Investigation of the structure of fluoroalkylsilanes deposited on alumina surface

Applied Surface Science 258 (2012) 9849–9855

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Investigation of the structure of fluoroalkylsilanes deposited on alumina surface M. Cichomski a,∗ , K. Ko´sla a , W. Kozłowski b , W. Szmaja b , J. Balcerski b , J. Rogowski c , J. Grobelny a a

Department of Materials Technology and Chemistry, University of Łód´z, Pomorska 163, 90-236 Łód´z, Poland Department of Solid State Physics, University of Łód´z, Pomorska 149/153, 90-236 Łód´z, Poland c ˙ Institute of General and Ecological Chemistry, Technical University of Łód´z, Zeromskiego 116, 90-924 Łód´z, Poland b

a r t i c l e

i n f o

Article history: Received 13 March 2012 Received in revised form 5 June 2012 Accepted 11 June 2012 Available online 18 June 2012 Keywords: Fluoroalkylsilanes Nanotribology Atomic force microscopy Alumina surface X-ray photoelectron spectroscopy Time of flight secondary ion mass spectrometry Fourier transform infrared spectroscopy

a b s t r a c t This paper presents investigation of the structure of fluoroalkylsilanes on alumina surface by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), Fourier transform infrared (FT-IR) spectroscopy and atomic force microscopy (AFM). The fluoroalkylsilanes were prepared on alumina surface using the vapor phase deposition (VPD) method. XPS and ToF-SIMS measurements confirm the presence of fluoroalkylsilanes on the alumina surface. AFM was used to compare the frictional behavior of samples after modifying the alumina surface by fluoroalkylsilanes. Nanotribological studies of fluoroalkylsilanes films with different head groups on the alumina surface include, besides frictional studies, also adhesive forces investigations. It is shown that surface modification of alumina by fluoroalkylsilanes decreases coefficient of friction and adhesive forces values by more than 50% compared to unmodified alumina. In comparison to a non-modified alumina surface all tested silanes, particularly compounds with three reactive atoms in the head group, cause a decrease in the friction coefficients in nanofriction tests. The same effect was also observed for adhesion measurements. It was found that the alumina modified by trifunctional fluoroalkylsilanes have a higher degree of hydrophobicity, lower adhesion, and lower coefficient of friction than the monofunctional fluoroalkylsilanes covered alumina surface. © 2012 Elsevier B.V. All rights reserved.

1. Introduction With advances in micro-/nanoelectromechanical systems (MEMS/NEMS), fluorosilanes on alumina are of interest for a wide range of applications, including lubrication, corrosion, interfacial wetting, adhesion and friction [1–4]. Devices normally fabricated with alumina are the digital micromirror device (DMD), MEMS used in optoelectronics, and sensors for biomolecules such as the stacked planar affinity regulated resonant optical waveguide (SPARROW) [5–8]. The SPARROW responds to changes in the effective refractive index (the amount, periodicity and rate of optical power transfer between the waveguides) initiated by biomolecules attachment. Fluoroalkylsilanes in case of the DMD ensure against wear between micromirror surfaces, and in case of the sensor ensure selectivity. Nonfluorinated alkylsilanes have been also used as substrates for adsorption of cytochrome films of submonolayer thickness. Adsorption of the cytochrome to a silane-based self-assembled monolayer (SAM) is a strategy for preparing an oriented protein film [9]. Controlling the molecular orientation of proteins immobilization on different substrates is important in fields such as biosensing.

∗ Corresponding author. Tel.: +48 42 6355836; fax: +48 42 6355832. E-mail address: [email protected] (M. Cichomski). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.040

Chemical formula of organosilane single molecule is Rn SiX4−n where X is a hydrolyzable leaving group such as Cl, OCH3 or OH and R is an organic functional group (for example alkyl group – in case of alkylsilanes or fluoroalkyl group – then we have to deal with fluoroalkylsilanes). These compounds are formed by the chemical adsorption on an active solid surface as a self-assembled monolayer. They consist of three building groups: a head group that reacts with a substrate, a backbone molecular chain group and a terminal group that interacts with the outer surface of the film [10,11]. The optimal choice for each group will yield the monolayer with best performance, low adhesion, friction and wear [12–14]. The Si atoms in the structure of head group provide diatomic bond strengths of the chemical bond in forming layer, 242.7 kJ/mole for Si O (chemical adsorption bond) and 414 kJ/mole for Si C bond [5,15]. In case of these investigations, leaving group is Cl and R is ten carbon perfluoroalkyl chain. High values of presented dissociation energies provide strong bonding of molecules to the surface and thus durability of produced coatings. Previous studies confirm that fluoroalkylsilanes show good thermal and chemical stability [16–18] and possess unique properties what makes them good candidates for tribological research. In our study we used 1H, 1H, 2H, 2H perfluorodecyltrichlorosilane (FDTS) and 1H, 1H, 2H, 2H perfluoroalkyldimethylchlorosilane (PFMS) layers because previous investigations showed that

