Effect of sliding distance on dry sliding tribological behaviour of Aluminium Hybrid Metal Matrix Composite (AlHMMC): An alternate for automobile brake rotor – A Grey Relational

Original Article

Effect of sliding distance on dry slidingtribological behaviour of AluminiumHybrid Metal Matrix Composite(AlHMMC): An alternate for automobilebrake rotor – A Grey Relational approach

M Kumar, Megalingam Murugan A, V Baskaran and KSHanumanth Ramji

Abstract

In fact, aluminium alloys have impressive mechanical and physical properties which make them robust for number of

automobile applications. Reinforcing hard ceramic particles and solid lubricants with aluminium alloys gives balanced

mechanical, physical and tribological characteristics for cast composites. The effect of sliding distance on tribological

behaviour of Al6061-T6 alloy and its composite reinforced with hard ceramic constituent alumina (3 wt.%) and solid

lubricant graphite (3 wt.%) fabricated through stir casting technique is discussed in this work. The American Society for

Testing of Materials-G99-95 standard specimens prepared from cast composites are supposed to dry slide on the steel

disc of a pin-on-disc apparatus. It is observed from the results that, for all combinations of applied load, sliding velocity

and sliding distance aluminium hybrid metal matrix composite (AlHMMC) reveal superior tribological properties than the

Al6061 alloy. Experimental wear test results exhibit an increasing trend of specific wear rate of AlHMMC for increasing

load and sliding distance which is absolutely lesser than Al6061 alloy. However, the impact of sliding velocity evident

decrease in specific wear rate upto 4.18 m/s later it starts increasing with increase in sliding distance. The worn-out

specimen surfaces are examined for their elemental, compound characterization and morphological analysis through

energy dispersive X-ray detector spectroscopy and scanning electron microscope, respectively. Optimized process par-

ameters are obtained by grey optimization approach which yields improved specific wear resistance and coefficient of

friction. Improved results of hybrid composite offers an assurance to alternate existing cast iron brake rotor for the future.

Keywords

Aluminium hybrid metal matrix composite, dry sliding tribological behaviour, pin-on-disc, X-ray diffraction, scanning

electron microscope–energy dispersive X-ray detector spectroscopy

Date received: 17 February 2015; accepted: 28 July 2015

Introduction

In the recent history, metal matrix composites(MMCs) have made a predominant change in auto-mobile industries by their unique and exceptionalengineering (mechanical, physical and tribological)properties. Usage of aluminium in automotive indus-tries achieved reduced weight, fuel consumption andimproved fuel efficiency and green gas emission. Dueto the addition of various firm particle reinforcements(Al2O3, SiC, B4C, etc.) these composites exhibitgood mechanical properties. In the development ofMMCs, there are several stumbling blocks like poros-ity, inefficient wettability and unequal distributionof reinforcements. While fabricating composites,primary importance is given to attain homogeneousdistribution of reinforcements, in this way a novel

fabrication technique has been proposed to improvethe wettability between alloy and reinforcements.1,2 Inmost of the cases, the size and type of reinforcementdetermine the mechanical and tribological propertiesof the composites. The effect of type of reinforcementssuch as alumina fibre, SiC particle and SiC whisker onthe properties of MMCs, made by powder metallurgyrevealed that, particulate reinforcement results in

Department of Mechanical Engineering, Bannari Amman Institute of

Technology, Sathyamangalam, Tamilnadu, India

Corresponding author:

Megalingam Murugan A, Bannari Amman Institute of Technology,

Sathyamangalam, Tamilnadu 638401, India.

Email: [email protected]

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improving the wear resistance of MMCs.3

Basavarajappa and Chandramohan4 established acorrelation between dry sliding wear of compositesand wear parameters using multiple regressions.Finally, confirmation tests were conducted to verifythe experimental results foreseen from the mentionedcorrelations. Srivastava5 has found phenomenon thatwhen the wt.% of reinforcement increases, failurethrough crack occurs at the interfaces. This micro-scopic fracture happens mainly because of thecracks developed on each Al2O3 particulates. Heattreated Al6061-SiCp has shown reduced wear,improved tensile strength and micro hardness com-pared to Al6061 alloy.6 Increasing graphite wt.%upto 5% and SiC particle size reduces the porosityof Al6061/10SiC composites.7 The author also inves-tigated the wear mechanism as adhesive wearfor Al6061 alloy and abrasive delamination incomposites.

