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WS2 nanorods prepared by self-transformation process and their tribological properties as additive in base oil
Materials Science and Engineering A 454–455 (2007) 487–491
WS2 nanorods prepared by self-transformation process and their tribological properties as additive in base oil L.L. Zhang, J.P. Tu ∗ , H.M. Wu, Y.Z. Yang Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 28 May 2006; received in revised form 9 November 2006; accepted 15 November 2006
Abstract WS2 nanorods with 10–15 nm in diameters and 0.1–2 m in lengths were prepared successfully by self-transformation process from the precursor, which was obtained by high-energy ball milling. The tribological properties of WS2 nanorods as lubricating oil additive were investigated with a MMW-1 four-ball tribotester. By the addition of WS2 nanorods in base oil, the antiwear ability was improved and the friction coefficient was decreased. The oil with WS2 nanorods showed better tribological properties than the oil with 2H-WS2 . A combination of the rolling effect between the rubbing surface and the formation of a thin physical tribofilm on the substrate can explain the good friction and wear properties of WS2 nanorods. © 2006 Elsevier B.V. All rights reserved. Keywords: WS2 nanorods; Self-transformation; Lubrication additive; Tribological properties
1. Introduction In the past few decades, putting additives into oil to reduce friction coefficient and improve antiwear ability or to mend a worn surface have been widely applied in lubrication engineering. These studies refer to synthesis of additives, tribological properties, as well as tribological mechanisms [1–5]. It had been found that when some nanoparticles were added into the base oil, the results presented interesting friction reducing and antiwear properties [6–9]. Nanoscale materials such as graphite, MoS2 and WS2 are used as additives in liquid lubricants, the tribological properties of the lubricating oil have been enhanced [10–12]. The viewpoints about mechanisms of antiwear and friction reducing can be explained as follows: (a) rolling friction [13], (b) the nanoparticles serve as spacer, which eliminate metal to metal contact between the asperities of the two mating metal surfaces [14]. Nowadays, it is more widely believed that nanoparticles deposit on the friction surface and compensate for the loss of mass, which have been called it “mending effect” [7]. So far several methods have been reported to prepare nanoscale WS2 , such as gas–solid or gas phase reaction [15], pulsed laser [16], arc discharge [17], solvothermal route [18]
∗
Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.072
and precipitation method [19]. In this present work, WS2 nanorods were prepared successfully by self-transformation process in autoclave, which is effective, convenient, less energy-demanding and less material consuming to prepare nanomaterials. The tribological properties of WS2 nanorods as lubricating additive in base oil were investigated.
2. Experimental 2.1. Preparation of WS2 nanorods The precursor was prepared by mechanically milled a mixture of layered (2H-) WS2 powder (average size: 0.7 m) and S powder (average size: 2.5 m) with the mass ratio of 1:1. In this case, the addition of S was to produce an S atmosphere during the ball-milling process. All chemicals used in the present work are analytical grade without further purification. The ball milling was carried out in argon at room temperature in a QM-4F type planetary ball-milling machine. The agate pots with capacity of 250 ml and agate balls with diameters of 10–20 mm were used as milling mediums. After ball milling at a rotating speed of 450 rpm for 122 h, the precursor nanosheets were formed. The precursor powder was placed into a beaker, and a surface dispersant agent (polyethylene glycol (PEG) with average molecular weight 20000) was also added into it. After stirring,
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the resulting slurry was put into an autoclave (WHF type) of 250 ml capacity, which was filled with alcohol up to 80% of the total volume. The autoclave was maintained at 240 ◦ C for 24 h, and then cooled to the room temperature naturally. The products were filtered out, washed with alcohol and de-ionized water several times, and then dried at 60 ◦ C for 1 h. The phase identity of the materials was investigated using Rigaku D/max-rA X-ray diffractometer (XRD) with Cu K␣ ˚ at 40 kV, 80 mA. The XRD data were radiation (λ = 1.5406 A) ◦ collected between 10 and 70◦ of 2θ angels with a step interval of 0.02 and a scanning rate of 1◦ min−1 . The powder morphology and structure were obtained on a SIRION JY/T010-1996 fieldemission scanning electron microscope (SEM) and a JEM-2010 transmission electron microscope (TEM). 2.2. Tribological properties of WS2 nanorods as oil additive Fig. 1. XRD pattern of WS2 nanorods.
