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Tribological properties of serpentine, La(OH)3 and their composite particles as lubricant additives
Wear 288 (2012) 72–77
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Tribological properties of serpentine, La(OH)3 and their composite particles as lubricant additives Fuyan Zhao, Zhimin Bai ∗ , Ying Fu, Dong Zhao, Chunmei Yan School of Materials Science and Technology, China University of Geosciences (Beijing), 100083 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 2 June 2011 Received in revised form 11 February 2012 Accepted 13 February 2012 Available online 22 February 2012 Keywords: Particle shape Additives Lubricant Wear behavior
a b s t r a c t La(OH)3 nanoparticles and serpentine/La(OH)3 composite particles were synthesized via sol–gel method. The phase compositions, micro-morphology of the synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tribological properties of different samples were tested and compared by the MM-10W Multi-functional friction abrasion tester and MHK-500 Ring-block wear testing machine. The TEM figure indicates that La(OH)3 nanoparticles are granular, the average size of the particles is 50 nm. The SEM figure of the composite particles illustrates that La(OH)3 particles uniformly coat on the surface of serpentine particles. The results of the friction tests indicate that serpentine, La(OH)3 and serpentine/La(OH)3 composite particles all exhibit friction-reducing and anti-wear properties compared to the base oil. The oil containing the composite particles have the best friction-reducing, anti-wear and self-repairing properties. The friction coefficients are reduced by 24.63% and the diameters of friction spots are reduced by 41.88% compared to the base oil. The energy spectrum of tribosurface gives evidence of that the contact area of rubbing pair was repaired by composite particles during friction tests. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A lot of inorganic and organic nanoparticles used as oil additives have been synthesized and investigated in recent years. It is found that they improved the tribological properties of the base oil and displayed good friction-reducing and anti-wear characteristics. Sulfur and phosphorus containing compounds themselves, in fact, are excellent extreme-pressure and friction-reducing additives but can cause environment pollution [1]. They widely used as the oil additives in the past. As an environmental protection measure, the use of sulfur-, chlorine- and phosphorus-containing compounds as lubricant additives has been restricted, so developing new additives that pollute less has therefore become the target for researchers [2]. Recently nano-inorganic compounds have been widely used as additives in lubricating oil to increase the tribological behavior, such as elementary substance [3,4], oxides [5–10], hydroxides [11], borate [12,13,2] and rare earth compounds [14,1]. In recent years, the study about the use of inorganic mineral nanoparticles as additives in lubricating oil has received more and more attention. It is found that most of these phyllosilicates which have layer structures possess good antioxidant, environmentally friendly, anti-wear properties and high thermal stabilities.
∗ Corresponding author. Tel.: +86 10 82322974; fax: +86 10 82322974. E-mail address: [email protected] (Z. Bai). 0043-1648/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2012.02.009
Serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)6 Si4 O10 (OH)8 ) minerals. They may contain minor amounts of other elements including chromium, manganese, cobalt and nickel [15]. Recent researches indicate that through the addition of serpentine to base oil, the wear resistance ability was improved and the friction coefficient was decreased [16]. It is well known that rare earth metal materials often possess some special properties [1]. They are usually used as a catalyst in the metallurgical industry. Recently, rare earth compounds have been paid much attention in the field of tribology and perform excellent extreme pressure and antiwear properties [14]. However, previous researches mainly focused on adding single kind of particle as lubricant additive, little research has been done on the composite particles which containing two or more different kinds of materials used as lubricant additive. In this work, the serpentine/La(OH)3 composite particles were synthesized, which containing the micro-serpentine coated by La(OH)3 nanoparticles. Some wear testing machines were used to evaluate the antiwear and friction-reducing properties of the samples. The morphologies of the wear traces and the elemental distributions in the worn surfaces were observed and determined with SEM and energy dispersive spectrometry (EDS). Further, the wear scar diameters and the temperature changes of the oil were studied. It is the first to show the preparation and the tribological properties of the serpentine/La(OH)3 composite particles.
