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Macroscale superlubricity of multilayer polyethylenimine / graphene oxide coatings in different gas environments Prabakaran Saravanan, Roman Selyanchyn, Hiroyoshi Tanaka, Durgesh Darekar, Aleksandar Staykov, Shigenori Fujikawa, Stephen Matthew Lyth, and Joichi Sugimura ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06779 • Publication Date (Web): 16 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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ACS Applied Materials & Interfaces

Macroscale Superlubricity of Multilayer Polyethylenimine / Graphene Oxide Coatings in Different Gas Environments Prabakaran Saravanan1†, Roman Selyanchyn1†, Hiroyoshi Tanaka1,2, Durgesh Darekar1, Aleksandar Staykov1, Shigenori Fujikawa1,3 Stephen Matthew Lyth1,4*and Joichi Sugimura1,2

1

WPI International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, Fukuoka 819-0395, Japan 2

Research Center for Hydrogen Industrial Use and Storage (HYDROGENIUS), Kyushu University, Fukuoka 819-0395, Japan. 3

Center for Molecular Systems (CMS), Kyushu University, Fukuoka 819-0395, Japan 4

Department of Mechanical Engineering, University of Sheffield, S10 2TN, UK

*Correspondence to: Stephen Lyth (I2CNER), e-mail: [email protected] † These authors contributed equally to this work.

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Abstract

Friction and wear decrease the efficiency and lifetimes of mechanical devices. Solving this problem will potentially lead to a significant reduction in global energy consumption. We show that multilayer polyethylenimine / graphene oxide thin films, prepared via a highly scalable layer-by-layer (LbL) deposition technique, can be used as solid lubricants. The tribological properties are investigated in air, under vacuum, in hydrogen, and in nitrogen gas environments. In all cases the coefficient of friction (COF) significantly decreased after application of the coating, and the wear life was enhanced by increasing the film thickness. The COF was lower in dry environments than in more humid environments, in contrast to traditional graphite and diamond-like carbon films. Superlubricity (COF < 0.01) was achieved for the thickest films in dry N2. Microstructural analysis of the wear debris revealed that carbon nanoparticles were formed exclusively in dry conditions (i.e. N2, vacuum), and it is postulated that these act as rolling asperities, decreasing the contact area and the COF. Density functional theory (DFT) simulations were performed on graphene oxide sheets under pressure, showing that strong hydrogen bonding occurs in the presence of intercalated water molecules compared with weak repulsion in the absence of water. It is suggested that this mechanism prevents the separation graphene oxide layers and subsequent formation of nanostructures in humid conditions.

Keywords: polyethylenimine, graphene oxide, layer-by-layer, solid lubricant, superlubricity, nanoparticles, friction, wear, DFT

1. Introduction 2 ACS Paragon Plus Environment

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Understanding tribology at the nano-, micro- and macro-scales is essential. Friction decreases the energy efficiency of devices with moving mechanical parts, as well as reducing lifetimes. It has been estimated that one third of the fuel in automobiles is consumed simply to overcome friction.1 Minimizing frictional energy losses may contribute significantly to the efficiency and lifetime of modern machines, as well as helping to cut anthropogenic CO2 emissions in the fight against climate change. Lubricants such as oil are commonly used to reduce friction and wear. However, in many important applications (such as in the aerospace industry) the use of conventional liquid lubricants is limited, especially in extremes of temperature and pressure, or in reactive/corrosive environments. 2, 3 Alternative solid lubricants are therefore of great interest. State-of-art solid lubricants include graphite, molybdenum disulfide (MoS2), tungsten disulfide (WS2),

boron

nitride

(BN),

magnesium

silicate

(e.g.

