The Synthesis of a Superhydrophobic and Thermal Stable Silica Coating via Sol-Gel Process

A super-hydrophobic surface at a high temperature (400 °C) using the sol-gel method with tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) as precursors has been obtained. The effects of the coatings’ ages, deposited times and thicknesses on the hydrophobicity of the silica coatings have been analysed. The morphology, chemical composition, thermal degradation and hydrophobicity of the resulting surfaces have been studied by scanning electron microscopy (SEM), Fourier transfer infrared spectrometer (FT-IR), Thermogravimetry (TGA) and water contact angle (WCA) measurement. The results show that an average water contact angle of 149° after been cured at 400 °C for a coating aged for 5 days, and four deposition cycles using a dipping rate of 1000 mm/min was achieved.


Introduction
The phenomenon of superhydrophobicity, also known as the "Lotus Effect" in a material, occurs when water droplets roll easily on its surface with a slip angle of less than 10 • giving rise to a water contact angle (WCA) higher than 150 • . This phenomenon occurs due to the reduction of the water contact area, which leads to a reduction of adhesion [1,2].
Artificial superhydrophobic surfaces can be achieved by: (i) creating a rough surface on a hydrophobic material or (ii) chemically modifying a rough surface with a hydrophobic coating [5]. For this, different methods have been used: lithographing, templating method, phase separation, plasma treatment, sputter coating, solution-immersion process, chemical vapor deposition, physical etching and sol-gel reaction, hydrothermal synthesis, and others [6,7].
For these superhydrophobic coatings to be implemented to industry, they must be easily fabricated and durable during services. However, many superhydrophobic coating materials deteriorate at relatively low temperatures, 300-350 • C, due to raw materials' low thermal stabilities, giving rise to a surface morphology or chemistry damage of the coatings surface [21][22][23].
The sol-gel synthetic route is appropriate to obtain superhydrophobic coatings with good properties and good thermal resistance [24][25][26]. Traditionally, most of these thin films have been prepared by sol-gel method because of it is a simple process and allows one to change the parameters to obtain the desired products [5].
On the other hand, generally, the studies of superhydrophobic surfaces are focused on rigid solid substrates, such as silicon wafers, glass slides and metal surfaces. These materials must be selected considering both the treatment processes and their final application [3,27].
In this work, a porous superhydrophobic and-stable-at-high-temperatures (~400 • C) silica film was synthesized via sol-gel method. To enhance the water repellence of the silica films, methyltriethoxysilane (MTES) was used as a hydrophobic reagent in the sol-gel process. The paper describes the influence of the aging period, dipping times and the thickness of coatings on the superhydrophobic, morphological, chemical and thermal stability properties of the silica films obtained using glass substrates. This investigation provides the optimal parameters to achieve, via sol-gel, superhydrophobic coatings suitable for high-temperature applications.

Preparation of Superhydrophobic Coating
Silica particles were prepared through a process based on the Stöber method [28], using tetraethoxysilane (TEOS) as a starting material. TEOS was dissolved in ethanol. Separately, ammonia, water and ethanol were mixed. Then, two solutions were mixed, and heated at 60 • C under stirring for 90 min. Then, a solution of MTES and EtOH was added to the sol and the mixture was continuously stirred for 19 h at 60 • C with a condensation reflux device. The molar ratio of MTES:TEOS:H 2 O:EtOH:NH 4 OH was 0.16:0.24:4:14:1. The prepared white silica sol was kept for 2, 5 and 9 days under room temperature.
Glass substrates were vertically dipped using an immersion-extraction velocity of 200 mm/min into test beakers containing the sol solution for 0 and 5 min, repeating the procedure 1 and 4 times with a thermal treatment at 150 • C for 30 min after each process. Adding to this, the dipping velocity was also varied for the sol aged for 5 days; velocities of 500 and 1000 mm/min were used. Finally, the substrates obtained were annealed at 400 • C for 2 h. The thermal stability was confirmed by putting the SiO 2 coating samples in an oven at different temperatures (400, 450, 500 and 550 • C) for 2 h, and then, testing the water contact angles for each coating. In addition, the dipping rate was analysed in order to study the effect of the thickness on the hydrophobic properties.

