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Epoxy/Glass Fiber Laminated Composites Integrated with Amino Functionalized ZrO2 for Advanced Structural Applications Sudipta Halder, Soyeb Ahemad, Subhankar Das, and Jialai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09149 • Publication Date (Web): 29 Dec 2015 Downloaded from http://pubs.acs.org on January 8, 2016

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ACS Applied Materials & Interfaces

Epoxy/Glass Fiber Laminated Composites Integrated with Amino Functionalized ZrO2 for Advanced Structural Applications Sudipta Haldera*, Soyeb Ahemada, Subhankar Dasa, Jialai Wangb a Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar-788010, Assam, India. b Department of Civil, Construction and Environmental Engineering, The University of Alabama Engineering, USA *Author to whom correspondence should be addressed. E-mail: [email protected], Tele: +91 3842 241313, Fax: +91 3842 224797

ABSTRACT This work demonstrates the successful silanization of ZrO2 nanoparticles (ZN) and their incorporation in glass fiber/epoxy composites. Microscopic investigation under transmission electron microscope (TEM) elucidates anti-aggregation and size enhancement of silanized ZrO2 nanoparticles (SZNs). FTIR spectroscopy has been used to demonstrate the chemical nature of SZNs prepared. EDX results reveal the presence of Si onto SZNs. Incorporation of SZNs shows a strong influence on tensile and flexural properties of hybrid multi-scale glass fiber composite (SZGFRP) compared to that of the neat epoxy glass fiber composite (GFRP). A significant variation of tensile strength, stiffness, and toughness of ~ 27%, 62% and 110% is observed with respect to GFRP. Strength and modulus under bending are also enhanced to ~22% and ~38%, respectively. Failure mechanisms obtained from macroscopic and microscopic investigation demonstrate reduced interfacial delamination for SZGFRP. Additionally, increased roughness of the fiber surface in SZGFRP laminates produces better interfacial bonding arising from SZN incorporation in laminates. This symptomatic behaviour exposes the espousal of organically modified ZrO2 to enhance the interfacial bonding for their use in next generation hybrid laminates.

Keywords: Glass Fiber, ZrO2 nanoparticles, Surface silanization, Morphological analysis, Molecular architecture, Hybrid laminates, Mechanical properties, Fracture mechanisms

1 Introduction Harnessing of attractive properties from glass fibers for their use as advanced next generation fiber reinforced polymer (FRP) composites has been pursued tremendously in recent years by incorporating various nano filers1–4. In structural application FRPs are largely utilized in the form of laminates (made by impinging stacks of fibers with thermosets like epoxy). The glass fibers are largely used as reinforcement in FRP composites, owing to their low cost, ease of

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availability and reduced maintenance cost. Favourable properties such as strength and stiffness to weight ratios as well as high corrosion and fatigue resistance of glass fiber reinforced polymer (GFRP) composites has uplifted their usage in versatile applications5. However, in critical structural concepts related to aerospace industries, in-plane and through thickness properties of GFRPs, to a large extent, control their potentiality. For example, poor through-thickness properties can lead to delamination of the composite. Diffused intralaminar and interlaminar damage along with fiber breakage generally governs the in-plane properties of the GFRP composites5, but, reduced through thickness delamination resistance camouflages the full potentiality of the fibers in matrix6. In recent years, various effort demonstrates the potentiality of using nanofillers for GFRP composites to enhance the intralaminar and interlaminar damage resistance5,7–12. Most of the attempts are carried out by modifying the epoxy with bare carbon nanofillers such as multiwall carbon nanotubes (MWCNTs)7 and costly single-wall carbon nanotubes (CNTs)1 to alleviate the delamination process by enhancing the toughness of the matrix by tailoring the interfacial properties. These works postulate that for fillers such as CNTs and MWCNTs, their alignment, dispersion and interfacial bonding with the matrix predominantly tailors the structural properties13–15. Failure prevention in CNTs or MWCNTs inserted GFRPs is largely hindered due to compounding of the aforesaid difficulties. Wichmann et al.1 infused pristine CNT/epoxy blend in glass fiber fabrics and found pronounced agglomeration of CNTs leading to interfacial failure. However, a few reports demonstrates the positive influence of CNTs/MWCNTs in hybrid GFRPs. Godara et al.16 showed 89% enhancement in interfacial shear strength by sizing the glass fiber with CNTs, but, most of other reports demonstrate poor interfacial bonding due to the presence of weak van der Waals force and coulomb attraction amongst CNTs in polymer matrix17. These attraction forces lead to meso scale damage in hybrid multi-scale GFRP composites. Numerous works were carried out recently to overcome such meso scale damage by enhancing the dispersion and interfacial bonding using functionalized nano fillers18–22. Recently work of Garg et al.23 showed the reduction in tensile strength by 15% through the incorporation of functionalized CNTs in GFRPs, but the flexural strength was increased to 46% with respect to control. They postulated that the strong covalent bond between the CNTs and amino groups has reduced tensile strength. In another work, even after the application of electric field while processing of laminates using NH2 modified double-wall CNTs, failure was predominantly interlaminar in nature1. On the other hand, a large amount of success has been achieved for in situ grown CNTs on FRPs. Qian et al.24 showed that, varying the CVD grafting time of CNTs on silica fiber decreases the interfacial shear strength predominantly.


