Investigating the role of synthesized reduced graphene oxide and graphite micro-fillers on mechanical and fretting wear performance of glass fiber epoxy-based composite

Abstract

In the current study, graphite was exfoliated using a Hummer’s technique to produce graphene oxide (GO), which was then reduced with hydrazine hydrate (a reducing agent) to produce reduced graphene oxide (rGO) with high purity. The XRD, FT-IR, SEM, and TGA analysis confirmed the synthesis of rGO by stating its crystal phase, chemical functional group, morphology, and thermal stability. The objective of this study is twofold: firstly, to synthesize the reduced graphene oxide (rGO) cost-effectively, and secondly, to explore its potential as an additional filler in Glass Fiber Reinforced Polymer (GFRP) composites. Aiming to enhance their overall performance. The GFRP laminate composite was fabricated through the hand layup technique by varying the concentration of Gr (i.e., 0.5 wt% & 1 wt%) and rGO (i.e., 0.5 wt% & 1 wt%). The morphological study of the fracture surface revealed a proper dispersion of filler obtained at 0.5 wt% and by increasing the concentration to 1 wt% it reveals a clustering of fillers and formation of micro-voids. The result revealed that maximum improvement has been observed in the GFRP composite having 0.5 wt% rGO composite than neat GFRP composite. Incorporation of rGO micro-filler in GFRP laminate composite significantly improved the tensile strength, flexural strength, and in their modulus by 59.56%, 18.21%, 24.45%, and 22.75% respectively compared to neat GFRP laminate composite. In the case of 0.5 wt% graphite filler demonstrates an enhancement in tensile strength, flexural strength, and their modulus by 37.98%, ∼8%, 6.64%, and ∼5% respectively. In fretting wear test reveals that 1 wt% of graphite filler has better wear resistance than all other composites. The worn morphology revealed adhesive and abrasive wear as the predominant wear mechanism. The incorporation of rGO filler in GFRP makes it a promising material for industrial applications that demand high strength and superior wear resistance.

Keywords

rGO filler polymer composite hummers method mechanical and wear testing

Introduction

The exponential growth in engineering increased the demand for materials having a high strength-to-weight ratio. Towards fulfilling this need, research groups have developed a new generation of engineering materials known as glass fiber-reinforced polymer composite (GFRP).¹ Glass fibers have stimulated tremendous sapidity as reinforcement for polymer matrix due to their very high strength-to-weight ratio, lightweight, low cost, high mechanical properties, and superior heat and environmental resistance as a result it has gained a place in several industrial application viz. Marine and piping industries, aircraft industries such as rudder, elevator, door landing gear, etc as well as automobile and sport industries.² The poor interfacial bonding between the phases of GFRP composite restricts its broader utilization in engineering applications. This is because it can lead to delamination, burrs, fiber pull-outs, matrix spreading, poor machining, and cracking when subjected to load.^1,3,4 Delamination is indeed a significant concern and one of the most severe types of damage that can occur in laminate composites. It has a detrimental impact on the overall properties of the composite material. To overcome such flaws, organic or inorganic nanofillers such as TiO₂, Al₂O₃, carbon black, silica, clay, Graphene, CNT, MWCNT, and SiO₂ have been incorporated into epoxy resin. In addition to that, it also enhanced the mechanical strength of the composite.^5,6 A similar observation also has been depicted by suresha et al., where the composite having 7.5 wt% of graphite particulates showed 24% increment in tensile strength as compared to the parent material.⁷ A similar study was also conducted by shanti et. al., where the author observed that graphite-filled glass fiber composite exhibit more mechanical and thermomechanical strength compared to the composite without a filler.⁸ A literature review of the influence of graphite filler was conducted by Shiva Murthy et. al. on GFRP laminate composite, the specific wear test was strongly dependent upon the graphite filler content and applied normal load. The result reveals that in hybrid laminate composite at 3 wt% of graphite filler obtained the optimum value of mechanical and wear performance. Further, enhance the filler content to increase the wear-specific rate and deteriorates the mechanical performance.⁹

