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Materials and Design 63 (2014) 294–304

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Materials and Design

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Technical Report

Dry sliding wear behavior of heat treated hybrid metal matrix compositeusing Taguchi techniques

http://dx.doi.org/10.1016/j.matdes.2014.06.0070261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: #133, Coronation Road, Tiptur 572201,Karnataka, India. Tel.: +91 8134 252717, mobile: +91 98441 13298.

E-mail addresses: [email protected], [email protected](T.S. Kiran).

T.S. Kiran a,⇑, M. Prasanna Kumar b, S. Basavarajappa c, B.M. Viswanatha a

a Department of Mechanical Engineering, Kalpataru Institute of Technology, BH Road, NH 206, Tiptur 572201, Karnataka, Indiab Department of Industrial Automation Engineering, PG Center, Visvesvaraya Technological University, Mysore, Indiac Department of Studies in Mechanical Engineering, University BDT College of Engineering, Davangere 577004, India

a r t i c l e i n f o

Article history:Received 9 February 2014Accepted 3 June 2014Available online 17 June 2014

a b s t r a c t

Dry sliding wear behavior of zinc based alloy and composite reinforced with SiCp (9 wt%) and Gr (3 wt%)fabricated by stir casting method was investigated. Heat treatment (HT) and aging of the specimen werecarried out, followed by water quenching. Wear behavior was evaluated using pin on disc apparatus.Taguchi technique was used to estimate the parameters affecting the wear significantly. The effect ofHT was that it reduced the microcracks, residual stresses and improved the distribution of microconstit-uents. The influence of various parameters like applied load, sliding speed and sliding distance on wearbehavior was investigated by means and analysis of variance (ANOVA). Further, correlation between theparameters was determined by multiple linear regression equation for each response. It was observedthat the applied load significantly influenced the wear volume loss (WVL), followed by sliding speedimplying that increase in either applied load or sliding speed increases the WVL. Whereas for composites,sliding distance showed a negative influence on wear indicating that increase in sliding distance reducesWVL due to the presence of reinforcements. The wear mechanism of the worn out specimen wasanalyzed using scanning electron microscopy. The analysis shows that the formation and retention ofceramic mixed mechanical layer (CMML) plays a major role in the dry sliding wear resistance.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Life of machine component is an important design consider-ation. Various parameters affect the life of components and theselection of material directly influences the life significantly.The choice of material for a particular application varies depend-ing on the variables like cost, density, specific strength, modulusand operating condition. The majority of engine components,gear drives and so on in automotive and aerospace industries uti-lizes metals and alloys. The sliding and rotating componentsintended to work in lubricating conditions may eventually endup working in semi-lubricated or dry conditions. This will resultin higher operating temperature with increase in wear and leadto quicker replacement of components. Hence, wear is one ofthe major problems that need to be tackled in order to improvethe life of the component. Composite materials are the promisingalternate for alloys, specifically in dry operating conditions.Current work concentrates on the development of a hybrid

reinforced composite material that can improve the wear resis-tance in components. Historically addition of reinforcementshas shown significant improvement in tribological properties.However in some instances it has shown deterioration inmechanical properties.

Zinc–Aluminum (ZA) alloy is a competitive bearing alloy thatshows improvement in both mechanical and tribological proper-ties compared with phosphor-bronze, SAE 73, SAE 660 and castiron. The density of the latter are much higher compared withthe former element [1,2]. ZA alloy exhibits superior wear resis-tance at low speed-high load application even in the absence oflubricant, while there is a decline in wear resistance with increasein speed and rise in temperature [3,4]. Seah et al. [5] and Babicet al. [6] performed dry sliding wear behavior of ZA-27 alloy rein-forced with Gr particles. These composite specimens exhibitedenhanced wear resistance than the alloy. The smeared Gr particlesformed a protective layer on the specimen. Applied load wasdirectly proportional to the wear rate for both alloy and compositespecimen [5,6], while variation in sliding speed showed contrastresults in composite specimen [6]. The hardness decreased withthe addition of graphite [5,6] as it is a soft inclusion.

