From Production Skills to Scientific Knowledge,
and Onward to Fusion with Medical Science

Product Report and Basic Points about“KZR-CAD NANOZR”
Achieving High-strength Zirconia with
Excellent Impact Resistance

YAMAKIN CO., LTD.

Head Office: 3-7 Sanadayama-cho Tennoji-ku Osaka 543-0015, Japan
Biological Science Safety Laboratory:
Laboratory in the Department of Oral and Maxillofacial Surgery, Kochi Medical School, Kochi University
Kohasu, Oko-cho, Nankoku-shi, Kochi 783-8505, JAPANBranch Office: Tokyo, Osaka, Sendai, Nagoya, Fukuoka, JAPAN
P: +81-887-55-0281 F: +81-887-55-0053
E: contact@yamakin-gold.co.jp

Edited by YAMAKIN Ph. D. Group
INTERNATIONAL20190131

Index
1. Introduction ・・・・・・・・・・・・・・・・・・・・・・・・・ 2
2. Material characteristics ・・・・・・・・・・・・・・・・・・・ 6
3. Mechanical properties・・・・・・・・・・・・・・・・・・・・ 7
3.1 Basic characteristics of zirconia ・・・・・・・・・・・・・・・・・・ 7
3.2 Impact resistance confirmed by ball drop test ・・・・・・・・・・・・・ 15
3.3 Breaking coping test assuming clinical practice ・・・・・・・・・・・・・ 17
4. Chemical properties ・・・・・・・・・・・・・・・・・・・・・ 18
5. Hydrothermal deterioration resistance property ・・・・・ 19
6. Adhesion ・・・・・・・・・・・・・・・・・・・・・・・・・・・ 20
7. Biocompatibility ・・・・・・・・・・・・・・・・・・・・・・・ 22
8. Conclusion ・・・・・・・・・・・・・・・・・・・・・・・・・・ 24

Supervision
YAMAKIN Ph. D. Group
Teruo Anraku (Ph. D. in Engineering)
Hiroyuki Itoigawa (Ph. D. in Science)
Takahiro Kato (Ph. D. in Engineering)Takeshi Sakamoto (Ph. D. in Pharmacy)
Yuji Sato (Ph. D. in Philosophy)
Hidekazu Tanaka (Ph. D. in Engineering)
Ritaro Matsuura (Ph. D. in Agriculture)
Masatoshi Yamazoe (Ph. D. in Dentistry)
Hirohisa Yamamoto(Ph. D. in entrepreneur engineering)
Advisor of YAMAKIN Ph. D. Group
Bunichiro Yamada (Ph. D. in Engineering)
What is the YAMAKIN Ph. D. Group?
This is a group of experts in various specialized fields who combine their knowledge, experience andtechnical expertise to act as a prime mover in the continuous generation of innovation.

Basic points about “KZR-CAD NANOZR”
and Product Reportfracture toughness value 3). When stress is applied to Y-TZP, the crystal structure of tetragonal tomonoclinic crystal occurs with about 4% volume expansion, and compressive stress is generated around
it. This is called stress-induction phase transformation, and compressive stress acts on the tip of thegenerated crack, thereby preventing crack propagation. On the other hand, Y-TZP exhibits excellent

Yamazoe, M., Ph. D. in Dentistry, Executive Officer, Head Researcher, Academic Department
Kato, T., Ph. D. in Engineering, Executive Officer, Head Researcher, Development Department
Matsuura, R., Ph. D. in Agriculture, Senior Chief Researcher, Biological Science and Safety Laboratorychemical stability as a ceramic material, but in a hydrothermal environment, characteristics of
low-temperature degradation are known in which the phase transformation is promoted from a

Tanaka H., Ph. D. in Engineering, Chief Researcher, Development Department

tetragonal to a monoclinic crystal, degrading its mechanical properties. This phase transformation is

Narikiyo H., Master of Engineering, Project leader, Inorganic Material Development Section

considered as one of the causes of the deterioration of strength. In order to function stably in the harshenvironment of the oral cavity over the long term, excellent durability is required, and thislow-temperature degradability is one of the factors causing instability.

1. Introduction
In case of tooth loss due to caries, periodontal disease, or accidents, the role of dental prostheses is to use
artificial materials to restore the shape, mechanical function, and aesthetic function of missing teeth. In
the field of dental care, significant emphasis is placed on the appearance of the prosthesis, including
aesthetic blending with adjacent teeth and reproduction of the coloration of natural teeth.

Ceria-stabilized tetragonal zirconia-based nanocomposite ceramic (nano-zirconia) was developed by
Nawa, et al 4, 5). This material incorporates nano-sized alumina particles into the crystal grains oftetragonal zirconia (Ce-TZP), which are stabilized with ceria. Similarly, nano-sized Ce-TZP particles areincorporated into alumina crystal grains; as a result, nano-zirconia has high fracture toughness andexcellent resistance to low-temperature degradation 6). These characteristics enable a frame thickness of

Aesthetic dental treatment satisfies the public demand for whitened teeth, and contributes to improving

0.3 mm—similar to metal—to be made using nano-zirconia as the frame of the prosthesis and enables

quality of life. In such treatment, porcelain-fused-to-metal crowns have long been used for repairing

reduction of the cutting amount of abutment teeth.

teeth. However, because porcelain-fused-to-metal crowns have a metal frame, the gingiva near thecervical part is liable to appear dark and gingiva blackening may occur where the metal comes into

As described above, although nano-zirconia has excellent mechanical properties compared with Y-TZP, it

contact with the gingiva. Furthermore, since the prosthesis is continuously present in the harsh

has minimal translucency, which is important in tooth color reproduction. For this reason, it is essential

environment of the oral cavity over long periods, metal allergy due to the eluate from the metal frame is

to use porcelain materials for facing teeth, in combination with dental techniques to reproduce the color

regarded as a problem, and new methods of metal repair have been sought.

tones of natural teeth or adjacent teeth. By contrast, this opacity can negate the influence of the materialand color of abutment teeth, so there is the advantage that it can be done using same building techniques

The affinity and compatibility of dental materials to the living body are important issues, as well as thefunctional expression of these materials as a prosthesis. Biological harmfulness such as metal allergy iscaused by the biological or chemical change of materials in the oral environment, and the metalcomponents contained in the material may induce change in the body tissue by dissolution or physicalcontact 1).

as fabricating crowns made of porcelain-fused-to-metal. Therefore, nano-zirconia —which has excellent
mechanical properties and biocompatibility—appears to be of very high clinical value as a useful
substitute for metal. We would be pleased if this report helps to inform practitioners about technicalinformation such as why nano-zirconia is different from the yttria-type zirconia widely used in dentistry,
and why nano-zirconia exhibits such characteristics.

