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Available online at www.sciencedirect.com

Wear 265 (2008) 349–360

Dry sliding wear of fly ash particle reinforcedA356 Al composites

Sudarshan ∗, M.K. SurappaDepartment of Materials Engineering, Indian Institute of Science, Bangalore, India

Received 8 April 2006; received in revised form 10 September 2007; accepted 20 November 2007Available online 3 January 2008

bstract

In the present study aluminium alloy (A356) composites containing 6 and 12 vol. % of fly ash particles have been fabricated. The dry slidingear behaviour of unreinforced alloy and composites are studied using Pin-On-Disc machine at a load of 10, 20, 50, 65 and 80 N at a constant

liding velocity of 1 m/s. Results show that the dry sliding wear resistance of Al-fly ash composite is almost similar to that of Al2O3 and SiCeinforced Al-alloy. Composites exhibit better wear resistance compared to unreinforced alloy up to a load of 80 N. Fly ash particle size and its

olume fraction significantly affect the wear and friction properties of composites. Microscopic examination of the worn surfaces, subsurfacesnd debris has been done. At high loads (>50 N), where fly ash particles act as load bearing constituents, the wear resistance of A356 Al alloyeinforced with narrow size range (53–106 �m) fly ash particles were superior to that of the composite having the same volume fraction of particlesn the wide size range (0.5–400 �m).

2007 Elsevier B.V. All rights reserved.

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eywords: Fly ash; Dry sliding; Pin-on-disc; Wear debris

. Introduction

The strengthening of aluminium alloys with a dispersion ofne ceramic particulates has dramatically increased their poten-

ial in wear resistant and structural applications [1–4]. There is anncreasing interest in the development of metal matrix compos-tes (MMCs) having low density and low cost reinforcements.lthough these MMCs have better properties including high

trength, high stiffness and better wear resistance their usage isimited due to their high cost. Among the various discontinu-us reinforcements used, fly ash is one of the most inexpensivend low-density reinforcement available in large quantities asolid waste by-product during the combustion of coal in thermalower plants. There are some reports to indicate the use of flysh particle as a filler/reinforcement material in polymers andetals. Incorporation of fly ash particles reduces the cost and

ensity of aluminium and its alloys, which are energy intensive

aterials [5–7].Extensive studies on the tribological characteristics of Al

MCs containing reinforcements such as SiC and Al2O3 is

∗ Corresponding author. Tel.: +91 80 22932697; fax: +91 80 23600472.E-mail address: shan@platinum.materials.iisc.ernet.in ( Sudarshan).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.11.009

vailable in the literatures [1–13]. However, reports on frictionnd wear characteristics of fly ash reinforced AMCs are veryimited [5–7]. Rohatgi [5] has reported that the addition of flysh particles to the aluminium alloy significantly increases itsbrasive wear resistance. He attributed the improvement in wearesistance to the hard aluminosilicate constituent present in flysh particles.

. Materials and experimental procedure

The chemical composition of matrix alloy and fly ash par-icles are shown in Tables 1 and 2. A356 Al-fly ash particlesomposites containing 6 and 12 vol. % fly ash particles haveeen studied. As received fly ash particles having wide size range0.5–400 �m) and sieved fly ash particles having narrow sizeange (53–106 �m) were used as reinforcement. Particle sizenalysis was done using a computerized particle size analyzer,alvern® make laser light particle size analyzer (Mastersizer).omposites were fabricated using stir-cast technique. Compos-

tes having different particle sizes and volume fraction of fly ashre designated as follows: composites with 6 vol. % fly ash par-icle(sieved), (C6S), composites with 12 vol. % fly ash particlesieved), (C12S) and composites with 12 vol. % fly ash particle

350 Sudarshan, M.K. Surappa / W

Table 1Chemical composition of Al–Si alloy (A356) in weight percent

Si 7.08Mg 0.41Cu 0.06Fe 0.09Ti 0.13Al Balance

Table 2Chemical composition of fly ash in weight percent

SiO2 64.80Al2O3 24.01Fe2O3 5.23CaO 2.76MgO 0.90TiO2 0.50L

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OIa 0.87–1.33

a Loss on ignition.

as received), (C12AR). Fly ash from Raichur thermal powerlant (India) was used in this study and their typical scanninglectron microscope (SEM) micrographs are shown in Fig. 1.