Hybrid metal matrix composites (HMMCs) arevery appropriate for the applications where combinedstrength, damping property, thermal conductivitywith lesser density and high wear resistance are inev-itable.8,9 HMMCs are extensively used for focusedautomobile applications like brake drum, pistons,brake rotor,10 cylinder block, etc. In recent years,HMMCs are being produced by Al, Ti, Cu, Mg andtheir alloys principally reinforced with hard ceramicparticles like alumina and silicon carbide.11

Introducing graphite as a primary reinforcementincreases the wear resistance of composites by form-ing a protective layer between pin and counter faceand the inclusion of alumina as a secondary reinforce-ment also has a significant effect on the wear behav-iour.10 Soft reinforcements like talc are used incomposites12 for their extensive usage in automobiles.The composites had improved tensile and yieldstrengths than the Al6061 alloy. Increase in strengthin the composites is attributed to an increase in thedislocation together with an increase in the agingresponse.13 The friction and wear properties ofHMMCs are also improved by rising wt.% ofreinforcement at all practical conditions.14 Upto25wt.% of SiC and Al2O3 particulate-reinforced com-posites reduce the wear rate at room temperature andthe wear of the test specimens increases with theincreasing load and sliding distance. The coefficientof friction (CoF) slightly decreases by increasing thepercentage of reinforcements and micro hardness ofthe hybrid composite test specimens increases withincreasing volume fraction of particulates reinforce-ment. Umanath et al. discussed about wear resistanceof Al6061 alloy and composites, in his results he hasstated that Al6061/3% alumina/15% SiC gives betterwear resistance than its alloy at all loading condi-tions.15 The wear resistance of hybrid composite wasenhanced by adding small quantity of SiC and B4Cparticles.16 The conventional grey cast iron (CI) brakedrum can be replaced with Al–SiC MMC brake

drums in automobiles due to its improved wear per-formance and comparative CoF17–19,9 reviewed anddiscussed about dry sliding wear behaviour of alumin-ium–SiC and graphite composites. The author hadreported that the friction couples (disc and pad)should have relatively high and stable CoF.20 Inorder to achieve the desired brake friction couple per-formance, it was also found that the CoF hardly chan-ged with applied braking load and speed for brakedrums of any material and the CoF of the compositesis found to be higher than CI brake drum.21,22

Hence in this present work, a keen attempt hasbeen made to fabricate aluminium hybrid metalmatrix composite (AlHMMC) to get improved tribo-logical performance. This study also investigates theeffect of wear parameters (applied load, sliding vel-ocity and sliding distance) on the dry sliding wearbehaviour of the Al6061 alloy and Al6061/alumina/graphite hybrid MMCs. The significance of slidingdistance over applied load and sliding velocity ofAl6061 alloy and AlHMMC was also discussed.Moderate (not higher or lower) and stable CoF arerequired in AlHMMC that is actually desirable for akind of brake rotor applications.

Experimental strategy

Material selection

In the present study Al6061-T6 alloy was selected asmatrix material since it has wide application in auto-mobile industries due to its low weight, good strengthand fuel efficiency. The reinforcements were alumina(Al2O3) particles of average size 22–25mm with3wt.% and graphite (Gr) of average size 14–17mm of3wt.%. Since alumina being a hard and brittle ceramicparticle it can blend with soft Al6061 matrix and withsolid lubricant graphite by the way it enhances thestrength and stiffness of AlHMMC. The chemical com-position of Al6061 alloy matrix is given in Table 1.

Hybrid MMC preparation

Stir casting, the most popular liquid metallurgy tech-nique was used for fabricating near homogeneousHMMC at nominal cost.23 In this method about1 kg of Al-6061 alloy is reinforced with constantwt.% of Al2O3 (3%) and graphite (3%) to fabricateHMMC. In order to achieve good bonding betweenmatrix and reinforcements 1wt.% of magnesium wasadded. Considering porosity formation, 1wt.% ofhexacholroethane was added as a degasifying agent.The furnace temperature was measured and con-trolled accurately by a thermocouple with propor-tional-integral-derivative controller so as to achievethe supreme quality composite. A graphite cruciblewith a mild steel stirrer was used to fabricate the com-posites. A 1 Horse Power (HP) direct current (DC)motor was employed to ensure the stirring action of

2 Proc IMechE Part J: J Engineering Tribology 0(0)



the semi solid state metal. The molten metal was keptfor about 1 h at a temperature span of 750–800�C.Vortex was created by means of a electrical stirrerand utmost care was taken to add the preheated(600�C) weighed quantity of Al2O3 (3%) and graphite(3%) to the molten metal at a constant stirring speedof 450 r/min for about 10min. Subsequent to thereinforcement addition, the crucible with compositematerial (750�C) was tilted and poured into preheated(300�C) steel die (20 x 150mm) and allowed to coolfor about 2min.24

Mechanical properties of hybrid composites

Hardness. The hardness was measured by Rockwellhardness investigation to ensure resistance to perman-ent indentation of materials as defined by AmericanSociety for Testing of Materials (ASTM) E-18. BothAl6061 alloy and AlHMMC were tested and it wasobserved that Al6061alloy and AlHMMC (Al6061alloyþ 3% Grþ 3% Al2O3) have 39.2 and 57.125 asRockwell hardness values, respectively.