The WS2 nanorods and sorbitol monooleate (span-80) as a dispersing agent were mixed with base oil and the mixture was stirred to make an uniform suspension with T-18 high-speed dispersion machine for 20 min, and dispersed again by KQ218 type ultrasonic bath for 30 min. The oil containing layered (2H) WS2 (average size: 0.7 m) and dispersing agent was taken after the above process. It was found that the addition of 1.0 wt.% dispersing agent sorbitol monooleate to the oil produced the best dispersion stability by judging the absorbance. The tribological properties of the oil containing the WS2 nanorods were investigated using a MMW-1 four-ball tribotester, in comparison to the base oil and the oil with the addition of 2H-WS2 . The friction and wear tests were conducted at a rotating speed of 1200 rpm and at various loads of 170, 245 and 320 N, for the test duration of 30 min. The balls (diameter in 12.7 mm) used in the tests were fabricated from a quenched-and-tempered GCr15 bearing steel with a hardness of 61 HRC. The dependence of the wear scar diameter on friction time was also measured under constant load of 245 N. The friction coefficient was recorded automatically with a strain gauge equipped with tester. The steel balls were cleaned in petroleum ether and dried before the test. After testing, the balls were then cleaned in petroleum ether and distilled water for 30 min, respectively. The wear scar diameters on the steel balls were measured using an optical microscope to an accuracy of ± 0.01 mm. The morphology of the wear scar was examined using a SIRION JY/T010–1996 scanning electron microscopy (SEM).
Fig. 2. As shown in Fig. 2a, the precursor obtained from the ball milling is lamellar, and in the average size of 20 nm in thickness and 550 nm in diameter. After self-transformation process, it is shown that the product consists of a large number of straight and smooth nanorods (Fig. 2b). Basically, there is no particle
3. Results and discussions 3.1. Characterization of WS2 nanorods Fig. 1 shows the XRD pattern of the as-prepared material by the self-transformation process. All the peaks in the XRD pattern can be indexed to WS2 . The main peaks in the XRD pattern of as-prepared WS2 and 2H-WS2 (JCPDS No. 08–0237) are well matched but have some little differences. The broadening Bragg peaks for the as-prepared WS2 are due to their small particle size. The general morphologies of the precursor and the product through self-transformation in autoclave are presented in
Fig. 2. SEM micrographs of the precursor (a) and WS2 nanorods (b).
L.L. Zhang et al. / Materials Science and Engineering A 454–455 (2007) 487–491
489
thing observed. The surface dispersant agent (polyethylene glycol) plays an important role in the synthesis process. PEG has two hydrophilic groups but no hydrophobic ones in its molecular formula. When the precursor nanosheets are added to the surface dispersant agent, PEG will adsorb on the surface of the being dispersed precursor nanosheets, and form a hydrophil film around the nanosheets, leading to the space steric effect. Therefore, the precursor nanosheets changed into WS2 nanorods by the solvent-thermal induced effect. Fig. 3 shows the TEM images of the WS2 nanorods. The sizes of the WS2 nanorods are about 10–15 nm in diameters and 0.1–2 m in lengths (Fig. 3a and b). As shown in the high resolution TEM micrograph in Fig. 3c, lattice fringes of WS2 can be clearly seen. 3.2. Effect of WS2 nanorods on tribological properties Wear scar diameters (WSD) of four-ball test, running at various loads of 170, 245 and 320 N in the base oil, the oil with 2.0 wt.% 2H-WS2 and the oil with 2.0 wt.