ar de ipez et al. Journal of the European Ceramic Society 21(2001)245-250 BNperp samples. Apparent stresses were essentially A summary of all of the creep tests that were per- independent of temperature and strain rate. SEM formed on the FM and Si3N4 samples is shown as a log- revealed, however, that the deformation of the bn log plot in Fig. 4. The data points in this figure corre- samples was dominated by sequential fracture(Fig 3). spond to the range in which apparent steady states were As has also been observed in creep testing of high-tem- measured. below the minimum stress for each material perature Bi-based cuprate superconductors, which con- no deformation was recor above the maximum sist of platelike grains, apparent steady-state responses values, the high stresses led to accelerating deformation can be obtained, but deformation actually proceeds by (i.e. tertiary creep). For comparison, the Bn results bending of the grains with concomitant microcracking have also been included. Over the experimental tem- between grains. The finding of creep fracture, rather perature range, BN is substantially softer than the Si3N4 than steady-state creep, in these CSr tests was not sur- and FM samples and does not deform in the steady prising. Torsional creep experiments achieved steady state, as noted above state deformation only at temperatures >2000 C 17 The The FMperp samples did not undergo significant fact that the fracture stress was approximately indepen- plastic deformation under the testing conditions. At dent of temperature over the narrow range used in these 1400C, only two data points were recorded, at 12 and tests is expected 40 O0A44 10 0LA 10 1000 6.6 7 7,4 Fig. 4. CL tests on Si3N4 monoliths and FMs. The symbols are: O Fig. 2. Apparent steady-state stresses from CSR tests on extruded SiN 4-1500°C;△, AMpara-1400°C;▲ BNpara AMpara-1500°c口, FMperp-1400°C. The slope of the line is I BNperp,=2-10-6s-1;▲ BNperp,g=1×10-6s-1 he shaded region indicates range of BN response. (a) Jo JI pm Fig 3. SEM photomicrographs, showing fracture of BN from deformation:(a)low-magnification view and (b) high-magnification view, in which cleavage and bending of bN graBNperp samples. Apparent stresses were essentially independent of temperature and strain rate. SEM revealed, however, that the deformation of the BN samples was dominated by sequential fracture (Fig. 3). As has also been observed in creep testing of high-tem￾perature Bi-based cuprate superconductors, which con￾sist of platelike grains, apparent steady-state responses can be obtained, but deformation actually proceeds by bending of the grains with concomitant microcracking between grains.16 The ®nding of creep fracture, rather than steady-state creep, in these CSR tests was not sur￾prising. Torsional creep experiments achieved steady￾state deformation only at temperatures >2000C.17 The fact that the fracture stress was approximately indepen￾dent of temperature over the narrow range used in these tests is expected. A summary of all of the creep tests that were per￾formed on the FM and Si3N4 samples is shown as a log± log plot in Fig. 4. The data points in this ®gure corre￾spond to the range in which apparent steady states were measured. Below the minimum stress for each material, no deformation was recorded; above the maximum values, the high stresses led to accelerating deformation (i.e. tertiary creep). For comparison, the BN results have also been included. Over the experimental tem￾perature range, BN is substantially softer than the Si3N4 and FM samples and does not deform in the steady￾state, as noted above. The FMperp samples did not undergo signi®cant plastic deformation under the testing conditions. At 1400C, only two data points were recorded, at 12 and Fig. 2. Apparent steady-state stresses from CSR tests on extruded BN: *, BNpara, " : =210ÿ6 sÿ1 ; ~, BNpara, " : =110ÿ6 sÿ1 ; *, BNperp, " : =2ÿ10ÿ6 sÿ1 ; ~ BNperp, " : =110ÿ6 sÿ1 . Fig. 3. SEM photomicrographs, showing fracture of BN from deformation: (a) low-magni®cation view and (b) high-magni®cation view, in which cleavage and bending of BN grains are evident. Fig. 4. CL tests on Si3N4 monoliths and FMs. The symbols are: *, Si3N4 Ð 1400C; *, Si3N4 Ð 1500C; ~, FMpara Ð 1400C; ~, FMpara Ð 1500C; &, FMperp Ð 1400C. The slope of the line is 1; the shaded region indicates range of BN response. A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250 247