Post-print version submitted to Tribology Transactions

Suggested Citation

M. Gulzar, H.H. Masjuki, M.A. Kalam, M. Varman, NWM Zulkifli,

R.A Mufti, Rehan Zahid and R.Yunus " Dispersion stability and

tribological characteristics of TiO2/SiO2 nanocomposites

enriched bio-based lubricant" Tribology Transactions (2016).

The final publication is available at

http://www.tandfonline.com/doi/abs/10.1080/10402004.2016.12023

66

http://www.tandfonline.com/doi/abs/10.1080/10402004.2016.1202366http://www.tandfonline.com/doi/abs/10.1080/10402004.2016.1202366

Dispersion stability and tribological characteristics of TiO2/SiO2 nanocomposites

enriched bio-based lubricant.

M.Gulzar1*, HH Masjuki2*, MA Kalam3, M Varman4, NWM Zulkifli5, R.A. Mufti6,

Rehan Zahid7, R. Yunus8

1,2,3,4,5, 7 Centre for Energy Sciences, Department of Mechanical Engineering,

University of Malaya, Kuala Lumpur-50603, Malaysia

6 School of Mechanical and Manufacturing Engineering, NUST, Islamabad-44000.

8 Institute of Advanced Technology, University Putra Malaysia, Serdang, Selangor-43400,

Malaysia

*Corresponding authors

Address: Department of Mechanical Engineering, University of Malaya, 50603 Kuala

Lumpur, Malaysia.

Tel.: +60 3 79674448; fax: +60 379675317.

E-mail address: mubashir_nustian@hotmail.com, masjuki@um.edu.my

Abstract

The stable dispersion of nano-additives is highly desirable for the effective lubrication

performance of nanolubricants. The compatibility of base oil with selected nano additives is

required for uniform and stable dispersion. This research evaluated the dispersion stability and

tribological characteristics of nano-TiO2/SiO2 (average particle size: 50 nm) as an additive in

a bio-based lubricant. The wear protection and friction reducing characteristics of the

formulations were evaluated by four-ball extreme pressure tests and piston ring–cylinder liner

sliding tests. Surface analysis tools, including scanning electron microscopy, energy-

dispersive X-ray spectroscopy, and atomic force microscopy, were used to characterize worn

surfaces. Results showed that the nanolubricants demonstrated appreciable dispersion

capability in the absence of a surfactant, improvement in load-carrying capacity, anti-wear

behavior, and friction reduction capability.

Keywords: Biodegradable Base Stocks; Nanotribology; Antiwear Additives; Friction

Modifiers; Wear Mechanisms

1. Introduction

The hazardous effects associated with the use of conventional engine lubricants are becoming

a global concern; hence, bio-based base stocks have created a budding interest for use in

various applications. Biolubricants, which include plant oils, animal fats, or the chemical

modifications of such oils/fats, are widely regarded as environmentally benign because of

their superior biodegradability and renewable feedstock. Among bio-based lubricants, the

direct application of vegetable oils is unsuitable for prolonged use in internal combustion

engines and other applications that require appropriate thermal and oxidative stability

(Rudnick (1)). These deficiencies have been addressed via chemical modifications. Such

modifications results in synthetic esters with improved thermo-oxidative stability and high

viscosity indices but without the undesirable impurities found in conventional petroleum-

based oils (Kodali (2, Arumugam and Sriram (3, Zulkifli, et al. (4, Nagendramma and Kaul

(5, Konishi and Perez (6)). Therefore, polyol esters and complex esters have been regarded as

possible base oil candidates and have attained considerable importance in various

applications, such as in engine lubricants, gear oils, hydraulic oils, compressor oils, pump oils,

and turbine oils (Nagendramma and Kaul (5)) (Reeves (7)). Among these bio-based esters,

trimethylolpropane (TMP) esters, which exhibit comparable viscosity, high viscosity index,

and moderate thermo-oxidative stability, are attractive alternatives to synthetic base oils

(Zulkifli, et al. (4, Gulzar, et al. (8, Zulkifli, et al. (9, Habibullah, et al. (10)). The antiwear

and friction reduction characteristics of palm TMP ester are comparable with those of

mineral-based oils but considerably below those of fully formulated lubricants (Zulkifli, et al.

