Aus der Praxis für die Praxis Introduction Many efforts have been exerted to introduce new selflubricating polymeric materials for bearing applications, where external lubricant such as oil or grease can be excluded and the design can be simplified and maintenance cost can be reduced. Polymeric composites consisting of polyamide (PA6) filled by different types of vegetables oils such as (almond oil, camphor oil, castor oil, cress oil, flax seed oil, black seed oil, lettuce oil, olive oil, sesame oil, and sun flower oil) in concentration up to 10 wt.%, were tested, [1]. It was found that, as the oil content increased friction coefficient decreased. It seems that friction decrease was displayed due to oil transfer from the specimen to the counterface forming a thin layer which was responsible for the friction decrease. The minimum value of friction coefficient (0.15) was observed for flax seed oil specimens, at 10 wt.% oil content and 30 N normal load. Proposed polymeric composites consisting of high density polyethylene (PE), polypropylene (PP) and polystyrene (PS) and filled by fibres of polytetrafluoroethylene (PTFE) in concentration up to 25 wt.% as well as different types of vegetables oils such as corn oil, olive oil, paraffin oil, glycerin oil, castor oil and sun flower oil in concentration up to 10 wt.% were tested, [2]. PP composites filled by corn oil showed slight friction increase. Besides, friction coefficient displayed by PS and PE specimens filled by glycerin oil decreased with increasing oil content, while friction coefficient displayed by PP specimens showed consistent trend. It was noted that, PE filled with 7.5 % glycerin oil and 20 wt.% PTFE displayed the minimum value of friction coefficient (0.07). This friction coefficient values recommend those composites to be used as bearing materials. PE filled by glycerin oil displayed relatively lower friction values due its common known good lubricating property. PP composites showed the lowest wear values. Among the self-lubricating materials the so-called engineering polymers have increasing importance, [4]. Dry sliding and lubricated friction and wear behaviors of polyamide (PA) and ultra-high molecular weight high density polyethylene (UHMWPE) blend were studied, [2]. It was observed that, PA specimen demonstrated highest friction coefficient, while UHMWPE displayed the lowest in both dry-sliding and lubricated sliding test. The friction of PA could be sufficiently decreased by blending with UHMWPE. The friction and wear properties of polyamide 66 (PA66), polyphenylene sulfide (PPS) and polytetrafluoroethylene (PTFE) sliding against themselves under dry sliding and oil-lubricated conditions were studied, [5]. Experimental results showed that friction properties of the three sliding combina- Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 55 * Ing. Ayman Solyman Ibrahim Prof. Dr. Ing. Medhat Ibrahim Khashaba Prof. Dr. Ing. Waheed Yosry Ali Faculty of Engineering, Minia University P. N. 61111, El-Minia, EGYPT Friction of High Density Polyethylene Filled by Vegetable Oils A. S. Ibrahim, M. I. Khashaba, W. Y. Ali* The aim of the present work is to introduce new selflubricating polymeric materials for bearing applications. The proposed polymeric composites are consisting of high density polyethylene (HDPE) filled by different types of vegetables oils such as almond oil, camphor oil, castor oil, cress oil, flax seed oil, black seed oil, lettuce oil, olive oil, sesame oil and sun flower oil in content up to 10 wt.%. The friction coefficient and wear resistance displayed by the proposed composites are investigated at different values of applied load when sliding against steel. Experiments showed drastic friction decrease for HDPE filled by oil up to 1 wt.% followed by slight decrease with further increase of oil content. The oils that displayed the lowest values of friction coefficient are ranked as follows: lettuce, olive, sun flower, flax seed, sesame, black seed, cress, camphor, almond, and castor oils. The friction decrease was attributed to the presence of pores inside the HDPE matrix filled by oil which during friction leaked out to the sliding surface forming oil film. The ranking of the tested oils depended on their lubricating properties which were influenced by the adhesion of their molecules to the sliding surfaces, where their adhesion depends on their polarity. Polar molecules will form multilayers, which strengthen the adhesion of oil into the contact