Composites in the Alumina-Zirconia system: an engineering approach for an effective tailoring of microstructural features and performances

The aim of this PhD study is the development of composites in the alumina-zirconia system through a powder engineering approach which allows tailoring the compositional and microstructural features, and, as a consequence, the properties of the final materials. The experimental activities refer to two different projects. The first, named MITOR project, was devoted to the elaboration and mechanical characterization of macro-porous Alumina-Zirconia composites. It dealt with the development of a new method for the elaboration of composite cellular ceramics and with the investigation of the role of zirconia and its toughening mechanisms in porous materials, thus filling a gap in the scientific literature. The latter, a European Project named Longlife, was dedicated to the preparation and characterization of zirconia (stabilized with ceria)-based composites for dental and spine implants. The major aim was to overcome the drawbacks proper of yttria-zirconia-based materials, concerning their stability in moisture atmosphere as well as their low toughness. Ceria-zirconia-based composites should benefit from phase transformation toughening still keeping high strength. In addition, they should not suffering of surface degradation in presence of water. So, materials characterized by high strength, high toughness with a perfect reliability and a lifetime longer than 60 years were investigated. The first chapter collects a literature overview of the most relevant zirconia-containing materials, either dense and macro-porous, with particularly emphasis on the transformation toughening proper of such materials and its effects on the mechanical properties and stability. It is described the role of the elaboration route used to prepare the composite powders and the role of some parameters (such as nature of the dopant, microstructural features and phase composition) on the final properties. A brief introduction of the mechanical models, particularly the Gibson-Ashby model, and of the influence of the total porosity and pore size on the mechanical behavior of porous ceramics is also illustrated. The second chapter deals with the set-up of the elaboration process of composite powders through the surface modification of commercial powders with inorganic precursors of the secondary phases. This innovative approach insures a high degree of control of the size and distribution of the second-phase grains on the surface of the parent material. Alumina-based composite powders containing 10 vol% of un-stabilized zirconia as well as tri-phasic zirconia-based composite powders containing 8 vol% of alumina and 8 vol% of an aluminate phase, were developed and characterized in terms of phase evolution and thermal behaviour. In addition, the adopted elaboration route allowed tailoring the ceria amount inside the zirconia grains: four different zirconia stabilization degree were thus investigated. It was shown that the deep knowledge of all the involved mechanisms (such as raw powders dispersion, pH suspension, powder thermal treatments) is crucial for achieving a full control of the powders features and, consequelntly, of the final microstructures. The third chapter is related to the MITOR project and deals with the development of macro-porous alumina-zirconia bodies through a modified gel-casting method in which a sacrificial phase was used as pore former. The selected pore former agent allows tailoring the porosity features, such as the amount of porosity, the pores shape and size distribution. Bodies with porosity amounts ranging from about 60% to 80 vol% were produced and characterized in terms of their microstructures and mechanical properties. Their properties were compared with those obtained on pure alumina components produced by the same way. The compressive strength decreased with decreasing the relative density and, from a compositional point of view, the porous composites showed higher strength values as compared to the pure alumina ones. The well-known zirconia toughening mechanisms (transformation and microcracking toughening mechanisms) were investigated, revealing a poor influence on the mechanical properties. The improvement of the compressive strength in the composite materials can be reasonably due to their finer microstructure, being characterized by smaller grains and pores. The last part of this thesis, related to the Longlife project, describes the development of dense Ce-TZP tri-phasic composites by slip casting and pressureless sintering. Here, the main results of a full characterization in terms of phase composition, microstructure, mechanical (hardness, fracture strength and toughness) as well as physical properties (aging behaviour, transformability, optical properties) are presented. The adopted surface modification technique of a commercial Ce-TZP powder was successful in developing composites having highly homogeneous and complex microstructures characterized by a very good distribution of the secondary phases (round-shaped alumina and elongated aluminate grains) inside a fine zirconia matrix. A strong influence of the composition and sintering cycle on the microstructure and, consequently, on the mechanical properties was revealed. In particular, two completely different mechanical behaviors were observed: in some composites (when the strontium-aluminate phase is present), the strength was transformation driven and the t-m transformation phase took place well before failure. Instead, when a magnesium-aluminate is present, the tetragonal-monoclinic phase transformation took place only around the fracture surface where weak transformation bands can be observed. Two very promising composites with high fracture strength, of about 900 MPa, and high crack resistance were found. Furthermore, the investigated composites showed high transformability and no low temperature degradation in moisture atmosphere in the time-scale of medical applications. It was shown that these properties are strongly affected by the zirconia stabilization degree: it is necessary to carefully investigate the relationship between the final properties and the composition/microstructure architecture in order to reach the desired properties.