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However, significant technical and manufacturing challenges sometimes hinder their property enhancement14. The variation in property as depicted in numerous reports is attributed to the quality of CNTs which has a significant effect on the properties of composites25. In case of in situ grown CNTs on fibers, defect density in fibers arising from processing, results in wide scatting of properties26. The use of various nanofillers in the recent trend has been pursued to make a new generation epoxy/nano-material blend. These nanoparticle blend is found to uplift their property and lead to overcome the challenges in various application fields27,28. Moreover, there are many works reported on improvement of non-mechanical properties of polymers such as electrical conductivity29,gas barrier resistances30, dielectric31, corrosion protection in coatings32 etc. by blending with nanoparticles. The use of such nanofillers in GFRPs can instigate their applications in versatile fields of their usage. However, limited works explored the impact of non-carbon materials such as clay33 and organically modified SiO234 to enhance tensile and flexural properties of GFRPs. Uddin et al.35 concluded dramatic enhancement in both longitudinal and transverse tensile properties of GFRPs by incorporating bare inorganic nanoparticles. A conclusion has been drawn by Santos et al.34 stating enhanced tensile modulus by 11.6 % and tensile strength by 28.8 % of GFRPs incorporated with PDDA functionalized nano silica inclusion. Krushnamurty et al.33 demonstrated enhancement of the cross linking density and impact resistance by incorporating amino functionalized nanoclay. Although, these reports suggest the modest potentiality of inorganic nanofillers on mechanical property enhancement of hybrid GFRPs, but, most of the reports demonstrated the predominant interlaminar fracture, mainly attributed to poor matrix/fiber adhesion. This gives essences of scrutinizing other inorganic nanofiller such as ZrO2 to improve the delamination resistance of GFRPs. But, as per authors best of knowledge, no such academic research highlights the effect of pristine and organically modified ZrO2 on tensile and flexural behaviors of GFRP composites. Zirconia nanoparticles (ZNs) have been extensively used as nanoscale reinforcement agents for the fabrication of polymer nanocomposites due to improved strength, high stiffness36. However, uniform dispersion of ZNs into polymer resins remained a technological challenge, because of the high aggregation tendency of ZNs resulting in structural defects in corresponding nanocomposites. This disadvantage restricts the improvement of mechanical properties of nanocomposites37. Hence, surface functionalization of ZNs is essential to overcome the difficulties of aggregation when entrenched in viscous polymers. The enhancement in surface wettability of ZNs due to organic modification is found substantial



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and thus, reduces the aggregation effect of high density ZNs in polymers38. The organic modification results in formation of functional group elements onto pristine ZNs, which can improve the interfacial bonding strength between the fillers and the polymer resin. By considering this fact, present work successfully organically modifies pristine ZNs by reaction with silane-coupling agent (APTES). A comprehensive characterization of ZNs and silanized ZNs has been carried out to investigate the occurrence of functional group elements and their anti-aggregation behaviour. Further, pristine and silanized ZNs (at mass fractions of 0.5%, 1% and 2%) were introduced in epoxy resin. These advanced nano-epoxy resins were then used to fabricate hybrid multi-scale GFRPs. A facile and scalable approach such as combined high shear mixing and acoustic cavitation followed by hand layup method was used to fabricate the hybrid multi-scale GFRPs. The mechanical responses of these hybrid multi-scale GFRPs has been investigated to achieve our main objective for improving the performance of conventional epoxy/glass fibre laminates. The failure mechanism under inplane and bending conditions was investigated under the field emission scanning electron microscope (FESEM), which elucidates that the entrenchment of silanized ZNs is the mechanistic reason to enhance the mechanical behaviour of GFRPs. We believe, the enhancement in properties that has been achieved in this study opens a new trend to incorporate modified Zr based fillers in GFRPs for their espousal in high performance structural composites.

