Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical Ferrule

doi:10.4236/msa.2011.21001

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Materials Sciences and Applicatio ns, 2011, 2, 1-5

doi:10.4236/msa.2011.21001 Published Online January 2011 (http://www.SciRP.org/journal/msa)

Crystallinity, Microstructure and Mechanical

Strength of Yttria-Stabilized Tetragonal Zirconia

Ceramics for Optical Ferrule

Sung-Dai Kim1, Kyu-Seog Hwang2*

1Department of Quality Non-Destructive Testing, Seoul Sanggye Vocational School, Seoul, Korea; 2Department of Biomedical En-

gineering, Nambu University, Gwangju, Korea.

Email: [email protected]

Received October 8th, 2010; revised December 21st, 2010; accepted December 27th, 2010.

ABSTRACT

Yttria-stabilized zirconia ceramics were prepared by using different raw materials in order to compare commercially

available optical ferrule. Injection-molded cylindrical green compacts were sintered in air at 1350˚C, 1400˚C and

1450˚C for 2 hrs, followed by furnace cooling. Crystallinity, microstructure and mechanical strength of the sintered

body were eva luated b y using an X-ra y diffraction analyses , a field emission -scanning electron microscope, a universal

tester, and a micro-hardness tester, respectively. For practical usage, the sample B sintered at 1350˚C was favorable

because of high tetrago nality and good mechanical strength.

Keywords: Zirconia, Tetragonality, Mechanical Strength

1. Introduction

It is well known that yttria (Y2O3)-stabilized tetragonal

zirconia (ZrO2) polycrystal (Y-TZP) possesses excellent

mechanical properties and represents toughened zirconia

ceramics [1,2]. The relationship between microstructure

and mechanical properties in Y-TZP ceramics has been

studied extensively over the past decade [3-5]. Generally,

it has been found that Y-TZP ceramics have high strength

and fracture toughness, making them attractive candi-

dates for a number of demanding structural applications.

It is also well established that these desirable mechanical

properties are strongly influenced by grain size. For ex-

ample, maximum strength is usually achieved with a

small grain size (< 1 μm). Their excellent mechanical

properties are derived from the stress-induced martensitic

transformation of the metastable tetragonal to the mono-

clinic phase [6].

The synthesis of fully tetragonal, pure, nano-crystal,

uniformly aggregated, agglomerate free zirconia powder

is the main emphasis in the production of advanced ce-

ramics with a desirable microstructure and properties. The

small grain size of the nanomaterials has a pronounced

effect on many physical properties, such as increased

strength and hardness.

Throughout our previous work [7], we studied crystal-

linity and microstructure of Y-TZP ceramics as a func-

tion of the variation of raw materials provided by differ-

ent suppliers with different particle-properties. From pre-

vious results, we confirmed that the raw materials with

fine particle size and high tetragonality could be sintered

to dense Y-TZP at 1400˚C.

In this work, in order to compare commercially avail-

able sample A with two raw materials (denoted B and C),

which exhibited relatively good sinterability in our pre-

vious work [7], were selected for practical usage. Crys-

tallinity, microstructure and mechanical strength were

examined as a function of various sintering temperature.

2. Experimental

Yttria-stabilized zirconia (YSZ) powders with composi-

tion 97 mol% ZrO2-3 mol% Y2O3 have been provided by

three different suppliers denoted as A, B and C (see Ref.

[7]). Green bodies of the Y-TZP were prepared by mix-

ing aqueous binder and YSZ powder at 100 ~ 150˚C for

24 hrs. After mixing, green compacts were formed with a

ferrule ingot using a screw type injection-molding ma-

chine. Cylindrical specimens for optical ferrule were sin-

tered in air at atmospheric pressure at 1350˚C, 1400˚C

and 1450˚C for 2 hrs, followed by furnace cooling.

Crystallinity of the sintered specimens was determined

Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical

2 Ferrule

by X-ray diffraction (XRD) (Rigaku Co., D-Max-1200,

Jpn.) θ-2θ scans. The XRD patterns were recorded by

using CuKα radiation (λ = 1.54056 Å) generated at 40

kV and 30 mA in the 20˚ < 2θ < 40˚ range. For this pur-

pose the specimens were ground after sintering by agate

milling. Tetragon ality of the sin tered body was calculated

from below equation;

T (%) = It(111)/ Im(111) + Im(–111) + It(111)

where Im (111), Im (–111) and It (111) are the intensities

of monoclinic (111), monoclinic (–111) and tetragonal

(111) reflections, respectively. Surface morphology of

the fractured cross-section of the sintered samples was

examined by using a field emission-scanning electron

microscope (FE-SEM) (Hitachi Co., S-4700, Jpn.). Bend-

ing strength and Vickers’ hardness were examined by

universal tester (Instron 4302, Instron Co., England) and

microhardness tester (Shimadzu Co., HMV-2 series, Jpn.),

respectively.

