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Effect of addition of graphite particulates on the wear behaviour

in aluminium–silicon carbide–graphite composites

S. Suresha *, B.K. Sridhara

Department of Mechanical Engineering, The National Institute of Engineering, Mysore 570 008, Karnataka, India

a r t i c l e i n f o

Article history:Received 23 August 2009

Accepted 7 November 2009

Available online 12 November 2009

Keywords:

Hybrid composites

Design of experiments

Interaction

a b s t r a c t

Aluminiummatrix composites with multiple reinforcements (hybrid AMCs) are finding increased applica-tions because of improved mechanical and tribological properties and hence are better substitutes for sin-

gle reinforced composites. Few investigations have been reported on the tribological behaviour of these

composites with % reinforcement above 10%. The present study focuses on the influence of addition of

graphite (Gr) particulates as a second reinforcement on the tribological behaviour of aluminium matrix

composites reinforced with silicon carbide (SiC) particulates. Dry sliding wear tests have been performed

to study the influence of Gr particulates, load, sliding speed and sliding distance on the wear of hybrid

composite specimens with combined % reinforcement of 2.5%, 5%, 7.5% and 10% with equal weight % of

SiCand Gr particulates. Experiments are also conductedon composites with % reinforcement of SiCsimilar

to hybridcomposites forthe sake of comparison. Parametric studies based on designof experiments (DOE)

techniques indicate that the wear of hybrid composites decreases from 0.0234 g to 0.0221 g as the % rein-

forcement increases from 3% to 7.5%. But the wear has a tendency to increase beyond % reinforcement of

7.5% as itsvalue is 0.0225 g at.%reinforcement of 10%. This trend is absentin case of composites reinforced

with SiC alone. The values of wear of these composites are 0.0323 g, 0.0252 g and 0.0223 g, respectively,

at.% reinforcement of 3%, 7.5% and 10% clearly indicating that hybrid composites exhibit better wear char-

acteristics compared to composites reinforced with SiC alone. Load and sliding distance show a positive

influence on wear implying increase of wear with increase of either load or sliding distance or both.Whereas speed shows a negative influence on wear indicating decrease of wear with increase of speed.

Interactions among load, sliding speed and sliding distance are noticed in hybrid composites and this

may be attributed to the addition of Gr particulates. Such interactions are not present in composite

reinforced with SiC alone. Mathematical models are formulated to predict the wear of the composites.

2009 Elsevier Ltd. All rights reserved.

1. Introduction

Metal matrix composites are evolved as newer materials as an

out come of a steadily growing efforts to cater to the increasing de-

mand for lightweight, inexpensive, energy saving, stiff and strong

materials in aircraft, space, defense and automotive applications.

Aluminium matrix composites (AMCs) are emerging as promising

materials in this direction [1,2]. AMCs reinforced with SiC particu-

lates are known for higher modulus, strength and wear resistance

compared to conventional alloys. Garcia et al. have observed that

the specific wear rate of AA6061-SiC composites decreases with in-

crease in volume fraction and size of reinforcement [2]. Das et al.

have discussed on formation of mechanically mixed layer (MML)

consisting of debris and smeared and fragmented SiC particles.

SiC needles in MML and in the subsurface region are fragmented

into finer particles thus demonstrating the occurrence of subsur-

face damage during abrasive wear of LM13-SiC composites [3].

Sahin et al. have noticed significant increase in wear resistance

up to 10% addition of SiC and a stable value between 10% and

55% for Al–SiC composites produced by vacuum infiltration [4].

Both mechanical strength and wear resistance of composites

increase by adding SiC particulates to the matrix alloy. But the con-

sequent increase in hardness makes the machining difficult. Thus,

it is essential to look for ways to retain the advantageous influence

of SiC and simultaneously attending the problem of machining of

composites reinforced with SiC. Graphite particulates come handy

in this direction, the addition of which improves the machining as

well as wear resistance of Al–SiC composites. Al–SiC composites

reinforced with Gr particulates are referred as Al–SiC–Gr hybrid

composites. The outcome of some investigations on such hybrid

composites are briefed in the next few lines.

Rohatgi et al. have reported that the reduction in friction coef-

ficient of Al–10SiC–6Gr is due to the combination of increase in

bulk mechanical properties as a result of addition of SiC and forma-

tion of graphite film [5]. Ted Guo et al. have observed that wear of

0261-3069/$ - see front matter 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2009.11.015

* Corresponding author. Tel.: +91 9448800649; fax: +91 0821 2485802.

E-mail address: [email protected] (S. Suresha).

