JP2793635B2 - Silicon nitride powder composition - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a silicon nitride powder composition capable of producing a sintered body having high toughness and high bending strength at high temperatures. Silicon nitride is used as a high-temperature structural material in gas turbine members, nozzles, bearings, and the like.

[Conventional technology]

Conventionally, methods for producing silicon nitride powder include (1) metal silicon direct nitriding, (2) silica reduction nitriding, and (3)
The silicon halide method is known. Powder produced by these methods, even if the production history is different or the amount of metal impurities, the amount of oxygen or the particle size, and the specific surface area are almost the same,
There is a great difference in the sinterability of the powder and the characteristics of the sintered body after sintering, for example, the bending strength and the fracture toughness value.

Generally, the powder produced by the method (1) is easily sinterable but has a low high-temperature bending strength.
It is said that the powder of the method (3), which is difficult to sinter but has high high-temperature bending strength, exhibits intermediate performance.

Regarding the oxygen content, the powder of the method (1) generally undergoes a pulverizing step, so that the total oxygen content usually exceeds 2% by weight, and at least 1.5% by weight. When a process such as acid treatment is performed to remove impurities in the method (1), the total amount of oxygen is reduced, but it is still difficult to reduce the amount to less than 1.0% by weight. On the other hand, even in the powder of the method (2), since silica powder is used as a raw material, silica remains and the total oxygen content is 2% by weight.
It is common to exceed.

These powders are currently available. Naturally, the sinterability and the characteristics of the sintered body of the powder are affected not only by the amount of oxygen in the powder, but also by various other factors such as specific surface area, crystallinity, particle shape, and particle size (fine powder). At present, it is hardly known how the powder characteristics of each of the above-mentioned production methods relate to the characteristics of the sintered body.

As for the toughness and high-temperature strength of the sintered body, generally, the higher the toughness, the lower the high-temperature strength, and the higher the high-temperature strength, the lower the toughness. Until now, a sintered body of silicon nitride having high toughness and high-temperature strength has not been obtained. As to toughness enhancement, whisker enhancement by adding silicon carbide whiskers and the like, particle dispersion enhancement by adding dispersed particles, and the like are known. For example, in JP-A-64-33076, ceramic whiskers such as SiC having different coefficients of thermal expansion are mixed into silicon nitride to improve the toughness of the sintered body. But,
In the case of whisker reinforcement, there are problems such as dispersibility and orientation,
It is difficult to produce a large-sized sintered body, and it is very expensive in terms of cost. In addition, in recent years, problems such as carcinogenicity have been pointed out for SiC whiskers.

[Problems to be solved by the invention]

The high-temperature strength and the fracture toughness of silicon nitride sintered bodies have contradictory parts, and it is very difficult to obtain a sintered body in which both are excellent.

An object of the present invention is to provide a silicon nitride powder suitable for producing a silicon nitride sintered body that has solved the above-mentioned problems, and by producing the same powder by a metal silicon direct nitridation method, An object of the present invention is to provide an inexpensive method as compared with a powder produced by a production method such as a silicon halide method.

[Means for solving the problem]

That is, the present invention provides an oxygen content obtained from metal silicon powder as a raw material of 0.6% by weight or less and an average particle size of 0.3 to 0.8 μm.
m powder, consisting mainly of silicon nitride and Cr ₃ C ₂ , Cr ₂ N, T
A silicon nitride powder composition comprising one or more components selected from iB ₂ , TiN, NbB ₂ and TaN.

 Hereinafter, the present invention will be described in more detail.

The oxygen content in the silicon nitride powder composition of the present invention is 0.
6% by weight or less. If the oxygen content exceeds 0.6% by weight, the α → β transition that occurs during sintering is more likely to occur at low temperatures, and the amount of the grain boundary phase formed by the sintering aid increases. The nature increases. As a result, the number of β nuclei increases, and β
It becomes difficult to obtain columnar crystals, and it becomes impossible to obtain a sintered body having excellent high-temperature strength and toughness.

