INFLUENCE OF MAGNETIC FIELD ON THE ACTION MECHANISM OF LUBRICANT ADDITIVES

Zaini H., Alahmadi A., Ali W. Y. and Abdel-Sattar S.

          Faculty of Engineering, Taif University, Al-Taif, SAUDI ARABIA.

ABSTRACT

The present work investigates the influence of magnetic field on the friction coefficient displayed by the sliding of steel pin on aluminium disc lubricated by paraffin oil and dispersed by different lubricant additives such as zinc dialkyldithiophosphates (ZDDP), molybdenum disulphide (MoS₂), heteropolar organic based additive (CMOC), graphite (C), detergent additive (calcium sulphonate) (DA), polytetrafluroethylene (PTFE) and polymethyl methacrylate  (PMMA). Aluminium was used as friction counterface to reduce the magnetic force acting on the contact surfaces when the magnetic field was applying.

The present experiments showed that, for surfaces lubricated by paraffin oils free of additives friction coefficient increased with increasing applied load. As the magnetic field increased friction coefficient increased. In condition of application of magnetic field it was found that when the paraffin oil was dispersed by ZDDP, MoS₂, DA and PTFE friction coefficient increased, while COMC, C and PMMA showed significant decrease in friction coefficient. Besides, the lowest values of friction coefficient were observed for PTFE particles dispersed in the oil.

KEYWORDS

Magnetic field, oil additives, polytetrafluroethylene, polymethyl methacrylate, molybdenum disulphide, zinc dialkyldithiophosphates, graphite, friction coefficient, steel and aluminium.

INTRODUCTION

There is an increasing demand to save the frictional losses in various mechanical and electronic devices, where magnetic field are applied. It is important to consider the friction and wear of their components and to understand the influence of magnetic field on their performance in order to prevent premature failure and to achieve higher energy efficiency. The effect of magnetic field on friction and wear processes has been studied by a number of researchers. The influence of magnetic field on the friction coefficient displayed by sliding of steel pin on steel disc lubricated by paraffin oil and dispersed by different lubricant additives such as zinc dialkyldithiophosphates (ZDDP), molybdenum disulphide (MoS₂), heteropolar organic based additive (CMOC), graphite (C), detergent additive (calcium sulphonate) (DA), polytetrafluroethylene (PTFE) and polymethyl methacrylate  (PMMA) was discussed, [1]. It was observed a significant transformation in tool wear tests with the magnetic field, [2 - 7]. Abrasive wear without magnetic fields was transformed, in the presence of magnetic field, to fracture due to the shift of shear stress to the subsurface region. Also, a decrease in wear, a lower friction coefficient, increase in hardness of magnetised steel surface and a rise in temperature of rubbing surfaces were observed. In magnetic fields, strongly oxidised wear particles will pose a serious problem for the contact due to the paramagnetism of oxygen. Accelerated oxidation does affect not only wear particles but also contacting surfaces. More specifically, highly oxidised wear particles, affected by magnetic force operating between contacting surfaces, act both as abrasive and as lubricating agents depending on their conditions.

It was observed that,for sliding of steel pin against oil lubricated brass discs, magnetic field decreased friction coefficient for all the tested oils, [8]. Dispersing oil by polyethylene (PE) particles significantly increased friction coefficient. A drastic reduction of friction coefficient was observed for olive, castor and almonds oils, when dispersing the tested oils by polyamide (PA) particles. Sliding of steel pin on oil lubricated aluminium disc caused significant friction increase for all the tested oils. Drastic friction reduction was observed for castor, almonds, jasmine and camomile oils when dispersed by PE particles. Dispersing vegetables oils by PA particles showed relatively lower friction coefficient for olive and castor oils, while corn, almonds, camomile and jasmine oils showed relatively higher friction values. Sliding of steel pin on oil lubricated steel disc showed the highest values of friction coefficient. Dispersing the tested oils by PE particles did not decrease friction coefficient. Jasmine oil displayed relatively lower friction than the other tested oils.

