Self-Dispersed Crumpled Graphene Balls in Oil for Friction and Wear Reduction

Self-dispersed crumpled graphene balls in oil for friction and wear reduction

Xuan Doua, Andrew R. Koltonowa, Xingliang Heb, Hee Dong Jangc, Qian Wangb,1, Yip-Wah Chunga,b,1, and Jiaxing Huanga,1

aDepartment of Materials Science and Engineering, Northwestern University, Evanston, IL 60208; bDepartment of Mechanical Engineering, Northwestern University, Evanston, IL 60208; and cRare Metals Research Center, Korea Institute of Geoscience & Mineral Resources, Daejeon, 305-350, South Korea

Edited by Alexis T. Bell, University of California, Berkeley, CA, and approved December 24, 2015 (received for review October 23, 2015) Ultrafine particles are often used as lubricant additives because balls could be well dispersed in oil and have superior lubrication they are capable of entering tribological contacts to reduce friction properties. Such miniaturized crumpled structures were first re- and protect surfaces from wear. They tend to be more stable than alized with graphene-based materials using an aerosol capillary molecular additives under high thermal and mechanical stresses compression approach (6). Just as how a crumpled paper ball is during rubbing. It is highly desirable for these particles to remain made by isotropically compressing a sheet of paper with one’s well dispersed in oil without relying on molecular ligands. Borrowing hands, flat graphene-based sheets suspended in nebulized aerosol from the analogy that pieces of paper that are crumpled do not droplets are isotropically compressed during solvent evaporation, readily stick to each other (unlike flat sheets), we expect that ultrafine leading to the final crumpled ball morphology. The resultant sub- particles resembling miniaturized crumpled paper balls should self- micrometer-sized crumpled graphene balls indeed have properties disperse in oil and could act like nanoscale ball bearings to reduce analogous to those of crumpled paper, including strain hardening friction and wear. Here we report the use of crumpled graphene balls and aggregation resistance. The morphology of crumpled graphene as a high-performance additive that can significantly improve the balls is highly stable in the solid state and when dispersed in liquids. lubrication properties of polyalphaolefin base oil. The tribological They do not unfold or collapse even after heating or pelletizing. performance of crumpled graphene balls is only weakly dependent Because they consistently do not form intimate contact with each on their concentration in oil and readily exceeds that of other carbon other, their interparticle van der Waals attraction is so weak that ENGINEERING additives such as graphite, reduced graphene oxide, and carbon black. they can be individually dispersed in nearly any solvent, including Notably, polyalphaolefin base oil with only 0.01–0.1 wt % of crumpled graphene balls outperforms a fully formulated commercial lubri- lubricant oils, without the need for any chemical functionalization cant in terms of friction and wear reduction. (8). Despite their compact appearance, crumpled graphene balls have a great deal of free volume and solvent-accessible surface area lubrication | tribology | aggregation-resistant particles | graphene inside, making them an effective absorber of oil, which could be released upon compression, ensuring uninterrupted wetting of the contact area. These properties should make them highly desirable ubricants reduce friction between contacting surfaces and for tribological applications. In this work, we demonstrate that Lthus increase the energy efficiency of engines and other machines. They can also reduce wear, thereby extending the life crumpled graphene ball is indeed a superior friction modifier of tribological components. Many types of ultrafine particles, compared with other carbon additives such as graphite powders, chemically exfoliated graphene sheets, and carbon black (9–16). here loosely defined as submicrometer-sized particles, have been – studied as lubricant additives (1, 2) because they can enter Remarkably, base oil with just 0.01 0.1 wt % of crumpled graphene contact regions between sliding surfaces and protect them from balls is more effective in friction and wear reduction than a fully direct mechanical contact (2–4). This makes ultrafine particles formulated commercial lubricant. effective for reducing friction and wear in the boundary regime, such as during startup or low-speed operation of an engine, when Significance the lubricant film at these contacts is too thin to prevent direct metal–metal contact (2–4). Further, because of the high shear Aggregation is a major problem for ultrafine particle additives stresses and sometimes high local temperatures at these contacts, in lubricant oil because it reduces the effective particle con- molecular additives can rub off, decompose, or simply fail to centrations, prevents particles from entering the contact area provide a sufficiently thick coverage for friction reduction and of working surfaces, and leads to unstable tribological per- wear protection (2–4). Therefore, ultrafine particles are appealing formance. Molecular ligands can help the particles to disperse, by virtue of their size and their chemical and thermal stability under but they tend to degrade under the harsh tribological condi- tribological conditions. However, it is challenging to disperse ul- tions. Therefore, self-dispersed particles without the need for trafine particles in lubricating oils. Typically, this requires surface surfactant are highly desirable. We report here, for the first functionalization with surfactant-like substances, which themselves time to our knowledge, such type of ultrafine particles made of are prone to degradation under tribological conditions, leading to crumpled, paper-ball–like graphene, which indeed can self-disperse unstable lubrication properties (2). Therefore, it would be highly in lubricant oil, and exhibit stable and superior tribological desirable if ultrafine particle additives could remain self-dispersed in performances. lubricant oil without the use of ligands. This work was inspired by the analogy with crumpled paper. A Author contributions: X.D., Q.W., Y.-W.C., and J.H. designed research; X.D. and X.H. performed research; X.D. and H.D.J. contributed new reagents/analytic tools; X.D., crumpled piece of paper in the shape of a ball has a rough sur- A.R.K., X.H., Q.W., Y.-W.C., and J.H. analyzed data; and X.D., A.R.K., Q.W., Y.-W.C., and face texture, which reduces the area of contact when placed on J.H. wrote the paper. top of a flat sheet of paper or in an assembly of crumpled paper The authors declare no conflict of interest. balls, resulting in minimal adhesion. Because of the multiple This article is a PNAS Direct Submission. folding, crumpled paper balls become strain-hardened (and thus 1To whom correspondence may be addressed. Email: [email protected], stiffer) under mechanical stress, so they can largely maintain their [email protected], or [email protected]. – shape and shape-induced nonstick properties (5 7). One might This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. expect, then, that ultrafine particles in the shape of crumpled paper 1073/pnas.1520994113/-/DCSupplemental.

