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2452 Communications of the American Ceramic Societ Vol 84. No 10 Table L. Selected T / T Values for Al, O3 Temperature material (J/m) fracture test 172A2O314.8 Deflected,025±00212 Deflected0.11±0.02 1400 10.4 SiC 15.4 Deflected 0.63 + 0.11 t-- alumina(1300 c) 1400 10.4 SiC 15.0 Deflected 0.62+ 0.11 1400 Broke through sic(1400c) eady-state load from boint bend test was then con- erted nterfacial resistance using a closed form Hotpressing pressure II. Results and discussion Fig. 2. Interfacial fracture resistance (in J/m-)as a function of hot- pressing pressure(in MPa) for various processing temperatures. Values of The measured fracture resistances are plotted as a function of the Al2O, system that has no elastic mismatch are plotted for comparison. hot-pressing pressure in Fig. 2, and the results of the fracture experiments are summarized in Table I. For comparison, the maximum fracture resistance value at which crack deflection wa interfacial fracture resistance of 15.0 Jm- was obtained for another observed in an Al,O, /porous-Al, O, interphase system is also specimen processed under the same conditions. This value is listed(obtained at hot-pressing temperature and pressure of within experimental error and confirms the validity of the result. A 1300.C and 17.2 MPa, respectively ) In this Al,,,O, high degree of reproducibility with this type of specimen also has system, crack penetration through the interface onto the lower bar been observed was observed when the interfacial fracture resistance was in- Several other issues need to be considered in applying the creased to a slightly higher value by processing at the same He-Hutchinson criterion to the experimental observations. The temperature and a higher pressure of 20.8 MPa. This set of basic He-Hutchinson derivation is based on energy release rates ocessing parameters was used as preliminary processing condi- for a homogeneous specimen with no separate layer to characterize tions for the work conducted here. As shown in Fig. 2, the the thin interphase. The effect of the elastic mismatch of a thin SiC/porous-Al,O, specimens exhibited lower interfacial fracture interlayer has been analyzed for the limiting case where the film resistances than the AlO,/porous-Al2O, specimens that were thickness asymptotically goes to zero. For the small elastic ocessed under identical conditions. The easier crack deflection mismatch of o=0.06 in our system, this effect leads to a slightly observed with SiC bars in our work(compared with the Al,O,/ smaller threshold for the energy release rate ratio, which rous-Al,O, system) indicated that the interfacial bonding be- contrast with the experimental observations. (This value of a was tween the SiC and porous Al,O3 was relatively weak. calculated using the elastic constants for dense Al,O3. With The monolithic CVD-SiC used as the matrix material has roughly 20% porosity for the interlayer in our experiments, o is fracture resistances of 20.6-29 7 J/m- with an average value o slightly larger if the elastic modulus varies linearly with porosity. 25.1 J/m. Taking one-fourth of the fracture resistance of the The effect of finite interphase thickness on the deflection threshold CVD-SiC as the threshold value for the interfacial fracture also has been explored by treating the laminate as a three-layer criterion for crack deflection in the absence of residual stress creased to higher r. this effect flattens out after some decrease in would be between 5.2 and 7.4 J/m- with an average value of 6.3 the interphase thickness, and, in addition, is significant only for a J/m. For the actual system, where there are in-plane tensile high negative value of a(as opposed to the positive value of a in residual thermal stresses in the porous-Al,O, layer, the threshold our syster values for Ii would be slightly lower than in the unstressed system, Another issue is that the theoretical predictions were formulated according to predictions made by He et al. In contrast, the a pupative crack at the interface, while the measured fracture measured fracture resistance of 15.4 J/m- obtained at a tempera- resistance values were obtained for deflected cracks that can be ture/pressure combination of 1400.C and 10.4 MPa was signifi- several millimeters long. The He-Hutchinson criterion is in cantly higher than the predicted threshold values. A similar excellent agreement with the analogous data obtained with Al O porous-Al2O3 laminates, which suggests that there are no signif cant differences between the initial crack and the longer, deflected crack. These previous specimens were fabricated and tested in the same way as the Sic/porous-Al2O3 specimens, but this does not guarantee that the measured fracture resistance values accurately represent the behavior of a pupative crack in this case as well especially because some differences in the behavior of the inter- ohase were observed in these two systems. R-curve effects were not observed with either of these systems (i.e, th 0.002mm constant during the steady-state crack growth that used to obtain the values in Table I and Ref. 8). Thus fference between a pupative crack and the measured results would hav Fig. 1. Experimental setup to interfacial fracture res ape detection with this measurement under four-point bending. Main crack was initiated from Vickers strikes on The basic mechanics analysis for a homogeneous interface lay the lower bar and was subsequently arrested in the interface. predicts that crack deflection occurs at the upper interface of ourbending. This procedure is also described elsewhere.8,9 The steady-state load from the four-point bend test was then converted to an interfacial fracture resistance using a closed form solution.9,10 III. Results and