Reinforcing hard SiCp into soft aluminum alloy improves thewear resistance as well as hardness of the composite material

Table 1Process parameters used in the experiment.

Level Load, L (N) Sliding distance, D (m) Sliding speed, S (m/s)

1 15 1000 0.632 45 3000 1.883 75 5000 3.14

T.S. Kiran et al. / Materials and Design 63 (2014) 294–304 295

[7–10]. Composites with increased volume fraction and larger rein-forcement size increase the wear resistance. Particle pull out andfracture was the mechanism observed for smaller and larger rein-forcement size respectively [7]. A step by step increase in appliedload increased the wear rate, whereas a contrast result wasachieved in case of sliding speed by where the wear rate decreasedwith increase in speed [8]. The increase in SiCp content improvedthe hardness which reduced the wear rate significantly. Compos-ites reinforced with SiCp exhibited superior wear resistance overthe alloy as fractured particles ensured the participation in wearbehavior avoiding the exposure of alloy [9]. Wilson and Alpas[10] showed that incorporation of SiCp in Al alloy improves themild wear regime at higher load and speed compared to the unre-inforced alloy. Prasanna kumar et al. [11] and Ranganath et al. [12]evaluated the dry sliding wear behavior of ZA-27/garnet compositeand concluded that, increasing garnet content improved the wearresistance. Meanwhile the wear resistance dropped with anincrease in applied load and sliding speed.

Inclusion of only graphite as reinforcement improved wearbehavior (as it is a solid lubricant), reducing hardness [5,6] (softinclusion) while SiCp inclusion showed improvement in both wearand hardness [7–10]. The attempt to obtain the combined effect ofsolid lubrication and improved hardness attributed to the creationof hybrid composites. The effect of sliding speed in deciding thewear behavior of hybrid composites was evaluated by Basavarajap-pa et al. [13]. It was witnessed that, the specimen experiencedhigher wear rate followed by seizure behavior at higher speedsfor alloy, while there was a minor effect of increase in speed forhybrid composite reinforced with SiCp and Gr. On the contrary,Suresha and Sridhara [14,15] evaluated that as sliding speed wasincreased, wear loss was reduced for different combinations ofSiCp and Gr. Hardness reduces with inclusion of Gr particles inAl-SiCp composite specimen.

Basavarajappa et al. [16] used Taguchi’s technique to identifythe influence of wear parameters and concluded that slidingdistance is the major contributor followed by applied load andsliding speed. Graphite plays an important role in the formationof mechanical mixed layer (MML). Several researchers’ [17–19]studied the heat treated ZA-27 alloy followed by water quenchingto investigate the hardness, tensile and wear behavior. Heattreatment to ZA-27 alloy improved the distribution of microcon-stituents. Heat treatment resulted in reduction of the hardnessand tensile properties but had a positive effect on the dry slidingwear behavior [17–19]. The specimen heat treated for 5 h [18,19]and aged for 8 h [17] showed superior wear behavior over otherheat treatment and aging conditions. The addition of solid lubri-cant (Gr) with SiC particles in Al alloy proved to be positive onthe dry sliding wear behavior [20,21]. A detailed study on the for-mation of mechanical mixed layer (MML) and its advantages on theworn surface of the specimen were presented [22,23]. A statisticalapproach was used to find out the significance of the factors affect-ing the wear behavior of hybrid MMCs [24–26].

The previous studies on ZA-27 alloy have concentrated onutilization of SiCp and Gr particles separately. The current workconcentrates on the HT of ZA-27 alloy reinforced with SiCp andGr particles which were not investigated in earlier research worksto the best of author knowledge. The parameters that influence thewear behavior of heat treated ZA-27 alloy and ZA-27/9SiC–3Gr areevaluated by Taguchi technique in the present investigation.

