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 coag ulated by the introduction of salt to the system creating a plastic paste w ith rheological properties suitable for extrusion. Such coagulated pastes c ontaining an alumina content of 81 wt% (50.7 vol%) have been extruded to pr oduce sintered components possessing an average three point bend strength o f 445 MPa. This corresponds to an improvement of 35-45% over samples prepar ed by a flocculated processing route at the solids loading required for ext rusion. This increase in strength is attributed to the initial dispersion o f samples at the high solids loading, which serves to maximise suspension h omogeneity and leads to a smaller critical flaw size in the final sintered extrudate even though the microstructures and densities are very similar. A t present the coagulated pastes produce relatively soft extrudates which ca n be subsequently moulded into more intricate shapes. Further work is requi red to formulate stiffer pastes resulting from higher alumina contents. (C) 2000 Elsevier Science Ltd. All rights reserved.