Molecular-scale grinding of uniform small-size graphene flakes for use as lubricating oil additives

GUO Yu-fen, ZHANG Hui-tao, LIU Yue-wen, ZHOU Xu-feng*, LIU Zhao-ping,*

（1. Advanced Li-ion Battery Engineering Laboratory of Zhejiang Province, Key Laboratory of Graphene Technologies and Applications of Zhejiang Province and CAS Engineering Laboratory for Graphene, Ningbo Institute of Materials Technology & Engineering,Chinese Academy of Sciences (CAS), Ningbo 315201, China;2. National Graphene Innovation Center, Ningbo 315201, China）

Abstract： A variety of industrial preparation methods to obtain graphene from graphite have been developed, the most prominent of which are the chemical reduction of graphene oxide and intercalation-exfoliation methods. However, the low-cost, thin-layer,large-scale production of graphene with a radial dimension smaller than 1 μm (SG) remains a great challenge, which has limited the industrial development and application of small-scale graphene in areas such as textile fibers, engine oil additives, and graphenepolymer composites. We have developed a novel way to solve this problem by improved ball milling methods which form molecularscale grinding aids between the graphite layers. This method can produce uniform, small-size (less than 1 μm) and thin-layer graphene nanosheets at a low cost, while ensuring minimal damage to the internal graphene structure. We also show that using this SG as an additive in lubricating oil not only solves the current dispersion stability of graphene, but also reduces the friction coefficient by more than 27% and wear by more than 38.8%. The SG preparation method reported is simple, low-cost, and has a significant effect in lubricating applications, which is of great commercial value.

Key words: Molecular-scale grinding；Ball milling；Small-size graphene；Lubricating oil additives；Stable dispersion

1 Introduction

Graphene boasts fascinating physical and chemical properties, such as zero-gap semiconductor[1],high carrier concentration and mobility[2], quantum Hall effect[3], spin-dependent transport at room temperature[4], large theoretical specific surface area[5],transparency and conductivity (light transmittance of single-layer graphene reaches 97.7%)[6], high thermal conductivity[7]and excellent mechanical strength[8].Thus, it has attracted widespread attention from the scientific research community and industry. After years of research, scientists have developed a variety of graphene preparation methods[9-14], including chemical vapor deposition (CVD), chemical reduction of graphene oxide, intercalation-exfoliation method,mechanical exfoliation method, liquid phase exfoliation method, etc. Graphene prepared by different methods may display various characteristics that in turn dictates its field of application. For example,graphene prepared by CVD method[9]is suitable for photoelectric devices or for probing its fundamental physical properties, but is relatively expensive to be translated into industrial application. Graphene prepared by chemical redox method[10-12]alit convenient to formulate into composites with polymers, usually shows low conductivity and is not suitable for industrial application due to relatively high cost and environmental pollution problems. Low-defect thin-layer graphene can be obtained by intercalation-exfoliation method[13]the size of graphene prepared in this way is relatively large because the intercalation process is usually a dynamic process where intercalation and deintercalation occur at the same time. So when the size of graphite is small, it is more inclined to deintercalation, and it is difficult to form stable graphite intercalation compunds. Ball milling usually struggles in achieving thin-layer graphene[14-15]nerally produces relatively broken graphite microflakes. Although a smaller radial size can be obtained, the thickness of the obtained “graphene” is relatively thick,and some even have dozens of layers, which makes it difficult to manifest the properties of graphene. The liquid phase exfoliation method[16,17]usually uses ultrasonic exfoliation of graphite to prepare graphene,which consumes large amount of energy and low yield and has no commercial value at present. The high pressure microfluidization method can get small-sized thin-layer graphene[18-20]but has a extremely high requirements for the equipment. In addition, the operating pressure of the equipment is usually higher than 60 MPa, and the production capacity is relatively low.