Materials and Design 31 (2010) 1804–1812

Contents lists available at ScienceDirect

Materials and Design

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Al–10SiC–2–8Gr increases up to 5% Gr because of reduced fracture

toughness and then decreases due to the formation of thick solid

lubricant film which overrides the effect of fracture toughness

[6]. Riahi et al. have focused upon the influence of tribo layer of

MML containing primarily Gr on wear of Al–10SiC–4Gr hybrid

composites [7]. The investigations of Basavarajappa et al. on Al–

15SiC–3Gr composites have indicated that the degree of subsurface

deformation and thereby the wear rate in graphite composites isless than that of graphite free composites [8]. These investigations

emphasize that the use of multiple reinforcements in aluminium

matrix hybrid composites yield better tribological properties.

However, efforts are scarce on parametric studies on the tribo-

logical behaviour of aluminium matrix hybrid composites. In this

context, an attempt is made here to study the influence of graphite

particulates, load, % reinforcement, sliding speed and sliding dis-

tance on the tribological behaviour of Al–SiC–Gr hybrid

composites.

2. Experiments

Al–SiC–Gr and Al–SiC composites required for the investigation

are fabricated by stir casting [9]. LM25 is used as the matrix alloyand details of its composition is given in Table 1. Table 2 provides

the details of SiC and Gr particulates which are used as reinforce-

ments. Table 3 gives the details of hybrid composites. Al–SiC–Gr

hybrid composites with combined weight % reinforcement of

2.5%, 5%, 7.5% and 10% are used. In each of these composite as seen

in Table 3, % reinforcement of each of SiC and Gr is equal. The hard-

ness values of these composites are also indicated in the table.

There is reduction in hardness with increase in % reinforcement

and this is due to the addition of Gr. Similar details of Al–SiC com-

posites are given in Table 4. It can be observed that hardness in-

creases with % reinforcement of SiC [14]. The as cast composites

are of 10 mm diameter, 50 mm length from which Wear test spec-

imens of length 35 mm and 8 mm diameter are machined. The end

of the specimens are polished with abrasive paper of grade 600 andfollowed by grade 1000. Dry sliding tests are carried out as per

ASTM G99-95a test standards on pin-on-disc equipment the disc

of which is of EN31 steel with surface roughness, Ra 0.1. The pins

are cleaned with acetone and weighed before and after testing to

an accuracy of 0.0001 g to determine the amount of wear. The slid-

ing end of the pin and the disc surfaces are cleaned with acetone

before testing.

The experiments are designed based on central composites de-

sign (CCD) scheme of design of experiments (DOE) [10]. CCD is very

effective experimental technique in studies involving large number

of factors. A set of experimental design that can look at K factors in

‘n’ observations with each factor at two levels is called two level

factorial design, which can prove good and efficient when a linear

relationship prevails between the factors and the response. Three

or higher level experiments are mandatory when nonlinear

relationship exists, which ends up with increased cost and time

of testing. CCD is the most efficient method, alternative to 3k or

more factorial experimental designs. CCD can be used to study fac-

tors at five levels in reduced number of tests [10–13].

Load, % reinforcement, sliding speed and sliding distance are the

factors which influence the tribological behaviour of a composite.

Table 5 provides the experimental plan and the experimental

results obtained. Wear in terms of weight loss is the measured

response used to evaluate the tribological behaviour of Al–SiC

composite and Al–SiC–Gr hybrid composites. Table 6 shows the

factors and their levels employed in the experiments.

3. Analysis of results

The experimental results are analyzed with the help of MINI-

TAB14, a statistical analysis software, which is widely used in

many fields of engineering research. Results are analyzed by ANO-

VA with a confidence limit of 95% or P -value of 0.05. This implies

any factor with P -value equal to or <0.05 is significant. The signif-

icance of a factor can be confirmed by the main effects plot and

normal probability plot. The influence of % reinforcement on wearrelative to load, sliding speed and sliding distance are discussed

with the help of respective contour plots [11,13].

ANOVA results of Al–SiC composites are given in Table 7. Sec-

ond and higher order interactions are not considered for ANOVA.

The quadratic terms of the factors are included in the analysis in

order to incorporate the results of the composite part of CCD which

may have curvilinear effect on the response [11]. It can be observed

that the % reinforcement ( A), sliding speed (B), load (C ) and sliding

distance (D) are significant as the P -value for these is <0.05. The %

contribution of each of these terms is calculated by dividing their

respective sum of squares value by the total sum of squares and

these values are indicated in Table 8. Fig. 1 shows the main effect

plot of the effects of factors on wear of Al–SiC composites.

