JPH0733290B2 - Silicon nitride sintered body and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a silicon nitride sintered body having high strength at high temperature and a method for producing the same.

(Prior Art) Conventionally, as a silicon nitride sintered body to which an oxide of a IIIa-based element including a rare earth oxide is added, for example, in JP-B-48-7486, Si ₃ N ₄ 85 mol% or more and a IIIa-based element are disclosed. Si ₃ N ₄ mixed with at least one 15 mole% or less selected from oxides method for producing a sintered body sintered molded in a non-oxidizing atmosphere and in Japanese Patent Publication No. Sho 49-21091 of Is at least 50
A silicon nitride sintered body is disclosed, each of which comprises 50% by weight or less of at least one selected from oxides of Y ₂ O ₃ or La-based elements, and 0.01 to 20% by weight of Al ₂ O ₃ .

However, simply adding a rare earth element to silicon nitride does not yield a sintered body having high temperature and high strength, and addition of Al ₂ O ₃ promotes densification, but the grain boundary phase has a low softening point and high temperature strength. However, there was a problem in that

In order to solve this problem of high-temperature strength, the applicant of the present invention has disclosed in Japanese Patent Laid-Open No. 63-100067 that a rare earth element having a predetermined composition and a predetermined amount ratio is added to Si ₃ N ₄ powder to obtain a crystalline phase of a sintered body. A technique for identifying high temperature and high strength is disclosed.

(Problems to be Solved by the Invention) The silicon nitride sintered body disclosed in Japanese Patent Application Laid-Open No. 63-100067 can achieve high strength at a high temperature to some extent, but has a problem of lowering the strength at room temperature. It is considered that this is because a slight amount of the glass phase remains in the above composition even if the grain boundaries are crystallized. In this regard, in order to reduce the residual glass phase, when converted to total oxygen content in the silicon nitride raw material in the amount of SiO _2, the residual glass phase by increasing the ratio of the additive rare-earth oxide to SiO ₂ A method of making the composition difficult to achieve is conceivable, but this method has a problem that densification is difficult.

An object of the present invention is to solve the above-mentioned problems and to provide a silicon nitride sintered body capable of obtaining high strength from room temperature to high temperature and a method for producing the same.

(Means for Solving the Problems) The silicon nitride sintered body of the present invention comprises a rare earth element compound containing at least Y and / or Yb in an amount of 2.7 to 10 mol% and the balance Si ₃ N ₄
A sintered body obtained by adding 0.5 to 11 wt% of SiC and 0.5 to 3 wt% of a W compound and / or a Mo compound in an external amount to a composition comprising The grain boundary phase of the Si ₃ N ₄ particles of the body is characterized by a crystalline phase.

In addition, the method for producing a silicon nitride sintered body of the present invention is a rare earth element compound powder containing at least Y and / or Yb 2.7 to 1
0.5 to 11 wt% of SiC powder and 0.5 to 3 wt% of W compound and / or Mo compound powder in external amounts were added to a formulation consisting of 0 mol% and the balance Si ₃ N ₄ powder. It is characterized in that the raw material is mixed and shaped to obtain a compact, and the compact is fired in an N ₂ atmosphere to crystallize the grain boundary phase of the Si ₃ N ₄ particles.

(Operation) In the above-mentioned constitution, SiC and W compound such as WC and / or Mo compound such as Mo ₂ C are added to Si ₃ N ₄ powder containing a predetermined rare earth element compound such as rare earth oxide.
By firing in an N ₂ atmosphere and crystallizing, a sintered body with the grain boundary phase of Si ₃ N ₄ grains as a crystal phase is obtained, and this silicon nitride sintered body achieves high strength from room temperature to high temperature. I found that I could do it.

That is, the addition of the W compound and / or the Mo compound acts together with the rare earth element compound to promote the densification of the sintered body, and mainly has the effect of increasing the room temperature strength, while the addition of SiC densifies the sintered body. And promotes crystallization of grain boundaries and is mainly effective in increasing high temperature strength.
Due to the additive effect of the addition of the compound and the addition of SiC, a high-strength silicon nitride sintered body can be obtained from room temperature to high temperature.

In addition, the addition of the W compound and the Mo compound has an effect of reducing the coloring characteristic of the rare earth element, which tends to occur in the Si ₃ N ₄ sintered body.
Further, coloring with a rare earth element may change the color when exposed to an atmosphere containing oxygen, which is not preferable, but since the Si ₃ N ₄ sintered body has a different color due to the above-described effect, the color change can be ignored. effective. Furthermore, W compound, M
o The addition of the compound has the effect of reducing the color unevenness of the sintered body even with a thick product due to the synergistic effect with the addition of SiC.

The amount of rare earth oxide added is 2.7 to 10 mol%. 2.7 mol%
Below, a liquid phase sufficient for densification cannot be obtained, and at 10 mol% or more, densification becomes difficult even if a SiC, W compound or Mo compound is added. More preferably, it is 4.7-9 mol%. The optimum addition amount of the rare earth oxide usually varies depending on the silicon nitride raw material used, but it may be in the range of 2.7 to 10 mol%. Lu ₂ O ₃ , Tm ₂ O ₃ , and Er ₂ O ₃ may be used as rare earth oxides other than Y ₂ O ₃ and Yb ₂ O ₃ . In addition, the mol% is (molar amount of rare earth oxide) / {(molar amount of rare earth oxide) + (Si ₃ N
₄ mol amount)} was calculated.

