The phase composition and twin structure of PSZ crystals after heat treatment at 1600 ˚C have been studied in relation to the content of the stabilizing impurity (Y2O3) by X-ray diffraction and transmission electron microscopy. The measurement of both hardness and fracture toughness by microindentation has been carried out. Studies have shown that crystals of PSZ obtained by directional solidification of the melt consist of two tetragonal phases (t and t’), with varying degrees of tetragonality. The yttrium-enriched phase t’ is ‘‘untransformable’’ in contrast to the t phase, with a lower content of yttrium, which, under the influence of mechanical stress, undergoes a martensitic transition to the monoclinic form. Increasing the stabilizing impurity concentration leads to an increase in the volume fraction of the ‘‘untransformable’’ phase. Increasing the concentration of Y2O3 also affects the form and dispersion of the twin domains. In this work it is shown that the quantity of hardening (fracture toughness) is proportional to the content of the transformable t phase. This work was supported by grants from the company "OPTEC"
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3540

Borik, M.A.

Bublik, V.T.

Kulebyakin, A.V.

Lomonova, E.E.

Milovich, F.O.

Myzina, V.A.

Tabachkova, N.Y.

English

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 4, 04NABM03 (6pp) (2013) 
 
 
2304-1862/2013/2(4) 04NABM03 (6) 04NABM03-1  2013 Sumy State University 
Study of the Structure and Mechanical Properties of PSZ (Partially Stabilized Zirconia) after 
Heat Treatment at 1600 C 
  
F.O. Milovich1,*, N.Y. Tabachkova1, V.T. Bublik1, E.E. Lomonova2,  
M.A. Borik2, A.V. Kulebyakin2, V.A. Myzina2. 
 
1 National University of Science and Technology "MISIS", 117936 Moscow, Leninsky pr., 4 
2 Prokhorov General Physics Institute, Russian Academy of Sciences, 119991, Moscow, Vavilov Str., 38 
 
(Received 17 May 2013; published online 30 August 2013) 
 
The phase composition and twin structure of PSZ crystals after heat treatment at 1600 ˚C have been 
studied in relation to the content of the stabilizing impurity (Y2O3) by X-ray diffraction and transmission 
electron microscopy. The measurement of both hardness and fracture toughness by microindentation has 
been carried out. Studies have shown that crystals of PSZ obtained by directional solidification of the melt 
consist of two tetragonal phases (t and t’), with varying degrees of tetragonality. The yttrium-enriched 
phase t’ is ‘‘untransformable’’ in contrast to the t phase, with a lower content of yttrium, which, under the 
influence of mechanical stress, undergoes a martensitic transition to the monoclinic form. Increasing the 
stabilizing impurity concentration leads to an increase in the volume fraction of the ‘‘untransformable’’ 
phase. Increasing the concentration of Y2O3 also affects the form and dispersion of the twin domains. In 
this work it is shown that the quantity of hardening (fracture toughness) is proportional to the content of 
the transformable t phase. This work was supported by grants from the company "OPTEC" 
 
Keywords: Partially Stabilized Zirconia, Transmission Electron Microscopy, X-ray Diffraction, Materials 
of High Strength and Durability, Skull Melting Technique in a Cold Crucible. 
 
 PACS numbers: 61.10.Nz, 62.20.-x, 68.37.Lp 
______________ 
* filippmilovich@mail.ru 
 
 
1. INTRODUCTION 
 
Zirconia based materials have a variety of unique 
physicochemical, electrical and mechanical properties 
including high strength, hardness, impact toughness, 
wear resistance, low coefficient of friction, high melting 
point, chemical inertness, low heat conductivity and 
biocompatibility. These properties account for the wide 
range of applications, from wear resistant ceramic 
bearings to medical and surgical instruments [1-2]. 
Such materials can be used to produce components for 
devices functioning under extreme conditions: at high 
mechanical loadings, in aggressive media, at elevated 
temperatures, without lubrication, etc. This material, 
with its high fracture toughness due to the inherent 
phase transition similar to martensitic transformations 
in steels, has received the name ‘ceramic steel’ [3]. 
Such properties of PSZ crystals are related to fea-
tures of their phase composition, structure and pres-
ence of nano and micro twin structure [4]. Changing 
the growth conditions and process annealing PSZ crys-
tals affects the phase composition and structure, and 
thus the mechanical characteristics of the material. 
The heat treatment is carried out in the tempera-
ture range 1400-1600 C to investigate the structural 
stability of the solid solutions, which depends on the 
nature and concentration of the stabilizing oxide, 
method for producing the material. The outcome will 
depend on the temperature and duration of destabiliz-
ing annealing. In work [5] it is shown that the high 
mechanical properties of PSZ crystals remain at the 
annealing temperature 1400 C, with their smaller deg-
radation as compared to ceramics. Inasmuch as this 
annealing correspond operating temperatures of the 
material. Therefore the study of the influence of ther-
mal annealing on the structural stability of the solid 
solutions at these temperatures is extremely important 
for practical applications of the material. 
 