Discussion The measured fracture resistances are plotted as a function of hot-pressing pressure in Fig. 2, and the results of the fracture experiments are summarized in Table I. For comparison, the maximum fracture resistance value at which crack deflection was observed in an Al2O3/porous-Al2O3 interphase system is also listed (obtained at hot-pressing temperature and pressure of 1300°C and 17.2 MPa,8 respectively). In this Al2O3/porous-Al2O3 system, crack penetration through the interface onto the lower bar was observed when the interfacial fracture resistance was increased to a slightly higher value by processing at the same temperature and a higher pressure of 20.8 MPa. This set of processing parameters was used as preliminary processing conditions for the work conducted here. As shown in Fig. 2, the SiC/porous-Al2O3 specimens exhibited lower interfacial fracture resistances than the Al2O3/porous-Al2O3 specimens that were processed under identical conditions. The easier crack deflection observed with SiC bars in our work (compared with the Al2O3/ porous-Al2O3 system) indicated that the interfacial bonding between the SiC and porous Al2O3 was relatively weak. The monolithic CVD-SiC used as the matrix material has fracture resistances of 20.6–29.7 J/m2 with an average value of 25.1 J/m2 . 11 Taking one-fourth of the fracture resistance of the CVD-SiC as the threshold value for the interfacial fracture resistance, the maximum values allowed by the He–Hutchinson criterion for crack deflection in the absence of residual stress would be between 5.2 and 7.4 J/m2 with an average value of 6.3 J/m2 . For the actual system, where there are in-plane tensile residual thermal stresses in the porous-Al2O3 layer, the threshold values for i would be slightly lower than in the unstressed system, according to predictions made by He et al. 7 In contrast, the measured fracture resistance of 15.4 J/m2 obtained at a temperature/pressure combination of 1400°C and 10.4 MPa was significantly higher than the predicted threshold values. A similar interfacial fracture resistance of 15.0 J/m2 was obtained for another specimen processed under the same conditions. This value is within experimental error and confirms the validity of the result. A high degree of reproducibility with this type of specimen also has been observed.6,8 Several other issues need to be considered in applying the He–Hutchinson criterion to the experimental observations. The basic He–Hutchinson derivation is based on energy release rates for a homogeneous specimen with no separate layer to characterize the thin interphase. The effect of the elastic mismatch of a thin interlayer has been analyzed for the limiting case where the film thickness asymptotically goes to zero.12 For the small elastic mismatch of 0.06 in our system, this effect leads to a slightly smaller threshold for the energy release rate ratio, which is in contrast with the experimental observations. (This value of was calculated using the elastic constants for dense Al2O3. With roughly 20% porosity for the interlayer in our experiments, is slightly larger if the elastic modulus varies linearly with porosity.) The effect of finite interphase thickness on the deflection threshold also has been explored by treating the laminate as a three-layer system.13 With thinner interphases, the deflection limit is increased to higher i . This effect flattens out after some decrease in the interphase thickness, and, in addition, is significant only for a high negative value of (as opposed to the positive value of in our system). Another issue is that the theoretical predictions were formulated for a pupative crack at the interface, while the measured fracture resistance values were obtained for deflected cracks that can be several millimeters long. The He–Hutchinson criterion is in excellent agreement with the analogous data obtained with Al2O3/ porous-Al2O3 laminates, which suggests that there are no significant differences between the initial crack and the longer, deflected crack.8 These previous specimens were fabricated and tested in the same way as the SiC/porous-Al2O3 specimens, but this does not guarantee that the measured fracture resistance values accurately represent the behavior of a pupative crack in this case as well, especially because some differences in the behavior of the interphase were observed in these two systems. R-curve effects were not observed with either of these systems (i.e., the load was constant during the steady-state crack growth that was used to obtain the values in Table I and Ref. 8). Thus, any difference between a pupative crack and the measured results would have to escape detection with this measurement. The basic mechanics analysis for a homogeneous interface layer predicts that crack deflection occurs at the upper interface of our Fig. 1. Experimental setup to measure interfacial fracture resistance under four-point bending. Main crack was initiated from Vickers strikes on the lower bar and was subsequently arrested in the interface. Fig. 2. Interfacial fracture resistance (in J/m2 ) as a function of hotpressing pressure (in MPa) for various processing temperatures. Values of the Al2O3 system that has no elastic mismatch are plotted for comparison.8 Table I. Selected i /f Values for Al2O3 † and SiC Composites Temperature (°C) Pressure (MPa) Substrate material Interfacial fracture resistance (J/m2 ) Result of fracture test i /f 1300 17.2 Al2O3 14.8 Deflected 0.25 0.02 1300 20.8 Al2O3 Broke through 1300 17.2 SiC 0.5 Deflected 1300 20.8 SiC 2.9 Deflected 0.11 0.02 1400 7.8 SiC 8.9 Deflected 0.36 0.06 1400 10.4 SiC 15.4 Deflected 0.63 0.11 1400 10.4 SiC 15.0 Deflected 0.62 0.11 1400 17.2 SiC Broke through † Reference 8. 2452 Communications of the American Ceramic Society Vol. 84, No. 10