Assessing Nano Scratch Behavior of Epoxy Nanocomposite

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Assessing Nano Scratch Behavior of Epoxy Nanocomposite Toughened with Silanized Fullerene Subhankar Das, Sudipta Halder, Arijit Sinha, Muhammad Imam, and NAZRUL ISLAM KHAN ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00763 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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ACS Applied Nano Materials

Assessing Nanoscratch Behavior of Epoxy Nanocomposite Toughened with Silanized Fullerene Subhankar Dasa, Sudipta Haldera*, Arijit Sinhab, Muhammad Ali Imamc, Nazrul Islam Khana a

Department of Mechanical Engineering, National Institute of Technology Silchar, Silchar-788010, Assam, India. b

Dr. M.N.Dastur School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711103, India. c

The University of Alabama, Tuscaloosa, Alabama-35487, USA.

*E-mail: [email protected], Tele: +91 3842 241313, Fax: +91 3842 224797

ABSTRACT Fullerene (C60) is used to prepare a high-performance epoxy nanocomposite having enhanced antiscratch performance. Herein, C60 were oxidized and then silanized using 3-Aminopropyltriethoxysilane (APTES). Oxidation disaggregates the C60 to form 3D interconnected chain like networks. These networks were concealed after silanization. The formation of active functional groups on silanized C60 regulated the polymer assembly and its coordination that enhances scratch recovery index (~32%), wear resistance (~57%) and microhardness (~19 %) of epoxy composites. Interestingly, dual advantage due to significant enhancement in mechanical and fracture properties uplifts the potential use of silanized C60 as nanoreinforcement in epoxy. The tensile strength, elastic modulus, and fracture toughness was enhanced by ~ 28%, ~30%, and ~67.5% respectively for the epoxy nanocomposites. We provide fundamentals combined with the insight into the nature of toughening due to stiffness modulation by silanized C60 that deliberates the best synergy in epoxy nanocomposite to resist nanoscratch. Keywords: fullerene (C60); silanization; epoxy nanocomposite; nanoscratch; fracture toughness. INTRODUCTION Epoxy resin systems, widely used engineering thermosets in coatings, adhesives, sports, and leisure equipment. This wide range of application is instigated from their outstanding mechanical properties, high resistance to corrosion, excellent heat resistance, lightweight, low shrinkage, easy molding, and

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low cost 1–5. For an instant, sports helmet, tennis rackets, skis, golf equipment and hockey sticks made of the epoxy composite are highly attractive to sportspersons as they are safer, lighter, stronger, more reliable and better resistant to fatigue 6. However, an inherent high cross-linked network of epoxy offers poor resistance to crack propagation and thus inept to enhance their wear resistance behavior 4,5,7,8

. Subsequently, various softer organic partilces (rubber/elastomers)9–11, inorganic nanoparticles

(Al2O3, TiO2, SiO2, ZrO2) 5,12,13 and also carbon-based nanoparticles (CNT, graphene, carbon nanobead) 14–16 were incorporated in epoxy resin to uplift the material toughness 17. Each of these nanomaterials can impose features like high strength, enhanced stiffness, durability, reduced weight, and abrasion resistance in various sports equipment. Furthermore, the advanced epoxy composite having combined above said features can have immense potential for sports equipment 6,17. However, the presence of softer organic partilces reduces the hardness, tensile (modulus and failure strength) and thermo mechanical properties of epoxy composites9,10,18, whereas, high-density inorganic nanofiller (for example TiO2 nanoparticle density is 3.9 g/cm3) limits the flow behavior of epoxy which in turn restrict their thorough dispersion19. The same disadvantage can be cited for CNTs and GNPs having issues like entanglement, and self-aggregation behavior20. Fullerene (C60), a third allotropic modification of carbon, has a hollow shell structure (carbon atoms at the nodes of 20 hexagons and 12 pentagons arranged in a cage lattice) 16. C60 thus having less weight and the high specific surface area in conjunction with excellent mechanical strength due to the sp2 carbon bonding networks 21. Considering this aspect we hypothesize by taking into account anticompression performance, non-entangling behavior, and high electron affinity of C60 to make the high performance of epoxy composites for sports applications. However, the C60 applicability in epoxy resin was found limited in the past due to limited production and high cost 22,23. Recently, the advent of large quantity production of C60 from combustion-based mass production technology 22 paves the pathway to use C60 within the niche sports equipment and coating industries efficiently 24–27. Under the flagship of C60, reduced weight and twisting of racquet frames can be ACS Paragon Plus Environment

