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Preparation of metallic coatings with reversibly switchable wettability based on plasma spraying technology.

Abstract Plasma spray technology was used for the preparation of metallic coatings with reversibly switchable wettability. By spraying Fe, Cr, and Ni mixture powders with different sizes, a micro/submicro-dual scale morphology was obtained. The resultant metallic coating had a superhydrophilic nature, but could he switched into superhydrophohic by spraying an acetone solution of dodecanoic acid while the sprayed surface remained within a temperature range of 100-200[degrees]C, although dodecanoic acid itself has a hydrophilic nature. The obtained superhydrophohic coating remained stable if the temperature was below 200[degrees]C. The surface wettability could further he switched between superhydrophilicity and superhydrophohicity within a time-scale of only seconds by heating above 220[degrees]C and re-spraying the acetone solution of dodecanoic acid in the temperature range of 100-200[degrees]C. A chemisorbed molecular layer of dodecanoic acid was responsible for the decrease of the surface energy, and the Fourier transform infrared spectroscopy (FTIR) results suggested that heating the substrate can greatly facilitate the formation of the chemisorbed layer and preferential orientation of the dodecanoic acid molecule.

Keywords Plasma spray, Dual scale morphology, Superhydrophohicity, Reversibly switchable wettability

Introduction

Wettability is an important character of a solid surface. Surfaces with very high water contact angles (CAs), particularly greater than 150[degrees], are usually called superhydrophobic surfaces. (1), (2) The creation of a superhydrophobic surface has aroused great interest for both fundamental research and practical applications due to its potential applications in self-cleaning, (3) antibiofouling, (4) prevention of water corrosions, (5)anti-icing surfaces, (6), (7) etc. Previous studies have revealed that superhydrophobic surfaces require a unique combination of surface roughness and low surface energy. (8), (9) Artificial superhydrophobic surfaces are generally prepared by a two-step method. First, a surface with proper roughness is created by various technologies, such as chemical-etching, (10) laser-etching, (11) lithography, (l2) and Sol--Gel processing. (13) Subsequently, substances with a low surface energy, such as organic silane or organic fluoride, are deposited to provide water-repelling properties. However, many of these methods depend on complicated processes or special equipment, (14), (15) while the surface-energy modification processes usually involve immersing the samples in liquids which are time-consuming and costly, or even infeasible for large industrial objects such as the hull of an ocean ship. A convenient, cost-effective, and robust method for large-scale superhydrophobic surfaces is still imperative.

Plasma spraying (PS) is a process that combines the generation of a plasma jet, the injection and melting of surfacing materials within the plasma jet, and finally the formation of a coating through the impingement of the surfacing materials on the substrate. (16), (17) PS has been widely used in the industry due to its excellent performance (18-20) and suitability for various substrates and large area production. Recently, our group found that the surface of a plasma sprayed Fe-Cr-Ni mixture coating has a micro/submicro-dual scale structure under certain spraying parameters, which is very similar to the morphology of the lotus leaf. In this article, we report a novel and facile method to create superhydrophobic surfaces based on the PS technology. Furthermore, the wettability of the coating can be switched between superhydrophobicity and superhydrophilicity. The coating is metallic, thus may show better mechanical properties than normal polymer superhydrophobic coatings. This facile method, we hope, will help to realize large-scale production of superhydrophobic surfaces and smart devices with new industrial applications.

Experimental

Materials

Analytically pure dodecanoic acid (AR) and acetone (AR) were purchased from Beijing Chemical Reagents Company. The iron, nickel, and chromium powders were purchased from Beijing General Research Institute of Mining & Metallurgy with average sizes of 40, 40, and 74 gm, and purities of 98.5, 99.5, and 99.0%, respectively. The sprayed powder was a mixture of Fe, Ni, and Cr with the ratio 3:1:1 mixed by ball milling for 2 h under 600 rpm.

