Effects of nanotube size and roof-layer coating on viscoelastic properties of hybrid diamond-like carbon and carbon nanotube composites

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Effects of nanotube size and roof-layer coating on viscoelastic properties of hybrid diamond-like carbon and carbon nanotube composites Ping-Chi Tsai *, Yeau-Ren Jeng

*

Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan Advanced Institute of Manufacturing with High-Tech Innovations, National Chung Cheng University, Chia-Yi 621, Taiwan

A R T I C L E I N F O

A B S T R A C T

Article history:

Hybrid carbon nanobuffers are developed by exploiting the ultra-hardness and wear-resis-

Received 5 September 2014

tant properties of diamond-like carbon (DLC) coatings and the inherent viscoelasticity

Accepted 7 January 2015

properties of vertically aligned carbon nanotubes (VACNTs). The viscoelastic properties of

Available online 21 January 2015

carbon nanobuffers incorporating thin-walled and thick-walled CNTs, respectively, are characterized by means of nanoindentation dynamic mechanical analysis tests. It is shown that the thin-walled nanobuffer has a better damping performance than the thick-walled nanobuffer due to its buckling-driven friction and post-buckling behaviors; particularly under large displacements. In addition, it is shown that under large indenter displacements, the VACNT arrays with DLC coatings display the improved stress distributions and enhanced strain energy dissipation performances due to the load transfer on the top of VACNTs. Molecular dynamics (MD) simulations are performed to investigate the rooflayer effect on damping behavior and structural deformation of the coated and uncoated VACNTs under nanoindentation. The results confirm that the VACNT with a DLC coating exhibits the significantly damping characterizations than the non-coated VACNT. Overall, the results presented in this study reveal the potential for tuning the damping performance of CNT-based nanobuffers through a careful control of the CNT size.  2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Minimizing the effects of inertial impacts and oscillating forces is a fundamental design problem in all dynamic mechanical systems, including those at the nano-scale [1–3]. In systems characterized by reciprocating motion, effective energy absorbers are essential in improving their efficiency and prolonging their service lives. Recent advances in foamlike materials such as carbon nanotube (CNT)-polymer composites [4–6] and vertically aligned carbon nanotube

(VACNT) films [7–9] provide an unprecedented opportunity to efficiently and reliably inject damping characteristics into load-bearing applications. However, while CNT-polymer composites have a promising viscoelastic damping performance, the suffer a number of important disadvantages, including poor dispersibility [10,11], self-agglomeration weakness [12,13] and inevitable sliding interactions [14,15]. Notably, CNT-metallic matrix composites suffer similar disadvantages [16–18]. In contrast to CNT-based composites, pure CNT films transfer impact responses by means of a direct folding/

* Address: Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan. Fax: +886 5 2720589. E-mail addresses: [email protected] (P.-C. Tsai), [email protected] (Y.-R. Jeng). http://dx.doi.org/10.1016/j.carbon.2015.01.012 0008-6223/ 2015 Elsevier Ltd. All rights reserved.

164

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

buckling of the CNTs and frictional movements among the neighboring CNTs [7,19–21]. However, the viscoelastic properties of such CNT films are limited by the weak interactions among the tubes, which cause the CNT films to split easily under the effects of the impact response [22,23]. Furthermore, the wetting behavior of CNT arrays prompts a transition of the CNT film behavior from a viscoelastic response to an elastic response when exposed to a humid atmosphere and/or a chemically functionalized environment [24–26]. The viscoelastic behavior of CNT films under mechanical compression has been extensively examined in recent years [19–21]. However, relatively few investigations have examined the role played by buckling-driven interactions in mitigating the effects of impact or possible strategies for optimizing the damping performance of CNT films. Thus, the mechanisms responsible for impact resistance and energy absorption in CNT films during compression merit further investigation. The present study develops a hybrid nanobuffer material, in which the inherent viscoelastic properties of VACNTs are combined with the high-hardness and high-wear resistance properties of diamond-like coatings. The mechanical behavior of the nanobuffer material is investigated by means of a

nanoindentation dynamic mechanical analysis (nano-DMA) technique given the use of CNTs with both thin (3 nm) and thick (9 nm) wall thicknesses. Molecular dynamics (MD) simulations are then performed to further examine the effects of the diamond-like carbon (DLC) coating on the structural deformation and damping behavior of the hybrid carbon composites.

2.

Experimental

2.1.

Fabrication of hybrid carbon nanobuffers

As shown in Fig. 1(a), in preparing the VACNTs, a tantalum nitride (TaN) coating and a nickel (Ni) catalyst were deposited in turn on a Si (1 0 0) substrate using a physical vapor deposition (PVD) system with a sputtering power of 800 W and a chamber pressure of 6.4 · 103 Torr. The Ni layer was heated from 350 C to 800 C over a period of 5 min in a N2/H2 (10/90 sccm) plasma gas; causing it to ball up into nanoclusters in order to serve as catalyst particles for a subsequent growth of VACNTs (note that the presence of the TaN coating diminishes the surface mobility of the Ni catalyst particles;

Fig. 1 – The schematic illustrations and their corresponding SEM and HRTEM images acquired at various stages of sample synthesis process: (a) Ni catalyst nucleation on silicon substrate, (b) VACNT growth, and (c) DLC coating on VACNTs. (A color version of this figure can be viewed online.)

