Preparation of lanthanum trifluoride nanoparticles surface-capped by tributyl phosphate and evaluation of their tribological properties as lubricant additive in liquid paraffin

Applied Surface Science 292 (2014) 971–977

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Preparation of lanthanum trifluoride nanoparticles surface-capped by tributyl phosphate and evaluation of their tribological properties as lubricant additive in liquid paraffin Zhiwei Li ∗ , Xiao Hou, Laigui Yu, Zhijun Zhang, Pingyu Zhang Key Laboratory of Ministry of Education for Special Functional Materials, Henan University, Kaifeng 475004, PR China

a r t i c l e

i n f o

Article history: Received 30 September 2013 Received in revised form 16 December 2013 Accepted 16 December 2013 Available online 25 December 2013 Keywords: Surface-capped LaF3 nanoparticles Lubricant additive Preparation Tribological properties

a b s t r a c t LaF3 nanoparticles surface-capped by tributyl phosphate (denoted as TBP–LaF3 ) were prepared by in situ surface modification route. The size, morphology and phase structure of as-prepared TBP–LaF3 nanoparticles were analyzed by means of X-ray diffraction and transmission electron microscopy. The thermal stability of as-synthesized TBP–LaF3 nanoparticles was evaluated based on thermogravimetric analysis, and their tribological properties as additive in liquid paraffin were evaluated with a four-ball friction and wear tester. Moreover, the morphology of worn steel surfaces was analyzed with a scanning electron microscope, and the composition and chemical state of typical elements on worn steel surfaces were examined with an X-ray photoelectron spectroscope. Results show that as-synthesized TBP–LaF3 nanoparticles possess good thermal stability and excellent anti-wear and load-carrying capacities as well as good friction-reducing ability. This is because, on the one hand, TBP as the surface-modifier is able to improve the dispersibility of LaF3 nanoparticles in liquid paraffin and allows good adsorption of LaF3 nanoparticles on sliding steel surfaces. On the other hand, active P element of TBP can form tribochemical reaction film on sliding steel surfaces. As a result, the boundary lubricating film consisting of adsorbed LaF3 nanoparticles and tribochemical reaction film results in greatly improved friction-reducing and antiwear abilities as well as load-carrying capacity of the lubricant base stock and gives rise to significantly reduced friction and wear of the steel–steel sliding pair. © 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years, inorganic nanoparticles have drawn much attention, due to their unique catalytic, optical, semiconductive and magnetic properties [1–4]. For example, various inorganic nanoparticles such as metal nanoparticles [5–7], metal oxide nanoparticles [8,9], metal sulfide nanoparticles [10], and graphene nanoparticle [11,12] have been studied as lubricating oil additives to improve the friction-reducing and antiwear abilities as well as load-carrying capacity of base stocks. Particularly, rare earth (RE) nanoparticles have been extensively focused on in the field of tribology, due to their fairly low hardness, hexagonal crystal structure, high melting point, and good resistance to thermal and chemical attack [13,14]. To name a few, Liu et al. [15,16] found that surfacemodified mixed rare earth nanoparticles as lubricating oil additive exhibit good friction-reducing and anti-wear abilities as well as high load-carrying capacity, while the excellent tribological properties of oil soluble mixed rare-earth alkylsalicylate are attributed

∗ Corresponding author. Tel.: +86 37123881358; fax: +86 37123881358. E-mail address: [email protected] (Z. Li). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.089

