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The Effect of Temperature on the Tribological Behaviorof RBD Palm StearinTiong Chiong Ing a , A. K. Mohammed Rafiq b , Y. Azli b & S. Syahrullail ca School of Graduates Studies, Universiti Teknologi Malaysia 81310, UTM Skudai, Johor,Malaysiab Faculty of Biomedical Engineering & Health Science, Universiti Teknologi Malaysia 81310,UTM Skudai, Johor, Malaysiac Faculty of Mechanical Engineering, Universiti Teknologi Malaysia 81310, UTM Skudai, Johor,Malaysia

Available online: 25 Apr 2012

To cite this article: Tiong Chiong Ing, A. K. Mohammed Rafiq, Y. Azli & S. Syahrullail (2012): The Effect of Temperature on theTribological Behavior of RBD Palm Stearin, Tribology Transactions, 55:5, 539-548

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Tribology Transactions, 55: 539-548, 2012Copyright C© Society of Tribologists and Lubrication EngineersISSN: 1040-2004 print / 1547-397X onlineDOI: 10.1080/10402004.2012.680176

The Effect of Temperature on the Tribological Behavior ofRBD Palm Stearin

TIONG CHIONG ING,1 A. K. MOHAMMED RAFIQ,2 Y. AZLI,2 and S. SYAHRULLAIL3

1School of Graduates StudiesUniversiti Teknologi Malaysia 81310

UTM Skudai, Johor, Malaysia2Faculty of Biomedical Engineering & Health Science

Universiti Teknologi Malaysia 81310UTM Skudai, Johor, Malaysia

3Faculty of Mechanical EngineeringUniversiti Teknologi Malaysia 81310

UTM Skudai, Johor, Malaysia

The wide use of petroleum-based oils raises concerns with

regard to pollution, and the rising of awareness of greenhouse

gases has created a demand for the use of environmentally

friendly and biodegradable lubricants for industrial applica-

tions. Vegetable oils are one of the bio-oils that have been pro-

moted as a replacement for petroleum products, in part due

to their environmentally friendly characteristics; they are non-

toxic, biodegradable, and easy to dispose of. Many researchers

have performed studies on sunflower oil, corn oil, and soy oil,

but few have studied palm oil as a lubricant. Palm oil produced

in a high-throughput manner could fulfill the demand for bio-

based lubricants. In this study, the influence of temperature on

friction and wear performance for refined, bleached, and de-

odorized (RBD) palm stearin and additive-free paraffinic min-

eral oil is presented. The experiments were conducted using a

four-ball tribotester. Test temperatures of 55, 65, 75, and 85◦C

were used. The sliding speeds were set to 1,200 rpm. Experi-

ments were run for 1 h under a 392.4 N load. The results of

RBD palm stearin were compared with those of paraffinic min-

eral oil. The experimental results showed that the RBD palm

stearin had better performance compared to paraffinic mineral

oil in terms of reducing frictional constraints.

KEY WORDS

RBD Palm Stearin; Paraffinic Mineral Oil; Four-Ball Tri-botester; Friction Coefficient; Wear Scar Diameter

INTRODUCTION

The worldwide increase in concern over the health-relatedand environmental effects of petroleum oil, as well as its limitedsupply, has generated interest in the use of biodegradable prod-

Manuscript received July 18, 2011Manuscript accepted March 20, 2012

Review led by Yeau–Ren Jeng

ucts. Moreover, special attention has been paid to protecting theenvironment against pollution from petroleum-based lubricants.A survey by Delgado, et al. (1) found that nearly 12 million tonsof lubricant waste is disposed of in the environment annually.

Biodegradable oils are becoming an important alternative toconventional lubricants as a result of the increased awarenessof ecological pollution. Vegetable oil, including animal fat, wasused as a lubricant thousands of years ago. As described byNosonovsky (2), in ancient Egypt, vegetable oils were used in theconstruction of monuments. Thus, the use of vegetable oils as alubricant in the industrial sector is not a new idea. For the lastthree decades, the lubrication industry has been trying to formu-late environmentally friendly lubricants with technical character-istics equal to those of mineral oil. The advantages of choosingvegetable oil rather than oils from other sources are that theyare biodegradable, less toxic compared to petroleum-based oils,easily produced, and renewable. Vegetable oils have lubricatingabilities that are better than those of currently used mineral orsynthetic oils because of the large amount of unsaturated and po-lar ester groups they contain, as reported by Alla and Richard (3).Kalin and Vizintin (4) explained that these components maintainthe desired conditions during reciprocating sliding.

