research article graphene oxide nanosheets as effective

Hindawi Publishing CorporationISRN TribologyVolume 2013 Article ID 425809 9 pageshttpdxdoiorg1054022013425809

Research ArticleGraphene Oxide Nanosheets as Effective Friction Modifier forOil Lubricant Materials Methods and Tribological Results

Adolfo Senatore12 Vincenzo DrsquoAgostino12 Vincenzo Petrone2

Paolo Ciambelli12 and Maria Sarno12

1 Department of Industrial Engineering University of Salerno 84084 Fisciano Salerno Italy2 NANO MATES Research Centre University of Salerno 84084 Fisciano Salerno Italy

Correspondence should be addressed to Adolfo Senatore asenatoreunisait

Received 31 December 2012 Accepted 20 January 2013

Academic Editors J Antunes J H Jang Y Lu J Mao and P Sahoo

Copyright copy 2013 Adolfo Senatore et alThis is an open access article distributed under theCreative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

The tribological behaviour of graphene oxide nanosheets in mineral oil was investigated under a wide spectrum of conditionsfrom boundary and mixed lubrication to elastohydrodynamic regimes A ball-on-disc setup tribometer has been used to verify thefriction reduction due to nanosheets prepared by a modified Hummers method and dispersed in mineral oil Their good frictionand antiwear properties may possibly be attributed to their small structure and extremely thin laminated structure which offerlower shear stress and prevent interaction between metal interfaces Furthermore the results clearly prove that graphene plateletsin oil easily form protective film to prevent the direct contact between steel surfaces and thereby improve the frictional behaviourof the base oil This evidence is also related to the frictional coefficient trend in boundary regime

1 Introduction

In the field of tribological applications nanoparticles asadditives in base oil have been extensively investigatedThesestudies refer to synthesis and preparation of nanoscale par-ticles and their tribological properties and friction reductionmechanisms It has been found that when the nanoparticleswere added to base oil the extreme pressure property andload-carrying capacity were improved and friction coefficientwas decreased

In the past few years nested spherical supramolecules ofmetal dichalcogenide have been synthesized by the reactionof metal oxide nanoparticles with H

2S at elevated tem-

peratures Because of their nested fullerene-like structurethese species are known as inorganic fullerene-like (IF)nanoparticles The IF nanoparticles exhibited improved tri-bological behaviour compared to the microscale platelets fortheir robustness and flexibility The tribological properties offullerene-like nanoparticles as additives to liquid lubricantswere studied in [1ndash4]

Recently due to high load-bearing capacity low surfaceenergy high chemical stability weak intermolecular and

strong intramolecular bonding nanocarbon materials havereceived a great attention by tribology researchers In recentyears graphene platelets due to their unique structure andremarkable properties have been the focus of interest instudies on practical applications However few studies onthe tribological applications of graphene platelets have beenreported so far A number of researchers have reported thatgraphite [5] and some graphite derivatives [6 7] as wellas other lubricant materials [8ndash10] together have the abovedesirable properties Lin et al [11] investigated the tribologicalproperties of graphite nanosheets as an oil additive Thesematerials are characterized by weak interatomic interactionsbetween their layers (Van der Waals forces) low strengthshearing [12] In [13] the tribological behaviours of graphenesheets modified by oleic acid and dispersed in lubricant oilwere investigated using a four-ball tribometer

Huang et al [14] investigated the tribological propertiesof graphite nanosheets as an oil additive They found that thefrictional behaviour and antiwear ability of the lubricating oilwere improved when graphite nanosheets were added to theparaffin oil at the optimal concentration

2 ISRN Tribology

In particular to ensure uniform dispersion without anyagglomeration of the graphene oxide (GO) in the base oiltaking advantage of the surface ndashOHandndashCOOH introducedduring the GO preparation a functionalization with longchain compounds (ie aliphatic amine to obtain the amidederivative) enhances the dispersion in nonpolar solvents [11]

On the other hand the additives such as carbon and evenmore if functionalized with ndashOH and ndashCOOH groups thatincrease their polarity can be dispersed through the use of adispersant [13] avoiding further chemical reactions andusinga methodology well known to the lubricant industry

We chose at the best of our knowledge for the first timeto add graphene oxide using a dispersant

In the present study the tribological behaviour of gra-phene nanosheets in group I base mineral SN150 was inves-tigated under a very wide spectrum of conditions that isfrom boundary and mixed lubrication to the elastohydrody-namic (EHL) regimes To explore the performances of thenanosheets in the lubricating fluid a rotational tribometerwith a ball-on-disc setup has been employed

