Dental Materials Journal 23 (3): 297-304, 2004

Development of Al2O3 Fiber-reinforced Al2O3-based Ceramics

Yasuhiro TANIMOTO and Kimiya NEMOTODepartment of Dental Materials, Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1 Sakaecho Nishi, Matsudo, Chiba 271-8587, Japan Corresponding author, E-mail: tanimoto@mascat.nihon-u.ac.jp

Received March 29, 2004/Accepted May 14, 2004

The purpose of this study was to use a tape casting technique to develop an Al2O3 fiber-reinforced Al2O3-based ceramic ma-

terial (Al2O3-fiber/Al2O3 composite) into a new type of dental ceramic. The Al2O3-based ceramic used a matrix consisting

of 60wt% Al2O3 powder and 40wt% SiO2-B2O3 powder. The prepreg sheets of Al2O3-fiber/Al2O3 composite (in which

uniaxially aligned Al2O3 fibers were infiltrated with the Al2O3-based matrix) were fabricated continuously using tape casting

technique with a doctor blade system. Multilayer preforms of Al2O3-fiber/Al2O3 composite sheets were then sintered at a

maximum temperature of 1000•Ž under an atmospheric pressure in a furnace. The results showed that the shrinkage and

bending properties of Al2O3-fiber/Al2O3 composite exceeded those of unreinforced Al2O3 - hence demonstrating the positive

effects of fiber reinforcement. In conclusion, the tape casting technique has been utilized to successfully develop a new type

of dental ceramic material.

Key words: Al2O3 fiber-reinforced Al2O3-based ceramics, Sheet, Tape casting

INTRODUCTION

Dental ceramics are commonly used as esthetic re-storative materials for crowns, bridges, and fixed

partial dentures. Metal-ceramics have been especially popular for restorations in fixed prosthodontics1,2). However, the metals used in metal-ceramic restora-tions have the potential of causing allergic or toxic reactions within the soft or hard tissues. Therefore, there is extensive clinical research on developing metal-free restorations3,4) or metal-ceramic restora-tions using pure titanium with a high degree of biocompatibility5). Moreover, it is well known that metal-free restorations provide an esthetic promise unmatched by conventional metal-ceramic restora-tions6). This is because metal-ceramic materials are known to cause graying of the gingival margin due to metal show-through7).

Against this backdrop of heightened use of ce-ramics for restorative procedures and the constant demand for improved clinical performance, several new ceramic restorative materials and techniques have been developed to-date. For example, various kinds of new ceramics such as castable ceramics8,9) and CAD/CAM ceramics10,11), which have been intro-duced into dentistry since the 1980s have shown con-siderable potential as restorative materials. These new dental ceramic materials offer higher degrees of esthetics and biocompatibility than do those of con-ventional porcelain ceramics and metal-ceramics12). However, restorations made of these materials are not as strong as those that are supported with metal because of their intrinsic mechanical properties13).

To overcome the above-mentioned problems,

concerted efforts and detailed studies have been de-voted to developing fiber-reinforced composite materi-als - the achievements of which have become a center of attention14-18) in dentistry. It has been demonstrated through researches (including clinical experiments) that it is possible to use fiber-reinforced composites in dental appliances such as crowns, bridges, and frameworks for fixed partial dentures19-21).

Currently, one industrial method that is used to manufacture flat ceramics with precise thickness control and consistency is the tape casting tech-nique22-24). This method employs specially formu-lated ceramic slurry, which is cast by a doctor blade onto a flat sheet or carrier, and then dried on a green sheet. A key advantage of the tape casting technique is that the thickness of the films can be precisely controlled by adjusting the gap of the blades with a micrometer screw gauge. Armed with precision and consistency, tape casting products have found their applications in a wide variety of indus-tries, such as microelectronics, photovoltaic solar ap-plications, laminated composites, and rapid proto-typing. In dentistry, ceramic sheets can be poten-tially utilized in a wide range of dental applications, such as crowns, bridges, laminate veneers, cooping, as dental ceramics for many types of restoration in fixed prosthodontics and also as traditional dental ce-ramics.

