Recursos de colección

Estudo Geral - Universidade de Coimbra (19.630 recursos)

ESTUDO GERAL é a designação do repositório digital da produção científica da Universidade de Coimbra, com o objectivo de divulgar conteúdos digitais de natureza científica de autores ligados à Universidade de Coimbra. A sua criação insere-se no movimento de Acesso Livre à Literatura científica (Open Access), ao qual o Conselho de Reitores das Universidades Portuguesas aderiu em 2006 e que a Universidade de Coimbra subscreveu. À semelhança de outras grandes universidades nacionais e internacionais, a UC tem o maior interesse em aumentar a sua presença na rede informática mundial, sendo cada vez mais - e também por essa via - um emissor de conhecimento e cultura.

FCTUC Eng.Mecânica - Artigos em Revistas Internacionais

Mostrando recursos 1 - 20 de 215

  1. Frictional behavior of self-adaptive nanostructural Mo–Se–C coatings in different sliding conditions

    Gustavsson, F.; Jacobson, S.; Cavaleiro, A.; Polcar, T.
    Sliding properties of Mo–Se–C coatings with two different carbon content deposited by magnetron sputtering were investigated in different sliding environments (argon, nitrogen, dry and humid air). Both coatings had a structure that was identified as randomly oriented structures of MoSe2 embedded into amorphous carbon matrix. The worn surfaces, i.e. the wear tracks and the wear scars of the balls, were analyzed by optical microscopy, Raman spectroscopy and scanning electron microscopy. The material transferred to the ball steel surfaces was almost exclusively MoSe2, whereas the wear tracks on the coatings were more complex, with areas rich in MoSe2 and areas similar...

  2. Structural and optical studies of Au doped titanium oxide films

    Alves, E.; Franco, N.; Barradas, N. P.; Nunes, B.; Lopes, J.; Cavaleiro, A.; Torrell, M.; Cunha, L.; Vaz, F.
    Thin films of TiO2 were doped with Au by ion implantation and in situ during the deposition. The films were grown by reactive magnetron sputtering and deposited in silicon and glass substrates at a temperature around 150 °C. The undoped films were implanted with Au fluences in the range of 5 × 1015 Au/cm2–1 × 1017 Au/cm2 with a energy of 150 keV. At a fluence of 5 × 1016 Au/cm2 the formation of Au nanoclusters in the films is observed during the implantation at room temperature. The clustering process starts to occur during the implantation where XRD estimates the...

  3. Carbon-based coatings doped by copper: Tribological and mechanical behavior in olive oil lubrication

    Balestra, R. M.; Castro, A. M. G.; Evaristo, M.; Escudeiro, A.; Mutafov, Petr; Polcar, T.; Cavaleiro, A.
    We deposited C-based films doped with Cu and tested their sliding properties in olive oil as environment-friendly lubricant, which can be used in many mechanical systems, particularly in agriculture engineering. The coatings were deposited in a four unbalanced magnetron sputtering device combining C and C/Cu targets; argon (hydrogen-free films) and Ar/CH4 (hydrogenated films) atmospheres were used. Cu content of the films was in the range 5-14 at.%. The hardness of the films was almost constant whatever the Cu content was. On the other hand, hydrogen-free coatings were much harder (about 15 GPa) than hydrogenated ones (about 4 GPa). The coatings were oleophilic and their sliding properties were...

  4. Self-adaptive low friction coatings based on transition metal dichalcogenides

    Polcar, Tomas; Cavaleiro, Albano
    This paper deals with three fundamentally different concepts of self-lubricant coatings based on the transition metal dichalcogenides (TMD) and deposited by magnetron sputtering. The first two designs could be considered as traditional: the TMD films doped by carbon or nitrogen. The main attention is aimed at qualitative description of surface and sub-surface modification of the films as the result of the sliding process. Based on a thorough analysis of the worn surfaces, two features emerge: self-adaptability, where originally randomly oriented TMD phase becomes well oriented (i.e. with basal planes parallel to the surface), and limited role of carbon or nitrogen in the contact. The films doped with...

