Optimizing alumina reinforcement in kevlar-epoxy composites: a study on mechanical and tribological enhancements

Abstract

Kevlar-reinforced epoxy composites are widely employed in high-performance applications due to their excellent strength-to-weight ratio and impact resistance. However, limitations in wear resistance hinder their use under abrasive conditions. To address this, Alumina (Al₂O₃) was incorporated as particulate filler at 2%, 4%, and 6% by weight. Composite laminates were fabricated using hand layup followed by compression molding, with a constant Kevlar fiber content of 30 wt%. Mechanical testing was conducted as per ASTM D3039 (tensile), ASTM D790 (flexural), and ASTM D785 (hardness) standards, while wear behavior was evaluated under dry sliding conditions using ASTM G99 with a pin-on-disc setup at 10 N load, 1.5 m/s sliding velocity, and 500 m sliding distance. The addition of 4% alumina yielded optimal performance, resulting in a 28% increase in tensile strength (from 98 MPa to 126 MPa), a 21.6% increase in flexural strength (from 152 MPa to 185 MPa), and a 12.2% increase in hardness (from 82 HRB to 92 HRB) compared to the control. The wear rate was reduced from 3.12 × 10⁻⁴ mm³/N·m in the control to 1.89 × 10⁻⁴ mm³/N·m at 4% alumina, representing a 39.4% improvement. The coefficient of friction also decreased from 0.52 (control) to 0.46 at 4% alumina.

Keywords:
Alumina; Kevlar; Epoxy Composites; Wear Resistance; Mechanical Properties

1. INTRODUCTION

Materials and engineering composites have witnessed remarkable progress in recent decades thanks to their versatility and tailored performance characteristics. Among various composite materials, epoxy-based composites have gained significant attention due to their exceptional properties, including high strength-to-weight ratio, excellent chemical resistance, and superior mechanical properties. This makes them ideal candidates for demanding applications in the automotive, aerospace, and protective gear industries. However, despite their impressive performance, epoxy composites still face challenges related to wear and friction, especially when exposed to harsh operational environments [¹]. To overcome these limitations, considerable research efforts have been directed toward enhancing the properties of epoxy composites by reinforcing them with high-performance fibers and adding fillers to optimize their mechanical and tribological behaviors [²]. To address this, the importance of enhancing the mechanical and tribological properties of epoxy composites for high-performance applications should be clearly stated. The growing demand for lightweight, high-strength, and wear-resistant materials in aerospace, automotive, and protective applications has necessitated research into fiber-reinforced composites with optimized filler content. The inclusion of Alumina as a reinforcing filler is based on its exceptional hardness, wear resistance, and thermal stability. The scientific problem being addressed is the optimization of alumina content to achieve the best combination of tensile strength, flexural strength, and wear resistance while mitigating potential disadvantages such as filler agglomeration and increased brittleness. The assumption underlying this research is that a controlled addition of Alumina can significantly improve the tribological performance of Kevlar-reinforced epoxy composites without negatively impacting mechanical integrity [³].

Several types of fillers have been explored for epoxy composites, each providing distinct advantages depending on the filler type. Among these, ceramic particles such as Alumina (Al₂O₃) are widely recognized for their outstanding hardness, wear resistance, and thermal stability [⁴]. When incorporated into epoxy resin matrices, alumina fillers significantly improve the composites’ mechanical strength and resistance to wear, making them suitable for high-stress and high-wear applications. Alumina’s high stiffness also plays a crucial role in enhancing the composite’s load-bearing capacity, while its hardness contributes to reducing wear under sliding contact. Despite these benefits, challenges remain in optimizing the alumina loading levels in epoxy composites to achieve a balance between improved mechanical properties and manageable processing complexities [⁵].

A significant gap in the current body of knowledge pertains to the optimal alumina content in epoxy composites and its effects on both mechanical and tribological properties. While studies have demonstrated the beneficial role of Alumina, inconsistencies in the findings—such as varying effects of alumina content on wear, tensile strength, and flexural properties—point to the need for a more detailed exploration [⁶]. Additionally, the dispersal of alumina particles, their interaction with the matrix, and their impact on wear resistance and friction need further investigation to fully understand the underlying mechanisms. This presents an important area of exploration in the field of composite material engineering. In the development of high-performance composites, there exists a critical need to enhance their wear resistance and mechanical properties while maintaining a lightweight structure. Despite the widespread application of Kevlar-reinforced epoxy composites in aerospace, automotive, and protective equipment, their tribological performance remains a challenge, especially under dry sliding conditions. The incorporation of ceramic fillers such as Alumina offers a promising approach to addressing these limitations; however, the impact of alumina content on both wear resistance and mechanical strength has not been fully optimized [⁷]. This study seeks to determine the optimal alumina concentration that maximizes these properties without causing filler-induced embrittlement or processing challenges. The significance of this research lies in its ability to provide insights into filler-matrix interactions and the trade-offs associated with increasing alumina content, which are essential for improving the longevity and functionality of Kevlar-based composites in industrial applications. The introduction has been revised to include additional references that provide a broader context for the study. Recent studies on epoxy-based composites incorporating ceramic fillers, such as silicon carbide and titanium dioxide, have been reviewed to highlight the novelty of using Alumina as an alternative filler. Furthermore, the objectives of this research have been explicitly stated to emphasize its contribution to optimizing filler content for superior mechanical and wear performance [⁸].

In comparison to other thermosetting and thermoplastic matrices, epoxy-based composites have shown distinct advantages in terms of dimensional stability, surface adhesion, and crosslinking density. However, alternative matrices such as polyamide 6 (PA6) and polyamide 12 (PA12) have also gained attention due to their superior flexibility, chemical resistance, and inherent toughness. In a comprehensive review, researchers. investigated various reinforced PA6 and PA12 systems and reported that PA6 composites exhibited enhanced impact resistance and thermal stability, especially when reinforced with nanoclays or glass fibers. Their results show that while PA matrices offer excellent ductility and fatigue resistance, their water absorption behavior and lower thermal degradation thresholds pose limitations in high-temperature structural applications. In contrast, epoxy matrices provide better thermal endurance and interfacial strength with reinforcing fibers, which is critical for wear-dominated environments. Therefore, although PA6 and PA12-based composites are suitable for applications involving dynamic mechanical stress, epoxy composites remain preferable for load-bearing and abrasive service conditions where dimensional rigidity and surface hardness are required. Recent advancements in composite material research have focused on the integration of ceramic fillers to enhance mechanical and wear properties. Studies have demonstrated that alumina-reinforced polymer composites exhibit improved hardness and tribological performance due to the ceramic’s intrinsic wear resistance [⁹]. However, excessive filler content has been linked to aggregation and poor interfacial bonding, which can negatively impact tensile and flexural strengths. Previous research has explored silicon carbide, titanium dioxide, and boron nitride as alternative fillers; however, Alumina stands out due to its cost-effectiveness, high hardness, and compatibility with epoxy resins.

