Wear 256 (2004) 176–181

Synthesis and tribological behavior of surface modified(NH4)3PMo12O40 nanoparticles

Lei Suna,b,∗, Jingfang Zhoub, Zhijun Zhangb, Hongxin Danga,ba State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, PR China

b Laboratory of Special Functional Materials, Henan University, Kaifeng 475001, PR China

Received 20 November 2002; received in revised form 14 April 2003; accepted 7 May 2003

Abstract

Surface modified (NH4)3PMo12O40 nanoparticles were synthesized in a mixture solution of water–ethanol with cetyltrimethyl am-monium bromide as a modifying agent. The morphology, structure and thermal properties of modified (NH4)3PMo12O40 nanoparticleswere investigated by means of transmission electron microscopy, X-ray powder diffraction, Fourier transform infrared spectrum, thermo-gravimetric analysis, and differential scanning calorimetry. Their tribological behaviors were evaluated on a four-ball testing machine.The results show that the modified (NH4)3PMo12O40 nanoparticles as oil additives can improve the load-carrying capacity and antiwearproperty of the base oil. The rubbed surfaces were investigated by scanning electron microscopy and X-ray photoelectron spectroscopy.It can be inferred that a tribological reaction film was formed, which contributed to the good tribological properties of surface modified(NH4)3PMo12O40 nanoparticles.© 2003 Elsevier B.V. All rights reserved.

Keywords: (NH4)3PMo12O40; Nanoparticles; Tribological properties

1. Introduction

Nano-scale particles have received considerable attentionin recent years, since their physical and chemical proper-ties are quite different from those of the bulk materials orindividual molecules. The synthesis and tribological proper-ties of nanoparticles have been conducted in many scientificfields[1–4]. However, the application of inorganic particlesin tribology was restrained due to their poor oil solubility.In order to improve the compatibility with lubricating oil, asurface modification technique using organic compounds isemployed. Since organic compounds are long-chain hydro-carbons, the modified inorganic nanoparticles possess goodsolubility in lubricating oil. Up to now, the surface modifiednanoparticles of chalcogenides[5,6], metals[7,8], oxides[9], and rare earth compounds[10,11] used as lubricatingoil additives have been investigated.

Heteropoly compounds have been used extensivelyin catalyst, non-linear optical materials, and anti-cancermedicines[12], etc. But few reports on its use in tribol-ogy have been presented. Since the heteropoly compoundcontains many elements on one molecule, among them

∗ Corresponding author. Tel./fax:+86-378-285-2533.E-mail address: hnsunlei@sohu.com (L. Sun).

elements such as N, P, S, and Mo have tribological activ-ity, so heteropoly compound can certainly become a newkind of lubricating additive. In this paper, we synthesized(NH4)3PMo12O40 nanoparticles modified by cetyltrimethylammonium bromide (CTAB), and investigated their tribo-logical properties as lubricating additives in liquid paraffin(LP).

2. Experimental

2.1. Chemicals and equipment

The reagents used in the experiment are all analyticallypure, and they are as follows: perchloric acid, phosphoricacid, sodium molybdate, ammonium chloride and CTAB.The morphology observation of the (NH4)3PMo12O40particles was performed on HITACHI H-600 and JEOLTEM-2010 transmission electron microscopy (TEM). Thecrystalline structure of nanoparticles was characterized ona Philips X’pert pro X-ray powder diffraction (XRD). Toconfirm the modification structure, an AVATAR 360 Fouriertransform infrared spectrum (FT-IR) was used. The ther-mal properties of nanoparticles were evaluated on a PerkinElmer 7 thermal analyzer.

0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0043-1648(03)00386-7

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2.2. Synthesis of (NH4)3PMo12O40 nanoparticles

