nano-tribological behaviors of crown-type phosphate ionic liquid ultrathin films on self-assembled monolayer modified silicon

Surface & Coatings Technology 205 (2011) 4855–4863

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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t

Micro/nano-tribological behaviors of crown-type phosphate ionic liquid ultrathin films on self-assembled monolayer modified silicon Jibin Pu a, b, Dong Jiang a, b, Yufei Mo c, Liping Wang a,⁎, Qunji Xue a a b c

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China Graduate School of the Chinese Academy of Sciences, Beijing 100039, PR China School of Chemistry & Chemical Engineering, Guangxi University, Nanning 530004, PR China

a r t i c l e

i n f o

Article history: Received 3 February 2011 Accepted in revised form 22 April 2011 Available online 4 May 2011 Keywords: Ionic liquid AFM Ultrathin film Micro/nano-tribology Adhesion

a b s t r a c t A series of novel imidazolium-based crown-type phosphate ionic liquid ultrathin films were fabricated on silicon substrates modified by a self-assembled monolayer. The formation and surface properties of the films were analyzed by means of ellipsometric thickness measurement, X-ray photoelectron spectroscopy, and atomic force microscope. Meanwhile, the adhesive and nano-tribological behaviors of the films were tested by a homemade colloidal probe, and a ball-on-plate tribometer was used to evaluate their micro-tribological performances in the reciprocating mode. As a result, the dual-layer films show enhanced micro/nanotribological properties as compared to single-component ionic liquid films coated directly on the silicon surfaces, which is ascribed to synergic effect of the steady self-assembled underlayer as a load-carrying phase and the proper amount of flowable ionic liquid fraction with self-replenishment property. Hopefully, the novel dual-layer ultrathin films based on fluorine-free ionic liquids and self-assembled monolayer produced in the present work might find promising applications in the lubrication of micro-electromechanical systems. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Micro-electromechanical systems (MEMS) have obtained rapid development in many high technology areas due to their small mass and size, low power consumption and unit cost [1]. However, serious stiction and friction at the micro/nano-scale reduce the performances and the operating lifetimes of sliding contact MEMS devices [2,3]. Proper lubrication schemes are imperatively needed to provide long contact endurance and acceptable reliability for MEMS [4,5]. Over the past years, many self-assembled monolayers (SAMs) have been studied as lubrication films [6,7]. These films can provide load-carrying strength owing to high packing density and solid-like properties [8]. However, these monolayer films do not last long under repeated sliding due to their low molecular mobility [9,10]. When some of molecules are removed from the surface by mechanical rubbing, the films tend to fracture and break down. To utilize SAMs as MEMS lubricants, it is necessary to consider the mobile characteristics in addition to the strongly bonded characteristics [11]. Kato et al. investigated the tribological behaviors of mixed lubricants composed of bonded perfluorodecyltrichlorosilane SAMs and mobile perfluoroalkylpolyether (PFPE), and found that the mixed lubricant possessed improved tribological properties [12]. However, PFPE is often out of action due to degradation catalyzed by strong nucleophilic agents and

⁎ Corresponding author. Tel.: + 86 931 4968080; fax: + 86 931 8277088. E-mail address: [email protected] (L. Wang). 0257-8972/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2011.04.089

strong electropositive metals, which together with the high cost of PFPE can limit its applications in some fields [13,14]. Room-temperature ionic liquids (RTILs), broadly defined as synthetic salts typically composed of bulky organic cations and smaller anions that exist in the liquid state at room temperature [15], have received considerable interest in a wide range of research fields due to their unique physical and chemical properties, such as their negligible volatility, high thermo-oxidative stability and chemical stabilities, nonflammability, controlled miscibility with a large variety of organic and inorganic compounds, designable properties, etc. [16–22]. These favorable characteristics of RTILs are also just what high-performance lubricants demand in MEMS [23]. For instance, their high electrical and thermal conductivities can minimize the contact resistance between sliding surfaces as compared with other synthetic lubricants, including PFPEs and multiply-alkylated cyclopentane (MAC), and help dissipate heat during sliding [24]. The high polarity enables them to form effective adsorption film [25]. So far, extensive researches have shown that RTILs possessed excellent lubrication performance as potentially molecularlevel lubricants on polished silicon or diamond-like carbon surfaces [26–29]. Our group carried out earlier research on micro/nanotribological properties of ultrathin RTILs films, and concluded that these RTILs were suitable for thin film lubrication [30–33]. However, due to weak physisorption to substrates, insufficient antiwear performances of RTILs ultrathin films have limited their applications in MEMS. In order to improve their tribological performances, RTILs ultrathin lubrication films should basically have both an adhesive nature for avoiding the solid–solid contacts and a fluidic nature for replenishing into the