Randles and Wright (5) claimed that the long-chain fatty acidspresent in vegetable oils improve their intrinsic boundary lubri-cation properties. Vegetable oils have been shown to have goodlubricating abilities, because they give rise to a low coefficientof friction. However, according to Bowden and Tabor (6), eventhough the coefficient of friction is low, the wear rate is high. Thisbehavior is possibly due to a chemical attack on the surface by thefatty acids present in the vegetable oil. The metallic soap film rubsaway during sliding and produces nonreactive detergents that in-crease wear.

In recent work, palm oil has been widely tested for engineer-ing applications. The potential of palm oil as a fuel for diesel en-gines (Kinoshita, et al. (7); Bari, et al. (8)), hydraulic fluid (WanNik, et al. (9)), and lubricants (Obi and Oyinlola (10)) has beenconfirmed in previous studies. In the early 20th century, the Palm

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Oil Research Institute of Malaysia (PORIM; presently referredto as the Malaysian Palm Oil Board, MPOB) successfully cre-ated palm oil methyl ester from crude palm oil using transesteri-fication. The transesterification method shortened the molecularchain in the palm oil from about 57 to 20 molecules, thus improv-ing the palm oil by reducing its viscosity and making it less pol-luting. In addition, Masjuki and Maleque (11) claimed that thisprocess improved the thermal stability of the palm oil.

A study by Odi-owei, et al. (12) also showed that when scuff-ing takes place, thermal instability is responsible because a run-away increase in temperature takes place at some critical com-bination of load, speed, and material and lubricant properties.Under such conditions, a surface’s basic characteristics, such ashardness and texture, can change. Lubricant film failure criteria,which consider the critical temperature, relate failure load to tem-perature or sliding speed. As reported by Ravikiran and Jahanmir(13), traditionally, wear has been analyzed as a function of loadrather than pressure due to the belief that wear is controlled byload alone.

Many researchers (Masjuki and Maleque (11); Yunus, et al.(14); Waleska, et al. (15)) have been using various vegetable-based oil lubricants and additives, but there are limited referencesto refined, bleached, and deodorized (RBD) palm stearin as abase lubricant or additive. The present research focused on in-vestigating the lubricating properties of RBD palm stearin usinga four-ball tribotester. The RBD palm stearin exists in a semi-solid state at room temperature. Additive-free paraffinic mineraloil (known simply as paraffinic mineral oil) was used as the com-parative lubricant in this research. The experiments were run for1 h at 1,200 rpm under a 392.4 N load. The results showed thatRBD palm stearin demonstrated better lubricating propertiescompared to paraffinic mineral oil, especially in reducing the fric-tional constraint. However, RBD palm stearin produces higherwear at high temperatures compared to paraffinic mineral oil.

EXPERIMENTAL METHOD

Apparatus

The use of a four-ball wear machine, first described by Boer-lage (16), is an established technique for investigating lubricantcharacteristics. This instrument uses four balls: three on the bot-tom and one on top. The upper ball is held in a collet at the lowerend of the vertical spindle, which is driven by the motor. The bot-tom three balls are held firmly in a ball pot containing the lubri-cant being tested and are pressed against the upper rotating ball.The important components are the oil cup assembly, collet, lock-nut adaptor, and standard steel ball bearings. The components’surfaces were cleaned with acetone before the tests. A schematicdiagram of the four-ball tribotester is shown in Fig. 1.

Materials

The test balls used in this experiment were made of chrome al-loy steel and met AISI E-52100 specifications. The test balls hada diameter of 12.7 mm, an extra polish (EP) grade of 25, and ahardness of 64–66 HRC. At the start of each new test, four newballs were cleaned with acetone and wiped with a lint-free indus-trial wipe.

Fig. 1—Schematic diagram of the four-ball tribotester assembly.

Lubricants

The lubricants used for this experiment were RBD palmstearin and paraffinic mineral oil. The paraffinic mineral oil wasused for comparison with the RBD palm stearin. The volume oflubricant used in this experiment was 10 mL for each test lubri-cant. The density and viscosity of RBD palm stearin and paraf-finic mineral oil were measured and are shown in Table 1.