Characterization Techniques
Fourier transform infrared spectroscopy (FTIR, FT/IR-4700LE, JASCO International Co., Ltd., Tokyo, Japan) was used in order to obtain information about the carbon-hydrogen, silica-oxygen and functional group bonds. FTIR was operated in the 4000-400 cm −1 range and the average of three scans for each sample was taken for the peak identification.
A goniometer (SURFTENS Universal, OEG GmbH, Frankfurt, Germany) was used to observe the hydrophobicity of the silica coatings by measuring the contact angle (θ) and sliding angle (α) of a water droplet of 5 µL placed on the surface of the coated glass substrates. The sliding angle was measured by tilting the stage table of the goniometer. The average value of five measurements, made at different positions of the coating surface was adopted as the value of WCA.
Morphologies of the samples were observed using a scanning electron microscope Ultra Gemini-II de Carl Zeiss SMT (Thornwood, NY, USA). The thickness of the coatings was determined with a profilometer (Dektak 8, Karlsruhe, Germany).
Thermal degradation of coating material was carried out with thermal gravimetric analysis (TGA) in a TGA/DSC1 equipment from Mettler Toledo (GIEβen, Hesse, Germany) from room temperature up to 1000 • C at 10 • C/min.

Fourier Transform Infrared Spectroscopy
The effect of the addition of MTES and the evolution of the condensation of the sol were studied by FTIR. The Figure 1 shows the FTIR spectra of sol before and after the addition of MTES (incorporation of MTES to the reaction for 2, 5 and 9 days). A broad intense band is detected between 3000 and 3800 cm −1 , with a peak at 3323 cm −1 , due to O-H vibrations and a weak deformation vibrations at 1649 cm −1 . The 3323 cm −1 and Si-OH (953 cm −1 ) bands decrease gradually during condensation reaction. In addition, two sharp bands at 2971 cm −1 and 1272 cm −1 corresponding to the absorption vibration of C-H in methyl and Si-CH 3 groups, respectively, indicate the substitution of hydroxil groups for methyl groups when MTES is added to the reaction. C-H bondsweare detected at 2928 cm −1 in a symmetric stretching mode and at 2971 cm −1 in an asymmetric stretching mode. The presence of the methyl groups is beneficial for the hydrophobic properties.
The Si-O-Si symmetric stretching vibration mode observed at 1048 cm −1 and the asymmetric mode at 1084 cm −1 indicate the hybrid nature of the material, which is in good agreement with other works [20,29]. When MTES is incorporated to the reaction, the bands increase due to the increase of Si-OH groups, and therefore, more Si-O-Si groups are formed during the condensation steps. Morphologies of the samples were observed using a scanning electron microscope Ultra Gemini-II de Carl Zeiss SMT (Thornwood, NY, USA). The thickness of the coatings was determined with a profilometer (Dektak 8, Germany) Thermal degradation of coating material was carried out with thermal gravimetric analysis (TGA) in a TGA/DSC1 equipment from Mettler Toledo (GIEβen, Germany) from room temperature up to 1000 °C at 10 °C/min.

