Enhanced Wear Resistance of Transparent Epoxy Composite

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Enhanced Wear Resistance of Transparent Epoxy Composite Coatings with Vertically Aligned Halloysite Nanotubes Kenan Song, Dayong Chen, Roberta Polak, Michael F. Rubner, Robert E. Cohen, and Khalid A. Askar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11872 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 3, 2016

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ACS Applied Materials & Interfaces

Enhanced Wear Resistance of Transparent Epoxy Composite Coatings with Vertically Aligned Halloysite Nanotubes Kenan Song *,§,#

§

§,†

, Dayong Chen

§,‡

, Roberta Polak

†,§

, Michael F. Rubner

*,†,&,^

, Robert E. Cohen

, Khalid A. Askar *,◊

Department of Chemical Engineering, Massachusetts Institute of Technology (MIT), 77 Mass

Ave, Cambridge, Massachusetts 02139, United States †

Department of Materials Science and Engineering, Massachusetts Institute of Technology

(MIT), 77 Mass Ave, Cambridge, Massachusetts 02139, United States &

Center for Materials Science and Engineering, Massachusetts Institute of Technology (MIT),

77 Mass Ave, Cambridge, Massachusetts 02139, United States ‡

Department of Mechanical Engineering, Massachusetts Institute of Technology (MIT), 77 Mass

Ave, Cambridge, Massachusetts 02139, United States ◊

Department of Materials Science and Engineering, Masdar Institute of Science and

Technology, Abu Dhabi, United Arab Emirates

1 Environment ACS Paragon Plus


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ABSTRACT The influence of nanoparticle orientation on wear resistance of transparent composite coatings has been studied. Using a nozzle spray coating method, halloysite nanotubes (HNTs) were aligned in the in-plane and out-of-plane directions, and in various randomly oriented states. Nanoscratching, falling sand and Taber Abrasion tests were used to characterize the wear resistance at different length scales. Composites consistently displayed better wear resistance than pure epoxy. Samples with out-of-plane particle orientations exhibited better wear-resistant behavior than those with in-plane particle distributions. In nanoscratching tests, the out-of-plane orientation decreases the normalized scratch volume by as much as 60% compared to pure epoxy. In the falling sand and Taber Abrasion tests, out-of-plane aligned halloysite particles resulted in surfaces with smaller roughness based on stylus profilometry and SEM observations. The decrease in roughness values after these wear tests can be as large as 67% from pure epoxy to composites. Composites with higher out-of-plane particle orientation factors exhibited better light transmittance after sand impingements and other wear tests. This study suggests a useful strategy for producing material systems with enhanced mechanical durability and more durable optical properties. KEYWORDS scratching, falling sand, Taber Abrasion, wear, alignment, transparent, composite, HNTs

INTRODUCTION Halloysite nanotubes (HNTs) are naturally occurring aluminosilicate layers rolled into a hollow tubular nanostructure with high aspect ratios. Compared to carbon nanotubes they have better thermal stability, they form better dispersions in polymers, they are abundant and are very cheap1. It has been demonstrated that the hollow lumen of HNTs can be loaded with different compounds thus enabling them to serve as nanoreactors (to fabricate nanowires2 or nanoparticles3), nanocontainers4-8 (for the controlled release of corrosion inhibitors or protective agents) and drug delivery systems9-11. HNTs also have excellent mechanical properties individually and depending on the loading amount, aspect ratio of the tubes, dispersion quality and the compatibility between the HNTs and the polymer matrix, the overall mechanical properties of the polymer composite can be significantly enhanced12. Therefore, HNTs have attracted research attention as a good reinforcing material to strengthen and toughen polymer composites. HNTs have been loaded into several different polymers including epoxy13, polypropylene14, polyurethane15, and chitosan16. However, to the best of our knowledge there have not been any studies correlating the orientation of the filler material in the composite with tribological properties, particularly wear resistance. Our group has recently reported on the fabrication of epoxy-based transparent composite coatings consisting of controllably aligned halloysite nanotubes (HNTs).17 The alignment of the HNTs was facilitated by the hydrodynamic flow established during spraying and tailored by varying the viscosity of epoxy/HNT suspensions. Higher viscosities led to better nanoparticle orientations along the plane-normal direction, resulting in significant reinforcement improvements in stiffness and hardness. This process revealed the potential of nanoparticle


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orientation control in improving both modulus and hardness in material systems. Due to the refractive index matching between the HNTs and polymer matrix (n ~1.5 for both), the resultant composites display a high transparency of 90% even at a high halloysite concentration of 20 wt%. These transparent composites are very attractive for fabricating multi-functional coatings.18-21 It is known that surface wearing will decrease the transmittance and weaken or even eliminate desirable surface functions of transparent material systems.22 Therefore, it is important to characterize the wear behavior of these nanocomposite coatings in conjunction with a quantitative assessment of how wear influences their optimal performance. Development of transparent and wear-resistant composite materials is of great significance in maintaining mechanical durability of optical surfaces (e.g., lenses, photographic film, windshields, goggles, energy cell panels, and packaging).23 Currently utilized transparent polymers, such as polycarbonate (PC), polystyrene (PS), and poly methyl methacrylate (PMMA), are criticized for their susceptibility to scratching and abrasion.24 Inclusion of hollow or mesoporous silica nanoparticles using sol-gel coating methods has been used to improve the mechanical robustness in transparent coatings.25 For instance, a transparent coating based on a single-layer hollow silica was found to endure sand abrasion of 20 g falling from a 40 cm height,26 while fluorosurfactant-coated silica aerogels retained their super-hydrophobic or superoleophobic property upon several-cycle sandpaper wearing,27 3H-4H pencil scratching tests,28 or 500 laundry washing cycles.29 Even though most of these coatings display high light transmittance or well-controlled wettability, the fading mechanical reliability over time is still the main factor limiting their usage in practical applications. For example, photovoltaic modules operating in medium to high soiling environments are subject to airborne particle abrasion and repeated washing, and would lose the long-term mechanical durability and anti-reflection capability.30 Products, such as solar roadways consisting of specially engineered solar panels utilized in the Netherlands31 and launched in the U.S32, working under more severe working conditions have appeared. These applications will have to bear heavy traffic and require much higher wear resistance. Unlike the modulus and hardness values frequently measured from linear elastic regions in material systems, wear resistance parameters are more related to interactions between surfaces specifically the removal and deformation of material on a surface as a result of mechanical action of the opposite surface.33 The definition, measurement, and evaluation, as well as mechanism explanation of wear-resistance have been challenging.34 For instance, wear has been described variously in terms of abrasion, adhesion, corrosion, and erosion, and evaluated by gloss reduction, topography change, deformation, roughness, and weight loss.35 In addition, it is difficult to obtain acceptable accuracy and reproducibility at multi scales due to limitations in different instrument types36 and difficulties in micro- and nano-structural controls. The degree of wear in nanoparticle-filled composites depends on several factors, namely, (i) physical and chemical compositions of the materials, such as dispersion quality, interfacial interactions, particle volumes, and particle orientations.37 The measurement of wear resistance can become very unreliable for polymer composites.38-39 At the nano-scale, crystalline and amorphous regions in semicrystalline polymers and nanoparticles may generate divergent resistance to wearing.36 At micro- and macro-scale, multiple combinations between polymer chain conformations and nanoparticle configurations may produce dissimilar or opposite trends;36 and (ii) test environmental parameters, including scratching tip geometry, contact conditions determined by the applied normal and friction force, temperature, wearing rate, and so on.39



