Research Advancements in the Mechanical Performance and Functional Properties of Nanocomposites Reinforced with Surface-Modified Carbon Nanotubes: A Review

Abstract

This review summarizes the recent advancements in the mechanical properties of nanocomposites reinforced with surface-modified carbon nanotubes (CNTs). A range of matrices, namely, polymers, metals, and cement, is investigated, which have demonstrated increasing importance in a broad range of industrial sectors, such as 3D printing, automotive, construction, and coatings. The strengthening mechanisms that CNTs impart in composites are reviewed, and synergistic effects with their surface groups or co-additives are analyzed, including wettability, mechanical interlocking, and chemical bonding. Different mechanical and functional properties of the CNT-reinforced nanocomposites are analyzed, such as tensile strength, flexural strength, impact resistance, thermal conductivity, and electrical conductivity. The improvements in these properties for a variety of CNT-based composites are presented, and details on how these improvements were attained are discussed. The review concludes that surface modification of CNTs has proven to be of high importance, enhancing compatibility with various matrices and facilitating improvements in the nanocomposite properties. Suggestions for viable CNT-based composites for use in the studied applications are also provided.

1. Introduction

Carbon has always been an important part of human lives. Over the past century, the use and function of carbon have taken many roles in the world of material science. During the 1960s, scientists created new types of carbon materials, which included carbon fibers made from various materials, pyrolytic carbon formed from vapor, hard crystalline carbons, dense carbons made by pressing, compounds that could conduct electricity well, and thin carbon sheets resembling diamonds [1,2,3]. During the 1990s, the potential of carbon became even more prominent as the attention shifted toward nanocarbons, like fullerenes, carbon nanotubes, and graphene [4,5,6]. In the realm of advanced materials science, the development of nanocomposites has marked a significant leap forward, offering unprecedented improvements in mechanical, thermal, and electrical properties.

Among them, composites with nano-structured carbon materials are gaining popularity due to their exceptional strength, stiffness, and conductivity. When integrated into composite materials, CNTs can improve mechanical performance and push the boundaries of what is achievable with traditional composites [7,8,9,10].

Carbon nanotubes are composed entirely of carbon atoms arranged in a hexagonal lattice, and rolled into a tubular form, CNTs exhibit a unique combination of properties that is difficult to find in other materials. Their high aspect ratio, along with the inherent strength of the sp² carbon–carbon bonds, allow CNTs to exhibit extraordinary tensile strength and stiffness. Moreover, the delocalized π-electron cloud along the tube’s surface contributes to their unique electrical properties, making them excellent conductors of electricity [5,11,12,13,14].

The features that make CNTs exceptional pose challenges in the application of the product. The strong van der Waals forces between individual tubes lead to clusters, hindering their uniform dispersion within a matrix. Moreover, the static nature of the CNT surfaces complicates their interaction with other matrices, limiting mechanical load transfer [15,16]. Their impurities and weak affinity toward metals are also some of the bigger challenges [17,18]. To overcome these obstacles, surface modification of CNTs has been developed as a crucial strategy. By introducing functional groups or coatings onto the CNTs’ surface, their compatibility with different matrices can be significantly improved. This modification not only enhances the dispersibility of CNTs but also facilitates stronger interfacial bonding between the CNTs and the matrix, which is essential for effectively transferring stress and enhancing the mechanical properties of the composite [19,20,21,22].

The exploration of surface-modified CNTs in reinforcing various composite matrices opens up new avenues for engineering materials with tailored properties. This paper focuses on the advancements in the mechanical performance of nanocomposites reinforced with surface-modified carbon nanotubes. The application of such nanocomposites has a wide range of applications in industrial sectors, including 3D printing [23,24,25], automotive [26,27,28], construction, and coatings [29,30,31,32], highlighting their growing importance in modern engineering and materials science.

This paper aims to provide a comprehensive review of the latest advancements in the field of surface-modified CNTs as additives in polymeric, metallic, and cementitious composites. Various production and synthesis processes are highlighted to provide necessary context, followed by an exploration of the resulting composites’ innovative applications, existing challenges, and future potential. This review aims to offer a detailed overview of the current State of the Art in the field of CNT-reinforced nanocomposites of polymers, metals, and cement and provide an understanding of the mechanisms of enhancement in their mechanical and functional properties with the incorporation of CNTs.

2. Production of CNTs

Extensive research has been carried out on the most efficient way of producing CNTs. Single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) are the most common categories of carbon nanotubes.

2.1. Single-Walled CNTs (SWCNTs)

Single-walled carbon nanotubes (SWCNTs) are cylindrical nanostructures composed of a single layer of carbon atoms arranged in a hexagonal lattice, forming a tube-like configuration, which has a diameter of ~1–2 nm, and its length can reach tens of micrometers [33,34,35]. SWCNTs were first reported in 1993 in the work of Iijima and Ichihashi, which synthesized SWCNTs with a 1 nm diameter [35]. The unique structure of SWCNTs allows them to have exceptional mechanical, electrical, and thermal properties. They possess tensile strength and elastic modulus, far surpassing those of other materials, including steel [33]. Electrically, SWCNTs can be either metallic or semiconducting, depending on their chirality and diameter, which is defined by the orientation of the graphene sheet rolling direction, characterized by the chiral vector, which is determined by how the graphene sheet is rolled up to form the tube (Figure 1). The electronic properties of SWCNTs are highly sensitive to these formations, enabling a wide range of applications from nanoelectronic devices to conductive composites. SWCNTs are known for their low resistance and high current carrying capacity of up to ${10}^9{ \mathrm{A} \mathrm{cm}}^{-2}$[34,35,36,37]. They are also characterized by high thermal conductivity, allowing them to absorb heat effectively [34,35,36,37].

Various methods, including arc discharge [39,40], laser ablation [41,42], and chemical vapor deposition (CVD) [43,44,45], can be used to synthesize SWCNTs. Each method has advantages and limitations in terms of yield, purity, and control over the structural properties of the nanotubes. However, CVD has been most commonly used to synthesize SWCNTs in recent years due to the flexibility in producing large amounts of products and using various substrates, allowing growth in various forms, such as powder and thin films [45,46].

