wear behavior of aluminum and cnt

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LECopyright © 2012 by American Scientific Publishers

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Science of Advanced MaterialsVol. 4, pp. 1–8, 2012

(www.aspbs.com/sam)

Wear Behavior of Spark Plasma Sintered Al2124Aluminum Alloy Containing Carbon NanotubesA. M. Al-Qutub1, A. Khalil1, N. Saheb1�2�∗, Al-Aqeeli1�2, and T. Laoui1�2

1Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals,Dhahran-31261, Saudi Arabia2Center of Excellence in Nanotechnology, King Fahd University of Petroleum and Minerals,Dhahran-31261, Saudi Arabia

ABSTRACT

In this work, the wear behavior of spark plasma sintered Al2124 alloy and its composite containing 1 wt.% car-bon nanotubes (CNTs) was investigated at a constant sliding speed and different loads against AISI 4140 steelcounterface using pin-on-disk configuration. It was found that the addition of CNTs improved wear resistanceof Al2124 alloy under lower loads. At higher loads, remaining pores caused crack development and propaga-tion and consequently severe delamination resulting in poor wear resistance of the composite as compared tothe monolithic alloy. Mixed modes of wear were observed for the monolithic alloy and the composite. Underlower loads, the composite mainly displayed abrasion with some localized delamination whereas the monolithicalloy showed significant delamination. Under intermediate loads, adhesion was found to be dominant for thecomposite as compared to microploughing observed for the monolithic alloy. Under the highest applied loadof 25 N, the composite displayed severe subsurface fracturing and delamination in the form of large flakes ascompared to the monolithic alloy in which the delamination was less intense due to the formation of a stableoxide layer.

KEYWORDS: Metal Matrix Nanocomposites, Powder Processing, Spark Plasma Sintering, Wear.

1. INTRODUCTION

There has been significant research on improving wearresistance of aluminum and its alloys either by differ-ent types of heat treatments and coatings or by incorpo-rating certain percentage of a different material to formaluminum metal matrix composite (Al-MMC). The latterapproach has been more attractive and effective as theformer approach involves strengthening the material onlyfrom the exterior which may not remain effective with thepassage of time. In contrast, the latter approach involvesstrengthening the material as a whole on the continuumlevel so that the properties remain unaffected. Several typesof ceramic particles such as SiC1–4 and alumina5–8 havebeen tested as reinforcements for improving wear resis-tance of aluminum and its alloys. Due to higher hardnessand strength of the reinforcement phase, it has been com-monly observed that wear resistance of aluminum alloysincreased with the increase of the volume fraction of thereinforcement.

∗Author to whom correspondence should be addressed.Email: [email protected]: xx Xxxx xxxxAccepted: xx Xxxx xxxx

Recently, researchers used nanoparticles as reinforcingagents to improve wear resistance of aluminum alloys.One of the candidate nanoreinforcements for this purposewas carbon nanotubes (CNTs)9 because of its exceptionalstrength10 in addition to its light weight. However, littlework has been reported on the effect of CNTs additionon wear behavior of aluminum alloys. Moreover, wearmechanism and optimum CNT content have not been suf-ficiently studied and the data available are contradictory.Zhou et al.11 reported steady decrease in wear rate andfriction coefficient of aluminum up to 20 vol.% CNTs.Kim et al.12 found that only 1 wt.% CNTs was an opti-mum content for best tribological characteristics of alu-minum. Choi et al.13 reported that 4.5 vol.% CNTs was theoptimum content for lowest wear rate and friction coeffi-cient of aluminum. The difference in optimum CNT con-tent could be due to CNTs dispersion technique, compositefabrication method, and type of wear test used. The aimof this work is to investigate the influence of CNTs addi-tion on wear behavior of spark plasma sintered Al2124alloy under dry sliding against a steel counterface at a con-stant sliding speed and different loads using pin-on-diskconfiguration.

Sci. Adv. Mater. 2012, Vol. 4, No. xx 1947-2935/2012/4/001/008 doi:10.1166/sam.2012.1409 1

Wear Behavior of Spark Plasma Sintered Al2124 Aluminum Alloy Containing Carbon Nanotubes Al-Qutub et al.

