An estimated 30%-50% of all energy consumption is used to overcome frictional forces as a result of interacting surfaces. Lubrication of surfaces therefore plays a critical role in reducing waste and increasing energy efficiency by decreasing production and manufacturing costs. The use of lubricants at the contact surface in mechanical processes has long been considered a key strategy in minimizing material and energy dissipation arising from frictional processes. However,equally important in maintaining mechanical durability with increased efficiency is selecting ‘‘well-suited’’ lubricant materials for a particular system.

One of the most intriguing materials for tribology is graphite, which has been widely used as a solid lubricant in mechanical devices. Its lamellar structure and weak bonding between atomic layers facilitate shear between adjacent layers,which is at the origin of the low-friction characteristics. As the building block of the most common macroscopic graphite solid lubricant,graphene, besides its excellent electronic and mechanical properties,is also of interest for its lubricating property which can be used in micro/ nanoelectromechanical systems (MEMS/NEM S) in the future ^{[1, 2, 3]}.

Recently,functionalized graphene-IL nanocomposite films with excellent lubrication properties have been proposed for use in nanomechanical applications ^{[4, 5]}. The tribological properties of single layer graphene oxide (GO) sheets as additives in water-based lubricants showed that adding GO particles into water significantly improved lubrication and presented a very low friction coefficient of approximately 0.05 with no noticeable surface wear after 60,000 cycles of friction testing ^[6]. However, studies on the lubricating properties of graphene-based semisolid grease have never been found.

In the present work,a graphene-based semi-solid grease has been prepared by the highly dispersed mixing method. Moreover, the friction performance of the graphene-based semi-solid grease was studied. 2. Experimental

Graphene oxide (GO) in this study was fabricated from graphite powder by a modified Hummers method. In a typical synthesis procedure,the mixture of 2 g graphite powder and 1 g NaNO₂ was added to 48 mL H₂SO₄(98%) (in an ice bath),and then 6 g KMnO₄ was added gradually. The mixture was stirred continually for 4 h under an ice bath to keep the temperature at approximately 22 ℃, and then 92 mL deionized water was added to dilute the mixture; it was maintained at that temperature for 30 min. After that,a 30% H₂O₂ solution was added into the solution slowly while stirring until the suspension turned brilliant brown,indicating full oxidization of graphite. The as-obtained graphite oxide slurry was dispersed in deionized water and exfoliated into the GOvia sonication using an ultrasonic generator (28 kHz,600 W) for 4 h under stirring. The suspension turned black again due to the formation of GO sheets during ultrasonic treatment. Finally,the mixture was collected using a centrifuge,washed four times with 5% HCl,followed with deionized water repeatedly until the pH 7, and finally dried in a vacuum oven at 120 ℃ for 2 days. Graphene was prepared by hydrazine-reduced GO according to the procedure described in Ruoff’s work ^{[4, 5]}.

The graphene-based semi-solid grease with high dispersion was prepared in four steps. First,1 g prepared graphene prepared as described above was heavily dispersed in 30 mL benzenevia sonication using an ultrasonic generator (28 kHz,600 W) for 30 min under stirring. Then,100 g semi-solid grease (ordinary commercial lithium based grease) was melted by heating at 160 ℃ until its use in the third step. Next,the graphene dispersed solution described above was added dropwise into the melted grease under heavy stirring and held for 30 min. Finally,the obtained mixture was cooled down till room temperature. Likewise,the graphitebased and the graphene oxide-based semi-solid greases were obtained by similar procedures.

Morphologies of pure graphite,graphene oxide and graphene were observed by a transmission electron microscope (TEM) (Philips Tecnai12,Holland). The crystal structures of the samples were indicated using an X-ray diffractometer (Bruker D8 Advance, German). The friction performance tests were performed on a vertical multifunction friction testing machine (MWW-1,China). The wear scar diameter of the steel ball was observed by optical microscope. The operating load was 588 N and the rotation rate was 2000 rpm. 3. Results and discussion

Fig. 1 shows XRD patterns of graphite,graphene oxide and graphene. The typical peak of graphite appears at 2θ= 26.7°. However,the typical peaks of graphite oxide appear at 2θ= 10.5°, and the typical peak of graphite hardly disappears on XRD patterns. After graphite oxide was reduced to graphene,the peak at the range of 22-23° becomes a broadened peak,suggesting that graphite oxide was completely converted to graphene oxide before reduction. Additionally,the XRD pattern of graphene shows that the crystal structure of graphene is very consistent.

Fig. 1

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Fig. 1.XRD patterns of grahite,graphene oxide and grapheme.

Fig. 2 shows TEM images of graphite,graphene oxide,and graphene. Compared to the regular structure of graphite,the morphologies of graphene oxide and graphene present the curly flake structure,confirming the successful exfoliation of graphite viasonication. Furthermore,the thickness of graphene is visibly thinner than that of graphene oxide.

Fig. 2

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Fig. 2.TEM images of grahite (A),graphene oxide (B) and grapheme (C).

Fig. 3 shows the friction performance of graphite,graphene oxide,and graphene in semi-solid grease. It is clearly found that the frictional coefficient of the graphene oxide-based semi-solid grease drops toca.0.075,an approximate 30% decrease compared to the graphite based grease with ca. 0.105. Surprisingly,the graphene-based semi-solid grease shows outstanding lubricating properties,and the frictional coefficient approximately drops to 0.04-0.07. The coefficient decreases about 40%-60% in comparison with the graphite-based one. Additionally,the wear scar diameter of the steel ball surface lubricated with the graphite-based semisolid grease is scratched about 480mm (not shown),whereas those of the graphene oxide-based and the graphene-based semi-solid grease are about 210mm and 185 μm (not shown),respectively. The decrease is obvious,over 50%. It has been verified that graphene as a flat monolayer of carbon atoms tightly packed into a honeycomb two-dimensional lattice exhibited low friction and wear rates ^{[7, 8]}. Therefore,the graphene oxide and graphene sheets in semi-solid grease adsorb to the lubricated ball surface and act as protective coatings ^[6]. These results indicate that the application of graphene sheets as a lubricating additive is desirable for reducing friction and surface wear.

Fig. 3

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Fig. 3.The friction performance of grahite-based (A),graphene oxide-based (B) and graphene-based (C) semi-solid grease.

In summary,we demonstrated the excellent lubricating property of graphene by testing the graphene-based composite material. Compared to graphite,graphene-based grease exhibited excellent lubrication properties,reducingca.50% of the friction coefficient and over 50% of the wear. It is therefore a promising candidate for application as a lubrication material.

This work was supported by the talent introduction fund of Yangzhou University.