JPH09227235A - Silicon nitride sintered compact and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an Si3 N4 sintered compact usable at a high temp. of >=1,250 deg.C and having high high-temp. strength. SOLUTION: When a powdery mixture of Si3 N4 powder as a principal component with a 1st and a 2nd sintering aids is compacted and sintered to obtain an Si3 N4 sintered compact, the average particle diameter of the Si3 N4 powder is regulated to 0.1-1.0μm, powder of oxide of one or more kinds of the group IIIa elements is used as the 1st sintering aid and powder of oxide of one or more kinds of elements selected from among Zr, Hf, Nb, Ta and W is used as the 2nd sintering aid. The average particle diameter of the 1st sintering aid is regulated to 0.1-10 times that of the Si3 N4 powder. In the 2nd sintering aid, the total number of particles whose average particle diameter is 10-100 times that of the Si3 N4 powder is allowed to account for 5-50% and the 2nd sintering aid is added by <=10wt.%.

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 which is used in a high temperature environment and has excellent heat resistance, and a method for producing the same. Specifically, from among the silicon nitride powder (main component), the oxide of the Group 3a element that is the first sintering aid, and the second sintering aids Zr, Hf, Nb, Ta, and W A silicon nitride sintered body made of an oxide of a selected element, which is capable of achieving a higher temperature strength than ever before by setting the particle size and the addition amount of these elements in the optimum range. The present invention relates to a bound body and a manufacturing method thereof.

[0002]

2. Description of the Related Art Sintered silicon nitride is excellent in high temperature strength and toughness. Therefore, turbine rotor of gas turbine,
There have been many studies applied to structural members used in environments exposed to high temperatures and high pressures, such as nozzles, ducts, combustion chambers, and the like. For example, since the combustion efficiency of a gas turbine increases with an increase in the inlet temperature of the turbine, it has a higher high-temperature strength than a conventionally used heat-resistant alloy such as Inconel and has a high toughness among ceramic materials. Research on silicon sintered bodies aimed at improving the high-temperature strength has been actively conducted.

[0003] Silicon nitride is a substance having a high covalent bond,
Since the sinterability is not good as a simple substance, in order to improve the sinterability, it is generally performed by mixing a sintering aid such as aluminum oxide, yttrium oxide, magnesium oxide or cerium oxide. With a simple shape, it is possible to add a small amount of a sintering aid and accelerate the sintering by a method using hot pressing, but the sintering aid is at the grain boundaries of the silicon nitride sintered body, It exists in an amorphous state, which causes reduction in high temperature strength. Therefore, it has been attempted to crystallize the sintering aid existing in the amorphous state or to sinter with a small amount of the sintering aid to increase the high temperature strength.

On the other hand, studies have also been conducted to increase the high temperature strength of a silicon nitride sintered body by using a sintering aid having excellent high temperature strength. For example, Japanese Patent Application Laid-Open No. 61-201666 discloses a technique for improving high-temperature strength and toughness by dispersing hafnium oxide and zirconium oxide in a specific ratio in a silicon nitride matrix. In addition, JP-A-62-
Japanese Patent No. 153169 discloses improving the high temperature strength by mixing a specific amount of an oxide of a rare earth element and at least one selected from the group of Hf, Ta or Nb oxides, carbides or silicides in a specific ratio. Techniques for doing so are disclosed.

[0005]

However, sufficient sintering aids having high high-temperature strength are not always sufficient to obtain sufficient high-temperature strength. Therefore, Japanese Patent Publication No.
No. 7 discloses that by setting the average particle diameter and the maximum particle diameter of the silicon nitride powder and the sintering aid in a specific range, the strength of the conventional silicon nitride sintered body is 20 kg / mm ² or more, Techniques for improving are disclosed. However, the technique disclosed here is mainly aimed at improving room temperature strength, and is not sufficient for high temperature strength.

[0006]

In order to further improve the high-temperature strength of a conventional silicon nitride sintered body to which a sintering aid excellent in high-temperature strength is added, the inventors of the present invention have performed a sintering process excellent in high-temperature strength. The present invention has been completed as a result of further research, focusing not only on the addition of a binder component but also on the type of sintering aid mixed with silicon nitride powder and its average crystal grain size.

