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The wear of aluminium-based journal bearing materials under lubrication

Erol Feyzullahoglu *, Nehir S�akirogluKocaeli University, Faculty of Engineering, Mechanical Engineering Department, Kocaeli, Turkey

a r t i c l e i n f o

Article history:Received 20 August 2009Accepted 17 November 2009Available online 22 November 2009

Keywords:A. Aluminium alloysC. Mechanical alloyingE. Wear

a b s t r a c t

The aluminium-based alloys, nowadays, are developed to be used in high performance engine bearings.In this study, new Al-based bearing alloys, which are produced by metal mould casting, were developed;and tribologic properties of these alloys under lubrication were analyzed experimentally. Four differentaluminium alloys were carried out on pin on disc wear tester for that purpose. SAE 1040 steel was used asthe disc material in the wear tester. Friction tests were carried out at 0.231–1.036 N/mm2 pressures andat 0.6–2.4 m/s sliding speeds. Wear tests were carried out at 1.8 m/s sliding speed and at 70 N normalload. Friction coefficients and weight losses of the samples were determined under various working con-ditions as a result of the experiments. The morphographies of the worn surfaces were analyzed. Hardness,surface roughness, and surface temperature of the samples were measured. The results showed that thefriction and wear behaviors of the alloys have changed according to the sliding conditions. The effects ofthe elements except aluminium composing alloys on the tribologic properties were analyzed.Al8.5Si3.5Cu alloy has a lower friction coefficient value than other alloys. Al8.5Si3.5Cu and Al15Sn5Cu3Sialloys, on the other hand, have the highest wear resistance. Al15Pb3.7Cu1.5Si1.1Fe alloy is the most wornmaterial; and Al15Pb3.7Cu1.5Si1.1Fe alloy has the highest wear rate. As a result of the evaluations con-ducted, Al–Sn and Al–Si alloys, which include Si and Sn, can be preferred, among the aluminium alloysthat will work under lubrication, as the bearing material.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Aluminium can be used in same load ranges with tin-bronzescontaining white metal and lead in journal bearings today.Al-based bearing materials have higher fatigue strength than whitemetal bearings and can be used at higher working temperatures.The aluminium alloys have fine properties like low cost, resistanceto corrosive effects, co-activation with steel shafts, high thermalconductivity, fatigue strength, lightness and workability. Internalcombustion engine bearings have to operate under hard condi-tions, and bearings used in internal combustion engines areexpected to work when the lubricant film layer is very thin. Underthese conditions, the biggest problem becomes the low wear resis-tance. Thus, aluminium-based alloys are developed to use in highperformance engine bearings. Recent advances in engine technol-ogy have seen the introduction of new materials. In automotiveindustry, Al-based bearing materials have been lately used in thecrank shaft bearings. Improvement in the efficiency of the internalcombustion engine has caused an increase in the usage of alumin-ium alloys such as Al–Si.

The aim of this study is to develop new Al-based bearing alloysthat have better properties than classic bearing materials and

experimentally analyze the tribologic properties of these alloys un-der lubrication. Thus, four different aluminium-based bearingmaterials were produced and tested. The effects of the elements,except aluminium composing alloys, upon tribologic propertieswere analyzed under lubrication.

2. Research background

Davis and Eyre investigated the wear performance of a range ofbinary aluminium–silicon alloys produced by a novel melt-spraytechnique in 1994 [1]. The samples of the 11 wt.% silicon alloywere produced by conventional casting methods to elucidate theinfluence of silicon morphology on the wear resistance in theirstudy. Wear tests were carried out under dry friction and bound-ary-lubricated conditions on the pin on ring wear test machine.Raising the silicon content of the alloy was reflected an increasein both the wear resistance and transition load. Under boundary-lubricated conditions, the wear rate of the 11 wt.% Si alloy de-creased with a reduction in the silicon particle size; and thesand-cast alloy exhibited a superior performance. Pathak et al.examined the anti-seizure and antifriction properties of Al–Si–Pballoys produced by cast molding method under various lubricationconditions in 1997 [2]. In the study, it was observed that the alloysincluding 10.5–15 wt.% Si and 16 wt.% Si can operate withoutseizure under continuous thin film lubrication and boundary

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* Corresponding author. Tel.: +90 262 3033002; fax: +90 262 3033003.E-mail address: [email protected] (E. Feyzullahoglu).

