Synergistic effects in binary systems of lubricant additives - Springer Link

Tribology Letters 8 (2000) 193–201

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Synergistic effects in binary systems of lubricant additives: a chemical hardness approach Jean-Michel Martin a , Carol Grossiord a , Karine Varlot a , B´eatrice Vacher a and Jinichi Igarashi b a Ecole

Centrale de Lyon, Laboratoire de Tribologie et Dynamiques des Syst`emes, UMR 5513, BP 160, F-69131 Ecully, France b Central Technical Research Laboratory, Nippon Oil Co., Ltd., Yokohama 231, Japan

Received 27 March 1999; accepted 12 June 1999

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and detergent overbased calcium borate (CB) lubricant additives have been investigated by coupling analytical TEM and micro-spot XPS in the tribotester Optimol of SRV GmbH (mild wear conditions in boundary lubrication). Synergistic effects have been observed on both friction and wear data, especially in the Modtc/Zndtp combination. Results have been interpreted on the basis of a chemical hardness concept: the hard and soft acids and bases (HSAB) principle, stabilisation of hard–hard pairs and the maximum hardness principle. The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single sheets and the digestion of MoO3 , which is also formed, by the zinc polyphosphate glass. The final result of the tribochemical reaction is a tribofilm composed of MoS2 sheets embedded in a mixed Mo/Zn polyphosphate glass. The CB/Modtc mixture has a similar mechanism except that the oxide is not completely eliminated, due to the softer borate anion compared with the phosphate one. Keywords: zinc dithiophosphate, molybdenum dithiocarbamate, calcium borate, synergistic effects, transmission electron microscopy, X-ray photoelectron spectroscopy

1. Chemical hardness and friction in boundary lubrication Whereas dry friction of metals is more concerned with physical, metallurgical, and mechanical properties of materials in the bulk, boundary lubrication in the presence of additives is mainly governed by chemical processes on surfaces. The role of acid–base reactions in tribology and particularly tribochemistry has already been emphasized in the literature [1]. Under boundary lubrication with additives, acid–base reactions can occur at three levels: (i) chemical interactions between additives in the bulk lubricant, (ii) competition for adsorption of additives to surfaces, and (iii) tribochemical interactions in tribofilm formation. Once the classification of hard and soft species is known, chemical reactions and especially exchange reactions can be then predicted. For example, it has been shown that the origin of an antiwear/dispersant antagonism is due to the fact that zinc dithiophosphate (Zndtp) chemically reacts and forms a complex with PIB succinimide in the bulk lubricant phase [2] and this further can prevent the additive from being adsorbed on the surface. The chemical bonding between Zn2+ cations in the Zndtp molecule and nitrogen atoms in the succinimide molecule (electron donors) is typically an acid–base reaction between two chemical species. Also, the exchange reaction in the lubricant between Zndtp and molybdenum dithiocarbamate (Modtc) which has been evidenced recently in the literature [3] is favorable due to stabilization of hard–hard pairs (molybdenum dithiophosphate (Modtp) in this case because Mo6+ is a harder Lewis acid than Zn2+ ).  J.C. Baltzer AG, Science Publishers

The HSAB principle has been applied with success to chemical reactions in solution but the extension of the concept of chemical hardness by Pearson [4] indicates that it can be applied to many other situations even when no liquids are present (adhesion problems in metal–polymer interactions, for example). Adsorption of additives from the lubricant to the contacting surface can also be considered. Molecules containing soft function groups (succinimide or sulfide as bases, for example) will adsorb preferentially on soft solid surfaces (nascent metal surfaces are soft bases). The softer the surface, the more reactive it is. Nascent metal surfaces created by friction are chemically softer because excited states are present. For example, Mori [5] has shown that friction-activated gold can react immediately with alkanes. We have shown recently that friction-induced acid–base reactions can exist in ultrahigh vacuum conditions [6]. As described recently by Pearson in his book [4], chemical hardness could be understood as the ability of electron clouds of atoms to be deformed in an electric field (change in electron distribution). By comparison, physical (or mechanical) hardness is more concerned with changes in nuclei motion (or change in nuclear position) due to a mechanical action. An equilibrium system should have the greatest resistance for both of these properties. The more the polarizability of the large-sized electron cloud, the more covalent is the bonding and the softer is the species from the chemical point of view. Sliding between two atomically smooth crystal surfaces consists in interactions between electromagnetic fields of the atoms of the two counterfaces. Atomic resolution

