Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S Journal of the European Ceramic Society 23(2003)1987-1996 www.elsevier.com/locate/jeurceramsoc The measurement and characterization of the interfacial toughness of Si3 N4/BN composites by a three-point bending test Linhua Zoul a, b, * Yong Huang, Ruifeng Chen, Chang An Wanga, Dong-Soo Park astate Key Lab of New Ceramics and Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China Ceramic Materials Group, Korea Institute of Machinery and Materials, 66 SangNam-Dong, Chang-Won City, KyongNe th korea Received 20 June 2002: received in revised form 30 October 2002: accepted 11 November 2002 In this paper, a simpler method has been proposed for measuring and characterizing the interfacial toughness of Si3N4/BN composites. A sandwiching material, with one preset crack source connecting directly to a Bn interphase in the middle of a single half-Si3 N4 matrix, were designed and fabricated by sandwiching a thinner bn layer with two Si3 N4 green bodies obtained by die ompaction at room temperature. The bn layer had been made by tape casting. The sandwiched sample bars measured with 3x4x50 mm were cut and machined. The interfacial toughness of Si3 N4/BN composites was measured by three-point bending test for pure BN interphase, and interphases modified by different amounts of Si3 N4 or AlO3. The interfacial toughness values were calculated based on three-point bending fracture mechanics model. The interfacial toughness values we obtained were 38.20, 104.03 nd 116. 14 J/m2 for pure BN, BN+15vol %Si3N4 and BN+25vol %Si3 N4 interphases: 46.86, 53.90, 73.64 J/m2 for BN+16vo- L %ALO3, BN+36vol %Al2O3 and BN+63vol %ALO3 interphases, respectively. When the amounts of modified Si3N4 or Al,O3 increased the interphase was strengthened and crack deflection and propagation within interphase could not occur, the interfacial toughness was not obtainable for the corresponding samples, but it could be obtained by extrapolation based on the value obtained in each kind of interphase systems. The results show that the method is simple and effective for measuring and char- acterizing interfacial toughness, compared with some other related works in literatures C) 2003 Elsevier Science Ltd. All rights reserved. Keywords: Composites: Interfaces: Toughness; Measurement: Si3Nr-BN: Testing 1. Introduction >4000 J/m2. Huang et al. 8, also fabricated the materi als with the two structures, the materials had bending Si3N4 ceramic is a very promising, high-temperature strength of about 600-800 MPa and work of fracture structural material with its excellent mechanical prop- >4000 Jm. Among all the studies mentioned earlier erties, but due to its brittleness, its wide application has most of them lacked of interface design and control long been restricted. With the emergence of the two More often it was followed by manufacturing material, structures Si3 N4/ BN composites, i.e. laminated and measuring and characterizing material properties, and fibrous monolithic ceramics, the toughness of the finally evaluating interface by the properties of the Siy N4 has been greatly improved. Haiyan Liu et al. 5 materials. This was a very passive process to fabricate made the multilayer Si3N4/BN ceramics, which has 430 material. In order to design, tailor the material, and MPa of average bending strength and about 6500 Jm- optimize its bending strength and work of fracture, we of average work of fracture. Halloran et al. 6. 7 manu- must know how to characterize interphases with differ factured the Si3N4/BN composites with the two struc- ent compositions and give a quantitative evaluation on tures respectively, the materials had bending strength of its bonding strength. Usually, the fracture toughness of about 400-600 MPa and work of fracture values of this kind of composite was characterized indirectly by measuring the work of fracture. Sometimes. it was also characterized by Single Edge Notched Beam (SENB) method. However values obtained from both of the two methods cannot reflect the toughness of interface 0955-2219/02/S. see front matter C 2003 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(02)00430
The measurement and characterization of the interfacial toughness of Si3N4/BN composites by a three-point bending test Linhua Zou1,a,b,*,Yong Huanga ,Ruifeng Chena ,Chang An Wanga ,Dong-Soo Parkb a State Key Lab of New Ceramics and Fine Processing, Department of Material Science and Engineering, Tsinghua University, Beijing 100084, PR China bCeramic Materials Group, Korea Institute of Machinery and Materials, 66 Sang-Nam-Dong, Chang-Won City, Kyong-Nam, South Korea Received 20 June 2002; received in revised form 30 October 2002; accepted 11 November 2002 Abstract In this paper,a simpler method has been proposed for measuring and characterizing the interfacial toughness of Si3N4/BN composites. A sandwiching material,with one preset crack source connecting directly to a BN interphase in the middle of a single half-Si3N4 matrix,were designed and fabricated by sandwiching a thinner BN layer with two Si3N4 green bodies obtained by die compaction at room temperature. The BN layer had been made by tape casting. The sandwiched sample bars measured with 3450 mm3 were cut and machined. The interfacial toughness of Si3N4/BN composites was measured by three-point bending test for pure BN interphase,and interphases modified by different amounts of Si3N4 or Al2O3. The interfacial toughness values were calculated based on three-point bending fracture mechanics model. The interfacial toughness values we obtained were 38.20,104.03 and 116.14 J/m2 for pure BN,BN+15vol.%Si3N4 and BN+25vol.%Si3N4 interphases; 46.86,53.90,73.64 J/m2 for BN+16vol.%Al2O3,BN+36vol.%Al2O3 and BN+63vol.%Al2O3 interphases,respectively. When the amounts of modified Si3N4 or Al2O3 increased,the interphase was strengthened and crack deflection and propagation within interphase could not occur,the interfacial toughness was not obtainable for the corresponding samples,but it could be obtained by extrapolation based on the values obtained in each kind of interphase systems. The results show that the method is simple and effective for measuring and characterizing interfacial toughness,compared with some other related works in literatures. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Interfaces; Toughness; Measurement; Si3N4–BN; Testing 1. Introduction Si3N4 ceramic is a very promising,high-temperature structural material with its excellent mechanical properties,but due to its brittleness,its wide application has long been restricted. With the emergence of the two structures Si3N4/BN composites,i.e. laminated and fibrous monolithic ceramics,14 the toughness of the Si3N4 has been greatly improved. Haiyan Liu et al.5 made the multilayer Si3N4/BN ceramics,which has 430 MPa of average bending strength and about 6500 J/m2 of average work of fracture. Halloran et al.6,7 manufactured the Si3N4/BN composites with the two structures respectively,the materials had bending strength of about 400–600 MPa and work of fracture values of >4000 J/m2 . Huang et al.8,9 also fabricated the materials with the two structures,the materials had bending strength of about 600–800 MPa and work of fracture >4000 J/m2 . Among all the studies mentioned earlier, most of them lacked of interface design and control. More often it was followed by manufacturing material, measuring and characterizing material properties,and finally evaluating interface by the properties of the materials. This was a very passive process to fabricate material. In order to design,tailor the material,and optimize its bending strength and work of fracture,we must know how to characterize interphases with different compositions and give a quantitative evaluation on its bonding strength. Usually,the fracture toughness of this kind of composite was characterized indirectly by measuring the work of fracture. Sometimes,it was also characterized by Single Edge Notched Beam (SENB) method. However,values obtained from both of the two methods cannot reflect the toughness of interface 0955-2219/02/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(02)00430-2 Journal of the European Ceramic Society 23 (2003) 1987–1996 www.elsevier.com/locate/jeurceramsoc * Corresponding author. 1 Now at Korea Institute of Machinery and Materials. E-mail address: linhua_zou@hotmail.com (L. Zou).
