130 CHAPTER 5 SYNTHESIS AND CHARATERIZATION OF YTTRIA STABILIZED ZIRCONIA MINISPHERES 5.1 INTRODUCTION Yttria stabilized zirconia have been investigated for many years because of several advantages such as low processing temperatures, homogeneity, crack-free coating, low cost, high strength, high toughness, chemical stability, high melting temperature, ionic, electrical and optical properties in advanced ceramics. This material is used extensively for precision engineering applications. Among the various monolithic ceramics, yttria stabilized tetragonal zirconia polycrystalline ceramics (Y-TZP) have been regarded as a potential structural material. The unique combination of high strength (700–1200MPa), fracture toughness (2–10MPam 1/2 ) and chemical inertness makes them indispensable for many structural applications especially as grinding media (Ruiz and Readey 1996). They are also well known for its wear resistant property which could be best utilized for use as a grinding media. This chapter describes the synthesis of yttria stabilized zirconia minispheres by the sol-gel drop generation route and their characterization studies reveal the physical, structural and mechanical properties.
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130
CHAPTER 5
SYNTHESIS AND CHARATERIZATION OF
YTTRIA STABILIZED ZIRCONIA MINISPHERES
5.1 INTRODUCTION
Yttria stabilized zirconia have been investigated for many years
because of several advantages such as low processing temperatures,
homogeneity, crack-free coating, low cost, high strength, high toughness,
chemical stability, high melting temperature, ionic, electrical and optical
properties in advanced ceramics. This material is used extensively for
precision engineering applications. Among the various monolithic ceramics,
yttria stabilized tetragonal zirconia polycrystalline ceramics (Y-TZP) have
been regarded as a potential structural material. The unique combination of
high strength (700–1200MPa), fracture toughness (2–10MPam1/2) and
chemical inertness makes them indispensable for many structural applications
especially as grinding media (Ruiz and Readey 1996). They are also well
known for its wear resistant property which could be best utilized for use as a
grinding media.
This chapter describes the synthesis of yttria stabilized zirconia
minispheres by the sol-gel drop generation route and their characterization
studies reveal the physical, structural and mechanical properties.
131
5.1.1 Phase Relations of ZrO2 -Y2O3 System
The phase diagram proposed by Scott (1975) is shown in Figure
5.1. Ruhle et al (1984) has tried to determine more accurately the position of
t/t+c boundary. The major feature in this diagram is that approximately 2.5
mol% Y2O3 can be taken into solid solution, which is in conjunction with the
low eutectoid temperature, allows a fully tetragonal ceramic to be obtained
(Tetragonal Zirconia Polycrystals, TZP), provided with small grain size. A
large t+c field which permits the formation of a partially stabilized zirconia
(PSZ). Sintering is done at high temperatures, upto 1700oC, to retain
sufficient tetragonal in the solution for the generation of fine, metastable
tetragonal particles. The formation of tetragonal phase with respect to yttria
content differs significantly with phase diagrams presented by Masaki and
Kobayashi (1998) where the diagram shows the occurrence of only tetragonal
phase for 5 mol% of Y2O3 in the temperature 1200 to 1700oC.
5.1.2 Previous Reports on Yttria Stabilized Zirconia
A vast amount of research was conducted in the last few decades to
improve the toughness of ceramics. Hannink et al (2000) proposed a
phenomenon popularly known as ‘Transformation Toughening’ where the
high toughness of yttria tetragonal zirconia polycrystals (Y-TZP) ceramics
was attributed to the stress induced transformation of tetragonal (t-ZrO2)
phase to the monoclinic (m-ZrO2) phase in the stress field of propagating
cracks.
The Y2O3 content in Y-TZP has been assigned to be above 3 mol%
to obtain good stability of t-ZrO2. Tekeli and Erdogan (2002) examined the
effects of grain size on super plastic deformation and cavity formation in
3 mol% yttria-stabilized zirconia polycrystalline. Lange (1982) observed that
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the tetragonal structure in sintered body was strongly related to the
mechanical property because the enhanced toughness and strength which
were attributed to the stress-induced phase transition of tetragonal phase.
Tetragonal zirconia polycrystals containing 2–4 mol% yttria (Y-TZP) was
reported to be superplastic at temperatures above 1573 K.
Figure 5.1 Equilibrium phase diagram of ZrO2-Y2O3 system (Scott 1975)
Gupta et al (1978) also identified that the Y-TZP produced with a
submicrometer grain size was initially motivated by the pursuit of
transformation toughening. Ruiz (1996) also concluded that the small grain
structure is required to produce a high strength zirconia system that was
obtained by the addition of yttria with 5 mol%, which also set the crystal
structure in tetragonal phase.
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Gupta et al (1977) reported the fabrication of a dense, fine grained,
poly-crystalline zirconia ceramic containing up to 98% of the metastable
tetragonal phase, small addition(<5mol% yttria) of Y2O3 were used to retain
the metastable phase.
