14 Février – Thesis defense - Fotios Frangelakis

09 h30 Amphi - Institut d'Optique d'Aquitaine (IOA) / Talence

Controlling and upscaling laser induced surface morphology: from tens of microns to tens of nanometres.

Current industrial markets demand highly value-added products offering new features at a low-cost. Among the most desired functionalities are surface colouring and blackening, anti-icing, anti-biofouling, wear reduction and anti-reflectivity. Laser surface processing holds a virtually endless potential in surface functionalization since it can generate versatile surface properties by modifying surface morphology and chemistry. Nevertheless, developing functional surfaces for implementation in the industry requires action on three levels. The first is to connect the macro-scale surface properties (colour, mechanical resistance, chemical stability, ageing) and the micro & nano-scale morphology. The second is to increase the level of control over the laser induced morphology in the near micron and submicron scale. The third is to upscale the lab-developed process both in terms of processed area and cycle time. Functional textures found in nature can be used as a guideline for connecting the surface texture with the surface property. It is well established that different textures can enable different functionalities. Nevertheless, the level of control of the laser induced morphology has to be improved significantly in order to allow one to mimic nature’s examples. Increase of control requires an in-deep understanding of the physical mechanisms that lead to nanostructure formation. To this end, we carry out a comprehensive parametric study of fs processing on stainless steel. The impact of wavelength, overlap, fluence, dose, repetition rate, polarization and interpulse delay in the induced morphology was investigated.We investigate several techniques to achieve controlled laser structuring in the submicron regime. Ripples of a few tens of nanometres were obtained with a UV laser. Double pulses were employed to further control the submicron structures. Structures of different size and symmetry were obtained in different delays underlining the key role of the interpulse delay (Δτ). Homogeneous triangular and square 2D-LIPSS were obtained for Δτ smaller than 5 ps and 500 ps, respectively. Process parameters and particularly fluence and polarization were found to play also a role in the laser induced feature characteristics. In a complementary set of experiments, we show that similar results can be obtained for small delays with a robust setup of birefringent crystals. In the above micron regime, trains of single pulses were employed for controlling the surface morphology. Fluence, dose and repetition rate, were varied to show a systematic variation of spikes in the range of tens of micrometers. Combining our experimental results with simulation data we underline the key role of heat accumulation on the structures size. Finally, we proposed an upscaling strategy showing the possibility to exploit repetition rates up to 10 MHz for laser texturing.In the upscaling part, areas much larger than the spot size were textured homogenously using high repetition rate laser and innovative laser positioning systems. Nanometric ripples induced by UV laser act as a subwavelength grating. Laser induced nano roughness exhibits superhydrophobic properties. Uniform distribution of well-defined, sub-wavelength, 2D-LIPSS was successfully generated over ~1 cm2. The final surface exhibits multiple axis iridescence giving a holographic effect. Employing a 10 MHz laser surface was textured at a rate of ~ 1 min/cm2 almost 60 times faster compared to our starting point. Lastly, surface blackening was achieved at a rate of ~ 9 sec/cm2.In conclusion, valuable data were provided both in surface functionalization, in understanding and controlling of laser induced structuring and in upscaling a lab developed process. We believe that our results open the way for exploiting  fs laser texturing in everyday applications employing up to date laser sources and positioning systems.

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