Cast ingots machined to a size of 55 mm diameter wereomogenized at 530 ◦C for 2 h and extruded at an extrusionatio of 21:1. From these extruded rods, wear test specimensere machined. Shimadzu Microhardness Tester (M2000) wassed to measure the hardness of the matrices of the compos-tes and the unreinforced alloy at 25 gm load with dwell timef 10 s. Macrohardness measurements were made using Brinellardness Tester with 2.5 mm steel ball indenter and a load of2.5 kg.

Sliding wear tests were conducted in air at ambient temper-ture using a Pin-on-Disc Machine, (DUCOM’s Wear Frictiononitor, Model-TR 20). The wear test specimens had stepped

in geometry with a diameter of 4 mm at the rubbing end. Pinsf specimens (unreinforced and composite) were tested againstN32 hardened steel disc (62HRC). Prior to actual wear testsliding surfaces of test specimens were rubbed on 600 grit SiCmery paper to remove the machining marks. The surface of theisc was polished to a surface roughness of 0.1 ± 0.02 Ra, usingeries of abrasive papers. Experiments were conducted under

ry conditions, at room temperature (27 ◦C, relative humidity5 ± 1%). The sliding speed and sliding distance in the experi-ents were kept constant at 1 m/s and 3000 m, respectively and

wpv

Fig. 1. SEM micrograph of fly ash particles (a) preci

ear 265 (2008) 349–360

he loads were varied from 10 to 80 N. Each experiment wasepeated thrice and the averages of closely repeatable test val-es were taken. The wear rate was calculated using weight lossnd graphical methods. In the weight loss method, volume lossas calculated by dividing the density from weight loss (weightifference between sample before the test and after the test)alue. In the graphical method, volume loss was calculated byultiplying the height loss acquired by the linear variable dif-

erential transformer (LVDT) to the area of the contact surfacef the pin.

Worn surface, cross-sections of worn surface and the debrisenerated during wear tests were examined by SEM. Chem-cal characterization of worn surfaces and wear debris wereerformed using energy dispersive spectrometer (EDS). Wornurfaces were covered with protective nickel-plating prior toectioning and metallographic preparation.

. Results

Fly ash from Raichur thermal power plant (India) had a widearticle size distribution. The particle size of the fly ash, in thes-received condition, lies in the range from 0.5 to 400 �m.ence, as received particles were sieved and those particles,hich pass through 140 mesh and retained on 270 mesh were

hosen. Fifty percent of the as-received particles were less than9 �m and 80% of the particles were less than 152 �m, with anverage particle size of 89 �m. In the case of sieved particles−140# + 270#), 50% of the particles were less than 76 �m and0% of the particles were less than 104 �m and had an averageize of 81 �m.

Fly ash particles are spherical in shape. Most of the fly ashainly consists of solid particles. Relatively smaller amount

f partially solid or hollow spherical particles were also seenFig. 1b and c). Fly ash particles are generally in the form ofolid spheres known as precipitator fly ash (Fig. 1a) or in theorm of hollow spheres termed as cenosphere fly ash (Fig. 1b).s such by looking at them we cannot make out whether thearticle is precipitator or cenosphere. From the micrographFig. 1b) we can see a broken cenosphere particle, which iserhaps broken during particle collecting or handling. The wallhickness of these cenospheres varies from 1 to 13 �m. Occa-

ithin the wall of the cenosphere. There are few porous fly asharticles that are also present (Fig. 1c). Microstructure of trans-erse and longitudinal section of extruded unreinforced alloy

pitator (b) cenosphere and (c) porous particles.

