Effect of initial particle packing on the sintering of nanostructured transition alumina

The effect of forming method (cold isostatic pressing and slip casting) on particle packing and the consequent effects on densification, phase transformation and microstructural evolution were evaluated during sintering of a transition alumina powder (Nanotek®, particle size of 47 nm, δ and γ phases). It is well known that the transformation of transition alumina towards the stable α phase has a critical influence on the sintering behaviour. Therefore, correlation between microstructural evolution and shrinkage in compacts was established using dilatometry, Scanning Electron Microscopy and X-Ray Diffraction. By the two mentioned forming methods, green bodies with the same density were processed, in order to investigate only particle packing homogeneity and its effect on phase transition and sintering. For the same initial green density, the samples prepared by slip casting present a better homogeneity of particle packing, due to an optimal dispersion of particles in the slurry. This initial microstructure feature improves the particles rearrangement during the transition to α-alumina and hence enhances the transformation to the thermodynamic stable α-phase and the densification step.