JP3279885B2 - Method for producing alumina-based sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an alumina sintered body having excellent strength and toughness.
The present invention relates to a method for producing an alumina sintered body of a high temperature resistant structural material used in the space industry, smelting industry, chemical industry and the like, and used for gas turbines, engine parts and the like.

[0002]

2. Description of the Related Art Alumina sintered bodies have been attracting attention as structural members having high temperature resistance because of their excellent environmental resistance and strength. Further, in order to further improve the strength and the fracture toughness, various structural controls and composites have been attempted. For example, it is generally known to form a fine-grained sintered structure by firing at a low temperature or adding an auxiliary agent that suppresses grain growth. Also, an Al ₂ O ₃ —SiC composite material, an Al ₂ O ₃ —ZrO ₂ composite material, and a shape-anisotropic β-Al ₂ O ₃ crystal-dispersed alumina material are known (Japanese Patent Application Laid-Open No. 61-122164). JP-A-63-13
No. 9044, JP-A-63-134551, etc.), and such a composite material can improve strength and toughness as compared with a pure alumina sintered body.

[0003]

However, the above A
l In ₂ O ₃ -SiC composite material, for SiC dispersion containing non-oxide lacks oxidation resistance in high-temperature oxidizing atmosphere, Al ₂ O ₃ -ZrO ₂ composite material sharply strength at temperatures around 900 ° C. Therefore, it is desired to improve strength and toughness in a system that does not include a second phase such as SiC or ZrO ₂ .

Further, although the alumina-based sintered body having the fine-grained structure has improved strength, it has a problem that the fracture toughness tends to be rather lowered, and the toughness as well as the strength has not been improved. Further, the β-Al ₂ O ₃ dispersed alumina material has a low toughness because the Young's modulus of the β-Al ₂ O ₃ crystal is low and the shape growth anisotropy particles are difficult to grow due to a low grain growth rate. The contribution to the improvement is small, for example, the fracture toughness at room temperature is at most 3.2 to 3.4.
MPa · m ^1/2 . (JP-A-63-134551)
issue).

An object of the present invention is to solve these problems and provide an alumina-based sintered body having excellent strength and fracture toughness, and a method for producing the same.

[0006]

In order to simultaneously increase the room temperature strength and fracture toughness of alumina, the present inventor is effective in coexisting fine isotropic crystals and crystals having an anisotropic shape. In addition, anisotropic shaped crystals have high Young's modulus and have been studied based on the viewpoint that it is important to control the size and shape.
μm or less, isotropic Al ₂ O with aspect ratio of 1.5 or less
_An alumina sintered body having a mixed structure of _three crystal grains and an anisotropic Al ₂ O ₃ crystal grain having a major axis of 10 μm or more and an aspect ratio of 3 or more was found to have both excellent strength and fracture toughness. Reached.

Further, a metal oxide having a eutectic point with Al ₂ O ₃ of 1600 ° C. or less is added to and mixed with Al ₂ O ₃ powder. It has been found that when the temperature is raised at a rate of not less than / min, an alumina-based sintered body having the above-mentioned structural characteristics can be obtained, and the present invention has been achieved. In the above-mentioned manufacturing method, it has been found that, by heating by baking with microwaves, it is possible to realize a predetermined rate of temperature rise and to promote the formation of the structure, thereby leading to the present invention.

Further, during heating and firing by microwaves,
It has been found that a sintered body having the above-mentioned structural characteristics obtained by applying a pressure to the sintered body has improved denseness and shows more excellent strength and fracture toughness, and has reached the present invention.

[0009]

The alumina sintered body according to the present invention has a particle size of 3 μm.
m2 and fine isotropic Al ₂ O ₃ crystal grains having a major axis of 10
By coexisting anisotropic Al ₂ O ₃ crystal grains having a size of ₃ μm or more and an aspect ratio of 3 or more, an alumina sintered body excellent in both strength and fracture toughness can be obtained. Moreover,
Since it does not contain a second phase such as SiC or ZrO ₂ , A
The excellent high-temperature oxidation resistance and rapid strength deterioration of l ₂ O ₃ do not occur.

Also, a metal oxide having a eutectic point with Al ₂ O ₃ of 1600 ° C. or less is added to Al ₂ O ₃ powder and mixed. By raising the temperature in the temperature range at a rate of 8 ° C./min or more, the liquid phase component is rapidly generated during the temperature raising process, so that the liquid phase is non-uniformly formed. Growth can be caused to produce a sintered body in which isotropic Al ₂ O ₃ crystal grains and anisotropic Al ₂ O ₃ crystal grains coexist.

