Tribology in Industry Wear Properties of A356/10SiC/1Gr Hybrid

148

Vol.35,No.2(2013)148‐154

TribologyinIndustry

www.tribology.fink.rs

WearPropertiesofA356/10SiC/1GrHybridCompositesinLubricatedSlidingConditions

M.Babića,B.Stojanović

a,S.Mitrovića,I.Bobićb,N.Miloradović

a,M.Pantića,D.Džunić

a

aFacultyofEngineering,UniversityofKragujevac,Kragujevac,Serbia,bInstitutionInstituteofNuclearSciences“Vinca”,UniversityofBelgrade,Belgrade,Serbia.

Keywords:

HybridcompositesAluminiumSiCGraphiteWearLubricationMML

ABSTRACT

This paper presents basic tribological properties of A356/10SiC/1Gr hybridcomposites in conditions with lubrication. Hybrid composite specimen isobtainedbycompocastingprocedure. A356aluminiumalloy isusedasabasematrixalloy,reinforcedwith10wt%ofSiCand1wt%ofgraphite. Tribologicaltestsaredoneonadvancedandcomputersupportedtribometerwithblock‐on‐disc contact pair. By the experimental plan, test is conducted under threedifferentvaluesofslidingspeed,threedifferentvaluesofnormalload,differentslidingdistances,andalsodifferentlubricants.SEMandEDSareusedforwearanalysis.TheanalysishasshownthepresenceofMML,whichmeansthattherewastransferofmaterialfromsteeldisctocompositeblock.

©2013PublishedbyFacultyofEngineering

Correspondingauthor:

BlažaStojanovićFacultyofEngineering,UniversityofKragujevac,Kragujevac,SerbiaE‐mail:[email protected]. INTRODUCTIONAluminium is the most attractive material inautomotive, airplane, space and precise devicesindustry. Improvement of mechanical andtribological properties of aluminium can beachieved through aluminium reinforcement withthe proper material and through creatingcomposite material. The most effectiveimprovement of these properties is achievedthrough creating hybrid composites with two ormore types of reinforcements. By adding theceramic reinforcement, mechanical properties ofthematrixarechanged,butinthatcaseproblemofmachinability occurs. To improve machinability,thegraphite isaddedtocompositematerials that

are already reinforced with ceramic material.Presence of graphite reduces mechanicalproperties (hardness decreases), but tribologicalpropertiesareimproved[1‐5].Basavarajappa et al [6‐8] have studied thetribologicalbehaviourofhybridcompositeswithaluminum base Al2219 reinforced by SiC andgraphite. They studied the tribologicalproperties ofhybrid compositeswith5,10and15% SiC and 3%Gr obtainedwith process ofliquid metallurgy. The tribological tests showthatweardecreaseswithincreasingSiCcontentinthehybridcomposite.Withincreasingslidingspeedandnormalload,wearrateofcompositesis growing. Mahdavi and Akhlaghi [9,10] have

RESEARCH

M.Babićetal.,TribologyinIndustryVol.35,No.2(2013)148‐154

149

studied the tribological properties of Al / SiC /Gr hybrid composites obtained by In situPowderMetallurgy process. Aluminum alloy Al6061isusedasabasereinforcedwithgraphite9% and 0 ÷ 40% SiC. The tribological tests aredoneontribometerwithpinondisccontact,andthe composite with 20 % SiC has the bestproperties. Further increase of SiC leads toincreasedwearofhybridcomposites.Suresha and Sridhara [11‐14] have studied theeffect of SiC content and graphite on thetribological behaviour of hybrid Al/SiC/Grcomposites with aluminum base LM25 (Al‐Si7Mg0.5)obtainedbystircastingprocess.AmesandAlpashave[15]studiedthetribologicaltesting of hybrid composites with a base ofaluminum alloy A356 reinforced with 20 % SiCand3÷10%Gr.Thetribologicaltestsaredoneontribometerwith block on ring contact. Thewearrate of hybrid composites is significantly lowerthan the wear rate of the base material withoutreinforcements,especiallyatlownormalloads.Vencl et al [16,17] have studied the tribologicalbehaviour of hybrid composites with the A356matrixreinforcedwithSiC,Al2O3andgraphite.Thetribological tests are done on tribometerwith pinondisccontactandshowthatthewearandfrictioncoefficientdecreaseswithadditionofgraphite.This paper presents tribological behaviour ofhybridcompositeswithaluminumbaseofA356alloy reinforcedwith SiC and Gr obtainedwithcompocastingprocedure.The testsaredoneoncomputer aidedblock‐on‐disc tribometerunderlubricated sliding conditions by varying thecontactpairs(slidingspeedandnormalload).

