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.

I&D CEMUC - Artigos em Revistas Internacionais

Mostrando recursos 1 - 8 de 8

  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. Fatigue Behaviour of T Welded Joints Rehabilitated by Tungsten Inert Gas and Plasma Dressing

    Ramalho, A.; Ferreira, J. A. M.; Branco, C. M.
    This paper concerns a fatigue study on the effect of tungsten inert gas (TIG) and plasma dressing in non-load-carrying fillet welds of structural steel with medium strength. The fatigue tests were performed in three point bending at the main plate under constant amplitude loading, with a stress ratio of R = 0.05 and a frequency of 7 Hz. Fatigue results are presented in the form of nominal stress range versus fatigue life (S–N) curves obtained from the as welded joints and the TIG dressing joints at the welded toe. These results were compared with the ones obtained in repaired joints, where...

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