MXene-reinforced Elium ® composites: A comprehensive study on tensile and flexural strengths,fracture toughness,thermal properties and damage mechanisms

Materials

In this study, high-purity precursors and commercial-grade reagents were carefully selected to ensure consistency and reproducibility in the synthesis and fabrication of MXene/Elium^® nanocomposites.

Ti₃AlC₂ MAX phase (99%+ and 325 mesh) was sourced from Sigma-Aldrich, and hydrofluoric acid (40%) from Merck. Elium^® resin (viscosity: 100 mPa·s and density: 1.02 g·cm⁻³) and benzoyl peroxide, used as the curing agent, were supplied by Arkema Industries. All chemicals were employed as received.

Preparation of multilayer Ti₃C₂T_x

The synthesis of Ti₃C₂T_x was carried out through selective wet chemical etching of Ti₃AlC₂, following a modified methodology from Hajizamani et al. (2023). In this process, 1 g of Ti₃AlC₂ powder was gradually added to 10 mL of 40% HF solution ¹ and continuously stirred at 350 r/min at 35 °C for 24 h. After the reaction was completed, the product was repeatedly washed with ultrapure water using centrifugation at 4000 r/min. The acidic supernatant was removed, followed by the addition of ultrapure water and subsequent repeated centrifugation. The washing process was repeated until the pH of the supernatant exceeded 5. The final products were vacuum-dried at 60 °C for 24 h. The steps of the synthesis process are presented in Figure 1(a).

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Figure 1.

The flowchart of synthesis of two-dimensional transition metal carbides and nitrides (MXene) (a) and preparation of MXene/Elium composite materials (b).

Fabrication of MXene-modified Elium specimens

In this study, different weight percentages of MXene were incorporated into Elium^® resin to fabricate MXene-reinforced nanocomposite specimens. Following earlier reports, five different MXene fractions (0, 0.25, 0.5, 0.75, and 1 wt.%) were incorporated relative to the resin weight, with the neat resin sample considered as the control (Montazeri et al., 2024; Qu et al., 2024; Vijayananth et al., 2025).

The MXene content was limited to a weight ratio of 1.0% for two reasons: (i) higher loadings reported in the literature often lead to significant particle agglomeration, increased viscosity, and processing difficulties in liquid thermoplastic systems (Gao et al., 2020; Liu et al., 2021a; Sheng et al., 2019; Si et al., 2019) and (ii) preliminary trials with higher MXene contents in Elium^® resulted in poor dispersion and visible settling, indicating that a 1.0% weight ratio is a practical upper limit for achieving a reasonably homogeneous microstructure.

For each composition, 50 g of Elium^® resin was weighed and transferred into a beaker. Corresponding amounts of MXene powder were added gradually to the resin according to the targeted weight ratio. The mixtures were first sonicated using a probe-type ultrasonic homogenizer (Bandelin SONOPULS HD 2200, 20 kHz) at 100% amplitude, using a pulsed sequence of 15 s on and 15 s off for a total sonication time of 30 min. Subsequently, the dispersion process was continued by mechanical stirring at 1500 r/min for an additional 30 min to promote homogeneous distribution of MXene within the Elium resin matrix (Wang et al., 2024). To prevent thermal degradation of the resin due to local overheating, the mixing was performed in an ice bath, and intermittent rest periods were used to maintain the resin temperature below 50 °C throughout the process.

After homogenization, the mixtures were degassed in a vacuum oven at 50 °C for 15 min to eliminate entrapped air bubbles formed during sonication. Once degassing was completed, 2 wt.% benzoyl peroxide curing agent was added to the resin mixture as an initiator (Kurdiş et al., 2025). The resin-curing agent mixture was then gently stirred using a mechanical stirrer at 500 r/min, avoiding vortex formation to minimize secondary bubble entrapment.

Prior to casting, stainless-steel molds were preheated to 70 °C and coated with a polyvinyl-based release agent to ensure easy demolding. The preheated molds facilitated improved flow and wetting behavior of the resin during casting, reducing the viscosity mismatch between the mold surface and the resin. This approach helped suppress void formation and promoted more uniform curing at the mold–resin interface.

