S. Bueno et al. / Journal of the European Ceramic Society 28(2008)1961-1971 1969 Values of Gic in the R curves determined for the composites AlO with a relative notch depth of 0.6 Go(S D)(/m2) Goo(S D)(/m2) Jc(S D)(/m2) A10-1450214(1.6) 585(2.7) A10-1550 94(2.1 569(29) 53.1(23) Go: initial values and Goo: steady state values. The Jc values correspond Table 2. S D. standard deviatior The different non-brittle mechanical parameters calculated in this work did not follow the same trend as a function of the microstructure of the composites. Toughness values from the R curves for completely developed process zones, Koo and Goo seem to be slightly higher for the fine-grained material, A10-1450, that presented smaller Go and process zone width, h, but significantly higher Aa across which the toughness increase (AKR)occurred. On the contrary, Jic was significantly higher for the coarse-grained composite, A10-1550, with larger Go and h and smaller Aa. This discrepancy is due to the fact that Jiccon- stitutes a fracture criterion for materials where the toughening occurs along limited crack propagation. 29 Therefore, Jic will be closer to the toughness of composite Al0-1550, for which the major part(76%)of the total toughness increase(38%)occurred along one half (180 um) of the total crack growth before the steady state was reached(Fig. 6). On the contrary, significantly larger crack growth had to take place in the composite A10-1450 to reach the steady state. In this latter material, a crack growth of 230 um occurred before the 76% of the total KR increase(54%) was reached(Fig. 6) Fig. 7. Post-fracture observations of the zones that surrounded the notch and The work of fracture values for the composites(Table 2)were crack-tip region in the bend bars of Al0. Scanning electron micrographs of considerably higher than those for the monophase aluminas in polished and chemically etched (HF 10 vol So-3 min)surfaces. (a) Damaged agreement with the toughening mechanisms described, and sim ite sintered at 1450 C(b) Detail of microcracks in the composite sintered 1550°C grain size of 25 um( J/m2)0 and in porous aluminas with a mean grain size of 15-20 um(40 J/m-). In those coarse- grained aluminas the main toughening mechanisms identified, grain size as those of the composites, A-1450 0e is the critical crack bridging and crack branching, 2 were related to extensive stress to trigger the microcracked process zone grain boundary microcracking, at the expense of lower strength Inserting in Eq.(6) the average width values from Fig. 7 values (300 MPa, 25% inferior to strength for fine-graine (h=20 and 30 um, for A10-1450 and A10-1550, respectively) alumina). The same drawback strength-microcracking occurs and the Ko values from Table 2(2.8 MPam), critical stresses in the composites studied here, that present lower strengths (oc)of 163 and 133 MPa for A10-1450 and A10-1550, respec- than the previously studied composite material (28-36% lower tively, are obtained. These values are one order of magnitude Table 1) higher than that determined by acoustic emission (20 MPa) Moreover, work of fracture values substantially exceeded for a coarse-grained(A123, 20-40 um, dso=16 um) alumina those of both energy-fracture toughness that resulted from the material+/49 and for composite materials of zirconia-alumina extension of the principles of linear elastic fracture to situations in which significantly larger process zones(h=mm) than the where the inelastic deformation occurs prior to fracture, JIc and ones observed in the materials studied here were observed. Goo, (YwoF >Jic/2, Goo/2, Tables 2 and 3). This fact suggests On the contrary, they are similar to those reported for that there are additional non-linear phenomena occurring during zirconia-alumina composites with small microcrack process fracture of the composites that significantly contribute to the total zones(h=60-160 um). o Even though the size of the process energy consumption but not to resistance to crack initiation and zone for the above mentioned coarse-grained alumina was not that can be envisaged as follows. The efficiency of microcraks reported, its size should be larger than those observed in this in the toughening of the studied composites will be the result work. In fact, using Eq (6) and for Ko the range of values deter- of a compromise between their crack shielding and weakening mined by Seidel and Rodel#0 and Fett et al. 4:1.5-3MPam2, effects. For sufficient levels of microcrack density, microcrack process zones of h=384-1539 um are found could coalesce and link together with the crack front, leading toS. Bueno et al. / Journal of the European Ceramic Society 28 (2008) 1961–1971 1969 Fig. 7. Post-fracture observations of the zones that surrounded the notch and crack-tip region in the bend bars of A10. Scanning electron micrographs of polished and chemically etched (HF 10 vol.