Carbon Nanotube Reinforced Metal Matrix Composite (CNT-MMC) Systems

Several metal matrices have been reinforced with carbon nanotubes in order to study the feasibility of composite fabrication and their properties for potential applications. Figure 1.5 in Chapter 1 showed the pie chart of the percentage of publications on various metal matrices. Based on the number of total articles published in journals, it is observed that Ni-CNT composites have been researched the most for non-structural applications, whereas Cu-CNT and Al-CNT composites have received major attention for structural applications. Other matrices like Mg, Ti, and Si have also received some attention. Attempts have also been made to incorporate CNTs in novel materials like metallic glasses. The aim of this chapter is to provide the reader an idea of the various attempts made by researchers in the area of CNT-MMCs. In this chapter, w'e look into specific metal-CNT systems and summarize the majority of the work carried out to date. This work has been summarized in the form of exhaustive tables for each metal-CNT system. These tables list important information for each metal-CNT system such as: (1) composite processing technique, (2) CNT dispersion method used, (3) CNT content and quality of dispersion, (4) reaction at the CNT/matrix interface, if any and (5) material properties. Since the largest intended application of the CNT-MMCs is for structural purposes, most of the researchers have reported the mechanical properties of these composites. Hence, mechanical properties (elastic modulus, hardness, yield/tensile strength, strain to failure) of CNT-MMCs are also tabulated to highlight the role of CNT addition. It was discussed in Chapter 4 that mechanical testing and properties are highly dependent on the processing methods, which often dictates the shape and dimensions of the test samples made out of CNT- MMCs. The samples utilized for the mechanical property evaluation are often nonstandard in size. The sample size used to evaluate the mechanical properties of the metal matrices w'ith and without CNT reinforcements is also outlined in the table. The table includes a separate column that outlines the improvement in other properties, namely electrical, thermal, tribological, etc. A detailed discussion on the improvement of other properties such as wear, and thermal and electrical properties, is provided in Chapter 8. A short discussion for each CNT-MMC system is also included to highlight the most significant studies related to the mechanical properties. This chapter also provides the reader a clear understanding of the CNT-MMCs with the scope for further research.

Aluminum-Carbon Nanotube System

Aluminum and its alloys, being the most abundantly used non-ferrous structural materials, were the first choice for reinforcement with carbon nanotubes. Table 5.1 is a compendium of the research work carried out on Al-CNT composites. The first publication on metal-CNT system was on Al-CNT in 1998 [1]. The powder metallurgy route has been used extensively due to ease of dispersing the CNTs within the aluminum matrix. A secondary consolidation by deformation processing is advantageous in obtaining higher densities and improved CNT distribution. Hot extrusion has been used extensively because it results in highly dense composites. In addition, hot extrusion has been showrn to break CNT clusters and even align them along the extrusion direction. Several methods have been used for dispersing CNTs in aluminum powder such as dispersing in liquid medium by ultrasonic mixing, blending, mechanical milling, spray drying, NSD, and synthesizing CNTs on powder by CVD. Ball milling is found to result in moderate to good dispersion with poor to excellent mechanical properties.

Most of the studies (>60%) have used ball milling for the powder preparation. Dispersion of the CNTs and the presence of porosity are two major factors that affect the mechanical properties of the CNT-MMC. CNT clusters are very detrimental to the properties because they act as notches and areas of stress concentrations. Processes using inefficient methods for dispersion like mixing by stirring in alcohol [1], blending by roll mixing [2], etc. result in formation of CNT clusters in the final product. A decrease in the tensile strength by 9% was observed in a 10 vol.% CNT composite in which stirring was used to disperse the CNTs in the powder [ 1 ]. Further annealing the composites resulted in softening of the sample without CNTs, while the ones containing CNTs showed negligible softening. CNT reinforcement in an A1 alloy (AA 7075) matrix has also been found to improve the impact and flexural strength by a significant 90% and 125%, respectively [3]. Fatigue strength is also enhanced by incorporating CNT into A1 alloy (A12024) matrix, which is reported to be due to crack bridging by CNTs, during failure [4].

