Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 2341 because of catastrophic failure [Fig. 2(A)]. On the other 3. 2. Strength retention hand, FM exhibited noncatastrophic failure due to its The thermal shock resistance was observed by mea- unique architecture, comprised of strong Si3 N4 cell and suring the retention of the flexural strength after ther weak BN cell boundary, resulting in high apparent mal shock test, as shown in Fig. 4. For monolithic WOF [Fig. 2(B). Moreover, the apparent strength Si3 N4, the traditional thermal shock behavior of brittle retention after the first failure was above 50% of origi material was observed, that is, the flexural strength nal strength, showing the noncatastrophic. This non- decreased rapidly after thermal shock with temperature catastrophic nature was attributed to the extensive difference of 1000C [Fig. 4(A)]. However, the flexural crack interactions, such as crack delaminations and strength of fM after thermal shock test was not chan crack deflections, as shown in Fig. 3. For fibrous ged much [Fig. 4(B)l, showing the excellent thermal monoliths, the crack propagates through the weak cell shock resistance. Moreover, there was no critical tem- boundaries to reduce the applied stress. Similar crack perature (AT), at which the strength decreases propagations have been observed in many different catastrophically, up to 1400C. kinds of fibrous monoliths 1-5 3. 22. fracture behavior 3.2. Mechanical properties after thermal shock lithic Si N4 and FM were not basically changed. After thermal shock. the fracture behaviors of mor When a material(monolithic Si3 N4 or FM)is sub jected to a rapid decrease in temperature(AT), the sur- face of the component is placed under tension and the interior under compression. If the tensile stress devel oped on the surface exceeds the strength of the material the cracks are generated, leading to a rapid drop in flexural strength 11-15 Fig. 3. Optical photograph of crack propagation of the fibrous monolithic Si]N4/ BN ceramic after flexural testing. Extensive crack (B)Fibrous Monolith interactions. such as crack delamination and crack deflection. were Crosshead Displacement I mm I ≈600. Fig. 2. Flexural response of (A) monolithic Si3 N4 and (B)fibrous (B)Fibrous Monolith monolithic Si3N4/BN ceramic before thermal shock test. Monolithic Si3N4 showed brittle fracture, while fibrous monolith showed graceful racture due to unique architecture. Note, retained apparent stress after first drop is above 50%(B) Table 0600800100012001400 Summarized mechanical properties of monolithic Si3N4 and fibrous monolithic Si3N4/BN ceramic Temperature Difference[C I p(g/cc) E(GPa) MOR (MPa) WoF (kJ/m) Fig. 4. Flexural strength of (A)monolithic Si3 N4 and (B)fibrous 3N4/bn ceramic after thermal shock with Monolithic si3N43.27±0.1318±4832±4 differene Flexural strength of monolithic Si3N Fibrous monolith3.09±0.1276±3416±34 5.94±1.34 strophically after thermal shock with AT=1000C; howe monolith showed negligible decrease in flexural strength.because of catastrophic failure [Fig. 2(A)]. On the other hand, FM exhibited noncatastrophic failure due to its unique architecture, comprised of strong Si3N4 cell and weak BN cell boundary, resulting in high apparent WOF [Fig. 2(B)]. Moreover, the apparent strength retention after the first failure was above 50% of origi￾nal strength, showing the noncatastrophic. This non￾catastrophic nature was attributed to the extensive crack interactions, such as crack delaminations and crack deflections, as shown in Fig. 3. For fibrous monoliths, the crack propagates through the weak cell boundaries to reduce the applied stress. Similar crack propagations have been observed in many different kinds of fibrous monoliths.15 3.2. Mechanical properties after thermal shock When a material (monolithic Si3N4 or FM) is sub￾jected to a rapid decrease in temperature (
T), the sur￾face of the component is placed under tension and the interior under compression. If the tensile stress devel￾oped on the surface exceeds the strength of the material, the cracks are generated, leading to a rapid drop in flexural strength.1115 3.2.1. Strength retention The thermal shock resistance was observed by mea￾suring the retention of the flexural strength after ther￾mal shock test, as shown in Fig. 4. For monolithic Si3N4, the traditional thermal shock behavior of brittle material was observed, that is, the flexural strength decreased rapidly after thermal shock with temperature difference of 1000
C [Fig. 4(A)]. However, the flexural strength of FM after thermal shock test was not chan￾ged much [Fig. 4(B)], showing the excellent thermal shock resistance. Moreover, there was no critical tem￾perature (
Tc), at which the strength decreases catastrophically, up to 1400
C. 3.2.2. Fracture behavior After thermal shock, the fracture behaviors of mono￾lithic Si3N4 and FM were not basically changed, as Table 1 Summarized mechanical properties of monolithic Si3N4 and fibrous monolithic Si3N4/BN ceramic Samples
(g/cc) E (GPa) MOR (MPa) WOF (kJ/m2 ) Monolithic Si3N4 3.270.1 3184 83246 Negligible Fibrous monolith 3.090.1 2763 41634 5.941.34 Fig. 2. Flexural response of (A) monolithic Si3N4 and (B) fibrous monolithic Si3N4/BN ceramic before thermal shock test. Monolithic Si3N4 showed brittle fracture, while fibrous monolith showed graceful fracture due to unique architecture. Note, retained apparent stress after first drop is above 50% (B). Fig. 3. Optical photograph of crack propagation of the fibrous monolithic Si3N4/BN ceramic after flexural testing. Extensive crack interactions, such as crack delamination and crack deflection, were observed. Fig. 4. Flexural strength of (A) monolithic Si3N4 and (B) fibrous monolithic Si3N4/BN ceramic after thermal shock with temperature difference (
T). Flexural strength of monolithic Si3N4 reduced cata￾strophically after thermal shock with
T=1000
C; however, fibrous monolith showed negligible decrease in flexural strength. Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347 2341