Recent studies have explored various strategies to significantly enhance tensile strength and thermal stability, as detailed in the comprehensive review by researchers. These methodologies align with the current study’s approach of incorporating Alumina (Al₂O₃) as a particulate filler to augment composite performance. In this research, Alumina was integrated into the epoxy matrix at varying concentrations (2%, 4%, and 6% by weight) to evaluate its impact on mechanical and tribological properties. The composites were fabricated following standardized procedures, ensuring consistency and reproducibility. Mechanical testing adhered to ASTM standards, with tensile tests conducted according to ASTM D3039 and flexural tests following ASTM D790 protocols. The incorporation of Alumina aimed to enhance the performance of the epoxy composites. By integrating these optimization techniques and leveraging insights from recent technological advancements, this study seeks to contribute to the ongoing development of high-performance epoxy composites suitable for demanding applications.

The novelty of this study lies in the systematic evaluation of alumina concentrations to determine an optimal balance between mechanical strength and wear performance. To further distinguish the present investigation from prior research, it must be emphasized that while many studies have examined ceramic fillers such as silicon carbide, titanium dioxide, or boron nitride in epoxy matrices, limited attention has been given to the systematic optimization of alumina content specifically within Kevlar-reinforced system. In most earlier works, Alumina has either been used in conjunction with different fiber reinforcements or evaluated without correlating mechanical and tribological characteristics through statistical modeling. In contrast, this study not only isolates AAlumina’s influence within a Kevlar-epoxy framework but also integrates mechanical, microstructural, and tribological data with RSM to optimize filler content under varying loads. Such a holistic approach has not been previously reported for Kevlar-alumina systems, and the findings provide a more robust understanding of how alumina dispersion influences both mechanical integrity and wear resistance [¹⁰]. This contribution serves to bridge the gap between filler content optimization and practical application needs in high-stress composite environments. Table 1 provides a comparative overview of twelve recent studies on polymer composite materials, highlighting their constituent materials, focus areas, testing methodologies, key findings,

Table 1
Summary of recent studies on fiber-reinforced composites.

REFERENCE	MATERIALS USED	FOCUS AREA	METHODOLOGY	KEY FINDINGS	RELEVANCE TO STUDY
[11]	Coconut coir, rice husk, SiC, epoxy	Mechanical properties of natural fiber composites	Hand layup, ASTM tests	Improved tensile, flexural, impact, and hardness with ceramic fillers	Moderately relevant – Natural fillers, ceramic SiC; similar mechanical goals
[12]	Glass, carbon, Kevlar, basalt fibers	Mechanical and tribological performance	Box–Behnken design, Firefly algorithm, pin-on-disc test	Optimal stacking reduces wear; improved strength and sustainability	Highly relevant – Tribological optimization, RSM, hybrid stacking
[13]	Red brick dust, epoxy	Wear prediction using FEM	Taguchi method, ANSYS FEM, erosion wear test	Wear resistance improves with filler; FEM predictions match experiments	Moderately relevant – Ceramic filler (brick dust), FEM-based wear study
[14]	Kota stone dust, epoxy	Sliding wear behavior	Taguchi method, Grey Wolf Optimization	Effective utilization of KSD in composites	Moderately relevant – Ceramic filler (stone dust), sliding wear optimization
[15]	Rice straw, coir, bagasse, jute, neem wood powder	Wear and friction performance	Pin-on-disc test, SEM	Improved wear performance with increased neem wood filler	Moderately relevant – Waste fibers/fillers, wear performance focus
[16]	Tamarind seed, eelgrass, adamant creeper, epoxy	Mechanical and thermal stability of green composites	Hand lay-up, DMA, TGA	Enhanced mechanical and thermal properties with hybrid fiber	Highly relevant – Hybrid composites, thermal & mechanical optimization
[17]	Cork core, natural fiber skins, epoxy	Fracture and creep behavior	ASTM standards, Charpy, bending tests	Cork-natural fiber composites viable for kayak shells	Low relevance – Sandwich structures for sports gear
[18]	Basalt fiber, clamshell waste	Enhancing mechanical properties	Hand lay-up, mechanical tests	Clamshell filler improves tensile, flexural, and impact resistance	Highly relevant – Ceramic filler (clamshell), basalt fiber composite
[19]	Prosopis Juliflora bark powder, jute fabric, epoxy	Mechanical and thermal behavior	Compression molding, mechanical and thermal tests	15% PJ powder improves overall performance	Highly relevant – Natural filler (PJ bark), mechanical & and thermal evaluation
[20]	Kevlar, jute, vinyl epoxy	Stacking sequence in composites	Hand lay-up, mechanical tests, SEM	Kevlar-dominant stacking yields high strength	Highly relevant – Kevlar-epoxy stacking and mechanical properties
[21]	Borassus Flabellifer fibers	Morphological and tensile properties	Chemical treatment, SEM, XRD	Chemically treated fibers viable for composites	Low relevance – Fiber-only tensile study, no epoxy matrix
[22]	Manila hemp, silica fume, concrete	Mechanical and flexural behavior of high-strength concrete	Concrete mix designs, strength tests	1% fiber + 20% silica fume improves strength and flexural behavior	Low relevance – Concrete system, not epoxy-based composites

This study aims to address these gaps by evaluating the effects of varying alumina filler content (2%, 4%, and 6% by weight) in epoxy composites reinforced with Kevlar fibers. All raw materials used in this study were produced from commercial suppliers. The Bisphenol-A type epoxy resin (LY556) and triethylenetetramine (TETA) hardener (HY951) were supplied by Atul Ltd., Gujarat, India. Unidirectional Kevlar fibers (Kevlar 49, plain weave) were purchased from DuPont-authorized distributors in Chennai, India. High-purity alumina (Al₂O₃) powder with an average particle size of 50 µm and a purity of 99.5% was obtained from Sigma-Aldrich, Bengaluru, India [²³]. Acetone (analytical grade, ≥99.5%) used for dispersion and cleaning purposes was purchased from Merck Chemicals. All materials were used as received without further purification or chemical modification. Through a systematic approach, the study investigates how different alumina loading levels influence mechanical properties (such as tensile strength, flexural strength, and hardness) as well as tribological performance, focusing on wear and friction. By providing a comprehensive analysis of these properties, this research will help identify the optimal alumina content that maximizes both mechanical strength and wear resistance, ensuring the successful application of Kevlar-reinforced epoxy composites in industrial and engineering settings. These concentrations were chosen based on prior research indicating that moderate filler levels (2-6%) significantly enhance mechanical properties without causing excessive brittleness or processing difficulties. A higher content beyond 6% is known to cause particle agglomeration, leading to reduced strength and increased wear. These specific alumina concentrations were selected based on preliminary experimental trials and a review of previous literature indicating critical thresholds for filler dispersion and agglomeration. Initial pilot studies conducted by the authors with 1% and 7% alumina revealed that lower concentrations resulted in negligible mechanical enhancement, while higher concentrations led to severe agglomeration and compromised structural integrity. Furthermore, studies reported that filler loadings between 2% and 6% strike an effective balance between reinforcement efficiency and processability in particulate-filled polymer composites. Therefore, alumina levels of 2%, 4%, and 6% were chosen to systematically assess the performance window before agglomeration effects dominate, allowing for an accurate determination of the optimal reinforcement threshold in Kevlar-epoxy systems.

