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Article

Enhanced Mechanical Properties of Epoxy Composites Reinforced with Silane-Modified Al2O3 Nanoparticles: An Experimental Study

by
Ting Zhang
1,
Xujiang Chao
2,*,
Junhao Liang
1,
Bin Wang
1 and
Mengmeng Sun
1
1
School of Materials and Engineering, Xi’an Polytechnic University, Xi’an 710048, China
2
School of Mechanical Engineering, Northwestern Polytechnical University, Xi’an 710072, China
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(5), 252; https://doi.org/10.3390/jcs9050252
Submission received: 14 April 2025 / Revised: 14 May 2025 / Accepted: 16 May 2025 / Published: 19 May 2025
(This article belongs to the Special Issue Editorial Board Members' Collection Series: Mechanical Analysis of Composite Materials)
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Abstract

:
This study investigates the mechanical performance of epoxy resin composites reinforced with silane coupling agent-modified Al2O3 nanoparticles (m-Nano-Al2O3/epoxy). Three silane coupling agents (KH550, KH560, and KH570) were employed to functionalize the Al2O3 nanoparticles, and their chemical structures were confirmed via Fourier transform infrared spectroscopy (FTIR). The microstructure and elemental distribution of the composites were characterized using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Mechanical properties, including tensile strength and hardness, were evaluated using a universal testing machine and a Rockwell hardness tester, respectively. The incorporation of m-Nano-Al2O3 significantly enhances the mechanical properties of the epoxy matrix. Compared to pure epoxy, the KH570-modified composites demonstrate a remarkable 49.1% improvement in tensile strength and an 8.8% increase in hardness. These findings highlight the potential of surface-modified Al2O3 nanoparticles as effective reinforcements for high-performance epoxy composites.

1. Introduction

Epoxy resin, one of the most essential thermosetting polymers, has served as a fundamental matrix material in composites and structural applications for nearly half a century due to its high modulus, exceptional chemical resistance, processability, and commercial availability [1,2,3,4,5]. These attributes have enabled its widespread use in adhesives, coatings, laminates, and construction materials [6,7,8,9]. However, the intrinsic brittleness of epoxy resins remains a critical limitation for high-performance applications.
To address this issue, nanoparticle reinforcement has emerged as a promising strategy, and there are many relevant research papers in this area. Bekeshev et al. [10] prepared epoxy composite reinforced with aluminum nitride modified with amino-acetic acid. They found that the functionalization of the nanofiller has a significant effect on mechanical properties of epoxy nanocomposites. For example, the bending stress and bending modulus of epoxy nanocomposites increase by 35% and 80%, respectively. Amirbeygi et al. [11] investigated the effects of silane-modified graphene on the mechanical properties of epoxy-based nanocomposites They found that the highest values of ILSS and compressive strength were related to the 0.3 wt% SGr–epoxy nanocomposite. Ghani [3] et al. fabricated epoxy composites reinforced with nanoparticles (WC and TiO2) and carbon fiber to improve their mechanical performance especially impact strength. The results showed that the highest improvement in impact resistance by the addition of 1.5% TiO2 is found to be about 420% compared to pure epoxy. However, a key challenge still lies in the weak interfacial bonding between inorganic nanoparticles and the organic polymer matrix [12,13,14], which often leads to poor stress transfer and suboptimal mechanical performance [15,16]. Surface modification of nanoparticles via coupling agents can significantly enhance interfacial compatibility. Silane coupling agents, in particular, have gained prominence due to their bifunctional molecular structure: one end chemically bonds with inorganic particle surfaces (e.g., via –SiOH groups), while the other interacts with the organic matrix through physical or chemical linkages, effectively bridging the two phases [17,18].
In recent years, epoxy nanocomposites have garnered substantial research interest for their ability to improve mechanical properties without significantly compromising the resin’s crosslinking density [19,20,21,22]. Among various nanofillers, Al2O3 nanoparticles stand out due to their high stiffness and thermal stability [23,24,25]. However, their tendency to agglomerate necessitates effective surface modification to ensure uniform dispersion and strong interfacial adhesion. A variety of silane coupling agents can be used to carry out the surface modification of Nano-Al2O3.
In this study, we fabricate epoxy nanocomposites reinforced with silane-modified Al2O3 nanoparticles (KH550/KH560/KH570). The nanoparticles’ dispersion and interfacial bonding are characterized via scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) and assisted by a direct examination of the dispersibility of modified Nano-Al2O3 in epoxy resin through direct observation method in the course of the experiment, while the mechanical enhancements—particularly in tensile strength and hardness—are systematically evaluated. Our findings demonstrate that optimized surface modification significantly improves load transfer efficiency, offering a pathway to high-performance epoxy composites.