2. Design of experiments (DOE)

DOE is an important and powerful statistical technique thatevaluates the effect of multiple parameters simultaneously. Exper-iments have to be conducted in a sequence, with a series of steps,so that the process performance is better understood. A certain

combinations of factors and levels are considered and varied in astrategic manner. The results obtained are observed and analyzed,to find out the significant factors and preferred levels [27]. The datacan be acquired in an orderly way by DOE based on Taguchiapproach. There are three main phases in the Taguchi process: (i)the planning phase (ii) the conducting phase and (iii) the analysisphase. Among the three listed phases, planning phase is vitalwhere the factors and levels are decided. The results obtained fromexperiments are analyzed for better understanding of the influen-tial factors.

3. Experimental procedure

3.1. Specimen preparation and wear test

ZA-27 is identified as the matrix material and the reinforce-ments used are 9 wt% of SiCp with 45 lm and 3 wt% of Gr with25 lm in size. The composite specimen was prepared by stircasting method. The ZA-27 alloy was heated above its liquidustemperature of 500 �C. A aluminite coated stirrer was introducedin the molten slurry to homogenize the temperature. The mixtureof reinforcements were preheated and poured into the rotatingmolten slurry. To improve the wettability of reinforcements,1 wt% of magnesium was added along with the reinforcements.The molten slurry was stirred for 10 min, so that the reinforce-ments distribute uniformly in the alloy. The melt was later pouredinto permanent castings. The alloy and composite specimen weresubjected to T6 type of heat treatment in four steps: first, the spec-imen were heat treated at 370 �C for 5 h; second, the heat treatedspecimen were quenched in water at room temperature; third, thequenched specimen were aged at 180 �C for 8 h; fourth, the agedspecimen were quenched in water at room temperature.

The dry sliding wear behavior of specimen were evaluated withpin-on-disc apparatus at room temperature. The specimen weremachined as per ASTM: G99-05(2010) standards, with a dimensionof 8 mm diameter and 30 mm height. The specimen was pressedagainst the rotating EN32 steel disc of hardness 65HRc and loadwas applied on the specimen by cantilever mechanism. The discand specimen surface were cleaned with acetone before eachexperiment to remove any traces on the surface. The specimenwere weighed before and after wear test using an electronicweighing machine which can measure up to 0.1 mg. The differencein the weight was measured and volume loss was calculated. Theweight loss of the disc is not considered as the hardness of discwas more compared to specimen.

3.2. Plan of experiments

Wear tests of the base alloy and composite specimen wereconducted under dry sliding conditions for three parameters:Applied load, sliding speed and sliding distance with variation of3 levels as shown in Table 1. The experiments were planned basedon standard L27 orthogonal array (OA), consisting of 27 rows and13 columns. The 1st, 2nd and 5th columns were assigned toapplied load (L), sliding distance (D) and sliding speed (S) respec-tively in the Orthogonal Array, while the remaining columns wereassigned to their interactions. The present investigation is based onthe objective to study smaller – the-better wear response.

Fig. 1. Microstructure of as-cast (a) alloy, (b) composite, (c) EDX of alloy and (d) EDX of hybrid composite.

296 T.S. Kiran et al. / Materials and Design 63 (2014) 294–304

4. Results and discussion

4.1. Heat treatment

As the dendrites dissolve uniformly after HT, the microstructureof as-cast specimen is shown in Fig. 1. In as-cast alloy (Fig. 1a), the

Table 2Experimental design using L27 OA.

