Friction reduction by water-soluble ammonium thiometallates

F. Chinas-Castillo,a,* J. Lara-Romero,b G. Alonso-Nunez,c J. D. O. Barceinas-Sanchez,d and S. Jimenez-Sandovale

aMechanical Engineering Department, Instituto Tecnologico de Oaxaca, Oaxaca, Oax., MexicobChemical Engineering Department, Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Mich., Mexico

cMaterials Department Chemistry, CIMAV, Chihuahua, Chih., MexicodResearch Department, CIATEQ, A.C., Queretaro, Qro., MexicoeMaterials Department, CINVESTAV, Queretaro, Qro., Mexico

Received 22 March 2006; accepted 13 November 2006; published online 19 December 2006

Lubricant bases for metalworking applications make extensive use of water-soluble additives to reduce friction and wear. In

order to do such task, these additives must form a lubricating film that separates the contact surfaces thus imparting good surface

finishes to the worked parts. This paper presents a study on the tribological performance of aqueous solutions of ammonium

thiomolybdate and ammonium thiotungstate. Tests were carried out on a pin-on-disc tribometer for a steel–aluminum contact

while keeping load, entrainment speed, sliding distance, temperature, and concentration of the additive constant to study the

lubrication effect of these two salts. Chemical analysis of the wear track indicates the presence of an in-contact-formed solid film

enriched with MoS2 and WS that reduces friction markedly.

KEY WORDS: tribological, additive, water soluble, friction, wear, film forming

1. Background

Industrial lubricants play an important role mini-mizing surface asperity contact of machine elements.This task is enhanced by addition of friction and wear-reducing agents composed by polar molecules added tothe lubricant. Carboxylic acids, fatty acids, esters, andsome solid materials such as graphite and molybdenumdisulfide are some representative examples. Whenadditive-containing lubricant enters in the contact zone,the polar head of the molecules anchors on the metalsurface while the tail solubilizes in the fluid phase, thus,forming a tribological film that stops surface asperitycontact and facilitates sliding motion [1]. These mole-cules provide a cushioning effect for low loads anddiminish surface asperity interactions, consequentlyreducing friction. However, as load and metallic contactincreases, the protecting additive film formed on surfaceshould be more resistant and that leads to a higheradditive chemical reaction commonly through the use ofsulphur–phosphorus-based EP additives, which formorganometallic salts on the loaded surfaces that serve assacrificial films to protect against aggressive wear.Frictional heating on continuously modified surfacesactivate chemical reactions and interactions betweenlubricating additives and the corresponding surfaces.Tribological performance of the contact depends notonly on the operating conditions but also the mating

pairs, lubricating film formed, and chemical nature ofthe additive.

In the boundary lubrication regime, load is supportedcompletely by contact asperities in relative motion.Under these conditions, the friction is essentially vis-cosity-independent and the average film thicknessformed in this regime is thinner than the elastically-deformed surface roughness. The continuous asperitiesinteractions initially produce elastic deformation, thenplastic deformation, and finally mechanical fracture.

Organomolybdenum compounds and other orga-nometallic salts have been studied for many yearsbecause of their beneficial properties as friction mod-ifiers being used effectively either as powder, protec-tive coating, or lubricant additive in machine elementssuch as gears, bearings, and metalworking applications[2–6]. The layered-lamella structure of molybdenum isknown to give low shear and therefore reduce frictionin highly-loaded contacts. Sulphur-containing molyb-denum suspensions have been used as lubricant addi-tive in paraffinic oil [7]. Some studies have also beencarried out on nanoparticles of MoS2 suspended inmineral oil under boundary lubrication conditions andultrahigh vacuum [8–9]. Molybdenum dithiocarba-mates (MoDTC) and molybdenum dialkyldithiophos-phates (MoDTP) have largely been systematicallystudied either individually or in synergistic combina-tion with ZDDP, ZDTP (zinc dialkyldithiophosphate),or alkylated diphenylamines. Results indicate thatthese organomolybdenum compounds reduce friction*To whom correspondence should be addressed.

E-mail: fernando-chinas@mail.com

1023-8883/07/0500-0137/0 � 2006 Springer Science+Business Media, LLC

Tribology Letters, Vol. 26, No. 2, May 2007 (� 2006) 137

DOI: 10.1007/s11249-006-9179-4

and wear by forming a protective film composed ofMoS2 or enhance its antioxidant performance [10–15].

