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Tribological properties of vertically aligned carbon nanotube arrays
Accepted Manuscript Tribological properties of vertically aligned carbon nanotube arrays Clemens F. Schaber, Thorsten Heinlein, Gareth Keeley, Jörg J. Schneider, Stanislav N. Gorb PII: DOI: Reference:
S0008-6223(15)30039-7 http://dx.doi.org/10.1016/j.carbon.2015.07.007 CARBON 10079
To appear in:
Carbon
Received Date: Revised Date: Accepted Date:
30 January 2015 8 June 2015 1 July 2015
Please cite this article as: Schaber, C.F., Heinlein, T., Keeley, G., Schneider, J.J., Gorb, S.N., Tribological properties of vertically aligned carbon nanotube arrays, Carbon (2015), doi: http://dx.doi.org/10.1016/j.carbon.2015.07.007
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Tribological properties of vertically aligned carbon nanotube arrays Clemens F. Schaber a,* , Thorsten Heinlein b, Gareth Keeley b, Jörg J. Schneider
b,*
, Stanislav N.
Gorb a a
Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1–9,
24098 Kiel, Germany b
Technische Universität Darmstadt, Fachbereich Chemie, Eduard-Zintl-Institut für Anorganische und Physikalische
Chemie, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
Abstract Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction µ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of µ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect µ after a preconditioning by a few sliding cycles. However, a strong decrease of µ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.
*
Corresponding authors. Tel: +49 431 880-4506. E-mail: [email protected] (Clemens F. Schaber) Tel: +49 6151 16-3225. E-mail: [email protected] (Jörg J. Schneider) 1
1. Introduction Such animals as insects, spiders, and geckos have outstanding adhesive abilities enabling them to walk upside down on rough and smooth surfaces. Their attachment devices are highly ordered hair-like structures on the feet with very thin plate-like terminal contact elements (spatulae). These contact elements are brought into contact in a short shear motion to align them with the substrate. Geckos and spiders attach due to van der Waals forces, whereas insects additionally use various kinds of secretory fluids to establish capillary forces. Up to date a lot of efforts have been undertaken to imitate the animal models in different approaches and fabrication methods, ranging from a few micrometer long silicone rubber and polyimide nano-hairs [1,2], up to 60 µm long polyurethane hairs [3], PDMS pillars and fibrils [4,5], MEMS fabricated so-called organorods [6] to highly specifically, hierarchically, and threedimensionally shaped microfibrillar surfaces [7,8]. Recent studies on polystyrene nanorods and polyurethane synthetic arrays focused on tip modifications for more detailed and better realization of the biological prototypes [9–11]. CNTs are very stiff and stable with an average axial Young´s modulus of single walled and multi walled CNTs in the range from 1 to 1.25 TPa [12,13] and a bending modulus of 0.91 to 1.24 TPa [14]. With 5 to 20 nm, their thickness is similar to that of the terminal elements of biological dry hairy adhesive systems (5-10 nm) [15]. Therefore, the contact areas of single CNTs to a substrate are assumed to be similar to that of single contact elements on the gecko toe. In particular, CNTs oriented vertically (VACNTs), and bound firmly to their substrate at the base, are good candidates for designing gecko-mimetic attachment systems. Ge et al. [16] reported that patterned VACNT arrays imitating setal arrays of the gecko supported shear stresses nearly four times higher than the biological model, and adhered to a variety of surfaces, including Teflon. Qu et al. [17] found that VACNTs exhibited adhesive forces of approximately 100 N cm-2 and demonstrated that a 4 × 4 mm piece of the VACNT film could hold a 1480 g textbook on a glass surface due to shear forces generated by entangled nanotube segments on the top. Sethi et al. [18] suggested arrays of VACNTs as self-cleaning adhesives in which the tips of the CNTs act as spatulae like in the gecko setae. Elasticity and adhesive properties of VACNT films were measured using a variety of methods, such as scanning probe microscopy (SPM) [19], micro- and nanoindentation [20–22], 2
micro force testing using microspheres [23], compression tests [21], and microfabricated resonators [24]. The effective Young´s moduli of VACNT arrays reported in these studies range from 0.1 MPa to 300 MPa. Using a SPM with standard rectangular silicon probes, Yurdumakan et al. [19] found that 65 µm thick films of VACNT brushes can generate adhesion forces of 1.6 × 10-2 nN nm-2, which is 200 times higher than those of the gecko foot-hairs. Measurements of frictional properties of VACNT films are sparse, but the results demonstrated the strong potential of VACNTs for applications where high friction at moderate loads is desired. A 6 µm thick film of VACNTs showed high friction coefficients µ between 1.0 and 2.2 independent on scan speed and spherical probe diameter [25]. Interestingly, after these microtribological experiments no wear tracks were observed, which was explained by elastic deformation of the CNTs. Peeling tests on 500 µm thick VACNT arrays revealed very stable and time-independent shear forces of 20 N cm-2 [16]. Using a home-built shear device, Sethi et al. [18] reported a friction force of approximately 5 N for a 0.16 cm2 VACNT tape and pointed out a self-cleaning effect of the tape. For VACNT arrays with a thickness of 280 µm, experiments in the friction cell of a nanoindenter exhibited strong stick-slip behavior, a dependence of the shear stress on preloading, and an alignment of individual CNTs after the friction tests [20]. A problem using small or/and sharp tips in adhesion experiments is that CNTs do not come in contact with the actual tip, but rather directly adhere to the sides of the geometrical probe structures. This type of contact geometry effects in a strong contribution of friction between the CNTs and the walls of the probe tip on the resulting pull-off force, and may lead to overestimated adhesion values. Our study comprises a friction characterization of arrays of very long (1.1 mm) VACNT using large spherical probes with diameters of 1.5 mm and normal forces in the range between 280 µN and 3.9 mN in consecutive tests at the same site. As well changes of the effective elastic modulus (EEM) of the VACNT arrays were examined in repeated measurements at the same site, which aided in understanding the change of the frictional properties after the initial loading cycles.
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2. Experimental
2.1 Growth parameters of carbon nanotubes In order to obtain VACNTs for friction and elasticity measurements, we deposited 11.6 nm Al followed by 1.4 nm Fe on a silicon substrate (B doped <100>) coated with 600 nm SiO2. The water-assisted chemical vapor deposition (WACVD) to grow CNTs was started by heating the substrate to 850 °C in a gas mixture of Ar/H2 (total flow 1400 sccm, 40 % H2) in a tubular furnace with a quartz-tube (inner diameter 85 mm) at normal pressure. The actual synthesis of VACNTs was initiated by decomposition of the carbon source ethylene. We used a flow of 200 sccm ethylene and added a small amount of water (420 ppm) by adding an argon flow through a water-bubbler. The lengths of the CNTs were controlled by the growth time up to 20 min. In that way it was possible to synthesize VACNTs on large-area substrates of up to 16 cm2 and heights of 1.1 mm. 2.2 Microscopic, Raman spectroscopic, and thermogravimetric sample characterization For transmission electron microscopy (TEM), the CNTs were suspended in ethanol (p. a.) by ultrasonification. These samples were deposited on copper grids, coated with a film of holey lacey carbon, and viewed at an acceleration voltage of 200 kV in a CM20 transmission electron microscope (Koninklijke Philips N.V., Eindhoven, The Netherlands). The VACNT samples were inspected by scanning electron microscopy (SEM) before and after the mechanical tests. For low magnification images Hitachi TM3000 and for high magnification images Hitachi S-4800 scanning electron microscopes were used at acceleration voltages of 3 kV on uncoated samples. The size of the deformations induced by friction tests was quantified in the central line of the friction tracks using the Plot Profile tool of the software ImageJ 1.480 (Wayne Rasband, National Institutes of Health, USA), where the edges of the cracks appear clearly as numerical changes of contrast values. Thermogravimetry was performed in a Netzsch TG 209 N1 thermo gravimeter (NetzschGerätebau GmbH, Selb/Bayern, Germany) with a heating rate of 25°C min-1 under oxygen.