Mg3Si4O10(OH)2,

talc),

and

polytetrafluoroethylene (PTFE, Teflon®).2,4 These may be applied as surface coatings or as fillers in lubricating composites. Contact with solid lubricants generally results in transfer of a thin layer of material from the coating to the counterface, otherwise known as tribofilm formation. As a result, wear surfaces can display different chemistry or microstructure due to surface chemical reactions with the surrounding environment. Therefore, solid lubricant coatings may enhance friction or wear in one environment, but fail quickly in others. Addition of solid lubricants can also enhance friction or wear behavior in polymer composites due to control of shear resistance in the interface zone.5 Graphite is one of the most commonly utilized solid lubricants due to its low cost and high thermal stability. Coefficients of friction (COF) of as low as ~ 0.06 – 0.1 in humid air have been measured. However, the tribology of graphite is dependent on the presence of moisture in the air, which weakens the interlayer bonding and facilitates movement of the layers across each other and the substrate. Therefore graphite is not a suitable lubricant for use under vacuum, or in very dry environments.4

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Diamond-like carbon (DLC) is one of the most popular solid lubricants. It displays some of the lowest recorded friction (e.g. 0.001 – 0.05 for amorphous DLC in dry N2).4 DLC tribological coatings are commonly utilized on razor blades, engine parts, hard discs, scratch-resistant glass, medical devices and microelectromechanical (MEMS) systems. They are generally deposited using chemical vapor deposition (CVD) techniques, leading to extremely thin and well defined films with high adhesion. However, this deposition technique requires complex equipment, and therefore is not suitable for e.g. large area deposition at an industrial scale. Therefore, alternative carbon-based solid lubricants with similar performance but enhanced processability are needed. Graphene is an emerging solid lubricant which has been investigated for sliding applications in various environments, due its extremely high strength, the lamellar structure, and the wellestablished tribological properties of graphite and DLC.6 The majority of works investigate graphene as a simple additive in conventional liquid lubricants. For example, Dou et al. recently reported improvement of the lubrication properties of polyalphaolefin (PAO) oil upon the addition of crumpled graphene balls, with specific emphasis on particle morphology.7 Solventcasting of few-layer graphene onto 440C stainless steel can result in a significant reduction in the COF from ~1.0 to ~0.2, and an increase in wear lifetime by more than three orders of magnitude (air, 30% RH)8, 9 CVD-grown monolayer graphene transferred onto 440C steel was found to have a COF of ~0.2, and in hydrogen gas environment (at 900 mbar) the wear life was 40 times longer than in nitrogen. This was attributed to hydrogen passivation of defects, preventing further damage to the graphene structure.10 Superlubricity has been observed for graphene sliding against a diamond-like carbon (DLC) counterface (COF ~0.004),11 attributed to the formation of nanoscrolls which reduce the contact area between the surfaces. The vast majority of studies report a friction coefficient of ~0.2 for graphene or graphene oxide coatings on steel.7, 8


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Adhesion onto solid substrates remains a challenge for graphene, unless CVD is used. Graphene oxide (GO) has numerous functional groups which can attach more strongly to surfaces, as well as enabling further chemical interactions.12,13 However, several groups have demonstrated that friction increases upon hydrogenation, fluorination or oxidation of graphene.14 For example, Lee et. al showed that low friction occurs at sp2-rich subdomains in monolayer oxidized graphene systems, using atomic force microscopy (AFM).15 Despite this, Liang et.al reported improved tribological properties of GO deposited directly onto silicon substrates via electrophoretic deposition (COF = 0.05).16 These contradictory studies suggest that more information is needed about the tribology of GO systems. Layer-by-layer (LbL) self-assembly has recently been utilized to fabricate well-defined GO multilayers with strong adhesion to substrates and a high degree of thickness control (to within several nanometers). In this technique, the negative surface charge of GO is exploited to interact with positively charged polymers via electrostatic attraction. Thus, well-defined and strongly interacting multilayers of alternating positively and negatively charged materials can be built up on surfaces with complex geometries simply by dip-coating.17 This is a low cost, and scalable technique.18 In tribology, this technique has been used to deposit thin films of poly(sodium 4styrenesulfonate)-mediated graphene with polyethylenimine (PEI), resulting in a COF of ~0.2, and a marginal increase in wear life with increasing thickness.19, 20 Here we deposit polyethylenimine / graphene oxide (PEI/GO)n tribological coatings onto steel by the LbL technique. GO is chosen for its high strength and ease of processing, whilst PEI is used specifically to “glue” the GO layers together, enabling the creation of thicker coatings, and improving adhesion to the steel surface. Overall, very few articles have investigated the effect of gas environment on the friction of graphene or GO, and therefore we report the tribological behavior in air, vacuum, nitrogen, and hydrogen gas environments. We report extremely low COF and long wear lifetime, depending on the thickness and environment. The best results are



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obtained in dry conditions, in contrast to graphite, which has low COF in humid conditions and fails in dry environments. Microstructural characterization and computer simulations are performed to gain insight into the mechanism behind these significant results.