CARBON

8 6 (2 0 1 5) 1 6 3–17 3

thereby preventing their coalescence [27]). Fig. 1(b) shows that a VACNT film was then grown using a microwave-plasma chemical vapor deposition (MPCVD) system with an operating frequency of 2.45 GHz [28,29]. During the synthesis process, CH4 feed gas (14 sccm) was introduced into the chamber for 1 min while maintaining a constant substrate temperature of 800 C and reactor pressure of 30 Torr. Finally, as seen in Fig. 1(c), the DLC coating was deposited on the VACNT film using a plasma-enhanced CVD (PECVD) apparatus fed with hexane vapor and argon gas at 0.01–0.03 Torr. Deposition was performed for 1 h with a discharge voltage of 700 V [30]. The average DLC thickness was found to be approximately 1 lm. The growth properties and structural characteristics of CNTs are fundamentally dependent on the catalyst and pretreatment conditions. Thus, in the catalyst pretreatment, the etching temperature was carefully controlled in order to tune the wall thickness of the synthesized CNTs [27]. Fig. 2 shows that details of the CNTs after DLC deposition through the scanning electron microscopy (SEM) and highresolution transmission electron microscopy (HRTEM) images, it is observed that the DLC layers tightly lock the tips of the VACNT film. Note that the intermediate graphitic shells also were observed between the DLC and the CNT, where sep˚. aration distance is estimated at approximately 3.6 A

2.2.

Viscoelasticity of hybrid carbon nanobuffers

The nanoscale dynamic properties of the composite nanobuffers were evaluated by means of nano-DMA tests [31,32]. The nano-DMA technique takes into account both the elastic

165

contribution and the viscous contribution of the test material. In the nano-DMA technique, the storage modulus is computed as pffiffiffi Ks p E0 ¼ pffiffiffiffi ð1Þ 2 A where A and Ks are the projected contact area and dynamic stiffness, respectively. Meanwhile, the loss modulus is calculated as pffiffiffi x  Cs p pffiffiffiffi E00 ¼ ð2Þ 2 A where x is the frequency of the applied force and Cs is the damping coefficient. The nano-DMA tests were performed in the frequency mode. That is, the applied load was specified (thereby fixing the indentation depth) and the probe was oscillated across a range of frequencies. The dynamic damping characteristic of the nanobuffer films was then evaluated as tan d ¼

E00 x  Cs ¼ Ks E0

ð3Þ

The nano-DMA tests were performed using a Hysitron Triboscope (Hysitron Inc., Minneapolis, MN, USA) with a Berkovich tip at room temperature (22 ± 0.5 C) and a relative humidity of 45 ± 2%. The frequency range was specified as 0–20 Hz. In order to prevent creep effects, the indenter was held for 3 min at the point of maximum indentation prior to probe oscillation. It should be noted that the nano-DMA test may induce a slight harmonic response between real and imaginary signals, however such a relatively small deviation

Fig. 2 – Cross-sectional SEM and TEM images of the DLC coating on top of the CNTs (region of HRTEM image showing the ˚ ). (A color version of this lattice spacing of intermediate graphitic shells between CNT and DLC coating is approximately 3.6 A figure can be viewed online.)

166

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

is unlikely to affect the estimate of damping characteristics during the steady state harmonic loading process. Moreover, CNT arrays are also expected to have a significantly lower elastic stiffness (i.e. buckling response) than that of nanoDMA machine, and thus such a deviation between real and imaginary region can be reasonably ignore. A similar observation was also reported for the dynamic mechanical properties of nanomaterials via a continuous stiffness measurement by Pharr et al. [33] and Pathak et al. [34]. Ten indentation tests were performed for each sample. The results obtained for the storage and loss moduli in each test were then averaged in order to obtain a mean value for the sample. A similar technique has also been successfully utilized to ensure the accuracy and reliability of the mechanical responses of CNT-based composites [17,35]. In performing the tests, the machine frame stiffness (and other calibration data) was determined using the relevant protocols prescribed in the machine manual for the Triboscope system.

2.3.

Numerical simulation and model

To complement the experimental work, a series of MD simulations were used to investigate the effect of the DLC coating on the damping behavior and structural deformation of the CNT arrays from the atomistic point of view. Fig. 3 presents a schematic representation of the simulation model. A schematic cross-sectional view of the CNT bundle in the present MD study, which comprises six (5, 5) CNTs that surround a core (5, 5) CNT. Periodic boundary conditions (PBCs) are established in the x and y directions, respectively. A similar technique has also been utilized to investigate the phase transitions in argon-filled CNT bundles under high pressure by Shanavas and Sharma [36]. Additionally, the CNTs in the ˚ apart and their lengths CNT bundle were equally spaced 3.4 A sim were LCNT = 3.68 nm. The diameter of each (5, 5) CNT is Dsim CNT = 0.68 nm, and each CNT bundle contains approximately

4500 atoms. It is noted that the atoms at the bottom of each tube are fixed in space (representing end-constrained atoms). In this study, two sets of samples were made on which the external diameters of the CNTs were approximately 20 nm with the similar length (i.e. approximately 1 lm). Thereby, the CNTs have an aspect ratio close to 0.02, is given by exp

exp

gAR ¼

LCNT Lsim sim CNT exp ¼ K  gAR ¼ K  sim DCNT DCNT

ð4Þ

exp

where gAR is the aspect ratio of the CNTs in the present experexp iment with their lengths and external diameters of LCNT and exp exp DCNT , respectively. Applying Eq. (4), the gAR also is expressed exp in terms of an effective-analogous factor K by gAR ¼ K  gsim AR , is an aspect ratio of the nanotube in our MD simuwhere gsim AR lation. In fact, because CNTs with experimental sizes could not be simulated due to computational power limitations, the geometrically analogous nanotubes (i.e. 3.68 nm in length and 0.68 nm in diameter) presented in the current simulations were deliberately considered instead in order to focus on the coating effects on buckling behaviors of CNT bundles themselves. A similar simplification has also been utilized to investigate the structure deformations of CNTs by Marques et al. [37], Peng et al. [38], and Tsai et al. [17,39]. The indentation and retraction steps are both performed at a constant loading/ unloading rate of 25 m/s in the z direction. The velocity of the atoms at a constant temperature is described using the Nose´-Hoover thermostat [40,41]. Meanwhile, the NVT model is used to control the number of atoms N, the volume V, and the temperature T. A similar MD technique has also been utilized to investigate the mechanical responses of carbon-based nano-materials under nanoindentation in our previous works [42–44]. The buckling behaviors of CNT bundle under nanoindentation were simulated by the numerical solution of the Hamilton equations of motion using Gear’s fifth predictor–