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LE2. MATERIALS AND EXPERIMENTAL

PROCEDURES

Al2124 prealloyed powder, supplied by Aluminium Pow-der Co. Ltd., UK and 95 percent pure multi-walled carbonnanotubes (MWCNTs) were used in this work. Al2124alloy powders containing 0.5, 0.75, 1 and 2 wt.% CNTswere sonicated in a probe sonicator for 30 minutes usingethanol. The sonicated mixture was charged into cylin-drical stainless steel vials together with stainless steelsballs. A ball to powder weight ratio of 10:1 was usedand wet milling was carried out at room temperaturefor 1 hour at 200 rpm in argon atmosphere to preventthe oxidation of the powders. After drying, the powderswere further dry milled for 15 minutes to breakdown theagglomerates. The powders were consolidated using sparkplasma sintering (SPS) process. Sintering was carried outat 450 �C for 20 minutes under a pressure of 35 MPa.More details on the effect of processing on the disper-sion of CNTs and spark plasma sintering were reportedelsewhere.14–16 The density of sintered samples was mea-sured using Alfa Mirage electronic densimeter (modelMD-300s) having accuracy of 0.001 g/cm3 and quantifiedaccording to Archimedes principle. Vickers microhardnessof spark plasma sintered samples was measured using adigital microhardness tester (Buehler, USA) using a loadof 100 gf and dwell time of 12 seconds. The reported hard-ness values represent the average of 10 readings taken atdifferent locations across the specimen.Cylindrical pins having diameter of 6 mm and length

of 12 mm were prepared from sintered monolithic alloyand the composite containing 1 wt.% CNTs. The flat sur-face of the pin was carefully ground with 600 grit sizeabrasive paper and then polished using a 9 micron dia-mond polishing suspension. AISI 4140 steel disk with ahardness of 24 HRC was used as counterface. An aver-age Ra value of 0.3 microns was maintained for the diskthrough grinding it with alumina abrasive wheel. The sur-faces of the pin and disk were cleaned with ethanol beforerunning the tests. Pin-on-disk wear tests were conductedat room temperature using a pin-on-disk tribometer sat-isfying ASTM G99 standard. The tests were carried outunder dry sliding conditions at constant sliding speed of0.5 m/s and applied loads ranging from 5 to 25 N. Forall the tests, the sliding distance was kept constant at500 m for which steady state conditions prevailed. Eachunique test was conducted three times. A Tescan Lyra-3Field Emission Scanning Electron Microscope (FESEM)with integrated Energy Dispersive X-Ray Spectroscopy(EDS) facility was used for analyzing the worn spec-imens and the carefully collected debris to understandthe wear mechanisms. The worn specimens were washedwith ethanol via sonication for 5 minutes prior to SEMand EDS analysis. Microscopic examination of counter-face disk was also carried out to support the obtainedresults.

3. RESULTS AND DISCUSSION

3.1. Microstructure and Hardness

Figure 1(a) shows a pore free microstructure of sparkplasma sintered Al2124 monolithic alloy. A relative den-sity of 99.17% and a hardness of 116 HV were achieved.Addition of 0.5, 0.75, and 1 wt.% CNTs led to lowerdensification compared to the monolithic alloy as a resultof some remaining pores and agglomeration of CNTs.However, it increased the hardness to a maximum valueof 121 HV at 1 wt.% CNTs. Typical microstructureof Al2124 alloy containing 1 wt.% CNTs is shown inFigures 1(b) and (c). A relative density of 98.36% wasachieved. A further increase of CNTs content to 2 wt.%resulted in poor densification and sharp decrease in hard-ness; this is due to poor densification, presence of pores,and excessive CNT agglomeration. Poor densification andmechanical properties of aluminum reinforced with higherCNT content were reported in many studies due to exces-sive CNT clustering.12�13�17–21 The composite containing1 wt.% CNTs had the highest hardness and was selectedfor wear analysis along with the monolithic alloy.

3.2. Wear Rate

The wear rate with standard deviation for the Al2124monolithic alloy and the composite as a function of load ispresented in Figure 2. For both materials, the wear rate wasfound to increase with increasing load. Under lower loadsof 5 N and 10 N, the composite displayed better wear resis-tance as compared to Al2124 monolithic alloy whereas forloads of 15, 20, and 25 N, the Al2124 monolithic alloy dis-played better wear resistance compared to the composite.The wear rate of the composite almost doubled as the loadincreased from 10 N to 15 N, but no such drastic increasewas observed for the Al2124 monolith alloy up to a loadof 20 N. However, at a load of 25 N, significant increasein wear rate was observed. Also, the composite displayedvery high wear rate with very large standard deviation ascompared to the monolithic alloy for loads of 15, 20, and25 N. These results show that addition of 1 wt.% CNTsimproves the wear resistance of Al2124 alloy under lowerloads (mild conditions). At higher loads (sever conditions),the pores in the composite and CNT agglomerates serve assource of crack nucleation and growth causing excessivesubsurface fracturing and delamination leading to higherwear rate. Also, weak adhesion of CNTs with the matrixcan be another reason for the observed trend. It has beenshown1 that wear rate of materials fabricated through pow-der metallurgy is very sensitive to its percent porositybecause the pores serve as a source of crack nucleationwhich causes severe subsurface fracturing. The observedtrend could also be due to the difference in wear mecha-nisms of the two materials as discussed below.The better wear resistance of the composite compared

