Influence of TiO₂ nanoparticle modification on the mechanical properties of basalt-reinforced epoxy composites

Abstract

This research work aims to experimentally evaluate the influence of resin modification using Titanium dioxide (TiO₂) nanoparticles on the mechanical properties viz. flexural, tensile and Inter Laminar Shear Strength (ILSS) of basalt reinforced epoxy composites based on ASTM standards. The laminates were fabricated using a combination of hand lay-up and compression moulding techniques. Five different weight proportions of TiO₂ nanoparticles were considered ranging from 1% to 5% with an increment of 1% by weight. To assess the quality of fabrication, void fractions were evaluated and were found to be in the range of 1.17% to 3.98%. The mechanical properties of TiO₂ modified epoxy basalt composites were compared with composites without any TiO₂ nanoparticles in it. The results indicated a significant improvement in the mechanical properties due to the addition of TiO₂ nanoparticles. When compared to samples without TiO_2, the highest mechanical properties were observed in samples having 4% TiO₂ nanoparticles wherein an increase of around 60%, 40% and 70% was seen in flexural (526 MPa), tensile (420 MPa) and ILSS (30.6 MPa), respectively. A dip was observed in all the properties with further increase in nanoparticle content. Scanning electron microscopy (SEM) was carried out to analyse the fractured surface for dispersion of the nanoparticles and the failure mechanisms. SEM micrographs confirmed uniform dispersion of the nanoparticles while the major failure mechanisms observed were brittle fracture and fiber-matrix debonding. Results suggest that such composites can be used as a material in engineering applications wherein the loading is light to moderate.

Keywords:

• basalt fibers
• titanium dioxide
• resin modification
• epoxy
• mechanical properties

1. Introduction

The current age of material science has evolved towards sustainability and that has led to the development of materials with combined properties from multiple base materials (Verma, Negi, et al., 2019). Composites combine the desired properties of multiple constituent materials and allow for tailored properties in the final product. Composite materials can be classified as polymer matrix, metal matrix or ceramic matrix composites, based on the nature of the matrix material (Shehab et al., 2023). Polymer matrix composites are easier to manufacture, widely available and easier to alter the properties with the technology of the polymer industry (Hiremath et al., 2021). They can further be classified based on the kind of reinforcement used as—fibre-reinforced, particulate-reinforced, and flake-reinforced. Fibre-reinforced polymer composites (FRPCs) are the most widely used composites today (Kumar et al., 2019). The flexibility to modify the matrix and fibre gives a variety of combinations to choose from for different applications. The fibre materials used can be synthetic or natural materials, giving synthetic FRPCs, like glass, carbon fibre, etc., and natural FRPCs (Begum et al., 2020).

Natural fibre reinforced polymer composites are the step towards sustainable composite materials, and when paired with biopolymers can be completely biodegradable as well (Rastogi et al., 2020; Verma & Singh, 2018; Weyhrich et al., 2023; Yadav & Singh, 2022). They may be derived from plant-based sources, like jute, hemp, flax, cotton, etc., animal remains, like wool, bone, and silk, or from mineral sources, like basalt, asbestos, and ceramics (Gholampour & Ozbakkaloglu, 2020). Basalt is an igneous rock that possesses comparable properties to synthetic reinforcement materials, while being natural and more eco-friendly (Afolabi et al., 2020; F. Wang et al., 2023). Basalt fibres are manufactured from the Junker’s process (Tamás-Bényei & Sántha, 2023) and are easily available, with applications in construction, automobile brakes, housing insulation, and the petrochemical industry (Chowdhury et al., 2022). Basalt fibres have better thermal properties than glass fibre (Xing et al., 2023) comparable strength characteristics to E-glass, and a higher chemical resistance than most synthetic fibres (Rova et al., 2023). Also there are published research that indicate that basalt fibers have good wettability with the various polymer resins and their inclusion in the polymer composite result in the improvement in the mechanical properties of the composite (Jain et al., 2019; Singh et al., 2019).

