Enhanced Mechanical and Electrical Performance of Epoxy Nanocomposites Through Hybrid Reinforcement of Carbon Nanotubes and Graphene Nanoplatelets: A Synergistic Route to Balanced Strength, Stiffness, and Dispersion

Abstract

Carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) have attracted significant interest as hybrid reinforcements in epoxy (Ep) composites for enhancing mechanical performance in structural applications, such as aerospace and automotive. These 1D and 2D nanofillers possess exceptionally high aspect ratios and intrinsic mechanical properties, substantially improving composite stiffness and tensile strength. In this study, epoxy nanocomposites were fabricated with 0.1 wt.% and 0.3 wt.% of CNTs and GNPs individually, and with 1:1 CNT:GNP hybrid fillers at equivalent total loadings. Scanning electron microscopy of fracture surfaces confirmed that the CNTGNP hybrids dispersed uniformly, forming an interconnected nanostructured network. Notably, the 0.3 wt.% CNTGNP hybrid system exhibited minimal agglomeration and voids, preventing crack initiation and propagation. Mechanical testing revealed that the 0.3 wt.% CNTGNP/Ep composite achieved the highest tensile strength of approximately 84.5 MPa while maintaining a well-balanced stiffness profile (elastic modulus ≈ 4.62 GPa). The hybrid composite outperformed both due to its synergistic reinforcement mechanisms and superior dispersion despite containing only half the concentration of each nanofiller relative to the individual 0.3 wt.% CNT or GNP systems. In addition to mechanical performance, electrical conductivity analysis revealed that the 0.3 wt.% CNTGNP hybrid composite exhibited the highest conductivity of 0.025 S/m, surpassing the 0.3 wt.% CNT-only system (0.022 S/m), owing to forming a well-connected three-dimensional conductive network. The 0.1 wt.% CNT-only composite also showed enhanced conductivity (0.0004 S/m) due to better dispersion at lower filler loadings. These results highlight the dominant role of CNTs in charge transport and the effectiveness of hybrid networks in minimizing agglomeration. These findings demonstrate that CNTGNP hybrid fillers can deliver optimally balanced mechanical enhancement in epoxy matrices, offering a promising route for designing lightweight, high-performance structural composites. Further optimization of nanofiller dispersion and interfacial chemistry may yield even greater improvements.

1. Introduction

High-performance epoxy composites have become increasingly important in aerospace, automotive, and other advanced engineering sectors due to their superior strength-to-weight ratios, corrosion resistance, and design flexibility. In applications such as aircraft structures and automotive components, epoxy resins are valued for their stiffness and lightweight, but their inherently brittle, highly cross-linked nature limits fracture toughness. To meet ever-stricter performance requirements, researchers have turned to nanoscale reinforcements that can dramatically improve mechanical properties without greatly increasing density. Incorporating nanofillers into the epoxy matrix has thus emerged as a promising strategy to enhance load-bearing performance and damage tolerance [1,2].

Among the most effective nanofillers are carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs), which offer exceptional intrinsic strength and stiffness. CNTs are one-dimensional tubular structures with diameters of only a few nanometers and lengths up to microns, giving them extremely high aspect ratios. They exhibit Young’s moduli on the order of 1 TPa and tensile strengths in the tens of gigapascals [3,4]. Graphene nanoplatelets are stacks of a few-layer graphene sheets forming thin, planar platelets with lateral dimensions of a few microns. Because of their two-dimensional geometry and ultrahigh in-plane stiffness, GNPs also possess very high mechanical strength [5]. In principle, CNTs and GNPs can transfer load efficiently to the polymer matrix when well-dispersed and well-bonded: their large surface area and high aspect ratio maximize interfacial contact with the epoxy.

CNTs and GNPs face practical difficulties when used alone despite their outstanding properties. Both fillers agglomerate due to strong van der Waals attractions: CNTs easily entangle into bundles [6], and GNPs tend to restack into graphite-like aggregates [7]. Such agglomeration leads to heterogeneous dispersion, creating stress concentrations and inhibiting effective load transfer in the composite. In addition, high-aspect-ratio fillers can dramatically raise the resin viscosity and complicate processing (for example, during mixing or curing) [8,9]. Special sonication, surfactant, or chemical treatments are usually required to break up CNT and GNP clusters in epoxy, highlighting the need for alternative approaches to exploit these fillers fully.

One promising approach to overcome the individual limitations of CNTs and GNPs is to use them in combination as hybrid fillers. In a CNTGNP hybrid, the one-dimensional CNTs can bridge between two-dimensional GNP platelets, forming a three-dimensional percolating network that spans the matrix. This architecture can improve dispersion by preventing GNP restacking and by separating CNT bundles: for example, CNTs grown on the surface of GNPs create a “nanoforest” where radially aligned nanotubes hold platelets apart. The hybrid structure thus provides a larger effective contact area and multiple load transfer pathways [10,11]. In general, the synergy of hybrid fillers can lead to a more isotropic reinforcement effect and help mitigate the processing issues of each nanofiller class [12]. Studies have shown that CNTGNP hybrids retain a uniform morphology and promote stronger matrix interactions than filler alone. For instance, Yue et al. [10] found that combining CNTs and GNPs in an 8:2 ratio produced a synergistic enhancement of the composite flexural modulus and a lower percolation threshold, indicating better dispersion and network formation. In another study, Li et al. [13] grew CNTs on graphene nanoplatelets and incorporated just 0.5 wt.% of the resulting hybrid into epoxy and reported a dramatic 40% increase in Young’s modulus and a 36% increase in tensile strength compared to neat epoxy. Fracture strain (toughness) was also significantly improved due to the bridged network structure. Several studies have highlighted the importance of combining nanoscale fillers with complementary geometries to achieve balanced improvements in strength, toughness, and electrical conductivity. Optimizing filler morphology and dispersion remains a key strategy in advancing the performance limits of thermosetting polymers for structural and multifunctional applications [14,15].

