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A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure
Journal Pre-proof A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure
Masoud Safarpour, S. Alireza Hosseini, Fateme Ahadani-Targhi, Petr Vašina, Mostafa Alishahi PII:
S0257-8972(19)31229-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125239
Reference:
SCT 125239
To appear in:
Surface & Coatings Technology
Received date:
26 September 2019
Revised date:
4 December 2019
Accepted date:
6 December 2019
Please cite this article as: M. Safarpour, S.A. Hosseini, F. Ahadani-Targhi, et al., A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure, Surface & Coatings Technology (2019), https://doi.org/10.1016/ j.surfcoat.2019.125239
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© 2019 Published by Elsevier.
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A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure Masoud Safarpoura, S. Alireza Hosseinia,*, Fateme Ahadani-Targhia, Petr Vašinab, Mostafa Alishahia,* a
Department of Materials and Polymer Engineering, Faculty of Engineering, Hakim Sabzevari University, 96179-76487 Sabzevar, Iran
Department of Physical Electronics, Faculty of Science, Masaryk University, CZ-61137
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Brno, Czech Republic
Abstract
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This study explores the microstructural tuning of the AZ91 magnesium alloy subjecting to
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different heat-treatment procedures to produce the superhydrophobic surface with different adhesive properties. In this regard, a two-step chemical etching process was used to form
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hierarchical nano/microstructure, during which various phases were corroded with different
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rates. The surface morphology of samples was characterized using field-emission scanning
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electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and confocal laser microscopy (CLSM). In addition, the power spectral density (PSD) analysis was employed to evaluate the nano-scale and micro-scale components of the surfaces. The results revealed that the preferential dissolution of the matrix in favor of primary precipitates during etching treatment formed a hierarchical nano/microstructure on the surface of as-cast AZ91 sample and provided petal-type superhydrophobicity. Employing a combination of annealing and aging treatments resulted in a transition from petal to lotus-type superhydrophobicity by redistributing of the β-precipitates in the α-Mg matrix. The
Corresponding authors: * E-mail addresses: [email protected] (M. Alishahi), [email protected] (S.A. Hosseini).
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Journal Pre-proof mechanism of this transition was discussed in detail, and a direct correlation between the microstructure and the wettability behaviors of AZ91 Mg alloy was established. Keywords: AZ91 Mg alloy, superhydrophobicity, petal-state, lotus-state, β-precipitates, chemical etching.
1. Introduction The wettability of solid surfaces has so far attracted tremendous interest from both
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fundamental and applied perspectives [1-5]. It is well known that solid/liquid interaction depends on two parameters: surface energy and surface morphology [6]. In general, the
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behavior of water droplets on a surface, depending on the apparent contact angle (ACA)
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between the water droplets and the solid surface, can be divided into four categories as follow
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[7]: 1) superhydrophilic behavior with ACA of < 10°, 2) hydrophilic behavior with ACA of 10° < < 90°, 3) hydrophobic behavior with ACA of 90° <<150° and 4) superhydrophobic
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behavior with ACA of >150°. The superhydrophobic surfaces, depending on their adhesive
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properties, are also categorized into the lotus-state and the petal-state. The former
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corresponds to the low-adhesion superhydrophobic surface at which water droplet rolls off on the surface at a sliding angle lower than 10°, while the latter corresponds to the high-adhesion superhydrophobic surface at which water droplet strongly adheres to the surface (Cassie impregnated state) [8]. It has been reported that the adhesivity of water droplets on the surface is strongly dependent on surface chemistry, microstructure, and roughness [9, 10]. The surfaces exhibiting petal-type superhydrophobicity can be used for the no-loss transportation of small volumes of liquid [11] and surfaces with lotus-type superhydrophobicity have the potential to apply in various fields such as self-cleaning surfaces [12], anti-icing surfaces [13], and water management in fuel-cell stack [14].
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Journal Pre-proof Magnesium (Mg) alloys, which have high strength, low density, and good machinability are currently used in a wide range of structural applications such as aerospace, automotive, and electronics industries [15-17]. Since Mg alloys are intrinsically hydrophilic, various surface modification methods such as anodizing [18], micro-arc oxidation [19], wet chemical etching [20], and hydrothermal synthesis [21] have been utilized to provide the superhydrophobic properties on their surfaces. In general, the superhydrophobic surface on the metals forms in two steps: First, a suitable surface morphology is produced with hierarchical nano/micro
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structures and then a low surface tension layer is deposited on the surface [22, 23]. Accordingly, the chemical etching is a facile, low-cost and versatile technique for fabricating
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the desired surface morphology with a superhydrophobic characteristic on Mg alloys, in
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which an aggressive solution preferentially dissolves the high energy sites of Mg alloys and
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subsequently, provides a hierarchical nano/micro structure [17]. AZ91 is a popular cast Mg alloy with a nominal composition of Mg- 9 wt.% Al- 1 wt.% Zn.
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The microstructure of as-cast AZ91 Mg alloy consists of primary dendritic α-Mg grains,
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largely divorced β-particles, and α+β lamellar phase, where α-phase is the solid solution of
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aluminum (Al) in Mg and the β-phase is the Mg17Al12 precipitate [24]. The distribution of βprecipitates can be tuned by employing the proper heat-treatment procedure. Conventionally, β-precipitates are initially dissolved into the α-matrix during solid solution treatment and then, redistributed in the structure through an aging treatment [25]. The time and temperature of aging treatment are crucial parameters for determining the type, size, and distribution of β-precipitates [25, 26]. The difference in the Al content of α and β phases results in different electrochemical activities. The electrochemical potentials of the α and β phases in a sodium chloride solution have been reported to be -1.3 V and -1.6 V, respectively [25]. Thus, upon exposure of AZ91 to a corrosive environment, a micro-galvanic couple is formed between the α matrix and β-
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Journal Pre-proof precipitate. In such a couple, the α matrix acts as the anode and is preferentially corroded while the β-precipitate is cathodically protected. Therefore, due to the difference in the corrosion rates of α and β phases, the topography of AZ91 alloy surface can be adjusted using chemical etching treatment. Yin et al. [27] reported a superhydrophobic behavior for chemically-etched AZ31 Mg alloy modified by silicone-based agents and found that a uniform nano/micro-porous structure traps air pockets and hence, provides a lotus-type superhydrophobicity. In addition, Liu et al.
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[28] used a laser treatment followed by the chemical etching and surface modification treatments in the DTS silane agent to improve the hydrophobicity of AZ91D Mg alloy, and
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showed that the formation of crater-like surface features resulted in the existence of petal-
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type superhydrophobicity. Therefore, it seems that the shape and distribution of surface
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features on the metallic surface determine the type of superhydrophobic behavior. Long et al. [29] reported the transition from the petal to lotus state by tuning the copper topography
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from above 90° to ~1°.
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through laser micro-machining treatment where the sliding angle of water droplets decreased
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So far, reports on the wettability behavior of Mg alloys have mostly focused on the use of different mechanical, chemical, and surface treatments to develop the superhydrophobic surfaces [17-21] and there is no report of the use of heat-treatment to alter the wettability state of the Mg surface. In this paper, we introduce a facile, low-cost and scalable method for producing superhydrophobic Mg surface together with tunable adhesive properties, having promising potential for large-scale applications such as electronic device housings, automobile parts, sporting equipment, etc. [30]. In this method, the shape and distribution of the topographic features of AZ91 Mg alloy were tailored using various heat-treatment procedures and subsequent chemical etching to fabricate different hierarchical nano/micro structures.
