69● Highly translucent, high strength La-doped zirconia
ceramics.
Advanced Materials Research Laboratory

Isao YAMASHITA
Yuya MACHIDA
Shouichi YAMAUCHI(1750 ℃ ) is also required.

1．Introduction

Figure 1 shows the developed translucent zirconia

Transparent and translucent ceramics have beenceramics. Colorless translucent ceramics is obtained.

studied extensively since the seminal work on

The sample size is 1 mm thick with a diameter of116 mm. The optical and mechanical properties are

Subsequently, researchers have carried out many

tabulated in Table 1 Density of the sintered body is

studies to develop transparent ceramics, such as MgO,

6.08 g/cm3, calculated by the Archimedean method;

Y2O3, MgAl2O4 (spinel), and Y3Al5O12 (YAG).2 Improving

its density is almost same as that of 3Y-TZP. The total

light transmittance of zirconia has also been attracting

forward transmission of translucent zirconia at D65 is

3-5>60 % (thickness 1mmt), and the internal transmission

For example, fully stabilized zirconia (FSZ) is(without reflection loss) reaches 80 %. This translucency

extensively studied since cubic crystal structure canoverwhelms that of 3Y-TZP, and the translucency is

be transparent. Although highly transparent ceramics

comparable with that of translucent alumina ceramics. 6with transmittance value that reaches that of single

The bending strength is estimated to be about 800-translucent alumina polycrystal by Coble in the 1960s.

much attention because of its industrial importance.

5

crystal was developed for cubic zirconia ceramics , its

1000 MPa, much higher than that of cubic structure

mechanical properties are poor because transformation
toughening mechanism does not work. Its bendingstrength is about 300 MPa; fracture toughness is about
1.8 MPa·m0.5. On the other hand, tetragonal zirconia
polycrystal (TZP), such as 3Y, is well known as highstrength and high-toughness ceramics. However, withpoor transmission, its typical total forward transmission
at D65 is 36 % at 1mmt thickness. We have developed
novel translucent zirconia ceramics with both hightranslucency and high-strength. This is the first report
regarding our novel translucent zirconia ceramics. InFigure １La-doped translucent zirconia ceramics(∅16 mm in diameter and 1 mmt in thickness)Table１

Optical and mechanical properties of La-doped
translucent zirconia ceramics sintered at 1750 ℃.

this technical report, optical and mechanical properties
are briefly introduced.

2．La-doped translucent zirconia ceramics.
Highly translucent zirconia ceramics with highstrength was developed using lanthanum (La) as
a novel stabilizer. The sintering process of thetranslucent zirconia is similar to that of transparent
zirconia ceramics. 5 The ceramics was sintered using
hot isostatic pressing (HIP); high temperature sintering

Density
6.08 g ／ cm3
Total forward transmission［1mmt, D65］ 60 ％Internal transmission［1mmt, D65］
80 ％Three−point bending strength
850 MPa
Biaxial strength
1000 MPa
Fracture toughness
3.3 MPa·m0.5
Hardness［Hv10］
1200
Thermal conductivity
1.8 W ／ m·K

TOSOH Research & Technology Review Vol.60（2016）70transparent zirconia. 5

the transmission nearly reaches a theoretical value
estimated from reflection loss (≅ 26 %). The absorption
edge of the ceramics is estimated to be 250 nm, which

3．Optical properties

is almost same as that of YSZ single crystal. The total

Detailed transmission properties were measuredusing spectrophotometer and photogoniometer.

forward transmission of La-doped zirconia ceramics is
comparable to that of translucent alumina ceramics.

Schematic figure of the optical system is shown in

Scattered light profile of the transmitted light

Figure 2 The sample was ground to a thickness of 1

was also evaluated using photogoniometer (GP-200,mm and thoroughly polished on both sides to eliminate

Murakami Color Research Laboratory) . Halogen lamp

surface scattering for the subsequent measurements.

was used as light source for the photogoniometer.

The total forward transmission of ultraviolet-visible

Figure 4 shows normalized amplitude of scattered light.

(UV-VIS) region was measured using a double-beam

Scattered light mainly focuses on direct transmission

spectrophotometer (V-650, JASCO Corporation) withregion ( θ =0), displaying good transparency. Thus,a wavelength range of 200-800 nm. Figure 3 showsdeveloped La-doped zirconia ceramics owns excellent

wavelength dependence of total forward transmission

visible-light transmission properties.

of the ceramics. High total forward transmission invisible light region was observed; the transmission
at 600 nm is about 65 %. In the 700-800 nm region,

4．Mechanical properties
Three-point bending strength was measured
following JIS-R-1601 with a gauge length of 30 mm
and a cross-head speed of 0.5 mm/min. The fractureLighttoughness was estimated using the single-edge
Sampleprecracked beam (SEPB) method following JIS-R-1607.