1988 L Zou et al. Journal of the European Ceramic Society 23(2003)1987-1996 directly. Meanwhile, the reliability of the SENB method symmetrically from the centre, also a debonded layer is influenced by notch retard effect and the highly ani- between the through-thickness crack and the tip of an sotropy in composite due to the existence of multi weak interfacial crack cannot undertake load. The com- interfaces. So the interfacial toughness, i.e. the inter- pliance corresp onding to the loading stage is calculated facial strain-energy release rate or interfacial resistance, according to the remained section as shown in Fig. 1. In was proposed for characterizing the interfacial bonding addition, it was assumed that the value of the critical strength. Unfortunately, data on interfacial toughness interfacial toughness Gic is a constant, uniform of the composite have been seldom reported in litera- throughout the specimen. 1. tures. In addition. so far. there are few relia ble methods From Fig. 1. the formula for calculating the inter for measuring interfacial toughness. Although Kovar et facial toughness can be deduced. Because the interlay alo gave a method for measuring the interfacial tough- is just in the middle of the upper and lower Si3N4 ness, the specimens they used were different with what matrixes, it is exact the position of the neutral axis. In the model based on, with which interfacial toughness region ( the beam can carry stress, there exists corre- was calculated, o and the method was not so reliable. sponding deflection curve equation Based on the model of Charalambides et al., Phillips et al. II measured the interfacial toughness of a lam d-y1 Px nated SiC/C composite using an SiC/C/Sic sandwich dx2 (1) sample with a single interphase. However, the sample mensions ado pted by those researchers were too large where yi is the displacement from the neutral axis and (3.5×18×140mm3) for the sample to be easily man-∑ the beam stifness of region① ufactured and machined, making the method incon In region 2, only the half side Si3 N4 matrix layer is venient for characterizing interfacial toughness and also assumed to carry the applied stress, so it is related with limiting its flexibility. Besides the big size, the sample the following equatio was notched and precracked under three-point bending, with a short loading span, before the test, but that pro- dy2=-2Es cess is difficult to control and also makes it very difficult to obtain a crack starting from the tip of the notch and just exactly reaches the interphase or deflects toward where y2 the displacement from the neutral axis of the wo sides a little there lower Si3N4 matrix, Es the beam stifness of the matrix In this paper, we improved the method used by Phil- layer in region(2 lips et al., and gave a method that is suitable for mea There exist the following boundary conditions suring and characterizing interfacial toughness of the Si3/BN composite, which is simple and easy to be )ilr=0 extended. By using this method the interfacial toughness of the BN interphase systems strengthend by Si3 N4 and Al2O3 respectively was measured and characterized dx l=) d 2. Experimental d d 2. 1. Experiment prIne According to the partial differential equations of (1)and Based on the model of the fracture of laminated(2), and the earlier-mentioned boundary conditions, we composites under three-point bending given by Phillips can obtain the relationship between the displacement of et al., 2 a single- interlayer type of sandwiching material, the central loading point and the propagating crack with one preset crack source connecting directly to a bn length interphase in the middle of a single half-Si3 N4 matrix acterizing interfacial toughness. The upper and lower P PL+6Ee Es)(L-a was designed and fabricated for measuring and char- Si3N4 matrixes height were kept as equal as possible to fix the effect of h/h2 on the phase angle v, i.e. to make since y= CP, hence the ratio of shearing to opening stress-intensity factors tended to be constant. 3, 4 The three-point loading system of the sample is shown in Fig. 1. According to the model, 2 the through-thickness cracks occur in the centre of the beam and the interfacial cracks propagate so the crack length can be expressed by compliance
directly. Meanwhile,the reliability of the SENB method is influenced by notch retard effect and the highly anisotropy in composite due to the existence of multi weak interfaces. So the interfacial toughness,i.e. the interfacial strain–energy release rate or interfacial resistance, was proposed for characterizing the interfacial bonding strength. Unfortunately,data on interfacial toughness of the composite have been seldom reported in literatures. In addition,so far,there are few reliable methods for measuring interfacial toughness. Although Kovar et al 6 gave a method for measuring the interfacial toughness,the specimens they used were different with what the model based on,with which interfacial toughness was calculated,10 and the method was not so reliable. Based on the model of Charalambides et al.,10 Phillips et al.11 measured the interfacial toughness of a laminated SiC/C composite using an SiC/C/SiC sandwich sample with a single interphase. However,the sample dimensions adopted by those researchers were too large (3.5 18 140 mm3 ) for the sample to be easily manufactured and machined,making the method inconvenient for characterizing interfacial toughness and also limiting its flexibility. Besides the big size,the sample was notched and precracked under three-point bending, with a short loading span,before the test,but that process is difficult to control and also makes it very difficult to obtain a crack starting from the tip of the notch and just exactly reaches the interphase or deflects toward two sides a little there. In this paper,we improved the method used by Phillips et al.,11 and gave a method that is suitable for measuring and characterizing interfacial toughness of the Si3N4/BN composite,which is simple and easy to be extended. By using this method,the interfacial toughness of the BN interphase systems strengthend by Si3N4 and Al2O3 respectively was measured and characterized. 