surfaces. Polarity of oil molecules influenced the thickness of oil film. Besides, friction coefficient decreased with increasing normal load. Keywords Friction coefficient, high density polyethylene, vegetables oils, carbon steel Abstract T+S_4_16 02.06.16 12: 27 Seite 55 Aus der Praxis für die Praxis tions could be greatly improved by oil lubrication, where the antiwear properties of PTFE and PPS were improved by oil lubrication. An investigation of the tribology of three thermoplastic polymer composites based on polytetrafluorethylene, polyethylene terephthalate and polyamide, that are considered to be used as sliding bearings in nanopositioning, was carried out, [6]. It was observed that, the high Young’s modulus was found to be beneficial for the formation of a thin transfer film responsible of a low and stable coefficient of friction. Novel poly(phthalazinone ether sulfone ketone) (PPESK) resins have become of great interest in applications such as bearing and slider materials. Dry sliding wear, of polytetrafluoroethylene (PTFE) and graphite-filled PPESK composites against polished steel counter parts, was investigated, [7]. It was found that friction coefficient and wear rate of the PPESK composites decreased gradually with addition of fillers. Results showed the excellent dry tribological characteristics of the modified UHMWPE/ steel rubbing pair compared with the pure UHMWPE/ steel rubbing pair, [8]. It was suggested that the modified UHMWPE could be used in dry friction conditions with a relatively high sliding velocity. The influence of sea water composition on the tribological behavior of PTFE was studied, [9]. Results show that the friction process in sea water was relatively stable, the friction coefficient and the wear rate of PTFE were slightly lower and a little larger than those in distilled water, respectively. Wear and friction simulator with metal cylinder on flat polymer was developed to analyze the tribological behavior of tibial insert used in Total Knee Replacement (TKR), [10]. Tests were first carried out with polymethyl methacrylate polymer (PMMA) for which the tribological behavior has been well developed. High density polyethylene (HDPE) was also characterized. In fact HDPE has been firstly used in the tibial insert before the use of ultrahigh molecular weight polyethylene (UHMWPE). High performance engineering polymers ensure desired properties for journal bearings and give good tribological results, [11]. Tribological behaviors of polymer based PE, PA, POM, PTFE, and Bakelite bearings were investigated and evaluated. As a result, the highest wear resistance had occurred in PA and POM bearings, [12]. It was observed that the average friction coefficient showed that the PA46 + 15 % aramid fibres generally had the lowest values compared to the other types of samples. The friction and wear properties of the polyimide (PI) composites filled with differently surface-treated carbon fibers (20 vol.%), sliding against GCr15 steel under oillubricated condition, were investigated, [13]. Experimental results revealed that the treatment largely reduced the friction and wear of CF reinforced PI (CF/ PI) composites. The friction and wear behavior of carbon nanotube reinforced polyamide 6 (PA6/ CNT) composites under dry sliding and water lubricated condition was comparatively investigated, [14]. The results showed that CNTs could improve the wear resistance and reduce the friction coefficient of PA6 considerably under both sliding conditions, due to the effective reinforcing and self-lubricating effects of CNTs on the PA6 matrix. The friction and wear behaviour for polyoxymethylene homopolymers (POM-H) and polyethylene terephthalate with teflon additives (PET/ PTFE) was compared, [15]. An extensive investigation, of polymer gear (acetal and nylon) friction and wear behavior, was carried out, [16]. It was found that the surface temperature was the dominant factor influencing the wear rate and an initial relationship between gear surface temperature and gear load capacity has been established and further developed. The effects of resin content on the wear of woven roving glass fibreepoxy resin and glass fibre-polyester resin composite materials were examined, [17]. Glass fibre-epoxy resin composites generally showed higher strength and minimum wear when compared with glass fibre-polyester resin composites materials. The better adherence of a polymer transfer film onto a steel counterface was explained by higher attractive forces resulting from its high surface energy, while there was little adherence on stainless steel counterfaces in accordance with its lower surface energy and lower friction, [18]. After sliding of UHMWPE/ carbon, no wear debris was observed as its higher toughness allows for tearing of the surface without particle detachment. In the present work, the friction coefficient displayed by HDPE filled by different types of vegetables oils in content up to 10 wt.