Laser Surface Structuring Research Articles

Compound Structures of Periodic Holes and Curved Ripples Fabricated by the Interference between the Converging Surface Plasmon Polaritons and Femtosecond Laser

Non-cylindrical vectorial femtosecond lasers are employed to irradiate tungsten surfaces. Compound nanopatterns composed of periodic nanoholes and semi-circular curved ripples are produced by scanning the target relative to the laser beam. The tangential direction of the curved ripples is perpendicular to the local polarization direction of the vectorial femtosecond laser beam. Therefore, the formation mechanism of the curved ripples can be attributed to the interference between the incident femtosecond laser and the laser-induced surface plasmon polaritons (SPPs). We found that, in addition to the curved ripples, periodic nanoholes with an average diameter of 406 nm also appeared on the target surface, and they all tended to appear at the vertexes of the semi-circular curved ripples, i.e., the converging point of SPPs. Further experiments demonstrated that the location of the periodic nanoholes was totally determined by the polarization state of the incident femtosecond laser. Therefore, we deduced that the convergent SPPs induced by the non-cylindrical vectorial femtosecond laser interfered with the incident laser at the convergent point, leading to the generation of periodic nanoholes. The investigations in this work exhibited the important role of manipulating the propagation of SPPs in femtosecond laser surface structuring, which not only diversifies the surface patterns that can be produced by laser-induced periodic surface structuring (LIPSS) but also provides deep insights in the excitation and propagation dynamics of SPPs.

Research on the formation mechanism of the surface structure in transition regime of laser polishing 304 stainless steel

Laser polishing is an innovative surface finishing technology to reduce surface roughness through the interaction of laser and material. However, some new surface structures will be introduced into the polished surface, reducing the surface quality of the polished workpiece. This paper investigated the formation mechanism of a new type of surface structure that has not been studied before. The laser polishing experiments presented the complete evolution of the surface profile and size parameters of the surface structure with the laser power, and the reason for the formation of the surface structure was preliminary analyzed. Furthermore, a two-dimensional numerical model coupled with heat transfer, fluid flow, material vaporization, and volume expansion was established to further explore the formation mechanism of this type of surface structure. With the help of the established numerical model, the temperature field, flow field, material vaporization, and volume expansion were analyzed during the formation process of surface structure. The contribution of five factors (capillary force, thermocapillary force affected by surface-active elements, recoil pressure, material removal, and volume expansion) to the formation of the surface structure was investigated, illustrating that this type of surface structure is the result of the combined effects of these factors. In addition, the surface profile and size parameters of the surface structure were simulated, which were in good agreement with the experimental results. This work further improves the theory of laser polishing, which is of great significance to promote the application of laser polishing. Importantly, this paper demonstrates the potential of laser surface structuring under the combined action of the surface tension, recoil pressure, material removal, and volume expansion of the molten pool in laser processing.

Influence of laser surface structuring of steel sheets on the mechanical properties of electrodeposited ZnNi coatings

The influence of laser surface structuring on the formation and the internal strength of electrodeposited ZnNi coatings on steel sheets was investigated. Surface structuring by extended direct laser interference patterning (xDLIP) was used to modify the cohesive strength in the ZnNi layer. An SEM analysis of the crystal structures shows perpendicular nucleation during ZnNi layer formation and areas of crosslinked crystalline structures, which results in an interlocking of crystal structures within the layer. The interlocked crystalline structure leads to an increase of the cohesive strength in the ZnNi layer, which was observed in roller peel tests according to DIN 1464 for adhesively bonded joints.

