*Corresponding author. Tel. : +91 -9509642235 E-mail addres s: [email protected] .ac.in (Mayank Chouhan ) A Study of Tribological Behaviour of Nano -Fe 3O 4 Based Ferrofluid under Variable Magnetic Field Mayank Chouhan *, Lalit Thakur, Jaideep Gupta Mechanical Engineering Department, NIT Kur ukshetra, Haryana -136119, India. Abstract Ferrofluids are stable colloidal systems consisting of magnetic particles of about 20 to 30 nm in diameter, coated with surfactants and dispersed in a carrier liquid. In the present study, a ferrofluid with optimum concentration of Fe 3O4 in a Group 2 base oil was prepared by a mechanical mixing.
The morphology (shape and size), elemental composition and crystallinity of the Fe 3O4 nanoparticles were studied by SEM, EDS and XRD analysis. In this paper, a contro llable and variable magnetic field was generated in a solenoid which was fitted in the four -ball tester. With this modified four -ball tester, the tribological behaviour of the ferrofluid under varying magnetic field was studied. The worn surfaces of the st eel balls lubricated with ferrofluids under different magnetic field were observed by using optical microscopy.
The coefficient of friction was found to be increased with the increase in magnetic field and decreases with the weakening of the magnetic field . The results also showed that the ferrofluid in the presence of a magnetic field had much better anti -wear properties as compared to an optimum ferrofluid concentration without the magnetic field. Keywords : Ferrofluids, magnetic field, tribological, four -ball tester, wear, lubrication 1. Introduction Many technological advances have been made in the field of tribology. The problem, however, is how the tribological properties can be improved. Any improvement in friction reduction helps to save energy losses, and avoiding wear increases the life of the machine [1,2] . An attempt is made to reduce the friction and wear of conventional oils by adding oil – soluble additives. However, these existing lubricants have reache d their productivity limits [3,4] . Therefore, the search for a new lubricant with high energy efficiency led many researchers to use nanoparticles (magnetic and non -magnetic) a s additives to lubricants [5,6] . These lubricants are called nano -lubricants. Nano lubricants consist of three system components. Base oil, nanoparticles and surfactants for the preparation of interfaces be tweennanoparticles and oils  . Magnetic nanoparticles are the most promising material among the various nanoparticles to reduce friction and wear because their flow ability and properties can be adjusted by applying a magnetic field. In addition, it is insoluble in nonpolar base oil; less chemically active with another additive present in the base oil, less chemically active with the surface than oil additives, therefore more durable. Due to its nanometer size, it can enter the contact zone between the friction surfaces, thereby improving rolling friction. The stable dispersion of magnetic nanoparticles in a base oil is defined as a magnetic liquid. In tribology, lubricants are mainly used in gask ets, bearings, gears, etc. [8,9] Lee -Jun et al.  investigated the influence of a magnetic field on the tribological properties of Mn 0.78 Zn 0.22 Fe 2O4 dispersed in turbine oil by fixing a permanent magnet ( NdFeB) on a ball -pot arrangement. The direction of the applied magnetic field was perpendicular to the axis of rotation. The wear and sliding properties improved when a magnetic field of 12 mT was appl ied, while Deysarkar et al.  reported only a decrease in the area of wear scars under the influence of a magnetic field (by placing a permanent magnet around t he ball pot assembly) on a magnetite magnetic fluid. Zeqi et al.  studied the tribological properties of atricresyl phosphate (TCP: anti -oil additive, from 0 to 2.5%) disper sed in 150 SN mineral oil under a magnetic field applied from a coil wound on a ball pot . They observed the coefficient of friction (COF) upon increasing the magnetic field. They exhibited that the applicati on of a magnetic field to a magnetic fluid affect s the tribological properties. In present study , a ferrofluid with optimum concentration of Fe 3O4 in a Group 2 base oil was prepared by a mechanical mixing. The tribological properties of Nano -Fe 3O4 Based Ferrofluid under varying magnetic field were examined by using a modified four -ball tester. The wear preventive experiments were performed to evaluate the anti -wear performance of the ferrofluid under variable magnetic field. 