Surface-Tailoring and Morphology Control as Strategies for Sustainable Development in Transport Sector

Abstract

Surface wetting plays an important role in the corrosion protection processes of aerospace applications. Here, we demonstrate the use of ultrafast femtosecond (fs) laser processing techniques to tailor the wetting properties of aluminum (Al) substrates by creating diverse surface morphologies. Specifically, two distinct laser scanning methods—dot-hatching and cross-hatching—were employed to fabricate microstructures on the substrates. By varying the incident laser parameters, we confirmed that the resulting surface morphologies exhibit different wetting behaviors, spanning from hydrophilicity to hydrophobicity. Furthermore, time-resolved spreading tests validate that dynamic wetting behaviors can also be modified. This fs laser processing approach provides a straightforward, one-step fabrication method for effectively modifying the wetting properties of Al alloys.

1. Introduction

Metals, such as aluminum, titanium, and their alloys, are essential materials in critical industries such as aerospace, energy, and transportation due to their lightweight properties and high strength [1]. Aluminum alloys have been widely used in the aerospace industry in many structural components for aircraft, from fuselage frames to wing skins [1]. However, exposure to harsh environments, such as extreme temperatures, moisture, or corrosive chemicals, can lead to corrosion, which progressively erodes their surfaces and compromises their structural integrity over time. A variety of surface protection methodologies utilizing wet-chemical processes has been developed to effectively mitigate corrosion in various aluminum alloy substrates, contributing to improved durability and reliability in service [2,3,4,5].

To overcome this issue, surface engineering is essential for enhancing the durability of target materials and surfaces while preserving their bulk properties. Surface wettability determines how easily water can wet a surface, and, therefore, modifying it has shown great potential in reducing surface corrosion on metal substrates through water repellency or hydrophobicity [6]. Specifically, hydrophobic surfaces, typically defined by a water contact angle (WCA) higher than 90°, prevent water from lingering on the surface and initiating corrosion process. Surface coatings have been widely employed in conjunction with anodization processes to increase the WCA, thereby improving wetting properties and enhancing corrosion resistance [7,8].

Nevertheless, these coatings are susceptible to a variety of stressors including UV exposure, mechanical stresses, which can lead to fatigue failures, and thermal cycling, which can degrade their performance over time [8]. Additionally, many of the conventional surface protection methods employed in the contemporary aircraft industry rely on wet-chemical processes or the use of chemicals [2,3,4,5]. Despite the performance and self-healing properties of these coatings [9,10,11,12,13], there is a potential advantage in developing more sustainable solutions that utilize reduced quantities of chemicals and minimize reliance on wet chemistry-based processes.

Direct surface texturing, including chemical etching, involves creating micro- and nanoscale surface structures that enable the development of unique surface characteristics, including enhanced wettability and optical properties. Particularly, bio-inspired surface structures, like that of the lotus leaf, have shown an outstanding ability to increase the WCA, leading to hydrophobicity [14,15,16]. Hence, altering surface geometries eliminates the need for coatings or other chemical treatments, providing a more sustainable and environmental-friendly way to achieve the desired functionalities. Hierarchical surface structures with multiple length scales show increased performance over single scale microstructuring techniques. However, current state-of-the-art methods face challenges related to multi-step fabrication, process complexity, scalability, and durability which limit their practical implementation in industrial settings [7,8].

Laser ablation, a process involving the selective material removal from a target surface, offers a simple, one-step, chemical-free, and scalable method for rapidly modifying surface topologies at the nanoscale across macroscopic surfaces [17,18,19]. This technique presents significant opportunities for precise material modification and advanced surface engineering [20]. At the forefront of laser processing is femtosecond (fs) laser technology, characterized by its ultrashort pulse durations and highly limited energy deposition times [21,22]. Compared to longer-pulsed laser processing techniques (e.g., nanosecond lasers), fs lasers provide superior control over material removal and precise surface modification while minimizing heat-affected zones [23]. Importantly, fs laser processing on metallic substrates has been shown to alter wetting properties, potentially enhancing corrosion resistance in aerospace applications [24,25]. Femtosecond laser processing proved capable of modifying the contact angle of AMG aluminum alloy in a range of 34.6 to 131 degrees [26] and a range of 15 to 150 on 7075 aluminum alloy without post processing [27]. Angell et. al. reported contact angles up to 158 degrees on 6061 aluminum alloy through femtosecond laser processing and subsequent low-temperature annealing [28].

