MICROSTRUCTURE REFINEMENT OF SINTERED ALUMINA BY A 2-STEP SINTERING TECHNIQUE

For a few oxide ceramics, the use of an initial precoarsening step pri or to densification (referred to as two-step sintering) has been obser ved to produce an improvement in the microstructural homogeneity durin g subsequent sintering, In the present work, the effect of a precoarse ning step (50 h at 800 degrees C) on the subsequent densification and microstructural evolution of high-quality alumina (Al2O3) powder compa cts during constant-heating-rate sintering (4 degrees C/min to 1450 de grees C) was characterized in detail. The data were compared with thos e for similar compacts that were sintered conventionally (without the heat treatment step) and used to explore the mechanism of microstructu ral improvement during two-step sintering, After the precoarsening ste p, the average pore size was larger, but the distribution in pore size s was narrower, than those for similar compacts that were sintered con ventionally to 800 degrees C. In subsequent sintering, the microstruct ure of the precoarsened compact evolved in a more homogeneous manner a nd, at the same density, the amount of closed porosity was lower for t he compacts that were sintered by the two-step technique, in compariso n to the conventional heating schedule, Furthermore, a measurably high er final density, a smaller average grain size, and a narrower distrib ution in grain sizes were achieved with the two-step technique, The mi crostructural refinement that was produced by the two-step sintering t echnique is explained in terms of a reduction in the effects of differ ential densification and the resulting delay of the pore channel pinch -off to higher density.