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Journal Pre-proof The surface characteristics of samples were determined by field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and confocal laser microscopy (CLSM). In addition, the surface wettability of the samples was systematically investigated by measuring the ACA, sliding angle (SA), and bouncing behavior of the water droplet.
2. Experimental procedures
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2.1. Sample preparation
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In the present work, an as-cast AZ91 Mg alloy ingot was used as the raw material. The chemical composition of the alloy in weight percentage (wt.%) was measured to be Al: 7.69;
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Zn: 0.95; Mn: 0.20; Si: 0.03; Cu: <0.01; Fe <0.01 and Mg: balance. A tubular furnace with an
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argon gas atmosphere was employed for solution annealing treatment. All samples were initially annealed at 413 °C for 24 hours to dissolve the intermetallic phases, and then
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immediately quenched into the water to form a supersaturated solid solution phase.
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Subsequently, the quenched samples were treated with aging at 190 °C for 8, 16, and 24 hours
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to precipitate different shape, size, and distribution of the second phase in their structure. The samples were labeled according to the employed heat-treatment cycle, as shown in Table 1. Table 1. Designation of the samples and heat-treatment cycles. Sample Thermal treatment Conditions as-cast
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S-NA
Solid Solution @ 413 °C for 24 h
S-A8
Solid Solution → Aging @ 190 °C for 8 h
S-A16
Solid Solution → Aging @ 190 °C for 16 h
S-A24
Solid Solution → Aging @ 190 °C for 24 h
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Journal Pre-proof Prior to the chemical etching process, the samples were mechanically polished with 1000-grit SiC emery paper and ultrasonically cleaned with acetone, ethanol, and deionized water, in sequence. Subsequently, the samples were immediately etched in a two-step process [17], where they were first etched in 1 vol.% H2SO4 for 240 seconds and then immersed in 20 vol.% H2O2 aqueous solution for 120 seconds. After that, the samples were rinsed with deionized water and finally, were modified in an ethanolic solution stearic acid (0.05 M) at
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ambient temperature for 60 minutes.
2.2. Surface Characterization
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The crystal structure of as-cast sample was studied by X-ray diffraction (XRD) in standard
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Bragg–Brentano (theta-2theta) mode using a GNR Explorer diffractometer with a Cu-Kα radiation source (λ=1.5406 Å). The microstructure and surface morphology of the samples
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were examined by a Tescan MIRA3 FE-SEM. A confocal laser scanning microscope (CLSM,
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Olympus LEXT 3D Measuring Laser Microscope OLS4000) was utilized to analyze the
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roughness and surface topography of the samples. The free-ware Gwyddion code [31] and the Digital Surf MountainsMap® Premium software version 7 (trial version) [32] was used for
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image processing and analysis. A homemade contact angle instrument was employed to measure wettability. The ACA was measured for a deionized water droplet of volume 5 µL which recorded by a CCD camera. In addition, the sliding angle (SA) was measured by tilting the sample stage and recording when the drop began to move in the downhill direction. For each sample, both ACA and SA measurements were repeated at least three times at different regions on the surface. The bouncing behavior of selected samples was also evaluated using the water droplets with a volume of ~5 µL and a dropping height of ~20 mm at the temperature of 23°C. The droplets velocity at the point of impact was about 0.4 m.s-1. The dynamic behavior of the water droplets was recorded with time sequence of ~17 ms by a CCD camera. 6
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3. Results and discussion 3.1. As-cast sample The XRD pattern shown in Fig. 1 depicts that the as-cast sample consists of two phases: the α-Mg phase (ICCD: 00-035-0821), which is a solid solution of Al and Zn in Mg, and the precipitate of β- Mg17Al12 phase (ICCD: 00-001-1128) with an α-Mn type lattice. This structure is typical of AZ91 Mg alloys, where the β-precipitate is distributed in an α-matrix
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[33].
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Fig. 1. XRD scan for as-cast AZ91 Mg sample
The ACA value for the polished surface of the as-cast sample was 38°, which is in agreement with the typical ACA values for the smooth surface of metals [3, 34]. The smooth metal surfaces generally exhibit low ACA hydrophilic behavior due to their high surface free energy (SFE). The wettability behavior of smooth surface is usually represented by Young’s equation (Eq. 1) [35]: 𝛾𝑠𝑣 = 𝛾𝑠𝑙 + 𝛾𝑙𝑣 cos 𝜃𝐸
(1)
where γsl is liquid/solid interfacial energy or SFE, γlv is liquid/gas interfacial energy, γsv is solid/gas interfacial energy and θE is the ACA. Accordingly, the water droplet spreads out over the metal surface with high SFE, while the deposition of low SFE coatings on the 7
Journal Pre-proof surface of metals increases the water droplet ACA. It has been reported that the smooth Mg alloy modified by stearic acid showed a hydrophobic behavior with an ACA of 109° [17]. In addition, the hydrophobicity of modified surfaces can be further improved by tuning their surface roughness [36]. In particular, the formation of a hierarchical nano/micro roughness results in the trapping of air pocket between the surface and water droplets, and hence provides a superhydrophobic behavior with an ACA greater than 150°. Fig. 2 illustrates the ACA and SA measurements of the as-cast sample after chemical etching and surface
superhydrophobicity with the ACA of 150.9 ± 0.6°.
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modification treatments. It can be seen from Fig. 2a that the as-cast sample showed the
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The microstructural investigations shown in Fig. 3a revealed that the etched surface of the as-
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cast sample was composed of a numerous micro-features in a smooth matrix. However, a
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closer look at the surface microstructure showed that the micro-features and their surrounded region were covered by some needle-like and lint-like nano-features, respectively (Fig. 3b).
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The EDS measurements shown in Fig. 4(a) revealed that the Al/Mg atomic ratio in the matrix
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and micro-features are 0.29 and 0.59, respectively. Considering the preferential corrosion of
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Mg relative to Al, it can be stated that the matrix is a corroded α-Mg phase, consisting of more Al atoms compared to the intact α-Mg phase. In addition, EDS analysis showed the Al/Mg ratio for the micro-features is close to that of the β-Mg17Al12 precipitates. However, the presence of Zn in the precipitates indicated that some of the Al atoms in the Mg17Al12 compound were substituted by Zn and formed β-Mg17(Al, Zn)12 phase presumably since, compared to Mg, the atomic radius of Zn is closer to that of Al [37]. It has been reported that ternary intermetallic compound β-Mg17(Al, Zn)12 is formed in the Mg-Al-Zn system when the Al/Zn atomic ratio is greater than three [33, 37, 38]. Mathieu et al. found that the replacement of Al by Zn could reduce the electrochemical potential of β-precipitate from -1.3 V to -1.24 V, thereby increasing its chemical inertness [39]. The EDS elemental mapping image
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Journal Pre-proof displayed in Fig. 4(b) further confirmed that the observed micro-features and needle-like features on the surface of the as-cast sample contained a high concentration of Al compared to the matrix. In addition, the distribution of elemental Zn revealed a higher concentration of Zn in the precipitate compared to the matrix. This result is in consistent with the literature and indicates the tendency of Zn atoms to segregate to the precipitate phase [37]. It is well known that the β- precipitate is electrochemically more noble than the α-Mg phase [25]. Therefore, in the α/β couple, the α matrix acts as the anode, while the Al-rich β-
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precipitates are cathodic sites. Consequently, upon exposure of the as-cast sample to the H2SO4 solution in the first step of etching treatment, the α-Mg matrix is preferentially
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corroded and the β-precipitates are cathodically protected. Because of preferential dissolution
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of the α-Mg matrix, the β-precipitates protrude on the surface of the sample and form micro
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features.The preferential dissolution of α-phase in the α+β lamellar structure further explains the formation of needle-like structure observed in the area around the β-precipitate (Fig. 3b).
