COLLOIDAL PROCESSING FOR LAMINATED COMPOSITES OF MULLITE ALUMINA WOVEN FABRICS

A sol-gel-processed mullite powder and alumina woven fabrics were for-med into laminated composites (5 vol% alumina fabrics) by filtration of the aqueous mullite suspensions containing polyacrylic ammonium at pH = 8.8 which were poured into the spaces separated with alumina fabrics. The mullite suspensions with 30-55 vol% solids showed a Newtonian flow and formed high-density green compacts of 60-63% of the theoretical density (TD). The green composites were densified to near full density by hot-pressing at 1650-degrees-C for 1 h at 39 MPa in N2 atmosphere. The mechanical properties (strength, hardness and fracture toughness) of the dense composites (> 99% TD) were measured at room temperature. However, no significant improvements of strength and fracture toughness were obtained due to the formation of large cracks (> 100 mum) across the alumina fabrics and disappearance of long fiber characteristics after sintering.