EP4683751A1

EP4683751A1 - Plasma-polymer surface coating

Info

Publication number: EP4683751A1
Application number: EP24708853.7A
Authority: EP
Inventors: Camilo RENDON PIEDRAHITA; Patrick Choquet; Roberto Quintana; Kamal BABA
Original assignee: Luxembourg Institute of Science and Technology LIST
Current assignee: Luxembourg Institute of Science and Technology LIST
Priority date: 2023-03-20
Filing date: 2024-03-07
Publication date: 2026-01-28
Also published as: WO2024194034A1; LU503697B1

Abstract

A method for providing a surface with a coating is proposed. The method comprises directing a gas to the surface to be coated and generating an atmospheric plasma by excitation of the gas, atomizing a precursor medium so as to form precursor droplets, producing plasma-activated precursor medium by subjecting the precursor droplets to the atmospheric plasma so as to generate reactive moieties in and/or on the precursor droplets, depositing the plasma-activated precursor medium on the surface so as to form the coating thereon, the coating comprising a plasma polymer. The precursor medium comprises or consists of one or more silazanes, e.g., cyclic silazanes.

Description

PLASMA-POLYMER SURFACE COATING

Background of the Invention

[0001 ] The invention generally relates to a plasma-polymer surface coating, and, in a preferred aspect, to a fluorine-free superhydrophobic coating. In a specific aspect, the invention concerns a fluorine-free superhydrophobic coating featuring multiscale (or hierarchical) surface roughness.

[0002] Superhydrophobic coatings exhibit high water repellence and prevent the wetting of a surface. As used herein, the expression “superhydrophobic” designates coatings that show a water contact angle (WCA) of at least 150°. The water contact angle (sessile drop contact angle) is the angle formed between the liquid-solid interface and the liquid-vapor interface. It can be measured with a contact angle goniometer. Standard ISO 15989:2004 may be used.

[0003] Another qualitative measure to characterize the water repellence is the so- called hysteresis angle (HA) or contact angle hysteresis. The hysteresis angle corresponds to the difference between the advancing angle and the receding angle. The advancing angle can be measured by increasing the volume of a water drop (by pumping water into it through a needle) and monitoring the contact angle - the advancing angle corresponds to the maximum observed contact angle just before the contact surface between the drop and the surface suddenly increases. The receding angle can be measured by decreasing the volume of the water drop (by pumping water out of it through a needle) and monitoring the contact angle - the receding angle corresponds to the minimum observed contact angle just before the contact surface between the drop and the surface suddenly decreases.

[0004] High water repellence may be associated with a low contact angle hysteresis (< 10°) or a high contact angle hysteresis (> 10°), depending on the wetting mode of the surface. Different modes wetting a rough surface exist. These include, e.g., the Wenzel, Cassie, lotus and (rose) petal wetting modes. Certain superhydrophobic surfaces, like lotus leaves, display the droplet bouncing effect. This effect manifests itself by water drops bouncing or rolling off the surface rather than spreading out and wetting it. Water repellency of lotus leaves is characterized by a very high WCA (>160°), a very low HA (<10°) and a large number of rebounds before the droplet adheres to the surface. This kind of hydrophobicity may thus be termed “lotus-type superhydrophobicity”. On the other hand, rose petals may have a high WCA (>150°), a high HA (>10°), but no rebounds upon droplet impact. This is, therefore, sometimes referred to as (rose) petal effect or “petal-type superhydrophobicity”.

[0005] Superhydrophobic coatings have a wide range of applications, including selfcleaning surfaces, anti-icing and de-icing surfaces, corrosion protection, and reduction of drag and friction automotive, aerospace, and marine industries.

[0006] Various methods for creating superhydrophobic coatings have been devised, including chemical vapor deposition, physical vapor deposition, electrospinning, chemical etching, plasma etching, dip-coating deposition, spray-coating, laser texturing, and layer-by-layer self-assembly, among others. Significant research and development efforts are made in the field of superhydrophobic coatings to develop coatings that are durable, easy to apply, and environmentally friendly.

[0007] The paper “Hydrophobic and superhydrophobic surfaces fabricated using atmospheric pressure cold plasma technology: a review”, P. Dim itrakellis, E. Gogolides, Adv. Colloid Interface Sci. 2018, 254, 1-21 reports that atmospheric pressure plasma (APP) techniques can be employed to deposit superhydrophobic coatings. Atmospheric pressure plasma is generated at atmospheric pressure, as opposed to the high-vacuum conditions typically found in other plasma processing methods. APP can be used to deposit thin films and coatings, including superhydrophobic coatings, and is currently used in industry. In the APP deposition method, a precursor material is introduced into a plasma, where it is exposed to high- energy ions and radicals that generate reactive moieties that polymerize or decompose and deposit onto a substrate as a thin film. The APP technique can be cost-effective and is considered scalable, as it can be used to coat large surfaces or complex shapes.

