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Tribological investigations of perfluoroalkylsilanes monolayers deposited on titanium surface
Materials Chemistry and Physics 136 (2012) 498e504
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Tribological investigations of perfluoroalkylsilanes monolayers deposited on titanium surface Micha1 Cichomski* Department of Materials Technology and Chemistry, University of Łódz, Pomorska 163, 90-236 Łódz, Poland
h i g h l i g h t s < Titanium surface modification by perfluoroalkylsilanes was investigated. < The effectiveness of modification was monitored by the surface free energy. < The modification procedure correctness was characterized by ToF-SIMS, AFM, FT-IR measurements. < The tribological performance of modified titanium in differed scale was studied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2012 Received in revised form 3 July 2012 Accepted 19 July 2012
Therefore the present work reports a systematic study of titanium modification by fluoroalkylsilanes and surface characterization from the tribological point of view. The vapor phase deposition method was used to modify titanium surfaces by fluoroalkylsilanes and the influence of the used modifier on the tribological properties is presented. The modification procedure efficiency, surface structure and morphology were characterized by secondary ion mass spectrometry, infrared spectroscopy and atomic force microscopy. The effectiveness of modification of the titanium surface was monitored by the measurement of the wetting contact angle and the surface free energy. The increase of surface hydrophobicity was observed upon the modification by increasing the wetting contact angle and reducing the surface free energy. The tribological performance of various perfluoroalkylsilanes films on the titanium surface was investigated in mili- and nano-newton load ranges. Dependence of the adhesive force and coefficient of friction values obtained in nano- and micro-scale on fluoroalkyl chain length was observed. Nano- and micro-tribological measurements show that titanium modified by fluoroalkylsilanes has lower adhesion and coefficient of friction than unmodified one. The investigation also indicates a decrease of the friction coefficient with increasing fluoric alkyl chain length. It was found that the titanium modified by fluoroalkylsilanes with longer alkyl chains is a prime candidate for practical use as a lubricant in biomedical and electronic applications. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Monolayers Vapour deposition Atomic force microscopy Adhesion Friction Tribology
1. Introduction Titanium and its alloys have been extensively studied in the recent years due to their useful electrical and optical properties [1,2], such as a high refractive index [3], high dielectrical permittivity [4], semiconductivity [5], superior biocompatibility and corrosion resistance [6]. Because of the mentioned properties, these materials are used in many areas for example: in devices [7,8], biomaterials [9,10], optical cells, highly efficient catalysts and
* Tel.: þ48 42 6355836; fax: þ48 42 6355832. E-mail address: [email protected]. 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.07.017
implants [11,12], cardiovascular application [13,14]. Some limitations in the exploitation of devices are large adhesion, friction and wear. In the case of medical applications the following problems present difficulties: corrosion degradation in humid environment, inflammatory reaction on implant surface and wear resistance. The solution of these problems is to use self-assembled monolayers (SAMs) as protective coatings and lubricants [15,16]. Self-assembled monolayers are ordered molecular assemblies formed by the chemical adsorption of active molecules on a solid surface. Monolayers are composed of molecules which consist of a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film and a backbone chain group that connects the head and tail groups [17,18]. The interactions
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between backbone chain groups of different molecules, such as van der Waals forces or hydrogen bonding, contribute to the films stability and formation. The stability of SAMs on metal oxides is also determined by covalent bonds between molecules and the surface, and cross-links between neighboring molecules consisting of three or two reactive atoms in the head group [19,20]. In recent research, the properties of SAMs, such as tiols, phosphonic acids and silanes, deposited on different substrates (gold, alumina, silicon, copper) have been studied [21e25]. One of the best tribological properties present fluoroalkylsilanes [26,27]. These compounds have low values of surface free energy, adhesive force, coefficient of friction, high thermal stability and contact angle values, and are promising alternative for overcoming the limitations of electromechanical devices’ lifetime [28,29]. Mayer et al. used fluoroalkylsilanes to control adhesion in microelectromechanical systems [30], Kushmerick et al. showed that these compounds can play a role as antistiction layers in micromachines [31]. Much interest has been also attracted to the investigations of electrochemical or biological properties of the SAMs on metal electrodes and the fabrication of electrochemical sensors or biosensors for the detection of chemicals or biomolecules [32e34]. In this study, the formation of fluoroalkylsilanes with different chain length with ten and three carbon atoms, such as 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) and (3,3,3-trifluoropropyl) trichlorosilane (FPTS), on titanium surface was investigated. The properties of the fluoroalkylsilanes films deposited by the gas-phase method were characterized using time of flight secondary ion mass spectrometry (ToF-SIMS) and Fourier transform infrared (FT-IR) spectroscopy techniques. The influence of chain length on film tribological properties was also presented. The hydrophobicity, surface free energy, adhesive force and coefficient of friction values were obtained using static contact angle (SCAs) measurements, atomic force microscopy (AFM) and microtribometer. This paper reports the improvement of the frictional properties of titanium substrate modified by fluoroalkylsilanes. While these films have found widespread applications for example in MEMS technology, their tribological properties in both nano- and microscale for the titanium substrate have been presented for the first time. The investigations presented in this work are important because of the constantly growing demand for compounds acting as lubricants in miniaturized systems. 2. Experimental 2.1. Sample preparation Silicon pieces were cut from a commercial p-type Si wafer and cleaned using ethyl alcohol, deionized water and dry argon (Ar) gas. The substrates of pure (100) silicon pieces were attached to oneaxis rotary sample holder in a vertical geometry at magnetron sputtering chamber. After the attachment of the substrates to the sample holder the air from vacuum chamber was evacuated to the pressure below 103 Pa. Before ion-cleaning the chamber was heated with use of resistance heater during the period of 1 h and cooled down for 3 h. During argon ions cleaning and titanium coating deposition the substrates were rotated with radial velocity of 0.13 p rad s1. During 30 min of sputtering of the samples surfaces, a negative bias of the samples was increased from 0 to 2 kV. Titanium coating deposition started 10 min after argon ion cleaning. Argon pressure during magnetron sputtering of pure titanium target was 0.37 Pa. Mean power was 1 kW with 1.2 A magnetron current. Sample bias in pulsed mode (160 kHz base frequency, 4 kHz modulation frequency and 100% duty cycle) during deposition was equal to 100 V. The thickness of the prepared unmodified titanium was 100 nm.
