PhD degree awarded to Sabri Alamri

On September 4th, 2020, the Early Stage Researcher Sabri Alamri, from the Laser4Fun project, successfully defended his thesis for the degree of Doctor.

The work titled “Advanced microstructuring strategies of polymers using Direct Laser Interference Patterning” was carried out at Technical University Dresden and the Fraunhofer IWS, both in Germany, under the supervision of Professor Andres Lasagni.

We congratulate Sabri on his achievement!

PhD defense ceremony Marek Mezera on Oct. 16th 2020

Marek Mezera an Early Stage Researcher (ESR), will defend his PhD-thesis on October 16th, starting at 16:30 o’clock in room 4 of the Waaier building of the University of Twente in The Netherlands. Supervised by prof.dr.ir. Gert-willem Römer and Dr.ir. Dave Matthews, he carried out most of his work at the Chair of Laser Processing at the University of Twente. Below is a summary of the work and results of Marek.

Summary

The interaction of an object with its surroundings depends firstly on the object’s surface, which determines its surface properties due to its chemical properties and macro- and microscopic structures on the surface. Earth’s flora and fauna offers a vast collection of different specialized surface properties due to (hierarchical) micro- and nanostructures. These natural examples serve as a catalog for scientists and product designers who aim for replicating those properties. A one-step method to produce highly regular (hierarchical) nanostructures is to create Laser-induced Periodic Surface Structures (LIPSS). Although extensively studied in the last half century, LIPSS manufacturing did not emerge from an experimental status into industrial applications due to scientific and technical challenges. In this thesis, a picosecond pulsed laser source is evaluated for the potential of processing a large portfolio of LIPSS on different types of materials. Therefore, six research topics are addressed. First, the feasibility of processing LIPSS on polymers using a picosecond pulsed laser source was studied. It was found, that LIPSS can be indeed be processed on two most prominent polymers (polycarbonate and polystyrene) using picosecond pulsed laser sources. The wavelength range, at which LIPSS production was possible was found to be wider than for nanosecond pulsed laser sources, but not as wide as for femtosecond pulsed laser sources. However, the range of peak fluence levels and number of pulses impinging one spot needed to produce LIPSS are found to be similar for all laser pulse durations. Second, the influence of the bulk temperature of a thermoplastic polymer (polycarbonate) on the formation of LIPSS using a picosecond pulsed laser source was investigated. It was found that the peak fluence levels at which LIPSS form decrease with increasing bulk temperature. However, the bulk temperature did not influence the number of pulses impinging one spot necessary for LIPSS development. From this, it can be concluded, that the development of so-called Low Spatial Frequency LIPSS of type II (LSFL-II) is an accumulative process, depending on the number of pulses and that LIPSS formation is strongly affected by the local sample temperatures reached and by the involved strong variations of the polymer viscosity. Third, the formation of LIPSS on a pre-structured polymer (polycarbonate) was examined in order to achieve hierarchical micro-/nano-surface structures. It was found that LIPSS can be formed on top of various forms and sizes of micrometer-sized Direct Laser Interference Patterning (DLIP) structures (ridges) by selecting the laser beam polarization perpendicular to the pre-structured microscopic ridges. Since LIPSS only form in a narrow window of laser fluence levels, the growth of LIPSS only on top of the pre-structured microscopic ridges was limited by the non-normal angle of incidence of the laser radiation at the side walls of the microscopic ridges. Fourth, the feasibility of producing all types of LIPSS known to form on metals due to femtosecond laser pulses, by using a picosecond laser source, with different types of laser polarizations on a medical-grade cobalt-chrome-molybdenum alloy was explored. It was found that indeed all of LIPSS types which form due to femtosecond laser pulses on metals can be produced using picosecond laser pulses. Fifth, a mathematical model was developed to predict the homogeneity of large areas | i.e. larger than the area of a laser spot | of LIPSS depending on laser processing parameters and material constants. The model was validated by experiments on silicon. This model can also be used to optimize laser processing parameters to decrease the production time for large areas of LIPSS. It was concluded, that the model is a convenient tool, which can be exploited for determining the process parameters necessary for the production of large homogeneous areas of LIPSS at the shortest possible production time. Last, it was shown that the LIPSS production time for Low Spatial Frequency LIPSS of type I (LSFL-I) can be further decreased by defocused laser processing. That is, it was shown experimentally that LIPSS produced on silicon do not differ in periodicity and height when processing in focus, or below or above the focus of the laser beam. Hence, to further increase production rates of LIPSS, defocused laser processing is a viable approach.

