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An investigation on tribological properties of graphite nanosheets as oil additive
Wear 261 (2006) 140–144
An investigation on tribological properties of graphite nanosheets as oil additive H.D. Huang a , J.P. Tu a,∗ , L.P. Gan b , C.Z. Li b a
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Key Lab for Uultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China Received 5 April 2005; received in revised form 3 September 2005; accepted 22 September 2005 Available online 21 October 2005
Abstract Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The tribological behavior of the graphite nanosheets as additive in paraffin oil were investigated with a four-ball and a pin-on-disk friction and wear tester. The wear surfaces of the steel ball lubricated with the additive-containing paraffin oil were analyzed by means of scanning electron microscopy (SEM). It has been found that the graphite nanosheets as additive in oil at proper concentration show better tribological properties than pure paraffin oil. The load-carrying capacity and antiwear ability of the lubricating oil were improved. Moreover, the friction coefficient of the lubricating oil was decreased by the addition of the graphite nanosheets. The optimal concentration of the additive in paraffin oil is about 1.0 × 10−2 wt.%. © 2005 Elsevier B.V. All rights reserved. Keywords: Graphite nanosheets; Additive; Tribological properties; Paraffin oil
1. Introduction For tribology applications, nanoparticles as additives in base oil have been investigated widespreadly. These studies refer to synthesis and preparation of nanoscale particles, and their tribological properties and mechanisms. It has been found that when the nanoparticles were added to base oil, the extreme-pressure property and load-carrying capacity were improved and friction coefficient was decreased. At present, the viewpoint about mechanisms of nanoparticle additives is as follow: (1) ball effect [1,2]; (2) tribochemical reactions [3–5]; (3) adsorption film theory [6]. The results of these studies indicate that nanoparticles using as lubricating oil additive can improve the tribological properties of base oils. Commercially layered compound powders, usually as solid lubricants, dispersed in oil were also included [7–11]. The solid lubricants, such as MoS2 dispersed in oil exhibited beneficial effects by reducing the friction and wear [7,8,12]. Chu et al. [13] found that graphite existed on the rubbing surfaces stably and
formed composition film with the oil-soluble additives. Bartz [10] found that an “optimal concentration” existed, and the wear rate actually increased with increasing the concentration over the optimal point of the solid lubricant dispersed in oil under heavily loaded conditions, and the larger the size of the solid lubricant particles, the larger the wear rate of surface. With fine particles at heavily loaded condition, the lubricating effectiveness was improved. Bartz [10,11] have found that the addition of solid lubricant to mineral oil can have beneficial or detrimental effects on the antiwear performance of these oils depending on the other types of additives in oils, particle hardness and particle size of the solid lubricant. In this present work, the graphite nanosheets were prepared by stirring ball milling, and the tribological properties of the graphite nanosheets as lubricating additive in paraffin oil were investigated. 2. Experimental 2.1. Preparation of graphite nanosheets
∗
Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.09.010
The graphite nanosheets were prepared by stirring ball milling. Natural flake graphite powder (particle size: 48 m)
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as well as dispersing agent and protecting agent were added into distilled water. Ammonia was dropped into the slurry to adjust the acidic pH 9. The ratio between ball and material by weight was 8:1. The resulting slurry was continuously stirred and milled for 3 h, and the rotating speed was 250 rpm. The morphology and structure of the graphite nanosheets obtained were characterized with transmission electron microscopy (TEM, JEM-2010) and scanning electron microscopy (SEM, SIRION JY/T010-1996) and Powder X-ray diffraction (XRD, Rigaku D/Max-rA) using Cu K␣ radiation. 2.2. Tribological properties of graphite nanosheets as additive To provide dispersion stability of the additive in base oil (paraffin oil), a surfactant span-80 (sorbitol monooleate) was used to modify the surface of graphite nanosheets. After removal of water in the as-prepared graphite nanosheets with both the paraffin oil and span-80, the resulting oil slurry containing the graphite nanosheets and dispersing agent span-80 was mixed with paraffin oil and the mixture was stirred to make a uniform suspension with T-18 high-speed dispersion machine for 20 min, then dispersed again by ultrasonic bath. For comparison, the oil containing the as-received flake graphite powder and dispersing agent took after the above process. It is found that the addition of 1.0 wt.% span-80 to the oil could get the best dispersion stability by judging the absorbance. The load-carrying capacities, friction and antiwear properties of the oil with graphite nanosheets were examined on a MMW-1 four-ball, in comparison to the base oil and the oil with the addition of flake graphite powder. The maximum nonseized loads of the lubricating oil were determined according to the ASTM D2783 standard method. The friction and wear tests were conducted at a rotating speed of 1500 rpm and under a constant load of 245 N, for the test duration of 30 min. A dependence of the wear scar diameter on friction time was also measured under the constant load of 245 N, the operation procedure of which was given as follow: measured wear scar diameter after rubbing for given time, then went on friction for another given time, repeated the operation. The balls (diameter in 12.7 mm) used in the tests were made of GCr15 bearing steel (AISI 52100) with a hardness of 61 HRC. Before each test, the steel balls were cleaned in petroleum ether and dried. The balls after testing were cleaned using ultrasonic bath in ligroin and then in distilled water for 10 min, respectively. The wear scar diameters on the steel balls were measured using an optical microscope to an accuracy of ±0.01 mm. The worn surfaces of the balls after friction test were examined with SEM.
