Investigation of tensile properties, hardness, and morphology of h-BN and MoS₂ filler modified carbon fabric/epoxy composites

Abstract

The present study aims to improve the tensile and hardness properties of carbon fabric–reinforced epoxy composite (CFEC) by using hexagonal boron nitride (h-BN) and molybdenum disulfide (MoS₂) fillers. Magnetic stirring and ultrasonication were used to disperse fillers in epoxy resin. CFEC, h-BN filled CFEC and MoS₂ filled CFEC fabricated as per hand layup and vacuum bag technique. The tensile strength of CFEC was remarkably improved up to 6 wt.% h-BN and MoS₂ addition separately in the resin matrix. The maximum tensile strength of 486 MPa and Young’s modulus of 44 GPa were noticed in 6 wt.% h-BN incorporated CFEC displaying 59% and 47% improvements, respectively. The maximum toughness is shown by 6 wt.% MoS₂ incorporated CFEC indicating 102% improvement. The uniformly dispersed high-performance filler in the epoxy effectively transferred stress, improving tensile properties. However, filler agglomerates, stress-raising spots, and poor filler–matrix interaction beyond 6 wt.% filler reduced the tensile property improvement. The introduction of h-BN and MoS₂ in the composite increased hardness, with 6 wt.% h-BN added CFEC recording the highest hardness denoting 23% greater than the neat CFEC.

Keywords:

• Carbon fabric
• Epoxy resin
• Vacuum-assisted hand layup
• Tensile properties
• Toughness
• Hardness
• Fracture surface morphology

1. Introduction

The applications of carbon fiber–reinforced epoxy composite (CFEC) for load-bearing structural materials increase predominantly in aviation, defense, railway, and automotive segments because of their decent strength-to-weight ratio, stiffness, thermomechanical, and tailorable properties (Buragohain, 2017; Chawla, 2012; Glover, 2004; Newcomb, 2017; Song, 2016). CFEC is also used in boat structural components such as a deck and hull, where weight reduction and corrosion resistance are important (Han et al., 2020). High speed, better payload capacity, and easy maneuverability can be achieved by using CFEC as construction material in a naval ship (Gargano et al., 2017; Tawfik et al., 2017). Composites used in these applications are exposed to a variety of loading conditions in their service life. The tensile strength and toughness are extremely important apart from the thermomechanical properties of composites for better performance of structural components. It is known that carbon fiber–neat epoxy composite has moderate tensile (Baptista et al., 2016) and toughness (crack resisting ability) properties due to brittle epoxy matrix (Frigione & Lettieri, 2020). The hardness of the composite affects its machinability; machining becomes difficult with harder composites. Composites of too high hardness adversely affect the machined surface quality and tool life (Altin Karataş & Gökkaya, 2018; Rao et al., 2019; Sheikh-Ahmad & Davim, 2012). On the contrary, a material with high hardness shows better scratch and abrasion resistance. In general, composites that are harder than the abrasive material exhibit a lower wear rate (better wear resistance). Hence, a composite with excellent tensile properties and moderate hardness is required. The tensile property of epoxy-based composite can be upgraded by adding a solid filler into a matrix while fabricating the composite (Hiremath et al., 2021; Sudarshan, 2021). The epoxy matrix reinforced with optimum filler concentration improves the matrix and fiber–matrix interface properties (Jiang, 2022; Nayak et al., 2021; Praveenkumara et al., 2021; Rao et al., 2021), hence increasing the load-carrying capacity.

Research studies have reported the application of solid-lubricant filler-loaded composite for high-performance (Gantayat et al., 2015; Kavimani et al., 2021; R. Kumar et al., 2018; Ribeiro et al., 2020). The solid-lubricant filler has a lamellar structure that shears easily, improves wear resistance of tribological contact surfaces, reduces friction coefficient and lowers thermal degradation of composite (Ben Difallah et al., 2012; Donnet & Erdemir, 2006; Scharf & Prasad, 2013). Commonly used solid-lubricant fillers are graphite, molybdenum disulfide (MoS₂), and graphite-like hexagonal boron nitride (h-BN). A group of researchers determined the tensile property of solid-lubricant dispersed polymer composite and studied the filler effect on the obtained property. Kumar et al. (R. Kumar et al., 2018) examined the influence of micro graphite flakes (GF) on tensile properties of carbon fabric-phenolic composite (CFPC). The authors reported a tensile modulus of 40 wt.% GF added CFPC improved by 78% whereas, tensile strength of 20 wt.% graphite added CFPC improved by 5%. The authors also mentioned an increased trend of tensile modulus as the GF concentration in phenolic matrix varied from 10 to 40 wt.%. Kavimani et al. (Kavimani et al., 2021) reported eight times improvement in tensile strength when glass fiber/epoxy composite was added with hybrid 1 wt.% graphene oxide (GO) and 2 wt.% h-BN fillers, respectively. The interlocking of fillers improved interfacial adhesion and tensile property. However, the tensile strength is reduced for 1 wt.% GO and 3 wt.% h-BN hybrid combination due to the agglomeration of h-BN. It was also confirmed that the inclusion of hybrid GO and h-BN filler makes the composite rigid and brittle. Yu et al. (Yu et al., 2017) reported a three times reduction in tensile strength due to 44 vol% micro size h-BN platelet addition to epoxy because of h-BN aggregation. Liu et al. (Liu et al., 2016) found 13 and 22% growth in the tensile strength and modulus when 3 wt.% ball milled h-BN dispersed in polyether-ether-ketone (PEEK). The extent of tensile property improvement depends on the size, shape, quantity, and interfacial adhesion of filler with polymer, apart from the chemical compositions (Agunsoye et al., 2019; Frigione & Lettieri, 2020; Ramdani, 2019; Wypych, 2016). Sudheer et al. (Sudheer et al., 2013) reported a decrease in tensile strength, modulus, and percentage elongation by 17, 4, and 30% due to 10 wt.% MoS₂ incorporation in epoxy resin. The stress concentration and microvoid developments in the composite are responsible for the decline in tensile properties. Arshad et al. (Arshad et al., 2020) noticed an improvement in the tensile modulus by 7.4 times and tensile strength by 2 times due to 0.9 wt.% MoS₂ nanosheets addition to polystyrene matrix. The experimental investigation of BN-CFEC and MoS₂-CFEC by Rao et al. (Rao et al., 2021) demonstrated that h-BN and MoS₂ fillers of less than 8 wt.% are more beneficial for improving flexural and interlaminar shear strength of composites. Ulus et al. (Ulus et al., 2015) incorporated 0.5 wt.% multiwalled carbon nanotubes (CNTs) and 0.3 wt.% boron nitride nanoparticles (BNNPs) in the CFEC individually and hybrid combination. As per their study, the tensile strength was increased by 12%, 17%, and 20% whereas, the elastic modulus increased by 6%, 9%, and 10% because of BNNPs, CNTs, and hybrid BNNPs-CNTs addition to the epoxy. The solid-lubricant filler inclusion with epoxy inhibits epoxy chain mobility because of spatial confinement. Hence, the properties (tensile modulus and strength) which are associated with the polymer chain movement vary with filler addition (Rasul et al., 2021). Kumar et al. (K. Kumar et al., 2021) found that adding 4 wt.% zirconium dioxide (ZrO₂) to epoxy increased its tensile strength, modulus and toughness up to 34, 12, and 140%, respectively. The excellent bonding between ZrO₂ and epoxy allows for better stress distribution, which is responsible for augmentation in the property. The decreased tensile property of 8 wt.% ZrO₂-reinforced epoxy composite was caused by agglomeration, which deteriorated interfacial bonding between ZrO₂ and epoxy. According to Bello et al. (Bello et al., 2021), 10 wt.% eggshell nanoparticles (ESPs) loaded with recycled low-density polyethylene improved tensile strength by 68% owing to the effective load distribution from polyethylene to ESP. The more filler quantity not only reduces composite properties but also increases the viscosity of the filler-loaded epoxy, which affects the smooth flow and uniform spread of filler-loaded epoxy (K. Kumar et al., 2021; Sudheer et al., 2013). Consequently, there are more microvoids. Moreover, at high filler concentrations, uniform filler distribution becomes difficult resulting in a weak interface (G. Zhang et al., 2011). The material’s abrasive wear loss is inversely related to the product of stress and strain at failure (UTS×strain)⁻¹ according to Ratner-Lancaster model (Lancaster, 1968; Srinath & Gnanamoorthy, 2006).

