JPS61256963A - High strength alumina sintered body and manufacture - 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 A. Field of Industrial Application The present invention relates to a high-strength alumina sintered body and a method for manufacturing the same.

Prior Art Alumina is a chemically extremely stable substance, has good electrical insulation properties, high hardness, and is inexpensive compared to other ceramics, so it is used for integrated circuit substrates and packages.
It is most widely used among ceramics such as tips for cutting tools.

Furthermore, ceramics have recently attracted attention as materials for sliding parts of internal combustion engines because of their excellent wear resistance. However, since alumina has lower mechanical strength than other ceramics, it is not suitable as a material for parts of internal combustion engines or other mechanical structures used in particularly harsh environments. These ceramics for mechanical structures include:
Applied research is mainly being carried out on silicon nitride, silicon carbide, and partially stabilized zirconia, and alumina has not been mentioned as a candidate for the above materials.

As mentioned above, alumina is relatively inexpensive among ceramics, and if its mechanical strength is improved, it can be used as a material for mechanical structures used in harsh environments such as parts of internal combustion engines. It can be expected that its scope of application will further expand.

The mechanical strength of alumina sintered bodies depends on the porosity and grain size, as reported by Yamada in Ceramics 17.810-816.1982, and the grain size is made smaller and the porosity is lowered. The higher the density, the better.

In addition, there have been many attempts to improve the mechanical strength of alumina sintered bodies by combining a second phase;
e) by American Ceramic Site Bulletin
ty Bulletin) 63.256-262.
As reported in 1984, although the addition of a second phase is effective in improving fracture toughness, in most cases it is difficult to improve mechanical strength. For example, special public service in Showa 59
An example of an alumina sintered body composited with zirconia is given in Publication No. 25748, and also published in the Journal of Materials Science by Wahi et al.
nal of Malerials 5science)
15+875 to 885+1980, there are examples of alumina sintered bodies composited with titanium carbide, but all of them contain a fine equiaxed second phase, and as in the present invention, There has been no report of a method for improving mechanical strength by combining a relatively large plate-like dispersed phase with substantially the same composition as that of a steel.

C. Purpose of the Invention The object of the present invention is to provide an alumina sintered body that is fully satisfactory as a material for mechanical structures and a method for producing the same.

2. Structure of the invention, that is, the first invention of the present invention is to provide a matrix having an average particle size of 3 to 5 μm in a matrix consisting essentially of alumina particles with an average particle size of 5 μm or less.
The present invention relates to a high-strength alumina sintered body having a structure in which plate-shaped alumina particles of 25 μm and an average aspect ratio of 3 to 20 are substantially separated from each other and dispersed in an amount of 2 to 30% by volume. Further, the second invention of the present invention provides alumina powder with an average particle size of 1 μm or less, an average particle size of 3 to 25 μm, and an average aspect ratio of 3 to 25 μm.
20 plate-shaped alumina powder at 2 to 30% by volume of the total
The method for producing a high-strength alumina sintered body according to the first invention, which comprises blending and mixing the mixed powder so that It depends.

Note that the particle size of the plate-shaped alumina particles refers to the diameter of the plate-shaped alumina particles on a plane or a plane close to the plane, and does not refer to the thickness of the plate-shaped particles.

E. Effects of the Invention The mechanical strength (fracture strength) σf of brittle materials such as ceramics was described by Miyata in Ceramics.

20.3-11.1985, σ
Child = is expressed as IC/ (y − rl). Here, KIC is fracture toughness, a is the size of a defect that leads to fracture, and y is a constant determined by the shape of the defect and the object.

From the above formula, the mechanical strength of ceramics is fracture toughness KI
It can be seen that this can be improved by increasing c and decreasing the defect size a.

This is a means to reduce the defect size a, and the attempt at second phase composite is a means to improve IC in fracture toughness.

