JP3216973B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a high oxidation resistance,
The present invention relates to a silicon nitride sintered body having excellent bending strength at high temperatures and having a small amount of distortion due to creep, and a method for producing the same. The silicon nitride sintered body of the present invention can be used as a material or a material for structural parts of a heat engine such as a gas turbine.

[0002]

2. Description of the Related Art Silicon nitride sintered bodies are excellent in mechanical properties, heat resistance, corrosion resistance, etc., and have been applied to structural materials used for heat engines such as automobile engines and gas turbine engines. ing.

[0003] Silicon nitride is difficult to sinter because of its high covalent bonding property, and it is necessary to use sintering aids such as Al ₂ O ₃ , MgO and oxides of rare earth elements for sintering. is there. However, sintered bodies obtained using these sintering aids tend to have low flexural strength or creep resistance at high temperatures. Attempts have been made to suppress a decrease in the bending strength and the like by a method such as crystallization.

For example, Japanese Patent Application Laid-Open No. 6-183842 discloses a technique in which heat treatment is performed at a temperature higher than the firing temperature to make the crystal grains coarse and improve the creep resistance. Further, JP-A-6-87665 and JP-A-6-135706 disclose that the content of a specific element such as fluorine, chlorine, iron or the like in a silicon nitride powder obtained by nitriding a metal silicon powder is specified. A technique for improving the creep resistance by the following is disclosed. Furthermore, Japanese Patent Publication No. 61-315 describes a technique for obtaining a silicon nitride sintered body having particularly excellent high-temperature creep resistance by crystallizing grain boundaries.

[0005]

However, in the above-described method based on the coarsening of particles, although the creep resistance is improved, the size of defects is expected to be increased at the same time, and the strength may be lowered. Further, a method of reducing the amount of impurities such as fluorine and chlorine seems to be effective for improving the creep resistance, but it is a very expensive product and is not practical. Further, crystallization of grain boundaries is currently the best known and simple method, and although it is a very effective means for improving creep resistance, it has a problem in oxidation resistance and is not always an advantageous method. I can't say.

The present invention solves the above problems, and has high oxidation resistance, excellent bending strength at high temperatures,
Another object of the present invention is to provide a silicon nitride-based sintered body having a small amount of distortion due to creep, particularly at a high temperature, and a method for producing the same.

[0007]

Means for Solving the Problems A silicon nitride sintered body of the first invention is a silicon nitride sintered body containing at least a rare earth element as a sintering aid component. , The outermost layer (Cristopalite and Yttrium silicon)
Excluding those containing a kate) , a first thin layer, a second thin layer, and a main part, and the sintering aid component amount of the main part is 10
When 0, the amount of the sintering aid component in the outermost layer is 150 or more by weight, the amount of the sintering aid component in the first thin layer is 5 to 50 by weight, and The sintering aid component amount in the thin layer is 90 to 110 in weight ratio, the grain boundary phase in the second thin layer is crystalline, and the grain boundary phase in the main portion is crystalline. It is characterized in that quality and amorphous are mixed.

A silicon nitride sintered body according to a second aspect of the present invention is a silicon nitride sintered body containing at least a rare earth element as a sintering aid component. It is composed of a thin layer, a second thin layer and a main part, and when the amount of the sintering aid component in the main part is 100, the amount of the sintering aid component in the first thin layer is 5% by weight. -50, and the amount of the sintering aid component in the second thin layer is 90-110 by weight, and the grain boundary phase in the second thin layer is crystalline, and Is characterized by a mixture of crystalline and amorphous.

Further, in the silicon nitride sintered body of the third invention, the thickness of the first thin layer is 5 to 250 μm, and the thickness of the second thin layer is 10 to 350 μm. It is characterized by.