(4)). Therefore, the use of appropriate antiwear and friction-reducing additives in TMP esters

are required to improve their tribological performance. In this regard solid nanoparticles have

emerged as suitable alternatives to conventional additives. The major challenge in using

nanolubricants for various applications is to formulate stable suspensions based on

nanoparticles because less stable suspensions tend to form agglomerates and sediments over

long stationary periods (Demas, et al. (11)) (Thottackkad, et al. (12)). Therefore, an important

consideration in producing effective suspensions is ensuring compatibility between the

selected nanoparticles and base oils (Alves, et al. (13)). Pierre Rabaso et al. (Rabaso, et al.

(14)) reported that a homogenous dispersion would be required to avoid the progressive

starvation of contact in nanoparticles. Demas et al. (Demas, et al. (11)) highlighted the fact

that among five considered surfactants, some were not beneficial for uniform dispersions.

They reported that for PAO10 base oil, BN nanoparticles dispersed well in different

surfactants. By contrast, MoS2-enriched suspensions were less sensitive to the nature of

surfactants. Therefore, selecting a suitable surfactant is a challenge given that its effectiveness

depends on a number of factors, including its compatibility with a variety of base oils and

suitability with the considered nanoparticle type and nanoparticle morphology. For dispersion

stability of nanocomposites in lubricants, previous studies have used surface modified

nanocomposites (Jiao, et al. (15, Li, et al. (16, Ma, et al. (17)). On the other hand, dispersion

homogeneity and appreciable stability over a long period have been observed among

unmodified nano-SiO2-enriched lubricants (Jiao, et al. (15, Xie, et al. (18, Anoop, et al. (19)).

Therefore, nanocomposites that partly contain nano-SiO2 can exhibit an advantage in

dispersion stability in the absence of a surfactant. Previous studies related to the use of

nanocomposites as additives have involved mineral oils and synthetic oils. Meanwhile, the

dispersion stability and related tribological characteristics of nanocomposites in bio-based oils

still have to be investigated. To address this gap and to investigate the dispersion effectiveness

of nanocomposites that partly contain nano-SiO2, the present study involves palm oil-based

base stock enriched with nano-TiO2/SiO2.

In this research, palm TMP ester was used as the bio-based base stock. In the first part of the

study, the dispersion stability of the suspensions was evaluated by analyzing the static

stability as well as measuring the optical absorbance spectra of all the nanolubricant samples.

For this purpose, nano-TiO2/SiO2 and nano-TiO2 were added to the base oil to determine the

improvement in suspension stability induced by the presence of nano-SiO2 in nano-TiO2/SiO2.

In the next part of study, the tribological characteristics of the stable suspensions were

investigated. The load-carrying capability of the trial oils were investigated via four-ball tribo-

tests. The nanocomposite-based suspensions were also tested for piston ring–cylinder liner

interaction using a high-stroke reciprocating test rig. After the tribological tests, the worn

surfaces were analyzed via scanning electron microscopy (SEM), energy-dispersive X-ray

(EDX) spectroscopy, and atomic force microscopy (AFM).

2. Materials and Methods

2.1 Formulation of Nanolubricants

To improve thermo-oxidative stability, the transesterification process was used to chemically

modify vegetable palm oil methyl ester in the presence of TMP. The detailed procedure and

mechanism for producing palm TMP ester were adapted from our previous research (Gulzar,

et al. (8)). To understand the role of the TiO2/SiO2 nanocomposites in dispersion stability,

nano-TiO2/SiO2 and nano-TiO2 were dispersed in palm TMP ester in concentrations of 0.25

wt%, 0.50 wt%, 0.75 wt%, and 1 wt%. Commercially available nano-TiO2/SiO2 and nano-

TiO2 were supplied by M/S Sigma-Aldrich (M) Sdn. Bhd, Malaysia. The true size and density

of these nanoparticles were provided by the supplier. Figs. 1(a) and 1(b) show the

morphology and elemental composition of the nano-TiO2 and nano-TiO2/SiO2, respectively.

