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Investigation of the tribological behavior of 3-mercaptopropyl trimethoxysilane deposited on silicon
Wear 261 (2006) 730–737
Investigation of the tribological behavior of 3-mercaptopropyl trimethoxysilane deposited on silicon Bai Tao a , Cheng Xian-Hua a,b,∗ a
b
School of Mechanical and Power Energy Engineering, Shanghai Jiao Tong University, Shanghai 200030, China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Received 30 April 2005; received in revised form 22 January 2006; accepted 24 January 2006 Available online 4 April 2006
Abstract 3-Mercapto-propyl trimethoxysilane (MPTS) thin film was prepared on hydroxylated silicon substrates by a self-assembling process and the terminal −SH group in the film was in situ oxidized to −SO3 H group. Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the thin films. The tribological properties of the as-prepared thin films sliding against a steel ball were evaluated on a friction and wear tester. It was found that the macroscopic friction coefficients for coating times more than 1 h ranged between 0.1 and 0.2 whereas the value for short coating time was as high as 0.7. And the friction coefficient was higher for the SAM with functional group −SO3 H compared to the SAM having −SH group. Surface energy of the substrate can be obviously increased when the terminal group (−SH) of self-assembly monolayers was oxidized into sulfonate one (−SO3 H) which can reduce the adhesion force of the moving surfaces. It was also found that the frictional behaviors of MPTS coated silicon surface sensitive to applied load and sliding velocity. © 2006 Elsevier B.V. All rights reserved. Keywords: MPTS; Self-assemble film; Characterization of the film; Friction and wear behavior
1. Introduction Micro-electromechanical-systems (MEMS) technology has been receiving much attention in the past decade for its potential applications in areas such as communications, mechatronics and biomedicals [1,2]. However, currently many potential applications for MEMS are not really practical, as many studies have revealed the profound negative influence of stiction, friction and wear on the efficiency, power output, of microdevices [3]. Obviously, it is essential to supply super lubrication for MEMS for successful applications. Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by chemical adsorption of an active surfactant on a solid substrate surface [4,5]. SAMs can be spontaneously formed by immersion of an appropriate substrate into a solution of an active surfactant in an organic solvent. The molecularly thin feature, the relatively strong chemical bonded interface, and the simple preparation process make SAMs inher-
∗
Corresponding author. Tel.: +86 21 62932404; fax: +86 21 62933772-13. E-mail address: [email protected] (C. Xian-Hua).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.01.045
ently manufacturable and thus technologically interesting for building efficient lubricants. Some researchers use self-assembled monolayer (SAM) of MPTS to mediate the film deposition because it can be obtained expediently and has been previously used to make functional monolayers, and the thiol groups (−SH) can be oxidized into sulfonic acid groups (−SO3 H) easily [6,7]. A number of studies have been done on the tribological properties of different SAMs [8–10], but the study of the MPTS SAM on the tribological behavior is much lacking. The MPTS SAM films constructed by strong chemical bonds can be prepared by the self-assembly technique. Despite experimental as well as recent theoretical advances, a general understanding of MPTS SAM tribological investigation is still lacking. In this study, some work was done to research the preparation and tribological property of MPTS SAM on silicon substrates. In the paper, MPTS SAM was prepared and the tribological properties were investigated. Many means, such as X-ray photoelectron spectroscopy (XPS), friction and wear tester, atomic force microscopy (AFM), etc. had been applied to characterize the structure and tribological properties of thin films in the paper. In this study we report on the fabrication, tribological behavior
B. Tao, C. Xian-Hua / Wear 261 (2006) 730–737
of MPTS SAM, specially the sensitivity of the frictional behavior with respect to variables such as coating time, applied load, and sliding velocity. 2. Experimental 2.1. Sample preparation 2.1.1. Materials A single-crystal silicon wafer polished on one side was used as substrate for the SAM film transfer. MPTS were purchased from Sigma–Aldrich (Deisenhofen, Germany) and used as received. All other chemicals used in chemical manipulations were of reagent grade. Water was deionized water obtained from a Barnstead Nanopure apparatus. 2.2. Preparation of film Silicon substrates were immersed for 30 min in Piranha solution (H2 SO4 :H2 O2 = 7/3 v/v) at 90 ◦ C to make hydroxy radicals on the surfaces. Then the substrates were carefully rinsed with deionised water and dried. After that the hydroxide substrates were dipped into the dehydrated benzene solution containing 0.5 mM of MPTS solution for 12 h. At last the substrates were cleaned ultrasonically with chloroform, acetone and deionised water in turn to remove the other physisorbed ions or molecules and dried for 1 h at 120 ◦ C, then cooled in a desiccator. The oxidization of the −SH groups to the desired −SO3 H groups was carried out by dipping the substrates into the solution of 30% nitric acid at 80 ◦ C for 1 h, followed by washing with distilled water and dried in nitrogen. 2.3. Experimental apparatus and measurements XPS is a highly diagnostic tool for the assessment of the chemical state of elements [11,12]. In this paper XPS analysis was conducted on a PHI-5702 XPS system, using Mg K␣ radiation operating at 250 W and a pass energy of 29.35 eV. It is well known about the importance to investigate the film morphology in the research of the self-assembly technique. Atomic force microscopy (AFM) has been employed to study the morphology of RE films, because not only it has great vertical resolution but also it allows the measurement of other interesting parameters which can help us to find more value information about films, such as roughness, grain size and surface cross-section. The AFM topographic measurements and patterning of RE films were performed with a SPM-9500 unit (Shimadzu Corp., Japan). The interfacial property, particularly the wettability and the roughness of the solid surface, has very significant importance in the study of the preparation and the tribological properties of the thin films. The study of contact angles of the samples was on the OCA-20 measurement apparatus (DataPhysics Instruments GmbH). Contact angles have been measured on 10 locations on the all the samples. All results on each sample were averaged and the accuracy obtained by this method. The tribological properties of coated silicon substrate with MPTS SAM sliding against a ball made of steel were evalu-