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Raman spectroscopy was carried out using a Raman spectrometer LabRam HR8000 (Horiba, Ltd., Kyoto, Japan) equipped with a green laser at a wavelength of 514 nm and a power of the incoming laser of 0.5 mW. 2.3 Elasticity and friction testing
The silicon substrates carrying the VACNTs were cut to 10 × 10 mm squares and fixed on aluminum stubs using double-sided adhesive carbon platelets. The samples were placed below the cantilever of the micro force tester Basalt-01 (TETRA GmbH, Ilmenau, Germany), which was operated in either friction or elasticity mode [26–28]. For the friction tests a cantilever with spring constants of 241 N m-1 in the horizontal (friction) and 88 N m-1 in the vertical (normal force) direction was used. Each test comprised five consecutive sliding cycles back and forth on the same location on the VACNT array. For the estimation of the effective elastic modulus EEM the spring constant of the cantilever was 241 N m-1. The probes mounted on the tip of the cantilever were sapphire spheres with diameters of 1.5 mm for friction, and 1 mm for elasticity tests. Friction tests were performed over a sliding distance of 2 mm with a speed of 38.5 µm s-1 at different normal preloads ranging from 280 µN to 3.9 mN. Friction data was processed in the way that data points recorded shortly after the probe´s change of sliding direction were excluded from further analysis, because large quotients between friction and normal force around zero normal force led to practically invalid values of the friction coefficient. The EEM was calculated from the force-deformation curves of elasticity tests by fitting the Hertzian contact model [29]. In order to find the optimum values of parameters, the Levenberg-Marquardt algorithm implemented in a Matlab software tool written by Alexander Kovalev (Kiel University, Germany) for Matlab R2012b (The MathWorks, Inc., Natick, MA, USA) was used to minimize the χ2-distribution [30]. Details of the procedure can be found in appendix A. Since the surfaces of the VACNT samples were very delicate and easily modified by any mechanical contact, special care was taken to preserve them pristine before the tests.
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3. Results
3.1 Synthesis and purity of the VACNT arrays VACNTs were prepared by a water assisted chemical vapor deposition (CVD) process at 850°C on highly n doped Si wafer substrates with ethylene and H2 as growth precursors. The growth conditions (gas flow, heating and cooling rate, water flow, etc.) are described in [31-33]. TEM did not show deposition of amorphous carbon on the walls of the as-grown CNTs (Fig. 1a). This was consistent with the results of the thermogravimetric analysis, where amorphous contaminations would be oxidized starting at temperatures exceeding 300°C and giving rise to a significant weight loss (Fig. 1b). The ID/IG ratio for our CNT material was determined to be 0.63 from Raman spectroscopy (Fig. 1c), which is comparable to values from e.g. triple walled CNTs [34]. Since in contrast to TEM, Raman spectroscopy allows screening over the whole ensemble of the sample, this might indicate that a certain number of tubes with a wall number larger than two are indeed present in the sample. A more detailed quantification however is not possible.
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Fig. 1 – (a) Transmission electron micrograph showing a mixture of double- to six-walled CNTs with a diameter of 6 to 10 nm. (b) The thermo gravimetric analysis of a sample of CNTs synthesized by CVD-processing shows the weight loss with increasing temperature. (c) Raman spectrum of the CNTs used in this study for mechanical testing. The peaks at 1340 cm-1 and 1580 cm-1 are from disordered and graphitic carbon, respectively.
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3.2 Appearance of the pristine VACNT arrays At low magnification the surfaces of the VACNT arrays formed by the top ends of the CNTs looked (Fig. 2a,b) smooth. The VACNTs appeared to consist of long solid parallel rods, which can be mechanically separated into layers with a thickness less than 1 µm (Figs 2a-c). At higher magnifications the VACNTs turned out to have a felt-like structure (Figs 2d,e) with height differences of the single CNTs at their top-ends in the range of a few 100 nm (Fig. 2d). Single CNTs had a typical thickness of 10–20 nm corresponding to 3–4 walls (Fig. 2f). In top-view the surface of the arrays appeared more or less homogeneously with a tendency for grouping of several VACNTs to bundles (Fig. 3a). At even higher magnification, SEM showed a certain entanglement and horizontal orientation as well as direct contact between single CNTs in the topmost layer (Fig. 3b).
Fig. 2 – SEM micrographs of the structure of the VACNT arrays at different magnifications; (a-d) micrographs of the edge from the top surface (bottom left in the images) to the side (top right in the images) of a VACNT array; (e,f) side view of a VACNT array.
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Fig. 3 – Top view of the surface of the VACNTs before (a,b) and after (c,d) friction tests at two different magnifications (SEM images). The friction tests were performed at 3.9 mN mean normal force in vertical direction of the image in (c), and at 759 µN in horizontal direction in (d), respectively.
3.3 Friction tracks The surface structure of the VACNT arrays was heavily altered by the friction tests (Figs 3c,d, Fig. 4). At high magnification, the VACNTs in the top region appeared condensed together forming a crust-like structure on top of the single, still uncondensed VACNTs seen a few µ m deeper (Fig. 3c). Interestingly, this crust-like structure formed during the friction tests (five consecutive sliding cycles on the same spot) basically looked the same at different mean normal loads (Figs 3c,d). At low magnification in SEM, the sliding tracks observed after friction tests were cracks in the surface and clusters of horizontally condensed VACNTs. The marks ran perpendicular to 9
the sliding direction. The cracks could be subdivided into three characteristic sizes: large, medium and small. The distances between the large cracks decreased with decreasing normal load: 177 ± 11 µ m at 3860 µN (n=9) down to 125 ± 23 µ m at 759 µN (n=10), and 36 ± 8 µ m at 289 µN (n=10) (Fig. 4). Medium and small cracks were found in between the large cracks. The distances between the medium cracks were 11.8 ± 3.1 µm at 3860 µN, 13.8 ± 2.8 µm at 759 µN, and 13.4 ± 2.8 µm at 289 µN (n=10 each). Even more consistent was the distance between the small cracks with 4.3 ± 1.1 µ m, 4.0 ± 1.2 µ m and 4.0 ± 1.3 µm (n=10 each) at 3860 µN, 759 µN, and 289 µN, respectively. Interestingly, the remaining cluster surface to crack ratio measured along the central line of the friction track remained constant within 40–50% without any trend at all different mean normal forces tested. Although the behavior of the array surface was not observed in situ, very likely the formation of the cracks can be directly related to the sudden drops of friction observed in the force curves of the first sliding cycle (Figs 5a,b).
Fig. 4 – SEM images of cracks on the surface of the VACNT arrays caused by clustering of individual CNTs due to shear forces applied in the friction tests (five sliding cycles). (a) Patterns obtained after sliding with normal loads of 3860 µN, 759 µN, and 289 µN (from top to bottom). The lines mark large individual cracks. (b) The same friction tracks as in (a) at higher magnification. Here the lines mark exemplary medium size cracks. The inset shows small cracks. 10
Fig. 5 – (a) Friction force and normal force from five consecutive friction cycles with a normal preload of 470 µN. The shaded areas mark pauses in the movement of the probe, showing no relaxation of the CNT arrays under load. The colored arrows exemplary point to the force drops most likely leading to the formation of large (green) and medium (yellow) sized cracks in the surface. The orange arrows point to the two most prominent peaks of friction. The positions of the arrows very probably correspond to the positions of the cracks depicted in (b). (b) SEM image of the friction track from the experiment shown in (a). The large arrows point in the sliding directions corresponding to the force curves in (a). The small arrows most likely correspond to the positions of the small arrows in the force curve in (a).