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In addition, the formation of lint-like features at the top of β-precipitates can be attributed to
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the selective dissolution of Mg from β- Mg17(Al, Zn)12 precipitates during the first step of
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etching followed by surface oxidation in hydrogen peroxide solution (H2O2) in the second step of chemical etching treatment. The surface oxidation of α-matrix and β-precipitates was confirmed by EDS results, where significant oxygen content was detected in their composition.
Fig. 2. Digital photographs for a) the ACA measurement and b) pinning behavior of a water droplet on the surface of the as-cast AZ91 magnesium alloy 9
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Fig. 3. (a) FE-SEM micrographs from the surface of the as-cast sample after chemical etching
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treatment, (b) enlarged view from surface features
Fig. 4. (a) EDS spectra of the matrix and precipitate phases in the as-cast sample (b) elemental EDS mapping of Mg, Al, Zn and O elements obtained from the white dashed square (The scale-bar is valid for all the micrographs) 10
Journal Pre-proof The formation of the aforementioned hierarchical nano/microstructure together with a low SFE stearic acid coating explains the superhydrophobic wettability in the etched as-cast sample. However, Fig. 2b showed that in the as-cast sample water droplets strongly adhere to the surface and hence, is classified as a surface with petal-type superhydrophobicity. The petal-state depends on the size and distribution of surface features and can occur when the droplet impregnates into the pores, while the hills and their surrounded region are still dry [40, 41]. For more clarification, the CLSM was used to analyze the topography of the as-cast
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sample, as shown in Fig. 5a. The results indicate that the numerous micro-features (βprecipitates) with an average width of ~22 µm and an average hight of 14.8 µm were
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uniformly distributed on the surface, where the mean distance between these features was
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115.4 µm. As schematically shown in Fig. 5b, a combination of high surface features and
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wide smooth area in a hierarchical nano/microstructure allows water droplets to fill the pores
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and hence, provided the petal-type hydrophobicity.
Fig. 5. a) 3D CLSM image from the surface of the as-cast sample after chemical etching treatment b) schematic illustration of liquid-solid contact area
3.2. Heat-treated samples Fig. 6 shows the effect of various heat-treatment cycles on the results of ACA and SA AZ91 Mg alloys. It is observed that solution annealing treatment at 413 °C for 24 hours significantly decreased the ACA of S-NA sample to 91.6 ± 0.6°. The surface morphology of the S-NA sample is shown in Fig. 7a. It can be seen that some micro-features are still 11
Journal Pre-proof observed on the surface of the S-NA sample, indicating that the solution treatment was not long enough to completely dissolve the β-precipitates. However, the corresponding high magnification FE-SEM micrograph shows that a volcano-like morphology is observed on the smooth surface and the nano- needle-like features have been disappeared. It seems that all βprecipitates were dissolved in the α+β lamellar phase during solution annealing treatment and thus the hierarchical nano/microstructure vanished. In fact, the dissolution of β-precipitates increased the Al concentration in the matrix and resulted in less difference in the galvanic
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potential between the matrix and residual β-precipitate. Therefore, solution annealing reduces the preferential corrosion of matrix during chemical etching and provides a smoother surface.
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These findings are supported by CLSM results, where the S-NA sample showed an RMS
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value of 3.08 µm, which is lower than the RMS of the as-cast sample (4.62 µm). These
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microstructural changes resulted in a significant decrease in the ACA of the S-NA sample compared to the as-cast sample.
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After the solid solution treatment, the α-matrix becomes super-saturated with Al, and hence,
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the aging treatment can be used to redistribute the β-phase in the α-matrix [26]. Fig. 6 shows
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that the aging treatment after the solid solution increased the ACA to values comparable with that of the as-cast sample. However, unlike the as-cast sample, the aged samples showed lower SA values. In particular, the ACA (SA) values of S-A8, S-A16, and S-A24 samples were 147.2 ± 0.7° (20.7 ± 0.8°), 159.3 ± 0.9° (2.6 ± 0.6) and 151.4 ± 0.6° (6.7 ± 0.5°), respectively. These results can be explained by the redistribution of β-precipitates during aging, which provides different surface morphologies after chemical etching, as shown in Fig. 7b-d. Accordingly, CLSM micrograph of Fig. 7c shows that the S-A16 has an RMS value of 7.31 µm, and its surface morphology consists of a uniform distribution of the microfeatures with various sizes. A high magnification SEM micrograph (Fig. 7d) also revealed that a continuous network of nano-features covers the whole surface of the S-A16 sample and
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Journal Pre-proof forms a hierarchical nano/microstructures. In fact, the micro-features observed in S-A16 samples are originally the undissolved β-particles that can grow up during aging treatment. In addition to these large β-particles, cellular growth of alternating α/β layers can also occur during aging within the grains (continuous precipitation) or at grain boundaries (discontinuous precipitation) [38]. The preferential corrosion of the α layer in this lamellar α/β structure leads to the formation of a uniform distribution of nano-features on the entire surface. The size and morphology of nano-features strongly dependent on the duration of
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aging treatment. Fig. 7b shows that the aging for 8h was not long enough to form a distinguishable cellular network of α/β layers, hence the low-intensity nano-features were
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distributed over the S-A8 sample surface. In contrast, the aging treatment for 24h resulted in
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the growth of β-precipitate and the formation of a coarse-lamellar structure with the sub-
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micron features rather than nano-features (Fig. 7c). As a result, a combination of highintensity micro-features and high-intensity nano-features in the S-A16 sample offered very
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rough hierarchical nano/microstructures with the highest ACA and the lowest SA. The effect
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of various intensities of roughness components on the wettability behavior will be discussed
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specifically in the next section using power spectral density (PSD) analysis
Fig. 6. The results of ACA and SA measurements for different samples 13
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Fig. 7. 3D CLSM and high-magnification FESEM images for (a) S-NA, (b) SA-8, (c) SA-16, (d) SA-24 samples
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Journal Pre-proof 3.3. Petal-state to the lotus-state transition mechanism The wettability analysis showed that the aged samples exhibited superhydrophobicity behavior similar to the as-cast sample. However, unlike the as-cast sample, which indicated the petal-type superhydrophobicity, the water droplets could easily run at low tilted angles on the surface of aged samples. In this regard, the S-A16 and S-A24 samples showed SA values less than 10°, and hence they represent lotus-type superhydrophobicity. Therefore, it can be
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stated that a transition from petal to lotus state in AZ91 Mg alloy can be accomplished by
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employing a suitable heat-treatment procedure
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In this section, we employed PSD analysis over a square area of 120 µm × 120 µm to find a correlation between various roughness components on the surface and the wettability
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behavior of samples. The PSD function provides information on the roughness components
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as the deviation of height from a mean plane and evaluates the lateral distribution/distance at which the height change occurs by using the following equation [42]: 2
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1 PSD(𝐟) = lim |∫ z(𝐫) exp(−2ði𝐟. 𝐫)d𝐫| A→∞ A A
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where z(r) represents the profile height, f is the spatial frequency in the x- and y- directions, A is the surface area of the measured field, and r is position vector. Fig. 8 presents the calculated PSD curves for the as-cast and heat-treated samples. The low-frequency region of the PSD spectrum is related to the nano-scale component of the roughness (nano-features), and the high-frequency region represents the micro-scale component of the roughness (microfeatures). It is obvious that the S-A16 sample depicted a higher spectrum roughness over the whole spatial frequency spectrum. Therefore, the S-A16 sample had the highest intensity of micro- and nano-features among all studied samples. Such a rough hierarchical nano/microstructure provided a high ACA value of ~160°. A decrease in the intensity of micro-/nano-scale components of the PSD curve revealed a weaker hierarchical
