Die pressing and mechanical behaviour of silicon nitride

Ceramics are usually produced by pressing the powder in a die and sintering the resulting green body. A homogeneous density distribution after pressing is an important prerequisite to obtain equal shrinkage resulting in a final product which meets low tolerance limits. In this paper constitutive equations from micromechanical models are developed as a basis for the numerical simulation of die pressing of different Si3N4 powders. Also the mechanical behaviour (strength and fracture touhness) of sintered plates of these powders is presented. All silicon nitride powders contain 3wt percent Al2O3 and 5wt percent Y2O3 as sintering aids. The powders were ball milled and spray dried by a subcontractor, die pressed into plates with 90 mm diameter by IWM and sintered by LONZA.