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Fig. 1. Chemical structures of crown-type phosphate ionic liquids.

lubricant-depleted areas [34]. Based on this knowledge, there was an increasing interest in creating two-phase RTILs lubrication films which contains both reducing friction and load-carrying phases [35,36]. Bhushan et al. reported enhanced tribological properties of partial bonding two-phase RTILs films obtained by heating, aiming at their application in MEMS [34,37]. Mazyar et al. [38] have studied the effect of RTILs on the shear dynamics and tribological properties of contacting ordered alkylsilane SAM on SiO2 surfaces using molecular dynamics simulation, and found that RTILs as mobile phase could potentially repair a damaged area of the SAM coating during sliding, and restore the tribological properties of the film. So far, there are few reports on tribological properties of dual-layer ultrathin lubrication films which were constructed with SAMs and RTILs. In present study, a steady silane SAM was constructed on a hydroxylated silicon surface as load-carrying phase. RTILs of interest were then coated on as-prepared SAMs. The strategy leads to the best combination of tribological properties of RTILs. In addition, due to corrosion properties of most of RTILs with PF6− or BF4− anions on the substrate surface [39], which was also a key issue for the use of RTILs as lubricants, a new series of imidazolium-based crown-type phosphate ionic liquids with fluorine-free anion were selected as lubrication phase of the dual-layer ultrathin films. The micro/nanotribological behaviors of the dual-layer films based on the novel RTILs and SAMs were in depth investigated, aiming to acquire insights into their potential in resolving the tribological problems of MEMS.

Fig. 3. A typical voltage signal curve of contact force for adhesion force calculation.

([(n, n)OEt-Im][Peh], n = 1, 2, 3) were synthesized and purified according to reported method [40]. The molecular structure is shown in Fig. 1. All other reagents are analytical grade and were used as received. P-type polished single-crystal silicon (100) wafers (obtained from GRINM Semiconductor Materials Co., Beijing, China) were used as substrates. Deionized water was used for rinsing. 2.2. Preparation of the dual-layer thin films

Octadecyltrichlorosilane (OTS) was obtained from Aldrich Chemical Co. and used as received. A series of crown-type phosphate ionic liquids

The silicon wafers with dimensions of 10 mm× 10 mm× 0.5 mm were ultrasonicated in acetone followed by ethanol for 10 min, and hydroxylated in freshly prepared Piranha solution (a mixture of 7:3 (v/ v) 98% H2SO4 and 30% H2O2) at 90 °C for 30 min. The Piranha-treated silicon wafers were rinsed copiously with deionized water and dried in a stream of N2. These hydroxylated silicon wafers were then divided into two groups, one group was immersed into the fresh 5 mM OTS solution in anhydrous toluene for 12 h at room temperature. After being taken out from OTS solution and ultrasonicated in anhydrous toluene for 5 min to remove the physically adsorbed coupling agent, then, the OTSassembled silicon wafers were dipped into three kinds of 0.2% (w/v) solutions of crown-type phosphate ionic liquids in acetone for 3 min and withdrawn from them at a constant velocity of 60 μm/s, allowing to the formation of dual-layer thin films (denoted as OTS-IL1, OTS-IL2, and OTS-IL3). The other group was directly dipped into three kinds of 0.2% (w/v) solutions of crown-type phosphate ionic liquids in acetone for 2 min. The obtained single-component ionic liquid thin films with

Fig. 2. SEM image of the colloid probe.

Fig. 4. TGA curves of the crown-type phosphate ionic liquids.

