Forming of nickel compacts by a colloidal filtration route

Concentrated suspensions of metal Ni powders have been prepared in water by dispersing with a polyelectrolyte and tetramethylammonium hydroxide at alkaline pH. Optimisation of the suspensions was studied by measuring the rheological behaviour as a function of pH and volume fraction of solids. The maximum packing fraction has been calculated with the Krieger-Dougherty model and the fractal dimension from the yield stress values determined from flow curves. The shear thinning behaviour and the fractal dimension demonstrate that suspensions form a structure at rest that retards sedimentation and easily flows on shearing. Optimised suspensions were slip cast in plaster moulds leading to green densities above 50% of theoretical. The good stability and homogeneous packing during consolidation also allows a reduction in the sintering temperature to obtain dense compacts. A relative density above 97% of theoretical has been attained after heating at 900 °C/1 h in flowing argon.