Hinging on the vast potential of fiber-reinforced ceramic sheets in dental applications, the purpose of this study was to use the tape casting technique to develop an Al2O3 fiber-reinforced Al2O3-based ceramic material (Al2O3-fiber/Al2O3 composite) into a new

298 DEVELOPMENT OF FIBER-REINFORCED CERAMICS

type of dental ceramic.

MATERIALS AND METHODS

Fabrication of Al2O3-fiber/Al2O3 composite

Fig. 1 is a flowchart of the fabrication process of

Al2O3-fiber/Al2O3 composite. An Al2O3-based powder,

consisting of 60wt% Al2O3 powder (ƒ¿-Al2O3, AL45-

1, Showa Denko Co. Ltd., Tokyo, Japan) and 40wt%

SiO2-B2O3 glass powder (SNK-01, Senyo Glass Co.

Ltd., Osaka, Japan) with compositions (wt%) of 33

SiO2, 32 B2O3, 20 CaO, and 15 MgO, was used as a

matrix for the Al2O3-fiber/Al2O3 composite (Fig. 2).

First, an Al2O3-based aqueous slurry was pre-

pared by mixing 18g Al2O3 powder, 12g SiO2-B2O3

powder, 0.12g copolymer of acrylic acid and acrylic

acid ester (AQ-2559, Lion Co., Tokyo, Japan) as

dispersant, 6g distilled water (Wako Pure Chem.

Ind. Ltd., Osaka, Japan), and 6g ethanol (Wako

Pure Chem. Ind. Ltd., Osaka, Japan) in an Al2O3

container with two different sizes of Al2O3 balls (ƒÓ=

3, 5mm) for 18 hours with a planetary ball mill

(P5/2, Fritsch Japan Co. Ltd., Kanagawa, Japan).

Next, 4.8g copolymer of acrylic acid and acrylic acid

ester (HB-500, Lion Co., Tokyo, Japan) as binder, 0.6

g polyethylene glycol (PEG # 600, Lion Co., Tokyo,

Japan) as plasticizer, 0.03g n-alkyl alcohol (1020H,

Lion Co., Tokyo, Japan) as dispersing agent, 0.06g

ammonia water (25% ammonia solution, Wako Pure

Chem. Ind. Ltd., Osaka, Japan), and 1.5g ethanol

were added to the initially prepared Al2O3 slurry and

further mixed by ball milling for 2 hours. Finally,

the Al2O3 slurry was degassed using a rotary pump

for 30 minutes.

The reinforcement used was Al2O3 fiber (ƒÁ-Al2O3,

Alflex, Taimei Chem. Co. Ltd., Nagano, Japan) with

a mean diameter of 10ƒÊm (Fig. 3). The prepreg

sheets of Al2O3-fiber/Al2O3 composite (in which the

uniaxially aligned Al2O3 fibers were infiltrated with

the Al2O3 matrix) were fabricated continuously by a

doctor blade system (DP-150, Sayama Riken Corp.,

Saitama, Japan), as shown in Fig. 4. Here, the

Al2O3 fibers were fixed to a carrier film, which

moved at a speed of 20cm/min, so that the Al2O3 fi-

bers became impregnated by running through the

Al2O3-based slurry. The heights of blades A and B

were adjusted to 1.0mm and 1.5mm respectively.

The width of the Al2O3-fiber/Al2O3 composite sheet

was 150mm, whereby this width is limited only by

the size of the doctor blade machine. The sheet was

then dried at room temperature to remove the sol-

vents. A digital micrometer was used to measure the

thickness of the Al2O3-fiber/Al2O3 composite sheet at

varying locations on the cast tape. Monolayer pieces

were then cut into specimen geometry, laminated,

and pressed together to prepare multilayer preforms.

Then, before sintering, 5 plies of the Al2O3-fiber/

Al2O3 composite sheets were stacked and pressed

Fig. 1 A flowchart of the fabrication process of Al2O3-

fiber/Al2O3 composite.

Fig. 2 FE-SEM image of Al2O3-based powder used as a matrix for the Al2O3-fiber/Al2O3 Composite.