Fourier Transform Infrared Spectroscopy
The effect of the addition of MTES and the evolution of the condensation of the sol were studied by FTIR. The Figure 1 shows the FTIR spectra of sol before and after the addition of MTES (incorporation of MTES to the reaction for 2, 5 and 9 days). A broad intense band is detected between 3000 and 3800 cm -1 , with a peak at 3323 cm -1 , due to O-H vibrations and a weak deformation vibrations at 1649 cm -1 . The 3323 cm -1 and Si-OH (953 cm -1 ) bands decrease gradually during condensation reaction. In addition, two sharp bands at 2971 cm -1 and 1272 cm -1 corresponding to the absorption vibration of C-H in methyl and Si-CH3 groups, respectively, indicate the substitution of hydroxil groups for methyl groups when MTES is added to the reaction. C-H bondsweare detected at 2928 cm -1 in a symmetric stretching mode and at 2971 cm -1 in an asymmetric stretching mode. The presence of the methyl groups is beneficial for the hydrophobic properties.
The Si-O-Si symmetric stretching vibration mode observed at 1048 cm -1 and the asymmetric mode at 1084 cm -1 indicate the hybrid nature of the material, which is in good agreement with other works [20,29]. When MTES is incorporated to the reaction, the bands increase due to the increase of Si-OH groups, and therefore, more Si-O-Si groups are formed during the condensation steps. A band of medium intensity at 953 cm -1 corresponds to Si-OH groups. However, this band is not univocally atributed to Si-OH modes. It can be overlapped with the Si-O of unreacted alcoxy groups from TEOS and MTES [30]. A weak Si-OH peak before MTES addition is observable, due to a complete hydrolisis and condensation of TEOS. After two days of MTES incorporation, the peak is sharper as a result of the presence of hydroxyl groups formed during the hydrolysis step, decreasing with time because of the condensation reaction and the transformation of these groups into Si-O-Si species. A band of medium intensity at 953 cm −1 corresponds to Si-OH groups. However, this band is not univocally atributed to Si-OH modes. It can be overlapped with the Si-O of unreacted alcoxy groups from TEOS and MTES [30]. A weak Si-OH peak before MTES addition is observable, due to a complete hydrolisis and condensation of TEOS. After two days of MTES incorporation, the peak is sharper as a result of the presence of hydroxyl groups formed during the hydrolysis step, decreasing with time because of the condensation reaction and the transformation of these groups into Si-O-Si species.
The FTIR spectrum of the optimized hydrophobic SiO 2 coatings prepared by TEOS and MTES as precursors and aged for 5 days and cured at 400 • C is shown in Figure 2. The FTIR spectrum of the optimized hydrophobic SiO2 coatings prepared by TEOS and MTES as precursors and aged for 5 days and cured at 400 °C is shown in Figure 2. As in the FTIR of the sol, the broad absorption bands at around 1649 and 3323 cm -1 indicate the presence of -OH groups. The stretching vibrations of the Si-O-Si bonds are represented by the 1048 cm -1 and 1084cm -1 absorption bands. The presence of this peak confirms the formation of a network structure inside the coating. The peak at 1272 cm -1 indicates Si-CH3 bonds due to the presence of MTES in the reaction. Finally, the absorption peaks observed at around 808 cm -1 , correspond to Si-C bonds. Si-C absorption peak indicates the presence of the CH3SiO3 groups on the surface of silica particles linked by condensation reaction of Si-OH, resulting in higher hydrophobicity [13].

Morphological Study
The representative images of the coatings deposited on glass substrates and treated at 150 °C and 400 °C are shown in Figure 3.
As expected, by comparing images for three cases, when the silica solution is deposited four times, a darker layer of particles is formed. It can be seen how the layer obtained for the case of the sol aged for 9 days, is more heterogeneous, showing zones with a higher concentration of silica particles because the degree of agglomeration of these particles increases with aging period [31][32][33].
Using ammonia, during the hydrolysis and condensation of TEOS, silica nanoparticles (with several Si-OH) progressively generate. According with other works [34,35], the hydrolysed MTES monomers cross-condense with Si-OH groups on the surface of silica particles, and then, link them into the porous network.
The Figures 4 and 5 show the SEM micrographs of the silica coatings with different dip-times and aging periods, respectively, all of them having been thermally treated at 400 °C. These reveal the size and shape of the prepared spherical silica particles, with the size of approximately 530 nm being uniform. As in the FTIR of the sol, the broad absorption bands at around 1649 and 3323 cm −1 indicate the presence of -OH groups. The stretching vibrations of the Si-O-Si bonds are represented by the 1048 cm −1 and 1084 cm −1 absorption bands. The presence of this peak confirms the formation of a network structure inside the coating. The peak at 1272 cm −1 indicates Si-CH 3 bonds due to the presence of MTES in the reaction. Finally, the absorption peaks observed at around 808 cm −1 , correspond to Si-C bonds. Si-C absorption peak indicates the presence of the CH 3 SiO 3 groups on the surface of silica particles linked by condensation reaction of Si-OH, resulting in higher hydrophobicity [13].

