Effect of Glass Additions on the Indentation-Strength Behavior of Alumina

The effect of small calcium aluminosilicate (CAS) glass additions on the microstructure and flaw tolerance of alumina ceramics is investigated, and the results compared to a high-purity alumina. The high-purity alumina specimens were dense with microstructures consisting of a uniform grain size distribution and equiaxed grain morphology. Additions of only 1 wt% glass phase resulted in a bimodal grain size distribution containing large, elongated grains within a fine-grain matrix. Indentation-strength tests indicated enhanced flaw tolerance with the bimodal microstructure, even though both materials had nominally the same average grain size. The strength of unindented specimens was also observed to decrease with glass additions. Observations of crack paths show a greater propensity for bridging in the glass-containing alumina due to the presence of coarse, elongated grains and perhaps a lower grain boundary toughness. However, crack extension occurs transgranularly when the size of the coarsest grains becomes too large. This suggests that an optimum in flaw tolerance will be achieved with an elongated grain morphology and intermediate grain size.