[0008] Some superhydrophobic coatings contain fluorinated compounds which can be costly and potentially harmful to the environment or human health. Therefore, finding alternative materials that can provide similar performance may be beneficial in terms of cost and sustainability. However, generating superhydrophobic coatings without the use of fluorine-containing compounds is a challenging task. This is due to the difficulties in achieving and maintaining high water repellency, long-term durability, and effective adhesion. [0009] Several studies have reported the successful deposition/fabrication of superhydrophobic coatings through a single-step process. However, most of these methods still require post-treatment to solidify or cure binders, while other methods may not be easily scalable due to the need for specific conditions or specialized equipment.

Summary of the Invention

[0010] According to an aspect of the invention, a method for providing a surface (substrate) with a coating is proposed. The method comprises directing a gas (“plasma gas”) to the surface to be coated and generating an atmospheric plasma by excitation of the gas, atomizing a precursor medium so as to form precursor droplets, producing plasma-activated precursor medium by subjecting the precursor droplets to the atmospheric plasma so as to generate reactive moieties in and/or on the precursor droplets, depositing the plasma-activated precursor medium on the surface so as to form the coating thereon, the coating comprising a plasma polymer. The precursor medium comprises or consists of one or more silazanes, such as, e.g., cyclic silazanes.

[0011 ] It will be appreciated that plasma polymerisation is significantly different from conventional polymerisation. A plasma-polymer is a highly cross-linked film that significantly differs from linear polymers essentially consisting of repetitions of the same building units. In particular, a plasma polymer may be thought as a highly branched, highly cross-linked statistical covalent macromolecular network with various kinds of terminations.

[0012] The precursor medium (and thus the coating formed by plasmapolymerization, i.e. , the plasma polymer coating) may be fluorine-free. As used herein, the expression “fluorine-free” means a fluorine content of less than 0.01 % by weight. The possibility that (only) non-fluorinated compounds may be used in embodiments of the method according to the invention is significant, as these compounds have been linked to environmental and health concerns. In particular, polyfluoroalkyl substances (PFAS), which are currently used for may purposes, e.g., to render surfaces (e.g., outdoor wear, carpets) water and dirt repellent or anti-adhesive (such as pans and pots), potentially need to be replaced in the future by more environmentally sustainable substances. By eliminating the use of fluorinated compounds, a more environmentally responsible and potentially safer alternative to the creation of hydrophobic, dirt- repellent and/or anti-adhesive coatings may be provided in the framework of the present invention. It may also be appreciated that the fluorine-free coatings may be in applied in a single step without the need to perform additional post-treatments.

[0013] The one or more silazanes may comprise one or more cyclic organosilazanes. Examples of the cyclic silazanes include 1 ,3,5-trimethyl-1 ,3,5-trivinylcyclotrisilazane (V3N3), 1 , 3, 5, 7-tetravinyl-1 ,3,5,7-tetramethylcyclotetrasilazane (V4N4),

1 ,2,3,4,5,6,7,8-octamethylcyclotetrasilazane, 1 ,1 ,3, 3, 5, 5, 7, 7- octamethylcyclotetrasilazane, 1 , 1 ,3,3,5,5-hexamethylcyclotrisilazane, 1 ,3,5- triisopropylcyclotrisilazane, and 1 ,2,3,4,5,6-hexamethylcyclotrisilazane. Alternatively, or additionally, the one or more silazanes may comprise at least one of 1 ,3-di-n-propyl- 1 , 1 ,3,3-tetramethyldisilazane, 1 , 3-d iviny 1-1 , 1 ,3,3-tetramethyldisilazane, hexamethyl- disilazane, tetramethyldisilazane, and heptamethyldisilazane.

[0014] The precursor medium may be atomized in the form of droplets into the gas flow before or in the discharge region. Preferably, however, the precursor droplets are introduced into a post-discharge zone of the atmospheric plasma. In the latter case, the precursor medium is not directly exposed to the electric discharges and the reactive precursor moieties are generated through reactions and energy exchange with the plasma (the exited plasma gas). The precursor medium may be atomized into a carrier gas flow, which is then led into the plasma gas flow or the precursor medium could be atomized directly into the plasma gas flow. The plasma gas and the carrier gas could be of the same or different composition. The reactive precursor moieties may comprise, e.g., metastables, neutrals, and radicals generated through reactions between the precursor medium and the plasma. The presence of droplets suspended in the gas (aerosol) is considered important for the formation of a plasma-polymer having hydrophobic or superhydrophobic properties. The plasma-induced deposition of a precursor vapor gives smooth surface coatings.