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Journal Pre-proof nano/microstructure, which corresponds to a lower ACA value. Comparing the results revealed that there is a correlation between the measured ACA values of the samples and the trend observed in the PSD curves, where a higher spectral roughness yields higher ACA. However, the PSD results were unable to explain the adhesive properties of the surfaces. For example, as-cast and S-A24 samples both showed similar PSD curves and ACA values, however, the former showed the petal-type hydrophobicity and the latter exhibited the lotustype hydrophobicity. In fact, the adhesive properties of surfaces depend not only on the
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intensity of the micro/nano features (PSD values at different frequencies) but also on the density and distribution of these features. Bhushan et al. [43] showed that a high density of
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nano-features in a hierarchical nano/microstructure resulted in the formation of air-pocket
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between the micro- and nano-features, thus reducing the adhesion of droplets to a significant
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level. Accordingly, we proposed a schematic model to show how the wettability behavior is affected by the surface morphology (Fig. 9). As mentioned earlier, the observed micro- and
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nano-features on the surface of the as-cast sample are derived from the largely divorced β-
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particles and α+β lamellar phase, respectively. Thus, the micro-features were homogeneously
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distributed on the surface of the as-cast sample, but the nano-features were only observed around the micro-features, and the gap between the micro-features was smooth (Fig. 9a). In such a structure, as schematically illustrated, the Cassie impregnated wetting state is operative, at which the droplet can wet the interspace space between the micro-features but due to the formation of air pockets, it cannot wet the nano-features. This structure provides a high surface adhesion due to the large solid-liquid interfacial area and leads to petal-type hydrophobicity [40]. Unlike the as-cast sample, the nano-features were homogeneously distributed on the surface of the S-A16 sample and covered the micro-features interspaces (Fig. 9b). These nano-features had a homogenous distribution because they originated during the aging from the continuous and discontinuous precipitation of the β phase within the grains
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Journal Pre-proof and at the grain boundaries, respectively. Therefore, water can wet neither nano-features nor micro-features interspaces; therefore, the adhesive force between the water and surface is negligible, and water droplets can be easily rolled off even at low tilting angles. This means that the transition from petal to lotus state on AZ91 surfaces occurred by redistribution of precipitates during the aging treatment. Consequently, the heat treatment can be employed as an effective way to tune the microstructure and subsequently determine the wettability
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behavior of the surface.
Fig. 8. PSD curves for different samples
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Fig. 9. Schematic illustration of the liquid-solid contact area and the surface morphology of
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3.4. Bouncing behavior
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(a) as-cast (b) SA-16 samples
Drops of water that hit the hydrophobic surface can bounce off or stick to the surface. Along
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with the CA and SA measurements, the drop-bouncing behavior on the surface is a critical parameter to characterize the wettability of the surfaces especially for the lotus-based applications [44]. Crick and Parkin [45] found that there was a linear correlation between CA and the number of bouncing cycles, where the water droplets bounce off only on the surfaces with CA over 151°. As shown in Fig. 10, the number of bouncing cycles for the S-A16 sample surface with the lotus-type hydrophobicity is three times, while no bouncing occurs on the as-cast sample with petal-type hydrophobicity. The latter can be attributed to the adhesive behavior of the surface which dissipates the kinetic energy of the droplet [46]. Nine et al. [47] reported that the bouncing characteristics of a surface drop depend on the surface morphology and the 18
Journal Pre-proof distribution of roughness components. Consequently, the existence of air pocket between surface features and water drops causes the drop to bounce off over the surfaces with lotustype superhydrophobicity [48]. On the other hand, for the surfaces with petal-type superhydrophobicity, the surface interaction between water droplets and the top surface area together with the frictional force between the water droplets and the wall of micro-features
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causes high energy loss; thus, the water drops stick instead of bouncing [46, 48].
Fig. 10. Series of photographs of water droplet dropped from a height of 20 mm with an impact velocity of ~0.4 m.s-1 (a) sticking of the water droplet on the surface of the as-cast sample with the petal-type superhydrophobicity, (b) bouncing of the water droplet on the surface of S-A16 sample with the lotus-type superhydrophobicity.
4. Conclusion A hierarchical nano/microstructure on the surface of AZ91 Mg alloy was developed by the preferential dissolution of α-matrix in favor of β-precipitates during a chemical etching 19
Journal Pre-proof treatment. Accordingly, a combination of annealing and aging treatments was used to tailor the shape, size, and distribution of β-precipitates in the AZ91 microstructure and subsequently, fabricate various surface morphologies with different wettability properties. The PSD analysis revealed that a rough hierarchical nano/microstructure could provide superhydrophobicity properties, while a reducing the intensity of micro-features or nanofeatures surfaces may decrease PSD values and result in a significant decrease in ACA values. In addition, it was found that the distribution of nano-features determines the surface
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adhesive properties. Accordingly, the formation of an α+β lamellar structure around the largely divorced β-particles in the as-cast sample leads to an inhomogeneous distribution of
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nano-features and provides petal-type hydrophobicity, while an appropriate heat-treatment
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process can redistribute β-precipitates to form a homogeneous distribution of nano-features
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and subsequently create a lotus state on the AZ91 Mg alloy surface. Consequently, the results reported here provide a direct correlation between the microstructure and the wettability
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Acknowledgments
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desirable wetting properties.
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behaviors of AZ91 Mg alloy, which can be used to fabricate the AZ91 Mg surface with
The authors acknowledge financial support from grant 6919 (Students' Scientific Association of HSU). Additionally, the support from CEPLANT (Masaryk University, Brno, Czech Republic) is gratefully acknowledged. We also thank E. Ghanei and S.M. Katebi for their technical support to carry out part of this work at HSU. MA gives special thanks to R. Pouriamanesh, S. Ranjbar and A. Baghani for their help in editing the manuscript.
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Journal Pre-proof CRediT author statement Masoud Safarpour: Investigation; Writing - Original Draft S. Alireza Hosseini: Conceptualization; Methodology; Resources; Writing - Review & Editing; Supervision; Project administration Fateme Ahadani-Targhi: Investigation; Writing - Original Draft Petr Vasina: Resources; Writing - Review & Editing Mostafa Alishahi: Conceptualization; Methodology; Resources; Writing - Review &
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Editing; Supervision; Project administration
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Journal Pre-proof Graphical abstract
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Highlights:
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Development of superhydrophobic Mg surface with tunable adhesive properties Formation of hierarchical nano/microstructure by selective etching Correlation between adhesive properties and distribution of β-precipitates within αMg matrix Investigation of petal-state to the lotus-state transition mechanism
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Masoud Safarpour, S. Alireza Hosseini, Fateme Ahadani-Targhi, Petr Vašina, Mostafa Alishahi PII:
S0257-8972(19)31229-0
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125239
Reference:
SCT 125239
To appear in:
Surface & Coatings Technology
Received date:
26 September 2019
Revised date:
4 December 2019
Accepted date:
6 December 2019
Please cite this article as: M. Safarpour, S.A. Hosseini, F. Ahadani-Targhi, et al., A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure, Surface & Coatings Technology (2019), https://doi.org/10.1016/ j.surfcoat.2019.125239
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© 2019 Published by Elsevier.