It is preferable that the Si ₃ N ₄ raw material has a large α content from the viewpoint of sinterability. The oxygen content is preferably 1 to 3% by weight.

The added amount of SiC is 0.5 to 11% by weight as an externally added amount to the compound of silicon nitride and rare earth oxide. If it is 0.5% by weight or less, sufficient densification effect and crystallization promoting effect cannot be obtained, and if it is 11% by weight or more, SiC hinders densification. More preferably, it is 1 to 5% by weight. The SiC may be of α-type, β-type or relative crystal quality.

The addition amount of the W compound and the Mo compound is 0.5 to 3% by weight as an external amount with respect to the blend of silicon nitride and the rare earth oxide. If it is 0.5% by weight or less, there is no sufficient effect of increasing the room temperature strength. More preferably, it is 1-2% by weight.

In the production method of the present invention, first, a mixture of a raw material powder of silicon nitride, a rare earth oxide, SiC, a W compound and / or a compound of Mo is prepared. Next, the obtained mixture is molded into a predetermined shape to obtain a molded body. Next, the obtained molded body is 1700 to 2100 ° C, preferably 1900 to 2100 ° C in a normal pressure or N ₂ atmosphere.
It is fired at a temperature of 2000 ° C. and crystallized by a temperature decreasing process or a reheat treatment.

As impurities in the raw materials used, especially the cation elements Al, Fe, Mg, C
Since a, Na and K cause reduction in high temperature strength, it is desirable that the content be 0.5% by weight or less. More preferably, it is 0.1% by weight or less. It is better that the particle size of the raw material is smaller in terms of sinterability. 2
It is desirable that the thickness is less than μm. More preferably 1 μm
It is the following.

Regarding crystallization of grain boundaries, since SiC has a grain boundary crystallization promoting effect, sufficient crystallization can be performed during the temperature lowering process if the temperature lowering rate up to 1000 ° C is 100 ° C / min or less. If the crystallization of the grain boundary phase is not sufficient due to a faster cooling rate or due to other reasons, crystallization may be performed by re-heat treatment. Further, reheat treatment may be performed for the purpose of removing residual stress in the sintered body. The reheat treatment is preferably carried out at 1300 ° C to 1500 ° C. Examples of the W compound and the Mo compound include WC and Mo ₂ C, but other W or Mo metals, silicides or oxides may be used.

Further, heat treatment is performed at 1000 to 1500 ° C. in an oxidizing atmosphere to form a surface layer substantially made of a compound of rare earth element-Si—O and SiO _{2 on the} sintered body, so that the strength is further increased.

(Example) Hereinafter, an actual example will be described.

Example 1 Purity 97% by weight, impurity cation elements Al, Fe, Mg, Ca, Na, K total 0.1% by weight or less, oxygen content 2.2% by weight, average particle size
0.6 μm, BET specific surface area 17 m ² / g, α content 0.95 silicon nitride raw material powder, purity 99% by weight, rare earth oxides in Table 1 having an average particle size of 0.3 to 2.0 μm, purity 99% by weight, Average particle size 0.
4 μm, SiC with BET specific surface area of 20 m ² / g, purity 99% by weight, average particle size 0.4 to 3 μm, W compound with BET specific surface area 0.5 to 10 m ² / g or purity 99% by weight, average particle size 0.5 to 4 μm, Mo ₂ C having a BET specific surface area of 0.3 to 10 m ² / g was prepared and pulverized at the ratio shown in Table 1. After that, water is evaporated to granulate to a particle size of 150 μm,
The powder was used for molding. Next, a 50 × 40 × 6 mm compact was prepared and fired under the firing conditions shown in Table 1 to obtain silicon nitride sintered bodies Nos. 1 to 20 of the present invention. In addition, Si added to the sintered body of No. 7
C is α type, No. 11 is amorphous SiC, and other is β type SiC
Is. Further, using the same raw materials, the additives and the mixing ratio thereof shown in Table 1 were mixed, and the same pulverization, granulation and molding were carried out, and the baking was carried out under the baking conditions shown in Table 1. Got The rate of temperature decrease was basically 100 ° C / min up to 1000 ° C. However, No. 9 was heat-treated at 1400 ° C. for 6 hours in nitrogen at a cooling rate of 110 ° C./min.