to the monolithic alloy at lower loads is in agreement with

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Al-Qutub et al. Wear Behavior of Spark Plasma Sintered Al2124 Aluminum Alloy Containing Carbon Nanotubes

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LE(a)

20 µm

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Fig. 1. SEM micrographs of specimens sintered at 450 �C (a) Al2124,(b) composite, and (c) magnified part indicated by arrow in (b).

other studies which reported that aluminum metal matrixcomposites display better wear resistance at low loads andthere exists a critical load after which the reinforcementhas negative effect on the wear resistance of the matrix.Wang et al.22 studied the effect of adding 15 vol.% Ni3Alon the wear resistance of Al6092 alloy and found that upto a load of 91 N, the composite displayed better wearresistance, however as the load was increased to 140 N,the trend reversed. Korkut23 also reported a similar trendfor Al2024\SiFe\Alumina composite prepared by powdermetallurgy. He found that as the conditions switch from

Fig. 2. Wear rate as a function of load for the Al2124 alloy andcomposite.

mild to severe, the composite starts displaying higher wearrates as compared to the monolithic alloy. The authors ofthe above studies attributed this behavior to the brittle frac-ture and fragmentation of reinforcement at higher loads.Sudarshan et al.24 also reported similar trend while study-ing wear behavior of A356 aluminum alloy reinforced withfly ash particles. They found that the composite contain-ing lower proportion of fly ash particles displayed betterwear resistance as compared to monolithic alloy at lowerloads only. It was proposed that at higher loads, the inter-face between the matrix and the fly ash particle providespreferential path for the growth of subsurface cracks.Also, it can be noted that even under lower loads, the

improvement in wear resistance of Al2124 alloy uponCNT addition was not significant. At loads of 5 and10 N, the average wear rate of the composite decreased by0.5 mm3/km and 0.65 mm3/km, respectively, as comparedto the monolithic alloy. The same trend was observed byZhou et al.11 who reported marginal improvement in wearresistance of aluminum-CNT composites; where a wearrate of 0.0135 mg/m for monolithic aluminum was reducedto a minimum value of around 0.01 mg/m for aluminumreinforced with 20 vol.% CNTs. Kim et al.12 also reportedonly slight decrease in wear rate for aluminum-CNT com-posites. In their investigation, the wear rate of monolithicaluminum was found to be around 11.5 mg which reducedto a minimum value of about 9.5 mg for 1 wt.% CNTs. It isworth mentioning here that in the present work and otherworks,11�12 a pin-on-disk configuration was used. WhereasChoi et al.13 used ball-on-disk configuration and reporteddecrease in wear rate from 75 mg to 35 mg upon 4.5 vol.%CNT addition in aluminum.

3.3. Analysis of Worn Surfaces

Figure 3 shows SEM micrographs of worn surfaces ofAl2124 monolithic alloy and the composite under different

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Fig. 3. SEM micrographs of worn surfaces of monolithic alloy and composite.

loads and at different magnifications. At 5 N, the mono-lithic alloy is characterized by abrasion in addition tonoticeable amount of delamination. Also at higher mag-nification, tiny cracks can be observed in the delami-nated regions. On the other hand, at 5 N, the compositeis mainly characterized by abrasion with some localizeddelamination at few locations and subsurface cracking canbe hardly observed. This shows the strengthening effectof CNTs and explains why the wear rate of compositeis lower as compared to the monolithic alloy at lowerload. At 15 N, the monolithic alloy is characterized byploughing wear. The long scars represent subsurface plas-tic deformation caused mainly due to ploughing. In con-trast, at 15 N, this ploughing effect is not evident in caseof composite. The composite is mainly characterized bylocalized pits representing adhesion of material from thespecimen to the counterface. Also, as can be seen at higher

magnification, the surface has tiny white particles possi-bly due to the transfer of iron from the counterface. Sometiny cracks can be observed, especially around the adheredregions showing the onset of delamination. The switchingof wear mechanism from localized delamination to adhe-sion explains the sharp increase in wear rate from 10 N to15 N in case of composite. At the maximum applied loadof 25 N, in addition to abrasion caused by microdebris, themonolithic alloy is characterized by deep grooves showingdelamination due to the removal of thicker flakes. Also,at higher magnification, some cohesion of particles canbe observed inside the deep groove which may be repre-senting some oxide formation. In contrast, the morphologyof composite at 25 N is completely different. The sur-face is characterized by severe damage due to large scaledelamination. The tiny cracks developed in composite ata lower load propagated freely at a higher load. The long