to the formation of a boundary lubricating film mainly composed of alkylsalicylic acid, rare earth oxide, and complexes of rare earth metals on rubbed surfaces. Kong et al. [17] said that cerium borate nanoparticles synthesized via sol–gel precipitation method are able to greatly decrease the friction coefficient of base oil. Hao et al. [18] preliminarily discussed the tribological action mechanism of lanthanum borate nanoparticles surface-capped by triethanolamine monooleate, and they supposed that surface-capped lanthanum borate nanoparticles are able to deposit on the worn surface and form a wear resistance film thereby improving the tribological properties of base stock. Yu et al. [19], by making use of surfacecapping with coupling and grafting agents, successfully improved the dispersibility of nano-Y2 O3 in liquid paraffin. Naturally, the dispersion of inorganic nanoparticles in lubricating oils significantly affects their tribological properties [20], which is why surface modification technique is widely used to improve the solubility of rare earth nanoclusters in liquid paraffin. In this sense, dialkyldithiophosphate (DDP) as a kind of traditional surface-modifiers for lubricating oil additives is interesting, since its active elements S and P play positive roles in improving the tribological properties of lubricating base oils [21–24]. Unfortunately, element S is harmful to environment and human body as

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well [25]. Therefore, it is imperative to pursue novel S-free surface modifying agents for rare earth nanoparticles so as to acquire surface-capped rare earth nanoparticles as lubricant additives with improved environmental acceptability. In this respect, long carbon chain organic compound like oleic acid is of significance for the surface-modification of rare earth nanoparticles like nano-cerium borate, since it is able greatly improve the dispersibility of rare earth nanoparticles in liquid paraffin affording a transparent dispersion [17]. However, oleic acid as a surface-capping agent of rare earth compounds is less competitive in the field of tribology, because it does not contain friction active element P. Bearing those perspectives in mind and viewing that Pcontaining compounds as potential surface-capping agents of rare earth compounds allow competitive adsorption of rare earths on metal surface thereby improving antiwear ability [23,26,27], we select tributyl phosphate (denoted as TBP) to surface-modify LaF3 nanocluster. The present research, hopefully, is to enhance the dispersibility of LaF3 nanoparticles in liquid paraffin and improve the anti-wear, load-carrying and friction-reducing capacities of the base stock with the assistance of the synergistic effect between TBP and LaF3 nanoparticles (TBP contains friction active P element which can form tribochemical reaction film). This paper reports the preparation of TBP–LaF3 nanoparticles as well as their effect on the tribological properties of paraffin liquid and antiwear mechanism in relation to analysis of morphology and composition of worn steel surfaces by means of scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). 2. Experimental methods 2.1. Material processing and sample preparation Ammonium fluoride was purchased from Tianjin Kermel Chemical Reagent Company (Tianjin, China). Lanthanum nitrate was purchased from Tianjin Institute of Fine Chemicals Retrocession (Tianjin, China). Tributyl phosphate was purchased from Tianjin Deen Chemical Reagent Factory (Tianjin, China). All the reagents used are of analytical purity and used without further purification. Distilled water was used as the solvent and for washing as well. Typically, 130 mL of distilled water and 20 mL of absolute ethanol were added in a 250-mL flask. Into resultant mixed solvent were dissolved 2.22 g of ammonium fluoride and 5.5 mL of TBP under magnetic stirring to yield a transparent solution. The reaction solution was gradually heated to 70 ◦ C, followed by addition of 50 mL of 0.4 M/L lanthanum nitrate solution through a dropping