LASER4FUN: the European Network on short pulsed laser micro/nanostructuring of surfaces

Results of the work in the Laser4Fun project has been published as:

Antonio Ancona, Andrés Fabián Lasagni, Gert-willem R. B. E. Römer, Stefan Dimov, Rainer Kling, Thomas Kiedrowski, José Luis Ocaña, Elmar Bonaccurso, Carsten Werner, Andrés Escartín Barduzal. LASER4FUN: the European Network on short pulsed laser micro/nanostructuring of surfaces. 21st International Symposium on Laser Precision Microfabrication, 23 – 26 June 2020.

Abstract

Bio-inspired surface structures, containing features at the nanometer/micrometer scales, offer significant commercial potential for the creation of functionalized surfaces. To this extent, technologies to modify surfaces instead of creating composites or applying coatings on surfaces can offer new industrial opportunities. The aim of the LASER4FUN project was to structure surfaces embedding properties for industrial applications. The research programme resulted in insights and applications beyond the current state of the art through the development of new surface micro/nano-structuring/patterning methods by using emerging short pulsed and ultra-short pulsed laser technologies, i.e. Laser-Induced Periodic Surface Structures (LIPSS), Direct Laser Interference Patterning (DLIP), Direct Laser Writing (DLW) and two hybrid technologies. New laser-induced surface textures, and parameters to control these, were developed. Optimized laser-induced surface textures were developed for various surface functionalities for applications in the fields of e.g. tribology, aesthetics and wettability. In addition, methods, strategies and tools to allow production of these surface textured at high industrial production rates were developed. The project created an International Training Network (ITN) for Early Stage Researchers (ESRs) in the exciting field of laser-material processing, consisting of 14 doctoral students recruited by 10 international partners with wide experience in the field: 3 academic partners, i.e. Polytechnic University of Madrid (Spain), University of Birmingham (UK) and University of Twente (Netherlands), 4 research centers, i.e. Alphanov (France), Consiglio Nazionale delle Richerche (Italy), Fraunhofer Institute (Germany) and IPF-Leibniz-Institute fur Polymerforschung Dresden (Germany), and 3 industrial partners, i.e. Robert Bosch (Germany), Airbus (Germany), BSH Home Appliances (Spain). The close cooperation among multidisciplinary partners have fostered knowledge transfer to cross the Death Valley between science and the markets. Additionally, their participation in the LASER4FUN project have impacted highly the ESRs employability to knowledge-intensive companies/institutes that are the key for EU welfare. During the project duration more than 50 publications were prepared and more than 50 dissemination events (targeting both academica, industry and the general public) took place.

Link(s)

https://lpm2020.inventum.de

Hierarchical Micro-/Nano-Structures on Polycarbonate via UV Pulsed Laser Processing

Results of the work in the Laser4Fun project has been published as:

Marek Mezera, Sabri Alamri, Ward A.P.M. Hendriks, Andreas Hertwig, Anna Maria Elert, Jörn Bonse, Tim Kunze, Andrés Fabián Lasagni, Gert-willem R.B.E. Römer. Hierarchical Micro-/Nano-Structures on Polycarbonate via UV Pulsed Laser Processing on Polycarbonate. Nanomaterials 2020, 10 (06), 1184.

Abstract

Hierarchical micro/-nanostructures were produced on polycarbonate polymer surfaces by employing a two-step UV-laser processing strategy based on the combination of Direct Laser Interference Patterning (DLIP) of gratings and pillars on the microscale (3 ns, 266 nm, 2 kHz) and subsequently superimposing Laser-induced Periodic Surface Structures (LIPSS; 7–10 ps, 350 nm, 100 kHz) which adds nanoscale surface features. Particular emphasis was laid on the influence of the direction of the laser beam polarization on the morphology of resulting hierarchical surfaces. Scanning electron and atomic force microscopy methods were used for the characterization of the hybrid surface structures. Finite-difference time-domain (FDTD) calculations of the laser intensity distribution on the DLIP structures allowed to address the specific polarization dependence of the LIPSS formation observed in the second processing step. Complementary chemical analyzes by micro-Raman spectroscopy and attenuated total reflection Fourier-transform infrared spectroscopy provided in-depth information on the chemical and structural material modifications and material degradation imposed by the laser processing. It was found that when the linear laser polarization was set perpendicular to the DLIP ridges, LIPSS could be formed on top of various DLIP structures. FDTD calculations showed enhanced optical intensity at the topographic maxima, which can explain the dependency of the morphology of LIPSS on the polarization with respect to the orientation of the DLIP structures. It was also found that the degradation of the polymer was enhanced for increasing accumulated fluence levels.

