Femtosecond laser surface structures obtained with nanosecond laser pulses

 
Usually, a material itself defines the properties of its surface. For example, the wettability of surfaces depends on the chemical composition, the possibility for hydrogen bonding towards the surface and several other aspects. To change the surface property of a material, coatings with specialized paints are a routine technique, for example to increase the emissivity of radiators with radiator lacquers. Although coating a surface with a different material to optimize its surface properties is an unbeatably simple method, several problems can arise. The adhesion of the coated layer on the bulk material can be unstable and the layer may detach. Furthermore, the physical properties of the bulk material and the coating usually differ. For example, the acceptable temperature range can be drastically reduced, if painted coatings are applied. For applications in extreme environments such as space or nuclear power plants, certified materials are used and a layer of a second substance requires new certification.

Femtosecond laser surface structuring offers a different technique to control the surface properties of the bulk material. A complex nano- and microstructure changes the surface properties dramatically. A common example is the lotus leaf effect, which provides superhydrophobicity. Typical examples for femtosecond laser structured surfaces are metals covered by microcones (bumps or pillars). These cones can act as a light trap and the surface appears deep black (“black metals”) although the chemical composition of the surface is not changed by the structuring process. Consequently, adhesion is not a problem and heat stability as well as stability in extreme conditions such as radioactivity are just as good as the bulk material. Besides the absorbed light, several surface parameters can be addressed by femtosecond laser structuring. Examples include metal surfaces tuned from superhydrophilic to superhydrophobic or a thermal emissivity of 100%. The surface area is increased by roughly a factor of 100 during microstructuring. The large surface areas leads to further applications of femtosecond laser structured materials, such as highly efficient electrodes for water electrolysis and catalyst carriers, for example for the dehydrogenation of liquid organic hydrogen carriers.

While the surface structures of femtosecond laser treated materials are excellent, the technology is comparably expensive, sensitive to vibration and dust and nontrivial to scale towards high power systems, which require water-cooling and stable environmental conditions. A critical point of femtosecond laser setups is to handle the extreme peak laser output power during the femtosecond pulse length, which stresses optical parts. In contrast, nanosecond laser systems are routinely used in industry; they are robust, stable, air-cooled and insensitive to temperature changes and vibration. The less demanding requirements of nanosecond laser setups are easily explained by the longer pulse length by a factor of 100.000. Consequently, the laser peak power is reduced accordingly. While the longer pulse length prevents damage to optical parts, the impact on the target material significantly differs from femtosecond laser pulses, too. Femtosecond laser pulse irradiation on the target surface leads to phase explosion and immediate vaporization of the bulk material, accompanied by particle redeposition. Under these conditions, laser induced periodic surface structures (LIPSS) are formed. Typically, periodic ripples are formed first, which slowly evolve to cones with increasing number of pulses per spot on the surface. These surface structures provide the unique surface properties as discussed above. In contrast, nanosecond laser pulses lead to melting processes on the surface and molten droplets resolidifying on the surface in the form of larger trenches.

To achieve the surface structures obtained with femtosecond laser pulses with nanosecond laser pulse irradiation, we investigated the addition of reactive gases to the process gas atmosphere during the laser structuring process. In this case, a combination of laser ablation and etching processes forms the structural motif on the surface. With reactive gases yielding low-boiling products upon reaction with the bulk material, material ablation is strongly enhanced. For example, the addition of bromine to the process gas atmosphere during the laser structuring of titanium allows achieving a homogeneous microconical surface with picosecond laser pulses. Furthermore, the laser structuring process of aluminium towards a surface homogeneously covered with sharp microcones (see title image) can be achieved with nanosecond laser pulses in the presence of minimal amounts of iodine as reactive species. In both cases, the low boiling metal halide easily evaporates from the bulk metal surface. Due to the comparably long pulses, explosive material evaporation is nearly absent and the processing chamber, especially glass windows for the incident laser beam, stay clean and structuring of larger surfaces is only limited by the size of the processing chamber.

Surface structuring with nanosecond laser pulses in the presence of low amounts of reactive gases can provide a competitive pathway to surface structures usually obtained with femtosecond laser pulses. In particular, fibre laser setups are convenient laser sources to scale the process towards high-power setups and larger surfaces. The choice of the reactive gas depends on the target material and must be evaluated according to reactivity and physical parameters such as boiling point of the reaction product.