TECHNICAL FIELD

The invention relates to the field of coatings of the type used e.g. in wind turbine blades. BACKGROUND ART Wind turbine blades are exposed to rain erosion, ice formation and debris erosion that lead to surface deterioration modifying their aerodynamic profile and eventually to destruction of the blade. The result is a decrease in performance and consequently high cost repair. Common practice in prior art entails applying specific coatings onto the blades:

WO2008036074A2 describes the fabrication of hydrophobic and hydrophobic surfaces consisting of fluorinated polymers. Another example is EP2674613A2 that describe a paint coating for optimizing the efficiency of wind turbine blades consisting of a solution of nanoparticles of hydrophobic silicon oxides or fluorinated polyhedral oligomeric

sissesquioxanes, and nanoparticles of hydrophilic silicon oxides or titanium oxides. This paint combines erosion-resistant property with ice-resistant and grime-resistant properties. GB2483672A relates to a multilayered coating with a top surface based on an icephobic material able to be replaced in case of erosion. Another example is US20130078450A1 or WO201 1 147757A1 that relate to a method for the fabrication of hydrophobic coatings by means of cold gas spray for their application in wind turbine blades.

Nevertheless, the coatings mentioned above lack of synergy between hydrophobic, icephobic and wear-resistant properties.

Icephobic coatings market includes various transportation applications, all arctic - especially off-shore - building and energy applications and also heat exchangers and cooling applications. Off-shore industry includes oil and gas industry besides wind energy. Sea water sprays form ice on structures causing work hazards and machinery

malfunctions. Ships that operate in arctic conditions suffer from ice accretion into structures. This creates working hazards and even unbalance to ship's behavior. Also deck machinery may be not usable due to ice formation. They are usually removed by mechanical hammering, steam or even electrical heating. More important examples, among others, of the applicability of these coatings developed in this patent are Bridges (Cables, Railings), Communication Towers, Train Cars, Cars and trucks, Heavy

Equipment Exposed to Ice/Snow, Heat exchangers, Antennas, Guy Wires, Cables, also on bridges with cables, Satellite Dishes, Microwave Domes, Overpasses, Elevated Walkways and Roof Edges.

For the reasons stated above, it is needed to develop multifunctional coatings against atmospheric agents.

SUMMARY OF INVENTION

The invention relates to a process for obtaining of a dense hydrophobic icephobic wear- resistant coating using Cold Gas Spray (CGS) technique. The intrinsic properties of this kind of CGS coatings make them very interesting for different industrial sectors. For example, the deposition of the coating of the present invention directly onto wind turbine blades increases the operational availability of said blades, increases their life operation cycle in icing, corrosive and humid environments significantly reducing manufacturing times/costs, contributing to minimize the power losses and mechanical failures. The coatings of the invention act as a passive protection system (anti-icing) against ice formation and replace the active systems (de-icing solutions) acting in the case of the wind turbine blades by heating using sophisticated devices, costly in installation, maintenance and energy supply and increasing the final weight of the blade and loosing energy production.

The advantages of the invention can be summarized in the following issues:

• Use of the coating during more time with the corresponding savings in

maintenance, repairs, and replacements.

• Use of the coating in ice forming conditions.

· Exclusive and upgraded combination of hydrophobicity, icephobicity and erosion resistance properties.

Therefore, a first aspect of the present invention related to a process for obtaining a dense hydrophobic icephobic wear-resistant coating onto a substrate (herein the process of the invention), characterized in that it comprises the following steps:

a) preparing a powder comprising

• ceramic oxide particles selected from Ti0 ₂ , Ce0 ₂ and Zr0 ₂ , and/or carbide particles selected from Si and Ti, said particles in a weight percent between 40% and 85% based on the final weight of the powder,

• a ductile agent made of a fluorinated polymer selected from a perfluoroalkoxy alkane (PFA) and a polyvinylidene fluoride (PVDF) in a weight percent between 10% and 50% based on the final weight of the powder, and

• additive selected from a silicone and a polyamide in a weight percent between 5% and 10% based on the final weight of the powder; and

b) depositing the powder obtained in step a) onto a substrate by cold gas spray technique under the following spraying parameters:

· gas selected from nitrogen or air,

• gas temperature between 25 °C and 650 °C,

• gas pressure between 5 bar to 30 bar,

• standoff distance between 10 mm to 100 mm,

• and a gun velocity between 100 mm/s and 500 mm/s;

wherein the substrate is made of glass fiber reinforced polymeric matrix composite or it is a metallic substrate, optionally covered by a primer and/or a filler.