Morphological Study
The representative images of the coatings deposited on glass substrates and treated at 150 • C and 400 • C are shown in Figure 3.
As expected, by comparing images for three cases, when the silica solution is deposited four times, a darker layer of particles is formed. It can be seen how the layer obtained for the case of the sol aged for 9 days, is more heterogeneous, showing zones with a higher concentration of silica particles because the degree of agglomeration of these particles increases with aging period [31][32][33].
Using ammonia, during the hydrolysis and condensation of TEOS, silica nanoparticles (with several Si-OH) progressively generate. According with other works [34,35], the hydrolysed MTES monomers cross-condense with Si-OH groups on the surface of silica particles, and then, link them into the porous network.
The Figures 4 and 5 show the SEM micrographs of the silica coatings with different dip-times and aging periods, respectively, all of them having been thermally treated at 400 • C. These reveal the size and shape of the prepared spherical silica particles, with the size of approximately 530 nm being uniform.      As can be seen, the surface morphologies keep nearly the same after being immersed for 0 and 5 min. As the immersion times increase, however, the particle concentration increases, enhancing the surface roughness of the silica coating. In addition, after the aging, the SiO 2 particles begin to assemble, forming a nano-micro-binary structure ( Figure 5) due to the condensation reaction of OHgroup between SiO 2 particles [20].
Coatings 2019, 9, x FOR PEER REVIEW 6 of 13 As can be seen, the surface morphologies keep nearly the same after being immersed for 0 and 5 min. As the immersion times increase, however, the particle concentration increases, enhancing the surface roughness of the silica coating. In addition, after the aging, the SiO2 particles begin to assemble, forming a nano-micro-binary structure ( Figure 5) due to the condensation reaction of OHgroup between SiO2 particles [20]. However, the layer obtained by aging the sol for 9 days shows a surface more heterogeneous (Figure 3), being a handicap for its industrial application. Then, in order to improve the hydrophobic properties of the homogeneous obtained coatings, the coating aged for 5 days and with four immersions for 5 min was selected to study the effect of parameters such as dipping rate (and then, the thickness) on the hydrophobic properties.
Therefore, for this case, the glass substrates were dipped using faster dipping rates (500 and 1000 mm/min) for 5 min (four times). As is well known, the coating thickness increases with the coating cycle number and with the substrate velocity. Values of 520, 605-1150 and 774-1105 nm were obtained for the measured thickness of the coating using dipping rates of 200, 500 and 1000 mm/min, respectively. Therefore, in agreement with literature, the average coating thickness increases with increasing dipping rate [36][37][38].
The SEM images obtained for theses coatings are shown in Figure 6. Using dipping rates higher than 200, in this case 500 and 1000 mm/min, as it is observed for more aging days, there is a condensation reaction of OH-group between SiO2 particles giving rise to a nano-micro-binary structure. Nevertheless, the coating shows a homogeneous distribution of particles ( Figure 7).  However, the layer obtained by aging the sol for 9 days shows a surface more heterogeneous (Figure 3), being a handicap for its industrial application. Then, in order to improve the hydrophobic properties of the homogeneous obtained coatings, the coating aged for 5 days and with four immersions for 5 min was selected to study the effect of parameters such as dipping rate (and then, the thickness) on the hydrophobic properties.
Therefore, for this case, the glass substrates were dipped using faster dipping rates (500 and 1000 mm/min) for 5 min (four times). As is well known, the coating thickness increases with the coating cycle number and with the substrate velocity. Values of 520, 605-1150 and 774-1105 nm were obtained for the measured thickness of the coating using dipping rates of 200, 500 and 1000 mm/min, respectively. Therefore, in agreement with literature, the average coating thickness increases with increasing dipping rate [36][37][38].
The SEM images obtained for theses coatings are shown in Figure 6. Using dipping rates higher than 200, in this case 500 and 1000 mm/min, as it is observed for more aging days, there is a condensation reaction of OH-group between SiO 2 particles giving rise to a nano-micro-binary structure. Nevertheless, the coating shows a homogeneous distribution of particles ( Figure 7). As can be seen, the surface morphologies keep nearly the same after being immersed for 0 and 5 min. As the immersion times increase, however, the particle concentration increases, enhancing the surface roughness of the silica coating. In addition, after the aging, the SiO2 particles begin to assemble, forming a nano-micro-binary structure ( Figure 5) due to the condensation reaction of OHgroup between SiO2 particles [20]. However, the layer obtained by aging the sol for 9 days shows a surface more heterogeneous (Figure 3), being a handicap for its industrial application. Then, in order to improve the hydrophobic properties of the homogeneous obtained coatings, the coating aged for 5 days and with four immersions for 5 min was selected to study the effect of parameters such as dipping rate (and then, the thickness) on the hydrophobic properties.
Therefore, for this case, the glass substrates were dipped using faster dipping rates (500 and 1000 mm/min) for 5 min (four times). As is well known, the coating thickness increases with the coating cycle number and with the substrate velocity. Values of 520, 605-1150 and 774-1105 nm were obtained for the measured thickness of the coating using dipping rates of 200, 500 and 1000 mm/min, respectively. Therefore, in agreement with literature, the average coating thickness increases with increasing dipping rate [36][37][38].
The SEM images obtained for theses coatings are shown in Figure 6. Using dipping rates higher than 200, in this case 500 and 1000 mm/min, as it is observed for more aging days, there is a condensation reaction of OH-group between SiO2 particles giving rise to a nano-micro-binary structure. Nevertheless, the coating shows a homogeneous distribution of particles ( Figure 7).    The SEM images show that for calcination temperatures higher than 400 °C, the samples do not have a uniform size, with particle diameters being approximately, 220-710, 300-675 and 365-820 nm, for the samples calcined at 450, 500 and 550 °C, respectively. Then, as the temperature value of the thermal treatment increases, the size distribution is more heterogeneous. In addition, as can be observed, the particle concentration decreases slightly with the calcination temperature, which is caused by the adsorption between hydroxyl groups on the surface of SiO2 particles [39].   The SEM images show that for calcination temperatures higher than 400 °C, the samples do not have a uniform size, with particle diameters being approximately, 220-710, 300-675 and 365-820 nm, for the samples calcined at 450, 500 and 550 °C, respectively. Then, as the temperature value of the thermal treatment increases, the size distribution is more heterogeneous. In addition, as can be observed, the particle concentration decreases slightly with the calcination temperature, which is caused by the adsorption between hydroxyl groups on the surface of SiO2 particles [39]. The SEM images show that for calcination temperatures higher than 400 • C, the samples do not have a uniform size, with particle diameters being approximately, 220-710, 300-675 and 365-820 nm, for the samples calcined at 450, 500 and 550 • C, respectively. Then, as the temperature value of the thermal treatment increases, the size distribution is more heterogeneous. In addition, as can be observed, the particle concentration decreases slightly with the calcination temperature, which is caused by the adsorption between hydroxyl groups on the surface of SiO 2 particles [39].