Nanofillers have been used with basalt fibre reinforced polymer composites in an effort to improve the mechanical performance of the composite. Various kinds of nanofillers have been experimented with for this purpose—Carbon Nanotubes (CNTs), Zinc oxide (ZnO), Titanium dioxide (TiO₂), Nanoclay, Titanium carbide (TiC), Silicon carbide (SiC), graphene, etc. (Mohit et al., 2021; Othman & Hassan, 2023; Sharma et al., 2022).

Titanium dioxide or Titania, (TiO₂), is a white oxide ceramic and comes in three crystalline forms: rutile with a tetragonal structure, anatase also with a tetragonal structure, and brookite with an orthorhombic structure (Eddy et al., 2023). It is used extensively for white pigmentation as Titanium White in inks, paints, and plastics (T. Wang et al., 2022) and in the photovoltaic industry and sensors (Nunzi & De Angelis, 2022). As a nanofiller, TiO₂ is non-toxic, has photocatalytic properties, good optical and electronic properties, and is biocompatible (Brillas & Garcia-Segura, 2023; Tripathy & Biswas, 2022b).

The effect of TiO₂ as a nanofiller has been studied and its ability to enhance the properties of composites is well established (Hiremath et al., 2023). The outcome of incorporation of TiO₂ in neat epoxy resin systems has been studied extensively. Further studies, while varying the percentage of TiO₂ in the nanocomposite, have also been carried with composition ranges varying from 0 to 10 vol% and 0 to 10 wt% (Ilhamdi et al., 2022; Karthick et al., 2022; Li et al., 2022; Vidakis et al., 2022). The change in the tensile strength, flexural strength, and fracture toughness were concentrated on, with improvements observed with the addition of TiO₂ up to a certain critical concentration, after which there was a deterioration in the properties. Titanium dioxide as a nanofiller has also resulted in enhancement of properties in other polymer matrix systems (Cazan et al., 2021) like acrylic resins (Hasanen & S, 2020) polypropylene (Vidakis et al., 2021) polyethylene (Zapata-Tello et al., 2020) etc.

The effect of addition of TiO₂ on fibre epoxy composite materials has also been investigated to examine the change in properties with the introduction of both the nanofiller and a fibre reinforcement (Lokesh Yadhav et al., 2020; Natrayan et al., 2022; Prasad et al., 2021; Thipperudrappa et al., 2021). These studies were carried out on synthetic, natural, and hybrid fibre epoxy systems. Again, there was an improvement seen in the mechanical properties (tensile, flexural, and impact strengths), and a decrease in the moisture diffusion.

Basalt fibre epoxy composite materials (BFECs) have also been modified using nanofillers to strengthen the composite system (H et al., 2023; Joshi et al., 2022; Kishore et al., 2021; Vinay et al., 2022; Zheng et al., 2021). TiO₂ has also been experimented as a coupling agent with basalt fibres (Dou et al., 2023). The presence of TiO₂ increases the corrosive resistance and the mechanical properties of the fibres. The addition of TiO₂ nanoparticles in Basalt-polysiloxane composites has been studied with composition varying from 0 wt% to 3 wt% (Mishra et al., 2012). The optimum composition was found for 1.5 wt% TiO₂ in the fabricated composite.

The present study aims to prepare a polymer environment friendly composite material having superior properties. Epoxy resin as the matrix material is selected as it is one of the most widely used thermoset resin for the preparation of polymer composites in various engineering applications (Verma, Budiyal, et al., 2019). From published literature, it is found that TiO₂ nanoparticles can be used for surface treatment and/or as coupling agents for the basalt fibres. Also, to the best of the authors’ knowledge, TiO₂ as nanoparticles with basalt fibres as reinforcement in epoxy matrix has not been explored. Thus, the present work focuses on the use of natural basalt fibres along with nature friendly TiO₂ nanoparticles (Roy, 2022) to prepare the composite laminates. The TiO₂ nanoparticles are used to modify the epoxy resin and the effect of such modification on the mechanical properties of the composite laminate is studied.

