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Surface-modification in situ of nano-SiO2 and its structure and tribological properties
Applied Surface Science 252 (2006) 7856–7861 www.elsevier.com/locate/apsusc
Surface-modification in situ of nano-SiO2 and its structure and tribological properties Xiaohong Li a,b,c, Zhi Cao b, Zhijun Zhang b,*, Hongxin Dang a,b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000, PR China b Laboratory for Special Functional Materials, Henan University, Kaifeng, Henan 475001, PR China c Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 20 July 2005; received in revised form 23 September 2005; accepted 24 September 2005 Available online 4 November 2005
Abstract The preparation of a series of dispersible nano-SiO2 by surface-modification in situ was described in this paper. It is found that some silane coupling agents can be combined with nano-SiO2 by covalent bonds, which change the nanoparticle’s surface properties and make nano-SiO2 disperse well and steadily in many organic mediums. The structure of nanoparticles was characterized by transmission electron microscopy (TEM), infrared spectrum (IR), X-ray photoelectron spectra (XPS) and thermogravimetric analysis (TG). The dispersivity of these nanoparticles in organic solvents was measured by light transmittance. Considering such superior dispersion in oily solvents and very small size, we primarily investigated their tribological behaviors as additive in lubricant on wear testers. The results show that they can evidently increase anti-wear ability and reduce the friction coefficient of lubricant. # 2005 Elsevier B.V. All rights reserved. Keywords: Nanosilica; Surface-modification; Dispersion; Tribology
1. Introduction As other nanomaterials, agglomeration of SiO2 nanoparticles has been the most obstacles in their wide applications [1,2]. The traits of nanoparticles are hardly embodied in usage because they are usually dispersed in medium in aggregation of micro-size [3,4]. So the study of improving dispersivity of nano-silica in organic solvents has raised considerable interest recently. Generally, fumed silica made by a costly method has good dispersivity while precipitated silica particles are difficult to be dispersed into organic solvents [5]. Many investigators treat precipitated nano-silica particles with coupling agents, surfactants, aliphatic acids, etc., but hardly make them well dispersive in organic solvent. This is because the unsaturated dangling bonds and hydroxyl groups are extremely active and they are saturated as soon as the SiO2 is exposed to air. So the modifiers are difficult to react with the active groups of SiO2. Moreover, partial agglomeration usually occurs before the modification.
* Corresponding author. Tel.: +86 391 6634601; fax: +86 391 6634601. E-mail address: [email protected] (X. Li). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.068
Surface-modification in situ is a polyreaction-like where the hydrolysis product of inorganic salt is used as monomer and modifier as chain terminator [6]. In our past work, we synthesized a series of dispersible nano-SiO2 (DNS) by means of surface-modification in situ in liquid phase. The DNS nanoparticles were modified with hydrocarbon chains containing different functional groups in order to improve dispersion. Some alkanols and aliphatic acids in situ help dispersing SiO2 in organic solvents to a certain extent. For example, the nanoSiO2 modified by longer chain compounds such as iso-octanol and octylic acid can be dispersed in liquid paraffin (LP) transparently. But we have noticed that the modifiers can easily be broken off the nano-SiO2 when they encounter polar mediums because of their weak bonding strength with SiO2. So we turn attention to the silane coupling agents. In the present work, some silane coupling agents were found to make nano-SiO2 disperse well and steadily in many organic mediums. The structure of nano-SiO2 was characterized by transmission electron microscopy (SEM), infrared spectrum (IR), X-ray photoelectron spectra (XPS) and thermogravimetric analysis (TG) and the dispersivity was measured by light transmittance (LT) method. These particles show superior
X. Li et al. / Applied Surface Science 252 (2006) 7856–7861
dispersivity in many organic solvents such as diesel oil, liquid paraffin, CCl4 in which they can even appear totally transparent. Considering such nano-SiO2 particles’ small size and superior dispersivity in lubricants, we investigated their tribological behaviors used as additives. As expected, these nanoparticles could be imbedded into the micro-cracks on the surface of friction couples to prevent the cracks from further expanding and to increase the anti-wear ability. 2. Experimental 2.1. Preparation of the dispersible nano-SiO2 The reagents in the experiments such as sodium metasilicate, hydrochloric acid, liquid paraffin, carbon tetrachloride, etc., were analytical pure reagents (AR). Silane coupling agents were purchased from Nanjing Crompton Shuguang Organosilicon Co. in China. They were used without further treatment. Sodium metasilicate dissolved into deionized water at a certain concentration in a reactor equipped with a condensator and two dropping funnels of constant pressure. Placed the solution of hydrochloric acid dissolved in deionized water into one dropping funnel. A certain quantity of modifier dissolved in absolute alcohol was placed into the other funnel. At first, added half of hydrochloric acid solution to the reactor under stirring. Then the residual hydrochloric acid and modifier were added dropwise into the reactor simultaneously. When the solution reached the equivalent point of sodium metasilicate and hydrochloric acid, the reaction solution appeared turbid. The hydrolysis reaction lasted for 4 h at 60 8C. Then the obtained suspension was filtered. The filtered cake was washed repeatedly with mixed solution of deionized water and alcohol until Cl could not be found by checking with silver nitrate solution. The filtered cake was re-dispersed into a quantity of mixed solution to form emulsion. Finally, the emulsion was spray-dried at 140 8C of outlet temperature and white SiO2 nanoparticles with good flowability were obtained. We chose three types of silane coupling agents as modifiers. An unmodified sample was prepared in the same way without adding modifiers. 2.2. Characterization of the dispersible nano-SiO2 Transmission electron microscopy (TEM) analysis was performed with a JEM-2010 microscope. Infrared spectrum was obtained using a Nicolet 170sx Fourier transform infrared spectrophotometer. The thermogravimetric analysis was performed in ambient atmosphere on an Exstar-6000 thermoanalysis system at the heating rate of 10 8C/min from RT to 800 8C. XPS analysis was conducted on an AXIS ULTRA Xray photoelectron spectroscopy system with monochromatic Al radiation as source (power, 150 W, 40 eV). The specific surface area was measured by Brunauer–Emmett–Teller (BET) method. Dispersivity of DNS was observed by measuring light transmittance on a 721 spectrophotometer at 530 nm.