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achieved and also can mitigate the chipping and cracking effect of bowling balls 17. This effectiveness of C60 to enhance the performance of epoxy composites was initially studied by Jiang et al. 28 and Ogasawara et al. 29. Following their work, Rafiee et al. 21 also modified the epoxy resin with 1 wt% of fullerene C60 and reported ~20% enhancement in tensile strength and ~52% enhancement in fracture toughness compared to the baseline epoxy. In contrast, higher weight fraction (~5-10%) of inorganic nanoparticles such as Al2O3, TiO2, and SiO2 were required to achieve the same improvement 21. Jeyranpour et al. 30 used molecular dynamics simulation to obtain thermo-mechanical and mechanical properties of epoxy composites containing fullerene C60 and reported 19.5% enhancement in Young's modulus. Upadhyay et al. showed that the incorporation of fullerene C70 particles into epoxy resin improve its dry sliding friction and wear behavior 31. Though fullerene C60 is effective nanofiller to uplift the performance of sports goods, yet the full potential of that was unattained due to their inert surface and self-aggregation tendency which lead to poor miscibility in the epoxy network 32,33. To overcome such difficulties the surfaces of C60 need to be engineered with functional moieties. We expect C60 surface engineering with organosilane can improve the interfacial interaction for advanced epoxy composites 33–36. Considering this concept we inferred a potential candidate in the form of functionalized C60 for advanced functional material in epoxy nanocomposites (ENC). In our previous work 33, C60 was silanized with 3-aminopropyltriethoxysilane that is found boosting the molecular relaxation and thermal behavior of the epoxy system. In continuation of the previous work, present study scrutinized the anti-scratch performance of the ENC considering the trade-off between tensile and fracture performance. The toughening mechanisms were identified from the morphological analyses of fracture surfaces using FESEM analyses. Though few authors used non-functionalized C60 only to enhance the mechanical properties of ENC 21,28,29, however, as per best of authors knowledge the nanoscratch behaviour of ENC toughened with silanized fullerene was not addressed in literature and hence this work is expected to open new pathways for silanized C60 in the development of ENC



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enriched with multifunctional properties for sports applications, coatings, and advanced machinery where good strength and better scratch performance is required. EXPERIMENTAL SECTION Materials and method. The epoxy resin system comprises (Lapox® L-12) having viscosity of 9000 – 12000 mPa.s at 25 oC and density 1.1 – 1.2 g/cm3 and hardener (Lapox® K-6) with viscosity (5 – 10 mPa.s at 25 oC) with density of 0.95 – 1.1 g/cm3. Pristine fullerenes C60 (PC60) of purity 99 %, diameter 60-80 nm (Figure S1a) with density of 1.650 ± 0.05 g/cm3 supplied by Nanoshel LLC, USA is used as nanofillers. The PC60 were oxidized with conc. HNO3 and silanization by using 3-Aminopropyltriethoxysilane (99% purity, supplied by Alfa Aesar, India). The reaction procedure and attachment of silane oligomers onto oxidized fullerenes (OC60) is demonstrated schematically Figure 1. In particular processing, 0.5 wt% of silanized fullerenes (SC60) were blended in epoxy resin by stir mixing at1500 rpm (Remi Lab Stirrer, Model: RQT-124A/D) and degassed for half an hour. The hardener was added to the degassed mixture and again stirred for another few minutes with further degassing. The Final mixture was poured into silicon mold and cured at room temperature for 24 h. The process and possible reaction mechanism of SC60 with the epoxy network is schematically represented in Figure 1. The neat epoxy (NE) and resin containing PC60 (P-FEC) and OC60 (O-FEC) were also made with a similar procedure. Characterization. All the samples were firstly rough polished with different grades of sandpaper and fine polished by using diamond paste (0.25 micron). The nanoscratch measurements were carried out at room temperature on the polished surface using CSM NST: 50-133 nanoscratch tester with a diamond sphero-conical indenter (Ref: SB‐A63, the radius of the spherical portion of indenter R = 2 µm and the half-cone angle α = 60°). The experiment was performed inside a cantilever under a constant load of 10, 20, 30 and 40 mN over a scratch distance of 0.5 mm with a scratch speed of 1 mm/min. Three ACS Paragon Plus Environment