www.pnas.org/cgi/doi/10.1073/pnas.1520994113 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 Results and Discussion to the Hamrock–Dowson equation(19), this condition can be Dispersion and Aggregation-Resistant Properties of Crumpled Graphene met by applying a 10-N load to the pin while the linear sliding Balls. The tribological performance of crumpled graphene balls was speed of the disk relative to the pin is 10 mm/s. The maximum investigated in comparison with three other widely studied carbon Hertzian contact pressure was about 1 GPa (20, 21). To test if additives: graphite platelets, reduced graphene oxide sheets (r-GO, the crumpled graphene balls can sustain this high pressure, static also known as chemically modified graphene), and carbon black. compression experiments were performed first by using the same Powders of these carbon materials (0.01–0.1 wt %) were sonicated in pin-on-disk configuration with a 10-N load. As shown in Fig. the lubricant base oil (polyalphaolefins type 4, PAO4) until they S2A, crumpled graphene balls drop-casted onto a polished steel were fully dispersed with no residual solids remaining. All four disk formed a uniform film. The steel ball left a nearly circular additives can initially disperse in the base oil right after sonication contact area of around 300 μm in diameter. The SEM overview (Fig. 1A). However, agglomeration was apparent in the dispersions image (Fig. S2B) of this area shows that some patches of crumpled of graphite platelets, r-GO sheets, and carbon black powders after graphene balls were removed with the ball, exposing the surface of a few hours. After 20 h, crumpled graphene balls were still dis- the steel disk. However, crumpled graphene balls remaining in the persed in the oil, but the other three carbon materials were fully contact area did not appear flattened or severely deformed (Fig. S2 sedimented (Fig. 1B). The microstructures of the four carbon ad- C–E). The resistance of crumpled graphene ball to compression is ditives were observed with the scanning electron microscope attributed to its strain-hardening property due to the multiple folds (SEM). The sonicated graphite platelets are typically around 1–3 created within the ball during its formation. Upon further com- μm in lateral dimension and 40–60 nm in thickness (Fig. 1C). Al- pression, more folds can be generated, leading to increased stiffness. though they disperse initially, they are prone to aggregation due to Results shown in Fig. S2 suggest that crumpled graphene balls can their flat, disk-like shape, which can form intimate interparticle survive the high pressure while remaining their crumpled ball shape. contact and generate strong attraction. Similarly, the r-GO sheets also tend to restack to form large chunks a few hours after soni- Self-Dispersed Crumpled Graphene Balls as Friction Modifiers. Friction cation (Fig. 1D). The primary particles in carbon black powders are testing results are shown in Fig. 2 C–F. Steel surfaces lubricated with about 50 nm in diameter. They aggregate into micrometer-sized the base oil display a typical type of run-in behavior. Initially, the clusters, which can be broken down to submicrometer pieces by friction coefficient increases due to the wear of asperities at the sonication (Fig. 1E). Carbon black powders can stay dispersed for contact surfaces. Eventually, development of wear tracks increases about 5–10 h. The dispersion of crumpled graphene balls, which the contact area and reduces the contact pressure, hence lowering are around 500 nm in diameter, is most stable, because their shape the friction coefficient. The friction curve for r-GO is similar to that prevents them from forming tight stacking, hence preventing ag- of the base oil. Among all of the carbon-based additives, crumpled gregation (Fig. 1F). After a few days, all of the dispersed carbon graphene ball gives the lowest friction coefficient: only 0.01 wt % of additives shown in Fig. 1 A and B would sediment in the oil. Upon crumpled graphene is needed to reduce the friction coefficient by shaking, sedimented graphite platelets, r-GO sheets, and carbon 20% compared with the base oil. In practice, a good additive should black powders can temporarily suspend in oil. Optical microscopy maintain consistent performance over a range of concentrations so observation (Fig. S1) reveals that these three suspensions contain that local concentration fluctuations and/or material loss do not large, persistent micrometer-sized aggregates.Therefore,theywill disrupt the functionality of the additive. Therefore, tests were also precipitate again quickly. In contrast, crumpled graphene balls can conducted at higher loading, 0.1 wt %. This high concentration (Fig. remain finely dispersed. 2D) results in no significant improvement in friction performance A pin-on-disk tribometer (Fig. 2 A and B) was used to study for r-GO, whereas graphite and carbon black display poorer per- the tribological properties of the carbon-based additives in formance, probably due to easier aggregation at higher concentra- PAO4 base oils (16, 17). The pin and the disk were made of M50 tion. Such larger aggregates are likely to be poorly dispersed and steel ball (Ø 9.53 mm, surface roughness Ra ∼17 nm) and cannot protect contacting surfaces. In fact, interaction among these E52100 steel (Ø 30 cm, Ra ∼5 nm), respectively. To keep the larger aggregates could induce jamming during the friction test, tribological test in the boundary lubrication regime, testing pa- leading to increased friction coefficient (22). By contrast, crumpled rameters were chosen to ensure that the thickness of the lubri- graphene displays consistent friction behavior at this higher con- cant film was smaller than the surface roughness (18). According centration. Its self-dispersion and aggregation-resistant properties

