MODELING DENSIFICATION DURING SINTER-FORGING OF YTTRIA-PARTIALLY-STABILIZED ZIRCONIA

A model to predict the densification and microstructural development o f a nanocrystalline ceramic powder compact during sinter-forging has b een developed. In the model, densification is predicted by superimposi ng stress-assisted densification mechanisms with a plastic-strain-cont rolled pore closure mechanism. During densification, grain growth is m odeled with pore-controlled grain growth during intermediate stage sin tering and combination of normal (static) grain growth-and dynamic gra in growth during final stage sintering. Applied stress and strain are allowed to vary as a function of time, so that widely varying experime ntal conditions can be modeled. The model predictions are compared wit h 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 b y strain. The model predicts that large pores are efficiently closed o nly by plastic strain but that small pores are easily eliminated by di ffusional mechanisms. It is shown that grain size is minimized as a fu nction of density under conditions that promote high strain rates, so that large pores are quickly eliminated while small pores are still av ailable to control grain growth.