Fig. 1. SEM morphology of graphite nanosheets.
tron microscope was used to examine the morphology of the nanosheets. The powder of nanosheets was dispersed in benzene in an ultrasonic bath for 10 min, and deposited on a copper grid covered with a perforated carbon film. Fig. 2 shows a TEM image of the graphite nanosheets. The thickness of the graphite nanosheets is about 10–20 nm. Relation to the XRD pattern shown in Fig. 3, the graphite crystal peaks can be observed from the powder. This suggests that the obtained graphite nanosheets are well crystallized. 3.2. Effect of graphite nanosheets on antiwear properties Using solid lubricant such as graphite in liquid lubricants exists an “optimal concentration”. Less than this concentration of solid lubricant is insufficient to maintain protection against wear [9]. Fig. 4 shows the wear scar diameter as a function of the additive concentration of graphite nanosheets and the asreceived flake graphite in paraffin oil under a load of 245 N for 30 min at a speed of 1500 rpm. It can be seen from Fig. 4 that with the addition of graphite nanosheets to paraffin oil, the wear scar
3. Results and discussion 3.1. Characterization of graphite nanosheets Fig. 1 shows a typical SEM morphology of as-prepared graphite nanosheets. It can be seen that the graphite keeps the previous laminated structure after stirring ball milling, with the average diameter of 500 nm. JEM-2010 transmission elec-
Fig. 2. TEM image of graphite nanosheets.
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Fig. 3. XRD pattern of graphite nanosheets. Fig. 5. Dependence of wear scar diameter on friction time (four-ball, 1500 rpm, 245 N).
diameter of steel ball decreases more remarkably compared to the flake graphite. In other words, the graphite nanosheets could improve antiwear properties more than the flake graphite. With increasing the addition of graphite nanosheets further, however, the wear scar diameter decreases, so as the flake graphite. It can be seen that the optimum concentration of nanosheets and the graphite is both 1.0 × 10−2 wt.%, which shows the smallest wear scar diameter. The increase in wear scar diameter at high concentrations of the graphite nanosheets may be explained only by a mutual hindrance of the many particles available [9]. For the larger size of the flake graphite compared to the graphite nanosheets, it accumulated at the inlet, and could not penetrate into the contact easily, so its effect on improving antiwear properties was not remarkable. A correlation of wear scar diameter versus friction time, measured under a discontinuous test time, is given in Fig. 5. The wear scar diameter on the ball, running in the paraffin oil with 1.0 × 10−2 wt.% additive and 1.0 wt.% dispersing agent span80, was slightly smaller than that running in paraffin oil at initial stage of the test. After running 5 min, the difference in wear scar diameter on the balls lubricated with pure paraffin oil and
Fig. 4. Wear scar diameter as a function of additive concentration (four-ball, 1500 rpm, 245 N, 30 min).
