JP3126059B2 - Alumina-based composite 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 an alumina-based composite sintered body, and more particularly to an alumina-based composite sintered body having a dense structure and high hardness, strength and fracture toughness.

[0002]

2. Description of the Related Art In general, alumina ceramics are particularly thermally and chemically stable among many ceramics and have relatively high hardness and mechanical properties. . For example, it is used for wear-resistant parts such as electronic circuit boards and packages, powder equipment, and cutting tools. However, the strength and fracture toughness are lower than those of partially stabilized zirconia ceramics, and the hardness is lower than that of boride or carbide ceramics.

[0003] For the purpose of improving the strength of alumina ceramics, silicon carbide has recently been dispersed in alumina grains. According to this method, silicon carbide having a particle diameter of about 0.1 to 0.5 μm is added, and alumina particles are pressure-sintered using a hot press. However, the fracture toughness has not been sufficiently improved. In addition, because of pressure sintering, products with complicated shapes cannot be sintered, and therefore products with complex shapes have to be manufactured from simple shaped sintered bodies by cutting, etc. It is expensive and is not an industrially advantageous method.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an alumina-based composite firing having a sufficiently dense structure and high hardness, strength and fracture toughness. In providing unity.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, it has been found that fine particles of silicon carbide are mainly dispersed inside the particles of alumina and that the particles of silicon carbide are dispersed. It has been found that a normal-pressure sintered body that forms a network of defects that are linearly extended and that are not connected to the outside of the alumina particles is useful, and completed the present invention.

That is, the alumina-based composite sintered body of the present invention is a normal-pressure sintered body consisting essentially of 80 to 97% by volume of alumina particles and 3 to 20% by volume of silicon carbide particles. The particle size is 1.0 to 10.0 μm,
The particle size of the silicon carbide is 0.3 μm or less,
The silicon carbide particles are dispersed inside the alumina particles and at the grain boundaries, and the temperature rising rate during sintering is set to 400 ° C./hr or more and the temperature drop rate is set to about 1,000 ° C./hr, whereby the inside of the alumina particles is reduced. The above-mentioned means for solving the above-mentioned problem is that the defects arranged in a line between the silicon carbide particles form a network inside the alumina particles, and the porosity is 2% or less as a whole.

Hereinafter, the alumina-based composite sintered body of the present invention will be described in detail. The alumina-based composite sintered body of the present invention has an alumina particle having a particle diameter of 1.0 to 10.0 μm and a particle diameter of 0.3 μm dispersed inside and at the grain boundary of the alumina particle.
m or less of silicon carbide fine particles.
Here, "essentially" means that the alumina-based composite sintered body of the present invention substantially consists only of alumina particles and silicon carbide fine particles. That is, in a typical embodiment,
Although it is composed only of such alumina particles and silicon carbide fine particles, it means that the third component may be mixed in a small proportion as long as the effect of the invention is not impaired.

In the alumina-based composite sintered body of the present invention, alumina particles occupy 80 to 97% by volume, and silicon carbide particles occupy 3 to 20% by volume. Here, the mixture at such a ratio is that the ratio of silicon carbide in the sintered body is 3% by volume.
If it is less than the above, the hardness and strength of the obtained sintered body will not be sufficiently improved, and if it exceeds 20% by volume, the silicon carbide fine particles will not be able to fully enter the alumina particles and will be present in large amounts at the grain boundaries. This not only hinders sintering but also becomes a source of destruction. Alumina particles and silicon carbide fine particles are previously blended so that the alumina particles in the sintered body occupy 80 to 97% by volume of the whole and the silicon carbide fine particles occupy 3 to 20% by volume of the whole. By sintering, the defects arranged in a line between the silicon carbide particles inside the alumina particles form a network, and the entire porosity is formed in an alumina-based composite sintered body of 2% or less. is there.

In order to manufacture the alumina-based composite sintered body of the present invention, first, in order to enable normal pressure sintering, alumina particles and silicon carbide particles have extremely small average particle diameters and large specific surface areas, respectively. Therefore, one having excellent surface activity is prepared. Alumina particles have an average particle size of 0.05 μm
Hereinafter, it is a γ-type crystal having a BET specific surface area of 40 m ² / g or more, and the silicon carbide particles are a β-type crystal having an average particle diameter of 0.05 μm or less, a BET specific surface area of 40 m ² / g or more. Is preferred.

