Enhancement of Mechanical and Tribological Properties of MWCNT-Reinforced Bio-Based Epoxy Composites Through Optimization and Molecular Dynamics Simulation

Abstract

This study investigated the enhancement of the mechanical and tribological properties of MWCNT-reinforced bio-based epoxy composites through systematic experiments and analysis. Composites incorporating MWCNTs at varying weight percentages were evaluated for hardness, wear rate, interfacial shear strength, and friction coefficient under diverse load, sliding speed, and distance conditions. An optimal MWCNT content of 0.3–0.4% resulted in a maximum hardness of 4 GPa and a minimum wear rate of 0.0058 mm³/N·m, demonstrating a substantial improvement over the non-reinforced system. FTIR and XRD analyses confirmed robust interfacial bonding between the MWCNTs and epoxy matrix, while molecular dynamics simulations revealed cohesive energy density and stress distribution profiles. The Taguchi optimization identified the MWCNT weight percentage as the most influential parameter, contributing over 85% to wear rate reduction. Contour plots and correlograms further illustrate the parameter interdependencies, emphasizing the role of MWCNT dispersion in enhancing the composite properties. These findings establish that MWCNT-reinforced bio-based epoxy composites are promising candidates for high-performance and sustainable tribological applications.

1. Introduction

1.1. Bio-Based Epoxy Composites

Bio-based epoxy resins made from renewable resources, such as plant oils and lignin, have become an environmentally friendly option for petro-based polymers to overcome their detrimental effects [1,2,3,4]. Bio-based epoxies are meant to be identical to conventional epoxies in terms of both mechanical and chemical qualities, but with a less unfavorable influence on the environment. Bio-based epoxies have very limited applications in high-wear applications, particularly in terms of wear resistance, which restricts the overall quality performance. Bio-based epoxies are increasingly reinforced by nanoparticles to improve their mechanical characteristics, thereby paving the way towards the production of sustainable composites with improved durability for more widespread technical usage [5,6,7].

Bio-based epoxy composites, derived from renewable sources such as lignin and plant oils, offer sustainability benefits, including reduced carbon footprint and biodegradability. However, challenges such as lower thermal stability, moisture absorption, and inferior wear resistance limit their performance. Recent studies highlight that nanofiller reinforcement, such as functionalized CNTs, can enhance mechanical strength, wear resistance, and interfacial bonding, making these composites viable for automotive, aerospace, and biomedical applications [8,9,10].

This study addresses these challenges by optimizing MWCNT-reinforced bio-based epoxy composites by enhancing hardness, wear performance, and interfacial adhesion while maintaining environmental benefits.

1.2. Carbon Nanotube Reinforcement

Owing to their high aspect ratio, exceptional Young’s modulus of approximately 1 TPa, and tensile strength of up to 150 GPa, carbon nanotubes (CNTs) are among the most potent reinforcement methods for polymers [11,12]. Significantly, CNTs have been demonstrated to boost tensile strength, hardness, and wear resistance when utilized as a filler in an epoxy matrix owing to their excellent load transfer mechanisms at the CNT–matrix interface. However, the efficacy of CNT primarily depends on aspects such as dispersion quality, alignment, and interfacial contact with the matrix [13,14].

Several traditional synthetic epoxies exhibit tremendous promise for increasing wear resistance, friction, and structural stiffness with the addition of CNTs. Despite this, the role of CNTs in bio-based epoxies still constitutes a less-explored sector, particularly with regard to wear characteristics [15,16]. In this study, the gap was spanned by evaluating the wear performance of composites reinforced with CNTs in a bio-based epoxy system and analyzing the significance of molecular interactions at the CNT–matrix interface in affecting the wear behavior.

The purpose of this study is to increase the wear characteristics of bio-based epoxy composites using CNT reinforcement. It seeks to investigate the improvement in wear resistance from CNT addition and utilize MD simulations to understand the stress transmission and wear processes at the molecular level. By integrating experimental wear testing with molecular modeling, this work attempts to relate microscopic CNT–matrix interactions to macroscopic wear behavior, revealing insights into how CNTs affect wear processes in sustainable polymer systems.

1.3. CNT-Reinforced Epoxy Composites: Advances in Mechanical and Wear Properties

CNT-reinforced epoxy composites demonstrated a significant increase in tensile strength and stiffness owing to the relatively modest loadings of CNTs. Cha et al. (2017) [17] showed a 25% increase in tensile strength and a 30% improvement in Young’s modulus with just 0.5 wt.% CNT loading in synthetic epoxy, suggesting that CNTs are effective at low concentrations because of their high aspect ratios and surface area. The quick load transfer from the matrix to the CNTs is crucial for resisting tensile stress, thereby delaying fracture. The optimal content of CNT is one of the most crucial aspects that supports wear resistance, which distributes the stresses and decreases surface abrasion. According to Baig et al.’s study in 2018 [18], the inclusion of 0.3 wt.% CNT fillers reduced the wear rate of epoxy composites by more than 60% owing to the high hardness of the CNTs and crack-deflection ability inside the matrix.

Appropriate CNT loading is vital because adding too many CNTs leads to agglomeration, which lowers the reinforcing impact of CNTs. For example, Li et al. (2018) [19] determined the optimal concentration of CNTs to be around 0.2 wt.% in synthetic epoxy when the wear rate reductions and hardness improvements were highest. By improving interfacial bonding with functionalized CNTs to provide efficient reinforcement, there should be good interface bonding to permit the transmission of load from CNTs to the epoxy matrix. Carboxyl (–COOH)– and hydroxyl (–OH)-modified functionalized CNTs provide chemical bonding sites that increase compatibility with epoxy matrices [20].

In 2024, Sabet et al. [21] showed that the interfacial shear strength of carboxyl-functionalized CNTs was enhanced by approximately 40% compared to that of non-functionalized CNTs. Functionalized CNTs create stronger interfacial connections that offer better resistance to wear through the augmented load-carrying capacity of the composite. The homogeneous dispersion of CNTs remains an issue because of the inherent van der Waals forces. Advanced dispersion methods including ultrasonication and high-shear mixing have been employed to overcome agglomeration. Siddiqui et al. (2025) [22] produced a homogenous dispersion of CNTs in synthetic epoxy composites employing protracted sonication, and the findings demonstrated up to a 45% increase in wear resistance compared with non-sonicated composites. These investigations demonstrate that optimum dispersion is crucial, especially for bio-based matrices, as the interactions of CNTs vary considerably compared to synthetic matrices.

1.4. Role of Molecular Dynamics (MD) Simulations in Composite Analysis

Understanding the interfacial properties with MD simulations of atomic-scale interactions between CNTs and polymer matrices has been discovered to provide novel insights into interfacial bonding. For instance, MD simulations led Aklil et al. (2022) [23] and Nurazzi et al. (2021) [24] to forecast the interfacial shear strength of CNT-reinforced synthetic epoxy, and they found that the load transfer efficiency increased for better-bonded interfaces, which further influenced wear resistance. Additionally, through research based on atomic interactions, MD simulations have shown parameters that dictate how the material should behave during wear, affecting the cohesive energy and interfacial shear strength.

1.5. Simulating Wear Mechanisms in CNT Composites: The Wear Process May Be Regulated by MD Simulations

Recent studies have performed pull-out simulations and shear experiments to determine the cohesive energy and interfacial shear strength with notable connections to wear resistance. MD simulations for determining the pull-out forces of CNTs with trends suggest that higher levels of cohesive energy correlate with increased wear resistance due to stronger bonding between the CNTs and the matrix [25,26,27]. These simulations are of particular value to bio-based epoxy systems, where the interfacial characteristics are not yet significantly important for improving composite durability.

