Introduction

The discovery and research of carbon nanofillers has been an attractive area of interest to most researchers in the field of composites. The critical aspects of the progress and commercialization of these particles with respect to the production of nanocomposite technology were closely related to the properties of the element and its processing of the reinforcing elements. The researchers have worked endlessly to efficiently categorize materials based on their specific dimensions. After the discovery of fullerene and carbon nanotube (CNT), which is zero- and one-dimensional carbon nanofillers, respectively, many researchers have focused their work on the manufacturing of 1D nanoribbon from 2D crystals, as depicted in Fig. 1.

Figure 1
figure 1

Graphene is a 2D material for all the carbon structure of all other dimensionalities a wrapped up into 0D buckyballs, b rolled into 1D nanotubes, and c stacked into 3D graphite [1,2,3]

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Among various nanofillers, graphene has been used widely and frequently by many researchers due to its exceptional properties in various application fields such as polymer nanocomposite manufacturing, biomedical, optical, and tissue engineering. Recently, an increasing number of researchers have devoted themselves to the study of graphene and its reinforced nanocomposite molecules. Graphene is a 2D and a single-layered nanosheet of sp2-hybridized carbon atoms [4, 5], which has been significantly employed for increasing the mechanical and thermal properties of graphene/epoxy composites [6,7,8]. The detention of interfacial interactions between graphene and epoxy plays an important role in the designing of nanocomposites, especially for providing a significant improvement in the fracture strength. These properties of graphene have enabled it and its associated nanofillers to be considered almost the suitable nanofiller for a broad range of applications (Fig. 2). The applications of graphene and its nanofillers range from tissue engineering [9,10,11,12,13,14], cancer therapy [15, 16], production of antiviral materials [17, 18], medical imaging [19, 20], bio-sensing [21,22,23], and so on. Extensive literature survey pertaining to the graphene-family-based nanomaterials depicted that these were the thinnest of available materials which possess such exceptional mechanical [24,25,26], thermal [27, 28], optical [29], and electrical properties [30, 31].

Figure 2
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Schematic overview of properties, structure and applications of graphene-based nanomaterials [32]

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Two-phase polymer nanocomposites are composite materials in which the matrix material is reinforced by one or more separate and diverse nanomaterials to improve their overall performance. The most common material used as a matrix in the nanocomposites production is polymers such as epoxy, nylon, poly-epoxide, polyetherimide, ceramics including alumina, glass or porcelain, and metals such as iron, titanium, magnesium. The outstanding mechanical, thermal and electrical properties of graphene typically increase its application in the spectrum of nanocomposites.

Structural nanocomposites, in particular, are prepared with an aim to take advantage of the extraordinary properties of graphene including its mechanical strength, high surface area, and high aspect ratios [33]. The importance of the bonds present between graphene and the matrices for the application of effective load transfer without slipping of contact surfaces has been widely recognized. While preparing them, it is indeed desirable that a large fraction of the load applied to the composites has been efficiently transferred to graphene. This transfer can be effortlessly facilitated through the interaction or bond formation between the surface of graphene and the surrounding polymer matrix. It is a widely accepted notion that larger area of interfaces formed between matrix and nanofillers optimizes the overall tensile strength [34]. Evidently, it is clear that graphene is the best substance for bulk manufacturing of nanocomposites. In spite of this, the improved dispersion rates of this manufacturing process still possess as a challenge.

The strategy of hybrid reinforcement of the nanofillers with different dimensions and aspect ratios has revealed that to ensure effectiveness in achieving improved mechanical and thermal properties of composites [35, 36]. Figure 3 highlights the important factors of graphene like its dimension, orientation, interaction, dispersion, which play an essential role in the fabrication of its polymer nanocomposites. On the other hand, the reinforcement of the graphene family is still a challenge and the problem lies in the fabrication of multi-scale hybrids with optimized weight fraction based on bulk composites. The current research reviews many fabrication processes that were previously reported.

Figure 3
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Factors affecting the development of graphene-reinforced polymer nanocomposites during processing

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An update on the research pertaining to graphene-reinforced polymer nanocomposites, dealing extensively with the mechanical and thermal properties of graphene composites, is discussed in this study. The first section of the study mainly focuses on the fundamental properties of graphene family and its diverse properties including strength, stiffness, thermal conductivity, and coefficient of thermal expansion. Furthermore, authors have summarized research outcomes of recent experimental studies conducted on graphene- and modified graphene-reinforced nanocomposites for the evaluation of mechanical and thermal properties. The above-mentioned properties of hybrid nanocomposites (with graphene as a common filler) have also been consolidated and discussed in brief. Research gaps have been well identified, and directions for future work are also provided. Potential structural applications have been identified and summarized at the end of the review. Concluding tables are presented in this research to illustrate the fabrication processes of graphene composites in a variety of polymer matrices, thermal properties, and overall properties of hybrid nanocomposites.

Scope of review

Numerous studies on graphene- and modified graphene-reinforced composites have already been conducted by various researchers. Several reviews allied to different properties of nanocomposites have also been depicted with graphene possessing a pivotal role [37,38,39]. In this progress review, a holistic approach of the different fabrication processes in addition to the mechanical and thermal characterization of graphene- and modified graphene-based polymer nanocomposites has been discussed elaborately.

The first section of the review will discuss the mechanical and thermal properties of graphene and its related nanofillers. The second section contains detailed description of the different intricacies involved in the fabrication processes of polymer nanocomposites, along with a note on the mechanical and thermal characterization of the mentioned nanocomposites. The third section of this research will comprehensively discuss the mechanical and thermal properties of hybrid nanocomposites with graphene as common filler. The concluding part of the current progress review enlists a few macro-level potential structural applications of graphene polymer nanocomposites as well as hybrid nanocomposites. The conclusion and future challenges pertaining to graphene will also be briefly presented in this research.

Graphene

Graphene, a monolayer sheet of graphite, is considered as a material composed of carbon structures of all other dimensionalities as depicted in Fig. 1. Graphene-based nanostructured materials have been classified based on the presence of number of layers of the sheet, the orientation, surface modifications, or their total oxygen content (Fig. 4). Graphene has been identified as a latest member of the carbon-based nano materials when compared to fullerene and CNTs. Comparison of graphene to CNTs demonstrated that the use of graphene is more advantageous than CNTs, because of its larger surface area and ability to be easily modifiable by different molecules. Different properties of graphene that were experimentally evaluated by several researchers in the last decade are summarized in Table 1.