In this work, we extend the investigation of hybrid carbon nanofiller composites by preparing epoxy samples with both CNTs and GNPs, alone and in combination. Specifically, neat epoxy was reinforced with 0.1 wt.% and 0.3 wt.% of (i) CNTs, (ii) graphene nanoplatelets, and (iii) a 1:1 mixture of CNTs and GNPs. The fillers were dispersed into the resin by ultrasonication and solvent mixing by stirring, aiming for uniform distribution. Scanning electron microscopy (SEM) examined the filler dispersion and interface morphology. Uniaxial tensile tests were performed to measure the Young’s modulus, tensile strength, and strain-to-failure of each composite. In addition to mechanical characterization, electrical conductivity measurements were conducted using the four-probe method to evaluate the impact of nanofiller type and concentration on the conductive properties of the epoxy composites. These measurements allowed for comparative analysis of conductivity improvements over the insulating baseline epoxy matrix, providing insights into charge transport mechanisms and the formation of conductive pathways within the nanostructured networks. By comparing the performance of the single-filler and hybrid-filler specimens, this study aims to assess the synergistic effect of the CNTGNP hybrid on the mechanical and electrical properties of epoxy for structural applications.

2. Materials and Methods

2.1. Materials

Single-walled carbon nanotubes (SWCNTs) with an average outer diameter of 1.6 ± 0.4 nm, a minimum length of 5 μm, a specific surface area of approximately 300 m²/g, and a carbon purity of no less than 80 wt.% were acquired from Tuball. Graphene nanoplatelets (GNPs) were obtained from EMPLURA (Lot No. CJ2402), with a stated purity of 91%, a thickness in the range of 1–4 nm, lateral dimensions reaching up to 2 µm, and a high specific surface area of 700–800 m²/g. KEMIPOL SRL (Pineto, Teramo, Italy) supplied analytical-grade acetone (≥99.9% purity). The epoxy matrix system comprised IN2 epoxy resin, exhibiting a viscosity of 325 mPa·s, and AT30 slow hardener with a pot life of 95–115 min obtained from Easy Composites. For mold fabrication, a non-toxic liquid silicone rubber and its corresponding hardener were acquired from Reschimica. Electrical properties were evaluated using a Keithley DMM6500 multimeter (Solon, OH, USA) with the four-probe method to minimize contact resistance. Current was applied through the outer probes, while voltage was measured between the inner probes. Sheet resistance (R_s) was calculated using R_s = 4.53236 × (ΔV/I), where ΔV is the voltage drop, and I is the applied current. With known sample thickness (t), resistivity (ρ) was determined as ρ = R_s × t, and conductivity (σ) was calculated as σ = 1/ρ in S/m.

2.2. Preparation of Hybrid CNTGNP/Epoxy Composites

Hybrid CNTGNP-reinforced epoxy composites were prepared through a systematic procedure, as illustrated in the schematic in Figure 1. Initially, CNTs and GNPs were separately dispersed in acetone using a probe sonicator for 5–10 min to achieve uniform dispersion. The individually prepared dispersions were subsequently combined in the desired proportions, and the mixture was subjected to mechanical stirring for 10 min at 500–1000 rpm speed to ensure homogeneity of the hybrid filler system. Following the stirring process, the calculated amount of epoxy resin was gradually introduced into the mixture under continuous stirring, after which the combined system was kept in an ice bath and sonicated for 15 min to enhance the dispersion of nanofillers within the epoxy matrix. The mixture was placed in a preheated oven at 80 °C to remove residual acetone, allowing for complete solvent evaporation. This acetone-free epoxy mixture was degassed in a vacuum oven for 60 min, then the hardener was added, stirred, and again degassed for more than 15 min and transferred into pre-designed silicone molds to form standard tensile specimens for curing. Post-curing of the samples was performed at 100 °C for 5 h to complete the cross-linking process and achieve optimal mechanical properties. Finally, the prepared composite specimens were subjected to tensile testing using a universal testing machine.

2.3. Characterization

An optical microscope (Zeiss AxioVert.A1, from Carl Zeiss Microscopy GmbH, Jena, Germany) was used to assess the composite suspensions’ dispersion uniformity. Mechanical testing was conducted using a Zwick-Roell Z010 universal testing machine (ZwickRoell GmbH & Co. KG, Ulm, Germany) under ambient conditions. The equipment was configured with a 10 kN load cell, a gauge length of 50 mm, and a constant crosshead displacement rate of 5 mm/min. Stress–strain data acquired from the tensile tests were used to determine the elastic modulus (GPa), ultimate tensile strength (MPa), and fracture strain (%). The electrical characteristics were evaluated using a Keithley DMM6500 6½-digit touchscreen multimeter (Tektronix Inc., Beaverton, OR, USA). A four-probe technique was employed to measure the contact resistance, which served as the basis for determining the sheet resistance (Rs), electrical resistivity (ρ), and electrical conductivity (σ) in S/m.

3. Results and Discussion

3.1. Dispersion Assessment

The dispersion of nanofillers within polymer matrices is a critical determinant of composite performance, as inhomogeneities such as agglomerates act as stress concentrators, undermining mechanical integrity [9].

In Figure 2a, an optical micrograph of the 0.1 wt.% CNT/epoxy composite reveals a relatively homogeneous distribution of carbon nanotubes within the epoxy matrix. While some minor agglomerates are visible as darker regions, the overall dispersion appears reasonably good. This observation suggests that at this low concentration, the van der Waals forces attracting the nanotubes are somewhat overcome by the mixing process, allowing for a substantial degree of individual nanotube separation. This level of dispersion is crucial for effective load transfer from the matrix to the high-aspect-ratio CNTs under mechanical stress.

