C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 crack deflection (and/or crack arrest) may take place deflection and the presence of extensive fibre pull-out at within the matrix as well as at the interphase/fibre and room temperature and at 1300C are evident from the interphase/matrix interfaces both promoting the stress-displacement curves shown in Fig. 6a and from damage-tolerant behaviour. The occurrence of crack the SEM micrographs shown in Fig. 6b and c. Linear elastic deformation is recorded until the first matrix 235MP crack is formed at the maximum stress and then the dominant mechanisms are fibre pull-out and debonding until the composite fails in a non-catastrophic manner. TEM observations were carried out along with EDX analysis on a sample tested at 1300C to investigate in detail the microstructure of the zones between NdPO4 and fibre and between NdPOa and matrix, as shown in 230MP Fig. 7. This was also carried out in order to identify any possible reaction products caused by the reactions at the relevant interfaces. TEM observation coupled with EDX analysis of the interfacial regions(interphase/matrix and interphase/fibre)revealed that there was no reaction zone or new product formation during composite sintering or deflection, mm high temperature testing, as shown in Fig. 7a and b. It is also clear from both pictures that the interphase is very dense but bonding between interphase and matrix and between interphase and fibre is sufficiently weak, leading to the occurrence of fibre debonding, crack deflection and fibre pull out during composite failure The acceptable thermomechanical behaviour of the composite was also confirmed by the thermal cycling tests conducted from a high temperature of 1150C, as the results in Table 3 show. After up to 300 cycles, the samples retained a high flexural strength(>85% of the strength of the as-fabricated material). Moreover the fracture behaviour of the thermally cycled samples was quasi-ductile'"and similar to that of the as-fabricated specimens. A SEM micrograph showing the surface of thermally cycled sample is shown in Fig 8. No forma- tion of microcracks or evidence of other surface micro- structural damage can be observed, indicating that the conditions investigated(thermal cycling between room temperature and 1150oC in air)are not severe enough to cause damage in these composites. Whether or not the presence of a fine dispersion of closed pores in the matrix(Fig 5b )is beneficial for the enhancing the ther- mal cycling resistance of the composites remains to be investigated. Experiments are planned using more Table 3 Variation of flexural strength of mullite fibre- reinforced mullite matrix composites with NdPOa interphase after thermal cycling between room temperature and 1 150C for different number of cycles Number of strength(MPa) Fig. 6.(a)Stress-deflection curves for mullite fibre-reinforced mullite 0 at room temperature and at 1300C.(b)SEM micrograph of the fracture 220 surface of the composite tested at room temperature and(C) SEM 300 icrograph of the fracture surface of the composite tested at 1300oCcrack deflection (and/or crack arrest) may take place within the matrix as well as at the interphase/fibre and interphase/matrix interfaces both promoting the damage-tolerant behaviour.The occurrence of crack deflection and the presence of extensive fibre pull-out at room temperature and at 1300
C are evident from the stress-displacement curves shown in Fig.6a and from the SEM micrographs shown in Fig.6b and c.Linear elastic deformation is recorded until the first matrix crack is formed at the maximum stress and then the dominant mechanisms are fibre pull-out and debonding until the composite fails in a non-catastrophic manner. TEM observations were carried out along with EDX analysis on a sample tested at 1300
C to investigate in detail the microstructure of the zones between NdPO4 and fibre and between NdPO4 and matrix, as shown in Fig.7.This was also carried out in order to identify any possible reaction products caused by the reactions at the relevant interfaces.TEM observation coupled with EDX analysis of the interfacial regions (interphase/matrix and interphase/fibre) revealed that there was no reaction zone or new product formation during composite sintering or high temperature testing, as shown in Fig.7a and b.It is also clear from both pictures that the interphase is very dense but bonding between interphase and matrix and between interphase and fibre is sufficiently weak, leading to the occurrence of fibre debonding, crack deflection and fibre pull out during composite failure. The acceptable thermomechanical behaviour of the composite was also confirmed by the thermal cycling tests conducted from a high temperature of 1150
C, as the results in Table 3 show.After up to 300 cycles, the samples retained a high flexural strength (> 85% of the strength of the as-fabricated material).Moreover the fracture behaviour of the thermally cycled samples was ‘‘quasi-ductile’’ and similar to that of the as-fabricated specimens.A SEM micrograph showing the surface of a thermally cycled sample is shown in Fig.8.No forma￾tion of microcracks or evidence of other surface micro￾structural damage can be observed, indicating that the conditions investigated (thermal cycling between room temperature and 1150
C in air) are not severe enough to cause damage in these composites.Whether or not the presence of a fine dispersion of closed pores in the matrix (Fig.5b) is beneficial for the enhancing the ther￾mal cycling resistance of the composites remains to be investigated.Experiments are planned using more Table 3 Variation of flexural strength of mullite fibre-reinforced mullite matrix composites with NdPO4 interphase after thermal cycling between room temperature and 1150
C for different number of cycles Number of cycles Four-point flexural strength (MPa) 0 235 95 231 220 207 300 201 Fig.6.(a) Stress-deflection curves for mullite fibre-reinforced mullite matrix composites with NdPO4 interface obtained in flexure strength test at room temperature and at 1300
C, (b) SEM micrograph of the fracture surface of the composite tested at room temperature and (c) SEM micrograph of the fracture surface of the composite tested at 1300
C. 2340 C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342