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AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films
Journal of Colloid and Interface Science 317 (2008) 275–287 www.elsevier.com/locate/jcis
AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films Bharat Bhushan a,∗ , Manuel Palacio a , Barbara Kinzig b a Nanotribology Laboratory for Information Storage and MEMS/NEMS (NLIM), The Ohio State University, 201 W 19th Ave., Columbus, OH 43210, USA b Surfaces Research and Applications, Inc., 8330 Melrose Dr., Lenexa, KS 66214, USA
Received 7 August 2007; accepted 15 September 2007 Available online 21 September 2007
Abstract Ionic liquids (ILs) are considered as lubricants for micro/nanoelectromechanical systems (MEMS/NEMS) due to their excellent thermal and electrical conductivity. So far, only macroscale friction and wear tests have been conducted on these materials. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a crucial step to understand how these novel materials can efficiently lubricate MEMS/NEMS devices. To this end, the adhesion, friction and wear properties of two ionic liquids, 1-butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 ) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4 ), applied on Si(100), are investigated for the first time using atomic force microscopy (AFM). Data is compared to the perfluoropolyether lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated using AFM-based surface potential and contact resistance techniques. Thermally treated coatings containing a mobile lubricant fraction (i.e., partially bonded) were better able to protect the Si substrate from wear compared to the fully bonded coatings, and this enhanced protection is attributed to lubricant replenishment. © 2007 Elsevier Inc. All rights reserved. Keywords: Ionic liquids; Lubricants; Adhesion; Friction; Wear; Atomic force microscopy; Kelvin probe microscopy; Surface potential; Resistance
1. Introduction The commercialization of products based on microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) relies on a better understanding of the various mechanisms that control the performance and failure of these devices. Adhesion, friction and wear at the nanometer size scale become critical and can be detrimental to the efficiency, power output and reliability of MEMS/NEMS devices [1,2]. For example, adhesion is the major cause of the failure of accelerometers used in automobile air bag triggering mechanisms [3] and in micromirror components of commercial digital light processing (DLP) equipment [2,4–7]. Wear has been found to compromise the performance of NEMS-based atomic force microscopy (AFM) data storage systems [8,9]. In order to im* Corresponding author. Fax: +1 614 292 0325.
E-mail address: [email protected] (B. Bhushan). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.046
prove tribological performance, lubricants are applied to the MEMS/NEMS device surfaces. The ideal lubricant should be molecularly thick, easily applied, able to chemically bond to the micro/nanodevice surface, insensitive to environment, and highly durable [10–13]. 1.1. Ionic liquids as potential lubricants An ionic liquid (IL) is a synthetic salt with a melting point below 100 ◦ C. A room temperature ionic liquid is a synthetic molten salt with melting points at or below room temperature. One or both of the ions are organic species. At least one ion has a delocalized charge such that the formation of a stable crystal lattice is prevented and the ions are held together by strong electrostatic forces. As a result of the poor coordination of the ions, these compounds are liquid below 100 ◦ C or even at room temperature [14]. The number of combinations of anions and cations that can be used to produce ionic liquids is in the range of one million.
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Typical cations include imidazolium, pyridinium, ammonium, phosphonium and sulfonium as shown below, where R stands for an organic group.
Imidazolium
Pyridinium
NR+ PR+ SR+ 4 4 3 Ammonium Phosphonium Sulfonium Typical anions are tetrafluoroborate, BF− 4 , hexafluorophosphate, PF− , bis(trifluorosulfonyl) imide, (CF3 SO2 )2 N (“tri6 flamide”), and toluene-4-sulfonate, C7 H7 O3 S (“tosylate”) [14]. Dependent upon the substrate wettability and other functional requirements, a set of cations and anions can be combined. The ionic liquids were initially developed for used as electrolytes in batteries and for electrodeposition. Recent applications have geared these compounds as environmentally friendly solvents for chemical synthesis (“green chemistry”) where these liquids are used as substitutes for conventional organic solvents. Ionic liquids are considered as potential lubricants. Their strong electrostatic bonding compared to covalently bonded fluids, leads to very desirable lubrication properties. They also possess desirable properties such as negligible volatility, nonflammability, high thermal stability or high decomposition temperature, efficient heat transfer properties, low melting point, as well as compatibility with lubricant additives [15–17]. Unlike conventional lubricants that are electrically insulating, ionic liquids can minimize the contact resistance between sliding surfaces because they are conducting, and conducting lubricants are needed for various electrical applications (e.g., see [9]). These liquids can also be used to mitigate arcing, which is a cause of electrical breakdown in sliding electrical contacts [18]. In addition, ILs have high thermal conductivity which helps to dissipate heat during sliding [19]. The use of ionic liquids instead of hydrocarbon base oils (such as highly reformed mineral oils) has the potential to dramatically reduce air emissions. Perfluoropolyethers (PFPEs) are used in magnetic rigid disk and vacuum grease applications due to their high thermal stability and extremely low vapor pressure [10,12]. However, from the commercial standpoint, ionic liquids are cheaper than PFPEs by a factor of two or so, providing the motivation for comparing the tribological properties of the former with the latter. ILs are being considered for MEMS/NEMS applications because of their high temperature stability, electrical conductivity, and desirable lubrication properties. Ionic liquids with the hexafluorophosphate anion have been evaluated extensively using conventional friction and wear tests and were found to exhibit improved friction and wear properties compared to conventional lubricants [20–23]. The ionic liquid containing the octyl sulfate anion has been developed more recently and is of interest due to its resistance to hydrolysis [14]. Based on experience, anions are observed to affect tribological performance. Table 1 lists the physical and thermal properties
of selected ionic liquids compared to the PFPE lubricant ZTETRAOL. The durability of ionic liquid films on various metal and ceramic substrates has been investigated from the standpoint of film formation (wettability) and film removal (friction and wear), where it was found that certain cations and anions exhibit better wetting, friction reduction and wear resistance properties. In general, ionic liquids exhibit better wettability on noble metal and ceramic surfaces than on non-noble metal surfaces [24]. The flat imidazolium cation shows poorer wettability compared to bulkier cations such as ammonium and sulfonium. Among salts with the imidazolium cation, the presence of longer organic side chains lead to a reduction of the coefficient of friction [23,25]. An anion effect is also observed, where oxygen-rich anions show better substrate wettability and lower wear compared to other imidazolium salts [14]. Based on these findings, the ionic liquids of interest in this study are 1-butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 ) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4 ). Their chemical structures are shown in Fig. 1 and a summary of their properties is listed in Table 1. 1.2. Nanoscale characterization A suitable characterization technique is needed for MEMS/ NEMS that can resolve the small changes with length scale during sliding. Atomic force microscopy is well-suited for this purpose as it has the ability to measure material properties with high spatial resolution. An atomic force microscope (AFM), which is a typical example of a scanning probe technique, detects minute forces between the sample surface and the scanning tip attached to a cantilever [26,27]. An AFM tip sliding on the sample surface simulates a single asperity contact under light load conditions typically encountered during MEMS/NEMS operation. The comprehensive evaluation of a potential lubricant involves the study of adhesion, friction and wear. To date, investigations on the friction behavior of materials coated with ionic liquids have been conducted using the conventional ball-ondisk or pin-on-disk tribometers [16,17,20,22,23,25,28]. Adhesion and friction properties at the macroscale are different from the micro/nanoscale [26]. Knowledge of the latter is important in optimizing lubrication of MEMS/NEMS devices. AFM can be used to address this need as it can be configured to measure adhesion and friction forces at the micro/nanoscale. The surface potential, which is one of the electrical properties that can be measured with an AFM, pertains to the potential difference between two surfaces as they are brought together in electrical contact and thermal equilibrium is achieved. Surface potential mapping is a useful technique for wear detection. DeVecchio and Bhushan [29] have shown that a surface potential change is detectable even in cases where there is little or no surface damage, making this a powerful technique for studying the initiation of wear. The technique is well suited for wear studies of MEMS/NEMS device components because it has also been used for characterizing the wear of metals, ceramics and a conventional perfluoropolyether (PFPE) lubricant
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Table 1 Physical, thermal and electrical properties of selected ionic liquids and Z-TETRAOL
Cation Anion Molecular weight (g/mol) Tmelting (◦ C) Tdecomposition (◦ C) Density (g/cm3 ) Kinematic viscosity (mm2 /s) Pour point (◦ C) Specific heat (J/g K) Thermal conductivity at 25 ◦ C (W/m K) Dielectric strength at 25 ◦ C (kV/mm) Volume resistivity ( cm) Vapor pressure at 20 ◦ C (Torr) Wettability on Si Water contact angle
Miscibility with isopropanol Miscibility with water a b c d e f g h
1-Butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 )
1-Butyl3-methylimidazolium octyl sulfate (BMIM-OctSO4 )
C8 H15 N+ 2 PF− 6 284a 10c 300c 1.37a 281a (20 ◦ C) 78.7d (40 ◦ C) < −50e 1.44f (25 ◦ C) 0.15h – – <10−9 Moderatec 46◦ (untreated) 39◦ (partially bonded) 41◦ (fully bonded) Totala –
C8 H15 N+ 2 C8 H17 SO− 4 349a 28c 150c 1.07a 907a (20 ◦ C) – 1.82g (25 ◦ C) – – – <10−9 Moderatec 32◦ (untreated) 22◦ (partially bonded) 41◦ (fully bonded) Totala Totala
Proprietary ionic liquid
Z-TETRAOL
– – – 25c 210c 0.94 –
– – 2300b – ∼320b 1.75b 2000b (20 ◦ C)
– 2.3–3.6c (50 ◦ C) ∼0.2c – – <10−9 – –
−67b ∼0.20b (50 ◦ C) ∼0.09b ∼30b ∼1013 b ∼10−12 b – 54◦ (untreated) 83◦ (partially bonded) 88◦ (fully bonded) –
Totalc Partialc
Merck Ionic Liquids Database, Darmstadt, Germany, also: http://ildb.merck.de/ionicliquids/en/startpage.htm. Z-TETRAOL Data Sheet, Solvay Solexis Inc., Thorofare, NJ. [24]. [15]. [23]. [41]. [42]. [43].