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ated on a Kyowa DF-PM model one-way reciprocating friction tester at ambient conditions (23 ◦ C). The sliding velocity and stroke were 3 mm/s and 6 mm. The normal force was selected as 50 mN. The coefficient of friction and sliding passes were recorded automatically. The friction coefficient was an average of five test results. Prior to the friction and wear test, all the samples were cleaned in an ultrasonic bath with acetone for 10 min and then dried in hot air. 3. Results and discussion 3.1. Characterization of the film Fig. 1 shows the AFM images of hydroxide silicon substrate, the MPTS SAM and oxidized MPTS SAM. Fig. 1(b)–(d) shows a series of AFM images of the MPTS SAM on silicon substrates, which were prepared from solution of MPTS with different immersion times ranging from 1 min to 24 h. From 15 to 30 min, with increasing immersion time the surface coverage increase and the surface morphology changed significantly. As can be seen, this increase is caused by an increasing number of MPTS islands. From 30 to 60 min, the density of the islands decreased as aggregation occurred, and the substrate surfaces became densely covered. After 24 h a flat surface with a Ra roughness of approximately 0.5 nm can be observed indicating that perfect monolayer coverage is reached after a prolonged period of deposition. The mean roughness (Ra ) were 0.20, 0.527 and 0.159 nm, respectively. It implied that MPTS molecules had been absorbed on the substrate and MPTS SAM was sequential and welloriented compact structure. When the terminal groups (−SH) of SAM in the top-most layer to the air/silane were completely oxidized into sulfonate groups (−SO3 H) in 30% HNO3 solution, the surface roughness decreased. The possible reason is that the size of sulfonate groups is bigger than the terminal groups −SH which lead to the sulfonate terminal molecules provided a more densely packed arrangement than the −SH terminal molecules. From the Fig. 1, it is seen that the surfaces of the oxidized silicon and the sulfonated MPTS-SAM are very smooth and homogeneous with Ra in the range of 0.2–0.3 nm, which is consistent with that reported elsewhere [13]. The X-ray photoelectron spectroscopic data of the surfaces were applied to detect the chemical states of some typical elements in prepared MPTS films. The changes in elemental composition can show if the reagents were deposited on the wafer surface. The presence of SAM on silicon substrate was confirmed by XPS measurement. Fig. 2 shows XPS spectra acquired from the MPTS-coated silicon substrate. The S2p signal in the spectrum for the modified silicon substrate can qualitatively account for the existence of MPTS SAM on silicon surface. The S2p signal can be investigated on the films with different immersion times. The S2p peak is symmetric and centered around 163.8 eV which indicated that MPTS film had been successfully obtained in our work. With proper prehydration, the oxidized silicon substrate is capable of covalent bonding to the hydrolyzed silane molecules
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Fig. 1. AFM image of the bare cleaned silicon substrate (a) and AFM images of MPTS SAM acquired as a function of immersion time (b) 15min (c) 30min (d) 24h and (e) oxidized MPTS SAM.
via condensation reaction of silanol groups. Several papers had described the bonding mechanism for the reaction of oxidized silicon and hydrolyzed silane [14–16]. Contact angle measurement is an effective way to characterize the hydrophilicity and hydrophobicity of a given surface which is closely related to the molecules packing density of SAM. The contact angle of the MPTS SAM in the test was
about 70◦ . It indicated that MPTS film had been successfully obtained in our work which led to the reduction of the surface energy [7]. When the terminal groups (−SH) of self-assembly monolayers were oxidized into sulfonate ones (−SO3 H) by dipping in a solution of 30% HNO3 , the contact angle of samples decreased from 70◦ to 31◦ . It indicated that various ratios of −SH groups
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Fig. 2. XPS spectra of S2p of MPTS-SAM coated on silicon substrate and S2p of oxidized MPTS-SAM coated on silicon substrate.
were oxidized with increasing oxidization time [7]. When oxidizing over 1 h, the contact angle remained constant, which implied that the oxidization became saturated. 3.2. Friction and wear 3.2.1. Effect of coating time The tribological behavior of MPTS SAM deposited on silicon substrate was investigated using the equipment described above. The major variable in the specimen preparation was the coating time. The motivation was to identify the minimum coating time that would still result in satisfactory tribological performance. Coating times of 1, 10, 30 min, 3, 6, 12 h were used to prepare the specimens following the coating procedure described earlier. Fig. 3 shows the contact angle measurements for MPTS deposited on silicon substrates with respect to the coating time. The measurements were repeated more than 10 times for each coating condition and their average values were marked on the graph. The hydroxylated silicon surface was hydrophilic with the water contact angle about 1◦ , indicating the silicon surface
Fig. 3. Contact angle of MPTS SAM acquired as a functional of immersion time.
