Development of Silane-Based Coatings with Zirconia Nanoparticles Combining Wetting, Tribological, and Aesthetical Properties

Silane-based coatings with nanoparticles have been widely used in applications related to surface protection. Between them, the improvement of corrosion resistance by increasing the hydrophobicity is one of the main research goals. However, most coatings present problems of low wear resistance and poor aesthetic appearance. Therefore, the overall goal of this research is to manufacture hydrophobic sol-gel coatings based on silanes which comply with good tribological and aesthetical properties. In the present study, stainless steel plates were coated with a silane-based solution containing zirconia nanoparticles by dip coating. Water–ethanol solutions with silanes (methyltrimethoxysilane (MTS) and tetrathoxysilane (TEOS)) and different percentages of ZrO2 nanoparticles were prepared. Gloss, color, contact angle, surface energy, wear resistance, and thickness of coating were analyzed to elucidate the effect of zirconium oxide on the performance of the coatings. Results demonstrate that the ZrO2–silane coatings on stainless steel offer a balanced combination of properties: low wettability, high wear resistance, and similar color and brightness compared to pristine stainless steel.


Introduction
Inorganic sol-gel coatings based on metal/oxygen networks are widely used for protecting metals such as stainless steel, aluminum, and copper from oxidation and corrosion.These kinds of coatings promote high corrosion resistance, but interfacial adhesion, shrinkage, and oxidation of the substrate remain a problem.To avoid these problems, organic-inorganic hybrid coatings are investigated.Besides, organic-inorganic coatings have been found to impart more hydrophobic character than the inorganic, which improves corrosion resistance [1].
Nowadays, silane-based organic coatings are highly used due to the enhancement of surface-related properties of some materials.The most extended applications of these coatings are associated with the isolation and protection in corrosive environments [2,3], surfaces with hydrophobic properties [4,5], and improvement of adhesive characteristics [6,7].Other applications are focused on the flame-retardant effect of hybrid organic-inorganic coatings [8].Good wear resistance is required for most of these applications.However, the most commonly used coatings are based on materials with poor tribological properties.
Several reports concerning the use of nanoparticles and silanes study how they modify different properties of coatings, ranging from chemical, wear, and corrosion resistance to the antibacterial and photocatalytic activity.For instance, previous studies [9][10][11][12] have shown that the addition of ceramic nanoparticles in the coatings enhances significantly the durability of these coatings, and the puncture force.Benerjee et al. [13] developed silica nanoparticle-reinforced fluorinated coatings on glass by dip coating in order to increase hardness, wear resistance, and hydrophobicity of surfaces.
Zirconium oxide (ZrO 2 ) nanoparticles have shown excellent properties such as strength, fracture toughness, wear resistance, chemical resistance, and hardness [14].ZrO 2 nanoparticles have been researched as corrosion inhibitor in sol-gel coatings, enhancing the corrosion resistance of metallic substrates [15,16].Some works [17,18] show that the addition of ZrO 2 nanoparticles improves thermal resistance as well as corrosion resistance.ZrO 2 nanofilms made by atomic layer deposition on magnesium substrate also show an improvement on corrosion resistance [19].
There are some studies about tribological properties of coatings with zirconia [20,21].Yen et al. [22] observed less wear loss and lower friction coefficient of Co-Cr-Mo alloy specimens thanks to the electrolytic deposition of ZrO 2 .However, studies about wear resistance and the aesthetic appearance of silane-based coatings with zirconia nanoparticles, as it has been mentioned previously, could not be found in literature.Therefore, the aim of the present work is to manufacture sol-gel coatings meeting low wettability, high wear resistance and transparence using an economic and easy method.Hence, tribological properties, aesthetic appearance, wettability and coating thickness are studied in this research.In order to achieve it, different percentages of zirconia nanoparticles have been added to silane solutions which have been used to coat stainless steel plates.
This coating is designed for habitat industry according to Sudoe Project, which funds this work.It could be applied on steel of kitchen (countertops, extractor hoods or appliances) or bathroom (countertops).Nowadays, metals are used in interior and exterior design by architects and designers, where this type of transparent coating with low wettability and wear resistance would be very useful.

Materials and Preparation of Coatings
Silanes used to manufacture the coatings were tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTS), both supplied by ABCR GmbH & Co. KG (Karlsruhe, Germany).They were mixed with a mixture of water and ethanol in molar proportion 1:1, and adjusted to pH 4 with 0.1 M acetic acid.The molar ratio of the final solution corresponded to the formula (T 1 M 2 ) 1 E 3 W 3 , where T = TEOS, M = MTS, E = ethanol, W = water.Both silanes were used in a previous research [23].
ZrO 2 nanoparticles were supplied by M K Impex Corp., Mississauga, ON, Canada.They were 99.99% purity and their size was 45 nm, although they are normally bonded together, as Figure 1 shows, where agglomerations of about 0.5 and 1 µm are found, formed by smaller particles.Different amounts of ZrO 2 (1 wt.%, 2.5 wt.% and 5 wt.%) were added to the solution and this final mixture was stirred for 15 min and hydrolyzed for 24 h.nanoparticles in the coatings enhances significantly the durability of these coatings, and the puncture force.Benerjee et al. [13] developed silica nanoparticle-reinforced fluorinated coatings on glass by dip coating in order to increase hardness, wear resistance, and hydrophobicity of surfaces.Zirconium oxide (ZrO2) nanoparticles have shown excellent properties such as strength, fracture toughness, wear resistance, chemical resistance, and hardness [14].ZrO2 nanoparticles have been researched as corrosion inhibitor in sol-gel coatings, enhancing the corrosion resistance of metallic substrates [15,16].Some works [17,18] show that the addition of ZrO2 nanoparticles improves thermal resistance as well as corrosion resistance.ZrO2 nanofilms made by atomic layer deposition on magnesium substrate also show an improvement on corrosion resistance [19].
There are some studies about tribological properties of coatings with zirconia [20,21].Yen et al. [22] observed less wear loss and lower friction coefficient of Co-Cr-Mo alloy specimens thanks to the electrolytic deposition of ZrO2.However, studies about wear resistance and the aesthetic appearance of silane-based coatings with zirconia nanoparticles, as it has been mentioned previously, could not be found in literature.Therefore, the aim of the present work is to manufacture sol-gel coatings meeting low wettability, high wear resistance and transparence using an economic and easy method.Hence, tribological properties, aesthetic appearance, wettability and coating thickness are studied in this research.In order to achieve it, different percentages of zirconia nanoparticles have been added to silane solutions which have been used to coat stainless steel plates.
This coating is designed for habitat industry according to Sudoe Project, which funds this work.It could be applied on steel of kitchen (countertops, extractor hoods or appliances) or bathroom (countertops).Nowadays, metals are used in interior and exterior design by architects and designers, where this type of transparent coating with low wettability and wear resistance would be very useful.

