Epoxy resin is one of the most widely used engineering thermoset polymers due to its good mechanical strength, chemical resistance, and adhesion characteristics. In the present study, a commercial epoxy system consisting of epoxy resin (LY 556) and corresponding hardener (HY 951) was procured from a local supplier in Chennai, India. The epoxy and hardener were used in the recommended weight ratio of 10:1.

Rice husk biochar, a by-product of the rice processing industry, was collected in bulk from a local rice mill in Madurai, Tamil Nadu, India. Prior to composite fabrication, the collected biochar was oven-dried at 105 °C for 8 h to remove absorbed moisture. The dried biochar was subsequently ground using a mechanical grinder and sieved to obtain fine particles. The particle size distribution was characterised using dynamic light scattering (DLS), which indicated an average particle size of approximately 1,099 nm (≈1.10 µm). No chemical surface treatment was applied to evaluate the inherent reinforcing effect of the as-produced biochar. The processed biochar was stored in sealed containers to minimize moisture reabsorption prior to use.

For composite preparation, the required quantity of rice husk biochar was gradually introduced into the hardener and dispersed using manual hand stirring for approximately 20 min to promote uniform particle distribution. The epoxy resin (LY 556) was then added to the biochar-hardener mixture in a weight ratio of 10:1 (epoxy: hardener), followed by further manual stirring for 5–10 min to obtain a homogeneous mixture. The mixture was carefully poured into an open-die mould (Figures 1, 2) and allowed to be cured at room temperature for 8–12 h. No post-curing treatment was applied. The cured plates were subsequently demolded and conditioned under laboratory conditions prior to machining test specimens.

Tensile and flexural tests were assessed using a computerized universal testing machine (Make: Tinius Olsen Model: H25KL) operating in unidirectional loading mode. All tests were performed under quasi-static loading and atmospheric conditions, with a constant crosshead displacement rate of 1 mm/min to ensure uniform strain application. The applied load and corresponding displacement were continuously recorded using the integrated data acquisition system.

Tensile testing was conducted in accordance with ASTM D638 to evaluate the ultimate tensile strength, elongation at break (EAB), and stress-strain behaviour. Standard dog-bone specimens with a gauge length of 50 mm, width of 13 mm, and thickness of approximately 3–4 mm were used.

Flexural testing was performed following ASTM D790 using a three-point bending configuration to assess flexural strength and deformation response. The support span was maintained at 16 times the specimen thickness, and the loading nose radius was 5 mm. All tests were repeated on multiple specimens to ensure reproducibility, and the reported values represent the average of at least three measurements. To evaluate the statistical significance of the observed mechanical property variations, one-way analysis of variance (ANOVA) was performed at a 95% confidence level (p < 0.05). All results are reported as mean ± standard deviation based on three independent specimens for each composition.

The dry sliding wear behaviour of neat epoxy and rice husk biochar-reinforced composites was evaluated using a pin-on-disc tribometer under ambient laboratory conditions. The counterface disc was made of hardened EN31 steel, with a hardness of approximately ∼60 HRC and an average surface roughness of ∼0.2–0.4 µm Ra. The disc had a diameter of 120 mm.

The composite specimens were prepared in pin form with a contact face diameter of ∼10 mm. Prior to testing, both pin and disc surfaces were cleaned with ethanol to remove contaminants. All experiments were conducted under dry sliding conditions at room temperature (∼25 °C ± 2 °C) and relative humidity of ∼50 ± 5%. The following test parameters were maintained for all specimens: normal load of 40 N, sliding speed corresponding to 300 rpm, and a track diameter of 100 mm, resulting in a total sliding distance of 1,500 m. The coefficient of friction (CoF) was continuously recorded, and wear mass loss was determined using a precision analytical balance.

TGA was conducted to evaluate the thermal stability characteristics of neat epoxy and rice husk biochar-added epoxy composites. The samples were heated from room temperature to 600 °C at a constant heating rate of 10 °C/min under a nitrogen atmosphere (flow rate: ∼50 mL/min). The mass loss of the samples was continuously recorded as a function of temperature to determine the onset degradation temperature, decomposition profile, and residual char content. The selected temperature range is sufficient to capture the entire thermal degradation process of epoxy and the stabilising effect of biochar, while the heating rate ensures reliable thermal resolution without thermal lag effects.

