Positive effect of laser structured surfaces on tribological performance

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

Tobias Stark, Sabri Alamri, Alfredo I. Aguilar-Morales, Thomas Kiedrowski, Andrés Fabián Lasagni. Positive Effect of Laser Structured Surfaces on Tribological Performance. Journal of Laser Micro/Nanoengineering Vol. 14, No. 1, 2019.

Abstract

In recent years laser surface structuring has emerged as a viable tool to enhance surface function-ality. In this study, an ultrashort pulsed laser is used to create advanced surface structures in order to improve the tribological properties of steels. The objective is to decrease friction and wear especially for the mixed lubrication regime in which body and counterbody are in contact. We state the hypoth-esis that the high flow resistance in small channels helps to hold the lubricant in the tribocontact and therefore separates contacting asperities. Direct Laser Writing and Direct Laser Interference Pattern-ing methods are used to create cross-like structures of different channel sizes. Tribological properties of these structures are evaluated by means of a ball-on-disc tribometer. The surface structures and the wear behavior are analyzed with confocal microscopy. The results show a decrease of the coefficient of friction depending on the size of the channels. This behavior is discussed and compared to the hypothesis stated above.

Link(s)

Fabrication of inclined non-symmetrical periodic micro-structures using Direct Laser Interference Patterning

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

Sabri Alamri, Mikhael El-Khoury, Alfredo I. Aguilar-Morales, Sebastian Storm, Tim Kunze and Andrés F. Lasagni (2019) Fabrication of inclined non-symmetrical periodic micro-structures using Direct Laser Interference Patterning. Scientific Reports, volume 9, Article number: 5455.

Abstract

The direct fabrication of microstructures, having a non-symmetrical morphology with controllable inclination, presents nowadays a challenging task. Natural examples of surfaces with inclined topographies have shown to provide anisotropic functionalities, which have attracted the interest of several researchers in the last years. This work presents a microfabrication technique for producing microstructures with a determined and controllable inclination angle using two-beam Direct Laser Interference Patterning. Polyimide foils are irradiated with a 4 ns UV (266 nm) laser source producing line-like structures with a period varying from 4.6 µm to 16.5 µm. The inclinations, retrieved by tilting the sample with respect to the optical axis of the setup, are changed from 0° to 75°, introducing a well controllable and defined inclination of the structure walls. The structuring parameters (laser fluence, number of laser pulses and interference period) as well as the inclination of the microstructures are correlated with the global tilting of the sample. As a result, a determined laser fluence and number of pulses are necessary to observe a remarkable non-symmetrical morphology of the structures. In addition, the presence of structural undercuts is reported, which opens the possibility for developing new direction-dependent properties on polymeric materials. As an example, preliminary results on light diffraction are presented, showing a similar behavior as blazed diffraction gratings.

Link(s)

On the Interplay of DLIP and LIPSS Upon Ultra-Short Laser Pulse Irradiation

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

Sabri Alamri, Fotis Fraggelakis, Tim Kunze, Benjamin Krupop, Girolamo Mincuzzi, Rainer Kling and Andrés Fabián Lasagni (2019) On the Interplay of DLIP and LIPSS Upon Ultra-Short Laser Pulse Irradiation. Materials, 12(7), 1018.

Abstract

Controlling laser induced surface morphology is essential for developing specialized functional surfaces. This work presents novel, multi-scale periodic patterns with two-dimensional symmetry generated on stainless steel, polyimide and sapphire. The microstructures were realized by combining Direct Laser Interference Patterning with the generation of Laser Induced Periodic Surface Structures in a one-step process. An industrial, fiber femtosecond laser source emitting at 1030 nm with a pulse duration of 500 fs was utilized for the experiments. In the case of stainless steel, it was possible to create line-like or pillar-like surface patterns by rotating the polarization orientation with respect to the interference pattern. In the case of polyimide and sapphire, the absorption of the laser radiation was promoted by a multiphoton mechanism. In polyimide, grooves and pillars of several microns in depth were produced over an area much larger than the spot size. Finally, for sapphire, the simultaneous generation of interference-like pattern and laser induced periodic surface structures was realized. The results reported here provide valuable data on the feasibility to combine two state-of-the-art techniques with an industrial apparatus, to control the induced surface morphology.

Link(s)

PhD degree awarded to Gagandeep Singh Joshi

Gagandeep Singh Joshi, an Early Stage Researcher (ESR), defended his PhD-thesis on March 19, 2019 at 12:30 pm, in the Physics department at the University of Bari, Italy. Supervised by prof.dr. Antonio Ancona, a Senior Researcher at Institute of photonics and nanotechnology, National Research Council of Italy (CNR-IFN, Bari), and prof.dr. Giuseppe Carbone, Professor and Head of the Department of Mechanics, Mathematics and Management of the polytechnic university of Bari, Italy. Below are some pictures of the cermony as well as an English and Italian summary of the work and results of Gagandeep.

