Nano-hybridization effects of nano-silica and nano-graphene platelet on mechanical properties of E-glass/epoxy nanocomposites

Abstract

This study reveals the nano-hybridization effects of nano-graphene platelets (NGPs) and nano-silica (SiO₂ nanoparticle), having different structural geometries on the mechanical properties, nano and micro-scale failure behaviors, and nanoscale fracture mechanisms of E-glass/epoxy composites. Tensile, three-point bending, and Charpy impact experiments were applied to determine the mechanical behaviors of 0.5 wt.% NGPs, 4 wt.% nano-silica and 0.5 wt.% NGPs + 4 wt.% nano-silica nanohybrid filled E-glass/epoxy and neat E-glass/epoxy composite samples. Failure of composite samples was examined by microscopy and SEM analysis. FTIR analyses were conducted to interpret the chemical and physical interactions between the nanoparticles and epoxy resin. Nano-hybridization exhibited the highest tensile strength and three-point flexural force for the composite samples. However, the NGPs filled nanocomposites also exhibited the best static tensile toughness and impact energy absorption. The experimental data showed that it was statistically significant as a result of the one-way ANOVA analysis. Remarkably, nano-hybridization of nano-silica and NGPs showed different fracture mechanisms at the nano and micro-scales.

Keywords

nano-hybridization nano-silica nano-graphene platelets nanocomposites mechanical properties

Introduction

Glass fiber-reinforced polymer-based composites provide many advantages such as high electrical, thermal insulation, chemical corrosion resistance, mechanical properties, and low cost. Therefore, glass fiber-reinforced composites are utilized extensively in many commercial and military fields, such as air, marine and land vehicles, oil, liquefied and pressurized natural gas pipelines, and fuel tanks. Researchers are making efforts by performing many tests and applications to glass fiber-reinforced composites to show superior mechanical, physical, thermal performances, and to improve the damage formation and progression compared to other fiber-reinforced composites. One of the most outstanding applications is the use of nanofillers in fiber-reinforced composites. Studies are generally aimed to improve the mechanical properties, impact resistance by enhancing the interfacial adhesion of fiber/matrix with nanoparticle fillers.^1–7

Afrouzian et al.⁸ added the nano-silica at 0-3 wt.% ratios into glass fiber-reinforced composites. They found the best energy absorption and tensile strength at 0.5 wt.% nano-silica doped composites. Conversely, the best flexural strength for glass-reinforced composites was found with 3 wt.% nano-silica addition into the matrix. They determined that the fracture surfaces of nano-silica added composites were quite rough according to the SEM images. Jayabalakrishnan et al.⁹ examined the influence of nano-silica and ABR rubber on the mechanical properties of GFR nanocomposites. They ascertained that nano-silica increased the adhesion strength of the matrix/glass fiber interfaces in composites. Eslami-Farsani and Khosravi¹⁰ found the highest mechanical properties for flexural strength and energy absorption of GFR/epoxy anisogrid composites at 3 wt.% nano-silica among 0, 1 and, 5 wt.% They emphasized that nano-silica can change crack propagation pathways by deflecting cracks, according to SEM analysis. Bozkurt et al.⁴ studied the Charpy impact behaviors of un-notched and notched GFR/epoxy composites filled with various weight ratios of nano-silica. They reported that the optimum ratio is 1.5 wt.% in terms of higher impact energy absorption and toughness. Moreover, they determined that the addition of nano-silica contents (0.5–3 wt.%) increased the impact resistances of composites. Özbek¹¹ investigated the axial/lateral buckling behaviors of nano-silica filled Kevlar/Epoxy nanocomposites. Wang et al.¹² informed that nano-silica affected the fracture energies by extending the crack paths in their study. Tzetzis and Mansour¹³ investigated the toughening mechanisms of nano-silica on nanocomposites by applying theoretical analyses and experimental tests. They indicated two prime toughening mechanisms: the growth of plastic voids with debonding failures and shear banding in the matrix epoxy resin. Christy et al.¹⁴ observed the impact responses and tensile properties of 0, 1, 2, and 3 wt.% nano-silica filled epoxy nanocomposites. They determined that 3 wt.% doped epoxy nanocomposites showed the highest mechanical properties among other weight percentages of nano-silica additives. Kostagiannakopoulou et al.¹⁵ examined the toughness influences of nano-graphene oxides (GO) and platelets (0.5 wt.% addition) on CFRP composites by applying the DCB tests for determining the fracture toughness (Mod I). They reported that all types of nano-graphene increased the energy release rate of CFRP composites. Additionally, they found that nano-graphene platelets provided the most remarkable toughening improvement on CFRP composites. Yao et al.¹⁶ performed micro-drop tests on CNTs and nano-GO filled CFRP nanocomposites to determine the interfacial mechanical behaviors of fiber/matrix. They underlined that nano-GO and CNTs enhanced the interfacial shear strength by providing mechanical interlocking interactions of fiber/matrix. Li et al.¹⁷ reported that nano-graphene sheets and carbon nanotubes enhanced nano-reinforcements and matrix interactions. Besides, they expressed that nano-graphene sheets showed a more critical role than carbon nanotubes in delaying crack progression. Bansal et al.¹⁸ reported the characterization of different contents of nano-graphene platelets (NGPs) and carbon nanofibers. They assessed from the results that 0.5 wt.% NGPs decreased the viscosity of the resin and increased the heat for curing. Erklig et al.¹ examined the Charpy impact behaviors of nano-graphene platelets filled notched and un-notched glass fiber-reinforced epoxy composites. They emphasized that 0.5 wt.% nano-graphene platelets filled GFR/epoxy composites exhibited higher energy absorption among all contents. Shokrieh and Rafiee¹⁹ investigated the effective role of changing CNT lengths on the mechanical behavior of polymer-based composites. They evaluated that as the CNT lengths increased, tensile module of the polymer resin improved. Bulut²⁰ revealed that 0.1 wt.% nano-graphane platelets show the highest tensile, flexural strength, and impact absorption. Umer et al.² determined the rheology, cure kinetics, and flexural strength of graphene oxides filled GFR/Epoxy composites at different weight ratios. They found that 0.2 wt.% GO filled GFR/Epoxy composites achieved the highest flexural strength with an increase of 30%. Demirci et al.²¹ studied the fracture toughness of nano-silica added (4 wt.%) BFR(Basalt Fiber-Reinforced) nanocomposites. In addition, Demirci²² studied the low-velocity impact behaviors of 4 wt.% SiO₂ nanoparticles added basalt fiber-reinforced epoxy composites. Demirci et al.²³ detected the nano-hybridization behaviors of the combination of SiO₂ nanoparticles and MWCNTs in BFR composites by applying the tensile and low-velocity impact tests. They interpreted that the MWCNTs structure shapes were effective in the mechanical behaviors.