  5. Influence of tool shoulder geometry on properties of friction stir welds in thin copper sheets

    Galvão, I.; Leal, R. M.; Rodrigues, D. M.; Loureiro, A.
    The aim of this work is to study the influence of the shoulder geometry on friction stir welding of 1 mm-thick copper-DHP plates. The welds were produced using three different shoulder geometries, flat, conical and scrolled, and varying the rotation and traverse speeds of the tool. The flat shoulder tool proved to be inadequate for performing welds, because many defects were produced for all welding conditions. In turn, the scrolled shoulder tool is more effective than the conical one in the production of defect free welds. However, both geometries required a minimum rotational speed to avoid internal defects. For the same welding parameters, greater grain refinement,...

  6. Effects of the constitution of CrON diffusion barrier on the oxidation resistance and interfacial fracture of duplex coating system

    Li, W. Z.; Yi, D. Q.; Li, Y. Q.; Liu, H. Q.; Sun, C.
    The duplex coating system of a NiCrAlY overlayer with a CrON diffusion barrier with different phase contents was deposited by AIP method. The duplex coatings were characterized regarding their microstructure, and the ability of the diffusion barrier was evaluated. The oxidation resistance and the interfacial strength of the duplex coatings were investigated. The results indicated that the duplex coating system with a diffusion barrier with an O/N ratio ∼66 and a lower Cr2O3 phase content exhibited more excellent oxidation resistance and lesser interdiffusion than with a diffusion barrier with O/N ∼30.1. The exposed coating samples possessed an improved interfacial strength...

  7. An XPS study of Au alloyed Al–O sputtered coatings

    Figueiredo, N. M.; Carvalho, N. J. M.; Cavaleiro, A.
    The focus of this research is the X-ray photoelectron spectroscopy (XPS) analysis of thin films consisting of Au metal clusters embedded in a dielectric matrix of Al-O coatings. The coatings were deposited by co-sputtering an Al+Au target in a reactive atmosphere with Au contents up to 8 at.%. The Al-O matrix was kept amorphous even after annealing at 1000°C. In the as-deposited films the presence of Au clusters with sizes smaller than 1-2 nm (not detected by XRD) was demonstrated by XPS. With increasing annealing temperature, Au clustering in the dielectric matrix was also confirmed by XPS, in agreement with XRD results.

  8. Life-cycle assessment of a house with alternative exterior walls: Comparison of three impact assessment methods

    Monteiro, Helena; Freire, Fausto

  9. Thermal stability in oxidative and protective environments of a-C:H cap layer on a functional gradient coating

    Louro, C.; Moura, C. Wagner; Carvalho, N.; Stueber, M.; Cavaleiro, A.
    Three types of hydrogenated amorphous carbon (a-C:H) coatings were synthesized on stainless steel substrates by a Plasma Assisted CVD process, containing hydrogen contents in the range from 25 to 29 at.%. The effect of annealing up to 600 °C in two different environments on both the structure and the mechanical properties of the coatings were investigated by means of Differential Scanning Calorimetry/Thermogravimetry (DCS/TG), Raman Spectroscopy and Depth Sensing Indentation. The results indicate that the structural modifications occurred in the coatings in both protective and oxidative atmospheres up to 400 °C were due to a complex atomic rearrangement involving the dehydrogenation...

  10. Adhesion failures on hard coatings induced by interface anomalies

    Silva, Carlos W. Moura e; Alves, E.; Ramos, A. R.; Sandu, Cosmin S.; Cavaleiro, A.
    In this work, the lack of adhesion occurred during the up-scaling of the deposition of tribological coatings in a semi-industrial apparatus is interpreted. The adhesion problems were detected for both hard and self-lubricant coatings from W-Ti-N and W:C systems, respectively, when they were deposited in a 4 cathodes TEER® chamber by reactive unbalanced magnetron sputtering. In spite of cleaning the substrates surface by ion bombardment prior to deposition, by establishing a discharge close to the substrate, insufficient adhesion critical load values were measured by scratch-testing. A powerful set of complementary techniques was used to the detailed analysis of the interfaces in...

