Factors controlling the incubation in the application of ps laser pulses on copper and iron surfaces

For laser micro processing with short and ultra-short pulses the threshold fluence is affected by the incubation and changes with the number of pulses applied. In general the incubation effect is described by a power function including the incubation coefficient S. Beside the threshold fluence also the energy penetration depth is subject to the incubation effect; moreover it is a main cause for the change of the threshold fluence with increasing pulse number. The behavior of the threshold fluence can be explained by varying absorption (due to changes in the surface reflectivity), chemical changes of the surface (e.g. due to oxidation) or changes in the microstructure of the material whereas the behavior of the energy penetration depth could be explained by the latter two effects but should not be affected by a change in the absorption. To try to distinguish between these three effects a systematic ablation study with 10 ps pulses at 1064nm wavelength on copper and iron under different gases atmospheres and pressures was done. The results show on the one hand the change of the energy penetration depth is the main cause of the incubation and that on the other hand an adapted model better fits the trend of the threshold fluence and the penetration depth as a function of the number of pulses applied. The influence of the gas (air, oxygen, nitrogen and argon) is only marginal whereas a reduction of the pressure from normal atmosphere down to 50 mbar results in a 25% increase of the maximum removal rate. Induced changes in the microstructure were detected by a high resolution X-ray diffraction analysis on single crystal (111-orientation) copper and iron samples.

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