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

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.

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 [6–31]. 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 [6,7,10,32–39]. 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 [6–39]. 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 [6,16,22,26–31,41–49], molecular-level mixing [7,9,11,12,14,24,25,50–54], particle composite system mixing [15,55], friction stir processing [56], layer stacking [57], ball milling [8,10,17,19,20,22–24,32–36,38,50,53,58–74], 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 [8,10,17,19,20,22–24,32–36,38,50,53,58–74].

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 [7,9,11,12,14,24,25,50–54].

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 I_D/I_G 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 [6,16,22,26–31,41–49].

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) [10,19,26,27,29–31,39,40,47,49,62,67–70], spark plasma sintering (SPS) [6,7,9,11,12,14–16,18,21,23–25,42,44–46,48,50–52,55,64,75,79,80], hot or cold rolling [7,50,59,60,64], hot extrusion [18,20,23,40,58,79], high pressure torsion (HPT) [22,31–33,41,54,61,65,66,71,73], friction stir processing [56], hot isostatic pressing (HIP) [22,57], microwave sintering [43], laser engineered net shaping (LENS) [36–38,76] 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 [10,19,26,27,29–31,39,40,47,49,62,67–70].

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 [6,7,9,11,12,14–16,18,21,23–25,42,44–46,48,50–52,55,64,75,79,80].

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 [83–85]. 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% [18,22,39–41,50,56–59,79]. The most used blending method for Al/CNT composites is ball milling [19,20,22,23,50,58–62].

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

Reference	Blending method	Sintering method	CNT 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)18	NSD: 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 milling	HPT: 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 I_D/I_G 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% [6–12,14,15,32–35,42–44,50,52–55,63–71]. 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 [6–12,14–17,32–35,42–45,51–55,63–75]. 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) [6,12,14–16,32,33,42,54].

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.

Reference	Blending method	Sintering method	CNT content	Relative 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 milling	HPT: 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 milling	HPT: 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.	XXX	XXX	XXX
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) [26–31,36–38,47,76,78,80]. 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% [24–31,36–38,46–49,76,78,80]. The most used blending method for Ni/CNT composites is colloidal mixing and ball milling [24,26–31,36,38,46–49].

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

Reference	Blending method	Sintering method	CNT content	Relative 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.	LENS	10 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]	XXX	CPS: 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.	XXX	96.8-; 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 I_D/I_G ratio as observed with Raman spectroscopy. Furthermore, the purity index I_G’/I_D 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).