Tensile Properties
The mechanical characteristics of the composites displayed significant variations with changes in the concentration of additives. The control composite (CF-Ep) recorded a tensile strength of 680 MPa along with a tensile modulus of 19 GPa. An enhancement was observed when 0.75 wt.% of halloysite nanotubes (HNT) was added, resulting in a tensile strength increase to 820 MPa and a modulus rise to 27 GPa, indicating optimal reinforcement. However, as the additive concentration increased, a decline in performance was noted: 1.75 wt.% HNT decreased tensile strength to 650 MPa and modulus to 20 GPa, while a concentration of 2.75 wt.% further deteriorated these values to 540 MPa and 16 GPa, respectively. The improvement at 0.75 wt.% is linked to robust adhesive bonding at the interface between fibers and the matrix, enhancing stress transfer and reducing structural defects. Conversely, the unfilled CF-Ep composite exhibited weaker bonding and more defects, such as voids and fiber pull-out. The addition of HNT effectively addressed these issues, with 0.75 wt.% HNT yielding the best mechanical performance.
Enhanced Tensile Strength of H-CF-Ep Composites
For H-CF-Ep composites, the tensile strength improved to 810 MPa with 0.75% HNT, a nearly 17.90% increase from the unfilled CF-Ep composites, which had a strength of 687 MPa. Furthermore, the tensile moduli of the H0.75% composites were enhanced by 37% compared to the unfilled variants (19.6 GPa). The presence of HNTs facilitated a uniform distribution within the epoxy, promoting effective stress transfer from the epoxy to the carbon fibers. Improved dispersion reduces void content, thus amplifying the surface area available for even load distribution.
EDAX Analysis of Composites
The Energy Dispersive X-ray Analysis (EDAX) of both unfilled and HNT-filled CF-Ep composites highlighted the elemental composition. The characteristic peaks of carbon and oxygen were evident in the CF-Ep composites, confirming their presence. The HNTs, represented by their chemical formula (Al2Si2O5(OH)4nH2O), displayed pronounced peaks for aluminum and silicon, indicating their inclusion in the CF-Ep composites. The total weight percentages of aluminum and silicon in H0.75%, H1.75%, and H2.75% composites were recorded at 0.28%, 0.36%, and 2.51%, respectively, affirming that all H-CF-Ep composites remained within their specified weight limits of 0.75%, 1.75%, and 2.75%. This analysis also confirmed the even distribution of HNTs within the CF-Ep composites achieved during the ultrasonication process. The incorporation of HNTs enhanced the properties of CF-Ep composites primarily due to restricted molecular motion and decreased deformability of the epoxy matrix, along with the organized exfoliation of polymer chains within the interstitial spaces of the HNTs.
Flexural Properties
The flexural strengths of various HNT concentrations (H0.75%, H1.75%, H2.75%) and unfilled CF-Ep composites were assessed. Results indicated that H0.75% composites exhibited superior flexural behavior compared to H1.75%, H2.75%, and unfilled CF-Ep composites. The addition of HNTs improved flexural behavior up to a specific point, with H0.75% showcasing optimal results. However, increasing HNT content beyond this threshold resulted in decreased performance due to defects such as micro-voids, cracks, and subpar bonding. The HNTs contributed to a plasticizing effect, enhancing toughness and effective stress transfer between the carbon fibers and the epoxy matrix. The incorporation of 0.75 wt.% HNT led to a 29% increase in flexural strength and a 34% increase in flexural modulus compared to unfilled CF-Ep composites.
Interlaminar Shear Strength
The interlaminar shear strength (ILSS) of the composites, illustrated in the analysis, displayed noteworthy enhancements with the addition of HNTs. The H0.75%, H1.75%, and H2.75% composites recorded increases of 28%, 14%, and 14%, respectively, compared to the unfilled CF-Ep composites. This improvement is attributed to the uniform distribution of HNTs within the epoxy matrix, which bolstered fiber bonding and load transfer. Previous studies have corroborated similar findings, indicating that the inclusion of nano-fillers enhances ILSS through improved interfacial bonding and reduced void fractions.
