Development of Cu-Based Metal Matrix Composites Using

Development of Cu-Based Metal Matrix Composites Using
Development of Cu-Based Metal Matrix Composites Using
Silicon Carbide, E-Glass Fiber and Multiwalled Carbon Nanotubes
as Reinforcement
A Thesis submitted in partial fulfilment of the
requirements for the degree of
Master of Technology (Research)
By
Harshpreet Singh
(Roll Number-612MM3006)
under the supervision of
Dr. S. N. Alam
Department of Metallurgical and Materials Engineering
National Institute of Technology Rourkela
Rourkela, Pin-769008
Odisha, India
2015
Department of Metallurgical and Materials Engineering
National Institute of Technology Rourkela
Rourkela-769008, Odisha, India
CERTIFICATE
This is to certify that the thesis entitled “Development of Cu-Based Metal Matrix
Composites Using Silicon Carbide, E-Glass Fiber and Multiwalled Carbon Nanotubes
as Reinforcement” being submitted by Mr. Harshpreet Singh to the National Institute of
Technology Rourkela, for the award of the degree of Masters of Technology (Research) is a
record of bonafide research work carried out under my supervision and guidance. The results
presented in this thesis have not been submitted elsewhere for the award of any other degree
or diploma. This work in my opinion has reached the standard of fulfilling the requirements
for the award of the degree of Masters of Technology (Research) in accordance with the
regulations of institute.
Date:
------------------------------Dr. S. N. Alam
(Supervisor)
i
Acknowledgement
It is a privilege for me to express my profound gratitude and indebtedness to my supervisor
Dr. S. N. Alam, Metallurgical & Materials Engineering Department, National Institute of
Technology Rourkela. Without his efforts and guidance this work could not have been
possible. He has guided me at all stages during this research work. I will cherish all the
moments of enlightenment he has shared with me.
I would like to convey my sincere gratitude to Prof. S.C. Mishra, Head of the Department,
Metallurgical and Materials Engineering Department, National Institute of Technology
Rourkela, for constant guidance and encouragement. I also gratefully acknowledge the
support of Prof. B.C. Ray who motivated me and provided valuable suggestions during my
research work. I would also like to express my sincere thanks to Prof. B. B. Verma and Prof.
S. Sen for constantly encouraging me and helping me understand my research problem. I am
very much thankful to Dr. A. Basu for helping me in understanding my research problem and
giving valuable suggestions. I would also take the pleasure of thanking all my master scrutiny
committee members, Dr. D. Chaira, Dr. S. K. Sahoo and Dr. M. Masanta for assessing my
research work and providing me valuable suggestions throughout the work. I would also like
to thank Prof. Mushahid Husain, Centre for Nanoscience and Nanotechnology, Jamia Millia
Islamia, for helping me in the synthesis of multiwalled carbon nanotubes used in the
development of the composites.
I am also thankful to Mr. Rajesh Patnaik, Mr. U. K. Sahoo, Mr. S. Chakraborty, Mr. S.
Pradhan, Mr. S. Hembram and Mr.Arindam Pal of NIT Rourkela for their technical guidance
in conducting various experimental studies during the research work.
I am also thankful to my friends Lailesh Kumar, Deepanshu Verma and Deepankar Panda for
their help and support during my research work.
I am grateful to my brother Mr. Ishpreet Singh for his love, affection and understanding. He
has provided constant support thought my period of study. Special thanks to my parents for
motivating me and assisting me. Without their help and encouragement it would not have
been possible for me to undertake this work. I would like to thank all my friends for making
my stay at NIT Rourkela lively and without their help this work would not have been
possible.
Harshpreet Singh
Date:
ii
CONTENTS
Certificate
i
Acknowledgement
ii
Contents
iii
List of Figures
v
List of Tables
x
Abstract
xi
Chapter 1 Introduction
1
1.1Motivation and Background of the Present Investigation
2
1.2 Cu-Based Metal Matrix Composites
6
1.3 Scope and Objective of the Work
7
1.4 Thesis Outline
8
Chapter 2 Literature Review
10
2.1 Composites
11
2.2 Metal Matrix Composites
12
2.3 Cu-Based Metal matrix Composites
14
2.3.1 Cu-SiCp Composite
15
2.3.2 Cu-E-Glass fiber Composite
18
2.3.3 Cu-Multiwalled Carbon Nanotubes Composites
19
2.4 Processing Techniques for Metal Matrix Composites
2.4.1 Liquid State processing Techniques
22
22
2.4.1.1 Infiltration Process
23
2.4.1.2 Dispersion Process
23
2.4.1.3 Spray Process
24
2.4.1.4 In-Situ Process
24
2.4.2 Solid State processing Techniques
25
2.4.2.1 Diffusion Bonding
25
2.4.2.2 Powder Metallurgy
26
2.4.2.3 Mechanical Alloying
28
iii
Chapter 3 Experimental Details
32
3.1 Equipment used in the Present Investigation
34
3.2 Selection, Synthesis and Characterization of Raw Materials
44
i. Cu Powder
44
ii. SiCParticulates
45
iii. E-Glass Fiber
45
iv. Synthesis of Multiwalled Carbon Nanotubes (MWCNTs)
46
3.3 Synthesis and Characterization of Cu-Based Metal Matrix Composites
Chapter 4 Results and Discussions
48
50
4.1 Mechanical Milling of Cu
51
4.2 Cu-SiCp Composite
60
4.3 Cu-E Glass fiber Composite
72
4.4 Cu-Multiwalled CNT Composite
84
Chapter 5 Conclusions
94
References
Publications/Conferences
Bio Data
iv
List of Figures
Figure No.
Figure Description
Page No.
Chapter 1 Introduction
Fig.1.1 (a,b)
Profile for various materials
3
Fig.1.2
Types of Composites
4
Chapter 2 Literature Review
Fig.2.1
12
Fig.2.2
Relationship between the classes of materials showing
the evolution of composites
Different type of reinforcements in matrix
Fig.2.3
Cost band for different fibers
18
Fig.2.4
21
Fig.2.8(a)
Graph showing number of publications in different
years of CNT reinforced composites
Graph showing number of publications in different
years of CNT reinforced MMCs
Systematic setup for the pressure less liquid metal
infiltration
Simplified flowchart illustrating the sequence of
operations in powder metallurgy process
Schematic of uniaxial cold compaction
Fig.2.8(b)
Sintering mechanism using solid state diffusion process
27
Fig.2.9
Applications and advantages of Powder Metallurgy
28
Fig.2.10
Ball-Powder-Ball collision during high energy ball
milling
Schematic view of motion of the ball and powder
mixture
30
Fig.2.5
Fig.2.6
Fig.2.7
Fig.2.11
12
21
23
27
27
31
Chapter 3 Experimental Details
Fig.3.1
Work plan for present Investigation
33
Fig.3.2(a)
Planetary Ball Mill
35
Fig.3.2(b)
Schematic view of mechanism of ball milling
35
Fig.3.3(a)
Schematic Diagram of Uniaxial Pressing
35
Fig.3.3(b)
Uniaxial Hydraulic Press
35
Fig.3.4 (a)
Philip’s X'pert Pro high resolution x-ray diffractometer
36
Fig.3.4 (b)
Schematic diagram of the diffractometer
36
Fig.3.5(a)
Tubular furnace
37
Fig.3.5(b)
Schematic diagram of the tubular furnace
37
v
Fig.3.6
Instron 1195
38
Fig.3.7 (a)
Scanning Electron Microscopy
39
Fig. 3.7 (b)
Field Emission Scanning Electron Microscopy
39
Fig.3.8
High Resolution Transmission Electron Microscopy
40
Fig.3.9(a)
Experimental Setup for Density measurement
41
Fig.3.9(b)
Density Measurement Kit
41
Fig. 3.10 (a, b)
41
Fig.3.11(a, b)
Schematic figure of the Vickers pyramid diamond
indentation
Vickers Microhardness Tester
Fig.3.12
Ball-on-plate wear tester
43
Fig.3.13
Fourier Transform Infrared (FTIR) Spectroscopy
44
Fig.3.14 (a, b)
42
Schematic of a typical CVD furnace setup used for the
synthesis of carbon nanotubes
Chapter 4 Results and Discussion
47
Fig.4.1(a-b)
52
Fig.4.1(c)
X -ray diffraction plots of Cu milled for various periods
of time
Variation of crystallite size with milling time
Fig.4.1(d)
Variation of r.m.s. strain with milling time
52
Fig.4.1(e)
Variation of lattice parameter with milling time
52
Fig.4.2(a)
SEM images of unmilled Cu powder
55
Fig.4.2(b)
SEM images of milled Cu-powder after 5 h of milling
55
Fig.4.2(c)
SEM images of milled Cu-powder after 10 h of milling
55
Fig.4.2(d)
SEM images of milled Cu-powder after 15 h of milling
55
Fig.4.2(e)
SEM images of milled Cu-powder after20 h of milling
55
Fig.4.3(a-c)
HRTEM images of 20 h milled Cu
56
Fig.4.3(d)
SAD pattern of 20 h milled Cu
56
Fig.4.4(a)
Particle size analysis of unmilled Cu powder
57
Fig.4.4(b)
Particle size analysis of 20 h milled Cu powder
57
Fig.4.5(a)
58
Fig.4.5(c)
X-ray analysis of 20 h milled Cu at different
temperatures
Variation of Crystallite size with variation
temperature
Variation of strain with variation in temperature
Fig.4.5(d)
Variation of lattice parameter at different temperatures
58
Fig.4.6(a)
SEM image and EDX analysis of 20 h Cu heat treated at
200oC for 2 h
59
Fig.4.5(b)
vi
in
52
58
58
Fig.4.6(b)
SEM image and EDX analysis of 20h Cu heat treated at
400oC for 2 h
Fig.4.6(c)
SEM and EDX analysis of 20h Cu heat treated at 600oC
for 2 h
Fig. 4.7(a)
HRTEM image of 20 h milled Cu powder heat treated at
200oC for 2 h
Fig. 4.7(b)
SAD pattern of 20 h milled Cu powder heat treated at
200oC for 2 h
Cu-SiCp Composites
59
Fig.4.8(a)
61
Fig.4.9(a)
Optical micrographs of unmilled Cu-10 vol. % SiCp
composites
Optical micrographs of unmilled Cu-20 vol. % SiCp
composites
Optical micrographs of unmilled Cu-30 vol. % SiCp
composites
Optical micrographs of unmilled Cu-40 vol. % SiCp
composites
SEM images of unmilled Cu-10 vol. % SiCp composites
Fig.4.9(b)
SEM images of unmilledCu-20 vol. % SiCp composites
62
Fig.4.9(c)
SEM images of unmilled Cu-30 vol. % SiCp composites
62
Fig.4.9(d)
SEM images of unmilled Cu-40 vol. % SiCp composites
62
Fig.4.10(a)
EDX analysis of unmilled Cu-40 vol. % SiCp composite
at SiCp rich region
EDX analysis unmilled Cu-40 vol. % SiCp composite at
Cu rich region
XRD plots of various unmilled Cu-SiCp composites
62
Variation of Relative Density of unmilled Cu-SiCp
composites
Variation of microhardness of as-milled and unmilled
Cu-SiCp composites for different volume percent of
SiCp
Wear Characteristic of unmilled Cu and various
unmilled Cu- SiCp composites
SEM images of wear track of unmilled Cu
63
SEM images of wear track of unmilled Cu-40 vol.%
SiCp composite
High magnification SEM images of the wear track of
unmilled Cu-40 vol. % SiCp composite
Optical micrographs of as-milled Cu- 10 vol. % SiCp
composites
Optical micrographs of as-milled Cu- 20 vol. % SiCp
composites
Optical micrographs of as-milled Cu- 30 vol. % SiCp
composites
64
Optical micrographs of as-milled Cu- 40 vol. % SiCp
67
Fig.4.8(b)
Fig.4.8(c)
Fig.4.8(d)
Fig.4.10(b)
Fig.4.11
Fig.4.12(a)
Fig.4.12(b)
Fig.4.13(a)
Fig.4.13(b)
Fig.4.13(c)
Fig.4.13(d,e)
Fig.4.14(a)
Fig.4.14(b)
Fig.4.14(c)
Fig.4.14(d)
vii
59
60
60
61
61
61
62
62
63
63
64
64
65
67
67
67
Fig.4.15(a)
composites.
Higher magnification optical micrographs of as-milled
Cu-40 vol. % SiCp composites
SEM image of as-milled Cu- 10 vol. % SiCp composites
Fig.4.15(b)
SEM image of as-milled Cu-20 vol. % SiCp composites
68
Fig.4.15(c)
SEM image of as-milled Cu-30 vol. % SiCp composites
68
Fig.4.15(d)
SEM image of as-milled Cu-40 vol. % SiCp composites
68
Fig.4.15(e,f)
68
Fig.4.16
Higher magnification SEM images of as-milled Cu-40
vol.% SiCp composites
XRD plots of as-milled Cu-SiCp composites
Fig.4.17
Density plots of as-milled Cu-SiCp composites
69
Fig.4.18(a)
Wear Characteristic of as-milled Cu-SiCp composites
70
Fig.4.18(b)
SEM images of the wear track of as-milled Cu
70
Fig.4.18(c)
SEM images of the wear track as-milled Cu-40 vol.
%SiCp composite
High magnification SEM images of the wear track of asmilled Cu-40 vol. %SiCp sample
SEM images of wear debris from as-milled Cu-40 vol.
%SiCp composite
EDX analysis of the wear debris
70
Fig.4.14(e,f)
Fig.4.18(d, e)
Fig. 4.19(a-b)
Fig. 4.19(c)
Fig. 4.19(d)
67
68
69
70
72
72
SEM image of the wear debris selected for elemental
mapping.
Cu-E-Glass Fiber Composites
72
Fig.4.20(a-b)
Optical images of E-glass fiber used in composite
73
Fig 4.21 (a-c)
SEM image of E-glass fiber used in composite
73
Fig. 4.22
Load vs Displacement plot of E-glass fiber
74
Fig.4.23.1(a-b)
SEM image of Cu-10 vol. % E-glass fiber composite
sintered at 900oC for 1h
SEM image of Cu-20 vol. % E-glass fiber composite
sintered at 900oC for 1h
SEM image of Cu-30 vol. % E-glass fiber composite
sintered at 900oC for 1h
SEM image of Cu-40 vol. % E –glass fiber composite
sintered at 900oC for 1h
EDX analyses ofunmilledCu-40vol. % composite sintered
at 900oC for 1h
Relative Density plot of various sintered unmilled Cu-Eglass fiber composite
Vickers hardness plot of various unmilled Cu-E-glass
fiber composite
SEM images of the fracture surface of unmilled Cu- 10
vol. % E-glass fiber composite
74
Fig.4.23.2(a-b)
Fig.4.23.3(a-b)
Fig.4.23.4(a-b)
Fig 4.24 (a-b)
Fig 4.25 (a)
Fig 4.25 (b)
Fig. 4.26.1(a-b)
viii
75
75
75
76
77
77
78
Fig. 4.26.2(a-b)
Fig. 4.26.3(a-b)
Fig. 4.26.4(a-b)
Fig.4.27(a-d)
Fig.4.28 (a-d)
Fig 4.29 (a)
Fig 4.29 (b)
Fig.4.30.1(a-b)
Fig.4.30.2(a-b)
Fig.4.30.3(a-b)
Fig.4.30.4(a-b)
SEM images of the fracture surface of unmilled Cu- 20
vol. % E-glass fiber composite
SEM images of the fracture surface of unmilled Cu- 30
vol. % E-glass fiber composite
SEM images of the fracture surface of unmilled Cu- 40
vol. % E-glass fiber composite
Optical micrographs of as-milled Cu-E-glass fiber
composites for different vol. % of fiber used as
reinforcement (10, 20, 30 and 40 vol. %).
FESEM images of as- milled Cu-E-glass fiber
composites for different vol. % of fiber used as
reinforcement (10, 20, 30 and 40 vol. %)
Variation of relative density of various sintered asmilled Cu-E-glass fiber composites
Variation of Vickers hardness of various as-milled CuE-glass fiber composites
SEM image of fracture surface of as-milled Cu-10 vol.
% E- glass fiber composite
SEM image of fracture surface of as-milled Cu- 20 vol.
% E- glass fiber composite
SEM image of fracture surface of as-milled Cu-30 vol.
% E- glass fiber composite
SEM image of fracture surface of as-milled Cu-40 vol.
% E- glass fiber composite
78
79
79
80
81
81
81
82
83
83
84
Cu-MWCNT Composites
Fig.4.31
XRD of the MWCNTs synthesized by LPCVD process
86
Fig.4.32 (a-c)
FESEM images of MWCNTs synthesized by LPCVD
process
HRTEM images of MWCNTs synthesized by LPCVD
process
SAD pattern of MWCNTs synthesized by LPCVD
process
FTIR analysis of CNTs after acidic functionalization
87
89
Fig.4.36(a-c)
Optical micrographs of developed Cu-MWCNTs
composites
SEM images of developed Cu-MWCNTs composites
Fig.4.37
EDX analysis for Cu- 1 vol. % MWCNT composite
90
Fig.4.38
Elemental mapping for Cu-5 vol. % MWCNT
composite
X-ray diffraction plots of various Cu-MWCNT
composites containing different vol. % of CNTs
Variation of density of Cu- MWCNT composite with
different vol.% of CNT
Variation of microhardness of Cu- MWCNT composite
with different vol.% of CNT
Wear Characteristic of Cu- MWCNT composites
90
Fig.4.33 (a-c)
Fig.4.33 (d)
Fig. 4.34
Fig.4.35(a-c)
Fig.4.39
Fig.4.40
Fig.4.41
Fig. 4.42 (a)
ix
87
87
88
89
91
92
92
93
Fig. 4.42 (b)
Fig. 4.42 (c)
Fig. 4.42 (d)
FESEM images of the wear track of Cu-1vol.%
MWCNT composite
FESEM images of the wear track of Cu-2vol.%
MWCNT composite
FESEM images of the wear track of Cu-5vol.%
MWCNT composite
93
93
93
List of Tables
Figure No.
Table Description
Page No.
Table 2.1
Properties for different fibers
18
Table 3.1
Properties of E-glass fiber
46
Table 3.2
Properties of MWCNTs
47
Table 4.1
Particle size analysis of pure Cu at different milling
time
Results obtained from the tensile test of E-glass fiber
57
Table 4.2
x
74
Abstract
Metal matrix composites (MMCs) combine the ductility of metal and the toughness of the
reinforcement which makes it an excellent candidate material for advanced engineering
applications. The unique features of MMCs like high strength to weight ratio and high
stiffness per unit density results in improvement of the service performance. The decrease in
structural weight, increase in creep strength, high fatigue strength, high thermal stability,
enhancement in wear resistance and electrical conductivity, further makes it a potential
engineering material. Cu has been extensively used as a matrix due to its superior thermal and
electrical properties. However Cu has inadequate mechanical properties from the structural
application point of view. Incorporation of ceramic particles like oxides or carbides in Cu
would strengthen the matrix. Here in our study three reinforcements, SiC particles, E-glass
fibers and multiwalled carbon nanotubes (MWCNTs), which are very different in nature and
morphology have been used for developing Cu-based MMCs.
Over the last several decades, there has been considerable interest in the use of Cu-based
MMCs. However very limited literature is available on Cu-based metal matrix composites.
For many applications pure Cu cannot be used because of its low strength and reinforcing Cu
with ceramics or fibers is a viable option to overcome this limitation. Here Cu-based metal
matrix composites have been developed using SiCp, E-glass fiber and multiwalled carbon
nanotube (MWCNT) as reinforcements by powder metallurgy route. A systematic study of
the various mechanical properties of the composites developed was done. The hardness and
wear properties of the various composites were determined. The fracture surface of the
various composites was analyzed and the density of the composites was also determined.
Here in our study both the as-received Cu powder and 20 h milled nanostructured Cu powder
was used as the matrix for the composites. The as-received Cu powder was milled for 20 h in
a high energy planetary ball mill in order to form nanostructured Cu which was later used for
xi
the development of as-milled Cu-based metal matrix composites. The 20 h milled Cu powder
doesn’t show any contamination during milling from the milling media. The variation of the
crystallite size, strain and lattice parameter of Cu were found out from the x-ray analysis of
the milled powder after different intervals of milling time. Both x-ray diffraction analysis and
HRTEM images of the 20 h milled Cu powder confirmed that the size of the Cu crystallites is
less than 25 nm. Cu-E-glass fiber and Cu-SiCp composites with reinforcement contents of 10,
20, 30 and 40 vol. % were developed by powder metallurgy route. Multiwalled carbon
nanotubes (MWCNTs) were developed by using low pressure chemical vapour deposition
(LPCVD) process. FTIR offers a quantitative and qualitative analysis for organic and
inorganic samples which identifies chemical bonds in a molecule by producing an infrared
absorption spectrum. FTIR of the functionalized MWCNTs was done in order to determine
the different functional groups after acid-modification of the multiwalled carbon nanotubes.
The FTIR analysis shows stretching vibrations from carboxyl (C=O) and hydroxyl (-OH)
groups. Skeletal vibrations from unoxidized graphitic domains of the carbon nanotube
backbone were also observed. MWCNTs were added to the Cu matrix to develop Cu-1, 2 and
5 vol. % MWCNT nanocomposites. The composites were developed by uniaxial cold
compaction under a load of 665 MPa followed by sintering at 900oC for 1 h in Ar
atmosphere. The microstructure of the composites was analysed using an optical microscope,
scanning electron microscope (SEM), field emission scanning electron microscope (FESEM),
energy dispersive x-ray spectroscopy (EDX) and high resolution transmission electron
microscope (HRTEM). X-ray diffraction of the various composites was done in order to
determine the different phases in the sintered composites. Density of the composites was
determined using the Archimedes’ principle. Hardness was determined using a Vickers
microhardness tester. Wear properties of the various composites was analysed using a ballon-plate tribometer. Fractographic analysis of the various composites fractured in impact test
xii
was done in order to study the fracture behaviour of the samples. The results show that the
reinforcement is homogeneously distributed all over the Cu-matrix and the composites
prepared show good bonding between the Cu-matrix and the reinforcement. Improvement in
hardness and wear properties were observed with increase in the content of reinforcement in
the composites.
Keywords: Cu-based MMC, SiC particle, E-glass fiber, MWCNT, microhardness, wear
xiii
Chapter 1- Introduction
Chapter 1
Introduction
1
Chapter 1- Introduction
Introduction
1.1 Motivation and Background
The hunt for finding a material which can perform under adverse environmental conditions is
never ending since the birth of mankind. This has encouraged researchers to take up
challenges to find new materials having desired properties and applications. This very
concept is responsible for the creation of composite materials. Throughout the history of
mankind there is evidence of the use of composite type materials. One of the earliest manmade composite material was straw and mud combined to form bricks for constructing
houses. A composite material is developed by using two or more materials. In most cases the
two materials have very different properties and together they give the composite very unique
properties which are not found in the individual components. The mud can easily be dried
forming a brick shape to provide a building material. It has high compressive strength but it
breaks while bending due to its poor tensile strength. Straw is economic and readily available.
The straw is a very strong material but it has poor compressibility and can be crushed easily.
However, mixing of mud and straw together can provide the excellent building material that
are resistant to both squeezing and tearing. Another ancient composite material is concrete.
Concrete is a mixture of small stones, cement and sand. It has very good compressive
strength. In current times it has been found that addition of metal rods or wires to the concrete
can increase its tensile strength. Concrete containing such rods or wires are called reinforced
concrete. There are several such examples of composites which have been used by mankind
since early civilization [1-3].
Today composites are used as structural materials for building aircraft and spacecraft.
Composite are preferred for many reasons. They are stronger, lighter and less expensive in
most cases as compared to the traditional materials. In transportation lighter weight of
2
Chapter 1- Introduction
vehicles achieved by using composites leads to fuel saving. Although composites are very
efficient the raw materials used to develop the composites could be expensive. Composites
are manufactured keeping in mind the parameters like shape, durability, stiffness, cost etc.
The most widely used composite materials are fiber or particle reinforced composites having
a matrix of another material. These composites are often used in structural applications. The
matrix of structural composites serves dual purposes. Firstly it binds the reinforcement phase
in its place and secondly it distributes the stresses among the reinforcements under an applied
stress. The reinforcement withstands maximum load and provides us the desirable properties.
In composites a strong bond should exist between the reinforcing materials and the matrix.
The interface plays an essential role in manufacturing of composite materials. The interface is
the area of contact between the reinforcement and the matrix. The main consideration which
should be kept in mind while selection and fabrication of composites is that the constituents
should be chemically inert and non-reactive. Composites have their special place in the world
of materials. They are capable of giving high strength and toughness and have low density
which makes it a useful material for a wide range of application. Fig 1.1(a) shows the
strength vs density and Fig 1.1(b) shows the strength vs toughness of various classes of
materials like metals and alloys, ceramics, glasses, foams, rubbers, polymers etc.
(a)
(b)
Fig.1.1 (a, b) Profile for various materials
3
Chapter 1- Introduction
The major advantages of composite materials are their light weight and high toughness.
Composites are able to meet various design requirements with important weight savings as
well as high strength-to-weight ratio as compared to conventional materials.
Composites
(MMCs, PMCs, CMCs)
Particle- reinforced
Largeparticle
Structural
Fiber- reinforced
Sandwich panels
Dispersion strengthened
Laminates
Discontinuous (short)
Continuous (aligned)
Randomly oriented
Aligned
Fig.1.2 Types of composites
Composite materials are usually categorized by the type of reinforcements that are used in
the composites. The reinforcements are incorporated into the matrix in order to strengthen the
matrix. The different type of reinforcements that are used in composites are particulate