In recent years, all-ceramic prostheses that do not use metal, known as metal-free prostheses, have been
widely applied clinically in order to advance high-precision processing technology through the
CAD/CAM system and to reduce the risk of metal allergy 2). In general, due to their excellent chemical
stability and mechanical properties, ceramics have become widely used as an alternative material at sites
where metallic materials have previously been used, for purposes such as implant materials and artificial
joints. However, one disadvantage of ceramics is that their tensile strength and impact strength areinferior to metal materials. Since ceramic is a hard and brittle material, its long-term reliability is poor,and there are constraints on its use for parts requiring impact resistance.
Fracture toughness functions as an index for evaluating impact strength, that is, brittleness. Tetragonal
zirconia (Y-TZP), which uses yttria as a stabilizer, can be mentioned as a ceramic material with high―2―

Note: “KZR-CAD NANOZR” is an original product of YAMAKIN CO., LTD.

―3―

Product line-up and processing examples of “KZR-CAD NANOZR”"KZR-CAD NANOZR" has the greatest line-up of thicknesses (10 types) in the "KZR-CAD series," andits sturdy mechanical features and precise machining accuracy are deployed for use in a wide range ofapplications, including single crowns to long-span bridge frames, etc. This is a photograph of a typicalexample of this product in use.

Bridge frame made of NANOZRNANOZR frames with an access holeAppearance of KZR-CAD NANOZRNANOZR bridge (Left: frame; Right: building up with porcelain)Line-up of thicknesses of KZR-CAD NANOZR

Bone-anchored bridge frame and abutments(Fixed with a titanium base abutment and resin cement)

NANOZR disc and examples of processing

―4―

―5―

2. Material characteristics

3. Mechanical properties

The raw material powder of NANOZR is an original product manufactured by Daiichi Kigenso Kagaku
Kogyo Co., Ltd. It mainly consists of zirconium oxide (ZrO2), cerium oxide (CeO2) as a stabilizer, andalso alumina (Al2O3). The overall ratio is shown in Table 1. Since alumina is present at 10 wt% or more,
there is no translucency, and the color tone is pale yellow due to the effect of cerium oxide. The crystal
structure is the eutectic of tetragonal zirconia and alumina. In addition, toughening is achieved by theformation of a nanocomposite structure in which the respective nanoparticles mutually diffuse inzirconia and alumina particles. Figure 1 is an image of a nanocomposite structure, and Figure 2 is an
SEM image (S-3500N: Hitachi High-Technologies Corporation) of sintered NANOZR.

3.1 Basic characteristics of zirconia
For evaluation of the mechanical properties of zirconia products, "KZR-CAD zirconia" T, HT, and SHT—
which are yttria-type zirconia—and NANOZR were prepared as test specimens for comparison. As anevaluation of mechanical properties, sintered density, linear thermal expansion coefficient, bending
strength, Vickers hardness, and fracture toughness were evaluated, and impact tests were carried out by
dropping steel balls (ball drop test). Also, the fracture strength test of the sintered body modeled after asingle crown coping, assuming a molar tooth, was carried out.
Table 2 Material properties of each sintered zirconia
NANOZR

Table 1 Composition of NANOZR
Components

Ratio(wt%)*

ZrO2 + HfO2

76.6

CeO2

11.0

Al2O3

12.4

Fe2O3

0.03

HT

SHT

Al2O3
NANO Ce-TZP
Ce-TZP
NANO Al2O3

Note

Thickness of test
specimen: 0.5 mm

Appearance*XRF

Transmittance (%)

0

33

43

51

Thickness of test
specimen: 1 mm

Density (g/cm3)

5.54

6.06

6.07

6.03

By the Archimedean
MethodRelative density (％)

100

99.9

99.9

99.8

Linear thermal expansion
coefficient (×10-6/°C)

10.1

10.8

10.8

10.2

ISO 6872: 2015 8）

Biaxial flexure strength (MPa)

1270

1380

1370

850

ISO 6872: 2015 8）Three-point bending
strength (MPa)

1110

1280

1080

770

ISO 6872: 2015 8）

Vickers hardness (HV)

1240

1340

1370

1410

ISO 14705: 2016

12.1

4.5

4.3

2.6

ISO 15732: 2003

21.3

5.5

5.4

3.2

IF 法 11）

1.0

4.0

2.4

4.5

ISO 6872: 2015 8）

Fracture toughness(MPa•m1/2)

Figure 1 Image of a nanocomposite structure
Chemical solubility (µg/cm2)

A range of different appearances of sintered zirconia (thickness: 0.5 mm) is shown in Table 2. NANOZR
has a pale yellow color tone and is opaque so that lines drawn under the test specimen cannot be visually
recognized. On the other hand, T, HT, and SHT are white and translucent, and lines drawn under the test
specimens can be seen. In particular, it can be confirmed that SHT has higher light transmission than T
and HT. Since NANOZR, with a thickness of 1 mm, does not exhibit translucency, it is not suitable forcrown bridges as monolithic zirconia from an aesthetic point of view. However, sufficient aesthetics canbe obtained by using porcelain for zirconia in combination to build upon a crown, and firing it.

Ce-TZP
Al2O3
1 μm

Figure 2 SEM image of sintered NANOZR―6―

TThe density of each sintered zirconia specimen was measured by the Archimedes method. The relative
density of each test specimen was 99.8% or more, and it was judged that there was no deterioration inlight transmission due to residual pores.