2. Experimental

2.1. Materials

The epoxy resin (E-51, bisphenol-A type, epoxy equivalent weight 196) and curing agent (2-ethyl-4-methylimidazole) were procured from Shanghai Aotun Chemical Technology Co., Ltd. (Shanghai, China) and Shanghai Aichun Biological Technology Co., Ltd. (Shanghai, China), respectively. Nano-Al2O3 particles (<20 nm, γ-phase) and silane coupling agents (KH550: γ-aminopropyl tri-ethoxy silane, NH2CH2CH2CH2Si(OC2H5)3; KH560: γ-glycidyl ether oxy-propyl trimethoxy silane CH2CHCH2O(CH2)3Si(OCH3)3; KH570: γ-methacryloxy propyl trimethoxy silane; CH2=C(CH3)COO(CH2)3Si(OCH3)3 purity ≥ 99%) were supplied by Shanghai Aladdin Reagent Co., Ltd. (Shanghai, China). Solvents (xylene, ethanol, acetone) and acetic acid were analytical-grade reagents from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water (resistivity > 18 MΩ·cm) was prepared in-house.

2.2. The Surface Modification of Nano-Al2O3

The modification process (Figure 1) involved two key steps:
(1)
Pretreatment of Nano-Al2O3:
  • Raw nanoparticles (50 g) were thermally treated at 200 °C for 1 h to eliminate surface adsorbates, followed by ultrasonic cleaning in acetone (5 min) and drying at 60 °C.
(2)
Silane Grafting:
  • Hydrolysis: A~5 wt% silane/ethanol solution was acidified to pH 4 with acetic acid, then hydrolyzed at 30–35 °C for 2 h under stirring.
  • Grafting: 5 g of pretreated Al2O3 was dispersed in the hydrolyzed solution, and the reaction proceeded at 60 °C for 4 h.
  • Purification: Modified particles were collected by centrifugation (2000 RCF, 1 min), washed with ethanol to remove physiosorbed silanes, and vacuum-dried at 70 °C (4 h).
It should be noted that identical procedures were applied for KH550 and KH560 modifications.
Jcs 09 00252 g001
Figure 1. Schematic diagram of modification process of Nano-Al2O3 modified by the silane coupling agent.
Figure 1. Schematic diagram of modification process of Nano-Al2O3 modified by the silane coupling agent.
Jcs 09 00252 g001

2.3. Preparation of m-Nano-Al2O3/Epoxy Composites

The schematic diagram for preparing m-Nano-Al2O3-reinforced composites is shown in Figure 2. The m-Nano-Al2O3 was dispersed in acetone for 30 min. A total of 20 g epoxy resin was preheated and then added and stirred at 45 °C for 1 h to ensure uniform dispersion of m-Nano-Al2O3. A mixture of xylene diluent (9 wt%) and 2-ethyl-4-methylimidazole curing agent (4 wt%) was slowly added to the resin and stirred until well mixed. The mixture was then defoamed in a 70 °C water bath for 10 min and poured into a silicone mold while still hot. Finally, the m-Nano-Al2O3/epoxy composites were cured at 60–80 °C for 24 h in a vacuum.

