Ultrashort pulse laser ablation of steel in ambient air

This paper explores large area laser ablation of steel in air at ambient pressure and compares the influence of different pulse duration regimes from femtosecond to nanosecond. It is demonstrated that irradiation with 70 ns long pulses induces sufficient surface heat load to damage the steel by producing a network of microcracks on the entire surface which propagate deep into the bulk. In stark contrast, ultrashort pulse laser ablation maintains the integrity of the steel bulk structure. The first appearance of cracks on surfaces processed at the optimal fluence occurs when the pulse duration is increased to 15 ps. The laser fluence dependence of the ablation rate in the ultrashort pulse regime with 275 fs pulses is investigated and the ablation threshold is determined at 0.25 +/- 0.05 J/cm2. The optimal ablation regime in terms of volume removal is established to be an ablation efficiency of 0.20 +/- 0.02 mm3/min/W at a fluence of ~ 1.2 +/- 0.3 J/cm2 . Finally, the surface roughness for fluences from the ablation threshold up to 2.5 J/cm2 is analysed and it is demonstrated that the best smoothness of the laser processed surface occurs in the same range of fluences where ablation is most efficient.