JPH05148030A - Silicon nitride-silicon carbide composite sintered material - Google Patents

Abstract

PURPOSE:To obtain the subject sintered material having uniform room temperature strength while suppressing the formation of abnormal texture caused by Fe and reducing the lowering of the high-temperature strength. CONSTITUTION:The objective material is produced by adding 0.01-2wt.% (in terms of W metal) of a W compound to a sintered material containing 5-2,000ppm (in terms of metallic element) of Fe and produced by adding 1-100 pts.wt. of silicon carbide to 100 pts.wt. of a silicon nitride component composed mainly of silicon nitride and containing 0.5-10mol% (in terms of oxide) of a compound of Y or a rare-earth element.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride-silicon carbide composite sintered body having excellent strength at room temperature and high temperature, and more particularly to an improvement for reducing the influence of impurities.

[0002]

2. Description of the Prior Art Sintered silicon nitride is being applied to engineering ceramics, especially structural materials for heat engines, because it has excellent strength. However, it has excellent properties, but its strength decreases at high temperatures. I have a problem. The problem of deterioration of high temperature strength has been improved by changing the weight of the sintering aid, the firing atmosphere, the firing pattern, etc., but the definitive countermeasure has not yet been reached. Met.

On the other hand, the silicon carbide based sintered body has a characteristic that it has a lower strength than the silicon nitride based sintered body, but has almost no strength deterioration at high temperatures. A composite sintered body has been proposed in which silicon carbide is added and fired. Further, since this composite sintered body contains silicon carbide, the sinterability of the system is deteriorated. Therefore, a rare earth element oxide such as Y ₂ O ₃ or Al ₂ O ₃ is added to increase the density. Is being pursued.

[0004]

However, although the above-mentioned composite sintered body can reduce the deterioration of high temperature strength to some extent as compared with the silicon nitride based sintered body, Al ₂ O ₃
Since the high temperature strength tends to decrease in the presence of such low melting point oxides, it is desirable not to add these low melting point oxides.

However, although the high temperature strength can be improved to some extent in such a system to which Al ₂ O ₃ is not added, there is a problem in that the characteristics vary.

Therefore, when the cause of this variation was investigated, it was found that the variation was large especially in the room temperature region, and the fracture source of the sintered body which caused the abnormal breakdown was due to the abnormal structure. In addition, an analysis of this abnormal structure portion revealed that iron (Fe) was present, which caused a eutectic reaction with silicon (Si), and the rare earth elements added around it were excessively collected. I found it.

[0007]

[Means for Solving the Problems] The inventors of the present invention have studied in detail the method of reducing the adverse effect of Fe due to the additive on the above problems, and found that by adding W in a predetermined ratio. The present inventors have found that the abnormal structure that can be a destruction source can be significantly reduced, the abnormal deterioration of the strength can be prevented, and the variation of the characteristics can be reduced, and the present invention has been completed.

That is, the sintered body of the present invention is a silicon nitride component 1 containing silicon nitride as a main component and containing at least Y or a rare earth element in an amount of 0.5 to 10 mol% in terms of oxide.
Silicon carbide is added at a ratio of 1 to 100 parts by weight with respect to 00 parts by weight, and the total amount of Fe is 5 ppm by weight.
~ 2000ppm relative to the sintered body containing W
Is added in an amount of 0.01% by weight to 2% by weight.

The present invention will be described in detail below. In terms of composition, the sintered body of the present invention contains silicon nitride as a main component and further contains at least Y or a rare earth element as a sintering aid in an amount of 0.5 to 10.
Silicon carbide is added at a ratio of 1 to 100 parts by weight with respect to 100 parts by weight of a silicon nitride component contained in mol%, but a raw material of low purity is used, or in a mixing step in a manufacturing process. Fe may be mixed in the system. This Fe uses a high-purity raw material,
By performing the treatment to remove Fe, the amount becomes 5
When the content is very low, such as less than ppm, it does not affect the characteristics of the sintered body. However, if the content is 5 ppm or more, as described above, an abnormal structure that becomes a fracture source in the sintered body is generated. Are formed, thereby deteriorating the characteristics of the sintered body or causing variations in the characteristics.