Raman analysis on the steel ball worn surfaces was per-formed to investigate the presence of graphitic material onthe mating surfaces after tribological tests in order to verifythe formation of a protective film on the rubbing surfaces dueto the additive

2 Experimental

21 Nanosheets Preparation and Characterization

211 Materials Graphene oxide (GO) nanosheets were pre-pared at the laboratories of the NANO MATES centre by amodified Hummers and offeman method [15] The oxidationof graphite particles were obtained from Lonza [16 17] tographitic oxide accomplished with a water-free mixture ofconcentrated sulfuric acid sodium nitrate and potassiumpermanganate The entire process requires less than twohours for completion at temperatures below 45∘C With theaid of a further sonication step the oxidized graphite layerswere exfoliated from each other Then 30 H

2O2was added

to the suspension to eliminate the excessMnO4

minusThe desiredproducts were rinsed with deionized water The remainingsalt impurities were eliminated with resinous anion andcation exchangers The dry form of graphitic oxide wasobtained by centrifugation followed by dehydration at 40∘C

A polyisobutenyl succinic acid-polyamine ester was soni-catedwith theGOnanosheet in base oil To provide dispersionstability of the additive the weight ratio GOdispersant was35 The dispersant has a polar head that attaches itself to thesolid particles and a very long hydrocarbon tail that keeps itsuspended in oil

Once several dispersant polar heads have attached them-selves to a solid particle the dispersant can no longer combinewith other particles to form large aggregates enwrapping onenanoparticle to repel another and thereby form a uniformsuspension

212 Characterization Methods Scanning electron micro-scopy (SEM) pictures were obtained with an LEO 1525

microscope The samples without any pretreatment werecovered with a 250 A thick gold film using a sputter coater(Agar 108A) Raman spectra were obtained at room tem-perature with a micro-Raman spectrometer Renishaw inViawith 514 nm excitation wavelength (laser power 30mW) inthe range 100ndash3000 cmminus1 Optical images were collectedwith the optical microscopy connected on line with theRaman instruments XRD measurements were performedwith a Bruker D8X-ray diffractometer using CuK120572 radiationMicro X-ray diffraction patterns were obtained by a 120583X-raydiffractometer (Rint Rapid Rigaku Corporation)

22 Tribological Tests Description In this study the investi-gated tribopair was composed by an upper rotating disc anda lower ball specimen completely flooded in a temperature-controlled lubricant bath in line with the features of theused tribometer Wazau TRM100 The upper element ofthe tribopair was an X155CrVMo12-1 steel disc 60HRCroughness 119877

119886= 050 120583m and 105mm diameter and the

lower one was an X45Cr13 steel ball 52ndash54HRC and 8mmdiameter

The normal force to the balldisc contact was deliveredby a lever system and could be varied in the range of 0ndash100N its value was measured through a load cell placedunder the specimen holder The following average Hertzianpressures were used at the balldisc interface with normalload given in brackets 117 GPa (30N) 147GPa (60N)and 168GPa (90N) The tests were performed for threedifferent temperatures 25 50 and 80∘C and the lubricantaverage temperature has been kept constant through a NiCr-Ni thermocouple in the oil reservoir and an electric resistancedriven by a digital controller This control system allows acontrol range from room temperature up to 100∘C

Speed-sweep tests at constant load on broad sliding speedrange have been performed to cover different lubricationregimes with the aim of minimizing the modification of thetribopair steel surfaces For this reason the time extension ofeach test was limited to 16 minutes The disc speed rose upto 220ms in the first 4 minutes and dropped to zero in thefollowing 4 minutes This speed pattern was repeated twiceThe current design of this experiment allowed obtaining acomplete Stribeck frictional graph

The tests were performed for three different temperaturesthat is 25 50 and 80∘C All the samples have been stirredbefore each test for 20 minutes by means of a Turrax T25Digital Homogenizer with adjustable speed No chemicaldispersant agents were used in order to explore the benefitscoming from the pure addition of nanoparticles

Tests were also carried out to analyse the influence of GOaddition to the base oil onwear behaviour of the steel balldisctribopair through steady state rubbing tests see Section 322For these tests constant values for average hertzian pressuretemperature and speed were chosen

3 Results and Discussion

31 Graphite and GO Characterization Graphite chips andGO nanosheets are shown in Figures 1(a) 1(b) 1(c) and 1(d)respectivelyThe images at highermagnification (Figures 1(b)

