JPH0570237A - Production of silicon nitride sintered body - Google Patents

Abstract

PURPOSE:To solve the problem that the strength of a sintered body is lowered in service environment by the formation of a liq. phase and to obtain a sintered body having high strength at high temp. as a high temp. structural material. CONSTITUTION:When a molded body consisting of silicon nitride powder with a silica coating film formed on at least the surface and a sintering aid is sintered, the silica coating film is removed immediately before sintering and a silicon nitride sintered body is produced. The film removal is carried out by heat treatment in the temp. range of 1,200-1,700 deg.C in a nonoxidizing atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon nitride sintered body useful in the field of high temperature structural materials. More specifically, the present invention relates to improvement of a manufacturing process of a silicon nitride sintered body which can improve high temperature strength.

[0002]

2. Description of the Related Art Sintered silicon nitride (Si ₃ N ₄ ) is excellent in heat resistance, impact resistance, fracture toughness and strength, and is required to have high strength at high temperatures such as gas turbine parts and diesel engine parts. Used in parts. Various studies have been made to develop, maintain and improve the high temperature strength of this sintered body.

Since silicon nitride has a high covalent bond and cannot be easily solid-phase sintered, a liquid-phase sintering method in which an auxiliary agent is added and a liquid phase is generated at a firing temperature for densification is generally used. for that reason,
The obtained sintered body had a glass phase in which the liquid phase was vitrified at the grain boundary, and the high temperature creep including high temperature strength depended on the glass phase existing at this grain boundary. On the other hand, it has been proposed to control the composition of the glass phase, to crystallize the glass phase by heat treatment, or to increase the heat resistance of the glass phase itself. However, there is a limit to the high temperature and high strength due to the crystallization of the glass phase and the improvement of heat resistance, only by examining the kind and amount of the additive to be added.

In order to remove or reduce the amount of glass phase in this silicon nitride sintered body, 1400 silicon nitride raw material powder was added.
A method is known in which silica and oxygen are removed by heat treatment in a non-oxidizing atmosphere at a temperature of ℃ to 1800 ℃ for 30 minutes to 2 hours, and a sintered body is obtained by using this silicon nitride powder (Japanese Patent Publication No. 2-47426). Further, a method has also been proposed in which the heat-treated silicon nitride powder and a mixture of the silicon nitride powder and a sintering additive are heat-treated and pulverized (Japanese Patent Publication No. 2-47427). Further, a method has also been proposed in which a silicon nitride ingot obtained by a direct nitriding method in a silicon nitride raw material manufacturing step is heat-treated at 1500 ° C. to 1800 ° C. and then pulverized to obtain a powder (JP-A-2-248308). Bulletin). Although the purpose is different, a method has also been proposed in which the surface silica layer of the silicon nitride powder is positively subjected to a reduction treatment using ammonia or a mixed gas of ammonia and hydrocarbon to modify the surface of the silicon nitride powder (special feature. Kaihei 1-226768).

In addition, silicon nitride powder and a sintering aid are added,
A method of increasing the strength by mixing and molding and then preheating is proposed (Japanese Patent Publication No. 58-15463). In this preheating treatment, the compact is embedded in powder of aluminum nitride, boron nitride, silicon nitride or the like, and the temperature is 1500 ° C to 1850 ° C for 10 minutes to 5 minutes.
If performed in a non-oxidizing atmosphere for a period of time, then
Sintering is performed at 1500 ℃ ~ 1900 ℃. The reason why the strength of the sintered body changes depending on the presence or absence of the preheating treatment is that the additive can exist in the crystal phase.

[0006]

As described above, the reduction of silica in the conventional silicon nitride sintered body has been carried out at the raw material processing level, but in each step until the subsequent sintered body is obtained. For increasing silica, its removal and reduction was not considered.

As can be seen from the fact that silicon nitride itself does not exist in the natural world, it is not thermodynamically stable in the atmosphere, and silicon nitride has a characteristic that it is always oxidized into silica. In particular, the silicon nitride powder used as a raw material for the ceramics sintered body has a size of submicron (10 ⁻⁴ mm order) and is easily oxidized by moisture and oxygen in the air. Therefore, even after the heat treatment of the raw material, re-oxidation of silicon nitride in each manufacturing process until it becomes a sintered body requires great care such as powder operation in a non-oxidizing atmosphere.
There is a problem that processing under such an environment is not practical. Furthermore, when silicon nitride is oxidized in the middle of the process by some operation, there is a high possibility that all the batches will be defective.

