JPH09328355A - Alumina-based sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an alumina-based sintered compact having stable strength in an oxidizing atmosphere from a room temperature to a high temperature and improved toughness and to provide a method for production thereof. SOLUTION: A molding compact comprising alumina as a main component and containing at least Fe element is heated in an oxidizing atmosphere so as to subject the Fe element to solid solution treatment in alumina crystal. The solid solution is heat-treated in a temperature range of a reducing atmosphere and a compound oxide of the formula FeAl2 O4 is precipitated in the alumina crystal particle to give the objective alumina-based sintered compact comprising alumina crystal having <=6μm average particle diameter as a main phase and >=1vol.% based on the whole amount of the compound oxide of the formula FeAl2 O4 having <=0.5μm average particle diameter in the alumina crystal particle. Another metal oxide crystal phase, in an amount of 3vol.% based on the whole amount, different from the FeAl2 O4 crystal is added to the compact to suppress the granule growth.

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 excellent in high strength, high toughness and environment 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-based sintered body and a method for manufacturing 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. Recently, in order to expand the applications of this alumina sintered body, various composites have been tried in order to further improve its 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-63-139044, etc.), Al ₂ O ₃ -Ti
O ₂ composite material (see JP-A-7-257964)
Etc. are known, and such a composite material can improve strength and toughness as compared with a general alumina-based sintered body.

[0003]

However, the above A
In the l ₂ O ₃ -SiC nanocomposite composite material, A
Since the non-oxide SiC particles are dispersed in l ₂ O ₃ , there is a problem that the SiC reacts with the matrix Al ₂ O ₃ when it is kept at a high temperature for a long time.

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

Further, the 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 form Al ₂ TiO _5, which causes a problem that the material properties are unstable.

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

[0007]

Means for Solving the Problems As a result of repeated studies on the above object, the present inventor has found that FeAl is contained in alumina crystal grains.
_The above object can be achieved by dispersing the composite oxide represented by ₂ O ₄ as fine particles having an average particle size of 1% by volume or more in the total amount and an average particle size of 0.5 μm or less. The present invention has been found out. Further, when the intragranular dispersed phase or another metal oxide phase different from the alumina crystal phase is dispersed in the grain boundary of the alumina main crystal, the grain growth of alumina is suppressed and the strength of the sintered body is further improved. The present invention was discovered and the present invention was reached.

Further, as a method for producing the above-mentioned alumina sintered body, a formed body containing alumina as a main component and containing at least Fe element is heat-treated in an oxidizing atmosphere to form a solid solution of Fe element in the alumina crystal. After this, the solid solution is further heat-treated in a reducing atmosphere to obtain the above-mentioned Fe.
It has been found that the element can be precipitated in the alumina crystal grains in the form of FeAl ₂ O ₄ , and the present invention has been completed.

According to the present invention as described above, by dispersing fine particles of FeAl ₂ O ₄ in the alumina main crystal grains, stable high strength from room temperature to high temperature can be realized 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. The alumina crystals are prone to grain growth during firing, but the average grain size is 6 μm or less. Is advantageous for increasing the strength of the sintered body, and the average grain size of the alumina crystals is particularly preferably 4 μm or less. Therefore, this alumina main crystal has an average grain size of 6
If it exceeds μm, the desired strength cannot be obtained. This alumina main crystal phase is 50% by volume or more, especially 70% by volume in the total amount.
From the viewpoint of strength, it is suitable to exist in a proportion of ˜95% by volume.

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

If the amount of the FeAl ₂ O ₄ intragranular dispersed phase is small, the effect of improving the strength and toughness is small. Therefore, it is dispersed in a total amount of 1% by volume or more, particularly 3 to 10% by volume. Is desirable. In the present invention, the average grain size of the intragranular dispersed phase is limited as described above because the strengthening effect on the alumina crystal is reduced when the average size of the intragranular dispersed phase exceeds 0.5 μm, and in some cases precipitation occurs. This is because an excessive stress may be generated between the particles and the matrix crystal to cause cracks. The size of the intragranular dispersed phase is especially 0.3μ
m or less.