2. EXPERIMENT

2.1 TheprocedureforobtainingcompositesHybridA356/SiC/Grcompositesareobtainedbythemodified compo‐casting procedure (infiltration ofparticlesinthesemi‐solidifiedmeltA356alloy).sub‐eutecticAl‐SialloysEnAlSiMg0,3(A356alloy)isusedas a basis. Using compocasting procedure, particlereinforcements are easily infiltrated / trapped. Thissolvestheproblemofwettabilityontheborderbaseandreinforcements.Thecostofcompositeproducingwiththatprocessismuchlower.

a)

b)

Fig.1.The structure of: a) basematerial A356, andb)hybridcompositeA356/10SiC/1Gr.Figure 1 shows the structure of the basematerial A356 and the hybrid composite with10wt%SiC and 1wt%Gr. When mixingcomposites, particles of graphite have becomefragmentedwithregardtooriginalsizeof35µm.The picture shows the distribution of SiCparticulatereinforcements,thesizeof39um.

2.2 Planofexperimentanddescriptionof

equipmentTribological tests are done on advanced andcomputer supported tribometer with block‐on‐disc contact pair in accordancewithASTMG77standard.ContactpairconsistsofrotatingdiscofdiameterDd=35mmandbroadnessbd=6.35mm,andastationaryblockofsize6.35x15.75x10.16mm3. The discs are made of steel 90MnCrV8hardnessof62‐64HRCwithgrindedsurfaces.The tests were performed in lubricated slidingconditionsonthesampleswiththebeststructural,mechanicalandanti‐corrosiveproperties.


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Fig.2.Tribometer.Thevaluesofslidingspeed(0.25m/s,0.5m/sand1m/ s) and thenormal loads (40N,80Nand 120 N) are in accordancewith the plan ofexperiment.The testsareperformed for slidingdistanceof2400m.

Fig.3.Lubricationofthecontactpair.Alltestsusedthesamehydrauliclubricantwithimproved anti‐wear properties, viscosity VG46(ISO3848).Lubricant ishousedinasmalltank,andlubricationisdonesothatthebottomofthediscisimmersedtouptodepthof3mmintothesmall tank with lubricant, whose volume is 30ml. During rotation of the disc, oil iscontinuously introduced into the zone of thecontact and makes boundary lubrication ofcontactpair(Fig.3).Allexperimentswererepeated5 times,andthemean values of obtained values are taken asauthoritative.

3. RESULTSOFTRIBOLOGICALTESTS

Resultsof tribological testsofhybridcompositeA356/10SiC/1Gr and basic material A356 areshowninthefollowingdiagrams.

V=0.25 m/s, lubrication

0 500 1000 1500 2000 2500

Sliding distance, m

0

1

2

3

4

5

6

7

8

9

Wea

r, m

m3

x 10

-2

A356, F1=40N A356, F2=80N A356, F3=120N A356/10SiC/1Gr, F1=40N A356/10SiC/1Gr, F2=80N A35610SiC/1Gr, F3=120N

V=0.5 m/s, lubrication

0 500 1000 1500 2000 2500

Sliding distance, m

0

1

2

3

4

5

6

7

8

Wea

r, m

m3

x 1

0-2

A356, F1=40N A356, F2=80N A356, F3=120N A356/10SiC/1Gr, F1=40N A356/10SiC/1Gr, F2=80N A356/10SiC/1Gr, F3=120N

V=1 m/s, lubrication

0 500 1000 1500 2000 2500

Sliding distance, m

0

1

2

3

4

5

6

7

8

We

ar,

mm

3 x

10

-2

A356, F1=40N A356, F2=80N A356, F3=120N A356/10SiC/1Gr, F1=40N A356/10SiC/1Gr, F2=80N A356/10SiC/1Gr, F3=120N

Fig.4.Wearvolumeforallthreevaluesofslidingspeed.Diagramsofwearvolumeareformedonthebasisofwearscarwhichisobtainedbymeasuringafter150m,300m,1200m,2400m,andtheyaregivenforallthreevaluesofslidingspeed(Fig.4).It is obvious that the wear rate of the hybridcomposites A356/10SiC/1Gr is several timesless than the wear rate of the base materialA356.With increaseof sliding speed,wear rateof the hybrid composite A356/10SiC/1Gr andthe base material are decreases. Wear ratedependencehasalmostlineardependenceforallvaluesofthenormalloads(Fig.5).