2 Experimental and characterization Silanization of ZrO2 nanoparticles. In the present work, monoclinic zirconia nanoparticles (99%, NanoShell, UK) of near spherical shape with an average diameter of 50 nm was used as filler. 3-aminopropyltriethoxysilane (APTES, 99% Alfa Aesar, India) was used as silane coupling agent. The silanization of ZNs was done by the co-condensation method in the presence APTES. The possible APTES reaction with ZNs having the surface hydroxyl group due to strongly absorbed water from the atmosphere is schematically shown in Fig. 1. We have demonstrated the hydroxyl group attachment onto ZrO2 nanoparticles by eight OH groups. Similarly, nine OH-group attachment has been demonstrated in the scheme of the other ref

32

. It is likely that the presence of hydroxyl groups onto nanoparticle surfaces

influences their chemical reactivity as well as hydrophilic properties39. Silanization was performed by blending ZNs of 1.23 g with 1.217 g of APTES to achieve a ZN–APTES molar ratio of 1:0.55 in the presence of ethanol (20 mL) in a 150 ml round bottom flask. The system was heated in an oil bath to maintain a constant temperature of 75 0C with continuous


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agitation using a magnetic stirrer at 1200 rpm for 3 h. Resulting silanized ZrO2 nanoparticles (SZN) were centrifuged for filtration and washed later with ethanol (until the solution reaches a pH value of 7) followed by vacuum drying in a vacuum oven at 800 C for 2 h. The schematic silanization process is illustrated in Fig. 1.

Preparation of bidirectional laminated composites. The amalgamation of ZNs and SZNs, in the epoxy network consisting of diglycidylether of bisphenol-A resin (Lapox-L12) and triethylenetetramine hardener (TETA-K6) (Atul, India), was done through the process of high speed shear mixing followed by acoustic cavitation. In a particular preparation of epoxy nanocomposite, epoxy resin was blended with 0.5 wt% of SZNs. A homogeneous blend of SZN/epoxy resin was obtained by impeller agitation at 1600 rpm for 30 minutes. Resulting blend was subjected to high energy acoustic cavitation under an ultrasonic liquid processor (Q700, Qsonica, USA) with maximum output power of 700 W at a constant frequency of 20 kHz and amplitude of 70%. The temperature of the blend was constantly monitored and maintained at around 40 oC to avoid resin degradation by using a chilled water bath with circulating water. Hardener was introduced later, with a stoichiometric ratio of 12:1, followed by impeller stirring for 10 min at 1600 rpm and later degassed under a low vacuum in the range of 1.33 x 10-4 to 1.33 x 10-5 bars in a vacuum chamber. The aforesaid procedure was adopted for preparation of epoxy nanocomposites consisting of varying concentration of ZNs and SZNs (0.5, 1 and 2 wt%). Balanced woven roving bidirectional glass/epoxy system devours upheld its potential espousal in laminates over their unidirectional counterpart in laminated composites. In the present exploration, neat epoxy or epoxy resin containing a varied concentration of ZNs or SZNs was made impinged within the woven roving balanced bidirectional plies of E-glass fibers using hand layup technique. The system was degassed under low vacuum in the range of 1.33 x 10-4 to 1.33 x 10-5 bars for removal of entrapped air. Woven roving bidirectional Eglass fabric (WR-360 100CM) was acquired from Owens Corning Ltd. (India). The weight fraction of the E-glass fibers with a surface mass of about 360 g/m2 is 0.495 in the laminates. The geometric parameters of the fabricated plates is with 11 balanced orientation of 0/90 woven consecutive bidirectional plies of glass fibers. The laminated composites were manufactured as 280 mm x 120 mm panels having a thickness of about 3.5 mm. The neat epoxy glass fiber composite (GFRP) as well as ZNs and SZNs hybrid multi-scale glass fiber composite (ZGFRP and SZGFRP) laminates were later degassed by using a vacuum bag system consisting of a vacuum pump. The process of GFRP, ZGFRP and SZGFRP laminate



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preparation is schematically represented in Fig. 2. The curing process was carried out by keeping in room temperature for 1 hour followed by vacuum oven curing at 800C for 2 hours. The tensile specimens were cut from the panels having a dimension of 250 mm in length and 25 mm width according to ASTM D3039 standard using a water-cooled diamond saw.

Characterization and evaluation. The morphology and the average size of ZNs and SZNs were examined under the field emission scanning electron microscope (FESEM) (Zeiss, Supra55) at an acceleration voltage of 15 kV. Fracture behaviour of the laminated composites was identified under the optical microscope at 200X and FESEM at a high magnification with an accelerating voltage of 5 kV. FTIR spectra of ZN and SZN samples were recorded with PerkinElmer Spectrum 100 series to identify the presence of functional groups and molecular architecture. For the FTIR characterization, samples were mixed with high purity potassium bromide powder and finally converted to transparent thin films. The FTIR spectra were acquired by scanning the samples (64 scans) from 600 to 4000 cm-1 with a resolution of 4 cm1

. The tensile properties of the GFRP as well as ZGFRP and SZGFRP laminates were

investigated according to ASTM D3039 standard and the flexural properties were investigated according to ASTM D790 with the help of a computerized universal tensile machine (INSTRON, Model 8801) at a crosshead speed of 1 mm/min.