The present research aims at an in-depth understanding of tensile strength, modulus, and toughness of the carbon fabric/h-BN/epoxy (hybrid), carbon fabric/MoS₂/epoxy (hybrid), and carbon fabric/epoxy (neat) composites. Thereby, the effect of h-BN and MoS₂ fillers on the aforementioned properties was investigated. Also, the tensile test specimen fracture morphology was analyzed based on scanning electron microscope (SEM) images. To the best of author’s knowledge, research articles documenting the tensile, toughness, and hardness properties considering the above-mentioned combination of fiber and filler with an epoxy matrix are limited. There are limited studies that deal with fillers range from 2 to 8 wt.% at intervals of 2 wt.%. Hence, fillers from 2 to 8 wt.% were chosen in this work.

2. Experimental

2.1. Materials properties

Carbon fabric of twill weave patterns comprising 0 and 90 ° oriented fibers procured from M/S Arrow tech. tex., Mumbai. The carbon fiber bundle consists of 3000 filaments. The yarn has a fineness of 200 tex and a linear density of 5 per 10 mm. The thickness, density, elasticity modulus, and tensile strength of carbon fabric are 0.25 mm, 1.76 g.cm⁻³, 205 GPa, and 5089 MPa, respectively. Diglycidyl ether of bisphenol-A epoxy and amine hardener were supplied by M/S Atul Ltd., India. Epoxy has a density of 1.15 g.cm⁻³ and viscosity of 11,000 mPa.s (at 25 °C). The h-BN and MoS₂ are procured from M/S Sigma-Aldrich Ltd., India. Young’s modulus of h-BN and MoS₂ are 0.93 and 0.36 TPa, respectively (Mukhopadhyay et al., 2017). The density of h-BN and MoS₂ are 2.1 and 5.1 g.cm⁻³ respectively. The h-BN has good thermal conductivity, dimension stability, dielectric property, fire-retardant ability, and anti-oxidation lubricant (Duan et al., 2014; Ma et al., 2019). MoS₂ is a transition metal di-chalcogenide that exhibits noteworthy electronic, mechanical, barrier, and thermal properties (Arshad et al., 2020; Jing et al., 2020).

2.2. Composite preparation

The h-BN and MoS₂ fillers of 2, 4, 6, and 8 wt.% dispersed separately in the weighted quantity of epoxy matrix. The filler was added to epoxy and placed over a magnetic stirrer (rotational speed 500 rpm, temperature 60 °C, time 20 minutes) for uniform mixing. Later, the epoxy–filler solution was ultrasonicated for 30 minutes in a probe sonicator (frequency 20 kHz, amplitude 50%) where the h-BN/MoS₂ structure scission due to the cavitation effect generated by acoustic-wave energy. After stirring and sonication, the filler-epoxy solution was blended with the triethylene-tetraamine cure agent taken 12 wt.% of epoxy, and hand stirred for a few moments. Bottom peel-ply and carbon fabric are placed on mild steel mold, which is applied with a release agent. The fabric is chosen in such a way that warp and weft fibers are balanced to obtain identical in-plane properties. The resulting filler-loaded resin was manually applied with a brush over laid carbon fabric. Each laminate is composed of 10 stacked layers that are covered with peel-ply and porous breather fabric. The entire arrangement was wrapped in a vacuum bag film and connected to a vacuum pump. The vacuum pressure (350 mm of Hg) was applied to force out excess resin and compact the laminate. The composite laminate was then post-cured for 2 hours at 50 °C accompanied by room-temperature curing for one full day. Thus, produced laminates had a thickness ≈ 4 mm. The carbon fabric-neat epoxy was prepared without using filler. The complete details of composite ingredients and the designation of various composites are disclosed in Table 1. Figure 1 depicts a schematic of the composite material preparation method. Figure 2 shows the images of fabrication process for h-BN loaded CFEC. The same procedure is used to prepare MoS₂ loaded CFEC.

