A Critical Assessment of Nanometer‐Scale Zirconia Green Body Formation by Pressure Filtration and Uniaxial Compaction

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Abstract
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.

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