Abstract
Abstract. The deposition of hydrophobic fluorocarbon coatings from C 2 F 6 and C 2 F 6 -H 2 rf discharges on different substrates was examined. Polyester textile, glass and two different ceramic compounds were used as substrates. The effect of the total gas pressure, the rf power dissipation and the deposition time on the hydrophobic character of the samples was investigated. Films deposited on polyester textiles at low pressure (0.03 mbar) and power consumption (16 mW/cm 2 ) using pure C 2 F 6 presented the highest water contact angles (~150 o ).
Abstract
On the other hand, the addition of hydrogen was necessary in order to deposit stable hydrophobic coatings on glass and ceramic substrates. Coatings deposited on glass at intermediate deposition rates (~100 Å/min) and pressures presented the highest angles (~105 o ).
Abstract
Concerning the heavy clay ceramics, samples treated in low pressure (0.05 mbar) and low power (16 mW/cm 2 ) discharges showed the highest contact angles The deposition time was found to play an important role on the hydrophobicity and long term behavior of porous and rough substrates.
Introduction
Recently there is a growing research and technological interest on the production of hydrophobic and self-cleaning surfaces [1] . Hydrophobic surfaces find applications in several industrial sectors as in the textiles, automotive, glass, food packaging and building materials industries. Plasma Polymerization of fluorinated organic compounds has been widely employed as a method to produce these hydrophobic and super-hydrophobic surfaces in a reliable, fast and cost effective manner [2] .
Introduction
Nevertheless, the physical and chemical properties of plasma deposited fluorocarbon coatings depend on various parameters such as the precursor gas, the substrate material and the plasma conditions.
Introduction
The decomposition of fluorocarbon gases in electric discharges leads in a first step to the production of CF x (1<x<3) fluorine atoms and ions which can either react in the gas-phase or diffuse and can take part in the surface growth. CFx radicals are the building blocks of the grown fluoropolymer film, their concentration in the gas phase directly affecting the polymerization rate. On the other hand fluorine atoms etch the surface and thus the total deposition rate is a result of these two competing processes [3] . D' Agostino et al. [4] [5] proposed an ion activated model to describe the deposition mechanism of fluorocarbon coatings. According to this model the deposition occurs on specific surface sites which are activated via ion bombardment. Moreover, O' Keefe [6] et al have pointed out that, during the initial stages of the process, growth occurs in localized areas of the substrate thus allowing interaction between plasma and the uncovered substrate surface as well as with growing film areas. This situation alters the relative rates of deposition and etching on different substrates and in consequence can determine the F/C ratio and the CFx distribution in the coating.
Introduction
The deposition rate and the chemical composition of coatings deposited on a specific substrate are determined by the relative densities of CFx radicals and fluorine in the discharge. These quantities depend on the precursor gas and the process conditions. When the F/C ratio of the fluorocarbon gas is high the F/CFx ratio in the discharge is also high and the polymerization is inhibited. However, the monomer fragmentation can be reduced by adjusting input power and pressure thus enhancing the deposition rate [7] [8] [9] [10] .
Introduction
Further increase of the deposition rate can be achieved by adding hydrogen in the gas feed [3, [11] [12] [13] [14] . Hydrogen reacts with fluorine atoms producing HF which does not etch the material.
Introduction
Moreover, H abstracts F atoms from CF x radicals (mainly from CF 3 ) towards species with lower fluorination degree. Therefore, the fluorine content and consequently the composition of the coating can be controlled by the hydrogen content in the mixture. High hydrogen content leads to coatings with lower F/C ratio, higher cross linking and thus lower hydrophobicity.
Introduction
The purpose of this work is to define optimum PECVD conditions for the deposition of stable hydrophobic coatings on different substrates such as polyester textiles, common glass as well as ceramic compounds typically used as building materials. Pure C 2 F 6 and C 2 F 6 in H 2 mixtures were used as film precursors. The effect of the gas feed composition, the total gas pressure, the real power dissipated in the plasma and the deposition time were investigated in order to obtain optimal hydrophobic properties of the coatings on these very different substrates. The variation of the deposition conditions and gas mixture required to optimize the process for each of the substrates are also discussed.
Experimental
The film depositions were carried out in a high vacuum capacitively coupled parallel plate reactor operating at the radio-frequency (RF) of 27.12 MHz (continuous wave) that has been described in detail elsewhere [15] . Briefly, the upper electrode (RF) is connected to the power generator (Dressler Cesar 276) via a pi-type automated matching network while the bottom grounded electrode works as substrate holder.
Experimental
The chamber pressure is controlled via a throttle valve controller, while the gas flow rates are independently adjusted using mass flow controllers. Pure Hexafluoroethane (Linde, N3.5) or in mixtures with Hydrogen (BOC, N6.0) were used as precursor gases.