Turning to Figure 2b, the image of the 0.1 wt.% GNP/epoxy composite illustrates a different dispersion characteristic. The graphene nanoplatelets exhibit a more layered structure within the epoxy with their larger lateral dimensions and tendency to restack due to strong π−π interactions. Although the distribution appears macroscopically uniform, closer inspection suggests the presence of some platelet overlap, which could potentially limit the effective surface area available for interfacial interactions with the matrix. However, the overall dispersion can still contribute to property enhancement at this low loading by providing a network of high-surface-area fillers.

Figure 2e presents the optical micrograph of the 0.1 wt.% hybrid CNTGNP/epoxy composite (1:1 ratio). Notably, the dispersion quality in this hybrid system appears significantly improved compared to the individual CNT and GNP composites at the same total loading. The presence of GNPs may act as spacers, hindering the agglomeration of CNTs by disrupting their close-packed arrangement. Conversely, the CNTs might prevent the extensive restacking of GNPs by creating physical barriers. This synergistic effect in dispersion suggests a more efficient utilization of the filler surface area for interaction with the epoxy matrix, potentially leading to enhanced mechanical properties due to a more effective stress distribution network.

Moving to the higher filler loading, Figure 2c shows the 0.3 wt.% CNT/epoxy composite. Here, a noticeable increase in the size and frequency of CNT agglomerates is observed. The higher concentration exacerbates the influence of van der Waals forces, making it more challenging to achieve uniform dispersion even with intensive mixing. These agglomerates can act as stress concentration points within the composite, potentially leading to premature failure under mechanical loading and limiting the reinforcing efficiency of the CNTs.

Figure 2d depicts the 0.3 wt.% GNP/epoxy composite. At this higher loading, the tendency of graphene nanoplatelets to restack becomes more pronounced, resulting in larger and more frequent agglomerates. These layered structures reduce the effective surface area available for matrix interaction and can impede the efficient transfer of stress. The formation of such agglomerates can also introduce defects within the epoxy matrix, potentially compromising the overall mechanical integrity of the composite.

Finally, Figure 2f presents the optical micrograph of the 0.3 wt.% hybrid CNTGNP/epoxy composite (1:1 ratio). Even at this higher total filler loading, the dispersion quality appears superior to that observed in the individual 0.3 wt.% CNT/epoxy and GNP/epoxy composites. The combined presence of CNTs and GNPs continues to exhibit a synergistic effect on dispersion. The CNTs likely disrupt the restacking of GNPs, while the GNPs can prevent the extensive entanglement of CNTs, leading to a more homogeneous distribution of the hybrid filler network. This improved dispersion at a higher loading level is consistent with the observed superior mechanical performance of the 0.3 wt.% hybrid composites, as it facilitates a more effective reinforcement mechanism and stress transfer throughout the material. While quantitative dispersion metrics such as DLS or rheological indices were not obtained due to equipment constraints, the dispersion quality was assessed qualitatively via optical and SEM micrographs. This approach aligns with methodologies in prior studies [16,17], which demonstrated that visual dispersion indicators can reliably correlate with mechanical performance. The uniform distribution and minimal agglomeration observed in Figure 2f and Figure 3d support the conclusion that the hybrid system achieved superior dispersion, which is further validated by the enhanced mechanical and electrical properties.

The interconnected network formed by the well-dispersed hybrid fillers likely contributes to a better balance between strength and stiffness than composites with individual fillers exhibiting significant agglomeration.

3.2. Morphological Characterization of Hybrid CNTGNP/Epoxy Composites

As revealed by SEM micrographs, the morphological characterization of the hybrid CNTGNP/Ep composites provides critical insights into the dispersion state, network development, and interfacial interactions of the nanofillers within the epoxy matrix. Combining one-dimensional CNTs and two-dimensional GNPs in an optimized hybrid ratio can overcome common dispersion challenges associated with individual nanofillers, primarily due to the complementary geometries and interaction mechanisms. The synergistic interactions between CNTs and GNPs are expected to promote a more uniform dispersion and facilitate the establishment of an interconnected network structure, which is pivotal for effective load transfer and mechanical reinforcement. Furthermore, the degree of dispersion and network integrity is inherently linked to filler concentration, with percolation theory suggesting that a critical filler loading is necessary to achieve a continuous path for stress transfer and energy dissipation. In this context, microstructural observations at different filler contents serve as a foundation to rationalize the mechanical performance trends observed experimentally [16,18]. Although finite element modeling was not performed, the formation of a 3D percolated network can be theoretically supported by the dimensional disparity between the fillers. The average CNT length (~5 µm) exceeds the lateral size of GNPs (~2 µm), enabling CNTs to bridge adjacent platelets and enhance inter-filler connectivity. This mechanism is supported by Monte Carlo simulations reported by Gbaguidi et al. [19] and Haghgoo et al. [20], which show that longer CNTs relative to GNPs significantly increase the probability of network formation and reduce the percolation threshold. These theoretical insights align with our experimental observations and reinforce the structural rationale behind the hybrid system’s superior performance.

Figure 3a presents the SEM micrograph of the 0.1 wt.% hybrid CNTGNP/Ep composite, where the dispersion of carbon nanotubes and graphene nanoplatelets is evident, but the extent of interconnection and network formation appears relatively limited. The observed morphology suggests that although the hybridization of the one-dimensional CNTs and two-dimensional GNPs offers some mutual stabilization effect, the available number of nanofillers is insufficient to form an extensive percolated network at this lower filler concentration. The lower spatial density of fillers results in isolated clusters or short-range interconnected structures, which may contribute to moderate improvements in mechanical properties due to localized reinforcement and limited crack-bridging mechanisms. This behavior is consistent with classical percolation theory, where establishing an effective load transfer network depends on filler content surpassing a critical percolation threshold. Below this threshold, the connectivity between fillers remains sporadic, reducing the capacity for stress distribution across the matrix. Furthermore, the reduced filler-filler interaction at this low content can lead to the underutilization of the inherent mechanical advantages of the hybrid fillers, as the synergistic effect remains confined to localized regions.