That is, the silicon nitride sintered body according to the present invention is sintered after forming a mixed powder in which a first sintering aid and a second sintering aid are added to silicon nitride powder as a main component. The average particle size of the silicon nitride powder as the main component is 0.1 to 1.0 μm, and the first sintering aid is at least one oxide powder of a 3a group element. The second sintering aid is Zr (zirconium), Hf
(Hafnium), Nb (niobium), Ta (tantalum), and W (tungsten) as an oxide powder of at least one element, and the average particle size of the first sintering aid is The average particle size of the silicon nitride powder is 0.1 to 10 times, and the second sintering aid has a particle size of 10 to 100 times the average particle size of the silicon nitride powder. The total number of certain particles was set to 5 to 50%, and the addition ratio was set to 0 to 10% by weight (excluding 0% by weight).

As the silicon nitride powder used in the present invention, either one manufactured by the imide method or direct nitriding method can be used, but one manufactured by the imide method is more preferable in order to obtain high strength at high temperature. The average particle size of the silicon nitride powder is 0.1 to 1.0 μm. When the average particle size is 0.1 μm or less, grain boundary sliding easily occurs when the sintered body receives a load at a high temperature, the high-temperature strength decreases, and when the average particle size is 1.0 μm or more, sintering occurs. This is because grain growth occurs and the high-temperature strength decreases. The shape of the silicon nitride powder is preferably substantially spherical.

Here, the average particle size specified in the present invention means
The cumulative frequency is 50% in the particle size cumulative distribution curve in which the horizontal axis is the particle diameter and the vertical axis is the cumulative particle number (hereinafter referred to as D50).
Is the diameter of the particle.

To increase the strength of the sintered body, it is necessary to optimally adjust the average particle size of the sintering aid according to the average particle size of the silicon nitride powder used. That is, the average particle size of the oxide powder of the Group 3a element as the first sintering aid is 0.1 to 10 times the average particle size of the silicon nitride powder.
This is because if it is 0.1 or less or 10 times or more, it is difficult to uniformly mix with the silicon nitride powder and the sinterability deteriorates. Therefore, it is preferably 0.5 to 5 times or less, and more preferably has an average particle diameter approximately the same as the average particle diameter of the silicon nitride powder.

The addition amount of the oxide powder of the 3a group element is 0.1 to 10% by weight. If it is less than 0.1% by weight, the sinterability will be deteriorated and it will be difficult to substantially sinter it. More preferably, it is 1 to 5% by weight. When two or more kinds of 3a elemental oxides are added, the average particle sizes do not necessarily have to be the same, and may be different as long as the average particle size is within this range. It suffices if the total amount falls within this range.

Examples of the Group 3a element constituting the first sintering aid include Y, lanthanoid rare earth elements, and actinide rare earth elements, preferably Y, Yb,
Lu is an element selected from Lu. Of course, two or more kinds may be added simultaneously.

Zr, Hf, which constitutes the second sintering aid,
It is known that oxides of Nb, Ta and W enhance the high temperature strength of the silicon nitride sintered body. The particle size is 10 to 100 times the average particle size of the silicon nitride powder.
It is preferably doubled. However, Zr, Hf, Nb, Ta, W
Rather than keeping all the oxide particles of
When the total number of particles of oxides of r, Hf, Nb, Ta, and W is 100, the content is 5 to 50%.

The reason for this is that if the grain size is 10 times or less, the effect of suppressing grain boundary slip is lost at high temperatures and 100
This is because if it is more than twice, it becomes a notch defect in the silicon nitride powder, stress concentration occurs, and it becomes a fracture starting point. Therefore, the range is more preferably 5 to 50 times. When the total number of particles is less than 5% of the total, most of the added oxides act as sintering aids and cannot exhibit high-temperature strength. This is because the bulk phase increases and acts as a defect, resulting in a decrease in strength.

