Modification of Graphene Platelets and their Tribological Properties as a Lubricant Additive

ORIGINAL PAPER

Modi?cation of Graphene Platelets and their Tribological Properties as a Lubricant Additive

after modi?cation by infrared spectroscopy and ultraviolet–visible spectrophotometer revealed that the modi?cation led to an improvement in the dispersion of graphene platelets in base oil.The tribological behavior of the lubricating oil containing modi?ed graphene platelets (MGP)was further investigated using a four-ball machine.The results indicated that the oil containing only 0.075wt%of MGP clearly improved the wear resistance and load-carrying capacity of the machine.Scanning electron microscopy and energy dispersive performed to analyze the wear scar surfaces after friction con?rmed that the outstanding lubrication performance of MGP could be attributed to their small size and extremely thin lami-nated structure,which allow the MGP to easily enter the contact area,thereby preventing the rough surfaces from coming into direct contact.

Keywords Graphene platelets áSurface modi?cation áSEM áTEM áAdditive-deposited ?lms

1Introduction

Graphene platelets have been the focus of interest in both theoretical research and in studies on practical applications due to their unique structure and remarkable properties [1–5].Recent studies have shown that graphene platelets have

properties when they materials and distributed matrix [6–8].Graphene has also been used to fabricate a number of simple electronic devices,such as ?eld-effect transistors [9,10]and resonators [11],due to its fundamental electronic properties and semi-metallic nature.Moreover,graphene platelets are promising candidates as components in applications such as organic photovoltaic cells [12],energy-storage materials [13],‘‘paper-like’’materials [14,15],and liquid crystal devices [16].However,very few studies on the tribological applica-tions of graphene platelets have been reported.Peterson et al.[17]pointed out that effective solid lubricants gen-erally have a number of basic properties,such as thermal stability,low shear strength,surface adherence,and char-acteristics of lamellar structure.A number of researchers have reported that graphite [18]and some graphite deriv-atives [19,20]as well as other lubricant materials [21,22]together have the above desirable properties.Graphene platelets are the building block of the common macro-scopic solid lubricant graphite,and they also are charac-terized by possessing the above-mentioned advantages [23].Huang et al.[24]investigated the tribological prop-erties of graphite nanosheets as an oil additive.They found that the load-carrying capacity and antiwear ability of the lubricating oil were improved when graphite nanosheets were added to the paraf?n oil at the optimal concentration.Filleter et al.[25]explored friction and dissipation in epitaxial graphene ?lms,revealing that bilayer graphene as a lubricant outperforms even graphite due to reduced adhesion.Wang [26]suggested the thin ?lms of solid lubricant materials would be an outstanding solid lubricant at temperatures ranging from -270to more than 1,000°C.Based on these results,it can be inferred that graphene platelets would be an effective solid lubricant.

J.Lin áL.Wang áG.Chen (&)

Institute of Polymer and Nanomaterials,Huaqiao University,Quanzhou 362021,China e-mail:hdcgh@https://www.360docs.net/doc/de4461114.html,

Tribol Lett (2011)41:209–215DOI 10.1007/s11249-010-9702-5

Graphene platelets that are derived from graphite gen-erally bear hydroxyl and carboxyl functional groups on their sheet edges[2],which make graphene platelets strongly hydrophilic and easy to coagulate in oil.Hence,when graphene platelets are added to the base oil as a lubricant additive,it is necessary to ensure uniform dispersion without any agglomeration of graphene platelets in the base oil.Two main approaches are currently used to solve the suspension problem.One is that the graphene platelets and effective dispersant are mixed with the base oil.It has been proven that the dispersant can enwrap one nanoparticle to repel another and thereby form a uniform suspension[24, 27].Nevertheless,due to this action,excessive or too little dispersant may greatly affect the effectiveness of the sus-pension[27].The second approach is to modify the graphene platelets with a proper modi?er.It has been demonstrated[28–30]that modifying nanoparticles with the appropriate modi?er is an effective way to prevent the coagulation of nanoparticles in suspension.Chen et al.[31] reported that multi-walled carbon nanotubes(MWNTs) modi?ed with stearic acid under the in?uence of gravity could be suspended homogeneously in base lubricant for over6months at room temperature,whereas the non-modi?ed MWNTs were entirely coagulated at the bottom after2months.Structurally,MWNTs consist of multi-graphene cylinders with end seamless caps.

