Combination of Functional Nanoengineering and Nanosecond Laser Texturing for Design of Superhydrophobic Aluminum Alloy with Exceptional Mechanical and Chemical Properties Ludmila B. Boinovich,*,† Evgeny B. Modin,‡,§ Adeliya R. Sayfutdinova,† Kirill A. Emelyanenko,† Alexander L. Vasiliev,‡,∥ and Alexandre M. Emelyanenko† †
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Leninsky prospect 31 bldg. 4, 119071 Moscow, Russia National Research Centre “Kurchatov Institute”, Pl. Akad. Kurchatova 1, 123182 Moscow, Russia § Far Eastern Federal University, 8 Suhanova St., 690090 Vladivostok, Russia ∥ Shubnikov Institute of Crystallography, Federal Scientific Research Centre “Crystallography and Photonics”, Russian Academy of Sciences, 119991 Moscow, Russia ‡
S Supporting Information *
ABSTRACT: Industrial application of metallic materials is hindered by several shortcomings, such as proneness to corrosion, erosion under abrasive loads, damage due to poor cold resistance, or weak resistance to thermal shock stresses, etc. In this study, using the aluminum-magnesium alloy as an example of widely spread metallic materials, we show that a combination of functional nanoengineering and nanosecond laser texturing with the appropriate treatment regimes can be successfully used to transform a metal into a superhydrophobic material with exceptional mechanical and chemical properties. It is demonstrated that laser chemical processing of the surface may be simultaneously used to impart multimodal roughness and to modify the composition and physicochemical properties of a thick surface layer of the substrate itself. Such integration of topographical and physicochemical modification leads to specific surface nanostructures such as nanocavities filled with hydrophobic agent and hard oxynitride nanoinclusions. The combination of superhydrophobic state, nano- and micro features of the hierarchical surface, and the appropriate composition of the surface textured layer allowed us to provide the surface with the outstanding level of resistance of superhydrophobic coatings to external chemical and mechanical impacts. In particular, experimental data presented in this study indicate high resistance of the fabricated coatings to pitting corrosion, superheated water vapor, sand abrasive wear, and rapid temperature cycling from liquid nitrogen to room temperatures, without notable degradation of superhydrophobic performance. KEYWORDS: laser treatment, superhydrophobic coating, wear resistance, oxynitrides, nanocontainer, corrosion resistance, resistance to thermal stresses luminum alloys are commonly used in many fields of application such as the automotive industry, shipbuilding, aviation and space technology, construction, and medicine. In each of these applications it is highly desirable and, in many cases, necessary to impart peculiar properties to the surface of alloys depending on exploitation conditions. In particular, the simultaneous provision of high corrosion resistance, electrical insulation, antifouling properties, improved wear resistance, and self-healing together with water-repellent surface properties is required for materials used for medical and marine
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© XXXX American Chemical Society
applications. For aviation and space technology, improved wear resistance and anti-icing resistance come to the fore, together with water repellency. An approach based on a surface treatment that leads to achievement of the superhydrophobic state of any metallic surface has recently received a lot of attention. When applied to Received: July 3, 2017 Accepted: September 5, 2017 Published: September 5, 2017 A
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the laser treatment regime to obtain the desired chemical composition and structure of surface textured layer, described here for aluminum alloy, can be effectively used to design durable superhydrophobic coatings for a wide variety of materials.
