Research Article EFFECT OF SOLID LOADING AND BINDER ... I/IJAET... · minispheres (30 wt% solid loading and sintered at 1500 oC for 5 hours) The concentration of the binder in the

International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.I/ Issue III/Oct.-Dec.,2010/201-212

Research Article

EFFECT OF SOLID LOADING AND BINDER ADDITION ON

PREPARATION AND PROPERTIES OF CERIA STABILIZED

ZIRCONIA MINISPHERES J. Judes* and V. Kamaraj

Address for Correspondence

Department of Ceramic Technology, Anna University, Chennai – 600025.

*Email [email protected]

ABSTRACT

In order to reduce shrinkage and to improve sintered density of the zirconia minispheres, solid loading has been performed

by adding t-zirconia powder. The effect of solid loading on the density, transformation temperature, shrinkage,

microstructure and mechanical properties of zirconia minispheres are observed. When the solid loading is increased,

variation in the density is observed, however the shrinkage is optimized for better dimension control. Variation in binder

concentration has been made to study its effect on final product. A maximum sintered density of 98% TD has been

obtained for the samples prepared with 30% of solid loading and 35 wt% of PVA. Solid loaded zirconia minispheres have

been extensively characterized to establish a correlation between structural, physical and mechanical properties. Solid

loading on ceria stabilized zirconia minispheres improves the properties such as density, porosity, shrinkage, weight loss,

crystallization temperature, crystallite size, phase, grain size and mechanical properties.

KEYWORDS Zirconia Minispheres, Sol-Gel Synthesis, Zirconium Oxalate Sol, Solid loading, Ceria, Stabilization.

1 INTRODUCTION

It is observed from the previous investigation [1] on

formulation of near-net-shaped prototype ceria

stabilized zirconia minispheres that the binder

additive helps to retain the shape during body and to

impart green strength to the shaped component. For

compacting oxide materials, the most commonly

used binder is PVA because it has a strong affinity

of adsorption on oxide particles dispersed in water

[2]. Polymer molecules absorbed on the surface of

particles will bridge them together in the gel. This

imparts organic network between the particles which

results in high green density [3]. Relatively little

work has been reported on the properties of technical

ceramics plasticized with organic binders. Therefore

an attempt is made to study the effect of binder

content on the sintered density of the ceria stabilized

zirconia minispheres. The difficulty encountered in

forming minispheres using sol-gel processing is the

magnitude of the shrinkage, subsequent difficulty in

maintaining dimensional control and achieving near

theoretical density (TD). This restricts the use of

zirconium oxalate sol alone as a starting route for the

fabrication of dense and hard zirconia minispheres.

Wistrom and Clark (1984) have suggested that the

presence of solid particulates or fibers substantially

reduces the amount of shrinkage [4]. In order to

overcome the above difficulty, in the present

study, the solid loading is performed in the

ceria-zirconyl oxalate (CZO) sol by the addition

of pure t-ZrO2 powder to reduce the shrinkage

and to improve the sintered density of the final

product. The mixing of t-ZrO2 with CZO

reduces shrinkage by (a) replacing the lower

specific volume CZO with t-ZrO2 thereby

changing, the total specific volume upon

transformation and by (b) increasing the density

with improved particle packing. However, the

differential shrinkage due to the bimodal

particle size distribution of the powder may be a

problem when CZO is mixed with t-zirconia of



different particle size. Small amount of PVA is

also added to the sol to increase the plasticity

and the green strength. In this present

investigation, the effect of solid loading and

binder addition on shrinkage, transformation

temperature, sintered density and enhancement

of structural and physical properties of the ceria

stabilized zirconia minispheres are presented.