Further, according to the present invention, at the grain boundary of the alumina crystal, the alumina crystal or F is used as a third crystal phase.
It is desirable that another metal oxide different from eAl ₂ O ₄ is 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 the alumina crystal that is the parent phase and at the same time contribute to the improvement of the strength of the material. As the third crystal phase, an oxide containing a group 2A to 7A element of 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 compounds are excellent in stability at high temperature and grain growth suppressing effect.

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

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

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

At this time, if the precipitation treatment temperature is low, reduction of Fe ³⁺ ions cannot proceed, and if the precipitation treatment temperature is high, the alumina crystals of the mother phase and the intragranular precipitation phase undergo grain growth, resulting in FeAl.
₂ O ₄ tends to segregate at grain boundaries. Therefore, it is preferable that the precipitation treatment is performed in the temperature range of 1300 ° C to 1650 ° C.

Although any heating method such as resistance heating or high frequency heating can be used during the precipitation treatment, heating by irradiation with microwaves makes it possible to more uniformly distribute the intragranular precipitation phase, It is also effective in producing a large-sized sintered body.

Furthermore, according to the production method of the present invention, rare earth element oxides such as Y ₂ O ₃ and La ₂ O ₃ , alkaline earth oxides such as Ca and Mg, and H are added to the above system.
Periodic Table 4A to f, Zr, V, W, Mo, Mn, etc.
By adding the 7A group element oxide to the alumina powder in an amount of 1 to 20% by weight in terms of oxide, these oxide phases or a composite oxide phase of these oxides and alumina are added to the grain boundaries. It can be deposited.

[0020]

Example 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 ) powders were mixed at the composition ratio shown in Table 1 to 1 t / cm ^2.
After mold molding at a pressure of 3, a hydrostatic pressure treatment was applied at a pressure of 3 t / cm ² . As for the firing conditions, as shown in Table 1, firing was performed in the air at 1500 to 1700 ° C. for 5 hours. The deposition 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 sintered body thus obtained was ground to a shape specified in JIS-R1601 to prepare a bending sample. This sample was subjected to a 4-point bending transverse strength test at room temperature and 1400 ° C. in the atmosphere 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, the average grain size of the alumina matrix phase and the intragranular precipitation phase and the volume fraction of the intragranular and grain boundary dispersed phase were quantitatively measured from the scanning electron micrograph. The results are shown in Table 2.

[0022]

[Table 1]

[0023]

[Table 2]

From the results of Table 1 and Table 2, FeAl ₂ O
_{All of} the alumina-based sintered bodies in which ₄ is finely dispersed in the alumina crystal grains have room temperature strengths of 500 MPa or more and 1400
℃ strength is 350MPa or more, fracture toughness is 4.0MPa
m ^1/2 or more. Further, the sintered bodies of Samples Nos. 4 and 6 to 10 of the present invention having the intragranular dispersed phase and the grain boundary dispersed phase tended to have a smaller average particle size of alumina crystals, and the smaller the alumina crystal particle size, the smaller It can be seen that the strength also increases.

On the other hand, Sample Nos. 11 and 12 are sintered bodies with no addition or precipitation treatment.
No. 13 is a large grown intragranular precipitation phase, No. 14 and 1
No. 5 is a large-grown alumina crystal, and it can be seen that all of them have lower strength and toughness than the sintered body of the present invention.

[0026]

As described in detail above, according to the present invention, it is possible to produce an alumina-based sintered body having stable and high strength from room temperature to a high temperature of 1400 ° C. and having high toughness.

Claims (3)

[Claims] 1. An alumina crystal having an average particle size of 6 μm or less as a main phase, and an average particle size of 0.5 μm in the crystal grains of the alumina.
The total amount of the composite oxide represented by FeAl ₂ O ₄ below is 1
An alumina-based sintered body characterized by being dispersed and contained in a proportion of not less than volume%. 2. The FeAl at the grain boundary of the alumina crystal.
3% of other metal oxide crystal phases different from ₂ O ₄ crystals
The alumina-based sintered body according to claim 1, characterized in that it is contained in a proportion of not less than volume%. 3. A molded body containing alumina as a main component and containing at least Fe element is heated in an oxidizing atmosphere to effect solid solution of Fe element 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 the composite oxide represented by FeAl ₂ O ₄ is precipitated in the alumina crystal grains by heat treatment.