The effect of granule morphology and composition on the compaction behavior and mechanical properties of 92% alumina spray dried granules

Abstract

The compaction behavior and flexural strength of green compacts produced from four different types of 92% alumina spray dried granules were studied. Three of the granule types were produced in-house while the fourth is commercially-available. In addition, the decomposition behavior of the granules was studied by TGA in order to help explain the observed results. The morphology of the granules was observed using microscopy. Examination of the three granules produced in-house showed that they have several undesirable features; agglomeration, hollow granules and non-spherical granules. The morphology of the commercial granules showed them to be unagglomerated and spherical. The in-house granules with undesirable morphological features had a lower tap-density than the commercial granules. Bend bars were produced to a controlled green density of 2.20 g/cm³. The flexural strength of the green bars was measured in 4 point bending. It was found that the average flexural strength of bar produced with in-house granules was 4 MPa. The bars produced with the commercial granules had an average flexural strength of 1.19 MPa. The compaction behavior of the granules was studied from 25-300 MPa. It was found that the in-house granules consistently had a higher yield pressure than the commercial granules, resulting in the need for higher pressure to achieve the same green density. The TGA results found that the in-house granules have a higher percentage of volatile components in the form of polymeric additives such as dispersant, binder and lubricant, than the commercial granules. The excess polymeric additives, particularly binder, can explain the observed increase in flexural strength of the in-house vs. commercial granules.