- Email: [email protected]
Preparation and tribological properties of surface-modified nano-Y2O3 as additive in liquid paraffin
Applied Surface Science 263 (2012) 655–659
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and tribological properties of surface-modified nano-Y2 O3 as additive in liquid paraffin Lin Yu ∗ , Lin Zhang, Fei Ye, Ming Sun, Xiaoling Cheng, Guiqiang Diao School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 23 September 2012 Accepted 25 September 2012 Available online 2 October 2012 Keywords: Nano-Y2 O3 Surface modification Coupling-grafting Lubricant additive
a b s t r a c t Surface-modified nano-Y2 O3 was prepared by a coupling-grafting method with vinyl methylerichlorosilane and methyl methacrylate as the coupling agent and grafting agent, respectively. The prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), transmission electron micrograph (TEM) and thermal gravimetric analysis (TGA). The tribological properties of the surface-modified nano-Y2 O3 as additive in liquid paraffin were evaluated with a four-ball tester. The results show that the nano-Y2 O3 keeps its original crystalline structure after surface modification, and the modified nano-Y2 O3 forms a core–shell structure with an average particle size of 24.5 nm. The maximum non-seizure load (PB value) and sintered load (PD value) increase by 25% and 26.9%, respectively, when compared with those of liquid paraffin, and the wear scar diameter (WSD) also decrease by 21% when 0.10% surface-modified nano-Y2 O3 was added. The protective inorganic–organic film formed by nano-Y2 O3 and organic modifiers plays an important role in the improved tribological properties of liquid paraffin. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the applications of inorganic nanoparticles have drawn much attention because of their unique properties including catalytic, optical, semiconductive, magnetic and antifriction [1]. Previous studies have shown that many inorganic nanoparticles such as metals [2], metal sulfide [3], metal oxides [4], metal carbonate [5], etc. can be used as additives in lubricating oils to increase the tribological properties. Among the various inorganic nanoparticles, rare earth (RE) nanoparticles have attracted particular interest as additives in lubricating oils due to their special physical and chemical properties [6,7]. It is found that the excellent tribological performance of RE nanoparticles can be attributed to the formation of a boundary lubricating film mainly composed of organic acid, oxide of rare earth and complex of rare earth metals on the rubbed surface [8,9]. Y2 O3 has a unique 4f electronic structure that gives it excellent performance in the fields of catalysis, ceramics, luminescence, coatings and so forth [10]. Recently, Y2 O3 has been introduced into ferritic steel to enhance the antiwear performance [11]. However, Y2 O3 used as additive in lubricating oils to enhance the tribological properties has not been reported. Currently, the dispersion of inorganic nanoparticles in lubricating oils is still a challenge for application of nano-additives [12].
∗ Corresponding author. Tel.: +86 10 39322202; fax: +86 20 39322202. E-mail addresses: [email protected], [email protected] (L. Yu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.130
Similarly, because nano-Y2 O3 particles have small particle sizes and high surface energy, they are prone to aggregation which limits their applications in the polar system. In order to obtain better dispersibility in lubricating oils, surface modification is usually required to introduce an organic layer on the surface of nanoparticles [13–16]. In this paper, we for the first time use coupling and grafting agents to modify the surface of nano-Y2 O3 and improve its dispersibility in liquid paraffin. The surface-modified nano-Y2 O3 was characterized by TEM, XRD and TGA techniques and its tribological properties as additive in liquid paraffin were evaluated by a four-ball tester. The results show that surface-modified nano-Y2 O3 exhibits good load carrying capacity, extreme pressure properties and antiware performances. 2. Materials and methods 2.1. Preparation of surface-modified nano-Y2 O3 All the chemical reagents were analytical grade and used as received. Nano-Y2 O3 was prepared by a precipitation method with polyvinyl alcohol as the dispersant [17]. Surface-modified nanoY2 O3 was prepared by coupling with vinyl methylerichlorosilane, followed by grafting with methyl methacrylate. Briefly, 1.0 g nanoY2 O3 was mixed with 50 mL toluene in a three-neck flask and ultrasonically dispersed for 1 h. Then, a certain amount of glacial acetic acid and vinyl methylerichlorosilane were added and continued ultrasonication for 1 h. The mixtures were centrifuged,
656
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
Scheme 1. Formation mechanism of surface-modified nano-Y2 O3 with enhanced dispersibility.
