A critical assessment of nanometer-scale zirconia green body formation by pressure filtration and uniaxial compaction

Green body formation with nanoscale powders poses unique challenges. This report discusses two specific green-forming procedures: pressure filtration and dry-powder compaction. Green compacts were formed from suspensions of hydrothermally derived YTZP prepared in the presence of bicine, which acts as a protective colloid and dispersant. However, due to capillary forces encountered when drying filter-pressed, nanoporous compacts, green bodies could not be recovered intact. Dry-powder compaction is explored as an alternative green-forming technique. The effect of powder recovery procedure on compaction behavior and microstructural homogeneity are discussed. Strong capillary forces also act on nanoparticles during pan-drying, yielding high-strength aggregates which are difficult to deform during compaction. Drying particles from a lower surface tension liquid, such as ethanol, is found to have little effect on aggregate strength. Freeze-drying is demonstrated as the preferred powder recovery technique since particles are kept in a dispersed state during drying, and compression of agglomerates by capillary forces is avoided. Compaction behavior of recovered powders is examined with a model that describes powder consolidation as an activated process. From this analysis, it is demonstrated that granule deformation and rearrangement contribute equally to the compaction process in pan-dried nanopowders. It is also shown that primary particle rearrangement accounts for the majority of compaction in freeze-dried nanopowders.