To further explain the improvement of fracture toughness in detail, the method is as follows.
Annual Review of Materials Science (by Wiederhorn)
Annual Review of Material
s 5science) 14.373~403.1984
As summarized in the following, the problem can be divided into two types: nonlinearization of the crack tip and formation of a shielding zone at the tip of the crack. Among these, from the viewpoint of improving fracture toughness by making the crack tip nonlinear, it is desirable to introduce a second phase with a large aspect ratio (for example, fibrous). This increases the size of defects that adversely affect mechanical strength, so improvement in fracture toughness is difficult to immediately lead to improvement in mechanical strength.

As a result of intensive research, the inventors discovered that plate-shaped alumina particles with an average particle size of 3 to 25 μm and an average aspect ratio of 3 to 20 were isolated from each other and dispersed in an alumina matrix consisting of fine crystal grains, thereby creating an alumina sintered body. It was found that the mechanical strength of the steel can be improved. The reason for this is that the use of plate-shaped alumina particles makes the crack tip non-linear and increases fracture toughness, but compared to fibers, it is easier to isolate them from each other and disperse them in the substrate, thus suppressing the introduction of large defects. It is thought that this improves mechanical strength. Here, the reason why the introduction of plate-shaped alumina particles makes the crank tip non-linear is that the propagation path of fracture is the grain boundary, and the crack tip takes a shape along the grain boundary.

Next, each phase, raw material, and manufacturing method in the present invention will be explained.

As described above, the finer the particle size of the alumina particles constituting the matrix, the smaller the defects. If this exceeds 5 μm@, defects become large and the mechanical strength of the sintered body decreases, so 5 μm is set as the upper limit.

The plate-shaped alumina particles to be dispersed in the matrix may be of α-type crystal form, but may also be of other crystal form. As for the shape, it is preferable that the aspect ratio is 3 to 20 on average. If this value is less than 3, the degree of nonlinearity at the crack tip will be small and no effect of increasing fracture toughness can be expected. If it exceeds 20, plate-like alumina particles will come into contact with each other, causing a decrease in density and an increase in defect size. Since the mechanical strength is reduced, the aspect ratio is set to 3 to 20 on average.

The average particle size of the plate-shaped alumina particles needs to be sufficiently larger than that of the alumina particles constituting the matrix from the viewpoint of producing a high-density sintered body. If this is less than 3 μm, there will be many opportunities for the plate-shaped alumina particles to come into contact with each other, making it difficult to obtain a high-density sintered body. On the other hand, if this is larger than 25 μm, it will itself act as a defect that adversely affects the strength. However, the mechanical strength decreases. Therefore, the average particle size of the plate-shaped alumina particles is in the range of 3 to 25 μm.

If the proportion of plate-shaped alumina particles in the matrix is less than 2% by volume, the effect of improving mechanical strength is not significant;
When the amount exceeds 0% by volume, plate-shaped alumina particles come into contact with each other, causing a decrease in density and an increase in defect size of the sintered body, resulting in a decrease in mechanical strength. Therefore, the amount of dispersion should be in the range of 2 to 30% by volume.

The alumina sintered body according to the present invention is essentially alumina
However, there may be small amounts of other components contained as impurities in the raw materials. Furthermore,
As with ordinary alumina sintered bodies, a small amount of magnesia (
It is preferable to contain MgO).

The fine alumina powder used as a raw material for the base material is conveniently in the α-type crystal form, but other crystal forms such as the δ-type may also be used. It is desirable that the purity is 98% or more.

As mentioned above, the particle size of the alumina particles constituting the matrix needs to be sufficiently smaller than the particle size of the plate-shaped alumina particles dispersed in the matrix, and the average size is 5 μm or less.
In addition, since it is desirable to have an appropriate balance between the two, and on the other hand, it is necessary to take into account the growth of crystal grains during sintering, the alumina powder used for forming the base should have an average grain size of 1 μm. It is best to use the following.

As the molding method, any of the usual molding methods such as press molding, slurry casting, injection molding, and extrusion molding can be employed. Of course, hot pressing can also be used.