As silicon nitride, the amount of oxygen as an impurity is preferably about 1 to 3% by weight, and the amount of other impurities is preferably extremely small. Both α-type and β-type are used without any particular limitation. Can be. The "sintering aid component" contains at least a "rare earth element", and as the rare earth element, Y, Sc, La, Ce, Pr, N
Elements such as d, Gd, Tb, Dy, Er, and Yb are included. The silicon nitride-based sintered body of the present invention, after weighing the required amount of each raw material powder of silicon nitride and a sintering aid, blending, adding an appropriate binder or the like to the mixed powder, and then molding, Firing, followed by heat treatment, may be obtained with or without removing the outermost layer.

As the sintering aids constituting the sintering aid components, in addition to those containing rare earth elements, Mg, Al, V, Mo, W and the like commonly used as sintering aids are used. What contains each element can also be used. Most of these sintering aids are composed of oxides, and when they are not oxides, most of them are converted to oxides by firing and become sintering aid components. Specifically, for example, Er ₂ O ₃ , Yb
₂ O ₃ , Al ₂ O _3, WO _3, etc., or a double oxide or a silicate compound thereof, and usually, a crystalline or amorphous Form a grain boundary phase. The raw material powder of the sintering aid may be an oxide or a powder that can be converted to an oxide during the firing process, for example, a powder of a salt such as a carbonate or an acetate, or a powder of a hydroxide.

The above-mentioned molding can be carried out by a known method, and can be formed into a desired shape by a method such as press molding, cast molding, extrusion molding and injection molding. The firing of the molded article is, for example, 1600 to 1600.
It is performed in a temperature range of 2300 ° C. in an atmosphere of nitrogen gas, a mixed gas of nitrogen gas and hydrogen gas or an inert gas, or the like. As the firing method, normal pressure firing method, gas pressure firing method,
Hot isostatic pressing, hot pressing, etc.
Further, a reaction sintered body obtained by firing a silicon powder and a rare earth element compound, or a system in which silicon nitride powder is added thereto in a nitrogen atmosphere, can be fired again by the above firing method.

When the amount of the sintering aid component in the “outermost layer” is less than 150 by weight, the amount of the sintering aid component in the thickness direction increases as the weight ratio in the first thin layer becomes 50 or less. A sintered body unevenly distributed cannot be obtained. The thickness of the outermost layer is not particularly limited. When the heat treatment is performed so that the thickness of the first thin layer falls within a predetermined range, the thickness is usually 5 to 5 mm.
The thickness is about 20 μm.

Further, in the "first thin layer", when the grain boundary phase is amorphous, the ratio of the sintering aid components in the first thin layer is low. In combination, the oxidation resistance is improved, and accordingly, the creep resistance is further improved, which is preferable because the generation of strain due to creep is suppressed.

If the weight ratio of the amount of the sintering aid component in the first thin layer is less than 5, the bonding force between the silicon nitride particles is reduced. If the weight ratio exceeds 50, the resulting silicon nitride The oxidation resistance of the sintered body decreases, and the amount of strain due to creep increases. If the thickness of the first thin layer is less than 5 μm, both the oxidation resistance and the creep resistance decrease, and
If it exceeds μm, the bending strength is inferior.
This thickness is preferably 45 to 110 μm, and within this range, a silicon nitride sintered body having more excellent bending strength and the like can be obtained regardless of the type of the sintering aid.

Further, in the "second thin layer", the grain boundary phase is a D phase represented by R ₂ Si ₂ O ₇ and R ₁₀ Al ₂ S
It is composed of “crystalline” such as an A phase represented by i ₃ O ₁₈ N ₄ , a J phase represented by R ₄ Si ₂ N ₂ O ₇ , and an H phase represented by R ₂₀ Si ₁₂ O ₄₈ . . Therefore, the creep resistance is further improved.

The weight ratio of the amount of the sintering aid component in the second thin layer is in the range of 90 to 110, but is usually about the same as the average value of the main part, and the weight ratio is about 100.
The major difference between the second thin layer and the main part lies in the grain boundary phase. In the main part, the amorphous part is clearly present together with the crystalline part. Thus, the second thin layer has a great feature in this point. The thickness of the second thin layer is 1
If the thickness is less than 0 μm, the creep resistance decreases, which is not preferable.

