234 research outputs found
Micro-mechanical testing of afvanced ceramics: tools, procedures and first results
Micro-mechanical testing has lately become a more accessible tool for understanding deformation, strengthening and
failure mechanisms at small scales. It has been found that the often considered intrinsic or “intensive” properties of
materials, i.e. not size dependent, start to exhibit an extrinsic behaviour if the volume of material tested is reduced down
to the level of the micro- or nano-scale. This is true at least for metals, where diverse experimental approaches have
shown that the ultimate strength strongly increases in enough small material volumes in the micro-nano range.
In ceramics, the small scale testing approach has received much less attention probably because of the absence of
dislocation-controlled deformation mechanisms. Even though, it is the only direct method for the study of the
mechanical behaviour of ceramics in thin coatings, superficial layers induced by surface degradation processes as in
wear, corrosion, etc. Besides, in ceramics with a grain size dependent transformation toughening mechanism, such as
zirconia-based ceramics, a clear effect is expected when testing at the micro-scale.
In this work the methodology of micro-mechanical testing is presented and is applied to yttria-stabilized zirconia.
Advantages and limitations of the technique are discussed and details about the combination of FIB-machining and
nanoindentation testing are illustrated. At the same time, first results of the strength in compression of zirconia micropillars
are presented and the failure mechanism is discussed.Postprint (published version
Hydrogen-rich gas production by steam reforming and oxidative steam reforming of methanol over La0.6Sr0.4CoO3-d: effects of preparation, operation conditions, and Redox cycl
La0.6Sr0.4CoO3-d (LSC) perovskite, as a potential catalyst precursor for hydrogen (H2)-rich production by steam reforming of methanol (SRM) and oxidative steam reforming of methanol (OSRM), was investigated. For this purpose, LSC was synthesized by the citrate sol–gel method and characterized by complementary analytical techniques. The catalytic activity was studied for the as-prepared and prereduced LSC and compared with the undoped LaCoO3-d (LCO) at several feed gas compositions. Furthermore, the degradation and regeneration of LSC under repeated redox cycles were studied. The results evidenced that the increase in the water/methanol ratio under SRM, and the O2 addition under OSRM, increased the CO2 formation and decreased both the H2 selectivity and catalyst deactivation caused by carbon deposition. Methanol conversion of the prereduced LSC was significantly enhanced at a lower temperature than that of as-prepared LSC and undoped LCO. This was attributed to the performance of metallic cobalt nanoparticles highly dispersed under reducing atmospheres. The reoxidation program in repetitive redox cycles played a crucial role in the regeneration of catalysts, which could be regenerated to the initial perovskite structure under a specific thermal treatment, minimizing the degradation of the catalytic activity and surface.Peer ReviewedPostprint (published version
Mechanical reliability of dental grade zirconia after laser patterning
The aim of this work is to test the mechanical properties of dental zirconia surfaces patterned with Nd:YAG laser interference (¿¿=¿532¿nm and 10¿ns pulse). The laser treatment produces an alteration of the topography, engraving a periodic striped pattern. Laser-material interaction results mainly in thermal effects producing microcracking, phase transformation and texturization. The role of such microstructural modifications and collateral damage on the integrity and mechanical performances has been assessed.
Laser patterned discs of zirconia doped with 3% mol yttria (3Y-TZP) have been tested before and after a thermal treatment to anneal residual stresses and revert phase transformation. Both groups of samples behave in a similar manner, excluding residual stresses and phase transformation from the origin of properties modification.
