Nanodispersed Ferrofluid Oil Lubricity Improvement with Processing Methods

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Procedia Engineering 206 (2017) 606–610

International Conference on Industrial Engineering, ICIE 2017 International Conference on Industrial Engineering, ICIE 2017

Nanodispersed Ferrofluid Oil Lubricity Improvement with Nanodispersed Ferrofluid Oil Lubricity Processing Methods Improvement with Processing Methods A.N. Bolotov, I.V. Gorlov, V.V. Novikov*, A.N. Bolotov, I.V. Gorlov, V.V. Novikov*, Tver State Technical University, 22, A. Nikitin Emb., Tver 170026, Russia Tver State Technical University, 22, A. Nikitin Emb., Tver 170026, Russia

Abstract Abstract The article proposes and justifies the use of technological methods of improvement of nanodispersed magnetic oil lubricating properties. is shownand thatjustifies if the ferrofluid oiltechnological producing process includes additional operations, which are aimed at removal The article Itproposes the use of methods of improvement of nanodispersed magnetic oilthe lubricating of excess surfactant stabilizer and big magnetic agglomerates from its composition, it allows decreasing friction and wear of properties. It is shown that if the ferrofluid oil producing process includes additional operations, which are aimed at the removal lubricated surfaces bystabilizer tens of percent. of excess surfactant and big magnetic agglomerates from its composition, it allows decreasing friction and wear of The authors have chosen lubricant oil that had been synthesized based on dioctyl sebacate for the research due to its lubricated surfaces by tensferrofluid of percent. good lubricating low volatility and oil viscosity and a wide temperature range of operability. remove the excess The authors haveproperties, chosen ferrofluid lubricant that had been synthesized based on dioctyl sebacateTofor the research due of to the its surfactant stabilizer dissolved in a ferrofluid oil dispersion medium the dispersion medium replacement method proposed by good lubricating properties, low volatility and viscosity and a wide temperature range of operability. To remove the excess ofR.E. the Rosenzweig was used. To remove magnetic units from magnetic the authors usedreplacement magnetic separation. surfactant stabilizer dissolved in a large ferrofluid oil dispersion medium thefluid dispersion medium method proposed by R.E. In boundary was friction surfaces lubricated by ferrofluid there might be theirtheabrasive agglomerates from single-domain Rosenzweig used. To remove large magnetic unitsoilfrom magnetic fluid authorswear used by magnetic separation. magnetic particles. this connection, experimentally proves the necessity of modifying magneticfrom liquid unit friction In boundary frictionInsurfaces lubricatedthe by article ferrofluid oil there might be their abrasive wear by agglomerates single-domain surfaces using MAO-technology to protect them from abrasion wear. magnetic particles. In this connection, the article experimentally proves the necessity of modifying magnetic liquid unit friction © 2017 The Authors. Published by B.V. surfaces using MAO-technology to Elsevier protect them abrasion wear. © 2017 The Authors. Published by Ltd. from Peer-review under responsibility of Elsevier the scientific committee of the International Conference on Industrial Engineering. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the International Conference on Industrial Engineering Keywords: ferrofluid lubricaiting oil; of friction; wear; stabilizer; microarc oxidation. Peer-review under responsibility the scientific committee of the International Conference on Industrial Engineering. Keywords: ferrofluid lubricaiting oil; friction; wear; stabilizer; microarc oxidation.

1. Introduction 1. Introduction Recently the problem of applying nanotechnologies to improve reliability of various vehicles and machinery draws attention of many of researchers. It’s known that some nanodispersed liquids might used for Recently the problem applying nanotechnologies to improve reliabilitymagnetic of various vehicles andbemachinery lubricating tribounits that researchers. work in hydrodynamical and some boundary friction mode [1-4]. liquids Such magnetic draws attention of many It’s known that nanodispersed magnetic might beliquids used are for lubricating tribounits that work in hydrodynamical and boundary friction mode [1-4]. Such magnetic liquids are

* Corresponding author. Tel.: +7-482-278-8880. E-mail address:author. [email protected] * Corresponding Tel.: +7-482-278-8880.

E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier B.V. Peer-review the scientific committee 1877-7058 ©under 2017responsibility The Authors. of Published by Elsevier B.V.of the International Conference on Industrial Engineering . Peer-review under responsibility of the scientific committee of the International Conference on Industrial Engineering .

