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Cite this: Chem. Commun., 2014, 50, 469
Fabrication of novel graphene–fullerene hybrid lubricating films based on self-assembly for MEMS applications†
Received 30th September 2013, Accepted 1st November 2013
Jibin Pu,a Yufei Mo,bc Shanhong Wana and Liping Wang*a
DOI: 10.1039/c3cc47486k www.rsc.org/chemcomm
The novel graphene–C60 hybrid films have been fabricated successfully on silicon surfaces by a multistep self-assembly process, and showed synergistic effects beyond individual performance in micro/ nano-tribological behaviors. It is expected that the graphene–C60 hybrid films may find wide applications as high performance lubricating films in MEMS.
Micro-electromechanical systems (MEMS) have been expected to trigger the creation of a new and future prominent industry. However, the micromachines have been considered machines incapable of movement because of a marked increase in friction. Thus, the realization of a novel lubricity system and/or a novel lubricant, making it possible for the micromachines to move easily, is strongly desired. Graphene, a novel and fascinating two-dimensional (2D) atomically thin carbon nanomaterial,1 has attracted increasing attention in recent years in view of its prominent properties such as superior electronic and mechanical properties,2 extremely high thermal conductivity and large theoretical specific surface area.3 Especially, as a basic building block of a macroscopic solid lubricant graphite/ graphene was found to be an effective solid lubricant due to its weakly stacked lamellar structure, and can prevent wear of the contact surfaces in motion due to its intrinsic strength.2,4,5 So far, many investigations have been focused on the solid lubricating film of graphene, which has a prominent improvement in terms of the friction coefficient.6–9 Filleter et al.10 suggested that epitaxial graphene films could be used to further reduce friction on silicon carbide surfaces. Kim and coworkers11 reported that the CVD-grown graphene films could a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. E-mail: [email protected]; Fax: +86-09314968163; Tel: +86-09314968080 b Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, P. R. China c School of Engineering & Applied Science, The George Washington University, Washington, DC 20052, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cc47486k
This journal is © The Royal Society of Chemistry 2014
effectively reduce the adhesion and friction forces between contact surfaces on the micro- and nanoscale. Another allotropic carbon nanomaterial, fullerene C60, have been considered to be super solid lubricants due to their spherical molecular structure, good stress resistance and stiffness, much lower surface energy, good chemical stability, etc.12,13 However, many of the physisorbed C60 films reported thus far have exhibited ‘‘island’’ formation and shown low mechanical stability under high pressure due to insufficient coverage or aggregation by sublimation of C60 onto a solid surface.14 Thus the formation of mechanically stable, ordered C60 films firmly attached to a solid substrate is considered to be a necessary step in the exploitation of their potential tribological properties. In light of the aforementioned advantages of two carbon nanomaterials, the hybridization of graphene with C60 will produce a new class of carbon-based nanocomposites which are expected to possess remarkable hybrid properties, such as more intriguing tribological properties. However, to the best of our knowledge, few studies have paid attention to the fabrication of the graphene–C60 hybrid nanomaterials because it is still a great challenge to develop facile and efficient synthetic approaches for graphene–C60 hybrids. In a few reports,15–17 C60 derivatives (such as pyrrolidine fullerene) were only grafted onto the edge of graphene via a coupling reaction between the ester group (or hydroxyl group) of C60 derivatives and the –COOH group of graphene oxide (GO), which could not effectively show hybridization effects between graphene and C60. Herein, we have developed a multistep self-assembly method to fabricate a graphene–C60 hybrid film on silicon surfaces, in which pristine C60 without involving any surfactant was chemisorbed uniformly on the whole surface of the amine-functionalized graphene bonded on a silicon surface via the nucleophilic addition reaction (see ESI1†), as shown in Fig. 1. In the self-assembly process, we gained an insight into changes in surface chemical components by water contact angle (WCA) measurement (see ESI3†). First, a selfassembled monolayer of (3-aminopropyl)triethoxysilane (APTES) was covalently anchored onto the silicon wafer via Si–O–Si covalent bonding. GO was then covalently attached onto the APTES-SAM surface through certain chemical reactions between epoxy–carboxyl
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Fig. 1 The schematic diagram of the construction process of the graphene–C60 hybrid film.
