JP2001019538A - Alumina-based composite ceramic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an alumina-based composite ceramic having a fracture strength and a fracture toughness hardly being lowered even at >=1,400 deg.C, and usable as a functional material at a high temperature. SOLUTION: This alumina-based composite ceramic is obtained by dispersing a yttria-alumina composite fiber in a matrix of a polycrystal α-alumina. A mixed aqueous solution of yttrium ion and aluminum ion in a YAG composition is precipitated by a homogeneous precipitation method. Acetic acid and an organic binder is added to the product, and the obtained mixture is spun and baked to provide the yttria-alumina composite fiber. The obtained yttria-alumina composite fiber is mixed with an alumina slurry, and the resultant material is compacted and baked to provide the objective alumina-based composite ceramic.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention has excellent oxidation resistance,
The present invention relates to an alumina-based composite ceramic having a small decrease in fracture strength and fracture toughness even in a high temperature region of 0 ° C. or higher.

[0002]

2. Description of the Related Art Alumina ceramics used as a structural material are excellent in oxidation resistance, hardness, abrasion resistance, chemical durability and the like, and are less expensive than other oxide-based fine ceramics. However, for alumina ceramics, 1000 ° C
The breaking strength rapidly decreases from the vicinity, making it difficult to use as a high-temperature material.

As a ceramic material used at a high temperature, non-oxide ceramics, such as SiC, is available from Journal of the Ceramic Society (Journal of the Ceramic).
Society of Japan), 102, 95
7-960 (1994), SiC fiber reinforced S
The iC matrix material is disclosed in Vol. 103, pp. 191-194 (1995). Further, a SiC whisker / Si ₃ N ₄ composite material is disclosed in Japanese Patent No. 2798701, and research and development are proceeding toward practical use.

However, when a non-oxide ceramic is used in a high temperature region in an oxidizing atmosphere of a practical environment, an amorphous layer is formed by oxidation at a bulk surface, a grain boundary, or an interface between a fiber and a matrix. For this reason, it is difficult to obtain satisfactory high-temperature characteristics, which is a major problem in practical use.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a ceramic having a small decrease in fracture strength and fracture toughness at high temperatures and a method for producing the same.

[0006]

The present invention is characterized by an alumina-based composite ceramic in which a yttria-alumina composite fiber is dispersed in a polycrystalline α-alumina matrix. The addition amount of the yttria-alumina composite fiber to the polycrystalline α-alumina is preferably 3 to 20 parts by volume, particularly preferably 5 to 20 parts by volume, relative to 100 parts of the polycrystalline α-alumina. The method for producing an alumina-based composite ceramic of the present invention is characterized in that a yttria-alumina composite fiber is uniformly dispersed in an alumina slurry in which polycrystalline α-alumina is previously slurried, and then molded and sintered. The viscosity of the alumina slurry is preferably from 500 to 1000 cps.

[0007]

The alumina-based composite ceramics of the present invention in which the yttria-alumina composite fibers are dispersed in a polycrystalline α-alumina matrix is 1400 ° C.
Even under the above-mentioned practical environment, the decrease in fracture strength and fracture toughness is small. Therefore, it can be used as a functional material at high temperatures by taking advantage of the characteristics of alumina ceramics such as oxidation resistance, hardness, wear resistance, and chemical durability. The fracture strength and fracture toughness of alumina-based composite ceramics vary depending on the amount of yttria-alumina composite fiber added. When the addition amount is in the range of 0 to 20 vol%, the fracture strength and fracture toughness of the alumina-based composite ceramic increase with the addition amount. If it exceeds 20 vol%, the dependence of fracture strength and fracture toughness on the amount of addition is small, and if it is less than 3 vol%, the effect is small. Therefore, the addition amount of the yttria-alumina composite fiber is preferably 3 to 20 vol%, particularly 5 to 20 vol%.
vol% is preferred.

In the method for producing an alumina-based composite ceramic according to the present invention, the yttria-alumina composite fiber is uniformly dispersed in an alumina slurry in which polycrystalline α-alumina is previously slurried, and then molded and sintered. This prevents damage to the yttria-alumina composite fiber during mixing, and provides an alumina-based composite ceramic in which the yttria-alumina composite fiber is uniformly dispersed.
If the viscosity of the alumina slurry is higher than 1000 cps,
When mixed with the fiber, it is difficult to make a uniform slurry, and if it is lower than 500 cps, the fiber tends to separate and settle. Therefore, the viscosity of the alumina slurry is 500 to 1
000 cps is preferred.

