Study on the selectivity of calcium carbonate nanoparticles under the boundary lubrication condition

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Study on the selectivity of calcium carbonate nanoparticles under the boundary lubrication condition Nan Xu, Ming Zhang, Weimin Li, Gaiqing Zhao, Xiaobo Wang, Weimin Liu

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S0043-1648(13)00440-7 http://dx.doi.org/10.1016/j.wear.2013.07.010 WEA100791

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Received date: 25 February 2013 Revised date: 9 July 2013 Accepted date: 19 July 2013 Cite this article as: Nan Xu, Ming Zhang, Weimin Li, Gaiqing Zhao, Xiaobo Wang, Weimin Liu, Study on the selectivity of calcium carbonate nanoparticles under the boundary lubrication condition, Wear, http://dx.doi.org/10.1016/j. wear.2013.07.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Study on the selectivity of calcium carbonate nanoparticles under the boundary lubrication condition Nan Xu, Ming Zhang, Weimin Li, Gaiqing Zhao, Xiaobo Wang*, Weimin Liu State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Abstract In this study, three kinds of calcite calcium carbonate nanoparticles (CCNP) with different average diameters as eco-friendly grease additives were successfully fabricated via the carbonation method. The morphologies and phase compositions of the CCNP were determined via XRD, FTIR and TEM. The effects of the concentration and size of synthesized products on the tribological properties of grease have been investigated via an Optimol-SRV IV oscillating friction and wear tester (SRV) and MicroXAM 3D non-contact surface mapping proler. The results show that the tribological properties of grease can be improved significantly by addition of CCNP. There exists an optimum CCNP concentration, where the grease can exhibit simultaneously optimal anti-wear and friction-reducing properties. Meanwhile, the final tribological performances of the grease not only depend on the mechanical properties of tribofilm formed with CCNP, but also lie on the perfection of grease fibre structure. More importantly, it is interesting to observe a selectivity of nanoparticles with different size for the test conditions. That is, the larger CCNP exhibits optimal performance under higher frequency, while for the smaller CCNP, it happens under higher load and lower frequency. Keywords: Sliding wear; Lubricant additives; Boundary lubrication; Surface analysis 1 

1. Introduction Friction between dry and some types of lubricated bearing surfaces is unavoidable and is an important reason for failure of mechanical components. Over the past decade, the search for the new additives with excellent tribological performances has attracted considerable interest. One example of such materials is nano-scale material. To our knowledge, different kinds of nanoparticles as grease or oil additives, such as metals [1-3], metal oxides [4-6], metal sulphides [7-9], rare earth compounds [10-12], and borides [13-15], has been investigated, due to their unique physical and chemical properties. They could greatly improve the anti-wear and extreme pressure properties, reduce friction coefficient, and even retard the thermoinduced oxidation of lubricant [16-20]. However, most of the reported nanoparticles either contain sulphur and phosphorus atoms or involve heavy metals, which is a potential threat to the environment. Therefore, green additives have received more attention recently, such as calcium carbonate. To date, Ji et al. [21] indicated that the calcite calcium carbonate with diameter of 45nm exhibited excellent tribological performance in the lithium grease. Zhang et al. and Jin et al. [22-23] found that calcium carbonate nanoparticles as oil additives could also dramatically improve the load-carry capacity, anti-wear and friction-reducing properties in lubricating oil. Up to now four different lubricantion mechanisms of nanoparticles have been reported, that is, colloidal effect, rolling effect, protective film, and third body [7, 10, 24-31]. However, limitations can be lack of investigations on the influences of the phase composition and size of nanoparticles, which would determine the mechanical properties of tribofilm and further affect the final tribological properties. Meanwhile, for better understanding the lubrication 2 