3.4 Friction force The curves of friction force vs. time exhibit strong stick-and-slip behavior. The friction force decreased rapidly from the first to the second cycle. Such a decrease was less pronounced from 11
the second to the fifth cycle (Fig. 5a). For the calculation of the coefficient of friction µ, only the force values of the initial direction (red in Fig. 5a) were taken into account. Two different approaches for the determination of µ were applied. In the first approach, µ was calculated by dividing the mean values of friction force by the mean values of normal force. In the second approach, all the friction data points were plotted versus the actual normal force. The slopes of the linear regression lines then gave the values of the coefficient of friction for each cycle (Fig. 6).
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Fig. 6 – Determination of the coefficient of friction µ from the friction tests. (a) Forces from five consecutive loading cycles and corresponding regression lines at a normal preload of 470 µN at the beginning of the test. (b) Coefficients of friction obtained from the slope of the regression lines depicted in (a).
The friction values obtained in different tests with different preloads could best be sorted according to the mean values of actual normal force. The small differences of normal preload in the range of approximately 100 µN did not necessarily result in corresponding changes of actual normal and friction force. Likely, this fact was due to small irregularities of the VACNT surface, which resulted in different contact areas at the beginning of a test. Using the conventional calculation of the coefficients of friction from the mean values of friction and normal forces, the values of µ appeared to be overestimated with values as high as
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11.3 at the first, a drop to 4.8 at the second, and still 3.3 at the fifth testing cycle, exemplary at the mean normal force of 278 µN. The same experiment, evaluated by linear regression, yielded friction coefficients of 7.8 at the first, a decrease to 4.3 at the second, and 3.5 at the fifth cycle. The strong decrease of the frictional coefficient µ from the first to the second sliding cycle, and a stabilization within the consecutive cycles, were observed at all the different normal loads tested (Fig. 7).
10 3860 µN 880 µN 759 µN 673 µN 511 µN 289 µN 278 µN
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Fig. 7 – Coefficients of friction revealed in successive cycles of friction tests at different normal loads.
In general, an increase of the coefficient of friction with decreasing normal force was observed. Consistently, the values are larger than 2 at the level of micro-Newton normal loads. Interestingly, the coefficient of friction did not decrease proportionally, when the normal force reached the milli-Newton regime (3.86 mN). The values of µ stabilized at high levels with increasing numbers of sliding cycles. Control experiments with the sapphire ball sliding on glass yielded values of µ of 0.74 ± 0.03 (n=5), and 1.11 ± 0.05 (n=5) on aluminum, which are well in the range of literature values [35].
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3.5 Effective elastic modulus (EEM) The force-distance indentation curves obtained from loading the VACNTs perpendicular to their surface strongly indicated plastic deformation of the sample by the strong hysteresis between the first loading and unloading curves (Fig. 8a). The corresponding penetration into the surface of the arrays also indicates plastic deformation, because penetration at the first indentation was always larger than that at the consecutive tests (Fig. 8b). A negative pull-off peak of adhesion force was never observed. The EEM determined from the penetration-corrected force-distance curves using the Hertz model for non-adhesive elastic contact increased from 3.6 ± 0.8 MPa (N=5, n=5) at the first measurements on the pristine surface to 6.0 ± 1.9 MPa (N=5, n=5) at the second measurements on the same spots. At all further repetitive measurements the EEM remained constant at 6.2 ± 2.0 MPa (N=5, n=15).
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Fig. 8 – (a) Force distance curves at indentation into the surface of a VACNT array for EEM testing (five consecutive indentations on the same spot; raw data without penetration correction) (b) Relative penetration of the sapphire sphere into the VACNT surface at EEM testing. The small filled symbols indicate data from single experiments. The red open circles show the mean values of the five experiments (± standard deviation).
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4. Discussion 4.1 Shear adhesion The extremely high values of both friction force and coefficient of friction µ at the initial sliding cycles of friction tests (Figs 5–7) can be explained by the contribution of adhesive forces between the tip layer of the VACNTs and the sapphire sphere of the micro force tester. As shown in figures 2b,c and 3a,b, in the pristine array surface the CNTs are evenly distributed and very probably contact the sphere of the force tester with the sides of their tips, which leads to adhesive van der Waals forces between the probe and many individual contact elements causing the high friction. As also shown in the EEM measurements, the CNTs deform plastically during the first loading cycle. Most likely, the plastic deformation in direct contact with the sapphire sphere leads to the condensation of the top-layer of the CNTs forming the crust seen on the VACNT array surface after the friction experiments (Figs 3c,d). In the crust the single CNTs are condensed together and form a more homogeneous surface contacting the probe in the subsequent sliding cycles. The decrease in the number of potential contact points and the loss of compliant single CNTs adhering to the probe results in the abrupt decrease of friction force from the first to the second sliding cycle. These findings are in agreement with the theory of contact splitting known from contact mechanics of fibrillar adhesives, where arrays with smaller contact elements result in greater adhesive strength [36]. Likely, the condensation of the CNTs, and thereby the crust, gets even more stable in sliding cycles 2 to 5, as may be read from the slight decrease of friction forces and the coefficients of friction (Figs 5–7). 4.2 High friction The coefficients of friction calculated from the static and sliding friction forces divided by the actual normal forces were in the same range and are well described by the linear regression fitting (Fig. 6). After the very high values of the coefficient of friction µ in the range from about 8 at the first sliding cycle, the values of µ decrease down to still respectable values larger than 2 during the consecutive loading cycles. To our knowledge, the initial sliding cycles have not been considered in much detail before. 15
The previously reported coefficient of friction of 1.0–2.2 were measured on 6 µm high VACNT arrays, less than 5 µN normal forces, and gold tips with radii less than 30 µm [25]. Such short VACNTs deformed elastically under very low normal forces and had small contact areas during friction tests. In our study, the 1.1 mm long VACNTs deformed at least partially in the plastic regime. The elastic deformation of short VACNTs also explains, why Kinoshita et al. [25] did not observe wear tracks in the VACNT surface after the tests. Also, the range of friction coefficients for the 6 µm thick VACNT films were close to the values of 2.2–3.5 we found in our experiments for the fifth and consecutive sliding cycles, when the surface of the VACNT array had already compacted and thus stiffened under normal loading in the micro-Newton range. Friction coefficients, obtained in our tests with 3.86 mN normal force, were consistently smaller than those measured in the micro-Newton regime of loading forces. In the milli-Newton regime, the influence of adhesion on friction is generally less pronounced [37], resulting in friction coefficients down to 0.9 at the fifth sliding cycle. From the comparison of our findings with those by Kinoshita et al. [25] the effect of increase of both friction and shear adhesion with increasing thickness of the VACNT film, and the length of the single CNTs, respectively, seems to be very likely. The longer the CNTs in the array, the more mechanically compliant are their tips to external forces, leading to larger contact area of the single CNTs with the opposing surface. 4.3 Absence of pull-off forces The absence of normal adhesive forces in our experiments is consistent with a previous study using a similar testing setup consisting of a spherical probe [25], in spite of different testing regimes and different VACNT morphology. The presence of pull-off forces found by other authors can be explained with different testing methods and VACNT arrangements [19,20,22,38]. Testing of adhesion of VACNTs by pressing with a plate led to an anisotropic alignment of the tips of the 200–500 µm long CNTs in parallel to the test surface [20]. This kind of deformation led to attachment of the CNTs to the test substrate by their sides, and that is why normal pull-off forces, presumably caused by shear adhesion between the single CNTs and the substrate, increased with preload. The latter was later predicted by Finite Element Analysis [38]. Chen et al. [22] reported significant pull-off forces 16
from micro-patterned 170–230 µm high VACNT pillar-like arrays, and an influence of pillar aspect ratio and spacing on both EEM and adhesion strength. Mechanical testing on 65 µm thick films of VACNT brushes using sharp SPM tips resulted in adhesion forces of 1.6 × 10-2 nN nm-2, which was 200 times higher than those of the gecko foot-hairs [19]. These values appear to be overestimated, since very likely most of the CNTs adhered with their sides to the sides of the cantilever tip with the typical radius of curvature smaller than 10 nm and indentation depths up to 250 nm in these tests. 4.4 Increase of EEM after mechanical preconditioning The EEMs of 3.6 ± 0.8 MPa for the pristine samples and stable 6.2 ± 2.0 MPa for the preconditioned VACNTs matched well with the broad range of EEM from 0.1 to 300 MPa reported in the literature [20–24]. The values closest to those obtained in our experiments were measured in tests using microsphere probes with radii of up to 25 µm, resulting in an EEM of up to 2.8 MPa of the top surface and up to 1.4 MPa of the base of VACNT arrays [23]. EEMs from 0.25 to 1.6 MPa obtained in different regimes of compression tests on 200–500 µm thick VACNT bundles were rather close to the above values [20]. The latter authors explained the changes of the elastic behavior during one test with different regimes of engagement of the nanotubes with the test surface. In the above mentioned micropatterned VACNT pillars, nanoindentation with a spherical sapphire tip (radius 400 µm) yielded EEMs from 0.1 to 10 MPa dependent on both the pillar aspect ratio and spacing [22]. A study, using nanoindentation with a standard Berkovich indenter, reported an EEM of 50 MPa on up to 100 µm thick VACNT arrays [21]. In the latter study, the authors pointed out that assumptions had to be made to account for the transition of the Berkovich tip´s shape from spherical to pyramidal during indentation. Finally, EEMs found using microfabricated resonators integrated with the VACNTs were 8–300 MPa for 0.5–100 µm thick films [24]. However, this delicate method focuses more on the simultaneous measurement of thermal and mechanical properties of the VACNT films, and is very sensitive to differences in the distribution density of the CNTs within the array.