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Fig. 4. Friction coefficient of MPTS SAM acquired as a functional of immersion time.
was thoroughly hydroxylated. The data points were concentrated near “1◦ ” when the coating times for several specimens were much less than 1 min. It was evident that the contact angle was low at about 10◦ ∼ 50◦ for short coating times, but increased over 72◦ for specimens coated for longer than 2 h. From these results, it may be concluded that good coverage of the MPTS film on the silicon surface can be achieved in 2 h of coating time. The sliding velocity and stroke were 3 mm/s and 6 mm. Fig. 4 shows the friction coefficient measurements with respect to MPTS coating time under 50 mN normal load. The friction coefficient, the value dropped from about 0.8 for very short coating times to about 0.2 for coating times beyond 12 h. The degree of drop was quite significant and the lubricating effect of the MPTS film was clearly demonstrated even for relatively high loads of 50 mN. As in the case of the contact angle measurement, the friction coefficient value was low despite incomplete coverage of the MPTS film on the silicon surface. This is perhaps due to the large contact area between the steel ball and the silicon surface. Thus, for the large contact area, it is plausible that the island-type deposits of MPTS film on silicon were sufficient to cause boundary lubrication effects. The sliding interfaces as well as the wear track of the MPTS coated silicon surface were observed using AFM. Fig. 5 shows the micrographs of silicon surfaces coated with MPTS for 1, 10, 30 min, 1, 2, 12 h, respectively, after 100 sliding pass. It distinctly showed that the MPTS coating was not effective for the 1 min coating case. The wear tracks were quite heavily contaminated with residues of the film and wear particles. On the contrary, the sliding interface as well as the wear track of the 1 h coating case was intact. Only a very thin line can be observed on the wear track. From the experimental results, it was concluded that even though the contact angle measurements indicated that the silicon surface was covered with MPTS film partially after 1 h coating, for the case of macroscopic contact applications where the contact area was relatively large, effective boundary lubrication condition can be achieved even with discontinuous MPTS film where the coating time was more than 1 h.
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Fig. 5. AFM images of surfaces of RE film sliding against steel ball at different sliding pass acquired as a functional of immersion time (a) 1 min, (b) 10 min, (c) 30 min, (d) 1 h, (e) 2 h, and (f) 12 h.
3.2.2. Effect of oxidized ratio from −SH groups into −SO3 H To understand the influence of oxidization of the MPTS SAM on the microscale friction behavior, we compared mixed SAM with different ratios of −SH and −SO3 H groups and learned the influence of different functionality (−SH versus −SO3 H). Xiao et al. [7] found the ratio of the groups in oxidized monolayers
were about 10%, 80% and 100% after oxidizing for 10, 30 min and 1 h, respectively. In the test, the samples were prepared with oxidization times ranging from 0 to 90 min. The sliding velocity and stroke were 3 mm/s and 6 mm and the normal force was selected as 50 mN in the test. Fig. 6 shows the variation of the friction coefficient as a function of oxidizing time for these samples. From Fig. 6, the friction coefficient is higher
B. Tao, C. Xian-Hua / Wear 261 (2006) 730–737
Fig. 6. Friction coefficient of MPTS SAM acquired as a functional of oxidizing time.
for SAM that has functional group −SO3 H when compares to the mixed self-assembly monolayers. The friction coefficient of SAM having −SH group was the lowest in all the samples. The difference in the frictional behavior of these SAMs may be due to the different surface energy values of the −SH and −SO3 H terminal groups. The friction coefficient values of different energy for different surfaces were expected to be different. Low surface energy can reduce stiction. It was noted that the contact angles of the MPTS SAM was 70◦ . When the terminal groups (−SH) of self-assembly monolayers were oxidized into sulfonate ones (−SO3 H) by dipping in a solution of 30% HNO3 , the contact angle of samples decreased from 70◦ to 31◦ which indicated that surface energy of the substrate can be obviously increased when the terminal groups (−SH) of self-assembly monolayers are oxidized into sulfonate ones (−SO3 H) which can reduce the adhesion force of the moving surfaces. 3.2.3. Effect of load and sliding velocity Though at relatively light loads the strong bonding of the MPTS SAM to the silicon surface can withstand the shear due to frictional force, the film would fail if the load was sufficiently high. In order to identify the effect of load on the effectiveness of MPTS, the friction coefficient of film was obtained for 12 h coated MPTS silicon for normal loads between 10 and 200 mN. The experiments were conducted at room temperature at about 30% relative humidity. Fig. 7 shows that the friction coefficient decreases with increasing load. The effect of sliding velocity on the friction coefficient of MPTS SAM was investigated. Sliding length was set to 10 mm and normal load was set to 50 mN. The velocity was varied from 50 to 200 mm/min. Fig. 8 shows the experimental result for 12 h MPTS coated silicon. From Fig. 8, it can be seen that the friction coefficient decreases with increasing sliding velocity. The result indicated that the friction coefficient of MPTS SAM depended on the sliding velocity. The chemically adsorbed self-assembled molecule on a substrate is just like assembled molecular spring anchored to the substrates and a tip sliding on the surface of SAM is like a tip
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Fig. 7. Friction coefficient of MPTS SAM acquired as a functional of load.