reinforcement, flake reinforcement and fibers. Fibers could be of various types like random
fiber, short fiber and continuous fiber etc. Fig.1.2 shows various types of composites [4,5].
At present metal matrix composites (MMCs) have generated a wide interest because of its
high strength, stiffness and fracture toughness. Beside this they can also resist elevated
temperatures in corrosive atmospheres. In MMCs both the metal and alloys used as matrices
4
Chapter 1- Introduction
and the reinforcement need to be stable over a range of temperature and should be nonreactive. The choice of the reinforcement depends on the matrix material and the application
of the MMC. The strength to weight ratios of resulting composites can be higher than most of
the metals and alloys. Several factors such as melting point, physical and mechanical
properties of the composites at various temperatures determine the service temperature of the
composites.
Metal matrix composites have opened up unlimited possibilities for developing innovative
materials. MMCs can be used as constructional and functional materials. Powder metallurgy
is one of the modern material processing technologies used for the development of MMCs.
The major advantage of composites is their low cost and improved performance. The
property of MMCs is determined by the property of the reinforcement. The reinforcements
can have different objectives as per the desired application of the MMCs. The precondition
here is the improvement of the properties of component. The objectives for development of
metal matrix composites are to improve properties like yield strength, tensile strength, creep
resistance, fatigue strength, thermal shock resistance, Young’s modulus and corrosion
resistance. The reinforcement not only serves a purely structural part of the MMCs but it also
enhances the physical properties of the composites such as wear resistance, friction
coefficient and thermal conductivity. The addition of reinforcements like particles, fibers,
whiskers and wires in composites show significant improvement in mechanical properties.
The reinforcement usually adds rigidity and greatly impedes crack propagation in the
composites. Thin fibers as reinforcements can provide high strength to the matrix and can
greatly improve the composites overall properties. Composite materials can be useful in
several applications and can lead to the evolution curve for modern materials [5,6].The
present applications and market prospects for metal matrix composites are primarily in
military and aerospace industries. MMC components have been developed for use in jet
5
Chapter 1- Introduction
engines, missiles and aircrafts. Particulate-reinforced MMCs are used as covers for missile
guidance systems. The composite piston is capable of giving better wear resistance and high
temperature strength compared to the cast iron piston. Other applications of MMCs include
cutting tools and circuit-breaker contacts. Metal matrix composites combine both metallic
properties such as toughness and ductility of the matrix and the ceramic properties of the
reinforcement such as high melting point and high modulus and strength at elevated
temperatures which enables the use of the MMCs at high temperatures. From the study of the
consumption of composite materials for several applications it can be concluded that MMCs
are not excessively expensive for a cost sensitive application. Metal matrix composites
generally consist of metals and alloys of metals like Cu, Al, Mg or Ti reinforced with ceramic
particulates, whiskers or fibers. The choice of the reinforcement is very important in
determining the cost and mechanical properties of the MMC that is being developed for a
suitable application. MMCs provide advantageous mechanical properties due to the presence
of reinforcement having high modulus and strength. These properties are very important for
any load-bearing structural applications. However, it should be noted that the properties like
fracture toughness and ductility of the metal matrix composites deteriorates as compared to
the monolithic material as the ductility and toughness of most ceramic reinforcements are
very low. Therefore, it is apparent that the matrix alloys having higher ductility and fracture
toughness are desirable for MMC applications [7].
1.2 Cu-Based Metal Matrix Composites
Cu shows high formability, high resistance to oxidation and corrosion and has a special place
among all metals because of its high electrical (5.96×107 S/m) and thermal conductivity
(401 W/m.K). So, the most universal application of Cu is where high electrical and thermal
conductivity are desired. The modulus of Cu is 130 GPa and its yield strength is117 MPa. Its
6
Chapter 1- Introduction
ultimate tensile strength is 210 MPa. There has been considerable interest in academics as
well as industries in the use of Cu-based metal matrix composites in past few decades. Cu is
an outstanding material for electrical applications whose competence can be enhanced by
refining its mechanical properties. Pure Cu cannot be used in several applications due to its
low strength and high ductility. Therefore it has become essential to improve the properties of
pure Cu for its use in cutting-edge technological applications. Cu has high thermal
conductivity and is used as a structural material for cooling. In order to increase its high
temperature properties different reinforcements are being used. Very limited literature is
available on Cu-based metal matrix composites (MMCs). The mechanical strength of copper
can be improved either by age hardening or by particle dispersion strengthening. The agehardenable Cu alloys are prone to precipitate coarsening at high temperatures which results in
the degradation of strength. Matrix strengthening can be done by incorporation of
reinforcement like continuous or discontinuous fibers, whiskers, wires and particulates. Cubased metal matrix composites are used for manufacturing hybrid modules, electronic relays,
electrically conducting springs and other electrical and electronic components [8,9].
Cu-based composites developed by powder metallurgy route have vast applications in
manufacturing of tribological engineering parts such as bearings and bushes. Cu-based
MMCs have applications in the area where good wear resistance without loss of electrical and
thermal conductivity of the matrix is needed. Many applications depend on the surface
property of the product so it is essential to modify the surface of the product by reinforcing
with ceramic particles to achieve desired properties.
1.3 Scope and Objective of the Present Work
Cu-based metal matrix composites by the addition of different reinforcement such as E-glass
fiber, SiC particle and multiwalled carbon nanotubes were developed by powder metallurgy
route. The objectives of the present investigation are:
7
Chapter 1- Introduction
i.
To develop Cu-based metal matrix composites using E-glass fiber, SiC particle and
multiwalled carbon nanotubes by powder metallurgy route. Here in our study both as received Cu and 20 h milled nanostructured Cu as have been used as matrix for the
development of Cu-based metal matrix composites in order to study the effect of
nanostructured Cu on sinterability and densification.
ii.
Synthesis and characterization of multiwalled carbon nanotubes (MWCNTs) using
low pressure chemical vapour deposition (LPCVD) method for use as reinforcement
in Cu-based composites.
iii.
To develop Cu-SiCp, Cu-E-glass fiberand Cu-MWCNTs composites and to study
their microstructure and properties. The various properties like hardness, density and
wear were studied for all the composites developed. Fracture surfaces of the various
composites were also analyzed.
1.4 Thesis Outline
The thesis contains five chapters. The 1st Chapter, ‘Introduction’, attempts to provide an
insight to the work carried out and highlights the background and motivation for the present
work. The 2nd Chapter, ‘Literature Review’, is dedicated to an extensive study of the work
carried out by other investigators in the field. The work carried out by them has been referred
wherever necessary to explain and support the experimental findings. The 3rd Chapter,
‘Experimental Details’, explains the various experimental procedures adopted in the present
investigation. The various instruments and the prescribed experimental norms have been
explained in detail in this chapter. The 4th Chapter, ‘Results & Discussions’, shows the
various results in the form of tables, graphs, optical, SEM and HRTEM images, fractography
etc. The results have been analysed and explained in the present chapter. Finally, on the basis
8
Chapter 1- Introduction
of the experimental findings useful conclusions have been drawn which are listed in the
5th Chapter, ‘Conclusions’.
………………………………………
9
Chapter 3- Experimental Details
Chapter 2
Literature Review
10
Chapter 2- Literature Review
Literature Review
2.1 Composites
A composite can be well-defined as a combination of two or more materials that results in
improved properties as compared to those of the individual components. The two constituents
in a composite are matrix and reinforcement. In contrast to metallic alloys, each material
holds its separate mechanical, chemical and physical properties. The reinforcement is usually
harder, stronger and stiffer than the matrix. The reinforcement phase provides the strength
and stiffness. The main advantages of the composite materials are their high strength and
stiffness, combined with low density. Composites are today used in several industries in order
to develop high-performance products economically.
During 20th century, arrival of the composites as a distinct classification came into existence.
The major advantage of modern composite materials is their strength and light weight.
Selection of appropriate combination of matrix and reinforcement is necessary to develop a
new material that meets the requirement for a specific application. In matrix-based structural
composites, the matrix serves dual purposes. It binds the reinforcement phase in place and
also distributes the stresses between the constituent reinforcement materials under an applied
force. Composite materials comprise some of the most advanced engineering materials today.
Fig.2.1 shows the relation between the classes of materials showing the evolution of
composites. Fig.2.2 shows the different types of reinforcements that can be incorporated
inside the matrix. The reinforcements can be a particulate, short fibers or continuous fibers.
The type of reinforcement and its orientation can alter the properties of the developed
composite [1-4, 10, 11].
11
Chapter 2- Literature Review
Metal filled plastics
(particulate and fiber fill)
Metals and
Alloys
Steels, Al alloys,
Cu and brasses,
Ti etc.
Plastics
Resins,
thermoplastics,
rubbers, foams
etc.
Metal-matrix composites,
Ceramic-matrix composites
Ceramics and
GlassesGlass,
ceramics,
concrete
Fiber reinforced plastics
(including GRP, CFRP etc.)
Fig.2.1 Relationship between the classes of materials showing the evolution of composites
Fig. 2.2 Different type of reinforcements in matrix
2.2 Metal Matrix Composites
Metal matrix composite (MMC) are materials comprised of two different constituents one
being a metal acting as a continuing matrix and the other material being an organic
compound or a ceramic material contributing as reinforcement. Metal matrix composites are
excellent materials for structural applications in automotive and aerospace industries owing
to their high strength and thermal stability. In recent years MMCs have found improved
application due to their excellent properties. The major advantages of the MMCs include
greater strength, low density, improved high temperature properties, low coefficient of
thermal expansion, and resistance to thermal softening, improved abrasion and wear
resistance. When fine ceramic or other hard particles are embedded in the soft metal matrix to
12
Chapter 2- Literature Review
form MMCs the properties of the metal matrix can be substantially improved or strengthened.
The variation in reinforcement particle size and shape alters the overall chemistry and
character of the microstructure and mechanical performance of the composite. The significant
shift in metal matrix composite knowledge began in the middle of 1980’s with replacement of
continuous reinforcement. The low cost composites offers high strength, stiffness and fatigue
resistance with a minimal increase in density over the base alloy. Most metals and alloys
could be used as matrices and they require reinforcement materials which should be stable
over a range of temperature and non-reactive with the matrix. In case of composites
reinforced with hard particles, interfacial bonding between the matrix and particle should be
strong. If the reinforcement is not well bonded to the matrix, the reinforcement elements
cannot contribute to the properties like strength and wear resistance. Since the matrix phases
are generally softer than the reinforcement phases, the extent of debonding of the
reinforcement phase can play a critical role in wear behaviour of the composite. So the choice
of reinforcement must be made judiciously in order to develop composites for a particular
application [12,13]. At present MMCs have found application in many areas of our daily life.
MMCs are being used to replace the conventional materials in numerous applications. They
are used in a wide range of applications like automobiles to sport equipment. MMCs with
high stiffness and strength could be used in applications in which weight reduction is a
dynamic factor. MMCs are used to develop high-speed machineries and high-speed rotating
shafts. Good wear resistance, with high specific strength, favours the use of MMCs in
automotive engine and brake parts. Tailorable coefficient of thermal expansion and thermal
conductivity make them favourable candidates for precision machinery, and electronic
packaging [14].
13
Chapter 2- Literature Review
2.3 Cu-Based Metal Matrix Composites
Cu and its alloys are one of the main groups of profitable metals. Cu is one of the most
significant materials for thermal and electronic applications. They are extensively used
because of their excellent electrical (5.96×107 S/m) and thermal conductivities
(401 W·m−1·K−1) , exceptional resistance to corrosion, ease of fabrication and low cost. The
coefficient of linear thermal expansion (CTE) of Cu (16.6 x 10-6 K-1) is lower than that of Al
(22.2 x 10-6 K-1). Cu-matrix composites are promising applicants for applications in electrical
sliding contacts. Cu-based metal matrix composites are also promising candidates for magnet
design and robotics because of their excellent combination of strength and electrical
conductivity. Cu-based MMCs can also be used to create high performance substrates for
microelectronics packaging. However, it has several other good properties like good
corrosion resistance, high ductility, high toughness etc. All these properties make Cu-based
MMCs a very significant material which has a wide range of applications. Cu has a melting
point of 1083.4°C and its density is 8.96 gm/cc. It’s Young's modulus is 130 GPa. It’s yield
strength is 117MPa while its tensile strength is 210MPa. It’s Poisson's ratio is 0.36. The
mechanical strength of Cu can be enhanced either by age-hardening or by particle dispersion
strengthening. The age-hardenable Cu alloys are subjected to precipitate coarsening at high
temperatures which results in deterioration of its strength. The most general application of Cu
is where high electrical and thermal conductivity are needed. Therefore it has become
essential to improve the properties of pure Cu for its use in cutting-edge technological
applications. There has been substantial interest in academics as well as industries in the use
of Cu-based metal matrix composites in past few decades. Cu has high thermal conductivity
and is used as a structural material for cooling. In order to increase its high temperature
properties diverse reinforcements are used. Cu-matrix composites have a superior
combination of thermal and electrical conductivity as well as high strength. They display
14
Chapter 2- Literature Review
significantly improved mechanical and tribological properties. These exceptional properties
make these composites suitable for sliding electrical contact applications in which high
electrical and thermal conductivity as well as increased wear resistance are necessary [15,16].
Cu composites produced by powder metallurgy route are widely used in tribological parts
like bearing and bushes. Composites based on Cu-Sn alloys can behave as self-lubricating
materials under various conditions such as excessive temperature and load. Cu-based
composites are used in the area where improved wear resistance is required with minimal loss
of thermal and electrical conductivity. The durability of the component in various
applications depends on surface properties. Therefore, it is appropriate to modify the surface
of the component by reinforcing with ceramic particles while the inner matrix remains ductile
and tough. Pure Cu is not used as a bearing material due to its poor mechanical and hardness
pproperties. Today self-lubricated sintered bearings and plastic materials are being used
where continuous lubricating is impossible. Cu-based composites prepared by powder
metallurgy route from Cu, Sn and solid lubricant MoS2 powders are being used for this
purpose [17].Like other metals or alloys Cu and its alloys also soften at high temperature.
This is why reinforcing Cu with ceramic particles or carbon fibers is one of the finest
solutions to overcome this problem. Although Cu has very good thermal and electrical
properties. Very limited literature is available on Cu-based MMCs. Here in this work a very
systematic investigation on Cu-based MMCs using SiCp, E-glass fiber and multiwalled
carbon nanotubes (MWCNTs) as reinforcement was carried out in order to find out the effect
of addition of three different types of reinforcement in the Cu matrix.
2.3.1 Cu-SiCp Composites
Cu shows high formability, high resistance to corrosion and oxidation and is a very good
thermal and electrical conductor. These properties make Cu an excellent candidate for
applications where high thermal and electrical conductivity are desired. The major limitations
15
Chapter 2- Literature Review
of Cu are its low strength and poor wear resistance. In order to improve these properties
discontinuous reinforcements can be incorporated in the Cu matrix. SiCp could be used as a
reinforcement to achieve the desired properties. Cu-SiCp composites combine together the
high ductility and toughness of Cu and the high strength and modulus of SiC reinforcements.
SiC has a melting point of 2730°C and its density is 3.20gm/cc. It’s specific heat is 0.66
J/g.K. It’s elastic modulus is 450 GPa. The hardness of SiC is 20.5 GPa which is comparable
to that of corundum and diamond. SiC has high chemical resistance. It’s coefficient of
thermal expansion is 4.0×10−6/K N and its thermal conductivity is 250 W/mK. It has a
fracture toughness of 2.94 MPa.m1/2. Particulates like SiC behave as an outstanding inclusion
because of their expectable isotropic behaviour in composites. These properties make SiC
particulates a desired reinforcement that can be incorporated in the Cu matrix. With the
incorporation of SiC particulates as reinforcement in the Cu matrix the high-temperature
mechanical properties can be enhanced. The wear resistance of pure Cu can also be improved
with the addition of SiC particulates in the Cu matrix. Particulate-reinforced Cu matrix
composites may have many evident advantages compared to Cu alloys. These kinds of
materials are considered to be favourable candidates for applications where properties like
high conductivity, high mechanical property and good wear resistance are required [18].
Several researchers have reported on Cu-SiCp composites. Yih and Chung [19]have
fabricated Cu composites containing 33-54 vol.% SiC whiskers by hot pressing. The whiskers
were coated with Cu prior to pressing. They reported that the resulting composites display
several good properties such as low porosity, high hardness, low electrical resistivity and
high thermal conductivity. Tjong et al.[20]reported that dry sliding wear of a Cu composite
with 20 vol.% of SiC produced by hot isostatic pressing technique shows better wear
resistance than pure Cu. They have indicated that the SiC particle is the major load bearing
component and also established that delamination is the leading wear mechanism in this
16
Chapter 2- Literature Review
composite. Schubert et al.[21] studied materials that could dissipate the heat generated in
electronic packages and according to their study Cu-SiC composites could be used
successfully for this application. They developed the Cu-SiC composites by powder
metallurgy route. These composites were prepared by pressure-assisted sintering using a hot
press. The authors investigated that enhancement in bonding strength and thermophysical
properties of the composites could be achieved by vapour deposition of Mo on SiC powder.
Dhokey and Paretkar [22] studied the wear mechanism in Cu-20 vol. % SiCp reinforced
composite. They studied the wear behaviour of Cu-SiCp composites in terms of its thermal
and mechanical characteristics. Dimensional equation between the collaborative variables is
stated in their study to relate their effect on the wear parameters of the material. The
composites were fabricated by powder metallurgy route and were sintered in N2 atmosphere.
They have reported results of mechanical, physical and microstructural characterization of the
developed composites. It was found from the tribological studies conducted by them that
there was a reduction in wear rate with increase in sliding speed. Efe et al. [23] studied the
effect of sintering temperature on the properties of developed Cu-SiCp composites. Cu-based
metal matrix composites were developed with different wt. % of SiC particles by powder
metallurgy method. Their study shows that that SiC particles are distributed uniformly in the
Cu matrix. It has been concluded from their study that with the increase in the content of
SiCp the hardness of the composite increases but the relative density of the composite
decreases. The highest electrical conductivity was achieved with the lower percentage of
SiCp at a sintering temperature of 900oC. Akramifard et al. [24] developed composites using
pure Cu sheets reinforced with 25 µm SiC particles by friction stir processing (FSP). For
achieving the uniform distribution of reinforcing SiC particles in the Cu matrix, a net of holes
were drilled on the surface of the pure Cu sheets. From their study it was concluded that the
17
Chapter 2- Literature Review
SiC particles improved the wear resistance of the composites and there was a rise in the
average friction coefficient of pure Cu.
2.3.2 Cu-E-Glass Fiber Composites
Glass fibers are one of the most versatile and useful industrial materials known. They are
easily produced from the raw materials which are obtainable in abundant supply. E-glass
fibers show excellent mechanical properties. They have a tensile strength of 3500 MPa and a
hardness of 6000MPa. It’s Young’s modulus is 85 GPa and it’s compressive strength is 5000
MPa. Typically glass fibers belong to two different categories, the low cost general purpose
fibers and the premium special purpose fibers. E-glass fibers come in the class of general
purpose fibers. The general purpose glass fibers are less costly as compared to the premium
category fibers. Fig.2.3 below shows the cost band for different fibers.
Table 2.1 Properties of different type of
glass fibers
Fig. 2.3 Cost band for different fibers
E-glass fibers are found to be one of the most suitable materials for the development of
composites due to its low cost and high strength. Glass fibers are used ina wide range of
applications. Glass fibers offer excellent properties from high strength to fire resistance.
Glass fiber is a dimensionally stable engineering material. It does not stretch or shrink after
exposure to extremely high or low temperatures. The maximum elongation of E-glass fiber at
break is 4.8 % with a 100 % elastic recovery when stressed close to its point of rupture. The
density of E-glass fiber is 2.58 gm/cc. It’s coefficient of thermal expansion is
5.3× 10-6/oC [25].
18
Chapter 2- Literature Review
E-glass fiber has been used extensively as reinforcement in polymer based composites.