and amine groups. Subsequently, the GO films assembled on silicon wafers were amine-functionalized by (N-[3-(trimethoxylsilyl)propyl]ethylenediamine) DA molecules (labeled as a-GO). After this, abundant primary and secondary amino groups on amine-functionalized graphene could undergo N–H addition reactions across the CQC bonds in C60 which fused two six-membered rings.13,18 Chemical bonding of the C60 moieties to the surfaces of graphene not only provided discrete monolayers of C60, but also organized assemblies in which the C60 moieties would remain immobilized on the surface of graphene. Finally, the target APTES–a-graphene–C60 hybrid films on silicon wafers were obtained after heat treatment. We characterized the morphology of the films by AFM. As shown in Fig. 2a, the APTES–GO film was quite homogenous, where GO nanosheets showed the irregularly sheet-like character over the substrate. The section view (Fig. 2f) analysis suggested that the thickness of the GO sheet was about 1 nm, indicating the few-layer GO with 2–4 layers. After grafting with DA molecules, the resultant APTES–a-GO (Fig. 2b) became more compact, in which GO nanosheets were not so easy to differentiate from the matrix, and showed fuzzy surface morphology due to the presence of DA molecules. In Fig. 2c, the surface of the APTES–a-graphene–C60 film was still flat on the micrometer scale with rarely occurring macroscopic corrugations. The detailed view of the APTES–a-graphene–C60 film showed a number of regular spherical features on the graphene surface (Fig. 2d), which demonstrated that tiny C60 aggregates with an average size of B15 nm scattered uniformly on graphene nanosheets by means of addition reactions between the amino-functionalized
Fig. 2 Tapping-mode AFM images of (a) APTES–GO film, (b) APTES–aGO film, (c) APTES–a-graphene–C60 film and (e) APTES–C60 film on Si substrates. (d) High-magnification image of (c). AFM section view of (f) APTES–GO film and (g) APTES–C60 film.
470 | Chem. Commun., 2014, 50, 469--471
Fig. 3 FTIR spectra of APTES-SAM, GO, APTES–GO and APTES–agraphene–C60 films.
graphene and C60 molecules (see ESI4†). The close-packed alkyl chains can locate the C60 molecules in a certain region, the selfaggregation will be limited and their scales were decreased. When C60 molecules were directly grafted onto the APTES-SAM surface, the grainy morphology of the resultant APTES–C60 film (Fig. 2e) reflects the surface domain microstructure composed of C60 clusters. To demonstrate the actual occurrence of chemical reactions among APTES, GO and C60, we obtained the FTIR spectra of the films (Fig. 3). After GO was assembled to the APTES-SAM surface, the epoxide nC–O band intensities significantly decreased due to the ring-opening of epoxy groups under the nucleophilic attack of the amines, the carboxylic CQO stretching vibration at 1725 cm 1 almost disappeared, and the new broad bands emerged at 1613/1513 cm 1 which could be associated with nCQO and dN–H of zwitterionic COO–NH3+ linkages.8 These results clearly indicate that GO has been bonded successfully to the APTES-SAM surface via the zwitterionic COO–NH3+ linkages which would convert finally the covalent amide linkage (–NHCQO) after heat treatment. In the FTIR spectrum of APTES–a-graphene–C60, the increase of methylene nC–H and nSi–O–Si and dN–H band intensities validated amino-functionalization of graphene by a DA silane coupling agent. The new C60 characteristic peaks at 1429 cm 1 and 1187 cm 1 in the spectrum of the resultant APTES–a-graphene–C60 indicated that C60 was successfully attached chemically to the amine-functional GO by nucleophilic addition reactions.19 FTIR, XRD, TGA, Raman spectroscopy and XPS were also conducted to further identify C60 grafted onto amino-functionalized graphene (see ESI5–9†). In order to estimate the anti-stiction and wear-resistant behaviors of these films, the adhesion behaviors (Fig. 4a) and nano-tribological performances of the films were investigated using the colloidal probe (ESI2†). The superhydrophilic hydroxylated Si surface showed strong adhesion because of the large capillary force caused by the adsorbed water, which has been reflected by its static water contact angles (ESI3†). Once APTES–GO was generated thereon, the adhesive force decreased about 66% compared with that on the hydroxylated Si. The water contact angle measurements revealed that the APTES–GO film has lower surface energy than hydroxylated Si, resulting in a low adhesive force. However, the amino-terminated APTES–a-GO film
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Fig. 4 (a) Adhesive forces and (b) nanofriction coefficients (NFC) between the microsphere of the colloid probe and the surfaces of the films. The inset shows the SEM image of the colloid probe. (c) Variation in microfriction coefficient with sliding time for the films.