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2.3. Tribological behavior of the dispersible nano-SiO2 The properties of anti-wear and friction-reduction were evaluated on a RFT-III Reciprocating Tribotester under the pressure of 30 N with the rotating speed at 600 rpm for 3 h (where the plate sample was sliding against a 45# steel column) and a MS-800 four-ball Tribotester under two pressures of 20 N for 60 min and 40 N for 30 min, respectively, at 1450 rpm (where hardness of the GCr15 steel balls was 5961 HRC). Morphologies and element distribution of the wear scars were recorded by a JSM-5600 LV scanning electron microscope and the formed coextruded film on the wear scars was analyzed on a KEVEX energy dispersive X-ray analysis spectrometer (EDS). 3. Results and discussion 3.1. Formation mechanism of the dispersible nano-SiO2 Surface-modification of nano-SiO2 in situ is a condensationlike polymerization where the hydrolysis product of sodium metasilicate is used as monomer and silane coupling agent as chain terminator. The condensation polymerization of hydrolysis products is a process of chain growth and surfacemodification is analogous to end stopping of chain. Here, three types of silane coupling agent were chosen as modifiers. Their molecular formulas are listed in Scheme 1. These modified nano-SiO2 particles are called DNS-A, DNS-E and DNS-D, respectively, according to the functional groups of silane coupling agents. We hypothesize that the condensation-like polymerization is carried through in the mode given in Scheme 2, where R stands for the –CH3 and the R1 stands for the organic chain with functional group of silane coupling agent. First, sodium metasilicate is hydrolyzed under the existence of hydrochloric acid to form silicic acid. Condensation polymerization happens among the silicic acids at three dimensions and the Si and O are bonded to each other to form the defective three-dimensional structures by tetrahedron. Large numbers of hydroxyls are left on the surface of particles. In the mean time, silane coupling agent is hydrolyzed to produce –OH. As soon as the particles are formed, the –OH or – OMe of coupling agent reacts with –OH of SiO2. The organic chains substitute portion of active group of SiO2 and cause the steric hinder, which prevents SiO2 from continuously growing up or agglomerating. They are competition reactions. Controlling the reaction conditions, we can obtain the nano-SiO2 particles ‘‘capped’’ with organic compound [6,7].
Scheme 1. The molecular formulas of modifiers.
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Fig. 2. FTIR spectra of DNS: (1) DNS-A; (2) DNS-E; (3) DNS-D.
Scheme 2. Formation mechanism of the dispersible nano-SiO2.
The size of nanoparticles may be conditioned by changing the concentration of sodium metasilicate and modifier. Either higher concentration of sodium metasilicate or lower concentration of modifier can make original particles bigger and conglutinative. To ensure both higher yield and dispersion, a typical formula was used as the following: 0.3 mol/L sodium metasilicate, 0.72 mol/L hydrochloric acid and 0.05 mol/L modifiers. 3.2. Structural characterization of the dispersible nanoSiO2 The TEM photographs of the typical DNS and unmodified SiO2 (UM-SiO2) dispersed in absolute alcohol are given in Fig. 1. The UM-SiO2 particles are severely aggregated while the morphology of several types of DNS looks alike and they are all well distributive with size of 15–20 nm. TG/DTA curves show that the total weight loss of each kind of DNS within 15–800 8C is no more than 13%. It can be
estimated that they contain about 87–90% silica, less than 3.0% water and 7–10% organics. Their specific surface area, measured by BET method, is 110 20 m2/g. Silane coupling agent molecules cover the surface of nano-SiO2 and greatly reduce the amount of exposed –OH, which decreases the surface adsorption capacity of the nano-SiO2. In fact the measured value by BET cannot reflect the actual specific surface area of DNS. From the FTIR spectra of DNS in Fig. 2, we can see that the stretching vibration peaks of Si–OH reaches near 1100 cm 1, the asymmetric stretching vibration near 810 cm 1, the symmetric stretching vibration and bending vibration near 470 cm 1 of Si–O–Si. The bands near 1390 cm 1 correspond to the asymmetric stretching vibration peakof C–H. The peaks of 1720 and 1560 cm 1 correspond to stretching vibration of carbonyl of DNS-D and bending vibration of N–H of DNS-A, respectively. Disappearance of the absorption band of Si–O–C in 1080 cm 1 shows that the Si–O–C does not exist. Further considering their superior dispersivity and stability in solvent, we may conclude that these coupling agents are bonded on the nano-silica by covalent bonds. The –OH of new born nano-SiO2 reacts with –Si(OMe)3 or –OH produced from coupling agent to form modified nano-SiO2 with organic compounds on the surface.