Test Procedures

In these experiments, the test temperatures, which were ex-pected to influence the friction and wear characteristics of lubri-cant, were evaluated. The experiment was carried out for 1 h un-der a 392.4 N load and with a spindle speed (rotational speed)of 1,200 rpm. The three bottom balls in the wear test were eval-uated, and the average diameter of the circular scar formed wasmeasured. The lubricating ability of the RBD palm stearin wasalso evaluated based on the frictional torque produced in com-parison with the paraffinic mineral oil. Prior to the experiments,all parts in the four-ball (upper and lower balls and oil cup) werecleaned thoroughly using acetone and wiped using a lint-free in-dustrial wipe. No trace of solvent remained when the parts wereassembled or when the lubricant was introduced. The steel ballbearings were placed into the oil cup assembly and the oil cupwas tightened using a torque wrench to prevent the bottom steelballs from moving during the experiments. The upper spinningball was locked inside the collet and tightened into the spindle.The test lubricant was introduced into the oil cup assembly. The

TABLE 1—DENSITY OF RBD PALM STEARIN AND PARAFFINIC

MINERAL OIL

Test Oil

Properties RBD Palm Stearin Paraffinic Mineral Oil

Density (kg/m3) 862.0 848.0Viscosity at 40◦C

(mPa·s)47.0 96.5

Viscosity at 75◦C(mPa·s)

17.5 21.5

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researcher confirmed that the oil filled all of the voids in the testcup assembly. The oil cup assembly components were installedinto the frictionless disc in the four-ball machine and, to avoidshock, the test load was applied slowly. The lubricant was thenheated to the desired temperature. When the set temperature wasreached, the drive motor, which was set to drive the top ball at thedesired speed, was started. After the 1-h test period, the heaterwas turned off and the oil cup assembly was removed from themachine. The test oil was then drained from the oil cup and thescar area was wiped using a tissue. The bottom balls were placedon a microscope base that was designed to hold the balls duringmicroscopic evaluation.

Friction Evaluation

A 20-kg beam-type load cell was used to measure the frictionaltorque. The load cell was fitted at a distance of 80 mm from thecenter of the spindle. The coefficients of friction in these experi-ments were calculated according to IP-239 standardsas shown inEq. [1], where µ is the coefficient of friction, T is the frictionaltorque (kg·mm), W is the applied load (kg), and r is the distancefrom the center of the contact surface on the lower balls to theaxis of rotation, which was determined to be 3.67 mm. A similarcalculation was used by Husnawan, et al. (17) and Thorp (18).

µ = T√

63Wr

[1]

The coefficient of friction plays a major role in determiningthe transmission efficiency via moving components where less re-sistance contributes to higher efficiency. Therefore, in terms oflubrication, less friction is desirable.

Wear Tests

The tests were carried out at a load of 40 kg (392.4 N) and at1,200 rpm for 1 h from 55 to 85◦C in 10◦C increments. The wearscar diameter of the three balls was measured using a CCDmicro-scope and scanning electron microscopy (SEM).

Flash Temperature Parameter

The flash temperature parameter (FTP) was calculated for allexperimental conditions. The FTP is a single number that is usedto express the critical flash temperature at which a lubricant willfail under given conditions. For the conditions used in the four-ball test, the following relationship was used:

FTP = Wd1.4

, [2]

where W is the load (kg) and d is the mean wear scar diame-ter (mm). These findings were explained in detail in previous re-search conducted by Bhattacharya, et al. (19) and Lane (20).

RESULTS AND DISCUSSION

Coefficient of Friction

The performance of RBD palm stearin as a lubricant was in-vestigated using a four-ball tribotester under various bulk tem-peratures. Figure 2 shows the coefficient of friction of the ballbearings lubricated with RBD palm stearin (denoted PS) andparaffinic mineral oil (denoted P2). It can be seen that the coeffi-cient of friction obtained increased when the bulk oil temperature

Fig. 2—Distribution of the coefficient of friction.

was increased. This effect was due to the decreased lubricant vis-cosity caused by the increased temperature, as reported by Clark,et al. (21).