% WS2 nanorods are given in Table 1. The wear scar diameters of the balls running in the oil with WS2 nanorods were evidently smaller than those running in other lubricating oils, under different loads for 30 min. The dispersing agent did not change the wear scar diameter. Therefore, the oil with WS2 nanorods possessed higher antiwear resistance than the base oil and the oil with 2H-WS2 . In order to investigate the concentration giving the best wear resistance, several concentrations of WS2 nanorods were tested with a constant load of 245 N for 30 min. Fig. 4 shows the wear scar diameter as a function of the additive concentration of 2HWS2 and the WS2 nanorods. It can be seen that with increasing the WS2 nanorod additive in the base oil, the wear scar diameters of the steel balls decrease more remarkably than that with the addition of 2H-WS2 . In other words, the WS2 nanorods could improve the antiwear properties than the 2H-WS2 . The WSD is a presentation of wear amount. A correlation about wear scar diameter versus sliding time, measuring under a discontinuous test time is given in Fig. 5. The wear scar diameters on the balls, running in the oil with 2.0 wt.% WS2 nanorods, were in comparison with those running in the base oil and the oil with 2H-WS2 . At the initial stage of the test, the difference of the WSD between the base oil and the oil with 2H-WS2 was little. After running 10 min, the WSD running in the oil with 2H-WS2 was smaller than the base oil. Furthermore, during the whole sliding, the WSD of the oil with WS2 nanorods was always much smaller than that of the oil with 2H-WS2 . It can be concluded that the WS2 nanorods possess superior antiwear properties to 2H-WS2 . The friction coefficients of the base oil, the oil with 2.0 wt.% 2H-WS2 and the oil with 2.0 wt.% WS2 nanorods are shown in Fig. 6. It can be seen that the oil with WS2 nanorods gives a
Fig. 3. (a) TEM image of WS2 nanorods and (b) HRTEM of WS2 nanorods. (c) Lattice fringes of WS2 nanorod.
more stable and smaller friction coefficient than the others. The addition of WS2 nanorods in base oil strongly reduces the friction coefficient. Moreover, the friction coefficient is relatively stable throughout the test, which is not the case with base oil. The evidences of reducing the friction coefficients and improving the antiwear properties of WS2 nanorods dispersed
Table 1 WSD comparison of the ball running in base oil, oil with 2H-WS2 , and oil with WS2 nanorods after running for 30 min Base oil Load (N) WSD (mm)
170 0.527
Oil with 2H-WS2 245 0.623
320 0.689
170 0.473
245 0.515
Oil with WS2 nanorods 320 0.635
170 0.385
245 0.409
320 0.508
490
L.L. Zhang et al. / Materials Science and Engineering A 454–455 (2007) 487–491
Fig. 4. Wear scar diameter as a function of additive concentration (245 N, 30 min).
Fig. 5. Dependence of wear scar diameter on the friction time (245 N, 30 min).
in base oil can be confirmed by the SEM observation. In Fig. 7a, it can easily be found that after rubbing with the base oil for 30 min, the wear scar is evidently rough with many thick and deep furrows. But the wear scar lubricated with oil containing the WS2 nanorods (Fig. 7c) is flat and smooth, compared to
Fig. 7. The worn surfaces of steel balls: (a) lubricated with base oil, (b) lubricated with 2H-WS2 and (c) lubricated with WS2 nanorods.
Fig. 6. Variation of friction coefficients for base oil, oil with 2H-WS2 and oil with 2 wt.% WS2 nanorods.
that with the base oil (Fig. 7a) and the oil containing 2H-WS2 (Fig. 7b). In this work, it has been shown that under a certain condition of WS2 nanorods, the good antiwear ability has been obtained, and at the same time, a very low friction coefficient is reached. It can be explained as follows: Firstly, the WS2 nanorods enter the contact with the oil and roll between the two rubbing surfaces. Secondly, during the sliding, because of the high contact pressure creating stressed zones of traction/compression, a thin physical tribofilm is formed on the metal substrate, as shown in Fig. 8.
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491
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 50471044) and the Research Fund of the Doctoral Program of Higher Education (20050335038). References
Fig. 8. A physical tribofilm on the rubbing surface.
