Grain Size Distributions and Strength Variability of High‐Purity Alumina

Abstract

Two batches of high‐purity, alumina ceramics having an equiaxed grain morphology were manufactured with a mean grain size of approximately 10 μm. A low‐temperature heat treatment performed prior to sintering on one batch of specimens resulted in a narrow grain size distribution. The other batch of specimens was fired in a conventional manner and exhibited a broader grain size distribution. Mechanical tests performed on these specimens indicated little difference in the flaw tolerance of T‐curve behavior of the aluminas, despite the presence of coarser grains in the conventionally fired alumina. Observations of cracks in these high‐purity aluminas revealed that large grains ruptured transgranularly and therefore did not act as effective bridging sites. Strength tests on polished specimens indicated that the alumina with the broad grain size distribution exhibited greater strength variability than the alumina with a narrow grain size distribution. A simple analysis shows that, because of the shallow T‐curve behavior observed in both aluminas, stable crack extension should not occur from natural flaws. The strength of these high‐purity aluminas is therefore controlled by the initial flaw size and the initial toughness. The increased strength variability in the alumina with the broad grain size distribution is rationalized in terms of a wider distribution of critical flaws.