Preparation of the superhydrophobic surface

In this work, we took copper sheets with a dimension of 50 x 50 x 5 [mm.sub.3] as the substrate. Prior to spraying, the copper substrate plates were thoroughly ultrasonically cleaned with acetone and distilled water, dried by compressed nitrogen gas, and then sand blasted with alumina powder (60-grit). Subsequently, the feedstock was deposited onto the substrate using PS equipment (APS-2000K, China) under the spraying conditions listed in Table 1. To obtain a superhydrophobic surface, an acetone solution of dodecanoic acid (1 g/100 mL) was sprayed onto the hot PS coating surface through a spraying gun with a flow rate of about 0.07 mL/(s [cm.sub.2]) for 3 s. The temperature of the sample was measured by an infrared thermodetector.
Table 1: Spray parameters

Classification           Condition

Plasma current           440 A
Plasma voltage           44 V
Prima gas flow rate      Ar: 40 L/min
Secondary gas flow rate  [H.sub.2]: 10 L/min
Carrier gas flow rate    6 L/min
Powder feeding rate      20 g/min
Spray distance           100-120 mm


Evaluation of thermal stability and reversible wettability

To evaluate the thermal stability of the superhydrophobic surfaces, the samples were heat-treated in the air in a muffle furnace at various temperatures for 10 min. CAs of water on these heat-treated samples were then measured at room temperature.

In the wettability switching tests, a superhydrophobic sample was switched to be superhydrophilic by heat-treating at 220[degrees]C for 10 min. To recover the superhydrophobicity, the sample was heated to 200[degrees]C and then the acetone solution of dodecanoic acid was sprayed onto the hot surface with a flow rate of about 0.07 mL/(s [cm.sub.2]). After 3 s of spraying, the sample was allowed to cool down naturally. CAs of water on the treated sample were then measured at room temperature.

Characterization

The microstructure and elemental distribution of the coatings were observed by using an FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDX). The CAs and sliding angles (SAs) were measured by the sessile-drop method with distilled water (5 [micro]L) on a DataPhysics OCA20 CA system at room temperature. A CA value was obtained by the average of at least five values at different positions of the same sample. IR spectra of films was recorded with a Magan-IR 560 Fourier transform infrared spectroscopy (FTIR) using KBr pellets (films are physically scraped from substrates and pressed into salt pellets). (21)

Results and discussion

Surface morphology of the prepared coating

Figure 1 shows the SEM images of the plasma sprayed Fe-Ni-Cr coating. We can see in Fig. la that islands of 20-50 [micro]m in diameter, with an average distance of about 20-50 [micro]m, are distributed across the surface. Figure lb is the magnification of a local area in Fig. la, showing numerous smaller islands with diameter about several micrometer spread over the large islands. This dual scale structure is very similar to the morphology of the lotus leaf with a micro/nano-binary structure, (22) which contributes positively to the superhydrophobicity of the surface.

[FIGURE 1 OMITTED]

Traditionally, a deposit (called a splat) in the shape of a disk is expected to provide the best bonding between a coating and its substrate. However, the surface morphology of such a coating is not appropriate for creating superhydrophobicity. Based on current understanding, a dual-scale morphology is necessary. On the other hand, although the dual-scale morphology shown in Fig. 1 can be easily produced by plasma spray, the detailed formation mechanism and related properties have never been a subject of research. Based on the established understanding on the coating morphology produced by plasma spray, the formation of this particular dual-scale morphology may be attributed to the following factors. Larger particles in the powder mixture are only partly melted under the selected spraying parameters. The melted surface of these particles can stick well to the substrate, and the hard core of the particles forms the main part of the big islands. (23) On the other hand, fully-melted particles will splash over the whole surface and break into even smaller droplets, which form the smaller islands on the surface. (20), (24) Also, some tiny particles in the mixture powder may not be able to penetrate into the flame center, (23) therefore are not fully melted due to low flame temperature, which would be another reason for the formation of smaller islands.

Figure 2 shows the back-scattered electron (BSE) image of the cross section of the coating. As we can see from Fig. 2a, many microdefects, such as pores, microcracks, and un-melted particles are generated in the coating. The presence of these microdefects can reduce the mechanical properties of the coatings, such as the elastic modulus and microhardness, etc. Moreover, the presence of the oxides within the coating, as shown in Fig. 2b, will further deteriorate the mechanical properties.

[FIGURE 2 OMITTED]

Obviously, a dual-scale coating does not possess peak bonding strength and wear resistance comparable to coatings with a typical lamellar structure. However, the superhydrophobic surfaces prepared by the existing methods are usually easy to be damaged if mechanically challenged, (8), (25) because the conventional superhydrophobic surfaces are usually polymers and the morphology is usually in nanoscale. The coating surface we report here is metallic and the morphology is in micron scale, thus may well have a better mechanical stability compared to the conventional superhydrophobic surfaces.