Nanomaterials have attracted much attention in the field of lubricants because of their large specific surface area and small size[21-24]. The stable chemical properties and excellent self-lubricating property of graphene make it suitable as an efficient and green lubricating oil agent[25-28]. However, so far it remains a challenge to solve the dispersion of graphene in lubricating oil. Since graphene settles in lubricating oil, it is difficult to protect the friction pair from damage in time as the friction progresses. In addition, graphene is more likely to agglomerate during the friction process, which eventually nullifies its protective effect[29-32]. It’s well known that lubricating oil, especially vehicle lubricating oil, contains complex components generally including base oil, detergent-dispersants, clean, antiwear agent, antioxidant, extreme pressure antiwear agent, oily agent, tackifier, pour point depressant, antifoam additives, etc. Each additive is added in a certain proportion, and when a certain additive is added too much, it will destroy other properties of the oil. Therefore, as an anti-wear agent additive, the lower the quantity of graphene that is added in the finished engine oil, the lesser impact it will have on other properties of the original engine oil[33].

Here, a method for preparing small-sized graphene byin-situsynthetic molecular-scale grinding aid-assisted ball milling at the edge of the graphite layer is reported. The graphene prepared by this method not only is of fewer layers, but small radial size of less than 1 μm. In addition, the method is simple, inexpensive and easily applicable for industrial application. Applying the graphene prepared by this method to lubricating oil additives not only solves the problem of dispersion stability of graphene in the lubricating oil system, but also reduces the friction coefficient by more than 27%, and reduces wear by more than 38.8%, which is of a very large commercial significance.

2 Experimental method

2.1 Preparation for small-sized graphene flakes

40 g of small-diameter graphite powder was added to 1.5 L of NaOH aqueous solution with a concentration of 1.8 mol/L by mechanically stirring until no dry powder was left, and then the mixture was subjected to ball milling for 30 min at a speed of 1 000 r/min. 1 L of CaCl2aqueous solution with a concentration of 1.1 mol/L was added slowly to the graphitealkali mixed solution while the ball mill remained in operation, and ball milling continued for another 3 h.After ball milling, the slurry was neutralized by adding dilute HCl solution under mechanical stirring until the pH of the solution reached ～7, then the slurry was washed with deionized water to remove the salt ions adsorbed by graphene, and finally freezedried to obtain SG powder. However, in view of the low efficiency of freeze-drying, small-sized graphene was also collected by centrifugation, so as to obtain SG mud with a solid content of about 60%, which was used as a raw material for further exploration.

2.2 Preparation of graphene lubricating oil additive

10 g of SG mud was added to 100 mL of 98%H2SO4by stirring. Then 20 mL of 50% H2O2solution was added, and the mixture was stirred for another 1 h before the reaction was quenched by ice water. The solid product was washed with deionized water through repeated centrifugation until it was neutral,and acidified graphene (ASG) mud with a solid content of 40% was finally obtained by high-speed centrifugation. 5 g of ASG mud was added into 200 g of monoalkenyl succinimide (T151), and the mixture was stirred and heated to 200 °C, and kept warm for 3 h until the water contained in it was completely removed. Then a graphene lubricating oil additive with a graphene content of 1% was obtained.

2.3 Test of graphene lubricating oil

Commercial basic oil PAO-10 and fully-synthetic engine oil was used as reference oils, in which 1%of graphene lubricating oil additives were added. A four-ball machine was used for friction and wear tests.The addition ratios of additives to reference oil are 1∶2 000, 1∶1 000, 1∶200, 1∶100 and 1∶10, corresponding to the addition amounts of graphene in the whole lubricating oil being 5×10-6, 1.0×10-5,5.0×10-5, 1.0×10-4and 1‰. The testing equipment is the MS-10A four-ball machine produced by Xiamen Tianji Automation Co., Ltd. The test method adopts the Petrochemical Industry Standard of the People’s Republic of China-Determination of Lubricating Oil Anti-wear Performance (Four-Ball Machine Method)SH/T 0189-92B. Test conditions: temperature of 75 °C, pressure of 392 N, test time of 60 min, friction speed of 1 200 r/min. The equipment software can directly read the change of the friction coefficient over time. After the test, the three small balls at the bottom were cleaned, the size of the wear spots on them was tested to compare the wear.

