JPH01176281A - Method for forming inorganic fiber-reinforced ceramics - Google Patents

Abstract

PURPOSE:To minimize residual stress strain in forming and obtain a compact having excellent fracture toughness by a press forming, such as cold isostatic press (CIP) forming, by using a mixture of inorganic fibers with ceramic powder having an average diameter at a specific ratio to the average diameter of the inorganic fibers. CONSTITUTION:(A) Inorganic fibers, e.g., SiC, Si3N4, Al2O3, or whiskers of graphite or potassium titanate, having <=30 average aspect ratio are mixed with (B) ceramic powder selected from SiC, Si3N4, Al2O3, SiO2 and ZrO2 having the average diameter of <=2/3 based on that of the component (A) and <=40% variation coefficient of particle diameter distribution at 0.05-0.40 weight ratio (A/B). The resultant mixture is then press-formed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for molding inorganic fiber-reinforced ceramics, and more particularly to a method for molding inorganic fiber-reinforced ceramics with significantly improved fracture toughness.

[Prior art and its problems] In recent years, there has been a great increase in demand for highly functional gas turbines, jet engines, and the like. There are great expectations for the development of heat-resistant materials for the blades and nozzles used in these heat engines under the most severe conditions.

Therefore, ceramics are attracting attention as a material with excellent heat resistance.

Although ceramics have great heat resistance.

Its fracture toughness is extremely low, so attempts have been made to strengthen it by filling it with fibers.

Generally, the fracture toughness of ceramics is 3 to 5 MPae
m ””, which is 200MPa1m for general carbon steel.
Fracture toughness is extremely low compared to ``''.

The main cause of this low fracture toughness is microcracks existing in the ceramic. Cold isostatic pressing (CIP), hot isostatic pressing (H
There are various molding methods such as IP) and hot press molding.

However, pressure forming of ceramics has its limitations, and the desired improvement in fracture toughness cannot be seen.

Therefore, attempts have been made to strengthen ceramics with inorganic fibers in order to increase their fracture toughness.

In general, the characteristics of inorganic fiber reinforcement are said to be that it inhibits fracture or increases fracture energy by pinning (preventing crack growth), crack defleximin (promoting crack bending), and pullout (pulling out fibers).

However, residual stress due to bending of fibers during pressure molding and CI
There was a problem that P was difficult.

Therefore, an object of the present invention is to minimize residual stress distortion during molding, and to enable simple and effective CIP molding and pressureless sintering processes for inorganic fiber-reinforced ceramics. It is.

[Means for achieving the above object] The structure of the present invention for achieving the above object comprises inorganic fibers having an average aspect ratio of 30 or less and an average diameter of 2/3 or less of the average diameter of the inorganic fibers. This is a method for molding inorganic fiber-reinforced ceramics, characterized by press-molding a mixture with a ceramic powder having the following properties.

The inorganic fibers in the present invention have an average aspect ratio of 30 or less, preferably 5 to 15.

The reason why such a low aspect ratio is preferable is that residual stress distortion during molding can be suppressed.

In order to adjust the aspect ratio of the inorganic fibers in the present invention within the above range, for example, known crushing means such as a show crusher, a gyratory crusher,
A crusher such as a roll crusher, edge runner, hammer crusher, bowl mill, jet mill, disc mill, etc. can be employed. Which pulverizer to use depends on the type of inorganic fiber.

In the present invention, the inorganic fibers include crystalline or crystalline short fibers! Both whiskers and whiskers can be used.

Examples of the inorganic fibers include glass fiber, high silicate fiber, alumina silicate fiber, asbestos, potassium titanate fiber, carbon fiber, alumina fiber, zirconia fiber, silicon carbide fiber, silicon nitride fiber, silicon carbide whisker, and nitride fiber. Examples include silicon whiskers, alumina whiskers, graphite whiskers, potassium titanate whiskers, and magnesia whiskers.

The various types of inorganic fibers mentioned above can be used alone or in combination of two or more.

In the present invention, the whisker is preferable as long as it has the aspect ratio, and at least one selected from the group consisting of silicon carbide, silicon nitride, alumina, graphite, and potassium titanate is particularly preferable.

Further, the particle size distribution of the ceramic in the present invention is usually 40% or less, preferably 20% in terms of coefficient of variation (CV).
It is as follows.

What is important in the present invention is that the average particle size of the ceramic is 2/3 or less of the average diameter of the inorganic fibers, preferably 172 or less of the average diameter of the inorganic fibers. This is because it has the effect of increasing the fluidity of particles during molding and suppressing distortion during molding.

Furthermore, if the aspect ratio of the inorganic fiber exceeds 30 or the average diameter of the ceramic exceeds 2/3, residual stress due to deformation of the inorganic fiber may occur when sintering at high pressure by HIP, hot press, etc. As a result, the fracture toughness of the inorganic fiber-reinforced ceramic cannot be improved. Furthermore, when once molded by CIP, the deformation recovers when the pressure drops, causing the molded product to crack or crumble.

Examples of the ceramics include oxides such as alumina, silicon oxide, zirconium oxide, tin oxide, indium oxide, titanium oxide, iron oxide, thallium oxide, ferrite, rare earth iron garnet, tungsten oxide, and tellurium oxide, zinc sulfide, and indium oxide. Examples include chalcogenides 1 such as antimony and molybdenum sulfide, nitrides such as titanium nitride, thallium nitride, aluminum nitride, niobium nitride, and silicon nitride, and carbides such as silicon carbide 1, titanium carbide, and zircon carbide.

