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

132

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.

133

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

12.77 260 to 428 Endothermic

Phase formation (or) Crystallization --- 472 Exothermic

140

200 400 600 800 1000 1200 1400 16000

10

20

30

40

50

60

70

%%

Temperature (oC)

Shrinkage Weight loss Porosity

Figure 5.4 Percentage of variation of porosity, weight loss and shrinkage

with sintering temperature for 5Y-ZrO2 minispheres

5.3.2 Effect of Heating Rate and Soaking Period

The weight losses during the decomposition of 5Y-ZrO2 with

heating rates of 5-10oC/h using linear heating method are given in Table 5.3.

High value of weight loss in the water-removal stage is observed for the slow

heating rate (5oC/min). The starting decomposition temperature increases as

the heating rate is increased. As the heating rate increases, the heat flow is

also increased which inturn broadens the decomposition temperature interval.

Decomposition at a heating rate of 5oC/min occurres over a temperature range

of 150oC, whereas decomposition at a heating rate of 10oC/h occurres over a

temperature range of 210oC. As a result, shifting the synthesis process to

high-temperature zone leads to coarser, aggregated powder. The

crystallization temperature increases to higher temperatures as the heating rate

is increased (Table 5.3). The temperature interval of crystallization also

becomes broader. It is observed that the crystallization temperatures are

141

dependent on the surface area of Y-ZrO2. For slow heating rate, the zone of

crystallization is 470-484oC. But for heating rates of 8-10oC/h, the completion

temperature of zirconia synthesis is higher and the time required for oxalate

decomposition is shorter. In such heating regimes, the surface area of the

powder has no time to develop and the temperature becomes reasonable for

the formation of presintered agglomerates of coarser primary particles. The

temperature of crystallization in such heating regimes is 490-530oC. It is also

observed that 5Y-ZrO2 with the finest particles began to decompose earlier

than the coarser powders and complete their decomposition at slightly higher

temperatures.

Table 5.3 Effect of heating rate on the surface area, particle size distribution,

crystallization temperature and weight loss % during water removal for 5Y-ZrO2 minispheres sintered at 1500oC

Heating rate

(ºC/min)

Surface area

(m2/gm)

Weight loss % -water removal

Particle size distribution

(nm)

Crystallization temperature

(ºC)

5 7 8

10

23 19.7 18.2 5.3

11.4 9.3 7.4 6.1

6 8

15 48

470 484 498 560

In order to study the effect of soaking time on the sinterability, two

soaking periods (2 and 5 hours) have been adopted. From the Table 5.4, it is

observed that the sintered density of the slow heating rate samples reaches

around 96% of theoretical density (TD) for a soaking period of 5 hours

whereas the density of the fast fired samples reaches only around 94% of TD

for the same period of soaking period. The same trend has been observed for

the samples with a soaking period of 3 hours also.

142

The slow heating rate facilitates the evolution of residuals before

the pores start to close and ultimately reaches the near TD. The results

observed in the present study agree very well with the results obtained by

Kim and Kim (1992).

Table 5.4 Effect of heating rate and soaking time on the sintered

density of 5Y-ZrO2 minispheres sintered at 1500oC

Heating rate Soaking period (hours)

Density (g/cc)

Theoretical density (%)

Slow(5oC) 3 5

5.76 5.81

94.43 96.56

Fast(10oC) 3 5

5.55 5.59

90.98 93.61

5.3.3 Surface Area Analysis

Specific surface area has been estimated for 5Y-ZrO2 using BET

technique with nitrogen following the procedure elaborated in chapter 4. The

dried gel powders have been sintered at different temperatures for 450, 550,

700, 850oC for 5hrs. Assuming the particles are spherical in shape, the

average crystallite sizes, specific surface area of 5Y-ZrO2 have been

calculated following the procedures given in chapter 2 and 4. It is found that

the surface area of 5Y-ZrO2 decreases with the increase in sintering

temperature which may be due to the increase of the crystallite size

(Figure 5.5). Table 5.5 shows the experimental data obtained for the surface

area, crystallite size and density for various sintering temperatures. It is

observed that the density and crystallite size increase with sintering

temperature whereas the surface area decreases.

