Adaptive Silicon Oxycarbide Coatings With Controlled Hydrophilic or Hydrophobic Properties

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DOI: 10.1002/adem.201500364
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Adaptive Silicon Oxycarbide Coatings With ControlledHydrophilic or Hydrophobic Properties**

By Boris Reznik,*,1 Jordan Denev and Henning Bockhorn

The study presents a method for producing of adaptive silicon oxycarbide (Si–O–C) coatings frompyrolyzed polymethylsilsesquioxane as well as a dedicated technique for molecular design of thepolymer precursor. The coating texture is spontaneously formed as a result of crack propagation duringshrinkage of the solidified polymer. The coatings contain periodic grooves and in situ formed scrolledfilaments. The synthesized coatings undergo a transition between hydrophobic and hydrophilic states.The scrolled fibers exhibit thermal adaptive behavior. The coatings are obtained on different substratesfrom the polymer precursor fractions exhibiting different molecular weights.

1. Introduction controlled fabrication of surfaces with these riblet-like

Metallic and ceramic materials are typically considered as“dead matter.” However, at elevated temperatures (approx.650–1100 �C) tailored surfaces have the potential to react toeffects from the environment in a very specific way and, thus,may provide special functionalities to a technical component.Therefore, the development of “living” or adaptive materials,which can exhibit a reversible shape change due totemperature or pressure variations, is an important scientificand technical challenge. Furthermore, the use of materials forreducing energy losses can be realized by decreasing thehydrodynamic surface friction and hence also the drag.[1,2] Itis known, that the skin of fast swimming sharks displays aprominent riblet structure on its surface. These riblets turnedout to reduce the drag loss significantly.[3] However, due to thedifficulties in production of such shaped surfaces theadvantages of these structures cannot be fully utilized forhigh-temperature applications. Therefore, the Priority DFG-Programme 1299 “Adaptive Surfaces for High-TemperatureApplications” targets to find technical solutions allowing a

[*] Dr. B. Reznik, Dr. J. Denev, Dr. H. BockhornKarlsruhe Institute of Technology (KIT), Engler-Bunte-Insti-tute, Combustion division, 76131 Karlsruhe, GermanyE-mail: [email protected]

1 The present address: Division of Structural Geology andTectonophysics, KIT, Institute of Applied Geosciences, 76131Karlsruhe, Germany

[**] This work was supported by the German DFG-Priorityprogram 1299: “Adapting surfaces for high temperatureapplications.”Mr. H. Weickenmeier is thanked for the technicalassistance. We thank to M. Ströbele and F. Parhat for theconducting of pyrolysis experiments. We thank also to Dr. C.Eberl for the nanoindentation tests.

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textures.Silicone oxycarbide derived from polysiloxanes is an

attractive coating material for high-temperature applicationsincluding fuel-injection pumps, heat transfer tubes, ignitionplugs, and thermal shields exhibiting anticorrosionbehavior.[4–6] The advantages of the polymer precursor routeto glasses or ceramics over a traditional ceramic processingroute are: i) polymers can generally be converted tometastable ceramics at temperatures less than 1 200 �C; ii)polymers can be readily purified; iii) ceramics coatingsexhibiting complex shapes can be fabricated; iv) porousceramics coatings can be used as catalysts, adsorbents as wellas supports for heterogeneousmetal catalysis; and v) chemicaland physical properties of the derived coatings can be tunedby designing preceramic polymers and controlling thepolymer pyrolysis.[4] However, up to now, no studies havebeen carried out focusing on the high-temperature perfor-mance of Si–O–C coatings which exhibit adaptive properties.The following results demonstrate that bymeans of controlledsurface structuring adaptive properties of this material can beachieved.

