Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

27

N. Rajesh Jesudoss Hynes*, R. Sankaranarayanan*, M. Kathiresan†, P. Senthamaraikannan‡, Anish Khan§,¶, Abdullah Mohamed Asiri§,¶, Imran Khank *Department of Mechanical Engineering, Mepco Schlenk Engineering College (Autonomous), Sivakasi, Tamil Nadu, India, †Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, Tamil Nadu, India, ‡Department of Mechanical Engineering, Kamaraj College of Engineering and Technology, Virudhunagar, Tamil Nadu, India, §Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, ¶Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia, kApplied Science and Humanities Section, University Polytechnic, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, India

Chapter Outline 27.1 Introduction 806 27.2 Synthesis and characterization of carbon nanotubes

809

27.2.1 Synthesis of carbon nanotubes 809 27.2.2 Carbon nanotube characterization 810

27.3 Mechanism behind carbon nanotubes

812

27.3.1 Single-walled carbon nanotubes 812 27.3.2 Multiwalled carbon nanotubes 815

27.4 Composites made of carbon nanotubes 815 27.5 Techniques related to the fabrication of CNT-based composites 27.5.1 27.5.2 27.5.3 27.5.4 27.5.5

818

Friction stiring 818 Plasma-assisted spark sintering 818 Dispersion technique via spreading 818 Mechanical stirring and casting technique 818 Milling technique 819

27.6 Factors and response analysis related to CNT-reinforced aluminum-based metal matrix composites 819 27.6.1 27.6.2 27.6.3 27.6.4

Time consumed for milling 819 Quantity of CNT 820 Microhardness 820 Friction and wear tendency of aluminium/CNT composites 822

Nanocarbon and its Composites. https://doi.org/10.1016/B978-0-08-102509-3.00027-4 © 2019 Elsevier Ltd. All rights reserved.

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Nanocarbon and its Composites

27.7 Factors and response analysis related to CNT-reinforced copper-based metal matrix composites 822 27.7.1 27.7.2 27.7.3 27.7.4 27.7.5 27.7.6

Implication of CNT percentage with the relative density of the MMC 822 Implication of CNT percentage with the hardness (Hv) of the MMC 824 Implication of CNT percentage with the grain size of the MMC 825 Implication of CNT percentage with the strain-hardening exponent of the MMC 825 Implication of CNT percentage with the young’s modulus of the MMC 825 Implication of CNT percentage with the yield (0.2% proof ) strength of the MMC 826

27.8 Conclusion 827 References 828 Further reading 830

27.1

Introduction

The presence of metal matrix composites is tremendous with various reinforcements. But the application of CNT as a reinforcement in the metal matrix composite is still under research, which began a couple of decades ago [1]. Metals dominate as structural materials, even today where CNT-based composites can be a potential alternative in the field of automobiles, aerospace, sports-based industries, and many more. The combination of strength, light weight, and stiffness makes them the most desirable applicants [2]. The area related to functional and structural aspects readily accepted CNTs after their discovery [3]. The exploration of CNTs by Iijima and their extraordinary properties turned industries and researchers toward CNTs [4]. The typical example is the excellent stiffness of the CNTs that is competitive to the diamond and even stiffer than a diamond. The magnitude of Young’s modulus is of the terapascal (TPa) range and the achievement up to 0.06 TPa of tensile strength is possible. The antiphon by CNT toward the deformation is very much impressive. This is one of the desired states for a material, as hard materials fail mostly with 1% strain or less than 1%. This phenomenon results from the spread of defects and dislocations. But this is not so in the case of CNT, as it can withstand tensile strain until 15% [4] before fracture [4]. Moreover, the excellent qualities of CNTs make researchers refer tto hem as the ultimate reinforcement with successful implementation into the metal matrix medium [5, 6]. CNTs are also known for their electronic properties, and those make them an appealing material in the nanotube-based applications and the respective studies. The size along with the symmetric nature of the nanotubes influences its properties. Good symmetric orientation of the nanotube structure with a very small size enhances its lattice, electronic, and magnetic properties. The other beneficial part is the exceptional quantum effects. Experimental results as well as theoretical values proved CNT’s exceptional electronic properties. In case thermal properties are a concern, individual multiwalled CNTs (MWCNTs) possess the extraordinary thermal conductivity of about 3000 W/(m K) whereas single-walled CNTs (SWCNTs) possess 6000 W/(m K). Even at room temperature, aligned SWCNT films hold 200 W/(m K). There are other properties that add values to CNTs with