The reason for using γ-type crystals as alumina particles is that γ-alumina particles usually undergo a phase transition to α-type by heating at 1100 ° C. or more, and at that time, about 100 times the grain growth occurs. The reason for using β type as silicon carbide particles is that β-silicon carbide is cubic and the unit cell is cubic, so that it can be introduced into the alumina particles in random directions. This is because, since it is incorporated, the characteristics to be expressed are uniform without any directionality.

Next, these powders are mixed in such a ratio that alumina particles account for 80 to 97% of the whole and silicon carbide fine particles account for 3 to 20% by volume of the whole. Here, the mixing at such a ratio is such that if the ratio of silicon carbide in the sintered body is less than 3% by volume, the hardness and strength of the obtained sintered body are not sufficiently improved, %, Silicon carbide particles do not fit into the alumina particles and are present in large amounts at the grain boundaries, which not only hinders sintering but also easily becomes a source of destruction. Further, at the time of mixing, since it is necessary to collect and uniformly mix the fine particles, it is desirable to apply fine vibrations by ultrasonic waves in addition to ordinary wet mixing.

Next, when the above mixed powder is heat-treated at about 1200 ° C., the alumina is transformed into α-form and becomes 0.5 to 1.0.
The grains grow to a particle diameter of μm. At this temperature, silicon carbide hardly grows in grain size, so it is taken into the alumina particles. Alumina powder having a particle size of 0.5 to 1.0 μm containing silicon carbide inside the particles synthesized as above is molded by a known method, and then sintered at 1800 ° C. or more for 1 hour or more. The above-mentioned alumina powder can be sintered under normal pressure, unlike a simple mixed powder. At this time, the heating rate was 400 ° C / h.
r or faster. The reason is that the low-temperature region in which a neck is formed between particles is quickly terminated, and the temperature quickly shifts to a high-temperature region in which grain growth occurs, and the grains are grown to a particle size of 1.0 to 10.0 μm to completely convert the silicon carbide particles into alumina particles. This is to take it inside.

When the diameter of the grown alumina particles is less than 1.0 μm, the dispersion of the silicon carbide particles inside the alumina particles becomes insufficient. On the other hand, when the diameter of the grown particles exceeds 10 μm, uniform growth becomes difficult and abnormalities occur. Pores are generated due to grain growth and the porosity becomes larger than 2%, and the mechanical properties of the obtained sintered body become insufficient.

The silicon carbide particles taken into the alumina particles have a tensile stress in the tangential direction of the silicon carbide particles due to a difference in thermal expansion from alumina when cooled from the sintering temperature.
A compressive stress is generated in the normal direction. Since the compressive stress in the normal direction goes to the grain boundary of alumina, it compresses and strengthens the grain boundary. For this reason, it is difficult to break from the grain boundaries, and the breaking source is inside the particles, and the size of the breaking source, which is one of the factors that determine the strength, becomes smaller than the particle diameter, leading to improvement in strength.

After normal pressure sintering at 1800 ° C. or more for 1 hour or more and cooling at a cooling rate of about 1000 ° C./hr, the bond of alumina atoms is broken by tensile stress in the tangential direction and defects are generated. Since the tensile stress is relieved in a portion where a defect is first generated, stress is concentrated from another portion, and the defect generated in the other portion is moved to form a stacked linear shape. Further, it combines with the defects of the adjacent silicon carbide particles to gradually form a network. At this time, if the cooling rate is too slow, the defects grow to the outside of the alumina particles and are released to the compressive stress of the grain boundaries, so that the grain boundaries are not strengthened. On the other hand, if the cooling rate is too fast, there is a danger that the alumina itself will crack due to a difference in thermal expansion between the outside and the inside. FIG. 1 shows a schematic view of the structure of the sintered body of the present invention.

[0016]

In the alumina-based composite sintered body of the present invention, alumina particles serve as a matrix, and silicon carbide fine particles are dispersed therein. Is formed.

The silicon carbide fine particles taken into the alumina particles generate stress toward the grain boundaries of the alumina particles due to a difference in thermal expansion from the alumina particles during cooling from the sintering temperature, thereby strengthening the grain boundaries. . Therefore, intragranular fracture is dominant and the fracture source is inside the particle, so that the size of the fracture source is smaller than the particle diameter, which leads to improvement in strength. Here, in the production of the alumina-based composite sintered body of the present invention, as a sintering method, unlike conventional pressure sintering, normal-pressure sintering is employed, and when cooling at a specific cooling rate, it occurs. Thermal stress is
In addition to going to the grain boundaries, some are released around the silicon carbide particles to break the bonds of the atoms in the alumina particles and create defects. These defects are moved between the silicon carbide fine particles by the tensile stress and are arranged in a line to form a network. FIG. 2 shows a transmission electron micrograph of one example. It is desirable that the linear defects exist inside the alumina particles and do not reach the grain boundaries. The reason is that if the defect reaches the grain boundary, most of the generated thermal stress is released, so that strengthening the grain boundary does not lead to improvement in strength.