Test Load L, (N) Distance D, (m) Speed S, (m/s) Wear volume loss inmm3

Alloy Composite

1 15 1000 0.63 1.4 0.52 15 1000 1.88 1.6 0.83 15 1000 3.14 2.2 1.24 15 3000 0.63 1.5 0.75 15 3000 1.88 2.4 1.06 15 3000 3.14 3.2 1.67 15 5000 0.63 2.5 0.98 15 5000 1.88 3.3 1.29 15 5000 3.14 4.6 1.810 45 1000 0.63 1.7 0.911 45 1000 1.88 2.3 1.112 45 1000 3.14 2.7 1.513 45 3000 0.63 2.7 1.414 45 3000 1.88 2.7 1.715 45 3000 3.14 4.1 2.116 45 5000 0.63 3.8 1.317 45 5000 1.88 4.6 1.918 45 5000 3.14 6.1 2.619 75 1000 0.63 3.5 1.520 75 1000 1.88 4.6 2.121 75 1000 3.14 6.5 2.822 75 3000 0.63 4.7 1.923 75 3000 1.88 5.9 2.624 75 3000 3.14 7.7 3.225 75 5000 0.63 5.9 2.726 75 5000 1.88 7.9 3.827 75 5000 3.14 9.5 4.1

aluminum rich (A) a-phase and zinc rich (B) g-phase can be clearlydifferentiated as white and black regions respectively. Theeutectoid (C) a + g phase is rarely visible which is the vital phasefor tribological applications [17–19]. The SiC and graphite particlesare shown in Fig. 1b. As the Heat treatment was carried out, themicrostructure was fully transformed into eutectoid phase, givingadded advantage to both alloy and composite specimen. Themicrocracks and residual stresses present in the as-cast specimenwere reduced by HT process facilitating improved wear resistance.EDX of the base alloy and hybrid composite are shown in Fig. 1cand d respectively, which confirms the presence of reinforcements(Fig. 1d). Since, all the specimen considered were heat treated, theadvantage of HT on as-cast is not discussed in the present work.

4.2. Hardness

Vicker hardness test was performed on the heat treated alloyand composite specimen. The results showed a slight increase inhardness of composite (108 HV) compared with the alloy(106 HV). The reason for the slight increase is due to the presenceof soft Gr particle that hindered the hardness value [14,15]. Onemore factor that influenced the reduction in hardness value is heattreatment [17–19].

4.3. Wear test

The dry sliding wear experiments were conducted as per the OAand the results are tabulated as shown in Table 2. For better under-standing of the various factors considered L (applied load, in N), D(sliding distance, in m), S (sliding speed, in m/s) and their interac-tions, it is required to develop an analysis of variance (ANOVA). Theexperimental results were analyzed using commercial softwareMINITAB, which is used in DOE applications. The effects and orderof significance of the design parameter with their interactions areto be studied on the wear behavior. The analysis was carried outfor a confidence level of 1%.

Table 3Analysis of variance for alloy.

Source Degrees of freedom Sum of squares Adjusted sum of squares Adjusted mean of Squares F-ratio P-value Percentage (%) of contribution

L 2 68.019 68.018 34.009 553.16 0.000 55.86D 2 26.605 26.605 13.303 216.37 0.000 21.35S 2 20.099 20.098 10.049 163.45 0.000 15.93L * S 4 2.761 2.761 0.690 11.23 0.002 0.66L * D 4 1.128 1.128 0.282 4.59 0.032 –D * S 4 0.901 0.901 0.225 3.67 0.056 –Error 8 0.492 0.492 0.061 6.20Total 26 120.005 ‘ 100

S = 0.620278, R-Sq = 93.6% and R-Sq(adj) = 91.7%.

Table 4Analysis of variance for hybrid composite.

Source Degrees of freedom Sum of squares Adjusted sum of squares Adjusted mean of Squares F-ratio P-value Percentage (%) of contribution

L 2 13.040 13.040 6.520 397.83 0.000 56.67S 2 4.602 4.602 2.301 140.41 0.000 19.25D 2 3.469 3.469 1.734 105.83 0.000 14.22L * D 4 0.891 0.891 0.223 13.59 0.001 1.63L * S 4 0.304 0.304 0.076 4.64 0.031 –D * S 4 0.109 0.109 0.027 1.66 0.251 –Error 8 0.131 0.131 0.016 8.23Total 26 22.547 100

S = 0.229643, R-Sq = 95.3% and R-Sq(adj) = 93.9%.

Table 5Response table for means: smaller is better.