Static and Sliding Water Contact Angle
The wetting behaviour of superhydrophobic surfaces depends on their chemical composition and surface microstructure.
The hydrophobic behaviour of silica coatings deposited on glass slides as a function of the duration of the dipping and the number of coatings was analysed by water contact angle (WCA). In all the cases the dipping speed was of 200 mm/min. As is well known, when a water droplet contacts a porous silica coating and spreads flat instantaneously on the surface, this coating shows a great superhydrophilic nature. The images of the static water contact angles measured for each experimental condition, are shown in Figure 9.

Static and Sliding Water Contact Angle
The wetting behaviour of superhydrophobic surfaces depends on their chemical composition and surface microstructure.
The hydrophobic behaviour of silica coatings deposited on glass slides as a function of the duration of the dipping and the number of coatings was analysed by water contact angle (WCA). In all the cases the dipping speed was of 200 mm/min. As is well known, when a water droplet contacts a porous silica coating and spreads flat instantaneously on the surface, this coating shows a great superhydrophilic nature. The images of the static water contact angles measured for each experimental condition, are shown in Figure 9. The Table 1 lists the values of the static water contact angles and sliding angles (SA) obtained for all samples as a function of the duration of the dipping and the number of coatings.   As can be observed, the coatings obtained by dipping one time exhibit smaller values of contact angle than the obtained dipping four times for the three cases, showing that the number of times that the substrate is immersed has an important influence on the hydrophobic surface properties of the material under study. This behaviour can be explained due to the increase of the particle concentration with the immersion times ( Figure 5) that enhances the surface roughness. Adding to this, an increase of the contact angle values was obtained by increasing the time during the substrate is dipped, especially for the samples aged for 5 and 9 days with four immersions, even though SEM did not show a significant difference of the particle concentration on the substrate surface ( Figure 4). These results are in good agreement with the results obtained by H. Yang et al [33].
In this study, the superhydrophobicity was reached when the number of assemblies increased up to four cycles, showing a water contact angle of 149° ± 1° for the sol aged for 9 days and a sliding angle of nearly <5°. This could be attributed to the increase of surface roughness [40]. The deposition of the aggregated silica microspheres on the glass substrates provides a rough surface, and the grooves between particles and surface roughness increases with the number of deposition cycles. The Table 1 lists the values of the static water contact angles and sliding angles (SA) obtained for all samples as a function of the duration of the dipping and the number of coatings. Table 1. Values of water contact angle (WCA) and sliding angle (SA) obtained for samples as a function of the duration of the dipping and the number of coatings. 1 time 108 ± 1 20 ± 1 115 ± 1 40 ± 1 116 ± 1 65 ± 2 117 ± 2 58 ± 2 118 ± 1 33 ± 1 120 ± 1 28 ± 1 4 times 123 ± 1 53 ± 1 121 ± 1 51 ± 2 124 ± 1 41 ± 2 132 ± 1 35 ± 1 137 ± 1 31 ± 1 149 ± 1 <5 As can be observed, the coatings obtained by dipping one time exhibit smaller values of contact angle than the obtained dipping four times for the three cases, showing that the number of times that the substrate is immersed has an important influence on the hydrophobic surface properties of the material under study. This behaviour can be explained due to the increase of the particle concentration with the immersion times ( Figure 5) that enhances the surface roughness. Adding to this, an increase of the contact angle values was obtained by increasing the time during the substrate is dipped, especially for the samples aged for 5 and 9 days with four immersions, even though SEM did not show a significant difference of the particle concentration on the substrate surface ( Figure 4). These results are in good agreement with the results obtained by H. Yang et al. [33].
In this study, the superhydrophobicity was reached when the number of assemblies increased up to four cycles, showing a water contact angle of 149 • ± 1 • for the sol aged for 9 days and a sliding angle of nearly <5 • . This could be attributed to the increase of surface roughness [40]. The deposition of the aggregated silica microspheres on the glass substrates provides a rough surface, and the grooves between particles and surface roughness increases with the number of deposition cycles. These rough structures of the films allow them to trap enough air in the grooves between particles, giving rise to a repellence of water.
On the other hand, when the coating thickness increases, the surface roughness and the hydrophobicity increase, but the transparency and mechanical stability decrease. In addition, as has been above mentioned, the layer obtained by aging the sol for 9 days showed a surface more heterogeneous. Figure 10 shows the water contact angles measured for the sample aged for 5 days with four rounds of dipping using a dipping rate of 1000 mm/min (cured at 400 • C).
Coatings 2019, 9, x FOR PEER REVIEW 9 of 13 These rough structures of the films allow them to trap enough air in the grooves between particles, giving rise to a repellence of water. On the other hand, when the coating thickness increases, the surface roughness and the hydrophobicity increase, but the transparency and mechanical stability decrease. In addition, as has been above mentioned, the layer obtained by aging the sol for 9 days showed a surface more heterogeneous. Figure 10 shows the water contact angles measured for the sample aged for 5 days with four rounds of dipping using a dipping rate of 1000 mm/min (cured at 400 °C).  Table 2 lists the values of the water contact angles obtained for the coatings aged for 5 days and dipped for 5 min four times for the samples using different dipping rates. As is shown, using higher ripping rates, higher values of the water contact angles were obtained. The above results suggested that the aging period, the deposition cycles, the time during the sample is dipped and the velocity used in the dip-coating process have great effects on the properties of the silica coatings that are critical for their desired application. The best results were obtained for the coating aged for 9 days and dipped for 5 min 4 times using a dipping rate of 200 mm/min (Table  1 and Figure 4c) and for the aged for 5 days and dipped for 5 min 4 times using a dipping rate of 1000 mm/min (Table 2 and Figure 5c). However, this last coating presents a superficial morphology more homogeneous.

