SCIENCE CHINA Technological Sciences • RESEARCH PAPER •

November 2010 Vol.53 No.11: 2972–2975 doi: 10.1007/s11431-010-4142-x

Preparation of biomimetic hydrophobic coatings on AZ91D magnesium alloy surface LIU Yan1,2*, LI Liang1, LU GuoLong1 & YU SiRong2 1

Key Laboratory for Engineering Bionics, The Ministry of Education, Jilin University, Changchun 130022, China; 2 College of Materials Science and Engineering, Jilin University, Changchun 130022, China Received August 10, 2010; accepted August 28, 2010

The hydrophobic coating has been a promising technology for improving surface performance. The surface performance of magnesium alloy has been limited in application. Furthermore, the hydrophobic of magnesium alloy is rarely investigated because magnesium alloy is an active metal alloy. In this paper, inspired by microstructure character of typical plant leaf surface such as lotus, the biomimetic hydrophobic coatings on AZ91D magnesium alloy surface were prepared by means of wet-chemical combining electroless. The samples were immersed into AgNO3 solution in wet-chemical method firstly. Then, biomimetic hydrophobic coatings were prepared by electroless after wet-method pretreatment. The microstructure was observed by SEM and the contact angles were measured by contact angle tester. The results indicated that the biomimetic hydrophobic coatings with uniform crystalline and dense structure could be obtained on AZ91D magnesium alloy surface. The results of contact angle revealed that the biomimetic nano-composite coatings were hydrophobic. The wet-chemical method treatment on the AZ91D magnesium alloy substrate provided a rough microstructure, thus improving adhesion of the coating and the substrate. biomimetic, hydrophobic, coatings, magnesium alloy Citation:

1

Liu Y, Li L, Lu G L, et al. Preparation of biomimetic hydrophobic coatings on AZ91D magnesium alloy surface. Sci China Tech Sci, 2010, 53: 2972−2975, doi: 10.1007/s11431-010-4142-x

Introduction

The surface properties of materials are very important, and researchers attempt all sorts of methods to obtain materials surfaces with required properties. With the development of bionic engineering, researchers are paying an increasing attention to biological surface in order to understand how the nature can solve engineering problems. The extensive investigations on biological surfaces have revealed that these surfaces have many unusual properties. The “lotus-effect” is a typical phenomenon that the natural surface structure as blueprint is used to design and fabricate engi*Corresponding author (email: [email protected]) © Science China Press and Springer-Verlag Berlin Heidelberg 2010

neering materials surfaces. The binary microstructure of lotus surface endows super-hydrophobicity, which can adapt to environmental conditions well [1–3]. In recent years, the super-hydrophobic biomimetic surfaces have been studied extensively due to the necessity of self-cleaning materials, micro-fluid device and others [4–6]. The bio-inspired super-hydrophobic surfaces are prepared by many methods according to physical and chemical principles, such as lithography [7], template method [8], sublimation [9], electrochemical methods [10], layer-by-layer methods [11, 12], bottom-up approach for fabrication of nano-arrays [13] and so on. However, researchers usually fabricate hydrophobic films on metal materials and inorganic materials surfaces with the stable chemical property. Consequently, reactive metal and their alloy surfaces are tech.scichina.com

www.springerlink.com

LIU Yan, et al.

Sci China Tech Sci

rarely investigated. Magnesium is one of the lightest engineering materials. Thus, it is expected that magnesium and its alloys will be used in aerospace, aircraft, automobile, and railway applications [14]. A hydrophobic coating would be a promising technology for improving surface performance. Jiang et al. [15] fabricated a super-hydrophobic surface on a Mg-Li alloy through chemical etching, followed by immersion and annealing processes using fluoroalkylsilane (FAS) molecules. Similarly, Ishizaki et al. [16] created a super- hydrophobic surface on a magnesium alloy by immersion in a cerium nitrate aqueous solution (20 min). Jun et al. [17] created a stable biomimetic super-hydrophobic surface on magnesium alloy fabricated by microarc oxidation pretreatment and followed by chemical modification based on lotus effect. Li et al. [18] prepared magnesium thin films by bias magnetron sputtering. In this paper the biomimetic hydrophobic surface was prepared by wet-chemical method combining electroless on AZ91D magnesium alloy substrate.

