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CHAPTER 2
LITERATURE SURVEY
2.1 INTRODUCTION
This chapter covers the literature review of results obtained from
investigations on the dry sliding wear and abrasive wear behaviour of
Aluminium Metal Matrix Composites (AMMCs). Apart from the matrix
alloy, reinforcement used, manufacturing methods, hybrid composites, this
survey includes the developments in the statistical modelling of abrasive and
dry sliding wear of metal matrix composites.
2.2 ALUMINIUM ALLOYS USED IN AMMCs
Most of the earlier studies have focused on developing MMCs with
commercially available aluminum alloys. Commercial aluminium alloys were
selected for MMCs because they offer good mechanical properties, easily
available and many of them are suitable for heat treatment. The common
series of wrought aluminium alloys are 1000 (Pure aluminium), 2000 (Al-Cu),
3000(Al-Mn) 4000 (Al-Si), 5000 (Al-Mg) 6000 (Al-Si-Mg), 7000 (Al-Zn-
Mg) and 8000 (Al-Li). Table 2.1 shows the some of the wrought aluminium
alloy used as matrix material. Among the cast aluminium alloys, aluminum-
silicon, aluminum-copper, and aluminum-magnesium alloy systems are
broadly used for MMC applications (Table 2.2).
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Table 2.1 Wrought aluminium alloys used as matrix
Wrought
Aluminium
Alloy
Reference
1060 Rosenberger et al. (2009)
1050 Lim et al. (1999)
1061 Rosenberger et al. (2005)
2024 Abdel-Azim et al. (1995), Kok (2005), Kok and Ozdin (2007),
Narayan et al. (1995), Yilmaz (2007)
2618 Sakthivel et al.(2008)
2219 Basavarajappa et al. (2007)
2009 Sannino and Rack (1995)
2014 Sahin and Murphy (1996), Modi (2001)
2124 Izciler and Muratoglu (2003), Muratoglu and Aksoy (2006)
2011 Sahin and Ozdin (2008)
6061 Ramesh et al. (2005), Yang (2003),
Straffelini et al. (1997), Al-Qutub et al. (2006),
Sharma et al. (2001), Wilson and Alpas (1996)
6063 Natarajan et al. (2009)
8090 Gomez-del Rio et al. (2009), Bauri and Surappa (2008)
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Table 2.2 Cast aluminium alloys used as matrix
Cast
Aluminium AlloyReference
A206 Rohatgi et al.(2010)
Al-2 Mg. Mandal et al.(2006)
A359 Hasim et al. (2001)
A356 Rajan et al.(2007), Li and Tandon (1997),
Akhlaghi et al. (2004), Wilson and Alpas (1997)
Ravikiran and Surappa (1997), Riahi and Alpas(2001), Wilson and Alpas (1996), Chen and Alpas(1996), Yalcin and Akbulut (2006),Bindumadhavan et al. (2001)
Al-10Si Acilar and Gul (2004)
Al-22Si Moustafa (1995)
Pure Al.+4.5 % Cu Kwok and Lim (1999)
ADC 12 Iwai et al. (2000), Mondal and Das (2006)
Al-12 Si Liu Yao-hui et al. (2004)
Al-20Si-3Cu-1Mg Bialo et al. (2000)
Al-13%Si+1.13Mg+0.88 %Cu
Liu et al. (1997)
Pure Al. Mandal et al. (2004)
Al-2Mg Mandal et al. (2008)
Al-5Mg Daoud et al. (2003)
Al /1-8wt.% Si Ahatchi et al.(2004)
Al-4.5wt.% Cu Das et al. (2007)
Al-Si12Fe Singh et al. (2001), Sevik and Kurnaz (2006)
Al-Si12Cu Das et al.(2008), Akbulut et al.(1998)
Sawla and Das (2004)
Al-(8,12,16%)Si-0.3Mg Rajesh Sharma et al. (2007)
2.3 REINFORCEMENTS USED IN AMMCs
Generally, the following are the requirements for a reinforcement:
low density, compatibility with matrix alloy, chemical compatibility, thermal
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stability, high compression and tensile strength, economic efficiency. Most of
the previous work is focused on Discontinuously Reinforced Aluminium
MMCs (DRAMMCs) because of their greater ease of manufacture, lower
production costs, and relatively isotropic properties. In DRAMMCs,
commonly used reinforcements are silicon carbide (SiC) and boron carbide
(B4C) particulate. Titanium carbide (TiC) is being investigated for high-
temperature applications (Jerome et al. 2010, Shipway et al. 1998). Table 2.3
shows the some of the previous works in AMMCs using different
reinforcement for investigations.
Table 2.3 Reinforcements used in aluminium MMCs
Reinforcement Type Reference
ParticulateAbdel-Azim et al. (1995), Kok (2005),Narayan et al. (1995), Yang (2003),Straffelini et al. (1997), Das et al. (2007)
Alumina (Al2O3)
FibresMoustafa (1995), Iwai et al. (2000),Liu et al. (2004), Cao et al. (2001), O'rourke et al. (1996), Jiang and Tan (1996)
Particulate
Akhlaghi et al. (2004), Sakthivel et al. (2008),Hasim et al. (2001), Acilar and Gul (2004),Wilson and Alpas (1997), Rosenberger et al.(2009), Basavarajappa et al. (2007), Kwok and Lim(1999), Ravikiran and Surappa (1997), Lim et al.(1999), Gomez-del Rio et al. (2009), Bauri andSurappa (2008), Bindumadhavan et al.(2001),Das et al. (2008)
Silicon Carbide(SiC)
FibresAkbulut et al.(1998), Sahin (1998), Liu et al.(1998), Srinivasa Rao and Upadhyaya (1995)
Boron Carbide(B4C)
FibresRosenberger et al. (2005),Sahin and Murphy (1996)
Titanium carbide(TiC)
Particulate Sheibani et al. (2007)
Silica Sand Particulate Rohatgi et al. (2010)Steel fibres Fibres Mandal et al.(2006)
Fly ash ParticulateRajan et al.(2007),Ramachandra and Radhakrihna (2005)
Granite Particulate Singh et al. (2001)TiO2 Particulate Ramesh et al. (2005)Short Glass fibre Fibre Sharma et al. (1998)TiB2 Particulate Natarajan et al. (2009), Tjong and Ma (1997)
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A comparison of properties of aluminum oxide or alumina (Al2O3)
with other reinforcements is given in Table 2.4 (Smith 2001). Due to better
mechanical strength and hardness alumina used in wearing environments.
