Colloidal Processing and Mechanical Properties of Whisker‐Reinforced Mullite Matrix Composites

Aqueous colloidal suspensions in the two systems of CVD‐processed ultrafine mullite powder (<0.1 μm), ‐Si 3 N 4 whisker and ‐mullite whisker, were prepared near the isoelectric point of mullite (pH 7.0) to prevent cracking during drying of wet green compacts consolidated by filtration. The freeze‐dried porous green compacts were hot‐pressed with a carbon die at 1500°C for 1 h at a pressure of 39 MPa in N 2 atmosphere. The relative densities of the mullite matrix composites with whiskers of 0 to 10 vol% were in the range of 95.2% to 99.8%. Increasing the fraction of Si 3 N 4 whisker increased the density, flexural strength, and fracture toughness of the hot‐pressed composites. On the other hand, addition of the mullite whisker increased the fracture toughness but decreased the density and strength of the composites.