1000 R35/4 sxS 25 kv Fig. 5. The crack path in 55 um thick alumina layer of Y-ZrO2/ Al2O3 composite (inverted image) 8§°封 Fig. 4. Y-ZrO 一= of crack path during fracture of layered posite dependent on the type of layer where notch ha one:(a)the end of the notch in zirconia layer tes perpendicularly to the layer, (b)the end of the notch in alumina layer-the crack immediately deflects Table 2. mean value of crack o n angle a gradient of 1888 compressive stresses in alumin of Y-ZrO2/Al2O3 com- posite as a function of na layer thickness Thickness of alumina layer(un Mean value of crack 022±562±890 deflection angle°) Gradient of c 13-250-81581188-4 stresses,△a(MPa) sR28/1叫 alumina layers are not only a function of barrier 85H layer thickness but a position across the layer also Maximum of compressive stress equalled 280 MPa is observed at the interface and it is independent on Fig. 6. The crack path in alumina layer of Y-ZrO2/Al2O3 alumina layer thickness. The minimum of stress is composite as a function of layer thickness: (a)19. 5 um and (b) achieved in the center of the layer. However the stres 8.2 um(inve in minimum is dependent on alumina layer thickness ( see Table 2). In the case of 60 um thick barrier layer type of alumina powder and at the same tempera the stress minimum equals 88.3 MPa. As it was said ture of sintering and caused by crystallographically earlier, this value is exactly equal the residual stresses anisotropic thermal expansion of the alumina only measured in alumina pellet prepared from the same It means also that the presence of compressivealumina layers are not only a function of barrier layer thickness but a position across the layer also. Maximum of compressive stress equalled 280 MPa is observed at the interface and it is independent on alumina layer thickness. The minimum of stress is achieved in the center of the layer. However the stress in minimum is dependent on alumina layer thickness (see Table 2). In the case of 60m thick barrier layer the stress minimum equals 88.3MPa. As it was said earlier, this value is exactly equal the residual stresses measured in alumina pellet prepared from the same type of alumina powder and at the same tempera￾ture of sintering and caused by crystallographically anisotropic thermal expansion of the alumina only. It means also that the presence of compressive Fig. 4. Character of crack path during fracture of layered Y±ZrO2/Al2O3 composite dependent on the type of layer where notch has been done: (a) the end of the notch in zirconia layerÐ the crack propagates perpendicularly to the layer, (b) the end of the notch in alumina layerÐthe crack immediately de¯ects. Table 2. Mean value of crack de¯ection angle and gradient of compressive stresses in alumina layer of Y±ZrO2/Al2O3 com￾posite as a function of alumina layer thickness Thickness of alumina layer (mm) 10 25 40 60 Mean value of crack de¯ection angle () 0 22‹5 62‹8 90 Gradient of compressive stresses,
 (MPa) 13.2 50.8 158.1 188.4 Fig. 5. The crack path in 55 m thick alumina layer of Y±ZrO2/Al2O3 composite (inverted image). Fig. 6. The crack path in alumina layer of Y±ZrO2/Al2O3 composite as a function of layer thickness: (a) 19.5m and (b) 8.2 m (inverted image). 258 H. Tomaszewski et al