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Friction and Wear Performance of TetraalkylammoniumThiomolybdates in Aqueous SolutionsJ. Lara-Romero a , F. Chiñas-Castillo b , G. Alonso-Núñez c & Y. Gochi-Ponce ba Chemical Engineering Department , Universidad Michoacana de San Nicolás de HidalgoMorelia , Mich. , Mexicob Mechanical Engineering Department , Instituto Tecnológico de Oaxaca , Oaxaca , Oax. ,Mexicoc Center for Nanoscience and Nanotechnology (CNyN) , Universidad Nacional Autonoma deMexico , Ensenada , B.C. , MexicoPublished online: 31 Jul 2008.

To cite this article: J. Lara-Romero , F. Chiñas-Castillo , G. Alonso-Núñez & Y. Gochi-Ponce (2008) Friction and WearPerformance of Tetraalkylammonium Thiomolybdates in Aqueous Solutions, Tribology Transactions, 51:4, 526-532, DOI:10.1080/10402000802071293

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Tribology Transactions, 51: 526-532, 2008Copyright C© Society of Tribologists and Lubrication EngineersISSN: 1040-2004 print / 1547-357X onlineDOI: 10.1080/10402000802071293

Friction and Wear Performance of TetraalkylammoniumThiomolybdates in Aqueous Solutions

J. LARA-ROMEROChemical Engineering Department

Universidad Michoacana de San Nicolas de HidalgoMorelia, Mich., Mexico

F. CHINAS-CASTILLOMechanical Engineering Department

Instituto Tecnologico de OaxacaOaxaca, Oax., MexicoG. ALONSO-NUNEZ

Center for Nanoscience and Nanotechnology (CNyN)Universidad Nacional Autonoma de Mexico

Ensenada, B.C., Mexicoand

Y. GOCHI-PONCEMechanical Engineering Department

Instituto Tecnologico de OaxacaOaxaca, Oax., Mexico

Aqueous solutions of three tetraalkylammonium thiomolyb-

dates (R4N)2MoS4 (R = methyl, propyl, or hydrogen) were

prepared and their tribological performance was tested using a

pin-on-disc tribometer on a steel-aluminum contact. Tests were

performed at the same conditions of load, entrainment speed,

sliding distance, temperature, and concentration of the solu-

tion in order to compare the activity (lubrication effect) of the

thiomolybdates. Although there is a friction reduction for the

three salts compared to pure water, a significant difference in the

friction coefficient is observed, depending on the alkyl group.

SEM/EDAX and Raman analysis of the wear tracks reveal the

formation of a solid film rich in molybdenum and sulfur, which

indicates the in situ formation of a MoS2 film.

KEY WORDS

Tribological; Additive; Water Soluble; Friction; Wear; FilmForming

BACKGROUND

Friction modifiers and mild antiwear agents are polarmolecules added to lubricants for the purpose of minimizing thesurface asperity contact that may occur in a given machine ele-ment. Esters, fatty acids, and some solid materials such as graphite

Manuscript received March 28, 2006Manuscript approved March 12, 2008

Review led by Cyril Migdal

and molybdenum disulfide are some examples used for thesepurposes. These additives are primarily formed by molecules thathave a polar head and a lubricant-soluble tail. When the frictionmodifier containing the lubricant comes into contact, the polarhead of the molecules anchor on the metal surface forming a low-shear tribological film that prevents surface asperity contact andfacilitates the sliding motion (Hsu (1)). As long as the frictionalcontact is light, these molecules provide a cushioning effect thatreduces the surface interactions, thus reducing friction. However,as the load and the metallic contact increases, the strength of theadditive and the chemical reaction process must increase. Thisleads to the use of sulphur-phosphorus based EP additives, whichform organo-metallic salts on the loaded surfaces that serve assacrificial films to protect against aggressive surface damage. Fric-tional heating on continuously modified surfaces boost chemicalreactions and interactions between lubricating additives and thecorresponding surfaces in the contact zone. Characteristics of thelubricating films formed in the contact depend on the tribologicalmating pairs, the chemical nature of the additive, and the operatingconditions.

