Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Alumina-zirconia nanostructured composites with highly improved properties for biomedical applications

The addition of a fraction of zirconia to alumina results in a “composite” material of increased toughness. The word "composite" refers to the combination, on a macroscopic scale, of two or more materials, different for composition, morphology and general physical properties.

Several processing routes have been proposed to obtain ZTA composites. Conventional methods include the mechanical mixing of the powder and/or attrition milling, followed by freeze-drying and/or hot pressing. Other researchers developed a method which involved the hydrolysis of zirconium alkoxides in a dispersed alumina slurry. However, using these processing techniques, it has been proved almost impossible to reach a fine and homogeneous microstructure. The development of new processing protocols in non-aqueous media has allowed preparation of composites with a significantly narrower particle size distribution of zirconia than conventional methods where nanophases are formed in situ on the alumina particle surface during sintering and located at grain boundaries in the final solid. With this method, it is possible to obtain high density ZTA nanocomposites (NZTA) with a very homogeneous microstructure, nearly the same hardness as alumina, a higher fracture toughness, high hydrothermal stability and high crack resistance.

The compressive residual stress field caused by the presence of a small volume fraction of evenly distributed zirconia nano- particles is responsible for the drastic change in the overall resistance to Slow Crack Growth of the alumina-zirconia nanocomposite. This result opens a new avenue o developing oxide ceramic based nano-structured composites for structural applications since they offer crack resistance similar to covalent materials without their major drawbacks associated to processing as well as machining. Concerning processing of bulk nanostructured composite materials, up to now only one approach was pursued: powder processing route, wherein nanoparticles of the material are first synthesized by some convenient chemical or physical method and then mechanically or wet mixed, and finally consolidated by pressureless or pressure-assisted sintering. When it is sought to obtain a dense composite formed by a matrix and a homogeneous distribution of a nanosized second phase, many problems arise for two reasons. First, in conventional powder processing, it is essential to synthesize nanoparticles that have to be nonagglomerated and preferably monodispersed. Second, the obtention of a homogeneous distribution of second phases on the nanometric scale is quite complex.

Produced using the specially developed nanostructured powders and processing technology, the developed material contains numbers of zirconia nanoparticles distributed uniformly among the alumina grains. The distribution of nanoparticles at both grain boundaries and intragranular position can be tailored by using the adequate dopants with a homogeneous molecular distribution.

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Consejo Superior de Investigaciones Científicas (CSIC)
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