Materials Science Forum Vols. 514-516

Abstract: Adobe is commonly found in Aveiro's ancient constructions. Preservation and rehabilitation of those constructions, some of them with architectural and historical interest, has been forgotten for many years. As a result, in Aveiro region, the majority of existing constructions in adobe is structurally weak, and, in several cases, they are in the threshold of ruin. Rehabilitation and/or strengthening urge. Despite some efforts has been made, a great difficulty for technicians working on the rehabilitation of these constructions relies on the lack of knowledge on adobe's mechanical behaviour. In fact, in order to properly describe the structural behaviour of those constructions, there is a need to investigate the mechanical behaviour of adobe. Hence, this paper is based on a study intended to characterise the behaviour of adobe brick units. Specimens were prepared from selected representative constructions of the Aveiro region. The prepared specimens were tested in order to evaluate their mechanical behaviour in compression and tension.

Abstract: The purpose of the present paper was to investigate the effect of interlayer on the maximum contact stresses in the critical regions in a hard coating under static contact condition using finite element analysis. Four different elastic moduli and nine different thickness of interlayer were used. Modeling results showed that the interlayer did not reduce the maximum shear stress at the coating/substrate interface, whether it was thick or thin. When the thin interlayer was presented, whether it was soft or hard, the maximum tensile stress on surface and maximum von Mises stress within coating were decreased.

Abstract: In this work, the structural evolution and damage of PET during stretching is assessed by wide- and small-angle X-ray scattering (respectively, WAXS and SAXS) experiments in specimens pre-deformed at different strain levels (ex-situ characterization). Injection moulded PET rectangular tensile specimens were stretched (at 2 mm/min) into the plastic domain in a universal test machine at different strain levels at room temperature. The structure of the central zone of the deformed specimens was then characterized by WAXS and SAXS experiments using an X-ray synchrotron source. PET was initially (before stretching) amorphous. A strong molecular orientation in the stretching direction is quickly developed for the initial plastic deformation levels, evidenced by strong equatorial WAXS reflections. This orientation rapidly levels off, remaining constant during further stretching. The WAXS patterns are accompanied with no reflections on SAXS, evidencing a local ordering phenomenon, typical of nematic liquid-crystalline structures. The SAXS patterns evidence the occurrence of some voiding in the cold drawing regime just after yielding. These anisotropic voids are oriented perpendicular to the stretching direction, as in a craze-like structure. The void size drastically increases at the onset of the strong strain hardening behaviour.

Abstract: A sputter deposition chamber inserted into the six-circle Huber diffractometer of the materials research station of the ROssendorf BeamLine (ROBL-CRG) at ESRF allowed to perform in-situ experiments during film growth of Ni-Ti. It is equipped with Kapton windows for X-Ray Diffraction (XRD) and specular Reflectivity (XRR) measurements. By following in situ the evolution of the structure of the growing film, we reveal intermediate “states” which cannot be seen/revealed ex situ, because those states occurred only during the growth but were no longer visible after deposition. Vertical Bragg-Brentano large-angle scattering geometry was employed to study the different trends of structural transformations taking place during deposition. Ni-Ti films exhibiting a non-uniform phase content across the film thickness could be produced by varying the power of co-sputtering Ni-Ti plus Ti. A significant decrease of IB2{110}/IB2{200} was observed when a bias of -45 V was applied.

Abstract: When atomic force microscopy is used to retrieve nanomechanical surface properties of materials, unsuspected measurement and instrumentation errors may occur. In this work, some error sources are investigated and operating and correction procedures are proposed in order to maximize the accuracy of the measurements. Experiments were performed on sapphire, Ni, Co and Ni-30%Co samples. A triangular pyramidal diamond tip was used to perform indentation and scratch tests, as well as for surface visualization. It was found that nonlinearities of the z-piezo scanner, in particular the creep of the z-piezo, and errors in the determination of the real dimensions of tested areas, are critical parameters to be considered. However, it was observed that there is a critical load application rate, above which the influence of the creep of the z-piezo can be neglected. Also, it was observed that deconvolution of the tip geometry from the image of the tested area is essential to obtain accurate values of the dimensions of indentations and scratches. The application of these procedures enables minimizing the errors in nanomechanical property measurements using atomic force microscopy techniques.

Abstract: Quaternary GaInAsSb films alloys were grown by MOVPE technique on GaSb substrates with different growth conditions such as substrate orientation and thickness. The composition of the films determines their bandgap, and also how well they are lattice matched to the substrate. It is thus essential to determine it accurately, which is not a trivial task in this system. The composition of the samples was studied with a combination of Particle Induced Xray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS) experiments. The RBS experiments were done with a 2 MeV 4He+ or H+ ion beam, according to the thickness of the films, and were used to determine the thickness of the samples. The PIXE experiments were performed at grazing angle conditions and provided accurate elemental composition information. It was found that for thin layers (300 nm) there is a dependence of In incorporation into the matrix according to the substrate orientation, although this tendency was not found for thicker films (24m).

Abstract: Induced defects in silver polycrystalline samples irradiated with 4 keV Ar+ were characterised with slow positron implantation spectroscopy. The implanted gas was found to interact with ion irradiation defects. The evolution of the defects and gas-defect interactions were followed through a multi-step isochronal annealing treatment. Two different defected regions were detected. A region near to the surface, due to a distribution of vacancy-like defects produced by irradiation, and a deeper one due to coalescence of Ar. The deeper defects evolve with thermal treatments and probably produce cavities which are not easily recovered.

Abstract: The thin and textured coatings present a double difficulty for characterization by conventional X-ray diffraction. Their shallow depth reduces the diffracted intensity and allows the interference of the underlying material. Frequently they present a crystallographic texture which limits the number of orientations that provide good intensity and induces anisotropy effects on their mechanical behavior. Reliable results can be determined using diffraction geometry of lowincidence angle. This paper describes the application of the technique to several films, characterized by thicknesses of the order of 1 μm and crystallographic textures. Examples are proposed of chromium films applied by PVD on molybdenum substrates, decorative electroplated coatings, and aluminum coatings used for interconnections in microelectronic circuits. The Cr films are 1.5 μm thick and exhibit a strong <100> fiber texture. The decorative coatings were studied both on the nickel undercoat and in the Cr top layer. Results are presented for chromium where tensile stresses and a <110> fiber texture were observed. The Al films are 1.0 μm thick. Some samples were heattreated at different annealing temperatures. Tensile stresses were always observed, which increase in the annealed samples.

Abstract: The conventional Bragg diffraction geometry, normally used to characterize the residual surface stress state, it is not suitable to evaluate surface treated materials and thin films. The X-ray path lengths through a surface layer or thin film are too short to produce adequate diffraction intensities in relation to the bulk or the substrate. Another limitation of the conventional technique appears when a residual stress gradient is present in the irradiated surface. The technique only enables the evaluation of the mean value of this gradient. In these cases, a recently proposed Pseudo-Grazing Incident X-ray Diffraction method would be better applicable. In this study, the Pseudo-Grazing Incidence X-ray Diffraction is applied to characterize the residual stress depth profiles of several AISI 4140 samples, which were prepared, by mechanical polishing and grinding, in order to present different surface roughness parameters, Ra. The experimental results lead to the conclusion that the surface roughness limits the application of the Pseudo-Grazing Incidence methodology to a minimum X-ray incident angle. This angle is the one that enables a mean X-ray penetration depth with the same order of magnitude of the sample surface roughness parameter, Ra.