Dynamic Structural Color Display Based on Femtosecond Laser Variable Polarization Processing

AbstractVivid structural colors, inspired by butterflies, which can display iridescent colors dynamically when white light illuminates the surface with subwavelength period ripples, have attracted significant interest for its potential application in the fields of information storage, anti‐counterfeiting, etc. However, there is still a huge demand for the development of multipattern dynamically transformable display surfaces using a versatile method. A new scheme based on femtosecond laser combined with variable polarization processing has been proposed. This scheme enables a surface with a structural color function that can be easily prepared by adjusting the polarization of the laser via a half‐wave plate. Four types of structural units with designated periodic orientations are written on the same sample to exhibit four different colorful images. Through experiment and analysis, it is demonstrated that different colors can be displayed by changing the observation angle, and a multi‐pattern display can be realized by rotating the sample. The lithography‐free inscribed patterns show a strong azimuthal angular sensitivity to color and its brightness. This work demonstrates that femtosecond laser surface structuring provides a versatile platform to create various micro/nanostructures on a solid surface, and this technique can be used for the biomimetic lithography‐free processing of any material using ablation.

Experimental study of the Ti-implant surfaces structured by the ytterbium-doped pulsed fiber laser

Relevance. Currently, there are several basic techniques for the dental implant surface structuring. Laser treatment is an extremely promising technique for the surface structuring. This technology allows creating regular implant surface without using chemicals and in just one technological step. The purpose was to present study aimed to compare and evaluate in vivo the stability and osseointegration of dental implants with 2 different surfaces structured by ytterbium-doped pulsed fiber laser operating at 1064 nm.Materials and methods. 60 dental implants were placed in the study. 2 types of dental implant surfaces, namely holes and parallel grooves, were created by the ytterbium laser operating at 1064 nm. A polished dental implant (without laser surface structuring) was also included in the experiment for comparison. The study was carried out on 15 laboratory animals (male rabbits, weight 3.5-4 kg). The implants were placed in the tibia. 4 implants with different surface types but of the same diameter and length were placed in each rabbit.Results. Laboratory animals were sacrificed 1.5 and 3 months after the surgery. The stability of the implants was assessed by RFA (Resonance Frequency Analysis), based on the registration of resonance electromagnetic oscillations of the implant and the surrounding bone when they are exposed to the electromagnetic field (Osstell ISQ). Also, nondecalcified bone blocks were histologically examined using a confocal laser scanning microscope (Carl Zeiss LSM 780) and histomorphometry was performed (BIC-index: Bone-to-implant contact). Bone blocks were prepared according to a special technique: they were soaked and embedded into the plastic and synthetic resin. The obtained blocks were cut into sections, 40-50 µm thick, and stained with toluidine blue.Conclusion. Laser surface structuring of the dental titanium implants is a promising technique. 59 in 60 (98.3%) implants were osseointegrated, there were no signs of inflammation in the bone tissue. The present results allow further studying of dental implants with various surface designs, structured by ytterbium laser.

Secondary electron yield reduction by femtosecond pulse laser-induced periodic surface structuring

The electron-cloud phenomenon is one cause of beam instabilities in high intensity positive particle accelerators. Among the proposed techniques to mitigate or control this detrimental effect, micro-/nano-geometrical modifications of vacuum chamber surfaces are promising to reduce the number of emitted secondary electrons. Femtosecond laser surface structuring readily allows the fabrication of Laser Induced Periodic Surface Structures (LIPSS) and is utilized in several fields, but has not yet been tested for secondary electron emission reduction. In this study, such treatment is carried out on copper samples using linearly and circularly polarized femtosecond laser pulses. The influence of the formed surface textures on the secondary electron yield (SEY) is studied. We investigate the morphological properties as well as the chemical composition by means of SEM, AFM, Raman and XPS analyses. Surface modification with linearly polarized light is more effective than using circularly polarized light, leading to a significant SEY reduction. Even though the SEY maximum is only reduced to a value of ~1.7 compared to standard laser-induced surface roughening approaches, the femtosecond-LIPSS process enables to limit material ablation as well as the production of undesired dust, and drastically reduces the number of redeposited nanoparticles at the surface, which are detrimental for applications in particle accelerators. Moreover, conditioning tests reveal that LIPSS processed Cu can reach SEY values below unity at electron irradiation doses above 10−3 C/mm2.