Materials and Design 31 (2010) 2532–2539

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lubrication while seizure occurs in alloys including 5–11.5 wt.% Siunder hydrodynamic lubrication conditions. Pb, which is added tothe Al–Si alloy, decreases the interface friction; thus, increases theresistance against seizure and decreases the coefficient of friction.It was understood that Si in the alloy is more important than Pb fordecreasing the friction and providing resistance against seizure un-der dry sliding or lubrication.

Zhu et al. in 2000 stated that mechanical alloying plays animportant role in developing the micro structure and wear proper-ties of Al–Pb alloys [3]. Wear properties are determined by mea-suring volumetric wear and friction coefficients by using a blockon disc type wear test device under dry sliding and lubrication. Itwas determined that the wear properties of Al–Pb alloys preparedby mechanical alloying method are better than alloys produced bypowder metallurgy or casting. It was also understood that wearrate decreases by increasing Pb content.

In 2003, Voong et al. examined the wear properties of Al–Si al-loys, used in the crank shaft bearings of internal combustion en-gines, under lubrication [4]. The lubrication of traditional Cr-bearing steel and Al–Si alloy-based components was investigatedusing two fully formulated lubricants, which have the same viscos-ity grade and phosphorous levels, in this study. A pin on reciprocat-ing plate wear tester was used for the tribologic experiments. Inthis research, it was found that, in a completely ferrous-based sys-tem, fully formulated lubricants are effective in reducing wear andfriction and a relatively thick anti-wear film was formed. The wearfilm was absent in the completely non-ferrous systems. Pathak andMohan used two compositions of conventional aluminium base al-loys and equal amounts of Sn and Pb in their research [5]. They em-ployed impeller mixing and chill casting technique for thepreparation of the samples. Tribologic behavior of the alloys wasstudied under lubrication. In this research, it was found that Al–Sn and Al–Pb alloys slightly differed in mechanical properties.Al–Pb alloy bushes showed marginally lower friction than the con-ventional ones. It was observed in the examination that the exis-tence of Sn and Pb (as their amount increases) in Al-basedbearing alloys decreases the coefficient of friction.

Das and Biswas examined the tribologic properties of Al–Si al-loys under boundary lubrication by using lubricants containingadditives in 2004 [6]. They explored the effect of base oil and addi-tivated engine oil on the friction and wear. They analyzed the datain terms of the formation of a mechanically mixed layer at theinterface and the corrosive action of additive addition. Fang et al.examined the effect of the lubricant, used in interface during theco-working of steel and aluminium alloys, on the wear behaviorsof the material [7]. They used a new type of environmentallyfriendly lube additive-amide type modified rapeseed oil. The effectof lubricant on the friction and wear behavior of steel–aluminiumalloy systems was researched with a four-ball wear test machineand Optimol SRV friction tester. In this study, the results showedthat the modified rapeseed oil as an additive could obviously de-crease the wear rate and friction coefficient.

In 2005, Pathak and Mohan examined the tribologic propertiesof aluminium bearing alloys under lubrication [8]. They preparedAl–Pb and Al–Sn alloys by impeller mixing and bottom dischargechill casting technique. They studied the sliding wear characteris-tics of these alloys under lubrication in detail. They also imple-mented a debris and worn surface analysis. It was found in thisstudy that the wear rate of Al–Pb alloys is lower than Al–Sn alloys.They showed that Al–Pb bearing alloys exhibited a superior wearresistance with slightly lower mechanical properties than that ofAl–Sn alloys. This case was based on the production method ofsamples and composition of samples. Le et al. examined the fric-tion properties under lubrication during micro contact betweenaluminium alloy and steel [9]. They used a ball-on-flat reciprocat-ing micro-tribometer to measure the friction coefficient between

aluminium alloy and a steel ball. They investigated the effects ofload, surface roughness, lubricants and boundary additives. Theyfound that the friction coefficient was significantly reduced bythe addition of a lubricant. Observations of the wear tracks and ballsurface showed that the material transfer from aluminium to theball was reduced in the presence of the lubricant.