Fig. 3 FE-SEM image of Al2O3 fiber used as a rein-forcement for the Al2O3-fiber/Al2O3 composite.

TANIMOTO & NEMOTO 299

Fig. 4 Schematic illustration of tape casting process for Al2O3-fiber/Al2O3 composite sheet fabrication.

under a pressure of 21MPa. Finally, the multilayer

preforms of the Al2O3-fiber/Al2O3 composite sheets

were sintered at a maximum temperature of 1000•Ž

under an atmospheric pressure for 4 hours in a fur-

nace (MSFT-1520-P, Nikkato Corp., Tokyo, Japan).

After sintering, Al2O3-fiber/Al2O3 composite samples

with a fiber content of about 6vol% were obtained.

Apart from the reinforced specimens, an unreinforced

Al2O3 sample was prepared so that it could serve as

control while the effects of fiber reinforcement were

being investigated.

Particle size analysis

To confirm the effectiveness of ball milling for the

Al2O3-based slurry, particle size distribution of the

Al2O3-based slurry was measured using a laser dif-

fraction particle size analyzer (SALD-7000, Shimazu

Corp., Kyoto, Japan). All adjustable measurement

parameters - such as flow rate through optical cell,

stirrer speed, and ultrasonics - were kept constant

at the same level for all samples. The mean particle

size was defined as half of the cumulative volume.

Observation by field-emission scanning electron mi-

croscopy

After vacuum-drying and platinum-sputtering of the

specimen surface, the surface appearance of the

Al2O3-fiber/Al2O3 composite was observed with a

field-emission scanning electron microscope (FE-SEM,

JSM-6340F, JEOL, Tokyo, Japan) at an acceleration

voltage of 5kV.

X-ray diffraction

The developed Al2O3-fiber/Al2O3 composite was

characterized by X-ray diffraction (XRD, Į-2Į

diffractometer, Geigerflex, Rigaku Corp., Tokyo,

Japan), which had an X-ray source of Cu Kƒ¿ and a

power of 40kV•~30mA. Samples were placed in

an aluminum holder. All collected X-ray spectra

were corrected by using pure silicon (99.95%) as an

external standard.

Sintering shrinkage measurement

The specimens used for measuring sintering shrink-

age were plates measuring 25-mm long (X-axis), 15-

mm wide (Y-axis), and 2-mm thick (Z-axis). The

unidirectional fiber orientation was the X-axis. Be-

fore and after sintering, the length of the side of

each specimen was measured by a digital micrometer.

The linear shrinkage Ls along the X-, Y-, and Z-axes,

and the volume shrinkage VS were then calculated

from the following equations:

Ls(%)=(Lb-La)/Lb•~100 (1)

Vs(%)=(Vb-Va)/Vb•~100 (2)

where Lb is the length of the sample before sin-

tering, La is the length of the sample after sintering,

Vb is the volume of the sample before sintering, and

Va is the volume of the sample after sintering. The

experimental values are the average of five measure-

ments (n=5).

Three-point bending test

The specimens used for bending test were rectangular

bars measuring 18-mm long, 4-mm wide, and 3-mm

thick. Three-point bending tests were performed at

a constant loading speed of 0.5mm/min at a span

length of 16mm by use of a computer-controlled

Instron testing machine (TCM500CR, Minebea,

Tokyo, Japan). The bending strength F and bending

modulus E were calculated from the following formu-

las:

F=(3/2)(PL/bh2) (3)

E=(1/4)(L3/bh3)k (4)

where P is the maximum load, L is the span

length, b is the specimen width, h is the specimen

thickness, and k is the slope at the initial stage in

the load-deflection curve. The experimental values

are the average of 12 measurements (n=12). In ad-

dition, the continuous fiber is oriented in the longitu-

dinal direction.

Statistical analysis of data

The data were examined with an analysis of variance

(ANOVA) and tested by Scheffe's multiple compari-

sons test among the means at p=0.05.

