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Friction and wear properties of different types of graphene nanosheets as effective solid lubricant Yitian Peng, Zhuoqiong Wang, and Kun Zou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00422 • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 23, 2015
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Friction and wear properties of different types of graphene nanosheets as effective solid lubricant Yitian Peng∗1, Zhuoqiong Wang2, Kun Zou1 (1.College of Mechanical engineering, Donghua University, Shanghai 201620, China, 2. School of Mechanical engineering, Southeast University, Nanjing 211189, China) ABSTRACT Friction and wear properties of graphene nanosheets prepared by different processes as solid lubricant on silicon dioxide have been comparatively studied via calibrated atomic force microscopy. The effect of normal load, humidity and velocity on the friction was also investigated. All kinds of graphene nanosheets possess friction-reduction properties at nanoscale. Mechanically exfoliated graphene nanosheets exhibit ultra-lubrication and zero wear under high pressure due to perfect graphitic structure and hydrophobic surface. Defects in chemical vapor deposited graphene nanosheets decrease the anti-wear and friction-reduction capability. The graphene oxide nanosheets (GOS) show the weakest friction-reduction properties on account of destroyed graphitic structure and hydrophilic surface. The reduced graphene oxide nanosheets(RGOS) possess better friction reduction than GOS in virtue of hydrophobic surface properties. Both RGOS and GOS have weak anti-wear properties due to the destroyed graphitic structure. Anti-wear properties correlated strongly with the structure and friction depends mainly on the structure and surface property. KEYWORDS: atomic force microscope; graphene nanosheets; friction; wear; solid lubricant
*Corresponding author. Tel.: Fax: +86 21 67874297. E-mail address:
[email protected] (Y.T. Peng). 1
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1. Introduction With the advancement of nanotechnology, problems caused by friction and wear in nano-electromechanical systems (NEMS) have become a big trouble. Reducing friction and wear-related mechanical failures in moving NEMS has gained increased attention due to their adverse impacts on efficiency, durability, and environmental compatibility. Liquid lubricants are hard to be used because of strong capillary forces that exist in the presence of liquid. Novel materials, coatings, and solid lubricants that potentially reduce nanoscale friction and wear are needed for the application of NEMS. Graphene being two-dimensional graphitic carbon material was confirmed to be one of the strongest materials in the world[1]. Graphene has favorable properties such as high chemical inertness, extreme strength, and easy shear capability on its densely packed and atomically smooth surface[2]. In addition, since the atomically thin nature and it is ultrathin even with multilayers, graphene can coat nano-scale objects simply by dispensing graphene solution make it a potential low friction and wear resistance coating with rotating and sliding contacts[3,4,5]. Graphene can significantly reduce the nanoscale friction and suppress wear as an ideal solid lubricant[6]. However, the friction and wear properties are strongly dependent on the specific structure, surface chemistry and elastic/plastic properties, on the chemical environment in which the measurements are performed, and on the sliding load and speed of the interface[7-11]. Many production methods are available for graphene until now[12,13,14]. Depending on the type and the quality, the surface properties, thickness, and the defect density of graphene vary a great deal. Mechanical exfoliation method, chemical vapor deposition(CVD) and reduction of chemical exfoliated graphene oxide nanosheets are three most usual processes for the preparation of graphene. The mechanical, physical, chemical and surface properties of graphene nanosheets prepared by the 2
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above three processes are quite different. Atomic force microscopy (AFM) has been a very useful tool in nanotribological studies because it can measure normal and lateral force between a sharp tip and a surface simultaneously[15]. Dienwiebel et al showed that orientational misfit between graphene flakes and graphite substrates would greatly affect the friction force, where the superlubricity state can be achieved if the misfit results in an incommensurate state with AFM experiments and theoretical simulations[13]. The surface chemistry change the tribological performance of graphene attracts strong interests of researchers[14-17]. The friction of disordered graphene chemically modified with oxygen, fluorine or hydrogen, is found to be much larger than that of pristine graphene by Jae-Hyeon Ko etc. [18]. Yalin Dong et al. further explored the atomic-level origin of the enhanced friction on chemically modified graphene by MD simulation and found hydrogenation increased friction significantly [19]. Additionally, nanoscale friction of graphene nanosheets is also related to some other factors, such as sliding velocity, applied normal load and relative humidity of experimental conditions [20, 21]. The number of layers and degree of defects in the graphene, nature of sub-surface on which graphene is bounded, preparation method collectively play an important role to determine its friction properties[22-27]. It has been reported that the frictional forces exhibited on graphene are much lower than substrates and decrease with the increasing number of layers for loosely-bound or suspended graphene[22].The thickness dependence of friction on graphene is attributed to van der Waals interactions between the AFM tip and graphene surface, the effect of electron-phonon coupling, as well as the puckering effect which is more dominant with fewer graphene layers[22, 23]. Zwörner et al. have shown that nanofriction increases logarithmically with the sliding velocity (GOS>RGOS>CGNS>MEGNS when the normal load is less than 10 nN. However, the friction force of four types of graphene nanosheets changes the order to
