ORIGINAL PAPER

MoS2 Films Formed by In-contact Decompositionof Water-soluble Tetraalkylammonium Thiomolybdates

Fernando Chinas-Castillo Javier Lara-Romero Gabriel Alonso-Nunez Juan de Dios Oscar Barceinas-Sanchez Sergio Jimenez-Sandoval

Received: 25 June 2007 / Accepted: 19 December 2007 / Published online: 8 January 2008

Springer Science+Business Media, LLC 2008

Abstract Synthesis and tribological evaluation of three

tetraalkylammonium thiomolybdate (R4N)2MoS4 (R =

methyl, propyl, or ammonia) aqueous-based lubricant

additives on a ball-on-disk tribometer was carried out for a

steelaluminum contact. Tests were performed at the same

conditions of load, entrainment speed, sliding distance,

temperature, and concentration of MoS2 to compare the

activity (lubrication effect) of the thiomolybdates prepared.

A friction reduction is observed for the three salts com-

pared to pure water; however, significant differences in

friction coefficient are observed depending on the alkyl

group. SEM/EDAX and Raman analysis of the wear tracks

reveal the in-contact formation of a MoS2-lubricating film,

rich in molybdenum and sulfur.

Keywords Tribological Additive Water-soluble Friction Wear Film forming

1 Background

Lubricant additives, such as friction modifiers and mild

antiwear agents, are added to lubricants for the purpose of

minimizing surface asperity contact that may occur in a

given machine element. Dichalcogenides (i.e., graphite,

molybdenum disulfide), esters, and fatty acids are repre-

sentative additives typically used for these purposes. Their

molecules have a polar head and a lubricant-soluble tail,

and when an additive-containing lubricant enters into the

contact, the polar head anchors on metal surfaces forming a

low-shear tribological film that prevents surface asperity

contact and facilitates sliding motion [1]. As long as the

mechanical contact is not heavily loaded, these molecules

provide a cushioning effect that reduces surface interac-

tions and thus reduce friction. However, as load and

metallic contact increase, the strength of the additive and

the chemical reaction process must increase. This leads to

the use of sulfurphosphorus-based EP additives, which

form organometallic salts on the loaded surfaces that serve

as sacrificial films to protect against aggressive surface

damage. Frictional heating on continuously modified sur-

faces boosts chemical reactions and interactions between

lubricating additives and the corresponding surfaces in the

contact zone. Characteristics of the lubricating films

formed in the contact depend on tribological mating pairs,

chemical nature of the additive, and operating conditions.

Considerable asperity contact is present in the boundary

lubrication regime where the contacting surfaces are no

longer separated by the lubricant. Under these conditions,

friction characteristics of the mating pairs are entirely

determined by the properties of the solids, and any lubri-

cating film formed at the interface and the friction

coefficient is essentially independent of fluid viscosity. The

average film thickness formed in this regime is thinner than

F. Chinas-Castillo (&)Mechanical Engineering Department, Instituto Tecnologico de

Oaxaca, Oaxaca, Oaxaca, Mexico

e-mail: fernando-chinas@mail.com

J. Lara-Romero

Chemical Engineering Department, Universidad Michoacana de

San Nicolas de Hidalgo, Morelia, Michoacan, Mexico

G. Alonso-Nunez

Materials Department Chemistry, CIMAV, Chihuahua,

Chihuahua, Mexico

J. D. O. Barceinas-Sanchez

Research Department, CIATEQ, A.C., Queretaro, Queretaro,

Mexico

S. Jimenez-Sandoval

Materials Department, CINVESTAV, Queretaro, Queretaro,

Mexico

123

Tribol Lett (2008) 29:155161

DOI 10.1007/s11249-007-9292-z

the elastically deformed surface roughness. Continuous

asperities, interactions initially cause elastic deformation,

then plastic deformation, and finally, mechanical fracture.