2. Materials and methodology

2.1. Materials

For fabrication of composites, plain woven basalt fabric with density as 2.6 g/cc and areal density as 300 gsm was employed. As matrix material, epoxy-Bisphenol A-epichlorohydrin (MY 740) and cyclo-aliphatic amine (HY 951) (room temperature cure system) were used. The resin is a transparent liquid with a density of 1.15–1.20 g/cc and has a viscosity in the range of 10,000–14,500 mPa.s at 250 °C. The viscosity of the corresponding liquid hardener is reported to be in the range of 50–80 mPa s and has a specific gravity is 1.59. Titanium dioxide (TiO₂) with a size in the range of 30–50 nm having a density of 4.26 g/cc was obtained from Merck, India, and is used as the nanofiller in this work.

2.2. Fabrication of composite laminates

The basalt fibre-reinforced epoxy with TiO₂ nanoparticles composite laminates (BET) were manufactured using hand lay-up and compression moulding techniques. The laminates were fabricated by varying the weight content of TiO₂ nanoparticles in the matrix. Table 1 explains the designation and composition of the six fabricated composite panels of dimensions 270 mm $\times$140 mm $\times$ 4 mm. A control composite panel with epoxy and 0 wt% TiO₂ nanoparticles was fabricated, while the other five were fabricated with epoxy modified weight content of nanoparticles varied with 1, 2, 3, 4, and 5 wt%. This particular range of wt% addition of TiO₂ nanofiller is selected based on the literature review, which suggested that beyond this range, there is a greater chance of agglomeration and proper dispersion of particles in the composite may not take place, resulting in poorer performance and unsuitable results (Abd El-Baky et al., 2022; Bulut et al., 2020; Joshi et al., 2022; Saravanan et al., 2020).

Table 1. Composition of the fabricated composites

Composite Laminate	Basalt Fibre (wt%)	Epoxy (wt%)	TiO2 (wt%)
BET0	50	50	0
BET1	50	49	1
BET2	50	48	2
BET3	50	47	3
BET4	50	46	4
BET5	50	45	5

The fabrication of composites was performed using 19 layers of basalt fibre, with a fabric size of 270 mm x 140 mm rectangles, to form a 4-mm-thick panel. These layers were weighed and an equal quantity of epoxy resin was prepared. Due to the high surface energy from nanoparticles, it was important to first create proper separation and ensure that subsequent dispersion of the particles in the resin was uniform (Shen et al., 2020). Agglomeration of nanoparticles in the resin would result in improper dispersion in the resin and the fabricated panels would generate inaccurate results. To achieve this, a measured amount of TiO₂ nanoparticles were first added to acetone in a 1:10 ratio and mixed at room temperature with a magnetic stirrer.

The addition of acetone facilitates the reduction in surface energy owing to the fact that it is an organic solvent, allowing for separation of the particles. The viscosity of the resin also reduces with the addition of acetone (Zhang et al., 2012) and this further allows for a uniform modified resin to be obtained. The acetone-nanoparticle mixture was then sonicated before the required amount of epoxy was added to it. The stirring at room temperature and sonication was repeated. To remove the added acetone, the mixture was heated at 70 °C and magnetically stirred till all the acetone was evaporated and only epoxy and TiO₂ particles remained. The epoxy was allowed to cool before it was used for fabrication. This step was followed by hand layup of the composite laminates. A corresponding amount of hardener in the ratio of 10:1 (resin-to-hardener) was added to the cooled resin and stirred for a minute with a wooden spatula/stick. The wet laid-up panels were cured under pressure in a compression moulding machine with 4 mm spacers (Nayak et al., 2020). An epoxy and basalt fibre panel was also fabricated as a comparison to gauge the effect of the modification of TiO₂. The cured panels were left to harden further for a week and were labelled as per the designation in Table 1. They were also cleaned for cutting and testing. The fabrication process of the BET composites is shown in Figure 1.

Figure 1. Fabrication of BET composite laminates.