Figure 4
figure 4

Characterization of the structure of graphite, a monolayer of GO, and monolayer of rGO using a, b SEM, c TEM [40]

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Table 1 Physicochemical properties of graphene family nanomaterials
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Tensile strength and Young’s modulus

Single-layer graphene has been evidently found to be superior nanofiller that was ever tested. The strength of the 2D material can vary considerably due to the presence of Stone–Wales and vacancy defects embedded in it. The breakage strength of graphene was measured by the atomic force microscopy (AFM) and was found to be 130 GPa [41]. Lee et al. [41] depicted the presence of brittle fractures of graphene that tend to occur at critical intrinsic strength value of 130 GPa. It has also been revealed that graphene can be easily bent, which meant that it can be exploited further for structural modifications and other applications. Use of AFM nanoindentation for identifying the nonlinear elastic response of intrinsic breaking strength was estimated to be 98 GPa. It has also been reported that graphene under shear loading develops wrinkles inside it which leads to a failure strength of 60 GPa. However, the shear strength deformation of graphene is approximately 97 GPa [42]. Zandiatashbar et al. [43] experimentally demonstrated the effect of the topological defects on the strength of graphene. Breaking strength of 100 GPa has been estimated for pristine graphene, while 80 GPa [44] and 75 GPa [45] values were reported for armchair and zigzag-type graphene, respectively. This variation was primarily observed due to the mechanics of critical covalent bonds that periodically distributed along the grain boundary due to which the bonds weakened led to an overall decrease in the strength of graphene. Rasool et al. [46] measured the fracture strength of graphene membranes and found it to be 94 GPa at a fracture force of 6.8 µN when applied on the strongest single crystal membrane of graphene. AFM has been widely used for estimating the breaking strength of graphene revealing that the formation of out-of-plane ripples effectively mitigated graphene in-plane stress [47]. It was found that 100 GPa was the breaking strength, in spite of that the upper bound was reported as 35 GPa. This difference was observed due to the weakening effect of grain boundaries. Clear distinction of these structures can be seen through AFM images only. Rasin Grantab et al. [44] suggested that graphene is one of the stiffest (Young’s modulus 1 TPa) and strongest (strength 100 GPa) materials, which can be further applicable in the field of nano-electromechanical systems (NEMS) as pressure barriers. Wei et al. [45] depicted the detailed arrangement of defects and eventually revealed its effect on the strength and stiffness of graphene. It was found that the maximum tensile stress and stiffness was a function of the tilt angle for both armchair and zigzag with 110 GPa and 0.980 TPa, respectively. This was found to be due to the pentagon-heptagon ring disinclination dipoles which are exactly opposite to the grain the boundary strength. Song Z et al. [48] performed a series of tensile tests on single crystalline graphene membranes to reveal an extremely high Young’s modulus at 1 TPa, while tensile strength remained as 130 GPa. The exceptional plane structures and geometries of the single-layer graphene were examined for their stiffness and breaking strength, and it was found to be 1.1 TPa and 125 GPa, respectively [49, 50].

Single-layered graphene has still been considered as one of the strongest fillers; however, further modification of graphene in terms of covalent modification can be a viable solution for enhancing the strong interaction between graphene and the polymer of interest. Graphene can be modified by using covalent and non-covalent methods. Non-covalent methods have weak interactions between functionalities and the graphene surface, and thus these methods are not generally preferred over covalent functionalization techniques. Covalent bond entangled graphene molecule has been widely used in providing the strong interactions between graphene surfaces and the functionalized groups. However, the originality of graphene typically gets distorted which leads to a big compromise of the thermal and mechanical properties of graphene. Suk et al. [51] used AFM to measure the Young’s modulus and the tensile strength of rGO platelets and estimated it as 207.6 ± 23.4 GPa and 39.7–76.8 MPa, respectively. These experimentally measured values were found to have a lower effective mechanical property, which was attributed to the poor cross-linking bonds between graphene surfaces and the modifier. Gong et al. [52] performed a series of tensile and bulge tests on rGO graphene papers with the thickness in the range of 0.5 to 100 µm. These were high when compared to pristine graphene (0.335 nm). The researchers observed a decline in the Young’s modulus and the fracture strength ranging from 44.6 to 8.5 GPa and from 170.2 to 40 MPa, respectively. A number of studies have shown a relative decline in the mechanical properties of GO paper with varying thicknesses of 2.5–25 µm [53] and 50–60 nm [54]. However, Park et al. [55] performed the mechanical characterization of one, two, and three overlapped layers of GO platelets using AFM. In another work, the DMA results showed that the adhesive characteristic of pDop significantly improves the tensile strength and elongation of pDop-rGO paper by 35 and 70%, respectively [56]. Lee along with their co-authors prepared ethylenediamine and 1,4-butadinol-modified GO (GO-N-GO and GO-O-GO, respectively) using chemical interlinking of GO papers. The mean strength of GO-N-GO and GO-O-GO papers improved by 36 and 10%, respectively, when compared to GO paper [57].

Thermal properties

Graphene possesses not only mechanical properties, as discussed in the proceeding section, but also thermal properties, especially thermal conductivity, which is valuable for thermal management in mechanical and microelectronics devices. Graphene acquires unique thermal properties due to the repetitive structure, robust, and anisotropic bonding, and the low mass of the carbon atoms [28]. The thermal conductivity of graphene was measured in the range of 2000–5000 W m−1 K−1 at room temperature by using a Raman method for both heating and temperature sensing [27, 58,59,60]. Baladin et al. [27] measured thermal conductivity of single-layered suspended graphene in the range of 4800–5300 W m−1 K−1, which is much higher than 2310 W m−1 K−1 for natural diamond [61], 3000 W m−1 K−1 for MWCNT [62], and 3500 W m−1 K−1 for SWCNT [63]. At this point of time, the values of graphene discussed above were approximately ten times of values set for conventional materials such as copper. Jang et al. [64] measured the thermal conductivity of single- and multilayer (ultrathin 7–10 layers) graphene. The researchers observed that for single-layer graphene, the thermal conductivity increases with the increase in the number of layers while showing suppression below 160 W m−1 K−1 at room temperature. Cai et al. [60] used CVD method for determining the thermal conductivity of single-layer graphene at 3000 W m−1 K−1, which was lower than that of other cited researches. This may be due to the lower suspension of graphene on substrate roughness.

The coefficient of thermal expansion (CTE) of CVD-grown graphene was estimated by Shaina et al. [65] by using temperature-dependent Raman spectroscopy. They obtained negative value of CTE of graphene (− 3.75 × 10−6 K−1) for all the temperature ranges within 90 to 300 K. The value reached zero when the temperature was recorded to be at less than 150 K. Storch et al. [66] studied 2D Young’s modulus and CTE of tensioned graphene membranes through the static displacement of membrane. The experiments were conducted by using laser interferometry, and it was found that the modulus eminently declines with increasing temperature. This results into a positive temperature-dependent thermal expansion.

