Broadband Ultra-Black Surfaces Fabricated by Femtosecond Laser Processing and Spray Coating

Ultra-black surfaces with broadband antireflection are highly desirable for stray-light suppression, infrared stealth, solar energy utilization, and thermal-management applications. Herein, we report a spray-coating-assisted femtosecond laser strategy for fabricating hierarchical ultra-black surfaces on 2A12 aluminum alloy. Periodic microstructures were first generated by femtosecond laser scanning to provide geometrical light trapping and multiple internal reflections. An ultra-black coating containing carbon black and carbon nanotubes was then deposited onto the laser-structured surface, introducing strong broadband absorption while preserving the laser-induced trapping architecture. Theoretical analysis and numerical simulations reveal that increasing the microstructure aspect ratio strengthens the graded-index effect and prolongs the optical path, while carbon-based absorbing components further enhance extinction and dissipative loss. By optimizing the laser fluence, scanning speed, scanning interval, number of repeated times, and spray-coating cycles, the resulting laser/spray hybrid surface achieves an average total hemispherical reflectance of 2.14% over 0.2-14.8 μm. It also maintains an average specular reflectance below 0.6% at incident angles from 30 to 60°. In addition, the hybrid surface exhibits an average contact angle of 159.40° and a photothermal conversion efficiency of 85.45% under 1.5-sun irradiation. After five abrasion cycles and ten tape-peeling cycles, independent surfaces retained average total hemispherical reflectance values below 3.2%. These results demonstrate that the integration of femtosecond laser processing and spray-deposited absorbing coatings provides a practical route for constructing broadband low-reflectance interfaces on aluminum alloys and other engineering materials.