2. Experimental Methods Coating deposition. Branched polyethyleneimine (PEI) (Mw ~25000 g mol-1, purity >99%, Sigma-Aldrich) was dissolved in water to make a 0.2 wt% solution. The resulting pH of the PEI solution was 10.3, measured by digital pH meter (SevenEasy, Mettler Toledo). Graphene oxide aqueous solution (4 mg ml-1, Sigma-Aldrich, monolayer content >95%) was diluted to make a 0.1 mg ml-1 aqueous solution which was used for film preparation. According to the manufacturer, the oxygen content in the GO is >36%. The pH of the 0.1 mg ml-1 solution was 3.75. Deionized pure water (18.3MΩ cm−1) was obtained by reverse osmosis followed by ion exchange and filtration (Millipore, Direct-QTM) and used for solutions and washing purposes. Electrostatic layer-by-layer deposition was utilized for the alternate adsorption of oppositely charged molecular species onto the steel substrates (Supporting Information Scheme S1). Different substrates were used for film deposition: steel (SUS 440C); silicon wafer; quartz glass; and gold-coated glass for friction tests, SEM, UV-Vis, and FTIR characterization, respectively. Before film deposition, substrates were washed in ethanol and dried under air flow. Next the substrates were treated by oxygen plasma using a plasma etching system (FA-1, SAMCO, Japan) in order to create a negatively charged surface for deposition of first layer of cationic PEI. Plasma treatment was performed under an RF power of 55W, and an O2 flow of 10 ml min1

. The treatment time was 5 min for the steel substrate and 2 min for the silicon wafer, quartz

glass and gold-coated glass substrates. After plasma treatment, the substrate was alternatively immersed (for 15 min) into aqueous solutions of PEI (0.2 wt%) and GO (100 mg/L). The


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substrates were rinsed in pure water and dried under nitrogen gas flow after each immersion step. To investigate the effect of the coating thickness in macro-tribological tests, three different coatings were fabricated, with 5, 10 and 15 bilayers of PEI/GO, respectively. Deposited films are denoted as (PEI/GO)n, where n is the number of bilayers. In each case, the outermost layer was graphene oxide.

Characterizations of (PEI/GO)n coatings. The LbL (PEI/GO)n coating deposition was monitored using a quartz crystal microbalance (QCM), and characterized using FT-IR-RAS, UV-Vis, and Raman spectroscopies, as well as water contact angle measurements. The morphology of the coatings and worn particles was investigated using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Adsorption of the PEI and GO layers at each deposition cycle was monitored by QCM on electrodes with a fundamental frequency of 9 MHz (USI System, Fukuoka, Japan). The frequency of oscillation was measured after thorough drying of the coating and measured in ambient atmosphere. Changes in frequency were used to calculate the mass change using the modified Sauerbrey equation.21 FT-IR-RAS measurements of the 5, 10 and 15 cycle (PEI/GO)n coatings were performed using a Nicolet iN10 MX Scanning FTIR Microscope (Thermo Scientific, USA), with a liquid nitrogen-cooled MCT (Mercury–Cadmium–Telluride) detector. The spectra were averaged from 128 scans, with a resolution of 2 cm-1 and normalized to the spectrum of gold-coated glass. All measurements were conducted at room temperature under ambient air. UV-Vis absorption spectra of the (PEI/GO)n coatings (on quartz glass substrates) were recorded after each deposition cycle at room temperature with a UV–vis–NIR double-beam spectrophotometer V-670 spectrophotometer (Jasco, Japan).



