JP3340025B2 - 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-based sintered body having high strength, high toughness and excellent environmental resistance and a method for producing the same, and relates to a high-temperature structural material used in an oxidizing and reducing atmosphere. The present invention relates to an alumina sintered body and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a structural member, an alumina-based sintered body has been widely used as a member for various industrial machines because of its excellent environmental resistance and strength characteristics. In recent years, in expanding the use of the alumina-based sintered body, various composites have been attempted to further improve the strength and fracture toughness. For example, Al ₂ O ₃ —SiC nanocomposite composite material (see JP-A-61-122164, etc.), Al ₂ O ₃ -ZrO ₂ composite material (see JP-A-63-139944, etc.), Al ₂ O ₃ -Ti
O ₂ composite material (see JP-A-7-257964)
According to such a composite material, strength and toughness can be improved as compared with a general alumina-based sintered body.

[0003]

However, the above A
In the l ₂ O ₃ -SiC nano-components Jie Tsu door composite material, A
Since the non-oxide SiC particles are dispersed in l ₂ O ₃ , there is a problem that if the high-temperature state is maintained for a long time, the SiC reacts with the mother phase Al ₂ O ₃ .

The Al ₂ O ₃ —ZrO ₂ composite material is 9
Since there is a problem that the strength is sharply reduced at a temperature of around 00 ° C., there is a problem that it is not suitable for use in a state where a stress acts at a high temperature.

Further, Al ₂ O ₃ —TiO ₂ composite material is
At a temperature of 1300 ° C. or higher, Al ₂ O ₃ and TiO ₂ react with each other to produce Al ₂ TiO _5, and there has been a problem that the characteristics of the material are unstable.

Accordingly, an object of the present invention is to provide an alumina-based sintered body having stable strength from room temperature to a high temperature and having high toughness, and a method for producing the same.

[0007]

Means for Solving the Problems The inventors of the present invention have studied the above object, and as a result, have found that FeAl
_The above object is achieved by dispersing the composite oxide represented by ₂ O ₄ as fine particles having an average particle diameter of 0.5% or less in an amount of 1% by volume or more of the total amount. And found the present invention. Further, when another metal oxide phase different from the intragranular dispersed phase or the alumina crystal phase is dispersed at the grain boundaries of the alumina main crystal, alumina grain growth is suppressed, and the strength of the sintered body is further improved. The present invention has been found.

Further, as a method for producing the above-mentioned alumina-based sintered body, a molded body containing alumina as a main component and containing at least an Fe element is subjected to heat treatment in an oxidizing atmosphere so that the Fe element is dissolved in the alumina crystal. After the heat treatment, the solid solution is further subjected to a heat treatment in a reducing atmosphere to obtain the Fe
The present inventors have found that the element can be precipitated in the alumina crystal grains in the form of FeAl ₂ O ₄ , and have reached the present invention.

According to the present invention, by dispersing fine particles of FeAl ₂ O ₄ in alumina main crystal grains, stable high strength can be realized from room temperature to high temperature, and high toughness is imparted. be able to.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The alumina-based sintered body of the present invention is mainly composed of alumina crystals. Alumina crystals tend to grow during firing, but have an average particle size of 6 μm or less. Is advantageous for increasing the strength of the sintered body. In particular, it is desirable that the average particle size of the alumina crystal is 4 μm or less. Therefore, this alumina main crystal has an average particle size of 6
If it exceeds μm, the desired strength cannot be obtained. This alumina main crystal phase accounts for 50% by volume or more, especially 70% by volume in the total amount.
It is appropriate from the point of strength that it is present at a ratio of up to 95% by volume.

According to the present invention, as the second crystal phase, a composite oxide phase represented by FeAl ₂ O ₄ is formed as fine particles having an average particle size of 0.5 μm or less in the alumina crystal grains. Distributed. FeAl ₂ in the alumina crystal grains
When the O ₄ phase is dispersed as fine particles, the alumina crystal itself, which is the parent phase, is strengthened, and the strength, toughness, and high-temperature strength of the entire sintered body are improved.