2.2. Multi-Walled CNTs (MWCNTs)

Multi-walled carbon nanotubes (MWCNTs) are cylindrical nanostructures composed of multiple layers of carbon atoms arranged in a hexagonal lattice, consisting of several SWCNTs nested within each other. In 1991, Iijima reported for the first time the synthesis of graphitic carbon needles (MWCNTs) that were 4–30 nm in diameter and up to 1 micrometer in length [47]. The structure of MWCNTs can be envisioned as multiple graphene sheets rolled into concentric cylinders, with the interlayer spacing approximately close to the distance (~0.35 nm) between graphene layers in graphite; the diameter of MWCNTs usually ranges between 3 and 30 nm; however, it often grows above the value of 30 nm (Figure 2) [48,49,50,51].

MWCNTs have exceptional tensile strength of 50–150 GPa and elastic modulus, although these properties can vary depending on the number of walls and the defects within and between the layers. The presence of multiple walls enhances the durability and impact resistance, making MWCNTs easier to work with in various applications compared to SWCNTs [51,52].

MWCNTs are known for their high thermal conductivity (2.2 × ${10}^4$ S/cm) along the tube axis. This property, combined with their structural stability, makes MWCNTs suitable candidates for thermal management [53]. Among the MWCNT production methods, CVD is popular for producing MWCNTs with controlled growth parameters, allowing for the tailoring of dimensions and properties for specific applications.

3. Composite Matrices and Applications

3.1. Polymers

Polymers are macromolecules composed of repeating monomers linked by covalent bonds. Their widespread application, customizable properties, and easy manufacturing process have made them indispensable in modern society [54]. However, there are several limitations to how polymers can be utilized in some sectors because of their lack of ductility and electrical and thermal conductivity. Researchers have been investigating ways to improve mechanical properties, such as the incorporation of mechanically superior additives [55,56,57].

Traditionally, carbon and glass fibers have been used as fillers for polymers to enhance their mechanical performance [58,59,60,61,62]. Although these conventional methods improve the mechanical properties of the polymers, CNT-based polymers have shown exceptional potential, as seen in Figure 3. MWCNTs with consistent alignment, orientation, and surface defects can also be used as fillers, with the developed nanocomposites showcasing exceptional mechanical performance and improved thermal conductivity, which are particularly beneficial for automobile tires and other transportation applications, as they result in enhanced fuel efficiency and improved fatigue resistance [63,64,65,66,67].

A study led by Wu et al. investigated the properties of carbon nanotube (CNT)/polypropylene (PP) composites and the improvement of electromagnetic interference (EMI) shielding effectiveness and several mechanical properties [68]. The author used the solid-phase molding method to restrict the diffusion of CNTs and maintained the polymer in a solid phase during the molding process. Elevated pressure and temperature were used to fabricate the CNT/PP composites with a segregated structure [68].

PP granules were mixed with water/ethanol, and CNTs were dispersed in ethanol with ultrasonication. Then, the two mixtures were combined and further ultrasonicated. After filtration, the CNT-coated PP granules were dried and compression-molded. This process resulted in CNT/PP-I. For comparison, CNT-coated PP granules were molded under regular solid-phase conditions (CNT/PP-II) and regular molding conditions (CNT/PP-III). Additionally, CNT/PP composites were prepared using solution mixing (CNT/PP-IV) and melt mixing (CNT/PP-V). The researchers found that in CNT/PP-I, carbon nanotube paths are more tightly packed compared to CNT/PP-II, which suggests that higher pressure and temperature help create better conductive CNT networks in the solid-phase molded composites. However, when the molding temperature surpasses the melting point of PP, some CNTs start to diffuse into the melted polymer regions due to lower viscosity, thus disrupting the segregated structure [50,68].

Among all the different compositions, CNT/PP-I shows the highest electrical conductivity. For instance, at 2.0 wt%, CNT, CNT/PP-I, and CNT/PP-II reach an electrical conductivity of 70 S/m, which is significantly higher than CNT/PP-III and CNT/PP-IV. It is also much greater than CNT/PP-IV by 7 orders of magnitude, as seen in Figure 4.

Compared to CNT/PP-II with a regular segregated structure, CNT/PP-I demonstrates significant enhancements in compressive, tensile, and flexural properties, including a 133% increase in compressive strength and a 65% increase in modulus (Figure 5). These improvements were caused by the elimination of voids and compacting of the segregated structure in CNT/PP-I. Overall, CNT/PP-I shows superior mechanical properties compared to the other CNT/PP composites.

As mentioned previously, one of the major drawbacks of polymers is their lack of thermal conductivity. However, some thermoplastic polymers are able to address this challenge to some extent. Polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyamides, polyesters, and polyurethanes are some known examples of such polymers [69]. Polyvinylidene fluoride (PVDF) is also a thermoplastic polymer known for its thermal and chemical stability, thus being an excellent material for the chemical industry and electronics [70,71,72,73]. It is also environmentally friendly and exhibits superior mechanical properties, thus attracting attention from the industrial and research community [71,72,73]. Still, its low thermal conductivity makes it undesirable in several applications [68]. To address this limitation, numerous thermal conductive fillers have been tested that resulted in increased thermal conductivity [74,75,76,77,78].

Recently, Guo et al. investigated if the incorporation of MWCNTs and graphene (GE) in PVDF can improve its thermal conductivity [79]. The study successfully demonstrated a significant increase in the thermal conductivity of PVDF composites, which was achieved through a well-dispersed network of MWCNTs and graphene. Different types of MWCNTs were studied: without acidification (p-MWCNTs), with acidification (a-MWCNTs), and with both acidification and silanization (s-MWCNTs). It was found that s-MWCNTs were more uniformly dispersed compared to a-MWCNTs, and both were more efficiently dispersed than p-MWCNTs [79].