The modified resins were carefully poured into the preheated molds and left to cure at ambient temperature for 24 h. Subsequently, a secondary curing stage was performed at 100 °C for 8 h to enhance the degree of polymerization and reinforce the mechanical behavior of the produced composites (Şenyurt et al., 2025). Figure 1(b) illustrates the flowchart outlining the production pathway of MXene/Elium composites. After post-curing, the samples were demolded, and the edges were gently polished using 300-grit sandpaper to remove surface defects and minor scratches that could act as stress concentrators during mechanical testing.

Materials characterization

XRD patterns were recorded using a PANalytical EMPYREAN diffractometer with a θ–θ geometry and Cu K_α radiation (λ = 1.5406 Å) generated under an applied voltage of 45 kV and a current of 40 mA. Data were collected in the range of 5° ≤ 2θ ≤ 90° with a step size of 0.026° in continuous scan mode. Fracture surface micrographs of tensile specimens were obtained using an environmental FE-SEM (ZEISS EVO LS10). An excitation voltage of 2 kV was applied in low-vacuum mode (0.8 torr) to suppress charging phenomena. Before scanning, the samples underwent iridium sputtering with a sputter coater from Edwards S150B, which involved a vacuum environment and a plasma exposure period of 60 s. The TGA curves of the MXene/Elium composite were meticulously assessed using a TG-DSC synchronous thermal analyzer (Setaram – Labsys Evo). This analysis was conducted across a comprehensive temperature range extending from ambient conditions to 600 °C, with the sample subjected to a controlled heating rate of 10 °C/min. TGA was conducted to assess changes in weight, thermal stability, and decomposition behavior as a function of temperature.

Tensile experiments were carried out according to ASTM D638 Type IV standard at a crosshead speed of 5 mm/min. The tensile specimens were prepared in the standard Type IV geometry with a gauge length of 25 mm. Flexural tests (ASTM D790) were performed at a crosshead speed of 2 mm/min, while fracture toughness measurements (ASTM D5045) were conducted at 10 mm/min using single-edge-notch bending (SENB) specimens. The SENB specimen dimensions were B = 5 mm, W = 20 mm, and a = 7.8 mm (a/W ≈ 0.39). For each configuration, five specimens were tested. All mechanical tests were conducted under extension control at ambient laboratory conditions (23 ± 2 °C and 50 ± 5% RH) using a Shimadzu AGS-X universal testing machine fitted with a 10-kN load cell. Error bars in the figures represent the standard deviation of the measurements.

Particle size analysis of MXene based on the FE-SEM images

Figure 2 shows the FE-SEM image of MXene particles at 1000× magnification with a 20 µm scale bar (a) and the corresponding particle length distribution obtained from ImageJ analysis (b). The high-resolution micrograph served as the basis for quantitative particle size measurement. ImageJ was calibrated using the 20 µm reference bar, and particle lengths were measured along their major axis. The resulting histogram reveals the predominant size range of MXene flakes, providing an initial quantitative assessment of their morphology.

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Figure 2.

FE-SEM image of MXene particles at 1000× magnification with a 20 µm scale bar (a) and the corresponding particle length distribution obtained by ImageJ analysis (b).

XRD of MXene

Upon detailed examination of the XRD patterns presented in Figure 3, the Ti₃AlC₂ MAX phase exhibited characteristic peaks typical of its layered structure, while the Ti₃C₂T_x MXene displayed a distinct shift in its diffraction profile. The (002) reflection moved from 9.50° in Ti₃AlC₂ to 8.93° after etching, corresponding to an increase in the basal spacing from ∼9.3 to 9.9 Å according to Bragg's law (nλ = 2d sin θ), confirming the removal of the Al layer and the successful formation of Ti₃C₂T_x MXene (Azam et al., 2024; Naguib et al., 2015; Wang et al., 2017). The characteristic aluminum peak at 2θ ≈ 38°, visible in the Ti₃AlC₂ sample, disappeared completely in the MXene pattern (Çelik et al., 2024), further validating the transformation and effective exfoliation during synthesis.

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Figure 3.

XRD patterns of MAX and MXene.

