effect of particle size on wear of particulate reinforced aluminum alloy composites at elevated...

Effect of Particle Size on Wear of Particulate ReinforcedAluminum Alloy Composites at Elevated Temperatures

Suresh Kumar, Ratandeep Pandey, Ranvir Singh Panwar, and O.P. Pandey

(Submitted March 18, 2013; in revised form June 22, 2013)

The present paper describes the effect of particle size on operative wear mechanism in particle reinforcedaluminum alloy composites at elevated temperatures. Two composites containing zircon sand particles of20-32 lm and 106-125 lm were fabricated by stir casting process. The dry sliding wear tests of thedeveloped composites were performed at low and high loads with variation in temperatures from 50 to300 �C. The transition in wear mode from mild-to-severe was observed with variation in temperature andload. The wear at 200 �C presented entirely different wear behavior from the one at 250 �C. The wear rateof fine size reinforced composite at 200 �C at higher load was substantially lower than that of coarse sizereinforced composite. Examination of wear tracks and debris revealed that delamination occurs after run inwear mode followed by formation of smaller size wear debris, transfer of materials from the countersurfaces and mixing of these materials on the contact surfaces. The volume loss was observed to increasewith increase in load and temperature. Composite containing bigger size particles exhibit higher loss undersimilar conditions.

Keywords metal matrix composite, SEM, wear debris, wearmechanism

1. Introduction

The mechanism of wear encountered in a sliding system andparameters affecting the wear provides a better understandingfor fabricating materials with enhanced wear resistance.Aluminum alloy matrix composites (AMCs) are widely usedin engineering applications due to high specific strength, highcorrosion resistance, ease of fabrication, and low cost (Ref 1-5).AMCs with ceramic particles reinforcement are suitablecandidate for materials used in tribological applications. Theparticle size and nature of reinforced ceramics have pronouncedeffect on microstructural and tribological properties of AMCs.The wear characteristics greatly depend on reinforcementvolume and size of the reinforced particles. The dry slidingun-lubricated wear of the composites is a complex processinvolving not only mechanical but also thermal and chemicalinteractions between the surfaces in contact. In sliding systemthe drastic change in wear rate occurs by delamination. Though,it is not the only reason but mostly observed under severe wearconditions. The study of wear surface indicates that parametersaccelerating delamination wear are of great importance forsliding systems to check the severe wear of the materials.

Studies on the tribological characteristics of AMCs con-taining varying percentage and various reinforcements such as

Al2O3, SiC, TiO2, TiC, B4C, TiB2, and fly ash of various sizesare available in the literature (Ref 4-6). Apart from these thenumber of work discussing the wear performance of fine andcoarse particle reinforced composites also exists (Ref 1, 5-12).However, report on the wear behavior of fine and coarse sizereinforced composites with variation in temperature is limited(Ref 13). Moreover, study of wear behavior of fine and coarseparticles reinforced composites in the light of delaminationmechanism is not reported. In this work, emphasis is made tounderstand the effect of reinforced particle size on type of wear,particularly delamination wear in discontinuous particle rein-forced aluminum alloy composites. Apart from this otheroperative mechanism is also discussed to correlate the entirewear behavior with variation in load and temperature.

The mechanism of delamination wear was proposed by Suh(Ref 14), on the basis of series of experiments. Suh andJahanmir (Ref 15) have explained various factors affecting thedelamination wear of materials. They have given the reasonsfor delamination where the removal of large wear particlesoccur by the process of plastic deformation of the surface layerwhich occurs by subsurface crack nucleation followed by crackpropagation. Kanchanomai et al. (Ref 16) investigated the roleof porosity on delamination wear of metal injection molded316L stainless steel and reported that at low sliding speed effectof porosity on delamination wear is not pronounced ascompared to high sliding speed. Similarly delamination wearis correlated with porosity by Simchi and Danninger (Ref 17) insintered plain iron. Huang et al. (Ref 9) investigated the sizeeffect of SiC particles on the tribological behavior of SiCreinforced magnesium AMCs prepared by melt-stirring tech-nique. Their results show variable wear rate at different particlesize. However, hardness is observed to increase with increasingparticle size. Mondal et al. (Ref 10) conducted abrasive weartest of SiC particles reinforced aluminum alloy composites as afunction of applied load, reinforcement size, and volumefraction. They found that the wear resistance of composite

Suresh Kumar, Ranvir Singh Panwar, and O.P.Pandey, School ofPhysics and Materials Science, Thapar University, Patiala 147004Punjab, India; and Ratandeep Pandey, Department of MechanicalEngineering, BBSBEC, Fatehgarh Sahib, Patiala 140407, India.Contact e-mail: [email protected].