funnel within 3 h. Then the mixed solution was naturally cooled to room temperature while aqueous ammonia was added to adjust its pH around 10. Precipitate, the crude product, was collected by filtration and fully washed with distilled water and absolute ethanol, followed by drying at 80 ◦ C in air for 12 h affording white powders as the desired final product. 2.2. Apparatus and experimental method X-ray powder diffraction (XRD) patterns were collected with an X’Pert Pro diffractometer (Cu K␣ radiation,  = 0.15418 nm; voltage 40 kV, current 40 mA). A JEM-2010 transmission electron microscope (TEM) was performed to observe the morphology of as-obtained TBP–LaF3 nanoparticles. A drop of the dispersion of TBP–LaF3 nanoparticles in absolute alcohol was dripped onto a copper grid covered by carbon film and then fully dried at ambient temperature to afford the sample for TEM analysis. Fourier transform infrared (FT-IR) spectrum was measured with an AVATAR360 FT-IR spectrometer. The as-prepared TBP–LaF3 nanoparticles were mixed with KBr powder and pressed into a pellet for measurement. Background correction was made using a reference blank KBr pellet. The hermogravimetric analysis (TGA) was carried out on a DSC6200 thermal analyzer. About 5 mg of samples heated from 30 ◦ C to 700 ◦ C at a heating rate of 10 ◦ C/min in flowing nitrogen. The tribological properties of as-prepared samples as lubricating additive in liquid paraffin were investigated using an MSR-10A four-ball apparatus made by Jinan Testing Machine Factory (Jinan, China). GCr15 bearing steel (SAE-52100) balls (diameter 12.7 mm, hardness HRC 61-64) purchased from Shanghai Bearing Factory (Shanghai, China) were used to assemble the frictional pair. The friction and wear tests were conducted at a rotary speed of 1450 rev/min and ambient temperature of about 25 ◦ C for 30 min; and three repeated tests were run under each pre-set condition so as to minimize data scattering. At the end of each test, the wear scar diameter (WSD) of the three lower balls was measured using an optical microscope at an accuracy of 0.01 mm. The average wear scar diameter of the three lower balls is calculated and reported in this paper. The PB (maximum non-seizure load, the load at which the measured wear scar diameter is not more than 5% greater than the compensation value at that load) values of liquid paraffin containing as-prepared products were evaluated according to Chinese national standard method GB/T 3142-82 (similar to ASTMD 27831998). The morphology of worn steel surfaces was observed with a JSM-5600 scanning electron microscope (SEM). An AXIS ULTRA multifunctional X-ray photoelectron spectroscope (XPS) was

Fig. 1. TEM images of nano-LaF3 (a) and TBP–LaF3 (b).

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Fig. 2. XRD patterns of nano-LaF3 and TBP–LaF3 .

performed to determine the composition and chemical states of typical elements in the worn surfaces of the steel balls, with which Mg-K␣ radiation was used as the excitation source and the binding energy of contaminated carbon (C1s: 284.8 eV) was used as the reference. 3. Results and discussion 3.1. Characterization of as-prepared TBP–LaF3 nanoparticles Fig. 1 shows the TEM images of nano-LaF3 and TBP–LaF3 . It is seen that both samples have a hexagonal structure, and their average particle size is in the range of 10–30 nm. Besides, surface modification of nano-LaF3 particulates by TBP helps to reduce their agglomeration and size. This is because TBP molecules anchored on the surface of nano-LaF3 particulates are able to reduce the surface energy of nano-LaF3 thereby preventing agglomeration. In the meantime, TBP molecules anchored on the surface of nanoLaF3 also help to decrease the collision rate of LaF3 crystal nucleus thereby retarding the growth of LaF3 nanoparticles. Fig. 2 shows the XRD patterns of nano-LaF3 and TBP–LaF3 (the standard XRD spectrum of LaF3 powder is also presented for a comparison). It can be seen that the XRD patterns of nano-LaF3 and TBP–LaF3 can be indexed to hexagonal structure phase of LaF3 (Joint Committee on Powder Diffraction Standards (JCPDS) File No. 32-0483). In the meantime, no other phases are detected by XRD, which indicates that surface modification by TBP does not change the crystal structure of nano-LaF3 . Fig. 3 illustrates the FT-IR spectra of nano-LaF3 and TBP–LaF3 . Both nano-LaF3 and TBP–LaF3 show broad absorption bands around 3421 cm−1 and 1642 cm−1 and weak broad absorption bands at 1400–1355 cm−1 , which are assigned to O H vibration and asymmetric stretching vibration of CO3 2− group, respectively; and both O H and CO3 2− groups are related to water and CO2 absorbed on the surface of the samples. Besides, sample nano-LaF3 shows a peak at about 561 cm−1 , and this absorption peak is assigned to the characteristic stretching vibration of La F bond. Moreover, sample TBP–LaF3 shows two new absorption peaks around 2962 cm−1 and 2875 cm−1 which are attributed to the asymmetric and symmetric C H stretching vibrations of methyl as well as a peak at 1468 cm−1 that is attributed to the C H deformation vibrations of methylene. These absorption peaks, in association with the asymmetric stretching vibration peak of C O at 1269 cm−1 and the peak of PO4 3− group at 1029 cm−1 , confirm that nano-LaF3 particulates are successfully surface-modified by TBP to afford TBP–LaF3 particulates.