suring carrier distribution in silicon device structures [35]. This resistance measurement technique has been used in conjunction with surface potential measurements in detecting wear on silicon after the perfluoropolyether (PFPE) lubricant Z-TETRAOL is removed from the surface [32]. 1.3. Objective of this research
Fig. 1. Chemical structures of the BMIM-PF6 , BMIM-OctSO4 and ZTETRAOL molecules.
[30,31]. More recently, the change in surface potential has been used to demonstrate how the presence of both immobile and mobile lubricant fractions on a silicon substrate provide enhanced wear protection compared to substrates containing only the immobile lubricant [32]. It has also been shown that surface potential measurements can elucidate the mechanism of triboelectric charging of human hair [33,34]. Recent advances in atomic force microscopy instrumentation have enabled the measurement of resistance by monitoring the current resulting from an applied DC bias voltage between tip and sample. This technique, known as scanning spreading resistance microscopy (SSRM) is originally developed for mea-
In this study, AFM-based adhesion, friction and wear measurements are presented for the first time for silicon substrates coated with the ionic liquids of interest. Conventional ball-onflat data is used in conjunction with AFM experiments in order to compare friction and wear properties at the microscale and nanoscale. Lubricant removal and surface wear is detected using surface potential and contact resistance measurements. Data from these measurements are compared with that from the perfluoropolyether (PFPE) lubricant Z-TETRAOL. 2. Experimental details 2.1. Materials and sample preparation The room temperature ionic liquids used in this study are 1butyl-3-methylimidazolium hexafluorophosphate, abbreviated as BMIM-PF6 (Merck, Darmstadt, Germany) and 1-butyl3-methylimidazolium octyl sulfate, abbreviated as BMIMOctSO4 (Merck, Darmstadt, Germany). Data from these ionic
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liquids are compared with the perfluoropolyether (PFPE) lubricant Z-TETRAOL (Solvay Solexis Inc.), which is the model lubricant used to develop the test methodology. The lubricants were applied on single-crystal Si(100) (phosphorus doped) with a native oxide layer on the surface using the dip coating technique. The method and the apparatus used have been described elsewhere [11,12]. Briefly, silicon substrates with dimensions of 10 × 10 × 0.5 mm were cut from Si (100) wafers. The wafers were ultrasonicated in acetone followed by isopropanol for 10 min each. Then, the cut wafer was vertically submerged into a beaker containing a dilute solution of the lubricant for 10 min. The solutions of the various lubricants are 0.2% (v/v) BMIMPF6 in isopropanol, 0.2% (v/v) BMIM-OctSO4 in isopropanol and 0.1% (v/v) Z-TETRAOL in HFE 7100 (a solvent consisting of isomers of methoxynonafluorobutane (C4 F9 OCH3 )). All the solutions were mixed vigorously and allowed to stand for at least an hour prior to use. The dip coating procedure is as follows. The silicon wafers were pulled up from solution with the aid of the motorized stage set at constant speed of 5 mm/s to obtain films of desired thickness. The lubricated sample used without post thermal treatment is referred to as untreated. Partially bonded samples were prepared by heating at 150 ◦ C for 30 min after dip coating, while the fully bonded samples were heated at 150 ◦ C for 30 min and washed in isopropanol (HFE 7100 in the case of Z-TETRAOL) to remove the mobile fraction. The ionic liquid-coated silicon samples were then measured with an ellipsometer in the fixed refractive index option (NFXD option, Gaertner Scientific L115C Ellipsometer Instruction Manual). Since the silicon wafer has a native oxide layer, a refractive index of 1.46 (CRC Handbook of Materials Science) was used in determining the thickness of the deposited ionic liquid. The coating thickness was found to be about 0.5, 1.5 and 2.5 nm for the fully bonded, partially bonded and untreated ionic liquid samples, respectively. Since ionic liquids do not have polar end groups (as compared to Z-TETRAOL), chemical bonding of the lubricant to Si is not obvious. However, it appears that thermal treatment leads to bonding of the ionic liquid because some coating is left after washing with isopropanol [27]. The thickness of the Z-TETRAOL coatings used in the study was about 1, 3 and 7 nm for the fully bonded, partially bonded and untreated samples, respectively.
2.2. Nanoscale adhesion, friction and wear measurements Adhesion and friction experiments at the nanoscale were carried out using a commercial Nanoscope IIIa MultiMode AFM (Veeco Instruments, Santa Barbara, CA) in ambient conditions (22 ◦ C, 45–55% RH) over a 2 µm scan line with the normal load ranging from 5 to 130 nN and a scan rate of 1 Hz. Square pyramidal Si3 N4 tips with a nominal 30–50 nm radius mounted on triangular Si3 N4 cantilevers with a spring constant of 0.58 N/m were used. The friction force was calibrated by the method described by Bhushan [26]. The adhesive force was calculated from the horizontal intercept of the friction versus normal load curves at a zero value of the friction force. A wear test based on an established procedure was used to monitor the change in friction force [11]. With the AFM in contact mode, a Si3 N4 tip with a nominal 30–50 nm radius mounted on a triangular Si3 N4 cantilever (spring constant of 0.58 N/m) was used to repeatedly scan a 2 µm line at a normal load of 70 nN and a scan rate of 0.1 Hz. The change in friction force is indicative of lubricant removal and surface wear. In order to be able to use higher loads for high wear needed in electrical measurements, lubricated surfaces were worn using a diamond tip with an apex angle of 60◦ and a tip radius of approximately 100 nm attached to a stainless steel cantilever with a spring constant of 10 N/m. Wear scars with dimensions of 5 × 5 µm2 were created and scanned for 20 cycles at a load of 10 µN. This experiment was performed for the untreated, partially bonded and fully bonded lubricant samples. This is the wear test used for subsequent surface potential and resistance measurements, described below. 2.3. Microscale friction and wear measurements For completeness, friction measurements at the microscale were conducted based on an established procedure of using a ball-on-flat tribometer under reciprocating motion [36,37] as shown in the schematic in Fig. 2. A sapphire ball with 3 mm diameter and surface finish of about 2 nm RMS was fixed on a stationary holder. A normal load of 500 g (4.9 N) was applied, and the frictional forces were measured with semiconductor strain gages that were then digitized and collected on a personal
Fig. 2. Schematic of the ball-on-flat tribometer used for microscale friction and wear measurements. Blowup shows the interface between the ball and surface of interest and the connections used for electrical measurements.