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A transition from petal-state to lotus-state in AZ91 magnesium surface by tailoring the microstructure Masoud Safarpoura, S. Alireza Hosseinia,*, Fateme Ahadani-Targhia, Petr Vašinab, Mostafa Alishahia,* a
Department of Materials and Polymer Engineering, Faculty of Engineering, Hakim Sabzevari University, 96179-76487 Sabzevar, Iran
Department of Physical Electronics, Faculty of Science, Masaryk University, CZ-61137
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Brno, Czech Republic
Abstract
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This study explores the microstructural tuning of the AZ91 magnesium alloy subjecting to
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different heat-treatment procedures to produce the superhydrophobic surface with different adhesive properties. In this regard, a two-step chemical etching process was used to form
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hierarchical nano/microstructure, during which various phases were corroded with different
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rates. The surface morphology of samples was characterized using field-emission scanning
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electron microscopy (FE-SEM) equipped with energy dispersive spectroscopy (EDS) and confocal laser microscopy (CLSM). In addition, the power spectral density (PSD) analysis was employed to evaluate the nano-scale and micro-scale components of the surfaces. The results revealed that the preferential dissolution of the matrix in favor of primary precipitates during etching treatment formed a hierarchical nano/microstructure on the surface of as-cast AZ91 sample and provided petal-type superhydrophobicity. Employing a combination of annealing and aging treatments resulted in a transition from petal to lotus-type superhydrophobicity by redistributing of the β-precipitates in the α-Mg matrix. The
Corresponding authors: * E-mail addresses: [email protected] (M. Alishahi), [email protected] (S.A. Hosseini).
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Journal Pre-proof mechanism of this transition was discussed in detail, and a direct correlation between the microstructure and the wettability behaviors of AZ91 Mg alloy was established. Keywords: AZ91 Mg alloy, superhydrophobicity, petal-state, lotus-state, β-precipitates, chemical etching.
1. Introduction The wettability of solid surfaces has so far attracted tremendous interest from both
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fundamental and applied perspectives [1-5]. It is well known that solid/liquid interaction depends on two parameters: surface energy and surface morphology [6]. In general, the
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behavior of water droplets on a surface, depending on the apparent contact angle (ACA)
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between the water droplets and the solid surface, can be divided into four categories as follow
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[7]: 1) superhydrophilic behavior with ACA of < 10°, 2) hydrophilic behavior with ACA of 10° < < 90°, 3) hydrophobic behavior with ACA of 90° <<150° and 4) superhydrophobic
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behavior with ACA of >150°. The superhydrophobic surfaces, depending on their adhesive
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properties, are also categorized into the lotus-state and the petal-state. The former
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corresponds to the low-adhesion superhydrophobic surface at which water droplet rolls off on the surface at a sliding angle lower than 10°, while the latter corresponds to the high-adhesion superhydrophobic surface at which water droplet strongly adheres to the surface (Cassie impregnated state) [8]. It has been reported that the adhesivity of water droplets on the surface is strongly dependent on surface chemistry, microstructure, and roughness [9, 10]. The surfaces exhibiting petal-type superhydrophobicity can be used for the no-loss transportation of small volumes of liquid [11] and surfaces with lotus-type superhydrophobicity have the potential to apply in various fields such as self-cleaning surfaces [12], anti-icing surfaces [13], and water management in fuel-cell stack [14].
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Journal Pre-proof Magnesium (Mg) alloys, which have high strength, low density, and good machinability are currently used in a wide range of structural applications such as aerospace, automotive, and electronics industries [15-17]. Since Mg alloys are intrinsically hydrophilic, various surface modification methods such as anodizing [18], micro-arc oxidation [19], wet chemical etching [20], and hydrothermal synthesis [21] have been utilized to provide the superhydrophobic properties on their surfaces. In general, the superhydrophobic surface on the metals forms in two steps: First, a suitable surface morphology is produced with hierarchical nano/micro
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structures and then a low surface tension layer is deposited on the surface [22, 23]. Accordingly, the chemical etching is a facile, low-cost and versatile technique for fabricating
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the desired surface morphology with a superhydrophobic characteristic on Mg alloys, in
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which an aggressive solution preferentially dissolves the high energy sites of Mg alloys and
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subsequently, provides a hierarchical nano/micro structure [17]. AZ91 is a popular cast Mg alloy with a nominal composition of Mg- 9 wt.% Al- 1 wt.% Zn.
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The microstructure of as-cast AZ91 Mg alloy consists of primary dendritic α-Mg grains,
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largely divorced β-particles, and α+β lamellar phase, where α-phase is the solid solution of
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aluminum (Al) in Mg and the β-phase is the Mg17Al12 precipitate [24]. The distribution of βprecipitates can be tuned by employing the proper heat-treatment procedure. Conventionally, β-precipitates are initially dissolved into the α-matrix during solid solution treatment and then, redistributed in the structure through an aging treatment [25]. The time and temperature of aging treatment are crucial parameters for determining the type, size, and distribution of β-precipitates [25, 26]. The difference in the Al content of α and β phases results in different electrochemical activities. The electrochemical potentials of the α and β phases in a sodium chloride solution have been reported to be -1.3 V and -1.6 V, respectively [25]. Thus, upon exposure of AZ91 to a corrosive environment, a micro-galvanic couple is formed between the α matrix and β-
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Journal Pre-proof precipitate. In such a couple, the α matrix acts as the anode and is preferentially corroded while the β-precipitate is cathodically protected. Therefore, due to the difference in the corrosion rates of α and β phases, the topography of AZ91 alloy surface can be adjusted using chemical etching treatment. Yin et al. [27] reported a superhydrophobic behavior for chemically-etched AZ31 Mg alloy modified by silicone-based agents and found that a uniform nano/micro-porous structure traps air pockets and hence, provides a lotus-type superhydrophobicity. In addition, Liu et al.
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[28] used a laser treatment followed by the chemical etching and surface modification treatments in the DTS silane agent to improve the hydrophobicity of AZ91D Mg alloy, and
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showed that the formation of crater-like surface features resulted in the existence of petal-
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type superhydrophobicity. Therefore, it seems that the shape and distribution of surface
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features on the metallic surface determine the type of superhydrophobic behavior. Long et al. [29] reported the transition from the petal to lotus state by tuning the copper topography
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from above 90° to ~1°.
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through laser micro-machining treatment where the sliding angle of water droplets decreased
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So far, reports on the wettability behavior of Mg alloys have mostly focused on the use of different mechanical, chemical, and surface treatments to develop the superhydrophobic surfaces [17-21] and there is no report of the use of heat-treatment to alter the wettability state of the Mg surface. In this paper, we introduce a facile, low-cost and scalable method for producing superhydrophobic Mg surface together with tunable adhesive properties, having promising potential for large-scale applications such as electronic device housings, automobile parts, sporting equipment, etc. [30]. In this method, the shape and distribution of the topographic features of AZ91 Mg alloy were tailored using various heat-treatment procedures and subsequent chemical etching to fabricate different hierarchical nano/micro structures.