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LEdistance crack propagation caused severe subsurface frac-turing and delamination and hence extremely high wearrate for the composite. Figure 4 shows the overall mor-phology of worn surfaces of the monolithic and the com-posite at an applied load of 25 N. From Figure 4(a), it isevident that the monolithic alloy underwent plastic defor-mation due to ploughing effect. As seen in Figure 4(c), thecorners of the monolithic alloy specimen are characterizedby plastic deformation and thermal softening representingductile nature of the monolithic alloy. In contrast, as evi-dent from Figure 4(b), the composite is characterized bysevere subsurface fracturing and delamination, especiallyat the corners, Figure 4(d), representing the brittle natureof the composite. EDS analysis of worn surfaces at dif-ferent loads are presented in Figure 5. There is no signifi-cant difference in the composition of the monolithic alloyand the composite at 5 N. However, at higher loads, theamount of oxygen is much higher in the monolithic alloyas compared to the composite which shows the forma-tion of stable oxide layer on the monolithic alloy surface.This oxide layer might served as a protective layer causinglower wear rate of the monolithic alloy as compared tothe composite. Also, at higher loads, the composite’s sur-face is mainly comprised of aluminum, especially at 25 Nwhere almost 90 percent of the surface is comprised ofaluminum with negligible proportion of iron. This showsthat due to excessive fracturing and delamination in thecomposite at higher load, stable iron oxide layer failed toform and if some oxide layer formed or some iron adheredfrom the counterface, it failed to survive because of thefrequent fracturing and delamination of material from thespecimen’s surface.

3.4. Analysis of Debris

Figure 6 shows the morphology of debris formed at dif-ferent loads in case of monolithic alloy and composite.At 10 N, the debris of monolithic alloy is mainly char-acterized by larger flakes, Figure 6(a), whereas the debrisof composite is characterized by smaller flakes in addi-tion to tiny particles, Figure 6(b). This is agreement withthe SEM analysis of worn surfaces where delaminationwas more pronounced in case of monolithic alloy at lowerloads. At 20 N, the opposite trend is observed. While themonolithic alloy debris is mainly composed of delaminatedflakes, Figure 6(c); the composite’s debris is characterizedby very large flakes due to excessive subsurface damageof the specimen, Figure 6(d). This is also in agreementwith the SEM analysis of worn surfaces where at higherloads, the composite displayed excessive fracturing anddelamination due to long distance crack propagation.The composition of debris shown in Figure 6 was ana-

lyzed through EDS and shown in Figure 7. These resultsare in good agreement with those shown in Figure 5. Themonolithic alloy debris contains significant proportion ofiron and oxygen. This shows the formation of thick iron

1 mm

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Fig. 4. SEM micrographs of worn specimens at a load of 25 N of(a) Al2124 alloy and (b) composite, (c) and (d) edges of specimens shownin (a) and (b), respectively.

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LE Fig. 5. EDS analysis of worn surfaces of (a) Al2124 alloy and (b) com-posite pins under different loads.

oxide layer on the specimen during steady state wear pro-cess. As the test continued, the oxide layer is removedfrom the surface due to weak adhesion with the specimenand hence became part of the debris. On the other hand,there is only slight formation of iron oxide layer in caseof composite and this layer is not stable at all due to fre-quent fracturing and delamination of composite’s surfaceat higher loads. Also, the proportion of aluminum in com-posite’s debris is much higher as compared to monolithicalloy which represents frequent delamination of aluminumin the form of large flakes. This may be attributed fromone side to the cracks developed due to CNT agglomeratesand from other side to the weak adhesion between CNTsand the matrix at some locations in the composite.

3.5. Analysis of Counterface

The microscopic analysis of counterface at an appliedload of 10 N and 20 N is shown in Figure 8. While thecounterface is mainly characterized by abrasion in case ofmonolithic alloy, Figures 8(a) and (c), the counterface incase of composite is also characterized by adhered lumpsfrom the specimen, marked with arrows in Figures 8(b)and (d). Also it can be seen that the size of the adheredlumps increased with the increase of the load. This is inagreement with the SEM results of worn surfaces wheresigns of adhesion in case of composite were very clear.This shows that adhesive wear is also prominent in thecomposite.

