BR112020006656B1 - METHOD OF APPLICATION OF A HYDROPHOBIC COATING TO A SURFACE OF A METALLIC SUBSTRATE - Google Patents

Abstract

The present invention relates to a hydrophobic coating and a method for applying such a coating to a surface of a metallic substrate. The method may include anodizing a nanoporous layer of anodic metal oxide on the surface; the cathodization of yttrium oxide nanoparticles on the surface; applying a hydrophobic ceramic coating composition to the surface by an application method selected from the group consisting of: pouring, dipping and spraying; and heating the coated surface to a curing temperature of about 150°C to about 300°C for at least 2 hours.

Description

FIELD OF THE INVENTION

[0001] The present invention relates generally to hydrophobic coatings, and more particularly to hydrophobic coatings comprising anodic and rare earth oxides, and methods of applying such a coating to a surface of a metallic substrate.

BACKGROUND

[0002] The control of the humidification properties of surfaces has been the object of scientific investigation. Most existing hydrophobic surfaces are based on low energy surface polymers, such as fluoroalkylsilane, or patterned roughness on low length scales. Both strategies have significant drawbacks. For example, fluorinated polymers lack abrasion resistance and are easily degraded by ultraviolet light. Similarly, high roughness coatings are often brittle and are poorly suited to harsh environments. Furthermore, these coatings are often based on complex manufacturing techniques that are not easily scalable.

[0003] Aeronautical, automotive and other transparency applications pose additional challenges. For these applications, a hydrophobic coating must maintain high hardness and resistance to attack by acids and bases. Furthermore, these applications may involve metallic substrates, with coefficients of thermal expansion and elastic modulus that are incompatible with many existing hydrophobic coatings.

[0004] It should be appreciated that there is a need for a scalable method of applying an enhanced hydrophobic coating that has an environmentally robust hydrophobic capability. The coating must be robust to environmental degradation, mechanical abrasion and repeated stress, while exhibiting inherently low surface energy without additional surface patterning. For applications involving metallic substrates, the coating must maintain hardness and resistance to acid and alkali attack, while also maintaining a permanent bond to the metallic surface as the surface thermally expands and contracts. The present invention satisfies these needs and provides other related advantages.

