Open access peer-reviewed chapter

Carbon Nanotube (CNT)-Reinforced Metal Matrix Bulk Composites: Manufacturing and Evaluation

By Sebastian Suárez, Leander Reinert and Frank Mücklich

Submitted: October 23rd 2015Reviewed: April 21st 2016Published: June 29th 2016

DOI: 10.5772/63886

Downloaded: 1748

Abstract

This chapter deals with the blending and processing methods of CNT-reinforced metal matrix bulk composites (Al/CNT, Cu/CNT and Ni/CNT) in terms of solid-state processing, referring mainly to the research works of the last ten years in this research field. The main methods are depicted in a brief way, and the pros and cons of each method are discussed. Furthermore, a tabular summary of the research work of the mentioned three systems is given, including the blending methods, sintering methods, the used amount of CNTs and the finally achieved relative density of the composite. Finally, a brief discussion of each system is attached, which deals with the distribution and interaction of the CNTs with the matrix material.

Keywords

  • Carbon nanotubes
  • metal matrix composites
  • reinforcement effect
  • solid-state processing.

1. Introduction

Composite materials have been in the spotlight of material science and engineering for a long time already. They provide the capability of tailoring their properties by managing very simple variables such as the reinforcement fraction or the processing parameters, among others. Their application area is found in a wide range of dissimilar fields, ranging from bioengineering [1,2] up to aerospace [3]. The applicability relies on the proper selection of both, the matrix phase and the reinforcing phase. By matrix, the phase with the largest volume fraction is meant, whereas the opposite is valid for the reinforcing phase. In the former – according to the specific application – polymers, ceramics or metals are used. Polymer-matrix composites are mainly used in applications where lightweight is required, working in environments that do not present high temperatures. On the opposite, ceramic matrices are used where inertness under high temperatures is required as well as high mechanical properties. Finally, metal matrix composites (MMC) lie in between both application fields, presenting in most cases tailored microstructures (and subsequently, tailored physical properties) and, in certain cases, lightweight.

Reinforcing phases can be of very dissimilar nature. The most widespread phases are usually ceramic fibres and/or particles (i.e. Al2O3, TiC), which show very good mechanical properties, thus enhancing the overall mechanical properties of the composite. However, when considering the transfer properties (electrical and thermal), their ceramic nature plays a detrimental role. Furthermore, the fact that the material being subjected to improvement is a metal, with usually very good transport properties, the task becomes indeed non-trivial.

In recent years, the appearance of carbon nanotubes (CNTs) has opened an interesting new field. Since CNTs show intrinsically outstanding physical properties, the aforementioned drawback brought by ceramic reinforcements might be straightforwardly overcome. Yet, the predicted physical properties of CNTs are only realisable if the CNT is in a “perfect” structural state. By “perfect” structural state, it is meant that there are (a) no structural defects on its lattice, (b) no exo- or endohedral contaminants (synthesis/catalysis residues) and (c) the CNTs are in an isolated state (no CNT agglomeration). Those three conditions are quite challenging to achieve in the praxis.

When it comes to CNTs, different synthesis methods have to be considered. CNT synthesis renders unavoidably contaminants (sulphur/amorphous carbon) and catalyst particles. The most common synthesis methods employed are chemical vapour deposition (CVD), arc discharge and laser ablation. The CVD synthesis is the one with the highest capacity to be scaled to industrial quantities and provides a better control on the morphology of the obtained CNTs [4,5] in comparison with other standard synthesis methods. Yet, the defect state is usually high and should be thoroughly analysed before the application.

Most of the recent research works are leaning towards commercially available MWCNTs as starting material for the composite production. Some of the most recurring suppliers are Nanocyl (Belgium), Nanolab Inc. (USA), Iljin Nanotechnology Co. Ltd (Korea), Bayer MaterialScience (Germany), Chengdu Organic Chemicals Co. Ltd (China), Chinananotech Co. Ltd (China) and Hanhwa Nanotech Co. Ltd (Korea) to name just a few [631]. But also self-produced CNTs are used for composite manufacturing. For this purpose, catalytic chemical vapour deposition (CCVD) is the most common way to synthesize CNTs that are used as reinforcement phase in composites [6,7,10,3239]. The MWCNTs generally show a purity between 90% [10] and 99.5% [19], but mostly around 95%. The diameter ranges from 10 nm to 80 nm with a length of 0.5 µm–50 µm [639]. All the information about CNT-MMCs in this book chapter is derived from the research works of the last ten years, including some relevant exceptions.

1.1. Blending methods

Considering that commercial CNTs are usually delivered in agglomerated form, different methods have to be employed to disaggregate and blend them with the metallic matrix material. There are a lot of dispersing and blending processes like magnetic stirring [40], nanoscale dispersion processing [18,21,41], colloidal mixing [6,16,22,2631,4149], molecular-level mixing [7,9,11,12,14,24,25,5054], particle composite system mixing [15,55], friction stir processing [56], layer stacking [57], ball milling [8,10,17,19,20,2224,3236,38,50,53,5874], dipping [75] or roller mixing [37,76]. However, three of them have to be pointed out as the most commonly used ways to disperse and blend CNT metal matrix systems. These three processing methods are ball milling, molecular-level mixing and colloidal mixing. The methods will be briefly commented in the following paragraphs.

Ball milling

It is usually performed using a planetary or attrition ball mill. The mixing is done by filling in the Metal powders and reinforcement phase together with some hard balls into mixing jars and rotating the jar with a certain rotational speed (Figure 1 a).

Figure 1.

Schematic draft of the ball milling process (a). The used balls are hitting the CNTs and the matrix powder material (b), welding and integrating the two components (c) and bounce of the powder particle to restart this process again at another spot (d).

During the rotational movement, the added balls are falling on top of the powder material (Figure 1 b), thus leading to a size reduction, particle welding and integration of CNTs into the matrix powder material (Figure 1 c and d) generated by the impact energy. Different ball materials and sizes, rotational speeds, balls to powder ratios, gas atmospheres and mixing times can be chosen as main mixing parameters. Usually, a process control agent, for example ethanol, is added in order to prevent cold welding of the matrix material powder particles. In other cases, like mechanical alloying, a cold welding of the particles is desired. Ball milling is known to produce a homogenous distribution of reinforcement phase in metal matrix composites as particle agglomerates are segmented, and dispersed CNTs are partially welded together with the matrix powder material (Figure 1 d). In general, high energy ball milling and low energy ball milling are distinguished in literature when it comes to the mixing process of CNTs with metal matrix material. This is because of the main drawback of the method, which is the increasing defect density of CNTs during the mixing process by the direct application of large contact pressures (up to 30 GPa) [77]. Using low energy ball milling, this unwanted effect can be reduced, but it will not vanish. The importance of a low defect density of CNTs will be discussed later on in entry 2.2 [8,10,17,19,20,2224,3236,38,50,53,5874].

Molecular-level mixing

For this mixing method, it is of utmost importance to functionalize the CNTs, for example with an acid treatment (Figure 2 a). After this, the CNTs can be dispersed in various solvents, for example using ultrasonic agitation to obtain a stable suspension.

Figure 2.

Schematic draft of the molecular-level mixing method. CNTs are functionalized by functional groups, which will be covalently bond onto the CNTs surface (a). After this, metal ions can electrostatically interact with the functional groups, thus coating the CNT surface with metal ions (b). These ions are transformed to a pure metal layer by calcination and reduction under temperature and H2, N2 or CO atmosphere (c). Finally, the CNTs are fully decorated or even integrated in the metal matrix material (d).

A metal salt is added and reduced by an added reducing agent to form a metal oxide CNT suspension with the CNTs acting as nucleation centres for the metal oxide formation (Figure 2 b). Finally, after washing off all the remaining chemicals, the powder is calcinated and then reduced, for example under hydrogen atmosphere to reduce it to the metal/CNT powder (Figure 2 c and d). The advantage of this process is that the CNT particles are embedded or coated by the metal matrix, thus resulting in very homogeneous distributions within the metal matrix (Figure 2 d). In many other mixing methods, the reinforcement phase can only be situated at the grain boundaries, offering weak interfacial bonding between the reinforcement phase and the matrix material and reducing the homogeneity of the distribution. For the molecular-level mixing process, this is not true, therefore being favoured for applications where a good connectivity of the reinforcement phase network is needed like thermal or electrical applications. The main drawback of the method is the required functionalization, which involves breaking up covalent carbon bonds to add functional groups to the surface of CNTs, thus diminishing their outstanding properties [7,9,11,12,14,24,25,5054].

Colloidal mixing

For colloidal mixing, the CNTs are dispersed using an ultrasonic bath, homogenizer or magnetic stirrer in a solvent (for the most part, ultrasonic agitation is used (Figure 3 a)). There are many different solvents that allow for a fine and stable dispersion of the CNTs, which is discussed in [81] (e.g. DMF or ethylene glycol). The dispersion grade and stability of the CNT suspensions are not only depending on the used solvent, but also on the surface of the used CNTs. Some research works functionalize the surface of the CNTs to obtain electrochemically stable dispersions, which works great but influences the physical properties of CNTs as already discussed for the molecular-level mixing. Other works avoid functionalization, thus using the pristine CNTs, retaining their properties in detriment of the dispersion quality.

Figure 3.

Schematic draft of the colloidal mixing method. First, the CNTs are dispersed in a liquid solvent using ultrasonic agitation, after which the matrix material powder is added (a). The two components are mixed again by ultrasonic agitation (c), and the solvent is evaporated (c) to finally obtain the mixed CNT/metal matrix powder (d).