In recent years, Das et al. examined the wear behaviors of alu-minium alloys under lubrication [10]. Sliding flat faces of steel pinson an Al–Si alloy under lubricated condition were used. They foundthat the wear rate at the 1–10 MPa regimes was very small, withinthe measuring instrument resolution, and also insensitive to con-tact pressure. This situation dealt with the minimal fragmentationof silicon particles and unfractured silicon particles in the subsur-face. Chen et al., on the other hand, examined the tribologic prop-erties of Al–Si alloys under lubrication [11]. They tested twoeutectic grade Al–Si alloys with similar silicon percentages(11 wt.% Si and 12 wt.% Si). The alloys differed in matrix hardnessbut had comparable silicon particle sizes and morphologies. Theyused a ball-on-disc tribometer under the light load in lubricatedconditions. It was shown that this was consistent with the observa-tion that the Al–11 wt.% Si with the higher matrix hardness suf-fered less surface damage – less significant amounts of particlesinking in and aluminium piling up – when compared with thedamage belonging to the Al–12 wt.% Si with softer matrix.

3. Experimental procedure

TE 53 Slim Model Multi-Purpose Friction and Wear Tester wasused in the analysis of friction and wear behaviors of test samples(Fig. 1). The wear tester was developed under a licence from theUnited Kingdom National Centre of Tribology. The wear testerwas produced in pursuance of ASTM G 77 and ISO/DIS 7148-2 stan-dards by Phoneix Tribology Ltd. The strain gauge transducer in thewear tester measures the friction forces at a contact point. Load cellcan measure forces up to 150 N (Fig. 2). The disc has a diameter of60 mm and was produced with 95 HRB hardness SAE 1040 steel[12].

In the experiments, samples were tested at various loads andspeeds under lubrication. It was conducted at 0.6, 1.2, 1.8, and2.4 m/s (200, 400, 600 and 800 rpm) sliding speeds and at 0.231,0.489, 0.630, 0.817, and 1.036 N/mm2 surface pressures. Sampleswere scaled during 10 h of work in order to determine the weightloss with the precision balance. As a consequence of this process,

Fig. 1. Pin on disc wear tester.

E. Feyzullahoglu, N. S�akiroglu / Materials and Design 31 (2010) 2532–2539 2533


68 km of sliding distance was obtained in each sample. The testswere carried out at 1.8 m/s speed and 70 N normal load while20 W/50 engine lubricant was used for the lubrication. The exper-iments were implemented at 3.3 cm3/h lubricant flow. During thestudy, friction force was recorded with a load cell; and temperatureof sample was measured with a infrared thermometer. The testswere conducted at 22 �C and in 50% RH humidity.

The chemical compositions of the samples were defined with aSpectro Spectramax Optical Emission Spectrometer. It incorporatesa microprocessor based control and measuring system. Variousprograms are available for low alloy steels, tool steels, stainlesssteels, cast irons, etc. in Spectrometer. The chemical analysis ofsamples are presented in Table 1.

Roughness measurements of worn surfaces of samples afterwear tests were performed with Mahr MP4i surface roughness tes-ter in pursuant of DIN 4768. It has along a trace length of 0.06 in.

Worn surfaces of the samples were analyzed as micro and thentheir photos were taken after the wear tests. Micro surface analysiswas carried out with an Olympus optical microscope.

Hardness measurements of the samples were carried out inWolpert hardness tester as HB and performed with 2.5 mm steelball diameter indenture and 612 N load.