The idea of performing preheat treatment before sintering does not mention the removal of silica and oxygen, which is proposed in Japanese Patent Publication No. S58-15463, and is different from the sintering process. Not clear.

The present invention has been made in view of the above circumstances, and not only removes silica contained in the raw material of silicon nitride, but also increases silica in each step from mixing the raw materials to forming a sintered body. It is also an object of the present invention to eliminate this and provide an excellent silicon nitride with little reduction in strength at high temperatures.

[0010]

The above object is to remove the silica coating just before sintering a molded body composed of a silicon nitride powder having a silica coating formed on its surface and a sintering aid. Was achieved by the method for producing a silicon nitride sintered body.

The raw material silicon nitride powder used in the production of the silicon nitride sintered body is trisilicon tetranitride, and the usual raw materials used in the production of the ceramic sintered body can be used. Silicon nitride includes α-type and β-type in addition to amorphous, but any of them may be used. This silicon nitride powder has a silica coating on the surface. Here, the silica on the surface of the silicon nitride grains is an amorphous or crystalline phase of silicon oxide (SiO ₂ , SiO) (cristobalite, etc.). The coating film may be formed entirely, or may be partially formed like a crushed material. In general, oxygen present in silicon nitride grains in a silicon nitride raw material powder is not only water and oxygen adsorbed on the powder surface but also silicon oxide (Si) forming the surface layer.
O _2, SiO) or the like, where is divided into three although the solid solution or trapped is possible ones and silicon nitride grains be removed by heating with hydrofluoric acid which is called silica. These ratios range from 0.01 wt% to 0.1 wt% for surface adsorbed oxygen and 0.1 wt% for surface layer silica.
3.0wt%, oxygen dissolved in the grains is 0.1wt% converted to silica
% To 2.5 wt%. The silicon nitride powder preferably has a small amount of oxygen in the raw material for the purpose of strengthening at high temperature by removing silica. Particle size is about 5 nm to 1 μm, usually 0.1 to
It may be about 0.8 μm.

The sintering aid can also be conventional. As typical sintering aids, alumina, magnesia, calcia, spinel, yttria, oxides such as rare earth oxides, nitrides such as aluminum nitride, and alkoxides or sols that are precursors thereof can be used. .. The amount of auxiliaries can also follow conventional methods, generally 0.1
% By weight, typically 35% by weight, usually 2% by weight to 12% by weight.

As a molding method for molding the raw material powder into a predetermined shape, generally used appropriate methods such as injection molding, extrusion molding, sludge casting, uniaxial pressing, CIP and the like are used.

As a method of removing the silica coating of the silicon nitride particles forming the molded body, 1200 to 17 in a non-oxidizing atmosphere is used.
A method of removing silica as SiO 2 gas by heating at 00 ° C., a method of previously adding carbon powder to a molded body and reducing the silica to silicon nitride by heating in a non-oxidizing atmosphere, In the above method, a method of using a phenol resin or the like that produces carbon by heating in place of the carbon powder can be used.

When the silica coating is removed by heating, the lower limit of the heating temperature is Si as a medium for removing silica.
It depends on whether the vapor pressure of O 2 gas is appropriate under flowing non-oxidizing gas or under reduced pressure. On the other hand, the upper limit of the heating temperature is
It is desirable that the temperature is lower than the temperature at which the auxiliary agent reacts with silica and silicon nitride to start liquid phase formation. When the liquid phase is generated, sintering through the liquid phase proceeds, and the silica component is continuously removed from the liquid phase by the heat treatment. In other words, it means that the sintering that started once stops halfway.
This finally makes the sintered body inhomogeneous. Therefore, it varies from 1200 ℃ to 1700 depending on the auxiliary agent and gas flow rate.
In the temperature range of ℃, preferably 1350 to 1650 ℃.
If the heating temperature is less than 1200 ° C, as shown in Fig. 1, S
The vapor pressure of iO gas becomes too low and the treatment time becomes unrealistic. At 1700 ° C. or higher, silica reacts with the sintering aid and silicon nitride to form a liquid phase.