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Therefore, it is of paramount importance to control both the material structure and test conditions for wear resistance consistency.40 In thin coatings, it has been challenging to achieve out-of-plane alignment of nanoparticles,17 thus the influence of such an alignment on anti-wearing behavior has rarely been studied.41-42 In this research, we demonstrate that nanoparticle orientation affects the wear resistance of the epoxy/halloysite nanocomposites produced in our previous work.17 Nano-scale scratching, micro-scale falling sand, and macro-scale Taber Abrasion characterizations were used to measure wear resistance. Nanoindentation was performed to reveal the correlation of wear volume to dissipation energy, plasticity index, and post-scratch recovery. Stylus profilometry, atomic force microscopy (AFM), and scanning electron microcopy (SEM) were used to characterize the surface roughness after a falling sand and Taber Abrasion test; UV-Vis spectra were used to determine changes in light transmittance. These combined studies reveal that composites with suitably aligned halloysite nanotubes exhibit improved wear resistance under a variety of aggressive mechanical challenges. RESULTS AND DISCUSSION Screening of transparent polymer coatings Commercially available transparent polymers include polycarbonate (PC), polystyrene (PS), poly methyl methacrylate (PMMA), and some epoxy systems. Among them, epoxy materials are well known for their superior temperature and chemical resistance as well as excellent adhesion to substrates such as glass and other polymers.37, 43-44 In addition, due to their excellent electrical insulation, thermal stability, low shrinkage, and low cost, epoxy resins have been widely used as coatings, adhesive materials, and resin matrices for advanced composites.43 In initial assessment tests, the scratch resistance of PC (Mw ~31750), PS (Mw ~35000), PMMA (Mw ~540000) and epoxy (EPO-TEK® OG142-112, see experimental section) coatings was measured using nanoscratching tests. The scratched volume normalized by the normal force and the scratching length was used to characterize wear resistance (see experimental section). The smaller the normalized scratch volume values, the more scratch-resistant the coatings are to wear. The friction coefficient is defined by the steady-state ratio between the tangential force and the normal force during scratching. The normalized scratch volumes and friction coefficient parameters are plotted in Figure 1 (representative plots of tangential force vs. time appear in Figure S1). The epoxy showed the highest scratch resistance and lowest friction coefficient. In addition, solvent resistance tests showed that PC, PS, and PMMA debonded from glass slides overnight in solvents including acetone, ethanol and water, while the epoxy coating only exhibited slight swelling on the glass-slide edge. To improve the wear resistance of these epoxy coatings, halloysite nanotubes were added as nanofillers. It has been found that composite materials filled with nano-scale particles (i.e., nanotubes, nanospheres and nanoplatelets) can exhibit a lower percolation threshold than materials loaded with traditional macro-scale fibers35. For example, glass fibers and carbon fibers increased mechanical and tribological properties to a maximum level at concentrations between 10 vol% to 20 vol%. In contrast, nanosilica and carbon nanotubes can significantly lower friction coefficients at concentrations as low as 0.5 vol%35. Nanoparticle-containing epoxy composites have been extensively studied regarding efficient mechanical reinforcement and toughening


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mechanism.45-47 Our previous study of epoxy/halloysite systems indicates that the mechanical percolation threshold is about 1.0 vol%.17 We therefore set out to determine how wear resistance was modified at these same low concentrations.

Figure 1. (a) Scratched volumes normalized by normal load and scratch length, and (b) friction coefficient for PC, PS, PMMA, and epoxy measured in scratching tests under a normal load of 200 µN with a scratch length of 10 µm. Nano-scale wearing of scratching Improved scratch resistance is indicated by smaller damage regions, more recovery, and higher energy-dissipation values. The nano-scale wear resistance of the epoxy composites was tested by using the Hysitron TriboIndenter. The variations of normalized scratch volumes from pure epoxy coatings to composites with different halloysite orientations are shown in Figure 2. The Krenchel orientation factor (ηo) was calculated17 and is listed along with the sample nomenclature used in this paper in Table 1. The results in Figure 2 displayed the usual load-correlated trend, i.e., higher normal loadings led to more severe damage to coatings, corresponding to a lower scratch resistance. The material with the lowest normalized scratch volume was found to be composite C (Krenchel orientation factor, η o=0.678), independent of the applied normal force. A further increase of the halloysite orientation did not improve the scratch resistance due to the presence of aggregates. This phenomenon has been explained in our previous study.17 The normalized scratch volumes were also studied using the analysis of variance (ANOVA) method. The differences among groups were significant, as shown in Table S2 and Figure S2. Our previous study17 showed that the mechanical properties do not change when the HNT concentrations go beyond the percolation threshold (1.0 vol%). Similar results were found with tribological behavior, that is, the scratch volumes under the same load did not change much with concentration beyond the mechanical percolation threshold (see Figure S3 and Figure S4). Table 1. Sample nomenclature17 Name Epoxy Composite A Composite B

Concentration of halloysite (vol%) 0 1 1


Krenchel orientation factor N/A 0.084 0.355


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Composite C Composite D

1 1

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0.678 0.795

Figure 2. Normalized scratch volume vs. normal force relationship for pure epoxy and 1 vol% HNT composites, with open square symbols (☐) standing for epoxy and filled marks for composites. Krenchel orientation factors for halloysite nanotubes (HNTs) are associated with processing compositions, as listed in Table 1. The surface morphology of the scratched samples was characterized by atomic force microscope (AFM) 3D imaging and is shown in Figure 3. Figures 3a1 and 3b1 show the scratching under a loading of 100 µN and Figures 3c1 and 3d1 depict the scratching under a 500 µN load for pure epoxy and composite C (ηo=0.678), respectively. Under a normal loading of 100 µN, the pure epoxy coating displayed more distinctive plough marks than those in the HNTs-filled composites, indicating more abrasion wearing (red arrows in Figure 3). The epoxy coating was so fragile that it cracked and broke away from the surface of the specimen; suggesting a brittle cleavage behavior. In contrast, the composite C (η o=0.678) showed a relatively smoother surface upon scratching. Scratching under the 500 µN load, as compared to the 100 µN load, fractured both pure epoxy and composites more severely. As shown in Figures 3c1 and 3d1, the accumulation of wear debris was higher than that revealed in Figures 3a1 and 3b1 on both sides of the sliding traces. Corresponding topographic (a2 to d2), phase images (a3 to d3), and profiles (a4 to d4) are plotted. The differences under specific loadings between epoxy and composite C (ηo=0.678) from the profile imaging clearly suggested better resistance to scratching in composites.


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Figure 3. 3D profile imaging from atomic force microscopy (AFM) for pure epoxy coating and 1 vol% concentrated composite C (ηo=0.678) under (a1-b1) 100 µN, (c1-d1) 500 µN. The scanning size is 5 µm · 5 µm. Corresponding topographic (a2 to d2), phase images (a3 to d3), and profiles (a4 to d4) are plotted.



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Figure 4a demonstrates the loading-unloading indentation curves of various samples, revealing the mechanical properties and energy dissipation capacities. The indentation depth was set to be 100 nm, less than one-tenth of the coating thickness. The hardness (H) and reduced elastic modulus (Er) of each coating were obtained from loading-unloading curves by applying the Oliver-Pharr method.48 As listed in Table 2, there is an increase in average H and Er from epoxy to composites; and the improvement was enhanced as the particles became more vertically oriented. It is well established that the elastic strain and plastic strain are proportional to H/Er and H3/Er2 values, respectivily.49 Therefore, composite C (ηo=0.678) is expected to present the best tribological performance due to a coexistence of the highest H/Er and H3/Er2 values. This is also consistent with the decreasing plasticity index (ψ, as defined by (wir)/(wir+wr), wir and wr are irreversible and reversible work done during mechanical tests, see experimental section of characterizations and Figure 4a) with better out-of-plane particle alignment as shown in Figure 4b and Table 2. A typical hysteresis loop appears as an insert in Figure 4a, and the energy converted from mechanical work to heat during the loading-unloading procedure is termed as dissipation energy, as calculated and plotted in Figure 4c.50 Dissipation energy values reflect the plasticity quantity, but it is important not to confuse plasticity values with plasticity index (Figure 4b). The increase of plasticity, correspondingly dissipation energy, may accompany a higher degree of elasticity increase and lead to decrease of plasticity index, which is the case in this study (reverse trends in Figure 4b and 4c). It is evident that as the particle orientation factor increases, the composites displayed more distinctive hysteresis loops (Figure 4a). For example, the dissipated energy of pure epoxy was 5.11 ×106 J/m3. In contrast, the composite B (ηo=0.355) and composite C (η 6 3 o=0.678) were 10.39 and 11.34 ×10 J/m , respectively (Figure 4c). These results indicate that the composites can dissipate more energy (larger area in the hysteresis loop) than the pure epoxy when the samples are deformed to the same level of displacement; moreover, in composites, nanoparticles with better out-of-plane alignment enlarged the energy dissipation capacities. For reaching the same strain, the increasing dissipated energy is beneficial for maintaining the material structural integrity because more energy will be converted into heat instead of transforming into cracking or degrading the impacted surface.50 This increasing trend in area of hysteresis loops from pure epoxy to composites is revealed further in multi-cycle loadingunloading tests under different strains, as presented in Figure S5. It can be seen clearly that when particles are aligned, the repeated loading-unloading tended to preserve the hysteresis loop area while pure epoxy displayed decreasing hysteresis loop size. Higher self-recovery percentage is another indicator of better wear resistance.51 Figure 4d illustrates the penetration depth recovery (PDR) vs. HNT orientation factor relationship. PDR was defined by the bounced-back-depth after scratching (after a 7 day period, see experimental section) divided by the in-situ scratching depth. The self-recovery depends on the soft components (epoxy) and the hard filler particles (HNTs). The temporary shape of the composites is fixed in the soft epoxy that consists of polymer chains, while the permanent shape is determined by the hard segment of nanotubes.52 Thus the self-recovery percentage of all samples increased significantly with the increase of nanoparticle orientation (Figure 4d). At the normal load of 100 µN, the self-recovery for pure epoxy is 60% while it is up to 80% for composite C ( ηo=0.678). These trends are consistent with the observed normalized scratch volume changes (Figure 2) and morphology/topography features (Figure 3). The higher the recovery percentage, the more wear-resistant a sample is.