In general, it may be considered that the geometrical structures and specific surface areas of nanoparticles cause an essential effect on the mechanical behavior of nanocomposites. Nanoparticles having different geometric shapes and sizes may exhibit different fracture mechanisms at the nanoscale. It can be thought that the diversity of shape structures of nanofillers has an essential effect on the increase in the mechanical strength of fiber-reinforced nanocomposites. Therefore, different mechanical and fracture properties can be obtained as a result of the hybridization of nanoparticles. In this study, the effects of NGPs (nano-graphene platelets) and nano-silica (SiO₂ nanoparticle) hybridization on the mechanical properties and nanoscale fracture mechanisms of E-glass fiber-reinforced nanocomposites were investigated. Tensile, three-point bending and Charpy impact experiments were applied to determine the nanoscale hybridization effects of spherical shaped nano-silica and platelet-shaped nano-graphene on mechanical behaviors. The static tensile toughness was calculated from the areas under the stress-strain curves of all composites. The failures of all composites were investigated with microscopy and SEM analysis. One-way analysis of variance (ANOVA) method was performed to define the significant values for statistical assessment.

Experimental procedure

Materials

The nanoparticles weight ratios were determined as 4 wt.% for nano-silica (SiO₂ nanoparticle), for 0.5 wt.% nano-graphene platelets (NGPs) according to previous studies and literature.^1,15,22–28 For nano-hybrid 4.5 wt.% mixing ratio, 0.5 wt.% nano-graphene platelets and 4 wt.% nano-silica were hybridized in the epoxy resin. All nanofiller ratios are depended on the weight percentage of the epoxy resin. Nano-silica has an average diameter of 15 nm, porous, and sphere geometry. Its surface area is 650 g/m² (Mknano). The size and thickness of nano-graphene platelets have 0.5–5 μm and 5–20 nm, respectively, and they have a surface area of 500 g/m² (Alfa Aesar). In this study, [0^o/90^o] oriented E-glass fabric (300 gr/m²) was preferred as the reinforcement material. A medium viscosity Hexion MGS L160 DGEBA epoxy resin and MGS H160 hardener in 100/36 weight ratio were used as the matrix resin material. The mechanical properties of epoxy matrix resin and E-glass fiber are given in Table 1 and Table 2, respectively.

Table 1.