mechanisms of nanoparticles, the materials, with the characteristic of easy-controlled phase composition and size, have more advantages. Through the investigation, the selectivity of nanoparticles with different sizes and phase compositons for the operating condition can be revealed, which is beneficial to further explore the lubrication mechanism. Therefore, it is attractive to have a study on the tribological performance of calcium carbonate nanoparticles with different size as grease additives. Herein, three kinds of CCNP with different average diameters were successfully fabricated by the carbonation method. Meanwhile, the effects of the size and concentration of CCNP on the tribological performances were also investigated, respectively. More importantly, we first report the selectivity of nanoparticles with different size for the test conditions in this study. 2. Experimental 2.1. Preparation of CCNP and lithium-calcium grease All of the starting materials were analytical grade and used without further purification. CCNP were synthesized via the carbonation method [32]. Typically, calcium hydroxide (5 wt.%) solution was firstly prepared by dispersing calcium oxide into distilled water (80 °C), which was kept overnight for aging. An ethanol solution with a certain content of oleic acid (OA) was added to the calcium hydroxide suspension above. Then, the mixture was stirred at room temperature under magnetic stirring and the gaseous CO2 at a flow rate of 1 L/min was carried into mixture solution with the reaction temperature of 20 °C. When the pH value of the solution reached 7, the reaction was stopped. The white precipitate was washed with deionized water and ethanol several times, respectively. The lithium-calcium grease was prepared as follows: proper amounts of 10cst mineral oil 3 

and 12-hydroxy-stearic acid were added in a container under stirring at 90 °C. Then, proper amounts of lithium hydroxide and calcium hydroxide aqueous solution were mixed and added into the above solution. After the saponification at 100-120 °C for 3 h, the mixture was heated to evaporate water at 160 °C, followed by addition of amounts of 10cst at 200 °C. Finally, the weight percentage of lithium-calcium soap is 12.5 wt.%. Then the mixture was ground on a triple-roller mill for three times. In order to study the effects of CCNP on the tribological properties of lithium-calcium grease, CCNP were added into base grease, mixed by mechanical stirring and ground for five times on the triple-roller mill. The synthesized greases were donated as H-x, x being equal to 0, 1, 3, 5, 7, or 9 (see Table 1 for details). 2.2. Friction and wear tests The boundary lubrication performances of CCNP, used as additive in the lithium-calcium grease, were investigated through an Optimol-SRV IV oscillating friction and wear tester. The SRV friction and wear test was conducted in a conventional reciprocating “ball-on-block” mode, with an oscillating upper ball (AISI E52100 steel, diameter 10mm, HV 710-730) and an fixed lower disc (AISI E52100, HV 710-730). A series of experiments, with applied load from 300 to 500 N and frequency from 10 to 40 Hz, were designed to investigate the tribologcial performances of CCNP as grease additive. The test temperature was kept at 25 °C. Every test under given testing condition was repeated three times to guarantee the accuracy of the testing above. For evaluating the anti-wear property, MicroXAM 3D non-contact surface mapping proler was employed to measure the wear volumes of the wear scar of lower discs. 2.3. Characterization The size and morphology of CCNP were observed with transmission electron microscope 4 

(TEM, Model JEOL JEM 1200, Japan). The phase composition and purity of CCNP was characterized by a X-ray diffractometer (XRD, Riga Ku D/max-RB, Japan) equipped with Cu K radiation (=1.54056 Å, 40 kV, 30 mA). Fourier transformation infrared (FTIR) spectra of synthesized CCNP were recorded on an IFS 66v/S FT-IR spectrometer (Bruker, Germany) using the KBr disk method. Rheological measurements were carried out on a HAAKE RS6000 (Germany) Rheometer with a coaxial cylinder sensor system (Z41Ti). In steady shear experiments, the shear rates were 0.01 and 100 s-1. 3. Results and discussion 3.1. Characterization of calcite calcium carbonate nanoparticles Calcite calcium carbonate nanoparticles were prepared via the carbonation method, with appropriate amount of OA as the surfactant to control the size of CCNP. The morphologies of synthesized products have been studied with TEM. Figure 1(a)-(c) show that most of cubic nanoparticles are uniform and well dispersed, with the average diameters of ca. 30 nm, 50 nm and 80 nm, which are denoted as CCNP30, CCNP50 and CCNP80, respectively. The X-ray diffraction pattern of the corresponding CCNP is displayed in figure 2(a). The image shows the main diffraction peak of the synthesized products at 2 = 29.4°, 48.5°, 47.5° and 39.4°, which is attributed to calcite calcium carbonate (CaCO3, JCPDS 25-1033), indicating that the main phase is calcite calcium carbonate. Figure 2(b) presents the FTIR spectrum of CCNP. The absorption bands at 2926 cm-1 and 2858 cm-1 are the anti-symmetric and symmetric C-H stretching vibrations of the -CH- groups in the hydrocarbon moiety, which reveal the presence of the long alkyl chains of the OA. Meanwhile, the bands at 1430 cm-1, 876 cm-1 and 714 cm-1 identify the presence of calcite phase [33]. 5 