Additional weak peaks at 2θ = 35.94° and 41.73° were attributed to titanium carbide (TiC), consistent with ICDD card 00-032-1383, which lists diffraction peaks at 35.97° and 41.71° (Lotfi et al., 2018). These results collectively confirm the successful synthesis of Ti₃C₂T_x MXene with expanded interlayer spacing and minor TiC residues.

Following the XRD evaluation, Gaussian peak fitting was applied to the major MXene reflections (8.93°, 18.32°, 27.68°, and 60.72°) to determine their full width at half maximum (FWHM). The derived βcosθ and 4sinθ values were used in a Williamson–Hall (W–H) analysis to distinguish size- and strain-induced broadening. The average crystallite size was calculated as 8.78 nm, with a corresponding microstrain of 0.0158, confirming nanoscale crystallinity and measurable lattice distortion within the MXene structure.

These structural features, particularly the shift of the (002) reflection toward lower angles and the disappearance of the Al-related peak, are commonly reported indicators of effective MAX phase etching and the formation of Ti₃C₂T_x MXene in HF-based synthesis routes.

SEM of MXene

The morphology of Ti₃AlC₂ (MAX phase) powder before and after the etching process was studied using FE-SEM. Typical MAX phase characteristics are demonstrated by the compact structure and closely stacked lamellas, as can be seen in the SEM in Figure 4(a). The morphology is indicative of the strong interlayer bonding nature of this layered compound material, Ti₃AlC₂. The SEM image of the Ti₃C₂T_x (MXene), acquired following the etching process, is displayed in Figure 4(b). The image clearly illustrates the distinct separation of layers, indicating successful delamination and the emergence of a more extensive, open structure. This morphology is characteristic of MXenes and arises from the selective removal of aluminum using a hydrofluoric acid-based solution. The high-magnification SEM image shown in Figure 4(c) gives a more accurate view of the MXene structure, which consists of the lamellar and well-patterned manner between the stacking lamellae. Such a complex structure is especially beneficial for a variety of applications due to its large surface area. Figure 4(d) presents the energy-dispersive X-ray spectroscopy (EDX) data for the MXene sample, which reveals the elemental composition. The material composition by EDX was recorded as titanium (57.5%), carbon (14.4%), oxygen (17.4%), and a small amount of aluminum (1.4%). The markedly reduced aluminum signal intensity confirms that the aluminum layers were effectively eliminated during the etching procedure, thus verifying the effectiveness of the etching process. Additionally, the presence of oxygen in the EDX spectrum suggests the existence of surface functional groups (–OH, –F, and –O) that are commonly reported for HF-etched Ti₃C₂T_x MXene. These surface characteristics enhance its dispersion capability by increasing the hydrophilicity of the material (Anasori et al., 2023). This analysis confirms the successful synthesis of MXene from Ti₃AlC₂, resulting in a product characterized by both flexibility and desirable elemental properties.

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Figure 4.

SEM images: (a) MAX phase, (b) low-magnification MXene, (c) high-magnification MXene, and (d) SEM-EDX MXene elemental analysis.

It should be noted that the MXene used in this study corresponds to multilayer Ti₃C₂T_x obtained after etching rather than fully delaminated nanosheets. Therefore, SEM analysis was primarily used to confirm the characteristic lamellar morphology and structural transformation from the MAX phase.

TGA and DSC analysis of MXene-reinforced Elium composite materials

Polymer composites are required to possess superior thermal insulation properties and exhibit resilience to elevated temperatures to minimize degradation during exposure to fire conditions. To evaluate their thermal stability, TGA curves were generated for pure Elium as well as for MXene-reinforced Elium composite materials at varying weight ratios of 0.25, 0.50, 0.75, and 1.00 wt.%, as shown in Figure 5. TGA is a critical analytical method used to identify the thermal decomposition temperature associated with a 10% weight loss, alongside the peak observed in the heat loss curve derived from DSC. These findings are considered a standard parameter for the evaluation of thermal stability in materials.

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Figure 5.

Neat Elium and different reinforced MXene/Elium composites TGA curves.