BRIEF DESCRIPTION OF THE INVENTION

[0005] The present invention is aggregated in a method of applying a hydrophobic coating to a surface of a metallic substrate, as well as in the hydrophobic coating formed by the method. In one embodiment, the method includes anodizing a nanoporous layer of anodic metal oxide on the surface; applying a hydrophobic ceramic coating composition to the surface, after anodizing the surface, by an application method selected from the group consisting of: pouring, dipping and spraying; and heating the coated surface to a curing temperature of from about 150°C to about 300°C for at least 2 hours. In another embodiment, the method also includes the step of catodizing yttrium oxide nanoparticles prior to applying the hydrophobic ceramic coating composition to the surface. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[0006] In one embodiment, the method includes anodizing a nanoporous layer of anodic metal oxide on the surface, and catodizing yttrium oxide nanoparticles on the surface after anodizing the surface. In another embodiment, the method also includes the steps of applying a hydrophobic ceramic coating composition to the surface, after cathodizing the surface, by an application method selected from the group consisting of: pouring, dipping and spraying; and heating the coated surface to a curing temperature of from about 150°C to about 300°C for at least 2 hours. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[0007] In any of the foregoing embodiments, the method may also include the step of cleaning the surface of the metal substrate with acetone prior to the anodizing step. In another embodiment, the anodic metal oxide can comprise anodic aluminum oxide. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[0008] In any of the foregoing embodiments, the anodizing step may comprise anodizing the surface in an acidic solution having a concentration of about 0.1M to about 0.3M, at an anodizing voltage of about 8 V to about 12 V for an anodizing time of about 20 minutes to about 90 minutes. In one embodiment, the acidic solution can have a pH of less than about 5. In another embodiment, the acidic solution can comprise an acid selected from the group consisting of: acetic acid, citric acid, hydrogen chloride, nitric acid and sulfuric acid. In a further embodiment, the acid can comprise sulfuric acid. In a further embodiment, the concentration can be about 0.2M. In yet another embodiment, the anodizing voltage can be about 10V. In one embodiment, the anodizing time can be about 30 minutes about 60 minutes. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[0009] In any of the foregoing embodiments involving sputtering, the sputtering step may comprise sputtering the surface onto a colloidal dispersion of yttrium oxide nanoparticles at a sputtering voltage of about 8 V to about 12 V , and a current of about 0.05 milliampere to about 0.15 milliampere, for a cathodization time of about 30 minutes to about 90 minutes. In one embodiment, the yttrium oxide nanoparticles can have an average particle size of about 10 nm and can be present in an amount ranging from about 2% to about 10% by weight of the colloidal dispersion. In another embodiment, the amount of yttrium oxide nanoparticles can be about 5% by weight of the colloidal dispersion. In a further embodiment, the cathode voltage can be about 10 V and the current can be about 0.1 milliamps. In a further embodiment, the cathodization time can be about 60 minutes. In one embodiment, the method may also include the step of removing additional nanoparticles from the surface after cathoding the surface. In another embodiment, the removal step can include cleaning the surface with isopropanol. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00010] In any of the foregoing embodiments that involve applying a hydrophobic ceramic coating composition to the surface, the method may also include the step of drying the coating composition on the surface of the substrate for about 1 hour. In one embodiment, the method can include heating the coated surface to a cure temperature of about 200°C. In another embodiment, the hydrophobic ceramic coating composition can comprise a yttrium compound, a dispersion of yttrium oxide nanoparticles, a water-soluble polymer and a solvent solution of deionized water and a water-soluble alcohol. In an additional embodiment, the yttrium compound can comprise yttrium acetate, the yttrium oxide nanoparticle dispersion can be present in an amount ranging from about 0.5% to about 1% by weight of the coating composition. , the water-soluble polymer may comprise polyvinyl alcohol in an amount of from about 1% to about 5% by weight of the coating composition, the water-soluble alcohol may comprise isopropyl alcohol, and the deionized water and soluble alcohol in water may be present in the solvent solution in a ratio of about 2:1. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00011] In any of the foregoing embodiments, the metallic substrate may comprise a metal selected from the group consisting of: aluminum, titanium and stainless steel. In one embodiment, the metal can comprise aluminum. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00012] The present invention is also framed in a hydrophobic coated substrate. In one embodiment, the substrate can include a metallic substrate having a surface, an anodic metal oxide nanoporous layer formed on the surface, and a hydrophobic ceramic coating bonded to the nanoporous layer. In another embodiment, the substrate can also include yttrium oxide nanoparticles embedded in the nanoporous layer. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00013] In one embodiment, the hydrophobic coated substrate may include a metal substrate having a surface, a nanoporous anodic metal oxide layer formed on the surface, and yttrium oxide nanoparticles embedded in the nanoporous layer. In another embodiment, the substrate can also include a hydrophobic ceramic coating bonded to the nanoporous layer. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00014] In any of the foregoing embodiments, the nanoporous layer of anodic metal oxide may include nanopipettes. In one embodiment, the nanopipettes can have an average diameter of from about 10 nm to about 100 nm. In another embodiment, the nanopipettes can have an average diameter of from about 20 nm to about 50 nm. In a further embodiment, the nanopipettes can have a minimum diameter of about 10 nm and a maximum diameter of about 100 nm. In one embodiment, the nanopipettes can have an average length of from about 100 nm to about 10 µm. In another embodiment, the nanopipettes can have an average length of from about 1.5 µm to about 8 µm. In a further embodiment, the nanopipettes can have a minimum length of about 100 nm and a maximum length of about 10 µm. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00015] In one embodiment, the yttrium oxide nanoparticles may have an average particle size of about 10 nm. In another embodiment, yttrium oxide nanoparticles can be embedded in nanopipettes. In a further embodiment, the hydrophobic ceramic coating can comprise yttrium acetate. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00016] In any of the foregoing embodiments, the metallic substrate may comprise a metal selected from the group consisting of: aluminum, titanium and stainless steel. In one embodiment, the metal can comprise aluminum. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00017] In any of the foregoing embodiments, the anodic metal oxide may comprise anodic aluminum oxide. Each feature or concept is independent but may be combined with any other feature or concept described in this patent application.

[00018] Other features and advantages of the invention should become apparent from the following description of preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[00019] Figures 1A to 1C are flowcharts showing methods of applying a hydrophobic coating according to any embodiment of the invention.

[00020] Figure 2A is a cross-sectional illustration of a nanoporous layer of anodic metal oxide formed on the surface of a metallic substrate, according to an embodiment of the invention.

[00021] Figure 2B is an illustration in top view of a nanoporous layer of anodic metal oxide formed on the surface of a metallic substrate, according to an embodiment of the invention.

[00022] Figure 3A is a cross-sectional illustration of a hydrophobic coated substrate according to an embodiment of the invention.

[00023] Figure 3B is a cross-sectional illustration of a hydrophobic coated substrate according to an embodiment of the invention.

[00024] Figure 3C is a cross-sectional illustration of a hydrophobic coated substrate, in accordance with an embodiment of the invention.

[00025] Figure 4 is a cross-sectional illustration of a hydrophobic ceramic coating applied to an aluminum substrate that has a native aluminum oxide layer.

[00026] Figure 5 is a scanning electron micrograph of an anodized aluminum oxide nanopipette layer, according to an embodiment of the invention.

[00027] Figure 6 is a photograph of a drop of water on an aluminum surface that has a hydrophobic ceramic coating reinforced with yttrium oxide nanoparticles, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES

[00028] Referring now to Figures 1A to 1C of the illustrative drawings, methods of applying a hydrophobic coating to a surface of a metal substrate according to embodiments of the invention are shown. With particular reference to Figure 1A, in one embodiment, the method may include the step 110 of anodizing a nanoporous layer of anodic metal oxide on the surface; step 120 of catodizing yttrium oxide nanoparticles onto the surface; step 130 of applying a hydrophobic ceramic coating composition to the surface; and step 140 of heating the coated surface.