Fig. 1. TEM photographs of SiO2 particles (a) UM-SiO2 and (b) DNS.
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Fig. 3. XPS spectra of SiO2 particles: (a) UM-SiO2 and (b) DNS-A.
Fig. 3 displays XPS spectra of Si2p on surface of UM-SiO2 and DNS-A. The binding energy of C1s (284.8 eV) is used as the reference. In spectrum of UM-SiO2, the peak of Si2p is at 103.8 eV, which is in agreement with Si2p in SiO2 (103.6 eV) [8]. As hydroxyl is electron withdrawing group, the binding energy of Si2p in Si–OH on the particle surface shifts to 104.9 eV. In XPS spectrum of DNS-A, two peaks of Si2p appear at 102.9 and 104.3 eV, respectively. The higher intensity Si2p peak at 102.9 eV is attributed to Si2p in Si-alkyl from the modifier where the group NH2 (CH2)3– is a strong donor group and therefore the binding energy is lower than SiO2 (103.6 eV). As silane coupling agent reacts with –OH and amount of –OH greatly decreases, the peak of Si–OH became weak. The peak at 104.3 eV, appearing at lower binding energy compared with the peak at 104.9 eV in UM-SiO2, is thought to be the overlapping of Si2p peaks of SiO2 (103.6 eV) and residual Si–OH (104.9 eV). The peak area is much smaller than that of Si– alkyl at 103.6 eV, which indicates that the nanoparticles have a SiO2 core and an organic surface layer. The XPS spectra of Si in DNS-E and DNS-D are similar with that of DNS-A. According to the results of IR and XPS, we can further affirm that the modifiers are bonded to the surface of SiO2 nanoparticles. 3.3. Dispersivity of the dispersible nano-SiO2 The better the nanoparticles are dispersed in dispersant, the higher the light transmittance (LT) of solution is. So the dispersivity of DNS can be measured by its light transmittance. The degree of dispersion is defined as the biggest concentration of nanoparticles in dispersant when the LT of the dispersion system is more than 80% at 25 8C. Under this condition, the system appears essentially clear. Adding a small quantity of unmodified SiO2 with KH-550, KH-560 and KH-570 in organic solvents such as liquid paraffin, carbon tetrachloride, we observed that these systems looked turbid and SiO2 precipitated soon. The LT of the solution of 0.5% SiO2 in CCl4 is only 21%. The dispersivity of surface-modified nano-SiO2 is listed in Tables 1 and 2. The LT of 0.5 wt% DNS solution in liquid paraffin is more than 88%. The results show that these surface modifiers evidently improve the nanoparticles’ dispersivity in mediums, especially in apolar hydrophobic. The organic compound changes the surface property of nano-SiO2 from
Table 1 Light transmittance of different concentrations of DNS in different solvents Tape
Concentration (wt%)
Diesel oil (%)
LP (%)
CCl4 (%)
GMO (%)
DNS-D
0.5 1.0 2.0 4.0
94.5 90.7 86.0 82.7
95.5 93.8 92.4 91.6
98.5 97.3 94.4 90.4
92.5 90.0 86.2 –
DNS-E
0.5 1.0 2.0 4.0
94.5 91.2 85.0 83.2
93.2 89.0 86.5 84.2
95.5 93.8 91.0 88.7
90.0 80.5 76.3 –
DNS-A
0.5 1.0 2.0 4.0
91.3 86.5 82.1 77.5
90.2 87.5 80.5 73.2
87.5 85.4 83.1 81.5
88.7 80.5 76.3 –
polarity to weak polarity so that DNS can be dispersed well in apolar solvents. The DNS-D possesses the best dispersivity and its degree of dispersion achieves 24% in CCl4. The solution of 2 wt% DNS-D in gas mobile oil (GMO) is still transparent though it is too sticky to flow. The reason that DNS-A’s dispersivity is inferior to DNS-H’s and DNS-D’s should be the existence of polar group –NH2. 3.4. Tribological characterization of the dispersible nano-SiO2 Since such nano-SiO2 has superior dispersivity in lubricants and it is so small in size we investigated their tribological behaviors used as additives. We added 0.3 wt% DNS particles to ST5W/30 gas mobile oil (GMO) under ultrasonic dispersion. These dispersion systems were still stable after 5 months. The tribological behaviors of GMO as well as the GMO with DNS (DNS/GMO) are given in Table 3. Table 2 Degree of dispersion of DNS in different solvents Sample
In GMO (%)
In LP (%)
In CCl4 (%)
DNS-D DNS-E DNS-A
>2 1.5 1.0
10 8.5 2.0
24.0 18.0 4
