Tribology International 33 (2000) 453–459www.elsevier.com/locate/triboint

Role of nitrogen in tribochemical interaction between Zndtp andsuccinimide in boundary lubrication

J.M. Martin a, C. Grossiorda,*, T. Le Mognea, J. Igarashib

a Ecole Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Syste`mes UMR 5513, BP 163, 69131 Ecully, Franceb Central Technical Research Laboratory, Nippon Mitsubishi Oil Corporation, Yokohama 231, Japan

Received 7 October 1999; received in revised form 27 March 2000; accepted 29 March 2000

Abstract

Tribochemical interactions between zinc dithiophosphate (Zndtp) and polyisobutene succinimide (PIBSI) were studied in the mildwear regime by a dual analysis approach. Both TEM analyses of wear fragments and inside wear scar XPS on tribofilms wereobtained in the same location of the wear track. In presence of the succinimide, there are notable modifications in the compositionof the zinc polyphosphate tribofilm which is classically formed in presence of Zndtp alone. Apart from amorphous zinc polyphosph-ate glass, the film contains also large amounts of oxidised species (oxides, sulphates and nitrates) together with residual succinimidefunction groups. The atomic ratio N/P is about 0.3. Moreover, compared to Zndtp, less tribofilm is produced due to some competitionin adsorption process. The results highlight the role of the tribochemical reaction to explain the antagonism between the twoadditives. The presence of some iron oxide in the tribofilm material indicates that the anti-abrasive wear mechanism of Zndtp ishindered by the presence of the succinimide. The lower content of the tribofilm in sulphide species also suggests a lower efficiencyto control the anti-seizure properties of the mixture of the two additives. 2000 Elsevier Science Ltd. All rights reserved.

Keywords:Zinc dithiophosphate; Succinimide; Antiwear mechanisms; Surface analysis; Antagonism; Tribochemistry

1. Introduction

Classically, antagonist interactions between Zndtp andsuccinimide dispersant additives have been observed attwo levels: (i) chemical interaction in the bulk lubricant[1,2] and (ii) competition to adsorption [3,4]. Chemicalinteractions in the bulk have been studied by31P NMR[5]. Results indicate that there is an acid-base reactionbetween nitrogen from the amine organic chain (softbase) of the succinimide and zinc atom in the Zndtp mol-ecule (soft acid). Due to some steric hindering, this canprevent Zndtp from being adsorbed onto the surface andthe tribofilm could hardly be formed in such situation.Another explanation is the modification of the compo-sition of Zndtp tribofilm due to the presence of nitrogen.This study is focused on the study of nitrogen-inducedZndtp tribofilm modifications by means of specific ana-lytical tools.

* Corresponding author.E-mail address:carol.grossiord@ec-lyon.fr (C. Grossiord).

0301-679X/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.PII: S0301-679X(00 )00073-6

2. Tribological parameters

AISI 52100 steel on steel combination was tested ina reciprocating friction and wear tester (Optimol tri-bometer from SRV GmbH) with selected additives inthe base oil, at a temperature of 60°C. The tribologicalconditions were: contact pressure 0.26 GPa, frequency50 Hz, stroke length 1.5 mm and duration of the test 30min. The surface roughness of the flat Ra is equal to 20nm. The friction coefficient was measured during the testand each test was reproduced three times to show thereproducibility. Due to the very low wear rate in thistest, iron oxide content in the tribofilm is used here asa measure of the antiwear efficiency of the lubricant [6].The Zndtp additive corresponds to secondary C3, C6type and the concentration of phosphorous in a GrIII/GrImixture base oil (B) is 950 ppm. The succinimide disper-sant (PIBSI in the following) is a PIBSI used at a con-centration of 1 wt%.

Three tests were compared with the following additiv-ations: B+PIBSI, B+Zndtp and B+Zndtp+PIBSI. Ana-

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lytical investigations were performed at the end of thefriction test and are detailed in the following.

3. Investigation of the tribochemical reaction by adual analysis approach

Our new strategy in analysing tribochemical processesis to compare in the same location of the wear scar, ana-lytical TEM data on wear debris and surface analysis byspatially-resolved XPS of the tribofilm underneath. Thisdual analysis has already been shown to better explainthe tribochemical reaction of the additives combinationwhich is tested [6].