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achieved in the friction force microscope (FFM) is good evidence for direct visualization of interactions between electron clouds of sliding individual atoms [7]. From the chemical point of view, electron charge, atom size, electronegativity, orbital overlap, and steric repulsion determine bond energies, and therefore contribute to friction forces between sliding atoms. Following this simple reasoning, sliding surfaces made of hard chemical species (typically ionic bonds) will need a lot of energy because electron clouds are difficult to deform chemically and valence electrons are held so tightly that loss of electrons is difficult. From a chemical point of view, that means that friction between such surfaces will give a rather high shearing force (phosphate/phosphate or oxide/oxide are good examples in our case). In contrast, soft species are made of atoms having large and easily deformable electron clouds, generating more covalent bonding. Smaller force interactions might be expected during sliding because surfaces made of soft bases materials are easily deformed, sheared and even oxidized (sulphide/sulphide case is a good one). An exception is high friction between metals (soft acids) due to the specificity of the metallic bond. In this latter case, friction will be high due to major contributions of plastic deformation and mechanically mixed layers [8]. Friction of diamond can be just the opposite of the rule because of the presence of dangling bonds. But if reconstruction of the diamond surface takes place by heating in vacuum, for example, therefore friction drops down to very low values [9]. Consequently, as a rule of thumb, and in absence of a lot of plastic deformation (mild wear lubricated conditions), we assume that friction between surfaces composed of chemically hard–hard species will give a high friction coefficient whereas friction between surfaces containing soft–soft species will give lower friction. The case of friction between surfaces containing both hard and soft pairs is very interesting and should be intermediate (example phosphate/sulfide). The HSAB principle gives additional information about the stabilization of hard–hard and soft–soft pairs and destabilization of hard–soft ones. At this stage, we assume that in the case where different species are present on sliding surfaces, the extension of the HSAB principle to tribology stipulates that friction-induced exchange reactions will take place and will lead to stabilization of harder (hh) and softer (ss) pairs. Finally, resulting (hh) against (ss) are favorable triboreactions in order to give the lowest dissipative structures (i.e., soft–soft pairs on both surfaces). This can be done by elimination of the (hh) species as wear debris, for example. For some of these acid–base reactions to occur, transfer phenomena are often necessary [10]. Of course, this refers to a stabilization of hard–hard or soft–soft pairs and this purely chemical and surface contribution to friction and wear reduction has to be added to other relevant contributions such as frictionoriented two-dimensional crystal structures (case of MoS2 , for example), atomically smooth and physically hard surfaces, low energy surfaces, etc. which are also favorable.

We are interested here in the interactions between antiwear, friction modifiers and overbased detergent additives, namely: Modtc, Zndtp, and micellar overbased calcium borate detergent (CB). We are especially interested in studying tribochemical interactions in binary systems: Zndtp/CB, Modtc/Zndtp and Modtc/CB which show some synergistic effects in both friction and wear reduction in mild wear conditions. The results are mainly based on the characterization of friction-induced tribofilms by analytical highresolution transmission electron microscopy (TEM) on wear fragments and inside wear scar X-ray photoelectron spectroscopy (XPS) on surfaces. We want to show that a chemical hardness concept can explain and predict the tribochemical reactions involved in these different cases.

2. Experimental The Zndtp additive corresponds to a secondary C3 , C6 type and the concentration of phosphorus in oil is 950 ppm. Calcium borate overbased detergent is used at 0.20 wt% in the synthetic base oil, a GrIII/GrI mixture in order to have better solubilisation of Modtc. The structure of micellar CB is described in the literature [11]. Typically, colloidal CB particles have a 5 nm amorphous calcium borate core surrounded by a salicylate detergent molecule (sulphurfree). The Modtc additive that we used here is mainly composed of di-sulfide-bis [oxo(dialkyldithiocarbamate)] molybdenum. It contains some impurities including 10 at% thiuram disulfide. The alkyl chains are C8 (2-ethylhexyl) and C13 . The ratio S/Mo (mass%) is equal to 1.3 (corresponding to 950 ppm Mo in oil). An AISI 52100 steel on steel combination was tested in a reciprocating friction and wear tester (Optimol of SRV GmbH) with additive in the base oil at a temperature of 60 ◦ C. The tribological conditions were: contact pressure 0.26 GPa, frequency 50 Hz, stroke length 15 mm, and duration of the test 30 min. This corresponds to boundary lubrication in mild wear conditions and it is expected that surface effects are preponderant in these conditions. Our strategy in analyzing tribochemical processes is to compare in the same location of the wear scar, analytical TEM data on wear debris and surface analysis by spatiallyresolved XPS of the tribofilm underneath. This dual analysis is thought to better explain the tribochemical reaction of the additive combination which is tested. Each tribofilm was carefully analyzed by XPS and analytical TEM carried out on wear fragments using the dual analysis method that we developed recently [12]. Wear debris were collected at the end of the SRV test and observed in the analytical TEM (JEOL 2010F TEM operating at 200 kV accelerating voltage). The observation mainly consists in examining wear debris collected inside the wear scar of the flat specimen. We used a holey carbon film mounted on a copper grid, covered itself by a very thin carbon film (approximately 5 nm thick) which is particularly suitable for high-resolution TEM and electron energy