The physical tribofilm could not only bore the load of the steel ball but also prevent from direct contact of two mating metal surfaces. Therefore, the antiwear ability of base oil with WS2 nanorods additive was improved, and the friction coefficient was decreased significantly and remained constant. 4. Conclusions WS2 nanorods with 10–15 nm in diameters and 0.1–2 m in lengths were prepared by self-transformation from the precursor in autoclave. The oil with addition of WS2 nanorods showed the best friction and wear properties, in comparison with the base oil and the oil with 2H-WS2 . From the characterization performed after friction tests, it indicated that the WS2 nanorods entered the contact with the oil and rolled between the two rubbing surfaces, and furthermore, a thin physical tribofilm formed on the rubbing surface, which could not only bear the load of the steel ball but also prevent them from direct contact. A combination of both effects can explain the good friction and wear properties of WS2 nanorods.
[1] J. Hershberger, O.O. Ajayi, G.R. Fenske, Scr. Mater. 53 (2005) 1449–1453. [2] L. Joly-Pottuz, F. Dassenoy, M. Belin, B. Vacher, J.M. Martin, N. Fleischer, Tribol. Lett. 18 (2005) 477–485. [3] F. Dalle Torre, H.Van Swygenhoven, R. Schaublin, P. Spatig, M. Vivtoria, Scr. Mater. 53 (2005) 23–27. [4] P.A. Hu, Y.Q. Liu, L. Fu, X.B. Wang, D.B. Zhu, Appl. Phys. A-Mater. 80 (2005) 35–38. [5] Z.W. Li, X.J. Tao, Y.M. Cheng, Z.S. Wu, Z.J. Zhang, H.X. Dang, Mater. Sci. Eng. A. 407 (2005) 7–10. [6] H.D. Huang, J.P. Tu, T.Z. Zou, L.L. Zhang, D.N. He, Tribol. Lett. 20 (2005) 247–250. [7] G. Liu, X. Li, B. Qin, D. Xing, Y. Guo, R. Fan, Tribol. Lett. 17 (2004) 961–966. [8] L. Sun, Z.J. Zhang, Z.S. Wu, H.X. Dang, Mater. Sci. Eng. A 379 (2004) 378–383. [9] C. Grossiord, J.M. Martin, T. Le-Mogne, T. Palermo, Surf. Coat. Technol. 108–109 (1998) 352–359. [10] C. Grossiord, K. Varlot, J.M. Martin, T. Le-Mogne, C. Esnouf, K. Inoue, Tribol. Int. 31 (1998) 737–743. [11] H.D. Huang, J.P. Tu, L.P. Gan, C.Z. Li, Wear 261 (2006) 140–144. [12] L. Cizaire, B. Vacher, T. Le-Mogne, J.M. Martin, L. Rapoport, A. Margolin, R. Tenne, Surf. Coat. Technol. 160 (2002) 282–287. [13] L. Rapport, Y. Bilik, Y. Feldman, M. Homyonfer, S.R. Cohen, R. Tenne, Nature 387 (1997) 791–793. [14] L. Rapoport, Y. Feldman, M. Homyonfer, H. Cohen, J. Sloan, J.L. Hutchison, R. Tenne, Wear 225–229 (1999) 975–982. [15] H.B. Yang, S.K. Liu, J.X. Li, M.H. Li, G. Peng, G.T. Zou, Nanotechnology 17 (2006) 1512–1519. [16] J.J. Hu, J.S. Zabinski, J.H. Sanders, J.E. Bultman, A.A. Voevodin, J. Phys. Chem. B 110 (2006) 8914–8916. [17] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430–4431. [18] Y.C. Wang, I.C. Leu, M.H. Hon, J. Cryst. Growth 237 (2002) 564–568. [19] Y.F. Zheng, X.C. Song, B. Liu, G. Han, Z.D. Xu, J. Inorg. Mater. 19 (2004) 653–656.
