Characterization of uniaxial compaction in spray dried ceramic powders

A model spray dried alumina powder system was studied with systematically varied binder content (2, 3, 4 and 5 wt%) and binder glass transition temperature (Tg). The Tg of the poly(vinyl alcohol) binder was adjusted between -32°C and +35°C by equilibration at five relative humidities. Compaction curves were constructed and dimensional changes on ejection were measured for all twenty compositions in uniaxial compaction at stresses from ∼6 MPa to ∼175 MPa. Compositions with low Tg during compaction resulted in the highest achievable densities and green strengths. However, green strength increased and achievable green density decreased with increasing binder content. Expansion on ejection were found to be dominated by the instantaneous axial springback (∼5--8%). Radial springback was less than ∼1% and the time-dependent relaxation was generally less than 0.5%.; A technique was developed with which the effect of sample size on the compaction curve was calculated. The force opposing compaction due to friction at the die wall and the intrinsic compaction curve of the material were calculated based on two measured compaction curves. With these two characteristics known, it became possible to predict compaction curves of the powder for different sample sizes. Calculated compaction curves calculated correlated to measured compaction curves with R2 > 0.99.; Additionally, a method was developed in which granule strength and intergranular bond strength during compaction was calculated. In a series of pellets pressed to a wide range of pressures, granule deformation and adhesion varied greatly between samples. Green strengths were measured by diametral compression and the fracture surfaces analyzed to determine the relative amounts of intergranular and intragranular fracture. A quadratic relationship was found between the overall green strength of the sample and the fracture mode. This curve was deconvoluted to the unique pair of linear functions that describe the intergranular bond strength and the intragranular strength throughout the compaction cycle. The 100% intergranular fracture, little or no consolidation has occurred, and the value of the intragranular strength line is a measure of the free granule strength. The free granule strength measured by this technique results in values much lower than the often-reported granule yield point measurement.