Sintering is carried out at 140°C in vacuum, in a non-oxidizing atmosphere or in air.
It is carried out at a temperature within the range of 0 to 1800°C. Sintering temperature is 14
When the temperature is lower than 00°C, the density of the obtained sintered body becomes low,
When the temperature exceeds 1800° C., the growth of alumina crystal grains becomes severe and the mechanical strength decreases in either case.

The feature of the present invention is that plate-shaped particles having substantially the same chemical composition as the base material are dispersed in the base material as described above, instead of combining the second phase with a chemical composition different from the base material. The average particle size of the alumina particles that make up the base material is 5 μm.
The plate-shaped (it is important that they are plate-shaped, not fibrous) alumina particles are dispersed in isolation from each other in this base material, with an average particle size of 3 to 25 μm, average aspect ratio of 3 to 2
0. By setting the dispersion amount to 2 to 30% by volume, the crack tip is made non-linear, improving fracture toughness, and the size of defects that lead to fracture is suppressed to a certain degree, improving mechanical strength. There is. Therefore, due to the improved mechanical strength, it is expected that the field of application of alumina sintered bodies will be expanded to include mechanical structural materials.

To, Example The present invention will be specifically explained below using Examples.

Plate-shaped alumina powder as shown in the table below is blended with α-type alumina powder with a purity of 99% or more and an average particle size of 0.7 μm,
Ethyl alcohol was mixed as a liquid mixture in a ball mill using a stick container and an alumina ball, wet mixed for 1 hour, and dried to obtain a mixed powder.

After press-molding these mixed powders, a compact of about 20 x 50 x 15 m was formed by isostatic hydrostatic pressure of 1,500 kg/-, and this compact was heated to 1,600° C. for 2 hours in an argon stream to sinter.

The average particle diameter of the alumina particles constituting the matrix of these sintered bodies was 1 μm.

A 3×4×36n bending test piece was taken from these sintered bodies using a diamond grindstone and a diamond blade, finished with a No. 200 diamond grindstone, and subjected to density measurement and bending test. The bending test was conducted at room temperature using a three-point bending test method with a fulcrum distance of 30 m and a crosshead speed of 0.5 n/win.

The table drawing is a sketch schematically showing a typical microscopic composition of the above-mentioned sintered body, taking 1lh4 as an example. Base made of fine alumina particles (grain boundaries are not shown)
1, plate-shaped alumina particles 2 are observed to be isolated and dispersed from each other.

For comparison, a similar test was conducted on an alumina sintered body having a single grain size obtained in the same manner as described above without adding plate-shaped alumina powder.

The test results are shown in the table above.

From the same table, it can be seen that the bending strength of the alumina sintered bodies of the examples is significantly improved compared to the conventional alumina sintered bodies of a single grain size for comparison. As can be seen from these results,
The alumina sintered body according to the present invention can also be applied as ceramics for mechanical structures such as internal combustion engine parts, for which the use of alumina sintered bodies has not been considered in the past. It makes it possible to further expand the field of application, and has great industrial utility value.

[Brief explanation of the drawing]

The drawing is a sketch schematically showing the microscopic structure of the alumina sintered body of the example. In addition, in the reference numerals shown in the drawings, 1...Material 2......Plate-shaped alumina particles.

Claims (1)

[Claims] 1. Platy alumina particles having an average particle size of 3 to 25 μm and an average aspect ratio of 3 to 20 are substantially separated from each other in a matrix consisting essentially of alumina particles having an average particle size of 5 μm or less. A high-strength alumina sintered body having a structure in which 2 to 30% by volume is dispersed. 2. Blend and mix plate-shaped alumina powder with an average particle size of 3 to 25 μm and an average aspect ratio of 3 to 20 to alumina powder with an average particle size of 1 μm or less so that the amount is 2 to 30% by volume of the whole, This mixed powder is molded, and this molded body is
A method for producing a high-strength alumina sintered body, which is sintered at a temperature within the range of 0 to 1800°C.