WS2 nanorods prepared by self-transformation process and their tribological properties as additive in base oil L.L. Zhang, J.P. Tu ∗ , H.M. Wu, Y.Z. Yang Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China Received 28 May 2006; received in revised form 9 November 2006; accepted 15 November 2006
Abstract WS2 nanorods with 10–15 nm in diameters and 0.1–2 m in lengths were prepared successfully by self-transformation process from the precursor, which was obtained by high-energy ball milling. The tribological properties of WS2 nanorods as lubricating oil additive were investigated with a MMW-1 four-ball tribotester. By the addition of WS2 nanorods in base oil, the antiwear ability was improved and the friction coefficient was decreased. The oil with WS2 nanorods showed better tribological properties than the oil with 2H-WS2 . A combination of the rolling effect between the rubbing surface and the formation of a thin physical tribofilm on the substrate can explain the good friction and wear properties of WS2 nanorods. © 2006 Elsevier B.V. All rights reserved. Keywords: WS2 nanorods; Self-transformation; Lubrication additive; Tribological properties
1. Introduction In the past few decades, putting additives into oil to reduce friction coefficient and improve antiwear ability or to mend a worn surface have been widely applied in lubrication engineering. These studies refer to synthesis of additives, tribological properties, as well as tribological mechanisms [1–5]. It had been found that when some nanoparticles were added into the base oil, the results presented interesting friction reducing and antiwear properties [6–9]. Nanoscale materials such as graphite, MoS2 and WS2 are used as additives in liquid lubricants, the tribological properties of the lubricating oil have been enhanced [10–12]. The viewpoints about mechanisms of antiwear and friction reducing can be explained as follows: (a) rolling friction [13], (b) the nanoparticles serve as spacer, which eliminate metal to metal contact between the asperities of the two mating metal surfaces [14]. Nowadays, it is more widely believed that nanoparticles deposit on the friction surface and compensate for the loss of mass, which have been called it “mending effect” [7]. So far several methods have been reported to prepare nanoscale WS2 , such as gas–solid or gas phase reaction [15], pulsed laser [16], arc discharge [17], solvothermal route [18]
∗
Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.11.072
and precipitation method [19]. In this present work, WS2 nanorods were prepared successfully by self-transformation process in autoclave, which is effective, convenient, less energy-demanding and less material consuming to prepare nanomaterials. The tribological properties of WS2 nanorods as lubricating additive in base oil were investigated.
2. Experimental 2.1. Preparation of WS2 nanorods The precursor was prepared by mechanically milled a mixture of layered (2H-) WS2 powder (average size: 0.7 m) and S powder (average size: 2.5 m) with the mass ratio of 1:1. In this case, the addition of S was to produce an S atmosphere during the ball-milling process. All chemicals used in the present work are analytical grade without further purification. The ball milling was carried out in argon at room temperature in a QM-4F type planetary ball-milling machine. The agate pots with capacity of 250 ml and agate balls with diameters of 10–20 mm were used as milling mediums. After ball milling at a rotating speed of 450 rpm for 122 h, the precursor nanosheets were formed. The precursor powder was placed into a beaker, and a surface dispersant agent (polyethylene glycol (PEG) with average molecular weight 20000) was also added into it. After stirring,
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the resulting slurry was put into an autoclave (WHF type) of 250 ml capacity, which was filled with alcohol up to 80% of the total volume. The autoclave was maintained at 240 ◦ C for 24 h, and then cooled to the room temperature naturally. The products were filtered out, washed with alcohol and de-ionized water several times, and then dried at 60 ◦ C for 1 h. The phase identity of the materials was investigated using Rigaku D/max-rA X-ray diffractometer (XRD) with Cu K␣ ˚ at 40 kV, 80 mA. The XRD data were radiation (λ = 1.5406 A) ◦ collected between 10 and 70◦ of 2θ angels with a step interval of 0.02 and a scanning rate of 1◦ min−1 . The powder morphology and structure were obtained on a SIRION JY/T010-1996 fieldemission scanning electron microscope (SEM) and a JEM-2010 transmission electron microscope (TEM). 2.2. Tribological properties of WS2 nanorods as oil additive Fig. 1. XRD pattern of WS2 nanorods.
