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)00150-3
Temperature-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 temperature (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 protection 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 homogeneous coating of ®bres, however, is an expensive process, 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 coworkers.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 ceramic matrix composites consisting of alumina ®bres and a matrix of Si3N4 or mullite.5ÿ7 Ceramic matrix composites consisting of highly porous mullite matrices and alumino silicate ®bres (3M, Nextel 720) recently have been fabricated by pressureless sintering of mullite-in®ltrated ®bre bundles in the Institute of Materials Research of the German Aerospace Center (DLR). To prevent ®bre degradation during processing, the sintering temperature of the composite was not allowed to exceed 1300C. Sintering activity of mullite precursors with stoichometric composition (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)
2492 M. Schmaicker et al Journal of the European Ceramic Society 20(2000)2491-2497 mullite was employed(68 wt. Al2O3) though some before grinding and polishing. SEM was performed glassy phase had to be accepted in the resulting mullite using a LEO Gemini 982 microscope equipped with a matrix. The porous mullite matrix composite shows field quasi-ductile fracture behaviour and strength values of Al2 D3 /SiO ra I cathode and an Oxford EDX system. The atio of matrix mullite crystals was deter more than 300 MPa in case of unidirectional reinforce- mined via lattice constant data I using fibre-free model ment with a fibre content of 45 vol%.8, 9 Since the samples. Lattice constants were obtained from careful fibre strength after 1300C heat treatment is >1500 X-ray diffraction(XRD) measurements using a Siemens MPa, the maximum strength of the composite is sig- D 5000 XRD machine. In selected specimens, the com- nificantly smaller than one would expect using the rule position of the submicron-sized matrix mullite crystals of mixture. Therefore, the strength of the composite was checked by eDX in combination with a transmis- seems to be controlled by fibre/matrix debonding pro- sion electron microscope(Philips EM 430 equipped with cesses rather than by direct fibre strength. Although the a Tracor EDX system) matrix contains more than 5% glassy phase, there is no Three-point bending tests (40 mm span) were per excessive creep deformation of the composite at elevated formed on 50x 5xl mm bars cut out of the lD-compo temperatures. Preliminary investigations show that the site material. At least eight specimens were tested for reep behavior of the composites is controlled by the each firing series creep resistivity of the fibres rather than by matrix properties. Similar results recently were reported by Deng investigating SiC-fibre/mullite matrix compo- The aim of the present study is the investigation of thermally induced reactions between the silica-rich matrix and alumina-rich Nextel 720 fibres Reactions between SiO(in the matrix)and o-AlO3(in the fibres can be expected according to the Al2Ox-SiO phase diagram when the thermal activation of the samples is sufficient. The reactions between fibres and matrix have implications for the mechanical properties of the com posite, which will be discussed in detail 2. Experimental 2.1. Materials processing Fig. I. Overview of the as-prepared alumino silicate fibre/porous mullite matrix composite( scanning electron micrograph from polished Green bodies of the porous mullite matrix composite ere fabricated by infiltration of fibre bundles with an aqueous mullite precursor (Siral, Ce once Germany) slurry and subsequent winding up on a mandrel. The infiltrated fibre tapes were removed from the mandrel in the moist stage, rolled in flat tapes, and sintered pres useless in air at 1300 C( 60 min). The fibre content of the ID-composite is approx. 45 vol % Details of the CMC processing are published elsewhere. CMC sam- ples were heat-treated at 1300 C(1000 h)and at 1400, 1500 and 1600 C(2 h)in air. 2.2. Characterization The microstructural development was monitored by means of scanning electron microscopy (SEM)on polished sections. Due to the high matrix porosity, the samples were infiltrated with a low-viscous epoxy resin Fig. 2. Detail of the as-prepared alumino silicate fibre/porous mullite f The composition of 3M Nextel 720 fibre is 85 wt %Al2O3,I matrix composite. Note the high-porous mullite matrix with small wt% SiOz. The fibre consists of a-Al2O3 plus mullite glassy pockets existing between the rectangular mullite crystals
