Micro-abrasion resistance of thermochemically treated steels in aqueous solutions: Mechanisms, maps, materials selection

The area of micro-abrasion is an interesting and relatively recent area in tribo-testing methodologies, where small particles of less than 10 μm are employed between interacting surfaces. It is topical for a number of reasons; its direct relation to the mechanisms of the wear process in bio-tribological applications, ease in conducting tests and the good repeatability of the test results. It has widespread applications in conditions used in the space and offshore industries to bio-engineering for artificial joints and implants. There have been many recent studies on the micro-abrasion performance of materials, ranging from work basic metals to nano-structured coatings. However, no significant work is reported on the micro-abrasion resistance of thermochemically treated steels. Hence, this paper looks at the performance of two thermochemically treated steels, Tenifer bath nitride stainless steel (T-SS) and vanadized carbon steel (V-CS) in such conditions with reference to the stainless steel (SS) by varying the applied load and sliding distance. The results indicated that T-SS demonstrates exceptionally poor resistance to micro-abrasion. It was observed that the heat treatment process and properties of the hardened layer (hardness and thickness) are extremely important in determining the micro-abrasion resistance of such steels. Finally, the results were used to develop micro-abrasion mechanism and wastage maps, which can be used to optimize the surface treated materials for micro-abrasion resistance

Effects of the second virial coefficient on the adiabatic lapse rate of dry atmospheres

We study the effect of the second virial coefficient on the adiabatic lapse rate of a dry atmosphere. To this end, we compute the corresponding adiabatic curves, the internal energy, and the heat capacity, among other thermodynamic parameters. We apply these results to Earth, Mars, Venus, Titan, and the exoplanet G1 851d, considering three physically relevant virial coefficients in each case: the hard-sphere, van der Waals, and the square-well potential. These examples illustrate under which atmospheric conditions the effect of the second virial coefficient is relevant. Taking the latter into account yields corrections towards the experimental values of the lapse rates of Venus and Titan in some instances.Comment: 12 pages, 8 figures. Comments are welcom

Tribo-Corrosion behaviour of TiCxOy thin films in bio fluids

In recent years, the development of thin film systems for decorative applications has attracted significant attention in scientific research. These decorative coatings require, not only an attractive appearance for market applications, but also an ability to protect the surface underneath. Because of this, corrosion, wear and their combined effects (termed tribo-corrosion) are particularly important for lifetime prediction. The tribo-corrosion behaviour of a range of single layered titanium oxycarbide, TiCxOy,coatings, produced by DC reactive magnetron sputtering, has been studied and reported as a function of electrode potential (-0.9 V, -0.5 V, 0.0 V and +0.5 V) and applied load (3, 6 and 9 N). The study was conducted in a reciprocating sliding tribosystem (Plint TE 67/E) in a bio fluid (an artificial perspiration solution) at room temperature. During the wear tests, both the open-circuit potential and the corrosion current were monitored. The results showed that electrode potential and load have a significant influence on the total material loss. The variations in Rp (polarization resistance) and Cf(capacitance) before and after sliding, obtained by Electrochemical Impedance Spectroscopy (EIS) were evaluated in order to provide an understanding of the resistance of the film in such conditions. Tribo-corrosion maps were generated, based on the results, indicating the change in mechanisms of the tribological and corrosion parameters for such coatings

Evaluation of corrosion resistance of multi-layered Ti/gass-ceramic interfaces by electrochemical impedance spectroscopy

Practical applications of metal/ceramic joints can be found in the biomedical field regarding the encapsulation of implantable telemetric devices, the fabrication of crowns and bridges for dental restoration, or in the production of drug delivery systems, biomedical sensors and electrodes. Most of metal/ceramic joints are produced by the active metal brazing technique, which originates a multi-layered interface which should be able of accommodating the abrupt electronic, crystallographic, chemical, mechanical and thermo-mechanical discontinuity that characterize these systems. Additionally, when considering biomedical applications, corrosion resistance becomes of prime importance. In this work, the corrosion resistance of Ti/glass-ceramic interfaces obtained by active metal brazing was evaluated by electrochemical impedance spectroscopy (EIS) tests. The electrochemical behaviour of the interface was monitored, as a function of time, in a simulated physiological solution at room temperature. In order to evaluate the contribution of each layer and galvanic interactions between them, to the degradation mechanism of the interface, individual samples, representative of reaction layers present at the interface, were fabricated and electrochemically ested. Results show that the corrosion behaviour, of the whole interface was strongly influenced by the chemical composition of its constitutive layers. Thus, layers containing high contents of both titanium and silver showed a polarisation resistance increase with the immersion time, as a result of the formation of a thermodynamically stable passive film. On the other hand, the copper rich layer, appears to be the main responsible for the interface degradation. In fact, for high immersion times, an instable passive film is formed and, as a consequence, large amounts of copper are released. Galvanic interactions between the copper and the silver rich layers where also dentified.Fundação para a Ciência e a Tecnologia (FCT), Portugal Portugal (projects POCTI/CTM/33384/2000 and SFRH/BPD/5518/2001)

Rhombomere of origin determines autonomous versus environmentally regulated expression of Hoxa3 in the avian embryo

