476 C Li et al. Materials Letters 57(2003)3473-3478 the eutectic temperature for the MYa type of sintering The load-displacement curves of both materials at aid is obviously lower than that for the LYa type of room temperature and at 1300C are shown in Fig 3 sintering aid, [9] therefore, the viscosity of the liquid From Fig. 3(a), both of MYA-LCs and LYA-LCs phase in the MYA-LCs at the sintering temperature is exhibit nonlinear fracture characteristics at room tem lower than that in the LYA-LCs. This brings about two perature, however, MYA-LCs has a higher load level effects. One is that the lower viscosity of the liquid but smaller area under the curve than those in LYA- phase is in favor of the migration of the liquid phase LCs. These agree well to the mechanical properties of from the Si3N4 matrix layers to the BN interlayers, the two laminated ceramics, as mentioned above which is good for the densification of the BN inter- The load increases linearly until a crack is initiated layers. This is the reason why the bending strength of from the tensile surface and propagates into the the MYA-LCs at the room temperature is a little bit nearby BN-containing interphase, where the crack is higher thatthat of the LYA-LCs, however, the denser deflected and arrested. Subsequent specimen deflec- interlayers are not good for the interface fracture tion causes the delamination cracks to propagate in the resistance to the fracture, so the work of fracture is interphase at an almost constant load. As displacement lower in the MYA-LCs than in the LYA-LCs. At 1300 of the specimen is continued, the load again begins to C, the grain boundary glassy phase starts to melt and increase linearly to a lower level until the uncracked dominates the mechanical properties of the laminated portion of the beam cannot support the applied load ceramics. Since the melting point of the glassy phase anymore. A crack then initiates in the Si3 N4 layer in MYA-LCs is lower than that in LYA-LCs [91, the close to the delamination crack and propagates until bending strength and work of fracture in LYA-LCs are is deflected in the next BN-containing interphase much higher than those in MYA-LCs Much energy is dissipated during this process. The 500u Fig 4. SEM micrographs showing zigzag crack path of two materials:(a) MYA-LCs and(b) LYA-LCsthe eutectic temperature for the MYA type of sintering aid is obviously lower than that for the LYA type of sintering aid, [9] therefore, the viscosity of the liquid phase in the MYA-LCs at the sintering temperature is lower than that in the LYA-LCs. This brings about two effects. One is that the lower viscosity of the liquid phase is in favor of the migration of the liquid phase from the Si3N4 matrix layers to the BN interlayers, which is good for the densification of the BN inter￾layers. This is the reason why the bending strength of the MYA-LCs at the room temperature is a little bit higher that that of the LYA-LCs, however, the denser interlayers are not good for the interface fracture resistance to the fracture, so the work of fracture is lower in the MYA-LCs than in the LYA-LCs. At 1300 jC, the grain boundary glassy phase starts to melt and dominates the mechanical properties of the laminated ceramics. Since the melting point of the glassy phase in MYA-LCs is lower than that in LYA-LCs [9], the bending strength and work of fracture in LYA-LCs are much higher than those in MYA-LCs. The load –displacement curves of both materials at room temperature and at 1300 jC are shown in Fig. 3. From Fig. 3(a), both of MYA-LCs and LYA-LCs exhibit nonlinear fracture characteristics at room tem￾perature, however, MYA-LCs has a higher load level but smaller area under the curve than those in LYA￾LCs. These agree well to the mechanical properties of the two laminated ceramics, as mentioned above. The load increases linearly until a crack is initiated from the tensile surface and propagates into the nearby BN-containing interphase, where the crack is deflected and arrested. Subsequent specimen deflec￾tion causes the delamination cracks to propagate in the interphase at an almost constant load. As displacement of the specimen is continued, the load again begins to increase linearly to a lower level until the uncracked portion of the beam cannot support the applied load anymore. A crack then initiates in the Si3N4 layer close to the delamination crack and propagates until it is deflected in the next BN-containing interphase. Much energy is dissipated during this process. The Fig. 4. SEM micrographs showing zigzag crack path of two materials: (a) MYA-LCs and (b) LYA-LCs. 3476 C. Li et al. / Materials Letters 57 (2003) 3473–3478