sliding on the top of ‘molecular springs or brush’ [17]. The orientation of the ‘molecular spring or brush’ under normal load reduces the shearing force the interface, which in turn reduces the friction force. The mechanisms responsible for the variation of the friction forces of MPTS SAM with different velocity and load were believed to be related to the reason above. In addition, it was believed that the results were related to the viscoelastic properties of SAM. 3.2.4. Friction coefficient as a function of sliding pass Fig. 9 shows the variation of friction coefficient as a function of sliding pass. It can be seen that MPTS-SAM recorded low initial friction coefficient about 0.2. It was seen that the wear life of the film exceeded 250 sliding pass. The friction coefficient of silicon substrate raised to a stable and high value (0.8) at about 120 sliding pass because some contamination was physiabsorbed on the substrate and contacting with the steel ball leaded to the increased friction coefficient of substrate when the contamination coating on substrate was wore out. Comparing the silicon substrate, MPTS-SAM reacted with the substrate by chemical
Fig. 8. Friction coefficient of MPTS SAM acquired as a functional of sliding speed.
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ing −SH group was the lowest in all the samples. That result was attributed to the hydrophobic and low surface energy properties of MPTS film. 5. The friction coefficient of the samples decreased with increasing load. And the friction coefficient of the samples decreased with increasing sliding velocity. The superior friction reduction and wear resistance of MPTES SAM were attributed to the orientation of the molecules adsorbed on a substrate and the viscoelastic properties of SAM. Acknowledgements
Fig. 9. Wear dependence of the friction coefficients.
bond. This reaction was thought to be helpful in increasing the bonding strength between the films and the substrates which can avoid sever brittle fracture happened on the surface of the films in the friction and wear test which led to the invalidation of the films and hence improving the tribological properties. Different SAMs may have different surface energy values. This will certainly result in different friction coefficients during the friction test. Low surface energy can reduce stiction, so the friction coefficient values of SAMs having lower surface energy are expected to be lower. It was noted that the contact angles of the hydroxide silicon substrate and the MPTS film were 1◦ and 72◦ , respectively, which indicated that surface energy of the substrate can be obviously reduced when MPTS SAM was coated which can reduce the adhesion force of the moving surfaces. The contact angle of MPTS SAM is lower than some other organic film such as OTS which the contact angles is 111◦ [18,19], so the friction coefficient of MPTS SAM is lower than that of silicon substrate whereas higher than that of OTS. The second factor that resulted in lower friction coefficient of MPTS SAM was that MPTS SAM had functional group −CH3 which had lower friction coefficient than other functional group, such as −COOH, −CH3 , −CF [8,9]. The other possible reason of the friction difference may be due to different surface roughness and different uniformity of the SAMs coated surface compared other SAMs, such OTS, and UTS [18]. 4. Conclusions 1. The results presented here demonstrated that MPTS SAM was self-assembled on silicon substrate. 2. The terminal groups (−SH) of SAM in the top-most layer to the air/silane could be completely oxidized into sulfonate groups (−SO3 H) in 30% nitric acid solution. 3. The macroscopic friction coefficients for coating times more than 1 h ranged between 0.1 and 0.2 whereas the value for shorter coating time was as high as 0.7. 4. The friction coefficient was higher for SAM that had functional group −SO3 H when compared to the mixed selfassembly monolayers. The friction coefficient of SAM hav-
Authors would like to thank for the National Natural Science Foundation of China (Grant No. 50475023) and “nano foundation of shanghai technology committee (0252nm014)” and “State Key Laboratory Fund (0102)” in state key laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Science support for the present work and the technical assistance of Instrumental Analysis Center of Shanghai Jiao Tong University in matters related to the AFM instrument. References [1] K. Komvopoulos, Surface engineering and microtribology for microelectromechanical systems, Wear 200 (1996) 305–327. [2] S.M. Spearing, Materials issues in MEMS, Acta Mater. 48 (2000) 179– 196. [3] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic, Dordrecht, 1998. [4] D. Appelhans, D. Ferse, H.J. Adler, W. Plieth, A. Fikus, K. Grundke, F.J. Schmitt, T. Bayer, Self-assembled monolayers prepared from -thiophenefunctionalized n-alkyltrichlorosilane on silicon substrates, Colloids Surf. A: Physicochem. Eng. Aspects 161 (2000) 203–212. [5] A. Nakasa, U. Akiba, M. Fujihira, Self-assembled monolayers containing biphenyl derivatives as challenge for nc-AFM, Appl. Surf. Sci. 157 (2000) 326–331. [6] J.Q. Wang, S.R. Yang, X.H. Liu, S.L. Ren, F. Guan, M. Chen, Preparation and characterization of ZrO2 thin film on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane, Appl. Surf. Sci. 221 (2004) 272–280. [7] Z. Xiao, L. Su, N. Gu, Z. Lu, Y. Wei, The growth of TiO2 thin films on mixed self-assembly monolayers from solution, Thin Solid Films 333 (1998) 25–28. [8] H.S. Ahn, P.D. Cuong, S.K. Park, Y.W. Kim, J.C. Lim, Effect of molecular structure of self-assembled monolayers on their tribological behaviors in nano- and microscales, Wear 255 (2003) 819–825. [9] I.H. Sung, J.C. Yang, D.E. Kim, B.S. Shin, Micro/nano-tribological characteristics of self-assembled monolayer and its application in nano-structure fabrication, Wear 255 (2003) 808–818. [10] K.H. Cha, D.E. Kim, Investigation of the tribological behavior of octadecyltrichlor-osilane deposited on silicon, Wear 51 (2001) 1169– 1176. [11] W.G. Zhang, W.M. Liu, Q.J. Xue, Characterization and tribological investigation of Al2 O3 and modified Al2 O3 sol–gel films, Mater. Res. Bull. 36 (2001) 1903–1914. [12] Y.J. Gao, Z.S. Wu, Z.J. Zhang, Q.J. Xue, Study on tribological properties of 2,5-dialkoxymethylthio-1,3,4-thiadiazoles, Wear 222 (1998) 129–134. [13] T.P. Niesen, M.R.D. Guire, J. Bill, F. Aldinger, Atomic force microscopic studies of oxide thin films on organic self-assembled monolayers, J. Mater. Res. 14 (1999) 2464–2474. [14] G.J. Kluth, M.M. Sung, R. Maboudian, Thermal behavior of alkylsiloxane self-assembled monolayers on the oxidized Si(1 0 0) surface, Langmuir 13 (1997) 3775–3780.