The TGA curves for the MXene/Elium matrix composites, displayed in Figure 5, reveal a notable trend: the thermal stability of the composites was found to increase with the proportion of MXene filler. This trend underscores the efficacy of incorporating MXene, which is renowned for its outstanding thermal stability, into Elium resins, thereby significantly enhancing the heat resistance characteristics of the developed materials (Wang et al., 2025). The results indicate that integrating MXene into the Elium resin matrix enhances its thermal stability and holds considerable promise for improving heat resistance.

Upon examination of Table 1, it is evident that the incorporation of MXene reinforcement significantly elevated the glass transition temperature (T_g), melting temperature, the temperature at which a 10% weight loss occurs, and the residual mass of Elium resin. A remarkable 39% increase was observed in the 10% weight-loss temperature of neat Elium, which rose from 204 °C to 284 °C. This enhancement indicates that the material exhibits delayed decomposition during exposure to fire and demonstrates improved thermal resistance by raising the decomposition temperature. The temperature at 10% weight loss (T₁₀), widely accepted as the onset of thermal degradation, is often used to evaluate thermal stability, particularly in nanocomposites where barrier effects and restricted chain mobility delay decomposition (Pešić et al., 2025). From a practical perspective, a higher T₁₀ delays matrix softening and degradation under elevated or localized temperatures, helping to preserve load-bearing capability and interfacial integrity during thermo-mechanical loading, which is critical for structural and damage-tolerant composite applications. While the residual mass for neat Elium resin was measured at 0.12%, Table 1 increased to 2.45% with the incorporation of MXene. An analysis of the melting temperatures of MXene-reinforced Elium composite materials with different weight ratios showed a significant increase compared to neat Elium resin.

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Table 1.

TGA/DSC values of MXene-reinforced Elium composite materials.

Sample	Glass transition temperatureRef: DSC, Figure 6Tg (°C)	% 10 weight loss temperatureRef: TGA, Figure 5T10 (°C)	Residual mass at 600 °CRef: TGA, Figure 5(%)
Neat Elium	95	204	0.12
0.25 MXene/Elium	97	258	0.35
0.50 MXene/Elium	99	272	0.85
0.75 MXene/Elium	103	278	1.09
1.00 MXene/Elium	107	284	2.45

TGA: thermogravimetric analysis; DSC: differential scanning calorimetry; MXene: two-dimensional transition metal carbides and nitrides.

As illustrated in Figure 6, the DSC curve for MXene at various weight ratios (0.25, 0.50, 0.75, and 1.00 wt.%) incorporated as a filler in Elium resin is presented. This analysis determines the T_g, offering insights into the material's structural characteristics and macromolecular arrangement. T_g represents the temperature at which a polymer changes from a flexible amorphous phase to a rigid and brittle state (Ekrem, 2021; Ekrem et al., 2024). T_g has been observed to increase in MXene-reinforced Elium composite materials when compared to neat Elium resin. This enhancement is attributed to the MXene, which restricts chain mobility within the polymer matrix and improves heat conduction, resulting in a more rigid structure (Bailey and Winey, 2020). Furthermore, due to the elevated surface area and the presence of functional groups in MXenes, robust interfacial interactions are established with Elium resin. These interactions effectively reduce molecular mobility, contributing to the increase in T_g. Indeed, the incorporation of 2D nano-reinforcement hinders the mobility of polymer chains during their maturation, facilitating greater bond formation among the polymer chains (Idumah, 2023).

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Figure 6.

Neat Elium and different reinforced MXene/Elium composites DSC curves.

The increases in both the glass transition temperature (T_g) and the 10% weight loss temperature (T₁₀) are closely associated with the interfacial interactions between MXene platelets and the Elium^® matrix. Surface terminations on Ti₃C₂T_x MXene (–OH, –O, and –F) enhance interfacial adhesion and restrict the mobility of polymer chains in the interphase region, which contributes to the observed increase in T_g by reducing segmental motion. In addition, the 2D layered structure of MXene acts as an effective thermal and mass transport barrier, delaying the diffusion of volatile degradation products and shifting T₁₀ toward higher temperatures. Similar interfacial- and barrier-controlled mechanisms have been reported for MXene-reinforced thermoplastic systems, where strong MXene–polymer interactions restrict chain mobility and delay thermal degradation, thereby improving thermal stability, in good agreement with the trends observed in the present Elium^®/MXene composites (Sanaka and Sahu, 2024; Shayan Asenjan et al., 2024).