[0011] If heating during firing is performed by microwaves, it is possible to achieve a high heating rate of 15 ° C / min or more, and further, by applying pressure simultaneously with microwave heating and firing, Promote densification,
A high-strength, high-toughness alumina sintered body can be produced.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The alumina sintered body of the present invention is structurally fine isotropic Al ₂ O
It is characterized in that _three crystal grains and shape-anisotropic Al ₂ O ₃ crystal grains coexist.

Here, the fine isotropic crystal is defined as having a major axis of 3 mm.
It refers to a crystal grain having an aspect ratio (major axis / minor axis) of 1.5 or less and 1.5 or less. It is known that the presence of the fine isotropic crystals contributes to the improvement of the strength of the sintered body. In the present invention, such fine and isotropic crystals form one of the entire sintered body.
It is important that it be present in a proportion of 5-80% by volume. If the amount of such fine crystals is less than 15% by volume,
If the effect of improving the strength is reduced, and if the amount is more than 80% by volume, cracks are easily developed along the grain boundaries, and the fracture toughness of the material is reduced. This isotropic crystal is desirably present in a proportion of 30 to 70% by volume of the whole sintered body.

Further, in the alumina-based sintered body of the present invention,
The existence of shape anisotropic Al ₂ O ₃ crystal grains is important. Here, the shape anisotropic Al ₂ O ₃ crystal grains have a major axis of 10 μm.
As described above, columnar or plate-like Al ₂ having an aspect ratio of 3 or more
O ₃ crystal. The presence of the columnar or plate-like Al ₂ O ₃ crystals in the fine Al ₂ O ₃ crystal structure remarkably improves the fracture toughness of the material due to the diffusion effect or bridging effect with respect to the development of cracks. However, if this columnar or plate-like crystal is less than 20% by volume, the effect of improving toughness is small, and if it exceeds 85% by volume, many abnormally grown crystals appear and the strength of the sintered body is significantly reduced. Therefore, the shape anisotropic Al ₂ O ₃ crystal grains are more preferably 20 to 85% by volume, especially 40 to 80% by volume of the whole sintered body.

The above alumina-based sintered body of the present invention is made of Al
_A metal oxide having a eutectic point with Al ₂ O ₃ of 1600 ° C. or less is added to the ₂ O ₃ powder, mixed, molded, and then the molded body is heated from room temperature to a sintering temperature by 8 ° C./min or more. It is manufactured by sintering after heating at a temperature rate. Here, the heating rate is an average heating rate from the room temperature to the firing temperature, and is calculated by dividing a difference between the firing temperature and the room temperature (25 ° C.) by a time from the room temperature to the firing temperature. Is done.

According to this production method, it is important to add a metal oxide having a eutectic point with Al ₂ O ₃ of 1600 ° C. or less. By the addition of the above metal oxide, a certain amount of liquid phase is generated during firing, and the anisotropic growth of crystals is promoted. If the eutectic point with Al ₂ O ₃ is 1600 ° C. or lower, either a single type of additive or a plurality of additives may be used. Although the type is not particularly limited, for example, Na, Mg, Ca, Sr, B
a, Ti, Fe, Mn, W, Si, and Periodic Table No. 3
One or more types selected from oxides of Group A elements. The amount of addition is adjusted according to the type of addition and the firing conditions.
It is desirable to add in the range of 1 to 10 mol%. In addition, some impurities in the Al ₂ O ₃ raw material can act on the added metal oxide to form a low melting point phase.

As the above-mentioned metal oxide, known raw materials, for example, oxide powder, oxide-forming powder such as nitrate, acetate and carbonate, metal powder, organic and inorganic substances containing the metal and their solutions can be used.

The above-mentioned metal oxide is mixed with Al ₂ O ₃ and molded into an arbitrary shape by a known molding technique, for example, die pressing, casting molding, extrusion molding, injection molding, cold isostatic pressing and the like. The molded body is fired by a known sintering method. According to the present invention, it is important that the rate of temperature rise from room temperature to the firing temperature during firing is 8 ° C./min or more. When the temperature is increased under such conditions, a liquid phase is unevenly generated in the sintered body. Although the growth of some shape anisotropic crystals is promoted in places where a large amount of liquid phase is generated, in the region where the liquid phase is small, isotropic crystals can be fired as fine grains, so that the shape is anisotropic crystals. A sintered body in which isotropic crystals are mixed is generated.

The firing temperature depends on the composition,
From the viewpoint of densification and structure formation, the
00 ° C. or lower, more specifically, 1200 ° C. to 1700
° C is good.

According to the production method of the present invention, it is effective to use a microwave as a heating means for the molded body. In a normal resistance heating method, since heat is transmitted from the outside to the inside of the sintered body, it is difficult to quickly increase the temperature of a large-sized sintered body. In contrast, with microwave heating,
Since the sintered body itself can generate heat, it is easy to realize a high temperature rising rate as in the present invention. Furthermore, utilizing the phenomenon of selectively absorbing microwaves and generating heat depending on the composition,
At the same time as the development of the anisotropic crystal is promoted, the firing time can be shortened, and the proportion of fine crystals can be increased. The microwave used as the heating means preferably has an output of 1 kW or more, particularly 5 kW or more.

