JPH0987009A - Alumina-mullite combined sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the strength and toughness of crystals themselves, to considerably improve resistance to cracking and plastic deformation at high temp. and to ensure high strength and high toughness over the range from room temp. to high temp. by dispersing metal oxide crystal grains different from alumina and mullite crystal phases in the grains of the alumina and mullite crystal phases of a combined sintered compact contg. the crystal phases as the principal crystal phases. SOLUTION: This sintered compact contains metal oxide crystal grains contg. one or more among Ti, Mg and Fe and preferably having <=500μm grain diameter preferably by 0.5-10wt.% (expressed in terms of oxide) of the total amt. The amt. of mullite in this sintered compact is not especially limited but is preferably 5-80wt.% of the total amt. A powdery mixture of alumina and mullite or silicon oxide with the above-mentioned metal oxide having different solubility limits in alumina and mullite in correspondence with different atmospheres is compacted in an arbitrary shape, the resultant compact is sintered in an atmosphere in which the solubility limit is increased and the sintered compact is heat-treated in an atmosphere in which the solubility limit is reduced.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to room temperature to 1,300
Alumina-mullite composite oxide sintered body that has excellent high temperature strength at ℃ and high toughness, especially for aviation related parts, refining parts, parts used in space environment, heat engine parts Is what is used.

[0002]

2. Description of the Related Art Conventionally, an alumina-based sintered body has been used as a typical ceramic material in various fields because of its excellent strength and chemical resistance. Further, since this alumina-based sintered body is insufficient in strength, fracture toughness, especially high temperature strength, various improvements have been made in order to improve these characteristics.

For example, a composite material obtained by adding silicon carbide or zirconia to alumina is disclosed in JP-A-61-2122164.
And Japanese Patent Laid-Open No. 63-139044. Further, a material in which β-alumina crystals having a shape anisotropy in which a rare earth element is contained is dispersed in alumina is also disclosed in JP-A-63-
Proposed in issue No. 134551. Further, a material in which mullite is mixed with alumina, for example, Japanese Patent Laid-Open No. 1-1195.
59 have been proposed.

[0004]

[Problems to be Solved by the Invention] However, the above A
Since the l ₂ O ₃ —SiC composite material contains non-oxide SiC in a dispersed state, it lacks oxidation resistance in a high temperature oxidizing atmosphere, and the Al ₂ O ₃ —ZrO ₂ composite material has a sharp strength at a temperature near 900 ° C. Therefore, it is desired to improve the strength and toughness in a system containing no second phase such as SiC or ZrO ₂ .

On the other hand, the composite material in which β-type alumina containing a rare earth element is dispersed is excellent in oxidation resistance, and the strength and toughness from room temperature to high temperature are superior to those of the unsintered alumina sintered body. 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.

Further, the alumina-mullite composite material is
Although the strength at room temperature and high temperature was improved as compared with the pure alumina sintered body, there were problems such as low fracture toughness.

[0007]

Means for Solving the Problems The inventors of the present invention have made extensive studies on a method for increasing the strength and toughness of an alumina-mullite composite material. As a result, in addition to the structural improvement of the sintered body, As a result of repeated studies from the viewpoint that it is important to increase the strength of the crystal phase itself that constitutes the sintered body, a metal oxide different from each crystal phase is formed in the grains of the alumina crystal phase and the mullite crystal phase. It has been found that high temperature strength and toughness can be improved by dispersing them as fine crystals.

That is, the alumina-mullite composite sintered body of the present invention is a composite sintered body having an alumina crystal phase and a mullite crystal phase as main crystal phases, and the particles of the alumina crystal phase and the mullite crystal phase. It is characterized in that metal oxide crystal particles different from each crystal phase are dispersed therein. The metal oxide crystal particles are 500 n
It has a particle diameter of m or less. Further, the metal oxide crystal particles contain at least one of titanium, magnesium, and iron.