Figure 1. Composite preparation depicted schematically.

Figure 2. Images of the fabrication process.

Table 1. Material combination and physical properties of the neat and hybrid composites

Material designation	Epoxy + hardener (wt.%)	Carbon fabric (wt.%)	Filler (wt.%)		Theoretical density (g.cm^−3)	Experiment-al density (g.cm^−3)	Void volume fraction (%)
			MoS2	h-BN
Neat-CFEC	50	50	-	-	1.4034	1.3842	1.3670
2BN-CFEC	48	50	-	2	1.4191	1.4072	0.8320
4BN-CFEC	46	50	-	4	1.4351	1.4166	1.2911
6BN-CFEC	44	50	-	6	1.4515	1.4283	1.5962
8BN-CFEC	42	50	-	8	1.4682	1.4426	1.7450
2MoS2-CFEC	48	50	2	-	1.4304	1.4074	1.6083
4MoS2-CFEC	46	50	4	-	1.4584	1.4262	2.2103
6MoS2-CFEC	44	50	6	-	1.4876	1.4509	2.4651
8MoS2-CFEC	42	50	8	-	1.5179	1.4788	2.5787

2.3. Density and void measurement

The density and void fraction of composites are estimated as per the ASTM D2374 (ASTM International, 2016). The theoretical density and void volume fraction (%) were calculated using equations (1)(1) $\frac{1}{\mathrm{Theoretical}\mathrm{density}}=\frac{{\mathrm{W}}_{\mathrm{CF}}}{\mathrm{Carbon}\mathrm{fiber}\mathrm{density}}+\frac{{\mathrm{W}}_{\mathrm{EP}}}{\mathrm{Epoxy}\mathrm{density}}+\frac{{\mathrm{W}}_{\mathrm{F}}}{\mathrm{Filler}\mathrm{density}}$(1) and (2(2) $\mathrm{Void}\mathrm{volume}\mathrm{fraction}\left(\mathrm{\%}\right)=\frac{\left(\mathrm{Theoretical}\mathrm{density}-\mathrm{Experimental}\mathrm{density}\right)\times 100}{\mathrm{Theoretical}\mathrm{density}}$(2) ) respectively. The experimental density is evaluated by dividing composite weight by volume.

(1) $\frac{1}{\mathrm{Theoretical}\mathrm{density}}=\frac{{\mathrm{W}}_{\mathrm{CF}}}{\mathrm{Carbon}\mathrm{fiber}\mathrm{density}}+\frac{{\mathrm{W}}_{\mathrm{EP}}}{\mathrm{Epoxy}\mathrm{density}}+\frac{{\mathrm{W}}_{\mathrm{F}}}{\mathrm{Filler}\mathrm{density}}$(1)

(2) $\mathrm{Void}\mathrm{volume}\mathrm{fraction}\left(\mathrm{\%}\right)=\frac{\left(\mathrm{Theoretical}\mathrm{density}-\mathrm{Experimental}\mathrm{density}\right)\times 100}{\mathrm{Theoretical}\mathrm{density}}$(2)

Where carbon fiber weight fraction (W_CF) = 0.5; carbon fiber density = 1.8 g.cm⁻³; epoxy weight fraction (W_EP) = 0.5-filler weight fraction; epoxy density = 1.15 g.cm⁻³; filler weight fraction (W_F) = 0.02 to 0.08; h-BN density = 2.1 g.cm⁻³; MoS₂ density = 5.06 g.cm⁻³.

2.4. FESEM morphology characterization

The morphology of fillers was studied through a field emission scanning electron microscope (FESEM) (MIRA-3 LMH, TESCAN). The fillers were gold-sputtered using a sputter coater (HHV, BT300). The energy dispersive spectroscope (EDS) (EDAX corporation, Inc.) is used to analyze elements or chemical composition of BN-CFEC and MoS₂-CFEC.

2.5. Tensile test details

Young’s modulus, tensile strength, and strain of different composites can be determined from the tensile test. These are required for designing composites. The composites’ tensile behavior was studied as per ASTM D3039 (ASTM International, 2017) in a computerized universal testing machine (Zwick/Roell Z100), and displacement was measured with an extensometer of resolution 0.12 µm. The cross-head speed of 2 mm.min⁻¹ was used for the test. The morphology of the tensile fractured specimens was inspected by using the SEM (ZEISS-EVO 18 and JSM-6380, JEOL), fracture surface was gold sputter-coated using coater (JFC-1600, JEOL, and Quorum SC-7620) to avoid sample charging during an inspection.

2.6. Hardness test details

Hardness denotes a material’s resistance to the indenter penetration under the action of a specific force that causes stretching, shear, and compression (Babenko et al., 2012). Knowledge about the hardness of a composite provides an approximate idea of its performance in wear and scratch tests. Harder composite is more resistant to scratches and wear. The bulk hardness of composites is measured using a Rockwell hardness tester on B-scale (Hitech-HIEPL). The hardened steel ball indenter (diameter 1.59 mm (1/16′′)) was used for the Rockwell hardness examination, which ensures even distribution of the load. A minor load of 10-kg force is applied to the specimen through the indenter, which eliminates the effect of backlash and surface asperities on the obtained hardness. Later, a major load of 100-kg force is applied. After 15 seconds, the major load is removed and the hardness number is noted down from the dial indicator. The depth of penetration of the ball indenter decides the hardness. The average hardness value reported was based on five indents performed at different locations on each sample. The indenter impression and the surrounding damage surface of each composite were inspected through an optical microscope (Olympus BX53M).

