WO2016176777A1 - Solution precursor plasma sprayed ("spps") ceramic superhydrophobic coatings, processes for applying the coatings and articles coated with same - Google Patents

Abstract

A method of forming a superhydrophobic coating on a surface of a substrate, said method comprising: preparing a surface of a metal substrate for coating; applying a coating solution to the metal substrate, preferably the coating solution comprising at least one rare earth metal or salt thereof, and at least one solvent; preferably the coating solution is applied to the substrate surface via solution precursor plasma spraying, until the coating is adhered to the surface of the substrate.

Description

TITLE

Solution precursor plasma sprayed ("SPPS") ceramic superhydrophobic coatings, processes for applying the coatings and articles coated with same.

FIELD OF THE DISCLOSURE

A method is disclosed for fabricating ceramic superhydrophobic coatings using rare earth metal salts. The method includes a deposition process on the surface of a substrate which deposition process is accomplished by the solution precursor plasma spray (SPPS) technique. The resulting coating material is the oxide form of the rare earth metal. The surface coated by the SPPS method results in a hierarchical structured surface which closely resembles the structure of superhydrophobic surfaces in nature, such as lotus leaves. All existing techniques to fabricate superhydrophobic surfaces require that the surface of the substrate be roughened or nano-patterned before deposition of a coating, or that the coating be roughened or nano-patterned after deposition The coating technique disclosed herein is the first method to produce a superhydrophobic surface with the desired surface topography without secondary patterning or roughening. The resulting coated surface has a water contact angle greater than about 150°, preferably as high as 165° and a roll-off angle between about 1° and about 10°, preferably less than about 5°. This method offers a rapid and economical way to fabricate a durable superhydrophobic surface at a large scale, with a wide range of potential applications, including but not limited to mainly in energy conservation such as enhancing heat transfer in steam condensers and boilers, reducing hydrodynamic drag in large pipes, and improving water turbine efficiency.

BACKGROUND

Since the development of electron microscopy in the last century, researchers have been able to study nano-textures of plant surfaces and animal skins. It has been found that a unique surface architecture provides a self-cleaning ability to leaves (e.g. lotus leaves), and allows the wings of insects to remain dirt-free and shed water when they fly in the rain. This type of surface is called a hydrophobic surface. Hydrophobic surfaces are characterized by a high water contact angle (>90°). When the water contact angle is higher than 150° with a roll off angle less than 10° the surface is considered to be superhydrophobic. Due to these properties, hydrophobic surfaces have a wide range of potential applications to benefit the environment in energy conservation, including friction drag reduction (Fukuda, K., Tokunaga, J., Nobunaga, T., & Nakatani, T.

(2000). Frictional drag reduction with air lubricant over a super-water-repellent surface. Journal of Marine Science and Technology, 5(3), 123-130); self-cleaning windows, windshields and roof tiles to save cleaning water; enhancement of condensation in steam power plants in order to increase the efficiency of electricity generation (Azimi, G., Dhiman, R., won, H., Paxson, A. T., & Varanasi, K. K. (2013). Hydrophobicity of rare-earth oxide ceramics. Nature Materials, 72(4), 315-320); and promotion of nucleation for pool boiling at low heat flux to enhance boiling heat transfer (Betz, A. R., Jenkins, J., Kim, C, & Attinger, D. (2013). Boiling heat transfer on superhydrophilic, superhydrophobic, and superbiphilic surfaces. International Journal of Heat andMass Transfer, 57(2), 733-741).

