Procedia Engineering 68 ( 2013 ) 152 – 157

Available online at www.sciencedirect.com

1877-7058 © 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysiadoi: 10.1016/j.proeng.2013.12.161

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* Corresponding author. Tel.: +6-010-231-2601 E-mail address: nurinmz@um.edu.my

Malaysian International Tribology Conference 2013 (MITC2013)

Experimental analysis of tribological properties of biolubricant with nanoparticle additive.

N.W.M. Zulkiflia*, M.A. Kalama, H.H. Masjukia, R. Yunusb aDepartment of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

bInstitute of Advanced Technology, University Putra Malaysia, 43400 Serdang, Selangor, Malaysia

Abstract

This study examined the tribological properties of two lubricating oils, paraffin oil and biolubricant added with TiO2nanoparticles used as additives. Biolubricant used in this experimental was derived from palm oil-based TMP (trimethylolpropane) ester. The TMP ester is produced from palm oil, which is biodegradable and has high lubricity properties such as a higher flash point temperature and VI (viscosity index). The friction and wear experiments were performed using four-ball machine tribotester. Experimental was carried out for 10 minutes under 40 kg, 80kg, 120kg, and 160kg at 1200rpm. Test temperatures were set at room temperature. The experimental results show that nanoparticles, TiO2 added to TMP ester exhibit good friction-reduction. The addition of TiO2 nanoparticles in the TMP ester at 160 kg decreased the friction coefficient by 15% respectively, and reduced the wear scar diameter by 11 %respectively, as compared to TMP ester without TiO2 nanoparticles. In conclusion, TMP ester added with nanoparticles is environmentally desired to mineral oil based lubricants, research to investigate the properties of nanoparticle added with TMP ester to make it technologically competitive as automobile lubricant, and should be encouraged.

© 2013 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia. Keywords: Wear behavior, Nanoparticles; Bio Lubricant; Extreme Pressure Properties

1. Introduction

Numerous researches have been investigated on the effect of nanoparticle added in lubricant [1-4]. In general, majority of the study agree that nanoparticle improved tribological property of lubricant by deposited at the contacted area and create a protective layer. There are several factors was to consider to use nanoparticle as an additive in lubricant such as the size, the shapes, the hardness, and the weight percentage of nanoparticle. The size of nanoparticle ranges is mostly around 2-120nm [4]. Liu et al [5] added the dialkyldithiophosphate (DDP) modified copper nanoparticles as additives in liquid paraffin. They found that the nanoparticles of smaller size were more likely to interact with the surfaces of the friction pairs to form a surface protective film, which increased anti-wear ability. Rapoport et al. [3, 6] reported that the friction properties of the inorganic fullerene-like (IF) particles in oil have the advantages of the spherical shape of IF opens the possibility for an effective rolling friction mechanism.

© 2013 The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.Selection and peer-review under responsibility of The Malaysian Tribology Society (MYTRIBOS), Department of Mechanical Engineering, Universiti Malaya, 50603 Kuala Lumpur, Malaysia

153 N.W.M. Zulkifl i et al. / Procedia Engineering 68 ( 2013 ) 152 – 157

TMP ester is a biolubricant derived from renewable sources such as palm oil and jatropha oil[7, 8]. The high performance and environmental friendly of esters offer benefits that can be exploited throughout the supply chain (gear oil, engine oil, and hydraulic oil). The esters are polar lubricants that they tend to transfer to metal surfaces and form physical bonds with the surface oxide layer [9].

However, only several research conducted on the effect of biolubricant added nanoparticle [1, 10]. Therefore this research has been conducted to study the effect biolubricant added nanoparticle at different load condition.

2. Experimental details

2.1. Lubrication preparation

Base oil used in this experiment as biolubricant is trimethylolpropane (TMP) ester. The TMP ester is produced from a palm oil methyl ester through transesterification. Transesterification eliminates the hydrogen molecule on the beta carbon position of the palm oil substrate, thus improving the oxidative and thermal stability of the TMP ester; an important lubricity property in vegetable oils [11]. In addition, TMP esters have good friction-reducing properties and acceptable anti-wear properties [12]The lubricants used for this experiment were TMP ester added TiO2. It is a challenging task to produce a uniform dispersion of TiO nanoparticles in biolubricant because of high surface energy of nanoparticles. Therefore they have the tendency to agglomerate and settle down. Hence, TiO nanoparticles were dispersed in solvent in order to prevent the nanoparticles from oxidizing with air and to form an additive solution. Glycol was used as the solvent for TiO2 nanoparticles. The additive solution containing 1% nanoparticles was then dispersed in biolubricant in ultrasonic bath for 8 hours. The lubricant comprised 90% TMP ester and 10% additive solution. In other words, the biolubricants modified with TiO2 were composed of 9.9% glycol and 0.1% nanoparticles. The percentage (%) was measured by weight. The density and viscosity of biolubricant were measured and shown in Table 1.