Low density, low reactivity with material and high temperature resistance as
well as the robustness that make this possible to be used in structural
applications. Alumina also cost effective and, the result being a reasonably
priced final product.
Table 2.4 Comparison of the properties of alumina with other
reinforcements
Properties Al2O3 SiC B4C TiC
Density (g/cm3) 3.92 3.21 2.52 4.93
Elastic modulus, (GPa) 350 430 450 345
Knoop hardness 2000 2480 2800 2150
Compressive strength (MPa) 2500 2800 3000 2500
Thermal conductivity (W/m.K) 32.6 132 29 20.5
Coefficient of thermal expansion (10-6K) 6.8 3.4 5 7.4
Specific thermal conductivity,
(W.m2/kg.K)
8.3 41.1 11.5 4.2
2.4 MANUFACTURING METHODS FOR AMMCs
Two common methods for aluminium MMCs manufacture are
Powder Metallurgy and liquid metal processing. Casting method is most
preferred method for production of aluminum MMCs. These include the sand,
gravity die, investment, and high-pressure die-casting. However, literature has
shown that several modifications are possible in the melting and casting
practice in order to produce high-quality castings from a composite. In
Table 2.5 shows the manufacturing methods used for MMC production.
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Table 2.5 Processes used for manufacturing aluminium MMCs
Process Reference
Liquid State Processing
Stir casting Abdel-Azim et al. (1995),Rohatgi et al. (2010)Kok (2005)Mandal et al.(2006)Hasim et al. (2001)Narayan et al. (1995)Basavarajappa et al. (2007)Ravikiran and Surappa (1997)Bauri and Surappa (2008)Bindumadhavan et al.(2001)Sharma et al. (2001)Mandal et al. (2008)Riahi and Alpas (2001)Yilmaz and Buytoz (2001)
Compocasting Akhlaghi et al. (2004)Rajan et al.(2007)Lim et al. (1999)Sharma et al. (1998)
Squeeze casting Moustafa (1995)Iwai et al. (2000)Sahin (1998)
Infiltration process: Acilar and Gul (2004)Liu Yao-hui et al. (2004)Akbulut et al.(1998)Liu et al. (1998)Sahin and Murphy (1996)Chen and Alpas (1996)
Spray deposition Gomez-del Rio et al. (2009)
In-situ processing Natarajan et al. (2009)
Solid State Processing
P/M processing Rosenberger et al. (2009)Kwok and Lim (1999)Unlu (2009)Sannino and Rack (1995)Al-Qutub et al. (2006)Muratoglu and Aksoy (2006)
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2.5 MICROSTRUCTURE AND PROPERTIES
Literature survey on evaluation of microstructure and properties of
composite materials are given in this section. The requirement of
microstructure is uniform particle distribution, less porosity and particle-
matrix interface and mechanical properties depends on the matrix properties,
wettability, amount of reinforcement and shape of reinforcing phase and the
size of the reinforcing particles.
Abdel-Azim et al. (1995), observed in 2024 Al / alumina
composites, matrix microstructure refinement is better than compare with
matrix alloy produced in vortex method. An increase in amount of alumina
particles in the matrix decreases the grain size.
Surappa’s (1997) review concluded that the wetting of the
reinforcement by the liquid metal, homogeneous mixing of the melt-particle
slurry as well as particle-solidification front interactions determines the final
particle distribution and matrix microstructure. Dendrite Arm Spacing (DAS)
and matrix grain size were affected by the presence of reinforcing particles.
Hasim et al. (1999, 2001) discussed the difficulties associated in
attaining a uniform distribution of reinforcement, good wettability between
substances, and a low level porosity in stir casting process. Variables such as
holding temperature, stirring speed, size of the impeller, and the position of
the impeller in the melt have important influence in the production of cast
metal matrix composites. The same author has observed that use of
magnesium enhances wettability, though increasing the content above
1 wt.% magnesium increases the viscosity of the slurry to the detriment of
particle distribution. Increasing the volume percentage of SiC particles in the
matrix alloy decreased the wettability.
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Akhlaghi et al. (2004) observed that in Semisolid–Liquid (SL)
processing uniform distribution of SiC particulates and lower porosity content
during the microstructure studies conducted on A356/SiC composites in
comparison to Semisolid-Semisolid (SS) processed samples, regardless of the
mould pre-heat temperature or the size and amount of the SiC particles.
Kok (2005) observed that in the case of 2024Al / alumina
composite, density of the composites increased with increasing weight
percentage. The wettability and the bonding force between aluminium alloy
and alumina particles were improved by the applied pressure after pouring
and porosity also decreased because of this pressure. The tensile strength and
hardness of MMCs increased while the elongation of composites decreased
with decreasing size and increasing weight percentage of the particles in stir
casting method.
Sakthivel et al. (2008) observed uniform distribution of particles
with little agglomeration of particles along with some porosity in the
microstructure of 2618 Al-SiC composites made by stir casting method. He
found that hardness and tensile strength of the composites increased with
decreasing size and increasing weight fraction of the reinforcement particles.
Rohatgi et al. (2010) reported that the dendrite arm space decreases
when the volume fraction of the particles increases in A206/Silica sand
composites in stir casting technique.
2.6 DRY SLIDING WEAR OF AMMCs
According to Sannino and Rack (1995), the important tribological
parameters that control the wear rate and coefficient of friction of
discontinuously reinforced aluminium composites under dry sliding
conditions are the reinforcement type, size, shape, orientation and
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reinforcement percentage referred to as the material factors. The applied load,
sliding velocity, sliding distance, environment and temperature as well as
counterpart material and these are collectively called as mechanical and
physical factors which also play an important role in controlling the
tribological behaviour of aluminium composites.
2.6.1 Testing Methods and Parameters
Wear test methods grouped into six categories: i) Machinery Field
Tests. ii) Machinery Bench Tests. iii) Systems Bench Tests iv) Components
Bench Tests v) Model Tests vi) Laboratory Tests (Horst Czichos 1992).
ASTM committee G-2 friction and wear test methods address many of the
most common occurrences of friction and wear in machinery. ASTM
G99-95a method describes a laboratory procedure for determining the wear of
materials during sliding using a pin-on-disc apparatus (Blau and Budinski
1999).