3.2. Tribological performance of the greases with different CCNP concentrations Under boundary lubrication condition, the friction-reducing and anti-wear properties are mainly dependent on the characteristics of nanoparticles, such as size, shape, concentration, phase and chemical compositions [21-23, 26-28]. Figure 3 reveals the influence of the concentration on the tribological properties of the grease with applied load of 300 N and frequency of 25 Hz. The dynamic friction curve corresponding to base grease shows violent fluctuations. After the initial short seizure at about the testing time of 5min, the friction coefficient always maintains at high level. With increasing the CCNP30 concentration, the friction-reducing property is obviously improved with stable and low friction coefficient for the entire test time. However, when the CCNP30 concentration is  3 wt.%, the friction coefficients are almost independent of the concentration, that is, give the similar values. Figure 3(b) displays the concentration-wear volume curves under different CCNP30 concentrations and the corresponding 2D images of wear scars of lower disc as shown in the inset. The results indicate that the addition of CCNP30 has a pronounced effect on improving the anti-wear property. Meanwhile, the wear volume show a tendency of decreasing at the beginning and increasing later with the increase of the CCNP30 concentration. In addition, figure 4 displays the 2D images of the wear scars of upper balls under different concentration. It can be observed that the wear is obviously decreased. The diameters of wear scars of upper balls are summarized in table 2. It shows a similar tendency of decreasing at first and then increasing, which is consistent with the results above. Under boundary lubrication condition, the CCNP30 are captured by the rubbing pairs and a tribofilm are formed through deposition or chemical reaction, which will be demonstrated in the later part of XPS analysis. The 6 

presence of tribofilm should contribute to the anti-wear properties. Herein, it can be assumed that the excessive CCNP30 content may favor the formation of the thicker tribofilm, which lead to higher friction coefficient value and lower wear volumes due to higher content of deposition. However, the assumption above is obviously contradictory to the results above. Therefore, it can be concluded that the tribological performance of grease under boundary lubrication condition is not just depending on the tribofilm properties. It has been known that the base grease is a kind of colloid system consisting of base oil and a cross-linked soap fibre network. A perfect soap fibre network plays an important role in the continuous transmission of nanoparticles additives from the supply zone to the transition zone, which further improves the formation of tribofilm in the next step. The presence of nanoparticles may have either beneficial or detrimental effects on the stability of the colloid system, which could further influence the tribological properties. Here, rheological data can provide more detailed information about the influence of nanoparticles on the grease microstructure. In our current study, the steady shear measurements were performed for the grease with CCNP as additives. The steady shear viscosities of base grease and the greases with different CCNP concentration as a function of time under constant shear rate are shown in figure 5. It has been known that the viscosity of grease could reflect the strength of microstructural network. The shear behaviors of greases with different CCNP concentration are similar to that of its base grease but exhibit remarkable viscosity increment under low shear rate (Figure 5(a)), which offer a powerful proof for the formation of a more stable colloid system. Whereas under high shear rate (100 s-1), it is interesting to observe that the viscosity of greases with CCNP as additives increases at first and then decreases with the 7 

