JPH07102994B2 - Method for producing fiber-reinforced ceramics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is an improvement in a method for producing a mechanical structural component or the like used in an environment of high temperature, corrosiveness, wear resistance, etc., using fiber-reinforced ceramics having high strength and toughness It is about.

[Conventional technology]

Gas turbine parts, diesel engine parts, turbocharger parts, heat exchanger parts used in the fields of energy, materials, transportation, etc. that require strength and toughness in environments with high temperatures, corrosiveness, wear, etc. The use of ceramics as a new material is considered, and research and development is underway.

Conventional ceramics existing as materials for such mechanical structural parts have a drawback that they have low toughness, and as a most promising method for strengthening toughness, a fiber reinforced method, particularly a reinforced method using short fibers, has been studied.

The method for producing short fiber reinforced ceramics (mechanical structural parts, etc.) that is currently performed is to mix ceramic powder serving as a matrix and ceramic short fibers, form the mixture, and then densify the matrix by sintering or the like. I am trying.

In such a conventional method, when the ceramic powder and the ceramic short fibers are mixed, a liquid such as water or an organic solvent, or
It is known that a molded product of good quality can be obtained by using a liquid of a hydrocarbon wax or a resin melt as a dispersant and performing molding by utilizing the fluidity of this liquid.

[Problems to be solved by the invention]

However, when water, an organic solvent, a molten wax, or a molten hydrocarbon resin is used as a dispersant used for improving mixing and molding in the conventional method, all of these dispersants are finally removed by evaporation or incineration. Will be done,
A large amount of voids remain between the powder of the molded product and the short fibers.

The more voids there are, the more difficult it becomes to densify in the next step, and if the densification is forcibly done, the short fibers will be damaged, and the properties of the final product (fiber reinforced ceramics) will be improved by the fiber reinforced method. There is a problem that is insufficient.

The present invention has been made in view of the problems of the prior art, and produces a product having excellent strength and toughness without causing a large amount of voids even if a dispersant is used for improving mixing and moldability. An object of the present invention is to provide a method for producing a fiber-reinforced ceramic that can be used.

[Means for solving problems]

In order to solve the above-mentioned problems, the method for producing a fiber-reinforced ceramics of the present invention comprises: mixing a ceramic short fiber, a ceramic powder and a liquid polymer ceramics precursor, molding the mixture to obtain a molded body, and heating the molded body. It is characterized in that the ceramic precursor is converted into ceramics, and then the compact is densified.

That is, a ceramic precursor of a liquid polymer is used as a dispersion liquid for dispersing the ceramic powder and the ceramic short fibers to improve the moldability, whereby the ceramic powder and the ceramic powder after molding are used. Fiber-reinforced ceramics (product) that excels in strength and toughness by converting the liquid dispersed in the gaps of short fibers into ceramics to reduce the porosity and facilitates subsequent densification.
Trying to get.

A specific example of the method for producing fiber-reinforced ceramics of the present invention will be described below with reference to the process chart shown in FIG.

(1) Mixing Step of Ceramic Short Fibers and Ceramics Powder The first step of the method for producing the fiber-reinforced ceramics is to mix the ceramic short fibers 1, the ceramic powders 2, and the liquid polymer ceramic precursor 3.

This ceramic powder 2 mainly constitutes a matrix of fiber reinforced ceramics, and in order to satisfy properties such as strength and toughness as a final product, silicon carbide, silicon nitride, alumina, mullite are required. ，
Use zirconia or one containing at least 70%.

Further, a sintering accelerator may be added to these ceramic powders 2, for example, boron, aluminum, or carbon is added to silicon carbide, or alumina or yttria is added to silicon nitride. , Seria, aluminum nitride, etc. are added to promote sintering.

