Development of dense, fine-grained bulk ceramics from nanosized zirconia powder using pressing technologies
Shaping of nanosized powders for producing bulk ceramics with a nanosized grain structure can be seen as a critical technological step until today. Nano powders show very low bulk weight and very limited flowability. Due to the very fine particles, compaction processes turn out to be difficult as the surface roughness of the die is higher than the particle diameter resulting in high friction coefficients. The high amount of air contained within the powder is also a problem for compaction processes. A coarsening of the microstructure due to exaggerated grain growth puts the question on use of nano powders and the related efforts in processing which are much higher in contrast to commercially available powders. It is emphasized that both problems microstructure development and processing techniques have to be treated equally. Yttria stabilized VP Zirconium Oxide 3-YSZ powder (Degussa GmbH) was transferred into a stabilized suspension and a granulate was prepared by spray drying adding adequate binder and lubricant content. The compaction behavior of the granular material was characterized by investigating the dynamic compaction curve, ejection forces, strain recovery inside and outside the die as well as by measuring the diametral compression strength. The granulate was processed to pellets and plates by dry compaction and subsequent isostatic pressing. After sintering and hot isostatic pressing ceramic parts with fine-grained microstructure and high mechanical strength which show a superplastic behaviour at elevated temperatures have been achieved.