traditional aluminum-based materials, this approach addresses many classic shortcomings of aluminum alloys, making it possible to impart the desirable multifunctional properties to the surface of these materials. Although the scientific community is optimistic about obtaining such coatings for a “material that maintains superhydrophobicity as it wears away”1 and for a wide class of metallic materials, including aluminum alloys, there are still a lot of problems related to the design of superhydrophobic coatings robust against mechanical loads and electrochemical impacts typical of really harsh exploitation conditions.2−7 In this study, using the aluminum-magnesium (AMG) alloy as an example of widely spread metallic materials, we will show that surface engineering based on the nanosecond laser chemical processing and texturing,8 followed by chemical surface modification by hydrophobic agents, can be used as a versatile and powerful tool for controlling the mechanical and chemical properties of a material with a superhydrophobic coating at the nanoscale. As shown in recent literature,9−16 laser surface texturing has received a wide application for manufacturing surfaces with multimodal (hierarchical) roughness and high curvature of texture elements. It is worth noting that the laser texturing is not the only method of physical treatment of the surfaces, allowing one to impart the hierarchical roughness to the material surface. Thus, for example, it was shown that highintensity ultrasound provides controllable metal processing with the manipulation of morphology, composition, crystallinity, and some mechanical properties of metal surfaces for different applications.17−19 However, the morphology of the coatings obtained on the basis of high-intensity ultrasound treatment17−19 and of the laser surface texturing employed here is very different. For the ultrasound treatment, the sponge-like textures with concave curvature of main texture elements (pores) are characteristic. In contrast, in the morphology of surfaces, produced by nanosecond laser ablation, prevails the micro- and nanoelements (particles and their aggregates) with convex curvature.9,11,12 As it was shown recently,20 the former type of textures is more appropriate for the design of superhydrophilic surfaces, while the latter gives essential advantages for obtaining the durable superhydrophobic coatings. The most important point of the approach for fabrication of superhydrophobic coatings developed here consists in the simultaneous effective use of surface laser texturing, applied to impart multimodal roughness, for task-oriented modification of the physicochemical properties of a thick surface layer of the material itself. This strategy for producing surfaces with multimodal roughness and appropriate chemical composition of texture elements is of particular importance for the fabrication of wear and corrosion-resistant coatings on aluminum alloys. Here it is worth noting that in general, aluminum alloys are not considered as intrinsically durable under abrasive wear or resistant to pitting corrosion in mineralized halide solutions. We will show that the appropriate choice of nanosecond laser surface texturing regime of AMG alloy results in the formation of multilayers with nanocavities, filled with hydrophobic agent and aluminum oxynitride nanoinclusions in different sublayers of the textured layers. The unique combination of nano and micro features of the hierarchical surface obtained in this study by a combination of physical and chemical methods for surface layer modification provides the surface of AMG alloy with high resistance to electrolyte corrosion and superheated water vapor, sand abrasive wear, and rapid temperature cycling from liquid nitrogen to room temperatures without notable degradation of superhydrophobic performance. It should be noted that the strategy of tuning
RESULTS AND DISCUSSION Surface Structure and Composition. Two types of samples with surface layers modified by nanosecond laser texturing under different treatment regimes will be discussed in this paper. We will compare the properties of samples textured with the same set of laser beam scanning parameters in the open atmosphere, where the content of nitrogen and oxygen is typical of ambient conditions. The sample treatment regimes differ from each other in the number of sequential laser passes over the sample surface. Hereinafter, the sample fabricated using a singlepass laser treatment will be referred to as an SPLT sample, and that obtained by 10 repetitive consecutive passes as an ILT (intensive laser treatment) sample. The secondary electron planview SEM images for the two sample types show the survey morphology of the surfaces (Figure 1a,b). SEM images of the top
Figure 1. Secondary electron plan-view SEM images of the surface of samples textured by (a) single-pass laser treatment (SPLT) and (b) intensive laser treatment (ILT).
and cross-section views of the initial sample are presented in Figure S1 in the Supporting Information. The difference in surface morphology of SPLT and ILT samples is better elucidated by cross-section imaging (Figure 2a,b). In each sample, the position of the initial surface before laser processing can be clearly traced (marked by thick white arrows in Figure 2a,b); this feature allows us to quantify changes in morphology. The heights of the redeposited material above the initial surface level h1 and the depths of grooves relative to the initial surface h2 of both samples are given in Table S1 in the Supporting Information. Laser treatment induced changes in the composition of the surface layer compared to the bare surface; minor for SPLT sample and significant for ILT sample. For the latter regime, the sequential cycles of rapid melting/cooling lead to surface layer quenching and grain refining. Indeed, in our recent study, broadening of the aluminum peaks in the sequence untreated alloy → SPLT sample → ILT sample was shown by comparison B
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Figure 2. Element (phase) distribution EDX maps for ILT (a) and SPLT (b) samples. Thick white arrows indicate the vertical position of initial surface of the sample; h1 and h2 are the height of the redeposited material above the initial surface level and the depth of grooves relative to the initial surface, respectively. The yellow color corresponds to the oxygen-containing layers. (c and d) The cross sections that were obtained using FIB cutting from the top regions marked by gray rectangles with the letter “t” in (b) and (a), respectively.