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Raman spectroscopy was performed on fresh and worn surfaces (after tribology tests), using a DXRTM Raman Microscope (Thermo Scientific, USA) with a wavelength of 532 ± 1 nm and a maximum laser power of 10 mW. The numerical aperture of the 20X lens was 50 µm and the laser beam diameter was 1 mm. Raman spectra were collected in the range 0 to 3500 cm-1, with an exposure time of 20 s and a laser power of 1 mW. The water-contact angle was measured after each (PEI/GO)n layer using a CA-W automatic contact angle meter (Kyowa Interface Science, Japan) equipped with an AD-31 auto-dispenser, and presented data was averaged from 10 separate measurements on the same coating. Morphology of the membrane surfaces, cross-sections, and wear debris after the friction tests were analyzed using field emission scanning electron microscopy (FESEM Hitachi S-5200, 5 kV). For cross-section observation, the samples were fractured in liquid nitrogen and dried under vacuum. To prevent charging by electron beam, all samples were coated with a thin platinum layer before observation, deposited using an ion sputterer (Hitachi E-1030). Average roughness (Ra) was measured by a profilometer (Dektak 6M) equipped with a 50 nm SuperSharp Tip and using the N-Lite low force sensor package (Veeco). High-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan) operated at 200 kV was used to characterize the wear particles after the friction tests conducted in various atmospheres.

Tribological Testing. Macro-scale friction tests were performed in air (~140 ppm of H2O), vacuum, dry hydrogen (H2) and dry nitrogen (N2) environments using a custom built multienvironmental tribometer. Schematics and a digital photograph of the tribometer setup are shown in Supporting Information Fig. S1 and S2, and have been reported in our previous work.22-25 All tests were performed at room temperature (25 oC). The normal load and linear speed are 0.5 N (contact pressure ~0.5 GPa) and 0.54 m/s respectively. The sliding distance (SD) per cycle is calculated to be ~0.13 meters. Elaboration of the specimen details and the


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experimental conditions can be found in Supporting Information Table S1. The steady-state coefficient of friction (COF) was calculated by averaging from the point where steady-state behavior was first observed until the end of the test, or until failure of the coating. Failure is defined at the point where COF >0.4, or abnormal fluctuation is observed. These criteria are based on our own observations: a COF >0.4 was generally associated with a sudden increase in COF to much higher values (e.g. 0.8 or 1) with large fluctuations in the frictional behavior. Each test was repeated at least three times, and average COF values are reported in all cases. Similar definitions of the COF have been used in the literature,26-28 and in our previous work.29-31

Density Functional Theory (DFT) simulations. Ground state DFT calculations of the interaction between GO layers under pressure and in various atmospheres were performed. Since the amount of PEI in the LbL films is small especially on the surface in thicker films, its influence was not considered at this stage, to reduce computing time. Graphene oxide bilayers were intercalated with water, hydrogen and nitrogen molecules in an infinitely repeating unit cell (in all three spacial dimensions), to simulate the macroscopic experimental results. The calculations were performed using the periodic plane wave DFT method, implemented in the Vienna Ab-initio Software Package (VASP).32-34 The general gradient approximation (GGA) was employed, and PAW pseudo potentials were used.35 The cut off energy was set to 400 eV, and the k-point sampling was 2 x 2 x 1.

3. Results and Discussion Film thickness characterization. LbL electrostatic self-assembly was used to deposit multilayer polyethylenimine / graphene oxide films (PEI/GO)n onto 440C steel substrates, with 9 ACS Paragon Plus Environment


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three different thicknesses (n = 5, 10, and 15). The chemical structures of PEI and GO are shown in Fig. 1a. Uniform thickness of the coatings was initially confirmed by the uniform change in surface color after several coating cycles (Fig.1b). Multiple characterization methods confirmed an increase in average thickness after each deposition cycle (Supporting Information Figs. S3-8). Figure 1c shows cross-sectional SEM images of the 5, 10, and 15 bilayer films deposited on silicon wafers, and the uniform layered structure of the coatings is clearly observed (approximately 50, 140 and 310 nm, respectively). The surface roughness increases with thickness from Ra = 132 Å, 157 Å, 253 Å for n = 5, 10, and 15, respectively, suggesting that there is some minor variation in film thickness over the sample area. Quantitative analysis of the gravimetric data (Supporting Information Fig S3 and S5) along with the thicknesses measured by SEM show that (PEI/GO)n coatings are mainly composed of GO and much less PEI; with a calculated density of ca. 0.5 g/cm3, which is also typical for dry GO powders. Tribology tests. Sliding friction tests were performed on the fresh (PEI/GO)n surfaces in different atmospheres (Fig 2). The COF for the bare steel substrate sliding against a steel ball in air is ~ 0.89, with similar behavior in all gas environments. All (PEI/GO)n coatings in air (∼ 140 ppm H2O) have a COF of ~ 0.17 (Fig. 2A), consistent with similar studies of GO in the literature for graphene based coatings.6, 10, 13, 16, 20 The wear life increases significantly with increasing thickness (from 3000, to 11,000, to 20,000 cycles for n = 5, 10, and 15 layers, respectively). Under vacuum quite different behavior is observed. For n = 5, the COF is ~ 0.2 (similar to the case in air), but the friction quickly increases and approaches that of bare steel. This suggests that the layer is easily compromised, exposing the underlying steel surface. However, for thicker coatings (n = 10, 15) a very low COF of ~ 0.08 is recorded up to around 10,000 cycles. In dry H2 (< 2 ppm H2O) (PEI/GO)5 has a COF of ~ 0.3 up until around 14,000 cycles, where it then increases to higher COF values (Fig. 2c and 2d show the same data at difference scales). For (PEI/GO)10 a much lower COF (~0.04) is observed up until around 5,000 cycles, after which


