R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... 141–148 ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES OF ALUMINA-MWCNT HYBRID NANOFILLER REINFORCED EPOXY COMPOSITES IZBOLJŠANJE MEHANSKIH, TOPLOTNIH IN ELEKTRI^NIH LASTNOSTI EPOKSIDNEGA HIBRIDA OJA^ANEGA Z NANO POLNILOM NA OSNOVI ALUMINIJEVEGA OKSIDA IN VE^ STENSKIH OGLJIKOVIH NANO CEVK R. Ramkumar 1 , C. Sabarinathan 2* 1 Department of Mechanical Engineering, St Xaviers Polytechnic College, Sivgangai, Tamil Nadu – 630302, India 2 Professor, Hindusthan College of Engineering and Technology, Coimbatore, Tamil Nadu – 641032, India Prejem rokopisa – received: 2022-11-08; sprejem za objavo – accepted for publication: 2023-02-03 doi:10.17222/mit.2022.684 In this work, alumina and multi-wall carbon nanotube (MWCNT) hybrid nanofiller reinforcing pure epoxy at varying weight fractions of (0.1, 0.2, 0.3, 0.4 and 0.5) w/% is investigated to enhance the mechanical, electrical and thermal properties. The po- rosity, tensile strength, electrical and thermal conductivity of epoxy hybrid nanocomposites are studied after the effects of the alumina-MWCNT hybrid nanofillers. The interfacial adhesion and mechanical interlocking between the hybrid nanofillers and epoxy are greatly increased with the addition of alumina and MWCNTs, thus leading to an improvement in the mechanical properties. Additionally, a uniform distribution of hybrid nanofillers results in a larger increase in the thermal and electrical con- ductivity. The presence of voids in specimens is gradually decreased when the nanofiller content is increased up to 0.3 w/%. The alumina-MWCNT reinforcement significantly improves the tensile strength, by 88 %, compared with pure epoxy. Similarly, the electrical and thermal conductivity increase by 85 % and 64 %, respectively, when compared with low weight fractions of the hybrid nanofiller. Agglomeration during the fabrication of nanocomposites is manageable but it is inevitable. During the forma- tion of chains and networks, the alumina-MWCNT reinforcement of pure epoxy greatly influences the thermal conductivity. This strategy provides a prospective new concept for the use of epoxy and its composites in structural and thermal engineering applications. Keywords: hybrid nanofillers, MWCNTs, alumina, mechanical properties Opisana je raziskava vpliva oja~itve ~iste epoksidne smole z delci aluminijevega oksida in nano delci iz ve~ stenskih ogljikovih nano cev~ic (MWCNTs; angl.: multi wall carbon nanotubes) na mehanske, elektri~ne in termi~ne lastnosti. Izdelali so hibridne polimerne nano kompozite z razli~nimi masnimi dele`i hibridnega nano polnila iz aluminijevega oksida in MWCNT (0,1, 0,2, 0,3, 0,4 in 0,5) w/%. Nato so dolo~ili poroznost, natezno trdnost, elektri~no in toplotno prevodnost epoksidnih hibridnih nano kompozitov ter dolo~ili vpliv koli~ine dodanega nano polnila na te latnosti. Medmejna adhezija in mehanska povezava med epoksijem in nano polnilom se je mo~no pove~ala z njegovim dodatkom, kar je precej izbolj{alo mehanske lastnosti izdelanih nano kompozitov. Dodatno sta se zaradi enakomerne porazdelitve delcev hibridnega nano polnila po prostornini epoksidne smole pove~ali tudi toplotna in elektri~na prevodnost kompozitov. Prisotnost praznin oziroma por v preizku{ancih se je postopno zmanj{evala s pove~evanjem vsebnosti nano polnila do 0,3 w/%. S hibridnim polnilom na osnovi Al-oksida in MWCNT oja~anim nano kompozitom se je v primerjavi s ~istim epoksijem mo~no pove~ala natezna trdnost (do 88 %). Podobno sta se izbolj{ali tudi elektri~na in toplotna prevodnost kompozitov za do 85 % oziroma 64 % v primerjavi z nizkim dele`em nano polnila. Aglomeracija med izdelavo kompozitov je neizogibna vendar jo je mo`no obvladati. Na toplotno prevodnost kompozitov najbolj vpliva tvorba verig in zamre`enje polnila s ~istim epoksijem. S predstavljeno strategijo izdelave nano polimernih kompozitov se ponuja nov koncept uporabe epoksija in njegovih kompozitov za strukturne in termi~ne in`enirske aplikacije. Klju~ne besede: hibridna nano polnila, ve~stenske ogljikove nano cevke, aluminijev dioksid, mehanske lastnosti 1 INTRODUCTION Polymer matrices and nanofillers with dimensions in the nanometre range have shown potential for various engineering applications. The high aspect ratio and sur- face-to-volume ratio of nanosized fillers lead to im- proved material properties and structural stability, as well as unique functions. The use of nanoscale graphite and ceramic by-products such as carbon nanotubes and alumina as fillers further enhances the advantages of polymer nanocomposites. Individual properties of a ma- terial are based on its constituents and the physical prop- erty can be improved with the reinforcement of fillers acting as composites so that it can be widely used in di- verse applications. 