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CHAPTER 1
INTRODUCTION
1.1 Background
“Surface finishing" is a generic term applied to a variety of processes for the
purpose of enhancing one or more properties of the surface of a metal. It is also applied
to a number of processes that involve the application of a metallic coating to a nonmetallic surface such as plastic, ceramic, epoxy, and even baby shoes. The art of
creating an altered surface on a metallic substrate dates back several centuries (1). There
are various processes to enhance the metal surface properties which are known as
surface finish. The concept of surface finish and altering the surface of materials has
been around for centuries, which includes grinding, painting, polishing and other nonsurface finishing processes such as heat treatment and tempering of metals.
Electroplating is considered as one of the most effective surface finishing processes,
which provides a thin surface coating over the substrate surface. The properties of
coating are superior to the substrate which results in protecting the substrate surface
Dennis and Such, 1993(1) . According to Zhong (2010), in 1950 decorative coatings
were introduced to toys and textile industry, since then its usage in engineering and
science areas provides an opportunity to improve the surface properties and
consequently increase sale of equipment and products. Alongside the industrial growth
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and technological advancements, new processes such as electroplating and numerous
methods of altering non-conductors have been introduced. Deposition of metallic
coating is considered as one of the most important applications of electroplating. This
process is carried out in the solution which contains the related metal salt to provide
sufficient metal ions in the plating solution. ‘Chemical’, ‘electrochemical’, and ‘laser’
deposition are three main sources to provide the metallic coating which are shown in
Figure 1.1.
Figure 1.1 Schematic main source of plating techniques (Kennai 2006)
Poyner (1997) described, electroplating which is also known as electrodeposition is one
of the most common processes for providing a coating. In this process, metal ions or
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complex metal ions in a chemical solution are transformed into solid metallic atoms onto
the surface of substrate, when an electrical pulse or current is applied (2).
This is more supported by Parthasaradhy (1989) who asserted, during the
electroplating process a metallic or composite coating has been produced on the surface
of material by applying electric current. The deposition is achieved by making the
material to be coated (cathode) negatively charged and immersed it into a solution which
contains the salt of the metal that is going to be deposited with the positive charge. The
metallic ions of the salt carry a positive charge and are thus attracted to the cathode
surface. When they reach the negatively charged surface, it provides electrons to reduce
the positively charged ions to metallic form. Electroplating has many advantages over
other techniques. It is relatively inexpensive regarding to the process and its equipment,
and also it is a safe and simple process
(2)
. This technique was used for decorative
purposes in the past, but nowadays it has become an important industrial technique
which can fulfill the requirements of a wide range of fields. Some of the functional
properties of this technique can be considered as corrosion and wear resistance, heat
resistance, tarnish resistance, electrical conductivity and solder ability (2).
Hadian (1990) said that one of the most popular electroplating methods in metal
finishing industry is ‘direct plating’. Direct electroplating has been achieved high
attention in recent years especially for providing the composite coating as it improves
the coating properties. The properties improvement is reported by many investigators
who asserted direct plating can provide a coating with fine grains, small grain size, high
purity and low porosity. Li et al (2007) also explained, direct plating is an effective
method for perturbing of adsorption and desorption process at cathode and plating
solution interface, which make it an economical process to provide a nanostructure
coating(3).
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The modern surface finishing era, however, began with the invention of the
galvanic cell in the early 1800s
(1)
. By the middle of the 19th century, silver, gold,
copper, and brass plating were commercially performed. In addition to electroplating,
numerous competitive methods of altering the surface of metals and non-conductors
have been added to the common definition of surface finishing since the 19th century.
Direct electroplating is the most common metal finishing process.
It utilizes a combination of a chemical solution formulated to contain metal ions
or complex metal ions to convert the metal ions in solution to solid metal atoms on the
surface of the substrate that when a current is applied. Plated metal coatings can be used
for a variety of purposes, including corrosion resistance, appearance, solder ability,
electrical resistance, electrical conductivity, vibratory bonding, abrasion resistance,
electroforming of a product, and as a matrix to hold abrasives such as diamonds and
carbides in cutting tools(2). The widest variety of metal surface properties can be
obtained through electroplating processes.