Here,we report on our use of the second approach to improve the stability of graphene platelets in oil.However,in our approach,the selection of an appropriate modi?er is the key.We therefore tested a large number of surface modi?ers, such as sodium dodecyl benzene sulfonate,stearic acid, dodecyl trimethyl ammonium chloride,oleic acid,sorbitan monooleate,as well as polysorbate and others,which involved a considerable research effort.Our results demon-strated that stearic and oleic acids are the most suitable modi?ers for graphene platelets.We then modi?ed the graphene platelets with a certain proportion of stearic and oleic acids and subsequently added the modi?ed graphene platelets(MGP)into the oil to enhance tribological properties of lubricating oil.The surface features of the graphene platelets before and after modi?cation were examined by Fourier transformed infrared spectroscopy(FTIR)and ultra-violet–visible spectrophotometry(UV–VIS).Scanning elec-tron microscopy(SEM)and energy dispersive spectrometry (EDS)were used to analyze wear scar surfaces after friction. 2Experimental

2.1Materials

The graphene platelets and natural?ake graphite(particle size:25l m)were provided by the Fujian Kaili Specialty Graphite Co.(China).Base oil SN350was supplied by the Shishi Zerun Lubricating Oil Co.(China).Stearic acid was purchased from Shanghai Reagent General Plant(Shang-hai,China).Oleic acid,cyclohexane,petroleum ether, ethanol,and acetone,all analytical reagent grade,were purchased from the Shantou Xilong Chemical Co.(Shan-tou,China).

2.2Chemical Modi?cation of Graphene Platelets

and Natural Flake Graphite

Stearic and oleic acids(mass ratio3:5)were mixed by sonication in40ml cyclohexane for30min.Dried graph-ene platelets were added to the above solution and the suspension sonicated for30min at ambient temperature. The suspension of graphene platelets was then stirred and re?uxed at80°C for5h,then cooled to ambient tempera-ture.To remove the residual stearic and oleic acids,the resulting graphene platelets were?ltered and washed with acetone.The?nal step was to dry the products under vacuum at100°C.For comparison,natural?ake graphite was also modi?ed using the above process.

2.3Stability Test of Modi?ed Graphene Platelets

The350SN base oil was used as base lubricant in this test. The MGP were dispersed in350SN base oil by sonication followed by stirring with a magnet for1h at80°C(rotate speed R=600r/min).To evaluate the stabilization of the MGP in base oil,following the dispersion of the MGP as described above,the MGP lubricant was diluted50times with pure base oil and precipitated by centrifugation at 1,000rpm.The supernatant?uid was then decanted at 30-min intervals to evaluate absorbance using a UV–VIS spectrophotometer.Based on the Lambert–Beer law, absorbance is proportional to concentration.The concen-tration of graphene platelets in the supernatant?uid is an indicator of the suspension property,with higher concen-trations associated with better suspension properties. Therefore,the dispersion stability of MGP in base oil was evaluated by assessing the absorbance.For comparison, pristine graphene platelets were obtained by the same process.

2.4Tribological Properties of MGP as an Oil Additive The maximum nonseized load(P B),friction coef?cient and the wear rate of the lubricating oil with MGP were tested on a MS-10A four-ball machine(Xiamen Tenkey Auto-mation Co.,China)and compared to those of the base oil and the oil with modi?ed natural?ake graphite(MNFG). The P B of the lubricating oil was determined according to the ASTM D2783standard method.The machine was

conducted at a drive shaft speed of 1,450rpm and in the temperature range 18–35°C.The friction and wear tests were carried out according to the ASTM D4172-82stan-dard method and conducted at a rotating speed of 1,200rpm and under a constant load of 147N for a test duration of 60min;a temperature of 75±2°C was maintained throughout the whole test process.The 12.7-mm diameter test balls used in this study were made of GCr15A bearing steel (AISI 52100)with a hardness of 64HRC.Before each test,all test-section components were cleaned ultrasonically with petroleum ether,rinsed in eth-anol,and dried.The wear scar diameters on the steel balls were measured using an optical microscope.The worn surfaces of the balls after the friction test were examined with SEM and EDS.