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perfluoroalkylsilanes with long chain are better lubricant medium for tribological applications than their unfluorinated and shorter analogs [14]. These layers are also distinguished by lower adhesion and coefficient of friction values than unmodified surface and surface covered by nonfluorinated alkylsilanes. Compounds possessing CF3 terminal group form more hydrophobic layers in comparison with other terminated compounds. Increase of hydrophobic properties ensures decreased capillary forces which in turn lead to lower values of adhesion forces. In this work we grow fluoroalkylsilanes films on alumina surface by vapor phase deposition (VPD). The studied fluoroalkylsilanes had different amount of the reactive attachment groups. We characterize the films by means of a contact angle analyzer for hydrophobicity, time-of-flight secondary ion mass spectrometry (ToF-SIMS), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) for identification of thin layers. The influence of the formed compounds on the nanotribological properties was measured by atomic force microscopy (AFM). These techniques were used to verify the process of the chemical modification and to determine the tribological properties of alumina surface modified by floroalkylsilanes compounds. The investigations presented in this work are important because of the constantly growing demand for compounds acting as lubricants which provide wear resistance in miniaturized systems made of alumina. For this reason we prepared and studied fluoroalkylsilanes tribological behavior on alumina surface. 2. Experimental 2.1. Materials and deposition procedures of perfluoroalkylsilanes films Modifiers were purchased from ABCR, GmbH & Co. KG, Karlsruhe. These chemicals were chosen to compare the effect of head group structure on tribological performance at the nanoscale. Commercial p-type Si wafers were ordered from Cemat Silicon S.A., Poland. Polycrystalline aluminum surface with the thickness of 100 ± 2 nm were deposited on oxidized Si (1 0 0) substrates, using the process of thermal evaporation at an incidence angle of 0◦ (with respect to the surface normal) in a system maintained at a base pressure of about 10−3 Pa. Silanes’ surface adsorption time parameter was optimized to produce the most hydrophobic surface. Static contact angle was measured to determine the degree of wetting properties. To remove an excess of the non-specifically and/or non-covalently bound silanes the surfaces were treated in a stream of inert gas. 2.2. Surface characterization 2.2.1. XPS measurements Samples were characterized by X-ray photoelectron spectroscopy which was carried out using Omicron ultrahigh vacuum (UHV) system working at the base pressure lower than 5 × 10−8 Pa, equipped with the EA 125 HR hemispherical analyzer with the resolution better than 0.8 eV. The XPS investigations were made using Mg K␣1,2 line with the power set at 75 W in all experiments. The two-point correction of the energy scale based on Au 4f7/2 (83.95 eV) and Ag 3d5/2 (368.22 eV) lines was applied to all spectra [19]. The XPSPEAK package was used to perform the quantitative analysis. 2.2.2. ToF-SIMS measurements The secondary ions mass spectra were recorded by a timeof-flight mass spectrometer manufactured by Ion-ToF GmbH, Muenster, Germany. The instrument is equipped with liquid metal

