Pressure infiltration of liquid aluminium into packed SiC particulate with a bimodal size distribution

High volume fraction composites were produced by infiltrating liquid aluminium into preforms made by mixing and packing SiC particles with average diameters of 170 and 16 mum. The maximum particle volume fraction (0.74) was attained for a mixture having 67% of coarse particles. The variation of the preform compactness with the percentage of coarse particles can be reasonably well understood in terms of a simple model. The experimental results indicate that the threshold pressure is mainly determined by the local compactness of fine particles, a result which is shown to be compatible with the model used to calculate the particle volume fraction. The permeability of the compacts was also evaluated by means of pressureless infiltration with an organic liquid (polyethylene glycol) and the results were compared with those obtained with aluminium. The two data sets qualitatively agree for low percentages of coarse particles. However, while the permeability obtained with the organic liquid shows a minimum for preforms having the maximum compactness (roughly half the value obtained for fine particles), in the case of aluminium, the permeability for that preform (and others having a compactness around the maximum) was significantly higher. This was ascribed to the pushing of particles during pressurised infiltration of aluminium, triggered by particle segregation. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.