3Results and Discussion

3.1Characterization of Graphene Platelets

Figure 1is a typical SEM image of graphene platelets and shows that the graphene platelets retain their original laminated structure,with an average diameter of 1.2l m.The thickness of the graphene platelets is about 10–15nm (Fig.2)as determined by transmission electron microscopy (TEM).

3.2Characterization of MGP

The FTIR spectrum of graphene platelets before and after modi?cation is shown in Fig.3.Pure graphene platelets only showed a weak peak at 1,629cm -1,which refers to C=C bonds.After modi?cation with stearic and oleic acids,the peak at 1,629cm -1was strengthened,likely due to the absorption of oleic acids on the graphene platelets.

Simultaneously,three new peaks at approximately 2,862,2,921,and 3,485cm -1were clearly observed.The peaks at 2,862and 2,921cm -1are attributed to CH 3and CH 2stretching mode,respectively,while the broad peak at 3,485cm -1can likely be attributed to -OH bonds.Coates [32]reported that the impact of hydrogen bonding could produce signi?cant band broadening as well as lower the mean absorption frequency,as shown in Fig.3.Therefore,we concluded that the surfaces of graphene platelets were chemically modi?ed by stearic and oleic acids.

Figure 4shows the suspension stability of the two lubricating oils as evaluated by UV–VIS spectrophotome-try,which measured the UV intensity of the lubricating

oil

Fig.1Scanning electron microscopy image of graphene

platelets

Fig.2Transmission electron microscopy image of graphene platelets

solution by evaluating the rate of nanoparticle sedimenta-tion.The relative concentration was calculated by the ratio of the particle concentration intensity of the supernatant ?uid at each measurement time divided by the initial con-centration intensity of the suspension.A value of1.0at the relative concentration means excellent stability of the lubricating oil solution without particle sedimentation. Figure4shows that the particle concentration of the two different suspensions was nearly equal before centrifuga-tion.Following a long period of centrifugation,rapid pre-cipitation was observed for the suspension with the pristine graphene platelets,indicating that unmodi?ed graphene platelets were heavily agglomerated in base oil.In contrast, little MGP precipitation was observed in the suspension with the MGP.This result indicates that the addition of MGP to the lubricating oil had produced an excellent stable suspension and that this improvement could be attributed to the effectiveness of the surface modi?cation.Following the

modi?cation of the graphene platelets with stearic and oleic acids,the hydrophilic segments of the stearic and oleic acids molecules were anchored onto the surface of the graphene platelets.When the MGP were dispersed in the base oil,the long hydrocarbon segments easily stretched into the base oil and therefore produced a typical steric hindrance effect [31],which effectively helped to separate the graphene platelets from each other.At the same time,the steric hin-drance force could conquer gravity and prevent the graph-ene platelets from coagulating.Therefore,the lubricating oil with MGP formed a uniform and stable suspension. 3.3Tribology Tests of the Lubricating Oil with MGP

or MNFG

Bartz[33]pointed out that an optimal concentration of solid additive,such as graphite,exists in liquid lubricant.In order to determine the concentration of additive giving the best tribological behavior,we tested several dispersions of MGP(0.015,0.035,0.055,0.075,0.095,and0.105wt%) and MNFG.Figure5shows the in?uence of MGP and MNFG concentration on maximum nonseizure load(P B)of the oil.The P B represents the load-carrying capacity of the lubricating oil;in the pure base oil with0wt%of graphite particles,the P B value was418.5N.There was a remark-able increase in the P B values of the lubricating oil with the addition of increased concentrations of MGP and MNFG, respectively,to the base oil.When the concentration of MGP and MNFG reached0.075wt%,both P B values reached their maximum,but the largest P B value of the oil with MGP was627.2N,which was much higher than that of oil with MNFG(523N).These results indicate that the oil with MGP had a better load-carrying capacity than the base oil and oil with MNFG.When the concentration was