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2. Experimental details Chemicals: Analytical reagent grade of lanthanum nitrate (La(NO3 )3 ·6H2 O), citric acid, sorbitol monostearate (span60); commercial CD15W-40 base oil with a kinematic viscosity of 110 mm2 /s at 313 K and 15 mm2 /s at 373 K. All of these agents are manufactured in China. 2.1. Preparation of materials The details about the preparation procedures were as follows: first, prepared 200 ml 0.1 mol/L La(NO3 )3 and citric acid solution using the deionized water, adjusted the PH of the solution to 7–8 by adding ammonia water. Then heated the solution to 353 K under stirring, and added 1 g serpentine particles. Keep the temperature at 363 K for 2 h, the sol with the colour of nattier blue was prepared. When all the water evaporated at the temperature of 373 K, it changed to yellow gel. When the gel was heated to 573 K, it combusted automatically, and changed to black powders. Calcined the powders in the muffle furnace at 873 K for 2 h, a white powder containing serpentine coated by La(OH)3 nanoparticles was obtained. By the same procedures as the above without addition of the serpentine, the La(OH)3 nanoparticles were synthesized. 2.2. Characterization of materials The phase compositions were characterized by X-ray diffraction (XRD, Rigaku Dmax12KW) with Cu K␣ X-ray source at a wavelength of 0.15406 nm, scanning speed 8◦ /min, tube voltage 40 kV, tube current 100 mA. The micro-morphology of the samples was investigated with scanning electron microscope (SEM, FEI Quanta600). The tribological properties of different samples were tested by the MM-10W multi-functional friction abrasion tester and MHK-500 Ring-block wear tester. Surfaces of the rubbing pair were examined after the friction tests by SEM and EDS to determine the presence of a tribofilm and analyse its chemical compositions. 2.3. Measurement of tribological properties In this study, MM-10W multi-functional friction abrasion tester was used to test the friction coefficient, temperature changes and wear scar diameters of different samples. The base oil is Changcheng CD15W-40 lubricating oil made in China. The balls were GCr15 steel ball which made of bearing steel and 12.7 mm in diameter with the hardness HRC59-61 (composition: 0.95–1.05 C, 0.25–0.45 Mn, 0.15–0.35 Si, ≤0.025 S, ≤0.025 P, 1.40–1.65 Cr, ≤0.10 Mo, ≤0.30 Ni, ≤0.25 Cu, ≤0.50 Ni + Cu). Before the test, the dispersing agent sorbitol monostearate (span60) and different samples with a mass ratio of 1:1 were added to the lubricating oil under stirring by an ultrasonic probe for 5 min, and then mixed by a high speed agitator for 30 min at the temperature of 333 K. The span60 here acted as a surfacemodifier to improve the dispersion capacity of the samples [15]. The particle concentration in the lubricating oil is 0.5 wt%, The test experimental conditions were: atmospheric environment, room temperature, normal load = 392 N, rotate speed = 1200 rpm and test duration = 30 min. At the end of each test, the steel balls were cleaned by petroleum ether, the wear scar diameters on the three stationary balls were measured by a optical microscope and the average wear scar diameter was calculated. Temperature changes of the oils containing different samples were also recorded during the tests. In order to detect the morphology and the elements presented on wear scar surfaces, The ring and the block were made by GCr15 bearing steel with the hardness of HRC59-61. The width of the ring is 13 mm and the diameter is 49.00 mm. The width and the length of
Fig. 1. XRD pattern and SEM image of serpentine: (a) XRD pattern of serpentine; (b) SEM image of serpentine.