extracted in acetone for 12 h to remove the unreacted monomers, and dried in a vacuum oven for 12 h to obtain the coupling product. For the second step, 1.0 g coupling product and 50 mL toluene were put into a three-neck flask and ultrasonically dispersed for 1 h. Then a certain amount of monomer methyl methacrylate as well as initiating reagent 2,2-azobisisobutyronitrile (AIBN) were mixed in toluene and put into a dropping funnel fixed at threeneck flask. After the air in the flask was removed by N2 flowing, the temperature of the flask was increased to 70 ◦ C. Then the mixtures of monomer and initiating agent were added to the flask at a speed of one drop per second. Finally, the products were centrifuged, extracted in acetone for 12 h to remove the unreacted monomers, and dried in a vacuum oven for 12 h to obtain the surface-modified nano-Y2 O3 . 2.2. Characterization of prepared samples The dispersibility test of surface-modified nano-Y2 O3 in liquid paraffin was performed to check the effect of surface modification. A certain amount of surface-modified nano-Y2 O3 was added to liquid paraffin in a glass tube to obtain a concentration of 0.25% and ultrasonically dispersed for 1 h, and then the mixtures were kept at room temperature for further observation. The morphology analysis was performed on transmission electron micrograph (TEM) analyzer (JEM-2010HR Japan). X-ray diffraction patterns of the samples were obtained with a Siemens D500 diffractometer fitted with a Cu tube operating at 40 kV and 30 mA. Fourier transform infrared spectra of the samples were carried out on spectrometer (Nicolet Co., Nexus-380, USA) with a resolution of 4 cm−1 . The content of coating polymer was determined by thermal gravimetric analysis (TGA) on thermal analyzer (Netzsch Co., STA409PC, Germany). Samples were heated in flowing air from room temperature to 600 ◦ C at a heating rate of 10 ◦ C/min. 2.3. Evaluation of the tribological properties The friction and wear tests were carried out on a MMW-1 fourball tester under the following conditions: 392 N, 1450 rpm, 30 min and ambient temperature. The balls were made of GCr15 bearing steel with a diameter of 12.7 mm and an HRC of 59–61. The maximum non-seizure load (PB value) and sintered load (PD value) were
obtained according to ASTM D2783. A microscope with an accuracy of 0.01 mm was used to determine the wear scar diameters (WSDs) of the three lower balls, and the results reported were the mean WSD of the three balls. The morphology and chemical composition of the scars of the ball were analyzed on a scanning electron microscope (Hitachi S-3400N) with an energy dispersive X-ray spectroscopy (EDS) analyzer (Oxford INCA300). 3. Results and discussions 3.1. Formation mechanism of surface-modified nano-Y2 O3 The good dispersibility of nano additives in lubricating oils is required in order to make full use of their tribological performance. The dispersibility test results (see Fig. S1 Supplementary data) show that the surface-modified nano-Y2 O3 can be readily dispersed in liquid paraffin to obtain homogenous solutions, which can keep unchanged for one week at room temperature, while the unmodified nano-Y2 O3 precipitates at the bottom of the glass tube. The improved dispersibility can be attributed to the decreasing polarity of nano-Y2 O3 due to the introduction of organic components after surface modification. We hypothesize that the surface modification of nano-Y2 O3 by the coupling-grafting method is carried out in the mode depicted in Scheme 1. First, vinyl methylerichlorosilane is hydrolyzed to vinyl methylerichlorosilic acid under the existence of acetic acid [18], and then vinyl methylerichlorosilic acid can react with the OH groups of nano-Y2 O3 , thus the unsaturated C C group is introduced onto the surface of nano-Y2 O3 . In the second step, the monomer methyl methacrylate and the C C group on the surface of nano-Y2 O3 can polymerize under the action of initiating agent. Finally, the organic compounds grafted nano-Y2 O3 with enhanced dispersibility in liquid paraffin can be obtained. 3.2. Characterization of nano-Y2 O3 and surface-modified nano-Y2 O3 Fig. 1 shows TEM images of nano-Y2 O3 and surface-modified nano-Y2 O3 . Both samples show a spherical structure with an average particle size in the range of 20–30 nm. After surface modification, the agglomeration of nano-Y2 O3 particles is reduced,
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
657
Fig. 1. TEM images of nano-Y2 O3 (a) and surface-modified nano-Y2 O3 (b).
indicating the improved dispersibility of surface-modified nanoY2 O3 . This is because the methacrylate on the nano-Y2 O3 surface can decrease the surface energy and thus prevent agglomeration. Moreover, the center of surface-modified nano-Y2 O3 particles is darker than the outside layers, which is indicative of the core–shell structure of metal oxides covered by polymers. XRD patterns of nano-Y2 O3 and surface-modified nano-Y2 O3 are presented in Fig. 2. It is clear from Fig. 2 that both diffraction peaks can be indexed as the cubic crystal structure of Y2 O3 (JCPDS 431036) [19]. No other phases were detected, indicating that the surface modification does not change the crystal structure of nanoY2 O3 . The average size of surface-modified nano-Y2 O3 particles estimated by Scherrer equation is 24.5 nm. The surface components of the surface-modified nano-Y2 O3 were analyzed by FT-IR. Fig. 3 illustrates the IR spectra of nano-Y2 O3 and surface-modified nano-Y2 O3 . In Fig. 3, the broad absorption bands around 3434 cm−1 and 1624 cm−1 are respectively designated as the O H stretching and O H deformation vibrations of the absorbed water as well as hydroxyl groups ( OH) on the surface of nano-Y2 O3 [20]. For the nano-Y2 O3 sample, a peak located at 561 cm−1 corresponds to the characteristic stretching vibration of Y O [21], and the peaks at 1524 cm−1 and 1384 cm−1 are attributed to C OH of the polyethylene glycol remained on the surface of nano-Y2 O3 during preparation. For the surface-modified nano-Y2 O3 sample, two new absorption peaks near 2959 cm−1 and 2853 cm−1 can be attributed to the C H bond bending vibration, and a peak at 1744 cm−1 belongs to the stretching vibrating of C O [13]. The peaks at 1542 cm−1 and 1451 cm−1 are assigned
to the symmetric and asymmetric C O stretching modes of ester group, respectively [22]. It can be concluded from the above results that nano-Y2 O3 has been successfully modified by vinyl methylerichlorosilane and methyl methacrylate. To estimate the amount of modifiers at the surface of nano-Y2 O3 , the samples were analyzed by TGA technique. Fig. 4 illustrates the TGA curves of nano-Y2 O3 , surface-modified nano-Y2 O3 . As can be
Fig. 2. XRD patterns of nano-Y2 O3 and surface-modified nano-Y2 O3 .
Fig. 4. TGA curves of nano-Y2 O3 and surface-modified nano-Y2 O3 .
Fig. 3. FT-IR spectra of nano-Y2 O3 and surface-modified nano-Y2 O3 .
