JPH06305838A - Production of sintered silicon nitride - Google Patents

Abstract

PURPOSE:To obtain a large-sized and thick-walled sintered material having improved reliability by baking a formed mixture of silicon nitride and a sintering assistant under specific baking condition to eliminate the open pores of the formed article and proceeding the sintering without causing the sintering difference between the surface and the core parts at a temperature and a pres sure higher than those of the baking stage. CONSTITUTION:A powdery mixture composed of silicon nitride as a main component and a sintering assistant (e.g. Y2O3, La2O3 and Er2O3) is formed. The formed product is baked in nitrogen atmosphere having an inert gas content of >=50% and a total pressure of 0.1-2atm at 1500-1700 deg.C to eliminate the open pores of the formed material. The baked product is further baked in nitrogen atmosphere at a temperature and a pressure higher than those of the primary baking process to proceed the sintering without causing the sintering difference between the surface and the core parts.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a silicon nitride sintered body suitable as a structural material composed of a large-sized product or a thick product or a material for a heat engine.

[0002]

2. Description of the Related Art Conventionally, a silicon nitride sintered material has attracted attention as a material excellent in strength and toughness, and its application is being promoted as a structural material and a heat engine material.

Since silicon nitride itself is difficult to sinter, the silicon nitride-based sintered body contains a sintering aid such as an oxide of a Group 3a group 3a element such as Al ₂ O ₃ or Y ₂ O _3. Addition is performed to generate a liquid phase at a temperature of 1600 to 2000 ° C. in a non-oxidizing atmosphere such as nitrogen, thereby promoting sintering and densification. Further, since silicon nitride is decomposed when the temperature during firing rises, a method of firing in a pressurized atmosphere with a nitrogen gas pressure of 2 atm or more is adopted.

In particular, recently, the firing method has been improved with the aim of increasing the density of the sintered body and further improving its characteristics. As an example, a multi-stage firing method has been proposed in which the open porosity is reduced to some extent under a low nitrogen pressure, and then the firing is performed in high-pressure nitrogen. According to this method, when the open pores are changed to closed pores by firing, high-pressure gas is not trapped in the closed pores, and a high density sintered body can be obtained.

[0005]

However, when liquid phase sintering is carried out in nitrogen, nitrogen is dissolved in the liquid phase component to increase the viscosity and melting point of the liquid phase, resulting in densification due to dissolution and reprecipitation. Was found to inhibit. Therefore, according to the above-described multi-stage firing method, when the open pores changed to closed pores, nitrogen gas was trapped in the compact while being at low pressure, and was trapped inside during subsequent firing in high-pressure nitrogen. Nitrogen is solid-dissolved in the liquid phase and the viscosity and melting point of the liquid phase rise, so internal sintering tends to be insufficient compared to the outside, and long-term firing is especially necessary when manufacturing large-sized products or thick products. There is a problem in that it is necessary or that the inner and outer densities and compositions are not uniform. Also, hot press method, glass capsule HIP
According to the method, it is possible to sinter the material in a relatively short time, but it is difficult to apply it to a product having a complicated shape and the cost becomes high, so that it is not possible to mass-produce it.

[0006]

[Means for Solving the Problems] The inventors of the present invention have repeatedly studied a method for preventing the dissolution of nitrogen in a liquid phase component in the sintering process, and as a result, have found that an inert gas is contained in the firing atmosphere. It was found that the introduction of nitrogen can suppress the solid solution of nitrogen in the liquid phase component. However, in that case, since the nitrogen pressure becomes low, the firing temperature cannot be raised, and the firing time must be lengthened. Therefore, according to the present invention, a sintered body having a uniform composition and structure is obtained in a short time by firing in high-temperature high-pressure nitrogen after sintering has progressed to a certain extent and for a large-sized product or a thick product. I found that I can do it.