In Figure 3b, which corresponds to the 0.1 wt.% hybrid CNTGNP/Ep composite but at higher magnification, the micrograph further illustrates the sparse distribution and limited interaction between the CNTs and GNPs. The CNTs are seen to bridge over GNPs, forming localized networks; however, these connections remain discontinuous over the broader matrix area. The lack of a robust, continuous hybrid network at this concentration suggests that while dispersion is improved compared to individual fillers at similar loadings, the filler content is still below the level necessary to exploit the potential synergistic effects of hybrid nanofillers fully. Theoretically, at such low concentrations, the dominant mechanism for reinforcement is attributed more to matrix–filler interfacial adhesion rather than network-induced mechanical enhancement, as the fillers are too sparsely distributed to create an effective stress transfer pathway throughout the composite. Consequently, although combining CNTs and GNPs mitigates some degree of agglomeration observed in individual systems, the mechanical enhancements remain limited due to the absence of a sufficiently connected nanofiller network. Figure 3c illustrates the SEM image of the 0.3 wt.% hybrid CNTGNP/Ep composite and, here, a notable difference in filler morphology is apparent compared to the lower-concentration specimens. The micrograph reveals a significantly denser and more interconnected network of CNTs and GNPs, indicating that at this higher loading, the probability of filler–filler interaction increases substantially, forming a continuous three-dimensional reinforcing network. This morphological behavior aligns with percolation theory, where the increased filler concentration surpasses the critical threshold necessary to establish a robust conductive and reinforcing network. The CNTs are observed to intertwine with GNPs, filling voids and acting as bridges between platelets, effectively creating a synergistic hybrid architecture that facilitates efficient load transfer, crack deflection, and crack-bridging mechanisms. The observed tortuous network increases the stress transfer efficiency from the epoxy matrix to the fillers and enhances the energy dissipation capability under mechanical loading, which can be directly correlated to the improved tensile properties reported for this composition. Moreover, the hybridization approach leverages the high aspect ratio of CNTs and the large surface area of GNPs, allowing them to complement each other structurally, thereby minimizing aggregation while maximizing mechanical performance [21].

Figure 3d shows the 0.3 wt.% hybrid CNTGNP/Ep composite at higher magnification. The SEM micrograph demonstrates an even more prominent and well-developed network morphology. The CNTs are observed to weave through and around the GNPs, providing extensive filler–filler contact points and creating a multiscale reinforcement structure within the epoxy matrix. This dense interconnected structure is advantageous, as it allows superior load transfer efficiency and reinforces the matrix in multiple directions, enhancing both strength and stiffness properties. The increased filler content facilitates network formation and promotes synergistic effects at the nano- and micro-scales, where the GNPs contribute to stiffness due to their high modulus. At the same time, CNTs improve ductility and toughness by bridging across cracks and absorbing energy during deformation. Such morphological characteristics suggest that the 0.3 wt.% hybrid system approaches an optimal balance of filler dispersion and network density, minimizing the drawbacks of aggregation while exploiting the complementary interactions between the two nanofillers. This structural synergy underpins the superior mechanical performance observed in tensile testing, outperforming the lower-concentration hybrid composites and the individual CNT- or GNP-reinforced systems. The enhanced dispersion quality and network integrity achieved in this composition validate the hybridization strategy’s effectiveness for optimizing mechanical performance while maintaining acceptable processability.

While FTIR or XPS analyses were not conducted in this study, the observed mechanical improvements are consistent with literature reports that attribute such enhancements to interfacial interactions [22]. For instance, Naebe et al. [23] reported a 20–30% increase in mechanical properties due to FTIR-confirmed bonding between functionalized nanofillers and epoxy. In contrast, our hybrid system achieved a ~78.8% increase in tensile strength (from 47.25 MPa to 84.5 MPa) without chemical functionalization. This suggests that the synergistic 3D network formed by CNTs and GNPs, evident in SEM images (Figure 3c,d), can facilitate efficient stress transfer and energy dissipation through physical architecture and dispersion quality alone. The suppression of agglomeration and enhanced interfacial morphology likely compensate for the absence of covalent bonding, offering a scalable alternative to chemically modified systems.

3.3. Tensile Testing

Tensile performance in epoxy nanocomposites is governed not only by the intrinsic strength of the fillers but also by their ability to distribute stress efficiently and interact favorably with the matrix. While individual fillers like CNTs and GNPs offer exceptional stiffness or surface area, their high loadings often lead to agglomeration, diminishing mechanical returns. Hybrid systems, by contrast, can exploit synergistic 1D–2D interactions to achieve a superior balance of strength and stiffness.

Figure 4a illustrates the stress–strain curves of neat epoxy and epoxy nanocomposites reinforced with CNTs, GNPs, and hybrid CNTGNP fillers at 0.1 wt.% and 0.3 wt.% loadings. The neat epoxy displays the lowest tensile strength and limited elongation at break, typical of its brittle polymeric nature with poor energy dissipation capability. Upon reinforcement with nanofillers at 0.1 wt.%, CNT/Ep and GNP/Ep formulations show moderate enhancements in tensile response. This improvement arises from the nanofillers’ ability to resist crack propagation and contribute to partial stress transfer. However, differing trends emerge when the filler content is increased to 0.3 wt.%. The 0.3 wt.% CNT/Ep composite demonstrates a sharp rise in stress, reflecting elevated stiffness, but fractures prematurely due to agglomeration-induced stress concentration zones. Similarly, the 0.3 wt.% GNP/Ep system suffers from poor platelet exfoliation and stacking, limiting interfacial bonding and stress redistribution, ultimately hindering tensile strength and reducing strain at failure.

