Transformation Toughening in Sol–Gel-Derived Alumina-Zirconia Composites

The microstructures of α-Al₂O₃ seeded sol–gel-derived alumina-zirconia composites containing 20 wt% unstabilized zirconia (processed from zirconium n-propoxide) were very fine, with submicrometer alumina grains and small, mainly intergranular zirconia particles, the latter having a critical size for the tetragonal-to-monoclinic phase transformation of 0.45 μm. The corresponding ratios of the toughening contribution to the matrix toughness are relatively low (δK_c/K₀<1). This finding is confirmed by an analysis of the tetragonal zirconia particle size dependence of the stress-induced transformation toughening.