Accepted Manuscript Tribological Properties of Carbon Nanotube-Polyethylene Oxide Composite Coatings Byung-Hoon Ryu, Anthony J. Barthel, Hae-Jin Kim, Hyun-Dai Lee, Oleksiy V. Penkov, Seong H. Kim, Dae-Eun Kim PII: DOI: Reference:
S0266-3538(14)00237-1 http://dx.doi.org/10.1016/j.compscitech.2014.07.007 CSTE 5874
To appear in:
Composites Science and Technology
Received Date: Revised Date: Accepted Date:
16 April 2014 3 July 2014 6 July 2014
Please cite this article as: Ryu, B-H., Barthel, A.J., Kim, H-J., Lee, H-D., Penkov, O.V., Kim, S.H., Kim, D-E., Tribological Properties of Carbon Nanotube-Polyethylene Oxide Composite Coatings, Composites Science and Technology (2014), doi: http://dx.doi.org/10.1016/j.compscitech.2014.07.007
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Tribological Properties of Carbon Nanotube-Polyethylene Oxide Composite Coatings Byung-Hoon Ryua, Anthony J. Barthelb, Hae-Jin Kima, Hyun-Dai Leea, Oleksiy V. Penkova, Seong H. Kimb*, and Dae-Eun Kima* a
Department of Mechanical Engineering, Yonsei University, Seoul, Korea b
Department of Chemical Engineering and Materials Research Institute, Pennsylvania State University, University Park, PA, USA *Corresponding authors: [email protected]
, [email protected]
The effect of polyethylene oxide (PEO) inclusion in carbon nanotube (CNT) coating on the tribological properties of the composite coating was investigated. CNT/PEO composite coatings with 12 different weight fractions were fabricated by using the spin-coating method on pre-cleaned Si (100) substrates. Wear tests were performed using a reciprocating tribo-tester under an applied load of 10 mN and a speed of 4 mm/s. A 1.6 mm diameter stainless-steel ball was used as the pin and the corresponding Hertzian contact pressure was calculated to be ~270 MPa. From the experimental results, it was shown that the steady-state friction coefficients for the CNT/PEO coatings gradually decreased from ~0.47 to ~0.3 as the concentration of PEO increased from 0 wt.% to 75 wt.%. The wear volume of the CNT/PEO composite coating rapidly increased with increasing amount of PEO when the weight fraction of PEO was comparatively low. The wear volume reached the maximum value for 87/13 wt.% of CNT/PEO coating and showed a continuous decrease in the wear volume with further increase in the PEO weight fraction. Morphological analyses of the wear pattern revealed that wear mechanism of the composite coating varied with respect to PEO concentration. The overall experimental results demonstrated that the tribological properties of CNT-based coatings could be significantly improved by adding an optimum amount of PEO. The reason for such an
improvement was attributed to the enhancement of interfacial forces within the composite coating due to the presence of PEO.
Keywords: A. Carbon nanotube; A. Polymer; B. Friction/wear; B. Interfacial strength; D. Scanning electron microscopy (SEM)
Carbon nanotube (CNT) has received a great deal of attention in various mechanical and electrical applications such as super capacitors [1, 2], hydrogen storage [3, 4] and wear reduction [5, 6] due to its superior electrical  and mechanical [8-12] properties. Individual CNTs can have exceptionally high Young’s modulus and tensile strength with values greater than 1 TPa  and 150 GPa  respectively. Also, the electrical resistivity . In addition to these unique
of CNT is extremely low, being less than
properties, CNTs can also have a very large aspect ratio that exceeds 1000 and they can be grown in the order of millimeter length, while the diameter can be in the range of a few nanometers. These geometric features and mechanical properties of CNTs make them attractive to be used as fillers for composite materials. A considerable number of studies have shown that inclusion of CNT as a filler material in a composite is effective in enhancing various properties of the bulk material [14-19]. It has been shown that, when mixed homogeneously in polymer matrices, CNT acts as a reinforcement material to improve the electrical [14-16] and mechanical [17-19] properties of the polymer composites. By using these prominent characteristics, numerous attempts
were made to utilize the CNT/polymer composites for a wide range of applications such as actuators , transparent flexible electronic thin film [21-23], and mechanically reinforced structures [24, 25]. Within the field of tribology, various works have been reported on various types of CNT-polymer based composite coatings in the past few years [26-30].
