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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

4.3.3 Microstructure

4.3.3 Microstructure Figure 4.13 a) to c) shows the microstructures of the milled AO powder and TO and MO in the as-purchased condition. The microstructure of MO powder was after storage for 12 months in a normal climate. (a) (c) Figure 4.13 Microstructures of the powders used for preform fabrication. a) AO in the milled condition. b) TO in the as-purchased condition. c) MO after storage. The low magnification image of the as-purchased AO powder in Figure 4.11 shows a rounded structure. The higher magnification image of the milled structure in Figure 4.13 a) shows small tabular grains with an aspect ratio significantly lower than unity as a result of milling 103 (b) which broke down the spherical agglomerates to the primary grains.

The particles of TO and MO were distinctively smaller and had a platelet morphology. On the surface of some agglomerates of MO, there were regions with very small round particles; one area is circled in Figure 4.13 c). 4.4. Pore forming additives (PFA) 4.4.1 Particle size distribution Two pore forming agents (PFA) were used: carbon fibres (PF) and cellulose particles (PC). The PF were ball milled to produce a fibre length which was dispersible in the aqueous ceramic slurry. The optimum milling time, which was a compromise between the maximum aspect ratio to keep the fibrous shape and the ability to be dispersed in the slurry, was found to be 1 minute (173) . The lengths and diameters of 1200 fibres were measured after milling. Cumulative Cumulative volume volume fraction / % (%) 100 80 60 40 20 0 1 10 100 104 PF PC PC: Particle size (µm) / PF: Fibre length (µm) PC: particle size /µm ; PF: fibre length /µm Figure 4.14 Size of the pore forming additives: Particle size of the cellulose (PC) and length distribution of the carbon fibre (PF). The mean diameter was 8 µm, with all measured fibres falling in the range between 7.5 and 8.5 µm. The volume fractions per fibre length class were calculated assuming the fibres to exhibit a cylindrical geometry with a diameter of 8 µm. This allowed the cumulative volume

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