tribochemistry in the analytical uhv tribometer

Tribology International 32 (1999) 617–626www.elsevier.com/locate/triboint

Tribochemistry in the analytical UHV tribometer

J.M. Martin a,*, Th. Le Mognea, M. Boehmb, C. Grossiorda

a Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Syste`mes, UMR 5513, 36 avenue Guy de Collongues, BP 163, 69131Ecully, France

b Pechiney, Centre de Recherches de Voreppe, Centr’Alp, BP 27, 38340 Voreppe, France

Abstract

The knowledge of tribochemical reactions in boundary lubrication in the presence of lubricant additives is fundamental in orderto improve chemical engineering of new molecules, multifunction compounds and to predict interactions between these additives.Experiments with in lubro, ex situ or post-mortem analyses are difficult and even dangerous to exploit because of numerous artefacts.We introduce two special approaches to simulate boundary lubrication tribochemistry. Basically, these approaches use a dedicatedanalytical UHV tribotester allowing in situ AES/XPS analysis of both counterfaces and introduction of partial pressure of gasesduring friction. The first model experiment is UHV friction on previously formed tribofilms from a lubricated test. The secondexperiment is gas phase lubrication with ethyl phosphate. The results reveal some fundamental aspects of the origin of friction-reduction by transfer of MoS2 single sheets. The reaction of friction-activated nascent surfaces with gases is also emphasised.2000 Elsevier Science Ltd. All rights reserved.

Keywords:Friction; Tribochemistry; UHV tribometry; Boundary lubrication; Additives

1. Introduction

During the last decade, increasing demands in themechanisms of lubricant additives and coatings haveresulted in the development of new friction machinescoupled with surface analysis capabilities [1,2]. Sincethe pioneering work of Buckley [3], ultrahigh vacuum(UHV) friction machines attracted a huge amount ofattention from tribologists. Many UHV tribometers havebeen built in the last 20 years to study space applicationsand resistance of coatings in vacuum and various atmos-pheres (see for example [4,5]). In our laboratory, wedeveloped a new analytical tribotester and new methodsdedicated to studies in the field of tribochemistry andboundary lubrication. Specifically, this tribometer allowsin situ Auger electron spectroscopy (AES)/X-ray photo-electron spectroscopy (XPS) surface analysis and imag-ing inside and outside wear scars of both friction coun-terfaces, to be performed. What is more, gas moleculescan be introduced into the chamber and this can be doneat any time of the duration of the friction test.

At first sight, experimental modelling of boundary

* Corresponding author. Tel.:+33-4-72-18-62-83; fax:+33-4-78-43-33-83.

0301-679X/99/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0301-679X(99 )00090-0

lubrication using UHV tribometry could be consideredvery audacious and to go too far. Several arguments,however, can be considered to justify this approach, thatwe have developed in more detail.

1.1. Using UHV friction on previously formedtribofilms

First we would like to emphasise that any attempt toinvestigate carefully the chemistry of friction surfaces byex-situ and post-mortem analyses is always questionable.Although many do, it is practically impossible, in alubricated wear test, to analyse properly the chemistryof the first atomic layers present on the two sliding coun-terfaces. The reason is that it is necessary to separatethe two bodies before analysis so that the worn surfacesimmediately interact for a certain time with the lubricantcontaining dissolved oxygen, as well as additives in sol-ution. As a result, at least carbon contamination of onemonolayer or more will immediately occur. More prob-ably oxidation will take place and even adsorption orfurther static chemical reaction of the additives with thetribofilm material is also likely to occur. After typicalcleaning procedures and before AES/XPS analysis, mostof the residual liquid lubricant will be effectivelyremoved, but the surface chemistry which has been

618 J.M. Martin et al. / Tribology International 32 (1999) 617–626

modified due to both contact with the environment andcarbon contamination by the solvent itself, is impossibleto eliminate completely. Therefore it is not possible tocertify that the surface chemistry of post-mortem tri-bofilms is really representative of that which it wasinside and during the lubricated contact. Even if nochemical reaction with the environment occurs, a car-bon-rich contamination layer will strongly attenuate theXPS photopeaks of interest. Unfortunately, it is wellknown that ionic etching of surfaces markedly changesthe chemical bonding of the top first atomic layerspresent on these surfaces. This is particularly drastic inthe case of molybdenum organic compounds which areknown to be chemically reduced by argon etching tonon-stoichiometric species and even to metallic molyb-denum in certain cases [6].

