Article Cite This: Langmuir 2018, 34, 7059−7066
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Laser Tailoring the Surface Chemistry and Morphology for Wear, Scale and Corrosion Resistant Superhydrophobic Coatings Ludmila B. Boinovich,* Kirill A. Emelyanenko, Alexander G. Domantovsky, and Alexandre M. Emelyanenko A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Leninsky prospect 31 bld. 4, 119071 Moscow, Russia
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S Supporting Information *
ABSTRACT: A strategy, combining laser chemical modification with laser texturing, followed by chemisorption of the fluorinated hydrophobic agent was used to fabricate the series of superhydrophobic coatings on an aluminum alloy with varied chemical compositions and parameters of texture. It was shown that high content of aluminum oxynitride and aluminum oxide formed in the surface layer upon laser treatment allows solving the problem of enhancement of superhydrophobic coating resistance to abrasive loads. Besides, the multimodal structure of highly porous surface layer leads to self-healing ability of fabricated coatings. Long-term behavior of designed coatings in “hard” hot water with an essential content of calcium carbonate demonstrated high antiscaling resistance with self-cleaning potential against solid deposits onto the superhydrophobic surfaces. Study of corrosion protection properties and the behavior of coatings at long-term contact with 0.5 M NaCl solution indicated extremely high chemical stability and remarkable anticorrosion properties.
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INTRODUCTION The technological application of aluminum−magnesium alloys in various areas is constantly growing. Currently, such alloys are actively used in the chemical industry for the manufacture of pressure vessels, in the production of water condensers, heat exchangers, water coolers, and freezers. In such applications, the surface of aluminum alloys undergoes intensive abrasive wear, becomes coated with scale, oil, deposits of salts and resins, oxidizes, etc. With the increase in the thickness of deposited layer, thermal and hydraulic resistance of equipment increases, heat exchange deteriorates, corrosion processes develop. Therefore, in the manufacture of equipment, it is necessary to impart additional functional properties for the materials surface that enable it to withstand the action of the factors noted above. One of the promising methods of increasing the service life of materials based on aluminum alloys is to impart their surface the superhydrophobic properties. Superhydrophobicity makes it possible to significantly improve the resistance to corrosion,1−5 biofouling,6−9 scale deposition,10 and to reduce the hydrodynamic resistance to flow.11,12 However, the main shortcomings of the superhydrophobic coatings are related to typically poor wear resistance13 and weak chemical stability14 of a layer of the hydrophobic agent atop of textured layers.14 Recently we have shown4 that the combination of laser chemical modification with laser texturing, followed by chemisorption of the fluorinated hydrophobic agent is a prospective way to significantly enhance the abrasive wear resistance with the simultaneous good chemical robustness of coatings in different exploitation conditions. In this study, we © 2018 American Chemical Society
will show that the developed approach can be effectively used for fabrication of long-standing wear resistant coating on the aluminum alloys for heat exchangers and water condensers. We will discuss, how the particular morphology, texture parameters, and composition allow enhancing the wear resistance and chemical stability.
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MATERIALS AND METHODS
Sample Preparation. The coatings were fabricated on the surface of aluminum−magnesium alloy with the composition (in weight %): Al 95.55, Mg 2.9, Mn 0.2, Cr 0.05, Cu 0.1, Fe 0.4, Si 0.4, Ti 0.1, Zn 0.2, and impurities 0.1. In this study, we used an Argent-M laser system (Russia) with an IR ytterbium fiber laser (wavelength 1.064 μm), which provides a wide choice of laser parameters, and a RAYLASE MS10 2-axis laser beam deflection unit (Germany). Laser treatment was performed at ambient conditions with humidity of 40−50% and temperature of 20−25 °C. Four types of laser surface treatment differing from each other by the number of sequential laser passes along the surface were used in this work. For each laser pass, we used a pulse duration of 50 ns, a repetition rate of 20 kHz, and a peak power of 0.95 mJ in TEM00 mode. The samples were raster scanned at a linear speed of 50 mm/s with a parallel line density of 50 mm−1. Laser beam diameter was focused onto a sample surface into a 40 μm wide (the 1/e2 level) focal spot with peak fluence of ≈19 J/cm2. This treatment, accompanied by laser ablation and subsequent deposition of nanoparticles, formed in the plasma, onto the treated surface, leads to the formation of Received: April 22, 2018 Revised: May 24, 2018 Published: May 25, 2018 7059
DOI: 10.1021/acs.langmuir.8b01317 Langmuir 2018, 34, 7059−7066
Article