[00029] In some embodiments, the method may omit one or more of these steps. For example, referring now to Figure 1B, in one embodiment the method may include the step 110 of anodizing a nanoporous layer of anodic metal oxide on the surface, the step 130 of applying a hydrophobic ceramic coating to the surface, and step 140 of heating the coated surface. Alternatively, referring now to Figure 1C, in another embodiment, the method may include the step 110 of anodizing a nanoporous layer of anodic metal oxide on the surface, and the step 120 of catodizing yttrium oxide nanoparticles on the surface. .

The Metallic Substrate

[00030] In one embodiment, the metallic substrate may comprise a metal selected from the group consisting of: aluminum, titanium and stainless steel. In another embodiment, the metal can comprise aluminum. These metallic substrates can present unique challenges to the design of durable hydrophobic coatings. For example, aluminum, titanium, and stainless steel have coefficients of thermal expansion and elastic modulus that are incompatible with many hydrophobic sol-gel coatings.

[00031] Furthermore, the self-passivation property of aluminum and titanium surfaces presents an additional challenge. It is known that metallic substrates such as aluminum and titanium develop a conformal oxide buildup on their surfaces. This oxide layer is formed within minutes of atmospheric exposure and generally reaches thicknesses not exceeding 10 nm. The naturally formed layer can be removed with light abrasion and mechanical deformation, making it difficult to maintain a surface coating with strong adhesion and durability.

Surface Anodizing

[00032] Anodizing is an electrochemical process that changes the surface chemistry of a metal, through oxidation, to produce an anodic oxide layer. Surprisingly, the nanoporous layer of anodic metal oxide formed on the surface by step 110 can serve to create a permanent bond between sol and gel coatings, such as a hydrophobic ceramic coating, and the surface of the metallic substrate. In one embodiment, the anodizing step 110 may comprise anodizing the surface in an acidic solution. In another embodiment, the acid solution can have a pH of less than about 5. In a further embodiment, the acid solution can comprise an acid selected from the group consisting of: acetic acid, citric acid, hydrogen chloride, nitric acid and sulfuric acid. In a further embodiment, the acid can comprise sulfuric acid.

[00033] In one embodiment, the acidic solution can have a concentration of about 0.1M to about 0.3M. In another embodiment, the concentration can be about 0.2M. , step 110 may be performed at an anodizing voltage of about 8 V to about 12 V for an anodizing time of about 20 minutes to about 90 minutes. In a further embodiment, the anodizing voltage can be about 10 V. In yet another embodiment, the anodizing time can be from about 30 minutes to about 60 minutes.

[00034] During the anodizing step 110, self-assembled nanostructures of anodic metal oxide 210, such as anodic aluminum oxide, can be grown on the surface 205, as illustrated in Figures 2A and 2B. In one embodiment, the anodizing parameters described above can produce a nanoporous layer of anodic metal oxide 210 that has an ordered arrangement of cylindrical pores, or nanopipettes 215. The pore diameters D, periodicity, and density distribution of the nanopipettes 215 can be controlled by adjusting the anodizing parameters described above. Adjustments to the anodizing parameters can be made to control the diameter D and length L of the nanopipettes 215 independently.

[00035] In one embodiment, the nanopipettes 215 can have an average diameter D of from about 10 nm to about 100 nm. In another embodiment, the nanopipettes 215 can have an average diameter D of from about 20 nm to about 50 nm. In a further embodiment, the nanopipettes 215 can have a minimum diameter D of about 10 nm and a maximum diameter of about 100 nm.

[00036] In one embodiment, the nanopipettes 215 can have a minimum diameter D of about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, or about 100 nm. In another embodiment, the nanopipettes 215 can have a maximum diameter D of about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, or about 100 nm.

[00037] In one embodiment, the nanopipettes 215 can have an average length L of about 100 nm to about 10 µm. In another embodiment, the nanopipettes 215 can have an average length L of from about 1.5 µm to about 8 µm. In a further embodiment, the nanopipettes 215 can have a minimum length L of about 100 nm and a maximum length L of about 10 µm.

[00038] In one embodiment, the nanopipettes 215 can have a minimum length L of about 100 nm, about 500 nm, about 1 μm, about 1.5 μm, about 2 μm, about 2.5 μm , about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about of 7 μm, about 7.5 μm, about 8 μm, about 8.5 μm, about 9 μm, about 9.5 μm, or about 10 μm. In another embodiment, the nanopipettes 215 can have a maximum length L of about 100 nm, about 500 nm, about 1 µm, about 1.5 µm, about 2 µm, about 2.5 µm, about about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about 7 μm, about 7.5 μm, about 8 μm, about 8.5 μm, about 9 μm, about 9.5 μm, or about 10 μm.

