materials

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

Michał Cichomski 1,* , Ewelina Borkowska 1, Milena Prowizor 1, Damian Batory 2 , Anna Jedrzejczak 3 and Mariusz Dudek 3

1 Department of Materials Technology and Chemistry, Faculty of Chemistry, University of Lodz, Pomorska 163, 90-236 Lodz, Poland; [email protected] (E.B.); [email protected] (M.P.) 2 Department of Vehicles and Fundamentals of Machine Design, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; [email protected] 3 Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego 1/15, 90-924 Lodz, Poland; [email protected] (A.J.); [email protected] (M.D.) * Correspondence: [email protected]; 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 effective 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 differ 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 effect 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 affinity 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 cm2 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 different 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 differences 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 σ = 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

1 (128 scans of the pure substrate as a background was recorded) at 8 cm− resolution, in the spectral 1 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 Kr˝ussGoniometr, Kr˝uss,Hamburg, Germany). Average water contact angle (WCA) values were obtained from measurements at five different 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 coefficient of friction is the average value obtained by measuring the friction force as a function of the normal load curve. A silicon nitride (Si3N4) ball, 5 mm in diameter, was used as a counterbody. The measurements were performed in technical dry friction conditions and repeated three times in three different 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 1 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 µm 2 µm. × 3. Results and Discussion

3.1. Effect 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 different lengths of alkyl chain as a function of their concentrations (Figure1). 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

FigureFigure 1.1. InfluenceInfluence ofof concentration of (3,(3, 3, 3-trifluoropropyl)3-trifluoropropyl) trichlorosilanetrichlorosilane (FPTS)(FPTS) andand 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane2H-perfluorodecyltrichlorosilane (FDTS)(FDTS) compoundcompound onon viscosityviscosity andand surface surface tension tension of of solutions. solutions.

InThese Figure results2 are presentedare related the to water the appearance contact angle of (WCA)critical and concentration surface free and energy interfacial (SFE) of tension SAMs created[38,39]. onFurthermore, DLC, Si-DLC the and authors Si substrates found that using this the activity above-described was higher perfluoroalkylsilanes for smaller concentrations solutions. of Forlinear the fluorosurfactants, investigated substrate which materials, affect thethe highestdecrease WCA in wassurface obtained tension. after A FDTS similar modification tendency with for 0.05%perfluoroalkylsilanes concentration, (126.8 was confirmed2.0 ) for Si-DLCin our measurem coating, (120.9ents. In2.0 addition,) for Si surface the strong and (115.1influence2.0 of )van for ± ◦ ± ◦ ± ◦ DLCder Waals coating, and while electrostatic achieving interactions a low value ofbetween SFE. The individual results described molecules above was allow observed, the conclusion which thatas a theconsequence alterations influenced of wettability the canlowest be connected indications with of thesurface fluctuations tension ofand surface higher tension values and of viscosity relative ofviscosity. the solution In our used. studies, The this highest phenomenon values of was WCA ob occurringserved for at the the solutions solution of concentrations perfluoroalkylsilanes used to formwith SAMsa concentration revealed the of highest 0.05% relativefor FDTS viscosity and 0.10% and the for lowest FPTS surface (indicating tension their (Figure higher1). For activity), SAMs obtainedrespectively. using FDTS, these values were observed at 1.35 of viscosity and 22.50 mN/m of surface tensionIn Figure of the used2 are presented solution. Forthe water comparison, contact FPTSangle layers(WCA) revealed and surface the mostfree energy promising (SFE) values of SAMs of WCAcreated (117.6 on DLC,2.0 Si-DLCfor Si-DLC) and Si atsu 1.24bstrates of viscosity using the and above-described 27.11 mN/m of perfluoroalkylsilanes surface tension of the solutions. solution. ± ◦ Additionally,For the investigated a critical substrate concentration materials, value, the highes abovet which WCA therewas obtained were no after significant FDTS modification changes in water with contact0.05% concentration, angle, viscosity (126.8 and ± surface 2.0°) for tension Si-DLC measurements, coating, (120.9 was ± 2.0°) observed. for Si surface This eff andect indicates(115.1 ± 2.0°) the changesfor DLC ofcoating, van der while Waals achieving and electrostatic a low value interactions of SFE. The between results described perfluoroalkylsilane above allowmolecules the conclusion and promotesthat the alterations the formation of ofwettability micelles orcan micelle-like be connect aggregates.ed with the It wasfluctuations found that of the surface same etensionffect is alsoand observedviscosity forof the surfactants solution [used.38]. The highest values of WCA occurring at the solution concentrations used to form SAMs revealed the highest relative viscosity and the lowest surface tension (Figure 1). 3.2.For FTIRSAMs Spectroscopy obtained using Analysis FDTS, of Investigatedthese values Surfaces were observed at 1.35 of viscosity and 22.50 mN/m of surfaceThe tension substrates of the with used di ffsolution.erent silicon For comparison, concentration, FPTS 0% layers (DLC), revealed 32% (Si-DLC) the most and promising 100% (Si wafer),values wereof WCA characterized (117.6 ± 2.0° using for Si-DLC) a FTIR at spectrometer 1.24 of viscosit (Figurey and3 27.11). The mN/m presented of surface results tension show of di thefferences solution. in 1 theAdditionally, amount and a critical intensity concentration of hydroxyl value, groups above obtained which between there were 3500 no and significant 3700 cm changes− . On the in water FTIR 1 1 1 spectrumcontact angle, for Si-DLC, viscosity C–H and vibrations surface attension 1300 cm measurements,− , 1387 cm− andwas 3105 observed. cm− are This visible. effect This indicates indicates the thatchanges Si-DLC of van contains der Waals more and hydrogen electrostatic in its interactions matrix than between non-incorporated perfluoroalkylsilane DLC coating. molecules The role and of hydrogenpromotes inthe the formation physical-chemical of micelles properties or micelle-like of diamond-like aggregates. carbonIt was found coatings that has the been same confirmed effect is also by Erdemirobserved [ 40for]. surfactants However, the [38]. influence of hydrogen on these materials will be described in the section 1 related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm− assigned to the 1 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.

