ISOTHERMAL SINTERING OF INORGANIC POWDERS
A. E. Kravchik
and S. S. Ordan’yan
Translated from Novye Ogneupory, No. 12, pp. 49 – 59, December, 2005.
Original article submitted September 23, 2005.
Results of a study of the isothermal sintering of specimens press-molded from powders of Ni, W, W + 0.5% Ni,
zirconium, and niobium carbides of different dispersity and composition are reported. Rate parameters of the
sintering process are calculated using the Ivensen phenomenological model. The activity of sintering is shown
to be controlled not by the total amount of free energy stored in the powdered material, but rather dependent
on the nanocrystalline structure of particles and processes that occur on the particle surface during sintering.
The sintering of powders molded under pressure into
components is notably a complex physicochemical process.
As predicted by theories of sintering, the principal factor to
control this process is the excess surface energy of the press-
molded specimen (component) [1, 2]. Likewise, the imperfec-
tion of the crystal lattice of powder particles is not a factor to
be overlooked . Viewed thermodynamically, the best sin-
tering is achieved in powders exhibiting a larger amount of
free energy. The free energy of powder particles may be repre-
sented as the sum of the surface (W ) and internal (V ) energies:
F = W + V. (1)
The surface energy of the molded component is deter
W = sS, (2)
where s is the surface tension of the material of the powder
particle and S is the total specific surface of the powder.
The internal energy, a major contributor to which is the
elastic energy of stress fields associated with dislocations
and point defects, can be evaluated by the average monolayer
faulting (degree of monolayer imperfection) Dd/d :
23 4 8
where E is Young’s modulus, n is Poisson’s ratio, and d is the
interplanar spacing in the crystal lattice.
The dislocation density in the material can be evaluated
from the relationship
where G is the shear modulus and b is the Burgers vector.
To single out in pure form the sintering effect due to each
individual component of the free energy is an arduous task at
present. Various interrelated physicochemical processes are
involved in the sintering of a press-molded component under
actual conditions. All the physicochemical processes can be
conventionally classified into major ones that determine
shrinkage and sintering, and minor (auxiliary) processes
which are concurrent with the major processes but are not di
rectly relevant to the sintering proper. The major processes
include chemical reactions on the surface and the interface,
the surface and bulk self-diffusion, and thermally activated
dislocation processes; the auxiliary processes include the re
covery and primary recrystallization.
The process of sintering of an actual molded component
can be conveniently divided into three stages classified in
terms of purely geometrical features. In stage 1, the powder
particles undergo mutual thermally stimulated adhesion. In
this stage, the powder particles retain their structural individ
uality. In stage 2, a porous solid with open porosity is
formed. In stage 3, the sintered solid retains mostly closed
pores, separated from each other. It stands to reason that all
the three stages cannot be definitely demarcated; in certain
cases, they may occur as concurrent events.
The molded component — an actual object in the powder
metallurgy — viewed from a physical standpoint is a “con
taminated” one, since during the sintering it is exposed to a
Refractories and Industrial Ceramics Vol. 46, No. 6, 2005
1083-4877/05/4606-0423 © 2005 Springer Science+Business Media, Inc.
Prikladnaya Khimiya Research Center of Russian Federation,
Federal State Unitary Enterprise, St. Petersburg, Russia; St. Pe
tersburg State Technological Institute (Technical University), St.