2.4. Characterization

Fourier transform infrared (FTIR) spectroscopy was performed using a Thermo Fisher Scientific IS50 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) in the range of 400–4000 cm−1 to characterize the m-Nano-Al2O3 powder samples. The surface and cross-sectional morphology of the m-Nano-Al2O3/epoxy composites, as well as the dispersion effect of m-Nano-Al2O3 in the epoxy, were observed using scanning electron microscopy (SEM, TEXTEST QUANTA-450-FEG, Thermo Fisher Scientific, Waltham, MA, USA).
The mechanical properties of the samples were tested using a universal testing machine (UTM5504, Jinan Hengsi Shengda Instrument Co., Ltd., Chongqing, China). Tensile tests were conducted at room temperature according to GB/T 1040.2-2006 [26] at a tensile rate of 8 mm/min, equipped with an extension meter. Results are the average of at least three samples. Specimens for tensile tests were machined from the cured m-Nano-Al2O3/epoxy composites, the dimensions of the sample are shown in Figure 3. The roughness of the composite was detected by a 3D laser confocal microscope (OLYMPUS, OLS5100-SAF, Olympus Corporation, Tokyo, Japan). The hardness of the composite surfaces was measured using a Vickers hardness tester (HV-10MPTA, Laizhou Weiyi Experimental Machinery Manufacture Co., Ltd., Laizhou City, China) with a load of 500 g (4.903 N) and a dwell time of 5 s.

3. Results and Discussion

3.1. FTIR Analyses of Surface Modification

The FT–IR spectra (Figure 4) confirm successful grafting of silane agents onto Al2O3 nanoparticles.
The infrared spectra of different silane coupling agents and Nano-Al2O3 before and after modification are shown in Figure 4. The characteristic peaks in these four FTIR spectra at 3452 cm−1 and 1631 cm−1 indicate the stretching vibration and bending vibration, respectively, of –OH groups on the surfaces of the Nano-Al2O3 particles [27,28]. However, after modification by KH550, the intensity of the peak at 3452 cm−1 decreases significantly, indicating a reduction in the number of –OH groups [16,28]. A peak for Si-O-Si appears at 1080 cm−1, indicating effective linkage between KH550 and the Nano-Al2O3 particles [29]. Compared with unmodified Nano-Al2O3, a sharp increase in the intensity peak of H–O–H at about 1631 cm−1 is observed, indicating the coupling reaction of Nano-Al2O3 particles with KH560 silane. The absorption peaks at 1687 cm−1 and 1421 cm−1 correspond to the C=O and Si–C–H vibrations of the silane coupling agent KH570, respectively, indicating that KH570 was successfully grafted onto the surface of Nano-Al2O3. The characteristic peak of C=O at 1687 cm−1 indicates that KH570 first undergoes hydrolysis, followed by dehydration and condensation to form oligomers, and then reacts with the-OH groups on the surface of the nanoparticles through dehydration to form partial covalent bonds [30,31]. Subsequently, the coupling agent coats the surface of the nanoparticles. Through FTIR infrared spectroscopy analysis, it can be analyzed that all the three silane coupling agents have reacted with the –OH groups on the Nano-Al2O3 particles, confirming successful grafting.

3.2. Dispersion Behavior

Nano-Al2O3 has a large specific surface area and high surface energy, which makes it prone to agglomeration. As a result, the dispersion of unmodified Nano-Al2O3 in the matrix is poor. After modification, the coupling agent combines with the hydroxyl groups on the surface of Nano-Al2O3 to form a hydrogen bond, thereby organically connecting the Nano-Al2O3 with the resin matrix, this enhances the affinity between Nano-Al2O3 and the resin matrix and improves its dispersibility.
To further determine the effect of different coupling agents on the dispersion of Nano-Al2O3, modified Nano-Al2O3 was dispersed in epoxy resin dyed with Congo red, Specifically, 2.5 g of Congo red was added to 20 g of epoxy resin. The mixture was stirred evenly at room temperature and then left undisturbed for a week. As shown in Figure 5, Nano-Al2O3 modified by KH550 and KH570 was well dispersed in the epoxy resin, while unmodified and KH560 modified Nano-Al2O3 exhibited poor dispersion. Among them, KH570-modified Nano-Al2O3 had the best dispersion effect, with no sedimentation observed at the bottom of the beaker. In contrast, KH560-modified Nano-Al2O3 had the worst dispersion effect, with a large amount of sedimentation appearing at the bottom of the beaker.
Based on the above analysis, the dispersion performance of Nano-Al2O3 modified with silane coupling agent KH570 in epoxy resin is the best, and the coating effect is also the best. Therefore, in the following experiments, the preparation of m-Nano-Al2O3/epoxy composite used KH570 modified Nano-Al2O3 particles as inorganic fillers.