First wear debris are collected at the end of the testand observed in the analytical TEM. The observationmainly consists in examining wear debris collected onthe flat specimen. These debris are thought to representvery small pieces of the additive-induced tribofilm whichis underneath but have participated to the shearing pro-cess. As regards to the specimen configuration and thevery low wear rate in the SRV test, we developed a newmethod to collect tribofilm fragments: we used a laceycarbon film mounted on a copper grid, covered itself bya very thin carbon film (approx. 5 nm thick).

At the end of the test, there was no cleaning procedureof the flat sample before collecting particles. The gridwas gently deposited in the middle of the rectangularwear track, directly in the residual used oil present onthe wear scar. Afterwards, the grid was picked up andimmersed in pure hexane for ten minutes in order toeliminate the residual oil from the carbon film. Solidsmall particles were found to remain stuck on the verythin carbon film and can be examined directly in theTEM. The advantage is here the possibility to examinewear debris supported by a very thin carbon film. This isparticularly suitable for high resolution TEM and EELSanalysis. With this collecting technique, there is practi-cally no perturbation of the flat specimen because onlya few particles are used [6].

EELS spectra and X-ray analysis were performed ona TEM operating at 120 kV accelerating voltage (PhilipsEM-420) with an under-saturated LaB6 filament,equipped with a parallel energy-loss spectrometer (GatanPEELS 666) placed under the column and an X-rayanalysis spectrometer (EDAX 9900). The energy resol-ution of the PEELS spectrometer was measured to be1.1 eV FWHM at the zero-loss peak and the collectionangle was set to 9.5 mrad. As far as quantification isconcerned, the energy dispersion was set to 0.5eV/channel and the acquisition time to 6 s to have a goodsignal/noise ratio.

After the collection of wear particles and before XPSanalysis, the flat was degreased by immersion severaltimes in pure hexane with ultrasonic cleaning, in orderto eliminate all the residual oil and therefore the remain-

ing free wear debris. Some analyses were also performedafter ion etching to observe the chemical compositionusing depth profiling. When using micro-spot XPS onthe flat specimen, the size of the X-ray probe was100×800>m (SSI spectrometer S-probe) so that the spa-tially-resolved analysis can be specifically obtainedinside the wear scar.

Several spectra were successively obtained for eachadditive combination:

1. Inside the wear scar, without any etching but just afterdegreasing the surface in pure hexane. Such a surfaceis generally contaminated with carbon and possiblyoxygen but this does not hinder the elements of theadditive.

2. After 30 s of argon etching (Ar+, 3 keV). The analysisin this case corresponds approximately to the removalof a 1 nm thick layer of material from the surface.

3. After 180 s of etching. Approximately 6 nm havebeen removed from the original surface. As a rule andin this particular case, the interpretation of XPS spec-tra is limited by the fact that chemical species maybe reduced by the ion bombardment. However, ironoxides and phosphates are usually not reduced to ironmetal in our working conditions.

We studied chemical species for the different elementsincluding phosphorous and nitrogen (from P2p and N1slines), sulphide or sulphate from the S2p line, iron oxide(phosphate) or metal from the Fe2p line. As far as oxy-gen O1s is concerned it is not easy to attribute the differ-ent contributions and results might be questionable.

Each spectrum was fit with Gaussian peaks using aleast-squares curve-fitting algorithm and after sub-tracting a Shirley background. Binding energies havebeen referenced to the C1s binding energy from carboncontamination (284.8 eV) to account for specimencharging.

4. Results

4.1. Friction data

The average values of stabilised friction coefficientsin the three cases (B+PIBSI, B+Zndtp, andB+Zndtp+PIBSI) are 0.10, 0.11, and 0.11, respectively.The friction coefficient is in the 0.1 range for the threeadditivations. It is not possible to find any difference dueto the presence of the dispersant additive in the lubricant.