Most research studies on these additives have beenconcentrated on their tribological performance whendispersed in lubricating oil, however, in recent yearsthe interest on their tribological performance on water-based fluids has also grown, especially in metalworkingapplications. Sulek and Wasilewski point out thataqueous solution of lauryl sulfates present good anti-seizure performance [16]. The lubricating characteristicsof water solutions of (NH)2MoS4 have also beenexplored which show a lubricity improvement andbetter wear resistance for aluminium surfaces [17]. Ithas also been found that both organometallic com-pounds and inorganic salts (e.g. sulfates, phosphatesand chlorides) have good tribological performance onfriction, wear, and seizure for metalworking and EPapplications [18–19]. Polymers have also been used aspartially soluble additives in water-based systems insynergistic combination with fullerene, enhancing theantiwear and anti friction characteristics of the basefluid [20–22]. In all these previous studies, the authorshave concluded that a protective film is formed on theinteracting surfaces which is in charge of reducingfriction and wear.

Recently Georges et al. carried out studies on thelubrication fundamentals of water-based lubricantsreporting that lamella nanostructures at the mechanicalcontact interface provide efficient lubrication undersevere contact conditions [23].

This paper presents some results on ammoniumthiomolybdate and ammonium thiotungstate water-soluble blends to lubricate a steel–aluminium contactunder boundary conditions.

2. Experimental procedure

2.1. Synthesis of ammonium thiometallates

Syntheses of the ammonium thiomolybdate andammonium thiotungstate salts were carried out in onestep following the method reported by Alonso et al.[24–26]. The preparation of a water-soluble ammoniumthiomolybdate (NH4)2MoS4, starting from ammoniumheptamolybdate (NH4)6M7O24 was carried out in anammonia/water solution with H2S flow at room tem-perature according to the next chemical reaction:

NH4ð Þ6Mo7O24 þ NH4OH=H2O

þ flow of H2S! NH4ð Þ2MoS4

The preparation of ammonium thiotungstate startingfrom ammonium metatungstate (NH4)6W12O39 wascarried out in an ammonia/water solution with H2S flow

at room temperature according to the next chemicalreaction:

NH4ð Þ6W12O39 þ NH4OH=H2O

þ flow of H2S! NH4ð Þ2WS4

The resultant precipitate is the ammonium thiometallatesalt (NH4)2MS4 (where M = Mo or W).

2.2. Substrate materials

The contact was formed by a 6 mm diameter pinmade of stainless steel 440C (upper specimen) loadedagainst a 50 mm diameter� 6 mm thick disk made ofaluminium alloy 6063 (lower specimen), respectively.These materials were selected considering the impor-tance of steel–aluminium mating pairs in metalworkingapplications. Surface roughness and hardness of thespecimens are indicated in table 1.

The disks were polished using a liquid suspensioncontaining 0.3 lm Al2O3 abrasive particles. Pin and diskspecimens were thoroughly cleaned in an ultrasoundbefore the tribological tests in boiling toluene, com-pletely rinsed in acetone and finally dried.

2.3. Friction tests

Friction tests were performed using a commercialpin-on-disk tribometer (Micro-Photonics). In thisapparatus a steel pin loaded against an aluminium diskforms the mechanical contact. In the pin-on-disc con-figuration, a stationary holder secures the pin firmlywhile the disk is on hold with a horizontal chuck drivenby a variable-speed electric motor. The contact thusformed is completely submerged in the test fluid. Alinear voltage displacement transducer attached to thepin holder monitors the friction coefficient and thereadings are recorded continuously through out the test.

The wear rate was obtained from LVDT sensor onthe pin. Figure 1 shows a picture of the test rig.

Tests were carried out at a constant temperature of30 �C and 60%±5% relative humidity and a deadweight of 10N was used in all the tests. The initialmaximum Hertzian pressure of 0.877 GPa generates acircular contact area of 0.017101 mm2. The aluminium

Table 1.

Mechanical properties of pin-on-disk specimens.

Specimen Material Brinell Hardness Roughness

(nm)

Pin Stainless Steel 440C 97 25

Disk Aluminium 6063 25 140

138 F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates

specimen rotates at a constant sliding speed of 1 mm s)1

for a period of one hour, running a total distance of3.6 m for each test. Therefore, according to the oper-ating conditions, the mechanical contact was operatingin the boundary lubrication regime.