the block are 12.30 mm and 19.00 mm, respectively. The MHK-500 Ring-block wear testing machine was used under the condition: load = 200 N, rotate = 400 rpm, test duration = 48 h. The particle concentration in the lubricating oil is 0.5 wt%. The morphology of the wear scar surface was studied by optical microscope and SEM, the elements presented on wear scar surfaces were analysed by EDS. 3. Results and discussions 3.1. Materials Fig. 1a shows the X-ray diffraction pattern of natural mineral serpentine raw materials. The diffraction peaks are all the index of antigorite which is one kind of the three structures of serpentine. So the serpentine raw sample is very pure. Fig. 1b shows the SEM image of serpentine. Serpentine is a kind of phyllosilicate. The SEM image indicates that serpentine particles are flaky, the range of the particle size is from 1 to 5 m. Fig. 2a shows the X-ray diffraction pattern of the synthesized La(OH)3 nanoparticles. The diffraction peaks in Fig. 2a are all the characteristic peaks of La(OH)3 . It indicates that the synthesized La(OH)3 particles are pure. Fig. 2b and c show the TEM image and SEM image of La(OH)3 , respectively. The images indicate that La(OH)3 nanoparticles are granular, the average particle size is about 50 nm. Particle size is equally distributed, no evident coagulation is found. Fig. 3a shows the XRD pattern of serpentine/La(OH)3 composite particles. The main phase compositions in the sample are
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Fig. 2. XRD pattern, SEM image and TEM image of La(OH)3 nanoparticles: (a) XRD pattern; (b) SEM image; (c) TEM image.
serpentine and La(OH)3 , there are no other impurities. Fig. 3b is the SEM image of serpentine/La(OH)3 composite particles. The image indicates that the surfaces of the serpentine particle are evenly coated by La(OH)3 nanoparticles. Fig. 3c is the
EDS image of serpentine/La(OH)3 composite particles of Fig. 3b. The result shows that the elements Mg, Si are the compositions of serpentine, and the element La indicates the existence of La(OH)3 .
Fig. 3. XRD pattern, SEM image and EDS image of serpentine/La(OH)3 composite particles: (a) XRD pattern; (b) SEM image; (c) EDS image.
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Fig. 4. The friction coefficient curves of different samples. Fig. 5. The average wear scar diameters of different samples.
3.2. Friction tests Fig. 4 illustrates a set of friction coefficient curves of lubricating oil containing different kinds of particles compared to the base oil. At the beginning, the friction coefficient of different samples have little difference of the base oil, they are all about 0.095. As the test duration increasing, the friction coefficient of the base oil rises quickly, at last the rate become steady stays between 0.14 and 0.15. The reason of this is that the particle additive in the oil have not taken effect on the friction coefficient at the beginning, the base oil plays the main role between the rubbing pair. It indicates that the friction reduction property of the base oil is not very good, as the increase of the oil temperature the properties of the base oil become worse, the friction coefficient increases too. The average friction coefficient of the base oil in the whole test time is 0.1287 (see Table 1). The friction coefficient of the lubricating oil containing La(OH)3 nanoparticles is a little higher than other samples at the beginning, not stable and has a great fluctuation. After a few minutes, the curve tends to be stable, finally stays between 0.105 and 0.100. The average friction coefficient of the lubricating oil containing La(OH)3 nanoparticles in the whole test time is 0.1006 (see Table 1). It is reduced by 18.03% compared to the base oil. The third curve is the lubricating oil containing serpentine particles. It illustrates that the serpentine displays a better friction-reducing and anti-wear properties at a longer test time. Serpentine needs time to deposit on and interact with the surface of rubbing pair. In contrast, the lowest friction coefficient is obtained by the base oil containing composite particles. It is reduced by 24.63% (see Table 1) compared to the base oil. The friction coefficient is stable during all the test time, the tribofilm might form on the surface at the beginning of the test. The rare earth element La here may act as the catalyst and accelerated the interaction of the serpentine and the surface of rubbing pair. Fig. 5 shows the variation of wear scar diameter of base oil containing different samples. Using base oil alone, the wear scar diameter is relatively large, it is 0.6237 mm. On the contrary, the wear scar diameter is smaller when using the base oil containing
Table 2 Temperature change of different samples.