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The next preparation step was the use of oxygen plasma to activate the surface before modification by compounds and to increase the amount of hydroxyl ions which are necessary for the reaction between fluoroalkylsilane compound and the substrate. 2.2. Modification procedure The modification of the titanium substrates was performed with FDTS and FPTS precursors. These chemicals were chosen to compare the effect of chemical structure on tribological performance at the micro- and nano-scale. The effects of carbon chain length as well as the number of fluoride atoms in the chain were investigated. All precursors were ordered from the ABCR, GmbH & Co. KG, Karlsruhe. The titanium surface activated by oxygen plasma was placed into a vapor phase deposition system and kept under low pressure (0.1 Pa) for 30 min. Then the sample was kept in the modifier vapor from 7 min to 2 h at room temperature and finally outgassed at low pressure for 1 h at 40 C to remove any of physisorbed and unreacted molecules [22,35]. The samples were removed from the vacuum chamber and transferred into a vacuum desiccator until characterization. Static contact angle was measured to determine the degree of the hydrophobicity. Based on the measured contact angle, optimum deposition time for FDTS and FPTS modifiers were selected [36]. The thicknesses of the deposited layers of FDTS and FPTS were 1.88 nm and 2.55 nm, respectively. These data were obtained from the ellipsometry measurements on a J.A. Woollam V-VASE ellipsometer. In order to obtain an accurate determination of the film thickness, a fit algorithm was used to first determine the optical constants of the deposited metal layer on the silicon substrate using the values of real part of the index of refraction and extinction coefficient of polycrystalline titanium. The fitted optical constants of the metal layer were assumed to remain constant as the fluoroalkylsilane layers were adsorbed on the top of it. To calculate the thickness of the adsorbed fluoroalkylsilane layer, a Cauchy dispersion model was used. 2.3. Measurement techniques 2.3.1. Contact angle and surface free energy measurements SCA were measured in air by a sessile-drop method using a contact angle goniometer. A drop of proper liquid (water, glycerine and diiodomethane) was deposited on the substrate with the use of microsyringe. The image of the droplet was obtained by a digital camera. The surface free energies were calculated using the Van OsseChaudhuryeGood method [37]. 2.3.2. ToF-SIMS investigations The secondary ions mass spectra were recorded with a ToF-SIMS IV mass spectrometer manufactured by Ion-ToF GmbH, Muenster, Germany. The instrument is equipped with Bi liquid metal ion gun and high resolution time of flight mass analyzer. Secondary ion mass spectra were recorded from an approximately 100 mm 100 mm area of the spot surface. During measurement the analyzed area was irradiated with the pulses of 25 keV Bi3 þ ions at 10 kHz repetition rate and an average ion current 0.14 pA. The analysis time was 100 s for both positive and negative secondary ions giving an ion dose below static limit of 1013 ions cm2. Secondary ions emitted from the bombarded surface were mass separated and counted in time of flight analyzer [38]. 2.3.3. FT-IR measurements Fourier transform infrared spectroscopy was performed using a BioRad 175C spectrometer equipped with a liquid nitrogencooled mercuryecadmiumetelluride (MCT) detector and grazing angle attenuated total reflectance (GATR) accessory manufactured
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by Harrick Scientific Products Inc. The GATR accessory used 65 grazing angle and a Ge ATR crystal for studies of adsorbed monolayers on the substrates. Each spectrum was taken by collecting 1000 scans (with 1000 scans of pure silicon as background) at a resolution of 8 cm1 in the spectral range 750 cm1e4000 cm1 in dry air flow. 2.3.4. Nanotribological characterization The adhesion and friction measurements were performed with a NT-MDT Solver P47 AFM apparatus operating in air under ambient conditions with a rectangular silicon nitride cantilever (the spring constant calibrated by the Sader method, k ¼ 0.72 N m1) [39,40]. The radius of curvature of the used tip was less than 20 nm. The adhesive force was obtained from the forceedistance curve after reckoning the pull-off force [41e44]. These data were arising from different points on the surface. The presented data for the adhesion force are the averaged data. The coefficient of friction was obtained from the slope of the friction force versus normal force plots. The friction force was calibrated using the method described by Ruan and Bhushan [45]. The technical parameters of measurements were the following: applied loads ranged from 5 to 100 nN, scan rate of 1 Hz, scan size of 1 mm 1 mm. Each measurement was repeated three times on different places of the sample surface. The average data are presented. Images were typically obtained in the tapping mode using silicon nitride cantilever (MikroMasch, NSC/Si3N4/AiBS), with spring constant k ¼ 14 N m1 and resonance frequency n ¼ 260 kHz. The typical scan area was fixed at 2 mm 2 mm. 2.3.5. Microtribometer measurements The micro-tribological investigations were carried out by a reciprocating ball-on-flat microtribometer [46]. Measurements were performed with the use of silicon nitride sphere with diameter of 5 mm over a normal force range from 30 to 80 mN. The ball moved parallel with respect to the sample surface with velocity of 25 mm min1 and traveling distance of 5 mm. The root mean square (rms) surface roughness of the ball was from 5 to 6 nm and the hardness was 14.7 GPa. The microtribometer was operating in ambient conditions (temperature and humidity). Measurements were performed at a technical dry friction and repeated three times in three at different places on the sample surface [28]. The adhesive force was calculated from the negative horizontal intercept of the friction force versus applied load curve (the negative applied load value where the friction force is zero) [47]. The typical curves are presented in Fig. 5a.
and in consequence to the surface. In Fig. 1 the contact angle is plotted against the deposition time for different chain length compounds (FDTS, FPTS), for time periods ranging from 7 min to 2 h. Contact angle values monitored for titanium surfaces that had been exposed to silanes vapors are highest for deposition time of 15 min for FDTS and 30 min for FPTS. These data show the sensitivity of the silanes formation process to the vapors exposure time and reveal the importance of different exposure time for the formation of FDTS and FPTS films which had different chain lengths. For further investigations was used surfaces which possessed deposition time of 15 and 30 min for FDTS and FPTS respectively, i.e. which were characterized by the highest contact angles values. An example of water droplets on FDTS and FPTS modified titanium surfaces obtained using optimum time deposition parameter and for the unmodified titanium surface is shown in Fig. 2. The unmodified surface is hydrophilic, which is observed in lowering of the water contact angle value in comparison with modified surface. After modification the surface becomes hydrophobic and the contact angle increases. An increase of the contact angle is a direct evidence of the presence of modifier molecules on the titanium surface. The surface free energy was calculated using the method of van Oss, Chaudhury and Good, by measurements of contact angles of two polar liquids (water, glycerol) and one apolar liquid (diiodomethane) [50]. This method yields the surface tension g as a summation of the dispersive component gLW based on LifshitzeVan der Waals interactions and the acidebase component gAB based on hydrogen bonding interactions. The acidebase term consists of the asymmetric components gþ for acid (electron acceptor) and g for base (electron donor) interactions. This term is expressed as
gAB ¼ 2 gþ g
1=2
The connection between the contact angle Q and components of the surface free energy is given by the YoungeDupre’ equation
1=2 1=2 1=2 ð1 þ cosQÞgL ¼ 2 gLW gLW þ2 gþ g þ2 g gþ L L L where: gLW dispersive component, gþ L L asymmetric component for acid and gL asymmetric component for base specific to the measurement liquid. Using this equation, values of the surface free energy were obtained for all films (Table 1). For titanium the surface free energy is
3. Results and discussion 3.1. Optimization of deposition time parameters and surface free energy studies Previous studies showed the relation between adhesive forces, coefficient of friction and contact angle values [28,46]. Investigations used SCAs for determining the deposition factors, such as immersion time, concentration of molecules or gas pressure, were performed by Bhushan, Liakos and Hoertz [36,48,49]. In the present studies water SCAs were used to determine time of the molecules vapor deposition on titanium. SCAs technique is sensitive to the order of the monolayer as a result of the higher energy of the eCF2e or eCH2e bonds (which are exposed when chains are disordered) compared with eCF3 or eCH3 group. The difference in values obtained for SCA results from the fact that in well-ordered monolayers the terminal group of the chain is oriented outwards, reducing access of the water drop to the chain
Fig. 1. Static contact angle values as a function of deposition time.
