ON STRUCTURAL FEATURES RESPONSIBLE
FOR THE MECHANICAL PROPERTIES OF CERAMICS
FROM ZIRCONIA PARTIALLY STABILIZED WITH Y
G. Ya. Akimov
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 15 – 17, December, 2000.
It is shown that Y-PSZ ceramics mainly fracture by an intercrystalline mechanism and the grain size fluctuates
from 0.2 to 0.8 mm, whereas Mg-PSZ ceramics mainly fracture by a transcrystalline mechanism and the grain
size fluctuates from 30 to 80 mm. The high level of mechanical properties is provided by compressive stresses
that appear in the transformation of the b¢-phase into the a-phase. In the Y-PSZ ceramics the transformation
concerns individual grains, and in the Mg-PSZ ceramics lenticular segregations of the b¢-phase are positioned
in the g-phase. The transformation is induced by tensile stresses of the elastic field of a crack, i.e., can be clas
sified as transformation toughening.
Ceramics from partially stabilized zirconia (PSZ) are un-
doubtedly one of the greatest scientific discoveries in the
field of materials science in the second half of the twentieth
century. Soon after the publication of , which described
the creation of a material called ceramic steel, hundreds of
works were devoted to the phenomenon of transformation
toughening, which determines the high level of mechanical
properties of PSZ ceramics.
It was established that the high-temperature b-phase
(tetragonal symmetry) can be preserved at room temperature
in the form of a metastable b¢-phase owing to the introduc
tion of oxides of virtually any rare earth element, CaO, or
MgO into ZrO
. However, the highest level of mechanical
properties was obtained only when the substitution ions in
lattice were Y
. By the data of various
authors and producers, the ultimate bending strength s
three-point flexure tests of ceramics from Mg- and Y-PSZ
can attain 1700 and 900 MPa, and the crack resistance K
can attain 19 and 17 MPa × m
, respectively [2, 3]. The high
parameters of the properties are provided by the martensitic
b¢ ® a transformation caused by the elastic field of the gen
erated and developing cracks (a is a monoclinic phase stable
under normal conditions). The transformation occurs with
about 5% volume expansion of grains or inclusions. This
growth of the volume creates compressive stresses that ham
per crack propagation. This effect is known as stress-induced
transformation (Fig. 1) or transformation toughening.
It should be noted that the high values of s
Mg- and Y-PSZ ceramics are provided by quite different
structures but the same physical process, i.e., a stress-in-
duced b¢ ® a transformation.
We investigated the structural differences of Mg- and
Y-PSZ ceramics and the processes determining their physi-
Mg- and Y-PSZ powders were fabricated by coprecipi
tation from chloride solutions. The obtained powders con
tained 9.1 mol.% MgO and 3 mol.% Y
. After a special
Refractories and Industrial Ceramics Vol. 41, Nos. 11 – 12, 2000
1083-4877/00/1112-0427$25.00 © 2001 Plenum Publishing Corporation
A. A. Galkin Physicotechnical Institute of the National Academy
of Sciences of Ukraine, Donetsk, Ukraine.
Fig. 1. Phase transformation of grains lying in the b¢-phase into
a-phase under the action of the stress of the elastic field of a crack.
The grains in the white background are positioned in the b¢-phase;
the hatched grains are positioned in the a-phase. The line denotes
the region in which the intensity of the stress field with radius r
sufficient for inducing the phase transformation.