Browsing by Author "Vilar, R."
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- Evidences for direct magnetic patterning via diffusive transformations using femtosecond laser interferometryPublication . Polushkin, N. I.; Oliveira, Vitor; Conde, O.; Vilar, R.; Drozdov, Y. N.; Apolinario, A.; Garcia-Garcia, A.; Teixeira, J. M.; Kakazei, G. N.The application of femtosecond laser interferometry to direct patterning of thin-film magnetic alloys is demonstrated. The formation of stripe gratings with submicron periodicities is achieved in Fe1-xVx (x=18-34wt. %) layers, with a difference in magnetic moments up to Delta mu/mu similar to 20 between adjacent stripes but without any significant development of the topographical relief (<1% of the film thickness). The produced gratings exhibit a robust effect of their anisotropy shape on magnetization curves in the film plane. The obtained data witness ultrafast diffusive transformations associated with the process of spinodal decomposition and demonstrate an opportunity for producing magnetic nanostructures with engineered properties upon this basis.
- Influence of femtosecond laser treated surfaces on the mode I fracture toughness of carbon-epoxy bonded jointsPublication . Ferreira Moreira, Raul Domingos; Oliveira, Vitor; Silva, Filipe Gonçalo Andrade Da; Vilar, R.; Moura, MarceloThe influence of laser surface treatments on mode I fracture toughness of carbon fibre reinforced composites bonded joints is addressed in this work. Five different surface treatments considering several combinations of laser fluence and scanning speeds were applied prior to bonding of the adherents, aiming to increase the adhesion and, consequently, the fracture resistance of the bonded joint. Mode I fracture characterisation considering Double Cantilever Beam tests was subsequently performed to assess the influence of the several treatments on the critical fracture toughness under pure mode I loading. A suitable compliance based data reduction scheme was used to obtain the resistance curves as function of the equivalent crack length. It was verified that a laser treatment based on an interference scheme is the most effective surface preparation concerning maximization of the mode I fracture energy. A finite element analysis including cohesive zone modelling was performed to validate the procedure and experimental conclusions.
- Influence of laser surface nanotexturing on the friction behaviour of the silicon/sapphire systemPublication . Alves-Lopes, I.; Almeida, Amelia; Oliveira, Vítor; Vilar, R.The aim of the present work was to investigate the influence of textures consisting of Laser-Induced Periodic Surface Structures (LIPSS) on the friction coefficient of silicon under unlubricated conditions, using nanoscale friction tests. The tests were performed on <1 1 1> single crystal wafers of p-doped silicon. Surface texturing was performed by a direct writing technique, using 560 fs pulses of 1030 nm wavelength radiation and parameters allowing large areas with a uniform LIPSS texture to be obtained. The tribological tests were performed in air using a ball-on-flat nanotribometer with 3 mm diameter sapphire balls as counterbodies. The wear scars were analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The friction coefficient of polished Si remains approximately constant during the tests and is independent of the applied load, with values in the range 0.12-0.14. The morphology of the wear scars indicated that the predominant material removal mechanism is mild oxidative wear, independently of the testing conditions used, which explains the low values of the friction coefficient measured. The LIPSS texture leads to an increase of the friction coefficient, in particular in the perpendicular sliding direction. The friction coefficient increases with the applied load and is larger for the perpendicular sliding direction than for the parallel sliding direction, the difference increasing with increasing load. The morphology of the wear scars showed that the predominant wear regime remains oxidative wear, but significant contributions of plastic deformation and ploughing appear. This plastic deformation in Si, which is brittle at room temperature, shows that significant surface heating is being caused by the interaction of the asperities in motion and under load. This temperature increase also accelerates surface oxidation and increases the wear rate. Surface temperature calculations showed that the temperature increase is larger for the perpendicular sliding direction than for the parallel one, which explains the difference of friction coefficient observed in the two directions.