A series of preliminary results of our research groupformed the basis of the development of the method forproducing Si–O–C coatings exhibiting adaptive (“living”)surface properties. First, in 2011[5] a spontaneous formation ofmicrometer-sized scrolled Si–O–C filaments was observed atthe surfaces of manually fractured free-standing flake-shapedglassy residues (flakes) obtained after pyrolysis of a poly-methylsilsesquioxane (PMS) powder. Later,[6] it was noticedthat after PMS pyrolysis, in a combustion porcelain boat(Figure 1a), a large amount of free-standing flakes are formed(Figure 1b). Furthermore, it was recognized, that the surface ofthe flake top (Figure 1c) is smooth while the flake bottomsurface, which was previously connected to the boat surface,

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Fig. 1. Optical micrographs illustrating the preliminary observation of SI–O–C coatings. (a) Centralpart of an as-received porcelain boat and polysiloxane powder (inset); (b) central part of the boatcontaining free-standing glassy-like flakes formed after a powder pyrolysis; (c) a flake with a smoothsurface; (d) the same flake exhibiting herringbone-shaped rough surface on its opposite site; (e)herringbone-shaped coating on the boat bottom observed after removing of the free-standing flakes. Inset:a magnified view of the coating taken by scanning electron microscope showing the textured surfacecomposed of grooves which are partly delaminated in the form of branched-off, curved filaments.

Fig. 2. Scheme illustrating the fabrication procedure of Si–O–C coatings and somephenomena influencing the quality of the coatings. (1) A PMS powder is loaded into aconcave substrate. (2) The powder is pyrolyzed in an inert atmosphere. (3) The followingtemperature drop provokes cracking within the solidifying polymer melt. (4) As theambient conditions are reached, the resulting polymerisate splits into two parts: free-standing delaminated flakes and a tiny coating adhered to the boat bottom.

B. Reznik et al./Adaptive Silicon Oxycarbide Coatings. . .

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is rough and contains elongated grooves exhibiting riblet-likemorphology (Figure 1d). Finally, it was discovered, that whenthe free-standing flakes are removed (Figure 1e), a thinherringbone-shaped or riblet-like coating is observed at thebottom of the boat (compare Figure 1e with 1a). Obviously,the propagating cracks induced by thermal stresses cut thesolidifying material into two parts: a thin coating adhered tothe boat (Figure 1e) and the free-standing delaminated flakes.Moreover, as a result of delamination, partly branched-offfilaments, similar to the previously reported in ref.,[5] areformed at the periodically arranged crest tips of the coating(Figure 1e, inset). It is surprising to note that the self-assembled Si–O–C morphology mimics the surface structureof the human skin.[7]

Evidently, the shape of the boat (substrate) may play acertain role in the process of crack formation duringsolidification of the polymer. To check this hypothesis, weprepared planar porcelain substrates being cut from the boats’walls. Despite using the same pyrolysis temperatures, nocoatings with periodic textured surfaces were observed on theplanar surfaces. The detailed understanding of the effect ofthe substrate shape on the self-texturing process needs a moredetailed, separate investigation which goes beyond the scopeof the present investigation. Therefore, as it is schematicallyshown in Figure 2, we attempted first to develop a coatingmethod for the boat-like, concave substrates.

For this purpose, several fundamental questions regard-ing the conditions influencing the coating formation have tobe posed: how does the pyrolysis temperature influencesthe morphology and the surface properties of coatings?What is the correlation between the substrate material andthe coating morphology? How can the molecular weightof the polymer precursor be related to the coating properties?

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In order to address these questions, in the presentstudy, pyrolysis experiments with PMS werecarried in a broad range of pyrolysis temperaturesusing different substrates as well as usingpolymer precursors with different molecularweight. As a result, we unambiguously demon-strate a possibility for creating adaptive “living,”temperature-sensitive Si–O–C coatings whichundergo transitions between hydrophobic andhydrophilic states. The discussed experimentalresults are grouped into the following mainparagraphs: bulk mechanical properties as afunction of pyrolysis temperature, microstructureand surfaces’ properties of adaptive coatingsobtained on porcelain substrates, molecular de-sign of PMS precursors toward achieving fine-tuned surface properties, and finally, microstruc-ture and surfaces’ properties of coatings obtainedon stainless steel substrates.