showed increased adhesive force because of their great hydrophilicity. As for APTES–C60 and APTES–a-graphene–C60 films with lower adhesive forces, aside from low surface energy of C60 molecules with a very stable closed-shell bond structure, another major reason for low adhesion was that the microbumps’ orderly structure of the C60 outer layer significantly reduced the contact area, thereby largely reducing the van der Waals force between the microsphere of the colloidal probe and the films. The nanotribological performances of these samples with linearly increased loads are shown in Fig. 4b. The nanofriction forces of these films changed in the order of their adhesive forces. This is easy to understand because the adhesive force has a marked effect on nanotribological properties. The C60-containing films exhibited a lower nanofriction coefficient (NFC) because of weak interaction (low adhesion) and highly stable spherical molecules. Among these, the more ordered APTES–a-graphene–C60 film showed a minimum NFC of 0.1. The improved nanolubricity was partly attributed to the cobblestone effect of the more ordered C60 outerlayer, which facilitated microsphere sliding because of the smaller contact area between the microsphere and the films. In addition, the presence of the few-layer graphene with excellent nanolubrication properties also contributed to the low NFC. The microtribological performances (ESI2†) of these thin films are very important for their application in protection layers. As shown in Fig. 4c, unlike its good nanotribological performances, the APTES–C60 film displayed poor microtribological properties characterized by high friction coefficient and a very short antiwear life of 138 s under the testing conditions of 0.1 N and 1 Hz. This might be because the C60 film directly bonded on the Si substrate film was not compact enough, and thus easily wore out. For the APTES–GO film, a reduced microfriction coefficient (MFC) (B0.25) and a lengthened antiwear life (B2600 s) were observed. The more compliant few-layer graphene could adhere to the Si surface under the applied load remaining localized over the same contact area. Consequently, the graphene layer prevented the direct contact between asperities on the steel ball counterpart and the Si surfaces and absorbed the energy generated by compression and shear during friction sliding. A possible
This journal is © The Royal Society of Chemistry 2014
Communication
explanation for the reduction in MFC may be that graphene interlayer sliding decreased the friction. Upon grafting C60, the resulting APTES– a-graphene–C60 film exhibited both reduction of MFC below 0.25 and a further increased antiwear life of above 1 h under the same testing conditions. The enhanced lubricating and antiwear properties may be attributed to the synergistic effect between the graphene layer as the load-carrying and wear resistant phase and the C60 outer layer as the friction reducer (mimicking ball bearing20). Under mechanical compression in a sliding interface, the C60 molecules would be readily detached, and may roll among hydrocarbon chains like many tiny ball bearings, leading to low friction and low wear. Such a micro-rolling effect of tiny C60 aggregates (o20 nm) was also confirmed by many researchers.14 The strong bonds between the carbon atoms in C60 molecules make C60 molecular structure very stable and not easily damaged, which is good for a long-lasting and effective lubrication. Thus, the wear resistance capacity of the APTES–a-graphene–C60 multilayer film was greatly enhanced. In conclusion, we have devised a novel approach to tether the fullerene C60 to amino-functionalized graphene employing an aminosilane self-assembled monolayer. The combination of graphene and C60 provides synergistic effects beyond individual performance. The resultant graphene–C60 hybrid film fabricated on the Si surface exhibited good friction reduction, load-carrying capacity, and antiwear ability due to the low surface energy, and the rolling effect of C60 molecules, together with high mechanical stiffness and interlayer sliding of graphene, which will make the carbon-based hybrid films possess potential in tribological applications for MEMS. The authors thank the National Natural Science Foundation of China (No 51105352 and 21373249) for financial support.