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Fig. 2. An example of water droplets on the unmodified Ti surface (a), Ti surface after FDTS modification (b) and after FPTS modification (c).
ski et al. [51] show that nonporous titania ob53.3 mJ m2. Piwon tained by the solegel method exhibits the surface free energy of about 68 mJ m2. The modification of this surface by FDTS and FPTS lowers the surface free energy by 40% compared to the surface free energy before modification [51]. Lee et al. [52] and Kubies et al. [53] present that the surface free energy values for titania film deposited on silicon are 64 mJ m2 and 47 mJ m2, respectively. Differences between the obtained and literature values are due to different preparation methods of the titania substrate. Fluoroalkylsilanes modified Ti surfaces show lower surface free energies than the unmodified Ti surface. It can also be observed that the molecules carrying longer perfluorinated chain (FDTS) exhibit a higher degree of hydrophobicity and lower values of the surface free energy than the molecules having short chain (FPTS). It was found that the calculated values of gΑΒ are consistent for films with longer chain and are dominant component in the obtained values of the surface free energy. For FDTS the acidebase component was 0.4 mJ m2. This value is considerably lower in comparison with the gΑΒ component for FPTS modified and unmodified surfaces. The obtained data show that the dominant component is the dispersive one. The results for perfluoroalkylsilane modified Ti surface showed that the water contact angle increased with a decrease in the surface energy, which is consistent with the general theory [37]. The differences in the obtained values, when we compare the unmodified Ti and fluoroalkylsilanes modified surfaces, are connected with eCF3 tail group ability in increasing of the contact angles. Trifluoromethyl group will give rise to a strong dipole of perfluoroalkylsilanes, the formation of films from CF3(CF2)n(CH2)2SiCl3 will generate an interface in which these strong dipoles are oriented near the contacting liquids. The interaction between the contacting liquids and these dipoles should give rise to a positive contribution to the surface free energy for the trifluoromethyl terminated films. Hydrogen bonding between the eCF3 tail groups and the hydroxyl groups of glycerol and water should also give rise to a positive contribution to the surface free energy for the CF3-terminated films. The presence of dipole interactions and/or hydrogen bonding can be used to explain the
Table 1 Water contact angle and surface free energy values of fluoroalkylsilanes deposited on titanium surface. Water contact Surface free Dispersive Acidebase angle (degree) energy (mJ m2) component component gAB (mJ m2) gLW (mJ m2) Unmodified Ti 73 2 Ti after FDTS 105 2 modification Ti after FPTS 95 2 modification
53.3 4.0 25.2 3.4
14.9 3.5 0.4 1.1
38.4 1.8 24.8 3.1
32.2 2.7
9.8 1.8
22.4 1.7
enhanced wettability of the perfluorinated films toward water and glycerol. In this case FDTS modified surface exhibits almost no contribution from the acidebase component gAB. It seems that the dipole interactions are predominantly responsible for the enhanced wettability. The difference in static contact angle values for FDTS and FPTS films on Ti is likely due to the layer penetration by water and interaction with the surfaces in the case of the films with shorter fluoroalkyl chain. In the case of the films with longer fluoroalkyl chain, water does not penetrate the Ti surface, so SCA values are higher. This is related to the formation of agglomerates during vapor deposition process in the case of FPTS layer. The mentioned effect will be discussed in more detail in the next section. 3.2. Surface characterization by ToF-SIMS and FT-IR measurements ToF-SIMS is a tool for obtaining chemical information (molecular and structural) on surfaces. ToF-SIMS has been successfully applied to studies of SAM systems such as thiols on Au, silanes and phosphonic acids on various oxides of metals [54e59]. This method was also used to monitor hydrolysis and condensation of 3-mercaptopropyltrimethoxysilane [22] and to analyze this compound modified by FDTS [26]. In the case of the present investigations ToF-SIMS has been used to prove the correctness of perfluoroalkylsilanes modification (Table 2). ToF-SIMS analysis shows the presence of CF3, CF, F and Si2O ions after Ti modification. Moreover, a proper modification of the titanium surface by silanes is evidenced by the decrease of the number of fragmented ions TiOH and TiO. Structure comparison for both fluoroalkylsilanes indicates that in the case of FPTS the number of fragmented ions is lower. In the case of FDTS modified Ti surface the results show a very strong increase in the amount of CF3 and CF fragmented ions compared to FPTS modification. It stems from the carbon chain length as well as from various types of polymerization that occur on the surface of titanium. In the case of FDTS compound horizontal polymerization occurs while for FPTS vertical polymerization takes place. The occurrence of polymerizations seems to be supported by the decrease number of TiOH and TiO ions (coverage of the unmodified surface). Furthermore, the appearance of Si2O ions confirms siloxane linkages formation between the modifier molecules in the case of FDTS (horizontal polymerization). The presence of polymerization in the vertical direction in the case of FPTS is evidenced by the increased number of OH ions. This happens because after exposure to the ambient atmosphere unreacted SieCl bonds undergo hydrolysis to form SieOH groups. The structure of fluoroalkylsilanes (FDTS and FPTS) on titanium was also characterized by infrared spectroscopy (Fig. 3). The peaks at 780 and 889 cml can be assigned to TieOeTi bond vibrations [60] and are present for the unmodified surface and also for both
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Table 2 ToF-SIMS secondary fragmentation ions analysis of fluoroalkylsilanes on titanium surface: before modification, after FDTS and FPTS modification. All values presented in the table are the number of counts divided by 103. Fragmented ions
CF3 CF2 F CF Si2O Si OH TiOH TiO
Positive ions
Negative ions
Before modification
After FDTS modification
After FPTS modification
Before modification
After FDTS modification
After FPTS modification
e e e e e e e 110.2 301.3
728 e e 839 e 937 e 14 13.6
2.7 e e 2.8 e 10 e 69 213
e e e e e e 477.2 e e
e 4.7 2064.4 28.5 120 e 173.6 e e
e e 133.2 0.6 3.4 e 772.6 e e
FDTS/Ti and FPTS/Ti surfaces. In the case of FPTS the mentioned peak at 780 cm1 is shifted to lower wavenumbers and is lower what is due to higher thickness of forming layer as the effect of vertical polymerization [61]. For the FDTS modified Ti surface the peak at 1065 cm1 due to SieOeSi asymmetric stretches reflects the presence of a siloxane network. FPTS also shows siloxane peaks at 1027 cm1 and 1069 cm1. The absence of the eOH peaks in FDTS and FPTS above 3000 cm1 suggests that the surface is covered by close packed silane films. Moreover, FDTS shows peaks due to asymmetric and symmetric stretches of CeF for eCF2 and eCF3 groups in the range of 1150e1366 cm1. For FPTS peaks in the range of 1216e1374 cm1 are due to asymmetric and symmetric stretches of CeF bonds for trifluoromethyl group. Peak at 1447 cm1 for FPTS is due to CeH asymmetric stretches. The obtained results confirm the presence of siloxane cross-links for compounds containing three hydrolyzable, reactive groups in the molecule. 3.3. Tribological properties At the first step, the quality of fluoroalkylsilane films formed on the titanium substrate was probed (Fig. 4). Based on AFM topography images root mean square (rms), the surface roughness Rq was defined and calculated as
" Rq ¼
N X
zi zaverage
2
#1=2 =N
i¼1
Fig. 3. FT-IR spectra obtained for the unmodified Ti surface (a), FDTS modified Ti surface (b) and FPTS modified Ti surface (c) in the spectral range of 750e4000 cm1 (left) and 750e1500 cm1 region zoom (right).
