Disengaged CNFs can form a transfer layer on the counterface and the friction reduction effect for CNF/epoxy composites can be attributed to the self-lubricating effects of CNF. However, at high load, the transfer film does not contribute much to wear mechanism as the transfer layers could be peeledoff due to load [32] which is evident from Fig. 6b. With the increase in CNF content, large number of microcracks were observed that induced disengagement of bigger chunks and breakage of wear tracks (Fig. 6a (xi,xii)). It resulted in lower load carrying capacity and obvious wear rate increase, which were found for 1 and 2 wt% CNF/epoxy composites at high load (30 N). The increased hardness and thermal conductivity of composites than those of pure epoxy restricts modulus drop and plastic deformation of these composites and induce more Hertzian contact stress as well as shear stress. Added to this, more amount of CNF 456

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Table 1 (a) EDAX analysis (element wt%) of steel ball after sliding (2 h) against pure epoxy and 0.6 wt% random CNF/epoxy composites; (b) Surface roughness of samples and counterface. a. EDAX analysis of counterface

Steel ball Against pure epoxy Against 0.6 wt% CNF/epoxy

C

O

Al

Si

Fe

Mo

Mn

Ni

Cr

Total

4.60 6.10 12.34

4.43 6.63 8.76

2.87 2.57 2.56

0 .61 0.43 0.41

58.03 55.37 52.42

2.03 1.88 1.78

2.23 2.23 1.62

9.09 8.70 8.31

16.11 16.09 11.01

100.000 100.00 100.000

b. Surface roughness of samples and counterface Surface Roughness (Ra, μm) Steel ball Pure epoxy 0.6 wt% random CNF/epo 0.6 wt% aligned CNF/epo 1 wt% random CNF/epo 1 wt% aligned CNF/epo

0.30 0.15 0.20 0.42 0.36 0.57

With higher speed, temperature of the contact point increased significantly. At 0.2 m/s sliding speed, pure epoxy and 2 wt% CNF/epoxy composite had an increase by 20 ± 1 °C and by 12 ± 0.6 °C at the contact point, respectively, over the room temperature (Fig. 5d). Because of very low heat dissipation capacity, pure epoxy and lower CNF concentration composites became compliant and plastically deformed. Their vibration and shock resistance capability also increased which contributed to higher wear resistance [18]. That led to a sudden drop in friction coefficient for epoxy and composites with lower CNF content. At 0.2 m/s, the friction coefficient of pure epoxy was found to be almost same as the friction coefficient of 2 wt% CNF/epoxy composite. As the CNF content increased, thermal conductivity and heat dissipation capability of composites also increased. As a result, plastic deformation and adhesion was no longer the dominant wear mechanism. In the case of higher CNF content composites (≥0.6 wt%), along with plastic deformation other factors such as improved hardness, presence of CNF and their lubricating effect also contributed towards the reduction in friction coefficient. Though plastic deformation occurred for pure epoxy

addition also led to agglomeration that resulted in microcrack initiation and propagation under fatigue loading and shear strength drop of composite [20,24]. Both of the reasons contributed towards the breakage of wear tracks, abrasion on composite and severe scratches on the counterface (Fig. 6a(xi),b). 3.4.3. Effect of sliding speed The variation in friction coefficient and specific wear rate with sliding speed at a constant load of 20 N is shown in Fig. 9. Friction coefficients for all samples showed a small reduction as the sliding speed increased from 0.05 to 0.1 m/s, however, reduced significantly as the speed increased to 0.2 m/s. Interestingly, the friction coefficient of epoxy was found to be lower than those of CNF/epoxy composites at 0.2 m/s speed. The variation in the specific wear rate with increase in sliding speed greatly depended on the CNF content: it decreased at lower concentrations (0–0.6 wt%), remained unchanged at intermediate concentration (0.8 wt%), and increased at higher concentrations (1 and 2 wt%).

Fig. 7. Tribological properties of CNF/Epoxy nanocomposites. Variation in (a) friction coefficient and (b) specific wear rate with increasing CNF concentration (randomly-oriented CNF/Epoxy composites) under different loads of 10, 20 and 30 N; constant sliding speed of 0.05 m/s.