The effect of magnetic field on the friction coefficient displayed by  sliding of steel pin against steel disc lubricated by paraffin oil and dispersed by different lubricants additives was investigated, [9].The experiments showed that, friction coefficient increased as the magnetic field increased due to the increase of the normal load caused by the magnetic force.The performance of ZDDP and Mo S₂ additives was not affected by the application of magnetic field. Besides, it was observed that magnetic field much affected the performance of oil dispersed by additives of electrical properties such as CMOC, DA and PTFE particles. The same trend of friction decrease was observed for PMMA particles dispersed in oil.

The friction and wear of polyethylene sliding against steel in the presence of magnetic field was investigated, [10]. It was found that, application of magnetic field decreases friction coefficient at dry sliding due to its influence to decrease the adherence of polyethylene worn particles into the steel counterface. Besides, the magnetic field favors the formation of oxide film on the contact surface, where it plays a protective role in dry friction, modifies the friction and changes wear from severe wear to mild. Lubricating the steel surface by oils caused significant reduction in friction coefficient, where the maximum reduction was displayed by paraffin followed by glycerine, almond, jasmine, corn, castor, olive and sun flower oils. Besides, wear of polyethylene test specimens shows relative decrease in the presence of magnetic field.

The effect of the magnetic field on the friction and wear of steel and brass sheets scratched by a steel insert at dry, lubricated by vegetable oils and dispersed by polymeric particles such as high density polyethylene (HDPE), polyamide (PA6) and polymethyl methacrylate (PMMA) was investigated, [11, 12]. Based on the experimental observations, it was found that olive oil displayed the lowest values of friction coefficient followed by castor oil, almonds, corn, chamomile and jasmine oils, where their polar molecules could significantly improve the wear resistance developed by their strong adsorption on the sliding surfaces. Application of magnetic field on the sliding surface caused significant friction reduction at dry sliding due to the enhanced ability of the oil molecules to orient themselves in relatively long chain adhered to the sliding surface and thus decreased the friction and wear.

The effect of applying external voltage on the sliding of copper, aluminium and polyethylene against steel surface lubricated by paraffin oil dispersed by polymeric particles such as polyethylene, polyamide and PMMA was tested, [13]. It was noticed that the friction coefficient and wear were significantly influenced by the generation of electric static charge on the contact surfaces which caused an attractive force imposed to the normal load. It was found that wear was more influenced by the electric static charge than friction coefficient.

It was shown that the magnetic field had no effect on friction coefficient observed for lithium grease without additives, [14]. Grease dispersed by high density polyethylene showed friction decrease. The lowest friction reduction was observed for polymethyl methacrylate. The strong adhesion of PTFE particles into the sliding surfaces significantly increased friction coefficient. It seems that PTFE particles were adhered to surfaces of inner and outer races as well as the balls. Changing the terminal of the voltage applied to the rotating shaft showed significant friction decrease for PMMA. Viscosity of the grease decreased with increasing the voltage. Friction coefficient decreased for HDPE and PTFE. The lowest friction values  were observed at 6 volts which indicated that increasing voltage across the sliding surface could significantly decrease friction coefficient.

In the presence of magnetic field around the tribocontact and in ambient air, the contact track was covered with very fine ferromagnetic particles, [15].  Friction and wear are influenced by the presence of oxide. The increase of oxide layer on the surface, the retention of passivated particles in the contact and their refinement by grinding modify the contact rheology which transits from metal/metal contact to oxide/oxide contact. It was noticed that magnetic field acts on the ferromagnetic contact surface by modifying their electrical and electronic behaviour. It increases the electronic speed in their orbits, [16], and creates the electrical fields and the electrical currents. Those electrical currents enhance the oxidation. Moreover, dislocations in subsurface of the materials in contact are influenced by contact shear stress field [17]. The stress field presents a decreasing gradient from the Hertzian point. The dislocations displace from the region of strong stress gradient to the region of weak gradient. The existence of magnetic field around the contact facilitates the movement of dislocations due to mechanical stress, decreasing the obstacles through the rearrangement of magnetic domains. This accumulation of dislocations leads to the increase of surface hardness.