In the past, extensive research has been performed to fabricate hydrophobic surfaces. In 2001 , a research group from Japan reviewed the different methods and materials used in processing superhydrophobic surfaces since the 1950's (Nakajima, A., Hashimoto, K , & Watanabe, T. (2001). Recent Studies on Super-Hydrophobic Films. Monatshefte fur Chemie, 132, 31 -41). Almost all of the techniques summarized in Nakajima, et al. (2001) involved coating or polymerizing a low surface energy material such as paraffin, organic polymers (polyvinylidene) or fluoropolymers (PTFE) on a substrate whose surface had been previously roughened, or roughening the low surface energy coating by plasma etching, air blast roughening or mechanical machining to further enhance the hydrophobicity. Recently, nature-inspired nano-patterned structures on silica, polymethyl methacrylate and polystyrene were manufactured using laser abrasion or soft lithography (Bhushan, B., Jung, T. C, & Koch, K. (2009). Micro-, nano- and hierarchical structures for superhydrophobicity, self-cleaning and low adhesion. Phil. Trans. R. Soc. A, 367, 1631-1672; and Fukuda, et al. 2001).

Despite the excellent water resistant properties of the hydrophobic surfaces produced by the above techniques, the coatings using silica or polymers are not stable at high temperature. Most of the polymers discussed above have a glass transition temperature less than 250°C. Durability is another issue; due to the mechanical properties of the polymers, the coatings produced by the above techniques cannot be used in a harsh environment. Many of the polymeric materials also suffer degradation and embrittlement when exposed to ultra-violet radiation for extended periods.

In 2013, rare earth oxides (REO) were proposed as a means of creating hydrophobic surfaces. Due to their unique electron configuration, REO surfaces exhibit a high water contact angle even when they are smooth. However, the link between this research and industrial applications has not been firmly established due to the difficulties of large scale fabrication, and complex processing procedures. In the publication by Azimi et al. (Azimi, et al. 2013), the REO powders were sintered to form a dense monolithic body whose surfaces were hydrophobic. In most applications of hydrophobic coatings, only the surface needs to be hydrophobic. However, the sintering technique requires the entire part to be produced from REOs, which increases the cost. The hydrophobic surfaces produced by this technique have water contact angles between 100° to 115°. Superhydrophobic surfaces were developed by depositing thin films of REOs on a surface which had previously been nano-textured (Azimi, et al. 2013). However, the nano-texturing process is hard to employ in large scale and more importantly, the bonding between the coating and the substrate was poor resulting in a non-durable coating.

Plasma spray deposition is a technique which has been widely used in industry to produce coatings due to its high deposition rate, near-net shape finishing, and most importantly, its ability to process almost all materials. Conventional plasma spray uses micro scale powder particles as the feedstock for the material to be deposited. Solution precursor plasma spray is a relatively new technique that, rather than using powder particles, uses a solution which decomposes during deposition to form the coating. The coating formation mechanisms for SPPS are different than for conventional plasma spray, and can result in nano- and submicron-structured coatings. This type of hierarchically structured surface is very desirable in fabricating hydrophobic surfaces since it captures the essence of self-cleaning leaves and wings in nature. The following describes a technique which may produce hierarchically structured superhydrophobic rare earth oxide coatings using the SPPS technique. SUMMARY

According to one aspect, there is provided a method of forming a superhydrophobic coating on a surface of a substrate, said method comprising:

i. Preparing a substrate surface, preferably a metal substrate, more preferably a stainless steel substrate, for coating, preferably pre-heating said substrate, optionally pretreating a surface of said substrate to allow for better adhesion;

ii. Applying a coating composition, preferably a suspension, more preferably a solution to said substrate, preferably said coating composition comprising at least one rare earth metal or salt thereof, preferably said at least one rare earth metal being selected from the group consisting of cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and combinations thereof, at least one solvent, preferably said at least one solvent being selected from water, alcohol and combinations thereof; preferably said coating solution is applied to said substrate via a plasma torch; more preferably via solution precursor plasma spraying, until said coating is adhered to said surface of said substrate, preferably said coating having a thickness of at least about 5 microns, more preferably from about 5 microns to about 50 microns, most preferably less than about 35 microns; preferably said solution is atomized by at least one atomizing gas, preferably argon nitrogen, and combinations thereof, and injected into said plasma, said plasma formed by at least one plasma gas selected from argon, nitrogen, hydrogen, helium and combinations thereof; preferably in one embodiment said plasma gas is 100% argon, 100% nitrogen, or any argon:nitrogen ratio thereof. Hydrogen and/or helium may also be added to the argon, nitrogen, or any mixture thereof from 0% to about 30%, preferably <30%. In one embodiment said plasma gas comprises hydrogen (10%- 14%), nitrogen (80%-72%) and argon (10%-14%); and

iii. Optionally vacuum treating said coated substrate, with the proviso of no surface treatment post coating of said substrate.