Thermal Stability
The effect of the calcination temperature was confirmed by putting the SiO2 superhydrophobic coating sample in an oven at 400, 450, 500 and 550 °C for 2 h, and then, it was tested by measuring the WCAs. The coatings were obtained by using a dipping speed of 1000 mm/min for those aged for 5 days and dipped four times.
The test results are shown in Figure 11, showing that above 400 °C the hydrophobicity decreases as coating curing temperature increases. In fact, when a calcination temperature of 550 °C is applied, the WCA decreased to nearly 0°, indicating that the wetting behaviour of the surface changed from superhydrophobicity to superhydrophilicity. This phenomenon, shown at relatively low temperatures (<600 °C) in other works about siloxane surfaces, was analysed in detail by Karapanagiotis et al., by studying the effects of thermal treatment temperature and time [41].  Table 2 lists the values of the water contact angles obtained for the coatings aged for 5 days and dipped for 5 min four times for the samples using different dipping rates. As is shown, using higher ripping rates, higher values of the water contact angles were obtained. The above results suggested that the aging period, the deposition cycles, the time during the sample is dipped and the velocity used in the dip-coating process have great effects on the properties of the silica coatings that are critical for their desired application. The best results were obtained for the coating aged for 9 days and dipped for 5 min 4 times using a dipping rate of 200 mm/min (Table 1 and Figure 4c) and for the aged for 5 days and dipped for 5 min 4 times using a dipping rate of 1000 mm/min (Table 2 and Figure 5c). However, this last coating presents a superficial morphology more homogeneous.

Thermal Stability
The effect of the calcination temperature was confirmed by putting the SiO 2 superhydrophobic coating sample in an oven at 400, 450, 500 and 550 • C for 2 h, and then, it was tested by measuring the WCAs. The coatings were obtained by using a dipping speed of 1000 mm/min for those aged for 5 days and dipped four times.
The test results are shown in Figure 11, showing that above 400 • C the hydrophobicity decreases as coating curing temperature increases. In fact, when a calcination temperature of 550 • C is applied, the WCA decreased to nearly 0 • , indicating that the wetting behaviour of the surface changed from superhydrophobicity to superhydrophilicity. This phenomenon, shown at relatively low temperatures (<600 • C) in other works about siloxane surfaces, was analysed in detail by Karapanagiotis et al., by studying the effects of thermal treatment temperature and time [41]. That phenomenon takes place because the -CH3 groups on the surface of SiO2 particles are replaced by -OH groups due to the decomposition of -CH3 groups after calcining at a high temperature [34,[41][42][43].