Initially, MWCNTs and GE aggregates were observed in the cross-sections, leading to poor dispersion within the PVDF matrix. However, when functionalized MWCNTs were introduced, the dispersion improved significantly, as seen in the scanning electron microscopy (SEM) images of Figure 6. Acidified MWCNTs combined with GE showed better dispersibility compared to non-functionalized MWCNTs. Silanization further improved the interfacial properties, enhancing compatibility between MWCNTs/GE and the PVDF matrix and promoting better dispersibility. Additionally, the filler content directly affected dispersion: increasing filler content improved contact between fillers and PVDF up to a point where excessive filler content led to aggregation, causing cracks and voids in the composite. Overall, functionalized MWCNTs played a crucial role in enhancing dispersion and improving thermal conductivity in the composite material [79].

The thermal conductivity and enhancement rates of PVDF composites containing either single or hybrid fillers of multi-walled carbon nanotubes (MWCNTs) and graphite (GE) were compared. GE/PVDF composites show the least enhancement in thermal conductivity due to the agglomeration of GE lamellae, reducing the contact area between fillers and the matrix. However, composites with both MWCNTs and GE exhibit significantly improved thermal conductivity. Specifically, p-MWCNTs/GE/PVDF with a 5 wt% loading demonstrates a 74.17% enhancement, outperforming composites with single fillers (see Figure 7). Modification of MWCNTs with the vinyl triethoxysilane (YDH-151) further enhances thermal conductivity, particularly with silanized MWCNTs and GE, reaching 1.46 W/(m·K) with a 10 wt% loading and a remarkable 711.1% enhancement. These results highlight the importance of compatibility in enhancing thermal conductivity, with the s-MWCNTs/GE/PVDF composite showing superior performance compared to non-functionalized MWCNTs/GE/PVDF and p-MWCNTs/GE/PVDF composites [79].

The electromagnetic properties of PVDF have also been tailored with the incorporation of carbon nanotubes. Cacciotti et al. developed microwave-absorbing composite films for electronic applications by adjusting electromagnetic properties through varying compositions [Cacciotti]. This study synthesized BaTiO₃ nanoparticles using a sol-gel process, combined them with PVDF and MWCNTs to create hybrid films, and further stacked the resulting hybrid material into multi-layer slabs. The films showed a significant increase in dielectric constant and a moderate increase in loss factor, which made them suitable for high-dielectric microwave applications [80].

The integration of mechanically superior materials, such as CNTs and graphene, into polymer composites has shown promising results in enhancing mechanical, electrical, and thermal properties. Notably, solid-phase molding techniques, along with filler functionalization, have demonstrated significant improvements in dispersion and functional properties [81]. These findings underscore the importance of filler selection, processing methods, and compatibility in tailoring the properties of polymer composites for diverse applications, ranging from automotive components to electronic devices.

Another emerging versatile thermoplastic is polyphenylene sulfide (PPS), which has gained popularity due to its easy manufacturing process, high-temperature stability, and chemical resistance. As a consequence, it is abundantly used in the automobile, aerospace, and chemical industries. However, as previously mentioned, pure PPS, like PVDF, is not characterized by good thermal conductivity and mechanical strength [82,83,84].

A study led by Pan et al. investigated the incorporation of CNTs in 3D-printed PPS. The experiment involved the preparation of Carbon Nanotube Molecularly Imprinted Polymers (CNT-MIPs) and polyphenylene sulfide (PPS) filaments. High-purity CNTs were combined with a polymer mix (including N-phenyl maleimide, methyl methacrylate, and other chemicals) and processed through heating, cooling, and purification steps to form CNT-MIPs. These CNT-MIPs were then mixed with PPS at various mass fractions, extruded into smaller filaments, and further processed into controlled-diameter filaments. The printing parameters were controlled, resulting in high shape accuracy of the printed samples, as seen in Figure 8 [85].

The temperature and heat flow curves, along with corresponding enthalpies and crystallinities of the composites, were analyzed. As the CNT content increased, so did the crystallinity values, with the 0.9 wt% CNT prints exhibiting almost double the crystallinity of pure PPS prints. Pure PPS exhibited an obvious glass transition around 90 °C, which became less pronounced with increasing CNT content. The formation of the glass transition peak is linked to the movement of amorphous chain segments in PPS. However, as the crystallinity of the samples increased with a higher CNT content, the number of amorphous molecular chains decreased, and some became constrained by crystallites [85].

The same study also investigated friction processes by analyzing friction coefficient curves under loads of 100 N and 196 N. During the running-in stage, the friction coefficient generally increased with sliding distance, especially under higher loads due to transfer film formation and changes in surface contact. However, the friction coefficient curve of the 0.9 wt% CNT composite showed smoother behavior due to increased PPS crystallinity from a high CNT content. This higher crystallinity led to stable wear under lower loads (Figure 9). The average friction coefficient decreased for certain CNT contents, particularly at 0.5 wt%, due to a shift from adhesive to abrasive wear. The wear rate also significantly decreased with CNT addition, mainly attributed to improved heat transfer, reduced contact area, and lubricating effects of CNTs on the friction surfaces [85].

Similarly, the mechanical properties also demonstrated a relative improvement. Tensile strength and modulus increased significantly with the addition of CNTs, particularly at 0.1 wt%, showing a 26% increase in tensile strength compared to pure PPS. The role of CNTs in load transfer, even at low concentrations, contributed to this enhancement due to their uniform dispersion and high compatibility with the matrix. Bending performance improved with the addition of CNTs up to 0.1 wt%, resulting in a 29% increase in flexural strength and a 37% increase in flexural modulus. However, impact strength showed slight fluctuations with varying CNT content, possibly influenced by changes in crystallinity and the reinforcing effect of CNTs [77].

Another major drawback of polymers is their high fire hazard susceptibility. As a result, their applications are limited in fields that require high fire safety standards. Research on 3D printing and flame-retardant polymers has grown significantly over the past two decades. Compared to traditional methods, like thermoforming and Fused Deposition Modelling (FDM), Selective Laser Sintering (SLS) is often preferred for creating flame-retardant polymers due to its ability to achieve precise dimensions [86,87]. In SLS, a laser is employed to manufacture objects from powdered materials by fusing particles on the surface of the object layer by layer [87].