of XRD patterns.21 Calculations using the Debye−Scherrer formula indicated a gradual decrease in Al crystallite sizes in the sequence ∼350 nm (for an untreated sample) → ∼311 nm (for SPLT sample) → ∼263 nm (for ILT sample). In this study, we analyzed the composition and the structure of both the top parts of textured samples and their side parts (within the grooves). Such separate analysis was important because of various possible types of contact between the superhydrophobic coatings and aggressive media at different contact times. Energy dispersive X-ray spectroscopy (EDX) elemental analysis and mapping reveal an oxygen-enriched layer atop of laser-modified surfaces (yellow areas in Figure 2a,b), which is induced by hightemperature oxidation of laser melted surface layers in the presence of atmospheric oxygen. For more accurate analysis of the composition of the surface layer and to avoid artifacts from mechanical polishing, several sections were prepared by focused ion beam (FIB) methods. Such sections for SPLT and ILT samples are shown in Figure 2c,d. The microstructure is characterized by the presence of a relatively dense layer directly on the surface of the aluminum alloy appearing as a bright band and a highly porous layer formed by redeposition of the material. The microstructure of the oxidized layers above the initial surface level is determined by the parameters of the ablation process. Thus, in the case of ILT sample, which was obtained by multiple laser passes, the amount of redeposited material on top of the ridges is approximately twice the value for SPLT sample obtained by the single-pass method (Table S1). The cyclicity of processing in the former case leads to the formation of several similar oxygen-containing layers (Figure 2d). The microstructure and composition of the surface layers for each sample were studied in more detail by transmission electron microscopy (TEM)/EDX/ electron energy loss spectroscopy (EELS) methods. Thin specimens were prepared for TEM by FIB from the side of the grooves from the areas marked by “s” in Figure 2a,b. The bright-field
TEM images of the structure of oxidized layers for these regions are shown in Figure 3a,b. The observed thickness of the formed structure in the case of SPLT sample is approximately 160 ± 10 nm (Figure 3a) and about 110 ± 10 nm for ILT sample (Figure 3b). These side surface layers can be divided into two parts differing in contrast, density, and morphology. The structure observed in the cross section of SPLT sample consists of a relatively dense layer with a thickness of ∼40 nm and a highly porous layer of redeposited material on the surface with a thickness of ∼120 nm. This redeposited layer is formed by nanoparticles deposited from the ablation plume formed during short laser pulses. The thickness of the outer layer of ILT sample (Figure 3b) is 40 nm with capsule-shaped pores, while the denser layer is much thicker, about 70 nm. Such capsule-shaped pores form in a deeper part of the redeposited layer due to cyclic melting/cooling during multipass laser treatment. The higher thickness of the dense part of the surface layer results in noticeably better corrosion-protective properties, as will be shown below. The pore size distribution for the surface layers of samples was estimated using segmentation of TEM/HRTEM images (Figure S4 in the Supporting Information). Quantitative EDX analysis in the areas marked in Figure 3a,b shows the formation of a layer with a high content of O and N (points 2 and 3) on the surface of the alloy (point 1). This composition of the textured layers has a very strong impact on the functional properties of the fabricated coating. For SPLT sample, the O content in the porous layer is more than twice as high relative to the dense layer (53 and 20 at. %, respectively, Figure 3c). In the case of multipass laser processing (ILT sample), the O content in the outer porous part (point 3 in Figure 3 b,d) is ∼30 at. %, which is less than in the corresponding layer of SPLT sample. However, the N content in the outer layers for both samples is the same (15−16 at. %). At the same time, the area enriched by both O and N propagates more deeply inside C
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Figure 3. TEM images of the oxidized layer structure for SPLT (a) and ILT (b) samples in cross sections prepared by FIB; markers (crosses) indicate points of EDX spectra collection. (c and d) EDX spectra at specified points for SPLT and ILT samples, respectively.
ILT sample. Several competing processes take place during multipass laser treatment, such as high-temperature interaction between the melted surface layers and droplets of molten Al and its suboxides in the laser ablation plume with atmospheric N2 and O2,22,23 simultaneous ablation, melting, recrystallization, and quenching of the Al alloy at the surface. Cyclic surface processing of ILT sample results in mixing of the O−N-enriched inclusions from the surface with the melted Al substrate, followed by recrystallization and quenching and eventually in formation of a thick layer enriched with oxygen and nitrogen. The structure of oxidized layers on top of the ridges in ILT sample formed during the process of material redeposition above the initial surface level is shown in more detail in Figure 4a. The observed periodic structures with density fluctuations are formed due to cyclic laser processing. Inspection of the EDX spectra obtained at points 1 and 2 (Figure 4) reveals a difference in the elemental composition, which may indicate the presence of a mixture of alumina (Al2O3) and aluminum oxynitride (AlON) phases. The capsule-shaped porous layers (point 1) show higher N content (Figure 4b). To clarify the phase composition of the oxidized layers on top of the ridges in ILT sample, we carried out EELS measurements,
which unambiguously indicate the presence of Al oxynitrides. The corresponding spectra are shown in Figure S2 in the Supporting Information. A high-resolution TEM (HRTEM) study was performed to analyze crystal structure, and the results are presented in Figure 5a together with the corresponding fast Fourier transforms from the selected regions (Figure 5 b,c). The results indicated that the AlON compound adopted a cubic crystal structure with unit cell parameter a = 7.89 Å. This corresponds to the data for Al2O3 (ICSD 66559) and AlON phases (ICSD 70032), which exhibited a cubic crystal structure (space group FD3̅m) with unit cell parameters a = 7.911 Å and a = 7.95 Å, respectively. Analysis of the selected area electron diffraction patterns (Figure S3) also indicated the cubic crystal structure of the layers, with space group FD3m ̅ and unit cell constant a = 7.9 Å. The observed nanocrystallinity of the oxidized layers on top of the ridges in ILT sample additionally confirms very fast kinetics of surface cooling and plays an important role in the mechanical properties of the fabricated coating. Hydrophobic Nanolayer and Water-Repellent Properties. The surface morphology of SPLT and ILT samples described D
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Figure 4. STEM image of ILT sample prepared from the region marked with arrows on the left insets. Periodic porous layers formed by the cyclic redeposition process (a). EDX spectra and the results of composition measurement at points 1 and 2 (b).
