Surface Functionalization (LiM 2019)

Application of femtosecond laser shock peening in Nitrogen gas for improvement of corrosion resistance of NiTi alloy in Hank's solution
Hao Wang, Evgeny Gurevich, Andreas Ostendorf

Femtosecond laser shock peening(FsLSP) in Nitrogen gas was performed on the NiTi shape memory ally without
protective coating. In this article X-ray diffraction (XRD), Energy Dispersive X-Ray (EDX), Scanning Electron Microscope
(SEM), three dimensional surface morphologies, wettability and corrosion properties were tested. The XRD results
showed that TiN and Ni4Ti3 coating can be found on the surface after femtosecond laser shock peening and the sample
Bragg diffraction peak became broader than that of the sample without FsLSP treatment, which also showed that the
surface residual stress could be generated by FsLSP. The wettability measurements showed that the FsLSP decreased the
contact angle of Hank's solution. The electrochemical corrosion property and immersion corrosion behaviour were
tested in Hank's solution, which was prepared in our laboratory. The corrosion results demonstrated that the corrosion
inhibitive properties of NiTi sample could be improved with FsLSP treatment.

Keywords: Femtosecond laser shock peening, NiTi shape memory alloy, wettability, corrosion inhibitive properties


Laser-induced Periodic Surface Structures (LIPSS) on stainless steel moulds for thermoplastic composite materials manufacturing
M. Feinaeugle, M. Mezera, G.R.B.E. Römer

Steel moulds are widely used as forming tools during the fabrication of fibre reinforced thermoplastic composite parts.
The aim of this work is to study sub-micron laser-induced surface modifications which help to control interfacial
properties, such as undesirable sticking, between the composite polymer and the steel mould material. Stainless steel
(AISI 430) samples were machined with a laser with wavelength of λ = 515 nm and 7 ps pulse duration. The periodicity of
the Laser-induced Periodic Surface Structures (LIPSS) was determined, via fast Fourier transform of scanning electron
microscope images, as a function of input parameters, such as effective number of laser pulses, pulse energy and
accumulated fluence. Above a certain threshold, LIPSS appear over a broad range of fluences. Spatial frequencies were
found to be between about λ/10 and ~λ for laser pulse energies of up to ~1 μJ. First results from experiments on
interaction of composite polymers and steel moulds are also shown.

Keywords: micro processing; surface functionalisation; laser-induced periodic surface structure; stainless steel; thermoplastic


Tailored femtosecond laser fabrication of reflective waveplates
A. San Blas, N. Casquero, M. Martínez-Calderón, J. Buencuerpo, L.M. Sanchez-Brea, J. del Hoyo, S.M. Olaizola, A. Rodríguez, M. Gómez-Aranzadi

Laser induced periodic surface structures (LIPSS) can be studied as waveplates due to the birefringence induced by the
nanoripples, with easy fabrication since they are fabricated in a one-step process by laser, where LIPSS geometry are
defined by the characteristics of the laser and the substrate. The optical properties of these waveplates are defined by
LIPSS parameters such as period, depth or width of the ripples. In this work we have measured the change in
polarization of the light reflected from LIPSS generated on stainless steel. Results show a gradual change in polarization
as the parameters employed in the fabrication vary. As shown in our experiments with a setup based in cylindrical
focusing lens, the fast fabrication of samples with applications as waveplates is demonstrated.

Keywords: LIPSS; femtosecond; nanostructures; polarization; waveplate; laser


Challenges and Opportunities for Laser-based Additive Manufacturing of Strain Sensors
Matthias Rehberger, Christian Vedder, Johannes Henrich Schleifenbaum

The use of printing and laser technology for additive manufacturing allows for rapid creation of individual sensor struc-
tures. Digital production technologies may substitute cost-intensive manual application processes of strain gauges. The
development of additive production of individualized, component-connected sensors is shown. Necessary layers and
structures of different materials are printed directly onto the component and then laser-treated (sintered, melted, hard-
ened, etc.). In the case of strain gauges the insulation layer, the measuring grid and the encapsulation are applied. In
contrast to conventional thermal processes, no complete heating of the component (furnace) or irradiation of the entire
surface (flash lamps) is necessary. The energy deposition can be controlled in terms of time and location. This makes the
selective coating of temperature-sensitive components possible. A new process chain for an automated way of sensor
creation and application offers the widespread use of strain sensors in a new generation of smart products.

