Direct Laser Interference Patterning Research Articles

Fabrication of Gradient Periodic Surface Structures on Stainless Steel Using Direct Laser Interference Patterning

In this work, the fabrication of gradient periodic line‐like structures with continuously changing spatial period on stainless steel by means of two‐beam direct laser interference patterning (DLIP) is studied. A 70 ps (picosecond [ps]) pulsed laser operating at 532 nm wavelength and 0.64 J cm−2 laser fluence is used to produce the gradient microstructures. By continuously changing the overlapping angle between the sub‐beams, the spatial period of gradient structures smoothly increases from 2.01 ± 0.09 to 3.99 ± 0.12 μm within a 500 μm long processed area. Different pulse‐to‐pulse overlaps are applied to study the influence of cumulated fluence on the topography of gradient structures. The surface topography is analyzed by optical confocal microscopy and is characterized by the roughness parameters Ra and Rz. Fourier transformations of the confocal images allows the verification of the variations of the spatial periods. In the experiments performed in this work, design rules are provided for fabricating complex microstructures in the future using DLIP‐based technology to achieve advanced surface function like anisotropic wetting or cell growth.

Influence of chemistry and topography on the wettability of copper

To understand the complex interplay of topography and surface chemistry in wetting, fundamental studies investigating both parameters are needed. Due to the sensitivity of wetting to miniscule changes in one of the parameters it is imperative to precisely control the experimental approach. A profound understanding of their influence on wetting facilitates a tailored design of surfaces with unique functionality.We present a multi-step study: The influence of surface chemistry is analyzed by determining the adsorption of volatile carbonous species (A) and by sputter deposition of metallic copper and copper oxides on flat copper substrates (B). A precise surface topography is created by laser processing. Isotropic topography is created by ps laser processing (C), and hierarchical anisotropic line patterns are produced by direct laser interference patterning (DLIP) with different pulse durations (D).Our results reveal that the long-term wetting response of polished copper surfaces stabilizes with time despite ongoing accumulation of hydrocarbons and is dominated by this adsorption layer over the oxide state of the substrate (Cu, CuO, Cu2O). The surfaces’ wetting response can be precisely tuned by tailoring the topography via laser processing. The sub-pattern morphology of primary line-like patterns showed great impact on the static contact angle, wetting anisotropy, and water adhesion. An increased roughness inside the pattern valleys combined with a minor roughness on pattern peaks favors air-inclusions, isotropic hydrophobicity, and low water adhesion. Increasing depth of the primary topography can also induce air-inclusions despite increasing peak roughness while time dependent wetting transitions were observed.

Diffraction-based approach for real-time monitoring of nanosecond direct laser interference patterning structure formation on stainless steel

Direct Laser Interference Patterning (DLIP) stands out as a versatile and cost-effective method for functionalizing material surfaces at high throughputs. Monitoring the dynamics of the structure formation can lead to a deeper understanding of the interplay between the main factors governing the process and ultimately to optimize the final texture. Here, the formation of gratings on stainless steel by DLIP with ns-pulses is studied using a diffraction-based approach, which measures the time-resolved reflectivity (TRR) of the sample. Measurements are performed for single pulses across different laser fluences. The melting dynamics are analyzed and compared with numerical results. By correlating the recorded signals with the structure depths, growth rates of 11 nm/ns and 57 nm/ns were estimated for fluences of 1.9 J/cm2 and 5.3 J/cm2, respectively. Furthermore, two growth regimes are identified. In the fast growth phase, the melting time increased from 73 to 380 ns for fluences of 1.9 J/cm2 and 5.9 J/cm2, respectively, showing a good agreement with the performed thermal simulations.

