E噩≈S Journal of the European Ceramic Society 22(2002)2333-2342 www.elsevier.com/locate/jeurceramsoc Mullite(Nextel M 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties CKaya,*, E.G. Butler, A Selcuk, A.R. Boccaccinib, M.H. Lewis a Interdisciplinary Research Centre(IRC) in Materials Processing, The University of Birmingham, Edgbas ha, B15 2TT UK dEpartment of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW72BP,UK Department of Physics, Centre for Advanced Materials, University of Warwick, Cor CV4 ZAL. UK Received 25 September 2001; accepted 8 December 200 Abstract A mullite matrix containing homogeneously distributed ultra-fine(70-350 nm) pores was reinforced with NdPOa-coated woven mullite fibre mats(Nextel M 720)leading to damage-tolerant composites with good high temperature(1300C)strength and ther mal cycling resistance. Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities. After sintering at 1200C for 3 h, the compacts had a density 86. 4% of theoretical density and showed damage-tolerant behaviour up to 1300C, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300oC, respectively. No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150 C for up to 300 cycles. The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1)the presence of a dense NdPOa interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores(<350 nm)within the mullite matrix. C 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Fibres; Mechanical properties; Microstructure-final: Mullite: Mullite fibres; Thermal cycling 1. Introduction temperature applications due to its good thermal shock resistance, low thermal expansion coeficient, high creep Continuous fibre-reinforced ceramic matrix compo- resistance and good chemical and thermal stability. I, I sites have attracted significant scientific and technologi- However, its low fracture toughness(1-3 M Pa m-1/ cal interest for high temperature structural applications and high flaw sensitivity need to be increased for struc due to their damage-tolerant behaviour, decreased flaw tural applications. One way to do this is by incorporating sensitivity and resistance to creep. The development high-strength continuous ceramic fibres into the mullite of oxide fibre-reinforced oxide matrix composites matrix in order to activate debonding, delamination, promising means of achieving lightweight, structural crack deflection, fibre bridging and fibre pull-out materials combining high-temperature strength with mechanisms that will contribute to a non-linear stress- improved fracture toughness, damage tolerance, ther- strain response and achievement of a high fracture mal shock and oxidation resistance. Significant research ffort is being expended in the optimisation of these It has been shown that catastrophic failure of oxide ceramic composites systems, with particular emphasis fibre reinforced oxide matrix composites can be pre being placed on the establishment of reliable and cost- vented using a very porous(up to 50 vol %)ceramic effective fabrication procedures.5-10 Mullite (3Al2O3. matrix with no particularly optimised interface between 2SiO2)is one of the ideal oxide matrix materials for high fibre and matrix. 3 However, most theoretical models and experimental findings suggest that for obtaining Corresponding author. Tel. +44-121-4143537: fax: +44- maximised damage-tolerance and fracture toughness in 4143441. fibre-reinforced brittle-matrix composites, tailored inter- E-mail address: c kaya(@ bham ac uk(C. Kaya) phases between fibre and matrix providing optimised 0955-2219/02/S. see front matter C 2002 Elsevier Science Ltd. All rights reserved. PII:S0955-2219(01)00531-3Mullite (NextelTM 720) fibre-reinforced mullite matrix composites exhibiting favourable thermomechanical properties C.Kayaa,*, E.G. Butlera , A.Selcukb, A.R. Boccaccinib, M.H. Lewisc a Interdisciplinary Research Centre (IRC) in Materials Processing, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK bDepartment of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP, UK c Department of Physics, Centre for Advanced Materials, University of Warwick, Coventry, CV4 7AL, UK Received 25 September 2001; accepted 8 December 2001 Abstract A mullite matrix containing homogeneously distributed ultra-fine (70–350 nm) pores was reinforced with NdPO4-coated woven mullite fibre mats (NextelTM 720) leading to damage-tolerant composites with good high temperature (1300
C) strength and ther￾mal cycling resistance.Electrophoretically deposited fibre preforms were placed in a high-load pressure filtration assembly, leading to formation of consolidated compacts with high green densities.After sintering at 1200
C for 3 h, the compacts had a density of 86.4% of theoretical density and showed damage-tolerant behaviour up to 1300
C, with flexural strength values of 235 MPa and 224 MPa at room temperature and 1300
C, respectively.No significant microstructural damage was detected after thermal cycling the samples between room temperature and 1150
C for up to 300 cycles.The thermomechanical test results combined with detailed electron microscopy observations indicate that the overall composite behaviour in terms of damage-tolerance, thermal capability and thermal cycling resistance is mainly controlled by two microstructural features: (1) the presence of a dense NdPO4 interphase but weak bonding with the matrix or fibre and (2) the presence of homogeneously distributed nano pores (<350 nm) within the mullite matrix. # 2002 Elsevier Science Ltd.All rights reserved. Keywords: Composites; Fibres; Mechanical properties; Microstructure-final; Mullite; Mullite fibres; Thermal cycling 1. Introduction Continuous fibre-reinforced ceramic matrix compo￾sites have attracted significant scientific and technologi￾cal interest for high temperature structural applications due to their damage-tolerant behaviour, decreased flaw sensitivity and resistance to creep.14 The development of oxide fibre-reinforced oxide matrix composites is a promising means of achieving lightweight, structural materials combining high-temperature strength with improved fracture toughness, damage tolerance, ther￾mal shock and oxidation resistance.Significant research effort is being expended in the optimisation of these ceramic composites systems, with particular emphasis being placed on the establishment of reliable and cost￾effective fabrication procedures.510 Mullite (3Al2O3 . 2SiO2) is one of the ideal oxide matrix materials for high temperature applications due to its good thermal shock resistance, low thermal expansion coefficient, high creep resistance and good chemical and thermal stability.11,12 However, its low fracture toughness (1–3 M Pa m1/2) and high flaw sensitivity need to be increased for struc￾tural applications.One way to do this is by incorporating high-strength continuous ceramic fibres into the mullite matrix in order to activate debonding, delamination, crack deflection, fibre bridging and fibre pull-out mechanisms that will contribute to a non-linear stress￾strain response and achievement of a high fracture energy.1 It has been shown that catastrophic failure of oxide fibre reinforced oxide matrix composites can be pre￾vented using a very porous (up to 50 vol.%) ceramic matrix with no particularly optimised interface between fibre and matrix.13 However, most theoretical models and experimental findings suggest that for obtaining maximised damage-tolerance and fracture toughness in fibre-reinforced brittle–matrix composites, tailored inter￾phases between fibre and matrix providing optimised 0955-2219/02/$ - see front matter # 2002 Elsevier Science Ltd.All rights reserved. PII: S0955-2219(01)00531-3 Journal of the European Ceramic Society 22 (2002) 2333–2342 www.elsevier.com/locate/jeurceramsoc * Corresponding author.Tel.: +44-121-4143537; fax: +44-121- 4143441. E-mail address: c.kaya@bham.ac.uk (C. Kaya).