Fig. 3 – Schematic representation of MD simulation model for nanoindentation testing of CNT with and without DLC coating (Note that schematic cross-sectional diagram of a CNT bundle of six (5, 5) CNTs surrounding a core (5,5) CNT, where the ˚ apart). (A color version of this figure can be viewed online.) nanotubes in the CNT bundle are spaced 3.4 A

CARBON

8 6 (2 0 1 5) 1 6 3–17 3

corrector algorithm [45] and a time step is kept fixed at 0.5 fs. The short-range atomic forces of the CNT were estimated for the covalent bonding between the atoms as dictated by the Brenner’s second generation reactive empirical bond order (REBO) potential [46], which is defined as EREBO ¼ VR ðrij Þ  bij VA ðrij Þ

ð4Þ

where VR(r) and VA(r) are repulsive and attractive terms, respectively. The bij term is used to include the reactive empirical bond order between the atoms. In addition, the Lennard-Jones (L-J) potential [47,48] are adopted to account for the long-range nonbonding van der Waals (vdW) interactions between the neighbor CNTs and for evaluation of the interspatial interactions in the CNT bundle. The L-J potential is defined as    rij 12 rij 6 ELJ ¼ 4n  ð5Þ r r where n denotes the well depth parameter and is 4.55 meV [49], and r is the collision diameter between two atoms. Therefore, the complete form of the potential employed can be given by XX Etotal ¼ ðEREBO þ EvdW Þ ð6Þ ij i

j>i

where EvdW is the contribution from L-J potential and is only nonzero as the covalent potential obtained by EREBO becomes zero for bond lengths exceeding 0.2 nm. Thus, EvdW can be evaluated as [50,51] 8 rij < 0:2 nm > <0 EvdW ¼ C3;k ðrij  rk Þ3 þ C2;k ðrij  rk Þ2 0:2 < rij < 0:32 nm ð7Þ > : 0:32 < rij < 1:0 nm EvdW where Cn,k are the cubic spline coefficients.

3.

167

Results and discussions

As shown in Fig. 4(a–e), the carbon nanobuffers have been developed by exploiting the ultra-hardness and wear-resistant properties of DLC coatings and the inherent viscoelasticity properties of VACNTs through a series of the sample preparation process. Since the specific composite structure of hybrid carbon nanobuffers plays a key role in their mechanical and damping responses, the viscoelastic properties of these nanobuffers containing thin-walled and thickwalled CNTs have been characterized by means of nanoDMA tests. As shown in Fig. 4(b and c), the CNTs have a length of approximately 2 lm. It is noted that the surface after the DLC depositions are flat and somewhat few CNTs are observed on the surface, indicating that the DLC deposited uniformly over the tops of the VACNT films. For our comparative studies on the viscoelastic properties of carbon nanobuffers incorporating thin-walled and thick-walled CNTs, Fig. 4(d) shows that the thick-walled CNT has an external diameter of 21 nm and an internal diameter of 3 nm. The wall thickness (9 nm) corresponds to approximately 26 walls, ˚ [52]. given the assumption of an inter-wall spacing of 3.4 A Meanwhile, for the thin-walled CNT (see Fig. 4(e)), the outer diameter is equal to approximately 20 nm, the internal diameter is equal to 12 nm, and the wall thickness (3 nm) corresponds to approximately 13 walls. The Raman spectra of the hybrid VACNT/DLC composites, DLC coatings and VACNTs have been displayed in Fig. 5. Raman spectra were obtained at room temperature by a DILOR triple-monochromator using k = 514.5 nm emission from an argon-ion laser at 150 mW. The Raman spectrum of DLC coating exhibits two broad features: (1) the D-band peak lay around 1350 cm1 resulting from a breathing mode (A1g-type model) of carbon sp2 sites with sixfold rings rather

Fig. 4 – Schematic illustration showing nanoindentation testing of a nanobuffer, (b) low-magnification SEM image of DLC/CNT nanobuffer film, (c) high-magnification SEM image of VACNTs, (d) HRTEM image of thick-walled CNT, and (e) HRTEM image of thin-walled CNT. (A color version of this figure can be viewed online.)

