JP3101972B2 - Alumina sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alumina sintered body having high strength at room temperature to 1200.degree. C. and high toughness. Particularly, it is used in aviation-related members, refining members, and space environment. Used for heat engine components and heat engine components.

[0002]

2. Description of the Related Art Conventionally, alumina-based sintered bodies have been used in various fields as a typical ceramic material because of their excellent strength and chemical resistance. Further, this alumina-based sintered body has a strength of at most 20 to 40 kg / mm ^2.
Therefore, various improvements have been made to achieve higher strength and higher toughness.

For example, a composite material obtained by adding silicon carbide or zirconia to alumina is disclosed in Japanese Patent Application Laid-Open No. 61-122164.
And Japanese Patent Application Laid-Open No. Sho 63-139044. Further, a material in which a β-type alumina crystal having a shape anisotropy in which a rare earth element is contained in alumina is dispersed is also disclosed in
No. 134551 has been proposed.

[0004]

However, the alumina-silicon carbide composite material has a problem that it lacks oxidation resistance in a high-temperature oxidizing atmosphere because it contains a non-oxide. Further, the alumina-zirconia composite material has an extremely low strength from around 900 ° C.
There was a problem that it could not be used at temperatures as high as 00 ° C.

Further, a composite material in which β-alumina containing a rare earth element is dispersed is excellent in oxidation resistance, and is superior in strength and toughness from room temperature to high temperature as compared with an alumina sintered body without addition. Although it is a material, the high-temperature strength at 1200 ° C. is at most about 380 MPa, and there is a problem that the strength is not sufficient.

[0006]

Means for Solving the Problems The present inventors have repeatedly studied a method of increasing the high-temperature strength and toughness of a composite material obtained by dispersing β-type alumina crystals in alumina, and as a result, As a result of repeated studies from the viewpoint that it is important to increase the strength of the crystal phase itself constituting the sintered body in addition to the structural improvement, the alumina crystal phase and β
By dispersing metal oxides different from each crystal phase as fine crystals in the grains of the type alumina crystal phase,
It has been found that high-temperature strength and toughness can be improved.

That is, the alumina-based sintered body of the present invention is a composite sintered body composed of an alumina crystal phase and a β-type alumina crystal phase. It is characterized in that metal oxide crystal particles different from each crystal phase are dispersed.
The metal oxide crystal particles have a particle diameter of 500 nm or less. Further, the metal oxide crystal particles include at least one of Ti, Mg, and Fe, and the β-type alumina crystal phase is a composite of alumina and an oxide of Group 3a or Group 2a of the periodic table. It is characterized by being composed of oxide and particles having an average aspect ratio of 3 or more.

Further, according to the method for producing an alumina-based sintered body of the present invention, the first metal oxide powder having a different solid solubility limit to alumina and β-type alumina due to a difference in atmosphere with respect to alumina powder. And a second metal oxide powder capable of producing β-type alumina by reaction with alumina, followed by mixing and molding, and then forming the molded body into alumina of the first metal oxide and β-type alumina. Baking in an atmosphere in which the solid solution limit is increased to generate β-type alumina crystals by the reaction between the second metal oxide and alumina, and the first metal oxide is formed of alumina crystals and β-type alumina; After preparing a sintered body having a solid solution in the crystal, the sintered body is heat-treated in an atmosphere in which the first metal oxide has a limited solid solubility in alumina and β-type alumina, and the alumina crystal is and A first metal oxide or a composite oxide of the first metal oxide and alumina or the second metal oxide is precipitated in the crystal grains of the type alumina; Is at least one of titanium oxide, magnesium oxide, and iron oxide, and the second metal oxide is a 2a of the periodic table.
And at least one of group 3a and group 3a element oxides.