% when sliding against steel was 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 Figure 1. The pin made of the tested composites is held in the specimen holder that fastened to the loading lever. Friction force can be measured by means of the load cell, fastened to the rotating carbon steel disc of 3.2 µm, Ra surface roughness. Friction tests were carried out under constant sliding velocity of 2.0 m/ s. Friction test was carried out under normal applied loads of 10, 20 and 30 N and lasted for 600 seconds. Friction coefficient was calculated as the average of the friction force divided by normal load. All measurements were performed at 25 ± 5 ºC and 30 ± 10 % humidity. The test specimen, in the form of a cylinder, was 10 mm diameter and 30 mm height. The diameter was reduced to 5 mm to contact the friction disc, Figure 2. The polymer used in the present work was high density polyethylene (HDPE). The polymer granulates, of 30 - 50 µm particle size, were mixed with different types of veget- 56 Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 T+S_4_16 02.06.16 12: 27 Seite 56 mal load. At 10 N load, friction coefficient slightly decreased with increasing oil content. At 20 and 30 N load, friction coefficient drastically decreased to minimum then showed slight increase with increasing oil content. It seems that friction was influenced by the HDPE transfer into steel surface, where the contact was partially polymer/ polymer. This behavior indicated that the adhesion of the molecules of almond oil was not enough strong to prevent polymer transfer into steel surface. The minimum value of friction coefficient (0.23) was observed at 1 wt.% oil content and 30 N load. Aus der Praxis für die Praxis ables oils in contents of 1, 2, 4, 6, 8, and 10 wt.%. The vegetables oils were almond, camphor, castor, cress, flax seed, black seed, lettuce, olive, sesame and sun flower oils. The mixture was compressed in the die and heated up to 110 °C by using hydraulic jack, Figure 3. Results and Discussion Friction coefficient displayed by HDPE filled by almond oil specimens, Figure 4, decreased with increasing nor- Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 57 Figure 2: Dimensions of the tested composites Figure 3: Preparation of the tested composites 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 4: Friction coefficient displayed by HDPE filled by almond oil 0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 12 Friction Coecient Oil Content, wt. % 10 N 20 N 30 N Figure 5: Friction coefficient displayed by HDPE filled by camphor oil Figure 1: Arrangement of friction test rig Friction coefficient displayed by HDPE filled by camphor oil slightly decreased with increasing oil content, Figure 5. It seems that friction decrease was displayed due to the oil transfer from the specimen to the counterface forming thin layer that was responsible for the friction decrease. The photomicrograph of the HDPE matrix, Figure 6, shows that the oil was trapped in pores after solidification of the HDPE. Those pores were working as reservoirs feeding oil into HDPE surface. A sketch is shown in Figure 7 illustrates the formation of oil film on the sliding surfaces. The film was fed by the oil stored inside the pores stored in the HDPE matrix. The strong adhesion of the oil molecules, known for vegetables oils, T+S_4_16 02.06.16 12: 27 Seite 57 Aus der Praxis für die Praxis experienced boundary lubricating film in which a low shear interfacial layer was formed on the sliding surfaces and easily removed by the shear instead of the contacting asperities. Figure 8 shows the relationship between friction coefficient displayed by HDPE filled by castor oil and oil content. It can be seen that friction coefficient decreased with increasing castor oil content. Values of friction coefficient were approaching that observed for almond and camphor oils filled composites. It is well known that when HDPE and steel surfaces are pressed or rubbed together, the surface of steel usually becomes positively charged, while HDPE