Compaction and sintering of ceramic powders
This Thesis describes a study of the compaction and the sintering of ceramic powders within the context of their "near net shape forming" into dense bodies. The operation of near net shape forming is the manufacture of ceramic parts with a required external dimensional tolerance combined with a defect free internal microstructure. The current study considers these requirements by focusing upon the fundamental facets of near net shape forming operations. The effects of the processing and material parameters on the compaction behaviour of agglomerated alumina powders have been investigated experimentally. It has been shown that the green compacts, formed from the agglomerated alumina powders by the uniaxial die pressing technique, possess certain density distribution patterns due to the frictional forces during the compaction process, and these density distributions depend primarily upon the properties of the agglomerates and the lubrication state of the die wall. The density distributions in the green compacts have been determined experimentally using the coloured layer technique and predicted using a first order model. Due to the inhomogeneous density distributions in the green compacts, the shrinkage of the compacts was not homogeneous. As a result, the shapes of the sintered compacts were not geometrically linear scaled replicas of the green compacts and the shape distortions in the sintered compacts have been characterised. It has been shown that there is a relationship between the extent of the shape distortions and the density distributions in the green compacts; that is, the more inhomogeneous green compacts the more is the shape distortions in the resulting sintered compacts. Sintering and grain growth equations, modified from established relationships, have been used for the sintering simulations in order to predict both the progression of the density and grain size of the sintered compacts. In order to predict the overall shape of the sintered compacts, a first order model has been developed by combining the predicted density distributions in the green compacts and the predicted density of the sintered compacts. The extent of the shape distortions have been reduced by optimising the properties of the agglomerates and the state of the die wall lubrication condition.