Further, according to the method for producing an alumina-mullite composite sintered body of the present invention, mullite or silicon oxide is different from alumina and metal oxides having different solid solution limit amounts to alumina or mullite are different depending on the atmosphere. And a step of molding a mixture of the mixed product, and the molded body obtained is fired in an atmosphere in which the solid solution limit amount of the metal oxide is increased, and an alumina crystal and a mullite crystal are used as a main crystal phase. And a step of producing a sintered body in which the metal oxide is solid-solved in the alumina crystal and the mullite crystal, and heat-treating the sintered body in an atmosphere in which the solid solution limit amount of the metal oxide is reduced. Then, in the alumina crystals and the mullite crystal grains, crystal particles of the metal oxide, or a crystal of a metal oxide in which the metal oxide is complexed with alumina or silicon oxide. Characterized in that comprising the step of precipitating the child. Further, the crystal particles of the metal oxide are characterized by containing at least one of titanium, magnesium and iron.

[0010]

OPERATION Alumina has stable characteristics from room temperature to high temperature especially in an oxidizing atmosphere, but at high temperature, dislocations are likely to move, so that it tends to be softened and plastically deformed. Also, at room temperature, fracture toughness is low because cracks easily propagate within the crystal. Therefore, by combining mullite with alumina, the room temperature and high temperature strength of the material have been improved, but it is still insufficient in terms of characteristics and there is a problem that fracture toughness is particularly low.

According to the present invention, by dispersing fine crystal particles made of a metal oxide other than alumina or mullite in the grains of the alumina crystal phase and the mullite crystal phase, the strength and toughness of the crystal itself is increased, The resistance to the propagation of cracks through the grains and the resistance to plastic deformation at high temperature can be greatly improved, and as a result, a sintered body having high strength from room temperature to a high temperature of 1300 ° C. and high toughness can be obtained.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The alumina-mullite composite sintered body of the present invention has a structure, as shown in FIG. 1, including a main crystal phase composed of an alumina crystal phase 1 and a mullite crystal phase 2, and the main crystal phases thereof. It is composed of metal oxide crystal particles 3 precipitated in the phase grains. Alumina crystal phase 1 has an α-type crystal structure. Here, the mullite crystal phase 2 is a composite oxide of alumina and silicon oxide, and is 3Al ₂ O ₃ .2Si.
Expressed as O ₂ .

Mullite crystals themselves have the property that their strength does not decrease even at high temperatures up to 1400 ° C. When added to alumina, the grain growth of alumina crystals during firing can be suppressed and a fine structure can be formed. Therefore, a sintered body having a higher strength from room temperature to a higher temperature than alumina can be obtained.

However, since the mullite crystal has a low Young's modulus and a low fracture toughness, it has no effect of suppressing the progress of cracks in the grain, and the composite sintered body with alumina has a problem of low fracture toughness. In addition, since the alumina itself softens at high temperature, that is, plastic deformation occurs due to the movement of dislocations, the high temperature strength is still insufficient.

According to the present invention, in the above-mentioned sintered body having the alumina crystal phase and the mullite crystal phase as the main crystal phases, as shown in FIG. 1, the crystal phases are different from each other within the grains. It is important that fine crystal particles 3 made of a metal oxide are dispersed. Here, the metal oxide crystal particles dispersed in the grains may be a metal oxide in which two or more kinds of metals are compounded, in addition to a single metal oxide,
It may be a composite oxide formed by reacting these metal oxides with alumina or mullite.

The metal oxide crystal grains to be dispersed in these grains contain a metal oxide whose solid solution limit amount changes with alumina or mullite depending on the atmosphere. This metal oxide is selected from the group consisting of Mg, Ca, Fe, Co, and 3a, 4a, 5a, and 6a group metals of the periodic table from the viewpoint of Young's modulus of the oxide and high stability at high temperature. It is preferable that at least one kind of element contained in the metal oxide crystal particles is contained in the metal oxide crystal particles because of easy control of solid solution-precipitation. It is desirable to contain at least one of Fe).

The solid solution in alumina is Mg is MgT.
It forms a large amount of solid solution in the form of iO ₃ .

