JP2000233979A - Silicon nitride sintered body and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a silicon nitride sintered body which is used as a high-temperature structural material capable of being adopted for use in a part required to be thermally insulated, and also to provide a production process for the sintered body. SOLUTION: This sintered body consists of: a surface layer which is a porous layer; an inner layer which is a dense layer having a lower porosity as compared with the surface layer; and an intermediate layer which is formed between the surface layer and inner layer and has such a composition that porosity of the intermediate layer is continuously increased in the direction toward the surface layer from the inner layer. This production process comprises: forming a first formed body by mixing powdery silicon nitride with a sintering aid and a solvent to obtain a mixture, thereafter evaporating the solvent in the mixture and forming the resulting mixture into a formed body, or alternatively, forming the above mixture before solvent evaporation into a formed body and thereafter evaporating the solvent in the formed body; forming a second formed body by mixing powdery silicon nitride with a solvent to obtain a mixture, thereafter evaporating the solvent in the mixture and forming the resulting mixture into a formed body, or alternatively, forming the above mixture before solvent evaporation into a formed body and thereafter evaporating the solvent in the formed body; then bringing such two second formed bodies into contact with the first formed body so as to interpose the first formed body between them; and in that state, sintering these first and second formed bodies.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body for a high-temperature structural material in which only the surface is made porous and heat-insulated, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, heat insulation has been studied as one means for increasing the thermal efficiency of an internal combustion engine or the like. Materials used for such high-temperature parts need to have high mechanical properties and low thermal conductivity,
It is desirable to have both of these characteristics. here,
The most effective way to lower the thermal conductivity of a material is to make it porous, which contains a lot of air with a large specific heat.However, porous materials generally have low strength, so they are applied to areas that require strength. Can not. Therefore, a method is widely used in which a high-strength material is used as a base, and a porous film is formed on the base material by thermal spraying or the like to provide heat insulation. However, in this method, stress may be generated inside the material due to a difference in thermal expansion or thermal conductivity between the base material and the film, and the film may be peeled.

On the other hand, among high-temperature structural materials, there are silicon nitride sintered bodies having high mechanical properties such as strength and toughness. However, these silicon nitride sintered bodies generally have a relatively high thermal conductivity and are generally used. Is not suitable for insulation. Furthermore, if it is attempted to make it porous for heat insulation, industrially advantageous thermal spraying or the like cannot be adopted because silicon nitride is decomposed. Therefore, silicon nitride ceramics, which are ceramics having the best mechanical properties of strength and toughness, cannot be used as a high-temperature structural material for members requiring thermal insulation because of their relatively high thermal conductivity.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems and to provide a silicon nitride sintered body as a high-temperature structural material which can be employed in a portion requiring heat insulation, and a method for producing the same. .

[0005]

Means for Solving the Problems A silicon nitride sintered body according to the present invention comprises, in order to achieve the above object, a surface layer which is a porous layer and a dense layer having a lower porosity than the surface layer. An inner layer, and an intermediate layer formed between the surface layer and the inner layer, wherein the composition of the intermediate layer is configured such that its porosity continuously increases from the inner layer side to the surface layer side. ing. The porosity of the surface layer is 2% to 10%, and the porosity of the inner layer is 0% to 2%. In addition, the method for producing a silicon nitride sintered body according to the present invention includes mixing a sintering aid and a solvent with silicon nitride powder and then evaporating the solvent, or molding and then evaporating the solvent to form a second mixture. 1 After preparing a molded body, mixing the solvent with the silicon nitride powder and evaporating the solvent,
Or, after molding, the solvent is evaporated to form a second molded body, and then the second molded body is brought into contact with the surface side of the first molded body. And baking the second molded body to form a porous layer on the surface side of the first molded body.