B. Tao, C. Xian-Hua / Wear 261 (2006) 730–737 [15] H. Brunner, T. Vallant, U. Mayer, H. Hoffmann, B. Basnar, M. Vallant, G. Friedbacker, Substrate effects on the formation of alkylsiloxane monolayers, Langmuir 15 (1999) 1899–1901. [16] Z.Q. Wei, C. Wang, C.F. Zhu, C.Q. Zhou, B. Xu, C.L. Bai, Study on singlebond interaction between amino-terminated organosilane self-assembled monolayers by atomic force microscopy, Surf. Sci. 459 (2000) 401– 412.
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[17] H.W. Liu, B. Bhushan, Investigation of nanotribological properties of selfassembled monolayers with alkyl and biphenyl spacer chains (invited), Ultramicroscopy 91 (2002) 185–202. [18] V. Depalma, N. Tillman, Friction and wear of self-assembled trichlorosilane monolayer films on silicon, Langmuir 5 (1989) 868–872. [19] E.S. Yoon, S.H. Yang, H.G. Han, H. Kong, An experimental study on the adhesion at a nano-contact, Wear 254 (2003) 974–980.
Investigation of the tribological behavior of 3-mercaptopropyl trimethoxysilane deposited on silicon Bai Tao a , Cheng Xian-Hua a,b,∗ a
b
School of Mechanical and Power Energy Engineering, Shanghai Jiao Tong University, Shanghai 200030, China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China Received 30 April 2005; received in revised form 22 January 2006; accepted 24 January 2006 Available online 4 April 2006
Abstract 3-Mercapto-propyl trimethoxysilane (MPTS) thin film was prepared on hydroxylated silicon substrates by a self-assembling process and the terminal −SH group in the film was in situ oxidized to −SO3 H group. Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the thin films. The tribological properties of the as-prepared thin films sliding against a steel ball were evaluated on a friction and wear tester. It was found that the macroscopic friction coefficients for coating times more than 1 h ranged between 0.1 and 0.2 whereas the value for short coating time was as high as 0.7. And the friction coefficient was higher for the SAM with functional group −SO3 H compared to the SAM having −SH group. Surface energy of the substrate can be obviously increased when the terminal group (−SH) of self-assembly monolayers was oxidized into sulfonate one (−SO3 H) which can reduce the adhesion force of the moving surfaces. It was also found that the frictional behaviors of MPTS coated silicon surface sensitive to applied load and sliding velocity. © 2006 Elsevier B.V. All rights reserved. Keywords: MPTS; Self-assemble film; Characterization of the film; Friction and wear behavior
1. Introduction Micro-electromechanical-systems (MEMS) technology has been receiving much attention in the past decade for its potential applications in areas such as communications, mechatronics and biomedicals [1,2]. However, currently many potential applications for MEMS are not really practical, as many studies have revealed the profound negative influence of stiction, friction and wear on the efficiency, power output, of microdevices [3]. Obviously, it is essential to supply super lubrication for MEMS for successful applications. Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by chemical adsorption of an active surfactant on a solid substrate surface [4,5]. SAMs can be spontaneously formed by immersion of an appropriate substrate into a solution of an active surfactant in an organic solvent. The molecularly thin feature, the relatively strong chemical bonded interface, and the simple preparation process make SAMs inher-
∗
Corresponding author. Tel.: +86 21 62932404; fax: +86 21 62933772-13. E-mail address: [email protected] (C. Xian-Hua).