One point that is controversially discussed is the impact of ultrasonic agitation on the defect density of CNTs. There are studies claiming for a rise in the defect density as a function of the time spent in the ultrasonic bath and others which report the opposite. The observed decrease in the defect density might be a misinterpretation of Raman spectra of disentangled CNTs (which would render improved ID/IG ratio stemming from the avoidance of intertube interactions). After the dispersion of CNTs in the solvent, the metal powder is added and mixed with the dispersed CNTs again by ultrasonic agitation, stirring or a homogenizer (Figure 3 b). Finally, the solvent is evaporated to obtain a dry mixed powder (Figure 3 c and d). A significant advantage of this method is that it can be very easily upscaled and still yield the same results [6,16,22,2631,4149].

1.2. Processing methods

After merging the CNT/metal composite powders, different densification methods are used for the consolidation of the final samples. Examples of these are cold pressed sintering (CPS) [8,14,17,20,27,28,34,35,47,53,63,74,78], hot uniaxial pressing (HUP) [10,19,26,27,2931,39,40,47,49,62,6770], spark plasma sintering (SPS) [6,7,9,11,12,1416,18,21,2325,42,4446,48,5052,55,64,75,79,80], hot or cold rolling [7,50,59,60,64], hot extrusion [18,20,23,40,58,79], high pressure torsion (HPT) [22,3133,41,54,61,65,66,71,73], friction stir processing [56], hot isostatic pressing (HIP) [22,57], microwave sintering [43], laser engineered net shaping (LENS) [3638,76] or a combination of those methods. Methods like cold or hot rolling, hot extrusion or HPT are often used for post-processing of the already consolidated samples [7,22,31,50,59,60,64]. The most often used methods for the production of metal matrix composites are CPS, HUP and SPS, which will be described briefly in the following.

Cold Pressed Sintering (CPS)

This is by far the simplest method to densify the blended powder. Using a uniaxial press or an isostatic press, the powder is pre-compacted to the desired shape (Figure 4).

Figure 4.

Schematic draft of pressureless sintering or cold pressed sintering. A die is filled with the powder material, which is then pressed uniaxial to a green pellet. The sample is then removed and sintered without pressure in a furnace under vacuum or inert gas atmosphere.

After this, the sample is sintered without further pressure under vacuum or an inert gas atmosphere to form the consolidated sample (Figure 4). Even though one heating–cooling cycle might be very time-consuming (the resistive furnaces usually have a very limited heating rate), it has the advantage that many samples of dissimilar shapes can be sintered at the same time, making this sintering method very time efficient. However, the densification mechanism is mainly based on lattice and grain boundary diffusion [81], and large porosities cannot be closed without applying pressure, thus resulting in a poor final density of the composite [8,14,17,20,27,28,34,35,47,53,63,74,78].

Hot Uniaxial Pressing (HUP)

In addition to lattice and grain boundary diffusion (mechanisms for pressureless sintering), plastic deformation and creep can be major sintering mechanisms. As the overall densification rate of a compact is a function of the sum of the active densification mechanisms, pressure-assisted sintering is much more effective than pressureless sintering. The application of an external pressure leads to an increase in the densification driving force and kinetics. As grain growth is not related to the applied external pressure, it is more effective in systems with a large grain growth to densification rate. To sum up, by using an external pressure, the sintering temperature as well as the sintering time can be reduced [81].

Figure 5.

Schematic draft of the hot uniaxial pressing method. A pre-compacted green pellet is inserted in a die, and uniaxial pressure is applied while the pellet is heated by induction under vacuum or inert gas atmosphere. The densified sample is finally removed.

When it comes to HUP, as with the CPS method, the mixed powder is usually pre-compacted using a uniaxial press or an isostatic press to obtain green pellets. After this, the green pellets are typically inserted in a die (often a steel die) where two punches (e.g. alumina) exert a uniaxial pressure onto the sample (Figure 5). The heating of the sample is usually conducted by induction, and thus, this system is very limited in the heating rate, rendering HUP as a very time-consuming sintering process. However, high pressures (several hundred MPa) can be applied with this method while sintering and almost full densification can be achieved (as punches with good mechanical properties can be used). To conclude, HUP is a time-consuming way to sinter samples, but it is also very effective when it comes to the final maximum densification of the sample [10,19,26,27,2931,39,40,47,49,62,6770].

Spark Plasma Sintering (SPS)

The active sintering mechanisms in SPS do not vary much from the active mechanisms in HUP. Yet, it provides a much quicker way to consolidate the composite powders [81].

Figure 6.

Schematic draft of the Spark plasma sintering method. A graphitic die is filled with the powder material, and a uniaxial pressure is applied via two graphite punches. A pulsed electric DC is applicated, which leads to the heating of the sample by its electrical resistance. The process is conducted under vacuum or inert gas atmosphere.

As with HUP, the mixed powders are pre-compacted using a uniaxial press or an isostatic press to obtain green pellets, which are inserted in a graphite die. With this method, graphite punches are used to allow for inducing a pressure at the sample and conducting a pulsed electric DC through the sample at the same time (Figure 6). The sample is heated by its electrical resistance, which depends on the used material. By controlling the used current, the heating rate can be adjusted. This method allows for a very high heating rate of several hundred °C/min, being therefore very time efficient. In contrast to HUP, the used pressure during sintering is much lower (typically about 50 MPa) because of the mechanically weak graphite punches. However, as with HUP, almost full densification can be reached with this method. Overall, this method offers a quick and effective way to consolidate the CNT-reinforced metal matrix powders, therefore being the most employed method in this area [6,7,9,11,12,1416,18,21,2325,42,4446,48,5052,55,64,75,79,80].

1.3. Potential applications

The main application of the CNT-reinforced metal matrix composites is in structural applications. This is due to the fact that most of the literature is devoted to the study of the mechanical properties of the composites. From the mechanical point of view, the addition of an intrinsically strong second phase would certainly improve the overall properties. Considering that, in the ideal state, the CNTs show a Young’s modulus of approximately 1 TPa and a maximum tensile strength of over 60 GPa [82], it is easy to trace their influence. Additionally, a set of factors influences the mechanical behaviour of these composites. First, it has been demonstrated that the addition of CNTs acts on the grain boundary mobility by hindering their displacement during grain growth [8385]. This effect influences the final microstructure (by refining it) and thus the mechanical behaviour (grain boundary strengthening). Second, a proper distribution of CNTs acts as an obstacle for dislocation movement, activating another strengthening mechanism known as particle dispersion strengthening (Orowan strengthening). Third, it has been shown that the CNTs present a very low or even negative coefficient of thermal expansion (CTE) in a wide temperature range [86,87]. When combined with high CTE materials (such as metals), this CTE mismatch acts also as a strengthening factor. Finally, considering the aforementioned values of the mechanical properties of the CNTs, the strengthening of the composite due to load transfer is expected to be significant. Summarizing, the addition of CNTs to MMCs is clearly expected to be beneficial in terms of the improvement of the mechanical behaviour, and subsequently, it is clear why most of the studies are focused towards this feature. The strengthening effect of the CNTs in MMCs is shown in Figure 4. There, the measured yield strength improvement of different systems is given as a function of the CNT volume concentration in the composite. The scattering of results is related to the different mixing and processing methods that were employed. In this context, it becomes clear that after a direct comparison of different research works involving a wide span of production methods for the same system, the task of correlating dissimilar results becomes non-trivial. Due to a lack of available information on the yield strength of Ni-CNT composites, this illustration provides only a summary of the mechanical reinforcement effect that was achieved so far with Al-CNT and Cu-CNT MMC systems.

Figure 7.

Yield strength improvement against the volume fraction of CNTs in (a) Al/CNT composites [18,20,56,57,60] and (b) Cu/CNT composites [7,9,11,53,64,66,70,75].

However, the transport properties occupy also a large amount of the research in this field. CNTs are predicted to have the highest thermal conductivity known (SWCNT: 6600 W/m.K [88], MWCNT: 3000 W/m.K [89]) and are expected to present a ballistic type of electrical conduction mechanism [90,91]. Both factors are of utmost importance when considering that the material to be improved is a metal (which usually shows very high thermal and electrical properties). However, it is critical to obtain individual CNTs after dispersion, since the transport properties could be reduced up to one order of magnitude If the CNTs are in agglomerated form [92]. In this regard, some parts of the research on Cu-CNT were aimed at the improvement of the thermal properties. Cho et al. observed an improvement of the thermal conductivity for very low CNT concentrations (up to 1 vol. %) [42]. For larger CNT concentrations, the agglomeration of CNTs starts to play a fundamental role in the conductivity decrease. The same effect was observed by Yamanaka et al. for Ni-CNT [46]. Inversely, Firkowska et al. tested several functionalization routes so as to improve the interface in Cu-CNT composites and thus the thermal transport [14]. They observed that for all cases, the degradation of the CNT’s intrinsic thermal properties was so relevant and that it was not able to increase the thermal conductivity of the composites in any case.

The electrical behaviour was also studied in several MMC-CNT systems, usually resulting in reduced conductivity. The major factor influencing this behaviour is the presence of agglomerates that, analysed from an electrical transport point of view, are seen as voids. This happens mainly due to the fact that the interfaces between CNT agglomerates and the metal matrices are weak. Only marginal improvements in conductivity were observed for low CNT concentrations in Ni/CNT composites [46].

The tribological properties of CNT-reinforced composites have also been reported in the literature. In most cases, a reduction in both the coefficient of friction (COF) and wear has been observed. In the case of Ni-CNT composites, the COF was reduced in margins from 40 [37] to 75 % [27]. Regarding the Cu-CNT system, the COF reductions ranged from 50 to 75 % [6,8,43,51,63]. For both systems, the reduction in wear losses achieved in certain cases up to 6 [51], 7 [27] and 8 [43] times that of the reference metal under the same experimental conditions.