1–3 Polymer composites are drawing a lot of attention due to their easy fabrication and cost ef- fectiveness compared to other composites. 4,5 Integration of nanofillers into a regular polymer matrix resulted in polymer nanocomposites, significantly employed in vari- ous engineering applications including aerospace, marine and automotive industries, construction, sports and en- Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 141 UDK 678.686:666.762.11 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(2)141(2023) *Corresponding author's e-mail: c.sabarinathan@gmail.com (C. Sabarinathan) ergy storage. 6 Nanosized fillers exhibit superior interface bonding with a polymer matrix, thus improving the structural stability and material property due to their high aspect and surface-to-volume ratios. 7,8 Pure epoxy exhib- its predominant properties when nanosized particles such as boron nitride, carbon nanotubes, silicon dioxide, alu- mina nitride, zinc oxide, alumina, graphite and hallosite nanotube (HNTC) are added as reinforcements. 9 There are many more engineering applications where alumina is introduced as a matrix element and such composites are widely employed due to their high strength, microstructural stability and elevated thermal resistance. 10 Alumina reinforcement of polymers pro- duces better thermal, mechanical and electrical proper- ties. With ceramic reinforcement, a higher wear resis- tance, exceptional hardness and good corrosion resistance can be achieved at a low cost. 11 In order to get particular customized and improved properties of a poly- mer, alumina-MWCNT reinforced epoxy hybrid polymer nanocomposites are used. A significant enhancement in the composite properties is achieved using hybrid nanofillers as the reinforcement, which have been com- pared with pure epoxy. The term hybrid composite indi- cates the use of more than one material as the reinforce- ment or particulates in multiple physical forms as a single reinforcement. The homogeneous mixture and in- terlock bonding architecture of the reinforcement in a polymer matrix can be employed to significantly en- hance the physical properties of the host matrix with synergistic strengthening effects. As compared to singu- lar or mono-reinforced nanocomposites, hybrid nano- composites exhibit improved properties. For the development of nanocomposites, MWCNTs are very attractive nanofillers due to their significant properties like thermal, electrical and noteworthy physi- cal properties. 12,13 Alumina has significant structural properties, providing unique physical, thermal and me- chanical properties when used as the reinforcement. Ep- oxy-based hybrid nanocomposites with ceramic and car- bonaceous nanofillers like alumina and multi-wall carbon nanotubes possess the properties of pure epoxy as well as the properties of the nanofillers. To improve the properties of a hybrid nanocomposite, a significant inter- facial bonding between the hybrid nanofillers and the polymer material is required. In epoxy-based hybrid nanocomposites, the hybrid nanofillers including carbon nanotubes and alumina are the best suitable reinforce- ments of pure epoxy. In the manufacturing of nano- composites, carbon nanotubes are widely used as nanofillers. The effect of the van der Waals attraction force in carbon nanotubes leads to the agglomeration of nanofillers. 14 Alumina has a 3D morphology and high as- pect and surface-to-volume ratios, which help it to inte- grate with a neat matrix and improve its properties. Due to the excellent mechanical properties, alumina is used as the strengthening reinforcement element in composites. Significant physical and mechanical properties and chemical stability of carbon nanotubes and alumina al- low the development of three-dimensional nanostructure building blocks. 