Hereinafter, the present invention will be described in detail. As shown in FIG. 1, the alumina-based sintered body of the present invention is composed of an alumina crystal phase 1, a β-type alumina crystal phase 2, and metal oxide crystal particles 3 precipitated in the crystal phase grains, as shown in FIG. Be composed. The alumina crystal phase 1 has an α-type crystal structure. Here, the β-type alumina crystal phase 2 is a composite oxide containing alumina as a main component and a Group 3a or Group 2a oxide of the periodic table, for example, La ₂ O ₃ .11Al
₂ O ₃ , Nd ₂ O ₃ .11Al ₂ O ₃ , CaO.6Al
₂ O ₃ , SrO · 6Al ₂ O ₃ , BaO · 6Al ₂ O ₃
And the like. Β reaction with these aluminas
Group 3a or 2
The group a oxide is defined as a “second metal oxide” in a manufacturing method described later.

[0010] The β-type alumina crystal itself exists as a columnar crystal having an average aspect ratio of 3 or more because the crystal itself grows in shape anisotropy. The minor axis diameter of this columnar crystal is about 0.5 to 5 μm. On the other hand, the alumina crystal phase 1 usually has an aspect ratio of 2 or less and an average particle size of 0.2.
Although it exists as granular crystals of 5 to 10 μm, in some cases, the alumina crystal phase may also exist as particles having an aspect ratio of 2 or more.

As described above, by dispersing the β-type alumina crystal phase as columnar crystals as described above, the toughness of the sintered body can be improved, and grain boundary sliding at high temperatures can be suppressed to some extent. The creep rate at high temperatures can be reduced. However, since β-type alumina has a small Young's modulus, the effect of suppressing the propagation of cracks through the grains is small,
In addition, there is no effect of suppressing softening of the crystal itself at high temperature, that is, plastic deformation due to dislocation movement.

According to the present invention, in a sintered body composed of the above-mentioned alumina crystal phase and β-type alumina crystal phase, as shown in FIG. It is important that the material crystal particles 3 are dispersed. Here, the metal oxide particles dispersed in the grains may be a single metal oxide or a composite oxide of two or more metals, or may be a composite oxide of these metal oxides and alumina or β-type alumina. May be a composite oxide with a metal oxide that forms

The metal oxide dispersed in these grains is a metal oxide whose solid solubility limit changes with respect to alumina or β-type alumina depending on the atmosphere, that is, “a first metal oxide” in a production method described later. Included are those defined as From the viewpoint of high Young's modulus and high stability at high temperature, Mg, Ca, F
e, Co, and at least one selected from the group consisting of metals of groups 3a, 4a, 5a, and 6a of the periodic table. Among these, titanium oxide (TiO ₂ ), oxide Magnesium (MgO) and iron oxide (Fe ₂ O ₃ ) are desirable. As for solid solution in alumina, MgO forms a large amount of solid solution in the form of MgTiO ₃ .

The metal oxide dispersed in the grains of the alumina crystal phase and the β-type alumina crystal phase has an average particle size of 500 n.
m or less. When the particle size of the dispersed phase is small, crystal consistency is maintained at the interface with the host crystal, and strain is generated, so that a greater strengthening and toughening effect can be exhibited. When the average particle size of the intragranular dispersed phase is 1 μm or more, the interaction between the crystals becomes small, and in some cases, a defect occurs at the phase interface due to thermal stress. The average particle size of the dispersed phase is desirably 200 nm or less.

In the alumina sintered body of the present invention, β
Group 2a of the periodic table for forming a type alumina crystal phase,
The Group 3a oxide is desirably contained at a rate of 1 to 6 mol% of the total amount, and the metal oxide precipitated in these crystal grains is the above-mentioned oxide amount of Ti, Mg and Fe ( TiO ₂ , MgO and Fe ₂ O ₃ ).
Desirably, the content is 5 to 5 mol%.

Next, a method for producing an alumina sintered body of the present invention will be described. According to the production method of the present invention, first, alumina is used as a starting material, and the first metal oxide powder having a different solid solubility limit to alumina and β-type alumina due to a difference in atmosphere is reacted with alumina. and a second metal oxide powder capable of producing β-type alumina.

As the first metal oxide, Mg, C
a, Fe, Co, and 3a, 4a, 5a of the periodic table,
It is selected from at least one oxide selected from the group of Group 6a metals.
₂ , Fe ₂ O ₃ and MgTiO ₃ , of which TiO ₂ is most desirable. The first metal oxide is added at a ratio of 0.5 to 5 mol% based on the total amount.

