R-curve behavior of in-situ-toughened Al2O3:CaAl12O19 ceramic composites

The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al2O3:CaAl12O19, or Al2O3: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 Al2O3:CA(6) composite were strongly bonded to the platelet Al2O3:CA(6) composite, demonstrated high strength over the range of tested flaw sizes.