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Table 3 Tribological properties of DNS in GMO Additive
On reciprocating tester m
0 DNS-D DNS-E DNS-A
0.0455 0.026 0.034 0.027
Dw (g) 0.0113 0.0036 0.0098 0.0035
On four-ball tester WSD (mm) 20 N, 60 min
40 N, 30 min
0.350 0.242 0.273 0.268
0.485 0.345 0.373 0.385
The results show that adding DNS into the GMO makes the frictional coefficient (m), wear quantity ( Dw) on a Reciprocating Tribotester and wear scar diameter (WSD) on a four-ball Tribotester decreased evidently. DND-D and DNS-A as additives in lubricant show superior anti-wear and frictionreducing ability. The SEM photographs of wear scar morphology of steel bars (applied with GMO with and without DNS) under the same load on the reciprocating tribotester are given in Fig. 4. Fig. 4b shows the wear scar morphology of the steel bar lubricated with GMO containing DNS-A (DNS-A/GMO). It is obvious that the scars with the DNS/GMO are much shallower and the surface is smoother. By examining the steel bars using EDS spectroscopy (Fig. 5) we can find that there are much more Si on scars lubricated with GMO containing DNS, which indicates that DNS particles are transferred and accumulated onto the surface of these scars during friction. From the distribution of elements on the surface of friction (Fig. 6), we can observe that the sites with denser Si have less Fe, which indicates that such nano-SiO2 can fill serious wearing micro-areas and play the role of self-repairing. A possible explanation is that an inorganic–organic coextruded film is formed on the rubbing contact area at a higher temperature because of the existence of surface modified nanoSiO2 [9]. The particles are imbedded into the micro-cracks owing to their very small size. Besides, SiO2 probably forms ceramic composites with metal during friction to increase the anti-wear ability [10]. At the same time, organic arrangement of
hydrocarbon chain on the surface of nano-SiO2 can also reduce the friction coefficient owing to ‘‘the brush mechanism’’ [11]. The formed film protects the contacting metal surfaces and fills the wearing scars, therefore reducing the friction and lessening the wear. The difference in tribological behavior of these nanoparticles should come from their different functional groups of modifier. The ester group of DNS-D by itself helps on wear reducing. And the double bonds seem to play a good role owing to the fact that they are liable to polymerize at higher temperature. The co-extruded nano-SiO2/polymer film possesses better anti-fraction capability than low-molecular weight compounds. Thus, DNS-D shows better tribological properties. The –NH2 of DNS-A is thought to make the nanoparticles having stronger absorption to the steel surface and accordingly it shows superior tribological properties.
Fig. 5. EDS spectra of wear scar of the steel bar surface: (a) lubricated with pure GMO and (b) lubricated with DNS-A/GMO.
Fig. 4. SEM photographs of wear scar of the steel bar surface: (a) lubricated with pure GMO and (b) lubricated with DNS-A/GMO.
X. Li et al. / Applied Surface Science 252 (2006) 7856–7861
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Fig. 6. Distribution of elements on the surface of friction with DNS-A/GMO: (a) Si and (b) Fe.
4. Conclusion A series of nano-SiO2 was prepared by means of surfacemodification in situ where silane coupling agents KH-550, KH560 and KH-570 were used as modifiers. Results show that these modifiers are combined to the surface of nano-SiO2 by covalent bonds and change the surface properties of nano-SiO2. These nanoparticles have superior dispersivity and stability in many types of organic solvents. The tribology study on these particles as additives in lubricant reveals their potential in lubrication. Acknowledgment This research is supported by a grant from the National Natural Science Foundation of China (20271016). References [1] M.L. Zhang, G.L. Ding, X.Y. Jing, X.Q. Hou, Preparation, modification and application of nanoscal SiO2, Appl. Sci. Technol. 31 (6) (2004) 64–66.