Examples of the second metal oxide include oxides of Group 2a and Group 3a elements of the periodic table. Among them, La ₂ is preferred because of its ease of reaction with alumina.
O ₃ is most preferred. This second metal oxide is added at a ratio of 1 to 6 mol% in the total amount.

The first and second metal oxides are not particularly limited as long as they can form oxides in the firing step. For example, metal powders, organic and inorganic substances containing the metal, and solutions of the metal powders may be used. Either may be used.

After adding and mixing these powders, the mixed powders are formed into an arbitrary shape by a desired molding means, for example, a die press, a cold isostatic press, an extrusion molding or the like.

Next, the molded body is subjected to a known sintering method, for example, a hot press method, a normal pressure sintering method, a gas pressure sintering method, a hot isostatic pressing (HIP) treatment after these sintering, and a glass sealing. Thereafter, HIP treatment is performed to obtain a dense sintered body having a theoretical density ratio of 95% or more.

According to the present invention, the firing at this time is a condition under which the second metal oxide and the alumina react with each other to form β-type alumina crystals, and the firing atmosphere is the first metal oxide. It is necessary that the atmosphere be such that the solid solubility limit of the product in alumina or β-type alumina increases. The firing temperature at this time is 1300 ° C. to 1800 ° C., particularly 1400 ° C.
The temperature is preferably 1750 ° C. from the viewpoint of solid solubility, denseness of the host crystal and suppression of grain growth.

Thereafter, the sintered body in which the first metal oxide is solid-dissolved as described above is heat-treated in an atmosphere in which the first metal oxide has a limited solid solution limit. The firing temperature at this time is suitably 1100 to 1700 ° C. By treating under such conditions, supersaturated solid solution elements can be uniformly precipitated in the form of oxides or composite oxides in the alumina and β-type alumina crystal grains.

In the above solid solution-precipitation step, for example, when TiO ₂ is used as the first metal oxide, the solid solution step is controlled to a reducing atmosphere such as H ₂ and the deposition step is controlled to an oxidizing atmosphere in the atmosphere. When an oxide of Ti / Mg (Ti: Mg = 1) or Fe is used, the solid solution step may be an oxidizing atmosphere such as air, and the precipitation step may be a reducing atmosphere.

[0025]

Alumina has a stable property from room temperature to a high temperature, especially in an oxidizing atmosphere. However, at high temperatures, dislocations tend to move, so that alumina tends to soften and plastically deform. Further, at room temperature, cracks are easily developed even in the crystal, and thus the fracture toughness is low. Although the fracture toughness and high-temperature strength of the material were improved to some extent by the composite of β-type alumina having shape anisotropy, it was still insufficient.

According to the present invention, the strength and toughness of the crystal itself are increased by dispersing the metal oxide in the grains of the alumina crystal phase and the β-type alumina crystal phase, and the resistance to the propagation of the cracks through the grains. And high resistance to plastic deformation at high temperatures. As a result, the high strength and high toughness from room temperature to high temperature of 1200 ° C are achieved by the synergistic effect of the dispersion strengthening effect and the high strength and high toughness by the shape anisotropic particles. A sintered body can be obtained.

[0027]

EXAMPLE As raw material powders, alumina (Al ₂ O ₃ ), lanthanum oxide (La ₂ O ₃ ), neodymium oxide (Nd
₂ O ₃ ), titanium oxide (TiO ₂ ), magnesium oxide (MgO), iron oxide (Fe ₂ O ₃ ), barium oxide (B
aCO ₃ ), calcium oxide (CaCO ₃ ), and strontium oxide (SrCO ₃ ), and the composition ratio was as shown in Table 1. The mixture was molded at a pressure of 1 t / cm ² , and then 3 t / cm ² . Hydrostatic pressure treatment was applied under pressure. And
The obtained molded body was subjected to solid solution (sintering) and precipitation treatment under the conditions shown in Table 1.

The obtained sintered body is processed into a mirror surface, the structure is observed with a scanning electron microscope, identified as a crystalline phase by X-ray diffraction, and a precipitated phase other than the α-alumina crystalline phase is identified. The results are shown in Table 2. In addition, the average particle size of the metal oxide observed in an electron micrograph and precipitated in an alumina phase or a β-type alumina phase,
The average aspect ratio of β-type alumina was measured and is shown in Table 2. FIG. 1 shows the structure of the sintered body obtained in Example 1 after the precipitation solid solution treatment.