[0009]

EXAMPLE A yttria-alumina composite fiber is produced, for example, by a urea method which is one of the uniform precipitation methods. For example, a mixed aqueous solution of yttrium and aluminum
(Y ₃ Al ₅ O ₁₂ ) is prepared and precipitated by adding urea and the like. To this precipitate, an organic acid, here acetic acid, and an organic binder, here polyethylene oxide, are added, and a solvent such as water or alcohol, here water, is evaporated and concentrated to a spinnable viscosity. This is pulled out from the nozzle and spun to obtain a yttria-alumina composite fiber.
It is fired at 0 ° C. or the like to obtain a yttria-alumina composite fiber. The yttria-alumina composite fiber is blown by a jet stream to blow off the spinning solution.
A spinning method in which a spinning solution is spun using a centrifugal force caused by high-speed rotation, a pulling method in which the spinning solution is pulled up using a glass rod or the like, or an extrusion method in which a spinning solution is extruded, can be used. The composition of the produced yttria-alumina composite fiber is not limited to YAG, and the yttria: alumina ratio is 20 to 80 mol%:
It may be 80 to 20 mol%, and in addition to YAG, YAP
(YAlO ₃ ), YAM (Y ₄ Al ₂ O ₉ ), etc.
Yttria-alumina composite fibers may be used, and Y ₂ O ₃ particles, Al ₂ O ₃ particles, or the like may partially exist. The obtained yttria-alumina composite fiber generally has a diameter of 10 to 100 μm, the aspect ratio of the crystals in the yttria-alumina composite fiber is close to 1, and the fiber of the polycrystalline yttria-alumina composite oxide in which these crystals are aggregated is obtained. Become.

[0010] The mixing of the yttria-alumina composite fiber into the alumina is carried out while stirring the alumina which has been previously slurried in order to prevent damage to the fiber during stirring.
It is preferable to add a yttria-alumina composite fiber. The solvent to be mixed with alumina for slurrying is preferably water or an alcohol such as methanol or ethanol. As the organic binder, polyethylene oxide, polyvinyl alcohol, celluloses and the like can be used. The concentration of alumina with respect to the total amount of the slurry was 7
The amount of the organic binder is preferably 0.5 to 2.0 parts by weight based on 100 parts by weight of alumina, and the slurry viscosity is preferably 500 to 1000 cps. As a result, sedimentation and separation of the yttria-alumina composite fiber can be prevented, and a slurry optimal for composite can be obtained. If the slurry viscosity is higher than 1000 cps, a uniform slurry will not be obtained when the yttria-alumina composite fiber is added. If the slurry viscosity is lower than 500 cps, the yttria-alumina composite fiber will settle and separate. When the slurry concentration is higher than 85% by weight, a uniform slurry is not obtained. When the slurry concentration is lower than 75% by weight, the yttria-alumina composite fiber sediments and separates.

A mixed slurry of alumina and yttria-alumina composite fiber is formed into a desired shape or a green sheet by slip casting or the like. CIP (normal temperature isostatic press), press, doctor blade,
It can also be formed by extrusion, injection molding, and the like. In CIP and press molding, it is preferable to use a mixed slurry of alumina and yttria-alumina composite fiber as granules with a spray dryer or the like.

After the molded body is degreased, it is sintered by normal pressure sintering, hot pressing, HIP (hot isostatic pressing) or the like to obtain an alumina-based composite ceramic. In the case of normal pressure sintering, the atmosphere is vacuum (10 ⁻³ torr or less), argon, hydrogen, or the atmosphere, and the sintering temperature is 1600 to 1
800 ° C or the like. In the case of performing the HIP treatment, it is preferable to calcine beforehand and to set the relative density to about 98%, for example, 95 to 9%.
Preferably, it is 9%. In the case of hot pressing, the atmosphere is preferably vacuum (10 ⁻³ torr or less) or argon, and the processing temperature is preferably 1400 to 1600 ° C.,
The pressure is preferably from 10 to 100 MPa. In the case of HIP, the atmosphere is preferably argon or the like, and the processing temperature is 130 ° C.
0 to 1500 ° C. is preferable, and the pressure is 50 to 150 MPa.
a and the like are preferable.