the additive-containing oil increased with sliding time. These results further indicate that graphite nanosheets increased the wear resistance of the oil and showed excellent antiwear properties. The result of oil with flake graphite is also given in this figure. It seems that during the initial stage, the wear scar diameter of the balls lubricated by oils with both additives are nearly the same, but as the sliding continues, the difference of wear scar diameters increases. It appears that graphite nanosheets is superior to the flake graphite. 3.3. Effect of graphite nanosheets on maximum nonseized load Maximum nonseized load (PB ) represents the load-carrying capacity of oil. The PB value of paraffin oil is 371 N and is not changed by the addition of 1.0 wt.% dispersing agent span-80. After addition of graphite nanosheets and flake graphite, respectively, in paraffin oil with different amounts, the PB values are measured and the results are shown in Fig. 6. As adding the
Fig. 6. Effect of graphite nanosheet and flake graphite concentration on maximum nonseizure load of the oil.
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Fig. 7. Effect of graphite nanosheets and flake graphite on friction coefficient of the oil (four-ball, 1500 rpm, 245 N, 30 min).
graphite nanosheets and flake graphite to paraffin oil, respectively, PB values enlarge remarkably. The oil with graphite nanosheets showed higher maximum nonseized load than the oil with the flake graphite, namely the graphite nanosheets could strengthen the load-carrying capacity of the oil more. Furthermore, a maximum of PB is shown in Fig. 6. The PB rises as the concentration of graphite nanosheets increases below 1.0 × 10−2 wt.%, but when the concentration is higher than 1.0 × 10−2 wt.%, the PB decreases on the contrary. This result means that excessive graphite nanosheets result in a decrease in maximum nonseized load of the oil. A possible explanation is that some coagulation of the graphite nanosheets were formed owing to the friction effect, which made the friction unstable or caused vibration and resulted in a decrease in the maximum nonseized load.
3.4. Effect of graphite nanosheets on friction coefficient Results of friction tests of the paraffin oil and the oil with 1.0 × 10−2 wt.% graphite nanosheets as well as 1.0 wt.% dispersing agent span-80 compared to the flake graphite are shown in Fig. 7. It can be seen that the lubricating oil with graphite nanosheets gave a smaller and more stable friction coefficient than pure paraffin oil and the oil with the flake graphite. The larger friction coefficient of paraffin oil can result from a real contacting area of the rubbing surface owing to wear, and the reason why the values of friction coefficient for the flake graphite is greater than that for graphite nanosheets, seems to be due to the damming and the starving actions of the flake graphite accumulated and cohered in the front of the leading edge of a steel ball as its size is larger, so that it could not easily penetrate into the interface with paraffin oil. Stable friction coefficient of the oil with additive can be explained in that at a given concentration, the graphite nanosheets more easily will penetrate into the interface with paraffin oil and form continuous film in concave of rubbing face [14], which can decrease shearing stress, therefore, give a low friction coefficient.
Fig. 8. The worn surface of steel ball: (a) lubricated with paraffin oil; (b) lubricated with oil containing graphite nanosheets; (c) lubricated with oil containing flake graphite.
3.5. Surface analysis The evidence of reducing the friction and wear of graphite nanosheets dispersed in paraffin oil can be confirmed by the results of SEM. Fig. 8 shows SEM images of the rubbing surface lubricated by paraffin oil, oil with graphite nanosheets and flake graphite. It can be found that the worn surface lubricated by paraffin oil shown in Fig. 8(a) is evidently rough with many thick and deep furrows, but the worn surface lubricated by oil with graphite nanosheets is comparably rather smoother and the furrows are rather shallower (Fig. 8(b)). As shown in Fig. 8(c), the rubbing surface lubricated by oil with flake graphite is smoother than that lubricated by paraffin oil, but its furrows are thicker than that lubricated by oil with graphite nanosheets. Under sliding conditions, the graphite nanosheets accumulates at the inlet, and the graphite nanosheets have more of a tendency to form a film. Fig. 9 shows the graphite nanosheets
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Acknowledgement This work was supported by a program of the Shanghai Bureau of Science and Technology (No. 0352 nm 037). References
Fig. 9. The graphite nanosheets deposited on the rubbing surface.
distributed on the rubbing surface. The graphite can stably exist on the rubbing surface lubricated by paraffin oil [13], implying that the graphite nanosheets form a physical deposition film on the rubbing surface and prevent them from direct contact. For the rubbing surfaces those lubricated by oil with nanosheets are smooth, it is possible that the nanosheets in contact with smooth surfaces are more likely to slip [12]. As the surface roughness increases, the nanosheets are easily to be drawn into the contact, and the result in Fig. 9 has proved this outcome. 4. Conclusions Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The wear resistance and load carrying capacity of paraffin oil can be improved and its friction coefficient can be decreased by the addition of the graphite nanosheets. There is an optimal content of graphite nanosheets in the lubricating oil, which gives the highest maximum nonseized load and antiwear ability. The graphite nanosheets formed a physical deposition film on the rubbing surface, which not only could bear the load of the steel ball but also prevent them from direct contact, resulting in a decrease in friction and wear.