Fig. 1. Dispersion properties of four carbon additives in the lubricating oil. Dispersion of four carbon additives, all at 0.1 wt %, in PAO4 (A) immediately after sonication and (B) 20 h after sonication; (C–F) SEM images of drop-casted powders of graphite, r-GO, carbon black, and crumpled graphene balls, respectively. The crumpled graphene balls stay dispersed due to their aggregation-resistant properties. The solid content for all of the dispersions is 0.1 wt %.

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Fig. 2. Coefficient of friction using PAO4 base oil with and without carbon additives. (A) Schematic illustration and (B) photo showing the pin-on-disk geometry of the tribometer; C and D show the variation of coefficient of friction as a function of time using PAO4 base oil and with 0.01 wt % and 0.1 wt % carbon-based additives, respectively. The corresponding bar charts in E and F show the friction values averaged over the entire duration of the test. For both concentrations, crumpled graphene balls are found to be the most effective carbon additive for friction reduction.

are primarily responsible for this consistently low friction over this sheets, which did not reduce the coefficient of friction greatly range of concentration. (Fig. 2), also failed to provide effective wear reduction; it showed the After the friction tests, the wear surfaces were imaged by same apparent decrease in wear coefficient as the base oil when SEM. Some carbon-based particles were left on the wear track. tested at a higher concentration for a longer time. Similar to what Severe aggregation was observed for all carbon-based additives was observed for friction coefficient, the wear reduction performance (Fig. 3 A–C) except crumpled graphene balls (Fig. 3D). Intact crumpled graphene balls can be seen filling the grooves of the wear tracks and are separated from each other clearly. Evi- dently, the strain-hardening property of crumpled nano- structure (6) prevents excessive deformation and damage during friction.