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stages were involved in the current study. Firstly, a pre-scan to initiate the depth profile of the scratching surface, then, the running of the scratch test to obtain the penetration depth and finally the post-scan to obtain the residual depth after load removal 37,38. The recovery index was obtained from the nanoscratch measurements as Scratch recovery index (ƞs) =

( )

(1)

Where Pd = penetration depth and Rd = residual depth. The plastic energy is expressed as Plastic energy (P.E) = x

(2)

Where Fn = corresponding normal load, Rd = residual depth of the scratched surface after removal of load and the coefficient of friction (COF) was obtained from the ratio between tangential load (FT) and normal load (Fn). The scratch tracks on the samples were analyzed to find out the scratch widths. The average value of these scratch widths (b) was used to calculate scratch hardness (Hs) using Equation (3) as described below:

=

(3)

The Hs values were further utilized to obtain the wear-coefficient (k) of the samples following the Archard's equation (Equation (4)) 39–41. The wear rate (Q) used in Equation 4 was calculated from the geometry of the indenter and the nanoscratch data (penetration depth Pd) 39,42. =

(4)

= ! × $ − &'(!) * + ,2./&!01 − (1 − &'(!)34 + 01 − (1 − &'(!)3 56(!*7 # (5) The wear resistance coefficient (Rw) 39 was estimated using Equation (6) as shown below: 8 =

(6)



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The tensile testing of NE and ENC were carried out following the ASTM-D638 (Type-V) standard under a universal testing machine (Instron: 5969), 50 kN load cell and crosshead speed of 1 mm min-1. At least 5 specimens were tested from each batch. The measurements of the SENB test were carried out at a crosshead speed of 1mm/min according to the standard of ASTM D5045. Fracture toughness (KIC) was determined from SENB test using the relationship 43 9:; =

A

) @(>)

(7)

where, PQ is the maximum load from the load-displacement curve; B(6 mm), W(12 mm) and a (6 mm) A

are the specimen’s dimension and @( ) is a factor related to the geometry of the specimen, calculated > using the equation. @

A

>

$=6

?

F

F

F

F

A C.EE G$ G$.HI.EI GJ.K G$ )L $ F F M/ > J $ $ G

(8)

G

Field emission SEM (Zeiss, Supra 55VP) at an accelerating voltage of 5 kV was employed to observe the morphology of Fullerenes, tensile fracture surfaces and nano-scratch tracks. Vicker’s hardness test (Model: S Auto, Make: OMNITECH, India) was performed on polished sample at 10 g of load to obtain the micro hardness. Transmission electron microscopy (TEM) of C60 is performed on JEM-2100 HRTEM, JEOL, Japan, operated at 200 kV. RESULTS AND DISCUSSIONS Transmission electron microscope (TEM) and field emission scanning electron microscope (FESEM) were used to investigate the agglomeration behavior of the C60 (Figure 2 and Figure S1, supporting information). TEM showed the transformation of agglomerated PC60 (Figure 2a) into the chain-like interconnected network (Figure 2b) by the generation of defect sites at the outer faces of OC60 (see black arrow in Figure 2c) and their de-coagulation with thickness equivalent to two or three C60. Silanization is found to increase the thickness of OC60 chains due to the concealing effect by grafting silane oligomers (Figure 2d). FESEM characterization describes 3D interconnected highly porous ACS Paragon Plus Environment