The former is usually traced back to a solid lubricant activity of the CNTs during the experiments [37]. Due to their high mechanical properties, they tend to act as rolling second phases at the interface between the rubbing surfaces, thus avoiding the direct contact between the surfaces. In some cases, also the development of a graphitic layer is observed, acting as a solid lubricant [37]. The wear reduction is mainly correlated to the increase in the mechanical properties of the composite, which—as already stated—is generated by a stabilization of the microstructure due to grain boundary pinning. Moreover, Kim et al. stated that a reduction in the wear loss might be due to the reduction in the grain peeling mechanism due to a CNT anchoring of the matrix grains [51].

2. Aluminium/CNT system

2.1. Solid-state processing

Al/CNT composites are mainly interesting, because of their high potential being a lightweight, reinforced material, which can be used in manifold applications. Therefore, Al/CNT composites have been in the focus of research since 1998 [40] and the interest is still growing.

There is a large variety of starting materials on the market that have been used to fabricate the composites. Some of the most mentionable suppliers for Al powders are ECKA Granules Japan Co. Ltd (Japan), Aluminium Powder Company Ltd (United Kingdom), Alpha Industries (South Korea) and AlfaAesar (Germany) [18,19,21,58,61,62]. The range of used powders goes from several µm up to 75 µm in mean particle size, having different particle shapes and various purity grades between >99% and >99.99% [18,22,3941,50,5659,79]. The most used blending method for Al/CNT composites is ball milling [19,20,22,23,50,5862].

A detailed overview of the research papers in the Al/CNT composite manufacturing can be found in Table 1.

Reference Blending methodSintering methodCNT content
Value wt%
Relative density
Value %
Kuzumaki et al. (1998) [40]Stirring: Stirring in ethanol at
300 rpm for 0.5h, drying
in vacuum furnace.
HUP and hot extrusion: Packed in an
Al case and preheated for 1.5 h at 873
K in vacuum (0.53 Pa) and then
compressed with 100 MPa in steel dies
for 60 min. The heating and loading
rates were 29.1 K/min and 10 MPa/
min, respectively. Then, the
composites are extruded at 773 K
(extrusion ratio = 25 : 1) at a speed of
10 mm/min
5 vol%
10 vol%
94% 96.2%
Xu et al. (1999) [39]Hand grinding: For > 30 min.HUP: At 793 K under a pressure of 25
MPa for more than 30 min.
1 wt%
4 wt%
10 wt%
XXX
Tokunaga et al. (2008) [41]Colloidal mixing process:
Mixing by sonication for 5min
and then evaporation of the
solvent.
HPT: At room temperature with an
applied pressure of 2.5GPa. The
rotation is initiated 5s after the load
application and terminated after 30
turns.
5 wt%98.4%
Kwon et al. (2009)18NSD: Consisting of commercial
gas atomized Al powder, CNTs
and natural rubber. The precursor
is heat treated at 500 °C for 2 h
in an argon atmosphere (1 l/min)
to evaporate the natural rubber.
SPS and hot extrusion: Sintering at
600 °C, holding time of 20 min, heating
rate of 40 °C/min and pressure of 50
MPa. The sintered compact was
extruded in a 60° conical die at 400 °C
with a pressure of 500 kN. The
extrusion velocity and the extrusion
ratio were fixed at 2mm/min and 20,
respectively.
5 vol%Sintered:
96.1% Hot
extruded:
98%
Esawi et al. (2009) [58]Ball milling: At 200 rpm for 3h
and 6h using 75 stainless steel
milling balls (10mm diameter);
giving a ball-to-powder weight
ratio of 10:1. The jars are
filled with argon and 2ml
of methanol is added.
Hot extrusion: Compacted at 475MPa.
Hot extrusion of the compact is
conducted at 500°C using an extrusion
ratio of 4:1.
2 wt%XXX
Kwon et al. (2010) [79]NSD: Consisting of commercial
gas atomized Al powder, MWCNTs
and natural rubber. The precursor
was heat treated at 500 °C
for 2 h in an argon atmosphere
(1 l/min) to evaporate the
natural rubber.
SPS and hot extrusion: Sintered at
480, 500, 560, and 600°C with a heating
rate and holding time of 40 °C/min and
20 min, respectively. A pressure of
50 MPa is used. SPSed compacts
were extruded in a 60° conical
die at 400 °C with a 500
kN press. The extrusion velocity
and extrusion ratio were fixed at
2mm/min and 20, respectively.
1 vol%Up to
96.8%
Choi et al. (2011) [59]Ball milling: A2024
chips are ball-milled at 500
rpm under argon atmosphere
for up to 48 h. The ball-to-chip
weight ratio was 15:1. A control
agent of 1.0 wt% stearic acid
is added. The composite powder
is then produced by ball-milling
18-h A2024 powder and CNTs
Hot rolling: Heated to a temperature
of 450 °C and then hot rolled
with every 12% reduction per a
pass; the initial thickness was
20mm, and the final thickness
was 1 mm.
1 vol%
2 vol%
3 vol%
XXX
Choi et al. (2011) [60]Ball milling: Aluminium
powder was solely ball-milled
for 18h or 12h and then mixed
with CNTs by ball-milling for 6h.
Or Aluminium powder is directly
mixed with CNTs by ball milling
for 6h.
Hot rolling: Heated to a temperature
of 450°C, 480°C or 530°C
and then hot rolled with every 12%
reduction per a pass; the initial
thickness was 20mm and the final
thickness was 1 mm.
1.5 vol%
3 vol%
4.5
vol% 6
vol% 7.5
vol% 9
vol%
96.3%-
99.9%
depending
on parame
ters.
Kwon et al. (2011) [19]Ball milling: For 3h under an
argon atmosphere; 360 rpm, Ø
10mm ball, 10:1 ball to powder
weight ratio, and 20 wt% heptane
was used as the process control
agent.
HUP: Consolidating at 500 °C
for 5min under a uniaxial
pressure of 57 MPa.
5 vol%
10 vol%
15 vol%
100%
Jiang et al. (2012) [20]Ball milling: Flake powder is
obtained by ball milling of near-
spherical powder with an initial
ball-to-powder weight ratio
of 20:1 and 423 rpm for several
hours in flowing Ar atmosphere with
water cooling. The as-prepared Al
nanoflakes are first surface
modified by PVA with 1700–1800
repeat units and then mixed with a
CNT suspension, which is then
heated in flowing Ar atmosphere at
500 °C for 2 h to remove the
PVA. 1kg blend in 8 hours.
CPS and Hot extrusion: Compacted
under 500 MPa pressure, and then
sintered in flowing Ar atmosphere
at 550 °C for 2h.
Then heated to 440 °C with
a heating rate of 10 °C/min within
a vacuum furnace installed with the
extruder. And extruded with an
extrusion ratio of 20:1 at a
ram speed of 0.5 mm/min.
0.5 vol%
2 vol%
After
CPS:
80-85%
After
hot
extrusion: >
99.5%
Nam et al. (2012) [50]Molecular-level mixing and ball milling: Poly-vinyl alcohol aqueous
solution and CNTs are mixed by
tumbler ball milling for 48 h,
followed by drying in vacuum at
100 °C. The PVA-coated CNTs, Cu(CH3COO)•2 H2O and 2
M NaOH aqueous solution are
dispersed in water and
heated to 80 °C
to form a CNT/CuO composite.
This is followed by vacuum
filtering and reduction at 300 °C
under a hydrogen atmosphere.
CNT/Cu composite powders
and Al powders are then mixed
using a planetary mill for 3 h
with rotation speed of 200 rpm;
the ball to powder ratio
was 10:1. The composition of the
Al–Cu matrix is Al-4wt%.
SPS and cold rolling: Sintered
at 500 °C for 5 min in a vacuum
of 10-3 torr under a
pressure of 50 MPa . The CNT/Al–Cu
composites are solution heat treated
at 550 °C for 12 h and quenched
in a water bath at room temperature.
They were then cold rolled to
achieve 5% plastic deformation. The
composites are finally aged at
130 °C for 0 to 24 h.
2 vol%
4 vol%
XXX
Kwon et al. (2013) [21]NSD: The precursor was heat-
treated at 500°C for 2 h in an
argon atmosphere (1 l/min) in
order to evaporate the
natural rubber.
SPS: Sintering at 600 °C, holding
time 20 min, heating rate 40 °C/min,
and pressure 50 MPa. Then heat-
treated in an alumina pan at 670 °C
and at 800 °C for 1 h in an argon
atmosphere (1 l/min) inside a tube
furnace.
5 vol%96%
Liu et al. (2013) [56]Friction stir processing (FSP):
Plates of aluminium alloy are
used as the substrate materials.
Holes with a depth
of 3.5 mm and a diameter of
0 mm, 2 mm, 4 mm, 6 mm,
8 mm and 10 mm respectively
were drilled in the aluminium
plates, which are then filled
with various quantities
of CNTs.
Friction stir processing: A
tilt angle of 2° is
applied on the fixed pin
tool during FSP. The pitch
distance was 0.3 mm. The
forward velocity of the
rotating pin is kept as a
constant of about 30 mm/min,
and a rotational speed of
950 rpm is used. Five
passes on the same position
were conducted on each plate.
1.6 vol%
2.5 vol%
4.4 vol%
5.3 vol%
6 vol%
XXX
Asgharzadeh et al. (2014) [61]Ball millingHPT: Pre-compacted under
pressure of ~1 GPa. Then,
the green compacts are compressed
and deformed under the
applied pressure of 6 GPa
at room temperature. The
rotation of the lower anvil
was started after 30 seconds
of load application
with a rotational speed of 1
rpm and was terminated after
various numbers of revolutions
up to 15.
3 vol%98.5% -
99.5%
Phuong et al. (2014) [22]Colloidal mixing and Ball milling: CNTs are functionalized
and dispersed in ethanol by
sonication. Then, Al powders
and CNT suspension were
mixed by mechanical stirring and
simultaneously heated in order
to evaporate most of the
ethanol. The slurry is then
processed in a high energy
ball mill for 2 h with 250 rpm.
Drying is accomplished
at 60 °C using a vacuum oven.
HIP and HPT: After cold
compaction, capsule free method
HIP is accomplished under
pressure of 103 MPa, at 600 °C
for 30 min. Straining was
carried out at room temperature,
by torsion to 5 revolutions, under
pressure of about 5 GPa and
at rotation speed of 2 rpm. The
specimens are then annealed in
vacuum for 3 h at temperatures
of 100 °C, 150 °C, 200 °C and 250 °C.
0.5 vol%
1 vol%
1.5 vol%
2 vol%
XXX
Isaza Merino et al. (2016) [57]Layer stacking:
CNTs are added to a solution
composed of 4 wt.%, PVA
dissolved in distilled water.
This mixture is
magnetically stirred at 1000
rpm during 1 h, followed by
a sonication at energies
between 40 and 60 kJ and
a power of 100W. Finally,
this mixture is cured at
room temperature in a Petri
dish for 8 days. The composite
sheets are cut into
small sections and stretched
at a rate of 2mm/min in a
tensile machine at 80 °C.
Then, two composite sheets
of polymer/CNTs are
alternately stacked with
three aluminium sheets.
HIP: In argon atmosphere .
The pressure and temperature are
raised gradually during 1.5 h
to 40MPa and 650 °C for 30
min. This allows for the
evaporation of PVA and aluminium
diffusion between the sheets
to finally produce the
consolidation of the composite.
0.5 wt%
2 wt%
XXX
Carvalho et al. (2016) [62]Ball milling: Al-Si
88-12 wt% and CNTs are
mixed using 12 steel milling
balls with 10-mm diameter,
presenting a ball-to-powder
weight ratio of 10:1. The
jar was placed in a
rotation device, and the mixing
was made with a constant
rotation speed of 40 rpm,
during 6 days (low-energy
ball milling).
HUP: Uniaxial load pressure
of 35MPa and a temperature of
550°C during 10min.
2 wt%
4 wt% 6
wt%
XXX
Chen et al. (2016) [23]Ball milling: Al
powder is milled with 2 wt.%
stearic acid. The powder
is sealed in a ZrO2 jar
together with 600 g ZrO2 media
balls (10mm in diameter) in an
argon gas atmosphere. The
rotation speed is 200 for 240
min. Then, Al-flake powder
is mechanically bathed in isopropyl
alcohol-based solution with ~1
wt.% zwitterionic surfactants and
1 wt.% CNTs in a plastic bottle
on a rocking ball milling machine
for 120 min. Thirty-gram ZrO2
media balls were added to
assist the coating of CNTs
on flaky powders.
SPS and hot extrusion:
Sintering at 800 K, 850 K and
900 K. The heating rate
is 20 K/min and the
holding time at target
temperatures is 60min. A
pressure of 30MPa is applied
on the sample under
a vacuum of ~5 Pa. Before
hot extrusion, the sintered
sample is preheated to 700
K and kept for 180 s under
an argon gas atmosphere.
Then, the billet is
immediately put into a
steel container) and extruded
through a die. The extrusion
ratio and the ram
speed are 37:1 and 3mm/s,
respectively.
1 wt%>99%

