JPS6042188B2 - Manufacturing method of silicon nitride molded body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a silicon nitride molded body having excellent oxidation resistance, and particularly to a method for manufacturing a silicon nitride molded body having excellent oxidation resistance even at temperatures around 800 to 1000°C.

In general, the method of producing a so-called reaction sintered silicon nitride molded body (hereinafter referred to as a "formed body") by molding and nitriding metallic silicon produces a molded body with excellent dimensions and precision, and it is possible to produce a molded body with a complex shape. It can be easily produced, and the molded product has a small coefficient of thermal expansion, excellent impact resistance, and can withstand temperatures of 1400°C.
It has the advantage that the strength hardly decreases up to ℃,
Attempts are being made to apply the method to various parts, such as parts for eyebrow motors, aircraft, high-temperature furnace materials, and precision machinery. However, since the molded body undergoes almost no dimensional change during its manufacture, it has the disadvantage that an apparent porosity of only 5 to 30% can be obtained, resulting in poor oxidation resistance.

The present inventors conducted various studies to improve its oxidation resistance.

The temperature conditions of the molded body were 40 to 50 to 600N700,
The temperature was changed from 80°C to 900N100°C to 110°C and 1200°C, and the oxidation time was set to 100N300 and 100 hours, and oxidation was carried out in the air, and the weight increase rate after the oxidation was investigated.

The results are shown in FIG. As is clear from FIG. 1, even when there is almost no free silicon, the weight increase due to oxidation is significantly large in a specific temperature range, that is, in the temperature range of 800 to 1050°C. The weight increase over time is 1000 for 300 hours from the start of oxidation.
℃ showed the maximum weight increase, and if oxidation was continued for a longer time, the weight increase reached the maximum at 900℃, and it was observed that the peak of weight increase shifted to the oxidation low temperature side.

Furthermore, some of the samples oxidized at 900° C. in the atmosphere for 100 hours or more developed fine racks. This 90
The relationship between oxidation time and bending strength up to 100 hours at 0°C is shown in Figure 3, where the bending strength slightly increases up to 20 hours.

This is Si0 in the pores of Si3N4. This is thought to be due to the formation of porosity, which lowered the porosity. The bending strength gradually decreases after 30 m, and the variation in bending strength increases after 100 hours when cracks are observed.

This is undesirable because it imposes temperature restrictions on the use of the molded body. For example, when used as a stud bolt for locking ceramic fiber, which is a furnace material,
Since stud bolts are used perpendicular to the temperature distribution, they can cause various problems such as falling off. The present inventors further investigated the cause of the abnormal weight increase at an oxidation temperature of 800 to 1050°C. In general, the reaction-sintered compact contains about 2% oxygen, but the oxygen is either in the form of Si2ON2, dissolved in α-Si3N4, or dissolved in β-SiAlON. It is not clear whether it exists in the form of SiO2 or SiO2, but it is thought that it exists in some form. When such molded bodies were oxidized in the presence of oxygen at a temperature of 700°C or higher for 100 hours, all of the molded bodies with cracks contained 4% or more of oxygen. In this case, the α/β ratios of Si2ON2 and Si3N4 hardly change, and chemical analysis shows that the nitrogen content decreases contrary to the increase in the acid content. Further, in this test, when X-ray diffraction was performed on the materials oxidized at a temperature of 1000 DEG C. or higher, a peak of .alpha.-SiO2 was observed in some products. The weight of the α-SlO2 peak is increased, and it is thought that the weight increase is mainly due to oxidation. The weight increase due to this oxidation increases as the apparent porosity increases.

Furthermore, if unreacted Si remains, the weight increase due to oxidation becomes even more severe. This is because unreacted Si reacts with oxygen and becomes SiO2, and Si is completely converted to SiO2.
If it becomes larger, the weight will increase by 2.1 times and the volume will expand by 2.3 times, which obviously causes strength deterioration. Furthermore, even in the case of a molded body in which there is no Yuzheng 1 at all, it is thought that the weight will increase according to the following formula, and. Since its theoretical volumetric expansion is about 86%, it is presumed that cracks occur due to expansion. Formula Sj3N4+302→
3Si02+2N2 On the other hand, even in this oxidation test, despite the increase in oxygen in the compact, the temperature was 1100
~1300°C! The surprising result was that there was almost no increase in weight or cracks in the heat-treated products. FIG. 2 shows the relationship between oxidation temperature and weight increase rate of the heat-treated product according to the present invention, and FIG. 3 shows the relationship between oxidation time and bending strength at 900°C.

The present invention was completed based on these findings, and the conventionally known molded product was heated in the atmosphere at a temperature of 1100 to 130°C.
By heat treatment at 0℃, the temperature is 800~105
The object of the present invention is to provide a method for producing an oxidation-resistant silicon nitride molded article that shows almost no increase in weight or generation of cracks even after continuous use for 100 C@ or more at 0.degree.

That is, in the present invention, a molded product obtained by reacting and sintering metallic silicon with nitrogen is heated at a temperature of 1100 to 130°C in the presence of an oxygen-containing gas.
It is characterized by heat treatment at 0°C.

The present invention will be further explained in detail below. In the present invention, for example, molded bodies such as stud bolts and furnace materials are heated to a temperature of 800 to 100.
This is an improved method for producing a silicon nitride molded body that can be used for a long period of time in the range of 50°C.
The reaction between oxygen and silicon nitride is suppressed by heat treatment at .degree. C. and forming a thin glassy film on the surface of the compact.