Mechanical properties of DGEBA epoxy resin.²⁹

Density (g/cm³)	1.13–1.17
Viscosity (mPas)	700–900
Tensile strength (N/mm²)	70–80
Modulus of elasticity (kN/mm²)	3.2–3.5
Compressive strength (N/mm²)	80–100

Table 2.

Mechanical properties of E-glass fiber.³⁰

Density (g/cm³)	2.62
Tensile strength (N/mm²)	3400
Modulus of elasticity (kN/mm²)	73
Elongation %	4.8

Production process of nanocomposites

The production stage of nanocomposite consisted of four stages. As a first stage, the nanofillers were weighed according to the weight percentage of the epoxy resin on a precision balance. In the second stage, each nanoparticle added to the epoxy resin was subjected to mechanical mixing for 5 min. Then, the ultrasonic mixing process by using a metal-probe sonicator (Bandalin HD 2220) was performed for 15 min to increase the dispersibility. The mixing process was completed by allowing the resin temperature to drop to room temperature so that the hardener could be added.^27,31–33 In the third stage, the wetting process of the fabrics was initiated by adding a hardener. Each E-glass fabric was wetted and stacked on top of one another until 10 layers were completed by applying the hand lay-up method. In the final stage, all nanoparticles filled E-glass/epoxy and neat E-glass/epoxy composites were produced by hot pressing process under pressure of 5 bar at 90^oC for 1 hour and final curing at 120^oC for 2 hours. Figure1 shows the production processes of nanoparticles filled and neat E-glass/epoxy laminated plates. The samples of tensile, three-point bending, and Charpy impact were cut from laminated composite plates according to ASTM D3039/D3039 M–08³⁴ (250 × 25 mm), ASTM D7264/D7264M–21³⁵ (13 × 100 mm), and ISO 179–1³⁶ (55 × 10 mm) standards. The edges of prepared samples were smoothed with a polishing machine (Metkon) with 600 grit sandpaper to minimize the possible notch effect and obtain the required measurements precisely for the experiment. Figure 1 presents a schematic diagram of the production processes of nanocomposites. The tensile, three-point bending and Charpy impact tests are presented in Figure 2, Figure 3, and Figure 4, respectively.

Figure 1.

The Production processes of nanoparticles filled E-glass/epoxy composites.

Figure 2.

Three-point bending tests of the composite sample.

Figure 3.

Tensile tests of the composite sample.

Figure 4.

Charpy impact tests of the composite sample.

Tests

FTIR analysis (Bruker Vertex 70 spectrometer) was carried out to determine whether there was any chemical or physical interaction between each nanoparticle and epoxy resin.

Three different test methods were performed to specify the mechanical behavior of nanoparticles and their nano hybrid filled E-glass/epoxy nanocomposites and neat E-glass/epoxy composites. Tension tests were applied to all types of composites with a displacement rate of 5 mm/min related to crosshead tension motion by using a universal tension & compression test machine, controlled by a servo electro-mechanical system with 10 kN load cell (KT-AS). Static tensile toughness values were determined by using the areas under tensile curves.

Three-point bending tests were performed on all types of composite samples at a rate of crosshead motion of 5 mm/min by using the universal tension & compression test machine (KT-AS). Charpy impact tests were employed for all types of un-notched composite samples on the universal Charpy impact device using a hammer load of 25 N.

Fracture surfaces of composite samples were investigated by using a SEM (Zeiss Evo 10) and a microscope (Nikon Eclipse MA 100). The microscope image resolution is 3200 × 2400 and 300 Dpi.

One-way ANOVA statistical analysis was performed to determine the significance of tests.

Results

Tensile and three-point bending, Charpy impact tests, FTIR, and statistical analyses were performed on 4 wt.% nano-silica, 0.5 wt.% NGPs and 4 wt.% nano-silica + 0.5 wt.% NGPs nano-hybrid (4.5 wt.%) filled and neat E-glass/epoxy composite samples. The test results of neat E-glass/epoxy composite samples without nanofillers were accepted as references. The test results were evaluated and interpreted by considering a reference sample.

FTIR analysis

Fourier transform infrared (FTIR) spectra of nano-silica, NGPs, and nano hybrid filled nanocomposite and composite samples are presented in Figure 5. The vibration peaks of the chemical groups obtained from the FTIR spectra analysis of the samples (400–4000 cm⁻¹) are summarized in the Table 3 below.^37–50

Figure 5.

FTIR spectra of nanoparticles and their nanohybrid filled nanocomposites.

Table 3.