Further, as mechanical properties, JIS-R16
The specimen was polished to the shape designated in No. 01 to prepare a bending specimen. This sample was subjected to a four-point bending strength test at room temperature and 1200 ° C. based on JIS-R1601. Further, the fracture toughness (K ₁ c) was measured by the Vickers indentation method. The results are shown in Table 2.

[0030]

[Table 1]

[0031]

[Table 2]

As is clear from the results shown in Tables 1 and 2, the strength and toughness of the sample No. 2 combined with the β-type alumina phase were compared with those of the alumina-only sintered body (sample No. 1). Both are improving. Further, a sintered body in which a metal oxide or a composite oxide is finely dispersed in the grains of the alumina crystal phase and the β-type alumina crystal phase obtained according to the present invention (Sample Nos. 5 to 5)
12) is superior to both Sample No. 1 and Sample No. 2, and is 600 MPa or more at room temperature and 1200 ° C.
In 400MPa or more, toughness 3.7MPam ^{0. 5} or more has been achieved.

It should be noted that, even in the composition system to which the second metal oxide was added, the sample No.
Samples Nos. 3 and 4 both have low strength and low toughness. In Sample No. 3, the composite oxides shown in Table 2 were precipitated not at the inside of the grains but at the grain boundaries.

[0034]

As described in detail above, according to the present invention, the metal oxide is dispersed in the grains of the alumina crystal phase and the β-type alumina crystal phase, whereby the strength and toughness of the crystal itself are increased, and Can greatly improve the resistance to propagation through the grains and the resistance to plastic deformation at high temperatures. As a result, the synergistic effect of the dispersion strengthening effect and the high strength and high toughness due to the shape anisotropic particles enables the high temperature from room temperature to 1200 ° C. A sintered body having high strength and high toughness can be obtained.

[Brief description of the drawings]

FIG. 1 is a view for explaining the structure of an alumina sintered body of the present invention.

[Explanation of symbols]

 1 Alumina crystal phase 2 β-type alumina crystal phase 3 Metal oxide crystal particles

Claims (6)

(57) [Claims] 1. A composite sintered body comprising an alumina crystal phase and a β-type alumina crystal phase, wherein a metal oxide different from each of the crystal phases is present in the grains of the alumina crystal phase and the β-type alumina crystal phase. Alumina-based sintered body characterized in that product crystal particles are dispersed. 2. The alumina-based sintered body according to claim 1, wherein said metal oxide crystal particles have a particle size of 500 nm or less. 3. The alumina-based sintered body according to claim 1, wherein the metal oxide crystal particles contain at least one of titanium, magnesium, and iron. 4. The particle according to claim 1, wherein the β-type alumina crystal phase is a particle having a composite oxide of alumina and an oxide of Group 3a or 2a of the Periodic Table and having an average aspect ratio of 3 or more. Alumina sintered body. 5. A second metal oxide capable of producing β-type alumina by reacting with alumina powder, a first metal oxide powder having a different solubility limit to alumina and β-type alumina due to a difference in atmosphere, and alumina. Adding, mixing and molding a metal oxide powder; and sintering the molded body in an atmosphere in which the solid solubility limit of the first metal oxide is increased to form the second metal oxide and The reaction with alumina produces β-type alumina crystals, and the first metal oxide comprises alumina crystals and β-type alumina crystals.
Producing a sintered body solid-dissolved in the alumina crystal; and heat-treating the sintered body in an atmosphere in which the solid solubility limit of the first metal oxide is reduced to obtain an alumina crystal and β-type alumina. Depositing a first metal oxide or a composite oxide of the first metal oxide and alumina or the second metal oxide in crystal grains. Manufacturing method. 6. The first metal oxide is at least one of titanium oxide, magnesium oxide and iron oxide, and the second metal oxide is group 2a, 3a of the periodic table.
The method for producing an alumina-based sintered body according to claim 5, wherein the alumina-based sintered body is at least one member selected from the group consisting of group-element oxides.