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(a)
(b) Fig. 3. (a) Schematic of the two passes of the AFM-based Kelvin probe technique [29] with the first pass in tapping mode for measuring surface height (left) and the second pass in lift mode for measuring surface potential (right). (b) Schematic of the AFM-based resistance measurement technique where the surface height is measured in contact mode and the resistance is measured by the current resulting from the applied DC bias voltage. (Schematic for (a) is for MultiModeTM AFM, while (b) is for DimensionTM AFM (Veeco Instruments, Inc.).)
computer. Typical test conditions are: stroke length = 2 mm, average linear speed = 4 mm s−1 , temperature = 22 ± 1 ◦ C, relative humidity = 45 ± 5%. Wear was characterized by imaging the resulting scar with an optical microscope and taking the cross section with a stylus profiler. The number of cycles to failure was determined by identifying the point where a sudden change in the friction force is observed. 2.4. Wear detection by surface potential and resistance measurements Surface potential measurements were taken with a Nanoscope IIIa MultiMode atomic force microscope equipped with the extender electronics module (Veeco Instruments Inc.). The extender enables the measurement of surface potential. Electrically conductive silicon tips coated with cobalt/chrome (MESP
probes, Veeco Instruments Inc.) were used. The schematic for this AFM-based Kelvin probe technique is shown in Fig. 3a. Surface potential is measured using a two pass method. The first pass is in tapping mode to measure surface height. In the second pass, the surface potential map is obtained in the socalled “lift mode” at a tip–sample separation (lift scan height) of 10 nm. In this mode, the piezo normally oscillating the tip in tapping mode is turned off and an oscillating voltage is applied directly to the conducting tip in order to generate an oscillating electrostatic force. This force has an amplitude described by the equation F = (dC/dz)vdc vac ,
(1)
where dC/dz is the vertical derivative of the tip–sample capacitance, vdc is the dc voltage difference between tip and sample and vac is the amplitude of the oscillating voltage applied to the
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tip. To measure the surface potential, the dc voltage is applied to the tip until the quantity vdc is equal to zero, giving zero oscillating force amplitude [29,30]. The nanoscale resistance measurements were taken with a Nanoscope IIIa Dimension 3000 atomic force microscope equipped with the scanning spreading resistance microscopy (SSRM) application module. The electrical resistance is measured between the conductive tip (MESP probes, Veeco Instruments Inc.) and a large current-collecting back contact while the tip is scanned in contact mode across the surface. A DC bias voltage is applied between the tip and the sample and the current is monitored using a logarithmic current amplifier built into the SSRM sensor, as shown in the schematic in Fig. 3b. In this study, the bias voltage is −10 mV. The resistance is obtained by calibrating the sensor against resistors with a range of resistance values. With the SSRM sensor, two resistances can be measured: the spreading resistance and the contact resistance. The former is the resistance defined by the distribution of the dopant atoms in the direction normal to the surface. The measured resistance is dominated by the spreading resistance when the applied load is high. In this study, the contact resistance of the tip–sample contact is predominant due to the low level of doping in the sample and the low loads that can be applied by the cobalt/chrome coated silicon tip [35]. Contact resistance measurements were performed at the microscale using a ball-on-flat tribometer under reciprocating motion. A steel ball with 3 mm diameter was used. As shown in the blowup in Fig. 2, the tribometer was modified so that a voltage of 5 V can be applied using a Wavetek power supply. The wear test was conducted for 100 cycles using the same test conditions described above (Section 2.3) while the resistance was simultaneously monitored with an Agilent 34401A multimeter. 3. Results and discussion 3.1. Surface topography, adhesion, friction and wear In Fig. 4a, the surface height images for the untreated sample (air dried) are compared with the two chemically bonded samples (partially bonded and fully bonded). Aggregates of varying sizes are observed on the untreated Z-TETRAOL and on all the ionic liquid coatings. These aggregates could have formed initially during preparation of the dilute solution. For ZTETRAOL, the long PFPE chains can orient in various configurations (such as coils), leading to aggregate formation. In ionic liquids containing the 1-butyl-3-methylimidazolium cation, it is believed that aggregate formation in solvents that are less polar compared to water aids in minimizing the charge density (charge delocalization) within the ions [38]. These aggregates are subsequently deposited on the silicon surface during the dip coating procedure. It is observed that the untreated lubricant surface has more prominent aggregates compared to the two chemically bonded samples, implying that the heat treatment promotes bonding to the Si substrate. Without the chemical bonding procedure, the lubricant molecules are less likely to attach to the substrate and would tend to attract each other in-
(a)
(b) Fig. 4. (a) Surface height images for the untreated, partially bonded and fully bonded films of BMIM-PF6 and BMIM-OctSO4 on silicon substrate. (b) Schematic for the attachment of BMIM-PF6 to the silicon substrate.
stead, such that dewetting is more likely. The immobilization of the ionic liquid, which is promoted by thermal treatment, occurs by reaction of the anion with the hydroxyl groups present on the silicon surface [39], as shown in Fig. 4b for BMIMPF6 . A similar mechanism is responsible for the attachment of BMIM-OctSO4 . For BMIM-PF6 on a mica substrate, aggregates of varying sizes were observed, which is a manifestation of the liquid phase [40]. In addition to this, zigzag structures, interpreted as the solid-like phase have been reported, indicating the possible coexistence of a liquid and solid phase. This was not observed on the ILs investigated in this study, which where all coated on silicon substrates. Fig. 5a is a summary of the adhesive force and coefficient of friction measurements on the ionic liquids. Z-TETRAOL and Si (100) data are provided for comparison. The adhesive force has been observed to decrease in the following order: untreated > partially bonded > fully bonded. This mobile fraction on the untreated sample facilitates the formation of a meniscus, which increases the tip–sample adhesion. The adhesive force is
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(a)
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(b)
Fig. 5. (a) Summary of the adhesive force and coefficient of friction and (b) durability data after 100 cycles for BMIM-PF6 and BMIM-OctSO4 at room temperature (22 ◦ C) and ambient air (45–55% RH). Data for the uncoated Si and Z-TETRAOL are shown for comparison. Schematic in (a) shows the effect of chemical bonding treatment and meniscus formation between the AFM tip and sample surface on the adhesive and friction forces.
highest in the untreated coating since it has the greatest amount of the mobile fraction among the three samples. Conversely, the sample with no mobile lubricant fraction available (fully bonded) has the lowest adhesive force. A different trend is observed in the coefficient of friction (μ) data. Both the fully bonded and partially bonded samples have lower μ values compared to the uncoated silicon. Friction forces are lower on the latter, implying that the mobile lubricant fraction present in the partially bonded samples facilitates sliding of the tip on the surface. However, μ values for the untreated samples are higher than the data for the heat treated coatings. Due to the lack of chemical bonding, the interaction of the lubricant to the substrate is weakened and dewetting can occur. Water and lubricant molecules are more likely to form a meniscus as the tip approaches the surface. This provides greater resistance to tip sliding, leading to higher coefficient of friction values. The lower portion of Fig. 5a is a schematic illustrating the role of meniscus formation in the adhesive and friction
forces obtained for the uncoated Si and the untreated, partially bonded and fully bonded lubricant-coated Si surfaces. Fig. 5b contains plots of the coefficient of friction as a function of the number of sliding cycles at 70 nN normal load. Only a small rise in the coefficient of friction was observed for both Z-TETRAOL and the BMIM-PF6 surfaces, indicating low surface wear. However, all three BMIM-OctSO4 samples exhibited gradual change in μ, implying that wear is taking place after approximately 25 cycles for both the untreated and fully bonded surfaces and 60 cycles for the partially bonded surface. In some cases, such as in the fully bonded BMIM-OctSO4 and the untreated Z-TETRAOL, a crossover is observed, where the coefficient of friction increases from its initial value and exceeds the μ of silicon after a certain number of cycles. This is attributed to the transfer of lubricant molecules to the AFM tip and the interaction of the transferred molecules with the lubricant still attached on the Si substrate, which will increase the friction force.