The pure epoxy showed a tensile strength of 22.7 MPa, and all the epoxy composites exhibited better tensile strength compared to pure epoxy as shown in Figure 3. The highest tensile strength of 29.2 MPa was obtained when the biochar concentration reached to 9 wt.%, which is 29% higher than the pure epoxy. In addition, the 3 wt.% and 6 wt.% biochar particles reinforced epoxy composite showed 8.5% and 20% improvement compared to pure epoxy, respectively. This increase in tensile strength was apparently due to the addition of the rice husk biochar powder. The mechanism behind this improvement may be that rice husk biochar hinders the mobility of the polymeric chain, helping to create a rigid structure (Zhang et al., 2018). The SEM images of pure epoxy and biochar composites indicated brittle fracture during the tensile test, and the river pattern formation on the fractured surface confirmed the brittle fracture. In addition, the river pattern and flat fractured surface support the brittleness of the pure epoxy. The reinforcement of rice husk biochar slightly reduced the brittle behaviour of the epoxy composites, indicating a shift from brittle fracture to a quasi-ductile nature. At a higher reinforcement level (9 wt.%), rice husk biochar acted as a physical barrier (as shown in Figure 5); consequently, the crack propagation path was interrupted, leading to the formation of microcracks, the disappearance of the river pattern, and the appearance of a long cleavage plane. The elongation at break (EAB) of pure epoxy was 0.8%, and this value steadily increased with the addition of rice husk biochar to the epoxy as shown in Figure 4. The EAB values for 3 wt.%, 6 wt.%, and 9 wt.% rice husk biochar epoxy composites are 0.91%, 1.01%, and 1.31%, respectively. This confirms that rice husk biochar functions as an active toughening agent and load absorber, rather than a brittle filler (Babu et al., 2020). SEM fractured images of the fracture surfaces during the tensile test showed distinct fracture mechanisms (Figure 5). Pure epoxy displayed noticeable river patterns, demonstrating brittle fracture behaviour. With the addition of rice husk biochar, the river patterns gradually disappeared, signifying a transition toward a more energy-absorbing fracture mechanism. This microstructural change correlated with an increase in the tensile strength of the rice husk biochar-reinforced epoxy composites. One-way ANOVA confirmed that the increase in tensile strength with biochar addition is statistically significant (p = 0.002 < 0.05), indicating that the observed enhancement arises from the reinforcing effect of rice husk biochar rather than experimental variability.

The flexural strength of the pure epoxy and different concentrations of rice husk biochar reinforced composites is assessed, and it is found that the pure epoxy showed the lowest flexural strength (corresponding flexural EAB) of 58.0 MPa (2.32%). The epoxy composites with 3 wt.%, 6 wt.% and 9 wt.% rice husk biochar showed the flexural strength (corresponding EAB) of 64.3 MPa (2.41%), 66.6 MPa (2.94%) and 70.1 MPa (2.88%), respectively (refer Figures 6, 7). The reinforcement of rice husk biochar particles considerably enhanced the flexural and deformation behaviour of epoxy composites. The flexural strength increased gradually from 58.0 MPa for neat epoxy to 70.1 MPa at 9 wt.% biochar, indicating effective stress transfer and resistance to bending-induced tensile failure. Simultaneously, the EAB increased steadily from 2.32% to 2.88%, confirming a transition from brittle to quasi-ductile fracture behaviour. This improvement is attributed to strong rice husk biochar and epoxy interfacial interactions, crack deflection, and enhanced energy dissipation, which delay crack origination and suppress unstable cleavage fracture during deformation. During three point bending test, the top and bottom surfaces of the specimen were subjected to compressive and tensile loads, respectively. The better rice husk biochar and epoxy interfacial adhesion enabled the effective load sharing under tensile load conditions and offered strong resistance to deformation on the compression side. The porous physical structure of biochar entangles the epoxy and leads to high energy absorption; as a result, the rice husk biochar reinforced epoxy composites exhibited higher flexural strength compared to pure epoxy. The increase in EAB of epoxy composites revealed an enlarged plastic effect due to the rice husk biochar reinforcement. Similarly, ANOVA results for flexural strength revealed statistically significant differences among the compositions (p = 0.007 < 0.05), confirming the beneficial role of biochar in improving bending performance.