3.3. Microstructural Evaluation

The dispersion of m-Nano-Al2O3 in epoxy resin will affect its mechanical strengthening effect. The optical photograph of m-Nano-Al2O3/epoxy composites is shown in Figure 6. SEM was used to observe the dispersion of KH570 to m-Nano-Al2O3 in the resin. The surface microstructure and elemental surface scanning of m-Nano-Al2O3/epoxy resin composites are shown in Figure 7.
As shown in Figure 6, with the increase in the filling amount of modified Nano-Al2O3, the number of bubbles and defects on the surface and inside of the composite gradually increases. The surface of the unfilled epoxy resin is very smooth and free of obvious defects. The surface of the 0.5 wt% m-Nano-Al2O3/epoxy resin composite is relatively flat. However, due to the low content of Nano-Al2O3, no Al element is detected. In the epoxy resin composites with 1.0 wt% and 1.5 wt% m-Nano-Al2O3 loadings, the nanoparticles are more evenly distributed in the epoxy matrix. As the content of m-Nano-Al2O3 increases to 2 wt%, particle aggregation occurs. This is because the steric hindrance effect exceeds the surface modification during curing, as shown in Figure 8e. When the filling amount of m-Nano-Al2O3 continues to increase to 3.0 wt%, pore defects appear on the surface of the epoxy resin composite material. As the distribution of nanoparticles becomes too dense, the number and size of pore defects also increase.

3.4. Mechanical Performance Optimization

The tensile strength of m-Nano-Al2O3 reinforced epoxy resin composites is shown in Table 1 and Figure 8. The appropriate addition of m-Nano-Al2O3 can effectively improve the tensile strength of composite. The average tensile strength of pure epoxy resin is 34.8 MPa. When 0.5 wt% m-Nano-Al2O3 is added, the average tensile strength of the composite gradually increases to 43.4 MPa. When the addition m-Nano-Al2O3 is increased to 1.0 wt%, the tensile strength reaches the maximum value of 51.9 MPa, which is 49.1% higher than that of pure epoxy resin. Continuing to increase the m-Nano-Al2O3 content results in a decrease in tensile strength from the maximum value. When 1.5 wt%, 2.0 wt%, and 3.0 wt% m-Nano-Al2O3 are added, the average tensile strength of the composite gradually decreases to 33.4 MPa, 27.1 MPa, and 21.7 MPa, respectively.
From the above test results, it can be seen that as the amount of m-Nano-Al2O3 added gradually increases, the tensile strength of the m-Nano-Al2O3/epoxy composite first increases and then decreases. When the amount of m-Nano-Al2O3 powder added to the composite is low (≤1.0 wt%), the m-Nano-Al2O3 can be more uniformly dispersed in the epoxy resin, as shown in Figure 7c. At the same time, m-Nano-Al2O3 restricts the deformation of the epoxy matrix by limiting the movement of the molecular chains of the epoxy resin matrix adjacent to the particles. Additionally, the uniformly dispersed m-Nano-Al2O3 can prevent the continuous expansion of cracks in the material. This is achieved through crack deflection, twisting, or even branching, which generates secondary cracks that deviate from the original main crack direction, thereby enhancing the toughness of the epoxy resin composite.
However, when the amount of m-Nano-Al2O3 powder added to the composite material exceeds a certain content (>1.0 wt%), the poor wettability between the m-Nano-Al2O3 powder and the epoxy resin causes the epoxy resin to wrap the m-Nano-Al2O3 aggregates, resulting in defects, such as voids and bubbles, forming in the composite during the thermal curing process as shown in Figure 7f.

