C. Kaya et al. Journal of the European Ceramic Society 29(2009)1631-1639 10000 Absolute Energy, (Jx10-6) Fig. 12. AE events of the composite subjected to tensile test showing the rela- tionship between amplitude and absolute energy Fig 10. FEG SEM micrograph of a composite specimen with NdPO4 interface tested under tensile loading. to be higher than the energy generated by the ceramic matrix. A closer examination of AE events shows that these high energy and nano-size colloidal yttria that have promoted the sintering aE events also have amplitude higher than 70 dB, as shown in kinetics of the matrix resulting in higher density values Fig. 12. Both energy and amplitude level of the AE signal indi- In our previous studies on CMCs with different oxide cate how much energy is released during the fracture process, matrices,8 the mechanical properties of the final components hence pointing to high strength-high stiffness materials such as have been linked to microstructural variations without consid- ceramic fibres. In total fourteen events have been detected with a ering in detail the possible effects of matrix pore size on the combination of high energy-amplitude and duration values(see properties. In the present work, the average pore size was mea- Figs. 11 and 12). Two of these events significantly differ from sured and found to be smaller than 100 nm for composites with the others as they have 100 times higher energy and duration val- two different interphases, as shown in Table 1. This fine poros- ues with the highest amplitude(78 dB)recorded during the test ity is related to the presence of ultrafine yttria particles(10nm) These two AE events correspond to final failure. Other 12 AE within the matrix that act as sintering additives and provide a events with a combination of high energy-duration and ampli more homogeneous matrix structure in terms of density and pore tude levels could be interpreted as the fracture of 12 fibre mats size distribution within the composite A fractured surface of a composite sample subjected to ten- Further analysis of high amplitude(70 dB) events shows sile test at room temperature is shown in Fig. 10 indicating the that there is a group of AE events with high amplitude and low presence of extensive fibre pull-out due to mechanisms, such as fibre/matrix debonding and crack deflection both initiated by the values of these ae events range between 20 and 60J 10- ature of the NdPO4 weak interface with event duration between 300 and 600 us. These groups Fig. 11 shows a detailed examination of AE parameters of of events could be deduced to be individual fibre fractures energy versus event duration. a distinct tail at energy val- within the fibre mats. The ultimate tensile strength of these ues higher than 100J x 10-8 and event duration higher than fibres could have been decreased(compared to the average 1000 us can be seen clearly. The AE events giving rise to the tail tensile strength) due to damage occurred during processing could be interpreted as fibre fractures because the fractureenergy Other AE events are categorised as matrix related events that generated by high strength and high modulus fibres is expected possibly arise from matrix cracking but also from interfacial When all AE activity is plotted against the total time of test (i.e, the extension of sample), it can be clearly seen that fibre mats fail towards the end of the tensile loading whereas matrix related events occur throughout the test, as shown in Fig. 13 This is a clear indication of damage-tolerant behaviour of the ceramic composites. In this composite system, the fibre mat has been treated to obtain a weak interfacial strength between the ceramic fibre and matrix. AE response shows that while the matrix crack propagates fibre mats stay intact in the crack wake leading to fibre bridging which leads to increased frac ture toughness. Although some individual fibre failures were observed, these did not compromise the overall integrity of the Absolute Energy, (Jx10") composite. It was also seen that when one fibre mat failed due to Fig. 11. AE events of the composite subjected to tensile test showing the rela- increased load transfer from the fractured ceramic matrix,other tionship between duration and absolute energy mats also failed simultaneously within a fraction of a secondC. Kaya et al. / Journal of the European Ceramic Society 29 (2009) 1631–1639 1637 Fig. 10. FEG SEM micrograph of a composite specimen with NdPO4 interface tested under tensile loading. and nano-size colloidal yttria that have promoted the sintering kinetics of the matrix resulting in higher density values. In our previous studies on CMCs with different oxide matrices,2,8 the mechanical properties of the final components have been linked to microstructural variations without consid￾ering in detail the possible effects of matrix pore size on the properties. In the present work, the average pore size was mea￾sured and found to be smaller than 100 nm for composites with two different interphases, as shown in Table 1. This fine poros￾ity is related to the presence of ultrafine yttria particles (10 nm) within the matrix that act as sintering additives and provide a more homogeneous matrix structure in terms of density and pore size distribution. A fractured surface of a composite sample subjected to ten￾sile test at room temperature is shown in Fig. 10 indicating the presence of extensive fibre pull-out due to mechanisms, such as fibre/matrix debonding and crack deflection both initiated by the nature of the NdPO4 weak interface. Fig. 11 shows a detailed examination of AE parameters of energy versus event duration. A distinct tail at energy val￾ues higher than 100 J × 10−18 and event duration higher than 1000s can be seen clearly. The AE events giving rise to the tail could be interpreted as fibre fractures because the fracture energy generated by high strength and high modulus fibres is expected Fig. 11. AE events of the composite subjected to tensile test showing the rela￾tionship between duration and absolute energy. Fig. 12. AE events of the composite subjected to tensile test showing the rela￾tionship between amplitude and absolute energy. to be higher than the energy generated by the ceramic matrix. A closer examination of AE events shows that these high energy AE events also have amplitude higher than 70 dB, as shown in Fig. 12. Both energy and amplitude level of the AE signal indi￾cate how much energy is released during the fracture process, hence pointing to high strength-high stiffness materials such as ceramic fibres. In total fourteen events have been detected with a combination of high energy-amplitude and duration values (see Figs. 11 and 12). Two of these events significantly differ from the others as they have 100 times higher energy and duration val￾ues with the highest amplitude (78 dB) recorded during the test. These two AE events correspond to final failure. Other 12 AE events with a combination of high energy-duration and ampli￾tude levels could be interpreted as the fracture of 12 fibre mats within the composite. Further analysis of high amplitude (≥70 dB) events shows that there is a group of AE events with high amplitude and low energy and duration values as shown in Fig. 12. The energy values of these AE events range between 20 and 60 J × 10−18 with event duration between 300 and 600 s. These groups of events could be deduced to be individual fibre fractures within the fibre mats. The ultimate tensile strength of these fibres could have been decreased (compared to the average tensile strength) due to damage occurred during processing. Other AE events are categorised as matrix related events that possibly arise from matrix cracking but also from interfacial failure. When all AE activity is plotted against the total time of test (i.e., the extension of sample), it can be clearly seen that fibre mats fail towards the end of the tensile loading, whereas matrix related events occur throughout the test, as shown in Fig. 13. This is a clear indication of damage-tolerant behaviour of the ceramic composites. In this composite system, the fibre mat has been treated to obtain a weak interfacial strength between the ceramic fibre and matrix. AE response shows that while the matrix crack propagates fibre mats stay intact in the crack wake leading to fibre bridging, which leads to increased frac￾ture toughness. Although some individual fibre failures were observed, these did not compromise the overall integrity of the composite. It was also seen that when one fibre mat failed due to increased load transfer from the fractured ceramic matrix, other mats also failed simultaneously within a fraction of a second.