2. Experimental details 2.1 Materials Fe 3O4 nanoparticles of size 10 -30 nm were procured from Nano Research lab (India). Oleic acid, Acetone and deionised water were purchased from LOBA Chemie (India). The Group 2 base oil was obtained from Fresh Lube (India) for the preparation of ferrofluids. First, the nanoparticles were coated with a surf actant, with 3 gm of Fe 3O4 nanoparticles with 5% w/v oleic acid, followed by heating of viscous solution at 65 °C for 10 min and ultrasonic dispersion for 20 minutes . The coated Fe 3O4 MNPs (Magnetite Nano Particles) was separated from the suspensions by a strong neodymium ma gnet. The precipitate was cleaned with de -ionized water and finally with acetone to remove water content. Lastly , the coated 3 wt. % of Fe 3O4 MNPs were dispersed in Group 2 N500 base oil by mechanical mixing procedure at 3000 rpm for 1 h our. 2.2 Characterization of particles The x -ray diffraction pattern of MNP was obtained by using powder X -ray diffractometer (Rigaku, model miniflex ii desktop x -ray) using CuK± (” = 0.15414 nm) radiation . The 2 angle was varied from 15° to 75° at a scan speed of 2° min -1. The morphology and elemental composition of the Fe 3O4 MNPs were studied by using scanning electron microscope (SEM), coupled with Energy -dispersive X -ray spectrography (EDS ). Figure 1 (a) X -ray diffraction pattern (b) SEM image (c) EDS elemental composition of Fe 3O4 magnetic nanoparticles 2.3 Four ball Tribological tester To investigate the anti -wear and friction reduction ability of this 3 wt .%. Fe 3O4 based magnetic fluid the tribological testing was performed as following condition: 1200 rpm, 15 minutes, 392 N and at room temperature. The four ball wear test was performed under two different loading conditions i.e. with varying magnetic field without the application of magnetic field. The balls used in the test (diameter 12.7 mm) were made of chromium -Element Weight % O K 36.96 FeL 60.12 NiL 1.41 MgK 0.76 AlK 0.42 S K 0.34 keV countsalloyed steel with HR C 60 -64 . At t he end of each test, the average wear scar area on the three lower balls was measured using an optical microscope with an accuracy of 0.01 mm. Finally, the average wear area was calculated as wear area in three identical tests. Figu re 2 Modified four ball tester A schematic diagram of a modified four -ball tester is shown in Fig . 2. A continuously adjustable field was produced by modifying a four ball tribological. The magnetic field was generated by a magneti c coil. The coil resistance was 12.9 ohms, and the current and voltage flowing through the coil were varied in the range of 0 -5 A and 0 -60 V. Due to the excessive heat generation in the solenoid coil during the experiment, the test time was restricted to 15 minutes under a varying magnetic field. 3. Results and Discussion 3.1 Characterization of the particles Figure 1 (a) shows that it is indicated on the X -ray image of nanoparticles that the synthesized samples consist of the cubic phase of Fe 3O4 (JCPDS 19 -629), i.e. magnetite. The expanding Bragg peaks for the resulting Fe 3O4 are due to their small particle size. The average particle size perpendicular to (311) 16.16 nm was calculated on the basis of the line broadening analysis accor ding to the Scherer formula  . The calculated particle size is in good agreement with the size specified by the suppliers. The morphology of nanoparticles was spherical. Energy -dispersive X -rays spectrography (EDS ) of elemental composition of Fe 3O4 nanoparticles without and with coated was p erformed as shown in Fig. 1( b-c). 