3.5. Tensile Fracture Analysis

It can be observed from Figure 9 that the color of the epoxy resin composites gradually becomes lighter as the amount of m-Nano-Al2O3 increases. When the amount of m-Nano-Al2O3 is less than 1.5 wt%, the surface of the composite material is smooth and free of obvious bubbles or voids. However, when the amount of m-Nano-Al2O3 is increased to 2.0 wt% and 3.0 wt%, as shown in Figure 6 and Figure 9, a large number of dense bubbles appear on the surface of the composite, making it uneven. After fracture, the composite exhibits brittle fracture characteristics. Due to the increased number of defects at 3.0 wt%, two fractures appear, while the rest of the samples show only one fracture characteristic.
From Figure 10a, it can be seen that the cross-section of pure epoxy resin is relatively simple, with a smooth and flat surface, exhibiting typical brittle fracture characteristics. In contrast, roughness of the m-Nano-Al2O3/epoxy composites is significantly higher than that of pure epoxy resin. When the content of m-Nano-Al2O3 is less than 1 wt%, scale-like stress expansion appears on the fracture surface, producing a clear river pattern under tensile load [1]. Stress dispersion occurs around the m-Nano-Al2O3 particles under the action of stress. The m-Nano-Al2O3 powders are uniformly dispersed in the epoxy matrix after curing. When cracks propagate to the m-Nano-Al2O3 particles under stress, the particles hinder the continued growth of the cracks.
However, as the content of m-Nano-Al2O3 continues to increase, the wettability between high-content m-Nano-Al2O3 and the matrix resin deteriorates, leading to some local agglomeration. This reduces the hardness of the composite material. As shown in Figure 10c–e, residues are observed on the cross-section surface after fracture. This is caused by the agglomeration of m-Nano-Al2O3 particles. When the addition amount reaches 3.0 wt%, the tensile strength drops sharply due to the agglomeration of nanoparticles, stress concentration, and the presence of bubble defects. This is consistent with the SEM analysis results.

3.6. Distribution of m-Nano-Al2O3

Distribution of nanoparticles is important for the performance of the composite. Before introducing into the matrix, the modified alumina was dispersed in acetone for 30 min. Acetone treatment can enhance the dispersion of alumina particles within the matrix, which is crucial for enhancing the performance of composite. After introduction into the matrix, the mixture was stirred at 45 °C for 1 h to ensure uniform dispersion of m-Nano-Al2O3. In addition, after modification, the hydroxyl groups of the coupling agent bond with the hydroxyl groups on the surface of m-Nano-Al2O3. An organic film encapsulates the m-Nano-Al2O3, creating significant steric hindrance. This prevents the aggregation of m-Nano-Al2O3 and enhances stability. Figure 11 is a micrograph of a fracture at a higher magnification for a microregion of the composite. As can be seen from Figure 11b–d, the dispersion of m-Nano-Al2O3 particles in the resin matrix is relatively uniform, with no agglomeration observed. Therefore, when the content of m-Nano-Al2O3 is not higher than 1.5%, it can be observed that the Al2O3 particles are relatively evenly dispersed within the matrix. When the content of m-Nano-Al2O3 rises to 2% and 3%, a small amount of agglomeration will occur. The yellow arrows in the Figure 11b–f all indicate m-Nano-Al2O3 particles. In particular, in Figure 11f, the area enclosed by the yellow dashed line is a region where alumina is more densely concentrated. Even if the m-Nano-Al2O3 amount is large, the absence of significant agglomeration indicates that our method for dispersing m-Nano-Al2O3 is feasible.