3 Results and discussion

Fig. 2 (a) SEM image of raw graphite flakes; (b, c) SEM images of graphene powder at different magnifications; (d) SEM image of dispersed graphene

The raw graphite powders had an average size of about 20 μm. The minimum size of ZrO2ceramic beads used in ball milling was 100 μm, which was far larger than the size of graphite flake, as shown in Fig. 1. Direct ball milling with ZrO2beads results in large graphite sheets being smashed. With the rotation of ZrO2beads, some shear forces can peel off graphite sheets, showing the state of smaller and thinner layers of graphite, but it is difficult to achieve graphene. As shown in Fig. 1, the edge of graphite is slightly curled in hot concentrated alkali solution.When graphite flakes and NaOH lye are milled together in the ball mill, the edge of graphite begin to curl with the increase of lye temperature, and a large amount of OH-is distributed around the edges. With the effect of ball milling shear force, OH-is brought into contact with the graphite interlayer. When CaCl2solution is added slowly, Ca2+in the solution quickly combines with OH-to form slightly soluble Ca(OH)2.The molecular size of Ca(OH)2is about 0.38 nm,which is close to the spacing between graphite layers.With the precipitation and aggregation of Ca(OH)2at the edge of graphite layer, graphite sheets ( Fig. 2a)are gradually peeled off under the shear force of ZrO2balls, thus obtaining thin SG (Fig. 2b-c). The Ca(OH)2selected here does not precipitate into its crystal form and has low hardness, therefore, does not damage the ZrO2grinding balls. In contrast, if CaCO3, which crystalizes easily, is selected as grinding aid, although its hardness is lower than that of ZrO2ceramic beads, it still wears ZrO2balls gradually under the condition of high-speed ball milling. This is detrimental for our purpose as ZrO2fragments have strong chemical stability and are difficult to remove (Fig. 3d).

From the SEM results in Fig. 2a, it can be seen that the size of the original graphite flakes is less than 20 μm, which is formed by stacking some graphite sheets with smaller sizes and has a relatively regular flake shape. Fig. 2b-c show the as-prepared SG under different magnifications. Different from the smooth and thick edges of raw materials, the SG sheets were found to be soft, curled and thin in a large field of vision, which shows that the preparation method of small-size graphene proposed in this paper can effectively obtain uniform thin-layer graphene. When the SG is fully dispersed, it can be seen that the expanded size of graphene is about 1-2 μm (Fig. 2d). Since some of them are multi-sheet superimposed, it can be speculated that the size of graphene in aqueous solution may be less than 1 μm.

Fig. 3 SEM images of graphene prepared with different methods.(a) Graphene prepared without adding any grinding aid; (b) Graphene prepared in sodium hydroxide alkali solution; (c) Graphene prepared using calcium hydroxide as grinding aid; (d) Graphene prepared using synthetic calcium carbonate as grinding aids

We also compared the morphology of graphene products prepared with different ball milling conditions. The first is to directly ball-mill the mixture of graphite raw materials and water without adding any grinding aids. Fig. 3a shows the SEM image of the sample after ball-milling for 4 h. It can be seen that the graphite sheet remains almost entirely unpeeled,and the as-prepared graphene sheet is still very thick,which cannot even be called graphene, but a slightly thin layer of graphite. Fig. 3b shows the results after milling graphite powder with NaOH alkali solution for 4 h. The graphene sheets are obviously thinner than those prepared without adding NaOH alkali solution, and the edge of graphite sheet is partially curled,which is more conducive to the complete exfoliation of graphene sheet in the later stage. Considering the strong adsorption of chloride ions, it is difficult to remove them completely. Therefore, when other conditions remain unchanged, a calcium acetate solution instead of calcium chloride is applied to prepare smallsized graphene, as shown in Fig. 3c. When Na2CO3solution replaces NaOH alkaline solution, CaCO3is formed as grinding aid, and some impurity nanoparticle can be found on the surface of SG (Fig. 3d).Due to the high hardness of CaCO3, the zirconium beads are worn to a certain extent. Because of the strong chemical stability of ZrO2, it is difficult to remove fragments coming off from ZrO2beads, which brings contaminants to graphene samples.