In another study, Gara et. al. used the different wt.% of exfoliated graphene oxide filler like 3, 6, and 9 wt% incorporated into a polymer composite. The results found indicate that the tensile, flexural, and impact strength increase upon the incorporation of 3 and 6 wt% of filler, whereas a decrease is observed at 9 wt%, except for impact strength, when compared to the base material.¹⁰ Umer et. al., study the processing of epoxy/glass fiber/GO composite and the implication that GO has on it. They observed that the incorporation of GO enhances the viscosity and reduced the curing time by decreasing the resin gel time. The result also demonstrates an enhanced flexural strength and flexural modulus by 30% and 21% respectively.¹¹ In another study Rathore et. al. The investigation highlights an economical and promising processing for enhancing GFRP composite by incorporation of (MWCNTs). The result reveals that when the addition of 1 wt% of MWCNT filler got optimum result in the fracture test, the maximum yield strength was 32.8% and modulus was 11.5% respectively increase as compared to neat GFRP composite at room temperature.¹² Additionally, one more literature review via Hossain et.al. on natural jute and synthetic glass fiber hybrid laminate composite by incorporation of graphene filler, for evaluation of mechanical properties. The result demonstrated that the asymptotic increment of mechanical properties when incorporated with the graphene filler such as tensile strength (146%), flexural strength (122%), and impact strength (144%) respectively with respect to neat jute, glass fiber laminate composite.¹³ Balu Maloth et. al. demonstrates the effect of surface modified TiO2 nanofiller in GFRP composite. The result demonstrated that the maximum tensile strength of the laminate composite is 541.8844 Kgf/cm² and the highest flexural strength of the composite is (33 Kgf/cm²), which is higher than the GFRP with only TiO₂ filler. When functionalization of TiO₂ powder with (ATBN) to increase the specific surface area of nano-powder and also attached one amine functional group which is help to better interaction bonding between filler to matrix.¹⁴ Additionally, the next review by Halder et.al. the effect of surface modification of ZrO2 nanofiller to enhance the mechanical properties. The incorporation of salinized ZrO2 nanoparticles (SZNs) demonstrates the strong influence on mechanical properties of hybrid GFRP laminate composite as compared to neat composite. The results indicate a substantial improvement in the tensile strength, stiffness, and toughness of approximately 27%, 62%, and 110%, respectively, compared to the neat GFRP composite. Whereas in the bending test, Strength and modulus are observed to improve by ∼22% and ∼38%, respectively compared to neat composite.¹⁵

Through an extensive review of the existing literature, it was found that there is a significant research gap in terms of the utilization and impact of reduced graphene oxide (rGO) on Glass Fiber Reinforced Polymer (GFRP) composites. Furthermore, the fretting wear behavior of GFRP composites has not been explored to the same extent as sliding wear. To address this research gap, the present study aims to investigate and compare the influence of rGO and graphite on the mechanical and tribological properties of GFRP composites. To fabricate the GFRP laminate composite, a hand layup technique was employed, incorporating both graphite and rGO fillers. The presence of these fillers in the GFRP composite led to improvements in tensile strength, flexural strength, and wear resistance compared to the neat GFRP composite. By conducting this investigation, the study aims to bridge the knowledge gap and shed light on the potential enhancements achieved by incorporating rGO and graphite fillers in GFRP composites.

Materials and methods

Materials

In this current research work, graphite powder was used as a filler material and also to synthesize GO and rGO. The necessary chemical agents required for the synthesis were also procured from various suppliers. Table 1 shows the details of the procured materials.

Table 1.

Synthesis and fabrication of GFRP composite required materials and their size/purity, supplier etc.

S. No.	Materials	Purity/Size	Supplier	Description
1	Graphite	99%/100 mesh	Bangalore fine chem India	Base materials
2	Sulphuric acid (H₂SO₄)	98%	High media, India	Intercalating agent
3	Potassium permanganate (KMnO₄)	99%	High media, India	Oxidizing agent
4	Sodium nitrate (NaNo₃)	99.8%	Alfa Aesar, Mumbai India	Oxidizing agent
5	Hydrogen peroxide (H₂O₂)	30%	Alfa Aesar, Mumbai India	Stop the oxidation
6	Hydrochloric acid (HCl)	35%–38%	Sigma Aldrich India	Eliminate the ions
7	Hydrazine hydrate (NH₂NH₂ · H₂O)	>80%	Sigma-Aldrich India	Reducing agent
8	E- glass fiber	200 GSM	Funique composite Pvt. Ltd, Delhi, India	Reinforcement fiber
9	Resin Araldite LY556, hardener HY906 and accelerator/Imidazole DY070	—	Hunstman advance materials Pvt. Ltd., India	Matrix materials

