Picosecond pulsed laser ablation of liquid covered stainless steel: Effect of liquid layer thickness on ablation efficiency

Under liquid laser ablation is a material removal technique in which a focused laser beam passes through a liquid layer on top of the surface of a sample to be processed. When compared to laser ablation without a liquid layer, material (re)deposition around ablated regions, as well as the size of the Heat Affected Zone (HAZ) is decreased. In addition, the ablation efficiency of the process, in terms of the amount of material removed per pulse, can be optimized by careful variation of the height of the liquid layer: a liquid layer height variation as small as a few tens of millimeters already has a measurable effect on the amount of ablated material. In studies reported in existing literature, the required liquid layer height is typically realized by pouring a pre-defined amount of liquid on top of the sample surface. Surface tension, however, causes the air-liquid interface at the boundaries of the domain to deviate from the planar interface away from the boundaries, which affects the accuracy with which the liquid layer height can be determined. To the best of our knowledge, these accuracy issues have not been studied in previous research. Therefore, an experimental set-up is proposed which circumvents the issues of a curved free surface. Next, a 7 picosecond pulsed laser source (M2≤1.3) at a wavelength of 515nm was employed to study the efficiency of laser ablation of stainless steel for a range of liquid layer heights. Our findings provide a more detailed quantification of crater depth as a function of liquid layer height than is available through existing literature.