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The Effect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological Features of Created Self-Assembled Monolayers

The Effect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological... materials Article The E ect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological Features of Created Self-Assembled Monolayers 1 , 1 1 2 Michał Cichomski * , Ewelina Borkowska , Milena Prowizor , Damian Batory , 3 3 Anna Jedrzejczak and Mariusz Dudek Department of Materials Technology and Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 163, 90-236 Lodz, Poland; ewelina.bystrzycka@chemia.uni.lodz.pl (E.B.); milena.prowizor@chemia.uni.lodz.pl (M.P.) Department of Vehicles and Fundamentals of Machine Design, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; damian.batory@p.lodz.pl Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; anna.jedrzejczak@p.lodz.pl (A.J.); mariusz.dudek@p.lodz.pl (M.D.) * Correspondence: michal.cichomski@chemia.uni.lodz.pl; Tel.: +48-42-635-5836 Received: 23 June 2020; Accepted: 27 July 2020; Published: 29 July 2020 Abstract: The presented article shows the influence of concentration of perfluoroalkylsilanes in solutions on tribological properties of self-assembled monolayers (SAMs) deposited on three surfaces with different silicon content in the millinewton load range. The SAMs were created using the liquid phase deposition (LPD) method with 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) and (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) solutions, for which viscosity and surface tension were estimated. Deposited layers were analyzed in terms of thickness, coverage, wettability, structure and coefficient of friction. The obtained results demonstrated that SAMs created on the silicon-incorporated diamond-like carbon (Si-DLC) coatings possess the best microtribological properties. Systems composed of perfluoroalkylsilane SAM structures deposited on Si-DLC coatings are highly promising candidates as material for microelectromechanical applications. Keywords: perfluoroalkylsilanes; viscosity; surface tension; self-assembled monolayers; thickness; microtribological properties 1. Introduction In recent years, nanotechnology has become one of the most dynamic developing fields of modern science [1–3], which allows structures to be obtained at the nanometer scale and new materials to be obtained with specific properties and consequently application of those materials for the miniaturization of devices [3–5]. The construction of devices with contact surfaces of the order of micro- and even nanometers resulted in the need to search for new nanomaterials as well as substances to protect their surfaces. These substances should have excellent tribological properties, i.e., low values of adhesion forces and friction coefficient, as well as the ability to reduce surface wear. Self-assembled monolayers (SAMs) made of perfluoroalkylsilanes exhibit such properties. In addition, the choice of perfluoroalkylsilanes for surface modification is due to their ability to form a siloxane network by which the formed layer is well ordered and resistant against various environmental factors. The uniform surface coverage, without any defects, reduces the contact area between two elements of tribological system which results in lower friction and wear. As literature data indicate, SAMs of perfluoroalkylsilanes protect the surface against corrosion and ensure their long functionality. Materials 2020, 13, 3357; doi:10.3390/ma13153357 www.mdpi.com/journal/materials Materials 2020, 13, 3357 2 of 15 Among the available methods of production of self-assembled monolayers (SAMs), the most e ective is liquid phase deposition (LPD). These structures are organic molecular assemblies which spontaneously form dense and ordered layers with a thickness of single nanometers on appropriate substrates [6–9]. Depending on the molecular design, SAMs provide a way to tailor surface properties such as wetting [10,11] adhesion [12,13], friction [14–16] and antimicrobial [17] and corrosion properties [18–20]. Their high attraction is also driven by the simple preparation procedure, the reproducible film quality and stability of the monolayer. Therefore, SAMs are presently very interesting for scientists in the nanotechnology area [1,6]. So far, most of the fundamental studies have been performed with alkane [21], aromatic thiols [22] and dithiols [23]. Currently, more and more interest from the application point of view in microelectromechanical systems (MEMS) is focused on compounds containing silicon in their structure [24,25]. In organosilicon structures, the bonds between silicon and carbon compared to carbon-carbon bonds are longer (186 pm compared to 154 pm) and characterized by weaker bond dissociation energy (451 kJ/mol vs. 607 kJ/mol). Moreover, due to higher electronegativity of carbon in comparison to silicon, the C–C and Si–C bonds di er in polarization (for C–Si equal to 2.55 vs. for C–C equal to 1.90). The result of these properties are Si–O bonds in chemical reactions, which are much stronger compared to C–O bonds (809 kJ/mol versus 538 kJ/mol) [26]. Another positive e ect of occurrence of energetically advantageous Si–O bonds is creation of perfluoroalkylsilanes network consisting of Si–O–Si linkages during the modification process. Additionally, surface modification by perfluoroalkylsilanes guarantees more satisfied physical-chemical properties than in the case of compounds that do not contain silicon in their structure [24,25,27]. Silicon plays a very important role in creating a self-assembled monolayer in the case of silicon-incorporated diamond-like carbon (Si-DLC) coatings. Our earlier studies [28,29] indicate that the more silicon that is in the DLC structure, the higher the anity of fluoroalkylsilanes to the surface. What is more, using Si-DLC in MEMS/NEMS systems is beneficial since the addition of silicon to the DLC coatings changes their surface roughness and adhesion forces, reduces compressive stress and improves wear resistance. Furthermore, the use of silicon may delay the graphitization process and thus increase the durability of DLC at elevated temperatures [30,31]. Perfluoroalkylchlorosilanes are representative candidates of organosilicon compounds used as modifiers to improve mainly the tribological features. Organic layers built of spontaneously adsorbed molecules on investigated surfaces exhibited growing potential as protective agents, which can be used in reducing friction, adhesion and wear, which was confirmed by other research [32,33]. Obtained results indicate their application potential in electromechanical systems as components of sensors and actuators used in many fields of technology and medicine [34]. In the presented study, the primary destination was to reveal the influence of the concentration of two kinds of alkyl chain silane compounds with different lengths in toluene solvent on their viscosity and surface tension and finally on the microtribological properties of created SAMs. The modifier/surface systems were created with use of two kinds of compounds: 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) and (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS). The modifications were performed on three surfaces: diamond-like carbon coating (DLC), silicon-incorporated diamond-like carbon coating (Si-DLC) and silicon. The surfaces were selected in such a way as to compare the influence of silicon content (the type of surfaces) on the effectiveness of modification. For studies with Si-DLC coating/surface systems, coatings with high concentrations of silicon were selected. We also for the first time presented the use of viscosity and surface tension measurement results to optimize the concentration of modifier solutions proper for the modification. The chemical composition of manufactured structures was obtained using Fourier transformed infrared spectroscopy (FTIR) analysis which allowed two important parameters to be determined: information about the coverage of the investigated surfaces and the thickness of the fabricated SAMs. The obtained thicknesses were confirmed using ellipsometry measurements. These parameters testified to the quality of the produced layers and allowed their physical-chemical properties to be determined. Materials 2020, 13, 3357 3 of 15 The ultrathin assembled monolayers produced using the LPD method were investigated in terms of wettability and tribological behavior in the millinewton load range. These tribological studies fill the gap between measurement results obtained in the nanoscale and macroscale region usually performed by other authors. 2. Materials and Methods 2.1. Materials As substrates for the creation of self-assembled monolayers, DLC and Si-DLC coatings deposited using the radio frequency plasma enhanced chemical vapor deposition (RF PECVD) method were selected. The coatings were synthesized on a single crystal p-type Si (100) wafer (Cemat Silicon S.A, Warsaw, Poland), which was also used as the model material. The RF PECVD process and physicochemical properties of carbon base coatings were described in detail in the publication of Jedrzejczak et al. [35]. For the investigations, DLC and Si-DLC coatings containing 32at.% of silicon, deposited at 600V negative substrate bias, were selected. 2.2. Pre-Treatment of Substrates Initially, surfaces were cut into 1.0 1.0 cm substrates and cleaned with ethanol (analytical grade, POCH S.A., Gliwice, Poland) in an ultrasonic bath to remove organic contaminents. Afterwards, the investigated surfaces were exposed to oxygen radio frequency (RF) plasma (Zepto plasma system, 40 Hz, 100 W, Diener Electronic, Ebhausen, Germany, 15 min at 50 W). The high hydrophilic character of the substrates after the treatment using plasma indicates the formation of a significant amount of polar surface silanols important to manufacture the covalent attachment of modifiers to the investigated surfaces [36]. 2.3. Preparation of Self-Assembled Perfluoroalkylsilane Layers The self-assembled monolayers were created from silane compounds FPTS and FDTS, on three (Si, DLC, and Si-DLC) surfaces, using the liquid phase deposition (LPD) method. The compounds were purchased from ABCR, GmbH & Co. KG, Karlsruhe, Germany. The FPTS and FDTS solutions at five di erent concentrations, 0.02%, 0.05%, 0.10%, 0.20% and 0.50% (wt%), were prepared by dissolving the compounds in toluene at room temperature under ambient conditions. Due to di erences in reactivity of both the perfluoroalkylsilanes, the modification was carried out in a time of 60 min for FDTS and 30 min for FPTS. Prior to the creation of SAMs, the FPTS and FDTS solutions were characterized. The surface tension investigations were performed using the Du Noüy ring method using a KSV Sigma703D (KSV Instruments Ltd, Helsinki, Finland) at 25 C. The force referred to as the wetted length acting on a ring as a result of the tension of the withdrawn liquid lamella when moving the ring from one phase to another is measured in this method. The reported values of the surface tension are averaged values of at least three measurements. The measured surface tension of the toluene used for the preparation of the perfluoroalkylsilanes solutions was  = 28.11 mN/m. The viscosity measurements were carried out using an Ostwald viscometer (Sibata, Nakane Soka, Japan) measuring the time for 1 mL of the prepared perfluoroalkylsilanes solution and toluene to flow through the capillary under the influence of gravity. The ratio of the average flow time of the investigated solutions and solvent from at least three measurements in relative viscosity are presented. 2.4. Characterization of Surfaces and Prepared Layers The characterization of pure surfaces and deposited perfluoroalkylsilanes films was conducted using Fourier transformed infrared spectroscopy (FTIR) (Nicolet iS50 FTIR, Thermo Fisher Scientific, Waltham, MA, USA). The spectrophotometer with an ultra-high sensitive, low noise, linearized mercury-cadmium-telluride (MCT) detector equipped with VariGATR— grazing angle ATR accessory dedicated for analysis of thin layers was applied. All spectra were recorded by collecting 128 scans Materials 2020, 13, 3357 4 of 15 (128 scans of the pure substrate as a background was recorded) at 8 cm resolution, in the spectral range of 700–4000 cm in the flow of dry air. Pure investigated substrates were used for background spectra. The obtained data was analyzed using OMNIC 9 software from Thermo Scientific. The thickness of the manufactured layers was determined using variable-angle spectroscopic ellipsometry (VASE) with a J. A. Woollam V-VASE ellipsometer (J.A. Woollam Co, Lincoln, CA, USA). Data was collected in the wavelength range 260–1300 nm at a 70 angle of incidence. A Cauchy dispersion model with constant parameters of 1.45, 0.01 and 0.0 was used to calculate the thickness of ultra-thin perfluoroalkylsilane layers. Hydrophobicity of the substrates was determined using the static contact angle measurements obtained at room temperature using a Drop Shape Analyzer (DSA25 Kruss ˝ Goniometr, Kruss, ˝ Hamburg, Germany). Average water contact angle (WCA) values were obtained from measurements at five di erent locations on the specimen surface. The values of contact angles for water, diiodomethane and glycerin were determined by the use of the Advance program. On the basis of these investigations, surface free energy (SFE) and its components were calculated using the van Oss Chaudhury-Good method [37]. The friction tests of the investigated surfaces were determined with the use of a ball-on-flat tribometer (T-23, ITEE, Radom, Poland), working in the millinewton load range. The coecient of friction is the average value obtained by measuring the friction force as a function of the normal load curve. A silicon nitride (Si N ) ball, 5 mm in diameter, was used as a counterbody. The measurements 3 4 were performed in technical dry friction conditions and repeated three times in three di erent locations on the sample surface. All specimens and counterbody movements as well as the load applying mechanism system were computer-controlled using Lab-View software. The applied normal loads were in the range of 30–80 mN. The sliding speed was 25 mm min over the distance of 5 mm. The topography of the wear tracks after the tribological tests was analyzed using scanning electron microscopy (SEM) (Nova NanoSEM 450, FEI, Hillsboro, WA, USA). The root mean square (RMS) of the surface roughness of samples was investigated using atomic force microscopy (AFM-Solver P47) (NT-MDT, Moscow, Russia) operating in an air-conditioned environment at RT and a relative humidity of 40 5%. The RMS of the surfaces was obtained from the scan area 2 m 2 m. 3. Results and Discussion 3.1. E ect of Physicochemical Properties of Solutions of Perfluoroalkylsilanes on Wetting Properties of SAMs The creation of SAMs using the LPD technique was preceded by the viscosity and surface tension measurements of the solution of two perfluoroalkylsilane compounds with di erent lengths of alkyl chain as a function of their concentrations (Figure 1). Based on the obtained data it was observed that the relative viscosity of the silane compounds solution prepared in toluene initially rises with increasing concentration (maximum for FDTS achieved at 0.05% and for FPTS at 0.10%) and next decreases with increasing content of perfluoroalkylsilane compounds, while for surface tension measurements a reversed behavior was observed. It was noticed that the minimum values of this parameter corresponded to the maximum values of relative viscosity. These results are related to the appearance of critical concentration and interfacial tension [38,39]. Furthermore, the authors found that this activity was higher for smaller concentrations of linear fluorosurfactants, which affect the decrease in surface tension. A similar tendency for perfluoroalkylsilanes was confirmed in our measurements. In addition, the strong influence of van der Waals and electrostatic interactions between individual molecules was observed, which as a consequence influenced the lowest indications of surface tension and higher values of relative viscosity. In our studies, this phenomenon was observed for the solutions of perfluoroalkylsilanes with a concentration of 0.05% for FDTS and 0.10% for FPTS (indicating their higher activity), respectively. Materials 2020, 13, 3357 5 of 15 Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Figure 1. Influence of concentration of (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) and 1H, 1H, 2H, Figure 1. Influence of concentration of (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) and 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) compound on viscosity and surface tension of solutions. 