Now the question is: how can we have a better evalu-ation of the in situ chemistry of the surfaces during slid-ing? Although some direct approaches by in situ analysisof a sliding contact by X-ray absorption spectroscopy(XAS) and Raman spectroscopy have been tested, itappears that the data generated are not yet exploitabledue to huge technological difficulties [7]. We choosehere a more indirect and simplified way to reach theinterface chemistry that we explain in the following.First of all, we assume that boundary lubrication isessentially due to action of polar molecules which arechemisorbed on the surface and which then penetrate thecontact zone. In a sense, boundary lubricated rubbingsurfaces are so close to each other that no free (nonadsorbed) molecules coming from the lubricant phasecan enter between these surfaces.

Our basic idea is now to assume that a steady-statelubricated test in the presence of additives can be exper-imentally modelled by a short friction test on the tri-bofilm which is formed, but in ultrahigh vacuum con-ditions. Another alternative way of thinking is to imaginethat the lubricant is suddenly removed and replaced byhigh vacuum for a few seconds during a running lubri-cated friction test. During this short period, it is likelythat the presence of the solid-like antiwear tribofilm willcontinue to insure the durability of the test and that thismodification will induce a very small perturbation inthe test.

Let us further imagine that during this short periodwhere friction occurs in UHV, we stopped the frictiontest. Then the two friction surfaces can be analysed byspectroscopic tools with good confidence because therewill be neither changes due to contamination nor toionic etching.

This represents an indirect way to analyse in situ thetribochemistry of the two counterfaces with limited tri-bological perturbations. In summary, it is then temptingto consider that such a friction experiment in ultrahighvacuum on such tribofilms may simulate a snapshot ofa boundary lubricated contact, but suddenly isolated

from its surrounding environment. Moreover, evaluationof the durability of this tribofilm can be easily obtainedin these conditions.

1.2. Gas phase lubrication

Second, an advantage of vacuum conditions is toallow partial pressures of gases to be introduced withvery accurate control measurements using a mass spec-trometer. Morecroft [8] observed that fresh surfaces suchas evaporated films decomposed paraffinic compoundsat ambient temperature. Moreover, it is well known thatfresh metallic surfaces generate many chemical reactionsin heterogeneous catalysis and this is of industrial use[9]. Mori et al. [10,11] reported that tribological nascentsurfaces created by scratching are able to decomposemolecules even at ambient temperature. Tribologicalnascent surfaces are believed to be far more reactivethan, for instance, single crystals. This is because theirreactivity may be enhanced by the presence of many sur-face defects.

Many tribologists have pointed out that due to thishigh chemical activity, interactions between nascent sur-faces and lubricant components may be very importantfor the formation of tribofilms and therefore for the pro-tection of tribological surfaces in boundary lubrication.

Our analytical tools enable an accurate definition ofthe tribological surfaces before and after friction experi-ments. Moreover, the surface can be generated in situand friction can be performed on specific oxides,hydroxides, or metallic surfaces. As a result, it is poss-ible to eliminate many of the confusing phenomenawhich may otherwise modify the reaction pathways bywhich additives are efficient. Gases with specific chemi-cal functional groups can then be introduced and thiscan simulate the tribochemical reactions of anti-wear andfriction modifying additives under boundary lubrication.

Of course the two approaches described here are notexclusive and the effect of selected gases on friction onpreviously formed tribofilms can also be considered withour apparatus. The application of the two in situapproaches is illustrated by studying the mechanisms ofaction of phosphate-based lubricant additives. Twomodel experiments are developed here:

1. UHV friction on lubricant-derived tribofilms frommolybdenum dithiophosphate (Modtp),

2. gas phase lubrication of steel by ethyl phosphate (etp).