[00039] This anodic metal oxide nanoporous layer 210 can enhance material interpenetration and increase the strength of mechanical and chemical bonding between the surface 205 and, for example, embedded yttrium oxide nanoparticles 220 (Figure 3C), a hydrophobic ceramic coating 230 (Figure 3B), or both (Figure 3A).

Surface Catodization

[00040] Referring again to Figures 1A and 1C, in one embodiment, the cathodization step 120 may comprise surface cathodization on a colloidal dispersion of yttrium oxide nanoparticles. In another embodiment, the yttrium oxide nanoparticles can have an average particle size of about 10 nm and can be present in an amount ranging from about 2% to about 10% by weight of the colloidal dispersion. In a further embodiment, the amount of yttrium oxide nanoparticles can be about 5% by weight of the colloidal dispersion.

[00041] In one embodiment, the cathodization step 120 comprises electrically connecting the metallic substrate to the negative (cathode) terminal in an electrodeposition assembly (not shown). In another embodiment, the surface of the metallic substrate is cathodized with a cathodizing voltage of about 8 V to about 12 V, and a current of about 0.05 milliampere to about 0.15 milliampere, for a time of about 30 minutes to about 90 minutes. In a further embodiment, the cathode voltage can be about 10 V and the current can be about 0.1 milliamps. In a further embodiment, the cathodization time can be about 60 minutes. In yet another embodiment, the cathodizing step 120 can also include the step of removing excess nanoparticles from the surface after it has been cathodized. In one embodiment, the removal step can include cleaning the cathodized surface with isopropanol.

[00042] Under these cathodization conditions, embodiments of this method can result in a hydrophobic coated substrate 200 as illustrated in Figure 3C. In one embodiment, the coated hydrophobic substrate 200 can include the metallic substrate having the surface 205, the nanoporous anodic metal oxide layer 210 formed on the surface 205, and the yttrium oxide nanoparticles 220 embedded in the nanoporous layer 210. In an additional embodiment, the yttrium oxide 220 nanoparticles can have an average particle size of about 10 nm. In yet another embodiment, yttrium oxide nanoparticles 220 can be embedded in nanopipettes 215.

[00043] These embedded yttrium oxide nanoparticles 220 can improve the hydrophobic ability of the surface 205 while also imparting robust performance against surface abrasion and deformation. Furthermore, when used in combination with a ceramic coating (Figure 3A), such as the ceramic coating discussed below, the nanoparticles 220 can allow the ceramic coating to cure at a lower temperature, which mitigates migration atomic and other changes in the mechanical properties of the metallic substrate.

Application of a Ceramic Coating

[00044] Referring again to Figures 1A and 1B, the coating composition can be applied to the surface, in step 130, by an application method selected from the group consisting of pouring, dipping and spraying. Selection of the appropriate method, or combination of methods, is normally understood by one normally versed in the prior art. For example, a flow or spray coating may be appropriate for large parts or complex shapes, or where two different coatings are required. Dip coating may be appropriate, for example, where an entire part is to be coated.

[00045] In one embodiment, the coating composition can include a yttrium compound; an additive selected from the group consisting of a cerium compound and a dispersion of yttrium oxide nanoparticles; a water-soluble polymer; and a solvent solution of deionized water and a water-soluble alcohol.

[00046] The following table presents chemical formulas for yttrium-based chemical reagents available for sol-gel synthesis. In one embodiment, yttrium is selected from the group consisting of yttrium acetate, yttrium carbonate, yttrium chloride, yttrium fluoride, yttrium hydroxide, yttrium metal, yttrium nitrate, yttrium oxalate, and yttrium sulfate. In a preferred embodiment, the yttrium is yttrium acetate.

[00047] In one embodiment, the cerium compound is water soluble. Examples of water-soluble cerium compounds include cerium bromide, cerium chloride and cerium nitrate. In another embodiment, the cerium compound is sparingly soluble in water. Examples of sparingly water-soluble cerium compounds include cerium acetate and cerium sulfate.

[00048] In other embodiments, the coating composition comprises an additive of a dispersion of yttrium oxide nanoparticles. The yttrium oxide nanoparticle dispersion is preferably compatible with the coating composition and therefore can be added at high levels without precipitation. In one embodiment, the yttrium oxide nanoparticle dispersion is present in an amount ranging from about 0.1% to about 5% by weight of the coating composition. In a preferred embodiment, the amount of yttrium oxide nanoparticle dispersion is from about 0.5% to about 1% by weight of the coating composition.

[00049] A preferred embodiment of the coating composition also comprises a water-soluble polymer. This water-soluble polymer component acts to increase the thickness of the final hydrophobic coating. The hydrophobic nature of the coating composition without the water-soluble polymer makes it resistant to high build buildup. The addition of a water-soluble polymer to the coating composition increases the final coating thickness to greater than about 50 nm, greater than about 75 nm, greater than about 100 nm, greater than about 125 nm, greater than about greater than 150 nm, greater than about 200 nm, greater than about 225 nm, or greater than about 250 nm.