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network after de-coagulation and subsequent silanization (Figure S1, supporting information). This interconnected network of SC60 is beneficial to pave the pathway for better transport of the resin system and can facilitate surface wetting also. In addition, reactive species of SC60s can facilitate the creation of interfaces at the juncture of epoxy-C60 system. We expect that if this happen could enhance the mechanical and nanoscratch behavior mainly instigated from longer time to molecular relaxation 44. Our previous work showed a significant increase in storage modulus by about 491% when 0.5 wt% of SC60 was incorporated in the epoxy system with respect to NE 33 (Figure S2, supporting information). To evaluate further, the epoxy system interaction ability was determined by investigating ∆Cp results from differential scanning calorimetry (DSC), and the results are shown in Figure 3. This findings corroborates significant increase in ∆Cp for ENC samples. Increased ∆Cp for all the ENC samples predicts ability of C60 to confine the epoxy network segment thus increases the entropy associated. Furthermore, CRR was determined to get a further insight information regarding limited network disruption, viscous response to stress, and steric limitations 45. The number and size of CRR subsystems of the epoxy is profoundly dependent on temperature. The characteristic volume (ζ3) and length (ζ) defines the CRR in terms of interphase induced segmental relaxation at Tg. Consequently, change in heat capacity at Tg 46, is used to determine ζ3 and ζ of ENC using Equation (9). ζI =

P QR ∆(/;T )

(9)

U(VQ)

where, δT represents the thermal fluctuation of an average CRR at Tg, ρ is the density (1.21 g/cm3), kB is the Boltzmann constant, and Cv is the heat capacity at constant volume, approximated by the constant pressure heat capacity (Cp) 36. NE has shown lower average CRR length (~0.795 nm) than ENC (Figure 3). This elevation of CRR suggest that C60 has ability to restrict the mobility of epoxy network segments by facilitating better interaction to help coordination with subsequent chain segments 47,48. PC60 with high CRR suggest interaction ability with epoxy network considering the aspect of its high electron affinity. This high electron affinity is camouflaged by the silane oligomers for SC60 but still



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giving encouraging CRR results reflecting covalent bonding with epoxy network resulting in chain segment mobility restriction and epoxy network coordination. This statement can also be judged by the increased ∆Cp for S-FEC compared to O-FEC. Considering this fact we hypothesized that SC60 in the epoxy network could increase the surface area of the de-cohesion zone under the load. Hence, assumed that SC60 has the capability to resist mechanical fracture as well as wear and mar resistance. To evaluate the potentiality of C60 on mechanical properties of ENC, tensile and fracture behaviors were examined. The variation of tensile stress-strain behavior and the tensile properties due to the addition of 0.5 wt% of PC60, OC60, and SC60 in epoxy and subsequent curing is shown in Figure S3 ( supporting information) and Figure 4a also tabulated in Table S1 (supporting information). The increase of tensile strength (σ) and Young’s modulus (E) by ~13% and ~20 % for P-FEC and ~ 16 % and ~ 27 % for O-FEC with respect to NE can be seen. Ogasawara et al. 29 reported a similar behavior where, ~13 % enhancement in σ was achieved for the 0.5 wt% addition of fullerenes (C60: 60%, C70: 25%, and other higher fullerenes: 15%) in epoxy resin. In our study, enhancement in σ and E for OFEC depicts the chain mobility restriction of epoxy networks due to the de-coagulated porous 3D interconnected network of OC60 49. However, there was a depression in failure strain (ε) and toughness (γ) by ~ 25 % and ~ 20 % (P-FEC) and ~31 % and ~24 % (O-FEC) compared to the NE indicates ineffectiveness monitor epoxy network flexibility 15. The interconnected porous network of OC60 even though necessitates convenient infusion of the epoxy system, but plays no role on stiffness modulation. With the advent of SC60, the σ and E was increased by ~ 28%, and ~ 30% respectively compared to that of NE. A slight drop in ε is observed, but γ was found increased for S-FEC. Work of others, Goyat et al. 15 showed 25 % enhancement in tensile strength for 2 wt % of carbon nanobead and Rafieee et al. 21 showed ~20 % enhancement with 1 wt % unmodified fullerene C60 (+ 99.5% purity). The obtained fracture toughness (KIC) and strain energy release rate (GIC) is illustrated in Figure 4b. The KIC and GIC were enhanced by ~16 % and ~18% respectively for P-FEC compared to that of NE, whereas for OFEC it was ~32 % and ~40 %. However, significant enhancement in KIC and GIC is observed for S-FEC ACS Paragon Plus Environment