loss spectroscopy (EELS) analysis. There was no cleaning procedure of the flat sample before collecting particles and the grid was gently deposited in the middle of the rectangular wear track, directly in the residual oil present on the wear scar. Afterwards, the grid was immersed in pure hexane for 10 min in order to eliminate the residual oil from the carbon film. Solid small particles were found to remain stuck on the very thin carbon film and can be examined directly in the TEM. With this collection technique, there is practically no perturbation of the flat specimen. After the collection of wear particles, the tribofilm was examined by micro-spot XPS. Before XPS analysis, the flat was degreased by immersion several times in pure hexane with ultrasonic cleaning, in order to eliminate all the residual oil and the remaining wear debris. When using microspot XPS on the flat specimen, the size of the X-ray probe was 100 × 800 µm (SSL XPS spectrometer) so that the spatially-resolved analysis can be specifically obtained inside the wear scar. Before etching, such a surface is generally contaminated with carbon and possibly oxygen but this does not hinder the detection of the elements of the additive. After 30 s of argon etching (Ar+ , 3 keV) the XPS analysis corresponds approximately to the removal of a 1 nm thick layer of material from the surface. Special attention has been paid to the Mo 3d photopeak. After 180 s of etching, approximately 10 nm have been removed from the original surface.

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responds to the steady-state value after 2000 cycles, at the end of the test. Wear was not measured directly on the flat samples due to very mild wear conditions. The iron content in the wear particles is a good indication of the antiwear action of the solution. Data in table 1 show that the effect of the Mo-compound friction modifying additives is quite clear: after a transient period of a few cycles only, the friction fst drops down below 0.05, in the so-called ultralow regime, whereas Zndtp additive gives friction above 0.1 (0.15). As far as friction is concerned, there are synergistic effects in the binary combinations (equimolar) Modtc/Zndtp, Modtc/CB. The mixture Zndtp/CB does not give friction reduction comparable with Modtc containing mixtures, only a small decrease to the CB level (0.1) is observed at the end of the test (table 2).

4. Tribofilm characterization by TEM and micro-spot XPS for individual additives Typically, tribofilms derived from the SRV tribotester are composed of both multilayers and composite nanomaterials with an average thickness less than 40 nm. First, let us consider the case of the additives when used separately (Zndtp, Modtc or CB). Here we give a summary of analytical data that we generated. XPS and TEM results will be more developed and detailed in the case of the binary systems. In the case of an additive used separately, results obtained by analytical TEM show that tribofilms are mainly composed of an amorphous phase (see table 1). Zinc polyphosphate glass is formed from Zndtp with a very low sulfur content and no iron is visible in the electron diffrac-

3. Friction results Friction data for the different additive combinations are shown in tables 1 and 2. The friction coefficient fst cor-

Table 1 Tribofilm composition and performance of individual additives in the SRV test. Antiwear

Friction

Matrix

Tribofilm composition Impurities

level

coefficient

Zndtp

Zinc polyphosphate Zn2 P10 O31 (> 80%)

ZnS or thiophosphate

Very good

0.15

Modtc

Carbon-based (C > 50 at%)

Fe2 O3 (10%) +MoS2 single sheets (2–3%) +MoO3 crystallites (2–3%)

Poor

0.05

Calcium borate Ca9 (B7 O15 )2 (> 80%)

Fe2 O3 (2–3%) + CaCO3

Good

0.10

CB

Table 2 Tribofilm composition and performance of binary systems of additives in the SRV test. Antiwear

