F Kaya/Ceramics International 33(2007)279-284 (b Fig 9. SEM micrographs of the composite samples with different porosity level and pore diameter: (a) porosity level of 20% with the average pore size of 90 nm and (b) porosity level of 30% with the average pore diameter of 200 nm. (b) 38O. sEM micrographs of the fractured composite samples after tensile tests: (a)porosity level of 20% with the average pore size of 90 nm and(b)porosity level of with the average pore diameter of 200 nm. The apparent difference in microstructure of the two are acting as crack initiation sites (as arrowed on the omposite plates is clearly seen from the SEM micrograph micrograph shown in Fig. 10b)and therefore the cracks that shown in Fig. 9. The microstructure shown in Fig. 9a represents are formed in these regions propagate within the brittle ceramic the structure of the composite with a pore size of 90 nm and matrix resulting in final failure of the composite sample in a lower porosity level of 20% whilst the microstructure shown in short time. The apparent difference in fracture behaviour of the Fig. 9b indicates the presence of large pores of about 200 nm two different composite samples after tensile tests is also and higher level of porosity up to 30%. The fracture behaviour clearly seen in Fig. 11 of the composite plates shown in Fig 9 is also different from As shown in Fig. I la, lower porosity level and pore size cause each other as shown in Fig. 10 some degree of fibre pull-out which indicates the presence of As shown in Fig. 10a, the results obtained from the tensile damage-tolerant behaviour with in the composite sample tests confirm that composite plate with lower porosity level and However, when the pore size and porosity level are quite high smaller pore size exhibit homogeneous fracture behaviour as 200 nm and 30%, respectively in this case, brittle fracture most of the fibres failed at the similar loads. However, the behaviours seen within the composite plate as shownin Fig 11b microstructure of the sample with higher porosity level (30%0) The microstructures shown in Figs. 10b and llb prove that large behaviour as shown in Fig. 10b which indicates that large pores cracks are acting as crack initiation sites or large fas s these and larger pore size(200 nm)shows very different fracture pores are responsible for the sudden failure of the sampl (b) Fig. 11. Micrographs of the fractured composite samples after tensile tests: (a) porosity level of 20% with the average pore size of 90 nm and (b) porosity level of 30% with the average pore diameter of 200 nm.The apparent difference in microstructure of the two composite plates is clearly seen from the SEM micrograph shown in Fig. 9. The microstructure shown in Fig. 9a represents the structure of the composite with a pore size of 90 nm and lower porosity level of 20% whilst the microstructure shown in Fig. 9b indicates the presence of large pores of about 200 nm and higher level of porosity up to 30%. The fracture behaviour of the composite plates shown in Fig. 9 is also different from each other as shown in Fig. 10. As shown in Fig. 10a, the results obtained from the tensile tests confirm that composite plate with lower porosity level and smaller pore size exhibit homogeneous fracture behaviour as most of the fibres failed at the similar loads. However, the microstructure of the sample with higher porosity level (30%) and larger pore size (200 nm) shows very different fracture behaviour as shown in Fig. 10b which indicates that large pores are acting as crack initiation sites (as arrowed on the micrograph shown in Fig. 10b) and therefore the cracks that are formed in these regions propagate within the brittle ceramic matrix resulting in final failure of the composite sample in a short time. The apparent difference in fracture behaviour of the two different composite samples after tensile tests is also clearly seen in Fig. 11. As shown in Fig. 11a, lower porosity level and pore size cause some degree of fibre pull-out which indicates the presence of damage-tolerant behaviour with in the composite sample. However, when the pore size and porosity level are quite high, 200 nm and 30%, respectively in this case, brittle fracture behaviouris seen within the composite plate as shown in Fig. 11b. The microstructures shown in Figs. 10b and 11b prove that large pores are responsible for the sudden failure of the sample as these cracks are acting as crack initiation sites or large flaws. F. Kaya / Ceramics International 33 (2007) 279–284 283 Fig. 9. SEM micrographs of the composite samples with different porosity level and pore diameter: (a) porosity level of 20% with the average pore size of 90 nm and (b) porosity level of 30% with the average pore diameter of 200 nm. Fig. 11. Micrographs of the fractured composite samples after tensile tests: (a) porosity level of 20% with the average pore size of 90 nm and (b) porosity level of 30% with the average pore diameter of 200 nm. Fig. 10. SEM micrographs of the fractured composite samples after tensile tests: (a) porosity level of 20% with the average pore size of 90 nm and (b) porosity level of 30% with the average pore diameter of 200 nm