[0015] Optionally, the precursor medium may comprise a suspension of solid particles in precursor liquid. Upon atomization, part of the precursor droplets may thus include the solid particles wetted with precursor liquid and other precursor droplets may consist of the precursor liquid.

[0016] The solid particles may have diameters in the range from 5 pm to 150 pm (-100/+2500 mesh), preferably in the range from 10 pm to 100 pm (-140/+1250 mesh), more preferably in the range from 10 pm to 90 pm (-170/+1250 mesh), yet more preferably in the range from 25 pm to 75 pm (-200/+500 mesh) and most preferably in the range from 37 pm to 63 pm (-230/+400 mesh). Mesh sizes herein refer to ASTM standard mesh sizes. The notation “-X/+Y” (where “X” and “Y” are mesh values) indicates that the particles pass the (coarser) X mesh sieve but remain on top of the (finer) Y mesh sieve.

[0017] The solid particles may be selected from ceramic particles, oxide particles, carbide particles, nitride particles, or metal particles. More specific examples of solid particles include poly(methyl methacrylate-co-ethylene glycol dimethacrylate particles, silica gel particles, alumina particles, titanium carbide particles, iron oxide particles, silicon particles, silicon carbide particles, zinc oxide particles, aluminum nitride particles, and carbon particles.

[0018] The concentration of the solid particles in the precursor liquid preferably lies in the range from 0.5 to 5 g/10 ml, more preferably in the range from 1 to 3 g/10 ml.

[0019] The plasma-polymer coating may be superhydrophobic, i.e., have a water contact angle (WCA) of at least 150°. Preferably, the coating shows a contact angle hysteresis of 10° or less. According to a preferred embodiment, the coating exhibits lotus-type superhydrophobicity.

[0020] The coating is preferably microstructured. More preferably, the coating has multiscale (also: dual-scale) surface roughness (hierarchical surface roughness). Multiscale surface roughness is characterised by a smaller-scale surface features that are superimposed on larger-scale surface features. It is known from the literature that multiscale roughness may contribute to the (super-)hydrophobic character of a surface.

[0021 ] It should be noted that the use of solid particles in the precursor medium may promote or facilitate the formation of multiscale roughness. The solid particles may indeed provide the larger-scale surface features while the smaller-scale surface features may be formed by the plasma-polymer. However, multiscale roughness may also be obtained without the solid particles, e.g., by adequately selecting the deposition parameters (such as, e.g., discharge power, plasma gas flow rate, carrier gas flow rate, precursor flow rate, droplet size, etc.)

[0022] A further aspect of the invention relates to water-repellent surface structure, comprising a substrate and a coating, the coating comprising a fluorine-free plasma polymer. [0023] The coating may comprise solid particles embedded in the plasma polymer. The solid particles preferably have diameters in the range from 5 pm to 150 pm, more preferably in the range from 10 pm to 100 pm, still more preferably in the range from 10 pm to 90 pm, yet more preferably in the range from 25 pm to 75 pm, and most preferably in the range from 37 pm to 63 pm. The solid particles preferably comprise ceramic particles, oxide particles, carbide particles, nitride particles, or metal particles. Other examples of solid particles include poly(methyl methacrylate-co-ethylene glycol dimethacrylate particles, silica gel particles, alumina particles, titanium carbide particles, iron oxide particles, silicon particles, silicon carbide particles, zinc oxide particles, aluminum nitride particles, and carbon particles.

[0024] Preferably, the coating is superhydrophobic.

[0025] The coating may have multiscale surface roughness.

[0026] The plasma polymer may be obtained from atmospheric-plasma polymerization of one or more precursors including one or more silazanes, preferably cyclic silazanes. Specifically, the one or more precursors may include one or more cyclic organosilazanes, e.g., at least one of 1 ,3,5-trimethyl-1 ,3,5- trivinylcyclotrisilazane, 1 , 3, 5, 7-tetravinyl-1 ,3,5,7-tetramethylcyclotetrasilazane, 1 ,2,3,4,5,6,7,8-octamethylcyclotetrasilazane, 1 ,1 ,3,3,5,5,7,7-octamethylcyclotetra- silazane, 1 , 1 ,3,3,5,5-hexamethylcyclotrisilazane, 1 ,3,5-triisopropylcyclotrisilazane, and 1 ,2,3,4,5,6-hexamethylcyclotrisilazane. Alternatively or additionally, the one or more precursors may include one or more of 1 ,3-di-n-propyl-1 , 1 ,3,3- tetramethyldisilazane, 1 , 3-d iviny 1-1 , 1 ,3,3-tetramethyldisilazane, hexamethyl- disilazane, tetramethyldisilazane, and heptamethyldisilazane.

