Fracture and contact damage behavior of some alumina-based ceramic composites

The present study was designed to (1) exploit the in-situ toughened concept in oxide ceramic composites and (2) develop an approach for increasing contact damage resistance.; First, the grain morphology of the CA{dollar}sb6{dollar} grains in alumina:calcium hexaluminate (CA{dollar}sb6{dollar}) system was tailored by using different precursor Ca containing powders: the use of calcium carbonate as the starting powder resulted in elongated platelet CA{dollar}sb6{dollar} grains; while calcium oxide produced equiaxed CA{dollar}sb6{dollar} grains. The results were discussed in terms of differences in the reaction mechanism.; Second, the effect of grain morphology on R-curve behavior was studied using the system. Indentation-strength results revealed that the composite containing platelet CA{dollar}sb6{dollar} grains (referred as platelet composite) showed improved R-curve behavior compared with that containing equiaxed CA{dollar}sb6{dollar} grains (referred as equiaxed composite). A simple grain pull-out model was established to estimate the toughness increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation strength measurements.; Finally, Hertzian indentation test was conducted on platelet and equiaxed composites and fine-grained alumina. The results demonstrated that both platelet and equiaxed composites exhibited a diffuse damage zone. While fine-grained alumina showed traditional cone cracking. Based on these results, a new philosophy for layer structural design for improving contact damage resistance has been proposed. The basic idea is that the material conducted damage zone was used as a inner layer to absorb mechanical energy by microfractures, while the material exhibiting cone crack was used to form an outer layer to provide hardness and wear resistance. The concept was demonstrated in alumina/alumina:calcium hexaluminate system. Hertzian contact results showed that the degree of cone cracking in the alumina layer was dramatically suppressed in such a laminate. The effect of top layer thickness on the capacity of inhibition of contact damage was also investigated. The results were discussed in terms of selecting optimum microstructural parameters for effectively suppressing contact damage. The Hertzian indentation-strength behavior of layer structure was measured. The result demonstrated that layer structure exhibited improved strength-degradation resistance comparing with monolithic alumina.