Stable Superhydrophobic Aluminum Surfaces Based on Laser-Fabricated Hierarchical Textures

Abstract

Laser-microtextured surfaces have gained an increasing interest due to their enormous spectrum of applications and industrial scalability. Direct laser interference patterning (DLIP) and the well-established direct laser writing (DLW) methods are suitable as a powerful combination for the fabrication of single (DLW or DLIP) and multi-scale (DLW+DLIP) textures. In this work, four-beam DLIP and DLW were used independently and combined to produce functional textures on aluminum. The influence of the laser processing parameters, such as the applied laser fluence and the number of pulses, on the resulting topography was analyzed by confocal microscopy and scanning electron microscopy. The static long-term and dynamic wettability characteristics of the laser-textured surfaces were determined through water contact angle and hysteresis measurements, revealing superhydrophobic properties with static contact angles up to 163° and hysteresis as low as 9°. The classical Cassie-Baxter and Wenzel models were applied, permitting a deeper understanding of the observed wetting behaviors. Finally, mechanical stability tests revealed that the DLW elements in the multi-scale structure protects the smaller DLIP features under tribological conditions.

Keywords: aluminum 1050; direct laser interference patterning; direct laser writing; single-and multi-scale textures; superhydrophobicity.

Figure 1

Schematic drawing of the used DLW setup, the local laser beam energy distribution (a) and the corresponding processing strategy (b). The used DLIP setup including the optical elements and the four-beam interference energy distribution (c) and the DLIP processing strategy depicting the dot-like interference pattern in the focal zone (d) are shown.

Figure 2

SEM images of the single-scale DLW and DLIP textures with a spatial period of 60 µm and 3.4 µm, respectively. The hexagonally arranged dot-like DLW textures were fabricated using a laser fluence of 6.56 J/cm², 20 pulses per dot and a repetition rate of 30 kHz. The pulse duration of the laser was 14 ns and the used wavelength was 1064 nm (a). The DLIP_shallow texture was fabricated using 2 pulses per DLIP area and a laser fluence of 0.82 J/cm² (b), the DLIP_deep texture was fabricated using 15 pulses per DLIP area and a laser fluence of 1.33 J/cm² (c). The laser wavelength, the pulse duration and the repetition rate were kept constant at 532 nm, 70 ps, and 30 kHz, respectively.

Figure 3

SEM images of the multi-scale textures resulting from DLW combined with a shallow DLIP pattern with a spatial period of 1.7 µm (a), 3.4 µm (b), and 4.8 µm (c) as well as the combination of DLW and deep DLIP pattern with the same spatial periods (d–f), respectively. For the DLW processing 20 pulses were used, each with a laser fluence of 6.56 J/cm². Details about the number of pulses and the laser fluence for the DLIP processing are given in Table 1

Figure 4

Overview of the structure depths of the DLW structure, the multi-scale DLW+DLIP structures, the DLIP structures within the DLW+DLIP textures as well as exemplary topography images taken with confocal microscopy of the deep multi-scale textures.

Figure 5

Evolution of static water contact angles for shallow homogeneous (a) and deep inhomogeneous (b) DLIP structures as well as multi-scale and DLW-containing structures with shallow (c) and deep (d) DLIP structure depths. The DLIP spatial period, in µm, is indicated by the number in the square brackets.

Figure 6

Static, advancing and receding water contact angles as well as the WCA hysteresis of the fabricated single-scale shallow (a) and deep (b) DLIP textures and of the multi-scale textures including shallow (c) and deep (d) DLIP elements measured 80 days after laser processing.

Figure 7

Static WCA calculated according to the Cassie-Baxter and Wenzel models and measured after 80 days for the produced single-scale structures (a) and for the multi-scale structures (b). The labels in the horizontal axes represent the spatial period of the DLIP textures.

Figure 8

SEM images of wear tracks achieved by a ball-on-disk test on a single-scale DLIP₃._{4µm deep} texture (a) and on a multi-scale DLW + DLIP₃._{4µm deep} treated texture (b). The red colored areas indicate the damaged zone. The used 100Cr6 ball had a diameter of 6 mm and the normal load was kept constant at 0.5 N.