Resumen de Mechanical properties of co-doped zirconia ceramics
Tetragonal polycrystalline zirconia, commonly stabilised with 3 mol% yttria (3Y-TZP), became one of the most interesting ceramics for biomedical applications due to its biocompatibility and high mechanical properties. Among them, its high fracture toughness should be highlighted, which is due to the stress-induced tetragonal-to-monoclinic (t-m) phase transformation near a crack tip. However, the tetragonal grains on the Surface can also spontaneously transform to monoclinic phase in a humid environment, phenomenon known as low-temperature degradation (LTD), which is an important issue for applications in which water is present.
Several methods have been proposed to increase the LTD resistance in zirconia, which range from improving the fabrication process in terms of grain size, density or residual stresses, to doping zirconia with other oxides, like magnesia or ceria. Particularly, ceria-stabilised zirconia (Ce-TZP) possesses higher LTD resistance and fracture toughness than 3Y-TZP, but lower fracture strength and hardness, partly because of its larger grain size. The approach proposed in this work consists in the improvement of mechanical properties of Ce-TZP by reducing grain size, without reducing LTD resistance. With this objective, starting from two compositions of Ce-TZP (10 and 12 mol% CeO2), different amounts of CaO and Y2O3 have been added to reduce the grain growth during sintering, and thus increasing the critical t-m transformation stress, and consequently flexural resistance and hardness. On the other hand, as flexural resistance is determined by fracture toughness from small superficial cracks, a novel reproducible methodology to produce superficial micro-notches by means of ultra-short pulsed laser ablation has been developed to measure this property. It has been highlighted that with the developed methodology very sharp cracks are produced, with the size in the order of natural cracks. Results show that small-crack fracture toughness is very different from values measured from methods using large cracks or from the indentation method.
This thesis is submitted for the degree of Doctor of Philosophy in the program "Materials Science and Engineering" at the Universitat Politècnica de Catalunya as a compendium of published articles. The research described in this work was carried out by the author during the period from December 2012 to October 2017 under the supervision of Prof. M. Anglada in the Department of Materials Science and Metallurgical Engineering at the Universitat Politècnica de Catalunya, and during 3 research stays during 2015, 2016 and 2017 (406 days in total) in the Department of Materials Science (MTM) at KU Leuven (Belgium) under the supervision of Prof. J. Vleugels. The work described in this dissertation is original, unless otherwise detailed references are provided.