2. Experimental section 2.1. Materials

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Fig. 5. (a) XPS scan survey spectra; (b) C1s core level spectra; (c) N1s core level spectra; (d) P2p core level spectra of the dual-layer films.

thickness of about 5 nm were defined as IL1, IL2, and IL3, respectively. All procedures mentioned above were carried out in a class-100 clean room at 20 °C and a humidity of 18%. 2.3. Characterization of the thin films The thermal properties of three kinds of crown-type phosphate ionic liquids were examined on TGA-7 thermogravimetric analyzer (PerkinElmer, USA) in flowing N2. The thicknesses of thin films were measured using an L116E ellipsometer equipped with a He–Ne laser source (632.8 nm) at an incident angle of 50° (Gaertner, USA), and the averages were obtained from 10 measurement locations on each specimen. Chemical compositions and element chemical state of the thin films were analyzed on a PHI-5702 X-ray photoelectron spectrometer (XPS, PerkinElmer, USA) with an excitation source of Mg Kα radiation (hυ = 1253.6 eV) at take-off angle of 35°, and the vacuum degree of chamber was about 5 × 10 − 8 Torr during testing. The binding energies of the target elements were determined at a pass energy of 29.35 eV with a resolution of about ±0.3 eV; the binding energy of C1s (CH2) at 284.8 eV was used as reference. The surface morphologies of the thin films were observed with a nanoscope IIIa multimode atomic force microscope (AFM) in tapping mode (Veeco, USA). 2.4. Tribological properties of the thin films The nano-friction and adhesion of the thin films were measured using a homemade colloidal probe mounted on the same AFM in contact mode. The colloidal probe (normal force constant 2 N/m) was prepared by gluing microsphere with a radius of 28 μm onto a tipless cantilever. Scanning electron microscope (SEM) image of the colloidal probe is shown in Fig. 2. When the microsphere slid at scan rate of 1 Hz over a

60 μm scan line, the voltage signals of the lateral torsion of the cantilever were continuously measured as a function of linearly increasing external loads. No attempt was made to calibrate the torsional force constant; the output voltages were directly used as the relative frictional force. Each curve represented an average over three separate measurement locations. The same colloidal probe was used to obtain the adhesive behaviors of the thin films. Based on voltage signal curve of contact force shown schematically in Fig. 3, the adhesive forces between the microsphere of colloidal probe and the film surfaces were calculated using the following equation [41]: FAdhesion = Kc ðN = mÞ·Δ X ðnmÞ; Δ X = Sz ðnm = VÞ × Sensitivity × Zp ðVÞ;

where Kc is the normal force constant of the colloidal probe, ΔX is deformation of the cantilever, and Zp is the horizontal distance between points A and B, i.e., the vertical displacement of the piezotube. To avoid the influence of lubricants which might transfer to microsphere surface, the pre-scan of the colloidal probe was carried out on a cleaved mica surface to remove the adsorbed lubricant. All of above experiments were carried out at a relative humidity of 15% at room temperature. Micro-tribological tests were carried out on a UMT-2MT tribometer (CETR, USA) in a ball-on-plate reciprocating mode. Commercially availed steel balls with a diameter of 3.18 mm and an RMS roughness of about 8.2 nm over a scanning area of 1 μm × 1 μm were used as the stationary upper counterparts, while the specimens coated by various thin films were mounted on a lower reciprocating table with a traveling distance of 5 mm. The measurements were performed three times for each test condition. The antiwear life of the thin films refers to the sliding time at which the friction coefficient rises sharply, corresponding to lubrication failure of the thin films. The wear scar morphologies of the thin films were observed with MicroXAM 3D non-contact interferometric microscope with phase

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Fig. 6. AFM images of various surface over a scanning range of 1 μm × 1 μm. (a) IL1; (b) OTS-IL1; (c) IL2; (d) OTS-IL2; (e) IL3; (f) OTS-IL3; (g) OTS-SAMs.