Low stress abrasion studies (three body) generally employ the use
of a Rubber Wheel Abrasion Test (RWAT) conducted to a specific standard
of ASTM-G65 (Modi et al. (2001), Izciler and Muratoglu (2003), Prasad et al.
(1992)). High stress abrasion studies (two body) are characterized by a
specimen pin sliding under an applied normal load against a fixed abrasive
medium by pin-on-disc apparatus (Deuis et al. 1996). The various researchers
conducted abrasive wear test on pin-on-disc type apparatus. (Das et al. (2007),
Kok and Ozdin (2007), Muratoglu and Aksoy (2006), Rajesh Sharma et al.
(2007), Yilmaz and Buytoz (2001)).Some of the research work done have
used a reciprocating type abrasive wear tester (Mondal and Das (2006), Sawla
and Das (2004), Singh et al. (2002)). These tests have been conducted varying
the materials parameters and operational parameters.
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2.6.2 Effect of Reinforcement Type
Rosenberger (2010) studied the AA1061 alloy reinforced with
Al2O3, B4C, Ti3Al and B2Ti processed by P/M technique. Composites
reinforced with alumina showed the largest amount of wear among the
composites due to the fact that the size of the alumina particles is one order of
magnitude smaller than the other reinforcements. The amount of wear for the
composites reinforced with particles of Ti3Al, B2Ti and B4C were practically
the same.
Yilmaz (2007) studied the effects of volume fraction and size of
SiCrFe, CrFeC, and Al2O3 particulates on the abrasive wear rate of compo-
casted Al 2024 metal matrix composites. The lowest wear rate was obtained
for composites having CrFeC, SiCrFe and Al2O3 particulates together. This
low wear rate was attributed to the chemical bond between intermetallic and
matrix as well as due to uniform distribution of the particulates in the matrix.
Miyajima and Iwai (2003) studied the effect of dry sliding wear of
SiC whisker, Al2O3 fiber and SiC particle reinforced with Al 2024 and
ADC12 aluminium alloys, in the initial wear regime, the fibres were larger in
diameter and length than the whiskers, so that the fibres were more effective
in decreasing the initial severe wear as compared with whiskers. The particle
reinforcement effectively prevented the plastic flow and the adhesion of
matrix material since the particle shape was of great advantage on carrying
contact load compared to whisker and fibre reinforcements.
2.6.3 Effect of Reinforcement Orientation
Sahin (1999) noticed that the incorporation of boron fibres to form
MMCs greatly reduced the wear rates compared to unreinforced aluminium
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alloy for both Normal (N) and Parallel (P) fibre orientations. At low speeds in
the normal fibre orientation, boron oxide and iron boride were formed.
However at high speeds the amount of aluminium increased and boron
decreased in the tribofilms was identified by X-ray photoelectron
spectroscopy (XPS).
Liu and Ogi (1999) observed that the wear behavior of alumina
continuous fibre reinforced in the Al-4.43%Cu composites against a steel
counterface under a dry sliding conditions, depended strongly on the fibres
orientations. Among the 45°, 90°, and 135° orientations, the composites
volume loss wear was lower for 135o while for anti-parallel orientation of
fibres with the sliding direction, the largest wear rate was observed for all
sliding distances.
2.6.4 Effect of Reinforcement Size
Sannino and Rack (1995) observed that wear resistance increased
with an increase in reinforcement particulate size in Al2009/SiC composite.
Scanning electron microscopy showed that adhesion-induced tribo-fracture
and micro-cutting, micro-ploughing and wedge formation were the
predominant wear mechanisms at smaller (4, 10, and 13 m) reinforcement,
while particulate cracking induced subsurface delamination occurred in the
large ceramic (29 m) reinforced composites attributed to an increase in
particulate size and consequent increase in the volume loss.
Bindumadhavan et al. (2001) observed that in A356/SiC composite
with Dual Particle Size (DPS) composite (47 and 120 m) showed better wear
resistance than the composite having only small (47 m) sized particles. In
these DPS composites, larger SiC particles help to carry a greater portion of
the applied load, thereby reducing the load on the smaller SiC particles as
well as on the base metal. The larger SiC particles also help to shield the
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smaller SiC particles from the gouging action of the abrasive, thereby aiding
the smaller particles to continue performing their wear resisting function.
Al-Qutub et al. (2006) observed that addition of 10 vol. %, of sub-
micron (0.7µm) alumina particles improved the wear resistance of the 6160
aluminium alloy by up to 45% compared to the unreinforced alloy.
Increasing the alumina content improved the wear resistance up to 145%.
Hardness test indicates that the hardness of the composite increased linearly
with the percentage of added alumina particles.
Sevik and Kurnaz (2006) concluded that Al-Si12/alumina
composites with larger alumina particles (125 m) showed better wear
resistance than the composites containing small particles (44 m). In the case
of 125 m, the particles were deeply embedded in the matrix and thus it is
very hard to pull out the particles from the matrix. In composites with smaller
particles (44 m), the depth of embedding was not so deep and the particles
can easily be taken out from their matrix.
2.6.5 Effect of Reinforcement Volume
Yalcin and Akbulut (2006) observed that both wear rate and
friction coefficient of the A356 alloy decreases with increasing SiC particle
content (5-20 vol.%). However, specimens reinforced with 15 and 20 vol.%
SiC when tested at 5 N applied load showed an increase in the friction
coefficient. It is believed that this increase was caused by poor interfacial
bonding between the matrix and SiC particles. Poor bonding, associated with
particle segregation can cause particle transfer from the matrix to the WC ball
and disc interface generating vibration.
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Sharma (2001) observed that the wear resistance of Al6061-garnet
composites are superior to that of unreinforced matrix alloy. The wear
resistance of composites increased with increasing percentage of garnet (4-12 %).
The average coefficient of friction of the Al6061 composite was observed to
be lower than that of matrix alloy. Formation of a Mechanically Mixed Layer
(MML) was responsible for the decrease in the wear-rate and friction
coefficient of the MMCs.
Sahin and Murphy (1996) observed that the hardness of the MMCs
and the matrix alloy increased linearly and their densities decreased linearly
with volume percent boron fibre (0-32 vol.%). The average wear rate of a 32
vol.% fibre composite in normal orientation was reduced by about 84% in
comparison with matrix alloy. The coefficient of friction gradually increased
with fibre content reaching maximum and decreased for all composite
materials.
