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

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

Mechanical behavior of PVD coatings deposited on ADI

In this work, the mechanical behavior of the coated ADI system is studied. For this purpose, single layer Ti and bilayer Ti/TiN and TiAl/TiAlN coatings, with different thicknesses, were deposited by the PVD technique of cathodic arc deposition on high strength ADI substrates using industrial and experimental devices. The characterization of the coatings includes the measurement of the layer thicknesses by the spherical abrasion method, the determination of existing phases and residual stresses by x-ray diffraction and the evaluation of the surface topography using a stylus profilometer. The analysis of the mechanical properties of the coatings comprises the measurement of the surface hardness by micro indentation tests and the evaluation of the scratch resistance. The scratch tests were performed on a scratch tester equipped with a Rockwell indenter. A progressive load from 1 to 100 N, a load rate of 99 N/min, a speed of 5 mm/min and a scratch length of 5 mm were employed. The influence of the coatings characteristics on the scratch resistance of the coated samples and the friction coefficients obtained are evaluated. The results show that Ti coated samples had tensile residual stresses, the lowest surface hardness and the lowest critical loads for scratch adhesion strength. The samples coated with Ti/TiN and TiAl/TiAlN had highly compressive residual stresses and the highest critical loads, while the hardness was higher for TiAl/TiAlN. On the other hand, the evolution of the friction coefficient was similar for all the coated samples.Fil: Colombo, Diego Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Márquez, Adriana Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física del Plasma. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física del Plasma; ArgentinaFil: Marro, Fernando García. Universitat Politécnica de Catalunya; EspañaFil: Llanes, Luis Miguel. Universitat Politécnica de Catalunya; EspañaFil: Sosa, Amadeo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Duplex stainless steels for thermal energy storage: characterization of oxide scales formed in carbonate salts at 500 °C

Next generation concentrated solar power (CSP) plants promise a higher operating temperature and better efficiency. However, new issues related to the corrosion against protection of the construction alloys need to be solved. In this work, two different duplex stainless steels grades, namely 2205 (DS2205) and 2507 (DS2507), were evaluated for their compatibility with the eutectic molten salt mixture of Li2CO3-K2CO3-Na2CO3 at 500 °C in air for thermal energy storage applications. Corrosion tests combined with complementary microscopy, microanalysis and mechanical techniques were employed to study the oxide scales formed on the surface of the duplex steels. The corrosion tests evidenced that the attack morphology in both duplex steels was a uniform oxidative process without localized corrosion. DS2507 presented a better corrosion resistance than DS2205, due to the formation of thinner, compact and continuous oxide layers with higher compositional content in Cr, Ni and Mo than DS2205. The oxide scales of DS2507 showed more remarkable mechanical integrity and adhesion to the metallic substrate.Postprint (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

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

Stability of nanocrystalline spark plasma sintered 3Y-TZP

Spark plasma sintered 3Y-TZP has been investigated with respect to hydrothermal ageing and grinding. The sintering was performed between the temperatures of 1,100 and 1,600 °C for a soaking time of 5 minutes and the resulting materials were obtained with grain sizes between 65 to 800 nm and relative densities between 88.5 to 98.8%. Experiments on hydrothermal ageing in water vapour at 131 °C, 2 bars during 60 hours shows that phase stability is retained, elastic modulus and hardness of near surface region measured by nanoindentation does not change in fine grain (<200 nm) materials, in spite of porosity. In ground specimens, very small amount of transformation was found for all grain sizes studied.Peer ReviewedPostprint (published version

Spherical indentation of polycrystalline cubic boron nitride (PcBN): contact damage evolution with increasing load and microstructural effects

The extremely severe conditions at which cutting tools are exposed in hard machining operations require tool materials with outstanding mechanical properties. Excellent combinations of high hardness and fracture toughness are aimed to enhance wear and contact damage resistance; and thus, reliability of the tool. Here, spherical indentation is used to induce controlled damage, under monotonic loads, and to assess the resulting damage scenario of polycrystalline cubic boron nitride (PcBN). In doing so, two grades with different content of cBN particles as well as chemical nature of the binder are investigated. Systematic and detailed inspection of residual imprints, by means of optical and scanning electron microscopy, is conducted to document critical loads for damage emergence and evolution through different stages, as a function of increasing load and micro- structural assemblage. It is found that H-PcBN grade, with higher content of cBN particles and a metallic binder, exhibits higher resistance to crack nucleation as well as a more gradual transition between distinct damage scenarios. This enhanced response is linked to the concomitant increase of hardness and fracture toughness observed for that grade, as compared to the L-PcBN one, consisting of a significantly lower amount of superhard particles bound by a binder of ceramic nature. In-depth inspection of interaction between fissures and micro- structure indicates the existence of four different crack paths for both PcBN grades - transgranular through cBN particles, across the binder phase, and intergranular along either cBN/cBN or cBN/binder interfaces. Relative frequency of these crack paths as well as operative toughening mechanisms define the effective crack growth resistance of each PcBN grade. In this regard, a higher fracture toughness is assessed for H-PcBN, as a result of crack bridging by metallic ductile ligaments together with the effective barrier-like role played by substructural features within cBN particles. Meanwhile, L-PcBN grade shows a rather low crack growth resistance, in direct correlation with the intrinsic brittle nature of the contiguous ceramic binder through which crack extension predominantly takes place. These relative differences in resistance to crack nucleation and growth become key for understanding the enhanced contact damage behaviour determined for the grade with higher content of cBN particles in this study.Peer ReviewedPostprint (published version