Modeling densification during sinter-forging of yttria-partially-stabilized zirconia

A model to predict the densification and microstructural development of a nanocrystalline ceramic powder compact during sinter-forging has been developed. In the model, densification is predicted by superimposing stress-assisted densification mechanisms with a plastic-strain-controlled pore closure mechanism. During densification, grain growth is modeled with pore-controlled grain growth during intermediate stage sintering and combination of normal (static) grain growth-and dynamic grain growth during final stage sintering. Applied stress and strain are allowed to vary as a function of time, so that widely varying experimental conditions can be modeled. The model predictions are compared with experimental data for the sinter-forging of 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) at 1050 and 1100 degrees C. These predictions are made assuming a bimodal pore size distribution where pores smaller than the grain size are not allowed to be closed by strain. The model predicts that large pores are efficiently closed only by plastic strain but that small pores are easily eliminated by diffusional mechanisms. It is shown that grain size is minimized as a function of density under conditions that promote high strain rates, so that large pores are quickly eliminated while small pores are still available to control grain growth.