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(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 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm−1. On the FTIR spectrum for Si-DLC, C–H vibrations at 1300 cm−1, 1387 cm−1 and 3105 cm−1 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(a) properties of diamond-like carbon coatings(b) has been confirmed by Erdemir [40]. However, the influence of hydrogen on these materials will be described in the FigureFigure 2.2. TT WaterWater contactcontact angleangle (WCA)(WCA) andand surfacesurface freefree energyenergy (SFE)(SFE) ofofself-assembled self-assembled monolayersmonolayers section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm−1 assigned (SAMs)(SAMs) createdcreated on three surfaces surfaces with with different different contents contents of of silicon silicon as as function function a concentration a concentration forfor (a) −1 to the( FPTSaSi-OH) FPTS and bond and (b) ( bFDTS. and) FDTS. 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. 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 the amount and intensity of hydroxyl groups obtained between 3500 and 3700 cm−1. On the FTIR spectrum for Si-DLC, C–H vibrations at 1300 cm−1, 1387 cm−1 and 3105 cm−1 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 section related to the tribological behavior. Moreover, the occurrence of a signal at 938 cm−1 assigned to the Si-OH bond and a signal at 864 cm−1 assigned to the Si–C bond was observed. This confirms that both silicon and oxygen are incorporated in the structure of Si-DLC coating.

FigureFigure 3. 3. Fourier transformedtransformed infrared infrared spectroscopy spectroscopy (FTIR) (FTIR) comparative comparative study study of diamond-like of diamond-like carbon carbon(DLC), (DLC), silicon-incorporated silicon-incorporated DLC (Si-DLC) DLC (Si-DLC) and Si and surfaces. Si surfaces.

FTIRFTIR spectroscopy waswas also also used used for thefor characterization the characterization of the surface of the after surface chemical after modification. chemical modification.It was found thatIt was the found most interestingthat the most region interesting for observing region the for fluoroalkyl observing layer the structure fluoroalkyl due layer to the 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 different concentrations (0.02–0.50%) are presented in Figure4.

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

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structure due to the major changes in spectra was from 700 to 1500 cm−1 and in the region 2000–3000 Materialscm−1. The2020 typical, 13, 3357 spectra in these regions for the Si-DLC coating modified using FPTS and FDTS7 of 15in five different concentrations (0.02–0.50%) are presented in Figure 4.

Figure 4. FTIRFTIR of oftypical typical spectra spectra for (a for) Si-DLC/FPTS (a) Si-DLC/ FPTSand (b and) Si-DLC/FDTS (b) Si-DLC in/FDTS the ranges in the of ranges 1500–700 of 1 1 1 1500–700cm−1, 2800–2000 cm− , 2800–2000 cm−1 and 3000–2800 cm− and 3000–2800cm−1. cm− .