Band assignments of nano-silica, nano-graphene platelets (NGPs), nano-hybrid filled and unfilled composites.

Bands (cm^–1)	Assignments
3400	O–H stretching
2960	Alkyl units (C–H and –CH₂)
2162	Combination of O-H stretching
1607–1513	Aromatic C = C stretching and N-H bending primary amine
1455	CH2 stretching
1369	C—H symmetric stretching
1235	–C–C–O–C stretching
1180	Aromatic C-O stretching
1106–1104	Aromatic C-O stretching
1036–1930	C–O–C stretching
824	Aromatic H–C
611	Alkyne C-H bend stretching
463	Si–O bending stretching

When the FTIR spectra in Figure 5 are examined, it can be observed that no new band was formed except for 463 cm⁻¹ peak vibration. The absence of a different band formation may be more effective physical interaction than chemical interaction.^40,51 However, it was seen in the FTIR spectra that the peak intensity increased with nanofillers. It was observed that the peak vibration intensities of NGPs and nano hybrid filled composite samples increased in the range of 1235–1824 cm⁻¹ more than nano-silica filled and neat samples. C-O and C-O-C stretching vibrations of chemical groups were considered to be high in NGPs and nano-hybrid composites. Therefore, this may be a sign of interaction between epoxy matrix and nanofillers.^44,45,52 It can be interpreted that the decrease in peak density for nanofillers added samples reduces the autocatalytic effect due to agglomeration during curing proceses.^40,53 As seen in Figure 5, new band formation, which indicates the Si-O bending stretching groups was observed at 463 cm⁻¹ in nano-silica and nano-hybrid added samples. A new band and peak formation can also be thought of as a chemical interaction rather than a physical interaction between epoxy and nanoparticles.^40,48,50,51

Tensile tests

Figure 6 shows the tensile strength versus strain (mm/mm) curves of nano-silica, NGPs, and nano hybrid filled and neat E-glass/epoxy composites. When Figure 7 was investigated, nano-silica + NGPs nanohybrid, NGPs, nano-silica filled, and reference samples showed mean tensile strength values of 515 MPa, 506 MPa, 490 MPa,³³ and 375 MPa,³³ respectively (Figure 2). Although nano-silica + NGPs hybrid nanoparticles filled samples showed the highest tensile strength value, E-glass/epoxy sample as the accepted reference exhibited the lowest mean value of tensile strength. When the reference composite samples compared to the nanoparticles filled E-glass/epoxy, composites, the highest tensile strength was achieved with an increase of 37% in SiO₂ + NGPs nano-hybrid filled E-glass/epoxy nanocomposites. Chennareddy et al.⁴⁴ achieved a high increment in tensile strength by adding MWCNTs to GFR-laminated epoxy composites. Vemuganti et al.⁴⁵ determined that MWCNTs addition into pultruded GFR/vinyl ester bars provided a 20% increase in tensile strength. Demirci et al.²³ determined that nano-hybridization of nano-silica and MWCNTs showed an adverse effect compared to SiO₂ nanoparticle added Basalt Fiber-Reinforced (BFR) nanocomposites for the tensile and low-velocity impact responses. However, they found that nano-hybridization increased the tensile strength of BFR nanocomposites compared to MWCNTs addition. They commented that the lengths of MWCNTs revealed the poor effects on the mechanical behaviors in nano-hybridization. In this study, it is thought that nano-hybridization of nano-silica and NGPs shows the improvement in the mechanical behavior of composites.

Figure 6.

Tensile strength-strain curves of nano-silica, nano-graphene platelets, nano hybrid filled, and neat E-glass/epoxy composites.

Figure 7.

Tensile strength of nano-silica, nano-graphene platelets, nanohybrid filled nanocomposites.

When the samples’ strains (mm/mm) were compared with reference values (Figure 8), nano-silica, nano hybrid, and NGPs filled samples showed increments of approximately 14.5%, 32%, and 46%, respectively. NGPs filled E-glass/epoxy composite showed the highest strain value.

Figure 8.

Tensile strain of nano-silica, nano-graphene platelets, nanohybrid filled nanocomposites.

Static tensile toughness

The static tensile toughness values of nanocomposites are compared with each other in Figure 9. As a result of the comparison, it is seen from Figure 4 that E-glass/epoxy reference sample showed the lowest static tensile toughness with a value of 21.9 kJ/m³. The static tensile toughness of nano-silica, nano-silica + NGPs nano hybrid, and NGPs filled nanocomposites were 30.8, 33.7, and 36.8 kJ/m³, respectively. NGPs filled nanocomposites achieved the highest static tensile toughness value with an increase of 68% compared to the reference sample.

Figure 9.

Comparison of the static tensile toughness of nano-silica, NGPs, nano-silica + NGPs nanohybrid filled nanocomposites. NGPS, nano-graphene platelets.

Three-point bending tests

Figure 10 and Figure 11 show the force-displacement results and the force mean values of nano-silica, NGPs, nano hybrid filled, and reference composite samples, respectively. In Figure 11, the nano hybrid filled nanocomposite sample shows the highest force increase of 69%. It was observed that NGPs filled nanocomposite samples exhibited a mean 61% increase in load. It was found that nano-silica filled nanocomposite put forward a 53% increase compared to the reference composite sample.

Figure 10.

Force-displacement curves of nano-silica, nano-graphene platelets, nanohybrid filled nanocomposites for three-point bending tests.

Figure 11.

Force mean values of nano-silica, nano-graphene platelets, nanohybrid filled nanocomposites for three-point bending tests.

When Figure 12 was investigated, the displacement values of NGPs filled nanocomposite sample showed the highest increase of 85% compared to the reference sample. Nano-silica and nano hybrid addition provided 38% and 66% increments in the displacements, respectively.

Figure 12.

Displacement mean values of nano-silica, nano-graphene platelets, nanohybrid filled nanocomposites.

Impact tests

The absorbed impact energies of nano-silica, NGPs and nano-silica + NGPs nano hybrid filled, and reference composite samples are presented in Figure 13. Nano-silica, NGPs, and nano hybrid nanoparticles contributed the increases of %58, 119%, and 76%, respectively. NGPs filled nanocomposite showed the highest impact absorption impact energy with an increase of 119% compared to other nanoparticles filled and reference composites. Erklig et al.¹ determined that 0.5%NGPs provided an increment of 41% in the impact energy. It was determined that NGPs increased the impact resistance as a result of Charpy impact tests. It has been interpreted that the improvement of fiber/matrix interfaces by NGPs plays an essential role in increasing the impact absorption.^27,54

Figure 13.

The Absorbed impact energies of nano-silica, nano-graphene platelets, nano hybrid filled nanocomposites.

Statistical analysis

One-way ANOVA analysis was performed for each different test of nano-silica, NGPs, nano-hybrid filled, and reference composite samples in order to determine the significant values for a statistical assessment. The test results and standard deviations are shown in Table 4. The one-way ANOVA analysis results of tensile strength, bending force, and impact energy versus nanofillers are given in Table 4, Table 5, and Table 6, respectively. In the ANOVA analysis; SS, Df, Ms, p-value, and R² are defined as the sum of the square, degree of freedom, mean square, significant differences, and coefficient of determination, respectively.^55,56

Table 4.

Mean values with deviations of experimental results of all composite samples.

Nanofillers (wt.%)	Tensile strength (MPa)	Dev	Flexural force(N)	Dev	Impact energy (J)	Dev
0 wt.%	375	±20.5	552	±33	8.8	±0.59
4 wt.% nano-silica	490	±15.9	845	±32	13.9	±0.63
4.5 wt.% nano-hybrid	515	±13.1	928	±42	15.5	±0.6
0.5 wt.% nano-graphene platelets	506	±13.8	890	±25	19.3	±0.65

Table 5.

One-way ANOVA results of tensile strength versus nanofillers (wt.%).

Source	DF	SS	MS	p-value	R²
Nanofillers (wt.%)	3	38,211	127,370	0,000	9363%

Table 6.

One-way ANOVA results three-point flexural force versus nanofillers (wt.%).

Source	DF	SS	MS	p-value	R²
Nanofillers (wt.%)	3	264,043	88,014	0,000	9659%

The most crucial value showing that the tests are statistically significant in the one-way ANOVA analysis of the experiments performed is the significant value given in the table as a p-value. In order to accept that differences between samples are statistically significant, P significance values lower than 0.05 are required.^56–58 It is seen in Table 5, Table 6, and Table 7 that P significant values are lower than 0.05 in the one-way ANOVA analysis of tensile, three-point bending, and Charpy impact experiments of nanocomposite samples. Therefore, it reveals that each applied experiment is statistically significant. In addition, the other coefficient that is important in the analysis of variance is the R² value.⁵⁶ R² ratios in tensile, three-point bending, and Charpy impact experiments were 93.63%, 96.59%, and, 97.77%, respectively. As a general assessment, experiments can be seen to be acceptable.

Table 7.

One-way ANOVA results of impact energy versus nanofillers (wt.%).

Source	DF	SS	MS	p-value	R²
Nanofillers (wt.%)	3	169,764	565,879	0,000	9777%

Failure analysis

Microscope analysis

Figures 14(a), (d), (g) and (j) show the macro images of nanoparticles filled and reference composite samples. Delamination and fiber fracture failures were clearly seen in macro views of nano-silica filled nanocomposite and reference composite samples. More dense failures were seen in the reference composite sample than nano-silica filled nanocomposite sample. It can also be seen that the delamination and fiber fracture failures in Figure 14(g) and Figure 14 (j) were less than nano-silica filled nanocomposite and reference composite samples.

Figure 14.

Macro and micro-failures of nano-silica, nano-graphene platelets and nanohybrid filled composite samples for three-point bending tests. (a) E-glass/epoxy, (b) E-glass/epoxy, (c) E-glass/epoxy, (d) SiO₂/E-glass/epoxy, (e) SiO₂/E-glass/epoxy, (f) SiO₂/E-glass/epoxy, (g) NPGs/E-glass/epoxy, (h) NPGs/E-glass/epoxy, (i) NPGs/E-glass/epoxy, (j) SiO₂ + NPGs/E-glass/epoxy, (k) SiO₂ + NPGs/glass/epoxy, (l) SiO₂ + NPGs/glass/epoxy.

When Figure 14 (b) and Figure 14(c) were examined, it was easily seen that the delamination failures spread widely to the GFR-laminated layers. When considering the stress distribution due to bending, they are subjected to compression stress in the upper layers and tensile stress in the lower layers. As seen in the microscope image in Figure 14(b), it is understood that there is delamination damage stemming from the bleaching and separation between the fibers in the upper layers due to compression stress. As a result of the tensile stress in the sub-layers during the load, it was observed that the failures progressed as delamination resulting in fiber fracture. As seen in Figure 14(c), debonding failure called as fiber/matrix interface separation failures effectively formed delamination damages.²⁷ Meanwhile, it was seen in microscopy images that the failures progressed as radial cracks between the layers. As a consequence of bending experiments, it was interpreted that the dominant damage in E-glass/epoxy reference samples was delamination, which spread over layers and fiber breakage failures.

Nano-silica filled nanocomposite samples are given in Figure 14(e) and Figure 14(f) with two different scales. It was seen that the nano-silica fillers restricted failures in the upper layers exposed to compression stress and the lower layers exposed to tensile stress compared to neat E-glass/epoxy reference sample. It was interpreted that the predominant failure with the nano-silica addition showed to change from delamination to fiber breakage. It was thought that Figure 14(h) and Figure 14(i) supported to interpretation that the nano-graphene platelets limited the delamination failure that occurred even more. More localized delamination and debonding were observed from Figure 14(h) and Figure 14(i). The predominant failure was seen as fiber breakage. It was commented from decreasing delamination failure that the results of tensile and bending tests proved the dominant failure type as fiber breakage. It can be a clue that nano-graphene platelets addition had the highest static toughness compared to others. The hybrid nanofillers (NGPs and SiO₂) showed the most restriction of delamination and debonding failures, as observed from Figure 14(k) and Figure 14(l). Besides, the most localized and minor debonding failures were detected in the fracture region. It was commented that nano-hybridization increased the adhesion strength at the fiber/matrix interface. In other words, it can be expressed that the nanoparticles increased the adhesion on the interfaces of fibers/matrix. It was also considered that the hybridization of nano-silica and nano-graphene platelets improved the load-carrying capacity, the load transfer ability from the epoxy resin to fiber, and strength of interfaces of fiber/matrix in nanocomposites.^{3,21,23,27,59–61} In addition, the increasing intensity of peak and new band formation in FTIR analysis can also be a sign of the interfacial adhesion of fiber/matrix.^44,45