After step (b), a dense hydrophobic icephobic wear-resistant coating deposited onto a substrate consisting of a homogeneous distribution of hard ceramic oxide particles selected from Ti0 ₂ , Ce0 ₂ and Zr0 ₂ , and/or carbide particles selected from Si and Ti, said particles being of micrometric and nanometric size, said particles distributed

homogeneously inside a ductile agent.

The ductile component has the purpose to plastically deform during the impact onto the substrate, while the ceramic particles give the desired functional properties to the final product.

In a preferred embodiment, the substrate is covered by a primer and/or a filler. The primer and the filler layers ^' purpose is to improve particle cohesive adhesion.

In another preferred embodiment, the process of the present invention is characterized in that it comprises a previous step (a) of grinding or sandblasting the substrate.

The sand blasting is done at 4 bar with fine alumina (grit 080), obtaining a roughness of R _£ = 3 μηι, R _z = 15 μηι, R _max = 20 μηι.

In another preferred embodiment of the process of the present invention, the substrate is covered by a glue layer. The correct application of the glue is a very important process step to produce a coating with good bonding properties. If the glue is too thin, the coating will not attach to the substrate. If the glue is too thick, it will flow during spraying and leave an irregular surface. Preferably, the glue layer has a thickness of between 20 and 100 μηη, more preferably of between 20 and 50 μηη.

A preferred example of that glue is Araldite Standard 2K. The application is done by mixing, apply to surface, distribute and wipe off excess glue to ensure a thin and homogenous glue layer

In another preferred embodiment of the process of the present invention, the ductile agent of step (a) is a PVDF. PVDF exhibits better results in terms of particle cohesion because it was able to melt and plastically deform during the CGS process. Higher particle compaction and coating densification are obtained using PVDF as ductile agent.

In another preferred embodiment of the process of the present invention, the ceramic oxide particle of step (a) is Zr0 ₂ . It was noticed that Zr0 ₂ gave better results compared with initial Ti0 ₂ and Ce0 ₂ . This fact is due to the thermal diffusivity of zirconium oxide material, which couples better with, for example, the PVDF polymer resulting in denser coatings with higher wear properties.

In another preferred embodiment of the process of the present invention, it comprises a further step of:

(c) thermomechanical treatment by grinding the coating obtained in step (b) under the following conditions:

• temperature between 80 °C and 200 °C,

• pressure between 5 bar and 15 bar,

• and a period of between 1 h and 20 h. After step (c), it is obtained a dense hydrophobic icephobic wear-resistant coating deposited onto a substrate consisting of a homogeneous distribution of hard ceramic oxide particles selected from Ti0 ₂ , Ce0 ₂ and Zr0 ₂ , and/or carbide particles selected from Si and Ti, said particles of micrometric and nanometric size, said particles distributed homogeneously inside a ductile agent.

The term "hydrophobic" refers herein to coating having a contact angle of a water droplet between 90 and 150°. In another preferred embodiment of the process of the invention, step (c) is performed in an autoclave or in a vacuum bag. Another preferred embodiment of the process of the invention relates to the process that further comprises a step (d) of polishing the coating obtained in step (c).

The coating obtained after step (c) has a Ra value of between 1 and 10 μηη and the coating after step (d) has a Ra value of between 0.1 and 1 μηη.

The coating obtained after step (b), step (c) or step (d) has preferably a thickness between 100 μηη and 1 mm.