Table 1.

Summary of blending and sintering methods for the production of Al/CNT composites.

2.2. Distribution and interaction with the matrix material

In all the analysed articles, CNT agglomeration is observed, being the size and amount of those agglomerates strongly related to the chosen dispersion method. Specifically, in those reports where ball milling is used, a significant clustering is noticed. This might be due to the use of non-functionalized CNTs that tend to easily re-agglomerate during processing. Yet, the utilization of functionalized CNTs in ball milling mixed blends would not bring any further improvement, since the structural quality of the CNTs would be significantly lower. However, despite the presence of unavoidable agglomerates, very fine cluster dispersion and a homogeneous distribution can be observed. Interestingly, one would assume that the formation of clusters would be detrimental to the proper densification of the composites. However, as already depicted in Table 1, the final densities of the composites were never below 94%. The formation of clusters is one of the most challenging problems to overcome when dealing with CNTs, since a trade-off between an optimal initial agglomerate disentanglement and the lowest possible damage to the CNT structural state must be found. There are two ways to observe the amount of agglomerates in the composites. Certain reports focus on the evaluation by electron microscopy on the polished surface of the samples, whereas other authors evaluate fracture surfaces of the composites. Both approaches are valid, in the sense that they show a good overview of the agglomeration. However, SEM imaging of the surface would allow a further quantification of the agglomerates size and distribution by segmenting the C-containing phases. Bakshi proposed an interesting methodology to quantify the dispersion, based on distance calculation algorithms from electron micrographs, which can be found in Bakshi et al. [93].

If we focus our attention on the CNT degradation during blending and after densification, we observe dissimilar results obtained by Raman spectroscopy, even for the same manufacturing method. For example, in samples densified by SPS (a technique known to exert strong thermomechanical stress on the CNTs), negligible variation of the ID/IG ratio is reported [23,79] as opposed to an increase in the defect density in other reports [18]. It is therefore very difficult to discern whether the sintering process has indeed any sort of influence on the CNT degradation. It has been already reported that given the strong stresses from SPS, CNTs are usually either damaged or degraded within the matrix material [94]. When strong shear forces are applied as in the case of HPT, sharp increases in the D band intensity are measured, depicting a modification of the graphitic structure. In all cases, an increased amount of structural damage would favour the reactivity of the CNT shells.

The CNT degradation is a major issue when working with metals which are strong carbide builders [95]. It is well known that aluminium tends to form a stable carbide (Al4C3) in a wide compositional range [96], being of brittle nature and having the major drawback of being water soluble [97]. Some authors mention that the formation of this carbide would not be as detrimental as expected [18,58], since it only sacrifices the outermost CNT layer, providing an optimal interface with the matrix. However, despite being able to efficiently transfer the applied load, the utilization of the composite in structural applications in humid environments would lead to an interfacial degradation. Another way to obtain this carbide is by transitioning from Al2O3 to Al4C3 [98]. Yet, this phase transition occurs only under certain circumstances that are rarely achieved in solid-state processing.

3. Copper/CNT system

3.1. Solid-state processing

Reports on the solid-state processing of Cu/CNT systems deal with a variety of different raw materials, blending methods and sintering techniques, therefore sticking out in the research field of CNT-reinforced metal matrix composites.

When it comes to the used Cu starting material, a great many of commercially available powders or chemicals are employed, making it very hard to compare the Cu/CNT systems of different publications. The materials used depend directly on the blending and production process that has been used. The range goes from nanosized to almost 100 µm sized Cu powder particles, having different particle shapes (spherical, dendritic) and various purity grades between >99.5% and >99.95% [612,14,15,3235,4244,50,5255,6371]. The most used blending methods are ball milling, molecular-level mixing or colloidal mixing, each having their advantages and disadvantages as discussed in the introduction [612,1417,3235,4245,5155,6375]. But especially for Cu, molecular-level mixing is employed very often, using copper compounds in solvents that have to be chemically treated in order to become pure copper (e.g. copper(II) acetate monohydrate, Cu(II) sulphate pentahydrate or simply CuO). Some prominent suppliers for the Cu starting materials are Sigma-Aldrich (USA), Alfa Aesar GmbH & Co. KG (Germany), Chang Sung Co. (Korea), Junsei Chemical Co. Ltd (Japan), Kojundo Chemical Lab. Co. Ltd (Japan), TLS Technik GmbH (Germany) or New Materials Research Co. Ltd (China) [6,12,1416,32,33,42,54].

Therefore, to review the progress in Cu/CNT composites, a detailed comparison of blending methods, sintering techniques and achieved final relative composite densities has to be conducted, which can be found in Table 2.