Table 1 lists the properties of the nano-TiO2/SiO2 and nano-TiO2 used in this study. Nano-

additives were dispersed in palm TMP ester using an ultrasonic probe for 30 min. The

physicochemical properties of all the nanolubricants are provided in Table 2.

Fig.1 (a) SEM micrograph and EDX of nanoTiO2/SiO2 (b) SEM micrograph and EDX of

nanoTiO2.

Table 1 Nanomaterial properties

Properties

Appearance

(Color)

Appearance

(Form)

Average

Particle Size

(nm)

Morphology Purity

(%) Molecular

Weight (g/mol)

TiO2/SiO2 White Powder 37 Nearly spherical 99.8 139.95

TiO2 White Powder 26 Nearly spherical 99.5 79.87

Table 2 Physicochemical properties of Palm TMP ester and nanolubricants.

Lubricant Sample Properties

Viscosity VI Density TAN TBN

40 oC

(cSt)

100 oC

(cSt)

15 oC

(g/cm3)

(mgKOH/g)

Palm TMP ester Oil A 40.031 9.155 221 0.9011 0.44 0.37

Palm TMP ester + 0.25% TiO2/SiO2 Oil B 40.143 9.261 223 0.9079 0.44 0.37

Palm TMP ester + 0.50% TiO2/SiO2 Oil C 40.252 9.319 220 0.9094 0.46 0.38

Palm TMP ester + 0.75% TiO2/SiO2 Oil D 40.559 9.447 220 0.9117 0.46 0.40

Palm TMP ester + 1% TiO2/SiO2 Oil E 40.771 9.591 221 0.9119 0.47 0.40

Palm TMP ester + 0.25% TiO2 Oil F 40.111 9.248 221 0.9044 0.44 0.38

Palm TMP ester + 0.50% TiO2 Oil G 40.207 9.294 223 0.9065 0.45 0.38

Palm TMP ester + 0.75% TiO2 Oil H 40.412 9.412 222 0.9081 0.45 0.39

Palm TMP ester + 1% TiO2 Oil I 40.655 9.561 222 0.9096 0.45 0.40

2.2 Dispersion Analysis

To analyze the effectiveness of the TiO2/SiO2 nanocomposite-enriched suspensions,

nanolubricant samples of nano-TiO2 were also used for comparison. Dispersion stability was

determined via the sedimentation method and ultraviolet–visible (UV–Vis) spectrophotometry

absorbance measurements to characterize the colloidal stability of the dispersions

quantitatively. For the sedimentation method, nanolubricant samples with the same volume

were placed in test tubes at room temperature without any disturbance or movement as

indicated by Amiruddin et al. (Amiruddin, et al. (20)) and Azman et al. (Azman, et al. (21)).

Photographs of the test tubes were taken using a camera to observe the stability of the

nanolubricant samples at specific time intervals. In addition to visual observation via the

sedimentation method, UV-Vis spectral analysis was performed to present the quantitative

analysis of nanolubricant samples. The degree of absorbance is proportional to the amount of

particles per unit volume, and thus, this measure can be used to denote variations in

supernatant particle concentration in the solution with time (Amiruddin, et al. (20, Yu and Xie

(22)).

For UV–Vis spectral analysis, a SPEKOL 1500 UV-Vis spectrophotometer was used, and

tests were conducted at a wavelength of 429 nm, with a repeatability of 1 nm. Palm TMP ester

and nanolubricant samples were placed in glass cuvettes with a capacity of 3.5 mL, and palm

TMP ester was used as the reference solution. UV–Vis spectrophotometer absorbance test was

performed immediately after the ultrasonic dispersion of nanoparticles in the lubricant. Time

rate changes in the absorbance level of visible light were recorded to measure the dispersion

capability of the samples.