Design of experiments (DoEs)

DoE is a significant and influential statistical tool tostudy the effect of multiple factors at a time. Itinvolves a series of steps for conducting the experi-ments to yield an improved process performance.DoE requires a certain number of combinations offactors and levels in order to observe the results. Asper Taguchi’s approach the DoE process has threemain phases: the planning phase, the conductingphase and the analysis phase.25,26 Analysis of theexperimental results uses a signal to noise ratio (S/Nratio) to assist in determining the best process param-eters.27,28 In this present research, L27 orthogonalarray (OA) is incorporated to analyse the experimen-tal results of dry sliding performances (wear & fric-tion) of Al6061 alloy and AlHMMC by varying theinput control parameters like load, sliding velocityand sliding distance. In L27 OA, it has 27 rows cor-responding to the number of runs and 13 columnswhich indicate the number of influencing factors andtheir interactions.29,30 The input control parametersand their levels are given in Table 2.

Dry sliding wear testing of hybrid composites

Wear tests were conducted on pin-on-disc apparatusas per ASTM G99-95 standard to investigate the dry

sliding wear characteristics of Al6061 alloy andAlHMMC specimens. The specimen pins of 10mmdiameter and the 30mm height were machined andprepared from the cast composite bars and they aremetallographically polished. Each specimen wascleaned in acetone and dried thoroughly beforetesting.

With an electronic balance of 0.0001 g precisionevery specimen was weighed before and after test tofind out mass loss (wear loss). Figure 1 shows theexperimental setup of pin-on-disc apparatus inwhich the wear studies were carried out. A stationarypin specimen was clamped and set pressed to slide ona hard EN31 steel rotating disc (62 RockwellHardness C-Scale (HRC)). The specimens had thetrack diameters (sliding distance) as mentioned inTable 2. Linear variable differential transducer(LVDT) was used to measure friction coefficientvalues and were recorded in CPU by DataAcquisition System (DAQ). Specific wear rate orvolumetric wear rate was calculated by volume losswith sliding distance. Mitutoyo SJ-201 (P) surfaceroughness tester was used to confirm the surfaceroughness (Ra) values of 0.26mm, 0.34 mm and0.43mm for Al-6061 alloy, AlHMMC and EN31disc, respectively.

Determination of specific wear rate and CoF

During the process of dry sliding wear tests, theweight loss and frictional force values were noted.Using equations (1), (2) and (3) the specific wearrate and CoF values were calculated.

Specific wear rate ¼�m

ð�� Fn�DÞð1Þ

�m ¼ mass loss gð Þ ¼ weight before wear

� weight after wear ð2Þ

Table 1. Chemical composition of Al6061 alloy.

Chemical

elements of

Al6061 Si Fe Cu Mg Cr Zn Ti Mn Al Others

Percentage 0.40–0.80 0.70 max 0.15–0.40 max 0.80–1.20 0.04–0.35 0.25 max 0.15 max 0.15 max 94.0 0.15 max

Table 2. Control parameters and levels.

Level

Load,

L (N)

Sliding velocity,

V (m/s)

Sliding distance,

D (m)

1 20 2.06 800

2 30 4.18 1600

3 40 6.28 2400

Kumar et al. 3



Coefficient of friction ð�Þ ¼ frictional force =

applied load ð3Þ

Normally for brake rotor applications, frictioncoefficient was expected to be stable and moderate.High CoF will cause more wear loss due to excessivefriction on parent material as well as counter material.Controversially, if the CoF is very low it also leads toslippage between contacting surfaces which is everundesirable for brake rotor considerations, since thisresearch deals about generation of novel material forautomobile brake rotor the friction coefficient must bestable and moderate.

Results and discussions

Dry sliding wear behaviour of AlHMMC

The effect of sliding velocity, applied load with respectto sliding distance on dry sliding wear behaviour ofAl6061 alloy and AlHMMC is discussed here. TheDoE was used to find out significant combinationsof input parameters that are important to achievethe reduced wear rate and average CoF. The pinswere slided on the EN31 steel disc at a track radiusof 0.1m for the sliding distances of 800m, 1600m and2400m, respectively.