2.3. Testing of mechanical properties of composite laminates

The composite laminates were subjected to tests to evaluate the mechanical properties as per ASTM standards (ASTM D2344/D2344M–22, 2022; ASTM D3039/D3039M–17, 2017; ASTM D7264/D7264M − 21, 2021). Samples were cut from the fabricated panels using abrasive water jet machining (AWJM) to find the tensile strength, flexural strength and interlaminar shear strength (ILSS) of each fabricated panel. Five test specimens for each failure mode were tested at room temperature. Specimen sizes according to the following standards and cross-head speeds are given in Table 2. The specimens were then cleaned and marked for testing. Tensile testing was carried out on a Shimadzu make universal testing machine of 100 kN capacity, while flexural testing and ILSS testing were carried out on a ZWICK ROELL Z020 with a 20kN capacity. A pre-load of 1N was applied on all the specimens to ensure proper positioning of the specimen during each test. Flexural strength was found using a three-point bending setup with a standard span-to-thickness ratio of 16:1, while the ILSS was found using a short beam shear test setup with a span-to-thickness ratio of 4:1. The void test is conducted as per ASTM D2734–16 (ASTM, 2016). Agarwal and Broutman equation (Eq. 1(1) ${\rho }_{\mathit{Ct}\mathit{\hspace{0.17em}}}=\frac{1}{\left(\frac{w_f}{{\rho }_f}\right)+\left(\frac{w_m}{{\rho }_m}\right)+\left(\frac{{\mathit{w}}_{\mathit{np}}}{{\rho }_{\mathit{np}}}\right)}$(1) ) is used to determine the theoretical density of the laminate $\left({\rho }_{\mathit{ct}}\right)$.

Table 2. Specimen dimensions and test parameters

Mechanical Test	Standard	Specimen Dimension (l $\times$ b)	Cross-head speed
Tensile Strength	ASTM D3039	250 mm x 25mm	2 mm/min
Flexural Strength	ASTM D7264	80 mm x 13mm	1 mm/min
ILSS	ASTM D2344	25 mm x 8mm	1 mm/min

(1) ${\rho }_{\mathit{Ct}\mathit{\hspace{0.17em}}}=\frac{1}{\left(\frac{w_f}{{\rho }_f}\right)+\left(\frac{w_m}{{\rho }_m}\right)+\left(\frac{{\mathit{w}}_{\mathit{np}}}{{\rho }_{\mathit{np}}}\right)}$(1)

Here $\mathit{w}$ and $\mathit{\rho }$ denote the mass fraction and density, while the suffixes$\mathit{f}$, $\mathit{m}$ and $\mathit{np}$ are used to depict the fibre, matrix and nanofillers, respectively. A digital density balance (Contech CAS-234) is used to assess the experimental density of the laminates$\left({\rho }_{\mathit{ce}}\right)$. The void fraction is determined using Eq. 2(2) $\mathit{Void}\mathit{fraction}=\mathit{\hspace{0.17em}}\left(\frac{{\mathit{\rho }}_{\mathit{Ct}\mathit{\hspace{0.17em}}}-{\rho }_{\mathit{Ce}\mathit{\hspace{0.17em}}}}{{\rho }_{\mathit{Ct}\mathit{\hspace{0.17em}}}}\right)\times 100\mathit{\hspace{0.17em}}$(2) .

(2) $\mathit{Void}\mathit{fraction}=\mathit{\hspace{0.17em}}\left(\frac{{\mathit{\rho }}_{\mathit{Ct}\mathit{\hspace{0.17em}}}-{\rho }_{\mathit{Ce}\mathit{\hspace{0.17em}}}}{{\rho }_{\mathit{Ct}\mathit{\hspace{0.17em}}}}\right)\times 100\mathit{\hspace{0.17em}}$(2)

2.4. Scanning electron microscopy (SEM) analysis of composite laminates

Analysis of the failure mechanism was carried out through SEM images of the fractured specimens. SEM images of the fabricated composites were obtained using a special edition ZEISS EVO18 microscope. The variable pressure mode was used to operate the microscope at 20 kV while the morphology was observed at varied magnifications ranging from 500× to 3000 × .