Further, silicon carbide is formed of, for example, aluminum nitride and a solid solution having a composition of (SiC) _1-x (AlN) _x (X = 0 to 1), and silicon nitride is formed of, for example, alumina and aluminum nitride. In the meantime, (Si ₃ N ₄ ) _1-x (Al ₂ O ₃ · AlN) _x , (x
= 0 to 0.67), zirconia is known to form a solid solution with yttria, magnesia, ceria, etc., which can change mechanical properties, thermal properties, corrosion resistance, etc. Therefore, a ceramic powder having a solid solution composition containing silicon carbide, silicon nitride, zirconia, or the like as a main component may be used as the ceramic powder 2.

Also, mechanical properties may be similarly improved by dispersing other ceramic powder such as titanium carbide in silicon carbide, silicon nitride or alumina in zirconia or silicon carbide. However, it is also possible to use, as the ceramic powder 2, a material containing silicon carbide, silicon nitride, alumina, etc. as a main component and other ceramic powder for dispersion strengthening added thereto.

Next, as the ceramic short fibers 1, it is desirable to use short fibers made of a ceramic similar to the ceramic powder 2 serving as a matrix in terms of strength and thermochemical stability.

Further, the ceramic short fibers 1 have a high elastic modulus equal to or higher than that of the matrix, and it is desirable to reduce the load on the matrix as much as possible when stress is applied.

Furthermore, the ceramic short fibers 1 need to be sufficiently stable against heating in the sintering which is a post-process.

Therefore, a ceramic short fiber 1 that satisfies these conditions
As for the matrix containing silicon carbide as the main component, silicon carbide or carbon short fiber is used, and for the matrix containing silicon nitride as the main component, silicon carbide or silicon nitride short fiber is used. For the matrix containing alumina, mullite or zirconia as the main component, short fibers of silicon carbide, silicon nitride or alumina can be used, respectively.

As such a ceramic short fiber 1, whiskers (whisker-shaped crystals made of a single crystal) having an appropriate diameter and length are used.
However, it is not limited to this, but if the requirements for strength and thermal stability are satisfied, short fibers made of polycrystalline or long fibers made of polycrystalline are suitable. It is also possible to use short fibers cut into various lengths.

Next, the liquid polymer ceramics precursor 3, which is the most important mixture in the present invention, will be described.

The ceramic precursor 3 of the liquid polymer is a liquid and polymeric substance among substances that can be thermally decomposed or reacted to be converted into ceramics by heating.

As such a liquid polymer ceramics precursor 3,
Examples of substances that can be converted into silicon carbide include polycarbosilane, polysilane, and a mixture of a polysiloxane and a hydrocarbon-based polymer, which are converted into silicon carbide by heating in a non-oxidizing atmosphere.

The liquid polymer ceramics precursor 3 that can be converted into silicon nitride includes polysilazane, polysilane, a mixture of polysiloxane and a hydrocarbon polymer, etc., which can be converted into silicon nitride by heating in a nitrogen or ammonia atmosphere. To do.

Further, as those that can be converted to alumina, polyaluminoxane and the like can be converted to silica,
A mixture of polyaluminoxane and polysiloxane is one that polysiloxane or the like can be converted to mullite.

Such a liquid polymer ceramics precursor 3 generally has a very high viscosity as the degree of polymerization progresses and loses fluidity, making it very difficult to uniformly mix it with the ceramic powder 2 and the ceramic short fibers 1. And subsequent molding becomes difficult.

Therefore, the liquid polymer ceramics precursor 3 should be at least 10,000 pois at the mixing temperature of the ceramic powder 2 and the ceramic short fibers 1 and the molding temperature.
Use a liquid having a viscosity of e or less, preferably 1,000 poise or less.

In order to obtain such a low viscosity range, the degree of polymerization of the liquid polymer ceramic precursor 3 itself is set to a low range,
Alternatively, a low molecular weight compound having compatibility with the liquid polymer ceramics precursor 3 is added as a plasticizer.

As the plasticizer, a liquid polymer ceramic precursor 3
A low-molecular-weight liquid having the same composition as the above may be used, or a normal hydrocarbon liquid such as phthalic acids and furans may be used.