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Sudarshan, M.K. Surapp

nd composites (wear test specimen) are shown in Fig. 2. Fly asharticles and eutectic silicon in the matrix are aligned along the

xtrusion direction while during the extrusion. These extrudedomposites show no significant agglomeration/segregation of flysh particles. Hardness of unreinforced alloy and the compos-

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ig. 2. Optical micrographs of wear test specimen; A356 Al alloy (a) transverse, (b12S composite (e) transverse, (f) longitudinal section and C12AR composite (g) tra

ear 265 (2008) 349–360 351

tes in the extruded condition are shown in Table 3. Resultshow considerable increase in both the bulk hardness and

icrohardness.Fig. 3 presents the original LVDT records with sliding dis-

ance. LVDT values fluctuates more at low loads (Fig. 3a, c,

) longitudinal section; C6S composite (c) transverse, (d) longitudinal section;nsverse and (h) longitudinal section.

352 Sudarshan, M.K. Surappa / W

Table 3Hardness and density of A356 Al alloy and its composites

Material Hardness Density (g/cm3)

Microhardness(VHN)

Macrohardness(BHN)

A356 Al 51.1 (1.2) 48.5 (1.0) 2.68C6S 59.5 (1.1) 54.8 (0.5) 2.64C12S 55.6 (2.5) 54.2 (0.4) 2.59C12AR 54.3 (0.8) 52.4 (0.9) 2.60

Standard deviation is shown in parentheses.

etuaWeshur

Fig. 3. Displacement vs. distance graphs of A356 (a and b), C6S (c an

ear 265 (2008) 349–360

and g) compared to higher loads (Fig. 3b, d, f and h) in allhe materials tested. In the graphical method, a graph of vol-me loss versus sliding distance is plotted. Volume loss showslinear relationship with sliding distance in most of the cases.ear rates of A356 Al alloy and their composites in the as-

xtruded condition are shown in Figs. 4 and 5. Both the graphs

how the same trend but in the graphical method wear rate isigher compared to weight loss method. At low load (10 N)nreinforced alloy and the composites show almost similar wearates. At 50 N, the unreinforced alloy and 12 vol. % reinforced

d d), C12S (e and f) and C12AR (g and h) at low and high loads.

Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360 353

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CuburmvsFwpebmrweAl-5 vol. % fly ash composite. They observed that the abrasive

ig. 4. Effect of load on wear rate of A356 Al alloy and its composites by weightoss method.

omposites show deviation in the wear curve. Further 6 vol. %einforced composite exhibits this deviation at much lower load20 N).

In the case of unreinforced alloy, the wear rate was mild upo loads of 50 N. Wear rate increases significantly when the loadas increased from 50 to 80 N. It was also observed that thereas a gross material transfer from pin to the steel disc, at theseigh loads. Patches of Al could be seen on the wear tracks and its an indication of possible melting and subsequent smearing ofin material onto the steel counterface. The amount of materialransferred onto the disc was more pronounced in the load rangeetween 50 and 80 N. This implies that there is a transition fromild to severe wear as the load increases beyond 50 N. At higher

oad (80 N) the test was stopped after 30 min, since the entirein wears out.

Up to 20 N, C6S composites show lower wear rate comparedo unreinforced alloy and 12 vol. % fly ash reinforced (C12ARnd C12S) composites (Fig. 7). C6S composite shows transi-ion from mild wear to severe wear as the load increases beyond0 N. After that wear rate increases gradually up to 65 N thenncreases steeply. At 50 N, composite C6S shows higher wear

ate compared to the unreinforced alloy. Wear test on C6S com-osite was stopped after 40 min at higher load (80 N), since thentire pin wears out. Here also material transfer from pin to disc

ig. 5. Effect of load on wear rate of A356 Al alloy and its composites byraphical method.

