Influences on incubation in ps laser micromachining of steel alloys

Efficient and accurate generation of micro holes and feature geometries is one object of investigation in ultrashort pulse laser processing. Different analytical models exist to describe the relationship between energy input and removed volume. These models relate to the ablation threshold and the energy penetration depth representing material-dependent parameters. Both parameters are influenced by incubation. Against this background, incubation effects depending on the number of pulses applied on two steel alloys are presented in the paper. The ablation threshold and the energy penetration depth are analyzed by the zero-damage method and an analytical model based on Beer's law describing the crater depth. The pulse frequency is chosen to f_P = 100 Hz to investigate incubation excluding the influence of temporal effects between subsequent pulses. The used ultrashort pulsed laser has a pulse duration of 10 ps and a wavelength of 1064 nm. Craters are generated with defined pulse numbers between N = 1 and N = 1000. Two commonly used steel alloys—the cold working steel 1.2379 and the austenitic stainless steel 1.4301—are investigated and compared. Additionally, the first one is investigated at two levels of hardness. All sample surfaces have the same initial average roughness S_a ≈ 0.02 μm which was realized by manual polishing. The comparison of two incubation models shows higher coincidence of the experimental results and the model proposed by Ashkenasi than for the model published by Jee, especially for pulse numbers N > 100. The linear optical penetration depths which are determined by ellipsometry measurements are larger than the energy penetration depths which are analyzed for the ablation regimes of the gentle phase. The main reasons for the pulse-number dependent decrease of the ablation threshold are investigated. Absorption changes due to surface modifications such as oxidation and generation of different surface morphologies which are observed by scanning electron microscope. The obtained results are the basis for further studies on incubation including temporal multipulse effects.