mullite was employed (68 wt.% Al2O3) though some glassy phase had to be accepted in the resulting mullite matrix. The porous mullite matrix composite shows quasi-ductile fracture behaviour and strength values of more than 300 MPa in case of unidirectional reinforcement with a ®bre content of 45 vol.%.8,9 Since the ®bre strength after 1300C heat treatment is >1500 MPa,17 the maximum strength of the composite is signi®cantly smaller than one would expect using the rule of mixture. Therefore, the strength of the composite seems to be controlled by ®bre/matrix debonding processes rather than by direct ®bre strength. Although the matrix contains more than 5% glassy phase, there is no excessive creep deformation of the composite at elevated temperatures. Preliminary investigations show that the creep behavior of the composites is controlled by the creep resistivity of the ®bres rather than by matrix properties. Similar results recently were reported by Deng investigating SiC-®bre/mullite matrix composites.10 The aim of the present study is the investigation of thermally induced reactions between the silica-rich matrix and alumina-rich Nextel 720 ®bres.y Reactions between SiO2 (in the matrix) and a-Al2O3 (in the ®bres) can be expected according to the Al2O3±SiO2 phase diagram when the thermal activation of the samples is sucient. The reactions between ®bres and matrix have implications for the mechanical properties of the composite, which will be discussed in detail. 2. Experimental 2.1. Materials processing Green bodies of the porous mullite matrix composites were fabricated by in®ltration of ®bre bundles with an aqueous mullite precursor (Siral, Condea, Germany) slurry and subsequent winding up on a mandrel. The in®ltrated ®bre tapes were removed from the mandrel in the moist stage, rolled in ¯at tapes, and sintered pressureless in air at 1300C (60 min). The ®bre content of the 1D-composite is approx. 45 vol.%. Details of the CMC processing are published elsewhere.8 CMC samples were heat-treated at 1300C (1000 h) and at 1400, 1500 and 1600C (2 h) in air. 2.2. Characterization The microstructural development was monitored by means of scanning electron microscopy (SEM) on polished sections. Due to the high matrix porosity, the samples were in®ltrated with a low-viscous epoxy resin before grinding and polishing. SEM was performed using a LEO Gemini 982 microscope equipped with a ®eld emission cathode and an Oxford EDX system. The Al2O3/SiO2 ratio of matrix mullite crystals was determined via lattice constant data11 using ®bre-free model samples. Lattice constants were obtained from careful X-ray diraction (XRD) measurements using a Siemens D 5000 XRD machine. In selected specimens, the composition of the submicron-sized matrix mullite crystals was checked by EDX in combination with a transmission electron microscope (Philips EM 430 equipped with a Tracor EDX system). Three-point bending tests (40 mm span) were performed on 5051 mm bars cut out of the 1D-composite material. At least eight specimens were tested for each ®ring series. Fig. 1. Overview of the as-prepared alumino silicate ®bre/porous mullite matrix composite (scanning electron micrograph from polished cross-section). Fig. 2. Detail of the as-prepared alumino silicate ®bre/porous mullite matrix composite. Note the high-porous mullite matrix with small glassy pockets existing between the rectangular mullite crystals. y The composition of 3M Nextel 720 ®bre is 85 wt.% Al2O3, 15 wt.% SiO2. The ®bre consists of a-Al2O3 plus mullite. 2492 M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497
M. Schmaicker et al. Journal of the European Ceramic Society 20(2000)2491-2497 3. Results of the matrix. Moreover, small glassy pockets become visible between the mullite crystals. Microstructural Fig. I gives an overview of the as-prepared Nextel 720 details of heat-treated samples are given in Fig 3 and 4 lumino silicate fibre/porous mullite matrix composite. With increasing temperature, gradual coarsening of the Fig 2 shows the mullite matrix in higher magnification: fibre compounds, a-Al2O3 and mullite, occurs. Beside it clearly demonstrates a very high porosity(a50 vol % coarsening, dissapearance of a-Al2O3 in the fibre rim area can be observed in the 1600C sample. These a Al2O3-free zones are formed only when fibres and matrix are in direct contact (Fig. 4). For comparison Nextel 720 fibres alone were fired at 1600C(Fig. 5).N depletion of a-Al,O, occurs in the fibre