When two or more kinds of these oxide powders are added, the particle size distribution does not necessarily have to be the same, and the total number of particles having a particle size of 10 to 100 times is within this range. I wish I had it. In order to increase the strength of the sintered body, it is necessary to appropriately adjust the particle size of the oxide powder of these elements within the range shown above according to the particle size of the silicon nitride powder used.

The amount of the oxide of the element selected from Zr, Hf, Nb, Ta and W is from 0 to 10% by weight.
(However, 0% by weight is not included) is suitable. These elements are effective even in very small amounts, but when added in large amounts, the average particle size is large, so that the sinterability deteriorates significantly. Therefore, the amount of addition must be 10% by weight or less. More preferably, it is 1 to 5% by weight. These oxide powders are
When adding more than one kind, it is sufficient that the total amount of addition is within this range.

On the other hand, the method for producing a silicon nitride sintered body of the present invention is a mixed powder comprising the above-mentioned silicon nitride powder as the main component, the first sintering aid and the second sintering aid. After molding, in an atmosphere containing 1 to 9 kgf / cm2 of nitrogen gas, 1
100-200 after primary sintering at 600-1800 ° C
Secondary sintering is performed in an atmosphere containing 0 kgf / cm ² of nitrogen gas at a temperature equal to or lower than the primary sintering temperature. The average particle diameters, addition ratios, etc. of the silicon nitride powder, the first sintering aid and the second sintering aid are the same as those described above.

The silicon nitride powder and the oxide powder which is the sintering aid must be mixed uniformly before molding. Examples of the apparatus used for mixing include a ball mill and a stirring mill, and any of wet pulverization and dry pulverization may be used. Examples of the solvent for performing wet pulverization include ethyl alcohol and methyl alcohol.

After the mixing of the silicon nitride powder and the sintering aid, processes such as drying, granulation, and sieving are performed according to the molding method. Examples of the molding method include press molding, slip cast molding, high-pressure injection molding (normal injection molding), low-pressure injection molding, cold isostatic pressing (CIP), hot pressing, and the like. However, the present invention is not necessarily limited to this molding method, and any of the molding methods is effective. In addition, in order to secure the degree of freedom of the shape of the molded body and the production efficiency and obtain a molded body with high density, after press molding, C
It is preferable to perform IP or low pressure injection molding.

The molded body was molded in an atmosphere containing nitrogen gas 16
Primary sintering is performed in the temperature range of 00 to 1800 ° C. When the sintering temperature is 1600 ° C or lower, the sintering reaction is difficult to proceed,
If the temperature exceeds 1800 ° C., the crystal grains of silicon nitride become coarser and the strength decreases. Further, the reason why sintering is performed in an atmosphere containing nitrogen gas is to prevent decomposition of silicon nitride, and the pressure is set to 1 to 9 kgf / cm ² . If it is 1 kgf / cm ² or less, the decomposition of silicon nitride occurs, and if it exceeds 9 kgf / cm ² , the pores are confined in the sintered body and it becomes difficult to densify the sintered body.

In the course of the primary sintering, the sintered body is densified, and in the subsequent secondary sintering, the sintered body is uniformly pressed from the surface by the pressurized atmosphere. And strength.

The primary sintering is performed until the relative density becomes about 90% or more. The time required for primary sintering depends on the size of the sintered body,
It varies depending on the wall thickness, but is about 1 to 24 hours. More preferably, it is 4 to 12 hours. If the sintering time is shorter than 1 hour, densification does not sufficiently proceed, and the effect of secondary sintering cannot be obtained. If it is longer than 24 hours, excessive crystal grain growth tends to occur.

The subsequent secondary sintering is performed at a temperature equal to or lower than that of the primary sintering in an atmosphere containing nitrogen gas,
This is performed in a temperature range of 1600 ° C. or more. When the temperature exceeds the primary sintering temperature, the crystal grains of silicon nitride become coarser, and the coarse grains cause a decrease in strength. If the temperature is not higher than 1600 ° C., the sintering reaction hardly proceeds. The ambient pressure containing nitrogen gas is
100-2000 kg / cm ² is suitable. Atmospheric pressure is 1
If it is less than 00 kg / cm ² , the sintering reaction does not easily proceed, and it is not sufficiently densified. When it exceeds 2000 kg / cm ² , the densification reaches saturation, and further densification reaction does not proceed. Conversely, if the atmospheric pressure is further increased, excessive coarse grains tend to grow.

