Applying Ultrashort Pulsed Direct Laser Interference Patterning for Functional Surfaces

Daniel Wyn Müller, Tobias Fox, Philipp G. Grützmacher, Sebastian Suarez & Frank Mücklich

Surface structures in the micro- and nanometre length scale exert a major influence on performance and functionality for many specialized applications in surface engineering. However, they are often limited to certain pattern scales and materials, depending on which processing technique is used. Likewise, the morphology of the topography is in complex relation to the utilized processing methodology. In this study, the generation of hierarchical surface structures in the micro- as well as the sub-micrometre scale was achieved on ceramic, polymer and metallic materials by utilizing Ultrashort Pulsed Direct Laser Interference Patterning (USP-DLIP). The morphologies of the generated patterns where examined in relation to the unique physical interaction of each material with ultrashort pulsed laser irradiation. In this context, the pattern formation on copper, CuZn37 brass and AISI 304 stainless steel was investigated in detail by means of a combination of experiment and simulation to understand the individual thermal interactions involved in USP-DLIP processing. Thereby, the pattern’s hierarchical topography could be tailored besides achieving higher process control in the production of patterns in the sub-μm range by USP-DLIP.

Highly specific surface properties in nature, like the well-known lotus and shark skin effects, are closely related to surface structures in the micrometre and nanometre length scale, often involving hierarchical patterns1. In fact, such biomimetic surface patterns have already proven to provide unique properties in several technical systems including the manipulation of contact mechanics and optical properties like light diffraction and absorption2,3,4. Patterns on the threshold between the micro- and the nanometre scale also showed to provide promising surface properties for medical products, as they can tailor the adhesion of both, cells and germs5,6,7. In this context, current research projects investigate the intricacies to prevent biofilm formation by surface patterning of different solid materials on the International Space Station (ISS), which endanger its crew in terms of both, health and damage to critical components8.

The predominant impact on the unique properties of patterned surfaces is defined by the scale and morphology of the surface features also including sub-patterns, which have to be adjusted specifically for each application. For instance, in antibacterial applications, feature sizes in the sub-μm length scale showed to be most effective against bacterial adhesion7,8,9,10. The processing methodology, dealing with such delicate surface modification, needs to ensure high machining precision as well as processing speeds and costs, which are able to compete with the classical methods of surface engineering. Besides lithographic methods, laser interference-based techniques have proven their worth in generating precise surface patterns, as they provide high processing speeds with little to no need for preparation and post-processing11. Direct Laser Interference Patterning (DLIP) using short pulsed laser sources was successfully applied for the controlled creation of periodic surface patterns in the micrometre scale, but has struggled to obtain sub-μm patterns on conductive materials3,4,5,6,7,12. Applying pulse durations close to or below the threshold of 10?ps in DLIP (which defines the ultrashort regime), allows for the generation of smaller pattern periodicities on a wider set of materials, even reaching below the μm scale range on metals12,13,14,15,16. When using ultrashort laser pulses (USP), sub-μm patterns can also be applied on several metals, semi-conductors and polymers by the formation of Laser Induced Periodic Sub-Structures (LIPSS)10,12,17,18,19. LIPSS generation is assumed to originate from the superposition of the incident laser pulse and a superficial plasmon wave within the irradiated substrate, inducing line-like sub-structures, which are oriented perpendicular to the laser pulse polarisation and exhibit a periodicity close to the laser wavelength. In addition to their use as primary pattern, LIPSS might be used as additional sub-patterns, rendering the morphology of DLIP generated patterns hierarchical16. Avoiding their formation, on the other hand, is often hard to achieve, especially on low conductive metals and semi-conductors. Hence, pattern morphologies on different materials generated by USP-DLIP might vary strongly in relation to the physical kinetics involved in topography formation, which need to be understood to efficiently tailor surface functionalities.

The current study investigates the material specific interaction in response to USP-DLIP for the creation of hierarchical structures in the micro- and sub-micrometre scale on ceramic, polymer and metallic materials involving primary pattern formation as well as sub-pattern generation. Since metals showed the most intricate response to processing by USP-DLIP in the experiments, the complex thermal surface kinetics involved in the pattern formation were examined by a combination of experiment and thermal simulation using a Two Temperature Model (TTM). Thus, the thermal and kinetic response of the noble metal copper, as well as the alloys CuZn37 brass and AISI 304 stainless steel on USP-DLIP could be determined in relation to a range of laser fluences and pulse accumulation. The knowledge of the material´s response allows for precise tailoring of the pattern’s topographical parameters and the creation of optimised patterns in the sub-μm scale.

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