Level Wear response of alloy Wear response of composite

L D S L D S

1 2.522 2.944 3.078 1.078 1.378 1.3112 3.411 3.878 3.922 1.611 1.800 1.8003 6.244 5.356 5.178 2.744 2.256 2.322Delta 3.722 2.411 2.100 1.667 0.878 1.011Rank 1 2 3 1 3 2


Tables 3 and 4 shows the ANOVA results for the WVL of alloyand composites respectively. It can be noted from column 7 ofTables 3 and 4 that the p-value is zero for applied load, slidingspeed and sliding distance, which indicates that these play a majorrole in the wear volume loss and have statistical significance.Table 3 shows that applied load (p = 55.86%) had a great influenceon wear loss of the alloy, while sliding distance (p = 21.35%) andspeed (p = 15.93%) showed less influence on the WVL. The interac-tions (L * S) had a negligible influence (p = 0.66%) on the WVL,while the other two interactions (L * D and D * S) had no effect onthe wear behavior. It can be observed from Table 4 that appliedload (p = 56.67%) has highest influence followed by sliding distance(p = 19.25%) and speed (p = 14.22%). The influence of interaction(L * D) is negligible (p = 1.63%) on wear volume loss. Thus load isan important factor that controls the WVL of both alloy and com-posite materials.

4.4. Analysis of control factors

The response table for WVL of alloy and composite is presentedin Table 5, to analyze the influence of the control factors. Analysis ofcontrol factors will give the additional important information aboutthe nature of the process under consideration. The highest differ-ence of control factors indicates the strongest influence on WVL.

It can be seen from Table 5 that the strongest influence on WVLwas applied load, followed by sliding distance and sliding speedrespectively in case of alloy. In case of composite, applied load

was the most influential factor and sliding speed was the secondmost influential factor followed by sliding distance. Fig. 2(a andb) shows the interaction plot for alloy and composites. Three levels(low, medium and high) are considered in the experimentation anda straight line can be drawn for second and third column. In thefirst column of Fig. 2(a and b), there is a sudden increase in theslope after 45 N, which shows that increase of applied load willaffect the wear performance of the specimen (Fig. 2a). The increasein sliding distance has positive effect on the composite as the lineshows a reduction in slope (Fig. 2b), while the alloy (Fig. 2a) showsno change in the wear behavior. The reason for the reduction inslope of composite specimen is the smearing of reinforcementsand formation of protective layer inhibiting the WVL. Hence, assliding distance is increased, the wear resistance improves margin-ally for composite (Fig. 2b).

Fig. 3(a and b) shows the main effects plot for means of alloyand composite respectively. The rise in slope of lines indicatesthe increase in WVL due to increase in applied load from 45 to75 N, which can be analyzed that the wear phenomenon hasentered severe wear from mild wear.

4.5. Regression analysis

To ascertain the correlation between the factors (applied load,sliding speed and sliding distance) and responses (volume loss),multiple linear regression equations were generated usingMINITAB software. The regression equations are as follows:

WearðalloyÞðmm3Þ ¼ 0:078þ 0:0266Lþ 1:84e�4Dþ 0:020S

þ 0:5e�5L�Dþ 0:0111L�Sþ 1:06e�4D�S

ðR-Sq ¼ 93:59% R-SqðadjÞ ¼ 91:66%Þ ð1Þ

WearðCompositeÞðmm3Þ¼0:137þ0:00987L�1:8e�5Dþ0:154S

þ0:4e�5L�Dþ0:00332L�Sþ3:3e�5D�S

ðR-Sq¼95:32% R-SqðadjÞ¼93:92%Þ ð2Þ

Eqs. (1) and (2) refers to the linear regression equation for cal-culating volume loss by substituting the values of variables of alloyand composite respectively. The positive sign of the co-efficients

Fig. 2. Interaction plots for wear volume loss (mm3) of (a) alloy and (b) composites.


refers to increase in the wear volume loss with increase in theirassociated variables. While negative sign indicates that WVLdecreases with increase in the associated variables. The negativesign in Eq. (2) indicates that as sliding distance is increased, wearresistance is increased due to the smearing of reinforcements thatact as ceramic mixed mechanical layer (CMML). However, theeffects of interactions are relatively insignificant.