Malvern Nanoparticles and Zata Potentiometric Analyzer was used to measure the size of graphene products. We used wet method for testing, and ethanol was selected as dispersing solvent. Graphene mud was dispersed in ethanol solvent with a graphene content of about 1 ‰, and ultrasonically treated for 5 min to fully break up the agglomerated graphene before particle size measurement. As shown in Fig. 4, the particle size of graphene was basically distributed between 0.6 and 1 μm, which accords with the above SEM analysis results. It proves that the preparation method proposed in this work can effectively prepare thin-layer graphene with small size.

Fig. 4 Particle size distribution of SG

Fig. 5 (a) XPS spectra and (b) Raman spectra of the raw graphite and SG

In Fig. 5a, the C1s high-resolution XPS spectrum is presented. From the spectrum, the raw graphite shows a similar peak shape with SG. Also for SG,the peak at 284.5 eV corresponding to the C in C—C bond is large while the peaks at 286.1 eV and 287.5 eV that correspond to C in C—O bond and C＝O bond are very small. All informations from the XPS spectrum indicate that the as-prepared SG is of high quality and almost unoxidized. In Fig. 5b, the Raman spectrum of graphite shows a smallDpeak at 1 350 cm-1attributed to the edge and the randomly distributed initial defects. Two of the other large peaks are theGpeak at 1 580 cm-1corresponding toE2g-symmetry phonons at the Brillouim zone center of sp2-bonded carbon atoms and the 2Dpeak at 2600-2800 cm-1originating from a second-order two-phonon process. For pristine graphite, the ratio of theDband toGpeak intensity (ID/IG) is 0.06. Compared with raw graphite, SG shows a relatively obviousDpeak withID/IG=0.43, indicating that there are large defects in the sample, which is mainly due to the small size and abundant edges of the obtained graphene.

Due to its small size and unstable edge defect structure, SG is acidified through chemical treatment in the mixture of concentrated sulfuric acid and hydrogen peroxide, making its edge form C—O—HO3S covalent bond[34]. Because T151 is alkaline, it can be grafted on the edge of SG during high temperature stirring, which imparts strong dispersion capability in base oil (Fig. 6). The lubricating oil additive is uniform as a whole with a graphene content of 1%, and graphene showed no precipitating phenomenon(Fig. 7).

To check the dispersion stability of graphene lubricating oil additives in basic white oil, a simple test was did. The dispersion was prepared by adding the lubricating oil additives into the basic white oil and stirred evenly with a glass rod, which was then stood for 1 month to observe the dispersion stability of the lubricating oil. From Fig. 8, we can see that the smallsized graphene lubricating oil additives in the base oil are all in a very stable state. In Fig. 8, the pictures from left to right correspond to the lubricating oil additive (graphene content of 1%), dispersions with lubricating oil additive and PAO ratio of 1∶10(graphene content of 1‰), 1∶100 (graphene content of 1.0×10-4), 1∶1 000 (graphene content of 1.0×10-5), and base white oil, respectively. No precipitation was formed in each dispersion solution, indicating the stable dispersion of the as-prepared smallsized graphene lubricating oil additive in the basic oil,which fully meet the needs of practical applications.

Fig. 6 Schematic diagram of SG grafting T151

Fig. 7 Optical photos of SG lubricating oil additive

Fig. 8 Optical photo of SG lubricating oil additive dispersed in base oil(PAO) after standing for 30 days. The graphene content from left to right is 1%, 1 ‰, 1.0×10-4, 1.0×10-5, and 0, respectively

The dispersion stability of graphene lubricating oil additives in PAO was characterized by a multiple light scattering instrument. In order to improve the accuracy of the test, a PAO dispersion with a graphene content of 1.0×10-4was used for the test with the duration of 12 h under the room temperature. The main measurement probe of the multiple light scattering instrument collects the data of transmitted light and scattered light, which scans once every 20 μm on a length of 55 mm, and the scanning results over time are plotted as a curve. It can be seen from Fig. 9 that the ΔTSI value is less than 0.01, which further confirms the stable dispersion of small-sized graphene in lubricating oil.

Frictional performance tests of graphene lubricating oil are conducted on a four-ball machine with the graphene lubricating oil additives added to the basic white oil and synthetic engine oil.