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A proposed recovery mechanism is illustrated in Figure 4e. In Figure 4e, the vertically aligned nanoparticles sustained major normal loading in the force-controlled nanoscratching test, experiencing much smaller strain than that in horizontally aligned particles. For vertical nanoparticles going through the same strain as epoxy, the modulus is calculated by Ecomposite=EHNT-axialVHNT+ EEpoxyVEpoxy, where E is the modulus and V is the volume fraction. However, for horizontal nanoparticles experiencing the same stress as epoxy, the modulus is computed following Ecomposite=1/(1/EHNT-radialVHNT+ 1/EEpoxyVEpoxy) ≈ EEpoxy. The HNT modulus is on the order of several GPa in the radial direction (EHNT-radial), two orders of magnitude smaller than that along the axial direction (up to 560 GPa, EHNT-axial).53 Therefore, the indentation tip penetrates the epoxy materials when HNTs are aligned horizontally, resulting in cracking and other failures with breakage due to the weakness of polymer and the nanotube along axis normal to the load.54 On the contrary, out-of-plane aligned particles experience bending and buckling upon normal loading and recover more to the original state when the load is released. (Figure 4e). Table 2. Average mechanical properties of coatings in loading-unloading indentation cycle Krenchel orientation H3/Er2 H/Er Ψ (%) H (GPa) Er (GPa) factor, ηo (GPa) Epoxy 0 0.2 4.8 4.2 · 10-2 3.5 · 10-3 76.7 Composite A 0.084 0.4 6.3 6.3 · 10-2 1.6 · 10-3 70.8 -2 -3 Composite B 0.355 0.5 6.6 7.6 · 10 2.9 · 10 69.2 Composite C 0.678 0.6 7.6 7.9 · 10-2 3.7 · 10-3 68.0 -2 -3 Composite D 0.795 0.4 6.8 5.9 · 10 1.4 · 10 69.3 Note: H and Er are the hardness and reduced modulus from nanoindentation. Ψ, plasticity index, is defined as the ratio between irreversible work and total work done in a loading-unloading cycle. See experimental section. Coatings



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Figure 4. (a) Loading-unloading cycle in indentation, (b) plasticity index (Ψ, defined by wir/(wir+wr) as shown in a), (c) dissipation energy, and (d) penetration depth recovery (PDR) vs. Krenchel orientation factors of halloysite. Open marks stand for pure epoxy while the closed symbols are composites with HNTs concentration of 1.0 vol%. The figures display nanoparticle orientation-related trends, i.e., higher out-of-plane alignment of the HNTs demonstrated larger dissipation energy, lower plasticity index, and better self-recovery after scratching. (e) Scratching in composites with out-of-plane and in-plane nanoparticle alignment (schematic to scale, with semi conical tip diameter of 1 µm, HNTs average length of 2 µm and spacing of ~685 nm between nanotube centers). Single halloysite nanotube (length ~ 400 nm, out diameter ~ 40 nm and inner diameter ~ 15 nm) bending has been measured using transmission electron microscopy (TEM)55, showing an intact structure upon 90o bending and pure elastic deformation up to 5o bending.


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During the tribology test, friction coefficient variations can reflect micro- or nano-structural modifications in the coatings. To exclude the influence of surface roughness, all the sample surfaces were characterized using stylus profilometry as shown in Figure S6, which indicates limited variations among all samples. Figure 5 shows the friction coefficient as a function of particle orientation factors for both pure epoxy (empty marks) and 1 vol% halloysite composite samples (filled symbols). The friction coefficient parameters show an increase trend as in-plane aligned nanoparticles are included in epoxy. When HNTs are more out-of-plane aligned, the friction coefficient decreases. Such increasing-decreasing trends remain similarly in all loading conditions. Comparatively, higher loadings allow the indenting tip to penetrate deeper into samples, leading to an increase of friction coefficient. The effect of friction is to add a compressive stress to the front edge of the contact and intensify the tensile stress at the back edge with fracture events. Thus, a lower value of the friction coefficient correlates with improvements in scratch resistance.56 Similar research results of vertical nanoparticle alignment improving wear-resistance have been observed in single-wall carbon nanotubes57, multi-wall carbon nanotubes58-59, and carbon nanofibers60. However, in nanoparticle reinforced composites the influence of nanoparticle alignment on friction coefficient has been studied61 much less extensively.

Figure 5. Friction coefficient (µ) vs. Krenchel orientation factor relationship shows the influence of particle alignment on friction coefficient. In-plane orientations of HNTs increased the friction coefficient while out-of-plane HNTs alignments caused µ to decrease or remain similar as compared to µ in epoxy. Open marks are pure epoxy and closed symbols are composites. Micro-scale wearing of falling sand Surface Morphology Changes In outdoor applications, coated surfaces may need to survive harsh conditions such as sand and hailstorms. To simulate such events, falling sand tests were performed using sand grains ranging



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from 100 to 300 µm in diameter, impinging the surface from a height of h=36 inches (36in=91.4 cm) following ASTM D968. The conditions correspond to an impinging energy of ~10-10 J per grain and an impact velocity of 15.4 km/h. The ASTM D968 test employs a total volume of sand V=100 cm3.62 Before the falling sand tests, the roughness values (root mean square, RMS) for all coated surfaces were smaller than 20 nm (Figure S6). After the ASTM standard test, coating surfaces were roughened by different degrees. Figure 6a reveals lower roughness in halloysite composites compared to pure epoxy coatings. Particle alignment had no effect on the falling sand resistance. These coatings were further tested under more intense conditions of falling sand, as shown in Figure 6b (h=36 inches, V=100 cm3 and Temperature T=65 oC, sand grain velocity 15.4 km/h), Figure 6c (h=60 inches, V=500 cm3 and Temperature T=25 oC, sand grain velocity of 21.6 km/h), and Figure 6d (h=60 inches, V=500 cm3 and Temperature T=65 oC). Higher volume and increased height of the falling sand as well as elevated test temperature increased the surface roughness. Among these tests, composites exhibited lower roughness compared to the unmodified epoxy. These surface roughness variations were also confirmed by contact angle measurements, as shown in Figure S7. Vertically aligned particles protected interior surface structure from sand abrasion, and this effect became more distinctive in more severe falling sand conditions (Figures 6b to 6d). The decrease of roughness can be up to 67% from epoxy to composites (Figure 6a), and roughness decreases range between 40% and 60% (Figures 6b to 6d). SEM images were taken on sand impinged coated surfaces. For the case of the more severe conditions with 500 cm3 sand falling from 60 inches at a surface temperature of 25 oC. The morphologies of the worn surfaces are displayed in Figure 7. The worn surface of epoxy was rougher than those of the composites. Some of the sand particles were entrapped in the impinged surfaces, as evidenced by the Energy-dispersive X-ray spectroscopy (EDS) mapping in Figure S8.


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Figure 6. Falling sand induced surface roughness vs. HNT alignment relationships plotted for (a) the ASTM D968 standard set-up with a falling sand height of 36 inches and sand volume of 100 cm3 at room temperature, modified ASTM D968 standard set-up with (b) falling sand height of 36 inches and sand volume of 100 cm3 at elevated temperature of 65 oC, (c) falling sand height of 60 inches and sand volume of 500 cm3 at room temperature of 25 oC, and (d) falling sand height of 60 inches and sand volume of 500 cm3 at elevated temperature of 65 oC. Roughness was measured by stylus profilometry and extracted using root mean square (RMS) method. Open marks are pure epoxy and closed symbols are composites with 1.0 vol% HNTs.