200 µm

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Fig. 6. SEM micrographs of debris formed in case of monolithic alloy(left) and composite (right) at an applied load of 10 N (a) and (b) and20 N (c) and (d).

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Fig. 7. EDS analysis of debris of (a) Al2124 alloy and (b) compositeunder different loads.

In order to verify that these lumps represent adhesionfrom the specimen to the counterface, the lumps weremechanically peeled off from the counterface and theircomposition was analyzed through EDS and the resultsare shown in Figure 9. As evident, the lumps are mainlycomposed of aluminum and this confirms that the adhe-sion actively occurred from the pin to the counterface incase of composite. Significant proportion of oxygen wasalso found in the lump which shows formation of alu-minum oxide. The continuous abrasion of the specimensby these adhered lumps having significant amount of hardaluminum oxide could be another reason for higher wearrate of composite at higher loads.The wear mechanisms observed for the Al2124-CNT

composite in the present work can be compared to thoseobserved by Zhou et al.,11 Kim et al.12 and Choi et al.13

for Al-CNT composites. Zhou et al. found oxidation as themain wear mechanism for Al-CNT composite. They pro-posed the formation of alumina layer on the composite andits subsequent delamination causing abrasion between thespecimen and the counterface. This single specific wearmechanism may be due to the fact that all tests were per-formed under a constant load and sliding speed with vary-ing CNT content. On the other hand, Kim et al. observedmixed abrasive and adhesive wear for Al-CNT compos-ites with varying CNT content at a fixed load and slid-ing speed. They also reported minimal oxidation wearfor the composite containing 1 wt.% CNTs. Choi et al.reported microploughing and delamination as the dominant

(b)

(d)

(c)

(a)

Fig. 8. Microscopic images (X20) of worn track on counterface in caseof monolithic alloy (left) and composite (right) at an applied load of 10 N(a) and (b) and 20 N (c) and (d).

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LE Fig. 9. EDS analysis of peeled off lumps from the counterface in caseof composite specimens.

wear modes for Al-CNT composites. At higher loads,they observed more delamination which is in agreementwith the present work where excessive delamination wasobserved for composite at the maximum applied load of25 N. The different modes of wear of Al-CNT compositesreported in literature may be due to different CNT con-tents, composite fabrication method, and wear test param-eters (speed and load). However, the present work clarifiesthat the wear behavior of Al-CNT composites is largelyinfluenced by the applied load and there exist a criticalload at which the pores inside the composite become activeresulting in long distance crack propagation and subse-quent delamination of the material. As a consequence,CNTs could have a negative influence on wear resistanceof aluminum and its alloys under severe conditions.

4. CONCLUSION

Pin on disk wear tests at a constant sliding speed of0.5 m/s and varying loads from 5 to 25 N againstAISI 4140 steel counterface showed that CNTs is can-didate reinforcement for improving wear resistance ofAl2124 alloy under lower loads. At higher loads, remain-ing pores caused crack development and propagation and

consequently severe delamination resulting in poor wearresistance of the composite as compared to the mono-lithic alloy. Mixed modes of wear were observed for themonolithic alloy and the composite. Under lower loads,the composite mainly displayed abrasion with some local-ized delamination whereas the monolithic alloy showedsignificant delamination. Under intermediate loads, adhe-sion was found to be dominant for composite as com-pared to microploughing observed for the monolithic alloy.Under the highest applied load of 25 N, the compositedisplayed severe subsurface fracturing and delamination inthe form of large flakes as compared to the monolithicalloy in which the delamination was less intense due tothe formation of a stable oxide layer.

Acknowledgment: The authors would like to acknowl-edge the financial support for this work provided by KingAbdul Aziz City for Science and Technology (KACST)through research project number ARP-28-122.

References and Notes

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38, 2928 (2007).19. A. Esawi and M. Elborady, Comp. Sci. Tech. 68, 468 (2008).20. A. M. K. Esawi, K. Morsi, A. Sayed, M. Taher, and S. Lanka, Comp.

Sci. Tech. 70, 2237 (2010).21. J. Z. Liao, M. J. Tan, and I. Sridhar, Mater. Design 31, S96 (2010).22. Y. Wang, W. M. Rainforth, H. Jones, and M. Lieblich, Wear

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