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4.5 Clustering Up to now fiber condensation was drawn to be a limiting factor reducing adhesion because of the reduction of the attainable aspect ratio and due to the increase of the packing density of the fibers [7,39,40]. We show that due to the self-organization (clustering) the friction performance of the VACNTs caused by shear adhesion decreased, but the mechanical properties of the VACNTs get durably more stable at still reasonable high levels of friction. Due to the stabilization of the mechanical properties fiber condensation becomes from foe to friend, because it can help to improve long-term performance of the structures. As we see from the friction and indentation curves, plastic deformation occurs mainly during the first loading of the sample. Most likely, the clusters in the friction tracks are established at the very first sliding cycle, as was also shown by numerical modeling [41]. Because of the bending modulus of CNTs in the range between 0.91 and 1.24 TPa [14] and the appearance of the friction tracks, we assume that during the sliding cycles the CNTs rotate around and/or buckle at their base leading to the formation of the clusters.
5. Outlook The frictional coefficients of long VACNT arrays are among the highest reported in the literature so far. The formation of stable clusters of VACNTs has strong potential for the development of CNT-based friction-enhancing surfaces, which can be used for applications where shear adhesion is desired, e.g. pick and place techniques. However, the dramatic change in both the EEM and friction force during the first few handling cycles has to be kept in mind, so that a preconditioning of the VACNT surfaces is necessary, and controlled procedures for this purpose have to be established. Although we have studied fairly long VACNTs, it is not obvious that these are always optimum. Shorter VACNTs might be even better suited for applications due to their higher stiffness. However, on the other hand, their alignment is not as perfect as with the longer ones, since the entanglement, which is intrinsic due to the overall CNT growth process, is still present in the structures giving rise to a significant lesser organization of the CNTs. Concerning the 18
mentioned dry adhesion purposes, in our first qualitative experiments, substrate areas of 1 cm × 1 cm composed of 2 mm long double walled VACNTs (similar in diameter to the ones used in this study) have shown to hold weights up to 500 g for extended periods of time. They thus seem better suited for dry adhesion purposes compared to shorter VACNT arrays with significantly shorter individual CNTs (e.g. below 500 µm). Regarding the mechanical stability and strength of such VACNT arrays, it is known that they can deform mechanically under load/stress. This deformation is often followed by crack formation and buckling of the macroscopic VACNT array structure. This detoriation is obviously dependent on the overall force applied and the particular force direction (e.g. on-top loading or laterally impinged shear stress). Nevertheless it has been shown that if VACNTs are loaded with compression forces impinging in a vertical manner, they can severely deform but do show full recovery to their original morphology (e.g. macroscopic length) in a highly reproducibly manner without showing signs of fatigue even after thousands of load/stress cycles [24,43,44]. The compliance of the VACNT arrays studied herein to the counterpart surface makes them well suited for better contact formation especially with rough surfaces. Hybrid materials as [VACNTs/polymer] are an interesting future material alternative to bare VACNT arrays for our studies combining positive effects of polymers with the stiffness of the CNTs. In such hybrids the interface polymer/CNTs [45,46] might play an important role on the mechanical stiffness of the array surface of the hybrid and should thus allow an alteration of crack formation and plastic deformation behavior.
Acknowledgements We thank Alexander Kovalev for the Matlab software tool for the evaluation of the EEM and constructive discussions. We also thank the Karl and Marie Schack-Stiftung, Frankfurt/Main, Germany, for the generous financial support. This work was supported by a joint grant of the German Research Foundation DFG (Bionik program, projects GO 995/10-1 and SCHN 375/201).
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Appendix A. According to [30], the effective elastic modulus EEM (analogous to the elasticity modulus of a homogeneous material) [42] was calculated using the Hertz model [29]. The formula used for fitting the force–deformation ൫ܨሺݐሻ − ߜ ሺݐሻ൯ curves was as follows. 4 ܨሺݐሻ = ܧ ܴଵ/ଶ ߜ ሺݐሻଷ/ଶ 3 with 1 1 1 = + ܴ ܴ௦ ܴ and 1 1 − ߭௦ଶ 1 − ߭ଶ = + ܧ ܧ௦ ܧ where ܧ is called the reduced modulus. ܧ௦ , ߭௦ , and ܴ௦ are the effective elastic modulus, the Poisson´s ration, and the radius of the sample, respectively. ܧ , ߭ , and ܴ are the same parameter for the probe. The material of the probe (sapphire sphere) was considerably stiffer than the material of the sample (VACNT array) ሺܧ ≫ ܧ௦ ሻ, and the radius of curvature of the sample´s surface was considerably larger than the radius of curvature of the probe´s tip ሺܴ௦ − ܴ ሻ. Therefore, ܴ௦ can be considered to be an infinite radius 1 1 − ߭௦ଶ ≈ ܧ ܧ௦ and ܴ ≈ ܴ Because we do not know the Poisson´s ratio of the VACNT arrays, we assumed them to be incompressible (Poisson´s ratio = 0.5).
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S0008-6223(15)30039-7 http://dx.doi.org/10.1016/j.carbon.2015.07.007 CARBON 10079
To appear in:
Carbon
Received Date: Revised Date: Accepted Date:
30 January 2015 8 June 2015 1 July 2015
Please cite this article as: Schaber, C.F., Heinlein, T., Keeley, G., Schneider, J.J., Gorb, S.N., Tribological properties of vertically aligned carbon nanotube arrays, Carbon (2015), doi: http://dx.doi.org/10.1016/j.carbon.2015.07.007
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Tribological properties of vertically aligned carbon nanotube arrays Clemens F. Schaber a,* , Thorsten Heinlein b, Gareth Keeley b, Jörg J. Schneider
b,*
, Stanislav N.
