JPS6379762A - Manufacture of silicon nitride base sintered body - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing a sintered body mainly composed of silicon nitride, and in particular, to a method for manufacturing a sintered body containing silicon nitride as a main component. Regarding the manufacturing method.

[Conventional technology]

Silicon nitride sintered bodies have excellent compressive strength, wear resistance, and heat resistance, and are one of the fine ceramics that are currently attracting attention in quantity. It is currently being applied to industrial products such as automobile turbochargers.

This silicon nitride sintered body is manufactured by a conventional method.

First, metal oxides and nitrides (e.g. aluminum oxide, magnesium oxide, silicon oxide, yttrium oxide, aluminum nitride) are used as sintering aids, and organic binders (e.g. polyvinyl alcohol, wax, polyacrylic acid, etc.) are used as forming aids. CMC) and a peptizer (for example, sodium silicate, polyacrylate, CMC salt) are added to the silicon nitride powder and mixed. Next, this mixture is molded into a desired shape by die molding, rubber press molding, cast molding, injection molding, extrusion molding, or the like. Next, after removing organic matter and pre-processing by calcining as necessary,
Pressure: 1 to 100 atmospheres, temperature: 1600 to 1,000 atmospheres in N2 atmosphere
It is sintered at 2,000°C and then subjected to post-processing to match the dimensions and make it into a product.

[Problem that the invention seeks to solve]

The silicon nitride sintered body obtained by the above conventional manufacturing method has a dimensional accuracy of around ±1% at most when unprocessed after sintering.
If greater precision is required, machining must be performed. This causes a significant increase in cost, and is a major obstacle to expanding its uses.

The above-mentioned variations in dimensions are due to the following reasons. In other words, the dimensional accuracy when removed from the mold during molding is quite high, but in order to make this into an m-dense sintered body, it is usually necessary to cause volumetric shrinkage of 30 to 50%, and in this process it is necessary to shrink uniformly. This is because it is difficult to progress.

For example, injection molding? ! The particle filling rate that allows molding of rough shapes is 55 to 65% for dry molded products, so in order to make this into an m-dense sintered body, a volumetric shrinkage of 43 to 35% is required. . In addition, even in rubber press molding and slurry casting molding, which allow relatively dense molding, the particle filling rate of the dry molded product is 60 to 70%, so in this case as well,
Requires volumetric shrinkage of 0-30%. If this is converted into a linear shrinkage rate, it will be 20 to 13%. With such significant densification, uniform shrinkage is difficult and warping occurs in some areas, resulting in dimensional accuracy of approximately ±1%.

The present invention has been made in view of the above circumstances, and is intended to provide a method for producing a silicon nitride sintered body having excellent mechanical strength and dimensional accuracy.

(Means for Solving the Problems) The method for producing a silicon nitride sintered body according to the present invention involves molding a powder mainly composed of silicon nitride, then converting a part of it into silicon oxide, and then forming a non-oxidizing sintered body. It is characterized by sintering in an atmosphere.

As a method for silicon oxidation in the present invention, a simple method of firing at a temperature of 600 to 1300° C. in an air atmosphere can be used, for example.

However, any other method may be used.

In the present invention, in order to further increase the particle filling rate of the dry compact, we do not add silicon oxide as a sintering aid to silicon nitride, but instead use a forming aid and other sintering aids as necessary. It is preferable to perform molding drying by adding . In that case, it is preferable to use a nitrogen atmosphere of 1 to 100 atm and 1600 to 200°C as conditions for converting a part of the silicon nitride into silicon oxide.

In the present invention, the method for molding silicon nitride powder is as follows:
General molding methods such as die molding, rubber press molding, slurry casting, injection molding, and extrusion molding can be used. Among these, rubber press molding and slurry casting molding, which allow the most precise molding, are preferred.

[Effect]

In the method of the present invention, after the silicon nitride powder is molded, a part of it is oxidized and converted into silicon oxide before sintering. This oxidation to silicon oxide is accompanied by volumetric expansion, but the dimensions of the molded body do not change during this process, so the particle filling rate of the molded body increases.

