Hydrothermal Ageing of Cracked 3Y-TZP

En la circona dopada con 3% molar de itria, el vapor de agua puede inducir la transformación de la fase tetragonal a monoclínica en la superficie. Dicha transformación se extiende hacia el interior acompañada por la aparición de microfisuras, lo que induce una pérdida de la integridad estructural. Este fenómeno de degradación a baja temperatura (LTD) es una clara desventaja para la utilización del material en atmósferas húmedas. Sin embargo, el efecto del agua sobre grietas de indentación no ha sido estudiado. Este trabajo estudia la influencia de LTD en la resistencia mecánica de dichas grietas. Los resultados muestran sorprendentemente un incremento de la resistencia mecánica en probetas indentadas sometidas a largos tiempos de degradación; al contrario del comportamiento en probetas sin indentar. Para explicar este comportamiento, se ha evaluado también la influencia de las tensiones residuales y el tratamiento de recocido adecuado para eliminarlas. Finalmente, se plantean los mecanismos que explican el aumento de la resistencia mecánica observado. // In zirconia stabilised with 3% molar of yttria, the water vapour can induce the transformation from tetragonal to monoclinic phase at the surface. This transformation propagates to the bulk with the apparition of microcracks; which induce a loss in the structural integrity. This low temperature degradation (LTD) is a clear disadvantage for the application of the material in humid environments. On the other hand, the effect of the water on indentation-induced cracks has not been previously studied. This work studies the change in mechanical strength of indentation cracks submitted to water vapour. The results show how, after long degradation times, there is surprisingly an increase in mechanical strength. This is the opposite behaviour as the one expected in normal specimens which are not indented. To explain the observed behaviour here, the influence of the residual tensions was evaluated included annealing treatments. Finally, the mechanisms explaining the observed increase in strength are discussed.Peer ReviewedPreprin

Propiedades mecánicas a tracción

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Propiedades mecánicas a flexión a temperatura ambiente y a 150 ºC para un material compuesto denominado FERROTEX

Determinar las propiedades mecánicas a flexión a temperatura ambiente y a 150 ºC para un material compuesto denominado FERROTEX.Preprin

Influence of boron content on the fracture toughness and fatigue crack propagation kinetics of bainitic steels

The relatively good combination of high strength and ductility makes bainitic steels a candidate to replace many other steels in industrial applications. However, in service, ductility and strength are not up to standard requirements. In many industrial components, toughness and fatigue performance are also very relevant. In the present study, bainitic steels with varying content of boron were fabricated, with the aim of analyzing the fracture toughness and changes in the fatigue life. The results show that a relatively small change in the boron content can cause a notable variation in the fracture toughness of bainitic steels. The maximum value obtained in fracture toughness was for the steel with the highest boron content. It was observed that the amount of interlath martensite constituents decreases in steels with the addition of boron, leading to the promotion of the presence of void coalescence and a remarkable rise in the toughness of bainitic steels. An increase on the fatigue life of the bainitic steels with an increase in the boron content was also observed, through analysis by means of Paris’ law. A comprehensive micrographic study was carried out in order to examine the mechanics of fatigue crack growth in the bainitic steels, revealing small longitudinal cracks in bainitic steels that lack boron. These cracks tend to disappear in bainitic steels that contain boron. To elucidate this behavior, micrographs of the surfaces generated by the crack growth process were taken, showing that several nano-cracks appeared between the bainite laths. It is finally argued that this high-energy consumption process of nano-crack nucleation and growth is the reason for the improved toughness and fatigue life observed in bainitic steels.Peer ReviewedPostprint (author's final draft

Enhanced reliability of yttria-stabilized zirconia for dental applications

An increasing number of dental applications based on yttria-stabilized zirconia (3Y-TZP) have been developed in recent years as a result of the advances and versatility of dry-processing and soft machining at the pre-sintered state. Nonetheless, the long-term surface stability of these materials in humid environment is still a matter of concern and may limit its application. In this work, a simple method to prevent hydrothermal degradation on zirconia surface is studied in detail. This method involves the infiltration of pre-sintered parts with optimized solutions containing Ce salts, leaving unchanged the other processing steps, allowing the diffusion of Ce during conventional sintering. Several pre-sintering conditions, solution concentrations and sintering temperatures were studied and characterized, obtaining working parameters for the production of zirconia parts with mechanical properties similar to standard 3Y-TZP and high resistance to hydrothermal aging. This optimal combination was obtained with the 1150 ºC pre-sintering temperature, 50 wt% solution and sintering at 1450 ºC, leading to a superficial CeO2 content of about 3 mol%.Postprint (author’s final draft

Weibull characterization of the flexural strength of hydrothermally degraded 3Y-TZP zirconia

The flexural strength of 3% mol yttria-stabilized tetragonal polycrystalline zirconia (3Y-TZP) with a ~0.30m grain size was studied under 4-point bending test. The material was evaluated both as received and after water vapour exposure for long times of 100hr and 200hr at 131ºC in autoclave. Results were systematically analyzed by Weibull statistics and fractography. It was found that the mean flexural strength and Weibull modulus change from 960MPa and m=10.9 in the as received condition to 916MPa (m=19.5) and 860MPa (m=19.8) for 100 and 200 hours ageing, respectively. This increase in m can be explained by a change in the origin of fracture from natural flaws in the original material to original defects in the degraded surface layer which extend under loading to a depth equal to its thickness. This layer can be clearly appreciated during fractographic analysis after long degradation times. In the present case the thickness of the degraded layer was on average equal to 8.5m after 100 hours of hydrothermal exposure.Peer ReviewedPostprint (published version