It is known that, during friction on metals or dielectric couples, part of the energy consumed turns into electrical energy. In the second part of the study, electrical phenomena induced by friction will be examined in situ. Hence, the appearance of the potential difference generated by lubricated friction will be studied.  This electrical effect leads to an embrittlement of friction surfaces and also involves a greater reactivity of surfaces with the lubricant. Because of triboelectrification, the charged surfaces can interact with each other due to the direct electrostatic forces, [18]. Since these forces are strong and effective, they contribute a major part of the adhesion force.

By applying an electric field between the rubbing surfaces, the oxidation of the rubbing surface at anode side is enhanced, and suppressed on the cathode side surface. The oxide film formed on the anode surface being harder than the bulk steel, the rubbing surface at the anode side was little worn, but it at cathode side was abrasively worn considerably. The application of an electric field, however, is considered to promote the breakdown of EHL film formed. Therefore, the effect of the application of an electric condition tested, [19]. The influence of applying electric field on the tribological behaviour of steel in a vertical magnetic field produced by an AC or DC electric current was investigated. The effect of a magnetic field on both oxidation and concentrations of dislocations on the surface is presented, [20]. Experiments show that a magnetic field applied through the sliding contact leads to decrease the wear rate.

In the present work, the effect of magnetic field on the friction coefficient displayed bysliding of steel pin against aluminium disc lubricated by paraffin oil and dispersed by different lubricants additives is investigated.

EXPERIMENTAL

Experiments were carried out using pin-on-disc wear tester. It consists of a rotary horizontal steel disc driven by variable speed motor. The details of the wear tester are shown in Fig. 1. The bearing steel pin (AISI 52100) is held in the specimen holder that fastened to the loading lever. Friction force can be measured by means of the load cell, where the pin holder and magnetic field were assembled. The aluminium disc, made of aluminium base alloy (SAE 770, Sn 6.25 %, Cu 1.0 %, Ni 1.0 %) was fastened to the rotating disc. Its surface roughness was about 0.67 µm, R_a. Test specimens were the rollers of cylindrical rolling bearing in the form of cylindrical pins of 6 mm diameter and 12 mm length. Friction and wear tests were carried out under constant sliding velocity of 2.0 m/s and 20 N applied load. Every experiment lasted 30 minutes. All measurements were performed at 25 ± 5 ºC and 30 ± 10 % humidity.

Fig. 1 Arrangement of the test rig.

Fig. 2 Application of the magnetic field.

The lubricant used in the experiments was paraffin oil (SAE 30). Fine particles of C, MoS₂ and CMOC of particle size less than 1.0 µm were dispersed into the paraffin oil at 5.0 wt. % concentration. Besides, ZDDP and detergent additive DA were used. Polymeric powder such as PTFE and PMMA of (30 – 50 µm) particle size were used as dispersant in the oil. Magnetic field was applied by a coil assembled above the pin holder of 0.2 and 0.4 mG flux intensity, Fig. 2.

RESULTS AND DISCUSSION

The friction coefficient displayed by the steel pin sliding against aluminium disc under application of magnetic field is discussed, Figs. 3 - 10. Friction coefficient slightly increased with increasing applied load, Fig. 3. Based on the values of the sliding velocity and load the contact was considered as mixed lubrication, where the contact was partially metal to metal and partially hydrodynamic film. Therefore,   friction increase might be attributed to the plastic deformation of the asperities of the contact surfaces which was responsible for increase of the contact area. Mixed lubrication occurs between boundary and hydrodynamic lubrication. The fluid film thickness is slightly greater than the surface roughness, so that there is very little asperity contact, but the surfaces are still close enough together to affect each other. In a mixed lubrication system, the surface asperities themselves can form miniature nonconformal contacts.