1.6. Use of CNT-Modified Bio-Based Epoxy Composites

Bio-based epoxy composites are somewhat ecologically benign; however, in general, they suffer from inferior mechanical and tribological qualities compared with their synthetic equivalents. Thus far, the literature regarding the research of CNT-reinforced bio-based composites is quite limited and on the rise. Hiremath et al. (2023) [28] showed an increase of 35% in tensile strength, along with a 20% improvement in wear resistance, when only 0.3 wt.% CNTs were added to the bio-based epoxy, which implies that CNTs can enable the performance requirements for bio-based composites to match or even exceed those of their synthetic counterparts. As a result, CNT-reinforced bio-based epoxies could provide a promising solution for both sustainability- and durability-demanding applications.

1.7. Dispersion and Interfacial Challenges Overcome in Bio-Based Matrices

Bio-based epoxies often have a poorer chemical affinity for CNTs compared to synthetic resins, and, thus, represent a problem for dispersion and interfacial bonding [15,29]. For bio-based systems, such functionalization and optimum dispersion strategies, which have previously been described as useful in synthetic epoxies, are a prerequisite to achieving similar advantages. Further studies, especially those based on MD simulations, are crucial for quantifying these interfacial properties and to acquire greater knowledge of how CNTs affect the wear behavior of bio-based composites under actual circumstances [30,31].

Several obstacles remain to be overcome in bio-based epoxies, while pioneering efforts have been made to employ CNT reinforcement in synthetic epoxy composites. Most of the early investigations were focused on synthetic epoxy systems and did not provide an appropriate understanding of the impacts of CNTs inside sustainable matrices [32,33].

The present work bridges the gaps with the experimental analysis of the wear performance of CNT-reinforced bio-based epoxy composites, complemented by MD simulations, by examining the interfacial behavior, shear strength, and cohesive energy. This approach provides deep insight into wear mechanisms at the molecular level and is beneficial for the design and development of high-performance eco-friendly composite materials.

This study enhances bio-based epoxy composites by integrating MWCNT reinforcement, molecular dynamics (MD) simulations, and experimental validation. Unlike previous works, it establishes a direct link between nanoscale stress distribution and macroscopic wear behavior, optimizes key parameters using Taguchi design, and improves interfacial bonding for enhanced durability. The focus on sustainable bio-based matrices makes this research highly relevant for advanced composite applications.

2. Micromechanical Modeling for Wear Analysis

The micromechanical modeling of a three-phase Representative Volume Element (RVE) is fundamental for understanding the wear behavior of MWCNT-reinforced bio-based FormuLITE epoxy composites [34,35]. In this model, the composite is conceptualized as a system containing three primary phases: MWCNTs, a polymer matrix (FormuLITE epoxy), and an interphase region. Based on the literature [36,37,38,39], the following equations are derived. The interphase represents the bonding characteristics between the MWCNTs and the matrix, allowing the assessment of the contribution of each phase to the overall wear performance of the composite.

2.1. RVE Definition and Volume Fraction Calculation

The RVE is divided into three distinct regions: MWCNTs, interphase, and FormuLITE matrices. Depending on the length of the MWCNT relative to the RVE, two configurations were defined: the MWCNT spans the entire length of the RVE [40,41,42,43]. Short MWCNTs: MWCNTs lie within the RVE but do not span their entire length. For simplicity, each MWCNT is modeled as a solid cylinder with a radius R_CNT. The volume fractions of each phase within the RVE are calculated as follows:

$$V_{CNT}={\displaystyle \frac{\pi R_{CNT}^2{L}_{CNT}}{a^2{L}_{RVE}}}$$

(1)

$$V_{\mathit{interphase}}={\displaystyle \frac{\pi {\left(R_{CNT}+t_{\mathrm{interphase}}\right)}^2{L}_{CNT}-V_{CNT}}{a^2{L}_{RVE}}}$$

(2)

$$V_{matrix}=1-V_{CNT}-V_{\mathrm{interphase}}$$

(3)

where $R_{CNT}$ is the radius of MWCNT, $L_{CNT}$ is he length of MWCNT, t_interphase is the thickness of the interphase, a is the transverse side length of the RVE, and L_RVE is the length of the RVE. For a long MWCNT, L_RVE = L_CNT. For a short MWCNT, the aspect ratio α (defined as the ratio of MWCNT length to RVE length) dictates the dimensions as follows.

$$\alpha ={\displaystyle \frac{L_{CNT}}{L_{RVE}}}$$

(4)

2.2. Three-Phase Mori–Tanaka Model

The Mori–Tanaka model was applied to evaluate the effective stiffness of the composite, considering the MWCNT, interphase, and FormuLITE matrix phases. The composite’s effective stiffness tensor C_nc can be calculated as follows:

$$C_{nc}=V_{matrix}{C}_{matrix}{A}_{matrix}+V_{\mathrm{interphase}}{C}_{\mathrm{interphase}}{A}_{\mathrm{interphase}}+V_{CNT}{C}_{CNT}{A}_{CNT}$$

(5)

where C_matrix, C_interphase, and C_CNT are the stiffness tensors of the matrix, interphase, and MWCNT, respectively. V_matrix, V_interphase, and V_CNT are the volume fractions of each phase. A_matrix, A_interphase, and A_CNT are the strain concentration tensors. The strain concentration tensor A_phase for each phase is defined as follows:

$$A_{\mathrm{phase}}={\left(I+S_{\mathrm{phase}}\cdot \left(C_{matrix}^{-1}-C_{\mathrm{phase}}^{-1}\right)\right)}^{-1}$$

(6)

where S_phase is the Eshelby tensor and I is the fourth-order identity tensor.

2.3. Strain Concentration Tensors and Eshelby Tensor

The Eshelby tensor S_phase depends on the Poisson’s ratio ν_matrix of the FormuLITE matrix and the inclusion shape. For cylindrical inclusions, the main components of the Eshelby tensor are as follows:

$$S_{11}=S_{22}={\displaystyle \frac{3{\alpha }^2-1}{4\left(1-{\nu }_{matrix}\right)\left({\alpha }^2-1\right)}}$$

(7)

$$S_{33}={\displaystyle \frac{5-4{\nu }_{matrix}}{8\left(1-{\nu }_{matrix}\right)}}$$

(8)

$$S_{12}=S_{13}=S_{23}={\displaystyle \frac{1-2{\nu }_{matrix}}{4(1-{\nu }_{matrix})}}$$

(9)

where $\alpha ={\displaystyle \frac{L_{CNT}}{D_{CNT}}}$ is the aspect ratio of MWCNT.

2.4. RVE-Based Micromechanical Model for Wear Properties

The RVE-based model divides the RVE into regions, with and without MWCNTs, for precise wear property calculations. The average compliance matrix S_RVE is given below.

$$S_{RVE}={\displaystyle \frac{1}{L_{RVE}}}{\int }_0^{L_{RVE}}S\left(x\right)dx$$

(10)

This can be split into two segments. L_c: length with MWCNT reinforcement and L_e: length without MWCNT (matrix only):

$$S_{RVE}={\displaystyle \frac{L_c{S}_{nc}+L_c{S}_{matrix}}{L_{RVE}}}$$

(11)

where $S_{nc}=C_{nc}^{-1}$ is the compliance matrix of the CNT–interphase matrix composite calculated using the Mori–Tanaka model.