[1] S. Tarasov, A. Kolubaev, S. Belyaev, M. Lerner, F. Tepper, Study of friction reduction by nanocopper additive to motor oil, Wear 252 (2002) 63–69. [2] Tao Xu, Jiazheng Zhao, Kang Xu, Qunji Xue, Study on the tribological properties of ultradispersed diamond containing soot as an oil additive, Tribol. Trans. 40 (1) (1997) 178–182. [3] J.X. Dong, Z.S. Hu, A study of the anti-wear and friction-reducing properties of the lubricant additive, nanometer zinc borate, Tribol. Int. 31 (5) (1998) 219–223. [4] Weimin Liu, Shuang Chen, An investigation of the tribological behaviour of surface-modified ZnS nanoparticles in liquid paraffin, Wear 238 (2000) 120–124. [5] Shuang Chen, Weimin Liu, Laigui Yu, Preparation of DDP-coated PbS nanoparticles and investigation of the antiwear ability of the prepared nanoparticales as additive in liquid paraffin, Wear 218 (1998) 153–158. [6] Qunji Xue, Weimin Liu, Zhijun Zhang, Friction and wear properties of a surface-modified TiO2 nanoparicle as an additive in liquid paraffin, Wear 213 (1997) 29–32. [7] J. Gansheimer, R. Holinski, Molybdenum disulfide in oils and greases under boundary conditions, ASME J. Lubric. Technol. 95 (1973) 242–248. [8] Hengzhou Wo, Kunhong Hu, Xianguo Hu, Tribological properties of MoS2 nanoparticles as additive in a machine oil, Tribology 24 (1) (2004) 33–37. [9] W.J. Bartz, Solid lubricant additive-effect of concentration and other additives on antiwear performance, Wear 17 (1971) 421–432. [10] W.J. Bartz, Some investigation on the influence of particle size on the lubrication effectiveness of molybdenum disulfide, ASLE Trans. 15 (1972) 207–215. [11] W.J. Bartz, J. Oppelt, Lubricating effectiveness of oil-soluble additives and molybdenum disulfide dispersed in mineral oil, Lubric. Eng. 36 (10) (1980) 579–585. [12] C. Cusano, H.E. Sliney, Dynamics of solid dispersions in oil during the lubrication of point contacts. Part I. Graphite, ASEM Trans. 25 (2) (1982) 183–189. [13] Shufeng Chu, Zhishan Jin, Qunji Xue, Study of the interaction between natural flake graphite and oil soluble additives, Tribology 17 (4) (1997) 340–347. [14] T. Hisakado, T. Tsukizoe, H. Yoshikawa, Lubrication mechanism of solid lubricants in oils, Lubric. Technol. 105 (1983) 245–253.