where zaverage is the average of the height values within a given area, zi is the individual height value and N is the number of pixels within the given area. The surface roughness for the unmodified Ti surface was 1.7 nm. The surface roughness values for FDTS and FPTS, were measured as 2.6 nm and 1.9 nm, respectively. The obtained rms surface roughness data suggest that fluoroalkylsilane layers have an effect on the surface roughness, and consequently have an effect on the hydrophobicity and surface free energy. With increasing silanes chain length, the surface roughness and water contact angle increase, and the surface free energy decreases. In other words we can say that the factors determining the change in contact angle values, and in consequence in surface free energy and coefficient of friction, are the surface roughness and chemical composition. Fig. 5 shows the coefficient of friction and adhesive force measured using microtribometer and AFM for fluoroalkylsilanes deposited on Ti substrates. The adhesion and coefficient of friction in both nano- and micro-scale were always higher for the unmodified surface compared to those for fluoroalkylsilanes modified Ti surfaces. Bhushan and Cichomski [36] show that if the thickness of the layer is less than 5 nm, the substrate affects the obtained value of the adhesive force. In the present studies, the thickness of fluoroalkylsilanes layers is about 2e3 nm, thus titanium substrate has an influence on the adhesion force values. However, the contribution of the titanium substrate to the obtained values of adhesion force is difficult to determine because we do not compare two or more different substrates. Among the fluoroalkylsilanes films, FDTS showed a lower adhesive force and coefficient of friction than FPTS. The observed values of coefficient of friction for FPTS and FDTS reveal the similar trend to the results obtained for the surface free energy. Ti surface coated by fluoroalkylsilanes is more hydrophobic, has a lower surface free energy and adhesion, and in consequence a lower coefficient of friction in comparison with the unmodified surface. As we can see in Fig. 5 and Table 1 an impact on the coefficient of friction value have the dispersive and acidebase components of the surface free energy. The dispersive component plays dominant role due to the higher values compared to acidebase component. This is likely to result from the fact that friction processes in micro- and nano-scale are determined by capillary effects, electrostatic interactions and adhesion. In the presented investigations the adhesion force arises mainly from capillary forces [62e64]. The capillary forces depend on surface hydrophilicity, so for fluoroalkylsilanes modified Ti surface they become smaller because FDTS and FPTS form films with surface hydrophobic properties. Different tribological properties for Ti coated with FDTS and FPTS result from the difference between chain length. The relationship between chain length and surface properties, such as hydrophobicity, adhesion and friction behavior, confirmed investigations performed for alkylsilanes [65,66]. Trifunctional silanes
M. Cichomski / Materials Chemistry and Physics 136 (2012) 498e504
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Fig. 4. AFM images of titanium (a), FDTS modified titanium (b) and FPTS modified titanium surface (c).
with short chain compared to their longer chain analogs, are more reactive and are capable of polymerizing which gives a lot of possible surface structures, formation of gauche defects and disorder in the formed layer. This disorder has an influence on the frictional measurements.
The obtained values in the nano-scale are much lower than those in the micro-scale because the friction force is a sum of the adhesional component and friction force component consumed on plastic deformation which are scale-dependent. This is connected with different counterpart radii, contact stresses and shear strength
Fig. 5. Coefficients of friction and corresponding components of the surface free energy, adhesive forces and friction forces vs normal loads curves in (a) micro-scale and (b) nanoscale.
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during sliding [67,68]. In the nano-scale the difference in coefficient of friction values also results from the pressure exerted by the tip, which more easily deforms the monolayer formed by reaction between the substrate and shorter chain molecules than the longer ones. Monolayers deformation during tip sliding on the surface is the reason for an increase of the shear force, and in consequence produces higher friction force. In nano-scale 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 [69]. The contact stress at nano-scale, in spite of small tip radii, generally does not exceed the sample hardness that minimizes plastic deformation. Average contact stresses in micro-contacts are generally lower than these in AFM contacts, however, a large number of asperities come into contact that go through some plastic deformation [41,70]. In nano-scale studies, the indentation hardness is higher than that at the micro-scale due to the small contact areas and very low loads used in AFM conditions. The lack of plastic deformation and improved mechanical properties reduce the degree of wear and friction. 4. Conclusions In this paper, gas-phase preparation method of FDTS and FPTS films on Ti surface was presented. The films structure and tribological properties have been studied using FT-IR spectroscopy, ToFSIMS, AFM, microtribometer and SCA measurements. The results obtained using contact angle, ToF-SIMS and FT-IR techniques proved the correctness of the gas-phase modification method. From studies performed in this paper, it can be concluded that the chain length has an impact on film wettability, surface free energy, coefficient of friction and adhesive force. Fluoroalkylsilanes with three carbon atoms in chain compared to ten carbon atoms in chain compounds, are more reactive and are capable of vertical polymerizing what confirmed FT-IR and ToF-SIMS investigations. Shorter chain molecules provided weaker interactions between the backbone chain groups, larger number of structure defects, and in consequence higher values of the adhesive force and coefficient of friction in nano- and micro-scale. Both silanes FDTS and FPTS provided hydrophobic properties of the modified surface, lower values of the surface free energy and improved titanium tribological properties. Titanium surface modification with the aid of fluoroalkylsilanes may find application in tribological systems where anti-frictional and anti-adhesion properties play a crucial role. Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education within Research Grant No. NN 50755153. References [1] A. Ferreira da Silva, I. Pepe, J.L. Gole, S.A. Tomas, R. Palomino, W.M. de Azevedo, E.F. da Silva Jr., R. Ahuja, C. Persson, Appl. Surf. Sci. 252 (2006) 5365. [2] H.P. Deshmukh, P.S. Shinde, P.S. Patil, Mater. Sci. Eng. B 130 (2006) 220. [3] Y. Liu, Ch. Lu, M. Li, L. Zhang, B. Yang, Colloid Surf. A: Physicochem. Eng. Aspects 328 (2008) 67. [4] F. Jin, H. Tong, L. Shen, K. Wang, P.K. Chu, Mater. Chem. Phys. 100 (2006) 31. [5] M.K. Nowotny, P. Bogdanoff, T. Dittrich, S. Fiechter, A. Fujishima, H. Tributsch, Mater. Lett. 64 (2010) 928. [6] E. Eisenbarth, D. Velten, M.M. Uller, R. Thull, J. Breme, Biomaterials 25 (2004) 5705. [7] S. Kataria, N. Kumar, S. Dash, A.K. Tyagi, Wear 269 (2010) 797. [8] S.J. Bull, Solid State Phenom. 159 (2010) 11. [9] T.R. Rautray, R. Narayanan, K.-H. Kim, Prog. Mater. Sci. 56 (2011) 1137. [10] V. Biehl, T. Wack, S. Winter, U.T. Seyfert, J. Breme, Biomol. Eng. 19 (2002) 97.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Tribological investigations of perfluoroalkylsilanes monolayers deposited on titanium surface Micha1 Cichomski* Department of Materials Technology and Chemistry, University of Łódz, Pomorska 163, 90-236 Łódz, Poland