ReferenceBlending methodSintering methodCNT contentRelative density
Tu et al. (2001) [63] Ball milling: CNTs
are milled for 8 h in
an organic liquid. Then,
CNTs are sensitized, activated
and finally coated by
electroless nickel. Copper-
and nickel-coated CNTs
are mixed for 30 min
in a ball mill.
CPS: Isostatically pressed
at a pressure of 600 MPa at
100 °C for 10 min under
vacuum. Then sintered at 800 °C
for 2 h.
4 vol%
8 vol%
12 vol%
16 vol%
97.5%
97.5%
97% 95%
Chen et al. (2003) [35] Ball milling: CNTs are
sensitized, activated and
then coated by electroless
nickel for 15 min. Then,
copper powder and nickel-
coated CNTs are mixed by
ball milling for 30 min.
CPS: Isostatically pressed
at a pressure of 600 MPa at
100 °C for 10 min
under vacuum. Then
sintered at 800 °C
for 2 h.
4 vol%
8 vol%
12 vol%
16 vol%
97.5%
97.5%
97% 95%
Kim et al. (2006) [64] Ball milling: Cu
powder and CNTs are
mixed through high energy
ball milling process for
24h with 150rpm.
SPS and cold rolling:
Pre-compacted under a
pressure of 10 MPa and
then sintered at 700 °C
for 1 min in vacuum (0.13 Pa)
under a pressure of 50
MPa with a heating rate
of 100 °C/min. Finally,
the samples are cold rolled
up to 50% reduction, followed
by full annealing at 650 °C
for 3 h.
5 vol%
10 vol%
99.3%
99.1%
Kim et al. (2007) [51] Molecular-level mixing process: CNTs are
purified and functionalized
by acid treatment. Then,
Cu acetate monohydrate
is added and sonicated
for 2h. The solution is
vaporized with magnetic
stirring at 100 °C, followed
by calcination at 350 °C
in air. and reduction
under N2 atmosphere.
SPS: Sintered at 550°C
for 1 min in vacuum( 0.1 Pa)
with a pressure of 50MPa
and a heating rate of 100
K/min.
5 vol%
10 vol%
99% 99%
Kim et al. (2008) [9] Molecular-level mixing process:
CNTs were purified under
sonication for 10 hours and
then functionalized. Then,
20 mg of CNTs are sonicated
in 300ml ethanol for 2 hours.
Cu(CH3COO)•2 H2O is added and
sonicated again for 2 hours.
After drying at 250 °C
in air and calcinating at 300 °C
in air, the powder is
reduced under a hydrogen
atmosphere.
SPS: Pre-compacted
under a pressure of 10 MPa
and then sintered at 550°C
for 1 minute in vacuum
(0.13 Pa) with a pressure
of 50 MPa. The heating
rate is 100°C/ min.
5 vol%
10 vol%
98 ±
2% 98 ±
1.5%
Kim et al. (2008) [11] Molecular-level mixing process: CNTs are
cleaned and functionalized.
Then, 10.5 mg of CNTs are
dispersed in 100 ml of
oleylamine in an ultrasonic
bath for 3 h. 3 g of
copper acetate monohydrate
is added and the solution
is purged with argon
for 1 h. Then, the
mixture is heated to 523 K
for 10 min with a heating
rate of 10 K/min. After
cooling, the powders
are reduced at 573 K
for 2 h under hydrogen
atmosphere.
SPS: Sintering at 823 K
for 1 min in vacuum (0.13 Pa)
with a pressure of 50 MPa
and a heating rate of 100
K/min.
5 vol%XXX
Daoush (2008) [12] Molecular-level mixing process:
Copper (II) sulphate pentahydrate,
tri-sodium citrate monohydrate
and CNTs are stirred
with 500 rpm magnetic stirrer
for 2 h at room temperature
to suspend the CNTs in the
solution. The equivalent
formaldehyde amount is added
with a rate of 0.1 ml/min within 30
min. The solution is filtered,
washed and vacuum dried at
100°C for 2 h and finally
reduced at 400°C for 30
min under hydrogen atmosphere.
SPS: Sintered at 600°C
for 1 min with a pressure of
20 MPa under vacuum
(0.13 Pa).
10 vol%
20 vol%
30 vol%
40 vol%
97.9%
97% 96.
7% 96%
Chai et al. (2008) [15] Particle Composite System mixing: CNTs are cleaned
in nitric acid in the
ultrasonic bath. Mixing condition
is 5000 rpm in rotary speed
for 40 min.
SPS: Sintered at 600 °C
for 5 min. The heating rate
is 100 °C/min and a
pressure of 50 MPa is applied.
5 vol%
10 vol%
15 vol%
98.4%
98.6%
96.1%
Li et al. (2009) [65] Ball milling: In an
argon protected environment
at room temperature. The
mass ratio of ball to
powder is 10:1, and the
milling time is 5 h.
HPT: Pre-compacted with
500 MPa in air at room temperature.
Then consolidated under 6 GPa for
5 revolutions at room
temperature.
1 wt%XXX
Li et al. (2009) [66] Ball milling: In an
argon protected environment
for 5 h
HPT: Pre-compacted
and then consolidated under
6 GPa for 5 revolutions at
room temperature.
1 wt%XXX
Chu et al. (2010) [55] Particle Composite System mixing: CNTs are
purified in nitric acid in
an ultrasonic bath, then
filtered,washed and dried
at 120°C. The mixing
is performed with 5,000
rpm for 40 min.
SPS: Sintered at 550°C-650°C
in vacuum (<5 Pa) with a
pressure of 40-60 MPa for
5-10 min. The heating
rate is 100°C/ min.
5 vol%
10 vol%
96.2%
- 99.2%
95.3%
- 98.9%
Cho et al. (2010) [42] Colloidal mixing process:
CNTs are sonicated in acid
solution at 323 K for 24h. Then,
Cu powder and CNTs were
sonicated separately in
ethanol solution for 1 h,
blended, stirred for 30 min
and oven-dried at 323 K.
The powders are finally
heat-treated at 623 K
for 1 h in Ar–5% H2 atmosphere.
SPS: Sintered at 823 K
for 1 min under a uniaxial
pressure of 50 MPa. The
heating rate is 50 K/min.
0.5 vol%
1 vol%
1.5 vol%
2 vol%
3 vol%
5 Vol%
10 vol%
96.8
- 99.0%
Uddin et al. (2010) [67] Ball milling: Milled in Ar
atmosphere and for different
hours with 50 stainless steel
balls (diameter of each
ball: 10 mm, ball to powder ratio
10:1, milling speed: 200 rpm). For
bronze composites, the chemical
composition is Cu 79%, Sn 10%, Zn
3% and Ni 8%.
HUP: Sintering at 750 °C
with pressure of 40 MPa for
Cu–CNT composites in Ar-
atmosphere, whereas 800 °C
and 40 MPa for Bronze–CNT composites.
Cu 0.1
wt% Cu
0.5 wt%
Cu 1
wt% Cu
2 wt%
Cu 4
wt%
Bronze
0.1 wt%
Bronze
0.5 wt%
Bronze
1 wt%
Bronze
2 wt%
Bronze
4 wt%
96.8%
96.8%
93.7%
91%
82.6%
95.9%
93.8%
88.9%
89.9%
85.6%
Xu et al. (2011) [8] Ball milling: Milled
for 5 h in an organic liquid.
CPS: Cold pressed at 200
MPa, then sintered at 850 °C
in vacuum atmosphere for 5 h.
After cooling, a second
pressing at 600 MPa and a
second sintering are performed.
10 vol%96%
Jenei et al. (2011) [33] Ball millingHPT: Pre-compacted by
cold pressing. Then consoli
dated at RT and 373 K with a
pressure of 2.5 GPa and 10
revolutions.
3 vol%97%
Kim et al. (2011) [52] Molecular-level mixing process: CNTs are cleaned
and functionalized. Then, the
CNTs are dispersed within
ethanol by sonication. Cu
(CH3- COO)• 2H2O is added to
the CNT suspension and sonicated
for 2 h. After vaporization
at 333–373 K and calcination at 573 K
in air, the powders are reduced
at 523 K for 4 h under hydrogen
atmosphere.
SPS: Sintered at 823 K f
CPS: Pre-compacted using a
hydraulic press 130 kN. Then s
intered in a tube furnace at 600 °C in vacuum (10.6 Pa) or 1
min in vacuum (0.13 Pa) with a
pressure of 50 MPa.
5 vol%
10 vol%
XXX
Rajkumar et al. (2011) [43] Colloidal mixing process:
CNTs are purified and oxidized
using nitric acid treatment by
sonication for 10 min at 60°C
Then, the CNTs are sensitized
and activated. The activated
CNTs are introduced into the
electroless copper bath and
stirred for 30 min while using
sonication. The coated CNTs are
dispersed in ethanol with
vigorous sonication for 10min
after which the copper
powder is added. The solution
is stirred and evaporated at 120°C.
The powder is finally mixed in
an electric agate
pestle mortar for 2 h.
Microwave Sintering: Pre-
compacted in a hydraulic press.
Sintering at 800°C with
the soaking time of 5 min
with a ramp rate of 12 °C/min.
5 vol%
10 vol%
15 vol%
20 vol%
95.5%
96%
96.5%
94%
Guiderdoni et al. (2011) [44] Colloidal mixing process:
CNTs are dispersed in deionized
water using a sonotrode for a
few seconds. The Cu powder
is added. After further
sonication of 1 min, the
dispersion is immersed in liquid
N2 for 2 min and freeze-
dried at- 40 °C for 48 h in vacuum
(12 Pa).
SPS: Sintered in vacuum
( <10 Pa) at 700°C with a
heating rate of 100 °C/min for 6 min.
The pressure of 100 MPa is
gradually applied within the
first minute of the dwell and
maintained during the remaining
5 min. Samples are naturally
cooled.
0.5 vol%
1 vol% 2
vol% 3
vol% 4
vol% 5
vol%
10 vol%
16 vol%
95% 96%
98% 96%
93% 93%
88% 78%
Pham et al. (2011) [34] Ball milling: The CNTs are
treated in acid at 60 °C for
4 h. Then, the CNTs are dispersed
in acetone. The Cu powder
and the CNTs dispersed in
acetone are mixed through a
high-energy ball milling
process for 6 h at 300 rpm.
CPS: Sintering at
temperatures of 850, 900 and 950° C
for 2 h in argon atmosphere.
0.5 wt%
1 wt%
1.5 wt%
2 wt%
2.5 wt%
3 wt%
3.5 wt%
XXX
Firkowska et al. (2011) [14] Molecular-level mixing process:
CNTs are functionalized by polymer
wrapping and dispersed in 1 wt%
Poly(sodium 4-styrenesulfonate)
by sonication for 2 h and
stirring overnight. Then,
CNTs are oxidized, washed and
suspended in water by ultrasonic
treatment. Copper acetate
is added. After 12 h, the
suspension is heated up to 70 °C
to evaporate the solvent. After
calcination, the mixture is
finally reduced under H2 at
350 °C for 2 h
CPS: Pre-compacted using
a hydraulic press 130 kN. Then
sintered in a tube furnace at
600 °C in vacuum (10.6 Pa)
for 60 minutes. SPS: Pre-
compacted under a pressure
of 6 kN. Then sintered at 600 °C for
5 min in a vacuum a
pressure of 50 MPa and a
heating rate of 100 °C/min.
0.2 wt%
1 wt%
3 wt%
10 wt%
XXX
Xue et al. (2012) [7] Molecular-level mixing process:
Dispersion of CNT in ethanol
(1 g/l) by sonication for 30 min.
Cu(CH3COO)•2H2O (2g/ml) is
dissolved in NH3•H2O (40%),
mixed and sonicated for 30
min with the CNT dispersion. After
solvent vaporization and
calcination, the powder is
reduced at 250 °C for 2 h under
hydrogen atmosphere.
SPS and hot rolling:
At 550 °C (vacuum 0.1 Pa)
under a pressure of 50 MPa
for 5 min with a heating
rate of 100 °C/min. Then
hot rolled up to 50% reduction
at 650 °C.
5 vol%XXX
Shukla et al. (2012) [10] Ball milling: Using stainless
steel balls of Ø 3/8 inch
(ball to powder ratio 5:1) under
argon with 200 rpm
for 20h.
HUP: Sintering at 700 °C
with 30 MPa pressure under
vacuum (0.001 Pa) for 30
minutes.
5 vol%93.4%
Chu et al. (2013) [68] Ball milling: Mixed
with Cu–Ti powders
with 1200 rpm for 2h. Ball-to-
powder weight ratio is 10:1.
Alcohol is added.
HUP: At 760 °C for 20 min
under an uniaxial pressure
of 40 MPa
5 vol%
10 vol%
15 vol%
99% 98%
96%
Guiderdoni et al. (2013) [6] Colloidal mixing process:
CNTs are dispersed in deionized
water using a sonotrode (a few s
econds). Cu powder is then
added and sonicated for
one minute. The dispersion
is immersed in liquid N2
for 2 min and freeze-dried at -40 °C
for 48 h in vacuum (12 Pa).
SPS: At 700°C (vacuum <10
Pa) under a pressure of 100
MPa for 6 min with a heating
rate of 100 °C/min.
Then natural cooling.
5 vol%
8,4 vol%
17,1 vol%
33,2 vol%
96% 85%
82% 73%
Jenei et al. (2013) [32] Ball millingHPT: Pre-compacted by cold
pressing and consolidated at RT
and 373 K with 2.5 GPa and 10
revolutions.
3 vol%97%
Lal et al. (2013) [53] Molecular-level mixing process and Ball milling: CNTs are
functionalized in acidic solution.
After washing and drying,
they are dispersed in Sodium
dodecyl sulphate (SDS). And
coated with copper. Copper
powder is ball milled for
10 h at 500 rpm using
stainless steel balls (5 mm).
Ball to powder ratio is 10:1
and ethanol is taken as a
process control reagent. Copper-
coated CNTs are milled together
with previously milled Cu
powder for 5 h at 250 rpm.
CPS: Pre-compacting with 500
MPa pressure. Then sintering at
900°C for 4 h under vacuum
(1.33 Pa).
0.5 wt%
1 wt%
1.5 wt%
XXX
Tsai et al. (2013) [54] Molecular-level mixing process:
CNTs are functionalized and
treated with 250 ml of oleylamine at
room temperature for 2.5 h. 3 g of
copper (II) acetate monohydrate
are added and the mixture
is sealed and purged in an
argon-protected environment
for 4 h. After heating to 523 K
with 10K/min for 15 min, the
powder is cooled, calcinated
at 573 K in air for 2 h and then
reduced in a N2 atmosphere.
HPT: Rotation speed of 1
rpm with a load of 2.5 GPa
at room temperature for 5
revolutions. The rotation is
initiated 5 s after the
load application.
5 vol%XXX
Shukla et al. (2013) [69] Ball milling: Using stainless
steel balls of 10 mm diameter
(ball to powder ratio 5:1) under
argon atmosphere at 200 rpm
for 20 h.
HUP: Sintering at 700°C for 30
min using 30 MPa uniaxial stress
under vacuum (0.001 Pa). The
compacts produced underwent
delamination in the centre
on cooling and were repressed
at 725°C under same conditions.
0.2 vol%
SWCNT 5
vol% SWCNT
10 vol% S
WCNT 5
vol% MWCNT
10 vol%
MWCNT
95% 99%
97% 96%
98%
Chu et al. (2013) [70] Ball milling: Mixed with
Cu–0.76wt.% Cr alloying powder
under argon atmosphere with
a rotary speed of 1200 rpm
for 120 min. A ball-to-powder
weight ratio of 10:1 is used,
and alcohol is added.
HUP: Pre-compacted to a
green density of 75% and then
consolidated at 750 °C for 15
min with a heating rate
of 50 °C/min and a
pressure of 40 MPa .
5 vol%
10 vol%
15 vol%
XXX
Yoon et al. (2013) [71] Ball milling: Using stainless
steel balls for 1 h (ball-to-
powder weight ratio was 15:1)
and 15 rpm.
HPT: Under room temperature
and 6 GPa using 10 revolutions
at a constant speed of 1 rpm.
5 vol%
10 vol%
99% 99%
Sule et al. (2014) [45] Colloidal mixing process:
Copper powder and CNTs are
dispersed separately in 60 ml
ethanol using sonication.
CNTs were sonicated for 1 h,
stirred for 5 min, sonicated
again for 30 min and stirred
again for 3 min. Copper powder
is sonicated in ethanol for 1 h
and stirred for 2 min. The
slurries are mixed together,
sonicated for 1 h and stirred
for 3 min. After drying
using a rotary evaporator,
the powder is annealed for
30 min at 550 °C with a
heating rate of 5 °C/min under
inert Ar atmosphere to
reduce the oxygen content.
SPS: Sintered at 650°C under
a vacuum of 1 hPa. A constant
pulse-to-pause ratio of 10:5
(10 ms on and 5 ms off) is
applied under a pressure of
50 MPa.
1 vol%97%
Barzegar Vishlaghi
 (2014) [72] Ball milling: CNTs are
sonicated in acetone for 20
and cleaned in nitric acid
for 1 h. Cu and Fe powder is
milled for 15h. Then, CNTs
are added and milled a
gain for 15h using chromium
steel balls under argon
atmosphere with 300 rpm and
a ball to powder weight ratio
of 20:1.
XXXXXXXXX
Imai et al. (2014) [75] Dipping process: Dipping
Cu­0.5 Ti powder into a
zwitterionic surfactant water
solution (3-(N,N-dimethyl
stearylammonio) propanesulfonate)
with CNTs. The powders are
then dried at 353 K for 7200
s in air. To remove surfactant
films, powders are heated to 873K
in H2 and Ar atmosphere.
SPS: Sintered at 1223 K for
1800 s under 30 MPa in
vacuum (6 Pa). The compacts
are preheated at 1073K
for 800s with a heating rate of
1K/s in Ar atmosphere.
After preheating, immediately
extruded by using hydraulic
press under an extrusion
ratio of 12.8.
0.19 wt%
0.34 wt%
XXX
Akbarpour et al. (2015) [73] Ball milling: In Ar
atmosphere for 3 h at 200 rpm
using 10 mm balls, with a 10:1
ball to powder weight ratio
and 0.5 wt% of stearic acid.
HPT: Pre-consolidated
at a load of 150 MPa and a
temperature of 973 K with
a holding time of 30 min.
Then, HPT process at room
temperature with 6 GPa and
various numbers of revolutions.
4 vol%99.6%
Chen et al. (2015) [17] Ball milling: The CNT
and NbSe2 are coated with copper
by electroless plating. CNTs and
NbSe2 are mixed with Cu
powder mechanically.
CPS: Powders are cold
pressed with 200 MPa
and sintered at 750 °C for 2 h
in N2 shielding. The sintered
compact is crushed, repressed
at 400 MPa and sintered again
at 750 °C for 2 h in N2
shielding.
1-4 vol% CNT with various
amounts of copper-coated NbSe2
XXX
Sule et al. (2015) [16] Colloidal mixing process: CNTs
are sonicated in ethanol for 1 h
and stirred for 5 min using a
magnetic stirrer. Then, Ru
powder is added, combined with
further sonication for 1 h
and stirring for 3 min. Copper
powder was ultrasonicated in
ethanol for 1 h and stirred for
2 min. The slurries are mixed
together and are sonicated for 1 h
and stirred for 3 min. After
drying, the powders are further
blended in a Turbula T2F mixer
for 1h at a speed of 101 revolutions
per minute and finally annealed
for 30 min at 550 °C with a heating
rate of 5 °C/min under inert
Ar atmosphere to reduce the
oxygen content.
SPS: Sintering at 600 or 650°C
with a heating rate of 80°C/min,
a holding time of 5 min and a
pressure of 50 MPa.
1 vol%
CNT
(600°C)
2 vol%
CNT
(600°C)
0.5 vol%
CNT
0.5 vol%
Ru
(600°C)
l vol%
CNT
0.5 vol%
Ru
(600°C)
2 vol%
CNT
0.5 vol%
Ru
(600°C)
l vol%
CNT
0.5 vol%
Ru
(650°C)
2 vol%
CNT
0.5 vol%
Ru
(650°C)
98.28%
98.15%
97.72%
96.77%
96.15%
97.63%
97.08%
Varo et al. (2015) [74] Ball milling: Flake
Cu–CNT nanocomposite
powders are synthesized
by 5h of ball milling, using a
rotation speed of 300rpm; the
ball-to-powder ratio is 5:1.
CPS: Pre-compacted with 200
MPa of uniaxial pressure at
room temperature. Then, the
green pellet is warm-pressed
at 500 MPa for 1 h at 500°C
and finally
sintered in a tube furnace
at 950°C for 2 h under argon
atmosphere.
0.5 wt%
1 wt%
0.15 wt%
0.2 wt%
0.25 wt%
3 wt%
5 wt%
Decreasing density with increasing
CNT content.