Notes and references 1 A. K. Geim and K. S. Novoselov, Nat. Mater., 2007, 6, 183. 2 C. Lee, X. Wei, J. W. Kysar and J. Hone, Science, 2008, 321, 385. 3 A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao and C. N. Lau, Nano Lett., 2008, 8, 902. 4 C. Lee, Q. Li, K. William, X. Z. Liu, H. Berger, R. W. Carpick and J. Hone, Science, 2010, 328, 76. 5 Z. Deng, A. Smolyanitsky, Q. Li, X. Q. Feng and R. J. Cannara, Nat. Mater., 2012, 11, 1032. 6 M. S. Won, O. V. Penkov and D. E. Kim, Carbon, 2013, 54, 472. 7 D. Berman, A. Erdemir and A. V. Sumant, Carbon, 2013, 54, 454. 8 J. F. Ou, J. Q. Wang, S. Liu, B. Mu, J. F. Ren, H. G. Wang and S. R. Yang, Langmuir, 2010, 26, 15830. 9 J. B. Pu, S. H. Wan, W. J. Zhao, Y. F. Mo, X. Q. Zhang, L. P. Wang and Q. J. Xue, J. Phys. Chem. C, 2011, 115, 13275. 10 T. Filleter, J. L. McChesney, A. Bostwick, E. Rotenberg, K. V. Emtsev, T. Seyller, K. Horn and R. Bennewitz, Phys. Rev. Lett., 2009, 102, 086102. 11 K. S. Kim, H. J. Lee, C. Lee, S. K. Lee, H. Jang, J. H. Ahn, J. H. Kim and H. J. Lee, ACS Nano, 2011, 5, 5107. ¨thi, E. Meyer, H. Haefke, L. Howald, W. Gutmannsbauer and 12 R. Lu ¨ntherodt, Science, 1994, 266, 1979. H. J. Gu 13 S. L. Ren, S. R. Yang and Y. P. Zhao, Langmuir, 2004, 20, 3601. 14 P. Y. Zhang, J. J. Lu, Q. J. Xue and W. M. Liu, Langmuir, 2001, 17, 2143. 15 D. S. Yu, K. Park, M. Durstock and L. M. Dai, J. Phys. Chem. Lett., 2011, 2, 1113. 16 Y. Zhang, L. Q. Ren, S. R. Wang, A. Marathe, J. Chaudhuri and G. G. Li, J. Mater. Chem., 2011, 21, 5386. 17 X. Y. Zhang, Y. Huang, Y. Wang, Y. F. Ma, Z. F. Liu and Y. S. Chen, Carbon, 2008, 47, 313. 18 K. Chen, W. B. Caldwell and C. A. Mirkin, J. Am. Chem. Soc., 1993, 115, 1193. 19 J. Fabian, Phys. Rev. B, 1996, 53, 13864. 20 K. Miura and S. Kamiya, Phys. Rev. Lett., 2003, 90, 055509.
Chem. Commun., 2014, 50, 469--471 | 471
Published on 01 November 2013. Downloaded by Gebze Institute of Technology on 15/05/2014 02:30:13.
COMMUNICATION
View Journal | View Issue
Cite this: Chem. Commun., 2014, 50, 469
Fabrication of novel graphene–fullerene hybrid lubricating films based on self-assembly for MEMS applications†
Received 30th September 2013, Accepted 1st November 2013
Jibin Pu,a Yufei Mo,bc Shanhong Wana and Liping Wang*a
DOI: 10.1039/c3cc47486k www.rsc.org/chemcomm
The novel graphene–C60 hybrid films have been fabricated successfully on silicon surfaces by a multistep self-assembly process, and showed synergistic effects beyond individual performance in micro/ nano-tribological behaviors. It is expected that the graphene–C60 hybrid films may find wide applications as high performance lubricating films in MEMS.