2. EXPERIMENTAL 
 
Yttrium oxide stabilized (concentration range 2.8 –
4.0 mol. %) PSZ crystals were grown using directional 
melt crystallization in a cold crucible at a 10 mm/h crys-
tallization rate. The phase composition and structure of 
the PSZ were studied using X-ray diffraction on a 
Bruker D8 instrument and transmission electron mi-
croscopy (TEM) on a JEM-2100 microscope at a 200 kV 
accelerating voltage. The specimens for the electron mi-
croscopy study were prepared as follows. Wafers were 
cut from the crystals so that the plane of the wafer was 
oriented either orthogonally to the <100> crystal axes. 
The specimens were ground to a 200 μm thickness. 
3 mm diameter discs were cut out of the source material 
by ultrasonic cutting, followed by the dimpling the center 
of the discs, and finally the specimens were thinned by 
ion beam etching. Hardness and fracture toughness were 
measured by microhardness indentation. Then, the 
plates ZrO2, stabilized from 2.8 to 4.0 mol. % Y2O3, an-
nealed in air at 1600 C. Conditions of heat treatment are 
– 6 hours heating, 14 hours ageing, 6 hours cooling. 
 
3. RESULTS AND DISCUSSION  
 
3.1 Research of the structure PSZ crystals by 
transmission electron microscopy 
 
All the specimens had a well developed twin domain 
structure. Fig. 1 (a, b, c) shows a typical example of the 
 
F.O. MILOVICH, N.Y. TABACHKOVA, V.T. BUBLIK, ET AL. PROC. NAP 2, 04NABM03 (2013) 
 
 
04NABM03-2 
twined structure for samples of compositions: 2.8, 3.0 
and 3.2 mol. % Y2O3. It is seen that the dispersion and 
morphology of the structure does not change signifi-
cantly with an increase in the concentration of stabiliz-
ing impurities to 3.2 mol. % Y2O3. TEM study of the 
PSZ crystals showed that, according to the electron 
diffraction patterns, all the specimens are single crys-
tals. Fig. 1a (bottom right) shows the diffraction pat-
tern of the samples cut perpendicular to the axis of the 
crystal <100>. 
 
 
 
Fig. 1 – TEM images of the twin structure typical for 
sample with the concentration of the stabilizing impurity (a) 
- 2.8 mol. % Y2O3, (b) - 3.0 mol. % Y2O3, (a) - 3.2 mol. % 
Y2O3 
 
A more detailed high resolution TEM study showed 
that large domains consist of smaller twinned domains. 
Fig. 2 shows a TEM image of 10–15 nm sized twins. 
Also, high resolution TEM identified coherent mono-
clinic phase inclusions in the structure (Fig. 4). After 
annealing at 1600 C in samples with Y2O3 to 
3.2 mol. % formed regions of the monoclinic phase in 
the form of a domain, width  20 nm, which is coherent 
with the conjugate phase of the tetragonal domains 
(Fig. 3).  
 
 
 
Fig. 2 – High-resolution image of twin domains in the ZrO2 
2.8 mol.% Y2O3 sample. 
 
 
 
Fig. 3  – High-resolution TEM image of a coherent 
boundary between tetragonal (t) and monoclinic (m) phase of 
the sample after annealing at 1600 C 
 
Unlike to the crystals after growth, where there was 
a small inclusion monoclinic phase dimensions of about 
10 nm with indistinct boundaries (Fig. 4). The occur-
rence of coherent monoclinic phase inclusions in these 
specimens can be caused by the low concentration of 
the stabilizing impurity and its local fluctuations dur-
ing crystallization. These inclusions may serve as nu-
clei for the formation of the monoclinic phase regions 
during subsequent heat treatment of crystals at a tem-
perature corresponding to the two-phase region in the 
phase diagram. The morphology and relative position of 
twin domains in samples of 3.7 - 4.0 mol. % Y2O3, was 
 STUDY OF THE STRUCTURE AND MECHANICAL PROPERTIES … PROC. NAP 1, 04NABM03 (2013) 
 
 
04NABM03-3 
different from the crystals with lower concentrations of 
2.8 - 3.2 mol. % Y2O3. The twin structure becomes more 
homogeneous and dispersed (Fig. 5). At higher stabiliz-
ing impurity concentrations (3.7–4 mol. % Y2O3), no 
twinning hierarchy was observed, atomic plane traces 
inside twin domains were not broken, and minimum 
twin domain sizes could be identified in diffraction con-
trast images. 
 