JMEPEG �ASM InternationalDOI: 10.1007/s11665-013-0642-8 1059-9495/$19.00

Journal of Materials Engineering and Performance

varies inversely with square of the reinforcement size. Das et al.(Ref 11) have compared the wear properties of alumina andzircon sand reinforced AMCs and reported that the decrease inparticle size improves wear resistance. Celebi et al. (Ref 12)reported that increase in particle size of SiC from 1 to 5 lmshows increase in hardness of the copper matrix compositefabricated by powder metallurgy route. In this present work, wehave analyzed the effect of particle size on delamination wear,which is the major cause of loss of materials and also the otherwear mechanism occurring during the dry sliding wear ofparticle-reinforced composites.

2. Experimental

2.1 Materials and Methods

In the present work, Al-Si alloy (LM13 piston alloy) of neareutectic composition is used as matrix material. High purityzircon sand (ZrSiO4) obtained from Indian rare earths Ltd.Mumbai (India) is used as reinforcement. The chemical compo-sitions of the alloy (in wt.%) is: 11.8 Si, 0.3 Fe, 1.2 Cu, 0.4 Mn,0.9 Mg, 0.2 Zn, 0.02 Ti, 0.9 Ni, 0.02 Pb, balance Al. Thecomposition (in wt.%) of the zircon sand is: 65.3 ZrO2,32.8 SiO2, 0.27 TiO2, and 0.12 Fe2O3.

Composites containing 15 wt.% reinforcement were devel-oped by stir casting route. The detail of experimental processfor preparation of composites is described in our earlier work(Ref 18). Required quantity of alloy was melted in a graphitecrucible in an electric furnace. This molten mass was stirredusing a graphite impeller at a speed of 630 rpm. After theformation of vortex in the melt, the zircon sand particles werecharged inside the vortex at the rate of 15-20 g/min. intothe melt during stirring. Zircon sand particles of fine grade(20-32 lm) and coarse grade (106-125 lm) were selected forpresent work. Before charging the zircon sand particles, it waspreheated at 400 �C to remove moisture and other volatilesubstances. The stirring was continued for another five minuteseven after completion of the particle feeding. This was done toensure homogeneous distribution of the sand particles insidethe melt. The molten mass was finely poured into the metalmold and allowed to solidify at room temperature. Duringfabrication of composite, the amount of LM13 alloy, stirringduration, and position of stirrer in the crucible were keptconstant to minimize the contribution of variables related tostirring on distribution of reinforced particles.

The amount of 15 wt.% reinforcement of zircon sand isdone on the basis of our earlier work using same compositionswhere tribological and mechanical properties obtained has beenoptimized (Ref 19). In order to compare and correlate the effectof reinforcement particle size on delamination wear and wearrate, two composites containing a total of 15 wt.% fine andcoarse reinforcement were fabricated and have been designatedas composite-A and composite-B, respectively.

2.2 Material Characterization

The microstructural analysis has been done with the help ofboth optical (Eclipse MA-100, Nikon) and scanning electronmicroscope (SEM) (JEOL, JSM-6510LV, Japan) at variousmagnifications. Energy Dispersive X-Ray Spectroscopy (EDS)analysis of samples was also done at different phases. Beforeoptical observation the sample was mechanically polished

and etched by Keller�s reagent to obtain better contrast.Microhardness of the different phases was measured by Vickerhardness testing machine (Mitutoyo, Japan) which was done oneach set of sample by taking minimum of five indentations persample at 100 gf load.

