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 2016.8:35552-35564. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/28/19. For personal use only.

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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, P.O. Box 54224 Abu Dhabi, United Arab Emirates S Supporting Information *

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-ofplane 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

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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, form better dispersions in polymers, and are abundant and very cheap.1 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 systems.9−11 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 enhanced.12 Therefore, HNTs © 2016 American Chemical Society

have attracted research attention as a good reinforcing material to strengthen and toughen polymer composites. HNTs have been loaded into several different polymers including epoxy,13 polypropylene,14 polyurethane,15 and chitosan.16 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 epoxybased 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/ Received: September 19, 2016 Accepted: December 2, 2016 Published: December 2, 2016 35552

DOI: 10.1021/acsami.6b11872 ACS Appl. Mater. Interfaces 2016, 8, 35552−35564

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ACS Applied Materials & Interfaces

composites.38,39 At the nanoscale, crystalline and amorphous regions in semicrystalline polymers and nanoparticles may generate divergent resistance to wearing.36 At micro- and macroscale, 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 are also factors.39 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-ofplane alignment of nanoparticles;17 thus, the influence of such an alignment on antiwearing 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 Nanoscale scratching, microscale falling sand, and macroscale 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 postscratch 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.

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 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 multifunctional 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 superhydrophobic or superoleophobic property upon several cycles of 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 antireflection capability.30 Products, such as solar roadways consisting of specially engineered solar panels utilized in The Netherlands31 and launched in the U.S,32 working under more severe 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 the mechanism of 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 multiscales due to limitations in different instrument types36 and difficulties in micro- and nanostructural 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

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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 ∼ 31 750), PS (Mw ∼ 35 000), PMMA (Mw ∼ 540 000), and epoxy (EPO-TEK OG142-112; see the 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 nanoscale particles 35553


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ACS Applied Materials & Interfaces Table 1. Sample Nomenclature17 name epoxy composite composite composite composite

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.

A B C D

concentration of halloysite (vol %)

Krenchel orientation factor

0 1 1 1 1

N/A 0.084 0.355 0.678 0.795

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 Figures S3 and S4). The surface morphology of the scratched samples was characterized by atomic force microscopy (AFM) 3D imaging and is shown in Figure 3. Figure 3a1,b1 shows the scratching under a loading of 100 μN, and Figure 3c1,d1 depicts 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 (ηo = 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 Figure 3c1,d1, the accumulation of wear debris was higher than that revealed in Figure 3a1,b1 on both sides of the sliding traces. Corresponding topographic images (a2 to d2), phase images (a3 to d3), and profiles (a4 to d4) are shown. 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. 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, respectively.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), where wir and wr are irreversible and reversible work done during mechanical tests; see Characterizations and Figure 4a) with better out-of-plane particle alignment as shown in Figure 4b and Table 2.

(i.e., nanotubes, nanospheres, and nanoplatelets) can exhibit a lower percolation threshold than materials loaded with traditional macroscale fibers.35 For example, glass fibers and carbon fibers increased mechanical and tribological properties to a maximum level at concentrations between 10 and 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 with regard to efficient mechanical reinforcement and toughening mechanisms.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. Nanoscale Wearing of Scratching. Improved scratch resistance is indicated by smaller damage regions, more recovery, and higher energy-dissipation values. The nanoscale 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

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.

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, ηo = 0.678), independent of the applied normal force. A further increase 35554


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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 and (c1−d1) 500 μN. The scanning size is 5 μm × 5 μm. Corresponding topographic images (a2 to d2), phase images (a3 to d3), and profiles (a4 to d4) are shown.

decrease of plasticity index, which is the case in this study (reverse trends in Figure 4b,c). 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 (ηo = 0.678) were 10.39 and 11.34 × 106 J/m3, respectively (Figure 4c). These results indicate that the composites can dissipate

A typical hysteresis loop appears as an inset 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 the 35555


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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 symbols stand for pure epoxy while the filled symbols are composites with HNT 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 semiconical 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, outer diameter ∼40 nm, and inner diameter ∼15 nm) bending has been measured using transmission electron microscopy (TEM),55 showing an intact structure upon 90° bending and pure elastic deformation up to 5° bending.

Table 2. Average Mechanical Properties of Coatings in the Loading−Unloading Indentation Cyclea Krenchel orientation factor, ηo

coatings epoxy composite composite composite composite

A B C D

0 0.084 0.355 0.678 0.795

H (GPa)

Er (GPa)

0.2 0.4 0.5 0.6 0.4

4.8 6.3 6.6 7.6 6.8

H3/Er2 (GPa)

H/Er 4.2 6.3 7.6 7.9 5.9

× × × × ×

−2

10 10−2 10−2 10−2 10−2

3.5 1.6 2.9 3.7 1.4

× × × × ×

10−3 10−3 10−3 10−3 10−3

Ψ (%) 76.7 70.8 69.2 68.0 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 the Experimental Section. a

