Study on Lubrication Properties of Modified Nano ZnO in Base Oil

Transcription

1 Lubrication Research China Petroleum Processing and Petrochemical Technology 2011,Vol. 13, No. 3, pp September 30, 2011 Study on Lubrication Properties of Modified Nano ZnO in Base Oil Qian Jianhua; Zhang Yu; Wang Lingling; Xing Jinjuan (Liaoning Key Laboratory of Synthesis and Application of Functional Compounds, Bohai University, Jinzhou ) Abstract: ZnO nanoparticles with an average size of 125 nm were prepared via homogeneous precipitation method and were characterized by SEM. The products were surface-modified by the surfactant SDS. Surface-modified nano particles were added at a mass ratio of 1.0%, 2.0%, 3.0%, and 4.0%, respectively, in base oil and their friction and wear behaviors were evaluated on a MRS-10D type four-ball wear tester. After four-ball wear tests, the morphology of the rubbing surfaces was evaluated with metallographic microscope. It was revealed that the modified nano ZnO had excellent behavior for improving anti-wear property and friction coefficient, which could greatly reduce the friction of machine parts. Key words: nano ZnO; homogeneous precipitation; surface modification; additive; lubrication properties 1 Introduction Nano-scale ZnO has been attracting considerable interest because of its unique structure and performance [1-4]. And a large number of studies on tribological characteristics of nano-particles as base oil additives have been carried out [5-7]. Nano ZnO has large surface area, high surface energy, strong adsorption, high diffusion, easy sintering, low melting point and other outstanding characteristics. When the nano ZnO is used as the additive in base oil, there will be different friction reduction and wear resistant effects on lube oils. Friction researchers proposed the views of adsorption, penetration and tribo-chemical reaction for the friction reducing and anti-wear mechanism of nano-particles [8-11].This new type of lubricating materials not only can form a layer of film in the rubbing surface to reduce the friction coefficient, but also play a role in self-repairing to fill and repair the friction surface to a certain degree [12]. Nano-sized base oil additives, which are suitable for the heavy load, low speed, high-temperature work, have outstanding extreme pressure resistance performance, excellent anti-wear resistance and better lubrication properties. In addition, nano ZnO is a widely used nano-oxides and its preparation method is simple, and can already be prepared in mass production scale. Nano ZnO used as an additive will not only improve the tribological properties of base oil, but also can significantly reduce the cost of lube oil. However, nano ZnO is hydrophilic-oleophobic with poor oil solubility. It needs to rely on the role of dispersant, strong agitation or ultrasonic dispersion to disperse nano ZnO particulates in the base oil [13-17]. In this paper, nano ZnO was prepared by homogeneous precipitation method using lauryl sodium sulfate (SDS) as the surfactant, and the oil solubility, anti-corrosion and tribological properties of prepared nano ZnO used as lubricant additive were studied. 2 Experimental 2.1 Preparation and characterization of additive Using urea as the homogeneous precipitant, zinc nitrate (0.8 mol/l) and urea (2.8 mol/l) were mixed in a flask. Then upon stirring and heating, the surfactant SDS was added to the system with the temperature being maintained at 95 for 3 hours. Finally the precursor was treated by filtration and washed with water and ethanol several times, and then the samples was obtained after the precursor was calcined at 400 for 3 h. The morphology of ZnO particles was investigated by scanning electron microscopy (SEM). Corrresponding Author: Dr. Qian Jianhua, Telephone: ; qianjianhualn@163.com 69

2 China Petroleum Processing and Petrochemical Technology 2011,13(3): Oil solubility performance tests of the additive The additives were introduced into base oil at a mass fraction of 1.0%, 2.0%, 3.0%, and 4.0%, respectively, then the system was treated with ultrasonic vibration for 20 min and was subjected to settling for 10 days, and finally the dissolution situation was studied. 2.3 Corrosion tests of the additive Corrosion tests of the additive were conducted according to the test method GB for copper strip corrosion of petroleum products at (100 ± 1) for 3 h. Then the corrosion situation was checked by comparing copper stripe against the standard shade guide after the tests. 2.4 Friction and wear tests and morphology of wear scar of the additive The base oil for testing was a commercial refined base oil. Tribological performance was tested by a four-ball friction test machine. The diameter of steel ball was 12.7 mm and its hardness was 59~61HRC. The steel ball needed to be cleaned by petroleum ether before the test using ultrasonic means to remove the rust on the surface. According to the method GB/T3142 the tribological properties of base oil samples with different contents of additives were determined under a speed of 1450 r/min at room temperature for 30 min. Test balls needed to be cleaned in petroleum ether using ultrasonic means for 15 min and dried, then the worn surface morphology was analyzed by a metallographic microscope. 3 Results and Discussion 3.1 Characterization of synthesized nano ZnO The SEM images of ZnO are shown in Figure 1. It can be seen from the micrographs that the average size of ZnO particles was 125 nm reaching the nanometer level. Figure 1 SEM images of ZnO particles prepared with SDS surfactant 3.2 Oil solubility and anti-corrosion properties of the additive The oil solubility and copper strip corrosion test results are shown in Tables 1 and 2. It can be seen from Table 1 that the oil samples were still clear and un-stratified after having settled down for 10 days with the additive being added to base oil at a mass fraction of 1.0%, 2.0%, 3.0%, and 4.0%, respectively. The results showed that the additive in base oil had a good oil solubility performance. And it can be seen from Table 2 that the corrosiveness of base oil and the tested oil sample containing 3.0% additive both reached a rating of 1b. Table 1 Experimental results of oil solubility of additive Additive content(w), % Base oil+zno 1.0 Dissolved, not stratified 2.0 Dissolved, not stratified 3.0 Dissolved, not stratified 4.0 Dissolved, not stratified Table 2 Copper stripe corrosion tests of additive Samples Copper strip corrosion, rating Base oil 1b Base oil+zno (3.0 m%) 1b 70