A study led by Chen et al. focused on enhancing the fire safety and mechanical properties of polymer nanocomposites made from polyamide 12 (PA12) by reinforcing them with modified carbon nanotubes (CNTs) and fullerene-like tungsten disulfide (WS2) nanofillers through SLS, as shown in Figure 10.

The SEM and TEM analyses show that the shapes of chemically modified MWCNTs and WS2 nanoparticles revealed rough surfaces and spherical/elliptical morphologies, respectively. Thermogravimetric analysis (TGA) shows that modifications led to earlier thermal degradation due to unstable functional groups but improved interfacial compatibility and bond strength, enhancing fire safety performance in laser-sintered composites [88].

The tensile strength of pure PA12 was 42 MPa, with an elongation at a break of 14.5%. When combined with CNTs (PA12/3C), the strength increased to 46 MPa (a 9.5% increase), with elongation rising to 16.7% (a 2.2% increase) [80]. This improvement is attributed to the formation of a network structure between CNTs and PA12, increasing cohesive energy density. However, as the CNT content approaches 5%, strength decreases (44 MPa for PA12/5C) due to hydrogen bond saturation, leading to filler agglomeration and reduced energy dissipation. The introduction of PEGylated IF-WS2 enhances tensile strength further, with PA12/3C/1W showing a 19.4% improvement. IF-WS2’s functional groups inhibit stacking, improve dispersion, create hydrogen bonding networks, and enhance mechanical properties. Moreover, hydrogen bonds prevent polymer chain movement, further improving tensile characteristics [88].

The results show that the addition of H-CNTs and P-IF-WS2 increased the materials’ flame resistance, evidenced by a decrease in Time to Ignition (TTI) and the Fire Growth Index (FGI). The heat release rate (HRR) and total heat release (THR) also decreased with nanocomposite addition, indicating improved fire safety. The smoke release rate (SRR) and total smoke release (TSR) were reduced, especially in PA12/3C/1W, suggesting better smoke suppression. Additionally, the limiting oxygen index (LOI) tests show increased LOI values, thus confirming enhanced flame retardancy with the addition of H-CNTs and P-IF-WS2. This was attributed to the formation of a hydrogen-bonded network structure, which inhibits thermal movement and reduces plasticizing effects during combustion [88].

The ongoing advancements in polymer composites through the integration of nanomaterials, like CNTs, are leading to materials that not only meet but exceed the traditional performance metrics of polymers. The use of solid-phase molding, functionalization of fillers, and innovative manufacturing techniques, like SLS, are critical in realizing composites that are mechanically superior and thermally conductive while also being flame resistant. This holistic improvement in polymer properties opens new doors for their application across various high-demand industries, enhancing their functionality and safety profiles.

The conclusions of the composites investigated in this section are summarized in

Table 1. Electrical conductivity performance. (Summary of the work of Wu et al. [69]).

Composites	Formulation	Output
CNT/PP-I	Solid-phase molding with CNT-coated PP granules	Highest electrical conductivity (70 S/m at 2 wt% CNT)
CNT/PP-II	Regular solid-phase molding	High electrical conductivity but lower than CNT/PP-I
CNT/PP-III	Regular molding	Lower conductivity than CNT/PP-I and CNT/PP-II
CNT/PP-IV	Solution mixing	Conductivity is lower by seven orders of magnitude than CNT/PP-I
CNT/PP-V	Melt mixing	Conductivity is significantly lower than CNT/PP-I and CNT/PP-II

,

Table 2. Thermal conductivity enhancement in PVDF Composites with MWCNTs and graphene. (Summary of the study of Hong et al. [79]).

Composites	Formulation	Output
PVDF composite	PVDF without fillers	Minimal thermal conductivity
p-MWCNTs/GE/PVDF	Pristine MWCNTs and graphene	Moderate thermal conductivity Improvement
a-MWCNTs/GE/PVDF	Acidified MWCNTs and graphene	Better dispersion and enhanced thermal conductivity
s-MWCNTs/GE/PVDF	Silanized MWCNTs and graphene	Highest thermal conductivity improvement (711.1%)

,

Table 3. Study on CNT-MIPs/PPS for 3D printing. (Summary of the study of Pan et al. [85]).

Composites	Formulation	Output
CNT-MIPs	CNTs mixed with polymer components for MIPs	Increased crystallinity and improved shape accuracy
CNT-MIPs/PPS	CNT-MIPs added to PPS filaments	Enhanced tensile strength and flexural modulus
CNT-MIPs/PPS (0.9 wt% CNTs)	Higher CNT content	Improved friction coefficient and wear resistance

and

Table 4. Fire safety and mechanical properties of PA12 nanocomposites. (Summary of the study of Ding et al. [88]).

Composites	Formulation	Output
PA12	Pure polymer, no fillers	Moderate tensile strength and low flame resistance
PA12/3C	PA12 with 3% CNTs	9.5% increase in tensile strength and slight flame resistance improvement
PA12/3C/1W	PA12 with 3% CNTs and 1% WS2	19.4% improvement in tensile strength and enhanced flame resistance

.

3.2. Metals

Metals are a class of elements characterized by their high electrical and thermal conductivity, malleability, ductility, and typically lustrous appearance. Metals such as aluminum, titanium, and magnesium are used in automotive, aerospace, and marine applications due to their strength-to-weight ratios, improving fuel efficiency and performance. However, sometimes, virgin metal alloys are not able to live up to expectations as technology progresses rapidly and requires high specific strength, elastic modulus, and stiffness in addition to enhanced functional characteristics [89,90,91,92,93]. As a response, the use of metal matrix composites (MMCs) has been at the forefront of many new applications, and there have been numerous research studies on the improvement of the mechanical properties of MMCs with micro- and nanofillers [94,95,96,97,98]. Due to their high carrier mobility (80,000 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$) and a current carrying capacity that reaches ${10}^{9 }{\mathrm{A} \mathrm{cm}}^{-2}$ (${10}^3$ times more than copper), CNTs have been utilized to synthesize metal nanocomposites for commercial use [99,100]. CNTs can be combined with metals using methods such as powder metallurgy and ball milling. Powder metallurgy preserves the structural integrity of CNTs by avoiding damage caused by the high energy forces present in ball milling [101,102,103,104,105,106]. This technique led to increased tensile strength and corrosion reduction in metals and opened new doors for applications for metal-CNT composites. Nevertheless, it is important to note that at high temperatures, CNTS may deform due to thermal stress. To avoid this, preventive measures can be applied, such as adjusting the heating rate and controlling the process environment [101,102,103,104,105,106].