A second aspect of the present invention relates to a dense (this means exhibing a porosity of less of 1 %), superhydrophobic, icephobic and wear resistant coating deposited onto a substrate consisting of a homogeneous distribution of hard ceramic oxide particles selected from Ti0 ₂ , Ce0 ₂ and Zr0 ₂ , and/or carbide particles selected from Si and Ti, said particles being of micrometric and nanometric size, said particles distributed

homogeneously inside a ductile agent, and obtained according to the process of the present invention described above, characterized in that

it is obtained according to the process comprising steps a) and b), it has a thickness of between 100 μηη and 1 mm, and

it has a surface roughness Ra of between 0.1 and 5 μηη. Another aspect of the present invention relates to a dense (this means exhibing a porosity of less of 1 %), hydrophobic, icephobic and wear resistant coating deposited onto a substrate consisting of a homogeneous distribution of hard ceramic oxide particles selected from Ti0 ₂ , Ce0 ₂ and Zr0 ₂ , and/or carbide particles selected from Si and Ti, said particles of micrometric and nanometric size, said particles distributed homogeneously inside a ductile agent, and obtained according to the process of the present invention described above, characterized in that

it is obtained according to the process comprising steps (a) to (c) or steps (a) to (d),

it has a thickness of between 100 μηη and 1 mm, and

· it has a surface roughness Ra of between 0.1 and 5 μηη.

The next aspect of the present invention relates to the use of the icephobic hydrophobic wear-resistant coatings mentioned above as coating of a wind turbine blade.

Further aspect of the present invention relates to a wind turbine blade characterized in that it comprises the dense hydrophobic icephobic wear-resistant coating as described above, deposited onto a substrate, said substrate being a blade of the wind turbine and said substrate being a metallic substrate such as aluminium or steel.

The dense hydrophobic icephobic wear-resistant coating as described above may have other uses as describes in the next paragraphs.

After the prohibition of tin-based anti-fouling biocides in international shipping in 2003 and substitute systems which proved proven to be less effective, new anti-fouling coatings with superior wear resistance, which can be scrubbed, have been increasingly accepted by the international maritime sector over the last decade. Hydrophobic coatings have crossed from advanced experimental stages to active commercial use on subsea zones of ships, such as hulls, which have been proven not only to reduce bio-fouling but also reduce drag and friction in the contact between the hull and the water. This in turn allows for greater speeds to be reached and in turn means reductions in time and fuel necessary to cover the necessary distance. Early trials indicated increases of between 6%- 10% with a corresponding reduction in fuel costs and emissions of C0 ₂ , NO _x and other gaseous pollutants. There have also been positive results with the reduction of fouling and less time and expense required to cleaning and repairs. Therefore, another aspect of the present invention relates to the use of the dense hydrophobic icephobic wear-resistant coating mentioned above as anti-fouling coatings when deposited on subsea zones of ships.

Furthermore, when a hydrophobic coating is applied, water simply rolls of the surface is question, taking with it both organic and inorganic dirt and not allowing contamination to fix itself and stain. Not only do these coatings improve the aesthetic appearance of the building but they also reduce the time, resource and potentially environmentally damaging chemicals used in cleaning processes. This can be applied to both macro architecture such as entire buildings and offices, and roofs and facades to micro architecture such as individual rooms and areas, for example, bathrooms and hygienic working areas. When combined with other technologies within the architecture of the building such as solar energy (photovoltaic or solar thermal), it can improve medium and long term energy efficiency levels. Therefore, another aspect of the present invention relates to the use of dense hydrophobic icephobic wear-resistant coating mentioned above as self-cleaning architecture.

Moreover, when aircraft pass through clouds of ice cold water droplets at high altitudes, areas of the plane such as the nose, the leading edges of the wings and the engine nacelles are prone to develop collections of water which if unchecked can go on to form layers of ice. These layers of ice can in turn change the aerodynamic characteristics of the aircraft causing controls difficulties and possibly even stalling. Hydrophobic coatings may prevent water droplets of any ice from settling on the fuselage before it can freeze and form ice. Therefore, another aspect of the present invention relates to the use of the dense hydrophobic icephobic wear-resistant coating mentioned above as aircraft coatings.