Table 1 The average friction coefficients of different samples. Additive
Average friction coefficients
Base oil Serpentine La(OH)3 Composite powder
0.1287 0.1006 0.1088 0.0970
serpentine or La(OH)3 , they are 0.5265 mm and 0.5261 mm, respectively. Similar with the results of friction coefficient, the composite particle displays the smallest wear scar diameter among all the samples, it is just 0.3625 mm, reduced by 41.88% compared with the base oil. The larger the wear scar diameter is, the more sever the wear is [17]. Therefore, the oil with the composite particle possesses the best wear resistance property. It is known that when surfaces slide relatively, almost all the energy dissipated in friction appears in the form of heat at the interface. This frictional heat raises the temperature of specimen and oil [1]. So the less the temperature change the smaller energy dissipated. Table 2 is the temperature changes of the base oil containing different samples. It is noted that the base oil containing composite particles causes the lowest temperature change with regard to the rest of samples. It is similar with the results of the friction coefficient and the wear scar diameter. All these illustrate the good tribological properties of the composite particles. The optical microscope, SEM and EDS images of the wear scar surface after Ring-block wear test are shown in Fig. 6. The surface of the block lubricated by base oil has wide and deep furrows and grooves in sliding direction, it indicates that the abrasive wear of the block surface is severe. There are not so many furrows in the surface of the block lubricated by the oil containing composite particles, just few grooves can be found. This result is in accordance with the best tribological behaviors of oil containing composite particles. The EDS results indicate that the surface of the block contains the elements of O and Si which are only contained in the serpentine, but the Mg and La element are not found. Oil containing serpentine nano-particles can form a tribofilm of multi-apertured oxide layer on worn surface [1]. In this study, the oil containing composite particles also formed a tribofilm containing O and Si. In the serpentine the non-bridging oxygen atoms in [SiO4 ] point to the same direction and connect with Mg2+ , and consequently form Mg–O octahedral layers. During the sliding the serpentine deposited on the surface of the block, then the instantaneous high-temperature made the ion exchange reaction of iron
Additive
Initial temperature (◦ C)
Final temperature (◦ C)
Temperature variation (◦ C)
Base oil Serpentine La(OH)3 Composite powder
28.0 32.5 37.8 39.1
59.8 58.2 63.1 60.7
31.8 25.7 25.3 21.6
Reduced by (%)
21.83 18.03 24.63
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Fig. 6. The surface topography and the EDS of the block after Ring-block wear test: (a) base oil; (b) oil containing composite particles: (1) optical microscope image, (2) SEM image, and (3) EDS image.
atoms and Mg2+ in the serpentine occurred, so the Mg element is not found in the surface of the block. The element La could not be detected on the wear surfaces for two reasons, the first reason is that the La element of nano-La(OH)3 act basically as a catalyst to accelerate the reaction. The second reason is that La element occupied so small proportion of the composite that the level of the deposition was too low to be detected.
friction-reducing and anti-wear properties. And the lubricating oil containing serpentine/La(OH)3 composite particles exhibited better tribological and self-repairing properties than the lubricating oil containing only a single kind of particle. (3) A tribofilm containing O, Si and Fe elements formed on the surface of the rubbing pair by the ion exchange reaction between iron atoms and Mg2+ of the serpentine, the lanthanum acted as catalyst during the reaction.
4. Conclusion (1) Nano-granular La(OH)3 and composite particles of serpentine coated by La(OH)3 were synthesized via sol–gel method. (2) The lubricate oil containing serpentine, La(OH)3 , and serpentine/La(OH)3 composite particles presented excellent
Acknowledgement This research was supported by the National Natural Science Foundation of China (51044011).
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[9] H. Kato, K. Komai, Tribofilm formation and mild wear by tribo-sintering of nanometer-sized oxide particles on rubbing steel surfaces, Wear 262 (2007) 36–41. [10] A. Hernández Battez, et al., CuO, ZrO2 and ZnO nanoparticles as antiwear additive in oil lubricants, Wear 265 (2008) 422–428. [11] Z. Zhang, W. Liu, Q. Xue, Study on lubricating mechanisms of La(OH)3 nanocluster modified by compound containing nitrogen in liquid paraffin, Wear 218 (1998) 139–144. [12] Z.S. Hu, J.X. Dong, G.X. Chen, Preparation and tribological properties of nanometer SnO and ferrous borate as lubricant additives, in: 98’ Asia International Tribology Symposium, Peking, China, 1998. [13] 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 (1998) 219–223. [14] L.G. Yu, Y.F. Lian, Q.J. Xue, The tribological behaviors of some rare earth complexes as lubricating additives, Wear 2141 (1998) 151–155. [15] H.L. Yu, Y. Xu, P.J. Shi, H.M. Wang, Tribological behaviors of surface-coated serpentine ultrafine particles as lubricant additive, Tribol. Int. 43 (2010) 667–675. [16] Y. Yu, J.L. Gu, F.Y. Kang, X.Q. Kong, Surface restoration induced by lubricant additive of natural minerals, Appl. Surf. Sci. 253 (2007) 7549–7553. [17] Z.S. Hu, et al., Preparation and tribological properties of nanometer magnesium borate as lubricating oil additive, Wear 252 (2002) 370–374.