2973

November (2010) Vol.53 No.11

followed by rinsing with a large amount of distilled water (50 mL) for three times. 2.4

Electroless

After being rinsed in distilled water, the samples were immersed in the electroless solution for plating Ni-P deposition layer. The electroless solution was taken in a 1000 mL glass beaker which was kept at constant temperature by a thermostat. The bath composition and all operation parameters for electroless Ni-P deposition are listed in Table 2. 2.5

Surface characterization

The surface morphologies of the obtained samples were observed by the scanning electron microscope (JSM5500LV, Japan Electronic). Static water contact angles of the prepared surfaces were estimated with a contact angle meter (JC2000A Powereach, China).

3

Results and discussion

2 Experimental 2.1

Sample preparation

The AZ91D magnesium alloy was chosen as experimental material, whose chemical composition was shown in Table 1. The test plates were cut into 10 m×10 mm coupons and polished to 1.5 μm mirror finish. The surface was first ground with No.1000 SiC paper, followed by No.1500, No.2000, and finally 1.5 μm diamond suspension in oil. The polished specimens were degreased by sonicating in acetone for 10 min, and then methanol for 10 min with a final distilled water rinse. 2.2

Acid activation

Acid activation was performed by immersing the polished, degreased samples in an aqueous solution of 125 g/L CrO3 and 110 mL/L HNO3 at room temperature for 30 s. This step was followed by rinsing in distilled water. 2.3

Wet-chemical method

Following acid activation, the samples were immediately immersed into the AgNO3 solution with different concentrations (from 0.01 to 0.05 M) for a certain time (from 5 s to 2 min) at room temperature (about 298 K). This step was Table 1

The typical SEM images of the products obtained with different concentrations of AgNO3 solution and different reaction times are shown in Figures 1 and 2. The different microstructures were formed due to variety of experiment conditions. As seen in Figure 1(c), under a higher magnification SEM images, the porous structures were clearly shown when concentration of AgNO3 was 0.05 mol/L and reaction time was 5 s. However, Figure 2(c) showed some little dendritic structures when concentration of AgNO3 was 0.01 mol/L and reaction time was 30 s. It is well known the magnesium alloy is an active metal and the speed of reaction is very high when it is immersed into AgNO3 solution. When the AgNO3 solution was 0.05 mol/L, the speed of reaction was too fast for the silver on the magnesium alloy to grow, so only the porous structure was formed after the magnesium alloy was corroded. When the AgNO3 solution was 0.01 mol/L, the reaction time became slow, allowing a preferential growth of dentrites on the magnesium alloy substrate, which were formed after Ag deposition. These results indicate that the concentration of AgNO3 solution and the reaction time play an important Table 2 The operation parameters of electroless Ni-P Operation

Electroless Ni-P

The composition of AZ91D magnesium alloy

Plating bath compostion NiSO4·6H2O

16 g/L

NaH2PO2·H2O

14 g/L

NaC2H3O2

13 g/L

HF (40%)

12 mL/L

Materials

Mg

Al

Zn

Mn

Si

Fe

Ni

NH4HF2

8 g/L

AZ91D

Bal.

8.534

0.522

0.208

0.016

0.002

<0.001

Stabilizer

0.001 g/L

Condition Temperature

pH6.4±0.2

82±2°C

2974

LIU Yan, et al.

Figure 1

Figure 2

Sci China Tech Sci

November (2010) Vol.53 No.11

The SEM images of the products obtained by AgNO3 solution (0.05 mol/L, 5 s). (a) 500×; (b) 2000×; (c) 5000×.