Agrawal’s group has studied synthesis of Al-CNT composites by thermal spray processes, namely plasma spraying [2,5]. HVOF [6,7], and cold spraying [8]. Various aspects like interfacial phenomena, quantification of CNT dispersion, and mechanical properties have been studied at length. Al-CNT composite prepared by plasma spraying of blended powders also showed very small increase in the tensile strength (4%) for 12.5 vol.% CNT addition [9]. Tensile tests on plasma spray formed bulk tensile samples using spray-dried powder and indicated that there is a 23% and 17% decrease in elastic modulus, 24% and 25% decrease in fracture strength, and 29% and 45% decrease in fracture strain by addition of 5 and 10 wt.% CNTs [10]. Improper milling also leads to poor dispersion and has been shown to reduce the strength by 52% for just 2.4 vol.% CNT addition [11]. Deterioration in the hardness in shock wave consolidated Al-CNT composite w'ith up to 5 vol.% CNT addition has been observed [12]. All of these studies showed the presence of CNT clusters in the microstructure, which indicates that they are detrimental to the properties of the composites.

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

5 and 10

vol.%

Stirred in ethanol (0.5 h, 300 rpm), dried in vacuum and preheated (600°C . 1.5 h, vacuum), compacted (100 MPa), hot extruded (500°C. extrusion ratio—25)

Diameter.: 3 mm Gauge length: 15 mm

at 873K ~ 80 MPa (for 0-100 h of annealing)

(0-100 h) 90-40 MPa (for 0-100 decreasing with annealing time)

Non-homogeneous distribution of CNT in matrix is reported

Ш

1,4, and 10wt.%

Hand ground (30 min), hot pressed (525°C, 25 MPa)

Agglomerate of CNT at grain boundary is reported

Electrical resistivity drops at low temperature

[97]

5wt.%

SWNT

Ultrasonicated in alcohol (30 min), pressed (1.5 GPa). hot pressed (260-480°C. 1 GPa. 30 min)

H: 2.89 GPa

H: 1.6 GPa

[99]

10 wt.%

Blended and mixed, ball milled, plasma sprayed (28 kV)

H: 146 VHN

85 VHN

Dangling CNT and entrapment of CNT in successive splats is found

[2]

0.8 and 1.6 vol.%

Precursor with natural rubber, roll milled, compressed in mold, heated (800°C, 1 h. nitrogen atmosphere)

Compression test, dimensions not mentioned

Al-1.6 vol.% CNT oYSIT): 225 MPa

aYS(T) < 50 MPa

Good dispersion and good adhesion of CNT in matrix is reported

[14]

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

0-20

vol.%

SWNT

Mixed, compacted (1.5 GPa), room temperature, consolidated at high temperature and pressure (3S0°C, 1 GPa, 30 min, vacuum)

Homogeneous dispersion of CNT in matrix is reported

Coefficient of thermal expansion reduces w ith SWNT addition

[1001

10 wt.%

Blended and mixed, ball milling (48 h), high velocity oxy-fuel and plasma sprayed (28 kV)

H(PSF): 1,57 GPa H (HVOF): 1.72 GPa

H: 1.0 GPa (conventionally casted)

CNT are found coated with Al-Si resulting into good bonding

[b]

0.5-2 vol.% CNT 1-2 vol.% SWNT

Mixed by ball milling (5 min, 200 rpm), compacted (120 kN), sintered (580°C, 45 min), and hot extruded (560°C)

Al: 2 vol.% CNT aVS(Tl: 99MPa ctts: 150 MPa Al: 2 vol.% SWNT oYS(T): 98.7 MPa ars: 181 MPa

[1011

A1 deposited by magnetic sputtering on vertically aligned film of CNT

A14Cj formation at interfaces is found

[91]

10 wt.%

Blended and mixed by ball milling (48 h), HVOF (high velocity oxy fuel forming) and plasma sprayed (28 kV)

Formation of SiC at interface is found

[7]

5 wt.%

CVD growth of CNT on A1 powder (using Ni catalyst), pressed (600 MPa), sintered (640°C. 2 h), re-pressed (2 GPa)

H: 0.65 GPa als: 398 MPa

H: 0.15 GPa oTS: 140 MPa

Homogeneous dispersion and good interfacial bonding of CNT with matrix is reported

[13]

2 wt.%

Ball milling (1-48 h, 200 rpm, argon atmosphere)

Good distribution after 48 h of milling. CNT embedded in deformed Al particle

[102]

0-20

vol.%

Al-Mg-CNT-mixed by ball milling (7 h. 300 rpm. argon atmosphere), pressed to preform, pressureless infiltration of Al. (800°C. 5 h. nitrogen atmosphere)

ex gvioAl: 15 vol.% CNT H(Brinell): 175

H(Brinell): 106

Fully dispersed and embedded CNTs in matrix are found

Decrease in coefficient of friction and wear loss with CNT addition

[103]

1 wt.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in Al case (460°C. extrusion ratio—25)

E: 102.2 GPa оте: 521.7 MPa %e: 17.9 MPa H: 136 MPa

E: 72.3 GPa ars: 384.5 MPa %e: 18.8 H: 104

Homogeneous distribution, good bonding with Al matrix, short pull out of CNTs are found, resulting in better elongation

[104]

2 and 5 vol.%

Layers of pure Al. and Al-CNT. green compacted, shock wave consolidated (7 GPa initial pressure)

D638 type: V ASTM Gauge length: 9 mm

Al-2 vol. % CNT H: 39 HRE* Al-5% vol. CNT H: 33 HRE* оте: 20 MPa, %e: 2 (*Rockwell hardness in E scale)

H: 40 HRE*

avs(T): 120 MPa %e: 6.5

Non homogeneous dispersion. CNT agglomerates, weak bonding with matrix is reported

[12]

5 wt.%

Mixed and ultrasonicated (30 min), dried (120°C. vacuum), cold isostatic pressed

CNT react with Al to form AI4Cj above Tm of Al (656.3°C)

[90]

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

0-2 wt.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in A1 case (460°C. extrusion ratio = 25)

Not mentioned

Al-1 wt.% CNT 520 MPa E: 102 GPa

%e: 19%

cTS: 385 MPa E: 72 GPa %e: 20

Reported homogeneous dispersion of CNTs with good bonding, bridging across crack, short pullouts, form A14Cj phase at interface (~656.3°C)

1105]

2-5 wt.%

Mechanically alloyed using ball milling

After 48 h of milling, CNT gets embedded in plastically deformed A1 particles

1106]

1 wt.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in A1 case (460°C. extrusion ratio = 25)

Damping specimen, gauge side 1 x 7 x 38 mm

Damping capacity storage modulus: 82.3 GPa (at 400°C) 98 GPa (at room temperature)

Damping capacity storage modulus: 71 GPa (at room temperature)

Reported homogeneous dispersion of CNTs with good bonding, bridging across crack, short pullouts, form AI4C3 phase at interface (~656.3°C)

1107]

1 wt.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in A1 case (460°C. extrusion ratio = 25)

Dog bone shape sample Gauge length: 15 mm

H: 136 MPa aYS(T): 336 MPa TS: 474 MPa E : 88 GPa

HV : 104 MPa

aYS(T): 289 MPa nTS: 384 MPa E: 71 GPa

Reported homogeneous dispersion of CNTs with good bonding, bridging across crack, short pullouts, and formation of AI4C, phase at interface (~656.3°C)

1108]

5 vol.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in A1 case (460°C. extrusion ratio = 25)

CNT transforms to nano AI4Cj needle shaped particle, mainly found at grain boundaries of A1 particle

[109]

1 wt.%

Mixed and ultrasonicated. cold isostatically pressed (300 MPa. 5 min), hot extruded in A1 case (460°C. extrusion ratio = 25)

CNTs found

homogeneously embedded in matrix, short pullout length

Coefficient of thermal expansion decreases with CNT addition

[110]

0.5, 1.0, and 2.0 wt.%

Mixed by blending (300 rpm), encapsulated, hot rolled (50% reduction per pass), vacuum sintered (300°C, 3 h). air sintered (550°C. 45 min)

Mechanical properties were measured with resonance frequency

Al-0.5 wt.% CNT Ysm: 100MPaars: 150 MPa E: 60 GPa

aYS(T): 70 MPa оте: 130 MPa E: 50 GPa

CNT dispersion worsens with increase in CNT amount and CNT clusters are formed

[11]'

Blended and ball milled, compacted, sintered, hot extruded (560°C)

CNTs are pinned at subgrain boundaries

E of CNTs has been calculated by assessing bending of CNTs at subgrain boundaries