We have investigated the pattern and regulation of Hoxa3 expression in the hindbrain and associated neural crest cells in the chick embryo, using whole mount in situ hybridization in conjunction with DiI labeling of neural crest cells and microsurgical manipulations. Hoxa3 is expressed in the neural plate and later in the neural tube with a rostral border of expression corresponding to the boundary between rhombomeres (r) 4 and 5. Initial expression is diffuse and becomes sharp after boundary formation. Hoxa3 exhibits uniform expression within r5 after formation of rhombomeric borders. Cell marking experiments reveal that neural crest cells migrating caudally, but not rostrally, from r5 and caudally from r6 express Hoxa3 in normal embryo. Results from transposition experiments demonstrate that expression of Hoxa3 in r5 neural crest cells is not strictly cell-autonomous. When r5 is transposed with r4 by rostrocaudal rotation of the rhomobomeres, Hoxa3 is expressed in cells migrating lateral to transposed r5 and for a short time, in condensing ganglia, but not by neural crest within the second branchial arch. Since DiI-labeled cells from transposed r5 are present in the second arch, Hoxa3-expressing neural crest cells from r5 appear to down-regulate their Hoxa3 expression in their new environment. In contrast, when r6 is transposed to the position of r4 after boundary formation, Hoxa3 is maintained in both migrating neural crest cells and those positioned within the second branchial arch and associated ganglia. These results suggest that Hoxa3 expression is cell-autonomous in r6 and its associated neural crest. Our results suggest that neural crest cells expressing the same Hox gene are not eqivalent; they respond differently to environmental signals and exhibit distinct degrees of cell autonomy depending upon their rhombomere of origin

Efficient calculation of formation energies of kink-pairs in BCC crystals

En materiales con estructura cristalina BCC el movimiento de dislocaciones de tipo tornillo a baja temperatura está asociado con la formación y el crecimiento de escalones en la línea de la dislocación. Para entender la movilidad de dislocaciones de tipo tornillo en estos materiales es muy importante la correcta predicción de las energías de nucleación de estos escalones. El cálculo a nivel atomístico de la mecánica de dislocaciones constituye un problema complicado desde el punto de vista tanto numérico como computacional. Este trabajo se centra en el cálculo de las energías de formación de distintas configuraciones de escalones dobles en cristales BCC y en ausencia de cargas exteriores. En nuestro modelo, basado en la teoría discreta de dislocaciones desarrollada por Ariza y Ortiz, la energía almacenada se calcula de forma eficiente mediante un algoritmo de programación basado en NVIDIA Compute Unified Device Architecture (CUDA). Los resultados obtenidos presentan un buen acuerdo con los calculados utilizando primeros principios y potenciales atomísticos, y los correspondientes a la teoría elástica de dislocaciones.Motion of screw dislocations in BCC materials at low temperature is believed to be related to the formation of mobile kinks on the dislocation line. Therefore, the accurate prediction of kink nucleation energies is required to fully describe mobility of screw dislocations in these materials. Studies of fundamental dislocation processes at atomic length scale are numerically and computationally intensive problems. This work studies the calculation of zero-stress formation energies of kink-pair configurations for BCC crystals. Our model for stored energy associated to a dislocation line configuration is based on the theory of discrete dislocations of Ariza and Ortiz. Its value is computed efficiently using an algorithm developed on the NVIDIA Compute Unified Device Architecture (CUDA). Results confirm those obtained using atomistic potentials and first principles calculations, and those based on the continuum theory of dislocations.Peer Reviewe

Uso de RDF y bases de datos de metadatos nativas dentro del proyecto Omnipaper

Este artículo describe el trabajo realizado para la creación de un prototipo para la búsqueda de información en archivos digitales distribuidos utilizando la tecnología RDF y una base de datos nativa. El articulo reseña los prerrequisitos para la descripción y normalización de la información de los archivos distribuidos, luego los criterios para la selección de la base de datos nativa, muestra las funcionalidades del prototipo creado y al final tiene una síntesis de las lecciones aprendidas y el trabajo futuro.Comissão Europeia - IST (OmniPaper - Smart Access to European Newspapers

Corrosão de interfaces ti/al2o3 em solucões fisiologicas simuladas

Nos últimos anos o estudo das interfaces metal/cerâmico (M/C) tem despertado grande interesse devido às diversas utilidades que estas podem oferecer em diferentes campos de aplicação prática. Um destes campos é o relativo à área das aplicações biomédicas. De entre os vários processos existentes para a produção de uniões M/C, as técnicas de união por difusão e de brasagem activa são algumas das mais utilizadas. Em qualquer uma destas técnicas, a alta temperatura envolvida no processo é um parâmetro que favorece o desenvolvimento de reacções químicas complexas que dão origem a interfaces multi-camadas cujas propriedades físico-químicas são complexas. Esta complexidade da interface, unida ao facto, já conhecido, de que as propriedades mecânicas, físicas e químicas, incluindo a resistência à corrosão, dependem da composição química e da microestrutura da região interfacial, fazem com que seja necessário um melhor conhecimento da natureza e das características físico-químicas desta região, de forma a poder controlar mais estritamente as propriedades da interface. O principal objectivo deste trabalho é o estudo do comportamento à corrosão de interfaces Ti/Al2O3 produzidas pela técnica de união por brasagem activa. Para efectuar este estudo para além da interface, foram produzidas ligas com composições químicas similares a cada camada interfacial, as quais foram testadas através de ensaios electroquímicos de corrente directa, nomeadamente, polarização potenciodinâmica, resistência à polarização e corrosão galvânica. Neste trabalho, será descrito o comportamento à corrosão da interface, assim como o efeito que a interacção entre as diferentes camadas presentes na interface produz sobre o comportamento à degradação geral da interface Ti/Al2O3.Fundação para a Ciência e a Tecnologia (FCT), Portugal (Projectos POCTI/CTM/33384/2000 e SFRH/BPD/5518/2001)