Further, the silicon nitride sintered body of the present invention does not have the outermost layer having a high oxide ratio as in the second invention,
It is preferable to use a sintered body composed of the first and second thin layers and the “main part”. This is because the outermost layer is a layer substantially composed of an oxide, so that the variation in bending strength at high temperatures may be large, and the stability of high-temperature characteristics can be increased by removing the outermost layer.

Such a silicon nitride sintered body is obtained by molding and firing a composition of silicon nitride powder and a sintering aid powder containing at least a rare earth element, as in the sixth invention, and then subjecting the composition to an oxidizing atmosphere. , At a temperature of 1100 to 1600 ° C for 30 hours or more to produce a silicon nitride-based sintered body having the outermost layer according to claim 1, and then removing the outermost layer.

When the atmosphere for the heat treatment is a non-oxidizing atmosphere such as a nitrogen gas atmosphere or an inert gas atmosphere, the effect of diffusion of the sintering aid to the surface of the sintered body is poor, and the sintering aid is composed of the sintering aid. A layer having few grain boundary phases, that is, a first thin layer is not sufficiently formed. If the heat treatment temperature is lower than 1100 ° C., the heat treatment effect is insufficient, the outermost layer and the first and second thin layers specified in the present invention are not formed, the oxidation resistance is reduced, and the silicon nitride obtained is reduced. In some cases, the amount of distortion due to creep of the high-quality sintered body increases. When this temperature exceeds 1600 ° C., even if the processing time is shortened, thermal deterioration of the material cannot be suppressed, and the bending strength is reduced. Also book
As shown in the fifth invention and the seventh invention, the heat treatment temperature is
It can be 1300 to 1600 ° C.

Further, the heat treatment time is generally longer within the above range if the processing temperature is lower, and shorter if the processing temperature is higher. However, such adjustment is not necessarily required, and the silicon nitride-based sintered body of the present invention can be obtained even if the treatment is performed at a relatively high treatment temperature for a long time.
If the treatment time is less than 30 hours, the desired sintered body cannot be obtained even if the treatment is performed at a relatively high temperature.

In the silicon nitride sintered body of the second invention, the sintering aid component has a low quantitative ratio in the surface layer portion thereof and is preferably amorphous and has a thickness of about several tens to several hundreds μm. In the first thin layer and the inside thereof, the quantitative ratio of the sintering aid component is substantially the same as the average value of the main portion, is crystalline, and has the same thickness as the above. And two thin layers.
With such a configuration, oxidation resistance is improved,
A silicon nitride sintered body having excellent creep resistance at high temperatures and the like and small distortion due to creep can be obtained.

In the silicon nitride sintered body of the second invention having the above surface structure, the sintering aid is diffused to the surface of the sintered body by subjecting the fired material to a heat treatment in an oxidizing atmosphere such as under air. It can be manufactured by doing. When such a heat treatment is performed, the amount of the sintering aid component, that is, the amount of oxidation, is reduced on the first thin layer having a low quantitative ratio of the sintering aid component constituted by the sintering aid. The outermost layer having many substances is further formed, and the silicon nitride sintered body of the first invention is obtained. In the silicon nitride sintered body of the present invention, the main purpose is to suppress distortion by improving creep resistance at high temperatures, and in this respect, the silicon nitride based sintered body of the first invention in which the outermost layer is left as it is. However, considering the variation in strength, the silicon nitride sintered body of the second invention from which the outermost layer is removed is more preferable.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The silicon nitride sintered body of the present invention uses a commonly used silicon nitride powder, and as a sintering aid,
A specific amount of Er ₂ O ₃ powder, V ₂ O ₅ powder and WO ₃ powder, or a combination of Yb ₂ O ₃ powder, Al ₂ O ₃ powder and AlN powder, etc., are mixed, wet-mixed and pulverized, and then dried. It is obtained by forming and firing by a conventional method, and then performing heat treatment in an air atmosphere at a specific temperature range for 30 hours or more, particularly 50 hours or more. This silicon nitride-based sintered body can be used as it is, but if the outermost layer having a high amount ratio of the oxide as a sintering aid component is removed according to a conventional method, silicon nitride having a small variation in strength at high temperatures is obtained. Quality sintered body.