3. Results and discussion

3.1. Density and void fraction analysis of the laminates

It can be seen from Figure 2 that there is variation in the experimental densities of the BET laminates which is caused by the presence of voids, matrix pores and/or air sacs within the laminate (D.-C. Kim et al., 2023). Such defects result in the reduction of experimental density of the laminates in comparison to the theoretical density. The theoretical density of the BET laminates increases as the TiO₂ nanofiller loading quantity varies within the laminate and has a linear relationship. As the proportion of TiO₂ increases in each laminate from BET1 to BET5, there is an obvious increment in the density of the laminate. It is observed from Figure 2 that the variation in the theoretical and experimental density is comparatively less for BET2, BET3 and BET4 laminates. This can be seen as an indication of relatively lesser voids the absence of voids, air sacks and other defects mainly due to uniform dispersion of the TiO₂ nanofillers within the resin that also creates uniform interfaces across the volume of the laminate.

Figure 2. Density of BET composite laminates.

Figure 3 indicates the computed void fraction for the BET laminates. The void fraction considering all the laminates lies in the range of 1.17% to 3.98% which for the fabrication process followed, is acceptable (DiLandro et al., 2017; Priya & Rai, 2005). It can be seen from Figure 3 that BET0 laminate which has no TiO₂ filler content is having a void percent of 3.57%. This can be attributed to the inclusion of air sacks that are generated within the laminate by the entrapment of air during the preparation of the laminates (Tretiak et al., 2023). There can be generation of voids within the laminate as the solvents (alcohol, amine and ether groups) vaporize during the exothermic curing reaction (Fu & Yao, 2022). The decrement in percent voids within the laminates due to the inclusion of 1, 2, 3 and 4 wt.% TiO₂ is due to the reason of strong attachment of TiO₂ nanofillers to the polymer chain that in turn gets into any air sacks that are generated during the curing and/or preparation of the laminates. The void fraction slightly increases at 3 and 4 wt.% TiO₂ addition which can be attributed to the generation of micro-voids at the interfacial regions that are created when the aspect ratio of the nanofillers increases due to cluster formation or agglomeration. Among the TiO₂ filled BET laminates, the BET5 laminate has the highest void fraction of 3.98% as the TiO₂ nanofillers agglomerate and tend to form clusters that have aspect ratio in micrometers. Simultaneously, there is reduction in the resin quantity which causes the viscosity of the resin to increase (Tripathy & Biswas, 2022a). This makes the resin hard to flow during the hand layup process that in turn result in the creation of resin-rich and resin-deficient areas within the composite laminate. This creates unevenly distributed voids within the laminate.

Figure 3. Void percent in BET composite laminates.

3.2. Behaviour of the laminate under bending loads

Flexural strength and modulus values for the BET composite laminates are shown in Figure 4. The hybridization obtained through the inclusion of TiO₂ nanofillers in the basalt-reinforced epoxy composites has resulted in a significant improvement of the flexural strength as well as modulus of the hybridized BET laminates in comparison to the BET laminates with 0 wt.% of TiO₂ nanofiller. Also, it is clear from Figure 4 that the wt.% loading of the nanofiller also plays a key role in the final strength and modulus values of the BET laminates. The strength and modulus of the TiO₂ filled BET laminates subjected to bending forces, increases with increase in addition to the TiO₂ nanofiller up to 4 wt.%. The flexural strength of the nanofiller filled BET laminates increases by approximately 19%, 34%, 50% and 60% respectively for BET1, BET2, BET3 and BET4 laminates in comparison to the flexural strength value of the unfilled BTE0 laminate. Also, the flexural modulus for BET1, BET2, BET3 and BET4 laminates improves roughly by 10%, 53%, 54% and 59%, respectively, in comparison to the flexural modulus of BET0 laminate. This increasing trend in the strength and modulus values of the TiO₂ filled BET laminates registers a halt with 5 wt.% inclusion of the nanofiller. The inclusion of hard ceramic TiO₂ nanofiller into the resin results in the formation of a rough surface topography which in turn improves the adhesion of the fibers with the resin (Thipperudrappa et al., 2021). The formation of such rough surface topography due to the addition of TiO₂ nanofiller into epoxy is reported by Thipperudrappa et al., where the authors studied the surface topography of TiO₂/glass fibre/epoxy composites through the atomic force microscope (Thipperudrappa, Kini, et al., 2019). Another study by Hiremath et al. reports the increment in surface roughness with the increase in the ZnO nanofiller content within the epoxy matrix (Hiremath et al., 2022). The most common failure mechanism in a composite laminate subjected to external bending forces is the delamination (Carraro et al., 2019). When the fibers tend to strongly adhere to the matrix, the successive laminate layers experience an effective and efficient transfer of the applied loads from matrix to fibers via the hard TiO₂ nanofiller interface. Thus, the delamination failure is considerably reduced.

