15

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).

16

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)

17

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

18

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)

19

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.

20

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)

21

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.

22

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

23

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.

24

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

25

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

26

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.

27

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

28

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

29

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

30

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.

31

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

32

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))

33

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))

34

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