―7―

The coefficient of thermal expansion was measured according to the standards for dental ceramicmaterial, ISO 6872: 2015 8). For the test, four zirconia sintered specimens (4.0 mm × 4.0 mm × 20.0 mm)
were prepared and temperature was raised from room temperature to 520 °C with heating at a rate of 5
°C/min and loading 147 mN with a thermomechanical analyzer (TMA 8310: Rigaku Corporation). The
average coefficient of thermal expansion from 50 °C to 500 °C was then measured. The measurement
results of the liner thermal expansion coefficient are given in Table 2.
Biaxial flexure test was carried out according to ISO 6872: 2015. The ten mirror-polished sinteredzirconia specimens (diameter 14.0 mm, thickness 1.3 mm) were prepared and the test was carried out
with a precision universal testing machine (Autograph AG-X: Shimadzu Corporation). As the test
conditions, three hardened heat-treated steel balls with a diameter of 4.0 mm were positioned 120° aparton a support circle with a diameter of 11.0 mm. From the central upper surface of a test piece placed
concentrically on this steel ball, the end face of a cylinder with a diameter of 1.4 mm was loaded onto thetesting machine at a cross-head speed of 1.0 mm/min.

For comparison, the three-point bending test was conducted according to ISO6872: 2015. Thirty mirrorpolished sintered zirconia specimens (4.0 mm in width, 20.0 mm in length, 1.2 mm in thickness, chamferedby 1.0 mm) were prepared and a small desktop testing machine (EZ-Graph: Shimadzu Corporation) wasused for testing.
As a test condition, a load was applied at cross-head speed of 1.0 mm/min using a jig having a support
point distance of 14.0 mm, a support rod and a load plunger diameter of 1.6 mm.
The three-point bending strength was calculated by the following equation (2). The test result is given in
Figure 4.
3Pl
(2)
σ＝
2wb2
where
σ is the three-point bending strength [MPa].
P is the breaking load [N].
l is the test span [mm].
w is the width of the specimen [mm].
b is the thickness of the specimen [mm].

The biaxial flexure strength was calculated by the following equation (1). The Poisson's ratio ν was set to0.31 for each zirconia specimen. The results of the test are given in Figure 3.
-0.2387P(X-Y)

(1)

b2

where
σ is the biaxial flexure strength [MPa].
P is the total load causing fracture [N].
X is (1+ν) ln(r2/r3)2 +[(1-ν)/2](r2/r3)2.
Y is (1+ν)[1+ln(r1/r3)2]+(1-ν)(r1/r3)2.
b is the specimen thickness at fracture origin [mm].
whereν is the Poisson's ratio.
r1 is the radius of support circle [mm].
r2 is the radius of loaded area [mm].
r3 is the radius of specimen [mm].

1500

1000

500

0

NANOZRT

HT

Vickers hardness HV was tested according to ISO 14705: 2009 9), stipulating the hardness test method forfine ceramics. Using a microhardness tester (MVK-EⅡ: Akashi Seisakusho, Ltd.), a Vickers indenter waspressed into a mirror-polished sintered zirconia specimen (10.0 mm × 10.0 mm × 1.0 mm) under the test
force of 9.807 N and a loading time of 15 sec, then diagonal length of the indentation was measured. The
test was carried out by press-fitting into 5 points on each sintered zirconia specimen, and the value wascalculated from the measured values and the following equation (3) (Figure 5).

1000

HV = 0.1891 ×
500

0NANOZRTHT

SHT

Figure 3 Biaxial flexure strength of each sintered zirconia specimen

―8―

SHT

Figure 4 The three-point bending strength of each sintered zirconia specimen

1500

Biaxial flexure strength (MPa)The three-point bending strength (MPa)

σ＝

F
d2

where
HV is the Vickers hardness.
F is the load force [N].
d is the diagonal length of the indentation [mm].

―9―

(3)

Test specimen

Vickers indentation

L

1000

C
W

Vickers hardness (HV)

B

1500

B

500

L ：Overall length of test (40 mm)
B ：Thickness of test specimen (3.0±0.1 mm)
W ：Width of test specimen (4.0±0.1 mm)
C ：Chamfering (0.1～0.3 mm)

0

NANOZRTHT

SHT

Figure 5 Vickers hardness of each sintered zirconia specimen

Figure 6 Size of test specimen

SupportPre-crack

S/2

While various methods for measuring fracture toughness value have been reported, in the ISO 6872: 2015
standard for dental ceramic materials, the Single-Edge V-Notch Beam (SEVNB) method is recommended,although it is not stipulated as an essential test. The Single-Edge Pre-cracked Beam (SEPB) method,
Surface Crack in Flexure (SCF) method, and Chevron Notched Beam (CNB) method are also given as
alternative methods for this test: however, it is considered that obtaining values by the crack length fromthe corner of the Vickers indentation is inadvisable.
On the other hand, the ISO 15732: 2003 10) standard for the test method for room-temperature fracture
toughness of fine ceramics specifies the SEPB method and the IF (Indentation Fracture) method to obtainfrom the crack length from the corner of the Vickers indentation.
The SEVNB method, recommended in the dental ceramic material standard ISO 6872: 2015, is very heavily influenced by the way in which the V notch tip is formed, and it is difficult to conduct testing with good
reproducibility. Therefore, evaluation was carried out by the SEPB method, which was introduced as analternative method in an annex of ISO 6872: 2015 mentioned above, and which was adopted in ISO 15732:
2003.
The fracture toughness test by the SEPB method was conducted with reference to ISO 15732: 2003. Figure
6 shows the shape of test pieces and Figure 7 is an image of a test jig.

SupportW

Fracture toughness value was mentioned at the beginning as an index to evaluate impact strength—that is,brittleness —tetragonal zirconia using yttria or ceria as a stabilizer shows a particularly high value among
ceramics materials. This is due to the stress-induced phase transformation mechanism, and in ceramics it
is a feature found only in zirconia.

Test specimen

D

NANOZR has a structure of intergranular nanocomposites in which nano-sized Ce-TZP and Al2O3 grainsinterpenetrate. Its biaxial flexural strength was 1270 MPa, and the three-point bending strength was 1110
MPa. Vickers hardness HV showed comparable values for each sample.

S/2
L

D：Diameter of support (4.0 mm)
S：Support point distance (16.0±0.2 mm)
Figure 7 Testing jig
For the test, the procedures until the sintering of the test specimens were conducted in-house, and the procedures from the adjustment of the specimens to testing were carried out at JFCC (Japan Fine Ceramics Center).
For introduction of pre-crack, the indentation load of Vickers indenter at 98 N was added for T, HT, and SHT.
It was impossible to introduce pre-crack on NANOZR at 98 N; as a result, a pre-crack initiation point was
introduced at 490 N and pop-in crack was introduced as pre-crack using a compression jig and an anvil.
Fracture toughness value KIc was calculated using equation (4) (Figure 8).