143

400 500 600 700 800 90020

25

30

35

40

45

50

55

60

Surfa

ce a

rea

(m2 /g

)

Sintering temperature (oC)

Figure 5.5 Surface area as a function of sintering temperature for 5Y-ZrO2

Table 5.5 Variation of surface area of 5Y-ZrO2 as a function of

temperature

Firing Temperature

(oC)

Specific Surface area

(m2g-1)

Density

(g/cc)

Average Crystallite Size

(nm)

450 550 700 850

56.4 47.2 30.4 23.3

5.17 5.25 5.72 5.85

10 12 17 22

5.3.4 XRD Analysis

X-ray diffraction studies have been carried out to determine the

crystal structure and phase identification of 5Y-ZrO2 minispheres sintered at

300°C to 1500°C for 5 hours in steps of 200oC (Figure 5.6). It is observed that

the synthesized minispheres are amorphous at room temperature and up to

144

300oC, but crystallize into tetragonal crystal structure after sintering at 470°C

and retains the tetragonal phase upto 900°C. The intensity of the tetragonal

diffraction peaks are found to be increasing with sintering temperature. It is

well known that the transformation toughening depends directly on the

t-phase content and therefore materials which contain 100% tetragonal phase

are capable of possessing very high toughness. The role of stabilization of the

t-phase of zirconia by yttrium ion shows that the presence of these ions which

reduces the over crowding of the oxygen around the zirconium ion and hence

relieves the strain energy associated with it.

When a low valency dopant cation, such as Y3+, is introduced into

the ZrO2 lattice, oxygen vacancies are created for the charge balance. This

oxygen vacancy association with Zr4+ cations reduces the effective

coordination number of Zr4+ cations below 7. To maintain its effective

coordination number close to 7, as dictated by the covalent nature of the Zr-O

bond, the ZrO2 lattice assumes a crystal structure, which offers 8-fold (higher

than 7) coordination number (typically tetragonal or cubic structures) and

simultaneously incorporates the generated oxygen vacancies into the lattice as

the nearest neighbors to Zr4+ cations. Hence, the effect is reflected in the

stabilization of the tetragonal phase in Y-ZrO2. Thus, the stabilization of

tetragonal phase in the nanocrystalline 5Y-ZrO2 minispheres within the

sintering temperature range of 400-900oC is a result of doping the ZrO2 lattice

with Y3+ cations. Srinivasan et al (1992) have also experienced the same

transformation as observed in the present study.

Further increase in temperature, tetragonal to monoclinic phase

transformation occurs at 1100oC. The fraction of monoclinic phase (m-ZrO2)

is identified as only 0.29 at 1100oC. The monoclinic phase is dominant for the

spheres sintered at 1500oC. The decrease in the t-phase content on increase of

sintering temperature may indicate the sluggishness of the diffusion of yttria

145

into zirconia. From the earlier investigations (Ramadass et al 1983), it is

evident that the partially stabilized zirconia with a mixture of m and t phases

was observed even for composition upto 11 mol% of yttria addition for the

sintering temperature of 1500oC and the existence of the t-phase in yttria

stabilized sphere is metastable and might be due to compressive stress

developed during sintering. When the tetragonal material reverts back to the

monoclinic phase, the transformation results in micro cracking which

weakens the sphere’s strength. The microcrack is evident in the SEM

micrograph of 5Y-ZrO2 minispheres sintered at 1500°C (Figure 5.7).

Figure 5.8 shows the SEM micrograph of 5Y-ZrO2 minispheres sintered at

1500oC and 900oC. It is observed that the minisphere sintered at 900oC is free

from cracks. The average crystallite size for 5Y-ZrO2 is determined by the

major diffraction line of XRD patterns using Scherer’s and Warren’s

equation. The decrease in width of the dominant tetragonal spectral lines in

the XRD pattern indicates the increase in the crystallite size of zirconia

minispheres. It is observed that the crystallite size increases from 10 to 54 nm

when the sintering temperature is increased from 500oC to 1500oC

(Table 5.6).