2. Experimental 2.1. Experiment principle Based on the model of the fracture of laminated composites under three-point bending given by Phillips et al.,12 a single-interlayer type of sandwiching material, with one preset crack source connecting directly to a BN interphase in the middle of a single half-Si3N4 matrix, was designed and fabricated for measuring and characterizing interfacial toughness. The upper and lower Si3N4 matrixes height were kept as equal as possible to fix the effect of h1/h2 on the phase angle ,i.e. to make the ratio of shearing to opening stress-intensity factors tended to be constant.11,13,14 The three-point loading system of the sample is shown in Fig. 1. According to the model,12 the through-thickness cracks occur in the centre of the beam and the interfacial cracks propagate symmetrically from the centre,also a debonded layer between the through-thickness crack and the tip of an interfacial crack cannot undertake load. The compliance corresponding to the loading stage is calculated according to the remained section as shown in Fig. 1. In addition,it was assumed that the value of the critical interfacial toughness Gic is a constant,uniform throughout the specimen.11,12 From Fig. 1,the formula for calculating the interfacial toughness can be deduced. Because the interlayer is just in the middle of the upper and lower Si3N4 matrixes,it is exact the position of the neutral axis. In region ,the beam can carry stress,there exists corresponding deflection curve equation: d2y1 dx2 ¼ Px 2Sc ð1Þ where y1 is the displacement from the neutral axis and c the beam stiffness of region . In region ,only the half side Si3N4 matrix layer is assumed to carry the applied stress,so it is related with the following equation: d2y2 dx2 ¼ Px 2Ss ð2Þ where y2 the displacement from the neutral axis of the lower Si3N4 matrix, s the beam stiffness of the matrix layer in region . There exist the following boundary conditions: y1jx¼0 ¼ 0 dy1 dx jx¼ð Þ La ¼ dy2 dx jx¼ð Þ La y1jx¼ð Þ La ¼ y2jx¼ð Þ La dy2 dx jx¼L¼ 0 According to the partial differential equations of (1) and (2),and the earlier-mentioned boundary conditions,we can obtain the relationship between the displacement of the central loading point and the propagating crack length. y ¼ P 6Ss L3 þ P 6 1 Sc 1 Ss ð Þ L a 3 ð3Þ since y ¼ CP,hence C ¼ 1 6Ss L3 þ 1 6 1 Sc 1 Ss ð Þ L a 3 ð4Þ so the crack length can be expressed by compliance. 1988 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996
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 some
a ¼ L þ c c s 6sC L3 1=3 ð5Þ According to interfacial fracture mechanics,meanwhile considering here the crack is assumed to be propagating 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 specimen is very thin,so the beam stiffness can be approximately 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 compositions were prepared by incorporating BN with different 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
1990 L. Zou et al. /Journal of the European Ceramic Society 23(2003)1987-1996 water,glycerin, and paraffin, milled, and incorporated thinner BN interfacial sheet. The samples then were into a 20 wt polyvinyl alcohol solution; this mixture stacked, placed in a graphite die, and sintered, by hot was milled again and then degassed, under vacuum, at pressing, at 1820C for 1.5 h in an atmosphere of N2, 1.013x 10 Pa pressure. The homogeneous slurry was under a pressure of 22 MPa. Initially, the heating rate used for tape casting, and green sheets 40-60 um thick was slow, to allow the binder in the interlayer tape to obtained pyrolyze and burn out below 500C. Strict control of gle-interface samples were prepared by sandwich- the heating rate was not necessary, because the inter- products of the Si3 N4 matrix around a facial layer was so thin. After sintering, the thickness of the interfacial layer was 15 to 30 um 2.3. Experimental method 3×4×50 machined. Because the phase angle of loading, v, defined as the angle having a tangent equal to the ratio of the shearing to the opening stress-intensity factors, 3 was influenced by sample dimensions, that value varied with the thickness ratio, h/h? Test errors resulting from fuctuation of the sample dimensions were reduced and the effect of phase angle y on interfacial toughness was fixed in the present study by making the thickness of the upper and lower beams as identical as possible. The KIMM 28kU"5nx3,009 sample was notched along the source of the crack to a certain depth, where it was near the interphase. The Fig. 2. SEM micrograph of the BN interphase crack loading system as illustrated in Fig. I was adopted to Displacement u(mm) Displacement, u(mm) 40 20 0 0.000.010.02003004005006007 Displacement, u(mm) Fig. 3. The three-point bending test and interfacial toughness calculation results for the bn+ Si3N4 interphase system. (a) BN;(b)bn+ 15vol %Si3 N4;(c)BN+ 25vol %Si3 N4: (d)BN 50vol %Si3N4
water,glycerin,and paraffin,milled,and incorporated into a 20 wt.% polyvinyl alcohol solution; this mixture was milled again and then degassed,under vacuum,at 1.013 105 Pa pressure. The homogeneous slurry was used for tape casting,and green sheets 40–60 mm thick was obtained. Single-interface samples were prepared by sandwiching two green products of the Si3N4 matrix around a thinner BN interfacial sheet. The samples then were stacked,placed in a graphite die,and sintered,by hot pressing,at 1820 C for 1.5 h in an atmosphere of N2, under a pressure of 22 MPa. Initially,the heating rate was slow,to allow the binder in the interlayer tape to pyrolyze and burn out below 500 C. Strict control of the heating rate was not necessary,because the interfacial layer was so thin. After sintering,the thickness of the interfacial layer was 15 to 30 mm. 2.3. Experimental method Test samples measuring 3 4 50 mm3 were machined. Because the phase angle of loading, , defined as the angle having a tangent equal to the ratio of the shearing to the opening stress-intensity factors,13 was influenced by sample dimensions,that value varied with the thickness ratio, h1/h2 .Test errors resulting from fluctuation of the sample dimensions were reduced and the effect of phase angle on interfacial toughness was fixed in the present study by making the thickness of the upper and lower beams as identical as possible. The sample was notched along the source of the crack to a certain depth,where it was near the interphase. The Fig. 2. SEM micrograph of the BN interphase crack. loading system as illustrated in Fig. 1 was adopted to Fig. 3. The three-point bending test and interfacial toughness calculation results for the BN + Si3N4 interphase system. (a) BN; (b) BN + 15vol.%Si3N4; (c) BN + 25vol.%Si3N4;(d) BN + 50vol.%Si3N4. 1990 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996