[0024]

Embodiments of the present invention will be described below. It should be noted that these examples do not limit the present invention. (Example 1) Silicon nitride powder (A34 × 0 manufactured by Ube Industries)
5) The average particle size is 0.5 μm and 480 g, and the particle size corresponding to a cumulative frequency of 50% on the cumulative distribution curve is 0.94 μm.
Ytterbium oxide (YHC3 made by Japan Yttrium)
940620) 12.5 g (2.5% by weight) and hafnium oxide (Soegawa Rikagaku 33238A) 325mesh
(Average particle size 5 μm, maximum particle size 50 μm, FIG. 1 shows a particle size distribution curve measured by a laser diffraction / scattering particle size distribution analyzer. D50 * 10 to D50 * 100 is 48%.)
7.5 g (1.5% by weight) and ethanol 5 as a solvent
00 g was mixed for 60 hours in a ball mill containing balls made of silicon nitride. The mixed powder was dried using a rotary evaporator and then passed through a 325 mesh sieve,
After press molding with a molding pressure of 200 kg / cm ^{2 with} a mold, 40
CIP (cold isostatic pressing) was performed at a pressure of 00 kg / cm ^{2 to} obtain test pieces of 50 (length) × 32 (width) × 8 (thickness).
The test piece was first sintered at 1800 ° C. for 4 hours in an atmosphere containing 9 kgf / cm ² of nitrogen gas to obtain a sintered body having a relative density of 92%, and further at 1800 ° C. for 2 hours in an atmosphere containing nitrogen gas. At 200 kgf / cm ² to obtain a sintered body having a relative density of 95% or more. This sintered body at 1250 ° C and 1450 ° C,
A JIS 1601 three-point bending strength test was performed. Table 1 shows the test results.

(Example 2) Silicon nitride powder (manufactured by Ube Industries)
A34 × 05) Ytterbium oxide (YHC3940620 manufactured by Japan Yttrium) 25 g (2.5% by weight) having an average particle size of 0.5 μm and 960 g and a particle size of 0.94 μm corresponding to a cumulative frequency of 50% on the cumulative distribution curve.
And hafnium oxide (the same material used in Example 1))
15 g (1.5% by weight) and ethanol 10 as a solvent
00 g was mixed for 60 hours in a ball mill containing balls made of silicon nitride. After drying this mixed powder using a rotary evaporator, 866.88 g was weighed and added with paraffin (153.05 g), stearic acid (17.53 g) and a dispersant (9.83 g) at 90 ° C. in a kneader.
Kneaded for hours. Then, molding was carried out in a mold having a mold temperature of 41 ° C. under a molding pressure of 6 kg / cm ² to obtain a 50 × 31 × 10 compact. This molded body was degreased in air at 500 ° C., and then primary sintered at 1800 ° C. for 8 hours in an atmosphere containing 9 kgf / cm ² of nitrogen gas to obtain a sintered body having a relative density of 92%. 1
200 kg in an atmosphere containing nitrogen gas at 800 ° C for 2 hours
Sintering was performed at f / cm ² to obtain a sintered body having a relative density of 95% or more. This sintered body was measured at 1250 ° C and 1450 ° C according to JIS1.
A 601 three-point bending strength test was performed. Table 1 shows the test results.

(Example 3) A test piece was prepared by the same method as in Example 1 except that the amount of ytterbium oxide added was 1% by weight and the amount of hafnium oxide added was 1% by weight. The results obtained are shown in Table 1.

(Example 4) A test piece was prepared in the same manner as in Example 1 except that the amount of ytterbium oxide added was 1.5% by weight and the amount of hafnium oxide added was 1.5% by weight. Table 1 shows the result of the strength test conducted by the method.