The fracture strength of the alumina-based composite ceramics is measured by, for example, a four-point bending test (JIS 1604) under an atmospheric pressure of 10 ⁻³ torr or less. Similarly, the fracture toughness value of these alumina-based composite ceramics is set to 1
At an atmospheric pressure of 0 ⁻³ torr or less, for example, SEVNB
It is measured by a three-point bending test by the method. The SEVNB method is a modification of the SEPB method specified by JIS, in which a V-shaped notch is formed on a sample piece (VAMAS round process).
Robin on Fracture Toughness Measurement of Ceramic Matrix Composite,
VAMAS Technical Report (VAMAS Rou
nd Robin on Fracture Tug
hness Measurement of Ceram
ic Matrix Composite, VAMAS
Technical Report) 32, p6 (1
997, September)).

The highest values of fracture strength and fracture toughness are obtained by HIP, followed by hot pressing and normal pressure sintering. In particular, the fracture strength of alumina-based composite ceramics due to HIP is almost the same as that of alumina ceramics even at room temperature. It increases by about 15% in comparison. As the temperature rises,
Alumina ceramics have a remarkable decrease in fracture strength and fracture toughness.
In this case, the temperature is about 1/3 to 1/4 of the room temperature. On the other hand, H
In any of IP, hot pressing, and normal pressure sintering, alumina-based composite ceramics have a small decrease in fracture strength and fracture toughness due to temperature rise. For example, even at 1400 ° C., the decrease in fracture strength and fracture toughness from room temperature is only on the order of 10%, and it can be practically used even at 1500 ° C. or higher, for example, 17%.
Can be used up to about 50 ° C.

In the alumina-based composite ceramic, the fracture strength and fracture toughness change depending on the amount of the added yttria-alumina composite fiber. As shown in FIG. 3, the addition amount of the yttria-alumina composite fiber is 0 to 20 vol.
In the range of 1%, the fracture strength at room temperature and the fracture toughness value increase with the addition amount. Although not shown in the drawing, this is the same at other temperatures, and the fracture strength and the fracture toughness increase with the addition amount even at a high temperature when the addition amount is in the range of 0 to 20 vol%.
For example, 3 vol. Of yttria-alumina composite fiber
% Of composite ceramics (formed by HIP method)
The fracture toughness value at 200 ° C. was about 3.5 MPa√m. Addition amount of yttria-alumina composite fiber is 2
If it exceeds 0 vol%, the dependence of the fracture strength and fracture toughness on the amount of addition becomes small. For example, the fracture toughness value at 1200 ° C. of the alumina-based composite ceramics with HIP added with 30 vol% is almost the same as that with 20 vol% added. there were. In view of the cost of adding the yttria-alumina composite fiber, the amount of the yttria-alumina composite fiber to be added is 3 parts by volume with respect to 100 parts of the raw material powder α-alumina.
To 20 parts, especially 5 to 20 parts, that is, 3.4 to 23 by weight.
Parts, especially 5.7 to 23 parts.

Alumina-based composite ceramics were cooled to room temperature and 1
The yttria-alumina composite fiber portion of the cross section broken at 400 ° C. was observed with a scanning electron microscope. In the fracture at room temperature, intragranular fracture occurs as shown in FIG.
In this case, grain boundary fracture occurs as shown in FIG. Cross-sections of alumina ceramics without addition of yttria-alumina composite fiber at room temperature and 1400 ° C. were observed with a scanning electron microscope. As shown in FIGS. 6 and 7, the fractures were observed at room temperature and 1400 ° C. Destruction occurs. As described above, in the ceramics to which the yttria-alumina composite fiber is added, the fracture surface in the yttria-alumina composite fiber shifts from the inside of the grain to the grain boundary as the temperature rises, thereby causing crack deflection.