168

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

Fig. 5 – Raman spectra of the DLC coating, the CNT forest, and hybrid DLC/CNT composite.

than chains, in which the breathing mode at the low-frequency domain of 185 cm1 has also been utilized to estimate the chirality [53], diameter distribution [54], and wall thickness of specific CNTs [55,56], and (2) the G-band peak closed to 1540 cm1 taking place the in-plane bond-stretching motion of pairs of carbon sp2 atoms whether in rings or chains [57,58]. Moreover, an intensity ratio of ID/IG was also calculated to the visible Raman data to estimate the sp3 carbon content [59,60]. For the DLC coating with an intensity ratio of ID/IG = 0.29, where this procedure gives the sp3 content of approximately 70%. This suggest that this case of DLC coating fabricated through a plasma-enhanced CVD (PECVD) has a significant structure change from sp2 to sp3bonding phase due to hydrogen effusion behaviors [61,62]. In addition, the VACNT spectrum shows two prominent peaks

lined at 1340 cm1 (D-band) and 1580 cm1 (G-band), respectively. The D-band for CNTs is related to defects, disordered graphite, and even amorphous carbon, whereas the G-band (E2g-type model) is related to well-ordered crystalline graphite [63,64]. The Raman spectrum of VACNT/DLC composite is a combination of both DLC and VACNT Raman characteristics. Specifically, the presence of D-band in VACNT/DLC spectrum is integrated the narrower VACNT D-band with the broader DLC one, and furthermore the G-band also shows a broader feature involving the DLC G-band and the VACNT one. This implies that the DLC coating locks the tips of the VACNTs to form a hybrid carbon composite. A similar observation was also reported for the mechanical behavior of CNT doped DLC composites by Wei et al. [65]. Fig. 6(a and b) shows the variation of storage modulus (E 0 ) and loss modulus (E00 ), respectively, as a function of the loading frequency for the nanobuffer films containing thin-walled and thick-walled CNTs. Note that the indentation depth is equal to 20 nm in every case. It is seen that both moduli are significantly insensitive to the loading frequency, irrespective of the number of walls in the CNT structure. This phenomenon might be attributed to the fact that the frictional losses between the individual CNTs during movement would prevent their structure recoverability to induce a ‘‘solid’’ continuous matter [66]. In practice, the nanobuffer films can be thought of as a dense array of homogeneous pillars (VACNTs) supporting a rigid roof (DLC film). When subjected to a compressive load, the DLC roof distributes the stress uniformly over the CNT pillars; thereby resulting in nearly constant material properties because of entanglement and agglomeration of the CNTs during movement [7]. Thus, the insensitivity of the viscoelastic properties of the nanobuffer films to the dynamic loading frequency is reasonably expected. A similar frequency-independent behavior was also observed in the nanomechanical characterizations of CNT-based composites

Fig. 6 – (a) Storage and (b) loss moduli as function of loading frequency at constant displacement of 20 nm for nanobuffers containing thin- and thick-walled CNTs, respectively. (A color version of this figure can be viewed online.)

CARBON

8 6 (2 0 1 5) 1 6 3–17 3

performed by Suhr et al. [67] and Teo et al. [23]. In addition, it is observed that for a constant frequency, the nanobuffer film containing thin-walled CNTs has both a higher storage modulus and a higher loss modulus than that containing thickwalled CNTs. In general, when aligned CNT arrays are mechanically compressed, the frictional interaction between neighboring CNTs caused by the anisotropic buckling of the tubes leads to a viscoelastic behavior of the CNT array rather than an elastic response [68–70]. The dynamic mechanical response of the present CNT-based nanobuffers can be broadly divided into two state, namely (1) abrupt softness and (2) gradual stiffness. In the abrupt softness state, the higher porosity and wider foam-like structures of the nanobuffers containing thin-walled CNTS result in large and sudden sliding interactions between the bent CNTs. Therefore, it is reasonable that the loss modulus is larger than that of the nanobuffer containing thick-walled CNTS. However, beyond a certain compression point, the thin-walled CNTs tends to intertwine with one another as a result of the larger interspatial gaps, and thus a confinement-induced stiffness effect occurs [71–73]. Consequently, for the nanobuffer containing CNTs with thinner walls, the change in orientation from an isotropic morphology to an anisotropic morphology during compression leads to an increased storage modulus compared to that of the nanobuffer containing CNTs with thicker walls. Note that a similar observation was reported in the compression of multi-walled CNT hairs by Ge et al. [74]. Fig. 7(a and b) shows the variation of the storage and loss moduli, respectively, as a function of the indenter displacement given a constant loading frequency of 4 Hz. It is seen that for both films, the storage modulus reduces with an increasing indentation, whereas the loss modulus increases. This result is reasonable since as the Nano-DMA process proceeds, the disordered and tangled arrangement of the CNT structures limits the ability of the nanobuffer to store energy, while the frictional effects caused by the rubbing together of

169

neighboring tubes in the array results in significant energy losses [75,76]. The results presented in Fig. 7(a and b) also reveal that the nanobuffer containing the thin-walled CNTs has a greater flexibility than that containing the thick-walled CNTs; particularly under large displacements. This phenomenon can be attributed to the fact that the thick-walled CNTs experience a greater difference in the structural deformations of the inner and outer walls, respectively. In general, slender columns experience both buckling and bending instabilities under a sufficiently large compression. At a certain compressive stage, the outer wall of a thicker CNT may buckle while the inner wall remains stable. The inner wall thus serves as a restraint in preventing the outer tube from dramatic deformation in the transverse direction [77]. By contrast, thinwalled CNTs are prone to an abrupt buckle in the sideways direction and the formation of a kink at the center of tube when subjected to axial compression. It can thus be inferred that the damping properties of thin-walled CNTs are intrinsically related to the anisotropic friction induced between adjacent CNTs as the tubes buckle. It is noted that a similar phenomenon was reported by Tsai et al. [78] and Schaper et al. [79] in their in situ observations of the buckling behavior of individual CNTs. To better understand the effect of the DLC coating on the deformation behavior of the composite nanobuffer films, further nano-DMA tests were performed using coated and uncoated VACNT films, respectively. Fig. 8 presents the corresponding results obtained for the variation of the loss tangent (tan d) with the indentation displacement. In order to isolate the roof-layer effect on damping behavior of VACNT array, all of the CNT samples were prepared using identical fabrication conditions in order to minimize inherent errors arising from CNT geometry and morphology. Although the potential discrepancy between the different samples might still be induced as due to material properties, the present comparative studies obtained through performing error

Fig. 7 – (a) Storage and (b) loss moduli as function of loading displacement at constant loading frequency of 4 Hz for nanobuffer films containing thin- and thick-walled CNTs, respectively. (A color version of this figure can be viewed online.)