Table 2.

Summary of blending and sintering methods for the production of Cu/CNT composites.

3.2. Distribution and interaction with the matrix material

The only stable copper carbide known is the copper acetylide Cu2C2 [99]. It is usually observed as a transition phase during the purification of Cu and after the reaction of cuprous oxide with water [99]. In the studied Cu-CNT composites, no carbide was detected whatsoever. However, in certain cases, CuO and Cu2O were observed as a result of the selected manufacturing process. This issue is overcome by utilizing reagents (such as EDTA) or a reducing atmosphere as a post-processing method.

Regarding interfacial features, there is a publication that reported on the influence of adsorbed oxygen on the CNT surface on the interfacial strength, observing an improved interface but a reduction in the transport properties [14]. Theoretical simulations have demonstrated that the addition of oxygen-containing functional groups results in improved interfaces. The assertion is based on the fact that chemically active oxygen on CNT surfaces might improve the binding of metals with CNTs by enhancing the electron exchange between the metal and the carbon atoms or by directly interacting with the metal [100].

In some cases, the presence of alloying elements in the matrix degraded the CNTs into carbides as for example: the presence of Al leads to the formation of Al4C3 [50], and the presence of Cr leads to the formation of Cr3C2 [70]. Another approach was considered by mixing the CNTs with a Cu-Ti alloy. Chu et al. showed that the CNTs reacted in the presence of Ti, resulting in a degradation of the CNTs into a TiC interphase, despite having only 0.85 wt% Ti in the mixture [68]. This reaction between the CNTs and the carbide forming alloying elements is usually referred to as an interfacial improvement, leaving aside the fact that the CNTs are likely degraded into a hardly improving second phase in the composites.

On the other side, there are reports in which a seamless interface was achieved even avoiding any phase formation, as shown by Cho et al. [42]. This coherent interface would then result in the reduction of detrimental features such as thermal resistance.

In certain cases, XRD is used to observe the possible interphase formation; however, it would not be sufficient to resolve it since the volume fraction of the formed phases would be low. HR-TEM is the most suitable way to characterize these interphases with the aid of SAED.

A very interesting way to overcome the reaction between the CNTs and the alloying elements was addressed by decorating the CNTs with Cu nanoparticles [14,43]. In this case, a good distribution was achieved, rendering improvements in hardness and thermal resistance, despite reducing the CNTs intrinsic properties.

The analysis of the agglomeration and distribution of CNTs was, in all cases, qualitatively reported. As a general case, the CNT distribution was acceptable, with low reagglomeration activity. Both, the reaction with alloying elements as well as the covalent functionalization of the CNTs (as in MLM) tends to reduce significantly the intrinsic properties of the nanotubes, thus hindering an optimal exploitation of the CNT usage.

Regarding the adhesion of the reinforcement to the matrix, the best outcomes are observed when covalent functionalization is used (MLM) or an interphase is formed. In the case of non-functionalized CNTs, the adhesion to the matrix is poor, mainly due to a poor metal-CNT wettability [95].

4. Nickel/CNT system

4.1. Solid-state processing

For Ni/CNT, as for the other composite materials, there is a large variety of starting materials that have been used to fabricate the composites. Suppliers that are found very frequent for Ni powders are Alfa Aesar (Germany) and Crucible research (USA) [2631,3638,47,76,78,80]. The range of used powders reaches from 120 nm up to 149 µm in particle size, having a dendritic or spherical morphology and showing a purity of 99.8% to 99.99% [2431,3638,4649,76,78,80]. The most used blending method for Ni/CNT composites is colloidal mixing and ball milling [24,2631,36,38,4649].

A detailed overview of the research papers in the Ni/CNT composite manufacturing can be found in Table 3.