Materials and Preparation of Coatings
Silanes used to manufacture the coatings were tetraethoxysilane (TEOS) and methyltrimethoxysilane (MTS), both supplied by ABCR GmbH & Co. KG (Karlsruhe, Germany).They were mixed with a mixture of water and ethanol in molar proportion 1:1, and adjusted to pH 4 with 0.1 M acetic acid.The molar ratio of the final solution corresponded to the formula (T1M2)1E3W3, where T = TEOS, M = MTS, E = ethanol, W = water.Both silanes were used in a previous research [23].
ZrO2 nanoparticles were supplied by M K Impex Corp., Mississauga, ON, Canada.They were 99.99% purity and their size was 45 nm, although they are normally bonded together, as Figure 1 shows, where agglomerations of about 0.5 and 1 µm are found, formed by smaller particles.Different amounts of ZrO2 (1 wt.%, 2.5 wt.% and 5 wt.%) were added to the solution and this final mixture was stirred for 15 min and hydrolyzed for 24 h.The substrate used for coating was an austenitic stainless steel (EN 1.4301).This steel was cut in rectangular plates of 6 × 2 cm 2 .The stainless steel plates were coated with the nanoparticle-silane The substrate used for coating was an austenitic stainless steel (EN 1.4301).This steel was cut in rectangular plates of 6 × 2 cm 2 .The stainless steel plates were coated with the nanoparticle-silane solution by dip coating.Withdrawal speed used was 400 mm/min and immersion time was 1 min.Then, plates were dried at 110 • C for 1 h.

Surface Tension
Surface tension of solutions used for coatings was measured in order to determine their wettability on the stainless steel.Pendant drop method was used to determine it, employing a goniometer OCA 15 plus (Dataphysics, Neurtek Instruments, Éibar, Spain).Surface tension of silane solutions was measured after they were hydrolyzed (for 24 h).
Drops were formed in two different environments: air and cyclohexane.Thus, it was possible to calculate polar and dispersive components of surface tension with SCA20 software (version 1.0).

Fourier Transform Infrared Spectroscopy
A Brucker Tensor 27 spectrometer from Brucker Optik GmbH (Ettlingen, Germany) equipped with a Golden Gate ATR accessory from Specac (Orpington, UK) was used to obtain the infrared spectra of pristine reagents (pure silanes, solvent (EtOH/H 2 O) and nanoparticles) as well as silane solutions containing ZrO 2 nanoparticles.Measurements were carried out with attenuated total multiple reflection technique (ATR).The ATR-IR spectra were recorded taking 100 scans with a resolution of 4 cm −1 and wavenumber ranging from 4000 to 650 cm −1 .The background corresponded to a spectrum obtained by using the equipment without any sample.

Dynamic Light Scattering
Dynamic light scattering (DLS) using Zetasizer Nano-ZS, Malvern Instruments (Worcestershire, UK) was used to determine the size of agglomerates in the silane solution as a function of the percentage of nanoparticles.DLS measurements were carried out in the solutions with 1 wt.%, 2.5 wt.%, and 5 wt.% of zirconia.These measurements were recorded 24 h after the preparation of the solution and after stirring for, at least, 15 min.

X-ray Fluorescence
A Spectro Xepos energy dispersive X-ray fluorescence (XRF) spectrometer (Spectro Analytical Instruments, Kleve, Germany) with a palladium anode of 50 W was used to qualitatively determine the composition of coatings, measuring Si and Zr percentages.These measurements were made on the coated plates.This spectrometer was provided by Ametek Materials Analysis Division (Kleve, Germany).

Coating Thickness
To measure coating thickness, scratch tester Elcometer 3000 (Rochester Hills, MI, USA) and roughness tester (Hommelwerke GmbH, Villingen-Schwenningen, Germany) were used.It was possible to estimate the thickness with the roughness profile of the plates with a scratch that pierced the coating.For that, scratches with different loads were performed until the device detected contact with metal.
Furthermore, Olympus DSX500 opto-digital microscope (Olympus Industrial, Essex, UK) allow checking the value of the coating thickness measured by scratch.

Fourier Transform Infrared Spectroscopy
Coated surfaces were analyzed by IR spectroscopy with the same spectrometer (Brucker Optik GmbH (Ettlingen, Germany), using the reflection-absorption infrared reflectance spectroscopy (RAIR) technique.The spectra were recorded with a resolution of 4 cm −1 from 4000 to 400 cm −1 , taking 128 scans at an angle of 80 • .A stainless steel sample without coating was used as a background.

