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Langmuir 1999, 15, 244-249
Humidity and Temperature Effect on Frictional Properties of Mica and Alkylsilane Monolayer Self-Assembled on Mica Fang Tian, Xudong Xiao,* and M. M. T. Loy Department of Physics, The Hong Kong University of Science and Technology, Hong Kong, China
Chen Wang and Chunli Bai Institute of Chemistry, The Chinese Academy of Sciences, Beijing, 100080, China Received August 10, 1998. In Final Form: October 21, 1998 The effects of humidity and temperature on tribological properties of mica and of the octadecyltriethoxysilane (OTE) monolayer self-assembled on mica are studied by friction force microscopy under controlled environments. As the humidity increases, the friction force is observed to decrease for mica and to increase for OTE at low loads at room temperature. Despite their hydrophobicity, water can penetrate into OTE films to alter their molecular chain ordering and to detach the OTE molecules from the mica substrate. The occurrence of OTE film damage is manifested as a crossover point in the friction force curves. Heating the samples in the temperature range of 20-80 °C induces a negligible effect for both mica and OTE in low humidity. However, in high humidity, a distinctive change in friction was found. Most of the heating effect can be understood by a “local humidity” concept although various discrepancies still exist.
Introduction Boundary lubrication is important to many modern technologies, including magnetic storage and micromachines.1 For industrial applications, a good boundary lubricant has to meet certain criteria such as strong adhesion to the substrate, significant reduction of the friction between the two moving surfaces, and endurance in different environments. While self-assembled monolayers (SAMs) can usually meet the first two requirements, very little is known about how their performance is affected by humidity and temperature of the environment. There are several different types of molecules that could form self-assembled layers.2 For example, alkanethiols self-assemble on a variety of metal substrates including gold, copper, and silver by forming S-metal bonds. Alkylsilanes (CH3(CH2)nSiR3) self-assemble on substrates such as oxidized silicon, mica, and glass. While on substrates such as oxidized silicon and glass, the strong adhesion comes from the formation of siloxane bonds between their surface hydroxyl groups and the active end group SiR3 (R ) Cl, -OCH2CH3, etc.) of alkylsilanes, for mica, the strong mechanical durability is due purely to the siloxane network among the alkylsilane molecules.3,4 Besides the common van der Waals interactions among the hydrocarbon chains, the most important difference between the SAMs formed by alkanethiols and alkylsilanes is the existence of siloxane bridges (-Si-O-Si-) that cross-link the adjacent molecules in the latter but have no equivalent in the former. Thus, it has been established * Corresponding author: fax (852)2358-1652; tel (852)2358-7494; e-mail,
[email protected]. (1) Persson, B. N. J. Sliding Friction: Physical principles and Applications; Springer-Verlag: Berlin, Heidelberg, 1998. Bhushan, B.; Israelachvili, J. N.; Landman, U. Nature 1995, 374, 607. (2) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (3) Xiao, X. D.; Hu, J.; Charych, D. H.; Salmeron, M. Langmuir 1996, 12, 235. (4) Lio, A.; Charych, D. H.; Salmeron, M. J. Phys. Chem. B 1997, 101, 3800.
that the alkylsilane monolayers are molecularly flat with a thickness determined by the molecular chain lengths5 but without long-range ordering due to the distortions of the headgroups near the substrates.3,4 Much is known about the effects of temperature6-8 and water8-10 on the formation of alkylsilane monolayers. However, only a few studies exist concerning the influence of temperature and water on the monolayers after their formation. In particular, a structural transition induced by thermal treatment was observed in alkylchlorosilanes11,12 and alkanethiol13 monolayers. The transition is irreversible since in the former case the alkyl chain of the octadecyltrichlorosilane (OTS) monolayers was found to aggregate and form islands of multilayer thicknesses after heating at 150 °C for 5 h,12 and in the latter case the octadecanethiol (ODT) monolayer was found to desorb partially from Au(111) after incubating at 120 °C for several hours.13 In this paper, we report how humidity and temperature affect the tribological properties of mica and of the N-octadecyltriethoxysilane (OTE) monolayer self-assembled on mica. While testing the humidity effect on mica is a repeat of previous measurements, testing the temperature effects on mica as well as both the humidity and temperature effects on OTE are new. For bare mica, (5) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1991, 7, 532. (6) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (7) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (8) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (9) Carson, G. A.; Granick, S. J. Mater. Res. 1990, 5, 1745. (10) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (11) Cohen, S. R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986, 90, 3054. (12) Yeh, M. C.; Kramer, E. J.; Sharma, R.; Zhao, W.; Rafailovich, M. H.; Sokolov, J.; Brock, J. D. Langmuir 1996, 12, 2747. (13) Wang, J.; Caffrey, M.; Bedzyk, M. J.; Penner, T. L. J. Phys. Chem. 1994, 98, 10957.