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which is ~67.5 % and ~116.5 % with respect to NE. Our study hence confers the amelioration effect of silane functionalization on the improvement of tensile properties at low filler content. The enhanced performance arises due to stiffness modulation by the covalent bonding between SC60 and epoxy matrix juncture. This result elucidates the efficiency of SC60 to enhance the strength, stiffness and fracture resistance of ENC. To clearly explain the reason for the improvement in mechanical properties, the fracture surfaces were examined under the FESEM (Figure S4a-d, supporting information). The topography of the fracture surfaces was evaluated and the toughening effect of fullerene onto the epoxy network is described in this section. The fracture surface for NE (Figure S4a) is found relatively smooth and glassy, representing brittle failure 7. The FESEM higher magnification investigation of the squared area (Figure S4a, supporting information) of NE (Figure 5a) depicts the absence of large-scale plastic deformation during fracture that signifies low fracture toughness. Compared to the NE the fracture surfaces (Figure S4b-d, supporting information) of P-FEC, O-FEC and S-FEC exhibit much rougher surface topography due to the flow of fracture in a meandering way. Hence, crack deflection can be said as the most apparent failure mechanisms inspired by the spherical shape of rigid C60, and are comparable with the toughening mechanism of epoxy composite having inorganic nanoparticles such as TiO2, SiO2 50–52. The higher magnification image of P-FEC (Figure 5b) showed hemispherical holes or random nanocavities at the fracture surface attributed to the de-bonding of agglomerated PC60 (see adjacent to the black arrow in Figure 5b also enlarged in right corner). This suggest poor resin infusion due to aggregated PC60 that resulted in inevitable de-cohesion within the composite matrix. Hence the particle pull-out mechanism club with crack deflection that partly enhances the mechanical properties of P-FEC. However, no such de-bonding phenomenon was observed for O-FEC and S-FEC (Figure 5c, d). These results indicates homogeneous mixing with better surface wetting of the chain like networks of OC60 and SC60 with epoxy matrix. However, compared to O-FEC, S-FEC possess higher strength and toughness, caused by the enhanced matrix yielding behavior that produces tortuous fracture profile ACS Paragon Plus Environment


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(Figure 5d). Apart from that, decreased de-cohesion zones between SC60 and matrix extrapolated from modulated stiffness at the juncture (shown enlarged in Figure 5c, d) postulates smaller random nanocavities that increases matrix volume energy thus restricting the creation of the new fracture surface under the load. Inspired by the increment in the storage modulus, tensile strength, stiffness and fracture properties, we performed the nanoscratch (NS) test on ENC, to scrutinize their resistance against scratching. In this test we focused to determine the volume deformation resistance of ENC to identify their potentiality for various industrial applications 8,31,53. The NS test was performed at different loads (10 mN, 20 mN, 30 mN, and 40 mN). Two important parameters (1) scratch recovery index (ƞs) and (2) plastic energy (P.E) are evaluated accordingly (Figure 6a, b). After performing nanoscratch, the indenter is retarded following the same path to determine the depth recovered by the material with respect to the actual penetration depth (Pd). Scratch recovery index measures the material ability to recover its deformation elastically after scratch has been formed 39,54. This parameter is dependent on Pd and signifies the elastic behavior of the material 39,54. The ƞs is found higher in case of P-FEC and O-FEC compared to NE for all the loads (Figure 6a). The enhancement in ƞs is mainly attributed to the hard and high modulus C60. It was postulated by few researchers that enhancement in material yield strength and elastic modulus can help material to perform well against scratching 44,55–57. We also found that C60 led ENC to achieve higher yield strength and higher modulus and subsequently improves their recovery index. The highest enhancement in ƞs is observed for S-FEC providing an indication of better scratch resistance. Here, the self-depth recovery is found higher in case of S-FEC, which can be clearly connected to higher restriction to molecular relaxation at room temperature due to stiffness modulation at the juncture of S-C60 and epoxy network. Furthermore, the inability of the material to recover elastically, when the load is removed, said as plasticity is used to measure plastic energy (PE). The PE acts exactly opposite with respect to the ƞs (Figure 6b). NE shows maximum PE, whereas the S-FEC demonstrates the lowest. In Figure 6a, the value of ƞs shows almost a plateau toward a load around 20 ACS Paragon Plus Environment