Scanning Electron Microscopy
In order to confirm the presence of ZrO 2 agglomerates on the surfaces, scanning electron microscopy (SEM) equipment (Philips XL-30 SEM, Eindhoven, The Netherlands) was used, coupled with an energy dispersive X-ray spectrometry (EDS) semiquantitative analyzer.The samples were analyzed after gold sputtered coating with Polaron SC7610 (VG Microtech, Uckfield, UK).

Color and Gloss
CIELAB system measures color in a three-dimensional space, defined by the coordinates "L*", "a*", and "b*".The first one corresponds to luminosity: 0 is black and 100 is white; coordinate "a*" measures variation between red (+a*) and green (−a*); and "b*" gives changes between yellow (+b*) and blue (−b*).Aesthetical appearance was characterized using color and specular gloss measurements, as it is required by architects and designers.L*a*b* coordinates were measured with a ColorEye XTH portable colorimeter, (X-Rite Pantone ® , Grand Rapids, MI, USA).
Finally, variation of color (∆E) was calculated (Equation ( 1)) in order to measure the module of the vector from the reference to the measurement.As a color reference, plates without coating were selected.∆E values lower than 10 are considered acceptable as it was assumed in previous works [24,25].
Specular gloss was measured using a Dr. Lange, REFO 3 reflectometer (Düsseldorf, Germany) equipment, with an angle of 85 • .At least three measurements were carried out for each coating.

Contact Angle
Hydrophobicity was determined by water static contact angle measurements.OCA 15 Plus from DataPhysics (Neurtek Instruments, Eibar, Spain) goniometer was used for doing these measurements and contact angles were determined using sessile drop method with the SCA20 software.The volume of the drops were 3 µL, and at least ten drops were measured for each sample.

Wear Resistance
The tribological tests were conducted at room temperature and 30% relative humidity (RH) with a pin-on-disk tribometer TDP/10/SCM (Microtest, Madrid, Spain).This device was equipped with a flat circle of P1000 sandpaper that wore the surface.This test directly provides the coefficient of friction as a function of the sliding distance or, in this case, number of cycles.
The parameters selected for the test were a normal load of 5 N and 100 cycles at a sliding speed of 60 rpm.The coatings were able to endure this test were subjected to another test with a normal load of 10 N at 60 rpm and the necessary cycles to produce the failure of the coatings.Three tests were carried out for each surface.