69 Ga+ primary ion gun and high-resolution time-of-flight mass ana-

lyzer. During the analysis, a 40 ␮m × 40 ␮m area of the sample surface was irradiated with the pulses of 25 keV ions at 10 kHz repetition rate and an average ion current 2.5 pA. The analysis time was 20 s giving an ion dose of approximately 2 × 1013 ions/cm2 . Secondary ions emitted from the bombarded surface are mass separated and counted in time-of-flight analyzer. Positive secondary ions mass spectra were recorded with high resolution typically greater than 5900 (calculated as peak maximum at the 29 m.u., divided by peak width at half height) with the primary ion pulse width 650 ns [20]. 2.2.3. FT-IR spectroscopy measurements The surface chemical modification of the films was studied by FT-IR spectroscopy. The used instrument was a Biorad 175C spectrometer equipped with a liquid nitrogen cooled mercury–cadmium–telluride (MCT) detector. The monolayers on the alumina surface were analyzed using grazing angle attenuated total reflectance (GAATR) accessory from Harrick Scientific Products Inc. The angle of incidence was 65 ± 1◦ . All spectra were recorded by collecting 600 scans (with 600 scans of pure silicon as background) at 8 cm−1 resolution, in the spectral range 750–4000 cm−1 in dry air flow. 2.2.4. Contact angle and surface free energy measurements The surface hydrophobicity was measured with a contact angle goniometer in the laboratory atmosphere at (45 ± 5%) relative humidity and (22 ± 2) ◦ C. A water, glycerine and diiodomethane droplets were placed on the substrate with the use of microsyringe, and the static contact angles were measured on both sides of the droplet. The image of the droplet was obtained by a digital camera using Motic 2.0 software. The surface free energies of alumina surfaces modified by fluoroalkylsilanes were calculated using the Van Oss–Chaudhury–Good method [21]. The Van Oss–Chaudhury–Good theory separates the surface energy of solids and liquids into components. It includes the dispersive component and subdivides the polar component as being the sum of two more specific components: the surface energy due to acidic interactions (␥+ ) and due to basic interactions (␥− ). The acid component theoretically describes a surface’s propensity to have polar interactions with a second surface that has the ability to act basic by donating electrons. Conversely, the base component of the surface energy describes the propensity of a surface to have polar interactions with another surface that acts acidic by accepting electrons. The principle equation for this theory is: (1 + cos)L = 2( LW LLW )

1/2

+ 2( + L− )

1/2

+ 2( − L+ )

1/2

where LLW dispersive component, L+ asymmetric component for acid and L− asymmetric component for base specific to the measurement liquid. 2.2.5. Nanotribological measurements The AFM measurements were carried out by a Solver P47 apparatus (NT-MDT) operating in air under ambient conditions. Nanotribological measurements were performed using a rectangular Si3 N4 cantilever having a spring constant k = 0.62 N/m, calibrated by the Sader method [22,23]. The adhesive force was obtained from the force-distance curve as calculation of the pull-off force [24]. Frictional forces were calculated from the distance between two branches of the frictional loop obtained when the tip was moving laterally forwards and backwards during scanning of the sample surface [25]. Measurements were performed several times by using the same tip for a relatively short time to minimize the influence

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of external conditions such as humidity/temperature changes. The scan size was fixed at 1 ␮m × 1 ␮m. The coefficient of friction was obtained from the slope of the friction force versus normal force plots. The friction force was calibrated by the method described by Ruan and Bhushan [26]. The technical parameters of the measurements were the same as those in our previous work [14].

3. Results and discussion

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an identical number of carbon atoms in the backbone chain as well as an identical trifluoromethyl terminus group. Due to their identical chain structures, they should have similar length. The lengths of these two types of fluoroalkylsilanes were obtained by the theoretical model determined using HyperChem 7.5. For FDTS and PFMS they were found to be 1.64 nm and 1.63 nm, respectively [14]. The experimental ellipsometry data show that FDTS has thicker layer (2.6 nm) than that of PFMS (1.9 nm), and that predicted by the theoretical calculation. This is found to be due to cross-linking of FDTS molecules, in agreement with the results reported by Booth et al. [31].