Thermal Degradation of the Superhydrophobic Coating
The thermogravimetric analysis (TGA) curves for the coatings aged for 5 days and cured at different temperatures are shown in Figure 12. The TGA curves had a main weight loss below 200 °C due to the physically adsorbed water molecules of the environmental humidity. After that, in the coating materials cured at 400 °C and 450 °C, the weight decreased gradually up to 750 °C, by means of the degradation of organic groups. For the coatings cured at 500 °C the weight loss curve is flat up to 600 °C, due to the fact that methyl groups were degraded during the thermal treatment of the coating. The total weight loss for the coatings cured at 400 °C and 450 °C is at about 8%, and for the coatings cured at 500 °C and 550 °C, is less than 5%, which indicates that they are thermally stable and inorganic.
For the sample cured at 400 °C, the weight-loss phenomena with respect to the increase in the temperature up to 500 °C corresponds to the degradation of -CH3, and the SiO2 coating finally develops a superhydrophilic nature. This behaviour has been previously observed, showing a WCA near to 0°. As can be observed, as the curing temperature increases, the silica products show less loss of mass because they have already lost mass during the curing heating. That phenomenon takes place because the -CH 3 groups on the surface of SiO 2 particles are replaced by -OH groups due to the decomposition of -CH 3 groups after calcining at a high temperature [34,[41][42][43].

Thermal Degradation of the Superhydrophobic Coating
The thermogravimetric analysis (TGA) curves for the coatings aged for 5 days and cured at different temperatures are shown in Figure 12. The TGA curves had a main weight loss below 200 • C due to the physically adsorbed water molecules of the environmental humidity. After that, in the coating materials cured at 400 • C and 450 • C, the weight decreased gradually up to 750 • C, by means of the degradation of organic groups. For the coatings cured at 500 • C the weight loss curve is flat up to 600 • C, due to the fact that methyl groups were degraded during the thermal treatment of the coating. The total weight loss for the coatings cured at 400 • C and 450 • C is at about 8%, and for the coatings cured at 500 • C and 550 • C, is less than 5%, which indicates that they are thermally stable and inorganic.
For the sample cured at 400 • C, the weight-loss phenomena with respect to the increase in the temperature up to 500 • C corresponds to the degradation of -CH 3 , and the SiO 2 coating finally develops a superhydrophilic nature. This behaviour has been previously observed, showing a WCA near to 0 • . As can be observed, as the curing temperature increases, the silica products show less loss of mass because they have already lost mass during the curing heating.

Conclusions
The superhydrophobic SiO2 surface was successfully prepared with TEOS and MTES as precursors by a sol-gel method. The effect of the number of sol-gel coatings, the aging period and the dipping rate on the hydrophobic properties and thermal stability were analysed. Homogeneous silica coatings with static water contact angles as high as 149° were obtained for the sample aged for 5 days, dipped four times with a dipping rate of 1000 mm/min and cured at 400 °C. These thermally stable superhydrophobic coatings are potentially suitable for various industrial applications.

Conclusions
The superhydrophobic SiO 2 surface was successfully prepared with TEOS and MTES as precursors by a sol-gel method. The effect of the number of sol-gel coatings, the aging period and the dipping rate on the hydrophobic properties and thermal stability were analysed. Homogeneous silica coatings with static water contact angles as high as 149 • were obtained for the sample aged for 5 days, dipped four times with a dipping rate of 1000 mm/min and cured at 400 • C. These thermally stable superhydrophobic coatings are potentially suitable for various industrial applications.