658
L. Yu et al. / Applied Surface Science 263 (2012) 655–659 Table 1 The PB and PD values of the surface-modified nano-Y2 O3 as additive in liquid paraffin.
Fig. 5. Wear scar diameter and friction coefficient as a function of surface-modified nano-Y2 O3 concentration.
seen from Fig. 4, the nano-Y2 O3 was relatively stable in air and only slightly decomposed. Slight weight loss (2.6%) is probably due to vaporization of water physically adsorbed at the surface or the water formed from condensation of OH groups at the nano-Y2 O3 surface. For the sample of surface-modified nano-Y2 O3 , a sharp weight loss is observed at about 300 ◦ C and continues to 600 ◦ C, possibly due to a large scale of thermal decomposition of the organic groups on the nano-Y2 O3 surface, and the curve indicates a mass content of 22.2% modifiers on the surface of modified nano-Y2 O3 . 3.3. Tribological properties of surface-modified nano-Y2 O3 The PB and PD values of liquid paraffin containing different concentrations of surface-modified nano-Y2 O3 are shown in Table 1. It can be seen from Table 1 that the surface-modified nano-Y2 O3 as additive in liquid paraffin can considerably increase the PB and PD
Concentration (wt%)
PB (N)
PD (N)
0 0.05 0.10 0.25 0.50 1.00
400 430 470 500 480 480
1236 1236 1569 1569 1569 1569
values of liquid paraffin. The PB values increase with the increasing concentration of surface-modified nano-Y2 O3 , and reach the maximum value of 500 N with 0.25% surface-modified nano-Y2 O3 , increasing by nearly 25% compared with that of liquid paraffin. But the PB values decrease when the concentration is higher than 0.25%. Similarly, the PD values increase by 26.9% to 1569 N when the concentration of surface-modified nano-Y2 O3 reaches 0.10%, but does not change as the concentration further increases. Therefore, the surface-modified nano-Y2 O3 exhibits good load carrying capacity and extreme pressure property, and the optimum concentration of surface-modified nano-Y2 O3 in liquid paraffin is 0.10–0.25%. Fig. 5 illustrates the wear scar diameters and friction coefficients of the steel balls lubricated by liquid paraffin containing different concentrations of surface-modified nano-Y2 O3 at a load of 392 N. It can be seen from Fig. 5 that the wear scar diameter and friction coefficient decrease gradually with increasing concentration of surface-modified nano-Y2 O3 , but higher concentrations of surface-modified nano-Y2 O3 in liquid paraffin lead to increase in scar diameter as well as friction coefficient. For instance, when 0.10% of surface-modified nano-Y2 O3 is added into liquid paraffin, the best antiwear performance is obtained, namely, the wear scar diameter decreases by 21% to 0.67 mm with respect to that of liquid paraffin (0.88 mm). Meanwhile, the lowest friction coefficient is achieved with liquid paraffin containing 0.25% surface-modified nano-Y2 O3 .
Fig. 6. SEM images of wear scar of the steel balls lubricated by liquid paraffin (a, b) and liquid paraffin containing surface-modified nano-Y2 O3 (c, d).
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
Fig. 6 presents the SEM images of the wear scar of steel balls lubricated by liquid paraffin and liquid paraffin containing 0.10% surface-modified nano-Y2 O3 . It is clear from Fig. 6 that the rubbing surface lubricated only by liquid paraffin is quite rough with obviously wide and deep furrows and grooves in sliding direction (Fig. 6a and b). On the contrary, the rubbing surfaces shown in Fig. 6c and d are smoother than those in Fig. 6a and b, and the furrows and grooves on the surface lubricated by liquid paraffin containing 0.10% surface-modified nano-Y2 O3 are highly inhibited, which is indicative of good anti-wear properties for surfacemodified nano-Y2 O3 . EDS spectra of wear scars of the steel balls lubricated by only liquid paraffin and liquid paraffin containing surface-modified nano-Y2 O3 are shown in Fig. S2 (see Supplementary data). It can be seen that there is additional element of Y on the scar of steel balls lubricated by liquid paraffin containing surfacemodified nano-Y2 O3 compared with that lubricated by only liquid paraffin. The results testify that surface-modified nano-Y2 O3 is transferred and accumulated onto the scars of steel balls during friction. It is observed from the above results that the addition of surfacemodified nano-Y2 O3 greatly enhances the antiwear properties of liquid paraffin. The antiwear mechanism could be explained as follows: firstly, the highly dispersed surface-modified nanoY2 O3 in liquid paraffin would homogenously adsorb on the wore surface through the organic groups. At high temperature and pressure caused by the friction, surface-modified nano-Y2 O3 could be imbedded into the micro-cracks of the wore surface to form a protective inorganic–organic film [16]. On the one hand, the spherical structure of nano-Y2 O3 could not only carry a proportion of the load, but also partially change sliding friction into rolling friction to enhance the load carrying as well as friction reducing properties [23,24]; on the other hand, the organic arrangement of hydrocarbon chain on the surface of nano-Y2 O3 could also reduce the friction owing to “the brush mechanism” [18]. It can be suggested that the protective film formed by nano-Y2 O3 and organic modifiers plays an important role in the improved tribological properties of liquid paraffin. 4. Conclusions In summary, surface-modified nano-Y2 O3 was prepared through a coupling-grafting process and its tribological performance as additive in liquid paraffin was studied. The surface-modified nano-Y2 O3 forms a core–shell structure with an average particle size of 24.5 nm, and it can be well dispersed in liquid paraffin. Liquid paraffin containing surface-modified nano-Y2 O3 exhibits good load carrying capacity, extreme pressure properties and antiware performance. Acknowledgments This work is supported by Natural Science Foundation of Guangdong Province (10251009001000003) and 211 Subject Construction Foundation of Guandong Province.