The average particle size is very important especially when normal-pressure sintering is employed, and it is generally said that it is preferable that the average particle size be small. The average particle size of the silicon nitride powder composition of the present invention,
It is in the range of 0.3 to 0.8 μm. If it exceeds 0.8 μm, the solubility of silicon nitride in the composite oxide caused by the reaction of sintering aids such as yttrium oxide, aluminum oxide, magnesium oxide, etc. with oxygen contained in the silicon nitride powder composition will decrease, and this will be sufficient. Will not be dense. On the other hand, 0.
If the thickness is less than 3 μm, the solubility of silicon nitride in the grain boundary phase formed by the sintering aid increases, and as a result, the number of β nuclei increases and a sufficiently grown β columnar crystal with a high aspect ratio can be obtained. Can not be done.

In general, the amount of oxygen in the silicon nitride is reduced, in a typical sintering aid _{Y 2 O 3, Al 2 O} 3 system, SH Hanpushiya and K.
H. Jacques, SHHampshire, KHJack, Proc. Of Brit. Ceram. Soc., Vol. 31, pp. 37-49 (198
1)] As described above, sintering becomes difficult because a sufficient amount of liquid phase cannot be obtained. In other words, sintering is performed by adding a sintering aid such as MgO, which has a phase boundary reaction rate-determining effect and generally has an effect of accelerating diffusion. In this case, the sinterability is improved, but the β columnar crystal having a high aspect ratio as described above still has a problem.

In contrast, in the present invention, silicon nitride and Cr ₃ C ₂ , Cr ₂ N, T
By combining at the time of synthesis one or more components selected from iB ₂ , TiN, NbB ₂ and TaN (hereinafter referred to as “subcomponents”), the composition has a low oxygen content. Instead, it can be sufficiently densified even in a general sintering aid Y ₂ O ₃ or Al ₂ O ₃ system. That is, the auxiliary component in the silicon nitride powder composition dissolves in the grain boundary phase formed by the added sintering aid during sintering, changes the composition of the grain boundary phase, and dissolves the silicon nitride in the grain boundary phase. And promotes densification. In addition, such a grain boundary phase simultaneously delays the α → β transition accompanying sintering of silicon nitride to a high temperature side, and as a result, β columnar crystals having a large aspect ratio developed in the sintered body are generated, High temperature strength and toughness in the properties of the sintered body can be improved. The proportion of the subcomponent with respect to 100 parts by weight of silicon nitride is preferably 1 to 10 parts by weight.

To manufacture the silicon nitride powder composition of the present invention, after the powder of the metallic silicon powder and subcomponents were mixed ball mill, a V-type mixer or the like, and formed into bulk density of about 0.8 g / cm ^3, a nitrogen, Nitriding is performed in an atmosphere of ammonia, hydrogen, or the like at a temperature of about 1300 to 1450 ° C. to synthesize an ingot. The obtained ingot is subjected to a conventional method, for example, coarse or medium crushing, and then wet or dry crushed with a ball mill, a vibration mill, a jet mill, an attritor mill or the like to obtain a silicon nitride powder composition. As for the particle size of the powder, the above-mentioned average particle size should be taken into consideration, and the powder should be treated under appropriate conditions including a pulverizer.

A major feature of the use of the silicon nitride powder composition of the present invention is that a large number of β columnar crystals having a large aspect ratio generated by the α → β transition are observed. That is, the size of the β columnar crystal is heterogeneous, but the heterogeneity is, for example, several tens μm.
It is homogeneous in the unit area, and its structure plays an important role in the development of high-temperature strength.

In general, the discovery of high-temperature strength mainly focuses on strengthening the grain boundary phase, for example, selecting a sintering aid capable of synthesizing a high-melting grain boundary phase, crystallization of vitreous, etc. Was the approach. Therefore, the present inventors have conducted various experiments on powder side approaches, for example, from powders having different oxygen, powders having different specific surface areas, powders having different crystallinity, etc. under a constant sintering aid. As a result, in the metal silicon direct nitriding method,
The composition of the grain boundary phase is changed by adding a secondary component to the metal silicon powder and nitriding, and the thickness of the β columnar crystal in the sintered body,
The aspect ratio is controlled and the ridgeline of the β columnar crystal is sharp β columnar crystal, in other words, β crystal with high crystallinity
The toughness of the sintered body is achieved by precipitating columnar crystals and strengthening the grain boundary phase itself. More specifically, in the field of view of the (30 μm) × (25 μm) sintered body, if the average aspect ratio of the five β columnar crystals, which are considered to have a large aspect ratio, is 10 or more, the high-temperature strength develops 250 MPa or more per unit density. The present invention has been found to be possible, and if it can be sintered to a value close to the theoretical density, expression of 700 MPa or more at a temperature of 1200 ° C. is sufficiently possible.