0043-1648/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2006.01.045
ently manufacturable and thus technologically interesting for building efficient lubricants. Some researchers use self-assembled monolayer (SAM) of MPTS to mediate the film deposition because it can be obtained expediently and has been previously used to make functional monolayers, and the thiol groups (−SH) can be oxidized into sulfonic acid groups (−SO3 H) easily [6,7]. A number of studies have been done on the tribological properties of different SAMs [8–10], but the study of the MPTS SAM on the tribological behavior is much lacking. The MPTS SAM films constructed by strong chemical bonds can be prepared by the self-assembly technique. Despite experimental as well as recent theoretical advances, a general understanding of MPTS SAM tribological investigation is still lacking. In this study, some work was done to research the preparation and tribological property of MPTS SAM on silicon substrates. In the paper, MPTS SAM was prepared and the tribological properties were investigated. Many means, such as X-ray photoelectron spectroscopy (XPS), friction and wear tester, atomic force microscopy (AFM), etc. had been applied to characterize the structure and tribological properties of thin films in the paper. In this study we report on the fabrication, tribological behavior
B. Tao, C. Xian-Hua / Wear 261 (2006) 730–737
of MPTS SAM, specially the sensitivity of the frictional behavior with respect to variables such as coating time, applied load, and sliding velocity. 2. Experimental 2.1. Sample preparation 2.1.1. Materials A single-crystal silicon wafer polished on one side was used as substrate for the SAM film transfer. MPTS were purchased from Sigma–Aldrich (Deisenhofen, Germany) and used as received. All other chemicals used in chemical manipulations were of reagent grade. Water was deionized water obtained from a Barnstead Nanopure apparatus. 2.2. Preparation of film Silicon substrates were immersed for 30 min in Piranha solution (H2 SO4 :H2 O2 = 7/3 v/v) at 90 ◦ C to make hydroxy radicals on the surfaces. Then the substrates were carefully rinsed with deionised water and dried. After that the hydroxide substrates were dipped into the dehydrated benzene solution containing 0.5 mM of MPTS solution for 12 h. At last the substrates were cleaned ultrasonically with chloroform, acetone and deionised water in turn to remove the other physisorbed ions or molecules and dried for 1 h at 120 ◦ C, then cooled in a desiccator. The oxidization of the −SH groups to the desired −SO3 H groups was carried out by dipping the substrates into the solution of 30% nitric acid at 80 ◦ C for 1 h, followed by washing with distilled water and dried in nitrogen. 2.3. Experimental apparatus and measurements XPS is a highly diagnostic tool for the assessment of the chemical state of elements [11,12]. In this paper XPS analysis was conducted on a PHI-5702 XPS system, using Mg K␣ radiation operating at 250 W and a pass energy of 29.35 eV. It is well known about the importance to investigate the film morphology in the research of the self-assembly technique. Atomic force microscopy (AFM) has been employed to study the morphology of RE films, because not only it has great vertical resolution but also it allows the measurement of other interesting parameters which can help us to find more value information about films, such as roughness, grain size and surface cross-section. The AFM topographic measurements and patterning of RE films were performed with a SPM-9500 unit (Shimadzu Corp., Japan). The interfacial property, particularly the wettability and the roughness of the solid surface, has very significant importance in the study of the preparation and the tribological properties of the thin films. The study of contact angles of the samples was on the OCA-20 measurement apparatus (DataPhysics Instruments GmbH). Contact angles have been measured on 10 locations on the all the samples. All results on each sample were averaged and the accuracy obtained by this method. The tribological properties of coated silicon substrate with MPTS SAM sliding against a ball made of steel were evalu-
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ated on a Kyowa DF-PM model one-way reciprocating friction tester at ambient conditions (23 ◦ C). The sliding velocity and stroke were 3 mm/s and 6 mm. The normal force was selected as 50 mN. The coefficient of friction and sliding passes were recorded automatically. The friction coefficient was an average of five test results. Prior to the friction and wear test, all the samples were cleaned in an ultrasonic bath with acetone for 10 min and then dried in hot air. 3. Results and discussion 3.1. Characterization of the film Fig. 1 shows the AFM images of hydroxide silicon substrate, the MPTS SAM and oxidized MPTS SAM. Fig. 1(b)–(d) shows a series of AFM images of the MPTS SAM on silicon substrates, which were prepared from solution of MPTS with different immersion times ranging from 1 min to 24 h. From 15 to 30 min, with increasing immersion time the surface coverage increase and the surface morphology changed significantly. As can be seen, this increase is caused by an increasing number of MPTS islands. From 30 to 60 min, the density of the islands decreased as aggregation occurred, and the substrate surfaces became densely covered. After 24 h a flat surface with a Ra roughness of approximately 0.5 nm can be observed indicating that perfect monolayer coverage is reached after a prolonged period of deposition. The mean roughness (Ra ) were 0.20, 0.527 and 0.159 nm, respectively. It implied that MPTS molecules had been absorbed on the substrate and MPTS SAM was sequential and welloriented compact structure. When the terminal groups (−SH) of SAM in the top-most layer to the air/silane were completely oxidized into sulfonate groups (−SO3 H) in 30% HNO3 solution, the surface roughness decreased. The possible reason is that the size of sulfonate groups is bigger than the terminal groups −SH which lead to the sulfonate terminal molecules provided a more densely packed arrangement than the −SH terminal molecules. From the Fig. 1, it is seen that the surfaces of the oxidized silicon and the sulfonated MPTS-SAM are very smooth and homogeneous with Ra in the range of 0.2–0.3 nm, which is consistent with that reported elsewhere [13]. The X-ray photoelectron spectroscopic data of the surfaces were applied to detect the chemical states of some typical elements in prepared MPTS films. The changes in elemental composition can show if the reagents were deposited on the wafer surface. The presence of SAM on silicon substrate was confirmed by XPS measurement. Fig. 2 shows XPS spectra acquired from the MPTS-coated silicon substrate. The S2p signal in the spectrum for the modified silicon substrate can qualitatively account for the existence of MPTS SAM on silicon surface. The S2p signal can be investigated on the films with different immersion times. The S2p peak is symmetric and centered around 163.8 eV which indicated that MPTS film had been successfully obtained in our work. With proper prehydration, the oxidized silicon substrate is capable of covalent bonding to the hydrolyzed silane molecules
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Fig. 1. AFM image of the bare cleaned silicon substrate (a) and AFM images of MPTS SAM acquired as a function of immersion time (b) 15min (c) 30min (d) 24h and (e) oxidized MPTS SAM.
