tribological properties of ws2 nanoparticles under mixed lubrication

Wear 255 (2003) 785–793

Tribological properties of WS2 nanoparticles under mixed lubrication

L. Rapoporta,∗, V. Leshchinskya, I. Lapskera, Yu. Volovik a, O. Nepomnyashchya,M. Lvovskya, R. Popovitz-Birob, Y. Feldmanb, R. Tenneb

a Department of Science, Holon Academic Institute of Technology, P.O. Box 305, Holon 58102, Israelb Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel

Abstract

Recently, it has been established that WS2 and MoS2 nanoparticles (inorganic fullerene-like, IF) mixed with oil, and impregnated intoporous matrix of powdered materials appear to enhance the tribological properties of mating surfaces in definite loading range in comparisonto typical metal dichalcogenide solid lubricants. The main results have been obtained under relatively low pressures. It is important toevaluate the tribological properties of IF when the concentrated contact is obtained. The effect of the IF in oil was studied using pin-on-disktester in the regime of mixed lubrication. The interaction between the full film and the asperity contact fractions has been considered andthe time evolution of the friction force was evaluated. The states of the mating surfaces and the nanoparticles before and after the frictiontest were studied by transmission electron microscopy (TEM), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy(XPS). It was established that the IF nanoparticles mixed with oil allow to decrease the fraction of straight asperity contact under mixedlubrication regime and thus improve their tribological properties. TEM analysis showed that the shape of the IF nanoparticles is preservedunder low loads. It was found that some of the IF nanoparticles were damaged after the friction at the maximal load of 420 N. The IFnanopaticles appear to form a protective film allowing increased load capacity of the rubbed pairs. The role of the IF solid lubricants as apart of a third body is discussed in this work. The mechanism of friction and wear of the IF nanoparticles are discussed.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Mixed lubrication; IF nanoparticles; Asperity contact; Friction behavior

1. Introduction

Layered materials such as graphite, MoS2 and WS2(platelets of the 2H polytype) are used both as solid lu-bricants and as additives in liquid lubricants. Minimumtangential resistance is commonly associated with shearingof the weak inter-layer (typically van der Waals, vdW)bonds in these materials. Over the past few years, inorganicfullerene-like (IF) supramolecules of metal dichalcogenideMX2 (M = Mo, W, etc.; X = S, Se), materials withstructures closely related to (nested) carbon fullerenes andnanotubes have been synthesized[1,2]. The main favorablebenefit of the hollow WS2 nanoparticles was attributed pre-viously to the following three effects, named: (a) rollingfriction [3,4], (b) the IF nanoparticles serve as spacer,which eliminate metal to metal contact between the asper-ities of the two mating metal surfaces[3,4], and (c) thirdbody material transfer[5]. IF–WS2 nanoparticles appear tohave excellent tribological properties within a definite loadrange (PV ∼ 150 N m/s) in comparison to typical metal

∗ Corresponding author. Tel.:+972-3-5026616; fax:+972-3-5026619.E-mail address: [email protected] (L. Rapoport).

dichalcogenides[6]. Sliding/rolling of the IF nanoparticlesin the interface between rubbed surfaces seems to be themain friction mechanism at low loads, when the shape ofnanoparticle is preserved. It was found that the beneficialeffect of IF nanoparticles increased with the load. Exfoli-ation of external sheets of IF nanoparticles was found tooccur at high contact loads (∼1 GPa)[7]. The transfer ofdelaminated IF nanoparticles appears to be the dominantfriction mechanism at severe contact conditions. Thus, oneof the main goals of this work was to study friction andwear behavior of the IF nanoparticles under severe contactconditions.