Table 1- Density of TMP ester added nanoparticle and pure TMP ester is shown

Sample Viscosity

VI Density (g/cm3) 40°C (cSt) 100°C(cSt)

Pure TMP ester 101.86 ±1.53 14.46±0.35 146 0.8549±0.0174

TMP ester added with nanoparticle

108.75±1.22 15.08±0.28 145 0.8553±0.0168

2.2 Friction and wear tests

The COF of TMP ester and TMP ester with added nanoparticle as an additive were investigated through a series of fourball friction tests. The four-ball wear tester consists of three balls held stationary in a ball pot plus a fourth ball held in a rotating spindle as shown in Fig.1. The balls used in this study were steel balls, AISI 52-100, 12.7 mm in diameter, with 60-64 Rc hardness. These balls were thoroughly cleaned with toluene before each experiment. The sample volume required for each test was 10 ml. The test conditions were the load is varied from 40 kg to 120 kg, operating temperature is at room temperature, rotational speed of 1200 rpm and operation time is 10minutes. The wear produced on the three stationary balls is measured under a calibrated microscope and reported as the WSD or calculated volume.

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Fig.1 Schematic of four-ball test machine

3. Results and Discussion

3.1 Coefficient of friction

Fig 2-4 illustrates the effect of nanoparticle in TMP ester reduced the COF compared to pure TMP ester. As can be seen in the Fig.2, the load increased, COF increased. The higher percentage reduction of COF as nanoparticle added in TMP ester was at 160kg which around 15%. All COF values in this experiment were in a mixed lubrication regime which the lubricant thickness around 3 to 25nm [13]. Therefore it is believed that as in the mechanical entrapment theory, which as the nanoparticle penetrates in the contact area and embedded in the contact area and create additional protective layer because have similar to lubricant film thickness [14]. There are three possible mechanism occurred, the nanoparticles may be molten and welded on the shearing surface, another is nanoparticle reacted with the specimen to form a protective layer, or tribo-sintered on the surface [15]. However, nanoparticles also had adverse effect as increasing wear or friction [16].

In general it can be seen that COF increased as the load increase. The increased of the load will produce higher stress concentration in localized region. Therefore, this situation can lead to localized plastic deformation and continued by initiation and abrupt propagation of crack which resultant in spall formation[17]. In addition to that the sudden abrupt COF changed from 80kg to 160 kg is because of fluid film was not developed here. As can be seen in Fig.3 and Fig.4, at 160kg there were sudden changes in COF at the beginning of the test because of the running in period. During this running in period, a high contact pressure between two surfaces caused wear and the plastic deformations of the peaks and surfaces, the profile contact of the surface is gradually improved and the surface pressure, and COF decreases so it enters a stable wear stage.

Fig.2. The effect of load on COF for biolubricant with nanoparticle

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Fig.3. COF as a function of time for pure TMP ester.

Fig.4 COF as a function of time for TMP ester added nanoparticle

3.2 Wear scar diameter

Fig.5 illustrates the effect of nanoparticle on WSD. Based on Fig.5, as the load increased the WSD increased. The nanoparticle improved the WSD at all load. Based on Fig.6 (a), it can be seen that WSD was dominated only by minimal abrasive wear. The wear scar was almost invisible; it is believed that the nanoparticle successful created a protective layer. As the load increased, the WSD is increased. In addition to that, the adhesive wear starts to take place at load more than 80kg. Fig. 6(e) shows that adhesive wear occurred in the middle of the ball, therefore there was boundary lubrication occurred here. As the two surfaces contacted because of the high-pressure load which caused the plastic deformation of the surfaces. For Fig. 6 (g) and (h), a severe wear occurred at high load. The nanoparticle may molten and welded on the shearing surface and protect the ball at high load.

Fig.5 WSD as a function of load for biolubricant

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TMP 1%-40kg

(a)

TMP-40kg

(b)

TMP 1%-80kg

(c)

TMP 80kg

(d)

TMP 1%-120kg

(e)

TMP -120kg

(f)

TMP 1%-160kg

(g)

TMP -160kg

(h)

Fig.6. Surface morphology as WSD of the ball using SEM

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4. Conclusions

The following conclusion can be drawn from the results presented above: For all load, TMP ester added with nanoparticle reduced COF up to 15% at high load. Nanoparticle also improved WSD of the TMP esters especially at low load (40kg) by creating an additional

protective layer.

Acknowledgment

The authors would like to thank the University of Malaya, which made this study possible through the research grant FRGS FP020/2011A, PV070/2011B and high impact research grant no UM.C/HIR/MOHE/ENG/07.

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