Synthesis of rGO from Gr

A hummer technique was employed to synthesis rGO from graphite. The Figures 1 and 2 illustrates the detailed steps involved and chemical route for Hummer’s methods. The mixture of pristine graphite powder/NaNO3 was prepared in a weight ratio 2:1. The mixture was added to a beaker containing 46 mL of H2SO4 at 15°C, resulting in a suspension. Gradually, KMnO4 powder, serving as an oxidation agent, was added to the suspension while continuously stirring. The weight of KMnO4 was three times that of the graphite mixture. The process involved three steps. Firstly, a low-temperature reaction took place, keeping the mixture below 20°C for 2 h with continuous stirring. The second step involved a mid-temperature reaction, maintaining the mixture at 40°C for 90 min after complete dissolution of KMnO4. Finally, a high-temperature reaction occurred by slowly adding deionized water to the mixture, resulting in the release of a significant amount of heat upon dilution of concentrated H2SO4. After 30 min, hot water and 30% H2O2 aqueous solutions were added with continuous stirring. The resulting bright yellow suspension was filtered while still hot using qualitative filter paper, followed by washing the solid mixture with diluted HCl (9 vol%) to eliminate ions and then with distilled water to neutralize the P^H. Finally At last it dried in a vacuum oven at 70°C for 24 h to obtain GO.^16,17 Further, 400 mg of GO was dispersed in 400 mL of water using 30 min of ultrasonic treatment, resulting in a homogeneous brown GO aqueous suspension. The suspension's P^H was adjusted to 10 by adding NH3⋅H2O. A certain amount of hydrazine hydrate was added to the suspension and heated at 80°C for 24 h in a reflux setup, maintaining a weight ratio of hydrazine hydrate to GO at 10:7. A black flocculent substance gradually precipitated out of the solution. The product was obtained by filtering it with qualitative filter paper.¹⁸ Finally, the resulting black product (rGO) was washed with methanol and water, then dried at 80°C for 24 h.

Preparation of laminate composite

The GFRP composite with Gr/rGO at different weight percentages was fabricated using the traditional hand layup technique. For this, the required amounts of resin, graphite, and rGO micro-fillers were individually weighed and then mixed using a mechanical high-shear mixer for 20 min at 2000 r/min. Hardener and catalyst were added to the resulting viscous slurry, and the mixture was stirred again using the high-shear mixer for 30 min at 2000 r/min to ensure a homogeneous solution. The slurry was then put in a vacuum desiccator to eliminate air bubbles that could cause defects during the pre-curing of the composite. After selecting a steel flat plate as the mold, a releasing agent was applied to its surface. Then, a peel ply was added, followed by E-glass fibers and the slurry mixture comprising epoxy resin, hardener, catalyst/accelerator, and filler, which was applied over the glass fibers. A roller was used to evenly distribute the slurry mixture between the reinforcement fibers. This process was repeated up to 10 layers, and then another peel ply was added for easy removal of the laminate from the flat plate mould. The mould was then pre-cured in an oven at 120°C for 2 h, followed by post-curing at 160°C for 8 h. The prepared sample and the layout for the hand layup technique were shown in Figure 3.

Characterization

To analyse the crystal size and crystal phase, a Buker D8 advanced X-ray powder diffraction (XRD) instrument was used, which employed Cu Kα radiation (λ = 0.15418 nm) and a scan rate of 5°/min-1. The functional groups present in the synthesized rGO powder were observed by recording FTIR spectra using a Vertex V70 Fourier-transform infrared spectrometer. The spectra were obtained in the range of 400-4000 cm-1 with a resolution of 2 cm-1. Additionally, the thermal stability of the in-house developed Gr and rGO was investigated through a Thermogravimetric analysis using an STA 449F3-1053-M TGA instrument. The analysis was carried out under a nitrogen environment with a heating rate of 10°C/min over a temperature range of 26°C to 800°C. To investigate the mechanical properties like tensile and flexural strength an INSTRON-made UTM machine was utilized by maintaining the crosshead speed at 2 mm/min with varying the load. The flexural properties were calculated from load-displacement curves of flexural testing using the following equations.¹⁹

F l e x u r a l s t r e n g t h (F S) = \frac{3 P L}{2 B t^{2}}

(i)

F l e x u r a l M o d u l u s (F M) = \frac{L^{3} P}{4 d B t^{3}}

(ii)

W o r k o f f r a c t u r e (W O F) = \frac{W}{B t}

(iii)

Where P is the maximum applied normal load(N) at the maximum point of the load-displacement curve. L is the support span, B is the specimen width, t is the specimen thickness (mm), and d is the test specimen displacement (mm) at load P, W (J) is the area under the loaddisplacement curve and represents the work done by the applied force to deflect and fracture the specimen. Work of fracture (WOF) is measured in KJ/m2.