However, report on use of E-glass fiber as reinforcement in metal matrix composites is
limited. Zak et al. [26] studied a rapid engineering process for the development of polymerbased composite parts using short discontinuous fibers as reinforcements. The mechanical
testing of these composite specimens showed up to 60% improvement in the modulus values
compared to unreinforced layered specimens. In their paper author has reported the use of a
UV-laser-based system for the selective solidification of the composite liquid. Schutte [27]
investigated the durability of glass-fiber/polymer composites. It was reported that
environmental attack by moisture can degrade the strength of the glass fibers and the fibers
can plasticize, swell, or produce microcracks in the matrix and degrade the fiber/matrix
interface by either chemical or mechanical attack.
2.3.3 Cu-Multiwalled Carbon Nanotubes Composites
Carbon nanotubes have emerged as promising reinforcement for a variety of nanocomposites
because of their sharp geometry, mechanical strength, chemical stability and electrical
conductivity since their discovery in the early 1990s. It is a tube-shaped material made of
carbon with diameter in nanometric scale. Single walled carbon nanotubes (SWCNTs) consist
of a single layer graphene sheet wrapped to form a tube structure having diameters at
nanoscale. Several experiments and simulations reported that CNTs have surprising
mechanical properties. They have elastic modulus of 0.3-1TPa, tensile strength of the order of
10-60 GPa and thermal conductivity of up to 3000 W/mK. The strength of carbon nanotubes
is approximately 100 times superior to that of steel of the same diameter. Carbon nanotubes
come in two principal forms, single walled carbon nanotubes (SWCNT) and multiwalled
carbon nanotubes (MWCNT).The density of multiwalled carbon nanotubes (MWCNTs) is
2.60 gm/cc and their specific surface area is about 200-400 m2/g.
19
Chapter 2- Literature Review
Cu-based metal matrix composites having carbon nanotubes as reinforcement are used for
structural applications and functional materials because of their high strength and excellent
electrical and thermal conductivity. CNTs are promising candidates that could be used as
nanoscale reinforcement in Cu-based metal matrix composites. It has been reported in
literature that with the addition of carbon nanotubes the bulk properties of Cu could be
improved. The Cu-based MMCs reinforced with CNTs have superior mechanical properties
and are more thermally stable compared to pure Cu. Carbon nanotubes act as a filler material
which reduces the thermal expansion coefficient of the Cu matrix. With the addition of CNTs
the bulk electrical conductivity of the Cu composites can also be modified. The two main
types of CNTs are single-walled carbon nanotubes (SWCNTs) and multiwalled carbon
nanotubes (MWCNTs). SWCNTs are an allotrope of sp2 hybridized carbon similar to
fullerenes. The structure of SWCNTs is that of a cylindrical tube comprising six-membered
carbon rings which are similar to graphite. On the other hand MWCNTs have several
concentric tubes. Here we have used MWCNTs as a reinforcement for developing CuMWCNT composites[28,29].Fig.2.4 shows the number of publications for the past few years
on CNT reinforced composites. It can be seen from the figure that the bulk of the research has
been done on polymer-based composites reinforced by CNTs. In past few years there is a
significant increase in the publications on metal matrix composites reinforced by CNTs.
Fig.2.5 shows the number of publications in the area of for various CNT reinforced metal
matrix composites using CNTs as reinforcement between 1997 to 2007. The figure shows
that the number of publications in this area has increased several times since 2003 making the
topic for current research [30].
20
Chapter 2- Literature Review
Fig.2.4 Graph showing number of
publications in different years of CNT
reinforced composites
Fig.2.5 Graph showing number of
publications in different years of CNT
reinforced MMCs
Several researches have reported that with the addition of carbon nanotubes the strength and
toughness of the material can be enhanced. Li et al. [31] studied the properties of Cu-CNTs
composites. They have reported that the composites developed shows high strength and good
ductility. It was investigated from the pillar testing that the strength and plastic strain of the
composites could be as large as 1700 MPa and 29 % respectively. From the results it is
evident that addition of 1wt. % CNTs could lead to an increase in the strength, stiffness and
toughness of the material. Microstructural analysis discloses that in the composites, CNTs
could be either distributed at the grain boundaries or inside the Cu grains.
Trinh et al. [32] studied the calculation of friction coefficient of Cu-CNT composite. They
developed the composites by powder metallurgy route and the friction coefficients were
evaluated. From their study it was concluded that the coefficient of friction of the developed
composites decreases with the increase in the mass fraction of CNTs in the composites.
Lal et al.[33] investigated an alternative method for the dispersion of CNTs in the Cu matrix.
In their work they used the molecular level mixing technique coupled with high energy ball
milling followed by powder metallurgy to synthesize the Cu-CNTs composites. It is observed
that there has been an increase in the mechanical properties over pure Cu and the method
used shows a distinct advantage for the synthesis of Cu-CNTs composites.
21
Chapter 2- Literature Review
It could be concluded form the above research papers that the most important factors in
developing Cu-MWCNTs composites is the homogeneous dispersion of the CNTs in the Cu
matrix, interfacial bonding between the CNTs and the Cu matrix and the retention of
structural integrity.
2.4 Processing Techniques for Metal Matrix Composites
Manufacturing is a very comprehensive area and include numerous processes such as
machining, fabrication and joining. The fabrication approach of a composite part depends
mainly on three factors: (i) the nature of the matrices and reinforcements, (ii) the shape and
sizes of products and (iii) their end use. There are numerous kinds of composite materials
which cover a wide spectrum of applications ranging from an engine valve to an aircraft
wing. The fabrication technique varies from one product to the other.
There are mainly two types of processing techniques. They are,
i.
Liquid State Processing Techniques
ii.
Solid State Processing Techniques
2.4.1. Liquid State Processing Techniques
The liquid state processing technique includes the ease of handling liquid metal related to the
powder. There is lesser cost involvement for obtaining liquid metals as compared to metal
powder and this technique also gives us the possibility of creating various shapes by using
several methods available in casting industry. Liquid state processing also suffers from a
number of limitations like incomplete control of the processing parameters and unwanted
chemical reactions at the boundary of the liquid metal and the reinforcement [34].
A brief description of the various liquid state processing techniques is given below:
22
Chapter 2- Literature Review
2.4.1.1 Infiltration Process
The liquid infiltration process comprises infiltration of a particulate or fibrous reinforcement
by a liquid metal. In this process the molten matrix is infiltrated in a pile of continuous or
discontinuous reinforcements. It is then allowed to solidify between the inter-reinforcement
spaces. This process of developing MMCs is not straight forward because of the difficulty of
wetting the reinforcement by the molten metal. The reinforcement can be pre-mixed with the
matrices prior to casting in the case of discontinuous reinforcement. The several techniques
available for pre-mixing the metal and the reinforcement are injection gun, dispersion of
reinforcements in a mildly agitated melt, mechanical agitation and centrifugal dispersion. To
improve wettability and to control the interfacial reactions a fiber coating is applied prior to
the process to achieve better results. However, it could be disadvantageous if the fiber coating
is exposed to air leading to surface oxidation.
Fig.2.6 Schematic diagram showing the setup of the pressure less liquid metal infiltration
technique
Fig.2.6 shows the schematic diagram of pressure less liquid metal infiltration technique for
developing MMCs. It can be used with reactive metal alloys such as Al-Mg to infiltrate
ceramic preforms. For Al-Mg alloys, the process takes place between 850 -1000°C in a N2rich atmosphere and typical infiltration rates are less than 25 cm/h.
2.4.1.2 Dispersion Process
Dispersion process is a liquid state processing technique in which the reinforcement is
incorporated in loose form into the metal matrix. To combine the two phases a mechanical
force is required and this can be achieved by stirring as most systems have poor wettability.
23
Chapter 2- Literature Review
The major advantage of this process is its low cost. In addition this process can be adopted
during both casting and extrusion. The simplest type of dispersion process is the vortex
method in which the liquid is stirred and the reinforcement particles are added during stirring.
The main disadvantage of this process is the presence of porosity resulting from gas
penetration during the process. The other drawbacks are the reaction between the matrix and
the reinforcement that takes place due to long interaction time and clustering that can occur
during mixing.
2.4.1.3 Spray Process
Monolithic alloys were produced initially by spray forming techniques. However, with the
advancement in technology particle reinforced MMCs are also being developed by this
process. One of the examples of this process is the co-spray process in which the heated SiC
particles are injected inside the molten Al alloy using a spray gun. SiC particles upto 20 vol.
fraction with aspect ratio 3-4 are incorporated in the Al alloy by this method. An optimum
particle size is needed for the process to be efficient as very fine particles and whiskers are
very difficult to transfer. The co-sprayed MMCs are subjected to scalping, consolidation and
several secondary finishing processes to form the wrought composite material. It is a liquid
metallurgy process and it is fast and automated. As the time of flight is very short there is no
possibility of formation of any toxic materials. The major advantage of this process is its
flexibility and the ease with which different types of composites can be developed. However,
this process is quite expensive because of the high cost of the equipment.
2.4.1.4 In-Situ Process
In-situ process is one of the widely used liquid state processing technique in which the
reinforcement is formed in-situ. Composites in this process are developed in a single step
from the starting alloy thus minimizing the efforts to form composites by combining different
constituents as generally done in the development of typical composites. One of the examples
24
Chapter 2- Literature Review
of in-situ processing is unidirectional solidification of eutectic alloys. Unidirectional
solidification of a eutectic alloy typically results in one phase being distributed in the form of
fibers or ribbon in the matrix phase. Several parameters such as spacing and relative size of
the reinforcement can be precisely controlled by controlling the solidification rate. In this
process the volume fraction of the reinforcement remains constant throughout the process.
The solidification rate in practice, however, is limited to a range of 1-5 cm/h because of the
need to maintain a stable growth front which requires a high temperature gradient.
2.4.2 Solid State Processing Techniques
Solid state processing techniques are one of the most preferred techniques to develop metal
matrix composites. In these techniques the MMCs are developed as a result of bonding
between the matrix metal and the dispersed reinforcement phase due to mutual diffusion
occurring between them at elevated temperatures and pressure. In solid state sintering there is
a reduction of undesirable reactions at the interface of the matrix and the reinforcement phase
as compared to liquid state fabrication techniques as the processing temperature is below the
melting point of both the metal matrix and the reinforcement. Metal matrix composites
formed by this process can further be deformed by rolling and extrusion [34].
Brief description of the various solid state processing techniques are given below:
2.4.2.1 Diffusion Bonding
Diffusion bonding is a common solid state processing technique for joining similar and
dissimilar metals. It is generally carried out at a higher temperature where the inter diffusion
of atoms between the metals takes place easily leading to bonding of the atoms. There are
several advantages of this method, one being the capability to develop a wide range of metal
matrices and other being the control of fiber fraction and their orientation. High temperature
and pressure is used during the diffusion process. Vacuum hot pressing is one of the
important steps in the diffusion bonding process for the development of metal matrix
25
Chapter 2- Literature Review
composites. Hot isostatic pressing (HIP) is preferred for diffusion bonding. Using HIP it will
be relatively easy to apply high pressures at elevated temperatures. It also enables the
development of products having variable geometries. Diffusion bonding also has several
disadvantages such as long processing time, requirement of high processing pressure and
temperature. Due to need of high processing temperature and pressure the process becomes
costly.
2.4.2.2 Powder Metallurgy
Powder metallurgy is one of the preferred methods of solid state processing technique for the
development of metal matrix composites. It is a process for producing useful products using
metal powders. It is one of the most important techniques through which particulate materials
are consolidated to finished products. Nowadays powder technology is used to develop
components providing exceptional properties that are desired in highly advanced aerospace
and nuclear energy industries. Automobile industries are also one of the major consumers of
powder metallurgy products. There are several significant reasons for using powder
metallurgy as the processing technique by the industries such as the creation of complex
components like tungsten filament, porous self-lubricating bearings etc. This process
minimizes or eliminates the scrap and machining losses leading to high volume production of
components. This process is economical, saves energy and raw materials. It also enables mass
production of quality precision components. Fig.2.7 shows the flowchart of the sequence of
operations in the powder metallurgy process. This process involves the combination of
blending the metal powders and other constituents followed by compaction to produce the
desired shape. The green compacts developed are then sintered at higher temperatures usually
below the melting point of the major constituent to develop a product of desired structure,
density and properties.
26
Chapter 2- Literature Review
Fig.2.7 Simplified flowchart showing the sequence of operations in powder metallurgy
For solid state sintering the sintering temperature is kept below the melting point of all the
constituents. Wherever, for liquid state sintering the sintering temperature is kept above the
melting point of any of the constituents. In hot pressing the two stages of compaction and
sintering are combined into one single step. Powders can also be rolled continuously and
sintered to produce strips and other flat products. Powder metallurgy process enables the
manufactures to develop products that are very consistent and predictable in behaviour.
Powder metallurgy products have a wide range of applications [35-37].
(a)
(b)
+
Fig.2.8 (a) Schematic of uniaxial cold compaction (b) Schematic diagram showing the
sintering mechanism using solid state diffusion process
27
Chapter 2- Literature Review
Fig. 2.8(a) shows the schematic diagram of a uniaxial cold compaction machine. Uniaxial
cold compaction involves the compaction of powder in a die by applying pressure in a single
axial direction through the punch. Fig. 2.8(b) shows the sintering mechanism using solid state
diffusion process. In solid state sintering the densification is attained through changes in
particle shape, deprived of particle rearrangement.
Fig.2.9 Applications and Advantages of Powder metallurgy
There has been an increasing interest in powder metallurgy with the expansion of various
industries since 1950. Several advantages and application of this technique are shown in
Fig.2.9. Nearly 90% of powder metallurgy products are used in transportation markets.
Several technologies are being developed for reducing fuel consumption. The automotive
industry is in the trend of developing lightweight technology and engine downsizing for
environmentally friendly vehicles. To achieve this reduction, powder metallurgy products,
which are components of the latest systems, are also required to have higher
performance[38].
2.4.2.3 Mechanical Alloying
Mechanical alloying (MA) is used as one of the preferred method for powder processing. In
this technique production of homogenous materials is done starting from blended elemental
28
Chapter 2- Literature Review
powder. John Benjamin and his colleagues at the Paul D. Merica Research Laboratory of the
International Nickel Company (INCO) developed the process around 1966. This technique
was the result of an extended search to develop nickel-base super alloy for gas turbine
applications. MA is one of the solid state processing techniques which comprises repeated
welding and fracturing of powder particles in a high-energy mill. However, in earlier days
this process had been used to develop oxide-dispersion strengthened (ODS) nickel and iron
base super alloys for applications in aerospace industry. It is a complex process and hence
involves optimization of a number of variables to achieve the desired product. Mechanical
Alloying (MA) is described as a high energy milling process in which powder particles are
exposed to repeated cold welding, fracturing, and rewelding. Due to the high energy of the
impacts a large amount of strain is introduced in the powder particles. As a result a large
number of dislocation and other defects are generated which act as fast diffusion paths. The
initial process of mechanical alloying starts with the mixing of powders in the desired
proportion. Then this powder is loaded in the mill with the grinding medium. Several
grinding media like tungsten carbide (WC), hardened chrome steel, stainless steel, zirconia
etc. are available. The powders are then grinded for the desired time until a steady state is
reached. Process control agents (PCA) like toluene could be used during milling. The powder
obtained after milling is then consolidated into a bulk sample and sintered at a desired
temperature to obtain the desired microstructure and properties [39-49].
Some of the important parameters of mechanical alloying that have an effect on the final
constitution of the powder are:
1. The yield or the amount of powder obtained from milling is determined by factors like the
milling speed, milling time and the amount of process control agent used. The process
control agent has a lubricating effect that minimizes the cold welding effect. Without the
use of the PCA, the powder being milled welds on the milling chamber vessel walls,
29
Chapter 2- Literature Review
resulting in a lesser fraction of the charge being recovered. However, a large amount of
PCA could also affect the kinetics of the mechanical alloying process. Therefore, an
optimum amount of PCA should be used during milling in order to ensure a higher yield
and shorter downtime.
2. type of mill (High energy planetary ball mill, spex mill, cryogenic mill etc.)
3. milling media (Zirconia, tungsten carbide, hardened chrome steel, stainless steel etc.)
4. milling speed (can be varied)
5. milling time (can be varied)
6. size of the balls used for milling (Various sizes of ball are available. If the balls are too
large then the number of breaking contacts will be low and as result the grinding capacity
will also be lower. Whereas, if the balls are too small, the grinding efficiency could be
decreased as the contacts may be too weak to break the particles.)
7. ball-to-powder weight ratio (1:10 , 1:20 , etc.)
8. milling atmosphere (Dry or wet milling. For wet milling toluene could be used as the
process control agent. Argon inert gas atmosphere can also be used.)
Ball Surface
Powder Particles
Fig.2.10 Ball-powder-ball collision during high energy ball milling
Fig.2.10 shows the collision of ball-powder-ball during high-energy ball milling. The powder
particles during the process are repeatedly flattened, cold welded, fractured and rewelded.
30
Chapter 2- Literature Review
Whenever the balls collide there is always some amount of powder particles that get trapped
in between the balls. Typically around 1000 particles with an approximate weight of 0.2 mg
could be trapped during every collision of balls. Fig.2.11 shows the motion of the balls and
the powder. As the rotation directions of the bowl and the turn disc are opposite in nature, the
centrifugal forces are alternately synchronized.
Balls
Movement of the supporting
disc
Centrifugal Force
Rotation of milling vial
Powder particles
Fig. 2.11 Schematic view of motion of the ball and powder mixture
The powder particles get work hardened and fractured during the continuous collision of
balls. The powder particles get deformed during the process. Initially the particles have
strong affinity to weld together due to their soft nature resulting in the formation of larger
particle. With continued deformation, the particles get work hardened and fracture by a
fatigue failure mechanism. Fragments generated by this mechanism may continue to reduce
in size in the absence of strong agglomerating forces [50].
…………………………………….
31
Chapter 3- Experimental Details
Chapter 3
Experimental Details
32
Chapter 3- Experimental Details
Experimental Details
Introduction
This chapter describes the experimental procedure as adopted in the present investigation.
The equipment/instruments used to carry out the experiments are listed below indicating their
specific use in the project along with their specifications and particulars in details. A detailed
report is also provided on the raw materials procured and synthesized that have been used for
the fabrication of the Cu-based metal matrix composites. This chapter provides the detailed
step-wise methods adopted for the fabrication of the composites. The following work plan
has been adopted:
As received Cu powder
(99% purity)
Mechanical Milling of Cu
for 20 h with appropriate
parameters
Addition of Reinforcements
(SiCp, E- glass fiber and
MWCNTs)
Characterization of milled Cu
Blending of Cu powder
with reinforcements
Cold Uniaxial Compaction at
665 MPa
Sintering of samples at 900oC
for 1 h in Ar atmosphere
Characterization
Metallography
(OM, SEM, HRTEM)
(OM
Spectroscopy
(XRD, EDX, FTIR)
Mechanical Testing (Microhardness,
Density, Wear, Fractography)
Fig.3.1 Work plan for present investigation
33
Chapter 3- Experimental Details
3.1 Equipment used in the Present Investigation
1. Planetary Ball Mill
Here a high energy ball mill is used for the milling of elemental Cu to reduce its crystallite
size to nanometer scale. The as-milled nanocrystalline Cu is later used for the development of
Cu-based metal matrix composites. The powder particles during the process are repeatedly
flattened, cold welded, fractured and rewelded. Whenever the balls collide some amount of
powder particles get trapped in between the balls. The powder particles get work hardened
and fractured during the continuous collision of balls throughout the process. The powder
particles get deformed during the process. Initially the particles have strong affinity to weld
together due to their soft nature resulting in the formation of larger particles. With continued
deformation, the particles get work hardened and fractured by a fatigue failure mechanism.
Fragments generated by this mechanism may continue to reduce in size in the absence of
strong agglomerating forces.
There are several advantages of ball milling such as:
i.
Low installation cost
ii.
Low powder cost
iii.
Low grinding medium cost
The milling conditions used in the present investigation are:
Milling Conditions:

