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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

fulfilled when the total

fulfilled when the total porosity Φtot is equal to the open cell porosity Φop. As summarized in Table 4.1, the differences between Φtot and Φop (the closed cell porosities Φc) were in the range of 0.2 to 2.0% for the preforms fabricated with pore forming agent (PFA) and measured using the an method based on the Archimedes´ principle (159) . The lower value was for AOPC20 and the highest for AGPC15. The latter was sintered at relatively low temperatures (1000°C) using a glassy frit. As AOPC20 was fabricated using similar powder without a frit, the porosity of AGPC15 may be attributed to the closed cell pore formation during sintering. No reactions were found between the frit and alumina, which would lead to inaccuracies in bulk density and therefore unreliable porosity parameter calculations. However, the 2% measured closed porosity was not found in the microstructure of the MMCs. This may be a result of the fuzzy appearance of the reinforcing phase caused by the transparency of the glassy frit. Alternatively, the porosity may have become closed during the metal infiltration. Due to the fact that the MMC strength of AGPC15IS was higher than that of AOPC20IS and that residual porosity has a detrimental effect on strength (exemplified by TOPC10IS in Figure 4.77) closure of the residual porosity during infiltration has to be assumed. This is supported by the fact that the frit softens at the preform preheating temperature (154) (800°C), leading to plastic deformation of the preform. The high compression of AGPC15IS during DSQC (Table 4.8) supports this hypothesis. The Φc values were not confirmed by mercury intrusion porosimetry (MIP - section 4.6.2 - Table 4.2). In identical preforms, the difference between total porosity, Φtot , and intruded porosity, ΦHg , which is assumed to represent the closed cell porosity, was significantly higher, between 4 and 9%. The maximum error in the measured intrusion volume was less than 1% (153) . According to Rootare and Prenzlow (109) , the difference between the two values resulted from the compression of the preform samples during mercury intrusion 197

measurements which resulted in a lower apparent total porosity. The amount of compression is reported to correlate with the strength of the test sample (109) . This corresponds to the differences between Φtot and ΦHg. For example, Φtot-ΦHg was 9% in TOPC10 compared to 0% for TO which was produced without PFA (Table 4.2). TO had a ceramic volume content of 56% compared to 38% in TOPC10 (38%). The strength of TO was correspondingly higher than that of TOPC10 but was not evaluated in the present work. Beyer (53) reported the bending strength of a titania preform similar to TO to be 27.7±2.3 MPa compared to a value of 1.1 MPa found for TOPC10. The foamed preform AODY30 exhibited a relatively high compression strength of 245 MPa which was 35 times higher than that of FATO and 220 times higher than that of TOPC10 (Figure 4.33). Therefore, in a similar way to TO, a minor difference between Φtot and ΦHg should have been found. However, 1% difference was measured, which has to be attributed to closed cell porosity and not to preform compression. It is therefore concluded that MIP is not a suitable method to determine closed cell porosity as the difference between Φtot and ΦHg is an effect of both preform compression and effective closed cell porosity and could not be attributed to individual parameters. 5.1.4 Influence of the ceramic structure As reported by Legzdins et al. (41) , brittle interfacial phases are detrimental to elastic and plastic properties of the composite. It has been shown (Figure 4.60 a) that a non-contaminated interface between the matrix and the reinforcing phase was achieved with the non-reactive preform AOPC20. Nevertheless, it has also been shown that silicate binder additions in similar volume fractions as those used in Saffil fibre preforms did not have a detrimental influence on composite properties in particulate preforms. On the contrary, the higher σ0 values of AGPC15IS compared to AOPC20IS indicated an improved microstructure. This is 198

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