Recently, there have been significant efforts to develop micro-mechanical models for a better understanding of in-service performance of WC-Co components. However, reliable information about the mechanical properties of individual features like grains or interfaces is still lacking. In this work, micro-beam testing has been used for analyzing the fracture strength of different WC-WC interfaces in a WC-6.5 wt%Co alloy. The method is based on machining cantilever beams by using Focused Ion Beam so that a single WC-WC interface is isolated from the rest of the microstructure. This machining is carried out in order to have the selected interface at a certain distance for the fixed end and perpendicular to the cantilever axis. CSL2 boundaries and randomly oriented boundaries have been identified by means of EBSD and subsequently tested by nanoindentation until fracture. Load-displacement curves confirm that CSL2 boundaries are stronger than the others and post mortem analyses indicates that the fracture mechanisms are different depending on the orientation between adjacent WC grains. This approach could be used to investigate the intrinsic strength of other interfaces present in hardmetals (i.e. WC/Co, FCC carbides/Co, FCC carbides/WC) and how it is related with processing parameters or in-service conditions.

Microbeam testing is proposed as a new method for analysing the mechanical properties of individual microstructural features in WC-Co hardmetals; i.e. portions of WC grains or a single metallic ligament. Firstly, cantilever microbeams with dimensions below the microstructural scale of the material are machined by means of a focused ion beam (FIB). Afterwards, these beams are bended to fracture by means of an instrumented nanoindenter. In this way, both portions of WC grains and binder phase ligaments are broken while simultaneously recording the load and the vertical displacement of the nanoindenter tip. These cracking events are detected as sudden steps in the load vs. displacement curves. Afterwards, a scanning electron microscope is used to measure the distance from the main crack to the beam clamping. From these data, the stresses at which portions of cobalt ligaments and WC grains fail are estimated from linear elastic theory and FEM models.

Microelectronic industry is driven by the continuous miniaturization process conducing to the introduction of materials with better performance. These materials are subjected to stresses mainly due to thermal mismatch, microstructural changes or process integration which can be in the origin of mechanical reliability issues. To study these phenomena and even electromigration a good mechanical characterization of the materials is needed. This work aims at developing tests to assess fracture and elastoplastic behavior of thin Cu films. The tests developed are based on the deflection of microbeams (micromachined using a focused ion beam) using a nanoindenter. Different test geometries for microbeams have been evaluated and quantitative data have been obtained combining experimental results with analytical or numerical models, depending on the property under study. Microbeam response shows a strong dependence on the orientation of the grains close to the fixed end. Grain orientation has been measured by electron backscatter diffraction and the plastic behavior has been modeled by the finite element method using an in-house crystal plasticity subroutine. The effect of film thickness on fracture energy has been determined from tests of notched beams. (C) 2014 Elsevier B.V. All rights reserved.

The oxidation of highly porous ceramic matrix composites (PCMCs) based on different Tyranno® fibers has been analyzed by means of thermogravimetry and electron microscopy. Both uncoated fibers and PCMC materials exhibit parabolic kinetics between 900°C and 1250°C, these being faster for Ti-doped than for Zr-doped Tyranno fibers. Oxide layers in Ti-doped fibers are porous and partially crystalline, whereas in Zr-doped materials a significant fraction of relatively coarse ß-SiC grains is still found embedded in the amorphous silica matrix. On the other hand, the CVD-SiC coatings exhibit higher oxidation rates from the outer surface than from the inner one, a phenomenon that has been associated not only with the more difficult access of oxygen to the inner face but also with the highly <111> textured structure of these coatings, for which very different oxidation rates have been published for the inward and outward directions. Cracking phenomena observed above 1100°C for long dwelling times do not lead to an acceleration of the oxidation process, which could be due to the simultaneous crystallization of the amorphous silica layers

The continuous miniaturization process in the microelectronic industry, along with the introduction of Interlayer Dielectrics (ILDs) with poorer mechanical properties, makes necessary the development of characterization techniques to evaluate the mechanical performance of very thin films. This work presents a mechanical characterization technique for thin films based on membrane testing. Membranes, micromachined with anisotropic wet etching of Si, are tested to fracture using a nanoindenter to apply the load and register the provoked deflection. The technique is applied to the fracture characterization of two different ILDs with four thicknesses ranging from 100 nm to 500 nm. Combination of experiments and finite element simulations allows for the calculation of the strength of the materials from the fracture load. The technique permits to discriminate both ILDs and to establish clear thickness dependence: for both materials, 100 nm films show a significant lower strength while no effect of film thickness on strength is observed in the range between 200 and 500 nm. A sensitivity analysis of the outcome of the technique, the fracture stress, to the variability of the input parameters is presented, showing the robustness of the proposed approach: the experimental error in the fracture stress is smaller than the variation in the input parameters.

The continuous process of miniaturization in the microelectronics industry requires the introduction of new, thinner interlayer dielectric (ILD) materials with poorer mechanical properties. As a consequence, new mechanical characterization techniques are needed in the industry to evaluate very thin films. This work presents a new fracture characterization technique for thin films, called "dual tip indentation" (DTI). The technique takes advantage of a particular geometry of the indentation tip to provoke shallow and controlled cracking on the targeted brittle thin film. The technique is applied to the fracture characterization of two different ILD with four thicknesses, ranging from 100 nm to 500 nm. Further fractographic analysis, along with finite element modeling, shows that it is possible to extract intrinsic fracture properties from the fracture load. The technique allows one to discriminate between the ILD and, for both materials, 100 nm films show lower strength. No effect of film thickness on strength is observed in the range between 200 and 500 nm. The results from DTI compare well with those previously obtained for the same materials from membrane testing, taking into account the differences in volume tested.

Oxidation tests have been carried out on highly porous ceramic matrix fiber composites consisting of Tyranno Lox M fibers coated with Pyro C and CVD-SiC thin layers. TGA experiments carried out at 900 degrees C confirm that mass loss rates are higher for materials with thicker Pyro C layers. At higher temperatures (i.e. 1250 degrees C) such differences are not significant likely due to the interaction between SiC oxidation and Pyro C burnout. These tests clearly show that the oxidation kinetics of Tyranno fibers are much faster than those of CVD-SiC coatings. Therefore, the CVD-SiC coatings protect the Tyranno fibers against oxidation, although this is less effective as the thickness of the Pyro C layer increases. Finally, it has been found that the oxidation kinetics of the CVD-SiC layers are faster as the coating thickness increases and are different for the inner and the outer coating surfaces.

The fatigue behaviour of next generation high strength steels (sigma(UTS) = 950-1000 MPa) has been studied. Specifically, this study is focused on the initiation stage of fatigue micro-cracks. With this purpose, high cycle fatigue tests under uniaxial loading have been performed. During these tests, the deformation history of the specimen has been tracked by means of speckle interferometry. This technique allows monitoring the evolution of the displacement field and its derivatives on the specimen surface, so that it can be used as a tool for detecting microcracks in the first stages of crack initiation. The observation of the fracture surfaces provides complementary information about the localization of the initiation of failure thus, a correlation between the observations made by interferometry and the actual location of the fatigue nucleus and the evolution of the crack during its propagation can be established. Results appoint speckle interferometry as a promising technique for the detection of fatigue failures. (C) 2009 Elsevier Ltd. All rights reserved.

In order to meet the increasing needs from economic and social developments in future, the research on new generation steels with higher strength and longer duration, has become a worldwide issue. It is well known that various mechanisms to strengthen the steels exist, but grain refinement is the only method to improve both strength and toughness simultaneously. A ferrite grain size in the range of 1÷4 um and a steel microstructure characterised by a mixture of ferrite-pearlite and/or martensite, bainite microstructure, could give a very good combination of mechanical (strength, ductility, toughness, fatigue) and technological properties (machinability, cold/warm metal forming, etc.) for final application to automotive components.