Contact damage in artificially aged 3Y-TZP

Tetragonal zirconia polycrystals doped with 3% mol of yttria (3Y - TZP) are increasingly employed as implants in the human body, because they are biocompatible and have relatively high strength and fracture toughness. These properties are related to the small submicrometer grain size and to the operation of a toughening mechanism around cracks which is referred to as phase tra nsformation toughening. Another important feature of 3Y - TZP is the phenomenon of low temperature degradation (LTD) that takes place in contact with water at temperatures in an interval around 250 ºC. Water activates t - m transformation at the surface and pr ogresses inside the body producing a t - m surface transformed layer. As a result, roughness and microcracking are produced in the surface. In a first stage, for short times of ageing , this layer is responsible for degradation of surface mechanical propertie s; at a later stage, after long degradation times, it induces a strong reduction in the bulk strength. In this paper we present a study of the mechanical behaviour of hydrothermal degraded 3Y - TZP under monotonic and cyclic spherical indentation. The inves tigation is focused mainly on analyzing the mechanical response of the material under spherical indentation in terms of the degraded layer. Plastic deformation and cracking induced by spherical contact loading are studied under increasing loads in hydrothe rmally degraded and in as sintered specimens. The contact hardness and elastic modulus of the degraded layer are extracted from nanoindentation using continuous stiffness measurements. It is found that permanent damage induced in degraded specimens is alwa ys larger than in as sintered specimens. Critical loads to induce ring cracking in degraded specimens are lower than in as sintered specimensPostprint (published version

Estadística de la rotura de circona dopada con itria sometida a degradación hidrotérmica

En este trabajo se ha estudiado la influencia de la degradación hidrotérmica en la resistencia a flexión de circona tetragonal estabilizada con 3% molar de itria. El material se ha evaluado tanto en estado original como degradado en autoclave a 131ºC durante 100 y 200 h. Los resultados se han analizado utilizando la estadística de Weibull con el fin de obtener cada una de las distribuciones de probabilidad de rotura, y mediante fractografía. Los resultados muestran que la máxima caída en resistencia a flexión provocada por la degradación hidrotérmica no supera el 10% de la resistencia inicial. No obstante, el módulo de Weibull aumenta desde 10.9 en el material en estado original, hasta valores próximos a 20 como resultado de la degradación. Tanto la caída de la resistencia como el aumento de módulo de Weibull se explican en términos del espesor de la capa superficial monoclínica, el origen de la fractura y las tenacidades de la fractura en las fases monoclínica y tetragonal.Peer ReviewedPostprint (published version

Daño por contacto hertziano en un nanocompuesto Ce-TZP/Al2O3

El material compuesto Ce-TZP/Al2O3 es una cerámica nanoestructurada con granos de alúmina y circona tetragonal estabilizada con ceria. Actualmente, este tipo de compuestos están siendo desarrollados con el objetivo de sustituir a la circona dopada con 3% molar de itria de grado biomédico. En este trabajo, se ha estudiado el comportamiento de este compuesto bajo contacto monotónico aplicado mediante indentación esférica. Mediante espectroscopia micro-Raman se ha puesto en evidencia el cambio de fase local inducido por acción de las tensiones en la huella residual. Esto indica que el mecanismo de transformación de fase es máximo en los bordes de la huella, que corresponde a la zona donde el campo de tensiones de tracción es máximo de acuerdo con la teoría hertziana de la indentación esférica. Las conclusiones son que a pesar de que las grietas anillo aparecen para cargas de indentación esférica similares a las correspondientes a la circona dopada con itria, en el presente nanocompuesto su penetración hacia el interior del material es mucho menor, lo cual pone de manifiesto una mayor tolerancia al daño. Esta mayor tolerancia se relaciona a una mayor a tenacidad de fractura. // The composite material Ce-TZP/Al2O3 is a nanostructured ceramic with alumina grains and tetragonal zirconia stabilized with ceria. Currently, this type of composites is being developed with the objective to substitute biomedical grade tetragonal zirconia polycrystals doped with 3% molar yttria. In the present work, the microstructure and mechanical properties of such a composite have been studied. Additionally, the behaviour under monotonic contact loading has also been studied by spherical indentation at different load ranges. Raman spectroscopy revealed the local phase transformation induced by the locally applied stresses on the residual imprint. This indicates that the phase transformation mechanism is maximal at the border imprints, which corresponds to the zone where the traction stresses are maximal according to classical Hertzian theory. The conclusions are that while the crack rings appear at similar indentation loads as 3Y-TZP, the penetration of these cracks in the interior of the composite is much smaller, which means a higher tolerance to contact damage. This higher tolerance is related to its higher fracture toughness.Peer ReviewedPreprin

Mechanical response under contact loads of AlCrN-coated tool materials

The mechanical behavior under contact loading of systems consisting of PVD AlCrN film deposited onto two distinct hard substrates - cemented carbides and tool steel is studied by means of indentation testing techniques, under monotonic and cyclic condition. Experimental work includes assessment of critical applied loads for emergence of circular cracks at the coating surface, as well as evaluation of both surface and subsurface damage evolution. Results indicate that both coated systems are susceptible to mechanical degradation associated with repetitive contact load. Furthermore, significant differences on contact fatigue behavior between the two studied coated systems are evidenced under consideration of cracking evolution at top surface and penetration towards the substrate. In this regard, the intrinsic mechanical properties of the substrate are pointed out as key feature for rationalizing the experimental findings.Postprint (published version