[0006] Examples of the solvent include water, ethanol and the like.
Use non-aqueous solvents such as rucols and toluene
Of these, water is the best for safety and cost.
Is also preferred. In the production of the first and second molded bodies,
The mixing ratio of silicon nitride powder, sintering aid and solvent
In the mold, the silicon nitride is 85 to 92% by weight, and the sintering aid is
15 to 8% by weight is preferred. The second molded body uses a sintering aid
A silicon nitride containing 100% by weight is used. Ma
In the case of CIP molding, the cold isostatic pressure is 50 to 300 k.
gf / mm 2 And preferably 100
~ 200kgf / mm 2 Is preferred. The first molded body and
When firing the second molded body, firing temperature and firing
The preferred range of time is a baking temperature of 160 in a nitrogen atmosphere.
The temperature is 0 to 2000 ° C. and the baking time is 10 minutes to 10 hours. Up
The sintered body sintered by the above method is a dense layer
The interface between the porous layer and the surface layer is
Not only increases the mechanical strength, but also
Thermal stress generated at the interface due to relatively high thermal conductivity
Can be expected to reduce the problem of peeling of the formed film
can do.

Further, another aspect of the method for producing a silicon nitride sintered body according to the present invention is to mix a sintering aid and a solvent with silicon nitride powder and then evaporate the solvent, or form the solvent after mixing. Evaporate to produce a first molded body, and then contact the silicon nitride powder, to which no sintering aid has been added, with the surface of the first molded body. In this state, add these sintering aids. The porous layer is formed on the surface side of the first molded body by firing the silicon nitride powder and the first molded body that have not been formed. In the sintered state of the silicon nitride, after firing, the first molded body and the second molded body, or the first molded body and the silicon nitride powder to which the sintering aid is not added, are in the vicinity of the interface. May be joined depending on conditions. In this case, when the silicon nitride sintered body is used for an actual product or the like, the second molded body or the silicon nitride powder to which the binder is not added is removed, and only the first molded body is used. .

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The inventors of the present invention have proposed, as a means for reducing the thermal conductivity of a silicon nitride sintered body having advantageous properties as a high-temperature structural material, a sintering method instead of a film-forming method. The present inventors have found a method of forming a porous layer only in the vicinity of the surface of the sintered body in the process of manufacturing the body, and have reached the present invention. Specifically, in a silicon nitride molded body (hereinafter, referred to as a first molded body) molded by adding a sintering aid, a portion desired to be made porous is formed by a silicon nitride molded body to which no auxiliary agent is added (hereinafter, referred to as a first molded body). It is fired in a state where it is in contact with a silicon nitride powder without additive (hereinafter, referred to as a second molded body) or a silicon nitride powder without additive (hereinafter, referred to as an additive-free powder) to produce a silicon nitride sintered body.

Hereinafter, a silicon nitride sintered body according to the present invention and a
An embodiment of the manufacturing method will be described in detail. [Preparation of silicon nitride sintered body]Production of the first molded body Add sintering aid and solvent to silicon nitride powder, mix and granulate
To produce granulated powder. This granulated powder is put into a mold,
To make the first molded body
You. The above pressure is desirably isotropic, and is static at room temperature.
Cold isostatic pressing, which is formed by applying water pressure, is most preferred.
No. This is performed by cold isostatic pressing or CIP (Cold Isosta
tic Pressing). This cold
In hydrostatic molding, in order to increase the density of the molded body, usually
Is called a binder that has the effect of increasing the density of the molded body
Fats and oils are mixed into the granulated powder.
And the hydrostatic pressure is 50-300kgf
/ Mm 2 , Preferably 100 to 200 kgf / mm 2 so
Molding. Hydrostatic pressure is 50kgf / mm 2 Below is the shape
Can not hold, 300kgf / mm 2 Above
This is because Also, in order to make it porous,
The shape density should be as low as possible and within the range that can maintain the shape
Needs to be the lowest.