According to another aspect, there is provided a superphydrophobic coating on a substrate whenever produced by the method described herein.

According to yet another aspect, there is provided a rare earth metal solution as described herein for use in coating a surface of a substrate resulting in a superhydrophobic coating on said substrate surface.

Preferably, the superhydrophobic coating results in a water contact angle of from about 150° to about 165°.

Preferably, the superhydrophobic coating results in a roll off angle from about 1° to about 10°, preferably from about 1° to about 5°.

Preferably the superhydrophobic coating is from about at least about 5 microns, more preferably from about 5 microns to about 50 microns, most preferably less than about 35 microns. Preferably the atomizing gas has an slpm in the range of 0 to about 30. More preferably, if the spray angle is small, the use of an atomizing gas is optional.

Preferably, when the coated substrate is vacuum treated, the coated substrate is vacuum treated at from about 10 Pa to about 10^"7 Pa, and from about 12 to about 48 hours, preferably at 1 Pa. More preferably, vacuum treated at 1 Pa from about 12 to about 48 hours.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1(a) is a scanning electron microscope image of the top surface of the coating of Example 1 showing micro-scale clusters (50 μιη scale bar).

Figure 1(b) is a scanning electron microscope image of the top surface of the coating of Example 1 showing fine nano-scale particles on the clusters (2 um scale bar).

Figure 2 is a scanning electron microscope image of the cross-section of the coating of Example 1 (50 μηι scale). Figure 3 is an x-ray diffraction pattern of the plasma sprayed coating of Example 1 compared with the Yb₂0₃ reference (PDF card No. : 01-075-6635).

Figure 4 depicts the water droplets on the coated surface of Example 1 with the droplet size being 30 μΐ_^.

Figure 5 depicts the dynamic impact of water droplets on the coated surface of Example 1 (Scale bar: 2 mm).

Figure 6 depicts the cross section of the heat treated sample of Example 1, and its water contact angle (droplet size is 73 μί).

Figure 7 depicts the dropwise condensation on the stainless steel substrate coated with Yb2Cb by the SPPS technique of Example 1. Figure 8 is a scanning electron microscope image of the cross section of the coating of Example 2 (droplet size is 40 μί).

Figure 9(a) is a scanning electron microscope image of the top surface of Example 2 showing micro-scale clusters (50 μηι scale bar).

Figure 9(b) is a scanning electron microscope image of the fine nanoscale particles on the clusters of Figure 9(a) (2 μηι scale bar).

DETAILED DESCRIPTION

The following applies to both Examples 1 and 2 described herein. Solution Preparation:

Researchers have already studied the hydrophobicity of the rare earth metal oxides in the lanthanide group; all are hydrophobic in nature, except promethium oxide (Azimi, et al. 2013). The corresponding salt forms of rare earth metals may be dissolved into water, alcohol, or water and alcohol mixtures to serve as the feedstock of the SPPS deposition solution. The nitrate and chloride salts of the rare earth metals are recommended due to their high solubility in water at room temperature. When an alcohol, such as ethanol, is introduced into the solution, research has shown that the heat transfer rate between the plasma and solution is increased (Pateyron, B., Calve, N., & PawAowski, L. (2013). Influence of water and ethanol on transport properties of the jets used in suspension plasma spraying. Surface & Coatings Technology, 220, 257-260).

Introducing alcohol into the solution also decreases the surface tension of the solution, improving the secondary breakup of the atomized solution in the plasma plume, resulting in smaller size droplets. Smaller droplets require less total heat to evaporate the solvent and fully melt the solute which enhances the density of the coating (Fauchais, P., & Vardelle, A. (2012). Solution and suspension plasma spraying of nanostructure coatings, advanced plasma spray applications. In H. S. Jazi (Ed.), Advanced plasma spray applications (pp. 149-188) InTech). The heat of vaporization of alcohol is less than that of water; water/alcohol mixtures therefore require less total heat to evaporate the solvent which may increase the density of the coating.