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mN. Therefore, for the 20 mN load, the penetration and residual depth vs. scratch distance graph have been plotted (Figure S5, supporting information). The variation of COF along scratch distance during the scratch measurements at the 20 mN load is also shown in supporting information (Figure S6). The scratch hardness (Hs), wear rate (Q) and wear resistance (Rs) are measured at 20 mN load accordingly. The Hs was calculated from the NS data and the scratch width was determined using Equation (3) and shown in Figure 6c also tabulated (Table S2, supporting information). The lowest Hs value is observed for NE, whereas S-FEC showed the highest depicting ~ 22% and ~ 19.7% enhancement for P-FEC and O-FEC. Incorporation of PC60 and OC60 into the epoxy increases the resistance of the material against the penetration of sphero-conical indenter as schemed in Figure 7. Here, the enhancement in Hs is found trivial for O-FEC compared to P-FEC, despite the fact that OC60 are distributed in chain-like structures in the epoxy network. Probably the disordered structures of OC60 and ineffective interaction with epoxy led the sliding and rollover of OC60 (see the scheme in Figure 7c). Also, chances that oxidation of fullerene damage its cage-like structure and could limit its inherent strength and stiffness. This disadvantages make OC60 incapable to offer resistance during indenter progress. In contrast, enhancement of Hs by ~31% is observed for S-FEC. Silane molecule has the capability to conceal the defects generated during oxidation of carbonaceous fillers 36 observed in our case also as discussed earlier and reported by Jiang et al. also 58. This capability help to regain the inherent strength of fullerene molecules to perform stiffness modulation at the interfaces. Thus, SC60 are able to control strain localization, and hence restrict deformation of the matrix especially at the epoxy SC60 juncture. Furthermore, adjacent interfacial matrix can restrict the flexibility of SC60 particles to slide or roll over when indenter is moving (see the schemed in Figure 7d). Thus, unlike CNTs 40,41 and Halloysite nanotubes (HNTs) 54, if stiffness adjacent to SC60 and matrix is modulated it will undergo compression only, whereas CNTs and HNTs will undergo buckling, bending and radial compression. This orientation factor moderation to scratching could be neglected in our case and can be an effective solution for industries. The wear rate (Q) characterizes the wear resistance was calculated from the NS ACS Paragon Plus Environment


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data and scratch geometry using Equation (5) and the results are shown in Figure 6c. Lesser the wear rate, material is expected to have higher scratch resistance. The NE undergoes a heavy wear rate (Q=42.07 µm2) in comparison to the ENC. The wear rate of P-FEC and O-FEC are found as 30.51 µm2 and 30.95 µm2 respectively. On the other hand, the S-FEC shows the minimum wear rate (Q=26.66 µm2). The wear rate is further utilized to find out the wear resistance of the ENC. The resistance to wear of a material is depending on its hardness and wear coefficient. The wear resistance is found minimum for NE and maximum for S-FEC (Figure 6c). To investigate the penetration and scratch resistance behavior scratch tracts were analyzed under FESEM (Figure 8). As shown in Figure 7a, penetration depth and scratch resistance is depends upon normal force (Fn) and resistance to tangential force (Ft) during the sliding process. It expected to create compressive stress in the front of indenter tip and tensile stress behind thus creating a difference stress field. For all the epoxy system, matrix beneath the indenter is expected to deform both viscoelastically and viscoplastically due to this difference in stress field 59. The variation of coefficient of friction (COF, µs) along scratch distance (Figure S6, supporting information) is found localized throughout the length for NE. In this context, it is worthwhile to mention that the process of scratching takes place by a stick-slip manner and continuous sliding 37,40. The slip process occurred at the vicinity of the tip where micro-cracks were generated and piled-up materials were fractured. The sudden decrease in the resistance to Fn due to such slipping process, resulted in the formation of inverted spike (marked by square block (Figure S6, supporting information) 40. For P-FEC difference in µs with scratch distance is found lower at some places (encircled in Figure S6, supporting information) but mostly the difference is larger throughout the scratch tract. This behavior is found reduced for S-FEC. FESEM image in Figure 8a for NE shows ductile plowing with micron sized crater mainly due to the matrix plastic deformation and thus ineffective to reduce to penetration depth and scratch resistance. This type of deformations under spherical indenter is also reported by others 59,60. In addition, a liner border of the grove is observed (see scheme top right insert in Figure 8a). However, P-FEC demonstrates different ACS Paragon Plus Environment