Nano/Microstructuring of Nickel Electrodes by Combining Direct Laser Interference Patterning and Polygon Scanner Processing for Efficient Hydrogen Production

The present work utilizes direct laser interference patterning in conjunction with a polygon scanner system to fabricate micro‐ and nanostructures on the surface of nickel foils. A picosecond pulsed laser source with an average power of ≈470 W is used to pattern line‐like structures having spatial periods of 11.0 and 25.0 μm and a maximum structure depth of up to 15 μm. The scanning speed and the number of consecutive scans, ranging from 5 to 500, are varied in order to assess their influence on the structure formation process. Furthermore, the use of ultrashort pulses permits the production of hierarchical surface structures having up to four feature size levels because of incubation effects and growth of different types of laser‐induced periodic surface structures. 2D‐fast Fourier transforms are applied to scanning electron microscopy images for analyzing the evolution of features from the nano‐ to microscale. The method proposed here permits the reduction of 22% in the overpotential of the hydrogen evolution reaction, which linearly correlates to the drop in energy required to drive this chemical reaction.

The friction of surface textured cylinder liner segments modified by Direct Laser Writing and Direct Laser Interference Patterning processes

Direct Laser Writing (DLW) using ultrashort pulsed lasers and galvanometer scanning, has become an established process for surface texturing, allowing an extensive variety of feature geometries and texture designs to be produced. The minimum feature sizes that can be generated are however limited by factors such as the laser output power, beam diameter, beam wavelength and associated optics. To address the need to produce even smaller texture features, Direct Laser Interference Patterning (DLIP) has been developed, which utilises bespoke beam splitting and converging optics to produce periodic interference fringing and enabling surface textures in the micrometric size range to be produced.In this study, the texturing of cast iron cylinder liner segments using DLW and DLIP processes is explored, to assess their tribological performance in high efficiency internal combustion engines, such as for hybrid applications. To facilitate the designs of textures and to capture high resolution data on surface interactions at specific points in the combustion cycle, the segmentation of cylinder lines and piston rings is undertaken. The results of tribology tests covering combinations of high and low speeds and contact pressures, have shown that the mechanical honing of cylinder liners could potentially be replaced with the cleaner DLIP texturing as an alternative.Optimised texture designs using the DLW process achieved up to a 54 % reduction in kinematic friction between the piston ring and textured cylinder liner surfaces, compared with mechanically honed cylinder liner segments. These results have indicated that DLW texturing offers the best improvement in friction performance in the boundary and mixed lubrication regimes at higher contact pressures, at both lower and higher sliding speeds. The results also provide useful comparative data on the performance of meso and micrometric scale textures and how they can be best applied to cylinder liners.

Functionalization of Ti64 via Direct Laser Interference Patterning and Its Influence on Wettability and Oxygen Bubble Nucleation.

The nucleation of bubbles on solid surfaces is an important phenomenon in nature and technological processes like electrolysis. During proton-exchange membrane electrolysis, the nucleation and separation of the electrically nonconductive oxygen in the anodic cycle plays a crucial role to minimize the overpotential it causes in the system. This increases the efficiency of the process, making renewable energy sources and the "power-to-gas" strategy more viable. A promising approach is to optimize gas separation by surface functionalization in order to apply a more advantageous interface to industrial materials. In this work, the connection between the wettability and bubble nucleation of oxygen is investigated. For tailoring the wettability of Ti64 substrates, the direct laser interference patterning method is applied. A laser source with a wavelength of 1064 nm and a pulse duration of 12 ps is used to generate periodic pillar-like structures with different depths up to ∼5 μm. The resulting surface properties are characterized by water contact angle measurement, scanning electron microscopy, confocal microscopy, and X-ray photon spectroscopy. It was possible to generate structures with a water contact angle ranging from 20° up to nearly superhydrophobic conditions. The different wettabilities are validated based on X-ray photon spectroscopy and the different elemental composition of the samples. The results indicate that the surface character of the substrate adapts depending on the surrounding media and needs more time to reach a steady state for deeper structures. A custom setup is used to expose the functionalized surfaces to oxygen-oversaturated solutions. It is shown that a higher hydrophobicity of the structured surface yields a stronger interaction with the dissolved gas. This significantly enhances the oxygen nucleation up to nearly 350% by generating approximately 20 times more nucleation spots, but also smaller bubble sizes and a reduced detachment rate.