Substrate Preparation: The SPPS technique may be used to deposit coatings directly onto most metal substrates without any pre-treatment of the surface of the substrate. However, research has shown that an increase in the substrate temperature prior to plasma spray deposition may improve the adhesion strength (Pershin, V., Lufitha, M., Chandra, S., Mostaghimi, J. (2002). Effect of Substrate Temperature on Adhesion Strength of Plasma-Sprayed Nickel Coatings. Journal of Thermal Spray

Technology 12(3), 370-376). The pre-heatmg is believed to remove surface contamination, moisture, and enhance the contact of the molten particles of the coating material with the substrate by delaying their solidification. Roughening the surface of the substrates is known to improve the adhesion between the coating material and the substrate for conventional plasma sprayed coatings. The rough surface provides craters for the molten droplets to flow into before solidification, and subsequently improves the mechanical interlocking between the coating material and the substrate (ASM handbook; v.5 A; thermal spray technology. (2013). Reference and Research Book News, 28(5)).

Solution Precursor Plasma Spraying Process:

The prepared solution is fed to the plasma torch from a pressurized chamber or by a pump, where it is atomized and injected into the plasma by an atomizing gas. Argon commonly serves as the atomizing gas but nitrogen and combinations thereof are also suitable. The solvent is evaporated in the plasma, resulting in precipitation of intermediate products which then decompose to yield the material that deposits on the substrate to produce the coating. The gases used to generate the plasma are typically a mixture of argon and nitrogen, pure argon or pure nitrogen. Hydrogen or helium may be added (up to approximately 30%) to the plasma gas to increase plasma plume temperature and heat transfer between the plasma and the atomized solution. A robot may be used to carry either the plasma torch or the substrate, or both to assist in the coating process.

Post-treatment: Hydrophobic performance of the coating may be improved if the coated surfaces may be vacuum treated at approximately 1 Pa for about 12 to 48 hours. Hydrophobicity depends on the surface chemistry, and research has shown that the excess surface oxygen decreases the water contact angle (Khan, S., Azimi, G., Yildiz, B., & Varanasi, K. K. (2015). Role of surface oxygen-to- metal ratio on the wettability of rare-earth oxides. Applied Physics Letters, 106(6), 061601). Vacuum treatment may relax the coating surface and remove some adsorbed oxygen on the surface.

Coating Characterization:

Cross-sectional and Surface Microstructure:

The as-sprayed coatings were sectioned by a precision diamond saw, IsoMet 5000 (Buehler, ON, Canada). The sectioned samples were then mounted in epoxy under a vacuum of 30 mbar. A low viscosity epoxy Jetset Epoxy (MetLab Corp, ON, Canada) was selected to allow the epoxy to penetrate into the pores of the cross section of the coating. The mounted coating's cross section was subsequently polished using P320 silica grinding paper, followed in sequence by 45 μπι, 15 μπι, 6 μιη, and 3 μιη diamond disks. Then 1 μιη and 0.05 μιη diamond suspensions were used as the final stages of polishing. Between each polishing step, the surface was cleaned in an ultrasonic bath and dried by compressed air. Scanning electron microscopy (Hitachi SU 3500) was used to characterize the surface and cross-sectional microstructures. To avoid charging effects in the SEM, specimens of the top surfaces of the coatings were sputtered coated with gold and the polished cross sections were coated with carbon before observation. Crystal Phase Identification:

X-ray diffraction (XRD) was performed to study the phase of the coating material using a Miniflex600 (Rigaku, MI, USA). The coating was cut into 15 mm by 15 mm square specimens in order to fit the sample holder of the XRD machine. The measurements were performed over a range of 2Θ angle from 15° to 105°, and the patterns obtained were compared with standard reference patterns.