The test samples were prepared as per the ASTM standard for both flexural (ASTM D790) and tensile (ASTM 3039) tests. The test samples and the test setup utilized for this study have been shown in Figure 4.

The fretting wears test setup was utilized to investigate the fretting behavior of the developed material. A schematic diagram for the fretting wear has been illustrated in Figure 5. A steel cylindrical pin of size 4 mm dia. and 15 mm length has been selected as a counter specimen. GFRP composite of size (15 × 15 × 5) mm was used as the test sample. The test has been carried out by keeping the fretting parameters viz. Frequency, stroke, temperature, and time constant with varying the load as 10N, 30N, 40N, 50N. The Frequency, stroke, temperature, and time were selected as 10Hz, 0.7mm, room temperature, and 20 min respectively. The length of the specimen exceeds the array of reciprocating sliding distance (700 μm) because fretting wear in the gross sliding regime leads to a significant widening of the contact zone.²⁰ The evaluation of coefficient of friction (μ) by following the formula,

μ = \frac{F_{f}}{F_{n}}

(iv)

Whereas, and

F_{n}

is the normal applied load (N),

F_{f}

is the frictional force (N)

Result and Discission

X-ray diffraction (XRD) analysis

The X-ray diffractometer analysis was performed on graphite flakes and their derivatives, including graphene oxide (GO) and reduced graphene oxide (rGO) shown in Figure 6. The XRD pattern reveals important information

about the crystalline properties and crystal size of the particles. Graphite flakes exhibit two prominent peaks in the XRD pattern: (002) at approximately 2θ = 26.5° (d-spacing of approximately 3.36 Å) and (004) at 54.65° (d-spacing of approximately 2.678 Å).²¹ These peaks indicate the presence of a graphite structure. The (002) plane, specifically, represents the spacing between graphene layers in graphite. When GO undergoes ultrasonic waves, it undergoes exfoliation, increasing the interlayer spacing between the graphene layers. Consequently, the XRD pattern of GO exhibits a wider interlayer spacing compared to rGO. This is attributed to the presence of oxygenated functional groups and water molecules between the interlayer sheets of GO, which contribute to the increased spacing. The peak intensity at 26.50° decreases significantly, and there is a shift in the peak position from 13.15° (d-spacing of approximately 6.728 Å), indicating the disruption caused by oxidation and chemical exfoliation. The subsequent step involves the thermal reduction of graphene oxide, leading to the formation of rGO. This process eliminates the oxygenated functional groups from between the GO layers, resulting in a decrease in interlayer spacing. The XRD pattern of rGO shows a narrower peak that shifts towards higher reflected angles, specifically at 17.56° (d-spacing of approximately 5.048 Å).²² This shift indicates a reduction in interlayer spacing due to the removal of oxygenated functional groups. However, it should be noted that even after reduction, rGO retains some oxygenated functional groups at the sheet edges. These remaining functional groups promote better attachment of rGO to the epoxy matrix in polymer composites.

Fourier-transform infrared spectroscopy (FT-IR) analysis

The FT-IR spectra analysis was conducted to identify the functional groups present in graphite, graphene oxide (GO), and reduced graphene oxide (rGO) samples synthesized using Hummer’s method. In the synthesized GO and rGO samples, various functional group indications were observed. The hydroxyl (OH) groups from carboxylic acid or alcohol groups were identified between 3000 and 3500 cm-1 in the spectra. However, it was noted that these signals could also be influenced by absorbed water or the instrument’s crystal.²³ Other characteristic peaks observed in the GO and rGO spectra were C=O (1707.15 cm-1), C-O (1192 cm-1), and C-OH (1296 cm-1), indicating the presence of carbonyl and carboxylic acid groups in the samples.²¹ The characteristic peak of C=C (1558 cm-1) was only present in the GO curve, indicating the skeletal vibration of unoxidized graphite powder in the GO sample. In contrast, the pristine graphite curve did not exhibit any functional groups since it solely consisted of carbon elements. This explains why graphite filler does not establish a strong attachment to the epoxide group in the polymer matrix compared to rGO filler. Additionally, some errors in the curve within the range of 2000-2500 cm-1 were attributed to machine or human error during the analysis as shown in Figure 7.