2. Experimental Section

2.1. Materials and Pyrolysis Conditions
The coating precursor used for this research was the
Wacker–Belsil powder product (Wacker Chemie GmbH,Burghausen, Germany). The powder is a solid solvent-freepolymethylsilsesquioxane (PMS) with a [CH3SiO1.5]n basicstructure and falls within the silicone resin group. Thepyrolysis/coating experiments were carried out in a DuPont951(USA) thermobalance. Two types of substrates were used:i) glazed porcelain combustion boats (Morgan Technical,Ceramics, Haldenwanger, Germany) with an overall length of90mm, width of 12mm and height of 8mm; ii) stainless steel(1.4404 alloy) half-pipes with an overall length of 15 andwidth of 20mm. The substrates were placed in a furnaceand a constant flow of helium (100mlmin�1, purity 99.999%)was maintained. The temperature was ramped up at a rate

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of 10 �Cmin�1 to the preset pyrolysis temperature, whichwas between 400 and 1 000 �C. The entire setup was cooleddown naturally to room temperature at an initial rate of18–21 �Cmin�1. The overall number of probes produced wasover 300.

2.2. Molecular Design of the Polymer PrecursorIn order to study the effect of the molecular weight of

polymer precursor on the coating properties, the original PMSpolymer was separated into fractions exhibiting differentmolecular weight (Figure 3a). A detailed description of thefractionation procedure can be found elsewhere.[8] For thispurpose, a home-made fractionation cell equipped witha magnetic mixer and ceramics membrane was used(Figure 3b). Using a feeding pump, a 10% acetone–polymersolution was introduced into the cell with pressures lying

Fig. 3. Molecular design of the polymer precursor. (a) Fragments with different atomic moleweight distributions (MWDs) derived by gel permeation chromatography (GPC).

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between 0.5 and 10 bar. The fractionation into retentate andpermeate fractions (Figure 3b) occurs using porous ceramicmembranes (Tami GmbH, Germany) with different MWCOsof 50, 15, 8, 5 3, and 1 kD, whereMWCO is a molecular weightcut-off and refers to the lowest molecular weight solute(in daltons).[9] Gel-Permeations-Chromatography (GPC) wasused to control the molecular weight of the retentate fraction(Figure 3c) which was used as a coating precursor.

2.3. MicroscopyFirst, the microstructure of coatings was investigated

using a Hund-Wetzlar (Wetzlar, Germany) and a ReichertMEF4A (Leica, Germany) light microscopes equipped witha PL-B623C digital camera (PixeLink, Ottawa, Ontario,Canada). For improved three-dimensional visualization ofthe coatings macrostructure, a stack of single images acquired

cular units (amu) of polymethylsiloxane. (b) Fractionation cell with membrane. (c) Mass

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at different planes of focus was manipulated using Helicon
Focus software (Kharkov, Ukraine). A Hitachi S570 (Hitachi,Ltd., Tokyo, Japan) scanning electron microscope (SEM)equipped with silicon drift detector (SDD-SMAx) forenergydispersive X-ray spectrometry (EDS) was used forthe investigation of the film morphology and the chemicalcomposition changes along the film/boat interfaces. Prior tothe SEM observations, the samples were covered with an Auconductive layer.

2.4. Contact Angle MeasurementsStatic water contact angles were measured using a home-

build apparatus, which is similar to that described by Lamouret al.[10] Prior to the angle measurements, the boats werecross-sectioned through their center using a diamond saw. Tomeasure the static contact angle, a drop volume of 1–2mlwas formed on the tip of a syringe needle. The syringe wasfastened to a stand which reduces any irregularities that areproduced by manual drop deposition. The substrate was thenraised till it touched the drop using the coordinate control ofthe stage. After the short touching, the substrate with thedrop was positioned down back to its initial position. Afterapproximately 2–3 s, images were acquired using a video-camera. The contact angles formed by the water drop on thesurface were measured using the ImageJ software.[11] Theaccuracy of measurements was �1.5�.

2.5. Mechanical PropertiesThe mechanical properties of the pyrolyzed bulk samples

were tested using the Agilent MTS Nano Indenter XPequipped with a diamond Berkovich indenter tip. Allnanoindentation tests were conducted in the continuous-stiffness-measurement mode[12] using free-standing flakes(Figure 1c). Calibration tests for the area function of theindenter and for the load frame compliance were performedon a fused silica specimen to ensure measurement accuracy.The Young’s modulus and hardness were calculated using theapproach described by Pharr and Oliver.[12]

Fig. 4. Mechanical properties of the polymer-derived Si–O–C. (a) Nanoindentation loadtemperature. The value of the reference material, fused silica is also given.