3. Results and discussion

3.1. FESEM characterization of fillers

The FESEM image of h-BN particles shown in Figure 3(a) depicts disk-like morphology having a narrow thickness below 20 nm. The Gaussian fitting of h-BN particle size distribution histogram given in Figure 3(b) is drawn using Image-J 1.52b software. The size distribution histogram shows that the majority of h-BN particles have diameters ranging from 60 to 120 nm. The FESEM image of MoS₂ shown in Figure 3(c) reveals a thin sheet-like morphology having a narrow thickness below 10 nm. The MoS₂ is irregular in shape and its Gaussian peak fitting size distribution histogram shown in Figure 3(d) reveals size variation from 10 to 1800 nm. The maximum number of MoS₂ particles ranges in size from 200 to 400 nm, with an average size of 314 nm.

Figure 3. FESEM image and size distribution histogram: (a-b) h-BN particles and (c-d) MoS₂ particles.

3.2. Composites density and void fraction

Table 1 shows the density and void percentage of the neat-CFEC and filler dispersed CFECs. It is found that the theoretical and experimental densities of filler-dispersed CFEC are higher compared to neat composite. The density and void percentage of composite raised as the filler content increased. The composite density attained the maximum at a higher filler concentration of 8 wt.%. The density of MoS₂-CFEC is slightly greater than that of BN-CFEC because MoS₂ is denser than h-BN. The void in the composite affects tensile, compressive, shear, flexural, and fatigue strength (Mehdikhani et al., 2019; A. Zhang et al., 2016). The void in filler-dispersed CFEC was found to be affected by filler size distributions and filler concentration. Minimum void content of 0.83% was found in 2BN-CFEC. The addition of nitride filler, like h-BN, to a polymer matrix improves reinforcement-matrix adhesion and minimizes voids in the composite (A. Khan et al., 2022). The largest void content of 2.58% was found in 8MoS₂-CFEC. The smooth flow of the polymer is impaired as the filler quantity increases, resulting in poor carbon fiber wetting by viscous epoxy filler, and hence the voids are increased.

3.3. Tensile properties

The tensile stress-strain behavior of neat and hybrid composites is presented in Figure 4. Table 2 shows the average tensile test result with error variation based on five samples tested in each composition. The percentage change in UTS, Young’s modulus, and toughness of each composite group in comparison to baseline neat composite are presented in Table 2. All the hybrid composites showed better tensile strength and modulus than neat composite because of the superior mechanical properties of the h-BN and MoS₂ fillers. Tensile strength increased up to 6 wt.% filler and decreased after 6 wt.% filler loading in both types of composites. The better stress transfer in h-BN-loaded CFEC is expected due to hydrogen bonding between the nitrogen atoms of h-BN and the hydroxyl group of epoxy (Tsuji et al., 2019). Furthermore, improved interactivity of high-performance h-BN filler with epoxy effectively distributes the load from the epoxy to carbon fiber via filler media. Filler loadings of 2 and 4 wt.% may not be adequate to modify the entire epoxy matrix and interface properties, and therefore not able to witness tensile strength improvement as that of 6 wt.%. The 6BN-CFEC and 6MoS₂-CFEC exhibited the maximum ultimate tensile strength (UTS) of 487 MPa and 456 MPa, respectively, which were 59% and 49% higher than the neat-CFEC. The 6BN-CFEC exhibited higher tensile strength as well as higher Young’s modulus. The reason for an increase in tensile strength is also due to a well-distributed filler, which promotes fair fiber–filler–matrix interfacial adhesion and sustains maximum load (Chaudhry & Mittal, 2014). Tensile strength was reduced in 8BN-CFEC and 8MoS₂-CFEC. In the literature, decreased UTS was reported at higher loading of exfoliated MoS₂ added to polystyrene matrix (M. B. Khan et al., 2017) and h-BN added to sisal fiber/epoxy composite (Agrawal et al., 2020). This decrement in the improvement of tensile property at higher filler loading is due to agglomeration, which is likely caused by the matrix saturation, i.e., the inability of epoxy to bind all filler particles. As a result, during tensile loading, empty micro spaces/voids and cavities are generated. This inference is supported by the literature (Agunsoye et al., 2019). Moreover, the increased number of close particles resulted in stress concentration, which constitutes weak points and leads to composite failure at low load (Bello et al., 2018; K. Kumar et al., 2021). The inadequacy of resin bonding to the filler within the agglomerates causes their de-cohesion under the applied load (Baptista et al., 2016). Agglomerated filler hinders the interchain interactions (crosslinking) during polymer cure, reducing tensile strength (Frigione & Lettieri, 2020). Also, agglomerated filler behaves as a filler with a limited surface area, reducing the effective filler-matrix bonding (Azimpour-Shishevan et al., 2020). Yet, the UTS and Young’s modulus of 8 wt.% filler added CFEC was still higher than the neat-CFEC due to the strength and stiffness contribution of fillers to the composite.

Figure 4. A plot of tensile stress versus strain of different composites.

Table 2. Tensile strength, modulus, strain, and toughness of the neat and hybrid composites