Wear tests of the reinforced composites without lubricationwere performed under the ambient temperatures between 25and 300 �C and at a relative humidity between 35 and 50%using a wear and friction monitor (TR-20, Ducom, Bangalore).The cylindrical shaped samples (309 9 mm) of compositewere tested against the hardened EN32 steel disk havingchemical composition (0.14% C, 0.52% Mn, 0.18% Si, 0.13%Ni, 0.05% Cr, 0.06% Mo, 0.019% P, 0.015% S, balance Fe) andhardness 65 HRC. Before testing, each specimen was ultraso-nicated in acetone to remove the adherent greasy material. Wearrate was determined by measuring specimen height changeusing a linear variable displacement transducer (LVDT). Tostudy the wear behavior, wear rate was calculated using theformula, [W (mm3/m) = height change (mm)9 pin area (mm2)/sliding distance (m)]. The wear tests of specimen of allcomposites have been carried out up to 3000 meters of slidingdistance at a constant sliding velocity of 1.6 m s�1 and underfive different loads varying from 1 to 5 kg with variation intemperatures.

3. Results and Discussion

A thorough examination of wear rate, worn surfaces, andwear debris provide useful information regarding operativewear mechanism at different conditions in materials. The wearmechanism was understood after analyzing the wear track andwear debris under SEM along with EDS analysis.

3.1 Microstructural Analysis

The optical micrographs of composite-A reinforced with15 wt.% fine particles (20-32 lm) at different magnificationsare shown in Fig. 1(a) and (b). Figure 1(a) depicts the lowermagnification micrograph of composite -A which shows fairlyuniform distribution of reinforced particles in alloy matrix.Uniform distribution of second phase particles is required forachieving better wear resistance and mechanical properties.Near uniform distribution of particles in a molten alloy isachieved due to the high shear rate during stirring which alsominimizes the particle settling tendency (Ref 20, 21). However,agglomeration of particles is also observed which is visible atcertain places in Fig. 1. Figure 1(b) shows the micrograph ofthe composite where fragmented dendrites in the alloy matrixcan be seen, though limited dendritic growth in the particledepleted region is also visible. This growth has occurred due toclustering of zircon sand. Fine size zircon sand particles arepushed or engulfed by advancing solid-liquid interface creatingsufficient space inside the matrix, which leads to growth ofdendrite (Ref 21). Dendritic structure get modified duringcasting, which is influenced by many factors such as dendritefragmentation, restriction of dendritic growth by the particles,and thermal conductivity mismatch between the particles andmelt. Ceramic particle also act as a barrier for dendritic growthand this phenomena is more pronounced when cooling rate ishigh. Zhou and Xu (Ref 22) in their work reported that theparticle can be assumed to act as a barrier for the dendritic


growth. Figure 1(b) shows very less amount of interdendriticphase that corresponds to eutectic solidification, which iscomparably less than that of the particle diameter. It can be seenthat there is good bonding between the matrix and thereinforced particulates.

Dendritic fragmentation can be attributed to the shearing ofinitial dendritic arms by the stirring action. During particleaddition, local solidification of the melt occurs which isinduced by the particles as there is a temperature differencebetween the particle and the melt. It was also found that theperturbation in the solute field due to the presence of particlescan change the dendrite tip radius and the dendrite tiptemperature. These effects give rise to a dendrite to celltransition with increased density of particles in the melt. Alsothe length of the dendrite reduces in the presence of theparticles (Ref 22). Composite-B shows uniform and homoge-neous distribution of coarse particles inside the alloy matrix ascan be seen in Fig. 1(c) and (d). Good bonding between particleand alloy matrix is exhibited in the Fig. 1(d). The smoothinterface provides better mechanical and tribological propertiesas transfer of load occurs through the interface (Ref 11, 23).Figure 1(d) shows the presence of dendrites in areas away fromparticle. The second phase hard particle restricts the growth ofdendrite and modifies the matrix with more refined structureleading to improvement in strength (Ref 24-28). The siliconpossessing acicular morphology in the matrix acquires globularform in the vicinity of the particles. Kaur and Pandey (Ref 23)reported similar modification in silicon morphology in theirearlier work. The higher magnification micrograph (Fig. 2a)exhibits round morphology of eutectic silicon having finedistribution as colonies around the reinforced particles.

Figure 2(a) indicates a large nucleation area of eutectic siliconaround the fine ceramic particles in composite-A. Around thecoarser particles, the nucleation area of eutectic silicon can alsobe seen in composite-B, which is shown in Fig. 2(b). Overallanalysis of structure indicates that microstructure is refinedwhere eutectic silicon are having blunted and globular mor-phological features. This refinement may lead to bettertribological and mechanical properties in the composites.

3.2 Microhardness Measurement

The microhardness measurement of composites at differentphases has been carried out to know the effect of size ofreinforced particulates on the alloy matrix, which is given inTable 1. Microhardness measurement has been carried out onthe embedded zircon sand particles as well as in the vicinity ofparticles and matrix. High hardness is observed at zircon sandparticles. However, hardness decreases at interface and matrix.The high hardness at particle-matrix interface compared tomatrix indicates good bonding between particle and alloymatrix. Fine particle zircon sand reinforced composite showsbetter microhardness in comparison to coarse particle zircon-sand-reinforced composite at interface and matrix.

3.3 Wear Rate

3.3.1 Effect of Load on Wear Rate. The wear rate of fineand coarse particle reinforced composites measured at differentloads at room temperature is shown in Fig. 3(a) and (b),respectively. It depicts better wear resistance of compositereinforced with fine particles at all loads under dry sliding wearcondition. It is observed in Fig. 3(a) and (b) that wear behavior

Fig. 1 Optical micrographs of composite-A at (a) 1009, (b) 2009 and for composite-B at (c) 1009, (d) 2009


of the composites is similar at room temperature for all loads.However, the wear rate of both composites increases with theincrease in applied load which may be due to the increase in theactual contact area between composite sample and disk.Initially the wear rate is higher which corresponds to the runin wear. However, the steady-state wear approaches at nearly2000 m sliding distance. Run-in wear of composite-B is higheras compared to composite-A at all loads as shown inFig. 3(a) and (b). This indicates that fine particle reinforcedcomposite-A exhibits better wear resistance as compared tocoarse particle reinforced composites-B. These results are ingood agreement with the earlier reported results (Ref 10, 29).The results of the composites presented in Fig. 3(a) and (b) alsoshow the initial stage variation which indicate towards grindingof asperities on the contact sample surface as indicated by thehigh wear rate with respect to increased loads. Large effectivesurface area of the fine particles in the matrix is also responsiblefor the better wear resistance of composite-A in comparison to

composite-B. The shape of the zircon sand particles alsoplays an important role for the wear behavior of the composites(Ref 29). The sharp edge particles (fine) may also get insertedeasily in the matrix during the applied load as compared to thecoarse particles. Since, the coarse particles at these conditionsare protruded so they may get fractured and increase the wearrate of the material. Material removal in composites is due tothe indentation and plowing action of the sliding indenters(reinforced particles) (Ref 30). The hardness of the materialdetermines the depth of the indentation of the abrasive particles.The hardness of material increases due to the presence of hardceramic particles. Incorporation of hard zircon particles in thecomposites restricts such plowing action of sliding indentersand improves the wear resistance.

3.3.2 Effect of Temperature on Wear Rate. Wearbehavior of both composites at different temperatures and atlow load (1 kg) are shown in Fig. 4(a). The fine particlereinforced composite-A exhibits better wear resistance ascompared to coarse particle reinforced composites-B. Goodinterfacial bonding of reinforced particles with the matrix andhigher hardness as possessed by composite-A reveals betterresults of wear behavior. Moreover, for composite-B the wearrate is higher as compared to composite-A in the temperaturerange of 50-150 �C. It is observed that the wear rate of thecomposites slightly decreases with an increase in temperature

Fig. 2 Optical micrographs at high magnification (500 X) in(a) composite-A and (b) composite-B

Table 1 Variation of hardness at different phases incomposites

Composite

Microhardness, Hv

At matrix At interface At particle

Composite-A 91 126 711Composite-B 82 107 716

0

2

4

6

8

10

12

14

16

Wea

r ra

te

10-3

(m

m3

/m)

Sliding distance (meter)

1 kg 2 kg 3 kg 4 kg 5 kg

(a)500 1000 1500 2000 2500 3000

500 1000 1500 2000 2500 30000

2

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28

Wea

r ra

te

10-3

(m

m3

/m)

Sliding distance (meter)

1 kg 2 kg 3 kg 4 kg 5 kg

(b)

Fig. 3 Wear rate of composites against sliding distance at differentloads for (a) composite-A (b) composite-B


from 50 to 150 �C for both composites at 1 kg load. Since atlower load, the asperities on the surface of composites undergoplastic deformation so slight decrement is observed. Higheramount of stress is expected to act on the asperities, due to thegreater degree of their hardness and sharpness. Because of thehigher stress concentration on actual contact area, asperities areplastically deformed. The interesting point is that a sharptransition in wear rate is observed above 150 �C and ratebecomes minimum for both composites at 200 �C. Under theinfluence of load and temperature, the debris generated fromworn surface form a tribo-layer (Ref 13). The thickness oftribo-layer increases with increase in temperature, which ismore in case of higher load. As observed by Yang et al. (Ref 31),the tribo-layer consist of oxides of materials forming a layeredstructure known as mechanically mixed layer (MML). ThisMML exhibits higher hardness than matrix and affect the wearmode greatly. The decrease in wear rate is due to formation ofthe hard MML on the sliding surface, which protects the surfacefrom further wear. With further increase in temperature from 200to 300 �C, the wear rate increases sharply. In this temperaturerange the tearing of MML and exposure of new areas on thesurface for wear occurs. Beyond 200 �C the matrix becomes softcausing plastic deformation on the surface. Some of the sharpestasperities also get fractured due to combined action of normaland shear stress causing increase in wear rate of the bothcomposites (Ref 13).

Figure 4(b) depicts the variation in wear rate with slidingdistance at an applied load of 5 kg and different temperatures.This figure shows that wear rate of both composites increaseswith increase in temperature from 50 to 150 �C. The interasperities distance of composite-A is less which contains morevolume fraction of reinforced particles as compared to com-posite-B. The size of asperities in composite depends uponmany factors such as; size of reinforced particles, temperaturebetween contact surface and applied loads. The size of anasperity increases with increased applied load on the bothcomposites. Hence, at higher load, both composites showincrement in wear rate. At higher load of 5 kg, coarse particlesare responsible for higher material removal during sliding attemperature varying from 50 to 150 �C. However, the wear rateof both composites decreases with increasing temperature from150 to 200 �C. At this temperature, the formation of tribo-layeroccurs where the debris generated undergo mechanical grindingand form MML on the sliding surface causing protection ofsurface and the wear rate decreases as observed at low load also(Ref 13). Due to the presence of finer size reinforced particle,composite-A has smoother surface as compared to composite-B, hence an early MML of higher thickness is formed on thesurface of composite-A as compared to composite-B whichacted as a solid lubricant and helped to reduce the wear rate inboth composites. Both composites show the increment in wearrate with increase in temperature from 200 to 250 �C at 5 kgload. In this range of temperature, wear transition from mild-to-severe is attributed due to the appearance of gross plasticdeformation and thermal softening of the counter surface andworn-surface matrix. The gross plastic deformation on the wornsurfaces would occur at the higher temperature than the originalcritical surface temperature. The higher load would lead tolarger plastic deformation of the matrix, thus causing an easydelamination of MML. In this condition, the wear enters inthe severe wear regime under the higher load and temperature(Ref 31). This transition in wear rate is because of removal ofMML due to continuous sliding action, which results in direct

metal-to-metal contact, and exposing new surface to theenvironment. The critical transition (from mild to severe) inwear mode occurs above 200 �C for both composites, which isresponsible for higher wear rate. Zhang and Alpas (Ref 32)suggested that the critical transition in wear mode occurs attemperatures corresponding to 0.4Tm, where Tm is the meltingpoint of the alloy (Tm = 650 �C, melting temperature for LM13alloy). At this critical temperature, the thermally activateddeformation process becomes more active and leads tosoftening of the material adjacent to the contact surfaces. Wearrate for both the composites, decreases sharply with increasingtemperature around 300 �C at 5 kg load. At this temperature,the strain transfer phenomenon to the interfaces becomes lesseffective as the surrounding matrix alloy begins to soften. Atthis stage, the fracturing of the reinforced particles may occurmainly from crushing and grinding action against each otherand the steel counterface in the soft matrix alloy which leads tonegative or minimum wear rate of both composites. The overallwear rate of composite-A is minimum as compared tocomposite-B as shown in Fig. 4(b).