2. The AES/XPS analytical UHV tribotester

This tribometer (Fig. 1) associates friction experi-ments with very well controlled environment and surfaceanalytical tools with a few nanometers depth sensitivity.

619J.M. Martin et al. / Tribology International 32 (1999) 617–626

Fig. 1. Schematic of the ultrahigh vacuum analytical tribometer at ECL. (1) Pin-on-flat computer-controlled tribometer, (2) electron spectrometer,(3) electron gun for AES and SEM, (4) X-ray source for XPS, (5) ion gun for etching, (6) gas partial pressure inlet.

The device consists of two chambers in which a pressurelower than 1027 Pa can be achieved. The main chamberis a sphere with the samples in the centre, the surfaceanalytical tools on the top, and the tribometer in thelower part. A preparation chamber with several stagesfor surface treatments is also available but was not usedhere. Partial pressures of pure gases can be introducedinto the main chamber by a leak valve in order to modifythe environment surrounding the friction experiment.The pressure can be varied from 1027 to 10 Pa. At lowpartial pressure, the purity of the gas is monitored by aresidual gas analyser.

The tribometer is a reciprocating pin-on-flat frictionmachine (Fig. 1). The flat is fixed on a vertical shaftattached to a XYZ manipulator. The X-axis is motorisedfor linear reciprocating motion. A special transducer hasbeen developed to measure the force between 0.1 N and5 N on the vertical part. The temperature of the flat canbe controlled from2192°C (liquid nitrogen) to 550°C.The pin is held on a rotating shaft fixed to a horizontalXYZ manipulator, and another transducer measures theforce in the horizontal direction (0.1–5 N). The X-axisof the manipulator is motorised and is used to apply aload between the pin and the flat. The motors are com-puter controlled and the force signals are recorded bythe same computer [12]. Software has been developedto process the data and particularly for obtaining tribos-copic images [13].

Tools for surface analysis are attached to the top ofthe main chamber. Auger electron spectroscopy (AES)

has an electron probe size down to 0.5µm. X-ray photo-electron spectroscopy (XPS) is performed with a nonmonochromatised source and has a dual anode forMgKα or AlKα irradiation. The area of the X-ray spotis around 1 cm2. The electron spectrometer (VG 220i)includes a set of inlet and outlet lenses and an energyanalyser. The electrons emitted by the surface of thesample during AES or XPS analysis are collected by theinlet lenses, energy filtered by the analyser, and detectedby 6 chaneltrons at the outlet. The solid angle and theapertures of the inlet lenses can be changed thanks totwo diaphragms. In this way, it is possible to reduce theanalysed zone to a size down to 60µm, permitting insitu XPS analysis to be obtained inside the wear scars.The detection area can be accurately localised thanks toan optical alignment procedure.

Classically, ion etching is used for cleaning beforeanalysis or for depth profiling. The argon ion beam canbe scanned and has a 100µm diameter size. Scanningelectron microscopy (SEM) can be performed with theelectron gun and the secondary electron detector, withlateral resolution near 0.5µm. Scanning Augermicroscopy (SAM) also uses the electron gun in connec-tion with the electron spectrometer. SAM images areobtained by scanning the electron beam and recordingenergy-filtered Auger electrons by the spectrometer.Therefore, chemical elemental maps are obtained withlateral resolution of about 1µm. An alternative, lesstime-consuming, technique is to record only AES spectraat each point by scanning on one line (line-scan AES),


with the same resolution of 1µm. X-ray photoelectronmicroscopy (XPM) is performed by using special inletlenses to focus the image of the sample at the entranceof the analyser [14]. So, chemical maps can be recordedon a CCD camera with a lateral resolution lower than15 µm in a relatively short acquisition time.

All the analyses and imaging modes are not only poss-ible inside the wear scar of the flat specimen but alsoinside the pin wear scar, by rotating the pin holder. Thisunique capability is essential for studying the role oftransfer phenomena in boundary lubrication [15].