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3. Results and Discussion

3.1. Bulk Mechanical Properties as a Function of PyrolysisTemperature

The free-standing glassy flakes were used to studythe effect of the pyrolysis temperature on the chemicalcomposition and mechanical properties of the derivedmaterial (Figure 4). As it can be seen from Figure 4,increasing pyrolysis temperature correlates with the increas-ing values of Young’s modulus and of the hardness. Notethat the mechanical properties are approaching those ofthe fused silica. Hence, the applied pyrolysis parametersallow producing ceramics materials from a chip polymerprecursor.

3.2. Microstructure and Surfaces Properties of AdaptiveCoatings Obtained on Porcelain Substrates

3.2.1. Coating MorphologyFigure 5 presents observations of the surface morpho-

logy of coatings using a light microscope. The uncoveredboat surface appears in form of rough globular units(Figure 5a). The derived thin coatings are transparent forincident and reflected light. Therefore, the images of thecoatings (Figure 5b–e) can be viewed as an overlappingof globular units with elongated periodic grooves asschematically shown in Figure 5f. It is seen (Figure 5b–e)that developed grooves are present in the coatingsderived at a pyrolysis temperature of 600 �C and higher.However, due to the complex light reflections, the detailsof the film texture cannot be easily analyzed using lightmicroscopy.

Consequently, the further investigation of the film texturewas carried out using SEM (Figure 6–8).

As it can be seen from Figure 6 and 7, the increasingfabrication (pyrolysis) temperature leads to a decrease ofthe average distance between the grooves. The intervalbetween grooves was determined using boats cross-sectioned by a diamond saw. It is interesting to note that

-displacement curves. (b) Young’s modulus and hardness as a function of pyrolysis

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Fig. 5. Optical micrographs of an uncoated bottom porcelain boat (a) and SI–O–C coatings obtained after polymer pyrolysis at 400 (b), 600 (c), 800 (d), and 1 000 �C (e). (f) Schemeillustrating that due to the transparency of coatings both the boat bottom and the coating texture can be observed simultaneously in reflected light.


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no coating delamination was observed after the appliedcross-sectioning. This fact suggests a good adhesionbetween the Si–O–C coatings and the porcelain surface.As it was already mentioned above (Figure 1e, inset) aremarkable effect is observed at the crests of the grooves:the delamination of the crests occur in form of partlybranched-off fibers (Figure 6b, d, f). Similar fracture–induced scrolled fibers were observed previously by

Fig. 6. SEMmicrographs showing morphology of SI–O–C coatings obtained after polymer psection view of the grooves. Branched-off, curved filaments on the crests of the grooves ar


Hausmann et al.[5] in manually fractured bulk PMS derivedflakes.

By a short heating with e-beam (Figure 8) the fiber bendswithout fracturing (Figure 7a, b) and comes back to its initialposition after cooling in cases when coatings are synthesizedat 400 and 600 �C. This fact strongly indicates the adaptiveproperties of fibers. For example, this effect can be utilizedfor creating of adaptive coatings with a thermal adaptive

yrolysis at 600 (a, b), 800 (c, d), and 1 000 �C (e, f) in porcelain boats. Inset in (e): a cross-e imaged in (b, d, f) as well as shown schematically by the inset in (f).

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Fig. 9. Water contact angles as a function of pyrolysis temperature for coatings obtainedin porcelain boats. The inset placed at the bottom left is an optical micrograph of a waterdrop on a smooth top surface of a flake pyrolyzed at 800 �C illustrating the water contactangle measurement procedure. The inset placed at the top right shows schematically thecomparatively studied samples: flake top, flake bottom, and coating. Different anglesmeasured at the flake top and the flake bottom (or coating) strongly indicate that thegrooved self-texturing contributes significantly to the wetting behavior.

Fig. 7. The effect of the pyrolysis temperature on the distance between grooves as well ason the diameter of fibers.


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element or sensor regulating flow velocity in a reactorvolume.