increase of concentration (Figure 5(b)). Therefore, excessive CCNP concentration is more likely to cause the cross-linked network to be disrupted, especially at the rigorous test conditions, which may lead to the aggregation of soap fibre and hence limits the amount of CCNP supplied from supply zone to transition zone and further hinders the formation of tribofilm as shown in figure 6(c). Then it is not difficult to understand why there exists an optimum concentration for improving the friction-reducing and anti-wear properties simultaneously. Meanwhile, the presence of CCNP will not largely influence the typical properties of the synthesized greases (see details in Table 1). Therefore, optimum concentration of CCNP (5 wt.%) is observed depending on its excellent friction-reducing and anti-wear properties simultaneously without destructively disturbing the soap fiber network, which is also vital to lubrication reliability and effectivity in the boundary lubrication state. 3.3. Tribological performance of the greases containing CCNP of different sizes Based on our work above, the appropriated concentration of 5 wt.% was identified. In order to further investigate the influences of the CCNP size on the triobligical performance, the synthesized products with fixed concentration of 5 wt.% were chosen. In this part, the study focuses on the selectivity of nanoparticles with different size for the test conditions. Figure 7 shows the effects of the size of CCNP on tribological properties under different applied loads from 300 to 500 N and fixed frequency of 25 Hz. It can be observed that the greases with different additives almost give the same friction coefficient under low load. Further increasing the applied load, the smaller CCNP displays better friction-reducing property and load-carrying capacity. After the short seizure at about the testing time of 21 min, 8 

the friction coefficient corresponding to the CCNP30 maintains at stable higher value. However, when taking CCNP50 and CCNP80 as grease additives, the reciprocation movements between two rubbing contact cannot move on due to poor lubrication.

In

addition, the corresponding wear volumes are similar under low load, while the CCNP30 displays optimal anti-wear property under high load, as shown in figure 7(d). Figure 8 shows the tribological performance under different frequency (10 Hz, 25 Hz and 40 Hz) and fixed applied load of 400N. It can be observed that when the frequency was low, the dynamic friction coefficient curves coincide with each other, while under high frequency the CCNP with larger size exhibit better friction-reduction property. Meanwhile, the smaller CCNP displays the optimal anti-wear property under low frequency, while the larger CCNP exhibits excellent performance at high frequency, as shown in figure 8(d). Based on the work above, the selectivity of CCNP for the test conditions (applied load and frequency) was manifested. It can be concluded that the CCNP with larger diameter exhibited better performance under higher frequency, while the CCNP with smaller diameter owned more excellent performance under higher applied load and lower frequency. Figure 9 shows the SEM images of the wear scars, corresponding to figure 8(b). It can be observed that the wear scars with CCNP30 and CCNP50 as additives were smooth, while the wear scar lubricated by CCNP80 showed signs of grooves and gouging. These detailed results are in accordance with the tribological performances above. XPS analysis is used to further clarify the chemical states of elements on the worn surface and analyse the mechanism of friction-reducing and anti-wear properties. Figure 10 gives corresponding XPS spectra of C, O, Fe and Ca of the wear scar in figure 8. The peak of C1s at 284.6 eV is identied as C in air, 9 

while the peak at 288.6 eV is attributed to the C in carboxyl group. Meanwhile, the weak peak at high binding energy of 290.0 eV is associated with carbonate [34] and the main Ca2p peak appearing at 347.3 eV is attributed to calcium in calcium carbonate and the weak peak at 346.2 eV is attributed to calcium in calcium oxide [35]. The results above indicate the existence of calcium carbonate and calcium oxide. In addition, the O1s peak appearing at 529.6 eV is attributed to oxygen in iron oxide and calcium oxide, while the peak at 531.8 eV is associated with oxygen in a carboxyl group. The peak around 710.0 eV in the spectrum of Fe2p corresponds to iron oxide [36, 37]. It can be concluded that tribofilms were formed on the contact surface, which have the same chemical compositions, consisting of calcium carbonate, calcium oxide, iron oxides and other organic compounds. Therefore, the mechanism of protective film is operating in this study. It has been well-known that the tribological performances of the grease under boundary condition mainly depend on the intrinsic mechanical properties of the tribofilm. After crushed by the rubbing contact, the CCNP deposited and formed a continuous tribofilm and the grain structure of calcite calcium carbonate still exists, which has been demonstrated in the XPS analysis. In addition, the hardness and yield stress of nanocrystalline materials increase with decreasing the grain size, a phenomenon known as the Hall-Petch effect. The finer grains provide strength, and the coarser grains maintain or even enhance ductility [38]. Therefore, based on the same chemical compositions and phase compositions of tribofilms, the selectivity of CCNP for the test conditions may be due to the differences of the mechanical properties, which depending on the different sizes of the CCNP. 4. Conclusions 10 