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(a) Fig. 6. (a) Optical images and height profiles taken after 20 cycles and (b) summary of the coefficient of friction and number of cycles to failure from ball-on-flat tests on various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
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(b) Fig. 6. (continued)
In order to compare friction and wear properties at the microscale and the nanoscale, conventional ball-on-flat tribometer experiments were conducted on the same samples. Images and profile traces of the wear scars are shown in Fig. 6a. The coefficient of friction and number of cycles to failure are summarized in Fig. 6b. Both ionic liquids are showing enhanced durability compared to both the Z-TETRAOL-coated and the uncoated Si. The nanoscale data presented in Fig. 5 can be compared to μ values obtained from ball-on-flat (Fig. 6b) and four-ball friction tests [24]. The μ values of the untreated lubricant samples obtained by using AFM are lower than the μ obtained from either the four-ball or the ball-on-flat tests. This is attributed to the difference in the length scales of the test techniques. An AFM tip simulates a single asperity contact while the conventional friction test involves the contact of multiple asperities present in the test system. With regards to wear, the interface contact of the AFM and ball-on-flat techniques are different from each other such that one cannot expect both tests to show the same trend. On an AFM, the tip stress is very high such that material can be displaced more easily. This could account for the early damage in BMIM-OctSO4 using AFM (Fig. 5b). For a ball-on-flat test, the tip exerts a lower pressure on the surface and the coating is in a confined geometry. As a consequence, displacement of the coating is not as easy as in AFM, leading to enhanced wear resistance. 3.2. Wear detection by surface potential and resistance measurements Fig. 7a is a summary of the wear tests and the corresponding surface potential measurements on the ionic liquids. A bar plot summarizing the average surface potential change on the
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tested area is shown in Fig. 7b. In all cases, a smaller amount of debris has been generated compared to the uncoated silicon surface, indicating that the ionic liquids are providing wear protection. In general, the samples containing the mobile lubricant fraction (i.e., untreated and partially bonded surfaces) exhibit a lower surface potential change compared to the fully bonded sample, which only has immobile lubricant molecules. This is attributed to lubricant replenishment by the mobile fractions, which can occur in the untreated and partially bonded samples [32]. From the bar plot in Fig. 7b, it is also observed that the change in surface potential is generally lower in the ionic liquid coatings compared to the Z-TETRAOL coatings and the uncoated silicon. This indicates that any built-up surface charges arising from the wear test were immediately dissipated onto the conducting ionic lubricant coating surface. In the case of Z-TETRAOL and the uncoated silicon, the charges remained trapped in the test area, since both these materials are insulators. Based on these findings and previous observations [30,32, 33], a considerable surface potential change will be observed on the wear region when: (1) the lubricant has been fully removed from the substrate; (2) the native SiO2 layer has been abraded from the surface; (3) wear has caused subsurface structural changes; (4) charges build up as they are unable to dissipate into the surrounding material. Contact resistance images for the surfaces subjected to the wear tests are presented in Fig. 8a. The average change in the contact resistance of the wear region relative to the untested area is summarized in Fig. 8b. The fully bonded BMIM-PF6 and BMIM-OctSO4 both have appreciable contact resistance increase in the wear region. Since silicon is a semiconductor, it has much higher resistance compared to the surrounding ionic liquid. The resistance increase in the worn area implies that the substrate is exposed after the wear test. Partially bonded films did not get worn out from the substrate after the test, as evidenced by the lack of contact resistance change in the tested area. The untreated Z-TETRAOL and BMIM-OctSO4 -coated surfaces exhibited an observable resistance change, while BMIM-PF6 did not. This can be correlated to the durability data in Fig. 5b, where the untreated ZTETRAOL and BMIM-OctSO4 samples exhibited an increase in the friction force with time due to transfer of lubricant molecules to the tip. Easier lubricant removal means that the diamond tip (in the case of Fig. 8) can cause substrate wear much sooner, leading to the observed resistance increase in the tested area. However, the resistance image does not provide a clear contrast between Z-TETRAOL and the newly exposed substrate since both materials have high resistance values. Microscale contact resistance obtained from ball-on-flat tribometer testing is shown in Fig. 9, along with the corresponding coefficient of friction data. For the ionic liquid films, the initial resistance is slightly lower than that of uncoated silicon, confirming their conductive nature. For the Z-TETRAOL samples, the contact resistance is about the same magnitude as the uncoated silicon. But for the conducting ionic liquids, an increase in resistance corresponds to an increase in the coefficient of friction, indicating wear of the lubricant and exposure of the silicon substrate, similar to observations in the nanoscale. These
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(a)
(b) Fig. 7. (a) Surface height and surface potential maps after wear tests and (b) bar chart showing surface potential change for various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
results are consistent with the adhesion, friction and surface potential results with regards to wear detection and wear protection coming from the mobile and immobile lubricant fractions. The durability data and trends for the ILs obtained by using a steel ball are inferior from the results in Fig. 6, which was measured by using a sapphire ball. In Fig. 9, the partially
bonded samples still show the best durability, but in this case, Z-TETRAOL has the highest number of cycles to failure (opposite trend compared to Fig. 6b), as indicated by the point where the jump in the coefficient of friction is observed. This can be accounted for from the wetting properties of ionic liquids on different surfaces, studied by applying small drops (∼2 mm diameter) to surfaces using a Pt wire [24]. It was observed
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a steel ball, less wettability means less lubricant retention at the interface. This material wetting effect is possibly more significant for the ionic liquids than in Z-TETRAOL. But nonetheless, the durability of the partially bonded BMIM-PF6 is still close to its Z-TETRAOL counterpart, such that ILs are still viable lubricants comparable to PFPEs. 4. Conclusions The adhesion, friction and wear properties of the ionic liquids BMIM-PF6 and BMIM-OctSO4 were characterized by AFM. The following conclusions can be drawn from this study:
(a)
(b) Fig. 8. (a) Nanoscale contact resistance images after wear tests and (b) bar chart showing contact resistance change for various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison. Cases without the clear wear scar did not exhibit measurable change.
that ionic liquids have a tendency to wet nonmetal surfaces (e.g., Si3 N4 , SiO2 , glass) better than conventional metal surfaces (such as 440C, M50 and 52100 steel). For wear tests with
• Based on the surface height, adhesion and friction data, chemical bonding treatment facilitates attachment of the ionic liquids to the silicon substrate surface, leading to more uniform coatings and lowered adhesion force and coefficient of friction. All of the chemically treated lubricantcoated samples show favorable lubrication, as seen from the adhesive force and coefficient of friction being less than that of uncoated silicon in all cases. • Ionic liquids have comparable coefficient of friction with the lubricant Z-TETRAOL. The partially bonded coatings have the lowest coefficient of friction as they possess a desirable combination of lubricant bonded to the substrate as well as a mobile fraction which facilitates tip sliding. • All lubricants show favorable durability, with the partially bonded lubricant showing the best characteristics. In microand nanoscale experiments, the ionic liquids exhibit comparable durability with the lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. • The low post-wear surface potential change observed on the silicon coated with ionic liquids is indicative of enhanced charge dissipation compared to the Z-TETRAOLcoated and uncoated surfaces, which are poor conductors. Contact resistance data is consistent with the surface potential data with regards to identifying the role of the mobile and immobile lubricant fractions in protecting the surface from wear. Based on these micro- and nanoscale friction and wear measurements, these ionic liquids show strong potential as lubricants for MEMS/NEMS because they have desirable thermal and electrical conductivity as well as desirable tribological properties. Acknowledgments Financial support by Surfaces Research and Applications, Inc. (Lenexa, KS) on an U.S. Air Force Office of Scientific Research (AFOSR) Contract (No. FA9550-05-C-0182) is gratefully acknowledged. We would also like to thank Christine Malott for performing the ball-on-flat friction and wear tests.
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Fig. 9. Microscale contact resistance and coefficient of friction after ball-on-flat tests for 100 cycles on various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
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References [1] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic, Dordrecht, 1998. [2] B. Bhushan, Springer Handbook of Nanotechnology, second ed., SpringerVerlag, Heidelberg, Germany, 2007. [3] R.E. Sulouff, in: B. Bhushan (Ed.), Tribology Issues and Opportunities in MEMS, Kluwer Academic, Dordrecht, 1998, pp. 109–119. [4] S.A. Henck, Tribol. Lett. 3 (1997) 239–247. [5] M.R. Douglass, in: Proceedings of the 36th Annual International Reliability Physics Symposium, 1998, pp. 9–16. [6] H. Liu, B. Bhushan, Ultramicroscopy 100 (2004) 391–412. [7] H. Liu, B. Bhushan, J. Vac. Sci. Technol. A 22 (2004) 1388–1396. [8] J. Vettiger, P. Brugger, U. Despont, M. Dreschler, U. Durig, W. Haberle, M. Lutwyche, H. Rothuizen, R. Stuz, R. Widmer, G. Binnig, Microelectron. Eng. 46 (1999) 11–17. [9] B. Bhushan, K.J. Kwak, Nanotechnology, in press. [10] B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices, second ed., Springer-Verlag, New York, 1996. [11] H. Liu, B. Bhushan, Ultramicroscopy 97 (2003) 321–340. [12] Z. Tao, B. Bhushan, Wear 259 (2005) 1352–1361. [13] B. Bhushan, Z. Tao, Microsyst. Technol. 12 (2006) 579–587. [14] B.J. Kinzig, P. Sutor, W.G. Sawyer, A. Rennie, P. Dickrell, J. Gresham, in: ASME World Tribology Congress Proceedings, 2005, WTC2005-63744, pp. 509–510. [15] R.A. Reich, P.A. Stewart, J. Bohaychick, J.A. Urbanski, Lubr. Eng. 49 (2003) 16–21. [16] B.A. Omotowa, B.S. Phillips, J.S. Zabinski, J.M. Shreeve, Inorg. Chem. 43 (2004) 5466–5471. [17] C. Jin, C. Ye, B.S. Phillips, J.S. Zabinski, X. Liu, W. Liu, J.M. Shreeve, J. Mater. Chem. 16 (2006) 1529–1535. [18] W.E. Campbell, IEEE Trans. Comp. Hybrids Manuf. Technol. 1 (1978) 4–16. [19] M.E. Van Valkenburg, R.L. Vaughn, M. Williams, J.S. Wilkes, Electrochem. Soc. Proc. 2002 (2002) 19–22.