The coefficient of friction for the RBD palm stearin was lowercompared to paraffinic mineral oil at 55, 65, and 75◦C. However,at 85◦C, the coefficient of friction for the RBD palm stearin wasslightly higher compared to that for paraffinic mineral oil. Ac-cording to Sharma, et al. (22), the fatty acid chains in vegetableoil permits monolayer film formation on the sliding surface, whichprevents direct metal-to-metal contact.The frictional torque datawere recorded by computer, and the frictional coefficient was cal-culated using Eq. [1]. The results are illustrated in Fig. 2. The co-efficient of friction for the contact between the lubricated ballswith the paraffinic mineral oil and the RBD palm stearin was in-creased with increasing normal loads. However, the coefficientof friction for paraffinic mineral oil was higher compared to thatfor the RBD palm stearin. This is because the stearic acid in theRBD palm stearin helps maintain the lubricant layer; therefore,the RBS palm stearin provides a lower coefficient of friction com-pared to the paraffinic mineral oil. A similar finding was reportedby Syahrullail, et al. (23).

These results are similar to the results of other researchers,as shown in Table 2. As explained by Odi-owei, et al. (12), thereis a good possibility that the scuffing phenomenon will occur inthe first few minutes. Scuffing would cause an increment of load;however, this load would decrease when steady-state conditionswere achieved. This observation was also described by Masjukiand Maleque (11). Other researchers have claimed that the fattyacid in vegetable oil (whether it comes from pure vegetable oilor a blend of vegetable oil and minerals) would reduce the coeffi-cient of friction (Masjuki and Maleque (11); Waleska, et al. (15))and provide better antiwear performance (Yunus, et al. (14); Ad-hvaryu, et al. (24)). However, the oxidation of vegetable oil whenoperating at high temperature should be one of the most impor-tant parameters to be alerted (Abdalla and Patel (25)).

Mean Wear Scar Diameter

After the experiments, the test balls (ball bearings) were sep-arated from the oil cup and the wear scar on the surface of the

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TABLE 2—PREVIOUS FRICTION COEFFICIENT RESULTS FOR VEGETABLE OIL TESTED WITH A FOUR-BALL TRIBOTESTER

Wear Analysis Coefficient of Friction Lubricant Authors

Wear scar diameter 0.06–0.11: before scuffing SAE 10 mineral oil Odi-owei, et al. (12)SEM, wear particle micrograph 0.34–0.36: during scuffing

0.10–0.13: after scuffingAverage wear scar diameter

No micrograph of the worn surfaceBar graph of the coefficient of friction: palm kernel

methyl ester showed comparable wear andfriction properties to commercial hydraulic oil

Palm kernel oil Yunus, et al. (14)

Wear characteristicNo micrograph of the worn surface

Average coefficient of friction: soybean oil shows alow coefficient of friction

Refined and modifiedsoybean oil

Waleska, et al. (15)

Wear scar diameterMicrograph of the worn surface

Coefficient of friction: 5% palm oil methyl esterdecreased the coefficient of friction

Palm oil methyl ester Masjuki and Maleque(11)

Wear track analysisSEM analysis: wear morphology

Coefficient of friction: fatty acids enhanced theantiwear properties of vegetable oil

Modified soybean oil Adhvaryu, et al. (24)

Wear scar diameter No graph of the coefficient of friction Coconut oil, sunflower oil,rapeseed, palm olein

Abdalla and Patel (25)

No micrograph of the worn surfacePicture of swarf analysis

Vegetable oils show almost similar wear scardiameter compared to mineral oil

three ball bearings, which were locked at the bottom, were in-spected with a CCD camera. From the images, the diameters ofthe wear scars were measured and average values were calcu-lated. The distribution of the mean wear scar diameter (MWSD)was plotted as shown in Fig. 3.

At low bulk oil temperatures (55 and 65◦C), the wear scar di-ameter on the ball bearings for both the RBD palm stearin andthe paraffinic mineral oil increased with increasing temperature.The MWSD for RBD palm stearin was smaller compared to thatfor the paraffinic mineral oil. RBD palm stearin showed betterlubricity in preventing metal-to-metal contact, as shown by thelower wear scar diameter value.However, at the higher bulk oiltemperatures (75 and 85◦C), the performance of the RBD palmstearin decreased, as shown by the higher wear scar diametervalue compared to the paraffinic mineral oil. According to Isaac(26), the decrease in viscosity caused by the increase in tempera-ture is attributed to the decrease in the likelihood of oil moleculesforming thermally stable “molecular clusters.” This is because the