1 Absorption maxima inin thethe rangerange ofof 950–1100950–1100 cmcm−−1 are characteristiccharacteristic for Si–O–SiSi–O–Si stretchingstretching vibrations,vibrations, whichwhich indicateindicate horizontalhorizontal siloxane connectionsconnections and promote the formation of covalentlycovalently attached monolayers,monolayers, leading to aa self-organizationself-organization processprocess [[41].41]. It is also worth noting that thethe 1 obtained maximamaxima observedobserved atat aboutabout 900900 cmcm−−1 areare associated associated with a deformation deformation of vibrations obtained forfor C–HC–H stretchesstretches originatingoriginating fromfrom –CH–CH22–CH–CH22–, a fragment of the alkyl chain. Nevertheless, the most 1 essentialessential peakspeaks forfor ourour investigationsinvestigations (identified(identified inin thethe rangerange 1100–13501100–1350 cmcm−−1) were revealed between carboncarbon andand fluorine,fluorine, whichwhich correspondcorrespond toto asymmetricasymmetric andand symmetricsymmetric stretches.stretches. ItIt waswas establishedestablished thatthat thesethese observationsobservations areare clearclear evidenceevidence ofof thethe modificationmodification process.process. The intensity of C–FC–F peakspeaks increasesincreases withwith the the concentration concentration of perfluoroalkylsilaneof perfluoroalkylsilane used used for deposition for deposition of SAMs of onSAMs three on surfaces. three 1 1 Thesurfaces. areas The ofthese areas peaksof these (symmetric peaks (symmetric at 1250–1300 at 1250–1300 cm− and cm asymmetric−1 and asymmetric at 1100–1200 at 1100–1200 cm− ) werecm−1) integrated.were integrated. Based Based on these on these data, data, the ratio the ratio of symmetric of symmetric to asymmetric to asymmetric vibrations vibrations of C-F of bondsC-F bonds was calculated.was calculated. In every In every case case (Figure (Figure5), the5), the ratio ratio increases increases with with higher higher saturation saturation of of the the investigated investigated solution.solution. It It means means that that the the degree degree of of coverage coverage of ofthe the investigated investigated surfaces surfaces incr increases.eases. It was It wasfound found that thatthe most the most appropriate appropriate coverage coverage by compounds by compounds is when is when the ratio the ratio of investigated of investigated areas areas is close is close to one to one[42]. [ 42Comparing]. Comparing this thisfact with fact with the measurements the measurements of the of contact the contact angle angle (Figure (Figure 2), we2), can we canconclude conclude that thatin this in case, this case,the obtained the obtained layers layersshow the show highest the highest hydrophobicity. hydrophobicity. Confirmation Confirmation of the high of coverage the high coverageratio of the ratio analyzed of the analyzedsurfaces using surfaces SAMs using is SAMsvisible is on visible the SEM on theimage SEM of image the surface of the with surface the with highest the highesthydrophobicity hydrophobicity (Figure (Figure 6). EDS6). maps EDS maps of those of those area areass indicate indicate ~10 ~10 wt% wt% of of fluorine fluorine next next toto atomsatoms characteristiccharacteristic forfor modifiedmodified coatingscoatings/surfaces./surfaces.

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(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:

1 (a) h = nN ()υ −υ , (b) (1) 2 1 2 Figure 5. The area ratio of FTIR bands assigned to symmetric and asymmetric C-F stretching vibration Where,versusversus h—layers concentrationconcentration thickness; ofof perfluoroalkylsilanesperfluoroalkylsilanes n—refractive index usedused of duringduring the in thethevestigated liquidliquid phasephase surface/layer; depositiondeposition (LPD)(LPD) N—number processprocess of fringesforfor within ((aa)) FPTSFPTS a andandspectral ((bb)) FDTSFDTS region compounds.compounds. and υ1, υ 2—starting and ending points in the spectrum in cm−1.

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: 1 h = nN ()υ −υ , (1) 2 1 2 Where, h—layers thickness; n—refractive index of the investigated surface/layer; N—number of fringes within a spectral region and υ1, υ2—starting and ending points in the spectrum in cm−1.

Figure 6. The scanning electron microscopy (SEM) images of three different different surfaces after creation of FPTS and FDTS SAMs.