The pin-on-disc wear study on epoxy and epoxy composites showed that increasing the rice husk biochar concentration from 0 to 9 wt.% raises the CoF value from 0.22 (pure epoxy) to 0.29 (9 wt.% rice husk biochar). The highest CoF (0.31) and mass loss (0.76 mg) were observed at 6 wt.% biochar loading, representing increases of 41% and 33.33%, respectively, compared with neat epoxy (Figure 8).

SEM analysis of the worn surface of neat epoxy (Figure 9a) revealed the formation of a relatively smooth and continuous transfer film, indicating mild adhesive wear with limited third-body abrasion. In contrast, the biochar-filled composites (Figures 9b,c) exhibited rougher worn surfaces characterized by micro-ploughing grooves and particle detachment features. The porous and irregular morphology of rice husk biochar promotes mechanical interlocking with the matrix but also makes the particles susceptible to pull-out under sliding stress.

At the intermediate loading of 6 wt.%, the detached biochar fragments become entrapped within the contact interface and act as third-body abrasives, leading to increased micro-ploughing and friction. Therefore, the dominant wear mechanism at this composition is identified as particle pull-out-induced third-body abrasion. At higher filler loading (9 wt.%), the increased particle population at the interface promotes partial stabilization of the tribological layer, which slightly reduces the wear severity compared to the 6 wt.% composite, although the CoF remains higher than that of neat epoxy. This behaviour indicates a transition from transfer-film-dominated wear in neat epoxy to particle-interaction-controlled wear in the biochar-reinforced systems.

Thermogravimetric degradation and its mechanisms can be discussed using Figure 10. At the start of the TGA test, a lower mass loss was observed in pure epoxy between room temperature and 300 °C. The pure epoxy exhibits comparatively gradual and lower mass loss in the 100 °C–300 °C range due to the evaporation of absorbed moisture, release of residual low-molecular-weight species, and early-stage degradation of unreacted epoxy groups. In contrast, rice husk biochar-filled composites show relatively higher mass loss in this region because the fillers used in this study are carbon-based biochar and tend to absorb moisture. The captured moisture was released during the heating of the composite samples and resulted in initial mass loss. In addition, the higher mass loss noticed for the 9 wt.% rice husk biochar composite in this temperature zone is attributed to the thermal decomposition of labile surface functional groups (-OH, -COOH, -C=O) present on biochar, as well as the release of physically adsorbed moisture within its porous network (Kaynak et al., 2025). At higher biochar concentration, the impact of these thermally unstable groups becomes more noticeable, led to increased mass loss at lower temperatures despite improved overall thermal stability at higher temperatures. All samples recorded a pronounced mass loss between 300 °C and 350 °C, corresponding to the primary degradation stage of the epoxy cross-link network. This region is associated with the scission of ether and C-N linkages, breakdown of cross-linked epoxy chains, and rapid evolution of volatile degradation products. The presence of biochar does not prevent this intrinsic chemical degradation of the epoxy backbone but influences the subsequent degradation pathway. In the temperature range of 400 °C–550 °C, the rice husk biochar added epoxy composites exhibits better thermal stability than the neat epoxy. This improvement is attributed to the formation of a thermally stable char layer originating from biochar, which acts as an insulating barrier that slows heat transfer and mass diffusion. Moreover, biochar promotes carbonaceous residue formation and suppresses further chain scission by stabilising free radicals generated during degradation. These mechanisms collectively delay mass loss and enhance high-temperature thermal resistance (Gonzalez-Lopez et al., 2021).