High precision laser processing of steel surfaces with sub-ns-lasers

Ultrashort pulsed lasers like ps- and fs-lasers with single pulses and pulse bursts allow a high ablation rate combined with a superior quality for several materials like copper and brass as well as glass and dielectrics.In case of steel surfaces this known quality could not be transferred, because after a defined ablation depth the surface roughness increases. An increased roughness is mainly caused by a growing area of cone like protrusion structures (CLP) with increasing of the ablation depth. To avoid this phenomenon a melt concomitant ablation, due to some 100ps pulse duration was applied with a moderate pulse energy of 30 μJ. The results show no CLP structures and a decreased surface roughness in combination with an acceptable recrystallization of melt residues around the ablation area. The non-melt-free ablation process with residual molten volume caused by the longer pulse duration (compared to USP-lasers) has to be compensated through a higher laser power. This investigation shows an approach with a parallelization of several separate modulated laser spots for increasing the processing speed