Strengthening of Oxide Ceramics by Transformation‐Induced Stress

Alumina‐zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar‐shaped specimens consisted of three layers. The outer layers contained A1₂O₃ and unstabilized ZrO₂ while the central layer contained A1₂O₃ and partially stabilized ZrO₂ (with 2 mol% Y₂O₃). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al₂O₃‐ZrO₂ specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength‐limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.