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Figure 7. SEM images showed the influence of particle alignment on falling sand resistance. The HNTs concentration is 1.0 vol%. (a) Epoxy, (b) composite A (ηo=0.084), (c) composite B (ηo=0.355) , (d) composite C (ηo=0.678), and (e) composite D (ηo=0.795). (a2-e2) is the zoom-out of (a1-e1) regions. ASTM D968 standard set-up was modified with falling sand height 60 inches and sands volume 500 cm3 at room temperature of 25 oC.


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Figure 8. UV-Vis measurement of transmittance for (a) as-prepared samples, (b) 100 cm3 falling sand from 30 inches height at coating surface temperature of 25 oC, (c) 100 cm3 sand falling from 30 inches height at coating surface temperature of 65 oC, (d) 500 cm3 sand falling from 60 inches height at coating surface temperature of 25 oC, (e) 500 cm3 sand falling from 60 inches height at coating surface temperature of 65 oC, and (f) decrease of transmittance measured at wavelength of 500 nm for samples with different particle orientation factors. Optical Property Changes The composites showed high transparency up to about 91% even at a HNT concentrations of 20 wt% due to the refractive index matching between the epoxy matrix (1.537) and the HNTs (1.534).17 Increases in surface roughness and damage due to sand impingement would be expected to cause light scattering and thus decrease the light transmittance. Following the ASTM



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standard in the falling sand test (Figure 8b), 100cm3 sand falling from 36 inches did not influence the optical transmittance significantly. As shown in Figures 8c to 8e, the most severe falling sand test (corresponding to Figures 6b to 6d) brought the light transmittance down from 91% (Figure 8a) to 80-85% (Figure 8e). All of these UV-Vis transmittance tests show a consistent trend that higher out-of-plane particle alignment leads to a less damaged surface and better optical transmittance (Figures 8a to 8e). The corresponding decrease in transmittance is plotted in Figure 8f. This is also consistent with the roughness measurements and SEM morphologies in Figures 6 and 7, that is, more roughened surfaces scatter more light and lead to less transmittance. Macro-scale wearing of Taber Abrasion ASTM-D4060 standard Taber Abrasion tests were used to explore the macro-scale damage resulting from abrasion of the epoxy-based coatings. As shown in Figure 9a, 1 vol% composites with higher out-of-plane particle alignment tend to display better resistance to Taber abrasion. With the progression of more abrasion cycles, the roughness values kept increasing up to cycle 1000, after which more abrasion cycles did not further roughen the surface. The changes in the surface roughness were also confirmed by contact angle measurements for samples with 2000 cycles of abrasion, as shown in Figure S7. The weight loss per cycle showed the same trend as roughness variations; for example, at 50 abrasion cycles the composite C (ηo=0.678) lost 1 µg per cycle while pure epoxy coatings lost on average 13 µg (Figure 9b). With the increase of abrasion cycles, the differences between composites and pure epoxy became smaller due to the polishing effects. Figure 10 shows the SEM images of worn surfaces after 2000 abrasion cycles. Rougher and more deeply fractured surfaces were found in the pure epoxy coating, with scratches, gouges, and scoring marks on the worn surface; while composite C (ηo=0.678) was in contrast much smoother and intact. In tribology studies, researchers attribute the former failure mode to both cutting and plowing (i.e., material removal and plastic deformation) while the latter to only plowing (i.e., plastic deformation)38. Both the falling sand and the Taber Abrasion tests displayed consistent results proving that composites with tailored nanoparticle alignment are more wear-resistant. No significant differences were observed between pure epoxy and composites regarding their optical transmittance after Taber Abrasion tests (see Figure S9).

Figure 9. Taber abrasion generated (a) surface roughness vs. HNTs alignment for 10 cycles, 50 cycles, 500 cycles, 1000 cycles, and 2000 cycles when the thickness of the films were slightly


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abraded. Roughness was measured by stylus profilometry and extracted using root mean square method. (b) Weight loss per abrasion cycle.

(a1)

(a2)

50 µm (b1)

1 µm (b2)

50 µm (c1)

1 µm (c2)

50 µm (d1)

1 µm (d2)

50 µm (e1)

1 µm (e2)

50 µm

1 µm

Figure 10. SEM images showed the influence of particle alignment on Taber Abrasion resistance. The HNTs concentration is 1.0 vol%. (a) Epoxy, (b) composite A (ηo=0.084), (c)



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composite B (ηo=0.355), (d) composite C (ηo=0.678), and (e) composite D (ηo=0.795). (a2-e2) is the zoom-out of (a1-e1). Taber Abrasion was conducted with 1kg weight and 2000 cycles. Both the falling-sand test and Taber Abrasion tests have shown the consistent trend of improved wear resistance as long as vertical out-of-plane particle alignment can be achieved. The use of halloysite in outdoor environment has led to questions regarding toxicity and environmental issues. Halloysite nanotubes used in this research have been found to be nontoxic in natural usage and biocompatible in cellular and tissue environments21, 63-65, which opens up significant application opportunities. CONCLUSIONS The structural, mechanical and tribological properties of epoxy/HNT composite coatings with different nanoparticle alignments were investigated using nanoscratching, loading-unloading nanoindentation, falling sand, and Taber Abrasion methods to determine the preferable particle orientations for enhanced wear-resistance. Compared to unmodified epoxy, out-of-plane oriented nanoparticles resulted in lower plasticity index, higher self-recovery, and essentially unchanged friction coefficients. Consequently, composites with such alignment presented a lower scratching volume in nanoscratching. Moreover, at larger abrasion length scales, falling sand and Taber Abrasion tests demonstrate that lower abrasion roughness in nanocomposites can be achieved with a higher degree of out-of-plane particle alignment, consistent with the observation from nanoscratching. SEM and UV-Vis spectra obtained after testing provided evidence of smoother morphology and higher light transmittance for composites with vertical alignment compared to horizontal alignment of HNTs. In conclusion, this research demonstrates that the optimal nanoparticle arrangement for polymer-based composites to resist wear is vertical alignment. ABBREVIATIONS HNTs, halloysite nanotubes; AFM, atomic force microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy; PC, polycarbonate; PS, polystyrene; PMMA, (poly(methyl methacrylate)); PDR, penetration depth recovery; PD, penetration depth; RPSD, residue post-scratch depth;

EXPERIMENTAL SECTION (Materials and methods)

Experimental details have been included in our previous ACS publication17 (“Spray-coated halloysite-epoxy composites: a means to create mechanically robust, vertically aligned nanotube composites”) and here it is very briefly summarized.


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Materials: DragoniteTM HNTs clay was obtained from Applied Minerals (density 2.54±0.03 g⋅cm-3, inner diameter 10-20 nm, outer diameter 40-60 nm, and aspect ratio ranges between 20 and 200. BET pore volume 20%, surface area up to 100 m2⋅g-1, refractive index 1.534). Epoxy 142-112 (EPO-TEK® OG142-112, purchased from Epoxy Technology, Inc., density 1.18 g⋅cm-3) and acetone (purchased from VWR, density 0.79 g⋅cm-3) were used as obtained. To compare epoxy 142-112 used in this research with other transparent polymers, polystyrene (PS, lot # MKBS6957V from Sigma Aldrich, CAS #9003-53-6, linear formula [CH2CH(C6H5)]n, average molecular weight 35,000, softening point 123-128 °C, density 1.06 g/cm3 at 25 °C, clarity 90%) and poly(methyl methacrylate) (PMMA, lot # 010908024 from Sigma Aldrich, CAS # 9011-147, linear formula [CH2C(CH3)(CO2CH3)]n, average molecular weight 540,000, glass transition point from DSC is 105 °C, density 1.56 g/cm3 at 25 °C, clarity 92%) were used to compare wear resistance from scratching test. PS and PMMA were dissolved in chloroform at a concentration of 10 wt% and coated on glass slides. Polycarbonate film (trademark of SABIC innovative plastics, linear formula [COOC6H5C(CH3)2C6H5O]n, glass transition 145 oC, melting temperature 225 oC, density 1.20 g/cm3 at 25 °C, abrasive resistance 10-15 mg/1000 cycles rom ASTM D1044 tests, clarity 88%) was used as is in scratching test. The wear resistance for PC, PS, and PMMA was compared with epoxy to give an example. Processing: The thin-film coatings were fabricated using spray coating method and cured using UV flood lamp, as detailed in the previous study17. The nomenclature and some of the corresponding Krenchel orientation factor parameters for HNTs particles are as listed in Table 1. Since both mechanical and tribological properties show percolation threshold at 1.0 vol%, beyond which concentration the material properties do not increase any more, only 1.0 vol% related composites were analyzed in the current study.