Effect of applied load and sliding velocity onspecific wear rate

At the outset, the effect of applied load on specificwear rate of Al6061 alloy and AlHMMC has beendetermined from dry sliding wear tests’ result. Theoperating conditions are applied loads of 20N, 30Nand 40N for the sliding distances of 800m to 2400mwith an increment of 800m.32 The variations of spe-cific wear rate with applied load for Al6061 andAlHMMC are shown in Figure 2 with respect to thesliding velocities of 2.06m/s, 4.18m/s and 6.28m/s,respectively. At all the sliding velocities, AlHMMCshows less specific wear rate compared to Al6061

alloy but at 40N applied load, the specific wear rateis high for Al6061 and AlHMMC compared to 30Nand 20N applied loads, this may be caused by theadhesive wear occurred on the pin material due totemperature raise between contacting surfaces athigher loads similar wear behaviour which wasobserved by Natarajan et al.9 At 20N load,AlHMMC exerts almost 90% lesser specific wearrate than Al6061 alloy at all the sliding velocities.Figure 2(a) and (b) ensures that AlHMMC possesses69% higher hardness than Al6061 alloy which capit-alize less specific wear rate in AlHMMC. As theapplied load and sliding velocity increase, the vari-ation in specific wear rate decreases due to the thirdlayer formation between contact surfaces (Figure 2(d)and (f)). This protective layer maybe formed byextraction of alumina particles from composite thatform an oxide layer over the sliding surfaces, similarbehaviour was encountered as mechanically mixedlayer (MML) by Basavarajappa et al.19 At 40Napplied load and for maximum sliding distance(2400m) this MML exists and therefore it reducesthe specific wear rate for all sliding velocities(Figures 2(f) and 3(ii)).

The effect of sliding velocities (2.06, 4.18 and6.28m/s) on specific wear rate of Al6061 alloy andAlHMMC has also been discussed for various appliedloads and sliding distances as shown in Figure 4; ifsliding velocity increases, specific wear rate alsoincreases for both Al6061 alloy and AlHMMC irre-spective of applied loads at all sliding distances. FromFigure 4(a) and (b) at lower velocity (2.06m/s),Al6061 alloy revealed 40% more specific wear ratethan AlHMMC at the sliding distance of 2400mdue to lack of tribo layer formation betweenmating surfaces. When the sliding velocity increasesfrom 2.06m/s to 4.18m/s, the rate of change invariation of specific wear rate decreases due to theformation of protective layer (Figure 4(c) and (d));identical behaviour was observed in Radhika et al.15

At higher sliding velocity (6.28m/s) and maximumsliding distance (2400m) the percentage of variationin specific wear rate was reduced by 55% in

Figure 1. Dry sliding tribological test – experimental setup (pin-on-disc apparatus).




AlHMMC compared to Al6061 alloy, this is due tothe existence of tribo layer by the solid lubricantgraphite in AlHMMC (Figure 4(e) and (f)). The rateof specific wear is remarkably less in AlHMMC whencompared to Al6061 alloy, which is due to the pres-ence of hard particle (alumina) that acts as a loadbearing element at higher loads and the nature ofwear became abrasive in composite, such kind ofbehaviour was observed by Natarajan et al.9 Athigher sliding velocity and load the tribolayer createdby solid lubricant is destroyed that leads to slightincrease in specific wear rate.

Measurement of CoF

The CoF is a measure of friction between two con-tacting surfaces. Friction force used to build up insuch a direction that opposes the contact between sur-faces. CoF is generally calculated by the ratio of forceinduced and applied force. Typical CoF behaviourwith respect to time for AlHMMC is discussed inFigure 5(a) and (b). Since this work concentrates onthe application of AlHMMC to the automobile brakerotor application, required friction coefficient should

be moderate (average value between lower and higherfriction coefficient) and stable.

Effect of applied load and sliding velocity on CoF

The effect of applied loads on the CoF of Al6061alloy and AlHMMC is noted down in Figure 6 fromexperimental results. The CoF of AlHMMC is 46%lesser than Al6061 alloy for all sliding distances atlower load. At 6.28m/s and 20N applied load, thefriction coefficient was found reduced for increasingsliding distances for both Al6061 alloy andAlHMMC, this is due to the third layer formationbetween surfaces at lower velocities by abrasivenature of material removal (Figure 6(a) and (b)).From Figure 6(c), at 30N applied load at all slidingvelocities, the friction coefficient of Al6061 alloytends to decrease as the sliding distance increases,this can be due to the presence of protective layerbetween surfaces, whereas in AlHMMC when loadincreases to 30N the CoF also increased 40% than itwas in 20N (Figure 6(d)). This phenomenon hashappened due to the transfer layer created by graph-ite (solid lubricant) stays stable at lower loads,

Figure 2. Effect of sliding distance and sliding velocity on specific wear rate for an applied load of 20 N: (a) Al6061 alloy,

(b) AlHMMC; applied load of 30 N: (c) Al6061 alloy, (d) AlHMMC; applied load of 40 N: (e) Al6061 alloy, (f) AlHMMC.