Example 5 A test piece was prepared in the same manner as in Example 1 except that the amount of ytterbium oxide added was 1.5% by weight and the amount of hafnium oxide added was 2.5% by weight. Table 1 shows the result of the strength test conducted by the method.

(Example 6) A test piece was prepared in the same manner as in Example 1 except that the amount of ytterbium oxide added was 2.5% by weight and the amount of hafnium oxide added was 2.5% by weight. Table 1 shows the result of the strength test conducted by the method.

(Example 7) A test piece was prepared in the same manner as in Example 1 except that the amount of ytterbium oxide added was 3% by weight and the amount of hafnium oxide added was 5% by weight. The results obtained are shown in Table 1.

(Example 8) A test piece was prepared by the same method as in Example 1 except that the amount of ytterbium oxide added was 5% by weight and the amount of hafnium oxide added was 3% by weight, and the strength test was conducted by the same method. The results obtained are shown in Table 1.

Example 9 Yttrium oxide (manufactured by Japan Yttrium) having an average particle size of 0.95 μm was added in an amount of 1.5.
A test piece was prepared in exactly the same manner as in Example 2 except that the amount of hafnium oxide was changed to 2.5% by weight, and the strength test was conducted in the same manner.

Example 10 A test piece was prepared in the same manner as in Example 9 except that the amount of yttrium oxide added was 1% by weight and the amount of hafnium oxide added was 1% by weight. The results obtained are shown in Table 1.

Example 11 A test piece was prepared in the same manner as in Example 9 except that the amount of yttrium oxide added was 2.5% by weight and the amount of hafnium oxide added was 1.5% by weight. Table 1 shows the result of the strength test conducted by the method.

(Example 12) A test piece was prepared in the same manner as in Example 9 except that the amount of yttrium oxide added was 3% by weight and the amount of hafnium oxide added was 5% by weight. The results obtained are shown in Table 1.

(Example 13) Example 1 except that the amount of lutetium oxide (made by Japan Yttrium) having an average particle size of 1.0 µm was 2.5% by weight and the amount of hafnium oxide was 1.5% by weight. Table 1 shows the results of a test piece prepared by the same method as above and a strength test conducted by the same method.

Example 14 Ytterbium oxide (manufactured by Japan Yttrium) having an average particle size of 0.94 μm was added in an amount of 2.5% by weight, and hafnium oxide (a cumulative particle size distribution curve is shown in FIG. 2). D50 * 10 ~ D50 * 100 is 19
%) Was added in the same manner as in Example 1 except that the amount added was 1.5% by weight, and a strength test was conducted by the same method.

Example 15 A test piece was prepared in the same manner as in Example 14 except that the amount of hafnium oxide added was changed to 1.0% by weight, and a strength test was conducted by the same method. Show.

Example 16 A test piece was prepared in the same manner as in Example 14 except that the amount of hafnium oxide added was changed to 0.5% by weight, and a strength test was conducted by the same method. Show.

Comparative Example 1 Ytterbium oxide having an average particle size of 0.94 μm was added in an amount of 2.5% by weight, and hafnium oxide (a cumulative particle size distribution curve is shown in FIG. 3 is shown. D50 * 1
Table 1 shows the results of a test piece prepared in the same manner as in Example 1 except that the added amount of 0 to D50 * 100 was 4%) was 1.5 wt%. From Table 1,
The decrease in the three-point bending strength is remarkable. Comparing with Example 1, it is considered that the sintering aid having excellent high temperature strength such as hafnium oxide having a relatively large particle size contributes to the improvement of high temperature strength.

Comparative Example 2 A test piece was prepared in the same manner as in Example 1 except that ytterbium oxide was added in an amount of 4% by weight and hafnium oxide was not added, and a strength test was conducted in the same manner. Shown in 1. From Table 1, in particular, when the temperature at which the three-point bending test is performed is increased from 1250 ° C to 1450 ° C, the decrease in strength is remarkable. Zr, Hf, Nb, Ta, W
It can be seen that the high-temperature strength was increased by the addition of the oxide powder of an element selected from among the above.