Langmuir multimodal roughness on the aluminum alloy surface. The sample marked as S1 was treated by a single pass of the laser beam. The samples S2, S3, and S4 were treated with 4, 7, and 10 sequential laser passes, respectively. All as-prepared surfaces just after the laser treatment were superhydrophilic, which is verified by the rapid complete spreading of a water droplet along the surface. To transform the superhydrophilic state into the superhydrophobic one, we reduced the surface energy of the samples by chemisorption of fluorooxysilane. Additional pretreatment of the laser-textured surface was used before the chemisorption process, to enrich the surface with hydroxyl groups serving as chemisorption centers. To do this, the samples were exposed to UV−ozone treatment (Bioforce Labs) for 60 min. Exposure of the samples in the sealed vessel with saturated vapors of methoxy-{3-[(2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyl)oxy]-propyl}-silane for 1 h at T = 95 °C results in condensation of the hydrophobic agent inside the pores and to the formation of a thin layer atop the texture. Consequent drying for 60 min in an oven at 130 °C lead to the formation of a cross-linked layer of hydrophobic agent atop the laser-textured surface (Figure SI1 in the Supporting Information). Measurement Techniques. The morphology of the sample surface and cross-section was studied using a Supra 40 VP (Carl Zeiss, Germany) field emission scanning electron microscope (SEM) equipped with an INCA PentaFETx3 detector (Oxford Instruments, U.K.) for energy-dispersive X-ray spectroscopy (EDS). The SEM images were taken at 2−5 kV acceleration voltage, and the Everhart− Thornley detector was used for the detection of secondary electrons. EDS spectra were obtained at 5−8 kV acceleration voltage. We have analyzed the composition of the laser-treated surface layers in the areas corresponding to different distances from the surface because the variation in the composition has a very strong impact on the functional properties of the fabricated coating. To study the detailed composition, the quantitative EDX analysis was performed in the cross-section of each sample. Characterization of the wettability of the coatings was based on contact and roll-off angle measurements. We used a homemade setup based on digital video image processing of sessile droplets to analyze the droplet shape parameters15,16 to characterize wetting of the coatings. The initial contact angles and roll-off angles for 10 μL droplets were measured at 10 different surface locations for each sample. For measurement of the roll-off angle, after the droplet deposition and equilibration of droplet shape, the substrate holder was tilted smoothly until the droplet started to roll over the surface. To study the resistance of the superhydrophobic coatings to the cyclic impact of supersaturated and superheated water vapor, we have used a BTD17L-A autoclave (P&T medical). During each cycle, the samples placed inside the autoclave were subjected to supersaturated vapor treatment at T = 134 °C and pressure P = 2.1 bar for 5 min followed by exposure to saturated water vapor at T = 80 °C for 40 min. The antiscaling property of the superhydrophobic coatings was studied using homemade setup. In each experiment, the superhydrophobic samples and a bare sample were simultaneously placed in a closed vessel with a circulating solution to compare the efficiency of the superhydrophobic coating to mitigate scale fouling relative to a reference surface. The solution temperature was 70 °C, the circulation was provided by the peristaltic pump at a rate of 8.2 L/h. According to the literature,10,17 the test solution was prepared by dissolving 0.5 g/L NaHCO3 and 0.5 g/L CaCl2·6H2O in deionized water. The antiscaling property of samples was quantitatively characterized by variation in the contact and roll-off angles of samples subjected to above test for desired time intervals. The electrochemical properties of the fabricated coatings were studied using a potentiostat/galvanostat (Elins P50x+FRA 24M, Russia). Measurements were carried out at 23 °C in a three-electrode cell K0235 (Princeton Applied Research) with a 0.5 M NaCl aqueous solution as an electrolyte. A silver/silver chloride electrode (Ag/AgCl) filled with saturated KCl solution served as a reference electrode and a Pt mesh as a counter electrode.
Prior to the electrochemical measurements, the samples were immersed in the solution for a certain time. The potentiodynamic polarization curves were registered at a scan rate of 1 mV/s in the applied potential range from −550 to 300 mV. A sinusoidal perturbation signal with an amplitude of 10 mV (with respect to open circuit potential) was used for the electrochemical impedance spectroscopy measurements. Impedance spectra were acquired in the frequency range from 0.05 Hz to 100 kHz with a logarithmic sweep (20 points per decade). Corrosion potential, Ecorr, and current, icorr, were derived from the potentiodynamic polarization curves after Tafel extrapolation.
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RESULTS AND DISCUSSION As discussed in recent studies,4,18−21 laser surface treatment has several important advantages compared to many other surface texturing methods. Namely, the interaction of high power radiation with condensed media in the presence of atmospheric gases is accompanied by the chemical reactions. Surface compounds formed during surface treatment significantly affect the physicochemical properties of surface layers. Besides, an ablation of the material at high absorbed energies leads to the formation of a surface texture, whose characteristics are controlled by the parameters of laser treatment. Finally, the combination of grain refinement (caused by rapid laser heating) with high stresses (induced by rapid cooling of the surface layer) allows one to flexibly control the mechanical properties of the surface.18 Multiple surface raster scanning at appropriate parameters results in enhancement of desired functional properties.4 In this study, we have explored the difference in the properties of the designed surface texture, which appears after a varied number of laser passes. The values of the contact angles and the roll-off angles for asprepared samples (Table 1) indicate the remarkable superhydrophobicity for all samples with wettability parameters close to each other. Table 1. Wettability and Parameters of Surface Structure for the Superhydrophobic Samples Studied in this Work sample initial contact angle roll-off angle thickness of textured layer, μm thickness of the nitrogen-enriched layer, μm
S1 172 ± 2