Scanning electron microscope analysis

The morphology and particle size of untreated graphite, graphene oxide (GO), and reduced graphene oxide (rGO) were studied using scanning electron microscopy (SEM), as shown in Figure 8(a)–(c). Untreated graphite displayed a rough, sheet-like structure with a million layers of carbon atoms tightly connected by van der Waals forces shown in Figure 8(a). The SEM images of treated graphite i.e., graphene oxide (Figure 8(b)) showed expected smooth surfaces with wrinkles and folded regions. The wrinkle nature of graphene oxide is beneficial for maintaining a high specific surface area, allowing for easy reaction with the epoxide group of epoxy resin to form strong covalent bonds and enhance the mechanical properties of laminate composites. The hydrophilic nature of GO allows it to dissolve easily in liquids like water, in contrast to the hydrophobic nature of both pristine graphite and rGO, which is attributed to the presence of oxygenated functional groups. The development of oxygenate groups in the basal planes of graphene sheets, along with various flaws in GO sheets, may contribute to these observations.^24,25 Figure 8(c) demonstrated better aggregation and exfoliation of rGO compared to graphene oxide. The reduction in specific surface area in GO sheets, along with the presence of folded edges, directly affects the mechanical strength. In contrast, rGO sheets did not exhibit such characteristics, resulting in increased mechanical properties superior to other fillers. The reduced graphene oxide sheets were bonded together by weaker van der Waals forces due to the elimination of oxygen functional groups located at the edges of the sp² carbon. Consequently, the rGO sheets displayed higher aggregation with covalent bonds at the edge of the basal plane.

Thermogravimetric analysis

In order to evaluate the thermal stability of pure graphite, graphene oxide, and reduced graphene oxide samples, TGA tests were carried out. Figure 9 below shows the TGA and DTG curves of the prepared Gr GO, rGO, and their corresponding derivative. Figure 9 depicts the TGA and DTG curves of pristine graphite. The curve illustrates that as the temperature increases up to 800°C, there is a slight degradation in weight, amounting to approximately 3.86%. Hence it can concludes that pristine graphite is almost thermally stable. In the case of GO, the weight loss occurs in three stages initially at 101.01°C the weight loss was 9.083%. Evaporation of water molecules present in the GO sheet could be the reason for the weight loss.²⁶ Thereafter, the thermal breakdown of carboxyl, hydroxyl, and epoxide groups with oxygen-containing groups in the edge and basal planes resulted in 12.93% weight degradation at a temperature of 223.3°C. The total degradation of weight loss in the GO sample was 35.26% up to 800°C which is shown by the DTG curve, a similar trend has also been observed by Ossonon et.al.²⁷ The presence of oxygen-containing functional groups in graphene oxide (GO) directly influences the weight loss observed during thermal analysis. As the amount of these functional groups increases in the GO sheets, the thermal stability decreases. In contrast, reduced graphene oxide (rGO) demonstrates improved thermal stability compared to GO, as evidenced by a reduced weight loss of 6.41% at 100°C.²⁸ Lower weight decomposition due to most of the oxygen functional group being removed during reduction from GO to rGO. The total degradation of weight loss in the rGO sample is 27.62% up to 800°C which is shown by the DTG curve.

Mechanical characterization

Tensile properties

The tensile properties of the hybridized GFRP laminate composites were compared with those of the reference samples (NE-GFRP) to investigate the impact of fillers such as Graphite and rGO. The figure labelled as Figure 10, represents the relationship between tensile strength and wt.% of filler, demonstrating that the incorporation of different wt.% of filler has an impact on the tensile behaviour of the composite. It is clear from the figure that in contrast to neat GFRP, the inclusion of 0.5 wt% Gr enhances the tensile strength (σy), tensile strain (Ɛ), and tensile modulus (E) by 37.98%, 36.16%, and 6.64% respectively. In the case of rGO, these values increased by 60.3%, 41.2%, and 17.9% respectively. In order to study the influence of filler upon the tensile behaviour of the composite, the microstructure of fracture surfaces of the composite was analysed through SEM which is also represented in Figure 11. The micrographs revealed that the fibers are aligned properly for 0.5 wt% filler incorporated into GFRP laminate composite. In the case of 0.5 wt% rGO GFRP laminate composite, the graphene particles are evenly dispersed. Furthermore, it is evident that the epoxy resin coats the fibers, establishing good attachments between the fiber and matrix.²⁹ In case of rGO filler based GFRP laminate composite have better interfacial bonding between matrix and fiber, because rGO filler incorporated polymer matrix (it contains epoxide group) which is easily react to amine group by covalent bonding. This amine group comes from graphene oxide react with hydrazine hydrate (as a reducing agent) for synthesis of rGO. so that it is one possibility to enhance the better attachment between matrix and fiber. Furthermore, the strength enhancement observed in the 0.5% (rGO) based (GFRP) composite is primarily ascribed to the formation of a superior interface between rGO and the epoxy matrix. This improved interface results in better stress transfer and facilitates a more effective strengthening mechanism. The incorporation of 0.5 wt% of rGO in the hybrid GFRP composite also acts to prevent fiber/matrix debonding and slows down crack propagation, leading to overall improvements in the tensile properties. Additionally, the presence of a well-formed fiber/matrix interface serves to limit the elongation of crimped bundles, thereby enhancing stiffness and reducing the overall elongation of the 0.5% rGO/GF laminate shown in Figure 11(b).³⁰