Keywords: Laser sintering; strain sensors; Additive manufacturing; Surface Functionalization


Antibacterial Surfaces Produced By High-Average Power USP Laser
Laura Gemini, Marc Faucon, Adrian H.A. Lutey, Luca Romoli, Javier Graus, Javier Sanz, Jorge Fantova, Yasmina Coman, Rainer Kling

Laser-textured antibacterial surfaces were produced on 316L stainless steel and its polypropylene replica. Antibacterial
properties of steel samples were investigated by ISO-standard based tests: best results were observed with 1μm-size
surface features, showing reductions of 99.8% for E.Coli and 84.7% for S.Aureus. Textured polypropylene inserts were
produced by injection molding from the textured steel samples and contaminated by standard bacteria live/dead kit. A
visible improvement in biofilm attachment was reached on the textured inserts with respect to non-textured reference

Keywords: antibacterial surfaces; biofilm adhesion; USP laser texturing; surface functionalisation


Direct Laser Interference Patterning of antibacterial surfaces on stainless steel by means of ultrafast laser
Alexander Peter, Adrian H.A. Lutey, Sebastian Faas, Luca Romoli, Volkher Onuseit, Thomas Graf

Direct Laser Interference Patterning (DLIP) is a laser processing method that provides a tool for creating a wide field of
functional surfaces. In the present work, the generation of antibacterial surfaces on stainless steel is shown with a high
power capable DLIP setup. The used laser was an ultrafast laser with a wavelength of 1030nm and a pulse duration of
eight picoseconds. Two different topographies were produced, which were generated with two different polarization
orientation of the laser. Both topographies were investigated in their antibacterial behavior. The employed method for
assessing the bacterial retention is based on ISO standards for measurement of antibacterial performance. The resulting
topographies shows a retention of up to 99,8% for E. Coli and up to 79.1% for S. aureus bacteria.

Keywords: direct laser interference patterning; two beam interference; functional surfaces; antibacterial surfaces; ultrafast laser;
stainless steel;


GO-neutral red nanocomposite reduction by nanosecond, picosecond and femtosecond lasers
Romualdas Trusovas, Mantas Valiuševicius, Justina Gaidukevic, Jurgis Barkauskas, Gediminas Niaura

Graphene has attracted significant interest from researchers due to its unique physical properties. One of graphene
production methods is based on graphene oxide (GO) reduction which can be implemented by laser irradiation. Our
previous research has shown that the best quality laser-induced graphene (LIG) coating is obtained using a GO-neutral
red (GO-NR) nanocomposite precursor.
In this work, we performed GO reduction experiments using nanosecond, picosecond and femtosecond IR lasers
irradiation. Irradiated GO-NR films were investigated by Raman spectroscopy. It was shown that the best quality LIG was
formed by nanosecond laser irradiation using 26 J/cm2 irradiation dose. When this dose was applied, the Intensity ratio
of Raman spectral lines I(2D)/I(G), showing GO reduction quality was 0.9.

Keywords: graphene; graphene oxide reduction; Raman


High resolution pixel based direct laser patterning for surface functionalization
Stephan Bruening, Keming Du, Manfred Jarczynski, Arnold Gillner

Micro patterning with ultrashort laser pulses allows the production of micro and nano sized surfaces patterns, which
results in specific functional properties. For the generation of those functionalities on large surfaces a two step
processing is necessary in which rotating cylinders are laser structured and those cylinders are used in micro embossing
to achieve processing speeds up to 20 m²/min. In order to realize embossing cylinders with seamless embossing
features, a moving ultra-short pulsed laser ablation process with multi beams is used. In this contribution direct
processing of negative nano and micro features are completely realized by a digital work flow beginning from a digital
data asset which is based on a pixel-to-pixel ablation in a fast surface scanning process (up to 40 m/s). Using high
repetition rate ultrashort pulsed laser ablation, a high productivity can be achieved with features sizes from 2 μm to
several 10 μm at areas of 2m² for adjustment of the surface properties.
A newly developed high compact ps-laser with
repetition rates of up to 1MHz and an average power of up to 500W was distributed into 16 independent modulated
parallel beamlets by a diffractive optical element (DOE).