ISRN Tribology 3

(a) (b)

(c) (d)

Figure 1 Graphite (a b) GO (c d)

and 1(d)) evidence the loss of the chips structure of originalgraphite to GO transparent and thin flakes

32 Tribological Characterization

321 Friction Coefficient The measured data are presentedaccording to the Stribeck curves representation that isfriction coefficient versus sliding speed Below are shown theresults obtained for the lubricant sample formulated with GOnanosheets for different levels of temperature and averagehertzian contact pressure The main properties of the baseoil were kinematic viscosity 297 cSt at 40∘C 51 cSt at 100∘Cdensity at 20∘C 087 kgm3

The set of graphs below (Figure 2) introduces the Stribeckcurves exhibited in the sliding tests by the SN 150 oil with01 wt GO nanosheets at different levels of average hertziancontact pressure and lubricant temperature along with theerror bars denoting the standard deviation around the rollingmean

Those results show the decrease of the friction coefficientfor increasing average hertzian contact pressure for the for-mulated sample the same behaviour has been observedfor the base oil according to a point-contact studied effectthe shear stress increases less in proportion to the contactpressure this leads to a slight reduction of friction [18] Asexpected for a given sample the minimum of the Stribeckcurvemoves right for increasing temperature due to the lowerviscosity Additionally for each sample the CoF increased

with the temperature at a given level of speed and contactpressure This observation could mainly be addressed to theeffect of the lower lubricant viscosity and the ensuing GOprecipitation at higher temperature

A comparison between the samples SN150 with 01 wtGO and the SN150 base oil shows that reduction of the fric-tion coefficient for the sample with graphene nanoparticles isremarkable (Figures 3(a) and 3(f))

The Stribeck graphs in Figure 3 for average hertzianpressure equal to 117 GPa 147GPa and 168GPa show ashape with a well-developed minimum that is considered thetransition from mixed lubrication regime to EHL regime forincreasing speed [19] This frontier divides the region withconcurrent phenomena of solid-to-solid contacts adhesionand interaction between friction modifier additives and steelsurface (mixed lubrication) from the other with predominantviscous stress and elastic deformation of the tribopair sur-faces (EHL) For instance the transition appears in the speedrange 030ndash040ms for the test at 25∘C and 050ndash060ms ata higher temperature of 80∘C

These tests confirm that using graphene nanoparticlesin lubricants enhances the friction reduction in boundarymixed and EHL lubrication regimes

For instance with average contact pressure of 117 GPaand temperature in the range 25ndash80∘C the average frictioncoefficient decreased by 20 of the base lubricant value buta similar average reduction could be observed for all the com-binations of the operating conditions Even in the heaviest

4 ISRN Tribology

022

02

018

016

014

012

010 05 1 15 2

Sliding speed (ms)

119879 = 25∘C

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(a)

022

02

018

016

014

012

010 05 1 15 2

119879 = 50∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(b)

022

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(c)

Figure 2 Stribeck curves comparison from the sweep speed test for three different average Hertzian contact pressures 117 147 and 168GPaat (a) 25∘C (b) 50∘C and (c) 80∘C Sample SN 150 oil with 01 wt GO nanosheets

Table 1 Friction coefficient in steady boundary and EHL lubrication conditions at 25∘C

Average Hertzian contact pressure 168GPa oil temperature 25∘C

Sample Average friction coefficient at 50mms slidingspeed

Average friction coefficient at 050ms slidingspeed Difference

Boundary regime EHL regimeSN150mdashbase oil 0158 0142 BenchmarkSN150 with 01 wt GO 0136 0118 minus14 minus17

test conditions (high pressure and temperature) and in fullydeveloped EHL regimewhere the viscous stress are prevalentthe CoF reduction is greater than 8 (Figure 3(f))

One-hour sliding tests were also performed to analyse theinfluence of the progressive wearing of the mating materialson the friction coefficient (Figure 4) The average frictioncoefficients measured during these steady state tests arepresented in Tables 1 and 2 According to ISOIEC Guide98-32008 the average friction coefficients are presentedwith expanded uncertainty equal to 5 times 10minus3 coverage factor119896 = 2

The lubrication regimes in these tables and Figure 4 areidentified according to the results of the Stribeck curves