It is known that severe contact conditions, which occur inthe mixed lubrication region, indicate the transition to scuff-ing and seizure. Mixed lubrication is an extremely impor-tant regime of liquid lubrication when both fluid film andboundary lubrication take place, see for example the reviewpaper[8]. Thin lubricant films (1–100 nm) are usually ob-tained under mixed lubrication conditions. If the separationbetween the contact surfaces is very small, nanoparticles offull-size cannot be accommodated at the interface. Severedeformation and fracture of IF nanoparticles are expectedto occur in this case. Therefore, different friction and wear

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0043-1648(03)00044-9

786 L. Rapoport et al. / Wear 255 (2003) 785–793

mechanisms (not sliding/rolling of IF) may become predom-inant over a mixed lubrication regime. The current paperevaluates the effect of IF–WS2 nanoparticles under mixedlubrication conditions.

It has been speculated for years that “third bodies” controlthe friction and wear behavior of low friction solid lubricants[9]. Third body in our case can be considered as a mixtureof oil, solid lubricant nanoparticles and wear particles. Thetribological role of the wear particles and powders of solidlubricants has been considered in the framework of a thirdbody lubrication model in the works of Godet (e.g. Ref.[10]), Berthier (e.g. Ref.[11]), and Heshmat (e.g. Ref.[12]).It is expected that the interaction between oil, spherical IFnanoparticles and the wear particles at the interface betweenthe rubbing surfaces determines essentially the friction andwear behavior of rubbed surfaces.

2. Experimental procedure

The average size of the IF–WS2 particles was close to100 nm (Fig. 1), while that of 2H–WS2 was 4�m. Samplesof the 2H–WS2 powder were milled for 24 h, leading toplatelets with an average size of 0.5�m. Pin-on-disk testwas performed at a sliding velocity of 0.6 m/s and loads of100–500 N. The materials of disk and flat pin were AISI1050 steel quenched and tempered up to a hardness of 420and 240 HB, respectively. Five drops of paraffin oil (60cSt at 30◦C) or oil thoroughly mixed with 5% of IF–WS2nanopowder were fed to the contact each 5 min during thetest. IF–WS2 nanoparticles with average size close to 100 nmwere synthesized according to a procedure described in Ref.[1]. The friction force and wear data were measured in thisexperiment.

In order to evaluate the role of a third body, the wear debrisand solid lubricant particles were rinsed out from the contactsurfaces of the samples by treating the sample in ultrasonic

Fig. 1. AFM image of IF–WS2 nanoparticle on the surface of pyrolytic graphite.

bath with hexane for 30 min. Residue particles from weartests were analyzed by transmission electron microscopy(TEM). The samples were dispersed inn-hexane using anultrasonic bath, three times for periods of 20 min. After eachperiod of ultrasonic treatment the supernatant was removedand a new portion of solvent was added. A drop from the fi-nal dispersion was placed on a carbon/nitro-cellulose coatedcopper electron microscope grid and blotted after 20 s. Inter-missions for examination of the rubbed sample with opticaltransmission microscope and scanning electron microscope(SEM), energy dispersive X-ray spectroscopy (EDS), X-rayphotoelectron spectroscopy (XPS), and with tip profiler weremade from time to time during the run-in period and theexperiment.

XPS measurements were carried out with an AXIS-HSKratos set-up, using a monochromatized Al K� X-ray source(hν = 1486.6 eV) and pass energies ranging from 20 to80 eV. The energy scale was calibrated, with reference to theC 1s line atEB = 284.8 eV. Curve fitting was applied usingGaussian–Lorenzian line shapes and a Shirley backgroundsubtraction. Argon ion beam sputtering, which is a destruc-tive depth profiling technique, was applied using beam en-ergy of 4 keV and 20 mA emission current. The incidentbeam angle was 45◦. A large area raster, 4 mm×4 mm, wasused to verify uniformity across the analyzed area. Sam-ples were sputtered at a rate of 3 nm/min as calibrated on aTa2O5/Ta reference.

3. Results

The time-dependence of the friction force for oil and oil+IF blend at load of 260 N is shown inFig. 2. The averagevalue of the friction coefficient in contact with oil+ IF wasclose to 0.014, while it was 0.02 for the sample rubbed withthe paraffin oil alone. A typical evolution of the frictionforce during 5 min cycles of lubricant feeding is presented

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Fig. 2. The time-dependences of friction force for oil and oil+ IF blendat load of 260 N.

