JPH1029867A - Silicon nitride sintered compact - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress grain boundary sip at high temp. and to improve high- temp. strength, creep characteristics, etc., by mixing and firing powdery silicon nitride, powdery oxide of a group IIIa element and powdery oxide of a specified element. SOLUTION: A powdery mixture of nearly spherical powdery silicon nitride (A) of 0.1-1.0μm average particle diameter with 0.1-10wt.% powdery oxide (B) of a group IIIa element whose average particle diameter is 0.1-10 times thatof the component A and >0 to 10wt.% powdery oxide (C) of Zr, Hf, Nb, Ta or W whose average particle diameter is 10-100 times that of the component A is primarily sintered at 1,600-1,800 deg.C in an atmosphere contg. gaseous nitrogen under 1-20kgf/cm<2> pressure and it is further sintered ion an atmosphere contg. gaseous nitrogen under 1,000-2,000kgf/cm<2> pressure to obtain the objective silicon nitride sintered compact consisting of a 1st phase of columnar β-silicon nitride, a 2nd phase of the component C and/or multiple oxide consisting of the components C, B and a 3rd phase of the component B. The 2nd phase exists among the grains of the 1st phase.

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 having excellent heat resistance for use in a high-temperature environment. For more information,
The present invention relates to a silicon nitride sintered body containing a second phase and a third phase of a sintering aid in a structure in addition to a first phase of silicon nitride.

[0002]

2. Description of the Related Art A silicon nitride sintered body has excellent 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 sinterability is not good by itself, sintering is generally performed by mixing a sintering aid such as aluminum oxide, yttrium oxide, magnesium oxide, and cerium oxide in order to improve sinterability. However, the high temperature properties are reduced due to the added sintering aid. Therefore, attempts have been made to add an oxide having a high eutectic temperature with SiO ₂ existing on the surface of silicon nitride as a sintering aid in order to improve high-temperature characteristics. However, there is a problem that the sinterability is reduced by an increase in the eutectic temperature. Further, if the shape is simple, it is possible to add a small amount of a sintering aid and promote sintering by a molding method using a hot press, but it is not suitable for a complicated part shape.

[0004] Therefore, studies have been made 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
No. 153169 discloses a high-temperature strength by mixing a specific amount of a rare earth element oxide and at least one selected from the group consisting of Hf, Ta or Nb oxide, carbide and silicide in a specific ratio. Techniques for improving are disclosed.

FIG. 3 is a structural diagram of a conventional silicon nitride sintered body. When a shear stress acts on silicon nitride particles in the state of FIG. Since only the binder is present, as shown in FIG.
It causes grain boundary slip. Therefore, the silicon nitride sintered body is excellent in heat resistance but inferior in toughness as compared with metal.

[0006]

However, sufficient sintering aids having high high-temperature strength are not always sufficient to obtain sufficient high-temperature strength. That is, an object of the present invention is to suppress grain boundary slip at high temperatures and improve high-temperature characteristics such as high-temperature strength and creep characteristics.

[0007]

Means for Solving the Problems In order to further improve the high temperature strength of a conventional silicon nitride sintered body to which a sintering agent excellent in high temperature strength has been added, the present inventors have proposed a firing method having excellent high temperature strength. As a result of research focusing not only on the addition of a binder component but also on the gaps between silicon nitride particles, the present invention has been completed.

That is, the silicon nitride sintered body according to the present invention
, A second phase and a third phase. The first phase is made of columnar β-type silicon nitride, and the second phase is Zr, Hf,
Oxides of at least one element selected from Nb, Ta and W, or composite oxides of these oxides and oxides of at least one element selected from group 3a elements The third phase is mainly composed of an oxide of at least one element selected from the group 3a elements, and the second phase is formed between the β-type silicon nitride particles forming the first phase. Existing structure. In addition, as said 3a group element,
It is preferable to use any one selected from Y (yttrium), Yb (ytterbium), and Lu (lutetium).

The oxide constituting the second phase is Zr, H
Since f, Nb, Ta, and W are rich, the melting point is high, and the oxide (composite oxide) constituting the second phase enters into the gap between the silicon nitride particles constituting the first phase. Since the compound (composite oxide) and the silicon nitride particles have different coefficients of thermal expansion, a compressive stress acts on the silicon nitride particles at a high temperature, so that even when a shear stress is applied, grain boundary slip hardly occurs.