3.1. XPS studies 3.2. ToF-SIMS investigations To verify the correctness of the modification procedure and also to confirm the presence of FDTS and PFMS agents on the surface, XPS measurements were performed. The obtained results of XPS investigations are summarized in Fig. 1. XPS spectra made for the unmodified alumina inform about the presence of oxidized forms on the surface (Fig. 1e). The presence of oxidized forms of aluminum on the surface enabled us to perform chemical modification of the Al by different compounds. The peaks attributed to metallic substrate Al 2p1/2 and Al 2p3/2 are located at 72.1 eV and 71.7 eV, respectively, and Al 2s1/2 at 116.6 eV. Shown in Fig. 1b and c, F 1s1/2 and FKLL peaks for fluorine were observed for the alumina surface modified by FDTS and PFMS compounds. These peaks indicate the successful attachment of fluoroalkylsilane molecules to the alumina surface. Fig. 1f and g present the XPS Al 2p region of FDTS and PFMS silanes modified alumina, respectively. These spectra show the AlOSi peaks which have been found at 75.5 eV for PFMS and 75.9 eV for FDTS. The peaks attributed to aluminum oxide are at 74.5 eV for both compounds modified substrate. AlOOH peaks are present at 72.4 eV and 72.5 eV for FDTS and PFMS, respectively. SiO peaks for Si 2p spectra are located at around 102.3 eV for both compounds (Fig. 1d). The appearance of AlOSi and SiO elements suggests the producing of chemical bond between the molecules and the substrate surface. In the case of FDTS silane with three reactive atoms in the head group ( SiCl3 ), at least two groups form bonds Metal O Si and one group produces polymerization Si O Si of the adjacent molecule. This is consistent with the increase of the intensity of ions AlOSi in the case of FDTS and also with the increase of the SiO element (Fig. 1d–g). Wang et al. [27] showed that the peak at 102.6 eV is assigned to Metal O Si linkages, whereas Ntais et al. [28] assigned the peak at 102.0 eV to siloxane cross-links. Concerning PFMS, the obtained results can be compared with XPS spectra for PFMS adsorbed on different substrates, such as alumina and copper, presented by Hoque et al. [29,30]. Those literature values correspond well with the results obtained in our investigations. In the case of FDTS compounds on Al, no XPS data are available in literature. In our previous study for different fluoroalkyl- and alkylsilanes, presenting also the data for FDTS on cobalt substrate, we observed peaks for the unique elements such as fluorine, F 1s1/2 and FKLL , higher intensities of C 1s, Si 2s1/2 , Si 2p1/2 and Si 2p3/2 peaks, and peak of Si 2p was observed at 102.8 eV which suggests the formation of Co O Si linkages [14]. Coverage of the alumina surface by FDTS and PFMS was confirmed by the XPS spectra (Fig. 1b and c), where peaks originated from fluorine atoms, F 1s and FKLL are present. Two peaks in the XPS spectra are taken into account to determine the coverage of the alumina surface by the molecules: F 1s 689 eV and C 1s 285 eV. Considering the intensity of the peaks of F 1s (Fig. 1h), and the peak height ratio F 1s to C 1s for both molecules, which is 10.4 for FDTS and 3.7 for PFMS, we can conclude that the packing of molecules associated with the surface of alumina in the case of FDTS is greater than for PFMS. The increased FDTS compound packing also demonstrates the fact that both silane molecules possess

ToF-SIMS measurements have been applied to studies of SAM systems, such as organophosphorus acids on various oxides of metals [32], as well as organosilanes on oxidized Al surface [33]. Ion fragmentation of SAMs verifies the presence of molecules and provides clues to elucidate the interaction between the molecular head group and the substrate. In our case in order to confirm the presence of modifying agents (FDTS and PFMS) on the surface, ToF-SIMS measurements were also performed. The obtained results are presented in Table 1. A large number of fragment ions containing fluoride (CF3 , CF2 , F) confirm modifications of the alumina surface. Another confirmation of the modification is reduction of the number of AlOH, AlO and OH ions. This fact is due to chemical reaction of these groups with Si Cl bond from fluoroalkylsilanes compounds. The appearance of AlOSi and SiO ions suggests the formation of Metal O Si linkages, and in the case of compounds with trifunctional groups at least two groups form bonds with the surface. Detection of the AlOSi positive ions which reveals chemical bonding between the organosilane molecule and the substrate was also reported by Abel et al. [33]. Ion fragmentation of fluoroalkylsilanes provides information about the interaction between the molecular head group and the substrate. A larger number of ions such as CF3 , CF2 , F and decreased number of AlOH, Al, AlO ions for FDTS compared to PFMS compound suggest that FDTS forms densely packed monolayer. The occurrence of Si2 O ions (Table 1) also provides confirmation of polymerization between neighboring molecules and consequently leads to increased packing density of the layer. The obtained ToF-SIMS data are in agreement with the XPS results presented earlier. 3.3. FT-IR characterization of fluoroalkylsilanes monolayers The chemical composition of the fluoroalkylsilanes on the alumina surface was investigated by FT-IR spectroscopy, which gave information about the various chemical bonds such as Si OH, C F and Si O Si. Infrared spectroscopy is also a tool which helps to understand the quality, molecular ordering and packing density of films deposited on the surface. One of the first works of FT-IR spectroscopy for monolayer was performed by Allara and Swalen [34]. Khatri and Biswas [35] used this technique to show molecular ordering of alkylsilanes films. In our investigations several characteristic peaks were observed in the range 750–1300 cm−1 indicating the presence of fluoroalkyl groups in the sample. Fig. 2 shows the FT-IR spectra of alumina, and alumina modified by FDTS and PFMS films. For unmodified substrate, at about 3400 cm−1 broad O H peaks are observed indicating hydrophilic nature of the alumina after plasma treatment. The intensity of O H peaks decreased for FDTS and PFMS modified surfaces, confirming an increase in the hydrophobicity of these surfaces (Fig. 2a–c). For Al the peak at around 822 cm−1 and the broad band at around 953 cm−1 are due to the scissoring vibration of O H bond and vibration modes of Al2 O3 , respectively [36]. These peaks are present in both modified

