Issue 78

R. Shivashankar et alii, Fracture and Structural Integrity, 78 (2026) 454-473; DOI: 10.3221/IGF-ESIS.78.26

Effect of Si 3 N 4 content on flexural properties of H/B F-Ep hybrid nanocomposites The flexural strength, flexural modulus and deflection at maximum load of H/B F-Ep hybrid nanocomposites with various Si ₃ N ₄ content of the surface-treated and untreated samples are presented in Tab. 4. The flexural strength of the untreated composite (S0ut) was 182.35 MPa and the modulus of 8.64 GPa, with a maximum deflection of 7.1 mm, whereas after alkali treatment of hemp fibers and silane treatment of basalt fibers, these values were enhanced to 218.44 MPa, 10.12 GPa, and 7.4 mm, respectively, confirming that the two treatments contributed to improving the bonding between the fibers and the matrix, as well as the transfer of stress.

Composites

FS (MPa)

FM (GPa)

Deflection at max Load (mm)

S0 UT S0 T

182.35 218.44 232.78 244.62 258.91 248.17

8.64 10.12 11.03 11.64 12.48 11.82

7.1 7.4 7.2 7.0 6.8 6.5

S1 S2 S3 S4

Table 4: Flexural properties of hybrid nanocomposites However, when Si ₃ N ₄ was added to this, the flexural response was further enhanced up to S3 (1.5 wt.%). The flexural strength of S1, S2 and S3 were found to be 232.78 MPa, 244.62 MPa and 258.91 MPa respectively and the corresponding modulus were found to be 11.03 GPa, 11.64 GPa and 12.48 GPa respectively. S3 showed approximately 42.0% and 44.4% greater flexural strength and flexural modulus when compared with S0ut. The S4 composite incorporating 2 wt. % Si ₃ N ₄ showed a little decrease in flexural properties, as seen by the flexural strength and flexural modulus dropping to 248.17 MPa and 11.82 GPa, respectively. Conversely, the deflection at maximum load gradually decreased from 7.2 mm (S1) to 6.5 mm (S4) as the nanoparticle content rose. S4 had the least amount of deflection, indicating that it was most resistant to bending deformation. Increased agglomeration (under the present processing conditions) was observed at 2 wt. % Si ₃ N ₄ . The critical agglomeration content, however, is not always the same and may depend on the ultrasonication time that may result in better dispersion and enable a higher amount of Si ₃ N ₄ to be incorporated more effectively. Hence these optimum loadings should be viewed in the light of the processing conditions used. Overall, the S3 composite with 1.5 wt. % Si ₃ N ₄ showed the highest flexural strength and flexural modulus, while the S4 composite showed the least amount of deflection at maximum load. The improvement of S0ut to S0t is due to the surface modification to improve the adhesion between the fibers and the matrix. Alkali treatment process cleans hemp fibers of hemicellulose, lignin, waxes and other impurities and causes the fibers to become rougher, because of which there is more mechanical interlocking between the fibers and the epoxy. Heat treatment of basalt fibers by silane increases the chemical bonding with the matrix. These modifications improve stress transfer and reduce fiber pull-out and debonding at the interfaces under flexural load. This further improvement with Si ₃ N ₄ is linked to matrix and interphase reinforcement. The well-dispersed nanoparticles can fill microvoids, enhance interphase and stress distribution. They may also act as crack pinning, crack deflection and crack bridging to delay crack propagation. Therefore, S3 showed the highest strength and modulus, revealing that a nanoparticle dispersion of 1.5 wt.% Si ₃ N ₄ was effective and it was a good reinforcement. The small drop in strength and modulus for S4 indicates that too much Si ₃ N ₄ loading had been applied resulting in loading levels being higher than the optimum under the current processing conditions. Greater attraction between particles may lead to agglomeration, resulting in localized stress and weaker areas, leading to a less efficient transfer of stress. Progressive decrease in deflection with increase in Si ₃ N ₄ content is further proof of the increase in composite stiffness due to the restriction of polymer-chain mobility by the stiff nanoparticles. However, the best loading could be different depending on the processing parameters like ultrasonication duration and efficiency of dispersion. Comparison with recent literature This flexural improvement that was observed in the present H/B F-Ep nanocomposites was in accordance with the recent studies of the interface modification and nanoparticle-reinforced polymer composite. Kiran et al. [26] and Praveena et al. [27] proved that the better the fiber–matrix adhesion the better the stress transfer and the flexural performance. In the same way, Khan et al. [28] claimed that bamboo/epoxy composites showed better flexural properties at optimum amount of ceramic filler loading. Dejene [30] demonstrated that too high loading of nanoparticles can lead to agglomeration and stress concentration localization at specific locations, which negatively affects the reinforcing effect. Effective nanoparticle dispersion and suitable surface modification are crucial for attaining optimal flexural performance, as evidenced by recent studies listed in Tab. 5 [31–34].

462