h i g h l i g h t s < Titanium surface modification by perfluoroalkylsilanes was investigated. < The effectiveness of modification was monitored by the surface free energy. < The modification procedure correctness was characterized by ToF-SIMS, AFM, FT-IR measurements. < The tribological performance of modified titanium in differed scale was studied.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 January 2012 Received in revised form 3 July 2012 Accepted 19 July 2012
Therefore the present work reports a systematic study of titanium modification by fluoroalkylsilanes and surface characterization from the tribological point of view. The vapor phase deposition method was used to modify titanium surfaces by fluoroalkylsilanes and the influence of the used modifier on the tribological properties is presented. The modification procedure efficiency, surface structure and morphology were characterized by secondary ion mass spectrometry, infrared spectroscopy and atomic force microscopy. The effectiveness of modification of the titanium surface was monitored by the measurement of the wetting contact angle and the surface free energy. The increase of surface hydrophobicity was observed upon the modification by increasing the wetting contact angle and reducing the surface free energy. The tribological performance of various perfluoroalkylsilanes films on the titanium surface was investigated in mili- and nano-newton load ranges. Dependence of the adhesive force and coefficient of friction values obtained in nano- and micro-scale on fluoroalkyl chain length was observed. Nano- and micro-tribological measurements show that titanium modified by fluoroalkylsilanes has lower adhesion and coefficient of friction than unmodified one. The investigation also indicates a decrease of the friction coefficient with increasing fluoric alkyl chain length. It was found that the titanium modified by fluoroalkylsilanes with longer alkyl chains is a prime candidate for practical use as a lubricant in biomedical and electronic applications. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Monolayers Vapour deposition Atomic force microscopy Adhesion Friction Tribology
1. Introduction Titanium and its alloys have been extensively studied in the recent years due to their useful electrical and optical properties [1,2], such as a high refractive index [3], high dielectrical permittivity [4], semiconductivity [5], superior biocompatibility and corrosion resistance [6]. Because of the mentioned properties, these materials are used in many areas for example: in devices [7,8], biomaterials [9,10], optical cells, highly efficient catalysts and
* Tel.: þ48 42 6355836; fax: þ48 42 6355832. E-mail address: [email protected]. 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.07.017
implants [11,12], cardiovascular application [13,14]. Some limitations in the exploitation of devices are large adhesion, friction and wear. In the case of medical applications the following problems present difficulties: corrosion degradation in humid environment, inflammatory reaction on implant surface and wear resistance. The solution of these problems is to use self-assembled monolayers (SAMs) as protective coatings and lubricants [15,16]. Self-assembled monolayers are ordered molecular assemblies formed by the chemical adsorption of active molecules on a solid surface. Monolayers are composed of molecules which consist of a head group that chemisorbs onto a substrate, a tail group that interacts with the outer surface of the film and a backbone chain group that connects the head and tail groups [17,18]. The interactions
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between backbone chain groups of different molecules, such as van der Waals forces or hydrogen bonding, contribute to the films stability and formation. The stability of SAMs on metal oxides is also determined by covalent bonds between molecules and the surface, and cross-links between neighboring molecules consisting of three or two reactive atoms in the head group [19,20]. In recent research, the properties of SAMs, such as tiols, phosphonic acids and silanes, deposited on different substrates (gold, alumina, silicon, copper) have been studied [21e25]. One of the best tribological properties present fluoroalkylsilanes [26,27]. These compounds have low values of surface free energy, adhesive force, coefficient of friction, high thermal stability and contact angle values, and are promising alternative for overcoming the limitations of electromechanical devices’ lifetime [28,29]. Mayer et al. used fluoroalkylsilanes to control adhesion in microelectromechanical systems [30], Kushmerick et al. showed that these compounds can play a role as antistiction layers in micromachines [31]. Much interest has been also attracted to the investigations of electrochemical or biological properties of the SAMs on metal electrodes and the fabrication of electrochemical sensors or biosensors for the detection of chemicals or biomolecules [32e34]. In this study, the formation of fluoroalkylsilanes with different chain length with ten and three carbon atoms, such as 1H,1H,2H,2Hperfluorodecyltrichlorosilane (FDTS) and (3,3,3-trifluoropropyl) trichlorosilane (FPTS), on titanium surface was investigated. The properties of the fluoroalkylsilanes films deposited by the gas-phase method were characterized using time of flight secondary ion mass spectrometry (ToF-SIMS) and Fourier transform infrared (FT-IR) spectroscopy techniques. The influence of chain length on film tribological properties was also presented. The hydrophobicity, surface free energy, adhesive force and coefficient of friction values were obtained using static contact angle (SCAs) measurements, atomic force microscopy (AFM) and microtribometer. This paper reports the improvement of the frictional properties of titanium substrate modified by fluoroalkylsilanes. While these films have found widespread applications for example in MEMS technology, their tribological properties in both nano- and microscale for the titanium substrate have been presented for the first time. The investigations presented in this work are important because of the constantly growing demand for compounds acting as lubricants in miniaturized systems. 2. Experimental 2.1. Sample preparation Silicon pieces were cut from a commercial p-type Si wafer and cleaned using ethyl alcohol, deionized water and dry argon (Ar) gas. The substrates of pure (100) silicon pieces were attached to oneaxis rotary sample holder in a vertical geometry at magnetron sputtering chamber. After the attachment of the substrates to the sample holder the air from vacuum chamber was evacuated to the pressure below 103 Pa. Before ion-cleaning the chamber was heated with use of resistance heater during the period of 1 h and cooled down for 3 h. During argon ions cleaning and titanium coating deposition the substrates were rotated with radial velocity of 0.13 p rad s1. During 30 min of sputtering of the samples surfaces, a negative bias of the samples was increased from 0 to 2 kV. Titanium coating deposition started 10 min after argon ion cleaning. Argon pressure during magnetron sputtering of pure titanium target was 0.37 Pa. Mean power was 1 kW with 1.2 A magnetron current. Sample bias in pulsed mode (160 kHz base frequency, 4 kHz modulation frequency and 100% duty cycle) during deposition was equal to 100 V. The thickness of the prepared unmodified titanium was 100 nm.
