In the Zirconia Toughened Alumina (ZTA), toughness enhancement is known to be related to the transformation toughening and microcracking toughening mechanisms induced by the transformation of $ZrO_2$ particles from tetragonal to monoclinic.
In this investigation, bar-shaped specimens which consisted of three layers are prepared to study the effects of residual compressive stress on the mechanical properties. The outer layers contained $Al_2O_3$ and unstabilized $ZrO_2$ while the central layer contained $Al_2O_3$ and stabilized $ZrO_2$ (with 5.10 wt% $Y_2O_3$). When cooled from the sintering temperature, some of zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form.
The transformation which induces to dilatational expansion led to the estabilishment of compressive stress in the outer layers and balances tensile stress in the bulk. Decrease of outer layer thickness (for a fixed total thickness) increases residual compressive stress. Because of residual compressive stress in the outer layers, the fracture toughness of outer layers of 3-layer's composites is 12.1 Mpa$\sqrt{m}$, which is increased to 50% above in comparison with 1-layer's specimens in ZTA. The 3-layer's composite is believed to exhibite greater fracture resistance in contact damage environment.