[2] X.H. Li, R. Sun, Z.J. Zhang, H.X. Dang, Progress of polymer/inorganic nanocomposites by chemical bonds, Prog. Nat. Sci. 14 (2004) 1. [3] Y.C. Ke, Polymer-Inorganic Nano-Composites, Chemical Industry Press, Beijing, 2002. [4] Z.W. Li, Y.F. Zhu, Surface-modification of SiO2 nanoparticles with olic acid, Appl. Surf. Sci. 211 (2003) 315–320. [5] S.M. Wang, Z.X. Xu, J. Fu, Technology of Preparation on Nano-Materials, Chemical Industry Press, Beijing, 2002. [6] M. Gutierrez, A. Henglein, Quantized colloids produced by dissolution of layered semiconductors in acetonitrile, Ultrasonics 27 (1989) 259–261. [7] M.W. Peterson, M.T. Nenadovic, T. Rajh, R. Herak, O.I. Micic, J.P. Goral, A.J. Nozik, Preparation of colloidal semiconductor solutions of MoS2 and WSe2 via sonication, J. Phys. Chem. 92 (1988) 1400–1402. [8] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation Physical Electronics Division, USA, 1992. [9] Y.Q. Huo, Y.T. Yan, X.X. Liu, Preparation and tribological properties of monodispersed nano-SiO2 particles as additive in lubricating oil, Tribology 25 (1) (2005) 34–38. [10] W.M. Liu, Application of nanoparticles in lubricants, Tribology 23 (4) (2003) 265–267. [11] Z.J. Zhang, J. Zhang, Q.J. Xue, Synthesis and characterization of a molybdenum disulfide nanocluster, J. Phys. Chem. 98 (1994) 12973– 12977.
Surface-modification in situ of nano-SiO2 and its structure and tribological properties Xiaohong Li a,b,c, Zhi Cao b, Zhijun Zhang b,*, Hongxin Dang a,b a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000, PR China b Laboratory for Special Functional Materials, Henan University, Kaifeng, Henan 475001, PR China c Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 20 July 2005; received in revised form 23 September 2005; accepted 24 September 2005 Available online 4 November 2005
Abstract The preparation of a series of dispersible nano-SiO2 by surface-modification in situ was described in this paper. It is found that some silane coupling agents can be combined with nano-SiO2 by covalent bonds, which change the nanoparticle’s surface properties and make nano-SiO2 disperse well and steadily in many organic mediums. The structure of nanoparticles was characterized by transmission electron microscopy (TEM), infrared spectrum (IR), X-ray photoelectron spectra (XPS) and thermogravimetric analysis (TG). The dispersivity of these nanoparticles in organic solvents was measured by light transmittance. Considering such superior dispersion in oily solvents and very small size, we primarily investigated their tribological behaviors as additive in lubricant on wear testers. The results show that they can evidently increase anti-wear ability and reduce the friction coefficient of lubricant. # 2005 Elsevier B.V. All rights reserved. Keywords: Nanosilica; Surface-modification; Dispersion; Tribology
1. Introduction As other nanomaterials, agglomeration of SiO2 nanoparticles has been the most obstacles in their wide applications [1,2]. The traits of nanoparticles are hardly embodied in usage because they are usually dispersed in medium in aggregation of micro-size [3,4]. So the study of improving dispersivity of nano-silica in organic solvents has raised considerable interest recently. Generally, fumed silica made by a costly method has good dispersivity while precipitated silica particles are difficult to be dispersed into organic solvents [5]. Many investigators treat precipitated nano-silica particles with coupling agents, surfactants, aliphatic acids, etc., but hardly make them well dispersive in organic solvent. This is because the unsaturated dangling bonds and hydroxyl groups are extremely active and they are saturated as soon as the SiO2 is exposed to air. So the modifiers are difficult to react with the active groups of SiO2. Moreover, partial agglomeration usually occurs before the modification.
* Corresponding author. Tel.: +86 391 6634601; fax: +86 391 6634601. E-mail address: [email protected] (X. Li). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.068
Surface-modification in situ is a polyreaction-like where the hydrolysis product of inorganic salt is used as monomer and modifier as chain terminator [6]. In our past work, we synthesized a series of dispersible nano-SiO2 (DNS) by means of surface-modification in situ in liquid phase. The DNS nanoparticles were modified with hydrocarbon chains containing different functional groups in order to improve dispersion. Some alkanols and aliphatic acids in situ help dispersing SiO2 in organic solvents to a certain extent. For example, the nanoSiO2 modified by longer chain compounds such as iso-octanol and octylic acid can be dispersed in liquid paraffin (LP) transparently. But we have noticed that the modifiers can easily be broken off the nano-SiO2 when they encounter polar mediums because of their weak bonding strength with SiO2. So we turn attention to the silane coupling agents. In the present work, some silane coupling agents were found to make nano-SiO2 disperse well and steadily in many organic mediums. The structure of nano-SiO2 was characterized by transmission electron microscopy (SEM), infrared spectrum (IR), X-ray photoelectron spectra (XPS) and thermogravimetric analysis (TG) and the dispersivity was measured by light transmittance (LT) method. These particles show superior
X. Li et al. / Applied Surface Science 252 (2006) 7856–7861