In summary, three kinds of CCNP, with different average diameters, were successfully prepared via the carbonation method. The influences of the size and concentration of CCNP were investigated via SRV test machine, which showed that the anti-wear and friction-reduction properties were obviously improved by the CCNP. There exists an optimum CCNP concentration, where the grease can exhibit simultaneously optimal anti-wear and friction-reducing properties. Meanwhile, the selectivity of CCNP for the test conditions (applied load and frequency) was manifested. The CCNP with larger diameter exhibited better performance under higher frequency, whereas the CCNP with smaller diameter owned more excellent performance under higher applied load and lower frequency. Here we demonstrated a concept that the final tribological performances of the grease not only depend on the mechanical properties of tribofilm formed with CCNP, but also lie on the perfection of grease fibre network. Finally, the chemical states of elements on the worn surface were analysized via XPS, which indicated that a boundary lubrication film was formed on the rubbing surface, composed of calcium carbonate, calcium oxide, iron oxide and some organic compounds. The excellent tribological performance might be attributed mainly to a tribofilm of calcium carbonate. Meanwhile, based on the same chemical compositions and phase compositions of tribofilms, the selectivity of CCNP for the test conditions may be due to the differences of the mechanical properties, which depending on the different sizes of the CCNP. Acknowledgment: This work was funded by the National 973 Program of China (Grant No. 2011CB706602) and National Natural Science Foundation of China (Grant No. 51205384). References [1] G. Liu, X. Li, N. Lu, et al., Enhancing AW/EP property of lubricant oil by adding nano 11 

Al/Sn particles, Tribology Letters 18 (2005) 85-90. [2] H. L. Yu, Y. Xu, P. J. Shi, et al., Tribological properties and lubricating mechanisms of Cu nanoparticles in lubricant, Trans. Nonferrous Met. Soc. China 18 (2008) 636-641. [3] M. Zhang, X. B. Wang, W. M. Liu, et al., Performance and anti-wear mechanism of Cu nanoparticles as lubricating oil additives, Ind. Lubric. Tribol. 61 (2009) 311-318. [4] A. H. Battez, J. E. F. Rico, A. N. Arias, et al., The tribological behavior of ZnO nanoparticles as an additive to PAO6, Wear 261 (2006) 256-263. [5] A. H. Battez, R Gonzalez., J. L. Viesca, et al., CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants, Wear 265 (2008) 422-428. [6] J. Q. Ma, M. W. Bai, Effect of ZrO2 nanoparticles additive on the tribological behavior of multialkylated cyclopentanes, Tribol. Lett. 36 (2009) 191-198. [7] S. A. Chen, W. M. Liu, L. G. Yu, Preparation of DDP-coated PbS nanoparticles and investigation of the antiwear ability of the prepared nanoparticles as additive in liquid parafn, Wear 218 (1998) 153-158. [8] S. Chen, W. M. Liu, Characterization and antiwear ability of non-coated ZnS nanoparticles and DDP-coated ZnS nanoparticles, Mater. Res. Bull. 36 (2001) 137-143. [9] X. H. Kang, B. Wang, L. Zhu, et al., Synthesis and tribological property study of oleic acid-modied copper sulde nanoparticles, Wear 265 (2008) 150-154. [10] J. F. Zhou, Z. S. Wu, Z. J. Zhang, et al., Study on an antiwear and extreme pressure additive of surface coated LaF3 nanoparticles in liquid parafn, Wear 249 (2001) 333-337. [11] L. B. Wang, M. Zhang, X. B. Wang, et al., The preparation of CeF3 nanocluster capped 12 