Experimental
Several plasma diagnostics were employed for investigating the process. More precisely, electrical (rf voltage and current) measurements were applied for the determination of the real power consumed in the discharge, the discharge current and impedance, using a reliable method as described in Ref. [16] . The deposition rate on glass substrates was measured in situ using Laser Reflectance Interferometry.
Experimental
The chemical composition of the deposited coatings was studied using X-ray Photoelectron Spectroscopy (XPS, SPECS LHS-10UHV), while the films morphology was monitored with Scanning Electron Microscopy (SEM, JEOL 6300). The hydrophobic character of the coatings was evaluated by measuring the wetting angle of the sample in contact with water (wca), while measurements of the rate of water droplet absorption were also performed (Krüss, DSA100S).
Experimental
Polyester textile, common glass and clay ceramic compounds, were used as substrates. Concerning the heavy clay ceramics two different compounds were used, a red one (porosity: 25%, 72.6%SiO 2 , 10.33% CaO) and a white one (porosity: 35%, 46.9%SiO 2 , 12.33 % CaO).
Results and Discussion
The deposition of hydrophobic coatings on polyester textiles, glass and ceramic substrates from pure C 2 F 6 and C 2 F 6 -H 2 rf discharges was investigated over a wide range of discharge powers and gas pressures. The effect of pressure was studied in constant power conditions and therefore a set of plasma electrical measurements was performed prior to the depositions to establish the proper electrical conditions leading to constant power consumption as pressure is varied. Figure 1 presents the applied RF voltage (left axis) required to maintain constant power consumption as well as the discharge current (right axis) for pure C 2 F 6 ( Fig. 1a) and C 2 F 6 -H 2 1:1 mixtures (Fig. 1b) , as a function of the gas pressure. The increase of pressure imposes a drop of the applied voltage in order to maintain constant power consumption (38 mW/cm 2 for pure C 2 F 6 and 73 mW/cm 2 for C 2 F 6 /H 2 mixture). This is mainly due to the enhancement of electron-molecule inelastic collisions with pressure. It is also worth noting that the values of the self-bias voltage are almost zero for all conditions presented here since the area of the powered electrode is almost equal to the sum of the areas of the substrate holder and the reactor walls. Moreover, the frequency of 27.12 MHz used in this study favors electrical symmetry.
Results and Discussion
The total discharge current drops with pressure either in pure C 2 F 6 discharges ( fig.1a ) or in C 2 F 6 -H 2 1:1 mixtures (Fig. 1b) . The discharge current and more precisely the current flow in the bulk of the plasma presents a linear relation to the electron drift velocity and the electron density [17] . The increase of pressure will result in an electron mobility drop and consequently in a drop of electron drift
Results and Discussion
velocity. An estimation of the variation of electron density with pressure can be done by using these electrical measurements and more precisely the discharge phase impedance and the resistive part of the total discharge impedance [18, 19] . Application of the analysis of refs [17, 19] . Therefore, it appears that the lower the fragmentation of the fluorinated gas precursor, obtained by lowering both pressure and power, the higher the water contact angles of the treated textiles.
Results and Discussion
As a further step for optimization, the effect of the process time on the wca values was investigated. Table 1 Table 2 . The surface content in -COOR groups (polyester textile) decreases with processing time while the % content in -CF x -groups increases. In addition, the fluorination degree is also enhanced (35% after 120 min) revealing that the coating density becomes higher, which in turn explains the higher wca values.
Results and Discussion
Even though the deposition of stable hydrophobic coatings on polyester textiles using pure C 2 F 6 was achieved, the results were not reproducible on glass and ceramic substrates. The glass and the ceramics coated under the same conditions with the textiles were either etched or the deposited hydrophobic coatings had bad adhesion with the substrates after the contact with water droplets. Therefore, the addition of H 2 in the mixture of C 2 F 6 was tested, in order to capture fluorine atoms and favor deposition [12] .
Results and Discussion
In the conditions of fig. 1(b) (pressure range 0.1-1.0 mbar and power density 73 mW/cm2 ) depositions were performed on glass and ceramics. Figure 5 presents the deposition rate (left axis) and the fluorine content of coatings deposited on the glass substrates (right axis). Both growth rate and fluorination degree present a dcameron@example.org mbar. The maximum of the deposition rate maybe again explained by the competitive processes of deposition from CFx radicals and etching from F atoms according to the ion activation model [13] . For pressures below 0.3 mbar, the deposition rate drops (55 Å/min) due to the lower gas density and consequently to the lower rate of species production. On the other hand, the increase of pressure above 0.3 mbar causes also a decrease of the deposition rate. At higher pressures the ratio of F atoms relative to polymerization species (CF x ) seems to be favored and according to the activation model [13] this will lead to a drop of the film growth rate.