3. Results and Discussion

Figures 1 and 2 show the XRD patterns for the sintered

Y-TZP specimens B and C at various sintering tempera-

tures, as comparing with specimen A sintered at 1400˚C.

It could be seen in Figures 1 and 2 that no other phases

except for tetragonal and monoclin ic Y-TZP phases were

detected by the XRD analysis. As shown in Figure 1,

a) B-1350

1400

1450

-1400

Inte nsity (ar b.units)

°C

m(110)

m(111)

m(-111)

t(111)

t(002)

t(200)

20 30 40

2θ(deg)

b) B-

c) B -

d) A

Figure 1. XRD patterns of the specimen B sintered at

1350˚C (a), at 1400˚C (b) and at 1450˚C (c), and the speci-

men A sintered at 1400˚C (d).

a) C- 1350°C

b) C -1400°C

c) C- 1450°C

d) A-1400°C

m(110)

m(111)

m(-111)

t(111)

t(002)

t(200)

Intensity (arb.units)

20 30 40

2θ(deg)

(a) C-1350˚C

(b) C-1400˚C

(d) A-1400˚C

Figure 2. XRD patterns of the specimen C sintered at

1350˚C (a), at 1400˚C (b) and at 1450˚C (c), and the speci-

men A sintered at 1400˚C (d).

specimen B showed the highest intensity of tetragonal

(111) reflection at 1400˚C, although, at 1450˚C, crystal-

linity was gradually decreased. On the contrary, for the

spec imen C, as the in crea se of si nter ing temp era tur e from

1350˚C t o 1450 ˚C, peak intensity corresponds to tetragonal

(111) reflection rather decreased, as shown in Figure 2.

-1350˚C (a) B

In order to more clearly compare differences of peak

intensities between the tetragonal and monoclinic, peak

intensities of tetragonal (111), monoclinic (110), mono-

clinic (111) and monoclinic (–111) were illustrated as

shown in Figure 3. As clearly shown in Figure 3, the

crystallinities of tetragonal (111) for the specimen B sin-

tered at 1400˚C and the specimen C sintered at 1350˚C

exhibited higher intensities than those of other samples.

However, although tetragonal peak intensity of specimen

B sintered at 1400˚C was comparable to that of specimen

A sintered at 1400˚C, which showed a dense and compact

microstructure in previous work [7], its peak intensities

of monoclinic reflections, especially (110), exhibited much

higher values than those of other samples.

(b) B-1400˚C

-1450˚C (c) B

(d) A-1400˚C In order to compare crystalline structure, we illustrated

change of tetragonality for the sintered specimens. As

shown in Figure 4, all the specimens, except specimen C

sintered at 1450˚C, showed high tetragonality after sin-

tering, i.e., above 70%. However, for the specimens B

and C, tetragonality was decreased as the increase with

sintering temperature from 1350˚C to 1450˚C. We as-

sume that low-temperature sintering was effective to in-

crease tetragonality of specimens B and C. The highest

Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical 3

Ferrule

m(110)

m(111)

m(-111)

t(111)

BBB CCCA

1350 °C 1400 °C 1450 °C 1350 °C 1400 °C 1450 °C 1400 °C

Specimen

6000

5000

4000

3000

2000

1000

Inten si ty (CPS)

Figure 3. XRD peak intensities of tetragonal or monoclinic

reflections [m(110): Monoclinic (110), m(111): Monoclinic

(111), m(–111): Monoclinic (–111) and t(111): Tetragonal

(111)].

B B B C C C A

1350°C 1400°C 1450°C 1350°C 1400°C 1450°C 1400°C

Specimen

Tetragonality(%)

Figure 4. Variation of tetragonality of specimens at various

sintering temperatures.

tetragonality about 79.95% was obtained from the speci-

men C after sintering at 1350˚C.