The physical mechanism behind the improvement in mechanical and thermal properties of these advanced composites is sp2 hybridization of graphene molecules so as to permit the bond rotation which finally provides the sufficient rigidity and stability to their subsequent molecule ions. At the same time, the enhancement in thermal conductivity is due to obstacle-free traveling of phonons at any point along the surface; this may be attributed to the regular and repetitive structure of graphene. The above discussion can also be justified through the experimentally obtained in-plane conductivity of a single-layered graphene sheet as 3000–5000 W m−1 K−1 while the cross-plane conductivity obtained as low as 6 W m−1 K−1 which may be due to the weak inter-plane vdW forces.

Properties of graphene-/modified graphene-reinforced composites

Mechanical properties

As mentioned above, graphene and modified graphene have excellent mechanical properties concerned with high elastic modulus, high tensile strength, fracture toughness, etc. These exceptional properties of graphene contributed to the development of inexpensive and high quality of nanocomposites. Most of the researches conducted on graphene-based epoxy nanocomposites are aimed at exploiting the remarkable mechanical strength.

Yazdi et al. [80] investigated mechanical properties of layered graphene/polyoxymethylene nanocomposites prepared by the spray method. The results of the study revealed that the addition of graphene up to 2.4 wt% increased Young’s modulus and tensile strength by 103 and 64%, respectively. Another study administered by Ma et al. [81] obtained the tensile strength of GO/PI (GO-reinforced polyimide (PI) composites developed with different loadings of GO using in situ polymerization and solution blending methods) with 3.0 wt% GO loading to be as 137.8 MPa. This value is 75.5% higher than that of a pure PI. It is inferred from the experiment that in situ polymerization accelerates a better dispersion of GO in the polymer matrix. Bortz et al. [82] monitored the effect of epoxide cure kinetics by keeping the cross-linking mechanism under observations through the use of DSC. An enhancement of 28–111% in fracture toughness and up to 1580% in uniaxial tensile fatigue life was witnessed when a small amount (≤ 1 wt%) of GO was added to the epoxy system. Kashyap et al. [83] performed a series of tensile tests on rGO-reinforced nanocomposites and observed an increment of 150% in elastic modulus and tensile strength of 0.3 wt% rGO/PVA nanocomposites. This is due to homogeneous distribution of fillers and improved load transfer capability between hydrogen bond contained by modifier and matrix. Mo et al. [84] performed the mechanical characterization on GO/PVA nanocomposites and found a tensile strength of 280 MPa and Young’s modulus as 13.5 GPa with 50% loading of modifiers. Another study utilized sulfonated poly(ether-ether-ketone) as a non-covalent surface modifier in the fabrication of non-covalent functionalized reduced graphene oxide (NFrGO)/cellulose acetate (CA) nanocomposites by using solution mixing technique [85]. Enhancements of about 102% (106.87 ± 2 MPa) and 143% (1.32 ± 0.3 GPa-3.21 ± 0.2 GPa) in tensile strength and modulus of NFrGO/CA, respectively, were observed for 1.5 wt% loading of NFrGO when compared with those of pure CA. This improvement is attributed to the good molecular-level dispersion of NFrGO sheets in the matrix of CA. Sainsbury et al. [86] used bisphenol-functionalized graphene nanosheets (f-GNS) to fabricate PBE/GO-based nanocomposites to facilitate the enhancement of 26 and 63% in Young’s modulus and ultimate tensile strength with regard to a filler loading of 0.25 wt%. Vlassiouk et al. [87] produced layered flat graphene laminates (type 1) and fibers (type 2) with the application of layer-by-layer assembly approach along with a systematic progression of layered structures, respectively, as shown in Fig. 5. The resulting composite structures indicated that the strength of graphene/PMMA laminates is double when compared to pure PMMA with only 0.13% load of graphene. However, mechanical properties of pure graphene scrolls (fibers) are less impressive (2.2 GPa strength and 0.3 TPa modulus) but offer a competitive edge in comparison with carbon fibers (CFs). An improvement in interfacial and tensile properties of CF/epoxy composites reinforced with single-layer GO sheets (5.0 wt%) was reported by Zhang et al. [88]. Alexopoulos et al. [89] evaluated the effect of the size of GNPs on the properties of epoxy/GNPs nanocomposites. The results revealed an increment of 15 and 12% in UTS and fracture toughness, respectively, at low filler concentrations (< 0.25 wt% GNPs) while 35 and 30% enhancement in Young’s modulus of nanocomposites with the addition of GO with particle size of 130 and 1200 μm, respectively [90]. The researchers further noticed a decline in mechanical properties with the incorporation of higher filler concentration (> 5 wt% GNPs) due to the particles agglomeration.

Figure 5
figure 5

a Scheme of laminates and fibers/scrolls, b 2D and G bands shift, and c Raman spectra of (Graphene/PMMA)16 sample [87]

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Recently, Chhetri et al. [91] reported a dispersion of 0.25 wt% of GNS in the epoxy matrix improves its fracture toughness, flexural strength, and flexural modulus by ~ 91, 46, and 71%, respectively, when compared to pure epoxy. However the tensile yield strength and Young’s modulus were obtained to be 66 MPa and 4.92 GPa, respectively, when PVA nanocomposites contained 1.0 wt% of GO [92]. Also, an improvement by 39 and 127% in the UTS and tensile toughness of poly(styrenesulfonate)–polyaniline (PSS)/rGO at a loading of 0.5 wt% was measured by Lin et al. [93]. Chen et al. [94] prepared a high-performance polymer composite by using a core–shell structured hybrid (SiO2-GO) as a new filler into the epoxy polymer matrix. Ultrathin GO shells-coated silica surfaces facilitated this enhancement. However, the functionalized GO (FGO) showed a homogeneous dispersion in the nanocomposites with high-density polyethylene (HDPE), where the stress and strain at the break of HDPE nanocomposites increased by 28.7 and 130%, respectively, with the incorporation of only 0.2 wt% FGO loading [95]. The improvements by 10.11 and 14.67% in the flexural modulus and strength of GO epoxy nanocomposites with the addition of 1.0 wt% of GO were henceforth witnessed [96]. Ni et al. [97] fabricated epoxy-based composites reinforced by 3D graphene skeleton (3DGS) by RTM method to avoid difficulty in dispersion and orientation of graphene sheets. The researchers implemented the direct mixing method and found that 3DGS/epoxy composites have superior mechanical properties and thermal stability as compared to EG/epoxy composites (Fig. 6).