4.2. Chemical analysis

Fig. 1 shows representative XPS general scans forB+Zndtp and B+Zndtp+PIBSI tests showing theelements present on the surfaces as chemisorbed films

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Fig. 1. XPS spectra of chemisorbed film (a) in the Zndtp case, and (b) the Zndtp+PIBSI case.

(the analyses were performed outside the wear track atthe end of the test). The spectra are dominated by surfacecontamination, nevertheless elements from the additivesare visible. The (B+Zndtp) chemisorbed film containsphosphorous and zinc and the iron signal is still visible(not shown). The (B+Zndtp+PIBSI) chemisorbed filmshows a lower phosphorous content and the nitrogenpeak becomes clearly visible. The data strongly suggestthat chemisorption of Zndtp is hindered by the presenceof PIBSI in the formulation.

The detailed elementary composition of tribofilms aresummed up in Tables 1 and 2. In the presence of PIBSI:(i) clearly, there is a decrease in the zinc and phosphor-ous content of the tribofilm, (ii) iron oxide wear debrisare present due to the PIBSI antagonist action, and (iii)the tribofilm contains nitrogen and more carbon in thebulk. The molecular composition was verified by decom-posing each peak into Gaussian functions. The O1s pho-topeak is particularly interesting because it contains

Table 1Elemental composition of Zndtp tribofilm at the top surface and after180 s of ion etching. The data were obtained after the treatment of theXPS spectra

Additivation Zndtp

Etching time (s) 0 180

C 52.8±5.5 19.0±2.0Elemental composition (%)O (oxide) / 8.8±0.8O (phosphate) 31.9±3.0 36.3±3.5P (phosphate) 9.5±0.9 13.4±1.2S (sulphide) 2.4±0.3 1.8±0.2Zn 3.4±0.4 9.9±1.0N / /Fe (metal) / 5.5±0.5Fe (oxide) / 5.3±0.6

Table 2Elemental composition of Zndtp+PIBSI tribofilm at the top surface andafter 180 s of ion etching. The data were obtained after the treatmentof the XPS spectra. Compared to Table 1, a decrease of zinc phosphatecontent is observed

Additivation Zndtp+PIBSI

Etching time (s) 0 180

Elemental composition (%)C 40.4±4.0 34.2±3.3O (oxide) 13.4±1.2 16.7±1.5O (phosphate) 28.5±3.0 23.5±2.2P (phosphate) 5.2±0.4 3.3±0.3S (sulphide) 2.0±0.3 1.5±0.2Zn 1.1±0.2 1.6±0.2N 2.3±0.3 1.7±0.2Fe (metal) 0.6±0.1 9.2±0.8Fe (oxide) 6.5±0.7 8.3±0.9

information on the presence of phosphate, but also oniron oxide chemical forms which is characteristic ofpossible abrasive wear contribution. Typical O1s spectrawith their decompositions are displayed in Fig. 2. Firstpeak near 530.1 eV characteristic of iron oxide is absentin the case of Zndtp but is still visible in the presenceof Zndtp+PIBSI. In the case where Zndtp is absent(B+PIBSI), the film is mainly composed of iron oxidesand/or hydroxides. Further peaks, near 531.7, 532.8 and534.2 eV are assigned to phosphate for Zndtp but caninterfere with other various oxidised species includinghydroxides, sulphates and nitrates for Zndtp+PIBSI.

The study of sulphur S2p photopeak in Fig. 3 showsthat it is only sulphide in the case of Zndtp but bothsulphate and sulphide are visible when succinimide isadded to the antiwear additive. The nitrogen N1s XPSsignal is more difficult to interpret and specific standardsare needed. Fig. 4 shows the comparison of N1s peaks

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Fig. 2. XPS O1s peak recorded at the top surface of (a) the PIBSI tribofilm, (b) the Zndtp tribofilm, and (c) the Zndtp+PIBSI tribofilm. Notethe absence of oxides in the case of the Zndtp tribofilm. When adding PIBSI (c), a small amount of oxide is detected.

Fig. 3. XPS S2p peak recorded at the top surface of (a) the Zndtp tribofilm, and (b) the Zndtp+PIBSI tribofilm. Note the presence of sulphatein case (b).