Wetting Behavior Measurements: The static water contact angle image was captured by a CCD camera (Sony XCD-SX900) with a horizontal microscope (Wild Heerbrugg 400076) at 5.8x magnification. The droplet was illuminated by a white-light projector from behind through a frosted glass. Image processing software (ADSA) was used to analyze the image of the static water droplet in order to determine the static water contact angle. The dynamic impacts of water droplets on the coating were captured by a high speed camera FASTCAM SA5 (Photron, CA, USA) at 4000 frames per second.

A condensation trial was performed to demonstrate a potential application of the coating and to assess the dropwise condensation. During the trial, coated substrates were attached to a water cooled heat exchanger while the coated surface was exposed to steam over a period of several minutes to 2 hours. The surface temperature of the coated substrate was kept at around 4°C during the condensation trial. The condensation trial followed the protocol as outlined in Azimi, et al. 2013.

Thermal Stability of the Coating:

As-sprayed coatings and vacuum treated coatings were heat treated in an oven at 1000 °C for 2 hours to examine their thermal stability. The oven was programmed to increase the temperature from room temperature to 1000°C at 10°C/min After two hours at 1000°C, the oven cooled by natural convection.

Example 1

Substrates of Type 304 stainless steel, 1 inch in diameter and 1/8 inch in thickness, were coated with Yb203 by the SPPS technique. The substrates were fabricated from cold-rolled steel by the supplier, and used with the surface finish as-received from the supplier. Prior to the plasma spray deposition, the substrates were pre-heated to 350°C.

The coating solution was prepared at room temperature by dissolving 99.999% Ytterbium nitrate pentahydrate (Pangea International, Shanghai, China) in distilled water. The concentration of the solution was 167.3 g of ytterbium nitrate per 100 g of water. An Axial III Series 600 plasma torch (Northwest Mettech Corp., North Vancouver, BC, Canada) was used to deposit the coating by the SPPS process onto the substrates. The nozzle size was 3/8 inch in diameter. Argon was used as the atomizing gas. The plasma gas consisted of a mixture of hydrogen (10%), nitrogen (80%) and argon (10%). A robot arm, which carried the torch, moved in a raster pattern, and in total 15 passes were performed on each substrate. The robot arm was set to a linear translation speed of 200 in/min and the vertical step size was 0.2 in. The spraying parameters are summarized in Table 1. During the plasma spraying deposition, the highest substrate temperature during the torch pass was measured to be 435 °C by a thermocouple in contact with the back surface of the substrate. These conditions resulted in a coating thickness of from 25 microns to 35 microns. We have found that a coating having a thickness less than 35 microns results in a water contact angle greater than 160°.

Table 1 : The spraying parameters used in Example 1

Examining the microstructure of the surface of the coating, a hierarchical microstructure was observed as expected. Figure 1(a) and 1(b) show an SEM image of the surface of the coating. As per Figure 1(a) micro-scale irregular clusters ranging from 5 microns to 10 microns in size were uniformly distributed on the surface. As per Figure 1(b) nano-scale particles were observed on the clusters.

In SPPS, the coating is, in general, formed by the stacking of irregular particles and molten splats, therefore, micro-scale agglomerates are commonly observed. The nano-scale particles may have resulted from the condensation of vaporized material. Another possibility is the nano- particles passed through the boundary layer to reach the substrate due to the thermophoresis force. This type of microstructure is very similar to the hierarchical structured hydrophobic surfaces observed in nature.

Figure 2 shows an SEM image of the cross-section of a coated substrate. Figure 2 also includes a magnified portion of the coating (10 μηι scale bar ). The coating is porous and a feathery structure is observed. The particles observed in the coating have irregular shapes which is an indication of incomplete melting. The coating appears to have been formed mainly by the sintering of incompletely melted particles. As the coating was built-up by the deposition of these particles, the porosity was enhanced by a shadowing effect leading to the formation of the feathery structure.