322 Wear Parameter At the end of each 1-hour steady statetest the worn surface of the steel ball has been measuredwith an optical microscope to acquire the wear scar diameter(WSD) The WSD values are reported in Tables 3 and 4According to ISOIECGuide 98-32008 the wear scar diame-ter (WSD) results are listed with expanded uncertainty equalto 20120583m coverage factor 119896 = 2

The antiwear property of GO as additive for liquidlubricants has been clearly exhibited in all the lubricationregimes

In particular the presence of graphene oxide allowedreductions of the ball wearscar diameter equal to 12 27and 30 in boundary mixed and EHL regime respectively

ISRN Tribology 5

0 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

) 119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(a)

0 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(b)

024022

02018016014012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(c)

024022

02018016014012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(d)

02

018

016

014

012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(e)

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(f)

Figure 3 Stribeck curves comparison from the sweep-speed test (SN150mdashblack SN150 with 01 wt GOmdashred)

Table 2 Friction coefficient in steady boundary and mixed lubrication conditions at 80∘C




Boundary regime Mixed regimeSN150mdashbase oil 0173 0163 BenchmarkSN150 with 01 wt GO 0141 0139 minus18 minus15

33 Raman Spectrum of the Worn Surface In Figure 5 theRaman spectra of graphite and graphene oxide nanosheets arereported In particular in the spectrum of graphite the mostprominent features [20] the so-called G band appearing at1582 cmminus1 [21] and the G1015840 or 2D band at about 2700 cmminus1using 514 nm excitation wavelength are collected The G1015840band at room temperature can be fitted with two Lorentzian

lines A broad D band due to disorder or edge [22] ofa graphite sample can be also seen at about half of thefrequency of the G1015840 band (around 1350 cmminus1 using 514 nmlaser excitation) The oxide graphene shows as expected animproved D band intensity and flattening of the 2D line anddisplays a shift to higher frequencies (blue shift) of a broaderG band [23] In Figure 5 the Raman spectrum collected on

6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)

Figure 4 Friction coefficient in a 1-hour steady state test with average hertzian contact pressure 168GPa and oil temperature 80∘C (a) Slidingspeed 50mms boundary regime (b) sliding speed 050 ms mixed regime (SN150mdash black SN150 with 01 wt GOmdashred)

Table 3 Wear scar diameter (WSD) after a 1-hour boundary and EHL regime test at 25∘C

Average hertzian contact pressure 168GPa oil temperature 25∘C

Sample Ball wear scar diameter (120583m) at 50mms slidingspeed

Ball wear scar diameter (120583m) at 050mssliding speed Difference

Boundary regime EHL regime

SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630

Pictures not to scale

minus12 minus30

the ball wear after a 2-hour test at 119901 = 168GPa 119879 = 25∘Cand 119907 = 05ms is also reported A notable fact is thatthe G band of this spectrum is located almost at the samefrequency as that in graphite while the G1015840 band exhibits asingle Lorentzian feature and the D band results are reducedevidencing the presence of a thin carbon film (ldquotribofilmrdquo) onthe wear scar surface

34 Discussion The incessant literature dispute on the fric-tion reduction mechanism introduced by nanoparticles aslubricant additives finds in the following list the more con-vincing physical explanations rolling-sliding ldquorigidrdquo motionstogether with flexibility properties [24ndash26] nanoadditiveexfoliation and material transfer to metal surface to form theso-called ldquotribofilmrdquo or ldquotribolayerrdquo [1ndash3] electronic effects

ISRN Tribology 7

Table 4 Wear scar diameter (WSD) after a 1-hour boundary and a mixed regime test at 80∘C



Ball wear scar diameter (120583m) at 050ms slidingspeed Difference

Boundary regime Mixed regime

SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27

in tribological interfaces [3 27 28] and surface roughnessimprovement effect or ldquomendingrdquo [29] along with the moreclassical hypothesis of surface sliding on lower shear stresslayers due to weak interatomic forces valid also for micro-scale additives used from decades

The experimental evidences of this paper could representa proof of the presence of a tribological film of reducedgraphene oxide which covers the whole worn surface of thesteel ball The since-start reduction of friction coefficient inFigure 4 could be addressed to the surface rubbing throughthe low shear stress GO layers The smoothly decreasingfriction coefficient in boundary regime in Figure 4(a) may berelated to the tribofilm development

The GO 01 wt concentration is quite lower than theusual one for inorganic nanoadditives [1 3] and carbonnanotubes [30] and in line with previous studies in whicholeic acid-modified graphene was used [13] This featureis welcomed in the preparation of fully formulated engineor gearbox lubricants since the GO addition could onlyslightly modify the delicate equilibrium achieved by oil

