Fabrication of hierarchical pillar-structured superhydrophobic surfaces on titanium alloy for underwater drag reduction via a hybrid micro/nanotexturing method

Constructing superhydrophobic micro/nanohierarchical surfaces is an efficient means of reducing hydrodynamic drag on underwater vehicles. Titanium alloys, owing to their high strength and corrosion resistance, have become essential structural materials for such applications. However, most existing surface-structuring approaches for titanium alloys rely on a single fabrication technique. This limitation constrains further improvements in drag-reduction performance. To address this challenge, this study proposes a multiprocess composite fabrication approach that integrates the advantages of three complementary techniques to produce micro/nanostructured superhydrophobic titanium alloy surfaces. Phase-controlled vibration microtexturing (PVMT) is first applied to generate microscale pillar structures; ultrafast laser nanotexturing (ULNT) is then used to machine nanoscale structures; finally, surface energy modification (EM) is conducted through fluorination treatment. The PVMT technique is newly proposed in this work. By precisely controlling the phase difference P between two cutting passes, the material removal behavior is regulated such that the residual material forms discrete micropillar arrays, which are favorable for enhancing drag-reduction performance. To verify the performance of this hybrid approach, titanium alloy samples were prepared under various process parameters, followed by wettability characterization and underwater drag measurements using a custom-built testing platform. The wettability results show that PVMT significantly improves hydrophobicity, increasing the static contact angle from 78.6° to 112.4°. When combining PVMT&ULNT&EM, the contact angle rises to 154.2°, demonstrating superhydrophobic behavior. Drag tests reveal that PVMT-generated micropillar structures can reduce drag by up to 12.6%. With additional ULNT and EM treatments, the drag-reduction rate increases to a maximum of 22.1%, among the highest drag-reduction values reported for titanium alloy surfaces. Additionally, the surface drag reduction performance is recoverable. These experimental results confirm the efficiency of PVMT in constructing functional micropillar arrays and validate the effectiveness of the PVMT&ULNT&EM composite process in fabricating hierarchical superhydrophobic surfaces for enhanced underwater drag reduction. This approach offers a promising route for functionalizing advanced underwater engineering materials.