Preparation of the second molded body The second molded body is prepared by adding no sintering aid, and otherwise is manufactured in the same procedure as the first molded body. Preparation of Silicon Nitride Sintered Body The second molded body is brought into contact with the surface of the first molded body, and in this state, the surface of the first molded body and the second molded body are fired to form a silicon nitride sintered body. Is obtained. Thereby, the surface layer of the first molded body, that is, the contact portion with the second molded body is made porous. The porosity of this surface layer is 2% to 10%, and the porosity of the inner layer not affected by the porosity is 0% to 2%. An intermediate layer is formed between the surface layer and the inner layer, and the porosity continuously decreases from the surface layer side to the inner layer side. [Solvent] As the solvent, water, alcohols such as ethanol, non-aqueous solvent systems such as toluene can be used, of which water is most preferable. [Sintering Aid] The sintering aid has a function of reacting and melting during heating to bond the silicon nitride particles to each other during firing.
Since silicon nitride has sintering resistance, a sintering aid is added. However, in the above-described firing process, if a part without an aid is made, the sintering is difficult to proceed, and the part is made porous.
Although the composition of the inner layer of the silicon nitride sintered body needs to be dense, the density of the molded body of the entire sintered body is low. Therefore, the inner layer portion is densified by increasing the amount of the auxiliary agent. Usually, the total amount of the sintering aid is about 5 to 8 wt%,
In the case of the present invention, a large amount of an auxiliary agent of about 8 to 15 wt% is added. If the amount is less than 8 wt%, the overall strength cannot be maintained because the density of the molded body is low, and if the amount is 15 wt% or more, the porosity near the surface does not progress. The type of the auxiliary agent is not particularly limited, but those having a lower viscosity at the time of sintering are preferable because they have a large effect of making the material porous.

[Principle (Action) of Porousization] In the first molded body, in the surface layer portion in contact with the second molded body or the additive-free powder, the auxiliary reacts and melts during firing, and the first The auxiliary component diffuses from the molded body into the non-added powder or the second molded body so as to be sucked, and undergoes mass transfer. Since the driving force contributing to sintering is reduced in the first molded body after the transfer of the auxiliary component, even if the inner layer reaches a temperature at which the inner layer is densified, the surface layer has the same density as that at the time of producing the molded body. Since there is almost no difference, the surface layer portion of the first molded body is more porous than the inner layer portion.

[0012]

Next, a method for manufacturing a silicon nitride sintered body will be described.
An example will be described.Production of the first molded body (Raw material powder) Silicon nitride powder as raw material powder is imide
Synthetic powder was used. However, instead of this imide synthetic powder,
Also, powders manufactured by other manufacturing methods such as direct nitriding
The same is true. A sintering aid is used when sintering silicon nitride.
Al commonly used 2 O 3 5 wt%, Y 2 O 3 5w
What mixed t% was used. (Manufacturing process) The above-mentioned raw material powder and sintering aid are mixed with water as a solvent.
Mix and disperse, then spray dry
Granulated. This granulated powder has a diameter of 60 mm and a thickness of about 10 mm
Is first molded using a mold, and then the hydrostatic pressure is set to 200 kgf.
/ Cm 2 Cold isostatic pressing to produce the first molded body
did.

[0013]Production of the second molded body As the raw material powder, use the same imide synthetic powder as in the first molded body.
Mix and disperse with water as a medium without adding sintering aid
The treatment was performed, and granulation was performed using spray drying. This structure
Using a mold to make the granules 60 mm in diameter and 10 mm in thickness
Primary molding, then hydrostatic pressure 200kgf / cm 2 Over
Two pieces of cold isostatic pressing are produced and used as a second molded body.
Was.Production of silicon nitride sintered body State where the first molded body is sandwiched from above and below by the second molded body
At the maximum pressure of 9.5kgf / cm
2 Baking in a nitrogen atmosphere at a temperature of 1800 ° C. for 2 hours
Thus, a silicon nitride sintered body was manufactured.

Inspection of silicon nitride sintered body From the silicon nitride sintered body obtained as described above, the first molded body disposed at the center is taken out, cut and polished so that the inside can be seen, and an engineering microscope is used. The condition of the pores was confirmed using the method. Further, a part of the first molded body was cut out to the size of a JIS test piece, and the strength was measured by a four-point bending test in which the distance between the lower spans was 30 mm and the distance between the upper spans was 10 mm using an Instron type testing machine. Was done.