Fig. 4(a and b) shows the normal probability plot for alloy andcomposite. These probability plots clearly indicates that thevalues lies closer to the normal probability line implying thatthe errors are distributed normally and the model is adequate.Thus the model formulated for prediction of volume loss of alloyand composite which are represented by Eq. (1) and Eq. (2) isadequate.

4.6. Response surface analysis

Response surface methodology (RSM) is a statistical methodthat make use of quantitative data from suitable tests conductedto determine and solve multi-variable equations. RSM, which isused to analyze the results and surface plots for alloy and compos-ites are shown in Figs. 5 and 6 respectively. WVL at any zone fromthe tests conducted can be predicted from the surface plots. FromFigs. 5 and 6 it is clear that applied load has the most dominanteffect on WVL for both alloy and composite. The remaining factors,sliding distance and sliding speed were less dominant compared toload. In Fig. 5, the interactions L * D and L * S show that the slope ofload is more compared to the other two factors, clearly indicatingthat applied load has more effect on the WVL.

Fig. 3. Main effects plot for means (a) alloy and (b) composite.


The magnitude of wear volume loss of alloy (Fig. 5), when com-pared with composite (Fig. 6) is nearly double, which confirms thewear resistance of composites and the presence of reinforcementsthat inhibit the WVL. The smeared and adhered reinforcements actas a medium preventing the specimen from excessive wear.

4.7. Determination of accuracy of wear volume loss

For each experiment in the design matrix, the WVL model ofEqs. (1) and (2) were used to calculate the theoretical wear volumeloss for alloy and composite. The results are summarized inTable 6.

The experimental values were compared with the calculatedvalues and the comparison is shown in Fig. 7. It can be noticed thatthe WVL values calculated from the multiple linear regressionmodel follows almost the similar trend as that of the experimentalvalues. The peaks of the alloy and composites reveal that the exces-sive wear was inhibited due to the addition of reinforcements. Theslopes of the alloy are higher while that of composites are lower

signifying the importance of reinforcements. The variation maybe due to the irregularities in the experiment like environmentalcondition, machine vibration or human errors.

4.8. Wear mechanism

Fig. 8(a and b) and Fig. 8(c–e) show the worn out surfaces ofalloy and composites respectively at a sliding speed of 1.88 m/s,sliding distance of 3000 m and at different applied load. The singlearrow shows the sliding direction of worn surface. It is evident thatthe surface of alloy (Fig. a and b) is rough with deep grooves com-pared with the composite specimen (Fig. 8c–e) with fine grooves.Fig. 8a and Fig. 8b shows the worn out surfaces of alloy at anapplied load of 15 and 75 N respectively. Due to the increase inapplied load, the morphology shows that the alloy (Fig. 8a and b)has experienced severe wear under the absence of reinforcements.The composites (Fig. 8c–e) show smooth surface in black region(double arrow) due to the presence of graphite that smears outduring sliding and acts as a layer, protecting the specimen from

Fig. 4. Normal probability plot of residuals of WVL (mm3) of (a) alloy and (b) composite.


direct contact with the disc, thus enhancing wear resistance[10,13–16]. The presence of SiCp and Gr are shown in Figs. 8eand 9(a–c). These reinforcements participate in the wear process,protecting the specimen from excessive wear.