Fig. 9 The dispersion stability of graphene

Fig. 10 The test curve of friction coefficient and the photos of wear scar test of basic white oil that containing graphene lubricating oil additive. The pictures below from left to right are wear scars of basic white oil, and white oil with 1.0×10-5 graphene and 5.0×10-5 graphene, respectively

In this experiment，adding graphene into the basic white oil PAO can effectively reduce wear and friction coefficient. As shown in Fig. 10, with the adding of graphene, the friction coefficient of PAO decreases, and the lubricity gets better. It can be seen that the diameter of the wear scar of the basic white oil is 0.738 mm. When the content of graphene is 1.0×10-5, the wear spot diameter is 0.698 mm, and when the content of graphene increases to 5.0×10-5,the wear spot diameter further reduces to 0.644 mm. It should be noted that only graphene was used as the anti-wear agent in the basic white oil PAO, and no other natural anti-wear agents, detergents, dispersants,anti-friction agents, oily agents and etc. were added,which can directly reflect the anti-wear and loss reduction effect of graphene.

Tests in commercial lubricating oils are necessary to evaluate the potential of the graphene lubricating oil additives for practical applications. The composition of commercial lubricating oil is more complex, so the testing result of graphene additive is commercially meaningful. Therefore, the graphene lubricating oil additive is added to a certain brand of fully synthetic engine oil to test its friction and wear performances. It can be seen from Fig. 11 that the addition of graphene can effectively reduce the friction coefficient and wear of the commercial engine oil.Since there are many types of additives in commercial engine oil, the protection of friction pairs is better and the wear of steel balls is lower. The calculated friction coefficient of pure engine oil is 0.095 μ, and the wear scar diameter is 0.362 mm. When the graphene filling content is 5.0×10-5, the friction coefficient of engine oil is 0.077 μ, and the wear spot diameter is 0.307 mm, which corresponds to a friction coefficient reduction of 19%, and the wear area reduction of 28%. When the graphene filling amount is 1.0×10-5, the calculated oil friction coefficient is 0.069 μ, the wear spot diameter is 0.283 mm, corresponding to a friction coefficient reduction of 27%, and a wear area reduction of 38.8%. The above analysis proves that significant anti-wear and loss performance can be obtained by adding only a small amount of graphene additive. These observations can be explained as follows: (1) The small-sized graphene plates are more likely to be adsorbed on the surface of the friction pairs, which can effectively reduce the direct contact of the friction pairs, thereby reducing friction coefficient. (2) The grafted graphene can promote the formation of the oil film between the friction pairs and slow down the rupture of the oil slick,thus better protecting the friction pairs. At the same time, due to the small size of the as-prepared graphene, it is easier to form a stable graphene dispersion in lubricating oil, raising their commercial application value in lubricating oil.

Fig. 11 The test curve of friction coefficient and the photos of wear scar test of synthetic oil that containing graphene lubricating oil additive. The figure below from left to right are the wear scars that with no graphene filler, and the graphene filler amount of 5.0×10-5, 1.0×10-5, respectively

4 Conclusion

This paper introduces a method for preparing small-sized graphene with low cost at a large scale by employing ball milling to generate in situ molecularscale grinding aids between graphite sheets; achieving highly efficient exfoliation of graphite sheets. This method can produce uniform graphene with fewer layers. The as-prepared graphene has a lateral size less than 1 μm and also has a lower content of oxygencontaining functional groups. Because of its abundant edge defects, small-sized graphene can be acidified and then easily grafted onto T151 to realize the stable dispersion of graphene in lubricating oil. In this research, when the added amount of graphene is only 1.0×10-5, it can reduce the friction coefficient and the wear of commercial engine oil effectively. The corresponding friction coefficient can be reduced by 27%,and the wear area can be reduced by 38.8%, which endows the small size graphene with a very large commercial application value.

Data availability statement

The data that support the findings of this study are openly available in Science Data Bank at https://cstr.cn/31253.11.sciencedb.09429 or https://doi.org/10.57760/sciencedb.09429.

Acknowledgments

This work was supported by the Postdoctoral Research Program of Zhejiang (ZJ2021004).