2.6.6 Effect of Normal Load
Acilar and Gul (2004) investigate that both the 10%, 30% SiC
particle reinforced with Al-10Si composites found that the wear rate of
composites increased with increasing sliding distance and applied load. The
damage to the surface of the composites increased with increasing load since
matrix materials did not have enough resistance and therefore volumetric
wear rate of the composites were higher.
Wilson and Alpas (1997) constructed wear transition maps for
A356 Al and A356 Al - 20% SiC against SAE 52100 steel, where load and
speed combinations for different wear rates and wear mechanism regimes
were identified using this wear maps. They found that the addition of SiC
particles to A356 Al extended the mild wear regime to higher speeds and
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loads thereby slowing down the severe wear. Addition of SiC particles to
A356 extended the mild wear regime to higher speeds and loads, thus slowing
down the transition to severe wear. SiC particles assisted the retention of an
oxide transfer layer on composite sliding surfaces which prevent metal-metal
contact and keep wear behaviour within the mild wear regime. When severe
wear starts in the composite, the formation of iron oxides from the steel
counter face acts as a lubricant and reduce the wear damage. An ultra-mild
wear rate regime is observed where SiC particles support the load, at low
sliding speeds and loads.
Moustafa (1995) found the wear rate of the Al-22Si matrix at a
normal load of 40 N to be 2.2 x10-3 mm3/m and composite at the same load to
be 0.2 x10-3 mm3/m. The load at which the transition occurs (the low wear
rate regime to the mild wear rate regime) was three times higher for the
composites. It was reported that “a critical load” for non-destructive wear
exists; below this load, the wear rate is mild and steady (i.e. the wear rate is
constant), and a severe wear rate occurs above this “critical load”. The
transition load in the case of the matrix was found to occur at 100 N while for
the composite, the load was 320 N. However, the wear rate in both cases at
the corresponding transition load was the same and had a value of 3.9 x10-3
mm3/m. This fixed wear rate was not expected and indicates that the transition
from mild to severe wear rate for this particular alloy and its composites takes
place at the same wear rate, but not at the same normal load. Improvement in
the transition load and the drastic decrease in wear rates in the case of the
composites were attributed to the presence of the alumina fibres. It was
proposed that the fibres supported part of the applied load, in addition to the
improving the yield strength of composites. He found that at a normal load of
40 N, the coefficient of friction of the matrix decreased to 0.33, while
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that of the composite decreased to 0.17. This reduction of the coefficient of
friction occurs as a result of the fibres and/or silicon particles protruding
above the surface. Al-22%Si alloy and composite exhibit the same wear
mechanisms, namely oxidation induced delamination, high strain induced
delamination, and sub-surface delamination.
Rosenberger et al. (2009) identified in two linear behaviours with
different slopes which change at 80 N in 1060Al-alumina P/M composites,
There is a pronounced change of slope at a load of about 80 N which indicates
two different wear regimes; moderate and severe. The main observation was
the presence of iron in the surface, which was also found in the surface of
samples wear at loads 4.9–52 N. On the other hand, iron is not present in
samples worn at higher loads between 81.4 and 91.2 N. Iron was transferred
from the wearing counterface by a mechanism of mechanical alloying which
results in the formation of a mechanically mixed layer on the wear surface.
2.6.7 Effect of Sliding Velocity
Basavarajappa et al. (2007) observed that the wear rate for
Al2219/15 wt.% SiC composites and Al2219/15 wt.% SiC/3%Gr hybrid
composites were almost unchanged with an increase in sliding speed up to
3 m/s. Beyond 3 m/s, the wear rate of the unreinforced alloy increased to
larger values than those of the composites and seizure was observed at a
sliding speed of 6.1 m/s. At a sliding speed of 6.1 m/s, wear by delamination
was observed to occur in the alloy with fragments from the pin being
transferred to the disc as well as to larger fragments. However, the wear rates
of the composites almost unchanged with increase in sliding speed up to
4.6 m/s after which there was increasing in wear rate.
Kwok and Lim (1999) noted that three distinct regimes
(demarcated by sliding speed) of friction and wear behaviour formed for
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Al+4.5%Cu+SiC P/M processed composites. These regimes are : low rate of
wear in Regime I (below 3 m/s); catastrophic failure of the composites occurs
readily in Regime II (3 to 8 m/s) when a certain critical load is exceeded; and
extensive melting of the composites takes place in Regime III (when the
sliding speed exceeds 8 m/s). The catastrophic failure observed in Regime II
occurs when a large amount of the specimen material very quickly adhere to
the counter face, making it impossible to continue with the test when this
happens. The extensive melting in Regime III has been attributed to the
increased bulk temperature of the composites and reaching the melting point
of the Al-alloy matrix. The results obtained suggest that a small particle size
leads to inferior high-speed wear resistance, with the composite experiencing
extensive melting even at a relatively low load is applied. The coefficient of
friction varies with sliding distance, and no unique value can be associated
with each sliding condition.
Ravikiran and Surappa (1997) observed that with increase in
sliding speed the numbers of SiC particles undergoing fracture decreased in
A356/SiC composites. The wear rate of the pin decreases with increasing
speed. At lower speeds (less than 2 m/s) pin surface experiences severe
damage resulting in a high wear rate. The area fraction of SiC particles
exposed on the pin surface increases with increasing speed. At higher speeds,
SiC particles protrude above the matrix due to the melting of a thin layer of
matrix material and bear almost the entire load. Formation of a high
temperature form of iron oxides takes place at high speeds, which forms a
protective layer both on the pin and the disc surfaces. Damage experienced by
the matrix decreases with increasing speed.
Singh et al. (2001) observed that while wear rate decreased initially
and then increased again with increasing sliding speed in Al-Si12 matrix
alloy. Wear rate reduced with increase in speed for the Al-Si12 / Granite
composite except at the maximum speed at high pressures.
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2.6.8 Effect of Sliding Distance
Lim et al. (1999), observed that amount of wear generally increases
with increasing sliding distance and the extent of wear generally becomes
greater with an increase in normal load in Al1050/SiC composite processed
by Rheocasting and P/M techniques. Also suggests that the rheocast
composites are generally superior in wear resistance compared to composites
fabricated using the powder metallurgy route incorporating mechanical
alloying over a range of sliding conditions. Furthermore, the rheocast
composites showed greater wear resistance under severe sliding conditions.