2H-perfluorodecyltrichlorosilane (FDTS) compound on viscosity and surface tension of solutions. In These resul Figure 2 ar ts a e pr re rela esented ted the to the a water ppeara contactnangle ce of (WCA) critical concent and surface ration and free ener int gye(SFE) rfacial t of e SAMs nsion cr [3eated 8,39]. Furthermore, the on DLC, Si-DLC and authors Si substrates found tha using t thi the s ac above-described tivity was higher perfluor for smal oalkylsilanes ler concentra solutions. tions of For linethe ar f investigated luorosurfactsubstrate ants, which materials, affect t the he dec highest reaWCA se in s was urfobtained ace tension after . A FDTS simi modification lar tendency fo withr 0.05% perfluconcentration, oroalkylsilanes wa (126.8 s conf  2.0ir)med i for Si-DLC n our mea coating, surem (120.9 ents. In  2.0 ad )di for tion, the strong i Si surface and (115.1 nfluence of  2.0 v ) for an DLC der Wa coating, als an while d elect achievi rostatng ic int a low eract val ions ue bet of SFE. ween Theind results ividu described al molecule above s wa allow s observed, the conclusion which a that s a the consequence alterationsinfluenced th of wettabilitye lowest in can be connected dications of with the surfac fluctuations e tensionof and surface higher tension values of relative and viscosity of viscosity. In the solution our used. studies, t The highest his phenomenon was ob values of WCA occurring served for the at the solution solution s o concentrations f perfluoroalkylsilane used tos form with SAMs a concentra revealed tion theof highest 0.05% f relative or FDTS a viscosity nd 0 a.n10% d the for lowest FPTS surface (indicat tension ing th (Figur eir hiegher 1). For activity), SAMs obtained respectively. using FDTS, these values were observed at 1.35 of viscosity and 22.50 mN/m of surface In Figure 2 are presented the water contact angle (WCA) and surface free energy (SFE) of SAMs tension of the used solution. For comparison, FPTS layers revealed the most promising values of WCA created on (117.6 D LC, Si-D 2.0 forLC and Si su Si-DLC) at 1.24 bstrat ofes u viscosity sing thand e above-descr 27.11 mN/ im bed perf of surface luoroa tension lkylsilof anes the so solution. lutions. For the investigated substrate materials, the highest WCA was obtained after FDTS modification with Additionally, a critical concentration value, above which there were no significant changes in water contact 0.05% concen angle,tr viscosity ation, (12 and 6.8 ± 2. surface 0°) for Si tension -DLC measur coatin ements, g, (120.9 was ± 2.observed. 0°) for Si su This rface e and ect indicates (115.1 ± 2. the 0°) for DLC coating, while achieving a low value of SFE. The results described above allow the conclusion changes of van der Waals and electrostatic interactions between perfluoroalkylsilane molecules and pr tha omotes t the al the tera formation tions of wet of micelles tability ca or micelle-li n be connect ke aggr ed egates. with the fl It was uctua found tions of that the surfa same ce tension a e ect is also nd viscosity of the solution used. The highest values of WCA occurring at the solution concentrations observed for surfactants [38]. used to form SAMs revealed the highest relative viscosity and the lowest surface tension (Figure 1). 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces For SAMs obtained using FDTS, these values were observed at 1.35 of viscosity and 22.50 mN/m of surface tension of the used solution. For comparison, FPTS layers revealed the most promising values The substrates with di erent silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), of WCA (117.6 ± 2.0° for Si-DLC) at 1.24 of viscosity and 27.11 mN/m of surface tension of the solution. were characterized using a FTIR spectrometer (Figure 3). The presented results show di erences in Additionally, a critical concentration value, above which there were no significant changes in water the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR 1 1 1 contact angle, viscosity and surface tension measurements, was observed. This effect indicates the spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates changes of van der Waals and electrostatic interactions between perfluoroalkylsilane molecules and that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of promotes the formation of micelles or micelle-like aggregates. It was found that the same effect is also hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by observed for surfactants [38]. Erdemir [40]. However, the influence of hydrogen on these materials will be described in the section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms that both silicon and oxygen are incorporated in the structure of Si-DLC coating. Materials 2020, 13, x FOR PEER REVIEW 6 of 15 Materials 2020, 13, x FOR PEER REVIEW 6 of 15 Materials 2020, 13, 3357 6 of 15 (a) (b) Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers (SAMs) created on three surfaces with different contents of silicon as function a concentration for (a) FPTS and (b) FDTS. 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces The substrates with different silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), were characterized using a FTIR spectrometer (Figure 3). The presented results show differences in −1 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR −1 −1 −1 spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of (a) (b) hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by Erdemir [40]. However, the influence of hydrogen on these materials will be described in the Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers −1 section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned (SAMs) (SAMs) cr create eated d on on three three su surfaces rfaces with with di di ffe er rent contents ent contents oof f sili silicon con asas fufunction nction a concent a concentration ration forfor (a) −1 to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms (FP a) FPTS TS and and (b) F (b) DFDTS. TS. that both silicon and oxygen are incorporated in the structure of Si-DLC coating. 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces The substrates with different silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), were characterized using a FTIR spectrometer (Figure 3). The presented results show differences in −1 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR −1 −1 −1 spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by Erdemir [40]. However, the influence of hydrogen on these materials will be described in the −1 section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned −1 to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms that both silicon and oxygen are incorporated in the structure of Si-DLC coating. Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like carbon carbon (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. FTIR spectro FTIR spectroscopy scopy was also used was also used for the forcharacterization the characteriof zat the ion o surface f the surf after chemical ace aftemodification. r chemical modif It wasicat found ion. It that was foun the mostd t inter hatesting the most region int for erest observing ing region thefor fluor observing t oalkyl layer he fl structur uoroa elk due yl lay to the er 1 1 major changes in spectra was from 700 to 1500 cm and in the region 2000–3000 cm . The typical spectra in these regions for the Si-DLC coating modified using FPTS and FDTS in five di erent concentrations (0.02–0.50%) are presented in Figure 4. Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like carbon (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. FTIR spectroscopy was also used for the characterization of the surface after chemical modification. It was found that the most interesting region for observing the fluoroalkyl layer Materials 2020, 13, x FOR PEER REVIEW 7 of 15 −1 structure due to the major changes in spectra was from 700 to 1500 cm and in the region 2000–3000 −1 cm . The typical spectra in these regions for the Si-DLC coating modified using FPTS and FDTS in Materials 2020, 13, 3357 7 of 15 five different concentrations (0.02–0.50%) are presented in Figure 4. Figure 4. FTIR of typical spectra for (a) Si-DLC/FPTS and (b) Si-DLC/FDTS in the ranges of Figure 4. FTIR of typical spectra for (a) Si-DLC/FPTS and (b) Si-DLC/FDTS in the ranges of 1500–700 1 1 1 −1 −1 −1 1500–700 cm , 2800–2000 cm and 3000–2800 cm . cm , 2800–2000 cm and 3000–2800 cm . Absorption maxima in the range of 950–1100 cm −1 are characteristic for Si–O–Si stretching Absorption maxima in the range of 950–1100 cm are characteristic for Si–O–Si stretching vibrations, which indicate horizontal siloxane connections and promote the formation of covalently vibrations, which indicate horizontal siloxane connections and promote the formation of covalently attached monolayers, leading to a self-organization process [41]. It is also worth noting that the attached monolayers, leading to a self-organization process [41]. It is also worth noting that the obtained maxima observed at about 900 cm −1 are associated with a deformation of vibrations obtained obtained maxima observed at about 900 cm are associated with a deformation of vibrations obtained for C–H stretches originating from –CH –CH –, a fragment of the alkyl chain. Nevertheless, the most 2 2 for C–H stretches originating from –CH2–CH2–, a fragment of the alkyl chain. Nevertheless, the most essential peaks for our investigations (identified in the range 1100–1350 cm −1) were revealed between essential peaks for our investigations (identified in the range 1100–1350 cm ) were revealed between carbon and fluorine, which correspond to asymmetric and symmetric stretches. It was established carbon and fluorine, which correspond to asymmetric and symmetric stretches. It was established that these observations are clear evidence of the modification process. The intensity of C–F peaks that these observations are clear evidence of the modification process. The intensity of C–F peaks increases with the concentration of perfluoroalkylsilane used for deposition of SAMs on three surfaces. increases with the concentration of perfluoroalkylsilane used for deposition of SAMs on three 1 1 The areas of these peaks (symmetric at 1250–1300 cm and asymmetric −1 at 1100–1200 cm ) wer−1 e surfaces. The areas of these peaks (symmetric at 1250–1300 cm and asymmetric at 1100–1200 cm ) integrated. Based on these data, the ratio of symmetric to asymmetric vibrations of C-F bonds was were integrated. Based on these data, the ratio of symmetric to asymmetric vibrations of C-F bonds calculated. In every case (Figure 5), the ratio increases with higher saturation of the investigated was calculated. In every case (Figure 5), the ratio increases with higher saturation of the investigated solution. It means that the degree of coverage of the investigated surfaces increases. It was found solution. It means that the degree of coverage of the investigated surfaces increases. It was found that that the most appropriate coverage by compounds is when the ratio of investigated areas is close to the most appropriate coverage by compounds is when the ratio of investigated areas is close to one one [42]. Comparing this fact with the measurements of the contact angle (Figure 2), we can conclude [42]. Comparing this fact with the measurements of the contact angle (Figure 2), we can conclude that that in this case, the obtained layers show the highest hydrophobicity. Confirmation of the high in this case, the obtained layers show the highest hydrophobicity. Confirmation of the high coverage coverage ratio of the analyzed surfaces using SAMs is visible on the SEM image of the surface with the ratio of the analyzed surfaces using SAMs is visible on the SEM image of the surface with the highest highest hydrophobicity (Figure 6). EDS maps of those areas indicate ~10 wt% of fluorine next to atoms hydrophobicity (Figure 6). EDS maps of those areas indicate ~10 wt% of fluorine next to atoms characteristic for modified coatings/surfaces. characteristic for modified coatings/surfaces. Materials 2020, 13, x FOR PEER REVIEW 8 of 15 Materials 2020, 13, 3357 8 of 15 Materials 2020, 13, x FOR PEER REVIEW 8 of 15 (a) (b) Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process for (a) FPTS and (b) FDTS compounds. Another factor determining the physical-chemical properties of the manufactured layers is their thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of the prepared layers [43–45]. These measurements were obtained based on the formula: (a) (b) h = nN() υ − υ , (1) 1 2 Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration Where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process −1 for (a) FPTS and (b) FDTS compounds. fringes within a spectral region and υ1, υ2—starting and ending points in the spectrum in cm . for (a) FPTS and (b) FDTS compounds. Another factor determining the physical-chemical properties of the manufactured layers is their thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of the prepared layers [43–45]. These measurements were obtained based on the formula: h = nN() υ − υ , (1) 1 2 Where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of −1 fringes within a spectral region and υ1, υ2—starting and ending points in the spectrum in cm . Figure 6. Figure 6. The The scanning electro scanning electron n microscopy ( microscopy (SEM) SEM) images images of three of three differen di erent t surfaces after surfaces after creation of creation of FP FPTS TS and and FDTS SAMs FDTS SAMs.. Another factor determining the physical-chemical properties of the manufactured layers is their A condition of determining the thickness of manufactured layers from FTIR investigations based thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. −1 the prepared layers [43–45]. These measurements were obtained based on the formula: In our studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, h = nN/(  ), (1) 1 2 where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of fringes Figure 6. The scanning electron microscopy (SEM) images of three different surfaces after creation of within a spectral region and  ,  —starting and ending points in the spectrum in cm . 