RESULTS

The particle size distribution of the Al2O3-based pow-

der before ball milling, and that of the slurry after

the first and second ball milling steps are presented

in Fig. 5. Before ball milling, the Al2O3-based powder

had a wide distribution of diameters in the range of

0.1ƒÊm to 100ƒÊm, and the mean particle size was 7.86

ƒÊ m, as depicted in Fig. 5(a). After 18 hours of ball

milling (first slurry), the size distribution of the

Al2O3-based powder in the slurry was not as large

and the mean particle size was 3.65ƒÊm (Fig. 5(b)).

In the second slurry, the mean particle size of the

Al2O3-based powder was reduced to 3.27ƒÊm (Fig.

5(c)).

Fig. 6 shows an Al2O3-fiber/Al2O3 composite sheet

fabricated in this study. The thickness of the sheet

was about 1.0mm. Fig. 7 is an FE-SEM image of

300 DEVELOPMENT OF FIBER-REINFORCED CERAMICS

(a)

(b)

(c)

Fig. 5 Distribution of particle size of Al2O3-based powder.

(a) Al2O3-based powder before ball milling

(b) Al2O3-based powder after first ball millig

(c) Al2O3-based powder after second ball milling

this composite sheet, which confirms that the Al2O3-

fiber/Al2O3 composite sheet is composed of Al2O3 fi-

bers and Al2O3-based powder. Fig. 8 is an FE-SEM

image of the surface of Al2O3-fiber/Al2O3 composite

after sintering at 1000•Ž. Fig. 8 confirms that no

Al2O3 powder remained on the surface and that the

surface was smooth. In addition, the Al2O3-fiber/

Fig. 6 Photograph of fabricated Al2O3-fiber/Al2O3 composite sheet.

Fig. 7 FE-SEM image of Al2O3-fiber/Al2O3 composite sheet.

Fig. 8 FE-SEM image of surface of Al2O3-fiber/Al2O3

composite after sintering at 1000•Ž.

TANIMOTO & NEMOTO 301

Al2O3 composite was densely sintered at the sintering

temperature of 1000•Ž.

Fig. 9 shows the XRD spectra of the Al2O3-

fiber/Al2O3 composite sheet and the Al2O3-fiber/

Al2O3 composite after sintering at 1000•Ž. The spec-

tra of the Al2O3-fiber/Al2O3 composite sheet con-

(a)

(b)

Fig. 9 XRD patterns of Al2O3-fiber/Al2O3 composite sheet

and Al2O3-fiber/Al2O3 composite after sintering at

1000•Ž.

(Al2O3; •›, 3Al2O3-2SiO2; •E, SiO2 (cristobalite); •¡).

(a) Al2O3-fiber/Al2O3 composite sheet before sin-

tering.

(b) Al2O3-fiber/Al2O3 composite after sintering.

firmed the typical Al2O3 peaks at 2-theta values of

25.62, 35.20, 37.82, 43.40, 52.57, 57.53, 61.35, 66.53, and

68.22 degrees. In contrast, the spectra of Al2O3-

fiber/Al2O3 composite after sintering at 1000•Ž had

3Al2O3-2SiO2 peaks at 2-theta values of 16.72, 26.58,

33.70, 36.90, and 41.80 degrees, and SiO2 (cristobalite)

peaks at 2-theta values of 22.02, 28.08, and 36.05 de-

grees (in addition to the typical Al2O3 peaks at 2-

theta values of 25.63, 35.20, 37.82, 43.40, 52.58, 57.53,

61.33, 66.55, and 68.23 degrees).

Table 1 shows the linear shrinkage and volume

shrinkage of Al2O3-fiber/Al2O3 composite and Al2O3

after sintering. There were significant differences in

the linear shrinkage of the X- and Z-axes between

Al2O3-fiber/Al2O3 composite and Al2O3 (p<0.05).

Also, there was significant difference in the volume

shrinkage between Al2O3-fiber/Al2O3 composite and

Al2O3 (p<0.05).

