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 e€ects 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) indicat￾ing 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 a￾Al2O3 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 a€ected 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 mulliti￾zation, the ®bre diameter increases (each volume unit of a￾Al2O3 reacting with silica forms 1.8 volume units of mul￾lite) 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 inves￾tigated 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