The influence of shaping method on the grain size dependence of strength in dense submicrometre alumina

The grain size dependence of the strength of pressureless sintered aluminas is investigated with specimens fabricated by unaxial pressing, cold isostatic pressing, pressure filtration, gel casting, and combinations thereof. The strength depends on the flaw population, and the observed grain size effect is different with different shaping approaches: with reduced grain sizes the diversity of measured strength averages broadens including increasingly high values for some of the shaping procedures only. Submicrometre grain sizes are, however, not an indispensable prerequisite for a high strength: grain sizes of 1-2 mu m are sufficient to achieve 800 - 900 MPa by pressureless sintering, but present microstructures with grain sizes < 1 mu m do not indicate a further increase of the strength. Under certain technological conditions, the highest strengths can be associated with a surprisingly small standard deviation. The measured data are discussed considering recent micromechanical investiga tions of the same materials.