170

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

Fig. 8 – Variation of loss tangent with indentation displacement for VACNT films with and without DLC coating, respectively. (A color version of this figure can be viewed online.)

minimizations are still adequate for generating qualitative estimates. In addition, the hardness and Young’s modulus of the DLC coating on top of the VACNT array are found to be approximately 26 and 210 GPa, respectively. These results are in good agreement with the known mechanical properties of DLC films by Chen et al. [80] and Erdemir et al. [81]. This

implies that the present DLC coating is rigid enough to transfer most of the load downward to the underlying CNTs. It is seen that for both samples, tan d is relatively insensitive to the indentation displacement for indentation depths greater than approximately 80 nm. Furthermore, it is seen that for all values of the indentation displacement, the value of tan d for the coated nanobuffer film is higher than that for the film with no DLC coating. In other words, the presence of the DLC coating prompts the strain energy to be dissipated rather than stored. This phenomenon supports the assertion above that the hard DLC coating spreads the compressive stress more uniformly over the underlying CNT structures, and therefore results in a significantly enhanced frictional interaction among neighboring CNTs in the array. By contrast, in the uncoated nanobuffer, significant stress concentrations are induced around the indented penetration zone as a result of the indenter geometry [82]. Due to these stress concentrations and the surface roughness of the CNT arrays [25,83,84], the significant friction effect is locally reduced, and hence the energy dissipation also reduces. To further evaluate the effect of the DLC coating on the damping behavior, a series of MD simulations have been performed to study mechanical responses and structural deformation of the CNT bundles. Fig. 9 shows the atomic morphologies of the coated (see Fig. 9(A–C)) and un-coated (see Fig. 9(a–c)) CNT bundles at various stages of the indentation process. Note that the strain, e, is calculated as the change in the initial length of the nanotube divided by the initial length as a flat punch (and/or an indenter) toward the top section of the CNT bundle, i.e. e¼

L0  LðeÞ DL ¼ L0 L0

ð8Þ

Fig. 9 – Atomic snapshots showing structural deformation of coated (A–C) and uncoated (a–c) CNT bundles at the various strains, respectively. (A color version of this figure can be viewed online.)

CARBON

8 6 (2 0 1 5) 1 6 3–17 3

where L0 is the initial length of the tube and DL is the axial displacement imposed to the nanotube. As shown in Fig. 9(A and a), at no strain of e = 0.00, both CNT bundles retain their original structures. However, as the strain is increased to e = 0.04 and e = 0.07 for two loading conditions, shell-buckling deformation [85] occurs in the coated film (see Fig. 9(B)), while Euler-buckling deformation [78] and sideways bending [86] occurs in the uncoated film (Fig. 9(b)). Finally, at a further strain of e = 0.08 and e = 0.14 for two loading conditions, the coated nanobuffer film undergoes excessively global buckling (Fig. 9(C)), while the uncoated film experiences slightly local buckling and splitting (Fig. 9(c)). It is noted that the MD simulation results are consistent with the theoretical observations reported previously for CNT bundles by Liew et al. [87,88]. Specifically, the simulation results suggest that the higher loss modulus of the DLC-coated film can be attributed to a large number of buckling-driven releases and further contact areas as a result of shell-buckling mechanism, which enhance the stress relaxation and energy dissipation effect [39].

4.

Conclusions

In summary, composite nanobuffer films comprising VACNT arrays with a DLC coating have been fabricated using PVD, MPCVD and PECVD techniques. The dynamic properties of nanobuffer films containing CNTs with wall thicknesses of approximately 3 nm and 9 nm, respectively, have been examined by a series of experimental nano-DMA tests and MD simulations. The results have shown that the nanobuffer with thin-walled CNTs has a better vicoelastic behavior than that with thick-walled CNTs, and therefore provides an improved damping performance; particularly under large compression displacements. In the present MD simulation, it has been shown that compared to an uncoated CNT array, the CNT array with a DLC coating has an improved energy dissipation performance since the coating distributes the compression stress uniformly over the CNT array and prompts multiple shell-buckling events as deformation proceeds. Remarkably, the experimental and numerical results show that the composite nanobuffer films developed in this study effectively combine the high hardness and high-wear resistance properties of DLC films with the inherent viscoelastic properties of VACNTs. As a result, they provide a promising damping material for systems characterized by reciprocating motion. Importantly, the results have also shown the potential for tuning the damping properties of the nanobuffer films through an appropriate control of the CNT wall thickness.

Acknowledgments The authors gratefully acknowledge the support provided to this research by National Science Council, Taiwan, NSC 1012120-M-194-002 and 101-2120-M-194-003. The support of AFOSR under Contract No. FA4869-06-1-0056 AOARD 064053 is also acknowledged.