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the International Conference on Industrial Engineering. 10.1016/j.proeng.2017.10.524

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usually called ferrofluid lubricating oil [5-8]. The main advantage of ferrofluid oil over traditional oil for lubrication is that the ferrofluid oil goes to the area of contacting friction surfaces as required under the influence of stationary thermomagnetic fields. Lubricating layer restoration occurs spontaneously. It does not require complicating tribounit design and using various extra devices to control oil movement. However, the results of ferrofluid oil triboengineering tests showed that their antifrictional and antiwear properties are not always high enough. One of the most effective methods of increasing antifriction and antiwear properties of ferrofluid oil is introducing additional oil additives and fillers, which do not worsen its colloidal stability. This method has been studied well [4,6,7] and is effectively used in practice. The aim of this work was to improve lubricating action of ferrofluid nanodispersed oil using technological methods. Works of many researchers, for example, the works of Professor D.N. Garkunov [9] show great potential of a similar approach to improving reliability and durability of friction units. The authors emphasize that the choice of tribounit elements for their technological improvement must be based on the system approach. In this context, it becomes clear that the term “lubricating effect” underlines the technological dependence of magnetic liquid tribounit properties not only on magnetic lubricant characteristics, but also on a friction material, surface topography, magnetic field sources, etc. The analysis of ferrofluid oil test results obtained according to the same process scheme, but from different test batches showed unacceptably large variation in lubricating and rheological properties. For example, a friction coefficient for surfaces lubricated by ferrofluid oils based on a diester of carboxylic acid and taken from different batches could vary by 20÷30%, the range of wear rate values reached 50÷100%. It has been found that triboproperties are highly sensitive to redundant content in the dispersion medium of surfactant stabilizer oil of a colloidal structure. A stabilizer activates negative processes of friction surface mechanochemical wear, metal alloy surfaces in particular. In addition, even after careful peptization magnetic oil always contains solid aggregates of monodomain magnetic particles with the size reaching 10-6M. The hardness of magnetite typical dispersed particles exceeds 5 GPa and significantly exceeds the hardness of many antifriction nonmagnetic alloys and especially polymers. According to ex-perimental data, the agglomerates of the dispersed particles increase the overall wear tribosurfaces because of their abrasive action. Hence, the first task of investigations was to include in the ferrofluid oil obtaining process some additional operations aimed at removing not adsorbed stabilizer from oil and separating large magnetic agglomerates, as well as assessing what changes of lubricants and other oil properties it will lead to. In the process of boundary friction of surfaces lubricated by magnetic oil there might form new agglomerates that stimulate abrasive wear under the influence of magnetostatic forces and as a result of mechanical destruction of magnetic particle solvation shells. Usually in magnetic liquid tribounits one friction pair element should be made of a magnetic material and the other of low magnetic readily available structural material (aluminum alloy, bronze, brass), with its hardness lower than magnetite hardness. Therefore, if it is possible to use abrasion resistant steel as a friction pair magnetic material, then it is necessary to protect counterface from wear technologically using solid antifriction coating. After a detailed analysis of various coating application technologies the authors have selected a microarc oxidation technology (MAO-technology) [10,11] for aluminum alloy surfaces. The main advantage of MAO-technology relating to magnetic liquid tribounits is that it allows obtaining high quality corundum coating on details with complex surface geometry. Hence, the second task of the researches was to prove applicability of microarc technology for applying protective coating on friction surface of magnetic liquid tribounits. 2. The object of research and experimental equipment For the research the authors have selected ferrofluid lubricating oil, which had been synthesized based on dioctyl sebacate. This carbonic acid diester is good as a dispersion medium due to its good lubricating properties, low volatility and viscosity, wide temperature range of performance. Ferrofluid oil synthesis was carried according to a traditional technology [12] that includes comprising magnetite precipitation (dispersed phase) of an iron salt solution, disperse phase dehydration with acetone and magnetite processing by a fatty acid solution in dioctyl sebacate at high temperature. In order to remove excess surfactant stabilizer dissolved in a ferrofluid oil dispersion medium the authors used a dispersion medium replacement method proposed by R.E. Rosenzweig [13]. The method is about dispersed phase revertive flocculation using the polar flocculation agent. Then, the liquid phase with a mixture of an initial