2.5. Wear Rate and Hardness Calculation Using the Effective Composite Properties

The wear rate W is estimated using Archard’s wear equation, where wear rate is inversely proportional to the composite hardness $H_{comp}$:

$$W={\displaystyle \frac{K\cdot F\cdot S}{H_{comp}}}$$

(12)

where K is the wear coefficient, F is the applied normal load, and S is the sliding distance. The effective composite hardness H_comp is calculated as a weighted sum of the hardness values of each phase.

$$H_{\mathit{comp}}=V_{\mathit{CNT}}{H}_{\mathit{CNT}}+V_{\mathit{interphase}}{H}_{\mathit{interphase}}+V_{\mathit{matrix}}{H}_{\mathit{matrix}}$$

(13)

Using the properties of FormuLITE, the matrix hardness H_matrix is estimated based on its tensile modulus, while MWCNT hardness and interphase hardness are estimated from the literature values or MD simulations.

2.6. Wear Efficiency Factor Calculation

The wear efficiency factor $\eta w$ evaluates the improvement in wear resistance due to MWCNT reinforcement:

$$\eta w={\displaystyle \frac{W_{\mathit{matrix}}}{W_{\mathit{composite}}}}$$

(14)

where $W_{\mathit{matrix}}$ is the wear rate of the pure FormuLITE epoxy and ${\mathrm{W}}_{\mathrm{composite}}$ is the wear rate of the MWCNT-reinforced composite.

3. Materials and Method

3.1. MWCNTs and Bio-Based Epoxy

The primary materials used in this study were carbon nanotubes (CNTs) and a bio-based epoxy matrix. The properties of the Multi-Walled Carbon Nanotubes (MWCNTs) considered in this study are listed in

Table 1. Properties of MWCNTs.

Property	Value	Unit	Description/Remarks
Typical Diameter	~120	nm	Represents the average diameter of MWCNTs.
Typical Length	Up to 1	mm	Defines the aspect ratio for reinforcement.
Aspect Ratio	~4000	-	Ratio of length to diameter (L/D).
Elastic Modulus	~900	GPa	Indicates high stiffness.
Tensile Strength	~45	GPa	Represents the maximum stress MWCNTs can endure.
Thermal Conductivity	~2500	W/(m·K)	Extremely high, making MWCNTs excellent thermal conductors.
Electrical Conductivity	~106	S/m	Suitable for electrical applications.
Density	~1.3–1.4	g/cm3	Low density for lightweight applications.
Hardness	~20	GPa	Extremely hard, contributing to wear resistance.
Specific Surface Area	~200–400	m2/g	High surface area aids in functionalization and bonding.
Thermal Stability	Up to 700	°C	Stable under high-temperature conditions.
Purity	~95	%	Typical purity level after synthesis.

. These MWCNTs were functionalized with carboxylic groups (–COOH) to enhance the dispersion and interfacial bonding with the epoxy matrix. The MWCNTs were dispersed in N,N-Dimethylformamide (DMF), a high-polarity solvent known for effectively stabilizing CNT dispersions and preventing agglomeration. The bio-based epoxy system FormuLITE was selected for its high bio-content, low viscosity, and mechanical durability. The epoxy resin and amine hardener, synthesized from cashew nut shell liquid (CNSL), were mixed at a ratio of 100:30 by weight. The mix viscosity at 25 °C was 700 cPs and the pot life was approximately 105 min.

Bio-based epoxy resin and hardener: FormuLITE 2500A and FormuLITE 2401B were sourced from Cardolite Specialty Chemicals India LLP, Mangalore, India. MWCNTs were obtained from Sigma-Aldrich (St. Louis, MO, USA) (sigmaaldrich.com, accessed on 13 February 2024). The dispersion solvent, N,N-Dimethylformamide (DMF), was procured from Emco Dyestuff Pvt. Ltd. Mumbai, India (emcochemicals.com, accessed on 13 February 2024), ensuring effective nanotube stabilization and uniform distribution in the epoxy matrix.

In this study, FormuLITE, a bio-based epoxy, was employed as the principal material. FormuLITE is an amine-cured epoxy formulation intended for high-performance resin matrices sourced from renewable sources. Both the resin and amine hardener are derived from CNSL (cashew nut shell liquid), a sustainable resource. This formulation has several advantages, including low viscosity; effective wetting qualities for diverse reinforcements; prolonged pot life; balanced mechanical characteristics; and resistance to heat, water absorption, acids, and alkalis. FormuLITE epoxy solutions are engineered for medium-to-large composite component producers and are compatible with methods such as wet lay-up, resin transfer molding (RTM), lamination, and vacuum infusion. Their intuitive design makes them appropriate for DIY enthusiasts to pursue professional outcomes.

Table 2. Properties of bio-based epoxy considered for this study.

Parameter	FormuLITE
Calculated bio-content	36.6
Mix ratio by weight	100:30
Mix ratio by volume	100:36
Mix viscosity at 25 °C (cPs)	700
Mix viscosity at 40 °C (cPs)	242
Pot life at 25 °C (min)	105
Pot life at 40 °C (min)	57
Tg (°C)	92
Tensile strength (MPa)	62
Tensile modulus (MPa)	2615
Elongation at Fmax (%)/Elongation at break (%)	4.8/6.4
Flexural strength (MPa)	92
Flexural modulus (MPa)	2262

shows the fundamental characteristics of the bio-based epoxy used in this study.

3.2. Sample Preparation

MWCNTs were dispersed in the bio-based epoxy matrix using a combination of mechanical and ultrasonic methods to ensure uniform distribution. Initially, the MWCNTs were sonicated in a suitable solvent for 30 min at an amplitude of 40%, using a high-power ultrasonic processor. The epoxy resin was added to the dispersed MWCNT solution, and the mixture was further homogenized at speeds ranging from 500 to 1200 rpm for 15 min. Degassing was performed in a vacuum chamber for 10 min to remove air bubbles. The hardener was then added and the final mixture was magnetically stirred for another 10 min. The prepared mixture was poured into molds and cured at room temperature for 24 h, followed by post-curing at 60 °C for 3 h.

3.3. Molecular Dynamics Simulation

Molecular dynamics (MD) simulations were conducted to investigate the cohesive energy density and stress distribution across the CNT–matrix interface. Simulations were performed using the LAMMPS software package with COMPASS force fields to model the interactions between the epoxy matrix and the functionalized CNTs. A simulation box containing a single MWCNT embedded in an epoxy matrix was created with periodic boundary conditions applied in all directions. The interfacial shear strength was calculated by gradually applying tensile stress along the MWCNT axis, while monitoring the force required to displace the CNT. The cohesive energy density was derived from the potential energy per unit volume. Heatmaps illustrating the stress distribution were generated using post-processing tools such as Ovito.

3.4. Nanoindentation Analysis

The hardness and modulus of the CNT-epoxy nanocomposites were evaluated using nanoindentation tests. A Berkovich nanoindenter was used to measure the depth-dependent hardness values of the samples. The tests were conducted under a maximum load of 500 mN with a loading and unloading rate of 50 mN/s. The hardness values were calculated using the Oliver–Pharr method, and the results were plotted as hardness versus indentation depth curves.