Link(s)

https://doi.org/10.3390/nano10061184

Design Rules for Laser‐Treated Icephobic Metallic Surfaces for Aeronautic Applications

Results of the work in the Laser4Fun project has been published as:

Vittorio Vercillo, Simone Tonnicchia, Jean‐Michel Romano, Antonio García‐Girón, Alfredo I. Aguilar‐Morales, Sabri Alamri, Stefan S. Dimov, Tim Kunze, Andrés Fabián Lasagni, Elmar Bonaccurso. (2020) Design Rules for Laser‐Treated Icephobic Metallic Surfaces for Aeronautic Applications. Advanced Functional Materials, 1910268.

 

Abstract

Ice accretion on external aircraft surfaces due to the impact of supercooled water droplets can negatively affect the aerodynamic performance and reduce the operational capability and, therefore, must be prevented. Icephobic coatings capable of reducing the adhesion strength of ice to a surface represent a promising technology to support thermal or mechanical ice protection systems. Icephobicity is similar to hydrophobicity in several aspects and superhydrophobic surfaces embody a straightforward solution to the ice adhesion problem. Short/ultrashort pulsed laser surface treatments are proposed as a viable technology to generate superhydrophobic properties on metallic surfaces. However, it has not yet been verified whether such surfaces are generally icephobic under representative icing conditions. This study investigates the ice adhesion strength on Ti6Al4V, an alloy commonly used for aerospace components, textured by means of direct laser writing, direct laser interference patterning, and laser‐induced periodic surface structures laser sources with pulse durations ranging from nano‐ to femtosecond regimes. A clear relation between the spatial period, the surface microstructure depth, and the ice adhesion strength under different icing conditions is investigated. From these observations, a set of design rules can be defined for superhydrophobic surfaces that are icephobic, too.

Link(s)

https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.201910268

PhD degree awarded to Antonio García Girón

On 28th February 2020, our Early Stage Researcher Antonio García Girón, from the Laser4Fun project, successfully defended his thesis for the degree of Doctor of Philosophy.

The work titled “Laser-based surface functionalisation: advances in durability and 3D processing” was carried out in the University of Birmingham under the supervision of Professor Stefan Dimov, studying some limitations of the laser surface functionalisation. The examination board was composed by Dr. C. Kong, Dr. S. Bigot and Dr. N. Gao, and the viva took place at the University of Birmingham in the morning on 28th February 2020.

Summary of Antonio’s research
Surface functionalization is gaining interests for industry and research due to the new attractive properties that can be “imprinted” on metal components, e.g. bacteria repellence or hydrophobicity among others. Considering the available alternative technologies to achieve such functional responses, direct laser writing is gaining a popularity due to its cost-effectiveness, selectivity and relatively short processing time. It allows surface properties to be modified or tuned by patterning and texturing at micron or submicron scales. However, laser surface functionalization has some limitations, too, such as the durability of the produced topographies and hence of their functionality, and also capabilities to apply it on free-form surfaces. In this context, the focus of the research presented in this thesis is on addressing these open issues. In particular, a combination of plasma surface alloying and laser patterning is proposed in order to increase hardness of produced functional surfaces, and thus to increase their wear resistance and durability. Also, a method to study the effects of the process disturbances in patterning 3D surfaces is proposed, especially on resulting topographies and their functional responses. All together, the research advances the knowledge in laser surface patterning and addresses key constraints for the broader use of this technology by industry.

Development of an Analytical Model for Optimization of Direct Laser Interference Patterning

Results of the work in the Laser4Fun project has been published as:

Bogdan Voisiat, Alfredo I. Aguilar-Morales, Tim Kunze and Andrés Fabián Lasagni. Development of an Analytical Model for Optimization of Direct Laser Interference Patterning. Materials 13 (200), 2020.

Abstract

Direct laser interference patterning (DLIP) has proven to be a fast and, at the same time, high-resolution process for the fabrication of large-area surface structures. In order to provide structures with adequate quality and defined morphology at the fastest possible fabrication speed, the processing parameters have to be carefully selected. In this work, an analytical model was developed and verified by experimental data, which allows calculating the morphological properties of periodic structures as a function of most relevant laser-processing parameters. The developed
model permits to improve the process throughput by optimizing the laser spot diameter, as well as pulse energy, and repetition rate. The model was developed for the structures formed by a single scan of the beam in one direction. To validate the model, microstructures with a 5.5 m spatial period were fabricated on stainless steel by means of picosecond DLIP (10 ps), using a laser source operating at a 1064 nm wavelength. The results showed a di erence of only 10% compared to the experimental results.