That is, the silicon nitride-based sintered body of the present invention is a silicon nitride-based sintered body in which a mixture of a main component of silicon nitride and a sintering aid is molded and then fired in a non-oxidizing atmosphere. In the manufacturing method, the firing is performed at a temperature of 1500 to 1700 ° C. in a nitrogen atmosphere containing 50% or more of an inert gas and having a total pressure of 0.1 to 2 atm until the open pores of the molded body disappear. It is characterized by comprising one firing step and a second firing step in which it is fired in a nitrogen atmosphere under a higher temperature and pressure than the first firing step.

The present invention will be described in detail below. According to the present invention, first, a silicon nitride powder and a sintering aid powder are prepared as raw material powders. The silicon nitride powder has an average particle size of 0.
Any α-type or β-type raw material having an amount of 3 to 1.5 μm, an oxygen amount of 3.0 wt% or less, and a metal impurity amount of 2.0 wt% or less can be used. In some cases, a silicon nitride component may be formed by nitriding after molding using silicon powder.

As sintering aids, Y ₂ O ₃ and La can be used.
₂ O ₃ , Er ₂ O ₃ , Yb ₂ O ₃ , Dy ₂ O ₃ , Ho ₂
O ₃ Periodic Table Group 3a element oxides, such as and, Al
Known auxiliaries such as ₂ O ₃ , MgO and SiO ₂ can be used. The amount of these auxiliaries is 0.5-2 in the total amount.
If it is added in an amount of 0% by weight and the amount of the auxiliary agent is less than 0.5% by weight, the sinterability is poor, and if it exceeds 20% by weight, the mechanical properties of the sintered body deteriorate.

Next, the above raw material powders are mixed in a predetermined ratio, and then the desired powder is formed into a desired shape by a well-known molding method, press molding, extrusion molding, injection molding, cold isostatic molding, casting molding or the like. After molding, the molded body is fired.

The firing process in the present invention mainly comprises two steps. First, as the first firing step, firing is performed at a temperature of 1500 ° C. or higher, which is a temperature at which a liquid phase is generated and sintering starts, under a low pressure of 0.1 to 2 atm including nitrogen. The total pressure at this time was limited to the above range,
If the pressure is lower than 0.1 atm, silicon nitride is likely to be decomposed. Therefore, the upper limit of the pressure is set to 2 atm in the closed pores when the open pores change to closed pores as sintering progresses. This is to reduce the pressure of the gas trapped in. Therefore, since the nitrogen pressure in this step is low and the temperature exceeds 1700 ° C., silicon nitride is decomposed, and therefore it is necessary to perform firing at 1700 ° C. or lower. In addition, this first firing step needs to be performed for a sufficient time to change the open pores of the molded body into the closed pores.

Further, according to the present invention, as the gas species to be trapped in the molded body when changing from the open pores to the closed pores, the conventional nitrogen gas is changed to an inert gas such as argon or helium. That is a major feature. Therefore, an inert gas such as argon or helium is introduced into the atmosphere in such a process together with nitrogen gas, and the concentration of the inert gas at this time is combined with the nitrogen gas pressure (total pressure).
It is necessary to control to 50% or more, especially 80% or more. This is because when the concentration of the inert gas is less than 50%, the amount of nitrogen dissolved in the liquid phase component when trapped in the closed pores of the molded body is large, and the object of the present invention cannot be achieved.

Next, the sintered body from which the open pores have disappeared in the first firing step is fired in a nitrogen atmosphere. The firing temperature in the second firing step needs to be higher than the firing temperature in the first firing step, and an appropriate temperature range varies depending on the composition, but is set to about 1700 ° C. or higher. If the temperature at this time is lower than the firing temperature of the first firing step, densification cannot be achieved. Further, considering that it is not economical to raise the firing temperature excessively, the upper limit of the firing temperature is set to 1950 ° C. Note that the atmosphere during this second firing step needs to be performed under a nitrogen pressure sufficient to suppress decomposition of silicon nitride as the firing temperature increases, so the nitrogen partial pressure is about 2 atm or more. Is set to. As for the atmospheric pressure, an inert gas or the like may be mixed as long as the nitrogen partial pressure is within the above range, and the total pressure may be a high pressure of 10 to 2000 atm. it can.

