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Individual role of the physicochemical characteristics of nanopatterns on tribological surfaces Prashant Pendyala, Harpreet Singh Grewal, Hong Nam Kim, Il-Joo Cho, and Eui-Sung Yoon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10123 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016
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Individual role of the physicochemical characteristics of nanopatterns on tribological surfaces Prashant Pendyala,1,# Harpreet S. Grewal,2,# Hong Nam Kim,1,# Il-Joo Cho,1 and Eui-Sung Yoon1,* 1
Center for BioMicrosystems, Korea Institute of Science and Technology (KIST), 02792, Seoul, Republic of Korea 2 Department of Mechanical Engineering, School of Engineering, Shiv Nadar University, 201314, India
[*] Correspondence: Dr. E. S. Yoon E-mail:
[email protected] Phone: +82-2-958-5651 Fax: +82-2-958-6910 [#] Prashant Pendyala, Harpreet S. Grewal and Hong Nam Kim contributed equally to this study.
Keywords: Nanopatterns, adhesion, friction, surface energy, nanopillar geometry
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Abstract Nanoscale patterns have dimensions that are comparable to the length scales affected by intermolecular and surface forces. In this study, we systematically investigated the individual roles of curvature, surface energy, lateral stiffness, material, and pattern density in the adhesion and friction of nanopatterns. We fabricated cylindrical and mushroom-shaped polymer pattern geometries containing flat- and round-top morphologies using capillary force lithography and nanodrawing techniques. We showed that the curvature, surface energy and density of the patterns predominantly influenced the adhesive interactions, whereas lateral stiffness dominated friction by controlling the geometrical interaction between the indenter and pillar during sliding. Interestingly, in contrast to previous studies, cylindrical and mushroom-shaped pillars showed similar adhesion characteristics but very different frictional properties. Using fracture mechanics analysis, we showed that this phenomenon is due to a larger ratio of the mushroom flange thickness (t) to the radius of the pillar stem (ρ), and we proposed a design criterion for mushroom patterns to exhibit a gecko-like effect. The most important result of our work is the discovery of a linear master curve in the graph of adhesion vs. friction for pillars with similar lateral stiffness values that is independent of curvature, material, surface energy, and pattern density. These results will aid in the identification of simple pattern parameters that can be scaled to tune adhesion and friction and will help broaden the understanding of nanoscale topographical interactions.
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1. Introduction
The multi-scale nature of roughness1 on surfaces causes the contacting regions called 'Asperities' to be ill defined.2-3 In particular, at the nanoscale, tribological phenomena are highly influenced by the complex interplay of intermolecular and surface forces, local geometry, and material properties. A deconvolution of the individual roles of the abovedescribed factors will facilitate the design of efficient nanotribological surfaces. In general, tribological studies have modeled the multi-scale roughness by using the concepts of superposed protuberances,4-5 signal analysis,6-7 and fractals8-11. Recent studies have effectively used fractal analysis of roughness to investigate adhesion,12-14 lubrication and contact mechanics10, 14-16 from the macroscale to the nanoscale. Applying the above-described concepts to experimental contact systems is still a cumbersome process. One of the methods for overcoming the complexity of multi-scale roughness is to limit the contact interactions to a few length scales by using well-defined surface geometry. Many classical tribological studies assumed that surfaces consist of spherical protuberances with a Gaussian, random or uniform distribution.4-5 The elastic deformations of the spherical contact geometry are described by the well-known Hertzian model.17 However, fabricating and testing surfaces with spherical protuberances was difficult for the pioneers. Bobji et al18 scaled up the contact model of surfaces with spherical protuberances to the problem of the interaction between a macroscopic spherical ball and an indenter to study the purely geometrical interaction of spherical asperities. Large numbers of studies have since investigated the tribology of uniform patterns with various geometries.19-25 Recently, Gong and Komvopolous used elastic-plastic contact analysis on layered patterned surfaces and showed how patterned surfaces are beneficial by reducing the maximum plastic strains and the size of the plastic zone in the sub-surface.25-27 3 ACS Paragon Plus Environment
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Extrapolation of the macroscopic studies to the nanoscale was found to be impossible because of the dominant effects of intermolecular and surface forces at the tiny scales. Depending on the relation between the range of forces and the elastic deformations caused by them, JKR28 (Compliant materials, strong adhesion forces) or DMT29 (stiff materials, weak adhesion forces) models describe the contact area. However, as described by Tabor30 et al., two extremes of the contact phenomena exist. A more generalized model was proposed by Maugis31 et al. using a Dugdale approximation, which is known as the Maugis-Dugdale (MD) model. A general equation that approximates the Maugis solution was proposed by Carpick et al.32. Based on the Maugis solution, Morrow et al. proposed a transition solution that considers the asperities that are not in contact but are within the range of adhesion.33 Although the above theories consider ideal geometries, the models have been successfully applied to varied micro/nano-contact problems13,
20,
34-35
and have broadened our
understanding of contact behavior at the nanoscale. In addition, a nanoscale contact is further complicated by the capillary forces, which are highly dependent on the minor deviations in geometry
36
and whose magnitude can be comparable to that of van der Waals forces37.