171

R E F E R E N C E S

¨ stu¨nel H, Roundy D, Arias TA, McEuen [1] Sazonova V, Yaish Y, U PL. A tunable carbon nanotube electromechanical oscillator. Nature 2004;431:284–7. [2] Hu¨ttel AK, Steele GA, Witkamp B, Poot M, Kouwenhoven LP, Van der Zant HSJ. Carbon nanotube as ultrahigh quality factor mechanical resonators. Nano Lett 2009;9(7):2547–52. [3] Loh OY, Espinosa HD. Nanoelectromechanical contact switches. Nat Nanotechnol 2012;7(5):283–95. [4] Suhr J, Koratkar N, Keblinski P, Ajayan P. Viscoelasticity in carbon nanotube composites. Nat Mater 2005;4:134–7. [5] Suhr J, Korakar N. Energy dissipation in carbon nanotube composites: a review. J Mater Sci 2008;43(13):4370–82. [6] Dai L, Chang DW, Baek JB, Lu W. Carbon nanomaterials for advanced energy conversion and storage. Small 2012;8(8):1130–66. [7] Cao A, Dickrell PL, Sawyer WG, Nejhad MNG, Ajayan PM. Super-compressible foam-like films of carbon nanotubes. Science 2005;310:1307–10. [8] Suhr J, Victor P, Ci L, Sreekala S, Zhang X, Nalamasu O, et al. Fatigue resistance of aligned carbon nanotube arrays under cyclic compression. Nat Nanotechnol 2007;2(7):417–21. [9] Liu L, Ma W, Zhang Z. Macroscopic carbon nanotube assemblies: preparation, properties, and potential applications. Small 2011;7(11):1504–20. [10] Yoon H, Yamashita M, Ata S, Futaba DN, Yamada T, Hata K. Controlling exfoliation in order to minimize damage during dispersion of long SWCNTs for advanced composites. Sci Rep 2014;4:3907-1–8. [11] Nish A, Hwang JY, Doig J, Nicholas RJ. Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers. Nat Nanotechnol 2007;2(10):640–6. [12] Ajayan PM, Tour JM. Materials science: nanotube composites. Nature 2007;447:1066–8. [13] Deng F, Ito M, Noguchi T, Wang L, Ueki H, Niihara K, et al. Elucidation of the reinforcing mechanism in carbon nanotube/rubber nanocomposites. ACS Nano 2011;5(5):3858–66. [14] Qian D, Dickey EC, Andrews R, Rantell T. Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites. Appl Phys Lett 2000;76(20):2868–70. [15] Chen X, Zheng M, Park C, Ke C. Strength of carbon nanotube– poly(methyl methacrylate) interfaces. Small 2013;9(19):3345–51. [16] Liu ZY, Xiao BL, Wang WG, Ma ZY. Singly dispersed carbon nanotube/aluminum composites fabricated by powder metallurgy combined with friction stir processing. Carbon 2012;50(5):1843–52. [17] Tsai PC, Jeng YR. Experimental and numerical investigation into the effect of carbon nanotube buckling on the reinforcement of CNT/Cu composites. Compos Sci Technol 2013;79:28–34. [18] Zhang Q, Huang JQ, Qian WZ, Zhang YY, Wei F. The road for nanomaterials industry: a review of carbon nanotube production, post-treatment, and bulk applications for composites and energy storage. Small 2013;9(8):1237–65. [19] Hutchens SB, Hall LJ, Greer JR. In situ mechanical testing reveals periodic buckle nucleation and propagation in carbon nanotube bundles. Adv Funct Mater 2010;20(14):2338–46. [20] Bradford PD, Wang X, Zhao H, Zhu YT. Tuning the compressive mechanical properties of carbon nanotube foam. Carbon 2011;49(8):2834–41. [21] Pathak S, Lim EJ, Abadi PPSS, Graham S, Cola BA, Greer JR. Higher recovery and better energy dissipation at faster strain rates in carbon nanotube bundles: an in-situ study. ACS Nano 2012;6(3):2189–97.

