Powder Processing for the Fabrication of Si₃N₄ Ceramics: I, Influence of Spray‐Dried Granule Strength on Pore Size Distribution in Green Compacts

Abstract

The effect of spray‐dried granule strength on the micro‐structure of green compacts obtained by isostatic pressing was quantitatively analyzed. The fracture strength of single granules of Si3N4 powder made with ultrafine A12O3 and Y2O3 powders was measured directly by diametral compression. It was found that fracture strength increased notably with the increasing relative density of the granule and the decreasing size of agglomerates in suspension before spray‐drying. Even when green bodies were prepared at an isostatic pressure of 200 MPa, intergranular pores, which negatively affected densification of the sintered bodies, occurred between unfractured granules. The volume and size of these pores in the green compacts increased with the increasing fracture strength of the granules. In the case of closely packed granules, an isostatic pressure of 800 MPa was required to completely collapse the intergranular pores. A simple equation was derived to calculate the isostatic pressure necessary for complete collapse of intergranular pores in the green compacts, and it was determined that granule strength must be kept as low as possible to obtain uniform green compacts.

Optimization of a Nanoparticle Suspension for Freeze Casting

“…Very desirably, freeze casting prevents defect formation by eliminating capillary force during drying and saves tremendous effort in binder removal. The technique is ideally suited for complex shapes with little tooling cost, which is a huge saving compared with dry pressing, isostatic pressing, or shock wave compaction 11–13 . Flexibility in forming complex shapes at minimal cost is also very suitable for rapid prototyping.…”

Correlation between slurry properties and structures and properties of granules

“…12,13 This in turns affect the compression response of the granules 14, 15 and thus the pore size distribution of green body. 16 Therefore, indirectly, slurry characteristics may control the structure and the properties on sintered bodies. Works have been reported on the effect of slurry characteristics of the particle packing.…”

Density Measurements of Single Granules Using the Atomic Force Microscope

“…The granule properties control not only the compaction behavior but they also have a strong influence on the reliability of the sintered material. [1][2][3] The density of the pressed body can often be related to the granule density, but very dense and strong granules may lead to incomplete deformation and residual intergranular pores, which can act as origins of fracture in the sintered part. [4][5][6] Variations in granule density can lead to poor repeatability in the filling of the die cavity, which results in poor dimensional tolerances of the sintered body.…”