Role of powder size, packing, solid loading and dispersion in colloidal processing of ceramics

The packing characteristics of various kinds of particle size distributions are described in terms of their variance and skewness. For a suspension comprising two or more powders of different size distributions, a search procedure is proposed for obtaining the composition that matches the mixture size distribution, to the maximum extent possible, with an ideal dense packing distribution, such as the Andreasen distribution. The method, which obviates the need for time consuming experimental search, is efficient and accurate when tested against published data for the density of green compacts. It is stressed that a dense packing size distribution by itself may not be sufficient to obtain high density green compacts. An example is included which demonstrates that it is necessary to concurrently control the rheology of the suspension by optimizing its solid loading and dispersion dosage. It is further pointed out that high green density does not necessarily guarantee an optimal sintering path, especially if it is achieved by incorporation of excessively coarse powder. This point is illustrated with the fabrication of zirconia–alumina composite ceramics by the colloidal processing route. This important system is interesting by virtue of the fact that the constituent powders differ not only in size but also in their surface properties in the suspension.