15,16 Multi-layer cylindrical graphene in the form of a tube structure with a diameter of less than 2 nm is called multi-wall carbon nanotubes. The modu- lus of elasticity and tensile strength of multi-wall carbon nanotubes in the axial direction can be up to 1 TPa and 63 GPa, respectively. Multi-wall carbon nanotubes pos- sess a large aspect ratio of about 1000:1. MWCNTs ex- hibit a thermal conductivity of 3500 N/mK at atmo- spheric temperature and the electrical resistivity is in the order of 10 –4 at room temperature. Furthermore, MWCNTs exhibit significant elasticity of up to 20 %. It is expected that an MWCNT-alumina hybrid provides an enhancement of various properties. In order to address the foremost property enhancement of a polymer, enor- mous investigations need to be carried out by researchers to achieve the mechanical properties required for ad- vanced structural applications without a loss in the toughness. However, the mechanical properties of ther- mosetting polymers such as the modulus of elasticity and yield stress are reduced during the addition of thermo- plastic polymers. Furthermore, enormous investigations of the rein- forcement of low- as well as high-aspect-ratio nano- particles in epoxy resins were carried out to improve the potential properties such as wear resistance, electrical re- sistivity and mechanical characteristics of polymers. 17 Carbon-nanotube and silver-particle reinforced epoxy became a good electrically conducting polymer. The ep- oxy reinforced with black carbon exhibits heat normal- ization during elevated-temperature applications. In the field of lightweight-structure applications, namely auto- motive, medical-instrument, aerospace, marine indus- tries, etc., nanofiller-reinforced polymer composites oc- cupy a significant space and also have a high strength-to-weight ratio. However, the use of fibres with nanofillers, macro- and nanosized organic and inorganic fillers, reinforcing epoxy has some advantages and re- strictions. But in recent days, the use of oxide nanofillers for reinforcing epoxy resins has gained significant atten- tion due to the chemical stability, high modulus value, cross-linking and aspect ratio. The use of carbon and ox- ide nanofillers (a hybridization filler) for reinforcing ep- oxy resin is gaining more attention for the development of a multifunctional structural composite material with enhanced mechanical characteristics. Three types of oxide nanoparticle hybridized in car- bon nanotubes, namely titanium oxide, silicon zirconium oxide and alumina oxide, are used to improve the me- chanical properties. 18 Homogeneous dispersions of hy- brid nanoparticles improve the properties of epoxy com- posites. A qualitative homogeneous dispersion of the nanoparticles in pure epoxy governs the toughening properties of the epoxy. To attain a homogeneous disper- sion of nanoparticles in the matrix, various dispersion methods are employed, namely mechanical stirring, solu- R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... 142 Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 tion mixing, shear mixing, melt mixing and ultrasoni- cation. These methods have been utilized to achieve dis- persion in the epoxy matrix. Initially, the reason for the change in mechanical properties is determined by ana- lysing the internal structures of nanocomposite speci- mens, such as filler alignment, filler distribution and in- terface between the filler and matrix. We place a significant emphasis on assessing the impact of internal voids or porosity on the interface area between the filler and matrix, while assuming no change in the interface adhesion. This investigation focusses on various weight fractions of MWCNT-alumina hybrid nanoparticles rein- forcing pure epoxy and investigates the mechanical prop- erties as well as electrical and thermal conductivity of the epoxy hybrid nanocomposite; a fractured surface morphological analysis is also presented. 2 EXPERIMENTAL PART 2.1 Epoxy as the polymer matrix In the current research work commonly used polymer epoxy resin is the matrix for the manufacturing of a structural material. The use of the epoxy matrix results in a dual-element system. The first element is epoxy resin and the second component is the curing agent or hard- ener. A multidimensional cross-linked network can be developed in epoxy resin when a solidification process is carried out using the curing agent. Cross-linked bisphenol A diglycidyl ether (BADGE) epoxy resin is frequently used for widely spread structural applications. Curing agents are broadly classified into carboxylic acid, anhydrides, amides and amines. Based on the required properties and applications, the curing agent is chosen. In room-temperature solidification processes amines are used generally. 