Fig. 2 shows the normal probability plot of standardized effectsof factors on wear of Al–SiC composites. The observation indicates

that % reinforcement and sliding speed have negative effect on

wear as they are on the left of the normal probability line indicat-

ing any increase of their value from low level to high level results

in reduction of wear. Other investigations, based on one factor- at-

a-time experiments, have revealed a little positive effect of sliding

speed on wear of Al–SiC composites in the range considered in the

present work [1]. The effect of other two factors, load and sliding

distance is positive on wear as they lie on the right to the line.

The effect of sliding distance is more predominant as evident from

Figs. 1 and 2. The effect of load is less than sliding distance as it is

close to the line. These are also evident from the values of % contri-

bution of factors as in Table 8. It can be observed that the value of %

contribution is the highest for sliding distance followed by slidingspeed, load and % reinforcement.

Table 1

Chemical composition of the matrix alloy LM25.

Element Si Mg Fe Cu Cr Zn Ni Mn

Content % 7.1 0.3 0.3 <0.012 0.004 0.002 0.01 0.28

Table 2

Details of reinforcements.

Reinforcement Hardness (GPa) Grain size (lm) Density (g/cm3)

SiC 24.5–29 10–20 3.22

Gr 0.25 70–80 2.09–2.23

Table 3

Details of Al–SiC–Gr hybrid composites.

Al–SiC–Gr hybrid composites

Combined % reinforcement 0.00 2.50 5.00 7.50 10.0

SiC 0.00 1.25 2.50 3.75 05.0

Gr 0.00 1.25 2.50 3.75 05.0

Hardness, BHN 67 72 70 68 66

Table 4

Details of Al–SiC composites.

Al–SiC composites

% reinforcement of SiC 0.00 2.50 5.00 7.50 10.0

Hardness, BHN 67 72 76 79 81

S. Suresha, B.K. Sridhara/ Materials and Design 31 (2010) 1804–1812 1805

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ANOVA is also carried out on experimental results of

Al–SiC–Gr hybrid composites. The main effect plot of the effects

of factors on wear of Al–SiC–Gr hybrid composites is shown in

Fig. 3. Fig. 4 shows the normal probability plot of the effects

offactors and their interactions on wear of Al–SiC–Gr hybrid

composites. The effect of factors A–D is similar as in Al–SiC com-

posites. Apart from the factors A–D, other terms like BC , BD, CD,

A A, B B and D D are also significant. The % contribution of

these terms is indicated in Table 9. There are interactions amongsliding speed, load and sliding distance and these may be attrib-

uted to the inclusion of Gr particulates in Al–SiC–Gr hybrid com-

posites. Other investigations, based on one factor-at-a-time

experiments, have revealed no effect of sliding speed on wear

of Al–SiC–Gr hybrid composites in the range considered in the

present work [8].

The increase of sliding speed results in formation of a mechan-

ically mixed tribo layer at a faster rate and thus reducing wear by

covering more area of contact. Increase in % reinforcement facili-

tates availability of more quantity of SiC and Gr for the formation

of tribo layer. This is evidenced by smear of very fine particles of

reinforcements over the disc surface during testing. Any increase

of load tries to weaken the tribo layer due to increase of pressure

over the tribo layer and consequent reduction of area covered bythe layer resulting in increase of wear. Wear increases with sliding

distance because of exposure of sliding end of the pin for more

duration of time for any values of % reinforcement, sliding speed

and load.

Mathematical models to predict wear are formulated by re-

sponse surface regression analysis. Response surface and corre-

sponding contour plots give wider insight to understand any

problem in general, and to optimize the factors influencing the re-

sponse in particular [11,13]. Table 10 shows the results of response

surface regression analysis for Al–SiC composites. The regression

model in terms of coded values is represented by Eq. (1). Table

11 shows the results of response surface regression analysis for

Al–SiC–Gr hybrid composites. The regression model in terms of

coded values is represented by Eq. (2). The adequacy of the modelcan be checked by normal probability plot of residuals and the plot

Table 5

Details of test combinations (tc) in coded and actual values of factors and corresponding experimental results.

tc A B C D % Reinforcement A Speed B Load C Dist D Wear Al–SiC (g) Wear Al–SiC–Gr (g)