Fig. 3—Wear scar diameter.

viscosity of oil molecules is proportional to the effective hydrody-namic volume of the oil molecule. More ordered molecular clus-ters will occupy a larger volume, resulting in higher viscosities.The study by Maleque, et al. (27) also showed that, at highertemperatures, the films formed by the fatty acid chains seem tobe less stable and cause comparatively higher wear compared toparaffinic mineral oil. Wu, et al. (28) and Masjuki and Maleque(29) claimed that this phenomenon could be related to the thin-ning of the fatty acid thin film, which acts as an effective barrieragainst metal-to-metal contact, resulting in the breakdown of thethin film at temperatures greater than 75◦C.

Flash Temperature Parameter

Figure 4 shows the effect of test temperature on the flash tem-perature for RBD palm stearin and paraffinic mineral oil. As seenin the figure, in general, the flash temperature for paraffinic min-eral oil increased with increasing test temperature. However, for

Fig. 4—Flash temperature parameter for RBD palm stearin and paraffinicmineral oil under various test temperatures.

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Fig. 5—Wear scars of ball specimens: (a) RBD palm stearin, 55◦C; (b) RBD palm stearin, 65◦C; (c) RBD palm stearin, 75◦C; (d) RBD palm stearin, 85◦C;(e) paraffinic mineral oil, 55◦C; (f) paraffinic mineral oil, 65◦C; (g) paraffinic mineral oil, 75◦C; and (h) paraffinic mineral oil, 85◦C. (color figureavailable online.)

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Fig. 6—Wear-worn surfaces observed using a CCD camera. (color figure available online.)

RBD palm stearin, the opposite results were seen. The maximumflash temperature for paraffinic mineral oil was obtained at a testtemperature of 85◦C, but for RBD palm stearin the maximumflash temperature was obtained at 55◦C.

According to Bhattacharya, et al. (19), the possibility of lubri-cant film breakdown is greater at low flash temperatures, whichindicates poor lubricity. Therefore, it was concluded that the fattyacid thin film became less stable at higher temperatures in slid-ing contact. Therefore, at the higher temperatures (75◦C andabove), paraffinic mineral oil showed better lubricating perfor-mance compared to RBD palm stearin.Pressure is the force (nor-mal load) per unit area. In this study, the area was the contactarea of the ball bearings, which was almost circular in shape. Be-cause the value of the normal load was constant (40 kg), the pres-

sure on the contact area depended on the wear scar diameter. Thelarger the wear scar diameter, the less pressure that was appliedto the contact surface. As shown in Fig. 3, at 85◦C the MWSDof the ball bearings lubricated with RBD palm stearin was largercompared to those lubricated with paraffinic mineral oil, whichmeans that the lubrication condition had collapsed and metal-to-metal contact occurred. Yu, et al. (30) stated that there is a highpossibility of plastic deformation during the sliding contact thatoccurs at high contact pressures, which generates heat on the slid-ing surface. Referring to Fig. 4, the flash temperature was highestat a test temperature of 85◦C. Based on this result and other stud-ies, the possibility of plastic deformation at higher temperatureswas greater for RBD palm stearin than for paraffinic mineraloil.

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Worn Surface Characteristics

Figure 5 shows the representative wear scar from the bottomball bearings for both RBD palm stearin and paraffinic mineraloil at different lubricant temperatures. For RBD palm stearin, itis clear that the wear scar diameter increased when the temper-ature increased. For paraffinic mineral oil, the largest wear scarwas at a lubricant temperature of 65◦C; however, the differencein the wear scar diameter was in the range of 0.1 mm (Fig. 3).

