L. Zou et al. Journal of the European Ceramic Society 23(2003)1987-1996 P/2 P/2 2a Fig. I. The schematic of loading and remained effective section for the three-point loading system. -(62C-L) where I, is the moment of inertia for the Siz N4 matrix a=l+ When the loading state is the plain strain case, the According to interfacial fracture mechanics, mean- Young,'s Modulus in the Eq(10) will be substituted by while considering here the crack is assumed to be pro- E=E/(1-v2), in which v is the Poissons ratio of the pagating symmetrically on either side of the central Si3 N4 matrix notch, interfacial toughness can be expressed as P2 dc 2.2. Specimen design and preparation A Si3N4/BN Si3 N4 sandwiched sample with a single From Eq. (4), an expression for the first derivative of BN interlayer and the thickness of the upper and lower C with respect to a can be written as: sides of the Si3N4 matrices as equal as possible was dc designed for the present study. On one side of the matrix, a crack source directly connecting to the BN interphase was preset during material preparation combining Eqs. (6)and(7), we can have First, a-Si3N4 powders (Founder High Technology G二(式)4 Ceramic Co., Beijing, China) combined with 8 wt% (8) Y203(>9.99% purity, Hokko Chemical Industry Co Tokyo, Japan), 2.5 wt %o Al2O3(>99.9% purity, Beijing Chemical Plant, Beijing)and 1.5 wt% MgO n our experiments, the interlayer of sandwich speci-(>99.9% purity, Beijing Hong Xing Chemical Plant, men is very thin, so the beam stifness can be approxi- Beijing) were milled in an ethanol medium. Then, 20 mately expressed as wt%SiC whiskers dispersed by ultrasonic in ethanol media (TWS-400, Hokko Chemical Industry)were 2e=ibEr'=le (9) added to the mixture, and the milling ste The twice-milled mixture was filtered and dried. then where h is the thickness of the specimen, e the Youngs sieved through a 60-mesh screen modulus of the Si3 N4 matrix, I s the moment of inertia of a green body with a Si3 N4 matrix was obtained by die the specimen compaction. The Bn interlayer was prepared by tape Substituting Eq.(9) into Eq. (8), meanwhile, casting. Mixed powders with different interfacial com- Es= IsE, the final interfacial toughness expression can positions were prepared by incorporating BN with dif- be deduced ferent amounts of a-Si3 N4 or Al2O3 powders and milling the mixtures in ethanol for 24 h. then filt eBi (10) drying, and sieving the milled mixtures through a 60- mesh screen. The sieved powders were mixed with somea ¼ L þ c c s 6sC L3   1=3 ð5Þ According to interfacial fracture mechanics,mean￾while considering here the crack is assumed to be pro￾pagating symmetrically on either side of the central notch,interfacial toughness can be expressed as: Gi ¼ P2 4b dC da ð6Þ From Eq. (4),an expression for the first derivative of C with respect to a can be written as: dC da ¼ 1 2 1 c 1 s
ð Þ L a 2 ð7Þ combining Eqs. (6) and (7),we can have Gi ¼ P2 8b 1 c 1 s
ð Þ L a 2 : ð8Þ In our experiments,the interlayer of sandwich speci￾men is very thin,so the beam stiffness can be approxi￾mately expressed as: c ¼ 1 12 bEh3 ¼ IcE ð9Þ where h is the thickness of the specimen, E the Young’s modulus of the Si3N4 matrix, Ic the moment of inertia of the specimen. Substituting Eq. (9) into Eq. (8),meanwhile, s ¼ IsE,the final interfacial toughness expression can be deduced. Gi ¼ P2 8Eb 1 Ic 1 Is
ð Þ L a 2 ð10Þ where Is is the moment of inertia for the Si3N4 matrix layer. When the loading state is the plain strain case,the Young’s Modulus in the Eq. (10) will be substituted by E’ ¼ E= 1 2 ,in which  is the Poisson’s ratio of the Si3N4 matrix. 2.2. Specimen design and preparation A Si3N4/BN/Si3N4 sandwiched sample with a single BN interlayer and the thickness of the upper and lower sides of the Si3N4 matrices as equal as possible was designed for the present study. On one side of the matrix,a crack source directly connecting to the BN interphase was preset during material preparation. First, a-Si3N4 powders (Founder High Technology Ceramic Co.,Beijing,China) combined with 8 wt.% Y2O3 (>9.99% purity,Hokko Chemical Industry Co., Ltd.,Tokyo,Japan),2.5 wt.% Al2O3 (>99.9% purity, Beijing Chemical Plant,Beijing) and 1.5 wt.% MgO (>99.9% purity,Beijing Hong Xing Chemical Plant,- Beijing) were milled in an ethanol medium. Then,20 wt.%SiC whiskers dispersed by ultrasonic in ethanol media (TWS-400,Hokko Chemical Industry) were added to the mixture,and the milling step was repeated. The twice-milled mixture was filtered and dried,then sieved through a 60-mesh screen. A green body with a Si3N4 matrix was obtained by die compaction. The BN interlayer was prepared by tape casting. Mixed powders with different interfacial com￾positions were prepared by incorporating BN with dif￾ferent amounts of a-Si3N4 or Al2O3 powders and milling the mixtures in ethanol for 24 h,then filtering, drying,and sieving the milled mixtures through a 60- mesh screen. The sieved powders were mixed with some Fig. 1. The schematic of loading and remained effective section for the three—point loading system. L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996 1989