3. Results and discussion 3.1. Anti-aggregation of APTES grafted ZNs and molecular architecture The most obvious evidence endorsing anti-aggregation due to APTES grafting and size enhancement of SZNs with respect to ZNs comes from the TEM investigation. The TEM images of ZNs and SZNs are shown in Fig. 3. The morphology of ZNs from the TEM image in Fig. 3(a) represents the presence of tightly agglomerated irregular shaped ZNs. The average particle size determined from TEM is 37±15 nm for ZNs as depicted in Fig. 3(c). This problem can be overwhelmed by controlled grafting of highly active hydrophilic ligand onto nanoparticle surfaces. The size enhancement of SZNs is observed from Fig. 3 (b). TEM analysis clearly reveals a partially interconnected network for SZNs. The average particle size determined from TEM is 50±15 nm for SZNs as depicted in Fig. 3(c). To understand the effect of APTES grafting on ZNs, their dispersion behaviour in DMF was monitored as depicted in Fig. 3 (d). It is found that hydrophilic ligand assisted SZNs to form a yellowish transparent solution with DMF after sonication for 20 min, as also reported in other refs 40–42. On the other hand, the ZNs in DMF remained cloudy. This strongly demonstrates the better


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dispersion ability of SZNs over ZNs. Hydrogen bonding between hydroxyl groups and amine ligand in SZNs probably transforms the cloudy solution into transparent yellowish dispersion. Investigation of ZNs as well as SZNs under FESEM in Fig. 4(a) and (b) provides a clear evidence of interconnected nanoparticle networks. These interconnected networks for ZNs are due to the aggregation of nanoparticles which, responsibly limits their usage in composites. This is due to their poor dispersion ability and interaction with matrix and filler. In case of SZNs, the tendency of APTES reverse attachment is presumably responsible for developing partial interconnection amongst SZNs. The surface energy may provide strong impact on the anti-aggregation behavior of the SZNs. The EDX analysis as depicted in the insert of Fig. 4, reveals the generation of a new deceptive peak of Si at 1.7 keV indicating the covalent attachment of siloxane group through the possible mechanism schematically demonstrated in Fig. 1. Increase in size of SZNs consequently indicates the presence of a layer of the active hydrophilic terminal group of ~10 nm onto SZN surfaces. Further confirmation of the active NH2 hydrophilic terminal group on ZNs comes from FTIR vibrational spectroscopy. Fig. 5 shows the FTIR spectra describing similarity and differences in molecular architecture of ZNs in their bare untreated state and in grafted state with the amino silane agent. The spectra of ZNs and SZNs both exhibit a broad spectroscopic vibration at 3340 cm-1. This vibration is mainly ascribed to the presence of the surface hydroxyl groups onto nanoparticle surfaces due to strongly absorbed water 43. This facilitates linking of the NH2 terminal group from APTES covalently during immobilization. However, N-H bands in spectra of SZNs, corresponding to asymmetric and symmetric stretching at wave numbers 3366 and 3296 cm-1, respectively, are not observed. This is attributed to the broad absorption band of the hydroxyl group shredding the narrow absorption bands of N-H, making them indiscernible easily. Several new absorption bands at 895, 961, 1040, 1135 and 1572 cm-1 are clearly observed. The presence of new bands in the spectrum between 8001400cm-1 and 2800-3000 cm-1, respectively, clearly confirms the successful grafting of SZNs by the amino silane agent

44

. In comparison to the spectrum of ZNs, newly appeared

absorbance band of 1572 cm-1 for SZNs is observed. This is primarily due to -NH2 group bending vibration, indicating the possible existence of the NH2 terminal group 44. Other bands at 895 and 961 cm-1 represent Si-O-Zr vibrations 45 and at 1040 and 1135cm-1 represent Si-OSi bonds

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, respectively. Hence, a concluding remark is generated for the successful

silanization with the establishment of the Si-O-Zr chemical bond between Si-O-Si around ZrO2 core surfaces of SZNs, as indicated in Fig.1 schematically. This modification on ZNs by binding highly active hydrophilic functional groups is predictive towards attenuation of



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aggregation by reducing the specific surface area and enhancing chemical reactivity towards fillers and matrix. 3.2. Mechanical response of hybrid multi-scale laminated composites The evidence validating the effectiveness of bare ZNs and SZNs incorporation on the mechanical response of the hybrid multi-scale laminates is obtained by investigating in-plane tensile and out of plane flexural behaviours. Tests were performed for ZGFRP and SZGFRP laminates with varying content (i.e. 0.5, 1.0 and 2 wt%) of ZNs and SZNs. The acquired results are represented as tensile stress-strain and flexural load displacement curves as elucidated in Fig. 6. A positive role of incorporating ZNs and binding highly active hydrophilic functional groups onto ZN is evident. The GFRP is the control laminate over which the property variation is compared.

3.2.1. Tensile properties Fig. 7 summarizes the deviation in in-plane tensile properties of ZGFRP and SZGFRP laminates with varying nanoparticle contents over control. All the data comprise statistical deviations, also summarized in Table S1 (see the supporting information). The tensile strength (σy), elastic modulus (E), strain (ε) and toughness (γ) of the control group are found as (255.89±5.65) MPa, (7.81±0.48) GPa, (0.022±0.001) mm/mm and (4.27±0.5) MJ/m3, respectively. The potential of the tensile property enhancement is noteworthy as indicated in Fig. 7. The tensile properties are increased noticeably till 1 wt% of bare ZNs incorporation in laminates. For 1 wt% of bare ZNs incorporation, σy, E, ε and γ, enhanced to (297.51±6.55) MPa, (9.98±0.43) GPa, (0.029±0.0015) mm/mm, and (6.84±0.52) MJ/m3, respectively, as can be seen in Fig. 7. Hence, the improvements were ~16%, ~27%, ~31% and ~60% in σy, E, ε and γ, respectively, with respect to that of control. However, lower content (0.5 wt%) of ZNs in ZGFRP exhibits no such appreciable enhancement of tensile properties. A slight decline in tensile properties is observed at higher nanoparticle content of 2 wt%. This decline could be attributed to exacerbation of agglomeration of bare high density ZNs. Agglomeration responsibly creates crack initiation sites at the interfaces between the matrix and fiber and within the matrix creating micro cracks46. Moreover, a high stiffness of bare ZNs in the matrix promotes stress concentration at ZN/epoxy interfaces and results faster propagation of such micro cracks, leading to delamination 47. On the other hand, as corroborated from Fig. 7, a dramatic increase in tensile properties for SZGFRP even at low nanoparticle content of 0.5 wt% is found as compared to that of control as well as ZGFRP laminates. Maximum σy, E, ε and γ were reached at 1 wt% of nanoparticle content as (323.54±7.51) MPa, (11.31±0.47)


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GPa, (0.032±0.0014) mm/mm, and (8.28±0.52) MJ/m3, respectively. This confirms dramatic increments in σy, E, ε and γ by ~26%, ~44%, ~45% and ~93%, respectively, with respect to that of control. Higher nanoparticle content (2 wt%) of SZNs incorporation resembles a marginal decrease in both the tensile strength and the elastic modulus, but, significant reduction in tensile strain and work of fracture is observed. The tensile strain has been enhanced significantly in comparison to composites made using bare ZNs and the control due to the creation of the Si-O-Zr layer along with the amine terminal group onto ZNs. This therefore indicates reduced obstruction for the molecular mobility within the highly crosslinked epoxy network. It demonstrates good dispersibility and enhanced interfacial interaction amongst SZNs and epoxy monomer through covalent bonding. However, in general increased tensile strength is largely attributed through the effect of ZrO2 nanoparticle concentration in the matrix48. However, at same nanoparticle concentration, enhancement in tensile strength for SZGFRP is symptomatic towards the dominant silanization effect over that of the concentration effect. Reduced tensile properties at high nanoparticle content for SZGFRP laminates suggest the possibility of affecting the epoxy network stoichiometry by enhancing the volume of the Si-O-Zr layer along with amine terminal groups. Excess enhancement of the interfacial region restricts the pronounced nanoparticle interaction between the matrix and fibers and thereby reducing the properties

38

. Hence, in concluding

remarks, we propose that SZNs can be one of the attractive tools to enhance the tensile behavior of hybrid laminates. However, property enhancement is largely dominated through the controlled formation of the Si-O-Zr layer along with the covalent attachment of desired hydrophilic groups and concentration of SZNs in the laminates.

3.2.2. Flexural properties The flexural strength (σz), flexural modulus (Eˈ) and work of fracture (γˈ) of ZGFRP and SZGFRP laminates varying with nanoparticle content is demonstrated through Fig. 8, also summarized in Table S2 (see the supporting information). All the data comprising statistical deviations. The flexural strength (σz), flexural modulus (Eˈ) and work of fracture (γˈ) was calculated according to the ASTM D 790 standard and by following the equation demonstrated in other work19. For the control group, σz, Eˈ and γˈ was (305.5±9.27) MPa, (10.54±0.44) GPa, and (9.42±0.41) MJ/m3, respectively. Fig. 8 clearly corroborates the results validating enhancement of flexural properties by incorporating either of bare ZNs and SZNs for all the levels. In case of ZGFRP laminates, SZN content of 1 wt% corresponds to maximum enhancement of σz, Eˈ and γˈ to values of (355.39±9.52) MPa, (13.49±0.45) GPa



