fabrication of superhydrophobic coating on stainless steel surface by ...

DOI 10.1007/s10946-015-9480-5

Journal of Russian Laser Research, Volume 36, Number 1, January, 2015

FABRICATION OF SUPERHYDROPHOBIC COATING ON STAINLESS STEEL SURFACE BY FEMTOSECOND LASER TEXTURING AND CHEMISORPTION OF AN HYDROPHOBIC AGENT P. N. Saltuganov,1,2 A. A. Ionin,2 S. I. Kudryashov,2,3 ∗ A. A. Rukhadze,4 A. I. Gavrilov,5 S. V. Makarov,2 A. A. Rudenko,2 and D. A. Zayarny2 1 Moscow

Institute of Physics and Technology (State University) Institutskiy per. 9, Dolgoprudny 141700, Moscow Region, Russia 2 Lebedev

Physical Institute, Russian Academy of Sciences Leninskii Prospect 53, Moscow 119991, Russia

3 National

Research Nuclear University MEPhI (Moscow Engineering Physics Institute) Kashirskoe shosse 31, Moscow 115409, Russia 4 Prokhorov

General Physics Institute Vavilov Street 38, Moscow 119991, Russia 5 Frumkin

Institute of Physical Chemistry and Electrochemistry Leninskii Prospect 31, bld. 4, Moscow 119071, Russia

∗ Corresponding

author e-mail: sikudr @ sci.lebedev.ru e-mail: pavel.saltuganov @ gmail.com Abstract

We determine the optimal regime of femtosecond IR laser nano- and micro-texturing of a stainless steel surface for subsequent multimodal surface relief texturing and chemical hydrophobization. We characterize the topological parameters of the modified surface and its wetting parameters after hydrophobization.

Keywords: femtosecond laser nano/microtexturing, stainless steel, superhydrophobic surface.

1.

Introduction

Superhydrophobicity is a property of solid material surfaces to exhibit extreme water repellency. Superhydrophobic surfaces demonstrate contact angles with water droplets typically higher than 150◦ [1]. Superhydrophobic materials exhibit a number of improved functional properties, such as water repellency, corrosion resistance, and self-cleaning of organic and nonorganic pollution. As a result, in recent years, the number of articles dedicated to different methods of fabrication of such superhydrophobic coatings on various surfaces has significantly increased [1–4]. Superhydrophobicity can be achieved by fabricating multimodal surface relief textures in the form of microscale structures covered by nanoscale structures, followed by chemisorption of hydrophobic chemical agent for lowering the surface energy. So far, different approaches were employed for preparing Manuscript submitted by the authors in English on January 21, 2015. c 2015 Springer Science+Business Media New York 1071-2836/15/3601-0081

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Journal of Russian Laser Research

Volume 36, Number 1, January, 2015

multimodal surface textures: chemical etching [5], electrochemical deposition [6], electroless replacement deposition [7], and photolithography [8]. Recently, femtosecond-laser ablative-surface texturing emerged as a promising method because of its high efficiency and lower costs. Fabrication of superhydrophobic durable functional coatings on construction material surfaces, e.g., on stainless steel surfaces under intense mechanical loads, is of a special interest. There is a number of studies describing fabrication of multimodal roughness texture on stainless steel surfaces for subsequent hydrophobization [2–4]. As the best example, water contact angles tending to 150–160◦ were achieved on a stainless steel surface exposed to femtosecond laser pulses, resulting in microcones covered by nanoscale asperities, at the expense of high total laser exposures at rather low processing speeds [2]. In this study, we search experimentally for efficient multimodal nano/microtexturing regimes for stainless steel surfaces exposed to ultrashort laser pulses. We carry out subsequent hydrophobization of the textured surface, processing highly superhydrophobic wetting properties.

2.