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Journal Pre-proof The surface characteristics of samples were determined by field-emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and confocal laser microscopy (CLSM). In addition, the surface wettability of the samples was systematically investigated by measuring the ACA, sliding angle (SA), and bouncing behavior of the water droplet.
2. Experimental procedures
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2.1. Sample preparation
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In the present work, an as-cast AZ91 Mg alloy ingot was used as the raw material. The chemical composition of the alloy in weight percentage (wt.%) was measured to be Al: 7.69;
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Zn: 0.95; Mn: 0.20; Si: 0.03; Cu: <0.01; Fe <0.01 and Mg: balance. A tubular furnace with an
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argon gas atmosphere was employed for solution annealing treatment. All samples were initially annealed at 413 °C for 24 hours to dissolve the intermetallic phases, and then
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immediately quenched into the water to form a supersaturated solid solution phase.
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Subsequently, the quenched samples were treated with aging at 190 °C for 8, 16, and 24 hours
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to precipitate different shape, size, and distribution of the second phase in their structure. The samples were labeled according to the employed heat-treatment cycle, as shown in Table 1. Table 1. Designation of the samples and heat-treatment cycles. Sample Thermal treatment Conditions as-cast
-
S-NA
Solid Solution @ 413 °C for 24 h
S-A8
Solid Solution → Aging @ 190 °C for 8 h
S-A16
Solid Solution → Aging @ 190 °C for 16 h
S-A24
Solid Solution → Aging @ 190 °C for 24 h
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Journal Pre-proof Prior to the chemical etching process, the samples were mechanically polished with 1000-grit SiC emery paper and ultrasonically cleaned with acetone, ethanol, and deionized water, in sequence. Subsequently, the samples were immediately etched in a two-step process [17], where they were first etched in 1 vol.% H2SO4 for 240 seconds and then immersed in 20 vol.% H2O2 aqueous solution for 120 seconds. After that, the samples were rinsed with deionized water and finally, were modified in an ethanolic solution stearic acid (0.05 M) at
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ambient temperature for 60 minutes.
2.2. Surface Characterization
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The crystal structure of as-cast sample was studied by X-ray diffraction (XRD) in standard
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Bragg–Brentano (theta-2theta) mode using a GNR Explorer diffractometer with a Cu-Kα radiation source (λ=1.5406 Å). The microstructure and surface morphology of the samples
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were examined by a Tescan MIRA3 FE-SEM. A confocal laser scanning microscope (CLSM,
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Olympus LEXT 3D Measuring Laser Microscope OLS4000) was utilized to analyze the
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roughness and surface topography of the samples. The free-ware Gwyddion code [31] and the Digital Surf MountainsMap® Premium software version 7 (trial version) [32] was used for
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image processing and analysis. A homemade contact angle instrument was employed to measure wettability. The ACA was measured for a deionized water droplet of volume 5 µL which recorded by a CCD camera. In addition, the sliding angle (SA) was measured by tilting the sample stage and recording when the drop began to move in the downhill direction. For each sample, both ACA and SA measurements were repeated at least three times at different regions on the surface. The bouncing behavior of selected samples was also evaluated using the water droplets with a volume of ~5 µL and a dropping height of ~20 mm at the temperature of 23°C. The droplets velocity at the point of impact was about 0.4 m.s-1. The dynamic behavior of the water droplets was recorded with time sequence of ~17 ms by a CCD camera. 6
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3. Results and discussion 3.1. As-cast sample The XRD pattern shown in Fig. 1 depicts that the as-cast sample consists of two phases: the α-Mg phase (ICCD: 00-035-0821), which is a solid solution of Al and Zn in Mg, and the precipitate of β- Mg17Al12 phase (ICCD: 00-001-1128) with an α-Mn type lattice. This structure is typical of AZ91 Mg alloys, where the β-precipitate is distributed in an α-matrix
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[33].
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Fig. 1. XRD scan for as-cast AZ91 Mg sample
The ACA value for the polished surface of the as-cast sample was 38°, which is in agreement with the typical ACA values for the smooth surface of metals [3, 34]. The smooth metal surfaces generally exhibit low ACA hydrophilic behavior due to their high surface free energy (SFE). The wettability behavior of smooth surface is usually represented by Young’s equation (Eq. 1) [35]: 𝛾𝑠𝑣 = 𝛾𝑠𝑙 + 𝛾𝑙𝑣 cos 𝜃𝐸
(1)
where γsl is liquid/solid interfacial energy or SFE, γlv is liquid/gas interfacial energy, γsv is solid/gas interfacial energy and θE is the ACA. Accordingly, the water droplet spreads out over the metal surface with high SFE, while the deposition of low SFE coatings on the 7
Journal Pre-proof surface of metals increases the water droplet ACA. It has been reported that the smooth Mg alloy modified by stearic acid showed a hydrophobic behavior with an ACA of 109° [17]. In addition, the hydrophobicity of modified surfaces can be further improved by tuning their surface roughness [36]. In particular, the formation of a hierarchical nano/micro roughness results in the trapping of air pocket between the surface and water droplets, and hence provides a superhydrophobic behavior with an ACA greater than 150°. Fig. 2 illustrates the ACA and SA measurements of the as-cast sample after chemical etching and surface
superhydrophobicity with the ACA of 150.9 ± 0.6°.
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modification treatments. It can be seen from Fig. 2a that the as-cast sample showed the
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The microstructural investigations shown in Fig. 3a revealed that the etched surface of the as-
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cast sample was composed of a numerous micro-features in a smooth matrix. However, a
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closer look at the surface microstructure showed that the micro-features and their surrounded region were covered by some needle-like and lint-like nano-features, respectively (Fig. 3b).
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The EDS measurements shown in Fig. 4(a) revealed that the Al/Mg atomic ratio in the matrix
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and micro-features are 0.29 and 0.59, respectively. Considering the preferential corrosion of
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Mg relative to Al, it can be stated that the matrix is a corroded α-Mg phase, consisting of more Al atoms compared to the intact α-Mg phase. In addition, EDS analysis showed the Al/Mg ratio for the micro-features is close to that of the β-Mg17Al12 precipitates. However, the presence of Zn in the precipitates indicated that some of the Al atoms in the Mg17Al12 compound were substituted by Zn and formed β-Mg17(Al, Zn)12 phase presumably since, compared to Mg, the atomic radius of Zn is closer to that of Al [37]. It has been reported that ternary intermetallic compound β-Mg17(Al, Zn)12 is formed in the Mg-Al-Zn system when the Al/Zn atomic ratio is greater than three [33, 37, 38]. Mathieu et al. found that the replacement of Al by Zn could reduce the electrochemical potential of β-precipitate from -1.3 V to -1.24 V, thereby increasing its chemical inertness [39]. The EDS elemental mapping image
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Journal Pre-proof displayed in Fig. 4(b) further confirmed that the observed micro-features and needle-like features on the surface of the as-cast sample contained a high concentration of Al compared to the matrix. In addition, the distribution of elemental Zn revealed a higher concentration of Zn in the precipitate compared to the matrix. This result is in consistent with the literature and indicates the tendency of Zn atoms to segregate to the precipitate phase [37]. It is well known that the β- precipitate is electrochemically more noble than the α-Mg phase [25]. Therefore, in the α/β couple, the α matrix acts as the anode, while the Al-rich β-
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precipitates are cathodic sites. Consequently, upon exposure of the as-cast sample to the H2SO4 solution in the first step of etching treatment, the α-Mg matrix is preferentially
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corroded and the β-precipitates are cathodically protected. Because of preferential dissolution
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of the α-Mg matrix, the β-precipitates protrude on the surface of the sample and form micro
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features.The preferential dissolution of α-phase in the α+β lamellar structure further explains the formation of needle-like structure observed in the area around the β-precipitate (Fig. 3b).