The various ceramics mentioned above can be used alone or in combination of two or more.

Among the various ceramics, at least one selected from the group consisting of silicon carbide, silicon nitride, alumina, silicon dioxide, and zirconium dioxide is preferred.

In the present invention, the inorganic fiber (A) and the ceramic (B) are mixed and then the mixture is subjected to pressure molding.

The mixing ratio is usually (A)/CB) (weight ratio) of 0.05 to 0.40, preferably 0. lO~0.3
Set to 0.

When the mixing ratio (A)/CB) is below the above range, the meaning of inorganic fiber reinforcement is lost, and when it exceeds the above range, defects are likely to occur inside the ceramic, which is not preferable.

As a method for mixing the inorganic fibers and the ceramics, conventionally known mixing methods or means can be employed.

Examples of mixing means include double anchor type, V type, horizontal cylinder type, double cone type, ribbon type, vertical screw type, batch type Muller type, continuous type Muller type, heat exchange type, single shaft rotor type, A mixer such as a turbine type can be used.

However, if the inorganic fibers break during mixing, adjust the aspect ratio of the inorganic fibers before mixing appropriately or make them thicker so that the aspect ratio after the mixing operation is the value specified in the present invention. It is best to select inorganic fibers with an aspect ratio of .

It goes without saying that in the present invention, there are no particular limitations on the sintering aid, binder, etc.

Thus, the mixture in the present invention can be molded into various molded bodies for use in high-temperature heat exchangers, nuclear reactors, MHD power generation, etc. by the above-mentioned pressure molding method.

[Example] Hereinafter, the present invention will be explained in more detail by showing examples.

(Example 1) Silicon carbide whiskers 25V cm (volume fraction) with an average particle size of 0.4 Bm and an average aspect ratio of 13, an average diameter of 1,
Op,,m Y2Os7. The powder with sV t% was suspended in alcohol, mixed in a V-type mixer for 20 hours, and then dried. The ceramic mixture has an inner diameter of 22φ and a length of ZO.
Fill the Oll rubber mold, seal it, and apply cold isostatic pressure (CIP).
An isostatic pressure of 4 tons/cm" was applied for 5 minutes.

Thereafter, the pressure was reduced and the molded body was taken out from the rubber mold, but no cracks were detected.

Further, the obtained molded body was subjected to atmospheric pressure sintering in an oxidizing atmosphere while being heated to 1,750°C. When the density of the sintered body was measured, it was found to be approximately the theoretical density.

(Comparative Example 1) Molding was performed in the same manner as in Example 1 except that silicon carbide whiskers having an average diameter of 0.44 m and an average aspect ratio of 50 were used. A large number of cracks were detected in the molded product obtained.

(Example 2) Silicon carbide whiskers 5Vt%, Si3N, powder 85.
Molding was carried out in the same manner as in Example 1 except that the proportions were 5%, Y2O, and 9.5Vt%. No cracks were detected in the obtained molded body.

Further, the obtained molded body was subjected to atmospheric pressure sintering in an oxidizing atmosphere while being heated to 1,750°C. When the density of the sintered body was measured, it was found to be approximately the theoretical density.

(Comparative Example 2) Molding was performed in the same manner as in Example 2, except that silicon carbide whiskers having an average diameter of 0.4 gm and an average aspect ratio of 50 were used. Cracks were detected in some places in the obtained molded body.

(Example 3) Silicon carbide whiskers 25Vr%, average diameter 0.4 End 111, average aspect ratio 50, average diameter 0.2 #LI
Il 5IJ467, SVt%, Y*Os7. with average diameter 1-0 μm. Example 1 except that SVt powder was used.
Cold isostatic pressing was performed in the same manner as above. No cracks were detected in the obtained molded body.

Further, the obtained molded body was subjected to atmospheric pressure sintering in an oxidizing atmosphere while being heated to 1,750°C. When the density of the sintered body was measured, it was found to be approximately the theoretical density.

(Example 4) Graphite whiskers ZSVt% with an average diameter of 0.3 gm and an average aspect ratio of 15, with an average diameter of 0.2 gm no 5
isN4 powder 1i7.5V f%, average diameter 1.0 (
7) Cold isostatic pressing was carried out in the same manner as in Example 1 except that powder of Y, 037, and 5% SVt was used. No cracks were detected in the obtained molded body.

The obtained molded body was subjected to atmospheric pressure sintering while being heated to 1,750° C. in a nitrogen atmosphere. When the density of the sintered body was measured, it was found to be approximately the theoretical density.

Claims (5)

[Claims] (1) An inorganic fiber-reinforced ceramic characterized by pressure-molding a mixture of inorganic fibers having an average aspect ratio of 30 or less and ceramic powder having an average diameter of 2/3 or less of the average diameter of the inorganic fibers. molding method. (2) The method for molding inorganic fiber-reinforced ceramics according to claim 1, wherein the pressure molding is cold isostatic molding. (3) The method for molding inorganic fiber-reinforced ceramics according to claim 1, wherein the inorganic fibers are whiskers. (4) Claim 1, wherein the whisker is at least one selected from the group consisting of silicon carbide, silicon nitride, alumina, graphite, and potassium titanate.
A method for forming inorganic fiber-reinforced ceramics as described in Section 1. (5) The method for forming an inorganic fiber-reinforced ceramic according to claim 1, wherein the ceramic powder is at least one selected from the group consisting of silicon carbide, silicon nitride, alumina, silicon dioxide, and zirconium dioxide. .