Table 2 shows the bending properties of both

Al2O3-fiber/Al2O3 composite and Al2O3 obtained from

three-point bending test. There were significant dif-

ferences in both the bending strength and bending

modulus between Al2O3-fiber/Al2O3 composite and

Al2O3 (p<0.05). Fig. 10 shows FE-SEM images of

the fracture surfaces of Al2O3-fiber/Al2O3 composite

and Al2O3 after the three-point bending test. The

FE-SEM image of the fractured Al2O3-fiber/Al2O3

composite revealed a reactive layer between the fiber

and matrix. Moreover, the fibers were bunched up

in the matrix (Fig. 10(a)). In contrast, the fracture

surface of Al2O3 was smooth (Fig. 10(b)).

DISCUSSION

In the present study, we used a tape casting tech-

nique to develop an Al2O3 fiber-reinforced Al2O3-based

ceramic material (Al2O3-fiber/Al2O3 composite) into a

new type of dental ceramic. The tape casting tech-

nique can be easily employed as a fast and non-

energy consuming procedure to mass produce ceramic

sheets with good uniformity and reproducible proper-

ties. Intrinsically, Al2O3-which is used as a ce-

ramic material-is a hydrophilic, low-friction,

thermodynamically and chemically stable material25).

Moreover, in this study, SiO2-B2O3 powder was added

to the Al2O3 powder to lower the sintering tempera-

ture to avoid degradation of the Al2O3 fibers during

Table 1 Sintering shrinkages of Al2O3-fiber/Al2O3 composite and Al2O3

Each experimental values (mean•}standard deviation) is the average of five measurements (n=5)

Values connected by vertical bars are significantly different from each other (p<0.05)

302 DEVELOPMENT OF FIBER-REINFORCED CERAMICS

Table 2 Bending properties of Al2O3-fiber/Al2O3 composite and Al2O3

Each experimental values (mean•}standard deviation) is the average of

12 measurements (n=12)

Values connected by vertical bars are significantly different from each

other (p<0.05)

(a)

(b)

Fig. 10 FE-SEM images of fracture surfaces of the Al2O3-fiber/Al2O3 composite and Al2O3 after three-point bending test.

(a) Al2O3-fiber/Al2O3 composite (b) Al2O3

sintering of the fiber-containing prepreg sheet23).

Use of the tape casting technique to prepare the

Al2O3-fiber/Al2O3 composite sheet requires control of

the size distribution of the original powder. In the

present study, ball milling was performed to achieve

this control. The Al2O3-based powder had two differ-

ent particle sizes (Fig. 5). A preliminary measure-

ment revealed that the mean particle sizes of

Al2O3 powder and SiO2-B2O3 powder were 1.89ƒÊm and

42.10ƒÊm respectively. Two steps of ball milling pro-

duced the appropriate particle size for tape casting.

The first step of ball milling was used to better dis-

perse the Al2O3-based powder; the second step was

used to furnish better forming ability and to improve

the handling properties of the Al2O3-fiber/Al2O3 com-

posite sheet. While preparing the flat ceramic sheets,

it was necessary to add a dispersant in the first

slurry, and a binder and plasticizer in the second

slurry. The dispersant used in the first slurry con-

sisted mostly of copolymer of acrylic acid and acrylic

acid ester. In the second slurry, the binder consisted

mostly of copolymer of acrylic acid and acrylic acid

ester while the plasticizer consisted mostly of poly-

ethylene glycol.