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Fig. 8. Effect of load; SEM images of wear tracks of 0.6 wt% CNF/epoxy nanocomposite at 0.05 m/s sliding speed under different loads (a) 10 N; (b) 20 N; and (c) 30 N. (Higher magnification images are shown as inset on left corner to reveal wear mechanisms).

and lower CNF content, the contact point temperature did not go beyond its glass transition temperature and severe ploughing or breakage was not observed that led to lower wear rate. As CNF content increased, abrasive particle generation on surface became increased thus changing the wear mechanism from adhesive to abrasive. Also, microcracks formation could be observed on track which was identified as fatigue-induced crack (Fig. 10c). Therefore, increased wear rate was observed for higher CNF content at 0.1 and 0.2 m/s (Fig. 9b). However, the wear rate was still lower than that of pure epoxy. SEM images (Fig. 10a,b,c) of 0.6 wt% CNF/epoxy at different sliding speed under a constant load of 20 N revealed the mechanisms better. It could be seen from the micrographs that the variation was not too much for 0.05 m/s and 0.1 m/ s. Micro-crack formation decreased sharply at 0.1 m/s with a few abrasive particles generated on the track. As speed increased, no transfer layer formation on the counterface surface was observed which was the result of peeling-off of layers due to high speed [32]. At the sliding speed of 0.2 m/s, wear tracks showed all type of failure patterns including fatigue induced micro-cracks, plucking due to adhesion and a few abrasive particles generated scratches (Fig. 10c). Though temperature increased noticeably at high speed, plucking effect and adhesion could not affect the whole track which may be attributed to reinforcing effect and thermal properties of CNFs. Better heat transfer and dissipation helped with comparatively lower temperature at contact and along with this, the exposed CNFs possibly acted as

lubricating interlayer thus improving overall wear and friction properties. 3.4.4. Effect of alignment The effects of alignment on the friction and wear properties of CNF/ epoxy composites were studied and compared with pure as well as randomly-dispersed composites. Fig. 11a,b showed the speed effect on friction coefficient and wear rate for 0.6 and 1 wt% random and aligned composites. Moreover, the detailed wear mechanisms were discussed with the micrographs of 0.6 wt% random and aligned composites (under 20 N load and 0.05 m/s sliding speed, Fig. 11c,d). The friction coefficient of 0.6 wt% aligned composite decreased by 15% and 27% when compared to 0.6 wt% random composite and pure epoxy, respectively. Similarly, the friction coefficient of 1 wt% aligned composite decreased by 35% and 52% compared to its random counterpart and pure epoxy, respectively. The highest wear rate reduction was found in the case of 1 wt% aligned composites (0.15 × 10−4 mm3/Nm) where it reduced by approximately 25 times compared to pure epoxy (3.95 × 10−4 mm3/Nm) and 7 times compared to its random counterpart (1.12 × 10−4 mm3/Nm). Wear rate for pure epoxy decreased marginally by 1.1 times with 0.6 wt% random CNFs (3.51 × 10−4 mm3/Nm), however reduction of ∼2.1 times was found when aligned CNFs was used (1.9 × 10−4 mm3/Nm). The detailed load variation effect on friction and wear of aligned composites along with the friction and wear performances comparison with random composites are shown in Fig. 12.

Fig. 9. Tribological properties of CNF/Epoxy nanocomposites. Variation in (a) friction coefficient and (b) specific wear rate with increasing CNF concentration (randomly-oriented CNF/Epoxy composites) at different sliding speeds of 0.05, 0.1 and 0.2 m/s; under a fixed normal load of 20 N.

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Fig. 10. Effect of speed; SEM images of wear tracks of 0.6 wt% CNF/epoxy nanocomposite under20 N load at different sliding speeds(a) 0.05 m/s; (b) 0.1 m/s; and (c) 0.2 m/s (Higher magnification images are shown as inset).

