260 H. Tomaszewski et al Table 3. Toughness of Y-ZrO2/Al2O3 composite as a function of barrier layer thickness and composition Composition of alumine Mixture of barrier layer alumina barrier layer (um) K1e,(MPam2)720±842±999±10.06±71± 0-150-550·760.33 0-41 Position across the layer, Hm layer as was shown by FEM calculations and rne d by Ho et al. seems to be helpful ele- ment in explana stress gradient in the crack deflection process observed in reported layered composites. The crack deflection in alumina barrier layers is a result of Position across the layer, um interaction of residual compressive stress acting in the plane parallel to the layers, and perpendicular Fig. 10. Frequency shift of the RI line and compressive stres- to the layer and notch plane, tensile stress, both ses in 40 um thick alumina layer of Y-Zroz/Al2O3 composite present in laminate after cooling from fabrication as a function of position across the layer temperature and the third one, tensile applied stress in bending of notched beam. For a maximum of compressive stress gradient(the minimum of compressive stress) observed in the case of 60 m thick alumina layer the tensile perpendicular stress begins to dominate and in a result the crack deflects in the center of the layer and propagates along the layer and then deflects back to the per pendicular direction when the compressive stress increases from the minimum in the center of the layer to the maximal value at the interface, as can be seen from Fig. 5. In the case of the thinnest alumina layer where the compressive stress across the layer becomes almost unchangable (the gra dient is in minimum) the crack propagates through the layer without deflection The results obtained for composites with barrier layers made of an oxide mixture instead of a pure 30 50 60 Al_03 prepared to minimize the larger shrinkage of Position across the lay Y-ZrO2(see Table 1)are good confirmation of the thesis on the role of compressive stress gradient in Fig. 11. Frequency shift of the Ri line and compressive stres- observed crack deflection. Residual stresses in ses in 60 um thick alumina layer of Y-ZrO /AlO3 composite these layers should be present also, but their dis- as a function of position across the layer tribution seems to be different. It is expected they have rather local character--alumina grains in a mixture are stressed by zirconia grains and vice were observed after cooling from fabrication tem versa. Although perpendicular stress exists in perature by Ho et al. >and it was found that for a this type of layers also, such a distribution of given residual stress, crack extension without any compressive stress should result in a lack of crack external stress will take place only when the layer deflection. As it shown at Fig 12, crack propagates hickness is greater than a critical value through the barrier layer without deflection inde In our case matrix and barrier layer thickness pendently on layer thickness. The magnitude of was equal (10 to 60 um) and cracks parallel to the frequency shift of the RI line and compressive layers were not found. But the presence of tensile stress in this case are slightly higher than in layers stress perpendicular to the alumina layer plane made of a pure alumina, but independent on posi with the maximum localized in the center of this tion across the layer(Fig 13)were observed after cooling from fabrication tem￾perature by Ho et al. 15 and it was found that for a given residual stress, crack extension without any external stress will take place only when the layer thickness is greater than a critical value. In our case matrix and barrier layer thickness was equal (10 to 60m) and cracks parallel to the layers were not found. But the presence of tensile stress perpendicular to the alumina layer plane with the maximum localized in the center of this layer as was shown by FEM calculations12,13 and con®rmed by Ho et al. 15 seems to be helpful ele￾ment in explanation of the role of compressive stress gradient in the crack de¯ection process observed in reported layered composites. The crack de¯ection in alumina barrier layers is a result of interaction of residual compressive stress acting in the plane parallel to the layers, and perpendicular to the layer and notch plane, tensile stress, both present in laminate after cooling from fabrication temperature and the third one, tensile applied stress in bending of notched beam. For a maximum of compressive stress gradient (the minimum of compressive stress) observed in the case of 60m thick alumina layer the tensile perpendicular stress begins to dominate and in a result the crack de¯ects in the center of the layer and propagates along the layer and then de¯ects back to the per￾pendicular direction when the compressive stress increases from the minimum in the center of the layer to the maximal value at the interface, as can be seen from Fig. 5. In the case of the thinnest alumina layer where the compressive stress across the layer becomes almost unchangable (the gra￾dient is in minimum) the crack propagates through the layer without de¯ection. The results obtained for composites with barrier layers made of an oxide mixture instead of a pure Al2O3 prepared to minimize the larger shrinkage of Y±ZrO2 (see Table 1) are good con®rmation of the thesis on the role of compressive stress gradient in observed crack de¯ection. Residual stresses in these layers should be present also, but their dis￾tribution seems to be di€erent. It is expected they have rather local characterÐalumina grains in a mixture are stressed by zirconia grains and vice versa. Although perpendicular stress exists in this type of layers also, such a distribution of compressive stress should result in a lack of crack de¯ection. As it shown at Fig. 12, crack propagates through the barrier layer without de¯ection inde￾pendently on layer thickness. The magnitude of frequency shift of the R1 line and compressive stress in this case are slightly higher than in layers made of a pure alumina, but independent on posi￾tion across the layer (Fig. 13). Fig. 11. Frequency shift of the R1 line and compressive stres￾ses in 60 m thick alumina layer of Y±ZrO2/Al2O3 composite as a function of position across the layer. Fig. 10. Frequency shift of the R1 line and compressive stres￾ses in 40 m thick alumina layer of Y±ZrO2/Al2O3 composite as a function of position across the layer. Table 3. Toughness of Y±ZrO2/Al2O3 composite as a function of barrier layer thickness and composition Composition of barrier layer Alumina Mixture of alumina and zirconia Thickness of barrier layer (m) 10 25 40 60 45 KIc, (MPa m1/2) 7.20‹ 0.15 8.42‹ 0.55 9.99‹ 0.76 10.06‹ 0.33 7.11‹ 0.41 260 H. Tomaszewski et al