3.2 Tribological behaviour of Nano -Fe 3O4 based ferrofluids In Fig. 3 shows the relationship between magnetic induction and wear scar area (WSA). WSA with 3% by weight ferrofluid based on nano -Fe 3O4 without magnetic field isapproximately 0.4413 mm 2. When the magnetic induction is 54 mT, the WSA is reduced by 28%. Then, with increasing magnetic inductio n, the WSA rises to 0.441 0 mm 2 at 74 mT . The results show that ferrofluids based on nano -Fe 3O4 have a broad magnetic induction range and can effectively improve the anti -wear properties of the base oil. Figure 3 Wear scar area as a function of the varying magnetic induction Figure 4 Coefficient of friction as a function of the varying magnetic induction In Fig. 4 shows the change in the friction coefficient as a function of the magnetic induction as a function of the test duration. The friction coefficient is lowest when the magnetic induction is 16 mT. The reduction in friction coefficient can be attribu ted to Nano -Fe 3O4 based ferrofluids with an adsorbent dispersant forms a protective film on rubbing scar area due to the shear effect  . Then, as the magnetic induction increases, the coefficient of friction increases. This can be explained as follows: With high magnetic induction, nanoparticles agglomerate easily and the viscosity of magnetic fluids increases with increasing magnetic induc tion. When a magnetic field is present, the suspended particles follow the direction of the magnetic line, increasing the flow resistance. It has also been 0.4413 0.398 0.371 0.378 0.3185 0.441 00.10.20.30.40.5 0 10 20 30 40 50 60 70 80 Wear Scar Area (mm2) Magnetic Induction (mT) 0.1119 0.0951 0.1197 0.122 0.1105 0.0962 0 0.020.040.060.080.10.120.14 0 10 20 30 40 50 60 70 80 Coefficient of Friction Magnetic Induction (mT)found that the coefficient of friction increases with increasing test duration in the absence of a magnetic field. However, when ferrofluids lubricated under a magnetic field, the coefficient of friction first increases and then practically does not stabilize. This can illustrate that under the action of a magnetic field, a film of nano -Fe 3O4 based ferro fluids easily forms in a lubricant on a worn surface. As already mentioned, this film can compensate for a scuffing scar to reduce the shear stress. Thus, it is easy to obtain a stabilizing friction coefficient under the action of a magnetic field. 3.3 Wor n surface analysis In order to investigate the lubricating mechanism of Nano -Fe 3O4 based ferrofluids under varying magnetic eld, the surface analysis of optical image was performed. Figure 5 Optical image of wear scar area under the varying magnetic field. Figure 5 shows the morphology of a worn surface lubricated with ferrofluids based on nano – Fe 3O4 under an alternating magnetic field. Under the influence of a magnetic field, an ferrofluid can quickly form a lubricating film on a worn surface, which can improve the lubricating properties of lubricating oil. As compared to lubrication on a worn surface without a magnetic field (see Fig. 5 (a)), the worn surface is smoother and the wear scar is smaller under the influence of a magnetic field, indicating that the magnetic field can reduce wear on the friction pair. The figures show that the magnetic field influences the tribological properties of the lubricant: when the magnetic induction is 0 mT (see Fig. 5 (a )), the wear on the worn surface is deep and there is little plowing; However, the worn surface is smooth and plowing almost disappears when the magnetic i nduction is 54 mT (see Fig. 5 (e )). At highmagnetic induction nanoparticles agglomerate easily. Thes e agglomerations can act as abrasive particles during lubrication so that slighter plowing (grooves) form on a worn surface when th e magnetic induction exceeds 74 mT. They will not seriously affect the tribological properties of the lubricant. Thus, it can be concluded that 101 mT is the optimum magnetic field strength for 3 wt% Fe 3O4 MNPs based ferrofluid; and magnetic nanoparticles as an additive in the base oil of Group 2 N500 in a magnetic field have excellent tribological properties. 4. Conclusion The present study provides useful information on the lubrication of -Fe 3O4 based ferrofluids in an amount of 3% by weight in a magnetic field. The following conclusion can be drawn. The method used to modified four -ball tester is feasible and a controlled varying magnetic field can be obtained. Ferrofluids based on MNPs have better wear resistance, load capacity and anti -friction ability in the magnetic field. The opt imal magnetic field is about 54 mT. The percent decrease in scar wear area is 28 % versus 3% by weight lubricant nano –Fe 3O4 based ferrofluids without magnetic field. 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