3.5. Wear Testing

The wear properties of the nanocomposites were assessed by using a pin-on-disc tribometer. Cylindrical composite pins with diameters of 10 mm were tested against a hardened steel disc under varying loads (5–15 N) and sliding speeds (200–600 rpm). The total sliding distance was set to 1 km for all the tests. The wear rate was calculated using Archard’s equation, which relates the wear volume to the applied load and the sliding distance. A post-test analysis of the worn surfaces was performed using Scanning Electron Microscopy (SEM) to identify wear mechanisms such as abrasion, adhesion, or delamination. To ensure repeatability and statistical reliability, each experiment was conducted five times, and the reported values represent the mean of these trials.

3.6. XRD, FTIR, and AFM Analysis

The structural characteristics of the MWCNT epoxy composites were analyzed using X-ray diffraction (XRD) with a Cu Kα radiation source (λ = 1.5406 Å) on a Rigaku SmartLab X-ray diffractometer. Scans were performed over a 2θ range of 10–80° with a step size of 0.02°. The epoxy matrix exhibited an amorphous broad peak, while the MWCNTs showed crystalline reflections corresponding to graphitic carbon. The functional groups in the functionalized MWCNTs and epoxy matrix were characterized using Fourier-transform infrared (FTIR) spectroscopy. The FTIR analysis was conducted on a Bruker Alpha II FTIR spectrometer (Bruker Corporation, Billerica, MA, USA). in the attenuated total reflectance (ATR) mode. Spectra were recorded in the 4000–400 cm⁻¹ range with a resolution of 4 cm⁻¹.

Atomic Force Microscopy (AFM) was used to evaluate the surface morphology and roughness of the MWCNT-reinforced bio-based epoxy composites. Specimens were prepared by sectioning them into 10 mm × 10 mm pieces, followed by gentle polishing with fine-grit sandpaper and ultrasonic cleaning in isopropanol for 10 min. AFM analysis was performed using a Bruker Dimension Icon AFM in tapping mode setup with a silicon nitride tip (radius of ~10 nm) to capture high-resolution height and phase images. Scans were conducted over areas of 5 μm × 5 μm and 10 μm × 10 μm at a resolution of 512 × 512 pixels. Surface roughness parameters such as average roughness (Ra), root mean square roughness (Rq), and maximum peak-to-valley height (Rz) were determined using NanoScope Analysis software (Version 1.9). The AFM results provided insights into the dispersion and interfacial bonding of MWCNTs within the epoxy matrix, which were correlated with the wear properties of the composites.

3.7. Taguchi Design for Wear Analysis

Table 3. Taguchi-based optimization of wear behavior in CNT-reinforced composites.

Exp. No.	Load (N)	Sliding Speed (m/s)	CNT Volume Fract. (wt.%)	Sliding Distance (m)	Hardness (GPa)	Interfacial Shear Strength (MPa)	Wear Rate (mm3/N·m)	Friction Coefficient (μ)
1	10	0.5	0.1	500	2.2	90	0.01	0.36
2	10	1	0.2	1000	2.5	100	0.0088	0.34
3	10	1.5	0.3	1500	3	120	0.0075	0.32
4	10	2	0.4	2000	3.5	140	0.0065	0.3
5	20	0.5	0.2	1500	2.6	110	0.008	0.33
6	20	1	0.3	2000	3.2	130	0.007	0.31
7	20	1.5	0.4	500	3.8	150	0.006	0.28
8	20	2	0.1	1000	2.3	95	0.0092	0.35
9	30	0.5	0.3	2000	3.4	135	0.0068	0.29
10	30	1	0.4	500	3.9	155	0.0062	0.27
11	30	1.5	0.1	1000	2.4	100	0.009	0.35
12	30	2	0.2	1500	2.9	115	0.0078	0.32
13	40	0.5	0.4	1000	4	160	0.0058	0.26
14	40	1	0.1	1500	2.8	110	0.0085	0.34
15	40	1.5	0.2	2000	3.1	125	0.007	0.3
16	40	2	0.3	500	3.7	145	0.0065	0.29

lists the Taguchi design of the experiments for the wear tests. The Taguchi L16 orthogonal array design was employed to evaluate the effects of the four critical input factors of load (F), sliding speed (v), MWCNT volume fraction (V_CNT), and sliding distance (S) on the wear behavior of the composite. Each factor was varied across four levels to systematically assess their individual and interactive influences on the output parameters: composite hardness (H_comp), interfacial shear strength (τ_ISS), wear rate (W), and friction coefficient (μ).

Wear tests were conducted using a pin-on-disc tribometer under dry conditions. A cylindrical pin made of a CNT-reinforced bio-based epoxy composite was tested against a hardened steel disc. Prior to each test, the pin and disc surfaces were cleaned with acetone to ensure consistent results. During the test, the applied load, sliding speed, and sliding distance were monitored and controlled based on the experimental design. The wear rate (W) was calculated using Archard’s law, while the friction coefficient (μ) was recorded directly by the tribometer. The output parameters, including H_comp and τ_ISS, were derived using micromechanical modeling and molecular dynamics simulations. The combination of experimental and modeling approaches ensured a comprehensive understanding of the wear behavior, providing reliable data for analysis.

4. Results and Discussion

4.1. XRD and FTIR Analysis

The FTIR spectra of Multi-Walled Carbon Nanotubes (MWCNTs) and bio-based epoxy composites (Figure 1) provide critical insights into their individual chemical structures and interfacial interactions. The MWCNT spectrum shows characteristic peaks, including a prominent band at 1718 cm⁻¹ corresponding to carbonyl (C=O) stretching, which confirms the presence of carboxylic functional groups introduced during functionalization. These groups play a vital role in enhancing the dispersion and compatibility of the MWCNTs within the epoxy matrix. Additionally, a broad band at 3430 cm⁻¹ was attributed to O–H stretching, indicating hydroxyl groups on the MWCNT surface that promote hydrophilicity and interfacial bonding. The region between 1680 and 1600 cm⁻¹ reflects C=C stretching, suggesting sidewall defects on the MWCNTs that further enhance the chemical reactivity and bonding potential. Peaks in the range of 1250–950 cm⁻¹, attributed to C–O stretching, confirm the presence of oxygen-containing functional groups that contribute to improved adhesion with the epoxy matrix [44,45].

The bio-based epoxy spectrum revealed complementary chemical features, such as a strong peak at 2960 cm⁻¹ corresponding to C–H stretching, which is indicative of the aliphatic chains in the epoxy resin that provide flexibility and mechanical integrity. The 1250–1000 cm⁻¹ region exhibited peaks associated with ether (C–O–C) functional groups, which are critical for polymerization and curing. A broad absorption band around 3430 cm⁻¹, overlapping with the O–H stretching region of MWCNTs, signifies hydrogen bonding between the epoxy matrix and nanotubes. Minor peaks near 1700–1600 cm⁻¹ indicate residual C=C bonds or defects, which could influence the thermal and mechanical properties of the composite.

The overlapping features in the FTIR spectra of the MWCNTs and bio-based epoxy confirmed the strong interfacial interactions. The carbonyl (C=O) and hydroxyl (O–H) groups on the MWCNTs facilitated hydrogen bonding and chemical reactions with the epoxy functional groups, particularly the ether (C–O–C) and hydroxyl groups [46]. The increased intensity of the peaks in the 1250–950 cm⁻¹ region in the composite spectrum, compared to the individual components, indicates the formation of new C–O bonds due to interfacial reactions. The broad O–H band around 3430 cm⁻¹ reflects additional hydrogen bonding, strengthening the interphase and improving the load transfer.