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Fig. 4. XPS N1s peak recorded at the top surface of (a) a deposit of PIBSI, (b) the PIBSI tribofilm, and (c) the Zndtp+PIBSI tribofilm. Note thepresence of nitrates in case (c).

for pure succinimide (PIBSI) as a deposit on steel,(B+PIBSI) and (B+Zndtp+PIBSI) tribofilms, respect-ively. First we show that nitrogen is present in the tri-bofilm composition at a relatively high concentration, theP/N atomic ratio is about 3 (see Table 1). Second wecan see from Fig. 4 that nitrate is present in the tribofilmassociated with a decrease of the nitrogen-containingPIB organic chain contribution (compared with puresuccinimide). The succinimide group seems to be presentinside the tribofilm structure. Moreover a small nitridepeak can be detected near 398 eV.

As far as Zndtp alone is concerned, the tribofilm iseffectively composed of long chain zinc polyphosphate,with some sulphide contribution certainly in the form ofZnS precipitates. Actually, the Auger spectra of zinc arethe sum of one peak assigned to Zn–O bond (oxide orphosphate) and another one to Zn–S. The structure of thetribofilm can be viewed as a phosphate glass of generalformula xZnO(12x)P2O5. with a mole fractionx of ZnOverifying the following equation [7]:

Bridging oxygen/non-bridging oxygen5P–O–P/P–O−

50.5(324x).

Quantification of the O1s spectrum gives a mole frac-tion near 0.51. The energies of the peaks depending onthe mole content of ZnO, our results are then in agree-

ment with literature on polyphosphate glasses forx near0.5 [7].

Unfortunately, the chain length of the polyphosphatepart of the (B+Zndtp+PIBSI) tribofilm is difficult to esti-mate and would be questionable because of the inter-ference of the P–O–P oxygen peak with some sulphatechemical forms. Similar experiments have been made inthe literature and analysed by IR and they show adecrease of the chain length of the polyphosphatechain [8].

4.3. Analysis of wear fragments

All wear particles have a similar morphology for agiven test and seem very homogeneous in chemical com-position. They are very thin and flaky (below 50 nmthick but thicker than the residual tribofilm).

As far as Zndtp alone is concerned, the wear particlesare mainly composed of zinc, phosphorus, oxygen, a fewatomic per cent of sulphur, and iron impurities. The par-ticle material is thought to be mainly a zinc polyphosph-ate glass and is completely amorphous by electron dif-fraction. Iron was practically not detected revealing verymild wear conditions.

Typical wear particles from (B+Zndtp+PIBSI) test aredisplayed in Fig. 5. Their chemical composition was

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Fig. 5. TEM image of wear particles from (a) PIBSI tribofilm, and (b) Zndtp+PIBSI tribofilm.

determined by coupling EELS, which is particularlysensitive to light elements and X-ray analysis. Theadvantage of X-ray analysis is to permit the contri-butions of phosphorus and sulphur to be clearly dis-tinguished, because they are superimposed on EELSspectra. Moreover, iron and zinc are more visible in X-ray analysis than in EELS. Fig. 6 shows a typical EELSspectrum of such a film fragment. The elemental ratiosZn/N are calculated from these spectra and is found tobe about 0.5. It is difficult to determine whether the par-ticles contain carbon or not because the signal on theEELS spectrum mainly comes from the thin carbon filmwhich supports the particles. Nevertheless, by comparinga spectrum on the wear debris with a spectrum besidethe wear debris, the contribution at the carbon K-edgeis found to be very similar in both cases, indicating thelow carbon content in the film material. Moreover, theiron impurities are not in the iron oxide form since nocharacteristic peak of Fe2O3 (532.0 eV) at the oxygenK-edge could be detected. This is slightly different fromthe analysis of the adherent tribofilm where some oxideis detected.