Ramesh et al. (2005) evaluated that wear coefficients were
evaluated by using Archard’s and Yang’s theoretical models for Al6061–TiO2
composites. Increased in volume fraction of TiO2 resulted in higher hardness
and lower wear coefficient of the composites. The wear coefficient of all the
Al6061–TiO2 composites studied decreased at higher loads and larger sliding
distances. At larger sliding distances, relatively higher rise in temperature of
the sliding surfaces resulted in softening of both the matrix alloy and the
composite pin surfaces leading to heavy deformation at higher sliding
distances. This contributes to higher wear losses of both the matrix alloy and
the composites.
Iwai et al. (2000) observed that the sliding distance or the time for
transition from severe to mild wear could be identified by two factors in
Al-Si10Cu2Fe/alumina fibres composites, i) an abrupt and steep reduction in
the frictional force, and ii) a change in the magnitude of displacement of pin
specimen. The wear curves were characterized by two distinct straight lines of
different slopes, which correspond to severe and mild wear conditions. Fibre
reinforcement effectively prevented the occurrence of severe wear and also
aided in decreasing the time/distance required for transition from severe to
mild wear. Aluminium alloy reinforced with alumina fibres could also
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improve dry sliding wear resistance. Reinforcements inhibit plastic flow and
restrict propagation of wear cracks. Both the duration of the severe wear
regime as well as the severe wear rate decrease with increase in the fibre-
volume fraction and, above certain value of volume fraction of reinforcements,
composites do not exhibit severe wear.
Yang (2003) to modelled the standard wear coefficients in both the
transient wear and steady-state wear of Al 6061–Al2O3/steel system. In the
case of 10% alumina composite, higher wear coefficient values were observed
compared to 15% alumina and 20% alumina reinforced composites attributed
to the presence of a lower volume fraction of alumina in its matrix.
2.6.9 Effect of High Temperature
Liu Yao-hui et al. (2004) observed that friction coefficient and
wear rate of Al-12Si monolithic alloy and Al-12Si/alumina/carbon composites
decreased slightly with increasing temperature up to 100oC. The trend
reversed beyond this temperature, finally leading to specimen seizure
indicated by a sharp rise in friction coefficient and wear rate.
(a) Friction coefficient (b) Wear rate
Figure 2.1 Influence of test temperature (Liu Yao-hui et al.(2004))
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Addition of 4 vol.% C to the Al–12Si /12 vol.% Al2O3 composites
reduced the friction coefficient, but was still larger than that of the monolithic
alloy as shown in Figure 2.1(a) indicating that the addition of reinforcements
reduced wear rate Al2O3 reinforcement in particularly decreased the wear rate
more effectively than carbon Figure 2.1(b). However, the wear rate of the
Al–12Si /4 vol.% C/ 12 vol.% Al2O3 hybrid composites was less than that of
remaining three compositions over the whole range of test temperatures and
the hybrid composites showed the best wear performance.
Gomez-del Rio et al. (2009) classified wear rate into mild and
severe wear in Al 8090-SiC composite. Relative wear rates under different
temperatures and sliding velocities are shown in Figure 2.2(a) and (b). Using
arbitrary value for relative wear, they determined the transition between both
regimes. The trend observed for the composite is similar under most of the
normal pressures, with a minimum wear occurring at 0.3–0.5 m/s within the mild
wear regime, after which a transition to severe wear occurred. Experiments
also indicated a delay in the transition for the composite material extending
the mild wear regime, an important advantage of reinforcing the matrix.
(a) Unreinforced alloy Al-8090 (b) Composite material
Figure 2.2 Variation of relative wear rates with test temperature
(Gomez-del Rio et al. (2009))
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The effect of temperature is quite similar, with minimum wear
around 100oC after which the transitions to severe wear occurred. During the
mild wear, both the pin and the steel disc got worn out and transfer of material
between the mating surfaces was observed.
Natarajan et al. (2009) studied the high temperature wear of
Al6063 – TiB2 composites. The wear rate of the monolithic alloy and
composites increased slightly with the increase in temperature up to 100oC.
The trend is reversed beyond this temperature, finally leading to specimen
seizure indicated by a sharp rise in wear rate up to around 200oC, and
thereafter the increase in wear rate becomes gradual. The wear rate decreased
with the increase in the amount of TiB2 for all test temperatures.
2.6.10 Effect of Counterpart Hardness
Bialo et al. (2000) studied effect of steel counter face (28, 40,
58 HRC) hardness with Al-20Si-3Cu-1Mg / alumina composite made by P/M
route. With soft steel as a mating material, material transfer from the
composite to the steel occurs and wear occurs due to abrasive action, while in
the case of harder steel counter face, delamination at the friction surface of the
composite was largely responsible for its wear. The friction coefficient of the
composite was also affected by the hardness of the counter face, in addition to
pressure. The coefficient of friction value was the lowest at the highest
contact pressure (3 MPa) and when the counter-specimen is softer. This is
probably because the detached material from the composite material is
pressed into the soft counter specimen, forming two-body abrasion giving a
corresponding low friction coefficient value. However, when the counter-
specimen is harder and the pressure is lower, the embedment of the
delaminated material would be difficult and as a result three-body abrasions
occurred and increase the friction coefficient.
35
2.6.11 Effect of Heat Treatment
The wide choice of alloy compositions, solution heat treatment
temperatures and times, quench rates, choice of artificial ageing treatment and
degree to which the final product has been deformed permit a wide range of
properties to be achieved. T4 (Solution heat-treated and naturally aged to a
substantially stable condition) and T6 (Cooled from an elevated temperature
shaping process and then artificially aged) heat treatment conditions are
mostly used for aluminium composites.
Li and Tandon (1997) observed that the wear resistance of the as-
cast Al356 / SiC composites can be moderately increased by the heat
treatment condition (T6 condition - solution heat treatment up to 540°C for
8 hrs and water quenched then artificially hardened at 150°C for 4 hrs),
although the increase is not significant when compared with the increase in
hardness due to the heat treatment. In-situ precipitation was found to occur
in the subsurface region during the sliding wear of the as-cast aluminium
composites. It was also found that coarsening of the age hardened precipitates
occurred during the sliding wear of aged composites. The in-situ precipitation
and the coarsening of precipitates accounted for changes in dislocation
structures in the subsurface created by the sliding wear.