（

KIc ＝

P×S
3
B ×W 2a×Y）｛（｝
2 W ） （W ）
×

3

a

a 1/2

a

a

a

(4)

× 2.15 - 3.93 W +2.7
［ 1.99 - W（1 - W ）
｛
（W ）｝
］
＝
a
a
（ ）
（1+2 W ）×（1- W ）
a
Y
W

2

3
2

where
KIc is the fracture toughness [MPa·m1/2].
P is the breaking load [N].
S is the support span [m].

― 10 ―

B― 11 ―

B is the specimen length [m].
W is the specimen width [m].
a is the pre-crack length [m].

Fracture toughness value (MPa•m1/2)

14
12
10
8
6
4

NANOZR

T

HT

SHT

2
0

NANOZRTHT

SHT

Figure 8 Fracture toughness value of each sintered zirconia specimen (SEPB method)(Tested by JFCC (Japan Fine Ceramics Center))In addition, in order to confirm the influence due to the difference in the test method, fracture toughness
value KIc, obtained from the crack length from the corner of the Vickers indentation, is calculated from
Niihara's equation 11) (5) and (6) by IF method. Using a hardness tester (HV-113: Mitutoyo Corporation), a
Vickers indenter was pressed into a mirror-polished sintered zirconia specimen (10.0 mm × 10.0 mm × 3.0mm) under a test force of 196 N and a loading time of 15 sec; the indenter made indentation and cracks,
and then length of the cracks was measured. The test was carried out by press-fitting into 5 points on eachsintered zirconia specimen.

Figure 9 Optical micrograph images of indentation caused by indentation fracture

Figure 9 shows the optical micrographs of indentation caused by indentation fracture. The length of thecrack from the corner of the indentation of NANOZR is shorter than that of T, HT, and SHT, which are
made of yttria-type zirconia, and the indentation is close to that of a plastically deforming metallic material. On observation, the vertical section of the indentation had a semi-elliptical shape from the corner ofthe indentation to the crack tip, and it was an initially generated Palmqvist crack. On the other hand, T,
HT, and SHT show long cracks from the corner of indentation compared to NANOZR. In observation of
the vertical section of the indentation, a median crack showing a semicircular shape from the crack tip tothe diagonal crack tip was confirmed. In particular, lateral cracks, which seems to have occurred at unloading, appear in a circle around the indentation, and median cracks and lateral cracks can be confirmed.
Figure 10 shows images of crack shapes caused by breakage, (a): Palmqvist-crack, (b): Median crack, (c):Lateral crack.

l

l
2a

(a)

P

c

P

P

2c

2d

2a

2a

(b)

(c)

Figure 10 Images of crack shapes caused by breakage(a): Palmqvist crack , (b): Median crack, (c): Lateral crack.

― 12 ―― 13 ―

In Palmqvist crack

（

）
（ ）

KIc
×
H × a1/2

（ ）

H 2/5
＝ 0.018 ×
E

l
a

-1/2

(5)

In median crack

（ ）

KIc
＝ 0.203 ×H × a1/2

c
a

- 32

3.2 Impact resistance confirmed by ball drop test
As an impact test (ball drop test), a sintered zirconia specimen processed to a diameter of 15 mm and a
thickness of 1 mm was fixed on a steel plate with resin cement (Super-Bond: Sun Medical), and steel ballsof different weights (7 g, 14 g, 24 g, 33 g, 65 g, 112 g, 175 g, 261 g, 372 g, and 508 g; Figure 12) were dropped
naturally from a height of 60 cm above the sintered zirconia specimen (Figure 13).

(6)
12 mm

KIc is the fracture toughness [MPa·m1/2].
a is the half of diagonal length of indentation [m].
c is the half of the crack length from the corner of the median crack [m].
l is the half of the crack length from the corner of the indentation of the Palmqvist crack [m].
E is Young's modulus [Pa].
H is the Vickers hardness.

7 g 14 g 24 g

50 mm

33 g

65 g

112 g

175 g

261 g

372 g

508 g

Figure 12 Steel balls used

In NANOZR, the form of the crack generated by the indentation is the Palmqvist crack, and furthermore,
the crack length is less than 1.5 times the diagonal length of indentation; thus, fracture toughness value KIc
was calculated according to Niihara’s equation (5).
On the other hand, in the yttria-based zirconia T, HT, and SHT, the form of crack was the median crack,and in each case, the crack length was larger than 2.5 times the diagonal length of the indentation, andfracture toughness was calculated from equation (6). Figure 11 gives the fracture toughness value calculated from Niihara's equation by the IF method.
60 cm

Fracture toughness value (MPa•m1/2)

30

25

20

15

10

Figure 13 Test method of impact test
50

NANOZRTHT

SHT

Figure 11 Fracture toughness value of each sintered zirconia specimen (IF Method)― 14 ―Firstly, starting from a small weight, a steel ball was dropped on each sintered zirconia specimen. The drop
impact load was repeated three times with the steel ball of the same weight; heavy steel balls were thensimilarly dropped with and the impact was applied until the sintered body was broken. Table 3 shows thetest results of repeated drop and impact loading. NANOZR was broken when a steel ball of 508 g was
dropped three times. T was broken at the third drop of 175 g, HT was broken at the first drop of 65 g, andSHT was broken at the first drop of 33 g.

