JPH05148031A - Sintered silicon nitride - Google Patents

Abstract

PURPOSE:To obtain the subject sintered material having high strength and uniform properties while suppressing the formation of abnormal texture caused by Fe, thereby reducing the breaking frequency caused by abnormal texture. CONSTITUTION:The objective material is produced by adding 0.01-2wt.% (in terms of W metal) of a W compound to a system composed mainly of silicon nitride and containing 0.5-10mol% (in terms of oxide) of a compound of Y and/or a rare-earth element and 5-2,000ppm (in terms of metallic element) of Fe as an impurity, molding the mixture and baking the produced molding at 1600-2000 deg.C in a non-oxidizing atmosphere.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength silicon nitride sintered body, and more particularly to an improvement for reducing variations in characteristics.

[0002]

2. Description of the Prior Art Sintered silicon nitride materials have been applied to high temperature materials as various engineering ceramics as materials excellent in high temperature strength.

Since such silicon nitride itself is difficult to sinter, it has hitherto been possible to form oxides of Group 3a elements of the periodic table such as Y ₂ O ₃ and various metal oxides such as Al ₂ O _3. It is added and fired.

Recently, the operating temperature of the silicon nitride sintered body is expanding to a high temperature range of 1400 ° C. or higher, and along with this, improvement of the high temperature characteristics of the sintered body is desired. Therefore,
From the viewpoint that the high temperature characteristics greatly change depending on the composition of the grain boundaries of the silicon nitride sintered body and the melting point of the grain boundaries due to the composition,
It has been proposed to suppress the addition of a low-melting substance such as Al ₂ O ₃ and to add Y ₂ O ₃ or an oxide of a rare earth element, or a sintered body of a system to which silicon oxide or the like is added, and to further crystallize the grain boundary phase. It has been proposed to increase the melting point of the grain boundaries by increasing the melting point.

[0005]

However, although the above-mentioned sintered body has excellent characteristics in bending strength and oxidation resistance as compared with the conventional products, the variation in the characteristics has not been solved yet. It has a drawback that it is difficult to obtain a sintered body having stable properties in terms of productivity.

Therefore, when the cause of this variation was investigated, it was found that an abnormal tissue was present in most of the tissue of the destruction source. Furthermore, 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. ..

[0007]

The inventors of the present invention have made a detailed study on a method for reducing the adverse effect of Fe as an impurity on the above problems. The present inventors have found that the addition of and containing the abnormal structure can remarkably reduce the generation of the abnormal structure, thereby improving the characteristic variation.

That is, the silicon nitride sintered body of the present invention contains silicon nitride as a main component, contains at least 0.5 to 10 mol% of Y or a rare earth element in terms of oxide, and Fe as an impurity in a weight ratio. It is characterized in that W is added and contained in a proportion of 0.01 wt% to 2 wt% with respect to the sintered body contained in a proportion of 5 ppm to 2000 ppm.

The present invention will be described in detail below. The composition of the sintered body of the present invention is mainly composed of silicon nitride, and further contains 0.5 to 10 mo of Y or a rare earth element as a sintering aid.
Although it is contained in an amount of 1%, Fe may be mixed in the system in a raw material having a low purity or in the mixing step in the manufacturing process. The amount of this Fe is 5 p
When the amount is very small, such as pm or less, the characteristics of the sintered body are not particularly affected, but when the amount is mixed in a ratio of 5 ppm or more, it becomes a destruction source in the sintered body as described above. An abnormal structure is formed, which deteriorates the characteristics of the sintered body and causes 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 upper limit of the amount of Fe is 2000 ppm, particularly 1000 ppm in weight ratio.

In the method for producing the sintered body of the present invention, silicon nitride powder is used as a starting material and at least Y or an oxide powder of a rare earth element is used as a sintering aid. Further, according to the present invention, it is important to add the compound of W to such a system in a ratio of 0.01 to 2% by weight in terms of W metal. As the W compound used,
Examples thereof include W oxides, carbides, and silicides.

These starting materials are mixed by the above-mentioned predetermined ratios and then molded by a known method. In addition to Fe being contained in the starting materials, Fe may be mixed in between the molding steps. .. 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, the molded body is fired at 1600 to 2000 ° C. in a non-oxidizing atmosphere such as nitrogen by a firing means such as atmospheric pressure firing, nitrogen gas pressure firing, and hot press firing. 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 method, 1
16 in an atmosphere containing nitrogen of 000 to 2000 atm
A method of firing at 00 to 2000 ° C., or after coating the surface of the molded body with BN, coating the glass powder on the surface, enclosing it in a glass capsule, or filling the inside with glass powder. After burying it in a heat-resistant container and melting the glass at high temperature to form a glass seal, 1000
A method of firing at 1600 to 2000 ° C. under ˜2000 atmospheric pressure is adopted.

