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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

compromise had to be

compromise had to be found, where there was sufficient porosity (60 to 70%) and strength as indicated by low edge fracture during handling. The processing parameters for AG with PC were previously investigated by Staudenecker (155) . The optimum PC fraction was found to be 15% giving a final porosity of 65%. Thus AGPC15 preforms were used for further characterisation and MMC processing. Linear regression of the data shown in Figure 4.17 indicates that AGPC, AOPC and TOPC exhibit similar gradients. The gradient for MOPC was significantly lower, indicating a different behaviour of the pore former PC in the MO preforms. Even though PF had a higher density (1750 kg/m³) than PC (1500kg/m³) AOPF showed similar gradient as AOPC in Figure 4.18, which has to be attributed to higher efficiency of the pore formation with PF. More detailed work may be performed in future to investigate the differences between PF and PC. Cellulose (PC) or carbon fibres (PF) were used in the AO preforms in order to investigate the influence of the pore former geometry and chemistry. For PC additions, the porosity was raised from 35% (AO) to 72% for AOPC40. Those with the highest values (AOPC30 and AOPC40) had low strength after sintering, making them unsuitable for further processing (circled data points in Figure 4.17). The AO preforms with PF could be fabricated with sufficient edge strength up to the maximum PFA addition investigated of 50%. The resulting total porosity was 79%. The porosity of AO in the AOPC curve was lower than that in the AOPF curve. This was due to a lower sintering temperature of 1500°C being used for AOPF. Therefore the remaining intragranular porosity fraction was higher. The porosity of TO preforms was 44% and therefore higher than that of AO sintered at 1500°C and 1600°C. For TOPC with 30 wt-% of pore former the preforms had inadequate 109

strengths, whereas with 10 and 20 wt-% PC the preforms had sufficient strength and 57% and 65% porosity respectively. The highest total porosity of 50% without PFA additions was found in the MO-preforms. The additions of PFA showed less effect in MOPC than in the other ceramics. The addition of 20 wt-% of PC resulted in an increase of 28% total porosity in AOPC compared to 12% for MOPC20. As TOPC did not show this behaviour, it has to be attributed to the chemistry of the preform. By altering the PFA content, the AOPF20 and TOPC20 preforms as well as AGPC15, provided materials in the target porosity range and therefore were taken forward for further characterisation and composite processing. It was wished to increase the porosity of MOPC20 from 62% to 65% (the centre of the range) by a reduction in sintering temperature which is presented in 4.5.3. In order to reach the target range with AOPC, other process parameters such as green compaction pressure and sintering temperature were varied, as presented in 4.5.2 and 4.5.3 respectively. The same variations were applied to the TOPC10 preforms. For the TO preforms, the influence of the pore structure on the preform infiltration behaviour was investigated. The pore former fractions influenced the ratio between the PFA-formed and intragranular porosities, thus giving different ratios of coarse and fine pores with the same total pore volume. The target was to get equal total porosity at two different pore former fractions. 4.5.2 Influence of green part compaction pressure The influence of the green compaction pressure on the porosity of AOPC20 and TOPC10 at sintering temperatures of 1600 and 1100°C respectively is presented in Figure 4.18. By reducing the compaction pressure from 100 MPa to 60 MPa, the AOPC20 preform could be 110

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