Reference Blending methodSintering methodCNT contentRelative density
Yamanaka et al. (2007) [46]Colloidal mixing process:
Stirred with an ultrasonic
homogenizer and a scroller
is used as a means to
obtain a homogenous mixture.
The slurry is dried for
24 h using a porous Al2O3
board.
SPS: Heating and cooling
rate of 50K/min with a holding
time of 1min. Sintered within
the range of 673–1073 K.
The sintering pressure is 50
MPa in vacuum (<5 Pa).
1 vol%
(673 K)
1 vol%
(773 K)
1 vol%
(873 K)
1 vol%
(973 K)
1 vol%
(1073 K)
2 vol%
(1073 K)
3 vol%
(1073 K)
4 vol%
(1073 K)
5 vol%
(1073 K)
10 vol%
(1073 K)
80% 94%
97% 97%
99% 99.5%
99.5%
99.9%
98.3%
97.5%
Hwang et al. (2008) [76]Roller mixing: Mixed
in a twin-roller mixer
consisting of two rolls
rotating in opposite
directions (one clockwise
and one anticlockwise).
This mixing is carried
out for 24 h.
LENS10 vol%XXX
Scharf et al. (2009) [37]Roller mixing: Mixed
in a twin-roller mixer
consisting of two rolls
rotating in opposite directions
(one clockwise and one
anticlockwise). This mixing
is carried out for 24 h.
LENS: Pulsed Nd:Yttrium
aluminium garnet laser (power
rating of 500 W, wavelength
of 1.064 nm) is focused on
the substrate to create a
melt pool into which the powder
feedstock is delivered through
Ar gas (mass flow rate 3.5 l/min).
Laser power of 400 W, 35 A current,
hatch width of 6.35 mm, layer
thickness of 0.01 in.
10 vol%XXX
Hwang et al. (2009) [36]Ball milling: For 48
hours using two different
sizes of tungsten carbide
balls of diameter 12.7
and 6.35 mm.
LENS: Pulsed Nd:YAG laser
(power rating of 500 W, wavelength
of 1.064 nm) is focused on the
substrate to create a melt
pool into which the powder
feedstock is delivered through
Ar gas. Laser power of 300 W,
30 A current.
5 wt%XXX
Singh et al. (2010) [38]Ball milling: For 48 hours
using two different sizes of
tungsten carbide balls of
diameter 12.7 and 6.35 mm.
LENS: Pulsed Nd:YAG laser
(power rating of 500 W, wavelength
of 1.064 nm) is focused on the
substrate to create a melt
pool into which the powder
feedstock is delivered
through Ar gas.
15 vol%XXX
Suárez et al. (2012) [47]Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound
bath with N,N-dimethylfor
mamide (DMF) and the
subsequent addition of Ni
powder. The CNT concentration
is 0.023 mg/ml. After 10min
in the ultrasound bath, the
resulting solution was
stirred to homogenize the
mixture and then dried
in a ventilated furnace.
The powder is finally
milled in an agate
mortar.
CPS and HUP: Uniaxially
pressed with a pressure of 990 MPa.
Then sintered in a vacuum (2 Pa)
tube furnace at 950°C
for 2.5 h. Or sintered in
a hot uniaxial press in vacuum
(0.2 Pa) at 750°C for 2.5h with
an axial pressure of 264MPa.
1 wt%93.7%
(CPS)
95.5%
(HUP)
Hwang et al. (2013) [25]Molecular-level mixing process:
CNTs were purified and
functionalized by using HF, acid
solution mixture of H2
SO4/HNO3. After
cleaning and drying, they are
dispersed in ethylene glycol
solution by ultra-sonication
and Ni(C2H3O2)2•4H2O
is added. Then 2 M-NaOH and
hydrazine are injected into
the mixed solution, heated to 70 °C
and kept 30 min. Finally, the
reduction process of fabricated
CNT/Ni composite powders is
performed in H2 + CO mixed
atmosphere for 2 h at 400°C.
SPS: Pre-compacted under
a pressure of 10 MPa. Sintered at
700 °C for 1 min under vacuum
(0.13 Pa) employing a pressure
of 50 MPa. The heating rate is
maintained at 100 °C/ min.
6 vol%98-99%
Suárez et al. (2013) [78]XXXCPS: Manufactured by cold
pressing at 990 MPa and sintering
at 950 °C for 2.5 h in vacuum. .
1 wt%
3 wt%
5 wt%
95-96%
Suárez et al. (2013) [26].Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound bath
with ethylene glycol and
the subsequent addition of Ni
powder. The CNT concentration
is 0.2 mg/ml. After 10min in the
ultrasound bath, the resulting
solution was stirred to homogenize
the mixture and then dried
in a ventilated furnace. The
powder is finally milled in
an agate mortar.
HUP: Cold pressed with
990 MPa. Then sintered in a hot
uniaxial press under vacuum
(0.2 Pa) at 750 °C for 2.5 h
with a 264MPa axial pressure.
1 wt%
2 wt%
3 wt%
5 wt%
95-97%
Sairam et al. (2014) [80]No mixing, only pure Ni
(high purity 99.99%) with a
size range from 1 to 5 μm.
SPS: Pre-compacted under a
pressure of 5 MPa. Then
sintered at 700, 850 or 1000 °C
for 5 min under a controlled
argon atmosphere under a
pressure of 50, 65 or 80 MPa.
The heating rate is maintained
at 100°C/min.
XXX96.8-
97.7%
(700°C)
97.8-99%
(850°C)
98.1
-99.2%
(1000°C)
Borkar et al. (2014) [24]Ball milling or molecular-level mixing: High energy ball milling
for 24 h at 400 rpm. For the
MLM process, CNTs are purified
and functionalized by acid
treatment. After cleaning and
drying, they are dispersed in
ethylene glycol by ultra-sonication
and Ni(C2H3O2)2•4H2O
is added. Then, 2 M-NaOH and
hydrazine are injected, the
solution is heated to 70 °C and
kept 30 min. Finally, the CNT/Ni
composite powders are reduced in
H2 + CO mixed atmosphere
for 2 h at 400°C.
SPS: Pre-compacted under a
pressure of 5 MPa. Then sintered at
800 °C for 5 min under controlled
argon atmosphere under a
pressure of 80 MPa. The heating
rate is maintained at 100° C/min.
5 vol%XXX
Nguyen et al. (2014) [48]Colloidal mixing process:
Functionalization of the
metal powders and CNTs by
zwitter-ionic surfactant,
3-(N,N-dimethylstearylammonio)-
propane-sulfonate (ZW).
Suspensions with varying
concentration of ZW (0.04-0.10
wt%) in de-ionized water
are heated to 353 K for 10
min with a fixed CNT
concentration (0.1 wt%) and
ultrasonicated for 15 min.
The metal powder is submersed in
100 ml of the 0.06 wt%
ZW-0.1 wt% CNT suspensions with
varying amounts of dwell
time from 1 to 6 h. The
ZW-CNT-metal powder
suspensions are then dried at
423 K in vacuum, and the
dried powders are subsequently
annealed in a vacuum
furnace at 693 K for 10 min to
decompose a majority of the
surfactant.
SPS: Consolidated at 550 or
650 °C in a vacuum environment
of 6 Pa, using a holding time
of 3 min and a pressure
of 200 MPa.
0.1 wt%93%
(550°C)
>97%
(650°C)
Suárez et al. (2014) [27].Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound
bath with ethylene glycol
and the subsequent addition of
Ni powder. The CNT concentration
is 0.2 mg/ml. After 10min in the
ultrasound bath, the resulting
solution was stirred to homogenize
the mixture and then dried in
a ventilated furnace. The powder
is finally milled in
an agate mortar.
CPS or HUP: Consolidated
with an axial pressure of 990 MPa.
Then densified by pressureless
sintering (CPS) under vacuum
(1 Pa) at 1223 K for 2.5 h. The
second set is densified in
a hot uniaxial press (HUP)
under vacuum (0.1 Pa) at
1023 K also for 2.5 h.
1 wt%
2 wt%
3 wt%
5 wt%
95.5%
(CPS)
98%
(HUP)
Suárez et al. (2014) [28].Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound bath
with ethylene glycol and the
subsequent addition of Ni
powder. The CNT concentration
is 0.2 mg/ml. After 10 min in the
ultrasound bath, the resulting
solution was stirred to
homogenize the mixture and then
dried in a ventilated furnace.
The powder is finally
milled in an agate
mortar.
CPS: Cold pressed at 990 MPa.
Then sintered at 950 °C for 2.5 h
in vacuum.
1 wt%
(6.5 vol%)
2 wt%
(12 vol%)
3 wt%
(18 vol%)
5 wt%
(27 vol%)
XXX
Suárez et al. (2014) [29].Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound bath
with ethylene glycol and
the subsequent addition of Ni
powder. The CNT concentration
is 0.2 mg/ml. After 10min in the
ultrasound bath, the resulting
solution was stirred to homogenize
the mixture and then dried in
a ventilated furnace. The powder
is finally milled in an
agate mortar.
HUP: Cold pressed with
990 MPa and then sintered in
under vacuum (0.2 Pa) for 2.5 h
at 750°C with a 264 MPa
axial pressure.
1 wt%
(6.5 vol%)
2 wt%
(12 vol%)
3 wt%
(18 vol%)
5 wt%
(27 vol%)
Up
to 99%
Rossi et al. (2014) [49].Colloidal mixing process:
Merging of the dispersion of
the CNTs in an ultrasound bath
with ethylene glycol and
the subsequent addition of
Ni powder. The CNT concentration
is 0.2 mg/ml. After 10min in the
ultrasound bath, the resulting
solution was stirred to homogenize
the mixture and then dried
in a ventilated furnace. The powder
is finally milled in an agate
mortar.
HUP: Cold pressed with 990
MPa and then sintered in under
vacuum (0.2 Pa) for 2.5 h at 750°C
with a 264 MPa axial
pressure.
1 wt%
(6.5 vol%)
2 wt%
(12 vol%)
3 wt%
(18 vol%)
XXX
Suárez et al. (2015) [31]Colloidal mixing process:
Dispersion of CNTs in ethylene
glycol (0.2 mg/ml) is mixed in an
ultrasound bath with the
metallic powders for 20 min and
then evaporated to obtain a
metal–CNT blend.
HUP and HPT: Sintered in vacuum
(0.1 Pa) for 2.5 h at 750°C at a
compacting pressure of 264 MPa.
The densified composites
were further processed at
room temperature inducing
severe plastic deformation
by HPT with 4 rotations
applying a 4 GPa pressure.
For the thermal stability
studies, the HPT-processed
samples are annealed at 573 K
with a heating rate of 20°C/min
and a dwell time of 3 h.
1 wt%
3 wt%
XXX
Reinert et al. (2015) [30]Colloidal mixing process:
CNT concentration in ethylene
glycol is 0.006 vol%. A
homogenizer (5-min
treatment) combined
with an ultrasonic
bath (20-min treatment)
is used for the
dispersion of the
CNTs. After that,
Ni powder is added and
mixed with the CNT
dispersion for 5 min
using the homogenizer.
Finally, the solvent is
evaporated at 150°C
in a furnace.
HUP: Pre-compacted at 990 MPa.
Then sintered in a hot uniaxial
press (axial pressure of 264
MPa) in vacuum (0.2 Pa) at 750°C
for 2.5 h.
6.5 vol%98%

Table 3.

Summary of blending and sintering methods for the production of Ni/CNT composites.

4.2. Distribution and interaction with the matrix material

As observed in the other metal-CNT systems, agglomeration is observed in all cases. If the different blending methods are considered, it is very unlikely to obtain a predominantly individual CNT dispersion rather than a homogeneous cluster distribution. Nevertheless, this cluster distribution provides also diverse improvements with regard to microstructural refinement and properties enhancement. Furthermore, when the initial agglomerate size (as-received state) is considered, all dispersion methods render very good disaggregation. Regarding the damage to the CNTs during processing, it is clear that highly energetic processing as ball milling increases the amount of defects on the CNTs, whereas milder processing routes as colloidal mixing present a fairly unmodified state of the CNTs structure. Interestingly, the sintering process tends to help in the defect healing [29]. The application of temperature in a non-reactive environment (vacuum sintering of non-carbide forming metal matrices) diminishes the ID/IG ratio as observed with Raman spectroscopy. Furthermore, the purity index IG’/ID also is reduced, evidencing some sort of contaminant removal (e.g. amorphous carbon).

Nickel does not form stable carbides [101]; however, it has been reported that under certain conditions it is possible to obtain Ni3C [102]. This carbide is brittle and, despite improving the interfacial cohesion to the CNTs, would have a detrimental influence on the transport properties of the composite. Due to its metastable nature, it is quite complex to detect it in the composite. It has been shown that the most suitable way is to assess the crystallographic lattice of the Ni in the vicinity of the CNT-containing areas by selected area electron diffraction [29,102]. In the case of a C-Ni reaction, the originally FCC Ni phase would transition to an intermediate Ni3C (hcp) and later would stabilize in an hcp Ni phase. Thus, if this hcp lattice is detected in the near region of the CNT zones, it would mean that the Ni3C was previously present. To the date, Ni3C in Ni-CNT systems has been only detected once by Hwang et al. [102] under a very specific set of synthesis conditions.

Yamanaka et al. and Nguyen et al. show that the application of SPS rendered a very smooth interface between the CNTs and the matrix [46,48]. This was also observed in HR-TEM for hot pressed samples [29]. In general, all the different approaches have resulted in seamless interfaces that are later translated in improved mechanical properties [26,48], thermal properties [46], tribological [27,37] and thermal expansion behaviour [47,78]. Additionally, a proper interface would favour a grain boundary drag that improves the microstructural control by achieving refined microstructures [26,28,48].

5. Outlook

Although a large amount of efforts was directed towards the development of CNT-MMCs systems, there is still a significant room for improvement. This statement is supported by the fact that as described, dissimilar results have been obtained by even using the same processing methods as well as the same type and amount of CNTs. This is generated by a scarcity of proper knowledge of each particular system. As an example, there is still an ongoing discussion in the community about the most suitable dispersion and blending methods for a certain application. Furthermore, it can be noticed that the impact of interphases on the physical properties of the composites is still not well understood. Thus, we foresee a very high potential to gain new insights on each particular system and subsequently achieve further developments in this field.

Acknowledgments

The present work is supported by funding from the Deutsche Forschungsgemeinschaft (DFG, projects: MU 959/38-1 and SU 911/1-1). The authors wish to acknowledge the EFRE Funds of the European Commission for support of activities. This work was also supported by the CREATe-Network Project, Horizon 2020 of the European Commission (RISE Project No. 644013).

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Sebastian Suárez, Leander Reinert and Frank Mücklich (June 29th 2016). Carbon Nanotube (CNT)-Reinforced Metal Matrix Bulk Composites: Manufacturing and Evaluation, Diamond and Carbon Composites and Nanocomposites, Mahmood Aliofkhazraei, IntechOpen, DOI: 10.5772/63886. Available from:

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