In the present invention, it is important that W is present in an amount of 0.01 to 2% by weight, particularly 0.05 to 2% by weight in terms of metal, in such a system containing Fe. Is. Due to the presence of W, the behavior of Fe in the sintered body can be fixed. Therefore, if the amount of W is less than 0.01% by weight, the occurrence of abnormal tissue cannot be suppressed and the variation in characteristics becomes large, and if the amount of W exceeds 2% by weight, W itself agglomerates. It becomes a destruction source, and becomes a factor that reduces the strength of the sintered body.

The effect of adding W is not sufficiently exhibited when the amount of Fe becomes too large, and the amount of Fe is 2000.
The upper limit is ppm, especially 1000 ppm.

In the present invention, Y which is a sintering aid is used.
Alternatively, the oxide conversion amount of the rare earth element and the amount of silicon carbide are limited to the above-mentioned ranges, because when the amount of Y or the rare earth element is less than 0.5 mol%, the sinterability is poor and the densification cannot be performed. If it exceeds%, the high temperature strength will decrease.
Further, if the amount of silicon carbide is less than 1 part by weight with respect to the silicon nitride component, the deterioration of high temperature strength becomes remarkable, and if it exceeds 100 parts by weight, it becomes difficult to obtain a dense body.

When the sintered body of the present invention is prepared, silicon nitride powder, silicon carbide powder as a starting material, and at least Y or an oxide powder of a rare earth element as a sintering aid is used. According to the present invention, the compound of W is 0.01 to 2% by weight in terms of W metal based on the system.
Is important to add. Examples of the W compound used include W oxides, carbides, and silicides.

These starting materials are mixed at the above-mentioned predetermined ratios and then molded by a known method. It is conceivable that Fe is mixed in the starting materials in addition to being contained in the starting materials. .. According to the present invention, the amount of Fe in the final molded body is 5 to 200 in terms of metal in consideration of these mixed amounts.
It is necessary to control to 0 ppm. Examples of the molding means include press molding, extrusion molding, injection molding, cast molding, cold isostatic molding and the like.

Next, this compact is fired at 1600 to 2000 ° C. in a non-oxidizing atmosphere such as nitrogen. Examples of the firing method include atmospheric pressure firing, nitrogen gas pressure firing, hot press firing and the like. Further, in the hot isostatic firing method as a special firing method, after densifying to a theoretical density ratio of 95% or more by the above-mentioned method, it is performed at 1600 to 2000 ° C. in an atmosphere containing nitrogen of 1000 to 2000 atm. How to bake,
Further, after applying BN to the surface of the molded body, applying glass powder to the surface, enclosing it in a glass capsule, or burying it in a heat-resistant container filled with glass powder, the high temperature After melting the glass below to form a glass seal, 1600-20 under 1000-2000 atmospheres
A method of firing at 00 ° C. or the like is adopted.

Further, the amount of Y or rare earth element converted to oxide is limited to 0.5 to 10 mol% by 0.5 wt%.
If it is less, a sufficient dense body cannot be obtained, and if it exceeds 10 mol%, mechanical properties such as high temperature strength are deteriorated. As the rare earth element, Er, Y, Sc, Ce, Yb, Dy, Tb,
Ho etc. are mentioned.

Further, according to the present invention, Al ₂ O ₃ , MgO, CaO, AlN, ZrO ₂ , Si is added to the above system.
Addition of O ₂ or the like in a proportion of 10% by weight or less does not affect the effects of the present invention, but among these, Al ₂
If low-melting-point substances such as O ₃ , MgO, and CaO are present to some extent, the adverse effect of Fe is not remarkable, and the high-temperature characteristics are deteriorated. In particular, in the composition system of 0.5% by weight or less, the effect of adding W is remarkable, and the characteristics are desirable.

[0018]

According to the present invention, when Fe is present in the system, silicon nitride and silicon carbide are decomposed during the sintering process, and a eutectic reaction with generated Si occurs. Although the reason is not clear, the rare earth elements are excessively gathered around the Fe-Si eutectic reaction product to form an abnormal structure different from usual.

For these behaviors, if W exists,
Generation of abnormal tissue due to the presence of Fe is reduced. The reason for this is that W produces a compound such as a silicide during the sintering process, and Fe is positively dissolved in this compound, so that the Fe element becomes inactive and the decomposition of silicon nitride is suppressed. It is thought to be done.

As a result, since the fracture starting point of the sintered body is due to the fracture source of the abnormal structure due to the presence of Fe, the surface of the sintered body is always present, so that the characteristics of the sintered body are improved and variations in the characteristics are reduced. A silicon nitride sintered body having stable characteristics can be manufactured.