2.2 Synthesis of alumina-MWCNT hybrid nanofillers Nanosized multi-wall carbon nanotubes were pur- chased from BT Corp. Pvt. Ltd., India. The MWCNT av- erage outer diameter was about 25–35 nm, the length was 15 μm and the specific surface area was 180–200 m 2 ·g –1 . The preparation of alumina-MWCNT hybrid nanofillers was accomplished based on the litera- ture review with a considerable modification based on hybrid applications. Auqa regia solution was used to clean all the glass ware used for this hybrid nanofiller preparation. The MWCNTs were first purified by dis- solving 2.5 g of them in 60 mL of concentrated nitric acid (75 w/%) at 90 °C for 18 h, filtering and washing them in ultrapure water, and then dissolving them in 50 mL of HF (40 w/%) at 70 °C for 24 h. To stabilize the turbid liquid pH value in a range of 7.0, ultrapure water was used for socking and drying, carried out at 100 °C for 12 h. Under 100 min –1 stirring condition, the purified MWCNTs were refluxed at 140 °C for 2 h with nitric acid (75 w/%) and sulphuric acid (80 w/%). The functionalized substance was filtered and socked in high-purity water to stabilize the pH value of 7.5 and kept in the oven for 24 h for drying. The required quan- tity of functionalized MWCNTs was dispersed in high-purity water and agitated with a magnetic shaker for6htoattain desirable dispersion. Afterwards, 7.8 g of Al(NO 3 ) 3 ·9H 2 O were appropriately dissolved in high-purity water. The Al(NO 3 ) 3 solution was dropwise added into dispersed functionalized MWCNTs. During the addition of successive drops, sufficient time allowed for the alumina to properly dissolve and occupy the functionalized MWCNT surfaces. After that, the suspen- sion was dried at 105 °C. The resulting synthesized CNT-alumina was kept in the oven at 130 °C overnight for the drying process. A scanning electron microscope was used to identify the CNT-alumina hybrid nano- powder surface, and XRD patterns (Figure 1) were taken at room temperature with 2 ranging from 0° to 90°, a step rate of 0.05° s –1 , an operating target voltage of 30 kV and a tube current of 100 mA. The prepared hy- brid nanocomposites were sealed in glass containers for subsequent testing. R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 143 Figure 1: SEM image and XRD pattern of CNT-alumina nanocomposites 2.3 Epoxy-alumina-MWCNT hybrid nanocomposite sample preparation The required quantity (ml) of epoxy was placed into a conical beaker and weighed with an electronic balance machine to calculate the weight of the epoxy. The epoxy was heated up to 60 °C and kept in a sonicator. The pro- cess frequency was set to 30 kHz. The synthesized hy- brid nanofillers were gradually added into pure epoxy, using different weight fractions of (0.1, 0.2, 0.3, 0.4 and 0.5) w/%. The pure epoxy and various weight fractions of hybrid nanofillers were labelled and presented in Ta- ble 1. The sonication process was continued for 30 min with a power of about 450–500 W and a frequency range of 100–150 KHz to ensure a homogeneous dispersion of hybrid nanofillers in the host matrix. The epoxy hybrid nanocomposite suspension was kept in a vacuum cham- ber for 2 h for evacuating the vacuum bubbles. Araldite HY951 was added in a volume ratio of 10:1. The epoxy hybrid nanocomposite solution was poured into an acrylic sample mould to make a test specimen and for each composition, five specimens were fabricated with- out changing the manufacturing method or conditions. 2.4 Porosity measurement The porosity clusters of the nanocomposites were computed using the visual inspection method. The epoxy alumina-MWCNTs hybrid nanocomposite specimens were cut into 16-mm diameter discs using a die punching machine as shown in Figure 2a. The discs were weighted with a 0.001 accuracy weighing machine to calculate the mass and thickness and they were measured using a micrometer, assuming that there was no porosity in the nanocomposite discs. For alumina-MWCNT nanocomposites with alumina volume fraction a and MWCNT volume fraction c the theoretical M t was cal- culated as M t =( a alumina + c mwcnt +( 1–( a + c ) epoxy ))v t where alumina , mwcnt and epoxy refer to the density of alumina, MWCNTs and epoxy, respectively, and v t is the volume of the disc. The porosity p is ex- pressed as p M M =− 1 m t (1) R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... 