3 Qian Jianhua, et al. Study on Lubrication Properties of Modified Nano ZnO in Base Oil 3.3 Maximum non-seizure load PB value of the additive The performance tests of the base oil and the base oil containing the additive were carried out in a four-ball test machine operating at room temperature, and the maximum non-seizure load (P B ) values were measured. The test results are presented in Table 3. It can be seen from the data listed in Table 3 that the P B value of the base oil was 295 N, and the P B value of the oil sample containing the additive increased with an increasing amount of additive. When the content of ZnO was 3.0%, the P B value reached a maximum and stabilized at that level, because there were extreme-pressure function elements S and Zn that could form a protective film under the high load conditions. The extreme pressure performance further improved because of the active elements concentration increased with an increasing additive content. It showed that the synthesized additive was a kind of good extreme pressure additives used in oil. Table 3 P B values of test oil samples with different contents of additive Additive content (w), % P B, N Anti-wear performance of the additive The changes in wear scar diameter of steel balls under load are shown in Figure 2 when the base oil containing 3.0% of additive was tested over a long duration of 30 min. As the load increased, the wear scar diameter increased under lubrication. The anti-wear effect of base oil containing additives was compared with the neat base oil. In the entire test, the wear scar diameter of base oil increased rapidly and the diameter of the oil containing additives increased slowly in the whole course of the test. Under high load conditions, the metal - metal sliding system formed between the two friction surfaces provided excellent extreme pressure and anti-wear properties and a boundary lubrication film containing Zn that was formed on the steel ball surface during the friction process. Nanoscale ZnO particles were deposited mainly in the form of a film during the friction process, because the surface energy and activity of the nano-particles became higher as the particle size decreased. It was more likely to form a nano-lubricating protective layer with low melting point and was prone to cover the metal wear surface. So the layer could bring about in-situ repair on the worn surface during the rubbing process. Figure 2 Changes of wear scar diameter with load The changes in wear scar diameter with a changing amount of additive in test oil under a load of 392 N and a period of 30 min are shown in Figure 3. The wear scar diameter decreased at first and then increased with an increasing content of additives in base oil. The wear scar diameter was the smallest when the additive content reached 3.0%. Since the adsorption of steel ball surface was saturated, the arrangement of molecules was closer and the protective film thickened with an increasing content of additive in base oil, so the anti-wear performance was enhanced. When the additive content in base oil reached 4.0%, the wear scar diameter increased and the anti-wear performance began to decline. This phenomenon was connected with the lower formation speed of lubrication film on the friction surface under a lower concentration of additive, which could not fulfill the consumption speed of lubricating film caused by wear and tear. Furthermore, nano-particles with very high surface energy at high concentrations were extremely liable to aggregate into large particles. These large particles could cause greater wear in the friction process, so that the wear scar diameter began to increase at an additive content of 4% in base oil. 71

4 China Petroleum Processing and Petrochemical Technology 2011,13(3):69-73 Figure 3 Changes in wear scar diameter with different amounts of additive 3.5 Friction reducing properties of the additive The changes in friction coefficient of the base oil and the oil containing 3.0 % additive with time are shown in Figure 4, when the oil samples were tested under a load of 392 N and a test duration of 30 min. It can be seen from Figure 4 that the friction coefficient of the additivecontaining base oil was significantly smaller than that of the neat base oil. The friction coefficient of additive was gradually decreased with an extension of test duration. During the friction test process, the real contact surface between the wear-affected areas increased because of the increasing wear and tear under the base oil lubricated conditions, thereby the friction coefficient showed a rising trend with time. On the other hand, the friction coefficient of the additive-containing oil sample decreased because the lubrication film between the rubbing surfaces can be formed rapidly thanks to the nano-scale ZnO added to the base oil. scar is shown in Figure 5 (a) after friction and wear testing of pure base oil. It can be seen from the micrograph that tumor tilting and pits were left on the worn surface because of the adhesion between deeper furrows after friction. The worn surface of metal lubricated by base oil containing 3.0% nano-scale ZnO additive was smooth and the adhesive abrasion level was lighter. There were no obvious tumor tilting and pits formed by bonding [Figure 5(b)]. Because the nano ZnO after modification can be adsorbed and deposited on the friction surface, leading to the formation of a discontinuous surface film with low shear strength in the rubbing surface contact zone. Therefore the film could isolate friction effectively and reduce the plowing and severe adhesion as a result of direct contact generated between the rubbing surfaces to provide good wear resistance performance. Figure 5 Morphology of wear scar of steel balls Figure 4 Changes in friction coefficients with time 3.6 Morphology of the steel ball wear scar The metallographic morphology of the steel ball wear 4 Conclusions Nano-scale ZnO was prepared by homogeneous precipitation method using lauryl sodium sulfate (SDS) as the 72