Aluminum is one of the most widely used metals after steel due to its high electrical conductivity and corrosion resistance; however, due to its relatively fragile nature, different wt% of copper, magnesium, and silicon are mixed with it to create alloys commonly used in the automotive industry and electrical goods [107,108,109]. The use of both SWCNTs and MWCNTs as reinforcement in aluminum matrices is important as carbon nanotubes provide improved strength and stiffness, high-temperature resistance, reduced density, controlled thermal expansion, enhanced electrical performance, and good wear and abrasion resistance [110,111,112].

Nayim et al. investigated the tribological and mechanical properties of aluminum matrix composites reinforced with CNTs and titanium carbide (TiC), prepared using the stir casting process [84]. In the initial (100% aluminum) composition (AlCTS02), only a few particles of CNTs and TiC were visible at 100× magnification. However, as the reinforcement content (wt% of CNT and wt% of TiC) increased in subsequent compositions (AlCTS03-AlCTS05), more particles were observed with uniform dispersion. According to the authors, higher reinforcement contents contributed to enhanced mechanical and tribological properties of the composites, as seen in Figure 11. The study investigated wear behavior using dry sliding pin-on-disc equipment with varying loads (10 N, 20 N, 30 N, and 40 N). The wear rate dropped dramatically as the concentration of CNTs increased. For instance, when 40N force was applied, AICTS01 had a wear rate of ~0.038 ${\mathrm{mm}}^3/\mathrm{km}$, whereas AICTS05 had a wear rate of ~0.006 ${\mathrm{mm}}^3/\mathrm{km}$, which is an 84.2105% decrease [113].

Another study led by Say et al. investigated the effect of CNT reinforcement on the mechanical properties and corrosion resistance of magnesium matrix composites [85]. Magnesium alloys AZ61(6.5 wt% Al, 1 wt% Zn, and 0.1 wt% Mn) and AZ91 (8.5 wt% Al, 0.8 wt% Zn, and 0.25 wt% Mn) were reinforced with varying amounts of carbon nanotubes (0.1, 0.2, and 0.5 wt%) using the powder metallurgy technique. The CNTs were synthesized using the chemical vapor deposition method [85]. It was observed that as the percentage of CNTs increases, the strength of the composites rises while elongation decreases. However, the addition of 0.1 wt% CNTs leads to a decrease in compression strength due to increased porosity in the matrix. The effect of CNT reinforcement is more significant in AZ61 series samples compared to AZ91 series, with a higher increase in compression strength observed [114].

The composites’ corrosion resistance varied with the amount of CNTs. Lower concentrations of CNTs (0.1 and 0.2 wt%) generally provided better corrosion resistance compared to higher concentrations (0.5 wt%). Specifically, composites with 0.2 wt% CNTs showed the best corrosion resistance across the tests (

Table 5. Corrosion properties of AZ61- and AZ91-based composites and unreinforced alloys calculated from PDS curves [114].

Matrix	CNT Ratio (wt%)	Corrosion Rate (mm/yr)
AZ61	Unreinforced	0.0336
AZ61	0.1	0.0388
AZ61	0.2	0.0324
AZ61	0.5	0.0678
AZ91	Unreinforced	0.0396
AZ91	0.1	0.0267
AZ91	0.2	0.0178
AZ91	0.5	0.0718

). Microstructural analysis reveals that CNT distribution was critical, with a more homogeneous distribution leading to better mechanical properties. Dislocations in the matrix, influenced by CNT distribution, played a significant role in strengthening the composites [114].

Another study created a high entropy alloy (HEA)/CNT nanocomposite by combining a high entropy alloy (HEA) made of AlCoFeMnNi with 1% CNTs using spark plasma sintering (SPS) [115]. Mechanical alloying effectively transformed the initial elemental powder mixtures into a solid solution of HEA within 40 h. The addition of CNTs during the final 5 h of alloying contributed to a finer crystalline structure in the nanocomposite (Figure 12). The crystallite size decreased from about 40 nm to around 10 nm after 50 h of milling, while lattice strain increased from 0.25 to 0.7 during the same period. Elemental powder peaks vanished after 40 h of milling, indicating the formation of an AlCoFeMnNi solid solution. The use of spark plasma sintering allowed for rapid consolidation of the powdered alloy, minimizing grain growth due to the short sintering duration. Different sintering temperatures (850 °C and 950 °C) demonstrated that higher temperatures helped reduce porosity significantly, thereby enhancing mechanical integrity and reducing defects within the sintered material [115].

As seen in Figure 12, the introduction of CNTs significantly increased the maximum shear strength and hardness of the HEA. The addition of CNTs significantly enhanced the ultimate tensile strength and hardness of the alloy from approximately 500 to over 650 VHN. The improvements are attributed to solid solution strengthening from carbon diffusion, dispersion hardening, the pinning effect of CNTs on dislocations, and crack bridging, all of which contribute to increased mechanical strength and ductility [115].

From the aforementioned studies, it is evident that the integration of CNTs into metal matrices represents a significant advancement in materials engineering, particularly for applications that demand high strength, stiffness, and enhanced functional properties. Metal matrix composites (MMCs) enhanced with micro and nanofillers, such as CNTs, are at the forefront of addressing these technological demands. The inclusion of CNTs in metal matrices not only boosts mechanical strength and stiffness but also improves high-temperature properties, electrical performance, and corrosion resistance while maintaining or reducing the overall weight of the composite. Studies have shown that even small additions of CNTs can significantly impact the wear resistance and mechanical integrity of composites. For instance, the incorporation of CNTs into aluminum matrices has dramatically reduced wear rates under high stress, demonstrating an 84% decrease in wear with higher CNT concentrations. Similarly, magnesium matrix composites reinforced with CNTs have shown varying mechanical and corrosion resistance based on the amount of CNTs added, with a noted increase in strength but a reduction in ductility as the CNT content increases.