3. UHV friction on Modtp-derived tribofilm

The additive used in this study is a molybdenum thi-omolybdyl dithiophosphate (Modtp) and some prelimi-nary data have already been described in previous papers[16,17]. Following the same procedure, Modtp tribofilmswere generated by sliding a cylindrical pin on a flat ina Cameron–Plint friction machine. The substrate is anAISI 52100 steel flat [18], polished with a 3µm diamondpaste, and immersed in a solution of Modtp at a concen-tration of 1% by weight in a PAO synthetic lubricantbase. The friction test was performed at 353 K under aload of 350 N for 1 h. The steady-state friction coef-ficient was recorded during the test and the typical valueis about 0.05. The test was reproduced five times toobtain several tribofilms under the same conditions. Atthe end of the test, the flat samples were washed in hex-ane and propanol. The solid antiwear film formed (calledModtp tribofilm in the following) covers a rectangulararea of 8 mm×4 mm and is approximately 60 nm thick.The UHV pin-on-flat friction test was directly performedon this tribofilm.

The friction curve (Fig. 2) recorded on the Modtp tri-bofilm at 1027 Pa (UHV conditions) presents an induc-tion period of a few cycles and the friction coefficient

Fig. 2. UHV friction curve recorded on a previously formed Modtptribofilm. Note the presence of the ultralow friction regime. For thefollowing surface analyses, the friction test was stopped after 20 cycles(friction coefficient near 0.04).

reaches a steady-state value of 0.04 (ultralow friction),which is very close to the value obtained in the lubricat-ing friction test. The induction period corresponds to theestablishment of a transfer film on the pin, originatingfrom the initial tribofilm on the flat [16].

To find any relationship between the friction-reducingproperties of Modtp tribofilm and the corresponding sur-face film chemistry, several friction experiments wereperformed and stopped at different stages of the test. Thepin wear scars were observed by various analytical tech-niques available as previously described: SEM, AESline-scans, AES elemental mapping, XPS micro-spotanalysis, and AES depth profiles. The results are herefocused on the nature of the transfer film on the pin after20 cycles of friction, when the friction coefficient wasat its lowest value (m=0.04). Fig. 3 presents severalimaging modes of the pin wear scar, i.e. optical, SEMand SAM maps. Using optical and in-situ scanning elec-tron microscope, the pin wear scar is hardly visible, onlysome debris can be seen at the corona of the Hertziancontact. SAM chemical maps of sulphur, molybdenum,phosphorous and oxygen were obtained exactly at thesame place where SEM was performed. An RGB imageis shown in Fig. 3 (Mo+S (A), P+O (B) and O+Fe (C)).The figure demonstrates that no phosphorous and practi-cally no oxygen are detected in the Hertzian contactzone, only a molybdenum sulphide film is present in thecontact zone. This transfer film could not be observedby ex-situ SEM indicating that it is very thin. The ratiobetween the peak heights of sulphur and molybdenum(S/Mo) in the AES derived spectrum is about 9 (notshown), in agreement with the value obtained when ana-lysing pure h-MoS2 [19]. Surrounding the wear scar, thewear debris contains mainly phosphorous and oxygen.

Fig. 4 shows high energy resolution spectra of theOKLL AES peaks extracted from line-scan sequenceacquisition. We observe the presence of a chemical shiftfrom 508.5 eV (phosphate bonding evidenced by a PLMM

line in this spectrum) to 512 eV (oxide bonding withouta phosphorous Auger peak). No charge effect is possiblebecause CKLL and FeLMM Auger lines are exactly at thesame energy in the two spectra. Although not referencedin the literature, this position of the OKLL peak is veryconvenient for giving information on the chemical stateof oxygen, and chemical maps indicate that phosphatemolecules are only localised in wear debris in the coronasurrounding the Hertzian contact zone.

