Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 shown in Fig. 5, that is, monolithic Si3N4 showed cata- ticles in the as-hot pressed material are already micro- strophic failure (not shown), while FM showed cracked. Hence, thermal shock damage seems to be noncatastrophic failure regardless of temperature d absorbed within the bn cell boundaries which would ference. Furthermore, with the increase in temperature decrease the cell boundary fracture resistance, enabling difference, more extensive crack interactions were easier crack deflection and higher WOF. The specimens observed. The increase in apparent stress after fist drop shocked with the highest temperature difference implies the midplane shear stress after thermal shock (AT=1400C)had the most extensive crack delamina- The apparent WOF of the FM specimens increased tions, as shown in Fig. 7(D). This remarkable increase remarkably after thermal shock test, as shown in Fig. 6. in crack delamination is attributed to not only pre- The WOF is higher if there is a large retained load once ferential crack propagation caused by thermal stress but fracture begins, and strongly depends upon the extent of also oxidized damage layer during exposure to air crack interactions and delamination. The thermally The change of surface morphology after thermal shocked specimens exhibited higher retained strength shock test is shown in Fig. 8. Up to the temperature and extensive crack delamination. Thermal shock difference of 1200oC, the surface was not damaged damage seems to be absorbed within the bn cell (not shown). However, with temperature difference of boundaries, which would decrease the cell boundary 1400C, the surface(both BN cell boundary and fracture resistance, enabling easier crack deflection and Si3N4 cell) was damaged to some extent due to the higher WOF oxidation 3.2.3. Crack propagation 3.2.4. Load-bearing capacit The increased crack interactions in the thermally The thermal stress developed on surface and interface shocked sample, manifested by crack path, are clearly of Si3 N4 and bn after the thermal shock affected the shown in Fig. 7A-D. After thermal shock, crack inter- flexural response of FM upon subsequent room tem- actions (crack delamination and crack deflection) perature testing, as shown in Fig. 9. The retained occurred more extensively compared to the specimen strength after the fist drop(Ist drop/lst peak) was not before the thermal shock(Fig. 3). Pronounced crack basically changed within the range between 40% and delamination occurred by the thermal shock of 800C 55%, meaning the excellent load-bearing capacity for [Fig. 7(A)], and further long crack delamination was actual applications. However, the normalized maximum observed after the thermal shock of 1200C [Fig. 7(C). strength (2nd peak/lst peak) increased after thermal The tendency for crack delamination in FM ceramics is shock test. This result means that the first peak was influenced by the interfacial crack resistance of the BN- caused by the crack initiation on the surface; thus, the ontaining cell boundary. 7. The increase in WOF after surface was slightly weakened due to the thermal stress thermal shock suggests that thermal shock reduces the Furthermore, the thermal stress developed in interface interfacial crack resistance of the cell boundary, which of Si, N4 and bn promoted extensive crack interactions is a composite of boron nitride and glass. The Bn par resulting in increased woF. A△T=800c (B)△T=1000c 0号 (c)△T=1200c D)△T=1400c 200400600800100012001400 Crosshead Displacement I mm] Temperature Difference[Cl lexural responses of fibrous monolithic Si3N4/BN Fig. 6. Work-of-fracture (woF) of fibrous monolithic Si3 N4/BN ermal shock with temperature difference(AT) of(A)80 800oC ceramic thermal shock with temperature difference (AT) oC,(C)1200C, and(D)1400C. All samples exhibited Fibrous monolith exhibited significant woF due to extensive crackshown in Fig. 5, that is, monolithic Si3N4 showed cata￾strophic failure (not shown), while FM showed noncatastrophic failure regardless of temperature dif￾ference. Furthermore, with the increase in temperature difference, more extensive crack interactions were observed. The increase in apparent stress after fist drop implies the midplane shear stress after thermal shock. The apparent WOF of the FM specimens increased remarkably after thermal shock test, as shown in Fig. 6. The WOF is higher if there is a large retained load once fracture begins, and strongly depends upon the extent of crack interactions and delamination. The thermally shocked specimens exhibited higher retained strength and extensive crack delamination. Thermal shock damage seems to be absorbed within the BN cell boundaries, which would decrease the cell boundary fracture resistance, enabling easier crack deflection and higher WOF. 3.2.3. Crack propagation The increased crack interactions in the thermally shocked sample, manifested by crack path, are clearly shown in Fig. 7A–D. After thermal shock, crack inter￾actions (crack delamination and crack deflection) occurred more extensively compared to the specimen before the thermal shock (Fig. 3). Pronounced crack delamination occurred by the thermal shock of 800
C [Fig. 7(A)], and further long crack delamination was observed after the thermal shock of 1200
C [Fig. 7 (C)]. The tendency for crack delamination in FM ceramics is influenced by the interfacial crack resistance of the BN￾containing cell boundary.7,8 The increase in WOF after thermal shock suggests that thermal shock reduces the interfacial crack resistance of the cell boundary, which is a composite of boron nitride and glass. The BN par￾ticles in the as-hot pressed material are already micro￾cracked.1 Hence, thermal shock damage seems to be absorbed within the BN cell boundaries, which would decrease the cell boundary fracture resistance, enabling easier crack deflection and higher WOF. The specimens shocked with the highest temperature difference (
T=1400
C) had the most extensive crack delamina￾tions, as shown in Fig. 7(D). This remarkable increase in crack delamination is attributed to not only pre￾ferential crack propagation caused by thermal stress but also oxidized damage layer during exposure to air. The change of surface morphology after thermal shock test is shown in Fig. 8. Up to the temperature difference of 1200
C, the surface was not damaged (not shown). However, with temperature difference of 1400
C, the surface (both BN cell boundary and Si3N4 cell) was damaged to some extent due to the oxidation. 3.2.4. Load-bearing capacity The thermal stress developed on surface and interface of Si3N4 and BN after the thermal shock affected the flexural response of FM upon subsequent room tem￾perature testing, as shown in Fig. 9. The retained strength after the fist drop (1st drop/1st peak) was not basically changed within the range between 40% and 55%, meaning the excellent load-bearing capacity for actual applications. However, the normalized maximum strength (2nd peak/1st peak) increased after thermal shock test. This result means that the first peak was caused by the crack initiation on the surface; thus, the surface was slightly weakened due to the thermal stress. Furthermore, the thermal stress developed in interface of Si3N4 and BN promoted extensive crack interactions, resulting in increased WOF. Fig. 5. Flexural responses of fibrous monolithic Si3N4/BN ceramic after thermal shock with temperature difference (
T) of (A) 800
C, (B) 1000
C, (C) 1200
C, and (D) 1400
C. All samples exhibited graceful fractures. Fig. 6. Work-of-fracture (WOF) of fibrous monolithic Si3N4/BN ceramic after thermal shock with temperature difference (
T). Fibrous monolith exhibited significant WOF due to extensive crack interactions. 2342 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347