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- Influence of femtosecond laser surface nanotexturing on the friction behavior of silicon sliding against PTFEPublication . Alves-Lopes, Isabel; Almeida, Amélia; Oliveira, Vítor; Vilar, RuiThe aim of the present work was to investigate the influence of laser-induced periodic surface structures (LIPSS) produced by femtosecond laser on the friction behavior of silicon sliding on polytetrafluoroethylene (PTFE) in unlubricated conditions. Tribological tests were performed on polished and textured samples in air using a ball-on-flat nanotribometer, in order to evaluate the friction coefficient of polished and textured silicon samples, parallel and perpendicularly to the LIPSS orientation. In the polished specimens, the friction coefficient decreases with testing time at 5 mN, while it increases slightly at 25 mN. It also decreases with increasing applied load. For the textured specimens, the friction coefficient tends to decrease with testing time in both sliding directions studied. In the parallel sliding direction, the friction coefficient decreases with increasing load, attaining values similar to those measured for the polished specimen, while it is independent of the applied load in the perpendicular sliding direction, exhibiting values lower than in the two other cases. These results can be explained by variations in the main contributions to friction and in the wear mechanisms. The influence of the temperature increase at the interface and the consequent changes in the crystalline phases of PTFE are also considered.
- 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.
- Wetting behaviour of femtosecond laser textured Ti-6Al-4V surfacesPublication . Cunha, Alexandre; Serro, Ana Paula; Oliveira, Vitor; Almeida, Amélia; Vilar, Rui; DURRIEU, Marie-ChristineThe aim of the present work was to investigate the wetting behaviour of biomedical grade Ti-6Al-4V alloy surfaces textured by a femtosecond laser treatment. The material was treated in ambient atmosphere using an Yb: KYW chirped-pulse-regenerative amplification laser with a wavelength of 1030 nm and a pulse duration of 500 fs. Four main types of surface textures were obtained depending on the processing parameters and laser treatment method. These textures consist of: (1) nanoscale laser-induced periodic surface structures (LIPSS); (2) nanopillars; (3) a bimodal roughness distribution texture formed of LIPSS overlapping microcolumns; (4) a complex texture formed of LIPSS overlapping microcolumns with a periodic variation of the columns size in the laser scanning direction. The wettability of the surfaces was evaluated by the sessile drop method using distilled-deionized (DD) water and Hank's balanced salt solution (HBSS) as testing liquids. The laser treated surfaces present a hydrophilic behaviour as well as a high affinity for the saline solution, with equilibrium contact angles in the ranges 24.1-76.2. for DD water and 8.4-61.8. for HBSS. The wetting behaviour is anisotropic, reflecting the anisotropy of the surface textures. (c) 2012 Elsevier B.V. All rights reserved.
- 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.
- 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.
- Ageing effects on the wettability behavior of laser textured siliconPublication . Nunes, B.; Serro, A. P.; Oliveira, Vitor; Montemor, M. F.; Alves, E.; Saramago, B.; Colaço, R.In the present work we investigate the ageing of acid cleaned femtosecond laser textured < 100 > silicon surfaces. Changes in the surface structure and chemistry were analysed by Rutherford backscattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), in order to explain the variation with time of the water contact angles of the laser textured surfaces. It is shown that highly hydrophobic silicon surfaces are obtained immediately after laser texturing and cleaning with acid solutions (water contact angle >120 degrees). However these surfaces are not stable and ageing leads to a decrease of the water contact angle which reaches a value of 80 degrees. XPS analysis of the surfaces shows that the growth of the native oxide layer is most probably responsible for this behavior. (C) 2010 Elsevier B.V. All rights reserved.
- Processing of poly(ionic liquid)-ionic liquid membranes using femtosecond (fs) laser radiation: Effect on CO2 separation performancePublication . Gouveia, Andreia S.L.; Oliveira, Vitor; Ferraria, Ana Maria; Rego, Ana; Ferreira, Maria João; Tomé, Liliana C.; Almeida, Amélia; Marrucho, I. M.Femtosecond (fs) laser micromachining on polymeric materials is a single-step, and contactless manufacturing technology. Knowing the potential of poly(ionic liquid)s (PILs) and their derived composite materials incorporating ionic liquids (PIL-IL) to design membranes with improved CO2 separation, we here explore for the first time the creation of microchannels on the surface of PIL-IL materials by laser ablation using femtosecond laser radiation. PIL-IL membranes composed of pyrrolidinium-based PILs containing the [NTf2](-) and [C(CN)(3)](-) anions and different amounts of their corresponding ILs (40 and 60 wt%) were prepared and micromachined using fs laser pulses varying the pulse repetition rate, scanning speed, and pulse energy. The morphology of the fs laser modified PIL-IL samples was investigated through scanning electron microscopy (SEM), while the influence of the fs laser processing on the membranes structure was analyzed by solid-state nuclear magnetic resonance (ssNMR), Fourier-transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The CO2/N-2 and CO2/H-2 separation performances of the irradiated membranes were also evaluated and compared to those of the non-irradiated. Depending on the parameters used, fs laser processing was successful in modifying the surface of PIL-IL membranes through the formation of microchannels around 55-60 mu m deep. Significant improvements in CO2, N-2 and H-2 permeabilities were achieved for the irradiated PIL-IL membranes, maintaining their CO2/N-2 and CO2/H-2 permselectivities.
- Wetting response of KrF laser ablated polyimide surfacesPublication . Oliveira, Vitor; Nunes, B.; Vilar, RuiThe wettability of polyimide surfaces microstructured using KrF laser radiation at fluences above the material ablation threshold was studied by static contact angle measurements. The laser-treated surfaces present a morphology consisting of conical features whose dimensions and areal density depend on the fluence. The effect of these parameters on the surface apparent contact angle depends on the wetting regime. When wetting occurs in the homogeneous regime, the apparent contact angle of the treated surfaces increases with the radiation fluence because the cone dimensions increase. In contrast, when wetting occurs in the heterogeneous regime, the apparent contact angle increases with the radiation fluence because the average distance between cones increases. The apparent water contact angle of the laser-treated surfaces can reach values as high as 162 degrees, as compared to 75 degrees for virgin polyimide.
- Transformations induced in bulk amorphous silica by ultrafast laser direct writingTransformations induced in bulk amorphous silica by ultrafast laser direct writingPublication . Oliveira, Vitor; Sharma, Sahendra P.; Herrero, Pilar; Vilar, RuiA transmission electron microscopy study of nanogratings formed in bulk amorphous silica by direct writing with an ultrafast pulsed laser with a radiation wavelength of 1030 nm and pulse duration of 560 fs is presented. The results achieved show that the nanogratings are composed of planar nanostructures with an average periodicity of 250 nm and typical thickness of about 30 nm, consisting of alternating layers of heavily damaged material and layers of material where a dense precipitation of nanocrystals occurred. The crystallization of silica to form these nanocrystals can be explained by the large pressures and temperatures reached in these regions as a result of nanoplasma formation and recombination. (C) 2013 Optical Society of America
- How short and light can a simple pendulum be for classroom use?Publication . Oliveira, VitorWe compare the period of oscillation of an ideal simple pendulum with the period of a more 'real' pendulum constituted of a rigid sphere and a rigid slender rod. We determine the relative error in the calculation of the local acceleration of gravity if the period of the ideal pendulum is use?d instead of the period of this real pendulum. © 2014 IOP Publishing Ltd..