Figures 5 and 6 show the FE-SEM images of frac-

tured cross section of the sintered specimens. As clearly

shown in Figure 6, all the specimens had well-crystal-

lized grains with 0.2 ~ 0.4 μm in size. Break down was

concurrently occurred at the inside and at the interface of

the grains for all the specimens. However, for the speci-

men C, some pores were found as shown in Figures

5(d-f).

Figure 7 shows the bending streng th of the specimens.

The highest bending strength was obtained for the stan-

dard sample A. Specimen C, which possessed some pores

as shown in Figures 5(d-f), exhibited low bending

strength, while the specimen B performed relatively high

bending strength. Low bending strength of the specimen

C was quite reasonable, since observable pores probably

corresponding to weak mechanical strength were recog-

a) B - 1350°Cb ) B -1400°Cc) B - 1450°C

d) C - 1350°Ce) C - 1400°Cf) C - 1450°C

g) A -1400°C

(a) B-1350˚C(b) B-1400˚C ( c) B-1450˚C

(d) C-1350˚C(e) C-1400˚C ( f) C-1450˚C

(g) A-1400˚C

m(–111)

1350˚C 1400˚C 1450˚C 1350˚C 1400˚C 1450˚C 1400˚C

Figure 5. FE-SEM images of fractured cross section of the

specimens (× 20,000).

a) B - 1350°Cb ) B -1400°Cc) B- 1450°C

d) C - 1350°Ce) C -1400°Cf ) C -1450 °C

g) A -1400°C

(d) C-1350˚C(e) C-1400˚C (f) C-1450˚C

(g) A-1400˚C

(a) B-1350˚C(b) B-1400˚C ( c) B-1450˚C

1350˚C 1400˚C 1450˚C 1350˚C 1400˚C 1450˚C 1400˚C

Figure 6. FE-SEM images of fractured cross section of the

specimens (× 50,000).

1300

1200

11 0 0

1000

900

800 B BBC CCA

1350°C 1400 °C 1450 °C 1350 °C 1400 °C 1450 °C 1400 °C

Specimen

Be nding st rengt h (MPa )

1350˚C 1400˚C 1450˚C 1350˚C 1400˚C 1450˚C 1400˚C

Figure 7. Bending strength of the specimens at various sin-

tering temperatures.

Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical

4 Ferrule

nized in sintered body, as shown in Figure 5.

Vickers’ hardness is plotted as Figure 8. It is found

that specimen B sintered at 1350˚C and specimen C sin-

tered at 1450˚C showed higher values. It should be

pointed out in this work that, for the specimen C, some

unclear problems, such as pores in sintered body and low

bending strength, were still existed, although their hard-

ness above ~1200 was sufficient for practical usage to

optical ferrule. Furthermore, for the specimen B, hard-

ness was rather lowered by high-temperature sintering.

We assume that hardness was probably decreased by ex-

istence of some amorphous-like phases in appearance, as

shown in Figure 5(c). Conclusively speaking, for the

specimen B, low-temperature sintering was favorable for

the sintered body with high tetragonality and good me-

chanical properties.

Figure 9 shows surface morphology (a) and photo-

graph (b) of prepared optical ferrule annealed at 1350˚C

by using raw material B. As shown in Figure 9(a),

well-defined small grains with below ~1μm were densely

formed. For our specimen B, it is difficult to find pores

after sintering, which generally exhibited at the grain or

at the grain boundary of sintered zirconia specimens. A

smooth specimen surface was obtained by polishing the

scratches.

4. Conclusions

In this work, two raw materials (B and C), which exhib-

ited relatively good sinterability in our previous work

were selected in order to compare with commercially

available sample A. Low-temperature sintering was ef-

fective to increase tetragonality of the specimens B and C.

For all the specimens, microstructures contained well-

crystallized grains with 0.2 ~ 0.4 μm in size were char-

acterized by FE-SEM. For practical usage, characterized

by FE-SEM. For practical usage, low-temperature sinter-

B BBC CCA

1350°C 1400 °C 1450 °C 1350 °C 1400 °C 1450 °C 1400 °C

Sp ecimen

Vi ckers hard ness ( Hv)

1600

1500

1400

1300

1200

1100

1000

Figure 8. Vickers’ hardness of the specimens at various sin-

tering temperatures.

a) b)

Figure 9. Surface morphology (a) and photograph (b) of

prepared optical ferrule sintered at 1350˚C by using raw

material B.

ing at 1350˚C was favorable for the specimen B because

of high tetragonality and mechanical strength. While, for

the specimen C, some unsoluble problems, such as pores

in the sintered body and low bending strength, was still

occurred.

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