The SEM images of the products obtained by AgNO3 solution (0.01 mol/L, 30 s). (a) 500×; (b) 2000×; (c) 10000×.

role in the formation of microstructure on the magnesium alloy substrate. Figure 3 showed the morphology of the electroless Ni-P coating. In the coating deposition process, nickel polycrystalline modules of predeposition formed the microscopic nodular and resulted in “edge effect” so that the current density increased and deposition rate accelerated. Thus, the role of feedback loop exerted, ultimately cellular organization of “cauliflower head” was formed on the surface coatings [19, 20]. Figure 4 showed the morphology of the biomimetic composite coating. The little dendritic structures make magnesium alloy substrate rough and enhance coalescent force between magnesium alloy and Ni so that the biomimetic coatings become fine and compact. Figure 5 showed the contact angles of different coatings. Surface wetting properties relies on the surface structure particles. It ws found that when the microscale bumps were used to form coating, the surface exhibited highly hydrophilic properties with a contact angle (CA) close to 108°(Figure 5(c)), which was obviously different from that of the untreated one (53°) (Figure 5(a)). Based on the morphology shown in Figure 5(c), it was believed that the structures of microscale bumps created more vacancies among individual nanostructures to trap air [21–24]. The coating was finer and more compact when the samples were pretreated in the 0.01 mol/L AgNO3 solution than in the 0.05 mol/L AgNO3 solution. Therefore, the contact angle of coating (pretreated in 0.05 mol/L AgNO3 solution) was only 87°. To understand the origin of the observed high hydrophobicity, we have described the CA in terms of the Cassie–Baxter equation [25], cosθr = f1cosθ0 − f2, where f1

Figure 3 The SEM image of electroless Ni-P coating.

Figure 4 The SEM image biomimetic composite coating (pretreated in AgNO3 0.01 mol/L).

LIU Yan, et al.

Sci China Tech Sci

November (2010) Vol.53 No.11

2975

Figure 5 The contact angles of different coatings. (a) The Ni-P coating (untreated); (b) the biomimetic composite coating (pretreated in 0.05 mol/L AgNO3 solution); (c) the biomimetic composite coating (pretreated in 0.01 mol/L AgNO3 solution).

and f2 are the fractions of solid surface and air contacting with liquid, respectively, θr and θ0 are the CAs on the bionic surface (108°) and on the Ni-P coating surface (53°), respectively. Since f1+f2=1, f1 is calculated to be 0.41. The low value of f1 suggests that the microscale bumps are responsible for the high hydrophobicity. In another word, this result indicates that the obtained microstructures can be used as highly hydrophobic and bionic surfaces.

8 9 10 11 12

4

Conclusions 13

1. The biomimetic composite coatings were fabricated by wet-chemical method combining electroless on AZ91D magnesium alloy surface. 2. The crystallization of biomimetic nanocompostie coatings were more uniform and the microstructure was denser than Ni-P electroless coatings (untreated). 3. The contact angles of the biomimetic composite coatings were measured and the maximum reached 108°, which is hydrophobic. The main reason for hydrophobicity is the microstructure of surface and the CA.

14 15 16

17 18

This work was supported by the National Natural Science Foundation of China (Grant Nos. 50635030, 50905071), the Program for the Development of Science and Technology of Jilin Province (Grant No. 20090539), and the Postdoctoral Science Foundation of China (Grant No. 20100471247).

19 20

1 2 3 4 5 6 7

Nosonovsky M, Bhushan B. Biologically inspired surfaces, boadening the scope of roughness. Advan Funct Mater, 2008, 18: 843–855 Koch K, Bhushan B, Barthlott W. Diversity of structure, morphology and wetting of plant surfaces. Soft Matter, 2008, 4: 1943–1963 Gao L C, McCarthy T J. The “lotus effect” explained: Two reasons why two length scales of topography are important. Langmuir, 2006, 22: 2966–2967 Min W L, Jiang B, Jiang P. Bioinspired self-cleaning antireflection coatings. Advan Mater, 2008, 20: 3914–3918 Nakajima A, Hashimoto K, Watanabe T, et al. Transparent superhydrophobic thin films with self-cleaning properties. Langmuir, 2000, 16: 7044–7047 Roach P, Shirtcliffe N J, Newton M I. Progess in superhydrophobic surface development. Soft Matter, 2008, 4: 224–240 Shiu J Y, Kuo C W, Chen P L, et al. Fabrication of tunable superhydrophobic surfaces by nanosphere lithography. Chem Mater, 2004, 16:

21 22 23 24 25

561–564 Vayssieres L. Growth of arrayed nanorods and nanowires of ZnO from aqueous solutions. Advan Mater, 2003, 15: 464–466 Nakajima A, Fujishima A, Hashimoto K, et al. Preparation of transparent superhydrophobic boehmite and silica films by sublimation of aluminum acetylacetonate. Advan Mater, 1999, 11: 1365–1368 Zhang X, Shi F, Yu X, et al. Polyelectrolyte multilayer as matrix for electrochemical deposition of gold clusters: toward super-hydrophobic surface. J Am Chem Soc, 2004, 126: 3064–3065 Zhai L, Cebeci F C, Cohen R E, et al. Stable superhydrophobic coatings from polyelectrolyte multilayers. Nano Lett, 2004, 4: 1349–1353 Niu J, Shi F, Liu Z, et al. Reversible disulfide cross-linking in layer-by-layer films: Preassembly enhanced loading and pH/reductant dually controllable release. Langmuir, 2007, 23: 6377–6384 Shi F, Niu J, Liu Z, et al. To adjust wetting properties of organic surface by in situ photoreaction of aromatic azide. Langmuir, 2007, 23: 1253–1257 Mordike B L, Ebert T. Magnesium-properties-applications-potential. In: Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing, 2001, 302: 37–45 Liu K S, Zhang M L, Zhai J, et al. Bioinspired construction of Mg-Li alloys surfaces with stable superhydrophobicity and improved corrosion resistance. Appl Phys Lett, 2008, 92: 183103 Ishizaki T, Saito N. Rapid formation of a superhydrophobic surface on a magnesium alloy coated with a cerium oxide film by a simple immersion process at room temperature and its chemical stability. Langmuir, 2010, 26: 9749–9755 Jun L A, Guo Z G, Fang J, et al. Fabrication of superhydrophobic surface on magnesium alloy. Chem Lett, 2007, 36: 416–417 Xiang X, Fan G L, Fan J, et al. Porous and superparamagnetic magnesium ferrite film fabricated via a precursor route. J Alloy Comp, 2010, 499: 30–34 Liu Y, Ren L Q, Yu S R, et al. Influence of current density on nano-Al3O2/Ni+Co bionic gradient composite coatings by electrodeposition. J U Sci Tech Beijing, 2008, 15: 633–637 Liu Y, Yu S R, Ren L Q, et al. Process parameters optimization of Ni-TiO2 nano-composite coating by electrodeposition on AZ91HP magnesium alloy. In: Conference Proceedings of 2009 International Institute of Applied Statistics Studies, Qingdao, 2009. 368–372 Feng L, Li S H, Li Y S, et al. Super-hydrophobic surfaces: From natural to artificial. Advan Mater, 2002, 14: 1857–1860 Tadanaga K, Morinaga J, Matsuda A, et al. Superhydrophobic-superhydrophilic micropatterning on flowerlike alumina coating film by the sol-gel method. Chem Mater, 2000, 12: 590–592 Pacifico J, Endo K, Morgan S, et al. Superhydrophobic effects of self-assembled monolayers on micropatterned surfaces: 3-D arrays mimicking the lotus leaf. Langmuir, 2006, 22: 11072–11076 Liu Y, Yu S R, Ren L Q, et al. Wettability of a biomimetic non-smooth coating by water. J Wuhan U Tech (Mater Sci Ed), 2009, 24(suppl.): 75–78 Cassie A B D, Baxter S. Wettability of porous surfaces. Trans Farad Soc, 1944, 40: 546–551