Organomolybdenum compounds and other organome-

tallic salts have been studied for many years because of their

beneficial properties as friction modifiers and used effec-

tively either as a powder, a protective coating, or a lubricant

additive in machine elements such as gears, bearings, and

metalworking applications [26]. Molybdenum is well

known for its lamella-layered structure and low-shear

strength that provide excellent friction reduction character-

istics in high-pressure contacts. Paraffin oil-soluble

suspensions of sulfur-containing molybdenum and nano-

particles of MoS2 dispersed in mineral oil have also been

used as lubricant additives and evaluated under boundary

lubrication conditions and ultrahigh vacuum [79]. The most

extensively studied class of organomolybdenum compound

is the molybdenum dithiocarbamate (MoDTC) and molyb-

denum dialkyldithiophosphate (MoDTP) either individually

or in synergistic combination with ZDDP, zinc dialkyldi-

thiophosphate (ZDTP), or alkylated diphenylamines to

reduce friction and wear by forming a protective film

composed of MoS2 or enhance its antioxidant performance

[1015].

Although most studies for these organomolybdenum

additives have been conducted to evaluate their performance

when dispersed in oil media, in recent years there has also

been a growing interest on evaluating their tribological

behavior on water-based fluids for metalworking applica-

tions. Experimental work carried out by Sulek and

Wasilewski has shown that aqueous solutions of lauryl sul-

fates present good antiseizure performance [16]. Maejima

and coworkers explored the lubricating characteristics of

water solutions of (NH)2MoS4 for aluminum surfaces and

reported a lubricity improvement and better wear resistance

[17]. Other studies have also found that inorganic salts (e.g.,

sulfates, phosphates, and chlorides) and organometallic

compounds have good tribological performance on friction,

wear, and seizure for metalworking and EP applications [18,

19]. Polymers have also been used as partially soluble

additives in water-based systems in synergistic combination

with fullerene, as they enhance the antiwear and antifriction

characteristics of the base fluid [2022]. All these previous

studies show some evidence that a protective film is formed

on the interacting surfaces, which is responsible for the

friction and wear reduction observed.

Recent studies carried out by Georges et al. [23] on the

mechanism of water-based lubricants indicate that lamella

nanostructures at the mechanical contact interface provide

efficient lubrication under severe contact conditions.

This paper presents some results on tetraalkylammonium

thiomolybdates as water-soluble lubricant additives working

under boundary conditions for steelaluminum surfaces.

2 Experimental Procedure

2.1 Synthesis of Tetraalkylammonium Thiomolybdates

The tetraalkylammonium thiomolybdate salts used in the

tests were prepared in a two-step synthesis following the

method reported by Alonso et al. [2426]. In the first step,

a water-soluble ammonium thiomolybdate (NH4)2MoS4 is

prepared from ammonium heptamolybdate (NH4)6Mo7O40in an ammonia/water solution with H2S flow at room

temperature according to the following chemical reaction:

NH46Mo7O40 NH4OH=H2O flow of H2S! NH42MoS4After this, the second step involves a rapid substitution of

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

radicals [R4N]+1, where R = methyl or propyl groups

according to the following chemical reaction:

NH42MoS4 2R4NBr ! R4N2MoS4 # 2NH4BrThe resultant precipitate is the tetraalkylammonium

thiomolybdate salt (where R = methyl, propyl, or ammonium)

called tetramethylammonium thiomolybdate [(Met)4N)]2MoS4, tetrapropylammonium thiomolybdate [(Pro)4N)]2MoS4, or tetraammonium ammonium thiomolybdate which

are also soluble in water.

The procedure described for the synthesis of the thio-

molybdate salts yields approximately 80%. The resulting

structures have been analyzed by UVvis, infrared, Raman,

and TGA-DTA by Alonso et al. [2628].

2.2 Substrate Materials

The materials used as specimens in the pin-on-disk tribom-

eter were selected considering the importance of steel

aluminum mating pairs in metalworking applications. The

upper specimen was a 6-mm diameter pin made of stainless

steel 440C, while the lower specimen was a 50-mm diame-

ter 9 6-mm thick disk made of aluminum alloy 6063,

respectively. Surface roughness for the specimens was

approximately 25 and 140 nm rms, while their Brinell

hardness values were approximately 97 and 25 HB for steel

pin and aluminum disk, respectively. The disks were mirror

polished using a liquid suspension containing 0.3-lm Al2O3abrasive particles. Specimens were thoroughly cleaned in an

ultrasonic bath in boiling toluene, completely rinsed in

acetone, and finally dried, previous to the tribological tests.