10.1021/la981008d CCC: $18.00 © 1999 American Chemical Society Published on Web 12/10/1998
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condensation of water from the humid environment reduces friction against the Si3N4 tip, which agrees with the previous result. Heating seems only to affect the “local humidity” on the mica surface. Despite their hydrophobicity, water can condense on OTE films from the humid environment. Penetration of water into the OTE films results in deterioration of the film quality and thus in a reduction in lubrication. Heating the OTE to high temperatures, although promoting water attack to the OTE monolayer, reduces the water content on the surface to result in less damage to the OTE films. Experimental Method Materials. N-Octadecyltriethoxysilane (OTE), (CH3CH2O)3Si(CH2)17CH3, was purchased from Aldrich Chemical Co., Inc. (Milwaukee, WI), and was filtered through a 0.2 µm PTFE filter prior to use. House-distilled water was passed through a fourcartridge Millipore µQF purification train producing a resistivity of 18.2 MΩ‚cm. Tetrahydrofuran (THF) and cyclohexane were of spectral quality. The glassware for preparation of the prehydrolysis solution and for the self-assembly was cleaned with chromic acid solution. The mica was cleaved just prior to exposure to the OTE solution. Monolayer Preparation. The preparation of the OTE monolayers on freshly cleaved mica was carried out as described previously.14 Briefly, prehydrolysis solutions were prepared by dissolving 0.23 µL of OTE in 20 mL of THF containing 5 mL of 1 N HCl. The solution was then stirred at room temperature for 3-5 days. Just prior to self-assembly, the hydrolysis solution was filtered through a 0.2 µm TPFE membrane. Then, it was further diluted with a ratio of 1:20 by cyclohexane and added to a clean Petri dish container. The freshly cleaved mica was immersed into the diluted solution for 5 min. Following the reaction, the samples were then rinsed with cyclohexane and dried under a stream of nitrogen, followed by two hours of baking at 120 °C. The OTE samples were stored in a desiccator before friction force microscopy (FFM) measurement. FFM Measurements. The experiments were conducted with a home-built Beetle-type atomic force microscope (AFM) controlled by RHK electronics. Commercially available V-shaped Si3N4 cantilevers with a nominal force constant of 0.5 N/m (Park instruments, USA) were used. As usual, a quadrant photodiode was used to measure both the friction force (cantilever torsion) and normal force (cantilever deflection) simultaneously. For the friction measurement, the feedback loop was disabled to allow changes of load via the applied voltages on the piezotubes. Typically, we measure friction in the loading/unloading cycles. While scanning in the x-direction, the cantilever tip starts from a noncontact position and approaches toward the sample surface scan-line after scan-line. When the attractive force acting on the AFM tip by the sample has a gradient exceeding the force constant of the cantilever, the tip snaps into contact with the surface and friction starts to appear. The external load is defined as zero at this point. Now, further contraction of the piezotubes scan-line after scan-line results in an increase of load. After a specific given load is reached, the unloading half-cycle starts. With extension of the piezotubes, the load decreases scan-line after scan-line. Because of the adhesion, the tip will not leave the surface when the external load reaches zero. Further tensile force (negative external load) is required to overcome the adhesion. When the tensile force of the cantilever reaches the value of the pull-off force, the tip snaps off the surface. At this point, the total load (external load + adhesion force) is zero. For a given load, the friction loop with the friction signals for both forward and backward scans is recorded. To compensate for the baseline change, the difference of the forward and backward friction signals is taken as twice the friction. This friction force is further averaged over x positions by excluding the static friction regions. A friction versus load curve can then be generated. Since the cantilever torsional force constant is unknown, we leave the friction force as an arbitrary unit. For the comparison to be valid, we use the same cantilever/tip during the experiment unless (14) Kessel, C. R.; Granick, S. Langmuir 1991, 7, 532.
