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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

preform compression. The

preform compression. The relative volumetric compression ciso was calculated as the ratio of volume change ΔV to the initial preform volume. The volumetric stiffness of the preform Eiso was calculated as the slope of the linear regression of the pressure- compression curve in the pressure range of 0.1 to 10 MPa. After reaching the maximum pressure, it was reduced stepwise to ambient pressure while measuring volume change to evaluate the permanent compression ciso,per after pressure release. 3.3.9. Reference preforms Two routes were used in preparation of the reference preforms: a foaming method developed by Dytech Corporation Ltd (Sheffield, UK) and a slurry vacuum filtration route for preforms supplied by Saffil Japan Ltd. These preform types were chosen as they represent the most important fabrication routes reported in the literature and because they were the only commercial preforms within the target ceramic per cent volume content range of 0.30 to 0.40. The processing of the foamed alumina preform was in accordance with the patent of Sambrook (162) . The target ceramic volume fraction of 0.30 could be reached by sintering the green part at 1550°C. The preforms were cut from plates to get the preferred geometry for infiltration which is 61 mm x 42 mm x 8 mm. The designation of this preform type was AODY30 which represents the alumina ceramics (AO), the supplier (DY) and the volume fraction of the ceramic phase (0.30). As reported by Hegeler et al. (119) , the ceramic volume fraction in chopped Saffil fibre preforms is limited to 0.25 by volume. Thus, to reach the target volume fraction of 0.30- 0.40, hybrid preforms made from Saffil RF fibres in combination with ceramic particles in a volumetric ratio of 1:1 were purchased. Initially, a chemically inert preform consisting of δ- alumina fibres and α-alumina particles was preferred. This type (Saffil designation RFX50AX-301) contained nominally a ceramic volume fraction of 0.30 but could not be used 79

as the measured mean value 0.23. The producer confirmed problems in processing, resulting in lower values. As a substitute a hybrid preform type was selected with the designation RF50TD-301, in the following designated as FATO. This consisted of Saffil chopped alumina fibres with titania particles having a mean ceramic volume fraction 0.29, which was close to the target range. The delivered preforms were 300mm x 300mm x 8mm and were then cut into standard size samples. The properties of the reference preforms are listed in Table 3.7. Table 3.7 Properties of purchased reference preforms, manufacturers data with exeption of density and ceramic volume fraction which were determined using procedures described in 3.3.5. Type α-alumina particles 80 AODY30 FATO 50% δ-alumina fibre RF + 50% TiO 2 particles Pore formation Slurry foamed Vacuum filtration Supplier Dytech LTD UK Saffil Japan LTD Suppliers designation POR-AL30 RF50TD-301 Theoretical density kg/m³ 3920 3830 Number of preforms 40 10 Density kg/m³ 1226 ± 59 1096 ± 22 Ceramics elastic modulus GPa 410 300 Ceramic volume fraction 0.31 ± 0.02 0.29 ± 0.01 3.3.10. Microstructural investigations SEM examinations were carried out on powder materials by spreading the powder on an adhesive carbon pad. The preform structures were investigated on sawn cross sections. For increased electrical conductivity, the samples were sputter coated with a gold-palladium alloy. 3.4. Infiltration Different infiltration modes were used to fill the pores of the preform. A constant pressure was applied using gas pressurization. In infiltrations using squeeze casting and high pressure

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