[1111

4 vol.%

Mixed and ball-milled (6/12 h). hot extruded (470°C, extrusion ratio =15)

Rectangular 2:1 length to width ratio

cls: 400 MPa H: 104.19 GPa

cls: 350 MPa E: 70.05 GPa

CNTs are found well dispersed, deeply embedded, aligned along the extrusion direction

[112]

5 wt.% SWNT

Mixed and ultrasonicated (5 min), high pressure torsion to form composite disks (2.5 GPa, 1 rpm, 30 turns)

Dog bone shape sample Length: 1mm Width: 1mm Thickness: 0.5mm

H: 76 VHN -215 MPa

Hardness: H: 43 VHN ars:~150 MPa

SWNTs are found sitting at grain boundaries

[113]

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

CNT preform fabricated by sintering ( 2500°C. 20 min, argon atmosphere), infiltration of liquid metal in preform by squeeze casting

NA

NA

NA

Claimed good wetting, i.e., good reinforcement and good dispersion

1114]

0.5 wt.%

Al-CNT composite powder synthesized by spray drying, cold sprayed (pressure difference = 2.9 MPa)

1115]

0.5. 1

wt.%.

Al-CNT composite powder synthesized by spray drying, cold sprayed (pressure difference - 2.9 MPa)

AI-0.5 wt.% CNT E-68.6 GPaAI-1 wt.% CNT E-68.1 GPa

Uniform dispersion of CNT in Al matrix is claimed

181

Friction stir welded (1500 - 2500 rpm)

H-213HVN

H: 140HVN

Good reinforcement but not completely uniform distribution is reported

1116]

5. 10 wt.%

Al-CNT composite powder synthesized by spray drying, plasma sprayed (22 kW)

Al-10 w't.% CNT E: 125 GPa H (nano): 2.89 GPa H (micro): 2.10 GPa

E: 90 GPa H (nano): 1.61 GPa H (micro): 0.87 GPa

Two phase microstructure - matrix having good distribution of CNT and CNT-rich clusters; aluminium carbide (AI4C.,) forms at CNT-AL interface

151

0.6 vol.%

Al-CNT composite powder synthesized by spray drying, cold sprayed (pressure difference: 2.9 MPa)

CNTs are reported uniformly dispersed in Al-Si matrix - the dispersion is quantified in this study

[П7]

5, 10 wt.%

Al-CNT composite powder synthesized by spray drying, plasma sprayed (22 kW)

Study on interfacial reaction: formation of AI4C, at interface with low Si content and SiC at higher Si content

[118]

5 vol.%

Composite powder prepared by NSD technique using natural rubber as precursor, SPS (600°C, 50 MPa. 20 min), hot extruded (400°C, 500 kN. extrusion ratio - 20)

Diameter: 3 mm Gauge length - 15 mm

оте: 194 MPa

85 MPa

Homogeneous and good dispersion of CNT in Al matrix, CNTs are oriented in matrix Al-carbide formation at Al-CNT interface

[15]

10 wt.%

Blended ball milled (48 h). plasma sprayed (28 kW)

Gauge length: 26 mm Width: 8 mm Thickness: 0.635 mm

E: 120.4 GPa oTS: 83.1 GPa Strain to failure: 8.8 x ,„-4

E: 67.5 GPa oTS: 79.8 GPa Strain to failure: 19.2 x К)-4

Some degree of CNT clustering and inhomogeneous distribution in matrix Thin layer of SiC formed at CNT-A1 interface

[91

1.5, 3,4.5. 6 vol.%

Ball milled (500 rpm. argon atmosphere), hot rolled (480°C, 12% reduction)

Gauge length: 12.5 mm Thickness:

1.5 mm 3-Point Bend Test Length: 15 mm Width: 3.75 mm Thickness: 1.88 mm

Al: 4.5 vol.% CNT E: 110.05 GPa oVS(Tl: 610 MPa KIt: 60.79 MPA. mm1'2

E: 70.063 GPa oVS(T,: 262 MPa K1C : 33.22 MPA.

mm 1/2

CNTs reported uniformly dispersed and embedded in Al matrix and aligned along rolling direction

[16]

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

0-2 wt.%

Mechanically milled (5 h. argon atmosphere), pressureless sintered (550°C, 3 h. vacuum), hot extruded (500°C , extrusion ratio - 16)