Micro-electromechanical systems (MEMS) have been expected to trigger the creation of a new and future prominent industry. However, the micromachines have been considered machines incapable of movement because of a marked increase in friction. Thus, the realization of a novel lubricity system and/or a novel lubricant, making it possible for the micromachines to move easily, is strongly desired. Graphene, a novel and fascinating two-dimensional (2D) atomically thin carbon nanomaterial,1 has attracted increasing attention in recent years in view of its prominent properties such as superior electronic and mechanical properties,2 extremely high thermal conductivity and large theoretical specific surface area.3 Especially, as a basic building block of a macroscopic solid lubricant graphite/ graphene was found to be an effective solid lubricant due to its weakly stacked lamellar structure, and can prevent wear of the contact surfaces in motion due to its intrinsic strength.2,4,5 So far, many investigations have been focused on the solid lubricating film of graphene, which has a prominent improvement in terms of the friction coefficient.6–9 Filleter et al.10 suggested that epitaxial graphene films could be used to further reduce friction on silicon carbide surfaces. Kim and coworkers11 reported that the CVD-grown graphene films could a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. E-mail: [email protected]; Fax: +86-09314968163; Tel: +86-09314968080 b Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, P. R. China c School of Engineering & Applied Science, The George Washington University, Washington, DC 20052, USA † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c3cc47486k
This journal is © The Royal Society of Chemistry 2014
effectively reduce the adhesion and friction forces between contact surfaces on the micro- and nanoscale. Another allotropic carbon nanomaterial, fullerene C60, have been considered to be super solid lubricants due to their spherical molecular structure, good stress resistance and stiffness, much lower surface energy, good chemical stability, etc.12,13 However, many of the physisorbed C60 films reported thus far have exhibited ‘‘island’’ formation and shown low mechanical stability under high pressure due to insufficient coverage or aggregation by sublimation of C60 onto a solid surface.14 Thus the formation of mechanically stable, ordered C60 films firmly attached to a solid substrate is considered to be a necessary step in the exploitation of their potential tribological properties. In light of the aforementioned advantages of two carbon nanomaterials, the hybridization of graphene with C60 will produce a new class of carbon-based nanocomposites which are expected to possess remarkable hybrid properties, such as more intriguing tribological properties. However, to the best of our knowledge, few studies have paid attention to the fabrication of the graphene–C60 hybrid nanomaterials because it is still a great challenge to develop facile and efficient synthetic approaches for graphene–C60 hybrids. In a few reports,15–17 C60 derivatives (such as pyrrolidine fullerene) were only grafted onto the edge of graphene via a coupling reaction between the ester group (or hydroxyl group) of C60 derivatives and the –COOH group of graphene oxide (GO), which could not effectively show hybridization effects between graphene and C60. Herein, we have developed a multistep self-assembly method to fabricate a graphene–C60 hybrid film on silicon surfaces, in which pristine C60 without involving any surfactant was chemisorbed uniformly on the whole surface of the amine-functionalized graphene bonded on a silicon surface via the nucleophilic addition reaction (see ESI1†), as shown in Fig. 1. In the self-assembly process, we gained an insight into changes in surface chemical components by water contact angle (WCA) measurement (see ESI3†). First, a selfassembled monolayer of (3-aminopropyl)triethoxysilane (APTES) was covalently anchored onto the silicon wafer via Si–O–Si covalent bonding. GO was then covalently attached onto the APTES-SAM surface through certain chemical reactions between epoxy–carboxyl
Chem. Commun., 2014, 50, 469--471 | 469
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Fig. 1 The schematic diagram of the construction process of the graphene–C60 hybrid film.