Laser Surface Structuring of Commercial Pure Titanium

Aim This Study existent laser surface structuring of commercial pure titanium (Ti) grade II dental implant,surface structuring of the titanium implant expected causing decrease the time of implant healing andaccelerated the periimplant bone formation. So the present study aimed was in vitro evaluation of commercialpure titanium when subjected to two laser surface structuring (dot and groove) in different laser scans andcomparing with titanium surface that not subjected to any surface structuring (control) by using opticalmicroscope, scanning electron microscope (SEM) and X-ray Diffraction analysis (XRD) test.Materials and Methods: CNC fiber laser machine laser was used for performing the structuring on thesurface of the commercial pure titanium (CP Ti) disks and then analyzed with the control group by opticalmicroscope, SEM and XRD test. The study groups are: polished CP Ti (control); (5, 15 and 25) CP Ti withdot design laser scans; (5, 15 and 25) CP Ti with groove design laser scans.Results the optical and SEM results showed a uniform homogenous structure on the surface of the CP Ti forthe two design of all tested laser scans (5, 15 and 25). The XRD results show an increasing in the titaniumoxide layer for the disks with laser structuring comparing to the control one which predicted to increasetitanium osseointegration while maintaining biocompatibility characteristics of the CP Ti.Conclusions: CNC fiber laser machine are successful method for CP Ti surface structuring. For both lasersurfaces structuring design, the 25 laser scan showed more promising results follow by 15 laser scan then 5laser scan and lastly the control group.

Insight into replication effectiveness of laser-textured micro and nanoscale morphology by injection molding

Abstract Detailed analysis and characterization of injection-molded polypropylene (PP) specimens produced with laser-textured molds has been performed. Ultrashort pulsed laser surface structuring was exploited to produce sub-micrometric surface features on injection molds over areas of 2400 mm2, including two-dimensional hole and cone arrays, and ridges parallel and perpendicular to the injection direction. Replication effectiveness was evaluated in terms of surface roughness parameters, Filling Volume Fraction (FVF) and Power Spectral Density (PSD), after which detailed comparison of the topography in the same region on plastic samples and molds was performed. A scatterometry setup was employed based on optical diffraction of a probe laser beam to assess replication fidelity over the entire surface of each PP sample with a measurement technique suitable for optimization and quality control in an industrial production environment. Of the tested surface morphologies, greatest replication effectiveness was achieved with LIPSS oriented parallel to the injection direction, where the arithmetic mean height of plastic samples (Sa = 37 ± 4 nm) was 97 % of the corresponding value of the mold (Sa = 38 ± 6 nm), while the average FVF was 86 ± 9 % and the PSD indicated ripple-like features with a spacing of λ ≃ 0.9 μm. Direct comparison of the topography in selected regions highlighted local variability in the transfer effectiveness of individual surface features. The investigation demonstrated that micro and nanoscale morphology can effectively be transferred via injection molding over large areas relevant to industrial applications, while careful attention must be paid to the size and nature of defects in relation to the specific functional surface under consideration.

Additive Manufacturing of Titanium with Different Surface Structures for Adhesive Bonding and Thermal Direct Joining with Fiber-Reinforced Polyether-Ether-Ketone (PEEK) for Lightweight Design Applications