AAnother condition factor of determining determining the the thickness physical-chemical of manufactured properties layers of the from manufactured FTIR investigations layers is based their onthickness. Equation Here, (1) is FTIR the spectroscopypresence of infrared is one of interference the techniques [46], used which for occurs determination as a “fringing of the thicknesseffect” [47]. of Inthe our prepared studies, layers this effect [43–45 had]. These been measurements observed between were 2000 obtained and 2800 based cm on−1 the(Figure formula: 4). In this region,

1 h = nN/(υ υ ), (1) 2 1 − 2 where,Figureh—layers 6. Thethickness; scanning electronn—refractive microscopy index (SEM) of the images investigated of three surfacedifferent/layer; surfacesN—number after creation of fringesof 1 withinFPTS a spectral and FDTS region SAMs. and υ1, υ2—starting and ending points in the spectrum in cm− . A condition of determining the thickness of manufactured layers from FTIR investigations based on EquationA condition (1) is the of presence determining of infrared the thickness interference of manufactured [46], which occurs layers as from a “fringing FTIR investigations effect” [47]. Inbased our 1 studies,on Equation this effect (1) is had the beenpresence observed of infrared between interference 2000 and 2800 [46], cm which− (Figure occurs4). as In a this “fringing region, theeffect” starting [47]. In our studies, this effect had been observed between 2000 and 2800 cm−1 (Figure 4). In this region,

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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 Table1. calculations of thickness are shown in Table 1. Table 1. Thickness [nm] of SAMs created on different substrates using LPD method using the solutions Tableof FPTS 1. Thickness and FDTS [nm] compounds of SAMs at created five diff onerent different concentrations, substrates estimated using LPD in FTIRmethod measurements. using the solutionsRelative uncertaintyof FPTS and is lowerFDTS than compounds 2%. at five different concentrations, estimated in FTIR measurements. Relative uncertainty is lower than 2%. FPTS Concentration FDTS Concentration SubstrateSubstrate Material 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 Si6.63 5.07 8.76 4.46 8.65 5.095.18 9.898.81 16.455.23 6.613.29 5.814.07 7.39 4.70 8.82 6.21 7.97 Si 8.76 8.65 5.09 9.89 16.45 6.61 5.81 7.39 8.82 7.97

ToTo confirm confirm these these results, results, a aspectroscopic spectroscopic el ellipsometrylipsometry technique technique was was used used for for thickness thickness measurementsmeasurements (Figure 77).). The The obtained obtained results results indicate indicate a thin a layerthin oflayer SAMs, of especiallySAMs, especially for Si-DLC for/0.05% Si- DLC/0.05%FDTS and Si-DLCFDTS and/0.1% Si-DLC/0.1% FPTS systems. FPTS This systems. behavior This can behavior be explained can be by explained the differences by the in thedifferences structure inand the the structure tendency and in the orientation tendency relative in orientation to the investigated relative tosurface. the investigated It means thatsurface. shorter It means compounds that shorterlike FPTS, compounds with the like length FPTS, obtained with the from length the theoretical obtained from model, the determined theoretical using model, HyperChem determined 7.5 using equal HyperChemto 0.70 nm, tend7.5 equal to form to 0.70 a vertical nm, tend orientation, to form whicha vertical is connected orientation, with which the creation is connected of a multi-layer with the creationstructure of (thickness a multi-layer from structure 8.81 nm for (thickness 0.50% to 4.46from nm 8.81 for nm 0.10%). for 0.50% On the to other 4.46 hand,nm for longer 0.10%). compounds On the otherlike FDTS,hand, withlonger a compounds theoretical length like FDTS, equal with to 1.64 a th nmeoretical [48], create length horizontally equal to 1.64 oriented nm [48], structures create horizontallyrelative to the oriented investigated structures surface relative and, asto athe consequence, investigated better surface packed and, structuredas a consequence, coverage. better Thus, packedit can bestructured stated that coverage. both the Thus, type it ofcan the be modifierstated that and both the the solution type of concentration the modifier and has the a significant solution concentrationimpact on the has thickness a significant of the impact layer, as on can the be thickn seeness in Figureof the 7layer,. as can be seen in Figure 7.

