Results 1 - 9 of 9
Results 1 - 9 of 9. Search took: 0.016 seconds
|Sort by: date | relevance|
[en] This work aims to investigate if glassy surface layers can be obtained when glass forming alloys are submitted to laser surface treatment techniques. Three types of alloys, with different glass forming abilities, were investigated: Zr-, Mg- and Al-based alloys. X-ray diffraction analysis shows that vitreous phases can be formed in the Zr- and Mg-based alloys, when the treatments are made using laser scanning speeds larger than 2 m/s
[en] The purpose of this paper is to illustrate the potential of laser melting to improve the surface characteristics of plastic mould steels, using a typical plastic mould steel (DIN X43Cr12) as a case study . After laser surface melting the microstructure of this steel is formed by fine dendrites of austenite partially transformed into martensite. Although the equilibrium solidification phase is δ-ferrite, the formation of primary austenite is kinetically favoured and this phase tends to predominate at the high solidification speeds used in laser processing. It was observed that the volume fraction of retained austenite depends critically on the laser processing parameters, so that the microstructure can change from almost completely martensitic to almost completely austenitic by changing the laser processing parameters. Laser melted tool steels show remarkable secondary hardening after tempering at suitable temperatures. In DIN X42Cr13 the secondary hardening peak temperature after LSM (600 degree centigree) is 100 degree centigree higher than after conventional heat treatment (500 degree centigree), due to the presence of large amounts of retained austenite. It was observed that this phase only destabilizes above 600 degree centigrade, due to the precipitation of M7C3 and stress relieving. After destabilization, retained austenite transforms into martensite during cooling. Secondary hardening is due to the transformation of retained austenite into martensite and to the precipitation of M7C3 and M23C6 carbides. (Author) 14 refs
[en] Quasicrystals are a new class of ordinated structures with metastable characteristics room temperature. Quasicrystalline phases can be obtained by rapid quenching from the melt of some alloys. In general, quasicrystals present properties which make these alloys promising for wear and corrosion resistant coatings applications. During the last years, the development of quasicrystalline coatings by means of thermal spray techniques has been impulsed. However, no references have been found of their application by means of laser techniques. In this work four claddings of quasicrystalline compositions formed over aluminium substrate, produced by a continuous CO2 laser using simultaneous powders mixture injection are presented. The claddings were characterized by X ray diffraction, scanning electron microscopy and Vickers microhardness. (Author) 18 refs
[en] In this paper, we present the results of a systematic study directed toward submicrometric scale triboactivity of a range of hydrogenated diamond-like-carbon (H : DLC) films derived from source gases with different hydrogen-to-carbon ratios. The H : DLC films were deposited on Si substrates in a plasma enhanced chemical vapour deposition system. Specifically, we produced three kinds of H : DLC films, using pure acetylene, pure methane and 25% methane + 75% hydrogen as the precursor source gases. Samples were subjected to wettability and depth sensing ultramicroindentation tests, and micro-to-nanoscale friction and wear studies using a nanotribometer and an atomic force microscope. The results of our study revealed a very close correlation between the wettability and the tribo-mechanical response of the H : DLC films at micro-to-nanoscales and their hydrogen-to-carbon ratio, i.e. lower hydrogen-to-carbon ratio leads to higher hardness (H) and lower water contact angles. Moreover, our results indicated a strong correlation between the hardness of the films and the threshold for severe wear damage. This threshold can be expressed by the ratio between the average Hertzian contact stress and the hardness which, in this study, is close to unity.
[en] Niobium carbide thin films were prepared by pulsed laser ablation of a stoichiometric NbC target. XeCl (308 nm, 30 ns) and Nd:YAG (266 nm, 5 ns) lasers operating at a repetition rate of 10 Hz were used. Films were deposited on Si (100) substrates at room temperature either in vacuum or in an argon atmosphere (2 x 10-1 mbar). Different laser fluences (2,4 and 6 J/cm2) and different numbers of pulses (1 x 104, 2 x 104 and 4 x 104) were tested. For the first time, NbC films were prepared through a clean procedure without the addition of a hydrocarbon atmosphere. The phase constitution of the films, unit cell size, mean crystallite dimensions and preferred orientation are determined as a function of deposition conditions by X-ray diffraction. Complementary morphological and structural analysis of the films were performed by scanning electron microscopy, atomic force microscopy and Rutherford backscattering spectroscopy. (orig.)
[en] Previous work by the authors on micromachining of Al2O3-TiC ceramics using excimer laser radiation revealed that a columnar surface topography forms under certain experimental conditions. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations show that the columns develop from small globules of TiC, which appear at the surface of the material during the first laser pulses. To understand the mechanism of formation of these globules, a 2D finite element ablation model was developed and used to simulate the time evolution of the temperature field and of the surface topography when a sample of Al2O3-TiC composite is treated with KrF laser radiation. Application of the model showed that the surface temperature of TiC rises much faster than that of Al2O3, but since TiC has a very high boiling temperature, its vaporization is significant only for a short time. By contrast, the surface temperature of Al2O3 rises above its boiling temperature for a much longer period, leading to a greater ablation depth than TiC. As a result, a small TiC globule stands above the Al2O3 surface. The results of the model are compared with experimental measurements performed by AFM. After three pulses, the height of the globules predicted by the model is about 340 nm, in good agreement with the height measured experimentally, about 400 nm
[en] Mo-Se-C films were deposited by sputtering from a carbon target with pellets of MoSe2. In addition to the standard evaluation of their chemical composition, structure, morphology, hardness and cohesion/adhesion, the core objective of this paper was to analyze the tribological behavior of these films, particularly in the high-load regime. The carbon content varied from 29 to 68 at.% which led to a progressive increase of the Se/Mo ratio and the hardness. The friction coefficient of Mo-Se-C coatings clearly decreased with load from ∼0.15 to ∼0.05. The excellent friction properties were attributed to the formation of a thin molybdenum diselenide film on the top of the wear track of the coating and on the counterpart surface, while the role of the carbon in the sliding process is only secondary by increasing the coating hardness and thus its wear resistance
[en] The microstructure and magnetic domain configuration of splat-quenched YFe11Ti aggregates have been investigated by transmission electron microscopy, Lorentz microscopy and magnetic force microscopy. The splat-quenched material adopted essentially the ThMn12-type structure and displayed an equiaxed polycrystalline microstructure with a scattered presence of α-Fe(Ti). Lorentz microscopy and magnetic force microscopy showed that the critical size for single-domain behaviour in the splat-quenched aggregates is close to 100 nm. Annealing induced a decrease in coercivity justified by overall grain coarsening and additional α-Fe(Ti) formation. Lorentz microscopy evidenced stripe/maze patterns in coarse YFe11Ti grains; while coarse α-Fe(Ti) grains exhibited vortex configurations resulting from accommodation to the demagnetizing fields of adjacent YFe11Ti grains. Planar defects with twin appearance and thermal antiphase boundaries have been observed in YFe11Ti grains but showed no interaction with domain walls. On the other hand, grain boundaries and in particular grain-boundary triple joints exhibited a moderate pinning effect.
[en] In the present work we investigate the ageing of acid cleaned femtosecond laser textured <1 0 0> 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 > 120o). However these surfaces are not stable and ageing leads to a decrease of the water contact angle which reaches a value of 80o. XPS analysis of the surfaces shows that the growth of the native oxide layer is most probably responsible for this behavior.