The yttria-alumina composite fiber is not taken into the alumina matrix during sintering, and the effect of adding the yttria-alumina composite fiber occurs. Y
Since the aspect ratio of the AG crystal powder is about 1, even if the YAG crystal powder is dispersed in the alumina matrix, the deflection of cracks does not occur much. However, when YAG is added as a fiber, crack deflection occurs at a temperature exceeding 1000 ° C., and the surface fracture energy of the alumina-based composite ceramic can be increased. As shown in equation (1), the fracture toughness value is determined by the Young's modulus and the surface fracture energy. It is considered that the decrease in Young's modulus due to the rise in the temperature of alumina ceramics is compensated for by the increase in the surface fracture energy of the yttria-alumina composite fiber itself, thereby suppressing the decrease in fracture strength and fracture toughness. K _IC = (2 · E · r) ^1/2 (1) (K _IC : fracture toughness value, E: Young's modulus, r: surface fracture energy) Therefore, the fracture strength and fracture toughness value of alumina ceramics in a high temperature region are calculated. When the yttria-alumina composite fiber is added for the purpose of improvement, the fracture surface of the yttria-alumina composite fiber changes as the temperature rises, and the fracture strength and fracture toughness when the temperature rises are improved by increasing the surface fracture energy. You.

[0018]

[Test Example] Preparation of yttria-alumina composite fiber An yttria-alumina composite fiber was prepared by a method of adding an organic binder to a mixed precipitate of yttrium ions and aluminum ions obtained by a uniform precipitation method and spinning.
Dissolve Y ₂ O ₃ and Al (OH) ₃ in nitric acid, and add 0.5
An aqueous solution of yttrium ion and aluminum ion having a mol / l concentration was prepared. These aqueous solutions were mixed at an atomic ratio of yttrium: aluminum of 3: 5 so as to have a YAG composition. To this aqueous solution, urea corresponding to 20 times the mole number of all metal ions of yttrium and aluminum in the solution was added to synthesize a precipitate such as a basic carbonate or a hydroxide salt. After washing the obtained precipitate with ion-exchanged water, the precipitate is dispersed in a solvent such as ion-exchanged water or alcohol, and an organic acid corresponding to three times the number of moles of all metal ions of yttrium and aluminum, acetic acid in this case. Was added. Further, after adding an organic binder, here polyethylene oxide, at 10% by weight based on the amount of the metal oxide, the solvent of this mixed solution was removed by an evaporator, and the mixture was concentrated to a viscosity at which spinning was possible. The spun fiber is used as a yttria-alumina composite fiber precursor. Spinning may be performed by any method,
Here, the spinning solution was drawn out from the nozzle. After drying this yttria-alumina composite fiber precursor, it was baked at, for example, 1200 to 1800 ° C. in a vacuum, hydrogen, or the like, to prepare an yttria-alumina composite fiber. The composition of the yttria-alumina composite fiber thus obtained is not limited to YAG, but may be
Alumina ratio is 20 to 80 mol%: 80 to 20 mol
% Is preferable, and in addition to YAG, YAP (YAlO ₃ ) or Y
An AM (Y ₄ Al ₂ O ₉ ) fiber may be used. The obtained yttria-alumina composite fiber generally had a diameter of 10 to 100 μm, and the aspect ratio of crystals in the fiber was close to 1, resulting in a polycrystalline fiber in which these crystals were aggregated.

Alumina and yttria-alumina composite
Preparation of mixed slurry of Ivar α-alumina powder (AKP-3, trade name, manufactured by Sumitomo Chemical Co., Ltd.)
0) To 100 parts by weight, 0.5 parts by weight of a polycarboxylic acid ammonium salt (trade name: A-6114, manufactured by Toagosei Co., Ltd.) as a dispersant, and polyoxyalkylenepentaerythritol ether (manufactured by NOF CORPORATION, as a defoamer) Name CE-
457), 0.5 to 2.0 parts by weight of polyethylene oxide as an organic binder, and 1000 ppm by weight of MgO as a sintering aid. Ion-exchanged water was added to the mixture to adjust the weight concentration of alumina to 75 to 85% by weight based on the total amount of the slurry, and the mixture was mixed by a wet ball mill to obtain a slurry. The viscosity of the slurry is 500-100
0 cps is preferred. When the slurry concentration was more than 85% by weight, a uniform slurry was not obtained. When the slurry concentration was less than 75% by weight, the yttria-alumina composite fiber settled and separated. The alumina slurry was charged into a container, and the yttria-alumina composite fiber was added under stirring. The added amount of the yttria-alumina composite fiber is α-alumina powder 100
3,5,10,15,20,30 parts by volume ratio with respect to parts (3,5,10,15,20,30 vol%,
3.4, 5.7, 11, 17, 23,
34 parts).