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AFM-based nanotribological and electrical characterization of ultrathin wear-resistant ionic liquid films Bharat Bhushan a,∗ , Manuel Palacio a , Barbara Kinzig b a Nanotribology Laboratory for Information Storage and MEMS/NEMS (NLIM), The Ohio State University, 201 W 19th Ave., Columbus, OH 43210, USA b Surfaces Research and Applications, Inc., 8330 Melrose Dr., Lenexa, KS 66214, USA
Received 7 August 2007; accepted 15 September 2007 Available online 21 September 2007
Abstract Ionic liquids (ILs) are considered as lubricants for micro/nanoelectromechanical systems (MEMS/NEMS) due to their excellent thermal and electrical conductivity. So far, only macroscale friction and wear tests have been conducted on these materials. Evaluating the nanoscale tribological performance of ILs when applied as a few nanometers-thick film on a substrate is a crucial step to understand how these novel materials can efficiently lubricate MEMS/NEMS devices. To this end, the adhesion, friction and wear properties of two ionic liquids, 1-butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 ) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4 ), applied on Si(100), are investigated for the first time using atomic force microscopy (AFM). Data is compared to the perfluoropolyether lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. Wear at ultralow loads was simulated and the lubricant removal mechanism was investigated using AFM-based surface potential and contact resistance techniques. Thermally treated coatings containing a mobile lubricant fraction (i.e., partially bonded) were better able to protect the Si substrate from wear compared to the fully bonded coatings, and this enhanced protection is attributed to lubricant replenishment. © 2007 Elsevier Inc. All rights reserved. Keywords: Ionic liquids; Lubricants; Adhesion; Friction; Wear; Atomic force microscopy; Kelvin probe microscopy; Surface potential; Resistance
1. Introduction The commercialization of products based on microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) relies on a better understanding of the various mechanisms that control the performance and failure of these devices. Adhesion, friction and wear at the nanometer size scale become critical and can be detrimental to the efficiency, power output and reliability of MEMS/NEMS devices [1,2]. For example, adhesion is the major cause of the failure of accelerometers used in automobile air bag triggering mechanisms [3] and in micromirror components of commercial digital light processing (DLP) equipment [2,4–7]. Wear has been found to compromise the performance of NEMS-based atomic force microscopy (AFM) data storage systems [8,9]. In order to im* Corresponding author. Fax: +1 614 292 0325.
E-mail address: [email protected] (B. Bhushan). 0021-9797/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2007.09.046
prove tribological performance, lubricants are applied to the MEMS/NEMS device surfaces. The ideal lubricant should be molecularly thick, easily applied, able to chemically bond to the micro/nanodevice surface, insensitive to environment, and highly durable [10–13]. 1.1. Ionic liquids as potential lubricants An ionic liquid (IL) is a synthetic salt with a melting point below 100 ◦ C. A room temperature ionic liquid is a synthetic molten salt with melting points at or below room temperature. One or both of the ions are organic species. At least one ion has a delocalized charge such that the formation of a stable crystal lattice is prevented and the ions are held together by strong electrostatic forces. As a result of the poor coordination of the ions, these compounds are liquid below 100 ◦ C or even at room temperature [14]. The number of combinations of anions and cations that can be used to produce ionic liquids is in the range of one million.
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Typical cations include imidazolium, pyridinium, ammonium, phosphonium and sulfonium as shown below, where R stands for an organic group.
Imidazolium
Pyridinium
NR+ PR+ SR+ 4 4 3 Ammonium Phosphonium Sulfonium Typical anions are tetrafluoroborate, BF− 4 , hexafluorophosphate, PF− , bis(trifluorosulfonyl) imide, (CF3 SO2 )2 N (“tri6 flamide”), and toluene-4-sulfonate, C7 H7 O3 S (“tosylate”) [14]. Dependent upon the substrate wettability and other functional requirements, a set of cations and anions can be combined. The ionic liquids were initially developed for used as electrolytes in batteries and for electrodeposition. Recent applications have geared these compounds as environmentally friendly solvents for chemical synthesis (“green chemistry”) where these liquids are used as substitutes for conventional organic solvents. Ionic liquids are considered as potential lubricants. Their strong electrostatic bonding compared to covalently bonded fluids, leads to very desirable lubrication properties. They also possess desirable properties such as negligible volatility, nonflammability, high thermal stability or high decomposition temperature, efficient heat transfer properties, low melting point, as well as compatibility with lubricant additives [15–17]. Unlike conventional lubricants that are electrically insulating, ionic liquids can minimize the contact resistance between sliding surfaces because they are conducting, and conducting lubricants are needed for various electrical applications (e.g., see [9]). These liquids can also be used to mitigate arcing, which is a cause of electrical breakdown in sliding electrical contacts [18]. In addition, ILs have high thermal conductivity which helps to dissipate heat during sliding [19]. The use of ionic liquids instead of hydrocarbon base oils (such as highly reformed mineral oils) has the potential to dramatically reduce air emissions. Perfluoropolyethers (PFPEs) are used in magnetic rigid disk and vacuum grease applications due to their high thermal stability and extremely low vapor pressure [10,12]. However, from the commercial standpoint, ionic liquids are cheaper than PFPEs by a factor of two or so, providing the motivation for comparing the tribological properties of the former with the latter. ILs are being considered for MEMS/NEMS applications because of their high temperature stability, electrical conductivity, and desirable lubrication properties. Ionic liquids with the hexafluorophosphate anion have been evaluated extensively using conventional friction and wear tests and were found to exhibit improved friction and wear properties compared to conventional lubricants [20–23]. The ionic liquid containing the octyl sulfate anion has been developed more recently and is of interest due to its resistance to hydrolysis [14]. Based on experience, anions are observed to affect tribological performance. Table 1 lists the physical and thermal properties
of selected ionic liquids compared to the PFPE lubricant ZTETRAOL. The durability of ionic liquid films on various metal and ceramic substrates has been investigated from the standpoint of film formation (wettability) and film removal (friction and wear), where it was found that certain cations and anions exhibit better wetting, friction reduction and wear resistance properties. In general, ionic liquids exhibit better wettability on noble metal and ceramic surfaces than on non-noble metal surfaces [24]. The flat imidazolium cation shows poorer wettability compared to bulkier cations such as ammonium and sulfonium. Among salts with the imidazolium cation, the presence of longer organic side chains lead to a reduction of the coefficient of friction [23,25]. An anion effect is also observed, where oxygen-rich anions show better substrate wettability and lower wear compared to other imidazolium salts [14]. Based on these findings, the ionic liquids of interest in this study are 1-butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 ) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4 ). Their chemical structures are shown in Fig. 1 and a summary of their properties is listed in Table 1. 1.2. Nanoscale characterization A suitable characterization technique is needed for MEMS/ NEMS that can resolve the small changes with length scale during sliding. Atomic force microscopy is well-suited for this purpose as it has the ability to measure material properties with high spatial resolution. An atomic force microscope (AFM), which is a typical example of a scanning probe technique, detects minute forces between the sample surface and the scanning tip attached to a cantilever [26,27]. An AFM tip sliding on the sample surface simulates a single asperity contact under light load conditions typically encountered during MEMS/NEMS operation. The comprehensive evaluation of a potential lubricant involves the study of adhesion, friction and wear. To date, investigations on the friction behavior of materials coated with ionic liquids have been conducted using the conventional ball-ondisk or pin-on-disk tribometers [16,17,20,22,23,25,28]. Adhesion and friction properties at the macroscale are different from the micro/nanoscale [26]. Knowledge of the latter is important in optimizing lubrication of MEMS/NEMS devices. AFM can be used to address this need as it can be configured to measure adhesion and friction forces at the micro/nanoscale. The surface potential, which is one of the electrical properties that can be measured with an AFM, pertains to the potential difference between two surfaces as they are brought together in electrical contact and thermal equilibrium is achieved. Surface potential mapping is a useful technique for wear detection. DeVecchio and Bhushan [29] have shown that a surface potential change is detectable even in cases where there is little or no surface damage, making this a powerful technique for studying the initiation of wear. The technique is well suited for wear studies of MEMS/NEMS device components because it has also been used for characterizing the wear of metals, ceramics and a conventional perfluoropolyether (PFPE) lubricant