Gorb a a
Functional Morphology and Biomechanics, Zoological Institute, Kiel University, Am Botanischen Garten 1–9,
24098 Kiel, Germany b
Technische Universität Darmstadt, Fachbereich Chemie, Eduard-Zintl-Institut für Anorganische und Physikalische
Chemie, Alarich-Weiss-Str. 12, 64287 Darmstadt, Germany
Abstract Carbon nanotubes (CNTs) are a promising material for the fabrication of biomimetic dry adhesives. The dimensions of single CNTs are in the range of those of terminal elements of biological dry hairy adhesion systems, such as the setal branches on the toe of the gecko. Here, the tribological properties of densely packed arrays of vertically aligned and up to 1.1 mm long multi-walled CNTs (VACNTs) synthesized by chemical vapor deposition are examined. The coefficient of friction µ is as high as 5–6 at the first sliding cycle, and decreases down to stable values between 2 and 3 at the fourth to fifth sliding cycles. Such high values of µ can only be explained by the strong contribution of adhesion induced by applied shear force. After the tests, wear-induced deformations of the VACNT surface are observed, which strongly depend on the amount of normal force applied during the friction experiments. Interestingly, the plastic deformation of the VACNTs does not significantly affect µ after a preconditioning by a few sliding cycles. However, a strong decrease of µ during the initial wear cycles has to be taken into account for the development of applications, such as non-slip surfaces and pick-and-place techniques for manufacturing.
*
Corresponding authors. Tel: +49 431 880-4506. E-mail: [email protected] (Clemens F. Schaber) Tel: +49 6151 16-3225. E-mail: [email protected] (Jörg J. Schneider) 1
1. Introduction Such animals as insects, spiders, and geckos have outstanding adhesive abilities enabling them to walk upside down on rough and smooth surfaces. Their attachment devices are highly ordered hair-like structures on the feet with very thin plate-like terminal contact elements (spatulae). These contact elements are brought into contact in a short shear motion to align them with the substrate. Geckos and spiders attach due to van der Waals forces, whereas insects additionally use various kinds of secretory fluids to establish capillary forces. Up to date a lot of efforts have been undertaken to imitate the animal models in different approaches and fabrication methods, ranging from a few micrometer long silicone rubber and polyimide nano-hairs [1,2], up to 60 µm long polyurethane hairs [3], PDMS pillars and fibrils [4,5], MEMS fabricated so-called organorods [6] to highly specifically, hierarchically, and threedimensionally shaped microfibrillar surfaces [7,8]. Recent studies on polystyrene nanorods and polyurethane synthetic arrays focused on tip modifications for more detailed and better realization of the biological prototypes [9–11]. CNTs are very stiff and stable with an average axial Young´s modulus of single walled and multi walled CNTs in the range from 1 to 1.25 TPa [12,13] and a bending modulus of 0.91 to 1.24 TPa [14]. With 5 to 20 nm, their thickness is similar to that of the terminal elements of biological dry hairy adhesive systems (5-10 nm) [15]. Therefore, the contact areas of single CNTs to a substrate are assumed to be similar to that of single contact elements on the gecko toe. In particular, CNTs oriented vertically (VACNTs), and bound firmly to their substrate at the base, are good candidates for designing gecko-mimetic attachment systems. Ge et al. [16] reported that patterned VACNT arrays imitating setal arrays of the gecko supported shear stresses nearly four times higher than the biological model, and adhered to a variety of surfaces, including Teflon. Qu et al. [17] found that VACNTs exhibited adhesive forces of approximately 100 N cm-2 and demonstrated that a 4 × 4 mm piece of the VACNT film could hold a 1480 g textbook on a glass surface due to shear forces generated by entangled nanotube segments on the top. Sethi et al. [18] suggested arrays of VACNTs as self-cleaning adhesives in which the tips of the CNTs act as spatulae like in the gecko setae. Elasticity and adhesive properties of VACNT films were measured using a variety of methods, such as scanning probe microscopy (SPM) [19], micro- and nanoindentation [20–22], 2
micro force testing using microspheres [23], compression tests [21], and microfabricated resonators [24]. The effective Young´s moduli of VACNT arrays reported in these studies range from 0.1 MPa to 300 MPa. Using a SPM with standard rectangular silicon probes, Yurdumakan et al. [19] found that 65 µm thick films of VACNT brushes can generate adhesion forces of 1.6 × 10-2 nN nm-2, which is 200 times higher than those of the gecko foot-hairs. Measurements of frictional properties of VACNT films are sparse, but the results demonstrated the strong potential of VACNTs for applications where high friction at moderate loads is desired. A 6 µm thick film of VACNTs showed high friction coefficients µ between 1.0 and 2.2 independent on scan speed and spherical probe diameter [25]. Interestingly, after these microtribological experiments no wear tracks were observed, which was explained by elastic deformation of the CNTs. Peeling tests on 500 µm thick VACNT arrays revealed very stable and time-independent shear forces of 20 N cm-2 [16]. Using a home-built shear device, Sethi et al. [18] reported a friction force of approximately 5 N for a 0.16 cm2 VACNT tape and pointed out a self-cleaning effect of the tape. For VACNT arrays with a thickness of 280 µm, experiments in the friction cell of a nanoindenter exhibited strong stick-slip behavior, a dependence of the shear stress on preloading, and an alignment of individual CNTs after the friction tests [20]. A problem using small or/and sharp tips in adhesion experiments is that CNTs do not come in contact with the actual tip, but rather directly adhere to the sides of the geometrical probe structures. This type of contact geometry effects in a strong contribution of friction between the CNTs and the walls of the probe tip on the resulting pull-off force, and may lead to overestimated adhesion values. Our study comprises a friction characterization of arrays of very long (1.1 mm) VACNT using large spherical probes with diameters of 1.5 mm and normal forces in the range between 280 µN and 3.9 mN in consecutive tests at the same site. As well changes of the effective elastic modulus (EEM) of the VACNT arrays were examined in repeated measurements at the same site, which aided in understanding the change of the frictional properties after the initial loading cycles.
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2. Experimental
2.1 Growth parameters of carbon nanotubes In order to obtain VACNTs for friction and elasticity measurements, we deposited 11.6 nm Al followed by 1.4 nm Fe on a silicon substrate (B doped <100>) coated with 600 nm SiO2. The water-assisted chemical vapor deposition (WACVD) to grow CNTs was started by heating the substrate to 850 °C in a gas mixture of Ar/H2 (total flow 1400 sccm, 40 % H2) in a tubular furnace with a quartz-tube (inner diameter 85 mm) at normal pressure. The actual synthesis of VACNTs was initiated by decomposition of the carbon source ethylene. We used a flow of 200 sccm ethylene and added a small amount of water (420 ppm) by adding an argon flow through a water-bubbler. The lengths of the CNTs were controlled by the growth time up to 20 min. In that way it was possible to synthesize VACNTs on large-area substrates of up to 16 cm2 and heights of 1.1 mm. 2.2 Microscopic, Raman spectroscopic, and thermogravimetric sample characterization For transmission electron microscopy (TEM), the CNTs were suspended in ethanol (p. a.) by ultrasonification. These samples were deposited on copper grids, coated with a film of holey lacey carbon, and viewed at an acceleration voltage of 200 kV in a CM20 transmission electron microscope (Koninklijke Philips N.V., Eindhoven, The Netherlands). The VACNT samples were inspected by scanning electron microscopy (SEM) before and after the mechanical tests. For low magnification images Hitachi TM3000 and for high magnification images Hitachi S-4800 scanning electron microscopes were used at acceleration voltages of 3 kV on uncoated samples. The size of the deformations induced by friction tests was quantified in the central line of the friction tracks using the Plot Profile tool of the software ImageJ 1.480 (Wayne Rasband, National Institutes of Health, USA), where the edges of the cracks appear clearly as numerical changes of contrast values. Thermogravimetry was performed in a Netzsch TG 209 N1 thermo gravimeter (NetzschGerätebau GmbH, Selb/Bayern, Germany) with a heating rate of 25°C min-1 under oxygen.