Friction

Matrix

Tribofilm composition Impurities

level

coefficient

Zndtp/CB

Calcium/zinc borophosphate Ca4 Zn3 P4 B4 O21 (> 80%)

Fe2 O3

Good

0.10

Modtc/Zndtp

Zinc/molybdenum polyphosphate + carbon-rich zones

MoS2 single sheets + ZnS

Very good

0.04

Calcium borate Ca9 (B7 O15 )2 (> 80%)

Fe2 O3

Good

0.04

Modtc/CB

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tion spectroscopy (EDS) spectra. Carbon-based material is formed by Modtc in which some nanometer scale sulfide species are dispersed. In the case of Modtc, bi-dimensional

Figure 1. High-resolution TEM micrograph of a wear fragment obtained in the presence of Modtc as an additive. MoS2 flexible single sheets are visible together with MoO3 crystallites.

MoS2 single sheets have been clearly observed by highresolution TEM (HRTEM) and characterized by EELS [13] but MoO3 and iron oxides particles are also present indicating some wear of the steel surfaces. Figure 1 shows a HRTEM micrograph of a wear fragment in the case of a Modtc tribofilm showing the presence of many MoS2 single sheets and MoO3 precipitates in a carbon-rich matrix. The generation of MoS2 material is responsible for the friction reduction in the ultralow regime by a single sheet lubrication mechanism [13]. In the case of CB, the tribofilm is composed of a calcium borate glass with a low iron content. EELS and EDS analysis suggests a general formula for the different tribofilm materials in the case of CB and Zndtp. Results are summarized in table 1. XPS data are in agreement with TEM data and show that the elemental compositions of the tribofilms are quite homogeneous in depth and that the film thickness is approximately 40 nm. Some additional data, however, were obtained from XPS and detailed in the following (see figures 2–4): – For Zndtp, sulfur (about 3 at%) is in the sulfide form, possibly zinc thiophosphate in the polyphosphate network or/and zinc sulfide from AES lines in the XPS spectra (figure 2).

Figure 2. S 2p XPS deconvoluted spectra recorded on tribofilms for different additive combinations. Note that sulfur is pure sulfide only when Zndtp is present.


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Figure 3. O 1s XPS deconvoluted spectra recorded on tribofilms for different additives combinations. The abrasive contribution from metal oxides is eliminated by Zndtp and Modtc/Zndtp combinations.

– For CB, a few per cent of iron in the tribofilm is both in borate bonding (B 1s photopeak at 192.2 eV) and iron oxide form (O 1s peak), presented in figure 3. – For Modtc, MoS2 (Mo IV from Mo 3d deconvolution) is present at about 3 at% in the carbon-rich matrix, residual Modtc (Mo V), MoO3 (Mo VI) and iron oxides are also confirmed (figure 4). Sulfur appears in both sulfide and sulfate forms and some nitrate could also be present from the deconvolution of the O 1s peak (figure 3). Unfortunately, any nitrogen (N 1s) present in the film from the Modtc molecule would interfere strongly with the Mo 3p peak. The composition of the carbon matrix is not easy to define more accurately. 5. Tribofilm characterization for binary systems Binary systems have been studied following the same procedure as individual additives, as well as for TEM and XPS analysis. Results are summarized in table 2. First of all, the Zndtp/CB combination is a very interesting case because calcium and zinc borophosphate glass has been detected by EELS/EDS analysis of the tribofilm

Figure 4. Mo 3d XPS deconvoluted spectra recorded on tribofilms for different additives combinations and for two reference compounds (h-MoS2 and MoO3 powder). Note that a MoS2 -rich layer is present on the top surface, in the presence of Modtc/Zndtp.

material. Moreover, XPS profiling results indicate a gradient composition with a phosphorus-rich film first covering the steel surface, on which a boron-rich glass is present (table 3). The friction coefficient approaches the value of pure CB at the end of the test (0.1) whereas it is slightly higher (0.15) at the beginning of the test (similar to pure Zndtp). XPS analysis of the oxygen photopeak (figure 3) indicates that some residual iron oxide is still present in the glass. The ability of borate (and borophosphate) to digest iron oxide is less than that of the phosphate [12]. But the wear results are quite good in regard of the important decrease of the phosphorus content of the oil. The Modtc/Zndtp combination leads to both a composite and bilayer tribofilm material. Two glassy phases are depicted: (i) a mixed zinc/molybdenum phosphate glass, and (ii) a carbon-rich amorphous phase. The mixed phase is thought to be due to the action of Modtp which is known to appear in the lubricant phase by exchange reactions [3].