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Table 1 Physical, thermal and electrical properties of selected ionic liquids and Z-TETRAOL
Cation Anion Molecular weight (g/mol) Tmelting (◦ C) Tdecomposition (◦ C) Density (g/cm3 ) Kinematic viscosity (mm2 /s) Pour point (◦ C) Specific heat (J/g K) Thermal conductivity at 25 ◦ C (W/m K) Dielectric strength at 25 ◦ C (kV/mm) Volume resistivity ( cm) Vapor pressure at 20 ◦ C (Torr) Wettability on Si Water contact angle
Miscibility with isopropanol Miscibility with water a b c d e f g h
1-Butyl3-methylimidazolium hexafluorophosphate (BMIM-PF6 )
1-Butyl3-methylimidazolium octyl sulfate (BMIM-OctSO4 )
C8 H15 N+ 2 PF− 6 284a 10c 300c 1.37a 281a (20 ◦ C) 78.7d (40 ◦ C) < −50e 1.44f (25 ◦ C) 0.15h – – <10−9 Moderatec 46◦ (untreated) 39◦ (partially bonded) 41◦ (fully bonded) Totala –
C8 H15 N+ 2 C8 H17 SO− 4 349a 28c 150c 1.07a 907a (20 ◦ C) – 1.82g (25 ◦ C) – – – <10−9 Moderatec 32◦ (untreated) 22◦ (partially bonded) 41◦ (fully bonded) Totala Totala
Proprietary ionic liquid
Z-TETRAOL
– – – 25c 210c 0.94 –
– – 2300b – ∼320b 1.75b 2000b (20 ◦ C)
– 2.3–3.6c (50 ◦ C) ∼0.2c – – <10−9 – –
−67b ∼0.20b (50 ◦ C) ∼0.09b ∼30b ∼1013 b ∼10−12 b – 54◦ (untreated) 83◦ (partially bonded) 88◦ (fully bonded) –
Totalc Partialc
Merck Ionic Liquids Database, Darmstadt, Germany, also: http://ildb.merck.de/ionicliquids/en/startpage.htm. Z-TETRAOL Data Sheet, Solvay Solexis Inc., Thorofare, NJ. [24]. [15]. [23]. [41]. [42]. [43].
suring carrier distribution in silicon device structures [35]. This resistance measurement technique has been used in conjunction with surface potential measurements in detecting wear on silicon after the perfluoropolyether (PFPE) lubricant Z-TETRAOL is removed from the surface [32]. 1.3. Objective of this research
Fig. 1. Chemical structures of the BMIM-PF6 , BMIM-OctSO4 and ZTETRAOL molecules.
[30,31]. More recently, the change in surface potential has been used to demonstrate how the presence of both immobile and mobile lubricant fractions on a silicon substrate provide enhanced wear protection compared to substrates containing only the immobile lubricant [32]. It has also been shown that surface potential measurements can elucidate the mechanism of triboelectric charging of human hair [33,34]. Recent advances in atomic force microscopy instrumentation have enabled the measurement of resistance by monitoring the current resulting from an applied DC bias voltage between tip and sample. This technique, known as scanning spreading resistance microscopy (SSRM) is originally developed for mea-
In this study, AFM-based adhesion, friction and wear measurements are presented for the first time for silicon substrates coated with the ionic liquids of interest. Conventional ball-onflat data is used in conjunction with AFM experiments in order to compare friction and wear properties at the microscale and nanoscale. Lubricant removal and surface wear is detected using surface potential and contact resistance measurements. Data from these measurements are compared with that from the perfluoropolyether (PFPE) lubricant Z-TETRAOL. 2. Experimental details 2.1. Materials and sample preparation The room temperature ionic liquids used in this study are 1butyl-3-methylimidazolium hexafluorophosphate, abbreviated as BMIM-PF6 (Merck, Darmstadt, Germany) and 1-butyl3-methylimidazolium octyl sulfate, abbreviated as BMIMOctSO4 (Merck, Darmstadt, Germany). Data from these ionic
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liquids are compared with the perfluoropolyether (PFPE) lubricant Z-TETRAOL (Solvay Solexis Inc.), which is the model lubricant used to develop the test methodology. The lubricants were applied on single-crystal Si(100) (phosphorus doped) with a native oxide layer on the surface using the dip coating technique. The method and the apparatus used have been described elsewhere [11,12]. Briefly, silicon substrates with dimensions of 10 × 10 × 0.5 mm were cut from Si (100) wafers. The wafers were ultrasonicated in acetone followed by isopropanol for 10 min each. Then, the cut wafer was vertically submerged into a beaker containing a dilute solution of the lubricant for 10 min. The solutions of the various lubricants are 0.2% (v/v) BMIMPF6 in isopropanol, 0.2% (v/v) BMIM-OctSO4 in isopropanol and 0.1% (v/v) Z-TETRAOL in HFE 7100 (a solvent consisting of isomers of methoxynonafluorobutane (C4 F9 OCH3 )). All the solutions were mixed vigorously and allowed to stand for at least an hour prior to use. The dip coating procedure is as follows. The silicon wafers were pulled up from solution with the aid of the motorized stage set at constant speed of 5 mm/s to obtain films of desired thickness. The lubricated sample used without post thermal treatment is referred to as untreated. Partially bonded samples were prepared by heating at 150 ◦ C for 30 min after dip coating, while the fully bonded samples were heated at 150 ◦ C for 30 min and washed in isopropanol (HFE 7100 in the case of Z-TETRAOL) to remove the mobile fraction. The ionic liquid-coated silicon samples were then measured with an ellipsometer in the fixed refractive index option (NFXD option, Gaertner Scientific L115C Ellipsometer Instruction Manual). Since the silicon wafer has a native oxide layer, a refractive index of 1.46 (CRC Handbook of Materials Science) was used in determining the thickness of the deposited ionic liquid. The coating thickness was found to be about 0.5, 1.5 and 2.5 nm for the fully bonded, partially bonded and untreated ionic liquid samples, respectively. Since ionic liquids do not have polar end groups (as compared to Z-TETRAOL), chemical bonding of the lubricant to Si is not obvious. However, it appears that thermal treatment leads to bonding of the ionic liquid because some coating is left after washing with isopropanol [27]. The thickness of the Z-TETRAOL coatings used in the study was about 1, 3 and 7 nm for the fully bonded, partially bonded and untreated samples, respectively.
2.2. Nanoscale adhesion, friction and wear measurements Adhesion and friction experiments at the nanoscale were carried out using a commercial Nanoscope IIIa MultiMode AFM (Veeco Instruments, Santa Barbara, CA) in ambient conditions (22 ◦ C, 45–55% RH) over a 2 µm scan line with the normal load ranging from 5 to 130 nN and a scan rate of 1 Hz. Square pyramidal Si3 N4 tips with a nominal 30–50 nm radius mounted on triangular Si3 N4 cantilevers with a spring constant of 0.58 N/m were used. The friction force was calibrated by the method described by Bhushan [26]. The adhesive force was calculated from the horizontal intercept of the friction versus normal load curves at a zero value of the friction force. A wear test based on an established procedure was used to monitor the change in friction force [11]. With the AFM in contact mode, a Si3 N4 tip with a nominal 30–50 nm radius mounted on a triangular Si3 N4 cantilever (spring constant of 0.58 N/m) was used to repeatedly scan a 2 µm line at a normal load of 70 nN and a scan rate of 0.1 Hz. The change in friction force is indicative of lubricant removal and surface wear. In order to be able to use higher loads for high wear needed in electrical measurements, lubricated surfaces were worn using a diamond tip with an apex angle of 60◦ and a tip radius of approximately 100 nm attached to a stainless steel cantilever with a spring constant of 10 N/m. Wear scars with dimensions of 5 × 5 µm2 were created and scanned for 20 cycles at a load of 10 µN. This experiment was performed for the untreated, partially bonded and fully bonded lubricant samples. This is the wear test used for subsequent surface potential and resistance measurements, described below. 2.3. Microscale friction and wear measurements For completeness, friction measurements at the microscale were conducted based on an established procedure of using a ball-on-flat tribometer under reciprocating motion [36,37] as shown in the schematic in Fig. 2. A sapphire ball with 3 mm diameter and surface finish of about 2 nm RMS was fixed on a stationary holder. A normal load of 500 g (4.9 N) was applied, and the frictional forces were measured with semiconductor strain gages that were then digitized and collected on a personal
Fig. 2. Schematic of the ball-on-flat tribometer used for microscale friction and wear measurements. Blowup shows the interface between the ball and surface of interest and the connections used for electrical measurements.