Figure 6
figure 6

Mechanical properties of composites with different filler loadings: a tensile strength, b tensile stress–strain curve, c compressive strength, and d flexural strength and modulus [97]

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The degree of dispersion can be defined as the ratio of wt% of nanofillers to the total wt% of epoxy matrix, which affects directly or indirectly the performance of nanocomposites. Moreover, an optimize sonication time, power, and temperature are the factors responsible for homogeneous dispersion of graphene in epoxy pool [98, 99]. Wei et al. [99] reported that with the incorporation of 0.3 wt% graphene, the tensile strength and storage modulus of nanocomposites increased from 57.2 to 64.4 MPa and 1.66 to 2.16 GPa, respectively. However, the Tg value of nanocomposites uplifted from 93.37 to 99.08 °C (for 0.3 wt% of graphene loading). The results showed that the dispersion state depended on the function of sonication time and temperature. Further, a significant reinforcement effect of graphene on epoxy was observed. Apart from this, the orientation of reinforcing element in epoxy matrix also plays an important role in enhancing the overall properties of composites. Li et al. [100] reported about the level of spatial orientation of GO flakes in nanocomposites as shown in Fig. 7. They noticed an improvement in Young’s modulus of nanocomposites reinforced with fully aligned graphene, while an expected fall by 50% was seen with an inclusion of randomly oriented graphene. Li et al. [101] studied the impact of dopamine-rGO (pDop-rGO) on the mechanical properties of epoxy-based composites. pDop-rGO was prepared through simultaneous functionalization and reduction of GO via polydopamine coating. The effective modulus and strength of GO and pDop-rGO at different contents in epoxy composites are shown in Fig. 8.

Figure 7
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a Schematic illustration of the Cartesian coordinate system with the sample geometries used in the Raman spectroscopic analysis in the three directions, b view with the laser beam along the Z axis, and c view with the laser beam along the X and Y axes [100]

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Figure 8
figure 8

a Tensile and b flexural properties of epoxy composites with various contents of GO or pDop-rGO [101]

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Wu et al. [102] noticed up to 900% improvement in mode-I fracture toughness of GNPs/epoxy nanocomposites with ~ 0.8 vol% of transversely aligned GNPs. This improvement was attributed to multiple extrinsic and intrinsic toughening mechanisms including pinning, micro-cracking, deflection, rupture, and pull-out of the GNPs as shown in Fig. 9. Atif et al. [103] conducted an experiment to maximize the mechanical properties at 0.1 wt% of multilayered graphene. They found that whereas flexural and tensile strength were enhanced by 49% at 74–111 MPa and 43% at 46–65 MPa, respectively, an increment in flexural and Young’s modulus by 23% at 598.3–732.8 MPa and 26% at 610–766 MPa, respectively, was obtained. A study used 4, 4’-Methylene diphenyldiisocyanate (MDI) as a chemical modifier to modify graphene, fabricated epoxy/graphene, and epoxy/modified graphene nanocomposites. The results showed an increase in Young’s modulus of neat epoxy from 0.675 to 8.0 GPa with the addition of 0.5 wt% of modified graphene, i.e., 25% more than the unmodified graphene [104]. Bora et al. [105] reported an improvement in tensile strength and Young’s modulus of composites by 123 and 87%, respectively, with the addition of 3.0 wt% of rGO in epoxy. However, the flexural modulus of GNP/epoxy composites increased by 160% when 4.0 wt% of GNP of aspect ratio 300–1000 and thicknesses of 5–17 nm is added into epoxy [106]. Table 2 lists the details of mechanical properties at different filler contents of graphene- and modified graphene-reinforced polymer nanocomposites.

Figure 9
figure 9

SEM images a–d of an epoxy nanocomposites containing 1.08 vol% of GNPs transversely aligned to the direction of crack growth showing: a tortuous crack path; b micro-cracks at the crack tip; c micro-cracks inducing crack deflection and branching; d crack bridging and graphene pull-out. (Note b and c are magnified SEM images taken from the circular and rectangular regions in (a), respectively); e effects of vol% and alignment of GNP on the mode-I fracture energy of the epoxy nanocomposites [102]

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Table 2 Mechanical properties of graphene- and modified graphene-based epoxy nanocomposites
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Thermal properties

Numerous studies on thermal properties of different polymer nanocomposites have already been conducted in the last decade but investigations on melting temperature, glass transition temperature, thermal conductivity, and thermal expansion coefficient of the nanocomposites were found to be most pivotal.

Glass transition temperature

The glass transition temperature (Tg) is one of the significant viscoelastic properties of the epoxy and its composites, which is usually defined by a point. This point signifies the behavior of the polymer from brittle to glassy to a rubbery state. The Tg can also be understood more precisely by considering the amount of free energy required by the atoms to escape from their original lattice positions to other significant positions, which typically result in a glassy to rubbery state. Once the rubbery state of the molecule has been achieved by these atoms, they become stiffer, more brittle, and highly inflexible. Therefore, it is absolutely necessary to calculate the Tg either by conducting experiments or by performing graded simulations [134]. The processing of the polymers depends on the temperature range, which is determined by Tg. There are numerous experimental methods to evaluate the Tg, such as dynamic mechanical analysis (DMA), differential thermal analysis (DTA), thermo-mechanical analysis (TMA), and differential scanning calorimetric (DSC). In addition to these, there are three common theoretical approaches that are used for calculating Tg by MD simulations, namely (a) the energy-temperature method, (b) the mean squared displacement, and (c) the property temperature method.

Ribeiro et al. [135] investigated the Tg of GO- and GO-TEPA (tetraethylenepentamine)-reinforced epoxy composites using the DMA tests. They estimated that the Tg values increase from 15 to 20 °C (0.5 wt%) for GO and GO-TEPA epoxy composites, respectively, in comparison with pure polymer. Li et al. [136] incorporated GO into epoxy matrix to observe that the Tg of nanocomposites enhanced by 33.05 °C with the 1.0 wt% fraction of filler. However, for GO/epoxy nanocomposites, Tg shifted to a lower temperature range as compared to the neat resin, i.e., from 164.8 to 157.1 °C [137]. Pour et al. [138] determined the Tg of PVI-g-GO-reinforced epoxy from the DSC scan. The Tg value of neat epoxy increased from 101 ± 3.0 to 108 ± 4.1 °C with the insertion of 0.25 wt% loading of PVI-g-GO.

