1. Introduction
Photon–electron interaction is a crucial technique used in surface science, materials science, and condensed matter physics. This phenomenon enables the study of surface properties at an atomic level [
1]. Consequently, the material undergoes modifications in surface morphology, chemical composition, and surface properties [
2]. Nanosecond lasers find use in many industrial applications; therefore, picosecond and femtosecond lasers are useful in applications that require high precision and minimal damage to the surrounding material. All three types of lasers have their advantages and disadvantages and complement each other in various industrial applications [
3]. Laser-based surface treatments can modify the surface structure of a material, leading to changes in its surface energy and wetting properties. Limited research has been conducted to examine the surface’s long-term durability against organic contamination [
4]. Currently, there is a limited amount of research on the utilization of CuBr vapor lasers for aluminum treatment. One of the studies in this area focuses on titanium ablation using a CuBr laser [
5], while another study in 2021 achieved a record high average laser power of 140 W through the use of an atomically self-terminating CuBr vapor laser [
6]. However, there is a lack of literature on the use of CuBr vapor lasers for material treatment and investigating roughness and wetting properties, as this has not been explored in previous studies. The present study aimed to investigate the effects of Copper Bromide vapor nanosecond laser treatment on the surface morphology, roughness, and wetting properties of aluminum 1050 and aluminum 2219 alloys. In a recent report, A.O. Ijaola et al. [
7] conducted a review on the surface wetting properties of different materials research published between the years 2003 and 2022. The report provides a comprehensive and critical analysis of superhydrophobic surfaces that have been laser textured on different substrate materials. In another review article, M.M. Quazi et al. [
8] provided a summary of the surface treatment and modification of aluminum and its alloys using laser systems such as CO2, diode laser, Nd:YAG, Yb-Fiber, Ruby, and Excimer. The article also offers an overview of the various techniques employed in the laser surface treatment of aluminum and its alloys. D.V. Zaitseva et al. [
9] reported on the behavior of water droplets on laser-modified surfaces of aluminum alloy (AMg6). The surfaces were modified using laser ablation with a 1064 nm Ytterbium nanosecond pulsed fiber laser. The article provides details on the experiment and the observed behavior of the water droplets on the laser-modified surfaces. M.V. Rukosuev et al. [
10] conducted research on the use of femtosecond laser ablation as a potential method for producing hydrophobic micro- and nanostructures on austenitic stainless steel (AISI 316L) surfaces. The authors successfully generated hierarchical structures on the surface, which led to superhydrophobic properties. Additionally, laser ablation of the surfaces resulted in dual-scale hierarchical structures, further enhancing the hydrophobicity to a 167° contact angle (CA). The study demonstrates the potential of femtosecond laser ablation as a method for creating hydrophobic surfaces with micro- and nanostructures on AISI 316L. L. Ruiz de Lara et al. [
11] conducted a study on the formation of laser-patterned ultrahydrophobic aluminum surfaces and evaluated their corrosion resistance using polarization curves and open circuit potential measurements. The article describes the process of creating these surfaces using laser patterning techniques and provides details on the testing methods used to assess their corrosion resistance. The study highlights the potential of laser patterning to create ultrahydrophobic surfaces with improved corrosion resistance properties. In another research study, Rafieazad Mehran et al. [
12] showed that surface hydrophobicity does not necessarily improve the corrosion resistance of a sample. The article presents a critical evaluation of the relationship between surface hydrophobicity and corrosion resistance and highlights the need for further investigation to better understand this relationship. Zhenhui Chen et al. [
13] used a picosecond laser to modify the surface of a pure aluminum sheet with a wavelength of 1064 nm, a repetition rate of 1000 kHz, and a scanning speed of 0.4 m/s. The laser treatment resulted in a superhydrophilic and light absorbent surface, which the authors suggest may be suitable for enhancing the efficiency of solar desalination. The study suggests that picosecond laser-treated aluminum surfaces have significant potential for sustainable desalination applications. As such, the utilization of a CuBr vapor laser to achieve similar results could be a promising area for further research. To conclude, limited research exists on the effects of dual-wavelength CuBr laser treatment on materials, but this treatment has shown potential for enhancing the wetting and roughness properties of aluminum alloys, making it useful in industries such as aerospace, automotive, and construction.