(Evaluation of Tissue Inspection) FIG.
It is a figure which shows the structure of the obtained silicon nitride sintered compact.
In FIG. 1, as shown in FIG.
In addition, from the interface in contact with the second molded body to a depth of about 2 mm
The portion has many pores, indicating that a porous layer was formed.
Also, as shown in (b) and (c), about 2..
There are almost no pores in the part deeper than 7mm, and the dense layer
It can be seen that it was formed. And these porous layers and fine
Between the dense layer, that is, a depth of 2 mm to 2.7 from the interface
mm, it is an intermediate layer in which the number of pores changes gradually.
I understand. Also, as shown in Table 1, the porous layer on the surface side
Is higher than that of the dense portion on the inner side.
Although it is about 4% lower, the standard deviation showing the variation is almost the same.
Therefore, a decrease in strength characteristics can be suppressed to a minimum. What
The strength reduction rate in Table 1 indicates the average bending in the dense portion.
Strength (97.0 kgf / mm 2 ) And flat in the porous layer
Uniform bending strength (81.1kgf / mm 2 ) Difference,
Average bending strength (97.0 kgf / mm) 2 )
It is the ratio divided by.

[0016]

According to the present invention, the surface which needs to be thermally insulated can be spontaneously made porous during the normal sintering process only by contacting it with silicon nitride without an auxiliary agent, without any special process. Can be achieved. The silicon nitride sintered body according to the present invention can be applied in various fields as described below.

The present invention can be applied to, for example, small reciprocating engines, gas turbine components for power generation, and the like. Since a small reciprocating engine has particularly low thermal efficiency, increasing the efficiency is a measure for suppressing air pollution. In order to increase thermal efficiency, effective measures are to eliminate cooling by eliminating a cooling system equivalent to discarding energy, and to insulate the entire engine. For the non-cooling, a material that can withstand high temperatures may be applied,
Since thermal stress is generated due to pulse heat flux due to intermittent combustion peculiar to the reciprocating engine, it is necessary to have a small coefficient of thermal expansion or high thermal conductivity. In the case of general silicon nitride ceramics, the coefficient of thermal expansion is normal, but the thermal conductivity is high, which is advantageous in this respect.

However, high heat conductivity is disadvantageous for heat insulation. As a material having both of these contradictory characteristics, the silicon nitride sintered body according to the present invention can be applied. In this case, the dense surface is the inner surface of the cylinder or the like that receives direct thermal stress, and the porous surface having heat insulation is the outer surface. Thereby, not only the function of the structural material as the cylinder part constituent material, but also the function of the functional material with respect to heat inflow / outflow and heat insulation are provided. In this way, non-cooling and heat insulation essential for improving the thermal efficiency of the engine can be achieved.

2) Fields Needing Further Weight Reduction The silicon nitride sintered body according to the present invention can be applied to, for example, aircraft gas turbines, engine components for spacecraft, and the like. Porosity is advantageous in order to further utilize the feature of high specific strength of silicon nitride. However, in the sintered body according to the present invention, a decrease in strength can be suppressed to a minimum even if the sintered body is made porous. Great use value.

[Brief description of the drawings]

1A is a schematic diagram of a silicon nitride sintered body produced by a method according to the present invention, FIG. 1B is a diagram showing a surface-side structure of a first molded body, and FIG. Is a diagram showing the structure of the central part of the first molded body, (d) is a diagram showing the structure on the back side of the first molded body, (e) is a diagram showing the structure of the porous layer of (b),
(F) is a diagram showing the organization of the inner layer of (b), (c) and (d),
(G) is a figure which shows the structure of the porous layer of (d).

Claims (2)

[Claims] Claims: 1. It has a surface layer that is a porous layer, an inner layer that is a dense layer having a lower porosity than the surface layer, and an intermediate layer formed between the surface layer and the inner layer, A silicon nitride sintered body characterized in that the composition of the intermediate layer is configured such that its porosity increases continuously from the inner layer side toward the surface layer side. 2. A second molded body made of silicon nitride is brought into contact with a first molded body made of silicon nitride and a sintering aid, and the first molded body and the second molded body are fired in this state. A method for producing a silicon nitride sintered body, wherein a porous layer is formed in the vicinity of the interface between the first molded body and the second molded body.