Gagandeep Singh Joshi-2019.03.19-01-XS

 

Gagandeep Singh Joshi-2019.03.19-02-XS

Gagandeep Singh Joshi-2019.03.19-03-XS

English summary

Over the past decades, surface texturing has shown to be an emerging technique to control the friction and wear. It consists of fabricating a pattern of small dimples or grooves on the surface of the materials in a very controllable way, which causes the change in the surface topography. Lasers with their excellent beam quality promised noticeable advantages and improvements in high precision and material processing at the microscale. In order to control the friction, it is important to understand the mechanisms which occur during the conformal or non-conformal contact in dry and lubricated conditions.
In the present thesis, I am dealing with laser surface texturing to improve the tribological properties of the technological steels. I have fabricated the pad based on a design developed previously to my thesis, it consists of an anisotropic and non-uniform texture to maximize the thrust load of a square pad prototype. The experimental results, showed that the non-uniform micro-texture largely affects the friction characteristics of the contact. In particular, in agreement with the BTH predictions, the tribo-system shows friction properties that are strongly sensitive to the direction of the sliding speed, as a consequence of the micro-fluid dynamics which are designed to occur only in a specific sliding direction. Results suggest that the joint action of virtual prototyping (BTH lubrication theory) and ultrafast laser micro-prototyping can lead to unconventional and impressive results in terms of enhanced or tailored contact mechanics properties of the generic lubricated tribopair.
I have also investigated the effects of the micro surface texturing in the lubricated non-conformal point contacts. In this investigation, I have focused on a regime poorly investigated in literature, where the contact area and the micro-holes have a comparable size. In particular, I found that, depending on the void ratio, a significant friction reduction or, on the contrary, a deterioration of the frictional performances can affect the boundary and mixed lubrication regimes. This was due to the simultaneous occurrence of two competing effects. One was related to the stress intensification, due to the presence of the micro-hole edges on the contact topography, which leads to a consequent increase in wear and friction. On the other hand, micro-texture may play a positive role in the friction optimization given the possibility, offered by the micro-holes, to entrap wear debris and, then, to preserve a smoother interface between the contacting pairs.
Investigation of the wettability and spreading behavior of the lubricant on laser textured surfaces helps to understand more about its tribological performance. The tribological behavior of different textured surfaces have been studied using mineral oil as lubricant. Results show that textured surfaces have friction reduction un- der different lubrication regimes as compared to un-textured surfaces. Furthermore, static contact angle and spreading were evaluated with water, mineral oil and pure glycerol. Every considered texture showed a slight reduction of contact angle for the three liquids, compared with the polished surface. Mineral oil exhibited a more persistent spreading over the un-textured and textured surfaces, and more consistency in friction reduction. Mineral oil shows compatibility for low and high dynamic velocities in the case of textured and un-textured samples, whereas, glycerol is beneficial at specific velocities.