L. Zou et al. Journal European Ceramic Society 23(2003)1987-1996 G Displacement, u( mm) Displacement, u(mm) Fig 4. The three-point bending test and interfacial toughness calculation results for the bn Al2O3 interphase system. (a)BN+ 16vol %Al20 (b)Bn+ 36vol %Al2O3;(c)BN 63vol %AlO3:(d) AlO3 110 SiNA Modifier ▲ALO2 Modifier 80 30 SiNA or AlO3 vol% Fig. 5. The dependence of interfacial toughness on volume fraction of Si3 N4 or Al2O3 modifier
Fig. 5. The dependence of interfacial toughness on volume fraction of Si3N4 or Al2O3 modifier. Fig. 4. The three-point bending test and interfacial toughness calculation results for the BN + Al2O3 interphase system. (a) BN + 16vol.%Al2O3; (b) BN + 36vol.%Al2O3; (c) BN + 63vol.%Al2O3; (d) Al2O3. L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996 1991
1992 L. Zou et al. Journal of the European Ceramic Society 23(2003)1987-1996 Counts 15000 0000 5000 A Energy(kev) Fig. 6. The energy dissipation spectrum on any of one point in BN interphase carry out the tests, with 40 mm span. At least two or phases were obtained(Fig 3). When crack propagation three samples were tested for each type of interphase in interphase began, the interfacial toughness tended to composition for the loading experiments. A universal be a constant. Its values for the three interphases were materials testing machine (model 2000, Shimadzu obtained by taking an average value within the corre- Corp, Kyoto, Japan) was used. To reduce the friction sponding stable region, they were 35.42, 96.45, and between roller and sample surface, the aluminum foil 107.67 J/m, respectively. The interfacial toughness of was used for the purpose. According to the sample size, the specimen without crack deflection and propagation ne loading state was regard as plain strain case, the in interphase could not be obtained by the same way Poisson's ratio(v) was taken as 0.27.6 The load-displacement curves of the samples with samples that has the same composition with the interphases modified by different amount of Al2O3 matrix Si3N4 was also prepared by the same process for (Fig. 4)indicates that crack deflection and propagation measuring Youngs modulus by three-point bending occurred in the interphases of the BN+ 16vol %Al2O3, test. The sample is 4 mm wide and 20 ratio of span to BN+ 36vol %Al2O3 and BN+63vol %Al2O3 In addi thickness, the tests were conducted with 40 mm loading tion, the extent of the load descending was larger than span. The average value was obtained from the results that of the BN interphase modified by Si3 N4. Therefore of 20 samples 3. Results and discussion The experiment results indicate the crack deflection and propagation occurred inside the interphase, rather than at the interface between matrix Si3 N4 and BN interphase (Fig. 2), this also confirmed the results obtained by Kovar et al. 6 The load-displacement curves with different interphases of BN+ Si3 N4 system(Fig 3) how that crack deflection and interfacial crack propa gation occurred in the interphases of BN, BN+ 15vol%Si3N4 and BN+25vol%Si3N4. As the Si3N4 modifier increased to 50vol %, the crack in thickness went across the interphase directly, leading to catastrophic fracture. This was because the interfacial toughness now was too large to allow crack deflection and propagation in the interphase. According to the qs.(5)and (10), the interfacial toughne 9912828KU89半1NM placement curves of the samples with the BN, BN+ 15vol %Si3 N4 and BN+ 25vol %Si3N4 inter- Fig. 7. The SEM micrographs of BN interphase
carry out the tests,with 40 mm span. At least two or three samples were tested for each type of interphase composition for the loading experiments. A universal materials testing machine (model 2000,Shimadzu Corp.,Kyoto,Japan) was used. To reduce the friction between roller and sample surface,the aluminum foil was used for the purpose. According to the sample size, the loading state was regard as plain strain case,the Poisson’s ratio () was taken as 0.27.6 Samples that has the same composition with the matrix Si3N4 was also prepared by the same process for measuring Young’s modulus by three-point bending test. The sample is 4 mm wide and 20 ratio of span to thickness,the tests were conducted with 40 mm loading span. The average value was obtained from the results of 20 samples. 3. Results and discussion The experiment results indicate the crack deflection and propagation occurred inside the interphase,rather than at the interface between matrix Si3N4 and BN interphase (Fig. 2),this also confirmed the results obtained by Kovar et al. 6 The load-displacement curves with different interphases of BN+ Si3N4 system (Fig. 3) show that crack deflection and interfacial crack propagation occurred in the interphases of BN, BN+15vol.%Si3N4 and BN+25vol.%Si3N4. As the Si3N4 modifier increased to 50vol.%,the crack in thickness went across the interphase directly,leading to catastrophic fracture. This was because the interfacial toughness now was too large to allow crack deflection and propagation in the interphase. According to the Eqs. (5) and (10),the interfacial toughness versus displacement curves of the samples with the BN, BN+15vol.%Si3N4 and BN+25vol.%Si3N4 interphases were obtained (Fig. 3). When crack propagation in interphase began,the interfacial toughness tended to be a constant. Its values for the three interphases were obtained by taking an average value within the corresponding stable region,they were 35.42,96.45,and 107.67 J/m2 ,respectively. The interfacial toughness of the specimen without crack deflection and propagation in interphase could not be obtained by the same way. The load–displacement curves of the samples with interphases modified by different amount of Al2O3 (Fig. 4) indicates that crack deflection and propagation occurred in the interphases of the BN+16vol.%Al2O3, BN+36vol.%Al2O3 and BN+63vol.%Al2O3. In addition,the extent of the load descending was larger than that of the BN interphase modified by Si3N4. Therefore Fig. 7. The SEM micrographs of BN interphase. Fig. 6. The energy dissipation spectrum on any of one point in BN interphase. 1992 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996
