R ‐Curve Behavior of In‐Situ ‐Toughened Al 2 O 3 :CaAl 12 O 19 Ceramic Composites

The mechanical behavior of reaction‐sintered alumina: 30 vol% calcium hexaluminate (Al 2 O 3 :CaAl 12 O 19, or A1 2 O 3 : CA 6 ) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA 6 ) phase possessed a platelike morphology showed more‐pronounced R ‐curve behavior than a composite in which the CA 6 phase consisted of equiaxed grains. Toughness curves derived from the indentation‐strength data exhibited a “crossover,” such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA 6 resulted in enhanced toughening, compared to single‐phase alumina of comparable grain size, thus demonstrating the viability of the in‐situ ‐toughening approach. A simple grain‐pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation‐strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A1 2 O 3 :CA 6 composite were strongly bonded to the platelet A1 2 O 3 :CA 6 composite, demonstrated high strength over the range of tested flaw sizes.