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 des￾cending 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 in￾thickness 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