2 EXPERIMENTAL PROCEDURE

The methodology of CZO preparation was followed

as described in the previous investigations on

synthesis and characterization of ceria stabilized

zirconia minispheres [1]. The sol was then

dehydrated by heating at 80oC to remove excess

amount of water content. To enhance the

densification and to reduce the shrinkage and

transformation temperature, the CZO sol was solid

loaded using pure t-ZrO2 powders. In solid loading,

t-ZrO2 powders of 1µm size were mixed with the

CZO sol and stirred well to avoid mass segregation

and to improve homogenous dispersion. The solid

loaded CZO sol was then adjusted to a pH value of

1.5 using nitric acid. PVA was added to impart

green strength and plasticity in the sol to make it

appropriate for minisphere formation in the setting

solution [5]. The concentration of binder is varied to

analyze the effect on sphere formation and on the

properties of the final product. The PVA inter-chain

separation distance in solution decreases with

increase in degree of hydrolysis and causes

interaction between polymer-solvent and polymer-

polymer system. The viscosity of the binder solution

and the adhesion strength of the organic film on the

particle alter the internal friction and the ultimate

shear strength of the sol. When the submicron

zirconia powders are flocculated with an organic

binder in the system, the plasticity is developed.

When the sol is to be extruded to form the shaped

components, the friction at particle contacts should

not restrict the particle sliding in the sol. For a

particulate system, a particular concentration of

binder is required for easy plastic flow without

friction. A 500 ml beaker filled with 400 ml of

ammonia solution was taken as sphere container. At

the suitable viscosity (15cPs), the solid loaded gel

was added drop wise to a gelation container for the

formation of uniform minispheres. The gelled

minispheres were then dried at room temperature for

24 hours to remove excess ammonia and converted

into t-ZrO2 minispheres by sintering at 1500oC for 5

hours. It is observed that the variation in heating rate

has minimum effect on the final product [6]. To

study the effect of solid loading on preparation and

properties, different wt% (30, 50 and 70) of solid

loading was used. Figure 1 illustrates the flow chart

for the preparation of zirconia minispheres using

CZO sol with solid loading.

Figure 1: Flow chart for the preparation of

zirconia minispheres using CZO sol and zirconia

minispheres with solid loading



3. RESULTS AND DISCUSSION

3.1 Thermal Analysis

Figure 2 gives the onset temperature at which

phase transformation occurs in ceria stabilized

zirconia minispheres solid loaded with t-ZrO2

powder. It is observed from DTA curve that the

solid loading on 13 mol % ceria stabilized zirconia

(13Ce-ZrO2) minispheres lowers the

crystallization temperature. The second

exothermic peak observed for 13Ce-ZrO2

minispheres without solid loading is reduced from

420oC to 388

oC, when the 13Ce-ZrO2 minispheres

solid loaded with 30wt % of pure t-ZrO2 powders.

The temperature shift on t-ZrO2 transformation

indicates that the t-ZrO2 particles do indeed act as

nuclei. It is also observed that the crystallization

temperature marginally decreases with further

increase in solid loading which supports the claim

that solid loading do not affect the transformation

temperature beyond a saturation level. Changes in

oxalate decomposition temperature and volatiles

removal temperatures are also observed which

may be attributed to the procedural alterations

adapted in the present preparation route (Figure 1).

It is evident from the DTA curve that the net

percentage of weight loss decreases from 41.80 to

34.69 which implies that the solid loading (30 wt%)

improves the residual weight of the 13Ce-ZrO2

form 58.20 to 65.31 %. It is observed that the

hydroxylation procedure adopted in the present

study does have an impact on the percentage weight

loss in water removal stage. Weight loss on water

removal stage is observed as 6.81% which is nearly

3% less than that observed for 13Ce-ZrO2 without

solid loading. In addition, shift in the first

endothermic peak (~100oC) is also observed which

may be due to the removal of entrapped residual

ammonia and water. Variations in percentage of

weight loss and shrinkage have been analyzed with

increase in amount of solid loading. It is observed

that the weight loss percentage and shrinkage

decreases with increase of solid loading. Figure 3

shows the comparison on residual weight,

shrinkage and density of 13Ce-ZrO2 minispheres

sintered at 1500oC with various amount solid

loading. It is observed that the residual weight of

13Ce-ZrO2 minispheres increases to nearly 7 % for

30 wt % of solid loading and further increase in

amount of solid loading is not yielding considerable

increase in residual weight. Nearly 12% reduction

in shrinkage is observed for the addition of 50 wt%

of solid loading.

