Abstract: A possibility of improving tribological properties by femtosecond laser irradiation on an example of a steel surface by creating regularly arranged micro-grooved textures that contain self-organized microstructures  and  nano  laser-induced  periodic  surface  structures  (LIPSS)  was determined. The friction coefficient of the treated surface of a sample of cold-rolled 1.4301 stainless steel was evaluated using a CSM Instruments SA tribometer according to a Pin-on-disk test scheme at a specific pressure of 1 MPa. It was found that the coefficient of friction was reduced by 35% compared to the initial grinded surface. Such laser treatment can find application for using parts in conditions of dry or boundary friction, mainly in units, in which the use of lubricant is unacceptable or extremely undesirable.

Abstract: This study shows the influence of laser fluence and pulse number on the spatial frequency distribution of laser induced periodic surface structures (LIPSS) on a stainless steel surface. Also the transition of LIPSS to larger self-organized, periodic, cone-like structures has been investigated. The experiments were carried out using a Ti:Sapphire femtosecond laser system with 800 nm centre wavelength, a pulse duration of 30 fs and a repetition rate of 1 kHz. Experiments have been carried out on flat, cold-rolled stainless steel surfaces (1.4301) by variation of the laser output power and feed rate. It could be shown, that the transition of low spatial frequency LIPSS (LSFL) to high spatial frequency LIPSS (HSFL) is a continuous process, strongly depending on the laser single pulse fluence and the pulse number. At higher accumulated fluences the transition of LIPSS to larger self-organized structures could be observed. As a result, hierarchical structures were created with micrometer-sized cones at the bottom and nanometer-sized LIPSS on top. By further increasing the accumulated fluence, the grooves between the micro structures are widend until the ablation threshold of the alloy is reached. These hierarchical structures could be of considerable value in improving wetting properties of technical surfaces.

Abstract: Biomimetic technical surfaces are very interesting for a wide field of possible applications in material science and engineering. For example, changing the wetting and deicing properties of components used in cold environmental conditions can help to reduce ice or snow aggregation, and thereby improve functionality and operational stability. In this study we investigate the correlation between wetting and deicing behavior of micro- and nanostructured stainless steel samples (1.4301). The samples were modified using a Ti:Sapphire femtosecond laser system with 800 nm central wavelength, a pulse duration of 30 fs and a repetition rate of 1 kHz. We generated two fundamentally different types of hydrophobic and superhydrophobic structures by varying the laser fluence and the number of applied pulses, thereby creating hierarchical structures in the micrometer regime and laser induced periodic surface structures (LIPSS) in the nanometer regime. The static water contact angle has been measured to quantify wetting properties of laser treated samples. To determine the ice adhesion shear stresses at the ice/steel-interface, cuvette encased ice columns were frozen onto the structured samples and sheared off by a push rod, while recording the forces. Several icing/deicing cycles have been carried out to investigate a possible decline in wetting behavior due to wear or other mechanisms. We could show, that surfaces with hierarchical microstructures and superhydrophobic wetting behavior will lose its ability to repel the applied water while freezing. Larger structures with higher surface roughness lead to increased ice adhesion shear stresses compared to the initial unstructured surface. LIPSS on the other hand might be not as hydrophobic, but showed lower ice adhesion in the range of the reference sample.

Abstract: Femtosecond laser processing is a key technology for surface modification and can be used to trigger superhydrophobic behavior through a combination of topographical and chemical effects. Nevertheless, immediately after laser processing, steel samples appear superhydrophilic and evolve into a hydrophobic state over a period of several days to weeks. To reduce the wetting transformation time down to hours, the sample can be stored in a vacuum chamber. In the chamber, the absence of atmosphere and thereby water vapor leads to an increased adsorption of hydrocarbons by the surface. In this study, we investigate an alternative and less demanding approach for shortening the wetting transformation time significantly by exposing the laser treated samples to a hydrocarbon liquid/atmosphere. This not only leads to a faster transformation, but also to an improved deicing performance. Both effects are analyzed for various hydrocarbon liquids and different laser patterns with respect to the surface chemistry.