1638 C. Kaya et al. Joumal of the European Ceramic Sociery 29(2009)1631-1639 100000 temperatures(1100.C). 2 The innovative approach to matrix formulation adopted in the present work results in a sinter- active alumina with the addition of ultra-fine particles to promote 10000 sintering kinetics and avoiding the use of other detrimental sintering aids like silica. Electrophoretic deposition coating of 2D fibre mats with nano-sized particles was successfully applied and coated samples were fully impregnated with alu- 1000 mina matrix using low temperature warm pressing leading to formation of damage-tolerant CMCs with superior properties. The effectiveness of this new approach is evidenced in the outstanding properties achieved(flexural strengths of 279 and 266 MPa at room temperature and at 1300C, respectively, in four-point bending test and a room temperature tensile stre Fig. 13. AE events of the composite subjected to tensile test showing the rela of 142 MPa for the composite samples containing 40 vol. fibre tionship between duration and extension loading, NdPO4 interface and an overall porosity of 10%0) In addition, as a significant improvement, interlaminar shear strength of the multilayer composites was also improved to over Further proof of fibre bridging can be seen in the fractogra- 9 MPa compared to the value of 4 MPa obtained in our previous phy where exposed fibre ends indicate interfacial debonding studies. 2 Currently, thermomechanical properties in terms ind crack front deviation due to weak interfacial strength(see creep and fatigue behaviour as well as ballistic impact and frac ture toughness are under investigation. Based on the properties As a conclusion from the results presented Figs. 11-13, AE obtained, these composites could be candidate for use as shroud is confirmed to be a powerful NDT technique for ceramic com- seals or insulating layers for combustor chambers in aircraft posite characterisation providing that it is used in conjunction engines with other evaluation techniques, such as fractography, in order to draw a more complete picture of the relevant fracture mech nisms. Different fracture mechanisms such as matrix fracture 4. Conclusions and fibre failure could be identified using AE event parameters of energy, amplitude and duration. The identification of failure Nextel 720 M fibre-reinforced alumina matrix composites mechanisms such as interfacial separation could be rather diffi- with two different interface materials(NdPO4 and ZrO2)were cult as the low energy released by such a weak source could be manufactured using electrophoretic deposition, colloidal infil- easily masked by the ae burst caused by fracturing fibres. ae tration and subsequent warm pressing at 180C to consolidate results indicate that the fracture of these oxide matrix composites a green body in multilayer form. After pressureless sintering started with matrix crack initiation and propagation. Although at 1200C for 2 h, the final composite had 40 vol. fibre and a few individual fibre fractures were detected the majority of 10% porosity with an average pore size smaller than 100 nm. fibres did not fail until the final failure. Particularly, the fibre mats The samples with dense NdPO4 coating layer showed damage did not fail until the peak load was reached indicating the fibre tolerant fracture behaviour at room temperature and at 1300C bundles within the fibre mats stayed intact in the crack wake. during flexural strength test. Interlaminar shear strength of the This behaviour is attributed to the weak interfacial strength CMC samples was determined to be over 9 MPa with a ten- and fibre bridging, which in turn increases the toughness of the sile strength of 142 MPa. AE results indicated that fracture of composite. the composite started with matrix crack initiation and propaga Overall, the present investigation has shown the devel- tion Although a few individual fibre fractures were detect opment of an optimised and reproducible CMC fabrication majority of fibres did not fail until the final composite route through understanding the relationship between matrix Specifically, the fibre mats did not fail until the peak load was precursor chemistry, process parameters and final mechanical attained indicating that the fibre bundles within the fibre mats properties. The key to attain optimum mechanical properties stayed intact in the crack wake. This is due to the low interfacial in Nextel 720M fibre-reinforced alumina ceramic matrix com- strength and results in fibre bridging, which should increase the posites with NdPO4 interface is the development of a strong toughness of the composite and dense matrix capable of efficient load transfer between rein- forcement fibres in conjunction with retention of fibre properties after processing. The requirement for high integrity matrices Acknowledgements an lead to fibre degradation due to high sintering tempera ures necessary to achieve high densification overcoming back TUBITAK (Turkish Science and Technological Research stresses imposed by the fibres In the past, the addition of sil- Council), EU Commission and YildIz Technical University ica to alumina matrices as sintering aid has resulted in loss of Istanbul are acknowledged for financial support. Richard of high temperature mechanical properties, in particular time Brown is acknowledged for some parts of the experimental dependant properties (creep and fatigue), even at moderate work1638 C. Kaya et