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Fig. 3. FT-IR spectra of nano-LaF3 and TBP–LaF3 .

To estimate the amount of TBP modifier on the surface of LaF3 nanocore, we conducted TGA analysis of samples nano-LaF3 and TBP–LaF3 . Fig. 4 illustrates the TG curves of nano-LaF3 and TBP–LaF3 . It can be seen that sample nano-LaF3 is relatively stable and shows a slight weight loss of about 7%, possibly due to vaporization of physically adsorbed water and impurity. Sample TBP–LaF3 , however, shows a sharp weight loss from 150 ◦ C to 400 ◦ C, which should be closely related to the vaporization of physically adsorbed water and impurities as well as the thermal decomposition of organic surface-modifier. Based on relevant TGA analysis results, we can estimate that there is about 10.7% (mass fraction; the same hereafter) TBP modifier on the surface of TBP–LaF3 particulates. Fig. 5 shows a schematic diagram to illustrate possible growth mode of TBP–LaF3 nanoparticles. Briefly, the formation and growth of TBP–LaF3 nanoparticles roughly involve two consecutive steps. The first step involves the nucleation and growth of LaF3 nanocore by way of chemical reaction between La3+ cation and F− anion. At the second step, TBP molecules are attracted by and anchored onto the surface of nano-LaF3 during the in situ surface modification process thereby preventing the growth and agglomeration of nanoLaF3 and giving rise to desired TBP–LaF3 nanoparticles.

Fig. 4. TGA curves of nano-LaF3 and TBP–LaF3 .

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Fig. 5. Schematic diagram showing possible growth mode of TBP–LaF3 nanoparticles.

3.2. Tribological properties of TBP–LaF3 sample as an additive in liquid paraffin Fig. 6 illustrates the friction coefficients and wear scar diameters of the steel balls lubricated by liquid paraffin containing different concentrations of TBP–LaF3 (load: 300 N; speed: 1450 rev/min; time: 30 min; room temperature). It can be seen that the friction coefficient and wear scar diameter of the steel–steel sliding pair gradually decrease with increasing concentration of TBP–LaF3 in liquid paraffin, and higher concentrations of TBP–LaF3 correspond to larger friction coefficient and wear scar diameter. For instance, the lowest friction coefficient and wear scar diameter are obtained when 0.40% of TBP–LaF3 is added into liquid paraffin; and increase of additive concentration above 0.40% leads to slight rise of the friction coefficient and wear scar diameter therewith. Besides, when TBP is added alone as an additive in liquid paraffin, much larger friction coefficients and wear scar diameters are obtained as compared with TBP–LaF3 as the lubricant additive. This demonstrates that TBP–LaF3 could be promising lubricant additive of liquid paraffin.

Fig. 7 shows the variations of friction coefficient and wear scar diameter with different loads for liquid paraffin alone and for liquid paraffin containing 0.40% of TBP–LaF3 (speed: 1450 rev/min; time: 30 min; room temperature). With liquid paraffin alone, the friction coefficient and the wear scar diameter are relatively large, and at a load higher than 300 N, scuffing took place. On the contrary, TBP–LaF3 nanoparticles as an additive can effectively improve the antiwear ability and load-carrying capacity of liquid paraffin, though the friction coefficient and wear scar diameter of the steel–steel sliding pair considerably increase with increasing load. Besides, the friction coefficient rises to 0.108 at an applied load of 400 N, and it remains almost unchanged with further increase of the load above 400 N. 3.3. SEM analysis of worn surfaces Fig. 8 presents the SEM images of the wear scar of steel balls lubricated by liquid paraffin and liquid paraffin containing 0.40% TBP–LaF3 (load: 100 N; speed: 1450 rev/min; time: 30 min; room temperature). It is clearly seen that the worn steel surface 0.130