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ig. 6. Effect of particle size range on wears rates (C12S composite-narrow sizeange and C12AR composite-wide size range).

as observed at higher loads (above 50 N) however, much lessompared to the unreinforced alloy.

Twelve volume percent of fly ash reinforced (C12AR and12S) composites exhibit lower wear rates compared to thenreinforced alloy from 20 to 80 N loads. There was a negligi-le difference in the wear rates of C12AR and C12S compositesp to 50 N. Beyond 50 N, C12S composite shows lower wearate compared to C12AR composite (Fig. 6.). The difference isore predominant at higher loads. This indicates that at a given

olume fraction, wear resistance of composites having a narrowize range particles is superior to that with wide range of size.rom the Fig. 7, it is clear that trend in the variation of wear ratesith volume fraction at low loads (10 and 20 N) is different com-ared to that at high load (50–80 N). At low loads (10 and 20 N)ffect of additions of fly ash on the wear resistance is negligible,ut at high loads (50–80 N) and higher volume fraction it showsarginal decrease in the wear rate. At high loads (50–80 N) wear

ates of 12 vol. % reinforced composite shows two times lessear than the composite with 6 vol. % reinforcement. Rohatgi

t al. [14] have studied the abrasive wear properties of A356

ear resistance of Al-fly ash composite is similar to that ofl–alumina fiber composite and is superior to that of the matrix

lloy for loads up to 8 N (transition load). A comparison of dry

ig. 7. Effect of volume percent of fly ash particles (53–106 �m) on wear ratef A356 Al alloy (weight loss method).

354 Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360

Table 4A comparison of dry sliding wear in composite materials

Investigators Test configuration Test conditions Material Wear rate (mm3/m)

Alpas and Zhang [13] Block-on-ring v = 0.2 m/s A356 Al 1.8 × 10−3

L = 17.2 N A356 Al–10% SiC 1.5 × 10−3

A356 Al–13% SiC 1.5 × 10−3

v = 0.8 m/s A356 Al 2.2 × 10−1

L = 98 N A356 Al–10% SiC 3.2 × 10−3

A356 Al–20% SiC 2.7 × 10−3

Pramila Bai et al. [2] Pin-on-disc v = 0.5 m/sL = 61.6 N A356 Al 8.1 × 10−3

A356 Al–15% SiC 6.1 × 10−3

L = 81.7 N A356 Al 8.4 × 10−3

A356 Al–15% SiC 7.4 × 10−3

Zhang and Alpas [15] Block-on-ring v = 0.2 m/sL = 52 N 6061 Al 4.8 × 10−3

6061 Al–20% Al2O3 3.6 × 10−3

L = 98 N 6061 Al 9.5 × 10−2

6061 Al–20% Al2O3 5.9 × 10−3

Present authors Pin-on-disc v = 1 m/sL = 20 N A356 Al 1.2 × 10−3

A356 Al–6% fly ash (S) 0.9 × 10−3

A356 Al–12% fly ash (S) 1.0 × 10−3

L = 50 N A356 Al 1.7 × 10−3

A356 Al–6% fly ash (S) 3.3 × 10−3

A356 Al–12% fly ash (S) 1.5 × 10−3

L = 80 N A356 Al 9.4 × 10−3

A356 Al–6% fly ash (S) 9.1 × 10−3

A356 Al–12% fly ash (S) 3.4 × 10−3

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utanpcaites. Rohatgi et al. [14] have measured the friction coefficientduring abrasion wear test of Al alloy–fly ash composite, andreport lower coefficient of friction compared to that of the matrixalloy.

: velocity, L: load, S: sieved.

liding wear of A356 Al-fly ash composite with Al compos-tes reinforced with Al2O3 and SiC is shown in Table 4. Theesults indicate that the dry sliding wear resistance of Al-fly ashomposite is nearly similar to that of Al2O3 and SiC reinforcedl-alloy.At higher loads (above 50 N) graphical method yield higher

ear rates both in the unreinforced alloy and their compos-tes compared to weight loss method. This could possibly bettributed to two reasons: first weight loss method reveals theear rate of the pin only. In reality, the disc also undergoes

ome amount of wear, and this is not reflected in the wear ratesalculated using weight loss method. In the case of graphicalethod, the weight loss takes into account both pin and discear rates. Secondly material of the pin surface flows plasti-

ally and curls up along the edges during the wear test, hencehe wear rate calculated by graphical method will be more thanhat of weight loss method. The hardness of the pin materials much lower than the disc material, hence, the second rea-on explains the difference in wear rate. The amount of curlingncreases with the increase in the load. Wear of steel disc was

ore when 12 vol. % fly ash reinforced composites were testedt higher load (80 N), due to enhanced abrasive action of fly asharticles.

Friction force increases with increase in load, however,s not proportional to increment in the load. This could be

ainly due to higher temperatures generated at the interfacet higher load. Effect of load on the friction co-efficient of F

nreinforced alloy and their composites are shown in Fig. 8. Inhe as extruded condition, friction co-efficient of 12 vol. % flysh reinforced composites C12S and C12AR varies within thearrow range of 0.56–0.58. These values are much higher com-ared to the values for the unreinforced alloy (0.33–0.51). C6Somposite shows lower friction coefficient (0.36–0.49) in thes-extruded condition, compared to 12 vol. % fly ash compos-

ig. 8. Effect of load on friction coefficient of A356 Al alloy and its composites.

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Sudarshan, M.K. Surapp

. Discussion

One of the objectives of this investigation was to find out theffect of additions of fly ash particles on wear resistance of A356l alloy during dry sliding wear. Here graphical method wassed to verify the results obtained in weight loss method. Theomparison of wear rates obtained from weight loss and graph-cal method reveal that the magnitude of difference between thealues increases with the increasing load. This confirms that theurling effect (Fig. 9) is the main cause for higher wear ratesesulted in the graphical method. At loads up to 20 N, in bothhe methods, wear rates of unreinforced alloy and the compos-tes are nearly same. That means at low loads, curling is almostegligible (Fig. 9a, c, e and g).

The large difference in the wear rate obtained by the twoethods for 6 vol. % reinforced composite is similar to the unre-

nforced alloy. However, this difference is much smaller in the2 vol. % reinforced composites. This is attributed to the extentf curling (observed more in unreinforced alloy and 6 vol. %omposite). These results confirm that with increase in the rein-orcement content, the magnitude of the difference in wear ratesetween these two methods decreases.

Fluctuation in LVDT values is more at low loads in all theaterials tested (Fig. 3a, c, e and g). It is due to the pre-generatedear debris coming in between the mating surfaces which lifts

he pin. At higher loads however, these fluctuations are muchess due to the high pressure on the pin in the vertical direc-ion (Fig. 3b, d, f and h). At higher load the sudden spikesn the LVDT values indicates the generation of large debrisarticle which come in between the pin and disc. This debrisay be the delaminated flake. Delamination is more promi-

ent in composites compared to unreinforced alloy which isvidently shown by the more number of spikes in compositesFig. 3b, d, f and h) displacement versus distance graph at higheroads.

In the case of unreinforced alloy, adhesion is the wear mech-nism, however, in 12 vol. % reinforced composites, abrasions the main wear mechanism along with the adhesion. Com-osite with 6 vol. % reinforcement shows both abrasion (due toy ash) and adhesion (due to matrix) due to small percentagef reinforcement. At high load these small volume fraction ofy ash particles could not bear the load alone, so the exposedy ash particle either breaks or gets detached along with theatrix. This is conformed by Fig. 10a which shows broken fly

sh particles in the wear debris. This image clearly shows thatt high load the reinforcement particle break and detach fromhe matrix. Due to the abrasive action of the fly ash particle steelisc wear out and their debris were seen in the debris analysisFig. 10b). The groves seen in the iron flake in the Fig. 10b isue to the ploughing action of the exposed fly ash particles inhe matrix.

