C. Kaya, E.G. Butler /Joumal of the European Ceramic Society 29(2009)363-367 the specimen was at this temperature. Tensile tests were per- formed at room temperature. For flexural and tensile tests, constant cross-head speed of 0.5 mm/min was used For all the mechanical results reported, seven samples were used for each value by eliminating the highest and the lowest values and taking the average value of the remaining five readings 2.5. Microstructural characterisation Microstructural examinations were carried out on sintered Fiber and fractured composite samples using a high-resolution field emission gun(FEG)SEM(FX-4000, Jeol Ltd, Japan). Further detail observations were carried out using transmission electron microscopy (TEM, Jeol Ltd, Japan, 4000 FX TEM) operat- ng at 400 kV, and equipped with an energy dispersive X-Ray Fiber/matrix analysis(EDX) unit. Porosity(%)and pore size were mea- interface sured using a mercury porosimeter(Hg Por, Micromechanics Instrument Corp, USA)using a penetrometer weight of 62.79 g, NdPO head pressure of 4. 45 psia and penetration volume of 6. 188 mL Coating Archimedes technique was also used for density measurements 3. Results and discussion Fan wme 00 nm Fig. 2 shows the SEM microstructure of dip-coated unidirec tional Nextel 720 fibers with NdPO4 interface material after Fig 3. Transmission electron microscopy (TEM)image of the composite sample sintering at 600C for 0.5 h indicating that the coating layer containing NdPO4 interface indicating that there is no reaction between the is very homogeneous, dense and its thickness is less than 2 um. interface and the fiber When oxide/oxide composites are designed for high temperature pplications, it is fundamental that a compatible weak interface actually caused by the fiber damage during processing and there- between the reinforcement fiber and ceramic matrix should be fore the dense coating structure shown in Fig. 2 helps to protect provided in order to obtain a damage-tolerant behaviour due to the fiber's surface from faws during manufacturing steps, esp mechanisms of crack deflection, debonding and fiber pull-out cially during impregnation and consolidation within the hollow that all contribute to increasing the toughness. Dense interfaces are also desirable plastic tube subjected to heat. Fig. 2 also shows that the NdPO4 as the protect the fibers at high tempera- coating layer is non-porous and quite dense and there is no vis- ture against heat and oxygen diffusion which may cause grain ible evidence of reaction taking place between the coating layer growth and loss in mechanical properties. Furthermore, it is well and the fiber. Further examinations were also conducted using established that significant reduction in mechanical properties is transmission electron microscopy to confirm that there was no reaction zone at the interfaces between NdPOa and fiber as in the TEM micrograph shown in Fig 3. As shown in Fig 3, the inter facial zone between the fiber and the NdPO4 interface is very clear and there is no strong g at this point that proves the NdPO4 absence of any chemical reaction between the interface mate- coating rial and the fibers. TEM EDX analysis on the interfacial zone laver also indicated that there was no reaction product at that region that explained the absence of any undesirable reactions(see also Fig 5b showing the clear surface of the pulled out fibers that also proves the absence of any strong bonding between the fibers and the interface materials). This is very important to improve the flaw tolerance and work of fracture of th to obtain non-catastrophic mode of failure due to presence of an interface which is weak enough to deflect propagating cracks which will lead to substantial energy dissipation and improve ed unidirectionalNextelzz0M ber with fracture toughness. Fig. 3 also shows that the grain size of a crack-deflecting NdPO4 interface after sintering at 600C for 0.5 h indicating he homogeneous and dense structure of the coating layer with a thickness of coating microstructure(see Fig. 2)resulted from sinter-active starting particles of NdPO4C. Kaya, E.G. Butler / Journal of the European Ceramic Society 29 (2009) 363–367 365 the specimen was at this temperature. Tensile tests were per￾formed at room temperature. For flexural and tensile tests, a constant cross-head speed of 0.5 mm/min was used. For all the mechanical results reported, seven samples were used for each value by eliminating the highest and the lowest values and taking the average value of the remaining five readings. 2.5. Microstructural characterisation Microstructural examinations were carried out on sintered and fractured composite samples using a high-resolution field emission gun (FEG) SEM (FX-4000, Jeol Ltd., Japan). Further detail observations were carried out using transmission electron microscopy (TEM, Jeol Ltd., Japan, 4000 FX TEM) operat￾ing at 400 kV, and equipped with an energy dispersive X-Ray analysis (EDX) unit. Porosity (%) and pore size were mea￾sured using a mercury porosimeter (Hg Por, Micromechanics Instrument Corp., USA) using a penetrometer weight of 62.79 g, head pressure of 4.45 psia and penetration volume of 6.188 mL. Archimedes technique was also used for density measurements. 3. Results and discussion Fig. 2 shows the SEM microstructure of dip-coated unidirec￾tional Nextel 720TM fibers with NdPO4 interface material after sintering at 600 ◦C for 0.5 h indicating that the coating layer is very homogeneous, dense and its thickness is less than 2 m. When oxide/oxide composites are designed for high temperature applications, it is fundamental that a compatible weak interface between the reinforcement fiber and ceramic matrix should be provided in order to obtain a damage-tolerant behaviour due to mechanisms of crack deflection, debonding and fiber pull-out that all contribute to increasing the toughness. Dense interfaces are also desirable as they protect the fibers at high tempera￾ture against heat and oxygen diffusion which may cause grain growth and loss in mechanical properties. Furthermore, it is well established that significant reduction in mechanical properties is Fig. 2. SEM microstructure of dip-coated unidirectional Nextel 720TM fiber with a crack-deflecting NdPO4 interface after sintering at 600 ◦C for 0.5 h indicating the homogeneous and dense structure of the coating layer with a thickness of less than 2m. Fig. 3. Transmission electron microscopy (TEM) image of the composite sample containing NdPO4 interface indicating that there is no reaction between the interface and the fiber. actually caused by the fiber damage during processing and there￾fore the dense coating structure shown in Fig. 2 helps to protect the fiber’s surface from flaws during manufacturing steps, espe￾cially during impregnation and consolidation within the hollow plastic tube subjected to heat. Fig. 2 also shows that the NdPO4 coating layer is non-porous and quite dense and there is no vis￾ible evidence of reaction taking place between the coating layer and the fiber. Further examinations were also conducted using transmission electron microscopy to confirm that there was no reaction zone at the interfaces between NdPO4 and fiber as in the TEM micrograph shown in Fig. 3. As shown in Fig. 3, the inter￾facial zone between the fiber and the NdPO4 interface is very clear and there is no strong bonding at this point that proves the absence of any chemical reaction between the interface mate￾rial and the fibers. TEM EDX analysis on the interfacial zone also indicated that there was no reaction product at that region that explained the absence of any undesirable reactions (see also Fig. 5b showing the clear surface of the pulled out fibers that also proves the absence of any strong bonding between the fibers and the interface materials). This is very important to improve the flaw tolerance and work of fracture of the composite and also to obtain non-catastrophic mode of failure due to presence of an interface which is weak enough to deflect propagating cracks which will lead to substantial energy dissipation and improve fracture toughness. Fig. 3 also shows that the grain size of the NdPO4 interface is about 250 nm which explains the dense coating microstructure (see Fig. 2) resulted from sinter-active starting particles of NdPO4.