1 1 1 1 1 2.5 0.8 30 800 0.0088 0.0081

a 1 1 1 1 7.5 0.8 30 800 0.0069 0.0063

b 1 1 1 1 2.5 1.6 30 800 0.0045 0.0041

ab 1 1 1 1 7.5 1.6 30 800 0.0039 0.0026

c 1 1 1 1 2.5 0.8 60 800 0.0115 0.0090

ac 1 1 1 1 7.5 0.8 60 800 0.0091 0.0070bc 1 1 1 1 2.5 1.6 60 800 0.0085 0.0060

abc 1 1 1 1 7.5 1.6 60 800 0.0053 0.0049

d 1 1 1 1 2.5 0.8 30 1600 0.0211 0.0165

ad 1 1 1 1 7.5 0.8 30 1600 0.0156 0.0149

bd 1 1 1 1 2.5 1.6 30 1600 0.0115 0.0098

abd 1 1 1 1 7.5 1.6 30 1600 0.0088 0.0085

cd 1 1 1 1 2.5 0.8 60 1600 0.0240 0.0188

acd 1 1 1 1 7.5 0.8 60 1600 0.0187 0.0176

bcd 1 1 1 1 2.5 1.6 60 1600 0.0226 0.0156

abcd 1 1 1 1 7.5 1.6 60 1600 0.0142 0.0134

aa 2 0 0 0 0 1.2 45 1200 0.0119 0.0119

+aa 2 0 0 0 10 1.2 45 1200 0.0108 0.0081

ab 0 2 0 0 5 0.4 45 1200 0 .0171 0.0127

+ab 0 2 0 0 5 2.0 45 1200 0.0085 0.0078

ac 0 0 2 0 5 1.2 15 1200 0.0071 0.0055

+ac 0 0 2 0 5 1.2 75 1200 0.0152 0.0100

ad 0 0 0

2 5 1.2 45 400 0.0024 0.0020+ad 0 0 0 2 5 1.2 45 2000 0.0198 0.0196

Zero 0 0 0 0 5 1.2 45 1200 0.0133 0.0087

Zero 0 0 0 0 5 1.2 45 1200 0.0076 0.0084

Zero 0 0 0 0 5 1.2 45 1200 0.0093 0.0089

Table 6

Factors and their levels in CCD experimental plan.

Factors Levels

2 1 0 1 2

Reinforcement, % A 0 2.5 5 7.5 10

Sliding speed, m/s B 0.4 0.8 1.2 1.6 2.0

Load, N C 15 30 45 60 75

Sliding distance, m D 400 800 1200 1600 2000

Table 7

ANOVA results of wear of Al–SiC composites. Analysis of variance for W (Al–SiC), using

adjusted SS for tests.

Source DF Seq SS Adj SS Adj MS F P

A 1 0.0000432 0.0000432 0.0000432 10.47 0.008

B 1 0.0001197 0.0001197 0.0001197 29.02 0.000

AB 1 0.0000000 0.0000000 0.0000000 0.00 0.981

C 1 0.0001000 0.0001000 0.0001000 24.26 0.000

AC 1 0.0000046 0.0000046 0.0000046 1.12 0.312

BC 1 0.0000076 0.0000076 0.0000076 1.83 0.203

D 1 0.0005 30 2 0 .0 00 530 2 0 .0 00 53 02 1 28 .5 4 0.00 0

AD 1 0.0000119 0.0000119 0.0000119 2.89 0.117

BD 1 0.0000042 0.0000042 0.0000042 1.02 0.334

CD 1 0.0000093 0.0000093 0.0000093 2.26 0.161

ACD 1 0.0000004 0.0000004 0.0000004 0.09 0.773

A A 1 0.0000004 0.0000037 0.0000037 0.91 0.361

B B 1 0.0000092 0.0000130 0.0000130 3.16 0.103

C C 1 0.0000015 0.0000029 0.0000029 0.70 0.420

D D 1 0.0000027 0.0000027 0.0000027 0.66 0.435

Error 1 1 0.0000 45 4 0 .000 04 54 0 .0 00 004 1

Total 26 0.0008902

S = 0.00203090, R-Sq = 94.90%, R-Sq(adj) = 87.95%.

Table 8

Percentage contribution of significant factors affecting wear of Al–SiC composites.

Factor A B C D Error and others

% 4.85 13.45 11.23 59.56 10.91

1806 S. Suresha, B.K. Sridhara/ Materials and Design 31 (2010) 1804–1812

7/23/2019 Suresh a 2010


of residuals versus fit [11,13]. Figs. 5 and 6, respectively, show theplot of residual versus fitted values of the response for Al–SiC

composites and Al–SiC–Gr hybrid composites. It can be seen in

the plots that the points are randomly scattered without forming

any particular pattern and there by evidencing the model

adequacy.

W ¼ 0:010067 0:001342 A 0:002233B þ 0:002042C

þ 0:004700D ð1Þ

Here,

A ¼ ða 5Þ=2:5; B ¼ ðb 1:2Þ=0:4; C ¼ ðc 45Þ=15;

D ¼ ðd 1200Þ=400

A–D are coded values and a–d are actual values of factors.