3. Results and Discussion
In this section, the plotted data trends for wetting, roughness, and derived variable values that were obtained by the treatment of materials using a CuBr vapor laser with different processing parameters could be observed. Trend lines with a higher R-squared coefficient, indicating a stronger correlation were chosen for all plotted data. A difference in the laser-applied parameters for the two processed alloys appears because of the parameters’ optimizations, which were made to obtain hydrophobic and hydrophilic properties on different aluminum alloy materials.
Two models can describe the correlation between surface roughness and hydrophobicity in the context of surface wettability. The Wenzel model predicts an increase in the apparent contact angle for hydrophobic surfaces and a decrease for hydrophilic surfaces as surface roughness increases. However, this model cannot describe the behavior of laser-textured surfaces that change the wettability of the surface from hydrophilic to hydrophobic. The Cassie–Baxter model assumes that droplets do not wet rough surfaces completely due to trapped air packs within the surface interstices, resulting in higher apparent contact angles on rough surfaces compared with smooth surfaces. The measured contact angles in this study are expected to fit the Cassie–Baxter model rather than the Wenzel model [
12].
The average energy density (
Ē) delivered to the laser-treated samples and the roughness and contact angles are plotted in
Figure 4. The plotted logarithmic trendlines have an R-squared value which indicates both a strong correlation and, in
Figure 4b, a moderate-to-strong correlation between the variables.
The initial roughness of the aluminum 2219 alloy was R
a = 0.318 µm, R
z = 4.19 µm, and R
q = 0.387 µm, and the contact angle was 68°. After laser processing with an average energy density of 6 J/mm
2 delivered on aluminum alloy 2219, the roughness increased to R
a = 5.52 µm, R
z = 44.51 µm, and R
q = 6.88 µm, and the contact angle reached a maximum of 134° (
Figure 4a). By increasing the average energy density by 16.67 times up to 100 J/mm
2, the roughness R
a increased by 3.03 times, R
z increased by 2.30 times, and R
q increased by 2.95 times. This change also affected the contact angle, which decreased by 6.70 times up to 20°. The wettability relative to the initial material surface properties decreased by 1.97 times when an average energy density of 6 J/mm
2 was applied, resulting in a hydrophobic surface. Conversely, an increase in wettability by 3.40 times was achieved when an average energy density of 100 J/mm
2 was applied, resulting in a hydrophilic surface.
The initial surface roughness of the aluminum 1050 alloy was measured as R
a = 0.156 µm, R
z = 3.97 µm, and R
q = 0.201 µm, and the contact angle was 73°. After laser processing with an average energy density of 0.88 J/mm
2, the roughness increased to R
a = 2.04 µm, R
z = 24.094 µm, and R
q = 2.437 µm. As a result, the contact angle also increased to a maximum of 145° (
Figure 4b). Further increasing the average energy density by 181.81 times to 160 J/mm
2 caused a significant increase in roughness, with R
a increasing by 11.55 times, R
z increasing by 5.74 times, and R
q increasing by 11.48 times. This change also affected the contact angle, which decreased by 20.74 times, reaching a value of only 7°. As a consequence of laser processing, the surface wettability was altered relative to the non-treated surface. Specifically, when an average energy density of 0.88 J/mm
2 was applied, the wettability decreased by 1.99 times, resulting in a hydrophobic surface. In contrast, by increasing the average energy density to 160 J/mm
2, the wettability also increased by 10.43 times, resulting in a hydrophilic surface.