3.7. Surface Roughness Analysis

Figure 12 shows the surface roughness 3D images of epoxy composites reinforced with different contents of m-Nano-Al2O3. A comparison reveals that the roughness of neat epoxy, 0.5 wt% m-Nano-Al2O3/epoxy composite and 1 wt% m-Nano-Al2O3/epoxy composite are relatively low, the Sa are all 0.009 μm. Whereas the roughness of the 1.5 wt% m-Nano-Al2O3/epoxy composite, 2wt % m-Nano-Al2O3/epoxy composite and 3 wt % m-Nano-Al2O3/epoxy composite are higher, the Sa are 0.021 μm, 0.026 μm, 0.040 μm. This is because Al2O3 is particulate in nature, and its addition to the matrix increases the roughness. When the m-Nano-Al2O3 content is lower than 1 wt%, m-Nano-Al2O3 can be uniformly dispersed within the matrix and has a minimal impact on the roughness. However, when at a content above 1.5 wt%, m-Nano-Al2O3 tends to agglomerate, leading to an increase in the roughness of the composite. The agglomeration of Al2O3 is clearly visible in the 3D Figure 12e,f.

3.8. Hardness Test Analyses

Table 2 shows the Vickers hardness data of m-Nano-Al2O3/epoxy composite. D1 is the horizontal diagonal length of a diamond indentation, D2 is the transverse diagonal length of a rhombic indentation. Comparing the indentation dimensions, one can see that the length of the indentation horizontal and transverse diagonals is 219.34 and 215.519 μm, corresponding to 0.5 wt% m-Nano-Al2O3/epoxy, which has the lowest hardness among all the samples. On the other hand, when the length of the indentation horizontal and transverse diagonals is 202.526 and 205.538 μm, corresponding to 3 wt% m-Nano-Al2O3/epoxy, this has the highest hardness among all the samples. With the increase in alumina content, the hardness of the composite initially decreases and then continues to increase.
Figure 13 shows Vickers hardness indentation diagrams and hardness bar charts for m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 content. As can be seen, the addition of m-Nano-Al2O3 increases the hardness of the composites, which corresponds to the higher hardness and more uniform dispersion of m-Nano-Al2O3. With the increase in the amount of m-Nano-Al2O3, the Vickers hardness of the epoxy composites first decreases and then increases. Composites filled with 0.5 wt% exhibit a decrease in hardness. This phenomenon is caused by the decreased adhesion between the epoxy matrix and m-Nano-Al2O3 nanoparticles [2]. Composites filled with m-Nano-Al2O3 (1~3 wt%) show incrementally increasing hardness, which is attributed to the uniform distribution of alumina and its inherent high hardness.
The curvature of the indentation edge can reflect the degree of the hardness. The straighter the indentation edge, the greater the hardness. Comparing the six indentations, one can see that the curvature of the edges (red dashed dot of one side of a rhombus) in Figure 13a–c is noticeably greater than that in Figure 13d–f, so composites filled with m-Nano-Al2O3 (1.5~3 wt%) show greater hardness. This is also consistent with the hardness data in Table 2.
Additionally, during the thermal curing process, the epoxy network is strengthened. As shown in Figure 14, the curing agent promotes the formation of the epoxy network. With the increase in the network density, the amount of m-Nano-Al2O3 fillers also increases. A high filler ratio results in cluster formation in the composite filled with m-Nano-Al2O3 particles, which reduces the hardness value of the epoxy composite [32].

4. Conclusions

In this study, silane coupling agents KH550, KH560 and KH570 were used to modify the surface of Nano-Al2O3. The modified Nano-Al2O3-reinforced epoxy resin composites were then successfully prepared. The mechanical properties and structural morphology of the composites were characterized, and the effect of the filler amount on the properties of the composites was investigated. The following conclusions can be drawn:
  • The dispersion and stability of Nano-Al2O3 modified by the KH570 coupling agent in epoxy resin are the best. The KH570 modified Nano-Al2O3 contains active groups, such as Si-O-Si, which can combine better with epoxy groups and enhance the dispersion and stability of Al2O3 particles in the epoxy.
  • The dispersibility of nanoparticles is optimal when the content of Nano-Al2O3 is 1 wt%, and the tensile strength is 51.9 MPa, which is 49.1% higher than that of pure epoxy resin. When the content exceeds 1 wt%, the nanoparticles gradually accumulate, causing agglomeration and defects in the m-Nano-Al2O3/epoxy composites, and result in damage to the mechanical properties.
  • The addition of m-Nano-Al2O3 in the matrix can increase the hardness of the epoxy resin. The 3wt% m-Nano-Al2O3/epoxy has the highest hardness among all the samples because of the inherent high hardness of alumina, which is an 8.8% increase compared to pure epoxy resin.
These findings demonstrate that KH570-modified Al2O3 at 1 wt% loading significantly enhances epoxy resin’s mechanical performance while maintaining good dispersion, while 3 wt% loading has the highest hardness.

