S. Bueno et al. / Journal of the European Ceramic Society 28(2008)1961-1971 Kic values determined for the fine grain sized alumina The classical linear fracture toughness parameters, KIC and are inside the variability for crack-tip toughness, Ko,(1.5- Gic, are adequate to characterize fracture of the fine alumina, as 3.0MPam)reported by Seidel and Rodel+0 and Fett et al. +I revealed by the coincidence between JIc and GIC(Table 2),as for a series of aluminas with grain sizes in the range of 1-20 um. occurs for perfectly linear materials. 28 On the contrary, the KiC These authors determined Ko from"in situ"crack opening dis- increase for the alumina with larger grain size was accompanied placement(COD)measurements in a SEM and attributed the by Jic being slightly higher than GIC, revealing toughening. An significant scatter of data to charging of the crack edges. Also, increase in Kic with grain size has been reported for other alumi charging might lead to observed COD at the point of fracture nas with similar microstructures.35-36However, coarse-grained and, consequently, calculated Ko values, smaller than the real materials with mean grain sizes larger than 10-20 um, where ones intergranular fracture occurs for the largest grains(50 um) act Apart from the experimental facts discussed above, differ brid ges,25.- are required for significant toughening and ences between crack-tip toughness determined for different rising R-curve behaviour. total toughness would be the sum of contributions of intergran- monophase alumina materials with a mean grain size lower lia aluminas can be attributed to differences in fracture mode. as There are no reported values of work of fracture for dens ular grain boundary fracture, cleavage across the easy fracture 5 um. Even though the work of fracture values determined in the planes, and the increase in fracture surface due to deflection. semi-stable tests reached in this work might be slightly overesti- A significant amount of transgranular fracture, independent mated, to the authors knowledge they are the lowest everreported from the grain size of the material, was reported by Seidel and for dense fine-aluminas For the alumina with the smallest aver- Rodel-o(20%), whereas only the largest grains(>5 um) pre- age grain size(GA=3.5 um, Table 1), the value determined in sented transgranular fracture in the fine-grained alumina studied this work (ywoF 2 10J/m, Table 2)is higher than the value here(Fig 5a) (rr =6J/m-)reported by wiederhorn for the rombohedral plane AT g. 5. Fractographic observations showing the mode of fracture of the materials. Scanning electron micrographs of fracture surfaces of SENVB specimens(a and b), strength specimens(c) and indentation cracks (d).(a) Monophase alumina sintered at 1450C. The largest grains(>5 um) show transgranular fracture and the mall ones show intergranular fracture.(b) Characteristic intergranular fracture in A10 composites with microcracks perpendicular to the fracture surfaces pointed by arrows. Specimen sintered at 1450C.(c) Mostly transgranular fracture in A10 composites previously obtained without no nanoparticles at grain boundaries (AlOAT).(d) Characteristic paths of indentation cracks in the Al0 composites sintered at 1450C Intact alumina(A)and aluminium titanate(At) grains were bserved along the crack traceS. Bueno et al. / Journal of the European Ceramic Society 28 (2008) 1961–1971 1967 KIC values determined for the fine grain sized alumina are inside the variability for crack-tip toughness, K0, (1.5– 3.0 MPa m1/2) reported by Seidel and Rodel ¨ 40 and Fett et al.41 for a series of aluminas with grain sizes in the range of 1–20m. These authors determined K0 from “in situ” crack opening dis￾placement (COD) measurements in a SEM and attributed the significant scatter of data to charging of the crack edges. Also, charging might lead to observed COD at the point of fracture and, consequently, calculated K0 values, smaller than the real ones. Apart from the experimental facts discussed above, differ￾ences between crack-tip toughness determined for different aluminas can be attributed to differences in fracture mode, as total toughness would be the sum of contributions of intergran￾ular grain boundary fracture, cleavage across the easy fracture planes, and the increase in fracture surface due to deflection. A significant amount of transgranular fracture, independent from the grain size of the material, was reported by Seidel and Rodel ¨ 40 (20%), whereas only the largest grains (>5 m) pre￾sented transgranular fracture in the fine-grained alumina studied here (Fig. 5a). The classical linear fracture toughness parameters, KIC and GIC, are adequate to characterize fracture of the fine alumina, as revealed by the coincidence between JIC and GIC (Table 2), as occurs for perfectly linear materials.28 On the contrary, the KIC increase for the alumina with larger grain size was accompanied by JIC being slightly higher than GIC, revealing toughening. An increase in KIC with grain size has been reported for other alumi￾nas with similar microstructures.35–36 However, coarse-grained materials with mean grain sizes larger than 10–20 m, where intergranular fracture occurs for the largest grains (>50m) act￾ing as bridges,25,42–43 are required for significant toughening and rising R-curve behaviour. There are no reported values of work of fracture for dense monophase alumina materials with a mean grain size lower than 5m. Even though the work of fracture values determined in the semi-stable tests reached in this work might be slightly overesti￾mated, to the authors knowledge they are the lowest ever reported for dense fine-aluminas. For the alumina with the smallest aver￾age grain size (GA = 3.5m, Table 1), the value determined in this work (γWOF ∼= 10 J/m2, Table 2) is higher than the value (γf ∼= 6 J/m2) reported by Wiederhorn for the rombohedral plane Fig. 5. Fractographic observations showing the mode of fracture of the materials. Scanning electron micrographs of fracture surfaces of SENVB specimens (a and b), strength specimens (c) and indentation cracks (d). (a) Monophase alumina sintered at 1450 ◦C. The largest grains (>5m) show transgranular fracture and the small ones show intergranular fracture. (b) Characteristic intergranular fracture in A10 composites with microcracks perpendicular to the fracture surfaces pointed by arrows. Specimen sintered at 1450 ◦C. (c) Mostly transgranular fracture in A10 composites previously obtained without no nanoparticles at grain boundaries (A10AT).8 (d) Characteristic paths of indentation cracks in the A10 composites sintered at 1450 ◦C. Intact alumina (A) and aluminium titanate (AT) grains were observed along the crack trace.