Hardness
The Shore D hardness values of both unfilled and HNT-reinforced CF-epoxy composites revealed that the addition of HNTs, which possess a rigid structure, resulted in a significant hardness increase compared to the unfilled variant. The even distribution of HNTs enhances the stiffness of the composite surface, leading to improved resistance to indentation and reduced deformability of the matrix. Notably, the H1.75% composite recorded the highest hardness value, with Shore D hardness rising from 66.2 (CF-Ep) to approximately 79. Conversely, the H2.75% composite demonstrated a decrease in hardness due to the agglomeration of nanoparticles, which began to cluster and negatively impacted the material’s overall homogeneity and load-bearing capacity.
Tensile Fractured Morphology
Scanning Electron Microscopy (SEM) images of the tensile-tested composites indicated that the epoxy matrix was primarily covered with carbon fibers. However, micro-voids and fiber pull-outs were evident across all types, including unfilled, H0.75%, H1.75%, and H2.75% CF-Ep composites. The fractographic analysis identified primary failure mechanisms such as fiber pull-out, fiber breakage, and interfacial debonding. The fractured surfaces revealed evidence of bonding between the fiber and matrix, which could be mechanical or chemical in nature. The SEM analysis showed that HNTs were uniformly distributed in the CF-Ep composites up to a certain content, beyond which weak interfacial interactions and brittle cracking occurred. The analysis also suggested that the smooth surface in HNT-rich areas indicated strong packaging and minimal crack propagation, contributing to improved composite performance.
Differential Scanning Calorimetry
Differential Scanning Calorimetry (DSC) curves of the composites showed minimal exothermic transitions, with significant endothermic peaks associated with curing reactions observed around 342 °C for different composites. The H0.75% composite, in particular, exhibited the lowest endothermic peak, indicating the most efficient curing process. These findings suggest that the enhanced curing contributes to superior mechanical and thermal stability. The degree of curing followed a specific trend, consistently aligning with FTIR, Dynamic Mechanical Analysis (DMA), and Thermogravimetric Analysis (TGA) results.
Fourier Transform Infrared Spectroscopy
FTIR analysis assessed the addition and dispersion of HNT within the epoxy matrix, revealing characteristic peaks associated with functional groups of the epoxy resin. The H0.75% composite demonstrated superior mechanical performance, with significant peaks indicative of HNT presence. Peaks associated with hydroxyl and amine groups were noted, suggesting effective incorporation and interaction of HNT within the polymer matrix. The analysis underscored the successful addition and even distribution of HNTs, with the H0.75% composites exhibiting sharper absorption peaks, indicative of better mechanical behavior and cross-linking density.
Dynamic Mechanical Analysis
The dynamic mechanical analysis indicated the storage modulus of the composites varied with temperature and filler content. The unfilled CF-Ep composite showed an initial storage modulus that decreased with rising temperature. In contrast, HNT-filled composites exhibited a higher storage modulus across all temperatures, particularly for the 0.75% HNT variant, highlighting effective dispersion and strong bonding with the epoxy matrix. The loss modulus (LM) data demonstrated that the H0.75% composite achieved the highest LM values, while the damping coefficient curves indicated increased values for H0.75% compared to the unfilled composite, reflecting the impact of HNT addition on the epoxy matrix.
Thermogravimetric Analysis
Thermogravimetric analysis (TGA) curves illustrated the influence of HNT on the thermal stability of the composites. HNTs, recognized for their thermal barrier properties, were expected to enhance the thermal resistance of the material. The findings revealed that the H0.75% and H1.75% composites demonstrated improved stability, delaying the onset of degradation. However, the H2.75% composite exhibited unexpected results, degrading earlier than its counterparts due to agglomeration effects that compromised the structural integrity of the matrix. This underscores the necessity of optimizing filler content to ensure uniform dispersion and effective matrix-filler interaction for improved thermal durability.