In the wear tests, weights of the samples were measured by aPrecisa 125 A precision balance. The balance has 0.0001 g precisionand can measure maximum weight of 220 g.

Temperatures of the test samples, on the other hand, were mea-sured with Testo 925 infrared thermometer. This thermometer of-fers quick and reliable surface temperature readings. It can workbetween�18 �C and + 260 �C. It holds temperature readings for 7 s.

3.1. Tested materials

3.1.1. Al–Si bearing alloysAl–Si alloys are widely used in engineering applications. They

have good mechanical properties like excellent corrosion resis-tance, high fatigue strength, and low expansion characteristic [2].In general, the addition of Si into aluminium in normal loads in-creases the wear resistance. The main effect of Si particulates is

that they intensify the hardness. Hard Si particulates, covered bysoft and full matrix, increase the wear resistance [13]. Such ele-ments as Cu, Mg, and Pb were added to Al–Si alloys for several pur-pose: high performance bearing materials are developed by addinga soft metal with low melting temperature, like Pb, into Al–Si al-loys. In the previous studies, it was observed that wear rate ofAl–Si and Al–Si–Pb alloys decreases with the increasing Si and Pbcontents and their load carrying capacities also increase [2]. Itwas also observed that the addition of Cu and Mg into Al–Si alloysincreases the hardness and wear resistance [14].

3.1.2. Al–Pb bearing alloysThe aluminium alloys, containing lead, is an important alterna-

tive to aluminium alloys that also contain tin. Lead is more effec-tive in soft phase alloying than tin as it provides higher scratchand friction characteristics. Al–Pb alloys are the leading of Al-basedbearing materials; in which Pb is soft phase. The homogenizedspread of soft phase in aluminium matrix is generally quite essen-tial for the wear properties [15]. The engines today, which havehigh temperatures, require the use of materials with higher melt-ing point and soft phase rather than classic bearing materials likeCu–Sn–Pb or Al–Sn. This problem can usually be solved by usingAl–Pb alloys [16].

3.1.3. Al–Sn bearing alloysAl–Sn alloys, which have high resistance and excellent surface

properties, have been used as the bearing materials for a long time.The compatibility of Al–Sn alloy is close to the tin-based whitemetals; and the fatigue strength of white metals is higher. Sn isfound as Al and Sn composition zones in the aluminium alloys.In Al–Sn composition zones, free Sn grains disperse in the alumin-ium matrix.

3.2. Production of tested materials

The samples consist of four different types of aluminium alloys.Three different samples and commercial pure aluminium (97%)were used in the experiments:

Alloy-1: Al8.5Si3.5Cu alloy was produced after the material,kept in induction furnace, was casted into a cylindrical metaldie and then hardened with a rapid cooling. This material isan aluminium alloy containing mostly Si.Alloy-2: The alloy was at first melted by adding commercial Al–Si alloy and tin after induction furnace was heated up to 700 �C.After the tin was melted, temperature was increased over1000 �C and then copper was added. The molten metal wascasted into metal die and hardened by leaving it to cool.Al15Sn5Cu3Si alloy is an aluminium alloy that mostly containsSn.Alloy-3: At first, the alloy was produced by casting Al–Cu–Si–Fealloy, melted in induction furnace, into metal die, adding leadinto liquid alloy in the die, and then hardening it with the cool-ing method. Al15Pb3.7Cu1.5Si1.1Fe alloy is an aluminium alloythat mostly contains Pb.

Fig. 2. Load cell, sample and disc on wear tester.

Table 1Chemical analysis of samples (%).

