Sedimentation of Fine SiC Particles During Liquid Processing of Al/SiC Composites

Abstract

The settling of fine SiC particles (2.5 - 20 vol%) during liquid processing of Al/SiC composites has been studied by an electrical resistance technique. Settling rate data show that the settling of fine SiC in aluminum does not obey Richardson-Zaki relation, characteristic ofnon-flocculated systems. Rather the data reveal two distinct settling regimes, a (non-linear) rapidly decreasing settling rate regime (of higher than Richardson-Zaki predicted rates) and a linear(below Richardson-Zaki predicted rates) regime. The final settled bed in the aluminum-fine SiC with a concentration ofonly 0.28 volume fraction SiC, lacks the compactness ofrandomclose-packed arrangement (0.62 volume fraction), characteristic of non-flocculated systems. The system, in essence, cannot be non-flocculating and the suspicion is that there is an appreciable level of flocculation or clustering among these fine particles. At low SiC volume fractions (where settling rates are higher than predictions of the Richardson-Zaki relation) the flocs/aggregates probably settle individually. At high volume fractions of SiC (where settling is slower than expected), it is most probable that the aggregates settle together as a coherent network structure.