Diameter: 10 mm Gauge length: 30 mm

Al: 2 wt.% CNT oVS(T): 189.2 MPa ars: 243 MPa H - 73 VHN

aYS(T): 105 MPa ors: 159 MPa H: 49.2 VHN

Good dispersion reinforcement of CNT is reported in matrix with Al-carbide formation at AI-CNT interface

119]

0-2 wt.%

Mechanically milled (5 h. argon atmosphere), pressureless sintered (550°C, 3 h. vacuum), hot extruded (500°C , extrusion ratio - 16)

Reported uniform dispersion of CNT in Al matrix and formation an amorphous interface that causes better adhesion between Al and CNT

1119]

2 wt.%

Ball milled (3/6 h. 200 rpm. argon atmosphere), compacted (475 MPa), hot extruded (500°C, extrusion ratio - 4)

Diameter: 4 mm Length: 65 mm Gauge length: 20 mm

Ball milled: 3 h <%: 345 MPa %e: 5.7 Ball milled: 5 h oTS: 348 MPa %e: 7.9

Ball milled: 3 h <%: 284.5 MPa %e: 8.6 Ball milled: 5 h oTS: 348.5 MPa %e: 8.4

Good dispersion and alignment of CNT is reported in matrix

1120]

0-5 wt.%

Acid treatment of CNTs. ultrasonicated with Al ( 20 min), SPS (600 °C, 50 MPa. Ю min, vacuum)

Al-5 wt.% CNT H: 55 VHN

H: 45 VHN

Al-carbide formed at interface with 5 wt.% CNT

Coefficient of friction decreases and wear decreases with 1 wt% CNT With 5 wt.% CNT. properties become poor

11211

0-9.5

vol.%

CNTs suspended in acetone, sprayed on Al foil, sandwiched cold rolled (70% reduction), intermediate annealed (250°C, 1 h)

Length: 50 mm Gauge length: 15 mm Width: 4 mm Thickness - 50 pm

Al-9.5 vol.% CNT ars: 97 MPa Al-2 vol.% CNT E: 75 GPa

cTS: 28 MPa E: 47 GPa

Good dispersion and reinforcement of CNT in Al matrix is reported at 2 vol.% CNT. agglomeration of CNT in matrix occurs at higher CNT content

[122]

3 vol.%

Ball milled (6 h). hot rolled (480°C, 12% reduction)

Gauge length: 12.5 mm Width: 6 mm Thickness: 1 mm

oVSITl: 520 MPa

aYS(T): 400 MPa

[123]

2 wt.%

Mechanically alloyed using ball milling (12/24/48/72 h. 200 rpm. argon atmosphere)

Uniform dispersion of CNT is reported in ball-milled composite powder

[124]

0-6.5

wt.%

CNT grown on Al powder by CVD. pressed (600 MPa), sintered (640°C , vacuum, 3 h), further pressed (2 GPa), annealed (850°C, 2 h)

Gauge length: 20 mm Width: 5.5 mm Thickness: NA

Al-5 wt.% CNT E: 95.4 GPa cTS: 398 MPa H: 0.65 GPa

E: 71.1 GPa als: 140 MPa H: 0.15 GPa

Homogeneous dispersion of CNT is reported in matrix along with formation of AI4Cj at Al-CNT interface

observed

[125]

0-2 wt.%

Mechanical mixing of powders (2h, 200 rpm). uniaxial cold compacted (2 ton/cm2). sintered (580°C. 90 min), cold extruded (extrusion ratio = 2.25)

Diameter: 7 mm Gauge length: 10 mm

Al: 2 wt.% CNT oYsm: 176 MPa ars: 184 MPa H: 74 VHN

(Tysi i,: 91 MPa оте: 98 MPa H: 69 VHN

Uniform dispersion and preferential alignment of CNTs in the matrix and formation of AI4C, at Al-CNT interface is reported

[126]

0-2 wt.%

Mixing of powder in roller mill, spark plasma sintered (500°C, 20 min, vacuum), hot extruded (500°C. extrusion ratio = 9)

Diameter: Gauge length: 28 mm

Al-0.5 wt.% CNT oYSIT,: 96 MPa axs:

174 MPa H: 50 VHN

(TVS|T|: 66 MPa ctts: 153 MPa H: 45 VHN

CNTs are found distributed at Al grain boundaries and tend to form clusters at higher CNT contents

[127]

TABLE 5.1 (Continued)

Table of the Summary of the Work Carried Out in an AI-CNT System

CNT

Content

Composite Processing Technique

Tensile Test Sample Size

Mechanical Properties (UTS, YS, E, Strain to Failure, Hardness)

CNT Dispersion and Interface

Other Properties

Ref.

