Effect of barrier layer thickness and composition on fracture toughness oflayered zirconia/alumina composites

Composites of yttria or ceria-partially-stabilized zirconia with layers of either alumina or a mixture of 50% by volume of alumina and zirconia were f abricated by sequential centrifuging of powder suspensions. This method all owed formation of layers with thickness of 10 to 70 mu m. In both cases (Y- ZrO2 and Ce-ZrO2 matrices), a significant increase in fracture toughness, w ork of fracture and bending strength was observed only for composites with barrier layers made of a pure alumina. A crack deflection in alumina layer was found to be the main mechanism responsible for an increase in mechanica l properties. For confirmation this thesis, no increase in the transformati on zone width was observed. As it was shown, crack deflection angle was dep endent on alumina layer thickness. Higher deflection angles for a thicker a lumina layers were found. Explanation of this phenomenon was given by deter mination of residual stress distribution in barrier layers made by piezospe ctroscopy. A correlation between the crack deflection angle and the differe nce of stress between the layer boundary and the centre of the layer was no ticed. The residual stresses observed are a result of thermal expansion mis match between alumina and zirconia and thermal anisotropy of alumina. Shrin kage mismatch, especially in the case of Ce-ZrO2 and Al2O3, as a third sour ce of stress is suggested. (C) 2000 Kluwer Academic Publishers.