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 Al2O3results 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 spec￾trum (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 demon￾strates that the BN interphase had a porous structure. This is the reason that doping with Si3N4 or Al2O3 could strengthen the BN interphase. The earlier￾obtained 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