Contents lists available at SciVerse ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Tribological properties of serpentine, La(OH)3 and their composite particles as lubricant additives Fuyan Zhao, Zhimin Bai ∗ , Ying Fu, Dong Zhao, Chunmei Yan School of Materials Science and Technology, China University of Geosciences (Beijing), 100083 Beijing, PR China
a r t i c l e
i n f o
Article history: Received 2 June 2011 Received in revised form 11 February 2012 Accepted 13 February 2012 Available online 22 February 2012 Keywords: Particle shape Additives Lubricant Wear behavior
a b s t r a c t La(OH)3 nanoparticles and serpentine/La(OH)3 composite particles were synthesized via sol–gel method. The phase compositions, micro-morphology of the synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The tribological properties of different samples were tested and compared by the MM-10W Multi-functional friction abrasion tester and MHK-500 Ring-block wear testing machine. The TEM figure indicates that La(OH)3 nanoparticles are granular, the average size of the particles is 50 nm. The SEM figure of the composite particles illustrates that La(OH)3 particles uniformly coat on the surface of serpentine particles. The results of the friction tests indicate that serpentine, La(OH)3 and serpentine/La(OH)3 composite particles all exhibit friction-reducing and anti-wear properties compared to the base oil. The oil containing the composite particles have the best friction-reducing, anti-wear and self-repairing properties. The friction coefficients are reduced by 24.63% and the diameters of friction spots are reduced by 41.88% compared to the base oil. The energy spectrum of tribosurface gives evidence of that the contact area of rubbing pair was repaired by composite particles during friction tests. © 2012 Elsevier B.V. All rights reserved.
1. Introduction A lot of inorganic and organic nanoparticles used as oil additives have been synthesized and investigated in recent years. It is found that they improved the tribological properties of the base oil and displayed good friction-reducing and anti-wear characteristics. Sulfur and phosphorus containing compounds themselves, in fact, are excellent extreme-pressure and friction-reducing additives but can cause environment pollution [1]. They widely used as the oil additives in the past. As an environmental protection measure, the use of sulfur-, chlorine- and phosphorus-containing compounds as lubricant additives has been restricted, so developing new additives that pollute less has therefore become the target for researchers [2]. Recently nano-inorganic compounds have been widely used as additives in lubricating oil to increase the tribological behavior, such as elementary substance [3,4], oxides [5–10], hydroxides [11], borate [12,13,2] and rare earth compounds [14,1]. In recent years, the study about the use of inorganic mineral nanoparticles as additives in lubricating oil has received more and more attention. It is found that most of these phyllosilicates which have layer structures possess good antioxidant, environmentally friendly, anti-wear properties and high thermal stabilities.
∗ Corresponding author. Tel.: +86 10 82322974; fax: +86 10 82322974. E-mail address: [email protected] (Z. Bai). 0043-1648/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2012.02.009
Serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)6 Si4 O10 (OH)8 ) minerals. They may contain minor amounts of other elements including chromium, manganese, cobalt and nickel [15]. Recent researches indicate that through the addition of serpentine to base oil, the wear resistance ability was improved and the friction coefficient was decreased [16]. It is well known that rare earth metal materials often possess some special properties [1]. They are usually used as a catalyst in the metallurgical industry. Recently, rare earth compounds have been paid much attention in the field of tribology and perform excellent extreme pressure and antiwear properties [14]. However, previous researches mainly focused on adding single kind of particle as lubricant additive, little research has been done on the composite particles which containing two or more different kinds of materials used as lubricant additive. In this work, the serpentine/La(OH)3 composite particles were synthesized, which containing the micro-serpentine coated by La(OH)3 nanoparticles. Some wear testing machines were used to evaluate the antiwear and friction-reducing properties of the samples. The morphologies of the wear traces and the elemental distributions in the worn surfaces were observed and determined with SEM and energy dispersive spectrometry (EDS). Further, the wear scar diameters and the temperature changes of the oil were studied. It is the first to show the preparation and the tribological properties of the serpentine/La(OH)3 composite particles.