In the boundary lubrication regime the contacting surfaces arenot separated by the lubricant and thus, there is considerable as-perity contact in relative motion. The friction characteristics of themating pairs are determined by the properties of the solids and anylubricating film formed at the interface. Under these conditions,the friction coefficient is essentially independent of fluid viscosity.The average film thickness formed in this regime is thinner thanthe elastically deformed surface roughness. The continuous as-perity interactions cause elastic deformation, plastic deformation,and, finally, mechanical fracture.

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Friction and Wear Performance of Tetraalkylammonium Thiomolybdates in Aqueous Solutions 527

Organomolybdenum compounds and other organometallicsalts have been studied for many years because of their beneficialproperties as friction modifiers and their use effectively eitheras powder, protective coating, or lubricant additive in machineelements such as gears, bearings, and metalworking applications(Smorygo, et al. (2); Stipanovic, et al. (3); Hamaguchi, et al. (4);Hu, et al. (5); Jianqiang, et al. (6)). In the case of molybdenumit is well known that its lamellar, layered structure, and low shearstrength bring excellent characteristics to reduce friction in high-pressure contacts. As a lubricant additive, sulphur-containingmolybdenum suspensions have been used that can be soluble inparaffinic oil (Rastogi and Yadav (7)). Nanoparticles of MoS2

dispersed in mineral oil have also been evaluated under boundarylubrication conditions and ultrahigh vacuum (Cizaire, et al.,(8); Parenago, et al. (9)). The most extensively studied class oforganomolybdenum compounds are the molybdenum dithiocar-bamates (MoDTC) and molybdenum dialkyldithiophosphates(MoDTP) either individually or in synergistic combination withZDDP (zinc dialkyldithiophosphate), or alkylated dipheny-lamines to reduce friction and wear by forming a protective filmcomposed of MoS2 or enhance its antioxidant performance (Un-nikrishnan, et al. (10); Gatto and Devlin (11); Arai and Tomizawa(12); Shaub (13); DeVries and King (14); Bec, et al. (15)).

While most work for these additives has been focused ontheir performance when dispersed in oil media, in recent yearsthere has also been a growing interest on evaluating their tribo-logical behavior in water-based fluids for metalworking applica-tions. Sulek and Wasilewski have shown that aqueous solutions oflauryl sulfates present good antiseizure performance (Sulek andWasilewski (16)).The lubricating characteristics of water solutionsof (NH)2MoS4 have also been explored, which show a lubricityimprovement and better wear resistance for aluminum surfaces(Maejima, et al. (17)). It has also been found that both inorganicsalts (e.g., sulfates, phosphates, and chlorides) and organometalliccompounds have good tribological performance on friction, wear,and seizure for metalworking and EP applications (Xuedong, et al.(18); Jain and Shukla (19)). Polymers have also been used as par-tially soluble additives in water-based systems in synergistic com-bination with fullerene, enhancing the antiwear and antifrictioncharacteristics of the base fluid (Duan (20); Jiang, et al. (21); Lei,et al. (22)). In all these previous studies, it has been inferred thata protective film is formed on the interacting surfaces and it isresponsible for the friction and wear reduction observed.

Recent studies carried out by Georges et al. on the lubricatingmechanism of water-based lubricants indicate that lamellar nano-structures at the mechanical contact interface provide efficientlubrication under severe contact conditions (Hollinger, et al. (23)).

This paper reports some results on tetraalkylammoniumthiomolybdates as water-soluble lubricant additives working un-der boundary conditions for steel-aluminum surfaces.

