Mullite—Alumina Composites by Extrusion

The rheological properties of a paste containing chopped alumina fiber and particulate silica suspended in a gelled boehmite liquid phase have been evaluated using a physically based extrusion model. When sintered, the paste formed a mullite‐alumina fiber composite. Extrudates with fiber volumes up to 30% in the sintered product were prepared. During extrusion, the pressure drop was largely independent of extrudate velocity, fiber length, and the fiber concentration. All pastes showed significant yield behavior leading to good postextrusion shape retention. For any given fiber length, it was shown that there exists a critical volume fraction above which fiber‐fiber interactions are so great that both yield and wall shear stresses increase. At these high concentrations of fiber, inhomogeneities also increase. Up to the critical volume fraction, dispersed wet fibers produced lower extrusion parameters than when dry fibers were used as the starting material. The observed behavior is explained in terms of low viscosity liquid formation above the yield point of the boehmite gel.