[0027] In the present document, the verb “to comprise” and the expression “to be comprised of’ are used as open transitional phrases meaning “to include” or “to consist at least of”. Unless otherwise implied by context, the use of singular word form is intended to encompass the plural, except when the cardinal number “one” is used: “one” herein means “exactly one”. Ordinal numbers (“first”, “second”, etc.) are used herein to differentiate between different instances of a generic object; no particular order, importance or hierarchy is intended to be implied by the use of these expressions. Furthermore, when plural instances of an object are referred to by ordinal numbers, this does not necessarily mean that no other instances of that object are present (unless this follows clearly from context). When this description refers to “an embodiment”, “one embodiment”, “embodiments”, etc., this means that the features of those embodiments can be used in the combination explicitly presented but also that the features can be combined across embodiments without departing from the invention, unless it follows from context that features cannot be combined.

Brief Description of the Drawings

[0028] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 : is a schematic illustration of the deposition of a superhydrophobic coating using an atmospheric pressure plasma;

Fig. 2: shows confocal microscope images (a), (b) and SEM images (c), (d) of a plasma-polymer coating obtained from V4N4;

Fig. 3: is a graph of the FITR spectra of V4N4 before (top curve) and after exposure to plasma and deposition as a plasma polymer (bottom curve);

Fig. 4: shows scanning electron microscopy (SEM) images of a plasma polymer coating obtained from a suspension of spherical silica gel particles in V4N4;

Fig. 5: shows (a) an SEM micrograph of a scratch performed on the plasma-polymer coating of Fig. 4 and (b)-(d) cross-sectional SEM images of the coating;

Fig. 6: is a SEM image of a plasma polymer coating obtained using 1 ,2, 3, 4, 5, 6,7,8- OMCTS as the precursor medium;

Fig. 7: is a cross-sectional SEM image of the plasma polymer coating of Fig. 6.

Detailed Description of Preferred Embodiments

[0029] According to an embodiment, a hydrophobic coating, preferably a superhydrophobic coating is deposited on a substrate by an atmospheric pressure plasma jet using a non-fluorinated precursor.

[0030] The precursor medium may be atomized into a carrier gas or the plasma gas (e.g., dinitrogen, argon, or air) resulting in small droplets containing the precursor molecules. The precursor molecules include at least one silazane species, preferably at least one cyclic silazane species. Particularly preferred cyclic silazanes include 1 , 3, 5-trimethyl-1 ,3,5-trivinylcyclotrisilazane, 1 , 3, 5, 7-tetravinyl-1 ,3,5,7- tetramethylcyclotetrasilazane, 1 ,2,3,4,5,6,7,8-octamethylcyclotetrasilazane (OMCTS) and 1 ,1 ,3,3,5,5,7,7-octamethylcyclotetrasilazane (OMCTS). 1 , 1 ,3, 3,5,5- hexamethylcyclotrisilazane, 1 ,3,5-triisopropylcyclotrisilazane, and 1 , 2, 3, 4,5,6- hexamethylcyclotrisilazane may be used as alternatives or in addition to the former.

[0031 ] Further to the precursor liquid, the precursor medium may comprise solid particles suspended in the precursor liquid. Examples of solid particles include, without being limited thereto: carbide particles, nitride particles, or metal particles. More specific examples of solid particles include poly(methyl methacrylate-co-ethylene glycol dimethacrylate) particles, silica gel particles, alumina particles, titanium carbide particles, iron oxide particles, silicon particles, silicon carbide particles, zinc oxide particles, aluminum nitride particles, and carbon particles. The solid particles preferably have diameters in the range from 5 pm to 150 pm (-100/+2500 mesh), more preferably in the range from 10 pm to 100 pm (-140/+1250 mesh), still more preferably in the range from 10 pm to 90 pm (-170/+1250 mesh), yet more preferably in the range from 25 pm to 75 pm (-200/+500 mesh) and most preferably in the range from 37 pm to 63 pm (-230/+400 mesh). Poly(methyl methacrylate-co-ethylene glycol dimethacrylate) - powder, particle size 50 pm. The following examples of solid particles are considered particularly useful in the context of the invention: silica gel spherical - powder, particle size: 45-75 pm (-200/+400 mesh); alumina powder - powder, particle size: 45 pm; titanium carbide - powder, particle size: 50 pm; iron oxide - powder, particle size: 50 pm; silicon - powder, particle size: 45 pm; silicon carbide - powder, particle size: 17 pm; zinc oxide - powder, particle size: 45 pm; aluminum nitride - powder, particle size: 50 pm; carbon - powder, particle size 75 pm.