Further, due to the above addition effect of W, even when a relatively low-purity silicon nitride raw material is used, strength deterioration and characteristic variation can be reduced, so that the cost of the sintered body can be reduced. it can.

[0022]

Example Silicon nitride raw material (oxygen content 1.0%, BET specific surface area 10 m ² / g, α ratio 98% or more, Fe content 30 p
pm) and Y ₂ O ₃ , Yb ₂ O ₃ and Er ₂ as sintering aids.
Each powder of O ₃ , Ho ₂ O ₃ , and Dy ₂ O ₃ was further used, and further, silicon carbide powder (α type, BET specific surface area 19 m ² / g, Fe content 200 ppm) was used, and these powders were used.
After being weighed at the ratio shown in (1), the mixture was placed in a polypot and mixed with a silicon nitride ball for 72 hours using methanol as a solvent. The resulting mixture is dried and then granulated, 80 mm x 45
It was press molded into a size of 5 mm × 5 mm. The Fe content of the obtained molded body was measured by ICP emission spectroscopy. Furthermore, each molded product was placed in an atmosphere of nitrogen gas pressure of 10 atm.
It was fired at the temperature of. For sample No. 20 in the table, BN was applied to the surface of a molded product having the composition shown in Table 1 and baked in a glass bath at a high pressure of 2000 atm at 1600 ° C.

As a result of composition analysis of the obtained sintered body, it was confirmed that the composition of the sintered body did not substantially change from the composition of the molded body. Further, 20 pieces of each test piece were cut out, and 17 pieces of them were measured for bending strength at room temperature by JISR1601 and the minimum value and the maximum value were shown.
Furthermore, the fracture source was examined from the transverse folds, and the number of grains having abnormal grain growth due to iron was examined. Furthermore, 3
The strength of 1400 ° C. was measured on each test piece in the same manner as above. The results are shown in Table 1.

[0024]

[Table 1]

As is apparent from Table 1, impurities were mixed at a Fe content level of several tens of ppm in the production under the above-mentioned conditions. In No. 3, the average strength was small, and most of the fracture sources were due to the abnormal structure of Fe-Si.

On the other hand, when the W compound was added,
Addition of 01% by weight improves the average strength as shown in sample No.4, and as the amount of W is increased, the Fe-Si abnormal structure is reduced as a destruction source, and the variation in characteristics is improved. I understand that it will be done. However, in the sample No. 12, in which the amount of W exceeded 2% by weight, aggregation due to W was observed, which deteriorated the strength.

Further, in order to confirm the effect of adding W, an evaluation was made on a system in which Fe was added to the system to increase the amount of Fe.
The effect of W addition was extremely smaller than that of No. 1, and more Fe-Si abnormal structures became the fracture source, and the average strength decreased.
As a result, it was found that the variation can be improved even when the raw material contains about 2000 ppm of Fe.

Further, when the addition amount of Y ₂ O _{3 in} the sintered body was changed, the densification was not sufficiently achieved in Sample No. 15 in which the addition amount was less than 0.5 mol%, and 10 mol% When it exceeded, deterioration of strength was recognized.

The effect of the present invention can be recognized even when the hot isostatic firing method is used as the firing condition. Further, even if WSi ₂ or WC other than WO ₃ is used as the W compound, other rare earth elements are used. A similar effect was observed in.

Regarding the effect of adding silicon carbide, deterioration of high temperature strength was observed in sample No. 1 in which silicon carbide was not added at all. Sample No. 1 containing excessive silicon carbide
In No. 4, it was not densified and the characteristics were low.

[0031]

As described in detail above, according to the present invention, F
The addition of W to a system containing e suppresses the generation of an abnormal structure by Fe, suppresses the decrease in the high temperature strength of the silicon nitride-silicon carbide composite sintered body, and reduces the variation in the room temperature strength. You can

Claims (1)

[Claims] 1. To 100 parts by weight of a silicon nitride component containing silicon nitride as a main component and containing at least Y or a rare earth element in an amount of 0.5 to 10 mol% in terms of oxide, 1 to 1 part of silicon carbide is used.
0.01 wt% to 2 wt% of W with respect to a sintered body containing 100 parts by weight and Fe as an impurity in a total amount of 5 ppm to 2000 ppm by weight.
A silicon nitride-silicon carbide based composite sintered body characterized by being added and contained at a ratio of.