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their incorporation, namely chemical, optoelectronic, electrochemical, and thermoelectric This leads to the analysis of functional aspects and features of the composites that are made with CNTs [4]. The distinct structure of CNTs provides them special properties such as a low density around 2 g/cm3, higher strength around 0.04 TPa, extraordinary chemical stability, superior thermal stability, large aspect ratio in the range of around 100–1000, and excellent elastic modulus around 1 TPa [7, 8]. CNT is called “advanced filler” for composites due to the above combination of material properties [8]. CNT is nothing but graphite in the form of rolled sheets, and appears as a tube. The structure of graphite (CNT) is different from the diamond, as graphite forms a twodimensional sheet with an array of a hexagonal nature, whereas diamond possesses a three-dimensional cubic crystalline construction with a tetrahedron nature. Diamond holds each carbon atom with four immediate neighbors whereas CNT holds three immediate neighbors. The cylindrical shape is derived from the rolling process of graphite sheets. The atomic arrangement plays a vital role in deciding the properties of CNTs and this atomic arrangement is decided by the way it has been rolled. Apart from the atomic arrangement, the nanostructure, length of tubes, morphology, and diameter of the CNT are other influencing factors as far as properties of CNT are concerned. The existence of CNT is in two different modes, namely SWCNT and MWCNT [7]. Tube chirality aids in the description of CNT’s atomic structure. The other term, called helicity, can also be fruitful in the description of the atomic structure of CNTs. !

These can be defined through the vector called the chiral vector (C h), along with the chiral angle (θ). Through Fig. 27.1, it can be visually recognized that the rolling of graphite sheet is in line with dotted lines and the chiral vector comes in contact at

Armchair Zig-zag

→

r cto

Ch

l ve hira

C

→

a2

→

ma2

q, Chiral angle →

a1

→

na1

Fig. 27.1 Schematic diagram showing how a hexagonal sheet of graphite is rolled to form a carbon nanotube [7].

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Nanocarbon and its Composites

the tail. The rollup vector is another terminology given for the chiral vector. The clear understanding of this vector is possible via the following Eq. (27.1) [7]. !

Ch ¼ na1 + ma2

(27.1)

The integers, namely n and m, refer to the number of steps through the hexagonal lat! tice carbon bonds in a ziz-zag manner, whereas unit vectors are mentioned with a 1 and ! a 2, as shown in Fig. 27.1. The angle called the chiral angle decides the quantity of a tube’s twist. The presence of these limitations can be recognized through a 0 degree as well as a 30 degree chiral angle. In specific, 0 degree and 30 degree are termed as zizzag and armchair, respectively, and can be decided on the basis of the geometrical arrangement of the bonds of the carbon located circumferentially around the nanotube. The discrepancy among the ziz-zag and armchair can be visualized in Fig. 27.2. The rollup vector representation of the nanotube for the armchair and ziz-zag are (n,n) and (n, 0), respectively. The diameter of the nanotube can be derived with the help of the rollup vector, as the interatomic spacing is already a known value for the carbon atoms. Consequently, the term called chirality has an influential association with

Fig. 27.2 Illustrations of the atomic structure of (A) an armchair and (B) a ziz-zag nanotube [7].

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the CNT properties. More specifically, the electronic properties have been influenced strongly by the chirality. In general, graphite comes under the semimetal category. But the same can be considered as a metal or semiconductor, decided on by the chirality of the tube [9]. The implication of the chirality was recorded properly through various studies. Yakobson et al. [7, 10] researched the instability condition of the CNT’s outside limits of the linear response. Their research via simulation clearly indicated the capability of the CNTs in terms of resilience. Thus, the sustainability of the CNTs was more even at intense strained conditions. Going further, the structure did even experience any brittleness. As far as elastic stiffness is concerned, the chirality possessed less influence on it. The role of the CNT on the plastic deformation is still critical under the tensile conditions. This was explained through the Stone-Wales transformation and the same is shown in Fig. 27.3. That happens under stressed conditions of the armchair nanotube along the axial direction.

27.2

Synthesis and characterization of carbon nanotubes

27.2.1 Synthesis of carbon nanotubes The different methods through which CNT synthesis can be accomplished have been shown in Fig. 27.4. The very first CNT synthesis was carried out unintentionally by Iijima through arc discharge. But the situation is entirely different for the current scenario as wide varieties of methods are available for CNT synthesis. The categorization of these different methods can be mentioned through CNT’s properties, namely temperature, time, heat source, precursor, mechanism, atmosphere of reactions, etc. The widely accepted methods are laser ablation, arc discharge, and chemical vapor deposition for the synthesis of a carbon nanotube [11–13]. The primary observation was done initially by Iijima over multiwalled nanotubes. The research continued by involving single-walled nanotubes in synthesis within a decade. As mentioned earlier,

5

7

7 5

Fig. 27.3 The Stone-Wales transformation occurring in an armchair nanotube under axial tension.