As the silicon nitride powder used in the present invention, any of those produced by the imide method or the direct nitridation method can be used, but in order to obtain high strength at high temperatures, those produced by the imide method are more preferred. 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.

In order to enhance 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 diameter of the oxide powder of the Group 3a element as a sintering aid is 0.1 to 10 times the average particle diameter of the silicon nitride powder. 0.1
If the ratio is not more than 10 times or more, it is difficult to mix uniformly with the silicon nitride powder, and the sinterability deteriorates. Therefore,
Preferably, the average particle diameter is 0.5 to 5 times or less, and more preferably, the average particle diameter is substantially the same as the average particle diameter of the silicon nitride powder.

The addition amount of the oxide powder of the Group 3a element is 0.1
To 10% by weight. If it is less than 0.1% by weight, sinterability deteriorates and substantial sintering becomes difficult. If it exceeds 10% by weight, it causes a decrease in strength at high temperatures. More preferably, 1 to 5
% By weight.

The group 3a element constituting the first sintering aid includes Y, lanthanoid-based rare earth elements, and actinoid-based rare earth elements. Preferably, Y, Yb,
Lu is an element selected from Lu. Of course, two or more kinds may be added simultaneously.

Further, Zr, Hf, Nb,
The oxides of Ta and W are known to have an effect of increasing the high-temperature strength of the silicon nitride sintered body. The particle size is 10 times to 1 times the average particle size of the silicon nitride powder as the raw material.
It is preferably set to 00 times. Where Zr, Hf, Nb, T
The total number of particles of the oxides of Zr, Hf, Nb, Ta, and W is not limited to the above range.
When it is set to 0, it is set to include 5 to 50%.

The reason is that if the particle size is 10 times or less, the effect of suppressing grain boundary sliding at high temperatures is lost, and
If it is more than twice, it will become a notch defect in the silicon nitride powder, causing stress concentration and becoming 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.

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, an example of a method for producing a silicon nitride sintered body according to the present invention is as follows: a silicon nitride powder as a main component, an oxide powder of a Group 3a element, Zr, Hf, Nb, T
a, after forming a mixed powder composed of an oxide powder of an element selected from W, first sintering at 1600 to 1800 ° C. in an atmosphere containing 1 to 20 kgf / cm 2 of nitrogen gas; In an atmosphere containing 100 to 2000 kgf / cm2 of nitrogen gas, the temperature is set to a value equal to or lower than the primary sintering temperature.
Next, sinter.

The silicon nitride powder and the oxide powder as a sintering aid need to be uniformly mixed first before molding. Examples of an 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 order to secure the degree of freedom of the shape of the molded article and the production efficiency and to obtain a molded article having a high density, it is preferable to perform CIP after slip casting, low pressure injection molding or press molding.

As described above, the primary sintering of the compact in an atmosphere containing nitrogen gas in a temperature range of 1600 to 1800 ° C. is performed when the sintering temperature is 1600 ° C. or lower. If the temperature exceeds 1800 ° C., the crystal grains of silicon nitride become coarse and the strength decreases. The sintering is performed in an atmosphere containing nitrogen gas to prevent decomposition of silicon nitride, and the pressure is set to 1 to 20 kgf / cm ² . If it is 1 kgf / cm ² or less, decomposition of silicon nitride occurs,
If it exceeds 20 kgf / cm ² , the pores are sealed in the sintered body, and the densification of the sintered body does not easily progress.

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 the time is shorter 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. If it exceeds 2,000 kg / cm ² , the densification reaches saturation, and no further densification reaction proceeds. Conversely, if the atmospheric pressure is further increased, excessive coarse grains tend to grow.

