Diphasic mullite gel pressed at 1.5 GPa : Mechanical properties and microstructure of ceramics

Mullite gels containing 72–85 wt% Al2O3 were prepared by mixing commercial boehmite and silica sols and gelled by heating. Powdered gels were either dried at 60°C or calcined at 550°C, and cold isostatically pressed under 0.5, 1.0 or 1.5 GPa. The effect of pressure is greater for the calcined gels than for the dried gels. The bulk density and room-temperature flexural strength of ceramics increased with increasing compacting pressure, while the fracture toughness is independent of the pressure. Mullite containing 75 wt% Al2O3 and sintered at 1500°C for 3 h has a density which is about 99% of the theoretical density, a flexural strength of 523±40 MPa and a fracture toughness of 3.5 MPa m1/2.