A novel fabrication of superhydrophobic surfaces on aluminum substrate

Applied Surface Science 447 (2018) 363–367

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A novel fabrication of superhydrophobic surfaces on aluminum substrate Jiyuan Zhu College of Mechanical and Control Engineering, Guilin University of Technology, Guilin 541004, China

a r t i c l e

i n f o

Article history: Received 7 November 2017 Revised 30 March 2018 Accepted 2 April 2018 Available online 3 April 2018 Keywords: Superhydrophobic surfaces Cutting Contact angle Corrosion

a b s t r a c t Superhydrophobic surfaces were successfully fabricated by a simple machine cutting method to create the rough surface and using stearic acid to modify the surface. The products were characterized by scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) Spectroscopy, X-ray photoelectron spectroscopy (XPS) analysis and contact angle (CA) measurements. Potentiodynamic polarization curves were used to reveal corrosion resistance of the samples in 3.5% NaCl aqueous solution. The results showed that the obtained superhydrophobic surfaces all possessed water contact angles of more than 150° and enhanced corrosion resistance performance. A possible formation mechanism of the surface morphologies was proposed through geometric figure. This approach requires no complex processing equipments or time-consuming preparation and no toxic reagents are involved. So this novel and environment-friendly attempt might have promising practical applications. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction Superhydrophobicity is the water-repellent characteristic of material surfaces. Inspired by many natural examples such as lotus leaf and wings of cicada [1], artificial superhydrophobic surfaces, which are characterized by water contact angle (CA) larger than 150°, have attracted increasing attention due to their potential industrial applications in self-cleaning [2], corrosion resistance [3], fluid drag reduction [4] and oil/water separation [5] and so on. In recent years, surfaces with superhydrophobic property have been successfully explored on metallic surfaces by a variety of methods such as chemical etching [6], electro deposition [7], anodization [8], phase separation [9], sol-gel [10] and so on. Nevertheless, there are obvious limitations in their applications. These traditional methods usually involve harsh experiment conditions and complicated processes, making the fabrication process tedious and inefficient. The need for specialized reagents and sophisticated equipments in the experiments definitely leads to the increase of cost. And the frequent use of strong corrosive or toxic fluorinecontaining reagents will endanger the environment [11–13]. Hence, exploring simple, effective, inexpensive and eco-friendly approaches is an increasing demand for industrial applications and mass production of superhydrophobic surfaces [14]. As an important engineering material, aluminum and its alloys have been extensively used in the household, automobile, aircraft, aerospace and other industries due to their great advantages like low density, high specific strength, excellent heat and electrical

E-mail address: [email protected] https://doi.org/10.1016/j.apsusc.2018.04.014 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

conductivities and low-specific weight etc [15,16]. However, aluminum and its alloys are prone to corrosion in contact with water, especially in aggressive and corrosive environments [17]. So it is significant to improve their anticorrosion ability of aluminum and its alloys to extend their industrial applications. The fabrication of superhydrophobic coatings on them has been proved to be a promising application. By preparing the water-repelling superhydrophobic coatings on the metal surface, the breaking of oxide layer of metals slow down, thus restraining their further corrosion [18]. A number of investigations have been performed on the preparation of superhydrophobic coatings on aluminum and its alloys in order to improve their corrosion. Shi et al. [19] successfully prepared superhydrophobic coatings on aluminum substrates through acid etching followed by modifications with nano-silica and fluorosilane. Tests showed superhydrophobic coatings increased the corrosion potential of aluminum substrates. Liu et al. [20] fabricated a superhydrophobic aluminum alloy surface with good corrosion resistance by anodic oxidation and a selfassembly process. Xu et al. [21] reported superhydrophobic thin films fabricated on chemically cleaned aluminum alloy substrates by electrodeposition process in the ethanolic solution containing Ni+2 ions and stearic acid under applied DC voltage. Karthik et al. [22] presented an approach to fabricate a corrosion-resistant superhydrophobic composite coating on aluminum surface from the hybrid assembly of inorganic-organic layers via electroless copper deposition followed by surface modification with laurylamine. However, these methods are still subjected to certain limitations like complicated procedure, expensive materials and potential hazards to the environment caused by chemical reagents [16,23,24].

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In this work, we presented a general, simple and low-cost route to prepare superhydrophobic surfaces through a combination of common machine cutting and chemical modification. Uniform rough surfaces were created on aluminum substrates by cutting on a common lathe. Then by modifying the rough surfaces with stearic acid to reduce the surface energy, the as-prepared surfaces were superhydrophobic with water contact angles of more than 150°. Compared with the previous methods, our method is a novel and environment-friendly attempt. The whole experiment is facile and effective. It does not demand special experimental conditions or complex operation. Not any toxic chemical reagents are used. Therefore it is highly expected that the method will have promising applications for fabricating large-area superhydrophobic surfaces. 2. Experiment 2.1. Materials and preparation Lathe C6140 was used to process the end faces of 6061 Al alloy bars using 60° thread cutter. By adjusting the speed of spindle and feeding, feed rate per revolution set was 0.05 mm, 0.1 mm, 0.15 mm, 0.2 mm, respectively. After that, the samples were cut by cutter and in turn rinsed in acetone and ethanol solution three times. The as-prepared aluminum substrates were immersed in a mixed solution composed of ethanol and stearic acid (0.1 g stearic acid, 30 ml ethanol, 30 ml water) in a reaction kettle at 60 °C for 24 h. Then the samples were ultrasonically washed three times with deionized water and ethanol in turn and kept dry in air. 2.2. Characterization The morphological structures of the as-prepared surfaces were examined by field emission scanning electron microscopy (Phenom proX, Phenom-World BV). The chemical composition was evaluated by Fourier transform infrared spectroscopy (VERTEX 70, Bruker) and X-ray photoelectron spectroscopy (Axis Ultra, Kro-