If the amount of the intragranular dispersed phase composed of FeAl ₂ O ₄ is small, the effect of improving strength and toughness is small, so that it is dispersed at a rate of 1% by volume or more, particularly 3 to 10% by volume in the total amount. Is desirable. In the present invention, the average particle size of the above-mentioned intragranular dispersed phase is limited as described above. When the average size of the intragranular dispersed phase exceeds 0.5 μm, the strengthening effect on alumina crystals decreases, and in some cases, precipitation occurs. This is because excessive stress may be generated between the particles and the matrix crystal, and cracks may be generated. The size of the intragranular dispersed phase is particularly 0.3 μm.
m or less.

Further, according to the present invention, the alumina crystal or F
It is desirable that another metal oxide different from eAl ₂ O ₄ be present in a proportion of 3% by volume or more, particularly 5 to 40% by volume. These grain boundary dispersed phases can effectively suppress the grain growth of alumina crystals, which are the parent phase, and can also contribute to improving the strength of the material. As the third crystal phase, an oxide containing a group 2A to 7A element in the periodic table, for example, MgAl ₂
O ₄ , CaAl ₁₂ O ₁₉ , Y ₃ Al ₅ O ₁₂ , LaAl ₁₁ O
One or more selected from compounds such as ₁₈ , HfO ₂ , and V ₂ O ₅ are suitable, and these have excellent stability at high temperatures and an effect of suppressing grain growth.

Next, a method for producing the alumina sintered body of the present invention will be described. In order to produce the above-mentioned alumina-based sintered body, first, a compact containing alumina as a main component and at least Fe element is produced. This compact is made of a compound containing Fe element, for example, iron powder, F
iron oxide powder such as eO, Fe ₂ O ₃ , Fe ₃ O ₄ , Fe
Add organic and inorganic salts and their solutions containing
After sufficiently mixing and drying with a ball mill or the like, the mixture is molded into an arbitrary shape by a desired molding means, for example, a die press, a cold isostatic press, an extrusion molding or the like, and then fired. Where F
The amount of e added is 1% by volume of the above-mentioned FeAl ₂ O ₄ in the total amount.
To precipitate at the above ratio, 0.5 mol% or more is appropriate in terms of Fe ₂ O ₃ .

At the time of firing, first, the molded body produced as described above is heated at a temperature of 1300 to 1700 ° C. in an atmosphere or an oxidizing atmosphere having an oxygen partial pressure of 10 ⁻³ or more at a temperature of 1 to 1700 ° C.
When the heat treatment is performed for about an hour, the Fe element turns into Fe ³⁺ ions and forms a solid solution in the alumina crystal.

Then, the solid solution obtained by the above method is subjected to a heat treatment in a reducing atmosphere composed of, for example, a CO, H ₂ gas or a non-oxidizing mixed gas atmosphere containing CO and H ₂ . By heat treatment in a reducing atmosphere, the Fe element becomes Fe ²⁺
Reduced to ions. Since the amount of the Fe ²⁺ ions dissolved in alumina is smaller than that of the Fe ³⁺ ions, the Fe ions are separated from the alumina lattice crystal and react with the alumina. As a result, the alumina crystal grains are converted into FeAl ₂ O ₄ crystals. Will be deposited within.

At this time, if the precipitation temperature is low, the reduction of Fe ³⁺ ions cannot proceed, and if the precipitation temperature is high, the alumina crystal and the intragranular precipitation phase of the mother phase grow and the FeAl
₂ O ₄ tends to segregate at grain boundaries. Therefore, the above-mentioned precipitation treatment is preferably performed in a temperature range of 1300 ° C to 1650 ° C.

At the time of the precipitation treatment, any heating method such as resistance heating or high frequency heating is possible, but heating by microwave irradiation can distribute the intragranular precipitation phase more uniformly. It is also effective in producing a large-sized sintered body.

