Metal surface temperature induced by moving laser beams \ud

Whenever a metal is irradiated with a laser beam, electromagnetic energy is transformed into heat in a thin surface layer. The maximum surface temperature is the most important quantity which determines the processing result. Expressions for this maximum temperature are provided by the literature for stationary cases. In practice, however, moving beams are of more importance.\ud Based on a fast numerical algorithm which allows calculation of the induced temperature profile, the maximum surface temperature for stationary and moving laser beams is calculated. Next, two types of approximating functions are presented relating the scanning speed to the maximum surface temperature. Using dimensionless numbers, the results can be applied to different material

Combined simple cautious and robust control for parameter and disturbance bounded distributions

The qualities and drawbacks of two control methods to cope with process uncertainty are considered: cautious control which only uses statistics, and robust control which only uses the bounds of the process uncertainty. On the basis of results obtained for new simple methods for both, two new performance measures are introduced which use statistics as well as bounds of process parameters and disturbances, and therefore combine the qualities of cautious and robust control. These controllers are shown to outperform cautious and robust contro

Ultra‐short pulsed laser processing of sapphire

Synthetic crystalline sapphire is hard, transparent and inert to most chemical etchants. It is a popular substrate for numerous applications in e.g. semiconductor industry, microfluidics, smartphones and watches. However, sapphire is challenging to machine with traditional techniques such as mechanical dicing. Tightly focusing a femto‐ or picosecond pulsed laser beam inside the bulk of sapphire amorphized a volume in and near the laser focus (diameter ~ 1 micrometer). This amorphized region can be selectively removed by chemical etching in a subsequent step, resulting in hollow volumes and structures [1]. For the technique to be fully exploited, several scientific challenges still need to be overcome. To address these challenges, we combined an experimental and a theoretical approach study and optimize this two‐step method. Our numerical model allows simulation of the laser‐material interaction during short ulsed laser processing of sapphire [2]. Physical phenomena included in the 2D and timedependent model are the laser intensity distribution, free electron density, electron temperature and lattice temperature during and directly after the pulse. Simulation results show that avalanche ionization needs to be triggered for sapphire to absorb laser energy. Our experimental results show that the pulse energy and focus depth are the most dominant laser parameters. Further, the type of etchant used has a strong effect on the resulting structures, not only, in the bulk, but also on the surface of sapphire. Acknowledgement: The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska‐ Curie grant agreement No. 675063

Electro-optic and Acousto-optic Laser Beam Scanners

Optical solid state deflectors rely on the electro-optical or acousto-optic effect. These Electro-Optical Deflectors (EODs) and Acousto-Optical Deflectors (AODs) do not contain moving parts and therefore exhibit high deflection velocities and are free of drawbacks associated with mechanical scanners. A description of the principles of operation of EODs and AODs is presented. In addition, characteristics, properties and the (dis)advantages of EODs and AODs, when compared to mirror based mechanical deflectors, is discussed. Deflection angles, speed and accuracy are discussed in terms of resolvable spots and related quantities. Also, response time, damage threshold, efficiency and the type and magnitude of beam distortions is addressed. Optical deflectors are characterized by high angular deflection velocities, but small deflection angles. Whereas mechanical mechanical scanners are characterized by relatively small deflection velocities, but large deflection angles. Arranging an optical deflector and a mechanical scanner in series allows to take advantage of the best of both world

Laser-induced forward transfer (LIFT) of water soluble polyvinyl alcohol (PVA) polymers for use as support material for 3D-printed structures

The additive microfabrication method of laser-induced forward transfer (LIFT) permits the creation of functional microstructures with feature sizes down to below a micrometre [1]. Compared to other additive manufacturing techniques, LIFT can be used to deposit a broad range of materials in a contactless fashion. LIFT features the possibility of building out of plane features, but is currently limited to 2D or 2½D structures [2–4]. That is because printing of 3D structures requires sophisticated printing strategies, such as mechanical support structures and post-processing, as the material to be printed is in the liquid phase. Therefore, we propose the use of water-soluble materials as a support (and sacrificial) material, which can be easily removed after printing, by submerging the printed structure in water, without exposing the sample to more aggressive solvents or sintering treatments. Here, we present studies on LIFT printing of polyvinyl alcohol (PVA) polymer thin films via a picosecond pulsed laser source. Glass carriers are coated with a solution of PVA (donor) and brought into proximity to a receiver substrate (glass, silicon) once dried. Focussing of a laser pulse with a beam radius of 2 µm at the interface of carrier and donor leads to the ejection of a small volume of PVA that is being deposited on a receiver substrate. The effect of laser pulse fluence , donor film thickness and receiver material on the morphology (shape and size) of the deposits are studied. Adhesion of the deposits on the receiver is verified via deposition on various receiver materials and via a tape test. The solubility of PVA after laser irradiation is confirmed via dissolution in de-ionised water. In our study, the feasibility of the concept of printing PVA with the help of LIFT is demonstrated. The transfer process maintains the ability of water solubility of the deposits allowing the use as support material in LIFT printing of complex 3D structures. Future studies will investigate the compatibility (i.e. adhesion) of PVA with relevant donor materials, such as metals and functional polymers. References: [1] A. Piqué and P. Serra (2018) Laser Printing of Functional Materials. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA. [2] R. C. Y. Auyeung, H. Kim, A. J. Birnbaum, M. Zalalutdinov, S. A. Mathews, and A. Piqué (2009) Laser decal transfer of freestanding microcantilevers and microbridges, Appl. Phys. A, vol. 97, no. 3, pp. 513–519. [3] C. W. Visser, R. Pohl, C. Sun, G.-W. Römer, B. Huis in ‘t Veld, and D. Lohse (2015) Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer, Adv. Mater., vol. 27, no. 27, pp. 4087–4092. [4] J. Luo et al. (2017) Printing Functional 3D Microdevices by Laser-Induced Forward Transfer, Small, vol. 13, no. 9, p. 1602553

Picosecond laser machined designed patterns with anti-ice effect

Micromachining using ultra short laser pulses (USLP) has evolved over the past years as a versatile tool for introducing functional features in surfaces at a micrometric and even at a sub wavelength scale. Being able to control the surface topography at this level provides a method to change the wetting behavior of a great number of materials. In most cases, when a surface has a natural tendency to be wetted (high surface energy), increasing its roughness will increase the spreading of water over it, and when it is naturally hydrophobic this roughness can dramatically enhance the water repellency. In this study, anti-ice properties of water repellent laser machined materials are investigated. Therefore, a stainless steel substrate (AISI 304L) has been textured with regular hatched patterns, using UV and green laser pulses of 6.7ps. In order to decrease the surface energy, a thin hydrophobic\ud coating has been applied on top of these structures. Super-hydrophobic state has been reached for many of the samples, and small hysteresis values have been measured to confirm the socalled, self-cleaning, or “lotus effect” properties of the engineered surfaces

Towards Friction Control using laser-induced periodic Surface Structures

This paper aims at contributing to the study of laser-induced periodic surface structures (LIPSS) and the description of their tribological properties in order to facilitate the knowledge for contact mechanical applications. To obtain laser parameters for LIPSS formation, we propose to execute two D2-Experiments. For the transfer of results from static experiments to areas of LIPSS we propose the discrete accumulation of fluences. Areas covered by homogeneously distributed LIPSS were machined. Friction force of these areas was measured using a tribometer in a ball on flat configuration. The friction force was found to be higher on the structured area than on the initial surface