499
The next preparation step was the use of oxygen plasma to activate the surface before modification by compounds and to increase the amount of hydroxyl ions which are necessary for the reaction between fluoroalkylsilane compound and the substrate. 2.2. Modification procedure The modification of the titanium substrates was performed with FDTS and FPTS precursors. These chemicals were chosen to compare the effect of chemical structure on tribological performance at the micro- and nano-scale. The effects of carbon chain length as well as the number of fluoride atoms in the chain were investigated. All precursors were ordered from the ABCR, GmbH & Co. KG, Karlsruhe. The titanium surface activated by oxygen plasma was placed into a vapor phase deposition system and kept under low pressure (0.1 Pa) for 30 min. Then the sample was kept in the modifier vapor from 7 min to 2 h at room temperature and finally outgassed at low pressure for 1 h at 40 C to remove any of physisorbed and unreacted molecules [22,35]. The samples were removed from the vacuum chamber and transferred into a vacuum desiccator until characterization. Static contact angle was measured to determine the degree of the hydrophobicity. Based on the measured contact angle, optimum deposition time for FDTS and FPTS modifiers were selected [36]. The thicknesses of the deposited layers of FDTS and FPTS were 1.88 nm and 2.55 nm, respectively. These data were obtained from the ellipsometry measurements on a J.A. Woollam V-VASE ellipsometer. In order to obtain an accurate determination of the film thickness, a fit algorithm was used to first determine the optical constants of the deposited metal layer on the silicon substrate using the values of real part of the index of refraction and extinction coefficient of polycrystalline titanium. The fitted optical constants of the metal layer were assumed to remain constant as the fluoroalkylsilane layers were adsorbed on the top of it. To calculate the thickness of the adsorbed fluoroalkylsilane layer, a Cauchy dispersion model was used. 2.3. Measurement techniques 2.3.1. Contact angle and surface free energy measurements SCA were measured in air by a sessile-drop method using a contact angle goniometer. A drop of proper liquid (water, glycerine and diiodomethane) was deposited on the substrate with the use of microsyringe. The image of the droplet was obtained by a digital camera. The surface free energies were calculated using the Van OsseChaudhuryeGood method [37]. 2.3.2. ToF-SIMS investigations The secondary ions mass spectra were recorded with a ToF-SIMS IV mass spectrometer manufactured by Ion-ToF GmbH, Muenster, Germany. The instrument is equipped with Bi liquid metal ion gun and high resolution time of flight mass analyzer. Secondary ion mass spectra were recorded from an approximately 100 mm 100 mm area of the spot surface. During measurement the analyzed area was irradiated with the pulses of 25 keV Bi3 þ ions at 10 kHz repetition rate and an average ion current 0.14 pA. The analysis time was 100 s for both positive and negative secondary ions giving an ion dose below static limit of 1013 ions cm2. Secondary ions emitted from the bombarded surface were mass separated and counted in time of flight analyzer [38]. 2.3.3. FT-IR measurements Fourier transform infrared spectroscopy was performed using a BioRad 175C spectrometer equipped with a liquid nitrogencooled mercuryecadmiumetelluride (MCT) detector and grazing angle attenuated total reflectance (GATR) accessory manufactured
500
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by Harrick Scientific Products Inc. The GATR accessory used 65 grazing angle and a Ge ATR crystal for studies of adsorbed monolayers on the substrates. Each spectrum was taken by collecting 1000 scans (with 1000 scans of pure silicon as background) at a resolution of 8 cm1 in the spectral range 750 cm1e4000 cm1 in dry air flow. 2.3.4. Nanotribological characterization The adhesion and friction measurements were performed with a NT-MDT Solver P47 AFM apparatus operating in air under ambient conditions with a rectangular silicon nitride cantilever (the spring constant calibrated by the Sader method, k ¼ 0.72 N m1) [39,40]. The radius of curvature of the used tip was less than 20 nm. The adhesive force was obtained from the forceedistance curve after reckoning the pull-off force [41e44]. These data were arising from different points on the surface. The presented data for the adhesion force are the averaged data. The coefficient of friction was obtained from the slope of the friction force versus normal force plots. The friction force was calibrated using the method described by Ruan and Bhushan [45]. The technical parameters of measurements were the following: applied loads ranged from 5 to 100 nN, scan rate of 1 Hz, scan size of 1 mm 1 mm. Each measurement was repeated three times on different places of the sample surface. The average data are presented. Images were typically obtained in the tapping mode using silicon nitride cantilever (MikroMasch, NSC/Si3N4/AiBS), with spring constant k ¼ 14 N m1 and resonance frequency n ¼ 260 kHz. The typical scan area was fixed at 2 mm 2 mm. 2.3.5. Microtribometer measurements The micro-tribological investigations were carried out by a reciprocating ball-on-flat microtribometer [46]. Measurements were performed with the use of silicon nitride sphere with diameter of 5 mm over a normal force range from 30 to 80 mN. The ball moved parallel with respect to the sample surface with velocity of 25 mm min1 and traveling distance of 5 mm. The root mean square (rms) surface roughness of the ball was from 5 to 6 nm and the hardness was 14.7 GPa. The microtribometer was operating in ambient conditions (temperature and humidity). Measurements were performed at a technical dry friction and repeated three times in three at different places on the sample surface [28]. The adhesive force was calculated from the negative horizontal intercept of the friction force versus applied load curve (the negative applied load value where the friction force is zero) [47]. The typical curves are presented in Fig. 5a.
and in consequence to the surface. In Fig. 1 the contact angle is plotted against the deposition time for different chain length compounds (FDTS, FPTS), for time periods ranging from 7 min to 2 h. Contact angle values monitored for titanium surfaces that had been exposed to silanes vapors are highest for deposition time of 15 min for FDTS and 30 min for FPTS. These data show the sensitivity of the silanes formation process to the vapors exposure time and reveal the importance of different exposure time for the formation of FDTS and FPTS films which had different chain lengths. For further investigations was used surfaces which possessed deposition time of 15 and 30 min for FDTS and FPTS respectively, i.e. which were characterized by the highest contact angles values. An example of water droplets on FDTS and FPTS modified titanium surfaces obtained using optimum time deposition parameter and for the unmodified titanium surface is shown in Fig. 2. The unmodified surface is hydrophilic, which is observed in lowering of the water contact angle value in comparison with modified surface. After modification the surface becomes hydrophobic and the contact angle increases. An increase of the contact angle is a direct evidence of the presence of modifier molecules on the titanium surface. The surface free energy was calculated using the method of van Oss, Chaudhury and Good, by measurements of contact angles of two polar liquids (water, glycerol) and one apolar liquid (diiodomethane) [50]. This method yields the surface tension g as a summation of the dispersive component gLW based on LifshitzeVan der Waals interactions and the acidebase component gAB based on hydrogen bonding interactions. The acidebase term consists of the asymmetric components gþ for acid (electron acceptor) and g for base (electron donor) interactions. This term is expressed as
gAB ¼ 2 gþ g
1=2
The connection between the contact angle Q and components of the surface free energy is given by the YoungeDupre’ equation
1=2 1=2 1=2 ð1 þ cosQÞgL ¼ 2 gLW gLW þ2 gþ g þ2 g gþ L L L where: gLW dispersive component, gþ L L asymmetric component for acid and gL asymmetric component for base specific to the measurement liquid. Using this equation, values of the surface free energy were obtained for all films (Table 1). For titanium the surface free energy is
3. Results and discussion 3.1. Optimization of deposition time parameters and surface free energy studies Previous studies showed the relation between adhesive forces, coefficient of friction and contact angle values [28,46]. Investigations used SCAs for determining the deposition factors, such as immersion time, concentration of molecules or gas pressure, were performed by Bhushan, Liakos and Hoertz [36,48,49]. In the present studies water SCAs were used to determine time of the molecules vapor deposition on titanium. SCAs technique is sensitive to the order of the monolayer as a result of the higher energy of the eCF2e or eCH2e bonds (which are exposed when chains are disordered) compared with eCF3 or eCH3 group. The difference in values obtained for SCA results from the fact that in well-ordered monolayers the terminal group of the chain is oriented outwards, reducing access of the water drop to the chain
Fig. 1. Static contact angle values as a function of deposition time.