(a) (b)

FigureFigure 7. 7. EffectEffect of of concentration concentration solution solution of of perfluor perfluoroalkylsilaneoalkylsilane on thicknessthickness andand roughnessroughness (RMS) (RMS) of ofSAMs SAMs created created on on three three surfaces surfaces with with diff differenterent contents contents of silicon of silicon for modifiers:for modifiers: (a) FPTS (a) FPTS and (andb) FDTS. (b) FDTS. 1 Further analysis of FTIR spectra in the region 2800–3000 cm− (Figure4) shows di fferences in C–H symmetricFurther andanalysis asymmetric of FTIR spectra stretching in the vibrations region 2800–3000 that originated cm−1 (Figure from the 4) shows hydrocarbon differences part in of C– the Hsilane symmetric chain. and It was asymmetric found that stretchi these changesng vibrations depend that on originated the occurrence from ofthe ordered hydrocarbon (crystalline-like) part of the or silanedisordered chain. (liquid-like)It was found structures that these [49 changes]. The lack depe ofnd asymmetric on the occurrence C–H bonds of ordered or the occurrence (crystalline-like) of these orstretches disordered with (liquid-like) low intensity, structures like in the [49]. case The of Si-DLC lack of/0.05% asymmetric FDTS, indicatesC–H bonds a crystalline-like or the occurrence structure. of theseThis stretches structure with designates low intensity, well-packed like in layers the case with of goodSi-DLC/0.05% coverage, FDTS, which indicates showed a two crystalline-like low intensity 1 1 structure.asymmetric This stretching structure modes designates visible well-packed at 2939 cm −layersand with 2921 cmgood− , coverage, whereas the which liquid-like showed structure two low is intensitymanifested asymmetric by one or stretching more absorption modes bandsvisible with at 2939 high cm intensities−1 and 2921 and cm/or− vibrations1, whereas shiftedthe liquid-like to higher structurewavenumbers. is manifested Hence, aby liquid-like one or more structure absorption that occurs bands on defectswith high due intensities to disorder and/or was observed vibrations for a 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

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2943 cm−1). Therefore, the presence, position and intensity of these bonds decides the ordering of the 1 manufacturedsystem composed structures. of Si-DLC and 0.50% FDTS (one peak with high intensity at 2943 cm− ). Therefore, 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 surfacesThe are tribological shown in Figure properties 8. It was at millinewton found that loadthe Si-DLC (values coating of friction showed coefficient) better friction for all investigatedproperties insurfaces comparison are shown to the in DLC Figure and8. Itsilicon was found surface that (CoF the Si-DLC= 0.21, coating0.24 and showed 0.31 respectively). better friction This properties trend is in visiblecomparison not only to the for DLC unmodified and silicon surfaces surface (CoFbut al=so0.21, after 0.24 the and modification. 0.31 respectively). The Thischanges trend in is these visible propertiesnot only for between unmodified the analyzed surfaces but surfaces also after are the linke modification.d with the The following changes factors: in these properties(i) a presence between of hydrogen,the analyzed (ii) surfacesa presence are linkedof native with silicon the following oxide as factors: well, (i)(iii) a presence wettability of hydrogen, of the surface (ii) a presence and (iv) of thicknessnative silicon of deposited oxide as layers. well, (iii) wettability of the surface and (iv) thickness of deposited layers.

(a) (b)

FigureFigure 8. 8. CoefficientCoefficient of of friction friction (CoF) (CoF) and and work work of of ad adhesionhesion (WoA) (WoA) of of SAMs SAMs created created using using the the LPD LPD processprocess on on three surfacessurfaces withwith di differentfferent contents contents of siliconof silicon as a functionas a function of the concentrationof the concentration of solutions of solutionsof (a) FPTS of ( anda) FPTS (b) FDTSand (b compounds.) FDTS compounds.