Up to a load of 50 N both the 12 vol. % composite (C12ARnd C12S) show almost similar wear rate. At higher loads (65

nd 80 N) the composite C12AR shows much higher wear ratesompared to C12S composite. The reason is as received fly ashas the large particle size difference (0.5–400 �m) i.e., wideange of particles. This particle size ratio is very much higher

thao

ear 265 (2008) 349–360 355

∼800:1) compared to narrow size range particle which is anverage less than 2:1. At higher loads as the test proceeds,ither larger or smaller particles, nearer to the mating surfaceill detach or break away. As a result, the whole load trans-

ers into the next level of particles in the jerk fashion (becausef the large difference in the particle size) due to which thosearticles also break away along with the matrix. Even the unre-nforced alloy shows high wear rate at this loads (65 N andbove). In case of the composite reinforced with narrow sizeange particles (53–106 �m), that jerk fashion loading is almostbsent due to more uniform size particles compared to wideize range particle reinforced composites. If the fraction of flysh particles on the contact area is small, particle fracture andamage accumulation events start to occur in the matrix mate-ial. The large plastic strains in the deformed layer give rise tooid nucleation and subsurface crack propagation. In this pro-ess, eutectic silicon and fly ash particles play an important role.he interface between the matrix and the particle provides pref-rential path for the growth of subsurface cracks. The cracknally reaches the contact surfaces and causes the delamina-

ion of subsurface layer. Hence, wear rates of lower volumeraction composite (C6S) is similar to those observed in unre-nforced alloy at higher load. None of the 12 vol. % fly asheinforced composite showed a drastic increase in the wear ratesp to 80 N (highest load applied). That is why fly ash parti-le reinforcement is beneficial for improving the overall life ofribosystems.

The temperature at the interface between the disc and thein increases with increase in the applied load during theest, although it could not be measured. At higher load plasticeformation possibly involves strain softening and/or dynamicecrystallization. Additions of fly ash particle improve the ther-al stability of the unreinforced alloy. It may be one of the

easons for suppression of the severe wear in 12 vol. % fly asheinforced composites.

The decrease in friction coefficient during test with increas-ng load have been attributed to the accumulation of more wearebris in the space between the pin and disc, resulted in reducedffective depth of penetration and eventually the mechanismhanges from two body to three body abrasive wear. Dependingn the characteristics, the third body can exert either beneficialr detrimental effects on the tribological behaviour of the slid-ng couple. At higher load frictional heating lead to a significantncrease in oxidation and softening effect with a consequentrogressive reduction in the resistance to friction. Six volumeercent of reinforced composite does not show much varia-ion in the friction coefficient compared to unreinforced alloy.owever, 12 vol. % reinforced composites show higher friction

oefficient than the unreinforced alloy and this is contrary toesult shown by Rohatgi et al. [14] in their abrasion wear test.his could be explained as follows: in abrasion test conductedy Rohatgi et al., most of the situation is a three-body abra-ion instead of two-body abrasion (due to debris from either

he abrasive sheet or the specimen). In the abrasion test, twoard ceramic particles (reinforcement of the composite (fly ash)nd particles of the abrasive paper (SiC)) will interact with eachther in the mating surface which will definitely reduce the fric-

356 Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360

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ig. 9. SEM micrograph shows worn surface of the A356 Al alloy (a) 10 N an0 N, and C12AR composite (g) 10 N and (h) 80 N.

ion (bearing principle), it is not so in the unreinforced alloy.ven though in dry sliding wear test this three-body wear isresent, it is much lesser compared to abrasive mode of test-ng.

