H. Hadraba et al /Ceramics International 30 (2004)853-863 AL, O3 20 um AO」 20 um Fig. 6. Microphotographs of indentation cracks in alumina zirconia layered composite LC HP/3Y propagated parallel with the interface of alumina and zirconia(crack initiated in(a)alumina and(b) zirconia). was conducted such that the appearing crack propagated terial, again parallel with the interface(even a crack initiated from the Al]O3 or ZrO2 material in three different directions: in the ZrOz layer got deflected into the Al2O3 layer).As erpendicular, askew and parallel to the interface mentioned above, the direction of compression (in Al2O3) If the crack propagated perpendicular to the interface and and tensile (in ZrO2) residual thermal stresses in the lay was initiated in Al2O3 or ZrO2(see Fig 4), it propagated ers is parallel with the layer interface. In the tensile stress through the Al2o3/ZrO2 interface without changing the di- field, a crack initiated in ZrOz parallel with the layer in- rection of propagation terface got in the Zroz layer deflected perpendicular to the When the indentation crack propagated askew to the layer interface, where its propagation was made easier due Al2O3/ZrO2 interface (see Fig. 5) the direction of crack to the crack opening out in the tensile field. After crossing propagation got deflected. When passing through the inter- the interface it propagated in the Al2O3 layer parallel with face, a crack that was initiated in Al2O3 was deflected in the tensile stresses in this layer. Another possible explana the Zro layer from the layer interface( towards the normal tion for the crack deflection from ZrO2 into Al2O3 is the to the interface area) and thus its path in the Zroz layer was deformation of the stress field close to the Al2O3/ZrO2 in- shortened. A crack that was initiated in ZrO2 and was propa- terface. As can be seen from Figs. 5 and 6, near the interface gating askew to the Zro2/Al2O3 interface deflected towards the crack is not initiated from the indentation corner, it is he layer interface when passing through the interface shifted towards the Al2O3 layer (in the case of symmetri The authors of recent papers [6, 7] also described a simi- cal indentations with respect to interface position(Fig. 4)it lar indentation crack propagation askew to the interface of is initiated from the indentation corner). As Youngs mod- Al2O3 and Zro2 layers. Hatton and Nicholson [8] explaine ulus of AlO3 is almost twice that of ZrO(see Table 1) is behavior of Al2O3/ZrO2-based layered composites by the stress induced by the indentation tool was in Al2O3 al- the presence of residual thermal stresses in composite lay- most twice that in Zro2. It is thus possible that maximum ers. In the compression field, the crack propagated parallel stress(and, consequently, the point of crack initiation)was to the direction of compression while in the tensile field not in the indentation corner. In the case of indentation it propagated perpendicular to the direction of tension [8]. in ZrO2, the crack could have initiated even in the al2o This also shows in the behavior of indentation cracks pass- layer at a point close to the indentation corner. The crack ing through the AlO3/ZrO2 interface described above: in then propagated on the one hand towards the indentation(in the Zro2 layer the crack was deflected from the layer inter- ZrO2)and, on the other hand, parallel with the interface(in face and propagated perpendicular to the tensile tension in Al2O3)in order not to be closed by the acting compressive the layer while in the AlzO3 layer it was deflected towards stress. It is not the aim of the present work to give a de- the layer interface, i.e. parallel with the compression tension tailed description of mechanical properties of layered com- in the layer posites but important knowledge has been obtained through A crack initiated parallel with the Al2O3/ZrO2 interface the changes in the trajectory of a crack propagating at the see Fig. 6)always propagated preferably in the Al2O3 ma- la ayer interface. It is a proof of the strength of the bond of858 H. Hadraba et al. / Ceramics International 30 (2004) 853–863 Fig. 6. Microphotographs of indentation cracks in alumina/zirconia layered composite LC HP/3Y propagated parallel with the interface of alumina and zirconia (crack initiated in (a) alumina and (b) zirconia). was conducted such that the appearing crack propagated from the Al2O3 or ZrO2 material in three different directions: perpendicular, askew and parallel to the interface. If the crack propagated perpendicular to the interface and was initiated in Al2O3 or ZrO2 (see Fig. 4), it propagated through the Al2O3/ZrO2 interface without changing the di￾rection of propagation. When the indentation crack propagated askew to the Al2O3/ZrO2 interface (see Fig. 5) the direction of crack propagation got deflected. When passing through the inter￾face, a crack that was initiated in Al2O3 was deflected in the ZrO2 layer from the layer interface (towards the normal to the interface area) and thus its path in the ZrO2 layer was shortened. A crack that was initiated in ZrO2 and was propa￾gating askew to the ZrO2/Al2O3 interface deflected towards the layer interface when passing through the interface. The authors of recent papers [6,7] also described a simi￾lar indentation crack propagation askew to the interface of Al2O3 and ZrO2 layers. Hatton and Nicholson [8] explained this behavior of Al2O3/ZrO2-based layered composites by the presence of residual thermal stresses in composite lay￾ers. In the compression field, the crack propagated parallel to the direction of compression while in the tensile field it propagated perpendicular to the direction of tension [8]. This also shows in the behavior of indentation cracks pass￾ing through the Al2O3/ZrO2 interface described above: in the ZrO2 layer the crack was deflected from the layer inter￾face and propagated perpendicular to the tensile tension in the layer while in the Al2O3 layer it was deflected towards the layer interface, i.e. parallel with the compression tension in the layer. A crack initiated parallel with the Al2O3/ZrO2 interface (see Fig. 6) always propagated preferably in the Al2O3 ma￾terial, again parallel with the interface (even a crack initiated in the ZrO2 layer got deflected into the Al2O3 layer). As mentioned above, the direction of compression (in Al2O3) and tensile (in ZrO2) residual thermal stresses in the lay￾ers is parallel with the layer interface. In the tensile stress field, a crack initiated in ZrO2 parallel with the layer in￾terface got in the ZrO2 layer deflected perpendicular to the layer interface, where its propagation was made easier due to the crack opening out in the tensile field. After crossing the interface it propagated in the Al2O3 layer parallel with the tensile stresses in this layer. Another possible explana￾tion for the crack deflection from ZrO2 into Al2O3 is the deformation of the stress field close to the Al2O3/ZrO2 in￾terface. As can be seen from Figs. 5 and 6, near the interface the crack is not initiated from the indentation corner, it is shifted towards the Al2O3 layer (in the case of symmetri￾cal indentations with respect to interface position (Fig. 4) it is initiated from the indentation corner). As Young’s mod￾ulus of Al2O3 is almost twice that of ZrO2 (see Table 1), the stress induced by the indentation tool was in Al2O3 al￾most twice that in ZrO2. It is thus possible that maximum stress (and, consequently, the point of crack initiation) was not in the indentation corner. In the case of indentation in ZrO2, the crack could have initiated even in the Al2O3 layer at a point close to the indentation corner. The crack then propagated on the one hand towards the indentation (in ZrO2) and, on the other hand, parallel with the interface (in Al2O3) in order not to be closed by the acting compressive stress. It is not the aim of the present work to give a de￾tailed description of mechanical properties of layered com￾posites but important knowledge has been obtained through the changes in the trajectory of a crack propagating at the layer interface. It is a proof of the strength of the bond of