ol-gel matrices of BAS, NAs and CAS 743 250 MPa l=3.33 JLLL LLLLL Strain (arbitrary units Fig 3. Stress/strain plots for composites(a)5(BAS matrix),(b )10(BAS10%Li matrix)and (c)interslab polished section of composite 6(BAS matrix). See Table 1 for details (or Ca2+) ions of the matrix with the zircono- the CAs and NAS matrix composi able 1) silicophosphate glass issued from the pyrolysis of the although the preparation cycle (temperature cycle ZrSiPB precursor. We therefore have a multilevel dwell, relative volume of fibre and of interface and reinforced composite(fibre- and particle-reinforced matrix precursors, etc. )was not optimized. However previous studies have shown that alkaline earth and Composite 5 was made with a BAS matrix free of phosphorus ions react with the Sic fibre. This reaction lithium addition but exhibiting a large amount of promotes the formation of a carbon-rich fibre/matrix celsian with monoclinic symmetry. Usually the interphase which leads to a sliding interface, but too monoclinic celsian is obtained by lithium addition or large an attack lowers the safe fibre cross-section and after long annealing at high tcmpcraturc. 5. 10 Thc hence the mechanical properties. Optimization of the achievement of the monoclinic phase in this composite relative proportion of each precursor is required. The not due to the reducing atmosphere related to the SiAl interface precursor seems to preserve the presence of the graphite felts and resistor in the oven fibre 4, I>although a higher temperature is needed for and to contact with the Sic fibre but may be related to sintering the low sintering temperature (matrix of the BAs composite 6 hot-pressed at 1250C is hexagonal celsian) or to the use of the TBB+ SiAl interphase 3.2 Acid and alkaline corrosion precursor Table 1). The section polished parallel to the fabric (interlab 3.2.1 Celsian(BAS) matrix polished section of composite 6, Fig 3(c)shows a Figure 4 compares the microstructures of polished regular array of cracks. This phenomenon may be sections of composites after attack by boiling sulphuric related to the expansion mismatch between the Sic acid or molten sodium nitrate. Cracks at the grain fabric (--3 5X10/c) and hexacelsian (8 boundary and a darkening of the grain limit are 10"/C)expansion coefficients, and can be compared observed for BAS matrices. The origin of this to the value of the monoclinic phase expansion phenomenon can be found in the fast attack of the coefficient(2. 3 x 10-7/ C). 12 The rather large expan- grain boundary second phase or in the grain sion coefficient of anorthite(6X10/'C)seems to constriction associated with ion exchange. Comparison be compatible with that of SiC fibre since no cracks of Raman spectra recorded on various grains by the are observed. On the other hand, a crack array was Raman microprobe (analysed area -3 um X 3 um) found in the NAS-matrix composite(3). Note that the clearly shows the band-broadening characteristic of a thermal expansion coefficients of pyrolysed SiAl and loss of crystallinity and a strong reduction in the ZrSiPB interface precursors are very similar to that of intensity of the 110 cm band assigned to the iC fibre. 11 The mismatch between the thermal translational motion of Ba2+ ions in monoclinic celsian expansion coefficients of SiC fibre and NAS and CAs ( Fig. 5). This indicates leaching of Ba ions. A atrix may explain the lower mechanical properties of longer duration of the chemical attack leads to a moreSol-gel matrices of BAS, NAS and CAS 743 A 200MPa a) BO - 2 ,111 111111111) Strain (a rbitrary unita) & Fig. 3. Stress/strain plots for composites (a) 5 (BAS matrix), (b) 10 (BASlO%Li matrix) and (c) interslab polished section of composite 6 (BAS matrix). See Table 1 for details. A io- 250 MPa IO - (or Ca*+) ions of the matrix with the zircono￾silicophosphate glass issued from the pyrolysis of the ZrSiPB precursor. We therefore have a multilevel reinforced composite (fibre- and particle-reinforced composite). Composite 5 was made with a BAS matrix free of lithium addition but exhibiting a large amount of celsian with monoclinic symmetry. Usually the monoclinic celsian is obtained by lithium addition or after long annealing at high temperature.5,10 The achievement of the monoclinic phase in this composite is not due to the reducing atmosphere related to the presence of the graphite felts and resistor in the oven and to contact with the Sic fibre but may be related to the low sintering temperature (matrix of the BAS composite 6 hot-pressed at 1250°C is hexagonal celsian) or to the use of the TBB + SiAl interphase precursor (Table 1). The section polished parallel to the fabric (interlab polished section of composite 6, Fig. 3(c)) shows a regular array of cracks. This phenomenon may be related to the expansion mismatch between the SIC fabric ( - 35 X 10-7/“C)‘1 and hexacelsian (8 X lo-‘/“C) expansion coefficients, and can be compared to the value of the monoclinic phase expansion coefficient (2.3 X 10-7/“C).12 The rather large expan￾sion coefficient of anorthite (6 X 10-7/“C)13 seems to be compatible with that of Sic fibre since no cracks are observed. On the other hand, a crack array was found in the NAS-matrix composite (3). Note that the thermal expansion coefficients of pyrolysed SiAl and ZrSiPB interface precursors are very similar to that of Sic fibre.” The mismatch between the thermal expansion coefficients of SIC fibre and NAS and CAS matrix may explain the lower mechanical properties of the CAS and NAS matrix composites (Table l), although the preparation cycle (temperature cycle, dwell, relative volume of fibre and of interface and matrix precursors, etc.) was not optimized. However, previous studie@’ have shown that alkaline earth and phosphorus ions react with the Sic fibre. This reaction promotes the formation of a carbon-rich fibre/matrix interphase which leads to a sliding interface,’ but too large an attack lowers the safe fibre cross-section and hence the mechanical properties.’ Optimization of the relative proportion of each precursor is required. The SiAl interface precursor seems to preserve the fibre14,15 although a higher temperature is needed for sintering.ll 3.2 Acid and alkaline corrosion 3.2.1 Celsian (BAS) matrix Figure 4 compares the microstructures of polished sections of composites after attack by boiling sulphuric acid or molten sodium nitrate. Cracks at the grain boundary and a darkening of the grain limit are observed for BAS matrices. The origin of this phenomenon can be found in the fast attack of the grain boundary second phase or in the grain constriction associated with ion exchange. Comparison of Raman spectra recorded on various grains by the Raman microprobe (analysed area -3 pm X 3 pm) clearly shows the band-broadening characteristic of a loss of crystallinity and a strong reduction in the intensity of the llOcm-’ band assigned to the translational motion of Ba” ions in monoclinic celsian (Fig. 5).5 This indicates leaching of Ba” ions. A longer duration of the chemical attack leads to a more