Kumar et al. 5



whereas it wipes out at higher loads due to raise intemperature.9 At higher applied load (40N) and slid-ing velocity (6.28m/s), AlHMMC exhibits lineartrend of friction coefficient variation, which ensuresthe presence of certain transfer layer between pinand the disc surfaces which was created by abradedhard (Al2O3) and soft particles (graphite) ofreinforcements from pin specimen (Figure 6(f)) andidentical behaviour was addressed by Radhikaet al.15 CoF of AlHMMC is almost 54% lowerthan Al6061 alloy at lower load for all sliding velo-cities, which is desired to have a better wear resistivematerial for the foresaid application.

At 2.06m/s sliding velocity and 40N load, the CoFwas found less for both Al6061 alloy and AlHMMCcompared to lesser loads which reveals presence oftransfer layer.

The addition of alumina particle reduces the CoFby forming oxide layers (MML) between contact sur-faces at higher loads and such performance wasalready recognized by Deuis et al.8 (Figure 7(a)and (b)). When sliding velocity increases to 4.18m/s,

the friction coefficient raises by almost 70% inAlHMMC than it was in 2.06m/s. This phenomenonhas happened due to destruction of tribolayer formedby graphite particles because of heat generation athigher loads and sliding velocities for all sliding dis-tances (Figure 7(e) and (f). This layer peeling resultedin increase of the specific wear rate by excess frictionbetween surfaces similar phenomenon was noted andreported by Radhika et al.15

SEM investigations and analysis of worn-out specimens

Scanning electron microscope (SEM) was equipped tounderstand and examine the wear mechanism ofworn-out specimens of Al6061 alloy and AlHMMC.Energy dispersive X-ray detector spectroscopy (EDS)was introduced for morphological analysis and elem-ental compound analysis of worn-out samples.Spectrum results also detect a number of elements,viz. Al, O2, Fe, Si, Ca and compounds, viz. Al2O3,Fe2O3, SiO2, CaO. EDS results of worn-out samples

Figure 3. (i) FESEM morphology of AlHMMC; (ii) EDS spectrum (SiN4) of worn-out sample.




Figure 5. Typical coefficient of friction behaviour with respect to time of AlHMMC for (a) load 20 N, sliding distance (SD) 800 m, and

sliding velocity (SV) 2.06 m/s; (b) load 40 N, SD 2400 m and SV 6.28 m/s.

Figure 4. Effect of sliding distance and applied load on specific wear rate for a sliding velocity of 2.06 m/s: (a) Al6061 alloy, (b)

AlHMMC; a sliding velocity of 4.18 m/s: (c) Al6061 alloy, (d) AlHMMC; a sliding velocity of 6.28 m/s: (e) Al6061 alloy, (f) AlHMMC.

Kumar et al. 7



confirm the presence of added reinforcement elements(alumina and graphite (cementite) Figure 3(ii)) andtheir compound forms (Al2O3, Fe2O3) as MML inthe cast composite.

It was ensured while sliding that the pin had sur-face contact with the steel disc and it had left somescratches on the steel disc. The SEM micrographs ofAl6061 alloy and AlHMMC before wear tests are pre-sented in Figure 8(a) and (b), respectively. The micro-scopic study of Al6061 alloy reveals more materialremoval during wear process. In the case ofAlHMMC inclusion of hard ceramic particle, alu-mina, reduces the number of grooves on pin surface.It is observed from the worn surfaces of AlHMMCthat the amount of pin material removed as debriswas less compared to Al6061 alloy. As a result, thespecific wear rate and CoF of AlHMMC were signifi-cantly less due to incorporation of graphite asreinforcement compared to Al6061 alloy.10