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(a)
(b) Fig. 3. (a) Schematic of the two passes of the AFM-based Kelvin probe technique [29] with the first pass in tapping mode for measuring surface height (left) and the second pass in lift mode for measuring surface potential (right). (b) Schematic of the AFM-based resistance measurement technique where the surface height is measured in contact mode and the resistance is measured by the current resulting from the applied DC bias voltage. (Schematic for (a) is for MultiModeTM AFM, while (b) is for DimensionTM AFM (Veeco Instruments, Inc.).)
computer. Typical test conditions are: stroke length = 2 mm, average linear speed = 4 mm s−1 , temperature = 22 ± 1 ◦ C, relative humidity = 45 ± 5%. Wear was characterized by imaging the resulting scar with an optical microscope and taking the cross section with a stylus profiler. The number of cycles to failure was determined by identifying the point where a sudden change in the friction force is observed. 2.4. Wear detection by surface potential and resistance measurements Surface potential measurements were taken with a Nanoscope IIIa MultiMode atomic force microscope equipped with the extender electronics module (Veeco Instruments Inc.). The extender enables the measurement of surface potential. Electrically conductive silicon tips coated with cobalt/chrome (MESP
probes, Veeco Instruments Inc.) were used. The schematic for this AFM-based Kelvin probe technique is shown in Fig. 3a. Surface potential is measured using a two pass method. The first pass is in tapping mode to measure surface height. In the second pass, the surface potential map is obtained in the socalled “lift mode” at a tip–sample separation (lift scan height) of 10 nm. In this mode, the piezo normally oscillating the tip in tapping mode is turned off and an oscillating voltage is applied directly to the conducting tip in order to generate an oscillating electrostatic force. This force has an amplitude described by the equation F = (dC/dz)vdc vac ,
(1)
where dC/dz is the vertical derivative of the tip–sample capacitance, vdc is the dc voltage difference between tip and sample and vac is the amplitude of the oscillating voltage applied to the
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tip. To measure the surface potential, the dc voltage is applied to the tip until the quantity vdc is equal to zero, giving zero oscillating force amplitude [29,30]. The nanoscale resistance measurements were taken with a Nanoscope IIIa Dimension 3000 atomic force microscope equipped with the scanning spreading resistance microscopy (SSRM) application module. The electrical resistance is measured between the conductive tip (MESP probes, Veeco Instruments Inc.) and a large current-collecting back contact while the tip is scanned in contact mode across the surface. A DC bias voltage is applied between the tip and the sample and the current is monitored using a logarithmic current amplifier built into the SSRM sensor, as shown in the schematic in Fig. 3b. In this study, the bias voltage is −10 mV. The resistance is obtained by calibrating the sensor against resistors with a range of resistance values. With the SSRM sensor, two resistances can be measured: the spreading resistance and the contact resistance. The former is the resistance defined by the distribution of the dopant atoms in the direction normal to the surface. The measured resistance is dominated by the spreading resistance when the applied load is high. In this study, the contact resistance of the tip–sample contact is predominant due to the low level of doping in the sample and the low loads that can be applied by the cobalt/chrome coated silicon tip [35]. Contact resistance measurements were performed at the microscale using a ball-on-flat tribometer under reciprocating motion. A steel ball with 3 mm diameter was used. As shown in the blowup in Fig. 2, the tribometer was modified so that a voltage of 5 V can be applied using a Wavetek power supply. The wear test was conducted for 100 cycles using the same test conditions described above (Section 2.3) while the resistance was simultaneously monitored with an Agilent 34401A multimeter. 3. Results and discussion 3.1. Surface topography, adhesion, friction and wear In Fig. 4a, the surface height images for the untreated sample (air dried) are compared with the two chemically bonded samples (partially bonded and fully bonded). Aggregates of varying sizes are observed on the untreated Z-TETRAOL and on all the ionic liquid coatings. These aggregates could have formed initially during preparation of the dilute solution. For ZTETRAOL, the long PFPE chains can orient in various configurations (such as coils), leading to aggregate formation. In ionic liquids containing the 1-butyl-3-methylimidazolium cation, it is believed that aggregate formation in solvents that are less polar compared to water aids in minimizing the charge density (charge delocalization) within the ions [38]. These aggregates are subsequently deposited on the silicon surface during the dip coating procedure. It is observed that the untreated lubricant surface has more prominent aggregates compared to the two chemically bonded samples, implying that the heat treatment promotes bonding to the Si substrate. Without the chemical bonding procedure, the lubricant molecules are less likely to attach to the substrate and would tend to attract each other in-
(a)
(b) Fig. 4. (a) Surface height images for the untreated, partially bonded and fully bonded films of BMIM-PF6 and BMIM-OctSO4 on silicon substrate. (b) Schematic for the attachment of BMIM-PF6 to the silicon substrate.
stead, such that dewetting is more likely. The immobilization of the ionic liquid, which is promoted by thermal treatment, occurs by reaction of the anion with the hydroxyl groups present on the silicon surface [39], as shown in Fig. 4b for BMIMPF6 . A similar mechanism is responsible for the attachment of BMIM-OctSO4 . For BMIM-PF6 on a mica substrate, aggregates of varying sizes were observed, which is a manifestation of the liquid phase [40]. In addition to this, zigzag structures, interpreted as the solid-like phase have been reported, indicating the possible coexistence of a liquid and solid phase. This was not observed on the ILs investigated in this study, which where all coated on silicon substrates. Fig. 5a is a summary of the adhesive force and coefficient of friction measurements on the ionic liquids. Z-TETRAOL and Si (100) data are provided for comparison. The adhesive force has been observed to decrease in the following order: untreated > partially bonded > fully bonded. This mobile fraction on the untreated sample facilitates the formation of a meniscus, which increases the tip–sample adhesion. The adhesive force is
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(b)
Fig. 5. (a) Summary of the adhesive force and coefficient of friction and (b) durability data after 100 cycles for BMIM-PF6 and BMIM-OctSO4 at room temperature (22 ◦ C) and ambient air (45–55% RH). Data for the uncoated Si and Z-TETRAOL are shown for comparison. Schematic in (a) shows the effect of chemical bonding treatment and meniscus formation between the AFM tip and sample surface on the adhesive and friction forces.
highest in the untreated coating since it has the greatest amount of the mobile fraction among the three samples. Conversely, the sample with no mobile lubricant fraction available (fully bonded) has the lowest adhesive force. A different trend is observed in the coefficient of friction (μ) data. Both the fully bonded and partially bonded samples have lower μ values compared to the uncoated silicon. Friction forces are lower on the latter, implying that the mobile lubricant fraction present in the partially bonded samples facilitates sliding of the tip on the surface. However, μ values for the untreated samples are higher than the data for the heat treated coatings. Due to the lack of chemical bonding, the interaction of the lubricant to the substrate is weakened and dewetting can occur. Water and lubricant molecules are more likely to form a meniscus as the tip approaches the surface. This provides greater resistance to tip sliding, leading to higher coefficient of friction values. The lower portion of Fig. 5a is a schematic illustrating the role of meniscus formation in the adhesive and friction
forces obtained for the uncoated Si and the untreated, partially bonded and fully bonded lubricant-coated Si surfaces. Fig. 5b contains plots of the coefficient of friction as a function of the number of sliding cycles at 70 nN normal load. Only a small rise in the coefficient of friction was observed for both Z-TETRAOL and the BMIM-PF6 surfaces, indicating low surface wear. However, all three BMIM-OctSO4 samples exhibited gradual change in μ, implying that wear is taking place after approximately 25 cycles for both the untreated and fully bonded surfaces and 60 cycles for the partially bonded surface. In some cases, such as in the fully bonded BMIM-OctSO4 and the untreated Z-TETRAOL, a crossover is observed, where the coefficient of friction increases from its initial value and exceeds the μ of silicon after a certain number of cycles. This is attributed to the transfer of lubricant molecules to the AFM tip and the interaction of the transferred molecules with the lubricant still attached on the Si substrate, which will increase the friction force.