- Internal structure of the nanogratings generated inside bulk fused sílica by ultrafast laser direct writingPublication . Sharma, S. P.; Oliveira, Vitor; Herrero, P.; Vilar, R.The aim of the present work was to characterize the internal structure of nanogratings generated inside bulk fused silica by ultrafast laser processing and to study the influence of diluted hydrofluoric acid etching on their structure. The nanogratings were inscribed at a depth of 100 mu m within fused silica wafers by a direct writing method, using 1030 nm radiation wavelength and the following processing parameters: E = 5 mu J, tau = 560 fs, f = 10 kHz, and v = 100 mu m/s. The results achieved show that the laser-affected regions are elongated ellipsoids with a typical major diameter of about 30 mu m and a minor diameter of about 6 mu m. The nanogratings within these regions are composed of alternating nanoplanes of damaged and undamaged material, with an average periodicity of 351 +/- 21 nm. The damaged nanoplanes contain nanopores randomly dispersed in a material containing a large density of defects. These nanopores present a roughly bimodal size distribution with average dimensions for each class of pores 65 +/- 20 x 16 +/- 8 x 69 +/- 16 nm(3) and 367 +/- 239 x 16 +/- 8 x 360 +/- 194 nm(3), respectively. The number and size of the nanopores increases drastically when an hydrofluoric acid treatment is performed, leading to the coalescence of these voids into large planar discontinuities parallel to the nanoplanes. The preferential etching of the damaged material by the hydrofluoric acid solution, which is responsible for the pores growth and coalescence, confirms its high defect density. (C) 2014 AIP Publishing LLC.
- Large-area homogeneous periodic surface structures generated on the surface sputtered boron carbide thin films by femtosecond laser processingPublication . Serra, R.; Oliveira, Vitor; Oliveira, J. C.; Kubart, T.; Vilar, R.; Cavaleiro, A.Amorphous and crystalline sputtered boron carbide thin films have a very high hardness even surpassing that of bulk crystalline boron carbide (≈41 GPa). However, magnetron sputtered B-C films have high friction coefficients (C.o.F) which limit their industrial application. Nanopatterning of materials surfaces has been proposed as a solution to decrease the C.o.F. The contact area of the nanopatterned surfaces is decreased due to the nanometre size of the asperities which results in a significant reduction of adhesion and friction. In the present work, the surface of amorphous and polycrystalline B-C thin films deposited by magnetron sputtering was nanopatterned using infrared femtosecond laser radiation. Successive parallel laser tracks 10 μm apart were overlapped in order to obtain a processed area of about 3 mm2. Sinusoidal-like undulations with the same spatial period as the laser tracks were formed on the surface of the amorphous boron carbide films after laser processing. The undulations amplitude increases with increasing laser fluence. The formation of undulations with a 10 μm period was also observed on the surface of the crystalline boron carbide film processed with a pulse energy of 72 μJ. The amplitude of the undulations is about 10 times higher than in the amorphous films processed at the same pulse energy due to the higher roughness of the films and consequent increase in laser radiation absorption. LIPSS formation on the surface of the films was achieved for the three B-C films under study. However, LIPSS are formed under different circumstances. Processing of the amorphous films at low fluence (72 μJ) results in LIPSS formation only on localized spots on the film surface. LIPSS formation was also observed on the top of the undulations formed after laser processing with 78 μJ of the amorphous film deposited at 800 °C. Finally, large-area homogeneous LIPSS coverage of the boron carbide crystalline films surface was achieved within a large range of laser fluences although holes are also formed at higher laser fluences.
- Laser surfasse patterning using a Michelson interferometer and a femtosecond laser radiationPublication . Oliveira, Vitor; Polushkin, N. I.; Conde, O.; Vilar, R.We report on a simple method to obtain surface gratings using a Michelson interferometer and femtosecond laser radiation. In the optical setup used, two parallel laser beams are generated using a beam splitter and then focused using the same focusing lens. An interference pattern is created in the focal plane of the focusing lens, which can be used to pattern the surface of materials. The main advantage of this method is that the optical paths difference of the interfering beams is independent of the distance between the beams. As a result, the fringes period can be varied without a need for major realignment of the optical system and the time coincidence between the interfering beams can be easily monitored. The potential of the method was demonstrated by patterning surface gratings with different periods on titanium surfaces in air.