― 15 ―

Table 3 Impact resistance of each sintered zirconia specimen in repeating impact loading
Steel ballT

NANOZRHTSHT

Weight

1st

2nd

3rd

1st

2nd

3rd

1st

2nd

3rd

1st

2nd

3rd

7

○

○

○

○

○

○

○

○

○

○

○

○

14

○

○

○

○

○

○

○

○

○

○

○

○

24

○

○

○

○

○

○

○

○

○

○

○

○

33

○

○

○

○

○

○

○

○

○

×

―

―

65

○

○

○

○

○

○

×

―

―

―

―

―

112

○

○

○

○

○

○

―

―

―

―

―

―

175

○

○

○

○

○

×

―

―

―

―

―

―

261

○

○

○

―

―

―

―

―

―

―

―

―

372

○

○

○

―

―

―

―

―

―

―

―

―

508

○

○

×

―

―

―

―

―

―

―

―

―

3.3 Coping break test assuming clinical practice
The fracture strength test of the sintered body modeled after a single crown coping, assuming a molar
tooth, was carried out by the central loading method with a universal testing machine (EZ-Graph;
Shimadzu) (Fig.15). The thickness of each zirconia coping was 0.5 mm (NANOZR had thicknesses of 0.5mm and 0.3 mm), and the surface was buffed with 1 μm diamond paste.
Zirconia coping was fixed with resin cement (Super Bond; Sun Medical, cement space set to 100 μm). As
the test conditions, a steel ball with a radius of 2 mm was loaded at the center of the upper surface of thespecimen at a velocity of 1 mm/min, and the load until breaking was measured. A resin (KZR-CAD HR
Block 3 GAMMATHETA: YAMAKIN), which has an elasticity coefficient close to natural tooth, was used
for the abutment. The result of the test was that NANOZR exhibited high breaking strength at both coping
thicknesses of 0.5 mm and 0.3 mm, compared to yttria-based zirconia (Fig.16). It was recognized that
NANOZR has excellent mechanical characteristics combining the above instantaneous impact resistanceand high breaking strength.

Next, the steel ball was dropped once for each sintered specimen, that is, as a single impact loading. Threetest specimens were tested with steel balls of each weight. The impact force at fracture was calculated fromthe weight of the steel ball at the time of fracture by using the following formula (7), even it was the first
specimen of the 3 specimens. The falling velocity was fixed and the collision time was 1 ms. The result of
the test was that each zirconia test specimen was broken at the same weight as the previous repeated impact
loading. The impact force at the time of breakage of NANOZR was 1,742 N, and the impact resistance of
NANOZR was 2 to 15 times or more than that of the yttria-type zirconia (Figure 14).
F=m √2ghΔt

where
F is the impact force [N].
m is the weight of a steel ball [kg].
g is the acceleration of gravity [m/s2].

(7)

h is the falling distance [m].
Δt is the collision time [s].

Figure 15 Test specimen for breaking-strength test assuming a single crown coping, and the tester
3000

508 g
2500

2000

Breaking loading (N)1800

Impact resistance (N)

1600
1400
1200

175 g

1000

2000

1500

1000

800
600

500

65 g
33 g

400

0

200
0

NANOZRTHT

SHTNANOZR
0.3 mmNANOZR
0.5 mmT
0.5 mmHT
0.5 mmSHT
0.5 mmFigure 14 Impact resistance of each sintered zirconia specimen in a single impact loading

Figure 16 Breaking strength of each sintered zirconia specimen― 16 ―― 17 ―

4. Chemical properties

5. Hydrothermal deterioration resistance property

In the case of restorations to be placed in the oral cavity, chemical durability against saliva and food anddrink is required; ceramics like zirconia have a strong bond strength between atoms and have excellent
chemical durability. Metals dissolve as metal ions due to corrosion, accompanied in some cases by
discoloration; however, ceramic is chemically stable and can maintain good aesthetics over long periodsof time. As described above, corrosion of ceramic as an oxide in the oral cavity is thus unlikely, and it canbe said that ceramic restorations are extremely stable compared with metal- and resin-type restorations.
Its properties were verified by dissolution tests.

Although zirconia shows excellent chemical durability, it has been reported that phase transformation(tetragonal to monoclinic) progresses even at a low temperatures close to body temperature in thepresence of moisture 12-15), and deterioration under the presence of moisture in the oral cavity is aconcern.

The dissolution amounts of NANOZR, T, HT, and SHT were measured in accordance with ISO 6872:
2015. For the test, six sintered zirconia specimens (10.0 mm × 25.0 mm × 1.0 mm) were prepared, andthe total exposed surface area of the specimens was adjusted to 30 cm2 or more. Sintered specimens were
immersed in 100 mL of 4 vol% acetic acid aqueous solution at 80 °C for 16 hours, and the dissolution
amount test was conducted (Figure 17).

For evaluation of hydrothermal deterioration property, each mirror-polished sintered specimen (4.0mm × 20.0 mm × 1.2 mm) of NANOZR, T, HT, and SHT was heated at 134 °C. using an autoclave
apparatus (SM 310: Yamato Scientific Co., Ltd.), with water-vapor pressure at 0.2 MPa for 5 hours(equivalent to 15 to 20 years in water at 37 °C), and 30 hours (equivalent to 90 to 120 years)16, 17). The
hydrothermally treated specimen was measured by X-ray diffraction (CuKα, 40 kV, 20 mA, XRD,
Ultima IV: Rigaku Corporation) in order to investigate the presence or absence of crystal phasetransformation accompanying hydrothermal treatment, and the monoclinic ratio fm was calculated
from the Garvie-Nicholson equation (8) 18).
fm =Dissolution amount (μg/cm2)

100

（8）

×100

The monoclinic ratio of each of the hydrothermally treated test specimens was determined by X-ray
diffraction, and the results are given in Table 4.
The monoclinic ratio of the as-sintered NANOZR was 4%, but after the hydrothermal treatment, thevalue became 5.3% in 5 hours, and 5.7% in 30 hours and the increase in monoclinic crystal was slight.
On the other hand, the monoclinic ratio of each zirconia T, HT, and SHT, both untreated and treatedfor 5 hours, was 0%. By treatment for 30 hours, monoclinic crystals of 4.7% in T and 14.6% in HT wereconfirmed, but monoclinic crystals were not observed in SHT. The addition of alumina is known tosuppress hydrothermal deterioration 19). HT is considered to be easily transformed into the monoclinic

80

60

40

20

0

|）
+ l（111l（111）
mm|）+ l（101）+ l（111+ l（111）l（111）mm
mc

NANOZRTHT

SHT

Figure 17 Dissolution amount of each sintered zirconia specimen
Although this test is an evaluation of conditions (pH 2.5, 80 °C.) more severe than the oral environment(average pH 6 to 7, 37 °C.), the dissolution amount of every specimen is minimal, and much lower than thestandard reference value (100 μg/cm2 or less (Class 6)) in ISO 6872: 2015.