Figure 3 shows the XRD pattern of the plasma sprayed coating The sharp peaks in the XRD patterns indicate that the coating material is crystalline. As shown in Figure 3, the peaks in the pattern match with the Yb203 reference (PDF card No.: 01-075-6635) at all peak locations. No additional peaks corresponding to the substrate are observed, which indicates the coating material fully covered the substrate.

The coated surfaces were treated in a low vacuum chamber at 1 Pa for 48 hours. Then the treated coatings' wetting behavior was examined. A water contact angle of 161 °, which indicates the coating is superhydrophobic, was observed. On a smooth REO surface, the measured water contact angle was between 100° to 115° (Azimi, et al. 2013). Thus, the hierarchical microstructure of the coating formed by the SPPS method described herein enhanced the water contact angle by as much as 60%. The roll-off angle, another important characteristic of the wetting behavior, is so small (< 5°) that the water droplet rolls off at the slightest tilt. Figure 4 shows several water droplets of various sizes on the coating and, in the inset, a 30 μΐ. droplet for which the measured water contact angle was 161°.

The dynamic impact of a single droplet and the coalescence between two droplets were observed using a high-speed camera. Figure 5 shows the dynamic impacts of water droplets on the as- sprayed coating. For a single droplet impacting on the coating at a speed of 1.4 m/s, the droplet completely rebounded (Top panels of Figure 5).

When a second droplet landed beside a static droplet at the same velocity, it deformed and interacted with the first droplet. Then the two droplets combined into one and the combined droplet also recoiled. The combination of the hierarchical surface structure and the intrinsic hydrophobicity of the material give the coating excellent water repellent properties (Bottom panels of Figure 5). The thermal stability of the coating was also examined. After the heat treatment of the as- sprayed and vacuum treated coatings, the coatings remained firmly adhered on the substrate as shown in Figure 6. The measured water contact angle after vacuum treatment of the heat-treated coating increased to 165°. The surface and cross-sectional microstructures of the heat-treated coating showed no difference compared to the as-sprayed coating owing to the high melting point of the rare earth oxide. A thin oxidation layer was observed between substrate and coating, which was a result of the oxidation of the stainless steel substrate during the high temperature heat treatment. Vacuum treated as-deposited coatings remained superhydrophobic without further vacuum treatment after heating to temperatures as high as 250°C for 1 hour. During the condensation trial, dropwise condensation was observed on the coated surface over 2 hours of testing. Compared to filmwise condensation, dropwise condensation can significantly increase the condensation heat transfer. At the same temperature difference between the vapour and the surface, dropwise condensation is several more times effective than filmwise. In dropwise condensation, condensation occurs in droplets which grow and continuously fall from the surface maintaining direct contact of the water cooled surface with the vapour.

Filmwise condensation allows condensation to form as continuous film on the surface. The film runs down the surface gaining thickness as it falls. This film acts as a resistance to heat transfer, as heat must be conducted through this film. Figure 7 shows the dropwise condensation on the coated surface.

Example 2

The previous example demonstrated a porous coating produced using the SPPS technique. In this example, a dense coating is presented by changing the spraying conditions. Compared to the porous coatings, dense coatings in general offer better adhesion and abrasion resistance. This is because feedstock needs to be fully melted when it arrives at the substrate in order to have dense coatings. When the fully melted feedstock, in the form of molten droplets, arrives at the substrate, they will form pancake shaped splats, and subsequently solidify on the substrate. The solidified pancake splats have better contact with the substrate, thus better adhesion strength than the sticking of incompletely melted particles.