[00050] In one embodiment, the water-soluble polymer is selected from the group consisting of poly(n-vinylpyrrolidone), poly(vinylamine hydrochloride), polymethacrylamide, polyvinyl alcohol, polyacrylamide, poly(ethylene oxide-b-oxide) of propylene), poly(methacrylic acid), poly(ethylene oxide), poly(n-iso-propylacrylamide) and poly(2-vinylpyridine). In a further embodiment, the water-soluble polymer is polyvinyl alcohol. In one embodiment, the water-soluble polymer is present in an amount ranging from about 1% to about 10% by weight of the coating composition. In yet another embodiment, the amount of the water-soluble polymer is from about 1% to about 5% by weight of the coating composition.

[00051] A preferred embodiment of the coating composition also comprises a solvent solution of deionized water and a water-soluble alcohol. In one embodiment, the water-soluble alcohol is selected from the group consisting of isopropyl alcohol, methanol, ethanol, propanol and butanol. The following table gives the chemical formulas and water solubility levels of some water-soluble alcohols, but any other water-soluble alcohol can be used. In another preferred embodiment, the deionized water and the water-soluble alcohol are present in the solvent solution at a ratio of about 2:1.

[00052] In one embodiment, not shown, the method includes the step of placing the coating composition on the surface of the substrate to dry before the heating step 140. In an alternative embodiment, the method may comprise the step of drying the coating composition coating on the surface of the substrate before heating 140. In any case, the coating composition can be allowed to dry for about 1 hour, about 2 hours, about 3 hours, or until the coating composition is in the "green state". ".

Surface Heating

[00053] Referring again to Figures 1A and 1B, in one embodiment, the method may comprise the step 140 of heating the coated surface to a curing temperature of about 150°C to about 300°C for a time of Cure for at least 2 hours. In another embodiment, the curing time can be from about 2 hours to about 24 hours. In a further embodiment, the cure temperature can be about 200°C and the cure time can be at least 2 hours.

A hydrophobic coated substrate

[00054] Referring again to Figures 3A to 3C of the illustrative drawings, hydrophobic coated substrates 200 are shown in accordance with embodiments of the invention. With particular reference to Figure 3A, in one embodiment, the substrate can include a metallic substrate having a surface 205, a nanoporous anodic metal oxide layer 210 formed on the surface 205, yttrium oxide nanoparticles 220 embedded in the nanoporous layer 210, and a hydrophobic ceramic coating 230 attached to the nanoporous layer 210. The hydrophobic coated substrate 200 shown in Figure 3A can be produced by the method outlined in Figure 1A.

[00055] In some embodiments, the hydrophobic coated substrate 200 may omit one or more of these elements. For example, referring now to Figure 3B, in one embodiment, the hydrophobic coated substrate 200 can include the metallic substrate having the surface 205, the nanoporous anodic metal oxide layer 210 formed on the surface 205, and the hydrophobic ceramic coating 230 bonded to nanoporous layer 210. This hydrophobic coated substrate 200 can be produced by the method outlined in Figure 1B. Alternatively, referring to Figure 3C, in another embodiment, the hydrophobic coated substrate 200 can include the metal substrate having the surface 205, the nanoporous layer of anodic metal oxide 210 formed on the surface 205, and the yttrium oxide nanoparticles 220 embedded in the nanoporous layer 210. This hydrophobic coated substrate 200 can be produced by the method outlined in Figure 1C.

[00056] In one embodiment, the nanoporous anodic metal oxide layer 210 can include the nanopipettes 215. In another embodiment, the nanopipettes 215 can have an average diameter D of about 10 nm to about 100 nm. In a further embodiment, the nanopipettes 215 can have an average diameter D of from about 20 nm to about 50 nm. In a further embodiment, the nanopipettes 215 can have a minimum diameter D of about 10 nm and a maximum diameter D of about 100 nm.

[00057] In one embodiment, the nanopipettes 215 can have a minimum diameter D of about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, or about 100 nm. In another embodiment, the nanopipettes 215 can have a maximum diameter D of about 10 nm, about 15 nm, about 20 nm, about 25 nm, about 30 nm, about 35 nm, about 40 nm, about 45 nm, about 50 nm, about 55 nm, about 60 nm, about 65 nm, about 70 nm, about 75 nm, about 80 nm, about 85 nm, about 90 nm, about 95 nm, or about 100 nm.

[00058] In one embodiment, the nanopipettes 215 can have an average length L of about 100 nm to about 10 µm. In another embodiment, the nanopipettes 215 can have an average length L of from about 1.5 µm to about 8 µm. In a further embodiment, the nanopipettes 215 can have a minimum length L of about 100 nm and a maximum length L of about 10 µm.