with oleic acid by extraction method and application to lithium grease, Mater. Res. Bull. 43 (2008) 2220-2227. [12] L. B. Wang, B. Wang, X. B. Wang, et al., Tribological investigation of CaF2 nanoparticles as grease additives, Tribol. Int. 40 (2007) 1179-1185. [13] Z. S. Hu, J. X. Dong, G. X. Chen, et al., Preparation and tribological properties of nanoparticle lanthanum borate, Wear 243 (2000) 43-47. [14] Z. S. Hu, Y. G. Shi, L. G. Wang, et al., Study on antiwear and reducing friction additive of nanometer aluminum borate, Lubr. Eng. 57 (2001) 23-27. [15] Z. S. Hu, R. Lai, F. Lou, et al., Preparation and tribological properties of nanometer magnesium borate as lubricating oil additive, Wear 252 (2002) 370-374. [16] M. Akbulut, N. Belman, Y. Golan, et al., Frictional properties of conned nanorods, Adv. Mater. 18 (2006) 2589-2592. [17] R. D. Liu, X. C. Wei, D. H. Tao, et al., Study of preparation and tribological properties of rare earth nanoparticles in lubricating oil, Tribol. Int. 43 (2010) 1082-1086. [18] Z. W. Li, Y. F. Zhu, Surface-modication of SiO2 nanoparticles with oleic acid, Appl. Surf. Sci. 211 (2003) 315-320. [19] A. Erdemir, Review of engineered tribological interfaces for improved boundary lubrication, Tribol. Int. 38 (2005) 249-256. [20] I. Minami, T. Kubo, H. Nanao, Investigation of tribo-chemistry by means of stable isotopic tracers, part 2: lubrication mechanism of friction modiers on diamond-like carbon, Tribol. Trans. 50 (2007) 477-487. [21]X. B. Ji, Y. X. Chen, G. Q. Zhao, et al., Tribological properties of CaCO3 nanoparticles as 13 

an additive in lithium grease, Tribol. Lett. 41 (2011) 113-119. [22] M. Zhang, X. B. Wang, X. S. Fu, et al., Performance and anti-wear mechanism of CaCO3 nanoparticles as a green additive in poly-alpha-olen, Tribol. Int. 42 (2009) 1029-1039. [23] D. L. Jin, L. H. Yue, Tribological properties study of spherical calcium carbonate composite as lubricant additive, Mater. Lett. 62 (2008) 1565-1568. [24] F. Chinas-Castillo, H. A. Spikes, Mechanism of action of colloidal solid dispersions, Trans. ASME 125 (2003) 552-557. [25] L. Rapoport, Y. Feldman, M. Homyonfer, et al., Inorganic fullerene-like material as additives to lubricants: structure–function relationship, Wear 225 (1999) 975-982. [26] Q. J. Xue, W. M. Liu, Z. Zhang, Friction and wear properties of a surface-modied TiO2 nanoparticle as an additive in liquid paraffin, Wear 213 (1997) 29-32. [27] S. Chen, W. M. Liu, Oleic acid capped PbS nanoparticles: synthesis, characterization and tribological properties, Mater. Chem. Phys. 98 (2006) 183-189. [28] L. Rapoport, M. Lvovsky, I. Lapsker, et al., Friction and wear of bronze powder composites fullerene-like WS2 nanoparticles, Wear 249 (2001) 150-157. [29] L Rapoport, V Leshchinsky, M Lvovsky, et al., Superior tribological properties of powder materials with solid lubricant nanoparticles, Wear 255 (2003) 794-800. [30] L. Rapoport, O. Nepomnyashchy, r. ILapske, Behavior of fullerene-like WS2 nanoparticles under severe contact conditions, Wear 259 (2005) 703-707. [31] L. Rapoport, V. Leshchinsky, M. Lvovsky, et al., Friction and wear of powdered composites impregnated with WS2 inorganic fullerene-like nanoparticles, Wear 252 (2002) 518-527. 14 