The WS2 nanorods and sorbitol monooleate (span-80) as a dispersing agent were mixed with base oil and the mixture was stirred to make an uniform suspension with T-18 high-speed dispersion machine for 20 min, and dispersed again by KQ218 type ultrasonic bath for 30 min. The oil containing layered (2H) WS2 (average size: 0.7 m) and dispersing agent was taken after the above process. It was found that the addition of 1.0 wt.% dispersing agent sorbitol monooleate to the oil produced the best dispersion stability by judging the absorbance. The tribological properties of the oil containing the WS2 nanorods were investigated using a MMW-1 four-ball tribotester, in comparison to the base oil and the oil with the addition of 2H-WS2 . The friction and wear tests were conducted at a rotating speed of 1200 rpm and at various loads of 170, 245 and 320 N, for the test duration of 30 min. The balls (diameter in 12.7 mm) used in the tests were fabricated from a quenched-and-tempered GCr15 bearing steel with a hardness of 61 HRC. The dependence of the wear scar diameter on friction time was also measured under constant load of 245 N. The friction coefficient was recorded automatically with a strain gauge equipped with tester. The steel balls were cleaned in petroleum ether and dried before the test. After testing, the balls were then cleaned in petroleum ether and distilled water for 30 min, respectively. The wear scar diameters on the steel balls were measured using an optical microscope to an accuracy of ± 0.01 mm. The morphology of the wear scar was examined using a SIRION JY/T010–1996 scanning electron microscopy (SEM).
Fig. 2. As shown in Fig. 2a, the precursor obtained from the ball milling is lamellar, and in the average size of 20 nm in thickness and 550 nm in diameter. After self-transformation process, it is shown that the product consists of a large number of straight and smooth nanorods (Fig. 2b). Basically, there is no particle
3. Results and discussions 3.1. Characterization of WS2 nanorods Fig. 1 shows the XRD pattern of the as-prepared material by the self-transformation process. All the peaks in the XRD pattern can be indexed to WS2 . The main peaks in the XRD pattern of as-prepared WS2 and 2H-WS2 (JCPDS No. 08–0237) are well matched but have some little differences. The broadening Bragg peaks for the as-prepared WS2 are due to their small particle size. The general morphologies of the precursor and the product through self-transformation in autoclave are presented in
Fig. 2. SEM micrographs of the precursor (a) and WS2 nanorods (b).
L.L. Zhang et al. / Materials Science and Engineering A 454–455 (2007) 487–491
489
thing observed. The surface dispersant agent (polyethylene glycol) plays an important role in the synthesis process. PEG has two hydrophilic groups but no hydrophobic ones in its molecular formula. When the precursor nanosheets are added to the surface dispersant agent, PEG will adsorb on the surface of the being dispersed precursor nanosheets, and form a hydrophil film around the nanosheets, leading to the space steric effect. Therefore, the precursor nanosheets changed into WS2 nanorods by the solvent-thermal induced effect. Fig. 3 shows the TEM images of the WS2 nanorods. The sizes of the WS2 nanorods are about 10–15 nm in diameters and 0.1–2 m in lengths (Fig. 3a and b). As shown in the high resolution TEM micrograph in Fig. 3c, lattice fringes of WS2 can be clearly seen. 3.2. Effect of WS2 nanorods on tribological properties Wear scar diameters (WSD) of four-ball test, running at various loads of 170, 245 and 320 N in the base oil, the oil with 2.0 wt.% 2H-WS2 and the oil with 2.0 wt.% WS2 nanorods are given in Table 1. The wear scar diameters of the balls running in the oil with WS2 nanorods were evidently smaller than those running in other lubricating oils, under different loads for 30 min. The dispersing agent did not change the wear scar diameter. Therefore, the oil with WS2 nanorods possessed higher antiwear resistance than the base oil and the oil with 2H-WS2 . In order to investigate the concentration giving the best wear resistance, several concentrations of WS2 nanorods were tested with a constant load of 245 N for 30 min. Fig. 4 shows the wear scar diameter as a function of the additive concentration of 2HWS2 and the WS2 nanorods. It can be seen that with increasing the WS2 nanorod additive in the base oil, the wear scar diameters of the steel balls decrease more remarkably than that with the addition of 2H-WS2 . In other words, the WS2 nanorods could improve the antiwear properties than the 2H-WS2 . The WSD is a presentation of wear amount. A correlation about wear scar diameter versus sliding time, measuring under a discontinuous test time is given in Fig. 5. The wear scar diameters on the balls, running in the oil with 2.0 wt.% WS2 nanorods, were in comparison with those running in the base oil and the oil with 2H-WS2 . At the initial stage of the test, the difference of the WSD between the base oil and the oil with 2H-WS2 was little. After running 10 min, the WSD running in the oil with 2H-WS2 was smaller than the base oil. Furthermore, during the whole sliding, the WSD of the oil with WS2 nanorods was always much smaller than that of the oil with 2H-WS2 . It can be concluded that the WS2 nanorods possess superior antiwear properties to 2H-WS2 . The friction coefficients of the base oil, the oil with 2.0 wt.% 2H-WS2 and the oil with 2.0 wt.% WS2 nanorods are shown in Fig. 6. It can be seen that the oil with WS2 nanorods gives a
Fig. 3. (a) TEM image of WS2 nanorods and (b) HRTEM of WS2 nanorods. (c) Lattice fringes of WS2 nanorod.