(NH4)3PMo12O40 nanoparticles modified by CTABwere prepared in our laboratory. Na2HMo12O40·14H2Owas synthesized according to literature[13]. We synthe-sized a series of samples which have different contentsof (NH4)3PMo12O40. Preparation methods of all sam-ples are essentially the same, the difference being in theratio of NH4Cl to CTAB used. A typical procedure ofpreparation of sample with (NH4)3PMo12O40 of 75% isas follows: first, 0.24 g NH4Cl and 40 ml distilled waterwere added into a 250 ml flask, the solution was heatedto 60◦C with stirring; 0.55 g CTAB which was previouslydissolved in 10 ml of ethyl alcohol was added, the solutionof Na2HMo12O40·14H2O (3.74 g) in 50 ml distilled waterwas then dropped into the flask with vigorous stirring, im-mediately, a yellow precipitate was produced. After 1 h ofcontinuous stirring and 12 h of standing, the yellow precip-itate was collected by filtering, and was rinsed with ethylalcohol and distilled water, respectively, and was desiccatedat room temperature. The yellow powder (sample a) wasthe expected surface modified (NH4)3PMo12O40 nanopar-ticle. Non-modified (NH4)3PMo12O40 particle (sample b)was prepared with the same procedures as above, exceptthat CTAB was not used.

2.3. Test of tribological properties

The maximum non-seizure load (pB value) is the high-est applied load measured, under which the system didnot seize between the upper ball and the three lower balls.It is a representation of the load-carrying capacity of oil.In this work, thepB value and the tribological propertiesof (NH4)3PMo12O40 nanoparticles were evaluated on afour-ball testing machine. ThepB value was determined ac-cording to GB 3142-82, which is similar to ASTM D2783.The φ12.7 mm balls used in the test were made of GCr15

Fig. 1. FT-IR absorption spectra of sample a and sample b.

bearing steel (composition: 0.95–1.05% C, 0.15–0.35% Si,0.24–0.40% Mn,<0.027% P,<0.020% S, 1.30–1.67% Cr,<0.30% Ni and<0.025% Cu) with a HRC of 61–64. Thebase stock was chemically pure paraffin oil, with a boilingpoint over 300◦C, density of 0.835–0.890 g/cm3, and aviscosity at 50◦C of 10.28 mm2/s. Samples as oil additiveswere dispersed ultrasonically in the LP and tested on thefour-ball testing machine with a speed of 1450 rpm. Beforeeach test, the balls and specimen holders were cleaned inpetroleum ether, and then dried in hot air. At the end ofeach test, the wear scar diameter (WSD) of the three lowerballs was measured on a digital-reading microscope to anaccuracy of 0.01 mm. Then the average WSD of the threeballs was calculated.

2.4. Analysis of the rubbed surface

The morphology of the rubbed surface was observed on aJEOL JSM-5600LV scanning electron micrograph (SEM).The binding energies of some typical elements on theworn surface were measured on a Perkin-Elmer PHI-5702multi-technique surface X-ray photoelectron spectrometer(XPS). The pass-energy was 29.35 eV, the Mg K� was usedas the excitation source, and the binding energy of C 1s(284.6 eV) was used as the reference. The binding energiesof the elements tested were measured to an accuracy of±0.3 eV.

3. Results and discussion

3.1. Dispersion capacity

The surface modified (NH4)3PMo12O40 nanoparticles canbe dispersed in several organic solvents, including chlo-roform, benzene, methylbenzene, and LP, whereas bare(NH4)3PMo12O40 particles can hardly be dispersed in these

178 L. Sun et al. / Wear 256 (2004) 176–181

Table 1The load-carrying capacity of (NH4)3PMo12O40 nanoparticles in differentadditive contents

w (%)

0.00 0.12 0.25 0.50 1.00

pB (N) 274 510 540 560 627

solvents. So it is concluded that after the surface modifica-tion with CTAB, the dispersion capability of (NH4)3PMo12O40 is improved. The improvement in the dispersion capa-bility enabled the surface modified (NH4)3PMo12O40 to beused as an additive in lubricating oils.

3.2. The FT-IR analysis of the nanoparticles

Fig. 1 is the FT-IR spectra of sample a and sample b.The Keggin style heteropoly acid anions have strong char-acteristic absorption bands in the range of 1100–700 cm−1.It can be seen fromTable 1 that sample a and sample bhave similar absorption bands of Keggin style as comparedwith Na2HMo12O40·14H2O [13], which indicates that the[PMo12O40]3− bone structure was not destroyed after theformation of ammonium salt. Besides the above character-istic frequencies of the heteropoly acid anion, in sampleb there is a broad band at 3212 cm−1, and a sharp bandat 1414 cm−1, which are the feature absorption of –NH4

+group, this indicates the formation of (NH4)3PMo12O40.Sample b has also a broad band at 3433 cm−1, which isattributed to the stretching vibration of –OH, and the ab-sorption band at 1629 cm−1 is assigned to the deformationvibration of H–O–H. These are the evidences of the exis-tence of water of adsorption water in/on sample b.