Ball Mill: Fritsch P5

Milling Medium: Wet (Toluene)

Vials and Balls: Hardened chrome steel

Milling Speed: 300 rpm

Diameter of Balls: 10mm

Ball to powder weight ratio = 10:1
34
Chapter 3- Experimental Details
(a)
(b)
Fig.3.2 (a) Planetary Ball Mill (b) Schematic diagram showing the mechanism of ball milling
Fig. 3.2(a) above shows the Fritsch planetary ball mill. Fig. 3.2(b) shows the schematic
diagram of the milling process and the behaviour of powder particles during milling. The
speed used for the current investigation is 300 rpm with ball to powder ratio of 10:1.
Planetary ball mill (pulverisette-5) manufactured by Fritsch Germany is used for the above
purpose.
2. Cold Uniaxial Hydraulic Press
Uniaxial die pressing is one of the ideal methods which is commonly used to manufacture
near net shaped components. It is used for making the green samples which were later
sintered in the tubular furnace. Uniaxial pressing includes the compaction of powder in a die
by applying pressure in a single axial direction through the punch or piston. The presses used
are generally mechanical or hydraulic and the pressing cycle repeats at 6 to 100 times/min.
(a)
(b)
Fig.3.3 (a) Schematic Diagram of Uniaxial Pressing (b) Uniaxial Hydraulic Press
35
Chapter 3- Experimental Details
In the present study a die with 15mm diameter is used and the applied stress is 665 MPa for
all the samples. The press used was designed by Soil lab with maximum capacity of 1130
MPa.
3. X-Ray Diffraction
A diffractometer is a measuring instrument for analyzing the structure of a material from
the scattering pattern
created
when
a
beam
of radiation or
particles
(such
as x-
rays or neutrons) interact with it. X-ray diffraction of the milled Cu powder after several
intervals of milling has been carried out to study the different phases formed during milling.
The variation of crystallite size, r.m.s. strain and lattice parameter with milling time was also
calculated. A Philip’s X'pert Pro high-resolution x-ray diffractometer has been used for the
purpose. The maximum scanning range (2θ) for the instrument is 0 to 160o.The radiation used
in x-ray diffraction was Cu-Kα having wavelength of 1.5409 Å. Ni was used as filter.
(b)
(a)
Fig.3.4 (a)Philip’s X'pert Pro high resolution x-ray diffractometer (b) Schematic diagram of
the diffractometer
4. High Temperature Horizontal Tubular Furnace
A high temperature tubular furnace is used for sintering the green samples. The sintering of
the samples was done in an inert atmosphere of Ar gas. A tubular furnace is an electric
heating device used to conduct synthesis and purification of inorganic compounds. The
design comprises of a cylindrical cavity surrounded by heating coils that are rooted in a
36
Chapter 3- Experimental Details
thermally insulating matrix. Temperature can be controlled via response from a
thermocouple. The samples were heated in a crucible in a tubular furnace in the presence of
inert argon gas atmosphere. The casting temperature in the furnace was maintained at 900 oC
with the holding time of 1 h for all the samples. A vacuum and control atmosphere furnace
manufactured by Naskar & Company with maximum attainable temperature of 1750oC is
used for the above investigation.
(a)
(b)
Fig.3.5 (a) Tubular furnace (b) Schematic diagram of the tubular furnace
5. Instron-1195
A universal testing machine (UTM) is used to determine the tensile stress and compressive
strength of the materials. It is used to perform several standard compressive and tensile tests
on components, structures and materials. TheInstron-1195 (Universal Testing Machine)
machines is used to calculate the mechanical properties of the components by carrying
different tests such as tension, impact, compression and torsion tests. Here it has been used
for carrying out the tensile test of E-Glass fiber to determine its mechanical properties.
During the test the control system and software records the values of load applied,
displacement and other parameters. The Instron 1195 manufactured by Instron Ltd. with load
carrying capacity between 0.1 kN to 100 kN is used for the present investigation.
37
Chapter 3- Experimental Details
Fig.3.6 Instron 1195
.
6. Scanning Electron Microscopy (SEM) and Field Emission Scanning Electron
Microscopy (FESEM)
Scanning Electron Microscope (SEM) uses high energy beam of electrons that are directed at
the specimen to produce a two dimensional image of a specimen. The electrons which are
produced from the hot filament are accelerated by magnetic and electric fields. The signals
produced provide the information about the surface topography composition and other
properties. In the present investigation pure Cu and the composites developed were
characterized and studied. The electron beam is concentrated to a fine probe to produce the
images. The electron beam is scanned in a raster scan pattern and the beam’s position is
combined with the detected signal to produce an image. A resolution of less than 1 nm is
achievable in a SEM. Every point on the sample emits signals in the form of electromagnetic
radiations. Selected portions of this radiation, usually secondary electron (SE) and
backscattered electron (BSE) are collected by a detector. The signals are amplified and
displayed on a monitor. The subsequent image is generally straight forward to interpret, at
least for topographic imaging of objects at low magnifications.
38
Chapter 3- Experimental Details
(a)
(b)
Fig.3.7 (a) Scanning electron microscopy (b) Field emission scanning electron microscopy
A JEOL-JSM-6480LV has been used in the present investigation. For energy dispersive x-ray
spectroscopy (EDX) analysis, an INCAPentaFET-x3 x-ray microanalysis system with a highangle ultra-thin window detector and a 30 mm2 Si (Li) crystal was used.
The composites developed were also analyzed using a FEI Nova Nano FEG-SEM 450
FESEM. FESEM produces clearer, less electrostatically distorted images with spatial
resolution down to 1.5 nm which is 3 to 6 times better than the conventional SEM. FESEM
uses field-emission cathode in the electron gun of a SEM. A field-emission cathode in the
electron gun provides narrower probing beams at low as well as high electron energy,
resulting in both improved spatial resolution and minimized sample charging and damage.
High quality images are obtained with negligible electrical charging of the samples using an
accelerating voltage in the range of 0.5 to 30 kV. Also in FESEM the need for conducting
coatings on insulating materials is virtually eliminated.
7. High Resolution Transmission Electron Microscopy (HRTEM)
High resolution transmission electron microscopy (HRTEM) is a technique that uses a beam
of electrons which are transmitted through thin specimens and it interrelates with the
specimen as it passes through it. From the interactions of electrons transmitted through the
specimen an image is formed. This image is then magnified and focussed with the help of
imaging device. The magnified image is focussed on a fluorescent screen on a layer of
39
Chapter 3- Experimental Details
photographic film. This is then detected by the CCD camera. In our study the 20 h milled
nanostructured Cu powder and MWCNTs were observed under a HRTEM. The samples were
dispersed by ultrasonication in acetone solution. The dispersed powder is then taken out using
a pipette and drops of powder dispersed in acetone is put on a carbon coated copper grid for
seeing the samples in the HRTEM. Selected area diffraction (SAD) pattern of all the samples
have also been taken using the HRTEM. A Philips CM12 TEM has been used for analysing
the samples and the accelerating voltage of 120 kV has been employed.
Fig.3.8 High resolution transmission electron microscopy
8. Density Measurement
Archimedes’ principle was used for the measurement of density of samples. Precise
measurement of the density of sintered samples was carried out. There are several errors in
density measurement of the sintered samples usually due to the interconnected pores and the
disconnected pores present in the samples. Liquid gets into the interconnected pore and
affects the density measurement. To remove this error three different weight measurements
were taken, namely weight in air, weight of the sample dipped in liquid and weight of the
sample in air after soaking in liquid for a long time. The liquid used is distilled water (ρ =1.0
gm/cc). The density of the sample was calculated using the following formula.
40
Chapter 3- Experimental Details
… (1)
Density of the composite =