Further, according to the production method of the present invention, a rare earth oxide such as Y ₂ O ₃ and La ₂ O ₃ , an alkaline earth oxide such as Ca and Mg, and H
Periodic Tables 4A to 4A to f, Zr, V, W, Mo, Mn, etc.
By adding the Group 7A element oxide to the alumina powder at a ratio of 1 to 20% by weight in terms of oxide, the oxide phase or the composite oxide phase of these oxides and alumina is added to the grain boundary. Can be deposited.

[0020]

EXAMPLES As raw material powder, alumina (Al ₂ O ₃ ),
Iron oxide (Fe ₂ O ₃ ) and yttrium oxide (Y ₂ O
₃ ), magnesium hydroxide (Mg (OH) ₂ ), lanthanum oxide (La ₂ O ₃ ), and hafnium oxide (HfO ₂ ) powder were mixed to a composition ratio shown in Table 1, and 1 t / cm ²
And then subjected to hydrostatic pressure treatment at a pressure of 3 t / cm ² . As shown in Table 1, firing conditions were firing at 1500 to 1700 ° C. for 5 hours in the air. The precipitation heat treatment was performed at 1300 ° C. to 1600 ° C. in a hydrogen atmosphere. Also,
Precipitation treatment of some samples was performed by microwave heating.

The obtained sintered body was polished to a shape specified by JIS-R1601, to prepare a bending specimen. This sample was subjected to a four-point bending strength test at room temperature and 1400 ° C. in the air based on JIS-R1601. Further, the fracture toughness (K ₁ c) was measured by the Vickers indentation method. The crystal phase in the sintered body was identified from the X-ray diffraction measurement data. Further, from the scanning electron micrograph, the average particle size of the alumina matrix phase and the intragranular precipitated phase and the volume fraction of the intragranular and intergranular dispersed phases were quantitatively measured. The results are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

From the results in Tables 1 and 2, it was found that FeAl ₂ O
_The alumina-based sintered body in which ₄ was finely dispersed in the alumina crystal grains had a room temperature strength of 500 MPa or more and 1400
℃ strength of 350MPa or more, fracture toughness of 4.0MPa
m ^1/2 or more. Also, the sintered bodies of Samples Nos. 4 and 6 to 10 of the present invention having a grain boundary dispersed phase together with an intragranular dispersed phase tend to have a smaller average particle size of alumina crystal. It can be seen that the strength is also increased.

On the other hand, Samples Nos. 11 and 12 are sintered bodies without addition or precipitation treatment.
No. 13 shows that the intragranular precipitation phase grew greatly, and Nos. 14 and 1
Sample No. 5 shows large growth of alumina crystals, and it can be seen that each of them has lower strength and toughness than the sintered body of the present invention.

[0026]

As described in detail above, according to the present invention, an alumina sintered body having stable and high strength and high toughness from room temperature to a high temperature of 1400 ° C. can be produced.

Claims (3)

(57) [Claims] A main phase is an alumina crystal having an average particle size of 6 μm or less, and an average particle size of 0.5 μm
The composite oxide represented by the following FeAl ₂ O ₄ was contained in the total amount of 1
An alumina-based sintered body characterized by being dispersed and contained in a proportion of at least volume%. 2. The method according to claim 1, wherein said FeAl
Another metal oxide crystal phase different from ₂ O ₄ crystal
2. The alumina-based sintered body according to claim 1, wherein the content of the sintered body is not less than% by volume. 3. A molded body containing alumina as a main component and containing at least an Fe element is heated in an oxidizing atmosphere to subject the Fe element to solid solution treatment in alumina crystals, and then the solid solution is subjected to a temperature range in a reducing atmosphere. A method for producing an alumina-based sintered body, characterized in that a heat treatment is carried out in step (a) to precipitate a composite oxide represented by FeAl ₂ O ₄ in alumina crystal grains.