The effect of magnetic field on the performance of paraffin oil depends on the paraffinic molecules which are approximately linear and consequently they are more effective than other hydrocarbons in preventing solid contact. This allows for the formation and persistence of a relatively thicker film. Since the molecules are polar the opposite ends are attracted to form pairs of molecules which are subsequently incorporated into the viscous surface layer. At the interface with the substrate the attractive force of the free end of the molecules to the substrate is sufficient to firmly bond the entire layer. It seems that magnetic field increased the bond of the oil layer and disturbed the adhesion of the oil film into the interfaces. Aluminium is a paramagnetic material which is only attracted in the presence of an externally applied magnetic field. Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum. The presence of polar groups in oil structure enhanced the orientation of oil molecules to form a surface film able to inhibit metal-to-metal contact. This behaviour leads to relatively thick oil film but the adhesion between the film and aluminium substrate is weak. As a result of that, as the magnetic field increased friction coefficient slightly increased. Sliding of steel against aluminium charged the contact surfaces so that they could interact with each other due to the direct electrostatic forces. Since these forces are strong and effective, they contribute significant increase of the adhesion force.

Dispersing oil by ZDDP additive caused significant friction decrease due to its decomposition to zinc polyphosphate and a mixture of alkyl sulphides which are the precursors of the antiwear action of ZDDP, Fig. 4. The performance of ZDDP additive was not affected by application of the magnetic field. The values of friction coefficient were much higher than that displayed by sliding steel pin against aluminium disc when the oil was free of ZDDP. This behaviour confirmed that the magnetic field increased the applied load by the extra magnetic force. Friction coefficient showed relatively higher values in the presence of the magnetic field. The friction increase might be from the electrostatic charge generated from the friction of steel on aluminium which disturbed the action of magnetic filed.

MoS₂ is frequently used as thermally stable extreme-pressure agents. MoS₂ is stable up to 350 °C in air and up to 1,200 °C in a vacuum. It provides lubricity by physically forming solid films. It does not corrode substrate metals, unlike chemically reactive extreme-pressure agents such as organic sulfur compounds. When the oil was dispersed by MoS₂ friction coefficient is shown in Fig. 5.  Friction coefficient showed relatively lower values in the presence of the magnetic field. The friction decrease might be from the common good lubricating properties of MoS₂. Under the influence of moving surfaces, aggregates are disrupted and the solid MoS₂ fills in the pits and valleys, thereby reducing roughness, increasing the true area of contact and providing a reservoir of lubricant. Its drawback, as an antiwear additive dispersed in oil, is in its insolubility which causes flocculation and settling during use.

Fig. 3 Friction coefficient displayed by the sliding of steel pin against oil lubricated aluminium disc.

The effect of magnetic field on performance of MoS₂ was insignificant. Generally, friction values observed for oil dispersed by MoS₂ showed higher values than that observed for oil and oil dispersed by ZDDP. This performance will be enhanced if the particles are strongly adhered to the contact area and the shear strength of the solid lubricant is less than the adhesion to the substrate. In addition, a film of solid lubricant should be built up of sufficient thickness to cover the contact area completely, and sliding takes place between two smooth oriented layers of lamellar solid lubricant.

CMOC is a copper base additive. Its mechanism of action depends on the selective migration. Copper does not form carbides and also being soft and good heat conductor.    The mechanism of action of the CMOC additive is to keep the two contacting surfaces away from each other by forming a protecting layer on the sliding surfaces. The adherence of CMOC particles, which are smaller than 1.0 µm size, into the sliding surfaces is enough strong due to their electronic properties. The degree of adherence of solid lubricant into the sliding surfaces is responsible for the friction and wear reduction. The magnetic field drastically decreased friction coefficient, Fig. 6. It seems that the polarity of CMOC particles strengthened their adherence into the contact area. Under the action of the magnetic field the force of adhesion significantly increased causing proper surface coating which caused the friction decrease. It should be mentioned that, adhesion force was affected mainly by the electrostatic force generated by friction between steel and aluminium surfaces.

Fig. 4 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by ZDDP.

Fig. 5 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by MoS₂.

Fig. 6 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by CMOC.

Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by C is shown in Fig. 7, where significant friction increase was observed. As already mentioned, not all lamellar solids are capable of interlamellar sliding at low shear stresses. The nature of bonding between the C lamellae is a weak Van der Waals bonding acting between lamellae. The friction increase might be from the relatively high adhesion between lamellae. The lubrication mechanism of C is believed to be a result of the relatively free movement of adjacent layers in these substances.  In general, however, C films fail at lower loads and exhibit shorter lifetime than MoS₂ film. The limiting contact stresses for C are a little over half that of MoS₂. Application of magnetic field caused slight friction reduction. This behaviour is attributed to the electronic properties of C. Dispersion of C particles into the oil distributed the electric static charge on the sliding surfaces and consequently adhesion among C particles, aluminium  and steel surfaces increased.

Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by DA showed significant friction decrease, Fig. 8. It seems that the polarity of DA molecules is responsible for the decrease of friction coefficient. Application of the magnetic field might modify the orientation and adherence of the additive particles into the sliding surfaces. Slight increase in friction coefficient was caused by the magnetic field. As the magnetic field increased friction coefficient increased. The lowest friction coefficient value was 0.03 at 16 N applied load.

Fig. 7 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by graphite.

Fig. 8 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by DA.

It is well known that the magnetic field affects polar molecules, which contain ionisable groups. The observed changes in the properties of polymers are attributed to the catalytic effect of the magnetic field on the molecules.The ability of polymers to reduce friction depends on their adherence to the sliding surfaces. The friction of the polymers with the steel surfaces produces electric static charge on both the surfaces of polymers and steel. The sign andamount of the charge depends on the location of the polymeric materials and steel in the triboelectric series. When two different materials are pressed or rubbed together, the surface of one material will generally gain some electrons from the surface of the other material. The material that gains electrons has the stronger affinity for negative charge of the two materials, and that surface will be negatively charged after the materials are separated. The other material will have an equal amount of positive charge. The amount and polarity of the charge on each surface can be measured for insulating materials. The triboelectric series predict which will become positive or negative and how strong the electric charge will be.

The good lubricating properties observed for PTFE additive can be from its ability to form a layer on the sliding surfaces. The very low intermolecular forces of PTFE allow the layer to slide over the friction surface with a minimum of interaction and hence a minimum of friction. PTFE particles dispersed in the oil were much influenced by the magnetic field, Fig. 9, where friction coefficient increased with increasing the intensity of the magnetic field. The same trend of friction decrease was observed for PMMA particles dispersed in oil, Fig. 10. The values of friction coefficient were relatively higher than that observed for PTFE.

Fig. 9 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by PTFE.

Fig. 10 Friction coefficient displayed by the sliding of steel pin against aluminium disc lubricated by oil dispersed by PMMA.

Adherence of PTFE into aluminium surface might be attributed to the relatively high amount of electric static charge generated from PTFE when rubbed both aluminium and steel, where PTFE gained negative charge and steel as well as polyamide surfaces gained positive charge. Besides aluminium surface gained extra positive charge when slid on steel.

CONCLUSIONS

1. At surfaces lubricated by paraffin oils free of additives friction coefficient increased with increasing applied load. As the magnetic field increased friction coefficient increased.

2. Dispersing oil by ZDDP additive caused significant decrease of friction. Magnetic field increased friction coefficient.  

3. Dispersing MoS₂ in the lubricating oil caused significant friction decrease. Like ZDDP, magnetic field increased friction coefficient. 

4. Magnetic field decreased friction coefficient when the oil was dispersed by CMOC.

5. In the presence of C dispersing the lubricating oil friction coefficient slightly increased with increasing applied load. Application of magnetic field caused slight friction decrease.

6. Friction coefficient displayed by oil dispersed by DA showed significant friction decrease. Application of the magnetic field caused slight friction increase.

7. The lowest values of friction coefficient were observed for PTFE particles dispersed in the oil. Magnetic field significantly increased friction coefficient. Surfaces lubricated by oil dispersed by PMMA showed friction decrease with increasing the magnetic field.

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