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scratch behavior forming irregular scratch tract with micron craters. This irregular scratch tract is caused due to different stress field beneath as well as at the sides of the indenter during its movement. This type of scratch generally develops due to the easy de-bonding of agglomerated hard particles in thermosetting matrix also reported by kurkcu et al. 59 and Tahmassebi et al. 61 with micro and nanoparticles. Thus, easy de-bonding of aggregated PC60 results in piling up of matrix material due indenter progression and hence form edge cracking with irregular scratch tracts (schemed in top right corner of Figure 8b). Whereas, Figure 8c reveals a dissimilar scratch tract for S-FEC for all indenter load variation. High magnification FESEM image of 20 mN load scratch tract of S-FEC demonstrates ripple marks (Figure 8d) also schemed in top right cornor in Figure 8d. It can be seen that more nanocracks are present for S-FEC indicating harder material surface and will happen only if the stifness is modulated at the juncture of SC60 and epoxy. This modulated stifness at the interface enables chipping of the epoxy matrix due to the bridging effect under differential stress field leading to higher indenter retardation. However, the scratching behavior may vary depending upon the scratch conditions or type and geometry of the scratch indenter 60,62. The ability of the ENC to resist penetration against external load is further evaluated from the Vickers hardness test 41,53. The Vickers hardness (Hv) of NE is measured as 19.71 ± 0.24. For P-FEC and OFEC, the Hv is found akin to that of the NE (Figure 6d). Uneven distribution of fullerene C60 due to their agglomeration might have limited the improvement in Hv. In contrary, a significant improvement in Hv (~ 19 %) is found for S-FEC. This improvement for S-FEC is mainly attributed to the homogeneous distribution and formation of percolating interface between SC60 and epoxy network 33,41. Comparing the limited reports 53,63, we found similar improvement in hardness but having decreased mechanical properties. For example, Kin-tak Lau et al. 63 showed ~19% improvement in hardness of epoxy having 2 wt% of CNTs but ~ 32 % decrease in flexural strength. Similarly, bare nanodiamonds had increased the hardness by ~20% but limit the fracture toughness by ~8% with 0.5 wt% of addition



53

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. This work elucidates SC60 as advanced functional material to improve resistance to scratch and

