Effect of Internal Lubricants on Defects in Compacts Made from Spray‐Dried Powders

Abstract

The role of internal lubricants in the closure of large intergranular pores during dry-pressing was investigated. Alumina was spray-dried with and without an internal lubricant to yield granules with similar characteristics other than lubricant effects. Green and sintered microstructures were evaluated at different compaction pressures. The defects were quantified by evaluating the fracture surface of ϳ90% dense sintered compacts. The samples that contained an internal lubricant had higher green densities and fewer defects at comparable compaction pressures. The internal lubricant did not cause any significant reduction in green strength or increase in springback.

Dry bag isostatic pressing for improved green strength of surrogate nuclear fuel pellets

“…Internal lubricants (zinc stearate) also reduce the incidence of these cracks [17]. Shinohara et al [16] used special optical techniques to examine green compact fabricated from spray dried powder and found intergranular pores and cracks as a source of flaws.…”

“…The yield strength of all sample sets was on the order of 0.1 MPa, which is a very low value and much below the pressing pressure of 207 MPa. Thus granules are sufficiently soft that no remnants of the granules are expected to be left after pressing [16,17].…”

Fracture properties of spray-dried powder compacts: Effect of granule size

“…Friction between the powder and punch may slightly exaggerate the number of defects compared with the interior of the compacts. Nevertheless, the pressed surface still provides a good indication of the large intergranular pores and granule boundaries 29 . Two typical defects are clearly visible in Fig.…”

“…4). Apparent yield pressure, P y , is believed to be a measure of the granule strength 29,30,33 . Five experiments were conducted for each powder.…”

“…Corrections for springback and the deformation were made, as described by Mort et al 28 . The apparent yield pressure, P y , was determined from the intersection of two straight lines fitted to the low-pressure and high-pressure legs of the compaction diagram [29][30][31][32] (Fig. 4).…”