Zaman et al. [139] synthesized two types of epoxy nanocomposites reinforced with GNP and surface-modified GNP. They found that the interface modifications had improved the Tg of neat epoxy from 94.7 to 108.6 °C, depicting a 14.7% increment (Fig. 10). However, the incorporation of 0.041 vol% (0.1 wt%) of graphene sheet into polypropylele (PP) enhanced the Tg of the nanocomposites by approximately 2.5 °C [140]. An unprecedented shift in the value of Tg to over 40 °C for poly(acrylonitrile) and 30 °C for PMMA was obtained at 1.0 and 0.05 wt% of the f-GNSs [141]. Layek et al. [142] also reported that the increase in the Tg from loss of modulus data was not significant. However, while performing estimations using the tanδ plot, it was observed that there was a 21 °C improvement in Tg and higher effects of damping in the 5.0% of PMMA-functionalized graphene (FG)-reinforced nanocomposites. In spite of this, the pure epoxy resins (EPs) showed a Tg value of 153.9 °C. An insertion of thermally reduced GNPs and FG into epoxy increased the Tg to 167.6 and 169.8 °C, respectively [143]. Chen et al. [94] observed that the Tg increase occurred at 197 °C for pure EP, whereas it was considerably shifted to lower temperatures (194.4 °C) for nanocomposites loaded with 0.1 wt% of GO. The Tg of epoxy-based (~ 81.5 °C) nanocomposites with the incorporation of 1.0 wt% of GO of particle size of 300 and 1200 μm was calculated as 87.8 and 90.7 °C, respectively [90]. Hence, after reviewing the pieces of literature, it is concluded that the Tg values of the epoxy nanocomposites depend on the curing extent, cross-linking density, content and type of filler, and the interfacial bonding between the filler and matrix [93, 97, 128, 129].

Figure 10
figure 10

Damping behavior of nanocomposites a epoxy/GP and b epoxy/m-GP [139]

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Thermal conductivity

The thermal conductivity of the epoxy nanocomposites was enhanced by the addition of carbon nanofillers [144]. However, the increment in the thermal conductivity can be more effectively done by the addition of 2D graphene nanosheets except 1D rod-like CNT [145]. According to the Kaptiza resistance, the lattice vibrations and free electron interactions between the two or more contacted surface were found to be responsible to carry out the transfer of the thermal energy. Therefore, the weak interactions at nanofiller or epoxy interfaces remarkably affected the thermal resistance [146]. Hence, a strong nanofiller or epoxy interfaces was required to achieve reasonable thermal conductivity [147]. The thermal conductivity determined by considering the orientation and properties of fillers, the morphology of the polymer nanocomposites, and their interaction with the fillers and the epoxy matrices is depicted in Fig. 11.

Figure 11
figure 11

Thermal conductivity of composites depends on multiple factors that need to be considered when designing new materials [148]

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Kostagiannakopoulou et al. [149] investigated the effect of MWCNTs and GNPs on thermal conductivity of nanoreinforced polymers and nanomodified CPRFs. They revealed that the addition of 15 wt% of GNPs to the epoxy improves the thermal conductivity of nanoreinforced polymers and nanomodified CPRFs by 176 and 48%, respectively. Xiao et al. [150] prepared poly(vinylidene fluoride) PVDF/CNT/GNP and PVDF/GNP/CNT composites by adding GNPs into the PVDF/CNT and CNTs into PVDF/GNP composites. It was found that the addition of 1.0 and 20 wt% of GNPs in the PVDF/CNT enhanced the overall thermal conductivity of the PVDF/CNT/GNP up to 0.31 and 1.92 W m−1 K−1, respectively. The results of this study demonstrated that the PVDF/CNT/GNP composites exhibit a higher thermal conductivity compared to PVDF/GNP, PVDF/CNT, and PVDF/GNP/CNT composites. This may be attributed to the improved 3D network made in between solid particles and polymer matrix, which further leads to a higher percolation threshold for nanocomposites. Gu et al. [151] fabricated the GNPs and f-GNPs/E-51 nanocomposites using the casting method. They found that the thermal conductivities of f-GNPs/E-51 nanocomposites were found to be increased with the increasing f-GNPs content and also higher than the GNPs/E-51 nanocomposites, whereas thermal conductive coefficient of f-GNPs/E-51 incorporated with 30 wt% of f-GNPs was 8 times higher than the pristine E-51. Teng et al. [152] formulated the localized pyrene groups on poly(glycidyl methacrylate) (Py-PGMA) to make an effective covalent bonding and cross-linked structure in Py-PGMA-GNS/epoxy composites. This research revealed that 16.4% increment of the thermal conductivity of 1.0 phr Py-PGMA-GNS/epoxy composites compared to 1.0 phr pristine GNS/epoxy composites. Kim et al. [153] suggested that the addition of GO and Al(OH)3-coated graphene (Al-GO) enhanced the thermal conductivity of epoxy composites. The thermal conductivity of pure DGEBA EP was found to be 0.188 W m−1 K−1. Figure 12 depicts a comparison between the thermal conductivities of GO/epoxy and Al-GO/epoxy composites prepared with 0.0, 1.0, and 3.0 wt% fillers at room temperature. Shan et al. [154] formulated GO-reinforced epoxy nanocomposites. The researchers examined that with the addition of 0.5 wt% of GO, the thermal conductivity was found to be enhanced from 54.63 × 10−3 to 136.01 × 10−3 W m−1 K−1 with the increase in the temperature from 6 to 16 K, respectively. Aradhana et al. [155] reported that the incorporation of 0.5 wt% of GO in the epoxy adhesive resulted in a thermal conductivity of 0.56–0.63 W m−1 K−1, i.e., enhancement of 211 and 174% at temperatures 30 and 90 °C, respectively.

Figure 12
figure 12

Thermal conductivities of epoxy composites of neat epoxy, graphene oxide, and Al(OH)3-FG [153]

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Olowojoba et al. [133] reported that the thermal conductivity of thermally rGO/epoxy composites and 2.0 wt% insertion of rGO resulted in a maximum thermal conductivity (0.264 W m−1 K−1) at 60 °C. Zhao et al. [156] introduced the graphene foam into poly-dimethylsiloxane (PDMS) and reported that the thermal conductivity was found to be increased by 88% for graphene foam/PDMS composites as compared to pure PDMS. Wu et al. [102] measured the effect of the alignment directions of the GNPs both parallel and transverse on the thermal conductivities of epoxy nanocomposites. They also suggested that the increment in the thermal conductivity by approximately 60% in the case of transverse alignment of GNPs, as compared to unmodified epoxy polymers (Fig. 13). Zhang et al. [157] designed and fabricated vertical and horizontal graphene film-reinforced epoxy nanocomposites. With the incorporation of 44 vol% of graphene into epoxy, thermal conductivity of horizontal graphene film/epoxy nanocomposites was found to be increased to 0.81 W m−1 K−1. However, the corresponding values were noticed as 384.9 W m−1 K−1 for vertical nanocomposites, which was observed at two orders of the magnitude higher than that of the horizontal nanocomposites.

Figure 13
figure 13

Effects of the volume content and alignment of the GNPs on the thermal conductivities of the epoxy nanocomposites [102]

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Coefficient of thermal expansion

A low CTE and good stability were among the critical factors that a composite needs to possess, specially designed for such climatic situations where the temperature changes dramatically. This may be used to strengthen by laying the composite coatings on the surfaces of aircrafts, aerospace craft, and buildings [158].