The relative density, the crystal phase of the grain boundary phase, the four-point bending strength at room temperature and 1400 ° C. of these sintered bodies were measured as follows, and the results are shown in Table 1. The relative density of the sintered body was determined by first measuring the bulk density by the Archimedes method and calculating the ratio between the bulk density and the theoretical density. The theoretical density was calculated from the formulation powder composition and the formulation density. The density of the formulation is Si ₃ N ₄ : 3.2g / c
m ³ , Y ₂ O ₃ : 4.8g / cm ³ , Yb ₂ O ₃ : 9.2g / cm ³ , Tm ₂ O ₃ : 8.8g / c
m ³ , Lu ₂ O ₃ : 9.4g / cm ³ , Er ₂ O ₃ : 8.6g / cm ³ , SiC: 3.2g / cm ³ , W
^{C: 15.7g / cm 3, Mo} 2 C: 9.2g / cm 3, W: 19.2g / cm 3, Mo: 10.2g / cm
³ , WO ₃ : 7.2g / cm ³ , MoO ₃ : 4.7g / cm ³ , WSi ₂ : 9.4g / cm ³ , MoS
i ₂ : 6.3 g / cm ³ was used. Four-point bending strength is JIS R-1601
The measurement was performed according to the "bending strength test method for fine ceramics". The grain boundary crystal phase was obtained from the results of X-ray diffraction using CuK α-rays. In Table 1, J is a caspidin structure crystal, H is an apatite structure crystal, K is a wollastonite structure crystal, and L is a crystal. Is Ln ₂ SiO ₅ (Ln: rare earth element), S is Ln ₂ Si ₂
It is a crystal represented by O ₇ .

The result of X-ray diffraction of the silicon nitride sintered body No. 6 of the present invention is shown in FIG. Of these, a is β-Si ₃ N ₄ , b is the J phase, which is a crystal with a caspidin structure, c is an H phase, which is a crystal with an apatite structure, d is WSi ₂ , and e is β-SiC. The results of the chemical analysis of the sintered body agreed with those calculated from the addition ratio.

As is clear from Table 1, rare earth oxide + SiC + (W, M
o Compound) sintered bodies Nos. 1 to 20 have high strength at room temperature and 1400 ° C. On the other hand, No. 21, 24 of addition of SiC only
~ 26 or No.22,23,27 ~ 32 with only (W, Mo compound) added
Is low intensity.

Example 2 The same raw materials as in Example 1 were used and blended and pulverized in the proportions shown in Table 2. After that, water is evaporated to granulate to a particle size of 150 μm,
The powder was used for molding. Next, a 50 × 40 × 6 mm molded body was produced and fired under the firing conditions shown in Table 2 to obtain silicon nitride sintered bodies No. 33 to 51 of the present invention. No. 48 to 51 are 1000 respectively
C. × 5 hrs, 1200 ° C. × 2 hrs, 1300 ° C. × 1 hr, 1500 ° C. × 1 hr Heat treatment was performed in an oxidizing atmosphere. In addition, using the same raw materials, the additives shown in Table 2 and the mixing ratio thereof were mixed, and the same raw materials were pulverized, granulated, molded, and fired under the firing conditions shown in Table 2. Got Note that SiC was β type. The temperature decreasing rate up to 1000 ° C was 100 ° C / min.

Bulk density of these sintered bodies, crystal phase of grain boundary phase, room temperature and
Four-point bending strength at 1400 ° C. was measured by the same method as in Example 1, and the results are shown in Table 2.

From Table 2, it can be seen that the effect of adding WC or Mo ₂ C is an increase in room temperature strength in comparison with Example No. 34 and Comparative Example No. 52, and Example No. 45 with Comparative Example No. 52, respectively. Recognize. SiC
The effect of addition is that Example No. 34 and Comparative Example No. 53 or No. 5
It can be seen from the comparison with 4 that the strength increase is 1400 ° C.

(Effects of the Invention) As is clear from the above description, according to the silicon nitride sintered body of the present invention and the method for manufacturing the same, a predetermined amount of a predetermined rare earth element is contained in the Si ₃ N ₄ powder. Since SiC and W compounds and / or Mo compounds are added and fired in an N ₂ atmosphere, the grain boundary phase of Si ₃ N ₄ particles can be made into a crystalline phase, and high-strength nitriding from room temperature to high temperature is possible. A silicon sintered body can be obtained.

[Brief description of drawings]

FIG. 1 is a graph showing the result of X-ray diffraction of the silicon nitride sintered body No. 6 of the present invention.

Claims (2)

[Claims] 1. A composition comprising 2.7 to 10 mol% of a rare earth element compound containing at least Y and / or Yb and the balance of Si ₃ N ₄ is added in an amount of 0.5 to 11 wt% of SiC and an external amount of 0.5. ~ 3wt
% W compound and / or Mo compound, wherein the grain boundary phase of Si ₃ N ₄ particles of the sintered body is a crystalline phase. body. 2. A SiC powder of 0.5 to 11 wt% as an external distribution and an external distribution to a formulation composed of 2.7 to 10 mol% of a rare earth element compound powder containing at least Y and / or Yb and the balance of Si ₃ N ₄ powder. After mixing raw materials obtained by adding W compound and / or Mo compound powder in an amount of 0.5 to 3 wt% and molding to obtain a molded body,
A method for producing a silicon nitride sintered body, which comprises firing a compact in an N ₂ atmosphere to crystallize a grain boundary phase of Si ₃ N ₄ particles.