2.3 Friction Tests

Tribological friction tests were carried out on a commercial

pin-on-disk tribometer (Micro Photonics-Tribometer). In

156 Tribol Lett (2008) 29:155161

123

this rig, a steel pin loaded against an aluminum disk forms

the mechanical contact. The pin is firmly secured to a

stationary holder for the pin-on-disk configuration, and the

disk is attached to a horizontal chuck driven by a variable-

speed electric motor and completely submerged in the test

fluid. A linear voltage displacement transducer attached to

the pin holder continuously monitors and records friction

force of the tribocontact.

A picture of the test rig is shown in Fig. 1.

All tests were performed at a constant temperature of

30 C and 60 5% relative humidity. A dead weight of10 N was used in all the tests carried out. Under these

conditions, the maximum Hertzian pressure is 0.877 GPa

that generates a circular contact area of 0.017 mm2. During

the tests, the aluminum specimen rotates at a constant

sliding speed of 1 mm/s for a period of 1 h, running a total

distance of 3.6 m in each test. The test conditions selected

are proper of boundary lubrication regime.

The wear rate was obtained from an LVDT sensor on the

pin. The lubricant solution was prepared by adding the

amount of salt necessary to have a constant 0.3 wt%

molybdenum concentration in three times distilled water

and vigorously mixed for 10 min for each additive solution

evaluated (methyl, propyl, and ammonium salt).

2.4 Surface Analysis

Wear tracks on the flat and ball wear scars were examined

optically and further analyzed using several surface ana-

lytical techniques at the end of the pin-on-disk sliding tests.

The morphological and chemical characterization of the

sliding surfaces after the tribological tests was carried out

with a Scanning electron microscope (SEM) and laser

Raman spectrometer. Raman spectroscopy was performed

using a LabRam model of Dilor micro-Raman system

equipped with a 20-mW HeNe laser emitting at 632.8 nm,

a holographic notch filter made by Kaiser Optical Systems,

Inc. (model supertNotch-Plus), a 2569 q1024-pixel CCD

used as detector, a computer-controlled XY stage with a

spatial resolution of 0.1 lm, two interchangeable gratings(600 and 1,800 g mm-1), and a confocal microscope with

10, 50, and 1009 objectives. All measurements were car-

ried out at room temperature with no special sample

preparation.

Worn disk tracks were also examined using a thin window

energy dispersive X-ray spectrometer (EDX) housed in a

JEOL JSM5800 LV scanning electron microscope (SEM).

EDX spectra were obtained at beam energy of 10 keV, beam

current 2.0 nA, and detector take-off angle of 25.

3 Results and Discussion

3.1 Friction Tests

Figure 2 shows the friction coefficient as a function of time

of steelaluminum mating pairs three times with distilled

water as lubricant and for the tetraalkylammonium thio-

molybdate salts synthesized and used as a lubricant

additive at a constant molybdenum concentration for every

Fig. 1 Pin-on-disk tribometer

0 1000 2000 3000 4000 50000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

(b)

(c)(d)

(R4N)2MoS4 / water

(a)

rFic

oitn

c

off

eic

ien

,t

Time / seconds

Fig. 2 Friction coefficient versus time for (a) water, (b) propyl, (c)methyl, and (d) ammonium

Tribol Lett (2008) 29:155161 157

123

solution prepared. The friction coefficient for the speci-

mens lubricated with water is initially 0.2 but increases

with distance and stabilizes at a value of *0.53.A significant friction reduction was observed when

specimens where lubricated with the synthesized thiosalts,

recording friction coefficient values in the range of 0.08

0.17. The lowest steady-state friction coefficient was 0.08

corresponding to the specimens lubricated with ammonium

thiosalt (see Table 1).

These results suggest the formation of a protective

lubricating film at the contact interface. High temperature

causes the thiomolybdate salts to be transformed to

molybdenum disulfide [24]. This is achieved by a chemical

reaction via thermal decomposition caused by the severe

rubbing contact conditions present at the interface.