[0024]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that these examples do not limit the present invention. (Example 1) Silicon nitride powder (A34 × 0 manufactured by Ube Industries)
5) 480 g of an average particle diameter of 0.5 μm and ytterbium oxide (YHC3940620 manufactured by Yttrium Japan)
2.5 g (2.5% by weight), 7.5 g (1.5% by weight) of hafnium oxide (Soekawa Chemical Co. 33238A)
500 g of ethanol as a solvent was mixed for 60 hours in a ball mill containing silicon nitride balls. After drying this mixed powder using a rotary evaporator, 32
After passing through a 5-mesh sieve and press-molding with a mold at a molding pressure of 200 kg / cm ² , CIP (cold isostatic pressing) is performed at a pressure of 4000 kg / cm ² and 50 (vertical) × 32 (horizontal) × 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 was subjected to a JIS1601 three-point bending strength test at 1250 ° C. and 1450 ° C. Table 1 shows the test results. Further, the structure of this sintered body is shown by a micrograph (5000 times) of FIG. 1, a schematic diagram of FIG. 2A, and an enlarged view of a portion surrounded by a line segment b of FIG. 2A. Shown in (b).

As apparent from these figures, the silicon nitride sintered body according to the present invention comprises a first phase (matrix) composed of columnar β-type silicon nitride particles. A composite oxide of Hf oxide and Yb oxide constituting the second phase is interposed in the gap between the first phase and the composite oxide containing Yb oxide constituting the third phase as a main component. And the second phase.

Then, as shown in FIG. 2B, the interface between the first phase and the third phase is clear, but the interface between the third phase and the second phase is clearly defined. not present, but Y ² O ³ and HfO ³ system at the interface between the first phase and the third phase is rich Hf or the like becomes rich toward the second phase.

Example 2 The addition amount of yttrium oxide having an average particle size of 0.95 μm (produced by Nippon Yttrium) was 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. In the structure of the obtained sintered body, the composite oxide of the Hf oxide and the Y oxide constituting the second phase was interposed between the gaps between the β-type silicon nitride particles constituting the first phase. .

Example 3 Example 1 was repeated except that the amount of lutetium oxide having an average particle size of 1.0 μm (produced by Nippon Yttrium) was 2.5% by weight and the amount of hafnium oxide was 1.5% by weight. Table 1 shows the results of preparing a test piece in exactly the same manner as described above and performing a strength test in the same manner. In the structure of the obtained sintered body, a composite oxide of Hf oxide and Lu oxide constituting the second phase was interposed in the gaps between the β-type silicon nitride particles forming the first phase. .

(Comparative Example 1) A test piece was prepared in the same manner as in Example 1 except that the amount of ytterbium oxide was 4% by weight and hafnium oxide was not added. It is shown in FIG. From Table 1, it can be seen that, by increasing the temperature for performing the three-point bending test from 1250 ° C. to 1450 ° C., the strength is significantly reduced, and 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. In the structure of the obtained sintered body, as shown in FIGS. 2A and 2B, an amorphous phase was present in the gap between the β-type silicon nitride particles forming the first phase.

[0030]

[Table 1]

[0031]

As described above, the silicon nitride sintered body according to the present invention has a structure in which the gap between the columnar β-type silicon nitride particles forming the first phase is filled with the oxide forming the second phase. The oxide (composite oxide) particles and the silicon nitride particles have different coefficients of thermal expansion because the particles of the material (composite oxide) are allowed to enter, so that the compression effect acts on the silicon nitride particles at a high temperature, Even if shear stress acts, grain boundary slip hardly occurs.

[Brief description of the drawings]

FIG. 1 is a micrograph (5000 ×) showing the structure of one example of a silicon nitride sintered body according to the present invention.

FIG. 2A is a schematic diagram of the structure of the silicon nitride sintered body,
(B) is an enlarged view of a portion surrounded by a line segment (b) in (a).

FIG. 3A is a diagram showing a state before grain boundary sliding occurs in a structure diagram of a conventional silicon nitride sintered body, and FIG. 3B is a view showing a state after grain boundary sliding has occurred.

Claims (2)

[Claims] 1. A first phase composed of columnar β-type silicon nitride, Zr (zirconium), Hf (hafnium), and Nb
(Niobium), Ta (tantalum) and W (tungsten)
A second phase comprising an oxide of at least one element selected from the group consisting of: or a composite oxide of these oxides and an oxide of at least one element selected from the group 3a elements; Β-type silicon nitride particles comprising a third phase containing, as a main component, an oxide of at least one element selected from Group 3a elements, wherein the second phase forms a first phase. A silicon nitride sintered body characterized in that it is present between the two. 2. The silicon nitride sintered body according to claim 1, wherein the group 3a element is any one selected from Y (yttrium), Yb (ytterbium), and Lu (lutetium). Silicon nitride sintered body.