2.3 Analysis of Load-carrying Capacity

A standard four-ball test apparatus was used to evaluate the extreme load-carrying capacity of

all the samples. For these experiments, the material of the balls used was AISI 52100 steel.

The balls had a diameter of 12.7 mm and an HRC hardness of 64–66. A new set of balls was

used for each test, and the balls and pot were cleaned with toluene after each experiment. The

tests were performed according to the procedure indicated in the ASTM D2783 standard for

examining load-carrying capacity and anti-wear properties. The test conditions are

summarized in Table 3. Following the standard procedure, a starting load of 40 Kg was

applied, and the cantilever load was increased gradually until the balls were welded.

Commonly used wear parameters, namely, last non-seizure load (LNSL), initial seizure load

(ISL), and weld point (WP), were evaluated. Each test was repeated three times so as to

minimize data scatter.

Table 3. Test conditions for four-ball tests

Condition Value

Test Temperature (oC) 25±5

Test Duration (sec) 10

Spindle Speed (rpm) 1770 ± 30

Load (Kg) Varies, 10-sec/stage

Ball Material AISI 52100

Ball Diameter (mm) 12.7

Ball Hardness (HRC) 65

Ball Roughness, Ra (µm) 0.035

2.4 Piston Ring–Cylinder Liner Tests

To evaluate the tribological behavior of the lubricant samples for piston ring–cylinder liner

conjunction, the lubricants were tested using a high-stroke reciprocating test rig. Table 4

presents the material properties of the selected piston ring and cylinder liner. The cylinder

liner specimen was placed in a lubricant bath where oil temperature was controlled using the

control panel. Meanwhile, the piston ring specimen was placed in a stationary holder at the

top. Fig. 2 shows the schematics of the high-stroke reciprocating test rig as well as the

dimensions of the piston ring and the cylinder liner specimens. For this test, a stroke length of

84 mm and a contact width of 25.4 mm were selected to ensure conformal contact between

the piston ring and the cylinder liner. All tests were performed at a cantilever load of 160 N, a

speed of 500 rpm, a lubricant feed rate of 5 mL/hr, and a lubricant temperature of 70 °C. The

selected test conditions corresponded to a maximum speed of 2.29 m/s and a contact pressure

of ∼10 MPa as indicated by Gullac and Akalin (Gullac and Akalin (23)). Running in duration

lasted for 2 hours such that the sharp asperities on the new surfaces were shaved off before

each test. The friction coefficient was measured for the test duration of 2 hours. For the wear

test, the contact between the piston ring and the cylinder liner lasted for 6 hours, and the

related wear was measured using the weight loss method. The samples were ultrasonically

cleaned and weighed to an accuracy of 0.1 mg before and after each test. Each test was

repeated three times using a new piston ring and cylinder liner specimen each time.

Fig.2 (a) High stroke reciprocating test rig (b) Specimen geometry.

Table 4 Relevant properties of piston ring and cylinder specimens.

Specimen Material Surface

Treatment

Roughness (µm) Elastic

Modulus

(GPa) Root mean

square, Rq

Average,

Ra

Cylinder Liner Cast Iron Honing 1.215 1.01 200

Ring Cast Iron Chrome Coated 0.138 0.111 210

3. Results and Discussion

3.1 Dispersion Stability

Figs. 3 and 4 show the dispersion stability of the nanolubricants in terms of optical

absorbance and their static stability by camera photographs, respectively. To analyze the

effectiveness of the nanocomposite-enriched suspensions, a comparison of the optical

absorbance of nano-TiO2 and nano-TiO2/SiO2 is provided via Figs. 3 (a) and 3(b),

respectively. The optical absorbance profiles show that the increasing concentration of

nanoparticles from 0.25 wt% to 0.75 wt% raises the optical absorbance of nanoparticles for

both types of nanolubricants. Fig. 3(a) shows that the nano-TiO2-enriched suspensions are

less stable, such that the separation interfaces between the sediment and the supernatant are

sharp after being initially stable for few hours and then gradually decline with time. Such

trend of nanoparticle-based lubricants was reported by Demas et al. (Demas, et al. (11)) in the

absence of surfactants. A clear base oil layer was observed after day 3 for all the nano-TiO2-

enriched suspensions because of the continuous sedimentation and separation of

nanoparticles, as shown by Oil F, Oil G, Oil H and Oil I in Fig. 4. By contrast, Figs. 3(b) and

