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Microstructure-level residual stresses occur in polycrystalline ceramics during processing, as a result of thermal expansion anisotropy and crystallographic misorientation across the grain boundaries. Depending on the grain size, the magnitude of these stresses can be sufficiently high to cause spontaneous microcracking when cooled from the processing temperature. They are also likely to affect where cracks initiate and propagate under macroscopic loading. The magnitudes of residual stresses in untextured and textured alumina samples have been predicted using experimentally determined grain orientations and object-oriented finite-element analysis. The crystallographic orientations have been obtained using electron-backscattered diffraction. The residual stresses are lower and the stress distributions are narrower in the textured samples, in comparison with those in the untextured samples. Crack initiation and propagation also have been simulated, using a Griffith-like fracture criterion. The grain-boundaryenergy:surface-energy ratios required for computations are estimated using atomic-force-microscopy thermal-groove measurements.

Powder pressing, either uniaxially or isostatically, is the most common method used for high-volume production of ceramic components. The object of a pressing process is to form a net-shaped, homogeneously dense powder compact that is nominally free of defects. A typical pressing operation has three basic steps: (1) filling the mold or die with powder, (2) compacting the powder to a specific size and shape, and (3) ejecting the compact from the die. To optimize a pressing operation, experienced press operators generally understand and control parameters such as die-fill density, die-wall friction, packing density, and expansion on ejection.Die filling/uniformity influences compaction density, which ultimately determines the size, shape, microstructure, and properties of the final sintered product. To optimize die filling and packing uniformity, free-flowing granulated powders are generally used. Spherical granules (i.e., agglomerates or clusters of finer particles) range in size from ~44 to 400 μm with the average size being ~100–200 μm. They are typically produced from 0.5 to 10-μm median particle-size powders by spray drying a ceramic powder slurry. To produce processable powders, various organic additives are typically added to the slurry prior to spray drying. These include binder(s) for strength, plasticizers that produce deformable granules, and lubricants that mitigate frictional effects. Consistent batching and dispersion of the granulated feed are critical for reproducible and uniform die filling. Granule densities that are 45–55% of the theoretical density (TD), and bulk-powder and die-fill densities of 25–35% TD are typical for ceramic powders.

A theoretical model is developed for the description of the compaction of granular materials exemplified by granulated ceramic powders. Its predictions compare satisfactorily to results of uniaxial compaction tests of ceramic granules of lead magnesium niobate‐lead titanate (PMN‐PT), rutile, and spray‐dried alumina. The theory uses volume‐based statistical mechanics and an activation analogy to treat, in parallel, the rearrangement of granules and their deformation. Variation of the model incorporates a distribution of barriers to deformation, which can be considered to include the effects of statistical pressure distributions within the compact. Other curve‐fitting schemes available in the literature are shown to correspond to particular cases of the theory, and a justification of the equations used in those schemes is provided on physical grounds.

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For infiltration of a molten salt into porous zirconia (relative densities = 0.56 to 0.88), the infiltration depth was found to be a function of both infiltration time and initial compact density. The intrinsic liquid permeability of the porous Y-TZP was determined by extrapolation of gas permeability data. Permeability values, which ranged from 0.5 × 10 −18 to 25 × 10 −18 m 2 (∼0.5 to 25 microdarcy), could be used to predict the trend in infiltration depth with compact density, though they underestimated the absolute values. The Carman-Kozeny relationship, which relates the permeability to measurable microstructural parameters, was also evaluated. There was good agreement at relative densities <0.75, but not at the higher densities, likely due to the increasing tortuosity of the flow path as pores shrink and close.

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