Powder morphology and loading effects on the properties of an engineering ceramic

An investigation of starting powder morphology and loading effects on the properties of a yttria stabilised zirconia powder (3mol% Y₂O₃-ZrO₂) has been undertaken. A commercially available grade of spray dried powder (TZ3Y) was sieved to obtain a range of size distributions which constituted varying morphology. The sieved ranges were compacted using a conventional rigid die pressing method and by shock compaction through use of gas driven gun launched projectiles and explosively driven flyer plates. The three compaction methods provided a wide range of strain rate and pressure. The morphology of the sieved ranges was characterised through image analysis and the frictional behaviour characterised through flow and packing tests. Further characterisation involved the use of a nano-hardness indenter to measure the strength of individual powder agglomerates through indentation of polished cross-sections. Differences were observed in the flow behaviour, however agglomerate strength did not vary as function of agglomerate size. The rigid die loading responses of the sieved size ranges in terms of pressure - density relationships were monitored up to a pressure of 590MPa. The agglomerate size range was not found to affect the loading response, despite differences observed in flow and packing tests. An existing shock compaction model by Page et al. (1997) was modified such that the loading response, modelled using a modified Heckel relationship, could be altered according to the estimate post - shock temperature. This was possible as the Heckel relationship contained a term related to the particle strength, and temperature - strength data was available for TZ3Y. Comparison with the measured shock Hugoniout indicated that the model with the particle strength fixed showed better agreement than the variable strength model. Shock compacted samples produced in the gas gun were compared in the green and sintered states with rigid die samples pressed to 100MPa. Evidence of wave interaction effects leading to cracking were observed in the gun compacted samples and green densities were higher than rigid die samples. The increased green densities led to a reduction in the sintering temperature of between 100-200°C required to achieve a given hardness and density. The explosively compacted sampled contained regions of near full density without the need for subsequent sintering. This was however to the detriment of overall sample integrity as the conditions required to attain such properties also resulted in the formation of a heat affected porous phase. Starting agglomerate size was not noted to affect the properties of shock compacts.