At lower loads, the grooves were formed on theworn surfaces of AlHMMC in its sliding direction.Due to reinforcing particles’ ejaculation these grooveswere revealed. Figure 9(c) and (d) represents the wornsurfaces of Al6061 alloy and AlHMMC at a sliding

velocity of 2.06m/s, sliding distance of 800m for anapplied load of 20N. Alumina, the hard ceramic par-ticle, acts as an effective load bearing element thatproduces abrasive nature of wear mechanism andthis issue no longer extends its damage of the wearsurfaces because of the presence of graphite as softlubricant. At higher loads, the groove formation wasdeeper due to excess stress caused on the pin. Becauseof these grooves the reinforcing particles tend to comeout and had contact with disc material. Some quantityof material transfer from pin to disc is also observedat higher loads. Peeled out alumina particles were theroot cause for material transfer between pin and discthat increases the specific wear rate of AlHMMC tosome extent. For Al6061 alloy, the rate of specificwear was high due to more plastic deformationcaused by excess material removal. Similar kind ofmaterial transfer was identified and recorded as plas-tic deformation by Radhika et al.15

Figure 10(e) and (f) shows the worn surfaces ofAl6061 alloy and AlHMMC at an applied load of20N, sliding velocity of 6.28m/s at a sliding distanceof 800m and an applied load of 40N, sliding velocityof 6.28m/s at a sliding distance of 2400m,

Figure 6. Effect of sliding distance and sliding velocity on coefficient of friction for an applied load of 20 N: (a) Al6061 alloy, (b)

AlHMMC; applied load of 30 N: (c) Al6061 alloy, (d) AlHMMC; applied load of 40 N: (e) Al6061 alloy, (f) AlHMMC.




respectively. Since the saturation of specific wear ratewas found early at 800m for Al6061 alloy, it is man-datory to take that specimen for SEM evaluation. Butfor the case of AlHMMC the saturation of specificwear rate was noticed at 2400m of sliding distanceand 6.28m/s sliding velocity, so that the specimentaken for SEM evaluation. Figure 10(f) (L¼ 40N,V¼ 6.28m/s, sliding distance¼ 2400m). If the sliding

velocity increases, the amount of reinforcementspeeled out also increases due to the ploughing actionof steel counterface on the pin material. Due to theexcess removal of hard ceramic alumina reinforce-ment, over a period sliding distance, the removedmaterial acts asMMLwhich gives a higher hardness toAlHMMC8 and this MML is confirmed by X-raydetector’s (XRD’s) compound analysis (Figure 3(ii)).

Figure 7. Effect of sliding distance and applied load on coefficient of friction for a sliding velocity of 2.06 m/s: (a) Al6061 alloy, (b)

AlHMMC; a sliding velocity of 4.18 m/s: (c) Al6061 alloy, (d) AlHMMC; a sliding velocity of 6.28 m/s: (e) Al6061 alloy, (f) AlHMMC.

Figure 8. SEM images before wear test (a) Al6061 alloy and (b) AlHMMC.

Kumar et al. 9



Results of Venkataraman and Sundararajan11 exhibitsimilar results that specific wear rate and CoF increasewhen applied load increases and it decreases when slid-ing velocity decreases for all sliding distances.

Gray relational approach – A multipleperformance optimization

Grey relational analysis: An overview

The tribological behaviour of composite materials hasbeen optimized by number of significant tech-niques.28,29 The grey relational analysis (GRA) asso-ciated with the Taguchi method30,25 corresponds to anadditional path of approach for multi objective opti-mization. The grey theory is based on the randomuncertainty of small samples, which was then devel-oped into an evaluation technique to solve certaincomplex problems that have incomplete information.Description of systems depends on the informationavailable with them. For an instance, a systemwhich has complete information is known as a‘white’ system, whereas a system for which therequired information is completely unknown is a

‘black’ system. Any system lies between these limitsis a ‘grey’ system having poor and limited informa-tion.26 GRA, which is a normalization evaluationtechnique, is extended to solve the complex multi-per-formances.31 The objective of this present work is toimprove the specific wear resistance and friction coef-ficient of AlHMMC. Specific wear rate and the CoFare mainly influenced by the design or control param-eters like applied load, sliding velocity and amount ofdistance travelled or sliding distance. The first foursteps of GRA represent the conduct of experimentsas per DoE developed. The remaining steps of GRAare discussed below.

Normalization of S/N ratio

Data pre-processing is performed in prior in order tonormalize the experimental data collected from drysliding results. A normalization of the S/N ratio ischanging the raw data values into the range betweenzero and one. Three certain forms of S/N ratio arepresent in the optimization process and they arelarger-the-better, smaller-the-better and nominal-the-better. In the present study smaller-the-better is the

Figure 9. SEM images of worn-out specimens at an applied load of 20 N and sliding velocity of 2.06 m/s (c) Al6061 alloy and

(d) AlHMMC.