172

CARBON

8 6 ( 2 0 1 5 ) 1 6 3 –1 7 3

[22] Du AJ, Smith SC. Van der Waals-corrected density functional theory: benchmarking for hydrogen-nanotube and nanotube-nanotube interactions. Nanotechnology 2005;16(10):2118–23. [23] Teo EHT, Yung WKP, Chua DHC, Tay BK. A carbon nanomattress: a new nanosystem with intrinsic, tunable, damping properties. Adv Mater 2007;19(19):2941–5. [24] Wirth CT, Hofmann S, Robertson J. Surface properties of vertically aligned carbon nanotube arrays. Diamond Relat Mater 2008;17(7–10):1518–24. [25] Tsai PC, Jeng YR, Mao CP, Wu KT, Hong CN. Effects of surface morphology, size effect and wettability on interfacial adhesion of carbon nanotube arrays. Thin Solid Films 2013;545:401–7. [26] Eom K, Nam K, Jung H, Kim P, Strano MS, Han JH, et al. Controllable viscoelastic behavior of vertically aligned carbon nanotube arrays. Carbon 2013;65:305–14. [27] Jeng YR, Wen HC, Tsai PC. The effect of Ni catalytic nanoparticle on the growth of carbon nanotubes: a perspective from nanotribological characterization. Diamond Relat Mater 2009;18(2–3):528–32. [28] Kinoshita H, Ippei I, Sakai H, Ohmae N. Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films. Diamond Relat Mater 2007;16(11):1940–4. [29] Zanin H, Saito E, Ceragioli HJ, Baranauskas V, Corat EJ. Reduced graphene oxide and vertically aligned carbon nanotubes superhydrophilic films for supercapacitors devices. Mater Res Bull 2014;49:487–93. [30] Wachesk CC, Pires CAF, Ramos BC, Airoldi VJT, Lobo AO, Soares CP, et al. Cell viability and adhesion on diamond-like carbon films containing titanium dioxide nanoparticles. Appl Surf Sci 2013;266:176–81. [31] Bhushan B, Li X. Nanomechanical characterisation of solid surfaces and thin films. Int Mater Rev 2003;48(3):125–64. [32] Espinosa HD, Bernal RA, Filleter T. In-situ TEM electromechanical testing of nanowires and nanotubes. Small 2012;8(21):3233–52. [33] Pharr GM, Strader JH, Oliver WC. Critical issues in making small-depth mechanical property measurements by nanoindentation with continuous stiffness measurement. J Mater Res 2009;24(3):653–66. [34] Pathak S, Cambaz ZG, Kalidindi SR, Swadener JG, Gogotsi Y. Viscoelasticity and high buckling stress of dense carbon nanotube brushes. Carbon 2009;47(8):1969–76. [35] Li XD, Gao HS, Scrivens WA, Fei DL, Xu XY, Sutton MA, et al. Nanomechanical characterization of single-walled carbon nanotube reinforced epoxy composites. Nanotechnology 2004;15:1416–23. [36] Shanavas KV, Sharma SM. Molecular dynamics simulations of phase transitions in argon-filled single-walled carbon nanotube bundles under high pressure. Phys Rev B 2009;79:155425-1–8. [37] Marques MAL, Troiani HE, Yoshida MM, Yacaman MJ, Rubio A. On the breaking of carbon nanotubes under tension. Nano Lett 2004;4(5):811–5. [38] Peng B, Locascio M, Zapol P, Li S, Mielke SL, Schatz GC, et al. Measurements of near-ultimate strength for multiwalled carbon nanotubes and irradiation induced crosslinking improvements. Nat Nanotechnol 2008;3(10):626–31. [39] Jeng YR, Tsai PC, Fang TH. Experimental and numerical investigation into buckling instability of carbon nanotube probes under nanoindentation. Appl Phys Lett 2007;90(16):161913-1–3. [40] Nose´ S. A unified formulation of the constant temperature molecular dynamics methods. J Chem Phys 1984;81:511–9. [41] Hoover WG. Canonical dynamics: equilibrium phase-space distributions. Phys Rev A 1985;31(3):1695–7.

[42] Tsai PC, Jeng YR, Fang TH. Coalescence, melting, and mechanical characteristics of carbon nanotube junctions. Phys Rev B 2006;74:45406-1–45406-10. [43] Tsai PC, Fang TH. A molecular dynamics study of the nucleation, thermal stability and nanomechanics of carbon nanocones. Nanotechnology 2007;18:105702-1–7. [44] Tsai PC, Jeng YR. Theoretical investigation of thermally induced coalescence mechanism of single-wall carbon nanohorns and their mechanical properties. Comput Mater Sci 2014;88:76–80. [45] Haile JM. Molecular dynamics simulation: elementary method. New York: Wiley; 1992. [46] Brenner DW, Shenderova OA, Harrison JA, Stuart SJ, Ni B, Sinnott SB. A second-generation reactive empirical bond order (REBO) potential energy expression for hydrocarbons. J Phys: Condens Matter 2002;14:783–802. [47] Lennard-Jones LE. On the determination of molecular fields— I. From the variation of the viscosity of a gas with temperature. Proc R Soc Lond Ser A 1924;106(738):441–63. [48] Lennard-Jones LE. On the determination of molecular fields— II. From the equation of state of a gas. Proc R Soc Lond Ser A 1924;106(738):463–77. [49] Rappe AK, Casewit CJ, Colwell KS, Goddard III WA, Skiff WM. UFF, a full periodic table force field for molecular mechanics and molecular dynamics simulations. J Am Chem Soc 1992;114(25):10024–39. [50] Mao Z, Garg A, Sinnott SB. Molecular dynamics simulations of the filling and decorating of carbon nanotubules. Nanotechnology 1999;10(3):273–7. [51] Sinnott SB, Shenderova OA, White CT, Brenner DW. Mechanical properties of nanotubule fibers and composites determined from theoretical calculations and simulations. Carbon 1998;36(1–2):1–9. [52] Ebbessen W, Ajayan PM. Large-scale synthesis of carbon nanotubes. Nature 1992;358:220–2. [53] Damnjanovic´ M, Dobardzˇic´ E, Milosˇevic´ I. Chirality dependence of the radial breathing mode: a simple model. J Phys: Condens Matter 2004;16:505–8. [54] Sauvajol JL, Anglaret E, Rols S, Anglaret L. Phonons in single wall carbon nanotube bundles. Carbon 2002;40:1697–714. [55] Batra RC, Gupta SS. Wall thickness and radial breathing modes of single-walled carbon nanotubes. J Appl Mech 2008;75:61010-1–6. [56] Dresselhausa MS, Dresselhausb G, Saitoc R, Joriod A. Raman spectroscopy of carbon nanotubes. Phys Rep 2005;409:47–99. [57] Daimay LV, Colthurp NB, Fateley WG, Grasselli JG. The handbook of infrared and Raman characteristic frequencies of organic molecules. New York: Academic Press; 1991. 117–54. [58] Rodil SE, Ferrari AC, Robertson J, Milne WI. Raman and infrared modes of hydrogenated amorphous carbon nitride. J Appl Phys 2001;89(10):5425–30. [59] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys Rev B 2000;61(20):14095–107. [60] Robertson J. Diamond-like amorphous carbon. Mater Sci Eng Rep 2002;37:129–281. [61] Li H, Xu T, Wang C, Chen J, Zhou H, Liu H. Annealing effect on the structure, mechanical and tribological properties of hydrogenated diamond-like carbon films. Thin Solid Films 2006;515(4):2153–60. [62] Jeng YR, Tsai PC, Wu KT, Wang YM, Hong CN, Huang SM, et al. Effect of feed gas composition effects on the nanotribological properties of diamond-like carbon films. Thin Solid Films 2013;529:301–5. [63] May PW, Hohn S, Wang WN, Fox NA. Field emission conduction mechanisms in chemical vapor deposited diamond and diamond like carbon films. Appl Phys Lett 1998;72(17):2182–4.