Link(s)

https://doi.org/10.3390/ma13010200

Influence of Bulk Temperature on Laser-Induced Periodic Surface Structures on Polycarbonate

Results of the work in the Laser4Fun project has been published as:

Marek Mezera, Jörn Bonse, Gert-willem Römer. Influence of Bulk Temperature on Laser-Induced Periodic Surface Structures on Polycarbonate. Polymers 2019, 11(12), 1947;

Abstract

In this paper, the influence of the bulk temperature (BT) of Polycarbonate (PC) on the occurrence and growth of Laser-induced Periodic Surface Structures (LIPSS) is studied. Ultrashort UV laser pulses with various laser peak fluence levels F0 and various numbers of overscans (NOS) were applied on the surface of pre-heated Polycarbonate at different bulk temperatures. Increased BT leads to a stronger absorption of laser energy by the Polycarbonate. For NOS<1000 High Spatial Frequency LIPSS (HSFL), Low Spatial Frequency LIPSS perpendicular (LSFL-I) and parallel (LSFL-II) to the laser polarization were only observed on the rim of the ablated tracks on the surface but not in the center of the tracks. For NOS≥1000 , it was found that when pre-heating the polymer to a BT close its glass transition temperature ( Tg ), the laser fluence to achieve similar LIPSS as when processed at room temperature decreases by a factor of two. LSFL types I and II were obtained on PC at a BT close to Tg and their periods and amplitudes were similar to typical values found in the literature. To the best of the author’s knowledge, it is the first time both LSFL types developed simultaneously and consistently on the same sample under equal laser processing parameters. The evolution of LIPSS from HSFL, over LSFL-II to LSFL I, is described, depending on laser peak fluence levels, number of pulses processing the spot and bulk temperature.

Link(s)

https://doi.org/10.3390/polym11121947

Experimental investigation of processing disturbances in laser surface patterning.

Results of the work in the Laser4Fun project has been published as:

Antonio Garcia-Giron, Jean-Michel Romano, Afif Batal, Aleksandra Michalek, Pavel Penchev and Stefan Dimov. Experimental investigation of processing disturbances in laser surface patterning. Optics and Lasers in Engineering (2020) 126, 105900. doi: 10.1016/j.optlaseng.2019.105900.

Abstract

Laser surface patterning has attracted a significant interest from industry and research due to its promising applications in surface functionalisation. However, there are specific issues and limitations associated with the beam delivery, especially when processing 3-D surfaces and/or setting up routines for executing complex multi-axis processing strategies. In particular, there are common processing disturbances that affect the resulting surface topographies and profiles and their respective functional responses, i.e. geometrical distortions of resulting surface patterns, focal offset distance (FOD) and variations of beam incident angle (BIA). A method to investigate the effects of these factors in laser patterning 3-D surfaces is presented in this research, especially how their effects can be analysed independently by conducting empirical studies on planar surfaces. A pilot implementation of the proposed methodology is reported for producing channel-like patterns on stainless steel plates with a super-hydrophobic functional response. The results are discussed in detail to show how the effects of processing disturbances on topographies, profiles and areal parameters together with the respective functional responses of patterned planar surfaces can be analysed and then used to set constraints in pre-processing 3-D surfaces for follow up laser patterning.

Correlating nano-scale surface replication accuracy and cavity temperature in micro-injection moulding using in-line process control and high-speed thermal imaging.

Results of the work in the Laser4Fun project has been published as:

Federico Baruffi, Mert Gulcur, Matteo Calaon, Jean-Michel Romano, Pavel Penchev, Stefan Dimov, Ben R. Whiteside and Guido Tosello. Correlating nano-scale surface replication accuracy and cavity temperature in micro-injection moulding using in-line process control and high-speed thermal imaging. Journal of Manufacturing Processes (2019) 47, 367-381.
doi: 10.1016/j.jmapro.2019.08.017.

Abstract

Micro-injection moulding (μIM) stands out as preferable technology to enable the mass production of polymeric components with micro- and nano-structured surfaces. One of the major challenges of these processes is related to the quality assurance of the manufactured surfaces: the time needed to perform accurate 3D surface acquisitions is typically much longer than a single moulding cycle, thus making impossible to integrate in-line measurements in the process chain. In this work, the authors proposed a novel solution to this problem by defining a process monitoring strategy aiming at linking sensitive in-line monitored process variables with the replication quality. A nano-structured surface for antibacterial applications was manufactured on a metal insert by laser structuring and replicated using two different polymers, polyoxymethylene (POM) and polycarbonate (PC). The replication accuracy was determined using a laser scanning confocal microscope and its dependence on the variation of the main μIM parameters was studied using a Design of Experiments (DoE) experimental approach. During each process cycle, the temperature distribution of the polymer inside the cavity was measured using a high-speed infrared camera by means of a sapphire window mounted in the movable plate of the mould. The temperature measurements showed a high level of correlation with the replication performance of the μIM process, thus providing a fast and effective way to control the quality of the moulded surfaces in-line.