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In addition, the formation of lint-like features at the top of β-precipitates can be attributed to
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the selective dissolution of Mg from β- Mg17(Al, Zn)12 precipitates during the first step of
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etching followed by surface oxidation in hydrogen peroxide solution (H2O2) in the second step of chemical etching treatment. The surface oxidation of α-matrix and β-precipitates was confirmed by EDS results, where significant oxygen content was detected in their composition.
Fig. 2. Digital photographs for a) the ACA measurement and b) pinning behavior of a water droplet on the surface of the as-cast AZ91 magnesium alloy 9
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Fig. 3. (a) FE-SEM micrographs from the surface of the as-cast sample after chemical etching
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treatment, (b) enlarged view from surface features
Fig. 4. (a) EDS spectra of the matrix and precipitate phases in the as-cast sample (b) elemental EDS mapping of Mg, Al, Zn and O elements obtained from the white dashed square (The scale-bar is valid for all the micrographs) 10
Journal Pre-proof The formation of the aforementioned hierarchical nano/microstructure together with a low SFE stearic acid coating explains the superhydrophobic wettability in the etched as-cast sample. However, Fig. 2b showed that in the as-cast sample water droplets strongly adhere to the surface and hence, is classified as a surface with petal-type superhydrophobicity. The petal-state depends on the size and distribution of surface features and can occur when the droplet impregnates into the pores, while the hills and their surrounded region are still dry [40, 41]. For more clarification, the CLSM was used to analyze the topography of the as-cast
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sample, as shown in Fig. 5a. The results indicate that the numerous micro-features (βprecipitates) with an average width of ~22 µm and an average hight of 14.8 µm were
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uniformly distributed on the surface, where the mean distance between these features was
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115.4 µm. As schematically shown in Fig. 5b, a combination of high surface features and
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wide smooth area in a hierarchical nano/microstructure allows water droplets to fill the pores
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and hence, provided the petal-type hydrophobicity.
Fig. 5. a) 3D CLSM image from the surface of the as-cast sample after chemical etching treatment b) schematic illustration of liquid-solid contact area
3.2. Heat-treated samples Fig. 6 shows the effect of various heat-treatment cycles on the results of ACA and SA AZ91 Mg alloys. It is observed that solution annealing treatment at 413 °C for 24 hours significantly decreased the ACA of S-NA sample to 91.6 ± 0.6°. The surface morphology of the S-NA sample is shown in Fig. 7a. It can be seen that some micro-features are still 11
Journal Pre-proof observed on the surface of the S-NA sample, indicating that the solution treatment was not long enough to completely dissolve the β-precipitates. However, the corresponding high magnification FE-SEM micrograph shows that a volcano-like morphology is observed on the smooth surface and the nano- needle-like features have been disappeared. It seems that all βprecipitates were dissolved in the α+β lamellar phase during solution annealing treatment and thus the hierarchical nano/microstructure vanished. In fact, the dissolution of β-precipitates increased the Al concentration in the matrix and resulted in less difference in the galvanic
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potential between the matrix and residual β-precipitate. Therefore, solution annealing reduces the preferential corrosion of matrix during chemical etching and provides a smoother surface.
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These findings are supported by CLSM results, where the S-NA sample showed an RMS
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value of 3.08 µm, which is lower than the RMS of the as-cast sample (4.62 µm). These
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microstructural changes resulted in a significant decrease in the ACA of the S-NA sample compared to the as-cast sample.
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After the solid solution treatment, the α-matrix becomes super-saturated with Al, and hence,
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the aging treatment can be used to redistribute the β-phase in the α-matrix [26]. Fig. 6 shows
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that the aging treatment after the solid solution increased the ACA to values comparable with that of the as-cast sample. However, unlike the as-cast sample, the aged samples showed lower SA values. In particular, the ACA (SA) values of S-A8, S-A16, and S-A24 samples were 147.2 ± 0.7° (20.7 ± 0.8°), 159.3 ± 0.9° (2.6 ± 0.6) and 151.4 ± 0.6° (6.7 ± 0.5°), respectively. These results can be explained by the redistribution of β-precipitates during aging, which provides different surface morphologies after chemical etching, as shown in Fig. 7b-d. Accordingly, CLSM micrograph of Fig. 7c shows that the S-A16 has an RMS value of 7.31 µm, and its surface morphology consists of a uniform distribution of the microfeatures with various sizes. A high magnification SEM micrograph (Fig. 7d) also revealed that a continuous network of nano-features covers the whole surface of the S-A16 sample and
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Journal Pre-proof forms a hierarchical nano/microstructures. In fact, the micro-features observed in S-A16 samples are originally the undissolved β-particles that can grow up during aging treatment. In addition to these large β-particles, cellular growth of alternating α/β layers can also occur during aging within the grains (continuous precipitation) or at grain boundaries (discontinuous precipitation) [38]. The preferential corrosion of the α layer in this lamellar α/β structure leads to the formation of a uniform distribution of nano-features on the entire surface. The size and morphology of nano-features strongly dependent on the duration of
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aging treatment. Fig. 7b shows that the aging for 8h was not long enough to form a distinguishable cellular network of α/β layers, hence the low-intensity nano-features were
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distributed over the S-A8 sample surface. In contrast, the aging treatment for 24h resulted in
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the growth of β-precipitate and the formation of a coarse-lamellar structure with the sub-
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micron features rather than nano-features (Fig. 7c). As a result, a combination of highintensity micro-features and high-intensity nano-features in the S-A16 sample offered very
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rough hierarchical nano/microstructures with the highest ACA and the lowest SA. The effect
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of various intensities of roughness components on the wettability behavior will be discussed
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specifically in the next section using power spectral density (PSD) analysis
Fig. 6. The results of ACA and SA measurements for different samples 13
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Fig. 7. 3D CLSM and high-magnification FESEM images for (a) S-NA, (b) SA-8, (c) SA-16, (d) SA-24 samples
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Journal Pre-proof 3.3. Petal-state to the lotus-state transition mechanism The wettability analysis showed that the aged samples exhibited superhydrophobicity behavior similar to the as-cast sample. However, unlike the as-cast sample, which indicated the petal-type superhydrophobicity, the water droplets could easily run at low tilted angles on the surface of aged samples. In this regard, the S-A16 and S-A24 samples showed SA values less than 10°, and hence they represent lotus-type superhydrophobicity. Therefore, it can be
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stated that a transition from petal to lotus state in AZ91 Mg alloy can be accomplished by
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employing a suitable heat-treatment procedure
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In this section, we employed PSD analysis over a square area of 120 µm × 120 µm to find a correlation between various roughness components on the surface and the wettability
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behavior of samples. The PSD function provides information on the roughness components
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as the deviation of height from a mean plane and evaluates the lateral distribution/distance at which the height change occurs by using the following equation [42]: 2
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1 PSD(𝐟) = lim |∫ z(𝐫) exp(−2ði𝐟. 𝐫)d𝐫| A→∞ A A