3.2 Wear Mechanism

A linearly dependent penetration of the pin on the alumi-

num specimen as time passes is observed in Fig. 3. In this

graph, wear rate is represented by the slope. Table 1 pre-

sents wear rate values registered after testing the additives

prepared and the control fluid (three times distilled water).

A wear reduction of approximately one decimal point is

reached using the thiomolybdate salts as additives.

After the pin-on-disk surfaces were rubbed for 1 h in the

thiosalt solution at a constant load of 10 N, the worn speci-

mens were characterized and analyzed by SEM, EDX, and

Raman.

Representative SEM micrographs of wear track on disk

specimens lubricated with water, propyl, methyl, and ammo-

nium produced from sliding at 30 C are shown in Fig. 4ad.Some representative SEM micrographs of wear track on

disk specimens lubricated with water, propyl, methyl, and

ammonium thiomolybdates produced from sliding at 30 Ccan be seen in Fig. 5ad.

The wear track of the disk specimen lubricated with

distilled water at 30 C presented numerous parallelplowing grooves in the sliding direction in Fig. 5a. These

surface features indicate dominance of abrasive wear.

Table 1 Friction coefficient and wear rate for each lubricantevaluated

Friction coefficient Wear rate (mm/s)

Water 0.53 0.001 0.0112

Propyl 0.17 0.001 0.00166

Methyl 0.1 0.001 0.00187

Ammonium 0.08 0.001 0.00184

0 1000 2000 3000 4000 50000

10

20

30

40

50

(a)

(b) , (c)(d)

(R4N)2MoS4 / Water

Time / seconds

eW

ra

T

rack

eD

htp /

m

Fig. 3 Wear rate versus time for (a) water, (b) propyl, (c) methyl,and (d) ammonium

Fig. 4 SEM images of thecontact track for (a) water, (b)propyl, (c) methyl, and (d)ammonium. Image

magnification 279

Fig. 5 SEM images of thecontact track for (a) water, (b)propyl, (c) methyl, and (d)ammonium. Image

magnification 5,0009

158 Tribol Lett (2008) 29:155161

123

Figure 5bd reveals different surface features on the

wear track produced after being lubricated by the molyb-

denum thiosalts. The smooth regions and tiny cracks

observed in these figures indicate in this case that adhesion

and localized microcracking were the prevailing wear

mechanisms. This may be attributed to the effect of high

sulfide content and the formation of a mixed hard-brittle

phase of the tribofilm. Microcracking resulted in regions of

partial delamination of tribofilm and indicates that both

adhesion and cohesive shearing of the film control the wear

process. However, it appears that the chemical reactions of

the additive and the freshly-exposed surfaces replenished

the film expeditiously.

3.3 Chemical Analysis

The EDX analysis carried out rendered the atomic percent-

ages of molybdenum and sulfur present on the wear track of

the aluminum specimens. Table 2 shows the molybdenum-

to-sulfur relation found on the wear track of the disk speci-

mens lubricated with the molybdenum thiosalts prepared. In

all cases, an atomic ratio of approximately two is observed,

suggesting the formation of MoS2, which is responsible for

the friction and wear rate reduction noticed during the tri-

bological tests.

Figure 6 shows the characteristic distribution of alumi-

num, molybdenum, sulfur, and oxygen in a section of the

contact track of the aluminum specimen. This analysis

confirms the presence of sulfur and molybdenum especially

in zones where no aluminum debris was detected.

An EDX analysis was carried out on aluminum speci-

mens lubricated with tetraalkylammonium thiomolybdates

to determine the chemical composition of the laminated

debris formed on the track and inside the small crevices

observed. The analysis indicated that laminated debris is

composed primarily of molybdenum disulfide, which is

also present inside the cracks (see Figs. 7, 8).

Raman spectroscopy was carried out on wear particles at

different parts of the wear track. The spectra taken on the

samples analyzed were compared with those obtained from

a standard reference (Fig. 9).

Raman spectroscopy of the wear track revealed very

sharp peaks at approximately 402 and 376 cm-1 corre-

sponding to the E12g and A1g vibrational modes of

2H-MoS2 [29].