[0032] Atomization of the precursor medium and exposure of the resulting precursor medium droplets to plasma may result in a hydrophobic coating due to the formation of agglomerates and agglomerate clusters in the plasma jet and on the substrate being coated.

[0033] The water contact angle (WCA) of the resulting plasma-polymer coating may be entirely a function of the topological characteristics of the deposition, since the precursor medium need not comprise atoms decreasing the surface free energy (such as fluorine). The resulting coating may have a WCA of 120° or higher. The WCA may be impacted by the parameters of the plasma generation. When the precursor medium takes the form of a suspension containing precursor liquid and solid particles, the plasma-polymer coating may tend to have a higher WCA than one would obtain using the precursor liquid without solid particles therein. Solid particles suspended in the precursor liquid may promote the formation of a plasma-polymer coating having dual surface roughness. Dual surface roughness may contribute to the achievement of a superhydrophobic character, characterized by a WCA of 150° or more. In addition, low contact angle hysteresis and a droplet bouncing effect may be obtained.

[0034] Preferably, the plasma power, the treatment distance (the distance between the substrate to be coated and the plasma source), and the treatment speed (the relative speed between the substrate and the plasma source) are maintained substantially constant during the coating deposition process. The section of the process parameters may be of importance for the properties of the plasma-polymer coating.

[0035] It shall be appreciated that the proposed method may eliminate the need of fluorinated precursors and thus offers a more environmentally responsible and potentially safer alternative to fluorine-containing hydrophobic coatings. Another advantage of the proposed method is that hydrophobic or superhydrophobic coatings may be deposited in a single step without any additional post-treatment (such as, e.g., washing with a solvent, (additional) curing using heat and/or radiation, drying, solvent removal, texturing, etc.)

[0036] Fig. 1 schematically illustrates an embodiment of a method for depositing a fluorine-free superhydrophobic coating using an atmospheric pressure plasma generated by a plasma source 10. A precursor medium, e.g., a silazane (e.g., a cyclic silazane) or a suspension of solid particles (e.g., spherical silica particles, -200/+400 mesh) in silazane (e.g. cyclic silazane), is atomized and the resulting droplets 12 are injected into the post-discharge 14 of an atmospheric pressure plasma jet 16 and directed onto the substrate to be coated 18. The substrate 18 is moved relative to the plasma source 10 so that the distance between the plasma source and the substrate remains substantially constant. The coating 20 may be deposited in several passes.

[0037] The deposited coating 20 exhibits dual roughness. The dual particle size, and particularly the formation of the agglomerates develops the roughness of the coating and decreases its wettability. The coating features a substantially continuous thin film of plasma polymer that covers the surface and that binds the larger and smaller particles together. The illustrated method does not require any additional treatment steps, such as high temperature curing or solvent removal, after the coating has been applied.

[0038] Dual roughness of the deposited coating may be observed when using silazane as the precursor medium or the suspension of solid particles in silazane. However, improved hydrophobicity could be observed with the suspension than with the pure precursor. This is believed to be due to both the presence of the solid particles and the formation thereon of smaller agglomerates of plasma polymer resulting from the exposure of the precursor to the atmospheric pressure plasma.

[0039] Examples

[0040] Plasma polymer coatings were applied to one-side polished undoped silicon wafers (with a <100> orientation, obtained from Siegert Wafer GmbH) at ambient room temperature (24°C) and atmospheric pressure (1 atm) using a post-discharge plasma coating deposition method. It is worthwhile noting that the invention is not limited to a particular type of surfaces to be coated. The method is applicable to various flat or curved surfaces of different chemical composition (metal, metal oxide, ceramic, plastic, etc.) The tests were conducted with silicon wafers in order to have well-defined conditions on the surface of the substrates used in the different runs.

[0041 ] Suitable plasma sources include atmospheric pressure plasma sources generating a spot-shaped or a line-shaped plasma jet. The plasma source used in the experiments was the PlasmaSpot (trademark) device from Molecular Plasma Group (MPG). For industrial applications, the PlasmaLine (trademark) device from the same manufacturer may be used. The mentioned devices are configured to operate in postdischarge mode, which is preferred in the present context. Nevertheless, part or all of the precursor medium could be introduced into the discharge region of the plasma source. It should be noted that this may require different plasma conditions (power, flow rates, etc.) than those presented hereinafter, which relate to post-discharge configurations.

[0042] An ultrasonic atomizer from Sono-Tek may be employed for the injection of the precursor medium (the frequency was set to 120 kHz and the power was set to deliver 3.2 W). As an alternative, a venturi atomizer may be used. While various carrier and/or plasma gases may be used in the context of the invention, nitrogen was used in the tests. The volumetric flow rate was set to 1 slm (standard liters per minute, i.e., liters per minute of a gas at standard conditions for temperature and pressure (at 0°C and 1 bar)). The aerosol containing the precursor medium droplets was directed through a tube into the post-discharge. The plasma power was set at 300 W under nitrogen flow at 40 slm, and the distance between the plasma source was from 5 to 11 mm. The treatment speed (relative speed between the plasma source and the substrate) was 3 mm/s. The coatings of the examples were obtained with 30 passes. However, the number of passes may be varied. Less or more passes are possible, depending on the needs of the application.