F. Zhao et al. / Wear 288 (2012) 72–77
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2. Experimental details Chemicals: Analytical reagent grade of lanthanum nitrate (La(NO3 )3 ·6H2 O), citric acid, sorbitol monostearate (span60); commercial CD15W-40 base oil with a kinematic viscosity of 110 mm2 /s at 313 K and 15 mm2 /s at 373 K. All of these agents are manufactured in China. 2.1. Preparation of materials The details about the preparation procedures were as follows: first, prepared 200 ml 0.1 mol/L La(NO3 )3 and citric acid solution using the deionized water, adjusted the PH of the solution to 7–8 by adding ammonia water. Then heated the solution to 353 K under stirring, and added 1 g serpentine particles. Keep the temperature at 363 K for 2 h, the sol with the colour of nattier blue was prepared. When all the water evaporated at the temperature of 373 K, it changed to yellow gel. When the gel was heated to 573 K, it combusted automatically, and changed to black powders. Calcined the powders in the muffle furnace at 873 K for 2 h, a white powder containing serpentine coated by La(OH)3 nanoparticles was obtained. By the same procedures as the above without addition of the serpentine, the La(OH)3 nanoparticles were synthesized. 2.2. Characterization of materials The phase compositions were characterized by X-ray diffraction (XRD, Rigaku Dmax12KW) with Cu K␣ X-ray source at a wavelength of 0.15406 nm, scanning speed 8◦ /min, tube voltage 40 kV, tube current 100 mA. The micro-morphology of the samples was investigated with scanning electron microscope (SEM, FEI Quanta600). The tribological properties of different samples were tested by the MM-10W multi-functional friction abrasion tester and MHK-500 Ring-block wear tester. Surfaces of the rubbing pair were examined after the friction tests by SEM and EDS to determine the presence of a tribofilm and analyse its chemical compositions. 2.3. Measurement of tribological properties In this study, MM-10W multi-functional friction abrasion tester was used to test the friction coefficient, temperature changes and wear scar diameters of different samples. The base oil is Changcheng CD15W-40 lubricating oil made in China. The balls were GCr15 steel ball which made of bearing steel and 12.7 mm in diameter with the hardness HRC59-61 (composition: 0.95–1.05 C, 0.25–0.45 Mn, 0.15–0.35 Si, ≤0.025 S, ≤0.025 P, 1.40–1.65 Cr, ≤0.10 Mo, ≤0.30 Ni, ≤0.25 Cu, ≤0.50 Ni + Cu). Before the test, the dispersing agent sorbitol monostearate (span60) and different samples with a mass ratio of 1:1 were added to the lubricating oil under stirring by an ultrasonic probe for 5 min, and then mixed by a high speed agitator for 30 min at the temperature of 333 K. The span60 here acted as a surfacemodifier to improve the dispersion capacity of the samples [15]. The particle concentration in the lubricating oil is 0.5 wt%, The test experimental conditions were: atmospheric environment, room temperature, normal load = 392 N, rotate speed = 1200 rpm and test duration = 30 min. At the end of each test, the steel balls were cleaned by petroleum ether, the wear scar diameters on the three stationary balls were measured by a optical microscope and the average wear scar diameter was calculated. Temperature changes of the oils containing different samples were also recorded during the tests. In order to detect the morphology and the elements presented on wear scar surfaces, The ring and the block were made by GCr15 bearing steel with the hardness of HRC59-61. The width of the ring is 13 mm and the diameter is 49.00 mm. The width and the length of
Fig. 1. XRD pattern and SEM image of serpentine: (a) XRD pattern of serpentine; (b) SEM image of serpentine.
the block are 12.30 mm and 19.00 mm, respectively. The MHK-500 Ring-block wear testing machine was used under the condition: load = 200 N, rotate = 400 rpm, test duration = 48 h. The particle concentration in the lubricating oil is 0.5 wt%. The morphology of the wear scar surface was studied by optical microscope and SEM, the elements presented on wear scar surfaces were analysed by EDS. 3. Results and discussions 3.1. Materials Fig. 1a shows the X-ray diffraction pattern of natural mineral serpentine raw materials. The diffraction peaks are all the index of antigorite which is one kind of the three structures of serpentine. So the serpentine raw sample is very pure. Fig. 1b shows the SEM image of serpentine. Serpentine is a kind of phyllosilicate. The SEM image indicates that serpentine particles are flaky, the range of the particle size is from 1 to 5 m. Fig. 2a shows the X-ray diffraction pattern of the synthesized La(OH)3 nanoparticles. The diffraction peaks in Fig. 2a are all the characteristic peaks of La(OH)3 . It indicates that the synthesized La(OH)3 particles are pure. Fig. 2b and c show the TEM image and SEM image of La(OH)3 , respectively. The images indicate that La(OH)3 nanoparticles are granular, the average particle size is about 50 nm. Particle size is equally distributed, no evident coagulation is found. Fig. 3a shows the XRD pattern of serpentine/La(OH)3 composite particles. The main phase compositions in the sample are
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Fig. 2. XRD pattern, SEM image and TEM image of La(OH)3 nanoparticles: (a) XRD pattern; (b) SEM image; (c) TEM image.
serpentine and La(OH)3 , there are no other impurities. Fig. 3b is the SEM image of serpentine/La(OH)3 composite particles. The image indicates that the surfaces of the serpentine particle are evenly coated by La(OH)3 nanoparticles. Fig. 3c is the
EDS image of serpentine/La(OH)3 composite particles of Fig. 3b. The result shows that the elements Mg, Si are the compositions of serpentine, and the element La indicates the existence of La(OH)3 .
Fig. 3. XRD pattern, SEM image and EDS image of serpentine/La(OH)3 composite particles: (a) XRD pattern; (b) SEM image; (c) EDS image.
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Fig. 4. The friction coefficient curves of different samples. Fig. 5. The average wear scar diameters of different samples.
3.2. Friction tests Fig. 4 illustrates a set of friction coefficient curves of lubricating oil containing different kinds of particles compared to the base oil. At the beginning, the friction coefficient of different samples have little difference of the base oil, they are all about 0.095. As the test duration increasing, the friction coefficient of the base oil rises quickly, at last the rate become steady stays between 0.14 and 0.15. The reason of this is that the particle additive in the oil have not taken effect on the friction coefficient at the beginning, the base oil plays the main role between the rubbing pair. It indicates that the friction reduction property of the base oil is not very good, as the increase of the oil temperature the properties of the base oil become worse, the friction coefficient increases too. The average friction coefficient of the base oil in the whole test time is 0.1287 (see Table 1). The friction coefficient of the lubricating oil containing La(OH)3 nanoparticles is a little higher than other samples at the beginning, not stable and has a great fluctuation. After a few minutes, the curve tends to be stable, finally stays between 0.105 and 0.100. The average friction coefficient of the lubricating oil containing La(OH)3 nanoparticles in the whole test time is 0.1006 (see Table 1). It is reduced by 18.03% compared to the base oil. The third curve is the lubricating oil containing serpentine particles. It illustrates that the serpentine displays a better friction-reducing and anti-wear properties at a longer test time. Serpentine needs time to deposit on and interact with the surface of rubbing pair. In contrast, the lowest friction coefficient is obtained by the base oil containing composite particles. It is reduced by 24.63% (see Table 1) compared to the base oil. The friction coefficient is stable during all the test time, the tribofilm might form on the surface at the beginning of the test. The rare earth element La here may act as the catalyst and accelerated the interaction of the serpentine and the surface of rubbing pair. Fig. 5 shows the variation of wear scar diameter of base oil containing different samples. Using base oil alone, the wear scar diameter is relatively large, it is 0.6237 mm. On the contrary, the wear scar diameter is smaller when using the base oil containing
Table 2 Temperature change of different samples.