becomes negatively charged due to triboelectrification. The intensity of the generated charge depends on the pressure and velocity of rubbing. Once charged, the two surfaces attract each other in dry contact. In the presence of polar molecules of vegetables oils, steel and HDPE attract the oil to their surfaces forming multilayers of the oil molecules separating the two materials, Figures 9 and 10. This behavior would affect the contact and change it from dry 58 Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 Figure 10: Details of HDPE/ steel contact 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 11: Friction coefficient displayed by HDPE filled by cress oil Figure 6: Photomicrograph of the PE matrix filled by oil Figure 7: Sliding of PE filled by vegetables oils against steel 0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 12 Friction Coecient Oil Content, wt. % 10 N 20 N 30 N Figure 8: Friction coefficient displayed by HDPE filled by castor oil Figure 9: Illustration of HDPE/ steel contact T+S_4_16 02.06.16 12: 27 Seite 58 Aus der Praxis für die Praxis to mixed lubrication. The oil molecules are stuck strongly to the charged steel and HDPE surfaces due to the electric bond. Besides, the generation of the electric static charges on the sliding surfaces homogeneously distributes the oil molecules over the contact area. Based on that behavior, friction coefficient decreased in values depending on the adhesion force of the molecules to the sliding surfaces which control the thickness of the oil film formed during sliding. HDPE filled by cress oil showed slight friction decrease, Figure 11, where friction coefficient decreased with increasing cress oil content. The load had significant effect in reducing friction coefficient, where it drastically decreased with increasing the load. The effect of filling HDPE by flax seed oil on friction coefficient is shown in Figure 12. At 10 N normal load the effect of oil content on friction coefficient was insignificant. At 20 N and 30 N normal loads, friction coefficient slightly decreased with increasing flax seed oil content, while friction values displayed at 10 N showed consistent values. It seems that at relatively higher loads, HDPE was compressed squeezing the oil into the surface. Slight friction decrease was observed for HDPE filled by black seed oil specimens with increasing oil content, Figure 13. The minimum value of friction coefficient (0.25) was observed at 10 wt.% oil and 30 N normal load. The dependency of friction coefficient on the load can be explained on the basis that as the load increased, the trapped oil inside the pores was forced to be fed to the surface forming an oil film on the contact area leading to significant friction decrease. HDPE filled by lettuce oil specimens displayed relatively lower friction values than that observed for the above mentioned oils, Figure 14, which confirmed the good lubricating properties of lettuce oil due to the relatively stronger adhesion of their molecules into the sliding surfaces. The same trend was observed for the friction coefficient displayed by HDPE filled by olive oil, Figure 15. Like all the tested composites, friction coefficient drastically decreased for oil content up to 1 wt.% then consistent trend was prevailing for further oil increase. The decrease in friction might be from the presence of pores inside the HDPE matrix filled by oil which during friction leaked out to the sliding surface forming oil film, which would prevent HDPE transfer into the steel counterface. Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 59 0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 12 Friction Coecient Oil Content, wt. % 10 N 20 N 30 N Figure 12: Friction coefficient displayed by HDPE filled by flax seed oil 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 13: Friction coefficient displayed by HDPE filled by black seed oil 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 14: Friction coefficient displayed by HDPE filled by lettuce oil 0 0.1 0.2 0.3 0.4 0.5 0 2 4 6 8 10 12 Friction Coecient Oil Content, wt. % 10 N 20 N 30 N Figure 15: Friction coefficient displayed by HDPE filled by olive oil T+S_4_16 02.06.16 12: 27 Seite 59 Aus der Praxis für die Praxis Friction coefficient of HDPE filled by sesame oil showed relatively higher values, Figure 16. It is seen that, increasing oil content was insignificant. The minimum value of friction coefficient (0.24) was detected at 10 wt.