Chang [159] prepared hexadecylamine and 4-amino-N-hexadecylbenzamide f-GNS-based PI nanocomposites. The results of this research depicted that the CTEs values for PI and PI hybrids decrease with an increase in the graphene loading (0–10 wt%) over a temperature range of 50–200 °C. Wang et al. [160] observed that the CTE for neat EP was estimated around 8.2 × 10−5/°C. However, an inclusion of 1.0 wt% GO or SWNTs in EPs did not show an obvious reduction. Figure 14 shows the CTE of the EP, graphite/epoxy, GO/epoxy, and SWNT/epoxy composites at ramp rate 10 °C/min.

Figure 14
figure 14

Thermal expansion of graphene composites [160]

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Kim et al. [161] investigated the dynamic mechanical behavior of exfoliated graphite nanoplatelets (xGnP)–linear low-density polyethylene (LLDPE) nanocomposites. The results of the study indicated that TE and the CTE of xGnP-loaded composites were much lower than those of the control LLDPE in the ranges of 45–80 °C (299.8 × 10−6/°C) and 85–100 °C (365.3 × 10−6/°C). Wu et al. [162] added few GNPs to polyetherimide (PEId) thermoplastic polymers and measured the coefficient of linear thermal expansion (LTE) in three directions, i.e., 0º, 90º and Z direction. This study observed that the LTE in 0° and 90° direction decreases with an increase of substantial GNP loading. However, it was inferred that the LTE remains the same at concentrations lower than 3.0 wt% GNP loadings. The various factors pertaining to thermal properties at different filler contents of graphene- and modified graphene-reinforced epoxy nanocomposites are listed in Table 3. Finally, on the basis of literature discussed in previous sections, an optimized fabrication strategy with different ranges (numerical values) for each step of fabrication have been proposed as shown in Fig. 15.

Table 3 Thermal properties of graphene- and modified graphene-based epoxy nanocomposites
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Figure 15
figure 15

Fabrication strategy of graphene-reinforced polymer nanocomposites

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Properties of graphene-/modified graphene-reinforced hybrid composites

The previous section explored the mechanical and thermal properties of surface-modified-graphene-reinforced nanocomposites. While there have been several reports published on CNT and graphene as individual fillers, a scarcity of literature on the fabrication of nanocomposites with CNT and graphene as co-applicants, which changed their category from nanocomposites to hybrid nanocomposites, was noticed. However, the studies reviewed reported better mechanical and thermal properties of hybrid nanocomposites as compared to general nanocomposites. Furthermore, hybridization process leads to certain disadvantages concerned with the composition, degree of functionalization and interestingly the mixing of the two fillers in the polymer matrix.

Graphene/fiber hybrid composites

The performance of carbon fiber-reinforced composites (CFRP) is typically dependent on the properties of the fiber-matrix interface. To improve the interfacial properties in CF/epoxy composites, direct introduction of graphene or f-GNS dispersed in the fiber sizing onto the surface of the individual CFs has been performed. During interface improvements, single-layer and double-layer graphene sheets were observed in randomly selected regions. The interface-modified nanocomposites demonstrated completely different surface fractographs to the unmodified peers, corresponding to higher improvements in the resistance of the solvent absorption resistance and fracture toughness. CFRP composites have been evidently depicted themselves as light weight, high tensile strength and modulus materials significantly reducing the weight of the structures. These may be further improved by reinforcing the matrix material with an optimized wt% of nanofiller.

Han et al. [174] fabricated a hybrid GO/CF-reinforced epoxy composites in two steps: First, the GO was added with EP, and then the modified EP was brushed onto plies of CF fabrics, later stacked by a hot-pressing machine (Fig. 16). The results of this study indicated that the incorporation of 0.10 wt% of GO into EP enhanced the ILSS by 8.05%, with an increase in Tg by 5 °C. Hallad et al. [175] investigated the behavior of a polymer beam reinforced with graphene and CF at nano- and micro-level both individually and combined form reinforcements, respectively. The researchers performed a three-point load test on the beams to estimate the flexural strength. An improvement by 35% was observed as compared to plain control beams when graphene was used as reinforcement fillers in the polymer matrix.

Figure 16
figure 16

Fabrication process of GO/CFs-reinforced epoxy composites [174]

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Hu et al. [176] also fabricated FGO/regenerated silk fibroin (RSF) hybrid composites fibers with improved mechanical properties using brittle and weak RSF by performing the wet spinning method. The results of this experiment indicated a 58.7% improvement in the breakage force of the hybrid fiber composites when compared to the pure RSF silk fibers. Mahmood et al. [177] theorized the concept of inducing a piezo-resistive response of epoxy/glass composites through an engineered interphase. They procured E-glass fibers coated with GO by electrophoretic deposition (EPD) and rGO-coated fibers by subsequent chemical reduction reactions. They also performed three-point bending tests on the unidirectional composites with uncoated, GO-coated and rGO-coated GF. The results of this study revealed that the flexural modulus and flexural strength were increased by 19 and 9% for GO- and rGO-coated GF, respectively, when compared to the neat composites. Another research by Hua et al. [178] marked the preparation of GO/CNT hybrid-coated glass fiber (GF)/epoxy composites. It was found that the interfacial bond strength of the hybrid-coated fiber increased 128% as compared to the calculated value in terms of the rule of the mixtures from the results of a single GO and CNT coating. The SEM, TEM, and HRTEM images of GO/CNT hybrid coating of GF surfaces are depicted elaborately in Fig. 17.

Figure 17
figure 17

SEM image GO/CNT hybrid coating layer on GF surfaces b TEM image of CNT/GO hybrid coating on one GF surface by CNT implantation into GO, c magnified TEM image of (b), d HRTEM image of the GO/CNT hybrid coating layer on GF surfaces and e transverse tensile strength of GF/epoxy composites [178]

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Pathak et al. [179] reported that the inclusion of 0.3 wt% of GO in the CF-reinforced polymer hybrid composites improved the flexural modulus and flexural strength by 72 and 66%, respectively. Idumah et al. [180] prepared a GNP kenaf fiber hybrid PP composite. The researchers recorded that about 70 and 98% enhancement was observed in flexural strength and flexural modulus, respectively, while the thermal conductivity value increased by approximately 88% at 3.0 phr loading of the GNPs. Papageorgiou et al. [181] studied the effect of the GFs, GNPs and the hybrid fillers on the properties of PP. They performed tensile testing analysis on PP-GNP, PP-GF, and PP-GF-GNP composites and concluded that the addition of hybrid filler improved the Young’s modulus of these composites. The fractured images and mechanical properties of each of the composites are presented in Fig. 18.