1000 1500 2000 2500 3000

Graphite

GO

Reduced GO on the ball wear scar

Wavenumber (1cm)

(au

)

Figure 5 Raman spectra of graphite GO and reduced GO on theball wear scar after a 2-hour sliding test

manufacturers between the essential and ubiquitous additivesas antioxidant viscosity modifier pour point depressant andother minors

8 ISRN Tribology

4 Conclusions

The tribological tests confirm good reduction of friction andwear parameters in boundary lubrication mixed lubricationand EHL regimes achieved with mineral oils formulated withgraphene oxide (GO) nanosheets prepared at the laboratoriesof the NANO MATES research centre For instance withaverage contact pressure of 117 GPa and temperature in therange 25ndash80∘C the average CoF decreased by more than 20compared with the base lubricant value The sliding tests insteady state conditions have shown an average reduction ofCoF equal to 16 The frictional reduction benefit has beenproven at any level of oil temperature contact pressure andsliding speed

The best antiwear result has been observed on the ballsurface in the mixed lubrication and EHL regime with amarked average decreasing around 30 A tribological filmof reduced GO nanosheets after the tribological test coversthe ball wear scar

The tendency to the precipitation is a drawback for theformulated sample at the higher temperatures due to thelower lubricant viscosity

The good friction and antiwear properties of graphenesheets may possibly be attributed to their small structure andextremely thin laminated structure which offer lower shearstress and prevent interaction between metal interfaces

The results clearly prove that graphene platelets in oileasily form protective deposited films to prevent the rub-bing surfaces from coming into direct contact and therebyimprove the entirely tribological behaviour of the oil

Future works will be addressed toward methods fordispersion improvement of GO in oil addition to other basesand fully formulated oils search of optimal concentrationand testing at lower contact pressure to verify the existenceof activation threshold related to nanoparticles structuremodification

Acknowledgment

This research was partially supported by the project ldquoTri-bological study of nanoadditives for lubricants and interac-tions at the interface with the mechanical couplingStudiotribologico di nanoadditivi per lubrificanti ed interazioniallrsquointerfaccia con le superfici dellrsquoaccoppiamento meccanicordquofunded by the University of Salerno Italy

References

[1] RGreenbergGHalperin I Etsion andR Tenne ldquoThe effect ofWS2nanoparticles on friction reduction in various lubrication

regimesrdquo Tribology Letters vol 17 no 2 pp 179ndash186 2004[2] L Joly-Pottuz F Dassenoy M Belin B Vacher J M Martin

and N Fleischer ldquoUltralow-friction and wear properties of IF-WS2under boundary lubricationrdquo Tribology Letters vol 18 no

4 pp 477ndash485 2005[3] L Yadgarov V Petrone R Rosentsveig Y Feldman R Tenne

and A Senatore ldquoTribological studies of rhenium dopedfullerene-like MoS

2nanoparticles in boundary mixed and

elasto-hydrodynamic lubrication conditionsrdquo Wear vol 297no 1-2 pp 1103ndash1110 2013

[4] R Rosentsveig A Gorodnev N Feuerstein et al ldquoFullerene-likeMoS

2nanoparticles and their tribological behaviorrdquo Tribol-

ogy Letters vol 36 no 2 pp 175ndash182 2009[5] J Wintterlin and M-L Bocquet ldquoGraphene on metal surfacesrdquo

Surface Science vol 603 no 10ndash12 pp 1841ndash1852 2009[6] T Ramanathan A A Abdala S Stankovich et al ldquoFunction-

alized graphene sheets for polymer nanocompositesrdquo NatureNanotechnology vol 3 no 6 pp 327ndash331 2008

[7] P J Bryant P L Gutshall and L H Taylor ldquoA study ofmechanisms of graphite friction and wearrdquo Wear vol 7 no 1pp 118ndash126 1964

[8] R L Fusaro ldquoMechanisms of graphite fluoride [(CF119909)119899] lubri-

cationrdquoWear vol 53 no 2 pp 303ndash323 1979[9] T Jun and X Qunji ldquoThe deintercalation effect of FeCl

3-

graphite intercalation compound in paraffin liquid lubricationrdquoTribology International vol 30 no 8 pp 571ndash574 1997

[10] M R Hilton R Bauer S V Didziulis M T Dugger J MKeem and J Scholhamer ldquoStructural and tribological studiesof MoS