1 2 FPTS and FDTS SAMs. A condition of determining the thickness of manufactured layers from FTIR investigations based on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. In our A condition of determining the thickness of manufactured layers from FTIR investigations based studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, the starting on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. −1 In our studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, Materials 2020, 13, 3357 9 of 15 Materials 2020, 13, x FOR PEER REVIEW 9 of 15 and ending points and the number of fringes were determined. The results of the calculations of thickness the starting and ending points and the number of fringes were determined. The results of the are shown in Table 1. calculations of thickness are shown in Table 1. Table 1. Thickness [nm] of SAMs created on di erent substrates using LPD method using the solutions Table 1. Thickness [nm] of SAMs created on different substrates using LPD method using the of FPTS and FDTS compounds at five di erent concentrations, estimated in FTIR measurements. solutions of FPTS and FDTS compounds at five different concentrations, estimated in FTIR Relative uncertainty is lower than 2%. measurements. Relative uncertainty is lower than 2%. FPTS Concentration FDTS Concentration Substrate Material Substrate FPTS Concentration FDTS Concentration 0.02% 0.05% 0.10% 0.20% 0.50% 0.02% 0.05% 0.10% 0.20% 0.50% Material 0.02% 0.05% 0.10% 0.20% 0.50% 0.02% 0.05% 0.10% 0.20% 0.50% DLC 7.15 6.03 5.44 6.50 12.68 11.22 7.64 9.04 9.60 9.27 DLC 7.15 6.03 5.44 6.50 12.68 11.22 7.64 9.04 9.60 9.27 Si-DLC 6.63 5.07 4.46 5.18 8.81 5.23 3.29 4.07 4.70 6.21 Si-DLC 6.63 5.07 4.46 5.18 8.81 5.23 3.29 4.07 4.70 6.21 Si 8.76 8.65 5.09 9.89 16.45 6.61 5.81 7.39 8.82 7.97 Si 8.76 8.65 5.09 9.89 16.45 6.61 5.81 7.39 8.82 7.97 To confirm these results, a spectroscopic ellipsometry technique was used for thickness To confirm these results, a spectroscopic ellipsometry technique was used for thickness measurements (Figure 7). The obtained results indicate a thin layer of SAMs, especially for Si-DLC/0.05% measurements (Figure 7). The obtained results indicate a thin layer of SAMs, especially for Si- FDTS and Si-DLC/0.1% FPTS systems. This behavior can be explained by the di erences in the structure DLC/0.05% FDTS and Si-DLC/0.1% FPTS systems. This behavior can be explained by the differences and the tendency in orientation relative to the investigated surface. It means that shorter compounds in the structure and the tendency in orientation relative to the investigated surface. It means that like FPTS, with the length obtained from the theoretical model, determined using HyperChem 7.5 equal shorter compounds like FPTS, with the length obtained from the theoretical model, determined using to 0.70 nm, tend to form a vertical orientation, which is connected with the creation of a multi-layer HyperChem 7.5 equal to 0.70 nm, tend to form a vertical orientation, which is connected with the structure (thickness from 8.81 nm for 0.50% to 4.46 nm for 0.10%). On the other hand, longer compounds creation of a multi-layer structure (thickness from 8.81 nm for 0.50% to 4.46 nm for 0.10%). On the like FDTS, with a theoretical length equal to 1.64 nm [48], create horizontally oriented structures other hand, longer compounds like FDTS, with a theoretical length equal to 1.64 nm [48], create relative to the investigated surface and, as a consequence, better packed structured coverage. Thus, horizontally oriented structures relative to the investigated surface and, as a consequence, better it can be stated that both the type of the modifier and the solution concentration has a significant packed structured coverage. Thus, it can be stated that both the type of the modifier and the solution impact on the thickness of the layer, as can be seen in Figure 7. concentration has a significant impact on the thickness of the layer, as can be seen in Figure 7. (a) (b) Figure 7. Figure 7.Eff E ect ect of concentrati of concentration on solution of solution of perfluor perfluoroalk oalkylsilane ylsilane on thick on thickness ness and rou and roughness ghness (RMS) (RMS) of SAMs created on three surfaces with di erent contents of silicon for modifiers: (a) FPTS and (b) FDTS. of SAMs created on three surfaces with different contents of silicon for modifiers: (a) FPTS and (b) FDTS. Further analysis of FTIR spectra in the region 2800–3000 cm (Figure 4) shows di erences in C–H symmetric and asymmetric stretching vibrations that originated −1 from the hydrocarbon part of the Further analysis of FTIR spectra in the region 2800–3000 cm (Figure 4) shows differences in C– silane chain. It was found that these changes depend on the occurrence of ordered (crystalline-like) or H symmetric and asymmetric stretching vibrations that originated from the hydrocarbon part of the disordered (liquid-like) structures [49]. The lack of asymmetric C–H bonds or the occurrence of these silane chain. It was found that these changes depend on the occurrence of ordered (crystalline-like) stretches with low intensity, like in the case of Si-DLC/0.05% FDTS, indicates a crystalline-like structure. or disordered (liquid-like) structures [49]. The lack of asymmetric C–H bonds or the occurrence of This structure designates well-packed layers with good coverage, which showed two low intensity these stretches with low intensity, like in the case of Si-DLC/0.05% FDTS, indicates a crystalline-like 1 1 asymmetric stretching modes visible at 2939 cm and 2921 cm , whereas the liquid-like structure is structure. This structure designates well-packed layers with good coverage, which showed two low manifested by one or more absorption bands with high intensities −1 and/or−vibrations 1 shifted to higher intensity asymmetric stretching modes visible at 2939 cm and 2921 cm , whereas the liquid-like wavenumbers. Hence, a liquid-like structure that occurs on defects due to disorder was observed for a structure is manifested by one or more absorption bands with high intensities and/or vibrations shifted to higher wavenumbers. Hence, a liquid-like structure that occurs on defects due to disorder was observed for a system composed of Si-DLC and 0.50% FDTS (one peak with high intensity at Materials 2020, 13, x FOR PEER REVIEW 10 of 15 Materials 2020, 13, 3357 10 of 15 −1 2943 cm ). Therefore, the presence, position and intensity of these bonds decides the ordering of the system composed of Si-DLC and 0.50% FDTS (one peak with high intensity at 2943 cm ). Therefore, manufactured structures. the presence, position and intensity of these bonds decides the ordering of the manufactured structures. 3.3. Tribological Properties 3.3. Tribological Properties The tribological properties at millinewton load (values of friction coefficient) for all investigated The tribological properties at millinewton load (values of friction coefficient) for all investigated surfaces are shown in Figure 8. It was found that the Si-DLC coating showed better friction properties surfaces are shown in Figure 8. It was found that the Si-DLC coating showed better friction properties in in comparison to the DLC and silicon surface (CoF = 0.21, 0.24 and 0.31 respectively). This trend is comparison to the DLC and silicon surface (CoF = 0.21, 0.24 and 0.31 respectively). This trend is visible visible not only for unmodified surfaces but also after the modification. The changes in these not only for unmodified surfaces but also after the modification. The changes in these properties between properties between the analyzed surfaces are linked with the following factors: (i) a presence of the analyzed surfaces are linked with the following factors: (i) a presence of hydrogen, (ii) a presence of hydrogen, (ii) a presence of native silicon oxide as well, (iii) wettability of the surface and (iv) native silicon oxide as well, (iii) wettability of the surface and (iv) thickness of deposited layers. thickness of deposited layers. (b) (a) Figure 8. Coecient of friction (CoF) and work of adhesion (WoA) of SAMs created using the LPD Figure 8. Coefficient of friction (CoF) and work of adhesion (WoA) of SAMs created using the LPD process on three surfaces with di erent contents of silicon as a function of the concentration of solutions process on three surfaces with different contents of silicon as a function of the concentration of of (a) FPTS and (b) FDTS compounds. solutions of (a) FPTS and (b) FDTS compounds. The di erences in the hydrogen content of DLC and Si-DLC coatings and the Si surface were The differences in the hydrogen content of DLC and Si-DLC coatings and the Si surface were evaluated by analyzing the signals of C-H bonds (Figure 3). In the case of the Si-DLC coating, in the evaluated by analyzing the signals of C-H bonds (Figure 3). In the case of the Si-DLC coating, in the FTIR spectra three low intensity bands assigned to C-H vibrations are observed, whereas for pure DLC FTIR spectra three low intensity bands assigned to C-H vibrations are observed, whereas for pure only one strong band is visible. The area ratios of bands assigned to C–H vibrations and to other bands DLC only one strong band is visible. The area ratios of bands assigned to C–H vibrations and to other that occur in the spectra were calculated: 14% for Si-DLC and 28% for DLC coating. These results bands that occur in the spectra were calculated: 14% for Si-DLC and 28% for DLC coating. These allow the assumption that the relative content of hydrogen in DLC coating is two times higher than in results allow the assumption that the relative content of hydrogen in DLC coating is two times higher Si-DLC coating. Erdemir [40] indicate that highly hydrogenated carbon coatings during a tribological than in Si-DLC coating. Erdemir [40] indicate that highly hydrogenated carbon coatings during a test performed in the open air absorbed more oxygen species and water molecules than other surfaces. tribological test performed in the open air absorbed more oxygen species and water molecules than Therefore, one of the reasons for the occurrence of better microtribological performance obtained by the other surfaces. Therefore, one of the reasons for the occurrence of better microtribological lower coecient of friction for Si-DLC coating (0.214 0.011) compared to DLC coating (0.240 0.011) performance obtained by the lower coefficient of friction for Si-DLC coating (0.214 ± 0.011) compared is linked with the presence of native silicon forms (SiO ) [50]. Silicon oxides in combination with the to DLC coating (0.240 ± 0.011) is linked with the presence of native silicon forms (SiOx) [50]. Silicon presence of water in the form of water meniscuses enhance the formation of a Si(OH) layer at the top oxides in combination with the presence of water in the form of water meniscuses enhance the of the investigated surfaces according to the following reaction: formation of a Si(OH)4 layer at the top of the investigated surfaces according to the following reaction: SiO + 2H O → Si() OH , (2) SiO + 2H O ! Si(OH) , (2) 2 2 2 2 4 The appearance of SiOx and Si(OH)4 in this study is confirmed by the presence of Si–O and Si– The appearance of SiO and Si(OH) in this study is confirmed by the presence of Si–O and Si–OH x 4 OH signals in FTIR spectra. Figure 3 shows the differences in the presence of these vibrations between signals in FTIR spectra. Figure 3 shows the di erences in the presence of these vibrations between Si Si substrate and Si-DLC coating. It is seen that in the case of the silicon-incorporated diamond-like substrate and Si-DLC coating. It is seen that in the case of the silicon-incorporated diamond-like carbon carbon coating one peak characteristic for Si–OH with significant intensity is visible. It was coating one peak characteristic for Si–OH with significant intensity is visible. It was established by established by Bhushan that the amount of silicon oxides as well as the presence of a Si(OH)4 layer is Bhushan that the amount of silicon oxides as well as the presence of a Si(OH) layer is one of the main one of the main reasons determining microtribological behavior [51]. Therefore, in the case of silicon- reasons determining microtribological behavior [51]. Therefore, in the case of silicon-incorporated incorporated coating, presence of SiOx promotes the formation of a thin layer containing Si(OH)4 Materials 2020, 13, 3357 11 of 15 coating, presence of SiO promotes the formation of a thin layer containing Si(OH) particles in a x 4 carbon matrix, which reduces friction in comparison to DLC coating. However, the FTIR analysis showed that the Si(OH) bonds occur not only for Si-DLC coating but also for the Si surface. Results of microtribological tests showed that despite the presence of silicon oxides, silicon was the substrate with the highest value of friction coecient (0.308 0.011). This indicates that a form of presence of Si(OH) in surface layers determines their tribological properties. It appears the continuous monolayer of silicon oxide on the top of the silicon results in a high friction coecient, whereas Si(OH) particles distributed in carbon matrix composed of C-H bonds (the case of Si-DLC coating) ensures a low friction coecient. From the above-mentioned factors, the wettability of the surface has a significant impact on the microscale tribological behavior. If we look at the previously presented results of the wettability (Figure 2), it was found that the silicon surface has the lowest contact angle compared to Si-DLC and DLC. This is related to the hydrophilic character of the silicon, which can easily adsorb water from the environment. As is well known, the water has a negative influence on the friction, because between the counterbody and the surface, adhesive connections are formed which are dicult to break. In the case of modifications, it was supposed that the amount of silicon is equally important in the context of the effect of deposition of silane SAMs. It was found that silicon plays a role of anchoring center, which enhances the formation and ordering of silane structures. The tests in the millinewton load range exposed improving tribological properties by lowering the coefficient of friction after modification up to 0.132 0.011 for Si-DLC/0.05% FDTS system. It proves that thin perfluoroalkylsilane layers can provide good lubrication between Si N ball and hydrophobic surfaces. It was also demonstrated that the less effective 3 4 lubrication was obtained on more hydrophilic surfaces, which indicated lower RMS values (Figure 7) than in the case of the most hydrophobic systems (Figure 2). A possible increase of roughness after modification for less hydrophobic layers is caused by local aggregation of modifier compounds [51], which also decides the increasing thickness for these coatings and influences the values of the coefficient of friction (Figure 8). Moreover, it was found that the hydrophilic character of the modified systems favors partial forming of water meniscuses on the top of these surfaces. Hence, in the case of tribological measurements, where the ball loaded in the millinewton load range is in contact with a flat surface, the presence of attractive force is caused by meniscus force. This attractive force is called Laplace force (F ) and is expressed by the following equitation: F = 2R (cos  + cos  ), (3) L LV 2 where, R is the radius of the water sphere; is the surface tension of the liquid against air;  and LV 1 2 are the contact angles between liquid and surfaces [51–53]. Furthermore, it was established that Si N balls have a considerable radius, hence the Laplace 3 4 force and van der Waals impact is large. Therefore, meniscus force is the main factor determining adhesion in the presented investigations. Influencing these forces leads to alterations of the work of adhesion. The work of adhesion was calculated based on Equation (4). This equation introduced a quantity on real surfaces of the apparent contact angle of static water for a water drop of a solid surface using the Young-Dupre equation [51]: W = (1 +  ), (4) e LV e where,  is the equilibrium (Young’s) contact angle. The performed investigation (Figure 8) indicated a major reduction of the work of adhesion for systems modified using perfluoroalkylsilane in comparison to pure materials (67, 120 and 104 mJ/m respectively for DLC, Si and Si-DLC), which confirms the e ectiveness of the performed modifications. In the case of modification, it was also found that solution concentration has a significant e ect on the quality of the obtained monolayer and, as a consequence, on the values of adhesion work. Moreover, Liu and Bhushan [51] indicated the relationship between the structure chemistry of produced SAMs and their adhesive properties. It was presented that with higher hydrophobicity, the impact of van der Materials 2020, 13, 3357 12 of 15 Materials 2020, 13, x FOR PEER REVIEW 12 of 15 Walls force is smaller on the adhesive force. As a consequence, for hydrophobic surfaces, adhesive force is proportional to the work of adhesion [51–53]. In our measurements the work of adhesion for hydrophobic surfaces, adhesive force is proportional to the work of adhesion [51–53]. In our confirms the authenticity of the described phenomenon. Taking into account the concentration of measurements the work of adhesion confirms the authenticity of the described phenomenon. Taking perfluoroalkylsilanes, where modification is the most e ective, the work of adhesion was 23.86, 14.26 into account the concentration of perfluoroalkylsilanes, where modification is the most effective, the 2 2 and 11.75 mJ/m for FPTS and 17.79, 9.10 and 5.37 mJ/m for FDTS SAMs created on DLC, Si and Si-DLC 2 2 work of adhesion was 23.86, 14.26 and 11.75 mJ/m for FPTS and 17.79, 9.10 and 5.37 mJ/m for FDTS substrate materials, respectively (see in Figure 8). Comparing the type of modifier, the lowest value was SAMs created on DLC, Si and Si-DLC substrate materials, respectively (see in Figure 8). Comparing obtained for the system composed of Si-DLC coating and 0.05% FDTS. These studies suggest that FDTS the type of modifier, the lowest value was obtained for the system composed of Si-DLC coating and films can perform as thin, well-ordered, anti-adhesion films with good coverage, which significantly 0.05% FDTS. These studies suggest that FDTS films can perform as thin, well-ordered, anti-adhesion reduces the work of adhesion. films with good coverage, which significantly reduces the work of adhesion. The last parameter influencing the friction is the thickness of SAMs. Generally, high friction The last parameter influencing the friction is the thickness of SAMs. Generally, high friction is is caused by thick and non-ordered layers with visible traces of wear after microtribological tests, caused by thick and non-ordered layers with visible traces of wear after microtribological tests, especially for FPTS modification. For this perfluoroalkylsilane modification at the concentration of especially for FPTS modification. For this perfluoroalkylsilane modification at the concentration of 0.1% for the whole range of load and for all studied surfaces, wear tracks were registered (Figure 9). 