The grains and grain boundaries of sintered alumina contain tensile residual stresses resulting from anisotropic thermal expansion, Young’s modulus along the crystal axes,
and crystallographic misorientation across the grain bound-aries. Therefore, in the sintered polycrystalline alumina, it is conceivable that the large crack along a grain boundary created by the synergetic effects of both residual stresses and processing defects, will be equivalent to the grain size of the material and that the weakest crack generated along a boundary in the specimen will dominate the strength of the specimen. The fracture toughness of grain boundaries is usually lower than that within the grains. Hence, polycrystalline alumina ceramics exhibit a mainly intergranular fracture mode, as schematically shown in Fig.. Fig. also shows the scanning electron microscopy (SEM) observation of the fracture surface of monolithic alumina. Reduction of both the defect size along the grain boundaries and the tensile residual stresses in the matrix grains by dislocations result in improvement of the strength of nanocomposites. Several mechanical properties of nanocomposites are also improved for the same reason, such as hardness, wear resistance, creep resistance and thermal shock resistance. Davidge et al. reporteddrastic changes in the abrasive wear surfaces between monolithic alumina and nanocomposites, where the surface of monolithic alumina showed grain pullout. The nanocom-posites, however, showed ground or abraded surfaces because of the improved strength along the grain boundaries.