We propose to use laser irradiation to modify the surface topography of metallic substrates in ways that alter the surface's susceptibility to macromolecular adhesion. By varying the laser conditions, we will achieve a broad range of different surface topologies with both microscale and nanoscale features. We seek to explore whether the resulting topographical modifications can enhance surface performance for a range of applications in which macromolecular adhesion in a solvent environment is either desirable or undesirable.
Laser irradiation has been shown to induce microstructural and chemical changes in surfaces, which can enhance their optical absorption, tribological performance, corrosion-resistance, or hydrophobicity. The effect that a laser will have on a given target is complicated, and depends on the laser’s pulse duration and intensity, as well as on the initial microstructure and properties of the target. At low intensity, the laser may melt the surface, which can be used to induce surface alloying or to remove deleterious second-phase particles in alloyed metals. At higher instantaneous intensities, achieved with short pulse duration lasers, athermal processes begin to be important, and ejection of material via formation of a hot, dense plasma is observed. If performed in liquid, this creates shock waves that “peen” the surface, which can improve its corrosion resistance; in air or in vacuum, ablation of material can lead to surface microstructures qualitatively described as “spikes” or “bumps”, which improve hydrophobicity by reducing the ability of a water drop to spread out as it would on a flat metal surface. Hydrophobic surfaces have attracted great attention in recent years as potential “self-cleaning” surfaces, with improved resistance to biological contamination, and the similar behavior of the small-scale (~10 micron) “protrusions” on the leaves of the Lotus plant inspire and motivate the search for similar behavior in metal substrates. For corrosion problems in which extended contact with hygroscopic substances is likely, improved hydrophobicity may improve a coating’s corrosion resistance as well. Preliminary data in our lab has shown a factor-of-3 reduction in corrosion mass loss for steel samples treated with a laser-structuring process compared to untreated steel. We have also found, consistent with the literature, that the laser irradiations of steel induce features on several length scales: there are “macro” scale features determined by the size of the laser spot, but these are decorated with smaller sub-micron scale protrusions which are themselves decorated with tens-of-nanometer-scale roughness. The topography changes depending on the laser parameters – the intensity, the number of shots hitting an area, the spacing between shots, and so on.
Previous laser-structuring work has shown improvements in surface optical properties, tribological performance, corrosion resistance, and hydrophobicity. This work will increase the overall knowledge base about the range of performance enhancements that can be achieved by laser surface modification. If successful, this work could provide significant reductions in toxic chemical use by replacing chemical surface pre-treatments with laser pre-treatment. The proposed approach will also quite likely lead to dramatic cost reductions for such pre-treatments.