For the RBD palm stearin at lubricant temperatures of 55 and65◦C, the wear scar was circular. At lubricant temperatures of 75and 85◦C, the edge of the wear scar was slightly ragged and ob-scured by metal. With RBD palm stearin at high temperatures,the lubricant chain started to break down and metal-to-metalcontact occurred. In addition, the wear particles that existed be-tween the mating surfaces of the ball bearing caused adhesivewear.For paraffinic mineral oil at all lubricant temperatures thewear scar was circular. There was a slightly rough edge found inthe wear scar at a temperature of 75◦C. This observation is par-allel with the results plotted in Fig. 4 for the flash temperature.RBD palm stearin had a low flash temperature at high tempera-tures, which means that the possibility of lubricant breakdown ishigh.Micrographs of the worn surfaces, produced after 1-h testsand lubricated with the RBD palm stearin and the paraffinic min-eral oil under different test temperatures, are shown in Fig. 6.The RBD palm stearin and the paraffinic mineral oil were com-pared in regard to the effects of temperature on the wear surfaces.At a low working temperature, a thin lubricant film with parallelgrooves formed to prevent metallic contact and create smoothsurface regions. Some of the grooves were deep and the otherswere shallow grooves in between. In this region, abrasion wasseen as the dominant wear mechanism. At a high working tem-perature, the thin lubricant films were broken and adhesive wearappeared to be predominant. For both lubricants, material trans-fer occurred from one ball to another, and grooves were found onthe worn surface. For RBD palm stearin, the long-chain fatty acidmolecules that formed on the metal surface effectively reducedthe wear and friction. A similar finding was previously reportedby Farooq, et al. (31).

For experimental lubrication with paraffinic mineral oil, theball bearings were heated above the austenizing temperature(at the local contact point) and then rapidly cooled, due to thelubricant reservoir (collet). The breakdown of the hydrocarbonlubricant in the contact zone led to increased carbon content dueto the diffusion of the carbon and gases into heated deformedmaterial.Thus, an increased coefficient of friction resulted froman increase in the operating temperature. A similar finding waspreviously described by Jones and Scoot (32). Haseeb, et al. (33)found that the fatty acids in RBD palm stearin undergo an oxida-tion process to form inorganic oxides, such as Fe3O4, which playan important role in the formation of a lubricant film on rubbingsurfaces. This can be observed as the dark-colored zone in Fig.6. In extreme working temperatures, the color of the lubricantmay become darker due to the contamination of the inorganicoxides; analyses using Fourier transform infrared (FTIR) may beone of the best methods to find any impurities in a lubricant, asdescribed by Waleska, et al. (15).In addition, the stearic acid in

(a)

0 20 40 60 80 100Distance (micron)

Dep

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P2 (85oC)PS (85oC)

(b)

0 20 40 60 80 100Distance (micron)

Dep

th

P2 (65oC)PS (65oC)

(c)

0 20 40 60 80 100Distance (micron)

Dep

thP2 (75oC)PS (75oC)

(d)

0 20 40 60 80 100Distance (micron)

Dep

th

P2 (85oC)PS (85oC)

Fig. 7—Mutual comparison of the wear surface topography.

RBD palm stearin shows a greater affinity to be absorbed intothe steel surfaces, as reported by Farooq, et al. (31). This inter-action of the acid molecules with the steel surface resulted in theformation of chemically polymerized molecules, which play animportant role in reducing friction and wear during sliding. It isthe result of a tribochemical reaction on the metal surface, whichreduced the wear and friction at the low working temperaturesused in this experiment.Figure 7 compares the topographies ofthe worn surfaces for each experimental condition. The topog-raphy was measured perpendicular to the direction of rotation(perpendicular to the scar line), shown as line A-B in Fig. 6e. Inthis surface topography graph, the surface asperities, consistingof valleys and hills, were compared. At working temperatures of55 and 65◦C, the asperities on the worn surface for both the RBD

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Fig. 8—SEM micrograph of worn surface for all experimental conditions.

palm stearin and the paraffinic mineral oil were almost the same.At higher working temperatures (75 and 85◦C), the worn surfacesof the ball bearings lubricated with paraffinic mineral oil no sig-nificant changes in the surface topography pattern were observedcompared to paraffinic mineral oil at a low temperature. How-

ever, for the RBD palm stearin, there were a few areas where thehills collapsed, creating a slightly larger wear scar compared tolubrication with paraffinic mineral oil. When the upper and lowerball bearing make contact, in the microscopic view, the asperitiesjunctions (hills) on the ball bearing surfaces would contact each