mode (ADE, USA). All tests were conducted at a relative humidity of 12% at room temperature. 3. Results and discussion The thermal properties of three kinds of ionic liquids were examined by thermogravimetric analysis (TGA) between 20 °C and 500 °C with a heating rate of 10 °C/min, and high-purity nitrogen purge was used for all the measurements. As shown in Fig. 4, all tested ionic liquids show little weight loss below 200 °C, which corresponds to high decomposition temperature and a low vapor pressures, and hence meets the demand of high performance lubricant. Furthermore, ionic liquid [(2, 2) OEt-Im][Peh] exhibits best thermal stability among tested ionic liquids. XPS is a powerful tool to clarify the chemical composition and the chemical states of some typical elements in thin films. Fig. 5 depicts

the stacked XPS scan survey spectra and several characteristic elements spectra of the dual-layer films on silicon surfaces. Each scan survey spectra showsthe following four characteristic elements: carbon (C1s), oxygen (O1s), nitrogen (N1s) at 400.8 eV and silicon (Si1s, Si2p). Due to low atomic sensitivity of element P, there is not a visible P peak in scan survey spectra of dual-layer films; thus, the high-resolution data of P2p at 133.4 eV are presented in Fig. 5d. The presences of the P atoms originating from anion of ionic liquids and N atoms from the imidazolium ring indicate that crown-type phosphate ionic liquids were coated successfully on the OTS underlayer. As shown in Fig. 5b, there are two peaks arising from C1 XPS spectra. The first peak at 284.8 eV is assigned to the CH2 group in ionic liquids, while the second peak at 289.2 eV originates from the C atoms bonded to the O atoms (C–O) in the cation of ionic liquids. Meanwhile, it is observed that the intensity of C–O peak increases in the order of IL1,

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Fig. 7. Adhesive forces between the microsphere of the colloid probe and the surfaces of various films.

Fig. 8. Nano-friction force versus load curves of various films.

IL2 and IL3, which corresponds with their chemical structure and element content. AFM morphological images of various thin films are presented in Fig. 6. As seen from the Fig. 6a, IL1 spreads evenly on the silicon surface and the surface coverage was near 100%, indicating perfect wettability of IL1 on the hydroxylated silicon. IL2 could not form quite uniform and continuous film on silicon surface, and some aggregates are found in Fig. 6c. IL3 (Fig. 6e) distributed as tiny droplet in nanometer scale on the silicon surface. It is suggested that the agglomeration might come from the de-wetting of IL3 molecules on silicon surface. In de-wetting, a liquid film lowers its free energy by becoming thicker in some areas, and thinner, even vanishing, in other

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Fig. 9. Nominal friction coefficients (NFC) directly derived from Fig. 8.

areas [42]. However, as shown in Fig 6d and f, it is found that for duallayer films, the number and height of aggregates on the OTS-modified silicon surface noticeably decreased, corresponding to improved wettability. These observations indicate that the adsorption of the ionic liquid molecules on the OTS surface helps to improve the film quality. Fig. 7 is a summary of the adhesive force on various thin films. The adhesive force has been observed to increase in the following order: OTS b OTS-IL1 b OTS-IL2 b OTS-IL3 b IL1 b IL2 b IL3. On micro/nano-scale, the adhesion between the contact interfaces is produced under the combined action of van der Waals, capillary (meniscus), electrostatic and chemical bonding [2]. With the presence of a thin liquid film such as lubricant or adsorbed water layer at the contact interface, menisci form around the contacting and non-contacting asperities due to surface energy effects. The pressure inside the meniscus is lower than that outside the meniscus, which results in an additional pulling force acting on the contact interface. The pulling force can result in increased adhesive force. In short, when a hydrophilic film is introduced at the contact interface either through adsorption or by deposition, the adhesive force is mainly dominated by capillary (meniscus) resulting from condensation of the mobile molecule and adsorbed water thereon [43]. As shown in Fig. 7, the densely packed, highly ordered OTS-SAMs exhibit lowest adhesive force between the thin films and AFM spherical tip owing to the lack of the mobile fraction, significant reduction of adsorbed water and low surface energy. In addition, the single-component RTILs thin films directly coated on hydroxylated silicon surfaces exhibit higher adhesive force than the dual-layer films based on OTS-SAMs and RTILs, indicating that the dual-layer thin films have good adhesion-resistance. This is because polar RTILs are much easier to adsorb on polar hydroxylated silicon surface of high surface energy than on nonpolar OTS-SAMs of low surface energy under same preparation condition; in other words, the single-component RTILs films would contain more RTILs

Fig. 10. Schematic drawing of the nano-friction mechanism of single-component and dual-layer films.