2. MATERIALS AND METHODS

Epoxy composites reinforced with unidirectional Kevlar fibers and Alumina (Al₂O₃) as a filler were prepared and tested to evaluate their mechanical and wear properties. The epoxy resin, selected for its strong mechanical properties, excellent adhesion, and chemical resistance, served as the matrix material. A Bisphenol-A type epoxy resin was combined with a triethylenetetramine hardener in a 100:10 ratio by weight to initiate crosslinking. Unidirectional Kevlar fibers were chosen as the reinforcement material due to their exceptional strength-to-weight ratio, thermal resistance, and toughness. The Kevlar fiber content in the composite was maintained at 40% by volume (approximately 30% by weight) across all samples to ensure consistent reinforcement levels and facilitate meaningful comparisons between alumina filler concentrations. Alumina, a ceramic material known for its high hardness, excellent wear resistance, and thermal stability, was incorporated into the epoxy matrix in weight fractions of 2%, 4%, and 6% to examine its impact on the composite properties [²⁴]. Table 2 presents the weight percentages of Kevlar fiber, epoxy matrix, and alumina filler used in the fabrication of different composite samples. The alumina content was varied to study its effect on mechanical and tribological performance.

Table 2
Composition of Kevlar-reinforced epoxy composites with varying alumina content.

COMPOSITE CODE	KEVLAR FIBER (% wt)	EPOXY + HARDENER (% wt)	ALUMINA (AL2O3) (% wt)
Control	30	70	0
A2	30	68	2
A4	30	66	4
A6	30	64	6

To prepare the composite, the epoxy resin and hardener were mixed thoroughly to ensure a homogeneous mixture (Figure 1). Alumina powder with an average particle size of 50 microns was gradually added to the epoxy mixture to achieve the desired weight percentages. The alumina particle size of 50 microns was selected to achieve an optimal balance between effective reinforcement and processability. Particles smaller than 50 microns tend to agglomerate, whereas larger particles may lead to poor interfacial bonding. The final composite composition contained 30% Kevlar by weight, with the remaining 70% consisting of the epoxy-alumina matrix. The mixture was continuously stirred using a mechanical stirrer for 15 minutes at a controlled speed to ensure even dispersion of the alumina particles. Care was taken to avoid agglomeration, which could compromise the mechanical properties of the composite. In this study, no additional surface treatment was applied to the alumina particles prior to their incorporation into the epoxy matrix. However, it is acknowledged that the surface chemistry of Alumina plays a significant role in interfacial bonding [²⁵]. The native oxide surface of Alumina is known to possess hydroxyl groups, which can interact with the polar groups in epoxy resins, such as epoxide or hydroxyl moieties, via hydrogen bonding or van der Waals forces. While this natural affinity facilitates a baseline level of adhesion, it may be insufficient under high mechanical stress conditions or at elevated filler loadings. Surface functionalization techniques, such as silane treatment, have been reported to enhance interfacial compatibility by forming covalent bonds between the filler and matrix, thereby improving stress transfer and reducing debonding. Although untreated Alumina was used in the present work, the possibility of further property enhancement through chemical modification of the filler surface is recognized as a potential direction for future research. The preparation process followed a systematic approach to ensure homogeneous dispersion of Alumina within the epoxy matrix. To enhance the distribution of Alumina, the filler was first pre-mixed with acetone and ultrasonicated for 15 minutes before adding to the epoxy resin. To further control particle agglomeration during the dispersion process, the alumina-acetone suspension was subjected to bath ultrasonication at a frequency of 40 kHz and a power of 100 W for 15 minutes. This ultrasonic treatment was used to break down particle clusters by creating high-energy cavitation zones, thereby improving particle wetting and reducing van der Waals-driven aggregation. Following ultrasonication, no additional surface functionalization or silane treatment was applied to the alumina particles. However, the use of acetone as a dispersing medium facilitated temporary stabilization by lowering the surface tension between the alumina and epoxy resin. During mechanical stirring, the suspension was continuously agitated at 2000 rpm for 20 minutes at ambient temperature to promote uniform distribution [²⁶]. The combined approach of solvent-assisted dispersion and sequential ultrasonication-mechanical stirring was found to be sufficient for achieving visually uniform dispersion, as supported by the absence of large clusters in the SEM images. Nonetheless, future studies may consider incorporating dispersing agents or surface treatments to further enhance stability and bonding efficiency. To further elucidate the interfacial bonding mechanisms, a schematic representation is provided in Figure 2. The illustration demonstrates the interactions between Kevlar fibers, alumina fillers, and the epoxy matrix. Specifically, van der Waals forces are expected to act between the filler surface and the Kevlar fibers, while hydrogen bonding likely occurs between the hydroxyl-rich alumina surface and polar groups within the epoxy matrix. These interactions facilitate improved load transfer and energy dissipation across the interfaces, contributing to the enhanced mechanical strength and wear resistance observed in the composites. The schematic supports the morphological findings from SEM and EDS analysis and provides a theoretical basis for the observed property enhancements with increasing alumina content.

Figure 1
Prepared samples.

Figure 2
Kevlar/epoxy composite reinforced with alumina (Al₂O₃) particles.

The resin-alumina mixture was then subjected to mechanical stirring at 2000 rpm for 20 minutes to ensure uniform dispersion. Subsequently, the mixture was degassed in a vacuum chamber for 10 minutes to eliminate air bubbles before it was introduced into the mold containing unidirectional Kevlar fibers. Unidirectional Kevlar fibers were carefully aligned in a steel mold of standard rectangular dimensions as per ASTM standards. Alumina was introduced into the epoxy matrix through a controlled dispersion process involving ultrasonication and mechanical stirring, as described earlier. To ensure effective reinforcement, the unidirectional Kevlar fibers were carefully pre-tensioned and arranged parallel to the mold’s longitudinal axis before resin infusion [²⁷]. This alignment facilitated an optimal fiber-matrix interaction, enhancing load transfer and minimizing weak interfacial regions. The prepared epoxy-alumina mixture was then poured over the Kevlar fibers to ensure complete impregnation. To further clarify the fabrication process, an additional surface pre-treatment step for Kevlar fibers could be incorporated to improve fiber-matrix adhesion. Although in the present study, the fibers were used in their as-received state, future improvements may involve cleaning the fibers with acetone or applying a mild plasma treatment to enhance wettability and surface energy. Moreover, the curing process can be refined by employing a staged thermal profile wherein the composite is first pre-cured at room temperature for 24 hours, followed by post-curing at elevated temperatures (e.g., 80°C for 2 hours). This method has been shown to enhance crosslink density and improve thermal stability in epoxy systems. Additionally, the use of vacuum-assisted resin infusion during the layup phase is recognized as a potential enhancement to minimize void content and ensure uniform wetting of the Kevlar fibers by the alumina-loaded resin. These process refinements can reduce interfacial defects, improve mechanical interlocking, and potentially enhance both the mechanical and tribological properties of the final composite. A vacuum bagging technique was employed to remove air bubbles and ensure uniformity. Figure 3 illustrates the schematic representation of the specimen preparation process.

Figure 3
Composite preparation process.

Once the curing was completed, the specimens were removed from the mold and cut into the required dimensions for mechanical and wear testing. To ensure accurate mechanical and wear testing, the outermost resin-rich layers (breadskin) of the cured composite samples were carefully removed using fine-grit sandpaper before testing. This step helped expose the actual fiber-reinforced composite structure and eliminate inconsistencies caused by excess resin. Tensile testing specimens were prepared following ASTM D3039 standards, while flexural testing specimens adhered to ASTM D790 guidelines. For wear testing, specimens were shaped according to ASTM G99 standards, suitable for pin-on-disk testing. Dimensions of all specimens were measured using a micrometer to ensure precision [²⁸].