Results and Discussion
The % fluorination of the surface ( fig.5 , right axis) as determined from XPS measurements presents also a dcameron@example.org mbar. For pressures below 0.3 mbar, the ratio of F atoms to CF x precursors seems to be enhanced, according to the results of the deposition rate, as the consequent etching will reduce the fluorination degree. On the other hand, the increase of pressure will favor F abstraction by H atoms from CF x radicals (mainly by CF 3 ) leading to less fluorinated film precursors [11] [12] [13] [14] the red ceramic and these maxima were dcameron@example.org mbar for both substrates. The roughness of the treated ceramic is in μm range (Rms~1 μm) and enhances the apparent wca value, compared to treated glass (Rms~3nm) [22, 23] . It should be noted that the wca maximum values do not coincide to the maximum deposition rate (figure 5, left axis) or the fluorination degree ( figure 5, right axis) .
Results and Discussion
Nevertheless, XPS analysis revealed that the peak at 292 eV, corresponding to CF 2 groups, is more intense for samples dcameron@example.org mbar. It should be mentioned that F/C ratio of the samples varied between 0.3 and 0.6, which is much smaller than the corresponding ratio of crystalline Teflon (F/C = 2) or typical Teflon-like films. The presence of Hydrogen in the gas feed (50 %) leads to lower F/C content in the coating and also to lower wetting angles.
Results and Discussion
In order to further optimize the deposition process of stable coatings on ceramic substrates the effect of the deposition time (30 min, 60 min, 120 min and 240 m) on the hydrophobicity of the treated samples was studied. The experiments were carried out for two total gas pressures (0.05 mbar and 0.5 mbar) and two discharge rf power consumptions (16 mW/cm 2 and 73 mW/cm 2 ), while the C 2 F 6 :H 2 ratio was again 1:1. The morphological properties of the coatings were observed by SEM, while the hydrophobic character was also determined by measuring the apparent water contact angles and the absorption time of water droplets on the surface. Figure 6 presents the residence time of water droplets on treated ceramics divided by the corresponding time on a silicon wafer (which does not absorb water) as a function of the deposition time. For a deposition time of 30 min, the surface coverage is not uniform due to the complex shape of the samples and it is possible that parts of the surface remain hydrophilic. Nevertheless, the rate of absorption was smaller than that of untreated surfaces. The increase of the deposition time causes a further decrease of the absorption rate and after 60 min the droplets remain on the surface until they evaporate. At higher processing times the thickness of the fluorocarbon coating increases and gradually covers the pores of the substrate thus enhancing the hydrophobicity of the surface.
Results and Discussion
Additional heating of the substrate from the plasma due to the rather long time of the process may also affect the film structure and density. This is better illustrated in Figure 7 , where SEM images of untreated ceramic (Fig. 7a ) and coatings after 30 min (Fig. 7b) , 60 min (Fig. 7c ) and 240 min (Fig. 7d) are shown. . In both cases, the residence time of water droplets on the surface was even in that case high enough (>15 min),
Results and Discussion
indicating that the surfaces remained hydrophobic.
Conclusions
The deposition of fluorocarbon coatings on different substrates was examined and the optimum conditions which lead to hydrophobic coatings were determined for each type of substrate. The growth of hydrophobic coatings on textiles was achieved using pure C 2 F 6 as precursor gas, whereas the addition of hydrogen was necessary in order to improve the adhesion of the coatings on ceramic or glass substrates. In both cases, the effect of the gas pressure, power consumption and deposition time was investigated.
Conclusions
Concerning the textile substrates, the coatings deposited at the lowest gas pressure (0.05 mbar) and power (16 mW/cm 2 Watt) presented the highest wetting angles in contact with water, which reached the value of 148 o . By decreasing the gas pressure the density of F atoms was limited and thus the competitive etching process was suppressed. The lower power consumption led to higher wca values due to the decrease of the density ratio between F atoms and CF x radicals.
Conclusions
The total gas pressure and the power consumption showed a similar effect when hydrogen was added in the gas feed. When the total gas pressure increases, the deposition rate presents a maximum of 128 Ådcameron@example.org mbar, as a result of the competitive effect between etching and deposition.
Conclusions
However, the higher fluorination degree, gained at the higher deposition rate, did not coincide with optimum hydrophobic properties of the coatings. Films deposited on glass substrates at higher pressure Finally, the deposition of hydrophobic coatings on textiles and ceramics were strongly affected by their surface morphology. Because of the high porosity, high deposition times (120 min) were needed to achieve high wetting angles. In almost all cases the hydrophobic properties remained extremely stable over a long period of time. 

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