Sample Al Si Cu Sn Pb Fe Mg Mn Zn Ni Ti

Al8.5Si3.5Cu 86.2 8.50 3.51 0.02 0.04 0.88 0.13 0.11 0.49 0.04 0.02Commercial pure Al 97.3 0.62 0.26 0.02 0.01 0.9 0.4 0.07 0.22 0.18 0.02Al15Pb3.7Cu1.5Si1.1Fe 76.5 1.5 3.7 0.13 15.1 1.17 0.78 0.16 0.41 0.17 0.9Al15Sn5Cu3Si 73.6 3.1 5.1 15.2 0.37 0.95 0.27 0.13 1.2 0.04 0.02

2534 E. Feyzullahoglu, N. S�akiroglu / Materials and Design 31 (2010) 2532–2539


Alloy-4: This alloy was provided ready-made as commercialpure aluminium (97 wt.% Al).

Samples were dimensioned as in Fig. 3 with machining aftercasting.

4. Results

4.1. Microstructure of tested materials

The grain structure of Al8.5Si3.5Cu alloy was examined in theoptical microscope as displayed in Fig. 4a. The greyish areas incoarse grains, in the zone left, are the Si particulates. The Si partic-ulates separated in the structure and then they surrounded thegrains. The grains and grain limits of commercial pure Al wereidentified also in the optical microscope (Fig. 4b). It was observedthat the grains, which occurred recently after hardening, werequite big and they increased independently from each other.

As can be seen in Fig. 5a, Al15Pb3.7Cu1.5Si1.1Fe shows the typ-ical casting structure. The phase rich of aluminium are demon-strated in light colour while the phase rich of lead are in thedark. A typical dendritic structure appeared in Al15Sn5Cu3Si alloyas displayed in Fig. 5b. This dendritic structure is found in all alu-minium alloys. The Si particulates are dark grey, cornered, and inthe mesh, among dendrites, in the structure.

4.2. Hardness of tested materials

The hardness of samples is shown in Table 2. As seen can befrom the table, Al8.5Si3.5Cu alloy containing more Si was the hard-

est sample whereas the commercial pure Al was found to be thesoftest.

4.3. Friction measurements of tested materials

The relationship between surface pressure and the friction coef-ficient of alloys are shown in Figs. 6 and 7. In general analysis of the

φ60

15

13 13

Fig. 3. Overview of the sample.

Fig. 4a. Al8.5Si3.5Cu. Observed photographs of samples by optical microscopy(50�).

Fig. 4b. Commercial pure Al. Observed photographs of samples by optical micros-copy (50�).

Fig. 5a. Al15Pb3.7Cu1.5Si1.1Fe. Observed photographs of samples by opticalmicroscopy (20�).

Fig. 5b. Al15Sn5Cu3Si. Observed photographs of samples by optical microscopy(20�).



graphs, Al15Sn5Cu3Si alloy was found to have the highest frictioncoefficient value for all speeds. On the other hand, Al8.5Si3.5Cu andAl15Pb3.7Cu1.5Si1.1Fe had the lowest friction coefficient values inall speeds. The variation of the friction coefficient for minimumand maximum surface pressures, in comparison with the slidingspeed, was shown in Figs. 8 and 9. A decrease was observed forthe friction coefficients of alloys in maximum pressure (1.036 N/mm2) as the sliding speed increased. The highest friction coeffi-cients in either graph were found for Al15Sn5Cu3Si alloy whereasthe lowest friction coefficients were found for Al8.5Si3.5Cu alloy.

When we examined Figs. 6–9, friction coefficient of Al15Sn5-Cu3Si was found to be higher than other alloys; and similarlyAl8.5Si3.5Cu alloy’s was lower than the others.

4.4. Wear measurement of tested materials

The relationship between the weight loss and sliding distanceunder lubrication is shown in Fig. 10. The biggest weight losswas observed in Al15Pb3.7Cu1.5Si1.1Fe alloy whereas Al15Sn5-Cu3Si and Al8.5Si3.5Cu alloys were the least worn alloys. Theweight loss that occurred as a result of wear in alloys increases reg-ularly with the sliding distance.

Wear rate was estimated by Eq. (1) below [3]:

Ws ¼DV

F � Lð1Þ

where Ws is the wear rate, DV is the lost volume, F is the appliedload, and L is the sliding distance.