Examples of the non-oxidizing atmosphere gas include argon (Ar), ammonia (NH ₃ ) gas and the like, in addition to nitrogen gas. In this non-oxidizing atmosphere, it is preferable to maintain the nitrogen gas partial pressure at or above the equilibrium partial pressure when silicon nitride and metal silicon are in equilibrium, that is, above the nitrogen gas line in FIG. This is because when the partial pressure of nitrogen gas is less than this equilibrium partial pressure, decomposition of silicon nitride occurs. Next, the partial pressure of SiO gas in the non-oxidizing atmosphere is lower than the partial pressure under the partial pressure of nitrogen gas in the non-oxidizing atmosphere. For example, when the partial pressure of nitrogen gas is the equilibrium partial pressure, the line of SiO gas in FIG. It is preferable to keep it down. This is because if the partial pressure of SiO gas becomes equal to or higher than this partial pressure, the reaction of decomposing silica to generate SiO gas does not proceed smoothly.

In order to positively remove the SiO gas, which is a decomposition reaction product of silica, from the object to be treated, it is desirable to expose the object to be treated to a non-oxidizing gas flow in which the SiO gas is unsaturated. If the flow rate is too high, it is not desirable from the standpoint of fixing the object to be treated and economically. Therefore, the velocity of the non-oxidizing atmosphere gas is preferably 0.0001 to 200 cm / sec.

As another method for positively removing the SiO gas from the object to be treated, the effect is remarkable when the atmosphere is depressurized by the atmospheric pressure. By keeping the nitrogen gas in the atmosphere at a pressure higher than the equilibrium partial pressure value and lower than the partial pressure of SiO gas at the nitrogen partial pressure, the silicon nitride is not decomposed and the SiO gas can be effectively removed. it can. The level of this pressure reduction corresponds to at least the region surrounded by the saturation curve of SiO 2 gas and nitrogen gas in the temperature range of 1700 ° C. or lower in FIG.

However, in the case of a processing apparatus which does not have a gas flow structure, if the processing volume is sufficient so that the SiO 2 gas does not reach saturation even in a stationary atmosphere, it is possible to remove silica although the processing time is long.

The heat treatment time is generally 10 minutes to 50 hours, though it varies greatly depending on the heating temperature, the shape and size of the object to be treated and various other conditions.

The extent to which the coating film is removed is determined by the application of the silicon nitride sintered body and the like. The silica coating is removed immediately before sintering. Just before this, it is sufficient if it is arranged so that a silica film is not formed thereafter. Also,
As a general rule, the molded body on which the silica coating is removed is directly subjected to the sintering step, and the shape of the molded body is the same as that of the sintered body.

Sintering may be carried out by appropriately selecting from known methods. Usually, it is carried out in a non-oxidizing atmosphere. At this time, the partial pressure of nitrogen gas is such that carbon, silicon nitride and silicon carbide are in equilibrium. It is preferable to maintain at least the equilibrium partial pressure. Also, it is preferable to maintain the partial pressure of nitrogen gas in the non-oxidizing atmosphere at the equilibrium partial pressure when carbon, silicon nitride and silicon carbide are in equilibrium even during the temperature reduction after sintering. After removing the silica coating, the nitrogen partial pressure is set to be higher than the pressure at which carbon, silicon nitride, and silicon carbide are in equilibrium to prevent contamination of the silicon nitride sintered body from the heating element and crucible. ..

FIG. 3 shows a thermodynamic stability diagram at 1400 ° C. of the condensed phase in the coexisting system of silicon, nitrogen, carbon and oxygen.
The horizontal axis of the figure is the oxygen partial pressure, and the vertical axis is the nitrogen partial pressure. In the figure, silicon nitride (Si ₃ N ₄ ) and silica (Si
O ₂ ), silicon carbide (SiC) have three condensed phase stable regions, and the nitrogen partial pressure is determined at the boundary lines that form the surface, ab, bc, and bd lines in the figure where the two condensed phases are stable. If the oxygen partial pressure is determined, and vice versa, the nitrogen partial pressure is determined. Figure 3 where the three condensed phases meet at one point
At point b, the nitrogen partial pressure and oxygen partial pressure are uniquely determined. S
The equation representing the equilibrium of i ₃ N ₄ (s) / SiO ₂ (s) is represented by the equation (1). On the other hand, the equation representing the equilibrium of Si ₃ N ₄ (s) / SiC (s) is represented by the equation (2). 3SiO ₂ (s, l) + 2N ₂ (g) = Si ₃ N ₄ (s) + 3O ₂ (g) (1) Si ₃ N ₄ (s) + 3C (s) = 3SiC (s) + 2N ₂ (g) ( 2)

At 1400 ° C., when the nitrogen partial pressure becomes 0.56 atm or less, the reaction of the formula (2) proceeds to the right and silicon carbide is produced. FIG. 2 shows the relationship between the equilibrium nitrogen partial pressure and temperature. Therefore, the nitrogen gas partial pressure is kept higher than this partial pressure shown in FIG.

