C. Kaya et al. Joumal of the European Ceramic Sociery 29(2009)1631-1639 Table I The effects of interface materials on the properties of ceramic composite tested Interface material Four-point bending strength, MPa Tensile strength, (RT) MPa Interlaminar shear strength, MPa average pore size, nm ZrO? 266(RT,232(1300°C) a damage-tolerant behaviour at room temperature and compos- detail transmission electron microscopy observations were car- ite failure started with fibre failure followed by multiple matrix ried out. It is seen in Fig 9 that there is no reaction zone between cracking(the first and second peaks as arrowed in the graphs) NdPO4 and alumina matrix, the interface region is clean as evi- and eventually delamination and fibre pull-out took place. Sim- dent by the absence of reaction products, which was confirmed ilar results were observed on the samples tested at 1300C, as also by TEM EDX. It is also seen from the TEM micrograph in shown in Fig. 8b, confirming that the composite plates produced Fig 9 that the grain size of the NdPOa interface is about 200 nm in the present work showed damage-tolerant behaviour at both which should act as a dense barrier preventing oxygen diffusion room temperature and at 1300C. The greater loss in strength from the matrix through the fibres at high temperature in composites with zirconia interface was Table 1 also shows that tensile strength values of the com- attributed to the lower oxidation protection at high temperature posite plates with zirconia and NdPO4 interphases are 136 and offered by zirconia in comparison to NdPO4, which does not 142 MPa, respectively. It is also shown in Table I that interlami- prevent oxygen diffusion through the interface layer. For exam- nar shear strength values are 8.6 and 9. 4 MPa for the coated and ple, the density of the coating layer may play a role at high impregnated fibre samples with zirconia and NdPO4 interphase temperatures. Further experiments are now underway to clarify respectively. These results represent a significant improvement this behaviour and determine the possible reasons for it. It is of the present woven fibre-reinforced CMCs, which exhibit bet important to avoid the occurrence of any reaction between the ter mechanical properties in terms of room temperature tensile coating layer and the reinforcement fibres in order to obtain ideal strength and interfacial bonding strength than previously devel damage-tolerant behaviour by activation of crack deflection and oped composites. The tensile strength and interlaminar shear fibre debonding/pull-out mechanisms. To observe the interface strength are both improved by the addition of liquid binders region of a composite sample containing NdPO4 interface in Fiber failure Matrix Nextel 720M fiber pull-out 00 Displacement mm ZrO NdPO4 interfa 100 nm Displacement mm Fig 8. Load-displacement curves of composites with zirconia and NdPOa inter- containing NdPOa interface indicating that there is no reaction between the aces:(a) at room temperature and (b) at 1300C interface and the fibre1636 C. Kaya et al. / Journal of the European Ceramic Society 29 (2009) 1631–1639 Table 1 The effects of interface materials on the properties of ceramic composite tested. Interface material Four-point bending strength, MPa Tensile strength, (RT) MPa Interlaminar shear strength, MPa Average pore size, nm NdPO4 279 (RT), 266 (1300 ◦C) 142 9.4 80 ZrO2 266 (RT), 232 (1300 ◦C) 136 8.6 70 a damage-tolerant behaviour at room temperature and compos￾ite failure started with fibre failure followed by multiple matrix cracking (the first and second peaks as arrowed in the graphs) and eventually delamination and fibre pull-out took place. Sim￾ilar results were observed on the samples tested at 1300 ◦C, as shown in Fig. 8b, confirming that the composite plates produced in the present work showed damage-tolerant behaviour at both room temperature and at 1300 ◦C. The greater loss in strength at high temperature in composites with zirconia interface was attributed to the lower oxidation protection at high temperature offered by zirconia in comparison to NdPO4, which does not prevent oxygen diffusion through the interface layer. For exam￾ple, the density of the coating layer may play a role at high temperatures. Further experiments are now underway to clarify this behaviour and determine the possible reasons for it. It is important to avoid the occurrence of any reaction between the coating layer and the reinforcement fibres in order to obtain ideal damage-tolerant behaviour by activation of crack deflection and fibre debonding/pull-out mechanisms. To observe the interface region of a composite sample containing NdPO4 interface in Fig. 8. Load–displacement curves of composites with zirconia and NdPO4 inter￾faces: (a) at room temperature and (b) at 1300 ◦C. detail transmission electron microscopy observations were car￾ried out. It is seen in Fig. 9 that there is no reaction zone between NdPO4 and alumina matrix, the interface region is clean as evi￾dent by the absence of reaction products, which was confirmed also by TEM EDX. It is also seen from the TEM micrograph in Fig. 9 that the grain size of the NdPO4 interface is about 200 nm which should act as a dense barrier preventing oxygen diffusion from the matrix through the fibres. Table 1 also shows that tensile strength values of the com￾posite plates with zirconia and NdPO4 interphases are 136 and 142 MPa, respectively. It is also shown in Table 1 that interlami￾nar shear strength values are 8.6 and 9.4 MPa for the coated and impregnated fibre samples with zirconia and NdPO4 interphase, respectively. These results represent a significant improvement of the present woven fibre-reinforced CMCs, which exhibit bet￾ter mechanical properties in terms of room temperature tensile strength and interfacial bonding strength than previously devel￾oped composites.2 The tensile strength and interlaminar shear strength are both improved by the addition of liquid binders Fig. 9. Transmission electron microscopy (TEM) image of the composite sample containing NdPO4 interface indicating that there is no reaction between the interface and the fibre