Characterization of Solutions
Using pendant drop method [26], surface tension of ethanol/water and its mix with silanes were calculated.The surface tension values are shown in Table 1, not finding meaningful differences between the solutions without nanoparticles.The most relevant finding is related to the polar and dispersive components.An increase in the polar fraction is observed for silane-solvent solution.This can be understood by taking into account that water molecules present in solution hydrolyze silane molecules, appearing silanol groups (SiOH).Surface tension increases when ZrO 2 nanoparticles are added into silane solution, finding similar values for the different ZrO 2 amounts.The most important feature is the dramatic increase of dispersive fraction when ZrO 2 nanoparticles are present in the solution.For silane solution, 79% of the surface tension value is due to its polar characteristics, while it is only 20%-39% in silane-ZrO 2 solutions.Several researches [27][28][29] inform that the ZrO 2 powder dispersed in water can easily adsorb H + and OH − ions on its surface and form a hydration layer.This layer could be related to the disappearance of a portion of SiOH groups in the solution in presence of ZrO 2 , even they may form SiOSi siloxane bonds, with dispersive character [30], by reacting with each other.It could also be explained by the bonding between these charged ZrO 2 nanoparticles with the water molecules, by hydrogen bonds, which also affects the polarity of the solution.
Total surface energy of the stainless steel substrate is 51.45 mN/m, higher than the surface tension of all silane solutions.This implies that all silane solutions will wet the substrate properly [31].Furthermore, the contact angle of the silane and silane-ZrO 2 solutions on stainless steel were measured, obtaining values lower than 10 • .
Surface tension increases when ZrO2 nanoparticles are added into silane solution, finding similar values for the different ZrO2 amounts.The most important feature is the dramatic increase of dispersive fraction when ZrO2 nanoparticles are present in the solution.For silane solution, 79% of the surface tension value is due to its polar characteristics, while it is only 20-39% in silane-ZrO2 solutions.Several researches [27][28][29] inform that the ZrO2 powder dispersed in water can easily adsorb H + and OH − ions on its surface and form a hydration layer.This layer could be related to the disappearance of a portion of SiOH groups in the solution in presence of ZrO2, even they may form SiOSi siloxane bonds, with dispersive character [30], by reacting with each other.It could also be explained by the bonding between these charged ZrO2 nanoparticles with the water molecules, by hydrogen bonds, which also affects the polarity of the solution.
Total surface energy of the stainless steel substrate is 51.45 mN/m, higher than the surface tension of all silane solutions.This implies that all silane solutions will wet the substrate properly [31].Furthermore, the contact angle of the silane and silane-ZrO2 solutions on stainless steel were measured, obtaining values lower than 10°.
To determine the species in the solutions, ATR-FTIR technique was used.Figure 2 shows the normalized FTIR spectra of ZrO2 nanoparticles (1 wt.%) in EtOH/H2O (Figure 2a) and silane solution ((T1M2)1E3W3) (Figure 2b).Peaks around 3327 cm −1 correspond to freely vibrating OH groups and hydrogen bonded OH groups.Bands at 2972 cm −1 show the presence of C-H bonds, from ethanol and the bands at 1085 and 1050 cm −1 , which are related to C-O bonds [32].
The bands due to alcoxy groups (methoxy, SiOMe, and ethoxy, SiOEt) [33][34][35] overlap in the same region than the E-W solvent bands (C-O) [36], as shown in Figure 2. Therefore, following the hydrolysis process is not possible from the disappearance of these bands.However, the band related to silanol groups (SiOH) [37], lying around 920 cm −1 (Figure 2b), affirmed that the hydrolysis process had taken place.Moreover, the presence of this band, corresponding to silanol specimens, explained the observed increase in the polar fraction when the silanes are in solution (Table 1).The condensation process occurs due to the reaction between silanol groups, according to the reactions described in Peaks around 3327 cm −1 correspond to freely vibrating OH groups and hydrogen bonded OH groups.Bands at 2972 cm −1 show the presence of C-H bonds, from ethanol and the bands at 1085 and 1050 cm −1 , which are related to C-O bonds [32].
The bands due to alcoxy groups (methoxy, SiOMe, and ethoxy, SiOEt) [33][34][35] overlap in the same region than the E-W solvent bands (C-O) [36], as shown in Figure 2. Therefore, following the hydrolysis process is not possible from the disappearance of these bands.However, the band related to silanol groups (SiOH) [37], lying around 920 cm −1 (Figure 2b), affirmed that the hydrolysis process had taken place.Moreover, the presence of this band, corresponding to silanol specimens, explained the observed increase in the polar fraction when the silanes are in solution (Table 1).The condensation process occurs due to the reaction between silanol groups, according to the reactions described in previous works [20].The bands related to these groups appear in 1190-1000 cm −1 region, also overlapping with C-O vibration modes of EtOH/H 2 O. Silane-ZrO 2 spectrum (Figure 2b) shows that these signals (at 1085 and 1050 cm −1 ) have a shoulder shifted to high wavenumber (around 1160 cm −1 ) due to siloxane bonds.
Regarding to the silanol (Si-OH) bands in FTIR spectra of the coatings, it is needed to have a large number of SiOH groups in the solution for getting a good adhesion with the substrate, as it is demonstrated in previous works [6].In this way, hydrolysis time and pH were optimized to the values used in this work (1 h and pH 4).
Several studies [38,39] assign the bands at 3780-3660 and 1560 cm −1 to the absorption bands of ZrOH and Zr-H vibrations.These bands cannot be assigned because both of them appear slapped with the bands of solvent.
According to the supplier, the mean raw ZrO 2 particle size is 45 nm.DLS measurements can determine the agglomeration degree of these nanoparticles in the solvent (ethanol-water) and its ability to disperse them.Results of number size distributions are displayed in Figure 3, for the different percentages of ZrO 2 .Particles with size much higher than 45 nm are found.It indicates the occurrence of agglomeration when the particles are in the solvent.A bimodal distribution is observed for the solution with 1 wt.% of ZrO 2 , where two predominant particle sizes are observed, around at 1260 and 4780 nm.The distribution only has one peak for 2.5 wt.% and 5 wt.%ZrO 2 , centered at 3200 and 1500 nm, respectively.
The differences on agglomeration sizes are explained by the equilibrium between attractive and repulsive forces between the nanoparticles, which present a dipolar moment due to the differences of electronegativity of the elements that form them.The highest attractive forces are found for 2.5 wt.% ZrO 2 , being the repulsive forces increased for a higher percentage.It can be considered that the system works as ions (in this case dipoles) in an ionic bond.previous works [20].The bands related to these groups appear in 1190-1000 cm −1 region, also overlapping with C-O vibration modes of EtOH/H2O.Silane-ZrO2 spectrum (Figure 2b) shows that these signals (at 1085 and 1050 cm −1 ) have a shoulder shifted to high wavenumber (around 1160 cm −1 ) due to siloxane bonds.
Regarding to the silanol (Si-OH) bands in FTIR spectra of the coatings, it is needed to have a large number of SiOH groups in the solution for getting a good adhesion with the substrate, as it is demonstrated in previous works [6].In this way, hydrolysis time and pH were optimized to the values used in this work (1 h and pH 4).
Several studies [38,39] assign the bands at 3780-3660 and 1560 cm −1 to the absorption bands of ZrOH and Zr-H vibrations.These bands cannot be assigned because both of them appear slapped with the bands of solvent.
According to the supplier, the mean raw ZrO2 particle size is 45 nm.DLS measurements can determine the agglomeration degree of these nanoparticles in the solvent (ethanol-water) and its ability to disperse them.Results of number size distributions are displayed in Figure 3, for the different percentages of ZrO2.Particles with size much higher than 45 nm are found.It indicates the occurrence of agglomeration when the particles are in the solvent.A bimodal distribution is observed for the solution with 1 wt.% of ZrO2, where two predominant particle sizes are observed, around at 1260 and 4780 nm.The distribution only has one peak for 2.5 wt.% and 5 wt.%ZrO2, centered at 3200 and 1500 nm, respectively.
The differences on agglomeration sizes are explained by the equilibrium between attractive and repulsive forces between the nanoparticles, which present a dipolar moment due to the differences of electronegativity of the elements that form them.The highest attractive forces are found for 2.5 wt.% ZrO2, being the repulsive forces increased for a higher percentage.It can be considered that the system works as ions (in this case dipoles) in an ionic bond.Tang et al. [28] measured the particle size of ZrO2 nanoparticles in aqueous solution at different pH conditions, indicating that ZrO2 nanoparticles agglomeration is due to ZrO2 surface charging as a consequence of adsorption of ions on its surface.They suggested that these charged nanoparticles can easily agglomerate by formation of interactions (hydrogen bonds or van der Waals attractions) with each other.This explains that the particle size found is larger than that supposed from supplied powder (45 nm) due to agglomeration, and it should provide an understanding of why the polar contribution at surface tension for silane solution decreases in presence of ZrO2 nanoparticles.Tang et al. [28] measured the particle size of ZrO 2 nanoparticles in aqueous solution at different pH conditions, indicating that ZrO 2 nanoparticles agglomeration is due to ZrO 2 surface charging as a consequence of adsorption of ions on its surface.They suggested that these charged nanoparticles can easily agglomerate by formation of interactions (hydrogen bonds or van der Waals attractions) with each other.This explains that the particle size found is larger than that supposed from supplied powder (45 nm) due to agglomeration, and it should provide an understanding of why the polar contribution at surface tension for silane solution decreases in presence of ZrO 2 nanoparticles.

