Effect of Wettability Gradient on Tribological Behavior of Biomimetic Textured Surfaces

With the rapid development of modern industry, higher technical requirements are put forward for the friction reduction and wear resistance of the surfaces/interfaces of mechanical parts. Wettability regulation and biomimetic texture design are important methods to achieve friction reduction and wear resistance, but the coupling effects of wettability gradient and biomimetic textures on tribological behaviors have rarely been investigated. In this paper, femtosecond laser was used to fabricate biomimetic microdimples with diameter of 50 μm, spacing of 100 μm, and depth of 15 μm on the smooth surfaces of AISI 440C stainless steel. We also prepared an environmentally friendly superhydrophobic coating with tunable wettability, which was composed of TiO₂ and polyurethane-modified silicone. The coating was deposited in the microdimples to fabricate biomimetic textured surfaces with wettability gradient. To demonstrate the successful fabrication of the surfaces, the microscopic morphology and elemental composition were analyzed by scanning electron microscope (SEM) and energy dispersive spectrometer, respectively. The results showed that the superhydrophobic coatings were deposited in the biomimetic textures and the flat regions outside the textures were not covered by the coatings. Then, four biomimetic textured surfaces with different wettability gradients (contact angles of coatings = 156°, 120°, 50° and 0°, respectively) were constructed by UV irradiation. The tribological behaviors of the four surfaces under different loads (0.5, 1.0, 2.0, 4.0 N) and frequencies (3, 6, 9, 12 Hz) were comparatively studied. Reciprocating tribological experiments were performed under 20 μL water lubrication. The reciprocating stroke and test duration were set to 4 mm and 15 min, respectively. The wear rate was calculated after examining the profiles of wear scars using laser confocal microscopy. The microscopic morphologies of wear scars were characterized by SEM. The results showed that wear rates decreased with the increases in load and frequency. More interestingly, it was found that the biomimetic textured surfaces with the wettability gradient of low-adhesion superhydrophobicity-weak hydrophobicity were conducive to achieving high wear resistance under low load and high frequency. When compared with polished surfaces, 47.9% decrease in wear rate was obtained. The underlying anti-wear mechanism was elucidated. Boundary slip was easy to occur when a solid surface possessed superhydrophobic property. Therefore, for biomimetic textured surfaces with superhydrophobic coatings, the flow velocity of water in the textures was higher than that on the flat regions. This contributed to the generation of hydrodynamic pressure and the improvement in bearing capacity of lubricating film. When a load was applied, the contact between rough peaks decreased, leading to a decrease in wear rate. For biomimetic textured surfaces with hydrophobic, hydrophilic, and superhydrophilic coatings, boundary slip was not easy to occur, which was not beneficial to the generation of hydrodynamic pressure. In conclusion, when a special wettability gradient of low-adhesion superhydrophobicity/weak hydrophobicity was formed between biomimetic textures and flat regions, the hydrodynamic pressure was maximum, resulting in the lowest wear rate. This work clarified the effect of wettability gradients on the tribological property of biomimetic textures and provided an effective strategy to decrease surface wear.