3.3.3 Analysis of Wear Track. Microstructural study ofwear tracks of the sample has been done to understand themode of wear. The SEM micrographs of wear tracks of thecomposites-A tested at 5 kg load at 50, 200, and 250 �C are

50 100 150 200 250 3000

1

2

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5

6

7

8

Wea

r ra

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10-3

(m

m3 /m

)

Temperature (°C)

Composite- A Composite- B

(a)

50 100 150 200 250 3000

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Wea

r ra

te

10-3

(mm

3/m

)

Temperature (°C)

Composite- A Composite- B

(b)

Fig. 4 Wear rate against the temperature of the composites with(a) 1 kg and (b) 5 kg loads


shown in Fig. 5(a) to (c), respectively . One common featureobserved in these figures is the formation of grooves and ridgesrunning parallel to the sliding direction in composite. Voids areclearly seen around the particles from where microcracksare initiating, which is shown in Fig. 5(a). Though the particlesare strongly bonded but crack initiation occurs from theinterfacial areas which propagate further because of continuousplowing action leading to lot of grooves. The cracks initiatedpropagate further along these grooves and also perpendicular tothe grooves to meet another crack near other particle. Finally indue course of time these cracks are connected to each other.

At 200 �C, the propagation of cracks is at faster rate, whichis shown in Fig. 5(b) and void formation is also accelerated.

The propagating cracks get interlinked further causing defor-mation and delamination of the matrix locally. The poor wettingor bonding initiates void formation around the particle. Thereinforced fine particles are main load bearer and resist thewear. On the other hand, they are also acting as crack initiationsites, which causes delamination and increase in the wear rate.The formation of grooves may lead to removal of metal in theform of ligament or wire in the initial stage. However, whenthese grooves get inter connected through cracks, it may causeremoval of material in the form of chunk as shown in Fig. 5(b),where interconnected cracks are observed. Wear particleformation is seen by the propagation of the cracks. Theseparticles are of smaller size and may get crushed or leave thesystem without being trapped as can be seen in Fig. 5(c). In thisfigure, the crack propagation is along the sliding direction andalso in perpendicular direction of sliding, which results inmaterial removal by delamination.

Figure 6(a) shows the SEM micrograph of worn pincomposite-B reinforced with coarse size zircon sand particlesat a load of 5 kg at 50 �C. Delamination of matrix is observedmostly along the sliding direction. Craters grow along longi-tudinal direction. Void formation and crack propagation areencountered by the matrix but reinforced particles are still intactshowing good bonding characteristics.

The delaminated tribo-oxide layer which from MML in duecourse of time on the worn surface (Fig. 6b) is responsible fordecrement in wear rate at higher temperature (200 �C), asoxide layer avoids the direct contact between the specimen andcounter-surface (Ref 13). The plastic deformation of the surfacelayer and propagation of cracks can be seen in Fig. 6(b). Thewavy pattern of the grooves results from the plowing action ofparticles followed by plastic deformation during the wear testat higher load. At 250 �C, the plowing marks become moredeeper and cause damage in the form of craters as can be seenin Fig. 6(c), which causes significant increase in the wear rate.The SEM micrograph of sample (Fig. 6c) at operatingtemperature of 250 �C shows large plastic deformation inaddition to abrasion action of zircon sand particles showinghigh degree of flow of materials along the sliding direction,which generates large cavities due to tearing and delaminationof surface materials causing a transition from mild to severewear near about 250 �C (Ref 33). At higher load sub-surfacedelamination is produced by coalescence of these wear cracksas shown in Fig. 6(c). Several local delamination events areinterlinked to form long craters. Formation of MML is alsorevealed from the worn surface analysis as fine size debris offractured MML are adherent within the cavity created bydelaminated layer. Figure 6(d) shows the EDS spectra of wornsurface of composite-B at 250 �C at 5 kg load. Presence of theoxygen in the spectra supports the formation of oxide layerduring the sliding at this temperature.