Keywords: usp-laser, diffractive optical element DOE, acuosto optical modulator AOM, multi spot, parallel micro processing, 3D micro
structures, surface functionalization, metal sheet embossing, roll-to-roll micro embossing, nano imprint lithography (NIL), intaglio print


Increasing heat transfer of metals through periodical microstructures using Direct Laser Interference Patterning
Sabri Alamri, Frederic Schell, Tobias Steege, Andrés F. Lasagni, Tim Kunze

The increasing number of micro-electronic components per unit area led not only to an increase of computing power but
also to an increase of the device’s operating temperature. Heat sinks, equipped with macroscopic fins or pins, dissipate
heat by increasing the surface area. In this work, a microscopic approach for improving the heat transfer of metals is
presented, aiming to increase the surface area by fabricating periodical microstructures. Direct Laser Interference
Patterning has been used for fabricating microstructures on stainless-steel plates employing a nanosecond IR laser. A
statistical design of experiment was used for optimizing the structuring parameters and maximizing the surface area. The
heat dissipation properties, evaluated with a heat flux sensor, were correlated with the developed surface area, finding a
clear trend. In particular, the microstructuring permitted to increase the surface area up to 280%, which lead to an
increase of heat transfer by 23.8%.

Keywords: Heat Transfer; microstructures; DLIP;


Innovative method to enhance the control over the fabrication of LIPSS on metallic surfaces
M. Gómez-Aranzadi, N. Casquero, M. Martínez-Calderón, A. San Blas, S.M. Olaizola, A. Rodríguez

LIPSS are an emergent method of fabricating functional surfaces. A high repetition rate ultrashort pulsed laser was used
to generate LIPSS on metallic surfaces. Different combinations of the processing parameters were tested to study their
effect on the period of the LIPSS. Additionally, the influence of the repetition rate (laser frequency) on the LIPSS period
was also studied when the same number of pulses, polarization and peak fluences are applied. Finally, a chemical
analysis was performed in order to establish a correlation between the laser processing conditions and the resulting
chemical modifications (especially oxidation) in the ultrashort pulse regime.
As expected, the analysis shows a decrease
in the period of the LIPSS as the number of pulses increases, while the other processing parameters affect the period
slightly. Regarding the laser frequency, variations in the repetition rate –while the rest of the parameters are kept
constant- only slightly affect the period and morphology of the LIPSS. In the case of the chemical variations, two
different regimes were found, with Low Spatial Frequency LIPSS exhibiting no oxidation for low accumulated fluences
and High Spatial Frequency LIPSS presenting increased oxidation as the fluences accumulate.

Keywords: LIPSS, femtosecond laser, period, repetition rate


Laser material processing and functionalization with tailored focal intensity distributions
Anna Möhl, Frank A. Müller, Ulrike Fuchs, Stephan Gräf

Gaussian intensity profiles are widely used in the field of laser material processing. Nevertheless, there are applications,
where the inhomogeneous beam profile is not acceptable, and another intensity distribution would be much more
advantageous. We show that refractive beam shaping systems provide very good results for generating tailored focal
intensity distributions, e.g. top-hat or doughnut shaped profiles. Even though using just one beam shaping system the
width of the profiles is scalable. Beyond, the device is suitable for working with a scanner and F-Theta lens as commonly
used for material processing. Results of material processing of stainless steel are presented for different focal intensity
distributions and evaluated. In conclusion, the optimization of processing time and parameters are discussed.
Additionally, an application example is presented in which laser-induced periodic surface structures (LIPSS) are
generated that can be used for the functionalization of surfaces.

Keywords: Beam Shaping, Laser material processing, surface functionalization, cutting, drilling


Laser polishing of Aluminum and Polyamide for dissimilar laser welded assemblies
Mahdi Amne Elahi, Marcus Koch, Peter Plapper

In the present study, laser macro polishing on both Aluminum (Al) and Polyamide (PA) was investigated as a pre-
treatment process for laser welding. To apply the laser polishing on Al and PA we used continuous wave (CW) fiber and
CO2 lasers, respectively. The samples were then welded in an overlap configuration. The results show that applying laser
polishing in an atmospheric condition generates a nano-structured oxide layer generated on the Al surface. This layer
represents much lower and isotropic roughness. In addition, considering the polishing of PA surface, some significant
changes in the structure of the polished area are observed compared to as-received condition; however, there is no
significant alteration in the surface chemistry. The alteration of surface structure can be attributed to a different
crystallinity state. The modification of the Al surface is mainly responsible for improving tensile-shear load of the joint;
however, modification of the PA surface can also contribute to the further improvement of the tensile-shear load and
change the mechanism of failure.