CNT/polytetrafluoroethylene (PTFE) composites. CNT was added to PTFE powder solution with different volume fraction from 0 to 30% as a filler material. It was observed that the friction coefficient and wear rate decreased as the volume fraction of CNT increased. Galetz et al.  demonstrated the possibility of utilizing carbon nanofiber (CNF) reinforced ultra-high molecular weight polyethylene (UHMWPE) nanocomposites for tribological applications. It was found that CNF improved the mechanical properties of the composites such as elastic modulus and yield strength with significant decrease in the wear volume. Thus, it was proposed that the CNF-UHMWPE nanocomposites could be considered as a promising material for tribological applications such as artificial knee implants . Among the many materials that have been combined with CNT to form a composite, polyethylene oxide (PEO) has also been utilized to attain superior material properties. Due to its excellent properties such as good chemical stability in air, PEO has been considered as a good candidate for various applications such as medicine tablets , biosensor  and solid state electrolytes [33, 34]. Awasthi et al.  assessed the electrical and mechanical properties of CNT/PEO composites. From the experimental results, it was found that both electrical and mechanical properties were considerably enhanced with the increase of CNT weight fraction in the composite material. Park et al.  proposed various experimental methods for fabricating the CNT/PEO composites with superior electrical and mechanical properties. Previous works on CNT/PEO composites have
focused mostly on synthesis method and general electrical and mechanical properties of the composites [35-37]. The focus of this work is on the tribological properties of CNT/PEO composite coatings that have not been investigated previously. Given the superior mechanical properties of CNT/PEO composite material, it was expected that with adequate mixture of PEO and CNT, the tribological properties of CNT-based coatings may be improved further. With this motivation, the tribological properties of CNT/PEO composite coatings with 12 different weight fractions were investigated. Also, the wear mechanism of CNT/PEO composite coatings was suggested based on experimental results.
2. Experimental details
2.1. Specimen preparation In order to fabricate the CNT/PEO composite coatings, PEO powders (Aldrich, Mw,avg~ 100,000) were dissolved in a CNT solution. The CNT solution (BMS Tech.) was composed of 2 wt.% multi-walled CNTs with a diameter of about 20 nm that were uniformly dispersed in isopropyl alcohol (IPA). A commercial spin-coater (YS-100, YI Engineering) was used to deposit the CNT/PEO composite coatings on pre-cleaned Si (100) substrates. CNT/PEO solutions with various weight fractions were blended for 3 hours using an ultrasonic bath followed by magnetic stirring for another 3 hours in 90 °C DI water bath to form a homogeneous dispersion of CNT in the PEO matrix. Two techniques were assessed: static single-layer coating and dynamic double-layer coating. For the static technique, the solution was initially placed on a stationary Si substrate and the substrate was rotated at 6000 RPM for 60 seconds. For the dynamic technique, the solution was dropped twice on the Si substrate with 30 second intervals while spinning the substrate at 6000 RPM. The
solution was first dropped to cover the whole area of the Si substrate and the second drop was dropped to form a uniform coating . Fig. 1 shows the surface topography, measured by using a 3-D profiler (Bruker, DXT -A), of CNT/PEO composite coatings that were fabricated by two different spin coating methods. It was shown that the dynamic dual-layer coating method was more effective in obtaining a uniform coating than the static single-layer coating technique. Thus, the dynamic dual-layer coating method was used in this study. After spin coating deposition, all the specimens were dried in a desiccator for 24 hours to evaporate the remaining IPA solvent. Specimens with different compositions were synthesized to investigate the effect of CNT/PEO weight fraction on the tribological properties of the composite coatings. It should be noted that the specimens with A weight % CNT and B weight % PEO in the fully dried coating was indicated as A/B wt.% in the manuscript. The 12 different specimens that were used in the experiment are presented in Table 1. When the fraction of PEO was over 75 wt.%, the mixture solution was not uniformly deposited on the Si substrate due to relatively high viscosity of the solution. To overcome this problem the concentration of PEO was kept below 75 wt.%. Thickness of the coatings on all the specimens was ~ 0.7 to 0.8 μm with a similar average surface roughness value of ~160 nm as shown in Fig. 1 and Fig. 2.
