Investigation of the ablation rate of stainless steel by means of few ultrashort laser pulses up to 150 J/cm2

The availability of high power ultrafast lasers for material processing enables new applications such as deep hole drilling. To reach high ablation rates and high aspect ratios, fluences in the order of magnitude of 100 J/cm2 are required. As this range has rarely been described in literature, a detailed study of the ablation rate for stainless steel is presented here. At a mean fluence of ~1.3 J/cm2 a molten phase is observed in the center of the hole which then extends over the entire ablated area for increasing fluences. Due to the onset of a melt layer a melt supported ablation superimposes the anticipated second ablation regime and hence no logarithmic dependence of the ablation depth is visible for higher fluences. For fluences of 10 J/cm2 and higher the entire area is subject to a melt supported ablation. In addition, in this fluence range only the deposition of a few pulses at a higher repetition rate can boost the ablation rate up to a factor of 10 due to heat accumulation