0.1% for the whole range of load and for all studied surfaces, wear tracks were registered (Figure 9). Wear track analysis also shows that Si-DLC/0.05% FDTS presented better microtribological properties Wear track analysis also shows that Si-DLC/0.05% FDTS presented better microtribological properties in comparison to other modifications due to the lowest coecient of friction and lack of wear after in comparison to other modifications due to the lowest coefficient of friction and lack of wear after friction tests. This fact is due to the previously discussed occurrence of ordered (crystalline-like) or friction tests. This fact is due to the previously discussed occurrence of ordered (crystalline-like) or disordered (liquid-like) structures as well as layer thicknesses of 4.46 nm and 3.29 nm for FPTS and disordered (liquid-like) structures as well as layer thicknesses of 4.46 nm and 3.29 nm for FPTS and FDTS respectively. FDTS respectively. Figure 9. The SEM images of the surface of investigated layer structures after tribological tests: (a) Figure 9. The SEM images of the surface of investigated layer structures after tribological tests: DLC/0.1% FPTS, (b) Si/0.1% FPTS, (c) Si/0.1% FPTS, (d) DLC/0.05% FDTS, (e) Si/0.05% FDTS, (f) Si- (a) DLC/0.1% FPTS, (b) Si/0.1% FPTS, (c) Si/0.1% FPTS, (d) DLC/0.05% FDTS, (e) Si/0.05% FDTS, DLC/0.05% FDTS. (f) Si-DLC/0.05% FDTS. 4. 4. Con Conclusions clusions Two homolog perfluoroalkylsilanes (FPTS and FDTS) were used to form self-assembled monolayers Two homolog perfluoroalkylsilanes (FPTS and FDTS) were used to form self-assembled on monolay DLC,eSi-DLC rs on DLC, S and silicon i-DLC and substrates silicon subst via the rat liquid es via the phase liquid p deposition hase deposition technique. The technique. The di erent concentrations of perfluoroalkylsilanes solution used to form SAMs allow indication of the system different concentrations of perfluoroalkylsilanes solution used to form SAMs allow indication of the characterized system characterize by thed by t best coverage he best cover of theasurface ge of th with e surf the ace wi highest th the hi hydrophobic ghest hydrophobi properties.c properti Comparing es. these two modifications, it was found that FDTS, due to the tendency of vertical polymerization and Comparing these two modifications, it was found that FDTS, due to the tendency of vertical polymerization and more fluorine in the structure, allows manufacturing of thinner, well-covered and more crystalline-like layers with higher WCA than in the case of FPTS modification. Moreover, results of the coefficient of friction, work of adhesion and wear investigations are in a close Materials 2020, 13, 3357 13 of 15 more fluorine in the structure, allows manufacturing of thinner, well-covered and more crystalline-like layers with higher WCA than in the case of FPTS modification. Moreover, results of the coecient of friction, work of adhesion and wear investigations are in a close relationship and confirm the e ectiveness of modification using a 0.05% concentration of FDTS in toluene. The investigation also enabled the best material candidate to be chosen as a substrate for the liquid perfluoroalkylsilane modification process. It was found that the DLC coating containing silicon is more attractive than silicon wafer and pure DLC coating. This is caused by the presence in DLC coating of dispersed native silicon forms (SiO ) which constitute active centers for chemical bonds and as a consequence improved chemical modification. Concluding, the low values of adhesion and coecient of friction as well as the obtained high hydrophobicity registered, especially for Si-DLC modified using FDTS with a concentration of 0.05% (126.8  2.0 ), makes these systems attractive, especially for MEMS devices, where intensive tribochemical processes take place. Author Contributions: Conceived and designed the research coordinated the study, M.C.; chemical modification, viscosity, surface tension, tribological and wettability measurements of self-assembled layers, E.B., M.P. and M.C.; formation of Si DLC coating, A.J., M.D. and D.B.; writing, review and editing, M.C., E.B., M.P., A.J., D.B. and M.D. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Polish National Science Centre, research grant no. 2014/13/B/ST8/03114. Conflicts of Interest: The authors declare no conflict of interest. References 1. Dowling, A.P. Development of nanotechnologies. Mater. Today 2004, 7, 30–35. [CrossRef] 2. Teoh, G.Z.; Klanrit, P.; Kasimatis, M.; Seifalian, A.M. Role of nanotechnology in development of artificial organs. Minerva Med. 2015, 106, 17–33. 3. Han, B.; Cheng, G.; Wang, Y.; Wang, X. Structure and functionality design of novel carbon and faradaic electrode materials for high-performance capacitive deionization. Chem. Eng. J. 2019, 360, 364–384. [CrossRef] 4. Rehman, M.A.U.; Munawar, M.A.; Schubert, D.W.; Boccaccinia, A.R. Electrophoretic deposition of chitosan/gelatin/bioactive glass composite coatings on 316L stainless steel: A design of experiment study. Surf. Coat. Technol. 2019, 358, 976–986. [CrossRef] 5. Bashami, R.M.; Soomro, M.T.; Khan, A.N.; Aazam, E.S.; Ismail, I.M.I.; El-Shahawi, M.S. A highly conductive thin film composite based on silver nanoparticles and malic acid for selective electrochemical sensing of trichloroacetic acid. Anal. Chim. Acta 2018, 1036, 33–48. [CrossRef] 6. Love, J.C.; Estro , L.A.; Kriebel, J.K.; Nuzzo, R.G.; Whitesides, G.M. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev. 2005, 105, 1103–1169. [CrossRef] 7. Zheng, M.; Chen, Y.; Zhou, Y.; Tang, Y.; Lu, T. The e ect of the surface coverage of the phosphonic acid terminated self-assembled monolayers on the electrochemical behavior of dopamine. Talanta 2010, 81, 1076–1080. [CrossRef] 8. Chiadò, A.; Palmara, G.; Ricciardi, S.; Frascella, F.; Castellino, M.; Tortello, M.; Ricciardi, C.; Rivolo, P. Optimization and characterization of a homogeneous carboxylic surface functionalization for silicon-based biosensing. Colloid. Surf. B 2016, 143, 252–259. [CrossRef] [PubMed] 9. Cheng, Y.; Zheng, B.; Chuang, P.; Hsieh, S. Solvent e ects on molecular packing and tribological properties of octadecyltrichlorosilane films on silicon. Langmuir 2010, 26, 8256–8261. [CrossRef] 10. Cichomski, M.; Kosl ´ a, K.; Pawlak, W.; Kozłowski, W.; Szmaja, W. Stability and tribological investigations of 1H, 1H, 2H, 2H-perfluoroalkyltrichlorosilane on titania surface. Tribol. Int. 2014, 77, 1–6. [CrossRef] 11. Cichomski, M.; Kosla, ´ K.; Grobelny, J.; Kozłowski, W.; Kowalczyk, P.J.; Busiakiewicz, A.; Szmaja, W.; Balcerski, J. Nano-and microtribological characterization of silanes deposited on cobalt substrate. J. Alloy. Compd. 2010, 507, 273–278. [CrossRef] 12. Bhushan, B.; Cichomski, M. Nanotribological characterization of vapor phase deposited fluorosilane self-assembled monolayers deposited on polydimethylsiloxane surfaces for biomedical micro-/nanodevices. J. Vac. Sci. Technol. A 2007, 25, 1285–1293. [CrossRef] Materials 2020, 13, 3357 14 of 15 13. Xiang, H.; Komvopoulosa, K. E ect of fluorocarbon self-assembled monolayer films on sidewall adhesion and friction of surface micromachines with impacting and sliding contact interfaces. J. Appl. Phys. 2013, 113, 224505. [CrossRef] 14. Cichomski, M. Tribological investigations of perfluoroalkylsilanes monolayers deposited on titanium surface. Mater. Chem. Phys. 2012, 136, 498–504. [CrossRef] 15. Khatri, O.P.; Bain, C.D.; Biswas, S.K. E ects of chain length and heat treatment on the nanotribology of alkylsilane monolayers self-assembled on a rough aluminum surface. J. Phys. Chem. B 2005, 109, 2340–23414. [CrossRef] 16. Jabbarzadeh, A. Friction anisotropy and asymmetry in self assembled monolayers. Tribol. Int. 2016, 102, 600–607. [CrossRef] 17. Cichomski, M.; Prowizor, M.; Borkowska, E.; Piwonski, ´ I.; Jedrzejczak, ˛ A.; Dudek, M.; Batory, D.; Wronska, ´ N.; Lisowska, K. Impact of perfluoro and alkylphosphonic self-assembled monolayers on tribological and antimicrobial properties of Ti-DLC coatings. Materials 2019, 12, 2365. [CrossRef] 18. Szubert, K.; Wojciechowski, J.; Karasiewicz, J.; Maciejewski, H.; Lota, G. Corrosion-protective coatings based on fluorocarbosilane. Prog. Org. Coat. 2018, 123, 374–383. [CrossRef] 19. Pourhashema, S.; Rashidi, A.; Vaezi, M.R.; Bagherzadeh, M.R. Excellent corrosion protection performance of epoxy composite coatings filled with amino-silane functionalized graphene oxide. Surf. Coat. Technol. 2017, 317, 1–9. [CrossRef] 20. Cichomski, M.; Burnat, B.; Prowizor, M.; Jedrzejczak, A.; Batory, D.; Piwonski, ´ I.; Kozłowski, W.; Szymanski, W.; Dudek, M. Tribological and corrosive investigations of perfluoro and alkylphosphonic self-assembled monolayers on Ti incorporated carbon coatings. Tribol. Int. 2019, 130, 359–365. [CrossRef] 21. Wang, S.; Goronzy, D.P.; Young, T.D.; Wattanatorn, N.; Stewart, L.; Baše, T.; Weiss, P.S. Formation of highly ordered terminal alkyne self-assembled monolayers on the Au {111} surface through substitution of 1-decaboranethiolate. J. Phys. Chem. C 2019, 123, 1348–1353. [CrossRef] 22. Tsunoi, A.; Lkhamsuren, G.; Mondarte, E.A.Q.; Asatyas, S.; Oguchi, M.; Noh, J.; Hayashi, T. Improvement of the thermal stability of self-assembled monolayers of isocyanide derivatives on gold. J. Phys. Chem. C 2019, 123, 13681–13686. [CrossRef] 23. Cichomski, M.; Tomaszewska, E.; Kosla, ´ K.; Kozłowski, W.; Kowalczyk, P.J.; Grobelny, J. Study of dithiol monolayer as the interface for controlled deposition of gold nanoparticles. Mater. Charact. 2011, 62, 268–274. [CrossRef] 24. Kluth, G.J.; Sung, M.M.; Maboudian, R. Thermal behavior of alkylsiloxane self-assembled monolayers on the oxidized Si (100) surface. Langmuir 1997, 13, 3775–3780. [CrossRef] 25. Tripp, C.P.; Hair, M.L. Direct observation of the surface bonds between self-assembled monolayers of octadecyltrichlorosilane and silica surfaces: A low-frequency IR study at the solid/liquid interface. Langmuir 1995, 11, 1215–1219. [CrossRef] 26. Cottrell, T.L. The Strengths of Chemical Bonds; Butterworths: London, UK, 1958; pp. A21–A34. 27. Prowizor, M.; Lewkowski, J.; Rodriguez, M.M.; Morawska, M.; Cichomski, M. The modification of silicon surface by selected aminophosphonates to improve its tribological properties. Mater. Res. Express 2020, 6. [CrossRef] 28. Bystrzycka, E.; Prowizor, M.; Piwonski, I.; Kisielewska, A.; Batory, D.; Jedrzejczak, A.; Dudek, M.; Kozlowski, W.; Cichomski, M. The e ect of fluoroalkylsilanes on tribological properties and wettability of Si-DLC coatings. Mater. Res. Express 2018, 5. [CrossRef] 29. Cichomski, M.; Kisielewska, A.; Prowizor, M.; Borkowska, E.; Piwonski, ´ I.; Dudek, M.; Jedrzejczak, ˛ A.; Batory, D. The influence of self-assembled monolayers on tribological properties of Si-DLC coatings. Surf. Topogr. Metrol. Prop. 2009, 7. [CrossRef] 30. Choi, J.; Kawaguchi, M.; Kato, T.; Ikeyama, M. Deposition of Si-DLC film and its microstructural, tribological and corrosion properties. Microsyst. Technol. 2007, 13, 1353–1358. [CrossRef] 31. Zajickova, L.; Bursikova, V.; Perina, V.; Mackova, A.; Janca, J. Correlation between SiO content and properties of DLC: SiO films prepared by PECVD. Surf. Coat. Technol. 2003, 174–175, 281–285. [CrossRef] 32. Mayer, T.M.; de Boer, M.P.; Shinn, N.D.; Clews, P.J.; Michalske, T.A. Chemical vapor deposition of fluoroalkylsilane monolayer films for adhesion control in microelectromechanical systems. J. Vac. Sci. Technol. B 2000, 18, 2432–2440. [CrossRef] Materials 2020, 13, 3357 15 of 15 33. Kushmerick, J.G.; Hankins, M.G.; de Boer, M.P.; Clews, P.J.; Carpick, R.W.; Bunker, B.C. The influence of coating structure on micromachine stiction. Tribol. Lett. 2001, 10, 103–108. [CrossRef] 34. Wang, Y.; Zhang, Z.; Jain, V.; Yi, J.; Mueller, S.; Sokolov, J.; Liu, Z.; Levon, K.; Rigas, B.; Rafailovich, M.H. Potentiometric sensors based on surface molecular imprinting: Detection of cancer biomarkers and viruses. Sens. Actuators B 2010, 146, 381–387. [CrossRef] 35. Jedrzejczak, A.; Batory, D.; Dominik, M.; Smietana, M.; Cichomski, M.; Szymanski, W.; Bystrzycka, E.; Prowizor, M.; Kozlowski, W.; Dudek, M. Carbon coatings with high concentrations of silicon deposited by RF PECVD method at relatively high self-bias. Surf. Coat. Technol. 2017, 329, 212–217. [CrossRef] 36. Nordin, N.H.; Ahmad, Z. Monitoring chemical changes on the surface of borosilicate glass covers during the silanisation process. J. Phys. E Sci. Instrum. 2015, 26, 11–22. 37. Van Oss, C.J.; Chaudhury, M.K.; Good, R.J. Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chem. Rev. 1988, 88, 927–940. [CrossRef] 38. Yamabe, T.; Moroi, Y.; Abe, Y.; Takahashi, T. Micelle Formation and surface adsorption of N-(1,1-Dihydroperfluorofluoroalkyl)-N, N, N-trimethylammonium chloride. Langmuir 2000, 16, 9754–9758. [CrossRef] 39. Kovalchuk, N.M.; Trybala, A.; Starov, V.; Matar, O.; Ivanova, N. Fluoro- vs hydrocarbon surfactants: Why do they di er in wetting performance? Adv. Colloid Interface 2014, 210, 65–71. [CrossRef] 40. Erdemir, A. The Role of hydrogen in tribological properties of diamond-like carbon films. Surf. Coat. Technol. 2001, 146–147, 292–297. [CrossRef] 41. Banga, R.; Yarwood, J.; Morgan, M.; Evans, B.; Kells, J. FTIR and AFM studies of the kinetics and self-assembly of alkyltrichlorosilanes and (perfluoroalkyl) trichlorosilanes onto glass and silicon. Langmuir 1995, 11, 4393–4399. [CrossRef] 42. Kang, J.F.; Jordan, R.; Ulman, A. Wetting and Fourier transform infrared spectroscopy studies of mixed self-assembled monolayers of 4-methyl-4mercaptobiphenyl and 4-hydroxy-4-mercaptobiphenyl. Langmuir 1998, 14, 3983–3985. [CrossRef] 43. Dumin, D.J. Measurement of film thickness using infrared interference. Rev. Sci. Instrum. 1967, 38. [CrossRef] 44. Griths, P.R.; de Haseth, J.A. Fourier Transform Infrared Spectrometry; John Wiley & Sons: Hoboken, NJ, USA, 2007. 45. Mirabella, F.M. Modern Techniques in Applied Molecular Spectroscopy, 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 1998. 46. Mpilitos, C.; Amanatiadis, S.; Apostolidis, G.; Zygiridis, T.; Kantartzis, N.; Karagiannis, G. Development of a transmission line model for the thickness prediction of thin films via the infrared interference method. Technologies 2018, 6, 122. [CrossRef] 47. Ionescu, M.A.; Ionescu, C.; Ciuca, I. Polymer vapour deposition of parylene-N and parrylene-C on Si (111): Thin film characterization by FTIR and ellipsometry. Rev. Chim. 2014, 65, 1242–1244. 48. Cichomski, M.; Kosla, K.; Kozlowski, W.; Szmaja, W.; Balcerski, J.; Rogowski, J.; Grobelny, J. Investigation of the structure of fluoroalkylsilanes deposited on alumina surface. Appl. Surf. Sci. 2012, 258, 9849–9855. [CrossRef] 49. Zhang, Q.; Archer, L.A. Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers. Langmuir 2005, 21, 5405–5413. [CrossRef] 50. Jedrzejczak, A.; Kolodziejczyk, L.; Szymanski, W.; Piwonski, I.; Cichomski, M.; Kisielewska, A.; Dudek, M.; Batory, D. Friction and wear of a-C:H:SiO coatings in combination with AISI 316L and ZrO counterbodies. Tribol. Int. 2017, 112, 155–162. [CrossRef] 51. Liu, H.; Bhushan, B. Nanotribological properties and mechanisms of alkylthiol and biphenyl thiol self-assembled monolayers studied by AFM. Phys. Rev. B 2001, 63, 245412. 52. Bhushan, B. Principles and Applications of Tribology; Wiley & Sons: Hoboken, NJ, USA, 1999. 53. Israelachvili, J.N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, UK, 1992. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Materials Multidisciplinary Digital Publishing Institute

The Effect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological Features of Created Self-Assembled Monolayers