Italian summary

Nei decenni passati, la micro-tessitura superficiale ha dimostrato di essere una tecnica emergente per il controllo dell’attrito e dell’usura. Tale tecnica consiste nel fabbricare sulla superficie del materiale ed in maniera estremamente riproducibile un motivo formato da piccoli buchi e scanalature che causano il cambiamento della topografia superficiale. Le sorgenti laser, grazie alla qualità eccellente dei loro fasci, hanno permesso di ottenere notevoli vantaggi e miglioramenti nella precisione delle lavorazioni su scala microscopica dei materiali . Per controllare l’attrito è importante capire i meccanismi che intervengono durante il contatto conforme e non conforme sia in condizioni asciutte che lubrificate.
Nel presente lavoro di tesi, la tecninca del “texturing” superficiale al laser è stata utilizzata per migliorare le proprietà tribologiche dell’acciaio tecnologico. Ho fabbricato un cuscinetto seguendo il design sviluppato precedentemente alla mia tesi che consisteva in una tessitura anisotropica e non uniforme finalizzata a massimizzare il carico di spinta di un prototipo di cuscinetto quadrato. I risultati sperimentali hanno mostrato che le microtessiture non uniformi influenzano consistentemente le caratteristiche dell’attrito del contatto. In particolare, in accordo con le previsioni del modello teorico BTH, il sistema tribologico mostra proprietà di attrito fortemente dipendenti dalla direzione della di velocità di scorrimento; questa è chiaramente una conseguenza dell’effetto micro-fluidodinamico che, come da progetto, intervenire solo in una specifica direzione di scorrimento. I risultati suggeriscono che l’azione combinata della prototipazione virtuale (teoria della lubrificazione BTH) e della micro-prototipazione laser ultraveloce può portare a risultati impressionanti e non convenzionali in termini di miglioramento o adattamento delle proprietà meccaniche del contatto di un generico sistema tribologico lubrificato.
Inoltre, ho anche analizzato gli effetti della micro tessitura superficiale nei punti di contatto non conforme lubrificato. In questo studio, ho focalizzato l’attenzione su un regime poco trattato in letteratura, in cui l’area di contatto e i micro-fori hanno dimensioni confrontabili. In particolare, ho riscontrato che, a seconda della percetuale di vuoti, una significativa riduzione dell’attrito o, al contrario, un suo aumento possono influenzare i regimi di lubrificazione misto e “boundary”.
Questo è dovuto all’incidenza simultanea di due effetti contrastanti; quello negativo è relativo all’intensificazione dello stress, dovuto alla presenza di bordi dei micro-fori nella zona di contatto, che causa un conseguente incremento dell’usura e dell’attrito. L’altro positivo è collegato alla possibilità di ridurre l’attrito a seguito di una ottimizzazione della geometria della testurizzazione microscopisca; i fori micorscopici infatti possono intrappolare i detriti derivanti dall’usura e, quindi, preservare una superficie più liscia all’interfaccia della coppia di contatto.
L’analisi del comportamento di bagnabilità e diffusione del lubrificante su una superficie testurizzata al laser, aiuta a capire di più riguardo alle sue performance tribologiche. I comportamenti tribologici di differenti superfici testurizzate sono stati studiati usando come lubrificante l’olio minerale. I risultati mostrano che le superfici testurizzate, a differenza di quelle non- texturizzate, consentono una riduzione dell’attrito in diversi regimi di lubrificazione. Inoltre l’angolo di contatto statico e la diffusione del lubrificante sono stati valutati usando come campioni l’acqua, l’olio minerale ed il gricerolo puro. Ogni tessitura esaminata, se confrontata con una superficie liscia, ha mostrato una leggera riduzione dell’angolo di contatto per ciascuno dei tre liquidi utilizzati. L’olio minerale presenta una diffusione più persistente sia su superfici testurizzate che non, insieme ad una riduzione più consistente dell’attrito.
Mentre l’olio minerale è compatibile sia per velocità di scorrimento basse che alte e sia nel caso di campioni testurizzati che non texturizzati, il glicerolo produce delle prestazioni migliori solo nel caso di specifici valori di velocità.

Analysis and modelling of icing of air intake protection grids of aircraft engines

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

Vittorio Vercillo, Norbert Karpen, Alexandre Laroche, Javier Alejandro Mayén Guillén, Simone Tonnicchia, Raphaelde Andrade Jorge, Elmar Bonaccurso (2019) Analysis and modelling of icing of air intake protection grids of aircraft engines. Cold Regions Science and Technology, Volume 160, April 2019, Pages 265-272.

Abstract

Icing represents a major problem in the aviation industry. While icing of aerodynamic surfaces such as airfoils due to the impingement of supercooled liquid water droplets is widely studied, even if not yet fully understood, icing of supporting structures like protection grids of engine air intakes has been investigated to a lesser extent. An optimization of the design of the grids will help to reduce icing severity and delaying or avoiding loss of efficiency that could lead to hazardous situations. The present study investigates the icing behaviour of stainless steel protection grids in use on rotorcraft and turboprop engines. New experimental and analytical tools were developed to enable a quantitative study of grid icing under representative icing conditions in a lab-scale icing wind tunnel. The variation of the most relevant parameters like liquid water content of the cloud, airspeed, ambient temperature and mesh size of the grid allowed the identification of their influence on the icing behaviour. Further analysis of the experimental data led to the development and validation of a general physical model for ice accretion on grid structures.

Link(s)

PhD degree awarded to Fotis Fraggelakis

On February 14th, Fotis Fraggelakis successfully defended his thesis and was awarded the degree of PhD.

The now dr. Fraggelakis, was an Early Stage Researcher (ESR) of the Laser4Fun project. He was the the first PhD-candidate from the project to defend his PhD-thesis. The public defense ceremony took place on February 14th  at the Institute d’Optique d’Aquitaine (IOA) in Talence (France). He was supervised by dr. Inka Manek-Hönninger and dr. John Lopez of the University of Bordeaux, carried out most of his work at Alphanov under the supervision of dr. Rainer Kling of Alphanov, amongst others.

The PhD-defense committee consisted, among others, dr. Jörn Bonse of the Bundesanstalt für Materialforschung und -prüfung (BAM) in Berlin (Germany), prof. Antonio Ancona of the Consiglio Nazionale delle Ricerche (CNR), Istituto di Fotonica (IFN) of the University of Bari (Italy), and prof. Gert-willem Römer of the Chair of Laser Processing of the University of Twente (The Netherlands).

