Avoiding Starvation in Tribocontact Through Active Lubricant Transport in Laser Textured Surfaces

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

Tobias Stark , Thomas Kiedrowski, Holger Marschall and Andrés Fabián Lasagni. Avoiding Starvation in Tribocontact Through Active Lubricant Transport in Laser Textured Surfaces. Lubricants 2019, 7(6), 54.

Abstract

Laser texturing is a viable tool to enhance the tribological performance of surfaces. Especially textures created with Direct Laser Interference Patterning (DLIP) show outstanding improvement in terms of reduction of coefficient of friction (COF) as well as the extension of oil film lifetime. However, since DLIP textures have a limited depth, they can be quickly damaged, especially within the tribocontact area, where wear occurs. This study aims at elucidating the fluid dynamical behavior of the lubricant in the surroundings of the tribocontact where channel-like surface textures are left after the abrasion wear inside the tribocontact area. In a first step, numerical investigations of lubricant wetting phenomena are performed applying OpenFOAM. The results show that narrow channels (width of 10 μm) allow higher spreading than wide channels (width of 30 μm). In a second step, fluid transport inside DLIP textures is investigated experimentally. The results show an anisotropic spreading with the spreading velocity dependent on the period and depth of the laser textures. A mechanism is introduced for how lubricant can be transported out of the channels into the tribocontact. The main conclusion of this study is that active lubricant transport in laser textured surfaces can avoid starvation in the tribocontact.

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How to Tailor Structural Colors for Extended Visibility and White Light Generation Employing Direct Laser Interference Patterning

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

Storm, S., Alamri, S., Soldera, M., Kunze, T., Lasagni, A. F., How to Tailor Structural Colors for Extended Visibility and White Light Generation Employing Direct Laser Interference Patterning. Macromol. Chem. Phys. 2019, 1900205. 

Abstract

The appearance of a surface can be controlled by creating periodic microstructures designed to diffract light and produce structural colors. Nevertheless, since structural coloration is based on diffraction, the produced colors have a strong dependence on the viewing angle and absence of coloration takes place while tilting the samples. In this work direct laser interference patterning is used to firstly provide transparent polymer sheets a structural coloration with a high‐range observation angle, and secondly to demonstrate the combination of structural colors, producing a white coloring effect. The employed approaches are based on the fabrication of micro‐gratings with multiple periods in the same structured area and on the engineering of the diffraction orders of the diffraction spectrum. The patterned surfaces are characterized by confocal microscopy and angular spectrometry in reflection mode. The morphological characterization shows homogeneous surface patterns, while the spectral results demonstrate that combining four spatial periods on a single patterned surface, a white appearance is obtained over an angular observation range higher than 30°. The experimental results are supported by theoretical predictions by means of generalized formulas based on the diffraction of light.

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Video: Hydrophobiticy of DLIP functionalized surface on stainless steel

The Direct Laser Interference Patterning (DLIP) technique has been used to create a functionalized surface on stainless steel (black area). In combination with an additional post-treatment, the metal surface has extremely water-repellent properties with contact angles of over 150 degrees. The microscopic surface exhibits “miniature mountains” that create a superhydrophobic surface, similar to its natural pendent on a lotus leaf. The sample was placed under water (no wetting of the structured areas is observed) to emphasize the water repellency. Consequently, even if the surface gets wet, it actually stays dry in the laser processed areas. Finally, DLIP allows for self-cleaning properties on technological surfaces at industrially-relevant throughput.

Durability of superhydrophobic laser-treated metal surfaces under icing conditions

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

Vittorio Vercillo, José Tiago Cardoso, Daniel Huerta-Murillo, Simone Tonnicchia, Alexandre Laroche, Javier Alejandro Mayén Guillén, José Luis Ocaña, Andrés Fabián Lasagni, Elmar Bonaccurso. Durability of superhydrophobic laser-treated metal surfaces under icing conditions. Materials Letters: X 3 (2019) 100021.. 

Abstract

Ice accretion on external surfaces of aircraft due to the impingement of supercooled liquid water droplets can be tackled by the implementation of icephobic surfaces. Among these, superhydrophobic surfaces represent a promising solution, due to their water repellent nature. In the last decade, short/ultra-short pulsed laser technologies have been proposed as a one-step process to manufacture superhydrophobic surfaces. However, the effectiveness and durability of such surfaces in operational icing conditions has not yet been validated. In this work, we investigate ice adhesion strength and the chemical stability of metal alloys textured with a UV nanosecond laser via Direct Laser Writing.

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PhD degree awarded to Jose Cardoso and Daniel Huerta Murillo

On April 11, Jose Cardoso and Daniel Huerta successfully defended their PhD thesis and were awarded the degree of PhD.

Jose Cardoso and Daniel Huerta Murillo were PhD candidates and Early Stage Researchers in the Laser4Fun project. They were the third and fourth (out of thirteen) PhD candidates from the Laser4Fun project to be awarded with the PhD degree.

The doctoral thesis were developed on the Laser Centre under the direction of Dr. José Luis Ocaña and the public defense of the doctoral thesis took place on April 11th 2019 at the School of Industrial Engineering of the Polytechnical University of Madrid (ETSII – UPM). The defense court was formed by Dr. Jesus de Vicente y Oliva as President, Dr. Henryk Fiedorowicz, Dr. Francisco Javier Ester Sola and Dr. Antonio Gimenez Fernandez as Vocals and Dr. Angel Garcia Beltran as Secretary.

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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.

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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.

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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.

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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.

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