659
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.09.130. References [1] V.N. Bakunin, A.Y. Suslov, G.N. Kuzmina, O.P. Parenago, Recent achievements in the synthesis and application of inorganic nanoparticles as lubricant components, Lubr. Sci. 17 (2005) 127–145. [2] Y. Choi, C. Lee, Y. Hwang, M. Park, J. Lee, C. Choi, M. Jung, Tribological behavior of copper nanoparticles as additives in oil, Curr. Appl. Phys. 9 (2009) e124–e127. [3] K.H. Hu, X.G. Hu, Y.F. Xu, F. Huang, J.S. Liu, The effect of morphology on the tribological properties of MoS2 in liquid paraffin, Tribol. Lett. 40 (2010) 155–165. [4] H.J. Song, X.H. Jia, N. Li, X.F. Yang, H. Tang, Synthesis of ␣-Fe2 O3 nanorod/graphene oxide composites and their tribological properties, J. Mater. Chem. 22 (2012) 895–902. [5] X.B. Ji, Y.X. Chen, G.Q. Zhao, X.B. Wang, W.M. Liu, Tribological properties of CaCO3 nanoparticles as an additive in lithium grease, Tribol. Lett. 41 (2011) 113–119. [6] R. Liu, X. Wei, D. Tao, Y. Zhao, Study of preparation and tribological properties of rare earth nanoparticles in lubricating oil, Tribol. Int. 43 (2010) 1082–1086. [7] Z.F. Jia, Y.Q. Xia, Hydrothermal synthesis, characterization, and tribological behavior of oleic acid-capped lanthanum borate with different morphologies, Tribol. Lett. 41 (2011) 425–434. [8] R.D. Liu, D.H. Tao, J.H. Zhang, W.G. Cao, S.F. Fu, Study of the tribological performance of a rare earth naphthenate as a lubricant additive, Lubr. Sci. 17 (2004) 105–113. [9] R. Liu, D. Tao, Y. Zhao, Tribological performance of rare-earth alkylsalicylate as lubrication additive, Ind. Lubr. Tribol. 60 (2008) 9–13. [10] G. Jia, H. You, Y. Song, Y. Huang, M. Yang, H. Zhang, Facile synthesis and luminescence of uniform Y2 O3 hollow spheres by a sacrificial template route, Inorg. Chem. 49 (2010) 7721–7725. [11] S.K. Karak, C.S. Vishnu, Z. Witczak, W. Lojkowski, J. Dutta Majumdar, I. Manna, Studies on wear behavior of nano-Y2 O3 dispersed ferritic steel developed by mechanical alloying and hot isostatic pressing, Wear 270 (2010) 5–11. [12] D. Kim, L.A. Archer, Nanoscale organic–inorganic hybrid lubricants, Langmuir 27 (2011) 3083–3094. [13] L. Zhang, D.F. Zhang, Y.L. Jin, P. Zhang, S.L. Zang, Surface modification of nanoSiO2 by in situ grafted polymerization of butyl methacrylatel, Inorg. Mater. 42 (2006) 39–43. [14] X. Kang, B. Wang, L. Zhu, H. Zhu, Synthesis and tribological property study of oleic acid-modified copper sulfide nanoparticles, Wear 265 (2008) 150–154. [15] Y. Tai, J. Qian, Y. Zhang, J. Huang, Study of surface modification of nano-SiO2 with macromolecular coupling agent (LMPB-g-MAH), Chem. Eng. J. 141 (2008) 354–361. [16] L. Zhang, L. Chen, H. Wan, J. Chen, H. Zhou, Synthesis and tribological properties of stearic acid-modified anatase TiO2 nanoparticles, Tribol. Lett. 41 (2011) 409–416. [17] L. Zhang, L. Yu, X. Cheng, G. Diao, Preparation and lipophilic performance of nano-Y2 O3 modified with stearic acid, J. Chem. Ind. Eng. (China) 62 (2011) 880–885. [18] X. Li, Z. Cao, Z. Zhang, H. Dang, Surface-modification in situ of nano-SiO2 and its structure and tribological properties, Appl. Surf. Sci. 252 (2006) 7856–7861. [19] L. Mingxi, H. Yizhu, Y. Xiaomin, Effect of nano-Y2 O3 on microstructure of laser cladding cobalt-based alloy coatings, Appl. Surf. Sci. 252 (2006) 2882–2887. [20] Q. Chen, N.L. Yakovlev, Adsorption and interaction of organosilanes on TiO2 nanoparticles, Appl. Surf. Sci. 257 (2010) 1395–1400. [21] X. Shen, Y. Zhai, Preparation and optical properties of Y2 O3 /SiO2 powder, Rare Met. 30 (2011) 33–38. [22] Y. Gao, G. Chen, Y. Oli, Z. Zhang, Q. Xue, Study on tribological properties of oleic acid-modified TiO2 nanoparticle in water, Wear 252 (2002) 454–458. [23] Z.J. Zhang, J. Zhang, Q.J. Xue, Synthesis and characterization of a molybdenum disulfide nanocluster, J. Phys. Chem. 98 (1994) 12973–12977. [24] J. Qian, X. Yin, N. Wang, L. Liu, J. Xing, Preparation and tribological properties of stearic acid-modified hierarchical anatase TiO2 microcrystals, Appl. Surf. Sci. 258 (2012) 2778–2782.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Preparation and tribological properties of surface-modified nano-Y2 O3 as additive in liquid paraffin Lin Yu ∗ , Lin Zhang, Fei Ye, Ming Sun, Xiaoling Cheng, Guiqiang Diao School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China
a r t i c l e
i n f o