Figure 5 shows that the 3D morphology obtained by laser scanning microscopy provides a reliable representation of the correlation between surface irregularities and wetting properties. In the sample with CA 130° (
Figure 5D), the surface roughness creates micro-pockets in which air becomes caught, resulting in a high contact angle of the water droplet with a roughness of R
a = 5.91 µm, R
z = 45.03 µm and R
q = 7.39 µm. Moreover, a decrease in the CA between the water droplet and the sample is accompanied by an increase in the number of the micro-channels, which are interconnected and allow air to escape laterally during droplet–surface contact, thereby accelerating the wettability of the droplet to <10° where the trend for superhydrophilicity is observable (
Figure 5A) with a roughness of R
a = 11.86 µm, R
z = 94.43 µm, and R
q = 15.29 µm.
Figure 6 data show that for the laser-treated specimen with CA 140° (
Figure 6D), the roughness of the surface creates small pockets where air gets trapped, leading to a high contact angle of the water droplet. The roughness values for this surface are R
a 1.90, µm, R
z 23.047 µm, and R
q 2.41 µm. It should be noted that a decrease in the contact angle between the water droplet and the sample is accompanied by an increase in the number of micro-channels that interconnect and allow air to escape laterally during droplet–surface contact, thus accelerating the wettability of the droplet. This results in a trend towards superhydrophilicity, as observable in
Figure 5A. Therefore, the specimen shown in
Figure 6A has roughness values of R
a 4.77 µm, R
z 44.35 µm, and R
q 6.07 µm.
In
Figure 7, the pulse overlap (
Kov) is plotted together with roughness and CA. The trendlines for
Figure 7a,b were exponential, with an R-squared value indicating a strong correlation for the Al 2219 alloy and a moderate correlation for the Al 1050 alloy.
After laser processing with a pulse overlap of 98% on aluminum alloy 2219, the roughness increased to R
a = 11.86 µm, R
z = 94.43 µm, and R
q = 15.29 µm, and the contact angle reached CA 10° (
Figure 7a).
By reducing the pulse overlap 1.05 times until reaching 93%, the roughness of R
a reduced by 2.14 times, R
z reduced by 2.12 times, and R
q reduced by 2.22 times. This change also affected the contact angle, which increased by 13.40 times up to CA 134° (
Figure 7a). The wettability relative to initial material surface properties decreased 1.97 times when a pulse overlap of 93% was applied, resulting in a hydrophobic surface. Conversely, an increase in wettability by 6.8 times was achieved when a pulse overlap of 98% was applied, resulting in a hydrophilic surface with CA 10°. Adjusting the pulse overlap during laser processing of aluminum alloy 2219 can significantly alter surface roughness and wettability, with a 93% pulse overlap resulting in decreased roughness and hydrophobicity and a 98% pulse overlap resulting in increased wettability and hydrophilicity.
After subjecting aluminum alloy 1050 to laser processing with a pulse overlap of 99%, the surface roughness increased to Ra = 23.564 µm, Rz = 138.299 µm, and Rq = 27.965 µm, and the contact angle was CA 7°. When the pulse overlap was reduced by 1.22 times to 83%, the surface roughness reduced by 11.55 times for Ra, 5.74 times for Rz, and 11.48 times for Rq. This change also had an impact on the contact angle, which increased by 20.7 times to CA 145°, and the surface became hydrophobic, with a decrease in wettability by 1.99 times relative to the initial material surface wetting. On the other hand, by applying a pulse overlap of 99%, the surface became hydrophilic, and the wettability increased by 10.42 times relatively to the non-treated sample. Modifying the pulse overlap during the laser processing of aluminum alloy 1050 can lead to significant changes in surface roughness and wettability, with a decrease in pulse overlap resulting in decreased roughness and increased hydrophobicity and an increase in pulse overlap resulting in increased wettability and hydrophilicity.
The changes that occur during laser processing depend on the material physical properties, chemical composition, and the surface properties. The same laser processing parameters utilized on two materials from different groups results in different properties occurring on the surface, especially where the main factor is related to the physical process of such technologies.