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Y.H. Koh et al. Journal of the European Ceramic Society 24(2004)2339-2347 2343 3 mm 3 mm (c) (D) mm 3 mm Fig. 7. Optical photographs of crack propagation of fibrous monolithic Si3 N4/BN ceramic after thermal shock with temperature difference(An)of (A)800C,(B)1000C,(C)1200C, and (D)1400C during flexural testing. All samples showed extensive crack interactions, such delaminations and crack deflections 4. Discussion aligned in the transverse direction. Therefore mechan- ical properties, such as elastic modulus, coefficient of The fracture strength of thermally shocked monolithic thermal expansion(CTE) and Poissons ratio, of FM Si3N4 is strongly dependent on the magnitude of tensile sample show anisotropy, as described in Table 2 stress developed on the surface, that is, if the tensile The elastic modulus of monolithic silicon nitride was stress exceeds its strength, the cracks are generated on sig d the transverse modulus of the the surface, resulting in catastrophic drop in flexural FM was less than half the longitudinal modulus, due to trength. However, for FM sample, the fracture the Bn, which has a small c-axis modulus. The long- strength of FM sample is less sensitive to surface flaws: itudinal thermal expansion of the Fm was slightly less therefore, the resistance to crack propagation is a more than monolithic silicon nitride, decreased by the a-axis critical factor than the resistance to crack initiation, BN, while the transverse thermal expansion of the FM which is critical for brittle monolithic Si3 N4. However, was much larger, increased by the c-axis BN. Poisson's pre-existing cracks on BN cell boundaries after hot- ratios were estimated from rule-of-mixture by taking pressing(T=1740oC)also affects the flexural response, 0.27 and 0.2 for Si3N4 and BN, respectively resulting in crack interactions. Therefore, some factors, The magnitude of thermal stress induced by the same such as the magnitude of thermal stress on surface, exposure will be different, depending on the cell align thermal shock resistance parameter and pre-existing ment (longitudinal and transverse direction). The tradi cracks tional approach to evaluate the thermal shock resistance is based on quenching the specimen from an elevated 4.1. Magnitude of thermal stress on the surface(ors) temperature into a quenching media and measuring the fracture strength of the material. Neglecting the heat Considering the structure of this uniaxial FM (Fig. 1), transfer and size effects, the maximum tensile stress it is noted that the elastic modulus and thermal expan- (ors) generated on the surface of the specimen can be sion coeficient is different in the transverse and long according itudinal directions. In addition, the hexagonal BN is ors =(Ea/(1-v).AT strongly textured, with the high stiffness/low expan sion a-axis aligned preferentially in the longitudinal where E, a and v represent the elastic modulus, the direction and the lower stiffness/higher expansion c-axis coefficient of thermal expansion(CTE)and Poissons4. Discussion The fracture strength of thermally shocked monolithic Si3N4 is strongly dependent on the magnitude of tensile stress developed on the surface, that is, if the tensile stress exceeds its strength, the cracks are generated on the surface, resulting in catastrophic drop in flexural strength. However, for FM sample, the fracture strength of FM sample is less sensitive to surface flaws; therefore, the resistance to crack propagation is a more critical factor than the resistance to crack initiation, which is critical for brittle monolithic Si3N4. However, pre-existing cracks on BN cell boundaries after hotpressing (T=1740 C) also affects the flexural response, resulting in crack interactions. Therefore, some factors, such as the magnitude of thermal stress on surface, thermal shock resistance parameter and pre-existing cracks, are discussed. 4.1. Magnitude of thermal stress on the surface (sTS) Considering the structure of this uniaxial FM (Fig. 1), it is noted that the elastic modulus and thermal expansion coefficient is different in the transverse and longitudinal directions. In addition, the hexagonal BN is strongly textured,19 with the high stiffness/low expansion a-axis aligned preferentially in the longitudinal direction and the lower stiffness/higher expansion c-axis aligned in the transverse direction. Therefore mechanical properties, such as elastic modulus, coefficient of thermal expansion (CTE) and Poisson’s ratio, of FM sample show anisotropy, as described in Table 2. The elastic modulus of monolithic silicon nitride was significantly higher, and the transverse modulus of the FM was less than half the longitudinal modulus, due to the BN, which has a small c-axis modulus. The longitudinal thermal expansion of the FM was slightly less than monolithic silicon nitride, decreased by the a-axis BN, while the transverse thermal expansion of the FM was much larger, increased by the c-axis BN. Poisson’s ratios were estimated from rule-of-mixture by taking 0.27 and 0.2 for Si3N4 21 and BN,22 respectively. The magnitude of thermal stress induced by the same exposure will be different, depending on the cell alignment (longitudinal and transverse direction). The traditional approach to evaluate the thermal shock resistance is based on quenching the specimen from an elevated temperature into a quenching media and measuring the fracture strength of the material. Neglecting the heat transfer and size effects, the maximum tensile stress (TS) generated on the surface of the specimen can be calculated according to:11 TS ¼ ð Þ E=ð Þ 1 DT ð1Þ where E, and represent the elastic modulus, the coefficient of thermal expansion (CTE) and Poisson’s Fig. 7. Optical photographs of crack propagation 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 during flexural testing. All samples showed extensive crack interactions, such as crack delaminations and crack deflections. Y.-H. Koh et al. / Journal of the European Ceramic Society 24 (2004) 2339–2347 2343