E≈S Journal of the European Ceramic Society 20(2000)2491-249 Temperature-induced fibre/matrix interactions in porous alumino silicate ceramic matrix composites M. Schmucker * B. Kanka. H. Schneider German Aerospace Center( DLR), Institute of Materials Research, 51147 Koln, Germany Received 14 February 2000: received in revised form 2 May 2000: accepted 1l May 2000 Abstract The thermal stability of alumino-silicate fibre(Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600 C. In the as-prepared state the fibres consist of mullite plus a-Al2O3, while the porous mullite matrix includes minor amounts of a Sio2-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and a-Al2O3 at the rims of the fibres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about I um thick are formed at the periphery of the fibres. The mullite forming process is initiated above about 1500 C under short time heat-treatment conditions(2 h)and at much lower tem- perature(1300C)under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing fibre/matrix bonding. According to this study, the hermal stability of alumino silicate(Nextel 720)fibre/mullite matrix composites ranges between 1500 C in short-term and 1300.C in long-term heat-treatment conditions. C 2000 Elsevier Science Ltd. All rights reserved Keywords: Aluminosilicate fibres; Composites; Mullite; Reaction path; Thermal stability 1. Introduction fibre coating materials such as turbostratic bn or C are not stable in air at high temperatures. An alternative Oxide ceramics have a high potential for long-term approach for damage-tolerant all-oxide ceramic matrix high-temperature applications such as thermal protec- composites was reported by Lange, Evans and cow- ion systems in combustion chambers of gas turbine orkers 5-7 This material which has been designated as engines. Monolithic ceramics. however, are not suitable ceramic wood" consists of ceramic fibres embedded in for many applications due to their inherent brittleness. a matrix of high porosity. The concept makes use of the A promising way to achieve tough and damage-tolerant porous matrix as a surrogate of a porous fibre/matrix ceramics is the reinforcement of ceramic bodies by interphase and was demonstrated succesfully for cera- ramic long fibres. Long fibre reinforced composites mic matrix composites consisting of alumina fibres and may exhibit non-brittle fracture behavior if the bonding a matrix of Si3N4 or mullite 5-7 between fibres and the matrix is relatively weak so that Ceramic matrix composites consisting of highly por rack deflection and fibre pull-out do occur. 2 Weak us mullite matrices and alumino silicate fibres (3M fibre/matrix bonding is controlled by weak fibre/matrix Nextel 720) ly have been fabricated by pressure- interfaces, e. g. by low-toughness fibre coatings, porous less sintering of mullite- infiltrated fibre bundles in the fibre coatings, or by"fugitive layers".4 The homo- Institute of Materials Research of the German Aero geneous coating of fibres, however, is an expensive pro- space Center (DLR). To prevent fibre degradation dur cess, especially if chemical vapor deposition (CVD) ing processing, the sintering temperature of the techniques are taken into account. Moreover, suitable composite was not allowed to exceed 1300oC Sintering activity of mullite precursors with stoichometric com Corresponding author. Tel:+49-2203-6010-2462: fax: +49-2203. position(72 wt% Al2O3, 28 wt% SiO, ). however is low E-o ail addres martin sch in case of pressureless firing at 1300C. Thus, a mullite precursor slightly supersaturated in SiOz with respect 0955-2219/00/S. see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00150-3Temperature-induced ®bre/matrix interactions in porous alumino silicate ceramic matrix composites M. SchmuÈcker *, B. Kanka, H. Schneider German Aerospace Center (DLR), Institute of Materials Research, 51147 Koln, Germany Received 14 February 2000; received in revised form 2 May 2000; accepted 11 May 2000 Abstract The thermal stability of alumino-silicate ®bre (Nextel 720)/porous mullite matrix composites was investigated in the temperature range between 1300 and 1600C. In the as-prepared state the ®bres consist of mullite plus a-Al2O3, while the porous mullite matrix includes minor amounts of a SiO2-rich glass phase. Temperature-controlled reactions between the silica-rich glass phase of the matrix and a-Al2O3 at the rims of the ®bres to form mullite have been observed. At the end of this process, virtually all glass phase of the matrix is consumed. Simultaneously, alumina-free layers about 1 mm thick are formed at the periphery of the ®bres. The mullite forming process is initiated above about 1500C under short time heat-treatment conditions (2 h) and at much lower tem￾perature (1300C) under long-term annealing (1000 h). Subsequent to annealing below the thermal threshold, the composite is damage tolerant and only minor strength degradation occurs. Higher annealing temperatures, however, drastically reduce damage tolerance of the composites, caused by reaction-induced gradually increasing ®bre/matrix bonding. According to this study, the thermal stability of alumino silicate (Nextel 720) ®bre/mullite matrix composites ranges between 1500C in short-term and 1300C in long-term heat-treatment conditions. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Aluminosilicate ®bres; Composites; Mullite; Reaction path; Thermal stability 1. Introduction Oxide ceramics have a high potential for long-term high-temperature applications such as thermal protec￾tion systems in combustion chambers of gas turbine engines. Monolithic ceramics, however, are not suitable for many applications due to their inherent brittleness. A promising way to achieve tough and damage-tolerant ceramics is the reinforcement of ceramic bodies by ceramic long ®bres.1 Long ®bre reinforced composites may exhibit non-brittle fracture behavior if the bonding between ®bres and the matrix is relatively weak so that crack de¯ection and ®bre pull-out do occur.2 Weak ®bre/matrix bonding is controlled by weak ®bre/matrix interfaces, e.g. by low-toughness ®bre coatings,3 porous ®bre coatings, or by ``fugitive layers''.4 The homo￾geneous coating of ®bres, however, is an expensive pro￾cess, especially if chemical vapor deposition (CVD) techniques are taken into account. Moreover, suitable ®bre coating materials such as turbostratic BN or C are not stable in air at high temperatures. An alternative approach for damage-tolerant all-oxide ceramic matrix composites was reported by Lange, Evans and cow￾orkers.5ÿ7 This material which has been designated as ``ceramic wood'' consists of ceramic ®bres embedded in a matrix of high porosity. The concept makes use of the porous matrix as a surrogate of a porous ®bre/matrix interphase and was demonstrated succesfully for cera￾mic matrix composites consisting of alumina ®bres and a matrix of Si3N4 or mullite.5ÿ7 Ceramic matrix composites consisting of highly por￾ous mullite matrices and alumino silicate ®bres (3M, Nextel 720) recently have been fabricated by pressure￾less sintering of mullite-in®ltrated ®bre bundles in the Institute of Materials Research of the German Aero￾space Center (DLR). To prevent ®bre degradation dur￾ing processing, the sintering temperature of the composite was not allowed to exceed 1300C. Sintering activity of mullite precursors with stoichometric com￾position (72 wt% Al2O3, 28 wt.% SiO2), however, is low in case of pressureless ®ring at 1300 C. Thus, a mullite precursor slightly supersaturated in SiO2 with respect to 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00150-3 Journal of the European Ceramic Society 20 (2000) 2491±2497 * Corresponding author. Tel.: +49-2203-6010-2462; fax: +49-2203- 67310. E-mail address: martin.schmuecker@dlr.de (M. SchmuÈcker)