Recently, Patil et al (2009) synthesized nanocrystalline 8 mol%
yttria stabilized zirconia (YSZ) powder by the oleate complex route. Abhijit
Ghosh and Suri (2007) synthesized fully stabilized zirconia containing
8 mol% yttria fully stabilized zirconia (8Y-FSZ) in nanocrystalline form by
the coprecipitation method and concluded that the hardness and toughness
values were dependent on microstructure in low-temperature-sintered samples
with sintering density of more than 95% at a temperature as low as 1150oC by
following a conventional sintering schedule. Yueh-Hsun Lee (2005) also
synthesized 8 mol % yttria-stabilized zirconia (8YSZ) nanocrystallites at a
relatively low temperature using ZrOCl2. 8H2O and Y(NO3)3.6H2O as starting
materials in an ethanol–water solution by sol–gel process.
Densification of nanocrystalline yttria stabilized zirconia (YSZ)
powder with 8 mol% Y2O3, prepared by a glycine/nitrate smoldering
combustion method was investigated by Dahl et al (2007) using spark plasma
sintering, hot pressing and conventional sintering. In which the spark plasma
sintering technique was identified as superior by giving dense materials (96%)
with uniform morphology at lower temperatures and shorter sintering time.
Roebben et al (2003) investigated the Stiffness and internal friction of yttria
stabilized tetragonal zirconia ceramics with varying yttria content (2–3 mol%)
measured between room temperature and 1000 K with the use of the impulse
excitation technique (IET).
Allen Kimel and Adair (2002) studied the surface chemistry of
Y-TZP in aqueous suspension to promote dispersion and permit aqueous
processing of Y-TZP powders. Thome et al (2004) observations showed that
134
YSZ surfaces after annealing present a much more stable and uniformly
distributed step structure than other ceramic oxides, such as Al2O3.
Pandofelli (1991) observed that the stabilization of tetragonal
zirconia was attributed to the structural similarity of larger yttrium ion radius
compared with the zirconium ion radius and based on the formation of oxygen
vacancies resulting from presence of these trivalent cations. The observation
of metastable tetragonal ZrO2 phase below the m-t transition temperature was
reported by many works. It was shown that the stabilization of t-ZrO2 at low
temperatures was governed by several factors such as the crystallite size
effect, the presence of stabilizers, the presence of impurities, the structural
similarities between the tetragonal phase and the amorphous phase of
precursor.
Xu et al (2003) reported that the crystalline structures and catalytic
properties of yttria stabilized zirconia were generally dependent on synthesis
and thermal treatment. The t-phase was obtained at room temperature when
the crystallite size was very small in the nanosize range. Vasylkiv and Sakka
(2001) described a nonisothermal process for obtaining nanosized yttria
stabilized zirconia with the narrowest primary crystallite size distribution and
secondary aggregates. Kimel and Adair (2002) showed that t-m phase
transformation decreased with increasing yttrium content.
Fangj et al (1997) studied the preparation reaction between
zirconium oxy nitrite and oxalic acid to form zirconium oxalate in nanosized
microemulsion domains. Two synthesis routes, namely, a single-
microemulsion processing route and a double-microemulsion processing
route, were studied and compared. Nanocrystalline (1520 nm) 3 mol % yttria
stabilized zirconia (3YSZ) powder was synthesized via sol-gel technique by
Satyajit Shukla et al (2003). In this investigation, mixed alkoxide and
135
nonalkoxide precursors were used. Interestingly, it was observed that
nanocrystalline 3YSZ powder exhibit very low activation energy for the grain
growth, relative to the bulk counterpart.
Observations on yttria-stabilized zirconia by atomic force
microscopy (AFM) in contact mode were reported for the first time by Deville
and Chevalier (2003). Duclos et al 2002 had a direct analysis of the changes
in surface topography resulting from deformation of zirconia specimens using
AFM which confirmed the main role of grain-boundary sliding during creep
of these materials. The use of atomic force microscopy reported here allowed
the observation of the first stages of martensite relief growth and new
martensitic features.
Baklanova et al (2007) studied the peculiarities of the yttria-
stabilized zirconia interfacial coatings on NicalonTM fiber and phase
transformations within coating layer by Raman spectroscopy. The
microwave-laser hybrid sintering process was implemented by
Ramesh Peelamedu (2004) for the preparation of yttria stabilized zirconia.
Using this process, rapid sintering of 3Y-TZP pellets was achieved in a few
minutes. Microstructural investigations revealed that the microwave-laser
hybrid sintered pellets of 3Y-TZP had nanograins averaging about 20 nm.
Asuncin Fermnndez et al (2002) developed an advanced process on a
laboratory scale for the fabrication of transmutation fuels and targets by
partially-yttria-stabilized zirconia pellets using sol-gel method.