During the wear process, the asperity on the surface of therotating steel disc comes in contact with the surface of the speci-men (composite). Due to a large difference in the hardness of alloyand reinforcement, the asperities in the counterface are pressedinto the specimen and the soft surface of composites is scratched.Due to work-hardening of the surface layer of composite, the pro-jected Fe asperity may detach from the counterface and adhere onthe composite surface. Due to severe scratching on the counterface,large delamination cavities are formed as a result of fracture of thesurface material. The debris (Fig. 10) from both (specimen andsteel disc) materials are pushed down into the cavities and groovesof specimen, until it becomes flat as the surrounding surface. Theformation of debris from the counterface may be by two ways.First, the asperities on the surface of counterface break off andare pressed against the composite surface during sliding, but areobstructed by the surface material of composite. Secondly, the hardreinforcements that bear the load on the composite surface willcertainly scratch heavily the counterface surface. These resultsare in agreement with Basavarajappa et al. [30].

Due to friction between the specimen and rotating disc duringdry sliding, temperature rises leading the specimen to lower itsmechanical property [1,2]. Due to rise in temperature, the alloyloses its property of bonding with neighboring elements, resultingin thin plate like wear debris (Fig. 10a). Even though zinc rich (g)phase contributes in wear resistance, it is unable to withstand thehigher temperature due to higher applied load. The rise in the tem-perature was noticeable as the applied load was increased, whichcauses a negative effect on the performance of specimen. As thetemperature rises, the bonding within the matrix begins to fail,leading to severe wear and further changing to delamination withfurther increase in applied load [25,26,28–30]. For compositespecimen, the rise in temperature was negligible as the formationof protective layer secluded further exposure of new layer inhibitingthe severe wear at lower load (15 and 45 N). At higher load (75 N),the protective layer of composite specimen gradually exposednew material that were unable to retain and leading to severe wear.In composites, due to rise in temperature, reinforcements graduallystart separating from the alloy, resulting in the direct exposure tothe rotating disc. The presence of microcracks (Fig. 8e) on the wornout surfaces were observed. The effect of HT is that the residualstress and microcrack is greatly reduced which affects the wearbehavior positively [17–19]. The reinforcements that smear out

Fig. 5. Response surface plot for alloy.

Fig. 6. Response surface plot for hybrid composite.


during the lower applied load and sliding speed get retained on thespecimen. As the applied load and sliding speed increases theparticles projected will brake and act as third body and startsremoving the matrix material. The metal oxides are formed because

of the rise in temperature, the crushed SiCp particles, the smearedgraphite along with a matrix material crush between the pin anddisc forming a ceramic mixed mechanical layer (CMML) preventingthe specimen from excessive wear (Fig. 8e). As the applied load is

Table 6Experimental and calculated values of alloy and composites.

Test Load L, (N) Distance D, (m) Speed S, (m/s) Wear volume loss in mm3

Alloy Composite

Experimental Calculated Experimental Calculated

1 15 1000 0.63 1.4 0.93 0.5 0.482 15 1000 1.88 1.6 1.29 0.8 0.773 15 1000 3.14 2.2 1.66 1.2 1.074 15 3000 0.63 1.5 1.57 0.7 0.605 15 3000 1.88 2.4 2.21 1.0 0.986 15 3000 3.14 3.2 2.84 1.6 1.367 15 5000 0.63 2.5 2.22 0.9 0.738 15 5000 1.88 3.3 3.12 1.2 1.199 15 5000 3.14 4.6 4.03 1.8 1.6510 45 1000 0.63 1.7 2.08 0.9 0.9611 45 1000 1.88 2.3 2.86 1.1 1.3812 45 1000 3.14 2.7 3.65 1.5 1.8013 45 3000 0.63 2.7 3.03 1.4 1.3214 45 3000 1.88 2.7 4.08 1.7 1.8215 45 3000 3.14 4.1 5.13 2.1 2.3316 45 5000 0.63 3.8 3.98 1.3 1.6917 45 5000 1.88 4.6 5.29 1.9 2.2718 45 5000 3.14 6.1 6.62 2.6 2.8619 75 1000 0.63 3.5 3.23 1.5 1.4320 75 1000 1.88 4.6 4.43 2.1 1.9821 75 1000 3.14 6.5 5.64 2.8 2.5322 75 3000 0.63 4.7 4.48 1.9 2.0423 75 3000 1.88 5.9 5.95 2.6 2.6724 75 3000 3.14 7.7 7.42 3.2 3.3025 75 5000 0.63 5.9 5.73 2.7 2.6526 75 5000 1.88 7.9 7.46 3.8 3.3627 75 5000 3.14 9.5 9.21 4.1 4.07

Fig. 7. Experimental and calculated values of alloy and composites.