Liu et al. (1997) observed the microstructure of Al-SiMg and Cu
alloy (13%Si+1.13Mg+0.88Cu) reinforced with 1.55 wt% graphite after
LASER processing. Micro-hardness of the laser treated composites was about
ten percent higher than that of the normal treated composite (heating to
505oC, holding for 5 hrs, followed by water quenching into water and then
aging at 180oC) for remained higher for all the aging times. These
improvements achieved after laser processing come mainly from the
contributions of more homogenous and fine microstructure of the matrix. The
wear rate of the LASER processed composite is lower than that of the normal
36
heat treated composite because matrix ploughing was reduced after the laser
surface treatment, which results in an increase in the wear resistance.
Straffelini et al. (1997) reported that extruded 6061 Al-alumina
composite showed low matrix hardness and displays the lowest wear rate.
Other composites (i) aged at the T6 condition (ii) forged and aged at T6
condition and (iii) treated and over aged at 220oC, displayed higher wear rates
which increased as their hardness is increased. The extruded material
displays subsurface hardening which contributes to sustaining the surface
transfer layer, essentially constituted by iron oxides. The other composites
undergo delamination wear. Plate like debris particles originated from the
detached hard surface mixed layers, which are formed once a critical strain is
reached. The subsurface plastic flow also gives to a softening of the matrix,
which thus becomes unable to conveniently sustain the surface mixed layers.
Bauri and Surappa (2008) observed that unreinforced Al 8090 alloy
and Al8090/SiC composites exhibited mild wear upto 20 N. Beyond 20N,
transition from mild to severe wear occurred. Composites exhibited much
better wear resistance in the severe wear regime in the as-extruded condition.
Wear resistance of the unreinforced alloy increases due to ageing for all the
test loads. In case of the composites, wear resistance improved due to ageing
for loads upto 20 N. Beyond 20 N, wear resistance of the composites is lower
in peak-aged condition compared to extruded condition. Greater material loss
due to larger surface and subsurface deformation in peak-aged condition were
attributed as the main reasons to this. Ageing was attributed to reduction in
ductility in peak-aged condition and thus attainment of subsurface plastic
strain conditions for surface softening.
Narayan et al. (1995) studied the variation in wear with load for the
Al2024 unreinforced alloy and Al2024/Alumina composites in the T6
condition. Up to 30 N, the wear rate of the aged composite was found to be
37
negligible. From 30 N to 200 N loads, the wear rates of the two materials are
comparable. Beyond 200 N, the wear rate of the peak-aged unreinforced alloy
is slightly higher than that of the composite. The unreinforced aged alloy
eventually seized at a load of 230 N. The seizure event was accompanied by a
sudden increase in wear rate, heavy noise and vibration. The peak-aged
composite shows an increasing wear rate between 225 and 260 N, and beyond
that the wear rate gradually decreases.
2.7 WEAR OF HYBRID COMPOSITES
When a soft metal like aluminium slides on hard steel without any
external fluid or solid lubrication, aluminium will flow and adhere to the steel
surface and creating an interface of low shear strength. Transfer of aluminium
to steel will continue with sliding (Prasad and Asthana 2004). Wear debris
may form as a result of ploughing of the soft aluminium surface by the
asperities of the hard steel and flaking off of patches from the transfer film
may also occur. The use of aluminium metal matrix composites (MMC)
reinforced with a solid lubricant (graphite, molybdenum disulfide, antimony
tri-sulphide, copper sulphide, calcium fluoride etc.), hard ceramic particles
(silicon carbide, alumina, boron carbide, etc.) and short fibres can help reduce
friction and wear of the matrix. The development of aluminium MMCs
dispersed with solid lubricants is primarily directed towards overcoming the
principal drawbacks of aluminium as a tribological material. Rohatgi and
co-workers first introduced graphite as a solid lubricant in aluminium
matrices by casting routes, involving mixing the molten alloy with graphite
particles to make a uniform suspension in casting method.
Riahi and Alpas (2001) studied A356 aluminium alloy reinforced
with 10SiC/4%Graphite and 5%alumina/3%Graphite composites produced by
stir casting technique. They reported a mild wear regime range of loads
(5–420 N) and sliding speeds (0.2–3.0 m/s) for both composites. In the mild
38
wear regime, wear of graphitic composites was primarily controlled by the
formation of the tribo-layer as well as an oxidized surface layer on the contact
surfaces for both types of composites.
Chu and Lin (2000) studied 6061Al reinforced with 10% SiC and
natural graphite or electroless nickel coated graphite (0, 2, 5, 8, and 11 vol.%)
made by P/M route. The wear results showed that the use of the electroless
nickel film was significantly beneficial in lowering the wear rate of the
component, although it did not produce a great reduction in the wear rate of
composite with pure graphite. Similarly, friction coefficients were at relatively
lower levels when electro less nickel coated graphite was used.
Guo and Tsao (2000) studied self-lubricated 6061 aluminium
/10%SiC / graphite hybrid composites containing 2, 5 and 8 vol.% of graphite
synthesized by the semi-solid powder densification (SSPD) method. Result
of wear test showed that hardness and fracture energy decreased with increase
of graphite. Seizure occurred for aluminium alloy, but no seizure occurred for
Al/SiC and Al/SiC/Gr composites. Friction coefficient decreased as the
percentage of graphite addition increases. Amount of the graphite released
during the wear increases as the percentage of graphite addition increases.
Wear behaviour becomes more stable as the amount of graphite addition
increases.
Yunxin Wu (1997) observed that the friction and wear behavior of
Ni/MoS2 self-lubricating composites changed with the formation of a surface
lubricating film. The integrity of the lubricating film can act as a criterion of
the self-lubricating property of the composite. When homogeneous and
continuous lubricating film formed on the whole frictional surface, the
mechanical property of the composite did not decline enough to cause
remarkable decrease in the film's self lubrication. The optimum MoS2
39
concentration of Ni/MoS2 composite was determined as 60% under the test
conditions.
2.8 ABRASIVE WEAR
In this section literature survey related to abrasive wear of
aluminium-based MMCs is discussed.
Singh et al. (2002) observed that abrasive wear rate of the
composite and the matrix alloy increased with increase in applied load and
abrasive size in Al-Si12Fe /SiC composites. Wear resistance of the composite
was superior to that of matrix alloy for finer size abrasives, whereas the trend
reversed for coarser size abrasives. Wear rate also decreased with increase in
sliding distance for composites due to work hardening of wear surface,
clogging, attrition and shelling of abrasive particles.
