Study of femtosecond laser multi-scale textured steel surfaces on the wettability in relation to aging

Micro and nanometric double scale structures are produced on a steel material irradiated by a femtosecond laser in order to study the alteration of the surface properties, in particular wettability. Following a texturing, the latter develops indeed from a hydrophilic to a hydrophobic state, possibly a superhydrophobic one, according to a progression which depends on multiple factors, gathered under the term aging. Two types of grid patterns were performed on the samples to test the effect of the Gaussian laser shot overlap. In order to characterize not only the topology of the textured surfaces, but also the associated chemistry, four techniques were used, allowing for the analysis from the first atomic layers to those at 1 µm depth. In particular, carbon and oxygen levels were measured as a function of material, maturation time and storage conditions. We have identified two oxides, hematite and magnetite, whose presence is directly related to the energy density of the laser beam. We explain the mechanism of their formation by combining two pre-existing models: the appearance of a magnetite-active site induced by the laser and a martitization process.