Tensile tests of MXene-reinforced Elium composite materials

Figure 7(a) illustrates the stress–strain behavior of Elium matrix composites reinforced with varying amounts of MXene. The pure Elium matrix (0.00% MXene) demonstrates relatively low tensile strength (53.39 MPa) and limited deformation capacity (∼0.253 mm). Raising the MXene fraction contributed to considerable improvements in the composites’ mechanical properties. Notably, at 0.75 wt.% MXene content, both the tensile strength (69.45 MPa) and deformation capacity (0.31 mm) reach their peak values. This enhancement is attributed to the high surface area of MXene and its effective interfacial interaction with the matrix, which enables efficient stress transfer. However, at 1.00 wt.% MXene content, a decline in performance is observed, potentially due to the agglomeration of MXene within the matrix, which creates stress concentrations. These findings align with previous studies, which also report that moderate MXene content enhances mechanical properties, while excessive amounts can impair performance (Sliozberg et al., 2020; Srinivas et al., 2024). The results indicate that 0.75 wt.% MXene is the optimal concentration for achieving superior mechanical performance, emphasizing the importance of precise control over the MXene content in composites.

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Figure 7.

(a) Tensile stress–strain curves of Elium matrix composites reinforced with varying two-dimensional transition metal carbides and nitrides (MXene) concentrations and (b) variation in maximum tensile strength and strain as a function of MXene content.

Figure 7(b) presents the tensile strength and tensile strain values derived from the stress–strain curves in Figure 7(a), plotted as a function of MXene loading ratio. The tensile strength increases progressively with MXene content, achieving a maximum at 0.75 wt.%, followed by a slight decrease at 1.00 wt.%. This trend confirms the enhanced load transfer capability and mechanical reinforcement provided by MXene at optimal concentrations. The tensile strain, which represents deformation capacity, exhibits a relatively consistent increase up to 0.75%, with a minor reduction at higher MXene content. This reduction is likely caused by the agglomeration of MXene, which disrupts uniform stress distribution. These results highlight the balance between tensile strength and strain at the critical loading ratio of 0.75%, underscoring its significance in optimizing the mechanical performance of Elium matrix composites.

Flexural tests of MXene-reinforced Elium composite materials

Figure 8(a) illustrates the flexural stress–strain curves of Elium matrix composites reinforced with varying concentrations of MXene. The results reveal a notable enhancement in flexural strength with increasing MXene content up to 0.75 wt.%. The neat Elium matrix (0.00 wt.% MXene) exhibits a flexural strength of ∼82 MPa, whereas the composite with 0.75 wt.% MXene achieves a peak strength of 110 MPa, corresponding to an impressive 34% improvement. This increase is credited to the consistent dispersion of MXene within the matrix, facilitating effective load transfer and reinforcing the composite structure. However, at 1.00 wt.% MXene, a slight reduction in flexural strength is observed compared to the 0.75 wt.% sample. This decline is likely due to the agglomeration of MXene at higher concentrations, which introduces stress concentration points and reduces structural integrity. Such behavior aligns with previous findings on the mechanical properties of polymer composites with excessive filler content (Liu et al., 2021b; Prasad et al., 2024). In terms of flexural displacement, the results indicate a decrease as the MXene content exceeds 0.75 wt.%, suggesting a shift towards more brittle behavior. While the neat Elium matrix demonstrates lower ductility, composites reinforced with MXene show improved deformation capacity, particularly at concentrations below 0.50 wt.%. Despite the reduced ductility at higher MXene levels, the overall mechanical performance and load-bearing capacity of the composites are significantly enhanced, with the optimal balance observed at 0.75 wt.% MXene.

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Figure 8.

(a) Flexural stress–strain curves of Elium matrix composites reinforced with varying two-dimensional transition metal carbides and nitrides (MXene) concentrations and (b) changes in maximum flexural strength and displacement as a function of MXene content.