Chemical composition immediately after APS

The elemental distribution of the coating surface immediately after PS was investigated by EDX, as shown in Fig. 3. In the survey shown in Fig. 3a, we can see the sprayed materials have been oxidized significantly after PS. According to previous studies, (18), (20), (26), (27) oxidation mainly occurs during in-flight and the coating formation process. Oxidation extent also varies with the processing parameters involved. Moreover, the element composition of different locations on the coating surface shows that the oxygen content is closely related to the size of the protrusions. As shown in Fig. 3h, the oxygen contents in the areas 1, 2, and 3 are 26.25, 44.16, and 58.61 at.%, respectively, while the average oxygen content of the surface is 31.28 at.%. There are several factors that may contribute to the fact that the oxygen content increases with decreasing of the size of protrusions. (1) The PS of metals in atmospheric environment inevitably leads to oxide formation. The temperature in the core region of the plasma jet is usually above 8000 K, (28) which is high enough to melt the oxide. The oxide phase separates from the metal phase due to the difference in surface tension and forms an oxide nodule within the particle. (23) (2) The in-flight oxidation during PS increases with decreasing of the metal particle size.2n This is because the smaller particles have higher specific areas. (3) The smaller islands have a higher specific surface area, which may accelerate the on-substrate oxidation.

[FIGURE 3 OMITTED]

Wettability before and after surface modification

The wettability of the surface is influenced by the chemical composition and the surface topography. Immediately after PS, the surface of the prepared coating is mainly composed of metal oxide with high surface energy. Amplified by the surface roughness, the CA of the coating immediately after APS is about 0[degrees], showing superhydrophific property.

In order to achieve superhydrophobic property, the surface energy must he modified. Generally, perfluoro compounds and silicones are used as the modification material due to their extremely low surface energy. However, these materials are very expensive and the immersing process is very time-consuming, which usually needs several hours to several days. This is not feasible for applications of large-scale industrial production. Recently, many researches have found that an extremely low surface energy is not vital in fabricating superhydrophobic surfaces. (29), (30) For example, the wax on lotus leaves has an intrinsic CA value of ~74[degrees], (30) in contrast to the expected CA of greater than 90[degrees]. Corresponding to this finding, Wang et al. (31) used fatty acid longer than Cl, to modify copper film, and obtained a superhydrophobic surface. Yuan et al. (32) have successfully made a rough (Co.sub.3][0.sub.4] surface superhydrophobic by utilizing stearic acid as the modification material.

In this article, we utilized dodecanoic acid as the low surface energy material to modify the prepared coating. The difference is, we sprayed the acid on a still hot surface right after the PS process. We found that the residual heat in the sample can greatly facilitate the formation of a low surface energy layer. An acetone solution of dodecanoic acid (1 g/100 mL) was successively sprayed on the metallic surface while the surface was still hot (with a temperature range of 100-200[degrees]C) from the earlier PS with a flow rate of about 0.07 mL/(s [cm.sub.2]) for 3 s. The behavior of water droplets (5 pL) on the as-prepared surface are shown in Fig. 4. The CA is about 156[degrees] (Fig. 4a) and the SA is below 5[degrees], showing superhydrophobic characteristics. The water droplets on such a surface have nearly spherical shapes (Fig. 4b), which can drip off rapidly when the surface is slightly inclined. In addition to the results in this article, there has been other research1 (10), (33), (34) utilizing dodecanoic acid to produce superhydrophobic surfaces. These results prove that surface modification by extremely low surface energy materials is not vital in fabricating superhydrophobic surfaces, even on surfaces with a micron scale morphology.

[FIGURE 4 OMITTED]

Thermal stability and reversible wettability

Although a large number of technologies have been developed to generate superhydrophobic surfaces, the practical applications of superhydrophohic surfaces are still limited. One of the main barriers preventing their success is their lack of environmental stability. (8), (35) Up to now, researchers have conducted much work on the stability of superhydrophohic surfaces against various conditions, including chemical stability, (36) mechanical stability, (3) and underwater stability. (37), (39) Thermal stability, as one important requirement for many industrial applications, has not received much attention.