2.2. Tribological testing of CNT/PEO coatings Tribological behaviors of CNT/PEO composite coatings were investigated using a reciprocating tribo-tester under ambient conditions. Pre-cleaned 1.6 mm diameter stainless steel balls were used as the pin. All tests were performed under 10 mN applied loads at a sliding speed of 4 mm/s with a 2 mm stroke for 600 cycles. To calculate the Hertzian
contact pressure, the hardness and Young’s modulus were measured using a commercial ultra-nano hardness tester (CSM UNHT). The variation of measured value was negligible with respect to the weight fraction of PEO. The averaged hardness and Young’s modulus values were measured to be 3.75 kPa and 44.1kPa, respectively. Based on these values, the Hertzian contact pressure was estimated to be ~20 kPa for all specimens. Three repeated sliding tests were carried out for each specimen. The wear track morphology of the specimens was observed using a scanning electron microscope (SEM, JEOL 6210) equipped with an energy dispersive X-ray spectroscopy (EDS, OXFORD INCA Energy) system. A 3-D profiler was used to measure the wear volume and thickness of the composite coatings. X-ray photoelectron spectroscopy (XPS; Thermo Scientific KAlpha) was employed to investigate the chemical composition of coatings. Gaussian and Lorentzian functions were used to deconvolute the XPS spectra. In order to characterize the adhesion properties of the coating on the Si substrate, a modified nano-scratch tester (CSM Instruments, Switzerland) was utilized. Linear ramp loading from 40 mN to 132 mN was applied on a 1.6 mm stainless steel ball while sliding for 2 mm with a sliding speed of 2 mm/min. Critical loads were identified at locations where complete removal of the composite coatings occurred.
3. Results and discussion 3.1 Structure property of coatings XPS analysis was performed to investigate the structural properties of the coatings. Fig. 3 shows the C 1s peak of the representative specimens. Fig. 3 (a) displays the spectrum of CNTs with two major peaks corresponding to sp2 and sp3 hybrids and other peaks
representing the carbon-oxygen bonds. The spectrum of the specimen containing 57/43 wt.% coating is shown in Fig. 3 (b). For a PEO molecule, it has been reported that C 1s XPS peaks around 285 eV and 286 eV correspond to CH2 (polymer backbone) and C-O-C (ether type carbon component), respectively . Therefore, the increased intensities of the two peaks that appear at 285.3 eV and 286.2 eV were attributed to the presence of PEO molecules. Due to the higher concentration of PEO, Fig. 3 (c) shows higher intensities at 285.3 eV and 286.8 eV than Fig.3 (b). Furthermore, Fig. 3 (a) shows very small intensity at 289.2 eV, which corresponds to carboxyl group. This suggests that CNT has very little anchoring site for chemical reaction. Also, it has been known that PEO does not have any chemical reactivity . Based on these findings it may be postulated that there was no formation of chemical bond between CNT and PEO in the CNT-PEO composite. The crystallinity of the CNT-PEO specimens was also investigated by using the crosspolarization microscope. It was found that distinct crystalline structure could not be found with all the coatings that contained CNT. This result was in agreement with the work by Jin et al.  in which the crystallinity of PEO was reported to be significantly reduced when CNT was added by as little as 1 wt.%.