Figure 5.9 shows the phase transformations of yttria stabilized

zirconia minispheres with various concentration of yttria sintered at 1500oC

for 5 hrs. It is observed that the percentage of t-phase increases with yttria

content for a given sintering temperature. It is found that the zirconia

minispheres stabilized with 2 mol % of yttria possesses with fully monoclinic

phase whereas 5 and 8 mol % of stabilized zirconia minispheres are partially

stabilized with monoclinic and tetragonal phases. There is no much variation

in the content of monoclinic phase for both 5 and 8 mol % of stabilized

zirconia minispheres. The reason for the monoclinic phase domination may be

because of the occurrence of t to m phase transformation when the

minispheres are grounded. In the sintered minispheres, the compressive

146

stresses may hinder the free conversion of the t phase to the m phase on

cooling. Thus, the presence of the t phase in the sintered minispheres seems

to be stress induced.

It is observed that the 5Y-ZrO2 do not transform into monoclinic

phase completely in the higher temperature regions. Such saturation behavior

has been explained in terms of grain size, anisotropic coefficient of expansion

and compressive stress due to the transformation (Lilley 1990). Isa Yamashita

and koji Tsukuma (2005) suggested using Rietveld analysis that the

nontransformable tetragonal region is attributed to the formation of high

yttrium region but in contrast, the low yttrium region can be easily

transformed into monoclinic phase. Thus, the transformable fraction during

sintering is governed by the amount of low yttrium region.

Table 5.6 Variation of density and crystallite size with sintering

temperature for 5Y-ZrO2 minispheres

Sintering temperature

(oC)

Density of sintered spheres

(gm/cm-3)

Percentage of theoretical

density (%)

Average crystallite size

(nm)

Average grain size

(μm)

500 5.01 82.13 09.87 -

700 5.72 93.77 17.32 -

900 5.89 96.56 24.17 0.85

1100 5.85 95.90 30.19 1.26

1300 5.82 95.41 49.62 1.72

1500 5.81 95.25 54.32 2.61

147

20 30 40 50 60 700

306090 Dried sphere at 40oC

2 theta (deg)

0153045 300oC

0153045

t ttt

700oC

500oC

0306090

t

t

tt

t

04080

120

t

t

t

t t900oC

Inte

nsity

(a.u

)

04080

m t t

t

1100oC

03060 m

m

t

mm

mtt1300oC

0

4080

120t mt

t

t1500oC

Figure 5.6 XRD patterns for 5Y-ZrO2 minispheres with various

sintering temperatures. (t – Tetragonal, m – Monoclinic)

148

Figure 5.7 SEM micrograph of 5Y-ZrO2 minispheres sintered at 1500oC

Figure 5.8 SEM micrograph of zirconia minispheres

a) sintered at 1500oC b) sintered at 900oC

a) b)

149

20 30 40 50 60 70

0

20

40

60

80

100

m m

m 2Y-ZrO2

2 theta (deg)

020406080

100120140 m

m

m

tmt

t

t5Y-ZrO

2

Inte

nsity

(a.u

)

0

50

100

150

200

tm mt

t

t

m

m

8Y-ZrO2

Figure 5.9 XRD patterns for yttria stabilized zirconia minispheres with

various yttria concentrations sintered at 1500oC for 5 hours

150

5.3.5 FTIR Analysis

Figure 5.10 shows the FTIR spectrum of yttria stabilized zirconia

minispheres. The vibrational frequencies of all possible functional groups for

the minispheres dried at 40oC and sintered at 1500oC in the region 4000-

400 cm-1 are assigned. The peaks in the range 795 cm-1 and 517 cm-1 are

prominent which confirms the Zr-O vibration. The appearance of the peak at

1402cm-1 illustrates the presence of nitrates in the dried oxalate spheres. The

peak at 3152 cm-1 shows the presence of –OH stretch. The peaks in the range

of 910cm-1, 1096 cm-1, 1279 cm-1 and 1689 cm-1 are related to the volatile

compounds (O-C=O, NO, C-O & C=O groups). These vibrations gradually

disappear with the increase of sintering temperature. As the sintering

temperature is increased above 1500oC, they are almost undetected.