Author Contributions

Methodology, T.Z. and X.C.; Formal analysis, B.W.; Investigation, T.Z. Writing—original draft, T.Z., X.C., J.L. and M.S.; Writing—review & editing, X.C.; Funding acquisition, T.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the doctoral Research Start-up Fund Project of Xi’an Polytechnic University, and also funded by Department of Science and Technology of Shaanxi Province (No. S2021-YF-YBNY-0350).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The authors are grateful for the financial support from Department of Science and Technology of Shaanxi Province (No. S2021-YF-YBNY-0350).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. Preparation of m-Nano-Al2O3 reinforced/epoxy composite.
Figure 2. Preparation of m-Nano-Al2O3 reinforced/epoxy composite.
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Figure 3. Schematic diagram of tensile specimen size of composite.
Figure 3. Schematic diagram of tensile specimen size of composite.
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Figure 4. Infrared spectra of Silane Coupling Agents and Al2O3 before and after modification.
Figure 4. Infrared spectra of Silane Coupling Agents and Al2O3 before and after modification.
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Figure 5. Dispersion of m-Nano-Al2O3 in epoxy resin: (a) Unmodified Nano-Al2O3; (b) KH550 modified Nano-Al2O3; (c) KH560 modified Nano-Al2O3; (d) KH570 modified Nano-Al2O3.
Figure 5. Dispersion of m-Nano-Al2O3 in epoxy resin: (a) Unmodified Nano-Al2O3; (b) KH550 modified Nano-Al2O3; (c) KH560 modified Nano-Al2O3; (d) KH570 modified Nano-Al2O3.
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Figure 6. Partial details of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 contents: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2 wt% m-Nano-Al2O3; (f) 3 wt% m-Nano-Al2O3.
Figure 6. Partial details of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 contents: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2 wt% m-Nano-Al2O3; (f) 3 wt% m-Nano-Al2O3.
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Figure 7. SEM images of the surface of epoxy composites reinforced with m-Nano-Al2O3 with different filling amounts: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
Figure 7. SEM images of the surface of epoxy composites reinforced with m-Nano-Al2O3 with different filling amounts: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
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Figure 8. Stress/strain curves of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 contents: (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
Figure 8. Stress/strain curves of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 contents: (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
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Figure 9. Macroscopic view of m-Nano-Al2O3/epoxy composite samples before and after fracture: The blue dashed box contains the neat epoxy, 0.5 wt% and 1.0 wt% m-Nano-Al2O3/epoxy composites samples, and the red dashed box contains the 1.5%, 2%, and 3% m-Nano-Al2O3/epoxy composites samples. The green solid line is the dividing line in the middle of the samples.
Figure 9. Macroscopic view of m-Nano-Al2O3/epoxy composite samples before and after fracture: The blue dashed box contains the neat epoxy, 0.5 wt% and 1.0 wt% m-Nano-Al2O3/epoxy composites samples, and the red dashed box contains the 1.5%, 2%, and 3% m-Nano-Al2O3/epoxy composites samples. The green solid line is the dividing line in the middle of the samples.
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Figure 10. SEM image of the tensile cross-section of m-Nano-Al2O3/epoxy composite: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
Figure 10. SEM image of the tensile cross-section of m-Nano-Al2O3/epoxy composite: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