phase by hydrothermal treatment, since the content of alumina is less than that of T. SHT is a
PSZ-type zirconia, with a larger proportion of Y2O3 as a stabilizer than T or HT, and is a tetragonal andcubic eutectic. Therefore, as the proportion of tetragonal phase-transforming to monoclinic systemdecreases, the propagation of stress or strain due to the volume expansion accompanying phasetransformation is suppressed by cubic crystal; as a result, in SHT, the phase from tetragonal tomonoclinic transformation hardly occurred, and it is thought that this is why monoclinic crystals were
not observed.
Table 4 The monoclinic ratio after aging treatment with an autoclave under steam at 134°C
Item― 18 ―

Aging time (hr)
0

5

30

NANOZR

4.0

5.3

5.7

T

0

0

4.7

HT

0

0

14.6

SHT

0

0

0

― 19 ―

6. Adhesion
Stainless steel rod

NANOZR has high strength and toughness and is effective as a frame material for long span bridges.
Figure 18 shows an example of the use of a bone-anchored bridge frame made of NANOZR. Since
NANOZR can be designed with less thickness than yttria-type zirconia, reduced risk of fracture is expected.

Resin Cement
Primer, Bonding Material
Masking tapeAlso, clinical application as a frame for denture bases, utilizing the toughness of NANOZR, has been
proposed. It is also possible to bond a resin crown onto a denture base made of NANOZR or to buildwith resin directly on a frame.
In this section, we introduce the results of evaluating adhesion between resin material and NANOZR.

Zirconia

Figure 19 Image of bonded cross-section

Alumina particles;
particle size 50 μWhen a primer is added

1) Sand-blasting and washing

When a bonding material is added

2) Applying primer
and drying4) Applying and drying bonding
material; irradiation

7) Keeping in water at
37°C for 1 day

8) Tensile test

10N

ia
ad
irr

n

tio

ia

ad

tio

n

irr

irradiation

3) Masking tape witha hole of 3 mm

Figure 18 Bone-anchored bridge frame made of NANOZR(Fixed with a titanium-base abutment and resin cement)

5) Applying resin cement;
irradiation

Adhesive resin cement is used when adhering a resin crown to NANOZR frame. As a pretreatment,
sand-blast treatment is applied to the adhesion surface to increase the adhesion area. After that, it isessential to apply primer or a bonding material containing a phosphoric acid monomer or a silane
coupling agent or the like on the surface of the zirconia in order to chemically bond with the adhesive
resin cement and to resinify the surface of the zirconia.

These test specimens were installed in a small tabletop material tester (EZ-Graph: Shimadzu
Corporation), and the tensile bond strength was measured by a tensile test at a crosshead speed of 0.5
mm/min. In this test method, no peeling occurred between the stainless steel rod and the resin cement.

Figure 20 Method of tensile adhesion test

45

Tensile bond strength (MPa)

Adhesive resin cement and the surface treatment material shown in Table 5 were adhered to NANOZR
and the comparative specimen SHT by the method shown in Figure 20, so as to be their bonded
cross-section as in Figure 19, and test specimens were fabricated.

40

NANOZR

35

KZR-CAD Zr SHT

30
25
20
15
10
5
0
Super Bond

Table 5 Adhesive resin cement and surface treatment materials
Manufacturer

Sun Medical

3M

Tokuyama Dental

Product nameSuper Bond
(Powder-liquid)

RelyXTM
Ultimate
Resin Cement

G-CEM
CERASMART

G-CEM
LinkForce

Block HC
Cem

PANAVIA® V5

Duo-LinkTM

Maxcem Elite™
ChromaType ofresin cement

Combined withprimerCombined with
Combined with Self-adhesive
bonding material primer
type

Combinedwith primer

Self-adhesivetype

Combined with primer

Combined withbonding material

Self-adhesivetype

Primer,Bonding material

Super BondPZ PRIMER(2 liquids)

ScotchbondTM
Universal
AdhesiveTOKUYAMAUNIVERSALPRIMER(2 Liquids)

G-MULTIPRIMER

Unnecessary

CLEARFIL®
CERAMICPRIMER PLUS

All-Bond
Universal

Unnecessary

ESTECEMGC

Unnecessary― 20 ―

Shofu

Kuraray Noritake

BISCO

Kerr

6) Setting a stainless steel rod(5mm) on a test specimen
with resin cement

RelyXTM
Ultimate
Resin Cement

ESTECEM

G-CEM
CERASMART

G-CEM
LinkForce

Block HC
Cem

PANAVIA®
V5

Duo-LinkTM

Maxcem Elite™
ChromaFigure 21 Initial tensile bond strength of NANOZR and SHT with each resin cement.
As shown in Figure 21, the tensile bond strength between zirconia and resin cement is 10 MPa or more
for all adhesive resin cements. The high tensile bond strength of 25 MPa or more was obtained when"Super Bond," " RelyXTM Ultimate Resin Cement," and "Block HC Sem" were used for NANOZR.

― 21 ―

7. Biocompatibility

90

Number of cells (×104 cells/mL)

Depending on the type of zirconia, the tensile bond strength with each adhesive resin cement wasslightly different. Particularly with Block HC Cem, NANOZR showed tensile bond strength about
twice that of SHT. The difference in tensile bond strength is thought to be influenced by the materialof NANOZR and SHT, and the adhesive component of the resin cement, primer and bonding material.
In addition, NANOZR shows a bonding strength of 20 MPa or more even with a self-adhesive typeresin cement, and it can be said that high adhesive strength can be obtained without using a primer ora bonding material in combination.

80
70
60
50
40
30
20
10
0

Control

NANOZRTHT

SHTNANOZR is a dental medical device (ceramics for dental milling and machining). Any medical device
which comes into contact with the human body needs assessments of biological safety for the patient or
the user. These assessments include cytotoxicity, sensitization and genetic toxicity.

Figure 23 Influence of each dental zirconia on THP.1 Cells (Cell number)In this section, we describe the evaluation of the cytotoxicity of NANOZR on THP.1 cells and on humanacute monocytic leukemia cells by the trypan blue dye exclusion test 20), which is a direct contact test.