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materials Article The E ect of Physicochemical Properties of Perfluoroalkylsilanes Solutions on Microtribological Features of Created Self-Assembled Monolayers 1 , 1 1 2 Michał Cichomski * , Ewelina Borkowska , Milena Prowizor , Damian Batory , 3 3 Anna Jedrzejczak and Mariusz Dudek Department of Materials Technology and Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 163, 90-236 Lodz, Poland; ewelina.bystrzycka@chemia.uni.lodz.pl (E.B.); milena.prowizor@chemia.uni.lodz.pl (M.P.) Department of Vehicles and Fundamentals of Machine Design, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; damian.batory@p.lodz.pl Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; anna.jedrzejczak@p.lodz.pl (A.J.); mariusz.dudek@p.lodz.pl (M.D.) * Correspondence: michal.cichomski@chemia.uni.lodz.pl; Tel.: +48-42-635-5836 Received: 23 June 2020; Accepted: 27 July 2020; Published: 29 July 2020 Abstract: The presented article shows the influence of concentration of perfluoroalkylsilanes in solutions on tribological properties of self-assembled monolayers (SAMs) deposited on three surfaces with different silicon content in the millinewton load range. The SAMs were created using the liquid phase deposition (LPD) method with 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) and (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) solutions, for which viscosity and surface tension were estimated. Deposited layers were analyzed in terms of thickness, coverage, wettability, structure and coefficient of friction. The obtained results demonstrated that SAMs created on the silicon-incorporated diamond-like carbon (Si-DLC) coatings possess the best microtribological properties. Systems composed of perfluoroalkylsilane SAM structures deposited on Si-DLC coatings are highly promising candidates as material for microelectromechanical applications. Keywords: perfluoroalkylsilanes; viscosity; surface tension; self-assembled monolayers; thickness; microtribological properties 1. Introduction In recent years, nanotechnology has become one of the most dynamic developing fields of modern science [1–3], which allows structures to be obtained at the nanometer scale and new materials to be obtained with specific properties and consequently application of those materials for the miniaturization of devices [3–5]. The construction of devices with contact surfaces of the order of micro- and even nanometers resulted in the need to search for new nanomaterials as well as substances to protect their surfaces. These substances should have excellent tribological properties, i.e., low values of adhesion forces and friction coefficient, as well as the ability to reduce surface wear. Self-assembled monolayers (SAMs) made of perfluoroalkylsilanes exhibit such properties. In addition, the choice of perfluoroalkylsilanes for surface modification is due to their ability to form a siloxane network by which the formed layer is well ordered and resistant against various environmental factors. The uniform surface coverage, without any defects, reduces the contact area between two elements of tribological system which results in lower friction and wear. As literature data indicate, SAMs of perfluoroalkylsilanes protect the surface against corrosion and ensure their long functionality. Materials 2020, 13, 3357; doi:10.3390/ma13153357 www.mdpi.com/journal/materials Materials 2020, 13, 3357 2 of 15 Among the available methods of production of self-assembled monolayers (SAMs), the most e ective is liquid phase deposition (LPD). These structures are organic molecular assemblies which spontaneously form dense and ordered layers with a thickness of single nanometers on appropriate substrates [6–9]. Depending on the molecular design, SAMs provide a way to tailor surface properties such as wetting [10,11] adhesion [12,13], friction [14–16] and antimicrobial [17] and corrosion properties [18–20]. Their high attraction is also driven by the simple preparation procedure, the reproducible film quality and stability of the monolayer. Therefore, SAMs are presently very interesting for scientists in the nanotechnology area [1,6]. So far, most of the fundamental studies have been performed with alkane [21], aromatic thiols [22] and dithiols [23]. Currently, more and more interest from the application point of view in microelectromechanical systems (MEMS) is focused on compounds containing silicon in their structure [24,25]. In organosilicon structures, the bonds between silicon and carbon compared to carbon-carbon bonds are longer (186 pm compared to 154 pm) and characterized by weaker bond dissociation energy (451 kJ/mol vs. 607 kJ/mol). Moreover, due to higher electronegativity of carbon in comparison to silicon, the C–C and Si–C bonds di er in polarization (for C–Si equal to 2.55 vs. for C–C equal to 1.90). The result of these properties are Si–O bonds in chemical reactions, which are much stronger compared to C–O bonds (809 kJ/mol versus 538 kJ/mol) [26]. Another positive e ect of occurrence of energetically advantageous Si–O bonds is creation of perfluoroalkylsilanes network consisting of Si–O–Si linkages during the modification process. Additionally, surface modification by perfluoroalkylsilanes guarantees more satisfied physical-chemical properties than in the case of compounds that do not contain silicon in their structure [24,25,27]. Silicon plays a very important role in creating a self-assembled monolayer in the case of silicon-incorporated diamond-like carbon (Si-DLC) coatings. Our earlier studies [28,29] indicate that the more silicon that is in the DLC structure, the higher the anity of fluoroalkylsilanes to the surface. What is more, using Si-DLC in MEMS/NEMS systems is beneficial since the addition of silicon to the DLC coatings changes their surface roughness and adhesion forces, reduces compressive stress and improves wear resistance. Furthermore, the use of silicon may delay the graphitization process and thus increase the durability of DLC at elevated temperatures [30,31]. Perfluoroalkylchlorosilanes are representative candidates of organosilicon compounds used as modifiers to improve mainly the tribological features. Organic layers built of spontaneously adsorbed molecules on investigated surfaces exhibited growing potential as protective agents, which can be used in reducing friction, adhesion and wear, which was confirmed by other research [32,33]. Obtained results indicate their application potential in electromechanical systems as components of sensors and actuators used in many fields of technology and medicine [34]. In the presented study, the primary destination was to reveal the influence of the concentration of two kinds of alkyl chain silane compounds with different lengths in toluene solvent on their viscosity and surface tension and finally on the microtribological properties of created SAMs. The modifier/surface systems were created with use of two kinds of compounds: 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) and (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS). The modifications were performed on three surfaces: diamond-like carbon coating (DLC), silicon-incorporated diamond-like carbon coating (Si-DLC) and silicon. The surfaces were selected in such a way as to compare the influence of silicon content (the type of surfaces) on the effectiveness of modification. For studies with Si-DLC coating/surface systems, coatings with high concentrations of silicon were selected. We also for the first time presented the use of viscosity and surface tension measurement results to optimize the concentration of modifier solutions proper for the modification. The chemical composition of manufactured structures was obtained using Fourier transformed infrared spectroscopy (FTIR) analysis which allowed two important parameters to be determined: information about the coverage of the investigated surfaces and the thickness of the fabricated SAMs. The obtained thicknesses were confirmed using ellipsometry measurements. These parameters testified to the quality of the produced layers and allowed their physical-chemical properties to be determined. Materials 2020, 13, 3357 3 of 15 The ultrathin assembled monolayers produced using the LPD method were investigated in terms of wettability and tribological behavior in the millinewton load range. These tribological studies fill the gap between measurement results obtained in the nanoscale and macroscale region usually performed by other authors. 2. Materials and Methods 2.1. Materials As substrates for the creation of self-assembled monolayers, DLC and Si-DLC coatings deposited using the radio frequency plasma enhanced chemical vapor deposition (RF PECVD) method were selected. The coatings were synthesized on a single crystal p-type Si (100) wafer (Cemat Silicon S.A, Warsaw, Poland), which was also used as the model material. The RF PECVD process and physicochemical properties of carbon base coatings were described in detail in the publication of Jedrzejczak et al. [35]. For the investigations, DLC and Si-DLC coatings containing 32at.% of silicon, deposited at 600V negative substrate bias, were selected. 2.2. Pre-Treatment of Substrates Initially, surfaces were cut into 1.0 1.0 cm substrates and cleaned with ethanol (analytical grade, POCH S.A., Gliwice, Poland) in an ultrasonic bath to remove organic contaminents. Afterwards, the investigated surfaces were exposed to oxygen radio frequency (RF) plasma (Zepto plasma system, 40 Hz, 100 W, Diener Electronic, Ebhausen, Germany, 15 min at 50 W). The high hydrophilic character of the substrates after the treatment using plasma indicates the formation of a significant amount of polar surface silanols important to manufacture the covalent attachment of modifiers to the investigated surfaces [36]. 2.3. Preparation of Self-Assembled Perfluoroalkylsilane Layers The self-assembled monolayers were created from silane compounds FPTS and FDTS, on three (Si, DLC, and Si-DLC) surfaces, using the liquid phase deposition (LPD) method. The compounds were purchased from ABCR, GmbH & Co. KG, Karlsruhe, Germany. The FPTS and FDTS solutions at five di erent concentrations, 0.02%, 0.05%, 0.10%, 0.20% and 0.50% (wt%), were prepared by dissolving the compounds in toluene at room temperature under ambient conditions. Due to di erences in reactivity of both the perfluoroalkylsilanes, the modification was carried out in a time of 60 min for FDTS and 30 min for FPTS. Prior to the creation of SAMs, the FPTS and FDTS solutions were characterized. The surface tension investigations were performed using the Du Noüy ring method using a KSV Sigma703D (KSV Instruments Ltd, Helsinki, Finland) at 25 C. The force referred to as the wetted length acting on a ring as a result of the tension of the withdrawn liquid lamella when moving the ring from one phase to another is measured in this method. The reported values of the surface tension are averaged values of at least three measurements. The measured surface tension of the toluene used for the preparation of the perfluoroalkylsilanes solutions was  = 28.11 mN/m. The viscosity measurements were carried out using an Ostwald viscometer (Sibata, Nakane Soka, Japan) measuring the time for 1 mL of the prepared perfluoroalkylsilanes solution and toluene to flow through the capillary under the influence of gravity. The ratio of the average flow time of the investigated solutions and solvent from at least three measurements in relative viscosity are presented. 2.4. Characterization of Surfaces and Prepared Layers The characterization of pure surfaces and deposited perfluoroalkylsilanes films was conducted using Fourier transformed infrared spectroscopy (FTIR) (Nicolet iS50 FTIR, Thermo Fisher Scientific, Waltham, MA, USA). The spectrophotometer with an ultra-high sensitive, low noise, linearized mercury-cadmium-telluride (MCT) detector equipped with VariGATR— grazing angle ATR accessory dedicated for analysis of thin layers was applied. All spectra were recorded by collecting 128 scans Materials 2020, 13, 3357 4 of 15 (128 scans of the pure substrate as a background was recorded) at 8 cm resolution, in the spectral range of 700–4000 cm in the flow of dry air. Pure investigated substrates were used for background spectra. The obtained data was analyzed using OMNIC 9 software from Thermo Scientific. The thickness of the manufactured layers was determined using variable-angle spectroscopic ellipsometry (VASE) with a J. A. Woollam V-VASE ellipsometer (J.A. Woollam Co, Lincoln, CA, USA). Data was collected in the wavelength range 260–1300 nm at a 70 angle of incidence. A Cauchy dispersion model with constant parameters of 1.45, 0.01 and 0.0 was used to calculate the thickness of ultra-thin perfluoroalkylsilane layers. Hydrophobicity of the substrates was determined using the static contact angle measurements obtained at room temperature using a Drop Shape Analyzer (DSA25 Kruss ˝ Goniometr, Kruss, ˝ Hamburg, Germany). Average water contact angle (WCA) values were obtained from measurements at five di erent locations on the specimen surface. The values of contact angles for water, diiodomethane and glycerin were determined by the use of the Advance program. On the basis of these investigations, surface free energy (SFE) and its components were calculated using the van Oss Chaudhury-Good method [37]. The friction tests of the investigated surfaces were determined with the use of a ball-on-flat tribometer (T-23, ITEE, Radom, Poland), working in the millinewton load range. The coecient of friction is the average value obtained by measuring the friction force as a function of the normal load curve. A silicon nitride (Si N ) ball, 5 mm in diameter, was used as a counterbody. The measurements 3 4 were performed in technical dry friction conditions and repeated three times in three di erent locations on the sample surface. All specimens and counterbody movements as well as the load applying mechanism system were computer-controlled using Lab-View software. The applied normal loads were in the range of 30–80 mN. The sliding speed was 25 mm min over the distance of 5 mm. The topography of the wear tracks after the tribological tests was analyzed using scanning electron microscopy (SEM) (Nova NanoSEM 450, FEI, Hillsboro, WA, USA). The root mean square (RMS) of the surface roughness of samples was investigated using atomic force microscopy (AFM-Solver P47) (NT-MDT, Moscow, Russia) operating in an air-conditioned environment at RT and a relative humidity of 40 5%. The RMS of the surfaces was obtained from the scan area 2 m 2 m. 3. Results and Discussion 3.1. E ect of Physicochemical Properties of Solutions of Perfluoroalkylsilanes on Wetting Properties of SAMs The creation of SAMs using the LPD technique was preceded by the viscosity and surface tension measurements of the solution of two perfluoroalkylsilane compounds with di erent lengths of alkyl chain as a function of their concentrations (Figure 1). Based on the obtained data it was observed that the relative viscosity of the silane compounds solution prepared in toluene initially rises with increasing concentration (maximum for FDTS achieved at 0.05% and for FPTS at 0.10%) and next decreases with increasing content of perfluoroalkylsilane compounds, while for surface tension measurements a reversed behavior was observed. It was noticed that the minimum values of this parameter corresponded to the maximum values of relative viscosity. These results are related to the appearance of critical concentration and interfacial tension [38,39]. Furthermore, the authors found that this activity was higher for smaller concentrations of linear fluorosurfactants, which affect the decrease in surface tension. A similar tendency for perfluoroalkylsilanes was confirmed in our measurements. In addition, the strong influence of van der Waals and electrostatic interactions between individual molecules was observed, which as a consequence influenced the lowest indications of surface tension and higher values of relative viscosity. In our studies, this phenomenon was observed for the solutions of perfluoroalkylsilanes with a concentration of 0.05% for FDTS and 0.10% for FPTS (indicating their higher activity), respectively. Materials 2020, 13, 3357 5 of 15 Materials 2020, 13, x FOR PEER REVIEW 5 of 15 Figure 1. Influence of concentration of (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) and 1H, 1H, 2H, Figure 1. Influence of concentration of (3, 3, 3-trifluoropropyl) trichlorosilane (FPTS) and 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (FDTS) compound on viscosity and surface tension of solutions. 