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and (12.79±0.42) MJ/m3, respectively, which were improved by ~16%, ~27% and ~36% with respect to that of control. A similar increment trend in flexural properties of SZGFRP laminates, as observed from the tensile test of those laminates is noteworthy at a low nanoparticle content of 0.5 wt%. A significant increment in flexural properties is noticeable due to incorporation of SZNs up to 1 wt% of their content in the laminates. The values of σz, Eˈ and γˈ are found as (374.90±9.4) MPa, (14.48±0.46) GPa and (13.69±0.41) MJ/m3, respectively, leading to an improvement of ~22%, ~37% and ~45% with respect to those of control. A marginally reduction in flexural properties is observed at a higher nanoparticle content of 2 wt%. This enhancement in flexural strength in case of SZGFRP laminates at all the levels could be ascribed to minimised micro crack generation and propagation at the interfaces. This behaviour is accrediting limited agglomeration due to the presence of Si-O-Zr coated shell onto ZNs and a layer of hydrophilic ligand forming an interfacial network within the epoxy network for SZGFRP laminates. In addition, restriction to the deformation under bending (flexural modulus) is higher than the elastic modulus. The effect of dispersion along with a uniform stress field dominates the propensity of the catastrophic failure criterion for hybrid laminates under tensile loading

47

. In bending, the stress gradient through the

thickness, the inherent stiffness along with constituent interfaces between the fiber and the matrix promote the flexural modulus and flexural strain. In the case of ZGFRP, flexural modulus and strain enhancement is mainly controlled by the high stiffness of ZNs and the stress gradient through the thickness. Enhancement in the flexural modulus in SZGFRP laminates is symptomatic towards not only the enhanced inherent stiffness but also the presence of constituent interfaces. This may cause resistance for interfacial de-bonding to restrict the formation of micro cracks, their faster propagation and interfacial delamination. Therefore, grafting of APTES onto ZNs and incorporating them in the epoxy network has tremendous tendency to limit the adverse effect of delamination arising from micro cracks generated at the interfaces.

3.3. In-plane failure mechanism The relation of the tensile properties with the failure mechanisms for ZNs and SZNs integrated laminates is verified in detail through fractographic investigation. The macrograph of fractured specimens, as demonstrated in Fig. 9 (a1), (b1) and (c1) of the laminates, represents large variation in failure behavior. The failure behavior for the control group is largely governed by diffused intralaminar damage through ‘blooming’ of fibers similar to brush (Fig. 9(a1)). In contrast, bare ZNs insertion in laminates showed reduced ‘blooming’


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effect with partial fiber breakage (Fig. 9(b1)). On the other hand, failure for SZGFRP laminates is primarily fiber breakage along with through thickness matrix shearing. This failure behavior leads to total suppression of fiber blooming as can be seen in Fig. 9(c1). In general, interfacial bonding strength manages the stress transfer amongst fibers and matrix for tailoring the tensile properties of hybrid multi-scale laminates19,49. A reduced blooming effect for SZGFRP laminates could be attributed to the enhanced interfacial bonding strength at the interfaces by better wetting of SZN/epoxy monomer onto fibers. This mechanism can enhance crack crippling (CC) and through-thickness micro cracking (TMC), thereby restricting percolation of localized delamination. To verify this, interlaminar failure behaviour throughout the specimen gauge section is investigated under the optical microscope at low magnification as illustrated in Fig. 9 (a2), (b2) and (c2); while high magnification microstructural studies is done under FESEM, shown in Fig. 10. Fracture zone of control one as shown in the optical photomicrograph (Fig. 9(a2)) suggests the presence of minute flaw arising from nucleation sites at the juncture of interlaminar fibers. Crack further propagates by non-harmonizing the fiber bundles rendering them debonded (FDL) and debundled (FDB) with minor fiber breakage (FB) (indicated by red arrow in Fig. 9(a2)). This results in straight crack propagation, more prominently understood when investigated under FESEM as shown in Fig. 10(a1). A clean fiber surface is observed in Fig. 10(a2), demonstrating non crippling crack path by initiation of damage at weak interfaces between the epoxy and the fibers. This phenomenon presents a challenge on enhancing the FDL and FDB resistance throughout the laminate. In the case of ZGFRP laminates, optical photomicrograph shows crack movement through the interlaminar sites leaving non serpentine crack traces (indicated by red arrow in Fig. 9 (b2)), similar to that of control. However, delamination sites showed few CC marks, indicating partial matrix yielding (MY) due to ZN insertion. Evidence of FDL with partial matrix yielding due to the presence of ZNs is observed through FESEM, as shown in Fig. 10(b1). High magnification FESEM photomicrograph of fiber surface (Fig. 10(b2)) exemplifies fiber wetting behavior by ZN/epoxy monomer, but, not strong enough to enhance the interfacial bonding strength. The presence of aggregated ZNs along with the total de-bonding (TDB) around them catalyses localized delamination between the fiber and the matrix. A modest increase in toughness is mainly attributed to the presence of other features such as FB and fiber fragmentation (FF) throughout the specimen gauge section. Enhanced roughness, as can be seen from Fig. 10 (b1), at the delamination zone confirms this statement. In contrast, enhanced CC due to crack propagation through the yielded matrix rather than weak interface (shown by red arrow) as