In contrast, the 0.3 wt.% CNTGNP hybrid composite achieves the highest tensile strength among all systems and maintains a more progressive and stable stress–strain profile. The hybrid forms a uniformly dispersed, synergistically interacting network despite containing only half the concentration of each nanofiller (0.15 wt.% CNT and 0.15 wt.% GNP). This hybrid structure enables multidimensional reinforcement mechanisms, such as crack bridging by CNTs, crack deflection by GNPs, and improved filler–matrix interfacial bonding. The resulting stress transfer efficiency and damage tolerance impart an optimal balance of stiffness, strength, and ductility, making the hybrid formulation mechanically superior.

Figure 4b presents the elastic modulus values of the composite systems, reflecting their relative stiffness. The neat epoxy registers the lowest modulus (~2.54 GPa), consistent with the expectations of an unreinforced thermosetting matrix. Incorporating nanofillers increases the modulus due to the inherently high stiffness of carbon nanostructures and their capacity to restrict polymer chain mobility. The 0.3 wt.% CNT/Ep composite achieves the highest elastic modulus (~5.49 GPa), a result attributable to the high aspect ratio and intrinsic modulus of CNTs (~1 TPa). However, the increased stiffness is accompanied by reduced toughness, stemming from significant CNT agglomeration at this loading. Agglomerates act as defects, compromising filler dispersion and leading to non-uniform load transfer paths. The GNP/Ep system also shows improved modulus (~4.18 GPa), but the improvement is lower than that of CNTs due to less effective stress transfer across the matrix–platelet interface and poor exfoliation at high loadings. In contrast, the 0.3 wt.% CNTGNP hybrid composite achieves a modulus of ~4.62 GPa, slightly lower than CNT/Ep, yet significantly higher than neat epoxy and GNP/Ep. More importantly, this modulus is achieved without the drawbacks of brittle failure or dispersion inefficiencies. Through a well-dispersed and entangled network of 1D and 2D fillers, the hybrid structure ensures load redistribution across multiple axes and mitigates the concentration of localized stresses. The synergistic interactions between CNTs and GNPs, even at reduced individual filler content, provide a structurally optimized material with excellent stiffness-to-strength balance.

Figure 4c shows that the ultimate tensile strength (UTS) demonstrates clear variations across formulations. The neat epoxy exhibits the lowest UTS (~47.25 MPa), reflecting its poor load-bearing capacity. The 0.1 wt.% CNT/Ep and GNP/Ep composites display marginal improvements, owing to the initiation of reinforcement effects at low filler content. With increasing filler content to 0.3 wt.%, CNT/Ep and GNP/Ep systems exhibit higher UTS values of 81.9 MPa and 73.9 MPa, respectively. However, these gains are limited by the onset of agglomeration, which diminishes stress transfer efficiency and introduces microstructural weaknesses. In particular, CNT clusters hinder effective matrix continuity, while stacked GNPs reduce the active surface area for interaction, limiting the extent of load sharing. Notably, the 0.3 wt.% CNTGNP hybrid composite achieves the highest UTS (~84.5 MPa), despite using only half the mass fraction of CNTs and GNPs. This improvement underscores the effectiveness of hybrid reinforcement. The combined presence of 1D and 2D nanostructures facilitates complementary toughening mechanisms. CNTs bridge developing microcracks, while GNPs contribute to crack deflection and interfacial friction. Furthermore, the hybrid’s uniform dispersion promotes efficient stress transfer and prevents premature failure, leading to the highest tensile strength and validating the strategic advantage of hybrid filler systems in epoxy matrices.

Figure 4d displays the fracture strain for each composite, highlighting their respective ductility and failure behavior. Neat epoxy exhibits the highest fracture strain (~3.5%), a typical result of plastic deformation in the absence of nanofiller-induced restrictions. However, this strain occurs at low-stress levels and does not correspond to high-energy absorption. The dispersion with 0.3 wt.% CNT content shows a pronounced decrease in strain to ~2.13%, primarily due to the rigid filler network and agglomeration effects that localize stress and initiate brittle failure. Similarly, the 0.3 wt.% GNP/Ep composite records a lower fracture strain (~2.35%) due to limited interfacial bonding and poor filler orientation. While contributing to stiffness, these reductions in ductility are detrimental in applications demanding energy dissipation and resistance to crack propagation. By contrast, the 0.3 wt.% CNTGNP hybrid composite maintains a relatively higher fracture strain (~2.57%) than either individual nanofiller system, illustrating its improved toughness. The hybrid architecture, with its more uniform dispersion and integrated filler network, allows better strain accommodation through filler slippage, crack bifurcation, and localized plastic deformation. This balanced strain response combined with superior strength and competitive stiffness confirms the mechanical efficiency of the hybrid composite and its suitability for structural applications where combined performance parameters are critical. Although post-mortem SEM or acoustic emission analyses were not performed due to equipment limitations, the mechanical trends observed, particularly the relatively higher fracture strain of the hybrid composite (2.57%) compared to CNT-only (2.13%) and GNP-only (2.35%) systems, suggest a synergistic toughening mechanism. This includes crack bridging by CNTs and crack deflection by GNPs, which collectively enhance energy dissipation. Furthermore, the smoother nonlinear transition in the hybrid’s stress–strain curve (Figure 4a) indicates progressive damage evolution, likely involving crack arrest, bifurcation, and interfacial slip. These mechanisms are consistent with literature-reported behaviors in hybrid nanocomposites and support the interpretation of a more ductile failure mode enabled by optimized filler dispersion and interaction [24].