rim area Fig. 6 provides information on the temperature- dependent development of the chemical composition of the matrix in its entirety and of the matrix mullite crys tals, respectively. The bulk composition was determined by EDX analyses of relatively large(5 um diameter or more) matrix agglomerates, each of them containing numerous mullite crystals and glassy areas. The mullite 的以好 25 龙 Fig. 4. Alumino silicate fibre/porous mullite matrix composite heat treated at 1600C( h). Note that the aAlO3 free fibre rims are formed only in areas of fibre/matrix contact. Fig 3. Microstructural changes of alumino silicate fibre/porous mul lite matrix composite caused by thermal treatment. (a) as-prepared 1500.C, 2 h;(c)1600 C,2 h. Note that with increasing temperature, Fig. 5. Nextel 720 fibres fired without matrix at 1600 C. Embedding gradual coarsening of the fibre compounds occurs. At 1600.C, a the fibres in epoxy allows the preparation of a polished cross-section depletion of a-Al2O3 in the fibre rim area is observed No a-Al2O3 free fibre rims occur in contrast to Fig 3
3. Results Fig. 1 gives an overview of the as-prepared Nextel 720 alumino silicate ®bre/porous mullite matrix composite. Fig. 2 shows the mullite matrix in higher magni®cation: it clearly demonstrates a very high porosity (50 vol.%) of the matrix. Moreover, small glassy pockets become visible between the mullite crystals. Microstructural details of heat-treated samples are given in Fig. 3 and 4. With increasing temperature, gradual coarsening of the ®bre compounds, a-Al2O3 and mullite, occurs. Beside coarsening, dissapearance of a-Al2O3 in the ®bre rim area can be observed in the 1600C sample. These aAl2O3-free zones are formed only when ®bres and matrix are in direct contact (Fig. 4). For comparison, Nextel 720 ®bres alone were ®red at 1600C (Fig. 5). No depletion of a-Al2O3 occurs in the ®bre rim area. Fig. 6 provides information on the temperaturedependent development of the chemical composition of the matrix in its entirety and of the matrix mullite crystals, respectively. The bulk composition was determined by EDX analyses of relatively large (5 mm diameter or more) matrix agglomerates, each of them containing numerous mullite crystals and glassy areas. The mullite Fig. 3. Microstructural changes of alumino silicate ®bre/porous mullite matrix composite caused by thermal treatment. (a) as-prepared; (b) 1500C, 2 h; (c) 1600C, 2 h. Note that with increasing temperature, gradual coarsening of the ®bre compounds occurs. At 1600C, a depletion of a-Al2O3 in the ®bre rim area is observed. Fig. 4. Alumino silicate ®bre/porous mullite matrix composite heattreated at 1600C (2 h). Note that the a-Al2O3 free ®bre rims are formed only in areas of ®bre/matrix contact. Fig. 5. Nextel 720 ®bres ®red without matrix at 1600C. Embedding the ®bres in epoxy allows the preparation of a polished cross-section. No a-Al2O3 free ®bre rims occur in contrast to Fig. 3. M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497 2493
M. Schmaicker et al. Journal of the European Ceramic Society 20(2000)2491-2497 76 74 Matrix mullite crystal composition 8芝E°d式 Matrix bulk composition 67 1200 1300 1400 1600 1700 Annealing temperature [ oc] Fig. 6. Matrix bulk composition (i.e. mullite plus vitreous phase) and matrix mullite composition of alumino silicate fibre/porous mullite matrix composite plotted as a function of temperature Rhombs: matrix composition as determined by SEM-EDX Squares: mullite composition on basis of lattice constant data. Open circles: mullite composition analyzed by TEM-EDX. crystal composition was determined via mullite lattice 1600oC, however, causes a drastic change of the matrix constants by X-ray methods using fibre-free reference composition mullite crystals and matrix display vir matrix material and was checked in composite samples tually the same composition with about 72 wt. Al2O by EDX analyses in combination with transmission Fig. 7 shows the fibre/matrix interface area of a sam- electron microscopy. The matrix mullite crystals of the ple heat-treated at 1300@C for 1000 h. A reaction zone, as-prepared material are relatively alumina-rich (74.5 free in a-Al2O3, becomes visible in this material though wt% Al2O3) with respect to stoichiometric mullite(72 the zone's extension is smaller than oberved in the sam- wt% Al2O3). Treatment at higher temperatures leads to ple heat-treated at 1600C, 2 h. Load/deflection curves a gradual development towards the Al2O3/SiO2 ratio of of the porous mullite composites heat-treated at various stoichiometric