Alumina and yttria-alumina composite
Molding of Ivar Mixed Slurry A mixed slurry of sintered alumina and yttria-alumina composite fiber was formed into a green compact by slip casting. After drying this green compact at 130 ° C, it was degreased at 400-600 ° C. After degreasing, the compact was sintered under normal pressure under a vacuum atmosphere (10 ⁻³ torr).
sintering at 1700 ° C. to obtain an alumina-based composite ceramic. The molded body after completion of degreasing, which was produced in the same manner, was hot-pressed at 1450 ° C., 5 ° C. under an argon atmosphere.
Sintering was performed at a pressure of 0 MPa to produce an alumina-based composite ceramic. The molded body after completion of degreasing, similarly produced, was calcined to have a relative density of about 98%. This was sintered by HIP under an argon atmosphere at 1350 ° C. and a pressure of 100 MPa to produce an alumina-based composite ceramic.

Measurement of fracture strength and fracture toughness, scanning electron
The breaking strength of the alumina composite ceramic of microscopic observation Example 1
At an atmospheric pressure of 0 ^-3 torr or less, a four-point bending test (J
IS1604). Similarly, the fracture toughness values of these alumina-based composite ceramics were measured by a three-point bending test (SEVNB method) at an atmospheric pressure of 10 ⁻³ torr or less. The sintered alumina-based composite ceramic was broken at room temperature and 1400 ° C., and the cross sections were observed with a scanning electron microscope.

Preparation of Comparative Example The above α-
Alumina powder was sintered by normal pressure sintering, hot pressing, and HIP in the same manner as in the example to obtain alumina ceramics. The fracture strength and fracture toughness of this alumina ceramic were measured by the same method and under the same conditions as in the examples. The cross sections of the alumina ceramics broken at room temperature and 1400 ° C. were observed with a scanning electron microscope. Regarding the alumina ceramics, the tendency of changes in fracture strength and fracture toughness due to temperature rise was the same in any of the normal pressure sintering, hot pressing, and HIP production methods.

FIG. 1 and FIG. 2 show changes in the fracture strength and fracture toughness values of the alumina-based composite ceramics prepared with the addition amount of the yttria-alumina composite fiber being 10 vol% with temperature. Similarly, it shows the fracture strength and fracture toughness of the alumina ceramics obtained by the normal pressure sintering of the comparative example due to the temperature change. As shown in FIGS. 1 and 2,
The fracture strength and fracture toughness values were highest for HIP, followed by hot pressing and normal pressure sintering.
In particular, the fracture strength of the composite ceramic by HIP was almost the same as that of alumina ceramic even at room temperature. Also, the fracture toughness at room temperature was higher in the case of HIP than in normal pressure sintering or hot pressing, and increased by about 15% as compared with the fracture toughness of composite ceramics prepared by normal pressure sintering. As the temperature rises, the fracture strength and fracture toughness of alumina ceramics decrease significantly, and the tendency is remarkable at 1000 ° C. or higher. On the other hand, in any of HIP, hot pressing, and normal pressure sintering, the decrease in fracture strength and fracture toughness of alumina-based composite ceramics due to temperature rise is 10 times less than that at room temperature.
% Order, and it became smaller. FIG. 3 shows the room temperature fracture toughness value of the yttria-alumina composite fiber at 0 to 20 vol%. When the addition amount was in the range of 0 to 20 vol%, the fracture toughness at room temperature increased as the addition amount of the yttria-alumina composite fiber increased. Although not shown in the figure, the fracture strength and the fracture toughness increased as the amount of addition increased in the range of 0 to 20% by volume even at high temperatures. 2 yttria-alumina composite fibers
Even if it is added in excess of 0 vol%, the fracture strength and fracture toughness do not improve any more. For example, 1200 ° C. of alumina composite ceramics (formed by HIP method) added with 30 vol%.
The fracture toughness value of the sample was almost the same as that of the sample in which 20 vol% was added. From the viewpoint of the cost of adding the yttria-alumina composite fiber, the amount of the yttria-alumina composite fiber to be added is 3 to 20 parts by volume, particularly 5 to 20 parts, and 3% by weight based on 100 parts of the raw material powder α-alumina. .4 ~
23 parts, especially 5.7 to 23 parts, is preferred.