[00059] In one embodiment, the nanopipettes 215 can have a minimum length L of about 100 nm, about 500 nm, about 1 μm, about 1.5 μm, about 2 μm, about 2.5 μm , about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about of 7 μm, about 7.5 μm, about 8 μm, about 8.5 μm, about 9 μm, about 9.5 μm, or about 10 μm. In another embodiment, the nanopipettes 215 can have a maximum length L of about 100 nm, about 500 nm, about 1 µm, about 1.5 µm, about 2 µm, about 2.5 µm, about about 3 μm, about 3.5 μm, about 4 μm, about 4.5 μm, about 5 μm, about 5.5 μm, about 6 μm, about 6.5 μm, about 7 μm, about 7.5 μm, about 8 μm, about 8.5 μm, about 9 μm, about 9.5 μm, or about 10 μm.

[00060] In one embodiment, the yttrium oxide 220 nanoparticles may have an average particle size of about 10 nm. In a further embodiment, yttrium oxide nanoparticles 220 can be embedded in nanopipettes 215. In yet another embodiment, hydrophobic ceramic coating 230 can comprise yttrium acetate.

[00061] These embedded yttrium oxide nanoparticles 220 can improve the hydrophobic ability of surface 205 while also imparting robust performance against surface abrasion and deformation. Additionally, when used with the ceramic coating 230, the nanoparticles 220 can allow the ceramic coating 230 to cure at a lower temperature, which mitigates atomic migration and other changes in the mechanical properties of the metallic substrate.

[00062] In some embodiments, the resulting hydrophobic coated substrate 200 will exhibit water contact angles of greater than about 90°, greater than about 95°, greater than about 100°, or greater than about 105° . The hydrophobic coating will have a thickness of greater than about 50 nm, greater than about 75 nm, greater than about 100 nm, greater than about 125 nm, greater than about 150 nm, greater than about 200 nm, greater about 225 nm, or greater than about 250 nm. Furthermore, the hydrophobic coating will be robust to environmental degradation, mechanical abrasion and repeated stress. For example, in some embodiments, the hydrophobic coating will exhibit high hardness and resistance to attack by acids and bases.

[00063] It should be appreciated from the above description that the present invention provides a scalable method of applying a hydrophobic coating that exhibits an environmentally robust hydrophobic ability. Coatings produced by these methods are hydrophobic and resistant to environmental degradation, mechanical abrasion, repeated stress and attack by acids and bases. Additionally, the coatings are thick enough for robust performance, and the cure temperature is low enough to mitigate atomic migration and other changes in the mechanical properties of the metallic substrate. For all these reasons, the methods described in this patent application, and the resulting coatings, are ideal for aeronautical and automotive transparency applications.

[00064] The specific methods, devices and materials are described, although any methods and materials similar or equivalent to those described may be used in the practice or testing of the present invention. Unless otherwise defined, all technical and scientific terms used in this patent application have the same meanings as are generally understood by one normally versed in the state of the art to which the present invention belongs.

[00065] As used in this patent application, singular words such as "one" and "one" mean "one or more" unless clear intent is shown to limit the element to "one." The term "about" means ±2% of the value it modifies.

[00066] As used in this patent application, the term "hydrophobic" means that it does not have an affinity for water, and a surface is considered hydrophobic when the contact angle of water is at least about 80 degrees.

[00067] As used in this patent application, the term "water soluble" means that the compound is infinitely soluble in water, freely soluble in water, freely soluble in water, or soluble in water, as these terms are normally understood. A material is generally considered to be "very soluble" if about 1 gram of the material requires about 1 milliliter or less of solute to dissolve. A material is generally considered to be "freely soluble" if about 1 gram of the material requires about 1 milliliter to about 10 milliliters of solute to dissolve. A material is generally considered to be "soluble" if about 1 gram of the material requires about 10 milliliters to 30 milliliters of solute to dissolve. A material is normally considered "sparingly soluble" if about 1 gram of the material requires about 30 milliliters to about 100 milliliters of solute to dissolve.

[00068] Without further elaboration, it is believed that one skilled in the art, by using the above description, can make and use the present invention to the fullest extent. Other objects, features, and advantages of the present embodiments will become apparent from the specific examples below. The specific examples, while indicating specific embodiments, are provided by way of illustration only. Accordingly, the present invention also includes the various changes and modifications within the character and scope of the invention which may become apparent to those skilled in the art from this detailed description. The following examples are illustrative only, and are not limiting the disclosure in any way. EXAMPLE 1

[00069] A first series of samples was prepared using aluminum specimens, which had been cleaned with acetone. A ceramic coating was applied to the aluminum substrates via flow or spray coating. The ceramic coating composition (for this and all subsequent samples) included a yttrium compound, a dispersion of yttrium oxide nanoparticles, a water-soluble polymer and a solvent solution of deionized water and a water-soluble alcohol. The coated substrate was heat treated at about 300°C for about 2 hours at ambient atmosphere. An illustration of the first series of samples is shown in Figure 4, which shows the ceramic coating 230 on a native aluminum oxide layer 206 formed on the surface 205 of the aluminum substrate.