Characterization of Coatings
XRF results are summarized in Table 2.The presence of silane coating and ZrO 2 nanoparticles is verified from the silicon and zirconium percentages measured, respectively.It is observed that the amount of silane on the substrate increases when there is zirconia in the coatings.Silicon percentages higher than 18% are measured for ZrO 2 -silane coatings, while lower than 15% are found for the coating without ZrO 2 .This is coherent with the surface tension and DLS results argued above.If interactions are established between SiOH groups with each other and/or with ZrO 2 , the amount of silane-ZrO 2 solution on metallic substrate should be higher too, explaining the XRF results observed in Table 2.These results are directly related to coating thickness since the penetration depth of XRF is higher than it.To validate this theory, coating thickness was estimated using scratch and roughness measurements and using the optical profilometer (Table 2).Silane coating thickness (0 wt.% ZrO 2 ) could not be measured as it was too thin.Results show that the higher the zirconia percentage in the silane solution, the greater the amount of ZrO 2 agglomerates deposited on the substrate (XRF) and the thicker the coating formed.Thickness measured with opto-digital microscope present similar values than the estimation with scratch.In Figure 4, images taken with the microscope are shown.

Characterization of Coatings
XRF results are summarized in Table 2.The presence of silane coating and ZrO2 nanoparticles is verified from the silicon and zirconium percentages measured, respectively.It is observed that the amount of silane on the substrate increases when there is zirconia in the coatings.Silicon percentages higher than 18% are measured for ZrO2-silane coatings, while lower than 15% are found for the coating without ZrO2.This is coherent with the surface tension and DLS results argued above.If interactions are established between SiOH groups with each other and/or with ZrO2, the amount of silane-ZrO2 solution on metallic substrate should be higher too, explaining the XRF results observed in Table 2.These results are directly related to coating thickness since the penetration depth of XRF is higher than it.To validate this theory, coating thickness was estimated using scratch and roughness measurements and using the optical profilometer (Table 2).Silane coating thickness (0 wt.% ZrO2) could not be measured as it was too thin.Results show that the higher the zirconia percentage in the silane solution, the greater the amount of ZrO2 agglomerates deposited on the substrate (XRF) and the thicker the coating formed.Thickness measured with opto-digital microscope present similar values than the estimation with scratch.In Figure 4, images taken with the microscope are shown.Figure 5 shows the FTIR spectra for the different coatings.Peaks at 3327 cm −1 correspond to freely vibrating OH groups, hydrogen bonded OH groups or OH groups associated to SiOH, ZrOH, and ZrH bonds [33,34,36,40].Its presence in the coating can be related to OH groups of solvents or formed in hydrolysis and condensation reactions of silanes as subproduct (H2O) [34].Band at 2972 cm −1 is related to vibration bands C-H of alkoxy groups or solvent (occluded ethanol).Si-CH3 bond is assigned to the peak at 1277 and 781 cm −1 [6,38], being directly related to MTS silane.Bands between 1160 and 1050 cm −1 correspond to the condensation species (Si-O-Si) [33,41] by reaction between silanol groups when the coating is curing into the oven at 110 °C.C-O bands due to ethanol can be overlapped under this broad band of siloxane groups.Finally, peaks at 910 cm −1 are related to silanol groups (Si-OH) [6].Differences among spectra as a function of amount of ZrO2 have not been found.Morphological differences among surface coatings are clearly observed in Figure 6. Figure 6a,c,e shows that the amount of ZrO2 agglomerates observed on the surface increases with the ZrO2 percentage added into silane solution, as it was observed by XRF as well.Besides, different sizes of agglomerates are distributed on the whole surface.The average particle size estimated from these SEM images is around 2 µm, coinciding with DLS results (Figure 3).The images at higher magnifications (Figure 6b,d,f) show that these large agglomerates consist of smaller ones.Figure 5 shows the FTIR spectra for the different coatings.Peaks at 3327 cm correspond to freely vibrating OH groups, hydrogen bonded OH groups or OH groups associated to SiOH, ZrOH, and ZrH bonds [33,34,36,40].Its presence in the coating can be related to OH groups of solvents or formed in hydrolysis and condensation reactions of silanes as subproduct (H 2 O) [34].Band at 2972 cm −1 is related to vibration bands C-H of alkoxy groups or solvent (occluded ethanol).Si-CH 3 bond is assigned to the peak at 1277 and 781 cm −1 [6,38], being directly related to MTS silane.Bands between 1160 and 1050 cm −1 correspond to the condensation species (Si-O-Si) [33,41] by reaction between silanol groups when the coating is curing into the oven at 110 • C. C-O bands due to ethanol can be overlapped under this broad band of siloxane groups.Finally, peaks at 910 cm −1 are related to silanol groups (Si-OH) [6].Differences among spectra as a function of amount of ZrO 2 have not been found.Figure 5 shows the FTIR spectra for the different coatings.Peaks at 3327 cm −1 correspond to freely vibrating OH groups, hydrogen bonded OH groups or OH groups associated to SiOH, ZrOH, and ZrH bonds [33,34,36,40].Its presence in the coating can be related to OH groups of solvents or formed in hydrolysis and condensation reactions of silanes as subproduct (H2O) [34].Band at 2972 cm −1 is related to vibration bands C-H of alkoxy groups or solvent (occluded ethanol).Si-CH3 bond is assigned to the peak at 1277 and 781 cm −1 [6,38], being directly related to MTS silane.Bands between 1160 and 1050 cm −1 correspond to the condensation species (Si-O-Si) [33,41] by reaction between silanol groups when the coating is curing into the oven at 110 °C.C-O bands due to ethanol can be overlapped under this broad band of siloxane groups.Finally, peaks at 910 cm −1 are related to silanol groups (Si-OH) [6].Differences among spectra as a function of amount of ZrO2 have not been found.Morphological differences among surface coatings are clearly observed in Figure 6. Figure 6a,c,e shows that the amount of ZrO2 agglomerates observed on the surface increases with the ZrO2 percentage added into silane solution, as it was observed by XRF as well.Besides, different sizes of agglomerates are distributed on the whole surface.The average particle size estimated from these SEM images is around 2 µm, coinciding with DLS results (Figure 3).The images at higher magnifications (Figure 6b,d,f) show that these large agglomerates consist of smaller ones.Morphological differences among surface coatings are clearly observed in Figure 6. Figure 6a,c,e shows that the amount of ZrO 2 agglomerates observed on