It was important to study the resistance of the layer of chemisorbed fluorooxysilane to hydrolysis and desorption from the surface on continuous or prolonged contact with an aqueous medium. One of the most effective ways to determine the chemical stability of superhydrophobic surfaces is to characterize the behavior of their water-repellent properties after exposure to superheated and supersaturated water vapor. The experimental data of Figure S5 show negligible degradation of the superhydrophobic state for both samples after 30 cycles corresponding to 150 min of total contact time with supersaturated and superheated vapors and 1200 min of exposure to saturated vapors at T = 80 °C. The data prove the extremely high chemical stability of fabricated coatings in the harsh conditions under consideration. Since it was shown in the literature that the kinetics of hydrolysis of the Si−O−Me bond (where Me stands for any metal, such as Al in our case) is accelerated in highly concentrated alkali halide solutions,24,25 we studied the durability of superhydrophobic properties of two types of coatings fabricated in this study in continuous contact with a 3 M KCl solution for 24 h. In Figure 6, the time evolution of contact angle and surface tension of a droplet of solution is presented. For comparison, we also show the data measured for a droplet of solution deposited onto a polished AMG sample with a chemisorbed fluorooxysilane layer (curves 3 in Figure 6). Very high stability of the superhydrophobic state for both types of superhydrophobic samples during 24 h of contact with a 3 M KCl solution is evidenced by the constancy of the contact angle. In addition, the behavior of the droplet surface tension, which remained constant for the entire duration of the experiment for a droplet on ILT sample and almost constant for a droplet on SPLT sample, indicates high resistance of chemisorbed fluorooxysilanes to hydrolysis and desorption for the considered period. In contrast, the surface tension for a droplet of solution deposited onto the hydrophobic sample noticeably decreases in time. The slight increase in the contact angle at the initial stages of contact results from deterioration of the droplet surface tension. Subsequent diminution of the contact angle indicates the formation of tiny hydrophilic defects at the coating/ solution interface associated with desorption of fluorooxysilane molecules from the surface due to hydrolysis. The significantly higher stability of SPLT and ILT samples compared to the hydrophobic sample is related to two factors. On the one hand,
Figure 5. HRTEM image of the oxidized layer structure (a). Fourier spectra from the indicated regions showing different orientations of AlON (γ-Al2O3) nanocrystals (b and c).
above contains micro- and nanoelements of texture, indicating the formation of multimodal roughness. To intensify the very attractive physicochemical properties of the fabricated texture and to ensure very high stability of the superhydrophobic state of the coatings, we used chemisorption from the vapor phase of fluorooxysilane molecules with long fluorocarbon tails and three terminal functional groups. Controlled preliminary grafting of surface hydroxyl groups serving as chemically active centers allowed the formation of a wellorganized cross-linked 2D layer of fluorooxysilane molecules with low surface energy atop the textured layer. Contact angles of water droplets above 170° and roll-off angles of 1−2° (Table S2), measured for coatings fabricated according to the two laser treatment regimes, described above indicate achievement of the superhydrophobic state with a very small fraction of wetted area on contact of the fabricated coatings with aqueous media. Chemisorption of the same fluorooxysilane layer onto a finely polished (with mirror reflection) AMG alloy plate results in an advancing contact angle of 120.6 ± 1.8° and receding angle of 102.5 ± 2.2°, respectively. The above value of the quasi stationary advancing contact angle may be considered an estimate of the Young contact angle for the laser textured samples. E
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Figure 6. Evolution of the contact angle (a) and the surface tension (b) of a droplet of 3 M KCl aqueous solution in time of contact with SPLT (1) and ILT (2) superhydrophobic samples and with a hydrophobic sample (3).