As can be seen in Fig. 11, the lowest wear rate was found forAl15Sn5Cu3Si alloy while the highest wear rate was found forAl15Pb3.7Cu1.5Si1.1Fe alloy.

A rapid increase was observed to be occurring when the contactinitially begun for the temperatures of alloys at the ambient tem-

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Fric

tion

coef

fici

ent

Sliding speed (m/s)

Al8.5Si3.5CuCommercial pure Al Al15Pb3.7Cu1.5Si1.1Fe Al15Sn5Cu3Si

Fig. 8. Relationship between friction coefficient and sliding speed at 0.231 N/mm2

pressure.

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.60.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Fric

tion

coef

fici

ent

Sliding speed (m/s)

Al8.5Si3.5Cu Commercial pure Al Al15Pb3.7Cu1.5Si1.1Fe Al15Sn5Cu3Si

Fig. 9. Relationship between friction coefficient and sliding speed at 1.036 N/mm2

pressure.

Table 2Hardness of samples.

Sample Hardness (HB)

Al8.5Si3.5Cu 94Al15Pb3.7Cu1.5Si1.1Fe 79Al15Sn5Cu3Si 65Commercial pure Al 55

0.2 0.4 0.6 0.8 1.0 1.20.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Fric

tion

coef

fici

ent

Surface Pressure (N/mm2)

Al15Sn5Cu3Si Commercial pure Al Al8.5Si3.5Cu Al15Pb3.7Cu1.5Si1.1Fe

Fig. 6. Relationship between friction coefficient and surface pressure at 0.6 m/ssliding speed.

0.2 0.4 0.6 0.8 1.0 1.20.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

Fric

tion

coef

fici

ent

Surface pressure (N/mm2)


Fig. 7. Relationship between friction coefficient and surface pressure at 2.4 m/ssliding speed.



perature as can be seen in Fig. 12. Then, the increase in tempera-ture went down as the process continued. The highest temperature

increase here actualized for Al8.5Si3.5Cu and Al15Pb3.7Cu1.5-Si1.1Fe while the lowest temperature increase was observed inAl15Sn5Cu3Si and commercial pure al alloys.

4.5. Wear surfaces of samples

Photos of wear surfaces can be seen in Figs. 13a, 13b, 14a and14b. When the photos are examined, it can be seen that there oc-curred deeper wear traces on the surface of Al15Pb3.7Cu1.5Si1.1Fethan the other alloys.

4.6. Surface roughness

The surface roughness values of wear surfaces of alloys areshown in Table 3 and Fig. 15. The biggest surface roughness valuewas found for Al8.5Si3.5Cu alloy with Ra = 2.69 lm while the low-est roughness value was found for Al15Sn5Cu3Si alloy.

5. Discussion

It was found that the hardness value of commercial pure Al wasseen to be lower than other aluminium alloys when the hardnessvalues were analyzed in this study (Table 2). The alloy materialssuch as Si, Cu, and Fe are effective on the hardness values. Thehighest hardness value here was found for Al8.5Si3.5Cu alloy

0 10 20 30 40 50 60 700.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Wei

ght l

oss

(mgr

)

Sliding distance (km)

Al8.5Si3.5Cu Commercial pure Al Al15Pb3.7Cu1.5Si1.1FeAl15Sn5Cu3Si

Fig. 10. Relationship between sliding distance and weight loss at 1.8 m/s slidingspeed and 70 N load.

10 20 30 40 50 60 700.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Wea

r ra

te x

10-8

(mm

3 /Nm

m)

Sliding distance (km)


Fig. 11. Relationship between sliding distance and wear rate at 1.8 m/s slidingspeed and 70 N load.

0 10 20 30 40 50 605

10

15

20

25

Tem

pera

ture

(ºC

)

Time (minute)


Fig. 12. Relationship between time and temperature at 1.8 m/s sliding speed and70 N load.