W ¼

0:008667

0:000846 A

0:001796B

þ0:001271C

þ 0:004263D þ 0:000416 A A þ 0:000478B B

þ 0:000616D D þ 0:000519B C 0:000481B D

þ 0:000619C D ð2Þ

Here,

A ¼ ða 5Þ=2:5; B ¼ ðb 1:2Þ=0:4; C ¼ ðc 45Þ=15;

D ¼ ðd 1200Þ=400

A–D are coded values and a–d are actual values of factors.

4. Discussions

Contour plots are plots of constant wear and they employ acombination of any two factors. Overlaid contour plots are conve-

M e a n o

f W e a r ( A l - S

i C ) , g

10.07.55.02.50.0

0.020

0.015

0.010

0.005

0.0002.01.61.20.80.4

7560453015

0.020

0.015

0.010

0.005

0.000

200016001200800400

% Reinforcement Sliding speed, m/s

Load, N Sliding distance, m

Main Effects Plot for wear of Al-SiC composites

Fig. 1. Main effect plot of effects of factors on wear of Al–SiC composites.

Standardized Effect

P

e r c e n

t

12.510.07.55.02.50.0-2.5-5.0

99

95

90

80

70

60

50

4030

20

10

5

1

Effect Type

Not Significant

Significant

D

C

B

A

Normal Probability Plot of the Standardized Effects

(response is wear(Al-SiC), Alpha = .05)

Fig. 2. Normal probability plot of effects of factors on wear of Al–SiC composites.


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nient for comparison of two and more number of responses. The %

reinforcement is considered as the common factor and the overlaid

contour plots are drawn in combination with sliding distance, load

and sliding speed.

Fig. 7 is the overlaid contour plot of wear of Al–SiC composites

and Al–SiC–Gr hybrid composites in terms of % reinforcement and

sliding distance. It is evident from the plot that as the % reinforce-

ment increases, the wear of Al–SiC composites continuously de-

creases for a particular value of sliding distance. The weardecreases from 0.0323 to 0.252 g for an increase in % reinforcement

from 3% to 7.4% for a sliding distance of 1998 m. This trend prevails

up to % reinforcement of 10% at which the wear is 0.0223 g. This

can be attributed to the formation of a mechanically mixed layer

(MML). Reduction of wear is the consequence of the existence of

MML, which prevents the wear of matrix alloy. This behaviour is

in agreement with the reported observations on LM13–10SiC com-

posites [3].

The wear of Al–SiC–Gr hybrid composites decreases with in-

crease in % reinforcement up to a % reinforcement of 7.4%, but itexhibits an increasing trend beyond % reinforcement of 7.4% for a

M e a n o

f W e a r ,

g

10.07.55.02.50.0

0.020

0.015

0.010

0.005

0.0002.01.61.20.80.4

7560453015

0.020

0.015

0.010

0.005

0.000200016001200800400

% Reinforcement Sliding speed, m/s

Load, N Sliding distance, m

Main Effects Plot for wear of Al-SiC-Gr hybrid composites

Fig. 3. Main effect plot of effects of factors on wear of Al–SiC–Gr hybrid composites.

Standardized Effect

P e r c e n

t

3020100-10

99

95

90

80

70

60

50

40

30

20

10

5

1

Effect Type

Not Significant

Significant

D*D

B*B

A*A

CD

BD

D

BC

C

B

A

Normal Probability Plot of the Standardized Effects

(response is wear(Al-SiC-Gr), Alpha = .05)

Fig. 4. Normal probability plot of effects of factors and their interactions on wear of Al–SiC–Gr hybrid composites.

Table 9

Percentage contribution of significant factors affecting wear of Al–SiC–Gr hybrid composites.

Factor A B C D BC BD CD A A B B D D Error and others

% 2.84 12.76 6.40 71.92 0.71 0.61 1.01 0.23 0.54 1.34 1.64


7/23/2019 Suresh a 2010


sliding distance of 1998 m. This is evidenced from the plot as the

values of wear are 0.0234, 0.0221 and 0.0225 g, respectively, at.%

reinforcements of 3%, 7.4% and 10%. This is due to the solid lubri-

cation of Gr particles, which get released during sliding and form-

ing a tribo layer at the contact surfaces. The tendency for the wear

to increase beyond % reinforcement of 7.4% may be attributed

todecrease in fracture toughness. Wear studies carried out on

A356–10SiC composites with Gr reinforcement up to 8% has exhib-

ited a reverse trend, wherein the wear increases up to 5% of Gr and

then starts decreases. Decrease of fracture toughness and increased

effect of solid lubrication of Gr particles with increase in % rein-forcement of Gr particles are the two identified competitive factors

responsible for the observed trend of A356–10SiC composites [6].