To determine the thickness of the transfer film, anAES depth profile inside the pin wear scar was perfor-med (Fig. 5). To make a comparison, the same depthprofile was recorded simultaneously outside the wearscar, where the native oxide layer of steel is still present.Results indicate that the transfer film inside the wear scarsputters away in one-third the time of the native ironoxide layer. This oxide layer is approximately 3 nmthick, as evidenced by the detection of the metallic XPS


Fig. 3. Images of the pin wear scar after 20 cycles of UHV friction on Modtp tribofilm (see Fig. 2). (a) Optical image, (b) in situ SEM image,and (c) in situ Auger elemental mapping in the RGB mode. (A) Mo+S, (B) P+O (as phosphate), and (C) Fe+O (as oxide). Results indicate that amoly-sulphide film has been transferred from the tribofilm. In vacuum, the image is slightly distorted due to detector position.

Fig. 4. OKLL Auger spectra recorded (a) outside the pin wear scar (iron oxide), (b) inside the wear debris in the corona (phosphate), and (c) insidethe wear scar (no oxygen). Note the chemical shift between oxide (512 eV) and phosphate (508.5 eV) chemical bondings.

Fe2p photopeak through the layer. Although the sputterrates of iron oxides and molybdenum sulphide might bequite different, we conclude that the transfer film thick-ness is probably no more than one or two nanometersand is only composed of sulphur and molybdenum. Thiscould correspond to two or three MoS2 layers (each layeris 0.7 nm thick). What is more, the native oxide layeron steel has been totally removed and the transfer filmis found to be directly in contact with pure iron whichis underneath.

Finally, micro-spot XPS analysis was performedinside the pin wear scar and compared to data obtainedon a crystal of pure h-MoS2 (see Fig. 6). The stoichi-ometry of the Mo–S compound in the wear scar was

indeed very close to that of pure MoS2. The position ofthe Mo3d peak recorded inside the transfer film is closedfrom the one recorded on pure h-MoS2 but a shift ofabout 1 eV towards lower binding energy is observed.The same trend is observed for the S2p photopeak butseems to be lower (about 0.5 eV). A charge effect canhardly explain the data in the case of such a thin film onmetal and for different shift values in the same spectrum.Consequently, this phenomenon may be attributed to anelectron charge transfer from molybdenum atoms to itsneighbouring sulphur ones (due to Fe–S bonding), ingood agreement with data from a study on Fe–Mo–Scompounds [20].

This set of data obtained in the analytical tribometer


Fig. 5. AES depth profiles obtained in two areas: (A) inside the pin wear scar (moly-sulphide), and (B) outside the pin wear scar (iron oxide).Results indicate that the moly-sulphide film thickness is smaller than the oxide one and is estimated to 1 nm. Note also that the sulphide film isdirectly bound to pure iron (no native oxide layer in the interface).

Fig. 6. (a) Inside wear scar micro-spot XPS analysis of Mo3d photo-peak, and (b) Mo3d XPS peak of pure h-MoS2 for reference. Notethe 1 eV chemical shift attributed to interaction between sulphur andiron atoms.

speak to us directly and we can now state with goodconfidence that MoS2 single sheets have been transferredfrom the tribofilm material to the nascent iron on thepin. This MoS2 transfer is thought to be responsible forthe friction decrease by Modtp tribofilms. In a previouspaper [16], the authors have also studied the very firstcycles of friction and shown that the action of phos-phates was to remove the native oxide layer on the pin

surface. The overall mechanisms were explained on thebasis of the chemical hardness approach as described byPearson [21]. First an acid–base reaction occurs betweenFe3+ (hard acid) and PO3−

4 (hard base). In a second reac-tion, S22 (soft base) reacts with metal iron Fe0 (softacid).

4. Gas phase lubrication of steel by ethylphosphate

In this second application of UHV analytical tribome-try, the main issue was to lubricate the contact by introd-ucing the lubricant components as gases or vapoursdirectly inside the UHV chamber during friction. Mol-ecules can be adsorbed on surfaces prior to friction orintroduced as a vapour phase during friction. The secondalternative has been chosen since it enables the ambientpressure to be modified and therefore control of the mol-ecule concentration at the sample surface is possible.However, because the molecules have to be in a gaseousstate, only light molecular weight compounds can beconsidered.