Weight of the sample in air W1=Wair

Weight of the sample in liquid W2 = Wliquid

Weight of the sample soaked in liquid for a long time W3 = Wsoaked
(a)
(b)
W3
Fig.3.9 (a) Experimental setup for density measurement (b) Density measurement kit
9. Vickers Microhardness
A Vickers microhardness tester uses a diamond indenter in the form of right pyramid with a
square base. The angle is supposed to be 136o between the opposite faces. Generally the
results are reported in kg/cm2 which is proportional to the load divided by the square of the
diagonal of the indentation calculated from the test. The load on the Vickers microhardness
indenter can be taken usually from a few grams to kilograms. The resulting indentation is
measured and converted to a hardness value using a mathematical formula. The test samples
were polished before measuring the hardness.
(a)
(b)
Fig. 3.10 (a, b) Schematic figure of the Vickers pyramid diamond indentation
41
Chapter 3- Experimental Details
The lengths of the two diagonals X and Y of the indentation left on the surface of the material
after removal of the load are measured and their arithmetic mean L is calculated. In the
present study, a load of 100 gf was considered (F = 0.98 N) and the Vickers hardness number
is calculated using the following equations:
=
… (1)
…. (2)
Where F is the applied load (N), L is the diagonal of square impression (mm), X is the
horizontal length (mm) of the indentation and Y is the vertical length (mm) of the
indentation.
(a)
(b)
Fig. 3.11 (a, b) Vickers microhardness tester
The micro-hardness measurements were done using Lecco Vickers microhardness (LV 700)
with a diamond indenter in the present investigation.
10. Wear Test
Wear is the progressive loss of materials from contacting surfaces which are relative in
motion. It is basically the erosion of material from its base position on a solid surface. Wear
is linked to the interactions between two surfaces. Specifically it is the removal and
deformation of material on a surface due to the mechanical action of the opposite surfaces.
The relative motion between the two surfaces and the initial mechanical contact between
42
Chapter 3- Experimental Details
asperities are one of the important factors in wear. Wear test is carried out to find the wear
characteristic of the composites developed and to understand the wear mechanism.
Fig. 3.12 Ball-on-plate wear tester
A computerized ball on plate wear tester (TR-208-M1, DUCOM) equipped with a diamond
indenter is used in the present investigation. The samples were polished with emery before
doing the wear test. The load given was 20 N for a period of 10 minutes for all the wear tests
conducted.
11. Fourier Transform Infrared Spectroscopy (FTIR)
The Fourier transform infrared spectroscopy (FTIR) is one of the most important infrared
spectroscopy methods. In this process an infrared (IR) radiation is passed through a sample.
Some of the radiations are absorbed by the sample and some of them are transmitted by it.
The spectrum indicates the molecular absorption and transmission which is a distinctive
molecular fingerprint of the sample. The infrared spectroscopy is useful for numerous types
of analysis. Molecular spectroscopy involves the interaction of the electromagnetic radiation
with the molecules in the material. Molecular spectroscopy involves the absorption of
electromagnetic radiation by the material whose molecular structure we are attempting to
determine. The absorption spectra of the atoms and molecules can be related to its physical
properties such as electronic structure, atomic or molecular mass and molecular geometry.
There are several factors like length and strength of the bonds, mass of atom which governs
the frequency of these vibrations. Molecular vibrations are stimulated by bonds absorbing
radiation of the same frequency as their natural vibrational frequency. Fourier transform
43
Chapter 3- Experimental Details
infrared spectroscopy (FTIR) is one of the most versatile, fast, inexpensive, and conclusive
techniques for surface and bulk characterization.
Fig. 3.13 FTIR spectrophotometer
Here the spectra were collected in the 4000cm-1 to 400 cm-1 region with 8 cm-1 resolution, 60
scans and beam spot size of 10µm-100µm.The FTIR imaging was performed in AIM-800
Automatic Infrared Microscope (SHIMADZU).
3.2. Selection Synthesis and Characterization of Raw Materials
Cu powder, SiC particles, E-glass fiber and multiwalled carbon nanotubes (MWCNTs) were
used as the raw materials to carry out the present investigation. Research grade Cu powder,
SiC particulates and E-glass fiber were procured directly from the manufacturers. MWCNTs
were synthesized by us using low pressure chemical vapour deposition (LPCVD) process.
The raw materials procured and synthesized by us were analysed using various techniques.
The detailed specification of the chemicals procured and the synthesis process used to
develop them are given below.
i. Cu Powder
Copper (Cu) is a ductile metal having high thermal and electrical conductivity. Pure Cu is
soft and malleable and its surface has a reddish-orange colour when exposed to atmosphere.
Cu has a melting point of 1083.4°C and its density is 8.96 gm/cc. It is mainly used as a
conductor of heat and electricity, building material, and a constituent of various metal alloys.
Cu has high electrical conductivity (5.96×106 S/m) and also high thermal conductivity (401
W/m.K).The yield strength of Cu is117 MPa while its tensile strength is 210 MPa .The
Young's modulus of Cu is 130 GPa. Cu is an integral part of several industries. It is the third
44
Chapter 3- Experimental Details
most extensively used metal in industries. It’s resistance to corrosion; high thermal and
electrical conductivity and malleability guarantee that it will continue to be used for a long
time. Here for our experimental work Cu was procured from Loba Chemie India. The Cu
powder had a purity of above 99% and an average particle size of 27.61 µm.
ii.
Silicon Carbide (SiC) Particles
Silicon carbide is a compound of silicon and carbon having chemical formula SiC. SiCp is
formed of tetrahedral structure of carbon and silicon atoms having strong bonds in the crystal
lattice. This makes silicon carbide a very hard and strong material. SiC shows strong
resistance to chemicals and corrosion up to 800oC. In air, it forms a protective oxide layer
composed of silicon oxide at 1200oC and can it maintains it strength upto a temperature of
1600oC with no loss in strength. Significantly grains of silicon carbide (SiC) can be bonded
together at elevated temperatures to form a hard ceramic material that are efficiently used in
several applications like car brakes, clutches, ceramic plates and bullet proof vests possessing
high durability. SiC does not show melting at any known pressure. It is also highly
chemically inert in nature. SiC also has a very low coefficient of thermal
expansion (4.0×10−6/K) and experiences no phase transitions which prevent discontinuities in
thermal expansion. SiC has a melting point of 2730°C and its density is 3.20gm/cc. The
elastic modulus of SiC is 450 GPa and it has a hardness of 20.5 GPa. SiC was procured from
Search Chem for our experimental work. It had a purity of above 98% and its average particle
size was 220 mesh.
iii.
E-Glass Fiber
Glass fibers are among the best multipurpose industrial materials known today. They are
easily produced from raw materials, which are available in almost unlimited supply. They
have very useful bulk properties such as hardness, transparency, resistance to chemical
attack, stability, and inertness, as well as desirable fiber properties such as strength,
45
Chapter 3- Experimental Details
flexibility, and stiffness. The major constituents of E-glass fiber are SiO2 (52-56 wt. %), CaO
(16-25 wt. %), Al2O3 (12-16 wt. %) and B2O3 (5-10 wt. %). Apart from this it also contains
oxides of Na and K (0-2 wt. %), MgO (0-5 wt. %), oxides of Fe and Ti and fluorides. E-glass
(electrical) fibers have lower alkali content and are stronger than A-glass (alkali) fibers. Eglass fibers were initially used for electrical applications which why they were called E-glass
fibers. E-glass fiber is alkali free and was the first glass formulation that was used for
continuous filament formation. It is the largest type of glass fiber produced in the world. It
high stiffness and possesses good tensile and compressive strength. It also has good electrical
properties and has a relatively low cost [25].
Table 3.1 Properties of E-glass fiber
Property
Bulk Modulus
Compressive Strength
Hardness
Tensile Strength
Young's Modulus
Minimum Value
43 GPa
4000 MPa
3000 MPa
1950 MPa
72 GPa
Maximum Value
50GPa
5000MPa
6000MPa
3500MPa
85GPa
The fiber used in the present investigation has been manufactured by Saint Gobain.
iv.
Synthesis of Multiwalled Carbon Nanotubes (MWCNTs)
The Chemical Vapour Deposition (CVD) method is one of the preferred and suitable methods
to synthesize carbon nanotubes (CNT). The synthesized carbon nanotubes from CVD method
possess high purity and can be used for large scale production. The CVD method involves the
catalytic decomposition of the hydrocarbon with the aid of supported transition metal
catalysts (Fe, Co, Ni, etc.). It is a versatile process in which the gas phases are decomposed to
form reactive species. These species leads to particle or film growth. The CVD process can
be used to deposit a wide range of conducting, semiconducting and insulating materials [51].
46
Chapter 3- Experimental Details
(b)
(a)
NH3, C2H2, H2
Temperature = 900oC
Fig.3.14 (a, b) Schematic of a typical CVD furnace setup used for the synthesis of MWCNTs
A conventional horizontal quartz tube with dimensions approximately 1 m long and 50 mm in
diameter was used. The temperature was maintained at 900oC with holding time of 40 mins.
Subsequently, the synthesis was initiated by introducing a flow of 20 sccm acetylene (C2H2),
50 sccm ammonia (NH3) and 40 sccm hydrogen (H2) for 40 mins. Acetylene (C2H2) was used
as the carbon source and was fed into the tube furnace at a controlled rate. To prevent
oxidation and removal of gaseous by products argon (Ar) gas was fed at a controlled rate of
600 sccm during the whole heating-up, growth and cooling down periods. Fig. 3.14 (a,
b)shows the schematic of a typical CVD furnace setup used for the synthesis of MWCNTs.
Carbon nanotubes are the strongest and stiffest materials discovered in terms of tensile
strength and elastic modulus respectively. Single walled carbon nanotubes (SWCNTs) are
known to have a Young’s modulus value of ~1 TPa and a tensile strength of 1353GPa.Multiwalled carbon nanotubes (MWCNTs) was tested to have a tensile strength of 63
GPa. Standard single-walled carbon nanotubes can withstand a pressure up to 25 GPa without
deformation [52-55].
Table 3.2 Properties of MWCNTs
Properties
Specific Gravity
Elastic Modulus
Strength
Thermal Conductivity
Thermal Stability
Specific Surface Area
Values
1.8 g/cm3
0.3-1TPa
10-60GPa
3000 W m-1K-1
2800oC
200-400 m2/g
47
Chapter 3- Experimental Details
The current uses and applications of carbon nanotubes have been limited to be used as bulk
nanotubes. These bulk nanotubes are referred as the mass of unorganized carbon fragments.
Bulk nanotube materials may never achieve a tensile strength similar to that of individual
tubes, but such composites may, nevertheless, yield strengths sufficient for many
applications. The superior mechanical properties of CNTs are applicable for the development
of many products ranging from daily use items like sports gear to combat jackets and space
elevators. We have used the acid functionalization route for dispersion of MWCNTs.
Multiwalled carbon nanotubes were stirred using magnetic stirrer for 8 h in an acidic solution
containing H2SO4 and HNO3 in 3:1 ratio having 50 % acid concentration. CNTs were then
filtered and washed repeatedly with distilled water till the pH value of 7 is reached. Drying of
CNTs was carried out in a vacuum oven at 80oC for 5 h. The Van der Waal’s forces between
the CNTs are overcome by the presence of carboxyl and hydroxyl groups which were
introduced during functionalization of the carbon nanotubes preventing agglomeration of the
CNTs [56,57].
3.3. Synthesis and Characterization of Cu-Based Metal Matrix Composites
It is known that matrix strengthening can be done by incorporation of reinforcements like
continuous or discontinuous fibers, whiskers, wires and particulates. Here in our work three
different types of reinforcements, SiCp, E-glass fibers and MWCNTs have been used to
develop Cu-based MMCs. Cu-10, 20, 30 and 40 vol. % SiCp, Cu-10, 20, 30 and 40 vol. % Eglass fiber and Cu-1, 2 and 5 vol. % MWCNTs composites were developed by powder
metallurgy route. Green compacts of the composites were developed by blending the
constituents followed by uniaxial cold compaction under a load of 665 MPa. The green
compacts were then sintered at 900oC for a period of 1 h in Ar atmosphere. The
microstructure, composition, fractography and mechanical properties like hardness and wear
48
Chapter 3- Experimental Details
resistance of the various Cu-based MMCs were then determined. X-ray diffraction of the
various composites was done in order to determine the different phases in the sintered
composites. The microstructure of the composites was analyzed using an optical microscope,
scanning electron microscope (SEM) and field emission scanning electron microscope
(FESEM). Hardness of the composites was determined using a Vickers microhardness tester.
Wear properties of the various composites was analysed using a ball-on-plate tribometer. The
density of all the composites was determined using the Archimedes’ principle.
…………………………………….
49
Chapter 4- Results and Discussion
Chapter 4
Results and Discussion
50
Chapter 4- Results and Discussion
Results and Discussion
Introduction
Cu powder has been used in numerous industrial applications since many years. The most
popular application known till date is the development of self-lubricating bearings which was
the first major application and still accounts for about 70 % of the granular copper powder
used. The above application of Cu shows the ability to produce a component with controlled
surface-connected porosity. Pure Cu is generally used in electronics and electrical industries
because of their outstanding electrical conductivity (5.96 × 107 S/m) and thermal conductivity
(401 W/m K).The melting point of Cu is 1083.4°C and its density is 8.96 gm/cc. Cu is widely
used as an alloying element in several components to improve its mechanical properties and
control its dimensional changes during sintering. In order to increase high temperature
properties of Cu different reinforcements are being used. The mechanical properties of pure
Cu can only be improved by two processes either age hardening or particle dispersion
strengthening. Cu matrix can be strengthened by the incorporation of several reinforcements
like particulates, fibers, whiskers and wires. Cu matrix composites are used for
manufacturing hybrid modules, electronic relays, electrically conducting springs and other
electrical and electronic components.
4.1 Mechanical Milling of Cu
Elemental Cu powder was milled in a high-energy planetary ball mill in order to synthesize
nanocrystalline Cu. Mechanical milling is a convenient and promising process to produce
nanostructured powders. It is a very effective processing technique for the preparation of
nanocrystalline metallic and ceramic powders.
51
Chapter 4- Results and Discussion
It is a solid-state powder processing technique which involves repeated welding, fracturing,
and rewelding of powder particles in a high-energy ball mill. Heavy deformation of particles
takes place during milling and increase in lattice strain can be depicted by the presence of
peak broadening.
(a)
(b)
(d)
(c)
(e)
Fig.4.1 (a, b) X -ray diffraction plots of Cu milled for various periods of time and variation
of (c) crystallite size (d) r.m.s. strain and (e) lattice parameter of Cu with milling time
52
Chapter 4- Results and Discussion
The x-ray diffraction of Cu powder milled for various durations of time are shown in
Fig.4.1 (a). The various peaks in the x-ray diffraction pattern could be indexed to the different
crystallographic planes of Cu. Fig. 4.1(b) shows the (111) peak of the as-milled and unmilled
Cu samples. This plot shows shift of the (111) peak with milling time. The (111) peak of Cu
at 5 h of milling shows shift towards the lower 2θ angle. The shift of the peak towards the
lower 2θ angle is possibly due to the increase in lattice parameter of Cu upto 5 h of milling.
Beyond 5 h of milling the (111) peak of Cu shows significant broadening. No other peaks
could be detected in the x-ray pattern suggesting that there is no contamination from the
milling media. Voigt’s method was used for calculating the crystallite size of the milled Cu
powder.
In the Voigt’sapproach the crystallite size and lattice strain comprises both the Lorentzian
and Gaussian component convolutions varying in 2θ as a function of 1/cosθ and tanθ
respectively. It is expected that both the crystallite size and strain effects are considered by
the Voigt’s function. According to Balzar [58], integral breadths of the size and strain
components of Cauchy and Gaussian parts can be expressed as,
... (1)
… (2)
where
and
are the Cauchy and Gaussian components of the total integral breadths β
respectively. The term
and
integral breadth, respectively and
represents the Cauchy components of size and strain
and
are the corresponding Gaussian components.
After the components are calculated by the above equations, the maximum lattice strain (e)
and volume weighted crystallite size ( DV) , can also be determined using equations (3) and
(4) [59].
… (3)
… (4)
53
Chapter 4- Results and Discussion
Here,
and
are the integral breadths of a Voigt’s function comprising the Gaussian and
the Lorentzian components respectively. λ is the wavelength and θ is the diffraction angle.
After 20 h of milling the crystallite size of Cu was found to be 18 nm (Fig. 4.1(c)). There is a
gradual drop in the crystallite size with milling time. From Fig. 4.1(c) it is evident that Cu
could be reduced to nanometric dimension within 5 h of milling. Cold welding was also
evident between 5 to 10 h of milling which led to the increase in crystallite size. There is a
gradual increase in the lattice strain due to severe deformation of the milled powder and
strain reaches a maximum value after 20 h of milling when the crystallite size is smallest
(Fig.4.1 (d)). Precise lattice parameter of milled Cu was also determined using the NelsonRiley function. Fig. 4.1(e) shows the variation of lattice parameter of Cu with milling time.
The lattice parameter of Cu shows a slight increase with milling time [60-64].
The lattice strain measured from the x-ray diffraction line broadening increased continuously
upto 10 h of milling due to the increase in dislocation density in the grains. The dislocation
density is very high within the heavily strained regions. With further milling upto 15 h, the
lattice strain decreases as the crystal disintegrates into subgrains that are separated by lowangle grain boundaries. The dislocations annihilate and recombine to form small angle grain
boundaries separating the individual grains. This results in a decrease of the lattice strain.
During further processing, deformation occurs in shear bands located in previously unstrained
parts of the material. The grain size decreases steadily and the shear bands coalesce. The
small angle boundaries are replaced by higher angle grain boundaries, implying grain rotation
and random orientation of the grains. As a result dislocation-free nanocrystalline grains are
formed. The minimum grain size obtainable by milling has been attributed to a balance
between the defect and dislocation structure introduced by the plastic deformation of milling
and its recovery by thermal processes.The variation of lattice strain is also reflected in the
lattice parameter of Cu calculated from the x-ray diffraction plots using Nelson-Riley
54
Chapter 4- Results and Discussion
function in Fig. 4.1(e). The lattice parameter shows an initial increase upto 5 h of milling.
This increase of lattice parameter is probably caused by the grain expansion due to the
increase in the density of dislocations. As a result of the increase in the dislocation density
the dislocations are closer to each other resulting in repulsion between them as the potential
energy increases when the dislocations are closer. Between 5 to 10 h of milling the lattice
parameter of Cu was found to decrease which is possibly because of the grain compression
due to the presence of compressive stress fields within the nonequilibrium grain boundaries
in the nanocrystallites. This results in the shrinkage of the lattice parameter. In addition the
decrease in the lattice parameter could also be due to the oxidation of the nanocrystalline Cu.
Oxidation of the nanocrystalline Cu accelerates their fracture. Beyond 15 h of milling both
the strain and the lattice parameter of Cu shows an increase. This is possibly due to the strain
hardening of the Cu powder that takes place when milling is done for a longer period of time.
[65-74].
(b)
(a)
(c)
(d)
(e)
Fig. 4.2 SEM images of (a) unmilled Cu and Cu milled for (b) 5h (c) 10 h (d) 15h (e) 20 h
55
Chapter 4- Results and Discussion
Figs. 4.2 (a-e) show the SEM images of milled Cu powder at different intervals. Due to the
ductile nature of Cu it was possible to flatten Cu particles to flat flakes within 5 h of milling.
20 h of milling leads to thickening of the plates due to cold welding.
(a)
(b)
21.07 nm
22.20 nm
(d)
(c)
(220)
(200)
(111)
(222)
(311)
Fig. 4.3(a-c) HRTEM images and (d) SAD pattern of 20 h milled Cu
The results for HRTEM analysis of 20 h milled powder is shown in Figs. 4.3(a-c), which
indicates that the powder has been reduced to nanometric dimension after high-energy
milling for 20 h. The SAD pattern in Fig. 4.3 (d) shows complete ring patterns which suggest
that the 20 h milled Cu powder has nanometric dimension. The ring patterns could be indexed
to the various crystallographic planes of Cu. The most important physical property of
particulate samples is its particle size. Particle size measurement is routinely carried out
across a wide range of industries and is often a critical parameter in the manufacture of many
products. The particle size analysis was done using a particle size analyser [75].
56
Chapter 4- Results and Discussion
(a)
(b)
Fig.4.4 Particle size analysis for (a) unmilled Cu and (b) 20 h milled Cu powder
Figs.4.4 (a, b) show the particle size analysis for unmilled Cu and 20 h milled Cu powder.
The average size of the particles was found to be 25.59 µm after 20 h of milling. Table 4.1
shows the particle size analysis of pure elemental Cu at different milling time. The results
show that there is a sudden increase in particle size at the initial stage of milling. This is due
to the cold welding effect in Cu as Cu is soft and ductile. At later stage the size of the
particles get reduced due to the fracture of the particles. The lowest particle size of 25.59 µm
is attained after 20 h of milling [76].
Table 4.1 Particle size analysis of pure Cu at different milling time
Milling Time (hours)
0
5
10
15
20
