The use of self-organized micro/nanostructured surfaces formed using femtosecond laser surface processing (FLSP) techniques has become a promising area of research for enhancing surface properties of metals, with many applications including enhancing heat transfer. In this work, we demonstrate advantages of the use of dual-pulse versus single-pulse FLSP techniques to produce self-organized micro/nanostructures on copper. With the dual-pulse technique, the femtosecond pulses out of the laser (spaced 1 ms apart) are split into pulse pairs spaced < 1 ns apart and are focused collinear on the sample surface. Single-pulse FLSP techniques have been widely used to produce self-organized “mound-like” structures on a wide range of metals including a number of stainless steel alloys, aluminum, nickel, titanium, and recently on copper. Due to its high thermal conductivity, copper is used in many critical heat transfer applications and micro/nanostructured copper surfaces are desired to further improve heat transfer characteristics. Using single-pulse (pulses spaced 1 ms apart) FLSP techniques, self-organized microstructure formation on copper requires much higher pulse fluence than is commonly used for producing microstructures on other metals, which results in instabilities during laser processing (non-uniform surfaces), low processing efficiency, and limitations on the control of the types of structures produced. In this paper, we report results that demonstrate that the dual-pulse FLSP technique can be used to produce microstructures on copper more efficiently than using single-pulse FLSP, with better control of the surface structures produced. Cross-sectional subsurface microstructure analysis is also presented for single-pulse versus dual-pulse FLSP functionalized copper surfaces.
Femtosecond laser surface processing (FLSP) is a powerful technique used to create self-organized microstructures with nanoscale features on metallic surfaces. By combining FLSP surface texturing with surface chemistry changes, either induced by the femtosecond laser during processing or introduced through post processing techniques, the wetting properties of metals can be altered. In this work, FLSP is demonstrated as a technique to create superhydrophobic surfaces on grade 2 titanium and 304 stainless steel that can retain an air film (plastron) between the surface and a surrounding liquid when completely submerged. It is shown that the plastron lifetime when submerged in distilled water or synthetic stomach acid is critically dependent on the specific degree of surface micro- and nano-roughness, which can be tuned by controlling various FLSP parameters. The longest plastron lifetime was on a 304 stainless steel sample that was submerged in distilled water and maintained a plastron for 41 days, the length of time of the study, with no signs of degradation. Also demonstrated for the first time is the precise control of pulse fluence and pulse count to produce three unique classes of surface micron/nano-structuring on titanium.
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