5. Discussion

As already mentioned in previously [9], the antiwearmechanism of Zndtp alone can be decomposed in severalcontributions depending on the severity of the wear test:

1. In the mild wear conditions, the lubrication action ismainly due to rheological properties of the long-chainzinc polyphosphate film which is generated on the

Fig. 6. EELS spectrum of wear fragments presented in Fig. 5. No iron is detectable, but nitrogen, oxygen and zinc are displayed.

surface. Actually, the transition temperature of theglass phosphate is about 200–300°C whereas themelting points of metal oxides or sulphides is in the1000°C range. Some kinds of viscous flow of themagma state glass is thought to separate efficientlythe two metal counterfaces.

2. When tribological conditions become more severe,acid-base reactions govern the tribofilm composition.For example, any iron (or transition metal) oxideabrasive particles are immediately eliminated by thephosphate glass. Short-chain mixed Fe/Zn poly-phosphate are then formed and can continue to controlrheological properties in the contact zone.

3. In severe conditions (extreme pressure), nascent metalsurfaces can be produced by the removal of the tri-bofilm itself by the wear process. In this case, thesulphide species present in the tribofilm can quicklyreact with the metal (acid-base reaction) and adhesivewear can be avoided. The iron sulphide formed insmall precipitates that are highly dispersed in thephosphate glass.

The analytical data that we obtained in the case of themixture Zndtp+PIBSI can be interpreted in the light ofthe antiwear mechanisms of pure Zndtp. First there issome competition in the chemisorption of the two addi-tives to the steel surface and this leads to smaller quan-tities of phosphate prior to friction. As already shown inthe literature, an explanation could be the formation ofa complex due to the acid-base reaction between Zndtpand PIBSI in the bulk lubricant [10]. As a consequenceof the presence of both phosphate and PIBSI as adsorbedspecies, the tribochemical reaction generates a tribofilm

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which is found to be a mixture of short chain zinc poly-phosphate, succinimide organic species and oxidisedcompounds including sulphates, oxides and nitrates.

TEM analyses of the morphology of wear fragments(Zndtp+PIBSI and Zndtp) suggest that rheologicalproperties of the tribofilms and the mechanisms ofremoval are quite similar. At the opposite, XPS analysisof the solid residual tribofilm shows that it is thinner andcontains amounts of iron oxide. Therefore it is thoughtthat, in the mild wear regime, there will be small pertur-bation of the lubrication mechanisms due to the presenceof impurities in the phosphate glass. Of course, in moresevere operating conditions, our analytical data stronglysuggest that acid-base reactions will be hindered tocounteract the presence of abrasive iron oxides (by reac-tion with long chain polyphosphates) and nascent metalsurfaces (by reaction with sulphides). Consequently, theantagonism will be much more important in severe con-ditions, particularly to combat both abrasive andadhesive wear.

6. Conclusion

Tribochemical interactions between Zndtp and PIBSIhave been investigated in boundary lubrication by meansof the dual analysis approach. Both TEM characteris-ation of film fragments and inside-wear scar analysis byXPS of the residual film have been carried out, respect-ively. Results on the mixture have been compared to theeffect of individual additives in the base oil. Results arethe as follows:

1. Friction of Zndtp is not affected by the presence ofPIBSI in mild wear conditions. The steady-state fric-tion coefficient is around 0.1 in all cases and repro-ducible.

2. Chemisorption of Zndtp on steel at 60°C is hinderedby the presence of the PIBSI. The result is a decrease

of phosphorous content and the presence of nitrogenin the adsorbed film.

3. In presence of pure Zndtp, zinc polyphosphate is themain constituent of the tribofilm. This result is inagreement with previous data in the literature,

4. In presence of PIBSI, iron oxide tribofilm is producedand no nitrogen is found on the surface.

5. The tribofilm composition in the presence ofZndtp+PIBSI is complex. Zinc polyphosphate hasbeen identified but contains high level of impurities.Both oxidised species (including sulphates andnitrates) and residual succinimide organic compoundshave been depicted in the film composition. Ironoxide is present in the film but not in the wear par-ticles.

6. Although tribological properties of Zndtp are notstrongly disturbed by the presence of PIBSI in ourmild wear conditions, our data suggest that antagon-isms are predictable in more severe conditions. Bothreduction of abrasive wear by metal oxides and pas-sivation of nascent metal surfaces by sulphide specieswill be hindered. Experiments are under considerationto confirm this point.

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