2H-perfluorodecyltrichlorosilane (FDTS) compound on viscosity and surface tension of solutions. In These resul Figure 2 ar ts a e pr re rela esented ted the to the a water ppeara contactnangle ce of (WCA) critical concent and surface ration and free ener int gye(SFE) rfacial t of e SAMs nsion cr [3eated 8,39]. Furthermore, the on DLC, Si-DLC and authors Si substrates found tha using t thi the s ac above-described tivity was higher perfluor for smal oalkylsilanes ler concentra solutions. tions of For linethe ar f investigated luorosurfactsubstrate ants, which materials, affect t the he dec highest reaWCA se in s was urfobtained ace tension after . A FDTS simi modification lar tendency fo withr 0.05% perfluconcentration, oroalkylsilanes wa (126.8 s conf  2.0ir)med i for Si-DLC n our mea coating, surem (120.9 ents. In  2.0 ad )di for tion, the strong i Si surface and (115.1 nfluence of  2.0 v ) for an DLC der Wa coating, als an while d elect achievi rostatng ic int a low eract val ions ue bet of SFE. ween Theind results ividu described al molecule above s wa allow s observed, the conclusion which a that s a the consequence alterationsinfluenced th of wettabilitye lowest in can be connected dications of with the surfac fluctuations e tensionof and surface higher tension values of relative and viscosity of viscosity. In the solution our used. studies, t The highest his phenomenon was ob values of WCA occurring served for the at the solution solution s o concentrations f perfluoroalkylsilane used tos form with SAMs a concentra revealed tion theof highest 0.05% f relative or FDTS a viscosity nd 0 a.n10% d the for lowest FPTS surface (indicat tension ing th (Figur eir hiegher 1). For activity), SAMs obtained respectively. using FDTS, these values were observed at 1.35 of viscosity and 22.50 mN/m of surface In Figure 2 are presented the water contact angle (WCA) and surface free energy (SFE) of SAMs tension of the used solution. For comparison, FPTS layers revealed the most promising values of WCA created on (117.6 D LC, Si-D 2.0 forLC and Si su Si-DLC) at 1.24 bstrat ofes u viscosity sing thand e above-descr 27.11 mN/ im bed perf of surface luoroa tension lkylsilof anes the so solution. lutions. For the investigated substrate materials, the highest WCA was obtained after FDTS modification with Additionally, a critical concentration value, above which there were no significant changes in water contact 0.05% concen angle,tr viscosity ation, (12 and 6.8 ± 2. surface 0°) for Si tension -DLC measur coatin ements, g, (120.9 was ± 2.observed. 0°) for Si su This rface e and ect indicates (115.1 ± 2. the 0°) for DLC coating, while achieving a low value of SFE. The results described above allow the conclusion changes of van der Waals and electrostatic interactions between perfluoroalkylsilane molecules and pr tha omotes t the al the tera formation tions of wet of micelles tability ca or micelle-li n be connect ke aggr ed egates. with the fl It was uctua found tions of that the surfa same ce tension a e ect is also nd viscosity of the solution used. The highest values of WCA occurring at the solution concentrations observed for surfactants [38]. used to form SAMs revealed the highest relative viscosity and the lowest surface tension (Figure 1). 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces For SAMs obtained using FDTS, these values were observed at 1.35 of viscosity and 22.50 mN/m of surface tension of the used solution. For comparison, FPTS layers revealed the most promising values The substrates with di erent silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), of WCA (117.6 ± 2.0° for Si-DLC) at 1.24 of viscosity and 27.11 mN/m of surface tension of the solution. were characterized using a FTIR spectrometer (Figure 3). The presented results show di erences in Additionally, a critical concentration value, above which there were no significant changes in water the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR 1 1 1 contact angle, viscosity and surface tension measurements, was observed. This effect indicates the spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates changes of van der Waals and electrostatic interactions between perfluoroalkylsilane molecules and that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of promotes the formation of micelles or micelle-like aggregates. It was found that the same effect is also hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by observed for surfactants [38]. Erdemir [40]. However, the influence of hydrogen on these materials will be described in the section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms that both silicon and oxygen are incorporated in the structure of Si-DLC coating. Materials 2020, 13, x FOR PEER REVIEW 6 of 15 Materials 2020, 13, x FOR PEER REVIEW 6 of 15 Materials 2020, 13, 3357 6 of 15 (a) (b) Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers (SAMs) created on three surfaces with different contents of silicon as function a concentration for (a) FPTS and (b) FDTS. 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces The substrates with different silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), were characterized using a FTIR spectrometer (Figure 3). The presented results show differences in −1 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR −1 −1 −1 spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of (a) (b) hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by Erdemir [40]. However, the influence of hydrogen on these materials will be described in the Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers Figure 2. T Water contact angle (WCA) and surface free energy (SFE) of self-assembled monolayers −1 section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned (SAMs) (SAMs) cr create eated d on on three three su surfaces rfaces with with di di ffe er rent contents ent contents oof f sili silicon con asas fufunction nction a concent a concentration ration forfor (a) −1 to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms (FP a) FPTS TS and and (b) F (b) DFDTS. TS. that both silicon and oxygen are incorporated in the structure of Si-DLC coating. 3.2. FTIR Spectroscopy Analysis of Investigated Surfaces The substrates with different silicon concentration, 0% (DLC), 32% (Si-DLC) and 100% (Si wafer), were characterized using a FTIR spectrometer (Figure 3). The presented results show differences in −1 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm . On the FTIR −1 −1 −1 spectrum for Si-DLC, C–H vibrations at 1300 cm , 1387 cm and 3105 cm are visible. This indicates that Si-DLC contains more hydrogen in its matrix than non-incorporated DLC coating. The role of hydrogen in the physical-chemical properties of diamond-like carbon coatings has been confirmed by Erdemir [40]. However, the influence of hydrogen on these materials will be described in the −1 section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm assigned −1 to the Si-OH bond and a signal at 864 cm assigned to the Si–C bond was observed. This confirms that both silicon and oxygen are incorporated in the structure of Si-DLC coating. Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like carbon carbon (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. FTIR spectro FTIR spectroscopy scopy was also used was also used for the forcharacterization the characteriof zat the ion o surface f the surf after chemical ace aftemodification. r chemical modif It wasicat found ion. It that was foun the mostd t inter hatesting the most region int for erest observing ing region thefor fluor observing t oalkyl layer he fl structur uoroa elk due yl lay to the er 1 1 major changes in spectra was from 700 to 1500 cm and in the region 2000–3000 cm . The typical spectra in these regions for the Si-DLC coating modified using FPTS and FDTS in five di erent concentrations (0.02–0.50%) are presented in Figure 4. Figure 3. Fourier transformed infrared spectroscopy (FTIR) comparative study of diamond-like carbon (DLC), silicon-incorporated DLC (Si-DLC) and Si surfaces. FTIR spectroscopy was also used for the characterization of the surface after chemical modification. It was found that the most interesting region for observing the fluoroalkyl layer Materials 2020, 13, x FOR PEER REVIEW 7 of 15 −1 structure due to the major changes in spectra was from 700 to 1500 cm and in the region 2000–3000 −1 cm . The typical spectra in these regions for the Si-DLC coating modified using FPTS and FDTS in Materials 2020, 13, 3357 7 of 15 five different concentrations (0.02–0.50%) are presented in Figure 4. Figure 4. FTIR of typical spectra for (a) Si-DLC/FPTS and (b) Si-DLC/FDTS in the ranges of Figure 4. FTIR of typical spectra for (a) Si-DLC/FPTS and (b) Si-DLC/FDTS in the ranges of 1500–700 1 1 1 −1 −1 −1 1500–700 cm , 2800–2000 cm and 3000–2800 cm . cm , 2800–2000 cm and 3000–2800 cm . Absorption maxima in the range of 950–1100 cm −1 are characteristic for Si–O–Si stretching Absorption maxima in the range of 950–1100 cm are characteristic for Si–O–Si stretching vibrations, which indicate horizontal siloxane connections and promote the formation of covalently vibrations, which indicate horizontal siloxane connections and promote the formation of covalently attached monolayers, leading to a self-organization process [41]. It is also worth noting that the attached monolayers, leading to a self-organization process [41]. It is also worth noting that the obtained maxima observed at about 900 cm −1 are associated with a deformation of vibrations obtained obtained maxima observed at about 900 cm are associated with a deformation of vibrations obtained for C–H stretches originating from –CH –CH –, a fragment of the alkyl chain. Nevertheless, the most 2 2 for C–H stretches originating from –CH2–CH2–, a fragment of the alkyl chain. Nevertheless, the most essential peaks for our investigations (identified in the range 1100–1350 cm −1) were revealed between essential peaks for our investigations (identified in the range 1100–1350 cm ) were revealed between carbon and fluorine, which correspond to asymmetric and symmetric stretches. It was established carbon and fluorine, which correspond to asymmetric and symmetric stretches. It was established that these observations are clear evidence of the modification process. The intensity of C–F peaks that these observations are clear evidence of the modification process. The intensity of C–F peaks increases with the concentration of perfluoroalkylsilane used for deposition of SAMs on three surfaces. increases with the concentration of perfluoroalkylsilane used for deposition of SAMs on three 1 1 The areas of these peaks (symmetric at 1250–1300 cm and asymmetric −1 at 1100–1200 cm ) wer−1 e surfaces. The areas of these peaks (symmetric at 1250–1300 cm and asymmetric at 1100–1200 cm ) integrated. Based on these data, the ratio of symmetric to asymmetric vibrations of C-F bonds was were integrated. Based on these data, the ratio of symmetric to asymmetric vibrations of C-F bonds calculated. In every case (Figure 5), the ratio increases with higher saturation of the investigated was calculated. In every case (Figure 5), the ratio increases with higher saturation of the investigated solution. It means that the degree of coverage of the investigated surfaces increases. It was found solution. It means that the degree of coverage of the investigated surfaces increases. It was found that that the most appropriate coverage by compounds is when the ratio of investigated areas is close to the most appropriate coverage by compounds is when the ratio of investigated areas is close to one one [42]. Comparing this fact with the measurements of the contact angle (Figure 2), we can conclude [42]. Comparing this fact with the measurements of the contact angle (Figure 2), we can conclude that that in this case, the obtained layers show the highest hydrophobicity. Confirmation of the high in this case, the obtained layers show the highest hydrophobicity. Confirmation of the high coverage coverage ratio of the analyzed surfaces using SAMs is visible on the SEM image of the surface with the ratio of the analyzed surfaces using SAMs is visible on the SEM image of the surface with the highest highest hydrophobicity (Figure 6). EDS maps of those areas indicate ~10 wt% of fluorine next to atoms hydrophobicity (Figure 6). EDS maps of those areas indicate ~10 wt% of fluorine next to atoms characteristic for modified coatings/surfaces. characteristic for modified coatings/surfaces. Materials 2020, 13, x FOR PEER REVIEW 8 of 15 Materials 2020, 13, 3357 8 of 15 Materials 2020, 13, x FOR PEER REVIEW 8 of 15 (a) (b) Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process for (a) FPTS and (b) FDTS compounds. Another factor determining the physical-chemical properties of the manufactured layers is their thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of the prepared layers [43–45]. These measurements were obtained based on the formula: (a) (b) h = nN() υ − υ , (1) 1 2 Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration Where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process versus concentration of perfluoroalkylsilanes used during the liquid phase deposition (LPD) process −1 for (a) FPTS and (b) FDTS compounds. fringes within a spectral region and υ1, υ2—starting and ending points in the spectrum in cm . for (a) FPTS and (b) FDTS compounds. Another factor determining the physical-chemical properties of the manufactured layers is their thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of the prepared layers [43–45]. These measurements were obtained based on the formula: h = nN() υ − υ , (1) 1 2 Where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of −1 fringes within a spectral region and υ1, υ2—starting and ending points in the spectrum in cm . Figure 6. Figure 6. The The scanning electro scanning electron n microscopy ( microscopy (SEM) SEM) images images of three of three differen di erent t surfaces after surfaces after creation of creation of FP FPTS TS and and FDTS SAMs FDTS SAMs.. Another factor determining the physical-chemical properties of the manufactured layers is their A condition of determining the thickness of manufactured layers from FTIR investigations based thickness. Here, FTIR spectroscopy is one of the techniques used for determination of the thickness of on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. −1 the prepared layers [43–45]. These measurements were obtained based on the formula: In our studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, h = nN/(  ), (1) 1 2 where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of fringes Figure 6. The scanning electron microscopy (SEM) images of three different surfaces after creation of within a spectral region and  ,  —starting and ending points in the spectrum in cm . 