CHARACTERIZATION OF UNIAXIAL COMPACTION IN SPRAY DRIED CERAMIC POWDERS

A model granulated ceramic powder system was studied with systematically varied binder content and binder plasticity (binder glass transition temperature, Tg). A submicron α-alumina was used as the inorganic component and poly(vinyl alcohol)–۴ wt% glycerol was the base binder system. Four compositions were spray dried containing ~2, 3, 4 and 5 wt% binder (dry weight basis alumina). The resulting powders were classified and the 75– ۱۵۰ μm granule size range powders prepared for testing. These powders were conditioned at five different relative humidities to adjust binder Tg to five different values between – ۳۲°C and +35°C. All twenty compositions were tested in uniaxial compaction in a 6.34 mm diameter cylindrical steel die to stresses from ~6 MPa to ~175 MPa. Green strengths of the samples produced were measured using the diametral compression test. Compaction curves were constructed and springback on ejection was measured. A selection of similar samples was prepared and dimensional changes after compaction were measured as a function of time. It was found that the 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. Dimensional changes on ejection were found to be dominated by the instantaneous springback in the axial direction (~5–۸%). Radial springback was generally less than 1% and the total dimensional change due to timedependent relaxation was generally less than 0.5%.