S. Bueno et al. /Journal of the European Ceramic Society 28 (2008)1961-1971 which is the preferred cleavage plane in alumina monocrystals at 43. Toughening mechanisms in the composites room temperature. The fracture energies generally determined for polycrystals are higher than those for monocrystals due to the Toughness increase with crack extension(AKR =54% and contribution of intergranular fracture, in the same way as crack- 38% for A10-1450 and A10-1550, respectively, Fig. 6)was tip toughness values in polycrystals are higher than those of observed for both composites. Moreover, they presented inter the easy cleavage planes, as discussed above. Nevertheless, the granular fracture(Fig 5b), revealing the micrometric aluminium coincidence between the values of 2ywoF and Gic for this mate- titanate grains located mainly at alumina triple points and grain rial(A-1450)would reveal the absence of significant crack-size boundaries, and numerous microcracks, perpendicular to the dependent toughening phenomena fracture surfaces, surrounding alumina and aluminium titanate For the coarser alumina(Ga=5.5 um, Table 1), the work of grains. Microcracks were not observed at the polished sur- fracture values (20J/m", Table 2)also coincide with those faces and the materials presented reversible thermal expansion ported for aluminas with similar microstructures.3 In this behaviour, 45,46 thus, the microcracks observed at the fracture material, the fact that ywoF was slightly higher than Gic reveals surfaces should be formed during the fracture process. From the action of limited additional energy consuming processes due the fracture surfaces of fine-grained materials it is not possible to the interaction of the growing crack with the microstructure. to ascertain whether such features are due to pull out of grains Taking into account the materials properties and the stiffness of that have acted as bridges during the fracture process or actual the testing device, the selected loading geometry imen and crack sizes should lead to unstable crack growth for Bridges are easily differentiated along the path of indenta the four studied materials, according to Bar-On et al.39 Nev- tion cracks such as those shown in Fig. 5d in which relatively ertheless, semi-stable tests were obtained for the aluminas and small grains of alumina(2-3 um) and aluminium titanate were easier to obtain for A-1550 specimens than for the finer =1-2 um) that acted as frictional sliding bridges during frac- ained alumina. However, it was not possible to build the R- ture are observed. In general, crack bridging efficiency, in curve because it is not possible to calculate compliance at each terms of AKR and of the crack extension along which AKl unloading point for semi-unstable tests ases with bridge size. Conversely, the smaller For the composites, the brittle fracture parameters, KIc and KR increase and crack extension for the material with the GIc, were similar, and higher than those corresponding to the largest grain sizes(AKR=1.1 MPa malong a crack extension alumina with similar grain size(A-1450, Table2). On the con-△a≈360mand△KR1.5 MPam2 along△a全480m trary, the ratio JIC/GIC (Table 2)was slightly higher than l for the for A10-1550 and A10-1450, respectively, Fig. 6) suggest that composite sintered at 1450C and increased for that fabricated the main toughening mechanism was not bridging 1550C. Moreover, they presented rising R-curve behaviour Post-fracture examinations of the zones that surrounded the (Fig. 6)with KR increasing to the steady state value(Koo)over notch and crack-tip regions in tested bend bars(Fig. 7) showed a crack extension, Aa, of about 360 and 480 um for A10-1550 irregular shaped damaged zones(Fig. 7a)of widths =15-30 and A10-1450, respectively. Therefore, the fracture behaviour and 20-40 um for the composites sintered at 1450 and 1550C of these materials would be more adequately described by the respectively. Detailed observations of these damaged zones non-brittle fracture parameters, Jic and R curve than by Kic or revealed microcracking along grain boundaries( Fig. 7b). These GIC. Extrapolation from the R curves(Fig. 6)showed crack-tip observations demonstrate that microcracking acted as toughen- toughness, Ko, values similar for both composites and of the ing mechanism during fracture of the composites. In general same order as that of the alumina material with similar grain microcracking is associated with low resistance of the materials size(A-1450, Table 2) to the propagation of small defects and, therefore, low strength values. Data in Table I clearly show how the composites devel oped here present significantly lower strength values than the A10-1450 one fabricated from already reacted aluminium titanate. This material, in which no titanium segregation occurred at the alu- Al-1550 mina grain boundaries, presented mostly transgranular fracture and no microcracking, crack bridging being the only toughening mechanism observed. According to the microcracking model by Evans and Faber+ 0 and the work by Lutz et al. +o it is possible to relate the width of he microcracked zones and the value of the crack-tip toughness Frontal Ko, with the critical stress for microcrack initiation according to Extended zone △a[pm h ×(1+u)2× ad-displacement curves of notched specimens with a relative notch depth of 0.6 and considering the onset of crack propagation at the point where the where Ko can be taken as the constant matrix crack-tip intensity on-linear behaviour starts in the load-displacement curves factor, equal to Kic for the monophase alumina with similar1968 S. Bueno et al. / Journal of the European Ceramic Society 28 (2008) 1961–1971 which is the