Availableonlineatwww.sciencedirect.com SCIENCE E噩≈S ournal of the European Ceramic Society 24(2004)565-578 www.elsevier.com/locate/jeurceramsoc Thermal degradation of an oxide fibre(Nextel 720)/ aluminosilicate composite M.-L. Anttia,*E. Lara-CurziobR. Warren a Division of Engineering Materials, Luled University of Technology, 97187 Luled, Sweden mEtals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830-6064, USA Materials Science Group, Division of Innovation, Production and Management, Malmo University, 20506 Malmo, Sweden Received 5 January 2002: received in revised form 15 March 2003: accepted 6 April 200 The effect of thermal exposure on the microstructure and tensile stress-strain behaviour has been investigated for composites of woven continuous oxide fibres(Nextel 720)in a porous aluminosilicate matrix. The tensile tests were carried out on straight-sided centre hole notched plates with 0/90 and +45 orientations. The as-received material was slightly notch sensitive in that the net section fracture stress decreased somewhat with increasing hole diameter but much less than predicted for an ideally elastic, fully notch-sensitive material. After exposure at 1100C and for long time at 1000C in air the composite was embrittled. In the o composite this resulted in a reduced fracture strength, a reduced strain to failure as well as a reduced fracture toughness and damage zone size. After exposure for 100 h at 1 100C(the most extreme exposure applied) the material also became significantly more notch sensitive and had failure characteristics similar to those of a monolithic ceramic. The 45 composite was also embrit- tled which resulted in a reduced strain to failure but an increase in fracture strength. Density measurements and observations on the microstructure and fracture surfaces indicated that the embrittlement was due mainly to localised densification of the matrix and an increase in fibre/matrix bonding C 2003 Elsevier Ltd. All rights reserved Keywords: Composites; Fibres; Mechanical properties; Nextel fibres; Thermal degradation 1. Introduction the matrix and the crack deflects into a plane parallel to the loading direction. ' In particular oxide/oxide CFCCs It is now well established that continuous fibre rein- exploiting this principle attract interest as candidate forced ceramic composites(CFCCs)can be made to materials for use in combustors, exhibiting damage tol- have non-brittle fracture behaviour and improved erance combined with inherent oxidation resistance damage tolerance by introduction of a suitably weak Their anticipated ability to operate at higher tempera fibre/matrix interface which provides a more favourable tures than the superalloys used today is expected to lead path for the extension of matrix cracks than penetrating to an increased efficiency and a reduced need for cooling the fibre. To achieve this an interphase is often coated air, as well as a decreased emission of NO gases. The separately onto the fibres during processing and this life requirements of the material are high, the aim being involves both an increased production cost as well as an several hundred thousand hours at at least 1100C added complexity. It has recently been demonstrated When designing a structural component, the effect of that similar crack-deflecting behaviour can also be holes and notches on the mechanical properties is an achieved by means of a finely distributed porosity in the important practical factor to be taken into considera matrix instead of a separate interphase between matrix tion. For this reason the mechanical strength of CFCCs and fibres -Delamination has been shown to occur in has frequently been studied in terms of the notch strength and notch sensitivity. The present work con- Corresponding author. TeL : +49-920-492093; fax: +46-920 cerns the notch strength behaviour of a commercially 491084. available composite consisting of laminated, woven E-mail address: marta-lena antti(@ mb luth. se(M.-L. Antti) mullite/alumina fibres (Nextel 720) in a porous 0955-2219/03/S. see front matter C 2003 Elsevier Ltd. All rights reserved. doi:10.1016S0955-221903)00250-4Thermal degradation of an oxide fibre (Nextel 720)/aluminosilicate composite M.-L. Anttia,*, E. Lara-Curziob, R. Warrenc a Division of Engineering Materials, Lulea˚ University of Technology, 97187 Lulea˚, Sweden bMetals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830-6064, USA c Materials Science Group, Division of Innovation, Production and Management, Malmo¨ University, 20506 Malmo¨, Sweden Received 5 January 2002; received in revised form 15 March 2003; accepted 6 April 2003 Abstract The effect of thermal exposure on the microstructure and tensile stress–strain behaviour has been investigated for composites of woven continuous oxide fibres (Nextel 720) in a porous aluminosilicate matrix. The tensile tests were carried out on straight-sided, centre hole notched plates with 0/90
and 45
orientations. The as-received material was slightly notch sensitive in that the net section fracture stress decreased somewhat with increasing hole diameter but much less than predicted for an ideally elastic, fully notch-sensitive material. After exposure at 1100
C and for long time at 1000
C in air the composite was embrittled. In the 0/90 composite this resulted in a reduced fracture strength, a reduced strain to failure as well as a reduced fracture toughness and damage zone size. After exposure for 100 h at 1100
C (the most extreme exposure applied) the material also became significantly more notch sensitive and had failure characteristics similar to those of a monolithic ceramic. The 45 composite was also embrit￾tled which resulted in a reduced strain to failure but an increase in fracture strength. Density measurements and observations on the microstructure and fracture surfaces indicated that the embrittlement was due mainly to localised densification of the matrix and an increase in fibre/matrix bonding. # 2003 Elsevier Ltd. All rights reserved. Keywords: Composites; Fibres; Mechanical properties; Nextel fibres; Thermal degradation 1. Introduction It is now well established that continuous fibre rein￾forced ceramic composites (CFCCs) can be made to have non-brittle fracture behaviour and improved damage tolerance by introduction of a suitably weak fibre/matrix interface which provides a more favourable path for the extension of matrix cracks than penetrating the fibre. To achieve this an interphase is often coated separately onto the fibres during processing and this involves both an increased production cost as well as an added complexity. It has recently been demonstrated that similar crack-deflecting behaviour can also be achieved by means of a finely distributed porosity in the matrix instead of a separate interphase between matrix and fibres.13 Delamination has been shown to occur in the matrix and the crack deflects into a plane parallel to the loading direction.1 In particular oxide/oxide CFCCs exploiting this principle attract interest as candidate materials for use in combustors, exhibiting damage tol￾erance combined with inherent oxidation resistance. Their anticipated ability to operate at higher tempera￾tures than the superalloys used today is expected to lead to an increased efficiency and a reduced need for cooling air, as well as a decreased emission of NOx gases. The life requirements of the material are high, the aim being several hundred thousand hours at at least 1100
C. When designing a structural component, the effect of holes and notches on the mechanical properties is an important practical factor to be taken into considera￾tion. For this reason the mechanical strength of CFCCs has frequently been studied in terms of the notch strength and notch sensitivity. The present work con￾cerns the notch strength behaviour of a commercially available composite consisting of laminated, woven mullite/alumina fibres (Nextel 720) in a porous 0955-2219/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0955-2219(03)00250-4 Journal of the European Ceramic Society 24 (2004) 565–578 www.elsevier.com/locate/jeurceramsoc * Corresponding author. Tel.: +49-920-492093; fax: +46-920- 491084. E-mail address: marta-lena.antti@mb.luth.se (M.-L. Antti).