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80 N, C6S composite (c) 10 N and (d) 20 N, C12S composite (e) 10 N and (f)

.1. Worn surface of the pin

Some of the features of the worn surfaces of the as-extrudednreinforced alloy and their composite pins tested at low (10 N)

Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360 357

show

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Fig. 10. SEM micrographs of the C12AR composite wear debris

nd high loads (80 N) are shown in Fig. 9. In all the SEMicrographs, an arrow mark is shown to indicate the sliding

irection.Worn surface of unreinforced alloy in the extruded condition

hows intense plastic flow and curling up of material along thepecimen edges (Fig. 9a and b). The material flow and the curlingffect were mild at low loads (10, 20 and 50 N), and more severehen the load was increased to 65 N and above. The curled upaterial gets detached with the progression of wear test result-

ng in large amount of wear debris. At higher loads, the plasticow of the material is dominant due to the excess heat gener-tion. The smooth (dark) and rough crater (white) regions cane seen (Fig. 11a) on the worn surface and it runs parallel to theliding direction indicating that mechanism of wear is adhesive.ubsurface examination of specimen tested at high load show

resence of crack beneath the worn surface. This indicates thatt high loads wear occurs by delamination (Fig. 11c).

SEM micrographs reveal that the worn surfaces of the com-osites are different from unreinforced alloy. Excessive plastic

fgyi

Fig. 11. SEM micrographs of the worn surface of A356 Al alloy (a) showing cr

ig. 12. SEM micrographs of the worn surface of composites showing (a) craters and

ing broken fly ash particle (a) and iron flakes (b) at higher loads.

loughing and cutting can be seen in alloy matrix (Fig. 11b), buthis is somewhat less in the composites (Fig. 12b). In addition,rooves were shallow in the case of composites. The curlingffect and material flow were not observed in the compos-te materials at low loads (10 and 20 N) and were observednly at higher loads (50–80 N). The rough granular region andmooth regions were observed in the composites (Fig. 12a) sim-lar to the unreinforced alloy. During running-in period, fly asharticles were exposed due to wear of matrix. These exposedy ash particles plough the steel disc in the initial stages ofear process and subsequently worn out by the steel disc dur-

ng the continuous sliding. The grooves could be seen on theear tracks present on the steel disc. The removal of mate-

ial from the surface of the steel disc (Fig. 10b) should be dueo the abrasive action of hard fly ash particles on the pin sur-

ace. Small amount of material removed from the steel discets transferred to the surface of the composite pin. EDS anal-sis of the worn surface of the pins confirms the presence ofron.

aters and smooth region showing (b) the ploughing and (c) delamination.

smooth region (C6S), (b) the ploughing (C12AR) and (c) delamination (C12S).

358 Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360

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ig. 13. SEM micrographs showing the subsurface crack in (a) A356 Al alloy,

The debri particles are likely to act as the third-body abrasivearticles and could be responsible for the higher wear rate ofhe counterface. The fly ash particles trapped between the speci-

en and the counterface causes a microploughing on the contacturface of the composite. The continuous longitudinal groovesarallel to the sliding direction on the worn surfaces of the com-osites, shown in Fig. 12b, probably result from the ploughingction of fly ash particles. At higher loads, composites showelamination (Fig. 12c). Unreinforced alloy also shows delam-nation at higher loads (Fig. 11c). Examination of subsurfacehows (Fig. 13) that void nucleation around the second phasearticles (silicon or fly ash) in the deformed region and theirubsequent growth and linkage parallel to surfaces lead to theelamination of subsurface layers and finally to the material lossn the form of plate like debris. The additional abrasive wear

microploughing) component of the sliding wear caused by they ash particles could be the reason for the marginal decrease inear rate of 12 vol. % fly ash reinforced composites compared

o the composite, reinforced with 6 vol. % fly ash.

emeo

ig. 14. SEM micrographs of debris of the A356 Al alloy (a) at low load, (b) at high

2AR and (c) C12S composites (arrow marks indicating the subsurface crack).

.2. Wear debris

The material is transferred back and forth several times duringhe sliding process and eventually produces particles of wearebri. When the applied load results in stresses higher than theracture stress of fly ash particles, these particles loose theirbility to support the load. Consequently, the aluminium matrixomes in direct contact with the counterface and large plastictrains are imposed on the contact surfaces of the pin. The severeocalized deformation gives rise to crack formation, a processn which fly ash particle/matrix decohesion plays an importantole.

