Microstructure Refinement of Sintered Alumina by a Two‐Step Sintering Technique

Abstract

For a few oxide ceramics, the use of an initial precoarsening step prior to densification (referred to as two‐step sintering) has been observed to produce an improvement in the microstructural homogeneity during subsequent sintering. In the present work, the effect of a precoarsening step (50 h at 800°C) on the subsequent densification and microstructural evolution of high‐quality alumina (Al2O3) powder compacts during constant‐heating‐rate sintering (4°C/min to 1450°C) was characterized in detail. The data were compared with those for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructural improvement during two‐step sintering. After the precoarsening step, the average pore size was larger, but the distribution in pore sizes was narrower, than those for similar compacts that were sintered conventionally to 800°C. In subsequent sintering, the microstructure of the precoarsened compact evolved in a more homogeneous manner and, at the same density, the amount of closed porosity was lower for the compacts that were sintered by the two‐step technique, in comparison to the conventional heating schedule. Furthermore, a measurably higher final density, a smaller average grain size, and a narrower distribution in grain sizes were achieved with the two‐step technique. The microstructural refinement that was produced by the two‐step sintering technique is explained in terms of a reduction in the effects of differential densification and the resulting delay of the pore channel pinch‐off to higher density.