[0043] The distance between the plasma source and the substrate has an effect on the structure of the coating. It was determined experimentally that increasing the distance (from 5 mm to 11 mm in the tests made by the inventors) translated into reduced surface roughness: as the distance was increased, the area density of defined agglomerates and plasma polymer particles decreased. The coatings according to the examples discussed hereinafter were obtained with the plasma source spaced from the substrate by 5 mm. It should be noted, however, that the optimal distance may depend on other parameters of the deposition, such as, e.g., carrier gas and aerosol flow rates, plasma power, plasma source geometry, etc.

[0044] Water contact angle measurements were carried out with DSA100 Drop Shape Analyzer. Static and dynamic contact angles were determined by depositing symmetric water droplets (12 pl) onto the surface at five different locations. The sessile droplet method was employed to measure static water contact angles. The WCA was calculated automatically by the “advance” software from Kruss following droplet deposition. (Alternative contact angle goniometer and/or drop shape analysis software could be used.) Advancing and receding contact angles, used to determine the hysteresis angle (HA), were measured by fitting asymmetric water drops using the ellipse and tangent methods, respectively, depending on the droplet shape. The fitting methods are preferably chosen based on their ability to fit the contour of the droplet during measurement, The fitting method recommended for different measurement ranges may be found in the drop shape analysis software, the user manual thereof or in the technical or scientific literature relating to the specific fitting methods. The values reported herein correspond to the average of five independent measurements taken across the surface under investigation. [0045] Droplet bouncing measurements were performed using video recorded at 500 fps (frames per second). Droplets were released from a height of 9.8 mm above the surface, with a fixed diameter of 1 .5 mm. All frames were extracted using VideoPad Video Editor to determine the number of bounces. Imaged (free image processing software developed at the National Institutes of Health) was used to calculate the height of the first bounce as the distance from the surface to the center of the droplet.

[0046] Example 1

The precursor medium utilized in example 1 was 1 ,3,5, 7-tetravinyl-1 ,3,5,7- tetramethylcyclotetrasilazane (V4N4), obtained from Gelest. The precursor was atomized with an ultrasonic atomizer from Sono-Tek and deposited using an atmospheric-pressure plasma. The deposition led to the formation of small particles. The resulting coating exhibited dual roughness, consisting of individual small particles and agglomerates. Fig. 2 shows confocal microscope images of the obtained plasma- polymer coating ((a) and (b)) as well as SEM images thereof ((c) and(d)), at different magnifications. The small particles had diameters in the range of 3-5 pm, whereas the agglomerates reached diameters of 30-60 pm.

[0047] Fig. 3 shows Fourier Transform Infrared (FITR) spectra of V4N4 before (top curve) and after exposure to plasma (bottom curve). The initial V4N4 spectrum exhibits peaks at 3388 cm^-1 (N-H stretching), 3045 cm^-1 (stretching =CH2), 3004 cm^-1 (symmetric stretching =CH2), 2954 cm^-1 (asymmetric C-H), 1593 cm^-1 (stretching C=C), 1402 cm^-1 (stretching C=C), 1251 cm^-1 (symmetric C-H), 1176 cm^-1 (rocking N- H), 1008-939 cm^-1 (stretching N-H), 867-784 cm^-1 (rocking C-H) and 740 cm^-1 (stretching C-H). After plasma exposure, the peaks at 1176 cm^-1, 1004 cm^-1 and 945 cm^-1 have virtually disappeared, which indicating that most of the N-H bonds are lost during deposition. However, new peaks can be seen at 1022-1099 cm^-1, which are typically associated with Si-O-Si (stretching Si-O) bonds, and a broad peak at 3200 cm^-1 resulting from O-H bonds appeared. The spectra suggest that the initial ring structure of the cyclic silazane undergoes ring opening and that most of the N-H bonds are transformed into new Si-O-Si bonds or terminal Si-OH bonds. As a result, the final coating is formed by polycondensation reactions.

[0048] The wettability of the plasma polymer coating obtained in example 1 was characterized through the measurement of the WCA, which was found to be 121 ± 3°, placing the coating within the hydrophobic range. The low wettability is believed to be a result of the roughness effect alone, as V4N4 contains no atomic species could explain the reduction of the surface free energy.