Many pieces of machinery including engines, gas and steam turbines and heavy duty machinery or also pieces in Civil Engineering applications are at risk of corrosion caused by an excess of water caused by various production processes sitting on metallic surfaces and oxidizing. Other machinery exposed to extreme or simply outdoor climates need to use additional energy in order to prevent icing or de-ice and may also suffer similar problems to those explained above if residual water is not properly disposed of. Therefore, another aspect of the present invention relates to the use of the dense hydrophobic icephobic wear-resistant coating mentioned above in the manufacture of civil engineering or in the manufacture of machinery pieces.

Automotive industry may be interested in hydrophobic coating to increase the product life of car motors and car windscreens as well as improving aesthetical aspect of appearance. Therefore, another aspect of the present invention relates to the use of the dense hydrophobic icephobic wear-resistant coating mentioned above in the manufacture of car, train or truck parts.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skilled in the art to which this invention belongs. Methods and materials similar or equivalent to those described herein can be used in the practice of the present invention. Throughout the description and claims the word "comprise" and its variations are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the invention will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration and are not intended to be limiting of the present invention. BRIEF DESCRIPTION OF DRAWINGS

FIG. 1. Nanocomposite powder deposited onto a two-component compound of epoxy reinforced nature by a) Low Pressure and b) High Pressure CGS.

FIG. 2. Nanocomposite coatings deposited onto a two-component compound of epoxy reinforced nature. FIG. 3. 3D representation of the surface of the coating produced using Low Pressure CGS.

FIG. 4. a) Water droplet onto the as-sprayed surface of a nanocomposite coating, b) water droplet onto the polished surface of a nanocomposite coating.

FIG. 5. SEM micrographs of Ce0 ₂ + 15% PVDF + primer / substrate

FIG. 6. SEM micrographs of Zr0 ₂ + 25% PVDF + primer / substrate FIG. 7. SEM micrograph top surface vs thermomechanical treatment

FIG. 8. Scheme of the global process for growing of a ceramic-PVDF based composite coating DESCRIPTION OF EMBODIMENTS Example 1

The coatings are deposited by Cold Gas Spray onto fibre-reinforced composite (e.g. a glass fibre-reinforced composite with polymer-based matrix). The cold-sprayed particles indeed achieve good mechanical interlocking with the substrate by penetrating into its surface on account of their very high kinetic energy at impact, and by filling into small porosities and irregularities. The coatings consist of a quite homogeneous distribution of hard phase particles (e.g.

Ce0 ₂ ) of micrometric and sub-micrometric size (from =100 nm to =1 μηι) distributed inside a polymer matrix of a perfluoroalxkoxy alkane (PFA). Some polymer-rich areas are interspersed within the coating in order to increase damping capacity.

The structural features of the coatings are not altered as compared to those of the starting powders, with no degradation of either the polymer matrix or the oxide reinforcement. The Fourier-transformed infrared (FT-IR) spectra of Ce0 ₂ + PFA coatings showed that the absorption peaks corresponding to the main vibration modes of the -CF ₂ - group at about 1 150 and 1200cm ^"1, as well as the absorption peak of Ce0 ₂ occurring at about 620 cm ^"1 , having identical intensities both in the feedstock powder and coatings. Static and dynamic contact angles with distilled water are typically in the range of 100° to 135°, but values as high as 150° - 155° can be achieved depending on the exact formulation of the composite and of its surface roughness as it results from the deposition process. Advancing (dynamic) contact angles are in the same range. Deposition of the studied powders was possible in the range of parameter conditions already exposed. FIG. 1 shows two coatings obtained by Low Pressure CGS, FIG. 1 (a), and High Pressure CGS, FIG. 1 (b). Cross section OM images are shown in FIG. 2. The composite nature of the material is shown in these two micrographs. Whiter dots are Ce0 ₂ particles of sizes below 1 μηη, and the particles are in a PFA matrix. The coatings do not show porosity which remains in levels below 1 % throughout the entire coating.