An investigation on tribological properties of graphite nanosheets as oil additive H.D. Huang a , J.P. Tu a,∗ , L.P. Gan b , C.Z. Li b a
Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China b Key Lab for Uultrafine Materials of Ministry of Education, East China University of Science and Technology, Shanghai 200237, China Received 5 April 2005; received in revised form 3 September 2005; accepted 22 September 2005 Available online 21 October 2005
Abstract Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The tribological behavior of the graphite nanosheets as additive in paraffin oil were investigated with a four-ball and a pin-on-disk friction and wear tester. The wear surfaces of the steel ball lubricated with the additive-containing paraffin oil were analyzed by means of scanning electron microscopy (SEM). It has been found that the graphite nanosheets as additive in oil at proper concentration show better tribological properties than pure paraffin oil. The load-carrying capacity and antiwear ability of the lubricating oil were improved. Moreover, the friction coefficient of the lubricating oil was decreased by the addition of the graphite nanosheets. The optimal concentration of the additive in paraffin oil is about 1.0 × 10−2 wt.%. © 2005 Elsevier B.V. All rights reserved. Keywords: Graphite nanosheets; Additive; Tribological properties; Paraffin oil
1. Introduction For tribology applications, nanoparticles as additives in base oil have been investigated widespreadly. These studies refer to synthesis and preparation of nanoscale particles, and their tribological properties and mechanisms. It has been found that when the nanoparticles were added to base oil, the extreme-pressure property and load-carrying capacity were improved and friction coefficient was decreased. At present, the viewpoint about mechanisms of nanoparticle additives is as follow: (1) ball effect [1,2]; (2) tribochemical reactions [3–5]; (3) adsorption film theory [6]. The results of these studies indicate that nanoparticles using as lubricating oil additive can improve the tribological properties of base oils. Commercially layered compound powders, usually as solid lubricants, dispersed in oil were also included [7–11]. The solid lubricants, such as MoS2 dispersed in oil exhibited beneficial effects by reducing the friction and wear [7,8,12]. Chu et al. [13] found that graphite existed on the rubbing surfaces stably and
formed composition film with the oil-soluble additives. Bartz [10] found that an “optimal concentration” existed, and the wear rate actually increased with increasing the concentration over the optimal point of the solid lubricant dispersed in oil under heavily loaded conditions, and the larger the size of the solid lubricant particles, the larger the wear rate of surface. With fine particles at heavily loaded condition, the lubricating effectiveness was improved. Bartz [10,11] have found that the addition of solid lubricant to mineral oil can have beneficial or detrimental effects on the antiwear performance of these oils depending on the other types of additives in oils, particle hardness and particle size of the solid lubricant. In this present work, the graphite nanosheets were prepared by stirring ball milling, and the tribological properties of the graphite nanosheets as lubricating additive in paraffin oil were investigated. 2. Experimental 2.1. Preparation of graphite nanosheets
∗
Corresponding author. Tel.: +86 571 87952573; fax: +86 571 87952856. E-mail address: [email protected] (J.P. Tu).
0043-1648/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2005.09.010
The graphite nanosheets were prepared by stirring ball milling. Natural flake graphite powder (particle size: 48 m)
H.D. Huang et al. / Wear 261 (2006) 140–144
141
as well as dispersing agent and protecting agent were added into distilled water. Ammonia was dropped into the slurry to adjust the acidic pH 9. The ratio between ball and material by weight was 8:1. The resulting slurry was continuously stirred and milled for 3 h, and the rotating speed was 250 rpm. The morphology and structure of the graphite nanosheets obtained were characterized with transmission electron microscopy (TEM, JEM-2010) and scanning electron microscopy (SEM, SIRION JY/T010-1996) and Powder X-ray diffraction (XRD, Rigaku D/Max-rA) using Cu K␣ radiation. 2.2. Tribological properties of graphite nanosheets as additive To provide dispersion stability of the additive in base oil (paraffin oil), a surfactant span-80 (sorbitol monooleate) was used to modify the surface of graphite nanosheets. After removal of water in the as-prepared graphite nanosheets with both the paraffin oil and span-80, the resulting oil slurry containing the graphite nanosheets and dispersing agent span-80 was mixed with paraffin oil and the mixture was stirred to make a uniform suspension with T-18 high-speed dispersion machine for 20 min, then dispersed again by ultrasonic bath. For comparison, the oil containing the as-received flake graphite powder and dispersing agent took after the above process. It is found that the addition of 1.0 wt.% span-80 to the oil could get the best dispersion stability by judging the absorbance. The load-carrying capacities, friction and antiwear properties of the oil with graphite nanosheets were examined on a MMW-1 four-ball, in comparison to the base oil and the oil with the addition of flake graphite powder. The maximum nonseized loads of the lubricating oil were determined according to the ASTM D2783 standard method. The friction and wear tests were conducted at a rotating speed of 1500 rpm and under a constant load of 245 N, for the test duration of 30 min. A dependence of the wear scar diameter on friction time was also measured under the constant load of 245 N, the operation procedure of which was given as follow: measured wear scar diameter after rubbing for given time, then went on friction for another given time, repeated the operation. The balls (diameter in 12.7 mm) used in the tests were made of GCr15 bearing steel (AISI 52100) with a hardness of 61 HRC. Before each test, the steel balls were cleaned in petroleum ether and dried. The balls after testing were cleaned using ultrasonic bath in ligroin and then in distilled water for 10 min, respectively. The wear scar diameters on the steel balls were measured using an optical microscope to an accuracy of ±0.01 mm. The worn surfaces of the balls after friction test were examined with SEM.