4
Raman spectroscopy was carried out using a Raman spectrometer LabRam HR8000 (Horiba, Ltd., Kyoto, Japan) equipped with a green laser at a wavelength of 514 nm and a power of the incoming laser of 0.5 mW. 2.3 Elasticity and friction testing
The silicon substrates carrying the VACNTs were cut to 10 × 10 mm squares and fixed on aluminum stubs using double-sided adhesive carbon platelets. The samples were placed below the cantilever of the micro force tester Basalt-01 (TETRA GmbH, Ilmenau, Germany), which was operated in either friction or elasticity mode [26–28]. For the friction tests a cantilever with spring constants of 241 N m-1 in the horizontal (friction) and 88 N m-1 in the vertical (normal force) direction was used. Each test comprised five consecutive sliding cycles back and forth on the same location on the VACNT array. For the estimation of the effective elastic modulus EEM the spring constant of the cantilever was 241 N m-1. The probes mounted on the tip of the cantilever were sapphire spheres with diameters of 1.5 mm for friction, and 1 mm for elasticity tests. Friction tests were performed over a sliding distance of 2 mm with a speed of 38.5 µm s-1 at different normal preloads ranging from 280 µN to 3.9 mN. Friction data was processed in the way that data points recorded shortly after the probe´s change of sliding direction were excluded from further analysis, because large quotients between friction and normal force around zero normal force led to practically invalid values of the friction coefficient. The EEM was calculated from the force-deformation curves of elasticity tests by fitting the Hertzian contact model [29]. In order to find the optimum values of parameters, the Levenberg-Marquardt algorithm implemented in a Matlab software tool written by Alexander Kovalev (Kiel University, Germany) for Matlab R2012b (The MathWorks, Inc., Natick, MA, USA) was used to minimize the χ2-distribution [30]. Details of the procedure can be found in appendix A. Since the surfaces of the VACNT samples were very delicate and easily modified by any mechanical contact, special care was taken to preserve them pristine before the tests.
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3. Results
3.1 Synthesis and purity of the VACNT arrays VACNTs were prepared by a water assisted chemical vapor deposition (CVD) process at 850°C on highly n doped Si wafer substrates with ethylene and H2 as growth precursors. The growth conditions (gas flow, heating and cooling rate, water flow, etc.) are described in [31-33]. TEM did not show deposition of amorphous carbon on the walls of the as-grown CNTs (Fig. 1a). This was consistent with the results of the thermogravimetric analysis, where amorphous contaminations would be oxidized starting at temperatures exceeding 300°C and giving rise to a significant weight loss (Fig. 1b). The ID/IG ratio for our CNT material was determined to be 0.63 from Raman spectroscopy (Fig. 1c), which is comparable to values from e.g. triple walled CNTs [34]. Since in contrast to TEM, Raman spectroscopy allows screening over the whole ensemble of the sample, this might indicate that a certain number of tubes with a wall number larger than two are indeed present in the sample. A more detailed quantification however is not possible.
6
Fig. 1 – (a) Transmission electron micrograph showing a mixture of double- to six-walled CNTs with a diameter of 6 to 10 nm. (b) The thermo gravimetric analysis of a sample of CNTs synthesized by CVD-processing shows the weight loss with increasing temperature. (c) Raman spectrum of the CNTs used in this study for mechanical testing. The peaks at 1340 cm-1 and 1580 cm-1 are from disordered and graphitic carbon, respectively.
7
3.2 Appearance of the pristine VACNT arrays At low magnification the surfaces of the VACNT arrays formed by the top ends of the CNTs looked (Fig. 2a,b) smooth. The VACNTs appeared to consist of long solid parallel rods, which can be mechanically separated into layers with a thickness less than 1 µm (Figs 2a-c). At higher magnifications the VACNTs turned out to have a felt-like structure (Figs 2d,e) with height differences of the single CNTs at their top-ends in the range of a few 100 nm (Fig. 2d). Single CNTs had a typical thickness of 10–20 nm corresponding to 3–4 walls (Fig. 2f). In top-view the surface of the arrays appeared more or less homogeneously with a tendency for grouping of several VACNTs to bundles (Fig. 3a). At even higher magnification, SEM showed a certain entanglement and horizontal orientation as well as direct contact between single CNTs in the topmost layer (Fig. 3b).
Fig. 2 – SEM micrographs of the structure of the VACNT arrays at different magnifications; (a-d) micrographs of the edge from the top surface (bottom left in the images) to the side (top right in the images) of a VACNT array; (e,f) side view of a VACNT array.
8
Fig. 3 – Top view of the surface of the VACNTs before (a,b) and after (c,d) friction tests at two different magnifications (SEM images). The friction tests were performed at 3.9 mN mean normal force in vertical direction of the image in (c), and at 759 µN in horizontal direction in (d), respectively.
3.3 Friction tracks The surface structure of the VACNT arrays was heavily altered by the friction tests (Figs 3c,d, Fig. 4). At high magnification, the VACNTs in the top region appeared condensed together forming a crust-like structure on top of the single, still uncondensed VACNTs seen a few µ m deeper (Fig. 3c). Interestingly, this crust-like structure formed during the friction tests (five consecutive sliding cycles on the same spot) basically looked the same at different mean normal loads (Figs 3c,d). At low magnification in SEM, the sliding tracks observed after friction tests were cracks in the surface and clusters of horizontally condensed VACNTs. The marks ran perpendicular to 9
the sliding direction. The cracks could be subdivided into three characteristic sizes: large, medium and small. The distances between the large cracks decreased with decreasing normal load: 177 ± 11 µ m at 3860 µN (n=9) down to 125 ± 23 µ m at 759 µN (n=10), and 36 ± 8 µ m at 289 µN (n=10) (Fig. 4). Medium and small cracks were found in between the large cracks. The distances between the medium cracks were 11.8 ± 3.1 µm at 3860 µN, 13.8 ± 2.8 µm at 759 µN, and 13.4 ± 2.8 µm at 289 µN (n=10 each). Even more consistent was the distance between the small cracks with 4.3 ± 1.1 µ m, 4.0 ± 1.2 µ m and 4.0 ± 1.3 µm (n=10 each) at 3860 µN, 759 µN, and 289 µN, respectively. Interestingly, the remaining cluster surface to crack ratio measured along the central line of the friction track remained constant within 40–50% without any trend at all different mean normal forces tested. Although the behavior of the array surface was not observed in situ, very likely the formation of the cracks can be directly related to the sudden drops of friction observed in the force curves of the first sliding cycle (Figs 5a,b).
Fig. 4 – SEM images of cracks on the surface of the VACNT arrays caused by clustering of individual CNTs due to shear forces applied in the friction tests (five sliding cycles). (a) Patterns obtained after sliding with normal loads of 3860 µN, 759 µN, and 289 µN (from top to bottom). The lines mark large individual cracks. (b) The same friction tracks as in (a) at higher magnification. Here the lines mark exemplary medium size cracks. The inset shows small cracks. 10
Fig. 5 – (a) Friction force and normal force from five consecutive friction cycles with a normal preload of 470 µN. The shaded areas mark pauses in the movement of the probe, showing no relaxation of the CNT arrays under load. The colored arrows exemplary point to the force drops most likely leading to the formation of large (green) and medium (yellow) sized cracks in the surface. The orange arrows point to the two most prominent peaks of friction. The positions of the arrows very probably correspond to the positions of the cracks depicted in (b). (b) SEM image of the friction track from the experiment shown in (a). The large arrows point in the sliding directions corresponding to the force curves in (a). The small arrows most likely correspond to the positions of the small arrows in the force curve in (a).