Tribo-Electric Analysis of Multi-Cycle Wear in Asymmetric Laser-Structured Cu-Sn Connectors

One of the key objectives of the connector industry for the past years has been to reduce the insertion friction forces of detachable electrical connections, where the majority of approaches combine a particular connector design with the usage of lubricants. The necessity for a solution that uses less contact force to maintain a lasting electrically conductive connection without sacrificing the mechanical robustness of the connection during use is a significant challenge in this situation. The approach in this work is to create surfaces supporting asymmetric friction, meaning lower insertion than removal forces. By Direct Laser Interference Patterning (DLIP), asymmetric saw-tooth structures with different structural inclinations and periodicities were generated on Sn-coated Cu plates. It is determined that topographical interlocking is the primary operating mechanism providing the required anisotropy. Multiple insertion/removal cycles were simulated on the tailored surfaces to tribologically characterize them and to determine the evolution of the electrical contact resistance at the end of each insertion motion. The vast majority of the generated structures showed an improvement compared to the reference state, with lower insertion and higher removal forces while the electrical behavior is not significantly impacted.

Single‐ and Multiscale Laser Patterning of 3D Printed Biomedical Titanium Alloy: Toward an Enhanced Adhesion and Early Differentiation of Human Bone Marrow Stromal Cells

AbstractThis study explores the enhancement of biocompatible titanium‐based implants through surface functionalization for improved bone healing. Specifically, a near‐beta type Ti‐13Nb‐13Zr alloy is 3D printed using laser powder bed fusion and subsequently textured using nanosecond (ns) and picosecond (ps) direct laser interference patterning (DLIP) to create single‐scale and multi‐scale surface textures. On these textures, the cell behavior, morphology, metabolic activity and osteogenic differentiation potential of human bone marrow stromal cells are assessed using fluorescence microscopy and MTS assays. Moreover, tissue non‐specific alkaline phosphatase activity served as an early osteoblast production marker. Compared to untextured specimens, both types of textures exhibited higher metabolic activity and cell proliferation. Single‐scale ns‐DLIP textures encouraged cell extensions anchored in groove regions, while ps‐DLIP textures with hierarchical structures promoted cell extensions attaching to nanostructures on sidewalls. The groove width and nanotopographies in groove areas facilitated cell spreading. Surface topography, roughness, and surface chemistry (surface energy, wettability) influenced cell adhesion, proliferation, and differentiation. A comprehensive evaluation of DLIP‐generated surface textures, including their topography and chemical states, complements the factors affecting in vitro cell behavior. Overall, this research demonstrates the potential of surface‐functionalized 3Dprinted titanium for a novel generation of biocompatible implants.

Direct Laser Interference Patterning of Zirconia Using Infra‐Red Picosecond Pulsed Laser: Effect of Laser Processing Parameters on the Surface Topography and Microstructure

AbstractThis study investigates the influence of processing parameters when applying direct laser interference patterning (DLIP) on the morphology and microstructure of zirconia surfaces using a 10 ps‐pulsed laser source with 1064 nm wavelength. An experimental testing matrix is built with different values of laser fluence (5.7–18.2 J cm−2) and pulse overlap (66–98%). Surface morphology and microstructure are characterized by confocal microscopy and scanning electron microscopy. Homogeneous line‐like patterns with periodic spatial repetition of 5.0 µm, with varying depths, widths, and aspect ratio, are fabricated using proper processing parameters (5.7–7.6 J cm−2 and 92–96%). Structures with maximum depth of 1.5 µm and sharp edges are obtained (7.6 J cm−2 and 96% overlap). Ablated regions presented a morphology typical of photophysical ablation mechanism, with signs of molten material at the surface. Sub‐micrometric pores and nanodroplets are registered for all conditions, while sub‐micrometric cracks developed only for higher fluences. A processing window conducing to homogenous DLIP structures is set based on experimental data. Periodic structures with multiscale topographic features are successfully obtained on zirconia surfaces using DLIP technology in this study. These outcomes open new perspectives for fabrication of multifunctional zirconia surfaces for advanced biomedical and engineering applications.

