G.A. Gogotsi/ Ceramics international 29(2003)777-784 grain sizes( 4 um for Si3 N4 and >I um for Y-PSZ) as contradict the results of sintered SiC whisker-reinforced it was mentioned elsewhere [19]. It should be empha- Si3N4[18] tests. A similar tendency is also typical of Sic sized that only for notch root radii less than 5-7 um [7] ceramics [191 e Kle values for Y-PSZ ceramics agree with SEPb data The examination of the specimens fractured in the (similar behavior is also typical of fine-grained alumina tests revealed a fracture crack that propagated from the [20]. A similar conclusion follows from [6], where an points where "additional"stress concentrators were attempt was made to relate the V-notch radius values present. This observation also confirms the assumption required for the correct determination of Kle values to that the fracture of a loaded ceramic specimen starts the averaged values of this parameter obtained in the from a small crack ahead of a machined notch root [19] RRFT97 studies. As follows from Fig 3a, the decrease In this context, it is interesting to mention that K in Klc values for Si, N4 ceramics occurs with a decrease magnitudes are influenced by the sharpness of a notch in notch radii down to about 30 um, which does not root rather than by its shape [7] W 250 150 l00 Raman shift(cm") Raman shift(cm") Fig. 6. Raman spectra of Y-PSZ specimens fractured by the SEVNB method (a)and by indentation(b): nonfractured specimen surface (1), dia- mond saw surface (2), fracture surface(3), razor blade surface(4), indentation edge(5), and indentation center(6) Table 4 Kic values for ceramics obtained by SEVNB and SENB methods(MPa m/2) Test method Index p Brittleness measure, X SEVNB SENB 5.14±0.29 1.18±0 SiC+ 50%ZrB,+ 10%B, C 3.52±0.08 6.24士 0.564 5.17±0.06 9.12±0.29 Three 0.567 SSiC 4.42±0.23 Si3 N4+ 30%SiC+ 3%Mgo 2.49±0.16 0.920 Mg- PSZ (TSgrade) 944±0.1 10.2±0.27 0.925 s& coo Three 0.25 The width Table 5 KIe values for single crystals obtained by SEVNB and SENB methods(MPa m) Single crystals Peculiarity Test method Index p modulus(GPa) measure, X SENB SEVN Zirconia Partially stabilized (3%Y203 9.33±0.95 0.33±2.171 Alumina Specimen axis 45 to optical axis of crystal 403 2.31±0.34 0.94 Specimen axis 90 to optical axis of cryst 410 3.19±0.53 85±0.50 1.12grain sizes (4 mm for Si3N4 and >1 mm for Y-PSZ) as it was mentioned elsewhere [19]. It should be empha￾sized that only for notch root radii less than 5–7 mm [7] the KIc values for Y-PSZ ceramics agree with SEPB data (similar behavior is also typical of fine-grained alumina [20]. A similar conclusion follows from [6], where an attempt was made to relate the V-notch radius values required for the correct determination of KIc values to the averaged values of this parameter obtained in the RRFT’97 studies. As follows from Fig. 3a, the decrease in KIc values for Si3N4 ceramics occurs with a decrease in notch radii down to about 30 mm, which does not contradict the results of sintered SiC whisker-reinforced Si3N4 [18] tests. A similar tendency is also typical of SiC ceramics [19]. The examination of the specimens fractured in the tests revealed a fracture crack that propagated from the points where ‘‘additional’’ stress concentrators were present. This observation also confirms the assumption that the fracture of a loaded ceramic specimen starts from a small crack ahead of a machined notch root [19]. In this context, it is interesting to mention that KIc magnitudes are influenced by the sharpness of a notch root rather than by its shape [7]. Fig. 6. Raman spectra of Y-PSZ specimens fractured by the SEVNB method (a) and by indentation (b): nonfractured specimen surface (1), dia￾mond saw surface (2), fracture surface (3), razor blade surface (4), indentation edge (5), and indentation center (6). Table 4 KIc values for ceramics obtained by SEVNB and SENB methods (MPa m1/2) Material Test method Flexure (points) Index ’ Brittleness measure,  SEVNB SENBa Y-PSZ 5.140.29 9.540.47 Four 0.538 1 Soda limit glass 0.660.07 1.180.09 Four 0.562 1 SiC+50%ZrB2+10%B4C 3.520.08 6.240.37 Three 0.564 1 Si3N4 5.170.06 9.120.29 Three 0.567 1 SSiC 2.610.18 4.420.23 Four 0.590 1 Si3N4+30%SiC+3%MgO 2.270.14 2.490.16 Three 0.920 0.88 Mg-PSZ (TS-grade) 9.440.12 10.20.27 Four 0.925 0.41 La8.8Ca0.2CoO3 2.2 2.2 Three 10.25 a The width of notch is 0.2–0.3 mm. Table 5 KIc values for single crystals obtained by SEVNB and SENB methods (MPa m1/2) Single crystals Peculiarity Elastic modulus (GPa) Test method Brittleness measure,  Index ’ SENB SEVNB Zirconia Partially stabilized (3% Y2O3) 245 9.330.95 10.332.17 1 0.9 Alumina Specimen axis 45
to optical axis of crystal 403 2.310.34 2.450.29 10.94 Specimen axis 90
to optical axis of crystal 410 3.190.53 2.850.50 1 1.12 G.A. Gogotsi / Ceramics International 29 (2003) 777–784 781