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it rapidly increases and plateaus at ~ 0.2. This behavior is then stable until 25,000 cycles, where the test was stopped. In the case of (PEI/GO)15 an ultra-low friction regime (COF ~ 0.04) is observed up until ~ 22,000 cycles. There are several points where the COF briefly increases for a few thousand cycles before returning to the low COF regime, suggesting a degree of selfhealing in these films under H2 atmosphere. This may be related to the hydrogen passivation effect observed in earlier studies.10 In dry N2 (PEI/GO)5 again has a COF of ~ 0.2 up to around 3000 cycles, after which it rapidly increases as the layer is compromised. However, extremely low COF values of < 0.01 are observed for n = 10 and 15 after a “running-in” period of a few hundred cycles (Fig. 2e and 2f show the same data at different scales). For (PEI/GO)10 the COF is very low up until 3,000 cycles, where it begins to increase step-wise, but retaining very low COF until around 15,000 cycles. An extremely stable ultra-low friction regime is observed in (PEI/GO)15 over a much longer timescale, with a very gradual increase in COF over time, and a larger increase only after around 20,000 cycles. The test was stopped at 25,000 cycles. Summarizing these tribological results, steel has a COF ~ 0.9 in all environments, and the COF of (PEI/GO)5 in air is ~ 0.2, confirming similar observations in the literature. (PEI/GO)5 is of insufficient thickness to significantly affect the tribology over the long term in any environment. Thicker coatings improve the tribological properties and increase the wear life in all environments. Dry conditions result in lower COF and improved wear life, and the best results are obtained in dry nitrogen. Optical micrographs of the worn surfaces and counterface balls demonstrated much less damage to the coating in dry environments (Supporting Information Fig. S9-S12). The reasons behind the differences in COF depending on environment and thickness are herein investigated further. To the best of our knowledge, these results are the first reported experimental observation of superlubricity in graphene-based solid lubricants on steel. In a recent study, Berman et.al



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demonstrated superlubricity between a DLC-coated ball and SiO2 achieved by the addition of nanodiamonds to solution-casted graphene sheets.11 It was shown that nanodiamond particles enhanced formation of graphene nanoscrolls in dry atmosphere. Here, we similarly observe the lowest friction in the driest environments (H2 and N2). Therefore, as in Ref. (11), we assume that one possibility for the observed ultra-low friction has its origin at the microstructural level. Other possibilities could be environment-induced formation and replenishment of tribo-films on the counterface material, or facile shearing of the GO layers, as observed in graphite. In order to clarify the underlying mechanism, the wear tracks and debris were thoroughly investigated. Wear-track characterization. Optical micrographs of the tested samples show the apparent contact area between the counterface ball and surface (Supporting Information Figure S9-S12). Balls tested in air have a larger apparent contact area than those tested in N2 or H2. This observation suggests that the films are less compliant in dry environments. The reduced contact area in N2 suggests a reduction in interfacial shear strength, which may contribute to the observed low friction results. In addition, SEM images (Supporting Information Fig. S4) reveal some surface asperities, which are more pronounced in the 10 and 15-bilayer films than in the 5bilayer film. These are also reflected in the increase in surface roughness with increasing thickness. These may be caused by the fact that for n=15, much thicker layers of GO are deposited than for n=5. This is expected to lead to weaker binding at the surface, resulting in the observed wrinkles. This roughness is expected to have similar behavior to nanotextured surfaces, reducing the contact area. Overall, the micrographs of the pre-test surface and the posttest balls indicate a correlation between the reduction in contact area and the observed tribological behavior. SEM observation reveals that under all testing conditions, the (PEI/GO)n films do not remain intact, but break up into micron-scale particles. The debris particles collected after testing in air (∼140 ppm H2O) have quite large primary particle size, and form agglomerates. Rounded


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particles with amorphous appearance and little fine-structure are observed (SEM, Fig. 3a). In contrast, under drier conditions (vacuum, H2, and N2) the debris particles are generally smaller in size. The layered structure of GO is retained, and many smaller platelets appear in the images (SEM, Fig. 3b-d). The layered structure in N2 is much more apparent than in H2 or vacuum. Transmission electron microscopy (TEM) of the wear debris after testing in air reveals no significant microstructure beyond the level of the particles observed in SEM (Supporting Information Fig. S13). However, testing in dry conditions (Fig. 3e) reveals that the micron-scale particles are liberally coated with nanostructures, including nanoparticles, hollow nanoparticles, and nanotubes / nanoscrolls. The thickness of the walls of the nanoparticles is around 2-10 nm, suggesting that they are somehow formed by “rolling up” or scrolling of the (PEI/GO)n multilayers. The formation of such nanostructures simply by the application of friction and wear is highly usual. However, Berman et.al have shown that the kinetic energy imparted by a sliding ball on GO flakes detaches them from substrate or coating, and the imparted energy favors scroll formation by creating energy imbalance in the system.11 This hierarchal structure may be responsible for the observed superlubricity, via much-reduced contact area, or by acting as nanoscale bearings. The observed superlubricity is unlikely to be due to sliding friction alone, and is more likely due to a combination of sliding and rolling of the observed micro- and nanoparticles. The “running-in” period in nitrogen environment during which the COF drastically decreases over the first few hundred cycles corresponds to the breaking up of the (PEI/GO)n film into micron-scale particles, and the formation of the observed nanoparticles. Such nanostructures are observed even after 25,000 wear cycles and the fact that they survive in their hollow form suggests that they are reasonably strong and durable. Furthermore, the nanostructures observed in this work are similar to the observation previously reported by Berman et.al, where superlubricity was related to formation of graphene nanoscrolls in dry conditions.15



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To investigate the condition of the GO on the sliding surface, the friction test was stopped at 5000 cycles (before failure). Films tested in dry environments (i.e. vacuum, H2 and N2) showed much less wear (Fig. 4a). Micro-Raman characterization was performed on the fresh surfaces and on the wear track to probe the local chemical structure of the carbon. All fresh surface spectra show characteristic D and G bands typical for GO (Supporting Information Fig. S8).36, 37 The AD/AG ratio is ∼1.42 for the fresh surface of all (PEI/GO)n coatings and the pure GO, confirming that no significant changes in the structure of GO occur during the LbL process.38-40 Additionally, the AD/AG ratio on the wear track was determined after the sliding test (Figure 4b). The AD/AG ratio increases to ∼1.52 after the test in air, whereas in H2, N2 and vacuum, the AD/AG ratios are even higher (ca. 1.61). It has previously been reported that the D band intensity increases due to frictional sliding and milling processes, reflecting an increase in the number of defects.

38-42

This suggests that more significant changes occur in the chemical structure of GO

after friction in dry conditions compared with humid conditions. However, the defects introduced to GO are same for vacuum, H2 and N2. This is in agreement with the observation of significantly different microstructure, and the formation of nanoparticles in dry environments. DFT simulations. In order to lend further insight into this system at atomistic scales, density functional theory (DFT) simulations were performed for graphene oxide bilayers with intercalated water, hydrogen and nitrogen molecules. The unit cell is repeated infinitely in three spacial dimensions and therefore this can be used to approximate a macroscopic multilayer system. GO was modeled by introducing epoxy oxygen atoms, carboxyl groups, and hydroxyl groups to pristine graphene, and the interlayer spacing in the absence of intercalated molecules was optimized to 5.8 Å. Two gas molecules (nitrogen, water, hydrogen) were captured between the GO layers (Fig. 5a), and the resulting optimized interlayer distances were 7.0 Å, 7.1 Å, and 6.3 Å, respectively. The pressure experienced by the films in tribological tests was simulated by artificially reducing the cell parameter perpendicular to the plane of the GO sheets, with a step


