Titanium alloys, owing to their exceptional comprehensive properties, find extensive applications across numerous critical sectors. However, in low-temperature, high-humidity environments, their surfaces are prone to forming ice and frost layers, leading to diminished equipment performance. Furthermore, when utilized in components such as aircraft engine blades or compressor blades, they must endure micro-motion friction and impact wear caused by high-speed ingestion of dust, ice crystals, and water droplets, thereby shortening the service life of the components. Conventional deicing methods suffer from high costs and short-lived effectiveness. In recent years, superhydrophobic surface anti-icing technology has garnered significant attention, yet it commonly exhibits poor wear resistance. Research indicates that superhydrophobicity alone cannot entirely prevent icing; if surface microstructural features are too large, water droplets may linger and freeze. Although femtosecond lasers can form dense nanostructures to enhance anti-icing effects, wear resistance remains inadequate. Integrating laser nitriding technology enables the formation of high-hardness titanium nitride layers on surfaces, thereby enhancing durability. Femtosecond lasers produce minimal thermal effects, creating fine structures with low roughness and thin nitride layers; conversely, nanosecond laser nitriding generates thicker nitride layers. This study therefore integrates the advantages of both approaches. Frist, femtosecond lasers were employed to create dense micro-nano structures, and enhance the anti-icing performance of titanium alloys; Subsequently, nanosecond laser nitriding was applied synergistically to the textured surface to generate a thicker, harder titanium nitride coating for improved wear resistance. The resulting anti-icing surface ultimately achieved a thicker nitrided layer and superior durability.
In this study, a 10 mm × 10 mm× 2 mm TC4 alloy (Ti-6Al-4V) substrate was employed. High-purity nitrogen gas (99.99%) and inert argon gas served as the protective atmosphere. Texturing of the substrate was first performed with a femtosecond laser. Subsequently, the textured surface underwent nitriding via a nanosecond laser. The sample was placed within an acrylic nitrogen protective cover, positioned at a height aligned with the nitrogen inlet to ensure uniform gas flow across the surface. A 1wt.% fluorosilane ethanol solution was prepared. The pre-fabricated samples were immersed in this solution for 1 hour, then baked in a constant-temperature vacuum oven at 120 ℃ for 20 minutes to complete the low-surface-energy modification. Microstructure and property testing involved acquiring the three-dimensional surface morphology of the samples using a laser confocal microscope. The cross-sectional morphology of the nitrided titanium was observed with an optical microscope. Scanning electron microscopy (SEM) was employed for morphological analysis of the titanium nitride cross section and post-friction surfaces, supplemented by elemental composition determination via energy-dispersive X-ray spectroscopy (EDS). Phase composition was analyzed by X-ray diffraction (XRD), while surface composition characterisation was conducted by X-ray photoelectron spectroscopy (XPS). Static contact angle measurements were performed with a contact angle analyser. Visualisation of the droplet freezing process was achieved through a semiconductor cooling system. A 100 g weight was employed to apply a normal load, completing 10 cycles on 800-grit sandpaper, with each friction stroke measuring 40 cm. Contact angles were measured after each friction cycle, and wear resistance was analyzed by capturing topographical images of the worn areas.
A regular grid structure was fabricated with a femtosecond laser, exhibiting a contact angle of 165.8° and an icing time of 810 s at -24 ℃, demonstrating high hydrophobicity. Subsequent nanosecond laser nitriding rendered the grid surface roughened, with a nitrided layer thickness of 125 μm, a contact angle of 166.2°, and an extended freezing time of 818 s. Crucially, nanosecond laser nitriding did not disrupt the surface microstructure generated by femtosecond laser texturing. The nitrided layer produced by nanosecond laser nitriding exhibited a bilayer structure. The upper layer was a nitrogen-rich layer, primarily composed of TiN; the lower layer was a nitrogen diffusion layer, mainly consisting of α(N)-Ti. The laser-nitrided layer formed a robust metallurgical bond with the substrate. The microstructure of the nitrided layer comprised dendritic crystals and acicular martensite. After cyclic friction testing, the freezing times of all samples decreased. However, the laser-textured nitrided surface exhibited the smallest reduction in freezing time, at 420 s. This result indicated superior wear resistance for the laser-textured nitrided samples. It followed that when droplets resided within small grooves, Laplace pressure differences arose vertically due to variations in droplet radius, ultimately causing the droplet to eject from the substrate. When droplet dimensions were smaller than the characteristic length of the groove, surface curvature gradients generated Laplace pressure differences on the droplet, propelling it to move in a specific direction and accumulate on the curved surface. Based on the preceding analysis of microstructure surface wear characteristics, the wear process typically comprised three stages: removal of nanoscale structures, flattening of mesoscale topography, and brittle fracture of micrometre-scale protrusions.
Micro-and nanostructures fabricated on the TC4 surface via femtosecond laser texturing effectively enhance anti-icing performance. The nitrided layer produced by composite nanosecond laser nitriding exhibits increased thickness compared with the femtosecond laser-treated layer, alongside a rougher surface topography. This enhances both hydrophobicity and anti-icing capability, resulting in prolonged icing initiation time. Following cyclic friction testing, the surface microstructure remains largely intact, whilst the substrate exhibits pronounced friction marks. This indicates the nanosecond laser nitrided layer possesses superior wear resistance, exhibiting the least reduction in icing time and demonstrating superior anti-icing performance compared to both the TC4 substrate and laser textured surfaces.