For the tensile test, specimens were prepared according to ASTM D3039 with dimensions of 250 mm × 25 mm × 3 mm (length × width × thickness). A constant crosshead speed (feed rate) of 2 mm/min was maintained during testing using a universal testing machine (UTM). Each formulation was tested using five specimens, and the average of the three most consistent results was reported. For the flexural test, samples were fabricated in accordance with ASTM D790, with dimensions of 125 mm × 13 mm × 3 mm. A three-point bending fixture was employed with a support span of 100 mm and a crosshead speed of 1.5 mm/min. Again, five replicates were tested per group to ensure repeatability. The flexural test was conducted using a three-point bending fixture with a support span of 100 mm and a crosshead speed of 1.5 mm/min, as per ASTM D790. A schematic illustration showing the dimensions of the tensile and flexural specimens is provided in Figure 4 for clarity and reproducibility.

Figure 4
Dimensions of the specimens.

Mechanical properties, including tensile and flexural strengths, were evaluated using a universal testing machine (UTM). All mechanical tests were carried out using a fully automated Universal Testing Machine (UTM, Model: Instron 3382, 100 kN capacity) equipped with Bluehill software for data acquisition and analysis. Hardness was measured using a Rockwell hardness tester (Model: Mitutoyo HR-430MR), calibrated for the HRB scale. Wear testing was performed on a pin-on-disc tribometer (Ducom TR-20LE), with wear volume and frictional force monitored in real-time. SEM micrographs were captured using a ZEISS EVO 18 scanning electron microscope operated at an accelerating voltage of 15 kV[²⁹]. The interfacial binding between the Kevlar fibers, epoxy matrix, and alumina filler was verified through detailed microstructural and elemental analysis. (SEM) was employed to observe the fracture and worn surfaces of the composites, revealing the extent of fiber pullout, matrix cracking, and particle dispersion [³⁰]. A strong fiber–matrix interface was indicated by minimal fiber debonding and the presence of epoxy remnants on the fractured Kevlar surfaces, suggesting effective stress transfer.

Furthermore, the distribution of alumina particles within the matrix and their integration with the epoxy network were assessed through (EDS), which confirmed the uniform presence of aluminum and oxygen across the composite surface. This uniform elemental mapping supported the conclusion that the alumina particles were well bonded within the resin phase. Additionally, the absence of large voids or interfacial gaps in SEM images further indicated good interfacial adhesion. Although direct chemical characterization techniques such as FTIR or XPS were not employed, the morphological and elemental evidence from SEM-EDS analysis provided sufficient qualitative validation of interfacial bonding in the composite system. Quantitative microstructural analysis and image measurements were conducted using ImageJ software (NIH, USA), while statistical evaluation of mechanical and wear data was performed using Design Expert software (v13, Stat-Ease Inc.) for ANOVA and response surface modeling [³¹]. In tensile testing, the specimens were clamped in the machine and subjected to tensile loads along the fiber axis until failure.

. Each mechanical test was conducted three times for statistical accuracy, and average values were reported. For each experimental group, a total of five specimens were fabricated and tested to evaluate tensile strength, flexural strength, and hardness. Similarly, five samples were prepared and tested for pin-on-disk wear measurements. Among them, the three most consistent results were selected based on minimal deviation and were used for calculating average values and standard deviations. This approach ensured both statistical rigor and practical consistency in assessing the effect of alumina content on the mechanical and tribological performance of the composites [³²].To ensure clarity and evaluate the consistency of the mechanical test results, the standard deviation values for tensile strength, flexural strength, and hardness were calculated and are presented in Table 3. This statistical data allows for a better understanding of the variability associated with each alumina concentration and supports the reproducibility of the observed trends. The inclusion of standard deviation also aids in comparing the effectiveness of different filler loadings under identical test conditions, thereby strengthening the reliability of the reported mechanical performance [³³].

Table 3
Mechanical properties of Kevlar-reinforced epoxy composites.

ALUMINA CONTENT (% wt)	TENSILE STRENGTH (MPa) ± SD	FLEXURAL STRENGTH (MPa) ± SD	HARDNESS (HRB) ± SD
0 (Control)	98.0 ± 1.5	152.0 ± 2.2	82 ± 1.0
2	112.0 ± 1.3	170.0 ± 2.0	87 ± 1.2
4	126.0 ± 1.1	185.0 ± 1.8	92 ± 1.1
6	121.0 ± 1.4	178.0 ± 2.3	94 ± 1.3

Wear testing was conducted using a pin-on-disk apparatus, where a cylindrical composite pin was pressed against a rotating steel disk under a constant load. For the wear test, cylindrical samples of 10 mm diameter and 30 mm height were prepared in accordance with ASTM G99 standards. The samples were carefully polished using 600-grit silicon carbide paper to ensure a uniform contact surface before testing. The wear rate was determined by measuring the mass loss of the specimen after a specific sliding distance. Tests were performed for a sliding distance of 500 meters under a normal load of 10 N. The selection of the wear test parameters—namely a 500-meter sliding distance, a normal load of 10 N, and a sliding velocity of 1.5 m/s—was based on a combination of literature benchmarks and conditions representative of moderate-load tribological applications. These values were adopted to simulate typical service conditions encountered in automotive and protective equipment components, where contact loads and speeds fall within similar operational ranges [³⁴]. Furthermore, previous studies involving particulate-filled epoxy composites have consistently used sliding distances between 300–600 meters and normal loads of 5–30 N to ensure measurable and comparable wear loss without causing catastrophic material failure The chosen parameters were also aligned with the recommendations of ASTM G99 for steady-state dry sliding wear tests, ensuring reproducibility and relevance. By maintaining consistent conditions across all samples, the impact of alumina content on wear behavior could be reliably assessed without introducing confounding effects from extreme test variables. The friction coefficient between the pin and the disk was recorded throughout the test. The wear test was conducted at a sliding velocity of 1.5 m/s and a track diameter of 80 mm, following ASTM G99 standards. The wear rate was calculated using the Equation (1).

(1) $$WearRate=\frac{\mathrm{\Delta }m}{\rho \cdot L}$$

The wear testing setup is shown in Figure 5. A control sample containing only Kevlar and epoxy without alumina filler was also prepared and tested to establish a baseline comparison. This allowed for a direct assessment of the role of Alumina in improving wear resistance and mechanical properties.

Figure 5
Pin-on-disk setup for wear and friction tests.

The hardness of the composite specimens was measured using a Rockwell hardness tester, with measurements taken at five different locations on each specimen to account for variations in alumina distribution. The average hardness value was calculated for each alumina content level. To analyze the wear mechanisms, the worn surfaces of the specimens were examined using SEM. SEM micrographs provided insights into the distribution of alumina particles and the nature of wear damage, such as abrasion, delamination, or matrix cracking. EDX was used alongside SEM to study the elemental composition of the worn surfaces and the interaction between the alumina particles and the epoxy matrix [³⁵]. All tests were conducted in a controlled environment with a temperature of 25°C and relative humidity of 50% to ensure consistent conditions. Statistical analysis, including ANOVA, was performed to determine the significance of alumina content on tensile strength, flexural strength, wear rate, and hardness. Statistical analysis helped identify trends and provided a basis for conclusions regarding the effects of Alumina on the composite properties. To account for variability in wear test results due to manual impregnation, error bars representing standard deviations have been included in all wear rate graphs. Additionally, statistical analysis using ANOVA was conducted to determine the significance of variations in wear behavior. The materials and methods employed in this study systematically addressed the preparation and testing of epoxy composites reinforced with Kevlar fibers and varying alumina content. By combining mechanical, wear, and microstructural analyses, the study provided a comprehensive understanding of the role of Alumina in enhancing the performance of these composites [³⁶]. To provide a comprehensive characterization of the reinforcement materials, additional SEM images of raw alumina powder and Kevlar fibers have been included to illustrate their surface morphology. Furthermore, an EDAX analysis of alumina particles was conducted to confirm their purity and composition. The results indicate that Alumina consists primarily of aluminum and oxygen peaks, with minor impurities below 1%. This confirms that the selected filler maintains its structural integrity and composition within the epoxy matrix.

