Microstructure and mechanical properties of alumina-5vol.%zirconia nanocomposites prepared by powder coating and powder mixing routes

Zirconia toughened alumina (ZTA) nanocomposites are attractive structural materials which combine the high hardness and Young's modulus of the alumina matrix with an additional toughening effect by the zirconia dispersion. In this study two approaches to prepare ZTA are compared. For the first approach, an ultrafine alumina powder was coated with 5 vol% zirconia by a wet chemical method. For the second one, the reference material was prepared by intensively mixing and milling the same alumina with nanoscale zirconia powder. Samples were consolidated at 1350–1600 °C by hot pressing and their mechanical properties, microstructure and transformation behavior were compared. Toughness increments derived from different toughening mechanisms are also briefly discussed. Besides better sinterability, the mixed material exhibited a finer grain size of both matrix and dispersion and thus higher hardness and strength. The alumina matrix was under compressive hydrostatic residual cooling stress, whereas zirconia was under tensile one. The coated material, however, showed higher transformability, deeper transformation zones and thus higher fracture toughness. In addition, it contained more monoclinic zirconia so the matrix was under tension.