The lubricant solutions were prepared adding 0.1,0.2, and 0.3% wt/wt ammonium thiomolybdate in waterdistilled three times and vigorously mixed for 10 minbefore each tribological test. The same procedure andconcentrations were used for solutions of ammoniumthiotungstate salts

2.4. Surface analysis

Wear scars on the pin and disk were examined afterthe sliding tests with some surface analytical techniques.Scanning electron microscopy (SEM) and laser Ramanspectroscopy were used for morphological and chemicalcharacterization of films and of the sliding surfaces.Raman spectroscopy was performed using a LabRammodel of Dilor micro-Raman system equipped with a20 mW He–Ne laser emitting at 632.8 nm, a holo-graphic notch filter made by Kaiser Optical Systems,Inc. (model supertNotch-Plus), a 256� q1024-pixelCCD used as detector, a computer-controlled XY stagewith a spatial resolution of 0.1 lm, two interchangeablegratings (600 and 1800 g mm)1), and a confocalmicroscope with 10, 50, and 100� objectives. All mea-surements were carried out a room temperature with nospecial sample preparation.

Worn disks track with transfer films were alsoexamined using a thin window energy dispersive X-rayspectrometer (EDX) housed in a JEOL JSM5800 LVscanning electron microscope (SEM). EDX spectra wereobtained at a beam energy of 10 keV, beam current2.0 nA, and detector take off angle of 25�.

3. Results and discussion

3.1. Friction tests

Friction tests were carried out at different concen-trations ranging from 0.01% to 0.3% wt/wt of thewater-soluble ammonium thiomolybdate salt. Figure 2shows friction coefficient registered as function of timefor the steel–aluminium tribological pairs lubricatedwith (NH4)2 MoS4, containing water for every solutionprepared. Adding a very small concentration (0.01%) ofthe ammonium thiomolybdate solution to distilled waterproduced initially a friction coefficient of approximately0.47 in the first 30 min of the friction test. After that, thefriction coefficient drops down to a value of 0.1 andkeeps fairly constant through out the rest of the test.

Initial friction values for all concentrations wereabout 0.45–0.47. However, as concentration of theammonium thiomolybdate salt increases to 0.025% andup to 0.3% a faster friction coefficient drop down to 0.1is observed. This friction drop is directly related to thefilm-forming ability of the ammonium thiomolybdatesalt. For a 0.3% of additive concentration after 5 minthe friction falls to 0.1.

Figure 1. Pin-on-Disk tribometer.

Figure 2. Friction coefficient vs. time for (NH4)2MoS4.

Figure 3. Friction coefficient vs. time for (NH4)2WS4.

F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates 139

In the case of ammonium thiotungstate salt at 0.1%concentration, figure 3 shows that the initial frictioncoefficient of �0.5 remains constant until the end of thetest. For a 0.2% concentration, after 15 min of the test,the initial friction coefficient of �0.5 falls quickly andstabilizes at �0.05 for further 40 min, and then increasesabruptly to the initial value at the commencement of thetest.

At 0.3% concentration, the initial friction coefficientfalls after 12 min and stabilizes at �0.05 for the rest ofthe test.

These results suggest the formation of a protectivelubricating film at the contact interface product of thethermal decomposition of precursor salts used as addi-tives. This thermal decomposition is caused by thesevere operating conditions of the mechanical contact,which generate a high-temperature rise at the interface.In the case of ammonium thiomolybdate this film-forming rate is faster than in the case of ammoniumthiotungstate, even for smaller concentrations.

3.2. Wear mechanism

After the friction tests, the pin-on-disk worn surfacesproduced by being rubbed for 1 h in the thiosalts

solutions at a constant load of 10N, were characterizedand analyzed by SEM, EDX, and Raman.

Table 2 presents a general pattern on the wear-rateobserved in aluminium disk specimens. Wear rate isrepresented by the slope. In the case of (NH4)2MoS4 awear rate reduction is observed as concentration of theadditive augments in solution. The same tendency isevident for aqueous solutions containing (NH4)2WS4,however, in this case, a good wear reduction is registeredwhen concentration is superior to 0.3% of the additive.Ammonium thiomolybdate containing water solutionsexhibit a clear friction and wear reduction with relativelysmall quantities of the additive.