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where z(r) represents the profile height, f is the spatial frequency in the x- and y- directions, A is the surface area of the measured field, and r is position vector. Fig. 8 presents the calculated PSD curves for the as-cast and heat-treated samples. The low-frequency region of the PSD spectrum is related to the nano-scale component of the roughness (nano-features), and the high-frequency region represents the micro-scale component of the roughness (microfeatures). It is obvious that the S-A16 sample depicted a higher spectrum roughness over the whole spatial frequency spectrum. Therefore, the S-A16 sample had the highest intensity of micro- and nano-features among all studied samples. Such a rough hierarchical nano/microstructure provided a high ACA value of ~160°. A decrease in the intensity of micro-/nano-scale components of the PSD curve revealed a weaker hierarchical
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Journal Pre-proof nano/microstructure, which corresponds to a lower ACA value. Comparing the results revealed that there is a correlation between the measured ACA values of the samples and the trend observed in the PSD curves, where a higher spectral roughness yields higher ACA. However, the PSD results were unable to explain the adhesive properties of the surfaces. For example, as-cast and S-A24 samples both showed similar PSD curves and ACA values, however, the former showed the petal-type hydrophobicity and the latter exhibited the lotustype hydrophobicity. In fact, the adhesive properties of surfaces depend not only on the
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intensity of the micro/nano features (PSD values at different frequencies) but also on the density and distribution of these features. Bhushan et al. [43] showed that a high density of
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nano-features in a hierarchical nano/microstructure resulted in the formation of air-pocket
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between the micro- and nano-features, thus reducing the adhesion of droplets to a significant
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level. Accordingly, we proposed a schematic model to show how the wettability behavior is affected by the surface morphology (Fig. 9). As mentioned earlier, the observed micro- and
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nano-features on the surface of the as-cast sample are derived from the largely divorced β-
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particles and α+β lamellar phase, respectively. Thus, the micro-features were homogeneously
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distributed on the surface of the as-cast sample, but the nano-features were only observed around the micro-features, and the gap between the micro-features was smooth (Fig. 9a). In such a structure, as schematically illustrated, the Cassie impregnated wetting state is operative, at which the droplet can wet the interspace space between the micro-features but due to the formation of air pockets, it cannot wet the nano-features. This structure provides a high surface adhesion due to the large solid-liquid interfacial area and leads to petal-type hydrophobicity [40]. Unlike the as-cast sample, the nano-features were homogeneously distributed on the surface of the S-A16 sample and covered the micro-features interspaces (Fig. 9b). These nano-features had a homogenous distribution because they originated during the aging from the continuous and discontinuous precipitation of the β phase within the grains
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Journal Pre-proof and at the grain boundaries, respectively. Therefore, water can wet neither nano-features nor micro-features interspaces; therefore, the adhesive force between the water and surface is negligible, and water droplets can be easily rolled off even at low tilting angles. This means that the transition from petal to lotus state on AZ91 surfaces occurred by redistribution of precipitates during the aging treatment. Consequently, the heat treatment can be employed as an effective way to tune the microstructure and subsequently determine the wettability
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behavior of the surface.
Fig. 8. PSD curves for different samples
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Fig. 9. Schematic illustration of the liquid-solid contact area and the surface morphology of
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3.4. Bouncing behavior
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(a) as-cast (b) SA-16 samples
Drops of water that hit the hydrophobic surface can bounce off or stick to the surface. Along
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with the CA and SA measurements, the drop-bouncing behavior on the surface is a critical parameter to characterize the wettability of the surfaces especially for the lotus-based applications [44]. Crick and Parkin [45] found that there was a linear correlation between CA and the number of bouncing cycles, where the water droplets bounce off only on the surfaces with CA over 151°. As shown in Fig. 10, the number of bouncing cycles for the S-A16 sample surface with the lotus-type hydrophobicity is three times, while no bouncing occurs on the as-cast sample with petal-type hydrophobicity. The latter can be attributed to the adhesive behavior of the surface which dissipates the kinetic energy of the droplet [46]. Nine et al. [47] reported that the bouncing characteristics of a surface drop depend on the surface morphology and the 18
Journal Pre-proof distribution of roughness components. Consequently, the existence of air pocket between surface features and water drops causes the drop to bounce off over the surfaces with lotustype superhydrophobicity [48]. On the other hand, for the surfaces with petal-type superhydrophobicity, the surface interaction between water droplets and the top surface area together with the frictional force between the water droplets and the wall of micro-features
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Pr
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causes high energy loss; thus, the water drops stick instead of bouncing [46, 48].
Fig. 10. Series of photographs of water droplet dropped from a height of 20 mm with an impact velocity of ~0.4 m.s-1 (a) sticking of the water droplet on the surface of the as-cast sample with the petal-type superhydrophobicity, (b) bouncing of the water droplet on the surface of S-A16 sample with the lotus-type superhydrophobicity.
4. Conclusion A hierarchical nano/microstructure on the surface of AZ91 Mg alloy was developed by the preferential dissolution of α-matrix in favor of β-precipitates during a chemical etching 19
Journal Pre-proof treatment. Accordingly, a combination of annealing and aging treatments was used to tailor the shape, size, and distribution of β-precipitates in the AZ91 microstructure and subsequently, fabricate various surface morphologies with different wettability properties. The PSD analysis revealed that a rough hierarchical nano/microstructure could provide superhydrophobicity properties, while a reducing the intensity of micro-features or nanofeatures surfaces may decrease PSD values and result in a significant decrease in ACA values. In addition, it was found that the distribution of nano-features determines the surface
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adhesive properties. Accordingly, the formation of an α+β lamellar structure around the largely divorced β-particles in the as-cast sample leads to an inhomogeneous distribution of
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nano-features and provides petal-type hydrophobicity, while an appropriate heat-treatment
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process can redistribute β-precipitates to form a homogeneous distribution of nano-features
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and subsequently create a lotus state on the AZ91 Mg alloy surface. Consequently, the results reported here provide a direct correlation between the microstructure and the wettability
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Acknowledgments
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desirable wetting properties.
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behaviors of AZ91 Mg alloy, which can be used to fabricate the AZ91 Mg surface with
The authors acknowledge financial support from grant 6919 (Students' Scientific Association of HSU). Additionally, the support from CEPLANT (Masaryk University, Brno, Czech Republic) is gratefully acknowledged. We also thank E. Ghanei and S.M. Katebi for their technical support to carry out part of this work at HSU. MA gives special thanks to R. Pouriamanesh, S. Ranjbar and A. Baghani for their help in editing the manuscript.
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Superhydrophobicity Induced by Multiscale Hierarchical Nanostructures, ACS Nano, 6
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(2012) 7656-7664. https://doi.org/10.1021/nn3032547.
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Journal Pre-proof CRediT author statement Masoud Safarpour: Investigation; Writing - Original Draft S. Alireza Hosseini: Conceptualization; Methodology; Resources; Writing - Review & Editing; Supervision; Project administration Fateme Ahadani-Targhi: Investigation; Writing - Original Draft Petr Vasina: Resources; Writing - Review & Editing Mostafa Alishahi: Conceptualization; Methodology; Resources; Writing - Review &
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Journal Pre-proof Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Journal Pre-proof Graphical abstract
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Highlights:
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Development of superhydrophobic Mg surface with tunable adhesive properties Formation of hierarchical nano/microstructure by selective etching Correlation between adhesive properties and distribution of β-precipitates within αMg matrix Investigation of petal-state to the lotus-state transition mechanism
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