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 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 Eqs. (5) and (10),the interfacial toughness versus dis￾placement curves of the samples with the BN, BN+15vol.%Si3N4 and BN+25vol.%Si3N4 inter￾phases 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 corre￾sponding 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 addi￾tion,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