3.4 Friction force The curves of friction force vs. time exhibit strong stick-and-slip behavior. The friction force decreased rapidly from the first to the second cycle. Such a decrease was less pronounced from 11
the second to the fifth cycle (Fig. 5a). For the calculation of the coefficient of friction µ, only the force values of the initial direction (red in Fig. 5a) were taken into account. Two different approaches for the determination of µ were applied. In the first approach, µ was calculated by dividing the mean values of friction force by the mean values of normal force. In the second approach, all the friction data points were plotted versus the actual normal force. The slopes of the linear regression lines then gave the values of the coefficient of friction for each cycle (Fig. 6).
10
b
st
1 cycle 2nd cycle rd 3 cycle 4th cycle th 5 cycle
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coefficient of friction µ
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normal force [mN]
Fig. 6 – Determination of the coefficient of friction µ from the friction tests. (a) Forces from five consecutive loading cycles and corresponding regression lines at a normal preload of 470 µN at the beginning of the test. (b) Coefficients of friction obtained from the slope of the regression lines depicted in (a).
The friction values obtained in different tests with different preloads could best be sorted according to the mean values of actual normal force. The small differences of normal preload in the range of approximately 100 µN did not necessarily result in corresponding changes of actual normal and friction force. Likely, this fact was due to small irregularities of the VACNT surface, which resulted in different contact areas at the beginning of a test. Using the conventional calculation of the coefficients of friction from the mean values of friction and normal forces, the values of µ appeared to be overestimated with values as high as
12
11.3 at the first, a drop to 4.8 at the second, and still 3.3 at the fifth testing cycle, exemplary at the mean normal force of 278 µN. The same experiment, evaluated by linear regression, yielded friction coefficients of 7.8 at the first, a decrease to 4.3 at the second, and 3.5 at the fifth cycle. The strong decrease of the frictional coefficient µ from the first to the second sliding cycle, and a stabilization within the consecutive cycles, were observed at all the different normal loads tested (Fig. 7).
10 3860 µN 880 µN 759 µN 673 µN 511 µN 289 µN 278 µN
coefficient of friction µ
9 8 7 6 5 4 3 2 1 0 1
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Fig. 7 – Coefficients of friction revealed in successive cycles of friction tests at different normal loads.
In general, an increase of the coefficient of friction with decreasing normal force was observed. Consistently, the values are larger than 2 at the level of micro-Newton normal loads. Interestingly, the coefficient of friction did not decrease proportionally, when the normal force reached the milli-Newton regime (3.86 mN). The values of µ stabilized at high levels with increasing numbers of sliding cycles. Control experiments with the sapphire ball sliding on glass yielded values of µ of 0.74 ± 0.03 (n=5), and 1.11 ± 0.05 (n=5) on aluminum, which are well in the range of literature values [35].
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3.5 Effective elastic modulus (EEM) The force-distance indentation curves obtained from loading the VACNTs perpendicular to their surface strongly indicated plastic deformation of the sample by the strong hysteresis between the first loading and unloading curves (Fig. 8a). The corresponding penetration into the surface of the arrays also indicates plastic deformation, because penetration at the first indentation was always larger than that at the consecutive tests (Fig. 8b). A negative pull-off peak of adhesion force was never observed. The EEM determined from the penetration-corrected force-distance curves using the Hertz model for non-adhesive elastic contact increased from 3.6 ± 0.8 MPa (N=5, n=5) at the first measurements on the pristine surface to 6.0 ± 1.9 MPa (N=5, n=5) at the second measurements on the same spots. At all further repetitive measurements the EEM remained constant at 6.2 ± 2.0 MPa (N=5, n=15).
b
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penetration depth [µm mN-1]
a
12
force [mN]
10 8 6
st
1 2nd 3rd th 4 5th
4 2 0 -2
8 7 6 5 4 3 2
0
20
40
60
80
100
120
1
distance [µm]
2
3
4
5
indentation cycle
Fig. 8 – (a) Force distance curves at indentation into the surface of a VACNT array for EEM testing (five consecutive indentations on the same spot; raw data without penetration correction) (b) Relative penetration of the sapphire sphere into the VACNT surface at EEM testing. The small filled symbols indicate data from single experiments. The red open circles show the mean values of the five experiments (± standard deviation).
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4. Discussion 4.1 Shear adhesion The extremely high values of both friction force and coefficient of friction µ at the initial sliding cycles of friction tests (Figs 5–7) can be explained by the contribution of adhesive forces between the tip layer of the VACNTs and the sapphire sphere of the micro force tester. As shown in figures 2b,c and 3a,b, in the pristine array surface the CNTs are evenly distributed and very probably contact the sphere of the force tester with the sides of their tips, which leads to adhesive van der Waals forces between the probe and many individual contact elements causing the high friction. As also shown in the EEM measurements, the CNTs deform plastically during the first loading cycle. Most likely, the plastic deformation in direct contact with the sapphire sphere leads to the condensation of the top-layer of the CNTs forming the crust seen on the VACNT array surface after the friction experiments (Figs 3c,d). In the crust the single CNTs are condensed together and form a more homogeneous surface contacting the probe in the subsequent sliding cycles. The decrease in the number of potential contact points and the loss of compliant single CNTs adhering to the probe results in the abrupt decrease of friction force from the first to the second sliding cycle. These findings are in agreement with the theory of contact splitting known from contact mechanics of fibrillar adhesives, where arrays with smaller contact elements result in greater adhesive strength [36]. Likely, the condensation of the CNTs, and thereby the crust, gets even more stable in sliding cycles 2 to 5, as may be read from the slight decrease of friction forces and the coefficients of friction (Figs 5–7). 4.2 High friction The coefficients of friction calculated from the static and sliding friction forces divided by the actual normal forces were in the same range and are well described by the linear regression fitting (Fig. 6). After the very high values of the coefficient of friction µ in the range from about 8 at the first sliding cycle, the values of µ decrease down to still respectable values larger than 2 during the consecutive loading cycles. To our knowledge, the initial sliding cycles have not been considered in much detail before. 15
The previously reported coefficient of friction of 1.0–2.2 were measured on 6 µm high VACNT arrays, less than 5 µN normal forces, and gold tips with radii less than 30 µm [25]. Such short VACNTs deformed elastically under very low normal forces and had small contact areas during friction tests. In our study, the 1.1 mm long VACNTs deformed at least partially in the plastic regime. The elastic deformation of short VACNTs also explains, why Kinoshita et al. [25] did not observe wear tracks in the VACNT surface after the tests. Also, the range of friction coefficients for the 6 µm thick VACNT films were close to the values of 2.2–3.5 we found in our experiments for the fifth and consecutive sliding cycles, when the surface of the VACNT array had already compacted and thus stiffened under normal loading in the micro-Newton range. Friction coefficients, obtained in our tests with 3.86 mN normal force, were consistently smaller than those measured in the micro-Newton regime of loading forces. In the milli-Newton regime, the influence of adhesion on friction is generally less pronounced [37], resulting in friction coefficients down to 0.9 at the fifth sliding cycle. From the comparison of our findings with those by Kinoshita et al. [25] the effect of increase of both friction and shear adhesion with increasing thickness of the VACNT film, and the length of the single CNTs, respectively, seems to be very likely. The longer the CNTs in the array, the more mechanically compliant are their tips to external forces, leading to larger contact area of the single CNTs with the opposing surface. 4.3 Absence of pull-off forces The absence of normal adhesive forces in our experiments is consistent with a previous study using a similar testing setup consisting of a spherical probe [25], in spite of different testing regimes and different VACNT morphology. The presence of pull-off forces found by other authors can be explained with different testing methods and VACNT arrangements [19,20,22,38]. Testing of adhesion of VACNTs by pressing with a plate led to an anisotropic alignment of the tips of the 200–500 µm long CNTs in parallel to the test surface [20]. This kind of deformation led to attachment of the CNTs to the test substrate by their sides, and that is why normal pull-off forces, presumably caused by shear adhesion between the single CNTs and the substrate, increased with preload. The latter was later predicted by Finite Element Analysis [38]. Chen et al. [22] reported significant pull-off forces 16