4 show the stable trend of the nano-TiO2/SiO2-enriched lubricant samples. Such stable

dispersion, even in the absence of a surfactant, can be attributed to the nano-SiO2 in the

TiO2/SiO2 nanocomposite given that nano-SiO2 exhibits a homogenous and stable dispersion

in oil for long periods (Jiao, et al. (15, Xie, et al. (18)). Fig. 3(b) shows that 0.75 wt% nano-

TiO2/SiO2 provided the most suitable dispersion stability. However, this dispersion stability is

dependent on the suitable weight concentration of the nano-additives because as the

concentration of nano-TiO2/SiO2 is further increased to 1 wt%, the optical absorbance curve

drops rapidly after 24 hours given that an increase in agglomeration tendency is observed. For

1 wt% nano-TiO2/SiO2 enrihed nanolubricant, high rate of sedimentation was observed after

day 3 as shown by Oil E in Fig. 4.

Fig.3 Dispersion ability of nanolubricants in terms of optical absorbance (a) NanoTiO2

enriched suspension (b) NanoTiO2/SiO2 enriched suspension.

Fig. 4 Static stability of nanolubricant samples over time.

3.2 Tribological Properties

The tribological analysis showed that such suspensions present the same results as those of

blank TMP ester because of the complete separation of the liquid phase from the solid phase

in the nano-TiO2-enriched suspensions. Therefore, the friction and wear results of the blank

TMP ester are compared only with the nano-TiO2/SiO2 suspensions to demonstrate the

effectiveness of uniform dispersions on tribological performance.

3.2.1 Load Carrying Capacity

The series of experiment (ASTM D2783) was used to analyze the effect of nanocomposites

on the load-carrying capacity of the lubricant samples under extreme pressure (EP)

conditions. Fig. 5 shows the wear trends of the blank palm TMP ester and the nanocomposite-

based suspensions against gradually increasing loads until ball seizure occurs. The variation in

the loading values of the initial sudden increase in wear scar diameter (WSD) demonstrates

that the LNSL and ISL have been improved for the lubricants with a nano-TiO2/SiO2

concentration of 0.50 wt% and higher. For the concentration of 1 wt% nano-TiO2/SiO2, WSD

is reduced but no significant change is observed in the LNSL, ISL, and WP values. Therefore,

the suspensions with the 0.75 wt% and 1 wt% nano-TiO2/SiO2 concentrations present the

highest values for the considered performance parameters under EP conditions. Table 5 lists

the individual values of LNSL, ISL, and WP for the base oil and the suspensions.

Table 5 LNSL, ISL and WP values for oil samples.

Fig. 5 Load vs. wear scar diameter (WSD) of lubricant samples.

3.2.2 Piston Ring–Cylinder Liner Friction

Fig. 6 shows the effect of different concentrations of nano-TiO2/SiO2 on the friction

coefficient of the piston ring specimen sliding against the cylinder liner specimen. Fig. 6(a)

indicates that the blank TMP ester exhibits an initial friction reduction because of the high

polarity of the TMP ester, which creates a strong affinity to the metal and provides a barrier

between surfaces (Zulkifli, et al. (9)). However, friction starts increasing after 25 minutes of

sliding because of the absence of a friction modifier and the low viscosity of the base oil.

Similar trends have been reported for blank palm TMP ester in an earlier study (Zulkifli, et al.