[00070] A second series of samples was prepared using aluminum specimens, which had been cleaned with acetone. The second series of samples was subjected to an anodizing process in an acidic 0.2 M sulfuric acid solution. For this process, aluminum specimens were joined to the anode and a constant voltage of 10 V was applied for about of 30 minutes. Under these anodizing conditions, a nanoporous layer of anodized aluminum oxide was developed on the aluminum surface. The nanoporous structure included nanopipettes that have an average diameter of around 20 nm and an average length of around 1.5 µm.

[00071] A third series of samples was prepared under the same conditions as the second series of samples, except that voltage was applied for about 5 hours. Under these anodizing conditions, an anodized aluminum oxide nanoporous layer was developed on the aluminum surface, and the structure included nanopipettes that have an average diameter of about 20 nm and an average length of about 8 μm.

[00072] A fourth series of samples was prepared under the same conditions as the second series of samples, except that the acid solution comprised 0.2 M hydrochloric acid. Under these anodizing conditions, a nanoporous layer of Anodized aluminum was developed on the aluminum surface, and the structure included nanopipettes that have an average diameter of about 50 nm and an average length of about 500 nm.

[00073] The resulting anodized layers (for the second, third, and fourth samples) exhibited a low water contact angle, which was consistent with aluminum oxide. The layers were abrasion resistant as measured by hand abrasion tests with steel wool. As shown in Figure 5, cross-sectional scanning electron microscopy reveals an extremely fine structure. The anodized layer (zone 1) comprises high aspect ratio aluminum oxide crystalline grains, which are oriented perpendicular to the substrate, and which have an average diameter of about 20 nm and an average length of about 8 µm.

[00074] Then, the ceramic coating was applied to the aluminum substrates, in the second series of samples, by means of flow or spray coating, and the coated substrates were heat treated at about 300°C for about 2 hours to the ambient atmosphere. An illustration of the second series of samples is shown in Figure 3B, which shows the nanoporous anodic metal oxide layer 210 formed on the surface 205, and the hydrophobic ceramic coating 230 bonded to the nanoporous layer 210.

[00075] The first series of samples, which were coated on the native aluminum surface, were compared to the anodized aluminum oxide buffered coatings of the second series of samples. The following table illustrates the results. As shown, hydrophobic ceramic coatings applied directly to the native aluminum surface exhibited variable quality in surface finish and hydrophobicity. Additionally, ceramic coatings that were applied without the anodized layer were easily removed with hand pressure. On the other hand, the second series of samples had a smooth hydrophobic surface and exhibited significant abrasion resistance.

EXAMPLE 2

[00076] For a fifth series of samples, the aluminum specimens were cleaned according to the preceding examples, followed by anodizing in 0.2 M sulfuric acid at about 10 V for about 1 hour. After the anodizing process, the specimens were cleaned in deionized water. Anodized surface illustrations are shown in Figures 2A and 2B.

[00077] For a sixth series of samples, the following steps were added after anodizing. First, a colloidal aqueous yttrium oxide solution with an average particle size of 10 nm and a solids loading of 5% was prepared. The anodized aluminum specimens were then electrically connected to the negative (cathode) terminal on the electrodeposition set. This configuration was selected due to the natural positive surface charge of yttrium oxide in the acidic aqueous solution. A voltage of about 10 V and a current of about 0.1 milliamps was maintained for about 1 hour. An accumulation of additional nanoparticles became visible on the anodized surface. This buildup was removed by wiping the surface with isopropanol, leaving a smooth anodized surface. Yttrium oxide nanoparticles were embedded in the anodized surface as illustrated in Figure 3C.

[00078] The fifth and sixth grades were compared, and the following table illustrates the results. As shown, initial resistance testing of nanoparticle enhanced coatings indicated significant abrasion resistance. The surfaces were hydrophobic, with water contact angles measuring between about 80° and about 90°, which is a reduction from that of the hydrophobic ceramic coating of the second series of samples. This is presumably due to incomplete coverage of the surface with yttrium oxide. The coated specimens showed extremely robust performance against surface abrasion and creep, sustaining greater than 5% deformation with no reduction in water contact angle.

EXAMPLE 3

[00079] For a seventh series of samples, the aluminum specimens were cleaned, anodized and coated with the ceramic coating according to the process outlined for the second series of samples. The ceramic coating was cured at approximately 300°C.

[00080] For an eighth series of samples, the aluminum specimens were prepared in an identical configuration to the seventh series of samples, but with a reduced 200°C cure.

[00081] For a ninth series of samples, the aluminum specimens were prepared in an identical configuration to the eighth series of samples. However, before the hydrophobic ceramic coating was applied, the anodized specimens were cathodized in a colloidal dispersion of yttrium oxide nanoparticles, as described above in relation to the sixth series of samples. An illustration of the ninth series of samples is shown in Figure 3A, which shows the nanoporous anodic metal oxide layer 210 formed on the surface 205, the yttrium oxide nanoparticles 220 embedded in the nanoporous layer 210, and the bonded hydrophobic ceramic coating 230 to the nanoporous layer 210. Referring to Figure 6, the ninth series of samples exhibited a water contact angle greater than or equal to about 90°.