The IR spectrum of sample a is similar to sample b, butnew bands at 2923 and 2852 cm−1 are emerged as comparedwith sample b, which are attributed to the asymmetric andsymmetrical stretching vibrations of –CH2–. The absorptionband of 1473 cm−1 is attributed to the asymmetric stretchingvibration of –CH3; they are obviously caused by the longaliphatic chains. Combined with the dispersion properties ofsamples, we assume that quaternary ammonium is coatedon the surface of (NH4)3PMo12O40 nano-cores, which sup-presses further adsorption of H2O molecule.

3.3. The morphology and structure of nanoparticles

The TEM images of sample a (50 000×) and sample b(6000×) are shown inFig. 2. It can be seen that the aver-age grain-size of the unmodified particles is about 1.2�mwith serious aggregation, however, in the modified samplethe particle size is with a mean graininess of 20 nm, oc-curs in no obvious aggregating phenomenon. It is becausethat the organic modification agent is adsorbed on the sur-face of nanoparticles, which can reduce the surface energyof the nanoparticles, and prevent the further aggregation of

Fig. 2. TEM images of sample a and sample b (sample a: 50 000×;sample b: 6000×).

nanoparticles. The results show that the existence of organicmodification shell can control the size of particles, and im-prove the dispersity of the nanoparticles.

Fig. 3shows the XRD pattern of sample a. It can be seenthat the peaks of sample a matched well with the cubic lat-tice (NH4)3PMo12O40·4H2O standard positions which wereindicated with vertical lines. Combined with FT-IR analysis,it can be confirmed that the (NH4)3PMo12O40 nanoparticleshave been synthesized successfully.

3.4. The thermal analysis of nanoparticles

The TGA and DSC curves of sample a are given inFig. 4,the conditions of measurement are as follows: in air, the

L. Sun et al. / Wear 256 (2004) 176–181 179

Fig. 3. XRD pattern of sample a.

temperature is programmed at 10◦C/min. It can be seen fromthe TGA curve, the weight loss of sample a can be dividedinto two stages. The first stage is from 18 to 75◦C, the lossof weight is only 1.5%, which is caused by the loss of crystalwater. The second stage can be divided into two steps: first, adrastic loss of weight of sample a of 2% at 250–260◦C, thena loss of weight of 14% occurs in the range of 260–500◦C.No further loss of weight occurs in 500–700◦C. Correspond-ingly, one endothermic peak emerges from 250 to 500◦C.In this stage, the modified layer and the ammonium ion de-composed, and was oxidized to CO2, H2O and N2, whichcauses the loss of weight. The heteropoly acid anions af-ter losing the counter-cation cannot steady exist. It will de-compose to forming MoO3, MoP2O8, etc. [14]. There isa very strong endoergic peak at about 430◦C on the DSC

Fig. 4. TGA and DSC curve of sample a.

curve correspondingly. The results show that the decompo-sition temperature of (NH4)3PMo12O40 nano-cores is about300◦C.

3.5. Tribological properties

Fig. 5 gives the tribological properties of a series ofsamples which have different contents of (NH4)3PMo12O40and pure LP. The test condition is as follows: rotatingrate 1450 rpm, load 294 N, test duration 30 min, additivecontent 0.5% (weight fraction). The results show that thesurface modified (NH4)3PMo12O40 nanoparticles as oiladditives can improve the tribological properties of LP, es-pecially when the (NH4)3PMo12O40 content of sample is75% (sample a). This suggests that the antiwear property

180 L. Sun et al. / Wear 256 (2004) 176–181

Fig. 5. Tribological properties of samples which have different(NH4)3PMo12O40 content added in LP (four-ball testing machine, additivecontent: 0.5%, rotating rate: 1450 rpm, test duration: 30 min) and pure LP.

of nanoparticles is mainly attributed to the inorganic cores,namely (NH4)3PMo12O40. The sample a (content of 75%)has higher (NH4)3PMo12O40 content than other samples,so it exhibited the best tribological properties. The unmod-ified (NH4)3PMo12O40 cannot disperse in LP well, so itstribological properties is not the optimum.

