Surface nanostructuring of metals by laser irradiation: effects of pulse duration, wavelength and gas atmosphere

Surface modifications by nanostructuring present a new laser application for improvement of surface properties such as adhesion, mechanical characteristics or corrosion protection. In this study, we report the formation of nanoparticles by laser irradiation of a steel surface. The influence of laser parameters such as pulse duration (25-30 ns, 500 fs), wavelength (248 nm, 308 nm), and the background gas pressure (10 mbar-1 bar) on the formation of this back deposition layer composed of aggregated iron oxide nanoparticles were investigated. Scanning electron microscopy and atomic force microscopy were used to characterise the irradiated steel surface and the particle morphology deposited by backward flux. In the nanosecond laser ablation regime, films are formed by aggregated nanoparticles with well developed cauliflower like structures, the size and the morphology depending on the nature and pressure of the background gas. In the femtosecond regime, we observed the formation of micrometer sized structures at the steel surface. In particular, a non-conventional mechanism of nanocluster condensation and growth is revealed since two different ablation rates corresponding to two different predominant processes are observed. These analyses demonstrate the possibility of controlling the distribution and the size of particles by varying the laser parameters and the background gas pressure and nature.