5.2 EXPERIMENTAL PROCEDURE
5.2.1 Preparation of Yttria Stabilized Zirconia Minispheres
Solutions of zirconyl chloride and yttrium nitrate were mixed
together with 1 M concentration. An appropriate amount of oxalic acid (1M)
was slowly added with continuously stirred mixed solutions of metal cations
136
of yttrium and zirconium at room temperature. The yttria doped zirconyl
oxalate (YZO) gel was prepared in such a way that the final product of the
sintered minispheres contains 2, 5 and 8 mol % of yttria stabilized zirconia
(Y-ZrO2) minispheres. The stabilizer was added to the starting material with
ratio of (ZrO)2+ to (Y)3+ equal to 1M. A transparent sol and gel was observed
when the addition of oxalic acid was sufficient to form the zirconyl oxalate
gel. The dopants were uniformly distributed on the pore surface of the
zirconyl oxalate gel structure. During sintering, dopant ions were substituted
for zirconium ions in the crystal structure which favours the formation of the
stabilized zirconia as suggested by Tohge at al (1984). The preparation
procedure was identical to that of CZO and MZO gels explained in chapter 4
and 6. However, the time taken for the formation of clear CZO and YZO sol
was lower than that of MZO and undoped zirconium oxalate (ZO) sol.
Moreover, the nature of the transparency of sol and gel was very
much less compared with CZO and MZO gels. The characteristics of the
formation of YZO sol and gel were similar to those observed for pure
zirconium oxalate system as given in chapter 3. The transparency of the sol
and gel were found to be slightly opaque even though the yttrium ions were
distributed satisfactorily in the zirconium oxy-chloride aqueous solution.
At a suitable viscosity, the mixed gel was added drop wise to a
gelation container for the formation of uniform minispheres. The spheres
were then dried at room temperature for 24 hours. The green bodies were
sintered at 300, 500, 700, 900, 1100, 1300 and 1500C for soaking time of
5 hours with a heating rate of 5C/min. Figure 5.2 shows the photograph of
dried and sintered yttria stabilized zirconia minispheres.
137
5.3 RESULTS AND DISCUSSION
5.3.1 Thermal Analysis
Thermo Gravimetric Analysis (TGA) shows (Figure 5.3) three
major stages of weight losses for 5 mol% yttria stabilized zirconia (5Y-ZrO2)
dried minispheres. The first stage weight loss of around 11.54% is carried out
up to 150°C correspond to the loss due to residual ammonia and dehydration
of water. The second stage of weight loss of around 6.88% is observed
between 150°C to 230°C correspond to the release of nitrate. Third weight
loss of 12.77% is due to the decomposition of oxalate with simultaneous
binder removal process and elimination of CO and CO2 molecules in the
temperature range of 260°C to 428°C. The liberation of chlorine may be at
around 512°C. The residue weight of 59.13% is observed for the spheres
sintered at 1500oC. The porosity details are estimated from the shrinkage data
as explained in chapter 2 by assuming that there is no further shrinkage above
1500oC. Variations in percentage of weight loss, shrinkage and porosity with
gradual increase in temperature are studied in detail (Table 5.1). Figure 5.4
shows the effect of sintering temperature on percentage of variation of
porosity, weight loss and shrinkage for 5Y-ZrO2 minispheres.
From DTA studies, an exothermic and three distinct endothermic
peaks have been observed (Figure 5.3). The endothermic peak around 64oC is
due to the dehydration of water and residual ammonia as observed in the TGA
curve. The second endothermic peak around 216oC is due to the
decomposition of nitrate. The endothermic peak around 288oC is attributed to
the decomposition of oxalate. The liberation of chlorides has not been
observed in the DTA curve which may be due to the smooth release of the
same. The exothermic peak around 472oC is due to the crystallization of
zirconia in tetragonal phase, which is attributed by the phase change from
amorphous zirconia to a metastable tetragonal phase (Mercera 1990). Thermal
138
analysis data for 5Y-ZrO2 minispheres are furnished in Table 5.2. During
sintering the dopant ions are substituted for zirconium ions in the crystal
structure which favours the formation of tetragonal phase (t phase) and
subsequently facilitates transformation toughening. Gradual elevation in the
DTA curve beyond 900oC indicates the possible tetragonal to monoclinic
phase transformation.
Figure 5.2 Photograph of 5 mol% magnesia stabilized zirconia (5Y-ZrO2) minispheres a) dried at 40oC b) sintered at 1500oC
Table 5.1 Variations in percentage of weight loss, linear shrinkage and estimated porosity as a function of sintering temperature for 5Y-ZrO2 minispheres
Temperature (oC)
Shrinkage (%)
Weight loss (%)
Porosity (%)
300 500 700 900
1100 1300 1500
29.79 38.69 44.73 47.99 49.01 51.33 51.79
29.87 34.32 37.34 38.97 39.48 40.64 40.87
67.62 51.35 33.63 20.36 15.48 2.81 ~0.0
a b
mm
139
0 200 400 600 800 1000
60
70
80
90
100
TGA
Temperature (oC)
wei
ght (
%)
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
DTA
65oC
218oC
286oC
Exo
Endo
472oC
Tem
pera
ture
Diff
eren
ce (0 C/
mg)
Figure 5.3 TGA / DTA curves for 5Y-ZrO2 minispheres dried at 40oC
Table 5.2 Thermal analysis data for 5Y-ZrO2 dried minispheres
Thermal change % wt. loss observed at the
end of each stage
Temperature range (oC)
Type of reaction
Release of physisorbed water 11.54 30 to 150 Endothermic
Nitrate decomposition 06.88 150 to 230 Endothermic Binder burnout, Organic decomposition and elimination of CO