Further, according to the production method of the present invention, it is effective to apply pressure to the sintered body during the heating or firing by the microwave heating. In order to produce the sintered body of the present invention, it is important to suppress the growth of fine isotropic particles while developing the anisotropic particles. Therefore, lowering the firing temperature and shortening the firing time are preferable for forming the structure. However, if the firing temperature is lowered or the firing time is shortened, the sintered body may not be sufficiently densified. Thus, a sintered body having a controlled structure and a sufficient density can be obtained by applying pressure during heating and firing. As the pressure application, a gas pressure or a mechanical pressure can be used. For example, 50 kg / cm ²
As described above, in particular, a mechanical pressure of 200 kg / cm ² or more can be applied, or a gas pressure can be applied by using a hot isostatic firing furnace. Is valid.

[0022]

EXAMPLES A powder having a composition shown in Table 1 was prepared using alumina powder, magnesium oxide powder, titanium oxide powder, lanthanum oxide powder and silicon oxide powder having a purity of 99.9%. The eutectic temperature of each composition system was estimated to be 1300 ° C to 1400 ° C due to the presence of silicon oxide (about 500 ppm) as an impurity in the raw material. After press-molding the mixed powder at a pressure of 1 t / cm ² ,
Further, a hydrostatic pressure treatment was applied at a pressure of 3 t / cm ² . The above molded body was heated in the atmosphere from room temperature to a sintering temperature at a heating rate shown in Table 1 by a heating method shown in Table 1 and baked under the conditions shown in Table 1. For Samples Nos. 8 and 11, a pressure of 100 to 300 kg / cm ² was applied based on the hot press method.

The density of the obtained sintered body was measured, and after polishing to a mirror surface, the structure was observed with an optical microscope and a scanning electron microscope, and the isotropic Al having a major axis of 3 μm or less and an aspect ratio of 1.5 or less was obtained. The existence ratio of ₂ O ₃ crystal grains and anisotropic Al ₂ O ₃ crystal grains having a major axis of 10 μm or more and an aspect ratio of 3 or more was determined by image analysis using Luzex.

Further, each sintered body was polished to a shape specified by JIS-R1601, to prepare a bending specimen.
At room temperature based on JIS-R1601, 4
A point bending flexural strength test was performed. Further, the fracture toughness (K1c) was measured by the Vickers indentation method. Table 2 shows the results of the density, the structure quantification measurement, the strength and the fracture toughness measurement. FIG. 1 is a schematic view of the structure of the sintered body of sample No. 5.
It was shown to.

[0025]

[Table 1]

[0026]

[Table 2]

From the results in Table 2, it can be seen that the alumina-based sintered body obtained by mixing the isotropic Al ₂ O ₃ crystal grains and the anisotropic Al ₂ O ₃ crystal grains at a predetermined ratio obtained according to the present invention was obtained. , Showing superior strength and fracture toughness compared to conventional alumina-based sintered bodies and alumina-based sintered bodies other than the present invention, and has a room temperature strength of 5%.
Excellent characteristics of at least 00 MPa and toughness of at least 4.0 MPa · m ^1/2 were obtained.

[0028]

According to the present invention, Al ₂ O ₃ powder and Al ₂ O ₃
A compact formed of a mixture with a metal oxide having a eutectic point with O ₃ of 1600 ° C. or lower is heated at a high speed to a sintering temperature and sintered to form anisotropic Al ₂ O ₃ crystal grains. Al
An alumina-based sintered body excellent in both strength and fracture toughness in which ₂ O ₃ crystal grains coexist can be easily obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a structure of an alumina sintered body of the present invention.

Claims (3)

(57) [Claims] The eutectic point of Al ₂ O ₃ powder with Al ₂ O ₃ is 1
After adding and mixing a metal oxide of 600 ° C. or less, after forming, the temperature is raised from room temperature to a firing temperature at a temperature increasing rate of 8 ° C./min or more and firing is performed, and the major axis is 3 μm or less and the aspect ratio is 1.5 or less. 15 to 80% by volume of isotropic Al ₂ O ₃ crystal grains in the total amount,
Anisotropic A having a major axis of 10 μm or more and an aspect ratio of 3 or more
method for producing an alumina sintered body characterized by containing l ₂ O ₃ crystal grains in a ratio of 20 to 85 vol% in the total amount. 2. The method for producing an alumina-based sintered body according to claim 1, wherein the heating and firing are performed by heating with microwaves. 3. The method for producing an alumina-based sintered body according to claim 1, wherein the heating and sintering are performed while applying pressure.