Therefore, the oxidation rate decreases rapidly. In the present invention, the heat treatment temperature is 1100-1300°C, preferably 1150-1300°C.
It is important to set the temperature to 1250°C; below 1100°C, the silicon nitride molded body has little oxidation prevention effect.

For example, heat treatment at a temperature of 1000° C. causes oxidation on the surface and increases the weight, but since a thin glassy film is not formed, there is almost no oxidation prevention effect. Furthermore, when the temperature exceeds 1300°C, the oxidation prevention effect of the silicon nitride molded body decreases to 1300°C.
Although it is almost the same as 'C, the thickness of the film formed on the outer surface of the silicon nitride molded body is larger and the dimensions are larger.
Not preferred when precise fitting is required.

The lower the oxygen concentration, the longer the heat treatment will take for the oxygen-containing gas, so it is usually sufficient that the oxygen content is 1% by volume or more, and it may be a mixed gas of oxygen and an inert gas, argon or nitrogen, or air. is used, with air being particularly preferred.

The heating time varies depending on the object to be treated, the heating temperature, etc., but is usually 1 powder to 5 hours, preferably 3 powders to 3 hours, and the heating rate is preferably relatively rapid heating in a high temperature atmosphere of 1100 ° C. You can also put it directly into the

Since the internal pores of the molded body are open cells, a glassy thin film is formed on the surface and inside of the silicon nitride molded body when subjected to heat treatment. In addition, although it is preferable that the molded product be of high purity, even if it is poorly nitrided, such as having free Si of 1% by weight or less, the oxidation resistance will be improved if it is heat-treated under the conditions of the present invention. be done. Next, the molded body used in the present invention will be explained, and the molded body obtained by a conventionally known method is used.

As an example, an organic binder is added to metal silicon powder having an average particle size of 2 μm or less, and the mixture is molded using a molding machine.

In this case, a thermoplastic resin such as styrene resin or a water-soluble resin such as methyl cellulose or polyvinyl alcohol is blended as an organic binder. Moreover, various molding aids can be added as necessary. The mixing ratio is 1 (1) part by weight of metal silicon powder, 3 parts of styrene resin.
Examples include, but are not limited to, 0.5 to 15 parts of stearic acid, and 0.5 to 15 parts of butylbenzyl phthalate.

Next, these are molded using known molding methods such as extrusion,
Molding methods such as injection, press, and slip casting are used.

Next, this is heated to remove organic compounds such as the binder, and then reacted and sintered in a heating furnace.

The molded body thus obtained may be treated at high temperature in an oxygen-containing gas atmosphere by the method described above.

The silicon nitride molded body produced by the method of the present invention having the above structure has a reduced weight increase rate when used under high temperature heating conditions, while preventing a decrease in bending strength, compared to conventional molded bodies. Ru.

Therefore, the silicon nitride molded body is used as a material for various parts used at high temperatures, for example, at temperatures of 800 to 800℃.
When used as a stud bolt, which is a heat-insulating ceramic fiber fixture in a high-temperature furnace of about 1050° C., it has the advantage of being durable over a long period of time compared to conventionally used ceramic supports. Examples 1 to 5 and Comparative Example 1
Metallic silicon powder 1 crushed to an average particle size of 2μ or less by weight of ~4
00 parts, styrene resin 13 parts, stearic acid 3.0 parts,
Butylbenzyl phthalate was mixed in a proportion of 5.4 parts, and this was molded into 107WEφ×10077!17 by injection molding machine.
! A L stud bolt was molded.

Next, this was heated to 350°C at a temperature increase rate of 1 to 10'ClHr while adjusting to prevent cracks, blisters, etc., to volatilize the organic acid, and after removing more than 90% of it, it was heated to 1450°C in a nitrogen atmosphere. The temperature was raised to ℃ and sintered for 4 hours.

As a result of measuring the physical properties of the sintered body, the bending strength at room temperature was 220.
0k91Cfi1 had an apparent porosity of 13%.

Also, according to chemical analysis, the content of oxygen was 2% by weight. This was placed in an electric furnace and treated in the atmosphere under the heat treatment conditions shown in Table 1. The physical properties after the heat treatment are shown in Table 1. Next, the silicon nitride molded body after the above heat treatment was oxidized at a temperature of 5900° C. for 30 hours and 100 hours, and the physical properties after oxidation were measured. Table 2 shows the results.

In addition, the sintered bodies obtained by heat treatment at 1200°C for 4 hours in Comparative Example 1 and Example 5 were
, 80 to 900, 1,000, 1,100, and 1,200° C. for 100 (b) at each temperature of 1,100,130° C. are shown in Table 3a.

The stud bolts obtained in each example were used to attach the ceramic fiber molded body to the furnace wall.

As a result, after being used at 900°C for 6 months, it could be used without falling off. As is clear from the above results, the oxidation resistance of the product of the present invention at a temperature of 800 to 1050°C is improved by heat treatment at a heating temperature of 1100 to 1300°C as shown in Tables 1 to 3. Recognize.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between oxidation temperature and weight increase rate due to oxidation of conventional products, FIG. 2 is a graph showing the relationship between oxidation temperature and weight increase rate due to oxidation of heat-treated products according to the present invention, and FIG. The figure shows the heat-treated product according to the present invention and the conventional product at 900°C.
It is a graph showing the relationship between oxidation time and bending strength in .

Claims (1)

[Claims] 1. A method for producing a silicon nitride molded body with excellent oxidation resistance, which comprises heating a molded body obtained by reacting and sintering metallic silicon with nitrogen at a temperature of 1100 to 1300°C in an oxygen-containing gas atmosphere.