surface of SPLT sample indicates the appearance of wetting defects related to desorption of fluorooxysilane as a hydrophobic agent from the surface. Based on the wettability data, we can expect much better corrosion resistance of ILT sample compared to SPLT sample. To characterize the corrosion resistance of our samples, we studied the electrochemical parameters of samples immersed in a 3 M KCl solution for a specific time. Figure 7 shows the
the fraction of solid surface for the superhydrophobic samples contacting with liquid is on the order of a few percent, thus decreasing the probability of hydrolysis and desorption. On the other hand, thermal treatment during laser texturing of the surface suppresses the hydrolytic activity of the Me−O−Si bond for the chemisorbed molecules.21,26 Corrosion Resistance on Long-Term Contact with Highly Corrosive Electrolytes. For practical applications, it is very important to characterize the stability of the superhydrophobic samples subjected to immersion in electrolyte solutions for a more prolonged period. To elucidate the effect of peculiar nanotexture and composition, we monitored wettability parameters and corrosion properties for samples contacting with a 3 M KCl solution for 30 days. The corrosion resistance of metallic samples with superhydrophobic surfaces in corrosive media is determined by the transport properties of the layer of hydrophobic molecules, the barrier properties of the underlying material, and its interaction with the aggressive aqueous medium. As mentioned above, immersion of a superhydrophobic surface in an aqueous solution is accompanied by hydrolysis of the hydrophobic agent and its partial desorption from the surface with the formation of wetting defects. On the one hand, such defects lead to deterioration of the superhydrophobic state and to local or complete (in the case of high defect concentration) transition to the homogeneous wetting regime.24 This effect is enhanced by an increase in the concentration of aggressive ions. On the other hand, a mass transfer of components of the solution occurs through these defects, resulting in an extensive localized corrosive attack on the aluminum surface. This is why a flaw-free state of a layer of hydrophobic molecules is of crucial importance for longevity of the functional properties of superhydrophobic materials, and a detailed study of the evolution of wettability is necessary for better understanding of corrosion processes for metallic materials protected by superhydrophobic coatings. Significantly better chemical stability of the layer of fluorooxysilane chemisorbed onto ILT sample after 30 days of immersion in a 3 M KCl solution is shown by the higher contact angle (Figure S6). The roll-off angle of 17°, characteristic of ILT sample, is considerably lower than the value of ≈30° measured for SPLT sample, corroborating the preservation of the superhydrophobic state of ILT sample with a small number of wetting defects. In contrast, the high roll-off angle with droplet pinning when rolling on the
Figure 7. Polarization curves for different samples contacted with a corrosive 3 M KCl for 10 days: (1) superhydrophobic SPLT sample; (2) superhydrophobic ILT sample; (3) smooth hydrophobic sample, and (4) smooth untreated AMG alloy sample. Line (2′) is a polarization curve for a superhydrophobic ILT sample contacted with the same solution for 30 days.
polarization curves for bare (hydrophilic), smooth hydrophobic, and superhydrophobic SPLT and ILT samples. A comparison of the polarization curves for samples contacting with a corrosive solution for 10 days indicates enhancement of corrosion protection properties for the smooth hydrophobic sample compared to the bare aluminum alloy surface due to the presence of a layer of fluorooxysilane. Comparison of the polarization curve for the hydrophobic sample with the curve for superhydrophobic SPLT sample immersed in the solution for 10 days shows further notable improvement in corrosion resistance. However, we can conclude very high corrosion resistance only for the F
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Figure 8. Mapping of chemical elements performed by EDX microanalysis for the top region of ILT sample marked by a gray rectangle with the letter “t” in Figure 2a. Distribution of fluorine content indicates deep penetration of hydrophobic agent into the pores, whereas noticeable amounts of nitrogen supports the conclusion on formation of a thick surface layer enriched with aluminum oxynitrides.
superhydrophobic ILT sample, which shows a corrosion current of 5 × 10−9 A/cm2. Moreover, the appreciable difference between pitting and corrosion potentials, on the order of 0.7 V, indicates significant protective properties against pitting corrosion. To estimate the durability of corrosion protection properties of ILT sample, we measured the polarization curves for this sample up to 30 days of immersion in corrosive solution. More prolonged immersion of ILT sample did not cause a perceptible increase in the corrosion current, which after a month of continuous immersion in 3 M KCl remained