Fig. 13a. Al8.5Si3.5Cu. Observed photographs of worn surfaces by optical micros-copy (100�).

Fig. 13b. Commercial pure Al. Observed photographs of worn surfaces by opticalmicroscopy (100�).



which had the most Si content. This alloy contained 8.5 wt.% Si;thus, addition of Si into alloy can be stated to be increasing thehardness values [13]. Besides, the addition of Cu also has a greatsignificance for the increase in the hardness values of alloys[17,18].

The contacting surfaces are being separated completely by a lu-bricant film for the cases of hydrodynamic lubrication. In certainsections, the lubricant film between the surfaces might break dueto the load increase (surface pressure) [19]. In such cases, bound-ary lubrication may occur in certain sections of the surface [2,5].In this study, there were unstable fluctuations in the friction coef-ficient–surface pressure graphs. This can be attributed to the thin-ning or breaking of the lubricant film (Figs. 6 and 7). The frictioncoefficients of alloys do not change as the pressure on contact sur-face increases.

As for the sliding speed–friction coefficient relation, it was ob-served that the friction coefficients increased rapidly in the begin-ning of the process (Figs. 8 and 9). This was due to the transientmetallic contact occurred in the beginning of this sliding move-ment. It was observed that friction coefficient then decreased;and a more stable stance was obtained after the lubrication. Thus,a decrease would be observed in the friction coefficients of alloysin the maximum pressure as sliding speed rises. Especially underhigh pressures, the main reason of this decrease in the frictioncoefficient can be explained as the dispersion of the lubricant be-tween the surfaces due to speed completely.

When the friction coefficient graphs are examined, it can be ob-served that Al15Sn5Cu3Si had the highest friction coefficient whileAl8.5Si3.5Cu had the lowest friction coefficient. Thus, it can be sta-ted that the chemical properties of the samples are not very effec-tive on the friction behavior of the material under lubrication butthe material–lubricant interaction plays a very significant role onthe friction behavior. On the other hand, in means of frictionbehavior, the Al15Pb3.7Cu1.5Si1.1Fe alloy where Pb is the softphase was found to be better than Al15Sn5Cu3Si alloy where Snis the soft phase. This result is compatible with the results thatPathak et al. obtained from the studies they conducted under lubri-cation [2,5].

The Al8.5Si3.5Cu alloy, which was seen to have the highesthardness value in wear tests, is one of the least worn materials.This superior wear behavior of Al8.5Si3.5Cu alloy results from itsmicrostructure. The addition of Si into Al increases the wear resis-tance [13]. Hard Si phase provides an excellent wear resistance forthis alloy: the Si particulates covered by ductile and firm alumin-ium matrix increase the toughness of the material and provideresistance towards wearing by preventing plastic deformation[1,2]. The Cu content in the structure, on the other hand, increasesthe rate of intermetallic Si phase and wear resistance [13,14].Al15Sn5Cu3Si alloy lost 0.01% by weight, and thus were less worncompared with the other alloys. The most worn material wasAl15Pb3.7Cu1.5Si1.1Fe alloy which lost 0.2% of its weight(Fig. 10). The Pb phase, found as the soft phases in microstructureof Al15Pb3.7Cu1.5Si1.1Fe alloys, is ductile and low strength. Thisphase decreases the wear resistance by increasing the ductility ofalloy. This case complies with the experimental work of Davisand Eyre [1].

When the wear rates were evaluated, Al15Pb3.7Cu1.5Si1.1Fe al-loy had a higher wear rate than other alloys. The most worn mate-rial was also Al15Pb3.7Cu1.5Si1.1Fe alloy. There is a directrelationship between the wear rate and weight loss as can be seenby the wear rate formula. It is also shown in the wear rate graphthat the wear rate of Al15Pb3.7Cu1.5Si1.1Fe decreases with thesliding distance (Fig. 11).

Fig. 14b. Al15Sn5Cu3Si. Observed photographs of worn surfaces by opticalmicroscopy (100�).

