Powder preforms of zirconia, containing 2.85 mol% yttria, were sinter-forged in simple uniaxial compression at 1400°C by applying constant displacement rates to the specimens. Shear and densifcation strains and the uniaxial stress were measured as a function of time. In contrast with alumina and silicon nitride, zirconia appears 60 densijj by a dislocation mechanism. As a consequence, rhe densification rate is linked to the applied strain rather than to the applied hydrostatic pressure; the powder compact requires a critical amount of compressive strain to consolidate to full density, irrespective of the strain rate or the stress at which that strain is applied.SINTER, forging is a net-shape forming technique where powder preforms are compressed in such a way that the preform consolidates to full density and acquires the desired shape in one step. The design of the process requires a quantitative knowledge of the rate of densification and shear deformation in the powder compact when it is isothermally compressed. Recently we have obtained such information on several materials including alumina,' silicon nitride,' and composites.' In the present paper we report results on zirconia. DeJonghe and co-workers have applied a similar technique to a number of otherIn addition to providing insight into the mechanisms of shear deformation and densification, the sinter-forging experiments also provide a measurement of the sintering pressure, which is the intrinsic driving force for densification during free sintering. In hard materials such as alumina and silicon nitride, densification as well as deformation occurs by diffusional mechanisms: this behavior is characterized by a linear dependence between the rate of deformation or the rate of densification and the driving force. The driving force is the applied shear stress for shear deformation and it is the sum of the applied hydrostatic pressure and the intrinsic sintering pressure for densification. In the case of
Viscosity of porous glasses has been derived from the elastic stress analysis, using the viscous analogy. Viscosity as a function of porosity has been estimated for spherical as well as for arbitrary pore geometry. Since the pore geometry changes during sintering, a shape factor that varies with pore geometry has been considered to predict the viscosityporosity relationship. Viscosity as a function of porosity was measured on cordierite-type glass by isothermal sinter-forging experiments and data showed good agreement with the anaylsis. Experimental data from literature on viscosity as a function of porosity on two other glasses also show good agreement with the analysis. [
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