Composite code	UTS (MPa)	Gain in UTS (%)	Young’s modulus (GPa)	Gain in modulus (%)	Strain	Toughness (kJ.m^−3)	Gain in toughness (%)	Average (UTS*strain)^−1 (MPa^−1)
Neat-CFEC	305.97 ± 7.63	-	29.75 ± 0.41	-	0.00968 ± 1.6 × 10^−4	1451.68 ± 63.26	-	0.3376
2BN-CFEC	368.23 ± 4.92	20.35	36.84 ± 0.05	23.83	0.00996 ± 9.5 × 10^−5	1853.86 ± 12.74	27.70	0.2727
4BN-CFEC	447.86 ± 3.51	46.37	40.34 ± 1.75	35.59	0.01076 ± 1.4 × 10^−4	2390.27 ± 6.09	64.66	0.2075
6BN-CFEC	486.53 ± 16.99	59.01	43.76 ± 1.49	47.09	0.01151 ± 7.2 × 10^−6	2799.91 ± 98.21	92.87	0.1786
8BN-CFEC	311.83 ± 0.33	1.92	39.69 ± 0.28	33.41	0.00815 ± 3.6 × 10^−5	1298.24 ± 5.01	−10.57	0.3935
2MoS2-CFEC	386.50 ± 5.98	26.32	30.12 ± 0.28	1.24	0.01196 ± 2.5 × 10^−4	2252.45 ± 36.25	55.16	0.2163
4MoS2-CFEC	415.03 ± 1.15	35.64	38.15 ± 0.35	28.24	0.01118 ± 3.6 × 10^−5	2308.46 ± 15.10	59.02	0.2155
6MoS2-CFEC	455.67 ± 11.65	48.93	37.44 ± 0.03	25.85	0.01261 ± 9.9 × 10^−7	2943.44 ± 15.00	102.76	0.1740
8MoS2-CFEC	328.65 ± 2.39	7.41	35.17 ± 0.41	18.22	0.00953 ± 1.5 × 10^−4	1566.32 ± 32.93	7.89	0.3193

With the addition of filler to CFEC, the change in tensile stress is significantly greater than the change in strain. Therefore, Young’s modulus increases as stress increases up to 6% filler addition to CFEC. The improved Young’s modulus of filler dispersed CFEC compared to neat-CFEC attributed to constrained polymer chain mobility, better stress transfer at the interface, and increased stiffness of composite with filler inclusion in the matrix (Fu et al., 2008; Joy et al., 2020). At all filler concentrations, Young’s modulus of BN-CFEC is higher than MoS₂-CFEC. This difference in the improvement level of modulus is likely due to the difference in modulus (stiffness) between h-BN and MoS₂ fillers. The maximum Young’s modulus of 43.76 GPa was witnessed for 6BN-CFEC, indicating a 47% improvement. In MoS₂-CFEC, the highest Young’s modulus of 38.15 GPa was noticed for 4MoS₂-CFEC. A decrease in Young’s modulus detected as the filler concentration exceeds 6 wt.% indicates poor interface adhesion of filler with matrix and large agglomerates of filler particles (Wypych, 2016). Layer-wise filler slip under the action of tensile force also reduces composite Young’s modulus.

The tensile stress-strain response demonstrates that 6 wt.% filler-loaded CFEC fails at a slightly larger strain. The increased strain (elongation at break) in both filler added composites illustrates that filler domains may cause cavitation and polymer chains to yield during mechanical deformation (Joy et al., 2020). The tensile failure strain of 8BN-CFEC and 8MoS₂-CFEC although reduced but not significantly. This behavior is consistent with the literature (Shivamurthy et al., 2013), which reports that a higher concentration of graphite filler addition to glass fabric/epoxy composite results in lower tensile strain. The lower composite failure strain indicates poor filler–epoxy interface adhesion, increased stress concentration, and discontinuities in the epoxy (Joy et al., 2020; Sudheer et al., 2013).

The obtained trend of Young’s modulus, strength, and strain agree with the observed behavior of GO nanoflakes-filled carbon fabric/epoxy (Adak et al., 2018), nano h-BN-filled epoxy (Joy et al., 2020), and exfoliated h-BN added PEEK (Liu et al., 2016). Furthermore, the maximum tensile property attained at filler wt.% differed from that described in the literature, possibly due to differences in filler size, filler morphology, and processing conditions adopted.

The toughness signifies the amount of energy necessary for the complete fracture of a tensile specimen due to imposed stress (Chatterjee et al., 2011). It is estimated by integrating the absolute area under the stress–strain curve up to the fracture point in “Origin pro®” software. The key factors that contribute to the toughness of polymer nanocomposite include crack deflection, matrix deformation, fiber/filler debonding, fiber/filler pull-outs, and fiber/filler fracture (Adak et al., 2018; G. Zhang et al., 2011). 6MoS₂-CFEC exhibited the maximum toughness of 2943 kJ.m⁻³ indicating 102% higher compared to the neat-CFEC. The combined increase of failure stress and strain in 6MoS₂-CFEC causes improvement in the toughness (energy absorption capability). The 6BN-CFEC also improved the toughness by 93%. It is observed that the 6BN-CFEC is stiffer but less tough compared to the 6MoS₂-CFEC. The uniformly distributed high modulus fillers in epoxy efficiently transfer load and improve toughness. Previous research studies demonstrated that the h-BN nanofiller increases the toughness of polyethylene (Chaurasia et al., 2019) and epoxy (song et al., 2019). In another study, the authors claimed that adding MoS₂ to polystyrene is capable of improving toughness (Arshad et al., 2020). As the filler concentration surpasses 6 wt.%, clusters of filler aggregate formation likely reduce the toughness of composite (Chaurasia et al., 2019).

The reciprocal of the product of average UTS and strain was evaluated. It was found that the (UTS×strain)⁻¹ decreased up to 6 wt.% filler dispersed CFEC and then increased at 8 wt.% filler content. The larger (UTS×strain)⁻¹ factor is caused by a sudden decrease in strain, thus the composite exhibits brittle behavior. The graph presented in Figure 5 shows the average variation of UTS, strain, modulus, and toughness among BN-CFEC and MoS₂-CFEC at different filler concentrations. The change in crosslink density due to an immobilized network of epoxy around filler is the possible reason for the mobility change of epoxy molecules, which alters the stiffness (modulus), Tg, and toughness of composite (Sue et al., 2000; Wetzel et al., 2006).

Figure 5. UTS, strain, modulus, toughness trends in BN-CFEC and MoS₂-CFEC at different filler concentrations.

Figure 6(a-i) shows the photographs of tensile test specimens indicating the failure zone. All composite samples failed within the gage region. Neat-CFEC experiences edge delamination and angled failure apart from fiber breakage indicating brittle failure. The 2BN-CFEC, 8BN-CFEC, 4MoS₂-CFEC, and 8MoS₂-CFEC specimens undergo lateral cross-section failure. The 4BN-CFEC experienced lateral gage failure and fiber breakage. An angled failure mode was evident in 6BN-CFEC. 2MoS₂-CFEC exhibited fiber splitting and fiber pull-out. The 6MoS₂-CFEC specimen display damage at two different locations apart from damage near the grip.