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Nanocarbon and its Composites

Arc Discharge Sono-chemical or hydrothermal Electrolysis

Laser Ablation

Oxygen assisted

CNT synthesis methods

Microwave plasma

Chemical vapour deposition

Radio-frequency

Thermal

Plasma enhanced

Water assisted

Fig 27.4 Various methods of CNT synthesis [1].

multi- and single-walled carbon nanotubes were synthesized via chemical vapor deposition, laser ablation, gas-phase catalytic growth, arc-discharge, etc [7, 14]. The manufacturing of composites using carbon nanotubes involves a large quantity of CNTs. But a typical quantity of CNTs was not possible to be produced economically through laser ablation or arc discharge methods. Moreover, residues in terms of impurities such as amorphous carbon, nontubular fullerenes, and catalyst particles are also present as part of the outcome in the synthesis of CNTs. This leads to the inclusion of an additional step in the synthesis process for the purpose of purification so that CNTs can be separated from the impurities. But the gas-phase-based synthesis method manufacture CNTs with lower impurities. In addition to this positivity, this method suits the mass production of CNTs. The road doesn’t end here as chemical vapor deposition that is based on the gas-phase method also favors the large-scale production of nanotubes [7].

27.2.2 Carbon Nanotube Characterization The characterization at the micromechanical level for nanotubes possesses real challenges. A further challenge lies in the modeling at the nano level for analyzing the fracture as well as the elastic demeanor. Property measurements cannot be taken directly through micromechanical characterization. The specimen size is restricted to some standard sizes. The ambiguity lies over the data that are taken out of indirect measurement methods. A deficiency of knowledge prevails over the preparation of the specimen for testing and poor control over the distribution as well as alignment of nano-tubes. The characterization is an essential tool in the process of understanding the mechanical-based properties of CNTs; several attempts were made by various researchers in this regard. An inherent thermal vibration’s amplitude and a transmission electron microscope result help in obtaining the modulus of elasticity for the stand-alone multiwalled nanotubes. The value of 1.8 TPa on average was acquired from 11 samples. An atomic force microscope (AFM) can be handy in the process

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of directly measuring the stiffness as well as the strength of the MWCNTs (structurally isolated). Wong and colleagues were first involved in this type of measurement, which was carried out by pinning one end of the nanotube with the surface of the molybdenum disulfide. The tube was given a load with the help of the tip of the AFM. The elastic modulus of 1260 GPa was obtained by measuring the bending force that changed with respect to the displacement as a functional factor through the unpinned portion along its length. The strength average obtained was around 14 GPa under a bending condition [7]. The tendency of SWCNT assemblage is in the form of ropes. Salvetat and team studied those bundles of CNTs by measuring with the assistance of AFM to understand its properties. The analysis showed that the moduli along the axial as well as the shear direction decreases considerably as a CNT bundle’s diameter decreases. This implies the slippage of the CNTs as a bundle structure. Further analysis continued at the AFM for obtaining the elastic strain of the bundle of CNT. The elastic modulus decreased due to the slippage inside the bundle. A tensile test for the CNT was conducted by fixing MWCNT and SWCNT ropes at the two opposite tips of the AFM. Subsequently, the load was applied. MWCNTs experienced the failure via the outermost tube and the failure continued in the form of a pullout that occurred inside the nanotubes. The failure system of the MWCNT is referred to as a sword and sheath telescoping failure and the same is shown in Fig. 27.5. The tensile strength range

Fig. 27.5 Micrographs showing (A) the apparatus for tensile loading of MWCNTs and (B) the telescoping sword and sheath fracture behavior of the MWCNT [7].

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Nanocarbon and its Composites

was 11–63 GPa for the outermost layer whereas the elastic modulus range was 270–950 GPa [7]. Similar tests were performed by Xie et al. [15] on the MWCNT under tension. The resulting values from the experiments were 3.6 GPa strength and 450 GPa as modulus. These low values may be due to the occurrences of defects from the chemical vapor deposition-based CNTs.

27.3

Mechanism behind carbon nanotubes

The latest studies have suggested that the exceptional mechanical properties of CNTs such as the potential to withstand fracture strains, the higher modulus of elasticity, and the exceptional elastic strain make it an outstanding material [7]. Similar results were obtained by another theoretical analysis [7, 16, 17], even though the correlation among the experimental as well as theoretical was less in numbers. The discussion of mechanics of the MWCNT and SWCNT was carried out here, which provided the positive results toward the CNTs.

27.3.1 Single-walled carbon nanotubes Many studies related to the mechanical properties of single-walled carbon nanotubes (SWCNT) have been done. Commonly, the vibration modes with long frequency and rigidity of structure possess up to 400 atoms with a minimum count of 100. Empirical Keating Hamiltonian led to this count. The excellence of the SWCNTs was explained through conducting various comparison studies. One such study was the comparison of SWCNT with the iridium beam. The platform of comparison was the bending stiffness for both materials. The continuum Bernoulli-Euler theory related to beam bending was selected to figure out the bending stiffness of the iridium beam. Overney and team came to the conclusion by stating that the rigidity of the bending beam for the CNT exceeds the topmost values of any of the rest of the present materials. In addition, the behavior of the CNTs under the condition of the compressive load was examined experimentally with the aid of a simulation technique called molecular dynamics. The outcome of the simulation deduced the deformation mechanism of SWCNTs where the bent was observed at larger angles. The related experimental as well as theoretical results suggested that CNTs possess exceptional flexibility. To be specific in terms of values, the bending can be reversed entirely for angles above 110 degrees. This phenomenon occurs even though the shape is complex, namely a kink. The diagrammatic visualization is given in Fig. 27.6, which portrays the extraordinary resilience of CNTs in the large strain condition. If the representation of the thickness is mentioned commonly, the real stiffness due to bending for the SWCNT is considerably lower in comparison to the shell model with elastic continuity. The effective stiffness related to CNT bending can be considered as the influencing parameter in the absence of the represented thickness. This concept eases the modification of the equations of the elastic shell, which can be handy for the SWCNT. These calculated results based on the above