In this example, stainless steel substrates of the same characteristics as Example 1 were used. The substrates were roughened by PI 20 silica sandpapers to improve adhesion of the coating. The coating solution was also modified. The solvent contained 50 wt% of pure ethanol and 50 wt% of distilled water. The concentration of the ytterbium nitrate was changed to 143.4 g of ytterbium nitrate per 100 g of solution because of the low solubility of ytterbium nitrate in ethanol. The same plasma torch and atomizing gas were used in this example. The total plasma flow rate was increased from 250 slpm to 275 slpm. To further enhance the plasma flow rate, a 5/16 inch in diameter nozzle was used in this example. Since the torch limits the maximum nitrogen gas flow rate at 200 slpm, the plasma gas composition was changed to hydrogen (14%), nitrogen (72%) and argon (14%). The substrates were pre-heated to the same temperature as in Example 1 prior to deposition. The same robot program was used for the deposition, but only 10 torch passes were performed. The spraying parameters for Example 2 are summarized in Table 2. Compared to Example 1, the current of the torch was increased from 200 A to 250 A. The high current together with the high plasma flow rate used in this example increased the total torch power from 115 kW to 170 kW. Because of the high power of the plasma, a longer standoff distance of 90 mm was used to avoid potential damage of the substrates. Furthermore a lower feedstock flow rate was supplied to the hot plasma. These adjustments in spraying conditions enhanced the heat transfer between the plasma to the feedstock. The highest substrate temperature measured during the torch pass was 815°C. This increase in substrate temperature agreed with the expectation from the changes in the spraying conditions.

Table 2: The spraying parameters used in Example 2

Figure 8 presents the cross section of the coating under conditions in Example 2. The coating is relatively dense and of a thickness ranging from 10 microns to 25 microns. In this Example, the dense regions of the coating were mainly formed by molten droplets. In between the dense regions, incompletely melted particles were observed. From the examination of a single torch pass, these particles resulted from feedstock that travelled at the perimeter of the plasma plume. At the perimeter, the plasma plume temperature may be one order of magnitude lower than at the center, which results in lower heat transfer between plasma and feedstock. From the inset, higher magnification SEM image in Figure 8, the molten droplets can be seen to have flowed into a crater in the substrate surface formed by roughening of the substrate surface prior to deposition, which improves the mechanical interlocking between the coating and the substrate. Micro clusters were also observed in Figure 8. Based on the cross section image and single torch pass observations, the micro-scale clusters may be explained by the shadowing effect due to the stacking of the incompletely melted particles and molten droplets and the initial surface roughness of the substrate.

In this Example, after vacuum treatment, the water contact angle measured on the dense coating was 165°. Under the same vacuum conditions as used in Example 1, only 12 hours of vacuum treatment was necessary to relax the coating surface. Compared to the porous surface of Example 1, the dense surface of Example 2 had less surface area, which may be the reason for the decreasing vacuum treatment time required for the dense coating. The SEM images of Figures 9(a) and 9(b) show the topography of the coating. Micro-scale irregular clusters ranging from 10 microns to 30 microns in size were uniformly formed on the surface. On the clusters, nano-scale particles were also observed. This topography of the coating agrees with the cross sectional image of the coating. For the dense coating, the hierarchical structured topography was retained. Again, this multi-scale roughness, hierarchical structured top surface of the coating is very similar to the lotus leaf.

Several characteristics of the technique disclosed herein include:

1. No previous coating techniques have been reported that produce a robust and superhydrophobic ceramic coating rapidly and economically. A moderate plasma torch has a deposition rate of 60 mm/min in air whereas typical physical vapor deposition requires a vacuum environment with a deposition rate ranging from 0.1 μηι/ιηίη to 100 μηι/min. The disclosed coating technique may fabricate robust ceramic superhydrophobic coatings comprised of rare earth oxides rapidly at large scales. Based on our laboratory setup, it takes less than 20 minutes (in air) to coat a 1ft² area of a substrate with a coating having an average thickness at least 15μηι.

2. The superhydrophobic coatings fabricated from this coating technique demonstrated a hierarchical structured surface which closely resembles the structure of superhydrophobic surfaces in nature. A desired surface topography is automatically formed from this coating technique without post-deposition roughening or nano-patterning of the surface. Compared to smooth REO surfaces, this hierarchical surface of the coating increases the water contact angle as much as 65%.