A weak absorption at 2330 cm-1 of CO band is also observed. It has

been reported that the oxalate ion has a quadridentate structure with the

zirconium ion (Etienne et al 1990). It is made clear that the addition of

yttrium ions does not affect the structure of the zirconyl oxalate. Strong bands

at 498 and 600 cm-1 are attributed to Zr–O binding. FTIR spectrum for the

dried minispheres shows the characteristic bands of (OH)/H2O, which is

vanished with increase in sintering temperature. The results obtained in the

present study are in good agreement with the results obtained by Gangadevi et

al (1980).

5.3.6 Density Studies

The density variations with sintering temperature are listed in

Tables 5.5 and 5.6. Theoretical density (TD) is calculated following the

theory of Ingel and Lewis (1986). The density of sphere increases with

sintering temperature. It reaches the maximum value of 5.89 gm/cm3 (TD =

96%) for spheres sintered at 900oC (Figure 5.11).

151

4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

V (M

N)

V (

ZrO

) + V

(C

-C))

V s (Z

r-O)+

(O

-C=O

)

Sintered at 1500oC dried at 40oC

Wavenumber (cm-1)

% T

V s (C

-O)+

(O

-C=O

)

V (

NO

)

V (C

-O(s

))

V (N

O3)-

V s (C

=O)

-OH

(s)

Figure 5.10 FTIR spectrum of 5Y-ZrO2 minispheres

On further increase in sintering temperature, the density of the

sphere is found to be reduced which may be due to the volume expansion that

takes place during the phase transformation from tetragonal to monoclinic

phase and formation of microcrack. Figure 5.11 shows the dependence of

crystallite size with density. It is observed that the crystallite size increases

linearly with temperature which leads to uniform increase in the density of the

minispheres upto the sintering temperature 900oC. Sharpe increase in the

crystallite size is observed above 1100oC and subsequently reaches to 54 nm

at 1500oC.

It is also observed that the yttria concentration has an impact on the

density of the minispheres. Density of the minispheres increases with yttria

concentration and reaches a maximum for 5 mol % of yttria (Figure 5.12).

Further increase in yttria content leads to the reduction of density of the

152

minispheres which may be attributed to the critical grain size effect. Several

researchers have expressed the difficulty in sintering high density nano-sized

stabilized zirconia at lower temperature (Gaudon et al 2004; He et al 2005;

chen 2003). In most of the cases, agglomeration of the powder may be

responsible for such poor densification. The better densification of 5Y-ZrO2 in

the present study may be due to the better control over the agglomeration state

because of the adaptation of sol-gel synthesis.

Recent reports on better densification of yttria stabilized zirconia by

hydrothermal synthesis methods (Zhu and Fan 2005; Zych and Haberko

2006) report a density of 97% TD with 2.5% open porosity for their dry

pressed powder compact.

400 600 800 1000 1200 1400 1600

5.0

5.2

5.4

5.6

5.8

6.0

Den

sity

(gm

/cm

-3)

Temperature ( OC )

Density Crystallite size

15

20

25

30

35

40

45

50

55

A

vera

ge C

ryst

allit

e S

ize

(nm

)

Figure 5.11 Variation of density and average crystallite size with

sintering temperature for 5Y-ZrO2 minispheres

153

2 3 4 5 6 7 85.6

5.7

5.8

5.9

6.0

Den

sity

(g/c

m3 )

yttria content (mol%)