The metal oxide crystal particles dispersed in the particles of the alumina crystal and the mullite crystal phase have an average particle diameter of 50.
It is preferably 0 nm or less. If the grain size of the crystal grains is small, the consistency of the crystal is maintained at the interface with the main crystal phase, and strain is generated, so that a larger strengthening and toughening effect can be exhibited. If the average grain size of the crystal grains dispersed in the grains exceeds 500 nm, the interaction between the crystals becomes small, and in some cases, thermal stress causes defects at the phase interface. The average particle size of the crystal particles dispersed in the particle is 200 n
It is preferably m or less.

In the alumina-mullite composite sintered body of the present invention, the amount of mullite is not particularly limited, but it is desirable that the amount of mullite is 5 to 80% by weight based on the overall characteristics of strength and toughness. Further, the metal oxides precipitated in these crystal grains are 0.5 to 0.5 in the total amount in terms of oxides.
It is desirable that the content is 10% by weight. As the amount of metal oxide increases, a reaction phase due to this metal oxide may be generated at the grain boundaries, but this has little effect on the properties of the material.

According to the method for producing an alumina-mullite composite sintered body of the present invention, first, alumina and mullite or silicon oxide are mixed with alumina and a metal oxide having different solid solution limit amounts in mullite due to a difference in atmosphere. Prepare a mixed powder. Examples of such metal oxides include Mg, Ca, Fe, Co, and 3a and 4 of the periodic table as described above.
Examples thereof include at least one oxide selected from the group of a, 5a, and 6a group metals. Among them, titanium oxide (TiO ₂ ), magnesium oxide (MgO), At least Ti, Mg, Fe such as iron (Fe ₂ O ₃ ) or a composite oxide thereof, or a composite oxide thereof and alumina or mullite.
It is desirable that the metal oxide contains.

The mixed powder is not particularly limited as long as it can form an oxide in the firing step, and examples thereof include metal powders, carbonates, nitrates, acetates of the metal, and organic materials containing the metal. Inorganic substances and their solutions, sols, etc. may be used.

The mixed powder is mixed with a desired molding means, for example,
Mold into any shape by die pressing, cold isostatic pressing, extrusion molding, etc.

Next, this molded body is subjected to a known sintering method such as hot pressing method, normal pressure firing method, gas pressure firing method, microwave heating firing method, and hot isostatic treatment (HIP) after these firing. ) Treatment and HIP treatment after glass sealing to obtain a dense sintered body having a theoretical density ratio of 95% or more.

According to the present invention, the calcination at this time needs to be a calcination atmosphere in which the solid solution limit amount of the metal oxide in alumina or mullite is large. The firing temperature at this time is 1300 ° C to 1800 ° C, especially 14 ° C.
It is preferable that the temperature is 00 ° C to 1750 ° C from the viewpoint of solid solubility, denseness of host crystal, and suppression of grain growth.

Thereafter, the sintered body obtained by solid-solving the metal oxide obtained as described above is heat-treated in an atmosphere in which the solid solution limit amount of the metal oxide is reduced. The appropriate treatment temperature at this time is 1100 to 1700 ° C. By treating under such conditions, the supersaturated solid solution element can be uniformly precipitated in the alumina and mullite crystal grains in the form of an oxide or a complex oxide.

The above solid solution-precipitation step is carried out by using the solid solution step as H ₂ when TiO ₂ is used as the metal oxide.
And the like, and the deposition process is performed in an oxidizing atmosphere in the air. When an oxide of Ti / Mg (Ti: Mg = 1) or Fe is used, the solid solution step may be performed in an oxidizing atmosphere such as the air, and the precipitation step may be performed in a reducing atmosphere.

[0027]

[Examples] Mullite (Al ₂ O ₃ / SiO ₂ = 72/28 weight ratio), titanium oxide (TiO ₂ ), magnesium oxide (MgO), iron oxide with respect to alumina (Al ₂ O ₃ ) as raw material powder (Fe ₂ O ₃ ) was added and mixed in the composition ratio shown in Table 1, and after molding with a pressure of 1 t / cm ² ,
Further, hydrostatic pressure treatment was applied at a pressure of 3 t / cm ² . Then, the obtained molded body is solid-solved (calcined) under the conditions shown in Table 1,
A precipitation treatment was performed.