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size of 0.2 Å. At each step, the atomic coordinates were fully relaxed, but the cell parameters in all spatial directions were fixed. Similar potential energy surfaces are observed for vacuum, nitrogen, and hydrogen in the absence of water (Fig. 5b), with an increase in repulsive force as pressure is applied. However, in the presence of water, multiple local minima suggest hydrogen bonding, and a series of geometrical reorganizations of the water molecules in order to minimize energy. Discussion. Given the above findings, the tribological behavior of the films in different environments can be explained in terms of interlayer interactions, and film microstructure. In all environments, the film breaks up into particles under the action of friction which contributes to a relatively low COF of ~ 0.2. In air, moisture readily interacts with the films due to the hydrophilic nature of both PEI and GO. According to the DFT simulations, the presence of water results in strong hydrogen bonding between the layers under pressure, preventing separation of the sheets and the subsequent formation of smaller microstructures, as confirmed by TEM observation. In nitrogen, much less water is present. According to the DFT simulations this results in electrostatic repulsion between the GO layers, facilitating separation of the individual sheets into flakes and nanoparticles, as observed in the SEM and TEM images. This contributes to smaller contact area, and therefore low friction. In vacuum the same nanoparticles are observed, suggesting a similar mechanism due to the dry conditions. In hydrogen atmosphere, the simulations suggest that the situation should be similar to nitrogen or vacuum. However, the measured tribological properties were marginally worse than nitrogen, and no nanostructure was observed by TEM (Supporting Information Fig. S13). This is attributed to the formation of water by reaction between hydrogen gas and the oxygen surface functional groups. 43 In summary, we suggest that repulsion between the GO layers in different atmospheres also influences the tribological behavior. Low friction is caused not only by the formation of micro-



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and nanoparticles, but also by interlayer repulsion in various atmospheres. Both conditions are satisfied only in dry N2 atmosphere, in which (PEI/GO)n coatings on steel displayed superior tribological performance. Comparison with graphite and DLC. The tribological behavior of these (PEI/GO)n films is very different from that of graphite. GO displays superlubricity in dry environments, and higher friction in humid environments. Graphite, however exhibits high friction in dry environments and superlubricity in humid environments.44-49 Low friction in humid environments is the result of weakening of the binding force between the basal planes of graphite near surface by intercalation or chemisorbed water molecules which increases the interlayer spacing between bulk and near surface basal planes. 47, 49 In contrast in GO containing coatings, intercalation of water between two GO layers results in reverse effect due to creation of more bonds between basal planes. This is attributed to the highly hydrophilic nature of GO compared with the hydrophobic nature of graphite. However, in dry environments, hydrophobic gas molecules in the GO coating exhibit a similar effect as water in graphite, namely increasing the distance between basal planes. The frictional behavior of (PEI/GO)n coatings compares favorably with common solid lubricants such as CVD-grown hydrogenated amorphous diamond-like carbon (DLC) coatings in dry nitrogen (COF ~ 0.06)50 and hydrogen (COF ~ 0.01)51 environments, but with the advantage of a more generally applicable and scalable deposition technique. However, at present DLC is extremely durable (e.g. displaying wear coefficients of 10-7 to 10-10 mm3/Nm when tested in dry nitrogen or inert gas environments) which may be attributed to the highly dense nature of DLC, and improved adhesion. It is also important to note that the extreme disparity in hardness between DLC (~50 GPa)50 and graphene oxide (~ 0.3 GPa). Further work will concentrate on understanding and improving the durability of our system.