preferred cleavage plane in alumina monocrystals at room temperature.44 The fracture energies generally determined for polycrystals are higher than those for monocrystals due to the contribution of intergranular fracture, in the same way as crack￾tip toughness values in polycrystals are higher than those of the easy cleavage planes, as discussed above. Nevertheless, the coincidence between the values of 2γWOF and GIC for this mate￾rial (A-1450) would reveal the absence of significant crack-size dependent toughening phenomena. For the coarser alumina (GA = 5.5m, Table 1), the work of fracture values (∼=20 J/m2, Table 2) also coincide with those reported for aluminas with similar microstructures.38 In this material, the fact that γWOF was slightly higher than GIC reveals the action of limited additional energy consuming processes due to the interaction of the growing crack with the microstructure. Taking into account the materials properties and the stiffness of the testing device, the selected loading geometry and the spec￾imen and crack sizes should lead to unstable crack growth for the four studied materials, according to Bar-On et al.39 Nev￾ertheless, semi-stable tests were obtained for the aluminas and were easier to obtain for A-1550 specimens than for the finer grained alumina. However, it was not possible to build the R￾curve because it is not possible to calculate compliance at each unloading point for semi-unstable tests. For the composites, the brittle fracture parameters, KIC and GIC, were similar, and higher than those corresponding to the alumina with similar grain size (A-1450, Table 2). On the con￾trary, the ratio JIC/GIC (Table 2) was slightly higher than 1 for the composite sintered at 1450 ◦C and increased for that fabricated at 1550 ◦C. Moreover, they presented rising R-curve behaviour (Fig. 6) with KR increasing to the steady state value (K∞) over a crack extension, a, of about 360 and 480 m for A10-1550 and A10-1450, respectively. Therefore, the fracture behaviour of these materials would be more adequately described by the non-brittle fracture parameters, JIC and R curve than by KIC or GIC. Extrapolation from the R curves (Fig. 6) showed crack-tip toughness, K0, values similar for both composites and of the same order as that of the alumina material with similar grain size (A-1450, Table 2). Fig. 6. Characteristic R curves determined for the composites from the load–displacement curves of notched specimens with a relative notch depth of 0.6 and considering the onset of crack propagation at the point where the non-linear behaviour starts in the load–displacement curves. 4.3. Toughening mechanisms in the composites Toughness increase with crack extension (KR ∼= 54% and 38% for A10-1450 and A10-1550, respectively, Fig. 6) was observed for both composites. Moreover, they presented inter￾granular fracture (Fig. 5b), revealing the micrometric aluminium titanate grains located mainly at alumina triple points and grain boundaries, and numerous microcracks, perpendicular to the fracture surfaces, surrounding alumina and aluminium titanate grains. Microcracks were not observed at the polished sur￾faces and the materials presented reversible thermal expansion behaviour,45,46 thus, the microcracks observed at the fracture surfaces should be formed during the fracture process. From the fracture surfaces of fine-grained materials it is not possible to ascertain whether such features are due to pull out of grains that have acted as bridges during the fracture process or actual microcracks developed during fracture. Bridges are easily differentiated along the path of indenta￾tion cracks such as those shown in Fig. 5d in which relatively small grains of alumina (∼=2–3m) and aluminium titanate (∼=1–2m) that acted as frictional sliding bridges during frac￾ture are observed. In general, crack bridging efficiency, in terms of KR and of the crack extension along which KR occurs, increases with bridge size.4–5 Conversely, the smaller KR increase and crack extension for the material with the largest grain sizes (KR ∼= 1.1 MPa m1/2 along a crack extension a ∼= 360m and KR ∼= 1.5 MPa m1/2 along a ∼= 480m, for A10-1550 and A10-1450, respectively, Fig. 6) suggest that the main toughening mechanism was not bridging. Post-fracture examinations of the zones that surrounded the notch and crack-tip regions in tested bend bars (Fig. 7) showed irregular shaped damaged zones (Fig. 7a) of widths ∼=15–30 and 20–40m for the composites sintered at 1450 and 1550 ◦C, respectively. Detailed observations of these damaged zones revealed microcracking along grain boundaries (Fig. 7b). These observations demonstrate that microcracking acted as toughen￾ing mechanism during fracture of the composites. In general, microcracking is associated with low resistance of the materials to the propagation of small defects and, therefore, low strength values. Data in Table 1 clearly show how the composites devel￾oped here present significantly lower strength values than the one fabricated from already reacted aluminium titanate.8 This material, in which no titanium segregation occurred at the alu￾mina grain boundaries, presented mostly transgranular fracture and no microcracking, crack bridging being the only toughening mechanism observed.8 According to the microcracking model by Evans and Faber47 and the work by Lutz et al.48 it is possible to relate the width of the microcracked zones and the value of the crack-tip toughness, K0, with the critical stress for microcrack initiation according to Eq. (6): h = √3 12π × (1 + ν) 2 × K0 σc 2 (6) where K0 can be taken as the constant matrix crack-tip intensity factor, equal to KIC for the monophase alumina with similar