tos). The contact angles (CA) were measured by a Dataphysics OCA20 CA system at room temperature. The corrosion performance of the samples in 3.5% NaCl aqueous solution was investigated by potentiodynamic polarization by using an electrochemical workstation (IM6ex, Zahner) with a standard threeelectrode system equipped with an Ag/AgCl reference electrode, a platinum mesh as the counter electrode, and the sample with an exposed area of 1 cm2 as the working electrode. The scan rate for the potentiodynamic polarization curves was 5 mV/s. 3. Results and discussion 3.1. SEM Scanning electron microscopy (SEM) images show the morphologies of the as-prepared surfaces in Fig. 1. As shown in the figure, there exist deep spiral pits on the samples after cutting. They are well-ordered and micro-scale. The distances of these pits are equal to the different feed rates per revolution, which are between 50 microns and 200 microns. When f(feed) = 0.05, the distance is 0.05(Fig. 1(a)). When f = 0.1, it is 0.15(Fig. 1(b)) and so on. The cutting and friction of the tool covered the surfaces with submicron or nanosized asperities. So the surfaces exhibited uniform micro-nano structures. 3.2. Chemical compositions To prepare the surface, stearic acid is used to modify the samples. Fig. 2 shows the FTIR spectra of the as-prepared samples (a) and stearic acid powders (b). Adsorption peaks located at around 2919 cm
1 and 2850 cm
1 of superhydrophobic surfaces are assigned to the symmetric vibration of sbndCH2 and asymmetric vibration of sbndCH3 bond, which proves the presence of long chain aliphatic groups on the superhydrophobic surfaces. In the low frequency region, a new peak emerges at 1645 cm
1 and the carboxyl group (sbndCOOH) from stearic acid at 1701 cm
1 is no longer observed. This is due to the existence of Carboxylate,

Fig. 1. SEM images of the as-prepared surfaces with different feeds (a) f = 0.05; (b) f = 0.1; (c) f = 0.15; (d) f = 0.2 mm.

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O

a Superhydrophobic surface b Stearic Acid

Absorbance

2919

2850 1645

b

2919

4000

Intensity (a.u)

a

3500

3000

2850

1701

2500

2000

1500

1000

538

500

536

534

532

530

528

526

Binding energy(ev)

Wavenumber (cm-1)

Fig. 4. XPS spectra of O 1S. Fig. 2. FTIR spectra of the as-prepared samples: (a) spectra of aluminum substrate modified with stearic acid; (b) spectra of stearic acid powders.

C C 1s

Intensit (a.u)

Intensity (a.u)

O 1s

Al 2p

292 1000

800

600

400

200

0

Binding energy(ev)

290

288

286

284

282

280

Binding energy (ev) Fig. 5. XPS spectra of C 1S.

Fig. 3. XPS wide scan spectra of the superhydrophobic surface.

sbndCH3 and sbndCH2 on the superhydrophobic surfaces. FTIR spectra reveal that chemical reactions have taken place between aluminum substrates and stearic acid. To further verify the chemical compositions of the superhydrophobic surface, XPS wide scan spectra are presented in Fig. 3. XPS results confirm after the modification with stearic acid O, C, Al is present on the aluminum substrate. XPS spectra of O 1S in Fig. 4 shows only one peak. The peak at 529.2 eV is attributed to the O element from Al2O3, AlO (OH) and ester (sbndCOOsbnd) . XPS spectra of C 1S in Fig. 5 indicate that peak at 284.6 eV is due to the carbon atom in alkyl groups and peak at 287.5 eV is assigned to that in carbonyl group. 3.3. Surface wettability The surface wettability is evaluated by measuring the contact angles (CAs) of water droplets (4 lL) at the ambient temperature. Each value is an average of five different points on the same surface. Fig. 6 illustrates the CAs of water droplets on as-prepared aluminum substrates. As the feeds vary from 0.05, 0.1, 0.15, 0.2 mm (a–d), the corresponding water contact angles are, 155.1°, 153.5°,152.8°, 150.2°, respectively. It could be seen that the machine-processed surfaces become superhydrophobic as all the

Fig. 6. Digital photographs of water droplet (4 lL) on as-prepared substrates with different feeds: (a)–(d): f = 0.05, 0.1, 0.15, 0.2 mm.