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s=2400m,lubrication

0.00 0.25 0.50 0.75 1.00 1.25

Sliding speed, m/s

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

We

ar

rate

, m

m3 x

10

-5/m

A356, F1=40 N A356, F2=80 N A356, F3=120 N A356/10SiC/1Gr, F1=40 N A356/10SiC/1Gr, F2=80 N A356/10SiC/1Gr, F3=120 N

Fig.5.Wearratedependenceonslidingspeed.

With increase of normal load, wear rateincreases. This increase is particularlypronouncedatthebasematerialA356(Fig.6).

s=2400m,lubrication

0 40 80 120 160

Load, N

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

We

ar

rate

, m

m3 x

10

-5/m

A356, V1=0.25 m/s A356, V2=0.5 m/s A356, V3=1.0 m/s A356/10SiC/1Gr, V1=0.25 m/s A356/10SiC/1Gr, V2=0.5 m/s A356/10SiC/1Gr, V3=1.0 m/s

Fig.6.Wearratedependenceonnormalload.

Wear rate dependence on normal load andsliding speed for sliding distance of 2400m, isshowninFig.7.

Fig. 7.Wear rate dependence on normal load andslidingspeed.

After the tribological tests, SEM analysis isperformed forwear scarof basematerialA356and hybrid A356/10SiC/1Gr composite, whosemicro‐photosareshowninFig.8.

a)

b)

Fig. 8. SEM micro‐photos of wear scar: a) basematerialA356,b)hybridcompositeA356/10SiC/1Gr.4. ANALYSESOFOBTAINEDRESULTSTheanalysesoftheobtainedtribologicalresultsshowthatthewearrateorwearvolumeismuchlowerinthehybridcompositesA356/10SiC/1Grcomparedtothebasematerial.DecreaseofwearrateoccursduetotheeffectsofSiCfromhybridcompositeincontactwithasteeldisc.WearratedependenceonnormalloadandslidingspeedareshowninFigs.9and10asthe3Dplots.Wearrateisapproximatedbyexponentialfunctionwithahighcorrelationcoefficient.Both tested materials show that at least wearoccursatthemaximumslidingspeedof1m/sandtheminimumnormalloadof40N.


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Fig.9.Wearratedependenceonthebasematerial.

Fig. 10. Wear rate dependence on the hybridcompositesА356/10SiC/1Gr.

Fig.11.The accumulation of iron in the compositeA356/10SiC/1Gr,120N,0.25m/s.SEMmicroscopyshowsthatduetothecontactoftheSiCcompositesandSiphasesfromthebasicA356, wear of steel disc occurs. Fe particlesenter the surface layer of the composites and

leadtothecreationofmechanicallymixedlayer(MML). The formation of MML layer ischaracteristic of aluminum alloys reinforcedwith SiC [18‐23]. Iron accumulates around theSiCparticlestakingapositionofsmallparticleofgraphite. At some parts white lines appearenrichedwith ironoxides,whichare consistentwiththeslidingdirection(Fig.11).Figure12showstheSEMphotographofpartofthehybrid composite A356/10SiC/1Gr. Wear scar isobtainedbyslidingspeedof0.25m/sandnormalloadof120Ninconditionsoflubrication.Athigherloads, the dominantwearmechanism is abrasivewear. SiC particles (darker) and iron particles(bright colours) are clearly visible on the scar.ConfirmationoftheseassumptionsisobtainedbyEDSanalysis,asshowninthetwospectrums.ThefirstspectrumshowsthepresenceofSiCparticles,andtheparticlesofironanditsoxidescanbeseenonsecondspectrum.

Fig.12. EDS analysis A356/10SiC/1Gr, 120 N, 0.25m/s,SEM.


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5. CONCLUSIONWeartestsofhybridcompositesA356/10SiC/1Grshow their superior performance in relation tothe base material A356. Applied compocastingmodified procedure, in addition to low prices,confirmsthegooddistributionofreinforcementsinthecomposite.Wear rate on A356/10SiC/1Gr hybridcomposites is 3 ÷ 8 times lesser than thewearrate on the basematerial A356. It is especiallybigdifferenceofwearrateat the lowestslidingspeedof0.25m/sandmaximumnormalloadof120 N. Wear rate decreases with decrease ofnormalloadandincreaseofslidingspeed.SEM microscopy and EDS analysis confirm agood distribution of SiC reinforcements in thehybrid composite. Also, advent mechanicallymixed layer (MML) isobvious, respectively, theappearance of iron and its oxides in thehybridcomposite.ACKNOWLEDGMENTSThispaperpresentstheresearchresultsobtainedwithin the framework of the project TR–35021,financially supported by the Ministry ofEducationandScienceoftheRepublicofSerbia.REFERENCES

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Tribology in Industry Wear Properties of A356/10SiC/1Gr Hybrid