and amine groups. Subsequently, the GO films assembled on silicon wafers were amine-functionalized by (N-[3-(trimethoxylsilyl)propyl]ethylenediamine) DA molecules (labeled as a-GO). After this, abundant primary and secondary amino groups on amine-functionalized graphene could undergo N–H addition reactions across the CQC bonds in C60 which fused two six-membered rings.13,18 Chemical bonding of the C60 moieties to the surfaces of graphene not only provided discrete monolayers of C60, but also organized assemblies in which the C60 moieties would remain immobilized on the surface of graphene. Finally, the target APTES–a-graphene–C60 hybrid films on silicon wafers were obtained after heat treatment. We characterized the morphology of the films by AFM. As shown in Fig. 2a, the APTES–GO film was quite homogenous, where GO nanosheets showed the irregularly sheet-like character over the substrate. The section view (Fig. 2f) analysis suggested that the thickness of the GO sheet was about 1 nm, indicating the few-layer GO with 2–4 layers. After grafting with DA molecules, the resultant APTES–a-GO (Fig. 2b) became more compact, in which GO nanosheets were not so easy to differentiate from the matrix, and showed fuzzy surface morphology due to the presence of DA molecules. In Fig. 2c, the surface of the APTES–a-graphene–C60 film was still flat on the micrometer scale with rarely occurring macroscopic corrugations. The detailed view of the APTES–a-graphene–C60 film showed a number of regular spherical features on the graphene surface (Fig. 2d), which demonstrated that tiny C60 aggregates with an average size of B15 nm scattered uniformly on graphene nanosheets by means of addition reactions between the amino-functionalized
Fig. 2 Tapping-mode AFM images of (a) APTES–GO film, (b) APTES–aGO film, (c) APTES–a-graphene–C60 film and (e) APTES–C60 film on Si substrates. (d) High-magnification image of (c). AFM section view of (f) APTES–GO film and (g) APTES–C60 film.
470 | Chem. Commun., 2014, 50, 469--471
Fig. 3 FTIR spectra of APTES-SAM, GO, APTES–GO and APTES–agraphene–C60 films.
graphene and C60 molecules (see ESI4†). The close-packed alkyl chains can locate the C60 molecules in a certain region, the selfaggregation will be limited and their scales were decreased. When C60 molecules were directly grafted onto the APTES-SAM surface, the grainy morphology of the resultant APTES–C60 film (Fig. 2e) reflects the surface domain microstructure composed of C60 clusters. To demonstrate the actual occurrence of chemical reactions among APTES, GO and C60, we obtained the FTIR spectra of the films (Fig. 3). After GO was assembled to the APTES-SAM surface, the epoxide nC–O band intensities significantly decreased due to the ring-opening of epoxy groups under the nucleophilic attack of the amines, the carboxylic CQO stretching vibration at 1725 cm 1 almost disappeared, and the new broad bands emerged at 1613/1513 cm 1 which could be associated with nCQO and dN–H of zwitterionic COO–NH3+ linkages.8 These results clearly indicate that GO has been bonded successfully to the APTES-SAM surface via the zwitterionic COO–NH3+ linkages which would convert finally the covalent amide linkage (–NHCQO) after heat treatment. In the FTIR spectrum of APTES–a-graphene–C60, the increase of methylene nC–H and nSi–O–Si and dN–H band intensities validated amino-functionalization of graphene by a DA silane coupling agent. The new C60 characteristic peaks at 1429 cm 1 and 1187 cm 1 in the spectrum of the resultant APTES–a-graphene–C60 indicated that C60 was successfully attached chemically to the amine-functional GO by nucleophilic addition reactions.19 FTIR, XRD, TGA, Raman spectroscopy and XPS were also conducted to further identify C60 grafted onto amino-functionalized graphene (see ESI5–9†). In order to estimate the anti-stiction and wear-resistant behaviors of these films, the adhesion behaviors (Fig. 4a) and nano-tribological performances of the films were investigated using the colloidal probe (ESI2†). The superhydrophilic hydroxylated Si surface showed strong adhesion because of the large capillary force caused by the adsorbed water, which has been reflected by its static water contact angles (ESI3†). Once APTES–GO was generated thereon, the adhesive force decreased about 66% compared with that on the hydroxylated Si. The water contact angle measurements revealed that the APTES–GO film has lower surface energy than hydroxylated Si, resulting in a low adhesive force. However, the amino-terminated APTES–a-GO film
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Fig. 4 (a) Adhesive forces and (b) nanofriction coefficients (NFC) between the microsphere of the colloid probe and the surfaces of the films. The inset shows the SEM image of the colloid probe. (c) Variation in microfriction coefficient with sliding time for the films.