On the Corrosion Properties of Aluminum 2024 Laser‐Textured Surfaces with Superhydrophilic and Superhydrophobic Wettability States

AbstractIn this work, the mechanism of the corrosion behavior of laser‐treated aluminum is studied. Two different laser techniques are used to fabricate the samples, direct laser interference patterning (DLIP) and direct laser writing (DLW), using nanosecond laser sources. The DLIP treatment uses a two‐beam optical configuration producing line‐like periodic structures. The DLW technique is employed to produce non‐periodic structures on the Al‐surface with the same cumulated fluences as in DLIP. The surface topography is analyzed by confocal microscopy, and the formation of oxide layers is investigated by scanning electron microscopy of cross‐sections produced using a focused ion beam. Wetting measurements performed on the laser‐treated samples exhibit a contrasting behavior, leading to either superhydrophobic or superhydrophilic states. In the case of the DLIP treatment, the static water contact angle is increased from 81° up to 158°, while for DLW, it decreases to 3°. Electrochemical tests demonstrate a decreased corrosion rate after laser treatment. Additionally, findings indicate no correlation between wettability and corrosion reduction. Therefore, the improvement in corrosion resistance is mainly attributed to the oxide layer formed by laser treatment. Although similar corrosion rates are achieved for both treatments, surfaces produced with DLIP can be beneficial when additional surface properties are required.

Increasing Heat Transfer from Metal Surfaces through Laser-Interference-Induced Microscopic Heat Sinks.

With the increasing processing power of micro-electronic components and increasing spatial limitations, ensuring sufficient heat dissipation has become a crucial task. This work presents a microscopic approach to increasing the surface area through periodic surface structures. Microstructures with a periodic distance of 8.5 µm are fabricated via Direct Laser Interference Patterning (DLIP) on stainless steel plates with a nanosecond-pulsed infrared laser and are characterized by their developed interfacial area ratio. The optimal structuring parameters for increasing the surface area were investigated, reaching peak-to-valley depths up to 12.8 µm and increasing surface area by up to 394%. Heat dissipation in a natural convection environment was estimated by measuring the output voltage of a Peltier element mounted between a hot plate and a textured sample. The resulting increase in output voltage compared to an unstructured sample was correlated to the structure depth and developed interfacial area ratio, finding a maximum increase of 51.4%. Moreover, it was shown that the output voltage correlated well with the structure depth and surface area.

Fabrication of functional zirconia surfaces using a two‐beam interference setup employing a picosecond laser system with 532‐nm wavelength: Morphology, microstructure, and wettability

AbstractThe aim of this work was to evaluate the feasibility of the fabrication of microtextures in zirconia using the direct laser interference patterning (DLIP) technique. A green ultra‐short pulsed laser (532 nm, 10 ps) with a two‐beam interference setup was used to produce line‐like structures with a spatial period of 3 µm. For a fixed set of fluence and pulse‐to‐pulse overlap values (6.2 J/cm2, 81%), periodic structures were successfully created for different hatch distances. The average depth of the features ranged from 0.37 µm for a hatch distance of 14.4 µm up to 0.84 µm for a hatch distance of 12.4 µm. However, a further decrease in hatch distance did not result in an increase in depth since the region of the ridges is also ablated. Scanning electron microscopy analysis showed pores formation on the laser grooves, but no sign of micro‐cracking could be observed. Wettability tests showed an increase in hydrophobicity after DLIP. These results bring exciting perspectives on the fabrication of micro‐textures with DLIP on zirconia surface.

Single‐step Production of Photocatalytic Surfaces via Direct Laser Interference Patterning of Titanium

AbstractState of the art approaches to produce photocatalytic surfaces generally require multiple processing steps to achieve highly active surfaces. Following recent trends to facilitate the production of active surfaces, this work presents a single‐step method to create porous photocatalytic surfaces via direct laser interference patterning (DLIP) of a titanium substrate with pulses in the picosecond range. The resulting surfaces contain a variety of titanium oxides while both their composition and morphology can be controlled through the laser process parameters. This makes it possible to tailor these surfaces for specific applications such as antimicrobial surfaces, implant materials or water treatment. Surface characterization was executed by applying scanning electron microscopy complemented by focused ion beam cross‐sectioning and energy dispersive X‐ray spectroscopy as well as gracing incidence X‐ray diffractometry. The photocatalytic activity achieved by different laser parameters is assessed by methylene blue degradation under UV‐A light. As DLIP is already established in industrial applications, this approach could greatly facilitate the use of photocatalytic surfaces for water treatment or medical applications, as it does not require nanoparticle synthesis or additional coating steps.