The Raman analysis of the friction formed tribofilm

proved that the platelets observed in SEM micrographs are

in fact MoS2 sheets. The changing positions of the peaks

provide additional information about the microstructure.

Plate-like layers of MoS2 can align themselves parallel to

the direction of relative motion under high stresses; so they

can slide over one another with relative ease and thus

impart low friction.

Table 2 Atomic percentages of elements detected in the contact zone

% C % O % Mo % S Mo/S ratio

Water 8.20 3.25 0 0 0

Propyl 71.54 7.18 1.92 4.21 2.19

Methyl 29.96 15.43 2.08 4.42 2.12

Ammonium 8.43 37.11 8.72 19.08 2.18

Fig. 6 Mapping of aluminum,molybdenum, sulfur, and

oxygen in a section of the

contact track

Fig. 7 SEM micrograph ofaluminum specimen lubricated

with tetralkylammonium

thiomolybdates (a) 1209, (b)10,0009

Tribol Lett (2008) 29:155161 159

123

The thiomolybdate salts prepared in the present paper

are stable at temperatures lower than 150 C. In the ball-on-disk tribometer, contact surface temperatures are higher

than 150 C, and this causes the thiomolybdate salt tothermally decompose forming an in-contact solid molyb-

denum disulfide.

The use of tetraalkylammonium thiomolybdenum salts

(precursors of MoS2) as water-soluble lubricating additives

offers an important reduction in friction and wear for high-

pressure contacts. The authors believe that oxygen and carbon

content present in the tribofilm is responsible for the main

differences on the friction coefficient values observed among

the (methyl, propyl, and ammonium) ammonium thiomo-

lybdates tested, but this assumption requires a deeper analysis.

At present some tests are being conducted to elucidate

how the hydrocarbon chain influences the hydrolysis pro-

cess, and the findings will be presented in a future

communication.

4 Conclusions

This paper presents a tribological study on tetralkylam-

monium thiomolybdates. The effectiveness of these

additives to reduce friction and wear in aqueous solution

during sliding at a temperature of 30 C was evaluated on apin-on-disk tribometer. From the results of this study, the

following main conclusions can be drawn:

1. Tetralkylammoniumthiomolybdates exhibit goodfriction

and wear reduction properties in water-based systems.

0 2 4 60

2

4

6

8

10

SMo

Al

OC

EDAX / Ammonia

Film

tisn

etnI

y

Energy, eV0 2 4 6

0

2

4

6

8

10

SMo

Al

OC

EDAX / Ammonia

Substrate

tisn

etnI

y

Energy, eV

(b)(a)Fig. 8 EDX analysis of analuminum specimen lubricatedwith tetralkylammonium

thiomolybdates (a) on film, (b)on substrate

200 250 300 350 400 450 500

0

100

200

300

400

500

600

700

MoS2

RamanHe-Ne Laser / 632.8 nm

376

402

nIet

nsyti

Raman Shift / cm-1

Fig. 9 Raman spectra on the wear track of aluminum specimenlubricated with molybdenum thiosalt

160 Tribol Lett (2008) 29:155161

123

2. The surface examination of the rubbing zone indicated

that an in-contact non-homogeneous tribofilm contain-

ing molybdenum disulfide is formed on the rubbing

surfaces during the sliding process. Tetraalkylammo-

nium thiomolybdates are transformed to molybdenum

disulfide by high-temperature triboreduction in the

contact influencing friction and wear.

3. From the thiomolybdates evaluated in this study, the

tetraammonium ammonium thiomolybdate performs

better than the rest in both friction and wear.

Acknowledgments The authors wish to express their sincere thanksto the National Council for Science and Technology (CONACyT

Projects 46871 and 43634) for the financial support to carry out the

present work.

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MoS2 Films Formed by In-contact Decomposition of Water-soluble Tetraalkylammonium ThiomolybdatesAbstractBackgroundExperimental ProcedureSynthesis of Tetraalkylammonium ThiomolybdatesSubstrate MaterialsFriction TestsSurface Analysis

Results and DiscussionFriction TestsWear MechanismChemical Analysis

ConclusionsAcknowledgmentsReferences

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