The role of residual stresses in layered composites of Y-ZrO2 and A120 257 spectrometer gratings using steps of 0.204 wave- numbers and integrating over 0.5s intervals. The Tensile stresses Zro collected data were subsequently analyzed with curve-fitting algorithms(double Lorentz function) The line position was identified by simultaneously Compressive stresses fitting the Ri and R2 peaks using Nice Fit software package. By using an objective lens of 100x mag- Tensile stresses zro nifying power, a minimum spot size of 3 um dia meter could be achieved. It is known that both Ri and R, lines shift to smaller wavenumber with Fig. 3. Expected stress distribution in layered zircon increasing temperature, so a consistent calibration alumina composites. for the ruby was performed. Instrumental fluctua tions were compensated by monitoring an external compressive stress in alumina layer will oppose the of a neon discharge opening the crack. This expectation was confirmed lamp. Although the volume of material probed in by the tests of crack initiation in notched beams o the experiments was unknown, it was estimated composite studied. It occured that for the same hat the spectroscopic information was obtained layer thickness and bearing distance, 25% higher from a depth equalled spot size. force had to be used to initiate the crack in the C For determining the stresses in alumina, the RI sample where notch ended at the beginning of alu ne and piezospectroscopic coefficient(7-59 cm-/ mina layer in comparison to the sample where it GPa), for hydrostatic stress state, found by He and ended in zirconia layer. The character of the crack Clarke have been used path during fracture was also different in these The same method was used for calculating the samples. In the case of second sample initiated the same type of alumina powder and at the same cularly to the layer Ough zirconia layer perpend esidual stresses in alumina pellet prepared from crack propagated thi temperature of sintering. Although the average In the case of first sample (notch ended at the stress over the pellet must be zero, variations in begining of alumina layer), crack was deflected at stress from one grain to another being a result of the begining of its way through the alumina layer the difference in thermal expansion coefficient (see Fig 4) along its (ae=9.5 Further observation of controlled crack (aa=86x 10-6oC-)cause both a line shift and a showed that deflection of crack takes place only in broadening of the line due to superposition of alumina layer. In zirconia layer the crack deflects spectra from individual fluorescing volumes. This back to its original direction. It was found that the way was possible to measure the everage value of magnitude of the crack deflection is dependent on the line shift and calculate subsequent everage alumina layer thickness of composite. The values value of local residual stresses in polycrystalline of crack deflection angle (understood as a deflec alumina tion angle from direction perpendicular to the lay- The critical stress intensity factor, Klc, of com- ers)in a function of layer thickness are listed in posites was measured on notched beams described Table 2. As can be seen, crack deflection angle earlier by the method and relation proposed by increases with layer thickness. In 60 um thick alu- Evans mina layers crack defects at 90(Fig. 5). In layers with thickness of 10 um and lower crack deflection does not take place(Fig. 6) 3 Results and discussion At the crack front, deflection process in alumina layers is more complicated than it was shown in Thermal expansion mismatch(azro,= 12 x 10 Figs 5 and 6. Crack not only deflects but branches 9x 10-6C-)and shrinkage mis- also(see Fig. 7)what distinctly enhances the length match(see Table 1) between zirconia and alumina of the crack way and energy release during fracture lead after cooling from fabrication temperature to through the alumina layer. After crack front mov residual stress distribution in layered composites ing farther, only one branch of the crack is widely shown at Fig. 3. In the layer with lower a and opened but the rest of them is getting less visible lower shrinkage, the biaxial compressive stress is for microscopic observations expected and similarly, biaxial tensile stress in the Described above crack behaviour was observed in layer with higher a and shrinkage. Such a distribu- the bulk of the material studied and it seems to be a tion indicates that expected tensile stress in zirconia result of residual stresses present in barrier layers layer should promote opening the crack in the not- As can be seen from Figs 8-ll, the frequency shift ched beam during bending. On the contrary, the Ri line and subsequent compressive stressesspectrometer