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Fig. 1. XPS spectra from Al surface: (a) unmodified Al surface, (b) Al surface after FDTS modification, (c) Al surface after PFMS modification, (d) spectrum of Si 2p after FDTS and PFMS modifications, (e) spectrum of Al 2p for unmodified surface, (f) spectrum of Al 2p region after FDTS modification, (g) spectrum of Al 2p after PFMS modification, (h) spectrum of F 1s after FDTS and PFMS modifications.

surfaces, for FDTS modified Al at 814 cm−1 and 955 cm−1 , for PFMS at 818 cm−1 and 938 cm−1 . In the case of FDTS modified Al (Fig. 2b), bands observed at around 1243 cm−1 , 1216 cm−1 , 1154 cm−1 and 787 cm−1 are due to stretching and bending of C F bonds originating from fluoroalkyl chain [37,38]. Peak at 1115 cm−1 originates from C C stretching vibrations. The strong absorption peak at 1073 cm−1 corresponds to the Si O Si asymmetric stretching vibration [39]. The presence of this peak confirms the formation of a network structure inside the film. In the case of PFMS modified Al, peaks in range

1150–1250 cm−1 indicate the presence of C F bonds (Fig. 2c). They correspond well with the results reported in literature for films on alumina [37]. The peak at 1100 cm−1 is due to C C bonds in perfluoroalkyl chain. 3.4. Nanotribological studies The study performed with the aid of AFM allowed to determine tribological properties of modified surfaces. Comparing the results obtained for surfaces studied in this work, it can be concluded

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Table 1 ToF-SIMS secondary fragmentation ions analysis. All values presented in the table are the number of counts divided by 103 . Fragmented ions

CF3 CF2 F OH AlOSi SiO Si2 O AlOH AlO Al

Positive ions

Negative ions

Before modification

After FDTS modification

After PFMS modification

Before modification

After FDTS modification

After PFMS modification

– – – – – – – 11.5 – 330.1

800.5 156.3 – – 15.9 – – 2.4 – 37.7

20.6 2.2 – – 3.2 – – 5.4 – 59.9

– – – 288.7 – – – – 36.0 –

– – 1287.8 15.1 – 32.9 30.0 – – –

– – 848.9 24.9 – 7.4 – – 2.8 –

Fig. 2. FT-IR spectra obtained in the spectral range 750–4000 cm−1 (upper figure) and zoom of the spectral range 750–1300 cm−1 (bottom figure) for (a) unmodified Al, (b) FDTS film on Al surface and (c) PFMS film on Al surface.

that alumina modified by fluoroalkylsilanes shows higher contact angle (hydrophobic properties) and lower root mean square (rms) surface roughness values than unmodified Al (Fig. 3). Fluoroalkylsilanes modified alumina surfaces possess lower surface free energy than the unmodified surface. A value of the surface free energy is 63.7 mJ/m2 for alumina, for FDTS 26.6 mJ/m2 and for PFMS 27.6 mJ/m2 , respectively. These results show that the water contact angle increased with a decrease of the surface energy. The surfaces with lower surface free energies and higher contact angle (hydrophobic properties) exhibit lower values of

the coefficient of friction and adhesive forces compared to the unmodified surface (Fig. 3). The trifunctional fluoroalkylsilanes (FDTS) have a higher degree of hydrophobicity, lower adhesion, and in consequence lower coefficient of friction than the monofunctional fluoroalkylsilanes (PFMS). Fluoroalkylsilanes with different amount of the reactive leaving groups (FDTS and PFMS) have different tribological properties. Different results obtained for these two fluoroalkylsilanes are probably due to different packing density and molecular

Fig. 3. Nanotriblogical data: (A) contact angle and adhesive force, (B) coefficient of friction and surface roughness (rms) values.