Figure 5.12 Variation of sintered density with yttria concentration for

zirconia minispheres sintered at 1100oC

5.3.7 Microstructural Analysis

Figures 5.13 (a, b & c) show the surface morphology of 5 mol %

yttria stabilized zirconia minispheres sintered at different temperatures for a

soaking time of 5 hours. The microstructure of 5Y-ZrO2 minispheres shows

(Figure 5.13) less agglomerated grain growth, which results in moderate grain

size and very close homogeneous microstructure. Gradual increase in average

grain size with increase in sintering temperature is observed for 5Y-ZrO2. It

has been observed that the average grain size increased from 0.85 to 2.6μm

with increase in sintering temperature from 900oC to 1500oC which influences

the internal tensile stress produced due to the thermal expansion of zirconia.

When the grain size is above the critical level, the internal stress is equal to

the stress required for transformation. Thus, zirconia grain growth and the

154

internal tensile stress are responsible for tetragonal to monoclinic

transformation above 900oC. Substantial increase in grain size is seen at

higher sintering temperatures and longer sintering time. It is, therefore,

concluded that the temperature is one of the key factor to influence the grain

growth in the sintering process. The low sintering temperature prevents the

excessive grain growth during sintering which is critical for obtaining

nanocrystalline zirconia ceramics. It is observed that the extensive internal

cavities are formed during high temperature sintering and the amount of

cavitations is increased with increase in grain size. This behaviour is in good

agreement with the reports of Tekeli and Erdogan (2002). From the

micrographs, it is observed that the existence of the pores at the grain corners

shows the inability of the maximum removal of pores during sintering. The

exciting pores at the grain boundaries in early and intermediate stages are

pulled together to form a large pore at the final stage of sintering.

Figures 5.14 (a and b) show the microstructures of yttria stabilized

zirconia minispheres with varying yttria content. It is observed that the grain

size increases with the increase of yttria content. The reason for the grain

growth is considered to be the yttria segregation at grain boundaries. Yttria

enriched at boundaries acts as impedance for boundary migration (e.g. solute

dragging effect) and therefore inhibits the grain growth.

5.3.8 Mechanical Properties The sample preparation for determining the Vickers hardness has

been preformed following the procedure explained in chapter 2. Optical

photograph (Figure 5.15) shows the typical indentation mark for yttria

stabilized zirconia minisphere sample. Load is applied ranging form 0.5 to 2.0

Kg in steps of 0.5 Kg to produce the indentation marks on the surface of the

sintered sample.

155

Figure 5.13 (a) SEM micrograph of 5Y-ZrO2 minispheres sintered at

700oC

Figure 5.13 (b) SEM micrograph of 5Y-ZrO2 minispheres sintered at

900oC

Figure 5.13 (c) SEM micrograph of 5Y-ZrO2 minispheres sintered at

1100oC

a

b

c

156

Figure 5.14 SEM micrograph of minispheres sintered at 1100oC

a) 3Y-ZrO2 b) 8Y-ZrO2

Figure 5.15 Optical photograph of a typical indentation mark on the

surface of 5Y-ZrO2 minispheres (applied load of 0.5kg)

5.3.8.1 Previous Reports on Mechanical Properties of Yttria Stabilized

Zirconia

Gupta et al (1977) have first reported the fully tetragonal zirconia

polycrystals by the addition of 3 mol% of Y2O3 with zirconia sintered at less

than 1500oC and the bend strength for the sintered samples. Since, then many

reports have been published on the mechanical properties of this class of

a b

157

materials. Lange (1982) has done extensive work on the transformation

toughening due to the stress induced t to m phase transformation and

contribution to the fracture toughness for various compositions of Y2O3-ZrO2

ceramics.

Effects of high pressure on the fracture toughness and hardness for

Y-TZP have been reported by Noma et al (1988) stating that the toughness

increases with increase of pressure and also found that the hardness value

initially increases with pressure and temperature and then decreases. Hardness

value of HIPed Y-TZP samples as high as 13 GPa has been reported by

Tsukuma et al (1988) and have also studied the variation of fracture toughness

with respect to the composition of yttria and retention of cubic phase. They

have found that the 3 mol% of Y2O3 has higher fracture toughness value of

17MPam1/2 for HIPed samples.