Moreover, the development of high entropy alloy (HEA) composites with 1% CNTs using advanced manufacturing techniques, like spark plasma sintering (SPS), highlights the ability to enhance alloy properties through fine control of microstructural features [101]. This includes significant improvements in shear strength and hardness attributed to the uniform dispersion of CNTs within the matrix, which enhances load transfer capabilities and increases dislocation density, thereby improving the overall mechanical properties of the alloy.

Overall, the integration of CNTs into metal matrices not only meets but often exceeds the current technological requirements for high-performance materials, promising a new era of advanced composites with tailored properties for specific applications. The conclusions of the composites investigated in this section are summarized in

Table 6. Aluminum–CNT composites with TiC reinforcement. (Summary of the study of Nayim et al. [113]).

Composites	Formulation	Output
AlCTS01	100% Al (aluminum)	Baseline mechanical properties, wear rate ~0.038 mm3/km at 40N load
AlCTS02	99 wt% Al + 0.5 wt% CNT + 0.5 wt% TiC	Few CNT/TiC particles, minor improvement in tensile strength
AlCTS03	98.5 wt% Al + 0.5 wt% CNT + 1 wt% TiC	Moderate improvement in wear resistance
AlCTS04	98 wt% Al + 0.5 wt% CNT + 1.5 wt% TiC	Significant wear resistance improvement
AlCTS05	97.5 wt% Al + 0.5 wt% CNT + 2 wt% TiC	Maximum wear resistance, wear rate ~0.006 mm3/km (84.21% reduction)

,

Table 7. Magnesium alloy composites with CNT reinforcement. (Summary of the study of Say et al. [114]).

Composites	Formulation	Output
AZ61 Unreinforced	AZ61 alloy (6.5 wt% Al, 1 wt% Zn, and 0.1 wt% Mn)	Baseline corrosion rate: 0.0336 mm/yr
AZ61-CNT 0.1	AZ61 alloy + 0.1 wt% CNT	Decreased compression strength, corrosion rate: 0.0388 mm/yr
AZ61-CNT 0.2	AZ61 alloy + 0.2 wt% CNT	Best corrosion resistance, corrosion rate: 0.0324 mm/yr
AZ61-CNT 0.5	AZ61 alloy + 0.5 wt% CNT	Increased porosity, lower ductility, corrosion rate: 0.0678 mm/yr
AZ91 Unreinforced	AZ91 alloy (8.5 wt% Al, 0.8 wt% Zn, and 0.25 wt% Mn)	Baseline corrosion rate: 0.0396 mm/yr
AZ91-CNT 0.1	AZ91 alloy + 0.1 wt% CNT	Improved corrosion resistance, corrosion rate: 0.0267 mm/yr
AZ91-CNT 0.2	AZ91 alloy + 0.2 wt% CNT	Optimal balance, corrosion rate: 0.0178 mm/yr
AZ91-CNT 0.5	AZ91 alloy + 0.5 wt% CNT	Reduced ductility, corrosion rate: 0.0718 mm/yr

and

Table 8. High entropy alloy (HEA)/CNT nanocomposites. (Summary of the study of Bahrami et al. [115]).

Composites	Formulation	Output
HEA (AlCoFeMnNi)	Spark plasma sintering (SPS) at 850 °C	Baseline hardness ~500 VHN, crystallite size ~40 nm
HEA-CNT 1%	Mechanical alloying + SPS at 850 °C	Hardness improved to ~650 VHN, crystallite size reduced to ~10 nm
HEA-CNT 1% Higher Temperature	Mechanical alloying + SPS at 950 °C	Further hardness increase to ~670 VHN, reduced porosity

.

3.3. Cement

Cement composites are a class of engineered material systems predominantly composed of hydraulic cement mixed with water and aggregates, often incorporating admixtures and reinforcements such as fibers or nanoparticles. These composites are designed to exhibit tailored properties that meet specific performance criteria, which ordinary concrete may not offer [116,117]. Cement composites consist of a binder, usually Portland cement, supplemented by various additives, like fly ash, slag, and silica fume, which enhance the material properties through pozzolanic reactions [116]. The binder is mixed with water, initiating hydration that results in the hardening of the composite. Aggregates, both fine (sand) and coarse (gravel), are added to provide volume and strength. Advanced cement composites may also include high-performance admixtures, such as superplasticizers for workability, retarders for controlling the setting time, and accelerators for early strength gain [116,118,119].

Due to their wide application in building insulation, significant research has been undertaken on improving the quality and performance of cementitious composites. Reinforcements in cement composites can vary from macroscopic reinforcements (e.g., steel bars) to CNTs, graphene/graphene oxide, and polymer microfibers. These reinforcements are introduced to improve the tensile strength, ductility, and toughness of the material, properties that pure cementitious materials lack. In addition, graphene oxide and CNTs are used to add functional properties, such as conductivity and thermal stability [120,121,122,123].

Wang et al. investigated the properties of MWCNT-reinforced cement-based composites. The study successfully used N,N-dimethylformamide (DMF) as a dispersant for MWCNTs, gradually adding it to the MWCNT solution with mechanical stirring for 10 min [124]. Another MWCNT–cement solution without DMF was prepared with mechanical stirring for comparison. The mixture underwent 60 min of ultrasonication in a probe sonicator. Various concentrations of MWCNTs and defoamer suspensions were mixed [124]. Mechanical strength increased gradually with increasing MWCNT dosages, peaking at 0.04 wt% before declining. The maximum compressive strength and flexural strength reached 77.2 MPa and 11.2 MPa, respectively, representing a 2.9% and 21.7% increase compared to the control sample. Samples with dispersed MWCNTs (with DMF) exhibited better mechanical properties compared to those with individual MWCNT dosages (without DMF). MWCNTs enhanced flexural strength while minimally affecting compressive strength. The ratio of compressive strength to flexural strength decreased by 15.9% with 0.04 wt% MWCNTs, indicating improved fracture resistance [109].

The X-ray diffraction (XRD) patterns show that no new phases were generated, indicating that the structure of the final hydration product remained unchanged in MWCNT/cement composites. The ratio of the integral areas of the CH (18 degrees) and C3S peaks (31.4 degrees) was used as an indicator of hydration degree. The integral peak area ratio of MWCNT/cement composites increased gradually with higher MWCNT dosages, indicating an accelerated cement hydration reaction (Figure 13).

Wang et al. also showed that the addition of MWCNTs led to a reduction in the porosity of the cement composites, as confirmed by mercury intrusion porosimetry. This was a result of the MWCNTs inhibiting the extension of cracks and enhancing the compactibility of the composites. Furthermore, SEM revealed that MWCNTs were well-integrated into the cement matrix, acting as bridges across microcracks, thus improving the microstructure and overall durability of the composites [124].

Mousavi et al. also investigated the enhancement of mechanical properties of cement composites using CNTs and titania nanoparticles (TiO₂) as nano-reinforcements [111]. To synthesize titania nanoparticles, titanium tetrachloride and ethanol were mixed. The gel formed was dried at 120 °C for 6 h and then heated at 400 °C for 2 h to obtain titania powders. MWCNTs were produced by chemical vapor deposition and were subsequently functionalized with carboxyl groups via oxidation with nitric acid, washed, and dried to obtain MWCNTs-COOH [125].

MWCNTs and TiO₂ nanoparticles were simultaneously incorporated in cement slurries at numerous weight percentages [125]. Mechanical characterization of the composites indicated that the compressive strength ranged from 36.5 to 39.5 MPa, and flexural strength ranged from 7.8 to 10.3 MPa, showing greater scatter for compressive strength. The addition of nano-reinforcing agents significantly increased both the flexural and compressive strength of hardened cement, aligning with prior studies on nano additives that included MWCNTs and nano titania. Among tested specimens compared to the control, the highest increase in compressive strength (32.5%) was observed in the specimen where 1.5% of Portland cement was replaced with titanium nanoparticles, MWCNTs, and surfactants. The highest increase in flexural strength (41.8%) was seen in the specimen where 1.0% of Portland cement was replaced with titania nanoparticles, MWCNTs, and surfactants. This indicates that replacing 1–2% of cement with nanofillers significantly improves mechanical properties [125]. In the results shown in the Pareto charts in Figure 14, the most significant effects on compressive strength were the incorporation of titania nanoparticles and the interaction between titania nanoparticles and CNTs. These graphs showed how compressive strength varied with different combinations of titania nanoparticles, CNTs, and surfactant concentrations. Optimal combinations for maximum compressive strength depended on fixed values of one of these components while varying the others [125].

Another study by Li et al. explored the use of modified MWCNTs to enhance the properties of cement-based composites [126]. The experiment involved the incorporation of pristine MWCNTs (p-CNTs), carboxyl functionalized MWCNTs (c-CNTs), and low-temperature plasma-modified MWCNTs (m-CNTs) into ordinary Portland cement (P.O 42.5R) and standard sand. A polycarboxylate superplasticizer (SP) was used to disperse each of the MWCNT types and ensure proper fluidity of the mixtures. To ensure proper dispersion, MWCNTs were first dispersed in water with SP using ultrasonic vibration for 10 min at 400 W, with intervals to prevent overheating. The mixture was then combined with cement and sand in a standard mixer, poured into molds, compacted, and left to cure for 24 h before removal. Finally, specimens were cured under standard conditions for an additional 27 days before testing [126].

Further analysis of the surface characteristics was conducted using Fourier Transform Infrared Spectroscopy (FTIR) and Raman Spectroscopy of all three types of MWCNTs. The FTIR spectra of m-CNTs revealed characteristic peaks indicating the presence of oxygen-containing functional groups generated during plasma modification. Raman spectra showed an increase in defect size and amount after functionalization (carboxyl or plasma), with m-CNTs exhibiting the highest level of defects. Thermal stability tests showed that p-CNTs were highly stable up to 790 °C, while c-CNTs and m-CNTs experienced weight loss of around 3.6% and 7.9%, respectively, due to the elimination of oxygen-containing functional groups.

MWCNTs were found to have a positive impact on cement matrix strength compared to the reference group, validating the nano-reinforcement effect. Specifically, samples containing 0.10 wt% m-CNTs exhibited the highest flexural strength (9.6 MPa), representing a 37.1% increase over the reference cement sample and a 17.1% increase over c-CNT at the same percentage. Moreover, the highest compressive strength was observed in m-CNT20 (57.2 MPa), showing a 24.3% increase compared to the reference sample. c-CNTs demonstrated similar but weaker reinforcement effects than m-CNTs. The addition of 0.1 wt% c-CNTs significantly enhanced flexural and compressive strength by 15.7% and 16.9%, respectively, as seen in Figure 15 [126].

In samples without any nano-additives, loose stack structures of hydration products with numerous pores and microcracks were observed, facilitating crack initiation and propagation. Cement composites with p-CNTs showed agglomerates of carbon nanotubes, as demonstrated by SEM in Figure 16, acting as micro-defects and adversely affecting mechanical properties. However, in composites with c-CNTs, a crack-bridging mechanism was observed, where c-CNTs effectively hindered crack propagation, leading to improved mechanical behavior.

The above studies collectively demonstrate the substantial potential of nano-reinforcements, particularly CNTs, in enhancing the mechanical properties of cement composites. Through different experimental setups and optimization of material formulations, researchers have successfully showcased significant improvements in tensile strength, flexural strength, and porosity management. The incorporation of such nano-additives not only fortifies the cement matrix but also modifies its microstructural characteristics, leading to reduced porosity and enhanced durability. The findings underscore the effectiveness of nano-reinforcements in construction materials, offering promising avenues for the development of advanced, durable, and more sustainable building materials that meet the rising demands of modern infrastructure challenges.