2solid lubricant films having tailored metal-multilayer

nanostructuresrdquo Surface and Coatings Technology vol 53 no 1pp 13ndash23 1992

[11] J Lin L Wang and G Chen ldquoModification of grapheneplatelets and their tribological properties as a lubricant addi-tiverdquo Tribology Letters vol 41 no 1 pp 209ndash215 2011

[12] B Bhushan and B K Gupta Handbook of Tribology McGraw-Hill New York NY USA 1991

[13] W Zhang M Zhou H Zhu et al ldquoTribological properties ofoleic acid-modified graphene as lubricant oil additivesrdquo Journalof Physics D vol 44 no 20 Article ID 205303 2011

[14] H D Huang J P Tu L P Gan and C Z Li ldquoAn investigationon tribological properties of graphite nanosheets as oil additiverdquoWear vol 261 no 2 pp 140ndash144 2006

[15] W S Hummers Jr and R E Offeman ldquoPreparation of graphiticoxiderdquo Journal of the American Chemical Society vol 80 no 6p 1339 1958

[16] ldquoThe history of Lonzarsquos graphite powdersrdquo Industrial Lubrica-tion and Tribolog vol 27 pp 59ndash69 1948

[17] C Altavilla P Ciambelli M Sarno et al ldquoTribological andrheological behaviour of lubricating greases with nanosizedinorganic based additivesrdquo in Proceedings of the 3rd EuropeanConference on Tribology (ECOTRIB rsquo11) and 4th Vienna Inter-national Conference on Nanotechnology (VIENNANO rsquo11) FFranek W J Bartz A Pauschitz J Vizintin E Ciulli and RCrockett Eds pp 903ndash908 Vienna

[18] M Kalin I Velkavrh and J Vizintin ldquoThe Stribeck curve andlubrication design for non-fully wetted surfacesrdquoWear vol 267no 5ndash8 pp 1232ndash1240 2009

[19] M H Cho J Ju S J Kim and H Jang ldquoTribological propertiesof solid lubricants (graphite Sb

2S3 MoS

2) for automotive brake

friction materialsrdquoWear vol 260 no 7-8 pp 855ndash860 2006[20] LMMalardM A Pimenta G Dresselhaus andM S Dressel-

haus ldquoRaman spectroscopy in graphenerdquo Physics Reports vol473 no 5-6 pp 51ndash87 2009

[21] A C Ferrari ldquoRaman spectroscopy of graphene and graphitedisorder electron-phonon coupling doping and nonadiabaticeffectsrdquo Solid State Communications vol 143 no 1-2 pp 47ndash572007

[22] M A Pimenta G Dresselhaus M S Dresselhaus L GCancado A Jorio and R Saito ldquoStudying disorder in graphite-based systems by Raman spectroscopyrdquo Physical ChemistryChemical Physics vol 9 no 11 pp 1276ndash1291 2007

ISRN Tribology 9

[23] K N Kudin B Ozbas H C Schniepp R K Prudrsquohomme IA Aksay and R Car ldquoRaman spectra of graphite oxide andfunctionalized graphene sheetsrdquo Nano Letters vol 8 no 1 pp36ndash41 2008

[24] L Joly-Pottuz E W Bucholz N Matsumoto et al ldquoFrictionproperties of carbon nano-onions from experiment and com-puter simulationsrdquo Tribology Letters vol 37 no 1 pp 75ndash812010

[25] O Tevet O Goldbart S Cohen et al ldquoNanocompression ofindividual multilayered polyhedral nanoparticlesrdquo Nanotech-nology vol 21 no 36 Article ID 365705 2010

[26] O Tevet P Von-Huth R Popovitz-Biro R Rosentsveig H DWagner and R Tenne ldquoFriction mechanism of individual mul-tilayered nanoparticlesrdquo Proceedings of the National Academy ofSciences vol 108 no 50 pp 19901ndash19906 2011

[27] G Seifert H Terrones M Terrones G Jungnickel and TFrauenheim ldquoStructure and electronic properties of MoS

2

nanotubesrdquo Physical Review Letters vol 85 no 1 pp 146ndash1492000

[28] B V Derjaguin and V P Smilga ldquoElectrostatic component ofthe rolling friction force momentrdquo Wear vol 7 no 3 pp 270ndash281 1964