Figure 5 shows the wetting behavior of water droplets on as-prepared superhydrophobic surfaces after heat treatment at various temperatures. Although the surfaces were still superhydrophobic after being treated at temperatures below 200[degrees]C, the wetting behavior showed different trends. When the treatment temperature was below 140[degrees]C, the CA essentially was constant and the SA decreased slightly as the treatment temperature was increased. In the treatment temperature range of 140-220[degrees]C, the CA decreased slightly with temperature, then sharply from about 200[degrees]C. The SA increased substantially as the temperature increased.

[FIGURE 5 OMITTED]

Obviously, heat treatments below 200[degrees]C do not modify the morphology of the metallic coating. Therefore, the changes of surface wettability with heating temperature may be explained by the change in surface energy induced by the changes of packing density and packing orientation of the dodecanoic molecules.

Figure 6 shows the FTIR of the prepared coating after being treated under various conditions. It can be seen that several new peaks appear in the FTIR profile after surface modification by spraying the dodecanoic acid solution on the hot surface (as shown in Fig. 6b). The peaks around 2919 [cm.sub.-1] ([v.sub.a] [CH.sub.2]) and 2850 [cm.sub.-1] ([v.sub.8] [CH.sub.2]), indicate that a dodecanoic acid layer has formed on the coating surface. (40) The appearance of the asymmetric and symmetric stretch vibration peaks for [COO.sub.-] of carboxylate headgroup, typically around 1560 and 1403 [cm.sub.-1], respectively, (40), (41) I coupled with the absence of the peak around 1700 [cm.sub.-1] for carboxyl (COOH) stretch vibration, (42), (43) indicate that most dodecanoic acid molecules have been chemisorbed on the surface and form salts with the metal or metal oxide.

[FIGURE 6 OMITTED]

Results in Fig. 6 confirm that both physisorbed and chemisorbed dodecanoic acid molecules exist in the surface layer. This may well explain the variation of surface energy with temperature. Figure 5 shows that if the dodecanoic acid sprayed samples are heated to a temperature below 140[degrees]C, the CAs are almost constant while the SAs decrease slightly. This phenomenon may be explained by the desorption of some physisorbed dodecanoic acid molecules from the surface. Immediately after the spraying of dodecanoic acid solution on the hot substrate, the acetone solvent vaporized out quickly and most dodecanoic molecules (clusters) were chemisorbed onto the bare surface, while a few molecules may be physisorbed through hydrogen bonding interaction among head groups of acids and/or van der Waals interaction between alkyl chains. If the physisorbed molecules exist in such a way that the carboxylic groups orient at the periphery of the molecular layer(s) (as illustrated in Fig. 7a), the surface wettability will be enhanced due to the higher affinity of carboxylic groups to water. However, the physisorbed molecules are very weakly bound to the surface, which can be easily desorbed by solvent rinse or heating under a lower temperature (45) (such as below 110[degrees]C reported by Chandekar et al. (45)). Therefore, if the dodecanoic acid sprayed sample is heated to a low temperature (e.g., below 140[degrees]C) again, the physically adsorbed molecules will vaporize, leaving the chemisorbed molecules on the surface with methyl groups exposing outside (as Fig. 7b), which will decrease the surface wettability.

[FIGURE 7 OMITTED]

When the treatment temperature was in the range of 140-200[degrees]C, the CA decreased slightly while the SA increased substantially as the temperature increased. From the FTIR spectrum as shown in Fig. 6c, we can see the intensity of the peaks for carboxylate salt decrease slightly if heat treated in this temperature range. It seems that the chemisorbed molecules begin to desorb or decompose. A previous report has confirmed that the chemisorbed organic molecules (or self-assembled monolayers) will desorb or decompose when the temperature is above a certain value. (45) The decrease of the chemisorbed molecules will lead to the change of the packing density and the orientation of the molecular chain (illustrated in Fig. 7c). A loosely packed molecular layer will expose a substantial fraction of methylene groups and high-energy native metal surface in addition to methyl groups, and thus lead to the increase of surface energy and wettability.

If the treating temperature is too high, e.g., above 220[degrees]C, the chemisorbed molecules will be totally desorbed or decomposed in a very short time, as can be seen in Fig. 6d, where the peaks of carboxylate salt have completely disappeared. The removal of the low-energy dodecanoic acid layer and the exposing of high-energy metal or metal oxide surface elevate the surface energy rapidly, and thus lead the surface wettability change from superhydrophobicity to superhydrophilicity.