1987-1 the interfacial toughness versus displacement curves of The interfacial toughness for the BN+ Si3 N4 and these three interphases were obtained according to the BN+ AlO3 interphase systems are listed in Table l, the Eqs. ( 5)and(10)(Fig. 4). Similar with the BN+ Si3N4 system, there also existed an approximate constant Table I interfacial toughness region after the abrupt load des- The interfacial toughness measured by three-point bending cending for each specimen in the BN +Al2O3 system. two interfacial modifier systems with different interphase compositions The interfacial toughness of this three interphases were nterface BN+Si3N4 BN+ AlO3 obtained by taking an average value within each of syster these regions, they were 43.44, 49.97 and 68.27 J/m Volume fraction respectively. The interfacial toughness value increased f Si3 Na( vol % of AlO,(vol % with the increase of volume fraction of Al,O3, as the 50163663100 interphase consisted of pure AlO3, its strength was too high to allow inside crack propagation, the crack in- 35.4296.45107.67 43.44499768.27 thickness crossed the interphase directly resulting in brittle fractur 15 KU 2.05u Fig 8. The SEM interface fracture morphology of BN interphases modified by Si3Na-(a) Pure BN;(b) Bn 15vol Si3N4:(c)BN 25Vol% Si3N4;(d)BN+ 50Vol% Si3N4-
the interfacial toughness versus displacement curves of these three interphases were obtained according to the Eqs. (5) and (10) (Fig. 4). Similar with the BN+Si3N4 system,there also existed an approximate constant interfacial toughness region after the abrupt load descending for each specimen in the BN+Al2O3 system. The interfacial toughness of this three interphases were obtained by taking an average value within each of these regions,they were 43.44,49.97 and 68.27 J/m2 , respectively. The interfacial toughness value increased with the increase of volume fraction of Al2O3,as the interphase consisted of pure Al2O3,its strength was too high to allow inside crack propagation,the crack inthickness crossed the interphase directly resulting in brittle fracture. The interfacial toughness for the BN+Si3N4 and BN+Al2O3 interphase systems are listed in Table 1,the Fig. 8. The SEM interface fracture morphology of BN interphases modified by Si3N4. (a) Pure BN; (b) BN +15vol.% Si3N4; (c) BN + 25Vol% Si3N4; (d) BN + 50Vol% Si3N4. Table 1 The interfacial toughness measured by three-point bending test for two interfacial modifier systems with different interphase compositions Interface system BN+Si3N4 BN+Al2O3 Volume fraction of Si3N4(vol.%) Volume fraction of Al2O3(vol.%) Interphase composition 0 15 25 50 16 36 63 100 Interfacial toughness (J/m2) 35.42 96.45 107.67 – 43.44 49.97 68.27 – L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996 1993
1994 results show that the values of the former system was were wrapped by glass phase and some of pores were bigger than that of the later in the whole composition filled. On the other hand, the micrograph also demon ange(Fig. 5). The Si3 N4 strengthened interphase is strates that the bn interphase had a porous structure. much stronger than the Al2O3 strengthened interphase. This is the reason that doping with Si3N4 or Al2O3 This is agreement with the four-point bending test could strengthen the bn interphase. The earlier- obtained results show the interfacial toughness was Studies on the microstructure of BN interphase have considerably influenced by the quantity of the doped shown that grain boundary glass phase in Si3 N4 matrix Si3N4 or Al2O3. The regularities of the dependence of usually diffuses into the interhase, the same case was the interfacial toughness on volume percentage of Si3N4 observed in our samples. The energy dissipation spec- or Al2O3 added into the Bn interphase can be explained trum(EDS)on any of one point in pure BN interphase by the SEM micrographs of the fracture surface due to indicates(Fig. 6)that there existed Si, Y, Al, Mg, Ca interface delamination(Figs. 8 and 9). As the volume ind O elements and some glass phase in matrix Si3N4 percentage of Si3 N4 increases, the bn interphase was moved into the interphase. This was also confirmed by densified gradually and quickly, this led to the increase the SEM surface morphology of the BN interphase of the interfacial toughness. When the percentage Fig. 7), some of the plate like hexagonal Bn grains reaches to 50%, the interphase already seemed to be 202.05u (b Fig 9. The SEM interface fracture morphology of BN interphases modified by Al2O3 (a)BN+ 16vol %Al2O3;(b)BN 36vol %;(c)BN+ 63Vol% AlO3: (d)Pure Al2O3