SEM analysis

Figure 15(a) exhibits the scanning electron microscope (SEM) fractography of neat E-glass/epoxy composite. Fiber fracture, pull-out, and debonding were intensely seen on the fracture surface. In fiber-reinforced composites, it was commented that the matrix cracks reached these interfaces, which were the regions where the damage resistance was the lowest. And so, the matrix cracks caused fiber/matrix surface separation failures called debonding.^21,22 It was considered that these phenomena indicated insufficient fiber/matrix interface strength. It was evaluated that these failures triggered the formation of pull-out failures and caused to spread around of the fracture regions during the load on the composite samples.^22,27 When the SEM fractography of nano-silica filled nanocomposite (Figure 15 (b)) was analyzed, it was seen that there was fiber fracture, pull-out with pull-out sockets, debonding failures, and rough matrix cracks. When compared to the failures on the fracture surfaces of E-glass/epoxy samples, it was found that similar failures were observed, but the number of failures decreased considerably. It was thought that the increasing strain values by filling nano-silica increased the interface strength of composites. This situation was considered to perform a paramount role in reducing the number of failures. Local fiber breakages were also detected in the fiber bundles given in the SEM image. The fact that these breakages were semi-embedded in the matrix also attracted attention in failure investigations. This situation has strengthened the opinion that nano-silica increased the fiber/matrix interface strength.^21–23,27 Figure 15(c) indicates the SEM images of nano-graphene platelets on fracture surfaces of nanocomposites. It was determined that the debonding failures decreased and much rougher fracture surfaces and matrix cracks formed compared to nano-silica-added composites. It was observed that the fracture surfaces became rough, resembling a concave shape with the nano-graphene platelets additives, and the fibers were embedded more in the matrix than in the nano-silica additives. It can also be detected in SEM images that the glass fiber surfaces are coated with epoxy matrix. Here, it was thought that the interfacial adhesion increased with the contribution of nano-graphene platelets. A similar observation was also determined in the literature.^62,63 The fracture surface SEM image of nanohybrid filled nanocomposites is shown in Figure 15(d). When the SEM image was analyzed, it was seen that the damages of pull-out and debonding were considerably reduced compared to nano-silica filled nanocomposite sample and the reference composite sample. When the roughness on the fracture surfaces of nanohybrid composites was examined, the matrix cracks formed a concave shape and occurred fracture branching. Besides, SEM images led to the thought that this roughness was higher than nano-graphene platelets filled composites. It was figured that the hybridization of graphene nano platelets and nano-silica effectively improved the fracture mechanisms. It is considered that the toughness of nanoscale fracture mechanism was effective in the enhancements of the mechanical properties of nanocomposite samples. SEM interpretations supported these comments. It may be thought that nano-graphene platelets, nano-silica, and their hybrids had a retarding effect on cracks that progressed under tensile or bending load. There can be many effects on the delay in progressive cracks. These fracture mechanisms may be;

•bowing at the crack tip,

•increase in interface strength and interphase formation formed by the specific surface areas of nanoparticles,

•coating of matrices on fiber surfaces,

•formation of plastic deformation effect on the crack ends as a result of the pinning,

•formation crack bifurcations at the front of nanofillers and branching cracks that divide many paths.^{21–23,27,62,64,65}

Figure 15.

SEM fractography images of neat E-glass/epoxy composites (a), nano-silica (b), nano-graphene platelets (c), and nanohybrid (d) filled nanocomposites.

The nanoscale fracture mechanisms of nano-graphene platelets, nano-silica and their hybrids are schematized in Figure 17.

Figure 17.

Nanoscale fracture mechanisms of nano-silica and nano hybrid in nanocomposites.

It may be understood from Figure 16(a) that nano-silica provided the adhesion of the matrix material on the glass fiber by modifying the epoxy matrix. In FTIR analysis, S-O bending stretching chemical interaction is thought to support the adhesion of nanoparticles. It was interpreted that the branching of cracks formed concave shapes on the fracture surfaces and caused roughness on the surfaces.^21–23,27 It was inferred by considering Figure 16(b) that nano-graphene platelets had a wall effect in the crack delay due to their geometric difference compared to nano-silica during crack propagation. It was interpreted that if the cracks progressed to the nano-graphene in the form of platelets perpendicularly, nano-graphene platelets had a more blinding and pinning effect over the cracks than spherical-shaped nano-silica. Meanwhile, nano-silica was thought to reduce the delaying crack propagation compared to nano-graphene platelets due to the spherical structure on the progressing cracks. However, it may be evaluated that these effects of fracture mechanisms provided by nano-graphene platelets depend on which direction the cracks come to the nano-graphene platelets (Figure 17). Therefore, it is thought that if the cracks come in parallel to the nano-graphene platelets instead of perpendicular and angled directions, it may cause adverse effects for pinning and trapping, plastic deformation and crack branching. When the cracks come parallel to the nano-graphene platelets, they can cause to separate the sheets of nano-graphene platelets.⁶² This issue may not have an effect on delaying the progress of the cracks, but may cause it to accelerate on the contrary. It can be expressed these phenomena by schematizing in Figure 17. In Figure 16 the distribution of NGPs and nano-silica in the matrix is shown, it is thought that the dispersibility of nanoparticles was provided effectively. It may be seen from the different fracture planes in the SEM images that the nanoparticles were embedded and closed to each other in the epoxy resin at the partial regions of the fracture surfaces. Vemuganti et al.⁴⁵ reported that as the additive amount of nanoparticles increases in the matrix, the increased viscosity of the matrix could affect the dispersibility.