Article history: Received 24 March 2012 Received in revised form 23 September 2012 Accepted 25 September 2012 Available online 2 October 2012 Keywords: Nano-Y2 O3 Surface modification Coupling-grafting Lubricant additive
a b s t r a c t Surface-modified nano-Y2 O3 was prepared by a coupling-grafting method with vinyl methylerichlorosilane and methyl methacrylate as the coupling agent and grafting agent, respectively. The prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectra (FT-IR), transmission electron micrograph (TEM) and thermal gravimetric analysis (TGA). The tribological properties of the surface-modified nano-Y2 O3 as additive in liquid paraffin were evaluated with a four-ball tester. The results show that the nano-Y2 O3 keeps its original crystalline structure after surface modification, and the modified nano-Y2 O3 forms a core–shell structure with an average particle size of 24.5 nm. The maximum non-seizure load (PB value) and sintered load (PD value) increase by 25% and 26.9%, respectively, when compared with those of liquid paraffin, and the wear scar diameter (WSD) also decrease by 21% when 0.10% surface-modified nano-Y2 O3 was added. The protective inorganic–organic film formed by nano-Y2 O3 and organic modifiers plays an important role in the improved tribological properties of liquid paraffin. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In recent years, the applications of inorganic nanoparticles have drawn much attention because of their unique properties including catalytic, optical, semiconductive, magnetic and antifriction [1]. Previous studies have shown that many inorganic nanoparticles such as metals [2], metal sulfide [3], metal oxides [4], metal carbonate [5], etc. can be used as additives in lubricating oils to increase the tribological properties. Among the various inorganic nanoparticles, rare earth (RE) nanoparticles have attracted particular interest as additives in lubricating oils due to their special physical and chemical properties [6,7]. It is found that the excellent tribological performance of RE nanoparticles can be attributed to the formation of a boundary lubricating film mainly composed of organic acid, oxide of rare earth and complex of rare earth metals on the rubbed surface [8,9]. Y2 O3 has a unique 4f electronic structure that gives it excellent performance in the fields of catalysis, ceramics, luminescence, coatings and so forth [10]. Recently, Y2 O3 has been introduced into ferritic steel to enhance the antiwear performance [11]. However, Y2 O3 used as additive in lubricating oils to enhance the tribological properties has not been reported. Currently, the dispersion of inorganic nanoparticles in lubricating oils is still a challenge for application of nano-additives [12].
∗ Corresponding author. Tel.: +86 10 39322202; fax: +86 20 39322202. E-mail addresses: [email protected], [email protected] (L. Yu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.09.130
Similarly, because nano-Y2 O3 particles have small particle sizes and high surface energy, they are prone to aggregation which limits their applications in the polar system. In order to obtain better dispersibility in lubricating oils, surface modification is usually required to introduce an organic layer on the surface of nanoparticles [13–16]. In this paper, we for the first time use coupling and grafting agents to modify the surface of nano-Y2 O3 and improve its dispersibility in liquid paraffin. The surface-modified nano-Y2 O3 was characterized by TEM, XRD and TGA techniques and its tribological properties as additive in liquid paraffin were evaluated by a four-ball tester. The results show that surface-modified nano-Y2 O3 exhibits good load carrying capacity, extreme pressure properties and antiware performances. 2. Materials and methods 2.1. Preparation of surface-modified nano-Y2 O3 All the chemical reagents were analytical grade and used as received. Nano-Y2 O3 was prepared by a precipitation method with polyvinyl alcohol as the dispersant [17]. Surface-modified nanoY2 O3 was prepared by coupling with vinyl methylerichlorosilane, followed by grafting with methyl methacrylate. Briefly, 1.0 g nanoY2 O3 was mixed with 50 mL toluene in a three-neck flask and ultrasonically dispersed for 1 h. Then, a certain amount of glacial acetic acid and vinyl methylerichlorosilane were added and continued ultrasonication for 1 h. The mixtures were centrifuged,
656
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
Scheme 1. Formation mechanism of surface-modified nano-Y2 O3 with enhanced dispersibility.
