JPS61186257A - Ceramic structure - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a ceramic structure with high strength and reliability.

[Background of the invention]

In recent years, oxide ceramics such as aluminum oxide and zirconium oxide, and silicon carbide.

Ceramic structures using silicide ceramics such as silicon nitride and sialon are beginning to be put into practical use.

In particular, silicide-based ceramics have the advantages of high high-temperature strength and excellent oxidation resistance at high temperatures, making them ideal for structures that can be used in high-temperature environments such as high-temperature gas turbine parts and automobile engine parts. , a wide range of applications are expected.

However, these conventional ceramic structures have
When surface scratches are generated that are unavoidable during processing and handling, external stress is concentrated and the product tends to break, resulting in a lack of reliability.

The size of these surface scratches is usually 1 to 10% of the grain size of the sintered body.
It is thought to be about 3 times as large. To overcome this drawback, we created a layer with a small coefficient of thermal expansion, such as quartz glass, on the surface of these ceramics, and took advantage of the fact that compressive stress is applied to this layer at room temperature to reduce the effects of surface scratches. However, it has also been proposed to improve strength and reliability (for example,
Evans et al., J. Mater, Sci. 5
, 314 (1970) and Kirchner et al., J.
AvIl, Cer, Soc, 49t330 (
1966) etc.).

However, methods that utilize this difference in thermal expansion coefficients have two drawbacks: the above compressive stress decreases at high temperatures, and it is not very effective in improving reliability at high temperatures, where the use of ceramic structures is most strongly expected. There was a problem in that the layers could crack due to the difference in thermal expansion, resulting in a decrease in strength.

[Purpose of the invention]

The object of the present invention is to provide a ceramic structure which has high strength and high reliability over a wide temperature range from room temperature to high temperature, while eliminating the drawbacks of the prior art described in 1.

[Summary of the invention]

The ceramic structure of the present invention is characterized in that a surface layer having a grain size larger than the grain size of the ceramic constituting the inside of the structure is provided on the surface of the structure which is subjected to external stress during use. .

[Embodiments of the invention]

1 and 2 show the structure of the ceramic structure of the present invention. In the figure, ■ is the surface layer, and 2 is the inside.

This surface layer can be formed by adding a grain growth promoter to one surface layer and selectively growing grains only in the surface layer during sintering, or by heating only the surface layer of the structure that has been sintered and processed into the final shape. hand,
It can be formed by a method in which only the surface layer grows grains. In either method, compressive stress is generated in the surface layer as a result of differences in grain growth between the interior of the structure and the surface layer.

This compressive stress is based on the difference in the degree of grain growth and is not due to the difference in the coefficient of thermal expansion, so this compressive stress acts on the surface layer at both room temperature and high temperature within the operating temperature range of the structure. Moreover, this compressive stress cancels out the effect of external stress acting on surface scratches, making the ceramic of the present invention high in strength and reliability.

It is desirable that the average grain size of the ceramics constituting the surface layer is at least three times the average grain size of the internal ceramics. If the particle size of the surface layer is not sufficiently larger than the internal particle size, the compressive stress acting on the surface layer will be small, and the strength and reliability will not be sufficient. On the other hand, if the particle size of the surface layer becomes too large, surface scratches tend to become deep and the reliability of the structure tends to decrease. The grain size of ceramics used as a structure is usually about 0.5 to 20 μm, but it is desirable that the grain size of the surface layer is 1-00 times or less than this.

Further, the thickness of the surface layer needs to be larger than the depth of the surface scratches, and specifically, it needs to be five times or more the particle size of the particles constituting the surface layer. Moreover, if the thickness of the surface layer is too thick, the compressive stress will not reach the entire surface layer, resulting in a decrease in strength. As a result, the thickness of the surface layer needs to be 115 times smaller than the thickness of the structure. Note that the particle diameters of the inside and surface layers may change so as to continuously increase from the inside toward the surface. In particular, the thickness of the surface layer means the width of the region where the grain size is at least 3 times the average grain size of the central part.

It is particularly desirable that the ceramic particles constituting the surface layer have an anisotropic shape that extends in the direction of the maximum external stress that the surface receives during use. In this way, it is possible to generate compression and stress that is effective against external stress without making the average grain size of the surface layer larger than necessary, making the ceramic structure particularly reliable. It becomes sexual. In this case, if the long axis of the ceramic particles in the surface layer is elongated three times or more than the short axis, and the length of the long axis is three times or more the average particle diameter of the internal ceramic particles, Particularly effective.

The manufacturing method for ceramic structures with this type of structure is as follows:
The surface of the structure during or after sintering may be locally heated with a heat source such as a laser beam, and the heated portion may be moved in the direction of maximum external stress. For this purpose, a heat source such as a laser beam may be scanned, or the heat source may be stopped and the sample moved. If such a method is used, the grain growth part moves in the direction of the maximum external stress during grain growth in the surface layer, resulting in a structure in which the grains in the surface layer are aligned and elongated in the direction of maximum stress. It will be done. Note that this method is particularly effective in systems where grain growth occurs by liquid phase growth.

Ceramics to which the present invention is applied include oxide ceramics such as aluminum oxide, zirconium oxide, and magnesium oxide, and silicide ceramics such as silicon carbide, silicon nitride, and sialon. Among these, silicide-based ceramics have the advantage of high strength even at high temperatures, which is a feature of the present invention, and the compressive stress of the surface layer does not disappear even at high temperatures. It has the advantage of producing a strong and highly reliable ceramic structure.

Hereinafter, the present invention will be explained according to examples.

