Microconical surface structuring of aluminium tubes by femtosecond laser processing
Femtosecond laser microstructuring is a convenient technology for the targeted surface functionalization of various materials. Commonly, the structuring process is performed on planar surfaces. Here, we investigated femtosecond laser structuring of aluminium tubes. Process parameters, i.e. the number of pulses per spot on the surface and the line distance, have been transformed from a line-by-line process on planar samples towards a helical process. The process is based on laser treating the rotating tube while the laser beam is moved along the axis of the tube. A significant difference of the surface structure obtained on a cylinder in comparison to the planar geometry is revealed. With exactly matching process parameters, a strong increase of the dimensions of the surface structures on aluminium tubes has been observed. With a typical parameter set to achieve microconically structured aluminium, the cone height increases from 5 to 24 µm and the cone-to-cone distance from 13 to 59 µm. The structure sizes were found to be unaffected from the diameter of the tube within a range from 12 to 40 mm. A possible explanation for the increased structure size is given by altered particle redeposition. Two different parameter sets have been transformed from a planar geometry to the cylindrical geometry. Deep black aluminium tubes providing hydrophobicity with a water contact angle up to 148° and a thermal emissivity up to 87 % are demonstrated.