from micro-patterned 170–230 µm high VACNT pillar-like arrays, and an influence of pillar aspect ratio and spacing on both EEM and adhesion strength. Mechanical testing on 65 µm thick films of VACNT brushes using sharp SPM tips resulted in adhesion forces of 1.6 × 10-2 nN nm-2, which was 200 times higher than those of the gecko foot-hairs [19]. These values appear to be overestimated, since very likely most of the CNTs adhered with their sides to the sides of the cantilever tip with the typical radius of curvature smaller than 10 nm and indentation depths up to 250 nm in these tests. 4.4 Increase of EEM after mechanical preconditioning The EEMs of 3.6 ± 0.8 MPa for the pristine samples and stable 6.2 ± 2.0 MPa for the preconditioned VACNTs matched well with the broad range of EEM from 0.1 to 300 MPa reported in the literature [20–24]. The values closest to those obtained in our experiments were measured in tests using microsphere probes with radii of up to 25 µm, resulting in an EEM of up to 2.8 MPa of the top surface and up to 1.4 MPa of the base of VACNT arrays [23]. EEMs from 0.25 to 1.6 MPa obtained in different regimes of compression tests on 200–500 µm thick VACNT bundles were rather close to the above values [20]. The latter authors explained the changes of the elastic behavior during one test with different regimes of engagement of the nanotubes with the test surface. In the above mentioned micropatterned VACNT pillars, nanoindentation with a spherical sapphire tip (radius 400 µm) yielded EEMs from 0.1 to 10 MPa dependent on both the pillar aspect ratio and spacing [22]. A study, using nanoindentation with a standard Berkovich indenter, reported an EEM of 50 MPa on up to 100 µm thick VACNT arrays [21]. In the latter study, the authors pointed out that assumptions had to be made to account for the transition of the Berkovich tip´s shape from spherical to pyramidal during indentation. Finally, EEMs found using microfabricated resonators integrated with the VACNTs were 8–300 MPa for 0.5–100 µm thick films [24]. However, this delicate method focuses more on the simultaneous measurement of thermal and mechanical properties of the VACNT films, and is very sensitive to differences in the distribution density of the CNTs within the array.
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4.5 Clustering Up to now fiber condensation was drawn to be a limiting factor reducing adhesion because of the reduction of the attainable aspect ratio and due to the increase of the packing density of the fibers [7,39,40]. We show that due to the self-organization (clustering) the friction performance of the VACNTs caused by shear adhesion decreased, but the mechanical properties of the VACNTs get durably more stable at still reasonable high levels of friction. Due to the stabilization of the mechanical properties fiber condensation becomes from foe to friend, because it can help to improve long-term performance of the structures. As we see from the friction and indentation curves, plastic deformation occurs mainly during the first loading of the sample. Most likely, the clusters in the friction tracks are established at the very first sliding cycle, as was also shown by numerical modeling [41]. Because of the bending modulus of CNTs in the range between 0.91 and 1.24 TPa [14] and the appearance of the friction tracks, we assume that during the sliding cycles the CNTs rotate around and/or buckle at their base leading to the formation of the clusters.
5. Outlook The frictional coefficients of long VACNT arrays are among the highest reported in the literature so far. The formation of stable clusters of VACNTs has strong potential for the development of CNT-based friction-enhancing surfaces, which can be used for applications where shear adhesion is desired, e.g. pick and place techniques. However, the dramatic change in both the EEM and friction force during the first few handling cycles has to be kept in mind, so that a preconditioning of the VACNT surfaces is necessary, and controlled procedures for this purpose have to be established. Although we have studied fairly long VACNTs, it is not obvious that these are always optimum. Shorter VACNTs might be even better suited for applications due to their higher stiffness. However, on the other hand, their alignment is not as perfect as with the longer ones, since the entanglement, which is intrinsic due to the overall CNT growth process, is still present in the structures giving rise to a significant lesser organization of the CNTs. Concerning the 18
mentioned dry adhesion purposes, in our first qualitative experiments, substrate areas of 1 cm × 1 cm composed of 2 mm long double walled VACNTs (similar in diameter to the ones used in this study) have shown to hold weights up to 500 g for extended periods of time. They thus seem better suited for dry adhesion purposes compared to shorter VACNT arrays with significantly shorter individual CNTs (e.g. below 500 µm). Regarding the mechanical stability and strength of such VACNT arrays, it is known that they can deform mechanically under load/stress. This deformation is often followed by crack formation and buckling of the macroscopic VACNT array structure. This detoriation is obviously dependent on the overall force applied and the particular force direction (e.g. on-top loading or laterally impinged shear stress). Nevertheless it has been shown that if VACNTs are loaded with compression forces impinging in a vertical manner, they can severely deform but do show full recovery to their original morphology (e.g. macroscopic length) in a highly reproducibly manner without showing signs of fatigue even after thousands of load/stress cycles [24,43,44]. The compliance of the VACNT arrays studied herein to the counterpart surface makes them well suited for better contact formation especially with rough surfaces. Hybrid materials as [VACNTs/polymer] are an interesting future material alternative to bare VACNT arrays for our studies combining positive effects of polymers with the stiffness of the CNTs. In such hybrids the interface polymer/CNTs [45,46] might play an important role on the mechanical stiffness of the array surface of the hybrid and should thus allow an alteration of crack formation and plastic deformation behavior.
Acknowledgements We thank Alexander Kovalev for the Matlab software tool for the evaluation of the EEM and constructive discussions. We also thank the Karl and Marie Schack-Stiftung, Frankfurt/Main, Germany, for the generous financial support. This work was supported by a joint grant of the German Research Foundation DFG (Bionik program, projects GO 995/10-1 and SCHN 375/201).
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Appendix A. According to [30], the effective elastic modulus EEM (analogous to the elasticity modulus of a homogeneous material) [42] was calculated using the Hertz model [29]. The formula used for fitting the force–deformation ൫ܨሺݐሻ − ߜ ሺݐሻ൯ curves was as follows. 4 ܨሺݐሻ = ܧ ܴଵ/ଶ ߜ ሺݐሻଷ/ଶ 3 with 1 1 1 = + ܴ ܴ௦ ܴ and 1 1 − ߭௦ଶ 1 − ߭ଶ = + ܧ ܧ௦ ܧ where ܧ is called the reduced modulus. ܧ௦ , ߭௦ , and ܴ௦ are the effective elastic modulus, the Poisson´s ration, and the radius of the sample, respectively. ܧ , ߭ , and ܴ are the same parameter for the probe. The material of the probe (sapphire sphere) was considerably stiffer than the material of the sample (VACNT array) ሺܧ ≫ ܧ௦ ሻ, and the radius of curvature of the sample´s surface was considerably larger than the radius of curvature of the probe´s tip ሺܴ௦ − ܴ ሻ. Therefore, ܴ௦ can be considered to be an infinite radius 1 1 − ߭௦ଶ ≈ ܧ ܧ௦ and ܴ ≈ ܴ Because we do not know the Poisson´s ratio of the VACNT arrays, we assumed them to be incompressible (Poisson´s ratio = 0.5).
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