Surface delamination is also contributed to process of debrisormation. Visual observations indicate wear debris are darkn colour. Morphology and composition of wear debris were

xamined under SEM. Figs. 14 and 15 show the typical SEMicrographs and EDS analysis of the wear debris of the as-

xtruded unreinforced alloy. Two types of morphologies werebserved in the wear debris collected (fine particles and plate-

load, and composites (c) at low load (C12AR) and (d) at high load (C6S).

Sudarshan, M.K. Surappa / Wear 265 (2008) 349–360 359

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(

Fig. 15. EDS spectrum of debris of the A356 Al alloy (a) at low load,

ike flakes) at different loads. At 10 and 20 N loads, the wearebris was in the form of very fine particles (Fig. 14a). At0 N and above loads, the debris comprised of a mixture ofhe fine particles and shiny metallic plate-like flakes (Fig. 14b).t can be seen that the finer debris get agglomerated to parti-les of 0.6–9.0 �m size. At higher loads debris in the size range0–250 �m are confirmed. EDS analysis revealed the presencef small amount of iron in the wear debris (Fig. 15a and b). Moreron was present in the aggregates of ultra fine particles, and themount of iron was relatively lower in the plate-like debris.

The topography of the wear debris confirms that adhesionas a dominant wear mechanism. The debris produced in the

dhesive wear regime was in the form of flake-like thin sheetss shown in Fig. 14b. The debris particles in the material atigher wear rates were usually irregular flakes. These particlesppeared to be generated by the breaking off of the material athe edge of the worn surface.

The wear debris generated during the sliding wear of compos-tes was also dark in colour. It was observed that the wear debris

as very fine agglomerates (Fig. 14c) with size ranging from 0.1

o 8.0 �m at low loads (10 and 20 N). Plate-like debris was in theize range 30–450 �m in C6S composite, 41–230 �m in C12Somposite and 45–244 �m in C12AR composite. Further finer

(

high load, and C12AR composite (c) at low load and (d) at high load.

articles were observed at higher load (50 N and above). Thiss because of pre-generated debris grinded in between pin andisc at higher load. EDS analysis shows more amount of iron inhe wear debris (Fig. 15c and d) compared to unreinforced alloy,ue to the abrasive action of the fly ash.

At low loads the fly ash particles remain intact during wearest in order to support the applied load and act as effectivebrasive elements. When the applied load induces stresses thatxceed the fracture strength of fly ash particles, the particlesracture (Fig. 10) and largely lose their effectiveness as loadearing components. Higher volume fraction of fly ash particlesead to a better wear resistance at higher load.

. Conclusions

1) Incorporation of 6 vol. % of fly ash particles into A356 Alalloy results in decrease in dry sliding wear rates at lowloads (10 and 20 N).

2) Twelve volume percent of fly ash reinforced composites

show lower wear rates compared to the unreinforced alloyin the load range 20–80 N.

3) In the case of composites with 12 vol. % fly ash, narrowerthe particles size, lower is the wear rate.

3 a / W

(

(

(

R

[

[

[

[

[14] P.K. Rohatgi, R.Q. Guo, P. Huang, S. Ray, Friction and abrasion resistance

60 Sudarshan, M.K. Surapp

4) Significant increase in the friction co-efficient (from ∼0.49to ∼0.58) is observed when volume fraction of fly ash par-ticles increases from 6 to 12.

5) The friction co-efficient of 12 vol. % fly ash reinforced com-posites show higher value at all loads. C6S composite showshigher value only at higher load (65 and 80 N) compared tounreinforced alloy.

6) Adhesive wear is dominant in unreinforced alloy, whereasabrasive wear is predominant in composites. At higher load,subsurface delamination is the main mechanism in both thealloy as well in composites.

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