- Laser-induced diffusion decomposition in Fe-V thin-film alloysPublication . Polushkin, N. I.; Duarte, A. C.; Conde, O.; Alves, E.; Barradas, N. P.; García-García, A.; Kakazei, G. N.; Ventura, J. O.; Araújo, J. P.; Oliveira, Vitor; Vilar, R.We investigate the origin of ferromagnetism induced in thin-film (similar to 20 nm) Fe-V alloys by their irradiation with subpicosecond laser pulses. We find with Rutherford backscattering that the magnetic modifications follow a thermally stimulated process of diffusion decomposition, with formation of a-few-nm-thick Fe enriched layer inside the film. Surprisingly, similar transformations in the samples were also found after their long-time (similar to 10(3) s) thermal annealing. However, the laser action provides much higher diffusion coefficients (similar to 4 orders of magnitude) than those obtained under standard heat treatments. We get a hint that this ultrafast diffusion decomposition occurs in the metallic glassy state achievable in laser-quenched samples. This vitrification is thought to be a prerequisite for the laser-induced onset of ferromagnetism that we observe. 2014 Elsevier B.V. All rights reserved.
- Mode II fracture toughness of carbon–epoxy bonded joints with femtosecond laser treated surfacesPublication . Ferreira Moreira, Raul Domingos; Oliveira, Vitor; Silva, Filipe Gonçalo Andrade Da; Vilar, R.; Moura, MarceloMode II fracture toughness of carbon–epoxy bonded joints considering laser treated surfaces was evaluated using the End-Notched Flexure (ENF) test. Four different surface preparations were performed using distinct processing conditions before bonding the specimen arms. The Resistance-curves of the fracture tests were obtained considering an equivalent crack length based procedure. The influence of the different surface treatments on the mode II fracture behaviour was assessed comparing the average values of the fracture toughness. It was verified that a laser treatment employing two interfering laser beams, which produces a complex multi-scale surface morphology free of contaminants, is the best surface preparation regarding higher values of the mode II fracture energy. A cohesive zone analysis was used aiming to validate the followed procedures. The excellent agreement obtained between numerical and experimental results reveals that the used techniques are sound.
- Phase-change magnetic memory: Rewritable ferromagnetism by laser quenching of chemical disorder in Fe60Al40 alloyPublication . Polushkin, Nikolay I.; Oliveira, Vitor; Vilar, R.; He, M.; Shugaev, Maxim; Zhigilei, LeonidHigh-intensity laser irradiation can effectively couple to practically all kinds of materials, causing modification of their physical properties due to laser-induced phase transformations. This gives rise to a broad field of laser micro- and nanofabrication with its various technological applications. We demonstrate that, in a 40-nm-thick film of Fe60Al40 alloy, a short laser pulse is capable of (re)writing the ferromagnetism observed at room temperature (RT). The energy of the pulse generating a ferromagnetic region has to be sufficiently high to induce melting of the Fe60Al40 layer, while the ferromagnetic state can be erased by various kinds of lower-intensity thermal treatment. This cycling of RT ferromagnetism can be explained in terms of the chemical order (B2)-disorder (A2) phase transition in the Fe60Al40 crystal lattice, which is affected by laser-induced melting and rapid resolidification. Our finding has implications for the development of a magnetic memory technology that would use the reversibility of the modulus of the magnetization vector instead of its direction. This promises to circumvent the problem of the superparamagnetic limit for magnetic data storage density.
- Sub-micron structuring of silicon using femtosecond laser interferometryPublication . Oliveira, Vitor; Vilar, R.; Serra, R.; Oliveira, J.C.; Polushkin, N.I.; Conde, O.We report the fabrication of planar sub-micron gratings in silicon with a period of 720 nm using a modified Michelson interferometer and femtosecond laser radiation. The gratings consist of alternated stripes of laser ablated and unmodified material. Ablated stripes are bordered by parallel ridges which protrude above the unmodified material. In the regions where ridges are formed, the laser radiation intensity is not sufficient to cause ablation. Nevertheless, melting and a significant temperature increase are expected, and ridges may be formed due to expansion of silicon during resolidification or silicon oxidation. These conclusions are consistent with the evolution of the stripes morphology as a function of the distance from the center of the grating.