The difference in the trends may be attributed to the way the rein-

forcement of SiC and Gr is incorporated in the two composites.

That is, the % reinforcement of SiC remains unchanged for A356–

10SiC composites but it is equal to that of Gr for Al–SiC–Gr hybrid

composites used in the present investigation.

It can be noticed from the plot that the wear increases with slid-

ing distance for any amount of % reinforcement for both Al–SiC

composites and Al–SiC–Gr hybrid composites. This can be due to

unstable MML in Al–SiC composites. It has been noticed that, at

longer sliding distances, there is a tendency of an increase in sub-

surface microcracking and severe fracturing of the SiC particles.

The fractured SiC particles cause further scratching on the surface

of composites and make the MML unstable [3]. The increase in

wear of Al–SiC–Gr hybrid composites may be due to the weakened

tribo layer. Similar results have been reported on wear rates of

A356 Al–10SiC–4Gr hybrid composites, which showed a drastic in-

crease with the sliding distance and shortly after the removal of

tribo layers the composite seized against the steel counterface [7].

The amount of wear is considerably less for Al–SiC–Gr hybrid

composites than for Al–SiC composites as can be observed from

the plot at.% reinforcement of 2.0% and for a sliding distance of

550 m. Similar observation can be made at.% reinforcement of

6.0% and for a sliding distance of 799 m.

The overlaid contour plot of % reinforcement in combination

with sliding speed for wear of Al–SiC composites and Al–SiC–Gr

hybrid composites is shown in Fig. 8. It can be observed that the

wear decreases for both the composites as the % reinforcement in-creases for any value of sliding speed. The values of wear and the

trend at a sliding speed of 1.99 m/s are identical to those observed

for a sliding distance of 1998 m in Fig. 7. It is evident from the plot

that the wear decreases with increase in sliding speed for any

amount of reinforcement in both Al–SiC–Gr hybrid composites

Table 10

Response surface regression: wear (Al–SiC), W versus A–D. The analysis was done

using coded units. Estimated regression coefficients for W (Al–SiC).

Term Coefficients SE coefficients T P

Constant 0.010067 0.001127 8.932 0.000

A 0.00 13 42 0 .0 00 39 8 3.367 0.006

B 0.002233 0.000398 5.605 0.000

C 0.002042 0.000398 5.124 0.000

D 0.004700 0.000398 11.795 0.000 A A 0.000419 0.000423 0.991 0.341

B B 0.000781 0.000423 1.848 0.089

C C 0.000369 0.000423 0.872 0.400

D D 0.000356 0.000423 0.843 0.416

A B 0.000013 0.000488 0.026 0.980

A C 0.0005 38 0.00 04 88 1.101 0.292

A D 0.000863 0.000488 1.767 0.103

B C 0.000687 0.000488 1.409 0.184

B D 0.000513 0.000488 1.050 0.314

C D 0.000762 0.000488 1.562 0.144

S = 0.001952, R-Sq = 94.9%, R-Sq(adj) = 88.9%.

Table 11

Response surface regression of wear (Al–SiC–Gr), W versus A–D. The analysis was

done using coded units. Estimated regression coefficients for wear, W .

Term Coefficients SE coefficients T P

Constant 0.008667 0.000443 19.567 0.000

A 0 .000 846 0.00 01 57 5.401 0.000

B 0 .001 79 6 0.00 01 57 11.468 0.000

C 0.001271 0.000157 8.115 0.000

D 0.004263 0.000157 27.220 0.000

A A 0.000416 0.000166 2.502 0.028

B B 0.000478 0.000166 2.879 0.014

C C 0 .000 14 7 0.00 01 66 0.884 0.394

D D 0.000616 0.000166 3.706 0.003

A B 0.000031 0.000192 0.163 0.873

A C 0.00 001 9 0.0001 92 0.098 0.924

A D 0.000006 0.000192 0.033 0.975

B C 0.000519 0.000192 2.705 0.019

B D 0.0004 81 0 .0 00 19 2 2.509 0.027

C

D 0.000619 0.000192 3.226 0.007S = 0.0007672, R-Sq = 98.8%, R-Sq (adj) = 97.5%.

Fitted Value

R e s

i d u a

l

0.0250.0200.0150.0100.0050.000

0.004

0.003

0.002

0.001

0.000

-0.001

-0.002

-0.003

Residuals Versus the Fitted Values(response is W(Al-SiC))

Fig. 5. Plot of residuals versus fitted values of wear of Al–SiC composites.