TheThe differences differences in in the the hydrogen hydrogen content content of of DLC DLC and and Si-DLC Si-DLC coatings coatings and and the the Si Si surface surface were were evaluatedevaluated by by analyzing analyzing the the signals signals of of C-H C-H bonds bonds (Figur (Figuree 3).3). In In the the case case of of the the Si-DLC Si-DLC coating, coating, in in the the FTIRFTIR spectra spectra three three low intensity bandsbands assignedassigned to to C-H C-H vibrations vibrations are are observed, observed, whereas whereas for purefor pure DLC DLConly only one one strong strong band band is visible. is visibl Thee. The area area ratios ratios of bands of bands assigned assigned to C–H to C–H vibrations vibrations and to and other to other bands bandsthat occurthat occur in the in spectra the spectra were calculated:were calculated: 14% for14% Si-DLC for Si-DLC and 28% and for 28% DLC for coating.DLC coating. These These results resultsallow allow the assumption the assumption that the that relative the relative content conten of hydrogent of hydrogen in DLC in coating DLC coating is two is times two highertimes higher than in thanSi-DLC in Si-DLC coating. coating. Erdemir Erdemir [40] indicate [40] indicate that highly that hydrogenated highly hydrogenated carbon coatings carbon during coatings a tribological during a tribologicaltest performed test performed in the open in air the absorbed open air more abso oxygenrbed more species oxygen and waterspecies molecules and water than molecules other surfaces. than otherTherefore, surfaces. one ofTherefore, the reasons one for theof occurrencethe reasons of betterfor the microtribological occurrence of performance better microtribological obtained by the lower coefficient 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± tois DLC linked coating with the(0.240 presence ± 0.011) of is native linked silicon with formsthe presence (SiOx)[ of50 ].native Silicon silicon oxides forms in combination (SiOx) [50]. withSilicon the oxidespresence in ofcombination water in the with form the of waterpresence meniscuses of water enhance in the theform formation of water of meniscuses a Si(OH)4 layer enhance at the the top formationof the investigated of a Si(OH) surfaces4 layer at according the top of to the the investigated followingreaction: surfaces according to the following reaction: + → () SiOSiO2 + 2H 2OO SiSi(OHOH) ,, (2) (2) 2 2 → 4 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 SiOx and Si(OH)4 in this study is confirmed by the presence of Si–O and Si–OH OHsignals signals in FTIRin FTIR spectra. spectra. Figure Figure3 shows 3 shows the the di difffferenceserences in in the the presence presence of of these these vibrations vibrations between between Si Sisubstrate substrate and and Si-DLC Si-DLC coating. coating. It isIt seenis seen that that in the in casethe case of the of silicon-incorporated the silicon-incorporated diamond-like diamond-like carbon carboncoating coating one peak one characteristic peak characteristic for Si–OH for with Si–OH significant with significant intensity is intensity visible. It is was visible. established It was 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)4 layer is one of the main onereasons of the determiningmain reasons microtribological determining microtribological behavior [51]. behavior Therefore, [51]. in Therefore, the case of in silicon-incorporated the case of silicon- incorporated coating, presence of SiOx promotes the formation of a thin layer containing Si(OH)4

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coating, presence of SiOx promotes the formation of a thin layer containing Si(OH)4 particles in a carbon matrix, which reduces friction in comparison to DLC coating. However, the FTIR analysis showed that the Si(OH)4 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 coefficient (0.308 0.011). This indicates that a form of presence of ± Si(OH)4 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 coefficient, whereas Si(OH)4 particles distributed in carbon matrix composed of C-H bonds (the case of Si-DLC coating) ensures a low friction coefficient. 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 (Figure2), 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 difficult 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 Si3N4 ball and hydrophobic surfaces. It was also demonstrated that the less effective lubrication was obtained on more hydrophilic surfaces, which indicated lower RMS values (Figure7) than in the case of the most hydrophobic systems (Figure2). 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 (Figure8). 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 (FL) and is expressed by the following equitation: FL = 2πRγLV(cos θ1 + cos θ2), (3) where, R is the radius of the water sphere; γLV is the surface tension of the liquid against air; θ1 and θ2 are the contact angles between liquid and surfaces [51–53]. Furthermore, it was established that Si3N4 balls have a considerable radius, hence the Laplace 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]:

We = γLV(1 + θe), (4) where, θe is the equilibrium (Young’s) contact angle. The performed investigation (Figure8) indicated a major reduction of the work of adhesion for systems modified using perfluoroalkylsilane in comparison to pure materials (67, 120 and 104 mJ/m2 respectively for DLC, Si and Si-DLC), which confirms the effectiveness of the performed modifications. In the case of modification, it was also found that solution concentration has a significant effect 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 forcefor hydrophobic is proportional surfaces, to the adhesive work of adhesionforce is proporti [51–53].onal In ourto the measurements work of adhesion the work [51–53]. of adhesion In our confirmsmeasurements the authenticity the work of of adhesion the described confirms phenomenon. the authenticity Taking of the into described account phenomenon. the concentration Taking of perfluoroalkylsilanes,into account the concentration where modification of perfluoroalkylsilane is the most es,ff ective,where themodification work of adhesion is the most was effective, 23.86, 14.26 the 2 2 andwork 11.75 of adhesion mJ/m for was FPTS 23.86, and 14.26 17.79, and 9.10 11.75 and 5.37mJ/m mJ2 /form FPTSfor FDTS and SAMs 17.79, created 9.10 and on 5.37 DLC, mJ/m Si and2 for Si-DLC FDTS substrateSAMs created materials, on DLC, respectively Si and Si-DLC (see in Figuresubstrate8). Comparingmaterials, respectively the type of modifier, (see in Figure the lowest 8). Comparing value was obtainedthe type of for modifier, the system the composed lowest value of Si-DLC was obtained coating andfor the 0.05% system FDTS. composed These studies of Si-DLC suggest coating that FDTS and films0.05% can FDTS. perform These as studies thin, well-ordered, suggest that FDTS anti-adhesion films can films perform with as good thin, coverage, well-ordered, which anti-adhesion significantly reducesfilms with the good work coverage, of adhesion. which significantly reduces the work of adhesion. The last parameter influencing influencing the the friction friction is is the the thickness thickness of of SAMs. SAMs. Generally, Generally, high high friction friction is iscaused caused by by thick thick and and non-ordered non-ordered layers layers with with visibl visiblee traces traces of of wear wear after after microtribological tests, especially for FPTS modification.modification. For this perfluoroalkylsilaneperfluoroalkylsilane modificationmodification at the concentration of 0.1% for the whole range of load and for all studied surfaces, wear tracks tracks were were registered registered (Figure (Figure 99).). Wear tracktrack analysisanalysis also shows that Si-DLC/0.05% Si-DLC/0.05% FDTS presented better microtribological properties in comparisoncomparison to other modificationsmodifications due to the lowestlowest coecoefficientfficient of friction and lack of wear after friction tests. This fact is due to thethe previouslypreviously discusseddiscussed occurrence of orderedordered (crystalline-like)(crystalline-like) or disordered (liquid-like) structures as well as layer thicknesses of 4.46 nm and 3.29 nm for FPTS and FDTS respectively.

Figure 9. TheThe SEM SEM images images of of the the surface surface of ofinvestigated investigated layer layer structures structures after after tribological tribological tests: tests: (a) (DLC/0.1%a) DLC/0.1% FPTS, FPTS, (b) Si/0.1% (b) Si/0.1% FPTS, FPTS, (c) Si/0.1% (c) Si/ 0.1%FPTS, FPTS, (d) DLC/0.05% (d) DLC/0.05% FDTS, FDTS, (e) Si/0.05% (e) Si/ 0.05%FDTS, FDTS,(f) Si- (DLC/0.05%f) Si-DLC/0.05% FDTS. FDTS.

4. Conclusions Two homologhomolog perfluoroalkylsilanes perfluoroalkylsilanes (FPTS (FPTS and FDTS)and FDTS) were used were to formused self-assembled to form self-assembled monolayers onmonolayers DLC, Si-DLC on DLC, and Si-DLC silicon and substrates silicon viasubstrates the liquid via phasethe liquid deposition phase deposition technique. technique. The different The concentrationsdifferent concentrations of perfluoroalkylsilanes of perfluoroalkylsilanes solution solution used to formused SAMsto form allow SAMs indication allow indication of the system of the characterizedsystem characterized by the best by coveragethe best coverage of the surface of th withe surface the highest with the hydrophobic highest hydrophobic properties. Comparingproperties. theseComparing two modifications, these two modifications, it was found thatit was FDTS, found due tothat the FDTS, tendency due of to vertical the tendency polymerization of vertical and 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 coefficient of friction, work of adhesion and wear investigations are in a close relationship and confirm the effectiveness 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 (SiOx) which constitute active centers for chemical bonds and as a consequence improved chemical modification. Concluding, the low values of adhesion and coefficient 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.; Estroff, 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 effect 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 effects on molecular packing and tribological properties of octadecyltrichlorosilane films on silicon. Langmuir 2010, 26, 8256–8261. [CrossRef] 10. Cichomski, M.; Ko´sla, 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.; Ko´sla,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. Effect 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. Effects 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.; Piwo´nski,I.; J˛edrzejczak,A.; Dudek, M.; Batory, D.; Wro´nska,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.; Piwo´nski,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.; Ko´sla,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 effect 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.; Piwo´nski,I.; Dudek, M.; J˛edrzejczak,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 SiOx content and properties of DLC: SiOx 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 differ 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. Griffiths, 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:SiOx coatings in combination with AISI 316L and ZrO2 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.

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