Figure 18
figure 18

SEM images of fractured samples a–c: a PP-GNP20, b GFs in the PP-GF sample, c GFs coated with PP and GNPs in the PP-GF16-GNP20 specimen, d tensile modulus, and e fracture stress of the PP and the composites samples [181]

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Kwon et al. [182] examined the effect of thermally rGO/CNT hybrid coating on the CF-reinforced epoxy composites. The results of these experiments revealed that an enhancement of over 10% in the short-beam strength as compared to uncoated CF composites, whereas IFSS of the GO-coated fiber/PTFE composite rose from 49.45 to 71.91 MPa (44.4%) as compared to the untreated CF [183]. In another work, CF was coated with partially rGO by an anodic EDP. The result showed an improvement of 14% in ILSS of composites. This enhancement revealed the better interfacial adhesion and wettability between coated CFs and epoxy [184].

Kostagiannakopoulou et al. [185] used GNPs and MWCNTs hybrid nanofillers to develop carbon fiber-reinforced polymers (CFRPs). The hybrid doped CFRPs were prepared by using 0.5 wt% of GNPs with varying wt% of MWCNTs (i.e., 0.5 and 1.0 wt%). This study concluded that the fracture toughness was enhanced by 45 and 31% for unequal and equal amounts of GNPs and MWCNTs, respectively. Menbari et al. [171] used three different diamines, namely ethylenediamine (EDA), p-phenylenediamine (PPD) and 4,4′-diaminodiphenyl sulfone (DDS), for FGO and incorporated them into an epoxy/GF composite laminate. It was revealed that the 0.5 wt% of GO to the EP/GF composite enhanced the in mode-I fracture toughness by 27 and 21% for initiation and propagation stages, respectively. However, the inter-laminar fracture toughness of the PPD-FGO, the DDS-FGO, and the EDA-FGO was found to be increased by 69.5, 70, and 93%, respectively, when compared to the unmodified EP/GF composites.

Graphene/nanofiller hybrid composites

The chemical compatibility between the fillers plays an important function in enhancing the overall properties of hybrid epoxy nanocomposites. Recently, a number of experiments have been performed to assess the increment of the interphase between fillers and the matrix. Chemical functionalization and specific contact area based techniques have been employed to assess the reduction in the size of the particle fillers. The high aspect ratio and van der Walls interactions of graphene and its agglomerates have been used widely so that decoration of graphene with other nanofillers can be deployed in order to avoid this phenomenon. These steps typically come at cost but provide an ease in the fabrication process as compared to the functionalization of graphene for its improved interaction with other nanofillers to help expand the reactive sites of graphene and lead to the formation of fewer covalent bonds between the matrix and hybrid filler.

Min et al. [186] constructed GO nanosheet/carboxyl-functionalized MWCNTs (GO/MWCNTs) hybrid reinforced PI nanocomposites (Fig. 19). The tensile strength was found to be increased by 25.1% at 3:1 ratio of GO/MWCNTs as compared to the pure PI due to the homogeneous dispersion patterns and strong interfacial covalent bonds that are formed between GO/MWCNTs and the PI matrix. Al-Saleh et al. [14] reported that the CNT/PP nanocomposite typically exhibit superior tensile strength and better thermal conductivity as compared to the GNP/CNT nanocomposites and GNP/CNT/PP hybrids. The addition of CNT volume fraction (up to 5 vol%) in the GNP/PP nanocomposites has shown an increase in the toughness and the tensile strength. Abdollahi et al. [187] synthesized a hydrogel nanocomposite based on poly(acrylic acid) grafted onto amylose (PAA-g-amylose) containing different loadings of the GO. They analyzed thermo-mechanical behaviors of the composites by employing TGA, DSC, and tensile testing assessments. The results of this study showed that 124 and 26% improvement in the tensile strength and elastic modulus of the hydrogel nanocomposites, respectively, with 3.0 wt% GO loading. However, thermal stability was also found to be improved by 67 °C and Tg shifted higher temperature by 53 °C at 5.0 wt% GO loading as compared to the pristine hydrogel matrix. The mechanical properties of the CNTs + GNPs or epoxy nanocomposites with 0.1, 0.2, 0.3 and 0.4 wt% of CNTs + GNPs (at mixing ration 1:1) were investigated by Moosa et al. [188]. The research revealed that the UTS increases by 36% and the modulus of elasticity was found to be doubled than that of the neat epoxy at 0.4% of CNTs + GNPs hybrid filler.

Figure 19
figure 19

The flow diagram of the preparation of GO/CNT/PI films [186]

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Hawkins et al. [189] examined the fracture toughness of carbon-graphene-epoxy hybrid nanocomposite using single-edge-notch bending specimens. The results of this study showed significant improvement in the fracture toughness of hybrid nanocomposite by 11.4%, as compared to a conventional carbon–epoxy composite. This study also observed an effective dispersion of graphene throughout the epoxy matrix in addition to a toughening mechanism such as a crack pinning, de-bonding, pull-out, crack deflection and fiber bridging through systematic SEM analysis. Chatterjee et al. [190] investigated the effect of CNT/GNP hybrid fillers on the properties of hybrid nanocomposite. They reported that the fracture toughness and fracture modulus of the epoxy-based hybrid nanocomposite had improved considerably by 77 and 17% with a CNT/GNP ratio of 9:1. However, the addition of GNP/MWCNT (0.1:0.4) hybrid filler to the epoxy had significantly increased the tensile strength and modulus of nanocomposite by 21 and ~ 17%, respectively, as compared to the unfilled epoxy [191]. Wang et al. [192] examined the mechanical properties of the cross-linked hybrid GO/CNT aromatic PI nanocomposite. The results of this research showed a dramatic increment in the strength, modulus, and fracture toughness by 118, 94, and 138%, respectively, as compared to neat PI. Sahu et al. [193] prepared HDPE polymer matrix reinforced with an alternate and possible combination of nanodiamonds (0D), MWCNTs (1D), and GNPs (2D). The results of the research depicted that the incorporation of hybrid filler as 0.1 wt% of GNP with 0.1 wt% of nanodiamond in HDPE considerably improved the Young’s modulus and the hardness by 25.17 and 83.33%, when compared to pure HDPE. Young oh et al. [194] also reported that the large scale of GO sheets prompted various isolated GO aggregation islands in the composite due to a strong π–π interaction between the layers. However, the small-scale GO sheet can provide high-density cross-linking reactions and can be homogeneously incorporated with SWCNT. Ribeiro et al. [195] introduced a 2D hybrid nanofiller by combining hexagonal boron nitride and graphene oxide (GO/h-BN). They observed a 140, 177, and 32% increment in tensile strength, ultimate strain and elastic modulus of the epoxy nanocomposites, respectively, with an incorporation of 0.5 wt% hybrid filler. However, thermal conductivity was found to be enhanced up to 142% with respect to the pure epoxy for 2.0 wt% of GO/h-BN.

The mechanical and the thermal properties of hybrid nanocomposites have been reviewed in this research. The improvement in these properties has shown the possibilities and capabilities of this new type of materials in several advanced structural and temperature-based engineering applications. It was also observed that the elastic modulus of hybrid nanocomposites increases with an increase in the filler contents, but breaking strength of the composite was found to be more sensitive to aggregation between the two corresponding fillers. Elaborate information of the mechanical properties at different filler content of the graphene-reinforced hybrid nanocomposite is enlisted in Table 4.

Table 4 Mechanical properties of graphene- and modified graphene-based hybrid nanocomposites
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Potential applications

Major efforts for the development of polymer nanocomposites are focused on overall properties enhancement of polymer matrices. An optimized loadings of nanofillers in the polymer matrix pool are generally used for structural reinforcing applications, while low loading has been used for improving other properties such as thermal stability, viscoelastic and electrical conductivity.

Automotive applications

The high-speed racing cars require extraordinary properties like structural stiffness, compressive strength, heat resistant, and many more. Moreover, the development of lightweight materials especially for an automotive vehicle is the most highlighted area of today’s research. 10% reduction in weight can improve the fuel efficiency by 6–8%; this improvement certainly makes petrol, electrical and alternative vehicles more competitive. Toyota Motor Co. started the use nylon-6/clay polymer nanocomposites in 1991 when they manufactured timing belt covers as a part of the engine for their newly developed Camry cars [216]. At the same time, Japan developed nylon-6 polymer nanocomposites for engine covers used in Mitsubishi vehicles [217]. Recently, Ford motor company has announced to use graphene parts in their vehicles, starting with F-140 and Mustang by the end of 2018. Ford and their co-partners have exploited graphene for manufacturing the noisy components such as the pumps, fuel rail, chain driven gears or belt driven pulleys to be mounted on front engines [218].

Aerospace applications

Excellent structural strength and conductivity of graphene in polymer composites make them a potential candidate for aerospace applications. Graphene lighter but stronger than CF can be utilized to fabricate a material that can replace steel in the structure of aircraft by which the weight can be reduced and finally improvement in fuel efficiency may be achieved. On the contrary, the use of graphene-reinforced CFRP certainly leads to an increase in performance at lower mass which in turn improves the aircraft efficiency. Cui and their co-workers recently developed a high strength and tough TiAl nanocomposites from CF coated with graphene [219]. They also investigated that above-mentioned nanocomposites have high mechanical strength and low density which could be more helpful to meet out the typical requirements of aerospace industries. Furthermore, graphene-reinforced CFRP improves properties such as ILSS and damage tolerance followed in a decrease in delamination. As of now, the existing aerospace component manufacturing companies are aiming to develop different parts such as aircraft ribs, panels, fuselages, wings, fuel tanks and tail assemblies [220]. On the other hand, companies are more focused in developing the environment friendly nanocomposites so that the aircraft parts can be easily maintained or recycled.

Marine applications

In marine applications, the use of graphene-reinforced polymer nanocomposites protects them against corrosion and also provides an ease of fabrication with lightweight. Graphene-induced CFRP are used to design and fabricate different basic parts of a ship such as spars, hull, till, rudder, keels, masts, and a long upright and parallel (deck) poles. The overall structure, especially the chassis of high-power racing boats, is now made up of hybrid nanocomposites keeping in view of their high performance and safety. Fothergill Composites Inc., Bennington (UK) has modeled and designed a shell-type structure fabricated from nanoparticles reinforced CF with honeycomb aramid structure to save their drivers from any mishap [221].

Wind applications

Nanomaterials-based composites are increasingly used in manufacturing of windmill blades for their high structural strength with less weight properties. Wind turbine blades are the most important part to be exploited with regards to their material. Wind turbine blades are generally manufactured by using CF epoxy matrix composites which offered a significant resistance to fatigue failure occurred due to fatigue (cyclic) loading. However, nanomaterials are allowing for the enhancement of mechanical, fracture and fatigue properties of above-mentioned CF-based composites. The additives such as graphene, CNT, nanosilica for CF composites are under development for turbine blade application [222].

Conclusions and future outlook

The mechanical and thermal properties of graphene- and modified graphene-based nanocomposites have been reviewed thoroughly in the current study. The entire discussion of the properties of graphene has revealed that it plays an important role in the improvement in the overall properties of polymer nanocomposites which can be used in the different areas of applications. Various factors on which the quality of nanocomposites depends such as the purity level, surface area, filler content, cross-linking, degree of dispersion, number of layers, aspect ratio and the interaction between epoxy and fillers have been identified here. This review contains a critical analysis on the research investigations which have been exhaustively performed for assessing the mechanical and thermal properties of graphene and functionalized graphene-based nanocomposites in addition to the properties of hybrid nanocomposites with graphene as a common filler. Henceforth, this detailed study reveals the effect of almost all types of graphene on the thermo-mechanical performance of polymer matrix-based two-phase nanocomposites.

Furthermore, there are numerous challenges to be resolved before industry can go on with the mass production of such nanofiller-based nanocomposites. For example, the production of graphene with maximum purity is still a major issue because such impurities degrade the quality and finally properties of the composite materials. Based on the earlier discussion, the best quality of graphene can be measured in terms of their aspect ratio (X and Y dimension) and proper thickness among the graphene layers. In order to produce these advance composite materials, the designer has to ensure the homogenous dispersion of nanofillers in epoxy matrix, henceforth to minimize the cluster formations along with dislocation (failure) points during the testing procedures. Strong bonding between nanofillers and polymer matrix has also to be maintaining so as to maximize the load transfer. Surface functionalization of graphene may be a viable solution for the increase in interactions between nanofillers and corresponding matrix; however, dimensional difference between these two constituents may be another drawback to scale up the production. Despite these challenges, hybrid nanofillers (graphene and inorganic filler such as CNT) have now become very interesting and promising materials for counterbalancing some of the non-desirable properties of functionalized graphene in the composites. In spite of the different challenges involved with graphene, the manufacturing industries cannot ignore the use of graphene and modified graphene in commercial products such as helicopter rotor blades, fan blades, propellers, racing cars chassis, wheelhouse assemblies, engine covers, battery covers, tennis rackets, hockey sticks, and bicycles. As of now, the business impact of graphene-reinforced nanocomposites is expected to be increased in near future and it is also expected that it transforms from a fundamental material to an advanced engineering material which can offer the best solutions to industry and their consumers.