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and Al–SiC composites. The decrease of wear with increase of slid-ing speed may be because of the increased effectiveness of the

MML in Al–SiC composites and the tribo layer in Al–SiC–Gr hybrid

composites. That is, high sliding speed may results in quick forma-

tion of tribo layer (or MML) and there by the wear gets reduced.

The amount of wear is less for Al–SiC–Gr hybrid composites than

for Al–SiC composites as can be observed from the plot at.% rein-

forcement of 2.0% and for a sliding speed of 0.5 m/s. Similar obser-

vation can be made at.% reinforcement of 6.0% and for a sliding

speed of 1.1 m/s.

Fig. 9 depicts the overlaid contour plot of % reinforcement in

combination with load for wear of Al–SiC composites and Al–

SiC–Gr hybrid composites. It is clear from the plot that the wear

decreases with increase in % reinforcement. At a load of 74.9 N,

the values of wear and the trend are similar to those noticed fora sliding distance of 1998 m in Fig. 7 and also at a sliding speed

of 1.99 m/s in Fig. 8. It is observed from the plot that the wear in-creases with load for any amount of reinforcement. The increase in

wear of Al–SiC composites at high loads may be due to increased

pressure on the MML resulting in transverse and longitudinal

cracks of MML as observed in LM13–10SiC composite [3]. Similarly,

the increase of wear of Al–SiC–Gr hybrid composites with increase

of load may be due to the reduced effect of the tribo layer. Increase

in load weakens the existing tribo layer resulting in transition from

mild to severe wear as noticed in case of A356 Al–10SiC–4Gr hy-

brid composites [7]. The amount of wear is less for Al–SiC–Gr hy-

brid composite than for Al–SiC composite as can be observed

from the plot at.% reinforcement of 2.0% and for a load of 19 N. Sim-

ilar observation can be made at.% reinforcement of 6.0% and for a

load of 45 N.

The overlaid contour plots of Figs. 7–9 shows the effect of %reinforcement on wear of Al–SiC composites and Al–SiC–Gr hybrid

Fitted Value

R e s

i d u a l

0.0200.0150.0100.0050.000

0.0010

0.0005

0.0000

-0.0005

-0.0010

-0.0015

Residuals Versus the Fitted Values(response is W(Al-SiC-Gr))

Fig. 6. Plot of residuals versus fitted values of wear of Al–SiC–Gr hybrid composite.

% Reinforcement

S l i d i n g

d i s t a n c e ,

m

1086420

2000

1750

1500

1250

1000

750

500

Hold Values

Sliding speed, m/s 2

Load, N 75

W(Al-SiC), g

0.025

0.012

0.036

W(Al-SiC-Gr), g

0.005

Overlaid Contour Plot of W(Al-SiC), g, W(Al-SiC-Gr), g

% Reinforcement = 9.97388Sliding distance, m = 1998.36W(Al-SiC), g = 0.0223054W(Al-SiC-Gr), g = 0.0225624






Fig. 7. Overlaid contour plot of effect of % reinforcement and sliding distance on wear of Al–SiC composites and Al–SiC–Gr hybrid composites.


7/23/2019 Suresh a 2010


composites for all the values of sliding distance, sliding speed and

load within the range considered in the present investigation. It is

evident from these plots that, the amount of wear is the least for

Al–SiC–Gr hybrid composites at around % reinforcement of 7.5%

beyond which it has a tendency to increase up to % reinforcement

of 10%. But this trend is not observed in case of Al–SiC composites.

It is observed from the plots that the amount of wear is less up to %

reinforcement of 7.5% for Al–SiC–Gr hybrid composites than for Al–

SiC composites for any values of sliding distance, sliding speed and

load. Thus, it can be concluded that the optimal % reinforcement is

around 7.5% considering all the factors affecting wear of Al–SiC–Gr

hybrid composites. There is no such % reinforcement for Al–SiC

composites. The % contribution of % reinforcement is 4.85% in

Al–SiC composites and 2.84% in Al–SiC–Gr hybrid composites asindicated, respectively, in Tables 8 and 9. But still Al–SiC–Gr

hybrid composites exhibit better wear resistance than Al–SiC

composites.

5. Conclusions

Experiments are conducted on Al–SiC composites with rein-

forcement up to 10% and Al–SiC–Gr hybrid composites with com-

bined reinforcement up to 10% using pin-on-disc equipment.

Following are the conclusions of the investigation.