In short, in the alumina-based composite sintered body of the present invention, alumina having high thermal and chemical stability serves as a matrix, and silicon carbide having high hardness is dispersed therein, so that the hardness of alumina is improved. In addition, stress is generated toward the grain boundaries by intra-particle dispersion to strengthen the grain boundaries, thereby improving the strength. In addition, the cracks that have progressed through intragranular fracture interact with a network of linear defects formed between the silicon carbide particles, causing cracks to deflect and branch within the particles, resulting in fracture. The required energy is increased, resulting in improved fracture toughness values. Further, it is said that the size of the defect that usually causes fracture is several tens of μm or more, whereas in the sintered body of the present invention, the defect in the particle is smaller than the particle diameter, It is not a cause, and the defect network may lead to an increase in strength due to an increase in fracture toughness, but does not cause a decrease in strength.

Therefore, the alumina-based composite sintered body of the present invention is a ceramic material useful as a raw material for producing excellent mechanical parts.

[0020]

Hereinafter, the present invention will be described in detail with reference to examples. Examples 1-3 and Comparative Example 1 γ-alumina powder (average particle size: 0.004 μm) and β-alumina powder
Silicon carbide powder (average particle size: 0.02 μm) was mixed at a ratio shown in Table 1, wet-mixed in methanol vibrated by ultrasonic waves for 10 hours, dried, and heat-treated and pulverized to reduce the particle size. Adjust to about 0.5-1.0μm, 45 × 35 ×
It was formed into a 5 mm plate shape. Thereafter, this molded product was sintered under a nitrogen gas atmosphere at normal pressure under the sintering conditions shown in Table 1 to obtain an alumina-based composite sintered body. At that time, the rate of temperature rise to the sintering temperature was 400 ° C./hr, and the rate of temperature decrease from the sintering temperature was 1
000 ° C.

Comparative Example 2 An alumina-based composite sintered body was obtained in the same manner as in Example 1 except that a silicon carbide raw material powder having an average particle diameter of 0.3 μm was used.

Comparative Example 3 Sintering was performed by hot pressing at 1800 ° C. for 1 hour in a nitrogen gas atmosphere, and the other conditions were the same as in Example 1 to obtain an alumina-based composite sintered body.

A transmission electron micrograph of the composite sintered body obtained in Example 1 is shown in FIG. With respect to Comparative Examples 2 and 3, it was confirmed from transmission electron micrographs that there were no linear defects between the silicon carbide particles inside the alumina particles.

The densities of the sintered bodies obtained in Examples 1 to 3 and Comparative Examples 1 to 3 were measured by the Archimedes method using water displacement, and the relative densities were calculated from the theoretical densities and the actually measured values. Further, the three-point bending strength at room temperature was measured according to JIS-R1.
According to No. 601, the hardness and the fracture toughness were measured by a Vickers hardness meter and an indentation method, respectively. Table 1 shows the obtained results.

[0025]

[Table 1] From Table 1, it was confirmed that the sintered body according to the present invention had a higher density, a denser structure, and excellent strength, hardness, and fracture toughness as compared with the sintered body of the comparative example.

[Brief description of the drawings]

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

FIG. 2 is a transmission electron micrograph showing the fine structure of the alumina-based composite sintered body of the present invention.

[Explanation of symbols]

 1 Alumina grain boundary 2 Silicon carbide fine particles 3 Network of linear defects

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-60-210571 (JP, A) JP-A-64-87552 (JP, A) JP-A-61-174165 (JP, A) JP-A-4- 240157 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/10 C04B 35/64

Claims (1)

(57) [Claims] 1. A method according to claim 1, wherein 80 to 97% by volume of alumina particles and 3 to
An atmospheric pressure sintered body consisting essentially of 20% by volume of silicon carbide particles, wherein the alumina particles have a particle size of 1.0 to 10.0.
μm, the particle size of the silicon carbide is 0.3 μm or less, and the silicon carbide particles are dispersed inside the alumina particles and at the grain boundaries.
hr or more, and the temperature decreasing rate is about 1,000 ° C./hr, so that defects linearly arranged between the silicon carbide particles inside the alumina particles form a network inside the alumina particles, and Characterized in that the porosity is 2% or less.