dispersivity in many organic solvents such as diesel oil, liquid paraffin, CCl4 in which they can even appear totally transparent. Considering such nano-SiO2 particles’ small size and superior dispersivity in lubricants, we investigated their tribological behaviors used as additives. As expected, these nanoparticles could be imbedded into the micro-cracks on the surface of friction couples to prevent the cracks from further expanding and to increase the anti-wear ability. 2. Experimental 2.1. Preparation of the dispersible nano-SiO2 The reagents in the experiments such as sodium metasilicate, hydrochloric acid, liquid paraffin, carbon tetrachloride, etc., were analytical pure reagents (AR). Silane coupling agents were purchased from Nanjing Crompton Shuguang Organosilicon Co. in China. They were used without further treatment. Sodium metasilicate dissolved into deionized water at a certain concentration in a reactor equipped with a condensator and two dropping funnels of constant pressure. Placed the solution of hydrochloric acid dissolved in deionized water into one dropping funnel. A certain quantity of modifier dissolved in absolute alcohol was placed into the other funnel. At first, added half of hydrochloric acid solution to the reactor under stirring. Then the residual hydrochloric acid and modifier were added dropwise into the reactor simultaneously. When the solution reached the equivalent point of sodium metasilicate and hydrochloric acid, the reaction solution appeared turbid. The hydrolysis reaction lasted for 4 h at 60 8C. Then the obtained suspension was filtered. The filtered cake was washed repeatedly with mixed solution of deionized water and alcohol until Cl could not be found by checking with silver nitrate solution. The filtered cake was re-dispersed into a quantity of mixed solution to form emulsion. Finally, the emulsion was spray-dried at 140 8C of outlet temperature and white SiO2 nanoparticles with good flowability were obtained. We chose three types of silane coupling agents as modifiers. An unmodified sample was prepared in the same way without adding modifiers. 2.2. Characterization of the dispersible nano-SiO2 Transmission electron microscopy (TEM) analysis was performed with a JEM-2010 microscope. Infrared spectrum was obtained using a Nicolet 170sx Fourier transform infrared spectrophotometer. The thermogravimetric analysis was performed in ambient atmosphere on an Exstar-6000 thermoanalysis system at the heating rate of 10 8C/min from RT to 800 8C. XPS analysis was conducted on an AXIS ULTRA Xray photoelectron spectroscopy system with monochromatic Al radiation as source (power, 150 W, 40 eV). The specific surface area was measured by Brunauer–Emmett–Teller (BET) method. Dispersivity of DNS was observed by measuring light transmittance on a 721 spectrophotometer at 530 nm.
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2.3. Tribological behavior of the dispersible nano-SiO2 The properties of anti-wear and friction-reduction were evaluated on a RFT-III Reciprocating Tribotester under the pressure of 30 N with the rotating speed at 600 rpm for 3 h (where the plate sample was sliding against a 45# steel column) and a MS-800 four-ball Tribotester under two pressures of 20 N for 60 min and 40 N for 30 min, respectively, at 1450 rpm (where hardness of the GCr15 steel balls was 5961 HRC). Morphologies and element distribution of the wear scars were recorded by a JSM-5600 LV scanning electron microscope and the formed coextruded film on the wear scars was analyzed on a KEVEX energy dispersive X-ray analysis spectrometer (EDS). 3. Results and discussion 3.1. Formation mechanism of the dispersible nano-SiO2 Surface-modification of nano-SiO2 in situ is a condensationlike polymerization where the hydrolysis product of sodium metasilicate is used as monomer and silane coupling agent as chain terminator. The condensation polymerization of hydrolysis products is a process of chain growth and surfacemodification is analogous to end stopping of chain. Here, three types of silane coupling agent were chosen as modifiers. Their molecular formulas are listed in Scheme 1. These modified nano-SiO2 particles are called DNS-A, DNS-E and DNS-D, respectively, according to the functional groups of silane coupling agents. We hypothesize that the condensation-like polymerization is carried through in the mode given in Scheme 2, where R stands for the –CH3 and the R1 stands for the organic chain with functional group of silane coupling agent. First, sodium metasilicate is hydrolyzed under the existence of hydrochloric acid to form silicic acid. Condensation polymerization happens among the silicic acids at three dimensions and the Si and O are bonded to each other to form the defective three-dimensional structures by tetrahedron. Large numbers of hydroxyls are left on the surface of particles. In the mean time, silane coupling agent is hydrolyzed to produce –OH. As soon as the particles are formed, the –OH or – OMe of coupling agent reacts with –OH of SiO2. The organic chains substitute portion of active group of SiO2 and cause the steric hinder, which prevents SiO2 from continuously growing up or agglomerating. They are competition reactions. Controlling the reaction conditions, we can obtain the nano-SiO2 particles ‘‘capped’’ with organic compound [6,7].
Scheme 1. The molecular formulas of modifiers.
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Fig. 2. FTIR spectra of DNS: (1) DNS-A; (2) DNS-E; (3) DNS-D.
Scheme 2. Formation mechanism of the dispersible nano-SiO2.