Figure 6. Failure photograph of tensile tested composite samples: (a) neat-CFEC; (b) 2BN-CFEC; (c) 4BN-CFEC; (d) 6BN-CFEC; (e) 8BN-CFEC; (f) 2MoS₂-CFEC; (g) 4MoS₂-CFEC; (h) 6MoS₂-CFEC; (i) 8MoS₂-CFEC.

3.4. SEM image analysis of tensile specimens

The fracture types of composite in the tensile test were studied through SEM characterization. The fracture micrograph of neat-CFEC is presented in Figure 7(a-e). The transverse carbon fiber breakage is more severe than longitudinal fibers (Figure 7(a-b)) because the fibers oriented 90 ° to the loading direction break earlier than the loading direction fibers. In addition, the strength of fibers follows a statistical distribution as few fibers will endure slightly low-stress levels and break early compared to the others (Mallick, 2007). Figure 7(b) displays evidence of resin cusp formation visible as raised platelets on the fiber edges resulting from the brittle failure. In the literature, the presence of resin cusp in carbon/epoxy laminates has also been mentioned (Souza et al., 2019). When the applied tensile load exceeds the interface bond strength, the carbon fibers debond from the surrounding epoxy via a crack running at the fiber–matrix interface, as seen in Figure 7(c-d; Mallick, 2007). This denotes the incapability of the initiated matrix crack to propagate across the carbon fibers because of the weak interface in neat CFEC (Agarwal et al., 2015). Therefore, neat-CFEC has low toughness. Figure 7(c) shows deep holes developed because of fiber pull-out. A circled region in Figure 7(e) indicates the surface of bare carbon fibers, which reveals an adhesive failure with fiber pull-outs. A similar observation was reported in the literature (Jain et al., 2021).

Figure 7. (a-e) Scanning electron micrographs of tensile tested neat-CFEC.

The SEM micrograph of 4BN-CFEC taken at the fracture location is presented in Figure 8(a-f). Serration and matrix fracture (Figure 8(a)) and splintered carbon fibers (Figure 8(b)) are visible at a few locations on the fracture surface. The void formed (Figure 8(c)) due to carbon fiber pull-out. The river-like pattern reveals that 4BN-CFEC inhibits crack propagation and requires more deformation energy to fracture. Michalcová & Kadlec (Michalcová & Kadlec, 2016) made a similar observation for multiwalled CNTs added to CFEC. Figure 8(d) depicts the zone of intimate contact between carbon fibers and h-BN filled epoxy, accompanied by intact carbon fibers. However, 4BN-CFEC also exhibits transverse carbon fiber debonding (Figure 8(e)).

Figure 8. (a-f) Scanning electron micrographs of tensile tested 4BN-CFEC.

The fracture micrograph of 6BN-CFEC is presented in Figure 9(a-f). Figure 9(a) reveals the matrix microcrack and fiber imprint (step-like feature) on the matrix. Figure 9(b) shows projected carbon fibers. Figure 9(c) shows matrix flaking and carbon fiber–hBN filler–epoxy intimate contact zone (good bonding). The cross-sectional morphology view of 6BN-CFEC from Figure 9(d) reveals a carbon fiber/h-BN/epoxy interface with less fiber–matrix debonding. A similar finding was also reported for graphene nanoplatelets (GNPs) distributed CFEC (Jain et al., 2021). The path of debonded carbon fibers and microcrack propagation is confirmed in Figure 9(e). The stiff h-BN distributed epoxy provides resistance against crack propagation by deviating the crack flow and absorbing fracture energy. In the literature, identical observations have been made for h-BN filled epoxy (Ulus et al., 2014). Carbon fibers circumferentially surrounded by h-BN dispersed epoxy and crack branching marks are seen in Figure 9(f).

Figure 9. (a-f) Scanning electron micrographs of tensile tested 6BN-CFEC.

The fracture micrograph of 8BN-CFEC is shown in Figure 10(a-e). Good carbon fiber/h-BN/epoxy bonding is seen in Figure 10(a) at a few locations. Nevertheless, tiny pores/crazes (Figure 10(a)) are produced due to increased stress during tension/pull. The h-BN filler protruded and was present at the fracture surface as in Figure 10(b); this was perhaps caused by the addition of more h-BN concentration to epoxy, which reduced its ability to interact with epoxy. Material dislodges due to crack propagation and carbon fiber pull-out are found in the fracture micrograph of 8BN-CFEC (Figure 10(c)). The transverse carbon fiber debonding is evident in Figure 10(d). Hackle formation (Figure 10(e)) occurs when the microcrack growth in a composite is prevented.

Figure 10. (a-e) Scanning electron micrographs of tensile tested 8BN-CFEC.

The SEM image of a tensile tested 4MoS₂-CFEC is shown in Figure 11(a-d). The carbon fiber–dominated failure (Figure 11(a)) was witnessed instead of matrix failure in 4MoS₂-CFEC due to MoS₂ strengthened epoxy. Shallow cusp, matrix feature presents on the carbon fiber edges (Figure 11(b)). Cusp developed in the composite due to localized stress (Suresha et al., 2007). Epoxy resin shards cling to carbon fiber (Figure 11(c)) indicating good fiber–matrix adhesion. Figure 11(d) shows a cross-section view of the fracture region and demonstrates MoS₂ strengthened epoxy impeding crack growth. The tensile test fracture morphology of 6MoS₂-CFEC is shown in Figure 11(e-g). Splintered carbon fibers (Figure 11(e)) and fibers debonding (Figure 11(f)) are witnessed in 6MoS₂-CFEC. In 4MoS₂-CFEC (Figure 11(c)) and 6MoS₂-CFEC (Figure 11(e)), fillers are present as a coating on the carbon fiber surface. Therefore, a greater pull-out force is necessary to dislodge fiber from the matrix, thus exhibiting larger resistance to tensile loading. Nayak et al. made a similar observation for coconut shell powder modified glass/epoxy composite (Nayak et al., 2022). Carbon fibers circumferentially wetted by MoS₂ filled epoxy seen in Figure 11(g) illustrate intimate contact of the fiber–filler–matrix. Therefore, 6MoS₂-CFEC has better tensile strength and toughness. A similar inference was also reported for GNPs added CFEC (Jain et al., 2021).