Wear and surface damage of the specimens tested canbe divided into different stages. SEM micrographs of thewear scars of disk surface at the end of the test showedthat polishing, mild grooving, microcracks, and plasticdeformation were the dominant wear modes in thestudy.

Figure 4(a) and (b) show representative SEMmicrographs of wear track on disk specimens lubricatedwith ammonium thiotungstate and ammonium thio-molybdate solutions produced from sliding at 30 �C.

From SEM micrographs and friction results infigures 2 and 3, it is observed the presence of delamina-tion and adhesion together with the non-homogeneousbuild up of tribofilm on the surface as concentration ofthe additive increases. However, this is more noticeablein the case of ammonium thiomolybdate.

Figure 5(a) and (b) are micrographs of disk weartrack lubricated with the thiosalts at the same concen-tration of 0.1% and reveal different surface features.The smooth region and tiny cracks evidenced in thesemicrographs indicate that adhesion and localizedmicrocraking were the prevailing wear mechanisms.

Table 2.

Wear rate for a) 0.1, 0.2, 0.3% (NH4)2WS4 b) 0.01, 0.1, 0.3%

(NH4)2MoS4.

Wear Rate (mm s)1� 10)6)

(NH4)2WS4 (NH4)2MoS4

0.1% 0.2% 0.3% 0.01% 0.1% 0.3%

59.6 0.7 1.39 3 2.1 1.56

Figure 4. SEM images of (a) 0.1, 0.2, 0.3% (NH4)2WS4 (b) 0.01, 0.1, 0.3% (NH4)2MoS4.

140 F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates

This may be attributed to the effect of the sulfide contentand the formation of a mixed hard-brittle phase of thetribofilm. However, it appears that the chemical reac-tions of the additive and the freshly-exposed surfacesreplenished the film expeditiously.

Some SEM micrographs were taken after the first10 min of the pin-on-disk sliding test when the frictioncoefficient value was about 0.5 where an incipient filmformation was observed. However, in this part of thetest abrasive grooves and partial smearing were morenotorious in figure 6(a). Figure 6(b) was taken at theend of the test were the tribofilm had already formed onthe surface reducing the friction coefficient to 0.05.

Closer inspection of the damaged surfaces showedplasticity and microcracks. Figures 5 and 6 showmicroploughing and cracking on the surface. It can beseen that use of additive contributes with a reduction ofwear attributed to the protective surface layer forma-tion, which influences the reduction of mechanical areaof contact between the two surfaces. Under boundarylubrication conditions (NH4)2WS4 and (NH4)2MoS4undergo thermal decomposition under extreme condi-tions (high load and high temperature), forming a pro-tective film through the adsorption process, and so thethin layer prevents welding of surface asperities, andthus reduces adhesive wear of the mating surfaces. Their

Figure 5. SEM micrographs of (a) 0.1% (NH4)2WS4 (b) 0.1% (NH4)2MoS4.

Figure 6. SEM micrographs of 0.3% (NH4)2WS4 (a) after first 10 min (b) end of test.

F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates 141

effectiveness is determined by their ability to form aprotective film on sliding contacts.

At the start of the test, the original surface is deformedelastically with no wear fragments created during the firstfew minutes. However, during subsequent minutes, therepeated superficial shear plastically deforms the surfaceand gradually transports material towards the sides of thetrack (see figures 5 and 6). This transport creates tonguesof heavily-deformed material that eventually break looseformingflake-like fragments along thewear track.Duringthe test a lesser amount of large and thick wear fragmentswere the result of bulk shear and fracture. These frag-ments exhibit an intact structure of base material andsurface reaction products of the salts, especially MoS2.

As the fragments are heavily deformed and reintro-duced into the contact, it forms extensive layers oftribofilms. These layers also contain higher levels ofoxygen than the bulk material indicating to a large extentthat they are formed by addition of agglomerated wearfragmentsmixedwith reactionproducts andMoS2orWS.Note that even under these conditions of rather coarsesurface the friction coefficient stays on a low value.

Table 3.

Atomic percentages of elements in the contact zone for (NH4)2MoS4.