144 Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 Figure 2: Test samples for: a) porosity measurement, b) tensile test, c) electrical conductivity measurement and d) thermal conductivity measure- ment Table 1: Composites of pure epoxy reinforced with various weight fractions of alumina-MWCNT hybrid nanofiller Composition: Pure epoxy Label: PE Pure epoxy + 0.1 w/% of alumina MWCNT hybrid nanofillers PEHNF1 Pure epoxy + 0.2 w/% of alumina MWCNT hybrid nanofillers PEHNF2 Pure epoxy + 0.3 w/% of alumina MWCNT hybrid nanofillers PEHNF3 Pure epoxy + 0.4 w/% of alumina MWCNT hybrid nanofillers PEHNF4 Pure epoxy + 0.5 w/% of alumina MWCNT hybrid nanofillers PEHNF5 where M m = measured mass of the hybrid composites M t = theoretical mass of the hybrid composites The porosity is known for causing imperfection, thus affecting the stress concentration and leading to poor mechanical properties. 2.5 Tensile strength Epoxy hybrid nanocomposite specimens were fabri- cated in accordance with the ASTM D638 standard as shown in Figure 2b. They were subjected to a tensile test using a Shimadzu tensile test machine with a load of 0.1 N mm/min. The tensile test was performed at room temperature with an overhead speed of 5 mm/min. Five specimens with the same composition were used to get the average value of the tensile strength. 2.6 Electrical conductivity measurement The computer-controlled two-wire method was used to measure the electrical conductivity (Keithley 6221DC, Tektronix, USA) of the hybrid nanocomposite test sam- ples. The specimen size was (25 × 25 × 2) mm and the tested surface was coated with a silver functional coating to improve the conductivity as shown in Figure 2c. The functional coating was applied to the substrate through a physical vapor deposition process. This is done by charg- ing a sputtering cathode and creating a plasma, which causes the material to be ejected from the target surface with the following specifications: a vacuum of 3×10 –2 mbar, a voltage of 1.5 kV, a current of 30 mA, a deposition of 25 nm/min and a grain size of = 5 nm. A silver-coated specimen was connected to the circuit board to determine the electrical conductivity ( ) with the following equation: =L /RA (2) where L = Length of the test sample A = Test sample cross-sectional area R = Resistance 2.7 Thermal conductivity measurement The thermal conductivity of epoxy hybrid nano- composites was measured using a laser flash apparatus. The test sample dimension wasØ6mmandthelength was40mmassho wninFigure 2d. For each composi- R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 145 Figure 4: SEM images show the porosity of: a) PEHNF1, b) PEHNF2, c) PEHNF3, d) PEHNF4, e) PEHNF5 Figure 3: Porosity of the pure epoxy/alumina-MWCNT hybrid nano- composites tion, five samples were taken and the average values were recorded at room temperature. The thermal conduc- tivity was measured using the following equation: = ×C p × (3) where = Thermal diffusivity coefficient (m 2 /s) C p = Specific heat capacity at constant pressure (J/(kg-k) = Density of the composite (kg/m 3 ) 3 RESULTS AND DISCUSSION 3.1 Porosity analysis The porosity (or voids) was investigated on the nanocomposites with different weight fractions. The voids of alumina-MWCNT polymer nanocomposites were characterized by both observing their cross-sec- tional morphology and quantifying the amount of voids. A surgical blade was used to slice the open samples of pure epoxy with various volume fractions of alu- mina-MWCNTs for viewing the internal voids. For each form of nanocomposite, the porosity was inversely pro- portional to the volume fraction of hybrid nanofillers. This indicates that small spaces caused by air interfer- ence are created by filler-to-filler contacts, which are the main factors for generating voids. Equation 1 was used to calculate the porosity. The theoretical mass (m t ) was computed using the densities of each component (i.e., alumina = 2.960 g/cm 3 , MWCNTs = 1.670 g/cm 3 and epoxy = 1.165 g/cm 3 ) whereas the actual mass (m) was measured using a weighing scale. The characterization of alumina- MWCNT polymer nanocomposites was analysed by quantifying the amount of voids and observing their sur- face morphology. The presence of void in the specimens varies with the reinforcement of the nanofillers with dif- ferent weight fractions. Interestingly, it was observed that the voids were gradually decreasing when the nanofiller content increased up to 0.3 w/% (Figures 4a to 4c). Due to the excessive addition of hybrid nanofillers that bundled together in the host matrix, with the additions of 0.4 w/% and 0.5 w/% of the nanocomposites, the voids increased (Figure 3). To fur- ther understand the presence of porosity in the surface, a morphological analysis was performed on the samples (Figures 4d to 4e). 