Fabrication of multisymmetrical hierarchical structures by direct laser interference patterning with 2 beams

A novel method based on Direct Laser Interference Patterning with 2 beams allowing to pattern surfaces with 2 dimensional periodicity and presenting several symmetry axes has been developped. It consists in the picoseond laser writing of the same 1D pattern for different sample orientations. The resulting final morphology depends on the rotation angle between the patterning steps and the number of scans. This way, various 2D short and long range periodic structures can be obtained. We show that by increasing the number of symmetry axes, the sample orientation dependance of the reflectivity, measured at 2 different wavelengths, is reduced, approaching an isotropic behaviour beyond 3 axes. Moreover, as the number of symmetry axes increases, the reflectivity decreases reaching ∼10−3 %, which is explained by the increase of roughness.

Surface Modification of Brass via Ultrashort Pulsed Direct Laser Interference Patterning and Its Effect on Bacteria-Substrate Interaction.

In recent decades, antibiotic resistance has become a crucial challenge for human health. One potential solution to this problem is the use of antibacterial surfaces, i.e., copper and copper alloys. This study investigates the antibacterial properties of brass that underwent topographic surface functionalization via ultrashort pulsed direct laser interference patterning. Periodic line-like patterns in the scale range of single bacterial cells were created on brass with a 37% zinc content to enhance the contact area for rod-shaped Escherichia coli (E. coli). Although the topography facilitates attachment of bacteria to the surface, reduced killing rates for E. coli are observed. In parallel, a high-resolution methodical approach was employed to explore the impact of laser-induced topographical and chemical modifications on the antibacterial properties. The findings reveal the underlying role of the chemical modification concerning the antimicrobial efficiency of the Cu-based alloy within the superficial layers of a few hundred nanometers. Overall, this study provides valuable insight into the effect of alloy composition on targeted laser processing for antimicrobial Cu-surfaces, which facilitates the thorough development and optimization of the process concerning antimicrobial applications.

Online process monitoring of direct laser interference patterning using an infrared camera system

Direct laser interference patterning enables the fabrication of homogeneous microstructures in the sub µm range. Generally, the topography of fabricated structures is characterized ex-situ using for instance confocal microscopy or white light interferometry. These methods require at least few seconds to fulfill a measurement and therefore are not suitable for online process observation. In this study, an IR camera system is used for the first time, for capturing the average temperature during the DLIP treatment in real time. To determine a correlation between the average temperature and the topographical parameters, the treated surfaces are analyzed using confocal microscopy. A linear dependency between the applied laser fluence (in the range from 0.7 to 4.9 J/cm2) and the average measured temperature could be established. In addition, in this fluence range, significant changes in the surface roughness, skewness and kurtosis occurred, which could be associated to a certain process average temperature.

Multi-Scale Structuring of CoCrMo and AZ91D Magnesium Alloys Using Direct Laser Interference Patterning

In this work, the technique of Direct Laser Interference Patterning (DLIP) was used to fabricate micrometric structures at the surface of Cobalt-Chromium-Molybdenum and AZ91D magnesium alloys. Line-like patterns with spatial periods of 5 μm were textured using an ultra-short pulsed laser (10 ps pulse duration and 1064 nm wavelength) with a two-beam interference setup. The surface topography, morphology, and chemical modifications were analysed using Confocal Microscopy, Scanning Electron Microscopy, and Energy Dispersive Spectroscopy (EDS), respectively. Laser fluence and pulse overlap were varied to evaluate their influence on the final structure. Homogeneous structures were achieved for the CoCrMo alloy for every condition tested, with deeper structures (up to 0.85 μm) being achieved for higher energy levels (higher overlap and/or fluence). For high energy, sub-micrometric secondary structures, so-called LIPSS, could also be observed on the CoCrMo. The EDS analysis showed some oxidation after the laser texturing. Regarding the AZ91D alloy, deeper structures could be achieved (up to 2.5 μm), but more melting and oxidation was observed, forming spherical oxide particles. Nonetheless, these results bring new perspectives on the fabrication of microtextures on the surface of CoCrMo and AZ91D using DLIP.