Wear Reduction by Self-Dispersed Crumpled Graphene Balls. In addition to the substantial friction reduction, noteworthy improvements in wear reduction were also observed in our experiments. In Fig. 3, SEM images of the wear tracks already suggest that oil modified with crumpled graphene balls is qualitatively more effective in reducing wear than the other three carbon additives. Quan- titative examination of the wear tracks was conducted with white light interferometer, which generated a 3D map of the surface. Wear coefficients for oils modified with additives at two different concentrations are shown in Fig. 4 A and B. For both 0.01 wt % and 0.1 wt %, crumpled graphene ball is significantly more effective than other carbon additives for wear reduction. The apparent wear coefficient difference in pure base oil between the –– Fig. 3. SEM images of remaining carbon additives in the wear tracks after two tests arises from the different testing times because the most tribological tests. (A) Graphite, (B) r-GO, (C) carbon black, and (D) crumpled significant wear tends to occur at the beginning of the test, the wear graphene balls. Only crumpled graphene balls remain aggregation-free. coefficient calculated from the longer test should be lower. r-GO (Scale bar in the Inset, 200 nm.)

Dou et al. PNAS Early Edition | 3of6 Downloaded by guest on October 2, 2021 Fig. 4. Wear coefficients of PAO4 base oil with and without carbon additives. The bar charts in A and B compare the wear coefficients of the base oil itself and samples with 0.01 wt % and 0.1 wt % carbon additives, respectively. The corresponding 3D profile images of the wear tracks are shown in C and D.Itis evident that crumpled graphene balls can better protect the steel surface from wear.

of graphite platelets and carbon black additives also degrades at coefficients of friction. 5W30 has organic molecular friction higher concentrations. Meanwhile, varying the concentration has modifiers which bind to the metal surface and decrease adhe- comparatively little effect on the wear coefficient for the lubricant sion, making it effective for friction reduction. However, crum- with crumpled graphene balls. A detailed analysis of the wear tracks pled graphene balls are more effective in wear reduction. The in Fig. 4D is presented in Fig. S3. Compared with the other three difference is revealed by the wear track profiles shown in Fig. 5 C additives, crumpled graphene balls generated shallower (0–0.5 μm), and D. The surface lubricated by 5W30 still yielded deep and wide narrower (0–4 μm), and more uniform wear tracks. In contrast, wear wear tracks at tens of μm scale. However, crumpled graphene balls tracks generated by other carbon additives are deeper (1–3.5 μm) can provide better protection of the surfaces, leaving a much and wider (12–20 μm), with a broader size distribution. It is signifi- smoother wear track, as shown by the profile (Fig. 5D). cant that the use of crumpled graphene ball prevents the formation ofweartrackslargerordeeperthan10μm, because such wear tracks Conclusions tend to generate large debris that can inflict severe abrasive wear In summary, addition of crumpled graphene balls to PAO4 base (23–25). Compared with the use of the base oil, the self-dispersed oil results in superior friction and wear performance, due largely crumpled graphene additives are able to eliminate wear by ∼85% to their aggregation-resistant property. This unique property (red bars in Fig. 4). makes them more stable in the base oil than other carbon-based materials, such as graphite, r-GO, and carbon black. Aggregation Benchmarking Against Fully Formulated Commercial Lubricant. The makes other carbon-based materials lose their ability to prevent base oil modified with 0.1 wt % crumpled graphene balls was the contact of two surfaces, negatively impacting their tribological also tested for comparison with a PAO-based commercial lu- performance. In contrast to other carbon additives, whose tri- bricant 5W30 (Fig. 5A). Both 5W30 and crumpled-graphene- bological properties are sensitive to their concentration, crumpled ball–modified PAO4 outperform the base oil, with comparable graphene balls deliver consistently good performance between

4of6 | www.pnas.org/cgi/doi/10.1073/pnas.1520994113 Dou et al. Downloaded by guest on October 2, 2021 Fig. 5. Comparison of lubrication between crumpled graphene balls in PAO4 base oil and commercial lubricant 5W30. At 0.1 wt % of loading level, the PAO4 base oil modified by crumpled graphene balls outperforms the fully formulated lubricant 5W30 (additives up to 10 wt %) as shown in the comparison of (A) coefficient of friction, (B) wear coefficient, and (C and D) 3D profile images of the wear tracks.