[0014]

Since the atmosphere in the first firing step has conventionally been a nitrogen atmosphere, nitrogen gas is trapped in the closed pores when the open pores change to closed pores, and the nitrogen gas is trapped in the liquid phase in the second firing step. The solid solution raises the melting point of the liquid phase and the glass transition temperature to lower the internal sinterability, but according to the present invention, the atmosphere in the first firing step is mainly an inert gas atmosphere. Therefore, since the characteristics of the liquid phase are not changed by the gas trapped in the closed pores, and the internal sinterability is enhanced in the second firing step, the firing should be performed at a relatively low temperature and in a short time. Is possible.

As a result, even if it is a large-sized product or a thick product, there is no difference in the compactness and characteristics inside and outside the sintered body, and a uniform sintered body can be obtained.

[0016]

【Example】

Example 1 α ratio 95%, BET specific surface area 10 m ² / g, oxygen amount 1.
A composition in which 4% by weight of Y ₂ O ₃ and 4% by weight of Al ₂ O ₃ are added to a commercially available silicon nitride raw material produced by a direct nitriding method of ₂ % by weight, 5% by weight of Y ₂ O ₃ and Yb ₂ O ₃ after 5 wt% the added composition prepared, after drying the slurry to wet mixing and grinding, 60mm diameter at a pressure of 3 ton / cm ^2, thickness 4
Rubber press molding was performed into a shape of 0 mm. The obtained molded body was fired according to the conditions shown in Table 1. In addition, in order to suppress decomposition of silicon nitride during firing, 200 g / l of silicon nitride powder was spread and a compact was placed.

From the inside and the outside of the obtained sintered body, 3 ×
A JIS 4 × 40 mm test piece is cut out and polished.
A 4-point bending strength test was conducted based on R1601. Also,
The bulk density was measured by the Archimedes method. The results are shown in Tables 1 and 2.

[0018]

[Table 1]

[0019]

[Table 2]

According to Tables 1 and 2, in the sample No. 15 fired in the single firing step based on the conventional method, the difference between the inside and outside of the obtained sintered body is large, and especially the inside is insufficiently densified. . Further, in the case of multi-stage firing, in samples No. 1 and ₂ in which the first firing step was performed in N ₂ , the inside was not sufficiently densified, and the firing temperature was increased or the time was increased to perform sintering. When it was allowed to proceed, abnormal grain growth was observed in the sintered body, which was the source of destruction, and it was not possible to eliminate the non-uniformity inside and outside. This means that sample N with different composition
It was the same with o.16.

On the other hand, when the inert gas was introduced into the atmosphere during the first firing step, the internal / external difference could be reduced, but the sample No. 3 in which the amount of the inert gas was less than 50%,
In No. 17, the effect is not remarkable. Further, in sample No. 7 in which the total pressure in the first firing step was lower than 0.1 atm, the decomposition of silicon nitride was confirmed. On the contrary, in Sample Nos. 9 and 24 where the pressure exceeded 2 atm, the inside could not be densified.

On the other hand, in all the samples of the present invention, the inside and outside difference in relative density was within 2%, which confirmed that the inside was densified as compared with the comparative example. As a result, the difference in strength between the inside and the outside was within 50 kg / mm ^2, and the mechanical properties were excellent in uniformity.

[0023]

As described in detail above, according to the present invention,
Since the sintering can proceed without any difference between inside and outside, even when a large-sized or thick-walled sintered body is obtained, it is possible to obtain a homogeneous sintered body having no inside or outside difference in density or mechanical characteristics. Thereby, the reliability of the product using such a sintered body can be improved.

Claims (1)

[Claims] 1. A method for producing a silicon nitride-based sintered body, which comprises firing a mixture of a main component of silicon nitride and a sintering aid, and then firing the mixture in a non-oxidizing atmosphere. In a nitrogen atmosphere having a total pressure of 0.1 to 2 atm until the open pores of the molded body disappear until 1500 to
A first firing step of firing at a temperature of 1700 ° C., and a second firing step in a nitrogen atmosphere under higher temperature and pressure than the first firing step
A method of manufacturing a silicon nitride-based sintered body, comprising: a firing step.