Figure 23 shows cell proliferation of THP.1 cells cultured on each test specimen.
The number of cells cultured on each zirconia sample increased from 100,000 cells/mL at seeding to
600,000 cells/mL or more. In addition, there were no significant difference between the dental zirconia
samples and the reference.

Trypan blue

Living cell

100

80

Cell viability (%)

NANOZR, T, HT, and SHT were used as test specimens. Each test specimen was milled and sintered to adisc-like shape with diameter of 15 mm and a thickness of 1 mm, and then mirror-polished.
When cells are cultured on test specimens with cytotoxicity, the cell membranes are damaged by thecytotoxic component. Trypan blue can stain dead cells with damaged cell membranes, while it cannot
stain living cells. Therefore, the trypan blue dye exclusion test can evaluate the effect of a sample on cellproliferation and viability by counting clear cells (living cells) and stained cells (dead cells) with ahematocytometer (Figure 22).

60

40

20

Living cell0

Dead cell
Trypan blue

Control

NANOZRTHT

SHT

Figure 23 Influence of each dental zirconia on THP.1 Cells (Cell viability)

Dead cell

In addition, each test specimen also exhibited high cell viability (> 95%) as shown in Figure 24.
Figure 22 Principle of Trypan blue exclusion test
Each test specimen was placed in a well of a 24-well plate. One mL of THP.1 cells (100,000 cells/mL) was
seeded, and then incubated for 3 days at 37 °C under 5% CO2. Fifty μL of the cultured cells were mixed
with the same amount of trypan blue. Living cells and dead cells were individually counted by thehemocytometer. Cells cultured without test specimen were used as reference.

― 22 ―As described above, none of the zirconia test specimens, including NANOZR, showed cytotoxicity in cellnumber and cell viability of THP.1 cells.
As described in "4. Chemical Properties," zirconia is a material with excellent chemical durability. In the
dissolution test of “KZR-CAD NANOZR,” only ultra-trace amounts of zirconium were detected. Some
investigations have reported that the biological safety of zirconium is equal to or higher than that oftitanium. which has excellent biocompatibility 21). Therefore, it is considered that the results of thecytotoxicity test were based on the characteristics of the dental zirconia products, which excel not only inthe chemical durability of the material, but also in biological safety of the components.

― 23 ―

8. Conclusion

References

NANOZR showed higher values than yttria-based zirconia in terms of strength, especially fracture1) U Raap, M Stiesch, H Reh, A Kapp, T Werfel: Investigation of contact allergy to dental metals in 206
patients. Contact Dermatitis. 60(6):339-343, 2009.

toughness value, and in impact test. NANOZR is a material that enables a thickness of crown/bridge
frame of 0.3 mm, the same as metal, taking advantage of this characteristic.

2) S. BAN: Properties of zirconia for realization of all-ceramic restoration. The Journal of the Tokyo
Dental College Society, 107(6): 670-684,2007.
3) R. C. Garvie, R. A. Haaink, R. T. Pascoe: Ceramic steel. Nature, 258: 703-704, 1975.

The light transmission of NANOZR is lower than that of yttria-type zirconia. However, this is not only adisadvantage; it can cancel out the influence of the material and color tone of abutment teeth, so that it has also
the advantage that the same building-up technique can be performed as for porcelain fused to metal crown.
In terms of strength and aesthetics, NANOZR can be thought of as a material positioned between dental
bonding alloy and yttria-based zirconia. It can also be called a new choice in materials. In view of themetal allergy problem, there is a demand for metal-free materials, and NANOZR, which has properties
closer to metals than yttria-type zirconia, is expected to play an active part as a substitute material for
metals.

4) M. Nawa, N. Bamba, T. Sekino, K. Niihara: The effect of TiO2 addition on strengthening andtoughening in intragranular type of 12Ce-TZP/Al2O3 nanocomposites. J. Europ. Ceram. Soc., 18(3):209-219, 1998.
5) M. Nawa, S. Nakamoto, T. Sekino, K. Niihara: Tough and strong Ce-TZP/Alumina nanocomposites
doped with titania. Ceram. Int., 24(7): 497-506, 1998.
6) S. Ban, M. Nawa, Y. Suehiro, H. Nakanishi: Mechanical Properties of Zirconia/Alumina
Nano-Composite after Soaking in Various Water-Based Conditions, Key. Engin. Mater., 309-311,
1219-1222, 2006.
7) M. Nawa: Ceria-stabilized tetragonal zirconia based nanocomposite for biomedical applications, J. J.
Soc. Dent. Mater. Devic., 31(4): 289-292, 2012.
8) ISO 6872: 2015 - Dental ceramic materials.

In this report, we have mainly introduced the mechanical properties of NANOZR, but it has also beenreported that it exhibits excellent features in terms of biocompatibility, especially in connectivity withthe oral mucosal tissue. Further research is therefore desired in the future.

9) ISO 14065: 2000 - Test methods for hardness of fine ceramics.
10) ISO 15732: 2003 - Testing methods for fracture toughness of fine ceramics at room temperature.
11) K. Niihara, R. Morena, D. P. H. Hasselman: Evaluation of KIc of brittle solids by the indentation
method with low crack-to-indent rations. J. Mater. Sci. Lett., 1: 13-16, 1982.
12) J. Chevalier, B. Cales, J. M. Drouin: Low-temperature aging of Y-TZP ceramics. J. Am. Ceram. Soc.,
82: 2150-2154, 1999.
13) S. BAN: Zirukonia kei zairyou no shurui to tokusei (Kinds and characteristics of zirconia materials.
T. MIYAZAKI, H. MIURA, K. KIMURA (ed.): Design Operation Clinical Zirconia Restoration. The
International Journal of Dental Technology EXTRA ISSUE, Tokyo, Ishiyaku Pub, Inc. 22-37, 2010.
14) C. L. Maria, S. S. Susanne, A. Patrick, J. Mare, H. W. Anselm: Low-temperature degradation of a
Y-TZP dental ceramic. Acta. Biomater, 7: 858-865, 2011.
15) I. Yamashita, K. Tsukuma: Phase Separation and Hydrothermal Degradation of 3 mol% Y2O3 -ZrO2
Ceramics. J. Ceram. Soc. J., 113(8): 530-533, 2005.
16) J. Chevalier, J. M. Drouin, B. Cales: Low temperature ageing behavior of zirconia hip joint heads.
Bioceramics, 10:135-137, 1997.
17) J. Chevalier, L. Gremillard, S. Deville: Low-temperature degradation of zirconia and implications
for biomedical implants. Annu. Rev. Mater. Res., 37: 1-32, 2007.
18) R. C. Garvie, P. S. Nicholson: Phase Analysis in Zirconia Systems. J. Am. Ceram. Soc., 55(6):303-305, 1972.
19) H. Fujisaki, K. Kawamura, K. Imai: Zirconia powder for translucent dental zirconia: Zpex. Tosohresearch & technology review, 56: 57-61, 2012.
20) Correa GT, Veranio GA, Silva LE, Hirata Junior R, Coil JM, Scelza MF: Cytotoxicity evaluation of
two root canalsealers and a commercial calcium hydroxide paste on THP1 cell line by Trypan Blue
assay. J. Appl. Oral Sci., 17(5): 457-461, 2009.
21) M. FUJITA: In Vitro Study on Biocompatibility of Zirconium and Titanium. J. stomatol. Soc. Jpn, 60(1): 54-65, 1993.