Hybrid joints consisting of metals and fiber-reinforced polymer composites exhibit highly desirable properties for many lightweight design applications. This study investigates the potential of additively manufactured surface structures for enhancing the bond strength of such joints in comparison to face milled and laser structured surfaces. Titanium samples with different surface structures (as-built surface, groove-, and pin-shaped structures) were manufactured via electron beam melting and joined to carbon fiber-reinforced polyether-ether-ketone (PEEK) via adhesive bonding and thermal direct joining, respectively. Bond strength was evaluated by tensile shear testing. Samples were exposed to salt spray testing for 1000 h for studying bond stability under harsh environmental conditions. The initial tensile shear strengths of the additively manufactured samples were competitive to or in some cases even exceeded the values achieved with laser surface structuring for both investigated joining methods. The most promising results were found for pin-shaped surface structures. However, the hybrid joints with additively manufactured structures tended to be more susceptible to degradation during salt spray exposure. It is concluded that additively manufactured structures can be a viable alternative to laser surface structuring for both adhesive bonding and thermal direct joining of metal-polymer hybrid joints, thus opening up new potentials in lightweight design.

Determining Interface Fracture Toughness in Multi Layered Environmental Barrier Coatings with Laser Textured Silicon Bond Coat

In the high temperature combustion atmosphere inside of aircraft turbines, the currently used ceramic matrix composites require a protective environmental barrier coating (EBC) to mitigate corrosion of the turbine parts. Besides thermomechanical and thermochemical properties like matching thermal expansion coefficient (CTE) and a high resistance against corrosive media, mechanical properties like a high adhesion strength are also necessary for a long lifetime of the EBC. In the present work, the adhesion between an air plasma sprayed silicon bond coat and a vacuum plasma sprayed ytterbium disilicate topcoat was aimed to be enhanced by a laser surface structuring of the Si bond coat. An increase in interface toughness was assumed, since the introduction of structures would lead to an increased mechanical interlocking at the rougher bond coat interface. The interface toughness was measured by a new testing method, which allows the testing of specific interfaces. The results demonstrate a clear increase of the toughness from an original bond coat/topcoat interface (8.6 J/m2) compared to a laser structured interface (14.7 J/m2). Observations in the crack propagation indicates that the laser structuring may have led to a strengthening of the upper bond coat area by sintering. Furthermore, in addition to cohesive failure components, adhesive components can also be observed, which could have influenced the determined toughness.

Ultrafast fiber laser-induced fabrication of superhydrophobic and self-cleaning metal surfaces

Wetting properties of metal surfaces play important role in different practical and fundamental applications. In this work we investigate the variation of wetting properties of three different metals (aluminum, copper and galvanized steel) when ablated with ultrafast fiber lasers of femtosecond and picosecond pulse durations. The role of pulse duration and scanning speed used in the laser-surface structuring is analyzed. Scanning electron microscopy, atomic force microscopy, and attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) are employed to characterize the roughness and chemistry of the laser-structured surfaces. The degree of hydrophobicity and self-cleaning characteristics of three laser-structured metals are compared when aged in different post-ablation environments. Freshly laser-treated samples of aluminum, copper, and galvanized steel demonstrated superhydrophilic wetting response when their contact angles are measured immediately after laser ablation. The superhydrophilic characteristics of these freshly laser-structured surfaces are found to evolve to hydrophobic state after 30 days of ageing in ambient atmospheric conditions, while they take only 6 h to transform to superhydrophobic state when aged in low-pressure environment. Ultrafast laser-surface nanostructuring, coupled with vacuum ageing, proves to be an effective and rapid approach in achieving extreme superhydrophobic states in different metals, which makes it suitable for a wide range of self-cleaning applications.

Self‐Limited Ice Formation and Efficient De‐Icing on Superhydrophobic Micro‐Structured Airfoils through Direct Laser Interference Patterning

AbstractForward facing aerodynamic surfaces such as rotors and wings are susceptible to ice build‐up when exposed to atmospheric icing conditions. If not removed, accumulated ice on aircraft surfaces affects aerodynamics or rotation balance, which can ultimately lead to increased fuel consumption, reduced operational performance and to potentially hazardous situations. Laser surface structuring is proposed as an alternative technology to coatings for achieving icephobic properties and support anti‐icing and de‐icing processes on aerodynamic surfaces. However, to authors’ knowledge, no study available in the literature reports on the icing behavior of microtextured curved aerodynamic profiles and the effect of the laser surface treatment on the electrothermal heating used for ice protection systems. In this work, direct laser interference patterning is employed to fabricate hierarchical micro‐ and nanostructures directly on a non‐planar titanium airfoil. The anti‐icing performance of the laser‐treated airfoil is tested in an icing wind tunnel under simulated atmospheric conditions. The results demonstrate a self‐limiting ice growth, a decrease in the deicing electro‐thermal power up to 80%, and up 60% lower heating power necessary to keep the surface free of ice than on the reference airfoil.