The micrograph also indicates that fibre pullout, which causes increased stress transmission between the fibre and matrix, is what caused the voids seen in the failed specimen during the tensile test, as shown in Figure 11(a). The tensile strength of the composite is increased as a result of the correctly disseminated and uniform dispersion of the graphene in the epoxy.³¹ The composite having 0.5 wt% graphite filler showed an enhanced tensile behaviour than neat GFRP but a marginal deterioration in tensile properties was observed when compared with 0.5 wt% rGO GFRP composite. This could be due to the less specific surface area and no oxygen-containing functional group of Gr which results in poor bonding between the matrix and filler as shown in Figure 11(c). Moreover, the poor attachment of fiber and matrix also damage the composite and broke the fiber in a transverse direction under the application of high load.³² Both composites i.e., GFRP/1 wt% Gr and GFRP/1 wt% rGO shows an enhancement in the tensile behaviour compared to neat GFRP. However, in both cases a reduction in tensile strength (σy), tensile strain (Ɛ), and tensile modulus (E) by 2.53%; 12.29%; 3.99%, and 21.03%, 14.06%, 10.04 respectively, when compared with corresponding GFRP composite containing 0.5 wt% filler from Table 2.

Table 2.

Depicting the Tensile Properties of NE, Graphite, and rGO filler GFRP laminate composite.

Sample code	Tensile strength (MPa)	Tensile strain (%)	Tensile modulus (GPa)
Neat epoxy	200.43 ± 3.692	4.48 ± 0.03	7.513 ± 0.028
Gr. 0.5%	276.56 ± 5.963	6.10 ± 0.120	8.0127 ± 0.085
Gr. 1%	269.54 ± 6.227	5.35 ± 60.079	7.69 ± 0.076
rGO 0.5%	321.25 ± 4.204	6.33 ± 0.126	8.864 ± 0.042
rGO 1%	253.66 ± 1.00	5.44 ± 0.090	7.977 ± 0.094

Based on the aforementioned results and observations from Figure 10, it can be concluded that the optimal tensile behaviour was achieved when the filler concentration was at 0.5 wt%. Nevertheless, when the content of graphite and rGO fillers in the hybrid GFRP composite increases to 1 wt%, a decline in tensile properties is observed, although they still remain higher than those of the neat composite. This reduction in tensile properties at higher wt% of rGO/Gr fillers can be attributed to the intensification of agglomeration, the fractography image presented in Figure 11(d). The agglomeration process is responsible for creating crack initiation sites at the fiber/matrix interface and generating microcracks within the matrix. Additionally, it promotes stress concentration at the interface, leading to a faster propagation of microcracks. These combined effects result in the observed decrease in tensile properties at higher filler content.^33–35

Flexural properties

Flexural properties affected by variation of fillers it represented by a bar diagram with standard deviation which is demonstrated through Figure 12 (a,b) and also summarised in Table 3. All these data were comprising statically deviation. The flexural strength, strain, and modulus for NE- GFRP composite were observed as ∼320.66 MPa, ∼3.798, and 13.007 GPa respectively which significantly improved the mechanical properties such as ∼8%, ∼14%, and ∼5% for 0.5 wt% of graphite filler while ∼25%, ∼17% and 22% for the 0.5 wt% of rGO filler. When further enhance the filler quantity up to 1 wt% (both graphite and rGO) were observed that marginally reduced in mechanical properties instead of an increase due to lower dispersion of filler into the epoxy matrix and low adhesion between filler and matrix shown in Figure 13(b). The flexural strength lowering occurs due to the discontinuous distribution of stress between fiber and epoxy matrix, which is why no proper interlocking between them and should not resist the load transfer from matrix to fibers depicted in Figure 13(c).³⁶ However, there was no discernible impact of fillers on the flexural modulus of elasticity. Because the inclusion of graphite and rGO lowered the composites' elongation at break values. The bending stress was found maximum at 0.5 wt% for both graphite and rGO for laminate composite which is owing to better interfacial bonding between fiber to the polymer matrix shown in Figure 13(d). Both graphite and rGO GFRP laminate composite have the potential to transfer the load from epoxy matrix to fiber, and also improve the mechanical interlocking between them depicted in Figure 13(a).