3.2.2. Surface PropertiesFigure 9 shows the surface behavior of coatings evaluated

through the measurements of water contact angle. It isknown that the surface wetting is mainly controlled by thenature of chemical bonds and the surface texture.[13]

Frequently, these two effects overlap. In order to separatethem, the values of contact angles measured on smooth topsurfaces of free-standing flakes were compared with thosemeasured on the rough coatings. Figure 9 demonstrates aclear trend – increasing fabrication (pyrolysis) temperatureleads to a decrease of the values of contact angles. Note that,

Fig. 8. Thermal adaptive properties of Si–O–C filaments. (a, b) SEM imagesdemonstrating that after 25 s of heating with the electron beam, the filament tip is bendto 2.3mm from its initial position. The heating was carried out using 90 000 or 130 000magnifications. (c) Plot showing that the higher the pyrolysis temperature (the higherthe crystallization degree of the Si–O–C-derived material) the higher electron beamexposure is needed in order to move a filament from its initial position. The bending offilaments formed at 400 and 600 �C is reversible (s. the blue dashed region), i.e., thefilaments are sensitive to the temperature changes and provide adaptive functionality forthe coatings.

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due to the replicating properties of the flakes, the contactangle measured on the flake bottom surfaces is equal to thatmeasured on the coatings. This was confirmed by a seriesof measurements of flakes and coatings derived at differenttemperatures.

Furthermore, the angles measured at the flake bottoms andat the coatings are smaller than those measured at the smoothflake tops (Figure 9). Especially at 800 and 1 000 �C, thedifference between angles measured at smooth and roughsurfaces is 25 and 27 grades, respectively. Evidently, the higherthe pyrolysis, the higher the role of the surface self-texturingin the surface wetting. The presented results (Figure 9)indicate strongly that the proposed coating method allowsone to synthesize Si–O–C coatings which undergo a transitionbetween hydrophobic (u � 90�) and hydrophilic (u � 90�)states.

The hydrophilic behavior of coatings obtained at 800 and1 000 �C can be explained by taking into account the shapeand the distance of the grooves in the film. If each groove isregarded as a small capillary channel (see inset in Figure 3c),then the smaller the channels, the larger the capillarydriving force. Indeed, compared to coatings obtained at600 �C, the distance between grooves obtained at 800 and1 000 �C is notably smaller (Figure 6). Therefore, due to theenhanced capillary forces in small channels formed at800 and 1 000 �C, a quicker penetration of water into thechannels occurs, and, as a result, smaller contact waterangles are measured (Figure 8). These results (Figure 8) arein accordance with the data reported for polymer coatingswith periodic surface structures created as antifoggingand self-cleaning coatings[14–17] that is especially significantfor fluid drag reduction in laminar and turbulent flows.Compared to our ceramic Si–O–C coatings, the derivedcoatings reported in ref.[14] however, still exhibit polymer

& Co. KGaA, Weinheim DOI: 10.1002/adem.201500364ADVANCED ENGINEERING MATERIALS 2015,

Fig. 11. The effect of the polymer average molecular weight (Mn-values) on the coatingstructuring including distance between grooves, diameter of branched-off fibers and thewater contact angle.


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behavior, e.g., a low mechanical strength and a low thermalstability. Furthermore, in our case, the coating texture isa result of self-assembled patterning process which iscontrolled by the crack propagation during the polymersolidification and delamination steps (Figure 2). Corre-spondingly, no additional technological patterning stepsapplying complex templates[18] or laser patterning[19] areneeded.

3.3. Molecular Design of the Polymersss Precursor towardAchieving Fine-Tuned Surface Properties

The results for the molecular design of the polymerprecursor are shown in Figure 10 and 11. The increasingmolecular weight correlates well with the increasing distancebetween the coating grooves and the branched-off fiberdiameter (Figure 10). These parameters strongly affect thewetting behavior of the coatings which undergo transitionbetween a hydrophilic and a hydrophobic range (Figure 11).These examples (Figure 10 and 11), demonstrate a hugetechnological potential toward fine-tuning of the coatingtexture and surface properties. First of all, the initial PMSpolymer is an inexpensive precursor material. Second, othersiloxanes exhibiting different chemical and melting/solidifi-cation properties can be used. Third, a relatively simplefractionation cell (Figure 3) is needed to obtain precursorswith different molecular weights.