mullite. On the other hand, the matrix temperatures are plotted in Fig 8. Bending tests signal bulk composition of samples fired below 1500C, is non-brittle(damage-tolerant) fracture behavior of the relatively poor in Al_O3(ca 68 wt. Al2O3). Firing at composites up to 1500C though the maximum strength values slightly decrease with increasing temperature Firing at 1600 C, on the other hand, leads to brittle A 200 MPa Fig. 7. Fibre /matrix interface area of alumino silicate fibre/porous Fig 8. Load/ deflection curves of alumino silicate fibre/porous mullite mullite matrix composite heat-treated at 1300oC for 1000 h. Note the matrix composites fired at various temperatures(2 h)(A) as-prepared ormation of an a-Al,O3 free fibre rim (B)1400°C;(C)1500°C;(D1600°C
crystal composition was determined via mullite lattice constants by X-ray methods using ®bre-free reference matrix material and was checked in composite samples by EDX analyses in combination with transmission electron microscopy. The matrix mullite crystals of the as-prepared material are relatively alumina-rich (74.5 wt.% Al2O3) with respect to stoichiometric mullite (72 wt.% Al2O3). Treatment at higher temperatures leads to a gradual development towards the Al2O3/SiO2 ratio of stoichiometric mullite. On the other hand, the matrix bulk composition of samples ®red below 1500C, is relatively poor in Al2O3 (ca. 68 wt.% Al2O3). Firing at 1600C, however, causes a drastic change of the matrix composition: mullite crystals and matrix display virtually the same composition with about 72 wt.% Al2O3. Fig. 7 shows the ®bre/matrix interface area of a sample heat-treated at 1300C for 1000 h. A reaction zone, free in a-Al2O3, becomes visible in this material though the zone's extension is smaller than oberved in the sample heat-treated at 1600C, 2 h. Load/de¯ection curves of the porous mullite composites heat-treated at various temperatures are plotted in Fig. 8. Bending tests signal non-brittle (damage-tolerant) fracture behavior of the composites up to 1500C though the maximum strength values slightly decrease with increasing temperature. Firing at 1600C, on the other hand, leads to brittle Fig. 6. Matrix bulk composition (i.e. mullite plus vitreous phase) and matrix mullite composition of alumino silicate ®bre/porous mullite matrix composite plotted as a function of temperature. Rhombs: matrix composition as determined by SEM-EDX. Squares: mullite composition on basis of lattice constant data. Open circles: mullite composition analyzed by TEM-EDX. Fig. 7. Fibre/matrix interface area of alumino silicate ®bre/porous mullite matrix composite heat-treated at 1300C for 1000 h. Note the formation of an a-Al2O3 free ®bre rim. Fig. 8. Load/de¯ection curves of alumino silicate ®bre/porous mullite matrix composites ®red at various temperatures (2 h) (A) as-prepared; (B) 1400C; (C) 1500C; (D) 1600C. 2494 M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497
M. Schmaicker et al. Journal of the European Ceramic Society 20(2000)2491-2497 2495 Original fiber surace b Fig. 10. Schematic presentation of the mechanism of fibre/matrix interaction in alumino silicate fibre/porous mullite matrix composite Note that the fibre diameter gradually increases with the degreee o reaction(see also Fig 3). the reaction 2 Sio,+3 a-Al,O3=3Al,,. At the end of this process, the Sio2 phase of the matrix is fully consumed. The reaction is virtually complete above about 1600 C. since bulk matrix and matrix mullite compositions then are nearly the same(Fig. 6)indicat ing that no"free"Sio, phase is present any more Starting from a fibre content of 70 wt % the following phase ratio of the composite can be estimated on basis of fibre and matrix compositions for T=1500 C, i.e. just below the fibre/matrix reaction process: about 67 Fig. 9. Fra ces of alumino silicate fibre/porous mullite wt. mullite(occurring in the matrix and in the fibres). (b)1600°C(2h) about 2 wt. of free silica (occurring in the matrix), about 31 wt. a-Al,O3(occurring in the fibre) failure of the composites though the strength value If all free silica has been reacted to mullite the slightly increases. Fracture surfaces of the as-prepared Al2O3 fraction of the composite is reduced from a31 to composite and of a specimen heat-treated at 1600 C are 26 wt. since one mass unit of SiO2 consumes 2.57 shown in Fig 9. While the as-prepared ceramic matrix mass units of a-Al2O3. Thus, one sixth of the original composite exhibits a fracture surface with delamination fibre cross-section should be affected by the a-Al2O3 effects and fibre pull-out