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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

fabricated in the target

fabricated in the target porosity range. A compaction pressure of 20 MPa resulted in preforms with too low edge strength. Preform porosity Φ (%) Tot / % 80 70 60 50 40 30 0 20 40 60 80 100 111 AOPC20_1600°C TOPC10_1100°C Green part compaction pressure (MPa) / MPa Figure 4.18 Total porosity Φtot of preforms as a function of the powder compaction pressure. The points in circles indicate parameters leading to insufficient edge strength or cracking. A reduction in the compaction pressure of TOPC10 to 40 MPa resulted in preforms with the target porosity but the strength was insufficient and cracks were observed after sintering. A possible solution to this issue was to change the sintering profile, which is reviewed in section 4.5.3. 4.5.3 Influence of sintering temperature The influence of the sintering temperature on the total preform porosity is shown in Figure 4.19. The porosity of the TOPC10 titania preform reached the target range when the temperature was lowered to 1050°C. The sintering temperature was also increased to 1150°C in order to investigate the correlations. The regression shows a negative slope. In contrast, there was no significant change in the total porosity values when the sintering temperature of MOPC20 was reduced to 800°C. The subjective assessment of preform target range

strength showed no significant difference between the lowest and the highest sintering temperatures. The preforms used for further processing were sintered at 1300°C as suggested by the supplier. For comparison of MMC microstructures, one preform sintered at 800°C was also infiltrated and characterised. Preform porosity Φ (%) Tot / % 80 70 60 50 40 TOPC10_100MPa MOPC20_100MPa 30 800 900 1000 1100 1200 1300 Sintering temperature T (°C) sint sint / °C 112 target range Figure 4.19 Final total porosity Φtot as a function of the sintering temperature. As with all preform types, the target porosity range was reached by varying the PFA content, compaction pressure or the sintering temperature. These optimized parameter combinations were used to produce further preforms which were used to investigate the structure and infiltration behaviour. The manufacturing parameters and properties of each preform type are listed in Table 4.1. The pore former concentration is given as both weight and volume fraction. The latter was calculated using the densities of the pore former given in Table 3.5. The resulting total, closed cell and open cell porosities are shown, as well as the porosity attributed to the pore forming additive, ΦPFA , which was calculated taking into account the porosity of the preforms without PFA. It was found that the the closed cell porosity, Φc , ranged from 0.2% (AOPC20) to 2%

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