| The post-fracture tensile (PFT) test has demonstrated the ability to isolate the wake zone in ceramics for direct characterization at room temperature. In this study, the procedure is refined to elucidate characteristic stress-crack face separation relationships of three microstructures at temperatures ranging from 20{dollar}spcirc{dollar}C through 1170{dollar}spcirc{dollar}C. Two microstructures, a commercial 99.7% alumina and a nominally pure alumina, Lucalox, differ primarily in grain size and purity. In addition, a cubic magnesium aluminate spinel was evaluated over the temperature range of 20{dollar}spcirc{dollar}C through 600{dollar}spcirc{dollar}C.; Specific microstructural parameters were targeted for study: thermal expansion anisotropy (TEA), the effect of grain boundary phases, grain size, and the nature of the grain bridging mechanism. TEA contributions were studied at temperatures below 600{dollar}spcirc{dollar}C for both the alumina and spinel to avoid the effects of softened grain boundary phases expected at higher temperatures. In alumina, the average residual stresses arising from TEA diminish with increasing temperature, causing two effects evidenced by a general downward shift of the characteristic stress-displacement curves. Similar tests conducted on the cubic spinel exhibited no change in bridging efficiency over the same range.; A behavioral change, characterized by an increase in the limiting COD, appears in the stress-displacement results of both alumina materials at temperatures greater than 600{dollar}spcirc{dollar}C, and coincides with the expected softening point of a suspected grain boundary phase. Based upon studies of strain rate and time dependence, no evidence for viscous behavior was observed. Instead, topographic changes of the fracture surface near this temperature explain the increase in toughening behavior at high temperatures. This was confirmed through the PFT test by a series of tests maintaining a constant surface roughness.; An important part of this work relates elements of the microstructure to the observed behavior. The stiffness of wake ligaments provides some insight to those features of the microstructure that affect the bridging efficiency. Compliant mechanisms, such as ligament bending, asperity loading, and grain rotation, predict the relatively low measured stiffness, as compared with the conceptual frictional grain-pullout model. |