The major advantage of the tape casting tech-

nique is that the thickness of the ceramic sheet can

be adjusted precisely simply by varying the gap be-

tween the blades and the glass surface. Moreover,

tapes with large lateral dimensions can be cast (see

Fig. 4). In the present study, the thickness of the

prepared Al2O3-fiber/Al2O3 composite sheet was about

1.0mm, which closely corresponded with the differ-

ence in the heights of blades A and B. As a result,

the multi-filament Al2O3 fibers were well infiltrated

into the Al2O3-based slurry in the doctor blade sys-

tem. Fig. 7 shows a tight binding between the

Al2O3-based powder and Al2O3 fiber in the Al2O3-

fiber/Al2O3 composite sheet. Furthermore, the XRD

spectra of the Al2O3-fiber/Al2O3 composite sheet re-

vealed that the crystallographic structure was the

same as that of the original Al2O3 powder. These re-

sults indicate that tape casting technique produced

sheets with good uniformity and with properties

closely resembling those of the original powder

TANIMOTO & NEMOTO 303

materials. Conversely, the XRD spectra of the Al2O

3-fiber/Al2O3 composite after sintering at 1000•Ž had

the peaks of 3Al2O3-2SiO2 and SiO2 (cristobalite) in

addition to the typical Al2O3 peaks. This indicates

that SiO2-B2O3 had been transformed after sinteririing

at 1000•Ž because Al2O3-based ceramics contains SiO2-

B2O326,27). Fig. 8 shows that the Al2O3-fiber/Al2O3

composite was densely sintered at a temperature of

1000•Ž. Currently, there are a few reports about fiber-

reinforced laminated ceramics. For example, Yoshida

et al. prepared hot-pressed, silicon carbide fiber-

reinforced silicon carbide laminated composites28).

Hirata et al. prepared Si-Ti-C-O fiber-reinforced

mullite laminated composites29). However, their lami-

nated composites were fabricated under a pressure of

40MPa and at a higher sintering temperature of

1500-1750•Ž. In contrast, in this study, laminated

Al2O3-fiber/Al2O3 composite was fabricated under

pressureless sintering at a lower temperature of

1000•Ž.

The effect of the B2O3 powder as additives in the

crystallization of ceramics has been investigated by

several researchers30-32). For promising effect in

crystallizing the matrix phase and reducing the sin-

tering temperature (to avoid thermal degradation of

Al2O3 fibers), SiO2-B2O3 powder was added to Al2O3

powder in the present study. For tape casting tech-

nique to be successfully utilized in dental applica-

tions, lowered sintering temperatures and pressure-

less sintering are important factors because expensive

fabrication facilities are not required.

The volume shrinkage of Al2O3-fiber/Al2O3 com-

posite was 2.3 times less than that of Al2O3 (Table

1). The bending strength of Al2O3-fiber/Al2O3 com-

posite was 1.2 times greater than that of Al2O3. In

addition, the bending modulus of Al2O3-fiber/Al2O3

composite was 1.6 times greater than that of Al2O3

(Table 2). These results demonstrate that the

shrinkage and bending properties of Al2O3-fiber/

Al2O3 composite are an improvement over unrein-

forced Al2O3, hence clearly and plainly demonstrating

the benefits of fiber reinforcement.

A fiber-reinforced material contains a matrix im-

pregnated with fibers. It derives its strength from

the fiber's bending modulus and strength, which are

significantly greater than are those of the matrix

alone. Many companies have been introducing new

systems of dental materials using f ibers33,34) These

materials are generally fabricated manually, and re-

quire special facilities. In other words, these systems

use continuously oriented fibers pre-impregnated

with monomers ready for curing with heat or light

under pressure. In contrast, the Al2O3-fiber/Al2O3

composite fabricated in this study is an all-ceramic

material composed of Al2O3-fiber reinforcement and

Al2O3-based ceramic. Compared with other fiber sys-

tems, the fabrication process developed in this study

requires no special fabrication facilities because of the low sintering temperature and pressureless sin-tering.

In conclusion, the Al2O3-fiber/Al2O3 composite

produced in the present study has several distinct ad-vantages. It is adaptable to various types of dental ceramic materials such as crowns, bridges, and lami-nate veneers because of its superior producibility and excellent mechanical properties. Potentially, tape casting is expected to become an important technique for applications in the field of dentistry. Further, the characterizations and mechanical properties of Al2O3-fiber/Al2O3 composite reflect those of ceramics or fibers other than Al2O3. As for sintering tem-

perature, its influence on the technique should be further investigated.

ACKNOWLEDGEMENTS

The authors would like to acknowledge the contribu-tions of Taimei Chemicals Co. Ltd. for supplying the Al2O3-fiber reinforcement materials, and Lion Corp. Ltd. for supplying the binder employed in the pre-sent study.

This work was supported in part by a Grant-in-Aid for Encouragement of Young Scientists (B) (No.15791143) from the Japan Society for the Promotion of Science and by a grant from the Ministry of Edu-cation, Culture, Sports, Science and Technology to

promote 2001-Multidisciplinary Research Projects (in 2001-2005).

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