M. Cichomski / Materials Chemistry and Physics 136 (2012) 498e504
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Fig. 2. An example of water droplets on the unmodified Ti surface (a), Ti surface after FDTS modification (b) and after FPTS modification (c).
ski et al. [51] show that nonporous titania ob53.3 mJ m2. Piwon tained by the solegel method exhibits the surface free energy of about 68 mJ m2. The modification of this surface by FDTS and FPTS lowers the surface free energy by 40% compared to the surface free energy before modification [51]. Lee et al. [52] and Kubies et al. [53] present that the surface free energy values for titania film deposited on silicon are 64 mJ m2 and 47 mJ m2, respectively. Differences between the obtained and literature values are due to different preparation methods of the titania substrate. Fluoroalkylsilanes modified Ti surfaces show lower surface free energies than the unmodified Ti surface. It can also be observed that the molecules carrying longer perfluorinated chain (FDTS) exhibit a higher degree of hydrophobicity and lower values of the surface free energy than the molecules having short chain (FPTS). It was found that the calculated values of gΑΒ are consistent for films with longer chain and are dominant component in the obtained values of the surface free energy. For FDTS the acidebase component was 0.4 mJ m2. This value is considerably lower in comparison with the gΑΒ component for FPTS modified and unmodified surfaces. The obtained data show that the dominant component is the dispersive one. The results for perfluoroalkylsilane modified Ti surface showed that the water contact angle increased with a decrease in the surface energy, which is consistent with the general theory [37]. The differences in the obtained values, when we compare the unmodified Ti and fluoroalkylsilanes modified surfaces, are connected with eCF3 tail group ability in increasing of the contact angles. Trifluoromethyl group will give rise to a strong dipole of perfluoroalkylsilanes, the formation of films from CF3(CF2)n(CH2)2SiCl3 will generate an interface in which these strong dipoles are oriented near the contacting liquids. The interaction between the contacting liquids and these dipoles should give rise to a positive contribution to the surface free energy for the trifluoromethyl terminated films. Hydrogen bonding between the eCF3 tail groups and the hydroxyl groups of glycerol and water should also give rise to a positive contribution to the surface free energy for the CF3-terminated films. The presence of dipole interactions and/or hydrogen bonding can be used to explain the
Table 1 Water contact angle and surface free energy values of fluoroalkylsilanes deposited on titanium surface. Water contact Surface free Dispersive Acidebase angle (degree) energy (mJ m2) component component gAB (mJ m2) gLW (mJ m2) Unmodified Ti 73 2 Ti after FDTS 105 2 modification Ti after FPTS 95 2 modification
53.3 4.0 25.2 3.4
14.9 3.5 0.4 1.1
38.4 1.8 24.8 3.1
32.2 2.7
9.8 1.8
22.4 1.7
enhanced wettability of the perfluorinated films toward water and glycerol. In this case FDTS modified surface exhibits almost no contribution from the acidebase component gAB. It seems that the dipole interactions are predominantly responsible for the enhanced wettability. The difference in static contact angle values for FDTS and FPTS films on Ti is likely due to the layer penetration by water and interaction with the surfaces in the case of the films with shorter fluoroalkyl chain. In the case of the films with longer fluoroalkyl chain, water does not penetrate the Ti surface, so SCA values are higher. This is related to the formation of agglomerates during vapor deposition process in the case of FPTS layer. The mentioned effect will be discussed in more detail in the next section. 3.2. Surface characterization by ToF-SIMS and FT-IR measurements ToF-SIMS is a tool for obtaining chemical information (molecular and structural) on surfaces. ToF-SIMS has been successfully applied to studies of SAM systems such as thiols on Au, silanes and phosphonic acids on various oxides of metals [54e59]. This method was also used to monitor hydrolysis and condensation of 3-mercaptopropyltrimethoxysilane [22] and to analyze this compound modified by FDTS [26]. In the case of the present investigations ToF-SIMS has been used to prove the correctness of perfluoroalkylsilanes modification (Table 2). ToF-SIMS analysis shows the presence of CF3, CF, F and Si2O ions after Ti modification. Moreover, a proper modification of the titanium surface by silanes is evidenced by the decrease of the number of fragmented ions TiOH and TiO. Structure comparison for both fluoroalkylsilanes indicates that in the case of FPTS the number of fragmented ions is lower. In the case of FDTS modified Ti surface the results show a very strong increase in the amount of CF3 and CF fragmented ions compared to FPTS modification. It stems from the carbon chain length as well as from various types of polymerization that occur on the surface of titanium. In the case of FDTS compound horizontal polymerization occurs while for FPTS vertical polymerization takes place. The occurrence of polymerizations seems to be supported by the decrease number of TiOH and TiO ions (coverage of the unmodified surface). Furthermore, the appearance of Si2O ions confirms siloxane linkages formation between the modifier molecules in the case of FDTS (horizontal polymerization). The presence of polymerization in the vertical direction in the case of FPTS is evidenced by the increased number of OH ions. This happens because after exposure to the ambient atmosphere unreacted SieCl bonds undergo hydrolysis to form SieOH groups. The structure of fluoroalkylsilanes (FDTS and FPTS) on titanium was also characterized by infrared spectroscopy (Fig. 3). The peaks at 780 and 889 cml can be assigned to TieOeTi bond vibrations [60] and are present for the unmodified surface and also for both
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Table 2 ToF-SIMS secondary fragmentation ions analysis of fluoroalkylsilanes on titanium surface: before modification, after FDTS and FPTS modification. All values presented in the table are the number of counts divided by 103. Fragmented ions
CF3 CF2 F CF Si2O Si OH TiOH TiO
Positive ions
Negative ions
Before modification
After FDTS modification
After FPTS modification
Before modification
After FDTS modification
After FPTS modification
e e e e e e e 110.2 301.3
728 e e 839 e 937 e 14 13.6
2.7 e e 2.8 e 10 e 69 213
e e e e e e 477.2 e e
e 4.7 2064.4 28.5 120 e 173.6 e e
e e 133.2 0.6 3.4 e 772.6 e e
FDTS/Ti and FPTS/Ti surfaces. In the case of FPTS the mentioned peak at 780 cm1 is shifted to lower wavenumbers and is lower what is due to higher thickness of forming layer as the effect of vertical polymerization [61]. For the FDTS modified Ti surface the peak at 1065 cm1 due to SieOeSi asymmetric stretches reflects the presence of a siloxane network. FPTS also shows siloxane peaks at 1027 cm1 and 1069 cm1. The absence of the eOH peaks in FDTS and FPTS above 3000 cm1 suggests that the surface is covered by close packed silane films. Moreover, FDTS shows peaks due to asymmetric and symmetric stretches of CeF for eCF2 and eCF3 groups in the range of 1150e1366 cm1. For FPTS peaks in the range of 1216e1374 cm1 are due to asymmetric and symmetric stretches of CeF bonds for trifluoromethyl group. Peak at 1447 cm1 for FPTS is due to CeH asymmetric stretches. The obtained results confirm the presence of siloxane cross-links for compounds containing three hydrolyzable, reactive groups in the molecule. 3.3. Tribological properties At the first step, the quality of fluoroalkylsilane films formed on the titanium substrate was probed (Fig. 4). Based on AFM topography images root mean square (rms), the surface roughness Rq was defined and calculated as
" Rq ¼
N X
zi zaverage
2
#1=2 =N
i¼1
Fig. 3. FT-IR spectra obtained for the unmodified Ti surface (a), FDTS modified Ti surface (b) and FPTS modified Ti surface (c) in the spectral range of 750e4000 cm1 (left) and 750e1500 cm1 region zoom (right).