CAs are more than 150° after the modification with stearic acid. Therefore it is obvious that the substrates maintain superhydrophobicity.

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3.4. Corrosion resistance performance

a bare aluminum alloy b superhydrophobic surface

-2 -3 -4

b

-5

a

-6 -7 -8

-1.1

-1.0

-0.9

-0.8

-0.7

-0.6

Potential (vs. SCE/V) Fig. 7. Potentiodynamic polarization curves of bare aluminum alloy and superhydrophobic surface in 3.5 wt.% NaCl corrosive solution.

Table 1 Electrochemical parameters from polarization curves of different samples.

Fig. 7 show the potentiodynamic polarization curves of bare aluminum alloy and as-received superhydrophobic surface in 3.5 wt.% NaCl corrosive solution. Electrochemical parameters from polarization curves of different samples are presented in Table 1. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the untreated aluminum alloy are
0.831 V and 2.9227  10
5 A/cm2, respectively. The Ecorr and Icorr of the superhydrophobic surface are
0.883 V and 8.4138  10
6 A/cm2, respectively. The superhydrophobic surface exhibits a smaller Icorr and larger Ecorr. Compared with the bare aluminum alloy substrate, the superhydrophobic substrate has better corrosion resistance performance. It is generally believed that in corrosive solution, the metal substrate reacts quickly with corrosive ion and water. When the as-prepared superhydrophobic surface is immersed in corrosive solution, air is trapped on the micro-nano hierarchical structure of the superhydrophobic surface, which inhibited the contact between the aluminum alloy substrate and corrosive solution and hence protected the substrate. 3.5. Formation mechanism of the surface morphologies

2

Ecorr (V)

Icorr (A/cm )

a b


0.831
0.883

2.9227  10
5 8.4138  10
6

Generally speaking, the parameters of cutting layer will affect the formation of the surface morphologies. Feed movement of cutter leaves residual area on the machined surface. If the depth of cut is large (Fig. 8(a)), the linear cutting edge will determine surface

H

Specimen

Kr '

Kr

r

¦Á

H

Log (Current density/ A·cm -2)

-1

f

f

(a)

(b)

Fig. 8. Formation mechanism of the surface morphologies with (a) larger depth of cut (b) smaller depth of cut.

f

f

K r'

Kr'

Kr'

f

(a) Kr'=60¡ã

(b) Kr'=30¡ã Fig. 9. Kr0 effect on the surface morphology.

(c) Kr'=15¡ã

J. Zhu / Applied Surface Science 447 (2018) 363–367

roughness. With no consideration of the edge radius (r), the height of residual area can be calculated by the following equation:

H¼

f cot K r þ cot K 0r

ð1Þ

where H denotes residual area’s height, f refers to the amount of feed, Kr means tool cutting edge angle, and Kr0 is end cutting edge angle. If the depth of cut is small (Fig. 4 (b)), the arched cutting edge will determine the machined surface roughness. The height of cut area is assumed to be: 2  a f 2 a H ¼ r 1
cos ¼ 2r sin  2 4 8r

ð2Þ

where H means the height of residual area, r means the edge radius, ɑ is the angle of thread and f is the amount of feed. So shape of the tool and feed rate will affect the microstructure of the superhydrophobic surfaces. If the relative parameters are changed, surfaces with micro-nano textures could be well constructed. From the above SEM photo, it can be concluded that feed rate (f) plays a significant role to the surface microstructure. The adjustment of the feed rate results in different surface microstructure, thus affecting the wettability of the surface. The cutting edge angle of tool can affect the residual area and the surface microstructure too. The end cutting edge angle Kr0 can be an illustration. As is shown in Fig. 9, with the same feed, when Kr0 = 60°, residual area’s height (H) achieves the maximum value. When Kr0 = 15°, residual area’s height (H) achieves the minimum value. Therefore as Kr0 decreases, H reduces. Consequently, the residual area becomes smaller and the surface roughness will decrease. Kr and Kr0 have the same effect on the surface roughness. 4. Conclusions In conclusion, we present a novel and simple route to fabricate superhydrophobic surfaces on aluminum substrate. After being modified by stearic acid, the rough machine-cut surfaces with CAs more than 150° possess superhydrophobic property and good corrosion-resistant performance. The method proposed in our work causes no harm to the environment as it does not require any fluorinated chemicals or strong acid and alkali reagents. As the cutting is operated on a common lathe, high processing efficiency can be guaranteed. Therefore it provides a new strategy for a cost-effective and nontoxic method to prepare superhydrophobic surfaces, which will offer potential opportunities for large-area production and industrial applications of superhydrophobic surfaces without complex processing equipment or time-consuming preparation. References [1] T. Sun, L. Feng, X. Gao, L. Jiang, Bioinspired surfaces with special wettability, Acc. Chem. Res. 38 (2005) 644–652. [2] S. Nishimoto, B. Bhushan, Bioinspired self-cleaning surfaces with superhydrophobicity, superoleophobicity and superhydrophilicity, RSC Adv. 3 (2013) 671–690.

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