Fraggelakis2019.02.14-03XS

Fraggelakis2019.02.14-02XS

Fraggelakis2019.02.14-01XS

PhD defense ceremony Fotis Fraggelakis on Feb. 14th 2019

fotis.fraggelakisFotis Fraggelakis, an Early Stage Researcher (ESR), will be the first PhD-candidate from the Laser4Fun project to defend his PhD-thesis. The public defense ceremony will take place on February 14th , 2019 at 9:30 am, in The amphitheater of IOA, the Institute d’Optique d’Aquitaine (IOA) at the Rue François Mitterrand 33400 Talence (France). Supervised by dr. Inka Manek-Hönninger and dr. John Lopez of the University of Bordeaux, carried out most of his work at Alphanov under the supervision of dr. Rainer Kling of Alphanov, amongst others. Below is an English and French summary of the work and results of Fotis.

English summary

Current industrial markets demand highly value-added products offering new features at a low-cost. Among the most desired functionalities are, surface colouring and blackenning, anti-icing, anti-biofouling, wear reduction and anti-reflectivity. Functional textures found in nature indicate that those properties can be enable by textures in the micro and nanoscale. Laser surface processing holds a virtually endless potential in mimicking bio inspired textures by modifying surface morphology and chemistry. We investigate several techniques to achieve controlled laser structuring in the submicron regime such as polarization control and double pulse irradiation. Valuable data were provided both in the surface functionalization, in understanding and controlling of laser induced structuring and in upscaling a lab developed process. We believe that our results open the way for laser texturing exploitation in everyday applications exploiting up to date laser sources and positioning systems.

French summary

L’Industrie actuelle demande des produits à haute valeur ajoutée offrant des nouvelles fonctions à moindre coût telles que la coloration ou le noircissement de surface, la réduction des frottements, la génération de surface antiréflexion, antibactérienne, superhydrophobe ou anti-icing. Les surfaces fonctionnelles présentes dans la nature nous indiquent que ces propriétés uniques ne sont possibles par des texturations de surface à l’échelle micro et nanométrique adéquates. La technologie laser révolutionne le champ des possibles et permet de reproduire ces fonctions inspirées du monde du vivant en modifiant la morphologie et la chimie de surface. Nous avons étudié plusieurs techniques de texturation de surface par laser femtoseconde en jouant sur la polarisation et l’irradiation en double impulsion. Ces travaux de recherche apportent une contribution significative dans la compréhension des mécanismes et dans la capacité à produire de telles texturations sur des grandes surfaces

Controlling 2D laser nano structuring over large area with double femtosecond pulses

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

F. Fraggelakis, G. Mincuzzi, J. Lopez, I. Manek-Hönninger and R. Kling. Controlling 2D laser nano structuring over large area with double femtosecond pulses. Applied Surface Science, November 2018

Abstract

Laser surface texturing is an established way to introduce surface functionalities on solid surfaces. By means of femtosecond laser sources, it’s possible to texture a variety of solids such as metals, semiconductors and transparent materials, introducing features of different symmetry and size. Fabrication of laser induced periodic surface structure (LIPSS) which typically range in the near-submicron length scale can lead to strong modifications of surface wetting, tribological and optical properties. Controlling LIPSS morphology could enable us to mimic functional textures found in nature, introducing functionalities such as antireflectivity and bactericidity. Several works demonstrate the impact of polarization control and double-pulse irradiation on determining texture symmetry and size. Here we present a comprehensive study on controlling laser induced structures by double femtosecond pulse irradiation. The effect of pulse polarization and interpulse delay is elucidated and the generation of novel 2D surface morphologies is reported. A plausible interpretation of the structure formation mechanism is proposed in the frame of non-linear convection flow. A high average power industrial femtosecond laser source with a pulse duration of 350 fs operating at high repetition rate was employed for the experiments. Large areas were processed by combining different 2D morphologies generating a holographic pattern. We believe that our results provide a novel insight in controlling laser induced submicron morphology. Moreover, the presented surface texturing process is scalable in terms of processed area and cycle time and fully compatible with high repetition rates, demonstrating for the first time the feasibility to introduce double-pulse processing in an industrial environment.

Link(s)

Annual Technical Board and Supervisory Board Meeting in Bari, Italy (2018)

The 2018 Technical Board (TB) meeting and Supervisory Board (SB) meeting of the Laser4Fun project took place on October 4 and 5 in Bari, Italy. The meeting was hosted by prof. Antonio Ancona of IFN-CNR, Istituto di Fotonica e Nanotecnologie, Bari, Italy. The TB & SB discussed the progress of the project, results, management and reporting issues, as well as communication and dissemination topics. Also all Early Stage Researchers (ESRs) presented their progress and results to the SB. We were happy to conclude the project is well on track.

Technical Board (TB) meeting and Supervisory Board (SB) and all ESRs during the annual progress meeting in Bari, Italy.
Technical Board (TB) and Supervisory Board (SB) and all ESRs during the annual progress meeting in Bari, Italy.