% reinforcement, sliding speed, load and sliding distance affect

the wear. Interactions exist among sliding speed, load and slid-

ing distance in Al–SiC–Gr hybrid composites and such interac-tions do not exist in Al–SiC composites.

% Reinforcement

S l i d i n g s p e e

d , m

/ s

1086420

2.00

1.75

1.50

1.25

1.00

0.75

0.50

Hold Values

Load, N 75

Sliding distance, m 2000

W(Al-SiC), g

0.032

0.025

0.045

W(Al-SiC-Gr), g

0.024


% Reinforcement = 9.98031Sliding speed, m/s = 1.99731W(Al-SiC), g = 0.0223108W(Al-SiC-Gr), g = 0.0225916


% Reinforcement = 3.00584Sliding speed, m/s = 1.99731W(Al-SiC), g = 0.0324003

W(Al-SiC-Gr), g = 0.0234623




Fig. 8. Overlaid contour plot of effect of % reinforcement and sliding speed on wear of Al–SiC composites and Al–SiC–Gr hybrid composites.

% Reinforcement

L o a

d ,

N

1086420

70

60

50

40

30

20

Hold Values

Sliding speed, m/s 2

Sliding distance, m 2000

W(Al-SiC), g

0.025

0.01

0.035

W(Al-SiC-Gr), g

0.009


% Reinforcement = 9.98738Load, N = 74.9344W(Al-SiC), g = 0.0222890W(Al-SiC-Gr), g = 0.0225829






Fig. 9. Overlaid contour plot of effect of % reinforcement and load on wear of Al–SiC composites and Al–SiC–Gr hybrid composites.


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Increase of speed reduces wear by supporting mechanically

mixed tribo layer and increase of load increases wear by reduc-

ing the role of tribo layer. Wear increases with sliding distance

and it is the predominant factor affecting wear of both the

composites.

Mathematical models using response surface regression analysis

is formulated and overlaid contour plots representing the model

have been drawn with % reinforcement being the common factorin combination with sliding distance, sliding speed and load.

Analysis of overlaid contour plots envisages that, increase in %

reinforcement reduces wear up to around 7.5% of % reinforce-

ment and beyond this the wear has a tendency to increase for

Al–SiC–Gr hybrid composites. Thus, the optimal % reinforcement

can be around 7.5% for any value of sliding distance, sliding

speed and load within the range considered in the investigation.

Overlaid contour plots depict that wear continuously decreases

with increase in % reinforcement for Al–SiC composites and thus

not evidencing the possibility of any optimal % reinforcement.

In tribological applications demanding similar strength require-

ment, Al–SiC–Gr hybrid composites are best suited than Al–SiC

composites on account of lesser amount of wear.

References

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[2] Garcia-Cordovilla C, Narciso J, Louis E. Abrasive wear resistance of aluminiumalloy/ceramic particulate composites. Wear 1996;192:170–7.

[3] Das S, Mondal DP, Sawla S, Dixit S. High stress abrasive wear mechanism of LM13–SiC composite under varying experimental conditions. Metall MaterTrans 2002;33A:3031–44.

[4] Sahin Y, Acilar M. Production and properties of SiCp-reinforced aluminiumalloy composites. Composites 2003;34A:709–18.

[5] Rohatgi PK, Guo R, Kim JK, Rao S, Stephenson T, Waner T. Wear and friction of cast Al–SiC–Gr composites. In: Proceedings of materials solutions’97 on wearof engineering materials, Indianapolis, Indiana; 15–18, 1997, p. 205–11

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[7] Riahi AR, Alpas AT. The role of tribo-layers on the sliding wear behavior of graphitic aluminum matrix composites. Wear 2001;251:1396–407.

[8] Basavarajappa S, Chandramohan G, Mahadevan Arjun, Tangavelu Mukundan,Subramanian R, Gopalakrishnan P. Influence of sliding speed on the dry slidingwear behaviour and the subsurface deformation on hybrid metal matrixcomposite. Wear 2007;262:1007–12.

[9] Rohatgi PK, Liu Y, Ray S. Friction and wear of metal–matrix composites, ASMhand book, vol. 18; 2004, p. 801–11.

[10] Barker Thomas B. Quality by experimental design. New York: Marcel DekkerInc.; 1985.

[11] Kuehl Robert O. Design of experiments. USA: Duxbury; 2000.[12] Krishnamurthy L, Sridhara BK, Abdul Budan D. Comparative study on the

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[13] Montgomery Douglas C. Design and analysis of experiments. New Delhi: WileyIndia (P) Ltd.; 2007.

[14] Krishnamurthy L. Machinability studies on metal matrix hybrid composites.PhD thesis, Karnataka, India: Kuvempu University; 2009.


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