penetration in both nano and microscale. CONCLUSIONS Current findings edify the significant enhancement in anti-scratch performance without sacrificing the tensile strength, stiffness, and fracture properties of epoxy composite, if SC60 is used. Remarkable enhancement in tensile strength, elastic modulus and fracture toughness for ENC with only 0.5 wt% of SC60 is achieved. This improvement in mechanical properties of S-FEC confirms the ameliorating effect from interfacial interaction of SC60 with epoxy network. Fracture investigation showed enhanced matrix yielding with decreased de-cohesion zones between SC60 and matrix for S-FEC as compared to NE. Silane oligomers from SC60 covalently bonds with epoxy network resulting in chain segment mobility restriction and epoxy network coordination that enhances the anti-scratch performance. Proof by investigating the nanoscratch tracts under FESEM demonstrates generation of ripple marks with enhanced nanocracks for S-FEC, making them harder to scratch. This study projects the use of SC60 as an advanced industrial friendly reinforcement in epoxy for a wide variety of applications where both structural stability and anti-scratch permanence needs to be ensured. ASSOCIATED CONTENT Supporting Information is Available FESEM images of pristine and functionalized fullerene, Storage modulus vs temperature curves of epoxy nanocomposites, tensile stress-strain curves of epoxy nanocomposites, Tensile and SENB results of epoxy nanocomposites, FESEM fractography images of tensile fractured samples of epoxy nanocomposites, Variation of depth with scratch distance plots of epoxy nanocomposite as obtained from the nanoscratch test under the applied load of 20 mN, Variation of COF and scratch depth with scratch distance of epoxy nanocomposites, and Scratch hardness (at 20 mN load), wear coefficient (k), wear rate (Q), wear resistance coefficient (Rw), of the epoxy nanocomposites. AUTHOR INFORMATION ACS Paragon Plus Environment

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Corresponding Author *Email: [email protected], [email protected], Tele: +91 3842 241313, Fax: +91 3842 224797 ORCID id Subhankar Das: 0000-0002-1724-2722 Sudipta Halder: 0000-0002-0505-6172 Arijit Sinha: 0000-0002-2188-3717 Muhammad Ali Imam: 0000-0002-4465-5362 Nazrul Islam Khan: 0000-0003-3453-2200 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the Department of Science and Technology, India under DST-FIST program 2014 with Grant No. SR/FST/ETI-373/2014. The authors also want to thank NMHS project with Grant No. NMHS/2016-2017/SG 18/07 for financial support. This work was initiated under the project head ‘‘Synthesis and Fracture Property Evaluation of Polymer Nanocomposites” supported by National Institute of Technology Silchar, Assam, India (Project number (RC)/457/122).

Figure Captions Figure 1. Schematic representation of the oxidation of PC60 (Step-1) and silanization of OC60 (Step-2) and Fabrication of epoxy nanocomposite induced with SC60 and schematic representation reaction mechanism through covalent bond formation between epoxy network silane functional moieties present on SC60.



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Figure 2. TEM images showed the (a) aggregated spherical shaped PC60 (b) Chain networks of OC60, (c) enlarged view of OC60, representing the defects present on the structure by black arrow mark, and (d) chain like structure of SC60 with enhanced size. Figure 3. Representing variation in specific heat capacity (∆Cp) and cooperatively rearranging region (CRR) for epoxy nanocomposites in terms of chain length segment from glass transition temperature (Tg) from DMA results. Figure 4. (a) Tensile properties of epoxy nanocomposites, and (b) fracture toughness and strain energy release rate of epoxy nanocomposites. Figure 5. (a) FESEM fractography of tensile fractured samples of NE representing brittle failure, (b) PFEC is revealing deboning of aggregated PC60 as indicated by black arrow, (c) O-FEC is revealing formation of enhanced nano-cavities, and (d) S-FEC showing enhanced matrix yielding behavior that produces tortuous fracture profile. Figure 6. (a) Variation of scratch recovery index with respect to loads, (b) Variation of plastic energy with respect to loads, (c) Scratch hardness (Hs), wear rate (Q), and wear resistance coefficient (Rw) of the nanocomposite samples, and (d) Micro hardness results of the nanocomposite samples. Figure 7. (a) Scratching process of epoxy with sphero conical indenter where Fn represent normal load, Ft represent tangential load (b) Interaction of indenter tip with aggregated PC60 resulting in compression of PC60 during penetration and rotation of PC60 during sliding (c) Interaction of indenter tip with OC60 chains and (d) bridging of SC60 with epoxy network and their interaction with indenter. Figure 8. FESEM micrograph of scratch track of (a) NE, (b) P-FEC, (c) S-FEC at low magnification and (d) S-FEC at high magnification showing ripple mark with enhanced nano cracks.


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Figure 1

Figure 2



Figure 3

Figure 4


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Figure 5

Figure 6



Figure 7

Figure 8


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Assessing Nano Scratch Behavior of Epoxy Nanocomposite

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