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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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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Durability and Wear Resistance of Laser-Textured Hardened Stainless Steel Surfaces with Hydrophobic Properties

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

A. Garcia-Giron, J.M. Romano, A. Batal, B. Dashtbozorg, H. Dong, E. Martinez Solanas, D. Urrutia Angos, M. Walker, P. Penchev, S.S. Dimov (2019) Durability and Wear Resistance of Laser-Textured Hardened Stainless Steel Surfaces with Hydrophobic Properties, Langmuir 35(15) 5353-5363. doi:10.1021/acs.langmuir.9b00398.

Abstract

Hydrophobic surfaces are of high interest to industry. While surface functionalization has attracted significant interest, from both industry and research, the durability of engineered surfaces remains a challenge, as wear and scratches deteriorate their functional response. In this work, a cost-effective combination of surface engineering processes on stainless steel was investigated. Low-temperature plasma surface alloying was applied to increase surface hardness from 172 to 305 HV. Then, near-infrared nanosecond laser patterning was deployed to fabricate channel-like patterns that enabled superhydrophobicity. Abrasion tests were carried out to examine the durability of such engineered surfaces during daily use. In particular, the evolution of surface topographies, chemical composition, and water contact angle with increasing abrasion cycles were studied. Hydrophobicity deteriorated progressively on both hardened and raw stainless steel samples, suggesting that the major contributing factor to hydrophobicity was the surface chemical composition. At the same time, samples with increased surface hardness exhibited a slower deterioration of their topographies when compared with nontreated surfaces. A conclusion is made about the durability of laser-textured hardened stainless steel surfaces produced by applying the proposed combined surface engineering approach.

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Subwavelength Direct Laser Nanopatterning Via Microparticle Arrays For Functionalizing Metallic Surfaces

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

Jean-Michel Romano, Rajib Ahmed, Antonio Garcia-Giron, Pavel Penchev, Haider Butt, Olivier Delléa, Melissa Sikosana, Ralf Helbig, Carsten Werner, Stefan Dimov (2019) Subwavelength Direct Laser Nanopatterning Via Microparticle Arrays For Functionalizing Metallic Surfaces, Journal of Micro- and Nano-Manufacturing 7(1) 010901. doi:10.1115/1.4042964.

Abstract

Functionalized metallic nanofeatures can be selectively fabricated via ultrashort laser processing; however, the cost-effective large-area texturing, intrinsically constrained by the diffraction limit of light, remains a challenging issue. A high-intensity near-field phenomenon that takes place when irradiating microsized spheres, referred to as photonic nanojet (PN), was investigated in the transitional state between geometrical optics and dipole regime to fabricate functionalized metallic subwavelength features. Finite element simulations were performed to predict the PN focal length and beam spot size, and nanofeature formation. A systematic approach was employed to functionalize metallic surface by varying the pulse energy, focal offset, and number of pulses to fabricate controlled array of nanoholes and to study the generation of triangular and rhombic laser-induced periodic surface structures (LIPSS). Finally, large-area texturing was investigated to minimize the dry laser cleaning (DLC) effect and improve homogeneity of PN-assisted texturing. Tailored dimensions and densities of achievable surface patterns could provide hexagonal light scattering and selective optical reflectance for a specific light wavelength. Surfaces exhibited controlled wetting properties with either hydrophilicity or hydrophobicity. No correlation was found between wetting and microbacterial colonization properties of textured metallic surfaces after 4h incubation of Escherichia coli. However, an unexpected bacterial repellency was observed.

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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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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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Upscaling laser-induced periodic surface structures (LIPSS) manufacturing by defocused laser processing

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

Marek Mezera and G.R.B.E. Römer. Upscaling laser-induced periodic surface structures (LIPSS) manufacturing by defocused laser processing, Proc. SPIE 10906, Laser-based Micro- and Nanoprocessing XIII, 109060U (4 March 2019); doi: 10.1117/12.2510004

Abstract

Low spatial frequency Laser-induced Periodic Surface Structures (LSFL) have been created on single crystal silicon with picosecond laser pulses with a wavelength of λ =1030nm with varying laser spot diameters obtained by a defocused laser beam. The laser processing parameters have been adjusted theoretically and experimentally to obtain similar LSFL for all studied laser spot diameters. The periodicity and amplitude of the LSFL were measured by SEM and AFM analysis. It has been found that the periodicities of the LSFL do not change when LSFL were created with larger laser spot diameters. The amplitudes of the LSFL decrease with increasing laser spot diameters, although this correlation is not strong.

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DOI: 10.1117/12.2510004