It should be noted that raising the temperature is also effective as a method for lowering the viscosity, but this heating temperature range needs to be set to a range in which the liquid polymer ceramic precursor 3 is hardly decomposed.

Further, in order to mix the ceramic powder 2 and the ceramic short fibers 1 into the liquid polymer ceramic precursor 3 and to disperse them as uniformly as possible, a liquid such as a silane coupling agent, stearic acid and the surface of the ceramic should be used. It is desirable to add a substance that improves the wettability and the dispersibility.

In addition, upon mixing, kneading is performed while applying a high shear stress using a pressure kneader or the like to sufficiently disintegrate the agglomeration of the ceramic powder 2 and the ceramic short fibers 1 and to perform uniform dispersion. desirable.

(2) Molding step of mixture In the second step, the mixture of the ceramic short fibers 1, the ceramic powder 2 and the liquid polymer ceramic precursor 3 prepared in the first step is mixed with the liquid polymer ceramic precursor 3 having a sufficiently low viscosity. And molding under pressure at a temperature at which decomposition does not occur.

As this molding method, the extrusion molding method is suitable for molding into a part having a constant cross-sectional shape such as a pipe, and the transfer molding method or the injection molding method is suitable for molding into a three-dimensional shape part. ing.

(3) Step of heating molded body In the third official determination, the molded body molded into a desired shape is heated to a required temperature in an appropriate atmosphere, and the liquid polymer ceramic precursor 3 in the molded body is heated to the ceramic 4 Convert to.

For this reason, for example, when polycarbosilane is used as the liquid polymer ceramics precursor 3, it may be heated to 1200 ° C. or higher in a high-purity argon atmosphere. Sufficiently control the rate of temperature rise so that deformation or the like does not occur.

When the liquid polymer ceramics precursor 3 is converted to the ceramics 4 by heating in this way, the volume is contracted and the voids 5 are generated in the molded body. It is a much lower value than that by the law.

If the molecular weight of the liquid polymer ceramics precursor 3 to be used is too low, the proportion of the weight lost due to evaporation or thermal decomposition increases, which leads to an increase in porosity. It is desirable to use one that has a viscosity of 1 poise or more.

(4) Densification Step of Formed Body This step, which is the final step, removes voids 5 generated in the formed body by heating, and ceramics 4 converted from ceramic powder 2 and liquid polymer ceramics precursor 3
By densifying the matrix made of, the fiber-reinforced ceramics 6 which is a desired product is completed.

For this densification, the matrix is sinter-shrunk by pressure sintering or the like so as to be densified, and more desirably, the liquid polymer ceramic precursor 3 used in the molding step is again provided with the voids 5. And densification by repeating the conversion into ceramics 4 by heating. Then, if necessary, after this, a densification treatment by hot isostatic pressing is further applied to completely remove the remaining pores.

〔The invention's effect〕

As described above, according to the method for producing a fiber-reinforced ceramics of the present invention, the following effects are obtained.

Since the ceramic precursor of liquid polymer is used and the ceramic short fibers and the ceramic powder are mixed therein, the dispersion and the mixing can be made uniform.

Since the liquid polymer ceramics precursor improves the fluidity of the mixture of ceramics short fibers and ceramics powder, molding methods such as extrusion molding, transfer molding, and injection molding can be adopted for molding, which is complicated and precise. It becomes possible to form the shape and also to form it efficiently.

Since the mixed liquid polymer ceramics precursor for improving dispersion and moldability can be converted into ceramics by heating, the converted ceramics are arranged also in the gap between the compacts made of ceramic short fibers and ceramic powder, It is possible to obtain a fiber-reinforced ceramic that is dense and has excellent characteristics as compared with the conventional manufacturing method.

Hereinafter, the features of the present invention will be described in more detail with reference to examples.

Example 1 A carbon fiber was coated with silicon carbide by CVD,
Further, a long fiber having a diameter of about 100 μm coated with carbon and cut into a length of about 3 mm is used as a ceramic short fiber.