1 2 FPTS and FDTS SAMs. A condition of determining the thickness of manufactured layers from FTIR investigations based on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. In our A condition of determining the thickness of manufactured layers from FTIR investigations based studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, the starting on Equation (1) is the presence of infrared interference [46], which occurs as a “fringing effect” [47]. −1 In our studies, this effect had been observed between 2000 and 2800 cm (Figure 4). In this region, Materials 2020, 13, 3357 9 of 15 Materials 2020, 13, x FOR PEER REVIEW 9 of 15 and ending points and the number of fringes were determined. The results of the calculations of thickness the starting and ending points and the number of fringes were determined. The results of the are shown in Table 1. calculations of thickness are shown in Table 1. Table 1. Thickness [nm] of SAMs created on di erent substrates using LPD method using the solutions Table 1. Thickness [nm] of SAMs created on different substrates using LPD method using the of FPTS and FDTS compounds at five di erent concentrations, estimated in FTIR measurements. solutions of FPTS and FDTS compounds at five different concentrations, estimated in FTIR Relative uncertainty is lower than 2%. measurements. Relative uncertainty is lower than 2%. FPTS Concentration FDTS Concentration Substrate Material Substrate FPTS Concentration FDTS Concentration 0.02% 0.05% 0.10% 0.20% 0.50% 0.02% 0.05% 0.10% 0.20% 0.50% Material 0.02% 0.05% 0.10% 0.20% 0.50% 0.02% 0.05% 0.10% 0.20% 0.50% DLC 7.15 6.03 5.44 6.50 12.68 11.22 7.64 9.04 9.60 9.27 DLC 7.15 6.03 5.44 6.50 12.68 11.22 7.64 9.04 9.60 9.27 Si-DLC 6.63 5.07 4.46 5.18 8.81 5.23 3.29 4.07 4.70 6.21 Si-DLC 6.63 5.07 4.46 5.18 8.81 5.23 3.29 4.07 4.70 6.21 Si 8.76 8.65 5.09 9.89 16.45 6.61 5.81 7.39 8.82 7.97 Si 8.76 8.65 5.09 9.89 16.45 6.61 5.81 7.39 8.82 7.97 To confirm these results, a spectroscopic ellipsometry technique was used for thickness To confirm these results, a spectroscopic ellipsometry technique was used for thickness measurements (Figure 7). The obtained results indicate a thin layer of SAMs, especially for Si-DLC/0.05% measurements (Figure 7). The obtained results indicate a thin layer of SAMs, especially for Si- FDTS and Si-DLC/0.1% FPTS systems. This behavior can be explained by the di erences in the structure DLC/0.05% FDTS and Si-DLC/0.1% FPTS systems. This behavior can be explained by the differences and the tendency in orientation relative to the investigated surface. It means that shorter compounds in the structure and the tendency in orientation relative to the investigated surface. It means that like FPTS, with the length obtained from the theoretical model, determined using HyperChem 7.5 equal shorter compounds like FPTS, with the length obtained from the theoretical model, determined using to 0.70 nm, tend to form a vertical orientation, which is connected with the creation of a multi-layer HyperChem 7.5 equal to 0.70 nm, tend to form a vertical orientation, which is connected with the structure (thickness from 8.81 nm for 0.50% to 4.46 nm for 0.10%). On the other hand, longer compounds creation of a multi-layer structure (thickness from 8.81 nm for 0.50% to 4.46 nm for 0.10%). On the like FDTS, with a theoretical length equal to 1.64 nm [48], create horizontally oriented structures other hand, longer compounds like FDTS, with a theoretical length equal to 1.64 nm [48], create relative to the investigated surface and, as a consequence, better packed structured coverage. Thus, horizontally oriented structures relative to the investigated surface and, as a consequence, better it can be stated that both the type of the modifier and the solution concentration has a significant packed structured coverage. Thus, it can be stated that both the type of the modifier and the solution impact on the thickness of the layer, as can be seen in Figure 7. concentration has a significant impact on the thickness of the layer, as can be seen in Figure 7. (a) (b) Figure 7. Figure 7.Eff E ect ect of concentrati of concentration on solution of solution of perfluor perfluoroalk oalkylsilane ylsilane on thick on thickness ness and rou and roughness ghness (RMS) (RMS) of SAMs created on three surfaces with di erent contents of silicon for modifiers: (a) FPTS and (b) FDTS. of SAMs created on three surfaces with different contents of silicon for modifiers: (a) FPTS and (b) FDTS. Further analysis of FTIR spectra in the region 2800–3000 cm (Figure 4) shows di erences in C–H symmetric and asymmetric stretching vibrations that originated −1 from the hydrocarbon part of the Further analysis of FTIR spectra in the region 2800–3000 cm (Figure 4) shows differences in C– silane chain. It was found that these changes depend on the occurrence of ordered (crystalline-like) or H symmetric and asymmetric stretching vibrations that originated from the hydrocarbon part of the disordered (liquid-like) structures [49]. The lack of asymmetric C–H bonds or the occurrence of these silane chain. It was found that these changes depend on the occurrence of ordered (crystalline-like) stretches with low intensity, like in the case of Si-DLC/0.05% FDTS, indicates a crystalline-like structure. or disordered (liquid-like) structures [49]. The lack of asymmetric C–H bonds or the occurrence of This structure designates well-packed layers with good coverage, which showed two low intensity these stretches with low intensity, like in the case of Si-DLC/0.05% FDTS, indicates a crystalline-like 1 1 asymmetric stretching modes visible at 2939 cm and 2921 cm , whereas the liquid-like structure is structure. This structure designates well-packed layers with good coverage, which showed two low manifested by one or more absorption bands with high intensities −1 and/or−vibrations 1 shifted to higher intensity asymmetric stretching modes visible at 2939 cm and 2921 cm , whereas the liquid-like wavenumbers. Hence, a liquid-like structure that occurs on defects due to disorder was observed for a structure is manifested by one or more absorption bands with high intensities and/or vibrations shifted to higher wavenumbers. Hence, a liquid-like structure that occurs on defects due to disorder was observed for a system composed of Si-DLC and 0.50% FDTS (one peak with high intensity at Materials 2020, 13, x FOR PEER REVIEW 10 of 15 Materials 2020, 13, 3357 10 of 15 −1 2943 cm ). Therefore, the presence, position and intensity of these bonds decides the ordering of the system composed of Si-DLC and 0.50% FDTS (one peak with high intensity at 2943 cm ). Therefore, manufactured structures. the presence, position and intensity of these bonds decides the ordering of the manufactured structures. 3.3. Tribological Properties 3.3. Tribological Properties The tribological properties at millinewton load (values of friction coefficient) for all investigated The tribological properties at millinewton load (values of friction coefficient) for all investigated surfaces are shown in Figure 8. It was found that the Si-DLC coating showed better friction properties surfaces are shown in Figure 8. It was found that the Si-DLC coating showed better friction properties in in comparison to the DLC and silicon surface (CoF = 0.21, 0.24 and 0.31 respectively). This trend is comparison to the DLC and silicon surface (CoF = 0.21, 0.24 and 0.31 respectively). This trend is visible visible not only for unmodified surfaces but also after the modification. The changes in these not only for unmodified surfaces but also after the modification. The changes in these properties between properties between the analyzed surfaces are linked with the following factors: (i) a presence of the analyzed surfaces are linked with the following factors: (i) a presence of hydrogen, (ii) a presence of hydrogen, (ii) a presence of native silicon oxide as well, (iii) wettability of the surface and (iv) native silicon oxide as well, (iii) wettability of the surface and (iv) thickness of deposited layers. thickness of deposited layers. (b) (a) Figure 8. Coecient of friction (CoF) and work of adhesion (WoA) of SAMs created using the LPD Figure 8. Coefficient of friction (CoF) and work of adhesion (WoA) of SAMs created using the LPD process on three surfaces with di erent contents of silicon as a function of the concentration of solutions process on three surfaces with different contents of silicon as a function of the concentration of of (a) FPTS and (b) FDTS compounds. solutions of (a) FPTS and (b) FDTS compounds. The di erences in the hydrogen content of DLC and Si-DLC coatings and the Si surface were The differences in the hydrogen content of DLC and Si-DLC coatings and the Si surface were evaluated by analyzing the signals of C-H bonds (Figure 3). In the case of the Si-DLC coating, in the evaluated by analyzing the signals of C-H bonds (Figure 3). In the case of the Si-DLC coating, in the FTIR spectra three low intensity bands assigned to C-H vibrations are observed, whereas for pure DLC FTIR spectra three low intensity bands assigned to C-H vibrations are observed, whereas for pure only one strong band is visible. The area ratios of bands assigned to C–H vibrations and to other bands DLC only one strong band is visible. The area ratios of bands assigned to C–H vibrations and to other that occur in the spectra were calculated: 14% for Si-DLC and 28% for DLC coating. These results bands that occur in the spectra were calculated: 14% for Si-DLC and 28% for DLC coating. These allow the assumption that the relative content of hydrogen in DLC coating is two times higher than in results allow the assumption that the relative content of hydrogen in DLC coating is two times higher Si-DLC coating. Erdemir [40] indicate that highly hydrogenated carbon coatings during a tribological than in Si-DLC coating. Erdemir [40] indicate that highly hydrogenated carbon coatings during a test performed in the open air absorbed more oxygen species and water molecules than other surfaces. tribological test performed in the open air absorbed more oxygen species and water molecules than Therefore, one of the reasons for the occurrence of better microtribological performance obtained by the other surfaces. Therefore, one of the reasons for the occurrence of better microtribological lower coecient of friction for Si-DLC coating (0.214 0.011) compared to DLC coating (0.240 0.011) performance obtained by the lower coefficient of friction for Si-DLC coating (0.214 ± 0.011) compared is linked with the presence of native silicon forms (SiO ) [50]. Silicon oxides in combination with the to DLC coating (0.240 ± 0.011) is linked with the presence of native silicon forms (SiOx) [50]. Silicon presence of water in the form of water meniscuses enhance the formation of a Si(OH) layer at the top oxides in combination with the presence of water in the form of water meniscuses enhance the of the investigated surfaces according to the following reaction: formation of a Si(OH)4 layer at the top of the investigated surfaces according to the following reaction: SiO + 2H O → Si() OH , (2) SiO + 2H O ! Si(OH) , (2) 2 2 2 2 4 The appearance of SiOx and Si(OH)4 in this study is confirmed by the presence of Si–O and Si– The appearance of SiO and Si(OH) in this study is confirmed by the presence of Si–O and Si–OH x 4 OH signals in FTIR spectra. Figure 3 shows the differences in the presence of these vibrations between signals in FTIR spectra. Figure 3 shows the di erences in the presence of these vibrations between Si Si substrate and Si-DLC coating. It is seen that in the case of the silicon-incorporated diamond-like substrate and Si-DLC coating. It is seen that in the case of the silicon-incorporated diamond-like carbon carbon coating one peak characteristic for Si–OH with significant intensity is visible. It was coating one peak characteristic for Si–OH with significant intensity is visible. It was established by established by Bhushan that the amount of silicon oxides as well as the presence of a Si(OH)4 layer is Bhushan that the amount of silicon oxides as well as the presence of a Si(OH) layer is one of the main one of the main reasons determining microtribological behavior [51]. Therefore, in the case of silicon- reasons determining microtribological behavior [51]. Therefore, in the case of silicon-incorporated incorporated coating, presence of SiOx promotes the formation of a thin layer containing Si(OH)4 Materials 2020, 13, 3357 11 of 15 coating, presence of SiO promotes the formation of a thin layer containing Si(OH) particles in a x 4 carbon matrix, which reduces friction in comparison to DLC coating. However, the FTIR analysis showed that the Si(OH) bonds occur not only for Si-DLC coating but also for the Si surface. Results of microtribological tests showed that despite the presence of silicon oxides, silicon was the substrate with the highest value of friction coecient (0.308 0.011). This indicates that a form of presence of Si(OH) in surface layers determines their tribological properties. It appears the continuous monolayer of silicon oxide on the top of the silicon results in a high friction coecient, whereas Si(OH) particles distributed in carbon matrix composed of C-H bonds (the case of Si-DLC coating) ensures a low friction coecient. From the above-mentioned factors, the wettability of the surface has a significant impact on the microscale tribological behavior. If we look at the previously presented results of the wettability (Figure 2), it was found that the silicon surface has the lowest contact angle compared to Si-DLC and DLC. This is related to the hydrophilic character of the silicon, which can easily adsorb water from the environment. As is well known, the water has a negative influence on the friction, because between the counterbody and the surface, adhesive connections are formed which are dicult to break. In the case of modifications, it was supposed that the amount of silicon is equally important in the context of the effect of deposition of silane SAMs. It was found that silicon plays a role of anchoring center, which enhances the formation and ordering of silane structures. The tests in the millinewton load range exposed improving tribological properties by lowering the coefficient of friction after modification up to 0.132 0.011 for Si-DLC/0.05% FDTS system. It proves that thin perfluoroalkylsilane layers can provide good lubrication between Si N ball and hydrophobic surfaces. It was also demonstrated that the less effective 3 4 lubrication was obtained on more hydrophilic surfaces, which indicated lower RMS values (Figure 7) than in the case of the most hydrophobic systems (Figure 2). A possible increase of roughness after modification for less hydrophobic layers is caused by local aggregation of modifier compounds [51], which also decides the increasing thickness for these coatings and influences the values of the coefficient of friction (Figure 8). Moreover, it was found that the hydrophilic character of the modified systems favors partial forming of water meniscuses on the top of these surfaces. Hence, in the case of tribological measurements, where the ball loaded in the millinewton load range is in contact with a flat surface, the presence of attractive force is caused by meniscus force. This attractive force is called Laplace force (F ) and is expressed by the following equitation: F = 2R (cos  + cos  ), (3) L LV 2 where, R is the radius of the water sphere; is the surface tension of the liquid against air;  and LV 1 2 are the contact angles between liquid and surfaces [51–53]. Furthermore, it was established that Si N balls have a considerable radius, hence the Laplace 3 4 force and van der Waals impact is large. Therefore, meniscus force is the main factor determining adhesion in the presented investigations. Influencing these forces leads to alterations of the work of adhesion. The work of adhesion was calculated based on Equation (4). This equation introduced a quantity on real surfaces of the apparent contact angle of static water for a water drop of a solid surface using the Young-Dupre equation [51]: W = (1 +  ), (4) e LV e where,  is the equilibrium (Young’s) contact angle. The performed investigation (Figure 8) indicated a major reduction of the work of adhesion for systems modified using perfluoroalkylsilane in comparison to pure materials (67, 120 and 104 mJ/m respectively for DLC, Si and Si-DLC), which confirms the e ectiveness of the performed modifications. In the case of modification, it was also found that solution concentration has a significant e ect on the quality of the obtained monolayer and, as a consequence, on the values of adhesion work. Moreover, Liu and Bhushan [51] indicated the relationship between the structure chemistry of produced SAMs and their adhesive properties. It was presented that with higher hydrophobicity, the impact of van der Materials 2020, 13, 3357 12 of 15 Materials 2020, 13, x FOR PEER REVIEW 12 of 15 Walls force is smaller on the adhesive force. As a consequence, for hydrophobic surfaces, adhesive force is proportional to the work of adhesion [51–53]. In our measurements the work of adhesion for hydrophobic surfaces, adhesive force is proportional to the work of adhesion [51–53]. In our confirms the authenticity of the described phenomenon. Taking into account the concentration of measurements the work of adhesion confirms the authenticity of the described phenomenon. Taking perfluoroalkylsilanes, where modification is the most e ective, the work of adhesion was 23.86, 14.26 into account the concentration of perfluoroalkylsilanes, where modification is the most effective, the 2 2 and 11.75 mJ/m for FPTS and 17.79, 9.10 and 5.37 mJ/m for FDTS SAMs created on DLC, Si and Si-DLC 2 2 work of adhesion was 23.86, 14.26 and 11.75 mJ/m for FPTS and 17.79, 9.10 and 5.37 mJ/m for FDTS substrate materials, respectively (see in Figure 8). Comparing the type of modifier, the lowest value was SAMs created on DLC, Si and Si-DLC substrate materials, respectively (see in Figure 8). Comparing obtained for the system composed of Si-DLC coating and 0.05% FDTS. These studies suggest that FDTS the type of modifier, the lowest value was obtained for the system composed of Si-DLC coating and films can perform as thin, well-ordered, anti-adhesion films with good coverage, which significantly 0.05% FDTS. These studies suggest that FDTS films can perform as thin, well-ordered, anti-adhesion reduces the work of adhesion. films with good coverage, which significantly reduces the work of adhesion. The last parameter influencing the friction is the thickness of SAMs. Generally, high friction The last parameter influencing the friction is the thickness of SAMs. Generally, high friction is is caused by thick and non-ordered layers with visible traces of wear after microtribological tests, caused by thick and non-ordered layers with visible traces of wear after microtribological tests, especially for FPTS modification. For this perfluoroalkylsilane modification at the concentration of especially for FPTS modification. For this perfluoroalkylsilane modification at the concentration of 0.1% for the whole range of load and for all studied surfaces, wear tracks were registered (Figure 9). 