Figure 2: TGA/DTA curves for the dried 13Ce-ZrO2 with solid loading (30wt%)



Figure 3: Variations on residual weight, shrinkage and density of 13-ZrO2 minispheres with different

solid loading

3.2 Density Studies

The density of the 13Ce-ZrO2 minispheres varies

considerably with the increase in the solid

loading as shown in figure 3. A maximum of

98% TD is obtained for the minispheres formed

using ceria doped zirconium oxalate sol with 30

wt% of t-ZrO2 solid loading sintered at 1500oC

for 5 hours. The densification of the zirconia has

been achieved and depends greatly on the

intrinsic densification behavior of the t-ZrO2

powder. Solid loading in the range of 50-70wt%

leads to the marginal reduction in the density of

the final product which may be due to the

imbalance occurs in the ratio of residual weight

and shrinkage. The difference in densification

behavior is believed to be due to the degree of

differential shrinkage. The critical value of solid

loading is observed as 30 wt% for obtaining

maximum density (98%) with the moderate

shrinkage. Enhanced dimensional control has

been achieved due to the reduction in sintering

shrinkage of the final product. However, the

desired density can be obtained with reasonable

shrinkage by using appropriate addition (in wt %)

of t-ZrO2 powder. Table 1 gives the density of the

13Ce-ZrO2 minispheres prepared with and

without solid loading sintered at 1500oC for

different soaking time. The impact of soaking

time on density is also observed.

Table 1: Dependence of density with and without solid loading for different soaking period

Density (TD%) – Sintering temperature 1500oC

Soaking time (hours)

Type of sol

used for

forming

Green density

(TD%)

3 5 8

CZO 48 93.61 94.75 95.16

CZO with

solid Loading

(30 wt %)

57 94.32 97.72 98.14



3.3 Effect of Binder and Sintering Temperature

200 400 600 800 1000 1200 1400 1600

70

75

80

85

90

95

100

Density (TD%)

Temperature (oC)

Figure 4: Variation of density with sintering temperature (13Ce-ZrO2 minispheres prepared with

30 wt% solid loading)

5 10 15 20 25 30 35 40

80

82

84

86

88

90

92

94

96

98

100

Density (TD%)

PVA Content (wt%)

Figure 5: Dependence of sintering density with the concentration of PVA as binder for 13Ce-ZrO2

minispheres (30 wt% solid loading and sintered at 1500oC for 5 hours)

The concentration of the binder in the sol

strongly influences the green density of the

shaped minispheres which inturn, increases the

sintered density of the final product. Higher green

density gives more initial particle contacts and

smaller initial pores which requires less sintering

temperature to obtain final density. Figure 5

shows the dependence of sintered density with



the concentration of PVA as binder of 13Ce-ZrO2

minispheres (30wt% solid loading sintered at

1500oC for 5 hours). The density increases as the

binder concentration is increased. A maximum

density of 98% TD is obtained for the samples

prepared with 35 wt% of PVA. Further increase

in the concentration results in high friction in the

gel which is not ideal for drop formation in the

setting solution. Figure 4 shows the variation of

density with sintering temperature for 13Ce-ZrO2

minispheres prepared with 30wt% solid loading.

The green density of the minisphere is 57% TD

and it gradually increases with sintering

temperature and reaches a maximum of 98% TD

for the spheres sintered at 1500oC.

3.4 XRD Analysis

20 30 40 50 60 70

0

10

20

30

40

50

60

70

30 wt%

2 theta (deg.)