5 Qian Jianhua, et al. Study on Lubrication Properties of Modified Nano ZnO in Base Oil surfactant. Nano-sized ZnO particles were more easily dispersed in base oil because there was an organic modification layer on their surface. Nano-sized ZnO prepared thereby had good oil solubility and anti-corrosion properties to be used as base oil additive. Nano-scale ZnO particles could reduce the wear of direct contact area of friction by depositing on the rubbing surfaces and forming a layer of lubricating film on the contact surfaces. The friction reducing and anti-wear properties of base oil were significantly improved by adding the surface-modified nano-sized ZnO particles. Acknowledgement: This work was supported by Liaoning Provincial Office of Education for Innovation Team (Project number: 2006T001) and Liaoning Province of Key Laboratory Project: Project number: ). References [1] Cao Ming, Zheng Shiyuan, Zhang Hui, et al. Preparation and application of zinc oxide[j]. Journal of Western Chongqing Institute, 2003, 2 (4): (in Chinese) [2] Zhou Minjie, Zhu Haojun, Jiao Yang, et al. Optical and electrical properties of Ga-doped ZnO nanowires arrays on conducting substrates[j]. J Phys Chem C, 2009, 113(20): [3] Soares J W, Whitten J E, Oblas D W, et al. Novel photoluminescence properties of surface-modified nanocrystalline zinc oxide: Towards a reactive scaffold[j]. Langmuir, 2008, 24(2): [4] Han Mingjuan, Zhao Kongshuang. Dielectric behavior of suspensions of polystyrene-zinc oxide composite microspheres[j]. J. Phys. Chem. C, 2008, 112: [5] Ma Jianqi, Wang Xiaobo, Cui Ruomei. Effect of oil-soluble Cu particles as additive on the friction-reducing and antiwear ability of several kinds of commercial lubricating oils [J]. Lubrication Engineering, 2004, 163 (3): [6] Liu Weimin. Application of nanoparticles in lubricants[j]. Tribology, 2003, 23 (4): [7] Zhou Huidi, Yue Mei e, Chen Jianmin. Effect of nanometer lanthanum fluoride as filler on the tribological behavior and corrosion resistance of polyimide bound solid lubricating coating [J]. Tribology, 2004, 24 (3): [8] Huo Y Q, Yan Y T, Liu X X, et al. Preparation and tribological properties of monodispersed nano-sio 2 particles as additive in lubrication oil [J]. Tribology, 2005, 25 (1): (in Chinese) [9] Dong L, Chen G X, Li H F, et al. Tribological properties and self-healing action of SiO 2 /SnO 2 complex nanoparticles as an additive in a machine oil[j]. Tribology, 2004, 24 (6): (in Chinese) [10] Wang L B, Feng D P, Liu W M. Tribological properties of a steel-steel pair under the lubrication of lithium grease containing various nano-particulates as additives [J]. Tribology, 2005, 25 (2): (in Chinese) [11] Wang X L, Xu B S, Xu Y, et al. Study on friction and wear behavior and mechanism of nano-cu additive in lubrication oil[j]. Tribology, 2005, 27 (3): (in Chinese) [12] Weng Zhongwen; Xu Binshi; Ma Shining; Qiao Yulin; Zhang Wei. Progress in the Research and Application of Nanoscaled Materials Used in Surface Engineering [J]. China Surface Engineering, 2000, 13 (2): 5-9 (in Chinese) [13] Battez A H, Gonzdez R, Felgueroso D, et al. Wear prevention behavior of nanoparticle suspension under extreme pressure conditions[ J ]. Wear, 2007, 263: [14] Zhou J F, Wu Z S, Zhang ZJ, et al. Study on an antiwear and extreme pressure additive of surface coated LaF 3 nanoparticles in liquid paraffin [J].Wear, 2001, 249: [15] Ou Zhongwen, Liu Weimin, Xu Binshi, et al. Synthesis of dispersion stability of nano-chalcogenide in oily medium: China Patent, [P] (in Chinese) [16] Tang E J, Cheng G X, Ma X L, et al. Surface modification of zinc oxide nanoparticle by PMAA and its dispersion in aqueous system[j]. Applied Surface Science, 2006, 252: [17] Chen Shuang, Li Nan, Liu Wei-min. Investigation on the tribological behaviour of DDP-coated PbO nanoparticles[j]. Journal of Jilin Institute of Chemical Technology 2002, 19(3): 4-6 (in Chinese) 73