as
follows
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the
relative
humidity
increased
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30%
to
55%
R.H.:
Si>RGOS>GOS>CGNS>MEGNS. Interestingly, the friction force on RGOS increased and the friction force on GOS decreased when the relative humidity increased. The friction force on RGOS is even higher than that on GOS under different load at 55% R.H. It is worth noting that the reported thickness dependent friction behavior in the graphene nanosheets was negligible since the 14
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investigated graphene nanosheets were close in thickness [21]. 14
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CGNS 55%
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Fig.5. Frictional force of AFM tip sliding on SiO2/Si substrate (a), four types of graphene nanosheets (b) and on MEGNS and CGNS(c) versus normal load recorded at 30% and 55% R.H. The sliding velocity was 1µm/s, and the temperature was ~25°C. 15
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Also the frictional forces on MEGNS and CGNS versus normal load recorded at 30% and 55% R.H. were provided in Fig.5(c). It can be seen that the change of the friction on MEGNS and CGNS is very small when the humidity increase from 30% to 55% R.H. However the trend of friction force on MEGNS and CGNS are different. The friction force of AFM tip on MEGNS increase, on CGNS decrease with the increase of humidity. The experimental schematic diagram of AFM tip sliding on four types of graphene nanosheets can be described as Fig.6. As shown in Fig.6(a), both MEGNS and CGNS have high graphitic crystal structure. MEGNS possesses the best friction-reduction performance which can be ascribed to the low surface energy as well as its intrinsic perfect structure. The friction on MEGNS and CGNS are less sensitive to humidity because of perfect graphitic structure, producing a highly hydrophobic surface. The friction on CGNS is higher than on MEGNS denotes that the defects increase the friction force. The change of friction on MEGNS and CGNS is very small from the curve of friction versus load means it seem unaffected by the relative humidity change. The structure of MEGNS and CGNS are almost the same. The residue PMMA and contamination on surface of CGNS may be main reason for different friction force and trend of friction on MEGNS and CGNS as the increase of humidity. As shown in Fig.6(b), the crystalline structure of RGOS was destroyed though the surface properties keep hydrophobic. Physically, a surface dislocation, vacancies, or corrugation were created in RGOS during oxidation and reduction. The friction strongly depends on the structure of graphene nanosheets. Consequently, the destroyed graphitic structure of RGOS causes higher friction force than MEGNS. However, the friction on GOS is even higher than on RGOS at 30% R.H. though the ID/IG of GOS is smaller than that of RGOS. Most oxygenous groups on the surface of GOS were removed during reduction and convert the hydrophilic surface to hydrophobic. The 16
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hydrophilic defects increase adhesion force between the AFM tip and GOS which can work as a drag force when the tip slides on GOS laterally. Friction forces of GOS correlate directly with the adhesion forces. (a)
(c)
(b) Defects
Defects
Fig.6. schematic diagram of AFM tip sliding on the (a) MEGNS and CGNS, (b) RGOS, (c) GOS. Gray carbon structure represents graphilic structure, red and white balls symbolize oxygeous groups It is evident that GOS possess the highest and load-sensitive friction force due to the combined effect of destroyed structure and hydrophilic surface properties at 30% R.H besides the SiO2/Si substrate. Based on the above results and analysis, two critical factors lead to low friction of MEGNS, are perfect graphitic structure and hydrophobic surface. However, the friction force on GOS and RGOS changed in different way when the relative humidity increased from 30% to 55% R.H. Specifically, friction force on RGOS increased about 35%, while on GOS dropped almost half under the load of 10nN. The effect of relative humidity on friction force under different loads involves different mechanism. The oxygen containing functional groups contribute mainly to the difference. The amount of oxygenous group on the surface of GOS and RGOS is quite different. The hydrophobic surface of RGOS has few oxygenous groups and hydrophilic surface of GOS has numerous oxygenous groups. As the relative humidity increased to 55%R.H., more water molecular adhere on the surface of RGOS and increase the water capillary force, then increase the adhesion forces between AFM tip and RGOS. The increased adhesion force causes the increase of frictional force. However, the contrary tendency is observed on GOS at 55% R.H. As shown in Fig.6(c), abundant oxygenous groups existing on the surface of GOS contributed to the relative humidity sensitive friction. The absorbed water molecules on the hydrophilic 17
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carboxyl groups form continuous water meniscus at the interface of the GOS surface. The continuous water meniscus lubricates the interface and reduces the shear strength of the contacting interface, thus reduce the friction force on GOS. Similar result was reported [26]. It can be concluded that the friction force on GOS and RGOS behave different when the relative humidity increased. 3.3. Friction of graphene nanosheets and GOS as functional of humidity and velocity The friction force of AFM tip on graphene nanosheets could be combined affected rather than a single factor. Functional groups on the surface of GOS and RGOS have a significant impact on the humidity sensitive friction. The friction forces on the GOS and RGOS were found to initially increase logarithmically with the sliding velocity [24]. This relationship between the friction (FL) and sliding velocity (v) can be described by the expression: FL = FL 0 + FL1 ln