3.2 Friction and wear behaviors of CNT/PEO composite coatings Fig. 4 shows the steady-state friction coefficients as a function of the weight fraction of CNT/PEO. As the weight fraction of PEO increased, friction coefficient gradually decreased from 0.47 to 0.3. Fig. 5 shows the wear track of 5 representative specimens obtained by using the 3-D profiler. It was found that the surface asperities of the coatings were suppressed due to repetitive sliding of the steel ball and formed burrs along both sides of the wear track. As the fraction of PEO increased from 100/0 wt.% to 87/13 wt.%,
the width of the wear track also increased. It was noted that the width decreased steadily as the PEO fraction increased from 20 wt.% to 75 wt.%. The difference of the track width resulted in the change in the wear volume of the coating as shown in Fig. 6. The wear volume increased from ~20,000 μm3 to ~80,000 μm3 as PEO weight fraction increased from 0 to 13%. The wear volume gradually decreased and reached a steady value around 10,000 μm3 as the fraction of PEO increased to 75 wt.%. Fig. 7 shows the critical loads determined from the nano-scratch tests and subsequent failure of the composite coatings for a few selected specimens. As shown in Fig. 7 (a), CNT coating with the fraction of 100/0 wt.% was only partially removed after the scratch test. Thus, it could be estimated that the critical load for 100/0 wt.% was higher than 132 mN, probably due to relatively high adhesion between the CNT coating and the Si substrate. Unlike the scratch behavior of the pure CNT coating, complete removal of the coating was observed for CNT/PEO composite coatings in 13 ~ 50 wt.% concentration range of PEO. According to the optical microscope images shown in Fig. 7 (b), (c) and (d), the critical loads for CNT/PEO composite materials with the fraction of 87/13 wt.%, 74/26 wt.% and 50/50 wt.% were determined to be 47 mN, 122 mN and 126 mN, respectively. These data indicated that the adhesion between the CNT/PEO composite film and the substrate was very weak at low PEO concentration and then it gradually increased as the PEO concentration increased. When the fraction of PEO was above 50 wt.%, the composite coatings were partially removed, which was similar to the scratch behavior of the pure CNT coating as mentioned above.
3.3 Wear mechanism of CNT-PEO composite coatings From the experimental results it was found that the amount of wear volume increased as
the PEO weight fraction increased from 0 to 13 wt.% and decreased at higher weight fractions of PEO. Also, from the results of the scratch tests as shown in Fig. 7, the adhesion between the coatings and the Si substrate was the lowest when the CNT/PEO weight fraction was about 87/13 wt.% and then substantially improved as the PEO content increased. SEM analyses of the wear tracks were performed to understand the wear mechanisms of the CNT coatings. Fig. 8 (a) represents the SEM image of the pure CNT coating after the wear test. It shows that a groove was formed with burrs along both sides of the wear track. The magnified image shows that the burrs were created with agglomerated CNTs due to high contact pressure of the steel ball. Considering the results from SEM analysis and scratch tests, it was postulated that the CNTs were swept away from the Si substrate which resulted in the formation of a pile up, and a layer of compact CNTs along the wear track was created due to repetitive sliding motion of the steel ball. In Fig. 8 (b), it shows that the 87/13 wt.% coating was torn out and formed a wrinkled shape along the wear track. The PEO, as a host matrix material, appeared to be securely bound to the CNT fibers. However, the adhesion between the coating and the substrate was not sufficient to withstand the shear stress experienced during the wear test. Thus, it could be stated that 87/13 wt.% coatings were completely removed and formed a larger wear track width which resulted in the highest amount of wear volume. When the CNT/PEO weight fraction was above 87/13 wt.%, the adhesion between the coating and the Si substrate seemed to be strong enough to withstand the shear stress experienced during the wear test. In the case of the 74/26 wt.% composite coating (Fig. 8 (c)) though parts of the coating were torn off, the tear did not propagate to the surrounding area of the coating. From Fig. 8 (d) and (e) it can be seen that the coatings with higher concentration of PEO were not destroyed, but rather a relatively smooth and continuous