With CNT

Without CNT

2 vol.%

WC layer deposited on CNTs. melted with A1 and casted. aging treatment performed (natural and artificial at 185-195°C)

Al-CNT H: 40 MPa 2024AI alloy - CNT H: 120 MPa

H: 15MPa 2024A1 alloy H: 102 MPa

Homogeneous distribution of CNT in matrix is found

[128]

1 vol.%

Composite powder prepared through NSD method. SPS (480/500/560/600°C, 50 MPa. 20 min), hot extruded (400°C, 500 kN, extrusion ratio = 20)

Diameter: 3 mm (ICS 59.100.01)

оте:207 MPa %e: 21.5

52 MPa %e: 19.5

Formation of AI4C, at Al-CNT interface is observed

[18]

5, 10wt.%

Al-CNT composite powder synthesized by spray drying, plasma sprayed (22 kW)

Uniform distribution of CNT as well as presence of clusters in matrix are reported

Wear resistance increases with CNT content, but no effect is found on coefficient of friction

[129]

2.5 wt.%

Mixing by ball milling (90 min. argon atmosphere), spark plasma extrude (433°C, extrusion ratio = 16)

Compression test sample Disk shape Length/ Diameter = 1.5

ccs: 415 MPa H: 99 VHN

ocs: 377 MPa H: 74 VHN

[130]

Note: — Data not available or not applicable; E - elastic modulus; H - hardness; ctts - tensile strength; ocs - compressive strength; aYs(T> - yield strength in tension; %e - percentage elongation; K|C- fracture toughness.

Growing CNTs on Al powder surface by CVD method leads to better bonding and dispersion. Composites prepared from CVD Al-CNT powder by sintering followed by repressing lead to an increase in tensile strength by 184% and hardness by 333% in a 5 vol.% CNT composite as compared to unreinforced material [13]. An increase in the compressive yield strength by 350% has been reported for samples prepared by NSD that resulted in better dispersion [14]. Composites prepared by hot extrusion of compacts consolidated by SPS of NSD Al-CNT powders showed good dispersion due to breakup of CNT clusters. The composite containing 5 vol.% CNT was found to have a tensile strength 128% higher than the unreinforced material [15]. Ball milling has been used extensively to disperse the powders in the Al powder. Depending on the degree of dispersion, different studies reported different degrees of strengthening. Aluminum 4.5 vol.% CNT composite prepared by hot rolling of ball milled powders was shown to have a tensile yield strength of 620 MPa and fracture toughness of 61 MPa mm1'2, which are, respectively, 15 and 7 times more than that for aluminum [16]. Plasma sprayed aluminum composite coatings made by blended powder have been shown to improve the hardness by 72%, elastic modulus by 78%, marginal improvement in tensile strength, and 46% decrease in ductility with 10 wt.% CNT addition [13]. Sintering at 673 К for 72 h of the plasma sprayed Al-10 wt.% CNT coating has been reported to further increase the elastic modulus of the composite coating by 80%, which has been attributed to reduction in porosity and residual stress [17]. Al-12 vol.% CNT composite produced by plasma spraying of spray-dried powders shows 40% increase in the elastic modulus [5]. CNT addition results in an increase in the elastic recovery [5]. Al-1 vol.% CNT prepared by hot extrusion of SPS compacts have displayed tensile strengths up to four times (198 MPa) of aluminum (52 MPa) [18]. Strengthening has been observed irrespective of the formation of AI4C, [15,19]. Significant strengthening has been achieved in samples produced by hot extrusion technique because the technique can produce high densities and can lead to breakdown of CNT clusters [15]. These results show that homogeneous distribution of CNTs, strong bonding with the matrix, and high density are the key factors to control the mechanical properties of the aluminum-CNT composites.

 
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