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J.-M. Martin et al. / Synergistic effects in binary systems of lubricant additives Table 3 Bilayer structure of tribofilms in the case of binary systems of lubricant additives. Evaluation of atomic ratio of elements P, B and C. Etching time

30 s 180 s

Zndtp

CB

Modtc/Zndtp

Modtc/CB

P/C

B/C

P/C

Zndtp/CB B/C

B/P

P/C

B/C

0.64 0.69

0.61 0.72

0.67 0.69

0.25 0.13

0.37 0.18

0.27 0.33

0.27 0.45

Schematic of the tribofilm

Figure 5 shows a HRTEM picture of MoS2 sheets embedded in the phosphate glass. EDS analysis in the nanoprobe mode (electron beam size: 0.6 nm) indicates that MoS2 is not oxidized and is actually protected by the anti-oxidant phosphate matrix. XPS profiling shows that the phosphate film is at the steel interface. The oxygen O 1s photopeak (figure 3) indicates that no metal oxide is visible. So the synergistic effect on both friction (generation of MoS2 ) and wear (elimination of abrasive iron oxides) is clearly shown from the analytical point of view. CB/Modtc tribofilms also have a bilayer structure, as shown by XPS profiling. The borate-rich layer is first formed covered by the carbon-rich one (table 3). MoS2 single sheets are also depicted in the matrix by HRTEM. It is not easy to determine if a mixed calcium and molybdenum borate glass has been effectively formed in this case. The observation of a O 1s photopeak shows a small contribution of iron oxide in the film.

6. Friction and wear reduction mechanisms by stabilization of chemically soft–soft friction pairs In our framework, the tribochemical mechanisms by which Mo-compounds reduce friction can be explained on the basis of the HSAB principle (or chemical hardness concept) [4] and we propose here that this principle may be directly extended to tribological processes in boundary lubrication. Acid–base reactions are governed by three principles: (i) The well known HSAB principle. Basically, the principle states that hard acids (electron acceptors) prefer to coordinate with hard bases (electron donors) to produce ionic compounds whereas symmetrically, soft acids prefer to coordinate with soft bases to give more covalent species. (ii) The HSAB principle also predicts the stabilization of hard–hard and soft–soft pairs and destabilization of hard–soft ones by cation exchange reactions, for example. (iii) The principle of maximum hardness stipulates that an equilibrium system should have both maximum chemical and physical (or mechanical) hardness.

Table 4 Classification on the basis of the HSAB principle of different species. The ones present in the tribofilms are bold. Acid Hard

H+ ,

Fe3+ ,

RPO+ 2 ,

Base

Mo6+ ,

RSO+ 2 ,

NO+ ,

Na+ ,

CO2

Borderline

Fe2+ ,

SO2 , Zn2+

Soft

Cu+ , Fe0 , RH3 , RO+ , metal atoms, bulk metals

OH− ,

2− , H2 O, CO2− 3 , O 3− − PO4 , ROH, RO , SO2− 4 − 2− BO3− 3 , N3 , N2 , SO3

R2 S, S2− , RNC, C2 H4 , S2 O2− 3

Table 4 shows the Pearson classification of the different species concerned in this study. According to the HSAB 3− principle (i), phosphate (PO3− 4 ) and borate (BO3 ) anions are hard (h) bases and will prefer to react strongly with iron or molybdenum cations (Fe3+ , Mo6+ ) which are hard (h) acids. On the other hand, soft bases (s) such as sulfides (S2− ) will prefer to coordinate with pure metals (Fe0 ), typically soft acids (s). From these analytical results, we can give information on the chemical pathways by which friction induces chemical changes. Although our test with pure Zndtp was in the mild wear regime, the example of Zndtp tribochemistry is here developed in more detail to explain how abrasive wear by iron oxide particles can be eliminated by the glass zinc polyphosphate. A possible route for the elimination of 1 mol Fe2 O3 would be as follows: θ,τ ,P