In this work, we demonstrate ultrafast fs laser processing on aircraft-relevant 6061 aluminum alloy to alter wetting properties for aerospace applications. This fs laser processing technique can generate different surface morphologies under different laser processing parameters, leading to a wide range of wetting behaviors, which could enhance the corrosion resistance. Surface morphology characterizations show that the microstructures can provide excellent water-repellent properties (hydrophobicity) for both freshwater and saltwater droplets, compared to pristine Al substrates that give hydrophilic properties. Time-resolved images show that the dynamic wetting properties are modified as well as the static contact angles. Our study provides the insight that this fs laser processing technique can be successfully employed to alter surface properties where multifunctional surface properties are required to achieve better performances.

2. Materials and Methods

2.1. Materials

6061 Aluminum alloy substrates were purchased from McMaster Carr (Chicago, IL, USA). As-received materials were used for fs laser processing without additional surface polishing. Laser-processed surfaces were cleaned in an ultrasonic water bath before testing and stored for at least a month in air to stabilize any surface reactions [29,30].

2.2. Saltwater Solution Testing

NaCl was added to de-ionized water to achieve a 5 wt-% concentration. The pH was adjusted in the range between 6.5 and 7.2 either with sodium hydroxide or hydrochloric acid aqueous solution. This solution was used to analyze how the texturized surfaces interact with the solution used for corrosion tests in salt spray chambers in accordance with the ISO 9227 standard.

2.3. Ultrafast Fs Laser Processing

A 500 fs laser with 1030 nm wavelength operating at a 200 kHz repetition rate (Tangor, Amplitude, San Francisco, CA, USA) was synchronized with a galvano scanner (excelliSCAN 14, SCANLAB from SCANLAB GmbH, Puchheim, Germany) and motorized XYZ stages (A-311, XY air-bearing stages with L-310 vertical stage, PI-USA).

2.4. Surface Morphology Characterization

A 3-dimensional surface profiler (VK-X3000, Keyence, Osaka, Japan) was used to characterize fabricated surface morphologies. The Keyence Multi Surface Analysis software (https://opendrop.readthedocs.io/en/latest/index.html, accessed on 20 March 2025) was used to further analyze the structural features and to obtain numerical values for surface roughness.

2.5. Water Contact Angle Measurements

Water contact angle measurements were performed using a custom goniometer system equipped with a high-speed camera (FASTCAM Nova S12, Photron, Tokyo, Japan), a 12× zoom lens (Navitar, New York, NY, USA), and motorized XYZ stages. A 4 μL droplet of either deionized or a saltwater solution was precisely dispensed onto each fabricated surface using a needle for consistent application. Static contact angle measurements were averaged out of 20 angles taken from each side of 10 droplets. Numerical values of the contact angle were extracted using OpenDrop contact angle goniometry software (https://opendrop.readthedocs.io/en/latest/index.html, accessed on 20 March 2025). Dynamic droplet measurements were taken at 2000 frames per second (fps) and characterized after the point of contact.

There were 12 trials taken for the static contact angle per sample, but I used a minimum of 10 measurements per sample to eliminate any outliers, so there is slight variation in the angles used per surface (meaning 20 angles when looking at both sides of the droplet). These data are summarized in the Supplementary Material.