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Fig. 11. Variation in micro-friction coefficient with sliding time for various films at sliding frequency of 2 Hz. (a) IL1; (b) OTS-IL1; (c) IL2; (d) OTS-IL2; (e) IL3; (f) OTS-IL3; (g) OTS-SAMs.

molecules while the OTS-SAMs only adsorb a small amount of RTILs molecule thereon which diminishes meniscus by themselves. The difference of adhesive forces on three kinds of single-component ionic

liquid thin films can be related to their chemical structure and corresponding physical and chemical properties, which also result in difference of that on their dual-layer thin films.

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Fig. 12. Typical images and profile traces of the worn surfaces at the same test condition (a load of 400 mN and sliding frequency of 2 Hz). (a) IL2; (b) OTS-IL2.

To study the nano-friction properties of various thin films, the friction force versus linearly increasing normal load curves are showed in Fig. 8. The friction force given is in the form of a voltage signal, which should be proportional to the real friction force. [44] Therefore, the friction forces on various film surfaces can be compared. The linearity of the friction-load relationship suggests that a modified form of Amonton's law can be applied in which the lateral force (FL) is given by FL = μFN + F0, where μ is the friction coefficient, FN is the normal load, and F0 is the friction force when the external load is zero [45,46]. Therefore, the gradient of the force curve can be used as the friction coefficient. However, a linear fit of the data shown here can only produce nominal friction coefficients (NFC), which are also proportional to the real friction coefficient. From these NFC curves shown in Fig. 9, it can be clearly seen that three kinds of single-component ionic liquid thin films exhibit higher NFC which is corresponding to their high adhesive forces, and IL3 performs the worst nano-lubricity among them. Due to strong adsorption affinity of superhydrophile hydroxylated silicon, excess adsorbed water and lubricant molecules thereon are more likely to form a meniscus as the tip approaches the surface, which provides greater resistance for tip sliding and leads to higher friction coefficient. Nevertheless, the duallayer films greatly reduce the NFC. Especially, OTS-IL1 and OTS-IL2 exhibit much better lubricity. The results imply that the right amount of mobile lubricants facilitate sliding of the tip on the surface due to low shear strength at the RTILs–OTS interface and within RTILs layer it is not easy to produce meniscus effect [47]. In addition, ionic liquid lubricants may incorporate into defective or damaged sites of OTS underlayer and recover or repair their tribological properties. As for OTS-SAMs, it exhibited higher friction coefficient due to lack of flowable mobile phase and numerous defects in the SAMs. Fig. 10 is a schematic illustrating the role of meniscus formation in the adhesive and friction forces obtained for single-component and dual-layer thin films. In order to evaluate tribological properties at the micro-scale, conventional ball-on-flat friction experiments were conducted on both single-component ionic liquid thin films and their corresponding dual-layer thin films. Fig. 11 shows their microtribological properties as functions of sliding cycles against steel ball at sliding frequency of 2 Hz. The friction coefficients and durabilities of all thin films gradually decrease with increased normal applied load, which can be reasonably understood because the higher load would intensify the distortion of the organic molecules and accelerate formation of defects. As shown in Fig. 11a, c, and e, three kinds of singlecomponent ionic liquid thin films with same anion exhibit different microtribological behaviors due to the difference of their physical and chemical properties. It can be seen that compared with poor durability