[0025]

The present invention will be described below in more detail with reference to examples. EXPERIMENTAL EXAMPLES 1-9 89% by weight of silicon nitride powder having an average particle diameter of 0.6 μm and an α ratio of 97%, 8% by weight of Er ₂ O ₃ powder having an average particle diameter of 1 to ₃ μm, and V ₂ O having an average particle diameter of about 1 μm ₅ 1% by weight of powder and 2% by weight of WO ₃ powder having the same particle size were mixed and wet-pulverized using a silicon nitride ball mill. Thereafter, the pulverized material is dried, and this powder is formed into a shape of 55 × 55 × 25 mm.
isostatic press molding at a pressure of ton / cm ² , and sintering the obtained molded body at 1800 ° C. for 2 hours by a hot isostatic pressing method, and further measuring bending strength based on JIS R1604. The test piece was processed.

Thereafter, except for Experimental Example 6, the test pieces were heat-treated in a predetermined atmosphere under the conditions shown in Table 1. Characteristics of silicon nitride sintered body after heat treatment and second
Table 1 also shows the properties of the grain boundary phase in the thin layer. In Table 1, ** indicates that the numerical values and other limitations of the first and fifth inventions are out of the range. In addition, * indicates that the numerical value limit of the third invention is out of the range. In addition, the measuring method of each characteristic of Table 1 is as follows.

(1) Confirmation of the constituent layers of the outermost layer, the first and second thin layers, and the main portion; The grinding was performed by X-ray diffraction while grinding from the surface to the inside. (2) the weight ratio of the amounts of the sintering aid components in the first and second thin layers;
The quantitative analysis of the elements in the cross section was performed based on the peak intensity ratio of the EPMA analysis (electron probe microanalyzer).

(3) Increase in oxidation: After heating the test piece at 1300 ° C. for 100 hours in accordance with JIS R1609, the weight increase was measured, and the weight increase was determined by dividing the weight by the surface area of the test piece. (4) Flexural strength: A 4-point flexural strength based on JIS R1604 was measured at 1300 ° C. (5) Amount of strain: 1400 ° C. in accordance with JIS R1612
A creep test of four-point bending was performed under the conditions of a load stress of 200 MPa and 100 hours, and the creep test was performed based on the displacement at the load point. Strain amount = 6tδ ₂ / [(L + 2l) × (L−1)] (t: thickness of test piece, δ ₂ : displacement of load point, L: outer span, l: inner span)

Experimental Examples 10 to 18 In Experimental Example 1, the sintering aid was 8% by weight of Yb ₂ O ₃ ,
A silicon nitride sintered body was obtained in the same manner as in Experimental Example 1 except that 1% by weight of l ₂ O _{3 and 2} % by weight of AlN were used (however, no heat treatment was performed in Experimental Example 15), and a bending test was performed in the same manner. Into test specimens. Since the sintering aid system used here had slightly lower heat resistance than that of Experimental Example 1, the temperature was generally lowered and heat treatment was performed. In Table 2, the meanings of * and ** are the same as in Table 1. In addition, in Tables 1 and 2, the column of the amount ratio is the weight ratio of the amount of the sintering aid component in the first and second thin layers when the amount of the sintering aid component in the main part is 100. .