gratings using steps of 0.2±0.4 wave￾numbers and integrating over 0.5 s intervals. The collected data were subsequently analyzed with curve-®tting algorithms (double Lorentz function). The line position was identi®ed by simultaneously ®tting the R1 and R2 peaks using NiceFit software package. By using an objective lens of 100 mag￾nifying power, a minimum spot size of 3m dia￾meter could be achieved. It is known that both R1 and R2 lines shift to smaller wavenumber with increasing temperature, so a consistent calibration for the ruby was performed. Instrumental ¯uctua￾tions were compensated by monitoring an external reproducible spectral line of a neon discharge lamp. Although the volume of material probed in the experiments was unknown, it was estimated that the spectroscopic information was obtained from a depth equalled spot size. For determining the stresses in alumina, the R1 line and piezospectroscopic coecient (7.59 cmÿ1 / GPa), for hydrostatic stress state, found by He and Clarke10 have been used. The same method was used for calculating the residual stresses in alumina pellet prepared from the same type of alumina powder and at the same temperature of sintering. Although the average stress over the pellet must be zero, variations in stress from one grain to another being a result of the di€erence in thermal expansion coecient along its c-axis (c ˆ 9
5 10ÿ6Cÿ1) and a-axis (a ˆ 8
6 10ÿ6Cÿ1) cause both a line shift and a broadening of the line due to superposition of spectra from individual ¯uorescing volumes. This way was possible to measure the everage value of the line shift and calculate subsequent everage value of local residual stresses in polycrystalline alumina. The critical stress intensity factor, KIc, of com￾posites was measured on notched beams described earlier by the method and relation proposed by Evans.11 3 Results and Discussion Thermal expansion mismatch (ZrO2 ˆ 12 10ÿ6 Cÿ1; Al2O3 ˆ 9 10ÿ6Cÿ1) and shrinkage mis￾match (see Table 1) between zirconia and alumina lead after cooling from fabrication temperature to residual stress distribution in layered composites shown at Fig. 3. In the layer with lower  and lower shrinkage, the biaxial compressive stress is expected and similarly, biaxial tensile stress in the layer with higher  and shrinkage. Such a distribu￾tion indicates that expected tensile stress in zirconia layer should promote opening the crack in the not￾ched beam during bending. On the contrary, compressive stress in alumina layer will oppose the opening the crack. This expectation was con®rmed by the tests of crack initiation in notched beams of composite studied. It occured that for the same layer thickness and bearing distance, 25% higher force had to be used to initiate the crack in the sample where notch ended at the beginning of alu￾mina layer in comparison to the sample where it ended in zirconia layer. The character of the crack path during fracture was also di€erent in these samples. In the case of second sample initiated crack propagated through zirconia layer perpendi￾cularly to the layers. In the case of ®rst sample (notch ended at the begining of alumina layer), crack was de¯ected at the begining of its way through the alumina layer (see Fig. 4). Further observation of controlled crack growth showed that de¯ection of crack takes place only in alumina layer. In zirconia layer the crack de¯ects back to its original direction. It was found that the magnitude of the crack de¯ection is dependent on alumina layer thickness of composite. The values of crack de¯ection angle (understood as a de¯ec￾tion angle from direction perpendicular to the lay￾ers) in a function of layer thickness are listed in Table 2. As can be seen, crack de¯ection angle increases with layer thickness. In 60m thick alu￾mina layers crack de¯ects at 90 (Fig. 5). In layers with thickness of 10m and lower crack de¯ection does not take place (Fig. 6). At the crack front, de¯ection process in alumina layers is more complicated than it was shown in Figs 5 and 6. Crack not only de¯ects but branches also (see Fig. 7) what distinctly enhances the length of the crack way and energy release during fracture through the alumina layer. After crack front mov￾ing farther, only one branch of the crack is widely opened but the rest of them is getting less visible for microscopic observations. Described above crack behaviour was observed in the bulk of the material studied and it seems to be a result of residual stresses present in barrier layers. As can be seen from Figs 8±11, the frequency shift of the R1 line and subsequent compressive stresses in Fig. 3. Expected stress distribution in layered zirconia± alumina composites. The role of residual stresses in layered composites of Y±ZrO2 and Al2O3 257