of IL3 with low viscosity, IL1 and IL2 exhibit better antiwear performance for both their strong adhesion to substrate and good fluidity making them be able to rapidly flow back into the wear track, namely, a self-replenishment property [12,48]. Furthermore, as drawing a parallel between IL1 and IL2, it is noticed that IL2 which possessesnot only fluidity, but also adhesion to substrate, performs antiwear and friction-reducing effects even under a load of 300 mN. But when the normal applied load rose to 400 mN, the friction coefficient rose sharply to over 0.6 after tens of seconds, implying that IL2 thin film failed. As for OTS-SAMs shown in Fig. 11g for comparison, wear easily occurred after several hundred sliding cycles under the normal applied load of 100 mN and a sliding frequency of 2 Hz. It is suggested that OTS-SAMs typically behave like a solid and have no mobile lubricant to replenish the lubrication film as the SAMs fail. Once the applied load is beyond a critical value, the SAMs would be removed from the surface due to severe plastic deformation which leads to film failure, which may be the main reason for the poor loadbearing capacity and durability of such solid-like films compared to ionic liquid thin films. This indicates that only lubricant sharing adhesive and fluidic nature can show desirable tribological properties. In case of the mixed lubricant system, as shown in Fig. 11b, d and f, all of dual-layer thin films with both mobile ionic liquid uplayer and bonding OTS underlayer show stable and decreased friction coefficient with increased sliding cycles. Especially, OTS-IL1 and OTS-IL2 dual-layer thin films show dramatically enhanced durability compared to their corresponding single-component ionic liquid thin films in addition to low friction coefficients. This is attributed to the synergic effect of solid-like and liquid-like fractions in dual-layer thin films, and the more densely packed and ordered dual-layer structure can compensate defects of the film itself and reduce friction. When the mobile ionic liquids are mechanically disrupted or displaced from the contact zone, OTS underlayer can prevent the steel ball and silicon substrate surface from the solid–solid contact, and highly mobile ionic liquids can reflow into the wear track and replenish the lubricantdepleted zone. In addition, the viscoelastic OTS underlayer between sliding pairs acts as a shock absorber to reduce the amount of impact energy transferred to the substrate, which may allow for wear protection during reciprocating movement [12]. To further clarify the excellent friction behavior of dual-layer thin film, Fig. 12b shows the worn surfaces and cross-section maps of typical OTS-IL2 thin films at normal applied loads of 400 mN and a frequency of 2 Hz. That of IL2 worn surfaces is also shown in Fig. 12a for a comparison. It is observed that OTS-IL2 dual-layer thin film hardly exhibits visible wear scar after 3600 cycles, indicating that OTS underlayer provides wear protection, while silicon surface coating IL2 thin film has shown a deeper wear scar just after IL2 thin film failed

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bonding fraction to bear load and a mobile fraction to reduce friction coefficient. 4. Conclusion Three kinds of novel imidazolium-based crown-type phosphate ionic liquid ultrathin dual-layer films, together with their singlecomponent films, were formed successfully on hydroxylated silicon surface. According to their adhesion and micro/nano-tribology experimental results, ionic liquid ultrathin dual-layer films, particularly OTS-IL2, show optimal micro/nano-tribological properties. Moreover, the following conclusions can be drawn: the proper amount of mobile ionic liquid fraction in the dual-layer films facilitate sliding of colloid probe on the film surface due to low shear strength at the RTILs–OTS interface and within RTILs layer while it is difficult to form meniscus due to the small content, which helps to reduce nanofriction force, and the synergic effect of solid-like OTS underlayer and flowable ionic liquid fraction with self-replenishment property in the dual-layer film combines high load carrying capacity with low and stable micro-friction coefficient and further improves microtribological properties of the ionic liquid thin films. Acknowledgments The authors are grateful for the financial support from Natural Science Foundation of China (Grant No. 50823008) and National 973 Program (Grant No. 2011CB706603). References

Fig. 13. (a) XPS scan survey spectra; (b) N1s core level spectra; (c) P2p core level spectra of the OTS-IL2 dual-layer films before and after tribo-tests (a load of 400 mN and sliding frequency of 2 Hz).

under same load and sliding frequency. In addition, for dual-layer thin film, some regions of the slight wear tracks are filled with the lubricant, which is attributed to lubricant replenishment from the mobile fraction. The RTILs' replenishment could be further confirmed by stacked XPS scan survey spectra (Fig. 13a) and characteristic elements spectra of the ionic liquids (Fig. 13b and c) in OTS-IL2 duallayer film before and after tribo-tests. It is indicated that mobile RTILs possess a self-replenishment ability due to both P and N elements of the RTILs that have been found on rubbed surface though the XPS intensity is weaker. In addition, no obvious chemical shift was observed after sliding tests, indicating that there was no chemical change that occurred during the friction process. Briefly, the experimental observations presented here shed light on how to design ionic liquid thin films with best combination of tribological properties. The structure should be composed of both a load-bearing

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