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 35556


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ACS Applied Materials & Interfaces 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 multicycle loading−unloading 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 the 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 selfrecovery 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. 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‑axial × VHNT + EEpoxy × VEpoxy, 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‑radial × VHNT + 1/ EEpoxy × VEpoxy) ≈ EEpoxy. The HNT modulus is on the order of several GPa in the radial direction (EHNT‑radial), 2 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). During the tribology test, friction coefficient variations can reflect micro- or nanostructural 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 (open symbols) and 1 vol % halloysite composite samples (filled symbols). The friction coefficient parameters show an increasing trend as inplane aligned nanoparticles are included in epoxy. When HNTs are more out-of-plane aligned, the friction coefficient decreases. Such increasing−decreasing trends remain similar in all loading conditions. Comparatively, higher loadings allow the indenting

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 symbols are pure epoxy, and filled symbols are composites.

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 nanotubes,57 multiwall carbon nanotubes,58,59 and carbon nanofibers.60 However, in nanoparticle reinforced composites, the influence of nanoparticle alignment on friction coefficient has been studied61 much less extensively. Microscale 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 from 100 to 300 μm in diameter, impinging the surface from a height (h) of 36 in. (36 in. = 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) of 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 in., V = 100 cm3, temperature (T) = 65 °C, and sand grain velocity = 15.4 km/ h), Figure 6c (h = 60 in., V = 500 cm3, T = 25 °C, and sand grain velocity of 21.6 km/h), and Figure 6d (h = 60 in., V = 500 cm3, and T = 65 °C). 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 (Figure 6b−d). The decrease of 35557


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Figure 6. Falling sand induced surface roughness vs HNT alignment relationships plotted for (a) the ASTM D968 standard setup with a falling sand height of 36 in. and sand volume of 100 cm3 at room temperature, modified ASTM D968 standard setup with (b) falling sand height of 36 in. and sand volume of 100 cm3 at elevated temperature of 65 °C, (c) falling sand height of 60 in. and sand volume of 500 cm3 at room temperature of 25 °C, and (d) falling sand height of 60 in. and sand volume of 500 cm3 at elevated temperature of 65 °C. Roughness was measured by stylus profilometry and extracted using the root-mean-square (RMS) method. Open symbols are pure epoxy, and filled 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) Zoom-in of (a1−e1) regions. ASTM D968 standard setup was modified with falling sand height of 60 in. and sands volume of 500 cm3 at room temperature of 25 °C.

roughness can be up to 67% from epoxy to composites (Figure 6a), and roughness decreases range between 40% and 60% (Figure 6b−d). SEM images were taken on sand impinged coated surfaces. For the case of the more severe conditions with 500 cm3 sand falling from 60 in. at a surface temperature of 25 °C, 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. Optical Property Changes. The composites showed high transparency up to about 91% even at 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 standard in the falling sand test (Figure 8b), 100 cm3 sand falling from 36 in. did not influence the optical transmittance significantly. As shown in Figure 8c−e, the most severe falling sand test (corresponding to Figure 6b−d) 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-ofplane particle alignment leads to a less damaged surface and better optical transmittance (Figure 8a−e). 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. Macroscale Wearing of Taber Abrasion. ASTM-D4060 standard Taber Abrasion tests were used to explore the

macroscale 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 was attributed 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). Both the falling-sand test and Taber Abrasion tests have shown the consistent trend of improved wear resistance as long 35558


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Figure 8. UV−vis measurement of transmittance for (a) as-prepared samples, (b) 100 cm3 falling sand from 30 in. height at coating surface temperature of 25 °C, (c) 100 cm3 sand falling from 30 in. height at coating surface temperature of 65 °C, (d) 500 cm3 sand falling from 60 in. height at coating surface temperature of 25 °C, (e) 500 cm3 sand falling from 60 in. height at coating surface temperature of 65 °C, and (f) decrease of transmittance measured at wavelength of 500 nm for samples with different particle orientation factors.

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). (a2− e2) Zoom-in of (a1−e1). Taber Abrasion was conducted with 1 kg weight and 2000 cycles.

methods to determine the preferable particle orientations for enhanced wear-resistance. Compared to unmodified epoxy, outof-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.

Figure 9. Taber Abrasion generated (a) surface roughness vs HNTs alignment for 10, 50, 500, 1000, and 2000 cycles when the thickness of the films was slightly abraded. Roughness was measured by stylus profilometry and extracted using the root-mean-square method. (b) Weight loss per abrasion cycle.

as vertical out-of-plane particle alignment can be achieved. The use of halloysite in an 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 environments,21,63−65 which opens up significant application opportunities.

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EXPERIMENTAL SECTION

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. Materials. Dragonite HNTs clay was obtained from Applied Minerals (density 2.54 ± 0.03 g·cm−3, inner diameter 10−20 nm,

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 35559


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ACS Applied Materials & Interfaces 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 # MKBS6957 V from Sigma-Aldrich, CAS #900353-6, 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 # 9011-14-7, 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 the 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 °C, melting temperature 225 °C, density 1.20 g/cm3 at 25 °C, abrasive resistance 10−15 mg/1000 cycles from ASTM D1044 tests, clarity 88%) was used as is in the 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 the spray coating method and cured using a UV flood lamp, as detailed in the previous study.17 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.