Table 1 The average friction coefficients of different samples. Additive
Average friction coefficients
Base oil Serpentine La(OH)3 Composite powder
0.1287 0.1006 0.1088 0.0970
serpentine or La(OH)3 , they are 0.5265 mm and 0.5261 mm, respectively. Similar with the results of friction coefficient, the composite particle displays the smallest wear scar diameter among all the samples, it is just 0.3625 mm, reduced by 41.88% compared with the base oil. The larger the wear scar diameter is, the more sever the wear is [17]. Therefore, the oil with the composite particle possesses the best wear resistance property. It is known that when surfaces slide relatively, almost all the energy dissipated in friction appears in the form of heat at the interface. This frictional heat raises the temperature of specimen and oil [1]. So the less the temperature change the smaller energy dissipated. Table 2 is the temperature changes of the base oil containing different samples. It is noted that the base oil containing composite particles causes the lowest temperature change with regard to the rest of samples. It is similar with the results of the friction coefficient and the wear scar diameter. All these illustrate the good tribological properties of the composite particles. The optical microscope, SEM and EDS images of the wear scar surface after Ring-block wear test are shown in Fig. 6. The surface of the block lubricated by base oil has wide and deep furrows and grooves in sliding direction, it indicates that the abrasive wear of the block surface is severe. There are not so many furrows in the surface of the block lubricated by the oil containing composite particles, just few grooves can be found. This result is in accordance with the best tribological behaviors of oil containing composite particles. The EDS results indicate that the surface of the block contains the elements of O and Si which are only contained in the serpentine, but the Mg and La element are not found. Oil containing serpentine nano-particles can form a tribofilm of multi-apertured oxide layer on worn surface [1]. In this study, the oil containing composite particles also formed a tribofilm containing O and Si. In the serpentine the non-bridging oxygen atoms in [SiO4 ] point to the same direction and connect with Mg2+ , and consequently form Mg–O octahedral layers. During the sliding the serpentine deposited on the surface of the block, then the instantaneous high-temperature made the ion exchange reaction of iron
Additive
Initial temperature (◦ C)
Final temperature (◦ C)
Temperature variation (◦ C)
Base oil Serpentine La(OH)3 Composite powder
28.0 32.5 37.8 39.1
59.8 58.2 63.1 60.7
31.8 25.7 25.3 21.6
Reduced by (%)
21.83 18.03 24.63
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Fig. 6. The surface topography and the EDS of the block after Ring-block wear test: (a) base oil; (b) oil containing composite particles: (1) optical microscope image, (2) SEM image, and (3) EDS image.
atoms and Mg2+ in the serpentine occurred, so the Mg element is not found in the surface of the block. The element La could not be detected on the wear surfaces for two reasons, the first reason is that the La element of nano-La(OH)3 act basically as a catalyst to accelerate the reaction. The second reason is that La element occupied so small proportion of the composite that the level of the deposition was too low to be detected.
friction-reducing and anti-wear properties. And the lubricating oil containing serpentine/La(OH)3 composite particles exhibited better tribological and self-repairing properties than the lubricating oil containing only a single kind of particle. (3) A tribofilm containing O, Si and Fe elements formed on the surface of the rubbing pair by the ion exchange reaction between iron atoms and Mg2+ of the serpentine, the lanthanum acted as catalyst during the reaction.
4. Conclusion (1) Nano-granular La(OH)3 and composite particles of serpentine coated by La(OH)3 were synthesized via sol–gel method. (2) The lubricate oil containing serpentine, La(OH)3 , and serpentine/La(OH)3 composite particles presented excellent
Acknowledgement This research was supported by the National Natural Science Foundation of China (51044011).
F. Zhao et al. / Wear 288 (2012) 72–77
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