Figure 8(b) presents the relationship between MXene loading ratios and the corresponding maximum flexural strength and displacement, derived from the stress–strain curves in Figure 8(a). The flexural strength exhibits a clear upward trend with the incorporation of MXene, reaching its maximum at 0.75 wt.%. At 1.00 wt.%, the flexural strength plateaus or slightly declines, likely due to structural inhomogeneity and the formation of stress concentration sites caused by particle aggregation. The composite modified with 0.75 wt.% MXene achieves the highest displacement, ∼0.042 mm/mm, compared to 0.031 mm/mm for the neat Elium matrix. In contrast, the displacement for the 1.00 wt.% MXene composite reduces to around 0.04 mm/mm, further highlighting the increased stiffness and brittleness at higher MXene concentrations. This trend reflects the stiffening effect of MXene, which restricts the mobility of polymer chains, thereby reducing ductility while improving mechanical strength (He et al., 2023; Liu et al., 2022) and (Maurya et al., 2023; Song et al., 2024).

Fracture tests of MXene-reinforced Elium composite materials

SENB is conducted through a three-point bending test on a specimen that has been pre-notched. As depicted in Figure 9(a), the load–elongation curve maintains a slope that does not surpass 5%. Therefore, the maximum load, denoted as P_max, is utilized for the calculation of fracture toughness (K_IC). The K_IC values were determined from the P_max load by applying equations that are based on elastic stress analysis relevant to SENB specimens in the scope of the three-point bending test. Five specimens have been tested for each configuration.

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Figure 9.

(a) Load–displacement curves of single-edge-notch bending (SENB) test specimens and (b) variation in maximum K_IC fracture toughness and displacement.

Figure 9(a) demonstrates the load–displacement behavior of pure samples compared to that of samples reinforced with MXene at different concentrations. It is evident that the incorporation of MXene significantly enhances both the crack propagation load capacity and deformation capability of the samples. The pure sample exhibited the lowest load capacity, while the sample with 0.75% MXene content achieved the highest capacity. Notably, the improvement in load-carrying capacity became less pronounced beyond 0.75 wt.% MXene content, suggesting a saturation impact of the reinforcement.

Figure 9(b) highlights the impact of MXene content on the fracture toughness (K_IC) and displacement values. The fracture toughness of the samples increased consistently with higher MXene ratios compared to the pure sample. For instance, the concentration of 0.25 wt.% MXene resulted in an ∼9% improvement in K_IC, while 0.50 wt.% MXene contributed to a 25% improvement. The maximum improvement in fracture toughness, ∼31%, was observed for the sample containing 0.75 wt.% MXene. However, a slight reduction in the improvement trend was noted at 1.00 wt.% MXene, indicating potential agglomeration or saturation effects at higher concentrations.

Damage analysis of MXene-reinforced Elium composite materials

The fracture surfaces of the SENB specimens for neat Elium and the MXene/Elium composites are shown in Figure 10. These images help to connect the mechanical trends (the “Tensile tests of MXene-reinforced Elium composite materials,” “Flexural tests of MXene-reinforced Elium composite materials,” and “Fracture tests of MXene-reinforced Elium composite materials” sections) with the underlying damage mechanisms. Neat Elium (Figure 10(a)) exhibits a predominantly smooth and flat fracture surface with only limited roughness or crack deflection. Such morphology is typical of a relatively brittle, amorphous thermoplastic with low capacity for plastic deformation and energy dissipation, and it is consistent with the lower fracture toughness and flexural strength measured for the unfilled matrix (Beylergil et al., 2025a). In contrast, the composites containing 0.25–0.75 wt.% MXene (Figure 10(b) to (d)) display markedly rougher and more tortuous fracture paths, with clear evidence of crack deflection, branching, blunting, scarps, shear cusps, debris and river-mark regions (Jiang et al., 2024; Seyyed Monfared Zanjani et al., 2020). These features indicate that the propagating crack is repeatedly forced to change direction, branch, and interact with MXene platelets and the surrounding matrix. Each deviation of the crack front increases the effective fracture surface area and promotes additional plastic deformation in the matrix, which together enhance energy absorption. The progressive increase in surface roughness and the abundance of tortuous features correlate well with the improvements in tensile and flexural strength and with the higher fracture toughness values obtained at the intermediate MXene loadings.