SEM micrographs of wear tracks (Fig. 11c and d) and optical micrographs (Fig. 11e and f) of steel ball counterfaces were analyzed to explore the wear mechanisms. Comparatively rough surface with sign of adhesive wear, a few micro-cracks and abrasive wear particles were observed in the case of 0.6 wt% random CNF/epoxy composites (Figs. 11c and 9a). Also, a significant amount of scratches could be found on the steel ball counterface in this case (Fig. 11e). The wear track and counterface morphology revealed that the major wear mechanisms were adhesion and abrasion along with mild fatigue. Uneven surface along with plucking effect on wear track is a sign of adhesion and it caused maximum wear. The EDAX elemental analysis of counterface steel ball exhibited high amount of oxygen (Table 1a) that indicates a chance of abrasive oxide particle formation which could be one of the reasons behind abrasion. Moreover, when the CNFs are randomly dispersed in the epoxy matrix, there is a comparatively high chance of fiber disengagement during sliding against steel ball. Those disengaged nanofibers can also act as abrasive particles and show the blade effect and thereby result in a lot of scratches on the counterfaces (Fig. 11e). Besides all these effects, the friction coefficient and wear of 0.6 wt% random composites decreased compared to those of pure epoxy. An important reason for this was the transfer layer, formed on counterface by disengaged CNFs and worn materials layer, which partially prevented direct contact and the presence of solid lubricant CNFs which induced low friction. Furthermore, comparatively better hardness and load carrying capacity, lower adhesion than pure epoxy also contributed in obtaining better friction behavior in random composites. The aligned composites showed better performance than both pure epoxy and randomly dispersed composites. However, the mechanism in this case was identified to be different from the random one. Observation from SEM micrograph of wear track revealed relatively smooth surfaces with a few plucking marks and micro-cracks (Fig. 11d). Optical micrograph study of steel ball counterface merely showed visible scratch like the blade effect found in the case of random composites (Fig. 11f). That confirmed the absence of large abrasive particles on wear track. Major reason for the lack of abrasive particle formation could be explained by the alignment of CNFs inside epoxy matrix and the orientation, which was normal to the sliding surface. During sliding of composites against steel ball, disengagement could happen easily for random composites by the shear stress developed during sliding and disengaged CNFs acted as abrasive particles thus inducing more abrasive wear. Without the reinforcing effect of CNFs, load carrying capacity reduced and chances of adhesion wear increased for random. As a result, wear resistance did not improve much compared to pure epoxy. On the other hand, it was comparatively difficult to disengage or uproot vertically aligned CNFs bonded with epoxy matrix. After some initial wear, CNFs got exposed on the track and became bent on the surface by the applied load. The bent CNFs restricted direct contact between two adjacent surfaces, impeded the penetration of steel ball into the matrix and thus reduced further wear. It also helped in lowering friction coefficient because of self-lubricating property of CNFs. Under that

situation, disengagement of CNFs became comparatively less than the random one. Also, the adhesive wear was nearly absent in this case which could be attributed to lower surface energy, higher thermal conductivity, and structural integrity. In the case of aligned composites the thermal conductivity was found to be more than the random composites, which helped in better dissipation of the heat generated at the contact. This was also verified from the contact point temperature measurement (Fig. 5d). The combined effect of CNFs alignment, better heat dissipation, reduced contact between surfaces, lower friction of CNFs and less abrasive particle generation contributed towards an improvement of 25 times in wear rate. Under a Hertzian contact pressure of ∼57 MPa and 0.1 m/s speed, this improvement of wear rate is rather important. None of the literature exhibited this type of wear improvement for lower CNF concentration. Besides this, the alignment was able to reduce the increased wear for high CNF concentration (2 wt%) at higher load (Fig. 12). However, the existing limited wear performance of this aligned composite can be attributed to the lower number density of CNFs on the surface and use of discontinuous nanofibers. Tribology of aligned carbon nanotubes was studied previously by Wang et al. [18] where they used highly dense continuous aligned vapor-grown CNTs with epoxy. Superior wear properties (∼219 times enhancement) were found in their case which was attributed to the cushioning effect and lubricating effect of bent CNTs on worn surfaces. However, vapor grown CNTs were used as reinforcement in their case, production of which is time-consuming, and the large-scale application is difficult. The current study showed the improvement of friction and wear behavior of epoxy using electric field alignment of discontinuous carbon nanofibers, which is comparatively convenient in terms of applicability. In order to improve wear resistance polymer composites, use of short carbon fiber (SCF) along with other nanofillers have been reported.The wear resistance of epoxy composites was found as ∼22 times more than pure epoxy when reinforced with 15 wt% SCF, 5 wt% PTFE and 5 wt% graphite [33,34]. However, the current study showed almost similar improvement (∼25 times) in wear resistance of epoxy, with only 1 wt% aligned CNF content. This study has explored the possibility of getting high wear resistant polymer using low amount of discrete carbon nanofillers which may be extended to other nanofillers too. 4. Conclusions In this work, electric field aligned CNF/epoxy composites were fabricated with different weight percentages of non-functionalized CNFs (from 0.2 to 2 wt%) and friction/wear tests were carried out under different loads (10, 20 and 30 N) and speeds (0.05, 0.1 and 0.2 m/s). Wear behavior and wear mechanisms of aligned composites were compared with pure epoxy and random CNF/epoxy composites and the following conclusions are drawn from this study. 1. At 1 wt% CNF concentration, the aligned composites exhibited wear 459