5Zn(PO3 )2 + Fe2 O3 −−−→ Fe2 Zn3 P10 O31 + 2ZnO (ZnO, P2 O5 ) (Fe2 O3 , 3ZnO, 5P2 O5 ) (1) This is an illustration of principle (ii) of stabilization of hh and ss pairs as stated by Pearson [4]. Reaction (1) is a cation exchange process between Fe3+ (as Fe2 O3 ) and Zn2+ (as ZnO). The digestion of the oxide in the phosphate glass (having lower transition temperature) can already explain the benefit in terms of wear protection by some hydrodynamic or EHD lubrication by the molten glass. Moreover, hematite, and more generally all iron oxides, are much harder than zinc oxide (approximately twice in Moh’s scale [14]). The reaction can continue if more iron oxide


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Figure 5. HRTEM micrograph of a wear fragment from the Modtc/Zndtp combination. EDS analysis in the nanoprobe mode of the MoS2 sheet and the phosphate matrix, respectively. Note that MoS2 sheets are preserved from oxidation when embedded in the phosphate matrix.

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is liberated by the wear process, for example: 2Fe2 Zn3 P10 O31 + 3Fe2 O3 (Fe2 O3 , 3ZnO, 5P2 O5 ) θ,τ ,P

−−−−→

5Fe2 Zn(P2 O7 )2 + ZnO (Fe2 O3 , ZnO, 2P2 O5 )

(2)

θ,τ ,P

The ZnO in excess which is produced may also react with a long-chain zinc polyphosphate according to the following reaction and be eliminated, for example: θ,τ ,P

5Zn20 P38 O115 + 14ZnO −−−−→ 19Zn6 P10 O31 (20ZnO, 19P2 O5 ) (6ZnO, 5P2 O5 )

(3)

In extremely severe conditions, the global reaction would have been: θ,τ ,P

Zn(PO3 )2 + Fe2 O3 −−−−→ 2FePO4 + ZnO (ZnO, P2 O5 ) (Fe2 O3 , P2 O5 )

on the other hand), MoO3 and possible iron oxides will immediately react with zinc polyphosphate to produce mixed Mo/Zn/Fe polyphosphates well before any abrasive wear appears. For example, the digestion of 1 mol of molybdenum trioxide by the zinc phosphate can be written as follows:

(4)

The case of CB should be very similar to the case of phosphate except that borate is chemically softer than phosphate, and the ability to react with iron oxide and digest it in the glass network will be limited. This is the reason why some iron oxide content is visible in the tribochemical reaction products obtained in the presence of CB additive. XPS data on the O 1s photopeak recorded on the tribofilms formed with individual additives are shown in figure 3 and give strong evidence for that. The case of Modtc additive is much more complex. Actually, the molecule can be degraded by the friction process. A possible chemical pathway has been proposed [13]: [R2 NCS2 ]2 Mo2 O2 S2 → [R2 NCS2 ]2 + Mo2 S2 O2 The formation of thiuram disulfide comes from electron transfer in the thiomolybdyl part of the molecule. But due to the presence of molecular oxygen in the lubricant solution, a possible reaction could be: Mo2 S2 O2 + 12 O2 → MoS2 + MoO3 This is the mechanism of genesis of the molecular MoS2 unit, forming the growth of single sheets. Unfortunately, molybdenum trioxide is an abrasive powder material and the wear substrate will be worn even if MoS2 reduces friction. Moreover, the nascent iron surface formed by abrasion is likely to react with molecular oxygen in the base oil and abrasive iron oxide species will also be produced. Moreover, MoS2 may be further oxidized to sulfate by molecular oxygen and the anti-friction effect will be reduced. Actually, high melting point hard oxides cannot be eliminated when only Modtc is used. No reaction is predicted between these oxides and the carbon-rich film. The combination Modtc/Zndtp gives several new possibilities in terms of acid–base reactions. Although the two additives can act separately for a part (Zndtp producing the zinc phosphate glass on the one hand and Modtc producing both MoS2 and MoO3 in the carbon-rich amorphous phase