The obtained sintered body was processed into a mirror surface, the structure was observed by a scanning electron microscope, the crystal phase was identified by X-ray diffraction, and the precipitated phases other than the α-alumina crystal phase and the mullite crystal phase were identified. The results are shown in Table 2. Table 2 shows the average particle sizes of the metal oxides precipitated in the alumina crystal phase and the mullite crystal phase, which were observed by an electron micrograph. The structure of the sintered body obtained in this example after the precipitation solid solution treatment is shown in FIG. Further, as a mechanical property, a bending sample was prepared by polishing to a shape specified in JIS-R1601. This sample was subjected to a 4-point bending transverse strength test at room temperature and 1300 ° C. based on JIS-R1601. Further, the fracture toughness (K1c) was measured by the Vickers indentation method. The results are shown in Table 2.
It was shown to.

[0029]

[Table 1]

[0030]

[Table 2]

As is clear from the results shown in Tables 1 and 2, the strength of the sample No. 2 compounded with mullite is improved as compared with the sintered body of the sample No. 1 of alumina alone. Furthermore, the sintered bodies of Samples No. 5 to 9 in which the metal oxide or the composite metal oxide is finely dispersed in the grains of the alumina crystal phase and the mullite crystal phase obtained according to the present invention are the sample No. 1 and the sample No. All of them are superior to No. 2, and strengths of 540 MPa or more at room temperature and 350 MPa or more at 1300 ° C. and toughness of 3.6 MPam ^0.5 or more were achieved.

It is to be noted that even in the composition system in which the metal oxide was added, the sintered body samples No. 3 and 4 in which the metal oxide was not precipitated in the main crystal phase without undergoing the solid solution-precipitation step Also strength,
The toughness is low, and in sample No. 3, the complex oxides in Table 2 were precipitated not at the grain boundaries but at the grain boundaries.

[0033]

As described in detail above, according to the present invention, by dispersing a fine metal oxide in the grains of the alumina crystal phase and the mullite crystal phase, the strength and toughness of the crystal itself is increased and cracks are generated. It is possible to greatly improve the resistance against the propagation of the particles through the grains and the resistance to the plastic deformation at high temperature, and as a result, it is possible to obtain a sintered body having high strength and high toughness from room temperature to a high temperature of 1300 ° C.

[Brief description of drawings]

FIG. 1 is a diagram for explaining the structure of an alumina-mullite composite sintered body of the present invention.

[Explanation of symbols]

 1 Alumina Crystal Phase 2 Mullite Crystal Phase 3 Metal Oxide Crystal Particles

Claims (5)

[Claims] 1. A composite sintered body comprising an alumina crystal phase and a mullite crystal phase as main crystal phases, wherein metal oxides different from the respective crystal phases are present in the grains of the alumina crystal phase and the mullite crystal phase. An alumina-mullite composite sintered body, characterized in that crystal particles are dispersed. 2. The alumina-mullite composite sintered body according to claim 1, wherein the metal oxide crystal particles have a particle diameter of 500 nm or less. 3. The alumina-mullite composite sintered body according to claim 1 or 2, wherein the metal oxide crystal particles contain at least one of titanium, magnesium and iron. 4. A step of molding a mixture of alumina, mullite or silicon oxide, and a metal oxide having a different solid solution limit amount in alumina or mullite depending on a difference in atmosphere, and the obtained molded body containing the metal. Sintered by firing in an atmosphere in which the solid solution limit amount of the oxide is increased to include alumina crystals and mullite crystals as a main crystal phase, and the metal oxide is solid-solved in the alumina crystals and the mullite crystals. A step of producing a body, and subjecting the sintered body to a heat treatment in an atmosphere in which the solid solution limit amount of the metal oxide is reduced, so that crystal grains of the metal oxide are contained in the alumina crystals and the mullite crystal grains. Or a step of precipitating crystal particles of a metal oxide in which the metal oxide and alumina or silicon oxide are compounded. The method of production. 5. The crystal particles of the metal oxide contain at least one of titanium, magnesium and iron.
A method for producing the described alumina-mullite composite sintered body.