0

20

40

60

80

100

50 wt%

Intensity (a.u)

0

40

80

120

160

70 wt%

Figure 6: XRD patterns for ceria stabilized zirconia minispheres with various concentrations

of solid loading (30, 50 and 70 wt%) sintered at 1500oC for 5 hours



Figure 6 illustrates the XRD diffraction patterns

of solid loaded (30, 50 and 70 wt%) ceria

stabilized zirconia minispheres sintered at 1500oC

for 5 hours. All the three samples are observed

with fully stabilized tetragonal phase. It is

observed that the intensities of the dominant

tetragonal phase increases with the increase in

amount of solid loading. The crystallite size of 30

wt% solid loaded 13Ce-ZrO2 minispheres

sintered at 1500oC has been determined as 68nm

by using Scherer’s and Warren’s equation [7].

When the spheres were heated to 1500oC, it has

been observed that there is a complete retention

of tetragonal phase. The same assimilates the

observation of Srinivasan et al., (1992) [8]. The

(202) and (220) peaks are clearly separated in

sintered spheres at 1500oC, which confirms the

presence of tetragonal phase [9]. The valency of

ceria is either 3+ or 4

+ and of them Ce

4+ is more

stable. During the decomposition, the Ce3+

ions

oxidize to Ce4+ ions. Considering the ionic radius,

the possibility of the substitution of the Ce3+ ions

in ZrO2 crystal structure is highly remote at low

temperature and hence the only chance is for the

formation of Ce4+ ions due to the oxidation of

Ce3+ ions. These Ce

4+ ions substitute for Zr

4+ ions

in the ZrO2 crystal structure and favour the

formation of tetragonal phase. There may be

another chance for possibility for the formation of

Ce2O3 compound in the oxalate decomposition

step. When the oxygen partial pressure decreases,

CeO2 forms a series of oxides, of which Ce2O3 is

the more reduced form. But, the X-ray diffraction

patterns do not show the existence of Ce2O3

reflections. Li et al (1994) studied the role of

stabilization of the t-phase of zirconia by Ce4+

ions shows that the presence of these dopant ions

reduces the overcrowding of oxygen ions around

the zirconium ion and thereby relieving the strain

energy associated with it [10]. The increase in

crystallite size with solid loading is reflected in

XRD patterns as the decrease in the width of the

dominant tetragonal spectral lines [11].

Crystallite size of 84nm has been observed for

minispheres solid loaded with 70 wt%.

3.5 Microstructural Analysis

Figures 7 (a, b and c) show the SEM micrographs

of 13Ce-ZrO2 minispheres sintered at 1500oC for

5 hours with 30, 50 and 70 wt% solid loading

respectively. The microstructures reveal that two

different size grains are present. The smaller

grains of size ~1 micrometer and the large grains

of size ~4 micrometer are observed with reduced

porosity. The presence of two different grain

sizes is due to the difference in the size of the t-

ZrO2 powder (used for solid loading) added

directly and the tetragonal zirconia evolved from

zirconium oxalate sol used for binding the

powder. However, the grains are arranged more

compactly due to the difference in the grain sizes

as compared with minispheres without solid

loading which may be the major reason for the

increase in density. Further increase in amount of

solid loading (>30wt%) leads to the reduction in

compactness of grains because of overcrowding

which may be the reason for marginal decrease in

the density of the minispheres.



Figure 7: SEM micrograph of 13Ce-ZrO2 minispheres sintered at 1500oC with

a) 30wt%, b) 50wt% and c) 70wt% of solid loading.

3.6 Hardness

Figure 8 shows the variation in Vickers hardness

for the 13Ce-ZrO2 minispheres sintered at 1500oC

with solid loading. It is observed that the

hardness of the 13Ce-ZrO2 minispheres sintered

at 1500oC with 30 wt% solid loading varies from

6.8 to 9.1 GPa (applied load of 2 to 0.5 kg).