Result show that laser patterning induces a minor decrease in mechanical properties and surface integrity of 3Y-TZP surfaces. The biaxial strength decreases as a consequence of the damage induced by laser patterning. Fractographic observations identify preexisting defects enlarged by local laser interaction as the fracture origins. The Hardness and Young modulus of treated surfaces tested with nanoindentation also decrease slightly after laser treatment and this may be attributed to laser-induced microcracking.Peer ReviewedPreprin
Small scale fracture behaviour of multilayer TiN/CrN systems: Assessment of bilayer thickness effects by means of ex-situ tests on FIB-milled micro-cantilevers
TiN/CrN multilayered PVD coatings are known to exhibit outstanding micromechanical properties and wear resistance. On the other hand, information on their small scale fracture behaviour is rather scarce. The present work aims to address it by testing to failure FIB-milled microbeams of multilayer TiN/CrN systems with different bilayer periods (8, 19 and 25 nm). In doing so, these micrometric specimens are first FIB notched, and thus deflected by means of a nanoindentation system. It is found that multilayer architecture translates into a beneficial synergic effect regarding critical load for reaching unstable failure; and thus, on energy absorption at fracture. Such behaviour is associated with small scale crack deflection as main toughening mechanism.Peer ReviewedPostprint (author's final draft
Fracture mechanics analysis of hardmetals by using artificial small-scale flaws machined at the surface through short-pulse laser ablation
Laser ablation has become an innovative treatment for cemented carbides, regarding edge rounding and surface modification, aiming to improve their tribomechanical performance. Meanwhile, the precision offered for this technique has also positioned it as an effective mean to generate micronotches used for evaluation of mechanical properties in structural materials. However, similar approach has not been attempted for hardmetals; thus, it becomes the main objective of this work. Dimple-like and elongated micronotches are introduced in one fine-grained WC-11%wtCo grade. In doing so, laser processing parameters are first optimized to attain micronotches with appropriated geometry and size, i.e. similar to critical flaws identified in broken pristine specimens. Success of the implemented approach is then validated through subsequent flexural testing, fractographic inspection and fracture mechanics analysis of the results attained on samples containing surface micronotches, as far as laser-induced residual stresses are taken into consideration. In this regard, elongated micronotches are found to exhibit lower residual stresses, and postulate themselves as the optimal option of the two micronotch types studied. The suitability of laser ablation for shaping artificial small-scale flaws opens a new route for introducing controlled defects, alike those intrinsic to processing or induced during service, key aspect for further understanding damage tolerance issues in cemented carbides.Peer ReviewedPostprint (published version
A computational framework for micromechanical modelling of WC-Co composites
In this study, the mechanical behavior under monotonic loads of tungsten carbide-cobalt (WC-Co) composites is investigated extensively by analyzing (1) nanoindentation tests on WC particles and Co matrix, (2) nanowires made of WC-Co composites tested in tension and (3) micropillars made of WC-Co composites tested in compression. To this end, a novel computational framework consisting of two different microplane constitutive models developed for WC and Co phases are proposed. For the Co matrix, the microplane J2-plasticity, called the model MPJ2, and for the WC particles a modified version of the microplane model M7, called the model M7WC, are employed. Furthermore, finite element meshes in 3D obtained from experimental tomography reconstructions of WC-Co composites are employed to rule out any spurious geometric features that is likely to be encountered in artificially generated meshes. After calibrating the aforementioned models, it is shown that the finite element predictions not only confirm the extensive experimental observations but also shed further light into the mechanical behavior of these composites.Peer ReviewedPostprint (published version
Growth and thermal stability of TiN/ZrAlN: Effect of internal interfaces
Wear resistant hard films comprised of cubic transition metal nitride (c-TMN) and metastable c-AlN with coherent interfaces have a confined operating envelope governed by the limited thermal stability of metastable phases. However, equilibrium phases (c-TMN and wurtzite(w)-AlN) forming semicoherent interfaces during film growth offer higher thermal stability. We demonstrate this concept for a model multilayer system with TiN and ZrAlN layers where the latter is a nanocomposite of ZrN- and AlN- rich domains. The interfaces between the domains are tuned by changing the AlN crystal structure by varying the multilayer architecture and growth temperature. The interface energy minimization at higher growth temperature leads to formation of semicoherent interfaces between w-AlN and c-TMN during growth of 15 nm thin layers. Ab initio calculations predict higher thermodynamic stability of semicoherent interfaces between c-TMN and w-AlN than isostructural coherent interfaces between c-TMN and c-AlN. The combination of a stable interface structure and confinement of w-AlN to nm-sized domains by its low solubility in c-TMN in a multilayer, results in films with a stable hardness of 34 GPa even after annealing at 1150 °C.Peer ReviewedPostprint (author's final draft
Estudio de la morfologia de grietas generadas por nanoindentación y su efecto en el cálculo de la tenacidad de fractura
En el presente trabajo se ha utilizado la técnica de nanoindentación para inducir grietas en diferentes materiales
cerámicos y estudiar la morfología de grieta generada por la aplicación de un indentador tipo
Berkovich
. El estudio
morfológico de las grietas se ha llevado a cabo medi
ante pulido secuencial y la técnica de microscopía de haz de iones
focalizados (FIB/SEM). Para la geometría de indentador utilizada y
la morfología de grieta obtenida, se ha llevado a
cabo un estudio de la aplicabilidad de las expresiones más
comúnmente utilizadas para el cálcu
lo de la tenacidad de
fractura en materiales frágiles por i
ndentación. Previamente al estudio
de la validez de las ecuaciones, para poder
obtener una medida de K
c
fiable, se ha realizado una calibración de la
s ecuaciones para la geomet
ría de indentador tipo
BerkovichPostprint (published version
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