results show that the values of the former system was bigger than that of the later in the whole composition range (Fig. 5). The Si3N4 strengthened interphase is much stronger than the Al2O3 strengthened interphase. This is agreement with the four-point bending test results.15 Studies on the microstructure of BN interphase have shown that grain boundary glass phase in Si3N4 matrix usually diffuses into the interhase,16 the same case was observed in our samples. The energy dissipation spectrum (EDS) on any of one point in pure BN interphase indicates (Fig. 6) that there existed Si,Y,Al,Mg,Ca and O elements and some glass phase in matrix Si3N4 moved into the interphase. This was also confirmed by the SEM surface morphology of the BN interphase (Fig. 7),some of the plate like hexagonal BN grains were wrapped by glass phase and some of pores were filled. On the other hand,the micrograph also demonstrates that the BN interphase had a porous structure. This is the reason that doping with Si3N4 or Al2O3 could strengthen the BN interphase. The earlierobtained results show the interfacial toughness was considerably influenced by the quantity of the doped Si3N4 or Al2O3. The regularities of the dependence of the interfacial toughness on volume percentage of Si3N4 or Al2O3 added into the BN interphase can be explained by the SEM micrographs of the fracture surface due to interface delamination (Figs. 8 and 9). As the volume percentage of Si3N4 increases,the BN interphase was densified gradually and quickly,this led to the increase of the interfacial toughness. When the percentage reaches to 50%,the interphase already seemed to be Fig. 9. The SEM interface fracture morphology of BN interphases modified by Al2O3. (a) BN+ 16vol.%Al2O3; (b) BN +36vol.%; (c) BN + 63Vol% Al2O3; (d) Pure Al2O3. 1994 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996
L Zou et al. /Journal of the European Ceramic Society 23(2003)1987-1996 1995 lenser, and no eminent big pores existed(Fig. 8d), cor- of tending to be a constant. Obviously, these were con- respondingly, the interfacail toughness was too big to tradictory with the initial assumption for the three-point allow crack deflection and propagaton in the interphase. bending fracture model, and they were not correct However, in the case of the Bn interphase modified by results. To solve this problem, we concentrated all the Al_O3, the densification rate was not so fast as that of deviations from the effecting factors upon the Young Si3 N4 modified interphase. With the increase of volume Modulus in Eqs. (5)and(10). In this way, The E value percentage of AlO3, the Bn interphase was densified was successfully adjusted to offset the earlier-mentioned gradually and slowly, and the interfacial toughness also influence The adjustment was not finished until the inter increased. Even if the percentage approaches 63%, the facial toughness value almost tended to be a constant after interphase still seemed to be loosen and porous(Fig. 9c), the occurrence of crack deflection and propagation within this is the reason why crack deflection and propagation interphase. To obtain a preliminary e value that is near still could happen in this interphase. As the interphase to the prospective one, sometimes the critical condition. was pure Al2O3, the interfacial toughness was bi i.e. a=0 or a=L employed for the purpose enough to hinder the occurrence of crack deflection and according to Eq. (5). As a=0, it was correspondi propagation From the microstructure analysis, it is also the initiation point of interfacial crack propagation, clear that the strengthening effect of Si3N4 is much big- there was a sudden load descent in the load-displace- ger than that of Al2O3, at the same volume percentage ment curve For a=L, it was only applicable to the case of modifier, the interfacial toughness value of Si3N4 in which the interfacial crack reached the ends of the doped bn interphase is much higher that of Al_O3 loading span. Based on the obtained preliminary E doped one value, the prospective E value was determined easily to The earlier-mentioned results indicate that the make the interfacial toughness nearly tend to be a con- method we used to measure and characterize interfacial stant. Comparing with the four-point bending test to toughness is viable. Compared with the method Phillips measure the interfacial toughness, the three-point bend et al. used for measuring the interfacial toughness of a ing test measurement results would be less accurate laminated SiC/C composite, the present method allowed us to measure the interfacial toughness using smaller sized samples, which were much easier to obtain. Thus 4. Conclusions the method of presetting a crack directly connecting to the interphase is viable, making the experimental testing simpler and more flexible 1. By considerably reducing sandwiching sample For the method presented here, the most important size, and presetting a crack source connecting step is to determine the crack propagation length by directly to a BN interphase in the middle of a compliance method (5). According to the equation, the single half-Si3 N4 matrix, a new method was crack length is also determined by the moment of inertia presented for measuring interfacial toughness of of half Si3N4 matrix (Is. Because the matrix thickness the Si3N4/BN composite (h2) was obtained by measuring its thickness in different 2. The interfacial toughness of the Si3N4/BN com- positions under common optical microscopy and then posite with different interphases strengthened by taking an average value, this introduced some errors Si3N4 or Al,O3 were measured, the method turned due to irregularities of the matrix surface contacted to out to be viable, and is simpler and more flexible the interphase. Because the inside surface of Si3N4 compared with other method in literatures matrix layer was not absolutely flat and its thickness 3. The interfacial toughness for Si3 N4/BN compo- had some fluctuations after the specimen was fabri sites with bN, BN+ 15vol% Si3N4, and cated. The crack propagation length was considerably BN+25vol Si3N4 interphases was 35.42, affected by the fuctuation of Is and finally the inter- 96.45, and 107.67 J/m, respectively. The com facial toughness was influenced indirectly. Interfacial posite with BN+ 16vol Al2O3, BN+ 36vo- toughness was also influenced by the Is [see Eq(10)1 1 %Al,O3, and Bn+ 63 vol% Al,O3 interphases so there existed a double influencing factors in three was 43.44J /m, 49.97J/m- and 68.27J/m- respec- point bending method, this was the cause of giving rise tively. For the both interphase systems, as the to deviation of the result. Another important factor amount of the modifier Si3 N4 or AlO3 increased that affected the measurement results was the Youngs the interphase was over-strengthened, the crack modulus of the matrix. For several samples, the mea deflection and propagation sured E value could not stand for that of the real Si3N4 and resulted in brittle fracture matrix in the sample for measuring interfacial tough 4. The method was affected by the fluctuation of the ness, leading to a result that was not reasonable. Some- moment of inertia of half Si3 N, matrix (Is)due to times the crack propagation length was negative, or the irregularities of the matrix surface contacted to interfacial toughness value had a big fluctuation instead the interphase. Also it was influenced by the