the surface increases with the ZrO 2 percentage added into silane solution, as it was observed by XRF as well.Besides, different sizes of agglomerates are distributed on the whole surface.The average particle size estimated from these SEM images is around 2 µm, coinciding with DLS results (Figure 3).The images at higher magnifications (Figure 6b,d,f) show that these large agglomerates consist of smaller ones.Hydrophobic properties of these coatings were evaluated measuring water contact angles.Results are shown in Figure 7.All coatings exhibit lower wettability than pristine steel.Fei et al. [42] assure that presence of methyl groups from MTS decrease the surface energy of the surface.Previous studies using different silanes (acrylic-silane [6] and alkyl-silane [20]) show a more hydrophobic surface when silane coatings have non-polar groups (siloxane and alkyl groups).Such as it was observed by FTIR, siloxane groups are found on the coated surface (Figure 5), which are responsible of this raise of hydrophobic properties along with Si-CH3 groups (from MTS silane).The most important increase of water contact angles is observed for the coating without particles.Adding zirconia nanoparticles decreases contact angles respect to silane coating (0 wt.% ZrO2), between 10° and 20°.This decrease is higher for the polar liquids.This can be explained due to the polar character that ZrO2 agglomerates provide to the surface.Differences among the three ZrO2-silane coatings are not significant as shown in Figure 7.  Hydrophobic properties of these coatings were evaluated measuring water contact angles.Results are shown in Figure 7.All coatings exhibit lower wettability than pristine steel.Fei et al. [42] assure that presence of methyl groups from MTS decrease the surface energy of the surface.Previous studies using different silanes (acrylic-silane [6] and alkyl-silane [20]) show a more hydrophobic surface when silane coatings have non-polar groups (siloxane and alkyl groups).Such as it was observed by FTIR, siloxane groups are found on the coated surface (Figure 5), which are responsible of this raise of hydrophobic properties along with Si-CH 3 groups (from MTS silane).The most important increase of water contact angles is observed for the coating without particles.Adding zirconia nanoparticles decreases contact angles respect to silane coating (0 wt.% ZrO 2 ), between 10 • and 20 • .This decrease is higher for the polar liquids.This can be explained due to the polar character that ZrO 2 agglomerates provide to the surface.Differences among the three ZrO 2 -silane coatings are not significant as shown in Figure 7. Hydrophobic properties of these coatings were evaluated measuring water contact angles.Results are shown in Figure 7.All coatings exhibit lower wettability than pristine steel.Fei et al. [42] assure that presence of methyl groups from MTS decrease the surface energy of the surface.Previous studies using different silanes (acrylic-silane [6] and alkyl-silane [20]) show a more hydrophobic surface when silane coatings have non-polar groups (siloxane and alkyl groups).Such as it was observed by FTIR, siloxane groups are found on the coated surface (Figure 5), which are responsible of this raise of hydrophobic properties along with Si-CH3 groups (from MTS silane).The most important increase of water contact angles is observed for the coating without particles.Adding zirconia nanoparticles decreases contact angles respect to silane coating (0 wt.% ZrO2), between 10° and 20°.This decrease is higher for the polar liquids.This can be explained due to the polar character that ZrO2 agglomerates provide to the surface.Differences among the three ZrO2-silane coatings are not significant as shown in Figure 7.  Aesthetical aspects of the coated plates are studied using color and specular gloss measurements.Color coordinates (L*, a* and b*) and color difference to uncoated steel (∆E) are shown in the Table 3. ∆E is calculated from the differences of its component and it informs about the significance of color change.In general terms, ZrO 2 -silane coating promotes a slight loss of lightness (L*).This loss of lightness is due to the opacity provided by ZrO 2 agglomerates.A similar tendency was already observed in previous reports for silane-SiO 2 coatings [20].However, a* and b* coordinates do not display a clear trend, and the changes are not meaningful.For all coated surfaces, colorimetric differences (∆E) are lower than 10.It informs that the change of color related to clear steel is small.This value increases when silane and nanoparticles are together on the surfaces, as it was observed in previous researches [20].Significant specular gloss changes (Figure 8) are observed for the silane coating, increasing more than 60% respect to clear steel, from 54 to 89 GU.However, specular gloss changes are not meaningful for silane-ZrO 2 coatings with 1 wt.% and 2.5 wt.% of nanoparticles regard to clear steel; being more noticeable for the coatings with 5 wt.%ZrO 2 (values around 30 GU are measured).However, these changes are not visible, a slight opacity can be observed only with different tilts of the plates.In conclusion, the effect of the nanoparticles on gloss is adequate for the in-service conditions, according to the architects and designers requirements.Aesthetical aspects of the coated plates are studied using color and specular gloss measurements.Color coordinates (L*, a* and b*) and color difference to uncoated steel (ΔE) are shown in the Table 3. ΔE is calculated from the differences of its component and it informs about the significance of color change.In general terms, ZrO2-silane coating promotes a slight loss of lightness (L*).This loss of lightness is due to the opacity provided by ZrO2 agglomerates.A similar tendency was already observed in previous reports for silane-SiO2 coatings [20].However, a* and b* coordinates do not display a clear trend, and the changes are not meaningful.For all coated surfaces, colorimetric differences (ΔE) are lower than 10.It informs that the change of color related to clear steel is small.This value increases when silane and nanoparticles are together on the surfaces, as it was observed in previous researches [20].Significant specular gloss changes (Figure 8) are observed for the silane coating, increasing more than 60% respect to clear steel, from 54 to 89 GU.However, specular gloss changes are not meaningful for silane-ZrO2 coatings with 1 wt.% and 2.5 wt.% of nanoparticles regard to clear steel; being more noticeable for the coatings with 5 wt.%ZrO2 (values around 30 GU are measured).However, these changes are not visible, a slight opacity can be observed only with different tilts of the plates.In conclusion, the effect of the nanoparticles on gloss is adequate for the in-service conditions, according to the architects and designers requirements.The silane coating (0 wt.