3. RESULTS AND DISCUSSION

The results of this study highlight the significant influence of alumina (Al₂O₃) filler content on the mechanical and wear properties of Kevlar-reinforced epoxy composites. Key findings, including tensile strength, flexural strength, wear rate, and hardness, are discussed alongside microstructural analysis to elucidate the mechanisms responsible for the observed improvements. The role of alumina distribution and its interaction with the epoxy matrix and Kevlar fibers are explored in detail.

Figure 6 shows the SEM micrographs of the composite materials incorporating varying alumina (Al₂O₃) content (2%, 4%, and 6%). The images reveal distinct structural changes in the material’s surface morphology as the alumina content increases. At 2% alumina, the microstructure displays a relatively smooth surface, with some areas showing a uniform distribution of alumina particles. However, the dispersion of Alumina is not as pronounced, and some agglomeration of particles is visible, particularly at higher magnifications. This suggests that at low alumina loading, the reinforcement may not be fully integrated into the matrix, which could limit its potential for enhancing material properties [³⁷].

Figure 6
SEM image of the worn surface of the composite with 2% alumina.

As the alumina content is increased to 4% (Figure 7), the material exhibits a more pronounced and uniform distribution of alumina particles across the surface. The microstructure appears denser, with fewer visible agglomerates, indicating improved dispersion and integration of the alumina phase into the epoxy matrix. This may contribute to enhanced mechanical properties, such as increased hardness and wear resistance, as the alumina reinforcement is more evenly distributed to bear loads and resist wear [³⁸].

Figure 7
SEM image of the worn surface of the composite with 4% alumina.

At the highest alumina content (Figure 8), the SEM images show a dense packing of alumina particles, with the composite surface exhibiting a rougher texture and increased heterogeneity. The uniformity of alumina distribution seems to improve, although some regions still exhibit agglomeration, especially at the boundaries. . However, excessive alumina loading might lead to embrittlement or a reduction in the ductility of the composite, as indicated by the rougher surface morphology. These microstructural changes reflect the evolution of material behavior with increasing alumina content, offering insights into how alumina loading affects the surface properties and performance of the composites [³⁹]. The tensile strength of the composites exhibited an increasing trend with the addition of Alumina, peaking at 4% filler content before showing a slight reduction at 6%. The control specimen without Alumina displayed a tensile strength of 98 MPa, while composites with 2% and 4% alumina achieved tensile strengths of 112 MPa and 126 MPa, respectively. At 6% alumina content, the tensile strength decreased slightly to 121 MPa, likely due to filler agglomeration, which may have caused stress concentrations and localized weak spots within the matrix. These results align with previous studies demonstrating that moderate filler additions enhance the load transfer between the matrix and fibers, while excessive filler may impede the matrix’s bonding efficiency. Flexural strength followed a similar pattern, with the composite containing 4% alumina showing the highest strength at 185 MPa, compared to 152 MPa for the control specimen. The flexural modulus also increased with alumina content, indicating improved stiffness of the composite. These enhancements can be attributed to the uniform dispersion of alumina particles, which acted as barriers to crack propagation and improved load distribution under bending forces. From a fracture mechanics perspective, alumina particles can act as crack deflectors or crack-bridging agents within the epoxy matrix. When microcracks initiate under mechanical stress, the rigid ceramic particles interrupt their path, forcing the cracks to deviate, bifurcate, or blunt around the filler [⁴⁰]. This deflection mechanism increases the energy required for crack propagation, thereby enhancing fracture toughness and delaying failure. Additionally, the stiffening effect of Alumina contributes to an improved load-transfer mechanism, whereby applied stresses are more effectively distributed between the matrix and the Kevlar fibers. The presence of hard inclusions such as Alumina restricts plastic deformation zones around growing cracks, which reduces the probability of catastrophic failure. This dual role—crack mitigation and reinforcement—explains the observed improvements in tensile and flexural strength, as well as resistance to wear-induced surface degradation. However, at 6% alumina, the marginal reduction in flexural properties, from 185 MPa to 178 MPa, was attributed to filler aggregation, as observed in SEM analysis. Figure 9 further demonstrates the flexural performance of the composites.

Figure 8
SEM image of the worn surface of the composite with 6% alumina.

Figure 9
Stress-strain and flexural performance of Kevlar-reinforced epoxy composites with varying alumina content.

The wear resistance of the composites showed substantial improvement with the addition of Alumina. The wear rate, measured using the pin-on-disk apparatus, decreased significantly as the alumina content increased. The control specimen exhibited a wear rate of 3.12 × 10⁻⁴ mm³/N·m, while composites with 2%, 4%, and 6% Alumina recorded wear rates of 2.45 × 10⁻⁴, 1.89 × 10⁻⁴, and 1.92 × 10⁻⁴ mm³/N·m, respectively. The most notable improvement was observed at 4% alumina, where the wear rate decreased by approximately 40% compared to the control. The slightly higher wear rate at 6% alumina may have been caused by increased surface roughness and uneven filler distribution, as shown in the SEM micrographs of the worn surfaces [⁴¹]. Fractographic analysis of the tensile test specimens provided critical insights into the failure mechanisms and interfacial behavior as a function of alumina content. As shown in Figure 10, the control sample (0% alumina) exhibited pronounced fiber pullout and matrix cracking, indicating weak fiber–matrix adhesion. In contrast, the 4% alumina-reinforced composite revealed strong interfacial bonding with minimal void formation and tightly embedded fibers, which correlates with its superior tensile performance. The alumina particles were well integrated into the epoxy matrix, improving load transfer and restricting fiber debonding. At 6% alumina, although the matrix showed improved coverage, some regions exhibited microvoids and signs of filler agglomeration, potentially acting as crack initiation sites. These microstructural features affirm that the optimal interfacial integrity and fracture resistance were achieved at 4% alumina, beyond which the mechanical advantages were slightly offset by filler-induced defects.

Figure 10
SEM fractography images of Kevlar/epoxy composites with 0%, 4%, and 6% alumina filler content.