In the present invention, the amount of Y or rare earth element converted to oxide is limited to 0.5 to 10 mol%.
If it is less than 0.5 mol%, a sufficiently dense body cannot be obtained, and 1
This is because if it exceeds 0 mol%, mechanical properties such as high temperature strength are deteriorated. As rare earth elements, Er, Y, Sc, C
e, Yb, Dy, Tb, Ho and the like can be 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
Fe in the presence of low melting point substances such as O ₃ , MgO and CaO
Since the deterioration of the characteristics due to the presence of is not remarkable, the constitution of the present invention is effective especially in a composition system in which these low melting point substances are 1% by weight or less, particularly 0.5% by weight or less.

[0017]

According to the present invention, when Fe is present in the system, silicon nitride is decomposed during the sintering process and a eutectic reaction with generated Si occurs. 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,
The above behavior of Fe is reduced. The reason for this is not clear, but W produces a compound such as a silicide in the sintering process, and Fe is positively dissolved in this compound, so that the Fe element becomes inactive and silicon nitride is formed. It is considered that the decomposition of Fe is suppressed and, as a result, the generation of the Fe—Si eutectic reaction product is suppressed.

As a result, since the starting point of fracture of the sintered body is due to 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 are reduced, and stable characteristics are obtained. It is possible to produce a silicon nitride-based sintered body having the same.

Further, due to the above addition effect of W, the characteristics of the sintered body can be stabilized even when an inexpensive silicon nitride raw material having a relatively low purity is used, so that the cost of the sintered body can be reduced. It can also be planned.

[0021]

Example Silicon nitride raw material (oxygen amount 1.0%, BET specific surface area 10 m ² / g, α ratio 98%, Fe content 30 pp
m) and Y ₂ O ₃ , Yb ₂ O ₃ and Sc ₂ O as sintering aids.
₃ , Er ₂ O ₃ , Ho ₂ O ₃ , Dy ₂ O ₃ , CeO ₂ ,
Each powder of Tb ₂ O ₃ was weighed at the ratio shown in Table 1, put 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 to 80 mm x 45 mm x 5 mm
Was press molded. The Fe content of the obtained molded body was measured by ICP emission spectroscopy. Then, each compact was fired at a temperature shown in Table 1 in an atmosphere of nitrogen gas pressure of 10 atm. For sample No. 16 in the table, BN was applied to the surface of the molded body having the composition shown in Table 1 and the BN was applied in a glass bath.
It was fired at a pressure of 2000 atm at 600 ° C.

Elemental analysis of the obtained sintered body confirmed that the composition after sintering was substantially the same as the composition of the molded body. Also, 20 test pieces were prepared from this sintered body.
Each piece was cut out, and the bending strength at room temperature was measured by JISR1601 and the average value was examined. Furthermore, the fracture source was examined from the fracture surface of the transverse rupture piece, and the number of fracture sources of abnormal grain growth due to iron was examined. The results are shown in Table 1.

[0023]

[Table 1]

As is clear from Table 1, in the production under the above-mentioned conditions, in the sample No, 1 in which W was not added at all for the inclusion of impurities at the Fe content level of several tens ppm, the destruction source was Most of them were due to an abnormal structure of Fe-Si, and some of them had a very low strength of 460 MPa.

On the other hand, when the W compound was added,
Addition of 01% by weight reduces the proportion of the sintered body that breaks due to an abnormal structure as shown in sample No. 2, and as the amount of W increases, the average strength of the sintered body recovers and the variation in characteristics is improved. I understand that it will be done. However,
Aggregation by W was observed in the samples No, 10 in which the amount of W exceeded 2% by weight, which deteriorated the strength.

Further, in order to confirm the effect of adding W, evaluation was carried out for a system in which Fe was added to the system to increase the amount of Fe.
The effect of W addition was further reduced, and a decrease in strength was observed.

Further, when the addition amount of Y ₂ O _{3 in} the sintered body was changed, the sample No. 11 in which the addition amount was less than 0.5 mol% did not achieve sufficient densification and 10 mol% When it exceeded, deterioration of strength was recognized.

The effect of the present invention is recognized even when the hot isostatic firing method is used as the firing condition. Further, even when 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.

[0029]

As described in detail above, according to the present invention, F
By adding W to a system containing e, the generation of an abnormal structure due to Fe is suppressed, the destruction due to the abnormal structure is reduced, and it is possible to produce a sintered body having high strength and no variation in characteristics.

Claims (1)

[Claims] 1. A main component of silicon nitride, containing 0.5 to 10 mol% of at least Y or a rare earth element in terms of oxide, and Fe as an impurity in a weight ratio of 5 ppm to 20.
W is 0.
A silicon nitride-based sintered body, characterized by being contained in an amount of 01 wt% to 2 wt%.