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Figure 11. Micrograph of a fracture at a higher magnification: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
Figure 11. Micrograph of a fracture at a higher magnification: (a) Neat epoxy; (b) 0.5 wt% m-Nano-Al2O3; (c) 1.0 wt% m-Nano-Al2O3; (d) 1.5 wt% m-Nano-Al2O3; (e) 2.0 wt% m-Nano-Al2O3; (f) 3.0 wt% m-Nano-Al2O3.
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Figure 12. Surface roughness 3D images of m-Nano-Al2O3/epoxy composites reinforced with different contents of Al2O3: (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
Figure 12. Surface roughness 3D images of m-Nano-Al2O3/epoxy composites reinforced with different contents of Al2O3: (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
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Figure 13. Vickers hardness indentation diagrams and hardness bar charts of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 content. (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
Figure 13. Vickers hardness indentation diagrams and hardness bar charts of m-Nano-Al2O3/epoxy composites with different m-Nano-Al2O3 content. (a) Neat epoxy, (b) 0.5 wt% m-Nano-Al2O3, (c) 1.0 wt% m-Nano-Al2O3, (d) 1.5 wt% m-Nano-Al2O3, (e) 2.0 wt% m-Nano-Al2O3, (f) 3.0 wt% m-Nano-Al2O3.
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Figure 14. Structural schematic diagram of the cross-linked network in m-Nano-Al2O3/epoxy resin composites.
Figure 14. Structural schematic diagram of the cross-linked network in m-Nano-Al2O3/epoxy resin composites.
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Table 1. Tensile strength of m-Nano-Al2O3/epoxy composite.
Table 1. Tensile strength of m-Nano-Al2O3/epoxy composite.
m-Nano-Al2O3
Content/wt%		00.51.01.52.03.0
I32.736.351.230.723.720.5
II32.948.150.933.628.421.9
III38.845.853.635.829.122.6
Average value34.843.451.933.427.121.7
Standard deviations2.835.111.212.092.400.87
Enhanced strength024.749.1−4.0−22.1−37.6
Table 2. Vickers hardness of m-Nano-Al2O3/epoxy composite.
Table 2. Vickers hardness of m-Nano-Al2O3/epoxy composite.
SamplesD1 (μm)D2 (μm)HVHV
Average		Standard
Deviations
Neat epoxy212.462213.9920.420.40.4
205.583217.04720.8
210.933220.10420
0.5 wt% m-Nano-Al2O3/epoxy219.34215.51919.619.80.255
216.283213.22620.1
214.754217.81119.8
1 wt% m-Nano-Al2O3/epoxy203.291209.40521.821.40.543
206.348207.11221.7
210.169212.46220.8
1.5 wt% m-Nano-Al2O3/epoxy207.112200.23422.421.51.136
217.047210.93320.2
207.876204.81921.8
2 wt% m-Nano-Al2O3/epoxy207.876207.87621.521.50
215.519200.23421.5
200.998214.75421.5
3 wt% m-Nano-Al2O3/epoxy201.762204.81922.422.20.255
206.348205.58321.9
202.526205.58322.3
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Zhang, T.; Chao, X.; Liang, J.; Wang, B.; Sun, M. Enhanced Mechanical Properties of Epoxy Composites Reinforced with Silane-Modified Al2O3 Nanoparticles: An Experimental Study. J. Compos. Sci. 2025, 9, 252. https://doi.org/10.3390/jcs9050252

AMA Style

Zhang T, Chao X, Liang J, Wang B, Sun M. Enhanced Mechanical Properties of Epoxy Composites Reinforced with Silane-Modified Al2O3 Nanoparticles: An Experimental Study. Journal of Composites Science. 2025; 9(5):252. https://doi.org/10.3390/jcs9050252

Chicago/Turabian Style

Zhang, Ting, Xujiang Chao, Junhao Liang, Bin Wang, and Mengmeng Sun. 2025. "Enhanced Mechanical Properties of Epoxy Composites Reinforced with Silane-Modified Al2O3 Nanoparticles: An Experimental Study" Journal of Composites Science 9, no. 5: 252. https://doi.org/10.3390/jcs9050252

APA Style

Zhang, T., Chao, X., Liang, J., Wang, B., & Sun, M. (2025). Enhanced Mechanical Properties of Epoxy Composites Reinforced with Silane-Modified Al2O3 Nanoparticles: An Experimental Study. Journal of Composites Science, 9(5), 252. https://doi.org/10.3390/jcs9050252

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