Therefore, the shrinkage rate required during sintering is reduced, and the variation in shrinkage rate is also reduced.

For example, if silicon oxide is not added as a sintering aid, but a forming aid and other sintering aids are added as needed, and molding is done by rubber press molding or casting molding, which allows for the most dense molding, particle filling A molded body with a yield of 70% is obtained. This molded body is fired in an air atmosphere at, for example, 600°C to 1300°C,
When a part of the silicon nitride is converted into silicon oxide, volumetric expansion occurs without changing the dimensions of the compact, so that the particle filling rate increases to 73% or more. As a result, the linear shrinkage rate during sintering required to reach the theoretical density is 10% or less, and the variation in shrinkage is reduced, resulting in improved dimensional accuracy.

(Examples) Example 1 Silicon nitride powder having a particle size of 1 mm was dispersed in water together with a deflocculant (ammonium polyacrylate).

At this time, no other so-called sintering aids were added.

This dispersion in water was poured into a plaster mold and cast molding was performed. The obtained molded body was heat-treated at 500°C, treated with a deflocculating agent, and then heated at 1000°C in the air.
By performing the firing treatment for 5 hours, a part of the silicon nitride was converted into silicon oxide. The weight increase at this time is 3.5%,
It is estimated that about 12% silicon oxide was produced. Moreover, no dimensional change of the molded body occurred during this firing treatment.

Next, the partially siliconized molded body was subjected to a sintering treatment in a nitrogen atmosphere at a pressure of 10 atmospheres and a humidity of 1700° C. for 4 hours. The obtained sintered body had a porosity of 0% and a linear shrinkage rate of 5%.

Further, it was confirmed that the sintered body did not show any warping or the like, and had contracted in a negative manner.

Example 2 96% by weight of silicon nitride powder with a particle size of 1-mm and 4% by weight of yttrium oxide powder with a particle size of 0.8-mm were dispersed in water together with a binder (polyvinyl alcohol). This was granulated by a spray drying method, and then pressed and molded by a rubber press method at a pressure of 1.5 ton/d. The obtained molded body was heat-treated at 500° C. to remove the binder, and then fired at 950° C. for 3 hours in the air to convert a portion of the silicon nitride into silicon oxide. The weight increase due to this firing process was 2.0%, and it is estimated that about 7% was converted into silicon oxide. Moreover, no dimensional change of the molded body occurred due to firing.

Next, the above-mentioned partially oxidized molded body was heated in a nitrogen atmosphere at a pressure of 10 atm and a temperature of 1800°C for 4 hours.
Sintered for hours. The porosity of the obtained sintered body was approximately 0%,
The linear shrinkage rate was 9%. Further, no warping or the like was observed in the sintered body, and it was confirmed that the sintered body had shrunk in a negative manner.

Comparative Example 89% by weight of silicon nitride powder with a particle size of 1#IR, 6% by weight of silicon oxide powder with a particle size of 1, and 5% by weight of yttrium oxide powder with a particle size of O,a were mixed in water with a binder (polyvinyl alcohol). It was dispersed into After granulating this by spray drying method, 1.5
It was pressed and molded with a pressure of ton/d. The obtained molded body was heat-treated at 500°C to remove the binder.

Next, the above molded body was sintered in a nitrogen atmosphere at a pressure of 10 atmospheres and a temperature of 1900° C. for 4 hours.

The porosity of the obtained sintered body was approximately 0%, and the linear shrinkage rate was 14%.
Met. Moreover, some warpage was observed in the sintered body.

As is clear from comparing the results of the above example and comparative example, the shrinkage rate during sintering in the comparative example was 14%, while the shrinkage rate in all of the examples was 10% or less, and the size was high. Accuracy is obtained.

〔Effect of the invention〕

As described in detail above, the present invention provides remarkable effects such as suppressing the shrinkage rate during sintering and producing a silicon nitride sintered body with high dimensional accuracy.

Claims (1)

[Claims] 1. A method for producing a silicon nitride sintered body, which comprises molding a powder containing silicon nitride as a main component, converting a portion of the powder into silicon oxide, and then sintering the powder in a non-oxidizing atmosphere.