THE INFLUENCE OF NANOPARTICLES ON THE LUBRICATING PROPERTIES OF RAPESEED OIL

2010 STLE Annual Meeting & ExhibitionMay 16-20, 2010Las Vegas, USATHE INFLUENCE OF NANOPARTICLES ON THE LUBRICATING PROPERTIES OF RAPESEED OILT. MALIAR, H. CESIULIS , VILNIAUS UNIVERSITETAS, CHEMIJOS FAKULTETAS, NAUGARDUKO G.24, LT – 03225 VILNIUS S. ACHANTA, D.DREES, FALEX TRIBOLOGY N.V., WINGEPARK 23B, ROTSELAAR B-3110, BELGIUMINTRODUCTIONNowadays, many attempts are made to solve tribological issues related to lubricated contacts by the introduction of metallic and non-metallic nanopowders as additives. The main objective related with the usageof magnetic nanoparticles like Fe, Co, Ni in the tribocontact is to minimize friction. Another hypothesis is that these particles penetrate local microcracks thereby arresting them or even forming a protective tribolayer thus controlling wear and local temperature increase (this paper should appear in the introduction). Some other theories related to tribofilm formation and rolling effects were also reported. The present paper explores the effect of Fe based micro-/nano- particle additions on the tribological properties of lubricating oils when subjectedto unidirectional and bi-directional sliding conditions. The ranking of various unmodified (Base oil, rapeseed oil and mineral oil) and modified oils (with Fe particles) based on tribological peoperties was done by measuring friction and quantifying wear loss on the sliding surfaces. Further, a comparison between Fe micro-/nano- particle blended oil and commercially known glycerol monooleate (GMO) additive is also reported.MATERIALS AND METHODSIn all, 10 oils/suspensions with different combination of additives, surfactants, and Fe particles were prepared and tested. Such a study intended to study the effect of each additional chemical compound on the tribological properties of the oil. Finally, base oil with and without commercially known glycerol monooleate (0.5% wt) additive was used as a reference (see Table. 1). For convenience the prepared oils/suspensions are abbreviated as shown in Table 2 second column.Test samples AbbreviationBase oilBase oil + 0.5% GMORapeseed oil + 0.5% phenol antioxidantRapeseed oil + 0.5% phenol antioxidant + 0.5% OS-20 + 2.2% NaBH4 + 2.2% FeSO4Rapeseed oil + 0.5% phenol antioxidant + 0.5% OS-20 + 4.4% H2ORapeseed oil + 0.5% phenol antioxidant + 0.5% ENB 904 + 2.2% NaBH4 + 2.2% FeSO4Rapeseed oil + 0.5% phenol antioxidant + 0.5% ENB 904 + 4.4% H2OSAE 10 + 0.5% phenol antioxidantSAE 10 + 0.5% phenol antioxidant + 0.5% OS-20 + 2.2% NaBH4 + 2.2% FeSO4BOBO + GMO RORO + OS-20 + FeRO + OS-20 + H2ORO + ENB + FeRO + ENB + H2OSAE 10 SAE 10 + OS-20 + FeSAE 10 + 0.5% phenol antioxidant + 0.5% OS-20 + 4.4% H2OSAE 10 + 0.5% phenol antioxidant + 0.5% ENB 904 + 2.2% NaBH4 + 2.2% FeSO4SAE 10 + 0.5% phenol antioxidant + 0.5% ENB 904 + 4.4% H2O SAE 10 + OS-20 + H2O SAE 10 + ENB + Fe SAE 10 + ENB + H2OTable 1. List of tested oils (components expressed in wt %) * Base oil and base oil+GMO are used asreferencesTribological testsThe four ball wear experiments are commonly used to study the anti-wear properties of lubricants under uni-directional sliding contact [24]. For this study a four-ball wear test machine made by Falex Corporation, USA was used. The test set-up consists of a ½ inch diameter top steel ball rotating in the cavity of three identical steel balls in contact and clamped in a cup below, containing the test fluid. The test balls (100Cr6 steel, 12.7 mm diameter, 64–66Rc hardness) were thoroughly cleaned with acetone and ethanol before each experiment. Test fluid (15-20 ml) was poured in the test cup so that the bottom three balls remain immersed. The test speed was set at 1200 rpm and the test temperature was maintained at 75 0C. To understand the evolution in friction and wear, three tests were done on each oil at a load of 40 Kg for 1 hr, at 60Kg for 1 hour and final test at 60 kg for 2 hours. All these tests were duplicated. After the test, the circular wear scar diameters on the three stationary balls were measured with a Ziess® optical microscope. For each test, the six scar measurements were averaged and reported as Wear Scar Diameter (WSD).Reciprocating sliding tests at normal forces of micro- up to milliNewtons were done with a high precision microtribometer (MUST, Falex Tribology NV, Belgium) in a ball-on-flat contact configuration were performed. Reciprocating sliding tests were performed on vacuum arc remelted (AISI 52100 steel, 24 mm diameter, 7.85 mm thickness, 60 Rc hardness, roughness parameter is in range 0.5 µm< R2 < 0.65 µm ) against 3.175 mm 100Cr6 bearing steel ball. Before every test 2-3 ml of oil/suspension was dropped on the disk and spread uniformly using a spatula. The displacement amplitude during the tests was fixed at 5 mm. The normal force was varied 500 mN, 750 mN and 900 mN. Sliding speed were