Figure 4. Flexural strength and modulus of BET laminates.

Also, it is important to ensure that all the fiber surfaces get uniformly coated with the TiO₂ filled epoxy resin so that the strong interface is uniformly produced and the material behaves almost as an isotropic material. Achieving this condition is challenging when nanofillers are involved. Due to their increased surface area, the TiO₂ nanoparticles experience greater Vander Walls force of attraction and tend to form agglomerated clusters. Such clusters tend to get haphazardly distributed within the resin and the fibers become devoid of uniform coating of the TiO₂ filler. This results in the creation of weak interfaces that eventually hamper the ability of the laminate to sustain the applied loads. Thus, there is a reduction in flexural strength and modulus of the BET5 laminate.

3.3. Behaviour of the laminate under tensile loads

The behaviour of the BET laminates under tension is experimentally evaluated and the results are shown in Figure 5. The tensile strength of all the TiO₂ filled BET laminates improves in comparison with the tensile strength of the unfilled BET0 laminate. The tensile strength of BET1, BET2, BET3, BET4 and BET5 laminates increases by about 5%, 19%, 28%, 40% and 30% respectively when compared with the tensile strength of BET0 laminate. For a fiber reinforced laminate subjected to external loading, intially cracks are created due to matrix splitting. The cracks then propagate till they reach the interface region that is generated between the matrix and the fiber. The cracks, upon reaching the interface are either halted, deflected or grow continuously until the laminate fails due to fiber pull-out or due to the fracture of the fiber (Khandelwal & Rhee, 2020). In a uniformly filled fiber reinforced laminate, the cracks that get generated in the matrix generally tend to encounter the fillers. These fillers absorb the energy and lead to the creation of several microcracks. Such microcracks further grow and then finally encounter the fibers. The creation of such microcracks allows the material to absorb more energy as reported by Thipperudrappa et al., wherein the authors report that the presence of uniformly dispersed ZnO nanoparticles within the epoxy matrix resulted in the creation of microcracks which eventually improved the mechanical properties of the polymer composite laminate (Thipperudrappa, Ullal Kini, et al., 2019). Thus, generally, the filled fiber composite laminates have higher tensile strength in comparison to the unfilled laminates. The percentage increase in the tensile strength value of the filled laminates over the unfilled laminates is dependent upon factors such as uniform dispersion of the filler within the matrix and uniform coating of the filled resin over the fiber surface that will generate a strong interface. In TiO₂ filled BET composites, the improvement in the tensile strength indicates that there is an uniform dispersion of the TiO₂ nanofiller within the resin. Due to this, the TiO₂ nanoparticle gets strongly bonded to the polymer chain of the resin. The resin modification thus provides additional fortification to the otherwise brittle polymer chain. Also, TiO₂ nanoparticles, being ceramic in nature, tend to have rough surfaces. The increase in the surface roughness results in the increase in the contact angle which in turn improves the surface wettability. Such improvement in surface wettability through the increase in contact angle is reported by Kim et al. (Y. Kim et al., 2022). These rough surfaces tend to latch onto the fiber surface resulting in the improvement of the wettability of the resin with the fibers. These conditions favour the generation of strong interfaces through which several toughening mechanisms such as crack pinning, crack bridging and crack deflection can be induced to improve the strength of the laminate (Hiremath et al., 2023). The drop in tensile strength of BET5 laminate in comparison with the strength of BET4 laminate indicates that weaker interfaces are getting generated and there is a lack of additional toughening mechanisms. This is due to the nature of the nanoparticles that tend to agglomerate at higher wt.% loadings. These clusters adhere non-uniformly onto the polymer chain rendering the resin unfortified. When such resin is coated onto the fibers, instead of acting as fortifying elements, they tend to act as localised stress raisers. Thus, the tensile strength of the laminate reduces.