Element 0.3% 0.1% 0.025% 0.01%

Substr Film Substr Film Subst Film Subst Film

Al 62.14 19.9 47.01 22.59 69.12 36.72 46.91 47.18

O 16.91 27.41 20.64 37.11 12.12 39.1 28.85 26.85

Mo 3.42 9.89 3.95 8.72 1.19 4.69 1.79 2.78

S 8.04 25.93 13.06 19.08 3.68 8.56 4.02 5.42

Table 4.

Atomic percentages of elements in the contact zone for (NH4)2WS4.

Element 0.3% 0.2% 0.1%

Substr Film Subst Film Subst Film

Al 93.38 63.42 97.9 89.73 97.60 82.10

O 6.00 31.22 2.40 8.34 2.40 15.67

W 0.37 2.76 – 0.96 – 1.25

S 0.25 2.60 – 0.96 – 1.00

5 m5 m

Substrate

0 40

2

4

6

8

10

SMo

Al

OC

EDAX / Amm onia

Substrate

Inte

nsity

Energy, eV

5 ∝m

Film

5 ∝m5 µm

Film

0 20

2

4

6

8

10

SMo

Al

OC

EDAX / Ammonia

Film

Inte

nsity

Energy, eV

2 6

4 6

µ

a)

b)

Figure 7. SEM/EDX of (NH4)2MoS4 for (a) substrate (b) film.

142 F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates

3.3. Chemical analysis

The EDAX analysis carried out on the wear track ofthe aluminium specimens rendered the atomicpercentages of the molybdenum, tungsten, and sulfurpresent. Tables 3 and 4 show the molybdenum, tung-sten, and sulfur atomic percentage found on the weartrack of the disk specimens lubricated with the thiosaltsprepared. These results indicate also that S increasedwith increasing percentages of (NH4)2WS4 andNH4)2MoS4 in the water-based systems under study.For the (NH4)2MoS4, an atomic ratio of approximatelytwo is observed, suggesting the formation of MoS2,which is responsible for the friction and wear-ratereduction noticed during the tribological tests. For(NH4)2WS4, an atomic ratio of approximately one isobserved, suggesting the formation of WS.

EDX analysis was carried out on aluminium specimento determine the chemical composition of the laminateddebris formed on the track and inside the small crevicesobserved. The analysis reveals that laminated debris iscomposed of molybdenum disulfide, which is also presentinside the cracks (see figure 7(a) and (b)).

After friction test, Raman spectroscopy was carriedout on wear track of aluminium disk specimens lubri-cated with (NH4)2MoS4 and (NH4)2WS4. The spectrataken on the samples analyzed were compared withthose obtained from a standard reference.

Raman spectroscopy of the wear track of disk spec-imens lubricated with ammonium thiomolybdaterevealed very sharp peaks at approximately 402 and376 cm)1 corresponding to the E1

2g and A1g vibrationalmodes of 2H–MoS2 [27]. Raman characterization of thetribofilm formed and observed in figure 8 proved thatthese platelets are in fact MoS2 sheets. Under high shearstresses, plate-like layers of MoS2 align themselves par-allel to the direction of relative motion, sliding one overanother with relative ease and thus reduce friction. Aweak and meaningless spectral signal was obtained inthe case of ammonium thiotungstate probably caused bythe very small film formed in contact.

The results presented in this paper indicate that smallconcentrations of ammonium thiomolybdate salt inwater solution offer an important reduction in frictionand wear for high-pressure contacts. A solid molybde-num disulfide film is formed by thermal decompositionof the salts due to the high temperatures reached in themechanical contact zone. A higher concentration ofammonium thiotungstate salt is needed to observe asatisfying tribological performance.

4. Conclusions

The effectiveness of ammonium thiomolybdates andammonium thiotungstates in aqueous-based solution toreduce friction and wear during sliding at a temperatureof 30 �C was evaluated on a pin-on-disk tribometer.Characterization of surfaces was conducted using SEM/EDX and Raman, spectroscopy. From this study, someconclusions are pin pointed:

1. Ammonium thiomolybdates exhibits good frictionand wear reduction properties in water-based sys-tems.

2. Surface analysis of the wear track indicated that atribofilm containing molybdenum disulfide is formedon the rubbing surfaces non-homogeneously duringthe sliding process. This film is formed by in situthermal decomposition of (NH4)2MoS4 to MoS2.