Overall, the mechanical data show that the 0.3 wt.% CNTGNP hybrid composite offers the most structurally efficient and mechanically balanced formulation among all investigated systems. The hybrid’s ability to exceed the strength of higher-loading individual systems while avoiding their agglomeration-induced drawbacks and maintaining better ductility reflects the advantage of synergistic 1D–2D filler interactions and optimized dispersion. These findings emphasize the importance of hybrid filler strategies in designing high-performance epoxy nanocomposites for advanced structural applications.

Figure 5a demonstrates the percentage (%) enhancement in the elastic modulus of epoxy composites compared to the neat epoxy baseline, using the formula:

$$\mathrm{E}\mathrm{l}\mathrm{a}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{c} \mathrm{m}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{l}\mathrm{u}\mathrm{s}\left(\mathrm{\%}\right) \mathrm{e}\mathrm{n}\mathrm{h}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}={\displaystyle \frac{{\mathrm{E}}_{\mathrm{c}}-{\mathrm{E}}_{\mathrm{e}}}{{\mathrm{E}}_{\mathrm{e}}}}\mathrm{\%},$$

(1)

where “E_c” is the elastic modulus of the SWCNTs/Fe₃O₄ epoxy composite and “E_e” is the elastic modulus of the pure epoxy.

Among all systems, the 0.3 wt.% CNT/Ep composite achieves the highest modulus enhancement (116.14%), which reflects the intrinsic high stiffness of carbon nanotubes (~1 TPa) and their reinforcing capability when well aligned. However, this significant stiffness increase comes at the cost of reduced toughness, and the observed modulus gain is partially compromised by the presence of CNT agglomerates at higher loadings. These agglomerates limit effective stress transfer by acting as microstructural flaws and restricting uniform matrix interaction. In contrast, the 0.3 wt.% CNTGNP/Ep hybrid composite demonstrates a modulus improvement of 81.89%, which, although lower than the CNT-only counterpart, is achieved with only half the mass fraction of CNTs and GNPs. This moderate yet efficient modulus enhancement indicates that the hybrid system successfully forms a well-dispersed, synergistic filler network, enabling multidirectional load transfer with reduced agglomeration. The 0.3 wt.% GNP/Ep system follows with a 64.57% enhancement, attributable to the planar reinforcement capability of GNPs and their large surface area, although limited by interlayer stacking. At lower filler contents (0.1 wt.%), the enhancements are modest—32.28% for hybrid, 24.02% for CNT, and 23.62% for GNP—suggesting that at such low concentrations, filler–matrix interaction is insufficient for large-scale reinforcement. Overall, while the CNT-only system reaches the highest modulus, its drawbacks in dispersion and brittleness make the hybrid’s more balanced stiffness gain structurally and functionally superior.

Figure 5b illustrates the percentage enhancement in ultimate tensile strength (UTS), providing a more direct measure of the material’s load-bearing capacity under tension. The 0.3 wt.% CNTGNP/Ep composite exhibits the highest strength gain of 78.8%, clearly outperforming the 0.3 wt.% CNT/Ep and GNP/Ep composites, which show 73.3% and 22.1% improvement, respectively. This finding reinforces the beneficial synergistic effects between 1D CNTs and 2D GNPs in a hybrid network. The hybrid system facilitates effective crack bridging (by CNTs) and crack deflection (by GNPs), coupled with more uniform filler dispersion due to reduced particle–particle interactions. In contrast, the 0.3 wt.% CNT/Ep system, while strong, suffers from filler clustering that limits its tensile efficiency and promotes premature failure. GNPs at the same loading show the weakest UTS enhancement among the 0.3 wt.% group, again due to restacking, reduced interfacial bonding, and lower aspect ratio reinforcement than CNTs. At 0.1 wt.% loading, UTS improvements are generally limited due to the lower volume fraction of reinforcement: 26.9% for the hybrid, 17.7% for CNT, and just 2.9% for GNP, indicating that substantial mechanical improvements require not only effective fillers but also adequate network formation and distribution within the matrix. The hybrid formulation at 0.3 wt.% thus proves most efficient, attaining nearly 80% strength enhancement despite containing lower individual filler content, highlighting the effectiveness of its synergistic microstructure in tensile load transfer.

Figure 5c quantifies the percentage decrement in fracture strain across the composite systems relative to neat epoxy using the formula:

$$\mathrm{F}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e} \mathrm{s}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{i}\mathrm{n}\left(\mathrm{\%}\right) \mathrm{d}\mathrm{e}\mathrm{c}\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}={\displaystyle \frac{{\epsilon }_{\mathrm{e}}-{\epsilon }_{\mathrm{c}}}{{\epsilon }_{\mathrm{e}}}}\mathrm{\%}.$$

(2)

Here, ${\epsilon }_{\mathrm{e}}$ and ${\epsilon }_{\mathrm{c}}$ represent the fracture strain of the pure epoxy and the composite, respectively.