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CHARACTERIZATIONS The scratch test is a simple and rapid means to assess the abrasion resistance and interfacial adhesion of coatings with the substrate. Scratch tests include three procedures: (i) pre-scan under a small load, (ii) scratch, (iii) post-scan. A standard scratch test device measures both the tangential and normal loads generated during scratching and hence offers a good estimate of abrasion resistance of the material under study. The advantages of the scratch testing are (a) it induces the same stable stress field and deformation as does the abrading, (b) the results are reproducible, and (c) the critical loading (Fc), which means the load required to lift a coating off the substrate can be easily calculated from friction force. Here in this study, the tests were performed with a Rockwell-shaped diamond indenter tip (radius of semi-conical tip head is 1 um) continuously loaded with a normal force ranging from 100 to 500 µN. The transverse velocity of the sample to the diamond was 10 mm⋅min−1, and the loading rate, dFN/dx=10 N⋅mm−1. The scratches were 10 µm long. To ensure the homogeneity of HNTs and study the failure mode along various thicknesses in composites, 5 scratches with spacing of 2 µm were performed. Several relevant parameters are important in studying the composite fracture features and thus are defined in the following. During the scratch procedure, the tangential force, FT, is continuously measured and the friction coefficient is defined by,

µ=

FT FN

(Equation 1)

A plot of the friction coefficient against the sliding distance should approach an equilibrium value (after a short scratching-in period) as long as no drastic changes in either the scratch mechanisms or sample components during the course of the scratch occur. If, on the other hand,


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a transition from one scratch feature (e.g. microcracking) to another (e.g. plowing associated with material removal) takes place, this should be associated with a change in µ. An estimation of the normalized scratch volume, wr, can also be determined from the final parts of the scratches. When considering the loss in volume of the material over an average short distance of the scratch lave under an average normal load, FNave, that acted over this distance,

wr = ∆V / (FNavelave ) = Cr lave / (FNavelave ) = Cr / FNave

(Equation 2)

where Cr refers to the cross-section of the scratch shortly before its end. The cross section can be assumed to have the shape of a segment of a circle, with a chord length equal to w (residual width) and height equal to the scratch depth h, (Equation 3) Assuming further that FNave is nearly equal to FN at the end of the scratch, the specific scratch wear rate characterized by the normalized scratch volume will reduce to,

wr = Cr / FN

(Equation 4)

The penetration depth recovery (PDR) in polymers [13] can also be quantified via the progressive load scratch test. For this, the penetration depth (PD) during the test must be recorded, and the residual post-scratch depth (RPSD) some specific time after the test needs to be measured using AFM. As a result, the penetration depth recovery PDR during the scratch process can be obtained by,66

PDR = (PD − RPSD) / PD ⋅100%


(Equation 5)


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AFM measurements were used to check some of the residual post-scratch depths, and in the present case, 7 days between scratch testing and AFM scanning was spaced to make sure the relaxation of polymers at room temperature was completed.67 To quantify the morphology and topography of the scratched traces, AFM imaging in tapping mode (DimensionTM 3100, Digital Instruments Vecco Metrology Group) were used. Height and phase images were acquired with a line-scan rate of 0.5 Hz within a 5 µm by 5 µm region. The AFM cantilever exhibited a nominal probe radius of 10 nm, nominal resonant frequency of 344 kHz, and nominal spring constant of 40 N⋅m-1 (MODEL RTESP, Part # MPP-11100-10). The depth values of the scratches were calculated with respect to the unscratched surface via software Gwyddion (schematics as shown in Figure 11a). To prove that the roughness from scratched surface does not influence profile scanning, comparison between optical imaging in scratching test and AFM imaging were compared in Figure S10, and the trend showed high consistency. To detect the energy dissipation capabilities, loading-unloading tests were tested in nanoindentation measurements (TriboIndentator from Hysitron Inc. equipped with a Berkovich diamond tip, with semi conical tip diameter of 1 µm). The samples were indented to one tenth of the coating thickness (about 100 nm deep with indenting volume of 1.78 µm3) to exclude the influence from the substrate. Indentation tests were operated in a displacement-control mode; the displacement excitation is applied to the sample according to a programmed loading function while the force response is continuously monitored with a resolution of 1 nN. The loading function in this work consisted of a 5s linear loading and a 5s unloading segments with the same 40 nm/s loading/loading rates.68-69 A total of 9 indents with lateral spacing of 2 µm were taken to obtain average dissipation energy values (area between the loading-unloading curves) on both control and composite samples coated on glass slides. The plasticity index, which characterizes


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the relative plastic/elastic behavior of the material when it undergoes external stresses and strains, is displayed in Figure 4a (insert schematics). This index, ψ, is defined in terms of energy,

ψ=

wir wir + wr

(Equation 6)

where wir and wr represent the irreversible work done during the indentation and the reversible work recovered by viscoelastic processes during the unloading stage respectively. For the falling sand abrasive test, ASTM D968-15 standard was used and modified to examine the sand abrasion resistance. The amount of abrasive pre unit film thickness or roughness value is often reported as the abrasion resistance of the coating on the panel. The in-house customized sand abrasion tester is illustrated in Figure 11b. Following the ASTM D968-15 standard, 100 cm3 of fine sand (diameter 62.5 to 2000 µm) impacted the surface from a height of 36 inches at coating surface temperature of 23 ± 2 oC. The sands fall though a guide tube of diameter of 1 inch, while the substrate was tilted at 45o to the horizontal plane. More severe sand impingement conditions were modified based on this ASTM 968 standard: (i) 100 cm3 sand, falling height 36 inches, coating surface temperature 65 ± 2 oC, (ii) 500 cm3 sand, falling height 60 inches, coating surface temperature 23 ± 2 oC, (iii) 500 cm3 sand, falling height 60 inches, coating surface temperature 65 ± 2 oC. The sand is characterized by its size and has silicon dioxide content greater than 99%. A predictive description of coating breakage is not yet available because this is a time-dependent procedure. For example, the number of micro-cracks and dislocations increase according to previous stress distributions. Therefore it is more common to study breakage probabilities on coating surface, which can be characterized by the roughness of surfaces.70



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To quantify the stability of our surfaces towards the impact of particles in practical applications, such as solar cell panels enduring sand storms in dessert, we estimate the allowed maximum impact velocity, v. The energy of falling sand mainly dissipate on the coating surface,

4 wimpact = mgh = (ρ ⋅ π R 3 )gh 3 1 1 4 = mv 2 = ⋅ ( ρ ⋅ π R 3 )v 2 2 2 3

(Equation 7)

The calculated sand impact velocity falling from 60 inches is around 6 m/s=21.6 km/h. This is based on the assumption that the falling sand breaks our coating surface. It has been noticed in literature that for a 3 mm dry film thickness, many of the old coatings would require 20 to 200 L of silica sand to achieve a abrasion-through failure. Today’s coatings that are more durable may require up to 600+ liters of sand to wear through. Therefore for an extremely durable coating, the Taber Abraser is preferred as the falling sand method tends to be laborious and time-consuming with the handling of large quantities of abrasive. Taber Abrasion tests of coatings was following ASTM D4060-15 standard (Figure 11c). Abrasion resistance can be calculated in three ways: as loss in weight as a specified number of cycles, as loss in weight per cycle, or as number of cycles required to remove a unit of amount of coating thickness. In essence, these parameters are the same and therefore wear index windex, the loss in weight in unit of microgram per cycle, was used for characterizing the abrasion resistance from the following equation,

windex =

W N

(Equation 8)

where W is the weight of abrasive, and N is the Taber Abrasion cycles.


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Taber Abraser consists of a pair of pivoted arms, to which the abrasive wheels and auxiliary masses can be attached. Load of 1000 g was applied on each wheel. Resilient calibrase wheels (N0. CS-10, 12.7 ± 0.3 mm, external diameter 51.9 ± 0.5 mm) were used. As abrading wheels are used they slowly disintegrate exposing fresh abrasive grains. When testing certain materials however, the wheel faces may become clogged as a result of the adhesive character of particles worn off the specimen. This changes the abrasive characteristics of the wheel and may impact test results. To reduce this variation, the working surface of abrading wheels must be cleaned and refreshed prior to use at regularly defined intervals, every 100 cycles here in this study. Resurfacing medium, an S-11 abrasive disk (4 inches in diameter, 150 grit abrasive paper), was used for resurfacing the abrasion wheels. Two resurfacing of 50 cycles each were applied to ensure perfect contact of abrading faces with specimen surface. Under both resurfacing and Taber Abrasion modes, the vacuum suction force was adjusted to 100 and abrasion frequency is 60 cycles/min. The test temperature is 23 ± 2 oC and humidity is 20 ± 5%.