Many possibilities have already been envisaged withusing this approach, for example: oxygen gas at 10 Papartial pressure was found to lubricate silicon carbide byformation of silica and release of graphite-like material[22], hydrogen sulphide H2S was used to lubricate mol-ybdenum metal by creating a thin MoS2 film in the con-tact area [23,24], hexane and hexene were introduced


into the chamber and hexene was found to form poly-mer-like film on the surface of aluminium [25]. Phos-phate and sulphide gases can also be studied [11] buttheir reactivity towards nascent metal surfaces has notbeen investigated in detail by surface analysis.

Ethyl phosphate was commercial reagent gradeorganic compound. Because heavy molecular weightcompounds are difficult to introduce into the vacuumchamber, triethyl phosphate was used as a model forextreme pressure additives. The organic compound wasdegassed by a repeated freeze–thaw technique and themolecules were introduced into the vacuum chamberthrough a variable leak valve. In this way pressure inthe vacuum chamber can be varied from 10 nPa to 1 kPaand therefore the molecule concentration at the samplesurface can be varied. Below 1 mPa, the system wascontinuously pumped during introduction of the gas andthe partial pressure was measured with a Bayard–Alpertion gauge. At higher pressures the vacuum chamber wasisolated from the pumping system and the pressure wasmeasured by a capacitance manometer. The compositionof the admitted vapour was controlled at low pressureby using a mass spectrometer.

The flat sample was made of cold rolled and annealedsteel representative of roller material in metal workingapplication (IF–Ti steel: interstitial-free steel containingtitanium). The specimen was not highly polished and theroughness has a Ra of about 1.1µm. Prior to frictionexperiments and in order to completely remove the oxideand contaminant layers, the steel surface was sputter-cleaned by several cycles of in situ argon ion bombard-ment (5 kV, 2µA, 40 min). The cleanliness of the sur-face was verified by Auger spectroscopy (OKLL and CKLL

peak heights were less than 5% of FeLMM). Hence,chemical reactions inside the wear track and on sputteredmetallic surfaces can be compared. We used a hemi-spherical pin with a radius of 3 mm. The pin (AISI52100 steel) was only solvent-cleaned and its contami-nation was checked by AES prior the friction experi-ment.

All friction experiments were performed using a nor-mal load of 0.5 N and a sliding speed of 0.5 mm/s. Sev-ere mechanical conditions were applied (contacts wereelasto-plastic) in order to create nascent metallic sur-faces. The friction coefficient was recorded during 100cycles. Experiments were performed at different partialpressures of ethyl phosphate and for two different tem-peratures (25°C and 200°C) of the flat sample. At agiven temperature, the friction tests for different press-ures were carried out on different flat and pin samples.To investigate the chemical nature of the surfaces, bothin situ XPS and AES were performed. AES was parti-cularly appropriate for this study of the tribochemicalreactions inside wear scars of both the pin and the flat.All analyses were done in the same vacuum chamber asthe friction experiments (eventually after pumping).

In Fig. 7, we compare evolution of the mean frictioncoefficient against the pressure of ethyl phosphate gasin the chamber. Below a partial pressure of 1027 Pa thefriction behaviour of the sputter-cleaned surface is cata-strophic with a very high (1.5) and fluctuating frictioncoefficient. The wear track is severely worn and is typi-cal of the seizure between the two counterparts. Thewear scar on the pin is covered with an extensive met-allic iron transfer as evidenced by AES (not shown).Therefore, nascent metallic surfaces were effectivelycreated on steel surfaces but did not sufficiently reactwith the gas molecules at this low pressure to ensurelubrication of the system. In view of this disastrous wearand friction behaviour, any beneficial effect of introduc-ing molecules should be readily observable. A significantdecrease of the friction coefficient is observed at a tran-sition pressure of about 1 Pa for the two specimen tem-peratures. At higher pressure (10 Pa), the friction coef-ficient decreases down to 0.2, which actuallycorresponds to the value recorded in a lubricated testwith pure liquid ethyl phosphate in a surrounding ambi-ent air (see the value in Fig. 7 noted liquid etp) [25].