Thermal stability experiments give rise to the possibility of producing reversibly switchable wettability. Smart surfaces that could change reversibly between superhydrophobicity and superhydrophilicity have attracted particular attention due to their potential uses as controllable surfaces. (2), (9) Up to now, various methods have been reported using stimuli of either an energy beam such as light, (46) or external materials such as solvents (47) and pH regulators. (48) In this article, the superhydrophobic property can be easily lost by heating above 220[degrees]C, and can be regained just by spraying the dodecanoic solution on the hot surface with a temperature range of 100-200[degrees]C, as shown in Fig. 8a. The elevated temperature makes the switch quite fast and only seconds are needed. The low temperature and the short time duration will not change the properties of the metallic coating (but should be high enough to damage most polymer coatings, therefore it is only applicable for metallic coatings). This cycle has been repeated several times, and good reversibility of the surface wettability was observed. As shown in Fig. 8b, the CA of the superhydrophilic state for each cycle equals 0[degrees], while the CA value of the superhydrophobic state for each cycle is above 150[degrees].

[FIGURE 8 OMITTED]

Recently, Gu et al. (47) reported one strategy to achieve reversible wettability conversion of [WO.sub.3] surfaces between superhydrophobicity and superhydrophilicity. By controlling the process of adsorption/desorption of n-dodecanethiol associated with the Ag deposits on [WO.sub.3] nanostructure surfaces, rapid changes of reversible wettability between superhydrophilicity and superhydrophobicity were realized. They named that strategy as an ex situ one, distinguishing from the previous reported in situ strategy. In our work, the switch is realized by the self-assembly of dodecanoic molecules, assisted by heating the coating. Therefore, the strategy reported in our paper can also be labeled as an ex situ strategy. The elevated temperature makes the switch of the wettability finish within only seconds, depending on how fast the surface is heated. Due to the metallic nature of the coating, a short heating to 220[degrees]C will not affect the mechanical or other functional properties of the surface. It is expected that the technique will extend the application of superhydrophobic metallic surfaces and find applications in fabricating smart devices. For example, our experiments showed that the surface (or local part) can be switched back to hydrophilic in a very short time period just by heating. Then, the surface can be modified (for example, by a weak laser beam) to have a complicated surface pattern with different wettability. This surface might be useful to obtain directed liquid movement on surfaces, controllable separation systems, or other fields where a designed wettability (even for a very large area) is required.

Conclusions

In conclusion, micro/submicro-dual scale structure surfaces were fabricated through PS technology. A broad particle size distribution induces different molten behaviors of the particles and facilitates the formation of the dual scale morphology. The as-prepared sample immediately after the plasma spray shows superhydrophilic property. By spraying an acetone solution of dodecanoic acid on the hot coating surfaces within a temperature range of 100-200[degrees]C, the wettability of the coating can switch from superhydrophilicity to superhydrophobicity. The superhydrophobicity has a good thermal stability below 200[degrees]C, and is switchable between superhydrophobicity and superhydrophilicity by heat treating and spraying the acetone solution of dodecanoic acid on the hot surface. The dodecanoic acid layer formed on the surface contains both physisorbed and chemisorbed dodecanoic acid molecules. A mild heating (below 140[degrees]C) can facilitate the evaporation of the physisorbed molecules and further decrease the surface energy. Heating to a temperature range from 140 to 200[degrees]C for 10 min will make the chemisorbed molecules start to desorb or decompose, but a heating above 220[degrees]C will totally destroy the dodecanoic acid layer in a short time. The easy and time-saving method we used in this article should be less expensive and more suitable for large-scale production. It is expected that the technique will extend the application of superhydrophobic metallic surfaces and find applications in fabricating smart devices.

Acknowledgment This work was supported by the Doctoral Fund of Ministry of Education of China (No. 20110007110009).

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Z. Li, Y. Zheng *, L. Cui

Department of Materials Science and Engineering, China University of Petroleum, Changping 102249 Beijing, China

e-mail: yanjun_zheng@hotmail.com

[c] ACA and OCCA 2012

DOI 10.1007/s11998-011-9390-6
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Author:Li, Zhengfeng; Zheng, Yanjun; Cui, Lishan
Publication:JCT Research
Date:Sep 1, 2012
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