Electroplating is often also called "electrodeposition", and the two terms are used
interchangeably. As a matter of fact, "electroplating" can be considered to occur by the
process electrodeposition. Electrodeposition is the process of producing a coating,
usually metallic, on a surface by the action of electric current. The deposition of a
metallic coating onto an object is achieved by putting a negative charge on the object to
be coated and immersing it into a solution which contains a salt of the metal to be
deposited (in other words, the object to be plated is made the cathode of an electrolytic
cell). The metallic ions of the salt carry a positive charge and are thus attracted to the
object. When they reach the negatively charged object (that is to be electroplated), it
provides electrons to reduce the positively charged ions to metallic form. Figure 1.2 is a
schematic presentation of an electrolytic cell for electroplating a metal "M" from
an aqueous (water) solution of metal salt "MA".
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Figure 1.2 Schematic of an electrolytic cell for plating metal "M" from a solution of the
metal salt MA”
1.2 Nickel
The use of nickel has been traced as far back as 3500 BC, but it was first isolated
and classified as a chemical element in 1751 by Axel Fredrik Cronstedt, who initially
mistook its ore for a copper mineral. Its most important ore minerals are laterites,
including
limonite and garnierite,
and
pentlandite.
Major
production
include Sudbury region in Canada, New Caledonia and Norilsk in Russia
sites
(2)
. The metal
is corrosion-resistant, finding many uses in alloys, as a plating, in the manufacture of
coins, magnets and common household utensils, as a catalyst for hydrogenation, and in a
variety of other applications (2) .
Electrodeposited nickel is widely used in decorative and protective applications
where it can be applied to cheap mild steel, aluminum alloys and die-cast zinc to protect
them in corrosive environments. Dennis and Such stated, during 2000 about 90% of
nickel consumed in electroplating industry, was in the form of thin, corrosion resistant
and decorative coatings which applied to strong or cheaply produced substrates(4).
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Most of the nickel which is used for decorative purposes is in the form of nickelchromium composite system as the bright appearance is required (5). According to the
ASM Handbook, nickel is used as an undercoating of nickel-chromium coating in
decorative applications in order to enhance the corrosion resistance of system as
deposition of nickel itself provides a yellow cast which tarnishes easily.
Chandrasekar and Pushpavanam pointed out, nickel plating is applied in industry
to improve the surface properties such as ductility, wear and corrosion resistance and
also enhance surface hardness in the range of 150-700 Hv (6).
It is also used for engineering applications and in areas that fully bright finish is
not essential. One of the applications of nickel coating is in automotive industry,
especially in pistons, cylinder walls and transmission thrust washers and other parts
which are subject to friction, for the purpose of increasing the wear resistance. The
amount of nickel incorporated in different applications varies with 60% in nickel steels,
14% in nickel-copper alloys and nickel silver, and 9% used to make other super alloys
such as malleable nickel, nickel clad and Inconel(7). These, alongside the rest of the
applications are illustrated in Figure 1.3.
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3%
3% 2%
nickel steel
3%
nickel-copper & nickel
silver
6%
other supperalloys
9%
60%
plating
14%
nickel cast irons
heat and electric
resistance alloys
Figure 1.3 Nickel incorporation in different applications (Source: Hadian, 1990)
Nickel is used in many industrial and consumer products, including stainless
steel, magnets, coinage, rechargeable batteries, electric guitar strings and special alloys.
It is also used for plating and as a green tint in glass. Nickel is pre-eminently an alloy
metal, and its chief use is in the nickel steels and nickel cast irons, of which there are
many varieties (8). It is also widely used in many other alloys, such as nickel brasses and
bronzes, and alloys with copper, chromium, aluminum, lead, cobalt, silver, and gold.
The amounts of nickel used for various applications are 60% used for making nickel
steels, 14% used in nickel-copper alloys and nickel silver, 9% used to make malleable
nickel, nickel clad, Inconel and other super alloys, 6% used in plating, 3% use for nickel
cast irons, 3% in heat and electric resistance alloys, such as Nichrome, 2% used for
nickel brasses and bronzes with the remaining 3% of the nickel consumption in all other
applications combined(9) .