3.2 Mechanical property analysis The integration of alumina-MWCNT nanofillers into the epoxy matrix significantly improved the pure-epoxy properties. Five specimens were tested under same envi- ronment condition and the average results were plotted (Figure 5). It was found that the epoxy hybrid nanocom- posites exhibited a considerable improvement in the ten- sile strength compared with pure epoxy. 0.1 w/%, 0.2 w/% and 0.3 w/% hybrid nanofillers added to pure epoxy caused 26 %, 52 % and 88 % improvements in the tensile strength, respectively. Furthermore, the 0.4 w/% hybrid nanocomposite exhibited an improvement value of 47 % compared with pure epoxy, but also a decrease in the tensile strength when compared with the 0.3 w/% specimen. In fact, alumina-MWCNT hybrid nanocom- posites brought a strong reinforcement to the epoxy com- posite, helping to absorb the load and prevent a sudden fracture. Moreover, the 0.3 w/% nanofiller hybrid nano- composite significantly improved the load absorbing ca- pacity due to the homogeneous dispersion in the matrix and caused the predominant mechanical interlocking. Therefore, the 0.3 w/% alumina-MWCNT hybrid nan- filler interface exhibits good mechanical properties. To understand the hybrid nanocomposite effect on the interlocking mechanism, fractured surfaces of the speci- mens were subjected to a morphology analysis. A R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... 146 Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 Figure 6: SEM morphology analysis of: a) pure epoxy, b) PEHNF1, c) PEHNF2, d) PEHNF3, e) PEHNF4, f) PEHNF5 Figure 5: Tensile strength of pure epoxy and composites with differ- ent weight fractions of alumina-MWCNT hybrid nanofiller smooth, glossy surface with a stepped morphology was observed for the epoxy by FESEM. The pure-epoxy frac- tured surface is brittle (Figure 6a). The morphology analysis revealed that different cracks were induced into the hybrid nanocomposites. The cracks were character- ized based on the size of their openings and propagation. The PEHNF1 hybrid nanocomposite exhibits small groove cracks along with a zig zag surface, resulting in hybrid nanocomposite reinforcement, which improved the epoxy properties and changed the brittle phase into a partially ductile one (Figure 6b). Furthermore, the PEHNF2 hybrid nanocomposite fracture surface exhibits cracks with small openings and a wrapping morphology is observed. During the crack initiation and propagation, the load carrying capacity is likely to be improved due to the impact on the reinforcement. Further crack initiation and propagation are also restricted by the reinforcement elements (Figure 6c). Similarly, the PEHNF3 hybrid nanocomposite fracture surface exhibits smooth and tinny cracks, resulting in a homogeneous distribution and good interlocking mechanism that significantly improve the epoxy properties and load carrying capacity. Further, the formation of waves, like an extended surface, shows the elastic nature of the composite, which significantly increased the load carrying capacity; tinny cracks and good interlocking of the composite are observed (Fig- ure 6d). The PEHNF4 hybrid nanocomposite morphology analysis revealed wider cracks on the fracture surface due to the nonhomogeneous dispersion of the reinforce- ment due to the excessive addition of the hybrid nanofiller (Figure 6e). The crack phenomenon and struc- ture are considerably good when compared with pure ep- oxy, but showing a reduced load carrying capacity when compared with the PEHNF3 composite. Further addition of the hybrid nanofiller (PEHNF5 – 0.5 w/%) to pure ep- oxy exhibits a lower performance than pure epoxy. A large quantity of the hybrid nanofiller forms into a bun- dle due to the van der Walls force so that no homogenous distribution is achieved and the fracture surface morphol- ogy exhibits large cracks and bowl holes while nanofiller agglomeration also indicates the excessive addition of the nanofiller (Figure 6f). Obviously, the typical crack initiation and propagation were the fundamental factors that caused the failure of the hybrid nanocomposite spec- imen. The reinforcement of the alumina-MWCNT hybrid nanofiller added to pure epoxy significantly improved the load carrying capacity, and the fracture surface mor- phology indicates that the interface between the hybrid nanofiller and the epoxy was significant, improving the mechanical properties. Compared with pure epoxy, the load carrying capacity is improved up to the addition of 0.3 w/% of the hybrid reinforcement. It is suggested that the interface bonding strength is increased significantly. However, no noticeable change in the mechanical prop- erties is observed with the 0.4 w/% and 0.5 w/% hybrid reinforcements due to the saturation level of the rein- forcement and agglomeration formation. According to this research, an excessive addition of nanofillers leads to a change in the matrix material natural properties; the 0.5 w/% nanofiller composite also exhibits a poorer per- formance than pure epoxy. 