Effects of La2O3 and Nb2O5 dopants on the microstructural development and fracture toughness of Al2O3 ceramic

Previous research has shown that the inherently low fracture toughness of Al2O3 ceramics can be improved by in situ growth of a small volume fraction of anisotropic grains. In this study, 7.5La2O3-5Nb2O5-87.5Al2O3 (volume percent) composite ceramics were fabricated by microwave sintering, with the intention of tailoring alumina microstructural development to improve fracture toughness. The effects of sintering temperature and holding time on density and fracture toughness of the composite were investigated, and the growth and toughening mechanisms of columnar grains were discussed. The optimal sintering conditions for La2O3/Nb2O5 doped Al2O3 ceramics occurred at 1500 °C for 30 min, which is at least 100 °C lower than that of dense Al2O3. The phases present in the 7.5La2O3-5Nb2O5-87.5Al2O3 included in situ columnar grains of α-Al2O3 and LaAl11O18, and equiaxed grains of monoclinic LaNbO4. It is shown that a liquid phase induced the growth of columnar grains. The fracture toughness of the 7.5La2O3-5Nb2O5-87.5Al2O3 composite ceramic was improved by 116% compared with that of Al2O3, due to the anisotropic grain growth that enabled toughening mechanisms such as grain pull-out, crack deflection, crack branching, crack bridging, and domain switching.