via condensation reaction of silanol groups. Several papers had described the bonding mechanism for the reaction of oxidized silicon and hydrolyzed silane [14–16]. Contact angle measurement is an effective way to characterize the hydrophilicity and hydrophobicity of a given surface which is closely related to the molecules packing density of SAM. The contact angle of the MPTS SAM in the test was
about 70◦ . It indicated that MPTS film had been successfully obtained in our work which led to the reduction of the surface energy [7]. When the terminal groups (−SH) of self-assembly monolayers were oxidized into sulfonate ones (−SO3 H) by dipping in a solution of 30% HNO3 , the contact angle of samples decreased from 70◦ to 31◦ . It indicated that various ratios of −SH groups
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Fig. 2. XPS spectra of S2p of MPTS-SAM coated on silicon substrate and S2p of oxidized MPTS-SAM coated on silicon substrate.
were oxidized with increasing oxidization time [7]. When oxidizing over 1 h, the contact angle remained constant, which implied that the oxidization became saturated. 3.2. Friction and wear 3.2.1. Effect of coating time The tribological behavior of MPTS SAM deposited on silicon substrate was investigated using the equipment described above. The major variable in the specimen preparation was the coating time. The motivation was to identify the minimum coating time that would still result in satisfactory tribological performance. Coating times of 1, 10, 30 min, 3, 6, 12 h were used to prepare the specimens following the coating procedure described earlier. Fig. 3 shows the contact angle measurements for MPTS deposited on silicon substrates with respect to the coating time. The measurements were repeated more than 10 times for each coating condition and their average values were marked on the graph. The hydroxylated silicon surface was hydrophilic with the water contact angle about 1◦ , indicating the silicon surface
Fig. 3. Contact angle of MPTS SAM acquired as a functional of immersion time.
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Fig. 4. Friction coefficient of MPTS SAM acquired as a functional of immersion time.
was thoroughly hydroxylated. The data points were concentrated near “1◦ ” when the coating times for several specimens were much less than 1 min. It was evident that the contact angle was low at about 10◦ ∼ 50◦ for short coating times, but increased over 72◦ for specimens coated for longer than 2 h. From these results, it may be concluded that good coverage of the MPTS film on the silicon surface can be achieved in 2 h of coating time. The sliding velocity and stroke were 3 mm/s and 6 mm. Fig. 4 shows the friction coefficient measurements with respect to MPTS coating time under 50 mN normal load. The friction coefficient, the value dropped from about 0.8 for very short coating times to about 0.2 for coating times beyond 12 h. The degree of drop was quite significant and the lubricating effect of the MPTS film was clearly demonstrated even for relatively high loads of 50 mN. As in the case of the contact angle measurement, the friction coefficient value was low despite incomplete coverage of the MPTS film on the silicon surface. This is perhaps due to the large contact area between the steel ball and the silicon surface. Thus, for the large contact area, it is plausible that the island-type deposits of MPTS film on silicon were sufficient to cause boundary lubrication effects. The sliding interfaces as well as the wear track of the MPTS coated silicon surface were observed using AFM. Fig. 5 shows the micrographs of silicon surfaces coated with MPTS for 1, 10, 30 min, 1, 2, 12 h, respectively, after 100 sliding pass. It distinctly showed that the MPTS coating was not effective for the 1 min coating case. The wear tracks were quite heavily contaminated with residues of the film and wear particles. On the contrary, the sliding interface as well as the wear track of the 1 h coating case was intact. Only a very thin line can be observed on the wear track. From the experimental results, it was concluded that even though the contact angle measurements indicated that the silicon surface was covered with MPTS film partially after 1 h coating, for the case of macroscopic contact applications where the contact area was relatively large, effective boundary lubrication condition can be achieved even with discontinuous MPTS film where the coating time was more than 1 h.
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Fig. 5. AFM images of surfaces of RE film sliding against steel ball at different sliding pass acquired as a functional of immersion time (a) 1 min, (b) 10 min, (c) 30 min, (d) 1 h, (e) 2 h, and (f) 12 h.
3.2.2. Effect of oxidized ratio from −SH groups into −SO3 H To understand the influence of oxidization of the MPTS SAM on the microscale friction behavior, we compared mixed SAM with different ratios of −SH and −SO3 H groups and learned the influence of different functionality (−SH versus −SO3 H). Xiao et al. [7] found the ratio of the groups in oxidized monolayers
were about 10%, 80% and 100% after oxidizing for 10, 30 min and 1 h, respectively. In the test, the samples were prepared with oxidization times ranging from 0 to 90 min. The sliding velocity and stroke were 3 mm/s and 6 mm and the normal force was selected as 50 mN in the test. Fig. 6 shows the variation of the friction coefficient as a function of oxidizing time for these samples. From Fig. 6, the friction coefficient is higher
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Fig. 6. Friction coefficient of MPTS SAM acquired as a functional of oxidizing time.