Average Particle Size, D(v,0.5) (µm)
27.61
57.36
35.72
25.91
25.59
57
Chapter 4- Results and Discussion
(b)
(a)
(c)
(d)
Fig. 4.5 (a) X-ray analysis of 20 h milled Cu at different heat treating temperatures(b)
Variation of crystallite size with variation in heat treating temperature (c) Variation of strain
with variation in heat treating temperature (d) Lattice parameter at different heat treating
temperatures.
Fig.4.5 (a) shows the x-ray diffraction plots of 20 h milled Cu heat treated at 200, 400, 600oC
for 2 h in Ar atmosphere. Apart from the peaks of Cu few peaks of CuO and Cu2O are also
visible in the x-ray diffraction patterns of the 20 h milled samples heat treated at 200, 400,
600oC. This is possibly due to the oxidation of Cu during heat treatment. There is an increase
in crystallite size with the increase in heat treating temperature due to the grain growth of Cu
during heat treatment as can be seen in Fig (4.5(b)). The lattice strain in Cu decreases after
heat treatment as can be seen in Fig.4.5(c). There is a contraction in lattice parameter with the
increase in the heat treatment temperature as shown in Fig.4.5 (d). This is possibly due to the
58
Chapter 4- Results and Discussion
presence of a high concentration of oxygen in the Cu powder as the presence of oxygen leads
to the contraction of the lattice.
(a)
Element Weight% Atomic%
OK
4.17
14.74
Cu K
95.83
85.26
Totals
100.00
100.00
(b)
Element Weight% Atomic%
OK
7.69
24.86
Cu K
92.31
75.14
Totals
100.00
100.00
cps/eV
(c)
Cu
50
40
30
C norm.C Atom.
[wt.%] [at.%]
-------------------Cu
86.42
61.58
O
13.58
38.42
-------------------Total: 100.00 100.00
20
O
10
Cu
Cu
0
1
2
3
4
5
keV
6
7
8
Fig. 4.6 SEM and EDX analysis of 20h Cu (a) heat treated at 200oC for 2 h (b) heat treated at
400oC for 2 h (c) heat treated at 600oC for 2 h
Figs. 4.6 (a-c) show the SEM image and EDX analysis after heat treatment of the 20 h milled
Cu powder at 200oC, 400oC and 600oC respectively. EDX analysis confirms the presence of
59
9
10
Chapter 4- Results and Discussion
oxygen in the heat treated samples. This was also seen in the x-ray diffraction plots of the
heat treated samples in Fig.4.5 (a).
(a)
(b)
Fig. 4.7(a) HRTEM image (b) SAD pattern of 20 h milled Cu powder heat treated at 200oC
The HRTEM image in Fig.4.7 (a) suggest that the 20 h milled Cu powder still has nanomteric
dimension even after heat treatment for 2 h at 200oC. The SAD pattern inFig.4.7 (b) also
indicates the nanometric nature of 20 h milled Cu powder heat treated at 200oC for 2 h.
4.2 Cu-SiCp Composites
Cu is commonly used in electronics and thermal applications dueto its high electrical
(5.96×107 S/m) and thermal (401 W/m K) conductivity. It also shows good corrosion
resistance and high melting point (1083.4oC). However, the low mechanical strength of Cu
limits its application in several fields. Metal matrix composites reinforced with discontinuous
particles exhibits enhanced properties.Particle reinforced metal matrix composites have
significant improved properties due to the high strength and modulus of the reinforcement
particles like TiC, Al2O3, SiC, TiB2 etc. In the present study, the focus was towards the
development of Cu-based metal matrix composites using SiCp as reinforcement by powder
metallurgy route. Silicon carbide (SiC) is widely used as reinforcement because of its very
high thermal stability. Its coefficient of thermal expansion is 4.0×10−6/K. SiC also has high
thermal conductivity (16.7 W/m.K) and high hardness (20.5 GPa). Its melting point is above
60
Chapter 4- Results and Discussion
2600°C and it is also highly chemically inert in nature. The above properties mentioned
makes SiC particles a potential additive to Cu-based metal matrix composites. The Cu-SiCp
composites were prepared by powder metallurgy route. Cu containing different volume
fractions of SiC particles (10, 20, 30 and 40 vol. %) were developed by blending the
constituents followed by cold compaction of the samples in a uniaxial compaction machine
under a load of 665 MPa. Sintering of the samples was done at 900oC for 1 h in Ar
atmosphere. The optical micrographs of the various Cu-SiCp composites in Figs.4.8 (a-d)
show that the SiCp are homogeneously dispersed in the Cu matrix. The SiCp are found to be
less than 100 µm in size. Figs. 4.9 (a-d) are the SEM images of the various Cu-SiCp
composites developed using different vol. % of SiCp reinforcement in the Cu matrix. From
the microstructure it is clear that the SiC particulates are irregularly shaped and are
homogenously distributed in the Cu matrix [77-79].
(b)
(a)
SiC particle
Cu matrix
(c)
(d)
Fig.4.8 Optical micrographs of (a) unmilled Cu- 10 vol. % SiCp composite (b) unmilled
Cu- 20 vol.% SiCp composite (c) unmilled Cu- 30 vol. % SiCp composite (d) unmilled Cu40 vol. % SiCp composite
61
Chapter 4- Results and Discussion
(a)
(b)
SiC
particle
Cu
matrix
(c)
(d)
Fig. 4.9 SEM imagesof (a) unmilled Cu- 10 vol. % SiCp composite (b) unmilled Cu- 20
vol. % SiCp composite (c) unmilled Cu- 30 vol. % SiCp composite (d) unmilled Cu- 40 vol.
% SiCp composite
(a)
Element Weight% Atomic%
CK
20.69
30.13
OK
56.28
61.54
Si K
5.72
3.56
Cu K
17.31
4.77
Totals
100.00
100.00
(b)
Element Weight% Atomic%
CK
16.68
44.11
OK
8.86
17.59
Si K
1.71
1.93
Cu K
72.76
36.37
Totals
100.00
100.00
Fig. 4.10 EDX analysis of (a) unmilled Cu-40 vol. % SiCp composite at SiCp rich region
(b) unmilled Cu-40 vol. % SiCp composite at Cu rich region
62
Chapter 4- Results and Discussion
The EDX analysis in Fig. 4.10 suggests that the dark region in the SEM image corresponds to
the SiC particles and the light coloured region corresponds to the Cu matrix. The dark
coloured SiC particles are surrounded by the light coloured Cu rich regions. EDX analysis
shows the presence of oxygen in the sintered sample. This is due to the undesirable oxygen
present during sintering in the Ar atmosphere. The x-ray diffraction analysis of the various
unmilled Cu-SiCp composites in Fig.4.11 also show peaks corresponding to Cu2Owhich
confirms the oxidation of Cu by unavoidable oxygen present during sintering. Fig.4.11 shows
the x-ray diffraction plots of various unmilled Cu-SiCp composites having different vol. % of
SiCp. The x-ray diffraction plots show peaks corresponding to Cu and SiC. The higher
content of SiCp in the composites leads to more intense peaks corresponding to SiC. Peaks
corresponding to copper oxide (Cu2O) were also found in the composites.
Fig. 4.11 XRD plots of various unmilled Cu-SiCp composites
(a)
(b)
Fig. 4.12 (a) Variation of relative density of various unmilled Cu-SiCp composites
(b) Variation of microhardness of various as-milled and unmilled Cu-SiCp composites
63
Chapter 4- Results and Discussion
Fig. 4.12(a) shows the variation of relative density of the various unmilled Cu-SiCp
composites. There is a gradual decrease in the relative density with the increase in vol. % of
SiCp reinforcement in the Cu matrix. This is possibly due to the increase in the content of
brittle SiC particles in the soft Cu matrix. The variation of hardness in as-milled Cu-SiCp
composites and unmilled Cu-SiCp composites in Fig. 4.12(b) shows that the hardness of both
the unmilled Cu-SiCp composites and the as-milled Cu-SiCp composites increases with the
increase in the vol. % of SiCp reinforcement. However it can be seen from the Fig.4.12 (b)
that the as-milled Cu-SiCp composites show better hardness than the unmilled Cu-SiCp
composites. Milled Cu due to its smaller particle size leads to better sinterability and
densification resulting in higher hardness of the composites.
(a)
(b)
(c)
633µm
64
625µm
Chapter 4- Results and Discussion
(d)
(e)
Fig.4.13 (a) Wear Characteristic of unmilled Cu- SiC composites. SEM images of the wear
track of (b) unmilled Cu (c) unmilled Cu-40 vol. % SiCp composite. (d-e) High magnification
SEM images of the wear track of unmilled Cu-40 vol. % SiCp composite
Fig. 4.13 (a) shows the wear characteristics of the various unmilled Cu-SiCp composites. The
plot shows that the wear resistance of the Cu-SiCp composites are higher than that of the
native Cu. It can be observed that with the increase in vol. % of SiCp in the Cu matrix the
wear resistance of the composite increases significantly. Cu-40 vol. %SiCp shows the least
wear depth (Fig. 4.13(a)).Fig. 4.13(b) and Fig. 4.13 (c) shows the SEM images of the wear
track of unmilled Cu and unmilled Cu-40 vol. % SiCp composite respectively. Fig. 4.13 (d, e)
are the high magnification SEM images of the wear track of unmilled Cu-40 vol. % SiCp
composite. It was found that the hardness of Cu-SiCp composites increases with the addition
of SiCp due to the presence of the harder ceramic particles of SiC. The hard SiC particles
support the stresses between the contact surfaces preventing the large plastic deformations
and abrasions. This reduces the amount of worn material. The Cu-SiCp composites
containing the hard SiC particles show better wear resistance due to the formation of a
tribolayer at the interface. During wear rapid removal and regeneration of the tribolayer at the
interface takes place. The interface temperature also increases during the wear test and this
causes oxidation of the sample. The increase of interface temperature during the wear test
significantly enhances the loss of mass by oxidation, matrix softening, cracking and
delamination wear. In the initial stages of wear, fine particles are generated from the two
surfaces in contact by the micro-cutting and rubbing effects. Sharp SiC particle edges
65
Chapter 4- Results and Discussion
protrude out of the Cu-SiCp composite surface. The wear particles entrapped by the
contacting surfaces undergo a mechanical mixing process, which is very similar to the
mechanical alloying (MA) process. During milling some original particles fracture further
and expose atomically clean surfaces that come in contact with each other. With further
mixing, cold welding and particle fracturing takes place which leads to a steady state particle
size distribution in the mixture. Few fine particles are dislodged from the interface whereas
some agglomerate and pile up. Due to the pressing and flattening effect of the normal load
and frictional forces a compact mechanically mixed layer on the stationary and relatively soft
composite surface is formed. This layer acts as a protective layer for the composite. It should
be noted that due to the fine size of the debris particles and high temperature at the contact
the metal components like Cu can be oxidized very rapidly. The drastic reduction in wear rate
may be attributed to the enhancement in hardness of the composite reinforced by SiC
particles and greater reduction of direct load contact between the Cu-SiCp composite surface
and ball due to load bearing component action of the hard SiC particles. Fig. 4.13 (b,c) shows
the SEM images of wear track of unmilled Cu and unmilled Cu-40 vol. % SiCp composite
respectively. From the images it is evident that Cu-40 vol. % SiCp composites show less
width of wear track as compared to pure Cu [80-85].
(a)
(b)
SiC
particle
Cu
matrix
66
Chapter 4- Results and Discussion
(c)
(d)
(e)
(f)
Fig.4.14 (a-d) Optical micrographs of as- milled Cu-SiCp composites (10,20, 30 and 40
vol% of SiCp) (e, f) High magnification optical micrographs of as-milled Cu-40 vol. %
SiCp composite
Fig.4.14 shows optical micrographs of as-milled Cu-SiCp composites for different vol. % of
SiCp. From the micrographs it is evident that SiC particles are homogenously distributed all
over the Cu matrix. The high magnification optical micrographs of as-milled Cu-40 vol. %
SiCp composites in Fig. 4.14 (e, f) show the interface between the SiC particle and the Cu
matrix. Good interfacial bonding can be observed between Cu and SiCp.
(a)
(b)
SiC particle
67
Chapter 4- Results and Discussion
(c)
(d)
(e)
(f)
Fig.4.15 SEM image of (a) as-milled Cu-10 vol. %SiCp composite (b) as-milled Cu-20 vol.
%SiCp composite(c) as-milled Cu-30 vol. % SiCp composite and (d) as-milled Cu-40 vol.
%SiCpcomposite(e, f) High magnification SEM images of as-milled Cu-40 vol.% SiCp
composite. The inset image in (e) shows the EDX analysis of SiC particle
Figs. 4.15 (a-d) are the SEM images of as-milled Cu-SiCp composites for different vol. % of
reinforcement. The SEM images of the composites also show homogeneous dispersion of
reinforcement in the Cu matrix. The scanning electron micrographs given in Fig. 4.15show
the typical microstructural features of a composite. In the micrographs of the various CuSiCp composites a range of SiC particulate sizes could be seen. Good interfacial integrity
between the SiCp and the Cu matrix could also be observed. High magnification SEM images
of as-milled Cu-40 vol. % SiCp composites in Figs.4.15 (e, f) show the interface between the
SiC particle and the Cu matrix. EDX analysis inset in Fig.4.15 (e) confirms that the dark
particles are SiC. The SiC particles are embedded in the Cu matrix. The interface between the
reinforcement and the matrix plays a crucial role in determining the mechanical properties of
68
Chapter 4- Results and Discussion
the composites. The SEM images the Cu-SiCp interface suggests that the bonding between
the Cu matrix and the SiC particles is very strong.
Fig. 4.16shows the XRD plots of as-milled Cu-SiCp composites for different vol. % of SiC
particles. Peak corresponding to Cu2O could be detected in the x-ray diffraction plots of all
the sintered as-milled Cu-SiCp composites. This is because of the oxidation of Cu due to the
residual oxygen present in the sintering atmosphere.
Fig. 4.16 XRD plot of various as-milled
sintered Cu-SiCp composite
Fig. 4.17 Variation of Relative Density
plots of sintered as-milled Cu- SiCp
composite
Fig.4.17 shows the variation of relative density of as-milled Cu-SiCp composite for different
vol. % of reinforcement. The relative density gradually decreases with the increase in the
content of SiCp as the incorporation of hard and brittle material in the soft Cu matrix leads to
presence of voids in the composites. Fine particles of 20 h milled Cu produces finer pores
which diminishes earlier as compared to coarser pores formed by large sized Cu particles.
This is why composites developed from finer 20 h milled Cu powder leads to better
densification and hardness as compared to the composites developed from coarser unmilled
Cu powder.
69
Chapter 4- Results and Discussion
(a)
(b)
(c)
625µm
468µm
(d)
(e)
Fig.4.18 (a) Wear Characteristic of various as-milled Cu- SiCp composites. SEM images of
the wear track of (b) as-milled Cu (c) as-milled Cu-40 vol. %SiCp composite (d, e) High
magnification SEM images of the wear track of as-milled Cu-40 vol. %SiCp composite. Inset
image in (e ) shows the edge of the wear track of as-milled Cu-40 vol. %SiCp composite
The wear resistance of the pure as-milled Cu and the various as-milled Cu-SiCp sintered
composites were determined using a ball-on-plate tribometer. Fig. 4.18 shows the variation of
wear depth of the various as-milled Cu-SiCp composites. It has been found that the as-milled
Cu-SiCp composites exhibits better wear in comparison to pure Cu. As-milled Cu- 40 vol. %
SiCp composite shows highest wear resistance. The enhancement of wear resistance is
70
Chapter 4- Results and Discussion
possibly due to the strengthening of the composites because of the fine dispersion of the SiC
particles in the Cu matrix. The dispersion of the SiC particles, as a hard ceramic phase in the
Cu matrix, improves the wear resistance significantly. From Fig. 4.12(b) it is evident that
there is an enhancement of microhardness due to the dispersed SiC particles in the Cu matrix
and this consequently improves the wear resistance of the composite. The enhancement in the
wear resistance of the composites can also be attributed to the good bonding between the Cu
and the SiC particles. Bonding between the matrix and the reinforcement is known to play an
important role in the wear resistance. Reduction of direct load at the contact between the CuSiCp composite surface and the ball due to load bearing component action of the hard SiC
particles as compared to that of pure Cu also enhances the wear resistance of the Cu-SiCp
composites. The wear mechanism was found to involve a combination of abrasion and
delamination. Figs. 4.18 (b,c) are the SEM images of wear tracks of as-milled Cu and asmilled Cu- 40 vol.% SiCp composite.The width of the wear track in case of as-milled Cu- 40
vol.% SiCp composite was found to be much lower as compared to that of as-milled pure Cu
[86-88].
(a)
(b)
71
Chapter 4- Results and Discussion
(c)
(d)
(e)
(f)
(g)
Cu
Si
O
Fig. 4.19 (a, b) SEM images of wear debris from as-milled Cu-40 vol. %SiCp composite (c)
EDX analysis of the wear debris(d) SEM image of the wear debris selected for elemental
mapping. Elemental map of (e) Cu (f) Si (g)O
Figs.4.19 (a, b) are the SEM images of the wear debris from as-milled Cu-40 vol. % SiCp
composite. The EDX analysis of the wear debris in Fig. 4.19 (c)shows mainly the presence of
Cu and very small amount of Si. This suggests that the wear debris consists mainly of Cu and
has a very small amount of SiC in it. The elemental map of Cu and Si in the selected area
shown in the SEM image in Fig.4.19 (d) also suggests that the wear debris mainly consists of
Cu and there is only a very small amount of SiC in it. The oxygen in the wear debris is
possibly due to the oxidation of the sample by the unavoidable oxygen present during
sintering. Oxidation of the wear debris could also take place during the wear test [89, 90].
4.3 Cu-E-Glass Fiber Composites
E-Glass fibers are the most widely used glass fibers as reinforcement in the composites. Eglass fibers are alumina-borosilicate glass with less than 1% w/w alkali oxides and are mainly
used for glass-reinforced plastics. E-glass or electrical grade glass was originally developed
72
Chapter 4- Results and Discussion
for use as an insulators for electrical wiring which was later found to have excellent fiber
forming capabilities and is now widely used as the reinforcing phase in composites and is
commonly known as fiber glass. E-glass fibers exhibit useful bulk properties such as hardness
(6000 MPa), dimensional stability, resistance to chemical attack and high strength. It has a
tensile strength of 3500MPa and its Young’s modulus is 85 GPa. The compressive strength of
E-glass fiber is 5000 MPa. The maximum hardness of E-glass fibers is around 6000 MPa and
its density is 2.58 gm/cc.
(a)
(b)
Fig. 4.20 (a, b) Optical images of E-glass fiber used in composite
(a)
(b)
(c)
Fig. 4.21 (a-c) SEM image of E-glass fiber used in composite
Fig 4.20 and Fig 4.21 shows the optical and SEM images of the E-glass fiber used as the
reinforcement for development of Cu-E-glass fiber composites respectively. The fibers have
diameter ranging from 14.6 µm to 17.2 µm. The tensile test of the fiber was carried out in
Instron 1195. The fiber shows brittle fracture with elongation to failure of 4.134 %.Table 4.2
shows the results of the tensile test. The elongation to fracture for the E-glass fiber is very
low as compared to that of ductile metals.
73
Chapter 4- Results and Discussion
Load at peak = 0.0929 kN
Strain at peak = 2.962%
Strain at break = 4.134%
Fig.4.22 Load vs Displacement plot of E-glass fiber
Table 4.2 Results obtained from the tensile test of E-glass fiber
(The tensile test was performed for a bunch of 15 E-glass fibers)
Strain at peak (%)
2.962
Strain at Break (%)
4.134
Load at Peak (kN)
0.0929
E-glass fiber is highly brittle and shows almost flat fracture surface. The maximum
elongation to fracture for E-glass fiber has been reported as 4.8 % and it has 100 % recovery
when stressed below the point of rupture. The tensile test for the E-glass fiber used in the CuE-glass fiber composites has also shown a similar strain to failure value of 4.134 %. The
tensile strength of a single E-glass fiber was found to be ∼2336 MPa [25].
(a)
SE Image
(b)
BSE Image
Fig. 4.23.1 (a) Secondary Electron SEM image (b) Backscattered Electron SEM image of
unmilled Cu-10 vol. % E-glass fiber composite sample sintered at 900oC for 1h
74
Chapter 4- Results and Discussion
(a)
SE Image
(b)
BSE Image
E-glass fiber
Cu matrix
Fig. 4.23.2 (a) Secondary Electron SEM image (b) Backscattered Electron SEM image of
unmilled Cu-20 vol. % E-glass fiber composite sample sintered at 900oC for 1h
(a)
(b)
SE Image
BSE Image
Fig. 4.23.3 (a) Secondary Electron SEM image (b) Backscattered Electron SEM image of
unmilled Cu-30 vol. % E-glass fiber composite sample sintered at 900oC for 1h
(a)
SE Image
(b)
BSE Image
Fig. 4.23.4 (a) Secondary Electron SEM image (b) Backscattered Electron SEM image of
unmilled Cu-40 vol. % E-glass fiber composite sample sintered at 900oC for 1h
Fig. 4.23 shows the microstructure of various sintered unmilled Cu-E-glass fiber composites
developed containing different vol. % of E-glass fiber. Sintering was done at 900oC for 1 h in
Ar atmosphere. From the microstructural analysis it is evident that the composites fabricated
have random distribution of the reinforcement in the Cu matrix and shows very good bonding
75
Chapter 4- Results and Discussion
with the matrix. The fibers show good wettability with Cu and shows good bonding with the
Cu matrix. Back scattered electron SEM image shows very good wettability of Cu with Eglass fiber. Relative density of 85.8 % could be achieved from 10 vol. % Cu - E-glass fiber
composite [91,92].
(a)
Element
Weight%
Atomic%
OK
16.80
44.51
Cu K
83.20
55.49
Totals
100.00
100.00
Element
Weight%
Atomic%
OK
47.61
63.58
Mg K
4.74
4.17
Si K
30.13
22.92
Ca K
17.52
9.34
Totals
100.00
100.00
EDX analysis of the point
marked in SEM image
(b)
EDX analysis of the point
marked in SEM image
Fig. 4.24 (a, b) EDX analyses of unmilled Cu-40vol. % composite sintered at 900oC for 1h
Fig. 4.24(a) shows the EDX analysis of the matrix phase in the unmilled Cu-40 vol. %
composite. Fig. 4.24 (b) shows the EDX analysis of the E-glass fiber reinforcement. The EDX
analysis of the Cu matrix in Fig 4.24 (a) shows a small amount of oxygen in the matrix. This is
possibly due to the unavoidable oxygen present in the furnace during sintering. Spectrums have
been taken at different places to show the various phases in the composite. The EDX analysis
of the E-glass fiber in Fig. 4.24 (b) shows presence of elements like Mg (4.17 at. %), Ca (9.34
at. %), Si (22.92 at. %) and O (63.58 at. %).
76
Chapter 4- Results and Discussion
(a)
(b)
Fig. 4.25 (a) Relative Density plot of various sintered unmilled Cu-E-glass fiber composite
(b) Vickers hardness plot of various unmilled Cu-E-glass fiber composite
Fig. 4.25 (a) shows the variation of relative density of unmilled Cu-E-glass fiber composite
containing different vol. % reinforcement. The densities of all the specimens have been found
out using the Archimedes’ principle. The relative density increases initially with the increase
in the content of E-glass fiber in the unmilled Cu-E-glass fiber composite but it shows slight
decreases when higher vol. % of glass fiber was added. It should be noted that a high vol. %
of glass fiber could lead to a lower theoretical density of the unmilled Cu-E-glass fiber
composite. Fig. 4.25 (b) shows the variation of Vickers hardness test for unmilled Cu-E-glass
fiber composites containing different vol. % of E-glass fiber. The hardness of the Cu-E-glass
