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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

Apart from permeability,

Apart from permeability, the specific surface area of a preform plays an important role in preform infiltration. As reported by Mortensen (121) ,Si of Saffil fibre preforms, which is the area (m²) per unit volume of porosity (m³) in the preform, is given by: S = 977800 + 22217 ⋅V Equation 33 i sf In a later publication of the same author (122) , the estimated value of Si was four times higher than that obtained by using Equation 33. The differences in Si were not further discussed. 2.4.2. Sintered particle compacts One of the most common ways to fabricate MMCs is to press ceramic particles into a cavity and infiltrate the packed bed with a molten metal. Due to the simplicity, several research groups have concentrated on this method (103) . If the particles are assumed to have a spherical shape with an average diameter D, Si of the packed particles is calculated as: 6λV p Si = D( 1−V ) p 49 Equation 34 where Vp is the particle volume fraction and λ is a geometrical factor introduced to describe deviations from sphericity, but also takes account of surface roughness and of the actual particle size distribution. As no binder is added, removal of the pressed compact out from the pressing cavity can result in breakage. Thus preforms fabricated using this route are not consistent. To achieve consistent ceramic preforms which exhibit sufficient strength to withstand the subsequent infiltration step, the particles have to be sintered. This allows the total porosity to be influenced to some extent by changing the powder compaction and sintering temperature, although it is very difficult to achieve porosities above 50%. It is possible to alter the metal ligament diameter to some small extent by changing the size of the particles (52, 68) . With this simple fabrication route it is not possible to influence the pore structure independent of the ceramic microstructure.

2.4.3. Foamed preforms Another interesting approach to produce fully interpenetrating networks has been suggested by Lange et al. (123) who slip cast a ceramic suspension into a reticulated polymeric foam. After drying, burnout of the polymer and sintering, this leaves a negative replica of the foam with fully open porosity. By varying the polymeric foam, a variety of ligament diameters was achieved. The ceramic structure was not independent from the cell size of the foam and hence ceramic volume fraction of this method was limited to about 0.10. A related technique has been reported by Cichocki et al. (124) who compressed thermoplastic polymer foams and fixed this state by heating to 200 °C with subsequent cooling. By cutting the foams into, for example, wedge shapes prior to compression to a uniform thickness, porosity gradients could be achieved. Subsequent slip casting, burnout and sintering as above led to interpenetrating, functionally graded networks with open porosity that were successfully infiltrated with aluminium melts. Preform porosities between approximately 5% and 25% were possible with this method. Another foam-based approach has been reported by Peng et al. (125) who used a direct foaming and reticulation technique on suspensions of alumina powder and short Saffil alumina fibres. This led to an open-celled ceramic foam of approximately 94% porosity. In other work Peng et al. (54) synthesized bi-continuous MMCs using direct-foamed preforms with higher ceramic contents in the porosity range of 94% to 78%. By lowering the porosity level, the channels connecting the large cavities (the so-called “windows” as shown in Figure 2.15) were reduced in size. 50

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