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)
1
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.
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
°C
°C
°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
(c) C-1450˚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
0
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(%)
82
80
78
76
74
72
70
68
66
64
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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