[0025]

In the method of the present invention, silica is removed in principle by removing the SiO gas having a high vapor pressure generated by the decomposition of silica as shown in the formula (1). 2SiO ₂ (s, l) = 2SiO (g) + O ₂ (g) (1)

On the other hand, the decomposition reaction of silicon nitride can be expressed by equation (2). The equilibrium partial pressure of nitrogen in the equation (2) is shown in FIG. 1, and this equilibrium partial pressure of nitrogen increases as the temperature rises. In order to suppress the decomposition of silicon nitride, it is necessary to maintain the partial pressure of nitrogen gas in the non-oxidizing atmosphere at or above this equilibrium partial pressure. Si ₃ N ₄ (s) = 3Si (s, l) + 2N ₂ (g) ↑ (2)

Equation (3) is established in the coexistence of silicon nitride and silica in the solid phase. When the partial pressure of nitrogen gas in the non-oxidizing atmosphere becomes equal to or lower than the partial pressure obtained by the equation (2), a reaction in the right direction occurs in the equation (3), and silica is removed by SiO gas, but at the same time, silicon nitride also decreases. Will end up. Si ₃ N ₄ (s) + ₃ SiO ₂ (s) = 6 SiO (g) + 2N ₂ (g) ↑ (3)

FIG. 1 shows the vapor pressure of SiO gas produced by the reaction represented by the equation (1) under the lowest nitrogen gas partial pressure at which silicon nitride is not decomposed. The partial pressure of SiO gas also increases as the temperature rises. Nitrogen pressure higher than the equilibrium nitrogen gas partial pressure is
As can be seen from the equation (3), since the vapor pressure of SiO gas, which is a medium for removing silica, is suppressed, there is a problem that the treatment time for removing silica becomes long if the partial pressure of nitrogen is higher than the equilibrium partial pressure. is there.

The silicon nitride sintered body according to the present invention removes and reduces, just before sintering, silica produced by the oxidation of the silicon nitride raw material powder which is inevitable in each step up to the conventional raw material and the subsequent sintering. As a result, the liquid phase formation temperature rises and the sintering temperature is raised. Thus, conventionally, the problem that the strength is lowered due to the formation of a liquid phase in the environment where the sintered body is used is solved, and a material having high high temperature strength is obtained as a high temperature structural material.

[0030]

Example 1 A raw material powder consisting of 95% by weight of silicon nitride powder having a silica coating on its surface and 5% by weight of yttrium oxide powder was mixed, and a rod-shaped sample of 20 × 80 × 15 mm was uniaxially at 40 MPa. After pressing, CIP (cold isotropic pressing) of 300 MPa was performed for molding. This was heat-treated for 1 hour in a nitrogen gas atmosphere of 1400 ° C. in a gas flow of 0.1 cm / sec in the absence of SiO gas.
Pressurize this at 1400 ℃ to 60atm, then 1950 ℃, 80atm
Sintering was carried out for 2 hours in the nitrogen atmosphere, and the pressure was gradually lowered to the same pressure as the temperature was raised to be cooled to obtain a sintered body. The density of this sintered body was obtained by the Archimedes method, and the relative density (% TD) was obtained from the ratio with the theoretical density. Further, a plurality of 3 × 4 × 40 mm test pieces were produced from this sintered body according to the test method defined in JIS, and a 4-point bending test was performed at 1400 ° C.
Table 1 shows the results of these tests.

[0031]

[Table 1]

Comparative Example 1 A sintered body having the same shape was prepared by the same procedure as in Example 1, except that the heat treatment was not performed before sintering. With respect to these sintered bodies, the density measurement and the 4-point bending strength at 1400 ° C. were carried out in the same manner as in Example 1. The results are shown in Table 1. It is clearly seen that the heat treatment increases the hot strength at 1400 ° C.

Comparative Example 2 After the heat treatment in Example 1, this was sintered for 2 hours at 1950 ° C. in a nitrogen atmosphere of 30 atm. As a result, stains due to carbon contamination from the heating element and crucible on the cut surface of the sintered body, which were not found in Example 1, were observed.

