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.