Example 1 To SjC powder with a particle size of 0.5 μm, 3 wt% of He〇N powder with a particle size of 1 to 2 μm and 2 wt% of polyvinyl alcohol as a binder were added, and the mixture was molded into a disk shape of 60φ×20t. . Next, the molded body was placed in a graphite mold, and a pressure of 3
Hot press sintering was carried out at 00 kg/cJ and a temperature of 2000° C. for 1 hour. The average grain size of the obtained sintered body was about 2 μm.

Both sides of this disk were ground, and a hole was made in the center and notches were made in the periphery to produce an impeller having the shape shown in FIG. 3.

Next, this impeller was placed in an infrared image furnace and heated to 2200℃.
As schematically shown in FIG. 1, the surface layer 1 having a thickness of about 200 μm was grown to have an average grain size of 10 μm.

As a result of examining this impeller through a rotation test, the tensile strength of the blade part was approximately 40% before being heat treated in an infrared image furnace.
kg/rrtn", but the tensile strength after heat treatment improved to 55 kg/mn". Moreover, this strength hardly changed in the range from room temperature to 1400°C.

Furthermore, as a result of experiments with different heat treatment temperatures, it was found that when the average grain size in the surface layer was three times or more the grain size in the center, a remarkable effect on strength improvement was observed.

Example 2 Si3N4 powder with an average particle size of 0.5 μm has an average particle size of 0.1
Y2O33w with the same particle size as AQ2032wt% of pm
t% and an aqueous solution of Cr (No3) 3 in an amount of 5 wt% in terms of Cr were added and mixed well.

This mixture was then heat treated at 1350°C in N2,
Cr (NO3) a was reduced to Cp. Add 2 wt% hair of polyvinyl alcohol as a binder to this,
Molded to 0x10x10x100 and heated in 2 atmospheres of N2
Sintering was carried out at 00°C for 1 hour. The shape of the obtained sintered body is approximately 8×
8×8011I113, particle size was approximately 1 μm.

Next, this sintered body was set in the container shown in Fig. 4, and N2
The surface of the sintered body is irradiated with laser light under 5 atmospheres, and the surface of the sintered body is approximately 20μ
,7. Only the surface area of x2oμm is 1800-1900
By heating the sintered body to a temperature of
It was moved at a speed of μm/min. This operation was repeated while sequentially rotating the sample around the rotation axis.

As a result, as schematically shown in FIG. 2, a sintered body was obtained in which the particles of the surface layer 1 extended in the longitudinal direction of the sintered body. In addition, as a result of experiments with various laser power, scanning speed, and temperature of the irradiated part, it was found that: ■ The higher the temperature of the irradiated part and the slower the laser scanning speed, the larger the thickness of the surface layer and the average particle size of the surface layer. It was found that the aspect ratio (ratio of the length of the major axis to the length of the minor axis) of the particles in the zero surface layer increases as the laser scanning speed increases.

The relationship between the strength of the obtained sintered body at 1000° C., the thickness of the surface layer, the length of the long axis of the surface layer particles, and the aspect ratio is shown in FIGS. 5 and 6.

Further, the same raw material as in the above example was subjected to ih hot press sintering at 1800° C. to produce a disk of 60φ×5t, and after grinding both sides, a hole of 10φ was drilled in the center.

Next, by irradiating the laser beam from the center of the disk in the radial direction, a sintered body having a structure in which the particles in the surface layer on both sides of the disk extended in the radial direction from the center of the disk was produced.

As a result of measuring the strength of the disk by a rotation test, the tensile strength of the disk with an average particle size of 0.8 μm after hot pressing was 4-
The particle size in the long axis direction of the particles in the 50 μm thick surface layer was reduced to 7 μm by laser irradiation.
As a result of setting the grain size in the short axis direction to 2 μm, the tensile strength was improved to 80 kg/nn2. Further, this strength did not change at room temperature or at 1000°C.

Example 3 - Average particle size of approximately 500 ZrO2 particles (8% 'Y2
2 wt% of polyvinyl alcohol/coal was added as a binder to 4-OX 40 x 200
It was molded to a size of mn3 and sintered in air at 1600°C for 2 hours.

The shape of the obtained sintered body was 35X'35X170 field 3, and the particle size was about 10 μm.

Next, this sintered body is irradiated with a laser in the same manner as in Example 2, and the irradiated part is heated to 1800°C. As shown in FIG. A sintered body was produced.

The relationship between the strength of the obtained sintered body at 5000C, the thickness of the surface layer, the length of the long axis of the surface layer particles, and the aspect ratio is shown in FIGS.

〔Effect of the invention〕

As explained above, the ceramic structure of the present invention has the advantage of being higher in strength than conventional structures, and as a result, is highly reliable.

[Brief explanation of drawings]

1 and 2 are schematic diagrams showing the structure of a ceramic structure according to an embodiment of the present invention, FIG. 3 is a structural diagram of a ceramic structure obtained in an embodiment of the present invention, and FIG. Schematic diagram showing the structure of the experimental apparatus used in the examples of the present invention, No. 5
8 are characteristic curve diagrams showing the characteristics of samples obtained in Examples of the present invention. 1...Surface layer, 2...Inside, 10...Pressure container, 2
0... Gas inlet, 30... Rotating shaft, 40... Sample,
50...Zn5e window, 60...Laser light.

Claims (1)

[Claims] 1. A ceramic structure characterized in that the particle size of the ceramic particles constituting the surface layer on the surface of the structure which is subjected to external stress during use is larger than the particle size of the ceramic particles constituting the inside of the structure.