fiber composite increased with increase in vol. % of glass fiber in the composite. This is due
to the strengthening of Cu matrix by the glass fiber. The fracture surface of the various
samples was analysed using a SEM. Fractographs of unmilled Cu-E-glass fiber composites
are shown in Fig.4.26. It can be observed that the Cu matrix undergoes ductile fracture
showing the presence of dimples in the fractured surface as shown in Fig.4.26.1 (a). E-glass
fibers could be seen embedded in the Cu matrix.
77
Chapter 4- Results and Discussion
(a)
(b)
Dimples
Fiber Pull-out
Fig.4.26.1 (a, b) SEM images of fracture samples of unmilled Cu- 10 vol. % E-glass fiber
composite
Fiber pull out could be seen in the unmilled Cu-10 vol. % E-glass fiber composite sample in
Fig.4.26.1(b).This is possibly due to the low load bearing capacity when the vol. % of glass
fiber is lower. Higher stress on each fiber leads to pull-out of the fiber. The stress on the fiber
also leads to plastic deformation of the Cu matrix around the fiber which is evident in the
SEM image in Fig.4.26.1(a).The fiber tries to restrict the plastic deformation in the Cu
matrix.
(a)
(b)
Fiber
Fig. 4.26.2(a, b) SEM images of fracture samples of unmilled Cu- 20 vol. % E-glass fiber
composite
Fig. 4.26.2 (a, b) are the SEM images of the fracture surfaces of unmilled Cu-20 vol. % Eglass fiber composite. From the SEM images it is clear that the nature of fracture in the Cu
matrix is ductile whereas the E-glass fiber undergoes brittle fracture. The fractured E-glass
fibers have Cu adhered to its surface indicating good wettability and bonding between the Cu
78
Chapter 4- Results and Discussion
matrix and the glass fiber. Figs.4.26.3 (a,b) are the SEM images of the fracture surfaces of
unmilled Cu-30 vol.% of E-glass fiber composite. Here also we seeductile nature of fratcure
in the Cu matrix and brittle fracture in the E-glass fibers. The SEM image in Fig.4.26.3 (a)
show pull-out of the E-glass fiber. The SEM images also show the presence of higher amount
of pores in the sample. It was also found out earlier (Fig. 4.25(a)) that the relative density of
the unmilled Cu- 30 vol. % E-glass fiber composite (78.2 %) is less than the relative density
of unmilled Cu- 20 vol. % E-glass fiber composite (85.6%). Addition of E-glass fiber beyond
20 vol. % has led to a decrease in the relative density of the composite.
(a)
(b)
Cracks
E-glass fiber
Fiber pull out
Fig. 4.26.3 (a, b) SEM images of the fracture surface of unmilled Cu- 30 vol. % E-glass fiber
composite
(a)
(b)
Cracks
Dimples
Fiber pull out
Fig. 4.26.4 (a, b) SEM images of the fracture surface of unmilled Cu- 40 vol. % E-glass fiber
composite
Figs.4.26.4 (a,b) showthe fracture surface of unmilled Cu-40 vol.% of E-glass fiber
composite. The Cu matrix shows dimples on its fracture surface which suggests the ductile
nature of fracture in Cu matrix.Several cracks can be clearly seen in the Cu matrix. As can be
79
Chapter 4- Results and Discussion
seen from the Fig. 4.25(a) the relative density of the unmilled Cu-40 vol. % E-glass fiber
composite is even lower than that of the unmilled Cu-30 vol. % E-glass fiber composite.
Addition of glass fiber beyond 20 vol. % deteriorates the relative density even further.
Addition of glass fiber beyond 20 vol. % leads to poor sinterability and densification of the
composites. This is the reason why we see a large number of pores in the Cu matrix.
(a)
(b)
10 vol. % E-glass fiber
20 vol. % E-glass fiber
(c)
(d)
30 vol. % E-glass fiber
40 vol. % E-glass fiber
Fig.4.27 (a-d) Optical micrographs of as-milled Cu-E-glass fiber composites for different
vol. % of fiber used as reinforcement (10, 20, 30 and 40 vol. %).
(b)
(a)
10 vol. % E-glass fiber
20 vol. % E-glass fiber
80
Chapter 4- Results and Discussion
(c)
(d)
30 vol. % E-glass fiber
40 vol. % E-glass fiber
Fig.4.28 (a-d) FESEM images of as- milled Cu-E-glass fiber composites for different vol. %
of fiber used as reinforcement (10, 20, 30 and 40 vol. %)
From the Figs.4.27 (a-d) it is clearly evident that there is a homogenous dispersion of fibers
throughout the Cu matrix. The diameters of fibers are approximately 20µm or lower. It is
clearly evident from the FESEM images that the fibers are embedded in the Cu matrix.
FESEM images of as-milled Cu-E-glass fiber composites in Fig. 4.28 (a-d) also show the
random distribution of reinforcements throughout the Cu matrix.
(a)
(b)
Fig 4.29 (a) Variation of relative density of various sintered as-milled Cu-E-glass fiber
composites (b) Variation of Vickers hardness of various as-milled Cu-E-glass fiber
composites
Comparison of the plots in Fig. 4.29 (a) and Fig. 4.25 (a) it is evident that the relative density
of the various sintered as-milled Cu-E-glass fiber composites is higher than that of the
various sintered unmilled Cu-E-glass fiber composites. In the case of as-milled Cu-E-glass
fiber composites 20 h milled nanocrystalline Cu is used as the matrix which results in better
81
Chapter 4- Results and Discussion
sinterability and densification. Cu has a melting temperature of 1083oC. Nanocrystalline
nature of Cu could reduce the melting point of Cu. This is why as-milled Cu possibly softens
at the sintering temperature of 900oC and results in better sinterability and densification
compared to the unmilled Cu-E-glass fiber composites. Nanocrystalline Cu has higher surface
area per unit volume and this could reduce its melting point. The hardness of the composites
shown in Fig.4.29 (b) also shows a gradual rise with the rise in vol. % of E-glass fiber in the
composites.Fig.4.29 (b) shows the variation of Vickers microhardness of as-milled Cu-Eglass fiber composites. As-milled Cu-E-glass fiber composites possess better hardness
because of better densification during sintering at 900oC for 1 h. The fractured surfaces of the
samples were analyzed under SEM. Fractographs of as-milled Cu-E-glass fiber composites
are shown in Fig.4.30. The dimples on the matrix shown in Figs.4.30.1 (a, b) suggest the
ductile fracture of the Cu matrix. It can also be seen that fibers are embedded inside the Cu
matrix and has good binding with the matrix. The SEM image in Fig.4.30.1 (b) clearly shows
the brittle fracture of E-glass fiber.
(a)
(b)
Brittle Fracture
of Fiber
Dimples
Fig.4.30.1 (a, b) SEM image of fracture surface of as-milled Cu- 10 vol. % E-glass fiber
composite
Figs. 4.30.2 (a, b) show the SEM images of as-milled Cu-20 vol.% E-glass fiber composites.
The fibers are deeply penetrated insided the Cu matrix. There is a random distribution of
fibers inside the Cu matrix. Fiber pull-out can easily be seen in Fig.4.30.2 (b).
82
Chapter 4- Results and Discussion
(a)
(b)
Fiber
Pull-out
Fig.4.30.2 (a,b) SEM images of the fracture surface of as-milled Cu- 20 vol. % E- glass fiber
composite
(b)
(a)
Cracks
Fig.4.30.3 (a, b) SEM images of the fracture surface of as-milled Cu- 30 vol. % E glass fiber
composite
Figs.4.30.3 (a,b) show the SEM images of the fracture surface of as-milled Cu-30 vol.% Eglass fiber composite. The fibers show good interfacial integrity with the Cu matrix. Cracks
could be seen in the Cu matrix. However, in the case of as-milled Cu-30 vol. % E-glass fiber
composite fewer numbers of pores and cracks could be seen in the Cu matrix as compared to
the unmilled Cu-30 vol. % E-glass fiber composite. This can be seen by comparing the
fractographs of the two composites in Figs.4.26.3 (a, b) and Figs.4.30.3 (a, b).This is possibly
due to better sinterability and densification in the case of as-milled Cu composite as
compared to the unmilled Cu composite. It should be noted that the Cu-E-glass fiber
composites developed by using as-milled Cu have higher relative density values as compared
to the Cu-E-glass fiber composites developed by using unmilled Cu.
83
Chapter 4- Results and Discussion
(a)
(b)
Brittle
Fracture of
Fiber
Fig.4.30.4 (a, b) SEM image of fracture surface of as-milled Cu- 40 vol. % E- glass fiber
composite. Inset in Fig (b) shows the EDX of agglomerated finer Cu particle
Fig.4.30.4 shows the SEM image of the fracture surface of as-milled Cu-40 vol.% E-glass
fiber composite. The SEM images clearly show Cu adhered to the E-glass fiber surface. The
brittle fracture of the glass fiber could be clearly seen in Fig.4.30.4(a). In the case of asmilled Cu-40 vol. % E-glass fiber composite a large number of Cu rich spherical particles
could be seen. This is possibly due to the agglomeration of fine Cu particles which tend to
come together to form larger particles thereby reducing the surface area per unit volume.The
surafce area per unit volume (1/r) varies inversely with the radius of the particle (r). This is
due to the Gibb-Thomson effect whichpredicts that nanoparticles dissolve at a lower
electrochemical potential as compared to the bulk materials. The Gibbs-Thomson effect is
due to the reduction of local chemical potential caused by nanoscale curvature. This was not
seen in thecase of unmilled Cu composites due to the coarse nature of the Cu particles in the
matrix [93-97].
4.4 Cu-Multi Walled Carbon Nanotubes Composites
Carbon nanotubes are very unique material found till date. The small dimension, strength and
other remarkable properties make them a promising candidate for potential applications.
CNTs are tube shaped material and can be synthesized with high purity and yield. CNTs are
made of carbon having diameter measuring on nanometer scale. Carbon nanotubes typically
84
Chapter 4- Results and Discussion
have diameters ranging from <1 nm to 50 nm. CNTs show a unique combination of
properties like strength, stiffness and tenacity compared to other fibers and materials. They
show high thermal and electrical conductivity values comparable to other conducting
materials. CNTs possess extraordinary mechanical properties compared to carbon fibers.
Their elastic modulus value is in the range of 0.3-1 TPa, tensile strength is of the order of
10-60 GPa, thermal conductivity is 3000 W/mK and electrical conductivity is 106-107 S/m.
Carbon nanotubes have strength approximately 100 times greater than that of steel of the
same diameter. The density of multiwalled CNTs is 2.6 gm/cc and their specific surface area
is about 200-400 m2/gm. CNTs do not show any environmental or physical degradation like
thermal expansion, corrosion and sensitivity to radiation. In order to optimize carbon
nanotubes yield and quality, three main synthesis processes have been used, namely, arc
discharge, laser ablation and chemical vapour deposition (CVD). Amongst the above three
mentioned processes, CVD is the most promising route for bulk production of high purity
nanotubes. The CVD method involves the catalytic decomposition of the hydrocarbon with
the aid of supported transition metal catalysts (Fe, Co, Ni, etc.). It is a versatile process in
which the gas phases are decomposed to form reactive species. These species lead to particle
or film growth. A conventional horizontal quartz tube having dimensions approximately 1 m
long and 50 mm diameter was used. The temperature was maintained at 900oC with holding
time of 40 minutes. Subsequently, the synthesis was initiated by introducing a flow of 20
sccm acetylene (C2H2), 50 sccm ammonia (NH3) and 40 sccm hydrogen (H2) for 40 minutes.
Acetylene (C2H2) was used as the carbon source and was fed into the tube furnace at a
controlled rate. To prevent oxidation and removal of gaseous by products argon (Ar) gas was
fed at a controlled rate of 600 sccm during the whole heating-up, growth and cooling down
periods [98-101].The improvement of mechanical properties of Cu is important for its use in
many applications. Carbon nanotubes (CNTs), since their discovery, have been used as a
85
Chapter 4- Results and Discussion
reinforcement material for the fabrication of a variety of composites. CNTs provide a new
avenue for the reinforcement of copper based materials. Carbon nanotubes not only help in
retaining the properties of the Cu matrix, but also enhance the mechanical, electrical and
thermal properties of Cu. Cu-MWCNT composites would give significantly improved
mechanical as well as electrical properties.
(101)
Fig.4.31 XRD of the MWCNTs synthesized by LPCVD process
Fig. 4.31 shows the x-ray diffraction plot of MWCNTs synthesized by low pressure chemical
vapour deposition (LPCVD) process. The x-ray diffraction plot confirms the peaks
corresponding to MWCNTs. The x-ray diffraction plot confirms the crystalline nature of the
carbon nanotubes. Peaks indexed to (002), (100) and (101) planes reflects the hexagonal
structure of the carbon nanotubes. The presence of (002) peak in the x-ray diffraction plot,
suggests multiwalled nature of carbon nanotubes [102 ,103].
Figs. 4.32 (a-c) are the FESEM images of the carbon nanotubes prepared by the LPCVD
process. The average diameter of the MWCNTs is found to be in the range of 10-30 nm with
length of several micrometers. From the SEM images it can be seen that carbon nanotubes are
agglomerated. The agglomerated lumps of carbon nanotubes were dispersed by
ultrasonication.
86
Chapter 4- Results and Discussion
(a)
(c)
(b)
Fig.4.32 (a-c) FESEM images of MWCNTs synthesized by LPCVD process
(a)
(b)
(c)
(d)
Outer diameter
Inner diameter
Fig.4.33 (a-c) HRTEM images and (d) SAD pattern of MWCNTs synthesized by LPCVD
process
Figs. 4.33 (a-c) are the HRTEM images of the MWCNTs synthesized by LPCVD process.
From the HRTEM images it is evident that the carbon nanotubes synthesized are multiwalled
in nature. The average length of the nanotubes is approximately 2 μm. The SAD image in
Fig. 4.33 (d) shows complete ring pattern which indicates the nanometric dimension of the
MWCNTs.
87
Chapter 4- Results and Discussion
105
3440 cm-1-OH stretching
%T
102.5
100
97.5
1720cm-1C=O
Stretching
95
1650cm-1CNT
Backbone
92.5
90
87.5
85
82.5
4000
CNT 2
3500
3000
2500
2000
1750
1500
1250
1000
750
500
1/cm
Fig. 4.34 FTIR analysis of CNTs after acidic functionalization
Functionalization of MWCNTs is an essential part for the formation of the Cu-MWCNT
composites. The functional groups provide a point of bonding between the metal surface and
the MWCNTs. Fig. 4.34 shows the FTIR spectrum for the acid treated MWCNTs in range of
wave numbers (400-4000 cm-1). Stretching vibrations from carboxyl groups (C=O) could be
seen at around 1720 cm-1.The bands of the hydroxyl group (-OH) stretching vibrations are
seen at 3440 cm-1 in the FTIR plot. At 1650 cm-1 skeletal vibration from unoxidized graphitic
domains of the carbon nanotube backbone were observed. These results suggest that the
MWCNTs were successfully acid-modified [104].
The Cu-MWCNT composites were prepared by powder metallurgy route. Cu powder with
different volume fractions of MWCNTs (1, 2 and 5 vol. %) were developed by blending the
constituents followed by cold compaction of the samples in a uniaxial compaction machine
under a load of 665 MPa. Sintering of the samples was done at 900oC for 1 h in Ar
atmosphere. Microstructure analysis and the dispersion of acid functionalized MWCNTs in
Cu matrix were studied using both optical microscope and SEM. The optical micrographs in
Figs. 4.35 (a-c) show the homogeneous dispersion of MWCNTs in the Cu matrix. The SEM
images in Figs.4.36 (a-c) show the surface of the sintered samples. MWCNTs are clearly
visible on the surface of the Cu-MWCNT composites. The SEM images show the
88
Chapter 4- Results and Discussion
homogeneous distribution of MWCNTs in the Cu matrix and very less agglomeration of
MWCNTs is noted.
(a)
(b)
(c)
MWCNTs
Fig.4.35 Optical micrographs of (a) Cu-1 vol. % MWCNT (b) Cu-2 vol. % MWCNT and
(c) Cu-5 vol. % MWCNT composite
(a)
(b)
CNTs
CNT
(c)
Fig.4.36 SEM image of (a) Cu- 1 vol. % MWCNTs (b) Cu- 2 vol. % MWCNTs and
(c) Cu- 5 vol. % MWCNTs composite
Fig. 4.37 shows the EDX analysis for Cu-1 vol. % MWCNT composite. The EDX analysis was
done at the dark region marked in the SEM image in Fig. 4.37. The EDX analysis shows the
89
Chapter 4- Results and Discussion
presence of some percentage of Cu (22.90 at. %) as carbon nanotubes (77.10 at. %) are
surrounded by the Cu matrix.
Element
CK
Cu K
Totals
Weight%
38.89
61.11
100.00
Atomic%
77.10
22.90
100.00
EDX analysis of the point
marked in SEM image
Fig.4.37 EDX analysis of Cu- 1 vol. % MWCNT composite
(b)
(a)
C
SEM Image
(c)
(d)
Cu
O
Fig.4.38 (a) SEM image. Elemental map of (b) C (c) O (d) Cu in Cu-5 vol. % MWCNT
composite
Fig. 4.38 shows the elemental maps of C, O and Cu in Cu- 5 vol. % MWCNTs composite.
The elemental map of C in Fig.4.37 (b) shows the distribution of the MWCNTs in the Cu
matrix. The elemental map of O in Fig.4.38 (c) shows the distribution of oxygen in the Cu
rich matrix. Fig. 4.39shows the x-ray diffraction plots of the various Cu-MWCNT composites
90
Chapter 4- Results and Discussion
containing different vol. % of MWCNTs. The most intense (002) peak of carbon nanotubes at
2θ = 26.4o is not visible as the concentration of the MWCNTs in the Cu-MWCNT composites
is very small. Peak corresponding to Cu2Ocould be seen at 2θ =37.2o. This is due to the
possible oxidation of Cu by the residual oxygen present during the sintering.
Fig.4.39X-ray diffraction plots of various Cu-MWCNT composites
Fig. 4.40 shows the relative density of Cu-MWCNT composites for different vol. % of
MWCNT reinforcement. It can be observed that the relative density increases with the
increase in volume fraction of carbon nanotubes in the Cu matrix. This is possibly due to
filling of the micro-voids in the Cu matrix by the nanometric MWCNTs. However, it does
not show significant increase in densification after addition of 2 vol. % of MWCNTs in the
Cu matrix. The chemical bonding formed between the MWCNTs and the Cu matrix provides
homogeneous distribution of CNTs as well as high interfacial strength. It should be noted that
there is a good bonding between the Cu matrix and the MWCNTs due to the acidic treatment
of the MWCNTs during functionalization which produces active sites providing better
wettability.
91
Chapter 4- Results and Discussion
Fig.4.40Relative density plot of various
Cu-MWCNT composites
Fig.4.41 Variation of microhardness of
various Cu- MWCNT composites
Fig.4.41 shows the Vickers microhardness of various Cu-MWCNT composites. The hardness
values of Cu-MWCNT composites increase gradually with the increase in the amount of
carbon nanotubes in the Cu-MWCNT composites. Carbon nanotubes can withstand much
higher loads than the Cu matrix and hence there is a substantial increase in the hardness
values. Another important factor is the interfacial bonding between the MWCNTs and the Cu
matrix, which is aided by the functionalization of MWCNTs. Introduction of carboxyl and
hydroxyl groups on the surface of the carbon nanotubes during functionalization provides
binding sites for the Cu matrix to the MWCNTs [105,106].
(a)
92
Chapter 4- Results and Discussion
(b)
(c)
319 µm
(d)
208 µm
206.5µm
Fig. 4.42 (a) Wear Characteristic of Cu- MWCNT composites and FESEM images of the
wear track of (b) Cu- 1 vol.% MWCNT (c) Cu- 2 vol.% MWCNT and (d) Cu- 5 vol.%
MWCNTs composite
MWCNT reinforced Cu-based metal matrix composites were fabricated by powder
metallurgy method. The dispersion of CNTs in the Cu matrix plays important role in
enhancing the wear resistance of the Cu-MWCNTs composites. The wear characteristics for
Cu-MWCNTs composites shown in Fig. 4.42 (a) indicate that the wear resistance of the CuMWCNTs composites increases with increasing volume fraction of the MWCNTs in the
composite. The MWCNTs act as a lubricating carbon film. The low coefficient of friction of
the MWCNTs leads to higher wear resistance of the Cu-MWCNTs composite. With the
addition of MWCNTs there is reduction in direct contact between the Cu matrix and the
indenter. Figs. 4.42 (b-d) show the FESEM images of the wear tracks of Cu-1, 2 and 5 vol. %
MWCNTs composites. It can be seen that the width of the wear track reduces with the
increase in volume fraction of MWCNTs in the Cu-MWCNTs composite [107-110].
………………………………………
93
Chapter 5- Conclusions
Chapter 5
Conclusions
94
Chapter 5- Conclusions
Conclusions
This thesis reports the results of a systematic study of development and characterization of
Cu-based metal matrix composites using SiCp, E-glass fiber and multiwalled carbon
nanotubes (MWCNTs) as reinforcements. The conclusions drawn from the present
investigation are as follows:
1. Milling of elemental Cu powder for 20 h led to the formation of nanostructured Cu.
The crystallite size of Cu after 20 h of milling was found to be around 18 nm. This
was confirmed by both x-ray diffraction analysis and high resolution transmission
electron microscopy images.There is a gradual increase in the lattice strain with
milling time due to the severe deformation of the milled powder and the strain reaches
a maximum value of around 0.383 % after 20 h of milling when the crystallite size
was smallest.
2. X-ray diffraction analysis of 20 h milled Cu powder reveals that there has been no
trace of contamination from the milling media.
3. Heat treatment of the 20 h milled Cu powder showed growth in crystallite size of Cu
with the increase in the heat treating temperature. There was a gradual rise in the
crystallite size of 20 h milled Cu with the increase in the heat treatment temperature
and beyond the heat treating temperature of 400oC the crystallite size of Cu was found
to be above 100 nm.
4. The hardness of the Cu-SiCp composite increases with the increase in the content of
SiCp. As-milled Cu-SiCp composites showed higher hardness as compared to the
unmilled Cu-SiCp composites due to the better sinterability when milled Cu was used
as the matrix. The highest value of hardness was found to be 3.24 GPa for the asmilled Cu- 40 vol. % SiCp composites.
95
Chapter 5- Conclusions
5. Cu-based MMCs reinforced by SiCp have significantly enhanced the wear resistance
of the Cu-SiCp composites both in the case of unmilled and as-milled Cu-SiCp
composites. The rise in SiCp reinforcement content imparts higher wear resistance
values as compared to native Cu.
6. The hardness values of Cu-E-glass fibre composite increases with the increase in vol.
% of E-glass fiber in the composite both in the case of unmilled and as-milled Cu-Eglass fiber composites.
7. As-milled Cu-E-glass fiber composites shows better densification and sinterability
compared to the unmilled Cu-E-glass fiber composites due to the finer size of Cu
particles in the milled Cu powder. Better densification and sinterability has led to
higher hardness. The maximum hardness of 2.86 GPa was found in the case of asmilled Cu-40 vol. % E-glass fiber composite.
8. The relative density of the Cu-MWCNTs composites increase with the increase in the
volume fraction of carbon nanotubes in the Cu matrix. However, the Cu-MWCNTs
composites do not show significant increase in densification after addition of 2 vol. %
of MWCNTs in the Cu matrix.
9. The hardness of Cu-MWCNTs composites increases gradually with the increase in the
amount of carbon nanotubes in the Cu-MWCNTs composites. Cu-5 vol. % MWCNTs
composite showed highest hardness of 1.44 GPa.
10. Addition of MWCNTs to the Cu matrix improved the wear resistance of the CuMWCNTs composites. An increase in wear resistance of the Cu-MWCNTs
composites was seen with the addition of upto 5 vol. % of MWCNTs.
………………………………..
96
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Publications/Conferences

