Synthesis of Whisker Reinforced Mullite Matrix Composites Using Colloidal Processing

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Abstract
CVD-processed ultrafine mullite powder (3N4-whisker or mullite-whisker were dispersed in H2O-C2H5OH-C3H8O3 with a low surface tension at pH 4.0 to prevent cracking during drying of wet green compacts consolidated by filtration. The rheological behavior of suspensions, consolidation process of the solids by filtration, and the pore structures of green compacts were studied to understand the formation of crack-free dense powder compacts with uniformly dispersed whiskers in the nanometer size matrix mullite. The crack-free powder compacts were hot-pressed at 1500°C for 1h at 10 to 39MPa in N2 atmosphere or pressureless-sintered at 1600° to 1650°C for 4 or 10h in air or vacuum. The monolithic mullite (99.9% T.D.) and 10% mullite-whisker-added mullite (91.2% T.D.) with the average strengths of 468MPa and 242MPa, respectively, showed little degradation of strength at 1300°C. The composite with 10vol% Si3N4-whisker (99.8% T.D.) showed a high average strength (632MPa) at room temperature, while the strength decreased to 341MPa at 1300°C. The fracture toughness (1.68MPa⋅m0.5) of the monolithic mullite was increased to 2.94MPa⋅m0.5 by the addition of 10% Si3N4-whisker.

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