〔Example〕

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

Examples 1 to 10 and Comparative Examples 1 to 5 100 parts by weight of metal silicon powder (purity: 99.98% by weight) were mixed with an auxiliary component (commercially available) as shown in Table 1, and 0.5 kg of the mixed powder was added to the mixture. After molding into a 150 x 150 x 20t (mm) shape, the mixed atmosphere consisting of nitrogen and ammonia
Nitriding was performed at 1450 ° C. The obtained ingot was subjected to coarse pulverization, medium pulverization, wet pulverization and acid treatment to obtain a silicon nitride powder composition. The powder properties are shown in Table 1.

Next, 5 parts by weight of Y ₂ O _{3 having} an average particle size of 1.5 μm and Al ₂ O ₃ having an average particle size of 0.8 μm were added to the silicon nitride powder composition as a sintering aid.
Was added 2 parts by weight, 1,1,1-trichloroethane was wet-mixed in a 4-hour ball mill added, at a molding pressure of dried ^{100kg / cm 2 6 × 10 ×} 60mm shape and molding ,, 2700 kg it / cm ²
CIP molding at a molding pressure of These compacts were set in a carbon crucible and fired at 1800 ° C. for 4 hours in an N ₂ gas atmosphere to obtain a sintered body. After grinding, the obtained sintered body was subjected to relative density, fracture toughness (K _IC ), room temperature (σ _RT ) and 1200 ° C.
The three-point bending strength at (σ1200) was measured. Table 1 shows the results.

When the obtained sintered body was observed by SEM, Examples 1 to 10
Was a structure showing a grown β columnar crystal having a large aspect ratio. FIG. 1 and FIG. 2 show SEM photographs of Example 1 and Comparative Example 1 as one example.

The measured values shown in Table 1 were obtained by the following method.

(1) Oxygen (% by weight): Using a TC-136 O / N simultaneous analyzer manufactured by LECO.

(2) Specific surface area (m ² / g): determined by a one-point method of Cantersorb Jr. BET manufactured by Yuasa Ionics.

(3) Average particle size (μm): CAPA-700 manufactured by Dig Seisakusho
by.

(4) Relative density (%): by Archimedes' method.

(5) Three-point flexural strength (MPa): Autograph AG-2000A manufactured by Shimadzu Corporation.

(6) Fracture toughness (MPa · m ^1/2 ): According to IM method.

〔The invention's effect〕

The silicon nitride powder composition of the present invention is low in oxygen and excellent in sinterability, and can obtain a sintered body excellent in high-temperature strength and toughness.

This is because Cr ₃ C ₂ , Cr ₂ N, TiB ₂ ,
TiN, acts subcomponents NbB ₂ and TaN are in the grain boundary phase is a result of controlling the generation and Growth of β columnar crystals of silicon nitride.

[Brief description of the drawings]

FIGS. 1 and 2 show the structures of the particle structures of the silicon nitride sintered bodies obtained in Example 1 and Comparative Example 1, respectively.
It is a SEM photograph.

────────────────────────────────────────────────── (5) References JP-A-63-185865 (JP, A) JP-A-62-235260 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C04B 35/584-35/587 C01B 21/068

Claims (1)

(57) [Claims] 1. A powder having an oxygen content of 0.6% by weight or less and an average particle diameter of 0.3 to 0.8 μm obtained from a metal silicon powder as a raw material, comprising silicon nitride as a main component, Cr ₃ C ₂ , Cr ₂ N, TiB ₂ , TiN, Nb
B ₂ and one or more silicon nitride powder composition characterized by comprising a component selected from among TaN.