Microstructure and mechanical properties of mullite/ZrO2 composites

The sintering, microstructure and toughness of mullite/ZrO₂ composites with increasing amount of ZrO₂ (0 to 20 vol %) have been studied. A very active premullite powder has been used as matrix. TheK _IC values increase from 2.1 to 3.2 MN m^?3/2 as the volume fraction of zirconia increases from 0 to 0.2. The realtive fraction of tetragonal zirconia decreases as the volume fraction of ZrO₂ increases to reach ～0.1 in the sample with 0.2 volume fraction of ZrO₂. The presence of ZrO₂ enhances the sintering rate and end-point density of the composites. Finally, the increasing toughness in mullite/ZrO₂ composites has been explained by a grain boundary strengthening mechanism produced by a metastable solid solution (～0.5 wt %) of ZrO₂ in mullite.