v , where FL0 presents v1
the friction force needed to induce an irreversible jump of the tip at zero temperature[23]. Then the velocity dependence of friction force was further investigated under different relative humidities. Fig.7 presents the variation in the zero-load friction force on GOS and RGOS with lnv at 30% and 55%R.H. RGOS exhibited better friction-reduction capability than GOS at all sliding velocity at 30% R.H. There appears to be a critical sliding velocity (Vc) at which the highest frictional force was found and the velocity range can be divided into two regions. The Vc of GOS and RGOS were about 0.6 and 0.4mm/s, respectively. When the sliding velocity is lower than Vc, frictional force increased linearly with ln(v) was observed, and which could be explained by atomic scale stick-slip. The frictional force on GOS changed significantly with the sliding velocity, and the initial increase is much more rapid than RGOS. At the velocity >0.6mm/s, the friction force on GOS decrease with velocity sharply. The decreasing trend in friction at higher velocities could be due to tip jump during sliding. However, different with the phenomenon revealed in GOS, a plateau of the frictional force 18
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on RGOS was observed when the sliding velocity >0.4mm/s. The sliding velocity has a negligible impact on the friction of the RGOS surface when the speed is higher than Vc. When the velocity is high, the AFM tip could jump and the total amount of energy dissipated during sliding does not change significantly, causing an approximately independent of frictional force on sliding velocity. When the relative humidity increased to 55% R.H., both the slope of the friction-velocity curve and the friction force were affected. The friction forces on GOS are even lower than that on RGOS at all velocities at 55%R.H. This is in accordance with the results of friction versus load at 30% and 55%R.H. It can be seen that RGOS and GOS have the similar friction behavior with sliding velocity at 55% R.H. The friction forces increase with velocity up to Vc, beyond which it starts to decrease slightly. The Vc of GOS and RGOS were about 0.4 and 0.6mm/s, respectively.
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2.5 2.0 RGOS30%RH GOS30%RH RGOS55%RH GOS55%RH
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ln[v/(mm/s)]
Fig.7. Frictional force of AFM tip sliding on RGOS and GOS versus sliding velocity recorded at 0nN normal load under two different humidities (T~25ºC, RH=30% and 55%). The amount of hydrophilic groups on the GOS surface was confirmed to affect friction force that sensitivity to humidity. The adsorbed water molecules condensed on a hydrophilic surface form water meniscus at 55% R.H. The decrease of frictional force at a higher sliding velocity can be mainly attributed to lubrication effect of continuous water meniscus on the hydrophilic surface of the GOS. The Vc of GOS decreased from 0.6mm/s to 0.4mm/s when the relative humidity increased 19
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from 30% to 55%R.H. That means the higher relative humidity contribute to the form of continuous water meniscus at lower velocity. The surface of GOS is terminated with hydrophilic functionalities such as -COOH and -OH that are capable of forming networks of H-bonds. The continuous water meniscus can build up when sliding velocity increase to a certain value, thus the frictional force on GOS decreases. On the hydrophobic surface of RGOS with high contact angle, it is difficult for continuous water meniscus to form. The decrease of frictional force dependence on velocity is explained as a consequence of the AFM tip jump on surface to low the effect of adhesion force on friction. 3.4. Wear properties of four types of graphene nanosheets The wear evaluation of four types of graphene nanosheets was conducted with the calibrated AFM cantilever scanning over the same line over multi-sliding cycles at a rate of 1µm/s. The friction forces versus sliding cycles at constant normal loads were recorded.
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Fig.8. Friction force of AFM tip sliding on four types of graphene nanosheets including MEGNS(a), CGNS(b), RGOS(c) and GOS(d) versus sliding cycles recorded at ambient conditions (T~25ºC, RH=30%). Fig. 8(a) and Fig.8(b) show the variation of friction forces on MEGNS and CGNS as a function of sliding passes under the normal load of 500nN. The friction force is fairly stable with a constant value of about 2.8nN on MEGNS until the end of the 1000th sliding cycle. MEGNS consists of a plane of carbon atoms cannot be worn off gradually with increasing number of sliding cycles. It denotes that the MEGNS can withstand high shear force caused by high pressure AFM tip. The MEGNS have perfect graphitic structure and strong Van der Waals between the layers can effectively distribute the high pressure. Then MEGNS can remain relatively effective in reducing friction and resisting wear under high pressure. The CGNS has low initial friction force of 7.4nN. The friction force remains nearly unchanged until about 800th pass then increased. The friction force on CGNS at initial period is higher than on MEGNS. This is consistent with the friction test under low load because defect exists in CGNS. An evident increase in friction force after 800 cycles denotes that defect was extended after 800 cycles’ wear test. The CGNS could be destroyed because few defects exist in the structure cause stress concentration after 800 cycles. Fig.8(c) and Fig.8(d) show the variation of the friction force as a function of a number of sliding cycles on GOS and RGOS at the normal load of 200nN. The friction force increased to high level at the initial period of the test (