[32] Y. X. Chen, X. B. Ji, G. Q. Zhao, et al., Facile preparation of cubic calcium carbonate nanoparticles with hydrophobic properties via a carbonation rout, Powder Technol. 200 (2010) 144-148. [33] R. Gueta, A. Natan, L. Addadi, et al., Local atomic order and infrared spectra of biogenic calcite, Angew. Chem. Int. Ed. 46 (2007) 291-294. [34] C. D. Wager, W. M. Ring, L. E. Davids, et al., Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corporation Physical Electronics Division, Eden Prairie, 1979. [35] M. T. Costello, Study of surface lms of amorphous and crystalline overbased calcium sulphonate by XPS and AES, Tribol. Trans. 49 (2006) 592-597. [36] W. Lisowski, A. H. J. Vandenberg, M. Smithers, et al., Characterization of thin alumina lms prepared by metal-organic chemical vapour deposition (MOCVD) by high resolution SEM, (AR)XPS and AES depth proling Fresenius, J. Anal. Chem. 353 (1995) 707-712. [37] T. Hanawa, H. Ukai, K. Murakami, X-ray photoelectron spectroscopy of calcium-ion-implanted titanium, J. Electron. Spectrosc. Relat. Phenom. 63 (1993) 347-354. [38] C. S. Pande, K. P. Cooper, Nanomechanics of Hall-Petch relationship in nanocrystalline materials, Progress in Materials Science 54 (2009) 689-706.

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Table 1. Typical properties of synthesized grease with different CCNP concentrations. Sample

CCNP concentration

Dropping pointa

Cone penetrationb

code

(wt. %)

(°C)

(0.1mm)

H-0

0

207

271

H-1

1

207

269

H-3

3

206

267

H-5

5

209

268

H-7

7

208

263

H-9

9

207

265

a, test standard: ASTM D 566; b, test standard: ASTM D 217.

Table 2. The diameter of wear scars of upper balls Sample code CCNP concentration (wt. %) Diameter a (m) Diameter b (m) a

0

899

863

b

1

736

723

c

3

683

687

d

5

662

663

e

7

642

660

f

9

687

674

16 

Figure Captions: Figure 1.TEM images of (a) CCNP30, (b) CCNP50 and (c) CCNP80. Figure 2. Features of the synthesized CCNP (a) XRD spectra of CCNP and (b) FTIR spectra of CCNP (curve a, CCNP30; curve b. CCNP50; curve c, CCNP80). Figure 3. Dynamic friction coefficient curves (a) and the corresponding wear volumes (b) with different concentrations of CCNP30 as grease additive (load: 300 N; frequency: 25 Hz; duration: 30 min). Figure 4. 2D images of the wear scars of upper balls under different concentration (a, 0 wt.%; b, 1 wt.%; c, 3 wt.%; d, 5 wt.%; e, 7 wt.%; f, 9 wt.%). Figure 5. The viscosity of greases with different CCNP30 concentration as a function of shear time under different shear rate (a, 0.01 s-1; b, 100 s-1). Figure 6. Schematic pictures illustrating the transmission process of nanoparticles (a) and the influence of fibre network on this process ((b) and (c)). Figure 7. Dynamic friction coefficient curves ((a)-(c)) and the corresponding wear volumes (d) under different applied loads and fixed frequency (duration: 30 min). Figure 8. Dynamic friction coefficient curves ((a)-(c)) and the corresponding wear volumes (d) under different frequency and fixed applied loads (duration: 30 min). Figure 9. SEM images of wear scars lubricated by the grease with different additives ((a), (d)) CCNP30; ((b), (e)) CCNP50; ((c), (f)) CCNP80 (load: 400 N; frequency: 25 Hz; duration: 30 min). Figure 10. XPS spectra of elements on the wear scars lubricated by the grease with different additives (curve a, CCNP30; curve b, CCNP50; curve c, CCNP80; load: 400 N; frequency: 25 17 

Hz; duration: 30 min).

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Highlights: ٓ Thefinaltribologicalperformancesdependonboththemechanicalpropertiesof tribofilmandtheperfectionofgreasefibrenetworkstructure. ٓ Itisinterestingto observe a selectivity of nanoparticles with different size for the various test conditions. ٓ Larger CCNP exhibited optimal performance under higher frequency, whileforsmallerCCNP,ithappenedunderhigherloadandlowerfrequency.  

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