Effect of pulse number on the ablation rate of metals in PLA multipulse regime

Abstract

The pulsed laser ablation of aluminium, copper and titanium irradiated with 4.5 ns pulses at 355 nm, 532 nm and 1064 nm wavelengths is investigated in open air at normal atmospheric conditions. The effect of pulse number, which is varied in the range of 5 to 50, on ablation rate in these three wavelength regimes is determined. The results indicate a higher efficiency of the ablation in VIS and UV regimes as compared to IR regime which is characterised by a very small optical absorbtivity. The ablation rate is demonstrated to be approximately constant when increasing the pulse number up to a certain value which strongly depends on the thermal properties of the material. Further increase of pulse number leads to a progressive decrease of ablation rate. The most pronounced decrease was obtained at 355 nm in aluminium where the ablation rate corresponding to the 50 th pulse is about 20% of the ablation rate corresponding to of first pulse. The decay of the ablation rate with the pulse number is attributed to the superposition of two phenomena: the enhanced attenuation of the laser beam in the plasma plume which is confined within the crater, and the decay of the effective laser fluence at the target surface due to the gradual increment of the effective irradiated area with pulse number.