Chen and Brook (1989) have reported that the fracture toughness

value for conventionally sintered 3Y-TZP samples is 8MPam1/2 and the

hardness value of 13.3 GPa for both electro-refined and the co-precipitated

powders. Cutler et al (1992) have reported the toughness values of different

composition of Y2O3 zirconia samples either HIPed and then sintered or

HIPed directly. For the eletro-refined powder of 3.2 mol% of Y-TZP, 1392

MPa has been noticed by Hepworth and Pindar (1987). Microstructure–

toughness–wear relationship of tetragonal zirconia ceramics have been

recently investigated by Bikramjit Basu et al (2004).

5.3.8.2 Hardness

Hardness values of 5Y-ZrO2 minispheres vary from 10.98 to

7.7 GPa for the grain size around 0.8 to 2.6 μm with increase in sintering

temperature from 900 to 1500oC (Figure 5.16). It is observed that the smaller

158

grain size may be the major reason for the improved hardness values observed

in the present study. Cutler et al (1992) has recorded a 13.7 GPa for the hot

pressed 6 mol% of Y-ZrO2 samples where the observed phase is cubic and

hence there is no change of phase transformation during indentation which

results in higher hardness values.

It is observed that the hardness value decreases with the increase of

applied load, varying from 9.26 GPa (applied load of 0.5Kg) to 5.12GPa

(applied load of 2kg) for the sintering temperature of 1100oC (Figure 5.18). It

is also observed that the hardness value increases with the yttria concentration

for all the applied loads. The samples, which are sintered at 900oC show a

maximum hardness. Hardness increases linearly with increase in yttria content

and has a maximum of 10.16GPa for 8Y-ZrO2 minisphere sintered at 1100oC

(Figure 5.17). Harness is also found to be heavily dependent on the density of

the minispheres (Figure 5.19). It is observed that the 5Y-ZrO2 minispheres

with 96.56 TD sintered at 900oC possesses a maximum hardness of 10.98GPa

for the applied load of 0.5gms.

5.3.8.3 Fracture Toughness

The fracture toughness of compacts mainly depends upon the

sintering temperature and crystalline phase formed in the system. Figures 5.16

shows the fracture toughness values of 5Y-ZrO2 minispheres heat-treated at

different temperatures. It is observed that the fracture toughness reaches a

maximum of 9.148 MPam1/2 for 5Y-ZrO2 minispheres sintered at 1100oC and

further increase in temperature leads to the reduction in fracture toughness.

The reason may be that the spheres sintered at 1100oC are possessed with

higher fraction of t phase. Since, toughening of ZrO2 can be achieved through

martensitic transformation from tetragonal to monoclinic phase.

159

The fracture toughness of the zirconia increases with the addition of

Y2O3 upto 5 mol % of yttria concentration beyond which it decreases

(Figure 5.17). This behaviour may be attributed due to the increase of

non-transformable tetragonal phase in 8 mol% of the yttria concentration. It is

obvious that the fracture toughness of yttria stabilized zirconia varies from

6.573MPam1/2 to 9.148MPam1/2 for different yttria concentrations. The

3 mol% yttria composition has relatively low fracture toughness

(4.102 MPam1/2) apparently due to the large amount of monoclinic phase.

5.3.8.4 Wear Resistance

Figure 5.20 shows the increase in wear with increase in milling

time. 5Y-ZrO2 minispheres sintered at 1100oC possesses with as low as

0.07 % of wear for a milling time of 3 hours. Maximum of 0.36% of wear has

been observed for 9 hours of milling. The observed values are comparatively

higher than the wear loss values of 13Ce-ZrO2 minispheres sintered at

1500oC. Rate of grinding depends on the parameters such as structure of the

material, temperature, environment and sliding speed. It is observed that the

wear resistance mainly depends on the amount of transformable t-phase in the

sample.