[00082] The seventh, eighth and ninth grades were compared, and the following table illustrates the results. As shown, the ninth series of samples enhanced with nanoparticles exhibited increased cosmetic quality and greater water contact angles compared to the version without the yttrium oxide nanoparticles (eighth series) cured at the same temperature. Data show that high concentrations of yttrium oxide nanoparticles in anodic aluminum oxide pores cause enhanced subsurface nucleation and suppression of crystallization temperature. The seventh and ninth series of samples had smooth high quality coatings, which exhibited robust hydrophobicity. Therefore, the anodic aluminum oxide buffered layer embedded in nanoparticles allows for a lower temperature cure to obtain similar desirable properties. The lower cure temperature mitigates atomic migration and other changes in the mechanical properties of the metallic substrate.

[00083] The invention has been described in detail with reference only to the presently preferred embodiments. Those skilled in the art will appreciate that various modifications can be made without deviating from the invention. Therefore, the invention is defined only by the following claims.

Claims (16)

1. Method of applying a hydrophobic coating to a surface of a metallic substrate, characterized in that the method comprises: anodizing a nanoporous layer of anodic metal oxide on the surface; applying a hydrophobic ceramic coating composition to the surface, after anodizing the surface, by an application method selected from the group consisting of pouring, dipping and spraying; and heating the coated surface to a curing temperature of 150°C to 300°C for at least 2 hours; wherein the hydrophobic ceramic coating composition comprises yttrium compound and a dispersion of yttrium oxide nanoparticles. 2. Method according to claim 1, characterized in that the metallic substrate comprises a metal selected from the group consisting of: aluminum, titanium and stainless steel. 3. Method according to claim 2, characterized in that the metal comprises aluminum. 4. Method according to claim 3, characterized in that the anodic metal oxide comprises anodic aluminum oxide. 5. Method according to claim 1, characterized in that the anodizing comprises anodizing the surface in an acid solution having a concentration of 0.1 M to 0.3 M, at an anodizing voltage of 8 V to 12 V for an anodizing time of 20 minutes to 90 minutes. 6. Method according to claim 5, characterized in that the acid solution comprises an acid selected from the group consisting of: acetic acid, citric acid, hydrogen chloride, nitric acid and sulfuric acid. 7. Method according to claim 1, characterized in that the hydrophobic ceramic coating composition comprises: a water-soluble polymer; and a solvent solution of deionized water and a water-soluble alcohol. 8. Method according to claim 1, characterized in that it further comprises the step of catodizing yttrium oxide nanoparticles on the surface before applying the hydrophobic ceramic coating composition to the surface. 9. Method according to claim 8, characterized in that the curing temperature is 200°C. 10. Method according to claim 8, characterized in that the cathodization comprises the cathodization of the surface in a colloidal dispersion of yttrium oxide nanoparticles, at a cathodization voltage of 8 V to 12 V, and at a current of 0 .05 milliampere to 0.15 milliampere, for a cathodization time of 30 minutes to 90 minutes. 11. Method according to claim 10, characterized in that the yttrium oxide nanoparticles have an average particle size of 10 nm and are present in an amount ranging from 2% to 10% by weight of the colloidal dispersion. 12. Method of applying a hydrophobic coating to a surface of a metal substrate, characterized in that the method comprises: anodizing a nanoporous layer of anodic metal oxide on the surface; the cathodization of yttrium oxide nanoparticles on the anodized surface; applying a hydrophobic ceramic coating composition to the cathodized surface by an application method selected from the group consisting of pouring, dipping and spraying; and heating the coated surface to a curing temperature of 2000°C for at least 2 hours; wherein the anodizing comprises anodizing the surface in an acidic solution having a concentration of 0.2 M, at an anodizing voltage of 10 V for an anodizing time of 30 minutes to 60 minutes; wherein the cathodization comprises cathodizing the surface on a colloidal dispersion of yttrium oxide nanoparticles, at a cathodizing voltage of 10 V, and a current of 0.1 milliamps, for a cathodizing time of 60 minutes; wherein the hydrophobic ceramic coating composition comprises yttrium compound and a dispersion of yttrium oxide nanoparticles. 13. Method according to claim 12, characterized in that the metal comprises aluminum and the anodic metal oxide comprises anodic aluminum oxide. 14. Method according to claim 13, characterized in that the acid comprises sulfuric acid. 15. Method according to claim 14, characterized in that the yttrium oxide nanoparticles have an average particle size of 10 nm and the amount of yttrium oxide nanoparticles is 5% by weight of the colloidal dispersion. 16. Method according to claim 15, characterized in that the hydrophobic ceramic coating composition further comprises: polyvinyl alcohol in an amount of 1% to 5% by weight of the coating composition; a solvent solution of deionized water and isopropyl alcohol at a ratio of 2:1; wherein the yttrium compound comprises yttrium acetate; wherein the dispersion of yttrium oxide nanoparticles is present in an amount ranging from 0.5% to 1% by weight of the coating composition.

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