According to Das et al. (2007) abrasive wear resistance properties
of Al–4.5 wt% Cu alloy improved significantly by the addition of alumina
and zircon particles. Decrease in particle size improves wear resistance
property for both alumina and zircon reinforced composites since smaller
particle reinforced composite has higher hardness and is more efficient in
blunting the abrading surface. Zircon reinforced composite shows better wear
resistance than alumina reinforced composite due to its superior particle–
matrix bonding.
Das et al. (2008) observed that the wear rate of Al-Si12Cu / SiC
composite is less than that of the alloy and it decreased with increasing in SiC
content. Further, the wear rate of the composite increased with increasing size
of reinforcement. The composite suffered more wear than the alloy if the
abrasive size is higher than that of reinforcement size. The effect of abrasive
size was found to be insignificant when the abrasive size was less than 60 m.
40
The wear rate however increased almost linearly with applied load. Addition
of SiC particle as well as heat treatment provided considerable improvement
in wear resistance.
Izciler and Muratoglu (2003) concluded that the abrasive wear rates
of the Al2124/SiC composites increased with increasing applied load. Wear
rate of the composites tested with SiC abrasive particles showed higher value
than that of the composites abraded by Al2O3 abrasive particles. The Al2O3
abrasive particles showed less effect on specimen surface of composites than
the SiC abrasive particles due to their relatively higher hardness.
Kok and Ozdin (2007) observed that Al2O3 reinforcement
significantly improved the abrasion wear resistance in the Al 2024 /alumina
composites tested against different abrasives. The wear resistance of the
composites was much higher than that of the unreinforced aluminium alloy.
Wear volume loss of the matrix alloy and the composites almost increased
linearly with increasing the sliding distance. The wear resistance of the
composites increased with both increase in the Al2O3 particle content as well
as size.
Modi et al. (2001) observed that Al 2104 / alumina composites
experienced lower material loss than the matrix alloy for all the test
conditions employed. This lower material loss due to the work hardening of
the matrix as well as the protrusion of the dispersoid phase. Increase in the
extent of protrusion of the dispersoid phase with sliding distance offered a
higher degree of protection to the matrix. The composites exhibited
maximum wear loss with silicon carbide abrasive medium while, minimum in
the case of zircon and intermediate in case of sand.
Muratoglu and Aksoy (2006) observed the abrasive wear behaviour
of both as cast and aged 2124Al/SiC composites in the temperature range
41
20–200oC. Wear test results obtained showed that the weight loss of the aged
specimens was less than that of the non-aged specimens. It was also observed
that the better wear resistance was seen for specimens tested at room
temperature for both aged and non-aged specimens. There was little change in
wear rate above 50oC for both the aged and non-aged specimens. The contact
between SiC particles in the composite material and abrasive paper, resulted
in broken or loosened hard SiC abrasives getting to the soft layer under worn
surface at temperature between 50–200oC. This caused an increase in strength
of surface of the composite specimens and hence resulted little change in wear
rate.
Sawla and Das (2004) observed superior abrasive wear resistance
in case of Al-Si12Cu/SiC heat-treated composite over the cast composite. It
was also noticed that the wear surface and subsurface deformation of heat-
treated composite showed less damage, reduced crack propagation and lower
depth of deformation as compared to cast composite. This was attributed to
the combined effects of the reinforcement of SiC and heat treatment, which
resulted in an improvement in hardness and wear resistance of composite.
Yilmaz and Buytoz (2001) observed that wear rate of Al-0.86Mg-
0.46Si-0.15Cu/Alumina composites 80 grade SiC abrasive paper increased
than 220 grades SiC abrasive paper. Aluminium composites with lager
alumina size are more effective against abrasive wear than those with smaller
alumina size. In addition particulate size and volume fraction decreased the
effect of aging treatments. The aging treatment on aluminium based MMC is
more effective than graphite addition in improving the wear resistance.
2.9 STATISTICAL ANALYSIS
Based on the available literature on the research work carried out
on dry sliding wear and abrasive wear analysis of wear behaviour can be
42
classified under three categories; (i) experimental investigations (ii) wear
model developments and (iii) numerical simulations. It is obvious from the
literature review that most literature focus on the first category while the
second and third categories have received much less attention, probably due
to the increased complexity in developing wear models and/or numerical
simulations. Statistical methods have commonly been used for analysis,
prediction and/or optimization of a number of engineering processes. Such
methods enable the user to define and study the effect of every single
condition possible in an experiment where numerous factors are involved. A
review of the statistical methods have used for studying the tribological
behaviour is discussed in this section.
Davim (2000) studied the tribological behaviour of the brass/steel
pair using Taguchi method. The result showed that temperature factor and
also the velocity/load and load/temperature interactions have a great influence
on the coefficient of friction. The wear was highly influenced by the load
factor and only to a smaller extent by temperature.
Mondal et al. (1998) developed a factorial design of experiment to
describe the high stress abrasive wear behaviour of Al-Si10Cu2Fe /10%
alumina composites and derived empirical linear regression equations for
predicting wear rate within a selected experimental domain. These equations
qualitatively hold good for alloys and composites and explain relative
influences of the individual variables such as load and abrasive size on the
wear resistance of these materials. The effect of load and abrasive size on the
wear rate is relatively more in case of composite material than that of the
alloy
Basavarajappa and Chandramohan (2006) proposed a Taguchi
design of experiments to describe the dry sliding wear behavior of 2219 Al /
15% SiC and Al / 15% SiC / 3% Graphite hybrid composites. Empirical linear
43
regression equations were developed for predicting the wear volume loss for a
given set of experimental conditions. These equations illustrated that the SiC-
Gr composite exhibited higher wear resistance compared to SiC-reinforced
composites. Wear volume loss decreased with an increase in the sliding speed
for both of the composites, but increased with an increase in applied load and
sliding distance. The interaction effect between load and sliding speed was
predominant in Al/SiC/Gr reinforced composites with reference to sliding
distance.
Sahin and Özdin (2008) employed a factorial design of an
experiments to develop linear equations for predicting wear rate of Al/SiC
composites. The established equations demonstrated that 10 and 15 wt.% SiC
composite exhibited higher wear resistance than that of the Al2011 matrix
alloy. The wear rate of the matrix and composites increased with increasing
abrasive size, applied load and decreased with sliding distance. Among
various parameters, abrasive size was found to be more significant for
composite, and followed by load. For the matrix alloy, however, the applied
load was dominant, followed by the abrasive size. The interaction of load and
abrasive size was found to be more significant for both alloy matrix and its
composite.
