Study on the Fabrication of Coating-Free Superhydrophobic Aluminum Alloy Surfaces by Femtosecond Laser and Its Wettability Control Mechanism

This work systematically investigates the coupled effects of femtosecond laser parameters (wavelength: 515 nm, pulse width: 373 fs, laser fluence: 3.18-12.7 J/cm², repetition frequence: 100 kHz) and post-fabrication thermal treatment on the micro/nano-structure evolution and wettability of aluminum alloys. By varying the scanning spacing (20-80 μm) and laser fluence, diverse hierarchical surface morphologies were obtained. At a small scanning spacing of 20 μm, increasing laser fluence causes severe thermal accumulation and structural collapse, with the microstructure height decreasing from 42.68 μm to 20.30 μm and the water contact angle (WCA) dropping from 158.6° to 143.5°, indicating a degradation of the superhydrophobic state. In contrast, at larger spacings (60-80 μm), moderate fluence enhances microstructure depth and roughness, yielding peak WCAs of ~160°, while excessive fluence induces feature coarsening and partial loss of nanoscale textures, leading to reduced wettability. Nanoscale evolution shows that optimized laser conditions promote dense nanoparticle redeposition and stable ridge-like structures. These structures are accompanied by cotton-like features with pore diameters of 50-100 nm and coral-like porous features with pore diameters of 100-200 nm, whereas excessive laser etching damage these nano-structures. Among, a scanning spacing of 40 μm achieves this most robust hierarchical nano-structure, corresponding to a maximum WCA of 162.6°. These results clarify the role of femtosecond laser parameters in regulating micro/nano-structural formation and the subsequent modulation of wettability through thermal treatment, providing a reference for the fabrication of coating-free superhydrophobic aluminum alloy surfaces.