that signals energy-dissipating consumption corresponding to a 0.5 um zone on the fracture mechanisms, the 1600C sample produces a periphery of the 10 um thick fibre. However, due to the smooth fracture surface similar to those of brittle addition of SiO2 from the matrix and subsequent mullite- monolithic ceramic materials zation, the fibre diameter increases(each volume unit of a- AlO3 reacting with silica forms 1.8 volume units of mul lite)and the newly formed mullite zone becomes about I 4. Discussion um thick(see Fig. 10). The present results demonstrate that this is actually the case(Figs. 3c and 4) Microstructural and microchemical analyses of the Reactions between free silica and a-Al2O3 were inves- aluminum silicate fibre/porous mullite composite reveal tigated by several authors. - Johnson and Pask reactions between matrix and fibres above about described newly formed mullite directly at the a-Al2O3 500C.Obviously, the free silica phase of the matrix is Sio2 interface. Closer inspection by means of analytical transported towards the fibre surface and mullite is formed in a peripheral area of the fibres according to s A mullite composition of 73 wt. AlO3, 27 wt. SiO2 was assumed for mullite occurring in the fibres and in the matrix. Mullite crystals in as-received Nextel 720 fibres are much richer in Al2O3 but t For simplification, the glassy phase of the matrix is called"silica develop towards stoichiometric composition when heated at phase or"free silica"even though a small percentage of alumina 1500C.A similar tendency, although less pronounced, can be ted in this glass. observed in the sol-gel derived mullite matrix(see Fig. 6)
failure of the composites though the strength value slightly increases. Fracture surfaces of the as-prepared composite and of a specimen heat-treated at 1600C are shown in Fig. 9. While the as-prepared ceramic matrix composite exhibits a fracture surface with delamination eects and ®bre pull-out that signals energy-dissipating fracture mechanisms, the 1600C sample produces a smooth fracture surface similar to those of brittle monolithic ceramic materials. 4. Discussion Microstructural and microchemical analyses of the aluminum silicate ®bre/porous mullite composite reveal reactions between matrix and ®bres above about 1500C. Obviously, the free silica phase{ of the matrix is transported towards the ®bre surface and mullite is formed in a peripheral area of the ®bres according to the reaction 2 SiO2+3 a-Al2O3 )3Al2O32SiO2. At the end of this process, the SiO2 phase of the matrix is fully consumed. The reaction is virtually complete above about 1600C, since bulk matrix and matrix mullite compositions then are nearly the same (Fig. 6) indicating that no ``free'' SiO2 phase is present any more. Starting from a ®bre content of 70 wt.%, the following phase ratio of the composite can be estimated on basis of ®bre and matrix compositions for T=1500C, i.e. just below the ®bre/matrix reaction process:x about 67 wt.% mullite (occurring in the matrix and in the ®bres), about 2 wt.% of free silica (occurring in the matrix), about 31 wt.% a-Al2O3 (occurring in the ®bre). If all free silica has been reacted to mullite the total aAl2O3 fraction of the composite is reduced from 31 to 26 wt.% since one mass unit of SiO2 consumes 2.57 mass units of a-Al2O3. Thus, one sixth of the original ®bre cross-section should be aected by the a-Al2O3 consumption corresponding to a 0.5 mm zone on the periphery of the 10 mm thick ®bre. However, due to the addition of SiO2 from the matrix and subsequent mullitization, the ®bre diameter increases (each volume unit of aAl2O3 reacting with silica forms 1.8 volume units of mullite) and the newly formed mullite zone becomes about 1 mm thick (see Fig. 10). The present results demonstrate that this is actually the case (Figs. 3c and 4). Reactions between free silica and a-Al2O3 were investigated by several authors.12ÿ15 Johnson and Pask described newly formed mullite directly at the a-Al2O3/ SiO2 interface. Closer inspection by means of analytical Fig. 9. Fracture surfaces of alumino silicate ®bre/porous mullite matrix composites. (a) As-prepared; (b) 1600C (2 h). Fig. 10. Schematic presentation of the mechanism of ®bre/matrix interaction in alumino silicate ®bre/porous mullite matrix composite. Note that the ®bre diameter gradually increases with the degreee of reaction (see also Fig. 3). { For simpli®cation, the glassy phase of the matrix is called ``silica phase'' or ``free silica'' even though a small percentage of alumina is incorporated in this glass. x A mullite composition of 73 wt.% Al2O3, 27 wt.% SiO2 was assumed for mullite occurring in the ®bres and in the matrix. Mullite crystals in as-received Nextel 720 ®bres are much richer in Al2O3 but develop towards stoichiometric composition when heated at 1500C.17A similar tendency, although less pronounced, can be observed in the sol±gel derived mullite matrix (see Fig. 6). M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497 2495