In the laboratory, nickel is frequently used as a catalyst for hydrogenation,
sometimes raney
nickel,
a
finely
divided
form
of
the
metal
alloyed
with aluminum which adsorbs hydrogen gas. Nickel is often used in coins, or
occasionally as a substitute for decorative silver. The American 'nickel' five-cent coin is
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75% copper and 25% nickel. The Canadian nickel minted at various periods
between1922-81 was 99.9% nickel, and was magnetic (10). Various other nations have
historically used and still use nickel in their coinage.
The corrosion – resistant properties of nickel electrodeposits are often thought
of as being of use only for protecting consumers, items, large or small, where decorative
embellishment is the most important factor, However nickel plate has many applications
in the engineering field where its functional behavior, rather than its appearance, is the
main criterion. When nickel is electroplated for this purpose, the coating deposited are
usually thicker than for decorative corrosion- protective uses, and so these are termed
heavy nickel coatings, which may be arbitrarily define as those greater than 50 microns
thick. These were first used to reclaim components which had worn or corroded in
service, or which had in correctly machined during manufacture(10).
Nickel was used to build up either the whole or just the effected portion of the
unserviceable article to a size greater than that actually required. Heavy nickel coating
are now often applied to new iron or steel components to prevent their corroding or
otherwise suffering damage caused by the normal wear or tear experienced in certain
uses, the thickness uses varying from 50 to 500 micro meter according to service
condition(10).
Matching of such coating is frequently not necessary. These nickel coating
prevent the basis metals from being corroded and by preventing this attack they those
reduce the danger of corrosion products of these substrates being produced which could
be contaminate nickel electrodeposits ideal for food-handling plant. This ability to
prevent metallic contamination together with their non-toxity, renders whose products
must not be contaminated by metallic impurities particularly iron, also make use of thick
electrodeposits nickel. Certain cylinders which are subject to wear have their service life
greatly extended in this manner (11).
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Figure 1.4 Applications of Ni plating
1.3 Nano-Nickel Crystals
Most of the material properties have been changed when their structure turns into the
nano-sized. This is due to the different properties of nano-sized materials such as high
thermal and electrical conductivity, as well as high wear and corrosion resistance
(Chandrasekar and Pushpavanam). Using direct electroplating technique provides an
opportunity to produce a nano structure matrix. Because of the nature of nano-sized
materials, a higher thermal and wear resistance can be predicted for nano-nickel crystals
(12)
. Nano-nickel coatings can be potentially used as lubricants due to their low-friction
resistance. Therefore they have found applications in various industries ranging from
aerospace and marine to medical and chemical (12).
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Figure 1.5 Nano- Nickel Particles
1.4 Carbon Nano Tubes (CNT’s)
Carbon Nanotubes (CNTs; also known as Bucky tubes) are allotropes of
carbon with a cylindrical nanostructure. Nanotubes have been constructed with lengthto-diameter ratio of up to 132,000,000:1 which is significantly larger than any other
material. These cylindrical carbon molecules have novel properties that make them
potentially useful in many applications in nanotechnology, electronics, optics and other
fields of materials science, as well as potential uses in architectural fields. They exhibit
extraordinary
strength
and
unique electrical properties,
and
are
efficient thermal conductors(3) .Carbon Nanotubes are nanofillers with a very high
potential in different industrial applications, e.g. for static dissipative or conductive parts
in automotive or electronic industries. In figure below SEM image of multi wall carbon
nano tube has been shown (3).
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Figure 1.6 SEM Micrograph of MWCNT’s
For the effective use of carbon Nanotubes (CNTs) an excellent distribution and
dispersion is an essential precondition.