more stable and smaller friction coefficient than the others. The addition of WS2 nanorods in base oil strongly reduces the friction coefficient. Moreover, the friction coefficient is relatively stable throughout the test, which is not the case with base oil. The evidences of reducing the friction coefficients and improving the antiwear properties of WS2 nanorods dispersed
Table 1 WSD comparison of the ball running in base oil, oil with 2H-WS2 , and oil with WS2 nanorods after running for 30 min Base oil Load (N) WSD (mm)
170 0.527
Oil with 2H-WS2 245 0.623
320 0.689
170 0.473
245 0.515
Oil with WS2 nanorods 320 0.635
170 0.385
245 0.409
320 0.508
490
L.L. Zhang et al. / Materials Science and Engineering A 454–455 (2007) 487–491
Fig. 4. Wear scar diameter as a function of additive concentration (245 N, 30 min).
Fig. 5. Dependence of wear scar diameter on the friction time (245 N, 30 min).
in base oil can be confirmed by the SEM observation. In Fig. 7a, it can easily be found that after rubbing with the base oil for 30 min, the wear scar is evidently rough with many thick and deep furrows. But the wear scar lubricated with oil containing the WS2 nanorods (Fig. 7c) is flat and smooth, compared to
Fig. 7. The worn surfaces of steel balls: (a) lubricated with base oil, (b) lubricated with 2H-WS2 and (c) lubricated with WS2 nanorods.
Fig. 6. Variation of friction coefficients for base oil, oil with 2H-WS2 and oil with 2 wt.% WS2 nanorods.
that with the base oil (Fig. 7a) and the oil containing 2H-WS2 (Fig. 7b). In this work, it has been shown that under a certain condition of WS2 nanorods, the good antiwear ability has been obtained, and at the same time, a very low friction coefficient is reached. It can be explained as follows: Firstly, the WS2 nanorods enter the contact with the oil and roll between the two rubbing surfaces. Secondly, during the sliding, because of the high contact pressure creating stressed zones of traction/compression, a thin physical tribofilm is formed on the metal substrate, as shown in Fig. 8.
L.L. Zhang et al. / Materials Science and Engineering A 454–455 (2007) 487–491
491
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 50471044) and the Research Fund of the Doctoral Program of Higher Education (20050335038). References
Fig. 8. A physical tribofilm on the rubbing surface.
The physical tribofilm could not only bore the load of the steel ball but also prevent from direct contact of two mating metal surfaces. Therefore, the antiwear ability of base oil with WS2 nanorods additive was improved, and the friction coefficient was decreased significantly and remained constant. 4. Conclusions WS2 nanorods with 10–15 nm in diameters and 0.1–2 m in lengths were prepared by self-transformation from the precursor in autoclave. The oil with addition of WS2 nanorods showed the best friction and wear properties, in comparison with the base oil and the oil with 2H-WS2 . From the characterization performed after friction tests, it indicated that the WS2 nanorods entered the contact with the oil and rolled between the two rubbing surfaces, and furthermore, a thin physical tribofilm formed on the rubbing surface, which could not only bear the load of the steel ball but also prevent them from direct contact. A combination of both effects can explain the good friction and wear properties of WS2 nanorods.
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