Plastic forming of alumina from coagulated suspensions

A processing system has been investigated in which alumina suspensions are initially dispersed at very high solids loadings and then subsequently coagulated by the introduction of salt to the system creating a plastic paste with rheological properties suitable for extrusion. Such coagulated pastes containing an alumina content of 81 wt% (50.7 vol%) have been extruded to produce sintered components possessing an average three point bend strength of 445 MPa. This corresponds to an improvement of 35–45% over samples prepared by a flocculated processing route at the solids loading required for extrusion. This increase in strength is attributed to the initial dispersion of samples at the high solids loading, which serves to maximise suspension homogeneity and leads to a smaller critical flaw size in the final sintered extrudate even though the microstructures and densities are very similar. At present the coagulated pastes produce relatively soft extrudates which can be subsequently moulded into more intricate shapes. Further work is required to formulate stiffer pastes resulting from higher alumina contents.