Table 3.

Depicting the flexural Properties of NE, Graphite, and rGO filler GFRP laminate composite.

Sample code	Flexural strength (MPa)	Flexural strain (%)	Flexural modulus (GPa)	Maximum load (KN)	Work of fracture MJ/m²
Neat epoxy	319.664 ± 4.147	2.798 ± 0.011	13.007 ± 0.100	0.259 ± 0.005	25.204 ± 0.178
Gr. 0.5%	345.343 ± 3.815	3.205 ± 0.009	13.676 ± 0.034	0.298 ± 0.009	27.218 ± 0.126
Gr. 1%	302.128 ± 4.913	3.078 ± 0.0311	11.299 ± 0.051	0.22 ± 0.002	32.518 ± 0.049
rGO 0.5%	397.166 ± 4.425	3.266 ± 0.016	15.819 ± 0.037	0.329 ± 0.007	35.209 ± 0.053
rGO 1%	369.628 ± 5.988	3.115 ± 0.028	14.422 ± 0.244	0.31055 ± 0.005	37.726 ± 0.331

Abbreviations: GFRP, glass fiber reinforced polymer, NE, neat epoxy, Gr., pristine graphite and rGO, reduced graphene oxide.

The flexural modulus and strain of the GFRP laminate composite shows similar increments at the 0.5% rGO filler. The improvement of modulus and strain is due to an increase in the stiffness and strength of the fiber and matrix interface and also could be minimized the micro-crack generation and propagation at the interface between matrix and fiber. Moreover, the strong interface bonding between fiber and matrix, restricted the molecular chain movement along with the interface of the stress direction, thereby improvement of flexural modulus.³⁷ To elaborate the one more mechanical property like work of fracture (γ) corresponding to load, we observed that ∼26 MJ/m², 0.26 KN was neat GFRP laminate composite, whereas added 0.5% graphite filler into the epoxy matrix and then uplift the properties ∼28 MJ/m² (∼8%), 0.3 KN (∼15%) and ∼36 MJ/m2 (∼39%), 0.33 (∼16%) KN respectively for rGO filler. Whereas, increasing the wt.% of filler up to 1% observed improved properties such as ∼47%, and ∼19% respectively for rGO filler. The maximum WOF was obtained at 0.5 wt% of rGO filler, which is greater than the graphite filler with respect to the neat system, due to inherent stiffness and the constituent interface between polymer matrix and fiber to promote more absorption of energy. Incorporation of rGO filler into an epoxy matrix to resist the interfacial de-bonding to restrict the creation of micro-crack, there faster propagation and interfacial delamination.¹⁵

Fretting wear behaviour

Fretting wear is a significant concern in engineering applications where materials are exposed to small-amplitude vibrations. This phenomenon is particularly critical as it can lead to material degradation, surface damage, and even component failure over time.³⁸

Hence, the fretting wear behaviour of the prepared composites was analysed, and the results obtained from the machine interface were illustrated in Figure 14. The result revealed that as compared to the neat GFRP all other composites show a better resistance to wear. Among the composites, the one containing graphite (Gr) 0.5 wt% filler demonstrated the highest level of wear resistance. Figure 15 represents the worn micrographs of composites having graphite 1 wt% fillers. The micrographs reveal that the presence of graphite flakes creates a lubricating effect, reducing the adhesive and abrasive wear mechanisms that commonly occur in fretting wear. The flakes act as solid lubricants by forming a low-friction transfer film on the contact surfaces, reducing direct contact, and minimizing wear.³⁹ Microscopic examination of the worn surface of GFRP composites induced with graphite typically reveals a more polished and smoother surface compared to composites without graphite Figure 15(a).