The analytical tools enable the lubrication mech-anisms by gases to be better understood. At the end ofthe test at 10 Pa pressure and 300 K, AES spectra wereobtained on the flat inside and outside wear scar. Fig. 8shows a comparison of the two AES spectra. It clearlyindicates the presence of phosphorous and oxygen in thetribofilm material on the rubbing surface, whereas thesputter-cleaned surface is partially oxygen free(O/Fe|0.1, compared to iron oxide where O/Fe|1.6).One can notice that outside wear scar surfaces areslightly sulphurised (certainly due to residual gas con-taminants in the chamber from previous experiments). Itis interesting to notice that this phosphate film itself isnot contaminated by sulphur and that phosphate itselfdid not react at all with sulphide. Results clearly showthat a tribofilm has been formed but only in the con-tacting zone of the steel sample. So, there is a very local-ised reaction of the phosphate gas with the nascent steelsurface on the contact area. In Fig. 9, AES maps of PKLL ,CKLL , OKLL and FeLMM Auger lines illustrate the forma-tion of the patchy film on the plateaux of the slidingsurfaces on the flat sample. This particular patchy mor-phology of the film on the steel wear scar observed onthe flat is due to the effect of the initial roughness ofthe steel surface. It indicates that friction only occurredat the top of asperities. AES line-scans across the wearscars on both counterfaces (not shown) confirm that thesurface composition of the tribofilm is mainly composedof phosphorous, oxygen and iron. This set of data clearlydemonstrates that mechanical effect as friction isrequired for tribofilm formation, and that ion-sputteredfresh surfaces do not present the same chemical proper-ties as friction-activated ones. Similar data were obtainedwith the H2S/molybdenum system [24] and MoS2


Fig. 7. Gas phase lubrication by ethyl phosphate (etp). Evolution of the friction coefficient against the partial pressure of gas in the chamber.Friction in air corresponds to lubrication by liquid ethyl phosphate. Note the drastic friction reduction at gas pressure above 10 Pa.

Fig. 8. In situ AES analysis at the end of the friction test at 10 Pa of etp, (a) outside the flat wear scar, and (b) inside the flat wear scar. Sulphuris detected outside the wear scar whereas phosphorous and oxygen are mainly present inside the scar.

material was preferentially detected inside the wear scar.This is a clear demonstration of the tribochemical effectthat is thought to be due to accumulation of energy inlattice defects which can relax by formation of heat,exoelectron emissions or excited states on the surface.

Concerning the chemical nature of the phosphate film,the AES spectrum indicates that it contains a low carboncontent compared to the stoichiometric ethyl phosphatemolecule (C/P atomic ratio is 6). The tribochemical reac-tion with friction-activated iron is believed to have liber-ated the organic part of the phosphate molecule. TheXPS spectrum on the P2p photopeak clearly indicates aphosphate chemical bonding at 133.5 eV, the absence ofphosphide (129 eV), and the absence of iron oxides (O1speak at 530.4 eV, not shown). This is also confirmed bythe chemical shift observed on the OKLL Auger linebetween the oxide form outside the pin wear scar andthe phosphate form inside the pin wear scar (not shown

but like the spectra in Fig. 4). To conclude, the AES/XPSanalyses carried out inside and outside the wear scarspermit a clear understanding of the film formation tobe made.

At a pressure of 1024 Pa, approximately one mono-layer of ethyl phosphate is adsorbed in one second. At1023 Pa, however, this layer appears to have an inci-dence on the friction behaviour compared to UHV (thefriction coefficient goes down from 1.6 to 1 at 475 K).As a result, strongly bonded molecules are probablyrequired to have a notable influence in boundary lubri-cation. Only strongly bonded compounds can penetrateinto the contact and modify the friction behaviour.

