1490 V. Bianchi et al differential thermal analysis(dta)of the glass It can be noted that the cte of the glass powder(Setaram Micro ATD M5--linear increase ceramic is lower than that of the glass, which of5°Cmin),as815855°Cand945-1055° depends on the low Cte of the cordierite. The respectively glass transition temperature interval obtained with The coefficients of thermal expansion(CTE) of this technique is similar to that determined by the glass and the glass-ccramic have bcc DTA sured with a vertical dilatometer(Setaram TMA 2), in argon, with a linear heating or cooling rate 2.3 Compusites processing (C/min)on bars obtained by unidirectional Pre-preg sheets were prepared by using a slurry pressing and sintering in air for 2h at the required infiltration process of fibre tows. The impregnated temperature. The results are summarized in Table fibres are wound on a hexagonal mandrel. The volume fraction of fibres is nearly equal to 35%. After drying, the tapes were cut into 35 x 35 mm pieces, stacked and debound. Then the samples were densified in argon by hot-pressing 950-1250oC temperature interval with a pressure of 10 MPa(LPA-DVM Goliath) followed by linear increase or decrease in temperature (25.C/min). The unidirectional pressure was applied when the sintering temperature had been reached and was removed on cooling In order to optimize the density and the fracture strength of the composites, various thermal cycles have been performed. The conditions are summarized in Table 3 where in some cases. two temperature-time combinations are given. The first corresponds to the densification step under plied pressure. The second -- if any sponds to an additional pressureless crystallization step at a higher temperature. H山r 3 Composites Microstructure and Properties 3.1 Density and fibl The density has been measured by the hydrostatic technique For the P25 fibre-reinforced composites, after hot-pressing for at least 30 min, the open porosity ranges from 1. 5 to 3. 7%0, except for Pl, which was hot-pressed at a low temperature and the open porosity of which is about 24%. Indeed, at 950oC, the viscosity of the glass is too high to allow a vis cous flow in the tows Nevertheless. some bundles of fibres were not impregnated by the glass and the distribution of the fibres in the matrix is not homogeneous(Fig 9(a) The density of T400H fibre-reinforced composites increases gradually as the sintering tempcrature is FBER raised up to 1150C and the viscosity decreases bably due to the high ber of fibres tow(3000), the composites always exhibit some porosity (Table 4) and non-impregnated bundles Fig. 4. P25 fibre cross-sections. Bright field TEM image, of fibres can be pulled out during polishing taken from fibres within the same composite(P5). The matrix Fig 9(b). For sintering temperatures higher than appears dark. Clear areas within the fibres are isotropic parts while dark areas are anisotropic parts: (a)and (b) are two 1150oC, the porosity increases again. This latter examples of fibres with various corrugated surfaces. phenomenon could be related to a release of nitro
1490 V. Biunchi et al. differential thermal analysis (DTA) of the glass powder (Setaram Micro ATD MS-linear increase of SWmin), as 815-855°C and 945-1055”C, respectively. The coefficients of thermal expansion (CTE) of the glass and the glass-ceramic have been measured with a vertical dilatometer (Setaram TMA 92), in argon, with a linear heating or cooling rate (3Wmin) on bars obtained by unidirectional pressing and sintering in air for 2h at the required temperature. The results are summarized in Table 2 5.14 Fig. 4. P25 fibre cross-sections. Bright field TEM image, taken from fibres within the same composite (PS). The matrix appears dark. Clear areas within the fibres are isotropic parts, while dark areas are anisotropic parts; (a) and (b) are two examples of fibres with various corrugated surfaces. It can be noted that the CTE of the glassceramic is lower than that of the glass, which depends on the low CTE of the cordierite. The glass transition temperature interval obtained with this technique is similar to that determined by DTA. 2.3 Composites processing Pre-preg sheets were prepared by using a slurry infiltration process of fibre tows. The impregnated fibres are wound on a hexagonal mandrel. The volume fraction of fibres is nearly equal to 35%. After drying, the tapes were cut into 35 X 35 mm’ pieces, stacked and debound. Then the samples were densified in argon by hot-pressing in the 950-1250°C temperature interval with a pressure of 10 MPa (LPA-DVM Goliath) followed by a linear increase or decrease in temperature (25”Umin). The unidirectional pressure was applied when the sintering temperature had been reached and was removed on cooling. In order to optimize the density and the fracture strength of the composites, various thermal cycles have been performed. The conditions are summarized in Table 3 where, in some cases, two temperature-time combinations are given. The first corresponds to the densification step under applied pressure. The second - if any - corresponds to an additional pressureless crystallization step at a higher temperature. 3 Composites Microstructure and Properties 3.1 Density and fibre distribution The density has been measured by the hydrostatic technique. For the P25 fibre-reinforced composites, after hot-pressing for at least 30 min, the open porosity ranges from 1.5 to 3.7%, except for Pl, which was hot-pressed at a low temperature and the open porosity of which is about 24%. Indeed, at 95O”C, the viscosity of the glass is too high to allow a viscous flow in the tows. Nevertheless, some bundles of fibres were not impregnated by the glass and the distribution of the fibres in the matrix is not homogeneous (Fig. 9(a)). The density of T4OOH fibre-reinforced composites increases gradually as the sintering temperature is raised up to 1150°C and the viscosity decreases. Probably due to the high number of fibres in a tow (3000), the composites always exhibit some porosity (Table 4) and non-impregnated bundles of fibres can be pulled out during polishing (Fig. 9(b)). For sintering temperatures higher than 115O”C, the porosity increases again. This latter phenomenon could be related to a release of nitro-
Carbon-fibre-reinforced (YMAS) glass-ceramic matrix composites. I gen from T400H fibres. Such a release occurring the viscosity of the matrix may become too high when the matrix is still viscous (at 1050 for T7) because of crystallization, which could prevent the likely to prevent good adhesion between the fibres impregnation of the fibres and the matrix whereas, for hot-pressing at 1150C (T5), carbon pyrolysis is nearly achieved 3. 2 Matrix crystallization le matrix can closely take the forms of the X-ray diffraction equipment with a computer fibres. When hot-pressing occurs at 1200C (T6), search/match program was used to determine the ANISOTROPIC AREA 310m Fig. 5. P25 fibre cross-section, Lattice fringe TEM image. The comparison with Fig. 2 demonstrates the lower structural organization f P25 fibres relative to P55 fibres, while exhibiting similar textural features (anisotropic domains). Note the absence of interlayer decohesion
Carbon-jbre-reinforced ( YMAS) glass-ceramic matrix composites. I. 1491 gen from T400H fibres. Such a release occurring when the matrix is still vis’cous (at 1050” for T7) is likely to prevent good adhlesion between the fibres and the matrix wherea,s, for hot-pressing at 1150°C (T5), carbon pyrolysis is nearly achieved and the matrix can closely take the forms of the fibres. When hot-pressing occurs at 1200°C (T6), the viscosity of the matrix may become too high because of crystallization, which could prevent the impregnation of the fibres. 3.2 Matrix crystallization X-ray diffraction equipment with a computer search/match program was used to determine the Fig. 5. P25 fibre cross-section. Lattice fringe TEM image. The comparison with Fig. 2 demonstrates the lower structural organization of P25 fibres relative to P55 fibres, while exhibiting similar textural features (anisotropic domains). Note the absence of interlayer decohesions