The FTIR spectrum of the MWCNT-reinforced bio-based epoxy composite exhibits key spectral changes that highlight the interfacial interactions and chemical bonding between the two constituents. The O–H stretching band observed at ~3430 cm⁻¹, which appears in both pure epoxy and MWCNT spectra, is intensified in the composite. This increase indicates enhanced hydrogen bonding between the hydroxyl groups present on the MWCNT surfaces and those in the epoxy matrix, improving interfacial adhesion. The C–H stretching peak at ~2960 cm⁻¹ remains unchanged in the composite, confirming the structural integrity of aliphatic chains, which contribute to polymer flexibility and mechanical stability. The C=O stretching at ~1718 cm⁻¹, originally from functionalized MWCNTs, remains present in the composite spectrum, validating the presence of carboxyl functional groups that aid in nanotube dispersion and interaction with the epoxy network. Minor C=C stretching peaks between 1680 and 1600 cm⁻¹ suggest the presence of residual unsaturated carbon bonds, potentially originating from epoxy polymerization or nanotube surface defects. Additionally, the C–O–C and C–O stretching region between 1250 and 950 cm⁻¹ shows increased intensity in the composite, signifying new C–O bond formation due to the chemical interaction between MWCNTs and the epoxy matrix. These observed changes in the FTIR spectra confirm the effective incorporation of MWCNTs into the bio-based epoxy resin, enhancing the chemical stability, mechanical properties, and interfacial bonding of the composite.

These interactions enhance the mechanical performance of the composite by ensuring the better dispersion of functionalized MWCNTs, minimizing agglomeration, and promoting uniform stress distribution. Strong interfacial adhesion, confirmed by FTIR analysis, improved the hardness, wear resistance, and interfacial shear strength. Structural defects in MWCNTs further enhance their anchoring ability, validating their effectiveness in reinforcing bio-based epoxy composites and advancing sustainable high-performance materials.

The XRD patterns of the MWCNTs, bio-based epoxy, and their composite (Figure 2) further validate the structural integrity and interfacial interactions observed in the FTIR analysis. The MWCNT pattern exhibits a sharp peak at 26° (2θ), corresponding to the (002) plane of graphitic carbon, indicative of a high degree of crystallinity. The smaller peak at 43° (2θ) reflects the (100) in-plane crystalline order, confirming the structural integrity of the nanotubes [47]. In contrast, the bio-based epoxy showed a broad peak in the range of 20–25° (2θ), characteristic of its amorphous nature.

In the composite XRD pattern, the distinct MWCNT peak at 26° (2θ) was retained, albeit with slightly reduced intensity, indicating the successful incorporation of the nanotubes into the epoxy matrix. The broad amorphous peak of the epoxy was also evident, confirming that the matrix maintained its disordered structure. The slight broadening and reduced intensity of the MWCNT peak suggests partial dispersion and interfacial bonding, indicative of the effective integration of MWCNTs into the matrix.

4.2. Analysis of Mechanical and Interfacial Properties of CNT Bio-Based Epoxy

Figure 3 shows the cohesive energy density (J/m²), which is a critical parameter that quantifies the interaction strength between the MWCNT and the surrounding epoxy matrix. The curve exhibits a monotonically decreasing trend along the MWCNT length, starting from 0.8 J/m² at the MWCNT center to approximately 0.1 J/m² at the MWCNT ends. This gradient suggests that the central region of the MWCNT maintains stronger interfacial adhesion due to uniform load transfer and reduced edge effects. The lower values near the MWCNT boundaries could result from stress concentrations or partial load-bearing contributions. These observations emphasize that the interfacial bonding strength is highly localized and strongly influenced by the CNT’s aspect ratio and dispersion within the matrix.

The heatmap in Figure 4 reveals a non-uniform stress distribution along the MWCNT length and across its diameter, with stress peaking at the center of the MWCNT at approximately 1 MPa and symmetrically declining toward the ends. This distribution aligns with Eshelby’s inclusion model, which predicts the maximum stress at the center of the cylindrical inclusions under axial loading. The stress concentration factor (σ_max/σ_avg) at the CNT’s center is calculated to be 2.5, indicating a significant amplification of localized stresses. This stress localization enhances load transfer mechanisms, ensuring the effective utilization of MWCNT reinforcement. However, the reduction in stress transfer at the edges, where stress drops below 0.2 MPa, underscores the importance of optimizing MWCNT alignment and interphase bonding to maximize load-bearing capacity along the entire MWCNT length.

Figure 5a–e depicts the stress distribution profiles across the CNT–matrix interface for five distinct cases modeled using molecular dynamics simulations. These cases highlight the effects of MWCNT functionalization, interfacial bonding strength, and MWCNT volume fraction on the mechanical behavior of the composite. In Figure 5a, representing Case 1 (pristine MWCNT in the bio–epoxy matrix), the stress is localized at the midpoint of the MWCNT and decreases symmetrically toward its ends. This behavior aligns with Eshelby’s inclusion model, which predicts the maximum stress concentration at the center of the cylindrical inclusions. However, the absence of functionalization limits the interfacial adhesion, resulting in suboptimal stress transfer between the MWCNT and the matrix. Figure 5b, corresponding to Case 2 (functionalized MWCNT with weak interfacial bonding), shows a slight improvement in stress transfer efficiency compared to Case 1. The stress peaks were higher, indicating some enhancement in the interfacial adhesion owing to functionalization. However, weak interfacial bonding prevents the stress from being uniformly distributed along the MWCNT length, suggesting that stronger bonding conditions are required for effective reinforcement.

In Figure 5c, which illustrates Case 3 (functionalized MWCNT with optimized interfacial bonding), the stress distribution demonstrates significant improvement. The strong interfacial adhesion facilitated a more uniform stress transfer along the MWCNT length, with higher peak stresses observed at the midpoint. This optimized bonding condition exemplifies the ideal scenario for mechanical reinforcement, showcasing the role of functionalized MWCNTs in enhancing the composite properties. Figure 5d, associated with Case 4 (high MWCNT volume fraction with strong bonding), highlights the impact of the increased MWCNT concentration. The stress intensity was notably higher, reflecting the improved load transfer capability owing to strong interfacial bonding. However, the localized stress peaks suggest potential challenges in achieving uniform MWCNT dispersion at higher concentrations, which may affect the mechanical properties and wear resistance of the composite. Finally, Figure 5e, representing Case 5 (extreme MWCNT concentration with potential agglomeration effects), reveals uneven stress distribution and localized stress amplification. While the stress peaks are the highest among all cases, the uneven profile indicates agglomeration effects at extreme MWCNT concentrations. This scenario highlights the trade-offs between maximizing the reinforcement and maintaining uniform dispersion for optimal composite performance. These stress distribution profiles underscore the importance of MWCNT functionalization, interfacial bonding, and dispersion for achieving effective load transfer and mechanical reinforcement. The optimized conditions, as seen in Cases 3 and 4, demonstrate superior stress distribution, validating the molecular dynamics simulations for understanding and improving the mechanical behavior of CNT-reinforced bio-based epoxy composites.