M. Schmicker et al /Journal of the European Ceramic Society 20(2000)2491-2497 TEM, however, showed that the mullite nucleation sponds to 2 h firing at 152 respectively. The occurred inside the glassy phase rather than in direct microstructural observation is in good accordance to contact with the a-Al2O3 grains. Microstructural this estimation(Figs. 7 and 3c): samples annealed for evidence suggested that Al,O3 was solved in the coex 000 h at 1300 C do display reaction zones at the fibre isting non-crystalline silica phase and mullite nucleation rims but the zones are smaller than these of samples occurs as soon as Al2O3 supersaturation is reached. >In thermally treated for 2 h at 1600C the present study, mullite formation inside the glassy Load/ deflection curves of the porous mullite matrix pockets of the fibres has never been observed composites heat-treated at various temperatures signa mullite grains a priori are in contact with both lize a damage tolerant fracture behaviour up to 1500oC (matrix)and a-Al2O3(fibres), it is concluded that no (Fig 8). A 25% decrease in maximum strength occurs mullite nucleation took place. We believe, instead, that after firing at 1500C with respect to the starting growth of the pre-existing mullite grains by interdiffu- material. Interestingly enough, Nextel 720 fibres alone sion of Si+ and Al+ occurred. The driving force for undergo a 60%strength degradation after a 1500C interdiffusion of Si++ and AP+ in mullite crystals is heat-treatment. 17 Obviously, the strength of the com- obviously the occurring concentration gradient ranging posites fired up to 1500C is not controlled by direct from A72 wt. Al2O3(mullite in contact with SiO2)to fibre strength but fibre/matrix delamination will occur 4 wt. Al2O3(mullite in contact with a-Al2O3; see as the first step of failure. The damage tolerant fracture Fig. 11). According to this model, mullitization starts at behaviour of the composite after reaching maximum he interfacial area of fibres and matrix. The matrix load is controlled by crack bridging, multiple cracking thereby acts as a silica reservoir when the Sio2 located and fibre pull-out, which is demonstrated by a typical directly at the fibre/matrix boundary is consumed. fracture surface(Fig. 9a). Firing at 1600C, on the other During the reaction process, the viscous silica-rich hand, leads to the above described reactions in the fibre/ phase is presumably transported towards the fibre sur- matrix interfacial area and hence bonding between face by capillary forces. Reaction kinetics can be esti- matrix and fibres drastically increases. As a con- mated assuming @=700 kJ/mol as a typical value sequence, the material becomes brittle as can be clearly of activation energy for diffusional processes in mul- derived from the load /deflection curves(Fig. 8)and by lite. 6 Using this activation energy value, a temperature the resulting flat fracture surface( Fig. 9b) increment of 25 C corresponds to a doubling of the reaction rate. and 1000 h annealing at 1300 oc corre e It is an important result of this study that only little rmally-induced degradation of the porous matrix composite occurs unless interactions between fibres and matrix take place. This favorable behavior is explained by the fact that thermal activation of reactions between vitreous SiO and a-Al2O3 is considerably high According to the present study, the thermal stability of mullite matrix composites is estimated to be 1500C in the case of short-term application long-term applications(1000 h and more 1800- mullite Acknowledgements silica mullite alumina We thank the german Research Foundation. DFG mullite (Schn 297/15-1)for financial support 1700 References 1. Chawla, K. K, Composite Materials, Science and Engineerin Springer-Verlag, New York, 1987(pp. 134-149) 2. Chawla, K. K, Ceramic Matrix Composites. Chapman and Hall, London,1993(pp.162-195) 3. Morgan, P. E. D. and Marshall, D. B, Funct terraces for 76 wt %AL O 4. Keller. K. A, Mah, T, Parthasarathy, T.A poke. C. M. Fugitive interfacial carbon coatings for oxide/oxide composites. Fig. Il. Mullite stability region of the SiOx-Al2O3 phase diagram Ceram. Eng. and Sci. Proc., 1997, 14, 878-879. (after Klug et al.) 5. Lange, F. F. Tu. W. and Evans. C. A. G. Processing of damage