Fig. 6 gives the tribological properties of different ad-ditives and pure LP versus applied loads. (The conditionsare: contents of additives 0.5%, rotating rate 1450 rpm, testduration 30 min, the loads are set at 10 kgf (98 N), 20 kgf(196 N) to 60 kgf (588 N).) It can be seen that the failureload of LP is 392 N (the machine is seized at this load); thesame value of modifying agent, CTAB is 294 N, the seizureload of LP containing surface modified (NH4)3PMo12O40nanoparticles is increased to 588 N. On the other hand, atthe same applied load, the WSD of LP containing nanopar-ticles is smaller than that of pure LP, but when modifyingagent is added to LP, its antiwear property deteriorates,which indicates that the antiwear ability of modifying agent

Fig. 6. The relationship between tribological properties and load (four-ball testing machine, additive content: 0.5%, rotating rate: 1450 rpm, test duration:30 min).

Table 2Elements binding energy and atomic concentration on the wear scar

Element

Mo 3d N 1s P 2p O 1s

Binding energy (eV) 232.5 400.1, 398.4 133.2 531.8, 530.2Content (%) 1.39 8.98 0.20 89.42

is very poor, the possible reason is the adsorption ability ofelement nitrogen in quaternary ammonium salt is poor, andit is not easy to form boundary film. So we can concludethat the good antiwear property of nanoparticles mainly at-tributed to the (NH4)3PMo12O40 cores fromFig. 6. It alsocan be seen that the surface modified (NH4)3PMo12O40nanoparticles display friction reduction ability to someextent.

Table 1gives pB values of pure LP and containing thesample a. It can be seen that (NH4)3PMo12O40 nanoparticlesas lubricating additives can effectively improve the load ofbase oil. And the content of additives is higher, thepB valueis bigger, when the additive content is 1.00%, thepB valueis 2.3 times as compared with pure LP.

3.6. The preliminary investigation of lubricationmechanism of (NH4)3PMo12O40 nanoparticles

Fig. 7shows the morphologies of the worn surfaces lubri-cated by LP and LP with (NH4)3PMo12O40 nanoparticles(sample a). It can be found that the WSD of the latter issmaller and the worn surface is smoother, the rubbed effectis lower. XPS analysis of the boundary film formed on theworn surfaces was performed, and using the sensitivity fac-tor method, we can also obtain the relative atomic concen-trations of the elements on the worn surface. The results areshowed inTable 2. We can infer that the main substanceson the worn surface are (NH4)2Mo2O7 (Mo 3d: 223.50 eV),Fe2O3 (O 1s: 530.20 eV) and [P2O7]4− (O 1s: 531.80 eV, P

L. Sun et al. / Wear 256 (2004) 176–181 181

Fig. 7. SEM morphologies of the worn steel surfaces lubricated with LP and LP/(NH4)3PMo12O40 (four-ball testing machine, additive content: 0.5%,rotating rate: 1450 rpm, load: 294 N test duration: 30 min): (a) wear scar for LP (100×); (b) wear scar for LP (500×); (c) additive of (NH4)3PMo12O40

nanoparticle (100×); (d) additive of (NH4)3PMo12O40 nanoparticle (500×).

2p: 133.20 eV), etc. There are two peaks in binding energyof N, it is because of the different chemical environmentof [NH4]+ and organic amine ion. From thermal analysisit can be concluded that the decomposition temperature of(NH4)3PMo12O40 nanoparticles is about 300◦C. Under thehigh pressure and high temperature surroundings of tribo-chemical reaction, the nanoparticles can decompose andform to a boundary film containing above substances. Sowe confer that the lubrication effect of (NH4)3PMo12O40nanoparticles is attributed to the formation of this film.

4. Conclusions

Surface modified (NH4)3PMo12O40 nanoparticles weresynthesized in a mixture solution of water–ethanol, theirmorphology and structure were characterized by TEM,XRD, FT-IR, TGA and DSC, etc. The results show thatthe modified nanoparticles can disperse in organic solventwell, and have an average graininess of 20 nm. The tribo-logical properties of nanoparticles as lubrication additivesin LP were investigated, the results show that the syntheticnanoparticles have good antiwear and certain friction reduc-tion ability. Through the analysis of rubbed surface by XPS,we infer that a boundary film was formed in the tribologicalreaction.

Acknowledgements

The authors wish to acknowledge the financial supportof Natural Science Foundation of Henan province (grantno: 004030400) and Science Research Foundation of HenanUniversity.

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