Superhydrophobic Micro-nano Structure Strengthened by Laser Nitriding in Open-Air with Assisted Blowing

Picosecond laser-induced microstructure in ambient air enables superhydrophobic surfaces. However, conventional laser processing often results in microstructures that are susceptible to wear. In this study, a picosecond laser was used to simultaneously induce nitriding strengthening and form surface microstructures in an air environment. As the laser energy increased from 8 to 30 W, the water contact angle (WCA) decreased from 158° to 147°, whereas the surface hardness increased by a factor of 4.8, and the wear depth increased from 100 to 150 μm. By analyzing the stress at the three-phase contact line during droplet–surface interaction and examining the relationship between microstructure characteristics and contact angle, the critical height (H) of the superhydrophobic microstructure was determined to be 16.7 μm, with a diameter-to-pitch ratio (d/p) ranging from 0.536 to 0.580. Furthermore, by establishing a correlation model between microstructural parameters and the shear force (Fs), the intrinsic relationship between structural parameters and mechanical robustness was elucidated. Specifically, when the diameter-to-height ratio (d/H) of the micropillars is less than the critical diameter-to-height ratio (d/H*), and the d/p ratio of the micropillar array is greater than the critical diameter-to-pitch ratio (d/pc), a smaller d/p ratio resulted in stronger hydrophobicity and a higher load-bearing capacity of the microstructure. Moreover, compared with microscale structures, nanoscale structures were more effective in enhancing hydrophobicity. These findings provide valuable guidance for the design of durable superhydrophobic surfaces via laser processing.