used 0.05 mm/s, 0.5 mm/s and 2.6 mm/s, respectively. Each combination of speed and load was executed thereby performing 9 experiments per oil. All experiments were carried out in ambient air at 23°C and 50 % relative humidity.RESULTSFour ball wear testsA summary of all the average coefficients of friction for oils/suspensions is summarized in Fig. 1a. The average scar diameters obtained after the four ball wear test at 40/60 Kg load is given in Fig. 1b. The modified rapeseed oil with iron particles showed the lowest average coefficient of friction which was lower than the Base oil containing GMO additive. The reason for the friction reduction could be attributed to the decrease in the effective shear strength in the contact due to the third-body effect e.g., rolling effect or the ability of the particles to carry the oil film deeper into the contact [1]. In the case of mineral oil, the addition of Fe particles led to an increase in the coefficient of friction. Especially modified SAE 10 + OS-20 + Fe blend caused some corrosion on the balls which was not noticed with untreated SAE10 oil (Fig. 2a). It is hypothesized that Fe particles with OS-20 surfactant in mineral oil forms some kind of active complexes which may have caused corrosion [2]. This result indicates that the selection of surfactant is very important for the application of micro-/ nano- particles as lubricating oil additives. Although there were contrasting effect of Fe particles on rapeseed and mineral oil, the addition of Fe particles has reduced wear loss for both the oils. This effect is more pronounced for the test done at 60 Kg load (see Fig. 2b).(a)(b)Fig. 1 (a) Calculated average CoF (b) average scar diameter on stationary balls measured with optic microscope at 50x . Test conditions: 75 0C, 1200 rpm, 12.7 mm diameter 100Cr6 bearing steel balls.(a) (b)Fig. 2 Wear scars after FBW test (a) non-modified SAE 10 oil at 40 Kg load, (b) modified SAE 10 oil withFe particles blended with OS-20 surfactant at 60 Kg load. Test conditions: 75 0C, 1200 rpm, 12.7 mmdiameter 100Cr6 bearing steel balls.The four ball wear test results obtained on all the oils/suspensions listed in Table.1 are given in Fig. 3. For convenience, the results are separated into mineral oil based and Rapeseed oil based. Fig. 3a shows that wear volume (WV) is relatively less for RO + OS-20 + Fe and RO + ENB + Fe suspensions compared to non-modified RO, BO, BO + GMO and RO+OS+20/ENB+H 2O. For a test at 60 Kg and 2 hours, the wear volumes of RO + OS-20 + Fe and RO + ENB + Fe converge although at 40Kg RO + OS-20 + Fe gives better wear protection than RO + ENB + Fe. This difference between RO + OS-20 + Fe and RO + ENB + Fe could have been due to of the difference in the synthesized Fe particle sizes. The details of the wear loss on mineral oil based oils/suspensions are shown in Fig. 3b. The SAE 10 + OS-20 + Fe suspension showed the least wear loss compared to SAE 10, BO and modified BO + GMO. The mineral oil suspension with OS-20/ENB + H 2O gave a better wear protection than the oil itself whereas for rapeseed oil such suspensions made no difference. In contrast the SAE 10 + ENB + Fe gave a higher wear loss than BO + GMO. On comparison with rapeseed oil Fe particle suspensions, the obtained wear loss with SAE 10 is significantly high. In general, OS-20 surfactant with Fe particles seems to deliver a better wear resistance to both the rapeseed and mineral oils. It is believed that the wear reduction by the Fe particles could be due to the formation of protective film formation in the contact which needs a further investigation.Reciprocating sliding testsA graph of the average coefficient of friction against Sommerfield number estimate [3] (velocity/load ratio) recorded during reciprocating sliding tests using a microtribometer is shown in Fig. 4. For the rapeseed oil (Fig. 4a), the lowest coefficient of friction (CoF) was measured for modified RO + OS-20 + H 2O and none of the Feparticle suspensions have shown significant reduction in coefficient of friction. At V/L ratio of 0.003, RO + OS-20 + Fe suspension seems to have slightly lower coefficient of friction than commercial GMO. In the case of mineral oil, addition of surfactants or surfactants + Fe particles has only led to an increase in the coefficient of friction. Especially with Fe particle suspensions the coefficient of friction exceeded 0.4. Thus under reciprocating sliding conditions modification of mineral oils with Fe particles does not improve the friction behaviour. The observed trend in the coefficient of friction is in good agreement with the four ball coefficient of friction data where an increase in the friction force was also recorded in SAE 10 based suspensions. On observing the disk after the test, only SAE 10 based suspensions showed some damage in the wear track. It is believed that the iron particles settle down (due to gravity) on the surface and act as abrasives thereby increasing the friction force.