― 24 ―― 25 ―

Lineup《Previously Published Technical Reports》
Technical Report on ZEO CE LIGHT (Aug. 2002)
Technical Report on Luna-Wing (May 2007)
Technical Report on TWiNY ( Jul. 2010)

KZR-CAD NANOZR《Previously Published Safety Test Reports》
Vol. 1 Pursuing International Standards in Quality and Safety (Dec. 2004)

Vol. 2 ZEO METAL Series Elution Test and in Vitro Cytotoxicity Test ( Jun. 2005)

Vol. 3 Elution Test and in Vitro Cytotoxicity Test of Precious-Metal Alloys and Gold Alloys for Metal Ceramic Restoration Use (Dec. 2005)
Vol. 4 Biological Evaluation of Luna-Wing ( Jun. 2006)

Vol. 5 Report on Physical Properties and Safety of High-Carat Gold Alloys (Oct. 2007)

Vol. 6 Examination of the Biological Impact of the Physical Properties of Dental-Material Alloys and Gold Alloys for Hard Resin and Metal Ceramic Restoration

Diameter(Φ) 98.3mm

Use (May 2008)

Thickness ( t )10 mm 12 mm 14 mm 16 mm 18 mm 20 mm 22 mm 25 mm 30 mm 35 mm

NANOZR

●

●

●

●

●

●

●

●

●

●

Vol. 7 Report on the Physical Properties and Safety of the Gold Alloy Nexo-Cast (Oct. 2008)
Vol. 8 Biological Evaluation of the Hybrid Composite Resin, TWiNY ( Jun. 2010)

Vol. 9 Chemical and Biological Characteristics of Precious-Metal Alloys: Elution Characteristics Produced Through Mixture with Titanium (Feb. 2011)
Vol. 10 Physical Properties and Safety of the Precious-Metal Alloy for Metal Ceramic Restoration Use Brightis (Oct. 2011)
Vol. 11 Physical Properties and Safety of the Dental Adhesive, Multi Primer (Mar. 2014)

Vol. 12 Safety of the Dental Pulp Capping Material, TMR-MTA CEMENT ( Jan. 2018)《Previously Published Macromolecule Technology Reports》
Vol. 1 The Polymerization of Dental Materials: The Basis of Radical Polymerization (1) (Oct. 2009)

KZR-CAD Zr T/HT/SHT

Vol. 2 The Polymerization of Dental Materials: The Basis of Radical Polymerization (2) (Feb. 2010)
Vol. 3 The Polymerization of Dental Materials: Restoration-Material Monomers (1) (Mar. 2010)
Vol. 4 The Polymerization of Dental Materials: Restoration-Material Monomers (2) ( Jul. 2010)
Vol. 5 The Polymerization of Dental Materials: The Influence of Oxygen (Aug. 2011)
Vol. 6 The Polymerization of Dental Materials: Primers and Developers (Oct. 2012)

Vol. 7 Polymerization Silane Coupling Agent: Methacrylic Resin (Acrylic Resin) ( Jun. 2013)
Vol. 8 Shrinkage of Dental Composite in Polymerization (Nov. 2014)

Vol. 9 Application of Iodonium Salt as Initiator Component in Dental Material (Mar. 2017)

Diameter(Φ) 98.5mm
Color shadeVol.10 Application of Nanogel to Dental Resin and Adhesive ( Jun. 2018)

Thickness ( t )10 mm 12 mm 14 mm 16 mm 18 mm 20 mm 22 mm 25 mm 30 mm 35 mm
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《All Previously Published Science Reports》

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Vol. 1 Dental Surgery and Bisphosphonate Formulation (Aug. 2010)

SHT

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―

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―

―

―

―

HT-A1, HT-A2, HT-A3, HT-A3.5

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―

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T
HT

SHT-A1SHT-A2, SHT-A3, SHT-A3.5

White

ColoredVol. 2 Reactive Oxygen: Its Generation, Elimination and Effects (Nov. 2011)
Vol. 3 The Hypoxic World ( Jul. 2012)

Vol. 4 Recent Progress in the Regeneration of Tooth Material (Feb. 2014)
Vol. 5 Application of Fluoride and Its Effect (Oct. 2016)《Previously Published Product Reports》
Basic Information and Product Report on Zirconia (Feb. 2014)
Basic Information and Product Report on Titanium ( Jun. 2016)
Basic Information and Product Report on Hybrid Resin for CAD/CAM use (Sep. 2014)
Basic Information and Product Report on Dental Restorative Material (Sep. 2015)
Basic Information and Product Report on Dental Bonding Material ( Jan. 2016)
TMR-MTA CEMENT Product Report (Aug. 2017)
Multi Primer Series Product Report (Oct. 2017)
KZR-CAD HR BLOCK 3 GAMMATHETA Product Report ( Jan. 2018)

The actual color of the product, model and package may differ from the photographs due to printing ink and shooting conditions.
Manufactured by

1090-3 Otani, Kamibun, Kagami-cho, Konan-shi, Kochi, 781-5451 Japan

Editor: Takahiro Kato
Publisher: Shigenari Yamamoto
Date of publication: 31 JAN. 2019