% ZrO2) and the uncoated stainless steel have the highest friction coefficients (Figure 9), 0.39 and 0.36 respectively.Moreover, it remains almost constant during the test, being changes at the beginning of the test related to the formation of wear track and wearing of steel or silane.The friction coefficient for the silane coated plate is higher than this coefficient for stainless steel, possibly due to the sticking of the coating on the pin.Previous studies about the wear resistance of silane coatings found that the presence of SiOSi groups could be related to the tribological characteristics of the coatings, taking place at lower friction coefficients and higher wear resistances [43,44].The infrared studied shown before (Figure 5) confirms the presence of siloxane groups.The silane coating (0 wt.% ZrO 2 ) and the uncoated stainless steel have the highest friction coefficients (Figure 9), 0.39 and 0.36 respectively.Moreover, it remains almost constant during the test, being changes at the beginning of the test related to the formation of wear track and wearing of steel or silane.The friction coefficient for the silane coated plate is higher than this coefficient for stainless steel, possibly due to the sticking of the coating on the pin.Previous studies about the wear resistance of silane coatings found that the presence of SiOSi groups could be related to the tribological characteristics of the coatings, taking place at lower friction coefficients and higher wear resistances [43,44].The infrared studied shown before (Figure 5) confirms the presence of siloxane groups.The presence of zirconia reduces friction coefficient respect to the silane coating without ZrO2 and the uncoated surface, reaching 0.23 for coatings with 5 wt.% of ZrO2.The higher percentage of ZrO2 promotes the lower friction coefficient, possibly related not only to ZrO2 amount but to presence of agglomerates (as XRF and SEM have shown), being able to stand wear.A similar conclusion was drawn by Banerjee et al. [11] for fluorosilane coatings with colloidal silica nanoparticles, showing that increasing the concentration of added silica to the sol-gel matrix led to a harder and more wearresistant coating on the surface [45].
It is observed that the friction coefficient of coating with 1 wt.% of nanoparticles ascends after a determined number of cycles.It indicates that the coating is worn down after 28 cycles [46] and the zirconia particles are stripped off and they could act as a third body, incrementing the friction coefficient.Coatings with 2.5 wt.% and 5 wt.% of ZrO2 do not worn after 100 cycles with 5 N of load.For this reason, another assay was carried out for these coatings.
The comparison of the two coatings with 10 N of load is shown in Figure 10.The friction coefficient of the coating with 2.5 wt.% of ZrO2 shows an important increment after 1610 cycles with 10 N of load.It means that the coating is worn down in this moment.Coating with 5 wt.%ZrO2 is not worn down significantly until the 3340th cycle.After this number of cycles, an increase in friction coefficient takes place, indicating that the coating is disappearing.Hence coating with 5 wt.% of ZrO2 nanoparticles shows the highest values of wear strength.
The coating without nanoparticles does not present the increase in friction coefficient when it wears down.This is due to the lack of the third body effect when nanoparticles are not present in the coating.The presence of zirconia reduces friction coefficient respect to the silane coating without ZrO 2 and the uncoated surface, reaching 0.23 for coatings with 5 wt.% of ZrO 2 .The higher percentage of ZrO 2 promotes the lower friction coefficient, possibly related not only to ZrO 2 amount but to presence of agglomerates (as XRF and SEM have shown), being able to stand wear.A similar conclusion was drawn by Banerjee et al. [11] for fluorosilane coatings with colloidal silica nanoparticles, showing that increasing the concentration of added silica to the sol-gel matrix led to a harder and more wear-resistant coating on the surface [45].
It is observed that the friction coefficient of coating with 1 wt.% of nanoparticles ascends after a determined number of cycles.It indicates that the coating is worn down after 28 cycles [46] and the zirconia particles are stripped off and they could act as a third body, incrementing the friction coefficient.Coatings with 2.5 wt.% and 5 wt.% of ZrO 2 do not worn after 100 cycles with 5 N of load.For this reason, another assay was carried out for these coatings.
The comparison of the two coatings with 10 N of load is shown in Figure 10.The friction coefficient of the coating with 2.5 wt.% of ZrO 2 shows an important increment after 1610 cycles with 10 N of load.It means that the coating is worn down in this moment.Coating with 5 wt.%ZrO 2 is not worn down significantly until the 3340th cycle.After this number of cycles, an increase in friction coefficient takes place, indicating that the coating is disappearing.Hence coating with 5 wt.% of ZrO 2 nanoparticles shows the highest values of wear strength.
The coating without nanoparticles does not present the increase in friction coefficient when it wears down.This is due to the lack of the third body effect when nanoparticles are not present in the coating.The presence of zirconia reduces friction coefficient respect to the silane coating without ZrO2 and the uncoated surface, reaching 0.23 for coatings with 5 wt.% of ZrO2.The higher percentage of ZrO2 promotes the lower friction coefficient, possibly related not only to ZrO2 amount but to presence of agglomerates (as XRF and SEM have shown), being able to stand wear.A similar conclusion was drawn by Banerjee et al. [11] for fluorosilane coatings with colloidal silica nanoparticles, showing that increasing the concentration of added silica to the sol-gel matrix led to a harder and more wearresistant coating on the surface [45].
It is observed that the friction coefficient of coating with 1 wt.% of nanoparticles ascends after a determined number of cycles.It indicates that the coating is worn down after 28 cycles [46] and the zirconia particles are stripped off and they could act as a third body, incrementing the friction coefficient.Coatings with 2.5 wt.% and 5 wt.% of ZrO2 do not worn after 100 cycles with 5 N of load.For this reason, another assay was carried out for these coatings.
The comparison of the two coatings with 10 N of load is shown in Figure 10.The friction coefficient of the coating with 2.5 wt.% of ZrO2 shows an important increment after 1610 cycles with 10 N of load.It means that the coating is worn down in this moment.Coating with 5 wt.%ZrO2 is not worn down significantly until the 3340th cycle.After this number of cycles, an increase in friction coefficient takes place, indicating that the coating is disappearing.Hence coating with 5 wt.% of ZrO2 nanoparticles shows the highest values of wear strength.
The coating without nanoparticles does not present the increase in friction coefficient when it wears down.This is due to the lack of the third body effect when nanoparticles are not present in the coating.