[0049] Another atmospheric-pressure plasma deposition experiment using V4N4 as the precursor medium was performed as in example 1 but with a Venturi atomizer. It was found that the hydrophobicity of the resulting coating can be modified by controlling the deposition parameters, in particular, the choice of the atomizer. The choice of the atomizer should also be made depending on the type precursor medium. A Venturi atomizer may, e.g., not be suitable for atomizing precursor medium containing solid particles and another type of atomizer, e.g., an ultrasonic atomizer may be preferred in that case.

[0050] Scratch tests performed on the plasma polymer coating obtained in example 1 using a sharp element gave evidence that the coating consisted of a continuous thin film having a rough surface, rather than isolated plasma polymer particles on the substrate.

[0051 ] Example 2

[0052] The precursor medium utilized in example 2 was a suspension of solid particles in V4N4. The solid particles were silica gel spherical particles (of -200/+400 standard US mesh size) from Sigma-Aldrich. The precursor and the solid particles were mixed in a suspension comprised of 2.5 g of solid particles per 10 ml (10 cm³) of precursor. The precursor medium (the suspension) was drawn into a syringe (a 25 ml syringe from Hamilton was used in the examples) and pumped through the atomizer using a syringe pump. It should be noted that the content of solid particles in the suspension may be chosen differently. In the examples, the content in silica gel particles was chosen just below the limit were gel formation was observed. Apart from the precursor medium, the deposition parameters were the same as in example 1 .

[0053] Fig. 4 shows scanning electron microscopy (SEM) images of the plasma polymer coating obtained in example 2. The surface structure is comprised of larger agglomerates that are fully coated with smaller particles. The smaller particles have great similarity to those that could be observed in example 1 .

[0054] Fig. 5 shows an SEM micrograph of a scratch performed on the plasma- polymer coating obtained in example 2 (image (a)) and cross-sectional images thereof (images (b), (c) and (d)). It is apparent that the dual surface roughness can be attributed to the larger solid particles covered with small particles from the precursor. Fig. 5(c) shows larger plasma-polymer-coated particles, the surface thereof being covered with smaller plasma-polymer droplets. Fig. 5(d) evidences that the larger solid particles of the suspension are embedded within the plasma-polymer coating, which forms a continuous film on the surface of the substrate.

[0055] The wettability the plasma-polymer coating obtained in example 2 was examined using WCA measurements, hysteresis angle measurements and through the droplet bouncing effect. A WCA of 150° ± 2° and a HA of 17° ± 3° were measured. Additionally, it was observed that a water droplet bounced one time after impacting the coating which also confirms the superhydrophobic character. The droplet reached a maximum height of 1 .2 mm during the first rebound. The droplet bouncing effect may be important for certain applications. It can help to prevent water from accumulating on a surface, and thus to prevent damage that may be the consequence of such accumulation, e.g., corrosion or erosion. The droplet bouncing effect can help to keep a surface clean and dry, which may be beneficial, e.g., in the automotive or aerospace industries.

[0056] It is believed that the superhydrophobic character of the coating obtained in example 2 is solely due to the dual surface roughness, since the coating does not contain atomic species that could lower the surface free energy.

[0057] Example 3

[0058] The precursor medium utilized in example s was 1 ,2,3,4,5,6,7,8-OMCTS. Instead of an ultrasonic atomizer, a venturi atomizer was used to produce the precursor medium droplets. Apart from these differences, the deposition parameters were the same as in example 1 .

[0059] Fig. 6 is a SEM image of the plasma polymer coating obtained in example 3. The dual surface roughness is clearly visible. Fig. 7 shows a SEM cross-sectional image of the plasma polymer coating obtained in example 3. It can readily be appreciated that the coating forms a continuous film on the substrate.

[0060] The wettability the plasma-polymer coating obtained in example 3 was examined using WCA measurements, hysteresis angle measurements and through the droplet bouncing effect. A WCA of 169° ± 2° and a hysteresis angle less than 5° were measured. Additionally, 10 bounces of the droplet were observed in droplet bouncing measurements, which indicates that the surface coating had a lotus-like superhydrophobic character.

[0061] It is believed that the superhydrophobic character of the coating obtained in example 3 is solely due to the dual surface roughness, since the coating does not contain atomic species that could lower the surface free energy.

[0062] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

1 . A method for providing a surface with a coating, comprising: directing a gas to the surface to be coated and generating an atmospheric plasma by excitation of the gas; atomizing a precursor medium so as to form precursor droplets; producing plasma-activated precursor medium by subjecting the precursor droplets to the atmospheric plasma so as to generate reactive moieties in and/or on the precursor droplets depositing the plasma-activated precursor medium on the surface to form the coating thereon, the coating comprising a plasma polymer; wherein the precursor medium comprises or consists of one or more silazanes.

