Improving water-repellent and de-icing surface properties via femtosecond laser structuring

Abstract

The application-specific modification of technical surfaces to increase their functional performance is becoming ever more important due to the development of new technologies. In many industrial applications where surfaces are exposed to humid and cold climates, ice formation and accumulation are inevitable unless countermeasures are taken. On the wings of wind turbines, atmospheric icing not only affects the aerodynamic performance but can also cause mass shifts on the rotor, which induce vibrations that can compromise the system’s structural integrity. In addition to the resulting reduction in efficiency, there is a risk of ice being shed and thrown off the rotor blades, posing a threat to public health and safety. In severely affected areas, electro-thermal or aero-thermal heating systems are integrated into the rotor blades, which require a lot of energy to operate. Passive ice-repellent surfaces that reduce ice accumulation and promote de-icing would solve this problem. With laser structuring technology, it is possible to create superhydrophobic surfaces based on reducing surface energy and incomplete wetting.The present work deals with the laser structuring of surfaces to modify the wetting and de-icing properties to reduce freezing effects. A Titanium:Sapphire femtosecond laser is used, characterized by its ultra-short pulses and the associated high pulse energy. In experimental studies on stainless steel X5CrNi18-10 (1.4301 / AISI 304), the mechanisms and properties of so-called laser-induced periodic surface structures (LIPSS) are examined in more detail, which due to their nature and comparatively short processing time, have great potential for large-scale industrial application. Furthermore, the influence of the surface topography of different microstructures or superimposed hierarchical structures generated by selective laser ablation on their wetting and icing properties is considered based on the present work. In addition to the effect of pure laser structuring, different methods of chemical post-treatment (hydrocarbons, vacuum) were examined, which led to a significant increase in the contact angle, and a reduction in ice adhesion. During wind tunnel tests, hydrocarbon treated samples showed an improved water runback and a considerable time delay in ice bead formation on the leading edge.Since durability is essential for practical application in wind energy or aerospace sectors, experiments on the erosion resistance of laser-generated nano- and microstructures were carried out. Long-term field tests, where laser-structured samples were attached to the rotor blades of a small-scale wind turbine, confirmed the high resilience of different laser-generated structures under harsh alpine environmental conditions. Furthermore, tribological investigations showed the structure’s capability to reduce the coefficient of friction.In summary, this work combines a comprehensive investigation of the potential of laser structuring in terms of targeted application-optimized surface modification with comprehensive experiments on resistance to environmental and other wear conditions. From the empirically collected data, predictions about the wetting and icing behavior of laser-structured surfaces, as well as their resistance to wear and corrosion, should be derived in the future.