J. She et al. Materials Science and Engineering 4325 (2002)19-24 the crack deflection characteristics of the matrix. as discussed below Fig. 4 shows the flexural strength behavior of porous mullite/mullite composites with multiple infiltrations For comparison, the flexural response of the e un- infiltrated specimen is also shown in Fig. 4. As can be seen, all the curves exhibit an initial linear elastic region until a maximum flexural stress is reached. Beyond the stress maximum, the infiltrated specimens with N<4 show a gradual decrease in stress with further cross- head displacement. This behavior is typical of fib 200 inforcedceramic-matrix composites withweak fiber/matrix interfaces. As shown in Fig. 5(a), the com- posites exhibit extensive fiber pullout, indicating the effectiveness of the porous matrix as a crack deflection medium. However, some holes, which might be formed Fig. 3. Microstructure of the surface region of a porous mullite/mul- lite comp s200 15 um 9150 8 Fig 2 SEM micrographs of the polished section of a porous mullite 100 mullite composite after six infiltrations:(a)overview, (b) close view of he surface region, and (c) close view of the interior region some lumina bridges' are formed at the interparticle necks or at the contact points between the fibers and 0 the matrix. These alumina 'bridges' would result in the formation of closed pores, preventing further infiltra tion. In addition, such alumina "bridges may signifi Displacement(um) cantly strengthen the interparticle bonds as well as the Fig 4. Effect of multiple infiltrations on the flexural strength behav. fiber/matrix interfaces, weakening or even eliminating ior of porous mullite/mullite compositesJ. She et al. / Materials Science and Engineering A325 (2002) 19–24 21 Fig. 2. SEM micrographs of the polished section of a porous mullite/ mullite composite after six infiltrations: (a) overview, (b) close view of the surface region, and (c) close view of the interior region. the crack deflection characteristics of the matrix, as discussed below. Fig. 4 shows the flexural strength behavior of porous mullite/mullite composites with multiple infiltrations. For comparison, the flexural response of the un￾infiltrated specimen is also shown in Fig. 4. As can be seen, all the curves exhibit an initial linear elastic region until a maximum flexural stress is reached. Beyond the stress maximum, the infiltrated specimens with N4 show a gradual decrease in stress with further cross￾head displacement. This behavior is typical of fiber-re￾inforced ceramic-matrix composites with weak fiber/matrix interfaces. As shown in Fig. 5(a), the com￾posites exhibit extensive fiber pullout, indicating the effectiveness of the porous matrix as a crack deflection medium. However, some holes, which might be formed in the matrix as a result of fiber pullout, were not Fig. 3. Microstructure of the surface region of a porous mullite/mul￾lite composite after ten infiltration cycles. Fig. 4. Effect of multiple infiltrations on the flexural strength behav￾ior of porous mullite/mullite composites. some alumina ‘bridges’ are formed at the interparticle necks or at the contact points between the fibers and the matrix. These alumina ‘bridges’ would result in the formation of closed pores, preventing further infiltra￾tion. In addition, such alumina ‘bridges’ may signifi- cantly strengthen the interparticle bonds as well as the fiber/matrix interfaces, weakening or even eliminating