Fig. 10 shows optical images of wear scars obtainedat 300 K in UHV (1027 Pa), at 10 Pa of ethyl phosphategas and in liquid ethyl phosphate, respectively. It is veryinteresting to notice that wear scar morphologies undergaseous feed (Fig. 10b) are very close to those obtained


Fig. 9. SEM and SAM images of the pin and flat wear scars corresponding to Figs 7 and 8. The phosphate film is only present in the contactarea, due to friction-activated nascent surfaces.

Fig. 10. Optical images of the wear scar after steel on steel friction, (a) in UHV (1027 Pa), (b) with 10 Pa of ethyl phosphate gas, and (c) inliquid ethyl phosphate. Note the very similar morphology of wear scars obtained under gaseous feed and in the contact lubricated by liquidethyl phosphate.

in contacts lubricated by liquid ethyl phosphate inboundary conditions (Fig. 10c).

5. Conclusion

We present here new methods which attempt to exper-imentally model boundary lubrication. In particular, abetter understanding of the role of antiwear and frictionmodifying additives is expected. The approach is basedon in situ friction experiments in an analytical ultrahighvacuum tribometer where the contact is lubricated eitherby previously formed solid tribofilms or by low molecu-lar weight molecules. These gas molecules are used tosimulate the heavy lubricant components by their func-tional group.

The friction-reducing properties of Modtp additivewere studied thanks to this new UHV analytical tri-bometer. Recent improvement of this device has allowedfurther comprehension of the mechanisms.

In the reciprocating pin-on-flat tribometer, friction-reducing properties of Modtp additives were shown to beassociated with transfer mechanisms of nanometer thickMoS2 layers between the two counterfaces. Once trans-ferred (one or two single sheets originating from thetribofilm), the layer is found to be chemically bound tometallic iron by sulphur atoms. To be able to observeand characterise such a very thin layer, powerful andsensitive in situ analytical tools are needed. AES andmicro-spot XPS were successfully used here.

Elemental imaging allows the localisation of the trans-fer film and of the wear debris to be clearly seen. AsAES and XPS imaging are sensitive at nanometer scaledepth, they can reveal contrasts which can not be seen byoptical or even SEM imaging. The association of frictionexperiments in UHV and powerful analytical tools dataappears to be a very efficient way to highlight the mech-anisms of the friction-reducing effect of organic molyb-denum compounds.

The tribochemical reaction of triethyl phosphate with


iron nascent surfaces has been investigated in the UHVtribometer as a preliminary test. Under UHV (1027 Pa),no film formation is observed by surface analysis andfriction is very high above unity. Above a transition tem-perature, phosphate gas strongly reacts with nascent sur-faces liberating a mineral-rich iron phosphate film onlyon the sliding areas at the top of asperities. Amazingly,no detectable chemical reaction occurred outside thewear scar where sputtered metallic iron was in contactwith the gas.

Moreover, wear scar morphologies under gaseous feedare very close to those obtained in classical lubricatedboundary contacts. Consequently, our approach appearsto model accurately the action of molecules in boundarylubrication and is likely to be a powerful tool in the pre-diction and the understanding of the action mechanismsof boundary additives.

Future work will entail the study of mixtures of differ-ent products such as two active gases, in order to evalu-ate the effects of antagonisms and/or synergies betweenthese compounds. Since reactions are localised insidewear scars, the use of model oxide surfaces would clarifywhether the nascent metallic surface or the tribo acti-vation of the surface governs tribofilm formation.

Acknowledgements

This work was carried out with the financial aid ofthe Research project contract (CPR): “Mise en forme desmateriaux: contact outil–me´tal–lubrifiant”, betweenCNRS, Irsid Usinor, Pechiney Centre de Recherches deVoreppe, ECL (LTDS), INSA de Lyon (LMC), lNSMP(CEMEF), INPT (IMF), College de France (PMC),Universite d’Orsay (LMS) and CNRS (SCA). Theauthors acknowledge M. Belin for the friction experi-ments performed at ambient air. The authors thank theInstitut Franc¸ais du Pe´trole (Dr Th. Palermo) for finan-cial aid and the authorisation to publish the data.

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tribochemistry in the analytical uhv tribometer

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