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Figure 11. Wear resistance measurement (a) Nanoscratching, (b) Falling sand and (c) Taber Abrasion tests. Solvent resistance test were conducted by soaking coatings on glass substrates in acetone, methyl ethyl ketone (MEK), ethanol and water. UV-Vis transmittance and reflectance spectra were measured on a UV/VIS/NMR spectrometer (Lambda 1050 manufactured by PerkinElmer). The measurement wavelength is between 250 nm to 2500 nm, with 10 nm data interval. Profilometer (DektakXT-A stylus profilometer manufactured by Bruker) was used to measure the surface roughness. The measurement stylus is 2 µm type B tip. The scanned length is 300 µm with duration of 10 s and resolution of 0.1 µm/s. The stylus force is 2 mg. AFM measurements were used to examine the surface roughness values and topography profiles using the parameters as mentioned.


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A field-emission high-resolution scanning electron microscope (SEM, Zeiss Supra 25, accelerating voltage 5 kV with magnification of 0.5k, 1k, 5k and 10k) was used to image the film surficial topographies. All samples were sputter coated with a thin (i.e., 10 nm) gold/ palladium layer using a Gatan high-resolution ion beam coater. SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website via the http://pubs.acs.org at DOI: XXX. Figures S1-S9, Table S1-S2, and experimental details as well as corresponding result analyses with screening of transparent polymer coatings; overview on polymer composites for friction and wear application; nano-scale wearing of scratching; loading-unloading cycles for epoxy and composites; surface roughness for as-prepared samples; hysteresis angle measurements for samples after falling sand and Taber Abrasion tests; Energy dispersive X-ray spectrometry (EDS) of sand-impinged epoxy coating surface; Light transmittance of Taber Abrased coating surface. AUTHOR INFORMATION Corresponding Authors *

(M. F. R.) Email: [email protected]

*

(R. E. C.) Email: [email protected]

*

(K. A. A.) Email: [email protected]

Notes

The authors declare no competing financial interest.



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^

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TDK Professor of Polymer Materials Science and Engineering, MIT, 77 Mass Ave, Cambridge,

MA, USA, 02139. #

St. Laurent Professor of Chemical Engineering Department, MIT, 77 Mass Ave, Cambridge,

MA, USA, 02139. ACKNOWLEDGMENT

This work was funded under the Cooperative Agreement between the Masdar Institute of Science and Technology, Abu Dhabi, UAE and the Massachusetts Institute of Technology, Cambridge, MA, USA, Reference No. 02/MI/MIT/CP/11/07633/GEN/G/00. Discussions with Prof. Gareth McKinley, Dr. Alexander Barbati, and Ms. Setareh Shahsavari from Mechanical Engineering at MIT, Prof. Randal Erb from Mechanical and Industrial Engineering at Northeastern University, Prof. Richard A. Register at Princeton University have been very helpful.

REFERENCES 1. Liu, M.; Jia, Z.; Jia, D.; Zhou, C., Recent Advance in Research on Halloysite NanotubesPolymer Nanocomposite. Prog. Polym. Sci. 2014, 39, 1498-1525. 2. Fu, Y.; Zhang, L., Simultaneous Deposition of Ni Nanoparticles and Wires on a Tubular Halloysite Template: A Novel Metallized Ceramic Microstructure. J. Solid State Chem. 2005, 178, 3595-3600. 3. Fu, Y.; Zhang, L.; Zheng, J., In-Situ Deposition of Pd Nanoparticles on Tubular Halloysite Template for Initiation of Metallization. J. Nanosci. Nanotechnol. 2005, 5, 558-564. 4. Abdullayev, E.; Abbasov, V.; Tursunbayeva, A.; Portnov, V.; Ibrahimov, H.; Mukhtarova, G.; Lvov, Y., Self-Healing Coatings Based on Halloysite Clay Polymer Composites for Protection of Copper Alloys. ACS Appl. Mater. Interfaces 2013, 5, 4464-4471. 5. Joshi, A.; Abdullayev, E.; Vasiliev, A.; Volkova, O.; Lvov, Y., Interfacial Modification of Clay Nanotubes for the Sustained Release of Corrosion Inhibitors. Langmuir 2013, 29, 74397448. 6. Cavallaro, G.; Lazzara, G.; Milioto, S.; Parisi, F., Halloysite Nanotubes with Fluorinated Cavity: An Innovative Consolidant for Paper Treatment. Clay Miner. 2016, 51, 445-455.


Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7. Abdullayev, E.; Price, R.; Shchukin, D.; Lvov, Y., Halloysite Tubes as Nanocontainers for Anticorrosion Coating with Benzotriazole. ACS Appl. Mater. Interfaces 2009, 1, 1437-1443. 8. Joshi, A.; Abdullayev, E.; Vasiliev, A.; Volkova, O.; Lvov, Y., Interfacial Modification of Clay Nanotubes for the Sustained Release of Corrosion Inhibitors. Langmuir 2012, 29, 74397448. 9. Shutava, T. G.; Fakhrullin, R. F.; Lvov, Y. M., Spherical and Tubule Nanocarriers for Sustained Drug Release. Curr. Opin. Pharmacol. 2014, 18, 141-148. 10. Lvov, Y. M.; DeVilliers, M. M.; Fakhrullin, R. F., The Application of Halloysite Tubule Nanoclay in Drug Delivery. Expert Opin. Drug Delivery 2016, 13, 977-986. 11. Liu, M.; Chang, Y.; Yang, J.; You, Y.; He, R.; Chen, T.; Zhou, C., Functionalized Halloysite Nanotube by Chitosan Grafting for Drug Delivery of Curcumin to Achieve Enhanced Anticancer Efficacy. J. Mater. Chem. B 2016, 4, 2253-2263. 12. Rawtani, D.; Agrawal, Y., Multifarious Applications of Halloysite Nanotubes: A Review. Rev. Adv. Mater. Sci 2012, 30, 282-295. 13. Sun, P.; Liu, G.; Lv, D.; Dong, X.; Wu, J.; Wang, D., Simultaneous Improvement in Strength, Toughness, and Thermal Stability of Epoxy/Halloysite Nanotubes Composites by Interfacial Modification. J. Appl. Polym. Sci. 2016, 133, 1-9. 14. Khunova, V.; Kristof, J.; Kelnar, I.; Dybal, J., The Effect of Halloysite Modification Combined with in Situ Matrix Modifications on the Structure and Properties of Polypropylene/Halloysite Nanocomposites. eXPRESS Polym. Lett. 2013, 7, 471-479. 15. Jiang, L.; Zhang, C.; Liu, M.; Yang, Z.; Tjiu, W. W.; Liu, T., Simultaneous Reinforcement and Toughening of Polyurethane Composites with Carbon Nanotube/Halloysite Nanotube Hybrids. Compos. Sci. Technol. 2014, 91, 98-103. 16. Liu, M.; Zhang, Y.; Wu, C.; Xiong, S.; Zhou, C., Chitosan/Halloysite Nanotubes Bionanocomposites: Structure, Mechanical Properties and Biocompatibility. Int. J. Biol. Macromol. 2012, 51, 566-575. 17. Song, K.; Polak, R.; Chen, D.; Rubner, M. F.; Cohen, R. E.; Askar, K. A., Spray-Coated Halloysite-Epoxy Composites: A Means to Create Mechanically Robust, Vertically Aligned Nanotube Composites. ACS Appl. Mater. Interfaces 2016, 8, 20396–20406. 18. Bravo, J.; Zhai, L.; Wu, Z.; Cohen, R. E.; Rubner, M. F., Transparent Superhydrophobic Films Based on Silica Nanoparticles. Langmuir 2007, 23, 7293-7298. 19. Lvov, Y. M.; Shchukin, D. G.; Mohwald, H.; Price, R. R., Halloysite Clay Nanotubes for Controlled Release of Protective Agents. ACS Nano 2008, 2, 814-820. 20. Lvov, Y.; Wang, W.; Zhang, L.; Fakhrullin, R., Halloysite Clay Nanotubes for Loading and Sustained Release of Functional Compounds. Adv. Mater. 2016, 28, 1227-1250. 21. Lvov, Y.; Abdullayev, E., Functional Polymer–Clay Nanotube Composites with Sustained Release of Chemical Agents. Prog. Polym. Sci. 2013, 38, 1690-1719. 22. Dyett, B. P.; Wu, A. H.; Lamb, R. N., Toward Superhydrophobic and Durable Coatings: Effect of Needle Vs Crater Surface Architecture. ACS Appl. Mater. Interfaces 2014, 6, 95039507. 23. Aliofkhazraei, M., Characterization of Nanostructured Coatings. In Nanocoatings, 1st ed.; Springer Berlin Heidelberg: Berlin, 2011; Chapter 3, pp 77-110. 24. Wood, R. J., Tribo-Corrosion of Coatings: A Review. J. Phys. D: Appl. Phys. 2007, 40, 5502-5521. 25. Sidky, P.; Hocking, M., Review of Inorganic Coatings and Coating Processes for Reducing Wear and Corrosion. Br. Corros. J. 2013, 34, 171-183.