Comparative Example 3 After firing in Example 1, the gas pressure was lowered and cooling was performed at 1500 ° C. while reducing the pressure to atmospheric pressure. No discoloration was observed on the cut surface of the sintered body, but the surface of the sintered body reacted with silicon nitride due to the splash of carbon powder generated by the supply and exhaust of steam and gas from the heating element and crucible, and partly changed to a pale yellow-green color. Was there. In addition, in Example 1, such discoloration on the surface of the sintered body was not observed.

Examples 2 to 8 Sinters of the same shape were prepared by the same operations as in Example 1 under the composition and heat treatment conditions shown in Table 2. However, after heat treatment, the respective objects to be treated were 53 atm in Example 2, 64 atm in Example 3, 71 atm in Example 4, and 60 in Example 5.
Atm, 63 atm in Example 6, 67 atm in Example 7, and 60 atm in Example 8 were pressurized and then fired. In Example 8, the target object after the heat treatment was sintered at 1850 ° C. for 2 hours, and the cooling was gradually reduced to the same pressure as the temperature rising to be cooled to obtain a sintered body. With respect to these sintered bodies, the density measurement and the 4-point bending strength at 1400 ° C. were carried out in the same manner as in Example 1. The results are shown in Table 3.

[0036]

[Table 2]

[0037]

[Table 3]

Comparative Example 4 In Example 8, a sintered body having the same shape was prepared by the same operation except that the heat treatment was not performed before sintering. With respect to these sintered bodies, the density measurement and the 4-point bending strength at 1400 ° C. were carried out in the same manner as in Example 1. The results are shown in Table 4. It is clearly seen that the heat treatment increases the hot strength at 1400 ° C.

[0039]

[Table 4]

[0040]

As described above, according to the present invention, by removing or reducing the silica present in the compact before the sintering step by heat treatment, nitriding excellent in high strength even at a high temperature of 1400 ° C. A silicon sintered body is obtained. Therefore, it is possible to provide a structural material for a portion that requires high heat resistance and high strength, such as a ceramic gas turbine engine, and solves the conventional problem of strength reduction at high temperatures.

[Brief description of drawings]

FIG. 1 is a diagram showing an equilibrium nitrogen gas partial pressure for suppressing decomposition of silicon nitride and an equilibrium nitrogen gas partial pressure and a partial pressure of SiO 2 gas generated under the equilibrium nitrogen gas partial pressure.

FIG. 2 is a diagram showing an equilibrium nitrogen gas partial pressure when silicon nitride and silicon carbide are in an equilibrium relationship in the presence of carbon.

FIG. 3 is a thermodynamic stability diagram at 1400 ° C. of an aggregate phase involved in a reduction nitriding treatment of silica in the presence of carbon.

Claims (7)

[Claims] 1. Removal of the silica coating by heating in a non-oxidizing atmosphere immediately before sintering of a compact comprising a silicon nitride powder having a silica coating formed on at least the surface thereof and a sintering aid. Of producing a silicon nitride sintered body 2. The method according to claim 1, wherein the silica coating is removed by heat treatment in a temperature range of 1200 to 1700 ° C. in a non-oxidizing atmosphere. 3. The manufacturing method according to claim 2, wherein the partial pressure of nitrogen gas in the non-oxidizing atmosphere is maintained at the equilibrium partial pressure when silicon nitride and metallic silicon exist in equilibrium. 4. The method according to claim 3, wherein the non-oxidizing atmosphere is placed in a non-oxidizing gas flow. 5. The manufacturing method according to claim 3, wherein the partial pressure of SiO gas in the non-oxidizing atmosphere is kept lower than the partial pressure under the partial pressure of nitrogen gas. 6. The method is characterized in that after the silica film is removed, the nitrogen gas partial pressure is kept at a value equal to or higher than the equilibrium partial pressure when carbon, silicon nitride and silicon carbide are in equilibrium, and sintering is carried out in a non-oxidizing atmosphere. The method for manufacturing a silicon nitride sintered body according to claim 1. 7. The temperature is lowered after sintering by maintaining the partial pressure of nitrogen gas in a non-oxidizing atmosphere at a pressure equal to or higher than the equilibrium partial pressure when carbon, silicon nitride and silicon carbide are in equilibrium.
Or the method for manufacturing a silicon nitride sintered body according to item 6.