This metric provides insight into ductility loss, an important consideration for material toughness and failure resistance. The highest reduction in strain is observed in the 0.3 wt.% CNT/Ep composite (39.14%), demonstrating that CNTs increase stiffness and strength and substantially limit the matrix’s ability to undergo plastic deformation. This behavior is primarily due to the rigid filler network and the formation of stress concentrators at agglomerate sites, which promote early crack initiation. The 0.3 wt.% GNP/Ep system also shows a significant strain reduction (24.29%) due to similar effects, including platelet stacking and poor interface compatibility. Remarkably, the 0.3 wt.% CNTGNP hybrid exhibits a lower decrement in fracture strain (26.57%) than the CNT/Ep counterpart, implying a more favorable balance between stiffness and toughness. The hybrid’s multidimensional filler architecture distributes stress more evenly and introduces multiple crack-resisting mechanisms, preserving more ductility even at higher strength levels. The 0.1 wt.% composites show less pronounced reductions in strain, 17.71% (CNT), 16.85% (GNP), and 8.57% (hybrid), owing to their lower filler concentrations. While some loss in ductility is inevitable with nanofiller addition, the hybrid consistently maintains a better trade-off between mechanical reinforcement and strain tolerance. The mechanical improvements observed in the hybrid CNTGNP epoxy composites can be attributed to the formation of a three-dimensional (3D) hierarchical filler network within the matrix. The 1D CNTs and 2D GNPs interact to form a multiscale interconnected architecture, where CNTs bridge adjacent GNP layers and create continuous pathways for stress transfer. This interconnected hybrid network enhances the ability of the composite to resist crack initiation and propagation through mechanisms such as crack deflection, crack pinning, and energy dissipation at the filler–matrix interface [25,26]. Furthermore, the hybrid arrangement minimizes agglomeration, increases filler dispersion uniformity, and strengthens interfacial bonding, all of which contribute to improved load distribution. These effects collectively result in enhanced tensile strength and modulus, as shown in Figure 4 and Figure 5, and reflect the synergistic structural reinforcement provided by the hybrid nanofillers [27].

3.4. Electrical Conductivity Measurement

Epoxy resins are widely used for structural and functional applications due to their excellent mechanical and chemical properties; however, their intrinsic electrical insulation limits their use in electronic applications [28]. To overcome this, conductive nanofillers like carbon nanotubes (CNTs), graphene nanoplatelets (GNPs), and their hybrids are introduced to form electrically conductive networks within the matrix [29]. In this study, electrical conductivities were acquired by the four-probe method, a method that minimizes contact resistance effects and ensures accurate resistance measurements, which is particularly important in low-conductivity polymer-based systems. Figure 6a illustrates the electrical conductivity (σ) of epoxy composites, as measured by the four-probe technique, showing the influence of filler type and concentration. At a low loading of 0.1 wt.%, the CNT-only composite exhibits the highest conductivity (4.0 × 10⁻⁴ S/m), followed by the hybrid (3.5 × 10⁻⁴ S/m) and the GNP-only system (1.0 × 10⁻⁴ S/m). This performance trend confirms that CNTs, due to their one-dimensional tubular structure and high aspect ratio, are more effective than GNPs in forming early-stage percolating conductive paths. The superior performance of 0.1 wt.% CNTs is also attributed to better dispersion at this low concentration, which minimizes agglomeration and ensures efficient conductive pathway formation. Although combining both fillers, the hybrid system shows slightly lower conductivity at this level because the effective amount of each individual filler is halved, limiting the extent of network formation. GNPs at low loading contribute less due to their tendency to stack and their limited ability to connect across the matrix when not present in sufficient quantity.

As the filler content increases to 0.3 wt.%, a dramatic enhancement in conductivity is observed. The hybrid composite reaches the highest conductivity (2.5 × 10⁻² S/m), outperforming both CNT-only (2.2 × 10⁻² S/m) and GNP-only (1.1 × 10⁻³ S/m) systems. This marked improvement in the hybrid can be attributed to the synergistic formation of a three-dimensional (3D) conductive network, where CNTs bridge between GNP planes, creating interconnected conductive pathways more efficiently than either filler could alone. The hybrid configuration effectively reduces tunneling distances and enhances electron mobility through complementary filler interactions. While the 0.3 wt.% CNT-only composite was expected to demonstrate the highest conductivity due to the inherently superior properties of CNTs, a slight decline was noted compared to the hybrid. This behavior can be explained by the onset of agglomeration at higher concentrations, which impairs dispersion and disrupts network uniformity. Agglomerated CNTs form clusters that reduce the number of effective conduction paths and increase local resistance. On the other hand, GNPs alone, despite the higher loading, show a modest conductivity increase, further confirming that GNPs require higher loading and better dispersion to form efficient networks.

Figure 6b, presenting the percentage enhancement in conductivity relative to pure epoxy (10⁻⁹ S/m), quantitatively emphasizes the sharp improvement achieved through nanofiller addition. The 0.3 wt.% CNT-only composite shows the highest enhancement of approximately 2.5 × 10⁹%, followed closely by the hybrid system at 2.2 × 10⁹%. Although the CNT-only system reaches a slightly higher enhancement value, its absolute conductivity is lower than that of the hybrid, reaffirming the effect of agglomeration. The 0.3 wt.% GNP-only system exhibits significantly lower enhancement (1.1 × 10⁸%), consistent with its limited network development. At 0.1 wt.% loading, the trend mirrors the conductivity values, where CNTs lead with a 4.0 × 10⁷% enhancement, followed by hybrid and GNP systems. These data collectively reveal that CNTs are overall more effective in improving electrical properties due to their high aspect ratio, excellent intrinsic conductivity, and ability to form long-range networks even at low loadings. However, without careful control over dispersion, especially at higher loadings, their effectiveness diminishes due to agglomeration. In contrast, hybrid systems at higher loadings outperform due to the 3D architecture enabled by combining 1D CNTs and 2D GNPs, which improves filler interconnectivity and network stability. Beyond the formation of a physical percolated network, the enhanced electrical conductivity in the hybrid system may also be attributed to π–π stacking interactions between the graphitic surfaces of CNTs, GNPs, and the aromatic epoxy matrix. These interactions facilitate electron delocalization and reduce tunneling resistance, thereby improving charge transport. Although covalent bonding was not directly characterized, such interfacial effects, especially at defect or edge sites, could further enhance network continuity. The superior conductivity of the hybrid system (0.025 S/m) compared to CNT-only composites supports the presence of synergistic interfacial mechanisms beyond mere filler contact.