EXPERIMENTAL PROCEDURE

Synthesis of Tetraalkylammonium Thiomolybdates

Synthesis of the tetraalkylammonium thiomolybdate was car-ried out in two steps following the method reported by Alonso etal. (Alonso, et al. (24-26)). The first step is the preparation of a

water-soluble ammonium thiomolybdate (NH4)2MoS4 from am-monium heptamolybdate (NH4)6Mo7O40 in an ammonia/watersolution with H2S flow at room temperature according to thechemical reaction:

(NH4)6Mo7O40 + NH4OH/H2O + flow ofH2S → (NH4)2MoS4

After this, the next step involves a rapid substitution of [NH4]+1

ions from (NH4)2MoS4 by tetraalkyl ammonium radicals [R4N]+1

where R= methyl and propyl groups according to the water solu-tion reaction:

(NH4)2MoS4 + 2R4NBr → (R4N)2MoS4 ↓ +2NH4Br

The resultant precipitate is the tetraalkylammonium thiomolyb-date salt (where R = methyl or propyl), named tetramethylam-monium thiomolybdate [(Met)4N)]2MoS4 and tetrapropylammo-nium thiomolybdate [(Pro)4N)]2MoS4, which are also soluble inwater.

Substrate Materials

The upper specimen was a 6 mm diameter pin made of stainlesssteel 440C while the lower specimen was a 50 mm diameter × 6 mmthick disk made of aluminum alloy 6063, respectively. These mate-rials were selected considering the importance of steel-aluminummating pairs in metalworking applications. Surface roughness forthe specimens was approximately 25 nm and 140 nm rms whiletheir Brinell hardness values were approximately 97 HB and 25HB for steel pin and aluminum disk, respectively. The disks werepolished using a liquid suspension containing 0.3 µm Al2O3 abra-sive particles. Specimens were thoroughly cleaned before the tri-bological tests with ultrasound in boiling toluene and completelyrinsed in acetone and then dried.

Friction Tests

Tribological friction tests were carried out in a commercial pin-on-disk tribometer (Micro Photonics-Tribometer). The mechan-ical contact was formed between a steel pin loaded against analuminum disk. The pin was firmly secured to a stationary holderfor the pin-on-disk configuration while the disk was attached toa horizontal chuck driven by a variable-speed electric motor andcompletely submerged in the test fluid. Frictional force was moni-tored by a linear voltage displacement transducer attached to thepin holder and was recorded continuously. A picture of the testrig is shown in Fig. 1.

All tests were performed at a constant temperature of 30◦Cand 60% ± 5% relative humidity. A dead weight of 10 N was usedin all the tests carried out. Under these conditions, the maximumHertzian pressure of 0.877 GPa generates a circular contact areaof 0.0171 mm2. During the tests, the aluminum specimen rotatesat a constant sliding speed of 1 mm/sec for a period of 1 hour,running a total distance of 3.6 m. The test conditions selectedensure boundary lubrication regime in the contact.

The wear rate was obtained from the LVDT sensor on the pin.The lubricant solution was prepared by adding 0.3% wt/wt molyb-denum salt in three times distilled water and vigorously mixed for10 min for each additive evaluated.

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528 J. LARA-ROMERO ET AL.

Fig. 1—Pin-on-disk tribometer.

Surface Analysis

After the pin-on-disk sliding tests, wear tracks on the flat andball wear scars were examined optically and with several surfaceanalytical techniques.

Scanning electron microscopy (SEM) and laser Raman spec-troscopy were used for morphological and chemical characteriza-tion of films and of the sliding surfaces. Raman spectroscopy wasperformed using a LabRam model of Dilor micro-Raman systemequipped with a 20 mW He-Ne laser emitting at 632.8 nm, a holo-graphic notch filter made by Kaiser Optical Systems, Inc. (model

Fig. 2—Friction coefficient vs. time for a) water, b) methyl, c) propyl,d) ammonium.

TABLE 1—RESULTS OF FRICTION COEFFICIENT AND WEAR RATE FOR

EACH LUBRICANT EVALUATED

Water Propyl Methyl Ammonium

Frictioncoefficient

0.53 ± 0.001 0.17 ± 0.001 0.1 ± 0.001 0.08 ± 0.001

Wear rate(mm/s)

0.0112 0.00166 0.00187 0.00184

super Notch-Plus), a 256 X q1024-pixel CCD used as detector,a computer-controlled XY stage with a spatial resolution of 0.1µm, two interchangeable gratings (600 and 1800 g mm−1), and aconfocal microscope with 10, 50, and 100 X objectives. All mea-surements were carried out at room temperature with no specialsample preparation.