This improvement in wear resistance is attributed to the high hardness and thermal stability of Alumina, which enhanced the matrix’s resistance to abrasive forces and material removal. The enhancements in mechanical and wear properties due to alumina reinforcement can be theoretically explained by multiple synergistic mechanisms. Alumina, being a high-modulus ceramic, introduces rigid particulates into the epoxy matrix, which serve to hinder the mobility of polymer chains and improve load transfer efficiency at the filler-matrix interface. This reinforcement mechanism increases stiffness and tensile strength by reducing localized deformation under stress. In addition, the hardness of alumina particles contributes to wear resistance by forming a protective micro-barrier layer on the composite surface during sliding, which limits material removal through abrasion. The fine particle distribution further reduces the effective contact area subjected to friction, minimizing energy dissipation and surface damage. Moreover, the thermal stability of Alumina delays the onset of softening or degradation under frictional heating, thereby preserving structural integrity under tribological stress [⁴²]. Together, these mechanisms provide a comprehensive explanation for the improvements observed in the composite’s performance.

The coefficient of friction (COF) showed a slight decrease with increasing alumina content. The control specimen exhibited an average COF of 0.52, which dropped to 0.46 for the composite with 4% alumina. The reduction in COF suggests that Alumina acted as a lubricant during sliding, reducing the frictional forces between the composite pin and the steel disk. Hardness measurements demonstrated a steady increase with the addition of Alumina. The control specimen recorded an average Rockwell hardness of 82 HRB, while composites with 2%, 4%, and 6% alumina exhibited hardness values of 87 HRB, 92 HRB, and 94 HRB, respectively. This consistent increase highlights the role of alumina particles in reinforcing the matrix and resisting localized deformation under load. To verify the statistical significance of the observed increase in hardness with varying alumina content, a one-way analysis of variance (ANOVA) was performed at a 95% confidence level. The ANOVA results indicated a p-value of less than 0.05, confirming that the differences in mean hardness values among the composite samples were statistically significant [⁴³]. This supports the conclusion that alumina content has a measurable and consistent impact on the hardness of the Kevlar-reinforced epoxy composites. The outcome reinforces the assertion that the increase in surface resistance to indentation is not due to random variation but rather a direct result of alumina incorporation and its uniform dispersion within the matrix. The uniform distribution of Alumina, particularly at 4%, contributed to the material’s enhanced resistance to indentation, as seen in Figure 11.

Figure 11
Rockwell hardness comparison.

Microstructural analysis using SEM provided insights into the wear mechanisms and alumina distribution within the composites. The worn surface of the control specimen showed extensive matrix cracking and fiber exposure, indicating poor resistance to wear forces. In contrast, composites with Alumina exhibited reduced fiber pullout and matrix degradation. The SEM image of the composite with 4% alumina. Energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the presence and distribution of alumina particles on the worn surfaces. Although EDS was employed to confirm the elemental composition of worn surfaces, a full quantitative mapping of the wear debris was not conducted in this study. It is recognized that EDS-based wear debris analysis can offer detailed insights into localized elemental concentrations, material transfer, and third-body abrasion effects, thereby enabling a more precise classification of the wear mechanisms involved. The current analysis relied primarily on SEM surface morphology to identify dominant wear features such as matrix cracking, fiber pullout, and abrasive grooves. For more robust conclusions, future work should incorporate EDS mapping of debris fields and wear tracks to quantitatively assess material removal patterns and correlate them with filler distribution and particle detachment behavior. Such an approach would complement microstructural observations and further clarify the role of Alumina in altering tribological performance under dry sliding conditions. The EDX spectra for the 4% alumina composite revealed a uniform presence of aluminum and oxygen peaks, indicating consistent dispersion of Alumina. The analysis also showed a reduced presence of wear debris compared to the control, further validating the protective role of Alumina in the matrix [⁴⁴].

The statistical analysis supported the observed trends, with ANOVA results confirming the significant influence of alumina content on tensile strength, flexural strength, wear rate, and hardness. The p-values for all properties were below 0.05, indicating statistically significant differences between the groups. The optimal alumina content for the tested properties was determined to be 4%, as it provided the best balance between mechanical performance and wear resistance without the adverse effects of agglomeration. The balance between particle dispersion and interfacial load transfer mechanisms can theoretically explain the observed optimum at 4% alumina content. At this concentration, the alumina particles are sufficiently dispersed to form an interconnected load-bearing microstructure within the epoxy matrix, enhancing stress distribution and hindering crack propagation. Moreover, 4% loading offers a high enough filler surface area for effective interfacial bonding with the matrix, improving mechanical strength and wear resistance without oversaturating the matrix [⁴⁵]. When the concentration exceeds this threshold, particle agglomeration becomes energetically favorable, leading to the formation of localized stress concentrations and interfacial voids, which reduce the composite’s structural integrity. Hence, 4% represents a critical point where the reinforcing effect of Alumina is maximized while avoiding the detrimental effects associated with particle clustering and matrix discontinuity.

Figure 12 shows the relationship between the Coefficient of Friction (μ) and the applied load (N) for various samples. As the applied load increases from 5 N to 30 N, the coefficient of friction (μ) also increases for all samples. Sample 1 shows the highest initial frictional behavior with a value of 0.2262 at 5 N, which rises to 0.1382 at 30 N, indicating an increase of approximately 39%. This increase in friction can be attributed to the increased contact pressure between the materials as the load increases, causing more resistance. Sample 2, which initially has a friction coefficient of 0.1088 at 5 N, shows a smaller increase, reaching 0.0647 at 30 N, with only a 40% increase [⁴⁶]. This suggests that Sample 2 has better resistance to friction under higher load conditions. The differences across samples can be attributed to their varying material properties, such as surface roughness and material composition, which affect their ability to resist sliding motion under stress. Typically, materials with higher hardness and smoother surfaces tend to show lower increases in friction with load, as seen in Sample 2.

Figure 12
Coefficient of Friction (μ) vs. Applied Load (N).

Figure 13 illustrates the wear loss (g) as a function of the applied load (N). Wear loss increases with an increase in applied load. At 5 N, Sample 1 shows a wear loss of 1.131 g, which increases to 4.145 g at 30 N, a notable increase of approximately 267% [⁴⁷]. Similarly, Sample 2 shows a 257% increase, from 0.544 g at 5 N to 1.942 g at 30 N. This is consistent with the general expectation that higher loads result in higher wear, as increased pressure leads to more material deformation and removal. The increase in wear loss is a direct consequence of the material’s inability to withstand the higher mechanical forces applied to it, leading to surface degradation. Sample 1, showing the highest wear loss, is likely more prone to wear due to a combination of softer surface material and potentially greater roughness, which accelerates abrasion under higher loads. Sample 3 and Sample 4, which show lower wear loss values, demonstrate better resistance to wear, suggesting that their compositions or structures may be more robust under increased stress [⁴⁸].

Figure 13
Wear Loss (g) vs. Applied Load (N).

Figure 14 shows the correlation between wear loss (g) and the coefficient of friction (μ). A positive relationship is observed, with the coefficient of friction increasing as wear loss increases. Sample 1, which exhibits the highest wear loss of 4.145 g at 30 N, also has the highest coefficient of friction of 0.1382 at the same load. This confirms that materials exhibiting higher friction tend to suffer more wear, likely due to the increased resistance to sliding, which leads to more material being removed [⁴⁹]. Sample 2, with lower wear loss (1.942 g at 30 N), has a lower coefficient of friction (0.0647 at 30 N), suggesting that the material undergoes less wear due to its lower frictional resistance. The direct relationship between friction and wear loss can be attributed to the fundamental principle that higher friction increases the mechanical energy dissipated in the form of heat and wear, thus leading to material degradation. Additionally, the coefficient of friction is influenced by factors like surface hardness, lubrication, and the nature of the contact surfaces, which explain the varying behavior among the samples. In general, the data supports the expectation that materials with lower friction coefficients tend to have lower wear losses, highlighting the importance of optimizing [⁵⁰].

