INITIAL COARSENING AND MICROSTRUCTURAL EVOLUTION OF FAST-FIRED AND MGO-DOPED AL2O3

The effect of an initial coarsening step (50-200 h at 800 degrees C) o n the subsequent densification and microstructural evolution of high-q uality compacts of undoped and MgO-doped Al2O3 has been investigated d uring fast-firing (5 min at 1750 degrees C) and during constant-heatin g-rate sintering (4 degrees C/min to 1450 degrees C), In constant-heat ing-rate sintering of both the undoped and MgO-doped Al2O3, a refineme nt of the microstructure has been achieved for the compact subjected t o the coarsening step. A combination of the coarsening step and MgO do ping produces the most significant refinement of the microstructure. I n fast-firing of the MgO-doped Al2O3, the coarsening step produces a m easurable increase in the density and a small refinement of the grain size, when compared with similar compacts fast-fired conventionally (i .e., without the coarsening step). This result indicates that the acce pted view of the deleterious role of coarsening in the sintering of re al powder compacts must be reexamined, Although extensive coarsening a fter the onset of densification must be reduced for the achievement of high density, limited coarsening prior to densification is beneficial for subsequent sintering.