Keywords: laser welding; laser macro polishing; nano-structured oxide layer; crystallinity.


Laser texturing of superhydrophobic surfaces on stainless steel: influence of storage conditions on the anti-wetting transition
Wojciech S. Gora, Juan Pedro Godoy Vilar, Jonathan, D. Shephard, Duncan P. Hand

The superhydrophobic properties of natural surfaces (e.g. lotus leaves), have been a key driver behind research focused
on laser texturing to provide such properties on metallic surfaces via the creation of similar topological features. Anti-
wetting surfaces have associated self-cleaning and antibacterial characteristics and hence the ability to controllably
create such surfaces on metal parts would be beneficial in many industrial sectors, such as food, medical, and aerospace.
In this paper we focus on creating superhydrophobic surfaces on thin sheets of SS316L using a nanosecond pulsed fibre
laser, with typical parameters of 220 ns pulse duration, scanning speed of 150 mm/s and repetition rate of 25 kHz. As
observed by a number of researchers, immediately after laser processing the textured surfaces display hydrophilic
properties, with some of these surfaces subsequently transitioning to anti-wetting over a period of days or even weeks.
The duration of this transition (and the ultimate degree of hydrophobicity) depends not only on the laser texturing
parameters, but also on the sample storage conditions. In this paper we present a study of the impact of both laser
scanning parameters (line separation, pulse energy) and storage conditions on the degree of hydrophobicity, by periodic
measurement of the contact angle of the textured samples.

Keywords: laser texturing, hydrophobicity, superhydrophobic, antiwetting


Periodic structures for superhydrophobic performance of metallic surfaces produced by femtosecond pulsed laser
P.W. Butler-Smith ,T.L See, J. Liu, X. Hou

Superhydrophobic surfaces can be defined as those exhibiting a water contact angle of greater than 150° with high
water droplet mobility. Such surfaces typically benefit from air pockets formed under incident water droplets which are
partially suspended by hierarchical structures of acutely angled protruding micro-asperities. In this work, controlled
populations of regular pyramid structures measuring less than 100 micrometres across were formed in aluminium
substrates in defined patterns using a femtosecond pulsed laser. It has been demonstrated from the as-processed
surfaces that the water droplet contact angle was dependent on feature population, escalating to a level of
superhydrophobicity, with water contact angles up to 157° and roll off angle of as low as 2° being achieved. In addition,
investigations were undertaken on these laser processed surfaces on performance consistency and durability with the
view to upscale for applications in the aerospace, energy, medical, food and wider industrial sectors.

Keywords: Femtosecond laser, hydrophobic, contact angle, periodic structures


Picosecond Laser Based Surface Texturing of Silicon for Anti-Reflective Properties
Anup Sharma, Deepak Marla, Suhas Joshi

Surfaces of Silicon-based solar cells are modified to reduce reflectivity and enhance the absorption of light. Texturing
using ultrafast lasers is a potential technique for surface modification of solar cells. In order to fabricate superior
surfaces with low reflectivity and optimized electrical performance, it is essential to understand the impact of texture
characteristics. In this work, two dimensional surface textures were fabricated on Silicon surface using a picosecond
laser operated at different laser parameters and the reflectance was measured. The experimental data was used to
identify the set of optimal laser parameters and dimensions of surface structures to minimize the surface reflectivity of
Silicon. The height of the structures obtained were around 10 μm with a spacing of 25 to 30 μm. As a result, a wave like
profile was generated, which can enhance the absorption of photons on the surface and hence reduce reflectivity. Using
this technique, the surface reflectivity was reduced to as low as 5 %. In addition, the electrical behavior of the textured
surface was simulated using finite difference time domain (FDTD) method. The current density and power generation
characteristics of the textures were obtained and were found to be comparable with existing technology of anti-
reflective coatings.

Keywords: Surface texturing; Picosecond pulsed laser; Silicon wafers; Reflectivity; Current Density and power generation