Figure 6a presents the stress–strain behavior of the composite, providing additional insights into its mechanical properties. The stress values derived from the nanoindentation tests align well with the theoretical stress–strain behavior. A 20% increase in interfacial shear strength (τ_ISS) is observed for composites with higher MWCNT volume fractions, validating the role of MWCNT reinforcement in enhancing interfacial bonding and load transfer capabilities. Furthermore, the analysis integrates experimental data points with a fitted curve, ensuring the quantitative validation of the observed trends. The close agreement between experimental and modelled values, with minor deviations (<5%) across multiple trials, reinforces the reliability of the data and consistency of MWCNT dispersion and interfacial bonding quality.

Figure 6b illustrates the hardness profile of the CNT-reinforced epoxy composite as a function of the indentation depth. The initial surface hardness of 2 GPa was attributed to the significant reinforcing effect of the MWCNTs in the top layer of the matrix. As the indentation depth increased, the hardness gradually decreased to approximately 1.5 GPa, indicating a transition from a reinforced surface layer to a softer bulk material. This reduction represents a 25% decline in hardness, highlighting the importance of the MWCNT distribution in achieving effective surface-layer reinforcement. The higher surface hardness demonstrates the efficacy of MWCNTs in resisting localized deformation, which is crucial for improving wear resistance. The observed gradient also suggests that MWCNTs are more concentrated near the surface, likely owing to fabrication or dispersion techniques.

4.3. AFM Analysis

Figure 7 shows the AFM analysis of the worn specimens. The selected AFM images represent distinct wear conditions of moderate, high, better, and excellent based on their correlation with key wear parameters, such as wear rate (mm³/N·m), friction coefficient (µ), hardness (GPa), and interfacial shear strength (MPa). These images were specifically chosen because they provide clear and quantifiable insights into the wear behavior under varying test conditions. Figure 7a shows moderate wear surface conditions. This image corresponds to the moderate wear resistance observed under a load of 20 N, high sliding speed of 2 m/s, and MWCNT volume fraction of 0.1%. The AFM topography revealed pronounced wear tracks with moderate surface roughness and material removal. The relatively high wear rate (0.0092 mm³/N·m) and friction coefficient (0.35) indicate that the low MWCNT content and high sliding speed contributed to the increased wear. These conditions highlight the importance of the MWCNT volume fraction in improving wear resistance. Figure 7b shows high-wear surface conditions. The AFM image represents high wear conditions with a load of 30 N, sliding speed of 1.5 m/s, and MWCNT volume fraction of 0.1%. The surface exhibited deeper and wider wear tracks with high material removal and roughness. The high wear rate (0.009 mm³/N·m) and friction coefficient (0.35) are attributed to the combined effects of the higher load and low MWCNT reinforcement, leading to poor wear resistance.

Figure 7c shows better wear surface conditions. This image corresponds to better wear resistance achieved under a load of 20 N, a moderate sliding speed of 1 m/s, and an MWCNT volume fraction of 0.3%. The AFM topography reveals smoother wear tracks with reduced material removal, reflecting a balanced combination of test parameters. The wear rate (0.007 mm³/N·m) and friction coefficient (0.31) confirm the role of moderate MWCNT reinforcement and optimal test conditions in improving the wear behavior. Figure 7d shows excellent wear surface conditions. The AFM image highlights excellent wear resistance observed under a load of 40 N, a low sliding speed of 0.5 m/s, and an MWCNT volume fraction of 0.4%. The surface exhibits minimal wear tracks and smooth topography, indicating reduced material removal. The lowest wear rate (0.0058 mm³/N·m) and friction coefficient (0.26) validate the effectiveness of higher MWCNT reinforcement and a lower sliding speed in minimizing wear and promoting uniform stress distribution.

4.4. Taguchi Design of Experiments and Its Implications

4.4.1. Taguchi Design

Table 4. Response tables.

Response Table for Signal to Noise Ratios					Response Table for Means
Level	Load (N)	Sliding Speed (m/s)	MWCNT wt.%	Sliding Distance (m)	Level	Load (N)	Sliding Speed (m/s)	MWCNT wt.%	Sliding Distance (m)
1	−5.702	−5.709	−5.699	−5.706	1	28.91	31.78	25.38	34.68
2	−5.710	−5.704	−5.706	−5.705	2	31.14	31.79	28.90	29.22
3	−5.707	−5.708	−5.708	−5.704	3	32.43	31.79	34.03	29.23
4	−5.707	−5.705	−5.713	−5.710	4	34.68	31.79	38.83	34.03
Delta	0.008	0.005	0.014	0.006	Delta	5.77	0.01	13.45	5.46
Rank	2	4	1	3	Rank	2	4	1	3

Nominal is best (10 × Log10(Ybar²/s²)).

presents the response analysis for signal-to-noise (S/N) ratios and means, highlighting the impact of the load (N), sliding speed (m/s), MWCNT weight percentage (wt.%), and sliding distance (m) on the wear performance of MWCNT-reinforced bio-based epoxy composites. The results reveal that the MWCNT weight percentage is the most influential factor with the highest Delta values (0.014 for S/N ratios and 13.45 for means), underscoring its critical role in enhancing wear resistance and interfacial bonding. Load ranks second (Delta = 0.008 for S/N ratios and 5.77 for means), reflecting its significant influence on stress distribution and surface damage. The sliding distance (Delta = 0.006 for S/N ratios and 5.46 for means) has a moderate effect, while sliding speed shows minimal impact, indicating that wear behavior is predominantly governed by reinforcement and applied forces. These findings emphasize the importance of optimizing the MWCNT content and load conditions to achieve superior wear performance.

Figure 7b and Figure 8a depict the main effects of load, sliding speed, MWCNT weight percentage, and sliding distance on the wear performance of the MWCNT-reinforced composites. Figure 8a shows that the MWCNT weight percentage has the most significant influence, with a sharp increase in the mean values as the MWCNT content increases, confirming its critical role in enhancing wear resistance. The load also demonstrates a positive effect, with higher loads improving wear performance, likely owing to better reinforcement activation. The sliding speed shows minimal variation, indicating its limited impact, while the sliding distance exhibits a nonlinear trend, suggesting interactions with other factors. Figure 8b highlights the consistency of results through signal-to-noise ratios, where MWCNT weight percentage and load emerge as the most impactful parameters. The sliding speed has a negligible influence, and the sliding distance shows fluctuations, potentially due to the degradation of the reinforcement efficiency over extended distances. These findings underscore the importance of optimizing the MWCNT content and load conditions to improve wear resistance and reliability.

4.4.2. Analysis of Variance (ANOVA)

Table 5. General linear model for hardness.

Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-Value	p-Value
Load (N)	3	0.78688	15.00%	0.78688	0.26229	49.05	0.005
Sliding Speed (m/s)	3	0.00687	0.13%	0.00688	0.00229	0.43	0.748
MWCNT wt.%	3	4.40188	83.94%	3.25771	1.08590	203.08	0.001
Sliding Distance (m)	3	0.03271	0.62%	0.03271	0.01090	2.04	0.287
Error	3	0.01604	0.31%	0.01604	0.00535
Total	15	5.24437	100.00%

presents the general linear model for hardness, showing that the MWCNT weight percentage is the most significant factor, contributing 83.94% to the total variation, with a very high F-value (203.08) and a significant p-value (0.001). This underscores the dominant role of MWCNT reinforcement in improving hardness by effectively distributing the loads and enhancing the matrix properties. The contribution of the load was 15%, which was significant, but secondary to the MWCNT content. The sliding speed and sliding distance had minimal contributions (0.13% and 0.62%, respectively) and were statistically insignificant (p > 0.05). This suggests that the hardness is primarily influenced by the material composition rather than the external test parameters.

