Characterization of Uniaxial Compaction in Spray Dried Ceramic Powders
Open Access
- Author:
- Carneim, Robert David
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 18, 2000
- Committee Members:
- Gary Lynn Messing, Committee Chair/Co-Chair
Virendra Puri, Committee Member
Ian Roland Harrison, Committee Member
James Hansell Adair, Committee Member
John Richard Hellmann Jr., Committee Member - Keywords:
- binder
polyvinyl alcohol
granules
compaction
strength
compaction curves
modeling
friction - Abstract:
- A model granulated ceramic powder system was studied with systematically varied binder content and binder plasticity (binder glass transition temperature, Tg). A submicron alpha-alumina was used as the inorganic component and poly(vinyl alcohol)–4 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 75–150 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 –32°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 (~5–8%). Radial springback was generally less than 1% and the total dimensional change due to time-dependent relaxation was generally less than 0.5%. It was observed that compaction behavior was affected by sample size. A technique was developed which determines this effect. By measuring two compaction curves of a powder of different sample size, it was possible to calculate the force opposing compaction due to friction at the die wall. This allowed the calculation of the intrinsic compaction curve of the material, i.e., its compaction behavior in a frictionless die. With these two parameters known it became possible to predict compaction curves of the powder for different sample sizes. Compaction curves calculated in this manner predicted experimentally determined compaction curves with correlation coefficients greater than 0.99. To aid in the characterization of individual granules and their interaction during compaction, an analysis was developed that calculates granule strength and intergranular bond strength during compaction and the free granule strength. In a series of pellets pressed to a wide range of pressures, granule deformation and adhesion varied greatly between samples. The strengths of these samples were measured by diametral compression and the fracture surfaces were analyzed to determine the relative amounts of intergranular and intragranular fracture. A quadratic curve was found to describe the relationship between the overall green strength of the sample and the area fraction of intergranular fracture. By applying knowledge of the physical process at the 0, 50 and 100% intergranular fracture points along this curve, this curve was deconvoluted to the unique pair of linear functions that track the intergranular bond strength and the intragranular strength throughout the compaction cycle. The 100% intergranular fracture point corresponds to little or no consolidation of the powder; therefore, the value of the determined 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 due to the difference in loading of the granules.