The delaminated area in composite-A is smaller in size ascompared to composite-B at 50 �C for 5 kg load. At highertemperature (above 200 �C), composite-A shows more incre-ment in the delaminated area as compared to composite-B at thesame load (5 kg) conditions. However, with increased temper-atures the depth and size of grooves in composite-B is more incomparison to composite-A. Increment in the crack size is alsolarger in composite-B in comparison to composite-A at highertemperature.

3.3.4 Analysis of Debris. Scanning electron microscopeanalysis of the wear debris has been done to identify themode of wear mechanism. Wear debris for composite-A and

Fig. 5 SEM images of wear tracks of composite-A at 5 kg(a) at 50 �C (b) 200 �C and (c) 250 �C


Fig. 6 SEM images of wear tracks of composite-B at 5 kg at (a) 50 �C, (b) 200 �C, (c) 250 �C, and (d) EDS spectra of worn surface ofcomposite-B at 250 �C with 5 kg load

Fig. 7 SEM images of wear debris collected at 50 �C with 5 kgload (a) composite-A and (b) composite-B



composite-B collected at 5 kg load for 50 �C are shown inFig. 7(a) and (b), respectively. Figure 7(a) represents the lips-type debris, which is generated by microcutting of the pin at theinitial stage of wear (metallic wear). Apart from this partiallyoxidized (ridges) and delaminated debris are also seen. Ridgescan then be flattened by further contact and forms extrusions orlips. These lips are broken off and become thin flat wear flakesof irregular edges.

Figure 7(b) shows the larger size delaminated debris withfractured particles. The crack present in large size debris isresponsible for higher wear rate. An asperity plowing through asurface get cold welded to the contact surface and the weldshear off further, leaving some of the asperity stuck to thecontact surface. This is responsible for higher wear rate. Thinwear sheets or flakes get frequently elongated in the slidingdirection during sliding between two surfaces and this can bebetter explained by delamination due to plastic deformation ofsurface layers.

Figure 8(a) and (b) shows the SEM images of wear debriscollected at 300 �C of composite-A and composite-B at 1 kgload. Wear debris consists of very fine particles of less than32 lm as shown in Fig. 8(a). The features at this load consist offlake, curly, fine size, and agglomerated-type debris as can beseen in Fig. 8(a). The flake-type debris was derived fromdelamination wear and causes an increase in wear rate. Plate-like debris generated in delamination wear is not seen whichindicate that at this load abrasive, oxidative and adhesive wearmechanisms are responsible for material removal (Ref 34).

Figure 8(b) shows that the size of flakes becomes larger, whichindicates that the matrix becomes softer with increasingtemperature and causes the transition from mild to severewear. These wear debris indicate that the adhesive weardominate in the sliding direction during wear. Due to theadhesive nature at higher temperature for low load, metal ischipped out in the form of flakes as debris. Corrugated structureis also observed in Fig. 8(b) which is due to the continuousrubbing of specimen during sliding wear.

Wear debris for composite-A and composite-B collectedat 5 kg load for 200 �C are shown in Fig. 9(a) and (b). At thiscondition, oxidation and formation of MML occurs whichprotect the specimen from further wear. The wear rate decreasessignificantly. Figure 9(a) shows the flake-type delaminateddebris along with the layered structure. Layered structureobserved could be due to continuous rubbing caused byconstant sliding between the pin material and counterface. Ineach rotation, deforming forces help to get these cracksinterconnected (Ref 35). The delaminated larger size debrisare fractured and converted to smaller size. This is due to thepresence of high stress during wear test. Figure 9(a) shows thepresence of oxide (Al2O3) debris in round shape and theseoxide (Al2O3/Fe2O3) layers help to reduce the wear rate ofcomposite at higher load as they keep on rotating inside thegrooves (Ref 18). The oxide of Al and Fe is also generatedwhich could be formed due to the frictional heat generatedduring sliding wear. Figure 9(b) shows the spherical, dumbbell-shaped, and rod-type wear debris which are formed by several




Fig. 11 EDS spectra of debris collected at 250 �C with 5 kg load for (a, b) composite-A and (c) composite-B


sources, including entrapment of wear debris in surfacedepression zone, inclusions from base metal, local melting,and air born contamination. Figure 9(b) also shows feathery-type debris and this type of wear debris are formed due tostrong interfacial bonding between the coarse size particles withmatrix followed by delamination of harden surface whichfurther participate in getting them mechanically mixed.