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organization (packing), as shown in studies performed by Genzer et al. [40]. Two methyl groups in PFMS molecule produce a steric hindrance between neighboring molecules. This steric hindrance in attachment groups is responsible for decreased amount of CF3 and CF2 ions (Table 1), and in consequence for smaller quantity of monochlorosilane molecules and larger amount of OH ions on the Al surface. McElwee et al. [41] showed that monolayers prepared from monochlorosilanes exhibit a maximal grafting density of 2.8 molecules/nm2 , whereas grafting density for monolayers prepared from trichlorosilanes is about 40% higher. Stevans [42] studied alkylsilane self-assembled monolayers and the influence of cross-polymerization on their stability. Those investigations focused on possible packing of SAMs formed from alkyltrichlorosilanes and alkylmonochlorosilanes. It was shown that dimethylalkylmonochlorosilanes were not able to cover the silica surface with surface densities equal to the surface hydroxyl density, as opposed to alkyltrichlorosilanes which can react with all of the surface hydroxyls. Such research is essential if one is interested in altering or controlling the chemical nature of the surface, by bonding alkylsilane derivatives to metal oxide surfaces, and properties such as adhesion, coefficient of friction or wear. The results obtained by ToF-SIMS investigations reveal higher amount of the OH negative ions and AlOH positive ions for PFMS monolayers in comparison with FDTS monolayers (Table 1). An increase in the presence of these ions results in increase of hydrophilic properties for monochlorosilanes modified surface. This leads to an increase of the capillary and adhesive forces and in consequence to higher values of the obtained friction force and coefficient of friction. The friction force is a sum of the adhesional component Fadh and friction force component consumed on plastic deformation Fdef [43]. The frictional forces have a significant impact on obtained values of the coefficient of friction. The adhesional component consumed on shearing of the adhesive connections is composed of the Van der Waals forces, capillary, electrostatic, ionic and chemical forces. Numerous investigations showed that the capillary force is the most dominant one in case of not charged hydrophilic surfaces in relative humidity of more than 30% [44–50]. For more hydrophobic surface the capillary force is lower than for hydrophilic surface. Therefore on the hydrophobic surface water film is more difficult formed and consequently for the FDTS modified alumina surface the adhesion force is decreased as compared with that for the unmodified surface and surface modified by PFMS. In the case of Fdef forces, they depend on the plastic deformation and plowing. All the mentioned components depend on the real area of contact (which is dependent on surface roughness, hardness and elasticity modulus) and shear strength during sliding. In nanoscale contact, the small apparent area of contact reduces the number of particles trapped at the interface, and thus minimizes the third-body plowing contribution to the friction force. The friction forces are only due to the adhesional component and capillary forces as a result of the lack of plastic deformation and plowing. The trend in contact angle and adhesion results agrees well with the coefficient of friction data. As mentioned previously, in general the contributions to the friction force come from the adhesional component and from component consumed on plastic deformation. In the case of the measurements presented in this paper, however, the friction force is mainly related to the adhesive force.

4. Conclusions VPD method was applied to obtain uniform fluoroalkylsilane films on alumina surfaces. The results obtained by XPS, ToFSIMS and FT-IR techniques proved that the method of surface