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Fig. 27.6 TEM micrograph and computer simulation of nanotube buckling [7].

conceptualization were in line with the outcome of simulations based on molecular dynamics [7]. The implications of the structure of the CNT and chirality on the torsion, tension, and bending was researched by Vaccarini et al. [18]. They concluded their studies by stating that the implications of the chirality were very minimal on the tensile modulus. Nevertheless, there is torsional behavior of an asymmetric nature by the chiral tube in relation to the right and left twist. But this is not the case for the armchair as well as ziz-zag tubes, as there is torsional behavior of a symmetric nature. Lu [16] did a comprehensive examination for the SWCNTs as far as the elastic properties are concerned. The dynamics model of the empirical lattice nature was

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Nanocarbon and its Composites

adopted by Lu, which had fruitful results for calculating the graphite’s phonon spectrum as well as the elastic properties. The approximation of the atomic interactions of the SWCNT layer was carried out through the summation of harmonic potential pair-wise between atoms in the dynamics model of the empirical lattice nature. The conformal mapping technique can be used for the construction of a local structure for the CNT from the graphite sheet to the cylindrical surface. Lu attempted to analyze the influence of the chirality as well as the size of the CNT on the elastic properties. Lu was also involved in the comparison of CNT with diamond and graphite. The conclusion of the above study suggested the insensitivity of the elastic properties with respect to the chirality and size. The bulk, shear, and Young’s modulus for the CNTs were comparable to diamond. Another study is also in line with Lu’s results with little higher value of Young’s modulus. In contradiction to the findings of Lu, they discovered that the diameter as well as the structure are very sensitive toward the elastic modulus [7]. Along with the distinct elastic properties, the nature of inelasticity has drawn remarkable attention. The simulation of molecular dynamics displays the various morphological patterns when CNT experiences huge deformations. The reversible switching of shape resulted in an unanticipated energy release. The corresponding stress-strain curve was singularity in nature. This can be well elucidated using a continuum shell model. The precise understanding on the demeanor of the CNT outside the linear elasticity is made possible by the above model. The simulation of molecular dynamics for SWCNTs with various temperature and chirality was also executed [17]. This simulation suggested that CNTs possess extraordinary breaking strain and the same decrease as temperature decreases. The dislocation theory was also tested by Yakobson [10] on CNTs for discovering the relaxation mechanism of CNTs under a tensile load condition. CNT symmetry plays a vital role in its yield strength. There was a belief about the existence of intramolecular-based plastic flow. Ropes or well-aligned bundles of SWCNT can be manufactured via arc discharge as well as laser ablation methods. Subsequently, theoretical studies on these CNT bundles have been done. The exploration of elastic buckling of CNT ropes was done through selecting the honeycomb (elastic) model with modifications at the pressurized condition of large magnitude. Easy formulation is available to find the critical pressure with respect to the Young’s modulus of CNT. The ratio between wall thickness and radius also can be calculated with a simple formula. These studies explored the idea that SWCNT ropes are affected by buckling of an elastic nature whenever they experiences large pressure. The elastic buckling of CNT lead to the pressure depended abnormal vibrations and electrical resistivity [7]. Popov et al. [19] researched SWCNT’s elastic properties related to crystal lattices of a triangular nature with the assistance of a model called lattice dynamics of constant force analytically. They also involved armchair as well as zigzag types of CNTs for finding the different elastic constant. In specific, the crystals of CNT were involved. It brought clear findings on the dependency of various factors on the tube radius. This apparently revealed that the tube radius plays a significant role in deciding the bulk modulus, Poisson ratio, and elastic modulus. Even up to 38 GPa of bulk modulus was obtained by the SWCNT crystals with approximately 0.6 nm of radius.