Interestingly, the nanocomposite powders formed layers of more than 100 μηη.

Since wettability of surfaces depends on composition and surface roughness, confocal microscopy was used to measure surface roughness of the as-sprayed coatings. FIG. 3 shows a 3D representation of the surface of an as-sprayed coating. In this particular case, the surfaces show surface roughness of Ra = 2.2 μηη and Rq = 2.9 μηη. However, different powders used and different spraying conditions lead to very different surface roughness which can range from Ra = 1.0 μηη to Ra = 10.0 μηη and from Rq = 1 .5 μηη to Rq = 15.0 μηι.

It is well-known that PFA shows water contact angles above 90°, due to its low surface energy. Therefore, it is considered a hydrophobic material. However, superhydrophobic surfaces are only considered if contact angle is above 150°. Wettability measurements were carried out to check for the hydrophobicity of the Ce0 ₂ + PFA nanocomposite surfaces. FIG. 4 (a) shows a water droplet onto a surface obtained using Low Pressure

CGS. Contact angle is well above 150° demonstrating the Superhydrophobic nature of the obtained coatings. To further check the nature of hydrophobic behaviour of the Ce0 ₂ + PFA nanocomposite coatings, the samples were polished at a final 4000 grit SiC paper. The polished surfaces showed hydrophobic behaviour with contact angles around 120°. These measurements proved that hydrophobic properties of the as-sprayed coatings are due to surface chemistry plus surface roughness directly originated from spraying conditions. Hence, Superhydrophobic surfaces are directly obtained when spraying by Low and High Pressure CGS, nanocomposite powders consisting of a PFA matrix and ceramic oxide particles.

Surface chemistry is essential for the hydrophobicity of these compounds. Both as- deposited coatings (with surface roughness values of Ra ~ 2 - 5 μηη) and polished one (up to Ra < 0.1 μηη) are indeed hydrophobic and ice-phobic if the C-based structural units on the very surface of the coatings consist of at least 65% of C-F bonds as revealed by XPS analyses. Other types of C-based structures (e.g. C-OH, C-O, C=0, C-C groups), which are mainly due to contamination and/or to chemical alteration and degradation of the polymer, are in much lower amounts on the surface of hydrophobic coatings. To the contrary, hydrophobicity is lost ins both as-deposited and polished coatings if the amount of surface C-F groups falls around or below 50% of the overall C-related XPS signal.

On the other hand, almost no XPS signal from the ceramic reinforcement phase (e.g. no signal from Ce in Ce0 ₂ -reinforced composites) appears to the surface, indicating that (i) even the ceramic particles emerging to the outer surface are covered by a nanometre- scale polymer layer and that (ii) the reinforcement is not giving chemical contributions to hydrophobicity. The role of the ceramic phase is that of supporting most of the stresses under mechanical and impact loading, also shown by finite element (FE) simulations. As an erodent particle hits the surface of the Ce0 ₂ + PFA coatings, indeed, the matrix of the Ce0 ₂ + PFA composite transfers most of the load onto the ceramic particles, with "veins" of stresses irradiating from one particle to the other.

The matrix of the Ce0 ₂ + PFA composite coatings can be either a thermoplastic polymer with elastic-plastic deformation behaviour, or an elastomer, with hyperelastic deformation behaviour. The latter case is particularly favourable for erosion resistance as the hyperelastic deformation of the matrix eventually leaves the material in a stress-free state after the erodent has rebounded and after the kinetic energy transferred by the erodent to the material has been dissipated and dispersed in the form of an elastic wave. The size of the hard phase and its distribution within the matrix control the load transfer between the two constituents; namely, as the matrix is much more compliant than the hard phase, it can re-arrange around small, isolated hard phase particles without effective load transfer. The strengthening effect of the hard phase is therefore lost where particles are too isolated, as demonstrated by micro-scale FE simulations. Very fine particles with a distribution as homogeneous as possible are recommended.