Worn disk tracks with transfer films were also examined using athin window energy dispersive X-ray spectrometer (EDX) housedin a JEOL JSM5800 LV scanning electron microscope (SEM).EDX spectra were obtained at a beam energy of 10 keV, beamcurrent 2.0 nA, and detector take off angle of 25◦.

RESULTS AND DISCUSSION

Friction Tests

Figure 2 shows the friction coefficient as a function of time ofsteel-aluminum mating pairs, for three time distilled water as lu-bricant, and for the tetraalkylammonium thiomolybdenum saltssynthesized and used as lubricant additive at a constant molyb-denum concentration for every solution prepared. The friction

Fig. 3—Wear rate vs. time rate for a) water, b) methyl, c) propyl,d) ammonium.

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Friction and Wear Performance of Tetraalkylammonium Thiomolybdates in Aqueous Solutions 529

Fig. 4(a–d)—SEM images of the contact track for water, propyl, methyl and ammonium. Image magnification ×27.

coefficient for the specimens lubricated with three times distilledwater is initially 0.2 but increases with the distance and stabi-lizes at a value of ∼0.53. The steady-state friction coefficient onspecimens lubricated with thiosalts prepared is in the range of0.08-0.17.

The lowest steady-state friction coefficient was 0.08 corre-sponding to the ammonium thiosalt-lubricated specimens (see Ta-ble 1). A significant friction reduction was observed. These resultssuggest the formation of a protective lubricating film at the contactinterface as a product of the thermal decomposition of precursorsalts used as additives. This thermal decomposition is caused bythe severe operating conditions of the mechanical contact, whichgenerate a high temperature rise at the interface.

Wear Mechanism

Figure 3 indicates a linearly dependent penetration of the pinon the aluminum specimen as time passes. The wear rate is rep-resented by the slope. Table 1 presents the wear rate values reg-istered after testing the additives prepared and the control fluid(three times distilled water). Using the thiosalts as additives a wearreduction of approximately one order of magnitude is reached.

After the friction tests, the pin-on-disk worn surfaces producedby being rubbed for 1 h in the thiosalts solution at a constant load of10 N were characterized and analyzed by SEM, EDX, and Raman.

Figures 4(a-d) show representative SEM micrographs of weartrack on disk specimens lubricated with water, propyl, methyl, andammonium respectively, produced from sliding at 30◦C.

Figures 5(a-d) show the representative SEM micrographs ofwear tracks on disk specimens lubricated with water, propyl,methyl, and ammonium, respectively, produced from sliding at30◦C.

Numerous parallel ploughing grooves in the sliding directionwere observed on the wear track of the disk specimen lubricatedwith three times distilled, water at 30◦C in Fig. 5(a). These surfacefeatures indicate dominance of abrasive wear.

Figures 5(b–d) reveal different surface features on the weartrack produced in the presence of the molybdenum thiosalts. Thesmooth region and tiny cracks evidenced in these figures indi-cate that adhesion and localized microcracking were the prevailingwear mechanisms. This may be attributed to the effect of the highsulfide content and the formation of a mixed hard-brittle phase ofthe tribofilm. Microcracking resulted in regions of partial delami-nation of tribofilm and indicates that both adhesion and cohesiveshearing of the film control the wear process. However, it appearsthat the chemical reactions of the additive and the freshly exposedsurfaces replenished the film expeditiously.

Chemical Analysis

The EDAX analysis carried out on the wear track of the alu-minum specimens rendered the atomic percentages of the molyb-denum and sulfur present. Table 2 shows the molybdenum relationto sulfur found on the wear track of the disk specimens lubricatedwith the molybdenum thiosalts prepared. In all cases, an atomic ra-tio of approximately two is observed, suggesting the formation ofMoS2, which is responsible for the friction and wear rate reductionnoticed during the tribological tests.