TEM, however, showed that the mullite nucleation occurred inside the glassy phase rather than in direct contact with the a-Al2O3 grains. Microstructural evidence suggested that Al2O3 was solved in the coexisting non-crystalline silica phase and mullite nucleation occurs as soon as Al2O3 supersaturation is reached.15 In the present study, mullite formation inside the glassy pockets of the ®bres has never been observed. Since mullite grains a priori are in contact with both, SiO2 (matrix) and a-Al2O3 (®bres), it is concluded that no mullite nucleation took place. We believe, instead, that growth of the pre-existing mullite grains by interdiusion of Si4+ and Al3+ occurred. The driving force for interdiusion of Si4+ and Al3+ in mullite crystals is obviously the occurring concentration gradient ranging from 72 wt.% Al2O3 (mullite in contact with SiO2) to 74 wt.% Al2O3 (mullite in contact with a-Al2O3; see Fig. 11). According to this model, mullitization starts at the interfacial area of ®bres and matrix. The matrix thereby acts as a silica reservoir when the SiO2 located directly at the ®bre/matrix boundary is consumed. During the reaction process, the viscous silica-rich phase is presumably transported towards the ®bre surface by capillary forces. Reaction kinetics can be estimated assuming Q=700 kJ/mol as a typical value of activation energy for diusional processes in mullite.16 Using this activation energy value, a temperature increment of 25C corresponds to a doubling of the reaction rate, and 1000 h annealing at 1300 C corresponds to 2 h ®ring at 1525 C, respectively. The microstructural observation is in good accordance to this estimation (Figs. 7 and 3c): samples annealed for 1000 h at 1300C do display reaction zones at the ®bre rims but the zones are smaller than these of samples thermally treated for 2 h at 1600C. Load/de¯ection curves of the porous mullite matrix composites heat-treated at various temperatures signalize a damage tolerant fracture behaviour up to 1500C (Fig. 8). A 25% decrease in maximum strength occurs after ®ring at 1500C with respect to the starting material. Interestingly enough, Nextel 720 ®bres alone undergo a 60% strength degradation after a 1500C heat-treatment.17 Obviously, the strength of the composites ®red up to 1500C is not controlled by direct ®bre strength but ®bre/matrix delamination will occur as the ®rst step of failure. The damage tolerant fracture behaviour of the composite after reaching maximum load is controlled by crack bridging, multiple cracking and ®bre pull-out, which is demonstrated by a typical fracture surface (Fig. 9a). Firing at 1600C, on the other hand, leads to the above described reactions in the ®bre/ matrix interfacial area and hence bonding between matrix and ®bres drastically increases. As a consequence, the material becomes brittle as can be clearly derived from the load/de¯ection curves (Fig. 8) and by the resulting ¯at fracture surface (Fig. 9b). It is an important result of this study that only little thermally-induced degradation of the porous matrix composite occurs unless interactions between ®bres and matrix take place. This favorable behavior is explained by the fact that thermal activation of reactions between vitreous SiO2 and a-Al2O3 is considerably high. According to the present study, the thermal stability of the Nextel 720 ®bre reinforced porous mullite matrix composites is estimated to be 1500C in the case of short-term application (several hours) or 1300C for long-term applications (1000 h and more). Acknowledgements We thank the German Research Foundation, DFG (Schn 297/15-1) for ®nancial support. References 1. Chawla, K. K., Composite Materials, Science and Engineering. Springer-Verlag, New York, 1987 (pp. 134±149). 2. Chawla, K. K., Ceramic Matrix Composites. Chapman and Hall, London, 1993 (pp. 162±195). 3. Morgan, P. E. D. and Marshall, D. B., Functional interfaces for oxide/oxide composites. Mat. Sci. Engg., 1993, A162, 15±25. 4. Keller, K. A., Mah, T., Parthasarathy, T. A. and Cooke, C. M., Fugitive interfacial carbon coatings for oxide/oxide composites. Ceram. Eng. and Sci. Proc., 1997, 14, 878±879. 5. Lange, F. F., Tu, W. and Evans, C. A. G., Processing of damageFig. 11. Mullite stability region of the SiO2±Al2O3 phase diagram (after Klug et al.18). 2496 M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497