Zirconia minispheres stabilized with 3 mol% of yttria is observed

with high fraction of monocline phase which inturn have a destructive effect

in wear. Percentage of wear loss is found to decrease considerably above

5mol % of yttria concentration. Wear resistance of 5Y-ZrO2 minispheres

sintered above 1300oC decreases drastically, since the minispheres sintered

above 1300oC have reduction in fraction of tetragonal phase and density. It is

found that the 5Y-ZrO2 sintered below 700oC and above 1300oC do not

withstand for milling operation.

160

900 1000 1100 1200 1300 1400 15004

6

8

10

12 Fracture toughness Hardness Grain size

Sintering temperature (oC)

Frac

ture

toug

hnes

s(M

Pa/

m)

Har

dnes

s (G

Pa)

0

2

Avg

. Gra

in s

ize

(um

)

Figure 5.16 Variations in hardness, fracture toughness and grain size of

5Y-ZrO2 minispheres with sintering temperature

0 2 4 6 82

4

6

8

10

Fracture toughness Hardness

Concentration of yttria (mol%)

Frac

ture

toug

hnes

s (M

Pa/m

)

4

6

8

10

12

Har

dnes

s (G

Pa)

Figure 5.17 Variation of mechanical properties with yttria

concentrations for stabilized zirconia minispheres sintered

at 1100oC (applied load of 0.5 kg)

161

0.0 0.5 1.0 1.5 2.0 2.52

3

4

5

6

7

8

9

10

11

12

3Y-ZrO2 5Y-ZrO2 8Y-ZrO2

Load (Kg)

Har

dnes

s (G

Pa)

Figure 5.18 Variation of Vickers hardness with applied load and yttria

concentrations sintered at 1100oC for 5 hrs

95.0 95.2 95.4 95.6 95.8 96.0 96.2 96.4 96.6 96.8 97.02

4

6

8

10

12

14

Density (T.D %)

Har

dnes

s (G

Pa)

0.5 Kg 1 Kg 2 Kg

Figure 5.19 Variation of hardness with sintered density of 5Y-ZrO2

minispheres sintered at 1100oC for different applied loads

162

3 4 5 6 7 8 9

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

% o

f Wea

r

Milling time (hrs.)

Figure 5.20 Variation of percentage of wear with different milling time

intervals for 5Y-ZrO2 sintered at 1500oC for 5 hrs

It is also observed that water has a deterious effect on the wear

resistance of yttria stabilized zirconia minisphere because of the effect of

hydrothermal degradation.

5.4 CONCLUSION

Sol-gel derived yttria stabilized zirconia minispheres has been

successfully fabricated by drop generation technique. Formation of zirconium

oxalate sol has been demonstrated as a good starting route for the preparation

of yttria stabilized zirconia minispheres.

Effect of heating rate and surface area on the density for yttria

stabilized minispheres has been observed. The ideal sintering temperature for

the preparation of sol-gel derived yttria stabilized zirconia minisphere has

163

been identified as 900oC, which has the fully stabilized tetragonal phase

having 97% of theoretical density. The minimum composition required for the

fully tetragonal microstructure clearly depends on the sintering temperature

and grain size produced. Increase in the sintering temperature above 900oC

leads to the phase transformation and reduction in density. Presence and

periodic removal of volatile compounds have been observed by DTA and

TGA, which has been confirmed by FTIR studies. Impact of yttria

concentration and sintering temperature on the final product has been

analyzed in detail. Zirconia minispheres has been extensively characterized to

establish a correlation between physical, structural and mechanical properties.

It is found that the sintering temperature has huge impact on the density,

crystalline phase and microstructure of the sintered body.

It is concluded that the Y2O3 content and the grain size of the

sintered minisphere affect the mechanical properties of the end products.

Y-ZrO2 minispheres have higher hardness and lower fracture toughness

compared to Ce-ZrO2. Yttria doping can be developed as a method to secure

the minisphere from cracking and high temperature preparation process.