Figure 11. Scanning electron micrographs of tensile tested (a-d) 4MoS₂-CFEC and (e-g) 6MoS₂-CFEC.

The tensile test fracture surface of 8MoS₂-CFEC is shown in Figure 12(a-f). As presented in Figure 12(a), intralayer carbon fiber splaying (separation) occurs under tensile load, and it may be due to an abrupt loading effect. Carbon fiber splintering (Figure 12(b-c)) and broom-like fiber bundle distortion (Figure 12(b)) are noticed in the fracture morphology of 8MoS₂-CFEC. Interface debonding (Figure 12(d)) occurred might be due to weak carbon fiber–MoS₂–epoxy interface bonding as a result of MoS₂ agglomeration. The cross-section of fiber fracture and severe matrix rupture caused by overloading (8 wt.%) MoS₂ filler (Figure 12(e)). MoS₂-rich resin in between fibers is noticed in Figure 12(f).

Figure 12. (a-f) Scanning electron micrographs of tensile tested 8MoS₂-CFEC.

3.5. Microstructure of filler–epoxy samples

The uniformly dispersed filler in the matrix causes better filler–matrix interaction and can upgrade the resultant properties of the matrix by improving stress transfer from matrix to reinforcement (Nayak et al., 2021). From the prepared filler–epoxy composite sample without fiber, the microstructure of filler and its distribution in an epoxy matrix are investigated using SEM. Figures 13(a-f) represent SEM micrographs of MoS₂ and h-BN dispersed in epoxy resin at lower and higher filler concentrations. The region of well-impregnated MoS₂ filler by the epoxy resin is witnessed in 2 wt.% MoS₂-epoxy (Figure 13(a-b)), and it guarantees enhanced performance of the composite. MoS₂ dispersed throughout the epoxy resin at 8 wt.% filler concentration (Figure 13(c-d)). The circled region with yellow color in Figure 13(d) depicts a zone of evenly spaced MoS₂ particles. However, MoS₂ agglomeration is visible at some locations in the micrograph of 8 wt.% MoS₂-epoxy, as indicated by the red color circled region (Figure 13(c-d)) on account of filler particle colonies (Zhou et al., 2014). The site of agglomerated MoS₂ becomes a stress concentration point and results in poor filler–epoxy interaction, which lowers the composite tensile property (Azimpour-Shishevan et al., 2020). Also, microvoid formation is witnessed at higher MoS₂ concentration as shown by the orange color circled region in Figure 13(c). The SEM micrograph of 2 wt.% h-BN-filled epoxy in Figure 13(e) reveals widely spaced h-BN particles at a few spots with some areas exclusively being epoxy resin. However, as the h-BN quantity increased to 8 wt.%, clusters/agglomeration of h-BN filler (Figure 13(f)) lessened the composite’s load-carrying capacity because the effective interaction surface area of h-BN with epoxy was reduced. A similar observation has been reported for BN nanosheet dispersed polymers (Rasul et al., 2021).

Figure 13. SEM images: (a-b) 2 wt.% MoS₂-epoxy, (c-d) 8 wt.% MoS₂-epoxy, (e) 2 wt.% h-BN-epoxy and (f) 8 wt.% h-BN-epoxy composites.

3.6. Energy dispersive spectroscope analysis

The scanned zones for EDS analysis, along with a graph representing intensity (counts per second) versus energy (keV) are shown in Figures 14(a; BN-CFEC) and 14(b)(MoS₂-CFEC). The identified elements in weight% are shown in Figure 14. In BN-CFEC, carbon (C) is the main element witnessed in addition to oxygen (O), boron (B), and nitrogen (N) elements. Boron and nitrogen elements confirm the presence of h-BN particles. The EDS graph of MoS₂-CFEC confirms the existence of molybdenum (Mo) and sulfur (S) related to MoS₂ particles apart from carbon (C) and oxygen (O).

Figure 14. EDS graph with scanned area: (a) BN-CFEC and (b) MoS₂-CFEC.

3.7. Hardness

The Rockwell hardness number (RHN) variation with filler concentrations is displayed in Figure 15. The addition of stiff h-BN and MoS₂ in CFEC increased the RHN of the composites at all the tested filler concentrations. The RHN gradually increases with h-BN and MoS₂ content up to 6 wt.% then starts a decreasing trend for further filler addition in the composite. The increased hardness of filler added composite indicates that filler contributes its hardness as well as stiffness to the matrix. In addition, good bonding between filler-loaded matrix and fibers is responsible for improved composite hardness (Li et al., 2013). The greatest RHN of 88 (23%↑) was attained for 6BN-CFEC, indicating larger resistance exhibited by the composite to indenter penetration because of improved laminate interface bonding and remarkable stiffness as well as the hardness of h-BN. Thus, it produces a smaller depth of impression. This finding is supported by the literature (Liu et al., 2016), which reported a 34% increase in the ball indentation hardness of 5 wt.% h-BN added PEEK. RHN of 6MoS₂-CFEC improved from 72 (neat-CFEC) to 87. MoS₂ inclusion in polystyrene improved Brinell hardness by 20% as it exhibits resistance to deformation (M. B. Khan et al., 2017). In another study, it was reported that 10 wt.% MoS₂ addition to neat-epoxy and glass/epoxy improved Rockwell hardness by 3.9 (RHN 80) and 19.5% (RHN 92), respectively (Sudheer et al., 2013). In the present work, the hardness of CFEC added with 8 wt.% filler slightly decreased due to micro-voids and the possibility of agglomeration in the composite. The reduced Rockwell hardness was reported at increased boron nitride nanotube concentration loaded to polyurethane (Li et al., 2013). Furthermore, the hardness is location-dependent due to the random distribution of fillers in CFEC, composite anisotropy, and inhomogeneity.

