C Kaya et al. Journal of the European Ceramic Society 22(2002)2333-2342 2341 severe thermal cycling conditions, e.g. by using forced air cooling and higher cycle frequency 4. Conclusions Fibre a damage-tolerant mullite fibre(Nextel M 720)-rein forced mullite matrix composite with a weak NdPO4 interphase was successfully produced using electro- phoretic deposition and pressure filtration techniques followed by pressureless sintering. The composite pro duced is capable of retaining room temperature flexural 00 nm strength and damage-tolerance at elevated temperatures ult of the dense, chemically and th mally stable NdPO4 interphase and the weak bonding between this interphase and both the matrix and fibres. Thermal cycling from 1150C to room temperature did not result in appreciable damage of the composite, and the samples retained a high flexural strength and"composite behaviour after thermal cycling for up to 300 cycles. It is concluded that the dense NdPO4 interphase improves damage-tolerant behaviour at room and high tempera NdPO tures if:(1)the bonding at the interfaces interphase-fibre Matrix and interphase-matrix is weak enough, which leads to extensive fibre debonding, crack deflection and fibre pull-out, and (2) the mullite matrix contains porosity with homogeneously distributed fine pores 500nm Acknowledgements Fig. 7. Bright-field TEM images of the interfacial between(a Professors m.h. loretto and p bowen are acknowl NdPO, interphase-mullite fibre and(b)NdPO4 interphase-mullite edged for the provision of laboratory facilities at the matrix, both showing the absence of any reaction products or zone IRC and School of Metallurgy and Materials, at the University of Birmingham, respectively. Partial financial support by the European Commission under the contract numbers BRITe-euRaM Ct97-0609 and CT 95-0110 is also acknowledged. ARB acknowledges support of the Nuffield Foundation(London) References L. Evans. A. G. and Marshall. D. B. High toughness ceramics and ceramic composites. Progress in Materials Science, 1989, 33, 85-90 2. Warren, R. and Deng, S, Continuous fibre reinforced ceramic tes fo 3. Chawla. K he matrix composites. JOM-J. Min Met. Mater., 1995, 47, 19-21 4. Porter, J.R., Reinforcements for ceramic-matrix composites for elevated temperature applications. Mater. Sci. Eng, 1993, A166, 179-184. 5. Holmquist, M, Lund berg, R, Sudre, O, Razzell, A G, Molliex L. Benoit, J. and Adlerborn, J. Alumin a porous airco d component testing. J. Eur. Ceram. Soc., 2000, 20, 599-606 Fig 8. SEM micrograph of the surface of a thermally cycled sample. Interface howing no evidence of microcracking or of other form of superficial oxide fibre/oxide matrix composites. Int. Mater. Rev., 2000, 165-1severe thermal cycling conditions, e.g. by using forced air cooling and higher cycle frequency. 4. Conclusions A damage-tolerant mullite fibre (NextelTM 720)-rein￾forced mullite matrix composite with a weak NdPO4 interphase was successfully produced using electro￾phoretic deposition and pressure filtration techniques followed by pressureless sintering.The composite pro￾duced is capable of retaining room temperature flexural strength and damage-tolerance at elevated temperatures (1300
C), as a result of the dense, chemically and ther￾mally stable NdPO4 interphase and the weak bonding between this interphase and both the matrix and fibres. Thermal cycling from 1150
C to room temperature did not result in appreciable damage of the composite, and the samples retained a high flexural strength and ‘‘composite’’ behaviour after thermal cycling for up to 300 cycles.It is concluded that the dense NdPO4 interphase improves damage-tolerant behaviour at room and high tempera￾tures if: (1) the bonding at the interfaces interphase-fibre and interphase-matrix is weak enough, which leads to extensive fibre debonding, crack deflection and fibre pull-out, and (2) the mullite matrix contains porosity with homogeneously distributed fine pores. Acknowledgements Professors M.H. Loretto and P. Bowen are acknowl￾edged for the provision of laboratory facilities at the IRC and School of Metallurgy and Materials, at the University of Birmingham, respectively.Partial financial support by the European Commission under the contract numbers BRITE-EURAM CT 97–0609 and CT 95–0110 is also acknowledged.ARB acknowledges support of the Nuffield Foundation (London). References 1. Evans, A.G.and Marshall, D.B., High toughness ceramics and ceramic composites. Progress in Materials Science, 1989, 33, 85–90. 2.Warren, R.and Deng, S., Continuous fibre reinforced ceramic composites for very high temperatures. Silicates Industriels, 1996, 5(6), 99–107. 3.Chawla, K.K., The high-temperature application of ceramic matrix composites. JOM- J. Min. Met. Mater., 1995, 47, 19–21. 4.Porter, J.R., Reinforcements for ceramic–matrix composites for elevated temperature applications. Mater. Sci. Eng., 1993, A166, 179–184. 5. Holmquist, M., Lundberg, R., Sudre, O., Razzell, A. G., Molliex, L., Benoit, J. and Adlerborn, J., Alumina/alumina composite with a porous zirconia interphase, processing, properties and component testing. J. Eur. Ceram. Soc., 2000, 20, 599–606. 6. Chawla, K. K., Coffin, C. and Xu, Z. R., Interface engineering in oxide fibre/oxide matrix composites. Int. Mater. Rev., 2000, 45, 165–189. Fig.7.Bright-field TEM images of the interfacial zones between (a) NdPO4 interphase–mullite fibre and (b) NdPO4 interphase–mullite matrix, both showing the absence of any reaction products or zone in these regions. Fig.8. SEM micrograph of the surface of a thermally cycled sample, showing no evidence of microcracking or of other form of superficial damage. C. Kaya et al. / Journal of the European Ceramic Society 22 (2002) 2333–2342 2341