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27.3.2 Multiwalled carbon nanotubes The construction of multiwalled carbon nanotubes (MWCNTs) can be achieved by composing a number of single-walled carbon nanotubes of a concentric nature. The van der Waals forces of weak nature hold these nanotubes. As more nanotubes are involved in the MWCNT, the analysis part through modeling becomes complicated. But the bending stiffness and tensile strength constant were found out by Ruoff and Lorents [20] for MWCNTs based on graphite’s elastic properties. The thermal expansion behavior of CNTs is different from the graphite as well as classical carbon fibers where CNTs possess isotropic thermal expansion and the other two possess anisotropic expansion. Nevertheless, CNT thermal conductivity is clearly anisotropic. Moreover, thermal conductivity’s magnitude for CNT is far better than other existing materials. If the parameter’s combination is more, the sensitivity of the elastic properties is low. The typical parameters are the radius of the tube, the quantity of layers, and chirality. But the same elastic properties become equal for all existing nanotubes within the MWCNT if the radius is more than 1.0 nm. It also was found that the influence of van der Waals at the interlayer is negligible toward shear stiffness and tensile strength. There is another mechanism called continuum mechanics that was used first by Govindjee and Sackman [21] to evaluate MWCNT properties. To confirm the validity of the above approach, they employed the Bernoulli Euler bending for obtaining Young’s modulus. The assumptions related to continuum mechanics must be carefully handled, as suggested by Govindjee and Sackman. The dependency of the explicit nature of the properties of materials prevailed over the size, if the assumption was a continuum (cross-section). Another model called elastic shell was employed in the analysis of the implication of van der Waals forces over the buckling (axial) for the double-walled carbon nanotube. The results of theses analyses revealed that there was no progress in the strain resulting from the critical axial buckling. This lead to the proposal of another model called the multiple (column) model. This model incorporates the radial displacements in between the layers that are connected via the van der Waals forces. The same model was utilized to reveal the consequences of the displacement in between the layers on the column buckling. The findings of the above analysis disclosed that displacement in between the layers is of negligible influence until the van der Waals forces become strong [7].

27.4

Composites made of carbon nanotubes

The characterization of CNTs through experiments provided results with differences. But when it was all put together, the experimental as well as theoretical findings disclosed the extraordinary properties of CNTs. These qualities in terms of properties lifted the CNTs to the next level from the research stage to the application platform, and the same was tried in the CNT-based composites. There were several attempts made to produce polymer composites with CNTs as reinforcements. But typical experiments were done on the metal as well as ceramic-based composites with CNT combinations as a reinforcement medium [7].

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Nanocarbon and its Composites

The accomplishment of CNT-reinforced metal matrix composites (MMC) can be done through various methods, as mentioned in the various studies. Liquid-based metallurgy is one of these solutions for producing MMCs. The process called melting eases the mass-scale manufacturing of MMCs through classical casting techniques. Problems exist in this process as dispersion with homogeneity was a challenge and difficult to achieve. The other issue associated with this process, is the creation of harmful products in between the face. The wetting condition also hinders MMC manufacturing through this method. The process called melting has its own subcategories, namely laser deposition, melt stirring, and melt infiltration [22]. Similarly, melt deposits with a disintegration process are fruitful in producing CNT-based MMCs and the same can achieve excellent mechanical properties [23]. Thermal spraying is another distinct technique in the process of manufacturing MMCs with wide applications such as engines for automobiles, turbine blades for aerospace purposes, electronic equipment, and prostheses for orthopedic usage. The coating uses materials in the form of fibers, CNTs, particulates, etc., as mentioned by Bakshi et al. [24]. The CNTs as reinforcements in the drops of powdered aluminum were treated through plasma spraying over the surface of the substrate. The processing techniques do not end here as powder metallurgy is also an option for producing MMCs. In specific, it is the least costly technique. The simplicity and flexible nature of the process make it economical among the contemporary processes. Here, CNTs are dispersed with metal powders for blending that can be achieved through a mechanism called milling. Subsequently, different processes come into the picture, namely isostatic pressing (cold), sintering with plasma spark, isostatic or normal pressing (hot), and compacting as well as sintering. The above processes are kept as the main processes. There are secondary processes that are required for achieving the mechanical deformation. These secondary treatments or processes are mostly executed in the hot environment. Typical processes are rolling (hot), extrusion (hot), and forging (hot). The final output of these hot processes is in the well-consolidated form with a densely packed nature. In general, it cannot be concluded that all processes can achieve the higher mechanical properties. This phenomenon results from the problem called agglomeration that can prevail in the metal matrix reinforced with CNTs. This agglomeration leads to the creation of phases with a harmful nature, which consequently affects the load transfer due to insufficient properties. The high temperature environment creates the above-specified problem. But the reactivity is less for the magnesium composites in the comparison of metals, for example, aluminum [2, 23, 25]. The tendency of agglomeration makes the dispersion as well as incorporation of CNTs difficult in the MMCs. But this negativity can be solved to some extent by involving the manufacturing methods, namely thermal spraying, powder metallurgy, and melting processes [3, 4, 10–32]. The mechanical properties cannot be increased beyond certain levels through the powder metallurgy process in specific cases as a result of agglomeration. Similarly, formed aluminum carbides of a harmful nature cannot always be restricted due to the high temperature environment [2, 25, 26]. For eliminating this problem, a new

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measure was executed that ensures good dispersion and proper alignment of the CNTs in the matrix of MMCs. The reason behind the achievement of good dispersion and proper alignment is the mixture’s rheology and maintenance of process temperature at alower controlled level that is below 200°C. This was experimented with successfully in the polymer matrix composites with CNTs as reinforcements. For example, a polyvinyl alcohol (PVA) matrix with the combination of CNTs as reinforcement materials provided welldispersed and properly aligned composites [27]. A typical example of synthesis flow of a MWCNT-reinforced composite is shown in Fig. 27.7.