On the formation and features of the supra-wavelength grooves generated during femtosecond laser surface structuring of silicon

We have investigated the formation process of supra-wavelength quasi-periodic surface structures, commonly termed as grooves, produced during femtosecond laser surface structuring of a silicon target, in air.The experiments have been carried out at two wavelengths, namely 513 nm and 1026 nm, and in multi-pulse irradiation conditions. Both static, i.e., irradiating the same target location, and dynamic, namely scanning the beam along a line, configurations have been explored. We have also carried out simulations of the process by using a multiscale numerical model, based on the two-temperature model coupled to a fluid dynamics approach. The comparison of experimental and numerical results has allowed us to clearly establish the role of hydrothermal waves in the grooves formation process.

Direct femtosecond laser surface structuring with complex light beams generated by q-plates

Abstract Direct femtosecond (fs) laser surface structuring became a versatile way to generate surface structures on solid targets demonstrating a high degree of flexibility and controllability in creating different types of structures for many applications. This approach demonstrated an alteration in various properties of the surface, such as optical properties, wetting response, etc. This paper focuses on direct fs laser surface structuring using complex light beams with spatially variant distribution of the polarization and fluence, with emphasis on the results obtained by the authors by exploiting q-plate beam converters. Although striking scientific findings were achieved so far, direct fs laser processing with complex light fields is still a novel research field, and new exciting findings are likely to appear on its horizon.

Efficient production of design textures on large-format 3D mold tools

Laser surface structuring is becoming increasingly important in the industry for tool and mold making. While structured surfaces contribute to minimizing friction in combustion engines or to increasing efficiency of light-emitting diode-based lighting systems, surface texturing is evolving a quality feature of products with regard to optical and haptic properties. Currently used manufacturing processes for tool texturing like photochemical etching are limited in precision and in flexibility. To establish a digital process chain and to increase the design flexibility, laser ablation with (ultra) short pulse laser radiation is becoming an increasingly important technology. In the research project “eVerest,” all necessary parts of a laser texture processing are integrated into the machine and operating concept, e.g., the virtual design of the product including unrolling and visualization of the textures. Finally, new process strategies and advanced machine and system technologies are developed.

Cryogenic surface resistance of copper: Investigation of the impact of surface treatments for secondary electron yield reduction

The surface resistance of copper samples with an amorphous carbon (a-C) coating or with laser surface structuring, the surface treatments of choice for electron cloud suppression in critical cryogenic sectors of the high-luminosity upgrade of the Large Hadron Collider (HL-LHC), has been measured for the first time at a cryogenic temperature using the quadrupole resonator at CERN. Three different frequencies of relevance for evaluating beam impedance effects, namely, 400, 800, and 1200 MHz, have been investigated. No significant increase in surface resistance is observed for the a-C coating, compared to plain copper. In the case of laser structuring, the surface resistance depends on the direction of the surface currents relative to the laser-engraved groove pattern. The increase is minimal for parallel patterns, but in the perpendicular case the surface resistance increases considerably. Radio frequency (rf) heating from wake losses would then also increase in the HL-LHC case; however, the reduction in the power deposited onto the cold surfaces thanks to electron cloud suppression would still outweigh this effect.