Although one can find controllable factors in the design of efficient and functional interfaces, the relative importance of such factors at the nanoscale cannot be easily discerned because of the limited number of systematic studies38-39. One of the reasons for the small number of systematic studies is the difficulty of fabricating structures with identical intricate geometries with varying curvature, surface energy, hierarchy, stiffness and optimal characterization techniques. However, advances in technology now make it possible to fabricate and test asperity shapes that are more complex and have nearly atomic dimensions.40-41 For example, it is now possible to mimic the superhydrophobic and adhesive organelles in biological species, such as the hierarchically organized protuberances on insects, spiders, and lizards,42-47 whose unique properties can later be used for advanced surface assisted bio4 ACS Paragon Plus Environment
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medical applications, such as targeted drug delivery, environmental remediation and dentistry.48-51 Micropillars with flat, mushroom-, spatula-, tubular- and concave-shaped regions at the top have been fabricated and tested for various contact configurations.46, 51-52 Studies have shown a significantly high adhesion of mushroom-like pillars compared with flat, spatula, tubular and concave shaped pillars in dry and wet conditions.53-54 The reported differences in adhesion of the pillars with mushroom-like heads were found to be due to the stress-states at the edge of the pillars determined by the design characteristics of the flange and stem-like structure.51, 55-58 It is understood that low modulus materials are desirable for high adhesion contact because of their low compliance, which results in conformability with a rough surface, and because of their small elastic energy storage. However, the sizes of mushroom-like and other bio-inspired pillar structures in the above-described studies are all greater than 5-10 µm. Fabrication of such intricate structures was difficult at smaller scales. In this study, using a combination of capillary force lithography59 and nanodrawing60 methods, we fabricated cylindrical and mushroom-shaped nanopillar patterns with varying curvature, surface energy, pattern density, and lateral stiffness. By modifying or adding single additional cues from the general cylindrical pattern, we define the contribution of individual cues in the adhesion and friction behavior of various nanopillars. Furthermore, using the measured adhesion and friction values, we investigate the crucial physicochemical factors in adhesion and friction and present the topography-dependent relationship between them.
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2. Experimental details Table 1: Geometric dimensions of the cylindrical and mushroom-shaped patterns Diameter (nm)
Height (nm)
Curvature (nm)
Pitch (nm)
Flat
250
220
--
500 625 750 1000
Round
250
160
258
500 625 750 1000
550
--
500 625 750 1000
550
512
Pattern shape
Cylindrical
Flange
250
Stem
100
Flange
250
Stem
100
Flat Mushroom Round
500
-
-
-
Figure 1: Scanning electron microscopy images of the pillar patterns generated: (a) cylindrical round-top pillar, (b) cylindrical flat-top pillar, (c) mushroom round-top pillar, and (d) mushroom flat-top pillars. The detailed dimensions of the diameter (d), height (h) and pitch (p) are given in Table 1.