The CNTs properties like Nanotubes type
(single-, double, multi-walled), length, diameter, bulk density, and waviness are
dependent on the CNT synthesis conditions, e.g. Catalyst, temperature of synthesis,
and synthesis method used. The purity and functional groups on the surface of the CNTs
as well as mainly their entanglements and strength of agglomerates affect the
dispensability of CNTs in different media. In addition, due to strong Vander Waals
forces CNTs tend to agglomerate. Ultrasonication of CNT dispersions is a common tool
used to break up CNT agglomerates in solution based processing techniques (13).
Ultrasonication can be done by different ways: using either an ultrasonic bath or
in setting an ultrasonic sonotrode into the solvent. The tip of ultrasonic sonotrode
oscillates at a fixed frequency and produces a conical field of high energy in the fluid.
The solvent within this conicalfield undergoes nucleated boiling and bubble collapse
that is the primary mechanism by which ultrasonic energy disperses particles (14). This
may help to debundle Nanotubes by providing high local shear, particularly to the
Nanotubes ends. For the preparation of CNT dispersions, surfactants are quite often used
as additives (14).
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During the dispersion process the surfactant adsorbs on the Nanotubes surface.
Pores within the bundles or primary agglomerates help in the propagation of surfactant
adsorption. Finally, the bundles or agglomerates are ideally separated into individual
Nanotubes and are kept in homogeneous and stable suspension
.The final
(14)
configuration of sodium dodecyl sulfate (SDS) covered Nanotubes was described as a
cylindrical micelle with a Nanotubes in the center(14). The destruction of agglomerates in
aqueous suspensions using ultrasonic energy was described by different authors. Lu et
al.
(15)
reported that multiwall carbon Nanotubes (MWNTs) get shorter with ultrasonic
time.
Nadler et al. (16) described for aqueous dispersions containing Bay tubes C150P
agglomerates that with increasing ultrasonic time (1 min up to 16 h) a bimodal
agglomerate size distribution pass into afinally mono
modal distribution, whereas the
mean particle size decreased signi
ficantly as investigated using a disc centrifuge.
These very broad size distributions of the dispersions were explained with the presence
of mass fractions of exfoliated CNTs and residual agglomerates. It was not possible to
deduce results concerning the carbon Nanotubes length using the disc centrifuge. Yu et
al.
(17)
described the dispersion of multiwalled carbon Nanotubes in an aqueous sodium
dodecyl sulfate solution at different ultrasonic treatment times. With higher sonication
energy a better exfoliation and disentanglement of CNTs was found using UV–
visible
spectroscopy
and
transmission
electron microscopy. Figure below
shows the SEM Image of the agglomeration of MWCNT’s.
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Figure 1.7 Scanning Electron Microscopy Images of MWCNT’s (A) NanocylTM
NC7000,(B) Baytubes_C150P, (C) Future Carbon CNT-MW, (D) Graph strength_
C100.
Carbon Nanotubes are the strongest and stiffest materials yet discovered in terms
of tensile strength and elastic modulus respectively. This strength results from the
covalent sp² bonds formed between the individual carbon atoms. In 2000, a multi-walled
carbon Nanotubes was tested to have a tensile strength of 63 GPa. Since carbon
Nanotubes have a low density for a solid of 1.3 to 1.4 g·cm−3, its specific strength of up
to 48,000 kN·m·kg−1 is the best of known materials, compared to high-carbon steel's
154 kN·m·kg−1. Under excessive tensile strain, the tubes will undergo plastic
deformation, which means the deformation is permanent (13).
This deformation begins at strains of approximately 5% and can increase the
maximum strain the tubes undergo before fracture by releasing strain energy. CNTs are
not nearly as strong under compression. Because of their hollow structure and high
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aspect ratio, they tend to undergo buckling when placed under compressive, torsional or
bending𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 (13) .
Diamond is considered to be the hardest material, and it is well known that
graphite transforms into diamond under conditions of high temperature and high
pressure. One study succeeded in the synthesis of a super-hard material by compressing
SWNTs to above 24 GPa at room temperature. The hardness of this material was
measured with a nanoindenter as 62–152 GPa. The hardness of reference diamond
and boron nitride samples was 150 and 62 GPa, respectively. The bulk modulus of
compressed SWNTs was 462–546 GPa, surpassing the value of 420 GPa for
diamond(13) .