Laser Surface Structuring of Cemented Carbide for improving the Strength of Induction Brazed Joints

The effect of micro patterning of cemented carbide surface using nanosecond diode pumped solid-state pulsed laser on the strength of induction brazed carbide and steel joints has been investigated. Surface patterns increase the total surface area of the joint and, for an originally hydrophilic surface, increase the wettability of a liquid on a solid surface such that, instead of building droplets, the liquid spreads and flows on the surface. Microcomputed tomography (µ-CT) was used to observe the filler/carbide interface after brazing and to analyze the presence of porosity or remnant flux in the joint. Microstructures of the brazed joints with various surface patterns were analyzed using scanning electron microscopy. The strength of the joints was measured using shear tests. Results have shown that the groove pattern on the surface of carbide increases the joint strength by 70–80%, whereas, surface patterns of bi-directional grooves (grid) reduced the joint strength drastically. Dimples on the carbide surface did not show any improvement in the strength of the brazed joints compared to samples with no surface pattern.

Recent Advances in Femtosecond Laser-Induced Surface Structuring for Oil–Water Separation

Femtosecond (FS) laser-induced surface structuring is a robust, maskless, non-contact, and single-step process for producing micro- and nanoscale structures on a material’s surface, which remarkably alters the optical, chemical, wetting, and tribological properties of that material. Wettability control, in particular, is of high significance in various applications, including self-cleaning, anti-fouling, anti-icing, anti-corrosion, and, recently, oil–water separation. Due to growing energy demands and rapid industrialization, oil spill accidents and organic industrial discharges frequently take place. This poses an imminent threat to the environment and has adverse effects on the economy and the ecosystem. Oil–water separation and oil waste management require mechanically robust, durable, low-cost, and highly efficient oil–water manipulation systems. To address this challenge superhydrophobic–superoleophilic and superhydrophilic–underwater superoleophobic membrane filters have shown promising results. However, the recyclability and durability issues of such filters are limiting factors in their industrial application, as well as in their use in oil spill accidents. In this article, we review and discuss the recent progress in the application of FS laser surface structuring in producing durable and robust oil–water separation membrane filters. The wide variety of surface structures produced by FS laser nano- and micromachining are initially presented here, while the excellent wetting characteristics shown by specific femtosecond-induced structures are demonstrated. Subsequently, the working principles of oil–water separation membranes are elaborated, and the most recent advances in the topic are analyzed and discussed.

Vector vortex beams generated by q-plates as a versatile route to direct fs laser surface structuring

We report an experimental investigation on direct laser surface structuring with femtosecond vector vortex beams generated by means of q-plates with topological charges q = 1, 3/2, 2, 5/2. Structured light beams with spatially variant state of polarization and intensity are generated and applied to multi-pulse irradiation of a solid crystalline silicon target. The creation of a variety of surface structures, like laser induced periodic surface structures, multi-spot arrays and shaped ablation craters, is demonstrated by direct laser surface structuring with vector vortex beams at different values of q. The features of the surface structures are compared with the vector vortex beam characteristics at the focal plane, evidencing their relationship with the polarization and intensity profile of the laser beams. Our experimental findings show that vector vortex beams produced by q-plates can offer a valuable and versatile route to imprint unconventional surface structures on a solid target through a mask-free ablative process and step scan processing.

Laser surface structuring of diamond with ultrashort Bessel beams

We investigate the effect of ultrafast laser surface machining on a monocrystalline synthetic diamond sample by means of pulsed Bessel beams. We discuss the differences of the trench-like microstructures generated in various experimental conditions, by varying the beam cone angle, the energy and pulse duration, and we present a brief comparison of the results with those obtained with the same technique on a sapphire sample. In diamond, we obtain V-shaped trenches whose surface width varies with the cone angle, and which are featured by micrometer sized channels having depths in the range of 10–20 μm. By laser writing crossed trenches we are also able to create and tailor on the diamond surface pillar-like or tip-like microstructures potentially interesting for large surface functionalization, cells capturing and biosensing.