al. / Journal of the European Ceramic Society 29 (2009) 1631–1639 Fig. 13. AE events of the composite subjected to tensile test showing the rela￾tionship between duration and extension. Further proof of fibre bridging can be seen in the fractogra￾phy where exposed fibre ends indicate interfacial debonding and crack front deviation due to weak interfacial strength (see Fig. 10). As a conclusion from the results presented Figs. 11–13, AE is confirmed to be a powerful NDT technique for ceramic com￾posite characterisation providing that it is used in conjunction with other evaluation techniques, such as fractography, in order to draw a more complete picture of the relevant fracture mech￾anisms. Different fracture mechanisms such as matrix fracture and fibre failure could be identified using AE event parameters of energy, amplitude and duration. The identification of failure mechanisms such as interfacial separation could be rather diffi- cult as the low energy released by such a weak source could be easily masked by the AE burst caused by fracturing fibres. AE results indicate that the fracture of these oxide matrix composites started with matrix crack initiation and propagation. Although a few individual fibre fractures were detected the majority of fibres did not fail until the final failure. Particularly, the fibre mats did not fail until the peak load was reached indicating the fibre bundles within the fibre mats stayed intact in the crack wake. This behaviour is attributed to the weak interfacial strength and fibre bridging, which in turn increases the toughness of the composite. Overall, the present investigation has shown the devel￾opment of an optimised and reproducible CMC fabrication route through understanding the relationship between matrix precursor chemistry, process parameters and final mechanical properties. The key to attain optimum mechanical properties in Nextel 720TM fibre-reinforced alumina ceramic matrix com￾posites with NdPO4 interface is the development of a strong and dense matrix capable of efficient load transfer between rein￾forcement fibres in conjunction with retention of fibre properties after processing. The requirement for high integrity matrices can lead to fibre degradation due to high sintering tempera￾tures necessary to achieve high densification overcoming back stresses imposed by the fibres. In the past, the addition of sil￾ica to alumina matrices as sintering aid has resulted in loss of high temperature mechanical properties, in particular time dependant properties (creep and fatigue), even at moderate temperatures (1100 ◦C).12 The innovative approach to matrix formulation adopted in the present work results in a sinter￾active alumina with the addition of ultra-fine particles to promote sintering kinetics and avoiding the use of other detrimental sintering aids like silica. Electrophoretic deposition coating of 2D fibre mats with nano-sized particles was successfully applied and coated samples were fully impregnated with alu￾mina matrix using low temperature warm pressing leading to formation of damage-tolerant CMCs with superior properties. The effectiveness of this new approach is evidenced in the outstanding properties achieved (flexural strengths of 279 and 266 MPa at room temperature and at 1300 ◦C, respectively, in four-point bending test and a room temperature tensile strength of 142 MPa for the composite samples containing 40 vol.% fibre loading, NdPO4 interface and an overall porosity of 10%). In addition, as a significant improvement, interlaminar shear strength of the multilayer composites was also improved to over 9 MPa compared to the value of 4 MPa obtained in our previous studies.12 Currently, thermomechanical properties in terms of creep and fatigue behaviour as well as ballistic impact and frac￾ture toughness are under investigation. Based on the properties obtained, these composites could be candidate for use as shroud seals or insulating layers for combustor chambers in aircraft engines. 4. Conclusions Nextel 720TM fibre-reinforced alumina matrix composites with two different interface materials (NdPO4 and ZrO2) were manufactured using electrophoretic deposition, colloidal infil￾tration and subsequent warm pressing at 180 ◦C to consolidate a green body in multilayer form. After pressureless sintering at 1200 ◦C for 2 h, the final composite had 40 vol.% fibre and 10% porosity with an average pore size smaller than 100 nm. The samples with dense NdPO4 coating layer showed damage￾tolerant fracture behaviour at room temperature and at 1300 ◦C during flexural strength test. Interlaminar shear strength of the CMC samples was determined to be over 9 MPa with a ten￾sile strength of 142 MPa. AE results indicated that fracture of the composite started with matrix crack initiation and propaga￾tion. Although a few individual fibre fractures were detected the majority of fibres did not fail until the final composite failure. Specifically, the fibre mats did not fail until the peak load was attained indicating that the fibre bundles within the fibre mats stayed intact in the crack wake. This is due to the low interfacial strength and results in fibre bridging, which should increase the toughness of the composite. Acknowledgements TUBITAK (Turkish Science and Technological Research Council), EU Commission and Yıldız Technical University of Istanbul are acknowledged for financial support. Richard Brown is acknowledged for some parts of the experimental work