where zaverage is the average of the height values within a given area, zi is the individual height value and N is the number of pixels within the given area. The surface roughness for the unmodified Ti surface was 1.7 nm. The surface roughness values for FDTS and FPTS, were measured as 2.6 nm and 1.9 nm, respectively. The obtained rms surface roughness data suggest that fluoroalkylsilane layers have an effect on the surface roughness, and consequently have an effect on the hydrophobicity and surface free energy. With increasing silanes chain length, the surface roughness and water contact angle increase, and the surface free energy decreases. In other words we can say that the factors determining the change in contact angle values, and in consequence in surface free energy and coefficient of friction, are the surface roughness and chemical composition. Fig. 5 shows the coefficient of friction and adhesive force measured using microtribometer and AFM for fluoroalkylsilanes deposited on Ti substrates. The adhesion and coefficient of friction in both nano- and micro-scale were always higher for the unmodified surface compared to those for fluoroalkylsilanes modified Ti surfaces. Bhushan and Cichomski [36] show that if the thickness of the layer is less than 5 nm, the substrate affects the obtained value of the adhesive force. In the present studies, the thickness of fluoroalkylsilanes layers is about 2e3 nm, thus titanium substrate has an influence on the adhesion force values. However, the contribution of the titanium substrate to the obtained values of adhesion force is difficult to determine because we do not compare two or more different substrates. Among the fluoroalkylsilanes films, FDTS showed a lower adhesive force and coefficient of friction than FPTS. The observed values of coefficient of friction for FPTS and FDTS reveal the similar trend to the results obtained for the surface free energy. Ti surface coated by fluoroalkylsilanes is more hydrophobic, has a lower surface free energy and adhesion, and in consequence a lower coefficient of friction in comparison with the unmodified surface. As we can see in Fig. 5 and Table 1 an impact on the coefficient of friction value have the dispersive and acidebase components of the surface free energy. The dispersive component plays dominant role due to the higher values compared to acidebase component. This is likely to result from the fact that friction processes in micro- and nano-scale are determined by capillary effects, electrostatic interactions and adhesion. In the presented investigations the adhesion force arises mainly from capillary forces [62e64]. The capillary forces depend on surface hydrophilicity, so for fluoroalkylsilanes modified Ti surface they become smaller because FDTS and FPTS form films with surface hydrophobic properties. Different tribological properties for Ti coated with FDTS and FPTS result from the difference between chain length. The relationship between chain length and surface properties, such as hydrophobicity, adhesion and friction behavior, confirmed investigations performed for alkylsilanes [65,66]. Trifunctional silanes
M. Cichomski / Materials Chemistry and Physics 136 (2012) 498e504
503
Fig. 4. AFM images of titanium (a), FDTS modified titanium (b) and FPTS modified titanium surface (c).
with short chain compared to their longer chain analogs, are more reactive and are capable of polymerizing which gives a lot of possible surface structures, formation of gauche defects and disorder in the formed layer. This disorder has an influence on the frictional measurements.
The obtained values in the nano-scale are much lower than those in the micro-scale because the friction force is a sum of the adhesional component and friction force component consumed on plastic deformation which are scale-dependent. This is connected with different counterpart radii, contact stresses and shear strength
Fig. 5. Coefficients of friction and corresponding components of the surface free energy, adhesive forces and friction forces vs normal loads curves in (a) micro-scale and (b) nanoscale.
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during sliding [67,68]. In the nano-scale the difference in coefficient of friction values also results from the pressure exerted by the tip, which more easily deforms the monolayer formed by reaction between the substrate and shorter chain molecules than the longer ones. Monolayers deformation during tip sliding on the surface is the reason for an increase of the shear force, and in consequence produces higher friction force. In nano-scale 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 [69]. The contact stress at nano-scale, in spite of small tip radii, generally does not exceed the sample hardness that minimizes plastic deformation. Average contact stresses in micro-contacts are generally lower than these in AFM contacts, however, a large number of asperities come into contact that go through some plastic deformation [41,70]. In nano-scale studies, the indentation hardness is higher than that at the micro-scale due to the small contact areas and very low loads used in AFM conditions. The lack of plastic deformation and improved mechanical properties reduce the degree of wear and friction. 4. Conclusions In this paper, gas-phase preparation method of FDTS and FPTS films on Ti surface was presented. The films structure and tribological properties have been studied using FT-IR spectroscopy, ToFSIMS, AFM, microtribometer and SCA measurements. The results obtained using contact angle, ToF-SIMS and FT-IR techniques proved the correctness of the gas-phase modification method. From studies performed in this paper, it can be concluded that the chain length has an impact on film wettability, surface free energy, coefficient of friction and adhesive force. Fluoroalkylsilanes with three carbon atoms in chain compared to ten carbon atoms in chain compounds, are more reactive and are capable of vertical polymerizing what confirmed FT-IR and ToF-SIMS investigations. Shorter chain molecules provided weaker interactions between the backbone chain groups, larger number of structure defects, and in consequence higher values of the adhesive force and coefficient of friction in nano- and micro-scale. Both silanes FDTS and FPTS provided hydrophobic properties of the modified surface, lower values of the surface free energy and improved titanium tribological properties. Titanium surface modification with the aid of fluoroalkylsilanes may find application in tribological systems where anti-frictional and anti-adhesion properties play a crucial role. Acknowledgments This work was supported by the Polish Ministry of Science and Higher Education within Research Grant No. NN 50755153. References [1] A. Ferreira da Silva, I. Pepe, J.L. Gole, S.A. Tomas, R. Palomino, W.M. de Azevedo, E.F. da Silva Jr., R. Ahuja, C. Persson, Appl. Surf. Sci. 252 (2006) 5365. [2] H.P. Deshmukh, P.S. Shinde, P.S. Patil, Mater. Sci. Eng. B 130 (2006) 220. [3] Y. Liu, Ch. Lu, M. Li, L. Zhang, B. Yang, Colloid Surf. A: Physicochem. Eng. Aspects 328 (2008) 67. [4] F. Jin, H. Tong, L. Shen, K. Wang, P.K. Chu, Mater. Chem. Phys. 100 (2006) 31. [5] M.K. Nowotny, P. Bogdanoff, T. Dittrich, S. Fiechter, A. Fujishima, H. Tributsch, Mater. Lett. 64 (2010) 928. [6] E. Eisenbarth, D. Velten, M.M. Uller, R. Thull, J. Breme, Biomaterials 25 (2004) 5705. [7] S. Kataria, N. Kumar, S. Dash, A.K. Tyagi, Wear 269 (2010) 797. [8] S.J. Bull, Solid State Phenom. 159 (2010) 11. [9] T.R. Rautray, R. Narayanan, K.-H. Kim, Prog. Mater. Sci. 56 (2011) 1137. [10] V. Biehl, T. Wack, S. Winter, U.T. Seyfert, J. Breme, Biomol. Eng. 19 (2002) 97.
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