Figure 10. SEM images of worn-out specimens (e) an applied load of 20 N and sliding velocity of 4.18 m/s, sliding distance

(SD)¼ 800 m for Al6061 alloy: (f) an applied load of 40 N and sliding velocity of 6.28 m/s, SD¼ 2400 m for AlHMMC.




characteristic used whereas low specific wear rate andfriction coefficients are the required performances.Then the equation must be normalized as given byequation (4).

xi � kð Þ ¼maxx

oð Þi kð Þ � x

oð Þi kð Þ

maxxoð Þi kð Þ �minx

oð Þi kð Þ

ð4Þ

Determination of deviation sequences, �0i kð Þ

The deviation sequence �0i kð Þ is defined as theabsolute difference between the reference sequencex0*(k) and the comparability sequence xi*(k) afternormalization of raw data. It is calculated using equa-tion (5).

�0i kð Þ ¼ x�0 kð Þ � x�i ðkÞ�� ð5Þ

Determination of grey relational coefficient (GRC)

For all the combinations GRC gives the relationshipbetween the best and actual normalized S/N ratio. Ifthe two sequences have same results at all sequences,at that time their GRC is 1. The GRC can beexpressed by equation (6).

� x0 kð Þ, xi kð Þð Þ ¼�minþ ��max

�0i kð Þ þ ��maxð6Þ

� is the distinguishing coefficient that can be takeninto account in the range between 0 and 1.Generally it is considered as 0.5.19

Determination of grey relational grade (GRG)

GRG plays a very important role in evaluating themultiple performance characteristics. Average sumof the GRC’s produce GRG and it can be calculatedthrough equation (7).

� x0,xið Þ ¼1

m

Xmi¼1

� x0 kð Þ,xi kð Þð Þ ð7Þ

Where � x0, xið Þ represents GRG for the jth experi-ment and m is the number of performance or responsecharacteristics.

Determination of optimal parameters

For each experimental data GRG was calculated andconsidered for the further analysis. The best qualitycharacteristic – larger-the-better – has been used foranalysis, since it shows the best performance of all theprocesses. The GRG obtained is analysed by analysisof variance (ANOVA) and it identifies the statisticalsignificance of design parameter individually for aparticular response and the results of analysis weretabulated in Tables 3 to 6.

Prediction of GRG under optimal parameters

Next step to the evaluation of optimal parameters, itis necessary to determine and confirm the develop-ment of quality characteristics by the optimal para-metric combination through GRG. The predictedGRG can be calculated by equation (5) for findingthe optimal machining parameters.

�̂ ¼ �m þXqi¼1

ðb�i� �mÞ ð8Þ

Where �m denotes the sum of all means of GRG(total GRG), b�i is the mean GRG optimal level and qis the number of design parameters influencing theperformance characteristics. The author expressedthat the predicted GRG (optimal) value is equal tothe mean GRG plus the summation of the differencebetween the overall mean GRG and the mean GRGfor each of the significant factors at optimal level.30

The confidence intervals using predicted GRG arepresented in Table 7.

Taguchi based GRA explicit the significance ofindividual factors with confidence level.30 Factor-B:‘sliding velocity’ influences specific wear rate andCoF more than applied load for all the sliding dis-tances. Clear indication from ANOVA results is thesliding velocity has 39.6% of influence in the wear

Table 3. GRA-ANOVA results of SWR for Al6061 alloy and AlHMMC.

Al6061 alloy AlHMMC

A B C A B C

Level

1 0.85465 0.96031 0.83088 0.91508 0.94002 0.85722

2 0.69831 0.67607 0.74253 0.79195 0.8852 0.79568

3 0.635 0.55159 0.61456 0.69838 0.58019 0.7525

Delta 0.21965 0.40872 0.21632 0.2167 0.35983 0.10472

Rank 2 1 3 2 1 3

Average grey relational coefficient 0.729322 0.801803

Kumar et al. 11



rate of Al6061. In the case of AlHMMC ‘applied load’and ‘sliding velocity’ exert very positive influence onthe specific wear rate and CoF. Whereas variations insliding distance have significant impact on the perform-ance of Al6061 alloy and AlHMMC at higher loadsand velocities. Combined effect of sliding distance andapplied load of 43.67% and 42.50755% contribution ofsliding velocity and sliding distance influences the spe-cific wear rate and CoF noticeably. The sliding distanceacts as a cohesive parameter and plays a vital role withthe sliding velocity and applied load for the bettermentof wear resistance of AlHMMC.

