Underwater ablation using short and ultrashort laser pulses
Sietse van der Linden is a PhD student in the department Engineering Fluid Dynamics. (Co)Supervisors are prof.dr.ir. G.R.B.E. Römer and dr.ir. R. Hagmeijer from the faculty of Engineering Technology.
Surface structuring of materials has a wide spread of applications, ranging from biomedical stent production to the creation of lens systems spanning several hundreds of micrometers in length and width. A viable method to produce these structures is through laser ablation, during which a laser source is used to remove material by focussing a large amount of laser energy onto an area which may be as small as a few square micrometers. This procedure is characterised by a trade off between two competing interests: a short structure production time requires a high material removal rate and thus a high energy level per area, whereas structure quality demands low energy per area deposition to prevent structure deteroriation due to heat build up in the heat affected zone at and around the laser irradiated area. While advances in the application of laser ablation have managed to optimize this trade-off for many materials and structures, maximizing material removal rate whilst maintaining adequate surface structure quality is to this day one of the biggest hurdles in the pursuit of laser ablation as an economic surface structuring tool.
In this context, laser ablation of a target through a water layer rather than in ambient air has demonstrated to be a promising solution to overcome this hurdle: given favorable water layer and laser parameters, craters produced under a water layer have proven to be deeper and posses a significantly smaller heat affected zone than their in air ablated counter parts. The favorable water layer and laser parameters are as of yet largely unidentified though, and thus require a closer examination to fully unlock the advantages underwater laser ablation might offer.
To identify and explain advantageous underwater processing conditions, this thesis explores the production of single craters on stainless steel and silicon prodcued in ambient air and under a water layer using a 1060 nm, 33 nanosecond and a 515 nm, 7 picosecond laser source for 1 upto 50 consecutive pulses over a pulse frequency range of 1 Hz upto 1 kHz. Pulse energies were varied between 1 µJ and 10 µJ for the picosecond laser source and approximately 60 µJ to 140 µJ for the nanosecond laser source. To assess the influence of different water parameters on the ablation process, water layer thickness above the target materials was varied and a flow was introduced over the ablation region. Water layer thickness was varied from 1 to 10 mm and Reynolds flow numbers from 0 (stationary water) upto 100 were examined. The comparative analysis of craters produced under different conditions was performed morphologically, by examination via microscopy and quantitatively by means of a crater volume comparison.
Morphological analysis revealed arc and spike like structures formed around and in the crater respectively during ablation of stainless steel under a water layer whereas melt structures were observed for ambient air created craters. Crater volume results hint at the existence of three distinct pulse energy dependent crater formation regimes: A surface modification regime, a volume displacement regime and a volume removal regime. Water layer thickness variation showed mixed results when using the picosecond pulsed laser source and a strong increase in ablated volume for a water layer thickness of 1 mm relative to results obtained under thicker layers when using the nanosecond pulsed laser source. The results observed for the nanosecond pulsed laser source were attributed to layer thickness dependent shockwave back reflections from a water-fused silica interface onto the ablated region. Flow in general was found to have a positive influence on ablated volume upto a pulse frequency of 1 kHz. In situ stroboscopic imaging revealed the existence of persistent bubbles produced during the underwater laser ablation process which were shown, through a ray optics analysis, to have a detrimental effect on the intensity profile of subsequent incident laser pulses. For sub 1 kHz pulse frequencies the flow conditions considered were adequate to remove these bubbles away from the ablation zone but at 1 kHz the bubbles were found to linger in the laser beam path, negatively affecting material removal rate. Ultimately, most stainless steel material, on average approximately 95 µm3 was removed per incident laser pulse for a pulse frequency of 100 Hz and a pulse energy of 110 µJ using a flowing water layer of 5 mm characterised by a Reynolds number of either 1, 10 or 100 using the nanosecond pulsed laser source. For silicon, most material per incident laser pulse, approximately 110 µm3 per pulse, was removed using the picosecond pulsed laser source set to a pulse frequency of 1 kHz at 10 µJ using a stationary water layer of 5 mm thickness. In both scenario's, significantly more material was removed than for craters shot at identical laser conditions in ambient air.
The presented results show underwater laser ablation is a particularly effective material removal method for laser pulse frequencies upto 1 kHz under a water layer. At a pulse frequency of 1 Khz however, the effect of persistent bubbles largely negate the shockwave back reflection induced increased crater material removal. Whilst persistent bubbles seem an inherent part of the process and are thus unavoidable, further research into the propagation of ablation induced shockwaves might serve as a valid means to further increase material removal rates for pulse frequencies below 1 kHz. To this end, the quality of future work would benefit greatly from in situ analysis methods, such as Schlieren imaging and spectroscopy analysis.