Figure 6 indicates the characteristic distribution of aluminum,molybdenum, sulphur, and oxygen in a section of the contact trackof the aluminum specimen. This analysis confirms the presence ofsulphur and molybdenum especially in zones where no aluminumdebris was detected.

An EDAX analysis was carried out on the tetraalkylammo-nium thiomolybdates-lubricated aluminum specimen to deter-mine the chemical composition of the laminated debris formedon the track and inside the small crevices observed. The analysis

Fig. 5—SEM images of the contact track for a) water, b) propyl, c) methyl, and d) ammonium. Image magnification ×5000.

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530 J. LARA-ROMERO ET AL.

Fig. 6—Mapping of aluminum, molybdenum, sulphur, and oxygen in a section of the contact track.

Fig. 7—SEM micrographs of aluminum specimen lubricated with tetralkylammonium thiomolybdates, a) ×120, b) ×10,000.

Fig. 8—EDAX analysis of aluminum specimen lubricated with tetralkylammonium thiomolybdates a) on film, b) on substrate.

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Friction and Wear Performance of Tetraalkylammonium Thiomolybdates in Aqueous Solutions 531

TABLE 2—ATOMIC PERCENTAGES OF ELEMENTS DETECTED IN THE CON-TACT ZONE

Water Propyl Methyl Ammonium

C 8.20% 71.54% 29.96% 8.43%O 3.25% 7.18% 15.43% 37.11%Mo 0 1.92% 2.08% 8.72%S 0 4.21% 4.42% 19.08%Mo/S Ratio 0 2.19 2.12 2.18

reveals that laminated debris is composed of molybdenum disul-fide, which is also present inside the cracks (see Figs. 7 and 8).

Raman spectroscopy was carried out on wear particles and ondifferent parts of the wear track. The spectra taken on the samplesanalyzed were compared with those obtained from a standardreference.

Raman spectroscopy of the wear track revealed very sharppeaks at approximately 402 and 376 cm−1 (Fig. 9) correspond-ing to the E1

2g and A1g vibrational modes of 2H-MoS2 (Jimenez-Sandoval, et al. (27)).

Raman characterization of the tribofilm formed proved thatthese platelets are in fact MoS2 sheets. The changing positions ofthe peaks provide additional information about the microstruc-ture. Under high shear stresses, plate-like layers of MoS2 canalign themselves parallel to the direction of relative motion so

Fig. 9—Raman spectra on the wear track of aluminum specimen lubri-cated with molybdenum thiosalt.

they can slide over one another with relative ease and thus impartlow friction.

The results presented in this paper indicate that the use oftetraalkylammonium thiomolybdenum salts (precursors of MoS2)as water-soluble lubricating additives offers an important reduc-tion in friction and wear for high-pressure contacts. A solid molyb-denum disulfide film is formed by thermal decomposition of thesalts due to the high temperatures reached in the mechanical con-tact zone. The authors believe that the amount of oxygen presentin the tribofilm is responsible for the main differences in the fric-tion coefficient values observed among the thiomolybdates testedbut this assumption requires a deeper analysis.

CONCLUSIONS

This paper presents a tribological study on tetra alkylammo-nium thiomolybdates. The effectiveness of this additive in aqueoussolution to reduce friction and wear during sliding at a tempera-ture of 30◦C was evaluated on a pin-on-disk tribometer. From theresults of this study, the following main conclusions can be drawn:

1. Tetra alkylammonium thiomolybdates exhibit good frictionand wear reduction properties in water-based systems.

2. The surface examination of the rubbing zone indicated thata tribofilm containing molybdenum disulfide is formed onthe rubbing surfaces non-homogeneously during the slidingprocess.

3. Under friction conditions, tetraalkylammonium thiomolyb-dates is transformed to molybdenum disulfide by triboreduc-tion influencing friction and wear performance.

ACKNOWLEDGEMENTS

The authors would like to thank the National Council for Sci-ence and Technology (CONACyT) for their financial support un-der Projects 46871 and 43634 to carry out the present work.

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