Experimental Details

Stainless steel specimens were irradiated in air with 1030 nm, 200 fs (full width at half maximum) pulses from a Yb-doped fiber laser (Satsuma, Amplitude Systems) [9]. Laser pulses with pulse energy up to 12 μJ (TEM00 mode) were focused onto the 32 μm wide (the 1/e level) focal spot on the specimen surfaces using a 35 mm focal-length lens. In single-shot ablation studies, one specimen was arranged onto a three-dimensional motorized translation microstage to be raster scanned at the linear speed V ≈ 6 mm/s at different pulse energies in order to produce a series of craters. The diameters and depths of single-shot ablative craters on the surface were measured using a NewView 6400s (ZYGO) optical profilometer (white-light interferometer) equipped with a 50× objective lens. In the texturing studies, the laser pulse energy was kept at 2 μJ and the other steel specimen was raster scanned at the same speed in a number of overlapping lines with the overlapping step Δ = 25 μm in order to produce a homogeneous 1×1 cm wide surface texture with regularly spaced trenches, where the number of accumulated laser pulses per spot was N ≈1300. The resulting surface textures were characterized by means of scanning electron microscopy (SEM, JEOL 7001F) in the relief mode with magnification up to 500,000. The surface hydrophobization after laser texturing was performed by chemisorption of hydrophobic agent, methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)-oxy]-propyl}-silane. For this purpose, the samples were immersed in 2% solution of hydrophobic agent in 99%-decane (Acros Organics) for 2 hours and then dried in an oven at 140◦ C for 60 min [10]. Characterization of the wettability of the coatings was based on the contact and rolling-angle measurements. We used the method of digital video image processing of sessile droplets. The homemade experimental setup for recording optical images of sessile droplets and software for the subsequent determination of the droplet parameters using the Laplace curve fitting routine were described earlier [11, 12]. To characterize the wetting of different coatings, the initial contact angles for 10–15 μl droplets were measured on five different surface locations for each sample. To measure the rolling angle, 10 μl droplets were deposited on the surface. After the initial droplet, the shape was equilibrated, manipulation with an angular positioner allowed us to change the sample surface tilt in a controllable manner and detect the rolling angle by averaging over five different droplets on the same substrate.

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3.


Results and Discussion

Ablation efficiencies for stainless steel irradiated by single ultrashort laser pulses of subpicosecond (0.3 ps) and short picosecond (3.6 ps) durations were studied to choose the optimum multi-shot texturing conditions. The obtained single-shot craters show that above some threshold fluence value Fspal , which slightly differs with the increase in the pulse duration, a thin layer (x ≈ 12 nm, Fig. 1) of the material was removed via spallation from the steel surface irrespectively of the pulse duration [13]. This regime can be of interest for precise surface processing due to the fact that the spallation layer thickFig. 1. Single-shot crater depth dependence on fluence ness is not influenced by variation in laser fluence for laser pulse widths of 0.3 ps () and 3.6 ps (). The in the range Fspal < F < Ffrag (Fig. 1), where Ffrag arrows show the corresponding spallation (F ) and spal is the fragmentation ablation threshold related to fragmentation (Ffrag ) thresholds. explosive hydrodynamic expansion of supercritical material fluid [14]. In the latter regime, at the minimum pulse duration of 0.3 ps, material removal was very intense, resulting in crater depths approaching to 30 nm in the laser fluence range between 0.5 and 1.2 J/cm2 . In contrast, for higher pulse durations, such strong-ablation fragmentation regime was not observed (Fig. 1). As a result, it is the fragmentation ablation mechanism that was applied in study to fabricate multimodal (nano- and micro-scale) surface relief texture at an optimum pulse duration of 0.3 ps, enabling highly efficient ablative fabrication of microtrenches covered with nanoscaled products of subsurface cavitation [14, 15] and laser ablation (Fig. 2). In the case of the single-shot ablation, the spallation threshold and the fragmentation threshold at 0.3-ps pulse duration were measured to be Fspal ≈ 0.29 J/cm2 and Ffrag ≈ 0.55 J/cm2 . However, for the given scanning speed, the pulse repetition rate and pulse energy optimum surface texturing were obtained for the laser fluence set at F ≈ 0.23 J/cm2 in trench centers. This became possible due to cumulative reduction of the ablation thresholds as a result of incubation effects during multi-shot ablation. Specifically, according to the incubation model of Jee et al. [16], the multi-shot ablation fluence threshold FN is related to the single-pulse ablation threshold fluence F1 by a power law, FN = F1 N S−1 , where N is the number of accumulated laser pulses and S is the accumulation parameter. The value of S for stainless steel in a femtosecond laser system obtained from the experiments is 0.86 [17], resulting, for N ≈ 1300 at the given scanning speed, in an effective fragmentation threshold fluence Ffrag,N ≈ 0.20 J/cm2 , which is exceeded in the trench centers for a peak incident fluence F ≈ 0.23 J/cm2 . The employed cumulative ablation provided by the total exposure per spot of ∼300 J/cm2 , i.e., almost by an order of magnitude in comparison with ∼2200 J/cm2 in the previous study [2], yielded a contact angle of 145◦ for the texture with microcone surface asperities of similar size. The fabricated surface texture with a trench period of 25 μm, representing the scan step Δ and depth about 20 μm covered with submicron-size particles, is shown in Fig. 2 a and b. Other SEM images demonstrate this texture after its hydrophobization (Fig. 2 b–d), with multiple randomly placed micro-scale asperities covered by nano-scale particles present on the ridges and almost absent within the trenches.