The graphite flakes tend to form a protective layer on the surface, acting as a barrier against wear and preventing extensive damage shown in Figure 15(b) and (c). This layer reduces the severity of wear-induced microcracks and surface deformation, resulting in a smoother worn surface. The graphite flakes create tribo-film during fretting wear. Tribo-films are thin layers that form on the contact surfaces and provide lubrication and protection. The presence of graphite facilitates the formation of a more effective tribo-film, further reducing wear and improving the overall wear behaviour of the composite. Additionally, the presence of graphite flakes can enhance the load-bearing capacity of the GFRP composite. The graphite particles act as reinforcement, improving the mechanical properties and toughness at high load of the composite material shown in Figure 16(b). This reinforcement can reduce the severity of wear and prevent crack initiation and propagation, enhancing the overall durability of the material under fretting conditions.⁴⁰ The graphite flakes act as sacrificial materials, absorbing and dissipating the applied loads, thereby protecting the underlying GFRP matrix. The worn surface exhibits wrinkles under higher loads, likely attributed to the substantial plastic deformation resulting from the polymer material's commendable ductility. During the fretting wear test, the wear surface undergoes the formation of peaks and valleys as shown in Figure 16(a). Several high peaks undergo further rubbing, resulting in a smoother surface. The presence of ploughing or cutting stripes, as depicted in Figure 16(c), is noticeable on these peaks. Consequently, the primary fretting wear mechanisms observed in the neat GFRP composite are plastic deformation and abrasive wear.

At the bottom of the worn scar, wave-shaped stripes are formed due to plastic deformation. Furthermore, the presence of furrows on the worn surface is more prominent compared to the neat epoxy. The inclusion of hard nanoparticles aids in the easier formation of ploughing stripes as the steel ball slides over the specimen surface. In the case of graphite microparticles reinforced against steel, plastic deformation and abrasive wear remain the dominant mechanisms in the fretting wear process.

The addition of 1 wt% (rGO) into a material has been found to result in an enhanced wear loss shown in Figure 17(a). While this may seem counterintuitive, there can be several reasons behind this observation. One possible reason is the agglomeration of rGO particles. When rGO is added to a material, it tends to agglomerate, forming clusters or larger structures. These agglomerates can act as stress concentrators, leading to localized areas of increased wear. The agglomerates may create micro-voids or weak points in the material, making it more susceptible to wear damage as shown in Figure 17(b).⁴¹ Another reason for the enhanced wear loss could be inadequate dispersion of 1 wt% filler rGO within the matrix under the higher load. If the dispersion is not uniform, there may be regions within the material where the rGO concentration is higher or lower. This non-uniform distribution can affect the overall wear resistance of the material, leading to enhanced wear loss in certain areas as shown in Figure 17(c). When examining the microstructure of the worn surface, certain features may be observed. The presence of rGO agglomerates or clusters can be seen, indicating areas of localized wear. These regions may exhibit more severe wear damage compared to other types of laminate composite material. Furthermore, the presence of inadequate rGO dispersion may result in variations in wear patterns. Some regions may show more pronounced wear loss, while others may appear relatively unaffected. This non-uniform wear pattern can be attributed to the uneven distribution of rGO within the material.

Conclusion

In this study, the Hummer method was used to synthesize rGO from pristine Graphite. A hand layup technique was adapted to fabricate the GFRP/graphite and GFRP/rGO filler-based laminate composite. Thereafter tensile, flexural, and wear tests have been done. The following conclusion was drawn from the experiment as follows:

• A Hummer method was adopted for the synthesis of rGO. The SEM micrograph revealed that the graphene oxide sheet was thicker and pile-up than the rGO sheet at the edge. The XRD, FT-IR, SEM, and TGA analysis confirmed the synthesis of rGO by stating its crystal phase, chemical functional group, morphology, and thermal stability.

• The composite having 0.5 wt% rGO filler showed an enhanced tensile strength, flexural strength, and in their modulus by 59.56%, 18.21%, 24.45%, and 22.75% respectively compared to neat GFRP laminate composite.

• The analysis of the fracture surface microstructure indicated that at a filler concentration of 0.5 wt%, a homogeneous distribution of the filler was observed. However, as the filler concentration increased to 1 wt%, agglomeration of the filler particles and the formation of micro voids became evident.

• Fretting wear test revealed that GFRP/1 wt% Gr shows better wear resistance than all other composites as the Graphite forms a tribo-film on the worn surface which restrict the direct contact of the mating surface. The worn morphology revealed adhesive and abrasive were the predominant wear mechanism for all the prepared composites.

Footnotes

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

Thingujam Jackson Singh

Data Availability Statement

used to support the findings of this study are included within the article.

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