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26. Deng, X.; Mammen, L.; Butt, H.-J.; Vollmer, D., Candle Soot as a Template for a Transparent Robust Superamphiphobic Coating. Sci. 2012, 335, 67-70. 27. Jin, H.; Tian, X.; Ikkala, O.; Ras, R. H. A., Preservation of Superhydrophobic and Superoleophobic Properties Upon Wear Damage. ACS Appl. Mater. Interfaces 2013, 5, 485-488. 28. Geng, Z.; He, J., An Effective Method to Significantly Enhance the Robustness and Adhesion-to-Substrate of High Transmittance Superamphiphobic Silica Thin Films. J. Mater. Chem. A 2014, 2, 16601-16607. 29. Zhou, H.; Wang, H.; Niu, H.; Gestos, A.; Wang, X.; Lin, T., Fluoroalkyl Silane Modified Silicone Rubber/Nanoparticle Composite: A Super Durable, Robust Superhydrophobic Fabric Coating. Adv. Mater. 2012, 24, 2409-2412. 30. Krebs, F. C., Fabrication and Processing of Polymer Solar Cells: A Review of Printing and Coating Techniques. Sol. Energy Mater. Sol. Cells 2009, 93, 394-412. 31. Solaroad. http://en.solaroad.nl (accessed August 1st, 2016). 32. Solar Roadways: A Real Solution. http://www.solarroadways.com/Home/Index (accessed August 1st, 2016). 33. Ebert, D.; Bhushan, B., Transparent, Superhydrophobic, and Wear-Resistant Coatings on Glass and Polymer Substrates Using Sio2, Zno, and Ito Nanoparticles. Langmuir 2012, 28, 11391-11399. 34. Liu, J.; Notbohm, J. K.; Carpick, R. W.; Turner, K. T., Method for Characterizing Nanoscale Wear of Atomic Force Microscope Tips. ACS nano 2010, 4, 3763-3772. 35. Bhushan, B.; Gupta, B. K., Handbook of Tribology: Materials, Coatings, and Surface Treatments. 1st ed.; McGraw-Hill, New York, NY: Malabar, FL, 1991; Vol. 1, p 1168. 36. Friedrich, K.; Zhang, Z.; Schlarb, A. K., Effects of Various Fillers on the Sliding Wear of Polymer Composites. Compos. Sci. Technol. 2005, 65, 2329-2343. 37. Song, K.; Zhang, Y.; Meng, J.; Green, E. C.; Tajaddod, N.; Li, H.; Minus, M. L., Structural Polymer-Based Carbon Nanotube Composite Fibers: Understanding the Processing– Structure–Performance Relationship. Mater. 2013, 6, 2543-2577. 38. Myshkin, N.; Petrokovets, M.; Kovalev, A., Tribology of Polymers: Adhesion, Friction, Wear, and Mass-Transfer. Tribol. Int. 2006, 38, 910-921. 39. Biswas, S.; Vijayan, K., Friction and Wear of Ptfe—a Review. Wear 1992, 158, 193-211. 40. Carpick, R. W.; Salmeron, M., Scratching the Surface: Fundamental Investigations of Tribology with Atomic Force Microscopy. Chem. Rev. 1997, 97, 1163-1194. 41. Friedrich, K.; Varadi, K.; Goda, T.; Giertzsch, H., Finite Element Analysis of a Polymer Composite Subjected to a Sliding Steel Asperity Part Ii: Parallel and Anti-Parallel Fibre Orientations. J. Mater. Sci. 2002, 37, 3497-3507. 42. Goda, T.; Váradi, K.; Friedrich, K.; Giertzsch, H., Finite Element Analysis of a Polymer Composite Subjected to a Sliding Steel Asperity Part I Normal Fibre Orientation. J. Mater. Sci. 2002, 37, 1575-1583. 43. Gu, H.; Ma, C.; Gu, J.; Guo, J.; Yan, X.; Huang, J.; Zhang, Q.; Guo, Z., An Overview of Multifunctional Epoxy Nanocomposites. J. Mater. Chem. C 2016, 4, 5890-5906. 44. Azeez, A. A.; Rhee, K. Y.; Park, S. J.; Hui, D., Epoxy Clay Nanocomposites Processing, Properties and Applications: A Review. Composites, Part B 2012, 45, 308-320. 45. Guo, J.; Long, J.; Ding, D.; Wang, Q.; Shan, Y.; Umar, A.; Zhang, X.; Weeks, B. L.; Wei, S.; Guo, Z., Significantly Enhanced Mechanical and Electrical Properties of Epoxy Nanocomposites Reinforced with Low Loading of Polyaniline Nanoparticles. RSC Adv. 2016, 6, 21187-21192.


Page 30 of 36

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


46. Wei, H.; Wang, Y.; Guo, J.; Shen, N. Z.; Jiang, D.; Zhang, X.; Yan, X.; Zhu, J.; Wang, Q.; Shao, L.; Lin, H.; Wei, S.; Guo, Z., Advanced Micro/Nanocapsules for Self-Healing Smart Anticorrosion Coatings. J. Mater. Chem. A 2015, 3, 469-480. 47. Marouf, B. T.; Mai, Y.-W.; Bagheri, R.; Pearson, R. A., Toughening of Epoxy Nanocomposites: Nano and Hybrid Effects. Polym. Rev. 2016, 56, 70-112. 48. Oliver, W. C.; Pharr, G. M., An Improved Technique for Determining Hardness and Elastic Modulus Using Load and Displacement Sensing Indentation Experiments. J. Mater. Res. 1992, 7, 1564-1583. 49. Sakharova, N.; Fernandes, J.; Oliveira, M.; Antunes, J., Influence of Ductile Interlayers on Mechanical Behaviour of Hard Coatings under Depth-Sensing Indentation: A Numerical Study on Tialn. J. Mater. Sci. 2010, 45, 3812-3823. 50. Suhr, J.; Koratkar, N. A., Energy Dissipation in Carbon Nanotube Composites: A Review. J. Mater. Sci. 2008, 43, 4370-4382. 51. Haque, M. A.; Kurokawa, T.; Kamita, G.; Gong, J. P., Lamellar Bilayers as Reversible Sacrificial Bonds to Toughen Hydrogel: Hysteresis, Self-Recovery, Fatigue Resistance, and Crack Blunting. Macromol 2011, 44, 8916-8924. 52. Lendlein, A.; Kelch, S., Shape‐Memory Polymers. Angew. Chem. Int. Ed. 2002, 41, 2034-2057. 53. Guimaraes, L.; Enyashin, A. N.; Seifert, G.; Duarte, H. A., Structural, Electronic, and Mechanical Properties of Single-Walled Halloysite Nanotube Models. J. Phys. Chem. C 2010, 114, 11358-11363. 54. Wong, E. W.; Sheehan, P. E.; Lieber, C. M., Nanobeam Mechanics: Elasticity, Strength, and Toughness of Nanorods and Nanotubes. Sci. 1997, 277, 1971-1975. 55. Lu, D.; Chen, H.; Wu, J.; Chan, C. M., Direct Measurements of the Young's Modulus of a Single Halloysite Nanotube Using a Transmission Electron Microscope with a Bending Stage. J. Nanosci. Nanotechnol. 2011, 11, 7789-7793. 56. Kato, K., Wear in Relation to Friction—a Review. Wear 2000, 241, 151-157. 57. Miyake, K.; Kusunoki, M.; Usami, H.; Umehara, N.; Sasaki, S., Tribological Properties of Densely Packed Vertically Aligned Carbon Nanotube Film on Sic Formed by Surface Decomposition. Nano Lett. 2007, 7, 3285-3289. 58. Dickrell, P.; Sinnott, S.; Hahn, D.; Raravikar, N.; Schadler, L.; Ajayan, P.; Sawyer, W., Frictional Anisotropy of Oriented Carbon Nanotube Surfaces. Tribo. Lett. 2005, 18, 59-62. 59. Zhang, W.; Xu, B.; Tanaka, A.; Koga, Y., Frictional Behaviour of Vertically Aligned Carbon Nanotube Films. Carbon 2009, 47, 926-929. 60. Tu, J. P.; Zhu, L. P.; Hou, K.; Guo, S. Y., Synthesis and Frictional Properties of Array Film of Amorphous Carbon Nanofibers on Anodic Aluminum Oxide. Carbon 2003, 41, 12571263. 61. Kim, H.-J.; Kim, D.-E., Nano-Scale Friction: A Review. Int. J. Precis. Eng. Manuf. 2009, 10, 141-151. 62. Scrinzi, E.; Rossi, S.; Kamarchik, P.; Deflorian, F., Evaluation of Durability of NanoSilica Containing Clear Coats for Automotive Applications. Prog. Org. Coat. 2011, 71, 384-390. 63. Kryuchkova, M.; Danilushkina, A.; Lvov, Y.; Fakhrullin, R., Evaluation of Toxicity of Nanoclays and Graphene Oxide in Vivo: A Paramecium Caudatum Study. Environ. Sci.: Nano 2016, 3, 442-452. 64. Veerabadran, N. G.; Price, R. R.; Lvov, Y. M., Clay Nanotubes for Encapsulation and Sustained Release of Drugs. Nano 2007, 2, 115-120.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

