STRUCTURAL EVOLUTION IN CERAMICS TECHNOLOGY
A. V. Belyakov
and V. S. Bakunov
Translated from Novye Ogneupory, No. 2, pp. 55 – 62, February, 2006.
Original article submitted September 2, 2005.
The next technological process is preparation of the
molding mix, which involves the mixing of powders of vari
ous dispersities and compositions, introduction of a sacrifi
cial bond, and granulation. When mixed, the particles tend to
lower their surface energy by combining with each other or
with the bond. This behavior prevents a uniform distribution
of constituents and is undesirable. The granulation of highly
dispersed powders requires special conditions to be efficient.
This is achieved through self-organization (a principle under-
lying the design of certain granulators) or through organizing
a process capable of providing the inheritance of previous
structures, for examples, droplets in the spray drying. Granu-
lation of dry-ground highly dispersed powders is used with
the aim of increasing the bulk density or to minimize the ad-
dition of the bond .
Structurally, the granules should be capable of preserving
cumulative properties inherent in powder particles. In parti
cular, the interaction between the particles and the bond (nor
mally accompanied by a decrease in surface energy) should
be such that the cumulative energy could be saved, trans
ferred through subsequent operations, and remain unspent
until the sintering stage. The structure of granulated dense
ceramic should be such as to provide the degradation of
granules only in the final stage of preform molding.
During molding, it is important to provide continuity of
the preform, which is done by subjecting the material to plas
tic deformation; no impairment of structural integrity is to
lerated in this case. For this purpose, sacrificial bonds are
used with the aim of minimizing the friction between powder
particles. The requirements placed on the bonds may appear
controversial: the material must be capable of easy deforma
tion (small internal friction, low apparent viscosity) during
molding; however as soon as shaped, the material should im
prove substantially these properties.
We now consider three main molding techniques in order
of increasing the amount of bond required: semi-dry press
ing, plastic molding, and casting.
In semi-dry pressing, the required amount of bond is
minimum, which, however, necessitates a larger amount of
energy for molding. In a powder treated by pressing, locally
compacted areas arise that strive to dissipate the input me-
chanical energy; with increase in pressure, these areas in-
crease in size to be finally formed into framework structures.
These take up the main load and provide its transfer (dissipa-
tion) to the side walls of the pressing mold and facilitate,
owing to the framework voids, the removal of air from the
preform; for this reason, the framework should be regarded
as part of the dissipative structure. At the same time, the
voids that occur within the framework serve to accumulate
energy, which is spent on the formation of new interfaces. In
this sense, the framework may be regarded as a cumulative
system. The higher the rate at which energy is supplied and
the higher the powder dispersity, the higher the void ratio of
the framework, since the system falls short of time for uni
form densification, and for this reason the energy is accumu
lated, to be spent on the creation of voids.
The overpressed cracks are associated with an elastic af
tereffect that arises because of the forced accumulation of
elastic energy by the air entrapped in the pores and by the
mechanically strained particles and molding die material; as
the load is relieved, the accumulated energy is rapidly re
leased, which is accompanied by crack growth. In this effect,
a factor of primary importance is compressed air; still, with
the pressing carried out in vacuum, overpressed cracks are
achieved likewise, but at much higher pressures. In certain
techniques, the molded preform is removed from the die
tightly compressed between two punches, which makes it
Refractories and Industrial Ceramics Vol. 47, No. 2, 2006
1083-4877/06/4702-0110 © 2006 Springer Science+Business Media, Inc.
Continued. For the previous communication, see this journal,
No. 1 (2006).
D. I. Mendeleev University of Chemical Technology, Moscow,
Russia; Institute for High Temperatures, Russian Academy of Sci
ences, Moscow, Russia.