Table 6. General linear model for interfacial shear strength.

Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-Value	p-Value
Load (N)	3	1062.50	14.31%	1062.50	354.17	*	*
Sliding Speed (m/s)	3	0.00	0.00%	0.00	0.00	*	*
MWCNT wt.%	3	6337.50	85.35%	4750.00	1583.33	*	*
Sliding Distance (m)	3	25.00	0.34%	25.00	8.33	*	*
Error	3	0.00	0.00%	0.00	0.00
Total	15	7425.00	100.00%

In Table 6 ‘*’ indicates that the F-value and p-value could not be computed due to the absence of error variance in the model.

provides the general linear model for interfacial shear strength, highlighting MWCNT weight percentage as the most influential factor, contributing 85.35% to the interfacial shear strength, with a significant impact (indicated by high F-values). This emphasizes the strong bonding between the MWCNTs and matrix, which enhances the load transfer and shear resistance. The load also played a notable role, contributing 14.31%, as higher loads activated more reinforcement potential. The sliding speed and sliding distance contribute negligibly (0.00% and 0.34%, respectively), indicating their limited effect on the shear strength. The results validate the critical role of MWCNT content in optimizing the interfacial adhesion and load transfer mechanisms.

Table 7. General linear model for wear rate.

Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-Value	p-Value
Load (N)	3	0.000003	13.08%	0.000003	0.000001	181.00	0.001
Sliding Speed (m/s)	3	0.000000	0.79%	0.000000	0.000000	11.00	0.040
MWCNT wt.%	3	0.000021	85.11%	0.000016	0.000005	905.57	0.000
Sliding Distance (m)	3	0.000000	0.94%	0.000000	0.000000	13.00	0.032
Error	3	0.000000	0.07%	0.000000	0.000000
Total	15	0.000024	100.00%

outlines the general linear model for the wear rate, where the MWCNT weight percentage again emerged as the dominant factor, accounting for 85.11% of the total variation. The extremely high F-value (905.57) and significant p-value (0.000) confirmed its importance in reducing wear. The load is the next most significant factor, contributing 13.08%, as higher loads influence the material interaction at the interface. The sliding speed and sliding distance contributed minimally (0.79% and 0.94%, respectively), with marginal p-values, suggesting a secondary influence on wear performance. This analysis highlights the importance of optimizing the MWCNT content for wear resistance, while the load also plays a supportive role in determining wear rates.

Table 8. General linear model for friction coefficient.

Source	DF	Seq SS	Contribution	Adj SS	Adj MS	F-Value	p-Value
Load (N)	3	0.002319	16.63%	0.002319	0.000773	14.45	0.027
Sliding Speed (m/s)	3	0.000069	0.49%	0.000069	0.000023	0.43	0.748
MWCNT wt.%	3	0.011319	81.17%	0.008627	0.002876	53.78	0.004
Sliding Distance (m)	3	0.000077	0.55%	0.000077	0.000026	0.48	0.719
Error	3	0.000160	1.15%	0.000160	0.000053
Total	15	0.013944	100.00%

displays the general linear model for the friction coefficient, showing that it is primarily influenced by the MWCNT weight percentage, which contributes 81.17% of the total variation. This indicates that MWCNTs significantly alter surface interactions and reduce friction through improved reinforcement and interfacial properties. The load contributed 16.63%, reflecting its role in modifying the contact pressure and friction behavior. The sliding speed and sliding distance had negligible effects, with contributions of 0.49% and 0.55%, respectively, and statistically insignificant p-values. This result underscores the importance of MWCNT distribution and load as key factors in controlling the friction behavior in composite systems.

4.4.3. Binned Scatterplots

Figure 9 reveals that the wear rate decreased significantly as the load increased from 10 to 40 N. At lower loads, the wear rates were higher owing to the insufficient contact pressure, resulting in more abrasive wear. As the load increased, the MWCNT reinforcement became more effective, and a reduction in the wear rate was observed. This trend underscores the role of the load in enhancing the CNT–matrix interaction and optimizing the reinforcement mechanism. Figure 10 demonstrates that the wear rate varies with sliding speed, showing a moderate decrease as the speed increases up to 1.5 m/s, but stabilizes at higher speeds. Higher sliding speeds generate heat, which may enhance the interfacial bonding between the MWCNTs and epoxy matrix, reducing wear rates. However, excessive sliding speeds may lead to matrix degradation, which limits further improvements in wear resistance. Figure 11 illustrates a strong inverse relationship between the MWCNT weight percentage and wear rate. Increasing the MWCNT content from 0.1% to 0.4% led to a substantial reduction in wear rate, highlighting the effectiveness of MWCNTs as reinforcement agents. This trend confirms that higher MWCNT concentrations improve the load transfer and reduce the matrix deformation, thus enhancing the wear resistance. Figure 12 shows that the wear rate initially decreases with increasing sliding distance, but stabilizes beyond 1500 m. Shorter sliding distances exhibit higher wear rates due to surface roughness and initial wear stages. As the sliding distance increased, the CNT-reinforced surface layer became polished, reducing the wear rates. The stabilization of the wear rate at longer distances reflects the steady-state behavior of the composite under tribological conditions.

4.4.4. Influence of Factors Considered on the Results of Wear Test

Figure 13 illustrates the influence of load, sliding speed, MWCNT volume fraction, and sliding distance on hardness. The contour plot in Figure 13a highlights the relationship between load, sliding speed, and hardness. Higher hardness values (>3.5 GPa) were achieved at higher loads (30–40 N) and intermediate sliding speeds (1–1.5 m/s). At lower loads (10–20 N) and low sliding speeds (0.5 m/s), the hardness reduces significantly (<2.5 GPa). This trend can be attributed to the enhanced stress transfer efficiency and better MWCNT alignment under higher loads, whereas moderate speeds ensure minimal wear-induced softening. Figure 13b demonstrates that higher MWCNT volume fractions (0.3–0.4%) and increased loads (30–40 N) resulted in maximum hardness (>4 GPa). At low MWCNT fractions (0.1–0.2%) and low loads (10–20 N), the hardness remains below 2.5 GPa. This pattern highlights the synergistic effect of higher MWCNT content and applied load, ensuring uniform stress distribution and improved interfacial bonding, enhancing the mechanical properties of the composite [48,49]. Figure 13c indicates that higher loads (30–40 N) combined with intermediate sliding distances (1000–1500 m) resulted in peak hardness values (>3.5 GPa). At shorter sliding distances (500 m), the hardness decreased slightly because of the limited interaction between the asperities and reinforcement. Conversely, longer distances (>1500 m) at lower loads exhibited reduced hardness (<2.5 GPa) owing to the matrix degradation.

In Figure 13d, the contour plot reveals that high MWCNT content (0.3–0.4%) and moderate sliding speeds (1–1.5 m/s) are ideal conditions for achieving maximum hardness (>3.5 GPa). At lower MWCNT concentrations (0.1–0.2%) and sliding speeds (<0.5 m/s), the hardness remained low (<2.5 GPa). The consistent enhancement with higher MWCNT content is due to the improved load transfer mechanisms and effective dispersion. Figure 13e illustrates that intermediate sliding speeds (1–1.5 m/s) and sliding distances (1000–1500 m) contribute to high hardness values (>3.5 GPa). At low sliding speeds (0.5 m/s) and short distances (500 m), the hardness remains below 2.5 GPa. Excessive sliding distances (>1500 m) at higher speeds tended to slightly reduce the hardness, possibly because of wear-induced thermal effects. Figure 13f confirms that a higher MWCNT content (0.3–0.4%) and intermediate sliding distances (1000–1500 m) yielded maximum hardness (>4 GPa). At lower MWCNT fractions (0.1–0.2%) and shorter distances (500 m), the hardness values remain below 2.5 GPa. The combination of the optimal MWCNT content and adequate sliding distance ensures the effective stress transfer and structural integrity of the composite, which is reflected in the enhanced hardness [50,51].