Figure 16.

SEM fractography images of nanoscale fracture mechanisms of nano-silica (a), nano-graphene platelets (b) and nanohybrid filled (c) nanocomposites.

It is thought that nano-silica shows the same fracture mechanism effect regardless of the direction of the progressing cracks due to its spherical geometry. Therefore, it has been assumed that the hybridization of nano-silica with nano-graphene platelets will make it advantageous for composites due to increasing fracture mechanism effects. Even if the cracks progress parallel to the nano-graphene platelets, nano-silica may increase the blocking effect of the nano-graphene platelets (can also be expressed the wall effect) by altering the crack direction and branching cracks due to its spherical structure. It has been interpreted that the nano-hybridization can positively affect the separations of nano-graphene platelets.⁶² This phenomenon can be called the hybridization effects for nanoparticles that have different geometries. The different heights of fracture surfaces by provided nano-hybridization may increase the resistance of shear failures.⁶² It was thought that the experimental results and SEM analysis (Figure 15 and Figure 16) supported these comments.

The bowing effect of nano-silica and the wall effect of nano-graphene (blocking and pinning) is indicated in Figure 16(c). It was considered that the both of them affected the crack progression. The SEM images support the experimental results and comments.

Conclusion

This study investigated the nanoscale and nano-hybridization effects of nano-graphene platelets, nano-silica, and their nanohybrid combination on the mechanical properties for nanocomposites. According to tests, FTIR and microscopy, and SEM analysis, it was determined that the nano-hybridization of nano-graphene platelets (NGPs) and nano-silica (SiO₂ nanoparticle) improved the mechanical properties of nanocomposite samples.

Comments on experimental results and analyses are briefly given below.

The vibration intensities of the peaks of NGPs and nano-hybrid filled composite samples were observed in the FTIR analysis. In addition, the chemical interaction groups of Si-O bending stretching were seen in the nano-silica and nano-hybrid filled samples.

An increase of 37% was found on the mean tensile strength for NGPs + nano-silica nano hybrid filled nanocomposites. The highest average values in tensile strength were determined for NGPs + nano-silica nanohybrid filled nanocomposites. The addition of nano-silica provided at least 14.5% increase in strain. The highest strain increase (46%) in the tensile test was obtained by adding NGPs nanofillers. Nano hybrid filled nanocomposite showed a 32% increase in strain. The highest static tensile toughness value was obtained for NGPs filled nanocomposite by an increase of 68%. As a consequence of the Charpy impact test, the addition of NGPs and nano-silica increased impact energy absorption by 119% and 58%, respectively. Nano hybrid (NGPs and nano-silica) addition increased impact energy absorption by %76 for E-glass/epoxy composites. When flexural forces were examined, the nano-silica additive provided an increase of 53%, while an increase of 61% was achieved with the NGPs. It was observed that nanohybrid (NGPs and nano-silica) additive ensured the enhancement of 69% in the flexural load. The maximum displacement of bending was obtained in NGPs filled nanocomposites by 85%. Nano-hybrid (NGPs + nano-silica) and nano-silica additives for the maximum displacements showed increases of 66% and 38%, respectively. After the one-way ANOVA analysis, the test results were found to be statistically significant.

Generally, matrix cracks, radial cracks between layers, debonding, interlayer delamination, fiber breakage, and pull-out were observed in microscopy and SEM images. Microscopic and SEM examinations revealed that debonding and delamination failures were limited by nanoparticle additives. It was found that the nanohybrid (NGPs + nano-silica) additive limited the occurrence and progression of failures according to SEM and microscopy analyses. It was determined that there were intense pull-out failures and fiber fractures on the fracture surfaces of E-glass/epoxy composites. Likewise, it has been determined that delamination damages are more intense in nanoparticle unfilled composites. It was evaluated that the nano-hybridization effects reduced these failures by providing the fracture mechanism. Nano-hybrid additives were interpreted to further delay the progression of cracks by providing the nano hybrid effects on the nanoscale fracture mechanisms such as wall effect, blinding, branching, and plastic deformation.

Footnotes

Acknowledgments

This manuscript was prepared from the MSc thesis of Osman AYDOĞUŞ in the Department of Metallurgical and Materials Engineering of the Graduate School of Natural and Applied Science of Selçuk University, Konya, Turkey. Mehmet Turan Demirci is the advisor of Osman AYDOĞUŞ’s MSc thesis.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Mehmet Turan Demirci

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