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 = ΔV /(FNavelave) = Crlave/(FNavelave) = Cr /FNave

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 Cr ≐ h(3h2 + 4w 2)/(6w) ≐ h3/2w + 2wh/3

wr = Cr /FN

(4)

The penetration depth recovery (PDR) in polymers 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 postscratch 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 by66

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) prescan under a small load, (ii) scratch, and (iii) postscan. 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 semiconical tip head is 1 μm) 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, was 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, FT FN

(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

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μ=

(2)

PDR = (PD − RPSD)/PD × 100%

(5)

AFM measurements were used to check some of the residual postscratch 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 (Dimension 3100, Digital Instruments Vecco Metrology Group) was 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 the scratched surface does not influence profile scanning, comparisons between optical imaging in the scratching test and AFM imaging were shown 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 a semiconical tip diameter of 1 μm). The samples were indented to one tenth of the coating thickness (about 100 nm deep with an 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 5 s linear loading and a 5 s unloading segment with the same 40 nm/s loading/loading rates.68,69 A total of 9 indents with lateral

(1)

A plot of the friction coefficient against the sliding distance should approach an equilibrium value (after a short scratchingin 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, a transition from one 35560


Research Article


⎛ 4 ⎞ ⎞ 1 1⎛ 4 wimpact = mgh = ⎜ρ πR3⎟gh = mv 2 = ⎜ρ πR3⎟v 2 ⎝ 3 ⎠ ⎠ 2 2⎝ 3 (7)

The calculated sand impact velocity falling from 60 in. 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 the 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 an 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 the coatings followed the 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 the number of cycles required to remove a unit of the amount of coating thickness. In essence, these parameters are the same and therefore wear index, windex, the loss in weight in units of microgram per cycle, was used for characterizing the abrasion resistance from the following equation,

Figure 11. Wear resistance measurement. (a) Nanoscratching, (b) falling sand, and (c) Taber Abrasion tests.

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

ψ=

wir wir + wr

windex =

W N

(8)

where W is the weight of the abrasive, and N is number of the Taber Abrasion cycles. The Taber Abraser consists of a pair of pivoted arms, to which the abrasive wheels and auxiliary masses can be attached. A load of 1000 g was applied on each wheel. Resilient calibrase wheels (No. 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. A resurfacing medium, an S-11 abrasive disk (4 in. in diameter, 150 grit abrasive paper), was used for resurfacing the abrasion wheels. Two resurfacing rounds of 50 cycles each were applied to ensure perfect contact of abrading faces with the 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 °C, and humidity is 20 ± 5%. Solvent resistance tests 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 and 2500 nm, with a 10 nm data interval. A profilometer (DektakXT-A stylus profilometer manufactured by Bruker) was used to measure the surface roughness. The measurement stylus is a 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

(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 preunit 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 in. at a coating surface temperature of 23 ± 2 °C. The sands fall though a guide tube with a diameter of 1 in., while the substrate was tilted at 45° to the horizontal plane. More severe sand impingement conditions were modified on the basis of this ASTM 968 standard: (i) 100 cm3 sand, falling height of 36 in., coating surface temperature of 65 ± 2 °C, (ii) 500 cm3 sand, falling height of 60 in., and coating surface temperature of 23 ± 2 °C, (iii) 500 cm3 sand, falling height of 60 in., and coating surface temperature of 65 ± 2 °C. 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 timedependent procedure. For example, the number of microcracks and dislocations increase according to previous stress distributions. Therefore, it is more common to study breakage probabilities on the coating surface, which can be characterized by the roughness of surfaces.70 To quantify the stability of our surfaces toward the impact of particles in practical applications, such as solar cell panels enduring sand storms in the desert, we estimate the allowed maximum impact velocity, v. The energy of falling sand mainly dissipates on the coating surface, 35561



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to examine the surface roughness values and topography profiles using the parameters as mentioned. 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 highresolution ion beam coater.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11872. Figures S1−S9, Tables S1 and S2, and experimental details as well as corresponding result analyses with screening of transparent polymer coatings; overview of polymer composites for friction and wear application; nanoscale 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 sandimpinged epoxy coating surface; light transmittance of Taber Abrased coating surface (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.F.R.). *E-mail: [email protected] (R.E.C.). *E-mail: [email protected] (K.A.A.). ORCID

Khalid A. Askar: 0000-0003-2178-8485 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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, and Prof. Richard A. Register at Princeton University have been very helpful. M.F.R. is the TDK Professor of Polymer Materials Science and Engineering at MIT, and R.E.C. is the St. Laurent Professor in the Chemical Engineering Department at MIT.

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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 postscratch depth 35562


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