Fig. 3. A typical evolution of the friction force during 5-min cycles oflubricant feeding: (1) paraffin oil; (2) oil+ IF nanoparticles.

in Fig. 3. It may be seen that feeding of the lubricant to thecontact region led first to a decrease in the friction force.The shape of friction force curves resembles an S-shapewith different inclinations for oil and oil+ IF lubricants.The results of the statistical treatment of these curves atdefinite loads will be presented below. The wear of the pinrubbed with oil+ IF lubricant was lower in comparison tothe pin lubricated with oil only at all studied loads (Fig. 4).

Fig. 4. The effect of load on the wear rate of pin lubricated with oil and oil+ IF lubricants.

This effect becomes more predominant with load increasing,suggesting that the favorable role of the IF increases with theload. At a load of 475 N the wear rate of a pin lubricated byoil was more than six times higher in comparison to oil+ IFblend.

In another series of experiments, the lubricant feedingwas interrupted during the steady state friction regime andthe time interval to a friction force jump was evaluated. Itis seen that while the friction force for pair lubricated withoil started increasing after 5 min, 35 min passed before asimilar jump occurred in the case of oil+ IF nanoparticles(Fig. 5). The surface of the pin rubbed with oil+ IF wasfound to be covered by a film. XPS analysis revealed that thisfilm consisted of WS2. The loaded pins were subsequentlyrinsed carefully in ethanol and etched with 3% HNO3 inalcohol. The results of this process are presented inFig. 6.Considerable pitting corrosion on the surface rubbed withoil only is clearly observed, while the pin lubricated withoil + IF exhibited pitting corrosion only in regions, wherethe WS2 film was discontinuous.

The IF agglomerates were shown to get compressed andthey penetrate into the surface layers of soft pin (Fig. 7). Theobserved ploughing tracks can probably be associated withthe dragging of agglomerated and compressed IF nanopar-ticles as well as wear debris in the interface, especially athigh loads. As a result of this effect, the surfaces rubbedwith oil + IF were rougher in comparison to the surfaceslubricated with oil (Fig. 8). The average values of the rough-ness parameter (Ra) for the pin under steady state frictioncondition under a load of 420 N was 0.28 and 0.32�m foroil and oil+ IF, respectively.

Analysis of the solid lubricant particles by TEM, SEMand XPS after wear testing showed that the shape of theIF nanoparticles was preserved at loads up to∼300 N.However, some of the nanoparticles were damaged underfriction at load of 420 N (Fig. 9). Thin nanosheets of de-laminated IF are also observed. Severe damage occurredfor a few single fullerene-like nanoparticles only. In most


Fig. 5. The effect of interruption of oil and oil+ IF feeding to the contact on friction force under steady state friction.

of the damaged nanoparticles only a few external molec-ular sheets of the IF were damaged. This situation mightbe beneficial for the tribological behavior of the contact,since most of the damaged IF will continue to serve assolid lubricants, while the exfoliated layers of the damaged

Fig. 6. The surfaces of the pins after friction with the load of 420 N ((a) oil; (b) oil+ IF) and after etching with 3% HNO3 in alcohol ((c) oil; (d) oil+ IF).

nanoparticles will serve as a part of an efficient transferredfilm.

Raw atomic concentrations was analyzed also by XPSobtained on the surface, lubricated with pure oil and oil+IF–WS2 under load of 420 N pins are presented inTable 1.


Fig. 7. Penetration of the IF nanoparticles and their agglomerates into the surface layers of the soft pin.

Table 1Surface concentrations (at.%) of the steel pins after friction under load of 420 N with paraffin oil and paraffin oil+ IF–WS2

Lubricant Elements and binding energies (eV)a

W 4f S 2p Fe 2p O 1s C 1s

33.2 WS2 35.8 WO3 163 WS2 169 SO2 ∼710 Fe oxide 530.6 284.9

Pure oil 0 0.06 0.1 0.2 7.4 38 52.55oil + IF–WS2 0.1 1.35 0.2 0.96 2.24 25.5 69.6

a Most likely compound for the corresponded chemical state.