Figure 1. Friction versus load curves in the loading half-cycle for bare mica and the OTE monolayer self-assembled on mica. The solid line is a fitting with 0.04(L + 5.5)0.67 and the dashed line is a fitting with 2.08 × 10-7(L + 2.7)3.8 + 2.9 × 10-3(L + 2.7). specified otherwise. In this paper, we will present the friction versus load curves in the loading half-cycle only since the results of the unloading half-cycle are similar. Each presented curve represents an average over at least 15 different measurements. All the measurements were taken over molecularly smooth areas, over which the root mean square roughness is less than 0.95 Å. The experiments are carried out in a glovebox in which the humidity can be controlled by a combination of dry air flow and water evaporation. The temperature of the samples is controlled by a Peltier heater.
Results and Discussion 1. Load Dependence. We first compare the friction versus load curves for bare mica and the OTE monolayer self-assembled on mica obtained at room temperature and at low humidity (∼5%). As shown in Figure 1, the friction increases as the load increases for both cases but with significantly different load dependence. While the friction varies approximately as Ft0.67 (solid line; Ft is the sum of external load and adhesion force) in the case of bare mica (the data below 10 nN are not included in the fitting as will be discussed later), a stronger load dependence with a leading term Ft3.8 is observed for the case of the OTE monolayer (dashed line). It is also apparent from Figure 1 that a strong lubrication effect exists, consistent with the previous report.3 When L < 40 nN, the friction force on mica is reduced by about a factor of 5 of the OTE monolayer. While the friction versus load curve for mica is typical and results from an elastic contact between a spherical tip and a flat surface with a constant elastic modulus, the load dependence of the friction for the OTE monolayer cannot be explained by such a simple model. As discussed elsewhere,15 a load-dependent shear strength alone cannot account for the observation. We must adapt a loaddependent elastic modulus in order to fit the data. This can be justified as follows. If we assume that the alkylsilane monolayer on mica is not as compact as a solid, the alkyl chains may lie down easily in response to the initial stage of compression, which is equivalent to a small elastic modulus. As the load increases, it is increasingly more difficult to depress the alkyl chains further since there is less space in which they can conform. The compression becomes intramolecular rather than intermolecular, and an increase of modulus is expected. Since there is no change (15) Tian, F. et al. To be submitted for publication.
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Figure 2. Friction versus load curves in the loading half-cycle for mica for different values of relative humidity. The temperature is 21 °C.
Figure 3. Friction versus load curves in the loading half-cycle for the OTE monolayer self-assembled on mica for different values of relative humidity. The temperature is 23 °C.
in bonding and no creation of structural defects, the monolayer can recover and the above process is elastic.16 2. Humidity Effect. Now, we turn to the humidity effect. In Figure 2 and Figure 3, we show the friction versus load curves for a number of relative humidity values for mica and the OTE monolayer, respectively. In the case of bare mica, the friction force decreases gradually as the humidity increases, which is consistent with the previous report.17 Since mica is hydrophilic, water can condense more and more on the surface from the vapor as the humidity increases. The thickness of the water films was found to be thicker than one monolayer.18 Therefore, our observation indicates a strong lubrication effect of water on mica. However, the detailed mechanism of the lubrication remains unknown. In particular, knowing whether the water molecules remain in contact during the scan is crucial. If one assumes that water molecules are squeezed out of the contact area as is evident in the mica lattice images taken at high humidities, understanding the water lubrication becomes even more difficult. In the case of the OTE monolayers, however, the dependence of the friction on humidity is very different. (16) Garcia-Parajo, M.; Longo, C.; Servat, J.; Gorostiza, P.; Sanz, F. Langmuir 1997, 13, 2333. (17) Hu, J.; Xiao, X. D.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1995, 327, 358. (18) Homola, A. M.; Israelachvili, J. N.; McGuiggan, P. M.; Gee, M. L. Wear 1990, 136, 65.
Tian et al.