3. Tribofilms formed by in situ thermal decompositionof (NH4)2WS4 lubricated contact, is composed of anon-homogeneous WS film.

4. Film-forming abilities of ammonium thiomolybdateare more effective than film-forming abilities ofammonium thiotungstate even at very small concen-trations.

Acknowledgments

Authors of the present work express their sincerethanks to the National Council for Science and Tech-nology (CONACyT) of Mexico for their financialsupport under Projects 46871 and 43634.

200 250 300 350 400 450 500

0

100

200

300

400

500

600

700

MoS2

RamanHe-Ne Laser / 632.8 nm

376

402

Inte

nsity

Raman Shift / cm-1

Figure 8. Raman spectra on the wear track of aluminium disk

lubricated with (NH4)2MoS4.

F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates 143

References

[1] S.M. Hsu, Tribol Lett 3 (1997) 1.

[2] O. Smorygo, S. Voronin, P. Bertrand and I. Smurov, Tribol. Lett.

17 (2004) 723.

[3] A.J. Stipanovic and J.P. Schoonmaker, SAE Technical Paper No.

932779, SAE, Warrendale, Pa (1993).

[4] H. Hamaguchi, Y. Maeda and T. Maeda, SAE Technical Paper

No. 810316, SAE, Warrendale, Pa (1981).

[5] J. Hu, H. Zhu, W. Li, F. Li and Z. Zong, Wear 259 (2005)

519.

[6] H. Jianqiang, W. Xianyong, Y. Junbing, X. Feng, Z. Huanqin

and Z. Fazheng, Tribol. Trans. 48 (2005) 171.

[7] R.B. Rastogi and M. Yadav, Tribol. Int. 36 (2003) 511.

[8] L. Cizaire, B. Vacher, T. Le Mogne, J.M. Martin, L. Rapoport,

A. Margolin and R. Tenne, Surf. Coat. Technol. 160 (2002)

282.

[9] O.P. Parenago, V.N. Bakunin, G.N. Kuzmina, A.Y. Suslov and

L.M. Vedeneeva, Doklady Chem. 383 (2002) 86.

[10] R. Unnikrishnan, M.C. Jain, A.K. Harinarayan and A.K. Metha,

Wear 252 (2002) 240.

[11] V.J. Gatto and M.T. Devlin, US Patent 5650381 (1997).

[12] Arai K. and H. Tomizawa, US Patent 5605880 (1997).

[13] H. Shaub, US Patent 6306803 (2001).

[14] L. DeVries and J. King, US Patent 4370246 (1983).

[15] S. Bec, A. Tonck, J.M. Georges and G.W. Roper, Tribol. Lett. 17

(2004) 797.

[16] M.W. Sulek and T. Wasilewski, Tribol. Lett. 18 (2005) 197.

[17] M. Maejima, K. Saruwatari and M. Takaya, Surf. Coat. Technol.

132 (2000) 105.

[18] W. Xuedong, C. Haigang, Y. Shengrong and W. Dapu, Wear 250

(2001) 362.

[19] V.K. Jain and D.S. Shukla, Wear 193 (1996) 226.

[20] B. Duan, Wear 236 (1999) 235.

[21] G. Jiang, W. Guan and Q. Zheng, Wear 258 (2005) 1625.

[22] H. Lei, W. Guan and J. Luo, Wear 252 (2002) 345.

[23] S. Hollinger, J.M. Georges, D. Mazuyer, G. Lorentz, O. Aguerre

and N. Du, Tribol. Lett. 9 (2001) 143.

[24] G. Alonso, G. Aguirre, I.A. Rivero and S. Fuentes, Inorg. Chim.

Acta. 274 (1998) 108.

[25] G. Alonso, G. Berhault, A. Aguilar, V. Collins, C. Ornelas, S.

Fuentes and R.R. Chianelli, J. Catal. 208 (2002) 359.

[26] G. Alonso, G. Berhault and R.R. Chianelli, Inorg. Chim. Acta.

316 (2001) 105.

[27] S. Jimenez-Sandoval, D. Yang, R.F. Frindt and J.C. Irwin, Phys.

Rev. B 44 (1991) 3955.

144 F. Chinas-Castillo et al./Friction reduction by water-soluble ammonium thiometallates