[29] G Liu X Li B Qin D Xing Y Guo and R Fan ldquoInvestigationof the mending effect and mechanism of copper nano-particleson a tribologically stressed surfacerdquoTribology Letters vol 17 no4 pp 961ndash966 2004

[30] F Dassenoy L Joly-Pottuz J M Martin and T Mieno ldquoCar-bon nanotubes as advanced lubricant additivesrdquo in CarbonNanotubes V N Popov and P Lambin Eds vol 222 of NATOScience Series II Mathematics Physics and Chemistry pp 237ndash238 2006

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2 ISRN Tribology

In particular to ensure uniform dispersion without anyagglomeration of the graphene oxide (GO) in the base oiltaking advantage of the surface ndashOHandndashCOOH introducedduring the GO preparation a functionalization with longchain compounds (ie aliphatic amine to obtain the amidederivative) enhances the dispersion in nonpolar solvents [11]

On the other hand the additives such as carbon and evenmore if functionalized with ndashOH and ndashCOOH groups thatincrease their polarity can be dispersed through the use of adispersant [13] avoiding further chemical reactions andusinga methodology well known to the lubricant industry

We chose at the best of our knowledge for the first timeto add graphene oxide using a dispersant

In the present study the tribological behaviour of gra-phene nanosheets in group I base mineral SN150 was inves-tigated under a very wide spectrum of conditions that isfrom boundary and mixed lubrication to the elastohydrody-namic (EHL) regimes To explore the performances of thenanosheets in the lubricating fluid a rotational tribometerwith a ball-on-disc setup has been employed

Raman analysis on the steel ball worn surfaces was per-formed to investigate the presence of graphitic material onthe mating surfaces after tribological tests in order to verifythe formation of a protective film on the rubbing surfaces dueto the additive

2 Experimental

21 Nanosheets Preparation and Characterization

211 Materials Graphene oxide (GO) nanosheets were pre-pared at the laboratories of the NANO MATES centre by amodified Hummers and offeman method [15] The oxidationof graphite particles were obtained from Lonza [16 17] tographitic oxide accomplished with a water-free mixture ofconcentrated sulfuric acid sodium nitrate and potassiumpermanganate The entire process requires less than twohours for completion at temperatures below 45∘C With theaid of a further sonication step the oxidized graphite layerswere exfoliated from each other Then 30 H

2O2was added

to the suspension to eliminate the excessMnO4

minusThe desiredproducts were rinsed with deionized water The remainingsalt impurities were eliminated with resinous anion andcation exchangers The dry form of graphitic oxide wasobtained by centrifugation followed by dehydration at 40∘C

A polyisobutenyl succinic acid-polyamine ester was soni-catedwith theGOnanosheet in base oil To provide dispersionstability of the additive the weight ratio GOdispersant was35 The dispersant has a polar head that attaches itself to thesolid particles and a very long hydrocarbon tail that keeps itsuspended in oil

Once several dispersant polar heads have attached them-selves to a solid particle the dispersant can no longer combinewith other particles to form large aggregates enwrapping onenanoparticle to repel another and thereby form a uniformsuspension

212 Characterization Methods Scanning electron micro-scopy (SEM) pictures were obtained with an LEO 1525

microscope The samples without any pretreatment werecovered with a 250 A thick gold film using a sputter coater(Agar 108A) Raman spectra were obtained at room tem-perature with a micro-Raman spectrometer Renishaw inViawith 514 nm excitation wavelength (laser power 30mW) inthe range 100ndash3000 cmminus1 Optical images were collectedwith the optical microscopy connected on line with theRaman instruments XRD measurements were performedwith a Bruker D8X-ray diffractometer using CuK120572 radiationMicro X-ray diffraction patterns were obtained by a 120583X-raydiffractometer (Rint Rapid Rigaku Corporation)

22 Tribological Tests Description In this study the investi-gated tribopair was composed by an upper rotating disc anda lower ball specimen completely flooded in a temperature-controlled lubricant bath in line with the features of theused tribometer Wazau TRM100 The upper element ofthe tribopair was an X155CrVMo12-1 steel disc 60HRCroughness 119877

119886= 050 120583m and 105mm diameter and the

lower one was an X45Cr13 steel ball 52ndash54HRC and 8mmdiameter

The normal force to the balldisc contact was deliveredby a lever system and could be varied in the range of 0ndash100N its value was measured through a load cell placedunder the specimen holder The following average Hertzianpressures were used at the balldisc interface with normalload given in brackets 117 GPa (30N) 147GPa (60N)and 168GPa (90N) The tests were performed for threedifferent temperatures 25 50 and 80∘C and the lubricantaverage temperature has been kept constant through a NiCr-Ni thermocouple in the oil reservoir and an electric resistancedriven by a digital controller This control system allows acontrol range from room temperature up to 100∘C