Figure 1a shows the ultrafast fs laser processing setup employed to fabricate various surface morphologies. The system uses a 500 fs laser operating at a wavelength of 1030 nm and a repetition rate of 200 kHz, which is directed through a galvano scanner and focused onto Al substrates (see details in Section 2). For the substrates, 6061 aluminum alloy is selected due to its prevalent applications which include aerospace engineering. To control the incident laser power, a half wave plate and a polarizing beam splitter are integrated into the setup. The focused beam spot size is 30 μm.

3. Results and Discussion

When exposed to sufficiently high fluence fs laser pulses, metal surfaces undergo complicated ablation dynamics involving material removal through plasma plumes, nanoparticles, and melt expulsion [18,21]. The remaining molten material and redeposited particles subsequently solidify, forming hierarchical surface structures composed of micro- and nanoparticles. The resulting surface morphology is highly influenced by laser processing parameters, such as the number of pulses and the scanning strategies applied.

Accordingly, to achieve diverse surface morphology architectures, two laser scanning strategies are utilized, as shown in Figure 1b. The dot-hatching method (Figure 1b, top) involves firing a fixed number of pulses per location (PPLs) at the same spot, allowing more precise and selective material removal from the target surface. Conversely, the cross-hatching method (Figure 1b, bottom) scans the laser beam in perpendicular directions, enabling efficient processing of relatively larger areas with a given number of laser pulses. These methods represent alternative patterns that are both simple to program and implement using a commercially available fs laser source, and each has a tailorable characteristic morphology. The dot-hatching pattern forms an array of craters with structural profiles similar to that of the incident beam, and the cross-hatch creates more grid-like structures with high near-micron roughness from overlapping pulses. The finer details of these structures have a large effect on performance, and they can be tailored by changing the input variables for the patterning, such as the scan speed or PPLs, the laser power, and the hatch geometry (spacing).

Directly after laser processing, all manufactured structures showed a strong increase in hydrophilicity, due to the formation of a surface oxide layer. In order to minimize post processing steps, samples were aged in ambient air for over a month before contact angle testing. This allowed organic molecules to adsorb onto the structures, which lowers the free surface energy and increases the resulting measured contact angles. Our experience and the existing literature show that this final wetting state tends to be stable after the month of initial aging [31]. Samples underwent similar laser processing procedures and identical post processing, so any wetting discrepancies should be a result of topological differences.

Figure 2 shows the surface morphologies fabricated under different laser processing conditions and their corresponding wetting properties, determined by water contact angle measurements. Static contact angle measurements were averaged out of 20 angles taken from each side of 10 droplets. As shown in Figure 2a, the pristine Al substrate exhibits an inherent roughness of 0.33 μm and a contact angle of 96.8 ± 5.2°. The experimental values obtained in this study are consistent with the data documented for pristine Al 6063 alloys [32], which exhibit a comparable compositional profile to that of the Al 6061 alloy utilized in the current investigation, particularly in terms of their alloying elements and structural characteristics. Laser-fabricated Al microstructures produced with a laser power of 6 W, 500 PPLs, and 80 μm spacing show an ablation depth of 50 μm, a crater diameter of 60 μm, and a contact angle of 117.6 ± 4.6° (Figure 2b). Similarly, microstructures created using a laser power of 6 W, 200 PPLs, and 90 μm spacing have an ablation depth of 40 μm, a crater diameter of 60 μm, and a significantly lower contact angle of 30.5 ± 8.7°, as presented in Figure 2c. Contact angle hysteresis values were also obtained for these surfaces, and the surfaces had values of $∆\theta =8.2, 14.6, \mathrm{a}\mathrm{n}\mathrm{d} 7.6$ for Figure 2a–c, respectively. These results demonstrate that the wetting properties (hydrophobicity or hydrophilicity) of Al substrates are significantly influenced by the incident laser processing parameters.

Although the laser-patterned structures shown in Figure 2b,c look similar on the scale of the craters, subtle differences in the topology can induce modified wetting properties. More specifically, the area roughness of the hydrophilic surface was measured to be 6.87 μm, while the hydrophobic surface had a roughness of 13.44 μm. Both values are much greater than the value for the bare aluminum surface of 0.33 μm, but they elicit different wetting regimes. On a surface with a given surface energy, a sufficient projected area is needed to suspend a droplet, but increased roughness below that threshold tends to increase wettability [33].