3.4. Microstructure and Surfaces Properties of CoatingsObtained on Stainless Steel Substrates

The results obtained for steel substrates are shown inFigure 12 and 13. Also in this case, periodically texturedSi–O–C coatings with flexible branched-off fibers are derived

Fig. 10. SEMmicrographs showing morphology of SI-O-C coatings obtained after polymer800 �C in porcelain boats using polymers with different molecular weights (see the Mn-valueformation of the branched-off, curved filaments (arrows) on the crests of the grooves.


(Figure 12). In contrast to the coatings on porcelainsubstrates, the coatings on steel substrates are morehydrophobic (Figure 13). The fact can be explained bydifferent solidification-texturing conditions of the polymermelt (Figure 2) on porcelain and steel exhibiting verydifferent mechanical and thermal properties. It is alsointeresting to note that the fine Si–O–C coating texture(Figure 12c, d) ignores the relatively rough substrate texture(Figure 12b). This behavior suggests that relatively rough,non-special prepared concave steel substrates can be coveredwith Si–O–C coatings exhibiting enhanced surface function-ality. Therefore, the described method (Figure 2) can find abroad industrial application.

pyrolysis ats). Note the

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4. Conclusions

A method for producing adaptive Si–O–Ccoatings with controlled hydrophilic or hydropho-bic properties is presented. The coatings are formedas a result of crack propagation induced by stressesdeveloping during the solidification of the pyro-lyzed polymer. The propagating cracks cut thesolidifying material into two parts: free-standingflakes and a thin coating adhered to the substratesurface. As a result two surface texture elementsare simultaneously formed: i) periodic grooves andii) branched-off fibers on the crests between thegrooves. The curved mm-sized fibers exhibitthermal adaptive behavior.

Increasing pyrolysis temperatures lead to in-creasing values of Young’s modulus and hardness.The derived ceramics coatings are optically trans-parent and exhibit a good adhesion with concaveporcelain and stainless steel substrates.

Through the variation of the pyrolysis tempera-ture and the molecular design of the polymercoating precursor a fine-tuning of the surface

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Fig. 12. Si–O–C coatings obtained in stainless steel semi-pipes. (a) Optical micrograph of an uncoatedsemi-pipe. (b) SEM bottom view of an uncoated, original semi-pipe. (c) Coating obtained after polymerpyrolysis at 800 �C. (d) Coating obtained after polymer pyrolysis at 1 000 �C. Arrows in (c) and (d) markbranched-off filaments on the crests of the grooves.


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texturing and the coatings’ wetting behavior is achieved. It isestablished that the temperature increase or the molecularweight decrease of the polymer precursor lead to a decrease ofthe average values of the fiber thickness and of the distancebetween the grooves. These textural variations control thesurface behavior of coatings between hydrophobic andhydrophilic states.

The obtained results are a precondition for the develop-ment of a simple one-step and cheap fabrication methodof large-area transparent adaptive Si–O–C coatings with

Fig. 13. Water contact angles as a function of pyrolysis temperature for coatingsobtained on steel and porcelain substrates. Note that the synthesized coatings undergotransitions between hydrophobic and hydrophilic states.

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variable surface properties. The method ischaracterized by several advantages. The coat-ings with surfaces containing periodic groovesare synthesized in a single fabrication step.The coating can be easily obtained usinginexpensive standard laboratory equipmentand inexpensive chemicals. The coating can beobtained on a variety of technologically impor-tant materials such as porcelain and stainlesssteel. The derived adaptive SI–O–C coatings withvariable surface properties can be used in diverseapplications, including self-cleaning, drag re-duction, and condensation.[14–19] Although atpresent, the method is restricted to concavesubstrates, the convenience of the coatingmethod and the achievable coating propertiesmake it a promising candidate also for acommercial use.[6]

Article first published online: xxxxManuscript Revised: December 14, 2015

Manuscript Received: July 15, 2015

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Adaptive Silicon Oxycarbide Coatings With Controlled Hydrophilic or Hydrophobic Properties