(4)). The addition of nano-TiO2/SiO2 helps significantly reduce the friction coefficient

compared with the blank palm TMP ester, as shown in Figs. 6(a) and 6(b). Oil B

demonstrates a slight improvement, whereas Oil C, Oil D, and Oil E exhibit a significant

Oil A Oil B Oil C Oil D Oil E

LNSL/Kg 70 70 80 80 80

ISL/Kg 80 80 90 100 100

WP/Kg 160 160 165 170 170

enhancement of friction surfaces, such that low friction coefficients are observed throughout

the sliding time. Oil D, which contains 0.75 wt% nano-TiO2/SiO2, presents the lowest friction

coefficient. A further increase in wt% concentration resulted in agglomeration, which, in turn,

increases the friction coefficient than that of Oil D. The friction reduction mechanisms in the

presence of nanolubricants can be attributed to friction surface enhancement resulting from

the mending effect and the polishing effect. The mending effect is characterized by the

deposition of the nanoparticles on the friction surface, which compensates for the loss of

mass. The polishing effect is attributed to reduction of roughness of the interacting surfaces.

Such effects of surface enhancement caused by polishing and mending reduce friction (Lee, et

al. (24)) (Peng, et al. (25)), (Padgurskas, et al. (26, Choi, et al. (27, Yu, et al. (28)). The

friction reduction mechanism attributed to surface enhancement was investigated and further

discussed through SEM, EDX, and AFM analyses.

Fig. 6 (a) Friction coefficient as a function of sliding time for lubricant samples (b) Average

friction coefficient of lubricant samples.

3.2.3 Cylinder Liner Wear Loss

Fig. 7 presents the wear loss of the cylinder liner specimen that was tested to determine the

sliding wear characteristics of the specific lubricant samples. For the nanolubricants, mass

was compensated by filling in the pits and grooves on the sliding surfaces, which is related to

the mending effect of nano-additives. The blank palm TMP ester exhibited the highest weight

loss, whereas minimal improvement has been observed after repeated wear tests when Oil B

was used. A further increase in nano-TiO2/SiO2 wt% concentration to 0.5 wt% helped

suppress wear loss, and a significant improvement was observed for the surfaces tested using

Oil D (0.75 wt% nano-TiO2/SiO2). Oil D provided the most adequate protection against

sliding wear because wear loss was reduced by as much as 10.4% when Oil D was used

compared with the blank palm TMP ester. A further increase in wt% concentration was

unsuitable for the considered test conditions because the highest wear loss was observed when

wear tests are performed using Oil E. In the case where Oil E was used for lubrication, low

wear protection was observed compared with the AW behavior of Oil D. This undesirable

wear behavior of Oil E may be attributed to its poor dispersion stability in palm TMP ester.

By contrast, Oil D exhibited high dispersion stability (Fig. 3 and Fig.4). This result indicates

that the synthesized nanolubricants with appropriate nano-TiO2/SiO2 wt% concentrations are

suitable for sliding contact under the considered test geometry. The authors have observed

similar wear reduction under the same test conditions when nano-CuO and nano-MoS2 were

dispersed in palm TMP ester (Gulzar, et al. (8)).

Fig. 7 Wear of cylinder liner specimen after sliding wear tests.

3.3 Worn Surface Characterization

3.3.1 SEM and EDX Analysis

To improve the understanding of the wear mechanisms of interacting surfaces, Figs. 8(a) to

8(d) provide the micrographs of the worn surfaces of the piston cylinder. A surface

enhancement effect was observed among all the nanolubricant samples for the considered

piston cylinder specimen. Figs. 8(a) to 8(d) illustrate surface mending via the deposition of

nanoparticles on the piston cylinder surface. Surface characterization was further aided by

EDX analysis, as shown in Figs. 9(a) to 9(h), to investigate and understand the lubrication

mechanism. Fig. 8(a) shows the worn cylinder liner surface tested against the piston ring

specimen for Oil B (palm TMP ester + 0.25 wt% TiO2/SiO2). The surface analysis showed

that the surface was enhanced by the presence of 0.25 wt% nano-TiO2/SiO2; however, a large

number of shallow grooves and small pits remained untreated. The related EDX analyses of

the worn surface region and the selected spot “A” are provided in Figs. 9(a) and 9(b),

respectively. For the micrograph region in Fig. 8(a), the EDX spectrum presents an extremely

low concentration of the concerned elements (Si and Ti), as shown in Fig. 9(a). Meanwhile,

the EDX spectrum related to the selected spot “A” verifies the mending effect of nano-