higher than 0.075wt%,excessive MGP also resulted in a decrease in the P B value of the oil.One possible expla-nation is that some coagulation of graphene platelets and metallic debris occurred at higher concentrations of MGP/MNFG owing to the friction effect,which made the friction unstable or caused vibration,leading to a decrease in the maximum nonseized load [34].

Figure 6shows the wear rate of lubricating oils as a function of friction time,with MGP and MNFG concen-trations of 0.075wt%and a load of 147N for 60min at a speed of 1,200rpm.It can be seen that the wear rates increased with increasing friction time.However,the wear rate of the oil with MGP increased to a lesser degree and more gradually than that of the base oil and the oil with MNFG.A similar tendency was observed in Fig.7,which displays the friction coef?cient as a function of friction time.With increasing friction time,the friction coef?cients

Fig.8The worn surface of the steel ball and the corresponding energy dispersive spectrometry analysis.a Lubricated with base oil,b lubricated with base oil containing MNFG,c lubricated with base oil containing MGP

of both the base oil and the oil with MNFG increased remarkably;in comparison,the friction coef?cient of the oil with MGP was relatively steady throughout the test. Moreover,the friction coef?cient of the oil with MGP was much lower than that of the base oil and oil with MNFG. The excellent anti-wear properties of MGP may possibly be attributed to their small diameter and extremely thin laminated structure,which allows MGP to easily enter the contact area and deposit a continuous protective?lm that prevents direct contact with the rough face.The MNFG was unable to perform this ef?ciency due to the large size of its lamellae.

3.4Surface Analysis

The lubricating oil containing MGP presents excellent anti-wear and friction-reducing properties,which were con?rmed by the results of the SEM and EDS analysis. Figure8shows the SEM images of the rubbing surfaces lubricated by three lubricating oils as well as the corre-sponding EDS analysis.As shown in Fig.8a,the metal surface has been severely scratched due to the poor anti-wear property of the pure base oil.The EDS analysis showed that the content of C element on the metal surface was only3.6wt%,which may be due to the basal com-ponent of the steel ball and carbonizations of the oil layer. The rubbing surface shown in Fig.8b was smoother than that in Fig.8a,but it still showed obvious scratches and extensive furrows.Correspondingly,the EDS analysis displayed a content of C element of10.2%,just a little higher than that found in Fig.8a.This result indicates that natural?ake graphite was barely deposited on the rubbing surface,which con?rms that this graphite has dif?culty entering the contact area and,consequently,forming a continuous protective?lm.In contrast,it can be seen in Fig.8c that many graphene platelets were distributed on the metal surface and that the surface had nearly none of wear and scratch traces observed in Fig.8a and b.In addition the content of C element was up to27.86%,which is nearly ninefold and2.7-fold higher than that shown in Fig.8a and b,respectively.This result clearly proves that graphene platelets in oil easily form protective deposited ?lms to prevent the rubbing surfaces from coming into direct contact and,thereby,improve the entirely tribolog-ical behavior of the oil.

4Conclusions

The results of our study clearly demonstrate that the graphene platelets were effectively modi?ed by stearic and oleic acids,thereby enabling the MGP added as lubricant additive to stably disperse in oil.The wear resistance and load-carrying capacity of the lubricating oil were greatly improved with the addition of MGP at an optimal content of0.075wt%.The friction coef?cient of the oil with MGP was much lower than that of the base oil and oil with MNFG.In summary,the overall lubricious properties of the lubricating oil were notably improved,and this improvement can be attributed to the addition of MGP. Acknowledgments This work is supported by National Science Foundation of China(No.20574025),Natural Science Foundation of Fujian Province(E0820001),and Fujian Key Laboratory of Polymer Materials(Fujian Normal University).

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