The contour plots in Figure 13f and Figure 14a provide insights into the interfacial shear strength (ISS) under varying combinations of load, sliding speed, MWCNT weight percentage, and sliding distance. In Figure 14a, the ISS is observed to increase significantly with higher loads and sliding speeds, particularly beyond 30 N and 1.5 m/s, highlighting the optimal conditions for an effective load transfer between the MWCNTs and the matrix. Similarly, Figure 14b shows that the MWCNT weight percentage is a dominant factor, with ISS peaking at 0.4% MWCNT content and higher loads, which is attributed to enhanced interfacial adhesion and stress distribution.

Figure 14c reveals a consistent increase in ISS with higher loads and sliding distances, particularly beyond 1500 m and 35 N, emphasizing the impact of extended sliding interactions in strengthening the interphase region. Figure 14d illustrates that higher sliding speeds and MWCNT content synergistically improve the ISS, with pronounced effects at elevated MWCNT concentrations under dynamic loading conditions. In Figure 14e, ISS improves as sliding speed and distance increase, with notable gains at sliding speeds above 1.5 m/s and sliding distances exceeding 1500 m, indicating the reinforcing effect of prolonged dynamic interactions. Finally, Figure 14f demonstrates the interplay between the MWCNT weight percentage and sliding distance, where the ISS peaks at 0.4% MWCNT and sliding distances beyond 1500 m, underscoring the importance of the MWCNT concentration and operational conditions in enhancing the interfacial properties.

Overall, the results highlight the critical role of MWCNT weight percentage and load in determining ISS, with the sliding speed and distance contributing synergistically under dynamic conditions. These observations provide valuable guidance for optimizing material parameters to improve composite performance in diverse wear scenarios.

The correlogram in Figure 15 provides a comprehensive visualization of the interrelationships among the load (N), sliding speed (m/s), MWCNT weight percentage (%), sliding distance (m), hardness (GPa), interfacial shear strength (ISS, MPa), wear rate (mm³/N·m), and friction coefficient (μ). The heatmap employed a color-coded scale, where red indicates strong positive correlations, blue represents strong negative correlations, and lighter shades signify weaker correlations or negligible relationships. The analysis revealed a significant positive correlation between the MWCNT weight percentage and both the hardness (GPa) and interfacial shear strength (MPa), with correlation coefficients approaching 0.8. This aligns with the well-documented role of MWCNTs as reinforcements, which enhances the mechanical and interfacial properties of the composite. Similarly, the sliding distance showed a moderate positive correlation with hardness, reflecting the influence of the extended dynamic interaction on wear-induced work hardening.

A strong negative correlation was observed between the MWCNT weight percentage and wear rate (mm³/N·m), as well as between the hardness and wear rate. This indicates that higher MWCNT concentrations and improved hardness significantly reduced material loss during wear, validating the effectiveness of MWCNTs in enhancing wear resistance. Conversely, the wear rate showed a positive correlation with the friction coefficient, suggesting that higher wear rates are associated with increased frictional forces, likely due to surface degradation. Interestingly, the load exhibits a dual effect, showing moderate positive correlations with both hardness and interfacial shear strength, while also contributing to an increased wear rate under higher operational stresses. Sliding speed, however, demonstrates weaker correlations across most parameters, indicating that it has a more nuanced or secondary role than other variables.

The surface plots presented in Figure 16a–d provide an insightful visualization of the interrelationships between critical parameters influencing the mechanical and tribological performance of the composite.

Figure 16a illustrates the relationship between the hardness (GPa), interfacial shear strength (MPa), and wear rate (mm³/N·m). The plot reveals a region of high hardness coinciding with the increased interfacial shear strength and decreased wear rate, emphasizing the role of robust interfacial bonding in achieving superior wear resistance and mechanical strength. Figure 16b shows the interaction between hardness (GPa), interfacial shear strength (MPa), and friction coefficient (μ). The plot shows a decrease in the friction coefficient with increasing hardness and interfacial shear strength, suggesting that improved interfacial bonding and material hardness mitigated frictional losses during sliding wear. Figure 16c shows the correlation between interfacial shear strength (MPa), wear rate (mm³/N·m), and friction coefficient (μ). This highlights that a higher interfacial shear strength significantly reduces the wear rate and friction coefficient, validating the effectiveness of MWCNT reinforcement and interfacial engineering in enhancing the tribological performance of the composite. Figure 16d shows the interdependence of hardness (GPa), wear rate (mm³/N·m), and friction coefficient (μ). The plot indicates a region of low wear rate and friction coefficient corresponding to a high hardness, reflecting the superior performance of the composite under optimized conditions.

The results confirmed expectations, with 0.3–0.4 wt.% MWCNTs significantly improving hardness, wear resistance, and interfacial bonding. Some deviations were noted at higher CNT concentrations due to agglomeration effects, and there were slight differences in failure modes between MD simulations and experiments. Overall, this study validates MWCNT reinforcement for enhancing bio-based epoxy composites while highlighting areas for further optimization [52]. MWCNT additives enhance lubricating oils for gear systems, bearings, and hydraulic transmissions, improving wear resistance and efficiency in automotive and industrial applications. However, oil discoloration and nanoparticle toxicity pose challenges. Future research should focus on eco-friendly CNT modifications to balance performance with environmental safety.

5. Conclusions

This study demonstrates a significant enhancement in the mechanical and tribological properties of MWCNT-reinforced bio-based epoxy composites, offering a robust framework for optimizing their performance in wear-critical applications. The incorporation of MWCNTs at an optimal weight percentage of 0.3–0.4% resulted in a remarkable increase in hardness, achieving a peak value of 4 GPa, which is approximately 60% higher than that of non-reinforced systems. The wear rate exhibited a significant reduction, with a minimum value of 0.0058 mm³/N·m under high-load conditions of 40 N and sliding distances of 1000–1500 m, demonstrating enhanced durability. FTIR and XRD analyses confirmed the strong interfacial bonding between the MWCNTs and the bio-based epoxy matrix, whereas molecular dynamics simulations revealed the stress distribution and cohesive energy at the interface, validating the observed improvements. The interfacial shear strength increased to 160 MPa, demonstrating the critical role of MWCNT dispersion and bonding. The Taguchi optimization identified the MWCNT weight percentage as the most influential parameter, with a contribution of 85.11% toward wear rate reduction, followed by a load of 13.08%.

Contour plots and correlograms established clear relationships between parameters, illustrating that increased MWCNT content and load enhanced the hardness and interfacial shear strength while reducing the wear rate and friction coefficient. The low friction coefficient of 0.26 under optimal conditions highlights the potential of these composites for applications requiring low wear and friction. This comprehensive study not only validates the efficacy of MWCNTs in enhancing bio-based epoxy composites, but also provides quantifiable insights into parameter optimization, advancing the development of sustainable and high-performance materials for tribological applications.