The size of nanoparticles may be conditioned by changing the concentration of sodium metasilicate and modifier. Either higher concentration of sodium metasilicate or lower concentration of modifier can make original particles bigger and conglutinative. To ensure both higher yield and dispersion, a typical formula was used as the following: 0.3 mol/L sodium metasilicate, 0.72 mol/L hydrochloric acid and 0.05 mol/L modifiers. 3.2. Structural characterization of the dispersible nanoSiO2 The TEM photographs of the typical DNS and unmodified SiO2 (UM-SiO2) dispersed in absolute alcohol are given in Fig. 1. The UM-SiO2 particles are severely aggregated while the morphology of several types of DNS looks alike and they are all well distributive with size of 15–20 nm. TG/DTA curves show that the total weight loss of each kind of DNS within 15–800 8C is no more than 13%. It can be
estimated that they contain about 87–90% silica, less than 3.0% water and 7–10% organics. Their specific surface area, measured by BET method, is 110 20 m2/g. Silane coupling agent molecules cover the surface of nano-SiO2 and greatly reduce the amount of exposed –OH, which decreases the surface adsorption capacity of the nano-SiO2. In fact the measured value by BET cannot reflect the actual specific surface area of DNS. From the FTIR spectra of DNS in Fig. 2, we can see that the stretching vibration peaks of Si–OH reaches near 1100 cm 1, the asymmetric stretching vibration near 810 cm 1, the symmetric stretching vibration and bending vibration near 470 cm 1 of Si–O–Si. The bands near 1390 cm 1 correspond to the asymmetric stretching vibration peakof C–H. The peaks of 1720 and 1560 cm 1 correspond to stretching vibration of carbonyl of DNS-D and bending vibration of N–H of DNS-A, respectively. Disappearance of the absorption band of Si–O–C in 1080 cm 1 shows that the Si–O–C does not exist. Further considering their superior dispersivity and stability in solvent, we may conclude that these coupling agents are bonded on the nano-silica by covalent bonds. The –OH of new born nano-SiO2 reacts with –Si(OMe)3 or –OH produced from coupling agent to form modified nano-SiO2 with organic compounds on the surface.
Fig. 1. TEM photographs of SiO2 particles (a) UM-SiO2 and (b) DNS.
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Fig. 3. XPS spectra of SiO2 particles: (a) UM-SiO2 and (b) DNS-A.
Fig. 3 displays XPS spectra of Si2p on surface of UM-SiO2 and DNS-A. The binding energy of C1s (284.8 eV) is used as the reference. In spectrum of UM-SiO2, the peak of Si2p is at 103.8 eV, which is in agreement with Si2p in SiO2 (103.6 eV) [8]. As hydroxyl is electron withdrawing group, the binding energy of Si2p in Si–OH on the particle surface shifts to 104.9 eV. In XPS spectrum of DNS-A, two peaks of Si2p appear at 102.9 and 104.3 eV, respectively. The higher intensity Si2p peak at 102.9 eV is attributed to Si2p in Si-alkyl from the modifier where the group NH2 (CH2)3– is a strong donor group and therefore the binding energy is lower than SiO2 (103.6 eV). As silane coupling agent reacts with –OH and amount of –OH greatly decreases, the peak of Si–OH became weak. The peak at 104.3 eV, appearing at lower binding energy compared with the peak at 104.9 eV in UM-SiO2, is thought to be the overlapping of Si2p peaks of SiO2 (103.6 eV) and residual Si–OH (104.9 eV). The peak area is much smaller than that of Si– alkyl at 103.6 eV, which indicates that the nanoparticles have a SiO2 core and an organic surface layer. The XPS spectra of Si in DNS-E and DNS-D are similar with that of DNS-A. According to the results of IR and XPS, we can further affirm that the modifiers are bonded to the surface of SiO2 nanoparticles. 3.3. Dispersivity of the dispersible nano-SiO2 The better the nanoparticles are dispersed in dispersant, the higher the light transmittance (LT) of solution is. So the dispersivity of DNS can be measured by its light transmittance. The degree of dispersion is defined as the biggest concentration of nanoparticles in dispersant when the LT of the dispersion system is more than 80% at 25 8C. Under this condition, the system appears essentially clear. Adding a small quantity of unmodified SiO2 with KH-550, KH-560 and KH-570 in organic solvents such as liquid paraffin, carbon tetrachloride, we observed that these systems looked turbid and SiO2 precipitated soon. The LT of the solution of 0.5% SiO2 in CCl4 is only 21%. The dispersivity of surface-modified nano-SiO2 is listed in Tables 1 and 2. The LT of 0.5 wt% DNS solution in liquid paraffin is more than 88%. The results show that these surface modifiers evidently improve the nanoparticles’ dispersivity in mediums, especially in apolar hydrophobic. The organic compound changes the surface property of nano-SiO2 from
Table 1 Light transmittance of different concentrations of DNS in different solvents Tape
Concentration (wt%)
Diesel oil (%)
LP (%)
CCl4 (%)
GMO (%)
DNS-D
0.5 1.0 2.0 4.0
94.5 90.7 86.0 82.7
95.5 93.8 92.4 91.6
98.5 97.3 94.4 90.4
92.5 90.0 86.2 –
DNS-E
0.5 1.0 2.0 4.0
94.5 91.2 85.0 83.2
93.2 89.0 86.5 84.2
95.5 93.8 91.0 88.7
90.0 80.5 76.3 –
DNS-A