Fig. 1. SEM morphology of graphite nanosheets.
tron microscope was used to examine the morphology of the nanosheets. The powder of nanosheets was dispersed in benzene in an ultrasonic bath for 10 min, and deposited on a copper grid covered with a perforated carbon film. Fig. 2 shows a TEM image of the graphite nanosheets. The thickness of the graphite nanosheets is about 10–20 nm. Relation to the XRD pattern shown in Fig. 3, the graphite crystal peaks can be observed from the powder. This suggests that the obtained graphite nanosheets are well crystallized. 3.2. Effect of graphite nanosheets on antiwear properties Using solid lubricant such as graphite in liquid lubricants exists an “optimal concentration”. Less than this concentration of solid lubricant is insufficient to maintain protection against wear [9]. Fig. 4 shows the wear scar diameter as a function of the additive concentration of graphite nanosheets and the asreceived flake graphite in paraffin oil under a load of 245 N for 30 min at a speed of 1500 rpm. It can be seen from Fig. 4 that with the addition of graphite nanosheets to paraffin oil, the wear scar
3. Results and discussion 3.1. Characterization of graphite nanosheets Fig. 1 shows a typical SEM morphology of as-prepared graphite nanosheets. It can be seen that the graphite keeps the previous laminated structure after stirring ball milling, with the average diameter of 500 nm. JEM-2010 transmission elec-
Fig. 2. TEM image of graphite nanosheets.
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Fig. 3. XRD pattern of graphite nanosheets. Fig. 5. Dependence of wear scar diameter on friction time (four-ball, 1500 rpm, 245 N).
diameter of steel ball decreases more remarkably compared to the flake graphite. In other words, the graphite nanosheets could improve antiwear properties more than the flake graphite. With increasing the addition of graphite nanosheets further, however, the wear scar diameter decreases, so as the flake graphite. It can be seen that the optimum concentration of nanosheets and the graphite is both 1.0 × 10−2 wt.%, which shows the smallest wear scar diameter. The increase in wear scar diameter at high concentrations of the graphite nanosheets may be explained only by a mutual hindrance of the many particles available [9]. For the larger size of the flake graphite compared to the graphite nanosheets, it accumulated at the inlet, and could not penetrate into the contact easily, so its effect on improving antiwear properties was not remarkable. A correlation of wear scar diameter versus friction time, measured under a discontinuous test time, is given in Fig. 5. The wear scar diameter on the ball, running in the paraffin oil with 1.0 × 10−2 wt.% additive and 1.0 wt.% dispersing agent span80, was slightly smaller than that running in paraffin oil at initial stage of the test. After running 5 min, the difference in wear scar diameter on the balls lubricated with pure paraffin oil and
Fig. 4. Wear scar diameter as a function of additive concentration (four-ball, 1500 rpm, 245 N, 30 min).
the additive-containing oil increased with sliding time. These results further indicate that graphite nanosheets increased the wear resistance of the oil and showed excellent antiwear properties. The result of oil with flake graphite is also given in this figure. It seems that during the initial stage, the wear scar diameter of the balls lubricated by oils with both additives are nearly the same, but as the sliding continues, the difference of wear scar diameters increases. It appears that graphite nanosheets is superior to the flake graphite. 3.3. Effect of graphite nanosheets on maximum nonseized load Maximum nonseized load (PB ) represents the load-carrying capacity of oil. The PB value of paraffin oil is 371 N and is not changed by the addition of 1.0 wt.% dispersing agent span-80. After addition of graphite nanosheets and flake graphite, respectively, in paraffin oil with different amounts, the PB values are measured and the results are shown in Fig. 6. As adding the
Fig. 6. Effect of graphite nanosheet and flake graphite concentration on maximum nonseizure load of the oil.