65. Fakhrullina, G. I.; Akhatova, F. S.; Lvov, Y. M.; Fakhrullin, R. F., Toxicity of Halloysite Clay Nanotubes in Vivo: A Caenorhabditis Elegans Study. Environ. Sci.: Nano 2015, 2, 54-59. 66. Wang, Z. Z.; Gu, P.; Zhang, Z., Indentation and Scratch Behavior of NanoSio2/Polycarbonate Composite Coating at the Micro/Nano-Scale. Wear 2010, 269, 21-25. 67. Friedrich, K.; Sue, H. J.; Liu, P.; Almajid, A. A., Scratch Resistance of High Performance Polymers. Tribol. Int. 2011, 44, 1032-1046. 68. Hu, H.; Onyebueke, L.; Abatan, A., Characterizing and Modeling Mechanical Properties of Nanocomposites-Review and Evaluation. J. Miner. Mater. Charact. Eng. 2010, 9, 275-280. 69. Bhushan, B.; Li, X., Nanomechanical Characterisation of Solid Surfaces and Thin Films. Int. Mater. Rev. 2003, 48, 125-164. 70. Vogel, L.; Peukert, W., From Single Particle Impact Behaviour to Modelling of Impact Mills. Chem. Eng. Sci. 2005, 60, 5164-5176.


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Table of Figures Figure 1. (a) Scratched volumes normalized by normal load and scratch length, and (b) friction coefficient for PC, PS, PMMA, and epoxy measured in scratching tests under a normal load of 200 µN with a scratch length of 10 µm. Figure 2. Normalized scratch volume vs. normal force relationship for pure epoxy and 1 vol% HNT composites, with open square symbols (☐) standing for epoxy and filled marks for composites. Krenchel orientation factors for halloysite nanotubes (HNTs) are associated with processing compositions, as listed in Table 1. Figure 3. 3D profile imaging from atomic force microscopy (AFM) for pure epoxy coating and 1 vol% concentrated composite C (ηo=0.678) under (a-b) 100 µN, (c-d) 500 µN. The scanning size is 5 µm · 5 µm. Corresponding topographic (a2 to d2), phase images (a3 to d3), and profiles (a4 to d4) are plotted. Figure 4. (a) Loading-unloading cycle in indentation, (b) plasticity index (Ψ, defined by wir/(wir+wr) as shown in a), (c) dissipation energy, and (d) penetration depth recovery (PDR) vs. Krenchel orientation factors of halloysite. Open marks stand for pure epoxy while the closed symbols are composites with HNTs concentration of 1.0 vol%. The figures display nanoparticle orientation-related trends, i.e., higher out-of-plane alignment of the HNTs demonstrated larger dissipation energy, lower plasticity index, and better selfrecovery after scratching. (e) Scratching in composites with out-of-plane and in-plane nanoparticle alignment (schematic to scale, with semi conical tip diameter of 1 µm, HNTs average length of 2 µm and spacing of ~685 nm between nanotube centers). Single halloysite nanotube (length ~ 400 nm, out diameter ~ 40 nm and inner diameter ~ 15 nm) bending has been measured using transmission electron microscopy (TEM)55, showing an intact structure upon 90o bending and pure elastic deformation up to 5o bending. Figure 5. Friction coefficient (µ) vs. Krenchel orientation factor relationship shows the influence of particle alignment on friction coefficient. In-plane orientations of HNTs increased the friction coefficient while out-of-plane HNTs alignments caused µ to decrease or remain similar as compared to µ in epoxy. Open marks are pure epoxy and closed symbols are composites. Figure 6. Falling sand induced surface roughness vs. HNT alignment relationships plotted for (a) the ASTM D968 standard set-up with a falling sand height of 36 inches and sand volume of 100 cm3 at room temperature, modified ASTM D968 standard set-up with (b) falling sand height of 36 inches and sand volume of 100 cm3 at elevated temperature of 65 oC, (c) falling sand height of 60 inches and sand volume of 500 cm3 at room temperature of 25 oC, and (d) falling sand height of 60 inches and sand volume of 500 cm3 at elevated temperature of 65 o C. Roughness was measured by stylus profilometry and extracted using root mean square (RMS) method. Open marks are pure epoxy and closed symbols are composites with 1.0 vol% HNTs. Figure 7. SEM images showed the influence of particle alignment on falling sand resistance. The HNTs concentration is 1.0 vol%. (a) Epoxy, (b) composite A (ηo=0.084), (c) composite B (ηo=0.355) , (d) composite C (ηo=0.678), and (e) composite D (ηo=0.795). (a2-e2) is the



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zoom-out of (a1-e1) regions. ASTM D968 standard set-up was modified with falling sand height 60 inches and sands volume 500 cm3 at room temperature of 25 oC. Figure 8. UV-Vis measurement of transmittance for (a) as-prepared samples, (b) 100 cm3 falling sand from 30 inches height at coating surface temperature of 25 oC, (c) 100 cm3 sand falling from 30 inches height at coating surface temperature of 65 oC, (d) 500 cm3 sand falling from 60 inches height at coating surface temperature of 25 oC, (e) 500 cm3 sand falling from 60 inches height at coating surface temperature of 65 oC, and (f) decrease of transmittance measured at wavelength of 500 nm for samples with different particle orientation factors. Figure 9. Taber abrasion caused (a) surface roughness vs. HNTs alignment for 10 cycles, 50 cycles, 500 cycles, 1000 cycles, and 2000 cycles when partially the films were abraded through thickness. Roughness was measured by stylus profilometry and extracted using root mean square method. (b) Weight loss per abrasion cycle. Figure 10. SEM images showed the influence of particle alignment on Taber Abrasion resistance. The HNTs concentration is 1.0 vol%. (a) Epoxy, (b) composite A (ηo=0.084), (c) composite B (ηo=0.355), (d) composite C (ηo=0.678), and (e) composite D (ηo=0.795). (a2e2) is the zoom-out of (a1-e1). Taber Abrasion was conducted with 1kg weight and 2000 cycles. Figure 11. Wear resistance measurement (a) Nanoscratching, (b) Falling sand and (c) Taber Abrasion tests.

Table of Tables Table 1. Sample nomenclature17 Table 2. Average mechanical properties of coatings in loading-unloading indentation cycle

Table of Equations F (Equation 1) µ= T FN

wr = ∆V / (FNavelave ) = Cr lave / (FNavelave ) = Cr / FNave

(Equation 2) (Equation 3)

wr = Cr / FN

(Equation 4) (Equation 5)

PDR = (PD − RPSD) / PD ⋅100%

ψ=

wir wir + wr

(Equation 6)


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4 wimpact = mgh = (ρ ⋅ π R 3 )gh 3 1 1 4 = mv 2 = ⋅ ( ρ ⋅ π R 3 )v 2 2 2 3

windex =

W N

(Equation 7)

(Equation 8)



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TOC:


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Enhanced Wear Resistance of Transparent Epoxy Composite

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