Integrating sphere

The typical bending strength and fracture toughness

Detectorof the ceramics are estimated to be about 850 MPa and(a) Total forward transmission.

3.30 MPa·m0.5, respectively. Biaxial flexure strength
following ISO/DIS6872 is about 1000 MPa. Small
discrepancy between three-bending strength and biaxialθLight

strength was found. This may attribute to difference

Detectorof effective volume tested and surface flaws due toSample

machining process. Vickers hardness following JISθ: Scattering angle

R1610 is 1200, almost same as that of 3Y-TZP. Thermal(b) Photogoniometer.

Optical system of (a) total forward transmissionand (b) photogoniometer80

conductivity following JIS R1611 is estimated to be 1.8
W/m·K, slightly lower than as that of 3Y-TZP (2.4 W/100Theoreticallimit 74 %6080
Intensity (a.u.)

Total f orw ard transmission (% , 1mmt)

Figure ２40206040200200300

400500600700

800

0-30

Wavelength λ(nm)Figure ３

Total forward transmission (sample thickness 1mm)of La-doped zirconia ceramics

Figure ４-20-100
Scattering angle

10θ(deg)

2030

Scattered light profile of the transmitted lightat D65 (sample thickness 1mm)

東ソー研究・技術報告第 60 巻（2016）

m·K).

71

◎ Monoclinic phase

Bending strength and fracture toughness ofdeveloped zirconia are much higher than that ofalumina in bending strength.

7

We can conclude thatdeveloped novel translucent zirconia ceramics shows
excellent mechanical properties as compared toIntensity (arb. unit)transparent zirconia. It excels that of translucent◎
◎◎◎◎

◎

◎

previous transparent/translucent ceramics.

Fracture surface

4．Structural properties

800

600

200Surface-1

Raman shift (cm )In order to clarify a mechanism of its translucent
and strengthened nature, precise structural study

400

Figure ６

has been conducted by means of high-resolution
transmission electron microscopy (HR-TEM) and

Raman spectra of fractured surface and polishedsurface. Peak of monoclinic phase indicates as
double circle

Raman spectrometric methods. Figure 5 shows highresolution TEM-EDS mapping of lanthanum (La). The

cubic and tetragonal phases were observed fornano-sized tetragonal and cubic domain (<50 nm)

polished surface. Raman spectra of the fracture

were observed. Detailed structural studies show thesurface clearly shows that the tetragonal phasenovel zirconia ceramics was crystallized into fluorite

underwent transformation into monoclinic phase understructure, and tetragonal (P 42/nmc ) and cubic (Fm 3m )tensile stress. Therefore transformation toughening

phase were detected. The lanthanum was detected inmechanism is responsible for high-strength andcubic phase, but not in tetragonal phase. The phase

toughness of the La-doped translucent zirconia.

separation size is much smaller than that of visible lightwavelength. According to light scattering theory, there
is no scattering in nano-size scattering origin. The effectof the scattering must be negligible because scattering
4

efficiency Q sca reaches zero.

Conclusion
We have pioneered highly translucent zirconia

Thus, a reduction of theceramics with high strength using lanthanum (La) as

size of heterogeneous structure reduces effect of lighta novel stabilizer. Its bending strength is much higher

scattering; it can be attributed to the high translucency.

than that of cubic structure transparent zirconia, andFigure 6 shows Raman spectra of fractured andpolished surface. Additional peak due to monoclinic
phase was clearly found on fracture surface while

Cubic
phase

Tetragonal
phase

Figure ５

High resolution EDS mapping of La-atom. Blue
area represents cubic phase including La, while
black areas is tetragonal phase

Figure ７

Translucent La-doped zirconia made by ceramics
injection molding (CIM) technique (30 mm×25 mmand 1 mmt in thickness)

TOSOH Research & Technology Review Vol.60（2016）72

its translucency also overwhelms that of 3Y-TZP. The
translucent zirconia can be molded using ceramic
injection molding (CIM) technique (Figure 7) to makecomplex shaped bodies. The La-doped translucent
zirconia can be used as light transmittance material
where high mechanical properties are required.
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