2. The method as claimed in claim 1 , wherein the precursor medium has a fluorine content of less than 0.01 % by weight.

3. The method as claimed in claim 1 , wherein the one or more silazanes comprise one or more cyclic silazanes, preferably cyclic organosilazanes.

4. The method as claimed in claim 3, wherein the one or more cyclic silazanes comprise at least one of 1 ,3,5-trimethyl-1 ,3,5-trivinylcyclotrisilazane, 1 , 3,5,7- tetravinyl-1 ,3,5,7-tetramethylcyclotetrasilazane, 1 , 2, 3, 4, 5, 6,7,8- octamethylcyclotetrasilazane, 1 , 1 ,3,3,5,5,7,7-octamethylcyclotetrasilazane,

1 .1 .3.3.5.5-hexamethylcyclotrisilazane, 1 ,3,5-triisopropylcyclotrisilazane, and

1 .2.3.4.5.6-hexamethylcyclotrisilazane.

5. The method as claimed in any one of claims 1 to 4, wherein the one or more silazanes comprise at least one of 1 ,3-di-n-propyl-1 ,1 ,3,3-tetramethyldisilazane, 1 , 3-d iviny 1-1 , 1 ,3,3-tetramethyldisilazane, hexamethyldisilazane, tetramethyl- disilazane, and heptamethyldisilazane.

6. The method as claimed in any one of claims 1 to 5, wherein the precursor droplets are introduced into a post-discharge zone of the atmospheric plasma.

7. The method as claimed in any one of claims 1 to 6, wherein the precursor medium comprises a suspension of solid particles in precursor liquid.

8. The method as claimed in claim 7, wherein the solid particles have diameters in the range from 5 pm to 150 pm (-100+2500 mesh), preferably in the range from 10 m to 100 pm (-140+1250 mesh), more preferably in the range from 10 pm to 90 pm (-170+1250 mesh).

9. The method as claimed in claim 7 or 8, wherein the solid particles are selected from ceramic particles, oxide particles, carbide particles, nitride particles, metal particles, poly(methyl methacrylate-co-ethylene glycol dimethacrylate particles, silica gel particles, alumina particles, titanium carbide particles, iron oxide particles, silicon particles, silicon carbide particles, zinc oxide particles, aluminum nitride particles, and carbon particles.

10. The method as claimed in any one of claims 7 to 9, wherein the concentration of the solid particles in the precursor liquid lies in the range from 0.5 to 5 g/10 ml, preferably in the range from 1 to 3 g/10 ml.

11 . The method as claimed in any one of claims 1 to 10, wherein the plasma polymer has a fluorine content of less than 0.01 % by weight.

12. The method as claimed in any one of claims 1 to 11 , wherein the coating is superhydrophobic.

13. The method as claimed in any one of claims 1 to 12, wherein the coating is microstructured, the coating preferably having multiscale surface roughness (hierarchical surface roughness).

14. A water-repellent surface structure, comprising a substrate and a coating, the coating comprising a fluorine-free plasma polymer.

15. The surface structure as claimed in claim 14, wherein the coating comprises solid particles embedded in the plasma polymer.

16. The surface structure as claimed in claim 15, wherein the solid particles have diameters in the range from 5 pm to 150 pm, preferably in the range from 10 pm to 100 pm, more preferably in the range from 10 pm to 90 pm.

17. The surface structure as claimed in any one of claims 14 to 16, wherein the coating is superhydrophobic.

18. The surface structure as claimed in any one of claims 14 to 17, wherein the coating has multiscale surface roughness.

19. The surface structure as claimed in any one of claims 14 to 18, wherein the plasma polymer is obtained from atmospheric-plasma polymerization of one or more precursors including one or more silazanes, e.g., cyclic organosilazanes.

20. The surface structure as claimed in claim 19, wherein the one or more precursors include at least one of 1 ,3,5-trimethyl-1 ,3,5-trivinylcyclotrisilazane, 1 ,3, 5, 7- tetravinyl-1 ,3,5,7-tetramethylcyclotetrasilazane, 1 , 2, 3, 4, 5, 6,7,8- octamethylcyclotetrasilazane, 1 , 1 ,3,3,5,5,7,7-octamethylcyclotetrasilazane,

1 .1 .3.3.5.5-hexamethylcyclotrisilazane, 1 ,3,5-triisopropylcyclotrisilazane,

1 .2.3.4.5.6-hexamethylcyclotrisilazane, 1 , 3-d i-n-propy 1-1 ,1 ,3,3- tetramethyldisilazane, 1 ,3-divinyl-1 ,1 ,3,3-tetramethyldisilazane, hexamethyl- disilazane, tetramethyldisilazane, and heptamethyldisilazane.