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4. Conclusions In conclusion, the coefficient of friction and wear-life of (PEI/GO)n thin films deposited on steel by dip-coating were investigated as a solid lubricant in air, vacuum, hydrogen and nitrogen. The coatings resulted in a reduction of the coefficient of friction in all cases, although the thinnest films (n = 5) had short wear life. The tribological performance of GO-based coatings in dry environments was far superior to the performance in humid environments. Superlubricity (COF < 0.04) was observed in dry environments for thicker films, and especially in nitrogen environment. This was attributed to a large reduction in the contact area due to the formation of characteristic carbon nanoparticles in dry conditions, which were not observed in humid conditions. Density functional theory suggested that in the presence of intercalated water, a strong hydrogen bonding network forms under pressure, preventing the separation of GO sheets to form the observed nanostructures. These results open up the field for durable, scalable, and almost frictionless graphene oxide solid lubricants for mechanical engineering applications, especially in dry environments.

Acknowledgments This work was supported by World Premier International Research Center Initiative (WPI), MEXT, Japan. This study was also supported by a Grant-in-Aid for Research Activity Start-up from the JSPS (Grant No. 26889045 and No. UFG5H06471). Takeshi Daio, Hironori Kouno, Nobuhiro Yanai and Nobuo Kimizuka are gratefully acknowledged for the assistance with TEM measurements.



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Supporting Information Supporting Information presents scheme of coating deposition, elaborated tribological conditions and tribometer details, film growth characterizations such as Quartz crystal microbalance (QCM), Surface SEM images and UV-Vis spectroscopy results. Water contact angle data, optical images of the wear track and tested ball and TEM images of wear debris in all environments were also presented.

Author Contributions P.S., R.S and S.M.L. conceived the study. R.S. designed the coating and performed characterization experiments (QCM, UV-Vis, WCA, SEM and TEM). P.S. conducted all tribological tests, RAMAN measurements and wear track imaging. D.D. and A.S. performed and interpreted DFT calculations. S.F. helped to design, characterize and interpret the coating properties in various atmospheres, H.T. assisted to perform friction tests. J.S. and S.M.L. guided the interpretation of results. P.S., R.S. and S.M.L. wrote the manuscript. All authors reviewed the manuscript and discussed the results before submission.

Competing Financial Interests Statement The authors declare no competing financial interests


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Figure 1. (a) Chemical structures of PEI and GO used in the layer-by-layer (LbL) assembly. b) Photographs of the (PEI/GO)n-coated steel discs used in the tribological tests. c) Cross-sectional SEM images of the (PEI/GO)n coatings deposited on silicon wafers.



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Figure 2. Coefficient of friction (COF) versus number of sliding cycles (N) for bare steel, and (PEI/GO)n-coated steel. Data was obtained at room temperature from ball-on-disk sliding tests with an 8 mm diameter steel ball in different gas environments: a) in air (H2O ca. 140 ppm); b) in vacuum; c-d) in dry hydrogen gas (< 2 ppm H2O); and e-f) in dry nitrogen gas (< 2 ppm H2O). The load was 0.5 N and the linear speed was 0.54 m/s.


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Figure 3. Images of wear debris collected after friction tests. Scanning electron microscopy (SEM) in: a) air (H2O ca. 140 ppm); b) vacuum; c) hydrogen gas (< 2 ppm H2O); and d) nitrogen gas (< 2 ppm H2O). e) Representative transmission electron microscopy (TEM) after testing in vacuum (similar microstructure was observed after testing in nitrogen gas, whilst no nanoparticles were observed after testing in air or H2, Supporting Information Figure S13).



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Figure 4. Wear track characterization. a) Photos of the wear tracks after 5000 cycles in different atmospheres. b) Intensity ratios (AD/AG) calculated from averaged Raman spectra maps of GO; freshly prepared (PEI/GO)15; and the wear tracks after friction test in different environments. The optical microscopy image shows a typical spectra acquisition region over the wear track.


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Figure 5. Density functional theory (DFT) simulations of pressurized graphene oxide intercalated with water, nitrogen, and hydrogen molecules. a) Schematic representation of the optimized unit cells. b) Potential energy surfaces of the different systems, showing strong hydrogen bonding in the presence of water.



TABLE OF CONTENTS

Layer-by-layer 300 nm

steel

+ (PEI/GO)n Coefficient of friction

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0.20

Air+140 ppm H2O

0.15

Vacuum

H2

0.10 0.05

N2

0.00 0

5k 10k 15k Number of sliding cycles

20k

20 nm


Macroscale Superlubricity of Multilayer ... - ACS Publications

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