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Figure 10.

Damage mechanisms of fractured surfaces of SENB test specimens with 1000× magnification; (a) neat Elium, (b) 0.25 MXene/Elium, (c) 0.50 MXene/Elium, (d) 0.75 MXene/Elium, and (e) 1.00 MXene/Elium.

At 1.00 wt% MXene (Figure 10(e)), the fracture surface still shows crack branching and blunting, but microvoids and locally weakened regions around MXene-rich zones become more apparent. These voids are likely associated with particle agglomeration and imperfect wetting at higher filler contents. Such defects can locally concentrate stresses and provide easier paths for crack advance, which explains why the gain in fracture toughness and strength tends to level off at the highest loading, despite the presence of additional reinforcement. Taken together, the SEM observations indicate that there is an optimal MXene content at which crack deflection and matrix plastic dissipation are maximized without introducing agglomeration-induced defects.

The high-magnification SEM images in Figure 11 provide further insight into how MXene platelets interact with the Elium matrix at different filler contents, and how these interactions relate to the mechanical trends observed in the tensile, flexural, and SENB tests. At 0.75 wt.% MXene (Figure 11(a)), the platelets are well dispersed and show clear evidence of good wetting by the Elium matrix. The matrix flows uniformly around the MXene surfaces, and no interfacial gaps or debonded regions are observed. This morphology is consistent with the tortuous crack paths, crack deflection, and plastic deformation zones identified in Figure 10(b) to (d). The presence of well-anchored MXene layers forces the propagating crack to deviate repeatedly, leading to multi-planar fracture surfaces and enhanced energy absorption. These features match the improvements in tensile and flexural strength and the higher fracture toughness measured at this loading level. For instance, fracture toughness increases by ∼31% at 0.75 wt.% compared to neat Elium, while the corresponding gain at 1.00 wt% is clearly lower (Figures 7 to 9), in line with the transition from well-dispersed to agglomerated MXene morphologies. Similar dispersion-dependent toughening mechanisms have been reported for Ti₃C₂T_x-modified acrylic and epoxy matrices, where well-distributed MXene platelets act as effective barriers to crack growth and promote matrix shear deformation (Hu et al., 2024; Yuksel Yilmaz et al., 2025).

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Figure 11.

Damage mechanisms of fractured surfaces of SENB test specimens with 10,000× magnification; (a) 0.75 MXene/Elium and (b) 1.00 MXene/Elium.

In contrast, at 1.00 wt.% MXene (Figure 11(b)), localized platelet clustering and incomplete wetting become clearly visible. Regions containing densely packed MXene stacks exhibit interfacial voids and micro-gaps, indicating that the matrix cannot fully penetrate or adhere to agglomerated platelet bundles. These poorly wetted regions act as stress concentrators, accelerating crack initiation and enabling more brittle crack advance. This behavior is consistent with the mechanical results, where increasing the MXene content beyond 0.75 wt.% no longer provides additional gains in strength or toughness. Instead, a gradual decline or plateau becomes evident, in line with previous findings for MXene- and graphene-enhanced polymer composites showing that excessive nanofiller content promotes agglomeration, reduces load transfer and introduces premature fracture pathways (Liu et al., 2021b; Stern and Marom, 2024). The presence of pit-like defects around agglomerated MXene clusters in Figures 10(e) and 11(b) further supports this interpretation. These pits indicate localized debonding or insufficient resin infiltration, both of which reduce the composite's capacity to arrest or deflect cracks. When the crack encounters these weak zones, instead of branching or blunting, it advances more directly, lowering the effective fracture energy. In conclusion, the SEM observations confirm that the most effective MXene content is ∼0.75 wt.%, at which the platelets are well integrated into the matrix and actively contribute to crack deflection, shear deformation and energy dissipation. Higher additions disturb this balance and introduce defects that outweigh the potential reinforcement effect.