[0030]

[Table 1]

[0031]

[Table 2]

According to the results shown in Table 1, in the silicon nitride sintered bodies of Experimental Examples 1 to 5, the weight ratios of the amounts of the sintering aid components in the first and second thin layers were respectively different from those of the first invention. The thickness of each thin layer is within the range of the third invention. Therefore, the oxidation increase was 0.1 mg in each example.
/ Cm ² , which is a very small amount, and the amount of distortion is 0.1 in all cases.
It is less than 001. The bending strength is also 570-620M.
It is in the range of Pa, which indicates that the silicon nitride-based sintered body is excellent in oxidation resistance and creep resistance at high temperatures.
The grain boundary phase of the second thin layer is a crystalline D phase in all of Experimental Examples 1 to 5.

On the other hand, in Experimental Example 6, the first and second thin layers were not formed because the heat treatment was not performed, and the flexural strength was large, but both the oxidation increase and the strain amount were in Experimental Examples 1 to 5.
It is larger than. Further, in Experimental Example 7 in which the heat treatment temperature exceeded the upper limit, although the oxidation resistance was good, the strength was largely reduced, and the test piece was broken during the creep test, so that the strain amount could not be measured. . Experimental example 8 in which heat treatment was performed in a nitrogen gas atmosphere instead of an oxidizing atmosphere.
In this case, the first thin layer was not formed, and the amount of increase in oxidation and the amount of distortion were inferior. Further, in Experimental Example 9 in which the heat treatment temperature was lower than the lower limit, no heat treatment effect was exhibited, and neither the first nor second thin layers were formed and heat treatment was not performed as in the case where no heat treatment was performed. Similar results are obtained, and the grain boundary phase of the second thin layer is also amorphous.

According to the results shown in Table 2, the results of Experimental Examples 10 to 10 are shown.
In the silicon nitride sintered body of No. 14, both the weight ratio of the sintering aid component amount and the thickness of each thin layer were appropriate, and the characteristics were as shown in Experimental Example 1.
The results are exactly the same as those of Examples 1 to 5, and the bending strength is generally higher than that of Examples 1 to 5.

On the other hand, in Experimental Example 15 in which the heat treatment was not performed, the strength was sufficient, but the amount of increase in oxidation and the amount of distortion were further inferior to those in Experimental Example 6. In Experimental Example 16, the first thin layer having a very large thickness of 1000 μm was formed, the strength was reduced, and the test piece was broken on the way in the creep test, and the amount of strain could not be measured. Further, in Experimental Example 17 in which a heat treatment was performed in a nitrogen atmosphere, the result was similar to that of Experimental Example 8, but the oxidation resistance was further reduced. Further, in Experimental Example 18 in which the heat treatment temperature is as low as 1000 ° C., the oxidation resistance is improved as compared with Experimental Example 9 in which the treatment temperature is similarly low, but the strain amount is further increased.

EXPERIMENTAL EXAMPLES 19-24 Three types of silicon nitride sintered bodies were produced in the same manner as in Experimental Example 1 except that the raw materials having the same compositions as those in Experimental Examples 10-18 were used and the temperature and time of the heat treatment were changed. The samples were manufactured, and the variation in bending strength was evaluated when the outermost layer was left as it was and when it was removed (the number of test pieces under each condition was 15). The variation in strength is represented by a Weibull coefficient. This coefficient is a numerical value calculated in Weibull statistics that statistically processes the fracture behavior of ceramics. For ceramics in general, if this coefficient exceeds 20, the variation is small, and if it is significantly less than 20, the variation is large. I have.

[0037]

[Table 3]

According to the results shown in Table 3, there is almost no difference in the average value of the bending strength depending on the presence or absence of the outermost layer. However, there is a clear difference in the Weibull coefficients, which are 25, 32 and 26 when the outermost layer is removed, and 14, 16 and 11 when the outermost layer is left as it is.
In this way, when the outermost layer is removed irrespective of the heat treatment conditions, it is apparent that the variation in strength is clearly small.

