Tailoring suspension flow for the gelcasting of oxide and nonoxide ceramics

摘要

Gelcasting is a forming process in which a concentrated slurry of ceramic powders suspended in a solution of organic monomers is polymerized to form a green body of the powder held in a polymeric gel. The process is suited to the production of near-net-shape complex parts. The slurry should contain high solids loading ((approximately) 50 vol%) in order to minimize shrinkage and distortion during drying and, thus, to produce a relatively dense green body that facilitates sintering of the green part to full density. To produce complex-shaped bodies, however, the slurry has to be fluid at the high solids loading, to flow, and to fill complex-shaped molds. Consequently, there is the need to study the rheology of the gelcasting slurries of typical ceramic powders in order to understand the factors that govern the flow and pourability of these slurries. The rheology of gelcasting slurries of concentrated (30--60 vol%) alumina, zirconia, silicon nitride, and sialon powders with isoelectric points varying between pH 5 and 9, was studied by measuring the viscosity and yield stress as functions of pH. Typically, the rheology is non-Newtonian. The lowest values of viscosity and yield stress that permit the highest solids loading were obtained at high and low pH values, where the powders are colloidally stable, far from the isoelectric points. This relationship applies whether or not a polymer dispersant is present. The acrylamide monomer which is used in gelcasting slurries, lowers these non-Newtonian properties permitting the preparation of more concentrated slurries. Fluid and pourable gelcasting slurries of each powder had apparent viscosities and yield stresses less than 2.0 Pa and 50 Pa, respectively.