0.1% for the whole range of load and for all studied surfaces, wear tracks were registered (Figure 9). Wear track analysis also shows that Si-DLC/0.05% FDTS presented better microtribological properties Wear track analysis also shows that Si-DLC/0.05% FDTS presented better microtribological properties in comparison to other modifications due to the lowest coecient of friction and lack of wear after in comparison to other modifications due to the lowest coefficient of friction and lack of wear after friction tests. This fact is due to the previously discussed occurrence of ordered (crystalline-like) or friction tests. This fact is due to the previously discussed occurrence of ordered (crystalline-like) or disordered (liquid-like) structures as well as layer thicknesses of 4.46 nm and 3.29 nm for FPTS and disordered (liquid-like) structures as well as layer thicknesses of 4.46 nm and 3.29 nm for FPTS and FDTS respectively. FDTS respectively. Figure 9. The SEM images of the surface of investigated layer structures after tribological tests: (a) Figure 9. The SEM images of the surface of investigated layer structures after tribological tests: DLC/0.1% FPTS, (b) Si/0.1% FPTS, (c) Si/0.1% FPTS, (d) DLC/0.05% FDTS, (e) Si/0.05% FDTS, (f) Si- (a) DLC/0.1% FPTS, (b) Si/0.1% FPTS, (c) Si/0.1% FPTS, (d) DLC/0.05% FDTS, (e) Si/0.05% FDTS, DLC/0.05% FDTS. (f) Si-DLC/0.05% FDTS. 4. 4. Con Conclusions clusions Two homolog perfluoroalkylsilanes (FPTS and FDTS) were used to form self-assembled monolayers Two homolog perfluoroalkylsilanes (FPTS and FDTS) were used to form self-assembled on monolay DLC,eSi-DLC rs on DLC, S and silicon i-DLC and substrates silicon subst via the rat liquid es via the phase liquid p deposition hase deposition technique. The technique. The di erent concentrations of perfluoroalkylsilanes solution used to form SAMs allow indication of the system different concentrations of perfluoroalkylsilanes solution used to form SAMs allow indication of the characterized system characterize by thed by t best coverage he best cover of theasurface ge of th with e surf the ace wi highest th the hi hydrophobic ghest hydrophobi properties.c properti Comparing es. these two modifications, it was found that FDTS, due to the tendency of vertical polymerization and Comparing these two modifications, it was found that FDTS, due to the tendency of vertical polymerization and more fluorine in the structure, allows manufacturing of thinner, well-covered and more crystalline-like layers with higher WCA than in the case of FPTS modification. Moreover, results of the coefficient of friction, work of adhesion and wear investigations are in a close Materials 2020, 13, 3357 13 of 15 more fluorine in the structure, allows manufacturing of thinner, well-covered and more crystalline-like layers with higher WCA than in the case of FPTS modification. Moreover, results of the coecient of friction, work of adhesion and wear investigations are in a close relationship and confirm the e ectiveness of modification using a 0.05% concentration of FDTS in toluene. The investigation also enabled the best material candidate to be chosen as a substrate for the liquid perfluoroalkylsilane modification process. It was found that the DLC coating containing silicon is more attractive than silicon wafer and pure DLC coating. This is caused by the presence in DLC coating of dispersed native silicon forms (SiO ) which constitute active centers for chemical bonds and as a consequence improved chemical modification. Concluding, the low values of adhesion and coecient of friction as well as the obtained high hydrophobicity registered, especially for Si-DLC modified using FDTS with a concentration of 0.05% (126.8  2.0 ), makes these systems attractive, especially for MEMS devices, where intensive tribochemical processes take place. Author Contributions: Conceived and designed the research coordinated the study, M.C.; chemical modification, viscosity, surface tension, tribological and wettability measurements of self-assembled layers, E.B., M.P. and M.C.; formation of Si DLC coating, A.J., M.D. and D.B.; writing, review and editing, M.C., E.B., M.P., A.J., D.B. and M.D. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Polish National Science Centre, research grant no. 2014/13/B/ST8/03114. Conflicts of Interest: The authors declare no conflict of interest. References 1. Dowling, A.P. Development of nanotechnologies. Mater. Today 2004, 7, 30–35. [CrossRef] 2. Teoh, G.Z.; Klanrit, P.; Kasimatis, M.; Seifalian, A.M. Role of nanotechnology in development of artificial organs. Minerva Med. 2015, 106, 17–33. 3. Han, B.; Cheng, G.; Wang, Y.; Wang, X. Structure and functionality design of novel carbon and faradaic electrode materials for high-performance capacitive deionization. Chem. Eng. J. 2019, 360, 364–384. [CrossRef] 4. Rehman, M.A.U.; Munawar, M.A.; Schubert, D.W.; Boccaccinia, A.R. Electrophoretic deposition of chitosan/gelatin/bioactive glass composite coatings on 316L stainless steel: A design of experiment study. Surf. Coat. Technol. 2019, 358, 976–986. [CrossRef] 5. Bashami, R.M.; Soomro, M.T.; Khan, A.N.; Aazam, E.S.; Ismail, I.M.I.; El-Shahawi, M.S. A highly conductive thin film composite based on silver nanoparticles and malic acid for selective electrochemical sensing of trichloroacetic acid. Anal. Chim. Acta 2018, 1036, 33–48. [CrossRef] 6. Love, J.C.; Estro , L.A.; Kriebel, J.K.; Nuzzo, R.G.; Whitesides, G.M. Self-assembled monolayers of thiolates on metals as a form of nanotechnology. Chem. Rev. 2005, 105, 1103–1169. [CrossRef] 7. Zheng, M.; Chen, Y.; Zhou, Y.; Tang, Y.; Lu, T. The e ect of the surface coverage of the phosphonic acid terminated self-assembled monolayers on the electrochemical behavior of dopamine. Talanta 2010, 81, 1076–1080. [CrossRef] 8. Chiadò, A.; Palmara, G.; Ricciardi, S.; Frascella, F.; Castellino, M.; Tortello, M.; Ricciardi, C.; Rivolo, P. Optimization and characterization of a homogeneous carboxylic surface functionalization for silicon-based biosensing. Colloid. Surf. B 2016, 143, 252–259. [CrossRef] [PubMed] 9. Cheng, Y.; Zheng, B.; Chuang, P.; Hsieh, S. Solvent e ects on molecular packing and tribological properties of octadecyltrichlorosilane films on silicon. Langmuir 2010, 26, 8256–8261. [CrossRef] 10. Cichomski, M.; Kosl ´ a, K.; Pawlak, W.; Kozłowski, W.; Szmaja, W. Stability and tribological investigations of 1H, 1H, 2H, 2H-perfluoroalkyltrichlorosilane on titania surface. Tribol. Int. 2014, 77, 1–6. [CrossRef] 11. Cichomski, M.; Kosla, ´ K.; Grobelny, J.; Kozłowski, W.; Kowalczyk, P.J.; Busiakiewicz, A.; Szmaja, W.; Balcerski, J. Nano-and microtribological characterization of silanes deposited on cobalt substrate. J. Alloy. Compd. 2010, 507, 273–278. [CrossRef] 12. Bhushan, B.; Cichomski, M. Nanotribological characterization of vapor phase deposited fluorosilane self-assembled monolayers deposited on polydimethylsiloxane surfaces for biomedical micro-/nanodevices. J. Vac. Sci. Technol. A 2007, 25, 1285–1293. [CrossRef] Materials 2020, 13, 3357 14 of 15 13. Xiang, H.; Komvopoulosa, K. E ect of fluorocarbon self-assembled monolayer films on sidewall adhesion and friction of surface micromachines with impacting and sliding contact interfaces. J. Appl. Phys. 2013, 113, 224505. [CrossRef] 14. Cichomski, M. Tribological investigations of perfluoroalkylsilanes monolayers deposited on titanium surface. Mater. Chem. Phys. 2012, 136, 498–504. [CrossRef] 15. Khatri, O.P.; Bain, C.D.; Biswas, S.K. E ects of chain length and heat treatment on the nanotribology of alkylsilane monolayers self-assembled on a rough aluminum surface. J. Phys. Chem. B 2005, 109, 2340–23414. [CrossRef] 16. Jabbarzadeh, A. Friction anisotropy and asymmetry in self assembled monolayers. Tribol. Int. 2016, 102, 600–607. [CrossRef] 17. Cichomski, M.; Prowizor, M.; Borkowska, E.; Piwonski, ´ I.; Jedrzejczak, ˛ A.; Dudek, M.; Batory, D.; Wronska, ´ N.; Lisowska, K. Impact of perfluoro and alkylphosphonic self-assembled monolayers on tribological and antimicrobial properties of Ti-DLC coatings. Materials 2019, 12, 2365. [CrossRef] 18. Szubert, K.; Wojciechowski, J.; Karasiewicz, J.; Maciejewski, H.; Lota, G. Corrosion-protective coatings based on fluorocarbosilane. Prog. Org. Coat. 2018, 123, 374–383. [CrossRef] 19. Pourhashema, S.; Rashidi, A.; Vaezi, M.R.; Bagherzadeh, M.R. Excellent corrosion protection performance of epoxy composite coatings filled with amino-silane functionalized graphene oxide. Surf. Coat. Technol. 2017, 317, 1–9. [CrossRef] 20. Cichomski, M.; Burnat, B.; Prowizor, M.; Jedrzejczak, A.; Batory, D.; Piwonski, ´ I.; Kozłowski, W.; Szymanski, W.; Dudek, M. Tribological and corrosive investigations of perfluoro and alkylphosphonic self-assembled monolayers on Ti incorporated carbon coatings. Tribol. Int. 2019, 130, 359–365. [CrossRef] 21. Wang, S.; Goronzy, D.P.; Young, T.D.; Wattanatorn, N.; Stewart, L.; Baše, T.; Weiss, P.S. Formation of highly ordered terminal alkyne self-assembled monolayers on the Au {111} surface through substitution of 1-decaboranethiolate. J. Phys. Chem. C 2019, 123, 1348–1353. [CrossRef] 22. Tsunoi, A.; Lkhamsuren, G.; Mondarte, E.A.Q.; Asatyas, S.; Oguchi, M.; Noh, J.; Hayashi, T. Improvement of the thermal stability of self-assembled monolayers of isocyanide derivatives on gold. J. Phys. Chem. C 2019, 123, 13681–13686. [CrossRef] 23. Cichomski, M.; Tomaszewska, E.; Kosla, ´ K.; Kozłowski, W.; Kowalczyk, P.J.; Grobelny, J. Study of dithiol monolayer as the interface for controlled deposition of gold nanoparticles. Mater. Charact. 2011, 62, 268–274. [CrossRef] 24. Kluth, G.J.; Sung, M.M.; Maboudian, R. Thermal behavior of alkylsiloxane self-assembled monolayers on the oxidized Si (100) surface. Langmuir 1997, 13, 3775–3780. [CrossRef] 25. Tripp, C.P.; Hair, M.L. Direct observation of the surface bonds between self-assembled monolayers of octadecyltrichlorosilane and silica surfaces: A low-frequency IR study at the solid/liquid interface. Langmuir 1995, 11, 1215–1219. [CrossRef] 26. Cottrell, T.L. The Strengths of Chemical Bonds; Butterworths: London, UK, 1958; pp. A21–A34. 27. Prowizor, M.; Lewkowski, J.; Rodriguez, M.M.; Morawska, M.; Cichomski, M. The modification of silicon surface by selected aminophosphonates to improve its tribological properties. Mater. Res. Express 2020, 6. [CrossRef] 28. Bystrzycka, E.; Prowizor, M.; Piwonski, I.; Kisielewska, A.; Batory, D.; Jedrzejczak, A.; Dudek, M.; Kozlowski, W.; Cichomski, M. The e ect of fluoroalkylsilanes on tribological properties and wettability of Si-DLC coatings. Mater. Res. Express 2018, 5. [CrossRef] 29. Cichomski, M.; Kisielewska, A.; Prowizor, M.; Borkowska, E.; Piwonski, ´ I.; Dudek, M.; Jedrzejczak, ˛ A.; Batory, D. The influence of self-assembled monolayers on tribological properties of Si-DLC coatings. Surf. Topogr. Metrol. Prop. 2009, 7. [CrossRef] 30. Choi, J.; Kawaguchi, M.; Kato, T.; Ikeyama, M. Deposition of Si-DLC film and its microstructural, tribological and corrosion properties. Microsyst. Technol. 2007, 13, 1353–1358. [CrossRef] 31. Zajickova, L.; Bursikova, V.; Perina, V.; Mackova, A.; Janca, J. Correlation between SiO content and properties of DLC: SiO films prepared by PECVD. Surf. Coat. Technol. 2003, 174–175, 281–285. [CrossRef] 32. Mayer, T.M.; de Boer, M.P.; Shinn, N.D.; Clews, P.J.; Michalske, T.A. Chemical vapor deposition of fluoroalkylsilane monolayer films for adhesion control in microelectromechanical systems. J. Vac. Sci. Technol. B 2000, 18, 2432–2440. [CrossRef] Materials 2020, 13, 3357 15 of 15 33. Kushmerick, J.G.; Hankins, M.G.; de Boer, M.P.; Clews, P.J.; Carpick, R.W.; Bunker, B.C. The influence of coating structure on micromachine stiction. Tribol. Lett. 2001, 10, 103–108. [CrossRef] 34. Wang, Y.; Zhang, Z.; Jain, V.; Yi, J.; Mueller, S.; Sokolov, J.; Liu, Z.; Levon, K.; Rigas, B.; Rafailovich, M.H. Potentiometric sensors based on surface molecular imprinting: Detection of cancer biomarkers and viruses. Sens. Actuators B 2010, 146, 381–387. [CrossRef] 35. Jedrzejczak, A.; Batory, D.; Dominik, M.; Smietana, M.; Cichomski, M.; Szymanski, W.; Bystrzycka, E.; Prowizor, M.; Kozlowski, W.; Dudek, M. Carbon coatings with high concentrations of silicon deposited by RF PECVD method at relatively high self-bias. Surf. Coat. Technol. 2017, 329, 212–217. [CrossRef] 36. Nordin, N.H.; Ahmad, Z. Monitoring chemical changes on the surface of borosilicate glass covers during the silanisation process. J. Phys. E Sci. Instrum. 2015, 26, 11–22. 37. Van Oss, C.J.; Chaudhury, M.K.; Good, R.J. Interfacial Lifshitz-van der Waals and polar interactions in macroscopic systems. Chem. Rev. 1988, 88, 927–940. [CrossRef] 38. Yamabe, T.; Moroi, Y.; Abe, Y.; Takahashi, T. Micelle Formation and surface adsorption of N-(1,1-Dihydroperfluorofluoroalkyl)-N, N, N-trimethylammonium chloride. Langmuir 2000, 16, 9754–9758. [CrossRef] 39. Kovalchuk, N.M.; Trybala, A.; Starov, V.; Matar, O.; Ivanova, N. Fluoro- vs hydrocarbon surfactants: Why do they di er in wetting performance? Adv. Colloid Interface 2014, 210, 65–71. [CrossRef] 40. Erdemir, A. The Role of hydrogen in tribological properties of diamond-like carbon films. Surf. Coat. Technol. 2001, 146–147, 292–297. [CrossRef] 41. Banga, R.; Yarwood, J.; Morgan, M.; Evans, B.; Kells, J. FTIR and AFM studies of the kinetics and self-assembly of alkyltrichlorosilanes and (perfluoroalkyl) trichlorosilanes onto glass and silicon. Langmuir 1995, 11, 4393–4399. [CrossRef] 42. Kang, J.F.; Jordan, R.; Ulman, A. Wetting and Fourier transform infrared spectroscopy studies of mixed self-assembled monolayers of 4-methyl-4mercaptobiphenyl and 4-hydroxy-4-mercaptobiphenyl. Langmuir 1998, 14, 3983–3985. [CrossRef] 43. Dumin, D.J. Measurement of film thickness using infrared interference. Rev. Sci. Instrum. 1967, 38. [CrossRef] 44. Griths, P.R.; de Haseth, J.A. Fourier Transform Infrared Spectrometry; John Wiley & Sons: Hoboken, NJ, USA, 2007. 45. Mirabella, F.M. Modern Techniques in Applied Molecular Spectroscopy, 2nd ed.; John Wiley & Sons: Hoboken, NJ, USA, 1998. 46. Mpilitos, C.; Amanatiadis, S.; Apostolidis, G.; Zygiridis, T.; Kantartzis, N.; Karagiannis, G. Development of a transmission line model for the thickness prediction of thin films via the infrared interference method. Technologies 2018, 6, 122. [CrossRef] 47. Ionescu, M.A.; Ionescu, C.; Ciuca, I. Polymer vapour deposition of parylene-N and parrylene-C on Si (111): Thin film characterization by FTIR and ellipsometry. Rev. Chim. 2014, 65, 1242–1244. 48. Cichomski, M.; Kosla, K.; Kozlowski, W.; Szmaja, W.; Balcerski, J.; Rogowski, J.; Grobelny, J. Investigation of the structure of fluoroalkylsilanes deposited on alumina surface. Appl. Surf. Sci. 2012, 258, 9849–9855. [CrossRef] 49. Zhang, Q.; Archer, L.A. Interfacial friction of surfaces grafted with one- and two-component self-assembled monolayers. Langmuir 2005, 21, 5405–5413. [CrossRef] 50. Jedrzejczak, A.; Kolodziejczyk, L.; Szymanski, W.; Piwonski, I.; Cichomski, M.; Kisielewska, A.; Dudek, M.; Batory, D. Friction and wear of a-C:H:SiO coatings in combination with AISI 316L and ZrO counterbodies. Tribol. Int. 2017, 112, 155–162. [CrossRef] 51. Liu, H.; Bhushan, B. Nanotribological properties and mechanisms of alkylthiol and biphenyl thiol self-assembled monolayers studied by AFM. Phys. Rev. B 2001, 63, 245412. 52. Bhushan, B. Principles and Applications of Tribology; Wiley & Sons: Hoboken, NJ, USA, 1999. 53. Israelachvili, J.N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, UK, 1992. © 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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Published: Jul 29, 2020

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