FIGS. 4 and 5 show cross sections of the alumina-based composite ceramics of the embodiment, which were broken at room temperature and 1400 ° C.
3 shows a scanning electron micrograph of the yttria-alumina composite fiber portion. 6 and 7 show scanning electron micrographs of cross sections of the alumina ceramics of the comparative example broken at room temperature and 1400 ° C. As shown in FIG. 4, in the case of alumina-based composite ceramics, ceramic particles cannot be clearly identified by fracture at room temperature, and intragranular fracture has occurred. In the case of fracture at 1400 ° C., as shown in FIG. Can be clearly identified, and grain boundary fracture has occurred. 6 and 7 show the alumina ceramics of the comparative example.
As shown in Fig. 7, ceramic particles can be clearly identified both at room temperature and at 1400 ° C, and grain boundary fracture occurs. As described above, by the addition of the yttria-alumina composite fiber, the fracture surface was shifted from the inside of the grain to the grain boundary with the rise in temperature, and the crack was deflected. The yttria-alumina composite fiber was not incorporated into the alumina matrix during sintering, and produced the effect of adding the yttria-alumina composite fiber. By adding YAG in the form of fiber, crack deflecting effect was generated at a temperature exceeding 1000 ° C., the surface fracture energy of the alumina-based composite ceramics was increased, and a decrease in fracture strength and fracture toughness was able to be suppressed.

Alumina ceramics can be used as a high-temperature functional material by compounding it with a yttria-alumina composite fiber. Therefore, it is possible to take advantage of the excellent characteristics of alumina ceramics such as oxidation resistance, hardness, wear resistance, chemical durability, etc.Alumina ceramics cannot be used by compounding with yttria-alumina composite fiber Can enter the field.
For example, tubes for heat exchangers, filters for high-temperature exhaust gas,
It can be used for a heat shield plate of a high temperature furnace, a heat shield plate for an engine, a gas turbine member, and the like. Further, they are more advantageous than non-oxide ceramic materials in terms of material cost, process cost, oxidation resistance and the like.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing a relationship between a breaking temperature and a breaking strength in Examples.

FIG. 2 is a characteristic diagram showing a relationship between a fracture temperature and a fracture toughness value in Examples.

Fig. 3 25 of yttria-alumina composite fiber
Characteristic diagram showing fracture toughness value at ℃

FIG. 4 is a scanning electron micrograph showing the ceramic structure of the yttria-alumina composite fiber portion at a cross section in which the alumina-based composite ceramic of the example was broken at room temperature.

FIG. 5 shows that the alumina-based composite ceramics of Example
Scanning electron micrograph showing the ceramic structure of the yttria-alumina composite fiber section at the cross section broken at 00 ° C.

FIG. 6 is a scanning electron micrograph showing the ceramic structure of a fractured surface of a conventional alumina ceramic at room temperature showing the ceramic structure.

FIG. 7 shows a conventional alumina ceramic at 1400 ° C.
Scanning electron micrograph showing ceramic structure of fracture surface at

Continued on the front page (72) Inventor Hidehiko Tanaka 1-1 Namiki, Tsukuba, Ibaraki Pref. Science and Technology Agency Mutsu Materials Research Laboratory (72) Inventor Toshiyuki Nishimura 1-1 Tsunami, Tsukuba, Ibaraki Pref. F term (reference) 4G030 AA12 AA36 BA20 BA21 BA33 CA05 GA04 GA26 GA27 GA29

Claims (4)

[Claims] 1. A polycrystalline α-alumina matrix comprising:
Alumina-based composite ceramics in which yttria-alumina composite fibers are dispersed. 2. The alumina-based composite ceramic according to claim 1, wherein the amount of the yttria-alumina composite fiber is 3 to 20 parts by volume relative to 100 parts of the polycrystalline α-alumina. . 3. A method for producing alumina-based composite ceramics, which comprises uniformly dispersing yttria-alumina composite fibers in an alumina slurry in which polycrystalline α-alumina is previously slurried, followed by molding and sintering. 4. The viscosity of the alumina slurry is 500-500.
A method for producing an alumina-based composite ceramics, which is 1000 cps.