% oil content and 30 N normal load. Friction coefficient of HDPE filled by sesame oil showed relatively higher values, Figure 16. It is seen that, increasing oil content was insignificant. The minimum value of friction coefficient (0.24) was detected at 10 wt.% oil content and 30 N load. Slight friction decrease was observed for HDPE filled by sun flower oil specimens, Figure 17. At 10 and 20 N the friction coefficient slightly decreased with increasing sun flower oil content. The minimum value of friction coefficient (0.23) was observed at 4 wt.% oil content and 30 N load. Conclusions 1. The tested composites showed drastic friction decrease when the oil was added to HDPE matrix up to 1 wt.%, followed by slight decrease with further oil increase. 2. Friction coefficient displayed by HDPE filled by almond oil specimens decreased with increasing almond oil content at 10 N load, while at 20 and 30 N load, friction coefficient decreased to minimum then slightly increased with increasing oil content. 3. HDPE filled by lettuce oil specimens displayed relatively lower friction values than that observed for the tested oils which confirmed the good lubricating properties of lettuce oil. It seems that the lubricating properties of lettuce oil are due to the relatively stronger adhesion of the oil molecules into the sliding surfaces. The same trend was observed for the friction coefficient displayed by HDPE filled by olive oil. References [1] Ibrahim A. S., Khashaba M. I., Ali W. Y., “Friction coefficient displayed by polyamide filled by vegetables oils”, Journal of the Egyptian Society of Tribology, Vol. 11, No. 3, July 2014, pp. 34 - 44, (2014). [2] Khashaba M. I., Eatemad H. S., Youssef M. M., Ali Y. 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H., Kukureka S. N., “The wear and friction of polyamide 46 and polyamide 46/ aramid-fibre composites 60 Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 16: Friction coefficient displayed by HDPE filled by sesame oil 0 0 . 1 0 . 2 0 . 3 0 . 4 0 . 5 0 2 4 6 8 1 0 1 2 Friction Coecient O i l C o n t e n t , w t . % 1 0 N 2 0 N 3 0 N Figure 17: Friction coefficient displayed by HDPE filled by sun flower oil T+S_4_16 02.06.16 12: 27 Seite 60 Aus der Praxis für die Praxis in sliding-rolling contact”, Wear, Vol. 267, No. 1 - 4, 15 June 2009, pp. 669-678, (2009). [13] Cheng X. H., “Friction and wear properties of surfacetreated carbon fiber-reinforced thermoplastic polyimide composites under oil-lubricated condition”, “Materials Chemistry and Physics”, Vol. 108, No. 1, 15 March 2008, pp. 67 - 72, (2008). [14] Meng H., Sui G. X., Xie G. Y., Yang R., “Friction and wear behavior of carbon nanotubes reinforced polyamide 6 composites under dry sliding and water lubricated condition”, Composites Science and Technology, Vol. 69, No. 5, April 2009, pp. 606 - 611, (2009). [15] Samyn P., Schoukens G., “Experimental extrapolation model for friction and wear of polymers on different testing scales”, International Journal of Mechanical Sciences, Vol. 50, No. 9, September 2008, pp. 1390 - 1403, (2008). [16] Mao K., Li W., Hooke C. J., Walton D., “Friction and wear behaviour of acetal and nylon gears”, Wear, Vol. 267, No. 1 - 4, 15 June 2009, pp. 639 - 645, (2009). [17] Nirmal U., Yousif B. F., Rilling D., Brevern P. V., “An experimental investigation of wear of glass fibre-epoxy resin and glass fibre-polyester resin composite materials”, Wear, 16 February 2010, (2010). [18] Samyn P., De Baets P., Schoukens G., Van Peteghem A. P., “Large-scale tests on friction and wear of engineering polymers for material selection in highly loaded sliding systems”, Materials and Design, Vol. 27, pp. 535 - 555, (2006). Tribologie + Schmierungstechnik 63. Jahrgang 4/ 2016 61 expert verlag GmbH: Wankelstr. 13, 71272 Renningen Postfach 20 20, 71268 Renningen Tel. (0 71 59) 92 65 - 0, Fax (0 71 59) 92 65 -20 E-Mail expert@expertverlag.de Vereinigte Volksbank AG, Sindelfingen BIC GENODES1 BBV, IBAN DE51 6039 0000 0032 9460 07 Postbank Stuttgart BIC PBNKDEFF, IBAN DE87 6001 0070 0022 5467 07 USt.-IdNr. DE 145162062 Anzeigen: Sigrid Hackenberg, expert verlag Tel. (0 71 59) 92 65 -13, Fax (0 71 59) 92 65-20 E-Mail anzeigen@expertverlag.de Informationen und Mediendaten senden wir Ihnen gerne zu. Vertrieb: Rainer Paulsen, expert verlag Tel. (0 71 59) 92 65 -16, Fax (0 71 59) 92 65-20 E-Mail paulsen@expertverlag.de Die zweimonatlich erscheinende Zeitschrift kostet bei Vorauszahlung im Jahresvorzugspreis für incl. 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