denser,and no eminent big pores existed (Fig. 8d),correspondingly,the interfacail toughness was too big to allow crack deflection and propagaton in the interphase. However,in the case of the BN interphase modified by Al2O3,the densification rate was not so fast as that of Si3N4 modified interphase. With the increase of volume percentage of Al2O3,the BN interphase was densified gradually and slowly,and the interfacial toughness also increased. Even if the percentage approaches 63%,the interphase still seemed to be loosen and porous (Fig. 9c), this is the reason why crack deflection and propagation still could happen in this interphase. As the interphase was pure Al2O3,the interfacial toughness was big enough to hinder the occurrence of crack deflection and propagation. From the microstructure analysis,it is also clear that the strengthening effect of Si3N4 is much bigger than that of Al2O3,at the same volume percentage of modifier,the interfacial toughness value of Si3N4 doped BN interphase is much higher that of Al2O3 doped one. The earlier-mentioned results indicate that the method we used to measure and characterize interfacial toughness is viable. Compared with the method Phillips et al.11 used for measuring the interfacial toughness of a laminated SiC/C composite,the present method allowed us to measure the interfacial toughness using smallersized samples,which were much easier to obtain. Thus, the method of presetting a crack directly connecting to the interphase is viable,making the experimental testing simpler and more flexible. For the method presented here,the most important step is to determine the crack propagation length by compliance method (5). According to the equation,the crack length is also determined by the moment of inertia of half Si3N4 matrix (Is). Because the matrix thickness (h2) was obtained by measuring its thickness in different positions under common optical microscopy and then taking an average value,this introduced some errors due to irregularities of the matrix surface contacted to the interphase. Because the inside surface of Si3N4 matrix layer was not absolutely flat and its thickness had some fluctuations after the specimen was fabricated. The crack propagation length was considerably affected by the fluctuation of Is and finally the interfacial toughness was influenced indirectly. Interfacial toughness was also influenced by the Is [see Eq. (10)], so there existed a double influencing factors in threepoint bending method,this was the cause of giving rise to deviation of the result. Another important factor that affected the measurement results was the Young’s modulus of the matrix. For several samples,the measured E value could not stand for that of the real Si3N4 matrix in the sample for measuring interfacial toughness,leading to a result that was not reasonable. Sometimes the crack propagation length was negative,or the interfacial toughness value had a big fluctuation instead of tending to be a constant. Obviously,these were contradictory with the initial assumption for the three-point bending fracture model,and they were not correct results. To solve this problem,we concentrated all the deviations from the effecting factors upon the Young’s Modulus in Eqs. (5) and (10). In this way,The E value was successfully adjusted to offset the earlier-mentioned influence. The adjustment was not finished until the interfacial toughness value almost tended to be a constant after the occurrence of crack deflection and propagation within interphase. To obtain a preliminary E value that is near to the prospective one,sometimes the critical condition, i.e. a=0 or a=L,was employed for the purpose according to Eq. (5). As a=0,it was corresponding to the initiation point of interfacial crack propagation, there was a sudden load descent in the load–displacement curve. For a=L,it was only applicable to the case in which the interfacial crack reached the ends of the loading span. Based on the obtained preliminary E value,the prospective E value was determined easily to make the interfacial toughness nearly tend to be a constant. Comparing with the four-point bending test to measure the interfacial toughness,the three-point bending test measurement results would be less accurate. 4. Conclusions 1. By considerably reducing sandwiching sample size,and presetting a crack source connecting directly to a BN interphase in the middle of a single half-Si3N4 matrix,a new method was presented for measuring interfacial toughness of the Si3N4/BN composite. 2. The interfacial toughness of the Si3N4/BN composite with different interphases strengthened by Si3N4 or Al2O3 were measured,the method turned out to be viable,and is simpler and more flexible compared with other method in literatures. 3. The interfacial toughness for Si3N4/BN composites with BN,BN+ 15vol.% Si3N4,and BN+25vol.% Si3N4 interphases was 35.42, 96.45,and 107.67 J/m2 ,respectively. The composite with BN+ 16vol.% Al2O3,BN+36vol.%Al2O3,and BN+ 63Vol% Al2O3 interphases was 43.44J/m2 ,49.97J/m2 and 68.27J/m2 respectively. For the both interphase systems,as the amount of the modifier Si3N4 or Al2O3 increased, the interphase was over-strengthened,the crack deflection and propagation no longer happened, and resulted in brittle fracture. 4. The method was affected by the fluctuation of the moment of inertia of half Si3N4 matrix (Is) due to irregularities of the matrix surface contacted to the interphase. Also it was influenced by the L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996 1995