All Nanotubes are expected to be very good thermal conductors along the tube,
exhibiting a property known as "ballistic conduction", but good insulators laterally to the
tube axis. Measurements show that a SWNT has a room-temperature thermal
conductivity along its axis of about 3500 W·m−1·K−1;] compare this to copper, a metal
well-known for its good thermal conductivity, which transmits 385 W·m−1·K−1. A
SWNT has a room-temperature thermal conductivity across its axis of about 1.52
W·m−1·K−1, which is about as thermally conductive as soil. The temperature stability of
carbon Nanotubes is estimated to be up to 2800 °C in vacuum and about 750 °C
inair (14)
1.5 Problem statements
CNT’s are difficult material on which to produce adherent electroplated
deposition coating due to their hydrophobic behaviors. Carbon Nano Tubes instantly
agglomerates when they face aqueous solution because of their high surface energies. In
𝑆𝑆
fact because of their high surface over volume ratio ( %) they are thermodynamically
𝑉𝑉
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unstable in solution, hence at the moment they added to the solution, they will
accumulate to gather.
Figure 1.8 SEM image of CNT’s agglomeration.
Figure 1.8 shows the agglomeration of CNT’s. Experiments have been carried out to
make CNT’s suspend in the solution. There are two different techniques for suspension
of CNT’s in solution: 1. using ultra sonic bath to suspend and disperse CNT’s in
solution. 2. Using a certain surfactant which can surround CNT’s and forces them to
suspend, because these kinds of surfactants have hydrophilic behaviors such as SDS,
SDDBS and Benzyl alcohol.
Additionally the cathode surface must be as smooth and clear as possible to increase the
adsorption and nucleation of Ni particles on the surface; the suggested processes for
electroplating of Ni are listed as follows:
Recommendation pre-treatments:
•
•
•
•
Ultra Sonic Cleaning by Ultra Sonic Bath for 20 minutes
Grinding the surface by grinding paper from 200-4000
Polishing the cathode surface
Etching in sulfuric acid for 5 minutes
•
Making nickel solution (Watt bath), and heat the solution by heater about
50𝐶𝐶 0 .
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• Suspend CNT’s in the solution by applying benzyl alcohol as a surfactant.
• CNT’s were poured in to 1Lit nickel solution and then ultra sonic bath was
used for 8hours for dispersing the CNT’s.
•
Surfaces of anode and cathode should be micro roughened (6).
Figure 1.9 Surface image of soft copper, used as a cathode
Figure1.9. shows a surface image of soft copper which used as a cathode in direct
electrodeposition techniques. In electroplating technique, solution is designed to
generate oxygen gas which forms the coating by reaction with the basis metal ions, as
the basis metal dissolves. Direct Electrodeposition is a unique technique for producing
fine coatings. The thickness of the coating is depended on some crucial parameters such
as agitation, current density, intensity, distance between cathode and anode and etc. DC
rectifier is used to generate current in range of milliamps; consequently the first embryos
will nuclei in nano size. On the other hand by applying this rectifier and by changing
some variables such as agitation rate, temperature, current density and etc. the growth of
these nano embryos could be controlled as well. All this operation must be in nickel
solution which the combination and producing procedure of this solution will be
discussed in chapter two. In this electrodeposition project, stainless steel would be anode
and soft copper would be cathode. To prevent the polarization effect, the size of anode
would be 3 or 4 times bigger than the cathode.
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1.6 Scope
Recent work shows that it is hard to plate CNT’s on the surface because of
agglomeration problem; hence the main scope is to make a stable suspension of CNT in
Ni solution. Other characteristics of the direct electroplating of CNT’s on the nano-Ni
crystals are: Current density, and deposition time. The deposited metal is expected to be
Nano crystalline-hence the deposited metal would be characterized by using SEM,
FESEM, XRD, AFM and Micro hardness testing. By studying the interface between
Nano-Ni Crystals and CNT’s an effect will be made to put forward a hypotheses on
how, the hydrophobic carbon nano tubes(CNT’s) are deposited and grow on Ni in direct
electrodeposition technique