The amount of carbon on the surface lubricated with IF–WS2is found to be higher than on the surface rubbed with paraf-fin oil. It is expected that the rougher surface rubbed withIF nanoparticles can be a main cause of this effect. Substan-tial amount of tungsten and sulfur oxides is found on thesurface of the sample treated with IF–WS2. Traces of WS2are found as well. Depth profiling via Ar ion sputtering in-dicates mostly the following order of the layers. The top-most layer consists mostly of a film of physisorbed carbonand oxygen. Beneath this layer one finds a layer enrichedwith non-volatile sulfur–oxygen species, and further downtungsten oxide, tungsten sulfide and finally iron oxide. Thethickness of the layers containing tungsten and sulfur oxidescalculated using an attenuation approach was about 2 nm.

4. Discussion

It is known, the mixed lubrication regime may be de-scribed by two types of friction generating regions: hydro-

dynamic full film regions and asperity regions[8]:

f = Xhfh + XAfA (1)

wherefh, fA are the friction coefficients of the hydrody-namic and asperity contact andXh, XA are the fractions ofthe total contact load supported by fluid and by asperities,respectively. The fraction of the friction force (�Fτ), whichis raised due to increasing asperity contact mode is

Fτ

Fmax= Fτ − Fτ0

Fmax= �Fτ (2)

It may be seen that the kinetics of the friction force changeswith time. For oil and oil+ IF lubricants this curve forms anS-shape curve,Fig. 3, which can be described by Avraamiequation[13] as

�Fτ = 1 − exp(−B�τk) (3)

whereB is the coefficient which depends on the formationof asperity contact during oil depletion,k the coefficient of


Fig. 8. The surface of the pins after friction with (a) oil and (b) oil+ IF. Load P = 420 N.

the exponent, and the�τ the time (5 min in present seriesof experiment). The effect of a load on the coefficientB isshown inFig. 10. It can be seen that under a low load of150 N, the value ofB is same for the lubrication with pureoil and oil+ IF. However, for higher loading,B remains un-changed for the oil+ IF but, it increased in the case of pure

oil. The coefficientk varies in the range of 0.6–1.2 for thetwo lubricants. The results of the statistical treatment of theexperimental data were compared with the theoretical ap-proximation (Fig. 11). In order to compare the friction forcesupon the lubrication with oil or oil+ IF, the results are pre-sented as the change in the friction force,F with respect


Fig. 9. TEM picture of the IF–WS2 nanoparticles after the friction test with load of 420 N.

to their maximal valueFmax. A good agreement betweenthe theoretical calculations and the experimental data is ob-served. The depletion of the oil from the contact leads to aquick increase of the friction force, while the formation of

Fig. 10. The effect of a load on the coefficientsB for oil and oil+ IFlubricants.

an IF film on the metal surface, protects the contact surface,restricting the rise of the friction force. On the basis of thisanalysis, it may be assumed that the addition of IF nanopar-ticles to oil allows to decrease the fraction of straight asper-ity contact and thus to improve the tribological propertiesof pin-on-disk contact pair under mixed lubrication. It maybe expected that the feeding of a lubricant provides origi-nally a continuous full film contact and low friction force,Fig. 3. The thickness of the lubricant film decreases withtime leading to an increase of the asperity contact and thusto increasing the friction force. It may be assumed that theapplied load under the pin-on-disk test is shared betweenfull film lubrication and straight asperity contact and thus amixed lubrication is the dominant friction mechanism underthese conditions. When the thickness of the film becomessmaller than the characteristic size of the IF nanoparticles,IF–WS2 nanoparticles have to delaminate and/or they canbe preserved in the valleys of roughened surfaces. The de-gree of deformation/delamination of the nanoparticles de-pends on the film thickness, the thinner is the film, thelarger is the fraction of delaminated nanoparticles. It may


