SCIENTIFIC RESEARCH AND DEVELOPMENT
PORE COORDINATION NUMBER AND SINTERING
A. V. Galakhov
Translated from Novye Ogneupory, No. 3, pp. 22 – 27, March 2010.
Original article submitted October 13, 2009.
Using the model developed, describing sintering and recrystallization in an assembly of particles in contact,
occurrence of these processes is analyzed in highly coordinated pores, that are defects in actual
inhomogeneous molding of agglomerated powders. The sintering time of these pores to complete density de
pends both on the configuration parameters, and on the physical characteristics of a material. The results ob
tained may be useful in predicting sintering in inhomogeneously packed powder moldings
Keywords: ceramic technology, powder moldings, particle packing inhomogeniety, agglomerates, highly co
ordinated pores, sintering, recrystallization, physical process modelling.
In ceramic technology (as in any powder technology)
considerable importance attached to original operations for
powder raw material preparation. This relates to mechanical
grinding, granulation, screening, etc. The main aim of these
operations is to give the starting powder optimum qualities,
providing to the maximum extent preparation of uniform and
dense particle packing in a molded compact. The criterion
for packing uniformity is most complete contact of its com-
ponent particles. This uniformity guarantees absence of in
ternal cavities with a size exceeding the average particle size.
The reason for development of these cavities is presence
within the starting powders of multiparticle string forma
tions, that are called agglomerates; the cavities themselves
have a generally accepted term, i.e. inter-agglomerate poros
ity. Presence of inter-agglomerate pores in a molding has an
extremely unfavorable effect on subsequent sintering. If in
densely packed (intra-agglomerate) areas removal of
interparticle pores with low coordination (by pore coordina
tion number we understand the number of particles surround
ing it) occurs for a short time, after which sintering may
cease in principle avoiding recrystallization coarsening of
the structure, then for total removal of highly coordinated
pores this time is clearly insufficient. Obligatory lengthening
of the thermal regime, required for removing inter-agglomer
ate pores, unavoidably leads to marked grain growth in
sintered material, that has an unfavorable effect on strength
properties of the ceramic obtained; this is particularly impor-
tant for structural ceramics.
Considering the importance of this problem, an experi-
mental study of features of the effect of pore structure
inhomogeniety for a powder compact on sintering kinetics
and the associated recrystallization processes has been the
subject of a number of works. The earliest and most signifi-
cant results for transformation of a pore structure on
sintering compacts of agglomerated submicron ZrO
powder were obtained by the authors of . They established
by mercury porosimetry that in the initial stage of sintering
compacts, containing inter-agglomerate pores, on a back
ground of active compaction of intra-agglomerate areas,
there is not a reduction but a marked increase in inter-ag
glomerate pores. This leads to development of a considerable
number of defects in the form of coarse residual porosity,
that is not removed even with a significant increase in soak
ing . Studies performed by the authors in  on specimens
of powders containing artificially formed agglomerates,
showed presence of them in powder compacts markedly
worsens the mechanical properties of the material obtained.
In spite of the abundance of experimental material, data
for theoretical evaluation of processes that occur during
compact sintering containing highly coordinated pores are
extremely sparse. Work in  should be noted within which
this analysis was carried out from a thermodynamic position.
It was demonstrated in this work that powder particles may
form configurations, when their further compaction (pore re
moval) becomes unsuitable from an energy point of view.
Refractories and Industrial Ceramics Vol. 51, No. 2, 2010
1083-4877/10/5102-0083 © 2010 Springer Science+Business Media, Inc.
A. A. Baikov Institute of Metallurgy and Materials Science, Mos