extracted in acetone for 12 h to remove the unreacted monomers, and dried in a vacuum oven for 12 h to obtain the coupling product. For the second step, 1.0 g coupling product and 50 mL toluene were put into a three-neck flask and ultrasonically dispersed for 1 h. Then a certain amount of monomer methyl methacrylate as well as initiating reagent 2,2-azobisisobutyronitrile (AIBN) were mixed in toluene and put into a dropping funnel fixed at threeneck flask. After the air in the flask was removed by N2 flowing, the temperature of the flask was increased to 70 ◦ C. Then the mixtures of monomer and initiating agent were added to the flask at a speed of one drop per second. Finally, the products were centrifuged, extracted in acetone for 12 h to remove the unreacted monomers, and dried in a vacuum oven for 12 h to obtain the surface-modified nano-Y2 O3 . 2.2. Characterization of prepared samples The dispersibility test of surface-modified nano-Y2 O3 in liquid paraffin was performed to check the effect of surface modification. A certain amount of surface-modified nano-Y2 O3 was added to liquid paraffin in a glass tube to obtain a concentration of 0.25% and ultrasonically dispersed for 1 h, and then the mixtures were kept at room temperature for further observation. The morphology analysis was performed on transmission electron micrograph (TEM) analyzer (JEM-2010HR Japan). X-ray diffraction patterns of the samples were obtained with a Siemens D500 diffractometer fitted with a Cu tube operating at 40 kV and 30 mA. Fourier transform infrared spectra of the samples were carried out on spectrometer (Nicolet Co., Nexus-380, USA) with a resolution of 4 cm−1 . The content of coating polymer was determined by thermal gravimetric analysis (TGA) on thermal analyzer (Netzsch Co., STA409PC, Germany). Samples were heated in flowing air from room temperature to 600 ◦ C at a heating rate of 10 ◦ C/min. 2.3. Evaluation of the tribological properties The friction and wear tests were carried out on a MMW-1 fourball tester under the following conditions: 392 N, 1450 rpm, 30 min and ambient temperature. The balls were made of GCr15 bearing steel with a diameter of 12.7 mm and an HRC of 59–61. The maximum non-seizure load (PB value) and sintered load (PD value) were
obtained according to ASTM D2783. A microscope with an accuracy of 0.01 mm was used to determine the wear scar diameters (WSDs) of the three lower balls, and the results reported were the mean WSD of the three balls. The morphology and chemical composition of the scars of the ball were analyzed on a scanning electron microscope (Hitachi S-3400N) with an energy dispersive X-ray spectroscopy (EDS) analyzer (Oxford INCA300). 3. Results and discussions 3.1. Formation mechanism of surface-modified nano-Y2 O3 The good dispersibility of nano additives in lubricating oils is required in order to make full use of their tribological performance. The dispersibility test results (see Fig. S1 Supplementary data) show that the surface-modified nano-Y2 O3 can be readily dispersed in liquid paraffin to obtain homogenous solutions, which can keep unchanged for one week at room temperature, while the unmodified nano-Y2 O3 precipitates at the bottom of the glass tube. The improved dispersibility can be attributed to the decreasing polarity of nano-Y2 O3 due to the introduction of organic components after surface modification. We hypothesize that the surface modification of nano-Y2 O3 by the coupling-grafting method is carried out in the mode depicted in Scheme 1. First, vinyl methylerichlorosilane is hydrolyzed to vinyl methylerichlorosilic acid under the existence of acetic acid [18], and then vinyl methylerichlorosilic acid can react with the OH groups of nano-Y2 O3 , thus the unsaturated C C group is introduced onto the surface of nano-Y2 O3 . In the second step, the monomer methyl methacrylate and the C C group on the surface of nano-Y2 O3 can polymerize under the action of initiating agent. Finally, the organic compounds grafted nano-Y2 O3 with enhanced dispersibility in liquid paraffin can be obtained. 3.2. Characterization of nano-Y2 O3 and surface-modified nano-Y2 O3 Fig. 1 shows TEM images of nano-Y2 O3 and surface-modified nano-Y2 O3 . Both samples show a spherical structure with an average particle size in the range of 20–30 nm. After surface modification, the agglomeration of nano-Y2 O3 particles is reduced,
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
657
Fig. 1. TEM images of nano-Y2 O3 (a) and surface-modified nano-Y2 O3 (b).
indicating the improved dispersibility of surface-modified nanoY2 O3 . This is because the methacrylate on the nano-Y2 O3 surface can decrease the surface energy and thus prevent agglomeration. Moreover, the center of surface-modified nano-Y2 O3 particles is darker than the outside layers, which is indicative of the core–shell structure of metal oxides covered by polymers. XRD patterns of nano-Y2 O3 and surface-modified nano-Y2 O3 are presented in Fig. 2. It is clear from Fig. 2 that both diffraction peaks can be indexed as the cubic crystal structure of Y2 O3 (JCPDS 431036) [19]. No other phases were detected, indicating that the surface modification does not change the crystal structure of nanoY2 O3 . The average size of surface-modified nano-Y2 O3 particles estimated by Scherrer equation is 24.5 nm. The surface components of the surface-modified nano-Y2 O3 were analyzed by FT-IR. Fig. 3 illustrates the IR spectra of nano-Y2 O3 and surface-modified nano-Y2 O3 . In Fig. 3, the broad absorption bands around 3434 cm−1 and 1624 cm−1 are respectively designated as the O H stretching and O H deformation vibrations of the absorbed water as well as hydroxyl groups ( OH) on the surface of nano-Y2 O3 [20]. For the nano-Y2 O3 sample, a peak located at 561 cm−1 corresponds to the characteristic stretching vibration of Y O [21], and the peaks at 1524 cm−1 and 1384 cm−1 are attributed to C OH of the polyethylene glycol remained on the surface of nano-Y2 O3 during preparation. For the surface-modified nano-Y2 O3 sample, two new absorption peaks near 2959 cm−1 and 2853 cm−1 can be attributed to the C H bond bending vibration, and a peak at 1744 cm−1 belongs to the stretching vibrating of C O [13]. The peaks at 1542 cm−1 and 1451 cm−1 are assigned
to the symmetric and asymmetric C O stretching modes of ester group, respectively [22]. It can be concluded from the above results that nano-Y2 O3 has been successfully modified by vinyl methylerichlorosilane and methyl methacrylate. To estimate the amount of modifiers at the surface of nano-Y2 O3 , the samples were analyzed by TGA technique. Fig. 4 illustrates the TGA curves of nano-Y2 O3 , surface-modified nano-Y2 O3 . As can be
Fig. 2. XRD patterns of nano-Y2 O3 and surface-modified nano-Y2 O3 .
Fig. 4. TGA curves of nano-Y2 O3 and surface-modified nano-Y2 O3 .
Fig. 3. FT-IR spectra of nano-Y2 O3 and surface-modified nano-Y2 O3 .