Table 3Various surface roughness values of the samples.

Sample Ra (lm) Rq (lm) Rz (lm) Rmax (lm) Rt (lm)

Al8.5Si3.5Cu 2.69 3.7 16.9 22.8 23.4Commercial pure Al 1.8 2.31 10.7 12.5 13.7Al15Pb3.7Cu1.5Si1.1Fe 1.17 1.57 7.8 11.8 11.8Al15Sn5Cu3Si 1.03 1.35 7.48 11 11

Al8.5Si3.5Cu Commercial pure Al

Al15Pb3.7Cu1.5Si1.1Fe

Al15Sn5Cu3Si0.0

0.5

1.0

1.5

2.0

2.5

3.0

Surf

ace

Rou

ghne

ss R

a (μ

m)

Fig. 15. Surface roughness of the samples.

Fig. 14a. Al15Pb3.7Cu1.5Si1.1Fe. Observed photographs of worn surfaces by opticalmicroscopy (100�).



In the shaft-bearing systems, the lubricant in the interface pre-vents the metallic contact, and thus decreases friction. As a result,the heating caused by friction decreases and lubricant serves as acooler; decreasing the temperature. In this study, it was observedthat there was a rapid increase in temperature at the beginningbut this increase descended over time. Friction was quite severeat first. The increases in the temperature values occurred due tothe friction. The temperature increases of Al15Pb3.7Cu1.5Si1.1Feand Al8.5Si3.5Cu alloys were higher than the temperature in-creases of other alloys (Fig. 12). Al8.5Si3.5Cu alloy had the highesthardness and the highest surface roughness values whileAl15Pb3.7Cu1.5Si1.1Fe alloy was the most worn material. Thesefeatures also affected the increase in temperature.

When the photographs of worn sample surfaces were exam-ined, it was observed that the worn surfaces were generally simi-lar. There were deeper wear traces on the surface of the mostworn Al15Pb3.7Cu1.5Si1.1Fe alloy. Al15Pb3.7Cu1.5Si1.1Fe alloyshowed a lower wear resistance. Therefore, it had deeper weartraces. On the surfaces, there were sections that are worn due moreto the breaking of the lubricant film (Figs. 13a and 13b).

It was observed that Al8.5Si3.5Cu alloy, containing 8.5 wt.% Si,had the highest surface roughness value when the roughness val-ues of wear surfaces of samples were compared (Fig. 15). This con-dition resulted from the chemical compound and mechanicalproperties of the samples since hard Si particles also affect theroughness values.

6. Conclusions

The main conclusions drawn from this study are as follows:

1. Al8.5Si3.5Cu alloy have the lowest friction coefficient valuesunder lubrication and also possess superior tribologic behaviorthan the other alloys analyzed.

2. Al8.5Si3.5Cu and Al15Sn5Cu3Si alloys have the greatest wearresistance under lubrication with the least weight loss.Al15Pb3.7Cu1.5Si1.1Fe alloy, on the other hand, is worn morethan the others and shows a lower wear resistance.

3. Al8.5Si3.5Cu alloy, containing 8.5 wt.% Si, has the highest hard-ness value.

4. Al15Pb3.7Cu1.5Si1.1Fe alloy has the highest wear rate underlubrication.

5. Temperature increases in Al15Pb3.7Cu1.5Si1.1Fe andAl8.5Si3.5Cu alloys are more than the other alloys underlubrication.

6. Al8.5Si3.5Cu alloy has the highest surfaces roughness valueunder lubrication.

7. Under lubrication, Al–Sn and Al–Si alloys have better wearproperties than Al–Pb alloys.

8. There are deeper wear traces on the surface of the most wornAl15Pb3.7Cu1.5Si1.1Fe alloy under lubrication.

When the above mentioned conclusions are evaluated, it can bestated that, among the aluminium alloys that are to work underlubrication, the ones containing Si and Sn, Al–Sn and Al–Si alloys,can be preferred.

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