Deliverables Colloidal production technology for nanostructured powders The surface of submicronic alumina particles in stable slurry in alcohol media are used as "support" for hydrolysis reactions with different diluted alkoxydes. By controlling the surface state of the submicronic alumina particles (OH groups) in the stable suspensions, diluted alcoxides in alcohol media are added dropwise. Controlling the OH- groups present on the surface of the original powder, a hydrolysis reaction takes place on the surface of the particles being recovered by the alkoxyde. From this point, by controlling the processing parameters it is possible to control nucleation and growth on the surface (in situ formation of nanoparticles) in such a way that it is possible to synthesize the nanoparticles and densify the material in a single step with the purpose to obtain nanocomposites with a metallic and/or ceramic matrix avoiding nanoparticle handling. 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 of 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. Process for pressure casting of ceramics This was one of the basic researches to be performed in the project which had an important technological meaning because this route requires a minimum level of energy consumption as no evaporation of alcohol is needed. Nevertheless conventional and pressure casting should always be possible in aqueous media starting from powders obtained by the low risk processing route mentioned below. The conventional casting of non-aqueous suspensions presents a medium risk level compared to pressure casting. Taking into account the relative low number of implants to be produced, this technique could be considered an industrial feasible alternative. The slip casting of knee implants presents an important risk due to the no defects requirement of these structural pieces. This workpackage represented a key in the development of the project. In this workpackage some fundamental researches have be done in order to understand the general behaviour of non-aqueous suspensions with the aim to develop a technology for pressure casting of hip and knee implants. Up to now the new processing route proposed in this project, consisting on powder-alcoxide mixtures without hydrolysis, were used at laboratory scale, thus slurries were dried under stirring before milling and sieving. This process was not economically feasible at industrial scale and present many problems. This workpackage developed a new technology to use powderalcoxide suspensions in two different forming technologies: dry pressing after powder drying or casting of slurries with and without pressure. As this technology had to dry a slurry and it was very important to start from a solid concentration in the slurry as high as possible, it was also very important to understand the rheology of this kind of slurries. In the second case different problems had to be resolved: to select polymers with smaller open porosity in order to avoid the obstruction of moulds during casting to control the viscosity to facilitate casting and to avoid phase separation during the casting process. The classical wet processing route from powder mixtures was used as a reference in this project. Complex shapes, as it is the case of knee implants could be obtained by slip casting. By this method very high green densities could be achieved as well as an improved mechanical behaviour. The appropriate amount of second phases and dopants was one of the most important issues in this workpackage. The selection of the optimum amount of dopants had to be done taking into account not only the basic mechanical properties but also the machinability and surface finishing after polishing. The use of a post HIP was necessary in order to achieve theoretical densities. It was necessary to control the microstructural evolution and as a consequence the evolution of the whole properties of the obtained materials. Exploitable results 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. Colloidal production technology for nanostructured powders The surface of submicronic alumina particles in stable slurry in alcohol media are used as "support" for hydrolysis reactions with different diluted alkoxydes. By controlling the surface state of the submicronic alumina particles (OH groups) in the stable suspensions, diluted alcoxides in alcohol media are added dropwise. Controlling the OH- groups present on the surface of the original powder, a hydrolysis reaction takes place on the surface of the particles being recovered by the alkoxyde. From this point, by controlling the processing parameters it is possible to control nucleation and growth on the surface (in situ formation of nanoparticles) in such a way that it is possible to synthesize the nanoparticles and densify the material in a single step with the purpose to obtain nanocomposites with a metallic and/or ceramic matrix avoiding nanoparticle handling. Process for pressure casting of ceramics This was one of the basic researches to be performed in the project which had an important technological meaning because this route requires a minimum level of energy consumption as no evaporation of alcohol is needed. Nevertheless conventional and pressure casting should always be possible in aqueous media starting from powders obtained by the low risk processing route mentioned below. The conventional casting of non-aqueous suspensions presents a medium risk level compared to pressure casting. Taking into account the relative low number of implants to be produced, this technique could be considered an industrial feasible alternative. The slip casting of knee implants presents an important risk due to the no defects requirement of these structural pieces. This workpackage represented a key in the development of the project. In this workpackage some fundamental researches have ben done in order to understand the general behaviour of non-aqueous suspensions with the aim to develop a technology for pressure casting of hip and knee implants. Up to now the new processing route proposed in this project, consisting on powder-alcoxide mixtures without hydrolysis, were used at laboratory scale, thus slurries were dried under stirring before milling and sieving. This process was not economically feasible at industrial scale and present many problems. This workpackage developed a new technology to use powderalcoxide suspensions in two different forming technologies: dry pressing after powder drying or casting of slurries with and without pressure. As this technology had to dry a slurry and it was very important to start from a solid concentration in the slurry as high as possible, it was also very important to understand the rheology of this kind of slurries. In the second case different problems had to be resolved: to select polymers with smaller open porosity in order to avoid the obstruction of moulds during casting to control the viscosity to facilitate casting and to avoid phase separation during the casting process. The classical wet processing route from powder mixtures was used as a reference in this project. Complex shapes as it is the case of knee implants could be obtained by slip casting. By this method very high green densities could be achieved as well as an improved mechanical behaviour. The appropriate amount of second phases and dopants was one of the most important issues in this workpackage. The selection of the optimum amount of dopants had to be done taking into account not only the basic mechanical properties but the machinability and surface finishing after polishing. The use of a post HIP was necessary in order to achieve theoretical densities. It was necessary to control the microstructural evolution and as a consequence the evolution of the whole properties of the obtained materials. Searching for OpenAIRE data... There was an error trying to search data from OpenAIRE No results available