1996 L Zou et al. Journal of the eure eramic Society 23(2003)1987-1996 Youngs modulus of the Si3N4 matrix in the nitride/boron nitride ceramic. J. 4m. Ceram. Soc. 1996. 79 sandwiching sample due to its indirectly deter mination. However, this was solved by con 6. Kovar. Desiderio. Thouless. M. D. and Halloran. J W. Crack centrating all the deviations from the effecting deflection and propagation in layered silicon nitride/boron nitride factors upon the Youngs Modulus in Eqs. (5) 7. Kovar, D. King. B. H,Trice, R.W. and Halloran,J.w and (10). In this way, The e value was success Fibrous monolithic ceramics. J. Am. Ceram Soc. 1997 80 2471 fully adjusted to offset these influences. 8. Hai Guo, Yong Huang and Chang-an Wang, Preparation and Acknowledgements ng an Wang, Yong Huang. Qingfeng Zan, Hai Guo and Shengyou Cai, Biomimetic structure design-a possible approach The authors gratefully acknowledge Professor Z. D to change the brittleness of ceramics in nature. Materials science Guan for his much help in doing three-point bending Engineering C, 2000, 11, 9-12. tests; also this work was supported by National Natural 10. Charalambides. P. G. Lund. J. Evans. A. G R. M. A test specimen for determing the fracture resistance of Science Foundation of China(No. 59632090) bimaterial interfaces. J. Appl. Mech, 1989, 56. 77-82 I1. Phillips, A.J. Clegg, w.J. and Clyne, T.W., Fracture behavior of ceramic laminates in bending-lI. Comparison of model pre- References dictions with e data. Acta Metall. Mater. 1993. 41 819-827 12. Phillips, A.J., Clegg, w.J. and Clyne, T.w Fracture behavior 1. Clegg. W.J., Kendall. K, Alford, N. M., Birchall, D. and But- hates in bending-1. Modelling of crack propa- s. Nature. 1990 gation. Acta Metall. Mater., 1993, 41. 805-817 13. Clyne, T. w. and Phillips, A. J, Interfacial control and macro- 2. Baskaran, S, Nunn, S. D, Popovic, Dragan and Halloran, J. w scopic failure in long-fiber-reinforced and laminated inorganic Fibrous monolithic ceramics: 1. fabrication. microstructure composites. Composites Sci. Tech, 1994, 51, 271-282 and indentation behavior. J. Am. Ceram Soc., 1993, 76, 2209- 14. Cao, H. C. and Evans, A. G, An experimental study of the fracture resistance of bimaterial interfaces. Mechanics of materi. neolithic ceramics als,1989,7,295-304 Il, flexural strength and fracture bhavior of the slicon carbide/ 15. Linhua Zou, Interfacial toughness measurement and characteriza- graphite system. J. Am. Ceram. Soc., 1993, 76, 2217-2224. tion for Si3N4/ BN composites. Report of Postdoctoral Research, 4. Baskaran. S. and Halloran. J. W. Fibrous monolithic ceramics: Tsinghua University, Beijing, 2002. Ill, mechanical properties and oxidation beha 16. Trice. R. w. and Halloran, J W.. Influence of microstructure and carbide/ boron nitride system. J. Am. Ceram. Soc., 1994, 77 mperature on the interfacial fracture energy of silicon nitride/ boron nitride fibrous monolithic ceramics. Am. Ceran. Soc 5. Liu, H. and Hsu. S M, Fracture behavior of multilayer silicon 1999,82,2502-2508
Young’s modulus of the Si3N4 matrix in the sandwiching sample due to its indirectly determination. However,this was solved by concentrating all the deviations from the effecting factors upon the Young’s Modulus in Eqs. (5) and (10). In this way,The E value was successfully adjusted to offset these influences. Acknowledgements The authors gratefully acknowledge Professor Z. D. Guan for his much help in doing three-point bending tests; also this work was supported by National Natural Science Foundation of China (No.59632090). References 1. Clegg,W. J.,Kendall,K.,Alford,N. M.,Birchall,D. and Button,T. W.,A simple way to make tough ceramics. Nature,1990, 347,445–447. 2. Baskaran,S.,Nunn,S. D.,Popovic,Dragan and Halloran,J. W., Fibrous monolithic ceramics: I,fabrication,microstructure, and indentation behavior. J. Am. Ceram. Soc.,1993, 76,2209– 2216. 3. Baskaran,S. and Halloran,J. W.,Fibrous monolithic ceramics: II,flexural strength and fracture bhavior of the slicon carbide/ graphite system. J. Am. Ceram. Soc.,1993, 76,2217–2224. 4. Baskaran,S. and Halloran,J. W.,Fibrous monolithic ceramics: III,mechanical properties and oxidation behavior of the silicon carbide/boron nitride system. J. Am. Ceram. Soc.,1994, 77, 1249–1255. 5. Liu,H. and Hsu,S. M.,Fracture behavior of multilayer silicon nitride/boron nitride ceramic. J. Am. Ceram. Soc.,1996, 79, 2452–2457. 6. Kovar,Desiderio,Thouless,M. D. and Halloran,J.W.,Crack deflection and propagation in layered silicon nitride/boron nitride ceramics. J. Am. Ceram. Soc.,1998, 81,1004–1012. 7. Kovar,D.,King,B. H.,Trice,R. W. and Halloran,J. W., Fibrous monolithic ceramics. J. Am. Ceram. Soc.,1997, 80,2471– 2487. 8. Hai Guo,Yong Huang and Chang-an Wang,Preparation and properties of fibrous monolithic ceramic by in-situ synthesizing. J. Mater. Sci.,1999, 34,2455–2459. 9. Chang-an Wang,Yong Huang,Qingfeng Zan,Hai Guo and Shengyou Cai,Biomimetic structure design—a possible approach to change the brittleness of ceramics in nature. Materials Science & Engineering C,2000, 11,9–12. 10. Charalambides,P. G.,Lund,J.,Evans,A. G. and McMeeking, R. M.,A test specimen for determing the fracture resistance of bimaterial internfaces. J. Appl. Mech,1989, 56,77–82. 11. Phillips,A. J.,Clegg,W. J. and Clyne,T. W.,Fracture behavior of ceramic laminates in bending—II. Comparison of model predictions with experimental data. Acta Metall. Mater.,1993, 41, 819–827. 12. Phillips,A. J.,Clegg,W. J. and Clyne,T. W.,Fracture behavior of ceramic laminates in bending—I. Modelling of crack propagation. Acta Metall. Mater.,1993, 41,805–817. 13. Clyne,T. W. and Phillips,A. J.,Interfacial control and macroscopic failure in long-fiber-reinforced and laminated inorganic composites. Composites Sci. Tech.,1994, 51,271–282. 14. Cao,H. C. and Evans,A. G.,An experimental study of the fracture resistance of bimaterial interfaces. Mechanics of Materials,1989, 7,295–304. 15. Linhua Zou, Interfacial toughness measurement and characterization for Si3N4/BN composites. Report of Postdoctoral Research, Tsinghua University,Beijing,2002. 16. Trice,R.W. and Halloran,J.W.,Influence of microstructure and temperature on the interfacial fracture energy of silicon nitride/ boron nitride fibrous monolithic ceramics. J. Am. Ceram. Soc., 1999, 82,2502–2508. 1996 L. Zou et al. / Journal of the European Ceramic Society 23 (2003) 1987–1996