Figure 5. Tensile strength of BET laminates.

Elongation at break is another important parameter which can be used to analyse the behaviour of the BET laminates under tensile loading. Under tension, the fibers are getting pulled longitudinally and the common mode of failure is either by fiber pull-out or fiber rupture. Elongation at break represents the fracture strain and can be used to access the reinforcing fiber’s ability to resist deformation and/or rupture when it is subjected to tensile loads. Figure 6 shows the elongation at break for BET laminates. The elongation at break is increasing with the increasing wt.% addition of the TiO₂ nanofillers indicating that fiber bundles are rigidly held by the fortified resin. Under such circumstances, the fibers do not get pulled-out easily, thereby allowing the material to resist fracture. The elongation at break for BET5 laminate drops slightly in comparison to the BET4 laminate. This indicates insufficient wetting of the fortified resin to the fibers. Hence, the fibers in BET5 laminate can be pulled out easily. Also, at 5 wt.% loading of the TiO₂ nanofiller, the resin is not sufficiently fortified due to agglomerated clusters of TiO₂ nanofiller. This condition enables easy splitting of the matrix and the crack propagates without any hindrance to encounter the fibers. Under such situation, the fibers rupture and the laminate fails.

Figure 6. Elongation at break of BET laminates.

3.4. Behaviour of the laminate under shear forces

Interlaminar shear strength (ILSS) value determines the ability of the successive laminate panels to adhere to each other when subjected to parallel shearing forces. Thus, ILSS is the clear measure of the strength of the adherent interface. Figure 7 shows the ILSS of BET laminates. The ILSS value increases with increment in wt.% addition of TiO₂ nanofiller. The ILSS for BET1, BET2, BET3, BET4 and BET5 improve by 27%, 46%, 52%, 70% and 14% respectively in comparison to the BET0 laminate which does not have the TiO₂ nanofiller. When the laminates are subjected to parallel shear forces, the laminates should resist delamination and the successive laminate layers should adhere to each other strongly. The inclusion of TiO₂ nanofiller into the polymer chain creates sufficient roughness which help the fibres to lock together when compressed between the fortified resin layers. This condition helps the laminate to resist delamination by inducing fibre bridging mechanism when the laminate plies are sheared.

Figure 7. ILSS of BET laminates.

The roughness of the polymer chain produced by the inclusion of TiO₂ nanofillers also generates enough friction between the laminate layers so that the plies adhere to each other when subjected to shearing force. Xu et al. reported the generation of strong interlocking mechanisms between the fibers and the resin through the inclusion of nanofillers (Xu et al., 2022). However, we can see a sudden drop in the ILSS value for BET5 laminate in comparison to BET4 laminate. This suggests that the roughness of the polymer chain is not sufficient to generate a strong locking mechanism between the plies. At higher wt.% loading of the nanofiller, there is a strong tendency of the nanofiller to agglomerate. Such agglomerates tend to bind loosely with the polymer chain which renders the polymer chain with uneven roughness. Under such circumstances, the fibres cannot get wet appropriately. Also, the laminate will be composed of non-homogeneous surface texture which will be smooth in areas where the polymer chain is bare, and will be rough where there are clusters of TiO₂ that are loosely bonded with the polymer chain. When BET5 laminate is subjected to shearing forces, the area in the lamina that is smooth and is not having sufficient roughness tends to split easily thereby setting in delamination of the plies.