layer was formed on the wear track. Thus, it was found that depending on the concentration of PEO in the composite coating, the wear mechanism varied significantly. Based on the results of the wear tests, the wear mechanisms of CNT/PEO composite coatings were proposed as schematically illustrated in Fig. 9. Fig. 9 (a) indicates the wear mechanism of the pure CNT coating. The CNTs are swept away and piled up at both sides of the wear track. The dark black region of the SEM image (Fig. 8 (a)) shows the formation of burrs along the wear track that are composed of compressed CNTs, whereas the bright region corresponds to the fresh coating surface. When the fraction of PEO is insufficient to increase the adhesion between the coating and the Si substrate, the composite coating is worn out severely due to strong interlocking between the PEO matrix and the CNT fibers as shown in Fig. 9 (b). It should be noted that a large volume of CNT/PEO composite material is removed simultaneously, which results in the increase of the wear track width. As the fraction of PEO is increased, the adhesion between the coating and the Si substrate increases to the point where the coating is strong enough to survive the wear test. Thus, a smaller wear track is formed resulting in a lower wear volume as depicted in Fig. 9 (c). The wear characteristics of the CNT/PEO composite coating described above may be further explained by the relative bonding strength inside the composite coating and between the coating and the substrate. For CNT/polymer composite coating, it has been reported that relative concentration of CNT/polymer significantly affects the mechanical strength of the composite coating [42, 43]. Shi et al.  studied the mechanical strength of CNT/polystyrene (PS) composite coatings and found that high concentration of CNT could decrease the strength of the coating due to CNT agglomeration effect. In this regard, the presence of PEO in the CNT coating may enhance the strength of the composite coating due to increase in the interfacial area, which also increases the van der Waals
interaction between the CNT fibers and the PEO matrix. Also, PEO can increase the adhesion between the composite coating and the substrate . Mcswain et al.  proposed that PEO may serve as an ultrathin adhesive layer between two glassy surfaces and verified its good adhesive characteristic. Therefore, the adhesion to the silicon substrate is expected to be increased if PEO is mixed with CNT to form a composite coating. The increase in the adhesion between the CNT/PEO composite coating and the substrate as a function of PEO concentration can be seen from the results of the scratch tests shown in Fig. 7. These findings demonstrate that the development of a robust composite coating for various tribological applications could be achieved with good interfacial forces between the fiber and the polymer matrix as well as strong adhesion of the coating to the substrate. In addition to varying weight fraction of CNT/PEO to modify the interfacial forces as explored in this work, chemical modification of the CNTs and the polymer is expected to provide higher interfacial forces. Enhancement of interfacial force between the composite components should be accompanied by increase in the adhesion between the coating and the substrate to attain the most durable composite coating.
The tribological properties of CNT/PEO composite coatings that were fabricated by using the spin-coating method were investigated as a function of PEO fraction in the composite. The steady state average friction coefficient gradually decreased from 0.47 to about 0.3 as the weight fraction of PEO increased from 0 wt.% to 75 wt.%. The wear resistance of the composite coatings initially become worse than the pure CNT film as the
PEO content increased from 0 to 13 wt.%, and then substantially improved and became better than the pure CNT wear when the PEO weight fraction was higher than 50 wt.%. The wear characteristics could be explained by the variation in the adhesion properties between the two components of the composite coating as well as between the composite coating and the substrate. Addition of a small amount of PEO increased the PEO-CNT interfacial forces but with poor composite coating-substrate adhesion, resulting in severe wear. With an increase in the amount of PEO, the composite coating adhered securely to the substrate. The overall experimental results demonstrated that the tribological properties of CNT could be significantly improved by adding an optimum amount of PEO.
Acknowledgment AJB and SHK were supported by the National Science Foundation (Grant No. CMMI1131128). AJB conducted experimental work at Yonsei University through the NSF EAPSI fellowship (Grant No. 1209737). BHR, HJK, HDL, OVP, and DEK were supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0018289).
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Table 1. Composition of 12 different CNT/PEO composite coating specimens.
No. of specimen
Fraction ofCNT/PEO (%)