0.01 wt % and 0.1 wt % concentration. It was found that crumpled 20 min. Before testing, the polished 52100 steel disks and steel ball were son- graphene balls reduce friction coefficient and wear coefficient by icated in acetone for 5 min to remove any possible residual contaminants. about 20% and 85%, respectively, with respect to the base oil. The metal disk was then fixed tightly in the holder of the tribotester, and ENGINEERING Furthermore, base oil modified with crumpled graphene balls plastic pipettes were used to transfer 3 mL of freshly mixed lubricant solution onto the disk. The tests were conducted at a linear speed of 10 mm/s, a alone outperforms a fully formulated 5W30 lubricant in terms of constant vertical force of 10 N (maximum Hertzian contact pressure ∼1 GPa), friction and wear reduction. The combination of aggregation and ambient temperature and humidity. The experimental duration was resistance, self-dispersion, and mechanical properties of crumpled 2,000 s and 4,000 s, respectively, for the 0.01 wt % and 0.1 wt % concen- graphene particles makes them an attractive material for tribological tration of each carbon-based additive. Each sample was tested at least twice applications. under identical conditions.

Materials and Methods Characterization of Wear Tracks. Before each SEM observation, the metal disk Preparation of Tested Materials. Graphite was purchased from Sigma-Aldrich. was cleaned in hexane for 3 min to remove the residual lubricant oil, and was Carbon black was purchased from VWR. Lubricant PAO4 base oil was pur- then air-dried. SEM images were recorded using an LEO 1525 microscope. chased from Exxon-Mobil. GO was made by a modified Hummers method (26) Before optical profilometry, the steel disk was further sonicated in acetone to described previously (5, 27). Crumpled graphene balls were prepared by completely remove all of the debris and lubricant materials. A Zygo NewView capillary compression in rapidly evaporating aerosol droplets of GO sheets as 7300 optical surface profiler was used to identify and analyze the 3D to- reported previously (6). An ultrasonic atomizer (1.7 MHz, UN-511 Alfesa pography of the wear track. The wear volume was defined as the amount of Pharm Co.) was used to generate aerosol droplets of aqueous GO solution at metal removed from a single track in the course of an experiment, and was a concentration of 1.5 mg/mL. Nitrogen flow was used to carry those estimated by numerically integrating the surface height (from optical pro- droplets through a 400 °C tube furnace. Particles were collected at the end filometry) over the area at eight different points along the track. Wear of the tube furnace using a Millipore Teflon filter with 200-nm pore size (6). coefficient is given by the following equation: Those partially reduced crumpled GO particles were further reduced at À Á Wear volume m3 × Surface hardnessðPaÞ 700 °C in argon for 1 h. r-GO was synthesized by hydrazine reduction of Wear coefficentðKÞ = . GO in water and collected by filtration based on a previous report (28). Normal loadðNÞ × Sliding distanceðmÞ

Vickers hardness measurements of steel disks were determined to be 575 ± Tribology Tests. A pin-on-disk tribometer was used to study the tribological 10.4 kgf/mm2 (∼5.64 GPa) by a Struers Duramin microhardness tester. The properties. The steel disks for friction tests were machined from an E52100 measurements were repeated three times for each disk. steel bar, and the disk surfaces were machine-polished to a mirror finish with surface roughness Ra of around 5 nm measured by an interferometer. The steel balls, 3/8“ in diameter and made of M50 steel, were purchased from ACKNOWLEDGMENTS. We thank Drs. F. Guan, J. Luo, and K. Raidongia for their earlier exploration related to this work. The work was supported by the McMaster-Carr and used as received. Lubricant additives (graphite, carbon Office of Naval Research (ONRN000141310556). J.H. thanks the John Simon black, and crumpled graphene balls) were added to the PAO4 base oil Guggenheim Memorial Foundation for a Guggenheim Fellowship. H.D.J. = (density 0.82 g/mL) and sonicated for 30 min in a water-bath ultrasonic was supported by the Basic Research Project of the Korea Institute of Geo- cleaner UC-32D, 125 W. Due to its poor dispersibility, the filtered r-GO was science and Mineral Resources, funded by the Ministry of Science, ICT and tip-sonicated (150 W) for 10 min before sonicating in a water bath for Future Planning.

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