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other. Due to the normal load applied to the operational system,the contacted asperities would deform plastically. Then, thematerial would be removed or plucked from the asperities onthe wear surface. As explained by Williams (34), this adhesivewear phenomenon can usually be observed on a surface with a“blocky” shape or dimension (Figs. 7c and 7d).Worn material anddebris (including abrasive and wear particles) was swept out ofthe contact and contaminated the lubricant. Due to the rotationof the upper ball bearing, the lubricant swept the contaminant tothe next ball bearing contact point (ball bearing gap). Particlesthat were larger than the lubricant film thickness damaged thesurface. In general, at this stage, the wear was classified asabrasive wear due to the long wear groove found on the wornsurface.Further confirmation of the wear behavior of the ballbearing surfaces lubricated by RBD palm stearin and paraffinicmineral oil were obtained by analyzing the wear scar using SEMat appropriate magnifications, as shown in Fig. 8. Overall obser-vation of the worn surface of the ball bearings lubricated with theRBD palm stearin at a working temperature of 55◦C showed thatthere were uneven grooves, with varying scar depths, indicatingabrasive wear. At this working temperature (55◦C), fatty acidmolecules in the RBD palm stearin help maintain the lubricantlayer and establish the boundary lubrication condition, whicheffectively prevents metal-to-metal contact. The coefficient offriction was lowest value compared to other experimental condi-tions. A few grooves were spotted with metallic flakes dislodgedfrom the contact surface, as shown in Fig. 8a. The flakes werecontinuously deforming along the groove. According to Ge,et al. (35), the flakes were formed due to plastic deformationin the sliding contact area. At a working temperature of 65◦C(Fig. 8b), parallel grooves with various depths could clearly beseen; Singh and Gulati (36) reported similar parallel groovesresulting from stiff particulate debris that caused abrasion wear.Lubricant breakdown occurred at higher working temperaturesof 75 and 85◦C. Figures 8c and 8d clearly show the rough surfacefrom the discontinuous plastic deformation on the worn surfacecaused by adhesive wear, indicating breakdown of the lubricantfilm (Masjuki and Maleque (11)). In the most severe conditions,the metal surface would fuse or weld. As a result, the coefficientof friction for RBD palm stearin increased with an increase intemperature. Yufu, et al. (37) reported that vegetable oil couldoxidize at high temperatures, leading to corrosive wear.Forparaffinic mineral oil at working temperatures of 55 and 65◦C(Figs. 8e and 8f), light ploughs with a few spots of materialtransfer could be seen, indicating that abrasive wear was thedominant wear mechanism. At a working temperature of 75◦C(Fig. 8g), parallel grooves could be seen, with no material transferobserved. As reported by Ren, et al. (38), this could be associatedwith abrasion caused by the debris from the detached ball bear-ing surface. Kim, et al. (39) suggested that a polishing processcould occur in this situation, resulting in a reduced coefficientof friction (Fig. 2). At a working temperature of 85◦C (Fig. 8h),the worn surface showed both shallow and deep parallel grooveswith rough ploughing, indicating breakdown of the lubricantfilm. However, the condition was not as severe as that found forlubrication with RBD palm stearin; the coefficient of friction forthe latter was slightly higher than for the paraffinic mineral oil.

CONCLUSION

The tribological behavior of RBD palm stearin at four dif-ferent elevated temperatures was evaluated using a four-ball tri-botester machine. All results were compared with additive-freeparaffinic mineral oil. The findings can be summarized as fol-lows:

1. RBD palm stearin showed better lubricity compared to theparaffinic mineral oil at lower test temperatures, as shown bya low coefficient of friction and less possibility of lubricantfilm breakdown (evaluated using the FTP value). However,at higher test temperatures, the paraffinic mineral oil showedbetter performance.

2. In wear scar diameter analysis under several test temper-atures, the RBD palm stearin showed smaller wear scardiameters at the lower test temperatures. At higher testtemperatures, the RBD palm stearin showed larger wear scardiameters compared to the paraffinic mineral oil. This is dueto the fact that at high temperature, the lubricant film formedby fatty acids tended to be less stable or was more likely tobreak down.

3. Abrasive wear was the dominant wear mechanism for bothRBD palm stearin and paraffinic mineral oil at all workingtemperature; a few spots of adhesive wear were observed ata working temperature of 85◦C.

ACKNOWLEDGEMENTS

The authors thank the Faculty of Mechanical Engineering atthe Universiti Teknologi Malaysia for their support and cooper-ation during this study. The authors are also grateful for the Re-search University Grant from the Universiti Teknologi Malaysia,the Ministry of Higher Education (MOHE), and the Ministry ofScience, Technology and Innovation (MOSTI) of Malaysia fortheir financial support.

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