The importance of
porosity effects on properties which arise from its significant impact, as well
as from its pervasive occurrence in most fabrication processes whether it is
desired or not. Despite the frequent desire for no porosity for many
applications, pores pervasively occur due to both intrinsic and extrinsicfactors as well as physical and economical limitations. A basic step in
understanding the effects of porosity is to be aware of its sources and
occurrences. Intrinsic limitations on reducing porosity were outlined for
sintering (Fig. A), where most or all pores start as open, intergranular pores and
transition to closed, and even more in the transition to intragranular pores;
the latter reflects a marked change in shape, i.e., rounded polyhedral to
spherical pores.
Sources of
porosity,
Both inter- and
intragranular pores of similar scale can also be introdued by various
processes, e.g., differential diffusion, annealing of radiation damage or
dislocations (especially entangled ones from deformation)" stoichiometric
changes, and phase changes due to reaction of constituents or impurities
during, or after, sintering.
1.2 Extrinsic Sources
in Sintering
There are other extrinsic
aspects of processing that effect the amount and character of pores beyond the
more intrinsic sources of porosity noted above, especially in sintering. Thus,
more, and possibly somewhat larger pores commonly remain in sintered ceramics
not only due to intrinsic effects, but also due to gases left in the originally
open pores (e.g., due to firing in air rather than vacuum). Gases not soluble
in the surrounding ceramic are entrapped in closed pores since they cannot be diffused
out of the pore. Many other extrinsic sources of (commonly intergranular) pores
exist, especially for sintered bodies. These other sources of pores, i.e., from
the materials and forming operations used, contribute mainly to heterogeneity
of the size, shape, and spatial distribution of the porosity.
The powders used for
forming bodies often contribute larger, heterogeneous pores from heterogeneous
particle sizes, shapes, or spatial distributions . Larger pores typically occur
with agglomerates in the powder.