658
L. Yu et al. / Applied Surface Science 263 (2012) 655–659 Table 1 The PB and PD values of the surface-modified nano-Y2 O3 as additive in liquid paraffin.
Fig. 5. Wear scar diameter and friction coefficient as a function of surface-modified nano-Y2 O3 concentration.
seen from Fig. 4, the nano-Y2 O3 was relatively stable in air and only slightly decomposed. Slight weight loss (2.6%) is probably due to vaporization of water physically adsorbed at the surface or the water formed from condensation of OH groups at the nano-Y2 O3 surface. For the sample of surface-modified nano-Y2 O3 , a sharp weight loss is observed at about 300 ◦ C and continues to 600 ◦ C, possibly due to a large scale of thermal decomposition of the organic groups on the nano-Y2 O3 surface, and the curve indicates a mass content of 22.2% modifiers on the surface of modified nano-Y2 O3 . 3.3. Tribological properties of surface-modified nano-Y2 O3 The PB and PD values of liquid paraffin containing different concentrations of surface-modified nano-Y2 O3 are shown in Table 1. It can be seen from Table 1 that the surface-modified nano-Y2 O3 as additive in liquid paraffin can considerably increase the PB and PD
Concentration (wt%)
PB (N)
PD (N)
0 0.05 0.10 0.25 0.50 1.00
400 430 470 500 480 480
1236 1236 1569 1569 1569 1569
values of liquid paraffin. The PB values increase with the increasing concentration of surface-modified nano-Y2 O3 , and reach the maximum value of 500 N with 0.25% surface-modified nano-Y2 O3 , increasing by nearly 25% compared with that of liquid paraffin. But the PB values decrease when the concentration is higher than 0.25%. Similarly, the PD values increase by 26.9% to 1569 N when the concentration of surface-modified nano-Y2 O3 reaches 0.10%, but does not change as the concentration further increases. Therefore, the surface-modified nano-Y2 O3 exhibits good load carrying capacity and extreme pressure property, and the optimum concentration of surface-modified nano-Y2 O3 in liquid paraffin is 0.10–0.25%. Fig. 5 illustrates the wear scar diameters and friction coefficients of the steel balls lubricated by liquid paraffin containing different concentrations of surface-modified nano-Y2 O3 at a load of 392 N. It can be seen from Fig. 5 that the wear scar diameter and friction coefficient decrease gradually with increasing concentration of surface-modified nano-Y2 O3 , but higher concentrations of surface-modified nano-Y2 O3 in liquid paraffin lead to increase in scar diameter as well as friction coefficient. For instance, when 0.10% of surface-modified nano-Y2 O3 is added into liquid paraffin, the best antiwear performance is obtained, namely, the wear scar diameter decreases by 21% to 0.67 mm with respect to that of liquid paraffin (0.88 mm). Meanwhile, the lowest friction coefficient is achieved with liquid paraffin containing 0.25% surface-modified nano-Y2 O3 .
Fig. 6. SEM images of wear scar of the steel balls lubricated by liquid paraffin (a, b) and liquid paraffin containing surface-modified nano-Y2 O3 (c, d).
L. Yu et al. / Applied Surface Science 263 (2012) 655–659
Fig. 6 presents the SEM images of the wear scar of steel balls lubricated by liquid paraffin and liquid paraffin containing 0.10% surface-modified nano-Y2 O3 . It is clear from Fig. 6 that the rubbing surface lubricated only by liquid paraffin is quite rough with obviously wide and deep furrows and grooves in sliding direction (Fig. 6a and b). On the contrary, the rubbing surfaces shown in Fig. 6c and d are smoother than those in Fig. 6a and b, and the furrows and grooves on the surface lubricated by liquid paraffin containing 0.10% surface-modified nano-Y2 O3 are highly inhibited, which is indicative of good anti-wear properties for surfacemodified nano-Y2 O3 . EDS spectra of wear scars of the steel balls lubricated by only liquid paraffin and liquid paraffin containing surface-modified nano-Y2 O3 are shown in Fig. S2 (see Supplementary data). It can be seen that there is additional element of Y on the scar of steel balls lubricated by liquid paraffin containing surfacemodified nano-Y2 O3 compared with that lubricated by only liquid paraffin. The results testify that surface-modified nano-Y2 O3 is transferred and accumulated onto the scars of steel balls during friction. It is observed from the above results that the addition of surfacemodified nano-Y2 O3 greatly enhances the antiwear properties of liquid paraffin. The antiwear mechanism could be explained as follows: firstly, the highly dispersed surface-modified nanoY2 O3 in liquid paraffin would homogenously adsorb on the wore surface through the organic groups. At high temperature and pressure caused by the friction, surface-modified nano-Y2 O3 could be imbedded into the micro-cracks of the wore surface to form a protective inorganic–organic film [16]. On the one hand, the spherical structure of nano-Y2 O3 could not only carry a proportion of the load, but also partially change sliding friction into rolling friction to enhance the load carrying as well as friction reducing properties [23,24]; on the other hand, the organic arrangement of hydrocarbon chain on the surface of nano-Y2 O3 could also reduce the friction owing to “the brush mechanism” [18]. It can be suggested that the protective film formed by nano-Y2 O3 and organic modifiers plays an important role in the improved tribological properties of liquid paraffin. 4. Conclusions In summary, surface-modified nano-Y2 O3 was prepared through a coupling-grafting process and its tribological performance as additive in liquid paraffin was studied. The surface-modified nano-Y2 O3 forms a core–shell structure with an average particle size of 24.5 nm, and it can be well dispersed in liquid paraffin. Liquid paraffin containing surface-modified nano-Y2 O3 exhibits good load carrying capacity, extreme pressure properties and antiware performance. Acknowledgments This work is supported by Natural Science Foundation of Guangdong Province (10251009001000003) and 211 Subject Construction Foundation of Guandong Province.