 
 
Fig. 4 – High-resolution image of inclusion monoclinic 
phase in the ZrO2 2.8 mol.% Y2O3 sample after growth 
 
This suggests that twinning occurs simultaneously 
and is localized within small volumes. In samples with 
a stabilizing concentration of impurities 2.8 to 
3.2 mol. % Y2O3 twinning first goes to the larger do-
mains, which also twinning. 
 
 
 
Fig. 5 – TEM images of the twin structure typical for 
sample ZrO2 4.0 mol. % Y2O3 
 
3.2 Phase analysis 
 
In accordance with the ZrO2–Y2O3 phase diagram, 
PSZ crystals growing during melt synthesis initially 
have a cubic structure, and a phase transformations oc-
cur during cooling in the solid state. As the temperature 
decreases, the cubic phase becomes unstable and trans-
forms to a tetragonal modification. The slight atomic 
shifts, mainly affecting the oxygen ions, distort the 
symmetry of the initial structure. The oxygen ions shift 
relative to the perfect fluorite lattice positions (1/4, 1/4, 
1/4). Generally, the tetragonal phase lattice is slightly 
elongated along the c axis as compared to the cubic 
phase lattice. X-ray diffraction from the PSZ phase 
showed that the material has two tetragonal phases of 
the P42/mnc space symmetry group. The simultaneous 
occurrence of the (006) and (600) reflections in the dif-
fraction pattern is accounted for by twinning as will be 
shown below in the discussion of the transmission elec-
tron microscopy results. Analysis of the diffraction pro-
file was hindered by the superimposition of the split re-
flections due to the presence of the tetragonal lattice and 
Rα-doublet splitting; therefore we conducted an addi-
tional experiment with Rβ radiation which allowed us to 
definitely confirm that the splitting is due to the pres-
ence of the two tetragonal phases (Fig. 5a). 
 
 
 
Fig. 5a – X-ray diffraction patterns of the YSZ 2.8 mol. % 
Y2O3, 5b – X-ray diffraction patterns of the YSZ 4.0 mol.% 
Y2O3 
 
Phase constituent study of zirconia doped to different 
Y2O3 concentrations (2.8–4.0 mol. %) showed that all the 
specimens, regardless of the stabilizing impurity con-
tent, have two tetragonal zirconia modification phases 
with varying degrees of tetragonality. Both phases have 
a slightly distorted fluorite structure and differ in the 
ratio of their lattice parameters. For one tetragonal 
phase – t, the c/a ratio was 1.014–1.015, and for the oth-
er tetragonal phase – t’, the c/a ratio was close to 1, i.e. 
1.004–1.005 [6]. The yttrium rich t’ phase is not trans-
formable unlike the lower yttrium content t phase which 
undergoes a martensitic transformation to the monoclin-
ic state under mechanical stress [7]: this transformation 
may suppress the sources of stress concentration and 
increase the fracture toughness of the material. There is 
data [8] on a wide range of tetragonal phase modifica-
tions depending on the degree of tetragonality. The c/a 
 
F.O. MILOVICH, N.Y. TABACHKOVA, V.T. BUBLIK, ET AL. PROC. NAP 2, 04NABM03 (2013) 
 
 
04NABM03-4 
ratio may vary over a wide range from 1.005 (for the so-
called t’ phase, which does not transform under mechan-
ical stress) to c/a  1.035 for the readily transformable to 
monoclinic phase. For example, if the t’ phase forms dur-
ing the cubic to tetragonal phase transition, it will not 
transform to the phase even under intense grinding of 
the specimen. The phase with the higher c/a ratio will 
transform easier in an elastic stress field. Thus, the spec-
imens contain two tetragonal phases with c/a  1.004–
1.005 and c/a  1.014–1.015 [9]. The intensity of the 
diffraction peaks suggests that the higher tetragonal 
degree phase dominates in the crystals with a 2.8 mol. % 
Y2O3 stabilizing impurity concentration. However, as the 
stabilizing impurity concentration is increased, the vol-
ume fraction of the untransformable t’ phase increases 
gradually, and the volume fraction of the t phase de-
creases (Fig. 5a, b). 
Analysis of broadening of the diffraction peaks has 
shown that after annealing at 1600 C diffraction line 
width is reduced compared to the crystals after growth. 
On the width of the diffraction peaks can affect mi-
crostress and dispersion of coherent domains in do-
main-twinned structure. Comparison of the domain 
size on TEM images before and after annealing 1600 °C 
showed that a significant change in the dispersion did 
not happen. It can be assumed that the width of the 
diffraction peaks are not changed due to increased dis-
persion of the domain, but due to the removal of mi-
crostrain in the PSZ crystals after annealing. The mi-
crostresses may be partially removed by the formation 
of a more ordered domain structure. 
 
3.3 Mechanical properties Conclusion 
 
Microhardness measurements were carried out at a 
load of 500 g. The table shows that the microhardness not 
dependent on the concentration Y2O3. Also, a significant 
change was not found when compared microhardness PSZ 
crystals before and after annealing. To obtain the depend-
ence of fracture toughness from the content of Y2O3, chose 
the load at which a crack formed on the crystals of all 
compositions. A sample with the lowest impurity concen-
tration of the stabilizing 2.8 mol. % Y2O3, was the most 
resistant to cracking. This result can be explained by the 
high content in it (2.8 mol. % Y2O3) t tetragonal phase, 
which provides a transformational mechanism of harden-
ing of the material. Compared with the crystal growth 
PSZ after annealing at 1600 C resulted in a slight im-
provement of fracture toughness values for the crystal 
with an impurity concentration of the stabilizing 2.8 
mol. % - 3.2 mol. % Y2O3. This may be due to the relaxa-
tion of microstress in the PSZ crystals after annealing. 
4. CONCLUSION  
 
Studies of the structure, phase composition, micro-
hardness and fracture toughness of PSZ crystals after 
annealing at a temperature of 1600 C were carried 
out. TEM studies have shown that crystals of all com-
positions have complex twinned structure. Upon cool-
ing, the crystal phase transformation with different 
specific volumes (F → t) leads to the formation of elastic 
stress relaxation which occurs by twinning. A more 
thorough study TEM high resolution revealed that ma-
jor domains consist from small twin domains. It can be 
assumed that the process of twinning occurs as long as 
in the sample remain elastic stresses are sufficient for 
twinning. At the same time dislocation are not detected 
in the samples. 
Upon cooling, the crystal transition from the cubic 
single-phase to two-phase region, according to the state 
diagram, goes at a lower temperature with increasing 
concentration of yttrium, which affects on the character 
the twin structure. In samples of all compositions was 
found the presence of two tetragonal phases with dif-
ferent degrees of tetragonal (t and t'). Moreover, the 
volume fraction of «transformed» (t) phase decreases 
with increasing concentration of the stabilizing impuri-
ty. Reduction of t phase affects on the "transformation-
al" hardening when advancing microcrack induces t – 
m martensitic transition which absorbs energy stress 
and blocks the advancing microcrack eventually stop-
ping it. Measurement of the mechanical properties 
shows that the microhardness of crystals does not 
change for different compositions. But a noticeable de-
crease in the fracture toughness with increasing doping 
concentration was discovered. We believe that this is 
due to the decrease in the volume fraction of trans-
formable tetragonal phase. 
 
Table 1 – Dependence of microhardness and fracture tough-
ness PSZ crystals before and after annealing from the Y2O3 
concentration 
 
 
 
 
REFERENCE 
 
1. J. Chevalier, L. Gremillard, D.R. Clarke, A.V. Virkar, J. 
Am. Ceram. Soc. 92, 1901–1920 (2009). 
2. J. Eichler, J. RoË del, U. Eisele, M. Hoffman, J. Am. Ce-
ram. Soc. 90, 2830–2836 (2007). 
3. D.L. Porter, A.H. Heuer, J. Am. Ceram. Soc. 60, 183 (1977). 
4. T. Sakuma. J. Jpn. Inst. Met. 29, 879 (1988). 
5. R.P. Ingel, D. Lewis, B.A. Bender, P.W. Rice. J. Am. Ce-
ram. Soc. 65, 150 (1982). 
6. M.A. Borik, V.T. Bublik, M.A. Vishnyakova, E.E. Lomonova, 
V.A. Myzina, N.Yu. Tabachkova, J. Surf. Investig.-x-ra. 5, 
166–172 (2011). 
7. J. Chevalier, L. Gremillard, A.V. Virkar, D.R. Clarke, J. 
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State. 47, 1978–1981 (2005). 
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D.R.G. Mitchell, J.M. Cairney, C.G. Levi, J. Am. Ceram. 
Soc. 94, 168–177 (2011). 
Compositions Y2O3, % 
2,8 3,0 3,2 3,7 4,0 
The values of microhardness, HV (300 g) 
1410±40 1420±30 1400±20 1420±25 1450±40 
The values fracture toughness for PSZ , МPа∙m-1/2 
(10 kg) 
10±0,6 9±0,6 7±0,6 6±0,6 5±0,6 
The values fracture toughness for PSZ after anneal-
ing, МPа∙m-1/2 (10 kg) 
11±0,6 10±0,6 8±0,6 7±0,6 6±0,6 


Study of the Structure and Mechanical Properties of PSZ (Partially Stabilized Zirconia) after Heat Treatment at 1600 ˚C

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Study of the Structure and Mechanical Properties of PSZ (Partially Stabilized Zirconia) after Heat Treatment at 1600 ˚C

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