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observed under the optical microscope (Fig. 9(c2)), could explain the formation of tough interface between the fiber and the matrix for SZGFRP laminates. In addition, the presence of others features such as TMC, FF and FB are also observed. Damage prevention as evident under FESEM is judged from the abridged micro crack formation by keeping the adjacent fibers and matrix intact, as can be seen in Fig. 10 (c1) (indicated as “FMB”). Targeted reinforcement of the SZNs in the matrix has restricted the intralaminar as well as interlaminar diffused damage by attaching most of the fibers to the matrix. Reduced matrix de-bonded cracks and diffused micro cracks has generated ductile scalloped fracture feature (SFF) between the adjacent fibers. It appears from Fig. 10 (c2) that partial de-bonding (PDB) around the anti-aggregated SZN/epoxy monomer coated fibers along with nano crack tail predominantly enhances the interfacial adhesion. As described earlier, the reaction between the epoxy monomer and the Si-O-Zr layer of SZN predominantly strengthened the interfacial region. This creates crack healing effect resulting in PDB of SZNs from the fiber surface, thereby, suppressing localized delamination at interlaminar regions 50. Thus, in comparison to ZGFRP laminates, alleviated tensile properties at nanoparticle content of 1 wt% is probably attributed to micro crack suppression due to uniform dispersion of SZNs on fibers as well as in the matrix, as proposed in refs

51–54

. Moreover, the contact force between the matrix and

the fillers might be highly influenced by interfacial bonding between the modified matrix and the fibre. However, at SZN content of 2 wt% in laminates, their tensile properties are mildly deteriorated owing to the reduction in interfacial interaction between the SZNs and the matrix/fibers caused by the enhanced volume of the hydrophilic layer around SZNs. This probably reduces the interfacial strength between the fiber and matrix

55

. In brief, anti-

aggregated SZNs enhance the adhesion between the fibers and the matrix by strong interfacial interaction, resulting in composites with enhanced tensile properties.

3.4. Through thickness failure mechanism Under three point bending, resistance of the specimens to crack initiation and propagation is mainly characterized by the shear stress transferred from ply to ply through the resin matrix 14

. Fractographic investigating of control under FESEM (Fig. 11(a1)) showed interlaminar

damage by interlaminar de-cohesion (ILD) and shear fracture (SF). Poor wetting of the fibers is clearly evident from featureless smooth bare surfaces of the fibers (Fig. 11(a2)). The selected mid-plane fracture behaviors of the ZGFRP and SZGFRP are examined under FESEM at low magnification and high magnification as shown in Fig. 11 (b1), (b2), (c1) and (c2). A different featured fractured surface primarily governs the failure mechanism of


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ZGFRP as well as SZGFRP laminates containing 1 wt% of ZNs and SZNs with respect to control. Features corresponding to matrix cracking (MC) and matrix deformation (MD) at the juncture of the interlaminar zone with partial resin adhesion (PRA) are observed in the ZGFRP laminate. On the other hand, a reduced MC and enhanced MD as well as PRA are observed as dominating features on FESEM fractograph of the SZGFRP laminate. Resin fracture and interfacial de-bonding forming ductile scalloped features (DSF) primarily enhance the flexural properties of the SZGFRP laminates. It is well understood that, interfacial bond strength vitally enhances the stiffness and toughness of the base matrix and can enhance the interlaminar delamination toughness 56. This could deflect the micro cracks formation and resist the delamination to occur. The increased roughness on the fiber surface of the SZGFRP laminates is responsible for such behavior under FESEM (Fig. 11 (c2)). Nevertheless, the formation of agglomeration of ZNs in the ZGFRP laminates could hinder the matrix deformation and release of strain energy, leading to lower flexural properties. Thus, reduced intralaminar and interlaminar damage during bending is largely attributed to the interfacial bonding strength enhancement arising from SZN incorporation in laminates. Therefore, the coating of Si onto ZNs and a layer of hydrophilic ligand have the capability to elevate the interfacial interaction between the glass fibers and the base matrix. Interface formulation due to SZN addition thus demonstrates the enhancement of the interfacial sensitive properties of the laminates. Higher SZN content could reduce the interfacial interaction, leading to the reduction of the flexural behavior of laminates. Further research for harnessing evidences on the interfacial interaction behavior in laminates awaits the clear picture of such flexural property depreciation at high nanoparticle content.