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dispersion medium and an agent has been removed. At the same time, we removed surplus surfactant. The structure of dispersed particle solvation shells was not broken. After that, stabilized particles were peptized in pure dioctyl sebacate. We used magnetic separation to remove big magnetic units from magnetic fluid. For this purpose, ferrofluid oil cuvette was placed into an inhomogeneous magnetic field and held 1÷2 hours depending on oil layer height and viscosity. Magnetostatic forces caused Boltzmann redistribution of small magnet particles and agglomerates settled permanently at the bottom of a cuvette. To create an inhomogeneous magnetic field we used an open core with a tapered gap for a cuvette. A magnetic field was produced by SmCo5 alloy permanent magnets, a magnetic field gradient in elevation cuvette changed from 0.5 А/m2 to 3 А/m2. Research on oil lubricating properties included using a threeball frictional testing machine. To hold ferrofluid oil on a friction track, a mandrel between the balls has cylindrical magnets facing polar surface to countersample surface. Ferrofluid oil was accumulated and held by an inhomogeneous field around a magnet pole. When the balls rotated, a magnet transferred oil and spread it on a friction track. The load on the balls was created by a weight method and changed discretely from 20 to 100N. The moment of friction was recorded continuously, wear was determined discretely by the diameter of a wear scar on a ball surface. Ball samples for the experiments had 8 mm diameter, made of steel ShKh-15. Cylindrical countersample was made of steel 40X, the friction surface was polished for a roughness parameter Ra = 0,25÷0,35 micrometer. The tests had a sliding velocity 0.24 m/s and initial pressure at the contact of 1.25 GPa. In order to obtain coatings on the details of aluminum alloy magnetic liquid tribounits the authors applied standard MAO-technology [10]. When the surface oxide coating is forming, we also have a loose technological layer consisting of silicates, which was removed by grinding. The coatings had thickness about 100 micrometers and possessed micro hardness increasing in depth. The glassy outer layer has microhardness about 5 GPa, the layer at the boundary with aluminum micro hardness increases up to 12÷20 GPa. For experiments the authors used D16 alloy with hard and strong oxide coating and good adhesion base (about 4·108 N/m2), which is explained by strong chemical relations. The ceramic layer porosity was usually not more than 15%, the average pore size was 10-30 microns. The pore size increases with porosity reduce. Triboengineering properties of coatings obtained by MAO-technology were studied in an equipment [14], which simulates a radial magnetic liquid sliding bearings operating under boundary lubrication ferrofluid oil. The shaft was made of hardened magnet steel U8A, which had a surface microhardness about 7-8 GPa. On inner surfaces of bushing test samples the cover was produced by microarc oxidation using an axial cathode. Ferrofluid oil viscosity was determined by a rotational viscosimeter without applying a magnetic field. 3. Experimental results and discussion Table 1 shows the main test results on the effect of additional technological operations on ferrofluid oil properties. There are lubrication properties of the original oil and oil without non-adsorbed on surfactant stabilizer molecules dispersed particles and large magnetic assemblies. The numerical values in this table reflect the influence on friction and wear of a surfactant stabilizer and abrasive agglomerates, which are in excess in a ferrofluid oil dispersed medium. Table 1. Lubricating properties of the original and modified ferrofluid oil. Ferrofluid oil No.

Saturation intensity, kA/m

Viscosity, Pas

Friction coefficient

Wear scar diameter, mm

1.

Original oil

31

0,13

0,25

0,68

2.

Oil after magnetic separation

26

0,08

0,20

0,42

3.

Oil after dispersed medium change

29

0,09

0,19

0,43

4.

Oil after combined treatment

27

0,08

0,17

0,38

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All types of additional processing of ferrofluid oil give a positive result; friction and wear characteristics decrease significantly. At the same time, oil viscosity also reduces due to reducing concentration of a magnetic phase and a free surfactant. It has a positive impact on the operating capacity of magnetic tribounits, particularly tribounits that operate at high sliding velocities [14-17]. A slight decrease in ferrofluid oil magnetization after further processing steps does not significantly affect operation of tribounits. Thus, not exactly standard processing operations significantly improved ferrofluid oil lubricating properties. The friction coefficient of surfaces lubricated by ferrofluid oil dropped almost by 35%, wear dropped by almost 45% after removal of a free surfactant stabilizer and large agglomerates from their composition. Our previous research on ferrofluid oil properties [6,18-20] show that wear rate significantly reduces as a result of reducing the mechanochemical and abrasion oil activity in relation to tribosurfaces. Frictional force reduces due to decrease in non-elastic deformation processes at the surface of contacted materials. Table 2. Tribolengineering characteristics of bearings with different friction pair materials. Friction materials

U8A–BrAZh9-4

U8A–LZhMts 591-1

U8A– 12Kh18N9T

U8A–corrund

U8A–poliamide

Friction conditions

Frictional properties

Contact pressure, MPa

Sliding speed, m/s

Friction coefficient, f

Wear rate, .10 -9

1

0,6

0,11

0,9

3

1,1

0,11

3,1

5

1,5

0,12

4,2

1

0,6

0,12

1,2

3

1,1

0,13

3,9

5

1,5

0,13

5,6

1

0,6

0,10

0,8

3

1,1

0,15

1,7

5

1,5

0,13

3,4

1

0,6

0,07

0,05

3

1,1

0,09

0,18

5

1,5

0,09

0,25

1

0,6

0,05

1,4

Table 2 shows experimental data on triboproperties of the friction pair steel–MAO-coating. For example, the table presents similar properties of bearing with their bushings made of traditional non-magnetic materials. The data in Table 2 show that the friction pair of steel–MAO-coating has higher triboproperties at various loadspeed friction conditions. When loads are low the friction pair of steel–polyamide lubricated by ferrofluid oil has expectedly low friction, but poor wear resistance. Obviously, this is a result of not only poor abrasion resistance of a polymeric material, but also heavy wear of a shaft material by charged magnetite particles in a polymer. The above information leads to the conclusion that the most promising way is to make non-magnetic elements triboconjunctions from aluminum alloy and coat the surface with solid ceramic coating by the microarc oxidation method. Thus, all studied technological methods of ferrofluid oil and friction material modification lead to a significant increase of ferrofluid oil lubricating action in magnetic liquid tribounits. However, it should be noted that the use of these technological methods, which are aimed at improving ferrofluid oil composition and structure, increases the cost of ferrofluid oil. It also complicates a hardware design of a ferrofluid oil production line and therefore their use is mainly suitable for oils intended for lubrication of critical components operating under a boundary lubrication regime.

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