%–3 min) surfaces. (a) Damaged zone adjacent to one side of the crack (upper part of the image) in the compos￾ite sintered at 1450 ◦C. (b) Detail of microcracks in the composite sintered at 1550 ◦C. grain size as those of the composites, A-1450. σc is the critical stress to trigger the microcracked process zone. Inserting in Eq. (6) the average width values from Fig. 7 (h ∼= 20 and 30m, for A10-1450 and A10-1550, respectively) and the K0 values from Table 2 (2.8 MPa m1/2), critical stresses (σc) of 163 and 133 MPa for A10-1450 and A10-1550, respec￾tively, are obtained. These values are one order of magnitude higher than that determined by acoustic emission (20 MPa) for a coarse-grained (A123, 20–40m, d50 = 16m) alumina material47,49 and for composite materials of zirconia–alumina in which significantly larger process zones (h ∼= mm) than the ones observed in the materials studied here were observed.48 On the contrary, they are similar to those reported for zirconia–alumina composites with small microcrack process zones (h ∼= 60–160m).48 Even though the size of the process zone for the above mentioned coarse-grained alumina was not reported, its size should be larger than those observed in this work. In fact, using Eq. (6) and for K0 the range of values deter￾mined by Seidel and Rodel ¨ 40 and Fett et al.41: 1.5–3 MPa m1/2, process zones of h = 384–1539m are found. Table 3 Values of GIC in the R curves determined for the composites A10 with a relative notch depth of 0.6 G0 (S.D.) (J/m2) G∞ (S.D.) (J/m2) JIC (S.D.) (J/m2) A10-1450 21.4 (1.6) 58.5 (2.7) 45.9 (3.3) A10-1550 29.4 (2.1) 56.9 (2.9) 53.1 (2.3) G0: initial values and G∞: steady state values. The JIC values correspond to Table 2. S.D.: standard deviation. The different non-brittle mechanical parameters calculated in this work did not follow the same trend as a function of the microstructure of the composites. Toughness values from the R curves for completely developed process zones, K∞ and G∞ seem to be slightly higher for the fine-grained material, A10-1450, that presented smaller G0 and process zone width, h, but significantly higher a across which the toughness increase (KR) occurred. On the contrary, JIC was significantly higher for the coarse-grained composite, A10-1550, with larger G0 and h and smaller a. This discrepancy is due to the fact that JIC con￾stitutes a fracture criterion for materials where the toughening occurs along limited crack propagation.29 Therefore, JIC will be closer to the toughness of composite A10-1550, for which the major part (76%) of the total toughness increase (38%) occurred along one half (180 m) of the total crack growth before the steady state was reached (Fig. 6). On the contrary, significantly larger crack growth had to take place in the composite A10-1450 to reach the steady state. In this latter material, a crack growth of 230m occurred before the 76% of the total KR increase (54%) was reached (Fig. 6). The work of fracture values for the composites (Table 2) were considerably higher than those for the monophase aluminas in agreement with the toughening mechanisms described, and sim￾ilar to those determined in dense alumina materials with a mean grain size of 25 m (∼=50 J/m2) 50 and in porous aluminas with a mean grain size of 15–20m (∼=40 J/m2).51 In those coarse￾grained aluminas the main toughening mechanisms identified, crack bridging and crack branching,52 were related to extensive grain boundary microcracking, at the expense of lower strength values (∼=300 MPa, 25% inferior to strength for fine-grained alumina).53 The same drawback strength-microcracking occurs in the composites studied here, that present lower strengths than the previously studied composite material (28–36% lower, Table 1). Moreover, work of fracture values substantially exceeded those of both energy–fracture toughness that resulted from the extension of the principles of linear elastic fracture to situations where the inelastic deformation occurs prior to fracture, JIC and G∞, (γWOF > JIC/2, G∞/2, Tables 2 and 3). This fact suggests that there are additional non-linear phenomena occurring during fracture of the composites that significantly contribute to the total energy consumption but not to resistance to crack initiation and that can be envisaged as follows. The efficiency of microcraks in the toughening of the studied composites will be the result of a compromise between their crack shielding and weakening effects. For sufficient levels of microcrack density, microcracks could coalesce and link together with the crack front, leading to