Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 1404)·2° peak/1"peak 0000 }- 100um 4 micrograph of fibrous monolithic Si3N4/BN ceramic Temperature Difference[CI al shock with temperature difference (An of 1400C. re at temperature up to 1200C, the surface was not Fig. 9. Retained apparent strength of fibrous monolithic Si3 N4/BN hile after exposure at 1400 oC, the surface layer was ceramics; (A)2nd peak/lst peak and(B)Ist drop/lst peak from load- damaged by the oxidation of both Bn cell boundaries and Si3 N4 cell deflection curve. After thermal shock, the retained strength of 2nd material peak/lst peak increased, implying that the fracture initiated from sur- face defects generated by thermal shock. Note, the retained strengths after first drop(b)of the all samples are higher than 40%, suggesting excellent load-bearing capacity ratio, respectively. AT is the temperature difference between exposure and water temperature 4.2. Thermal shock resistance parameter The normalized thermal stresses(oN Ts), that is, ther- mal stresses with longitudinal and transverse direction The conditions for crack and propaga were divided by that of monolithic Si3 N4 are estimated, have been extensively analysed by hasselman et al. as described in Table 2. Lower tensile stresses were Opposing property requirements prevail, depending on developed on the surface of FM samples with 80 and whether the material is required to be resistant to crack 55% for longitudinal and transverse direction, respec- initiation(for which high strength and low stifness are tively. Considering temperature difference of 1200oC, essential) or resistant to strength degradation following ne tensile stress of 2000 MPa was developed on the a severe thermal shock (in which case low strength and surface of monolithic Si3 N4; this value is high enough to high stifness are beneficial). We consider the crack develop the cracks on the surface, resulting in cata- initiation parameter R and crack propagation para strophic drop in flexural strength [Fig. 4(A)]. However, meter R estimated for monolithic Si3 N4 and FM actual thermal stress needs consideration of the heat sample consisting only longitudinal direction despite transfer depending on heat transfer coefficien there was anisotropy in thermal stress. These para- quenching medium, the thermal conductivity(k)and the meters can be expressed as characteristic dimension of the sample. Moreover, the r=kor(I-v)/(Ea) value from Eq. (1)only suggests the condition for crack nitiation which is critical for brittle material (i.e. monolithic Si3 N4) and not for crack propagation, which more important for tough material (i.e, FM sample) R"=KRc/(G2·(-) Therefore, a new parameter for describing thermal where, or is the fracture strength and Kic is the tough shock resistance should be considered ness and k is the thermal conductivity of the material The fracture strength of monolithic Si3N4 is almost twice that of FM sample(Table 1). The thermal con- ized thermal stress (ON. ductivities are 37 and 53 W/m K for monolithic Si3N4 eveloped on the surface of monolithic Si3 N4 and fibrous monolithic and FM sample with longitudinal direction. The calcu lated crack initiation parameter R(17.7 kw/m)of monolithic Si3 n4 is only slightly larger than that (15.8 (GPa) (10-C N,Is kW/) of FM sample, implying the condition for crack initiations are almost the same. Therefore, the large 318 difference in behavior can not be explained by resistance Fibrous monolith(longitudinal) 276 0.25 3.8 0.80 Fibrous monolith( transverse) 270.126.7 0.55 to crack initiation. The toughness of FM sample twice that of monolithic Si3N4.23 The calculated crackratio, respectively.
T is the temperature difference between exposure and water temperature. The normalized thermal stresses (N,TS), that is, ther￾mal stresses with longitudinal and transverse direction were divided by that of monolithic Si3N4, are estimated, as described in Table 2. Lower tensile stresses were developed on the surface of FM samples with 80 and 55% for longitudinal and transverse direction, respec￾tively. Considering temperature difference of 1200
C, the tensile stress of 2000 MPa was developed on the surface of monolithic Si3N4; this value is high enough to develop the cracks on the surface, resulting in cata￾strophic drop in flexural strength [Fig. 4(A)]. However, actual thermal stress needs consideration of the heat transfer depending on heat transfer coefficient of the quenching medium, the thermal conductivity (k) and the characteristic dimension of the sample. Moreover, the value from Eq. (1) only suggests the condition for crack initiation which is critical for brittle material (i.e., monolithic Si3N4) and not for crack propagation, which is more important for tough material (i.e., FM sample). Therefore, a new parameter for describing thermal shock resistance should be considered. 4.2. Thermal shock resistance parameter The conditions for crack initiation and propagation have been extensively analysed by Hasselman et al.11,12 Opposing property requirements prevail, depending on whether the material is required to be resistant to crack initiation (for which high strength and low stiffness are essential) or resistant to strength degradation following a severe thermal shock (in which case low strength and high stiffness are beneficial). We consider the crack initiation parameter R0 and crack propagation para￾meter R0000 estimated for monolithic Si3N4 and FM sample consisting only longitudinal direction despite there was anisotropy in thermal stress. These para￾meters can be expressed as R0 ¼ k fð Þ 1  =ðÞ ð E 2Þ R0000 ¼ K2 IC= 2 f ð Þ 1-
ð3Þ where, f is the fracture strength and KIC is the tough￾ness and k is the thermal conductivity of the material. The fracture strength of monolithic Si3N4 is almost twice that of FM sample (Table 1). The thermal con￾ductivities are 37 and 53 W/m K for monolithic Si3N4 and FM sample with longitudinal direction. The calcu￾lated crack initiation parameter R0 (17.7 kW/m) of monolithic Si3N4 is only slightly larger than that (15.8 kW/m) of FM sample, implying the condition for crack initiations are almost the same. Therefore, the large difference in behavior can not be explained by resistance to crack initiation. The toughness of FM sample was twice that of monolithic Si3N4. 23 The calculated crack Table 2 The values for calculating the normalized thermal stress (N,TS) developed on the surface of monolithic Si3N4 and fibrous monolithic Si3N4/BN ceramic Samples E (GPa)  (106 /
C) N,TS Monolithic Si3N4 318 0.27 4 1 Fibrous monolith (longitudinal) 276 0.25 3.8 0.80 Fibrous monolith (transverse) 127 0.12 6.7 0.55 Fig. 8. SEM micrograph of fibrous monolithic Si3N4/BN ceramic after thermal shock with temperature difference (
T) of 1400
C. After exposure at temperature up to 1200
C, the surface was not damaged, while after exposure at 1400
C, the surface layer was damaged by the oxidation of both BN cell boundaries and Si3N4 cell material. Fig. 9. Retained apparent strength of fibrous monolithic Si3N4/BN ceramics; (A) 2nd peak/1st peak and (B) 1st drop /1st peak from load– deflection curve. After thermal shock, the retained strength of 2nd peak/1st peak increased, implying that the fracture initiated from sur￾face defects generated by thermal shock. Note, the retained strengths after first drop (B) of the all samples are higher than 40%, suggesting excellent load-bearing capacity. 2344 Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347