Speed-sweep tests at constant load on broad sliding speedrange have been performed to cover different lubricationregimes with the aim of minimizing the modification of thetribopair steel surfaces For this reason the time extension ofeach test was limited to 16 minutes The disc speed rose upto 220ms in the first 4 minutes and dropped to zero in thefollowing 4 minutes This speed pattern was repeated twiceThe current design of this experiment allowed obtaining acomplete Stribeck frictional graph

The tests were performed for three different temperaturesthat is 25 50 and 80∘C All the samples have been stirredbefore each test for 20 minutes by means of a Turrax T25Digital Homogenizer with adjustable speed No chemicaldispersant agents were used in order to explore the benefitscoming from the pure addition of nanoparticles

Tests were also carried out to analyse the influence of GOaddition to the base oil onwear behaviour of the steel balldisctribopair through steady state rubbing tests see Section 322For these tests constant values for average hertzian pressuretemperature and speed were chosen

3 Results and Discussion

31 Graphite and GO Characterization Graphite chips andGO nanosheets are shown in Figures 1(a) 1(b) 1(c) and 1(d)respectivelyThe images at highermagnification (Figures 1(b)

ISRN Tribology 3

(a) (b)

(c) (d)












4 ISRN Tribology

022

02

018

016

014

012

010 05 1 15 2

Sliding speed (ms)

119879 = 25∘C

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(a)

022

02

018

016

014

012

010 05 1 15 2

119879 = 50∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(b)

022

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(c)













ISRN Tribology 5

0 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

) 119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(a)

0 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(b)

024022

02018016014012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(c)

024022

02018016014012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(d)

02

018

016

014

012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(e)

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(f)









6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)







SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630


minus12 minus30



ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









ISRN Tribology 3

(a) (b)

(c) (d)












4 ISRN Tribology

022

02

018

016

014

012

010 05 1 15 2

Sliding speed (ms)

119879 = 25∘C

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(a)

022

02

018

016

014

012

010 05 1 15 2

119879 = 50∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(b)

022

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(c)













ISRN Tribology 5

0 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

) 119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(a)

0 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(b)

024022

02018016014012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(c)

024022

02018016014012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(d)

02

018

016

014

012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(e)

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(f)









6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)







SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630


minus12 minus30



ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









4 ISRN Tribology

022

02

018

016

014

012

010 05 1 15 2

Sliding speed (ms)

119879 = 25∘C

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(a)

022

02

018

016

014

012

010 05 1 15 2

119879 = 50∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(b)

022

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

119901 = 168GPa

119901 = 117GPa119901 = 147GPa

120583(mdash

)

(c)













ISRN Tribology 5

0 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

) 119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(a)

0 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(b)

024022

02018016014012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(c)

024022

02018016014012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(d)

02

018

016

014

012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(e)

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(f)









6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)







SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630


minus12 minus30



ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









ISRN Tribology 5

0 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

) 119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(a)

0 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 117GPa03

028

026

024

022

02

018

016

014

012

01

(b)

024022

02018016014012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(c)

024022

02018016014012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

Fric

tion

coeffi

cien

t (mdash

)

119901 = 147GPa

(d)

02

018

016

014

012

010 05 1 15 2

119879 = 25∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(e)

02

018

016

014

012

010 05 1 15 2

119879 = 80∘C

Sliding speed (ms)

SN150

Fric

tion

coeffi

cien

t (mdash

)

SN150 +

119901 = 168GPa

01 wt GO

(f)









6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)







SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630


minus12 minus30



ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









6 ISRN Tribology

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(a)

018

016

014

012

0 1800 3600

Time (s)

120583(mdash

)

(b)







SN150mdashbase oil

600 900

Benchmark

SN150 with 01 wt GO

530630


minus12 minus30



ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









ISRN Tribology 7






SN150mdashbase oil

620 910

Benchmark

SN150 with 01 wt GO

540660


minus13 minus27




1000 1500 2000 2500 3000

Graphite

GO


Wavenumber (1cm)

(au

)



8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









8 ISRN Tribology

4 Conclusions







Acknowledgment


References
















3-














2S3 MoS






ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









ISRN Tribology 9






2







RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









research article graphene oxide nanosheets as effective

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