Fig. 27.7 Synthesis flow of the Mg-PVA/MWCNTs composites [23].

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27.5

Nanocarbon and its Composites

Techniques related to the fabrication of CNT-based composites

Aluminum-based MMCs with CNT as reinforcement can be possible by different manufacturing processes, such as sintering (plasma spark), ball milling, and friction stir methods. These methods provide the composites with excellent strength as well as stiffness without compromising the light weight [1, 2]. Typical techniques are discussed below.

27.5.1 Friction stiring The concept behind this method lies behind the rotating tool of a nonconsumption nature and the same is pierced inside the workpiece. Rotational motion generates friction between the tool and job. This friction is the prime reason for heat build-up. The plastic deformation of the material is the consequence of the above occurrences which soften the material and started flowing around the nonconsumable tool. The above proceedings promote good dispersion of the material. Meanwhile, the solid state of the materials prevails that inhibits the formation of deformities [28]. The previous research mentioned that there was an increment in HV hardness through the friction stir technique [1, 29].

27.5.2 Plasma-assisted spark sintering The densification of CNT up to 1% in terms of volume and the relative density of aluminum powder until the 96.8 percentage is achievable through the sintering method. The improvisation in the tensile strength is the reason behind the curtailment of dislocation movements. In other words, tensile strength restricts the plastic deformation whenever experiences stress that is generated while controlling the layer especially at the boundaries [30].

27.5.3 Dispersion technique via spreading The layer formation can be achieved through this technique by laying the number of sheets followed by pressing and rolling, which achieves the bonding in between each layer in the formation of a single layer. This technique increased the tensile strength up to 66%. Meanwhile, the grain structure was refined to around 20 nm. This enhancement in the properties will have an impact on the even distribution of CNTs in the absence of porosity or clusters. Moreover, the above steps enhance the bondage between CNTs and aluminum. Finally, the good properties of graphite can be retained [31].

27.5.4 Mechanical stirring and casting technique This technique starts with the melting of aluminum in the form of a bar. The melting step is achieved through employing a furnace called a muffle. The MWCNT powder in the purified form is dispersed once the required melting of the aluminum bar is

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achieved. The dispersion starts mixing with the molten aluminum with the aid of a stirrer. Finally, the casting step begins where the molten aluminum, including the well-mixed MWCNT powder, gets a specific shape by pouring the mixture inside the side, followed by solidification. This technique reduced the tensile strength marginally in comparison to the CNT’s presence with lower quantity. The process is economical for preparing CNT-based MMCs. However, the uniform dispersion of CNTs cannot be accomplished in the complete matrix region. The reason behind the above problem is the uneven flow of the aluminum CNT mixture in the molten form while pouring inside the die. In addition to that, the stirring operation is unable to accomplish its purpose as solidification begins that makes the stirring impractical in the solid form [1].

27.5.5 Milling technique This milling mechanism converts the raw material into a smooth powder form with the aid of a number of small balls that mill the material through collisions among those balls [32]. This conversion into a powder form naturally helps in the even distribution of CNTs in the matrix medium. As a consequence of milling, the damage related to the structure as well as the morphology is minimized [32]. This powder obtained from milling is kept in an inactive environment before being applied in the process called hot extrusion that protects the powder from both oxidation and burning. This milled material is placed in the heated mold with a suitable temperature for extrusion. At the end, the required shape is obtained by extruding the material [1]. The best results in terms of performance on the mechanical ground can be possible through the even dispersion of the CNTs. Excellent bonding between the interlayers is also possible through good dispersion that assists in improving the load transfer from the aluminum matrix to the respective CNTs. The agglomeration phenomenon exists in the process due to the van der Waal forces [33]. However, the economical nature of the process and the easy access to the raw material make this technique a favorite [34]. Many studies have acknowledged the potential benefits of the ball milling process, namely homogeneous MMCs with uniform dispersion of CNTs and a smooth microstructure [33–35]. The good dispersion additionally prevents the powder from severe milling [36]. The possibilities of damage are also present by the aluminum powder on the CNTs during the milling operation [34]. Despite the criticality, the ball milling technique can be considered an excellent solution in handling agglomeration [35].