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lubricated by liquid paraffin alone is quite rough and shows wide and deep furrows as well as grooves along the sliding direction (Fig. 8(a and b)). Contrary to the above, the worn steel surfaces lubricated by liquid paraffin containing 0.40% TBP–LaF3 are smooth and show few furrows and grooves (Fig. 8(c and d)), which well corresponds to good anti-wear ability of TBP–LaF3 . 3.4. XPS analysis of worn surfaces The worn surfaces of upper steel ball lubricated by liquid paraffin containing 0.40% TBP–LaF3 were analyzed by mean of XPS so as to acquire more information about the tribochemical reactions involved during the sliding process. The XPS spectra of C1s, O1s, La3d, F1s, Fe2p and P2p on the rubbed steel surfaces are shown in Fig. 9 (load: 100 N; speed: 1450 rev/min; time: 30 min; additive concentration 0.4%). The C1s peak at 284.6 eV and O1s peak at

531.5 eV correspond to organic compounds adsorbed on the surface of the steel ball; and the Fe 2p peak at 709.9 eV illustrates that there exists Fe3 O4 on worn steel surface. Besides, only very weak F1s and La3d XPS signals are detected, which is because LaF3 particles are deposited on worn steel surfaces via adsorption rather than tribochemical reaction [23]. Moreover, the P2p peak at 133.4 eV, attributed to PO4 3− , is due to the adsorption of P-containing surface-modifying agent TBP on worn steel surface. Since lubricant additives containing friction active element P favor to effectively improve the tribological properties of base stocks by way of adsorption and tribochemical reaction [23–25], we can infer that TBP–LaF3 as an additive in liquid paraffin is able to form a boundary lubricating film consisting of the tribochemical reaction film of TBP containing friction active element P and the deposited film of LaF3 nanoaprticles on rubbed steel surface. As a result, friction and wear of the steel–steel contact is significantly reduced,

Fig. 8. SEM images of wear scar of the steel balls lubricated by liquid paraffin (a and b) and liquid paraffin containing TBP–LaF3 (c and d).

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Fig. 9. XPS spectra of Cu2p, Si2p, S2p and Fe2p of worn steel ball surfaces lubricated by liquid paraffin containing 0.4% TBP–LaF3 .

due to the significant improvement in the friction-reducing and antiwear abilities as well as load-carrying capacity of the lubricant base oil.

just the boundary lubricating film composed of the adsorption layer of the rare earth compound additive and the tribochemical reaction film of the surface-modifier that accounts for the significantly reduced friction and wear of the steel–steel sliding contact.

4. Conclusions Acknowledgments In situ surface-modification of LaF3 nanoparticles with TBP helps to reduce their size and prevent them from agglomeration. As-prepared TBP–LaF3 nanoparticles as a lubricant additive can effectively improve the friction-reducing and anti-wear abilities as well as load-carrying capacity of liquid paraffin. This is because, during the friction process, LaF3 nanocores can be released from TBP–LaF3 nanoparticles and deposited on rubbed steel surfaces to form an adsorption layer, while surface-modifier TBP containing friction active element P can participate in tribochemical reaction to form tribochemical reaction film on rubbed steel surfaces. It is

The authors acknowledge the financial support provided by the Ministry of Science and Technology of China (project of “973” plan; grant No. 2013CB632303) and the National Natural Science Foundation of China (grant No. 21371050). References [1] S.M. Pourmortazavi, M. Rahimi-Nasrabadi, M. Khalilian-Shalamzari, M.M. Zahedi, S.S. Hajimirsadeghi, I. Omrani, Synthesis, structure characterization and

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