The confidence intervals calculated for Al6061 andAlHMMC from GRA analysis are given in Table 7.The conformation experiments (A1B1C1) are alsocarried out to check whether the confirmation resultsfall within the confidence interval or not for bothAl6061 and AlHMMC.16 After conducting confirm-ation experiments, the new GRG for Al6061 is 0.8059and which is found well within the confidence interval(0.733024 to 1). For AlHMMC (A1B1C1), the new

GRG after confirmation run is 0.9668 which is alsowithin confidence interval of 0.897613 to 1.0697.33,34

By the way grey relational approach for this multiobjective optimization is successfully utilized and theoptimized control parameter combinations were sug-gested for improved wear and friction coefficients ofAl6061 alloy and AlHMMC.

Conclusions

AlHMMC holds 45% more hardness than Al6061alloy. Dry sliding tribological behaviour of Al6061

Table 4. GRA-ANOVA results of CoF for Al6061 alloy and AlHMMC.

Al6061 alloy AlHMMC

A B C A B C

Level

1 0.5431 0.65306 0.52281 0.77811 0.63727 0.62326

2 0.57688 0.54934 0.5887 0.52339 0.62684 0.63203

3 0.63029 0.54788 0.63876 0.46857 0.50596 0.51478

Delta 0.08719 0.10518 0.11596 0.30954 0.13132 0.11725

Rank 3 2 1 1 2 3


Table 5. GRA-ANOVA results of SWR and CoF for Al6061 alloy and AlHMMC.

Al6061 alloy AlHMMC

A B C A B C

Level

1 0.60339 0.3821 0.38043 0.8466 0.78865 0.74024

2 0.5979 0.2985 0.48688 0.65767 0.75602 0.71386

3 0.0752 0.59589 0.40919 0.58347 0.54307 0.63364

Delta 0.08719 0.10518 0.11596 0.26312 0.24558 0.1066

Rank 3 2 1 1 2 3


Table 6. GRA-ANOVA average GRC comparison.

Al6061 alloy AlHMMC

SWR CoF SWR & CoF SWR CoF SWR & CoF

Average grey relational coefficient 0.72932 0.58342 0.6563732 0.8018 0.59002 0.69591

Table 7. GRA-ANOVA confidence intervals for Al6061 and

AlHMMC.

Confidence interval Value

Al6061 0.733024 to 1

Al6061þ 3% Grþ 3% alumina 0.897613 to 1.0697




alloy and AlHMMC has been investigated throughthe GRA optimization technique and the followingconclusions are drawn:

. As the sliding distance increases the wear resistanceof AlHMMC increases from 20% to 50% with effectof applied load and sliding velocity compared toAl6061 alloy, due to incorporation of hard ceramic(Al2O3) and soft solid lubricant (Gr) reinforcements.

. For AlHMMC, stable and moderate friction coef-ficient are achieved at 20N applied load, 6.28m/ssliding velocity and 2400m sliding distancewhereas in Al6061 alloy it is attained at 40Napplied load, 4.18m/s sliding velocity and 2400msliding distance.

. For optimized parameters of 20N applied load,2.06m/s sliding velocity and 800m sliding distance,AlHMMC showed 40% less specific wear rate thanAl6061 alloy.

. Based on the friction coefficient results of GRA,sliding distance followed by sliding velocity andapplied load influences Al6061 alloy respectivelybut in AlHMMC it is influenced by applied load,sliding velocity and finally by sliding distance.

. Results derived from the specific wear rate ofGRA-ANOVA show that sliding velocity has aninfluence of 39.57% on Al6061 alloy and 42.51%on AlHMMC but applied load has more influenceon AlHMMC than sliding velocity for all slidingdistances.

. From both specific wear rate and CoF results ofGRA-ANOVA, sliding velocity (39.57%) later byapplied load and then sliding distance reveal a con-siderable influence for Al6061 alloy whereas in thecase of AlHMMC first by applied load (43.67%)followed by sliding velocity (42.51%) finally thesliding distance exerts a significant influence.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest withrespect to the research, authorship, and/or publication of

this article.

Funding

The author(s) received no financial support for the research,authorship, and/or publication of this article.

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Appendix

Notation

D sliding distance (m)Fn frictional force (N)�m mass loss�0i kð Þ deviation sequence,

�0i kð Þ ¼ x�0 kð Þ � x�i ðkÞ��

�min smallest value of �0i kð Þ�max maximum value of �0i kð ÞQ number of design parametersx0

(o)(k) reference sequencexi

(o)(k) comparability sequences; i¼ 1–27,k¼ 1–6

�m sum of all means of GRG or total GRGb�i mean GRG optimal levelm coefficient of friction� density (g/mm3)� distinguishing coefficient (between 0

and 1; generally it is considered as 0.5)




Effect of sliding distance on dry sliding tribological behaviour of Aluminium Hybrid Metal Matrix Composite (AlHMMC): An alternate for automobile brake rotor – A Grey Relational

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