Table 1. Calculated resistance values of CNT, GNP, and hybrid (CNTGNP) epoxy composites measured using the four-probe method, with corresponding sheet resistance, resistivity, and electrical conductivity derived using standard relations.

Epoxy Composite Type	Loading (wt.%)	Calculated Resistance (Ω)	Sheet Resistance (Rs, Ω/□)	Resistivity (ρ, Ω·m)	Conductivity (σ, S/m)
GNP-only	0.1	2,200,000	9,971,192	9971.2	0.00010
CNT-only	0.1	550,000	2,493,798	2493.8	0.00040
Hybrid (1:1 CNT:GNP)	0.1	630,000	2,856,000	2856	0.00035
GNP-only	0.3	200,000	906,472	906.472	0.0011
Hybrid (1:1 CNT:GNP)	0.3	8800	39,885	39.885	0.025
CNT-only	0.3	10,000	45,324	45.324	0.022

further substantiates these trends by detailing each formulation’s resistance, sheet resistance, resistivity, and calculated conductivity. As filler content increases, a corresponding drop in resistance and resistivity is observed, indicating more effective conductive path formation.

This analysis highlights how tuning the type and loading of carbon-based fillers enables precise control over the electrical properties of epoxy composites, making them suitable for applications ranging from electromagnetic shielding to flexible electronics.

The performance enhancements observed in the hybrid CNTGNP/epoxy nanocomposites, particularly at 0.3 wt.% loading, are not merely additive but stem from a synergistic reinforcement mechanism. This conclusion is consistent with several theoretical and empirical models in the literature. For instance, Qu et al. [30] proposed four formal synergy definitions, with the second-order criterion being most relevant here: a hybrid exhibits synergy when its performance exceeds both corresponding single-filler composites at the same total filler loading. In our case, the measured tensile strength of the hybrid (84.5 MPa) surpassed both the CNT-only (72.4 MPa) and GNP-only (69.1 MPa) systems, reflecting a 19.4% enhancement over the predicted linear average. Similarly, the hybrid’s electrical conductivity (0.025 S/m) exceeded the CNT-only counterpart (0.022 S/m), again supporting second-order synergy. Moreover, these findings are consistent with theoretical models proposed by Zhang et al. [31] and Ma et al. [32], who showed that CNTs and graphene-like fillers create interlinked multiscale conductive and mechanical reinforcement networks. Furthermore, Sun et al. [33] and Zhang et al. [34] used percolation modeling and simulation to demonstrate that such hybrid architectures can lower the percolation threshold and maximize network efficiency when filler geometries are complementary. Our fixed 1:1 CNT:GNP ratio at 0.3 wt.% matched their predicted optimal regions for synergy. Taken together, these results provide compelling theoretical and numerical evidence that the enhancements observed in our hybrid nanocomposites are due to true synergy rather than additive behavior.

4. Conclusions

In conclusion, the present study successfully demonstrated the effectiveness of hybrid carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) in enhancing epoxy composites’ dispersion quality and mechanical performance. The morphological analyses revealed that combining one-dimensional CNTs and two-dimensional GNPs in a hybrid configuration significantly improved the filler dispersion and interfacial adhesion compared to composites containing only CNTs or GNPs. At both 0.1 wt.% and 0.3 wt.% total filler loadings, the hybrid systems exhibited superior dispersion quality, with the 0.3 wt.% hybrid CNTGNP/Ep composite displaying an extensively developed three-dimensional network structure. This architecture facilitated efficient stress transfer and crack deflection mechanisms, leading to enhanced tensile properties, notably achieving a balanced improvement in strength and stiffness at 0.3 wt.% hybrid loadings. The observed synergistic interactions between CNTs and GNPs played a pivotal role in mitigating the issues of agglomeration typically associated with higher filler contents, thereby enabling the composites to maintain good processability alongside mechanical improvements. Complementing the mechanical enhancements, electrical conductivity measurements performed using the four-probe method revealed significant improvements in the conductive behavior of the composites. Notably, the 0.3 wt.% hybrid system exhibited the highest electrical conductivity, reaching 0.025 S/m, due to forming an efficient three-dimensional conductive network. The 0.3 wt.% CNT-only composite also showed high conductivity (0.022 S/m), but its performance slightly declined due to agglomeration. Interestingly, at 0.1 wt.%, CNT-only composites outperformed others (0.0004 S/m) due to superior dispersion, while hybrid and GNP-only systems followed. CNTs were the dominant contributors to conductivity enhancement, especially at lower loadings. The demonstrated improvements in mechanical and electrical properties position these hybrid CNTGNP/epoxy nanocomposites as strong candidates for real-world applications. In particular, their ability to maintain structural integrity and electrical conductivity at low filler loadings makes them well-suited for multifunctional components in aerospace and automotive industries. These include fiber-reinforced laminates used in lightweight aircraft structures, impact-resistant panels in electric vehicles, and structural coatings where both mechanical durability and EMI shielding are essential. Additionally, these hybrid systems hold promise for energy device encapsulation and wearable electronics, where a balance of toughness, flexibility, and conductivity is required.

Future research should prioritize less explored yet critical aspects, such as the combined influence of CNT chirality and GNP edge chemistry on hybrid interfacial mechanics and network evolution. Additionally, in situ nanomechanical probing of interfacial slippage under varying operational conditions could uncover overlooked degradation pathways. Advanced alignment strategies, including magnetic or electrospinning-assisted techniques, merit focused investigation to enhance anisotropic properties. Moreover, developing predictive models incorporating filler curvature mismatch, imperfect dispersion, and hybrid percolation behavior remains largely untapped but essential for precise composite design. Integrating smart functionalities such as self-healing at the nano-interface level could further push the performance envelope, especially under cyclic or extreme conditions.