Suppression of grain growth in sub-micrometer alumina via two-step sintering method

Two-step sintering (TSS) was applied to suppress the accelerated grain growth of sub-micron (∼150 nm) alumina powder. The application of an optimum TSS regime led to a remarkable decrease of grain size down to ∼500 nm; while the grain size of the full-dense structures produced by conventional sintering ranged between 1 and 2 μm. To find how important the temperatures at sintering steps might be, several TSS regimes were conducted. The results showed that the temperatures at both sintering steps play vital roles in densification and grain growth of the alumina compacts. Based on the results, the optimum regime consisted of heating the green bodies up to 1250 °C (first step) and then holding at 1150 °C for more than 60 h (second step). This yielded the finest microstructure with no deterioration of the densification. Heating at 1300 °C (first step) and then at 1200 °C (second step) was not a successful procedure. Lowering the temperature of the second step down to 1100 °C resulted in exhaustion of the densification at 88% -theoretical density. A nearly full-dense structure with an average grain size of 850 nm was obtained when the temperature of the second step was increased to 1150 °C. Empirical results show that not only the first step temperature has to be high enough to reach a structure containing unstable pores, but the second sintering temperature must also be high enough to activate the densification mechanism without grain growth. This means that a considerable densification at the first step does not imply enough second-step densification.