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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.

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