Increase in hardness with the addition of 30 wt %

solid loading may be attributed to the elevated

density (98%). During indentation, small grain

size of t-ZrO2 powder restricts tetragonal to

monoclinic phase transformation and hence the

hardness decreases.

The decrease in hardness with increase in solid

loading (above 30 wt %) may be due to the

marginal decrease in the sintered density. It is

observed that the variation of solid loading above

30 wt% have very little effect on the hardness.

b

)

c

)

a

)



1 2

7

8

9

Solid loading

30 wt%

50 wt%

70 wt%

Intentation Load (Kg)

Hardness (GPa)

Figure 8: Variation of Vickers hardness with applied loads and different solid loadings for 13Ce-

ZrO2 minispheres sintered at 1500oC

2 4 6 8 10

0.1

0.2

0.3

0.4

0.5

30 wt%

50 wt%

70 wt%

Milling time (hours)

% of wear

Solid loading

Figure 9: Variations in % of wear with different solid loadings and different milling time intervals.

3.7 Wear Resistance

Effect of solid loading on material loss of the

minispheres during grinding has been analyzed

which gives the extent of contamination in the

milled powder. 100gm of grinding media

(minispheres) are subjected to milling in a

planetary mill at the speed of 100 rpm for 9 hours

and the wear occurred for different milling

intervals are measured. It has been found that the

wear loss of the 13Ce-ZrO2 minispheres with



30wt% of solid loading decreases as the time of

the milling increases. Figure 9 shows the % of

wear occurred for different milling time interval

with variation in solid loading. It is observed that

the wear resistance of 13Ce-ZrO2 improves with

30 wt% of solid loading. Maximum wear of only

0.20% occurs during 9 hours of milling. It

indicates that the contamination during milling by

the milling media is minimal. The increase in the

wear resistance of the material is due to the high

density as reported by Zum Gahr et al (1993) [12].

Further increase in solid loading above 30 wt%

leads to the reduction in wear resistance and the

wear loss reaches nearly 0.37 % for 70wt% of

solid loading. It is also observed that the wear loss

increases with grinding time irrespective to the

amount of solid loading.

4. CONCLUSION

The solid loading using t-ZrO2 powder in the

zirconium oxalate sol reduces the shrinkage and

increases the sintered density of 13Ce-ZrO2

minispheres. It is also observed that the density of

the 13Ce-ZrO2 minispheres increases with the

binder content. Subsequently, the binder addition

improves the structural and physical properties of

the final product after sintering. The green density

and sintered density are observed as 57% and 98

%TD respectively. Maximum of 98% TD is

observed for 30 wt% of zirconia solid loading and

further increase in solid loading marginally

reduces the sintered density of 13Ce-ZrO2

minispheres sintered at 1500oC for 5 hrs.

The microstructure shows two different grain sizes

but the grains are arranged more compactly. A

maximum hardness of 9.1 GPa is obtained for 30

wt% of solid loading. Wear resistance studies

shows that only 0.2% wear has occurred even after

9 hrs of milling for 13Ce-ZrO2 minispheres

sintered at 1500oC with 30 wt% of solid loading. It

indicates that the solid loading has a positive effect

on wear resistance of 13Ce-ZrO2 minispheres.

Further increase in the solid loading leads to the

marginal decrease in density, hardness and wear

resistance of the minispheres. However, the

contamination by the milling media is found to be

very small.

A maximum density of 98% TD has been obtained

for the minispheres prepared using 30 wt% solid

loaded zirconium oxalate with addition of 35 wt%

PVA and sintered at 1500oC for 5hours. It shows

that the t-ZrO2 acts as nucleus which results in

high density.

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(vinyl alcohol) as a binder’, Am. Ceram. Soc.

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Research Article EFFECT OF SOLID LOADING AND BINDER ... I/IJAET... · minispheres (30 wt% solid loading and sintered at 1500 oC for 5 hours) The concentration of the binder in the