TiO2/SiO2 deposition on the tested surface. The increase in nanocomposites wt%

concentration enhanced the surface mending effect by filling in the shallow grooves and

polishing the surfaces, as shown in Figs. 8(b) and 8(c). Smooth surfaces were observed, and

minor pits replaced the scratched grooves. The smooth surfaces, as shown in Figs. 8(b) to

8(c), are related to the nanoparticle precipitation that occurs on the contact surfaces. These

regions, as well as spots “B” and “C” mentioned in the micrographs (Figs. 8(b) to 8(c)), were

investigated for elemental details via EDX spectroscopy, and the results are given in Fig. 9(c)

to 9(f). The spectrum of these regions exhibited a significant increase in the wt%

concentrations of Ti and Si on the tested surfaces, whereas the mending effect was observed

in spots “B” and “C.” The mending effect of material filling using nanoparticles has been

reported by various researchers (Padgurskas, et al. (26, Choi, et al. (27, Yu, et al. (28)). In

addition to the mending effect, surface polishing has also been reported for sliding tests by

Chang et al. in the SEM observations of nano-TiO2 as additive (Chang, et al. (29))(Chang, et

al. (27)). Such polishing effect was also verified by Peng et al. when nano-SiO2 (Peng, et al.

(30)) and nanometer-sized Al particles (Peng, et al. (25)) were used as lubricant additives.

Such surface enhancement effect of polishing and mending resulted in reduced friction and

improved wear protection capability (Peng, et al. (25, Chang, et al. (29)). For the Oil E, a

further increase in nanoTiO2/SiO2 concentration resulted in the random deposition of nano-

TiO2/SiO2 on the worn surface, as depicted in Fig. 8(d), whereas the polishing effect was not

prominent on the surface. The related EDX spectrum of the micrograph region is shown in

Fig. 9(g), whereas Fig. 9(h) highlights the mending effect in the EDX spectrum of spot “D.”

The dispersion stability tests indicated that the 1 wt% concentration of nano-TiO2/SiO2

resulted in non-uniform dispersion and high sedimentation as shown in Fig. 3 and Fig. 4.

Such non-uniformly distributed nanocomposites result in abrasive wear rather than facilitate

the lubrication phenomenon because of the high deposition on the small part of the worn

surface. Consequently, high wear loss is reported for tests performed using Oil E (palm TMP

ester + 1% TiO2/SiO2), as discussed in the previous section. Thus, uniform nanoparticle

dispersion is essential to improve wear protection.

Fig. 8. The morphology of the wear scar of cylinder line surfaces tested by nanolubricants (a)

wear scar by Oil B (b) wear scar by Oil C (c) wear scar by Oil D (d) wear scar by Oil E.

Fig. 9. EDX analysis of (a) region in Fig. 8a (b) spot ‘A’ in Fig. 8a (c) region in Fig. 8b (d)

spot ‘B’ in Fig. 8b (e) region in Fig. 8c (f) spot ‘C’ in Fig. 8c (g) region in Fig. 8d (h) spot ‘D’

in Fig. 8d.

3.3.2 AFM Analysis for Surface Roughness

The surface topography of the cylinder liner specimens was recorded using an atomic force

microscope (Ambios Q-scope®, USA). The mean surface roughness was assessed at contact

mode. Regions with a scan area of 10 μm × 10 μm were selected randomly to obtain surface

roughness values. Then, 3D images were acquired for each specimen. Figs. 10(a) to 10(d)

show the surface profiles of the cylinder liner specimen when tested using nanolubricants with

nano-TiO2/SiO2 wt% concentrations of 0.25 wt%, 0.50 wt%, 0.75 wt%, and 1 wt%,

respectively. Surface roughness decreases with increasing nano-TiO2/SiO2 wt%

concentrations from 0.25 wt% to 0.75 wt%, and a smoother surface was observed for