27.6

Factors and response analysis related to CNTreinforced aluminum-based metal matrix composites

27.6.1 Time consumed for milling The duration of milling is one of the prime influencing factors that affects the morphology of aluminum powder. The greater the milling time, the larger the surface area and the greater the generation of Al2O3. Consequently the properties will vary by an

Nanocarbon and its Composites

Relative increment (%)

820

50 45 40 35 30 25 20 15 10 5 0

Yield strength Ultimate tensile strength

4

6

10 8 Ball milling time (h)

12

Fig 27.8 Relative increments of ultimate tensile strength and yield strength of CNT/Al composites relative to the Al matrix [1, 32].

increment in the hardness level and a decrement in the ductility [37]. The considerable increment in the milling time can even double the modulus of elasticity and hardness at the nano level in comparison to aluminum in the pure form, as stated by M. Raviathul Basariya et al. The influence of the milling time can be explained graphically, as shown in Fig. 27.8. Similar improvements as far as mechanical performance was concerned were mentioned by Yoshida et al. The increment of around 285% was achieved in the performance by keeping the milling process throughout the day just short of 4 h [33]. The duration of 6 h was minimum enough to disperse CNTs in the aluminum matrix. Beyond that, the damage in the CNTs was the output. The tensile as well as yield strength improves when the duration of ball milling is higher. But elongation is a matter of concern that increases initially and shows a decrement [32].

27.6.2 Quantity of CNT The presence of CNTs in terms of amount or quantity inside the composite has significant implications in deciding the properties of the respective composites. Generally, the presence of CNTs in the composite is mentioned by the weight. The strength as well as modulus of elasticity has a direct linkage with the CNT’s presence. But the quantity of CNT cannot be increased beyond the saturation point, as mentioned by A.M.K. Esawi et al. A typical influence of the CNT percentage on the tensile strength is represented graphically in Fig. 27.9.

27.6.3 Microhardness The inclusion of nanotubes in the matrix medium is one of the deciding factors for the hardness of the composites. The marginal inclusion also improves the hardness and the same is realized through a Vickers hardness test. In comparison to aluminum

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300 Tensile strength (MPa)

Measured value

250 200 150 100 50 0 Pure

0.5

1 CNT content (wt%)

2

5

Fig 27.9 The effect of CNT content on the tensile strengths of the investigated composites [1, 32].

in the pure form, this small inclusion enhanced the hardness up to 300% [1]. The increment in the hardness contributes a lot in the process of strengthening the composite. This prevents the dislocation. Strain in the lattice structure can be controlled. It can achieve the work hardening during the milling process itself. The probability of agglomeration is less [37]. M. Raviathul Basariya et al. have found the importance of the microhardness through their research. They involved the milling time to analyze the influence of microhardness on the duration of milling. The outcome of the findings revealed that the hardness shows an increasing trend in line with the increase in the duration of milling, as shown in Fig. 27.10. There was considerable increment in the hardness in the initial period of milling. The work hardening phenomenon decreased the rate of increment of the hardness at the longer run. In addition to an increment in

Fig 27.10 Effect on hardness of unreinforced EN AW6082 and composite powders with increasing milling time [1, 37].

Microhardness (HV0.01)

500 400

EN AW6082 EN AW6082 MWCNT

300 200 100 0

10

20

30 40 Milling time (h)

50

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the hardness, the even dispersion of CNTs in the matrix due to high milling time prevents the deformation of the matrix, which enhances the strength of the composite, as stated by Bustamante et al. [38, 39].

27.6.4 Friction and wear tendency of aluminium/CNT composites The wear property was researched by Esawi et al. [40] through differing the quantity of CNT in the range of 0%–5% in terms of weight. The respective specimens were analyzed by being subjected to various loads and sliding speeds. As the load increased, the wear rate showed the increasing trend. But the coefficient of friction was in a downward trend. The CNTs that were not embedded with matrix, lower the wearing surface against the surface of rubbing. The wear characteristics enhanced a lot as a consequence of the above occurrence. A typical graph of wear rate against the quantity of CNT percentage in terms of weight is shown in Fig. 27.11. A fish bone representation (Fig. 27.12) provides wide knowledge on the possibilities through which wear characteristics, friction tendency, and mechanical properties can be boosted.

27.7

Factors and response analysis related to CNTreinforced copper-based metal matrix composites

27.7.1 Implication of CNT percentage with the relative density of the MMC The implication of CNT percentage with the relative density of the MMC is shown in Fig. 27.13. As the CNT content increases in the copper matrix medium, the relative density showed initial fluctuations and followed the lower percentage with less variation in the higher side of the CNT content in the matrix of the composite. 60

Wear rate (mg/km)

50 40 30 20 10 0

Pure aluminium

1% wt CNT

Fig 27.11 Wear rate versus CNT wt% [1, 40].

2.5% wt CNT

5% wt CNT

Inert atmosphere

1% Stearic acid Toluene 1% Ethanol + Stearic Acid Powder to ball ratio(8:1, 10:1, 5:1)

Methanol

Argon

Improvement in mechanical properties and friction and wear behaviour

CNT/Al composite fabricated by ball milling process

4-8 nm. Multi-walled CNTs Internal diameter CNT Aspect ratio

5% by weight 40 nm. Outside diameter

Single-walled CNTs

2% by weight

140 nm outside diameter

Type of CNT

Diameter of CNTs

CNT content by wt.