3.3 Electrical and thermal conductivity of hybrid nanocomposites Investigations were made into the electrical conduc- tivity (Equation 2) of the reinforced hybrid nanocom- posites as shown in Figure 7a. The electrical conductiv- ity of the alumina-MWCNT hybrid nanocomposites with filler weight percentages of (0.1, 0.2, 0.3, 0.4 and 0.5) w/% is (0.041, 0.057, 0.076, 0.063 and 0.037) S/m, respectively. Due to the uniform distribution of nano- fillers and the interlocking mechanism, the electrical conductivity of the hybrid nanocomposite significantly increased by 85 % with the 0.3 w/% filler addition. Fur- thermore, the epoxy matrix agglomeration and disconti- nuity caused the electrical conductivity of the 0.4 w/% and 0.5 w/% nanofiller composites to decrease signifi- cantly. Figure 7b displays the thermal conductivity of the composites. It was discovered (Equation 3) that when compared to the other nanocomposites, the hybrid nanocomposite with the 0.3 w/% filler addition has its thermal conductivity improved by 64 %. This outcome shows how an ideal reinforcement of the hybrid nano- R. RAMKUMAR, C. SABARINATHAN: ENHANCING THE MECHANICAL, THERMAL AND ELECTRICAL PROPERTIES ... Materiali in tehnologije / Materials and technology 57 (2023) 2, 141–148 147 Figure 7: a) Electrical conductivity, b) thermal conductivity of pure epoxy and nanocomposites with different nanofiller weight fractions composite enhances the underlying material characteris- tics. 4 CONCLUSIONS In conclusion, by comprehending the mechanical properties of composites, it is possible to predict a prod- uct performance more correctly. The considered nano- filler-reinforced polymer nanocomposites have higher or lower degrees of interior voids due the distribution of particles. Structural properties greatly affect the perfor- mance due the presence of voids, the so-called defects or one of the weaknesses of nanocomposites. In an effort to eliminate the voids during the sample preparation, more attention was devoted to the sonication process. Pure ep- oxy and alumina-MWCNT hybrid nanofiller reinforced nanocomposites were successfully prepared. The 0.3 w/% filler nanocomposite exhibited an increase in the tensile strength of 88 % when compared to pure epoxy. Both the interfacial adhesion and mechanical interlock- ing mechanism were greatly improved by adding the right weight fraction of alumina-MWCNT hybrid nanofiller to pure epoxy. Furthermore, the reinforcement of hybrid nanocomposites can increase the load carrying capacity, which leads to a ductile failure and results in an enhancement of the tensile strength. Similarly, the elec- trical and thermal conductivity of the 0.3 w/% nanofiller nanocomposite significantly improved, by 85 % and 64 %, due to the homogeneous distribution of the nano- filler in the host matrix. Considering different weight fractions of alumina-MWCNTs, additions of the hybrid nanofiller up to 0.3 w/% lead to a considerable improve- ment in all aspects. Further additions of the hybrid nanofiller, 0.4 w/% and 0.5 w/%, exhibit considerable re- ductions in the results due to the saturation level of the matrix. The FESEM morphology analysis helped us un- derstand the fracture surfaces of the hybrid nanocom- posites. This research work was focused more thor- oughly on the mechanical properties as well as electrical and thermal conductivity of the reinforced epoxy nanocomposites. The wear and erosion characteristics are associated with the proposed models, and our find- ings can provide an insight into the development of new composites for engineering applications. 5 REFERENCES 1 F. Hussain, M. Hojjati, M. Okamoto, R. E. Gorga, Polymer-matrix nanocomposites, processing, manufacturing, and application: an overview, J. Compos. Mater., 40 (2006) 17, 1511–1575, doi:10.1177/ 0021998306065289 2 Q. Gao, S. Wu, S. Lü, X. Duan, Z. Zhong, Preparation of in-situ TiB2 and Mg2Si hybrid particulates reinforced Al-matrix composites, J. 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