modification was successful. They provide information about the presence of fluorinated compounds on the alumina surface as well as the formation of chemical bonds between molecules and the substrate. Moreover, the used techniques provided useful and complementary information about the formation of Metal O Si linkages and horizontal fluoroalkylsilanes polymerization. The surfaces modified by the fluoroalkylsilanes films consistently have smaller roughness, larger water contact angles and lower coefficient of friction in nanoscale than the unmodified alumina surfaces. In the case of comparison of tribological properties in nanoscale, the trifunctional silanes exhibit a lower adhesion and coefficient of friction than the monofunctional ones. These results led us to conclude that covalently bonded fluorosilanes with trifunctional reactive groups have the improved properties of the hydrophobicity and lowest friction coefficient. Tribological tests performed on unmodified alumina surface and surface modified by fluoroalkylsilanes demonstrate superior performance of the FDTS layers. Therefore they are a prime candidate for practical use as a lubricant in MEMS systems which are made of aluminum. Acknowledgement This work was supported by the Polish Ministry of Science and Higher Education within Research Grant No. NN 507551538. References [1] B. Bhushan, Microelectronic Engineering 84 (2007) 387. [2] J.G. Kushmerick, M.G. Hankins, M.P. de Boer, P.J. Clews, R.W. Carpick, B.C. Bunker, Tribology Letters 10 (2001) 103. [3] T.M. Mayer, M.P. de Boer, N.D. Shinn, P.J. Clews, T.A. Michalske, Journal of Vacuum Science and Technology B 18 (2000) 2432. [4] D. Wang, Y. Ni, Q. Hu, D.E. Tallman, Thin Solid Films 471 (2005) 177. [5] B. Bhushan, Handbook of Nanotechnology, 3rd edition, Springer, Heidelberg, 2007, p.1575. [6] S.A. Henck, Tribology Letters 3 (1997) 239. [7] S. Miller, Lubricating Micro-machined Devices Using Fluorosurfactants, Patent No. 7738154 (2007). [8] A. Hozumi, B. Kim, T.J. McCarthy, Langmuir 25 (2009) 6834. [9] Y.Z. Du, S.S. Saavedra, Langmuir 19 (2003) 6443. [10] A. Ulman, Chemical Reviews 96 (1996) 1533. [11] F. Schreiber, Progress in Surface Science 65 (2000) 151. [12] B. Bhushan, T. Kasai, G. Kulik, L. Barbieri, P. Hoffmann, Ultramicroscopy 105 (2005) 176. [13] B. Yu, L. Qian, J. Yu, Z. Zhou, Tribology Letters 34 (2009) 1. [14] M. Cichomski, K. Ko´sla, J. Grobelny, W. Kozłowski, P.J. Kowalczyk, A. Busiakiewicz, W. Szmaja, J. Balcerski, Journal of Alloys and Compounds 507 (2010) 273. [15] B. Bushan, M. Cichomski, Journal of Vacuum Science and Technology A 25 (2007) 1285. [16] D. Devaprakasam, S. Sampath, S.K. Biswas, Langmuir 20 (2004) 1329. [17] G. Mani, D.M. Johnson, D. Marton, V.L. Dougherty, M.D. Feldman, D. Patel, A.A. Ayon, C.M. Agrawal, Langmuir 24 (2008) 6774. [18] L. Pan, H. Dong, P. Bi, Applied Surface Science 257 (2010) 1707. [19] M.P. Seah, Surface and Interface Analysis 31 (2001) 721. ´ J. Grobelny, M. Cichomski, G. Celichowski, J. Rogowski, Applied [20] I. Piwonski, Surface Science 242 (2005) 147. [21] H. Onoe, K. Matsumoto, I. Shimoyama, Journal of Microelectronic Systems 13 (2004) 603. [22] J.E. Sader, J.W.M. Chon, P. Mulvaney, Review of Scientific Instruments 70 (1999) 3967. [23] C.P. Green, H. Lioe, J.P. Cleveland, R. Proksch, P. Mulvaney, J.E. Sader, Review of Scientific Instruments 75 (2004) 1988. [24] X.D. Xiao, L.M. Qian, Langmuir 16 (2000) 8153. ´ [25] I. Piwonski, A. Kisielewska, Tribology Letters 45 (2012) 237. [26] J. Ruan, B. Bhushan, ASME Journal of Tribology 116 (1994) 378. [27] D. Wang, R.D. Oleschuk, J.H. Horton, Langmuir 24 (2008) 1080. [28] S. Ntais, V. Dracopoulos, A. Siokou, Journal of Molecular Catalysis A: Chemical 220 (2004) 199. [29] E. Hoque, J.A. DeRose, P. Hoffmann, B. Bhushan, H.J. Mathieu, Journal of Physical Chemistry C 111 (2007) 3956. [30] E. Hoque, J.A. DeRose, R. Houriet, P. Hoffmann, H.J. Mathieu, Chemistry of Materials 19 (2007) 798. [31] B.D. Booth, S.G. Vilt, J.B. Lewis, J.L. Rivera, E.A. Buehler, C. McCabe, G.K. Jennings, Langmuir 27 (2011) 5909. [32] H.Y. Nie, Analytical Chemistry 82 (2010) 3371. [33] M.L. Abel, R.P. Digby, I.W. Fletcher, J.F. Watts, Surface and Interface Analysis 29 (2000) 115.

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