Silicon carbide powder containing 0.5% by weight of boron and having an average particle size of 0.7 μm is used as the ceramic powder.

A mixture of these ceramic short fibers and ceramic powder with a mixing ratio of 40:60 was further added, and as a liquid polymer silicon carbide based ceramics precursor, the viscosity at 150 ° C was about 100.
After adding 17% by weight of polysilastyrene, which is 0 poise, and 2% by weight of dibutyl phthalate as a plasticizer, the mixture was kneaded at 150 ° C. in a kneading machine filled with argon.

This kneaded product was formed into a circular tube shape for a heat exchanger having an outer diameter of 40 mm and an inner thickness of 3 mm by using an extruder.

This molded product was heated to 1500 ° C in an argon atmosphere at 9.8 atm.
Was heated to convert the polysilastyrene in the molded body into silicon carbide.

The porosity of the molded body obtained by converting this polysilastyrene into silicon carbide was 38%.

In order to densify this molded body into a fiber reinforced ceramics, the polysilastyrene used as the liquid polymer ceramics precursor was pressure-impregnated, and the densification step of converting it into silicon carbide ceramics by heating was repeated three times.

As a result, a dense reinforced ceramic having almost no open porosity and reinforced with silicon carbide short fibers was obtained.

On the other hand, for the purpose of comparison, in the above production method, polystyrene, which is an ordinary hydrocarbon-based polymer, is used in place of polysilastyrene, and silicon carbide short fibers and silicon carbide powder are mixed by the same process. And extrusion.

The molded body molded in this way is heated to 600 in an argon stream.
When heated to ℃, most of the polystyrene in the molded body is pyrolyzed, undesired free carbon remains as a residue,
In addition, the porosity was as high as 45%.

After this, the compact was subjected to the densification step by pressure impregnation of polysilastyrene and heating in the same manner as above, but the presence of residual open porosity was observed even after three times of treatment, indicating that the densification was complete. It wasn't done.

By comparing these fiber-reinforced ceramics, it was confirmed that the production method of the present invention is much superior to the conventional method.

[Example 2] Using silicon carbide whiskers as ceramic short fibers and silicon nitride powder as ceramic powder, the ratio of the former to the latter was 35: 6.
A mixture of 5 was obtained.

And, controlling the degree of polymerization of hexamethyltricyclosilazane as a liquid polymer ceramics precursor, polysilazane having a viscosity at 150 ° C. of about 500 poise, with respect to a mixture of the above silicon carbide whiskers and silicon nitride powder. , 45% by volume, and kneading with heating was performed while filling nitrogen with a closed kneader.

This was made into a preformed product, which was then fluidized by pressurizing it in a mold heated to 170 ° C., and formed into a gas turbine stationary blade shape by transfer molding.

By this molding, a molded body was obtained in which the silicon carbide whiskers and the silicon nitride powder were bonded by polysilazane.

After that, the molded body was heated to 1400 ° C. in nitrogen containing hydrogen to convert the polysilazane into silicon nitride.

Next, a capsule containing Vycor glass as a main component was formed on this molded body, and pressure-sintered by a hot isostatic press at 1800 ° C. and 1500 atm.

As a result, a gas turbine stationary blade, which is a dense, high-strength and high-strength fiber-reinforced ceramics having a silicon nitride matrix reinforced with silicon carbide whiskers as a matrix, was obtained.

[Brief description of drawings]

FIG. 1 is a process drawing according to an example of the method for producing fiber-reinforced ceramics of the present invention. 1 ... Ceramic short fibers, 2 ... Ceramic powder, 3
...... Liquid polymer ceramics precursor, 4 …… Inverted ceramics, 5 …… Voids, 6 …… Fiber-reinforced ceramics.

Claims (1)

[Claims] Claims: 1. Ceramic short fibers, ceramic powder, and liquid polymer ceramics precursor are mixed, a molded body is molded, and then this molded body is heated to convert the ceramics precursor into ceramics, and then the molded body. A method for producing a fiber-reinforced ceramics, characterized in that: