Controllable multi-scale hydrophobic structures on titanium alloy by polarization-dependent femtosecond laser fabrication and magnetron sputtering

A significant challenge is to develop effective and controllable processes for fabricating hydrophobic metal surfaces with high mechanical properties. The combination of femtosecond laser direct writing (LDW) and gold nanoparticle magnetron sputtering (Au-MS) has successfully produced multiscale structures on Ti–6Al–4V (TC4) alloy, which possess excellent superhydrophobic and hardening properties. By adjusting the laser polarization to control micro/nanostructures, the dual-scale structure is transformed into a triple-scale structure, leading to a significant increase in the contact angle under proper scanning intervals. The Au-MS further fabricates a superhydrophobic surface, with a maximum contact angle of 153° and a minimum sliding angle of 4°. Surface hardness can be raised to 408 HV from 367 HV. This work reveals that the wetting mechanism for the dual-scale structures of TC4 alloy fabricated by laser direct writing is in the Cassie-Baxter state. While the triple-scale structures of Au-MS after linear polarization LDW (LPLDW) and the triple-scale structures of circular polarization LDW (CPLDW) with or without Au-MS are in Wenzel state, resulting in a different relationship between contact angle and surface structure. The flexible and high-efficiency fabrication of these multiscale structures has made TC4 alloy more competitive in self-cleaning and anti-freezing applications.