659
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsusc.2012.09.130. References [1] V.N. Bakunin, A.Y. Suslov, G.N. Kuzmina, O.P. Parenago, Recent achievements in the synthesis and application of inorganic nanoparticles as lubricant components, Lubr. Sci. 17 (2005) 127–145. [2] Y. Choi, C. Lee, Y. Hwang, M. Park, J. Lee, C. Choi, M. Jung, Tribological behavior of copper nanoparticles as additives in oil, Curr. Appl. Phys. 9 (2009) e124–e127. [3] K.H. Hu, X.G. Hu, Y.F. Xu, F. Huang, J.S. Liu, The effect of morphology on the tribological properties of MoS2 in liquid paraffin, Tribol. Lett. 40 (2010) 155–165. [4] H.J. Song, X.H. Jia, N. Li, X.F. Yang, H. Tang, Synthesis of ␣-Fe2 O3 nanorod/graphene oxide composites and their tribological properties, J. Mater. Chem. 22 (2012) 895–902. [5] X.B. Ji, Y.X. Chen, G.Q. Zhao, X.B. Wang, W.M. Liu, Tribological properties of CaCO3 nanoparticles as an additive in lithium grease, Tribol. Lett. 41 (2011) 113–119. [6] R. Liu, X. Wei, D. Tao, Y. Zhao, Study of preparation and tribological properties of rare earth nanoparticles in lubricating oil, Tribol. Int. 43 (2010) 1082–1086. [7] Z.F. Jia, Y.Q. Xia, Hydrothermal synthesis, characterization, and tribological behavior of oleic acid-capped lanthanum borate with different morphologies, Tribol. Lett. 41 (2011) 425–434. [8] R.D. Liu, D.H. Tao, J.H. Zhang, W.G. Cao, S.F. Fu, Study of the tribological performance of a rare earth naphthenate as a lubricant additive, Lubr. Sci. 17 (2004) 105–113. [9] R. Liu, D. Tao, Y. Zhao, Tribological performance of rare-earth alkylsalicylate as lubrication additive, Ind. Lubr. Tribol. 60 (2008) 9–13. [10] G. Jia, H. You, Y. Song, Y. Huang, M. Yang, H. Zhang, Facile synthesis and luminescence of uniform Y2 O3 hollow spheres by a sacrificial template route, Inorg. Chem. 49 (2010) 7721–7725. [11] S.K. Karak, C.S. Vishnu, Z. Witczak, W. Lojkowski, J. Dutta Majumdar, I. Manna, Studies on wear behavior of nano-Y2 O3 dispersed ferritic steel developed by mechanical alloying and hot isostatic pressing, Wear 270 (2010) 5–11. [12] D. Kim, L.A. Archer, Nanoscale organic–inorganic hybrid lubricants, Langmuir 27 (2011) 3083–3094. [13] L. Zhang, D.F. Zhang, Y.L. Jin, P. Zhang, S.L. Zang, Surface modification of nanoSiO2 by in situ grafted polymerization of butyl methacrylatel, Inorg. Mater. 42 (2006) 39–43. [14] X. Kang, B. Wang, L. Zhu, H. Zhu, Synthesis and tribological property study of oleic acid-modified copper sulfide nanoparticles, Wear 265 (2008) 150–154. [15] Y. Tai, J. Qian, Y. Zhang, J. Huang, Study of surface modification of nano-SiO2 with macromolecular coupling agent (LMPB-g-MAH), Chem. Eng. J. 141 (2008) 354–361. [16] L. Zhang, L. Chen, H. Wan, J. Chen, H. Zhou, Synthesis and tribological properties of stearic acid-modified anatase TiO2 nanoparticles, Tribol. Lett. 41 (2011) 409–416. [17] L. Zhang, L. Yu, X. Cheng, G. Diao, Preparation and lipophilic performance of nano-Y2 O3 modified with stearic acid, J. Chem. Ind. Eng. (China) 62 (2011) 880–885. [18] X. Li, Z. Cao, Z. Zhang, H. Dang, Surface-modification in situ of nano-SiO2 and its structure and tribological properties, Appl. Surf. Sci. 252 (2006) 7856–7861. [19] L. Mingxi, H. Yizhu, Y. Xiaomin, Effect of nano-Y2 O3 on microstructure of laser cladding cobalt-based alloy coatings, Appl. Surf. Sci. 252 (2006) 2882–2887. [20] Q. Chen, N.L. Yakovlev, Adsorption and interaction of organosilanes on TiO2 nanoparticles, Appl. Surf. Sci. 257 (2010) 1395–1400. [21] X. Shen, Y. Zhai, Preparation and optical properties of Y2 O3 /SiO2 powder, Rare Met. 30 (2011) 33–38. [22] Y. Gao, G. Chen, Y. Oli, Z. Zhang, Q. Xue, Study on tribological properties of oleic acid-modified TiO2 nanoparticle in water, Wear 252 (2002) 454–458. [23] Z.J. Zhang, J. Zhang, Q.J. Xue, Synthesis and characterization of a molybdenum disulfide nanocluster, J. Phys. Chem. 98 (1994) 12973–12977. [24] J. Qian, X. Yin, N. Wang, L. Liu, J. Xing, Preparation and tribological properties of stearic acid-modified hierarchical anatase TiO2 microcrystals, Appl. Surf. Sci. 258 (2012) 2778–2782.