Example 2 The initial feedstock consists in a composite material: a ductile agent of PVDF combined with a ceramic oxide such as Ce0 ₂ or Zr0 ₂ produced by spray drying technique. The ductile component has the purpose to plastically deform during the impact onto the substrate, while the ceramic oxide particles give the desired functional properties to the final product.

CGS process was used and spray parameters for many different composite powder mixtures were sprayed onto glass fiber reinforced polymeric matrix composite substrate material. Spraying distance, feeding rate, temperature, offset and number of passes were changed and optimized in order to produce hydrophobic,well-adhered and wear resistant surfaces. After spraying experiments, coated surfaces were analysed by SEM microscopy and wear jet erosion test. In addition, wetting behaviour was studied with static and dynamic contact angle measurements.

Selection of spray parameters used for CGS coatings are presented here:

· Gas temperature: 25 - 650 °C

• Gun velocity: 100 - 500 mm/s

• Spraying distance: 10 - 100 mm

• Pressure: 6 - 10 bar

• Spraying angle: 90°

The initial feedstock was made of PFA polymer and Ce0 ₂ . This composite powder presented a wide deposition window, was deposited by CGS onto glass fiber substrate and gave hydrophobic properties. CGS process was optimized with the use of different percentages PFA and the use of Araldite applied onto the substrate. Results were promising because hydrophobic characteristics of coatings were maintained and coatings adhesion was improved thanks to the effect of the primer bond coat. The type of failure changed from adhesive (between substrate and first coated layer) to cohesive (between each layer of deposited particles).

Finally, Ce0 ₂ and Zr0 ₂ with different particle sizes extending from nano to micron scale, were used as ceramic materials.

PVDF gave better results in terms of particle cohesion because it was able to melt and plastically deform more than for example PFA during the CGS process. Furthermore, it was noticed that Zr0 ₂ gave better results compared with Ce0 ₂ . This fact is due to the thermal diffusivity of zirconium oxide material, which couples better with the PVDF polymer resulting in denser coatings with higher wear properties. See FIG. 5 and 6.

Thermomechanical surface treatment: In order to improve wear characteristics of coatings, a superficial thermomechanical treatment was proposed and applied to the coatings.

The thermomechanical post-treatment step is the most important step to create coating properties necessary for the application under mechanical loads.

The as-sprayed surface presented a Ra value of 9.1 μηη. A surface roughness reduction process was carried out. After a first rapid grinding, a thermomechanical treatment was applied. This procedure reduced the Ra value up to 3.6 μηη Finally, a surface polishing gave a flat (Ra = 0,1 -0,4 μηι) and free of defects surface (FIG. 7).

At the laboratory scale and first steps of industrialization, thermomechanical treatments were also carried out in an autoclave. Several, times and temperatures have been used to improve the quality of the final coating. Compactions of more than 33% have been achieved with this final treatment. After several iterations, the best treatment has been found to be at 150 °C, 6 bar of pressure and 12 h, on surface. CGS coatings showed good adhesion, contact angles, and a good performance in both types of erosion tests. In particular, Zr0 ₂ -25% PVDF CGS coating shows:

• Contact angle of as-sprayed coating is 134° before UV and 140° after UV, being reduced to 95 ⁰ and 1 16 ⁰ after thermomechanical process

• Ice-adhesion is 80 KPa after thermomechanical process

· Jet erosion resistance of 13 h at 191 Km/h under ASTM G73-10

• Rain erosion resistance of 10 h 500 Km/h under ASTM G73

• Sand erosion resistance of 0,54 mg/300g under ASTM G76 Furthermore, this coating shows excellent resistance to UV treatment, to abrasion and to the jet, sand and rain erosion test. It is also worth mentioning their hydrophobic behaviour even after the UV treatment. The intrinsic properties of the CGS coatings, the easiness for these coatings to grow them thicker as well as the possibility to repair them on site makes them very interesting for industrial application, not only onto composite substrates, but also onto metallic ones.