823

Fig. 27.12 Fish bone diagram illustrating the parameters that improve the mechanical properties and friction and wear behavior of the aluminum matrix composite [1].

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

Process controlling reagent

Milling time (30 min-50 h)

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Nanocarbon and its Composites

99 Relative density (%)

98 97 96 95 94 93 92 91 90

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.13 Effect of CNT content on the relative density of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.2 Implication of CNT percentage with the hardness (Hv) of the MMC The implication of CNT percentage with the hardness (Hv) of the MMC is shown in Fig. 27.14. As the CNT content increases inside the copper matrix medium, the hardness of the MMC showed the drastic progress, and the respective influence of the CNT content is very much evident here.

190

Hardness (Hv)

185 180 175 170 165 160 155

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.14 Effect of CNT content on the hardness (Hv) of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

825

33 32 Grain size (mm)

31 30 29 28 27 26 25 24 0

0.02

0.04

0.06 0.08 CNT volume fraction

0.1

0.12

0.14

Fig. 27.15 Effect of CNT content on the grain size of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.3 Implication of CNT percentage with the grain size of the MMC The implication of CNT percentage with the grain size of the MMC is shown in Fig. 27.15. As the CNT content increases in the copper matrix medium, the grain size showed the deep initial fall, followed by minor fluctuations and finally the progress in the size of the grains in the composite.

27.7.4 Implication of CNT percentage with the strain-hardening exponent of the MMC The implication of CNT percentage with the strain-hardening exponent of the MMC is shown in Fig. 27.16. As the CNT content increases in the copper matrix medium, the exponent showed drastic progress, and the influence of the CNT content is very much evident here as far as the strain-hardening exponent of the composites is concerned.

27.7.5 Implication of CNT percentage with the Young’s modulus of the MMC The implication of CNT percentage with the Young’s modulus of the MMC is shown in Fig. 27.17. As the CNT content increases in the copper matrix medium, the Young’s modulus of the composite increases with a progressive trend. The influence of the CNT content is very much evident here and improves the mechanical property of the copper matrix-based composites.

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Nanocarbon and its Composites

Strain hardening exponent, n

0.38 0.35 0.32 0.29 0.26 0.23 0.2

0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.16 Effect of CNT content on the strain-hardening exponent of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

Young’s modulus (GPa)

12.5 11.5 10.5 9.5 8.5 7.5 6.5

0

0.02

0.04

0.06 0.08 CNT volume fraction

0.1

0.12

0.14

Fig. 27.17 Effect of CNT content on the Young’s modulus of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

27.7.6 Implication of CNT percentage with the yield (0.2% proof ) strength of the MMC The implication of CNT percentage with the yield (0.2% proof ) strength of the MMC is shown in Fig. 27.18. As the CNT content increases in the copper matrix medium, the yield (0.2% proof ) strength of the composite increases with a progressive trend

Yield (0.2% proof) strength (MPa)

Synthesis, properties, and characterization of carbon nanotube-reinforced metal matrix composites

827

195 190 185 180 175 170 165 160 155 150 0

0.02

0.04

0.06 0.08 0.1 CNT volume fraction

0.12

0.14

Fig. 27.18 Effect of CNT content on the yield (0.2% proof ) strength of the composites. Adapted from Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A, Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties, Mater Sci Eng A 2011;528:6727–6732.

similar to the Young’s modulus performance. The influence of the CNT content is very much evident here and improves the mechanical property of the copper matrix-based composites.

27.8

Conclusion

The extraordinary physical as well as mechanical properties of CNTs along with the light weight provide an excellent platform for carbon as an outstanding candidate for composite reinforcement. A broad spectrum of knowledge on CNT-reinforced composites can be acquired through analyzing the combination of thermal and mechanical attitudes of respective composites. However, the analysis and research are taking place at the scale of the nanometer. Because the reinforcement size is in the nano level, the approach toward analysis changes and fresh challenges come into the picture. Those challenges are in terms of characterization, methodology to be adopted, measurement techniques, fracture analysis, etc. Moreover, the atomic level synergy makes it compulsory to find out new mechanisms for conducting analysis. The benefits and challenges prevail simultaneously, for which the latter must be handled effectively to harvest the positivity of the CNTs in applying for composites. The entire route starts with an economic and efficient manufacturing method followed by incorporation and analysis. The quantity of CNT present inside the composite, the duration of milling, and the dispersion quality of the CNT in the matrix are some of the crucial and critical points to be observed for achieving optimum results through the performance of the composites. The same would enhance the strength of the composites.

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[40] Bastwros MMH, Esawi AMK, Wifi A. Friction and wear behaviour of Al–CNT composites. Wear 2013;307:164–73.

Further reading [41] Bhat A, Balla VK, Bysakh S, Basu D, Bose S, Bandyopadhyay A. Carbon nanotube reinforced Cu–10Sn alloy composites: mechanical and thermal properties. Mater Sci Eng A 2011;528:6727–32.