[0042]

[Table 1]

[0043]

In the silicon nitride sintered body according to the present invention, the average particle size of the group 3a element, which is the first sintering aid, is 0.1 to 10 times the particle size of the silicon nitride powder. Therefore, the powder is uniformly mixed with the silicon nitride powder and, after sintering, exists on the outer periphery of the silicon nitride powder particles to form a film having a constant thickness, and the silicon nitride powders are firmly bonded to each other, and the strength of the sintered body is improved. Further, as a second sintering aid having high strength at high temperature, by adding oxide powder of at least one element selected from Zr, Hf, Nb, Ta and W, silicon nitride sintering can be performed. Since the high temperature strength of the body is increased and 5 to 50% of particles having a relatively large particle diameter, which is 10 to 100 times the average particle diameter of the silicon nitride powder, is contained, the grain boundary of the silicon nitride sintered body is Since a refractory oxide phase having substantially the same size as the silicon nitride crystal grains is generated, the grains in the sintered body are less likely to cause grain boundary slip at high temperatures, and the high temperature strength is further improved.

On the other hand, the method for producing a silicon nitride sintered body according to the present invention is performed in an atmosphere containing 1 to 9 kgf / cm ² of nitrogen gas.
Densification is achieved by performing primary sintering in a temperature range of 1600 to 1800 ° C., and in subsequent secondary sintering, the density of the sintered body is further improved. Especially 100 ~ 2000kg of secondary sintering
By carrying out at a temperature lower than the primary sintering in an atmosphere containing nitrogen gas of f / cm ² , it is possible to prevent coarsening of crystal grains,
The density of the sintered body is improved and the strength is increased. By performing the sintering step in an atmosphere containing nitrogen gas, decomposition of silicon nitride can be prevented and the sintered body can be densified.

[Brief description of drawings]

FIG. 1 is a graph showing a particle size distribution curve of the sintering aid used in Examples 1 to 13.

FIG. 2 is a graph showing a particle size distribution curve of the sintering aid used in Examples 14 to 16.

FIG. 3 is a graph showing a particle size distribution curve of the sintering aid used in Comparative Example 1.

Claims (4)

[Claims] 1. A silicon nitride powder as a main component, an oxide powder of at least one 3a group element as a first sintering aid, and Zr (zirconium) as a second sintering aid. Obtained by molding and then sintering a mixed powder of oxide powder of at least one element selected from among Hf (hafnium), Nb (niobium), Ta (tantalum) and W (tungsten). The silicon nitride sintered body according to claim 1, wherein the average particle diameter of the silicon nitride powder as the main component is 0.
1 to 1.0 μm, the average particle size of the first sintering aid is 0.1 to 10 times the average particle size of the silicon nitride powder,
Further, in the second sintering aid, the total number of particles having a particle size of 10 to 100 times the average particle size of the silicon nitride powder is set to 5 to 50%, and the addition ratio thereof is A silicon nitride sintered body characterized by being 0 to 10% by weight (however, 0% by weight is not included). 2. The silicon nitride sintered body according to claim 1, wherein the group 3a element as the first sintering aid is selected from Y (yttrium), Yb (ytterbium) and Lu (lutetium). A silicon nitride sintered body characterized by being one of the following: 3. 1 to 9 kgf / after molding a mixed powder consisting of the silicon nitride powder as the main component according to claim 1, a first sintering aid, and a second sintering aid. After primary sintering at 1600 to 1800 ° C. in an atmosphere containing nitrogen gas of cm ² , in an atmosphere containing nitrogen gas of 100 to 2000 kgf / cm ² , the primary sintering temperature is equal to or higher than the primary sintering temperature. 2 at low temperature
A method of manufacturing a silicon nitride sintered body, which comprises performing subsequent sintering. 4. The method for manufacturing a silicon nitride sintered body according to claim 3, wherein the group 3a element as the first sintering aid is Y (yttrium), Yb (ytterbium) or Lu.
1. A method for manufacturing a silicon nitride sintered body, which is selected from (lutetium).