CARBON

8 6 (2 0 1 5) 1 6 3–17 3

[64] Marciano FR, Bonetti LF, Lima-Oliveira DA, Mello CB, Ueda M, Corat EJ, et al. Characterization of crystalline diamond incorporated diamond-like carbon films. Diamond Relat Mater 2010;19(10):1139–43. [65] Wei CH, Wang CI, Tai FC, Ting K, Chang RC. The effect of CNT content on the surface and mechanical properties of CNTs doped diamond like carbon films. Diamond Relat Mater 2010;19(5–6):562–6. [66] Hutchens SB, Needleman A, Greer JR. A microstructurally motivated description of the deformation of vertically aligned carbon nanotube structures. Appl Phys Lett 2012;100:121910-1–4. [67] Suhr J, Zhang W, Ajayan PM, Koratkar NA. Temperatureactivated interfacial friction damping in carbon nanotube polymer composites. Nano Lett 2006;6(2):219–23. [68] Cao A, Dickrell PL, Sawyer WG, Nejhad MNG, Ajayan PM. Super-compressible foamlike carbon nanotube films. Science 2005;310(5752):1307–10. [69] Xu M, Futaba DN, Yamada T, Yumura M, Hata K. Carbon nanotubes with temperature-invariant viscoelasticity from 196 to 1000C. Science 2010;330(6009):1364–8. [70] Maschmann MR, Zhang Q, Du F, Dai L, Baur J. Length dependent foam-like mechanical response of axially indented vertically oriented carbon nanotube arrays. Carbon 2011;49(2):386–97. [71] Herrera JMR, Terrones M, Terrones H, Dag S, Meunier V. Covalent 2D and 3D networks from 1D nanostructures: designing new materials. Nano Lett 2007;7(3):570–6. [72] Li Y, Qiu XM, Yang F, Yin YJ, Fan QS. Stretching-dominated deformation mechanism in a super square carbon nanotube network. Carbon 2009;47(3):812–9. [73] Fu Y, Carlberg B, Lindahl N, Lindvall N, Bielecki J, Matic A, et al. Templated growth of covalently bonded threedimensional carbon nanotube networks originated from graphene. Adv Mater 2012;24(12):1576–81. [74] Ge L, Ci L, Goyal, Shi AR, Mahadevan L, Ajayan PM, et al. Cooperative adhesion and friction of compliant nanohairs. Nano Lett 2010;10(11):4509. 4509-13. [75] Pathak S, Lim EJ, Abadi PPSS, Graham S, Cola BA, Greer JR. Higher recovery and better energy dissipation at faster strain rates in carbon nanotube bundles: an in-situ study. ACS Nano 2012;6(3):2189–97.

173

[76] Suhr J, Koratkar N, Keblinski P, Ajayan P. Viscoelasticity in carbon nanotube composites. Nat Mater 2005;4(2):134–7. [77] Pantano A, Boyce MC, Parks DM. Nonlinear structural mechanics based modeling of carbon nanotube deformation. Phys Rev Lett 2003;91(14):145504-1–4. [78] Tsai PC, Jeng YR, Huang YX, Wu KT. Buckling characterizations of an individual multi-walled carbon nanotube: insights from quantitative in situ transmission electron microscope nanoindentation and molecular dynamics. Appl Phys Lett 2013;103(5):53119-1–4. [79] Schaper AK, Wang MS, Xu Z, Bando Y, Golberg D. Comparative studies on the electrical and mechanical behavior of catalytically grown multiwalled carbon nanotubes and scrolled graphene. Nano Lett 2011;11(8):3295–300. [80] Chen LY, Hong CN. Diamond-like carbon nanocomposite films. Appl Phys Lett 2003;82:3526–8. [81] Erdemir A, Donnet C. Topical Review-Tribology of diamondlike carbon films: recent progress and future prospects. J Phys D: Appl Phys 2006;39:R311–27. [82] Gouldstone A, Chollacoop N, Dao M, Li J, Minor AM, Shen YL. Indentation across size scales and disciplines: recent developments in experimentation and modeling. Acta Mater 2007;55(12):4015–39. [83] Daraio C, Nesterenko VF, Jin S. Highly nonlinear contact interaction and dynamic energy dissipation by forest of carbon nanotubes. Appl Phys Lett 2004;85(23):5724–6. [84] Kesari H, Lew AJ. Effective macroscopic adhesive contact behavior induced by small surface roughness. J Mech Phys Solids 2011;59(12):2488–510. [85] Hu N, Nunoya K, Pan D, Okabe T, Fukunaga H. Prediction of buckling characteristics of carbon nanotubes. Int J Solids Struct 2007;44(20):6535–50. [86] Duan X, Tang C, Zhang J, Guo W, Liu ZF. Two distinct buckling modes in carbon nanotube bending. Nano Lett 2007;7(1):143–8. [87] Liew KM, Wong CH, Tan MJ. Buckling properties of carbon nanotubes bundles. Appl Phys Lett 2005;87(4):41901-1–3. [88] Liew KM, Wong CH, Tan MJ. Tensile and compressive properties of carbon nanotube bundles. Acta Mater 2006;54(1):225–9.