[0039]

In the silicon nitride sintered body of the first invention, since the outermost layer having a high oxide content ratio is left as it is, the variation in strength at high temperatures is slightly large, but it has sufficient performance for practical use. The obtained silicon nitride-based sintered body is obtained. Also, the second
In the silicon nitride-based sintered body of the present invention, the amount of the sintering aid component in the first thin layer is small, and the grain boundary phase in the second thin layer is crystalline. It is possible to obtain a silicon nitride sintered body having high bending strength at a high temperature and small distortion due to creep. Further, the first and second thin layers are
By setting the thickness specified in the third invention, a sintered body having more excellent high-temperature characteristics can be obtained.

As described above, the silicon nitride-based sintered body of the present invention has excellent high-temperature characteristics and a very small amount of strain due to creep at high temperatures. Furthermore, according to the method for manufacturing a silicon nitride-based sintered body of the present invention, the sintered body obtained by molding and firing is heat-treated under an oxidizing atmosphere under specific processing conditions,
By removing the outermost layer, a silicon nitride sintered body having excellent characteristics of the present invention can be easily manufactured.

Continuation of front page (56) References JP-A-59-116175 (JP, A) JP-A-61-191582 (JP, A) JP-A-6-100387 (JP, A) JP-A-6-227866 (JP) , A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/584-35/596 C04B 35/64

Claims (7)

(57) [Claims] 1. A silicon nitride sintered body containing at least a rare earth element as a sintering aid component, wherein the sintered body is composed of the outermost layers (cristoparite and yttrium lithium) in order from the surface.
Excluding those containing silicates) , a first thin layer, a second thin layer, and a main part. When the sintering aid component amount of the main part is 100, the sintering aid in the outermost layer is The amount of the sintering aid component in the first thin layer is 5 to 50 by weight, and the amount of the sintering aid component in the second thin layer is 150 by weight or more. 90 to 110, wherein the grain boundary phase in the second thin layer is crystalline, and the grain boundary phase in the main part is a mixture of crystalline and amorphous. Silicon nitride-based sintered body. 2. In a silicon nitride sintered body containing at least a rare earth element as a sintering aid component, the sintered body is formed from a first thin layer, a second thin layer, and a main portion in order from the surface. When the amount of the sintering aid component in the main part is 100, the amount of the sintering aid component in the first thin layer is 5 to 5 in weight ratio.
50, and the amount of the sintering aid component in the second thin layer is 90 to 110 in weight ratio, the grain boundary phase in the second thin layer is crystalline, and A silicon nitride sintered body characterized in that the grain boundary phase is a mixture of crystalline and amorphous. 3. The thickness of the first thin layer is 5 to 250 μm.
3. The silicon nitride sintered body according to claim 1, wherein the thickness of the second thin layer is 10 to 350 μm. 4. 4. According to JIS R1612, in air, 1
When a bending stress of 200 MPa is applied at 400 ° C.,
The silicon nitride-based sintered body according to any one of claims 1 to 3, wherein a strain amount after a lapse of 100 hours is less than 0.001. 5. A silicon nitride powder and at least a rare earth element
A composition with a sintering aid powder containing, was molded and fired
Then, in an oxidizing atmosphere at 1300-1600 ° C. for 30 hours or less
2. The silicon nitride according to claim 1, wherein said silicon nitride is obtained by heat treatment.
Substrate sintered body. 6. A silicon nitride powder and at least a rare earth element
A composition with a sintering aid powder containing, was molded and fired
After that, in an oxidizing atmosphere at 1100 to 1600 ° C. for 30 hours or less
The outermost layer according to claim 1 is provided by heat treatment.
A silicon nitride sintered body is manufactured, and then the outermost layer is removed.
A method for producing a silicon nitride-based sintered body, characterized in that : 7. The temperature of the heat treatment is from 1300 to 1600.
The method for producing a silicon nitride-based sintered body according to claim 6, wherein the temperature is ° C.