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Fig. 2. SEM images of periodical pattern of micro-trenches on the stainless steel surface textured by 0.3 ps laser pulses at F ≈ 0.23 J/cm2 and scanning speed V = 6.25 mm/s. The period of the ridges is about 25 μm. Front view of the surface after texturing (a) and side-view image of the surface after chemisorption of the hydrophobic agent (b). Multimodal roughness is observed on the surface in the form of microtrenches (c) covered by micro-sized aggregates of nanoscale particulates (d).

Wetting parameters of the final hydrophobized surface coating show a contact angle of 158.8 ± 4.0◦ (Fig. 3) and rolling angle of 21.8 ± 6.4◦ , with the former value significantly improved compared with the previous result of ≈145◦ [2].

4.

Conclusions

In conclusion, multimodal surface relief texture was fabricated in the scanning mode by IR subpicosecond laser pulses with parameters optimized from studies of single-shot femtosecond laser surface ablation of stainless steel. The texture is composed Fig. 3. Side-view optical image of a water microdroplet of regular surface microtrenches decorated by ag- on the superhydrophobic stainless steel surface. The gregates of nanoparticles. measured contact angle is Θ ≈ 158◦ .

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The ensuing hydrophobization of the surface texture provided significantly improved superhydrophobicity with a contact angle of 158◦ and a rolling angle of 20◦ .

Acknowledgments This study is supported by the Presidium of the Russian Academy of Sciences under Program No. 24. A.I.G. thanks the grant for the support of Leading Scientific Schools of the Russian Federation under Project NSh-2181.2014.3.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

L. B. Boinovich and A. M. Emelyanenko, Rus. Chem. Rev., 77, 583 (2008). S. Moradi, S. Kamal, P. Englezos, and S. G. Hatzikiriakos, Nanotechnology, 24, 415302 (2013). P. Bizi-Bandoki, S. Benayoun, S. Valette, et al., Appl. Surf. Sci., 257, 5213 (2011). B. Wua, M. Zhoua, J. Lia, et al., Appl. Surf. Sci., 256, 61 (2009). W. Song, J. J. Zhang, Y. F. Xie, et al., J. Colloid Interface Sci., 329, 208 (2009). N. Zhao, F. Shi, Z. Q. Wang, and X. Zhang, Langmuir, 21, 4713 (2005). Q. Wang, B. W. Zhang, M. N. Qu, et al., Appl. Surf. Sci., 254, 2009 (2008). L. Barbieri, E. Wagner, and P. Hoffman, Langmuir, 23, 1723 (2007). http://www.amplitude-systemes.com/satsuma-fiber-laser.html L. B. Boinovich, A. M. Emelyanenko, V. V. Korolev, and A. S. Pashinin, Langmuir, 30, 1659 (2014). A. M. Emel’yanenko and L. B. Boinovich, Instrum. Exp. Tech., 45, 44 (2002). A. M. Emel’yanenko and L. B. Boinovich, Colloid J., 63, 159 (2001). I. A. Artyukov, D. A. Zayarniy, A. A. Ionin, et al., JETP Lett., 99, 51 (2014). A. A. Ionin, S. I. Kudryashov, L. V. Seleznev, et al., J. Exp. Theor. Phys., 116, 347 (2013). A. A. Ionin, S. I. Kudryashov, A. E. Ligachev, et al., JETP Lett., 94, 266 (2011). Y. Jee, M. F. Becker, and R. M. Walser, J. Opt. Soc. Am. B, 5, 548 (1988). G. Raciukaitis, M. Brikas, P. Gecys, and M. Gedvilas, “Accumulation effects in laser ablation of metals with high-repetition rate lasers,” High-Power Laser Ablation VII, Proc. SPIE, 7005, 70052L (2008).

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