Conclusions
The austenitic stainless steel is successfully coated by a silane-based solution with zirconia nanoparticles.It is proved by FTIR, where the bands corresponding to silanes and zirconia were observed, and XRF, which shows an increasing percentage of zirconia.By DLS and SEM, it is observed that the nanoparticles clump to the larger aggregates in both, solutions and coatings.
The contact angle drastically increases with all the coatings, which means that wettability of surfaces decreases, protecting the surface from water and its effects, such as corrosion.
Related to aesthetical appearance, results show that gloss and color do not change significantly with the addition of a small percentage of nanoparticles (1 wt.% and 2.5 wt.%).However, for higher concentration (5 wt.%), gloss decreases drastically, although it is not appreciated with the naked eye.Therefore, it is possible to conclude that coatings with zirconia are optically transparent.
In terms of tribological properties, the friction coefficient of the coatings decreases with the addition of nanoparticles.It is also observed that coatings with a high percentage of nanoparticles stand more wear cycles.
In conclusion, coatings on stainless steel which combine wetting, tribological, and aesthetical properties were developed.

Figure 7 .
Figure 7. Contact angle of coatings with different liquids as a function of zirconia percentage added.

Figure 7 .
Figure 7. Contact angle of coatings with different liquids as a function of zirconia percentage added.Figure 7. Contact angle of coatings with different liquids as a function of zirconia percentage added.

Figure 7 .
Figure 7. Contact angle of coatings with different liquids as a function of zirconia percentage added.Figure 7. Contact angle of coatings with different liquids as a function of zirconia percentage added.

Figure 8 .
Figure 8. Variation of gloss as a function of the percentage of zirconia.

Figure 8 .
Figure 8. Variation of gloss as a function of the percentage of zirconia.

Figure 9 .
Figure 9. Friction coefficients of each surface obtained during wear test, as a function of number of cycles.

Figure 9 .
Figure 9. Friction coefficients of each surface obtained during wear test, as a function of number of cycles.

Coatings 2018, 8 , 15 Figure 9 .
Figure 9. Friction coefficients of each surface obtained during wear test, as a function of number of cycles.

Figure 10 .
Figure 10.Friction coefficient for coatings with 2.5 wt.% and 5 wt.% of ZrO2 with 10 N of load.Figure 10.Friction coefficient for coatings with 2.5 wt.% and 5 wt.% of ZrO 2 with 10 N of load.

Figure 10 .
Figure 10.Friction coefficient for coatings with 2.5 wt.% and 5 wt.% of ZrO2 with 10 N of load.Figure 10.Friction coefficient for coatings with 2.5 wt.% and 5 wt.% of ZrO 2 with 10 N of load.
Note: Percentage of polar and dispersive contributions indicated in parentheses.

Table 2 .
Analysis of the composition and estimated thickness of the coatings for the different ZrO 2 percentages by XRF and scratch and roughness measurements, respectively.

Table 2 .
Analysis of the composition and estimated thickness of the coatings for the different ZrO2 percentages by XRF and scratch and roughness measurements, respectively.

Table 4
summarizes these results.The friction coefficient when coatings are not worn down decreases when the percentage of ZrO 2 increases.It indicates that adding ZrO 2 makes the coatings harder.On the other hand, wear resistance clearly increases when the content of nanoparticles increases.

Table 4 .
Friction coefficient and number cycles to wear down for the coatings.