The microstructures of three different kinds of alumina-based ceramics, viz: (i) Coors AD85, (ii) alumina-anorthite, and (iii) alumina-aluminum titanate composites, have been tailored with the intent of altering their crack resistance (R- or T-curve) behavior. The influence of the two important microstructural parameters, viz: (i) internal residual stresses, and (ii) microstructural coarseness, on grain-localized crack bridging, a toughening mechanism responsible for the T-curve behavior in these materials in which intact grains exert frictional closure forces across the crack walls in the wake of an advancing crack tip, has been investigated. The residual stresses were tailored by crystallization of the intergranular glass, by quenching, and by the addition of a discrete second phase. The microstructural coarseness was tailored by homogeneous coarsening (scaling) and by heterogenous coarsening (incorporation of large grains within a fine-grain matrix).; In AD85 aluminas it has been shown that both crystallization of the intergranular glass and quenching have no effect on its flaw tolerance, a T-curve behavior derived mechanical property. On the other hand, in alumina-anorthite materials, crystallization of the intergranular anorthite glass resulted in a marked improvement in its flaw tolerance. Homogeneous microstructural coarsening of AD85 aluminas resulted in improved flaw tolerance, whereas the elongated nature of the grains in alumina-anorthite materials influenced its flaw tolerance only marginally. These results have been discussed with reference to the influence of the microstructural parameters on the bridging characteristics, in the light of an existing theoretical model.; It has been shown that addition of aluminum titanate to alumina results in improved flaw tolerance, whereas homogeneous coarsening of these microstructures results in a relatively weak material. However, heterogeneous microstructural coarsening resulted in composites with impressive flaw tolerance and T-curve properties. Direct evidence for active bridging in these composites has been presented. The processing of these tough ceramics has been discussed using a phenomenological model, and their mechanical properties have been discussed with reference to the influence of the microstructure on the bridging characteristics. A theoretical model describing bridging in heterogeneously coarsened (bimodal) microstructures has been presented.
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