Kok (2010) used Taguchi method to investigate the abrasive wear
behaviour of Al2O3 particle reinforced 2024 aluminium alloy cast composites
under different testing conditions. The results indicated that reinforcement
size was found to be the most influencing factor on abrasive wear, followed
by abrasive grain size. The wear rate of the composites increased with
increasing abrasive grain size and applied load while it decreased with
increasing reinforcement size and sliding distance.
Kumar and Balasubramanian (2010) developed a mathematical
model to predict the abrasive wear rate of AA7075 aluminium alloy matrix
44
composites reinforced with SiC particles. The model was developed using
Response Surface Method (RSM). The effect of volume percentage of
reinforcement, reinforcement size, applied load, sliding speed and abrasive
size on abrasive wear behaviour was analysed and it was inferred that the size
of abrasive exerted the greatest effect on abrasive wear.
Suresha and Sridhara (2010) investigates the dry sliding wear
behaviour of Al-Si7Mg matrix composites reinforced with graphite and 10%
SiC particulate. Using Central Composite Design (CCD), they studied the
effect of percentage of reinforcement, load, sliding speed and sliding distance
on stir cast Al / Gr, Al/SiC composites and Al/SiC/Gr hybrid composites,.
The result shows that hybrid composites exhibit better wear characteristics.
Increased speed reduces wear, while increase of either load or sliding distance
or both increases wear.
Sahoo (2007) studied the tribological performance of electroless
Ni–P coatings and optimization of tribological test parameters based on the
Taguchi method coupled with grey relational analysis. A grey relational grade
obtained from the grey relational analysis was used as performance index to
study the behaviour of electroless Ni–P coating with respect to friction and
wear characteristics. Grey relational analysis was done to find optimum test
parameter combination that yielded minimum friction and wear characteristics.
2.10 SUMMARY OF LITERATURE SURVEY
A detailed literature survey was undertaken on aluminium metal
matrix composites with emphasis on matrix, reinforcements, process,
characterization and correlation between the microstructure and the properties
like hardness, tensile strength and in particular dry sliding wear and abrasive
wear properties as well as statistical modelling studies. Based on the literature
survey, the following areas have been identified for further research.
45
Al-Si10Mg alloy is not yet tried as matrix material. This alloy is
easy to fabricate by means of stir casting process due to it is better casting
qualities. In particle reinforced composites, the properties of the MMCs was
observed to depend on reinforcement type, reinforcement particle size, nature
of interface, volume fraction of reinforcement. Alumina is a low cost and
easily available and good interface with aluminium matrix compare to silicon
carbide, so the alumina is selected as reinforcement. Work on Al-Si10Mg/
Alumina and Al-Si10Mg/Alumina/Molybdenum disulphide is also limited.
Multi-component composites consisting of a matrix phase
reinforced with different reinforcement are termed as hybrid composites.
Behaviour of hybrid composites under dry sliding and abrasive wear
behaviour is another open-ended area in which a lot of meaningful research
can be done. However, literature survey suggests that by incorporating soft
reinforcement into the matrix of particle reinforced composites, better results
may be achieved in the tribological properties. Literature survey also
suggested that Aluminium/Alumina/Molybdenum disulfide hybrid composites
not been attempted so far. Thus the priority of this work will be to prepare
Al-Si10Mg / Alumina MMC and Al-Si10Mg / Alumina / Molybdenum
disulfide hybrid composites and study the microstructure and properties with
special attention on tribological behaviour.
For production of metal matrix composite and each process has its
own merits and demerits. In particular, some are more expensive than others.
The manufacturer generally prefers the lowest cost route. Therefore,
stir-casting technique represents a substantial proportion of the MMCs in
commercial sectors today. Dry sliding and abrasive wear behaviour is a
complex wear phenomenon in which a number of control factors collectively
determine the performance output i.e. the wear rate and coefficient of friction
and mechanical properties; there is enormous scope in it for implementation
46
of appropriate statistical techniques for process optimization. However, not
many such studies have been reported so far. The present work aims to
address this aspect by adopting a systematic statistical approach (Taguchi
method) to optimize the process parameters and materials parameters leading
to minimum wear of the aluminium metal matrix composites and hybrid
composites chosen for the present investigation.
2.11 AIM OF THE PRESENT WORK
The following objectives were identified for present work, after
extensive literature survey:
1. To produce the following by stir casting method :
a. Al-Si10Mg Matrix alloy
b. Al-Si10Mg / 5wt.% Alumina and 10 wt.% Alumina
(hereafter referred to as Al2O3)
c. Al-Si10Mg / 5wt.% Alumina with 2 wt.% and 4 wt.%
Molybdenum disulphide (hereafter referred to as MoS2)
d. Al-Si10Mg / 10 wt.% Alumina with 2 wt.% and 4 wt.%
Molybdenum disulphide
2. To study the microstructure, density, hardness, and tensile
strength of the above composites.
3. To evaluate the effect of various parameters in dry sliding
wear behaviour of the above composites using Grey Relational
Analysis (GRA) and Taguchi Method.
4. To evaluate the effect of various parameters on abrasive wear
behaviour of hybrid metal matrix composites using Grey
Relational Analysis (GRA) and Taguchi Method.
47
5. To quantify the percentage of influence of dry sliding and
abrasive wear parameters using Analysis of Variance (ANOVA).
6. To evaluate the morphology of the worn surface of the
composites and provide qualitative description of wear
mechanism using Scanning Electron Microscope (SEM).
A flow chart showing the experimental procedure is shown in
Figure 2.3.
Figure 2.3 Experimental Flowchart
Literature Review
Selection of matrix alloy, reinforcements
and manufacturing method
Microstructure, density, hardness and
tensile strength studies
Fabrication of Composites
Dry sliding wear and
Abrasive wear studies on
the AMMCs and HMMCs
Identification of the input parameters and
response variables for dry and abrasive
wear tests
Selection of orthogonal array of
experiment and conducting experiments
and levels
Effect of parameters on response
variables
Linear Data Pre-processing
Calculating Grey relational
coefficient and grades
Analysis of Variance
Identify the optimum dry siding / abrasive
wear parameters
Prediction of optimum value
Confirmation experiment
Taguchi Method
Grey Relational Analysis
Wear surface study using SEM analysis
Conclusions