specimens tested using the nanolubricant with 0.75 wt% nano-TiO2/SiO2 (Fig. 10(c)). For all

the nanolubricants, the polishing effect provided a reduction in surface roughness. The surface

roughness of the nanolubricant with 1 wt% nano-TiO2/SiO2 was increased (Fig. 10(d))

compared with the nanolubricants dispersed in 0.5 wt% nano-TiO2/SiO2 (Fig. 10(b)) and 0.75

wt% nano-TiO2/SiO2 (Fig. 10(c)). For Oil E, a part of the cylinder surface benefits from the

polishing effect, whereas the other parts exhibit high rough asperity magnitudes, as observed

in Fig. 10(d). Such a behavior of Oil E can be attributed to non-uniform dispersion of nano-

additives which affected its surface enhancement ability by partially polishing the tested

surface. Overall, for all the nanolubricants, the tendency of reduced surface roughness can be

attributed to the polishing effect exhibited by nanocomposites and these results agreed with a

number of previous experimental studies (Thottackkad, et al. (12, Lee, et al. (24, Lee, et al.

(31, Arumugam and Sriram (32)).

Fig. 10 3D AFM images of cylinder liner specimen tested by using nanolubricants (a) Oil B

(b) Oil C (c) Oil D (d) Oil E.

4. Conclusion

The conclusions drawn from this research are provided as follows.

The results showed that nano-TiO2/SiO2 exhibited appreciable dispersion capability,

particularly at a concentration of 0.75 wt%, in the absence of a surfactant. Such

uniform dispersion did not only provide a stable suspension, but was also effective in

reducing friction and wear compared with blank palm TMP ester.

Palm TMP ester enriched with 0.75 wt% nano-TiO2/SiO2 exhibited improved

tribological characteristics under EP conditions and piston ring–cylinder liner

conjunction.

For sliding wear at medium loading, the wear loss of the cylinder liner samples

supported the trends observed in the sliding friction tests.

Surface analyses via SEM, EDX, and AFM confirmed the surface enhancement of the

worn cylinder liner via the mending and polishing effects of nano-TiO2/SiO2.

ACKNOWLEDGEMENT

The authors would like to thank the University of Malaya, which made this study possible

through the High Impact Research Chancellory Grant, Project title: ‘‘Development of

Alternative and Renewable Energy Carrier’’ Grant Number: UM.C/HIR/MOHE/ENG/60 and

UMRG Grant RP016-2012A.

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List of Figures

Fig.1 (a) SEM micrograph and EDX of nanoTiO2/SiO2 (b) SEM micrograph and EDX of

nanoTiO2

Fig.2 (a) High stroke reciprocating test rig (b) Specimen geometry.

Fig.3 Dispersion ability of nanolubricants in terms of optical absorbance (a) NanoTiO2

enriched suspension (b) NanoTiO2/SiO2 enriched suspension.

Fig. 4 Static stability of nanolubricant samples over time.

Fig. 5 Load vs. wear scar diameter (WSD) of lubricant samples.

Fig. 6 (a) Friction coefficient as a function of sliding time for lubricant samples (b) Average

friction coefficient of lubricant samples.

Fig. 7 Wear of cylinder liner specimen after sliding wear tests.

Fig. 8. The morphology of the wear scar of cylinder line surfaces tested by nanolubricants (a)

wear scar by Oil B (b) wear scar by Oil C (c) wear scar by Oil D (d) wear scar by Oil E.

Fig. 9. EDX analysis of (a) region in Fig. 8a (b) spot ‘A’ in Fig. 8a (c) region in Fig. 8b (d)

spot ‘B’ in Fig. 8b (e) region in Fig. 8c (f) spot ‘C’ in Fig. 8c (g) region in Fig. 8d (h) spot ‘D’

in Fig. 8d.

Fig. 10. 3D AFM images of cylinder liner specimen tested by using nanolubricants (a) Oil B

(b) Oil C (c) Oil D (d) Oil E.