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(a) Fig. 6. (a) Optical images and height profiles taken after 20 cycles and (b) summary of the coefficient of friction and number of cycles to failure from ball-on-flat tests on various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
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(b) Fig. 6. (continued)
In order to compare friction and wear properties at the microscale and the nanoscale, conventional ball-on-flat tribometer experiments were conducted on the same samples. Images and profile traces of the wear scars are shown in Fig. 6a. The coefficient of friction and number of cycles to failure are summarized in Fig. 6b. Both ionic liquids are showing enhanced durability compared to both the Z-TETRAOL-coated and the uncoated Si. The nanoscale data presented in Fig. 5 can be compared to μ values obtained from ball-on-flat (Fig. 6b) and four-ball friction tests [24]. The μ values of the untreated lubricant samples obtained by using AFM are lower than the μ obtained from either the four-ball or the ball-on-flat tests. This is attributed to the difference in the length scales of the test techniques. An AFM tip simulates a single asperity contact while the conventional friction test involves the contact of multiple asperities present in the test system. With regards to wear, the interface contact of the AFM and ball-on-flat techniques are different from each other such that one cannot expect both tests to show the same trend. On an AFM, the tip stress is very high such that material can be displaced more easily. This could account for the early damage in BMIM-OctSO4 using AFM (Fig. 5b). For a ball-on-flat test, the tip exerts a lower pressure on the surface and the coating is in a confined geometry. As a consequence, displacement of the coating is not as easy as in AFM, leading to enhanced wear resistance. 3.2. Wear detection by surface potential and resistance measurements Fig. 7a is a summary of the wear tests and the corresponding surface potential measurements on the ionic liquids. A bar plot summarizing the average surface potential change on the
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tested area is shown in Fig. 7b. In all cases, a smaller amount of debris has been generated compared to the uncoated silicon surface, indicating that the ionic liquids are providing wear protection. In general, the samples containing the mobile lubricant fraction (i.e., untreated and partially bonded surfaces) exhibit a lower surface potential change compared to the fully bonded sample, which only has immobile lubricant molecules. This is attributed to lubricant replenishment by the mobile fractions, which can occur in the untreated and partially bonded samples [32]. From the bar plot in Fig. 7b, it is also observed that the change in surface potential is generally lower in the ionic liquid coatings compared to the Z-TETRAOL coatings and the uncoated silicon. This indicates that any built-up surface charges arising from the wear test were immediately dissipated onto the conducting ionic lubricant coating surface. In the case of Z-TETRAOL and the uncoated silicon, the charges remained trapped in the test area, since both these materials are insulators. Based on these findings and previous observations [30,32, 33], a considerable surface potential change will be observed on the wear region when: (1) the lubricant has been fully removed from the substrate; (2) the native SiO2 layer has been abraded from the surface; (3) wear has caused subsurface structural changes; (4) charges build up as they are unable to dissipate into the surrounding material. Contact resistance images for the surfaces subjected to the wear tests are presented in Fig. 8a. The average change in the contact resistance of the wear region relative to the untested area is summarized in Fig. 8b. The fully bonded BMIM-PF6 and BMIM-OctSO4 both have appreciable contact resistance increase in the wear region. Since silicon is a semiconductor, it has much higher resistance compared to the surrounding ionic liquid. The resistance increase in the worn area implies that the substrate is exposed after the wear test. Partially bonded films did not get worn out from the substrate after the test, as evidenced by the lack of contact resistance change in the tested area. The untreated Z-TETRAOL and BMIM-OctSO4 -coated surfaces exhibited an observable resistance change, while BMIM-PF6 did not. This can be correlated to the durability data in Fig. 5b, where the untreated ZTETRAOL and BMIM-OctSO4 samples exhibited an increase in the friction force with time due to transfer of lubricant molecules to the tip. Easier lubricant removal means that the diamond tip (in the case of Fig. 8) can cause substrate wear much sooner, leading to the observed resistance increase in the tested area. However, the resistance image does not provide a clear contrast between Z-TETRAOL and the newly exposed substrate since both materials have high resistance values. Microscale contact resistance obtained from ball-on-flat tribometer testing is shown in Fig. 9, along with the corresponding coefficient of friction data. For the ionic liquid films, the initial resistance is slightly lower than that of uncoated silicon, confirming their conductive nature. For the Z-TETRAOL samples, the contact resistance is about the same magnitude as the uncoated silicon. But for the conducting ionic liquids, an increase in resistance corresponds to an increase in the coefficient of friction, indicating wear of the lubricant and exposure of the silicon substrate, similar to observations in the nanoscale. These
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(a)
(b) Fig. 7. (a) Surface height and surface potential maps after wear tests and (b) bar chart showing surface potential change for various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
results are consistent with the adhesion, friction and surface potential results with regards to wear detection and wear protection coming from the mobile and immobile lubricant fractions. The durability data and trends for the ILs obtained by using a steel ball are inferior from the results in Fig. 6, which was measured by using a sapphire ball. In Fig. 9, the partially
bonded samples still show the best durability, but in this case, Z-TETRAOL has the highest number of cycles to failure (opposite trend compared to Fig. 6b), as indicated by the point where the jump in the coefficient of friction is observed. This can be accounted for from the wetting properties of ionic liquids on different surfaces, studied by applying small drops (∼2 mm diameter) to surfaces using a Pt wire [24]. It was observed
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a steel ball, less wettability means less lubricant retention at the interface. This material wetting effect is possibly more significant for the ionic liquids than in Z-TETRAOL. But nonetheless, the durability of the partially bonded BMIM-PF6 is still close to its Z-TETRAOL counterpart, such that ILs are still viable lubricants comparable to PFPEs. 4. Conclusions The adhesion, friction and wear properties of the ionic liquids BMIM-PF6 and BMIM-OctSO4 were characterized by AFM. The following conclusions can be drawn from this study:
(a)
(b) Fig. 8. (a) Nanoscale contact resistance images after wear tests and (b) bar chart showing contact resistance change for various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison. Cases without the clear wear scar did not exhibit measurable change.
that ionic liquids have a tendency to wet nonmetal surfaces (e.g., Si3 N4 , SiO2 , glass) better than conventional metal surfaces (such as 440C, M50 and 52100 steel). For wear tests with
• Based on the surface height, adhesion and friction data, chemical bonding treatment facilitates attachment of the ionic liquids to the silicon substrate surface, leading to more uniform coatings and lowered adhesion force and coefficient of friction. All of the chemically treated lubricantcoated samples show favorable lubrication, as seen from the adhesive force and coefficient of friction being less than that of uncoated silicon in all cases. • Ionic liquids have comparable coefficient of friction with the lubricant Z-TETRAOL. The partially bonded coatings have the lowest coefficient of friction as they possess a desirable combination of lubricant bonded to the substrate as well as a mobile fraction which facilitates tip sliding. • All lubricants show favorable durability, with the partially bonded lubricant showing the best characteristics. In microand nanoscale experiments, the ionic liquids exhibit comparable durability with the lubricant Z-TETRAOL, which has high thermal stability and extremely low vapor pressure. • The low post-wear surface potential change observed on the silicon coated with ionic liquids is indicative of enhanced charge dissipation compared to the Z-TETRAOLcoated and uncoated surfaces, which are poor conductors. Contact resistance data is consistent with the surface potential data with regards to identifying the role of the mobile and immobile lubricant fractions in protecting the surface from wear. Based on these micro- and nanoscale friction and wear measurements, these ionic liquids show strong potential as lubricants for MEMS/NEMS because they have desirable thermal and electrical conductivity as well as desirable tribological properties. Acknowledgments Financial support by Surfaces Research and Applications, Inc. (Lenexa, KS) on an U.S. Air Force Office of Scientific Research (AFOSR) Contract (No. FA9550-05-C-0182) is gratefully acknowledged. We would also like to thank Christine Malott for performing the ball-on-flat friction and wear tests.
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Fig. 9. Microscale contact resistance and coefficient of friction after ball-on-flat tests for 100 cycles on various BMIM-PF6 and BMIM-OctSO4 coatings. Data for the uncoated Si and Z-TETRAOL are shown for comparison.
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