3.5. Fractography using SEM

Fractographic analysis is carried out to analyse the underlying fracture mechanism that has resulted in the failure of the laminate at various micro and nanoscales (Mahato et al., 2019). Figure 8 shows the SEM images of fractured BET0, BET1, BET2 and BET3 laminates. Figure 8(a) shows the fractured surface characterized by the presence of clean fiber surface indicating that no TiO₂ nanofiller is included. Also, due to the absence of TiO₂ nanofiller, there is no fortification of the resin. This has resulted in a neat crack propagation without any deflection. The lack of induced toughening mechanism in BET0 laminate has resulted in debonding of fibers, which is visualized as fiber imprints on the matrix. The surface of BET1 laminate shown in Figure 8(b) is characterized by evenly distributed TiO₂ nanofillers that adhere strongly to the polymer chain. This has rendered the surface rough, and this roughness is beneficial for achieving thorough interlocking of the resin and the fibers. Such strong interlocking mechanism induces strong interfaces between the fiber and the matrix as seen in Figure 8(c,d).

Figure 8. SEM images of BET0, BET1, BET2 and BET3 laminates.

Figure 9 shows the SEM images of the fractured BET4 and BET5 laminates. Figure 9(a) illustrates the fiber bridging mechanism that allows BET4 laminate to sustain the applied loads effectively. Figure 9(b) shows the fractured surface of BET4 laminate which is characterized by the fibers that are strongly bonded with the resin. This enables the BET4 laminate to transfer the stresses from the matrix to the fibers through the strong interfaces generated between the TiO₂ fortified resin and the basalt fibers. Figure 9(c) shows the fractured surface of the BET5 laminate which is composed of smooth fiber surface with no coating of the resin. This indicates that at higher wt.% loading, there is uneven distribution of the TiO₂ nanofiller within the resin. Hence, the fibers are not sufficiently coated with the rough resin fortified by TiO₂. Th uneven distribution of TiO₂ nanofillers in BET5 laminate is also evident by the presence of smooth matrix surface as shown in Figure 9(c). These factors have resulted in the induction of failure mechanism such as fiber debonding, interfacial debonding and brittle fracture as shown in Figure 9(c-d).

Figure 9. SEM images of BET4 and BET5 laminate.

4. Conclusion

Composite laminates were fabricated with plain woven basalt fabric with epoxy modified with TiO₂ nanoparticles in varying weight proportions. Void fractions of all the composites were found to be within the acceptable limits where they varied from 1.17% to 3.98%. To ascertain the influence of resin modification over the mechanical properties, composite laminate without any TiO₂ was also fabricated and their mechanical properties were compared. Results showed significant increase in all the mechanical properties namely, flexural, tensile and ILSS as a result of resin modification. The highest strength was seen in composites having 4% TiO₂ nanoparticles wherein the flexural strength was observed to be 526 MPa, tensile strength to be 420 MPa while the ILSS was 30.6 MPa. With increase in content of the nanoparticles, the strength decreased to 471 MPa, 389 MPa and 20.5 MPa in flexural, tensile and ILSS respectively but was still higher than the samples without nanoparticles. The elongation at break and flexural modulus also saw increasing trend similar to other mechanical properties. Good dispersion of nanoparticles was visible through SEM micrographs while the major failure modes that were observed were crack propagation through the composite, fiber matrix debonding and brittle fracture.

Acknowledgments

The authors would like to thank Manipal Academy of Higher Education, Manipal for the financial support through Seed Funding for UG/PG students’ research with seed money ID 00000796. The authors also wish to acknowledge the support rendered by Advanced Composite Material Testing Laboratory during fabrication and MIT Workshops in preparation of samples using Abrasive Water Jet Machine.

Disclosure statement

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

Additional information

Funding

The work was supported by the Manipal Academy of Higher Education [Seed Money ID 00000796].