Figure 14
Wear Loss (g) vs. Coefficient of Friction (μ).

Figure 15 shows the stress-strain behavior of Kevlar-reinforced epoxy composites with varying Alumina (Al₂O₃) content—0%, 2%, 4%, and 6%. The graph reveals that the inclusion of Alumina significantly enhances the mechanical performance, especially the tensile strength and stiffness of the composite. The control sample (0% Al₂O₃) shows maximum stress of 98 MPa at 2.5% strain, serving as the baseline. When 2% alumina is introduced, the peak stress increases to 112 MPa, representing a 14.3% improvement. This enhancement is attributed to the intrinsic hardness and rigidity of alumina particles, which improve load-bearing capacity and stress transfer within the epoxy matrix. At 4% alumina, the tensile strength reaches its maximum value of 126 MPa—a 28.6% increase over the control and a 12.5% increase compared to 2% alumina. This improvement is due to the more uniform dispersion of filler at this concentration, which provides effective reinforcement without inducing significant agglomeration. The alumina particles act as stress-transfer bridges between the matrix and the Kevlar fibers, improving both stiffness and strength [⁵¹]. However, with 6% alumina, the tensile strength slightly decreases to 121 MPa—a 4% drop from the 4% optimum. This reduction is caused by particle agglomeration at higher filler content, which leads to stress concentration points and microvoid formation, ultimately compromising the interfacial bonding. Despite this, the 6% sample still shows a 23.5% improvement over control, confirming that Alumina remains beneficial but must be optimized to prevent diminishing returns in mechanical performance.

Figure 15
Wear Loss (g) vs. Coefficient of Friction (μ).

The findings of this study highlight the potential of Alumina as an effective filler for improving the performance of Kevlar-reinforced epoxy composites. The observed improvements in tensile and flexural strengths, wear resistance, and hardness make these composites suitable for applications in harsh environments, such as aerospace, automotive, and protective gear. The study also underscores the importance of maintaining an optimal filler content to prevent issues related to agglomeration and uneven distribution, which can compromise the composite’s properties. The incorporation of Alumina into Kevlar-reinforced composites significantly enhanced their mechanical and wear properties. The optimal alumina content was identified as 4%, offering the best combination of strength, stiffness, wear resistance, and hardness [⁵²]. The microstructural analysis provided valuable insights into the wear mechanisms and the role of Alumina in reinforcing the composite. Future research could explore the effects of varying alumina particle sizes, alternative filler materials, and hybrid reinforcements to further optimize the performance of such composite systems. RSM is a statistical technique used for modeling and analyzing the relationships between multiple variables and their effect on a response or outcome of interest. In this study, RSM was employed to optimize the mechanical and tribological properties of Kevlar-reinforced epoxy composites filled with Alumina (Al₂O₃). The primary objective of using RSM was to identify the optimal alumina content that maximizes tensile strength, flexural strength, wear resistance, and hardness while simultaneously analyzing the interaction between the alumina content and applied load [⁵³].

A CCD was utilized to conduct RSM analysis, which is effective for studying the effects of several variables and their interactions. In this study, two factors were considered: alumina content (ranging from 2%, 4%, and 6%) and applied load (ranging from 5 N, 10 N, and 30 N). The key responses measured were tensile strength, flexural strength, wear rate, and hardness. Data was collected based on different combinations of the factors, leading to a total of 9 experimental runs. For each combination, the mechanical properties (tensile strength, flexural strength, and hardness) and tribological properties (wear rate) were measured and recorded. These measurements were used to fit a second-order polynomial model for each response. The general form of the model is given by Equation (2):

(2) $$Y=\beta 0+\beta 1X1+\beta 2X2+\beta 3X12+\beta 4X22+\beta 5X1X2Y=\beta 0+\beta 1X1+\beta 2X2+\beta 3X_1^2+\beta 4X_2^2+\beta 5X1X2$$

Once the models were fitted, ANOVA was performed to determine the significance of each factor and their interactions. The p-values for each factor and interaction term were calculated, and those with p-values less than 0.05 were considered statistically significant. This helped in identifying the most influential factors on the responses and evaluating the optimal alumina content and applied load that result in the best mechanical and tribological performance. The goal of the RSM analysis was to determine the optimal alumina content that maximizes the mechanical and tribological properties. The RSM model was used to predict responses at various combinations of alumina content and applied load. The optimal conditions were found to be at 4% alumina content, where the composites exhibited the highest tensile strength, flexural strength, wear resistance, and hardness [⁵⁴]. This optimal alumina content provided a balance between the mechanical properties and wear resistance, showing that excessive alumina loading (beyond 4%) led to a slight decrease in tensile strength and increased wear due to particle agglomeration. To visualize the results of the RSM analysis, 3D surface plots were generated for each response. These plots illustrate the relationships between the factors and responses. For tensile strength, the plot showed a marked increase up to 4% alumina, followed by a slight decrease at 6%, which could be attributed to the filler agglomeration. Similarly, flexural strength showed the highest performance at 4% alumina. Wear rate decreased significantly as the alumina content increased, with the best performance observed at 4%, while hardness increased steadily with increasing alumina content. These surface plots allowed for a clear understanding of how alumina content and applied load interacted to affect the composite’s properties [⁵⁵].

Figure 16 shows the relationship between tensile strength, applied load, and alumina content. The surface plot reveals that as alumina content increases, the tensile strength improves up to 4% alumina content, after which a slight decrease is observed at 6%. At 2% alumina, the tensile strength is 112 MPa, which increases to 126 MPa at 4% alumina, representing a 12.5% increase. This improvement can be attributed to the uniform dispersion of alumina particles, which enhances the load-bearing capacity and contributes to the material’s strength. However, at 6% alumina, the tensile strength drops slightly to 121 MPa, a decrease of approximately 4% from the 4% alumina composite. This reduction in strength can be due to filler agglomeration, which creates localized weak spots and prevents optimal bonding between the alumina particles and the epoxy matrix.

Figure 16
3D Surface Plot - Tensile Strength (MPa) vs. Applied Load and Alumina Content.

Figure 17 shows the flexural strength of the composite in relation to alumina content and applied load. The surface plot demonstrates that the flexural strength increases with the addition of Alumina, with the maximum value achieved at 4%. The composite with 4% alumina exhibits a flexural strength of 185 MPa, which is a 21.6% increase from the control specimen’s flexural strength of 152 MPa. This improvement is due to the reinforcement effect of alumina particles, which help in resisting bending forces. However, when alumina content reaches 6%, the flexural strength decreases slightly to 178 MPa, reflecting a 3.8% reduction compared to the 4% alumina composite. These results underscore the importance of maintaining a balance in filler content to maximize the composite’s strength under bending stress [⁵⁶]. Figure 18 shows the wear rate as a function of alumina content and applied load. The plot reveals that wear resistance improves significantly with increasing alumina content, peaking at 4% alumina. At 2% alumina, the wear rate is 2.45 × 10⁻⁴ mm³/N·m, and it decreases to 1.89 × 10⁻⁴ mm³/N·m at 4% alumina, reflecting a 23% improvement. This improvement acts as a barrier to wear and reduces material degradation during sliding contact. However, at 6% alumina, the wear rate slightly increases to 1.92 × 10⁻⁴ mm³/N·m, a 1.6% increase compared to the 4% alumina composite. This can be explained by the increased surface roughness and uneven filler distribution at higher alumina content, which may lead to increased wear under certain conditions. The optimal alumina content for wear resistance is found to be 4%, which provides the best balance of wear resistance and composite integrity [⁵⁷].

