EFFECT OF DISPERSING LITHIUM GREASE BY ALUMINUM OXIDE NANOPARTICLES AND CARBON NANOTUBES

EFFECT OF DISPERSING LITHIUM GREASE BY ALUMINUM OXIDE NANOPARTICLES AND CARBON NANOTUBES

Badran A.¹, EL-Abden S. Z.¹ Ali W. Y.¹ and Elzayady N.²

¹Department of Production Engineering and Mechanical Design, Faculty of Engineering,

Minia University, El-Minia, EGYPT.

²Mechanical and Aerospace Department, Institute of Aviation Engineering and Technology, Giza, EGYPT.

ABSTRACT

The effect of dispersing lithium grease by aluminum oxide (Al₂O₃) nanoparticles and carbon nanotubes (CNT) on the lubricating properties is discussed. Friction coefficient and wear displayed by the reciprocating sliding of bearing steel ball on aluminum (Al) sheet lubricated by lithium grease dispersed by Al₂O₃ nanoparticles and CNT.

It was found that friction coefficient displayed by the grease dispersed by CNT displayed the highest values followed by Al₂O₃, while CNT/Al₂O₃ showed the lowest values. Minimum values were observed at 0.4 – 0.6 wt. % of nanomaterial content. Minimum wear values were displayed by grease dispersed by CNT/Al₂O₃of 0.4 wt. % content. Further increase in Al₂O₃ content significantly increased wear due to their abrasive action. It was observed that combination of CNT/Al₂O₃ significantly reduced of friction and wear.

That can be explained on the bases that agglomeration of CNT increased their layers and shear stress. Al₂O₃ showed relatively lower increase in friction due to the ball bearing effect of the nanoparticles. The enhancing effect of CNT/Al₂O₃ may be attributed to the easy rolling of Al₂O₃ nanoparticles by the help of the low shear CNT.

KEYWORDS

Aluminum oxide nanoparticles, carbon nanotubes, lithium grease.

INTRODUCTION

Solid lubricants are extensively used to increase the efficiency of greases in different application. Polymeric particles dispersed in grease were proposed, [1, 2]. Their function is to adhere into the sliding surfaces, where the adhesion force depends on their deformed contact area that increases for soft polymers. Relatively harder polymers roll on the sliding surfaces decreasing friction and. Intensive care should be considered in selecting the solid lubricants suitable for dispersing greases. Rolling contact wear is familiar for rolling bearings, [3 - 5]. Increasing load carrying capacity of journal bearing running under mixed lubricating condition is to apply extreme pressure (EP) additives. Polytetrafluoroethylene (PTFE) was dispersing grease.

Type of the lubricants mainly influences the lubrication of sliding surfaces, [6 - 9]. In metal forming, the relative movement between the tools and the work piece is accompanied by friction that controls the surface quality. Applying the proper lubricants can reduce the drawbacks of the friction. Friction was measured by sensors during metal forming process, [10 – 12]. It was found that engineering materials are sensitive to the contact pressure. The possibilities to evaluate friction were discussed, [13, 14]. Recently, it was found that friction coefficient displayed by the grease dispersed by multiwall carbon nanotubes (MWCNT) showed lower values than that dispersed by silica nanoparticles, [15]. Wear resistance provided by silica nanoparticles was superior to that observed for MWCNT.

The present work discusses the effect of dispersing lithium grease by Al₂O₃ nanoparticles and CNT on friction coefficient and wear displayed by the sliding of bearing steel ball on aluminum sheet.

EXPERIMENTAL

The test rig used in the present experiments consists of bearing steel ball sliding on aluminum sheet in multiple passes under different load values, Figs. 1 and 2. Experiments were carried out to investigate the effect of the tested nanomaterials dispersing lithium grease on friction coefficient and wear. Weights of 2, 4, 6, 8, 10 and 12 N were vertically applied. Wear scar width was measured using optical microscope of ± 0.01 mm accuracy. The test was conducted manually under dry condition at room temperature. The stroke was 20 mm and repeated 10 times.

Fig. 1 Arrangement of the test rig.

Fig. 2 Details of the test process.

The dispersing additives used were Al₂O₃ nanoparticles of 30 – 50 ηm particle size and CNT of 10 - 30 µm length. The nanomaterials were added in different contents of 0.2, 0.4, 0.6, 0.8 and 1.0 wt. %.

RESULTS AND DISCUSSION

Friction coefficient displayed by the grease dispersed by CNT displayed the highest values followed by Al₂O₃, while CNT/Al₂O₃ dispersing the grease showed the lowest values, Fig. 3. Friction coefficient slightly decreased down to minimum then increased with increasing nanomaterial content. Minimum values were observed at 0.4 – 0.6 wt. % of nanomaterial content.  Further increase in nanomaterial content increased friction coefficient up to values higher than that recorded for grease free of nanomaterial. That behavior was more pronounced for CNT. It seems that agglomeration of CNT increased their layers and consequently shear stress increased.

Fig. 3 Friction coefficient displayed by the tested nanomaterials dispersed in grease.

Fig. 4 Wear displayed by the tested nanomaterials dispersed in grease.

Al₂O₃ showed relatively lower increase in friction due to the ball bearing effect of the nanoparticles. The enhancing effect of CNT/Al₂O₃ may be attributed to the easy rolling of Al₂O₃ nanoparticles by the help of the low shear CNT.

Wear of the test specimen measured by the wear scar width, Fig. 4, showed the same trend observed for friction. Minimum values were displayed by grease dispersed by CNT/Al₂O₃of 0.4 wt. % content. Further increase in Al₂O₃ content significantly increased wear due to their abrasive action.