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Fig. 7. Effect of graphite nanosheets and flake graphite on friction coefficient of the oil (four-ball, 1500 rpm, 245 N, 30 min).
graphite nanosheets and flake graphite to paraffin oil, respectively, PB values enlarge remarkably. The oil with graphite nanosheets showed higher maximum nonseized load than the oil with the flake graphite, namely the graphite nanosheets could strengthen the load-carrying capacity of the oil more. Furthermore, a maximum of PB is shown in Fig. 6. The PB rises as the concentration of graphite nanosheets increases below 1.0 × 10−2 wt.%, but when the concentration is higher than 1.0 × 10−2 wt.%, the PB decreases on the contrary. This result means that excessive graphite nanosheets result in a decrease in maximum nonseized load of the oil. A possible explanation is that some coagulation of the graphite nanosheets were formed owing to the friction effect, which made the friction unstable or caused vibration and resulted in a decrease in the maximum nonseized load.
3.4. Effect of graphite nanosheets on friction coefficient Results of friction tests of the paraffin oil and the oil with 1.0 × 10−2 wt.% graphite nanosheets as well as 1.0 wt.% dispersing agent span-80 compared to the flake graphite are shown in Fig. 7. It can be seen that the lubricating oil with graphite nanosheets gave a smaller and more stable friction coefficient than pure paraffin oil and the oil with the flake graphite. The larger friction coefficient of paraffin oil can result from a real contacting area of the rubbing surface owing to wear, and the reason why the values of friction coefficient for the flake graphite is greater than that for graphite nanosheets, seems to be due to the damming and the starving actions of the flake graphite accumulated and cohered in the front of the leading edge of a steel ball as its size is larger, so that it could not easily penetrate into the interface with paraffin oil. Stable friction coefficient of the oil with additive can be explained in that at a given concentration, the graphite nanosheets more easily will penetrate into the interface with paraffin oil and form continuous film in concave of rubbing face [14], which can decrease shearing stress, therefore, give a low friction coefficient.
Fig. 8. The worn surface of steel ball: (a) lubricated with paraffin oil; (b) lubricated with oil containing graphite nanosheets; (c) lubricated with oil containing flake graphite.
3.5. Surface analysis The evidence of reducing the friction and wear of graphite nanosheets dispersed in paraffin oil can be confirmed by the results of SEM. Fig. 8 shows SEM images of the rubbing surface lubricated by paraffin oil, oil with graphite nanosheets and flake graphite. It can be found that the worn surface lubricated by paraffin oil shown in Fig. 8(a) is evidently rough with many thick and deep furrows, but the worn surface lubricated by oil with graphite nanosheets is comparably rather smoother and the furrows are rather shallower (Fig. 8(b)). As shown in Fig. 8(c), the rubbing surface lubricated by oil with flake graphite is smoother than that lubricated by paraffin oil, but its furrows are thicker than that lubricated by oil with graphite nanosheets. Under sliding conditions, the graphite nanosheets accumulates at the inlet, and the graphite nanosheets have more of a tendency to form a film. Fig. 9 shows the graphite nanosheets
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Acknowledgement This work was supported by a program of the Shanghai Bureau of Science and Technology (No. 0352 nm 037). References
Fig. 9. The graphite nanosheets deposited on the rubbing surface.
distributed on the rubbing surface. The graphite can stably exist on the rubbing surface lubricated by paraffin oil [13], implying that the graphite nanosheets form a physical deposition film on the rubbing surface and prevent them from direct contact. For the rubbing surfaces those lubricated by oil with nanosheets are smooth, it is possible that the nanosheets in contact with smooth surfaces are more likely to slip [12]. As the surface roughness increases, the nanosheets are easily to be drawn into the contact, and the result in Fig. 9 has proved this outcome. 4. Conclusions Graphite nanosheets with average diameter of 500 nm and thickness about 15 nm were prepared by stirring ball milling. The wear resistance and load carrying capacity of paraffin oil can be improved and its friction coefficient can be decreased by the addition of the graphite nanosheets. There is an optimal content of graphite nanosheets in the lubricating oil, which gives the highest maximum nonseized load and antiwear ability. The graphite nanosheets formed a physical deposition film on the rubbing surface, which not only could bear the load of the steel ball but also prevent them from direct contact, resulting in a decrease in friction and wear.
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