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Fig. 11. Effect of AC electric field induced alignment on friction/wear properties of 0.6 wt% and 1 wt% CNT/epoxy composites (a) friction coefficient, (b) Specific wear rate comparison of pure epoxy, 0.6 wt% R,A and 1 wt% R, A with load variation (at 20 N load; R = randomly oriented and A = aligned composites); SEM micrographs comparison of wear track of (c) 0.6 wt% CNF/epoxy randomly-oriented (d) 0.6 wt% CNF/epoxy aligned composites; Optical micrograph of Counterfaces of 0.6 wt% CNF/epoxy (e) randomly-oriented and (f) aligned composites and the schematic of wear mechanisms.

rate reduction by approximately 25 times compared to pure epoxy and 7 times compared to randomly oriented CNF/epoxy composites. Similarly, the friction coefficient also reduced from 0.6 for pure epoxy to 0.44 and 0.24 for random and aligned epoxy/CNF composites, respectively. 2. Adhesive and ploughing wear mechanisms were dominant for pure epoxy which changed to a combined effect of adhesive, abrasive and fatigue wear for random CNF/epoxy composites. Disengaged CNFs and debris formation in random composites were replaced by the presence of bent CNFs on wear track, fewer numbers loosen CNFs and debris. Exposed and bent over CNFs on wear track prevented direct contact between steel ball and polymer thus contributed

towards reduced friction coefficient and wear rate, and the wear of the steel counterface was also negligible. The strong reinforcing effect, better heat dissipation property and hardness in aligned composites inhibit materials to be worn out. All of these mechanisms contributed towards the better wear performance of vertically aligned CNF/epoxy composites. 3. For 0.6 wt% aligned CNF/epoxy composites, thermal conductivity improved by 77% and 44%; electrical conductivity improved by 7 and 2 orders of magnitude compared to pure epoxy and random composites, respectively. Overall, CNFs was found to have a significant impact on the epoxy 460

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Fig. 12. Comparison of friction coefficient between random and aligned CNF/epoxy composites under (a) 10 N load; (b) 20 N load; and (c) 30 N load at 0.05 m/s sliding speed; (d) Specific wear rate improvement of aligned composites over random composites under 20 N load and 0.05 m/s speed.

system in terms of better tribological, electrical and thermal properties and the properties enhanced further, when CNFs were vertically aligned inside epoxy matrix. This aligned CNF/epoxy composite can have applications as high wear resistant polymer bearings, electrically conductive bushings as well as thermally stable coatings.

[4] [5]

Acknowledgements

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The authors are thankful to Prof. Jayashree Bijwe, IIT Delhi, for providing access to the probe sonicator. AC acknowledges the Ph.D. fellowship support from IIT Delhi. NVD acknowledges the partial support from Science and Engineering Research Board (SERB), Government of India, under Grant No. ECR/2015/456.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.triboint.2019.06.014.

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