Targeting new ways for large-scale, high-speed surface functionalization using direct laser interference patterning in a roll-to-roll process

Direct Laser Interference Patterning (DLIP) is used to texture current collector foils in a roll-to-roll process using a high-power picosecond pulsed laser system operating at either fundamental wavelength of 1064 nm or 2nd harmonic of 532 nm. The raw beam having a diameter of 3 mm @ 1/e 2 is shaped into an elongated top-hat intensity profile using a diffractive so-called FBS®-L element and cylindrical telescopes. The shaped beam is split into its diffraction orders, where the two first orders are parallelized and guided into a galvanometer scanner. The deflected beams inside the scan head are recombined with an F-theta objective on the working position generating the interference pattern. The DLIP spot has a line-like interference pattern with about 15 μm spatial period. Laser fluences of up to 8 J cm−2 were achieved using a maximum pulse energy of 0.6 mJ. Furthermore, an in-house built roll-to-roll machine was developed. Using this setup, aluminum and copper foil of 20 μm and 9 μm thickness, respectively, could be processed. Subsequently to current collector structuring coating of composite electrode material took place. In case of lithium nickel manganese cobalt oxide (NMC 622) cathode deposited onto textured aluminum current collector, an increased specific discharge capacity could be achieved at a C-rate of 1 °C. For the silicon/graphite anode material deposited onto textured copper current collector, an improved rate capability at all C-rates between C/10 and 5 °C was achieved. The rate capability was increased up to 100% compared to reference material. At C-rates between C/2 and 2 °C, the specific discharge capacity was increased to 200 mAh g−1, while the reference electrodes with untextured current collector foils provided a specific discharge capacity of 100 mAh g−1, showing the potential of the DLIP technology for cost-effective production of battery cells with increased cycle lifetime.

Optoelectronic performance of indium tin oxide thin films structured by sub-picosecond direct laser interference patterning

A route to increase the efficiency of thin film solar cells is improving the light-trapping capacity by texturing the top Transparent Conductive Oxide (TCO) so that the sunlight reaching the solar absorber scatters into multiple directions. In this study, Indium Tin Oxide (ITO) thin films are treated by infrared sub-picosecond Direct Laser Interference Patterning (DLIP) to modify the surface topography. Surface analysis by scanning electron microscopy and confocal microscopy reveals the presence of periodic microchannels with a spatial period of 5 µm and an average height between 15 and 450 nm decorated with Laser-Induced Periodic Surface Structures (LIPSS) in the direction parallel to the microchannels. A relative increase in the average total and diffuse optical transmittances up to 10.7% and 1900%, respectively, was obtained in the 400–1000 nm spectral range as an outcome of the interaction of white light with the generated micro- and nanostructures. The estimation of Haacke’s figure of merit suggests that the surface modification of ITO with fluence levels near the ablation threshold might enhance the performance of solar cells that employ ITO as a front electrode.

Fine-tuning effect of Direct Laser Interference Patterning on the surface states and the corrosion behavior of a biomedical additively manufactured beta Ti alloy

Direct laser interference patterning (DLIP) is applied on additively manufactured near-beta Ti-13Nb-13Zr using nanosecond (ns) and picosecond (ps) pulses to tune its surface properties. Multiscale surface chemical and microstructural analyses (AES, XPS, XRD, SEM, TEM, GD-OES, contact angle) of those DLIP states are performed for understanding the corrosion behavior in a physiological PBS solution. Increased beta-phase fractions and uniformly thick passive layers of ns-DLIP surfaces led to enhanced corrosion stability compared to ps-DLIP with defective surface oxide. Both DLIP states control the surface wettability, thereby limiting corrosion and metal ion release rates, which is beneficial for implant applications.