5Zn(PO3 )2 + MoO3 −−−−→ Zn3 MoP10 O31 + 2ZnO (ZnO, P2 O5 ) (3ZnO, MoO3 , 5P2 O5 ) By this way, abrasive wear is eliminated by this acid– base reaction. The presence of molybdenum in the zinc phosphate matrix by EDS/TEM (probe diameter 0.6 nm) in figure 5 is in agreement with such a chemical reaction. The atomic ratio Mo/Zn = 0.33 is in good agreement with the one calculated from the X-ray spectra of the matrix. A consequence of the maximum hardness principle (iii) is that chemically hard–hard pairs (Fe2 O3 , MoO3 in our case) are also mechanically harder than soft–soft pairs (ZnO or ZnS, MoS2 , etc. . .). Moreover, this prevents MoS2 sheets from oxidation and this is well visible in the XPS S 2p photopeak carried out on the tribofilm (figure 2) in the different cases (Modtc, Zndtp, and Modtc/Zndtp). It is clear that pure sulfide (absence of sulfate) is formed only when phosphate is present. 7. Conclusion Tribochemical interactions between binary systems of lubricant additives have been studied in the SRV tribometer by coupling analytical TEM on wear fragments and microspot XPS on wear surfaces. Three combinations have been investigated: molybdenum dithiocarbamate (Modtc)/zinc dithiophosphate (Zndtp), Zndtp/overbased calcium borate (CB), and Modtc/CB. Results are the following: (1) Zndtp and CB as individual additives are antiwear by the formation of a zinc phosphate glass and a calcium borate glass, respectively. Friction is in the 0.1 range. (2) Modtc is an efficient friction-reducing additive (f = 0.05) by the formation of MoS2 as single sheets. Molybdenum trioxide is also formed as an abrasive material and the additive is generally pro-wear. (3) The equimolar mixture Zndtp/CB is antiwear by the formation of a mixed calcium and zinc borophosphate tribofilm. Friction is about 0.1, similar to the value for the individual additives. (4) The Modtc/Zndtp combination is excellent in reducing both friction and wear at the lowest level. The mechanism involves the formation of a tribofilm composed of MoS2 single sheets embedded in a mixed molybdenum and zinc polyphosphate glass. (5) Modtc/CB interactions are very similar to the previous case with the formation of MoS2 in a calcium and molybdenum borate glass. However, some wear increase is observed compared to the Modtc/Zndtp case, due to the persistence of iron oxide abrasive species.


(6) All tribofilm material reactions observed can be explained on the basis of the HSAB principle, the stabilization of hard–hard pairs and the maximum hardness principle. For example, in the Modtc/Zndtp case, the acid–base reaction between MoO3 and the zinc polyphosphate glass is responsible for the elimination of the abrasive contribution and the preservation of pure MoS2 from oxidation. The main advantage of the chemical hardness concept is its powerful predictive character in a very simple way. Acknowledgement The authors would like to thank the Consortium Lyonnais de Microscopie Electronique (CLYME, INSA-ECLCNRS) for access to the FEG/TEM microscope and the firm “Science et Surface SA” for the micro-spot XPS analysis. References [1] C. Kajdas, in: Proc. ITC Yokohama Satellite Forum on Tribochemistry, Yokohama (1995) p. 31.

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[2] H. Mansuy, Ph.D. thesis, University Paris VI (1995). [3] M.D. Johnson, R.K. Jensen and S. Korcek, SAE Technical Paper No. 972860 (1997) 37. [4] R.G. Pearson, in: Chemical Hardness (Wiley/VCH, New York/ Weinheim, 1996). [5] S. Mori, in: Proc. ITC Yokohama Satellite Forum on Tribochemistry, Yokohama (1995) pp. 37–42. [6] C. Grossiord, J.-M. Martin, Th. Le Mogne and Th. Palermo, Surf. Coat. Techn. 108–109 (1998) 352. [7] S. Fujisawa, Y. Sugawara, S. Ito, S. Mishima, T. Okada and S. Morita, Nanotechnology 4 (1993) 138. [8] D.A. Rigney and J.E. Hammerberg, MRS Bulletin 23 (1998) 32. [9] M.N. Gardos, Tribol. Lett. 4 (1998) 175. [10] J.-M. Martin, Th. Le Mogne, C. Grossiord and Th. Palermo, Tribol. Lett. 3 (1997) 87. [11] J.-M. Martin, J.-L. Lavergne, B. Vacher and K. Inoue, Microsc. Microanal. Microstruct. 6 (1995) 53. [12] K. Varlot, J.-M. Martin, C. Grossiord, R. Vargiolu, B. Vacher and K. Inoue, Tribol. Lett. 6 (1999) 181. [13] C. Grossiord, K. Varlot, J.-M. Martin, Th. Le Mogne, C. Esnouf and K. Inoue, Tribol. Int. 31 (1998) 737. [14] D.R. Lide, ed., Handbook of Chemistry and Physics (CRC Press, New York, 1997) pp. 4–130.