Due to the water resistant properties, the superhydrophobic surfaces produce by the coating and coating technique of the present disclosure have a wide range of potential applications to benefit the environment, especially in energy conservation, including but not limited to:

1. Pipes: Fukuda, et al. studied the drag reduction on a flat superhydrophobic surface (Fukuda, et al. 2000). The friction resistance was reduced 80% at 4 m/s and 55% at 8 m/s on the superhydrophobic surface compared to a smooth surface. If large water pipes can be coated with a superhydrophobic coating, then the pump requirements can be significantly reduced for them.

2. Boilers: Researchers also looked into the effect of hydrophobic and hydrophilic surfaces on boiling heat transfer (Betz, et al. 2013). At low heat fluxes, a hydrophobic surface can promote more nucleation sites, thus enhance boiling heat transfer. Betz et al. also produced a biphilic surface, a surface having a mixture of hydrophobic and hydrophilic areas. They found that the highest boiling heat transfer coefficient is achieved by the biphilic surface since the hydrophobic surface decreases the nucleation temperature and the hydrophilic surface provides good water transportation from the cold region to the hot region.

Energy: superhydrophobic coatings may be applied to turbine blades to reduce the loss in efficiency due to the formation of a liquid film on the turbine blade surface resulting from water droplets entrained in the steam impacting on the blades. When a hydrophobic surface is used in a condenser, heat transfer was enhanced by promoting dropwise condensation (Azimi, et al. 2013).

Aircraft wings: because of the superior water repellent property of the superhydrophobic surfaces, superhydrophobic coatings on aircraft wings may potentially prevent ice formation and improve fuel efficiency and flight safety.

As many changes can be made to the preferred embodiment without departing from the scope thereof; it is intended that all matter contained herein be considered illustrative and not in a limiting sense.

Claims

A method of forming a superhydrophobic coating on a surface of a substrate, said method comprising:

Preparing a substrate, preferably a metal substrate, more preferably a stainless steel substrate, for coating, preferably pre-heating said substrate, optionally pretreating a surface of said substrate to allow for better contact;

Applying a coating solution to said substrate, preferably said coating solution comprising at least one rare earth metal or salt thereof, preferably said at least one rare earth metal being selected from the group consisting of cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium and combinations thereof, at least one solvent, preferably said at least one solvent being selected from water, alcohol and combinations thereof; preferably said coating solution is applied to said substrate via a plasma torch; more preferably via solution precursor plasma spraying, until said coating is adhered to said surface of said substrate; preferably said solution is atomized and injected into said plasma by at least one atomizing gas selected from argon, nitrogen, and combinations thereof; preferably injected into said plasma formed by at least one gas selected from argon, nitrogen, and combinations thereof; preferably at least one of hydrogen, helium and combinations thereof may be further added; preferably from about 0% to about 30% of hydrogen, helium and combinations thereof may be added to the at least one gas selected from argon, nitrogen and combinations thereof; in one embodiment said plasma gas is hydrogen (10%- 14%), nitrogen (80%-72%) and argon (10%-14%).

2. The method of claim 1 further comprising vacuum treating said coated substrate, with the proviso of no surface treatment post coating of said substrate.

3. The method of claim 2 wherein said vacuum treating is at about 10 Pa to about 10^"7 Pa , preferably at 1 Pa.

4. The method of claim 2 or 3 wherein said vacuum treating is from about 12 to about 48 hours.

5. The method of claim 1 wherein said solution is atomized at a slpm of from about 0 to about 30 slpm.

6. A superphydrophobic coated substrate whenever produced by the method described herein.

7. A rare earth metal solution as described herein for use in coating a surface of a substrate resulting in a superhydrophobic coating on said substrate surface.

8. The substrate of claim 6 with a water contact angle of from about 150° to about 165°.

9. The substrate of claim 6 or 8 with a roll off angle of from about 2° to about 10°.

10. The substrate of claim 6, 8 or 9 wherein the coated substrate has a coating from about at least about 5 microns.

11. The substrate of claim 10 wherein the coating is from about 5 microns to about 50 microns in thickness.

12. The substrate of claim 6, 8 or 9 wherein the coated substrate has a coating less than about 35 microns in thickness.