(a)(b)Fig. 3 Wear Volume results obtained from four ball wear tests on (a) rapeseed oil (b) mineral oil SAE 10based oils/suspensions and compared with Base oil + GMO. Test conditions: 75 0C, 1200 rpm, 12.7 mmdiameter 100Cr6 bearing steel ballsFig.4 Average coefficient of friction recorded in bi-directional sliding conditions on (a) rapeseed oils/suspensions (b) SAE 10 oils/suspensions compared with baseoil with and without GMO. Relativestandard error < 7.2%. V – Sliding speed, L – normal load.The possible presence of iron particles in the contact can be understood by observing the friction loops (tangential force versus distance) during the reciprocating tests. A comparison of friction loops for mineral SAE 10 oil and rapeseed oil with and without Fe particles is shown in Fig. 5. The friction loop recorded on SAE 10 oil with particles showed fluctuations compared to the untreated oil. The fluctuations in the friction force are known to arise due to different effects like third body effect, material/phase effects, and topographical effect [4]. In this case, the fluctuations could have been due to the Fe particles in the contact, and an evidence of such material/phase/third body effect is marked in Fig. 5a. It is interesting to notice that for rapeseed oil there is hardly any difference between the friction loops with and without iron particles. This definitely requires further research on how particles remain suspended in the oil through chemical interactions between surfactant and oils. The superior tribological performance of oils could be partly explained by bulky polymeric structure of(a)(b)monoalkyl polyethylene glycol ether compared to block copolymers. It is commonly known the positive tribological influence of metal-organic compounds which are bounded by surfactants of bulky chain structure [5].T a n g e n t i a l f o r c e (m n )Displacement (mm)(a) T a n g e n t i a l f o r c e (m n )Displacement (mm)(b)Fig. 5 Representative friction loops on (a) non-modified SAE 10 and modified SAE 10 with Fe particles suspended with OS-20 surfactant, (b) non-modified RO and modified RO with Fe particles suspended with OS-20 surfactant. Test conditions: vacuum arc remelted disc against 100Cr6 steel ball at 500 mNnormal load with 0.05 mm/s scan rateCONCLUSIONSIron micro-/ nano-particle were prepared in rapeseed and mineral oils using colloidal technique. The tribological properties of oils with and without surfactant/iron micro-/nano- particles were determined to verify the anti-wear and friction reduction offered by the iron micro-/nano-particle additives. In the four ball unidirectional tests, addition of Fe micro-/nano- particles has reduced the wear loss on the sliding surfaces. For rapeseed oil a decrease in the global coefficient of friction was obtained, but for mineral oil addition of surfactants and Fe micro-/nano- particles gave a higher coefficient of friction than the unmodified mineral oil. The exact reason for the increase in friction, but decrease in wear in the case of mineral oil must be further investigated. Rapeseed oil based suspensions gave a lower coefficient of friction and wear loss than the commercial base oil with glycerol monooleate (GMO) additive. It is hypothesised that Fe particles act as rolls and also provide lubrication locally in the contact acting as oil carriers. In summary, Fe micro-/nano- particles do reduce wear like anti-wear additives in high sped unidirectional test conditions.In the bi-directional tests, the addition of surfactants/Fe particles has no effect on the friction behaviour of the untreated rapeseed oil. For mineral oil a negative effect or an increase in friction was noticed, which was attributed to the abrasive activity of the iron particles which settle down in the absence of agitation. Some corrosion products were noticed on the disk with mineral oil /OS-20 based suspensions, which needs further investigation through analytical techniques. In the future work, surface analysis of the wear tracks using SEM, XPS, and XRD techniques is intended to investigate the formed reaction layers, role of surfactants, etc.REFERENCES[1] Wu YY, Tsui WC, Liu TC. Experimental analysis of tribological properties of lubricating oils with nanoparticle additives. Wear 262 (2007) 819–825.[2] Q. Sunqing, D. Junxiu and C. Guoxu, Tribological properties of CeF 3 nanoparticles as additives in lubricating oils, Wear 230 (1999), 35–38.[3] Bayer, R.G., , “Mechanical Wear Prediction and Prevention”, Marcel Dekker, USA, (1994) 657[4] Achanta.S, Liskiewicz.T, Drees.D, Celis.J.P. Friction Mechanisms at Microscale, Tribol. Intern., 42, (2009) 1792-1799. [5 Garkunov, D.N.: Scientific discoveries in tribotechnologies. No-wear effect under friction. Hydrogen wear of metals. Moscow: MAA Publishing house, 2007, p. 383。