The conclusions of the composites investigated in this section are summarized in

Table 9. MWCNT-reinforced cement composites. (Summary of the study of Wang et al. [124]).

Composites	Formulation	Output
MWCNTs + Cement (with DMF)	MWCNTs dispersed with DMF, mixed with cement using ultrasonication	Increased compressive strength (77.2 MPa) and flexural strength (11.2 MPa); DMF dispersion improved mechanical properties
MWCNTs + Cement (without DMF)	MWCNTs mixed with cement without DMF	Lower mechanical strength than with DMF; MWCNTs contributed to improved fracture resistance

and

Table 10. Modified MWCNTs in cement composites. (Summary of the study of Li et al. [126]).

Composites	Formulation	Output
Cement + p-CNTs	Pristine MWCNTs dispersed with superplasticizer	Improved strength over reference sample, but p-CNTs showed agglomeration, affecting properties negatively
Cement + c-CNTs	Carboxyl-functionalized MWCNTs dispersed with superplasticizer	Improved flexural (15.7%) and compressive strength (16.9%) through crack-bridging mechanism
Cement + m-CNTs	Plasma-modified MWCNTs dispersed with superplasticizer	Highest flexural strength (37.1% increase) and compressive strength (24.3% increase) due to enhanced dispersion and bridging

.

4. Future Research

As climate change and global warming increasingly pose challenges to modern society, the scientific community is also following suit. This trend is evident in the field of CNT research, which is increasingly focused on the development of sustainable materials and production methodologies. Significant efforts are dedicated to exploring bio-surfactants derived from organic sources that minimize environmental impact. These bio-based surfactants represent a promising avenue for reducing the ecological footprint of nanomaterial production and application, aligning with global sustainability objectives.

A recent study proposes using bio-approach to functionalize CNTs and carbon nanofibers (CNFs) using proteins such as soy protein isolate (SPI) and gelatin [127]. This method aims to enhance dispersibility and material properties. While SPI has shown promising effectiveness in improving carbon nanofiber dispersion, its potential as a bio-surfactant has only been qualitatively assessed so far, typically through optical or microscopic observation. However, there is a lack of quantitative analysis to systematically evaluate its efficiency in functionalizing nanofillers, controlling particle size and distribution, and enhancing the mechanical performance of bulk composites. Some studies have already focused on that aspect [127,128,129,130]. One of these studies by Huang et al. performed a quantitative analysis to measure the impact of soy protein isolate (SPI) as a bio-surfactant. The study found that the nanocomposites with SPI-treated CNTs showed notable improvements in tensile modulus, strength, and fracture toughness by 27%, 24%, and 32%, respectively, at a 1.0 wt% CNT loading. SPI outperforms sodium dodecyl sulfate (SDS) in terms of dispersion efficiency and stability. The SPI-CNTs exhibit less agglomeration and more stable particle size distribution, contributing to the uniform distribution of CNTs within the composite. These results highlight the potential of SPI as an effective and environmentally friendly surfactant for improving the dispersion and overall properties of CNT-reinforced composites, suggesting its suitability for industrial applications in fields requiring high-performance materials [130].

Another study proposes a new method inspired by mussels to enhance the interface between carbon fibers (CFs) and epoxy matrix in carbon fiber-reinforced polymer (CFRP) composites using polydopamine (PDA) and carbon nanotubes [131]. This method increases the roughness and wettability of CF surfaces, improving the bond with the epoxy matrix. The modification with PDA and CNTs increases the composite’s tensile strength by 14.73% and enhances the interfacial and interlaminar shear strength of CF/epoxy composites by 89.72% and 55.44%, respectively. This environmentally friendly method offers a simple way to enhance the properties of CFRP composites [131].

Sadiq et al. explored the development of a cost-effective and efficient pressure sensor using an environmentally friendly approach [117]. The sensor is composed of a conductive film of carbon nanotubes (CNTs), graphene, and Polydimethylsiloxane (PDMS), assembled in a three-layer structure. Detailed analysis using SEM and other techniques confirmed the fine dispersion of CNTs and graphene within the PDMS matrix and the sensor’s effective mechanical and electrical performance. This study demonstrates significant advancements in pressure sensor technology, emphasizing environmental sustainability and the potential for broad application of CNT-based composites in various technological fields [132].

The use of bio-surfactants, like soy protein isolate (SPI), has been shown not only to improve the dispersion of CNTs but also to enhance the mechanical properties of composites, marking a significant step forward in the application of sustainable materials in industrial processes. Furthermore, the adaptation of bio-inspired methods, such as using polydopamine, to improve the interface in carbon fiber-reinforced polymers exemplifies the integration of nature-inspired solutions to enhance material performance sustainably. These developments are complemented by advances in sensor technology, where environmentally friendly materials can be utilized to create efficient pressure sensors with broad application potential. Collectively, these studies demonstrate a critical shift toward more sustainable practices in materials science, aligning with global efforts to address environmental challenges while advancing technological innovation.

5. Conclusions

This review details the significant strides made in the field of nanocomposites reinforced with surface-modified CNTs, emphasizing their importance across various industrial sectors, including 3D printing, automotive, construction, and coatings. The exploration of different composite matrices, such as polymers, metals, and cement, highlights the versatility and wide-range applicability of CNT-enhanced materials. Surface modification of CNTs has proven to be a critical innovation, enhancing compatibility with various matrices and facilitating improvements in mechanical properties through more effective dispersion and, thus, efficient stress transfer. Future research should continue with optimization of the functionalization techniques for CNTs, expansion of applications of bio-based surfactants, such as soy protein isolate, and exploration of the synergy between different nano-reinforcements (hybrid materials) to optimize the properties of nanocomposites. The development of CNT-reinforced composites not only addresses the current demands for high-performance materials but also sets a foundation for future innovations that could revolutionize various industries.