Figure 10(a) and (b) shows SEM images of wear debriscollected at 250 �C of composite-A and composite-B at 5 kgload. Figure 10(a) shows the wavy pattern structure of weardebris that is created due to the presence of fine size particlesinside the matrix. A large size crack observed in the wear debrisis due to the plastic fracture which is responsible for higherwear rate of composite-A. Figure 10(b) represents the SEMimage of wear debris of composite-B at 250 �C for 5 kg load.Generation of longer multi-layered corrugated structure (shapedinto alternating parallel grooves and ridges) is attributed to theexistence of coarse size particles in matrix. With the applicationof load (applied load and tangential force) on the countersurface of the specimen (composite-B), in sliding condition,coarse particles penetrate through soft matrix at this particulartemperature. Hence, the coarse particles work as a nucleator oflonger multi-layered corrugated structure in soft matrix.

Debris collected from composite-A at 5 kg load and 50 �Care smaller in size as compared to composite-B. However,debris of composite-A are in curve shape, while debriscollected from composite-B are flat in shape. Grooves andcrakes are also observed on flat-shaped debris. At 300 �C with1 kg load, composite-A shows fine size flakes-type and curly-shaped debris but composite-B shows wavy pattern withcorrugated structure on debris. At higher temperature andhigher load conditions, size of debris for composite-A is alsoincreased, showing sharp edges. However, debris collectedfrom composite-B at same conditions, indicate irregular shapes(e.g., rod, spherical and dumbbell shape) with rough edges. Athigher load, the oxide layer readily delaminates, thus causes anincrease in wear rate. The plastic deformation of matrix is solarge that delamination would occur inside the bulk metal(Ref 13). In the case of the composites-B, it is clear that theimprovement in the wear properties is due to the formation ofthe MML. It is also clear that the coarse size reinforcement isthe key factor in the formation of thick MML, acting as adynamic element and the precursor during the initial transitionleading to the formation of MML.

3.3.5 EDS Analysis. Semi-quantitative chemical analysisof wear debris confirms the formation of MML where thedebris exhibit different chemical compositions. The analysisconcluded that wear debris are generated by various wearevents. EDS analysis revealed the presence of iron in the weardebris having varying percentage. EDS analysis of fine particlereinforced composite-A at 250 �C with 5 kg load is shown inFig. 11(a) and (b). Figure 11(a) represents the less content of Fein molten state debris which got melted during frictional heatgenerated between pin and steel disk. Figure 11(b) shows theiron rich delaminated wear debris which helps in reducing thewear rate of composite-A by acting as in situ solid lubricant.The beneficial effects of more weight percentage of Fe indelaminated wear debris are also reported by Alpas and Zhang(Ref 1). The less weight percentage of Fe in delaminated weardebris of composite-B is shown in Fig. 11(c). This EDS imagealso shows the presence of MML having less weight percentageof Fe in material on the surface of delimited debris, which isresponsible to increase in wear properties in composite-B. The

oxygen content of the wear debris may get reduced due to theexistence of bulk metal. Undoubtedly, the transition from mildto severe wear occurs with a rapidly increased wear rateat 250 �C and at 5 kg load.

4. Conclusion

(1) Microstructural analysis of the developed compositesexhibits uniform distribution of reinforced particles. Themorphology of silicon eutectic got modified from acicu-lar to blunt and round shape in the composite.

(2) In fine particle reinforced composite-A the nucleation ofeutectic silicon around the particles is more as comparedthe coarse particles in composite-B.

(3) Composite-A exhibits better wear properties in compari-son to composite-B at all loads and temperatures. How-ever, a transition in wear mode of both composite isobserved above 200 �C for all loads where material lossis due to delamination of mechanical mixed layer.

(4) SEM analysis of the worn surfaces and collected wear deb-ris also indicates that both adhesive and abrasive wearmechanism contribute for the wear of the both composites.

Acknowledgments

The authors are thankful to Armament Research Board(ARMREB), Defence Research and Development Organization(DRDO), India for providing financial support under the letterno. ARMREB/MAA/2008/105 for this study.

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effect of particle size on wear of particulate reinforced aluminum alloy composites at elevated...

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