0.5 1.0 2.0 4.0
91.3 86.5 82.1 77.5
90.2 87.5 80.5 73.2
87.5 85.4 83.1 81.5
88.7 80.5 76.3 –
polarity to weak polarity so that DNS can be dispersed well in apolar solvents. The DNS-D possesses the best dispersivity and its degree of dispersion achieves 24% in CCl4. The solution of 2 wt% DNS-D in gas mobile oil (GMO) is still transparent though it is too sticky to flow. The reason that DNS-A’s dispersivity is inferior to DNS-H’s and DNS-D’s should be the existence of polar group –NH2. 3.4. Tribological characterization of the dispersible nano-SiO2 Since such nano-SiO2 has superior dispersivity in lubricants and it is so small in size we investigated their tribological behaviors used as additives. We added 0.3 wt% DNS particles to ST5W/30 gas mobile oil (GMO) under ultrasonic dispersion. These dispersion systems were still stable after 5 months. The tribological behaviors of GMO as well as the GMO with DNS (DNS/GMO) are given in Table 3. Table 2 Degree of dispersion of DNS in different solvents Sample
In GMO (%)
In LP (%)
In CCl4 (%)
DNS-D DNS-E DNS-A
>2 1.5 1.0
10 8.5 2.0
24.0 18.0 4
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Table 3 Tribological properties of DNS in GMO Additive
On reciprocating tester m
0 DNS-D DNS-E DNS-A
0.0455 0.026 0.034 0.027
Dw (g) 0.0113 0.0036 0.0098 0.0035
On four-ball tester WSD (mm) 20 N, 60 min
40 N, 30 min
0.350 0.242 0.273 0.268
0.485 0.345 0.373 0.385
The results show that adding DNS into the GMO makes the frictional coefficient (m), wear quantity ( Dw) on a Reciprocating Tribotester and wear scar diameter (WSD) on a four-ball Tribotester decreased evidently. DND-D and DNS-A as additives in lubricant show superior anti-wear and frictionreducing ability. The SEM photographs of wear scar morphology of steel bars (applied with GMO with and without DNS) under the same load on the reciprocating tribotester are given in Fig. 4. Fig. 4b shows the wear scar morphology of the steel bar lubricated with GMO containing DNS-A (DNS-A/GMO). It is obvious that the scars with the DNS/GMO are much shallower and the surface is smoother. By examining the steel bars using EDS spectroscopy (Fig. 5) we can find that there are much more Si on scars lubricated with GMO containing DNS, which indicates that DNS particles are transferred and accumulated onto the surface of these scars during friction. From the distribution of elements on the surface of friction (Fig. 6), we can observe that the sites with denser Si have less Fe, which indicates that such nano-SiO2 can fill serious wearing micro-areas and play the role of self-repairing. A possible explanation is that an inorganic–organic coextruded film is formed on the rubbing contact area at a higher temperature because of the existence of surface modified nanoSiO2 [9]. The particles are imbedded into the micro-cracks owing to their very small size. Besides, SiO2 probably forms ceramic composites with metal during friction to increase the anti-wear ability [10]. At the same time, organic arrangement of
hydrocarbon chain on the surface of nano-SiO2 can also reduce the friction coefficient owing to ‘‘the brush mechanism’’ [11]. The formed film protects the contacting metal surfaces and fills the wearing scars, therefore reducing the friction and lessening the wear. The difference in tribological behavior of these nanoparticles should come from their different functional groups of modifier. The ester group of DNS-D by itself helps on wear reducing. And the double bonds seem to play a good role owing to the fact that they are liable to polymerize at higher temperature. The co-extruded nano-SiO2/polymer film possesses better anti-fraction capability than low-molecular weight compounds. Thus, DNS-D shows better tribological properties. The –NH2 of DNS-A is thought to make the nanoparticles having stronger absorption to the steel surface and accordingly it shows superior tribological properties.
Fig. 5. EDS spectra of wear scar of the steel bar surface: (a) lubricated with pure GMO and (b) lubricated with DNS-A/GMO.
Fig. 4. SEM photographs of wear scar of the steel bar surface: (a) lubricated with pure GMO and (b) lubricated with DNS-A/GMO.
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Fig. 6. Distribution of elements on the surface of friction with DNS-A/GMO: (a) Si and (b) Fe.
4. Conclusion A series of nano-SiO2 was prepared by means of surfacemodification in situ where silane coupling agents KH-550, KH560 and KH-570 were used as modifiers. Results show that these modifiers are combined to the surface of nano-SiO2 by covalent bonds and change the surface properties of nano-SiO2. These nanoparticles have superior dispersivity and stability in many types of organic solvents. The tribology study on these particles as additives in lubricant reveals their potential in lubrication. Acknowledgment This research is supported by a grant from the National Natural Science Foundation of China (20271016). References [1] M.L. Zhang, G.L. Ding, X.Y. Jing, X.Q. Hou, Preparation, modification and application of nanoscal SiO2, Appl. Sci. Technol. 31 (6) (2004) 64–66.
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