Fig. 5 Effect of ball bearing mechanism.

The mechanism of ball bearing depends on the size of the particles that should be bigger than the minimum film thickness to support the load, Fig. 5. Besides, the hardness of the particles should be higher than the hardness of the two sliding surfaces. In the presence of nanoparticles, that mechanism can be valid for the very fine surfaces. In extreme pressure, nanoparticles can perform better due to their interaction with the asperities of surface roughness. Further increase of Al₂O₃ nanoparticles increased wear as result of the increased abrasiveness. They act as rolling bearings separating the two contact surfaces, where metal to metal contact is prevented. The drawback is their fracture when the load is increased and the rolling action is retarded, [16 – 20].

Fig. 6 Carbon nanotubes dispersed in grease.

The enhancement of the wear resistance offered by CNT might be attributed to the fact that CNT is quite good solid lubricant. The lubrication mechanism of CNT depends on their nature as cylinders. Their nanosize enables them entering into and adsorbing on the asperities of the rubbing surfaces, Fig. 6. It was found that their carbon content displayed significant reduction in friction due to the carbon formation on the friction surface of low shear strength film that protected from further friction and wear.

Fig. 7 Adhesion of Al₂O₃ into the sliding surfaces.

CNT could be adsorbed onto the sliding surfaces, Fig. 8, forming physical adsorption film, Fig. 9. They provide the surface by chemical reaction film and promote the wear resistance [21 - 23]. The nanomaterials form thin layer of high plasticity on the sliding surface. The highly plastic film works as a viscous lubricant. The results revealed the presence of a transferred film. When the film is formed from a softer material than the substrate it prevents detention and tearing in depth.

Fig. 8 Adhesion of CNT into the sliding surfaces.

Fig. 9 Formation of the hardened layers.

Fig. 10 Formation of carbon-containing compounds.

The presence of hard nanoparticles between the sliding surfaces prevented their direct contact, Fig. 10. CNT deformed under the load, while Al₂O₃ caused severe plastic deformation on the specimen that leads to the increase in the scratch depth, Fig. 11. Al₂O₃ nanoparticles act such as ball bearings sliding and rolling on the sliding surfaces accompanied by a polishing effect. The ball bearing effect decreased the formation of the transferred layer, [27 - 30]. It was concluded that the friction of Al₂O₃ nanoparticles reduces the wear rate and enhances the morphology of the worn surface. The ball bearing mechanism offers wear and friction reduction by hard nanoparticles whilst the Al₂O₃ nanoparticles roll between the rubbing surfaces. In addition to that, Al₂O₃ nanoparticles polish the asperities and penetrate the contact area devolving the morphology of the rough surface.

Fig. 11 CNT and Al₂O₃ dispersed in grease.

In the present experiments, the friction coefficient and wear of CNT/Al₂O₃ nanoparticles were studied. It is expected that the surface is adhered by film of carbon-containing compounds. It seems that CNT/Al₂O₃ are transferred onto the surface. Based on that, nanoparticles change sliding friction into a combination of sliding friction as result of CNT and rolling friction because of Al₂O₃ nanoparticles.

The mechanism of action of CNT/Al₂O₃ depend on the fact that they enter the friction surfaces and form the third body that bear the shear force acting on the surface asperities. CNT dispersed in lithium is absorbed onto the sliding surface. They form protective film and prevent the direct contact between the asperities of the two surfaces. Besides, their relatively low shear strength causes interlayer sliding of relatively low friction. It is known that agglomeration of nanoparticles increases friction due to the reduced shear and ball bearing effects.

The shape of nanomaterial influences its performance in lubrication process. This can be explained on the bases of the nature of the contact between the nanoparticles and the contact area. Spherical nanoparticles have high load carrying capacity and EP properties due to their ball bearing effect, [31]. They have point contact with the sliding surfaces, while the line contact is resulted from interaction of CNT with the sliding surfaces. The morphology of nanoparticles is much affected by their internal nanostructure, [32]. The structure of CNT has a tendency to form tribo-films on the sliding surfaces and can withstand severe working conditions. It can be concluded that combination of CNT/Al₂O₃ results in significant improvement in the reduction of friction and wear. It seems that the agglomeration of nanoparticles decreased allowing the nanomaterial to behave in efficient way. 

CONCLUSIONS

1. Friction coefficient displayed by the grease dispersed by CNT displayed the highest values followed by Al₂O₃, while CNT/Al₂O₃ dispersing the grease showed the lowest values.

2. Friction coefficient slightly decreased down to minimum then increased with increasing nanomaterial content. Minimum values were observed at 0.4 – 0.6 wt. % of nanomaterial content. 

3. Minimum wear values were displayed by grease dispersed by CNT/Al₂O₃of 0.4 wt. % content. Further increase in Al₂O₃ content significantly increased wear due to their abrasive action.

4. The lubrication mechanism of CNT depends on the carbon formation of low shear strength film on the friction surface that protected from further friction and wear, while Al₂O₃ nanoparticles act as ball bearing and offer wear and friction reduction, where Al₂O₃ nanoparticles roll between the rubbing surfaces. Besides, Al₂O₃ nanoparticles polish the asperities and penetrate the contact area improving the morphology of the rough surface.

5. Further increase of Al₂O₃ nanoparticles increased wear as result of the increased abrasion.

6. Combination of CNT/Al₂O₃ significantly reduced friction and wear.

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