S.N.Alam, Harshpreet Singh, Development of copper-based metal matrix composites:
An analysis by SEM, EDS and XRD, Microscopy and Analysis, 28(4) (2014) 8-13.

Harshpreet Singh, Lailesh Kumar, Syed Nasimul Alam, Development of Cu
reinforced SiC particulate Composite, IOP Conference Series: Materials Science and
Engineering 75 (1) (2015) 012007.

Oral Presentation at IUMRS-ICA-2013 held at IISc Bangalore, Title “Development of
Cu based metal matrix composites by mechanical alloying route”.

Poster Presentation at 5th National Symposium for Research Scholars at IIT Bombay,
Title “Synthesis of Cu-E glass fibre Metal matrix Composites”.

Oral Presentation at NCPCM-2013 held at NIT Rourkela, Title “Synthesis and
Characterization of SiC particle reinforced Cu Matrix Composites”.

Oral Presentation at NCPCM-2014 held at NIT Rourkela, Title “Development of Cu
reinforced SiC particulate Composite”.
…………………………………
Bio Data
Name
Harshpreet Singh
Date of Birth
22.02.1990
Permanent Address
4M/2 Dabouli
Ratan lal Nagar
Kanpur
Uttar Pradesh
India
Pin: 208022
E-mail ID
[email protected]
Languages Known
English, Hindi, Punjabi
Educational Qualifications:
Year
Degree/Certificate
Institution
2012
Bachelor of Technology
(Mechanical Engineering)
Sunderdeep Engg. College
2008
S.S.C ( CBSE Board)
Harmilap Mission School,
Kanpur
2006
H.S.C (ICSE Board)
U P Kirana Public School,
Kanpur
Marks obtained
78.9%
1st Div. Honours
76%
1stDiv.
72.5%
1stDiv.
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