Using a combination of capillary force lithography and nanodrawing methods, we fabricated various types of pillar-type patterns: cylindrical and mushroom-shaped with a round or flat 6 ACS Paragon Plus Environment
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curvature at the top surfaces (Fig. 1a–d). We fabricated the cylindrical pillars by using both polymethyl methacrylate (PMMA) and polystyrene (PS), and the mushroom-shaped pillars were fabricated using PS alone. PMMA and PS are widely used in MEMS/NEMS devices61-62 and are often used for studying polymer-polymer and polymer-substrate interactions.63 In addition, hydrophobic and hydrophilic surfaces can be generated using PMMA and PS. Furthermore, the characteristics of PS and PMMA are such that the fabrication process does not need to be varied widely to obtain basic pillar structures. Further, as a first step of Bosch process, coating procedure of a thin polytetrafluoroethylene (PTFE) layer is easily optmized. Hence, the surface energy of all the above patterns was varied using a thin surface coating (~20 nm) of PTFE. The parametric details of the types of pillar patterns used in this study are presented in Table 1. The pattern densities calculated of the pillar asperities were 19.63%, 12.56%, 8.72%, and 4.90% for pitches of 500, 625, 750 and 1000 nm, respectively. The detailed fabrication procedure is described below and can be found elsewhere.60, 64-65
Fabrication of PMMA nanopillar patterns: In the first step, PMMA dissolved in toluene (20 %) was double spin-coated on a bare silicon substrate. Coated polymer films were then baked on a hot plate at 120 °C for 10 min to remove any residual solvent in the film. A polyurethane acrylate (PUA) mould was placed on the micropatterned surface under slight pressure (20 g/cm2) for conformal contact. The mould was then annealed at 120 °C (well above the glass transition temperature of PMMA) for 5 min. In the final step, the PUA mould was peeled off.
Fabrication of PS nanopillar patterns: In brief, a thin (~100 nm) layer of polymethyl methacrylate (PMMA), as an adhesion layer, was spin-coated on a clean silicon wafer, and the solvent was removed in an oven (70 °C) for 10 min. A 1-µm-thick polystyrene (PS) (Mw=2.8 × 105, Tg=100 °C, Aldrich) layer was then spin-coated on the PMMA layer and baked on a hot plate at 120 °C for 10 min to remove any residual solvent in the film. A PUA mould containing nanocavities, such as cylindrical holes, was placed on the PS layer under slight 7 ACS Paragon Plus Environment
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pressure (20 g/cm2) for conformal contact and then annealed at approximately 150 °C (well above the glass transition temperature (Tg) of PS) for 2 h. In this step, the molten PS polymer spontaneously filled the nanocavities via a capillary force. Varying the temperature and curing time could be used to generate different pattern shapes. The cylindrical nanopillars, which had identical dimensions as the nanocavity, were fabricated by detaching the PUA mould after the cooling process. The mushroom-shaped pillars were fabricated by vertically displacing the PUA mould before the cooling process was complete. This stretches the polymer within the cavities, yielding a flange-like head suspended on a stretched neck-like region, similar to a mushroom. The schematic of the fabrication of a mushroom-shaped pillar is shown in Fig. 2. The shape of the flange-like head can be altered to be flat or rounded by controlling the duration of heating. We coated all patterns with polytetrafluoroethylene (PTFE) using a C4F8 plasma activation method, which is the first step of the Bosch process. We used the following process parameters for the coating: RF power, 800 W; pressure, 15 mTorr; C4F8 gas flow, 110 sccm; and coating time, 7s. The thickness of the PTFE coating was measured using ellipsometry to be approximately 20 nm.
Controlling top-surface curvature: In the fabrication of the pillar patterns, control of the topsurface curvature was possible by tuning the temperature of the imprinting step of the fabrication process; in this step, the polymer is allowed to flow via capillary action into a structure with cavities of a mould. Higher temperatures (> Tg +20 °C, where Tg denotes the glass transition temperature) yielded a flat-top morphology owing to the ease of cavity filling, whereas lower temperatures (Tg 3, and the DMT model71 is applied when λ < 0.1. For intermediate λ values (0.1