Figure 15. Hardness versus filler concentration.

The optical microscope images of Rockwell hardness-tested composites surface are shown in Figure 16. No evidence of surface cracks visible from optical microscope images reveals composites have reasonably good bulk hardness. The image analysis confirms the difference in depth of indentation and damaged area (boundary) around the vicinity of indentation among various composites. The damaged area is computed through Image-J software, and the outcome is revealed in Figure 16. The minimum damaged area of 1.85 and 2.96 mm² were noticed in 6MoS₂-CFEC and 6BN-CFEC, respectively, due to a higher resistance exhibited by the aforesaid composites surface for indenter movement. The larger damaged areas of 3.95 and 3.74 mm² were detected for 8MoS₂-CFEC and neat-CFEC samples, respectively.

Figure 16. Optical microscope images of Rockwell hardness tested surfaces (a) neat-CFEC; (b) BN-CFEC and (c) MoS₂-CFEC.

4. Conclusions

• The tensile strength and Young’s modulus attained the maximum for 6BN-CFEC, exhibiting 59% and 47% enhancement, respectively. The increased stiffness, constrained epoxy chain mobility, and better degree of stress transfer due to filler inclusion in the CFEC increased Young’s modulus. The fracture morphology of 6BN-CFEC shows the intimate contact of carbon fiber/h-BN/epoxy in addition to carbon fiber imprint and debonded carbon fiber.

• 6MoS₂-CFEC has the highest toughness. The zone of intact carbon fibers and crack growth obstruction due to MoS₂ strengthened epoxy indicates improved energy absorption of 6MoS₂-CFEC.

• The tensile strength and toughness of 8 wt.% filler added CFEC not significantly improved due to agglomeration, which induces stress concentration and deteriorates the filler–matrix bonding. Therefore, the load-carrying capacity of this composite was limited.

• When compared to the neat CFEC, the Rockwell hardness of 6BN-CFEC and 6MoS₂-CFEC improved by 23% and 21%, respectively. The hardness improvement was due to a better fiber/matrix/filler interface and remarkable stiffness of the filler, which exhibits resistance to indenter penetration.

• The research work could be extended in the future to study the effect of h-BN and MoS₂ fillers on machinability, impact strength, residual stress, and tribological properties of CFEC. The tensile properties and toughness of CFEC blended with hybrid h-BN and MoS₂ fillers can be explored in future works.

Acknowledgements

The authors are grateful to the Manipal Academy of Higher Education for providing the infrastructural facility to perform research.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors received no direct funding for this research.

Notes on contributors

Yermal Shriraj Rao

Mr Yermal Shriraj Rao is currently pursuing a Ph.D. in the Department of Mechanical and Industrial Engineering at the Manipal Institute of Technology, MAHE, India. He holds a B.E. in Mechanical Engineering and M.Tech. in Machine Design. His research area of interest includes the characterization of polymer nanocomposites and machining studies of composites.

Basavannadevaru Shivamurthy

Dr Basavannadevaru Shivamurthy, Associate Professor (Senior Scale) in the Department of Mechanical and Industrial Engineering at the Manipal Institute of Technology, MAHE, India, has received a Ph.D. from the National Institute of Technology Karnataka (NITK) Surathkal in the field of Materials Engineering. He has more than 10 years of industrial experience in the polymer processing and research. For the last 21 years, he has been involved in teaching and research. Also, he has been an active researcher in polymer nanocomposites, tribology, ballistic materials, and EMI shielding materials. He has published several research articles in national and international journals of repute.

Nanjangud Subbarao Mohan

Dr Nanjangud Subbarao Mohan, Professor in the Department of Mechanical and Industrial Engineering at Manipal Institute of Technology, MAHE Manipal, India, received his Ph.D. from the Manipal Institute of Technology in the field of machining studies on glass fiber composites. He has 35 years of teaching and research experience. His research interest includes composite machining, hygrothermal aging studies on natural fiber composite, and polymer nanocomposites. He has published several research articles in national and international journals of repute.

Nagaraja Shetty

Dr Nagaraja Shetty, Associate Professor in the Department of Mechanical and Industrial Engineering at Manipal Institute of Technology, MAHE Manipal, India, received his Ph.D. in the machining of carbon fiber–epoxy composite from the National Institute of Technology Karnataka (NITK) Surathkal. He has 8 years of experience in teaching and research. His areas of interest include the machining study of polymer composites, design, and optimization. He has published several research articles in national and international journals of repute.

Sanjay Mavinkere Rangappa

Dr Sanjay Mavinkere Rangappa is currently working as a Senior Research Scientist/Associate Professor and also Advisor within the office of the President for University Promotion and Development towards International goals at King Mongkut’s University of Technology North Bangkok (KMUTNB), Thailand. He received a Post Doctorate from KMUTNB. His current research areas include natural fiber composites, polymer composites, and advanced material technology.

Suchart Siengchin

Prof. Dr.-Ing. habil. Suchart Siengchin is the President of King Mongkut’s University of Technology North Bangkok (KMUTNB), Thailand. He received his Dipl.-Ing. in Mechanical Engineering from University of Applied Sciences Giessen/Friedberg, Hessen, Germany, M.Sc. in Polymer Technology from University of Applied Sciences Aalen, Baden-Wuerttemberg, Germany, M.Sc. in Material Science at the Erlangen-Nürnberg University, Bayern, Germany, Doctor of Philosophy in Engineering (Dr.-Ing.) from Institute for Composite Materials, University of Kaiserslautern, Rheinland-Pfalz, Germany and Postdoctoral Research from School of Materials Engineering, Purdue University, USA. In 2016 he completed the Habilitation (Dr.-Ing. habil.) in Mechanical Engineering from Chemnitz University of Technology, Saxony, Germany.