4. Conclusions The successful amino functionalization onto ZNs resulted in binding highly active hydrophilic functional groups to attenuate aggregation and enhance chemical reactivity between the filler and the matrix. The effectiveness of bare ZNs and SZNs incorporation on the mechanical response of the hybrid multi-scale laminates is obtained by investigating inplane tensile and out of plane flexural behaviours. Amongst the range of mass fraction of SZNs considered in the presence study, highest mechanical properties was obtained at 1wt% of addition. The tensile strength, stiffness and toughness of the laminate was enhanced by ~27%, `62% and ~110%, respectively, and the flexural strength, flexural modulus and work of fracture was enhanced by ~22%, ~38% and ~45%, respectively, with respect to that of control. SZNs can be an attractive tool to enhance the properties of hybrid laminates under in-



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plane loading as well as bending. However, property enhancement is largely dominated through the controlled formation of the Si-O-Zr layer along with the covalent attachment of the desired hydrophilic groups. Reduced properties at high nanoparticle content for SZGFRP laminates suggest the possibility of affecting the epoxy network stoichiometry. In-plane failure mechanism is governed by blooming of individual fiber bundles in the GFRP laminates. The mechanistic and fractographic study suggest that coating of Si onto ZNs and a layer of hydrophilic ligand have the capability to elevate the interfacial interaction between the glass fibers and the base matrix. This leads to better wetting and enhanced interfacial property for the SZGFRP laminates. Features corresponding to matrix cracking and matrix deformation at the juncture of the interlaminar zone with partial resin adhesion for the ZGFRP laminate governs the flexural properties. This study demonstrates the viability of developing advanced multi-scale composites which can be utilised in advance structural applications, in particular for automobile and aerospace industries, by incorporating a very small concentration of amino functionalized zirconia nanoparticles.

Associated Contents Supporting information Tables showing in-plane tensile properties and flexural properties (mean ± standard deviation) of GFRP, ZGFRP and SZGFRP laminates.

Acknowledgements The authors thanks Department of Science and Technology, India under DST-FIST program 2014 with Grant No. SR/FST/ETI-373/2014. This work was initiated within the research support from National Institute of Technology Silchar.

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Captions Fig 1. Schematic of silanization process of ZrO2 nanoparticles and reaction with epoxy network.



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Fig 2. Schematic representing preparation of SZN incorporated composite laminates. Fig 3. (a) TEM image representing the morphology of ZNs, (b) Morphology of SZNs under the TEM, (c) average particle size of ZNs and SZNs and (d) dispersion behaviour in DMF of ZNs and SZNs. Fig 4. FESEM photomicrographs of (a) ZNs and (b) SZNs with their EDX inserted depicting presence of Si in SZN. Fig 5. FTIR spectroscopy of ZNs and SZNs. Fig 6. Representing (a) tensile stress-strain and (b) flexural load displacement curves of hybrid laminates. Fig 7. Histogram illustrating the deviation in in-plane tensile properties of hybrid laminates. Fig 8. Histogram illustrating the deviation in flexural properties of hybrid laminates. Fig 9. Macrograph of the fractured tensile GFRP (a1), ZGFRP (b1) and SZGFRP (c1) laminates at ZN and SZN content of 1 wt% confirming variation in blooming effect when (a2), (b2) and (c2) are their optical micrograph at 200X magnification confirming variation in crack crippling (CC) and illustrating intralaminar damage behavior. Fig 10. FESEM evidence of interlaminar damage mechanism of GFRP and ZGFRP and SZGFRP tensile laminates at ZN and SZN content of 1 wt% confirming FB, FDB and FDL for GFRP laminates (a1) and (a2), MY at the delamination sites and FDL for ZGFRP laminates (b1) aggregated ZN and TDB at high magnification of 60 KX for ZGFRP laminates (b2), FMB and SFF for SZGFRP laminates (c1) and non-aggregated SZN and PDB at high magnification of 60 KX for SZGFRP laminates (c2). Fig 11. FESEM evidence of through thickness damage mechanism of GFRP and ZGFRP and SZGFRP laminates at ZN and SZN content of 1 wt% confirming ILD and SF for GFRP laminates (a1) and (a2), MC between adjacent fibers for ZGFRP laminates (b1) PRA and MD for ZGFRP laminates (b2), reduced MC, enhanced MD and PRA and DSF for SZGFRP laminates (c1) and (c2).


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Fig . 1 O

O

HO HO

OH ZrO2

OH ZN

C9H23NO3Si Silanization, 75 0C Refluxed-3h (Ethanol medium)

Si

O O

H Si

OH OH

H

O

O

(APTES)

OH HO

NH2

Si

Si

O

ZrO2

Si

O H

O

O

O Si

H

H2N

O

H Si

n

Epoxy chain

H

O

O

N

O Si

O

Si O

Si O

O H

O

Si

O

O

O

O

O

ZrO 2

O

H Si

SZN/epoxy resin

O Si

re ac tio

O

N H

(TETA)

O

Si

H N

SZN

Si

O

Si

O O

O

OH O

O

n O

O

O

O

O

O

(DGBEA)

In sit u

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O

Fig. 2


NH2


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Fig. 3


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Fig. 4

Fig. 5



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Fig. 6

Fig. 7


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Fig. 8

Fig. 9



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Fig. 10


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Fig. 11



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