Fig. 8. SEM of worn surfaces of Alloy (a) 15 N, (b) 75 N


increased further, the layer formed will be destroyed at a faster rateleading to direct contact between the new surface of specimen anddisc resulting in higher WVL. The similar results were observed byother researchers [10,21,22,26,30].

The presence of Fe in Fig. 9(b and c) clearly shows that, therewas a formation of CMML on the surface on the specimen. But asthe applied load increased, there was a progressive increase inthe WVL. At lower load (15 N), the transfer of the disc materialonto the specimen surface was observed and the intensity of Fepeak was higher experiencing mild wear. As the applied load wasincreased (75 N), CMML formed on the specimen surface waseroded. The formation and removal of the CMML at lower appliedload is slow, hence retaining the protective layer. At higher appliedload, the removal rate of the protective layer is at a faster rate thanthe layer formation, leading to severe wear [22]. The study has

, hybrid composite (c) 15 N, (d) 45 N and (e) 75 N.

Fig. 9. EDX of worn surfaces of hybrid composite at load (a) 15 N, (b) 45 N and (c) 75 N.

Fig. 10. Wear debris at 75 N (a) alloy and (b) hybrid composite.


clearly indicated the instability and the consequent removal ofCMML resulting in high WVL and further causing transition frommild to severe wear.

The wear debris thrown out from the rotating disc is been pre-sented in Fig. 10 which shows the size of wear debris of alloy(Fig. 10a) and composite (Fig. 10b) at applied load of 75 N, sliding


speed of 1.88 m/s and sliding distance of 3000 m. The size of thewear debris proves that the extent of wear of alloy (Fig. 10a) expe-riencing delamination wear. The mechanical layer formed on thealloy surface were incapable of withstanding the higher load(75 N) and the layer were detached and thrown away as thin platelike particles (Fig. 10a). Whereas for composite specimen, thesmeared reinforcements were fragmented and crushed betweenthe specimen and rotating disc, forming a protective layer. Thewear debris of composites (Fig. 10b) exhibits mild wear with smallparticles thrown out from the rotating disc. The debris emergedout of the alloy measures up to 500 lm (Fig. 10a) and the averagesize of debris are nearly 200 lm. In case of composite specimen,the debris measured are below 100 lm (Fig. 10b). The size ofdebris explains the extent of wear in alloy (delamination) incomparison with the composite specimen.

5. Conclusions

The following conclusions were drawn:

(1) The microconstituents of heat treated materials are well dis-tributed and gets dissolved providing wear resistance by thezinc rich (g) constituent. The effort to reduce the residualstresses is attained by heat treatment. The microcracks pres-ent in the as-cast specimen which causes excessive wear arereduced by heat treatment resulting in superior wearresistance.

(2) The significant parameters in the wear analysis were foundfrom ANOVA. Applied load is the most significant factor fol-lowed by sliding distance and sliding speed in causing wearin case of the alloy. Similarly the contributions for compos-ites are applied load, sliding speed and sliding distance.The interactions show negligible contribution for both alloyand composite specimen.

(3) The metal oxides are formed because of the rise in tempera-ture, the crushed SiCp particles, the smeared graphite parti-cles along with a matrix material crush between the pin anddisc forming a ceramic mixed mechanical layer (CMML).

(4) The addition of solid lubricant (Gr) as secondary reinforce-ment along with SiCp improves the wear resistance by form-ing a CMML on the contact geometry. The formation andretention of CMML acts as a protective layer, thereby reduc-ing the wear volume loss in case of composites.

(5) The size of wear debris that emerged out of wear specimendemarcated the severity of wear in alloy while fine weardebris showed mild wear in composites.

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