  11. Mixed Protein Carriers for Modulating DNA Release

    Morán, M. Carmen; Pais, Alberto A. C. C.; Ramalho, Amilcar; Miguel, M. Graça; Lindman, Björn
    Aqueous mixtures of oppositely charged polyelectrolytes undergo associative phase separation, resulting in coacervation, gelation, or precipitation. This phenomenon has been exploited in forming DNA gel particles by interfacial diffusion. We report here the formation of DNA gel particles by mixing solutions of double-stranded DNA with aqueous solutions containing two cationic proteins, lysozyme and protamine sulfate. The effect of the lysozyme/protamine ratio on the degree of DNA entrapment, surface morphology, swelling−deswelling behavior, and kinetics of DNA release has been investigated. By mixing the two proteins, we obtain particles that display higher loading efficiency and loading capacity values, in comparison to those...

  12. Oxygen Nonstoichiometry, Mixed Conductivity, and Mössbauer Spectra of Ln0.5A0.5FeO3−δ (Ln = La−Sm, A = Sr, Ba): Effects of Cation Size

    Kharton, Vladislav V.; Kovalevsky, Andrei V.; Patrakeev, Mikhail V.; Tsipis, Ekaterina V.; Viskup, Alexandre P.; Kolotygin, Vladislav A.; Yaremchenko, Aleksey A.; Shaula, Aliaksandr L.; Kiselev, Evgeny A.; Waerenborgh, João C.
    Increasing the difference of the Ln3+ and A2+ cation radii in perovskite-type Ln0.5A0.5FeO3−δ (Ln = La, Pr, Nd, Sm; A = Sr, Ba) results in higher oxygen deficiency and lower oxygen-ionic and p-type electronic conductivities, determined using the oxygen permeation and total conductivity measurements at 973−1223 K. The relationships between the anion transport and A-site cation size mismatch remain essentially similar in air and under reducing conditions when most iron cations become trivalent, thus confirming critical influence of oxygen-vacancy trapping processes induced by the lattice strain. At low temperatures, analogous correlation is also observed for quadrupole splittings derived from the...

  13. Modelling of Magnetron Sputtering of Tungsten Oxide with Reactive Gas Pulsing

    Kubart, Tomáscaron; Polcar, Tomáscaron; Kappertz, Oliver; Parreira, Nuno; Nyberg, Tomas; Berg, Sören; Cavaleiro, Albano
    Reactive sputtering is one of the most commonly employed processes for the deposition of thin films. However, the range of applications is limited by inherent instabilities, which necessitates the use of a complex feedback control of reactive gas (RG) partial pressure. Recently pulsing of the RG has been suggested as a possible alternative. In this report, the concept of periodically switching the RG flow between two different values is applied to the deposition of tungsten oxide. The trends in the measured time dependent RG pressure and discharge voltage are reproduced by a dynamical model developed for this process. Furthermore, the...

  14. Shock activation of [alpha]-alumina from calcinated Al-rich sludge

    Farinha, A. R.; Ribeiro, J. B.; Mendes, R.; Vieira, M. T.

  15. Experiments with service-oriented architectures for industrial robotic cells programming

    Veiga, G.; Pires, J. N.; Nilsson, K.

  16. Optics of quasi-particle phase transitions in nanostructured Ag thin films

    Gnanavel, M.; Bharathi Mohan, D.; Sunandana, C.S.

  17. Tribological behaviour of W-Ti-N coatings in semi-industrial strip-drawing tests

    Severo, V.; Vilhena, L.; Silva, P. N.; Dias, J. P.; Becker, D.; Wagner, S.; Cavaleiro, A.

  18. Residual stresses analysis of ND-YAG laser welded joints

    Costa, J. M.; Pires, J. T. B.; Antunes, F.; Nobre, J. P.; Borrego, L. P.

  19. Fatigue crack growth in heat-treated aluminium alloys

    Borrego, L. P.; Costa, J. M.; Antunes, F. V.; Ferreira, J. M.

  20. Formability of similar and dissimilar friction stir welded AA 5182-H111 and AA 6016-T4 tailored blanks

    Leitão, C.; Emílio, B.; Chaparro, B. M.; Rodrigues, D. M.

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