The hydrophobic surface can suspend the droplet in a Cassie state, where it rests on a heterogeneous surface of both air and water, increasing the macroscopic contact angle and preventing water from saturating the surface following $cos\left({\theta }_{CB}\right)=\phi \left(cos\right(\theta )+1)-1$ [34]. Where φ is the solid fraction of the surface in contact with the liquid, θ is the intrinsic contact angle for a homogeneous surface of the material of interest, and θ_CB represents the contact angle of the air–surface composite. The increased roughness on the hydrophilic surface leads to a reduced contact angle, and the droplet is in a Wenzel state, where it penetrates into the morphology, and wetting scales directly with increased roughness following $cos\left({\theta }_W\right)=r\ast cos\left(\theta \right)$, where r is the surface roughness and θ_W is the macroscopic contact angle [35]. In terms of the contact angle hysteresis ($∆\theta$), the nanoscale protrusions leading to the highest contact angle also tend to increase the chance of droplet pinning at the edges, whereas the smoother crater structure does not have a significant effect on ∆θ.

Figure 3 depicts the results related to the contact angle measurements. The contact angle between the solid surface and the liquid can vary depending on the chemical composition of liquids and is a function of their surface tension. Of interest to this work, the metal corrosion process can be expedited when electrolytes, such as saltwater, are present [8]. Therefore, it is important to verify contact angle stability in the presence of saltwater. We characterize the water contact angles for freshwater and saltwater for the surface that is fabricated using the cross-hatching method shown in Figure 3a. This structure is clearly different from the topology of the craters created using the dot-hatch, but it also modifies the wetting performance. These different topologies may offer multifunctional improvements in other desirable material properties, and the processing can be tailored to achieve multiple goals simultaneously. The saltwater recipe was created to follow the ISO 9227 standard used for corrosion testing in the aerospace industry [36] but deposited as single droplets following the same experimental procedure as the freshwater tests. Figure 3b and c show that the use of saltwater does not affect the hydrophobic nature of the structure, and the saltwater has a greater contact angle than the freshwater droplet. Because the water repellency is maintained, the surface maintains improved corrosion resistance in more harsh, salty environments.

Laser processing can also be used to modify the dynamic wetting behavior of a surface. The hydrophilic and bare surfaces identified in Figure 2a,c were imaged at the moment of droplet contact using a high-speed camera at 2000 fps to examine the dynamic spreading behavior, and snapshots can be seen in Figure 4a. Although the spread upon contact is similar, the rate of spread is clearly modified in the latter frames. Figure 4b,c show the droplet width and contact angles measured as a function of time for each of these frames. The average rate of spread was also calculated using these data and averaged over both sides of each droplet. The bare surface had an average spread rate of 200 mm/s, and the hydrophilic surface had a rate of 480 mm/s over the first 6 ms after contact. For surfaces in the Wenzel regime, the roughness provides a capillary force that drives fluid into the structure, increasing the rate of spread [37]. Increasing the hydrophilicity on aluminum substrates can also impart corrosion resistance in certain environments, and this has been shown by corrosion tests [38].

4. Conclusions

In this work, we fabricated microstructured 6061 Al alloy to alter the wetting properties that can contribute to enhancing corrosion resistance. It was demonstrated using static and dynamic droplet experiments that multiple laser processing strategies can provide a wide variety of morphologies with different wetting properties. The performance seen in freshwater droplets carried over to saltwater experiments that were performed to simulate a more corrosive environment. Contact angle measurements showed that the surfaces could be tuned for either hydrophilic or hydrophobic behavior, and both have shown potential for corrosion resistance in different scenarios. Furthermore, the structures fabricated in this work can be created at high speeds and without the need for harsh chemicals or coatings that have increased environmental impacts. The performance of this process as a sustainable corrosion process will be addressed in a future publication.