First, as the humidity increases, the friction versus load curves start to have an additional characteristic in the measured load regime. Below ∼20% humidity, they are charaterized by a single load dependence of Ln with n > 1. Above ∼30% humidity, however, the friction versus load curves consist of two parts: one increases with load as Ln with n > 1 in the low load regime, and the other increases roughly linearly with load in the high load regime. The crossover point of the two regimes shifts from high load to low load as the humidity increases. Second, the humidity dependence of the friction in the two regimes is opposite. At loads below the crossover points, the friction gradually increases as the humidity increases. But at the loads above the crossover points, the friction decreases as the humidity increases, similar to the case of bare mica. As an example, consider the case at 50% humidity. Below ∼20 nN, which is the load at the crossover point, the friction increases rapidly with a power law. Above ∼20 nN, the friction changes its behavior and increases nearly linearly with load. This behavior is qualitatively in agreement with previous observations at a given humidity.3 The fact that the load at which the crossover occurs is much lower here than that in the previous experiment is possibly due to different tip radius and/or different OTE film qualities. It is surprising that a high humidity effect exists for OTE since this organic monolayer is hydrophobic and presumably should not be affected by water. The experimental results above indicate that such a simple view based only on the hydrophobicity is incorrect. Since the alkylsilane monolayer is not fully compact due to longrange disordering, condensed water can penetrate through the film to the OTE/mica interface. The accumulation of water at the interface weakens the links between OTE and mica and the links in the OTE and therefore deteriorates the quality of the OTE films. With load applied during scanning, the OTE layer can now be pushed away to allow tip-mica direct contact. As a result, the friction versus load curve changes its characteristic from a strong load dependence to a weak load dependence. The shift of the crossover point toward low load as humidity increases is also consistent with the above picture. The decrease of friction at loads above the crossover point as a function of humidity is thus the property of bare mica, giving that water lubricates mica as shown in Figure 2. To verify the above scenario, we image the OTE/mica sample at different loads at 49% humidity. At a low load of 8 nN, no long-range ordering appears in the image (Figure 4a), consistent with the structure of the OTE monolayer. At 24 nN, above the crossover point, however, the image reveals the mica lattice periodicity (Figure 4b). The damage to the OTE monolayer due to scratch from the tip is permanent as shown by Figure 4c, where a newly created “hole” is observed in a large-scale image at the low load. Our observation on the removal of OTE film by load is in agreement with Kiridena et al.19 It is now clear that the appearance of the crossover point is due to damage to the OTE monolayer, which results in tip contact with the OTE monolayer first at low loads and with the mica substrate later at high loads. The above picture is also consistent with the water effect during the organic monolayer formation studied previously. Water at the substrate surface is crucial for the formation of the self-assembled alkylsilane monolayers.6,8,12 It is water that provides hydroxyl groups in the substrate to link the alkylsilane monolayer via siloxane (19) Kiridena, W.; Jain, V.; Kuo, P. K.; Liu, G.-Y. Surf. Interface Anal. 1997, 25, 383.
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Figure 5. Friction versus load curves in the loading half-cycle for mica for various temperature in the range of 20-70 °C (a) at 5% relative humidity and (b) at 54% relative humidity. The values of the calculated “local humidity” are also indicated.
Figure 4. Topographic images of the OTE/mica monolayer obtained at 49% humidity. The image force and scan size are as follows: (a) 8 nN 10 × 10 nm2; (b) 24 nN and 9 × 9 nm2; (c) after imaging (b), zooming out, 2 nN and 500 × 500 nm2.
bonds -Si-O-Si- to the substrate. Baking at high temperature (∼120 °C) to get rid of the excess water was found to improve the film quality since the excess water
at the OTE/mica interface after monolayer formation can hydrolyze either the -Si-O-substrate connections or -Si-O-Si- bonds within the monolayer itself to reduce the stability and the ordering of the monolayer. 3. Temperature Effect: Mica. The friction versus load curves for bare mica in the temperature range of 20-70 °C in environments with about 5% and 54% humidity are shown as parts a and b of Figure 5, respectively. At 5% humidity, the friction was observed as independent of temperature within the experimental uncertainty. These results also indicate that the AFM cantilever is not affected by heating. The friction versus load curves have two distinctive regimes. At low loads ( 1. At high loads, it behaves as Ln with n < 1. The former is due to contamination or most likely water films on mica, and the latter represents the intrinsic property of mica. At 54% humidity, the friction strongly depends on the sample temperature. As the temperature increases, the friction increases. When the temperature is relatively low (