M. Schmaicker et al. Journal of the European Ceramic Society 20(2000)2491-2497 tolerant. oxidation resistant ceramic matrix 12. Johnson, S. M. and Pask, J. A, Role of impurities on formatic cursor infiltration and pyrolysis method. Mat. Sci. Engg., 1995. of mullite from kaolinite and Alo A195,145-150. l,1982,61,838-842 6. Tu, w, Lange, F. F and Evans, C. A G, Concept for a damage 13. Rodrigo, P D. D and Boch, P High purity mullite ceramics by tolerant ceramic composite with"strong"interfaces. J.Anm. reaction sintering. IntJ. High Tec. Cera. 1985. 1. 3 Ceram soc.1996.79.417-424. 14. Saruhan, B, Albers, W. Schneider, H. and Kaysser, w.A. 7. Levi. C. G. Yang. J.Y Dalgleish. B. J. Zok. F.w. and Evans Reaction and sintering mechanisms of mullite in the system A. G. Processing and performance of an all-oxide cer cristobalite/a-Al2O3 and amorphous Sio2/a-Al2O3. J. Europ posite. J. Am. Ceram Soc., 1998, 81, 2077-2086 996,16.1075-1081 8. Kanka. B and Schneider. H. Alumosilicate fibre/mullite matrix 15. Schmucker. M. Albers. W. and Schneider. H. Mullite formation composites with favourable high temperature properties. J. Eur. by reaction sintering of quartz and a-Al2O3,- J. Eur. Ceram Soc. Ceran.Soc,2000,20,619-623. 1994.14.441-48 9. GOring, J. Kanka. B. J. Steinhauser, U. and Schneider. H 16. Schneider. H. Okada, K. and Pask, J.A., Mullite and Mullite Thermal barrier coating on Nextel 720 fibre/mullite matrix com- Ceramics. John Wiley Sons, Chichester, 1994(p. 208) osites: potential for long-term high-temperature use in gas tur- 17. Schneider, H, Goring, J, Schmucker, M. and Flucht, F. bine. In Proc. 24th Annual Cocoa Beach Conference, in press mal stability of Nextel 720 alumino silicate fibres. In Te 10. Deng, Z..Y., Effect of different fibre orientations on compressive A. P. Glaeser, A. M.(eds ), Ceramic Microstructures Tomsia reep behavior of SiC fibre-reinforced mullite matrix composites at the Atomic Level. Plenum Press. New york. 1998 J.Eur. Ceran.Soc,1999,19.2133-2144 18. Klug. F. J. Prochazka. S. and Doremus. R. H. Alumina-silica 11. Cameron, W.E., Composition and cell dimensions of mullite. phase diagram in the mullite region. J. Am. Ceram. Soc., 1987, Am. Ceran.Soc.Btl1977,56,1003-1011. 70.750-759
tolerant, oxidation resistant ceramic matrix composites by precursor in®ltration and pyrolysis method. Mat. Sci. Engg., 1995, A195, 145±150. 6. Tu, W., Lange, F. F. and Evans, C. A. G., Concept for a damage tolerant ceramic composite with ``strong'' interfaces. J. Am. Ceram Soc., 1996, 79, 417±424. 7. Levi, C. G., Yang, J. Y., Dalgleish, B. J., Zok, F. W. and Evans, A. G., Processing and performance of an all-oxide ceramic composite, J. Am. Ceram Soc., 1998, 81, 2077±2086. 8. Kanka, B. and Schneider, H., Alumosilicate ®bre/mullite matrix composites with favourable high temperature properties. J. Eur. Ceram. Soc., 2000, 20, 619±623. 9. GoÈring, J., Kanka, B. J., Steinhauser, U. and Schneider, H., Thermal barrier coating on Nextel 720 ®bre/mullite matrix composites: potential for long-term high-temperature use in gas turbine. In Proc. 24th Annual Cocoa Beach Conference, in press. 10. Deng, Z.-Y., Eect of dierent ®bre orientations on compressive creep behavior of SiC ®bre-reinforced mullite matrix composites. J. Eur. Ceram. Soc., 1999, 19, 2133±2144. 11. Cameron, W. E., Composition and cell dimensions of mullite. Am. Ceram. Soc. Bull, 1977, 56, 1003±1011. 12. Johnson, S. M. and Pask, J. A., Role of impurities on formation of mullite from kaolinite and Al2O3±SiO2 mixtures. Ceramic Bull., 1982, 61, 838±842. 13. Rodrigo, P. D. D. and Boch, P., High purity mullite ceramics by reaction sintering. Int. J. High Tec. Ceram., 1985, 1, 3. 14. Saruhan, B., Albers, W., Schneider, H. and Kaysser, W. A., Reaction and sintering mechanisms of mullite in the systems cristobalite/a-Al2O3 and amorphous SiO2/a-Al2O3. J. Europ. Ceram. Soc., 1996, 16, 1075±1081. 15. SchmuÈcker, M., Albers, W. and Schneider, H., Mullite formation by reaction sintering of quartz and a-Al2O3,. J. Eur. Ceram. Soc., 1994, 14, 441±448. 16. Schneider, H., Okada, K. and Pask, J. A., Mullite and Mullite Ceramics. John Wiley & Sons, Chichester, 1994 (p. 208). 17. Schneider, H., GoÈring, J., SchmuÈcker, M. and Flucht, F., Thermal stability of Nextel 720 alumino silicate ®bres. In Tomsia, A. P., Glaeser, A. M. (eds.), Ceramic Microstructures Ð Control at the Atomic Level, Plenum Press, New York, 1998. 18. Klug, F. J., Prochazka, S. and Doremus, R. H., Alumina-silica phase diagram in the mullite region. J. Am. Ceram. Soc., 1987, 70, 750±759. M. SchmuÈcker et al. / Journal of the European Ceramic Society 20 (2000) 2491±2497 2497