showed increased adhesive force because of their great hydrophilicity. As for APTES–C60 and APTES–a-graphene–C60 films with lower adhesive forces, aside from low surface energy of C60 molecules with a very stable closed-shell bond structure, another major reason for low adhesion was that the microbumps’ orderly structure of the C60 outer layer significantly reduced the contact area, thereby largely reducing the van der Waals force between the microsphere of the colloidal probe and the films. The nanotribological performances of these samples with linearly increased loads are shown in Fig. 4b. The nanofriction forces of these films changed in the order of their adhesive forces. This is easy to understand because the adhesive force has a marked effect on nanotribological properties. The C60-containing films exhibited a lower nanofriction coefficient (NFC) because of weak interaction (low adhesion) and highly stable spherical molecules. Among these, the more ordered APTES–a-graphene–C60 film showed a minimum NFC of 0.1. The improved nanolubricity was partly attributed to the cobblestone effect of the more ordered C60 outerlayer, which facilitated microsphere sliding because of the smaller contact area between the microsphere and the films. In addition, the presence of the few-layer graphene with excellent nanolubrication properties also contributed to the low NFC. The microtribological performances (ESI2†) of these thin films are very important for their application in protection layers. As shown in Fig. 4c, unlike its good nanotribological performances, the APTES–C60 film displayed poor microtribological properties characterized by high friction coefficient and a very short antiwear life of 138 s under the testing conditions of 0.1 N and 1 Hz. This might be because the C60 film directly bonded on the Si substrate film was not compact enough, and thus easily wore out. For the APTES–GO film, a reduced microfriction coefficient (MFC) (B0.25) and a lengthened antiwear life (B2600 s) were observed. The more compliant few-layer graphene could adhere to the Si surface under the applied load remaining localized over the same contact area. Consequently, the graphene layer prevented the direct contact between asperities on the steel ball counterpart and the Si surfaces and absorbed the energy generated by compression and shear during friction sliding. A possible
This journal is © The Royal Society of Chemistry 2014
Communication
explanation for the reduction in MFC may be that graphene interlayer sliding decreased the friction. Upon grafting C60, the resulting APTES– a-graphene–C60 film exhibited both reduction of MFC below 0.25 and a further increased antiwear life of above 1 h under the same testing conditions. The enhanced lubricating and antiwear properties may be attributed to the synergistic effect between the graphene layer as the load-carrying and wear resistant phase and the C60 outer layer as the friction reducer (mimicking ball bearing20). Under mechanical compression in a sliding interface, the C60 molecules would be readily detached, and may roll among hydrocarbon chains like many tiny ball bearings, leading to low friction and low wear. Such a micro-rolling effect of tiny C60 aggregates (o20 nm) was also confirmed by many researchers.14 The strong bonds between the carbon atoms in C60 molecules make C60 molecular structure very stable and not easily damaged, which is good for a long-lasting and effective lubrication. Thus, the wear resistance capacity of the APTES–a-graphene–C60 multilayer film was greatly enhanced. In conclusion, we have devised a novel approach to tether the fullerene C60 to amino-functionalized graphene employing an aminosilane self-assembled monolayer. The combination of graphene and C60 provides synergistic effects beyond individual performance. The resultant graphene–C60 hybrid film fabricated on the Si surface exhibited good friction reduction, load-carrying capacity, and antiwear ability due to the low surface energy, and the rolling effect of C60 molecules, together with high mechanical stiffness and interlayer sliding of graphene, which will make the carbon-based hybrid films possess potential in tribological applications for MEMS. The authors thank the National Natural Science Foundation of China (No 51105352 and 21373249) for financial support.
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