for SAM that has functional group −SO3 H when compares to the mixed self-assembly monolayers. The friction coefficient of SAM having −SH group was the lowest in all the samples. The difference in the frictional behavior of these SAMs may be due to the different surface energy values of the −SH and −SO3 H terminal groups. The friction coefficient values of different energy for different surfaces were expected to be different. Low surface energy can reduce stiction. It was noted that the contact angles of the MPTS SAM was 70◦ . When the terminal groups (−SH) of self-assembly monolayers were oxidized into sulfonate ones (−SO3 H) by dipping in a solution of 30% HNO3 , the contact angle of samples decreased from 70◦ to 31◦ which indicated that surface energy of the substrate can be obviously increased when the terminal groups (−SH) of self-assembly monolayers are oxidized into sulfonate ones (−SO3 H) which can reduce the adhesion force of the moving surfaces. 3.2.3. Effect of load and sliding velocity Though at relatively light loads the strong bonding of the MPTS SAM to the silicon surface can withstand the shear due to frictional force, the film would fail if the load was sufficiently high. In order to identify the effect of load on the effectiveness of MPTS, the friction coefficient of film was obtained for 12 h coated MPTS silicon for normal loads between 10 and 200 mN. The experiments were conducted at room temperature at about 30% relative humidity. Fig. 7 shows that the friction coefficient decreases with increasing load. The effect of sliding velocity on the friction coefficient of MPTS SAM was investigated. Sliding length was set to 10 mm and normal load was set to 50 mN. The velocity was varied from 50 to 200 mm/min. Fig. 8 shows the experimental result for 12 h MPTS coated silicon. From Fig. 8, it can be seen that the friction coefficient decreases with increasing sliding velocity. The result indicated that the friction coefficient of MPTS SAM depended on the sliding velocity. The chemically adsorbed self-assembled molecule on a substrate is just like assembled molecular spring anchored to the substrates and a tip sliding on the surface of SAM is like a tip
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Fig. 7. Friction coefficient of MPTS SAM acquired as a functional of load.
sliding on the top of ‘molecular springs or brush’ [17]. The orientation of the ‘molecular spring or brush’ under normal load reduces the shearing force the interface, which in turn reduces the friction force. The mechanisms responsible for the variation of the friction forces of MPTS SAM with different velocity and load were believed to be related to the reason above. In addition, it was believed that the results were related to the viscoelastic properties of SAM. 3.2.4. Friction coefficient as a function of sliding pass Fig. 9 shows the variation of friction coefficient as a function of sliding pass. It can be seen that MPTS-SAM recorded low initial friction coefficient about 0.2. It was seen that the wear life of the film exceeded 250 sliding pass. The friction coefficient of silicon substrate raised to a stable and high value (0.8) at about 120 sliding pass because some contamination was physiabsorbed on the substrate and contacting with the steel ball leaded to the increased friction coefficient of substrate when the contamination coating on substrate was wore out. Comparing the silicon substrate, MPTS-SAM reacted with the substrate by chemical
Fig. 8. Friction coefficient of MPTS SAM acquired as a functional of sliding speed.
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ing −SH group was the lowest in all the samples. That result was attributed to the hydrophobic and low surface energy properties of MPTS film. 5. The friction coefficient of the samples decreased with increasing load. And the friction coefficient of the samples decreased with increasing sliding velocity. The superior friction reduction and wear resistance of MPTES SAM were attributed to the orientation of the molecules adsorbed on a substrate and the viscoelastic properties of SAM. Acknowledgements
Fig. 9. Wear dependence of the friction coefficients.
bond. This reaction was thought to be helpful in increasing the bonding strength between the films and the substrates which can avoid sever brittle fracture happened on the surface of the films in the friction and wear test which led to the invalidation of the films and hence improving the tribological properties. Different SAMs may have different surface energy values. This will certainly result in different friction coefficients during the friction test. Low surface energy can reduce stiction, so the friction coefficient values of SAMs having lower surface energy are expected to be lower. It was noted that the contact angles of the hydroxide silicon substrate and the MPTS film were 1◦ and 72◦ , respectively, which indicated that surface energy of the substrate can be obviously reduced when MPTS SAM was coated which can reduce the adhesion force of the moving surfaces. The contact angle of MPTS SAM is lower than some other organic film such as OTS which the contact angles is 111◦ [18,19], so the friction coefficient of MPTS SAM is lower than that of silicon substrate whereas higher than that of OTS. The second factor that resulted in lower friction coefficient of MPTS SAM was that MPTS SAM had functional group −CH3 which had lower friction coefficient than other functional group, such as −COOH, −CH3 , −CF [8,9]. The other possible reason of the friction difference may be due to different surface roughness and different uniformity of the SAMs coated surface compared other SAMs, such OTS, and UTS [18]. 4. Conclusions 1. The results presented here demonstrated that MPTS SAM was self-assembled on silicon substrate. 2. The terminal groups (−SH) of SAM in the top-most layer to the air/silane could be completely oxidized into sulfonate groups (−SO3 H) in 30% nitric acid solution. 3. The macroscopic friction coefficients for coating times more than 1 h ranged between 0.1 and 0.2 whereas the value for shorter coating time was as high as 0.7. 4. The friction coefficient was higher for SAM that had functional group −SO3 H when compared to the mixed selfassembly monolayers. The friction coefficient of SAM hav-
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