Fig. 11. The results of statistical treatment and theoretical approximation of the friction force data during 5-min cycle.

be assumed that under friction with a film thickness closeto the size of IF nanopowder (low loads) the shape of thenanoparticles is preserved and sliding/rolling of the spheri-cal IF nanoparticles at the interface seems to be the domi-nant friction mechanism. The oil plays a considerable rolein this case. When the film thickness is lesser than the sizeof nanoparticles (high loads, mixed lubrication), deforma-tion and destruction of the IF nanoparticles have to lead toformation of the transferred IF film on the contact surface.It may be expected that with load increasing, the amount ofdelaminated IF nanoparticles increases leading to formationof more stable solid lubricant film. In this case, the effectof oil is decreased. The IF–WS2 film protects the contactsurface and thus it enhances the wear resistance of contact

Fig. 12. The scheme of third body under friction with IF nanoparticles.

surfaces. Identification of WS2 on the contact surface (XPSand etching); preservation of the undeformed IF nanoparti-cles (TEM analysis), and formation of rough surface understeady friction with load of 420 N, allude to the fact that apart of the IF–WS2 is preserved undamaged in the valleys ofthe contact surface, while another part of the IF nanoparti-cles is delaminated and transferred to the underlying rubbedsurfaces. The IF nanoparticles in the valleys and the de-laminated transferred nanosheets on the surface of contactheights decrease the part of straight asperity contact undermixed lubrication and thus improves the tribological behav-ior of the contact. The decreasing pitting corrosion for thesamples rubbed with the IF nanoparticles (Fig. 6) confirmsthe presence of IF film on the contact surface. Thus, it may


be expected that the delaminated transferred nanosheets ofthe IF nanoparticles on the surface of the asperity crests de-crease the part of straight asperity contact under mixed lu-brication and improves the tribological properties of rubbedsurfaces.

Friction behavior of the IF nanoparticles can be under-stood better by making use of a third body model. Thescheme of a third body under friction of contact pair lubri-cated with oil+IF lubricant is shown inFig. 12. It is expectedthat under severe contact conditions, when the thickness ofthe lubricant film is lesser than that of the size of the IFnanoparticles, the third body consists not only of the oil andwear debris, but also of the delaminated nanosheets of IFnanoparticles. The straight asperity contact is limited by thepresence of IF nanoparticles, which are partially confined inthe valleys of the contacting surfaces and by the transferredfilms of IF at the asperity crests. It is expected that the firstbodies covered with the IF nanoparticles (in valleys or astransferred films) facilitate the shearing of the third body. IFnanoparticles can be furnished from the valleys to the con-tact surface, improving thereby the friction behavior.

5. Conclusions

1. The IF nanoparticles showed the best tribological prop-erties in comparison to pure paraffin oil.

2. The application of IF nanoparticles allows to decreasethe fraction of straight asperity contact in comparison tolubrication with oil only and thus to improve tribologi-cal properties of a pin-on-disk contact pair under mixedlubrication.

3. The surface of the pin rubbed with oil+ IF was found tobe covered by a film. The friction coefficient remainedat low values over long periods of time after interruptionof the lubricant supply.

4. The observed ploughing tracks are associated with theshear of agglomerated and compressed IF particles aswell as wear debris in the interface, especially at highloads.

5. The contacting surfaces were found to be rougher in thecase of oil+ IF than in contact with pure oil only. It isexpected that at very high loads, improvements in thefriction response can be associated with the transfer ofthin delaminated nanosheets.

Acknowledgements

We are grateful to Dr. Rita Rosentsveig for her help withthe synthesis of the IF–WS2 nanoparticles. This work wassupported by grants from the Israeli Ministry of Science(Tashtiot) and the US–Israel Binational Science Foundation.

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tribological properties of ws2 nanoparticles under mixed lubrication

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