Figure 17
3D surface plot - Flexural Strength (MPa) vs. Applied Load and Alumina Content.

Figure 18
3D Surfcae plot - Wear Rate (mm³/N·m) vs. Applied Load and Alumina Content.

Figure 19 shows the hardness of the composite as a function of alumina content and applied load. The surface plot indicates a steady increase in hardness with the addition of Alumina. The hardness value for the control specimen is 82 HRB, and it increases to 92 HRB with 4% alumina, representing a 12.2% improvement. This increase is due to the incorporation of Alumina, which reinforces the epoxy matrix and resists localized indentation and deformation. At 6% alumina, the hardness further increases to 94 HRB, a 2.2% improvement compared to the 4% alumina composite. This demonstrates that the addition of Alumina continues to enhance the hardness, contributing to better wear resistance and strength. The consistent increase in hardness highlights the positive effect of Alumina on improving the material’s resistance to external forces, making the composite more suitable for high-performance applications [⁵⁸].

Figure 19
3D Surface plot Hardness (HRB) vs. Applied Load and Alumina Content.

The surface plots reveal a clear relationship between alumina content and the mechanical and tribological properties of Kevlar-reinforced epoxy composites. Alumina addition enhances tensile strength, flexural strength, wear resistance, and hardness, with the optimal alumina content being 4%. However, excessive alumina loading beyond 4% can lead to diminished performance, particularly in terms of tensile strength and wear resistance, due to agglomeration and uneven particle dispersion. These findings underscore the importance of optimizing filler content to achieve the desired balance of mechanical strength and wear resistance in composite materials [⁵⁹,⁶⁰,⁶¹].

The RSM analysis successfully identified the optimal alumina content for improving the mechanical and tribological properties of Kevlar-reinforced epoxy composites. Alumina content of 4% was found to provide the best overall performance in terms of tensile strength, flexural strength, wear resistance, and hardness. However, excessive alumina content (6%) caused some reduction in the mechanical properties, likely due to agglomeration of particles. The study’s findings suggest that the incorporation of Alumina into the composite significantly enhances its wear resistance and hardness, making it a promising solution for high-performance applications. Future research could explore varying alumina particle sizes, alternative fillers, and hybrid composite systems to further enhance the properties of these materials [⁶²,⁶³,⁶⁴,⁶⁵]. The application of RSM in this study demonstrated its effectiveness in optimizing composite material properties by providing a systematic approach to explore factor interactions and optimize the composition. This methodology can be extended to other composite materials, paving the way for more efficient and targeted material development in the field of advanced engineering applications. Table 4 presents a comparison of mechanical properties—tensile strength, flexural strength, and impact strength—of Kevlar/epoxy composites reinforced with different fillers, including Alumina (Al₂O₃), silicon carbide (SiC), titanium dioxide (TiO₂), and hybrid reinforcements [⁶⁶].

Table 4
Comparative mechanical properties of kevlar/epoxy composites with various fillers.

COMPOSITE TYPE	FILLER TYPE & CONTENT	TENSILE STRENGTH (MPa)
Kevlar/Epoxy + 3% Al2O3 + 2% SiC	Hybrid Nanoparticles	Superior strength and rigidity compared to other compositions [67]
Kevlar/Epoxy + 4 wt% TiO2	Titanium Dioxide Nanoparticles	Enhanced mechanical strength compared to 2% and 6% TiO2 compositions [68]
Kevlar/Epoxy + 7 wt% SiC	Silicon Carbide Microparticles	289.22 [69]
Kevlar/Glass Fiber/Epoxy + Nano-Al2O3	Hybrid Fibers with Alumina Nanoparticles	Improved tensile strength and impact energy absorption compared to composites without nanoparticles [70]
Kevlar/Epoxy + Organoclay (Modified Bentonite)	Organoclay Nanoparticles	Improved tensile and bending strength due to better fiber–matrix adhesion [71]

5. CONCLUSIONS

This study systematically explored the enhancement of the mechanical and tribological properties of Kevlar-reinforced epoxy composites by incorporating Alumina (Al₂O₃) as a filler. The experimental results showed that the addition of Alumina significantly improved the tensile strength, flexural strength, wear resistance, and hardness of the composites. Specifically, the composite with 4% alumina exhibited the best overall performance.

• The incorporation of 4 wt% alumina into Kevlar-epoxy composites increased tensile strength from 98 MPa to 126 MPa, marking a 28% enhancement over the control.

• Flexural strength improved by 21.6%, rising from 152 MPa (control) to 185 MPa at 4 wt% alumina, indicating better load-bearing capacity under bending.

• Hardness values increased from 82 HRB to 94 HRB with 6 wt% alumina, showing a 14.6% gain in surface resistance.

• The wear rate was reduced by 39.4%, from 3.12 × 10⁻⁴ mm³/N·m (control) to 1.89 × 10⁻⁴ mm³/N·m at 4 wt% alumina, indicating improved abrasion resistance.

• SEM analysis revealed uniform dispersion and good interfacial bonding at 4 wt% alumina, while higher loading led to agglomeration and microstructural defects.

• Based on the measured improvements, the 4 wt% alumina composite is suitable for applications like automotive brake linings, aerospace panels, and protective gear.

Based on the enhanced mechanical strength, wear resistance, and thermal stability demonstrated by the alumina-reinforced Kevlar-epoxy composites, several potential applications have been identified. These composites are particularly suitable for use in lightweight armor panels and ballistic protection gear, where a high strength-to-weight ratio and energy absorption capability are critical. In the automotive sector, they can be employed in brake pad backings, clutch plates, and underbody shields, where wear resistance and thermal endurance are required. Additionally, aerospace components such as interior panels, structural stiffeners, and protective covers may benefit from the improved dimensional stability and surface hardness provided by the alumina reinforcement. These materials may also be used in industrial environments as wear-resistant liners, drive belts, and composite gears, where contact with abrasive surfaces is frequent. The inclusion of Alumina enhances their utility in scenarios demanding resistance to sliding wear and mechanical vibrations. The optimized composites developed in this study are well-positioned for deployment in demanding structural, tribological, and protective applications.

Future research should focus on exploring varying alumina particle sizes, hybrid fillers, and their interactions with other fibers to further enhance the performance of these composites. Additionally, the effect of different processing methods on the dispersion of fillers and the impact on composite performance warrants further investigation. The findings from this study contribute to advancing the design and application of high-performance epoxy-based composites in industries requiring superior mechanical and tribological properties.

6. ACKNOWLEDGMENTS

The author B. Saleh extends his appreciation to Taif University, Saudi Arabia, for supporting this work through project number (TU-DSPP-2024-91). This research was funded by Taif University, Saudi Arabia, Project No. (TU-DSPP-2024-91).

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Publication Dates

• Publication in this collection
  23 May 2025
• Date of issue
  2025

History

• Received
  26 Feb 2025
• Accepted
  14 Apr 2025